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**7**

*USA*

Martin E. Kordesch2 *1Ball State University 2Ohio University*

**Cathodoluminescence from Amorphous and**

**Rare Earth and Transition Metals**

Muhammad Maqbool1, Wojciech M. Jadwisienczak2 and

**Nanocrystalline Nitride Thin Films Doped with**

Rare earth (RE) ion luminescence has long been used in laser and optical fiber communications technology. Bulk RE doped oxides were widely used in color phosphors for Cathode Ray Tubes. The wide band gap (WBG) semiconductors and insulators have been used for visible emission at 300 K from RE ions since the reports first by Zanata (Zanatta and Nunes 1998) for Er in silicon nitride (photoluminescence) and then shortly thereafter by Steckl (Steckl and Birkhahn 1998) for Er in GaN. The III-nitrides were emerging as Light Emitting Diodes and semiconductors at the time, and it was reported that the luminescence intensity of the RE ions was improved by a wide band gap host. Silicon (band gap ≈ 1.1 eV) is not suited for most visible RE ion emission. Glasses and oxides were used for the infra red (IR), especially for the emission from Er3+ ion at ~ 1.5 μm. In 1998, RE ion incorporation into a crystalline host was often accomplished by ion implantation at low atomic concentrations, or by in situ doping (again with low atomic percentages) of the RE ions. It was believed that the quality of the host lattice was essential to the RE luminescence. However, Zanata and Nunes observed green room temperature luminescence from an Er-doped silicon nitride film deposited by reactive sputtering in nitrogen. Visible emission was observed with an estimated 10 at. % dopant concentration in an amorphous material. Both the amorphous and crystalline hosts discovered by the Zanata and Steckl groups set in motion the (enduring) pursuit of practical visible light

In 1999, Gurumurugan (Gurumurugan, Chen et al. 1999) sputter deposited amorphous AlN doped with Er (3.4 at.%) and observed the full range of Er emission lines in cathodoluminescence (CL) in amorphous AlN. There is no visible defect band in the large gap of AlN (~6 eV), in contrast to sputtered GaN which suffers from an intense, broad yellow emission from defects. The AlN band gap is almost identical with the crystalline AlN bandgap, and the bandgap was not reduced significantly by the RE dopant. The AlN film was transparent, amorphous, and could be deposited at 300 K. Gurumarugan et al. also showed that the amorphous AlN films could be heated to 1300 K without crystallization. The heating improved the RE emission intensity. Also, it was noted that in these films the intensity of some

emission devices using RE ions in wide band gap materials.

**1. Introduction**

### **Cathodoluminescence from Amorphous and Nanocrystalline Nitride Thin Films Doped with Rare Earth and Transition Metals**

Muhammad Maqbool1, Wojciech M. Jadwisienczak2 and Martin E. Kordesch2 *1Ball State University 2Ohio University USA*

#### **1. Introduction**

160 Cathodoluminescence

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quantum dots. J. Appl. Phys. 90, pp. 4489-4493.

thermal evaporation. Appl. Phys. Lett. 81, pp.757-759.

Oxide Quantum Rods. J. Am. Chem. Soc. 126, pp. 6206-6207.

and the defect physics of ZnO. Phys. Rev. B 63, pp. 075205(7).

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studies of stoichiometric and oxygen-deficient ZnO films. Appl. Phys. Lett. 78, pp.

Dimensional Nanostructures: Synthesis, Characterization, and Applications. Adv.

(2002). Controlled Growth of ZnO Nanowires and Their Optical Properties. Adv.

emission and optical properties in polyvinyl pyrrolidone surface modified ZnO

property of nitrogen-acceptor-bound states in ZnO. J. Appl. Phys. 99, pp. 046101(3).

ZnO rods formed by metalorganic chemical vapor deposition. Appl. Phys. Lett. 83,

frequency lateral electric fields in quantum wells. Appl. Phys. Lett. 78, pp. 1559-

Microwave plasma growth and high spatial resolution cathodoluminescent spectrum of

emissions influenced by O and Zn implantations in ZnO. Appl. Phys. Lett. 87, pp.

(1997). Lateral superlattices as voltage-controlled traps for excitons, Phys. Rev. B 56,

Wegscheider W.; & Abstreiter G. (1998). Spatially resolved exciton trapping in a voltage-controlled lateral Superlattice. Appl. Phys. Lett., Vol. 73, No. 2, pp. 154-156. Zimmermann S.; Wixforth A.; Kotthaus J. P.; Wegscheider W.; & Bichler M. (1999). A Semiconductor-Based Photonic Memory Cell. Science 283, pp. 1292-1295.

Rare earth (RE) ion luminescence has long been used in laser and optical fiber communications technology. Bulk RE doped oxides were widely used in color phosphors for Cathode Ray Tubes. The wide band gap (WBG) semiconductors and insulators have been used for visible emission at 300 K from RE ions since the reports first by Zanata (Zanatta and Nunes 1998) for Er in silicon nitride (photoluminescence) and then shortly thereafter by Steckl (Steckl and Birkhahn 1998) for Er in GaN. The III-nitrides were emerging as Light Emitting Diodes and semiconductors at the time, and it was reported that the luminescence intensity of the RE ions was improved by a wide band gap host. Silicon (band gap ≈ 1.1 eV) is not suited for most visible RE ion emission. Glasses and oxides were used for the infra red (IR), especially for the emission from Er3+ ion at ~ 1.5 μm. In 1998, RE ion incorporation into a crystalline host was often accomplished by ion implantation at low atomic concentrations, or by in situ doping (again with low atomic percentages) of the RE ions. It was believed that the quality of the host lattice was essential to the RE luminescence. However, Zanata and Nunes observed green room temperature luminescence from an Er-doped silicon nitride film deposited by reactive sputtering in nitrogen. Visible emission was observed with an estimated 10 at. % dopant concentration in an amorphous material. Both the amorphous and crystalline hosts discovered by the Zanata and Steckl groups set in motion the (enduring) pursuit of practical visible light emission devices using RE ions in wide band gap materials.

In 1999, Gurumurugan (Gurumurugan, Chen et al. 1999) sputter deposited amorphous AlN doped with Er (3.4 at.%) and observed the full range of Er emission lines in cathodoluminescence (CL) in amorphous AlN. There is no visible defect band in the large gap of AlN (~6 eV), in contrast to sputtered GaN which suffers from an intense, broad yellow emission from defects. The AlN band gap is almost identical with the crystalline AlN bandgap, and the bandgap was not reduced significantly by the RE dopant. The AlN film was transparent, amorphous, and could be deposited at 300 K. Gurumarugan et al. also showed that the amorphous AlN films could be heated to 1300 K without crystallization. The heating improved the RE emission intensity. Also, it was noted that in these films the intensity of some

Cathodoluminescence from Amorphous and

(Ebdah 2011), used with permission.

over the luminescent layer.

substrate can produce polycrystalline films and even epitaxy.

Nanocrystalline Nitride Thin Films Doped with Rare Earth and Transition Metals 163

Because the plasma is "hot", reactions are fast and complete; rapid thermal quenching by condensation onto a (relatively) cold substrate preserves the composition achieved in the plasma. Phase segregation can be avoided as long as the films are not heated to the point where the different phases are free to form. Alternatively, sputtering onto a heated

Fig. 2.1. RF magnetron sputtering system. a) actual system; b) schematic diagram. From

or alloy with the sputter target, this method could also be used.

Practical considerations can encourage the use of a particular target. Al sputtered in nitrogen is an ideal system. Only AlN forms from the reaction. The Al itself is inexpensive. Targets can even be made from industrial grade Al plate or bars (however, these materials usually contain small amounts of Cr, and chromium oxide CL is observed), making the use of multiple targets simple and convenient. In Kordesch's laboratory at Ohio University, multiple targets were fabricated from 2 inch diameter (50 mm) Al bars or from targets purchased from commercial vendors. Small plugs of the RE or transition metals were pressed into the aluminium targets. In this way, multiple dopants or multiple dopant percentages could be obtained with very simple methods. Zanatta used plates or chips of metal on the silicon target. For materials that do not melt,

Gallium was sputtered in our laboratory at Ohio University from a pressed powder target and also from a liquid metal target. In this case, the powder target was more difficult to use with regard to the dopant material, because there is no good mechanical contact between the powder and the metal insert. For the liquid target, some droplet ejection occurs during sputtering. The existence of metal droplets on the thin film surface is detrimental to device performance, especially if contact or insulating layers are deposited

of the Er3+ transitions *decreased* with rising temperature, and some *increased* with rising temperature.

The crystalline III-nitrides are now extensively used for LED's, lasers and solid state lighting, high power electronic devices and other passive electronics uses related to the thermal conductivity of AlN. The light emission from direct transitions in GaN and In-Al-Ga-N alloys has displaced the III-nitride RE devices in lighting applications. The low intensity and long lifetime of the RE3+ transitions have also hindered the application of RE luminescence for on-chip communications. Thin film materials doped with RE's are used for phosphors, and specialty applications such as in situ thermometry.

#### **2. Growth, doping and measurement techniques**

#### **2.1 Sputter deposition**

Radio frequency (RF) sputtering is a physical vapour deposition process that uses RF excitation to cause gas molecules to collide with a target, removing the target material by the mechanical impact of the sputtering gas. The RF sputtering process is well known and used for many industrial applications. The details can be obtained from several textbooks. RF magnetron sputtering is a variation most often used for the efficient deposition of thin films.

In the sputtering process, a target of a metal or insulator might be made from solid material or pressed powders. The sputter gas used is often Argon (Ar). The gas pressure is adjusted so that a plasma of Ar ions is created by the RF field above the target. In magnetron sputtering, a system of magnets is used to confine the RF field and the ions so that the target is used efficiently. In this example, the sputter gas is inert, and physically collides with the target surface to eject a target atom that is then condensed onto the substrate. In this simple example, the kinetic energy of the Ar ion is used to eject the target atom. No reaction takes place, and ideally the condensed film is composed only of the target atoms. For AlN or GaN, sputterd with Ar, the condensed film is ideally a stoichiometric film of AlN or GaN. Sputtered films are often disordered, because the process does not generate or require the input of large amounts of heat such as thermal evaporation or chemical vapor deposition. The substrate can be cold. As a consequence, the condensed atoms do not have sufficient energy on the substrate to organize into a crystalline lattice.

In reactive sputtering, the target material is sputtered in a reactive gas. The RF plasma creates ions of the reactive gas, so that impact with the target results in a compound of the target and sputter gas formed by a chemical reaction which then condenses onto the substrate. For nitrogen and aluminium, for example, the nitrogen decomposes into N ions, and reacts with the Al metal target to form AlN. For simple compounds, where there are few alternative compositions and structures, the stoichiometric product is often the most likely compound to be deposited. The purity of the initial components determines the composition of the deposited film. In the previous example, reactive sputtering of Al in nitrogen, water vapor or oxygen in the sputter gas can produce aluminium oxide along with AlN.

A very significant aspect of sputter deposition is the option to grow unusual phases or compositions that could not be grown in equilibrium processes at high temperature.

of the Er3+ transitions *decreased* with rising temperature, and some *increased* with rising

The crystalline III-nitrides are now extensively used for LED's, lasers and solid state lighting, high power electronic devices and other passive electronics uses related to the thermal conductivity of AlN. The light emission from direct transitions in GaN and In-Al-Ga-N alloys has displaced the III-nitride RE devices in lighting applications. The low intensity and long lifetime of the RE3+ transitions have also hindered the application of RE luminescence for on-chip communications. Thin film materials doped with RE's are used

Radio frequency (RF) sputtering is a physical vapour deposition process that uses RF excitation to cause gas molecules to collide with a target, removing the target material by the mechanical impact of the sputtering gas. The RF sputtering process is well known and used for many industrial applications. The details can be obtained from several textbooks. RF magnetron sputtering is a variation most often used for the efficient deposition of thin

In the sputtering process, a target of a metal or insulator might be made from solid material or pressed powders. The sputter gas used is often Argon (Ar). The gas pressure is adjusted so that a plasma of Ar ions is created by the RF field above the target. In magnetron sputtering, a system of magnets is used to confine the RF field and the ions so that the target is used efficiently. In this example, the sputter gas is inert, and physically collides with the target surface to eject a target atom that is then condensed onto the substrate. In this simple example, the kinetic energy of the Ar ion is used to eject the target atom. No reaction takes place, and ideally the condensed film is composed only of the target atoms. For AlN or GaN, sputterd with Ar, the condensed film is ideally a stoichiometric film of AlN or GaN. Sputtered films are often disordered, because the process does not generate or require the input of large amounts of heat such as thermal evaporation or chemical vapor deposition. The substrate can be cold. As a consequence, the condensed atoms do not have sufficient

In reactive sputtering, the target material is sputtered in a reactive gas. The RF plasma creates ions of the reactive gas, so that impact with the target results in a compound of the target and sputter gas formed by a chemical reaction which then condenses onto the substrate. For nitrogen and aluminium, for example, the nitrogen decomposes into N ions, and reacts with the Al metal target to form AlN. For simple compounds, where there are few alternative compositions and structures, the stoichiometric product is often the most likely compound to be deposited. The purity of the initial components determines the composition of the deposited film. In the previous example, reactive sputtering of Al in nitrogen, water vapor or oxygen in the sputter gas can produce

A very significant aspect of sputter deposition is the option to grow unusual phases or compositions that could not be grown in equilibrium processes at high temperature.

for phosphors, and specialty applications such as in situ thermometry.

**2. Growth, doping and measurement techniques**

energy on the substrate to organize into a crystalline lattice.

aluminium oxide along with AlN.

temperature.

**2.1 Sputter deposition**

films.

Because the plasma is "hot", reactions are fast and complete; rapid thermal quenching by condensation onto a (relatively) cold substrate preserves the composition achieved in the plasma. Phase segregation can be avoided as long as the films are not heated to the point where the different phases are free to form. Alternatively, sputtering onto a heated substrate can produce polycrystalline films and even epitaxy.

Fig. 2.1. RF magnetron sputtering system. a) actual system; b) schematic diagram. From (Ebdah 2011), used with permission.

Practical considerations can encourage the use of a particular target. Al sputtered in nitrogen is an ideal system. Only AlN forms from the reaction. The Al itself is inexpensive. Targets can even be made from industrial grade Al plate or bars (however, these materials usually contain small amounts of Cr, and chromium oxide CL is observed), making the use of multiple targets simple and convenient. In Kordesch's laboratory at Ohio University, multiple targets were fabricated from 2 inch diameter (50 mm) Al bars or from targets purchased from commercial vendors. Small plugs of the RE or transition metals were pressed into the aluminium targets. In this way, multiple dopants or multiple dopant percentages could be obtained with very simple methods. Zanatta used plates or chips of metal on the silicon target. For materials that do not melt, or alloy with the sputter target, this method could also be used.

Gallium was sputtered in our laboratory at Ohio University from a pressed powder target and also from a liquid metal target. In this case, the powder target was more difficult to use with regard to the dopant material, because there is no good mechanical contact between the powder and the metal insert. For the liquid target, some droplet ejection occurs during sputtering. The existence of metal droplets on the thin film surface is detrimental to device performance, especially if contact or insulating layers are deposited over the luminescent layer.

Cathodoluminescence from Amorphous and

**2.2 Cathodoluminescence measurements 2.2.1 CL at room temperature and above**

entrance slits of a monochromater using a lens.

filament is blocked by the aperture.

Fig. 2.3. Experimental setup for CL.

bonded to the N atom.

Nanocrystalline Nitride Thin Films Doped with Rare Earth and Transition Metals 165

considerable controversy over the role of oxygen in promoting the luminescence of RE ions in the nitrides. It is not known if the RE nitrides in a nitride host decompose or remain

Finally, it should be pointed out that no electronic doping is observed in the amorphous nitride films. Partially, this is due to the fact that most of the amorphous nitride films are insulators, with the exception of n-type GaN. Electronic doping with Mg or Si was not successful, because the dopants react to form nitrides in the plasma, making the dopants

For CL measurements at 300 K and above, a simple system build in a six-way stainless steel cross with copper sealed flanges was used. The system was pumped with a 450 L/sec turbomolecular pump. A CRT electron gun with deflection plates was adapted to a multipin electrical feedthrough. The electron gun was powered with a laboratory-built power supply. The electron beam voltage could be varied up to 2.8 kV. Focus controls allowed the beam to be focussed onto the sample and to observe the spot on the sample through a vacuum viewport for optimization. The CL spot on the sample was then focussed onto the

The sample was illuminated with the electron beam from one side, the viewport was at 90 degrees from the electron gun. The sample was placed at 45 degrees to both arms of the cross. An aperture was placed in front of the sample between the sample and electron gun to block the visible light spot from the filament. The deflectors on the electron gun make it possible to steer the electron beam through the aperture while the direct light from the hot

A commercial Luminoscope was also used for some CL measurements. The Luminoscope is meant to be used for geological specimens, and is equipped with a vacuum chamber that replaces the stage of a light-optical microscope. A window allows microscopic observation of

commonly used in crystalline GaN for example, ineffective in amorphous nitrides.

Fig. 2.2. Left: Target with (clockwise from top) Sm, Er, and Tb pieces during sputtering. Right: clockwise from top: Liquid metal Ga target in a copper cup, GaN powder target in a copper cup, aluminium target with Mn , Tb and Cu plugs, aluminium target with RE plug removed, and Tm dendrites pressed into the target. The circular depression in the targets at about ½ the radius is the "racetrack", where the plasma is confined by the magnets to improve the efficiency of the sputter process. All targets are 50 mm diameter.

In practice, the targets used were mostly 50 mm diameter, 6 mm thick disks of aluminium. The RE or transition metal plugs were obtained commercially as rods of 3 – 12 mm diameter. Plugs were cut from the rods and pressed into holes in the aluminium target. Manganese, for example, could only be purchased as a powder, so the powder was pressed into the 6 mm diameter hole. Some RE metals were obtained as dendrites or split fragments. These could be pressed into the softer aluminium target by force, even if voids were left around the RE material. Typical power levels were from 100-200 Watts RF (13.56 MHz) power at 2- 10 mTorr pressures of nitrogen. This is the equivalent of 5-10 Watts/cm2. The substrates could be heated from the back by using a quartz lamp with a parabolic reflector focussed onto the back of the substrate, or cooled by clamping the substrates to a copper cold finger cooled with dry ice. We have not used liquid nitrogen to cool the samples. First, the lower temperature will condense water onto the sample, and second, there are stresses that come from warming a film grown at low temperature to 300 K, just as there are stresses in films cooled from a high temperature deposition.

Aluminum, gallium, beryllium, scandium, boron, silicon, silicon nitride, aluminium oxide, zinc, hafnium, titanium and alloy targets were used. Dopant RE metals such as Er, Tb, Gd, Tm, Sm, Pr and Ho were used, as well as Cr, Cu, Mn, Mg, Si, C, Ti, Sn, Ag. The sputter target is always below the sample/substrate to avoid particulates formed in the sputter process from falling onto the substrate (see Fig. 2.1).

An important consideration for RE doping in the nitrides is the stability of the host material. Many RE nitrides decompose when in contact with water or humid air, which causes a reaction resulting in ammonia and a RE oxide. AlN, BN, GaN, SiN and ScN are not soluble in water. In our experiments, beryllium nitride was also stable in humid air. There is considerable controversy over the role of oxygen in promoting the luminescence of RE ions in the nitrides. It is not known if the RE nitrides in a nitride host decompose or remain bonded to the N atom.

Finally, it should be pointed out that no electronic doping is observed in the amorphous nitride films. Partially, this is due to the fact that most of the amorphous nitride films are insulators, with the exception of n-type GaN. Electronic doping with Mg or Si was not successful, because the dopants react to form nitrides in the plasma, making the dopants commonly used in crystalline GaN for example, ineffective in amorphous nitrides.

#### **2.2 Cathodoluminescence measurements**

164 Cathodoluminescence

Fig. 2.2. Left: Target with (clockwise from top) Sm, Er, and Tb pieces during sputtering. Right: clockwise from top: Liquid metal Ga target in a copper cup, GaN powder target in a copper cup, aluminium target with Mn , Tb and Cu plugs, aluminium target with RE plug removed, and Tm dendrites pressed into the target. The circular depression in the targets at about ½ the radius is the "racetrack", where the plasma is confined by the magnets to

In practice, the targets used were mostly 50 mm diameter, 6 mm thick disks of aluminium. The RE or transition metal plugs were obtained commercially as rods of 3 – 12 mm diameter. Plugs were cut from the rods and pressed into holes in the aluminium target. Manganese, for example, could only be purchased as a powder, so the powder was pressed into the 6 mm diameter hole. Some RE metals were obtained as dendrites or split fragments. These could be pressed into the softer aluminium target by force, even if voids were left around the RE material. Typical power levels were from 100-200 Watts RF (13.56 MHz) power at 2- 10 mTorr pressures of nitrogen. This is the equivalent of 5-10 Watts/cm2. The substrates could be heated from the back by using a quartz lamp with a parabolic reflector focussed onto the back of the substrate, or cooled by clamping the substrates to a copper cold finger cooled with dry ice. We have not used liquid nitrogen to cool the samples. First, the lower temperature will condense water onto the sample, and second, there are stresses that come from warming a film grown at low temperature to 300 K, just as there are stresses in films

Aluminum, gallium, beryllium, scandium, boron, silicon, silicon nitride, aluminium oxide, zinc, hafnium, titanium and alloy targets were used. Dopant RE metals such as Er, Tb, Gd, Tm, Sm, Pr and Ho were used, as well as Cr, Cu, Mn, Mg, Si, C, Ti, Sn, Ag. The sputter target is always below the sample/substrate to avoid particulates formed in the sputter

An important consideration for RE doping in the nitrides is the stability of the host material. Many RE nitrides decompose when in contact with water or humid air, which causes a reaction resulting in ammonia and a RE oxide. AlN, BN, GaN, SiN and ScN are not soluble in water. In our experiments, beryllium nitride was also stable in humid air. There is

improve the efficiency of the sputter process. All targets are 50 mm diameter.

cooled from a high temperature deposition.

process from falling onto the substrate (see Fig. 2.1).

#### **2.2.1 CL at room temperature and above**

For CL measurements at 300 K and above, a simple system build in a six-way stainless steel cross with copper sealed flanges was used. The system was pumped with a 450 L/sec turbomolecular pump. A CRT electron gun with deflection plates was adapted to a multipin electrical feedthrough. The electron gun was powered with a laboratory-built power supply. The electron beam voltage could be varied up to 2.8 kV. Focus controls allowed the beam to be focussed onto the sample and to observe the spot on the sample through a vacuum viewport for optimization. The CL spot on the sample was then focussed onto the entrance slits of a monochromater using a lens.

The sample was illuminated with the electron beam from one side, the viewport was at 90 degrees from the electron gun. The sample was placed at 45 degrees to both arms of the cross. An aperture was placed in front of the sample between the sample and electron gun to block the visible light spot from the filament. The deflectors on the electron gun make it possible to steer the electron beam through the aperture while the direct light from the hot filament is blocked by the aperture.

Fig. 2.3. Experimental setup for CL.

A commercial Luminoscope was also used for some CL measurements. The Luminoscope is meant to be used for geological specimens, and is equipped with a vacuum chamber that replaces the stage of a light-optical microscope. A window allows microscopic observation of

Cathodoluminescence from Amorphous and

**2.2.3 CL in the electron microscope**

obtained in scanning TEM, STEM.

Nanocrystalline Nitride Thin Films Doped with Rare Earth and Transition Metals 167

Cathodoluminescence attachments can be obtained for both scanning electron microscopes (SEM) and transmission electron microscopes (TEM). In the simplest SEM versions, a parabolic mirror is mechanically inserted into the specimen chamber above the sample, with a hole in the mirror for the passage of the electron beam. The light from the sample CL is reflected onto a photomultiplier tube and used to form a scanned image based on the CL yield. In advanced systems, and monochromater is added to the optical system so that spectroscopically resolved images can be generated. Similar systems are used in TEM, but without a CL image. Only spectroscopy is possible in the TEM. CL based images can be

Fig. 2.6. Left: GaN crystallites in the SEM. A bit of a-GaN is shown in the stippled circle. Right: The same area, showing CL from the a-GaN and c-GaN. This image was one of the

In the last two decades it has been shown that the incorporation of RE elements as dopant atoms into III-nitride semiconductors such as GaN, AlN and their alloys, both crystalline and amorphous, leads to a temperature-stable luminescence whose wavelength is nearly independent of the specific semiconductor host (O'Donnell 2010). Luminescence from these materials having amorphous morphology grown on variety of substrates, and doped by implantation or during the growth with Ce (Aldabergenova, Osvet et al. 2002) , Pr (Maqbool, Ahmad et al. 2007; Maqbool, Richardson et al. 2007; Maqbool and Ahmad 2009), Sm (Zanatta, Ribeiro et al. 2001; Weingartner, Erlenbach et al. 2006; Maqbool and Ali 2009), Eu (Aldabergenova, Osvet et al. 2002), (Weingartner, Erlenbach et al. 2006), (Caldwell, Van Patten et al. 2001), Gd (Maqbool, Ahmad et al. 2007),(Maqbool, Kordesch et al. 2009), Tb (Aldabergenova, Osvet et al. 2002), (Weingartner, Erlenbach et al. 2006),(Richardson, Van Patten et al. 2002), (Jadwisienczak, Lozykowski et al. 2000), Dy (Weingartner, Erlenbach et al. 2006), Ho (Maqbool, Ali et al.; Aldabergenova, Frank et al. 2006; Maqbool, Kordesch et al. 2009), Er (Gurumurugan, Chen et al. 1999; Chen, Gurumurugan et al. 2000; Dimitrova, Van Patten et al. 2000; Dimitrova, Van Patten et al. 2001; Zanatta, Ribeiro et al. 2005), Tm (Maqbool, Kordesch et al. 2009) and Yb (Weingartner, Erlenbach et al. 2006) has been reported so far. In particular, RE doped GaN-nitrides based electroluminescent devices (ELDs) have been shown to have a versatile approach for the fabrication of variety of electrically driven optical light

**3. Cathodoluminescence of rare earth doped amorphous nitrides**

first experimental observations of a-GaN. (Hassan 1998).

the sample surface during electron illumination. A gas discharge electron source is used to generate an electron beam of up to 30 kV. The Luminoscope can be used to take color micrographs of the sample surface. A drawback to spectroscopic measurements in the Luminoscope is the necessity to remove the gas discharge spectrum (He) from the CL spectra.

Fig. 2.4. Left: Lumiscope sample holder. Right: Lumiscope control and electronics.

#### **2.2.2 CL at low temperature**

Low temperature CL spectroscopic analysis was measured in our laboratory at cryogenic temperatures and excitation conditions using experimental setup shown in Fig.2.5. Typically samples were mounted on the cold finger of a closed-cycled helium refrigerator operating down to 6K. The CL was generated by the Staib Instruments, Inc. Electron Gun EK-20-R equipped with a beam blanker (repetition rate: DC to 1 MHz) electron gun system being in common vacuum (of 510-7 Torr) with the cryostat. The electron beam was incident upon the sample at a 45° angle from an electron gun. The CL depth of the excitation could be easily varied by varying the electron acceleration voltage between 500eV up to 20 keV.

Fig. 2.5. Front view of a low temperature cathodoluminescence experimental setup operating between 6K- 330K and electron acceleration voltage up to 20 kV. Insets show color emissions from different RE-doped III-nitrides.

#### **2.2.3 CL in the electron microscope**

166 Cathodoluminescence

the sample surface during electron illumination. A gas discharge electron source is used to generate an electron beam of up to 30 kV. The Luminoscope can be used to take color micrographs of the sample surface. A drawback to spectroscopic measurements in the Luminoscope is the necessity to remove the gas discharge spectrum (He) from the CL spectra.

Fig. 2.4. Left: Lumiscope sample holder. Right: Lumiscope control and electronics.

Fig. 2.5. Front view of a low temperature cathodoluminescence experimental setup

emissions from different RE-doped III-nitrides.

operating between 6K- 330K and electron acceleration voltage up to 20 kV. Insets show color

Low temperature CL spectroscopic analysis was measured in our laboratory at cryogenic temperatures and excitation conditions using experimental setup shown in Fig.2.5. Typically samples were mounted on the cold finger of a closed-cycled helium refrigerator operating down to 6K. The CL was generated by the Staib Instruments, Inc. Electron Gun EK-20-R equipped with a beam blanker (repetition rate: DC to 1 MHz) electron gun system being in common vacuum (of 510-7 Torr) with the cryostat. The electron beam was incident upon the sample at a 45° angle from an electron gun. The CL depth of the excitation could be easily varied by varying the electron acceleration voltage between 500eV up to 20 keV.

**2.2.2 CL at low temperature**

Cathodoluminescence attachments can be obtained for both scanning electron microscopes (SEM) and transmission electron microscopes (TEM). In the simplest SEM versions, a parabolic mirror is mechanically inserted into the specimen chamber above the sample, with a hole in the mirror for the passage of the electron beam. The light from the sample CL is reflected onto a photomultiplier tube and used to form a scanned image based on the CL yield. In advanced systems, and monochromater is added to the optical system so that spectroscopically resolved images can be generated. Similar systems are used in TEM, but without a CL image. Only spectroscopy is possible in the TEM. CL based images can be obtained in scanning TEM, STEM.

Fig. 2.6. Left: GaN crystallites in the SEM. A bit of a-GaN is shown in the stippled circle. Right: The same area, showing CL from the a-GaN and c-GaN. This image was one of the first experimental observations of a-GaN. (Hassan 1998).

#### **3. Cathodoluminescence of rare earth doped amorphous nitrides**

In the last two decades it has been shown that the incorporation of RE elements as dopant atoms into III-nitride semiconductors such as GaN, AlN and their alloys, both crystalline and amorphous, leads to a temperature-stable luminescence whose wavelength is nearly independent of the specific semiconductor host (O'Donnell 2010). Luminescence from these materials having amorphous morphology grown on variety of substrates, and doped by implantation or during the growth with Ce (Aldabergenova, Osvet et al. 2002) , Pr (Maqbool, Ahmad et al. 2007; Maqbool, Richardson et al. 2007; Maqbool and Ahmad 2009), Sm (Zanatta, Ribeiro et al. 2001; Weingartner, Erlenbach et al. 2006; Maqbool and Ali 2009), Eu (Aldabergenova, Osvet et al. 2002), (Weingartner, Erlenbach et al. 2006), (Caldwell, Van Patten et al. 2001), Gd (Maqbool, Ahmad et al. 2007),(Maqbool, Kordesch et al. 2009), Tb (Aldabergenova, Osvet et al. 2002), (Weingartner, Erlenbach et al. 2006),(Richardson, Van Patten et al. 2002), (Jadwisienczak, Lozykowski et al. 2000), Dy (Weingartner, Erlenbach et al. 2006), Ho (Maqbool, Ali et al.; Aldabergenova, Frank et al. 2006; Maqbool, Kordesch et al. 2009), Er (Gurumurugan, Chen et al. 1999; Chen, Gurumurugan et al. 2000; Dimitrova, Van Patten et al. 2000; Dimitrova, Van Patten et al. 2001; Zanatta, Ribeiro et al. 2005), Tm (Maqbool, Kordesch et al. 2009) and Yb (Weingartner, Erlenbach et al. 2006) has been reported so far. In particular, RE doped GaN-nitrides based electroluminescent devices (ELDs) have been shown to have a versatile approach for the fabrication of variety of electrically driven optical light

Cathodoluminescence from Amorphous and

defines which RE transitions can be excited.

**3.1 Rare earth doping**

Nanocrystalline Nitride Thin Films Doped with Rare Earth and Transition Metals 169

forbidden nature of these transitions results in long radiative lifetimes when the selection rules are relaxed. Placing trivalent rare earth ions in amorphous solids can still result in characteristic emission from these intra-4*f* transitions despite the lack of a crystalline host structure (O'Donnell 2010). In both cases RE luminescence depends very little on the nature of the host and the ambient temperature. The 4*f* orbitals of RE ions incorporated into semiconductors, including a-III-nitrides, are so deeply buried within the electronic shell that the energy levels of the 4*f*<sup>n</sup> configuration are only slightly perturbed compared to free ion energy levels indicating large independence from the chemical surrounding and thus the choice of the host material (Wybourne and Smentek 2007). RE ions when doped to a-III-nitrides are known to exhibit sharp and well-defined luminescence which arises mostly from internal transitions between 4*f* levels within RE3+ ion. Because of the shield provided by the outer 5*s*2 and 5*p*6 shells, the wavelength involved in these intra-4*f* transitions is less temperature sensitive and rather weakly depends on the a-III-nitride host. On the other hand the mechanism behind the activation of RE are known to be sensitive to a local host details such as atomic structure and optical band gap which

Doping of a-III-nitrides is generally achieved *in situ* ; however it can be also done by RE ions implantation (Aldabergenova, Frank et al. 2006). In the former case doping typically takes place during sputtering process in which RE-doped a-III-nitride is formed from Ga, Al and RE atoms removed from metal targets by momentum transfer from an RF plasma. The sputtering growth of a-III-nitrides is explored much less than other technologically important amorphous semiconductors e.g a-Si doped with RE impurities (Iacona, Franzo et al. 2009). Ion implantation has been used extensively to *ex situ* dope III-nitride crystalline semiconductors with variety of rare earth ions. Excellent review concerning this issue is available (O'Donnell 2010). There are a limited number of papers on RE ions implantation to a-III-nitrides and resulting luminescence spectra (Jadwisienczak, Lozykowski et al. 2000). Ion implantation is a well established technique, which provides good control of the concentration profile in the sample. However, because of the unavoidable damage associated with the implantation process, typically a high temperature annealing step is imperative. This is due to the fact that when heavy atoms like RE metals are implanted to an amorphous semiconductor they can introduce both a chemical change in the substrate, in that they can introduce a different element than the host or induce a nuclear transmutation, and a structural change, in that the morphology of the host can be damaged or even destroyed by the energetic collision cascades

It is known that the thermal anneal of the a-AlN films affects not only the luminescent properties of the films, but also their optical band gap and chemical characteristics. As a result of thermal treatments of RE-doped a-III-nitrides at increasing temperatures the optical-absorption edge of amorphous films may change indicating the Al and N bonding environment modification (Zanatta, Ribeiro et al. 2005). Based on the available experimental results it is clear that RE-doped a-III-nitrides subjected to isochronal thermal annealing treatments indeed most likely underwent partial removal of both structural and chemical disorder resulting in modification of RE ions local environments as it is shown in Figs. 3.1 and 3.2 for Er-doped a-AlN films (Gurumurugan, Chen et al. 1999; Jadwisienczak,

induced by implanted heavy RE ions (Kucheyev, Williams et al. 2004).

Lozykowski et al. 2000; Weingartner, Erlenbach et al. 2006).

sources with narrow line-width emissions from the ultraviolet to the infrared (Steckl 1999). Thus optoelectronic devices utilizing 4*f*n transitions appearing in the nitrides forbidden band gap "window" are practically viable with these semiconductors (O'Donnell 2010). Recently it was demonstrated that the low voltage-operation of current-injected red emission from a crystalline *p*-type/Eu-doped/*n*-type GaN epilayers light emission diode operating at room temperature (Nishikawa, Kawasaki et al. 2009; Kasai, Nishikawa et al. 2010; Nishikawa, Furukawa 2010; Dierolf 2011) together with demonstrations of stimulated emission from Eu3+ doped GaN and AlGaN layers on a Si substrate proved in principal that optoelectronic devices covering the UV, visible and IR regions might be fabricated (Park and Steckl 2004; Park and Steckl 2005; Park and Steckl 2006).

These new results suggest a novel way to realize III-nitride semiconductors-based red emitting, current driven light emitting devices, as well as other primary colors and their mixture, monolithic devices. In the past the feasibility of using the RE-doped amorphous IIInitrides (a-III-nitrides) for light emitting applications was also demonstrated (Dimitrova, Van Patten et al. 2000; Dimitrova, Van Patten et al. 2001; Richardson, Van Patten et al. 2002; Maqbool, Kordesch et al. 2009), (Kim, Shepherd et al. 2003; Kim and Holloway 2004). However, to make these devices commercially viable, the internal quantum efficiency of the active RE-doped layers has to be significantly improved. The future success of the RE-doped III-nitrides optoelectronics, both crystalline and amorphous based will most probably depend on engineering of multilayer structures. In these devices an enhancement of RE3+ ion luminescence intensity can be achieved through *e.g.* (a) modification of the RE3+ center environment and (b) localization of carriers in the vicinity of emitting RE3+ ion center. The former one can be achieved through engineering stress/strain parameters during the growth process, whereas the last one will results from carriers confinement in the quantum structures.

In general, the majority available research papers focus on RE-doped crystalline III-nitride semiconductors with less emphasis on a-III-nitrides used as hosts for optically, electrically and/or magnetically active RE dopants. The amorphous semiconductors including a-IIInitrides as hosts for RE doping have many of the desirable qualities of the crystalline materials; however they also offer unique features rooted in the nature of the amorphous matrix (Adachi 1999; Singh 2003; Street 2010). The most important are that a-III-nitrides can be achieved at higher growth rates and lower temperatures, substrate selection and resulting lattice mismatch is not a significant obstacle here, they do not easily recrystallize when subjected to thermal processing and that they can adapt the RE ions in concentrations far beyond those given by the solubility limit found in their crystalline counterparts. Furthermore, it is known that a-IIInitrides show a natural tendency to develop a state free gap [(Chen and Drabold 2002; Drabold 2010). Also a-III-nitrides, unlike other amorphous systems or glasses which look locally very similar to the crystal, apparently have local environments very different from the main crystalline morphology what make them potentially useful as hosts for optically and electrically active RE dopants. It was demonstrated that the RE ions luminescence intensity, when intended for optoelectronic applications, can be increased by proper structural tailoring of a-III-nitride matrices. At the same time the RE3+ ion excitation and de-excitation processes are more dependent on the local RE ion environment than in crystalline semiconductors due to the lack of long range ordering in amorphous host (Zanatta, Ribeiro et al. 2006). The *f-f* transitions are forbidden, but they are partially allowed when the wave functions are mixed with those of opposing parity. This always occurs on localized ions in non-central symmetrical lattices. The forbidden nature of these transitions results in long radiative lifetimes when the selection rules are relaxed. Placing trivalent rare earth ions in amorphous solids can still result in characteristic emission from these intra-4*f* transitions despite the lack of a crystalline host structure (O'Donnell 2010). In both cases RE luminescence depends very little on the nature of the host and the ambient temperature. The 4*f* orbitals of RE ions incorporated into semiconductors, including a-III-nitrides, are so deeply buried within the electronic shell that the energy levels of the 4*f*<sup>n</sup> configuration are only slightly perturbed compared to free ion energy levels indicating large independence from the chemical surrounding and thus the choice of the host material (Wybourne and Smentek 2007). RE ions when doped to a-III-nitrides are known to exhibit sharp and well-defined luminescence which arises mostly from internal transitions between 4*f* levels within RE3+ ion. Because of the shield provided by the outer 5*s*2 and 5*p*6 shells, the wavelength involved in these intra-4*f* transitions is less temperature sensitive and rather weakly depends on the a-III-nitride host. On the other hand the mechanism behind the activation of RE are known to be sensitive to a local host details such as atomic structure and optical band gap which defines which RE transitions can be excited.

#### **3.1 Rare earth doping**

168 Cathodoluminescence

sources with narrow line-width emissions from the ultraviolet to the infrared (Steckl 1999). Thus optoelectronic devices utilizing 4*f*n transitions appearing in the nitrides forbidden band gap "window" are practically viable with these semiconductors (O'Donnell 2010). Recently it was demonstrated that the low voltage-operation of current-injected red emission from a crystalline *p*-type/Eu-doped/*n*-type GaN epilayers light emission diode operating at room temperature (Nishikawa, Kawasaki et al. 2009; Kasai, Nishikawa et al. 2010; Nishikawa, Furukawa 2010; Dierolf 2011) together with demonstrations of stimulated emission from Eu3+ doped GaN and AlGaN layers on a Si substrate proved in principal that optoelectronic devices covering the UV, visible and IR regions might be fabricated (Park and Steckl 2004; Park and

These new results suggest a novel way to realize III-nitride semiconductors-based red emitting, current driven light emitting devices, as well as other primary colors and their mixture, monolithic devices. In the past the feasibility of using the RE-doped amorphous IIInitrides (a-III-nitrides) for light emitting applications was also demonstrated (Dimitrova, Van Patten et al. 2000; Dimitrova, Van Patten et al. 2001; Richardson, Van Patten et al. 2002; Maqbool, Kordesch et al. 2009), (Kim, Shepherd et al. 2003; Kim and Holloway 2004). However, to make these devices commercially viable, the internal quantum efficiency of the active RE-doped layers has to be significantly improved. The future success of the RE-doped III-nitrides optoelectronics, both crystalline and amorphous based will most probably depend on engineering of multilayer structures. In these devices an enhancement of RE3+ ion luminescence intensity can be achieved through *e.g.* (a) modification of the RE3+ center environment and (b) localization of carriers in the vicinity of emitting RE3+ ion center. The former one can be achieved through engineering stress/strain parameters during the growth process, whereas the last one will results from carriers confinement in the quantum structures. In general, the majority available research papers focus on RE-doped crystalline III-nitride semiconductors with less emphasis on a-III-nitrides used as hosts for optically, electrically and/or magnetically active RE dopants. The amorphous semiconductors including a-IIInitrides as hosts for RE doping have many of the desirable qualities of the crystalline materials; however they also offer unique features rooted in the nature of the amorphous matrix (Adachi 1999; Singh 2003; Street 2010). The most important are that a-III-nitrides can be achieved at higher growth rates and lower temperatures, substrate selection and resulting lattice mismatch is not a significant obstacle here, they do not easily recrystallize when subjected to thermal processing and that they can adapt the RE ions in concentrations far beyond those given by the solubility limit found in their crystalline counterparts. Furthermore, it is known that a-IIInitrides show a natural tendency to develop a state free gap [(Chen and Drabold 2002; Drabold 2010). Also a-III-nitrides, unlike other amorphous systems or glasses which look locally very similar to the crystal, apparently have local environments very different from the main crystalline morphology what make them potentially useful as hosts for optically and electrically active RE dopants. It was demonstrated that the RE ions luminescence intensity, when intended for optoelectronic applications, can be increased by proper structural tailoring of a-III-nitride matrices. At the same time the RE3+ ion excitation and de-excitation processes are more dependent on the local RE ion environment than in crystalline semiconductors due to the lack of long range ordering in amorphous host (Zanatta, Ribeiro et al. 2006). The *f-f* transitions are forbidden, but they are partially allowed when the wave functions are mixed with those of opposing parity. This always occurs on localized ions in non-central symmetrical lattices. The

Steckl 2005; Park and Steckl 2006).

Doping of a-III-nitrides is generally achieved *in situ* ; however it can be also done by RE ions implantation (Aldabergenova, Frank et al. 2006). In the former case doping typically takes place during sputtering process in which RE-doped a-III-nitride is formed from Ga, Al and RE atoms removed from metal targets by momentum transfer from an RF plasma. The sputtering growth of a-III-nitrides is explored much less than other technologically important amorphous semiconductors e.g a-Si doped with RE impurities (Iacona, Franzo et al. 2009). Ion implantation has been used extensively to *ex situ* dope III-nitride crystalline semiconductors with variety of rare earth ions. Excellent review concerning this issue is available (O'Donnell 2010). There are a limited number of papers on RE ions implantation to a-III-nitrides and resulting luminescence spectra (Jadwisienczak, Lozykowski et al. 2000). Ion implantation is a well established technique, which provides good control of the concentration profile in the sample. However, because of the unavoidable damage associated with the implantation process, typically a high temperature annealing step is imperative. This is due to the fact that when heavy atoms like RE metals are implanted to an amorphous semiconductor they can introduce both a chemical change in the substrate, in that they can introduce a different element than the host or induce a nuclear transmutation, and a structural change, in that the morphology of the host can be damaged or even destroyed by the energetic collision cascades induced by implanted heavy RE ions (Kucheyev, Williams et al. 2004).

It is known that the thermal anneal of the a-AlN films affects not only the luminescent properties of the films, but also their optical band gap and chemical characteristics. As a result of thermal treatments of RE-doped a-III-nitrides at increasing temperatures the optical-absorption edge of amorphous films may change indicating the Al and N bonding environment modification (Zanatta, Ribeiro et al. 2005). Based on the available experimental results it is clear that RE-doped a-III-nitrides subjected to isochronal thermal annealing treatments indeed most likely underwent partial removal of both structural and chemical disorder resulting in modification of RE ions local environments as it is shown in Figs. 3.1 and 3.2 for Er-doped a-AlN films (Gurumurugan, Chen et al. 1999; Jadwisienczak, Lozykowski et al. 2000; Weingartner, Erlenbach et al. 2006).

Cathodoluminescence from Amorphous and

Nanocrystalline Nitride Thin Films Doped with Rare Earth and Transition Metals 171

It is believed that when this takes place the dangling or broken bonds and/or the atomic rearrangements of the amorphous material may have great influence on the electronic states of amorphous host. The available reports show that the conducted x-ray diffraction analysis of as-grown or thermally treated RE-doped a-III-nitride thin films has not revealed any

Fig. 3.3. XRD spectra of a-AlN thin films doped with Tb3+ ions. (a) unannealed and (b) annealed for 30 min at 1050 ºC in nitrogen. Ref. (Jadwisienczak, Lozykowski et al. 2000).

AlN:Ho host was observed after annealing above 900 ºC. See Fig 3.4.

As reported by Zanatta et al. (Zanatta, Khan et al. 2007) a-SiN doped with Sm and Tb exhibit significant enhancements of 4*f*-shell radiative recombinations upon thermal annealing due to a decrease in the density of deep and/or tail defects (suppression of the nonradiative processes). A similar observation was made for a-AlN:Ho system (Aldabergenova, Frank et al. 2006). Furthermore, in that study, the growth of small crystallites in initially mostly a-

In order to avoid the implantation induced amorphous material morphological changes and defects due to ion implantation, simultaneous co-deposition of different RE metals during RF sputtering growth was demonstrated (Maqbool, Kordesch et al. 2009). This approach is an extension of a typical a-III-nitrides RF sputtering growth technique where RE ions concentration in resulting film is controlled by sputtering metals to target surface ratio. It is possible using this procedure to control the RE concentration in the sample over a wide range. Furthermore, this technique gave an opportunity to derive mixed RE-doped a-IIInitride systems where more complex interaction schemes defining energy transfer processes can be studied. However, there exist only limited papers reporting on this issue up to date (Maqbool, Kordesch et al. 2009). The single RE ion and multiple RE ions doping to a-IIInitrides should have typical doping effects in these hosts in the sense of controlling the electronic properties of the material and determining the Fermi level position. In contrast to unhydrogenated a-Si and hydrogenated a-Si:H where it was proposed that several different REs turned out to act as acceptors or donors (Tessler 1999) no similar postulate was considered in the case of RE-doped a-III-nitrides to date. It is uncertain at present if other than trivalent RE ions in a-III-nitrides can exist; however such a scenario is feasible

microscopic morphology changes in studied materials (see Fig. 3.3).

Fig. 3.1. Optical transmission spectra of undoped and Er-doped a-AlN thin films. Inset shows the photon energy dependence of the square of the absorption coefficient for undoped and Er-doped a-AlN thin films. After Ref. (Gurumurugan, Chen et al. 1999)

It is documented in the literature that thermally annealed amorphous semiconductors experience some structural reordering (not necessarily crystallization) (Gurumurugan, Chen et al. 1999). Figure 3.2 shows evolution of cathodoluminescence (CL) spectra of Tb-doped a-AlN films subjected to thermal annealing in nitrogen ambient up to 1100 ºC.

Fig. 3.2. Evolution of CL spectra in Tb-doped a-AlN films as a function of annealing temperature. After Ref. (Gurumurugan, Chen et al. 1999).

Fig. 3.1. Optical transmission spectra of undoped and Er-doped a-AlN thin films. Inset shows the photon energy dependence of the square of the absorption coefficient for undoped and Er-doped a-AlN thin films. After Ref. (Gurumurugan, Chen et al. 1999)

AlN films subjected to thermal annealing in nitrogen ambient up to 1100 ºC.

Fig. 3.2. Evolution of CL spectra in Tb-doped a-AlN films as a function of annealing

temperature. After Ref. (Gurumurugan, Chen et al. 1999).

It is documented in the literature that thermally annealed amorphous semiconductors experience some structural reordering (not necessarily crystallization) (Gurumurugan, Chen et al. 1999). Figure 3.2 shows evolution of cathodoluminescence (CL) spectra of Tb-doped aIt is believed that when this takes place the dangling or broken bonds and/or the atomic rearrangements of the amorphous material may have great influence on the electronic states of amorphous host. The available reports show that the conducted x-ray diffraction analysis of as-grown or thermally treated RE-doped a-III-nitride thin films has not revealed any microscopic morphology changes in studied materials (see Fig. 3.3).

Fig. 3.3. XRD spectra of a-AlN thin films doped with Tb3+ ions. (a) unannealed and (b) annealed for 30 min at 1050 ºC in nitrogen. Ref. (Jadwisienczak, Lozykowski et al. 2000).

As reported by Zanatta et al. (Zanatta, Khan et al. 2007) a-SiN doped with Sm and Tb exhibit significant enhancements of 4*f*-shell radiative recombinations upon thermal annealing due to a decrease in the density of deep and/or tail defects (suppression of the nonradiative processes). A similar observation was made for a-AlN:Ho system (Aldabergenova, Frank et al. 2006). Furthermore, in that study, the growth of small crystallites in initially mostly a-AlN:Ho host was observed after annealing above 900 ºC. See Fig 3.4.

In order to avoid the implantation induced amorphous material morphological changes and defects due to ion implantation, simultaneous co-deposition of different RE metals during RF sputtering growth was demonstrated (Maqbool, Kordesch et al. 2009). This approach is an extension of a typical a-III-nitrides RF sputtering growth technique where RE ions concentration in resulting film is controlled by sputtering metals to target surface ratio. It is possible using this procedure to control the RE concentration in the sample over a wide range. Furthermore, this technique gave an opportunity to derive mixed RE-doped a-IIInitride systems where more complex interaction schemes defining energy transfer processes can be studied. However, there exist only limited papers reporting on this issue up to date (Maqbool, Kordesch et al. 2009). The single RE ion and multiple RE ions doping to a-IIInitrides should have typical doping effects in these hosts in the sense of controlling the electronic properties of the material and determining the Fermi level position. In contrast to unhydrogenated a-Si and hydrogenated a-Si:H where it was proposed that several different REs turned out to act as acceptors or donors (Tessler 1999) no similar postulate was considered in the case of RE-doped a-III-nitrides to date. It is uncertain at present if other than trivalent RE ions in a-III-nitrides can exist; however such a scenario is feasible

Cathodoluminescence from Amorphous and

Nanocrystalline Nitride Thin Films Doped with Rare Earth and Transition Metals 173

hole, the isoelectronic trap is negatively or positively charged, and by Coulomb interaction it will capture a carrier of the opposite charge creating a bound exciton. There are three possible mechanisms of energy transfer (Lozykowski 1993). The first is the energy transfer process from excitons bound to structured isoelectronic centers to the core electrons. This takes place as a result of the electrostatic perturbation between the core electrons of the RE structured impurity and the exciton, effective-mass-like particles. The second mechanism is the transfer of energy to the core electrons involving the structured isoelectronic trap occupied by electron (hole) and free holes (electrons) in the valence (conduction) band. The third mechanism is the transfer through an inelastic scattering process in which the energy of a free exciton near a RE structured trap is given to the localized core excited states. If the initial and final states are not resonant, the energy mismatch must be distributed in some way, e.g., by phonon emission or absorption. If the atomic core excitations are strongly coupled to the host phonons, the energy transfer probability is likely to be higher (Lozykowski 1993). Strong phonon coupling may also be desirable in ensuring that relaxation down the ladder of the core excited state occurs quickly, thus preventing back transfer. However, for efficient radiative recombination, the phonon coupling should not be strong, in order to prevent core de-excitation by nonradiative multiphonon process. It is natural to assume that when isoelectronic impurity atoms, in this case RE ions, are incorporated in a-III-nitride host, they modify local vibrational properties in a definite way. Thus, there should be observed distinct vibrational frequencies associated with localized motion of the RE impurity in addition to the host lattice vibrations. In III-nitrides the threshold energy for electron damage in amorphous films is considerably smaller contrary to crystalline compounds (Zanatta, Ribeiro et al. 2005). Moreover, energetic electron irradiation may create defects that are not possible with photons with energies in the visible energy range. These additional defects, mainly broken bonds and atomic displacements, act as nonradiative centers that considerably reduce the luminescence efficiency of a-III-nitride semiconductor. On the other hand, RE ions are high efficiency recombination centers that effectively compete with other non-radiative processes taking place in a semiconductor host. As a result, when irradiating the RE-doped a-III-nitrides with up to a few keV energy electrons, most of the electron-hole pairs recombine preferentially through the RE ions. Luminescence after photon excitation, on the other hand, behaves in a different manner and is very strongly excitation wavelengths dependent (Gurumurugan, Chen et al. 1999;

Jadwisienczak, Lozykowski et al. 2000; Zanatta, Ribeiro et al. 2005).

In photoluminescence each absorbed photon with energy higher than the band gap produces a single electron-hole pair while in cathodoluminescence, high energy single electrons generate a huge number of ''hot'' e–h pairs, reducing the energy from tens of keV to zero (Jadwisienczak, Lozykowski et al. 2000). In general, the optical excitation process demands that the *4f* electrons are excited absorbing energy. In a-III-nitride materials with large band gap like a-GaN and a-AlN this can occur with a direct absorption to one of the transitions of upper energy of the RE3+ ion degenerated with the conduction band or via transfer of energy from a defect related Auger resonant process between the RE3+ ion states and the dipole formed by the *s*-like conduction band states and *p*-like dangling bond states. The PL of RE3+ ion in a-III-nitrides is obtained by an indirect excitation process via transfer of energy to the 4*fn* electron system from e-h pairs. In electron beam excitation (CL), however, the RE3+ ions are excited by direct impact with hot electrons, as well as by energy transfer processes from the generated e–h pairs or by impact excitation (or ionization)

especially if one considers a plethora of possible RE ions configurations in amorphous matrix as well as the fact that RE ions are strong oxygen and other impurities getters. Systematic RE ion doping studies with controlled co-dopant concentrations are needed to validate this consideration in the future.

Fig. 3.4. High resolution micrograph showing small AlN crystallites embedded in as grown a-AlN film. The insert shows the pertaining diffraction pattern. After (Aldabergenova, Frank et al. 2006).

#### **3.1.1 Rare earth excitation**

The excitation processes of RE ions in a-III-nitrides, similarly to crystalline hosts, can be generally divided into two categories: direct and indirect excitation processes (Jadwisienczak, Lozykowski et al. 2000; Lozykowski, Jadwisienczak et al. 2000). The direct excitation process occurs in selective excitation of 4*fn* electrons by photons (photoluminescence, PL) selective excitation or in cathodoluminescence (CL) and electroluminescence (EL) by collision with hot electrons. The indirect excitation process occurs via transfer of energy to the 4*fn* electron system from electron–hole pairs generated by photons with higher energy than the band gap (PL excited above band gap), injected in forward bias *p*-*n* junctions, or generated by hot carriers in CL and EL. An excitation mechanism in CL and EL involves direct impact excitation of RE3+ ions by hot electrons, as well as an energy transfer from the generated electron–hole pairs or by impact excitation (or ionization) involving impurity states outside the 4*f* shell, with subsequent energy transfer to this shell. The most important, from an applications point of view, is the excitation of the RE ions by energy transfer processes from electron-hole (e-h) pairs or excitons. This process most probably involves the rare earth ion isovalent traps (Lozykowski, Jadwisienczak et al. 2000). Since there is no charge involved, the isoelectronic center forms the bound states by short range central cell potential. After an isoelectronic trap has captured an electron or a

especially if one considers a plethora of possible RE ions configurations in amorphous matrix as well as the fact that RE ions are strong oxygen and other impurities getters. Systematic RE ion doping studies with controlled co-dopant concentrations are needed to

Fig. 3.4. High resolution micrograph showing small AlN crystallites embedded in as grown a-AlN film. The insert shows the pertaining diffraction pattern. After (Aldabergenova,

The excitation processes of RE ions in a-III-nitrides, similarly to crystalline hosts, can be generally divided into two categories: direct and indirect excitation processes (Jadwisienczak, Lozykowski et al. 2000; Lozykowski, Jadwisienczak et al. 2000). The direct excitation process occurs in selective excitation of 4*fn* electrons by photons (photoluminescence, PL) selective excitation or in cathodoluminescence (CL) and electroluminescence (EL) by collision with hot electrons. The indirect excitation process occurs via transfer of energy to the 4*fn* electron system from electron–hole pairs generated by photons with higher energy than the band gap (PL excited above band gap), injected in forward bias *p*-*n* junctions, or generated by hot carriers in CL and EL. An excitation mechanism in CL and EL involves direct impact excitation of RE3+ ions by hot electrons, as well as an energy transfer from the generated electron–hole pairs or by impact excitation (or ionization) involving impurity states outside the 4*f* shell, with subsequent energy transfer to this shell. The most important, from an applications point of view, is the excitation of the RE ions by energy transfer processes from electron-hole (e-h) pairs or excitons. This process most probably involves the rare earth ion isovalent traps (Lozykowski, Jadwisienczak et al. 2000). Since there is no charge involved, the isoelectronic center forms the bound states by short range central cell potential. After an isoelectronic trap has captured an electron or a

validate this consideration in the future.

Frank et al. 2006).

**3.1.1 Rare earth excitation**

hole, the isoelectronic trap is negatively or positively charged, and by Coulomb interaction it will capture a carrier of the opposite charge creating a bound exciton. There are three possible mechanisms of energy transfer (Lozykowski 1993). The first is the energy transfer process from excitons bound to structured isoelectronic centers to the core electrons. This takes place as a result of the electrostatic perturbation between the core electrons of the RE structured impurity and the exciton, effective-mass-like particles. The second mechanism is the transfer of energy to the core electrons involving the structured isoelectronic trap occupied by electron (hole) and free holes (electrons) in the valence (conduction) band. The third mechanism is the transfer through an inelastic scattering process in which the energy of a free exciton near a RE structured trap is given to the localized core excited states. If the initial and final states are not resonant, the energy mismatch must be distributed in some way, e.g., by phonon emission or absorption. If the atomic core excitations are strongly coupled to the host phonons, the energy transfer probability is likely to be higher (Lozykowski 1993). Strong phonon coupling may also be desirable in ensuring that relaxation down the ladder of the core excited state occurs quickly, thus preventing back transfer. However, for efficient radiative recombination, the phonon coupling should not be strong, in order to prevent core de-excitation by nonradiative multiphonon process. It is natural to assume that when isoelectronic impurity atoms, in this case RE ions, are incorporated in a-III-nitride host, they modify local vibrational properties in a definite way. Thus, there should be observed distinct vibrational frequencies associated with localized motion of the RE impurity in addition to the host lattice vibrations. In III-nitrides the threshold energy for electron damage in amorphous films is considerably smaller contrary to crystalline compounds (Zanatta, Ribeiro et al. 2005). Moreover, energetic electron irradiation may create defects that are not possible with photons with energies in the visible energy range. These additional defects, mainly broken bonds and atomic displacements, act as nonradiative centers that considerably reduce the luminescence efficiency of a-III-nitride semiconductor. On the other hand, RE ions are high efficiency recombination centers that effectively compete with other non-radiative processes taking place in a semiconductor host. As a result, when irradiating the RE-doped a-III-nitrides with up to a few keV energy electrons, most of the electron-hole pairs recombine preferentially through the RE ions. Luminescence after photon excitation, on the other hand, behaves in a different manner and is very strongly excitation wavelengths dependent (Gurumurugan, Chen et al. 1999; Jadwisienczak, Lozykowski et al. 2000; Zanatta, Ribeiro et al. 2005).

In photoluminescence each absorbed photon with energy higher than the band gap produces a single electron-hole pair while in cathodoluminescence, high energy single electrons generate a huge number of ''hot'' e–h pairs, reducing the energy from tens of keV to zero (Jadwisienczak, Lozykowski et al. 2000). In general, the optical excitation process demands that the *4f* electrons are excited absorbing energy. In a-III-nitride materials with large band gap like a-GaN and a-AlN this can occur with a direct absorption to one of the transitions of upper energy of the RE3+ ion degenerated with the conduction band or via transfer of energy from a defect related Auger resonant process between the RE3+ ion states and the dipole formed by the *s*-like conduction band states and *p*-like dangling bond states. The PL of RE3+ ion in a-III-nitrides is obtained by an indirect excitation process via transfer of energy to the 4*fn* electron system from e-h pairs. In electron beam excitation (CL), however, the RE3+ ions are excited by direct impact with hot electrons, as well as by energy transfer processes from the generated e–h pairs or by impact excitation (or ionization)

Cathodoluminescence from Amorphous and

Nanocrystalline Nitride Thin Films Doped with Rare Earth and Transition Metals 175

excited RE ions in semiconductors, including RE-doped a-III-nitrides, are phonon dependent (Jadwisienczak, Lozykowski et al. 2000). Furthermore, at sufficiently high concentrations of RE ions, nonradiative decay can occur via ion pair cross-relaxation or energy migration and finally transfer to quenching centers (Jadwisienczak, Lozykowski et al. 2000). For low RE concentration, where ion–ion pair relaxation or energy migration and transfer to quenching centers has low probability, excited RE electronic states gave up energy radiatively by purely electronic or phonon assisted transitions or nonradiatively by the emission of a single phonon or multiphonon. Nonradiative relaxation between RE3+ ion *J*-manifolds in solid state host, including reported CL kinetics of Tb-doped a-AlN , typically requires the participation of several phonons and occurs at slow rates, while relaxation between crystal field levels of a *J*manifold by single or two phonon processes is much faster. It is known that the rate of multiphonon emission is strongly dependent upon the number of phonons required to conserve energy, and hence on the size of the energy gap to the next lowest energy level. The detailed investigations of nonradiative decay of RE ions in a-III-nitrides have not been done yet; however it is expected that that multiphonon transitions involving the emission of a maximum of a few phonons will effectively compete with radiative transitions. Furthermore, it is expected that the temperature dependence of the decay rates will show that relaxation occurs mainly by high energy phonons. It is worth noting that energy separation of *J*-crystal field levels shows only small changes with the host (Jadwisienczak, Lozykowski et al. 2000; Jadwisienczak, Lozykowski et al. 2000). Differences in the rate of nonradiative decay from a particular RE energy level arise from the phonon energy spectrum of the host material and the strength of the RE ion–lattice coupling. Local vibrational modes related to RE ions, their complexes, or others impurities present in a-III-nitride host will also play an important role in the above-discussed

process; however these assumptions will have to be critically confirmed in the future.

Fig. 3.6. CL spectra of a-AlN:Tb recorded at cryogenic and room temperature. The inserts show the total integral intensity of the dominant 4*f*-shell transition lines as a function of temperature.

After Ref. [(Jadwisienczak, Lozykowski et al. 2000)].

involving other impurities (or complex defects) with subsequent energy transfer to the RE 4*f-*shell electrons. Generally the excitation by energetic electrons produces emission via all possible luminescence mechanisms available in a semiconductor. Another factor which may play a role in excitation and emission processes is the charged nature of excitation: uncharged photons in PL versus negatively charged electrons in CL. The excitation depth in PL and CL are also different due to the strong absorption of the excitation photons in a-IIInitrides within a few tens of nanometers layer; whereas the electron penetration depth is electron acceleration energy dependent and can reach up to a few hundreds nanometers (Jadwisienczak, Lozykowski et al. 2000). Figure 3.5 and Fig. 3.6 show examples of CL spectra of the rare earth (Tm, Tb, Dy, Sm, Eu, Er and Yb) doped a-AlN layers [(Gurumurugan, Chen et al. 1999; Jadwisienczak, Lozykowski et al. 2000; Weingartner, Erlenbach et al. 2006). The transitions corresponding to the strongest emission lines are indicated by their energy level assignments. It was reported in these studies that all RE-doped a-AlN layers show (even untreated) pronounced RE3+ ion optical spectra at room temperature.

Fig. 3.5. Survey of CL spectra of RE-doped a-AlN thin films. After Ref. (Weingartner, Erlenbach et al. 2006).

The RE ion excitation process responsible for observed EL from RE-doped a-III-nitrides is very similar to described above CL. Thus CL and EL spectra, in general, shows similar spectra features. It was reported that in cases of the Er and Tb-doped a-AlN alternating-current thinfilm electroluminescent devices the EL and CL spectra are very similar (Dimitrova, Van Patten et al. 2000; Dimitrova, Van Patten et al. 2001; Richardson, Van Patten et al. 2002).

Typical transient analysis of luminescence observed from RE-doped III-nitride crystalline semiconductors stimulated by different means provided detailed insight into energy migration and excitation processes observed in these technologically important materials. There are a very limited number of papers reporting on luminescence (CL, EL) kinetics of RE-doped a-III-nitride up to date (Richardson, Van Patten et al. 2002). It is known that the nonradiative decays of

involving other impurities (or complex defects) with subsequent energy transfer to the RE 4*f-*shell electrons. Generally the excitation by energetic electrons produces emission via all possible luminescence mechanisms available in a semiconductor. Another factor which may play a role in excitation and emission processes is the charged nature of excitation: uncharged photons in PL versus negatively charged electrons in CL. The excitation depth in PL and CL are also different due to the strong absorption of the excitation photons in a-IIInitrides within a few tens of nanometers layer; whereas the electron penetration depth is electron acceleration energy dependent and can reach up to a few hundreds nanometers (Jadwisienczak, Lozykowski et al. 2000). Figure 3.5 and Fig. 3.6 show examples of CL spectra of the rare earth (Tm, Tb, Dy, Sm, Eu, Er and Yb) doped a-AlN layers [(Gurumurugan, Chen et al. 1999; Jadwisienczak, Lozykowski et al. 2000; Weingartner, Erlenbach et al. 2006). The transitions corresponding to the strongest emission lines are indicated by their energy level assignments. It was reported in these studies that all RE-doped a-AlN layers show (even

untreated) pronounced RE3+ ion optical spectra at room temperature.

Fig. 3.5. Survey of CL spectra of RE-doped a-AlN thin films. After Ref. (Weingartner,

et al. 2000; Dimitrova, Van Patten et al. 2001; Richardson, Van Patten et al. 2002).

The RE ion excitation process responsible for observed EL from RE-doped a-III-nitrides is very similar to described above CL. Thus CL and EL spectra, in general, shows similar spectra features. It was reported that in cases of the Er and Tb-doped a-AlN alternating-current thinfilm electroluminescent devices the EL and CL spectra are very similar (Dimitrova, Van Patten

Typical transient analysis of luminescence observed from RE-doped III-nitride crystalline semiconductors stimulated by different means provided detailed insight into energy migration and excitation processes observed in these technologically important materials. There are a very limited number of papers reporting on luminescence (CL, EL) kinetics of RE-doped a-III-nitride up to date (Richardson, Van Patten et al. 2002). It is known that the nonradiative decays of

Erlenbach et al. 2006).

excited RE ions in semiconductors, including RE-doped a-III-nitrides, are phonon dependent (Jadwisienczak, Lozykowski et al. 2000). Furthermore, at sufficiently high concentrations of RE ions, nonradiative decay can occur via ion pair cross-relaxation or energy migration and finally transfer to quenching centers (Jadwisienczak, Lozykowski et al. 2000). For low RE concentration, where ion–ion pair relaxation or energy migration and transfer to quenching centers has low probability, excited RE electronic states gave up energy radiatively by purely electronic or phonon assisted transitions or nonradiatively by the emission of a single phonon or multiphonon. Nonradiative relaxation between RE3+ ion *J*-manifolds in solid state host, including reported CL kinetics of Tb-doped a-AlN , typically requires the participation of several phonons and occurs at slow rates, while relaxation between crystal field levels of a *J*manifold by single or two phonon processes is much faster. It is known that the rate of multiphonon emission is strongly dependent upon the number of phonons required to conserve energy, and hence on the size of the energy gap to the next lowest energy level. The detailed investigations of nonradiative decay of RE ions in a-III-nitrides have not been done yet; however it is expected that that multiphonon transitions involving the emission of a maximum of a few phonons will effectively compete with radiative transitions. Furthermore, it is expected that the temperature dependence of the decay rates will show that relaxation occurs mainly by high energy phonons. It is worth noting that energy separation of *J*-crystal field levels shows only small changes with the host (Jadwisienczak, Lozykowski et al. 2000; Jadwisienczak, Lozykowski et al. 2000). Differences in the rate of nonradiative decay from a particular RE energy level arise from the phonon energy spectrum of the host material and the strength of the RE ion–lattice coupling. Local vibrational modes related to RE ions, their complexes, or others impurities present in a-III-nitride host will also play an important role in the above-discussed process; however these assumptions will have to be critically confirmed in the future.

Fig. 3.6. CL spectra of a-AlN:Tb recorded at cryogenic and room temperature. The inserts show the total integral intensity of the dominant 4*f*-shell transition lines as a function of temperature. After Ref. [(Jadwisienczak, Lozykowski et al. 2000)].

Cathodoluminescence from Amorphous and

**3.2 Amorphous nitrides: Examples**

peaks excessively.

structure is possible.

Nanocrystalline Nitride Thin Films Doped with Rare Earth and Transition Metals 177

due to their large radii as well as gettering effect (O'Donnell 2010). It was theoretically shown that the Coulomb excitation of 4*f* electrons near interface of heterostructures, where certain degree of disorder is expected, is more effective than a similar excitation in the bulk semiconductors (Zegrya and Masterov 1995; Zegrya and Masterov 1996; Zegrya and Masterov 1998; Masterov and Gerchikov 1999). Also, as discussed above, the thermal annealing process of RE-doped a-III-nitride changes grain boundary, where RE ions most probably reside, resulting in change of RE ion local environments and their optical activities due to relaxed 4*f*-shell electron-lattice coupling. In that sense one may assume that RE3+ ions located at the surface of the a-III-nitride nano-grains may act as grain boundary activators. It has now been well established that semiconductor nano-clusters can act as efficient luminescence sensitizers for RE ions in wide band gap solid states (Iacona, Franzo et al. 2009). Moreover, it appears that it is not necessary for the semiconductor inclusions to be crystalline – amorphous nano-clusters are at least as efficient as nano-crystals (Kenyon and Lucarz). The customary observation is that absorption of photons by semiconductor nano-clusters results in excitation of RE ions via an efficient transfer mechanism to unexcited RE ions. Such a process is significant for two reasons: firstly, the effective absorption cross-section of the RE ion is increased by several orders of magnitude (Kenyon, Lucarz; Kik 2003), and secondly, it becomes possible to excite the RE ions via the broad-band absorption of the semiconductor nano-clusters (Kenyon, Chryssou et al. 2002; Kik 2003; Iacona, Franzo et al. 2009). Moreover, the prospect of engaging amorphous nano-clusters to activate RE ions in a-III-nitrides removes the prerequisite to control tightly the crystallinity and size distribution of nanocluster sensitizers thus greatly simplifying material processing. More studies are necessary to clear this point. It is our belief that the quenching mechanism should not depend on the form of sample preparation rather than on the final RE ion local symmetry

inducing site degeneracy required for observing peculiar intra 4*f*-shell transitions.

Most of the metal nitrides are M3+N3- compounds, so that no special considerations are necessary for RE emission. There is still some controversy about the site of the RE ion in crystalline nitrides. In an amorphous solid, bonding to the N as a (3+) ion is the most obvious choice. However, it is the case that sharp lines are observed from RE ions in the amorphous hosts, and in some cases the analogue of "crystal field splitting". There are probably several types of sites for the RE ion, but not enough to broaden the emission

Because the emission lines in RE ions are well known, in most cases the experimental task is to see which transitions are observed in each host. Post growth heat treatment of the asgown nitrides often improves the intensity of the RE emission in the nitrides. Because there is no "annealing" in the sense of crystal structure or long range lattice improvement, the heating process can cause the removal or transport of materials unrelated to the nitride structure. There could be removal of hydrogen, for example, or diffusion of oxygen. The reaction of water trapped in the film due to background gasses in the vacuum or in the heat treatment gas during post growth thermal treatment would include the removal of N atoms as ammonia, affecting the basic M-N bonds. Some rearrangement of the short range nitride

Fig. 3.7. CL spectra of a-AlN:Er recorded at cryogenic and room temperature. The inserts show the total integral intensity of the dominant 4*f*-shell transition lines as a function of temperature. After Ref. [(Gurumurugan, Chen et al. 1999)].

The local environment of the RE3+ ions, and consequent luminescence intensity, in a-IIInitrides can be greatly influenced by the presence of nitrogen atoms, intrinsic impurities, larger clusters and thermal treatments stimulating morphological changes at micro-scale. This in turn affects the optical activity of RE3+ ion by enhancing or quenching luminescence originating from 4*f*-shalls transitions. The vulnerability of amorphous semiconductors to defects generation in CL and EL is rather straight forward considering the threshold energy for electron damage in amorphous networks being much smaller (1 keV) comparing to crystalline semiconductors (100 keV) (Zanatta, Ribeiro et al. 2005). This fact creates an interesting research opportunity for RE-doped a-III-nitrides in the search for more effective energy transfer process between RE ions and amorphous host. It is known that the III-nitride quantum structures with dimensions close to the excitonic Bohr radius exhibit electronic and optical properties affected by the confinement of electrons in one, two, or three dimensions (Ihn 2010). The principal consequences of quantum confinement are an increase in the band gap energy and increased probability of radiative transitions. Confinement of carriers in real space causes their wavefunctions to spread out in momentum space, increasing the probability of radiative processes due to greater wavefunction overlap. Because of the modification of their band structure, doping such confined systems with RE3+ ions can also help to overcome some of the nonradiative deexcitation problems associated with RE-doped III-nitride. Quantum confinement affects also carrier lifetimes and their degree of localization in real space. This in turn modifies the Auger back-transfer processes that limit luminescence efficiency and increases the interaction probability between the confined carriers and the RE3+ ions. In general, the RE3+ ion radiative quantum efficiency strongly depends on the carrier mediated energy transfer processes, which have to compete with nonradiative recombination channels abundant in a-III-nitrides. It is known that RE3+ ions induce significant local site distortion due to their large radii as well as gettering effect (O'Donnell 2010). It was theoretically shown that the Coulomb excitation of 4*f* electrons near interface of heterostructures, where certain degree of disorder is expected, is more effective than a similar excitation in the bulk semiconductors (Zegrya and Masterov 1995; Zegrya and Masterov 1996; Zegrya and Masterov 1998; Masterov and Gerchikov 1999). Also, as discussed above, the thermal annealing process of RE-doped a-III-nitride changes grain boundary, where RE ions most probably reside, resulting in change of RE ion local environments and their optical activities due to relaxed 4*f*-shell electron-lattice coupling. In that sense one may assume that RE3+ ions located at the surface of the a-III-nitride nano-grains may act as grain boundary activators. It has now been well established that semiconductor nano-clusters can act as efficient luminescence sensitizers for RE ions in wide band gap solid states (Iacona, Franzo et al. 2009). Moreover, it appears that it is not necessary for the semiconductor inclusions to be crystalline – amorphous nano-clusters are at least as efficient as nano-crystals (Kenyon and Lucarz). The customary observation is that absorption of photons by semiconductor nano-clusters results in excitation of RE ions via an efficient transfer mechanism to unexcited RE ions. Such a process is significant for two reasons: firstly, the effective absorption cross-section of the RE ion is increased by several orders of magnitude (Kenyon, Lucarz; Kik 2003), and secondly, it becomes possible to excite the RE ions via the broad-band absorption of the semiconductor nano-clusters (Kenyon, Chryssou et al. 2002; Kik 2003; Iacona, Franzo et al. 2009). Moreover, the prospect of engaging amorphous nano-clusters to activate RE ions in a-III-nitrides removes the prerequisite to control tightly the crystallinity and size distribution of nanocluster sensitizers thus greatly simplifying material processing. More studies are necessary to clear this point. It is our belief that the quenching mechanism should not depend on the form of sample preparation rather than on the final RE ion local symmetry inducing site degeneracy required for observing peculiar intra 4*f*-shell transitions.

#### **3.2 Amorphous nitrides: Examples**

176 Cathodoluminescence

Fig. 3.7. CL spectra of a-AlN:Er recorded at cryogenic and room temperature. The inserts show the total integral intensity of the dominant 4*f*-shell transition lines as a function of

The local environment of the RE3+ ions, and consequent luminescence intensity, in a-IIInitrides can be greatly influenced by the presence of nitrogen atoms, intrinsic impurities, larger clusters and thermal treatments stimulating morphological changes at micro-scale. This in turn affects the optical activity of RE3+ ion by enhancing or quenching luminescence originating from 4*f*-shalls transitions. The vulnerability of amorphous semiconductors to defects generation in CL and EL is rather straight forward considering the threshold energy for electron damage in amorphous networks being much smaller (1 keV) comparing to crystalline semiconductors (100 keV) (Zanatta, Ribeiro et al. 2005). This fact creates an interesting research opportunity for RE-doped a-III-nitrides in the search for more effective energy transfer process between RE ions and amorphous host. It is known that the III-nitride quantum structures with dimensions close to the excitonic Bohr radius exhibit electronic and optical properties affected by the confinement of electrons in one, two, or three dimensions (Ihn 2010). The principal consequences of quantum confinement are an increase in the band gap energy and increased probability of radiative transitions. Confinement of carriers in real space causes their wavefunctions to spread out in momentum space, increasing the probability of radiative processes due to greater wavefunction overlap. Because of the modification of their band structure, doping such confined systems with RE3+ ions can also help to overcome some of the nonradiative deexcitation problems associated with RE-doped III-nitride. Quantum confinement affects also carrier lifetimes and their degree of localization in real space. This in turn modifies the Auger back-transfer processes that limit luminescence efficiency and increases the interaction probability between the confined carriers and the RE3+ ions. In general, the RE3+ ion radiative quantum efficiency strongly depends on the carrier mediated energy transfer processes, which have to compete with nonradiative recombination channels abundant in a-III-nitrides. It is known that RE3+ ions induce significant local site distortion

temperature. After Ref. [(Gurumurugan, Chen et al. 1999)].

Most of the metal nitrides are M3+N3- compounds, so that no special considerations are necessary for RE emission. There is still some controversy about the site of the RE ion in crystalline nitrides. In an amorphous solid, bonding to the N as a (3+) ion is the most obvious choice. However, it is the case that sharp lines are observed from RE ions in the amorphous hosts, and in some cases the analogue of "crystal field splitting". There are probably several types of sites for the RE ion, but not enough to broaden the emission peaks excessively.

Because the emission lines in RE ions are well known, in most cases the experimental task is to see which transitions are observed in each host. Post growth heat treatment of the asgown nitrides often improves the intensity of the RE emission in the nitrides. Because there is no "annealing" in the sense of crystal structure or long range lattice improvement, the heating process can cause the removal or transport of materials unrelated to the nitride structure. There could be removal of hydrogen, for example, or diffusion of oxygen. The reaction of water trapped in the film due to background gasses in the vacuum or in the heat treatment gas during post growth thermal treatment would include the removal of N atoms as ammonia, affecting the basic M-N bonds. Some rearrangement of the short range nitride structure is possible.

Cathodoluminescence from Amorphous and

exclusively due to Sm2+ ions (Dieke 1968).

**3.2.2 Amorphous aluminum nitride**

(Jadwisienczak, Lozykowski et al. 2000).

**<IP_ADDRESS> Erbium and terbium**

**<IP_ADDRESS> Europium**

(Zanatta, Ribeiro et al. 2004) **<IP_ADDRESS> Samarium and terbium**

2007).

Nanocrystalline Nitride Thin Films Doped with Rare Earth and Transition Metals 179

The CL spectra of the present RE-doped a-SiN films are shown in Fig. 3.8. In addition to the levels close to 490 nm these new transitions correspond to Er3+ ions (4G11/2 to 4I15/2 at 390 nm, 2H9/2 to 4I15/2 at 410 nm); Ho3+ ions (5G5 to 5I5 at 430 nm, 3H5 to 5I5 at 360 nm); Sm3+ (4K11/2 to 6H5/2 at 380 nm); and Pr3+ ions (1S0 to 1D2 at 370 nm, 1S0 to 1I6 or 3P1 at 395 nm). The variations observed in the luminescence intensity probably occur because of differences in the RE content; Er (0.5 at.%), Ho (0.3 at.%), Dy (0.3 at.%), Sm (0.5 at.%), and Pr (0.2 at.%).

Most of the luminescence features present in the 400–850 nm wavelength range (Fig. 3.9) correspond to optical transitions due to the Sm3+ and Tb3+ ions (Dieke 1968): Sm1 at ∼565 nm (4G5*/*<sup>2</sup> → 6H5*/*2), Sm2 at ∼605 nm (4G5*/*2 → 6H7*/*2), Sm3 at ∼650 nm (4G5*/*<sup>2</sup> → 6H9*/*2), Tb1 at ∼485 nm (5D4 → 7F6), Tb2 at ∼545 nm (5D4 → 7F5), Tb3 at ∼590 nm (5D4 → 7F4), and Tb4 at ∼625 nm (5D4 → 7F3). In addition to these, we also observe the Sm4 light emission at ∼725 nm (4G5*/*<sup>2</sup> →6H11*/*2), which is associated with the superposition of the 4G5*/*<sup>2</sup> → 6H11*/*2 transition due to Sm3+ ions and the 5D0 → 7F0, 5D0 → 7F1, and 5D0 → 7F2 transitions of Sm2+ ions. The infrared contribution Sm5 at ∼810 nm corresponds to the 5D0 →7F4 transition and is

Fig. 3.9. Smarium and Terbium CL in amorphous silicon nitride. From (Zanatta, Khan et al.

The first RE spectrum in amorphous Aluminum Nitride (a-AlN) was that of Gurumurugan et al. (Gurumurugan, Chen et al. 1999). The spectra are shown in Figures and 3.7

Europium doped a-AlN was studied by Caldwell et al. (Caldwell, Van Patten et al. 2001).

The most significant result of this study was that thermal activation of the RE ion in a-AlN, which was found generally to increase the luminescence yield by a factor of up to 100, could

Even though there can be much larger RE concentrations in sputter deposited nitride films compared to ion implanted films, there is no dramatic increase in intensity with higher RE concentrations. It must be assumed that a large number of the RE ions are not optically active in the as-grown amorphous films. Heat treatment or oxygen additions increase the number of active RE ions. Co-doping with other (3+) ions sometimes also improves the emission intensity of RE ion luminescence. One of the possibilities is that the (3+) impurity occupies the inactive sites and displaces the RE ions onto active sites. Optical interactions are also possible.

#### **3.2.1 Amorphous silicon nitride**

The bulk of studies on amorphous silicon nitride doped with RE ions are due to Zanata and co-workers. Silicon is multivalent, so that a-SiN may well result in a RE3+ ion without any further need for charge compensation.

#### **<IP_ADDRESS> Erbium, Holmium, Dysprosium Samarium and Praseodymium**

Zanatta and co-workers studied Pr, Sm, Dy, Ho, and Er in amorphous SiN films.

Fig. 3.8. Cathodoluminescence spectra of RE3+-doped a-SiN films deposited by cosputtering. The spectra have been acquired at room temperature by using 15 keV electrons and a current density of 10 nA. All spectra were normalized and vertically shifted for comparison. From (Zanatta, Ribeiro et al. 2004).

The CL spectra of the present RE-doped a-SiN films are shown in Fig. 3.8. In addition to the levels close to 490 nm these new transitions correspond to Er3+ ions (4G11/2 to 4I15/2 at 390 nm, 2H9/2 to 4I15/2 at 410 nm); Ho3+ ions (5G5 to 5I5 at 430 nm, 3H5 to 5I5 at 360 nm); Sm3+ (4K11/2 to 6H5/2 at 380 nm); and Pr3+ ions (1S0 to 1D2 at 370 nm, 1S0 to 1I6 or 3P1 at 395 nm). The variations observed in the luminescence intensity probably occur because of differences in the RE content; Er (0.5 at.%), Ho (0.3 at.%), Dy (0.3 at.%), Sm (0.5 at.%), and Pr (0.2 at.%). (Zanatta, Ribeiro et al. 2004)

#### **<IP_ADDRESS> Samarium and terbium**

178 Cathodoluminescence

Even though there can be much larger RE concentrations in sputter deposited nitride films compared to ion implanted films, there is no dramatic increase in intensity with higher RE concentrations. It must be assumed that a large number of the RE ions are not optically active in the as-grown amorphous films. Heat treatment or oxygen additions increase the number of active RE ions. Co-doping with other (3+) ions sometimes also improves the emission intensity of RE ion luminescence. One of the possibilities is that the (3+) impurity occupies the inactive sites and displaces the RE ions onto active sites. Optical interactions

The bulk of studies on amorphous silicon nitride doped with RE ions are due to Zanata and co-workers. Silicon is multivalent, so that a-SiN may well result in a RE3+ ion without any

**<IP_ADDRESS> Erbium, Holmium, Dysprosium Samarium and Praseodymium**

Zanatta and co-workers studied Pr, Sm, Dy, Ho, and Er in amorphous SiN films.

Fig. 3.8. Cathodoluminescence spectra of RE3+-doped a-SiN films deposited by co-

comparison. From (Zanatta, Ribeiro et al. 2004).

sputtering. The spectra have been acquired at room temperature by using 15 keV electrons and a current density of 10 nA. All spectra were normalized and vertically shifted for

are also possible.

**3.2.1 Amorphous silicon nitride**

further need for charge compensation.

Most of the luminescence features present in the 400–850 nm wavelength range (Fig. 3.9) correspond to optical transitions due to the Sm3+ and Tb3+ ions (Dieke 1968): Sm1 at ∼565 nm (4G5*/*<sup>2</sup> → 6H5*/*2), Sm2 at ∼605 nm (4G5*/*2 → 6H7*/*2), Sm3 at ∼650 nm (4G5*/*<sup>2</sup> → 6H9*/*2), Tb1 at ∼485 nm (5D4 → 7F6), Tb2 at ∼545 nm (5D4 → 7F5), Tb3 at ∼590 nm (5D4 → 7F4), and Tb4 at ∼625 nm (5D4 → 7F3). In addition to these, we also observe the Sm4 light emission at ∼725 nm (4G5*/*<sup>2</sup> →6H11*/*2), which is associated with the superposition of the 4G5*/*<sup>2</sup> → 6H11*/*2 transition due to Sm3+ ions and the 5D0 → 7F0, 5D0 → 7F1, and 5D0 → 7F2 transitions of Sm2+ ions. The infrared contribution Sm5 at ∼810 nm corresponds to the 5D0 →7F4 transition and is exclusively due to Sm2+ ions (Dieke 1968).

Fig. 3.9. Smarium and Terbium CL in amorphous silicon nitride. From (Zanatta, Khan et al. 2007).

#### **3.2.2 Amorphous aluminum nitride**

#### **<IP_ADDRESS> Erbium and terbium**

The first RE spectrum in amorphous Aluminum Nitride (a-AlN) was that of Gurumurugan et al. (Gurumurugan, Chen et al. 1999). The spectra are shown in Figures and 3.7 (Jadwisienczak, Lozykowski et al. 2000).

#### **<IP_ADDRESS> Europium**

Europium doped a-AlN was studied by Caldwell et al. (Caldwell, Van Patten et al. 2001).

The most significant result of this study was that thermal activation of the RE ion in a-AlN, which was found generally to increase the luminescence yield by a factor of up to 100, could

Cathodoluminescence from Amorphous and

**<IP_ADDRESS> Holmium and gadolinium**

Nanocrystalline Nitride Thin Films Doped with Rare Earth and Transition Metals 181

**Number Transition Wavelength**

AlN:Ho 1 5G5 → 5I8 362 0.355 2 5G4 → 5I8 394 0.530 3 5F1 → 5I8 461 0.300 4 5S2 → 5I8 549 1.000 5 5F3 → 5I8 659 0.277

Table 3.1. Summary of Ho3+ and Gd+3 ions emission from a-AlN:Ho and a-AlN:Gd.

1 0 1 0 (reference) 1 1 1.275 27.5 1 2 1.564 56.4 1 3 1.747 74.7 1 4 2.074 107.4 1 5 2.047 104.7 1 6 2.054 105.4

Table 3.2. Effect of Gd concentration on the luminescence of Ho 5S2 → 5I8 transition at 549

 6 5S2 → 5I7 758 AlN:Gd 1 6P7/2 → 8S7/2 314

> **Gd relative Concentration**

**(nm)**

**Relative Intensity of 549 nm peak**

**Relative Intensity**

**% increase in intensity**

Data for Ho and Gd in a-AlN are given by Maqbool (Maqbool 2005).

Fig. 3.12. CL from codoped Gd and Ho a-AlN films.

**Material Transition**

**Ho relative concentration**

nm.

be matched or exceeded by the addition of oxygen during the growth of the film by sputtering. The luminescence yield increased over 600 times with the addition of oxygen to the sputter gas.

Fig. 3.10. CL spectra of a-AlN:Eu processed as follows: a) no oxygen, and no heat treatment, b) no oxygen, heated to 923 K. c) grown with 1.6% oxygen in nitrogen, d) grown in 3.8% oxygen in nitrogen, e) grown in 20% oxygen, with the balance nitrogen. From (Caldwell, Van Patten et al. 2001).

Fig. 3.11. CL from some of the types of a-AlN:Eu films described in Fig. 3.10.

#### **<IP_ADDRESS> Holmium and gadolinium**

180 Cathodoluminescence

be matched or exceeded by the addition of oxygen during the growth of the film by sputtering. The luminescence yield increased over 600 times with the addition of oxygen to

> 5 D0 7 F3

5 D0 <sup>7</sup> F4

Fig. 3.10. CL spectra of a-AlN:Eu processed as follows: a) no oxygen, and no heat treatment, b) no oxygen, heated to 923 K. c) grown with 1.6% oxygen in nitrogen, d) grown in 3.8% oxygen in nitrogen, e) grown in 20% oxygen, with the balance nitrogen. From (Caldwell,

Wavelength (nm)

x 29

x 615

x 7

500 550 600 650 700

5 D0 <sup>7</sup> F1 5 D0 7 F2

Fig. 3.11. CL from some of the types of a-AlN:Eu films described in Fig. 3.10.

the sputter gas.

Van Patten et al. 2001).

0

1

A

B

C

D

E

2

3

Relative Intensity (a.u.)

4

5

Data for Ho and Gd in a-AlN are given by Maqbool (Maqbool 2005).

Fig. 3.12. CL from codoped Gd and Ho a-AlN films.

Table 3.1. Summary of Ho3+ and Gd+3 ions emission from a-AlN:Ho and a-AlN:Gd.

Table 3.2. Effect of Gd concentration on the luminescence of Ho 5S2 → 5I8 transition at 549 nm.

Cathodoluminescence from Amorphous and

**Material Transition**

Data for Tm in a-AlN are given by Maqbool (Maqbool 2005).

Table 3.5. Summary of Tm3+ ion emissions from Tm-doped AlN.

Table 3.6. Summary of Sm3+ ion emissions from Sm-doped a-AlN.

Data for Sm in a-AlN are given by Maqbool (Maqbool 2005).

**Number**

**Number**

**<IP_ADDRESS> Thulium**

**<IP_ADDRESS> Samarium**

**Material Transition**

**3.2.3 Amorphous gallium nitride**

Nanocrystalline Nitride Thin Films Doped with Rare Earth and Transition Metals 183

**Transition Assignment**

**Transition Assignment**

AlN:Tm 1 4G5/2 → 6H5/2 564 0.425

2 4G5/2 → 6H7/2 600 1.000

3 4G5/2 → 6H9/2 648 0.686

4 4G5/2 → 6H11/2 711 0.312

Amorphous GaN usually has a large defect luminescence in the yellow region of the visible spectrum. In Fig. 3.13, an a-GaN:Er thin film is shown in CL. This film was heated to 950 ºC in nitrogen for 30 minutes. The Er3+ ion transitions are clearly visible on the yellow defect emission band. The defect band problem is much less in crytsalline GaN, which was

extensively investigated by several groups, most notably Steckl and his coworkers.

AlN:Tm 1 D2 → 3H6 371 0.126 2 1D2 → 3F4 467 1.000 3 1D2 → 3H6 480 0.648 4 1D2 → 3H5 528 0.092 5 1G4 → 3F4 650 0.126 6 1D2 → 3H4 685 0.061 7 3H4 → 3H6 802 0.563 8 3H4 → 3H6 808 0.539

**Wavelength (nm)**

**Wavelength (nm)**

5 Gd impurity 314 0.276

6 Cr impurity 692 0.220

**Relative Intensity(a.u)**

**Relative Intensity(a.u)**

#### **<IP_ADDRESS> Praseodymium**

Data for Pr in a-AlN are given by Maqbool(Maqbool 2005).

Table 3.3. Summary of Pr3+ ion emissions from Pr-doped a-AlN.

Table 3.4. Effect of Gd concentration on the luminescence of Pr3+ ion transitions.

#### **<IP_ADDRESS> Thulium**

182 Cathodoluminescence

**Assignment**

**Gd Relative Concentration**

3P0 → 3H4 490 0 1 0 (reference)

3P0 → 3H6 618 0 1 0

3P0 → 3F2 649 0 1 0

Table 3.4. Effect of Gd concentration on the luminescence of Pr3+ ion transitions.

AlN:Pr 1 1S0 → 1D2 335 162 2 1S0 → 1I6 385 438 3 3P2 → 3H4 439 362 4 3P0 → 3H4 488 1002 5 3P0 → 3H4 504 1024 6 3P1→ 3H5 526 4577 7 3P0 → 3H5 573 767 8 3P0 → 3H6 618 643 9 3P0 → 3F2 652 1227 10 3P0 → 3F3 710 410 11 3P0 → 3F4 738 520

**Wavelength**

**Relative Intensity**

 1 1.58 58 4 4.37 337

 1 1.73 73 4 4.14 314

 1 1.66 66 4 3.4 240

**% Increase in Intensity**

**(nm) Intensity(a.u)**

**<IP_ADDRESS> Praseodymium**

Data for Pr in a-AlN are given by Maqbool(Maqbool 2005).

Table 3.3. Summary of Pr3+ ion emissions from Pr-doped a-AlN.

**(nm)**

**Pr Transition Wavelength**

**Material Transition Number Transition**

Data for Tm in a-AlN are given by Maqbool (Maqbool 2005).

Table 3.5. Summary of Tm3+ ion emissions from Tm-doped AlN.

#### **<IP_ADDRESS> Samarium**

Data for Sm in a-AlN are given by Maqbool (Maqbool 2005).

Table 3.6. Summary of Sm3+ ion emissions from Sm-doped a-AlN.

#### **3.2.3 Amorphous gallium nitride**

Amorphous GaN usually has a large defect luminescence in the yellow region of the visible spectrum. In Fig. 3.13, an a-GaN:Er thin film is shown in CL. This film was heated to 950 ºC in nitrogen for 30 minutes. The Er3+ ion transitions are clearly visible on the yellow defect emission band. The defect band problem is much less in crytsalline GaN, which was extensively investigated by several groups, most notably Steckl and his coworkers.

Cathodoluminescence from Amorphous and

luminescence is not visible.

**Material Transition**

Table 3.8. BN:Pr

**<IP_ADDRESS> BeN**

Data are given below in Table 3.8 (Maqbool 2005)

**Number**

studied with samarium (Zanatta, Richardson et al. 2007).

BeN sample is presented in the upper-right corner of Fig. 3.15.

Nanocrystalline Nitride Thin Films Doped with Rare Earth and Transition Metals 185