Absorption edge determination of thick GaAs wafers using surface photovoltage spectroscopy

Abstract

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 ≈700 μm) by performing SPS measurements at $\text{f⩾1}$ 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 $\text{f=3}$ kHz. Here, we demonstrate the capability of the SPS technique to measure large absorption coefficient (α) values for thick wafers by performing SPS measurements and normalizing this with the reported α 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 α values up to ${10}^4\hspace{0.5em}{\mathrm{cm}}^{\mathrm{-1}}$ for 700-μm-thick GaAs wafers, whereas TS could only measure α values up to about 15 ${\mathrm{cm}}^{\mathrm{-1}}.$ 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.