Quantum photoyield of diamond(111)—A stable negative-affinity emitter

Quantum photoyield and secondary-electron distributions are presented for an unreconstructed diamond (111) surface (type-$\mathrm{II}b$, gem-quality blue-white semiconductor). This chemically inert surface exhibits a negative electron affinity, resulting in a stable quantum yield that increases linearly from photothreshold (5.5 eV) to ∼20% at 9 eV, with a very large yield of ∼40%-70% for $13\lesssim h\nu \lesssim 35$ eV. For all photon energies, secondary-electron energy distributions show a dominant ∼0.5-eV-wide emission peak at the conduction-band minimum (${\Delta }_1^{min}=5.50\mathrm{}\pm 0.05$ eV above the valence-band maximum ${{\Gamma }_{25}}^{\prime }$). In contrast with recent self-consistent calculations [J. Ihm, S. G. Louie, and M. L. Cohen, Phys. Rev. B 17, 769 (1978)] no occupied intrinsic surface states with ionization energies in the fundamental gap (the Fermi level was 1 eV above ${{\Gamma }_{25}}^{\prime }$) were observed. Likewise, the measured photothreshold ($E_{\mathrm{vac}}-{{\Gamma }_{25}}^{\prime }$) is significantly smaller than calculated (7.0±0.7 eV).