Growth of (indium,aluminum)gallium nitride alloys by RF-plasma assisted molecular beam epitaxy for application in high electron mobility transistor structures.

摘要

In this thesis work, growth of (In,Al)GaN alloys by molecular beam epitaxy (MBE) was investigated with the goal of developing these materials for application in high electron mobility transistor structures (HEMTs). Growth of InGaN alloys by MBE was investigated in detail with the objective of implementing InGaN channels into HEMT structures to improve device performance. A growth diagram for InGaN growth based on III/V ratio during growth was developed. Control of indium composition was studied in detail and found to be highly dependent upon several growth parameters. Systematic studies resulted in demonstration of complete compositional control during growth of InGaN across the entire compositional range. Implementation of these layers into HEMT structures yielded inferior device properties due to an extremely high level of unintentional background carriers in the InGaN channel. Transport measurements were done on bulk InGaN for the first time demonstrating carrier concentrations as high as 1018 cm-2.; An all-MBE growth process for AlGaN/GaN HEMTs on SiC was also developed utilizing an AlN nucleation layer and a two-step growth process for the GaN to reduce and control threading dislocation density. The GaN growth process was structurally and electrically optimized to achieve semi-insulating HEMT buffers. Two methods were developed to reduce buffer leakage. The first was through implementation of carbon doping via CBr4, and the second was by optimization of the AlN nucleation layer growth conditions in unintentionally doped (carbon-free) structures. Optimization of the direct-growth process and elimination of buffer leakage led to record output power densities in MBE-grown AlGaN/GaN HEMTs and device performance which is on par with state-of-the art HEMTs grown by metalorganic chemical vapor deposition (MOCVD).