PASSIVATION OF GAAS(001) SURFACES BY INCORPORATION OF GROUP-VI ATOMS - A STRUCTURAL INVESTIGATION

We have investigated the relationship between interface chemistry, structure, and surface electronic states in the band gap of Se and Te-passivated GaAs(001). Metalorganic chemical vapor deposition was used to carry out the passivations. Chemical, structural, and electronic characterization of the surfaces was accomplished by means of x-ray photoelectron spectroscopy and diffraction, and low-energy electron diffraction. We find that the superior passivating capability of Se can be related to the structural quality of the interface created in the near-surface region. Se undergoes extensive, but diffusion-limited, anion exchange in the top five to seven anion layers. The majority of As atoms in the surface layer undergo anion exchange with Se, and the extent of anion exchange diminishes with increasing depth. Thus, a graded Se(x)As1-xGa/GaAs(001) heterojunction of high structural quality forms. In contrast, Te does not passivate GaAs(001), and this inability to passivate can also be related to interface structure. Te undergoes extensive anion exchange only in the surface layer; displacement of third-layer As atoms appears to be limited to approximately 10 at. %. Furthermore, the interface is partially disordered and approximately 1 monolayer-equivalent of unincorporated Te remains bound to the surface. The combined effect of these defects is to create a surface-state density sufficient to induce significant band bending.