Absorption edge determination of thick GaAs wafers using surface photovoltage spectroscopy

A procedure for choosing the appropriate chopping frequency (f) for the surface photovoltage spectroscopy (SPS) measurements in order to obtain the absorption related features is presented. We could obtain the absorption edge of thick n(+) GaAs wafer (thickness approximate to700 mum) by performing SPS measurements at fgreater than or equal to1 kHz at room temperature (300 K). The similar information for semi-insulating (SI) GaAs could not be obtained due to the carrier trapping at deep levels or surface states at 300 K. However, we could obtain the absorption edge of SI-GaAs by performing SPS measurements at 395 K at f=3 kHz. Here, we demonstrate the capability of the SPS technique to measure large absorption coefficient (alpha) values for thick wafers by performing SPS measurements and normalizing this with the reported alpha value at one wavelength in the above band gap region. For comparison, we also perform quasisimultaneous SPS and transmission spectroscopy (TS) measurements. The SPS technique could provide alpha values up to 10(4) cm(-1) for 700-mum-thick GaAs wafers, whereas TS could only measure alpha values up to about 15 cm(-1). An improved design of the sample holder for measuring the surface photovoltage in the chopped light geometry, which increases the signal strength by reducing the gap between the top electrode and the wafer in a controlled manner, is presented. This ensures that there is no sample damage or contamination. (C) 2002 American Institute of Physics.