Modulation spectroscopy is a powerful method for the study and characterization of a large number of semiconductor configurations, including bulk/thin film, microstructures (heterojunctions, quantum wells, superlattices, quantum dots), surfaces/interfaces and actual device structures in addition to semiconductor growth/processing. Furthermore, the influence of external perturbations such as temperature, electric fields, hydrostatic pressure, uniaxial stress, etc. can be investigated. This optical technique utilizes a very general principle of experimental physics, in which a periodically applied perturbation (either to the sample or probe) leads to sharp, derivative-like spectral features in the optical response of the system. Because of the richness of the derivative-like spectra, the information in the lineshape fits, room temperature performance and relative simplicity of operation this method is becoming increasingly more important as a tool to study these materials and structures. This article will review developments in the field during the last decade.

Modulation spectroscopy of semiconductors: bulk/thin film, microstructures, surfaces/interfaces and devices

In the presence of an electric field, the dielectric constant of a semiconductor exhibits Franz–Keldysh oscillations (FKO), which can be detected by modulated reflectance. Although it could be a powerful and simple method to study the electric fields/charge distributions in various semiconductor structures, in the past it has proven to be more complex. This is due to nonuniform fields and impurity induced broadening, which reduce the number of detectible Franz–Keldysh oscillations, and introduce uncertainties into the measurement. In 1989, a new structure, surface–undoped–doped (s‐i‐n+/s‐i‐p+) was developed, which allows the observation of a large number of FKOs and, hence, permitting accurate determination of electric fields. We present a review of the work on measuring electric fields in semiconductors with a particular emphasis on microstructures using the specialized layer sequence. We first discuss the general theory of modulation techniques dwelling on the approximations and their relevance. The cas...

Franz–Keldysh oscillations in modulation spectroscopy

Publisher Summary This chapter describes the application and flexibility of modulation spectroscopy, particularly electroreflectance (ER) and photoreflectance (PR), to study artificially structured materials (ASMs). The chapter explains the way measurements on microstructures have helped understand the properties of ASM and of the modulation mechanisms involved in electromodulation spectroscopy. The types of physical systems to be studied are of a multilayer variety whose building block is usually composed of two materials having different band gaps. This difference in band gaps can lead to the confinement of carriers in one of the two materials (that is, the carriers can become confined to a quantum well). With modulation spectroscopy, the derivative of the absorptivity or reflectivity with respect to some parameter is evaluated. This spectroscopy is sensitive to critical-point transitions in the Brillouin zone, while the resulting spectrum has sharp derivative-like features and a little, if any, featureless background.

Chapter 4 Photoreflectance Spectroscopy of Microstructures

Transmission and photoreflectance spectra in highly strained InGaAs-GaAs multiple quantum wells

Nanometer‐scale wires cut into a Si/Si0.87Ge0.13 multiple quantum well structure were fabricated and characterized by using photoluminescence and photoreflectance at temperatures between 4 and 20 K. It was found that, in addition to a low‐energy broadband emission at around 0.8 eV and other features normally observable in photoluminescence measurements, fabrication process induced strain relaxation and enhanced electron‐hole droplets emission together with a new feature at 1.131 eV at 4 K were observed. The latter was further identified as a transition related to impurities located at the Si/Si0.87Ge0.13 heterointerfaces.

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Bingshen Wang

Papers

Photoreflectance spectroscopy of GaAs doping superlattices