New III-V Semiconductor Alloys for Solar Hydrogen Production

Dilute III-V alloys have garnered immense interest as suitable materials for solar hydrogen generation due to their compositional tunable band gaps, high carrier mobilities and high absorption coefficients. Till date there are no III-V materials that can satisfy all the stringent criteria for photoelectrochemical water splitting. In this presentation, we report two new III-V materials GaSbN and GaSbP that addresses current materials bottleneck for photoelectrochemical water splitting is developed for photoelectrochemical water splitting. Applicability of the Ga(Sb x )N1−x alloys for practical realization of photoelectrochemical water splitting is investigated using first-principles density functional theory and experiments. Theoretical results predict that Ga(Sb x )N1−x , GaSbP alloys with 2 eV band gaps straddle the potential window at moderate to low pH values, thus indicating that dilute Ga(Sb x )N1−x alloys could be potential candidates for splitting water under visible light irradiation.1,2 Theoretical computations with Sb composition beyond 7% change the electronic band gap from direct to indirect. In the case of GaSbP a small amount of Sb incorporation will change the indirect GaP band gap to direct band gap.3 Experimental synthesis is carried out using metal organic chemical vapor deposition using trimethyl gallium (TMGa) and Trimethyl Antimony (TMSb) and ammonia. Crystalline GaSbxN1-x films were obtained at x values ranging from 0-5%. The synthesis was carried out on different planar substrates and GaN nanowires. Optical measurements confirm that severe band gap reduction occurs with incorporation of antimony in to GaN as predicted by the theoretical calculations. X-Ray Diffraction (XRD) results confirm the lattice expansion at small concentrations of antimony.  This work is the first successful attempt on the experimental synthesis of crystalline GaN and GaP based alloy in the low antimonide regime and photoelectrochemical data on activity, band edge energetics and stability show high suitability for direct solar water splitting. The fundamental insights gained from this work on the growth mechanisms and formation of stacking faults can be extended to other ternary alloy nanowires of interest. From a scientific point of view, the large band gap bowing and corresponding optical properties with very small alloying compositions is interesting and can be exploited for band gap engineering. This presentation will highlight our results with both synthesis and photoelectrochemical characterization of  GaSbN and GaSbP alloys.   Acknowledgements: Financial support from US Department of Energy (DE-FG02-07ER46375) and NSF (DMS1125909).   R.M. Sheetz, E. Richter, A.N. Andriotis, C. Pendyala, M.K. Sunkara and M. Menon, “Visible light absorption and large band gap bowing in dilute alloys of gallium nitride with antimony”, Phys. Rev. B, 84, 075304 (2011) S. Sunkara, V.K. Vendra, J.B. Jasinski, T. Deutsch, A.N. Andriotis, K. Rajan and M.K. Sunkara, “New Visible Light Absorbing Materials for Solar Fuels, Ga(Sbx)N1-x”, Adv. Mater., 26 (18), 2878-2882 (2014). H.B. Russell, A.N. Andriotis, M.Menon, J. Jasinski and M.K. Sunkara, “Direct Band Gap Gallium Antimony Phosphide (GaSbxP1-x) Alloys”, Submitted (2015).