Decoupling of thermo-electronic effect by traveling photothermal mirror method for characterization of thermal properties of semiconductors

Abstract

Interactions between super bandgap energy photons and semiconductor materials involve plasma waves in addition to thermal waves, resulting in a strong thermo-electronic coupling effect. Here, we propose an effective traveling photothermal mirror method to decouple the thermo-electronic effect for characterization of thermal properties of a semi-insulating GaAs wafer as a demonstration. A theoretical model is presented for describing the dynamic processes of the thermal and plasma waves in the sample as well as the heat coupling between the sample and the surrounding fluid under the excitation of a laser beam traveling at a constant velocity. Based on the solution to the diffusion equations, we obtain the phase shift introduced to another probe beam associated with the photothermal signal due to the thermoelastic and electronic-strain responses of the sample and the refractive index gradient of the fluid. The theoretical and experimental results reveal that the steady-state process in the laser–material interaction makes the electronic effect act as an insensitive constant background in the photothermal signal, and the thermodynamic process is governed by the thermal properties of the sample. The distinct advantage of the traveling photothermal signal being immune to the variation of the electronic transport parameters allows the thermal diffusivity of the sample to be accurately determined from the best fit to the signal, and the traveling excitation nature of the method provides a way for high-efficiency photothermal imaging to identify thermal defects.