Molecular Beam Epitaxy growth of III-Nitride heterostructures
Wide band gap Nitride heterostructures find applications both in extreme electronics and optoelectronics. On the one hand, high power and frequency electronics derived from Nitrides benefit from unusually favorable transport properties, such as the high electron sheet charge due to impurity-free polarization doping, and temperature and radiation resistance. For optoelectronics, the impressively wide range in band gap available (0.6 to 6 eV) and large exciton binding energy provide opportunities for generating solid state white lighting as well as novel optical devices. Because Nitride heterostructures exhibit large conduction band offsets (2 eV GaN/AlN, 1.68 eV InN/GaN, and 3 eV InN/AlN), the possibility exists to extend intersubband device technology from the mid-IR into the near-IR in GaN/AlN quantum wells, and possibly into the visible in InN/AlN quantum wells or dots.
Our group uses plasma-assisted molecular beam epitaxy (MBE) to grow III-Nitride hetereostructures of AlN/GaN, GaN/AlGaN, InGaN/GaN with p (Mg) or n (Si) doping. Polarization charge occuring along polar c-plane heterointerfaces in Nitrides allows impurity-free polarization doping of electrons. The system also has a high temperature Gd-doping cell. Gd-doping introduces magnetic moments and can generate ferromagnetism.
GaN is usually prepared using Ga-rich conditions for GaN/AlGaN plasma-assisted MBE growth, where the adatom incorporation occurs under a pure Ga liquid bilayer. At higher substrate temperatures beyond decomposition temperature for GaN, it is possible to produce smooth and high mobility GaN by layer-by-layer growth at high overpressures of active Nitrogen. If too much Nitrogen is used, self-assembled nanowires of GaN form.