High carrier lifetime bulk-grown 4H-SiC substrates for power applications

A thermal anneal process has been developed that significantly enhances minority carrier lifetime (MCL) in bulk-grown substrates. Microwave photoconductivity decay (MPCD) measurements on bulk grown substrates subjected to this process have exhibited decay times in excess of 35 mu s. Electron Beam Induced Current (EBIC) measurements indicated a minority carrier diffusion length (MCDL) of 65 mu m resulting in a calculated MCL of 15 mu s, well within the range of that measured by MPCD. Deep level transient spectroscopic (DLTS) analysis of samples subjected to this anneal process indicated that a significant reduction of deep level defects, particularly Z(1/2), may account for the significantly enhanced lifetimes. The enhanced lifetime is coincident with a transformation of the original as-grown crystal into a strained or disordered lattice configuration as a result of the high temperature anneal process. PiN diodes were fabricated employing 350 mu m thick bulk-grown substrates as the intrinsic drift region and thin p- and n-type epitaxial layers on either face of the substrate to act as the anode and cathode, respectively. Conductivity modulation was achieved in these diodes with a 10x effective carrier concentration increase over the background doping as extracted from the differential on-resistance. Significant stacking fault generation observed during forward operation served as additional evidence of conductivity modulation and underscores the importance of reducing dislocation densities in substrates in order to produce a viable bulk-grown drift layer.