Charge trapping and built-in field studies in electroreflectance of a GaAs structure

A form of contactless electroreflectance, vacuum electroreflectance (VER), is used to quantitatively compare photoreflectance (PR) with electroreflectance (ER). It is found that the amplitude of the VER signal for a given applied modulation taken under CW laser illumination drastically decreases with increasing laser intensity. This new effect is studied as a function of frequency, and is explained in terms of the screening of the a.c. applied field by the surface charge, due to the presence of extra charge carriers in the depletion region. This effect is not present in contact forms of ER. It is also shown from a systematic comparison of PR and VER data that even for the lowest laser intensities which give an observable PR signal, the nature of the modulation in PR lowers the measured built-in field by an amount well outside the uncertainties in the determination of the field.