Mechanisms for GaAs Surface Passivation by a Molecular-Beam Epitaxial Cap Layer Grown At 200-Degrees-C

A thin, undoped, molecular beam epitaxial (MBE) GaAs cap layer grown on top of an n‐type conductive layer significantly reduces the free‐electron depletion from the latter. By analyzing electron transfer to surface, interface, and bulk acceptor states in the cap, as a function of cap thickness, we show that either (1) the usual EC−0.7 eV surface states are absent, (2) a dense donor near EC−0.4 eV exists or (3) a high donor interface charge (∼5×1012 cm−2) is present. Any of these conclusions constitutes an important new aspect of low‐temperature MBE GaAs