Effect of substrate thickness, back surface texture, reflectivity, and thin film interference on optical band-gap thermometry

A model for the temperature-dependent shape of the absorption edge in GaAs is used to determine the effect of substrate thickness, back surface texture, and deposited dielectric films, on the temperature measurement accuracy, for both transmission and diffuse reflectance geometries of optical band-gap thermometry. A 5% change in substrate thickness, a 20% change in light scattering at the back of the substrate, or a 40% change in the residual absorption below the band edge all cause temperature errors of about 1°C. Band-gap thermometry is also sensitive to thin film interference during the growth of wide band-gap layers, which can cause the apparent temperature to oscillate. An algorithm is presented that uses the width of the knee in the spectrum to correct temperature errors caused by interference oscillations. This algorithm is suitable for real-time applications as the information needed to correct the knee position is obtained from the spectrum itself. The correction procedure is tested on data taken during growth of AlGaAs on GaAs. The interference oscillations in the deposited thin film reduced apparent temperature oscillations from about 1.5°C at 570°C to about 0.7°C (RMS).