Differential reflectance spectroscopy of semiconductors

Abstract

This thesis describes the development of Differential Reflectance (DR) Spectroscopy of semiconductors. While other modulation spectroscopies have found wide applications in semiconductor science and technology, the DR technique, introduced as early as 1969, concentrated mostly on the study of metallic compounds. In this work, DR spectroscopy was applied to study semiconductors and semiconductor microstructures relying on intrinsic or extrinsic spatial inhomogeneities present on or below the semiconductor surface. Two types of DR spectrometer were designed and constructed, and they were applied to study various ID-V and IT-VI compound semiconductors. These studies revealed that DR spectra of as-grown semiconductors exhibited sharp derivative-like features as a result of intrinsic inhomogeneities in various semiconductor parameters, such as surface field, strain, alloy composition, carrier concentration and layer thickness. The applications of DR spectroscopy for the determination of critical point energies, quantised energy states, alloy composition, carrier concentration and layer thickness were demonstrated on semiconductor quantum wells and heterostructures. In the case of extrinsic (or intentionally introduced) inhomogeneities, DR spectroscopy was found to be a sensitive technique to study the damage induced by various surface treatments which are commonly employed in semiconductor technology. The DR technique involved modifying one half of the sample, for example by ion implantation, hydrogenation or plasma-etching, while the other half was left unaltered to remain as a reference. When the reflectivity difference between the two halves was measured, DR signals provided useful information on the effects of such surface treatments. DR spectroscopy was also applied to profile the depth distribution of damage in semiconductors. Damage in GaAs was induced by various ion-assisted processes and damage profiles were obtained with a depth resolution of -60 A. The process of profiling involved a stepwise removal of the damaged layer and then, after each etch removal, measuring the integrated area under the DR spectrum profile in a narrow spectral region around the B1 + 4 1 critical point. The profiling of damage covering a wide range of damage levels and depths was demonstrated using this technique.