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Superlattices of alternating layers of semiconductors were first proposed1 in 1970, and since then a variety
of structures have been grown. Their technological importance has spurred considerable experimental and
theoretical work. The unique feature of quantum confinement of carriers has made possible unusual
devices. By combining various semiconductors and alloys of ffl-V, 11-TV and group IV materials, unusual
band lineups between neighboring layers have been obtained. Both lattice matched and strained layer
structures have been grown.
In this article we will focus on the electronic structure of the quantum well heterostructures under the
external perturbation of hydrostatic pressure. Pressure has been used extensively to investigate materials
in regions of phase space not otherwise accessib1. lu the study of quantum well structures, it has also
been used to move band edges in a controlled fashion, and alter band lineups, allowing the determination
of band offsets with an accuracy that was not possible without the use of pressure. As in bulk
semiconductors, optical techniques provide powerful tools in studying the electronic states in quantum
well heterostructures (QWH). Photoluminescence (PL) spectroscopy is only sensitive to spectral features
associated with energy states close to the bottom of the well due to rapid thermalization of carriers.
Photoluminescence excitation (PLE) is often limited by the availability of tunable lasers. Photoreflectance
(PR), on the other hand, can provide a rich structure due to both symmetry allowed and forbidden
transitions encompassing the entire quantum well. This sensitivity is due to the derivative nature of the
spectroscopy. Experiments can be carried out easily at different temperatures and over wide spectral
regions.
This article is organized as follows. In section 2 we will review some of the theoretical calculations of
electronic bands in quantum wells and discuss the changes expected under pressure. In Sec. 3, we
discuss the experimental details, including descriptions of the optical techniques used. Section 4 will deal
with studies of the quantized transitions in GaAs/GaixAlxAs and GaSb/A1Sb QWH under pressure,using
PR and PL. The examples are illustrative of the comparative merits of the two techniques.
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