Optical bandgap thermometry in molecular beam epitaxy

The temperature dependence of the optical absorption edge (Urbach edge) is measured in semi-insulating and n-type (n = 2*10¹⁸ cm⁻³) GaAs from room temperature to 700 °C. The characteristic energy of the exponential absorption edge is found to increase linearly with temperature, from 7.5 meV at room temperature to 12.4 meV at 700 °C, for semi-insulating GaAs. The temperature dependent part of the width of the Urbach edge for semiinsulating GaAs and InP is smaller than predicted by the standard theory where the width of the edge is proportional to the phonon population. The part of the width not characteristic of the phonon occupation number in semi-insulating GaAs and InP, is attributed to static fluctuations in the band edges due to point defects. The absorption edge in n-type GaAs and InP is broadened by fluctuations in the band edge caused by the electric fields and the strain fields of the ionized donor impurities. An optical method for measuring the temperature of a substrate material with a temperature dependent bandgap is presented. In this method the substrate is illuminated with a broad spectrum lamp; the bandgap is determined from the spectrum of the diffusely scattered light. Light with energies below the bandgap is transmitted through the substrate and reflected from the back surface of the substrate while light with energies above the bandgap is absorbed by the substrate. The front surface of the substrate is polished and its back surface is either rough or polished with a scatterer behind the substrate. The reflection of light from the front surface is specular and the light diffusely reflected at the back of the substrate is detected in a non-specular location. Substrate temperature is determined from the wavelength of the onset of the non-specular reflection. An algorithm is presented that utilizes the position and the width of the knee in the diffuse reflectance spectrum as a reference point for the onset of transparency and hence temperature of the substrate. A model is developed that relates the onset of transparency of the substrate to the optical bandgap and the width of the absorption edge of the substrate. From this model the sensitivity and absolute accuracy of the measurement for differences in substrate thickness, back surface texture, and conductivity is determined. The temperature sensitivity and reproducibility of the diffuse reflectance technique is better than 1°C when using GaAs substrates. Using the diffuse reflectance technique the temperature of GaAs substrates is profiled in a molecular-beam-epitaxy system with a spatial resolution of 3 mm and a thermal resolution of 0.4 °C. The effects of substrate doping, back surface textures, thermal contact to holder, and a pyrolytic boron nitride diffuser plate, on the temperature uniformity of radiatively heated substrates is explored. Both positive and negative curvatures are observed in the temperature profiles.Science, Faculty ofPhysics and Astronomy, Department ofGraduat