Application of wide-gap semiconductors to surface ionization: Work functions of AlN and SiC single crystals

For surface ionization purposes, a study of the work functions of SiC and AlN, both refractory wide-gap semiconductors, has been undertaken. Work function measurements have been performed in the 300–1600-K range using the Shelton retarding field method. Surface cleaning was carried out by heating in uhv to a high temperature using of a cw CO2 laser. Both n- and p-type 6H SiC single crystals with extreme bulk doping levels were investigated. For each doping type, the work functions have been found to be temperature independent. They exhibit only a slight variation from the n to the p type (4.75 and 4.85 eV, respectively) giving evidence, as in the case of Ge and Si, of Fermi level pinning at the surface by the intrinsic surface states. For an n-type AlN single crystal, the work function, measured in the high temperature range, remains constant as the temperature varies. The value of 5.35 eV obtained for AlN makes it a most attractive material for positive surface ionization applications.