Quantitative assessment of the photosaturation technique

The photosaturation technique is a well-known method for measuring the band-bending at semiconductor surfaces. It is based on the assumption that the bands can be flattened upon sufficiently intense illumination. The validity of this approach has been a subject of considerable dispute. A rigorous, quantitative examination of the method is presented. The physical mechanisms governing the photosaturation experiment are identified and analyzed using both an analytical and a numerical model. We show that while the technique is essentially valid, the illumination intensity required to obtain band flattening may be unrealistically high. Criteria for attaining photosaturation are formulated in terms of surface state parameters. Numerous pitfalls and sources of misinterpretation are pointed out. Specifically, a previously undiscussed pseudo-saturation due to surface states with significantly different thermal cross-sections, is described. A systematic approach to future experiments is suggested.