This document discusses photoreflectance spectroscopy of semiconductors. It provides an overview of semiconductor properties including crystal structure and doping, as well as defects in semiconductors. It then discusses optical properties such as absorption and photon energy. Modulation spectroscopy is defined as measuring changes in optical response caused by modifying measurement conditions. The technique is described as affecting semiconductor properties due to impurities and electric fields. Equipment used for modulation spectroscopy includes a laser, monochromator, photodetector, chopper, frequency generator, and lock-in amplifier.