Indium-Rich InGaN Films Grown on Ge Substrate by Plasma-Assisted Molecular Beam Epitaxy for Solar Water Splitting

Indium-rich InGaN films were grown on Ge(111) substrate by plasma-assisted molecular beam epitaxy (PAMBE). The influence of the indium flux on the structural properties, surface morphology, and photocurrent for water splitting has been investigated. Before the InGaN growth, 20 nm of GaN was deposited as a buffer layer. A streaky reflection high-energy electron diffraction (RHEED) pattern was observed for the GaN buffer growth. At the onset of InGaN growth, the streaks became spotty, indicating roughening of the surface and three-dimensional (3D) growth due to the lattice mismatch between GaN and InN. The indium composition in the InGaN structure was roughly fit to be around 50% from x-ray diffraction (XRD) ω–2θ measurements. Growth with excess indium supply led to segregation of metal indium on the surface. During cooling down, this metal indium transformed partially into InN. The crystal quality of the InGaN film decreased with increase of the indium flux. The photocurrent of the InGaN films used as photoelectrodes for water splitting also decreased with increase of the indium flux.