Dispersion-free AlGaN/GaN CAVET with low R_(on) achieved with plasma MBE regrown channel with Mg-ion-implanted current blocking layer

摘要

GaN-based power transistors are rapidly developing as a contender for application in next generation high efficiency power electronics. They are either lateral devices like HEMTs or vertical devices of the form of the Current Aperture Vertical Electron Transistor (CAVET) [1, 2]. A CAVET (Fig.1) is a vertical device with source and gate on top and a current aperture that allows current to flow vertically down from the source to the drain. It has a current blocking layer (CBL) to block current flowing vertically through any other path but the aperture. The process flow for fabricating a CAVET is illustrated in figure 2. One way to achieve the current blocking layer is by implanting the layer with [Mg]-ion. However the difficulty in using a Mg-implanted barrier layer was the unintentional diffusion of the Mg into the overlying channel region (see Fig.3). This caused a significant threshold voltage shift in devices and in many cases the electrons in the channel were trapped or completely depleted by diffusing Mg acceptors. In this paper we report on successfully addressing this critical problem by using MBE to re-grow the channel containing the 2DEG on top of the n drift region that is grown by MOCVD resulting in devices free of dispersion (80μs pulse width) and R_(on) less than 2.5 mΩ-cm~2. Here the advantages of MBE, that of a low growth temperature and hence suppressed Mg diffusion, is combined with the advantages of MOCVD, that of a high growth rate of high purity material necessary for the thick drift region to support high voltage required in power electronics. The base structure grown by Toyota Motor Corporation, Japan, consisted of a 0.3-μm-thick GaN (Si: 2×10~(16)/cm~3), 3-μm-thick n-GaN (Si: 2×10~(16)/cm~3) on n+GaN substrate. 10~(15)/cm~2 [Mg] was ion implanted at 80keV on the top GaN layer to form the current blocking layer. Prior to regrowth, the sample was degreased by standard solvent clean and outgassed in ultrahigh vacuum (1×10~(-9) Torr) at 400°C for 1 hour. The regrowth was conducted at 720°C under Ga-rich droplet conditions for smooth growth. The regrown structure was initiated with a 15-nm carbon-doped GaN layer [3], followed by a 135-nm-thick undoped GaN buffer, and capped with a 30-nm Al_(0.3)Ga_(0.7)N barrier layer. The carbon dopants were supplied by a gaseous CBr_4 source at a foreline pressure of 80 mTorr, which corresponded to a doping concentration of 6×10~(18) cm~(-3) as measured by secondary ion mass spectroscopy (SIMS). The 15nm C-doped GaN layer was grown to compensate for the residual Si, which could not be removed by UV-Ozone/Hf treatment standardized for MOCVD regrowth, as MBE regrowth involved significant loading time. The device had an optimized Ti/Al/Ni/Au non-alloyed contact at the backside (N-face of GaN) to access the drain region. Two sources (Ti/Au/Ni) to contact the [Si]-ion implanted GaN were placed on each side of the gate (Ni/Au/Ni) with a 1.5 μm gate to source distances