Contact potential difference (CPD) measurements are reported for GaAs, GaP, and InAs UHV-cleaved single crystals, and for GaAs epitaxially grown layers of (110) orientation. For the GaAs and InAs single crystals the CPD between n- and p-type samples of the same material was practically equal to the band gap. This means that the intergap surface state density must be <1012/cm2. For epitaxially grown GaAs layers the CPD appeared to be independent of the volume dope, i.e., the Fermi level is stabilized due to surface states in the forbidden zone. For GaP single crystals the energy bands appear to be flat up to the surface in p-type material, but for an n-type sample a band bending of 0.45 eV occurs.
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A 1976 study studied this question.