Ultra-violet photoelectron spectroscopy (UPS) and other surface-sensitive electron spectroscopies (Auger electron spectroscopy, low energy electron diffraction, electron energy loss spectroscopy and X-ray photoelectron spectroscopy) have been used to study the electronic properties of the cleavage (110) faces of GaP, InP and GaAs, both in their clean state and during oxygen adsorption.\ud UPS shows that filled surface states are absent from the bulk bandgap in all three compounds, and only the tall of the empty surface state distribution extends Into the bandgap on InP (110) and GaAs (110). The Fermi level of GaP(110) is pinned 1.50eV above the valence band maximum by a band of empty surface states which is also seen in synchrotron radiation-excited photoemission partial-yield measurements. The sticking coefficient for oxygen is very small on all three compounds studied here, exposures around 107 Langmuirs (10 Torr. sec) being necessary for monolayer coverage on the (110) faces. The Fermi level on GaP remains pinned after adsorption of 108 Langmuirs of oxygen, and for InP and GaAs pinning within the gap is induced by extrinsic states at coverages of less than one-thousandth of a monolayer.\ud The energies of some symmetry points in the bulk bandstructures of GaP and InP are determined unambiguously for the first time from a direct-transition analysis of the low-energy UPS data. The assignment of Auger peaks in clean GaP and the nature of the P-O interatomic Auger transitions and interface plasmons found during oxygen adsorption are discussed in detail. Electron beam-enhanced oxidation was observed, and a new mechanism proposed for this, involving adsorption of oxygen into the ceramic parts of the electron gun, followed by the projection of oxygen ions towards the sample
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