Synthesis and Characterization of P-N Junction Diodes in Beta-Silicon Carbide Grown on Titanium Carbide

A beta-SiC p-n junction fabrication process, including material synthesis, doping calibration, reactive ion etching, oxide passivation and metal contact formation, was developed and characterized. A metal-organic chemical vapor deposition (MOCVD) system, with an inverted vertical reactor, was designed and built. The MOCVD process, using single reactant source 1,2-disilylethane (DSE or Si_2C _2H_{10} ), was optimized and used to grow p-n junction structures on TiC substrates. Studies of beta -SiC films by Nomarski microscope, scanning electron microscope (SEM) and electron channeling contrast pattern (ECCP) showed that the optimum growth temperatures were in the range of 1240^circC to 1290 ^circC, and that the maximum beta-SiC epitaxial growth rate was 6 mum/hr. The beta-SiC epilayers grown under optimized growth conditions were free of antiphase domains and stacking faults. Electrical properties of undoped and in-situ doped beta-SiC epilayers were investigated by hot probe measurement, and by I-V and C-V measurements of Ag/beta-SiC Schottky diodes. Undoped and nitrogen doped beta-SiC epilayers showed n-type conduction. P-type beta -SiC was obtained by in-situ aluminum doping. As determined by C-V measurement, the background concentration of undoped beta-SiC was about 1 times 10^{18} cm ^{-3}. Reactive ion etching of beta -SiC was performed in CF_4/O _2/Ar and SF_6 plasmas. Etching properties were studied as functions of applied power, total pressure and O_2 concentration. Conditions for etching beta-SiC at useful rates (100-1000A/min.) were established. Studies of beta-SiC etching surface morphologies and wall profiles by Nomarski microscope and SEM showed that beta-SiC etching was highly anisotropic, and SF_6 etching resulted in much better surface morphology than CF_4/O _2/Ar etching. Two methods, thermal oxidation of beta -SiC and thermal oxidation of a Si/beta -SiC structure, were employed to grow SiO _2 on beta-SiC for surface passivation. The thermal oxidation of the Si/ beta-SiC structure included two steps: (1) Si CVD deposition on beta-SiC; and (2) thermal oxidation of the Si layer. Mesa type beta-SiC p-n diodes were fabricated by integrating the optimized material synthesis and device processing conditions. I-V characteristics of the beta-SiC diodes showed rectification with relatively high reverse leakage current, an ideality factor about 3.5 and series resistance on the order of 1kOmega. Both oxide growth approaches reduced the leakage currents of the diodes. However, Si/ beta-SiC oxidation appeared to be more effective. The cause of the leakage current was attributed to the high background impurity concentrations in the DSE reactant source and to the subgrains in the TiC _{rm x} substrates.