Study of semiconductor surfaces and interfaces using electromodulation

Abstract

The modulation spectroscopy technique of electromodulation (EM) is a major tool for the study and characterization of a number of semiconductor surfaces/interfaces and also for the evaluation of processinduced damage at surfaces. The most useful forms of EM are photoreflectance (PR) and contactless electroreflectance (CER) because they are sensitive to surface/interface electric fields and often yield the sharpest structure (third-derivative lineshape of the unmodulated spectrum in the case of bulk/thin-film material). For sufficiently high electric fields the PR/CER spectra may exhibit Franz-Keldysh oscillations, which are a direct measure of the relevant electric field. Furthermore, PR and CER require no special mounting of the sample and hence can be employed in situ (PR) or non-destructively on wafer-sized material. This article will discuss some recent applications of PR/CER for the investigation of a number of surfaces/interfaces, such as: process-induced effects on the surface of Si; Fermi-level pinning on a number of III-V materials, including low-temperature-grown GaAs (with excess As) and wurtzite GaN; Schottky barrier formation; process-induced damage (sputtering, reactive ion etching, chemically assisted ion beam etching) on III-V materials; homojunctions (GaAs/GaAs); heterojunctions (GaAlAs/GaAs, ZnSe/GaAs, CdTe/GaAs, AlInAs/InP, InGaAs/InP, InGaAs/InAlAs and wurtzite InGaN/GaN); and device structures such as heterojunction bipolar transistors and pseudomorphic high-electron-mobility transistors. A number of these studies have been performed in situ with an ultrahigh vacuum environment.

Study of optical properties of GaAsN layers prepared by molecular beam epitaxy

“…As shown in Fig. 2 these values are in good agreement with the GaAsN transitions observed in the PR spectra, which were obtained by fitting the experimental data to a third derivative functional form [4]. Fig.…”

“…From an analysis of the FKO is possible to determine the strength of the built-in internal electric field present in the samples. The FKO amplitude extrema occur at energies (E j ) given by [4] …”

Study of AlGaAs/GaAs quantum wells grown by molecular beam epitaxy on GaAs substrates subjected to different treatments

“…For sample M1, we also note a transition (marked by (*) in the figure) below the AlGaAs bandgap energy, presumably related to an impurity. In addition the PR spectrum of sample M1 shows Franz-Keldysh oscillations (FKO) above the AlGaAs bandgap energy region, which appear when a strong built-in internal electric field is present in the sample [7]. Employing the Franz-Keldysh model [7], we obtained a strength of E int =6 Â 10 6 V/cm for the built-in electric field in sample M1.…”

Optical characterization techniques for process monitoring

“…These features are discussed in detail in the paper by Pollak. 18 A combined technique called photo-spectroscopic ellipsometry 19 has been used to study directly the Seraphin coefficients and the relationship between the modulated spectral derivatives from photoreflectance and mathematically differentiated spectroscopic ellipsometry spectra.…”