UC Berkeley

A metal-semiconductor heterojunction is investigated by Auger and photoelectron spectroscopy to characterize the structural and electronic properties of the metallic film and to obtain the time-resolved electronic response induced by femtosecond laser excitation of the semiconductor material. The 3.5 monolayer (ML) Zn films deposited on p-type Si(100) at liquid nitrogen temperature grows in a layer-by-layer fashion. Electronic structure measurements by extreme ultraviolet (XUV) photoelectron spectroscopy indicate that the films are metallic in nature, creating a Schottky barrier at the 3.5 ML Zn/p-Si(100) interface. Utilizing a 35 fs, 800 nm pump pulse at a pump intensity of (2.5-6) × 109 W/cm2 to excite the Si and a time-delayed extreme ultraviolet pulse to probe the Zn, we observed large transient surface photovoltage shifts of 0.3-2.2 eV at carrier densities of (1.5-4.5) × 1020 cm-3. Three shifts are determined the Zn 3d core level, the photoemission onset, and the metallic Fermi level. The photovoltages increase with laser excitation intensity, and the Zn 3d core level exhibits the largest binding energy shifts due to pronounced screening of the core level. The large observed shifts are rationalized on the basis of the energetics of band flattening and carrier accumulation in the metallic layer of the Zn/p-Si(100) heterojunction at high carrier densities. The observed carrier recombination dynamics are biexponential in character, with similar time constants for both the Zn 3d and photoemission onset binding energy shifts. The Zn 3d core level shifts are also found to be sensitive to the electron temperature. These results show that core-level photoemission can be used to monitor valence electron dynamics, allowing separation of charge dynamics in heterojunctions and solids composed of multiple elements.