Anion alloying of III-V semiconductors for water splitting.

Abstract

The challenge of solar energy conversion and storage translates into the discovery and development of semiconductor materials that can transform sunlight energy into electricity and/or high-energy-density fuels in a cost-effective way. From the solar energy conversion perspective, cation-alloyed III-V solar cells hold record-breaking energy conversion efficiencies, while anion-alloyed III-V’s are particularly interesting for photoelectrochemical hydrogen generation. The major challenge for the adoption of III-V-based technologies lies in the high manufacturing costs of established synthesis techniques and the difficulty to control the composition of ternary alloys. One of the materials investigated in this work is GaSbzP1-z. Thermodynamic equilibrium and steady-state kinetics modeling of a Halide Vapor Phase Epitaxy (HVPE) reactor, facilitated process mapping for the growth of GaSbzP1-z free-standing films with antimony incorporation levels up to 6.7 at% and a direct band gap shift from 2.68 to ~2.0 eV. The Water Splitting performance of these zincblende alloys revealed a more cathodic photocurrent onset potential, and an increased charge transfer efficiency at the alloy-electrolyte interface when compared to sulfur-doped commercial gallium phosphide. For the first time, Plasma-Assisted Vapor Liquid Phase Epitaxy (PA-VLPE) was used to grow anion-alloyed GaSbxN1-x and GaBiyN1-y wurtzite nanowires. Gold or Copper coated substrates (silicon, sapphire, and stainless steel) enabled high antimony and bismuth incorporation levels since the dissolution of these species into the metals was favored at high growth temperatures. The nanowires showed a direct band gap shift from 3.4 to ~2.0 eV in the case of 5.6 at% antimony and 8.8 at% bismuth incorporation. Photo-electrochemical spectroscopy measurements showed efficient light absorption of 620 nm photons in the case of GaSb0.056N0.944 sample. On the other hand, samples with incorporation (x, y) below 1 at%, helped to confirm the existence of a band gap reduction discontinuity at such a low alloying levels. The most significant accomplishments of this work are the demonstration of anion substitution control in the growth of GaSbzP1-z alloys through HVPE and the demonstration of visible light absorbing GaSbxN1-x and GaBiyN1-y nanowires grown through PA-VLPE. These results apply to the growth of other anion-alloyed III-V systems in multi-junction high-efficiency photovoltaics, power devices, and solid-state laser applications.