Low-Cost Synthesis Methods for Single Crystal Quality III-V Alloys

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, 2], 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.<br/><br/>It has been extensively demonstrated that Molecular Beam Epitaxy (MBE) and Metal-Organic Physical Vapor Deposition (MOPVD) techniques are capable of producing multinary III-V 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 the incumbent Si solar cells manufacturing technologies [3]. In this context, it is necessary to develop novel synthesis methods with comparable growth quality capabilities, higher throughput, and lower operational costs than the afore mentioned ones. 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 [3].<br/><br/>More recently, the HVPE growth of GaSb_yP_1-y alloys has been studied with experimental and computational approaches, allowing the obtention of single crystal free-standing films with up to 6.7% antimony incorporation and growth rates around 500 µm/h. GaSb_yP_1-y alloys show lower onset potentials and higher fill factors compared to single crystal GaP when linear sweep voltammetries using 1M H₂SO₄ are carrier out. These results have been attributed to lower direct band gap transitions between 2.2 eV and 2.5 eV, and lower charge transfer resistance at the semiconductor/electrolyte interface of the GaSb_yP_1-y alloys versus pure GaP [4].<br/><br/>On the other hand, Vapor Liquid Phase Epitaxy (VLPE) is known for the low defect density materials it can produce and it has successfully been used for the growth of single crystal GaN at a growth rate of 1 µm/h with the use of ionic nitrogen gaseous species (plasma-assisted) and the careful control of their concentration in molten gallium by means of pulsing the applied radio frequency energy, preventing undesirable nucleation at the gallium/gas interface [5]. Furthermore, 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 GaSb_xN_1-x and GaBi_yN_1-y ternary 0-D or 1-D structures with up to 7% anion incorporation, leading to visible light absorption and band edge straddling of the Hydrogen Reduction and Oxygen Evolution electrochemical potentials.<br/><br/>References:<br/>[1] W. H. Cheng<i> et al.</i>, "Monolithic Photoelectrochemical Device for Direct Water Splitting with 19% Efficiency," <i>Acs Energy Letters, </i>vol. 3, no. 8, pp. 1795-1800, Aug 2018, doi: 10.1021/acsenergylett.8b00920.<br/>[2] S. Essig<i> et al.</i>, "Raising the one-sun conversion efficiency of III-V/Si solar cells to 32.8% for two junctions and 35.9% for three junctions," <i>Nature Energy, </i>vol. 2, no. 9, Sep 2017, Art no. 17144, doi: 10.1038/nenergy.2017.144.<br/>[3] K. T. VanSant<i> et al.</i>, "Toward Low-Cost 4-Terminal GaAs//Si Tandem Solar Cells," <i>Acs Applied Energy Materials, </i>vol. 2, no. 4, pp. 2375-2380, Apr 2019, doi: 10.1021/acsaem.9b00018.<br/>[4] S. J. Calero-Barney, W. Paxton, P. Ortiz, and M. K. Sunkara, "Gallium antimonide phosphide growth using Halide Vapor Phase Epitaxy," (in English), <i>Solar Energy Materials and Solar Cells, </i>Article vol. 209, p. 9, Jun 2020, Art no. 110440, doi: 10.1016/j.solmat.2020.110440.<br/>[5] D. F. Jaramillo-Cabanzo, J. B. Jasinski, and M. K. Sunkara, "Liquid Phase Epitaxy of Gallium Nitride," <i>Crystal Growth & Design, </i>vol. 19, no. 11, pp. 6577-6585, Nov 2019, doi: 10.1021/acs.cgd.9b01011.