The effects of sub-contact nitrogen doping on silicon carbide photoconductive semiconductor switches

Forming non-rectifying (ohmic) contacts to wide band gap semiconductors such as silicon carbide (SiC) requires a heavily doped subsurface layer to reduce the Schottky barrier height and allow efficient electron injection. Nitrogen, a common n-type dopant in SiC, was incorporated into a SiC sample using a laser enhanced diffusion process in which an impurity is incorporated into the semiconductor to very high surface concentrations (> 10²⁰ cm³) and very shallow depths ( 200 nm) with the use of a pulsed 266 nm laser. This paper evaluates the effects of nitrogen introduced through laser enhanced diffusion on the contact formation and the efficiency of silicon carbide photoconductive switches at low and high injection levels under different biasing conditions. Nine lateral switches were fabricated on a high-purity semi-insulating 4H-SiC sample; three with no sub-contact doping, three with sub-contact doping on only one contact, and three with sub-contact doping on both contacts. Results are presented for tests under pulsed laser illumination with sub-contact doping on only the anode, only the cathode, neither, and on both of the contacts.