Axial GaAs/Ga(As,Bi) Nanowire Heterostructures
Miriam Oliva Guanhui Gao Esperanza Luna Lutz Geelhaar Ryan B. Lewis



Axial GaAs/Ga(As,Bi) Nanowire Heterostructures]Axial GaAs/Ga(As,Bi) Nanowire Heterostructures


Paul-Drude-Institut für Festkörperelektronik, Leibniz-Institut im Forschungsverbund Berlin e. V., Hausvogteiplatz 5–7, 10117 Berlin, Germany

Paul-Drude-Institut für Festkörperelektronik, Leibniz-Institut im Forschungsverbund Berlin e. V., Hausvogteiplatz 5–7, 10117 Berlin, Germany
Present address: Material Science and NanoEngineering department, Rice University, 6100 Main Street MS 364, Houston, TX 77005, USA

Paul-Drude-Institut für Festkörperelektronik, Leibniz-Institut im Forschungsverbund Berlin e.V., Hausvogteiplatz 5–7, 10117 Berlin, Germany

Paul-Drude-Institut für Festkörperelektronik, Leibniz-Institut im Forschungsverbund Berlin e.V., Hausvogteiplatz 5–7, 10117 Berlin, Germany
Department of Engineering Physics, McMaster University, L8S 4L7 Hamilton, Canada
oliva@pdi-berlin.de


Bi-containing III-V semiconductors constitute an exciting class of metastable compounds with wide-ranging potential optoelectronic and electronic applications. However, the growth of III-V-Bi alloys requires group-III-rich growth conditions, which pose severe challenges for planar growth. In this work, we exploit the naturally-Ga-rich environment present inside the metallic droplet of a self-catalyzed GaAs nanowire to synthesize metastable GaAs/GaAs_1-xBi_x axial nanowire heterostructures with high Bi contents. The axial GaAs_1-xBi_x segments are realized with molecular beam epitaxy by first enriching only the vapor-liquid-solid (VLS) Ga droplets with Bi, followed by exposing the resulting Ga-Bi droplets to As_2 at temperatures ranging from 270 to 380^∘C to precipitate GaAs_1-xBi_x only under the nanowire droplets. Microstructural and elemental characterization reveals the presence of single crystal zincblende GaAs_1-xBi_x axial nanowire segments with Bi contents up to (10±2)%. This work illustrates how the unique local growth environment present during the VLS nanowire growth can be exploited to synthesize heterostructures with metastable compounds.




[
    Ryan B. Lewis
    Received ; accepted 
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§ INTRODUCTION

Alloying III-V semiconductors with Bi—the group V element which is the heaviest of all non-radioactive elements—has been a hot topic in recent years. This is largely due to the huge reduction of the III-V host bandgap energy per unit Bi incorporation compared to conventional/lighter alloying elements, making III-V-Bi alloys promising for extending the wavelength range of optoelectronic devices for a given III-V material system <cit.>. Owing to the large Bi mass, Bi-containing materials are also of interest for spintronics <cit.> and thermoelectrics <cit.>. 
However, the planar growth of III-V-Bi compounds such as GaAs_1-xBi_x is complicated by the requirements of low growth temperatures and group-III-rich growth conditions <cit.>. Such conditions often result in the formation of metallic droplets on the surface, which are highly detrimental for device applications <cit.>.

Nanowires (NWs) are ideally suited as nanoscale substrates for the growth of highly lattice-mismatched and metastable heterostructures. These structures have made it possible to form coherent heterostructures with materials which would be extremely defective in planar form <cit.>. Regarding III-V-Bi NWs, the growth of lattice-mismatched GaAs/GaAs_1-xBi_x core–shell NWs has only recently been attempted <cit.>. Bi has also recently been used as a surfactant during NW growth to alter the crystal structure of Au-catalyzed GaAs NWs from wurtzite to zincblende <cit.>, and to induce the self-assembly of InAs quantum dots on GaAs NW sidewalls <cit.>. Additionally, the Bi-catalyzed vapor-liquid-solid (VLS) growth of GaAs nanodiscs <cit.> and of in-plane Ga(As,Bi) NWs <cit.> has been observed. However, the incorporation of Bi in axial NW heterostructures—as well as in VLS growth of standing III-V NWs—has not been reported.

In this work, we exploit the Ga-rich environment at the tip of self-assisted VLS GaAs NWs, to realize GaAs_1-xBi_x axial NW segments with Bi contents in excess of 10% on Si substrates. Exposing GaAs NWs to Bi results in an accumulation of Bi only within the Ga droplets. Subsequently exposing these Ga-Bi droplets to an As_2 flux precipitates axial GaAs_1-xBi_x NW segments, with excess Bi remaining on top of the segment as metal droplets. Varying the temperature during precipitation between 270^∘C and 380^∘C, Bi contents up to (10±2)% are achieved in the segment. Microstructural and elemental characterization reveals that the GaAs_1-xBi_x segments with highest Bi concentration are single crystalline zincblende. These results illustrate how VLS NW synthesis can be exploited to realize heterostructures with metastable compounds.




§ EXPERIMENTAL




 §.§ Sample growth

Samples were grown by molecular beam epitaxy on p-type Si(111) wafers covered by native oxide. Standard effusion cells provided Ga and Bi fluxes, and valved crackers supplied As_2. Substrate temperatures were measured by a pyrometer calibrated to the oxide desorption temperature of GaAs(100). Prior to growth, the wafers were ramped to about 645^∘C and annealed for 10min. Ga droplets were then formed on the substrate by exposing the substrate to Ga for 30s at 1.32µmh^-1 GaAs equivalent growth rate, followed by a 220s flux interruption <cit.>. Vertical GaAs NW segments with diameter, length and density on the order of 30nm, 300–700nm and 0.1–2.5µm^-2, respectively, were obtained through simultaneous exposure of As_2 at 3.8µmh^-1 and Ga at 0.3µmh^-1 GaAs equivalent growth rate for 3min. Subsequently, the As_2 flux was decreased to 0.6µmh^-1 GaAs equivalent growth rate in order to enlarge the diameter of the NWs <cit.>, and the growth was continued for 10min while maintaining the substrate at 645^∘C. The resulting GaAs NWs had a total length of 0.3–1.8µm and a top diameter of 50–110nm. After the growth of GaAs NWs, the fluxes were interrupted and the substrate ramped to 420^∘C, and the sample was subsequently exposed to a Bi flux corresponding to a 0.2µmh^-1 growth rate for 3 min to enrich the Ga droplets with Bi. The substrate was then ramped to 270–380^∘C without the exposure to any fluxes. Axial GaAs_1-xBi_x NW segments were formed with a 30min exposure of the Ga-Bi droplets to As_2 at 0.5µmh^-1 GaAs equivalent growth rate. The growth of the samples depicted in figures <ref>a,b was stopped after the formation of GaAs NWs, and Ga-Bi droplets, respectively. Samples were rotated at 10rpm during growth. 



 §.§ Transmission electron microscopy

Transmission electron microscopy - energy dispersive X-ray spectroscopy (TEM-EDS) studies and morphological analysis were carried out on dispersed NWs. The samples were prepared by mechanically scratching the NWs from their substrates and subsequently transferring them to lacey carbon films with 300 mesh copper grids. TEM and scanning transmission electron microscopy (STEM) imaging, high-resolution TEM imaging, correlated EDS and mapping measurements were performed with a JEOL 2100F field emission microscope operated at 200kV. The microscope is equipped with a Gatan Ultra Scan 4000 CCD camera for image recording. High-resolution images were obtained by aligning the NWs along the [110] zone axis orientation, which was controlled by an electron diffraction pattern.

g_002 dark-field (DF) TEM investigations were carried out on cross-sectional TEM specimens, using a Jeol JEM 3010 microscope operating at 300kV equipped with a GATAN slow-scan CCD camera. The NWs were prepared in the two orthogonal [110] and [112] projections using conventional techniques, where the NWs are glued for mechanical stabilization and thinned by mechanical grinding, dimpling, and Ar-ion milling.




§ RESULTS AND DISCUSSION

Figure <ref> depicts the end portion of individual NWs after different stages of growth.

First, GaAs NWs were grown by the Ga-assisted VLS method in molecular beam epitaxy <cit.> (figure <ref>a). Next, the NWs were exposed to a Bi flux corresponding to a 0.2µmh^-1 growth rate of metallic Bi for 3min at a temperature of about 420^∘C (figure <ref>b). At these conditions, Bi has been shown to behave as a surfactant for growth on GaAs{110} NW sidewalls and planar substrates <cit.>. For the present case, assuming a constant ratio between a GaAs NW diameter and the diameter of its Ga droplet, we estimate that based on the analysis of 14 NWs by scanning electron microscopy (SEM), the Bi exposure increases the averaged diameter of the droplets by a factor of about 1.7, indicating an enrichment of the Ga droplet with Bi. This increase corresponds to a Bi/Ga volume ratio in the droplet of about 5. No evidence for Bi accumulation was observed by SEM on the NW sidewalls or substrate, indicating that Bi desorbed from these surfaces [see Supporting Information (SI)]. After enriching the Ga droplets with Bi, the NWs were exposed for 30min to an As_2 flux of 0.5µmh^-1 GaAs equivalent planar growth rate at substrate temperatures ranging from 270^∘C to 380^∘C. The resulting NW structure obtained for a substrate temperature of 300^∘C is shown in figure <ref>c. The As_2 exposure results in the formation of a precipitated segment below the droplet (compare figure <ref>b and <ref>c). 

According to the bulk Ga-Bi phase diagram, Ga and Bi form a homogeneous mixture for all Bi concentrations above the Bi melting point (271.4^∘C) <cit.>. Thus, the Ga-Bi droplet should be a homogeneous liquid mixture for all As_2 exposure temperatures, with the possible exception of the lowest temperature of 270^∘C. The addition of As_2 is expected to precipitate a GaAs-rich solid <cit.>. We note that GaAs/InAs axial NW heterostructures have previously been obtained by a related approach, where In droplets formed atop of GaAs NWs (where the Ga droplets had previously been converted to GaAs by As exposure) were subsequently converted into InAs axial segments by As exposure <cit.>. 




The microstructure of NWs grown by the above approach was investigated by TEM. Figure <ref> shows TEM micrographs of a single GaAs/GaAs_1-xBi_x NW hererostructure grown by exposing the Ga-Bi droplet to As_2 at 300^∘C [see SI for micrographs on additional NWs]. 
