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The GaAs-like longitudinal-optical (LO) phonon frequency in hydrogenated GaAsN (x = 0.01) layers - with different H doses and similar low-energy irradiation conditions - was investigated by micro-Raman measurements in different scattering geometries and compared with those of epitaxial GaAs and as-grown GaAsN reference samples. A relaxation of the GaAs selection rules was observed, to be explained mainly on the basis of the biaxial strain affecting the layers. The evolution of the LO phonon frequency with increasing hydrogen dose was found to heavily depend on light polarization, thus suggesting that a linear relation between strain and the frequency of the GaAs-like LO phonon mode should be applied with some caution. Moreover, photoreflectance measurements in fully passivated samples of identical N concentration show that the blueshift of the GaAs-like LO frequency, characteristic of the hydrogenated structures, is dose-dependent and strictly related to the strain induced by the specific type of the dominant N-H complexes. A comparison of photoreflectance results with the finite element method calculations confirms that this dependence on the H dose is due to the gradual replacement of the N-2H complexes responsible for the electronic passivation of N with N-3H complexes, which are well known to induce an additional and sizeable lattice expansion.

Raman scattering is an effective tool for the investigation of the strain state of crystalline solids. In this brief review, we show how the analysis of the GaAs-like longitudinal optical phonon frequency allowed to map the strain behavior across interfaces in planar heterostructures consisting of GaAsN wires embedded in GaAsN:H matrices. Moreover, we recently showed how the evolution of the longitudinal optical frequency with increasing H dose strongly depends on polarization geometry. In a specific geometry, we observed a relaxation of the GaAs selection rules. We also present new results which demonstrate how laser irradiation intensity–even at low levels–may affect the line shape of the GaAs-like spectral features in GaAsN hydrogenated materials.

Photoluminescence and photoreflectance spectroscopy are used to investigate the electronic properties of InxGa1−xAs1−yNy/GaAs heterostructures treated with low-energy atomic hydrogen. With increasing hydrogen doses, the InxGa1−xAs1−yNy optical gap blue-shifts until it reaches the value of a corresponding N-free reference sample. This effect is ascribed to the formation of N–H bonds, which leads to an electronic passivation of N and, hence, to a reopening of the material band gap. Thermal annealing of the samples restores the band gap the InxGa1−xAs1−yNy had before hydrogenation and gives an activation energy for the dissociation of N–H complexes equal to 2.28 eV.

Photoreflectance measurements have been performed in a number of InAs/GaAs single-quantum wells with nominal thickness L ranging from 0.6 to 2.0 ML. The InAs growth mode was investigated by analyzing the evolution, with increasing coverage, of the optical response associated with the InAs layer. For L⩽1.6 ML, the experimentally derived energies for the optical transition originating in the InAs are consistent with those evaluated in a simple square-well envelope-function scheme. The dependence of the photoreflectance line shape broadening on L is well described up to L=1.4 ML in terms of a disordered InAs/GaAs interface made by interconnected InAs and GaAs islands with a typical size of order 2 nm. For L=1.6 ML, the quantum well spectral features broaden abruptly and vanish for L=2 ML, suggesting the disappearance of the InAs 2D layer in favor of a predominant nucleation of large quantum dots.

We studied the optical properties of high quality GaSb layers, grown by molecular beam epitaxy, in the region of the fundamental gap E0 using thermoreflectance spectroscopy in the temperature range between 80 and 300 K. The experimental line-shapes were analyzed with a functional form model including excitonic effects. Taking advantage of the derivative-like nature of the thermoreflectance spectroscopy, an accurate determination of the temperature dependence of the energy gap E0(T) is obtained, which is well reproduced by the semi-empirical Varshni relation.

An optical study of the basic epitaxial structure for the fabrication of strain-induced sub-two-dimensional quantum systems was performed by comparing photoreflectance measurements with calculations of the energy positions and the relative intensities of the interband transitions originating in the different regions of the sample. It is shown that by taking advantage of the explicit calculation of the overlap integrals associated with the asymmetric well transitions, the complicated optical response of the whole heterostructure can easily be interpreted. This, in turn, enables the development of the optical investigation of the inner GaAs well (the active region of the device) by means of photoreflectance even at room temperature, as an alternative to the most commonly used low-temperature luminescence techniques.

Photoreflectance vs photoluminescence measurements of stress-induced quantum wires, obtained by patterning an InGaAs layer pseudomorphic grown on a GaAs well in wire-shaped stressors, are presented and compared. An excellent agreement was found between the experimental results obtained with the two techniques about the lateral confinement of the carriers and evidence is given at room temperature of quantization in two directions by means of photoreflectance.

We report absorption measurements on Ge nanoparticles in the range 0.6–6 eV. By reducing the dot average radius to about 13 A, one observes a relevant blue-shift of theE1+Δ1 andE2 peaks and a change of the oscillator strength of the two main structures. Simulations using effective-medium theories have been carried out and the embedding matrix contribution to the measured properties has been evaluated. The different behaviour of the absorption peaksE1,E1+Δ1 andE2 under quantum confinement has been related to their specific nature within the whole Brillouin zone.

Photoreflectance spectra of MBE grown GaAsAlGaAs MQWs in an intermediate electric field regime are analyzed by using a lineshape model accounting for both bulk and multilayer contributions to the Δ R R signal. The experimental spectra exhibiting the simultaneous presence of both Franz-Keldysh oscillations and quantum confinement related structures in the same spectral energy range (1.4–1.7 eV) were fitted to a lineshape model consisting of a linear combination of a modified version of the Aspnes asymptotic expression for subsidiary oscillations and first derivative functional forms typical of exciton photoreflectance in quasi-bidimensional systems. By comparing the optical data obtained in different surface field conditions it is shown that the proposed analytical procedure allows reliable information on MQW parameters to be obtained even when the spurious signal due to subsidiary oscillations originating in the buffer or substrate, is much greater than the optical response associated with quantum confinement.

Photoreflectance (PR) at different temperatures and spectroellipsometry (SE) at room temperature were used to study, in a systematic and complementary way, the optical response of a series of strained and relaxed InxGa1−xAs (x

The binding energy of the heavy-hole ground-state exciton in In0.25Ga0.75As1−yNy/GaAs single quantum wells (y=0, 0.011) was experimentally derived by photoreflectance measurements. We measured a binding energy of 6.6 and 8.5 meV for the N-free and the N-containing sample, respectively. The observed increase of the exciton binding energy can be accounted for by an increase of the exciton reduced mass of about 30% upon N introduction into the InxGa1−xAs lattice, consistently with recent experimental results and in agreement with earlier theoretical predictions.

GaAsN/GaAsN:H heterostructures were made by an in-plane selective hydrogen incorporation controlled by H-opaque metallic masks. The strain field and hydrogen distributions in GaAsN micro-sized wires thus obtained have been mapped by an all optical procedure that combines micro-Raman scattering and photoreflectance spectroscopy. The strain field is related to the formation of N-H complexes along the hydrogen diffusion profile with an ensuing expansion of the GaAsN lattice whose patterning generates an anisotropic stress in the sample growth plane. These results highlight a powerful non-invasive tool to simultaneously determine both the H diffusion profile and the related strain field distribution.

Photoreflectance (PR) and optical absorption (OA) have been used to study the energy-gap dependence on temperature in ${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As thin layers, metalorganic vapor-phase epitaxy grown on InP substrates under nearly lattice-matched conditions (\ensuremath{\Vert}\ensuremath{\Delta}a/${\mathit{a}}_{0}$\ensuremath{\Vert}\ensuremath{\le}${10}^{\mathrm{\ensuremath{-}}3}$). Both PR and OA spectra show a split structure in the band-gap energy region. Moreover, the energy-gap values at all temperatures, obtained by Franz-Keldysh-oscillations analysis of PR line shapes, are blueshifted with respect to the values of the perfectly lattice-matched alloy (x=0.472). The results are related to the valence-band splitting at k=0 generated by the biaxial strain in ${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As, due to the small lattice mismatch. We calculated the splitting and shift values using the orbital-strain Hamiltonian and accounting for compositional effects. The values are in agreement with the experimental results giving optical evidence of coherent growth of the alloy film.

The effect of hydrogen irradiation on the optical properties of GaAs1xNx/GaAs heterostructures was investigated using photoreflectance and reflectance techniques. Systematic measurements performed on both as-grown and hydrogenated samples for N-concentrations ranging from 0.0% to 3.5% and for H-implanted doses from 31018 to 61018 ions/cm2 have shown that (a) the H-induced widening of the energy gap is accompanied by a decrease of the refractive index of the H-treated samples with respect to the as-grown ones, resulting in an index mismatch that can be as large as 2% in the subgap spectral region; and (b) the presence of compressive strain in fully passivated GaAsN determines a decrease of the refractive index even below that of GaAs that can be eliminated via moderate thermal annealing. These findings are promising for the development of heterostructures with planar geometry, in which the simultaneous confinement of both carriers and photons, even on a nanometric scale, can be obtained in a single step process.

We report on Raman scattering experiments on InAs/Al(x)Ga(1-x)As quantum dot heterostructures with 0 0, we detected several lines originating from the Al(x)Ga(1-x)As alloy. These can be related to scattering from GaAs-like and AlAs-like phonons with q congruent to 0, and weaker scattering from disorder-activated phonons with q not equal 0. In particular, we identified a line at similar to 250 cm(-1) as due to disorder-activated longitudinal optical phonons in the alloy. This conclusion is different than the attribution of this line to scattering from dots and, consequently, we do not recognize the possibility of deriving any information about the actual composition of the dots from an analysis of this line as proposed by other authors.

We studied oxygen precipitation in silicon upon brief sample annealing at 1100°C by high resolution Fourier transform infrared technique. This technique permits the detection of small agglomerates of disc-shaped precipitates formed in the bulk of annealed silicon wafers. Precipitates are characterized by a peak at 1230 cm -1 in the infrared absorption spectrum. It is further demonstrated that only the disc-shaped precipitates can contribute to the absorption band mentioned. Moreover, the exact wavenumber position of the peak can give additional information about the chemical composition of the oxide constituting the discs.

In this communication we report on experimental results obtained from the combined use of ellipsometry and photoreflectance to investigate the optical response of typical MQW structures. The motivation for this approach is related to the need for a correct analysis of the lineshape of photoreflectance spectra. Ellipsometric data yield two important contributions about the nature and the degree of localization of the spectral structures in the excitonic region: (a) experimental curves instead of model profiles as starting reflectivity functions for derivations, (b) possible values of oscillator strengths of the transitions investigated. The energies of the main spectral features are compared with the quantized electronic levels obtained in the framework of the effective mass approximation in the envelope function scheme and a discussion of data previously reported in the literature is performed with the help of results obtained by the coupled techniques.

Interstitial oxygen profile across an epitaxial silicon and Czochralski silicon interface has been studied using high-spatial-resolution Fourier transform infrared spectroscopy. Systematic transmission measurements performed on a transversal wafer cross section evidenced oxygen contamination of the epilayer. This was due to solid-state outdiffusion from the substrate occurring during epilayer deposition. Oxygen diffusivity values resulting from the experiments suggest a mechanism scarcely influenced by the interface. Oxygen contamination is strictly related to the type of dopant present in the substrate and not to that present in the epilayer. The oxygen contamination of the epilayer (significant in n-type substrate samples) could explain the structural defects often observed in epitaxial layers by different techniques.

We report on a photoluminescence and photoreflectance study of metamorphic InAs/InGaAs quantum dot strain-engineered structures with and without additional InAlAs barriers intended to limit the carrier escape from the embedded quantum dots. From: (1) the substantial correspondence of the activation energies for thermal quenching of photoluminescence and the differences between wetting layer and quantum dot transition energies and (2) the unique capability of photoreflectance of assessing the confined nature of the escape states, we confidently identify the wetting layer states as the final ones of the process of carrier thermal escape from quantum dots, which is responsible for the photoluminescence quenching. Consistently, by studying structures with additional InAlAs barriers, we show that a significant reduction of the photoluminescence quenching can be obtained by the increase of the energy separation between wetting layers and quantum dot states that results from the insertion of enhanced barriers. These results provide useful indications on the light emission quenching in metamorphic quantum dot strain-engineered structures; such indications allow us to obtain light emission at room temperature in the 1.55 microm range and beyond by quantum dot nanostructures grown on GaAs substrates.

Oxygen precipitation in the bulk of silicon wafers was investigated by using micro‐Fourier transform infrared spectroscopy. It was found that even at 1100 °C annealing (in single step) SiO2 precipitates are formed in platelet shape, in the bulk, giving rise to characteristic absorption peak in the infrared spectrum at 1230 cm−1. Complete mapping of wafer cross section demonstrated that these precipitates are not distributed homogeneously but are agglomerated in irregularly shaped clusters and are easily detectable up to distance of about 100 μm from the back surface and from the epi‐substrate interface.

A high quality of epitaxial silicon layers grown on Czochralski single crystal substrates is required in semiconductor technology, since it directly influences device parameters and yield. Unintentional doping of these layers from the substrate is well documented in the literature. However, we found no reports on unintentional contamination of epi-layers due to the oxygen present in the substrate. The use of micro Fourier transform infrared spectroscopy in transversal mode is shown to be a powerful technique for oxygen profiling and precipitation studies at the same time. It is found that oxygen behaviour during epitaxial layer growth, even for lightly B and P doped substrates, is significantly different. Oxygen, that in some cases diffuses into the grown epitaxial layer, tends to form precipitates there. This is an important consequence of outdiffusion since it directly influences the quality of the epitaxial layer.

Spectroscopic ellipsometry (SE) from 0.8 to 5 eV and photoreflectance (PR) from 0.7 to 1.5 eV were used to study the interband optical response at room temperature of epitaxial GaAs1−xSbx layers with 0

Thermoreflectance spectroscopy was used to precisely determine the direct optical gap ${E}_{g}^{\ensuremath{\Gamma}},$ as a function of composition and temperature of a series of ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{Sb}$ layers $(0.0l~xl~0.5)$ epitaxially grown on GaSb. The experimental line shapes were fitted with a critical-point functional form including excitonic effects, to derive the direct gap and broadening parameter values. The relation between ${E}_{g}^{\ensuremath{\Gamma}}$ and x shows a x-dependent bowing, which was compared with previous results and theoretical models, leading to the conclusion that ${E}_{g}^{\ensuremath{\Gamma}}(x)$ curves in ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{Sb}$ alloys have a cubic polynomial form.

We calculate the binding energy of a two-dimensional exciton for a set of states labelled by the quantum numberm associated to the angular momentum in the direction perpendicular to the surface. The Frohlich electron-phonon and hole-phonon interactions are taken into account. The statem=0 is more bound with respect to that obtained by the Wannier exciton theory with the screening given by the static dielectric constant and the reduced mass calculated by the electron and hole polaronic masses. The opposite effect is found for the statem=1,2,... Whenm becomes large, the hydrogenic series is recovered with the screening and reduced mass defined above. Our results are compared with the experimental data concerning exciton luminescence GaAs/Al x Ga1−x As, CuCl/CaF2 and CdTe/Cd1−x Zn x Te quantum wells.

Systematic infrared transmission measurements have been performed on heavily Sb‐doped (up to 2×1018 atoms/cm3) silicon samples. The interstitial oxygen content is obtained analyzing the optical response near the 1107‐cm−1 oxygen absorption band with a new technique based on a curved baseline, accounting for free‐carrier absorption. The results are discussed and compared with those obtained for the same samples by a short baseline technique and by secondary‐ion mass spectroscopy (SIMS) analysis. Regression straight‐line fits, relative to two lots of 16 samples each, show a good linear correlation between infrared (IR) and SIMS data, although the IR estimates are consistently lower than those obtained by SIMS.

A Fourier transform infrared study of Fe‐donor B‐acceptor complexes formed in iron‐implanted B‐doped Czochralski‐grown silicon crystals is performed by monitoring the behavior of boron‐acceptor excitation spectrum lines. The effect of the iron presence on absorption spectra due to B‐acceptor hydrogenlike systems, related to electronic transitions from the ground to excited states associated with the silicon P3/2 valence band, was analyzed in a wide range of fluences. Low‐temperature optical data are reported and discussed comparing optical results with secondary‐ion mass spectroscopy and spreading resistance measurements, supporting the existence of a fluence threshold that controls iron diffusion into the bulk.

Different doses of iron ions were implanted in Czochralski grown single‐crystal silicon samples and subsequently annealed at 1000 °C for 2 h in a dry nitrogen atmosphere. The behavior of the implanted iron and of oxygen already present in the material was monitored. It was found that the existence of the remaining structural disorder after the annealing treatment plays a dominant role in iron and oxygen segregation into the disordered region. This confirms the theory which predicts that the structural disorder and related strain fields are dominant mechanisms for gettering of metallic ions.

This work deals with the strain relaxation mechanism in InGaAs metamorphic buffers (MBs) grown on GaAs substrates and overgrown by InAs quantum dots (QD). The residual strain is measured by using Raman scattering and X-ray diffraction, both in Reciprocal Space Map and in single omega-2 theta scan modes (omega and theta being the incidence angles on the sample surface and on the scattering planes, respectively). By relating the GaAs-like longitudinal optical phonon frequency omega(LO) of InGaAs MBs to the in-plane residual strain epsilon measured by means of photoreflectance (PR), the linear epsilon-vs.-omega(LO) working curve is obtained. The results of Raman and XRD measurements, as well as those obtained by PR, are in a very satisfactory agreement. The respective advantages of the techniques are discussed. The measurements confirm that strain relaxation depends on the thickness t of the buffer layer following a -t(-1/2) power law, that can be explained by an energy-balance model.

The effect of deuterium irradiation on the optical and strain properties of GaAsN/GaAs heterostructures was investigated by photoreflectance and reflectance techniques. The strain occurring in as-grown and deuterated GaAsN layers is monitored and measured by means of photoreflectance spectroscopy, highlighting the strain inversion after irradiation. By combining static and modulated reflectance results, evidence is given that the deuterium-induced recovery of the GaAs band gap as well as the strain inversion in GaAsN layers are accompanied by a 0.4%-0.8% reduction of the refractive index in the 1.31 and 1.55 mu m spectral windows of interest for fiber optic communications. These results anticipate a single step process to an in-plane confinement of carriers and photons. (c) 2007 American Institute of Physics.

We report a photoreflectance study conducted in the 0.7–1.2 eV photon energy range and at temperatures from 80 to 300 K of GaSb/Al0.4Ga0.6Sb single quantum wells grown by molecular beam epitaxy. We observed clear and well-resolved structures, which could be attributed to the interband optical transitions originating in both the GaSb buffer and the quantum wells, and which could be fitted by standard critical-point line shapes. Our results demonstrate that even unintentionally doped GaSb-based quantum systems can be studied and characterized by photoreflectance, especially at low temperatures.

The GaAs-like longitudinal optical phonon frequency in two hydrogenated GaAs1-N-x(x)/GaAs1-N-x(x):H microwire heterostructures-with similar N concentration, but different H dose and implantation conditions-has been investigated by micro-Raman mapping. In the case of GaAs0.991N0.009 wires embedded in barriers where GaAs-like properties are recovered through H irradiation, the phonon frequency in the barriers undergoes a blue shift with respect to the wires. In GaAs0.992N0.008 wires embedded in less hydrogenated barriers, the phonon frequency exhibits an opposite behavior (red shift). Strain, disorder, phonon localization effects induced by H-irradiation on the GaAs-like phonon frequency are discussed and related to different types of N-H complexes formed in the hydrogenated barriers. It is shown that the red (blue) character of the frequency shift is related to the dominant N-2H (N-3H) type of complexes. Moreover, for specific experimental conditions, an all-optical determination of the uniaxial strain field is obtained. This may improve the design of recently presented devices that exploit the correlation between uniaxial stress and the degree of polarization of photoluminescence. (C) 2014 AIP Publishing LLC.

Lattice-matched, single and multiple InGaP/GaAs/InGaP quantum wells (QWs) were grown at 600 °C by low-pressure metalorganic vapour phase epitaxy (LP-MOVPE), with the use of the tertiarybuthylarsine (TBAs) and tertiarybuthylphosphine (TBP) group-V sources. In order to enhance the interface abruptness, different gas switching sequences were exploited during the growth of the interface, and the best results were obtained by inserting a few monolayer-thick GaAsP interlayers (IL), at the direct GaAs-on-InGaP interface. Low-temperature photoluminescence (PL), high resolution X-ray diffraction, transmission electron microscopy and photoreflectance spectroscopy analysis were performed on the grown heterostructures, to correlate the adopted growth sequence with the interface properties and the QW optical transitions. Promising results were obtained, among which: (a) the suppression of the anomalous PL emission at low energy, (b) optical emission from the InGaP/GaAs/InGaP QWs, exhibiting a good correlation with theoretical expectations, (c) direct interface fluctuations within 1 nm.

We report on photoreflectance measurements performed in the 0.8–1.6 eV photon energy range in as grown and hydrogenated InxGa1-xAs1-yNy/GaAs single quantum wells grown on GaAs substrates by molecular beam epitaxy. In the hydrogenated samples, a blue-shift of all the QW spectral features and a surprising change with temperature in the nature of the lowest energy transition are found. These features are related to the interaction of H with N atoms. An increase in the binding energy of the heavy-hole exciton upon N introduction into the InxGa1-xAs lattice has been measured also and explained in terms of an increase in the electron effective mass.

The Al-free InGaP/GaAs heterostructure is an interesting alternative to AlGaAs/GaAs system in a wide range of micro- and optoelectronic applications [1-3]. Nevertheless, the required ML-abruptness of both normal and inverse InGaP-GaAs interfaces is not obtainable, due to both the As/P intermixing and the In memory effect during the MOVPE growth process in which a standard gas switch sequence (GSS) is adopted [4]. As a consequence, unintentional InGaAsP intermediate layers are formed at the interfaces, so that the optical properties of the InGaP/GaAs quantum wells (QW's) are affected by unexpected, low-energy photoluminescence (PL) contributions [5], whereas either the hindrance of the QW emission or its line-shape width increasing, are expected. In this work different lattice-matched InGaP/GaAs single and multi QW structures were grown on exact, S.I. (001)GaAs substrates at 600°C by low-pressure MOVPE, with the use of the TMGa, TMIn, TBAs and TBP precursors; GaAs quantum well thickness varied in the range 3-8 nm. In order to enhance the direct interface abruptness, different GSS's were exploited for the interface growing, and few ML-thick GaAsP interlayers (IL), were inserted at the GaAs-on-InGaP interface [6]. Low-temperature photoluminescence (PL), High resolution X Ray Diffraction (HRXRD), Transmission Electron Microscopy (TEM) analysis and Photo Reflectance Spectroscopy (PR) measurements were performed on the grown heterostructures for correlating the interface properties and the width of the QW emission with the parameters of the growth method adopted. A model to predict the expected PL energy emission from the GaAs QW's, referred to an ideal square QW's with finite potential barriers, was employed to compare the theoretical and experimental results.

The goal of this paper is to investigate the morphology and the structure of ion-beam synthesized β-FeSi 2 precipitates and the defects in the Si matrix by using transmission electron microscopy (TEM) and, in a few cases,conversion electron Mossbauer spectroscopy or Rutherford backscattering spectrometry. We shall point out how the different process parameters affect the resulting structure and the optimum process window for light emitting application of β-FeSi 2 . In particular, we will also show that within the optimum process window at least two different types of precipitates are obtained in separate regions: small ball-shaped precipitates in the surface region and large disc-shaped precipitates deeper in the sample. The latter are shown to display a very good interface to the Si matrix, as obtained by TEM analysis and dedicated molecular dynamics simulation. In the second part of this paper [Martinelli et al., Phys. Rev. B 66, 085320 (2002)], the nature and the origin of the 1.54-μm photoluminescence signal obtained from our best samples will be analyzed in detail.

Cd1-xMnxTe/CdTe superlattices were grown by molecular beam epitaxy with a CdTe buffer layer on GaAs(001) substrate. Photoreflectance spectra were performed on Cd1-xMnxTe/CdTe superlattices with high compositions of x=0.4, 0.8 at room temperature and liquid nitrogen temperature. The exciton transitions related to the heavy (H) and light (L) holes of 11H, 11L, 22H, and 33H are observed. After taking into account the strain-induced and quantum confinement effects, the theoretical calculations are in good agreement with the photoreflectance measurement results except x=0.8 of 33H. Photoluminescence measurements were also performed at room temperature and low temperature in order to compare with our photoreflectance results. Our results show that the photoreflectance spectroscopy technique is a powerful probe for the study of quantized state structures in superlattices systems. (C) 2002 American Institute of Physics

The energies of bipolaron states in three- and two-dimensional systems are calculated variationally, treating the electron-phonon interaction in the Fr\"ohlich approximation, and separating the relative motion from the center-of-mass motion. The bipolaron is bound if the electron-phonon coupling constant \ensuremath{\alpha} is larger than 6 in three dimensions and 2 in two dimensions, provided the ratio \ensuremath{\eta}=${\mathrm{\ensuremath{\epsilon}}}_{\mathrm{\ensuremath{\infty}}}$/${\mathrm{\ensuremath{\epsilon}}}_{0}$ is smaller than a critical value which depends on \ensuremath{\alpha}. The theory yields the free-polaron energies as given in the Lee, Low, and Pines approximation for arbitrary values of the electron-phonon coupling constant \ensuremath{\alpha}. For \ensuremath{\alpha} larger than \ensuremath{\sim}10 and \ensuremath{\sim}4 in the three- and two-dimensional cases, respectively, the lowest free-polaron self-energies are obtained in the strong-coupling approach; it is shown that a trial harmonic envelope wave function, localized both in the center-of-mass and relative coordinates, with a coherent phonon state gives a bound localized bipolaron state of lower energy. It is interesting for application to high-${\mathit{T}}_{\mathit{c}}$ superconductivity that the bipolaron bounds more easily in two dimensions than in three, and that the mean value of the pair radius is a few angstroms. Furthermore, bipolaron states obtained when the linear total momentum is conserved have intrinsically high mobility, which is also an important condition to make the bipolaron mechanism consistent with high-temperature superconductivity.

The electronic properties of ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{As}}_{1\ensuremath{-}y}{\mathrm{N}}_{y}/\mathrm{GaAs}$ single quantum wells have been investigated by photoluminescence and photoreflectance spectroscopy as a function of temperature. The introduction of nitrogen leads to a sizable slow down in the redshift of the ground state recombination energy with temperature. We explain the observed effects in terms of an anticrossing between states of the conduction band (CB) edge and a N-induced localized level resonant with the CB. The extent of this anticrossing, described by the matrix element ${V}_{\mathrm{MN}},$ is derived from the temperature dependence of the exciton recombination energy in a wide compositional range. The measured functional dependence of ${V}_{\mathrm{MN}}$ on nitrogen concentration is compared with results reported in the literature.

Oxygen redistribution near the interface between the epitaxial layer and substrate was monitored with micro‐Fourier transform infrared measurements in a transversal wafer cross‐section configuration. It has been shown that oxygen contamination of the epitaxial layer may induce the formation of SiO2 precipitates into the film, due to outdiffusion from the substrate. To our knowledge this is the first direct evidence of oxygen precipitation phenomena within the epitaxial layer.

The oxygen content near epitaxial layer‐substrate silicon interface was investigated using a micro Fourier transform infrared technique. Systematic measurements, performed in a transversal wafer cross‐section configuration, clearly indicated the presence of interstitial oxygen in the epitaxial layer when it was grown on n‐type substrates, while no evidence of oxygen in it was obtained (in concentration detectable by the infrared technique) when grown on p‐type substrates. Interstitial oxygen profiles, near the epitaxial layer‐substrate interface, obtained by analyzing the optical data, are reported and discussed considering different combinations of dopants in the substrate and in the epilayer.

Spectroscopic ellipsometry from 1.4 to 5 eV was used to systematically characterize epitaxial heterostructures AlxGa1-xSb/GaSb for different x concentrations (x≤0.5). The structures were grown by MBE at temperatures and beam equivalent pressure ratios which optimize their lowtemperature photoluminescence properties. Complex dielectric functions of AlxGa1-xSb versus x were derived, for the first time, from the ellipsometric spectra after mathematically removing the oxide overlayer effects. The spectra were analyzed with their second energyderivatives in term of standard analytical lineshapes: in particular the El, E1+Δ1 and E2 critical point energies, broadening and amplitude parameters were derived as a function of x. On this basis we verified that the energy-shift model is appropriate to interpolate for any x≤0.5, thus allowing a nondestructive optical diagnostic of layer thickness and composition of epitaxial heterostructures based on A1xGa1-xSb.

We measured photoreflectance (PR) spectra at different temperatures between 80 and 300 K, and optical absorption (OA) at 3 K on MOVPE grown Inl-xGaxAs nearly lattice-matched to InP. x-ray diffraction measurements gave a lattice mismatch δa/ao = -0.9.10−3 between ternary alloy and InP, corresponding to × = 0.485. We obtained the energy gap dependence on T from PR spectra. The blue shift of the gap was accounted for in terms of compositional difference with respect to the perfectly lattice matched alloy (× = 0.472), and elastic strain; moreover PR and OA showed evidence of the valence bands splitting at k = 0 due to interfacial strain, in fine agreement with theory.

The interstitial oxygen profile across the epitaxial silicon and Czochralski silicon interface has been determined using a micro Fourier transform infrared technique. Systematic measurements performed in transversal wafer cross-section configuration demonstrate the presence of interstitial oxygen in the epitaxial layer, clearly indicating that solid state outdiffusion from the substrate occurs during film preparation. Moreover, it is shown that oxygen contamination may produce precipitation phenomena inside the film.

The interstitial oxygen profile across the epitaxial silicon and Czochralski silicon interface has been studied using a micro Fourier transform infrared technique. Systematic transmission measurements performed in transversal wafer cross-section configuration demonstrated the presence of interstitial oxygen in the epitaxial layer, clearly indicating that solid state outdiffusion from the substrate occurs during film preparation. Furthermore the major consequence of outdiffusion, e.g. oxygen contamination of the epitaxial layer has been shown to produce precipitation phenomena inside the film, directly influencing its quality.