Abstract

When p-type semiconductor is cathodically biased very strongly, either deep depletion or inversion layer is formed. Inversion layer formation plays a significant role in the electron transfer kinetics at semiconductor/solution interface and it is of interest to probe the inversion layer formation. The measurements of surface conductance and impedance are most often used for this purpose. If the inversion layer is formed at p-type semiconductor surface and strong oxidant which can inject holes into the valence band exists, the electroluminescence (EL) should be observed. SO₄^.- which is generated in situ by one electron reduction of S₂O₈^-- is known to be a very strong oxidant and is often used to study EL of n-type semiconductors. When p-GaAs (Zn doped: 1.09 x 10¹⁹ cm^-3) in solutions containing 5 M NaOH and 0.15 M S₂O₈^-- is negatively biased, first the cathodic currents due to the reduction of S₂O₈^-- and SO₄^.- are observed between -0.5 V and ca. -2 V. When the potential becomes more negative than ca. -2 V, the cathodic current due to hydrogen evolution is observed. In this potential region, EL which has peak around 880 nm is observed. Since the peak energy is in good agreement with the energy gap of GaAs, this EL seems to be due to bandadge emission. Thus, when p-GaAs is biased more negative than ca. -2.0 V, the electrons are transferred from the valence band in the bulk to the conduction band in the surface by turneling and some of them are consumed for hydrogen evolution and are recombined radiatively with holes, emitting band edge EL. The lower the carrier density, the more negative the critical potential at which the current due to hydrogen evolution started to flow and the weaker the intensity of EL. These results suggest that the electron concentration in the conduction band at the surface is lower at p-GaAs of lower carrier concentration. This is reasonable since the barrier width for the turneling is wider at p-GaAs of lower carrier concentration.

© 1984 Optical Society of America