Electroreflectance and photoreflectance studies of surface Fermi level andsurface state densities of InPSIN+ structures

The built-in electric fields, sur face Fermi level, and surface state densi ty of a series of InP surface-intrinsic-n(+) structures are calculated from the Franz-Keldysh oscillations of the modulation spectra of photoreflectan ce (PR) and electroreflectance (ER). Those results indicate that for sample s with similar doping concentrations in the buffer layer and the substrate, the only built-in electric field across the top layer increases as their t op layer thickness decreases. The surface Fermi level, however, remains rou ghly constant. For samples with a semi-insulated substrate, two built-in el ectric field exist separately in the top layer and at the interface between the buffer layer and the substrate. In addition, ER spectra are taken with the modulation electric field across the top layer. Although the built-in electric field across the top layer derived from the ER spectra increases a s the top layer thickness decreases, the surface Fermi level, again, remain s roughly constant. An approach of room temperature photoreflectance, based on the thermionic-emission theory and the current transport theory, is emp loyed to estimate the surface state density. The surface state density of t he InP SIN+ structure is found to diminish with an increase in the thicknes s of the top layer. (C) 2000 American Vacuum Society. [S0734-2101(00)04305- 0].