SURFACE PHOTOVOLTAGE SPECTROSCOPY OF THIN-FILMS

The surface photovoltage (SPV) spectrum due to subband-gap illuminatio n of thin films is theoretically studied. It is shown that this SPV is inherently sensitive to buried interfaces just as it is sensitive to the external semiconductor surface. The different contributions to the SPV from all the optically active gap states present within a sample, consisting of a bulk substrate covered by a thin film, are analyzed. Analytical expressions are obtained in the low illumination intensity and the depletion approximation regime. The evolution of the SPV spect rum with film thickness is examined and is found to depend on both sit e and population of the gap states. Three modes of evolution are found , according to the relative importance of gap state population changes with film thickness. These modes are confirmed by a numerical simulat ion of a thin film of pseudomorphic InAlAs on InP substrates and by ex periments conducted on the same system. The approach is also applied t o the InP/In2O3 system, revealing gap state formation, followed by fil ling with electrons, thereby explaining previous observations of nearl y ideal I-V behavior at this junction. (C) 1996 American Institute of Physics.