Thermally reversible band bending for Bi/GaAs(110): Photoemission and inverse-photoemission investigations

Results of synchrotron-radiation photoemission, inverse-photoemission, and low-energy electron diffraction studies of the Bi/GaAs(110) interface over the temperature range 20 to 300 K are presented. At 300 K, the first Bi monolayer grows in zigzag chains along the [11¯0] direction to form an ordered (1×1) overlayer. This overlayer is semiconducting with a gap of 0.7 eV. Continued deposition results in Bi island growth atop the initial monolayer (ML) and conversion from semiconducting to semimetallic character. Bi deposition at 50 K results in layer-by-layer growth with no long-range order. Temperature- and coverage-dependent studies of band bending show symmetric behavior for n- and p-type GaAs(110) with matched bulk dopant concentrations. For 300-K deposition, the Fermi level ${\mathit{E}}_{\mathit{F}}$ moves rapidly toward final positions 0.74 and 0.98 eV below the conduction-band minimum for n- and p-type GaAs doped at ${10}^{17}$ ${\mathrm{cm}}^{\mathrm{-}3}$. For 50-K deposition, the bands remain nearly flat to coverages of 1–2 ML where ${\mathit{E}}_{\mathit{F}}$ moves rapidly toward midgap. Significantly, both techniques also show that Fermi-level movement is thermally reversible, such that the position of ${\mathit{E}}_{\mathit{F}}$ in the surface band gap depends on temperature. A small part of the ${\mathit{E}}_{\mathit{F}}$ movement is due to nonequilibrium processes involving the creation of electron-hole pairs by the incident photon or electron beam followed by the transport of the minority carriers to the surface region by the electric field in the depletion region. This surface voltage is most important at low temperature, and studies which vary the incident photon flux by over 3 orders of magnitude establish that the photovoltage has a small (≤100 meV), but non-negligible, effect on the band bending measured under ‘‘normal’’ experimental conditions. Hence, equilibrium processes control coupling of substrate and adsorbate-induced states in the GaAs band gap through the semiconductor depletion region and they determine the temperature-dependent interface band bending.