Activating Ion Implants in 4H-SiC by Annealing with an AlN or BN Cap

摘要

Planar SiC devices are fabricated using ion implantation because the rates of diffusion of dopants into SiC are too low even at temperatures as high as 1800℃ to be technologically useful. However, the implanted material must be annealed to activate the dopants at temperatures where the Si preferentially evaporates and severely roughens the surface as is shown in Fig. 1. An AlN film has the necessary properties to act as an annealing cap for temperatures up to 1600℃~4; it retains coverage of the SiC surface during the anneal, does not react with the SiC surface during the anneal, and can be removed selectively without harming the SiC surface after annealing. Slight shifts in the Bragg x-ray peaks show that the AlN films become more dense at the higher annealing temperatures, and TEM micrographs show that the acicular, c-oriented grains become larger. However, the AlN/SiC interface remains intact up to an annealing temperature of 1600℃. For the higher annealing temperatures required for implanted p-type dopants, a BN/AlN composite cap can be used where the BN cap is ion milled off and the remaining AlN is etched off preferentially. The as-deposited BN cap is composed almost exclusively of the turbostatic phase. This phase is a graphitic two dimensional phase composed of sp~2 bonded planes that readily glide over each other~5. This enables to BN to reduce any strains that would naturally occur due to differences in thermal coefficients of expansion and therefore reduce the chances for cracking. The BN film near the AlN interface regrows as the hexagonal phase via solid phase epitaxy, and its greater three dimensional strength keeps the AlN film from decomposing and evaporating.