Laser enhanced diffusion of nitrogen in high purity semi-insulating 4H silicon carbide substrates for non-rectifying contact formation to photoconductive semiconductor switches

Non-rectifying (ohmic) contacts are essential for efficient photoconductive semiconductor switch performance and maximizing breakdown voltage. Fabricating ohmic contacts requires a very heavily doped surface layer (>; 10¹⁸ cm^-3), and in silicon carbide (SiC) is typically done by ion implantation. The high energy ions from this process often cause surface and bulk damage, and a high temperature anneal is required to repair the crystal structure and activate the impurities. This paper investigates the use of a gas immersion laser enhanced diffusion system to selectively dope the SiC as an attractive, low cost alternative to ion implantation. A pulsed 260 nm laser with a peak irradiance of 69.9 MW/cm² was used to dope a high purity semi-insulating (HPSI) 4H-SiC sample with nitrogen to a depth of 150 nm, with measured a surface concentration greater than 10²⁰ cm^-3. Using a one dimensional thermal model, the experimental data was fit to diffusion coefficients that are orders of magnitude greater than typically seen in SiC. The gas immersion laser doping technique has been demonstrated as a viable alternative to ion implantation for selective area doping of SiC bulk photoconductive switches.