Electronic properties of nascent GaP(110)-noble-metal interfaces.

Abstract

The evolution of the interface properties of cleaved n-type and p-type GaP(110) surfaces with Ag, Cu, and Au overlayers was investigated by photoemission spectroscopy excited with synchrotron radiation. Silver was found to be relatively unreactive with GaP, although some evidence for a Ga replacement reaction was found. Cu and especially Au were observed to react strongly with GaP, with clear evidence of metal-Ga exchange reactions occurring for coverages of \ensuremath{\gtrsim}0.3 \AA{}. Phosphorus-metal reaction products include Cu-P and Au-P components characterized by a P 2p binding energy comparable to that of P in GaP. An additional reaction product was observed for Au, which was tentatively interpreted as elemental P. All phosphorus components tend to diffuse to the metal surface. The band bending, or specifically the position of the interface Fermi level ${\mathit{E}}_{\mathit{F}}$ relative to the band edges, was measured as a function of metal coverage. On n-type surfaces ${\mathit{E}}_{\mathit{F}}$ remained fairly constant for coverages to \ensuremath{\sim}1 \AA{}, but dropped lower into the band gap as the overlayer assumed metallic properties. On p-type surfaces ${\mathit{E}}_{\mathit{F}}$ increased for low coverages, but decreased at the onset of metallicity (overshoot). For coverages \ensuremath{\gtrsim}10 \AA{}, ${\mathit{E}}_{\mathit{F}}$ reached a common value on n- and p-type surfaces, which differed among the metals. Measured Schottky-barrier heights ${\mathrm{\ensuremath{\Phi}}}_{\mathit{B}}^{\mathit{n}}$ for n-type GaP(110) were 1.55 eV (Au), 1.37 eV (Ag), and 1.41 eV (Cu). The delocalization model was applied to calculate both the drop in ${\mathit{E}}_{\mathit{F}}$ with metallization, and ${\mathrm{\ensuremath{\Phi}}}_{\mathit{B}}^{\mathit{n}}$ for the three metals. Agreement with experiments supports the model and underscores the importance of defect and metallic states in Schottky-barrier physics.