Photoemission studies of molecular beam epitaxially grown GaAs (001) surfaces exposed to a nitrogen plasma

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Abstract
GaAs(001) surfaces prepared in situ by molecular beam epitaxy (MBE) have been investigated by reflection electron diffraction, valence-band photoemission and core level photoemission after exposures to nitrogen plasmas. The nitridation leads to disordered surfaces. The Fermi level (EF) which for the clean surface is situated at 0.4 eV above the GaAs valence-band maximum (VBM) shifts as a function of the plasma exposure towards a saturation value of 1.1 eV above VBM. Strong nitrogen derived peaks are found in the valence-band spectra at energies 5 to 8 eV below EF. As(3d) and Ga(3d) core level spectra show chemical shifts towards higher binding energies due to the nitridation. The shift is 0.7–0.8 eV for the Ga(3d) level. For the As(3d) level shifts in the range 1.4–2.2 eV are observed. The dependence of the chemical shifts and the linewidths on the plasma exposure and on subsequent heat treatments are explained by the presence of different As nitrides having different binding energies and volatilities, whereas only one Ga nitride (GaN) is formed by the nitridation.