Modulation techniques such as electroreflectance, photoreflectance and piezomodulation have become popular room temperature probes of the optical and electronic properties of two dimensional systems such as quantum wells and superlattices. Because of their derivative nature, they allow us to obtain sharp optical features related to interband transitions in the material under study. From this one can obtam transition energies of the microstructure of interest, from which quantum well widths, barrier heights and thicknesses (superlattice case) can be determined. As a direct consequence of the derivative nature of modulation spectroscopy (MS), information can be obtained about the response of the sample to the applied perturbation (used for the modulation), i.e. the electric field or optically induced free carriers. In this paper, we will review the application of MS to the study of optical and electronic properties of microstructures. We will consider not only the experimental implementation of the various techniques, but also discuss the nature of the electromodulation line shapes. From this it will be seen how these techniques can be applied to device structures. Examples from recent work will be presented.

We describe both reflectance anisotropy and electroreflectance measurements carried out to determine the physical origin of the anisotropies observed in the reflectance spectrum of (001) and (110) GaAs. We find an anisotropy component which depends on impurity concentration for both (001) and (110) surfaces [and on conductivity type for (001) GaAs]. This component is actually a bulk-related electro-optic effect produced by the electric field present at the semiconductor surface. This electric field is due to the pinning of the Fermi level at surface states. We find that a linear electro-optic effect is responsible for the impurity-dependent aniso tropies observed in GaAs (001), while a quadratic electro- optic effect is responsible for those observed in GaAs (110). We give an estimate for the linear electro-optic coefficients of GaAs at energies around the E1 and E1 + z transitions.

Electron Beam Electroreflectance (EBER), a low power-density form of cathodoreflectance, has been investigated for use in a wide variety of potential applications. Herein, we describe a phenomenological theory of EBER and provide experimental support for the inherent electroreflectance and thermoreflectance mechanisms. In addition to pair generation and heating proposed earlier, a new mechanism, electron charging of the surface, is identified from the 2f EBER signal of P-type semiconductors. Finally, we compare the potential advantages of EBER to those of photoreflectance.

Using both photoreflectance (PR) at 80 and 300 K and photoluminescence (PL) at 77 K, we have investigated the passivation by H-gun treatment of the surface, interface and volume defects in Ga83Al017As/GaAs/GaAs MBE-grown heterostructure on LEC substrate. Both amplitude of PR and phase delay angle between laser excitation and response were measured. After H-treatment, substantial changes were observed in all properties. Room temperature PR is indicative of the ready disappearance of surface and interface defects at early stages of hydrogenation. Deep trap passivation in the bulk gives rise to increased PL emission in both GaA1As and GaAs, the effect being large only for the latter. A concomitant low in the phase delay angle is detected. The optimum occurs when the total dose of H ions reaching the surface is 1017 cm2. When the largest doses of H are attained, a high density of new bulk defects develops in GaAs, which virtually wipe-out luminescence and reinstate a large phase delay.

Photoreflectance (PR) spectroscopy has been used to study the built-in surface potential in GaAs epitaxial film grown by molecular beam epitaxy (MBE). The PR signal amplitude ILIR/RI sensitively depends on modulation light power, built-in surface potential and temperature. From the analysis of the modulation light power dependence of IzlR/RI, the built-in surface potential of 0.47±0.09eV was determined for a MBE-grown GaAs(lOO) epitaxial film, and the increase of the surface potential by gold deposition was observed.

We report the use of rooni-Lemperature photorefleetance measuremenLs to deLenitine radial arid axial nonuniformity of low levels of indium in 3-inch diameter semi-insulaLing GaAs bulk materials grown by the liquid-encapsulaLed Cochralski method. These resulLs were compared with umc. Types of inhomogeneities are discussed in Lerms of indium segregation and the shape of the solid and liquid interface during crystal growth.

A1GaInP alloy grown by organometallic vapor phase epitaxy (OW/FE) was examined with various optical spectroscopic techniques; photoluminescence, electroref'lectance, Raman scattering, extended X-ray absorption fine structure, and with electron microscopy. It turns out that the band gap shrinkage observed in AlGaInP is caused by deviation of atomic position from the lattice sites of the normal zinc-blende lattice which is effected by the interaction of random distribution structure and long-range ordering. The long-range ordering is found to be specific to the (100) substrate plane. Anomally in low temperature photoluminescence which may be related to long- range ordering is also reported.

We discuss the application of thermorefiectance to two interesting physical systems: ferromagnetic CrBr3 and heavily doped silicon. This technique allows us to identify the physical mechanisms affecting the optical response below the transition phase of CrBr3 . In the case of heavily doped silicon we analyse the many-body effects of high doping on the E1 and E2 edges; the variation of some interesting related parameters is evaluated as a function of the dopant concentration (up to 6x 1021 cm3).

Reflectance difference spectroscopy (RDS) is a surface analysis technique that was invented in 1985 by Aspnes. Here, we give a summary of its application to crystal growth techniques which gave new and valuable real tinw information about the growth process. Also, this information was obtained in-situ, in molecular beam epitaxy (MBE) and organometallic chemical vapor deposition (OMCVD) crystal growing setups which are routinely used to produce high quality device structures. The application of RDS to OMCVD allowed us to develop a "textbook" like model of growth kinetics, which includes two independent microscopic mechanisms, i.e. adsorption (at -26 kcal I mole) of the reacting molecule (trimethylgallium (TMG) in the case of GaAs), followed by its decomposition (at 39 kcal/mole) on the growing GaAs surface. Our model includes an effect called steric hindrance, associated with the large size of the TMG molecule. This study represents the first direct quantitative evaluation of the catalytic effect of the GaAs surface for the decomposition of TMG. We discuss implications of the model both for growth in the ALE mode as well as for conventional OMCVD growth and comment on the relative importance of surface and gas phase reactions. The application of RDS to MBE revealed remarkable details about the complex intermediate steps that surfaces undergo during growth and enabled to extract directly surface dielectric functions. Finally, applications of the technique as well as results obtained in a number of laboratories where RDS is currently being developed are discussed.

Spectral ellipsornetry has been proven to be a very powerful tool to study the band structure and electronic properties of semiconductors. Line-shape analysis of the derivative spectra of the pseudo-dielectric function allows a precise determination of critical point parameters, such as amplitudes, energies, broadening and excitonic phase angles. Here we will review the application of the technique, showing examples of the effects of temperature, alloy and doping in the band structure of semiconductors and the effects of a new periodicity along the growth direction in semiconductor structures.

Using the contactiess modulation spectroscopy technique of photoreflectance, the temperature variations of the direct gap E0 of GaAs, InP, GaA1As, InGaAs have been measured at elevated temperatures up to 600°C. The parameters which describe the temperature dependence of the band gap energies have been evaluated. The ability to measure the band gap at elevated temperatures opens up many new possibilities for in-situ monitoring of MBE and MOCVD processes. In this paper, we review some of the recent developments in the use of photoreflectance at elevated temperatures.

Differential reflectance (DR) spectroscopy, applied to semiconductors, is shown to be equivalent in some cases to a contactless electro-reflectance technique. DR spectra are achieved by modifying one half of the sample surface or, in the case of semiconductor alloys, just relying on the inhomogeneities present. Our DR spectra of GaAs reveal sharp critical point structures and are comparable to the known electro-reflectance data. The DR spectra show a marked improvement in signal to noise ratio over photoreflectance spectra of the same samples. This new technique has also been used to characterize 111-V quantum well structures.

Differential Reflection Spectrometry utilizes two almost identical samples which are mounted stationary, side by side. The samples differe slightly in their surface properties, for example, in composition, crystal structure, etc. Unpolarized light is alternately deflected to one or the other specimen by means of a vibrating mirror. Electronic processing yields the difference in the reflectivities, that is, a first derivative-like spectrum. Examples for applications are given, such as in situ studies of corrosion and passivation in aqueous solutions, investigations of alloys, and studies of the fmer details of implantation damage in semiconducting materials.

Photoreflectance spectra from superlattices and quantum wells exhibit a variety of pecularities which may be used in order to derive electronic structure data, provided experimental lines shapes are properly understood. This problem is addressed in the present paper. The electric field effect on optical constants is expressed in terms of the Franz-Keldysh as well as Stark effect of mini- and subbands. Theoretical line shapes are given and compared with experiment.

The excitonic photoreflectance (PR) spectra of GaAs/A1GaAs multiple quantum wells, grown by the molecular beam epitaxial (MBE) technique, were investigated at oblique and near-normal incident angle with different polarized probe lights. The PR spectra have been measured at room temperature using He-Ne laser as a pumping beam in order to study the variations of the spectral line shapes. The experimental results show that the usefulness of the electromodulation to characterize the microstructure of the substrate may be enhanced if we take in account the polarization state of the probe light which is incident at larger oblique angle. The PR spectra were fitted by a third order derivative functional line shape, thus making it possible to determine the energy band gap, broadening parameters, amplitudes, and the phases of the spectral features precisely.

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.

The dependence of intersubband transitions on temperature and pressure in strained InGa1As/GaAs multiple quantum wells with different x and well widths has been investigated by use of the modulated reflection and absorption spectroscopies. The identifications for dislocation free at the interface will be discussed.

Photoreflectance measurements of InGaAs/InP superlattices grown by solid source molecular beam epitaxy show a well resolved ground state exciton and some of the higher confined transitions in the quantum wells. Ground state splitting possibly due to one monolayer variations in the layer thicknesses is observed. Strong unconfined transitions below the InP band gap are due to arsenic doping of the barriers caused by small amounts of arsenic leakage from the evaporation sources during the epitaxy.

Photoreflectance and photoluminescence experiments are carrIed out in (GaAs)/(AlAs) (n = 1-15) in the temperature range from 25 to 275 K. Weak signals of photoreflectance associated with the critical point of the pseudodirect transition, weakly allowed direct transition arising from the zone-folding effect, have been found as well as main signals associated with the direct allowed transitions. This assignment is supported by the temperature dependence of the photoluminescence intensity, which gives the transition probability ratio of the direct allowed to pseudodirect transition about 102. Temperature dependence of the critical point energies and luminescence peak energies shows a similar behavior to bulk semiconductors. These observations show also that the monolayer number n for which the crossover of direct and pseudodirect transitions occurs depends on temperature. In addition the broadening parameters determined from photoreflectance spectra are found to be almost independent of temperature and depend on the monolayer number n, which may be explained in terms of broadening induced by the layer number fluctuation.

InGaAs/InAlAs quantum wells provide large barrier height for electrons accommodating numerous confined states. Absorption and electroabsorption spectra of samples with 50 - 100 wells are compared. Heavy hole excitons respond strongly to the electric field which for the lowest state is in good agreement with the predicted red shift due to the quantum confined Stark effect. Higher confined excitons show spectra of similar strength and lineshape but not because of a red shift of the transitions but due to redistribution of oscillator strength. Superlattices with thin barriers develop minibands whose spectra are very different from those of uncoupled wells. Features arising from M0 and M1 singularities enable direct determination of the width of the minibands and determination of the conduction band disontinuity. Large electric fields destroy the minibands and quantum confined excitons reappear.

Resonance Raman Scattering is a unique form of modulation spectroscopy that uses the material's phonons rather than externally supplied agents to modulate the electronic structure. We review the applications of this technique to semiconducting materials. Special emphasis is given to research on quantum well and superlattice systems, particularly to phonon localization effects and to analysis of resonant profiles.

Photoreflectance and photoluminescence measurements are carried out to clarify optical properties of (GaAs)/(AlAs) (n = 1-15) short-period superlattices, placing main interest in the crossover of direct-indirect transition and zone-folded weak transition. Photoreflectance spectra of (GaAs)/(A1As) with n < 10 exhibit a weak structure below the main (strong) structure. Photoluminescence peaks appear at the photon energies corresponding to the critical points of these weak and strong structures. Energy band calculations are performed by using the empirical tight-binding method. Calculations of momentum matrix elements between the top valence band and lowest three conduction bands at the I' point show an existence of weakly allowed direct transition below the strongly allowed direct transition edge in (GaAs)/(AlAs)with n < 5. These results strongly suggest that the observed weak structures in the photoreflectance arise from the weakly allowed direct transition, indicating that the conduction band reflects the nature of the zone-folding effect (Xi).

We report investigations of layered structures whose modulated reflectance spectra not only have features that suggest the occurrence of spatially indirect transitions but also display anomalous electric field effects. The spatially indirect transitions can occur from either the GaAs conduction or valence band to quantum levels of carriers confined between narrow potential barriers. In addition to the usual band-to-band excitonic transitions, we discuss spatially direct transitions between the conduction band and the two-dimensional hole states that exist in a spacer layer separating heavily-doped GaAs regions from AlGa1_As barriers. Finally, we comment on spectral lineshapes asssociated with built-in electric fields that differ from those predicted using Franz-Keldysh theory. The energies of all transitions are compared with those calculated using potential profiles based upon the growth parameters.

We have employed the transverse electroreflectance technique to characterize multiple quantum well structures. A weak modulating electric field (1-100 V/cm) was applied transversally to the probe light beam, i.e., parallel to the quantum well layers. Photoreflectance and in-plane photoconductivity spectra can also be measured with this configuration. The transition energies measured were closely the same as those obtained from the photoreflectance and Schottky barrier electroreflectance spectra. The method can be applied for relatively highly resistive undoped layers grown on semi-insulating substrates. The modulation mechanism in transverse electroreflectance is not well understood.

Using photoreflectance spectroscopy, fifteen electronic transitions have been measured from a 60 period Si8Ge32 superlattice grown on a Si02Ge0•8 buffer layer on < 100 > Si. The superlattice transitions fit well to a third derivative functional form and most of their energies were determined using a one band envelope-function model, including strain effects. The temperature dependences of the E0 transition in bulk Ge and in the Si8Ge32 superlattice were also fit to a nonlinear functional form.

An electroreflectance study of several short-period Si/Ge superlattices and SiGe alloy layers grown by molecular beam epitaxy has been performed. Contactless Electron Beam Electroreflectance (EBER) measurements taken at 120K provide clear evidence of superlattice transitions. Comparisons between EBER spectra from sequentially stripped samples reveal superlattice transitions accompanied by those related to the substrate material and buffer layers. Results from the EBER measurements are compared with predictions from empirical pseudopotential calculations. The experimental features support the theoretical predictions.

Low field electroabsorption spectra are compared with absorption spectra and are used to determine accurately the band gap. The spectra show for larger fields F numerous Franz-Keldysh oscillations above the absorption edge which shift in perfect agreement with theory. This shift allows determination of built-in fields and the actual field-strength in the sample. Lattice mismatch removes the valence band degeneracy and causes for fractional change of the lattice constant za/a < 0 splitting into a doublet of the electroabsorption spectrum at small fields. The splitting disappears at higher fields and is never observed for La/a > 0. This different behaviour probably results from differences of the band structure in field direction.

We report results of photoreflectance (PR) investigations of GaAs/Alo.3Gao.7As hetero-n-i-p-i crystals with either GaAs-QWs ("type II") in the intrinsic region or GaAs-QWs interspersed in the n-region ("type I"). To understand the complex PR-spectra of these samples we changed several measurement parameters such as ac-pump-intensity, dc-pump-intensity, pump frequency and temperature. Especially the spectra of type II showed a strong temperature dependence. We compare these spectra to spectra calculated with the model of an infinite quantum well in an electric field. Because of the dielectric function being periodic in space the z-dependence of the dielectric function is taken into account.

Silicon S-doped GaAs samples grown by molecular-beam epitaxy with different dopant concentration and cap layer thickness were investigated by photoreflectance spectroscopy. The features observed above the GaAs fundamental energy gap are temptatively attributed to transitions involving continuous valence band states and quantum- confined states at the conduction band. These interband transition energies are in qualitative agreement with the self -consistent ones calculated taking into account the spreading of dopants.

We performed room temperature photo- and electro-modulation measurements on MBE-grown GaAs pnp and pin structures with large layer thicknesses. In these crystals the optical properties are expected to be dominated by the local field-induced changes of the dielectric function rather than by subband-transitions. The dominating effect in the pnp-structure turns out to be the spatial dependence of the varying refractive index, resulting in a characteristic interference pattern. For the case of the pin structure we clearly observe Franz-Keldysh oscillations changing in amplitude and width with the internal field. Theoretical calculations using effective mass theory are in very good quantitative agreement with the experimental results. This agreement can be achieved only by the inclusion of excitonic effects in the calculation of the field-dependent absorption.

The technique of photoreflectance (PR) spectroscopy has been applied to study the two-dimensional electron gas (2DEG) in a modulation-doped GaAlAs/GaAs heterojunction. We investigate the dependence on temperature of the PR results. At room temperature Franz-Keldysh oscillations-like features are observed. The energy spacing of this oscillations decreases with temperature. Theoretical self-consistent calculations have been carried out to determine the potential profile and sub-band energies in this system. The comparison of the values for the electric field estimated from the calculations and those extracted from the experiment suggests that the observed features are due to the electric field in the space charge region.

We report photoreflectance studies ofMOCVD grown doped GaAs at the higher energy transition E1( 2.9 eV). We are especially interested in the variation ofboth the energy position and the broadening parameter F of the E1 transition with doping concentration. Above 1 x 10'8cin3 for Si:GaAs and ' 7 X 1018 for Zn:GaAs, we observe an increasing overlap of B1 and E1 + Li structures. Evaluation of r based on curve fitting of the KramersKronig analysed data shows a nearly linear relation between F and the logarithm of carrier concentration. This observation has potential application in the determination of carrier concentration for heavily doped films.

Laser excitation above the bandgap of semiconductor materials like GaAs creates a high density of electrons and holes. These quasi particles form a quantum mechanical system in which optical nonlinearities arise as a result of many-body effects such as screening of the Coulomb potential, reduction of the bandgap, and filling of the band and of the states.' The refractive index change resulting from such processes may be employed to demonstrate a variety of devices such as nonlinear switches, modulators, and logic gates. These optical nonlinearities may be measured using various techniques such as four-wave mixing, interferometry, and modulation spectroscopy. In the latter technique, an analogy is established to the electroreflectance effect2'3 in which the optical properties of a semiconductor are modulated by the application of a low-frequency electric field. In the experiment reported here, the modulating element is the E-field of the pump beam. At a much higher frequency than in electroreflectance spectroscopy, the pump beam thus produces field-induced reflectance and transmittance changes from which the refractive index change is obtained.

Electric fields alter the optical properties of the various layers in a quantum well laser by the Franz-Keldysh effect or by the quantum confined Stark effect. The resulting features of an electroabsorption spectrum obtained by combination of dc and ac fields are characteristic for confined and unconfined states and allow the determination of transition energies, band gaps of bulk layers, internal fields and homogeneity of the sample. The results of two structures of different barrier height are compared with luminescence and photocurrent data and with model calculations.

We have studied the photoreflectance spectra at 300 K from a number of GaAs/Ga1AlAs heterojunction bipolar transistor (HBT) structures grown by molecular beam epitaxy and metal-organic chemical vapor deposition. From the observed Franz-Keldysh oscillations we have been able to evaluate the built-in dc electric fields, F , in the Ga Al As emitter as well as the n GaAs collector region. In dc 1-x x addition, the Ga1AlAs band gap (and hence Al composition) has been determined. The obtained values of Fd are in good agreement with numerically-computed values for the analyzed HBT structures, thus making it possible to deduce doping levels in these structures. The GaAlAs FKO have been correlated with device performance.

Photoreflectance (PR) was used to determine the surface damage caused by polishing on semi-insulating lnP:Fe substrates. PR measurements were performed between subsequent etching steps. The PR results on substrates, obtained from various vendors and laboratories, indicate that the exciton structure near the fundamental absorption edge transition is very sensitive to surface imperfections and the bulk resistivity of the substrates.

Extensive measurements and theoretical calculations have provided strong evidence for the creation of direct bandgap superlattice structures from indirect bandgap Ge and Si. Much of the experimental evidence for this conclusion is drawn from electroreflectance measurements. Interpretation of this experiment begins with the standard lineshape fitting analysis, but it is complicated by local electric fields and interference effects. We will discuss these effects and how they can be separated from properties intrinsic to the electronic structure.

Optical transitions in the range I .9—4.3eV have been observed in Si:Ge superlattices by spectroscopic ellipsometry (SE) and electron—beam electroreflectance (EBER). Interference is shown to play an important role in both techniques, leading to complications in • interpretation. Strong features may be produced both by multiple reflections from capping layers and from buried growth imperfections.

The electroreflectance lineshapes of 111/V semiconductors are modelled using the intermediate field theory and a multilayer reflection routine to include electric field variation. This introduces additional structure into the lineshape, which has previously been assumed to be from impurity or exciton effects. By using a thermal broadening parameter of F0=O.OO9eV we show that spectrum size as well as shape can be fitted successfully to experimental data so that depletion layer voltages can be measured.