(Invited) Dilute Anion Alloyed III-Nitride Nanowires for Photoelectrochemical Water Splitting

Abstract

The record efficiencies in photovoltaic devices and hydrogen production via photoelectrochemical water splitting have been achieved in both cases by employing complex architectures of III-V alloys 1, which has been enabled by the outstanding charge carrier transport properties, band gap and band edge engineering flexibility, as well as highly mismatch growth that can be achieved in III-V alloys by controlling their chemical composition.It has been extensively demonstrated that Molecular Beam Epitaxy (MBE) and Metal-Organic Physical Vapor Deposition (MOPVD) techniques can produce III-V multinary alloys with compositions beyond miscibility gaps due to their “far from equilibrium” nature. And although great progress has been made towards improving the growth rate in these techniques, III-V device synthesis cost is still significantly higher (one order of magnitude) than Si solar cells manufacturing technologies 2. In this context, it is necessary to develop novel synthesis methods that yield high crystalline quality III-V alloys, higher throughput, and lower operational cost. For instance, Hydride/Halide Vapor Phase Epitaxy HVPE-grown GaAs top cell incorporated in a GaAs/Si tandem cell is an example of an optimized photovoltaic device with 31.4% efficiency, showcasing the uttermost potential of this growth method 2.More recently, Halide Vapor Phase Epitaxy growth of GaSbzP1-z polycrystalline free-standing films with up to 6.7% antimony incorporation and growth rates around 500 µm/h has been reported3. Linear sweep voltammetry measurements in 1M H2SO4 using GaSbzP1-z photoanodes showed lower onset potentials and higher fill factors compared to single crystal GaP. These results have been attributed to the visible light absorbing capabilities (direct band gaps between 2.2 and 2.5 eV), and lower charge transfer resistance at the semiconductor/electrolyte interface.On the other hand, a novel method for gallium nitridation involving plasma-activated nitrogen species dissolution into molten gallium has enabled the growth of single crystal GaN4. Furthermore, dilute anion Ga-Nitride alloys have been synthesized with the same technique but including other V-group species ions, i.e., Sb+ and Bi+, that can substitute nitrogen forming ternary GaSbxN1-x and GaBiyN1-y.In this presentation, we show the progress with dilute anion alloyed III-Nitride nanowires, GaSbxN1-x and GaBiyN1-y, synthesized by dissolving gallium, antimony or bismuth and plasma-activated nitrogen species into copper or gold droplets. Antimony and bismuth incorporation levels determined through x-ray diffraction c-plane peak shift, x and y, reached levels as high as 5.6 and 8.8 at%, respectively, resulting in direct band gap energies around 2.0 eV, determined through diffuse reflectance and photocurrent spectroscopy measurements. The visible light absorbing nanowire films exhibited photocurrent onset potentials around 0.0 V vs RHE when tested in 1 M sulfuric acid solution and 3-electrode setup with Ag/AgCl reference and Pt counter electrodes. 2-electrode chronoamperometry measurements with p-Si and n-type dilute alloys showed stable photocurrent densities up to 8.5 mA*cm-2 in 30 h. These results show the promising potential of dilute anion alloyed III-Nitrides as stable photoanodes for water splitting applications, and demonstrate the advantage of this novel synthesis technique over MOCVD that in previous studies has enabled the obtention of 0-D or 1-D GaSbxN1-x structures with up to 7% anion incorporation, while compromising crystalline quality.5