HOT-ELECTRON REDUCTION AT N-SI/AU THIN-FILM ELECTRODES

Schottky barrier electrodes were prepared by deposition of thin films of Au on device quality n-Si. Barrier heights for fourteen day old films, determined by high frequency capacitance measurements at open circuit in the presence of 0.3M each K4Fe(CN)6/K3Fe(CN)6, were found to be 0.60 eV in the thickness range of 200 to 1000 angstrom. The exchange current densities were determined as a function of film thickness and aging time up to about one month after deposition. Ideal solid-state-controlled Tafel slopes of 60 mV/decade were observed for overpotentials up to 120 mV. Mott-Schottky plots confirmed the symmetry factor was unity. The ratio of the exchange current j(o) (100 angstrom)/j(o) (2000 angstrom) was about 100 for measurements made within 1 h after deposition. A similar increase in j(o) with decreasing thickness was observed at five and fourteen days, but the magnitude of the currents were lower by one to two orders of magnitude, with larger changes for thinner films. The lower currents after aging, were attributed to larger barrier heights, perhaps caused by changes in the interfacial layer such as oxidation. The observed dependence of exchange current on thickness after each aging period, was successfully modeled by assuming ballistic transport of hot electrons across the metal film. The ballistic mean-free path of electrons in our Au films, determined by application of the theory developed in Paper I, was 100 +/- 20 angstrom.