TIME-DEPENDENCE OF THE SURFACE FERMI-LEVEL OF GAAS IN ATMOSPHERE

This letter reports the time dependence of the surface Fermi level of GaAs grown by molecular-beam epitaxy and then exposed to atmosphere. T he sheet resistance of sample structures for field effect transistors alternately increased, decreased, increased, and decreased to become n early constant after about 500 h. These changes correspond to the surf ace Fermi level varying between 0.3 and 0.7 eV and finally settling 0. 7 eV above the valence band maximum. Comparison between annealed and u nannealed samples with low-temperature-grown GaAs layers showed that t he pinning of the surface Fermi level at 0.7 eV above the valence band maximum is caused by arsenic antisite defects. The result supports th e advanced unified defect model.