Cesium on GaP(110) surfaces: A confirmation of the metal-induced gap states-plus-defects model

Abstract

The formation of Cs–GaP contacts was studied at room temperature and at 80 K by recording Auger electrons, energy distribution curves (EDCs) of photoelectrons excited by He i radiation [ultraviolet photoemission spectroscopy (UPS)], and by measuring the contact potential with a Kelvin probe in the dark [contact-potential difference technique (CPD)]. While the samples held at room temperature show a saturation coverage of one monoatomic Cs layer, at low temperatures homogeneous growth of metallic films was achieved. The position of the Fermi level with respect to the valence-band maximum was obtained as the difference between the ionization energy from UPS and the work function from CPD measurements. This procedure accounts for any possible influence of surface photovoltage. After the occurrence of a Fermi edge in the EDCs recorded at 80 K, the Fermi level was found to become pinned on both p- and n-GaP(110). The corresponding barrier height φBn measures 1.00 eV. This result excellently agrees with what is predicted by the model, that the continuum of metal-induced gap states (MIGS) determines the barrier heights in abrupt Schottky contacts, and definitely rules out the Schottky–Mott model also for GaP. The barrier heights published so far for GaP Schottky contacts may again be explained by the MIGS-plus-defects model.