IN SITU STUDY OF FERMI-LEVEL PINNING ON N- AND P-TYPE GAAS (001) GROWN BY MOLECULAR-BEAM EPITAXY USING PHOTOREFLECTANCE

We have conducted an in situ study of the Fermi-level pinning behavior of n- and p-type GaAs (001) surfaces in the ultrahigh-vacuum environment of a molecular-beam-epitaxy chamber using photoreflectance. As-grown surfaces as well as the effects of a few monolayers of arsenic deposition/desorption were investigated. The measured barrier heights of the as-grown n (${\mathit{V}}_{\mathit{B},}$n)- and p (${\mathit{V}}_{\mathit{B},}$p)-type samples (relative to their respective band edges) were 0.61 V (midgap pinning taking into account the photovoltaic effect) and 0.33 V, respectively The in situ deposition of a few monolayers of arsenic had no effect on ${\mathit{V}}_{\mathit{B},}$n but caused ${\mathit{V}}_{\mathit{B},}$p to increase to 0.60 V, i.e., midgap pinning. The desorption of the arsenic layers brought ${\mathit{V}}_{\mathit{B},}$p close to its as-grown value but had no effect on ${\mathit{V}}_{\mathit{B},}$n. These observations, together with earlier studies on similar but air-stabilized samples, provides evidence that As plays a crucial role in the formation of the surface Fermi levels.