Capacitance-voltage profiling and the characterisation of III-V semiconductors using electrolyte barriers

This article is a personal review of the principles, capabilities, limitations and potential of the technique of electrochemical capacitance-voltage (C-V) carrier concentration profiling of compound semiconductors and the associated technique of photovoltage absorption spectroscopy. The profiling technique was developed by Ambridge and co-workers (1973-5, 1980) to overcome the depth limitation in depletion C-V profiling by using an electrolyte barrier to measure the carrier density and to etch the material in a controlled electrolytic process. The electrolyte also provides a transparent barrier which facilitates observation of absorption spectra, hence providing the added capability of band-gap profiling. The author summarises the basic principles of C-V profiling and analyses the balance between measurement accuracy and instrumental depth resolution, and considers the effect of series resistance. In reviewing the principles of electrochemical C-V profiling he pays particular attention to the electrolyte (Helmholtz) capacitance, the high electrolyte resistance and the definition of contact area. In considering these problems, and those of depth resolution and the influence of deep states, he takes account of the use of a fixed low reverse bias in electrochemical C-V profiling compared with an increasing bias in depletion profiling. The interpretation of photovoltage spectra from single layers and heterostructures is described and examples are given of band-gap profiling of laser structures. The article concludes with examples of the characterisation of multiple quantum-well structures including carrier density profiles and photovoltage spectra on structures with periods less than 200AA.