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

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Abstract
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.

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