*4.1.1. Shrinkage of GeO2<Ge-NCs>/SiNxOy bilayer coatings under ns PLA*

confinement effects, the efficient band gap value decreases with an increase of nanoparticle size, tending at large nanoparticle sizes to the bandgap value of bulk α-Ge. The *k*(*λ*) data for all examined films were plotted in the Tauc coordinates, with the square root of absorbance coefficient *α* being plotted along the vertical axis. This coefficient was obtained from the extinction coefficient by formula *αj*(*E*)=2*kj*(*E*)/*λ*; here, the parameter *j* is the number of a studied film in Fig. 8. Using the well-known formula for the absorption coefficient of indirect-band semiconductors, **α ½** ~ (*E - Eg*), and interpolation of the linear dependences of **α½** (*E- Eg*) to (*E- Eg*) → 0, we were able to evaluate the effective optical gap *Ej geff* in the GeO(solid) films and in the GeO**2**<Ge-NCs> heterolayers formed upon decomposition of the films. The variation of the effective optical gap in GeO(solid) films with the growth of annealing temperature T**an** is shown in Fig. 9. For comparative analysis of obtained *Ej geff*– values and for analysis of Ge-nanocluster sizes, the values of *Ej geff* for amorphous bulk germanium and for GeO**2**<Ge-NCs> heterolayers are also shown in Fig. 9, curves 5 and 6. For the effective optical *gap E5geff* in GeO**2**<Ge-NCs> heterolayers, a value of 1,15 eV was found, and the mean size of Ge NCs in the heterolayers proved to be ~5 -6 nm (according to TEM data). Since the value of *Ej geff* in annealed GeO(solid) films always remained appreciably higher than 1,15 eV, it can be concluded that a-Ge nanoparticles in those films were smaller than nanocrystal sizes in GeO**2**<Ge-NCs> heterolayers. These data have also allowed us to evaluate the sizes of a-Ge nanoparticles in other examined films.

**Figure 8.** Spectral dependences of optical constant (n(λ) and k(λ)) in the various films: 1, 2, 3, 4 – asdeposited GeO(solid) film and films annealed at 260, 290, and 320 oC, 5 – LP CVD GeO2<Ge-NC> heterolayer, 6 - thermal GeO2 film grown on a single-crystal Ge(111) substrate.

GeO2 Films with Ge-Nanoclusters in Layered Compositions: Structural Modifications with Laser Pulses 405

this spectral region where the maximum density-of-state value of optical vibrations in a-Ge falls. In the films under study, amorphous germanium is normally contained in the form of

The decrease of film thickness and increase of film material density after PLA as manifested in the Raman spectra of Si/GeO2<Ge-NCs>/SiNxOy films is accompanied with the growth of the intensity of the peak at 520 cm-1 due to light scattering by long-wave optical phonons

**Figure 10. a** - AFM image illustrating the shrinkage of the GeO2<Ge-NCs>**/**SiNxOy bilayer system in region 2 (see Fig. 7 b) after PLA of the sample with ns laser pulses (fluence E0=150 mJ/cm2); **b**  measurements of the shrinkage of the bilayer film in the region treated with one laser pulse, **с** – the same for two and four laser pulses. The arrows ↓, ↓↓ and ↓↓↓↓ indicate regions subjected to the single,

**Figure 11.** Effect of the number of laser pulses, laser fluence, and thickness of GeO2<Ge-NCs>

heterolayer on the shrinkage value of the Si/GeO2<Ge-NCs>/SiNxOy bilayer system.

double and four-time exposures.

nanoparticles dispersed in glassy GeO2 matrix (Knoss, 2008).

**Figure 9.** The effective optical bandgap E*effg* versus annealing temperature: **1** – in as-deposited GeO(solid) films (with Ge-NCs sized 0,2 - 0,4 nm) and after subsequent anneals (**2** - 260 oC, 6 min; **3** - 290oC, 4 min, **4** – 320oC, 4 min); **5** – E*effg* for a GeO**2**<Ge-NCs> heterolayer with 5,5-nm Ge NCs; **6** - E*effg* for bulk Ge.

According to AFM data, on a sample area irradiated with one laser pulse the shrinkage degree of the film was ~5 - 9%; this value increased up to ~23 - 34% under four laser pulses (Figs. 10 and 11). The cause for the film shrinkage was the low initial density of the film conditioned by the presence of many defects in the atomic network of GeO(solid). Similar data were also obtained for GeOx layers covered by a cap SiO2 layer (on a silicon substrate) in which the predominant part of the GeO(solid) component has already decayed during the growth of those layers. Notable film shrinkage is usually observed on anneals in loose films grown by CVD at low temperatures. Normally, thermal treatments modify the structure of CVD films and improve their quality. Anneals of multilayer structures activate both the shrinkage processes in individual layers of multilayer films and the chemical reactions proceeding among neighboring layers (Knoss, 2008). The high defect content alleviates mass transfer processes in the layers and rearrangement of their structure.

#### *4.1.2. Changes in the structure and properties of thin bilayer coatings under ns PLA*

Raman spectroscopy and microprobe IR-spectroscopy were used to analyze the structure of germanium inclusions in examined films. The non-destructive Raman method combined with calculations is a very informative tool for nano-object studies. The presence of Raman peaks at 301,5 and 520 cm-1 in the spectra is an indication of c-Si and c-Ge micro-inclusions present in the layers in which light is scattered by long-wave lattice phonons. It is in this way that micro- and nanoparticles are usually detected in the films of these materials. By the way, if the crystals are small-sized (~nm), then the spectral position of related Raman peaks exhibits a red shift in comparison with the spectral position of the peaks due to bulk c-Si and c-Ge. According to the peak shift value, one can determine the mean size of semiconductor nanocrystals in the films of interest using the phonon localization method (Volodin et al., 2005). The greater is the Raman shift of the peaks toward smaller wavenumbers in comparison with the position of the peaks in bulk c-Ge (301,5 cm-1), the smaller is the typical size of Ge nanocrystals in the films (Fig. 12). Broad scattering bands peaking at 275 - 280 cm-1, which are due to the presence of amorphous germanium, are also clearly seen in the Raman spectra of examined GeO(solid) films and GeO2<Ge-NCs> heterolayers since it is into this spectral region where the maximum density-of-state value of optical vibrations in a-Ge falls. In the films under study, amorphous germanium is normally contained in the form of nanoparticles dispersed in glassy GeO2 matrix (Knoss, 2008).

404 Laser Pulses – Theory, Technology, and Applications

bulk Ge.

**Figure 9.** The effective optical bandgap E*effg* versus annealing temperature: **1** – in as-deposited GeO(solid) films (with Ge-NCs sized 0,2 - 0,4 nm) and after subsequent anneals (**2** - 260 oC, 6 min; **3** - 290oC, 4 min, **4** – 320oC, 4 min); **5** – E*effg* for a GeO**2**<Ge-NCs> heterolayer with 5,5-nm Ge NCs; **6** - E*effg* for

**GeO(s)**

**1**

**1,0 1,5 2,0 2,5**

**Eg, eV**

transfer processes in the layers and rearrangement of their structure.

*4.1.2. Changes in the structure and properties of thin bilayer coatings under ns PLA*

Raman spectroscopy and microprobe IR-spectroscopy were used to analyze the structure of germanium inclusions in examined films. The non-destructive Raman method combined with calculations is a very informative tool for nano-object studies. The presence of Raman peaks at 301,5 and 520 cm-1 in the spectra is an indication of c-Si and c-Ge micro-inclusions present in the layers in which light is scattered by long-wave lattice phonons. It is in this way that micro- and nanoparticles are usually detected in the films of these materials. By the way, if the crystals are small-sized (~nm), then the spectral position of related Raman peaks exhibits a red shift in comparison with the spectral position of the peaks due to bulk c-Si and c-Ge. According to the peak shift value, one can determine the mean size of semiconductor nanocrystals in the films of interest using the phonon localization method (Volodin et al., 2005). The greater is the Raman shift of the peaks toward smaller wavenumbers in comparison with the position of the peaks in bulk c-Ge (301,5 cm-1), the smaller is the typical size of Ge nanocrystals in the films (Fig. 12). Broad scattering bands peaking at 275 - 280 cm-1, which are due to the presence of amorphous germanium, are also clearly seen in the Raman spectra of examined GeO(solid) films and GeO2<Ge-NCs> heterolayers since it is into

According to AFM data, on a sample area irradiated with one laser pulse the shrinkage degree of the film was ~5 - 9%; this value increased up to ~23 - 34% under four laser pulses (Figs. 10 and 11). The cause for the film shrinkage was the low initial density of the film conditioned by the presence of many defects in the atomic network of GeO(solid). Similar data were also obtained for GeOx layers covered by a cap SiO2 layer (on a silicon substrate) in which the predominant part of the GeO(solid) component has already decayed during the growth of those layers. Notable film shrinkage is usually observed on anneals in loose films grown by CVD at low temperatures. Normally, thermal treatments modify the structure of CVD films and improve their quality. Anneals of multilayer structures activate both the shrinkage processes in individual layers of multilayer films and the chemical reactions proceeding among neighboring layers (Knoss, 2008). The high defect content alleviates mass

**0 100 200 300 0,5**

**Ge-NCs size 5,5 nm**

**bulk Ge**

**Temperature, o**

**3**

**C**

**5 6**

**4**

**2**

The decrease of film thickness and increase of film material density after PLA as manifested in the Raman spectra of Si/GeO2<Ge-NCs>/SiNxOy films is accompanied with the growth of the intensity of the peak at 520 cm-1 due to light scattering by long-wave optical phonons

**Figure 10. a** - AFM image illustrating the shrinkage of the GeO2<Ge-NCs>**/**SiNxOy bilayer system in region 2 (see Fig. 7 b) after PLA of the sample with ns laser pulses (fluence E0=150 mJ/cm2); **b**  measurements of the shrinkage of the bilayer film in the region treated with one laser pulse, **с** – the same for two and four laser pulses. The arrows ↓, ↓↓ and ↓↓↓↓ indicate regions subjected to the single, double and four-time exposures.

**Figure 11.** Effect of the number of laser pulses, laser fluence, and thickness of GeO2<Ge-NCs> heterolayer on the shrinkage value of the Si/GeO2<Ge-NCs>/SiNxOy bilayer system.

in the Si substrate (Fig. 12). This effect is manifested as an increase of the transparency of the bilayer film for the excitation laser radiation in Raman measurements (514,5 nm Ar+ laser) falling onto, and reflected by, the substrate. The cause for the shrinkage effect are PLAinduced changes of optical constants of the films in the Si/GeO2<Ge-NCs>/SiNxOy bilayer system, as optical constants of Si substrate remained essentially unaltered in the spectral region around the excitation wavelength. Let us consider now possible reasons for such changes. Some minor increase in the refractive index of a very thin SiNxOy film due to its shrinkage and compaction cannot be the factor appreciably affecting the optical constants of the bilayer system. The absorption index of the film in this spectral region is initially very small, and the irradiation of the sample with laser radiation could not considerably increase its value. Hence, the increase in transparency of the bilayer system was most probably due to the change in the optical constants of the GeO2<Ge-NCs> heterolayer or, alternatively, it could be a result of the chemical interaction of the two layers in the interfacial region of the structure.

GeO2 Films with Ge-Nanoclusters in Layered Compositions: Structural Modifications with Laser Pulses 407

heterolayers, amorphous germanium phase usually forms a big amount of amorphous Ge nanoparticles growing in size during high-energy treatments of the system, and then such nanoparticles undergo crystallisation without interrupting their continuous growth. The growth of Ge nanoparticles proceeds primarily due to decay of nanocluster fractions with minimal sizes. The formation processes of α-Ge and c-Ge nanoparticles are also maintained by the decay of the atomic network of metastable GeO(solid), which supplies the

According to micro-IR-spectroscopy data (Fig. 13), the atomic network of the insulator matrix also undergoes modifications that proceed simultaneously with the transformation processes of germanium inclusions in the GeO2<Ge-NCs> heterostructure. Yet, the process turned out different than the expected formation of pure glassy GeO2 by reaction (3) during anneals of GeO(solid) films (Fig. 4 b). In the heterolayer region treated with one laser pulse the IR absorption band starts moving towards longer wavelengths instead of showing the expected trend to the maximum at 860-870 cm-1. On the other hand, the maximum IR absorption intensity in the region treated with two laser pulses decreases markedly, and the whole band notably widens. In the bilayer structure under study, in addition to the formation of GeO2 during decomposition of metastable GeO(solid), the GeO2<Ge-NCs> heterolayer and the SiNxOy film can react with each other in the interfacial region. The GeO2 matrix of the heterolayer, and the Si3N4 and SiO2 materials forming the cap SiNxOy layer, are involved in this reaction. In particular, the products of the reaction between GeO2 and Si3N4 proceeding at T>6500C (3GeO2 + Si3N4 → 3SiO2+Ge3N4), SiO2 and Ge3N4, can be dissolved by glassy GeO2 matrix. As described above, these processes finally yield a GeO2:SiO2:Ge3N4→ k:p:q glassy compound. Since pure Ge3N4 and glassy SiO2 exhibit IR absorption peaks respectively at about 770 cm-1 and 1070 cm-1, it can be expected that the peaks due to the Ge3N4:GeO2 and GeO2:SiO2 double glasses will tend to shift in opposite directions from the peak due to pure GeO2; i.e. the first peak will shift towards longer wavelengths and the second peak, towards shorter wavelengths. On the whole, the material of the GeO2 matrix is expended on the formation of both SiO2 and Ge3N4, and also on the formation of the threecomponent GeO2:SiO2:Ge3N4 → k:p:q compound. As a result, a predominant part of the cap SiNxOy layer may turn into Ge3N4 and SiO2 that will subsequently undergo dissolution in the upper part of the GeO2<Ge-NCs> heterolayer. Such transformation will be capable of reaching depths comparable with the thickness of the initial cap layer. This may become the reason for the observed modification of the IR absorption band in the irradiated film, including reduced absorption at the band maximum, shift of the band, and its strong

Of course, the properties of the upper layer in the initial bilayer structure will undergo dramatic changes. In particular, the optical constants and the optical bandgap will strongly change in this layer. Dissolution of wide-bandgap glassy SiO2 (*EgSIO2* ~ 9 eV) in the matrix of the GeO2<Ge-NCs> heterolayer will radically change the energy-band characteristics of the heterosystem. A similar behavior is also demonstrated by glass-like Ge3N4, which is close to GeO2 in terms of optical gap, *EgGe3N4* ~ 5-5,5 eV, yet differs from GeO2 in its high refractive index (1,90-2,00) and permittivity (~9-11). On the contrary, the refractive index of SiO2 (1,45)

is lower than that of glassy GeO2 (1,61-1,63) (Gorokhov, 2005).

nanoparticles with atomic germanium.

widening (Fig. 13).

Indeed, micro-Raman and micro-IR-spectroscopy data showed that such processes indeed proceed in the film system under study. Raman spectra taken from local areas of the film system irradiated with one, two, or four laser pulses (see Fig. 12 a) were measured with excitation laser beam focused to a ~1-µm diameter spot on the sample surface. From the registered changes in Raman spectra, we were able to judge the rate of release of Ge atoms during decomposition of the GeO(solid) structure, and also the formation of amorphous (α-Ge) and, then, crystalline (c-Ge) phases of Ge. The formation kinetics of α-Ge and c-Ge phases within the heterolayer is characterized by initially the highest amorphous germanium content of the layer system and by almost complete absence of c-Ge phase from it (see the initial spectrum in Fig. 12 a). After one laser pulse, the fraction due to α-Ge component sharply decreased in the system, and c-Ge phase emerged in quite large quantities. Two and four laser pulses gave rise to an increased amount of c-Ge phase at a comparatively slow regeneration of α-Ge component. During annealing in these

**Figure 12. a** - Raman spectra illustrating the influence of the number of ns laser pulses given to a local region on the sample (region 2 in Fig.7 b, E0 =150 mJ/cm2) on the structure of the GeO2<Ge-NCs> heterolayer in the Si/GeO2<Ge-NCs>**/**SiNxOy bilayer system; **b** – magnified fragment of the same spectra.

heterolayers, amorphous germanium phase usually forms a big amount of amorphous Ge nanoparticles growing in size during high-energy treatments of the system, and then such nanoparticles undergo crystallisation without interrupting their continuous growth. The growth of Ge nanoparticles proceeds primarily due to decay of nanocluster fractions with minimal sizes. The formation processes of α-Ge and c-Ge nanoparticles are also maintained by the decay of the atomic network of metastable GeO(solid), which supplies the nanoparticles with atomic germanium.

406 Laser Pulses – Theory, Technology, and Applications

structure.

in the Si substrate (Fig. 12). This effect is manifested as an increase of the transparency of the bilayer film for the excitation laser radiation in Raman measurements (514,5 nm Ar+ laser) falling onto, and reflected by, the substrate. The cause for the shrinkage effect are PLAinduced changes of optical constants of the films in the Si/GeO2<Ge-NCs>/SiNxOy bilayer system, as optical constants of Si substrate remained essentially unaltered in the spectral region around the excitation wavelength. Let us consider now possible reasons for such changes. Some minor increase in the refractive index of a very thin SiNxOy film due to its shrinkage and compaction cannot be the factor appreciably affecting the optical constants of the bilayer system. The absorption index of the film in this spectral region is initially very small, and the irradiation of the sample with laser radiation could not considerably increase its value. Hence, the increase in transparency of the bilayer system was most probably due to the change in the optical constants of the GeO2<Ge-NCs> heterolayer or, alternatively, it could be a result of the chemical interaction of the two layers in the interfacial region of the

Indeed, micro-Raman and micro-IR-spectroscopy data showed that such processes indeed proceed in the film system under study. Raman spectra taken from local areas of the film system irradiated with one, two, or four laser pulses (see Fig. 12 a) were measured with excitation laser beam focused to a ~1-µm diameter spot on the sample surface. From the registered changes in Raman spectra, we were able to judge the rate of release of Ge atoms during decomposition of the GeO(solid) structure, and also the formation of amorphous (α-Ge) and, then, crystalline (c-Ge) phases of Ge. The formation kinetics of α-Ge and c-Ge phases within the heterolayer is characterized by initially the highest amorphous germanium content of the layer system and by almost complete absence of c-Ge phase from it (see the initial spectrum in Fig. 12 a). After one laser pulse, the fraction due to α-Ge component sharply decreased in the system, and c-Ge phase emerged in quite large quantities. Two and four laser pulses gave rise to an increased amount of c-Ge phase at a comparatively slow regeneration of α-Ge component. During annealing in these

**Figure 12. a** - Raman spectra illustrating the influence of the number of ns laser pulses given to a local region on the sample (region 2 in Fig.7 b, E0 =150 mJ/cm2) on the structure of the GeO2<Ge-NCs> heterolayer in the Si/GeO2<Ge-NCs>**/**SiNxOy bilayer system; **b** – magnified fragment of the same spectra. According to micro-IR-spectroscopy data (Fig. 13), the atomic network of the insulator matrix also undergoes modifications that proceed simultaneously with the transformation processes of germanium inclusions in the GeO2<Ge-NCs> heterostructure. Yet, the process turned out different than the expected formation of pure glassy GeO2 by reaction (3) during anneals of GeO(solid) films (Fig. 4 b). In the heterolayer region treated with one laser pulse the IR absorption band starts moving towards longer wavelengths instead of showing the expected trend to the maximum at 860-870 cm-1. On the other hand, the maximum IR absorption intensity in the region treated with two laser pulses decreases markedly, and the whole band notably widens. In the bilayer structure under study, in addition to the formation of GeO2 during decomposition of metastable GeO(solid), the GeO2<Ge-NCs> heterolayer and the SiNxOy film can react with each other in the interfacial region. The GeO2 matrix of the heterolayer, and the Si3N4 and SiO2 materials forming the cap SiNxOy layer, are involved in this reaction. In particular, the products of the reaction between GeO2 and Si3N4 proceeding at T>6500C (3GeO2 + Si3N4 → 3SiO2+Ge3N4), SiO2 and Ge3N4, can be dissolved by glassy GeO2 matrix. As described above, these processes finally yield a GeO2:SiO2:Ge3N4→ k:p:q glassy compound. Since pure Ge3N4 and glassy SiO2 exhibit IR absorption peaks respectively at about 770 cm-1 and 1070 cm-1, it can be expected that the peaks due to the Ge3N4:GeO2 and GeO2:SiO2 double glasses will tend to shift in opposite directions from the peak due to pure GeO2; i.e. the first peak will shift towards longer wavelengths and the second peak, towards shorter wavelengths. On the whole, the material of the GeO2 matrix is expended on the formation of both SiO2 and Ge3N4, and also on the formation of the threecomponent GeO2:SiO2:Ge3N4 → k:p:q compound. As a result, a predominant part of the cap SiNxOy layer may turn into Ge3N4 and SiO2 that will subsequently undergo dissolution in the upper part of the GeO2<Ge-NCs> heterolayer. Such transformation will be capable of reaching depths comparable with the thickness of the initial cap layer. This may become the reason for the observed modification of the IR absorption band in the irradiated film, including reduced absorption at the band maximum, shift of the band, and its strong widening (Fig. 13).

Of course, the properties of the upper layer in the initial bilayer structure will undergo dramatic changes. In particular, the optical constants and the optical bandgap will strongly change in this layer. Dissolution of wide-bandgap glassy SiO2 (*EgSIO2* ~ 9 eV) in the matrix of the GeO2<Ge-NCs> heterolayer will radically change the energy-band characteristics of the heterosystem. A similar behavior is also demonstrated by glass-like Ge3N4, which is close to GeO2 in terms of optical gap, *EgGe3N4* ~ 5-5,5 eV, yet differs from GeO2 in its high refractive index (1,90-2,00) and permittivity (~9-11). On the contrary, the refractive index of SiO2 (1,45) is lower than that of glassy GeO2 (1,61-1,63) (Gorokhov, 2005).

GeO2 Films with Ge-Nanoclusters in Layered Compositions: Structural Modifications with Laser Pulses 409

Note that all the discussed processes can be realized using traditional heat treatments at T≥ 4000C. Yet, the duration of such heat treatments is typically greater than one minute, whereas, in our case, many atoms simultaneously participated in several structural-chemical rearrangement processes for only several tens of ns. To gain an insight into the whole picture of involved processes, let us focus on their most important features. First, AFM data on the shrinkage of the layered structure and IR-spectroscopy data on the modifications of its chemical composition reveal a gradual increase in the rate of structural-chemical transformations in the system from the first to last laser pulse (Fig. 11). Second, the dynamics of irradiation-induced changes in the optical properties of the bilayer film shows a distinct saturation. Those changes first occur rapidly and, then, their rate sharply decreases. The latter is evident from changes in the Raman spectra of the Si/GeO2<Ge-NCs>/SiNxOy system observed 275 to 300 cm-1 (Fig. 12 a) at the beginning and at the end ot its irradiation with a sequence of ns laser pulses. During the first pulse, the input energy was absorbed more readily by the bilayer film in comparison with subsequent pulses since the changes in the Raman spectra due to the first pulse were manifested more distinctly in comparison with the changes induced by the second pulse and, the more so, by the two subsequent pulse. The successive growth of the intensity of the Raman peak (520 cm-1) due light scattering by the lattice of Si substrate for Ar+ laser excitation (Fig. 12) also agrees with the general tendency in pulse-by-pulse modification of film-structure properties although, here, a more steady growth of peak intensity is observed. The effect is explained by an increase of bilayer film transparency, or reduced absorption of laser radiation by the film in this spectral region). A common feature for all pulses was that a predominant part of their energy dissipating in GeO2<Ge-NCs> was spent on rearrangement of its lattice structure that

proceeded mainly during the time intervals between the pulses.

An unusual thing here is that, with each subsequent radiation pulse, the Si/GeO2<Ge-NCs>/SiNxOy hetrosystem becomes more and more transparent to the probing Ramanspectrometer radiation. Initially, we expected quite a contrary thing, namely, an increase in the absorption of Ar+ laser beam radiation by the GeO2<Ge-NCs> heterolayer. Such an increase was expected to be a result of the considerable amount of free Ge atoms released in the heterolayer bulk. Ge atoms become the material for nucleation and growth of many Genanoparticles, which was to be manifested as an increased absorption coefficient of the heterolayer in the visible spectral region. For instance, such a growth of the absorption coefficient was observed during successive anneals of GeO2<Ge-NCs> heterolayers and GeO(solid) and GeOx films (Fig. 8). Similarly, during growth of silicon nanoparticles in the bulk of non-stochiometric SiOx and SixNy(H) films under chemical and laser treatments no enhanced transparency of such films was observed in the visible spectral region (Korchagina et al., 2012; Rinnert et al., 2001; Volodin et al., 2010). However, it proved to be a very difficult task to reveal the true nature of such an unusual effect in the experiments with the bilayer system under study, as in PLA-treated samples several different chemical and structural processes proceed simultaneously in the bulk of the GeO2<Ge-NCs> heterolayer and at its boundaries. It was required to simplify the complex of such structural and compositional modifications in the films. Therefore, we first undertook an analysis of material

**Figure 13.** Micro IR transmittance spectra illustrating the influence of the number of ns laser pulses given to a local region on the sample (region 2 in Fig.7b, E0 =150 mJ/cm2) on the structure of the dielectric in the Si/GeO2<Ge-NCs>**/**SiNxOy bilayer system.

The degree of compositional changes in the upper part of the layer will depend on material viscosity. In the described process, this degree will grow from one to the other irradiation pulse as regions with altered chemical composition will spread away from the interface across both films of the bilayer system. In turn, the compaction of the material in each layer will more and more impede the diffusion process in the intermixing region of the bifilm. Formation of fine cracks at the periphery of the squares irradiated with laser pulses serves an indication for the growth of viscosity in the upper part of the bilayer structure. The formation process of such cracks is defined by two factors: (i) growth of the internal tensile stress in the film due to material compaction (the shrinkage of the film in the direction normal to the film plane does not completely eliminate the lateral internal tensile stress in the film) and (ii) growth of viscosity in the compacted layer due to increase of strength characteristics of the material proceeding with simultaneous vanishing of defects from the atomic network and with modification of the chemical composition of the layer (Gorokhov et al., 1998). As a result, quite a thick intermediate layer (up to ~50-80 nm), in which the insulator matrix has a variable chemical composition and altered structural, mechanical, physical and other material properties, forms during the reactions at the interface between the GeO2<Ge-NCs> heterolayer and the SiNxOy film. There are no Ge nanoparticles in the upper part of this layer, but the concentration of such nanoparticles gradually increases as we approach the pure GeO2<Ge-NCs> heterolayer. The optical constants of the upper layer, as well as its optical gap value, also vary in a complex manner over the thickness of the layer. But, since glassy SiO2, a very broad-band material, presents one of the composite-glass components, the optical gap in the mixture will be everywhere wider than the optical gap in the regions where only narrow-band glassy components, having an optical gap not wider than 5 - 5,5 eV, are present. Hence, in the course of PLA treatments of the samples with KrF excimer laser pulses the upper multi-component layer will become increasingly more transparent to the excitation beam of the Ar+ laser (514,5 nm) in comparison with the bulk of the lower GeO2<Ge-NCs> heterolayer.

dielectric in the Si/GeO2<Ge-NCs>**/**SiNxOy bilayer system.

the lower GeO2<Ge-NCs> heterolayer.

**Figure 13.** Micro IR transmittance spectra illustrating the influence of the number of ns laser pulses given to a local region on the sample (region 2 in Fig.7b, E0 =150 mJ/cm2) on the structure of the

The degree of compositional changes in the upper part of the layer will depend on material viscosity. In the described process, this degree will grow from one to the other irradiation pulse as regions with altered chemical composition will spread away from the interface across both films of the bilayer system. In turn, the compaction of the material in each layer will more and more impede the diffusion process in the intermixing region of the bifilm. Formation of fine cracks at the periphery of the squares irradiated with laser pulses serves an indication for the growth of viscosity in the upper part of the bilayer structure. The formation process of such cracks is defined by two factors: (i) growth of the internal tensile stress in the film due to material compaction (the shrinkage of the film in the direction normal to the film plane does not completely eliminate the lateral internal tensile stress in the film) and (ii) growth of viscosity in the compacted layer due to increase of strength characteristics of the material proceeding with simultaneous vanishing of defects from the atomic network and with modification of the chemical composition of the layer (Gorokhov et al., 1998). As a result, quite a thick intermediate layer (up to ~50-80 nm), in which the insulator matrix has a variable chemical composition and altered structural, mechanical, physical and other material properties, forms during the reactions at the interface between the GeO2<Ge-NCs> heterolayer and the SiNxOy film. There are no Ge nanoparticles in the upper part of this layer, but the concentration of such nanoparticles gradually increases as we approach the pure GeO2<Ge-NCs> heterolayer. The optical constants of the upper layer, as well as its optical gap value, also vary in a complex manner over the thickness of the layer. But, since glassy SiO2, a very broad-band material, presents one of the composite-glass components, the optical gap in the mixture will be everywhere wider than the optical gap in the regions where only narrow-band glassy components, having an optical gap not wider than 5 - 5,5 eV, are present. Hence, in the course of PLA treatments of the samples with KrF excimer laser pulses the upper multi-component layer will become increasingly more transparent to the excitation beam of the Ar+ laser (514,5 nm) in comparison with the bulk of Note that all the discussed processes can be realized using traditional heat treatments at T≥ 4000C. Yet, the duration of such heat treatments is typically greater than one minute, whereas, in our case, many atoms simultaneously participated in several structural-chemical rearrangement processes for only several tens of ns. To gain an insight into the whole picture of involved processes, let us focus on their most important features. First, AFM data on the shrinkage of the layered structure and IR-spectroscopy data on the modifications of its chemical composition reveal a gradual increase in the rate of structural-chemical transformations in the system from the first to last laser pulse (Fig. 11). Second, the dynamics of irradiation-induced changes in the optical properties of the bilayer film shows a distinct saturation. Those changes first occur rapidly and, then, their rate sharply decreases. The latter is evident from changes in the Raman spectra of the Si/GeO2<Ge-NCs>/SiNxOy system observed 275 to 300 cm-1 (Fig. 12 a) at the beginning and at the end ot its irradiation with a sequence of ns laser pulses. During the first pulse, the input energy was absorbed more readily by the bilayer film in comparison with subsequent pulses since the changes in the Raman spectra due to the first pulse were manifested more distinctly in comparison with the changes induced by the second pulse and, the more so, by the two subsequent pulse. The successive growth of the intensity of the Raman peak (520 cm-1) due light scattering by the lattice of Si substrate for Ar+ laser excitation (Fig. 12) also agrees with the general tendency in pulse-by-pulse modification of film-structure properties although, here, a more steady growth of peak intensity is observed. The effect is explained by an increase of bilayer film transparency, or reduced absorption of laser radiation by the film in this spectral region). A common feature for all pulses was that a predominant part of their energy dissipating in GeO2<Ge-NCs> was spent on rearrangement of its lattice structure that proceeded mainly during the time intervals between the pulses.

An unusual thing here is that, with each subsequent radiation pulse, the Si/GeO2<Ge-NCs>/SiNxOy hetrosystem becomes more and more transparent to the probing Ramanspectrometer radiation. Initially, we expected quite a contrary thing, namely, an increase in the absorption of Ar+ laser beam radiation by the GeO2<Ge-NCs> heterolayer. Such an increase was expected to be a result of the considerable amount of free Ge atoms released in the heterolayer bulk. Ge atoms become the material for nucleation and growth of many Genanoparticles, which was to be manifested as an increased absorption coefficient of the heterolayer in the visible spectral region. For instance, such a growth of the absorption coefficient was observed during successive anneals of GeO2<Ge-NCs> heterolayers and GeO(solid) and GeOx films (Fig. 8). Similarly, during growth of silicon nanoparticles in the bulk of non-stochiometric SiOx and SixNy(H) films under chemical and laser treatments no enhanced transparency of such films was observed in the visible spectral region (Korchagina et al., 2012; Rinnert et al., 2001; Volodin et al., 2010). However, it proved to be a very difficult task to reveal the true nature of such an unusual effect in the experiments with the bilayer system under study, as in PLA-treated samples several different chemical and structural processes proceed simultaneously in the bulk of the GeO2<Ge-NCs> heterolayer and at its boundaries. It was required to simplify the complex of such structural and compositional modifications in the films. Therefore, we first undertook an analysis of material transformations proceeding during traditional heat treatments in a GeO(solid) film on Si substrate without cap layer (ISP SB RAS) and in a GeOx film on Si substrate protected by a cap layer formed from sputtered SiO2 (~100 nm) (Nancy University, France).

GeO2 Films with Ge-Nanoclusters in Layered Compositions: Structural Modifications with Laser Pulses 411

Note that the spectral position of the Raman peak due to Ge-nanocrystals in the GeOx film is no difference from the spectral position of the peak in bulk Ge crystals (301 cm-1). The latter finding is indicative of rather large mean sizes of Ge nanocrystals (~8-10 nm and

**Figure 14. a** –Raman spectra of GeO(solid) films of different thicknesses after decomposition of GeO(solid) into GeO2 and Ge nanoclusters and oxidation of the nanoclusters: **1** – Si substrate, **2** and **3** thick and thin areas of the GeO(solid) film, **4** - thick area of the GeO(solid) film where Ge nanoclusters

Thus, the experiments with GeO(solid) and GeOx films showed that the modification processes of their structures proceeding during traditional anneals are qualitatively similar to PLA-induced processes. All such processes (film shrinkage and densification, all decomposition stages of the metastable GeO(solid) atomic network, formation of a-Ge nanoparticles and their oxidation and crystallisation) do not make the layers more transparent for Ar+ laser excitation radiation. Among all the above-considered physicochemical processes in the analyzed one- and bilayer compositions, only reactions of the glassy GeO2 matrix in GeO2<Ge-NCs> heterolayers with Si3N4 cap layers could lead to such an effect. Besides, the decrease of the Raman-spectrometer beam absorption in the Si/GeO2<Ge-NCs>/SiNxOy system with increasing the number of ns KrF excimer laser pulses given to the sample (Fig. 12) could be explained assuming that a-Ge nanoclusters formed by short laser pulses in the GeO2<Ge-NCs> heterolayer were substantially smaller than a-Ge nanoclusters formed during traditional thermal treatments. Thus, the whole mass of free Ge atoms leaving the decaying atomic lattice of GeO(solid) during irradiations formed a

The latter is quite natural a situation under conditions of abrupt heating of the heterolayer not only with laser impulses, but also with the energy released during decay of the unstable atomic network of GeO(solid) undergoing decomposition during the discussed treeatments. However, due to the short width of laser pulses, the length of possible diffusion of free Ge toward Ge nanoclusters does not exceed 2-3 cluster diameters, as nanoclusters are separated with distances close to the mean radius of nanoclusters in studied heterolayers. Hence, in

were oxidized by the Ar + laser; **b** – magnified fragments of the same spectra.

greater) formed in this film during anneals.

multitude of very small c-Ge nanoparticles.

A GeO(solid) layer of uniform thickness 61 nm was characterized with the follows values of optical constants: n=1,86 and k=0 (obtained from many-angle ellipsometric measurements using an He-Ne laser with λ=632,8 nm at beam incidence angles 450, 500, 550, 600, 650, and 700). A broad band peaking at 280 cm-1 due to Ge nanoparticles was observed in the Raman spectra of the as-deposited film (Fig. 14). The film was thin and its decomposition just began; therefore the amplitude of this Raman band was very low, especially at the sample edges (spectrum 3) where the film was less heated by the evaporator during the film deposition process. The highest degree of decomposition of the GeO(solid) film was observed at the centre of the sample (spectrum 2). Note that 4-minute exposition of a local sample area to the excitation laser beam focused to a ~1-µm diameter spot on the film surface at the centre of the sample caused a strong reduction in the beam scattering by a-Ge nanoparticles (spectrum 4). This effect was due to photo-stimulated oxidation of the particles by air oxygen in the locally heated surface area. Here, the concentration of a-Ge nanoparticles decreased in the film; yet, the film thickness grew in value due to the formation of an additional amount of germanium dioxide in the film. A comparison of the Raman spectrum of Si substrate without a film (spectrum 1) with the other spectra in Fig. 14 shows that deposition of a GeO(solid) layer onto the film surface (spectrum 3), growth of the fraction of a-Ge in the film in the form of nanoparticles (spectrum 2), and transformation of part of a-Ge nanoparticles into glassy GeO2, i.e. increase in the volume of heterolayer matrix (spectrum 4) lead to multiply decreased amplitude of the peak at 520 cm-1 in the Raman spectrum of the Si/GeO(solid) system, this peak being due to light scattering by long-wave optical phonons in the Si substrate. The latter finding can be attributed to enhanced absorption of Ar+ laser radiation in the GeO(solid) film for all these cases.

The cap layer prevents evaporation of the GeOx layer in Si/GeOx/SiO2 structures (with layer thicknesses 100 nm/100 nm) annealed at temperatures T>4500C. As a result, the initial metastable atomic network of GeO(solid) in such structures undergoes decomposition with the emission of all the excessive germanium and its accumulation in a-Ga nanoparticles. On increasing the annealing temperature, such a-Ga nanoparticles transform in Ge nanocrystals. These processes can be easily monitored using Raman scattering measurements (see Fig. 15). The temperatures used to anneal the Si/GeOx/SiO2 sample were insufficient for the onset of a reaction between the GeO2 matrix in the decomposed GeOx film and the SiO2 cap layer; this sample was found to also not absorb the Ar+ laser radiation and undergo no heating due to the wide optical gap of the material. Hence, all the changes proceeding with the Raman scattering peak due to light scattering by phonons in Si substrate bear no relation with modification processes in the structure of the GeOx layer. On the other hand, it is seen that the process of germanium release in the GeOx film proceeding with the formation of a-Ge nanoparticles and their subsequent crystallisation successively reduces the amplitude of the peak due to Raman scattering in Si substrate. The same result was also observed in deposition of the initial GeOx layer, like in the case of GeO(solid) layer synthesis (see Fig.14). Note that the spectral position of the Raman peak due to Ge-nanocrystals in the GeOx film is no difference from the spectral position of the peak in bulk Ge crystals (301 cm-1). The latter finding is indicative of rather large mean sizes of Ge nanocrystals (~8-10 nm and greater) formed in this film during anneals.

410 Laser Pulses – Theory, Technology, and Applications

transformations proceeding during traditional heat treatments in a GeO(solid) film on Si substrate without cap layer (ISP SB RAS) and in a GeOx film on Si substrate protected by a

A GeO(solid) layer of uniform thickness 61 nm was characterized with the follows values of optical constants: n=1,86 and k=0 (obtained from many-angle ellipsometric measurements using an He-Ne laser with λ=632,8 nm at beam incidence angles 450, 500, 550, 600, 650, and 700). A broad band peaking at 280 cm-1 due to Ge nanoparticles was observed in the Raman spectra of the as-deposited film (Fig. 14). The film was thin and its decomposition just began; therefore the amplitude of this Raman band was very low, especially at the sample edges (spectrum 3) where the film was less heated by the evaporator during the film deposition process. The highest degree of decomposition of the GeO(solid) film was observed at the centre of the sample (spectrum 2). Note that 4-minute exposition of a local sample area to the excitation laser beam focused to a ~1-µm diameter spot on the film surface at the centre of the sample caused a strong reduction in the beam scattering by a-Ge nanoparticles (spectrum 4). This effect was due to photo-stimulated oxidation of the particles by air oxygen in the locally heated surface area. Here, the concentration of a-Ge nanoparticles decreased in the film; yet, the film thickness grew in value due to the formation of an additional amount of germanium dioxide in the film. A comparison of the Raman spectrum of Si substrate without a film (spectrum 1) with the other spectra in Fig. 14 shows that deposition of a GeO(solid) layer onto the film surface (spectrum 3), growth of the fraction of a-Ge in the film in the form of nanoparticles (spectrum 2), and transformation of part of a-Ge nanoparticles into glassy GeO2, i.e. increase in the volume of heterolayer matrix (spectrum 4) lead to multiply decreased amplitude of the peak at 520 cm-1 in the Raman spectrum of the Si/GeO(solid) system, this peak being due to light scattering by long-wave optical phonons in the Si substrate. The latter finding can be attributed to enhanced

cap layer formed from sputtered SiO2 (~100 nm) (Nancy University, France).

absorption of Ar+ laser radiation in the GeO(solid) film for all these cases.

The cap layer prevents evaporation of the GeOx layer in Si/GeOx/SiO2 structures (with layer thicknesses 100 nm/100 nm) annealed at temperatures T>4500C. As a result, the initial metastable atomic network of GeO(solid) in such structures undergoes decomposition with the emission of all the excessive germanium and its accumulation in a-Ga nanoparticles. On increasing the annealing temperature, such a-Ga nanoparticles transform in Ge nanocrystals. These processes can be easily monitored using Raman scattering measurements (see Fig. 15). The temperatures used to anneal the Si/GeOx/SiO2 sample were insufficient for the onset of a reaction between the GeO2 matrix in the decomposed GeOx film and the SiO2 cap layer; this sample was found to also not absorb the Ar+ laser radiation and undergo no heating due to the wide optical gap of the material. Hence, all the changes proceeding with the Raman scattering peak due to light scattering by phonons in Si substrate bear no relation with modification processes in the structure of the GeOx layer. On the other hand, it is seen that the process of germanium release in the GeOx film proceeding with the formation of a-Ge nanoparticles and their subsequent crystallisation successively reduces the amplitude of the peak due to Raman scattering in Si substrate. The same result was also observed in deposition of the initial GeOx layer, like in the case of GeO(solid) layer synthesis (see Fig.14).

**Figure 14. a** –Raman spectra of GeO(solid) films of different thicknesses after decomposition of GeO(solid) into GeO2 and Ge nanoclusters and oxidation of the nanoclusters: **1** – Si substrate, **2** and **3** thick and thin areas of the GeO(solid) film, **4** - thick area of the GeO(solid) film where Ge nanoclusters were oxidized by the Ar + laser; **b** – magnified fragments of the same spectra.

Thus, the experiments with GeO(solid) and GeOx films showed that the modification processes of their structures proceeding during traditional anneals are qualitatively similar to PLA-induced processes. All such processes (film shrinkage and densification, all decomposition stages of the metastable GeO(solid) atomic network, formation of a-Ge nanoparticles and their oxidation and crystallisation) do not make the layers more transparent for Ar+ laser excitation radiation. Among all the above-considered physicochemical processes in the analyzed one- and bilayer compositions, only reactions of the glassy GeO2 matrix in GeO2<Ge-NCs> heterolayers with Si3N4 cap layers could lead to such an effect. Besides, the decrease of the Raman-spectrometer beam absorption in the Si/GeO2<Ge-NCs>/SiNxOy system with increasing the number of ns KrF excimer laser pulses given to the sample (Fig. 12) could be explained assuming that a-Ge nanoclusters formed by short laser pulses in the GeO2<Ge-NCs> heterolayer were substantially smaller than a-Ge nanoclusters formed during traditional thermal treatments. Thus, the whole mass of free Ge atoms leaving the decaying atomic lattice of GeO(solid) during irradiations formed a multitude of very small c-Ge nanoparticles.

The latter is quite natural a situation under conditions of abrupt heating of the heterolayer not only with laser impulses, but also with the energy released during decay of the unstable atomic network of GeO(solid) undergoing decomposition during the discussed treeatments. However, due to the short width of laser pulses, the length of possible diffusion of free Ge toward Ge nanoclusters does not exceed 2-3 cluster diameters, as nanoclusters are separated with distances close to the mean radius of nanoclusters in studied heterolayers. Hence, in

small-sized a-Ge nanoparticles, due to the 3D quantum confinement effect, the effective optical gap will be substantially wider than that of larger nanocrystals formed during traditional anneals and, hence, their radiation absorption effect will not be so high as that of the larger Ge nanoparticles.

GeO2 Films with Ge-Nanoclusters in Layered Compositions: Structural Modifications with Laser Pulses 413

crystallization of the nanoparticles (Gallas et al., 2002). In such cases, the silica films undergo shrinkage. Here, all processes proceed like in GeO2<Ge-NCs> heterolayers. However, unlike in the latter case, in PLA-treated GeO2<Ge-NCs> heterolayers there is no growth of transparency for Ar+ laser radiation; on the contrary, according to (Korchagina et al., 2012), the absorption of 514,5-nm radiation increases. Today, the reason for the different behaviors demonstrated by the GeO2<Ge-NCs> and SiO2<Si-NCs> systems under PLA treatments remains unclear. In this effect, which depends on many factors, the contribution due to each

**Figure 16.** The spectral position of the Raman peak due to Ge nanocrystals versus nanocrystal size in GeO2<Ge-NCs> heterolayers: ● – data of (Nelin & Nilsson, 1972), ▲ - data calculated by the model of effective density of folded vibration states (Volodin et al., 2005). Using the Raman spectra in Fig. 12, one can determine the sizes of Ge NCs size formed in local areas of the GeO2<Ge-NCs>/ SiNxOy bilayer

Finally, we would like to emphasize here that irradiation of samples with 25-ns KrF excimer laser pulses (λ=248 nm) produces a very important effect on GeO2<Ge-NCs> heterolayers, namely, its allows one to decrease both the sizes and size dispersion of Ge nanoparticles formed in metastable GeO(solid) and GeOx layers. This process proceeds in the strongly densifying GeO2<Ge-NCs> heterolayer that consists of 70% vol. of GeO2 matrix, which, during the same irradiation treatment, is capable of controllably changing its chemical

Differences in laser radiation parameters, and also differences in other irradiation conditions, strongly affect the material modification process and its ultimate results. The

composition, structure, and physical, optical and electrophysical properties.

**4.2. Femtosecond treatments of samples with Ti-Sapphire laser**

system treated with one, two and four ns laser pulses.

factor changes under PLA differently in Ge- and Si-based heterosystems.

The sensitivity of our Raman spectrometer has allowed us to reveal in our experiments specific features directly defined by the sizes of Si and Ge nanocrystals in thin-film materials (Volodin et al., 2005). In large Ge nanocrystals, the spectral position of the Raman peak is at 301,5 cm-1, with its blue shift becoming notable at nanocrystal sizes < 6-7 nm. Data on the spectral position of the Raman peaks due to Ge nanocrystals hosted in a GeO2 matrix as calculated from two theoretical models (data of (Nelin & Nilsson, 1972) and data calculated from the model of effective density of folded vibration states (Volodin et al., 2005)) are shown in Fig. 16. Raman spectra of irradiated GeO2<Ge-NCs> heterostructures measured in the spectral region around the wavenumber 301,5 cm-1 are shown in Fig. 12 b. In these spectra, a red shift of Raman peaks, typical of Ge NCs, is clearly observed. According to model calculations, the sizes of Ge NCs increase from 1,4 – 1,8 nm to 1,8 – 2,3 nm (Fig. 16)

**Figure 15. a** - Raman spectra of a SiO2/GeOx/Si (100/100 nm) structure illustrating the precipitation of germanium in GeOx layers during anneals, and also the formation of a-Ge nanoparticles and their subsequent crystallisation: **1** - as deposited film; **2** and **3** – films annealed at 300 oC for 40 min and at 480 oC, 3 min + 530 oC, 1 min, respectively; **b** – the same spectra at a larger magnification.

with increasing the number of ns KrF excimer laser pulses given to local regions on the sample surface. This result confirms the previous assumption on the reason for transparency enhancement for Ar+ laser radiation in GeO2<Ge-NCs> heterolayers irradiated with 25-ns KrF excimer laser pulses (λ=248 nm).

Note that analogous treatments of non-stochiometric silicon-rich SiOx and SixNy(H) glassy films with high-energy KrF excimer laser pulses *(λ*=248 nm, *tp*=25 ns) activate a complex structure rearrangement process which begins with the emission of excessive Si atoms from the atomic network of the material due to breakage of Si-O bonds (in SiOx films) or Si-H and Si-N bonds (in SixNy(H) films). The process ends with the formation of many a-Si nanoparticles in the film bulk from released Si atoms followed with subsequent crystallization of the nanoparticles (Gallas et al., 2002). In such cases, the silica films undergo shrinkage. Here, all processes proceed like in GeO2<Ge-NCs> heterolayers. However, unlike in the latter case, in PLA-treated GeO2<Ge-NCs> heterolayers there is no growth of transparency for Ar+ laser radiation; on the contrary, according to (Korchagina et al., 2012), the absorption of 514,5-nm radiation increases. Today, the reason for the different behaviors demonstrated by the GeO2<Ge-NCs> and SiO2<Si-NCs> systems under PLA treatments remains unclear. In this effect, which depends on many factors, the contribution due to each factor changes under PLA differently in Ge- and Si-based heterosystems.

412 Laser Pulses – Theory, Technology, and Applications

the larger Ge nanoparticles.

small-sized a-Ge nanoparticles, due to the 3D quantum confinement effect, the effective optical gap will be substantially wider than that of larger nanocrystals formed during traditional anneals and, hence, their radiation absorption effect will not be so high as that of

The sensitivity of our Raman spectrometer has allowed us to reveal in our experiments specific features directly defined by the sizes of Si and Ge nanocrystals in thin-film materials (Volodin et al., 2005). In large Ge nanocrystals, the spectral position of the Raman peak is at 301,5 cm-1, with its blue shift becoming notable at nanocrystal sizes < 6-7 nm. Data on the spectral position of the Raman peaks due to Ge nanocrystals hosted in a GeO2 matrix as calculated from two theoretical models (data of (Nelin & Nilsson, 1972) and data calculated from the model of effective density of folded vibration states (Volodin et al., 2005)) are shown in Fig. 16. Raman spectra of irradiated GeO2<Ge-NCs> heterostructures measured in the spectral region around the wavenumber 301,5 cm-1 are shown in Fig. 12 b. In these spectra, a red shift of Raman peaks, typical of Ge NCs, is clearly observed. According to model

calculations, the sizes of Ge NCs increase from 1,4 – 1,8 nm to 1,8 – 2,3 nm (Fig. 16)

**Figure 15. a** - Raman spectra of a SiO2/GeOx/Si (100/100 nm) structure illustrating the precipitation of germanium in GeOx layers during anneals, and also the formation of a-Ge nanoparticles and their subsequent crystallisation: **1** - as deposited film; **2** and **3** – films annealed at 300 oC for 40 min and at 480

with increasing the number of ns KrF excimer laser pulses given to local regions on the sample surface. This result confirms the previous assumption on the reason for transparency enhancement for Ar+ laser radiation in GeO2<Ge-NCs> heterolayers irradiated with 25-ns

Note that analogous treatments of non-stochiometric silicon-rich SiOx and SixNy(H) glassy films with high-energy KrF excimer laser pulses *(λ*=248 nm, *tp*=25 ns) activate a complex structure rearrangement process which begins with the emission of excessive Si atoms from the atomic network of the material due to breakage of Si-O bonds (in SiOx films) or Si-H and Si-N bonds (in SixNy(H) films). The process ends with the formation of many a-Si nanoparticles in the film bulk from released Si atoms followed with subsequent

oC, 3 min + 530 oC, 1 min, respectively; **b** – the same spectra at a larger magnification.

KrF excimer laser pulses (λ=248 nm).

**Figure 16.** The spectral position of the Raman peak due to Ge nanocrystals versus nanocrystal size in GeO2<Ge-NCs> heterolayers: ● – data of (Nelin & Nilsson, 1972), ▲ - data calculated by the model of effective density of folded vibration states (Volodin et al., 2005). Using the Raman spectra in Fig. 12, one can determine the sizes of Ge NCs size formed in local areas of the GeO2<Ge-NCs>/ SiNxOy bilayer system treated with one, two and four ns laser pulses.

Finally, we would like to emphasize here that irradiation of samples with 25-ns KrF excimer laser pulses (λ=248 nm) produces a very important effect on GeO2<Ge-NCs> heterolayers, namely, its allows one to decrease both the sizes and size dispersion of Ge nanoparticles formed in metastable GeO(solid) and GeOx layers. This process proceeds in the strongly densifying GeO2<Ge-NCs> heterolayer that consists of 70% vol. of GeO2 matrix, which, during the same irradiation treatment, is capable of controllably changing its chemical composition, structure, and physical, optical and electrophysical properties.

#### **4.2. Femtosecond treatments of samples with Ti-Sapphire laser**

Differences in laser radiation parameters, and also differences in other irradiation conditions, strongly affect the material modification process and its ultimate results. The idea of to which extent the final effect due to PLA might differ in various irradiated systems can be grasped through making a comparison of PLA data for film structures involving GeO, GeOx layers, and GeO2<Ge-NCs> heterolayers obtained in treatments performed using two different lasers, a KrF excimer laser and a Ti-Sapphire laser.

GeO2 Films with Ge-Nanoclusters in Layered Compositions: Structural Modifications with Laser Pulses 415

Martsinovsky et al. (Martsinovsky et al., 2009) showed that, under the action of femtosecond laser pulses, intense photo-excitation of semiconductor surface, which dramatically modifies its optical response and provides conditions for generation of surface electromagnetic waves of various types, becomes possible. Martsinovsky et al. have also discussed the interrelation of electronic processes optically induced in the near-surface layer with the formation of periodical surface microstructures that were observed in experiments on irradiation of

Femtosecond PLAs were also used to crystallise silicon films. In PLA treatments, a critical point for thin films is proper choice of irradiation conditions since at high pulse energies such films may readily suffer ablation. At present, the issue of Si-nanocrystal formation in silicon nitride and silicon oxide films under nanosecond PLAs is a well-studied matter. A process for fabricating nanocrystals in dielectric films with the the help of nanosecond laser pulses was patented as involved in the production technology of non-volatile flash memory devices. Femtosecond treatments were used for modification of Si nanoclusters in SiO2 films (Korchagina et al., 2012). Yet, for femtosecond treatments structural changes in silicon clusters in SiNx and SiOx films have not been studied. As for germanium, Ge films, and Ge nanoparticles embedded in dielectric layers, to the best of our knowledge, only one reported communication on fs-laser-initiated crystallisation in thin layers of amorphous Ge is

On the whole, the physics underlying processes in film systems subjected to to femtosecond anneals still remains scantily understood. That is why it would be interesting to perform a study of such processes in samples treated with nanosecond and femtosecond laser pulses with revealing all involved characteristic features, also including possible athermal effect on Ge nanoparticles hosted in an insulator matrix, both with the aim of detecting new manifestations of PLA-induced effects in germanium-based materials and with the aim of comparison of similar processes in systems based on two chemically akin semiconductors, Si and Ge. Assumes that the results of these studies will prove also useful for applications.

Our study of fs PLA-treated thin-film systems based on germanium oxide layers was the first one devoted to the subject matter of interest. Therefore, the first task in such investigations was to develop a general concept of the complex effects and phenomena occurring in the film systems of interest subjected to PLAs under different conditions. In particular, we examined GeO2<Ge-NCs> heterofilms of different thicknesses grown by the LP CVD method on (bare and CVD SiO2-covered) silicon or quartz glass substrates. The films either had one layer and no cap layer or they were protected by a thin cap layer of PE CVD SiNxOy or CVD SiO2. In another set of samples, ~ 20 – 30 nm and ~200-250 nm thick, GeO2<Ge-NCs> layers were part of multilayer compositions where they were separated with thin (~7-10 nm) and thick (~100 nm) CVD SiO2 layers as shown in Fig. 17 a. Scanning was made with a focused fs-laser beam at a velocity of 100 µm/s. The frequency of laser pulses was 10 pulses per 1 µm. The laser fluence (*Epulse*) varied from 15 to 1 mJ in ~70-µm diameter

silicon targets.

presently available (Salihoglu et al., 2011).

*4.2.1. Femtosecond PLA of GeO2<Ge-NCs> heterolayers*

Phase transition mechanisms in solid materials under femtosecond laser treatments are radically different from analogous PLAs implemented using nanosecond pulses. These differences are most clearly manifested in the case of ultra-high-power femtosecond laser treatments, when nonlinear effects play an important role in the light absorption processes. There are many contributions devoted to the study of the interaction of femtosecond laser pulses with matter (Juodkazis et al., 2009). Results of an experimental study of the dynamics of electron-hole plasma formation and recombination of photo-induced charge carriers in Si under fs PLA at laser energy lower the threshold value for surface melting and ablation were reported by Ashitkov et al. (Ashitkov et al., 2004). During the action of a fs laser pulse, electron-hole plasma with electron concentration in the conduction band up to 1022 cm-3 forms, both thermally activated and athermal surface-layer melting processes being observed along with a transition of material to metallic state. In (Ashitkov et al., 2004), timedependent optical reflection was measured for heating-pulse energies.

In phase transitions under such conditions, athermal effects may emerge. Indeed, the duration of fs laser pulses is much shorter then the duration of the electron- phonon interaction in semiconductor (approx. 1-2 ps). That is why in silicon, during laser pulse the ''hot'' electron-hole plasma does not excite vibrational modes and relaxes insignificantly. The temperatures of the electron and atomic sub-systems are much different. According to theoretical calculations, the material becomes unstable when the electron concentration excited from the valence to conduction band reaches 9-20% of the atom concentration in Si lattice (Bok, 1981). This metastable state may relax to a more stable crystalline phase without melting yet with release of latent crystallisation heat. The process is similar to ''explosive'' crystallisation. In several picoseconds after the pulse, the phonon and electron temperatures should equalize. Post-pulse processes, e.g. post-pulse atomic diffusion in heated films, start manifesting. Very probably, diffusion can be stimulated by laser radiation not only via film heating, but also due to the breaking of valence bonds that occurs when electrons undergo excitation to the conduction band. Under nanosecond laser treatments, the time of Si film cooling to a temperature below the melting point is known to amount to about 100 nanoseconds (Sameshima & Usui, 1991). For fs laser treatments, the cooling time can be expected to be the same. This time can turn out sufficient for diffusion of excess silicon and for formation of nanocrystals only in the case of melt.

Zabotnov et al. (Zabotnov et al., 2006) showed that femtosecond laser annealing of Si surfaces leads to their nanostructuring, two types of formed nanostructures being observed. In one case, these are ordered lattices with a period somewhat shorter than laser radiation wavelength. The formation of such structures can be explained by the interference between the fs radiation waves and the non-equilibrium plasma excited by radiation in the surface layer. Here, ablation of Si atoms is accompanied with their rapid oxidation in air, and also with the formation of surface Si nanocrystals sized 3-5 nm.

Martsinovsky et al. (Martsinovsky et al., 2009) showed that, under the action of femtosecond laser pulses, intense photo-excitation of semiconductor surface, which dramatically modifies its optical response and provides conditions for generation of surface electromagnetic waves of various types, becomes possible. Martsinovsky et al. have also discussed the interrelation of electronic processes optically induced in the near-surface layer with the formation of periodical surface microstructures that were observed in experiments on irradiation of silicon targets.

Femtosecond PLAs were also used to crystallise silicon films. In PLA treatments, a critical point for thin films is proper choice of irradiation conditions since at high pulse energies such films may readily suffer ablation. At present, the issue of Si-nanocrystal formation in silicon nitride and silicon oxide films under nanosecond PLAs is a well-studied matter. A process for fabricating nanocrystals in dielectric films with the the help of nanosecond laser pulses was patented as involved in the production technology of non-volatile flash memory devices. Femtosecond treatments were used for modification of Si nanoclusters in SiO2 films (Korchagina et al., 2012). Yet, for femtosecond treatments structural changes in silicon clusters in SiNx and SiOx films have not been studied. As for germanium, Ge films, and Ge nanoparticles embedded in dielectric layers, to the best of our knowledge, only one reported communication on fs-laser-initiated crystallisation in thin layers of amorphous Ge is presently available (Salihoglu et al., 2011).

On the whole, the physics underlying processes in film systems subjected to to femtosecond anneals still remains scantily understood. That is why it would be interesting to perform a study of such processes in samples treated with nanosecond and femtosecond laser pulses with revealing all involved characteristic features, also including possible athermal effect on Ge nanoparticles hosted in an insulator matrix, both with the aim of detecting new manifestations of PLA-induced effects in germanium-based materials and with the aim of comparison of similar processes in systems based on two chemically akin semiconductors, Si and Ge. Assumes that the results of these studies will prove also useful for applications.

#### *4.2.1. Femtosecond PLA of GeO2<Ge-NCs> heterolayers*

414 Laser Pulses – Theory, Technology, and Applications

two different lasers, a KrF excimer laser and a Ti-Sapphire laser.

dependent optical reflection was measured for heating-pulse energies.

for formation of nanocrystals only in the case of melt.

with the formation of surface Si nanocrystals sized 3-5 nm.

idea of to which extent the final effect due to PLA might differ in various irradiated systems can be grasped through making a comparison of PLA data for film structures involving GeO, GeOx layers, and GeO2<Ge-NCs> heterolayers obtained in treatments performed using

Phase transition mechanisms in solid materials under femtosecond laser treatments are radically different from analogous PLAs implemented using nanosecond pulses. These differences are most clearly manifested in the case of ultra-high-power femtosecond laser treatments, when nonlinear effects play an important role in the light absorption processes. There are many contributions devoted to the study of the interaction of femtosecond laser pulses with matter (Juodkazis et al., 2009). Results of an experimental study of the dynamics of electron-hole plasma formation and recombination of photo-induced charge carriers in Si under fs PLA at laser energy lower the threshold value for surface melting and ablation were reported by Ashitkov et al. (Ashitkov et al., 2004). During the action of a fs laser pulse, electron-hole plasma with electron concentration in the conduction band up to 1022 cm-3 forms, both thermally activated and athermal surface-layer melting processes being observed along with a transition of material to metallic state. In (Ashitkov et al., 2004), time-

In phase transitions under such conditions, athermal effects may emerge. Indeed, the duration of fs laser pulses is much shorter then the duration of the electron- phonon interaction in semiconductor (approx. 1-2 ps). That is why in silicon, during laser pulse the ''hot'' electron-hole plasma does not excite vibrational modes and relaxes insignificantly. The temperatures of the electron and atomic sub-systems are much different. According to theoretical calculations, the material becomes unstable when the electron concentration excited from the valence to conduction band reaches 9-20% of the atom concentration in Si lattice (Bok, 1981). This metastable state may relax to a more stable crystalline phase without melting yet with release of latent crystallisation heat. The process is similar to ''explosive'' crystallisation. In several picoseconds after the pulse, the phonon and electron temperatures should equalize. Post-pulse processes, e.g. post-pulse atomic diffusion in heated films, start manifesting. Very probably, diffusion can be stimulated by laser radiation not only via film heating, but also due to the breaking of valence bonds that occurs when electrons undergo excitation to the conduction band. Under nanosecond laser treatments, the time of Si film cooling to a temperature below the melting point is known to amount to about 100 nanoseconds (Sameshima & Usui, 1991). For fs laser treatments, the cooling time can be expected to be the same. This time can turn out sufficient for diffusion of excess silicon and

Zabotnov et al. (Zabotnov et al., 2006) showed that femtosecond laser annealing of Si surfaces leads to their nanostructuring, two types of formed nanostructures being observed. In one case, these are ordered lattices with a period somewhat shorter than laser radiation wavelength. The formation of such structures can be explained by the interference between the fs radiation waves and the non-equilibrium plasma excited by radiation in the surface layer. Here, ablation of Si atoms is accompanied with their rapid oxidation in air, and also Our study of fs PLA-treated thin-film systems based on germanium oxide layers was the first one devoted to the subject matter of interest. Therefore, the first task in such investigations was to develop a general concept of the complex effects and phenomena occurring in the film systems of interest subjected to PLAs under different conditions. In particular, we examined GeO2<Ge-NCs> heterofilms of different thicknesses grown by the LP CVD method on (bare and CVD SiO2-covered) silicon or quartz glass substrates. The films either had one layer and no cap layer or they were protected by a thin cap layer of PE CVD SiNxOy or CVD SiO2. In another set of samples, ~ 20 – 30 nm and ~200-250 nm thick, GeO2<Ge-NCs> layers were part of multilayer compositions where they were separated with thin (~7-10 nm) and thick (~100 nm) CVD SiO2 layers as shown in Fig. 17 a. Scanning was made with a focused fs-laser beam at a velocity of 100 µm/s. The frequency of laser pulses was 10 pulses per 1 µm. The laser fluence (*Epulse*) varied from 15 to 1 mJ in ~70-µm diameter

spot and from 0.028 to 0.0018 mJ in 1-µm diameter spot. In the experiments, samples with GeO2<Ge-NCs> heterolayers were used. In addition, in some cases optical bench vibrations of ~1,5-µm amplitude in two dimensions, in the plane of the optical bench and in the direction normal to it, were implemented. A typical result for layered systems with GeO2<Ge-NCs> heterolayers on Si substrate scanned with Ti-Sapphire laser beam focused to a 70-µm diameter spot is shown in Fig. 17 b. From line to line, the laser fluence was decreased in 1-mJ steps within 15-mJ wide intervals of *Epulse*. Thus, the adopted femtosecond PLA regimes and conditions for examined films were different from their treatments with the nanosecond laser in terms of the majority of basic physical parameters (laser radiation wavelength, pulse duration, intervals between pulses, arrangement of the irradiation process). Besides, in the case of fs PLAs the adopted irradiation technique did not allow us to trace changes in the film structure following successive irradiations of the sample with a series of laser pulses.

GeO2 Films with Ge-Nanoclusters in Layered Compositions: Structural Modifications with Laser Pulses 417

laser are used to modify GeO2<Ge-NCs> heterolayers. Only Ge-nanoclusters, whose optical gap *Egeff(*Ge-NC) < 2 eV, absorbed femtosecond laser radiation with *λ*=800 µs in GeO2<Ge-NCs> heterolayers (see Figs. 8 and 9). Nanosecond KrF laser radiation with *λ*=248 nm is additionally absorbed by the GeO2 matrix, which constitutes 70% of the heterolayer volume, its optical gap being *Egeff*(GeO2) ~4,5-5 eV. The actions of the light emitted by the two lasers on the two material components in the heterolayers are radically different: while KrF laser radiation heats both the Ge-nanoclusters and the dielectric matrix, Ti-Sapphire lase radiation heats only Ge-nanoparticles while leaving the matrix cold. It is this difference that makes the effects due to PLA processes in GeO2<Ge-NCs> heterolayers under irradiation with the two lasers cardinally opposite. Namely, in GeO2<Ge-NCs> heterolayers treated with the fs laser the heated Ge-nanoclusters begin to react with the surrounding glassy GeO2 and, during this process, the initial heterostructure transforms into a totally different kind of solid with

*4.2.2. Formation of GeO2 nanofoam using fs PLA of GeO2<Ge-NCs> heterolayers*

Indeed, Ge-nanoclusters surrounded by GeO2 matrix were rapidly heated by fs laser radiation, to which the oxide was transparent, unless the chemical reaction (4) between both components in GeO2<Ge-NCs> heterostructure was initiated. The emitted germanium monooxide molecules (GeO(gas)) are initially localised in the vicinity of hot Genanoparticles surrounded by a cooler insulator (Fig. 18). Affected by the vapor temperature and pressure, the glassy GeO2 matrix gets heated and it extends when softening. On cooling, the GeO2<Ge-NCs> heterostructure turns into a swelled mass of nano-dimensional glass cells. Each of the initial Ge-nanoclusters, as its grows smaller and lighter, is to remain inside a formed nano-cells (Fig. 18 c). The most surprising thing here is that, in the outcome of nano-foam formation process, all Ge-nanoparticles should have a uniform minimal size independently of their size in the initial heterolayer. Due to the 3D quantum-dimensional effect, Ge-nanoclusters, as they grew smaller during their reaction with the GeO2 matrix, should stop absorbing the heating light according to the condition *hν* = *Egeff*(NCs). This technique allows us to obtain ensembles of isolated indirect gap semiconductors of very

The process is completed as follows. While the nano-cell sizes in the glass softened under the internal gas pressure of the gas inside grow in value, their glass walls start cooling. The latter is due to the fact that, first, the gas in the nano-cells volumetrically expands under near-adiabatic conditions and, hence, the gas temperature will decrease. In addition, the mechanical expansion of the nano-cells in the soft GeO2 nanofoam consumes the energy from the gas in cells. The gas gives off heat and its pressure decreases. Simultaneously, the nanocell-hosted Ge-nanoclusters, as they grow smaller, start absorbing less fs laser radiation (due to the 3D quantum-confinement effect). This reduces the efficiency of the reaction yielding new portions of GeO(gas) in the nano-cells. Thus, in the nanofoam the nano-cell walls start cooling at a sufficiently high internal pressure due to GeO(gas) vapor. Cooling of the GeO2 glass rapidly increases its viscosity, which prevents the nano-cells from being compressed with decrease of the internal GeO(gas)

fundamentally different physical properties.

small dimensions that also have a low size dispersion.

The first result was that optical microscopy was quite a sensitive technique allowing us to reveal both changes in the surface relief and the variations in the optical parameters of the material induced by structural and chemical modification processes proceeding in heterolayers under laser beam treatments, whereas the SEM method was efficient only when studying surface morphology modifications (cp. Figs. 17 a and b). The main fundamental difference in the PLA mechanisms for the two lasers is manifested when the

**Figure 17.** Surfaces of multilayer coatings involving alternating GeO2<Ge-NCs> heterolayers and SiO2 layers after fs PLA (beam diameter 70 µm): **a** – schematic of the multilayer coating (similar to that in Fig. 6); **b** – SEM image of the film coating treated with laser pulses of various energies; **c** – optical microscopy image of the same sample: (1) evolution of surface morphology at the centerline of laser spot path; (2) along coating edges there is no relief, although light stripes defined by the changes of optical properties of some layers inside the coating are clearly seen; (3) laser treated paths are separated with wide stripes where the film coating structure has remained unchanged (*Epulse* was not high enough); **d**, **e, f** and **g** – SEM images obtained at the various stages of the coating laser damage process implemented with laser energy increase: **d** – development of a wavy surface relief as a result of transformation of 3 thin GeO2<Ge-NCs> heterolayers in the top tier into nanofoam (*Epulse* = 8 µJ ); **e**  beginning of layer destruction in the top tier (*Epulse* = 9 µJ ); **f** – simultaneous damage of upper and middle tiers (*Epulse* = 10 µJ); **g** – simultaneous damage of all layers down to the Si substrate and the beginning of its damage by formed ripples (*Epulse* =13 µJ).

laser are used to modify GeO2<Ge-NCs> heterolayers. Only Ge-nanoclusters, whose optical gap *Egeff(*Ge-NC) < 2 eV, absorbed femtosecond laser radiation with *λ*=800 µs in GeO2<Ge-NCs> heterolayers (see Figs. 8 and 9). Nanosecond KrF laser radiation with *λ*=248 nm is additionally absorbed by the GeO2 matrix, which constitutes 70% of the heterolayer volume, its optical gap being *Egeff*(GeO2) ~4,5-5 eV. The actions of the light emitted by the two lasers on the two material components in the heterolayers are radically different: while KrF laser radiation heats both the Ge-nanoclusters and the dielectric matrix, Ti-Sapphire lase radiation heats only Ge-nanoparticles while leaving the matrix cold. It is this difference that makes the effects due to PLA processes in GeO2<Ge-NCs> heterolayers under irradiation with the two lasers cardinally opposite. Namely, in GeO2<Ge-NCs> heterolayers treated with the fs laser the heated Ge-nanoclusters begin to react with the surrounding glassy GeO2 and, during this process, the initial heterostructure transforms into a totally different kind of solid with fundamentally different physical properties.

#### *4.2.2. Formation of GeO2 nanofoam using fs PLA of GeO2<Ge-NCs> heterolayers*

416 Laser Pulses – Theory, Technology, and Applications

series of laser pulses.

spot and from 0.028 to 0.0018 mJ in 1-µm diameter spot. In the experiments, samples with GeO2<Ge-NCs> heterolayers were used. In addition, in some cases optical bench vibrations of ~1,5-µm amplitude in two dimensions, in the plane of the optical bench and in the direction normal to it, were implemented. A typical result for layered systems with GeO2<Ge-NCs> heterolayers on Si substrate scanned with Ti-Sapphire laser beam focused to a 70-µm diameter spot is shown in Fig. 17 b. From line to line, the laser fluence was decreased in 1-mJ steps within 15-mJ wide intervals of *Epulse*. Thus, the adopted femtosecond PLA regimes and conditions for examined films were different from their treatments with the nanosecond laser in terms of the majority of basic physical parameters (laser radiation wavelength, pulse duration, intervals between pulses, arrangement of the irradiation process). Besides, in the case of fs PLAs the adopted irradiation technique did not allow us to trace changes in the film structure following successive irradiations of the sample with a

The first result was that optical microscopy was quite a sensitive technique allowing us to reveal both changes in the surface relief and the variations in the optical parameters of the material induced by structural and chemical modification processes proceeding in heterolayers under laser beam treatments, whereas the SEM method was efficient only when studying surface morphology modifications (cp. Figs. 17 a and b). The main fundamental difference in the PLA mechanisms for the two lasers is manifested when the

**Figure 17.** Surfaces of multilayer coatings involving alternating GeO2<Ge-NCs> heterolayers and SiO2 layers after fs PLA (beam diameter 70 µm): **a** – schematic of the multilayer coating (similar to that in Fig.

6); **b** – SEM image of the film coating treated with laser pulses of various energies; **c** – optical microscopy image of the same sample: (1) evolution of surface morphology at the centerline of laser spot path; (2) along coating edges there is no relief, although light stripes defined by the changes of optical properties of some layers inside the coating are clearly seen; (3) laser treated paths are separated with wide stripes where the film coating structure has remained unchanged (*Epulse* was not high enough); **d**, **e, f** and **g** – SEM images obtained at the various stages of the coating laser damage process implemented with laser energy increase: **d** – development of a wavy surface relief as a result of transformation of 3 thin GeO2<Ge-NCs> heterolayers in the top tier into nanofoam (*Epulse* = 8 µJ ); **e**  beginning of layer destruction in the top tier (*Epulse* = 9 µJ ); **f** – simultaneous damage of upper and middle tiers (*Epulse* = 10 µJ); **g** – simultaneous damage of all layers down to the Si substrate and the

beginning of its damage by formed ripples (*Epulse* =13 µJ).

Indeed, Ge-nanoclusters surrounded by GeO2 matrix were rapidly heated by fs laser radiation, to which the oxide was transparent, unless the chemical reaction (4) between both components in GeO2<Ge-NCs> heterostructure was initiated. The emitted germanium monooxide molecules (GeO(gas)) are initially localised in the vicinity of hot Genanoparticles surrounded by a cooler insulator (Fig. 18). Affected by the vapor temperature and pressure, the glassy GeO2 matrix gets heated and it extends when softening. On cooling, the GeO2<Ge-NCs> heterostructure turns into a swelled mass of nano-dimensional glass cells. Each of the initial Ge-nanoclusters, as its grows smaller and lighter, is to remain inside a formed nano-cells (Fig. 18 c). The most surprising thing here is that, in the outcome of nano-foam formation process, all Ge-nanoparticles should have a uniform minimal size independently of their size in the initial heterolayer. Due to the 3D quantum-dimensional effect, Ge-nanoclusters, as they grew smaller during their reaction with the GeO2 matrix, should stop absorbing the heating light according to the condition *hν* = *Egeff*(NCs). This technique allows us to obtain ensembles of isolated indirect gap semiconductors of very small dimensions that also have a low size dispersion.

The process is completed as follows. While the nano-cell sizes in the glass softened under the internal gas pressure of the gas inside grow in value, their glass walls start cooling. The latter is due to the fact that, first, the gas in the nano-cells volumetrically expands under near-adiabatic conditions and, hence, the gas temperature will decrease. In addition, the mechanical expansion of the nano-cells in the soft GeO2 nanofoam consumes the energy from the gas in cells. The gas gives off heat and its pressure decreases. Simultaneously, the nanocell-hosted Ge-nanoclusters, as they grow smaller, start absorbing less fs laser radiation (due to the 3D quantum-confinement effect). This reduces the efficiency of the reaction yielding new portions of GeO(gas) in the nano-cells. Thus, in the nanofoam the nano-cell walls start cooling at a sufficiently high internal pressure due to GeO(gas) vapor. Cooling of the GeO2 glass rapidly increases its viscosity, which prevents the nano-cells from being compressed with decrease of the internal GeO(gas)

pressure. Thus, the individual nano-cells, as well as all of the nanofoam, will preserve their dimensions in the cooling process.

GeO2 Films with Ge-Nanoclusters in Layered Compositions: Structural Modifications with Laser Pulses 419

**Figure 19.** The simultaneous laser damage of two tiers of the multilayer system in Fig. 17 g during fs PLA (SEM image): **a** - radial Gaussian distribution of energy inside the laser beam; **b** - *E(1)(a)*, *E(2)(a)*, *E(3)(a)* - threshold energies for laser damage: (1) - in the top tier of the multilayer film system, (2) and (3) - in a thick SiO2 layer and in a thick GeO2<Ge-NCs> heterolayer of the middle top, respectively; 2*ra1,*  2*ra2*, 2*ra3* - width of damaged areas in the respective layers of the system; *E(s)* - threshold energy for the onset of the foaming process in thin GeO2<Ge-NCs> heterolayers of the top tier of the multi-layer film

The experiment revealed some interesting features of the nanofoam formation process. First of all, the process was indeed implemented. The latter was indirectly proved by several methods of structural analysis. For instance, at an early stage of the process a wavy relief not violating the continuity of the multilayer coating emerged on the coating surface. The formation of this relief was registered by optical microscopy and SEM (Figs. 17 c and d). With increasing the laser pulse energy, the top tier in the coating starts destructing, and the smooth thick SiO2 layer of the middle tier stands out. Then, the top tier of the coating detaches from the lower tier, tears and rolles, with all its thin layers becoming laminated (see Figs. 17 f and 19 b). This is the first stage of the laser damage of the whole multilayer system. Further on, the process laser damage develops involving more and more layers (Fig. 17 f and g and Fig 19 b), the boundaries of destructed zones in each layer spreading from the centerline of the laser treated path on the sample surface with growth of laser pulse energy. The ablated zone (band) in each layer is always wider than that in the lower layer. Therefore, SEM images allow observation of the structure and morphology of the uncovered

Ultra-short laser pulses of the investigated heterostructures cause structural rearrangements and chemical reactions as: films shrinkage, GeO(solid) layers decay on reaction (3), growth of Ge-nanoparticles in the heterolayer and their crystallisation, chemical reactions of GeO2 matrix of heterolayers with cap layers on their interface, possible crystallisation of GeO2 glass around Ge-nanoparticles and reaction of GeO vapour formation from GeO2<Ge-NCs>

surface of any exposed coating layer down to the Si-substrate surface.

system.

PLA results of multilayer film systems consisting of a conventional dielectric layers and GeO2<Ge-NCs> heterolayers illustrate in Fig.17 the complex effect of fs laser radiation on the system. The system consisted of nine layers belonging to two types: thick and thin SiO2 films and GeO2<Ge-NCs> heterolayers that alternated among each other (according to the diagram in Fig. 17 a). With increase of laser pulse energy, distinct indications of film structure modification started showing up, becoming manifested more and more clearly. The sample holder vibration accompanying the scanning process proved useful in revealing important details of the mechanisms underlying the impact of laser pulse radiation on the structure of irradiated films. Changes in film properties began to evolve in the volume of the films, and then they continued in surface morphological changes. To describe the observed processes, it was necessary to employ the whole complex of available experimental methods, each method, as a rule, allowing characterisation of a limited number (one or two) of the changing properties of the objects under study.

For instance, optical microscopy (Fig. 17 c) was sensitive to variations of optical parameters of the heterolayers under laser beam, manifested in color changes. This was an indication of the beginning of modification of material properties in some layers inside the coating. Those processes preceded the changes of the thickness and surface relief of the coating. SEM was effective only when studying surface morphology modifications (cp. Figs. 17 b, d, e, f and g). Besides, it has limited capabilities in height measurements of many relief details. However, when combined, the two methods were capable of revealing a complete picture of the dynamics of fs PLA induced changes in investigated multilayer structures on increasing the laser pulse energy.

**Figure 18.** Schematic illustrating the process of transformation of a GeO2<Ge-NCs> heterolayer in nanofoam-like glassy GeO2 in a film structure subjected to fs laser annealing: **a** - initial heterolayer structure; **b** - heating of Ge-nanoclusters by fs laser pulses, which initiates the formation of GeO(gas) around these clusters; **c** - final structure of nanofoam-like glassy GeO2.

their dimensions in the cooling process.

of the changing properties of the objects under study.

laser pulse energy.

pressure. Thus, the individual nano-cells, as well as all of the nanofoam, will preserve

PLA results of multilayer film systems consisting of a conventional dielectric layers and GeO2<Ge-NCs> heterolayers illustrate in Fig.17 the complex effect of fs laser radiation on the system. The system consisted of nine layers belonging to two types: thick and thin SiO2 films and GeO2<Ge-NCs> heterolayers that alternated among each other (according to the diagram in Fig. 17 a). With increase of laser pulse energy, distinct indications of film structure modification started showing up, becoming manifested more and more clearly. The sample holder vibration accompanying the scanning process proved useful in revealing important details of the mechanisms underlying the impact of laser pulse radiation on the structure of irradiated films. Changes in film properties began to evolve in the volume of the films, and then they continued in surface morphological changes. To describe the observed processes, it was necessary to employ the whole complex of available experimental methods, each method, as a rule, allowing characterisation of a limited number (one or two)

For instance, optical microscopy (Fig. 17 c) was sensitive to variations of optical parameters of the heterolayers under laser beam, manifested in color changes. This was an indication of the beginning of modification of material properties in some layers inside the coating. Those processes preceded the changes of the thickness and surface relief of the coating. SEM was effective only when studying surface morphology modifications (cp. Figs. 17 b, d, e, f and g). Besides, it has limited capabilities in height measurements of many relief details. However, when combined, the two methods were capable of revealing a complete picture of the dynamics of fs PLA induced changes in investigated multilayer structures on increasing the

**Figure 18.** Schematic illustrating the process of transformation of a GeO2<Ge-NCs> heterolayer in nanofoam-like glassy GeO2 in a film structure subjected to fs laser annealing: **a** - initial heterolayer structure; **b** - heating of Ge-nanoclusters by fs laser pulses, which initiates the formation of GeO(gas)

around these clusters; **c** - final structure of nanofoam-like glassy GeO2.

**Figure 19.** The simultaneous laser damage of two tiers of the multilayer system in Fig. 17 g during fs PLA (SEM image): **a** - radial Gaussian distribution of energy inside the laser beam; **b** - *E(1)(a)*, *E(2)(a)*, *E(3)(a)* - threshold energies for laser damage: (1) - in the top tier of the multilayer film system, (2) and (3) - in a thick SiO2 layer and in a thick GeO2<Ge-NCs> heterolayer of the middle top, respectively; 2*ra1,*  2*ra2*, 2*ra3* - width of damaged areas in the respective layers of the system; *E(s)* - threshold energy for the onset of the foaming process in thin GeO2<Ge-NCs> heterolayers of the top tier of the multi-layer film system.

The experiment revealed some interesting features of the nanofoam formation process. First of all, the process was indeed implemented. The latter was indirectly proved by several methods of structural analysis. For instance, at an early stage of the process a wavy relief not violating the continuity of the multilayer coating emerged on the coating surface. The formation of this relief was registered by optical microscopy and SEM (Figs. 17 c and d). With increasing the laser pulse energy, the top tier in the coating starts destructing, and the smooth thick SiO2 layer of the middle tier stands out. Then, the top tier of the coating detaches from the lower tier, tears and rolles, with all its thin layers becoming laminated (see Figs. 17 f and 19 b). This is the first stage of the laser damage of the whole multilayer system. Further on, the process laser damage develops involving more and more layers (Fig. 17 f and g and Fig 19 b), the boundaries of destructed zones in each layer spreading from the centerline of the laser treated path on the sample surface with growth of laser pulse energy. The ablated zone (band) in each layer is always wider than that in the lower layer. Therefore, SEM images allow observation of the structure and morphology of the uncovered surface of any exposed coating layer down to the Si-substrate surface.

Ultra-short laser pulses of the investigated heterostructures cause structural rearrangements and chemical reactions as: films shrinkage, GeO(solid) layers decay on reaction (3), growth of Ge-nanoparticles in the heterolayer and their crystallisation, chemical reactions of GeO2 matrix of heterolayers with cap layers on their interface, possible crystallisation of GeO2 glass around Ge-nanoparticles and reaction of GeO vapour formation from GeO2<Ge-NCs>

heterolayer components, also ablation — explosive damage of films and substrates (Korte et al., 1999). Many of these processes are also expressed in radiation of GeO2<Ge-NCs> heterolayers by fs Ti-Sapphire laser, just as in case of Kr excimer laser. But, as is seen from Fig. 17, the interaction character of fs laser radiation with the studied films and substrate was considerably different.

GeO2 Films with Ge-Nanoclusters in Layered Compositions: Structural Modifications with Laser Pulses 421

Successive onsets of structural-chemical modification processes of group I was observed in bilayer coatings given ns PLAs. In the case of fs PLA, successive (layer-by-layer) initiation of structural-chemical modification processes becomes distinctly pronounced during ablative destruction of multilayer coatings (see Fig. 19 b that shows a fragment of Fig. 17 f). It is seen that the laser pulse fluence absorbed by the multilayer coating is sufficiently high to exceed four energy threshold values, *E(s)*, *E(1)(a)*, *E(2)(a)*, and *E(3)(a)* (see Fig. 19 a). Three of the threshold energies are the thresholds for laser damage initiation in the two top tiers of the irradiated system (in the top tier and in the two layers of the middle tier, a thick SiO2 layer and a GeO2<Ge-NCs> heterolayer). The very first threshold energy is the non-ablative threshold *E(s)* of the foaming process of three thin GeO2<Ge-NCs> heterolayers in the top coating tier. The radial distribution of light energy in the laser beam obeys the Gauss law *E(r) = E0 exp(-2r2 / r02)* (Korte et al., 1999) as shown in Fig. 19 a; therefore, destruction of the layered structures begins at the centerline of the laser treated path on the film surface when the pulse energy exceeds the necessary energy threshold of the top tier. *E(1)(a)* (see Fig. 19 b). If the pulse energy is sufficient to exceed the damage thresholds of the first, second and subsequent film, the laser damage will be initiated in all these layers simultaneously. Yet, ablative removal of the lower- coating material is impossible without a preliminary removal of the upper films. That is why, in most cases multilayer coatings are damaged successively, layer by layer. In this case the positions of the boundaries in destructed areas of each of the layers are determined by the radial energy distribution in the laser-beam cross section. Qualitatively, the theoretical expectation for this regularity is illustrated by Fig. 19 a. The experimental data of Fig. 17 f, g, and Fig. 19 b, in principle, do not disagree with the stated concept, although a multi-mode laser beam structure in real cases often distorts the strictly circular pattern of radial energy distribution in laser spots on irradiated surfaces. Thus, the processes activated in PLA-treated films experimentally revealed the real energy

The particular case under consideration is interesting in that respect that, apart from the propagation of the laser damage process into depth in a multilayer coating involving two types of films of different origin, here there was also a process that can be classed to intralayer processes that can also proceed in layers covered by other films. This was the transformation process of fs PLA treated GeO2<Ge-NCs> heterolayers into a nanofoam. The formation of nanofoam in germanate heterolayers was not hindered by thin SiO2 layers that covered such heterolayers. Such SiO2 layers were weakly heated by the laser beam, and they do not change their properties due to chemical and structural modifications. Simultaneously, this process progressed along the centerline of the fs laser treated path when the energy threshold of nanofoam formation E(s) was overcome during the fs PLA. If the laser fluence many times exceeded the threshold energy *E(s)*, then the top tier was immediately torn off from the middle tier as a homogeneous film according to the common explosive machanism of film damage. Here, the chemical nanofoam formation process did not have sufficient time to evolve in the GeO2<Ge-NCs> heterolayers of this tier. Here, under fs laser irradiation the system of upper layers behaves as a solid film of homogeneous material. The thickness of this layer system at its edges along the line of damage rupture was almost no difference from the thickness of the whole layer on the substrate, and the

distribution round the laser-beam center.

Results of using PLA and fs lasers for the studied film coatings showed that a number of the above-mentioned structural rearrangement processes and chemical reactions excite one by one with laser fluence growth. In general, to activate a new (i)-process, it is necessary to have laser fluence, absorbed by a film coatings square unit, exceeding the energy threshold of (E(i)). Energy intervals among thresholds of different processes successively excited in films are usually so large that they can be definitely separated from each other. Therefore, the processes observed in multi-layer systems could be divided into two main groups. The first one consists of almost all the processes from the presented list, which proceed in the volume and interfaces of each of a multi-system's layers independently from processes in its other layers. Ablation processes belong to the second group as they, proceeding in one of a multi-system's layers, as a rule, determine all other processes in its lower layers.

#### *4.2.3. Laser damage in multilayer coatings*

In investigated heterostructures, ultra-short laser pulses induce various structural rearrangements and chemical reactions such as film shrinkage, decomposition of GeO(solid) layers according to reaction (3), growth of Ge-nanoparticles followed with their crystallisation in the heterolayer, chemical reactions of the GeO2 matrix with the cap layers in the interfacial region, possible crystallisation of GeO2 glass around Ge-nanoparticles and reaction (4) of GeO vapor formation from GeO2<Ge-NCs> heterolayer components, and also ablation and explosive laser damage of the films and substrates (Korte et al., 1999). Many of these processes are manifested in GeO2<Ge-NCs> heterolayers irradiated with fs Ti-Sapphire laser in a similar way like in the case of Kr excimer laser. However, as it is seen from Fig. 17, the interaction of fs laser radiation with examined films and Si substrate was largely different.

Results of using ns and fs PLA for treating examined film coatings showed that a sequence of the above-mentioned structural rearrangement processes and chemical reactions gets initiated on increasing the laser fluence. In general, for activation of a new (i-th) process, it is required to reach a laser fluence absorbed by a film coatings in unit square in excess of the energy threshold for the process (E(i)). The energy intervals between the thresholds of the various processes successively initiated in the films are usually so large that they can be definitely separated from each other. Therefore, the processes observed in our multilayer systems could be divided into two main groups. The first group consists of almost all the processes from the above process list, which proceed in the bulk and interfacial regions of each of the involved layers independently of the processes proceeding in other layers. Ablation and laser damage processes belong to the second group as they, proceeding in one of the layers, as a rule, govern the processes proceeding in lower layers.

*4.2.3. Laser damage in multilayer coatings*

different.

was considerably different.

heterolayer components, also ablation — explosive damage of films and substrates (Korte et al., 1999). Many of these processes are also expressed in radiation of GeO2<Ge-NCs> heterolayers by fs Ti-Sapphire laser, just as in case of Kr excimer laser. But, as is seen from Fig. 17, the interaction character of fs laser radiation with the studied films and substrate

Results of using PLA and fs lasers for the studied film coatings showed that a number of the above-mentioned structural rearrangement processes and chemical reactions excite one by one with laser fluence growth. In general, to activate a new (i)-process, it is necessary to have laser fluence, absorbed by a film coatings square unit, exceeding the energy threshold of (E(i)). Energy intervals among thresholds of different processes successively excited in films are usually so large that they can be definitely separated from each other. Therefore, the processes observed in multi-layer systems could be divided into two main groups. The first one consists of almost all the processes from the presented list, which proceed in the volume and interfaces of each of a multi-system's layers independently from processes in its other layers. Ablation processes belong to the second group as they, proceeding in one of a

In investigated heterostructures, ultra-short laser pulses induce various structural rearrangements and chemical reactions such as film shrinkage, decomposition of GeO(solid) layers according to reaction (3), growth of Ge-nanoparticles followed with their crystallisation in the heterolayer, chemical reactions of the GeO2 matrix with the cap layers in the interfacial region, possible crystallisation of GeO2 glass around Ge-nanoparticles and reaction (4) of GeO vapor formation from GeO2<Ge-NCs> heterolayer components, and also ablation and explosive laser damage of the films and substrates (Korte et al., 1999). Many of these processes are manifested in GeO2<Ge-NCs> heterolayers irradiated with fs Ti-Sapphire laser in a similar way like in the case of Kr excimer laser. However, as it is seen from Fig. 17, the interaction of fs laser radiation with examined films and Si substrate was largely

Results of using ns and fs PLA for treating examined film coatings showed that a sequence of the above-mentioned structural rearrangement processes and chemical reactions gets initiated on increasing the laser fluence. In general, for activation of a new (i-th) process, it is required to reach a laser fluence absorbed by a film coatings in unit square in excess of the energy threshold for the process (E(i)). The energy intervals between the thresholds of the various processes successively initiated in the films are usually so large that they can be definitely separated from each other. Therefore, the processes observed in our multilayer systems could be divided into two main groups. The first group consists of almost all the processes from the above process list, which proceed in the bulk and interfacial regions of each of the involved layers independently of the processes proceeding in other layers. Ablation and laser damage processes belong to the second group as they, proceeding in one

of the layers, as a rule, govern the processes proceeding in lower layers.

multi-system's layers, as a rule, determine all other processes in its lower layers.

Successive onsets of structural-chemical modification processes of group I was observed in bilayer coatings given ns PLAs. In the case of fs PLA, successive (layer-by-layer) initiation of structural-chemical modification processes becomes distinctly pronounced during ablative destruction of multilayer coatings (see Fig. 19 b that shows a fragment of Fig. 17 f). It is seen that the laser pulse fluence absorbed by the multilayer coating is sufficiently high to exceed four energy threshold values, *E(s)*, *E(1)(a)*, *E(2)(a)*, and *E(3)(a)* (see Fig. 19 a). Three of the threshold energies are the thresholds for laser damage initiation in the two top tiers of the irradiated system (in the top tier and in the two layers of the middle tier, a thick SiO2 layer and a GeO2<Ge-NCs> heterolayer). The very first threshold energy is the non-ablative threshold *E(s)* of the foaming process of three thin GeO2<Ge-NCs> heterolayers in the top coating tier. The radial distribution of light energy in the laser beam obeys the Gauss law *E(r) = E0 exp(-2r2 / r02)* (Korte et al., 1999) as shown in Fig. 19 a; therefore, destruction of the layered structures begins at the centerline of the laser treated path on the film surface when the pulse energy exceeds the necessary energy threshold of the top tier. *E(1)(a)* (see Fig. 19 b). If the pulse energy is sufficient to exceed the damage thresholds of the first, second and subsequent film, the laser damage will be initiated in all these layers simultaneously. Yet, ablative removal of the lower- coating material is impossible without a preliminary removal of the upper films. That is why, in most cases multilayer coatings are damaged successively, layer by layer. In this case the positions of the boundaries in destructed areas of each of the layers are determined by the radial energy distribution in the laser-beam cross section. Qualitatively, the theoretical expectation for this regularity is illustrated by Fig. 19 a. The experimental data of Fig. 17 f, g, and Fig. 19 b, in principle, do not disagree with the stated concept, although a multi-mode laser beam structure in real cases often distorts the strictly circular pattern of radial energy distribution in laser spots on irradiated surfaces. Thus, the processes activated in PLA-treated films experimentally revealed the real energy distribution round the laser-beam center.

The particular case under consideration is interesting in that respect that, apart from the propagation of the laser damage process into depth in a multilayer coating involving two types of films of different origin, here there was also a process that can be classed to intralayer processes that can also proceed in layers covered by other films. This was the transformation process of fs PLA treated GeO2<Ge-NCs> heterolayers into a nanofoam. The formation of nanofoam in germanate heterolayers was not hindered by thin SiO2 layers that covered such heterolayers. Such SiO2 layers were weakly heated by the laser beam, and they do not change their properties due to chemical and structural modifications. Simultaneously, this process progressed along the centerline of the fs laser treated path when the energy threshold of nanofoam formation E(s) was overcome during the fs PLA. If the laser fluence many times exceeded the threshold energy *E(s)*, then the top tier was immediately torn off from the middle tier as a homogeneous film according to the common explosive machanism of film damage. Here, the chemical nanofoam formation process did not have sufficient time to evolve in the GeO2<Ge-NCs> heterolayers of this tier. Here, under fs laser irradiation the system of upper layers behaves as a solid film of homogeneous material. The thickness of this layer system at its edges along the line of damage rupture was almost no difference from the thickness of the whole layer on the substrate, and the

edges of the layer were like those of glassy films under brittle fracture. In other words, the edges mostly looked like broken line segments (Fig. 20 b, area 1), which were similar to the edges of thick SiO2 layers after damage ruptures in the same multilayer coating (Fig. 20 b, areas 2 and 4).

GeO2 Films with Ge-Nanoclusters in Layered Compositions: Structural Modifications with Laser Pulses 423

measure in the SEM images the thicknesses of the foam-like layers, which were found to 1,5- 3 times exceed the initial thicknesses of the GeO2<Ge-NCs> heterolayer. In some areal parts of the foam-like layers (where the layers became uncoated following the laser damage of the SiO2 layer, Fig. 20), the foamy mass often increased, approximately by 2-4 times, the thickness of the GeO2<Ge-NCs> heterolayer. During of the most foamed layer, some individual fragments of that layer were often explosively thrown up onto the SiO2 layer. The thicknesses of those fragments were bigger than the three- or four-fold thickness of the SiO2

As a rule, AFM data on the sizes of important relief features of the film coatings (Fig. 21) complied with the data obtained by other measuring techniques. Thus, AFM measurements proved that the top tier tears off from the underlying SiO2 layer during laser damage ablation (Fig. 22). Besides, it is possible to determine the initial thickness (*h*) of the top tier (*h* ~50-60 nm) and the degree of its changes during fs PLA using the surface relief profile (Fig. 22 b) measured along the *p*-line (Fig. 22 c). The obtained value for the thickness of the top tier (*H*) in the immediate vicinity of the damage rupture (Fig. 22 a, area 2) is five times greater than the initial layer thickness (*h*). But the value H is not only the result of transformation of the three GeO2<Ge-NCs> heterolayers in the top tier into nanofoam. The laminating effect of the edges of the top-tier layers from the lower films in this area also contributed to the found value of H since after the tearing the free edges bent upwards.

The energy threshold of the foam formation process *E(s)* in the thin GeO2<Ge-NCs> heterolayers of the top tier during fs PLA was exceeded over the whole irradiated film surface (Figs. 21 and 22). Moreover, the rate of irradiation along the laser treated path (close to the line *q* in Fig. 22 a) varied periodically and at the points of maximum it reached the lower threshold energy for laser damage in this tier *E(1)(a)*. In accordance with these periods, crests in swollen and buckled regions of the top tier formed. On the average, the separation between crests is 2,4 - 2,5 µm (line *q* in Fig. 22 a). There are zones in between the crests, where there was a minimal foaming of GeO2<Ge-NCs> heterolayers. From the relief profile measured along the cutting line *q* (Fig. 22 d), we could determine the modulation frequency

**Figure 21.** 3D AFM image of fs laser treated path on the multilayer surface shown in Fig. 17 e.

layer.

**Figure 20.** SEM image of layer-by-layer destruction processes in a multilayer structure exposed to laser radiation (**a** – *Epulse* = 10 mJ , **b** – *Epulse* = 13 mJ) : **1** - the top tier of thin layers, **2, 3** – the middle tier of thick layers (SiO2 and nanofoamed GeO2<Ge-NCs> heterolayer, respectively), **4, 5** – the bottom tier (lower SiO2 layer and Si substrate, respectively.

However, laser damage took a different course if, under fs PLA, the laser fluence was only a little in excess of the threshold E(s) and the process of GeO2<Ge-NCs> heterolayer foaming had sufficient time to develop. The thicknesses of those layers increased appreciably together with the thickness of the whole top tier in the multilayer coating; the top tier swelled and began to gradually tear off from the underlying SiO2 layer. Then the areas of the top tier showing a most pronounced nanofoam formation suffered destruction accompanied by tearing off of its fragments (see Figs. 19 b and 20 a). Here, there are many thickened monolayer fragments of the top tier, exfoliated from the substrate incompletely and left in many areas over the edges of the layer along the centerline of the damage rupture zone. Those fragments roll in microtubes and laminate into very thin separate layers that constitute this tier (Figs. 19 b and 20 a).

During fs PLA, the nanofoam formation process of the thick GeO2<Ge-NCs> heterolayer in the middle tier of the film coating proceeded in a different way than that in thin GeO2<Ge-NCs> heterolayers of the top tier. The reason for this was that the thickness of GeO2<Ge-NCs> heterolayer and its mass allowed it to accumulate more laser energy per unit volume. Moreover, a longer time was required for the damage process to cause removal of the thin film layers and the upper thick SiO2 layer. Both factors allow the heterolayer to accumulate in its volume a considerable energy and considerably prolong the action of this energy transforming the heterolayer into nanofoam. Therefore, in all SEM images (Figs. 17 g, 19 b, 20 a and b) one can observe under the upper thick SiO2 layer removed by laser damage a substance that looks like a liquid light foamy mass.

#### *4.2.4. Some characteristic parameters of the nanofoam formation process*

The thicknesses of the layers in the bottom and middle tiers as measured in SEM images were close to the values obtained in as-synthesized films. We could also conveniently measure in the SEM images the thicknesses of the foam-like layers, which were found to 1,5- 3 times exceed the initial thicknesses of the GeO2<Ge-NCs> heterolayer. In some areal parts of the foam-like layers (where the layers became uncoated following the laser damage of the SiO2 layer, Fig. 20), the foamy mass often increased, approximately by 2-4 times, the thickness of the GeO2<Ge-NCs> heterolayer. During of the most foamed layer, some individual fragments of that layer were often explosively thrown up onto the SiO2 layer. The thicknesses of those fragments were bigger than the three- or four-fold thickness of the SiO2 layer.

422 Laser Pulses – Theory, Technology, and Applications

(lower SiO2 layer and Si substrate, respectively.

constitute this tier (Figs. 19 b and 20 a).

substance that looks like a liquid light foamy mass.

*4.2.4. Some characteristic parameters of the nanofoam formation process*

areas 2 and 4).

edges of the layer were like those of glassy films under brittle fracture. In other words, the edges mostly looked like broken line segments (Fig. 20 b, area 1), which were similar to the edges of thick SiO2 layers after damage ruptures in the same multilayer coating (Fig. 20 b,

**Figure 20.** SEM image of layer-by-layer destruction processes in a multilayer structure exposed to laser radiation (**a** – *Epulse* = 10 mJ , **b** – *Epulse* = 13 mJ) : **1** - the top tier of thin layers, **2, 3** – the middle tier of thick layers (SiO2 and nanofoamed GeO2<Ge-NCs> heterolayer, respectively), **4, 5** – the bottom tier

However, laser damage took a different course if, under fs PLA, the laser fluence was only a little in excess of the threshold E(s) and the process of GeO2<Ge-NCs> heterolayer foaming had sufficient time to develop. The thicknesses of those layers increased appreciably together with the thickness of the whole top tier in the multilayer coating; the top tier swelled and began to gradually tear off from the underlying SiO2 layer. Then the areas of the top tier showing a most pronounced nanofoam formation suffered destruction accompanied by tearing off of its fragments (see Figs. 19 b and 20 a). Here, there are many thickened monolayer fragments of the top tier, exfoliated from the substrate incompletely and left in many areas over the edges of the layer along the centerline of the damage rupture zone. Those fragments roll in microtubes and laminate into very thin separate layers that

During fs PLA, the nanofoam formation process of the thick GeO2<Ge-NCs> heterolayer in the middle tier of the film coating proceeded in a different way than that in thin GeO2<Ge-NCs> heterolayers of the top tier. The reason for this was that the thickness of GeO2<Ge-NCs> heterolayer and its mass allowed it to accumulate more laser energy per unit volume. Moreover, a longer time was required for the damage process to cause removal of the thin film layers and the upper thick SiO2 layer. Both factors allow the heterolayer to accumulate in its volume a considerable energy and considerably prolong the action of this energy transforming the heterolayer into nanofoam. Therefore, in all SEM images (Figs. 17 g, 19 b, 20 a and b) one can observe under the upper thick SiO2 layer removed by laser damage a

The thicknesses of the layers in the bottom and middle tiers as measured in SEM images were close to the values obtained in as-synthesized films. We could also conveniently As a rule, AFM data on the sizes of important relief features of the film coatings (Fig. 21) complied with the data obtained by other measuring techniques. Thus, AFM measurements proved that the top tier tears off from the underlying SiO2 layer during laser damage ablation (Fig. 22). Besides, it is possible to determine the initial thickness (*h*) of the top tier (*h* ~50-60 nm) and the degree of its changes during fs PLA using the surface relief profile (Fig. 22 b) measured along the *p*-line (Fig. 22 c). The obtained value for the thickness of the top tier (*H*) in the immediate vicinity of the damage rupture (Fig. 22 a, area 2) is five times greater than the initial layer thickness (*h*). But the value H is not only the result of transformation of the three GeO2<Ge-NCs> heterolayers in the top tier into nanofoam. The laminating effect of the edges of the top-tier layers from the lower films in this area also contributed to the found value of H since after the tearing the free edges bent upwards.

The energy threshold of the foam formation process *E(s)* in the thin GeO2<Ge-NCs> heterolayers of the top tier during fs PLA was exceeded over the whole irradiated film surface (Figs. 21 and 22). Moreover, the rate of irradiation along the laser treated path (close to the line *q* in Fig. 22 a) varied periodically and at the points of maximum it reached the lower threshold energy for laser damage in this tier *E(1)(a)*. In accordance with these periods, crests in swollen and buckled regions of the top tier formed. On the average, the separation between crests is 2,4 - 2,5 µm (line *q* in Fig. 22 a). There are zones in between the crests, where there was a minimal foaming of GeO2<Ge-NCs> heterolayers. From the relief profile measured along the cutting line *q* (Fig. 22 d), we could determine the modulation frequency

**Figure 21.** 3D AFM image of fs laser treated path on the multilayer surface shown in Fig. 17 e.

GeO2 Films with Ge-Nanoclusters in Layered Compositions: Structural Modifications with Laser Pulses 425

23 a were obtained at two closely located points (2 and 3) on one of the lines treated by the fs laser beam (Fig. 23 d and e, respectively). Yet, because of heterogeneity of the radial energy distribution in the beam, intensive formation of nanofoam mass from the GeO2<Ge-NCs> heterolayer proceeded at both points of this line during PLA. At point 3, the film coating was at an early stage of destruction, still remaining almost intact, whereas at point 2, damage was well activated. The latter is evident from the intensity of the Raman peaks of Ar+ laser beam due to Si-substrate lattice (in the corresponding spectra); at point 2 the film practically does not hinder the Ar+ laser beam to reach the substrate. In other words, at points 2 and 3 we observe an early and the final stage of the destruction process of the foamed layer. Nonetheless, despite the possibility of mixing up film fragments belonging to different areal parts of the film, Ge-nanocrystal sizes were not large, ~2,3 – 3,1 nm as calculated by model (1) and ~3,1 - 4,1 as calculated by model (2). At point 4 in Fig. 23 с the fs laser energy was slightly lower than that at points 2 and 3. Therefore, the film foamed without and violation of its continuity. Ge-NCs of the least diameter ~1,0 (model (1)) and 1,4

**Figure 23. a, b** - Raman spectra and **c, d, e** - photos of GeO2<Ge-NCs> heterolayers transformed to nanofoam under a SiNxOy cap layer on the Si substrate: **1** – non-irradiated films; **2** - strong laser damage of the films; **3** - laser damage at an early stage; **4** - the formation of foam without destruction of the cap

Thus, the both main features which are to accompany the formation of nanofoam from GeO2<Ge-NCs> heterolayers during their fs laser pulse treatments were observed in the experiment. Direct images of the studied material are supposed to be obtained with HR TEM, but additional information about the properties of the nanofoam was also obtained with the SEM method. Characterizing the most impressive effects produced in GeO(solid) films and GeO2<Ge-NCs> heterolayers by fs PLA, we would like to note that, here, the same structural and chemical modification processes like those in ns PLA treated samples proceed both consequently and in parallel in the material; those process include: decomposition of the metastable solid germanium monoxide atomic network with its transformation into the atomic network of glassy GeO2 oversaturated with Ge atoms; clusterization of released germanium into amorphous nanoparticles; and growth and crystallisation of α-Ge nanoparticles. The effect of film transparency increase is also common for both kinds of laser

nm (model (2)) were found here (spectrum 4 in Fig. 23 a).

layer.

**Figure 22. a** - AFM image of the surface of the multilayer coating shown in Fig. 21: *p* and *q* - cutting lines of the sample surface across and along the fs laser treated path, respectively; **b** – schematic of the sample cross section along the line *p* drawn along the rupture edge of the the top tier of the coating with swelled films (**1** and **4** - areas with unchanged relief, **2** - laser damage zone of the top tier, **3** - zone of the maximum change in relief at the edge of rupturing film during its swelling; **c** - relief of the film coating along the line *p*: *α* - the initial level of coating surface, *β* - surface of the upper thick SiO2 layer*, γ* - the highest level of coating relief changes, **d** - relief of the film coating along the line *q***.**

of laser beam energy connected with the sample vibration. An analysis of this relief in the described fs PLA conditions showed that, at this modulation frequency of laser irradiation, its energy changes within the interval between the threshold energies of the two processes *E(1)(a)* and E(s) during the period of ~ 0,025 sec. The thickness of GeO2<Ge-NCs> heterolayers may increase 3-4 times for the time equal to half the period.

As a result, the vibration experiment revealed: (i) a low threshold energy *E(s)* of the process; (*ii*) a high capability of GeO2<Ge-NCs> heterolayers to increase of their volume (by 3-5 times) during transformation into nanofoam material; (*iii*) high dynamic characteristics of the nanofoam formation process during increase of fs laser pulse energy.

Micro-Raman spectra of fs laser treated GeO2<Ge-NCs>heterolayers (Fig. 23) produce another evidence of their transformation into a nanofoam-like material. For simplification of the analysis, the spectra were registered in GeO2<Ge-NCs> heterolayers deposited onto a Si substrate protected by a 25-nm thick SiNxOy cap layer (the effect due to ns PLA was also studied on this sample). The effect due to laser fluence value exciting the nanofoam formation process in local regions of the GeO2<Ge-NCs>/SiNxOy bilayer system is shown in Fig. 23. In Fig. 23 a, in spectrum 1, which corresponds to non-irradiated film at point 1 (Fig. 23 e), only a broad Raman band due to light scattering by transversal optical phonons of α-Ge is shown, like in the case of ns laser treatment of this film system (Fig. 12).