Surface studies by modulation spectroscopy

Surface barrier and electronic surface states are important parameters for characterizing semiconductor surfaces. Qualitative information on the surface electric field can be deduced from electroreflectance studies. Energetic positions of surface states have been determined by spectroscopic methods using effect modulation techniques. Besides the field effects of surface conductivity and absorption, which are limited to low densities of surface states, new information on surface states was gained by investigating the spectral dependence of surface photoconductivity. Also surface phonons were detected in “spectral oscillations” of photoconductivity. The measurement of surface photovoltage at photon energies where the bulk is transparent promises a new tool for surface state research in the future. To demonstrate these modulation techniques examples are given for Ge, Si, ZnO, and CdS surfaces.