Formation, structure, and properties of GaN nanowire polytype heterostructures

Semiconductor polytype heterostructures, which consist of adjacent layers of lattice-matched materials differing only in their atomic stacking sequences, offer opportunities for exceptional electronic device performance. For example, interfaces between wurtzite (WZ) and zincblende (ZB) polytypes are expected to result in the formation of a polarization-doped 2-dimensional electron gas, with both high carrier concentration and high carrier mobility. In the case of GaN, we recently discovered a metal-mediated molecular-beam epitaxy (MBE) process to nucleate ZB GaN films and/or nanowire (NW) ensembles directly on silicon (Si) [1]. For both films and NW ensembles, reflection-high energy electron diffraction and x-ray diffraction reveal ZB-to-WZ transformations at thicknesses ~ 20 nm. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) reveals ZB NWs with axes oriented ~37° from the Si [001] surface normal. Interestingly, the NW axis orientation remains fixed as it transforms to the WZ polytype. Furthermore, for thicknesses in excess of 45 nm, alternating ZB and WZ polytypes are observed. To determine the electronic states and band-offsets at the ZB/WZ GaN NW interfaces, STEM electron energy loss spectroscopy (EELS) studies are in progress.