IN-SITU STUDY OF FERMI-LEVEL PINNING ON N-TYPE 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 environm ent of a molecular-beam-epitaxy chamber using photoreflectance. As-gro wn surfaces as well as the effects of a few monolayers of arsenic depo sition/desorption were investigated. The measured barrier heights of t he as-grown n (V-B,V-n)- and p (V-B,V-p)-type samples (relative to the ir respective band edges) were 0.61 V (midgap pinning taking into acco unt the photovoltaic effect) and 0.33 V, respectively The in situ depo sition of a few monolayers of arsenic bad no effect on V-B,V-n but cau sed V-B,V-p to increase to 0.60 V, i.e., midgap pinning. The desorptio n of the arsenic layers brought V-B,V-p close to its as-grown value bu t had no effect on V-B,V-n These observations, together with earlier s tudies on similar but air-stabilized samples, provides evidence that A s plays a crucial role in the formation of the surface Fermi levels.