BUILT-IN ELECTRIC-FIELD AND SURFACE FERMI-LEVEL IN INP SURFACE-INTRINSIC N(+) STRUCTURES BY MODULATION SPECTROSCOPY

The techniques of the photoreflectance and electroreflectance (ER) wer e used to study the built-in electric fields and the surface Fermi lev els of InP surface-intrinsic-n(+) (SIN+) structures. The substrates of SIN+ structures are either Fe-doped semi-insulated InP or Sn-doped N InP with the same doping concentrations as its buffer layer. The buil t-in electric field and the Fermi level were calculated from the Franz -Keldysh oscillations of the photoreflectance spectra. Our studies fou nd that for the samples with the same doping concentration in the buff er layer and substrate, the built-in electric field increases as their top layer thickness decreases. The surface Fermi level, on the other hand, remains approximately constant. For samples with a semi-insulate d substrate, the photoreflectance spectra indicate the simultaneous ex istence of two built-in electric fields, one in the top layer and the other at the interface region between the buffer layer and substrate. ER spectra were measured with the application of a modulation electric field across the top layer. The built-in electric field across the to p layer obtained from the ER spectra increases as the top layer thickn ess decreases while the surface Fermi level, again, remains approximat ely constant. (C) 1997 American Institute of Physics.