Processing challenges for GaN-based photonic and electronic devices

摘要

The wide gap mateirals SiC, GaN and to a lesser extent diamond are attracting great interest for high power/high temperature electronics.There are a host of device processing challenges presented by these materials because of their pyysical and chemical stability,including difficulty in achieving stable, low contact resistances,especially for one conductivity type. absence of convenient wet etch recipes, generally slow dry etch rates, the high temperatures needed for implant activation,control of suitable gate dielectrics and the lack of cheap, large diameter conducting and semi-insulating substrates. The relatively deep ionization levels of some of the common dopants (Mg in GaN; B, Alin SiC; P in diamond) means that carrier densities may be low at room temperature even if the impurity is electrically active- this problem will be reduced at elevated temperatue,and thus contact resisftances will be greatly improved provided the metallization is stable and reliable. Some recent work with CoSi_x on SiC and W-alloys on gaN show promise for improved ohmic contacts. The issue of unintentional hydrogen passivation fo dopants decreases in resistivity of diamond.Recent work on development of wet etches has found recipes for Aln (KOH), while photochemcial etching of siC and Gan has been reported. In the latter cases p-type materials is not etched,which can be a major liability in some devices. The dry etch results obtained with various novel reactors, including ICP, ECR and LE4 will be compared - the high ion densities in the former techniques produce the highest etch rates for strongly-bonded materials,but can lead to preferential loss of N from the nitrides and therefore to a highly conducting surface. This is potentially a major problem for fabrication of dry etched, recessed gate FET structures.