Band bending, Fermi level pinning, and surface fixed charge on chemically prepared GaAs surfaces

We compare the nature of the band bending under GaAs surfaces prepared by alkaline sulfides [Na2S⋅9H2O, (NH4)2S] with that under oxidized GaAs surfaces. We make the point that Fermi level pinning implies band bending, but band bending does not necessarily imply ‘‘pinning.’’ In either case, even weak light illumination substantially flattens the bands. On ammonium sulfide treated surfaces the fixed and trapped charge density in the dark is only ∼5×1011 electrons/cm2, but these few states are mostly neutralized at low-level forward injection. This behavior should not be confused with Fermi level pinning.