Issues and Examples Regarding Growth of AlN, GaN and Al_xGa_1−xN Thin Films via OMVPE and Gas Source MBE

Abstract

Organometallic vapor phase epitaxy (OMVPE) and molecular beam epitaxy (MBE) are the most common methods for the growth of thin films of A1N and GaN. Sapphire is the most common substrate; however, a host of materials have been used with varying degrees of success. Both growth techniques have been employed by the authors to grow AIN, GaN and Al_xGa_1−xN thin films primarily on 6H-SiC(0001). The mismatch in atomic layer stacking sequences along the growth direction produces double positioning boundaries in A1N and the alloys at the SiC steps; this sequence problem appears to discourage the two-dimensional nucleation of GaN. Films of these materials grown by MBE at 650°C are textured; monocrystalline films are achieved between 850°C (pure GaN) and 1050°C (pure A1N) by this technique and OMVPE. Donor and acceptor doping of GaN has been achieved via MBE without post growth annealing. Acceptor doping in CVD material requires annealing to displace the H from the Mg and eventually remove it from the material. High brightness light emitting diodes are commercially available; however, numerous concerns regarding metal and nitrogen sources, heteroepitaxial nucleation, the role of buffer layers, surface migration rates as a function of temperature, substantial defect densities and their effect on film and device properties, ohmic and rectifying contacts, wet and dry etching and suitable gate and field insulators must and are being addressed. Selected issues surrounding the growth of these materials with particular examples drawn from the authors' research are presented herein.