Optical characterization of semiconductors using photo reflection spectroscopy

Abstract

The study of semiconductor materials and more recently, artificially struc¬tured materials, is important for both scientific and industrial purposes. Many techniques have been developed to characterize the electronic properties of these materials. Optical characterization is a popular approach to this field of study, as the absorption or reflection of incident photons is directly related to the band structure of a semiconductor material. When measuring the absolute reflectivity or absorptivity, the resulting spec¬trum is often rather featureless, making it difficult to observe band structure features. This led to the invention of modulation spectroscopy techniques where the derivative of the absorptivity or reflectivity of a semiconductor ma¬terial with respect to some experimental parameter, is evaluated. Weak fea¬tures in the absolute spectrum are thus enhanced, making the identification of band structure features easier. This study describes the technique of photo reflection spectroscopy (com¬monly known as photoreflectance spectroscopy) where modulation is achiev¬ed optically. The theory behind photoreflectance spectroscopy is discussed in detail, whereafter the practical implementation is described. This is followed by measurements done on GaAs to do a basic comparison with published re¬sults. Finally, three different doping superlattices were investigated with this technique; including the influence that a-particle irradiation and consequent annealing have on the measured photoreflectance spectra.