InxGa1-xAs Nanowires on Silicon: One-Dimensional Heterogeneous Epitaxy, Bandgap Engineering, and Photovoltaics

We report on the one-dimensional (1D) heteroepitaxial growth of InxGa1-xAs (x = 0.2-1) nanowires (NWs) on silicon (Si) substrates over almost the entire composition range using metalorganic chemical vapor deposition (MOCVD) without catalysts or masks. The epitaxial growth takes place spontaneously producing uniform, nontapered, high aspect ratio NW arrays with a density exceeding 1 x 10(8)/cm(2). NW diameter (similar to 30-250 nm) is inversely proportional to the lattice mismatch between InxGa1-xAs and Si (similar to 4-11%), and can be further tuned by MOCVD growth condition. Remarkably, no dislocations have been found in all composition InxGa1-xAs NWs, even though massive stacking faults and twin planes are present. Indium rich NWs show more zinc-blende and Ga-rich NWs exhibit dominantly wurtzite polytype, as confirmed by scanning transmission electron microscopy (STEM) and photoluminescence spectra. Solar cells fabricated using an n-type In0.3Ga0.7As NW array on a p-type Si(111) substrate with a similar to 2.2% area coverage, operates at an open circuit voltage, V-oc, and a short circuit current density, J(sc), of 0.37 V and 12.9 mA/cm(2), respectively. This work represents the first systematic report on direct 1D heteroepitaxy of ternary InxGa1-xAs NWs on silicon substrate in a wide composition/bandgap range that can be used for wafer-scale monolithic heterogeneous integration for high performance photovoltaics.