Modulation spectroscopy utilizes a general principle of experimental physics, in which a periodically applied perturbation either to the probe or the sample leads to derivative-like features in the optical response of the sample. We review the application of the modulation techniques of electroreflectance, photoreflectance and piezomodulation to microstructure such as quantum wells, superlattices and modulation doped heterojunctions. It is shown that modulation spectroscopy is a powerful set of optical tools for the characterization of microstructures.

From a comparison of the thermoreflectance (first-derivative spectroscopy) and photoreflectance [electromodulation (EM)] spectra at 300K and 77K from a GaAs/Ga0.82A10.18As multiple quantum well structure we have obtained important information about the mechanisms of gm for uncoupled and coupled states. At 300K EM produces a third-derivative lineshape for the latter due to low-field modulation of tunneling states, while for the former a first-derivative is observed, related to modulation of exciton transitions. At 77K both coupled and uncoupled states yield a first-derivative lineshape due to these excitonic effects.

The lineshape of photoreflectance spectra from quantum well structures remains an unsolved problem, despite previous attemps to develop an adequate model using Seraphin coefficients for a bulk semiconductor. We show that the Seraphin coefficients in quantum well structures are substantially different from those in the bulk. We obtain analytical forms of the coefficients for a single quantum well, which show that the interference phase depends on the geometrical structure of the quantum well. The predictions are confirmed by our experimental photoreflectance data from quantum well samples. Our study suggests a new approach to the study of the lineshape problem of modulation reflectance.

We use the modulation techniques of electrolyte electroreflectance and photoreflectance to locally probe the semiconductor surface potential of various semiconductor/electrolyte interfaces. Changes in the surface potential are important in determining the rate of charge transfer in both electrode and electrodeless photochemical etching.

We have used the contactless and nondestructive technique of photoreflectance (PR) to study boron-ion implanted {100} CdTe both before and after annealing. The samples studied were implanted using 100 to 400 keV boron ions to a dosage of 1x10¹⁶/cm² and some were annealed in vacuum at 500C for 1 hour. The spectral measurements made at 77K in the vicinity of the direct gap, Eo, indicate three dominant phases with distinctly different line widths that may originate in a nearly crystalline, partially crystalline and heavily damaged regions in the sample. A study of the E1 spectra and the changes in the line shapes in the vicinity of E_o with varying the pump penetration depth from ~2500A using the 6328A He-Ne laser line to <400A using a near-ultra violet.beam yields some insight into the depth distribution and volume fraction of the three phases.

The effects of boron ion implants on the properties of undoped semi-insulating (100) GaAs single crystals have been studied. Raman scattering, Rutherford backscattering spectrometry (RBS) with ion channeling, double-crystal x-ray diffraction (DCD), photoreflectance spectroscopy, and electron paramagnetic resonance measurements have been used to provide a more complete description of the implant damage. Multiple energy implants with total doses up to 1.5x1016 ions/cm 2 generated uniformly damaged regions from the surface to various depths that exceed a micron. The boron implants caused substantial changes in the intensities and linewidths of the Raman spectra which were correlated with the damage indicated by the channeling measurements. Partial removal of the implant damage was observed upon annealing. While both furnace anneals at 850°C and 925°C rapid thermal anneals via quartz lamps can remove much of the lattice strain as seen by changes in the Raman and DCD results, the RBS and photoreflectance measurements indicated that considerable disorder remained.

Effective bandgap shrinkage due to the presence of a thin p+ diffused region on a unif-ormly doped n-type silicon constituting a p+n junction solar cell is measured by electro-lyte electro-reflectance spectroscopic method and reported here. Results show two prominent features in two bands (a) in the vicinity of indirect gap around 1.2 eV and (b) in the region above 3.2 eV. The peaks were found to shift with doping density in the p⁺ region. The observed shift was ~22 eV between two samples of doping densities 10¹⁸ and 10²⁰ cm^-3 in the p+ region.

The extensive development of instrumentation and methods of data analysis in the 1970's resulted in the application of spectroellipsometry (SE) to a wide variety of material and structural problems, and its advantages and limitations are now fairly well established. Here, I assess the present status of the field, concentrating on applications of interest to semiconductor technology. Dielectric function measurements and microstructural analysis are now mature fields, with e data accurate to within a few percent now available for most common semiconductors and alloys, in some cases as a function of doping and temperature. Methods for preparing clean, dielectrically abrupt surfaces have been a byproduct of this work. Thicknesses of the multilayer transparent or semitransparent films of silicon technology can be determined nondestructively to accuracies comparable to those obtainable with cross-sectional transmission electron microscopy and Rutherford backscattering, with compositional information provided in addition. Examples of analyses of compound semiconductor structures are given, including quantum confinement effects and the determination of surface electric fields, but the strong optical absorption of these materials restricts such analyses to outermost layers until infrared instruments become more widely available. Examples pertaining to growth and etching show that the most fertile area for further work lies in real-time, in situ monitoring and control of processing, although full capabilities will be realized only after the development of next-generation instrumentation that can make simultaneous measurements at a number of wavelengths.

Variable angle spectroscopic ellipsometry (VASE) is used to study the built-in electric fields in AlxGa1-xAs/GaAs heterostructures. VASE data, obtained with no electrical contacts on the sample, contain Franz-Keldysh (FK) lineshapes at the AlGaAs bandgap energy, which are due to the built-in field across the buried AlGaAs layer. Analysis of the lineshapes, using the FK theory together with a multilayer model, yields the approximate field profile in the layer. This, combined with a numerical solution of Poisson's equation, provides the approximate doping concentration. In addition, measurements made on samples with electrical contacts are described.

Variable angle of incidence spectroscopic ellipsometry (VASE) was used to determine the thickness, alloy composition, and growth quality of multilayer samples. These samples contained a single quantum well grown on a GaAs/AlGaAs superlattice. The superlattice layers were modeled as a bulk Al GaAs layer of unknown composition and thickness. A data fitting procedure allowed the variation of five layer thicknesses and two alloy compositions in a regression analysis. Extremely good data fits of the ellipsometric parameters * and A were realized in the 1.55 to 3.54 eV spectral range. Computer generations of the ellipsometric parameters were performed, and were compared with experimental results. The e-hh(1), e-lh (1), and e-hh(2) exciton transitions were observed in the VASE data measured at room temperature. VASE has thus provided a nondestructive and highly effective technique for characterizing intricate multilayered structures.

We discuss the first real-time spectroscopic measurements of (001) GaAs and AlAs surfaces during crystal growth by MBE. Reflectance-difference spectroscopy (RDS) is used to enhance the typically low sensitivity of optical probes to these surface phenomena. We describe a photoelastic-modulator optical-bridge configuration that achieves a sensitivity of 5 x 10^-5 to reflectance-difference (RD) signals under actual growth conditions, and use it to obtain the dynamic surface response for changes under various growth conditions. Comparison of RD and reflection high energy electron diffraction (RHEED) signals upon interruption of the As flux during otherwise normal growth of GaAs and AlAs show that RD signals are sensitive to either surface chemistry or surface structure according to photon energy. The spectral dependence of the chemically sensitive component is sufficiently pronounced so that Al- and Gaterminated surfaces can be distinguished. We use this capability to assess the competition between codeposited Al and Ga for the same surface-bonding sites. We also discuss the first observation of the RD analog of RHEED oscillations upon initiation of crystal growth, for both chemically and structurally sensitive conditions. Our results suggest that systematic investigations of the optical properties of growth surfaces will lead to a new understanding of crystal growth and new opportunities for the control of crystal growth processes.

Electronic and optical materials can and are being fabricated in a variety of forms including monotonic variation of their physical properties. A characteristic matrix for translating the optical parameters in media having linear gradients In complex refractive indices Is the basis for an algorithm for a program to calculate optical parameters for ellipsometry, reflectance and transmittance. Illustrative examples are given for three different types of gradient systems such as might be encountered in opto-electronic applications.

The optical properties of each semiconducting layer in amorphous solar cells were deduced from transmittance and reflectance measurements. Optical gaps, absorbances and photon efficiencies o structures were deduced. The influence on such properties of the different transparent conductive oxides used as windows in solar cells was obtained by comparing the same structures covered by Tin Oxide and Indium Tin Oxide respectively.

Raman scattering is a useful and non-destructive tool for characterization of semiconductor multilayered structures. This review paper is focused on the determination of the structure and composition as well as the optical, acoustical and acousto-optical properties of superlattices (or multiquantum wells). Most of the experimental results presented here concern the GaAs/AlAs superlattice system and are based on light scattering by folded acoustic modes and quantized optical modes. The Raman investigation of other superlattice systems (GaSb/ AlSb, GeSi/SisxGel-x etc...) as well as the scattering by interface modes are briefly reviewed. In certain application it is necesssary to locally destroy the multiquantum well structure in order to obtain optical and electrical confinements. The intentionally induced disordering can be also analyzed by Raman scattering.

The effect of reactive ion etching (RIE) on Si-doped molecular beam epitaxial (MBE) grown <100> GaAs has been studied by Raman scattering. The MBE samples were grown on doped substrates having a free carrier concentration of 1.5 x 1018 cm-3 . The ID phonon and coupled plasmon-ID phonon modes of reactive ion etched (RIE) GaAs were studied to determine both depletion widths and carrier concentrations. Measurable surface disorder, confined to within 10 nm of the surface, is observed for ion bombardment energies of > 300 eV. The Raman spectra of highly doped samples (N = 8 x 1018 cm-3 ), along with the I-V measurements of Schottky barriers made with RIE samples of lower free carrier concentration (N = 3 x 1017 cm-3 ), indicate that samples reactively ion etched with energies <200 eV provide a high quality GaAs surface.

Raman scattering, Rayleigh scattering and band edge photoluminescence are employed to examine the behavior of alloy disorder in AlxGa1-xAs alloys grown on GaAs(100) under reflection high energy electron diffraction determined growth conditions in molecular beam epitaxy. Evidence is found for the dependence of the short and long range disorder effects on the growth kinetics attendant to chosen growth conditions and on the Al concentration. At high Al concentration (xAl~0.8) evidence is found for the occurrence of atomic scale GaAs-like and AlAs-like regions. In addition, the "AlAs-like" TO mode is observed in the forbidden back scattering geometry and is likely due to strain and/or disorder related breakdown of the usual selection rules.

Lattice relaxation, elastic strain, and phonon shifts are studied in as-grown, MeV ion-implanted, and thermally annealed strained GaInAs layers on GaAs(001) substrates. The degree of lattice relaxation for the as-grown samples is discussed in terms of the measured in-plane lattice constants and the calculated critical thickness. For the 15 MeV Cl or 9 MeV P ion bombarded GaInAs/GaAs, the beam-induced elastic strains and beam-induced phonon shifts are measured and discussed for samples with different degrees of initial relaxation. Thermal annealing on the as-grown and the ion-implanted GaInAs layers indicates a substantial thermal loss of indium in the thin surface layers and a full recovery of radiation damage in the GaInAs layers by 500°C.

The strain distribution of Ge_xSi_1-x/Si strained layer superlattice (SLS) as a function of the distance from the interface has been studied by Raman Spectroscopy. A small angle bevel was made by angle lapping on a given thick Ge_xSi_1-x/Si SLS so that it is possible to probe the structure at different thicknesses. The Raman spectrum as a function of the distance from interface is then obtained. The results indicate that, as we move away from the interface, compression strain in the alloy layers decreases, tensile strain in the Si layers increases and lower concentration of crystalline defects are present as observed from linewidth measurement.

Photoluminescence, excitation and magneto-optical experiments are performed on GaAs/GaAs(001) layers grown by molecular beam epitaxy. Attention is paid to the growth-induced complex defects, particularly their related excitonic-emissions in the 1.504-1.511 eV region (g-v band). The influence of growth conditions on the incorporation of residual impurities is examined. It is shown that the shallow acceptor responsible for the bound exciton (BE) line around 1.511 eV (g line) involves the majority acceptor species (C, Be). An acceptor-isoelectronic association model describes the properties of the g acceptor complex defect whose experimental binding energy is smaller than that of the involved C or Be single substitutional acceptor. Among the previously characterized isoelectronic-like complex defects having respectively Td, C3V, Cs and C2v symmetries, the simplest one in cubic symmetry is found to be present only in samples grown at temperatures lower than 580- 600°C).

Photoluminescence data are used to obtain the concentration and temperature dependence of the bandgap energies of epitaxial layers of quaternary alloy In Ga Al As for Al concentrations less than 0.6. The samples were grown by MBE without an intervening buffer layer. For an In concentration of 0.1, the transition from a direct to an indirect bandgap material occurs at Al concentration of about 0.5. Coefficients of the temperature dependence of the bandgap energy are also presented.

Recently we found two new emissions denoted by 'g' and [g-g] in the low temperature photoluminescence (PL) spectra of acceptor-impurity incorporated GaAs. 'g is situated at an energy slightly below that of the bound exciton emissions. [g-g] is situated just below 'g' and shows a significant energy-shift towards the lower energy side with increasing acceptor concentration,[A]. Previously we proposed a model in which [g-g] was ascribed to the acceptor-acceptor pair formed by the overlapping of wave functions of the 2p state of the isolated acceptors. Although the calculated energy as a function of [A] showed qualitative agreement with the experimental results, it was always larger than the observed binding energy of [g-g]. In this paper it is indicated that the red shift of [g-g] with increasing [A], and its energy locking at a critical [A], can be well explained by phenomenologically taking into account the screening effect of the hole which is in the ground state. It was explicitly demonstrated that [g-g] is a very useful optical tool for the estimation of [A].

Transmission electron microscopy (TEM) has been extensively used to characterize crystalline defects in II-VI compound semiconductors in the systems Hg-Cd-Te, Hg-Zn-Te and Cd-Zn-Te. These compounds are currently of interest for infrared detector and substrate materials. Several examples of the use of TEM to understand and improve these materials are presented. The examples include the characterization of defects in materials grown by most of the techniques currently in use, the structure of anodic oxides grown on these materials, and defects induced during ion implantation.

Secondary Neutral Mass Spectrometry (SNMS) has been applied to the quantitative compositional analysis as a function of depth of AlxGa1-xAs material systems. The technique described utilizes Ar+ ions from a high frequency, low pressure plasma for sputtering the sample surface. Low primary ion energies (typically 200 eV), coupled with high sample current densities (1-2 mA/cm2) allow rapid, high resolution depth profiling since atomic mixing and "knock-on" effects are minimized. Sputtered neutrals are post-ionized in the plasma by electron bombardment. The well-known matrix effects which limit quantitation in Secondary Ion Mass Spectrometry (SIMS), are negligible in SNMS. The sensitivity factor for Al is found to be independent of sample composition, x.

Thin germanium-rich layers have been formed by the implantation of Ge into Si substrates, followed by a wet oxidation process. We have measured the germanium concentration profile of layers formed by this method for samples initially implanted to a dose of 1 x 10¹⁷cm^-2. Structural, optical, and electronic probes have been used, all of which yield similar results. The results from Ruth-erford backscattering (RBS), Auger spectroscopy (AES), Raman spectroscopy, and electroreflectance (ER) show that the germanium-rich layer is about 300A thick, and contains an average germanium concentration of 65%. The SiGe/Si interface is not sharp but diffuse, with a decreasing germanium concentration. In addition, the data suggest a thin layer of Si₁₃Ge₈₇ near the Si0₂/SiGe interface.

Scanning tunneling microscopy (STM) provides a means of directly imaging surface topography with atomic scale resolution. Scanning tunneling spectroscopy (STS) usually refers to the image-wise determination of the tunnel current variation with bias voltage for fixed tunnel gap width. Results can be related to the density of filled and empty surface electronic states. The association of energetic features with spatially localized features is a unique capability. Other variations of STS are inelastic tunneling imaging and barrier spectroscopic imaging. All these techniques are described and representative examples given to provide a survey of STS.

The In growth on cleaved GaAs(110) surfaces at room temperature (RT) and 80 K low temperature (LT) as well as the initial stage Schottky barrier formation at this interface has been studied using photoelectron spectroscopy. In grows as 3-D islands at RT, but in Stanski-Krastanov mode at LT. The size of the clusters has been estimated through an intensity study. The In core level spectra continuously shifts to high binding energy direction in the transformation from isolated atoms to bulk metal. The Fermi level pinning pattern shows a strong temperature dependence, which challenges current models of Schottky barrier formation at metal/semiconductor interfaces.

The electron spin resonance technique has been used for the identification of defects in silicon-based dielectric thin films deposited by plasma enhanced chemical vapor deposition. Characteristics of the signal due to Si dangling bonds are obtained for oxides, nitrides, and oxynitrides deposited below 500°C. The g-values and linewidths are primarily determined by the composition although defect configurations do not obey random bonding statistics in nitrides and oxynitrides. The defect densities depend essentially on the composition, the growth rate, and the substrate temperature. The good correlation with electrical measurements on MIS structures shows that, in the case of silicon nitride, ESR is a powerful technique for the optimization of deposition conditions for films to be used as low temperature gate insulators.