The GaAs-like longitudinal-optical (LO) phonon frequency in hydrogenated GaAs1−xNx (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 GaAs1−xNx 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.

The GaAs-like longitudinal optical phonon frequency in two hydrogenated GaAs1-xNx/GaAs1-xNx: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.

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.

We report on Raman scattering experiments on InAs/AlxGa1−xAs quantum dot heterostructures with 0≤x≤0.6. The samples were prepared by using molecular beam epitaxy (MBE) and atomic layer MBE for the growth of different layers. For x>0, we detected several lines originating from the AlxGa1−xAs alloy. These can be related to scattering from GaAs-like and AlAs-like phonons with q≅0, and weaker scattering from disorder-activated phonons with q≠0. In particular, we identified a line at ∼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.

The effect of hydrogen irradiation on the optical properties of GaAs1−xNx/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 3 × 1018 to 6 × 1018 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 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.

We present a complete study both by experiments and by model calculations of quantum dot strain engineering, by which a few optical properties of quantum dot nanostructures can be tailored using the strain of quantum dots as a parameter. This approach can be used to redshift beyond 1.31μm and, possibly, towards 1.55μm the room-temperature light emission of InAs quantum dots embedded in InGaAs confining layers grown on GaAs substrates. We show that by controlling simultaneously the lower confining layer thickness and the confining layers’ composition, the energy gap of the quantum dot material and the band discontinuities in the quantum dot nanostructure can be predetermined and then the light emission can be tuned in the spectral region of interest. The availability of two degrees of freedom allows for the control of two parameters, which are the emission energy and the emission efficiency at room temperature. The InAs∕InGaAs structures were grown by the combined use of molecular beam epitaxy and atomic layer molecular beam epitaxy; their properties were studied by photoluminescence and photoreflectance spectroscopies and by atomic force microscopy; in particular, by means of photoreflectance not only the spectral features related to quantum dots were studied but also those of confining and wetting layers. The proposed approach has been used to redshift the room-temperature light emission wavelength up to 1.44μm. The optical results were analyzed by a simple effective-mass model that also offers a rationale for engineering the properties of structures for efficient long-wavelength operation.

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.

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 ω-2θ scan modes (ω and θ being the incidence angles on the sample surface and on the scattering planes, respectively). By relating the GaAs-like longitudinal optical phonon frequency ωLO of InGaAs MBs to the in-plane residual strain ε measured by means of photoreflectance (PR), the linear ε-vs.-ωLO 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.

We show that the residual strain occurring in constant-composition metamorphic buffer layers of III–V heterostructures can be accurately predicted by the suitable design of the epitaxial structures and measured all optically by means of photoreflectance spectroscopy. This result allows one to single out the nonequilibrium models among those that have been proposed to predict strain relaxation. The resulting ∝t−1∕2 dependence of the residual in-plane strain on buffer thickness t can be used to design metamorphic buffers not only for 1.3–1.55μm emitting quantum dot structures, but also for sophisticated graded-composition metamorphic structures for different classes of devices.

Recently, a simple method has been developed to obtain inverted GaAs/AlGaAs quantum dots (QDs) below a quantum well (QW) via multi-step (hierarchical) self-assembly [1]. Here we report on the optical characterization of a series of structures GaAs QDs with different size. The study is performed by means of reflectance (R) down to T=15 K, photo- and thermo-reflectance (T>80 K) and by spectroscopic ellipsometry (at RT). Such measurements allow us to check the thickness and composition of the barrier layers and to complete the study of the electronic states involved in the emission properties of QDs and QW levels. QW states were well identified and their energies and shift with the size parameters well agree with the photoluminescence (PL) data. The identification of QD states seems less straightforward in R measurements and deserves further investigation.

Lattice-matched, single and multiple InGaP/GaAs/InGaP quantum wells (QWs) were grown at 600 degreesC 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. (C) 2003 Elsevier B.V. All rights reserved.

We report on photoreflectance (PR) measurements in the 0.8-1.5 eV photon energy range and at temperatures from 80 to 300 K of InAs self-assembled quantum dots (QDs) grown by Atomic-Layer Molecular Beam Epitaxy. The QDs are embedded in a In0.15Ga0.85As lower confining layer (LCL) with thickness ranging from 20 to 360 nm and in a 20 nm thick upper confining layer with the same composition, that we assume to be pseudomorphic to the LCL. The structures were previously characterized by spectroscopic ellipsometry, photo-luminescence and atomic force microscopy. PR spectra show clear and well-resolved spectral features due to both the QD ground-state transitions and the interband transitions between the topmost split valence bands and the lowest conduction band of LCLs. This allows to self-consistently study the effects on the QD emission energy of parameters such as thickness and composition of partiallyrelaxed LCLs, that determine the QD strain amount and the QD-CL band discontinuities. In this work, these two contributions to the tuning of the QD emission energy are separated by comparing experimental results to that calculated by means of a simple and yet valuable model for ground-state transitions in QDs. It is proved that QD strain (related to the CL-QD lattice-mismatch determined by the thickness-dependent LCL strain-relaxation) can be effectively used to tune the QD emission energy at room-temperature, in particular in the 1.3 μm window.

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 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.

Cd 1−x Mn x Te/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.

In this paper we present photoluminescence, photoreflectance, and absorbance measurements on silicon samples with beta-FeSi2 precipitates, as structurally characterized in the first part of this paper [M. G. Grimaldi , Phys. Rev. B 66, 085319 (2002)]. By comparing the photoluminescence measurements in different experimental conditions and with excitation energy above and below the silicon threshold, by considering the direct gap estimations by photoreflectance and absorption, we argue that the 1.54 mum photoluminescence peak in the spectra is produced by an indirect transition in the disc-shaped precipitates. However, the latter ones are predicted to be the most efficient configuration, acting as a trapping well for carriers generated in the silicon matrix, and displaying a high structural quality with no dangling bonds at the beta-FeSi2/Si interface. Our simple model, based on band lineup at the interface, is also able to explain the temperature quenching of the photoluminescence peak.