AlGaN/GaN MODFET regrown by rf-MBE on MOCVD templates

AlGaN/GaN devices are typically grown on foreign substrates such as SiC and sapphire due to lack of commercial bulk GaN. Metalorganic chemical vapor deposition (MOCVD) is a widely used method for growth of GaN templates for these structures even for other growth methods. Because the growth temperature during molecular beam epitaxy (MBE) is low, dislocation motion is hindered leading to a high dislocation density, particularly pure edge type, when grown directly on foreign substrates. On the other hand, low background doping, sharp interface and well-controlled growth rate allow MBE to grow high performance modulation doped field effect transistor (MODFET) structures on MOCVD GaN templates. However, the regrowth interface in this case has been reported to act as a parallel channel unless Zn-doped GaN templates were used. [J. Appl. Phys. v92 p338] In this paper we report on the control of the regrowth interface of GaN/AlGaN MODFETs by rf-assisted MBE on GaN templates prepared by MOCVD. We have found that the defective parallel channel at the regrowth interface could be effectively eliminated by a proper growth procedure and pre-cleaning using KOH combined with high temperature (800°C) thermal annealing in vacuum. Reflection high energy electron diffraction (RHEED) was used to monitor the interface quality to a first order during the initial growth stages. Electrical and structural properties at the regrowth interface were analyzed by capacitancevoltage (CV) measurements and transmission electron microscopy (TEM). Al_0.3Ga_0.7N/GaN MODFET structures grown under the optimized conditions exhibited a maximum transconductance of 230 mA/mm for a 1μm gate length.