TEMPERATURE-DEPENDENT COMPOSITION, ORDERING, AND BAND BENDING AT GAP(100) SURFACES

We have used soft x-ray photoemission spectroscopy (SXPS), Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED) to investigate thermally induced changes in atomic composition, bonding, and geometric ordering at GaP(100) surfaces prepared by molecular-beam epitaxy. Following growth, GaP(100) surfaces were coated with several monolayers of P, after which an approximately 2000 angstrom-thick As cap was applied to seal the structure for transfer to analytical chambers. Surface-sensitive SXPS core-level spectra and more bulk-sensitive AES spectra collected at in situ annealed surfaces indicate complex changes in P, As, and Ga atomic ratios throughout the 350-650-degrees-C desorption temperature range. Core-level photoemission intensity ratios reveal rapid loss of both P and As for surface annealing temperatures below 450-degrees-C, a dramatic increase (decrease) in the P (As) concentration in the 450-550-degrees-C range, and a relatively stable, stoichiometric GaP(100) surface composition for temperatures between approximately 550 and 640-degrees-C, above which thermally driven surface decomposition becomes noticeable. Existence of clear LEED patterns, (1 X 2) for desorption temperatures below approximately 450-degrees-C and (4 X 2)-c(8 X 2) above this temperature, indicates little sensitivity of GaP(100) surface reconstructions to details of surface stoichiometry and bonding. We propose that both this insensitivity and the abrupt changes in the P to As atomic ratios indicate strong competition between the two anions to form bonds with Ga surface atoms. In this context, compositional and structural details of decapped GaP(100) depend sensitively on the interplay of surface desorption, atomic diffusion from the substrate to the free surface, and the anion-cation chemical bonding at the surface. Overall, our work demonstrates the success of As and P capping combined with subsequent thermal desorption in producing stable GaP(100) surfaces with desired chemical and electronic properties.