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) were used to study the built-in electric fields and the surface Fermi levels 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 built-in electric field and the Fermi level were calculated from the Franz-Keldysh oscillations of the photoreflectance spectra. Our studies found that for the samples with the same doping concentration in the buffer 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-insulated substrate, the photoreflectance spectra indicate the simultaneous existence 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 top layer obtained from the ER spectra increases as the top layer thickness decreases while the surface Fermi level, again, remains approximately constant.