| The results of a study of the chemical vapor deposition (CVD) growth and characterization of SiC polytype heterojunctions are presented. Device quality α-SiC has been grown at 1580°C, with p-type background between 6 × 1014cm−3 and 5 × 1015cm−3. N-type device layers have also been grown using nitrogen as a dopant gas. These materials were characterized by various methods including photoluminescence, atomic force microscopy, scanning electron microscopy, secondary ion mass spectroscopy, reflection high energy electron diffraction, C-V and I-V measurements. The above results along with a pre-growth hydrogen/propane etch study were used as a baseline for developing a growth process for the SiC polytype heterojunctions. High quality heterojunctions have been obtained with almost no polytype inclusions. Characterizations were performed to study the heterojunctions, which included an oxidation study to map polytype inclusions in the layers grown under different conditions resulting in a high (>95%) polytype homogeneity. Transmission electron microscopy (TEM) shows no visible defects and/or columnar growth features, a sign of high quality crystalline films. TEM diffraction patterns obtained are very strong, further indicating high quality 3C-SiC epitaxial layers. X-ray rocking and reciprocal space mapping (RSM) were further used in characterizing the SiC polytype heterojunctions. The FWHM of the x-ray scans are very narrow, between 0.01° and 0.02°, an excellent figure further indicating high quality crystalline 3C-SiC epitaxial layers. Iso-intensity contours from the RSM show very little broadening in ω which is due to the mosaic nature of the samples. The ω-2&thetas; direction shows almost no broadening implying that there is almost no strain in the material. In addition, the electrical quality of the heterojunctions was obtained by the fabrication and characterization of MESFETs made by growing a 0.6 μm n-type 3C-SiC on a semi-insulating 4H-SiC substrate. Saturation currents of 8.2 and 18.75 mA were achieved with transconductances of 9.22 and 21.4 mS/mm for two device dimensions with aspect ratios of 24 and 33 respectively. Carrier mobility as high as 700 cm2/V-s has been achieved, which is close to the reported Hall mobilities for 3C-SiC (770 cm2/V-s). The saturation drift velocity obtained (1.9 × 107 cm/s) is also close to theoretical values. By using transmission line model measurements, Ni based ohmic contacts to this material has been calculated to be ∼5 × 10−5 Ω-cm2, which is two orders of magnitude lower than Ni based ohmic contacts to 6H-SiC at the same doping level. |
