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The infrared (IR) reflectivity of the cubic metal oxides MgO, MnO, and NiO has been measured at room temperature using the technique of oblique incidence. The use of this technique at three angles of incidence provides multiple sets of spectra (including both s- and p-polarizations) to analyze when compared to the standard normal incidence case, which has been used extensively in the past for these compounds. It is shown that the transverse optic (TO) and longitudinal optic (LO) mode phonon parameters can be determined with greater accuracy by using the factorized model for the fits, as compared with the popular classical model used previously, and by fitting the derivative of the reflectivity. Our results for the phonon mode parameters are similar to those found earlier, as could be expected, but are generally more precise. An analysis of the difference in frequency of the TO mode in antiferromagnetic NiO at room temperature for the two polarizations of reflected light revealed a TO mode splitting of about 4 cm−1, with the p-polarized light TO mode having the higher frequency: This small splitting is in agreement with theoretical predictions. This more precise IR method for revealing the phonon mode behavior in such magnetically ordered cubic metal oxides, which are prototypes of strongly correlated electronic systems and have been found to be Mott-Hubbard insulators, may be readily applied to any similar antiferromagnetic system.

Following up on some recent work that has been presented (Orapunt et al., J Appl Phys 119:065702-1-12, 2016), we report on the optical properties associated with a unique form of thin-film silicon that has been deposited onto optical quality fused quartz substrates through ultra-high-vacuum evaporation. For the purposes of this particular analysis, we focus on how the growth temperature influences the spectral dependence of the optical functions associated with these thin silicon films, for growth temperatures ranging from 98 to 572 °C. Through measurements of the specular reflectance spectrum at near normal incidence and the regular transmittance spectrum at normal incidence, we determine the spectral dependence of the refractive index, the extinction coefficient, the real and imaginary parts of the dielectric function, and the optical absorption coefficient for the 11 thin silicon films considered in this analysis. We find that generally the refractive index increases in response to increases in the growth temperature. The optical absorption spectral dependence is also observed to exhibit a fundamental transition in its functional behavior accompanying increases in the growth temperature. Some details, related to recently developed methods employed for the determination of the optical functions from measurements of the reflectance and transmittance spectra, are provided as a complement to this analysis.

The determination of the spectral dependence of thin-film optical functions from experimental measurements of the optical response, performed on thin-film/substrate structures, represents a problem of general interest to the thin-film community. In this paper, we present details related to thin-film optical function acquisition from experimental measurements of the reflectance and transmittance spectra, these being worked out through a case study performed on the optical functions associated with a series of thin-films of silicon that had been grown through ultra-high vacuum evaporation deposition on optical-quality-fused quartz substrates, this study being performed by Moghaddam et al. (J Mater Sci: Mater Electron 31:13186–13198, 2020). Following the presentation of our approach to deposition, details related to our optical response measurements, the uncertainty in our reflectance and transmittance results, the analytical framework within which the optical functions were acquired, how the ambient and the optical-quality-fused quartz substrate optical functions were modeled, and means through which unphysical solutions were identified and eliminated, are presented. We believe that the collection of these results will be of interest to those within the broader thin-film community.

An overall comparative study is made of the spin-phonon interactions in several rutile-structure transition-metal difluorides, specifically FeF₂, MnF₂, NiF₂, and CoF₂, in terms of recent developments obtained experimentally using inelastic light scattering spectroscopy and theoretically using a modified mean-field approach to estimate spin-pair correlation functions. New experimental data are presented here and interpreted within an extended and comprehensive theoretical treatment to yield estimates for the spin-phonon coupling coefficients and the relative magnitudes of the magneto-optical coupling coefficients. С использованием последних результатов экспериментально с применением спектроскопии неупругого рассеяния света и теоретически методом модифицированного среднеполевого подхода для оценки корреляционных функций спиновой пары проведено полное сравнительное исследование спин-фононных взаимодействий в нескольких дифлуоридах переходных металлов рутиловой структуры, в частности FeF₂, MnF₂, NiF₂ и CoF₂. С целью получения оценок коэффициентов спин-фононной связи и относительных величин коэффициентов магнитооптической связи представлены новые экспериментальные данные, которые интерпретируются в рамках всестороннего расширенного теоретического описания З використанням останніх результатів експериментально зі застосуванням спектроскопії непружного розсіяння світла та теоретично методом модифікованого середньопольового підходу для оцінки кореляційних функцій спінової пари проведено повне порівняльне дослідження спін-фононних взаємодій в декількох діфлуоридах перехідних металів рутилової структури, зокрема FeF₂, MnF₂, NiF₂ та CoF₂. З метою отримання оцінок коефіцієнтів спін-фононного зв’язку та відносних величин коефіцієнтів магнітооптичного зв’язку представлено нові експериментальні данні, які інтерпретуються в рамках всебічного розширеного теоретичного опису.

In this article we review in detail the magnetic properties of the simple cubic oxides NiO, MnO, and CoO, and also discuss the properties of the more complicated, but related, transition-metal oxides CuO and FeO. The magnetic ordering scheme and the several competing magnetic interactions strongly affect the lattice structure (giving rise to distortions) and hence the vibrational properties. The various interactions of the magnetic moments plus the spin-phonon coupling, which heavily influence the physical properties of these metal monoxides, are discussed thoroughly.

Using ultra-high-vacuum evaporation, thin-films of silicon were grown. This paper starts with a placement of our thin-films into the overall thin-film silicon continuum. Then, our thin-film’s Raman spectra are examined. In particular, two short-range order Raman related measures, namely the breadth and position of the Raman spectrum’s transverse-optic-peak, and an intermediate-range order Raman related measure, namely the ratio of the integrated transverse-acoustic and transverse-optic-peak intensities, are plotted as functions of the growth temperature. We then correlate these changes found in the nature of the order found within our thin-films of silicon with the changes observed in the spectral dependence of the refractive index, focusing on the maximum of the refractive index over the spectral range under consideration in this analysis. We find that the maximum of the refractive index increases in response to increased ordering. Finally, the potential device implications of these results are explored.

A number of thin silicon films are prepared through ultra-high-vacuum evaporation on optical quality fused quartz substrates with different growth temperatures. Through an analysis of grazing incidence X-ray diffraction results, a phase transition, from amorphous-to-crystalline, is found corresponding to increases in the growth temperature. The corresponding Raman spectra are also observed to change their form as the films go through this phase transition. Using a Raman peak decomposition process, this phase transition is then quantitatively characterized through the determination of the amount of intermediate-range order and the crystalline volume fraction for the various growth temperatures considered in this analysis. The possible device consequences of these results are then commented upon.

Light emission from Si nanostructures has been of great interest for some time now owing to the need for silicon-based light sources for applications in silicon photonics. Both Si and Ge possess indirect band gaps, which makes them very inefficient light emitters. Band gap engineering has been proposed as one way to overcome this limitation and although light emission with greatly improved efficiency has been obtained at low temperatures the emission at room temperature is still very weak, because of exciton dissociation. Recently, through employing novel band gap engineering computations, entirely new Si/Ge [M. d’Avezac, J.-W. Luo, T. Chanier, and A. Zunger, Phys. Rev. Lett. 108, 027401 (2012)] and Si [H. J. Xiang, B. Huang, E. Kan, S.-H. Wei, and X. G. Gong, Phys. Rev. Lett. 110, 118702 (2013)] supercell structures possessing direct gaps have been proposed. According to d’Avezac et al., a SiGe2Si2Ge2SiGen superstructure should have a direct and dipole-allowed gap of 0.863 eV, which is ideally suited for optical fiber data transmission applications. Here we report on the growth of such a structure and its optical properties. Two similar samples were prepared by different growth methods: molecular beam epitaxy (MBE) and solid phase epitaxy (SPE). In both samples the superstructure (terminated with n = 12 monolayers of Ge) was grown on a 30-nm thick buffer layer of Si0.4Ge0.6 on a 5 µm thick relaxed buffer layer of Si0.4Ge0.6on a 750 µm thick (001) Si substrate. The photoluminescence (PL) spectra were measured using a Bomem DA3 FTIR spectrometer, with the samples excited at low temperatures with either 70 mW of 405 nm or 35 mW of 458 nm laser light. Photoluminescence (PL) spectra obtained at 6 K with excitation at 405 nm are shown in Fig. 1. Similar spectra were obtained with 458 nm excitation. No PL was detected from the two samples in the energy range 1000-1850 meV or at room temperature. The sharp drop at low energy near 700 meV is due to the cut-off in the instrumental response. A strong low-energy PL doublet is seen, with peaks near 780 and 820 meV, together with a much weaker peak at 872 meV. The ratio of intensities of the strong and weak peaks is the same in both samples. The intensities of all three PL peaks decrease with increasing temperature up to 25 K, but the weak peak decreases in intensity faster than that of the strong peaks. The weak peak at 872 meV is most likely the dipole-allowed direct-gap transition expected at 0.863 eV in the superstructure. The small difference in energy between theory and experiment could be the result of a difference in strain within the layer in the sample compared with the ideal (perfect) modeled structure or from assumptions in parameter values in the model. The strong peaks at 820 and 780 meV are assigned to the no-phonon and transverse-optic-phonon emission lines, respectively, of the Si0.4Ge0.6 buffer layer. The ~40 meV separation between the two strong peaks is characteristic of the phonon energies in SiGe alloys. The energies of the peaks, however, are much lower than that expected for a bulk Si0.4Ge0.6 alloy (~0.97 eV). The energies and general appearance of these peaks is reminiscent of what has been obtained from PL studies of SiGe nanostructures imbedded in Si. It is therefore likely that this PL arises predominately at the Si0.4Ge0.6/superstructure interface where there is type-II band alignment. In conclusion, we have obtained experimental evidence of the predicted direct-gap optically-allowed transition in a special supercell comprised of a number of ultrathin layers of Si and Ge.

Experimental data are reported for the temperature and polarization dependence of the oneand two-magnon Raman light scattering in the rutile-structure antiferromagnet CoF2. The correspondingn Stokes and anti-Stokes Raman spectra are measured, emphasizing the temperature and polarization variations of the one-magnon frequency, line width, and integrated intensity. A Green's function method is employed to derive the excitation energies and their intensities over a broad range of temperatures (below the Néel temperature = 38 K) for a spin S = 3/2 anisotropic antiferromagnet with strong spin-orbit coupling. The exchange terms are analyzed using the random-phase approximation while single-ion anisotropy terms are treated exactly by generating a closed set of coupled Green's function equations. Good overall agreement between theory and experiment is obtained. © 2013 The Korean Physical Society.

HgTe plates have been grown by vapor phase epitaxy on (111) SrTiO3 substrates with a preferred orientation in the (111) crystalline direction, as indicated by x-ray diffraction. Examination of the plates using the micro-Raman mapping shows that the HgTe plates exhibit unusual strain patterns: the Raman peaks from the transverse-optical and longitudinal-optical phonons for the thicker (central) parts of the HgTe plates are at the same frequency as that of the bulk HgTe, while the Raman peaks for the thinner parts of the HgTe plates, which surround the thicker parts and can hardly be seen in a scanning electron microscope, are significantly larger in frequency. The full width at half maximum is smaller in the thinner areas than in the thicker parts. Theoretical analysis shows that the HgTe plates on SrTiO3 substrates suffer from compressive stress, and this may be sufficient to induce the three-dimensional topological insulator behavior in HgTe.

The development of a light emitter compatible with Si based complementary metal-oxide- semiconductor (CMOS) circuit technology and fast optical interconnects is important for the new generations of microprocessors and computers. Self-assembled Si/Si1-xGex nanostructures (NSs) with light emission in the important optical communication wavelength range of 1.3 – 1.55 μm are compatible with conventional CMOS processes. However, the predicted and experimentally confirmed long carrier radiative lifetimes in Si and Si/Si1-xGex NSs impede the demonstration of efficient light-emitting devices and lasers. Thus, engineering of Si/Si1-xGex heterostructures with controlled composition and interface abruptness is critical in producing the desired fast and efficient photoluminescence (PL) peaked at around 0.8-0.9 eV. In this paper we assess how the nature of the interfaces between SiGe NSs and Si in heterostructures strongly affects carrier mobility and recombination for physical confinement in one dimension (corresponding to the case of quantum wells), two dimensions (quantum wires), and three dimensions (quantum dots). The interface sharpness is influenced by many factors such as growth conditions, strain, and thermal processing, which in practice can make it difficult to attain the ideal structures required. This is certainly the case for NS confinement in one dimension. However, we demonstrate that axial Si/Ge nanowire (NW) heterojunctions (HJs) with a Si/Ge NW diameter in the range 50 – 120 nm produce a clear PL signal associated with band-to-band electron-hole recombination at the NW HJ that is attributed to a specific interfacial SiGe alloy composition. For three-dimensional confinement, the experiments outlined here show that two quite different Si1-xGex NSs incorporated into a Si0.6Ge0.4 wavy structure exhibit an intense PL signal with a characteristic decay time as much as 1000 times shorter than that observed in conventional Si/SiGe NSs. The experimentally observed non-exponential PL decay in Si/SiGe NSs is explained as being due to variations of the distances separating electrons and holes at the Si/SiGe heterointerface. The results demonstrate that an abrupt Si/SiGe heterointerface reduces the carrier radiative recombination lifetime and increases the PL quantum efficiency making these SiGe NSs promising candidates for applications in CMOS compatible light-emitting devices.

We present a review of recent studies of inelastic light scattering spectroscopy in two types of Si/SiGe nanostructures: planar superlattices and cluster (dot) multilayers including first- and second-order Raman scattering, polarized Raman scattering and low-frequency inelastic light scattering associated with folded acoustic phonons. The results are used in semi-quantitative analysis of chemical composition, strain and thermal conductivity in these technologically important materials for electronic and optoelectronic devices.

For Ge nanodots ∼20 nm in diameter grown by annealing a thin amorphous Ge layer deposited by molecular beam epitaxy on a mesoporous TiO2 layer on Si(001), photoluminescence (PL) was observed as a wide near-infrared band near 800 meV. Using a tight binding theoretical model, the energy-dependent PL spectrum was transformed into a dependence on dot size. The average dot size determined the peak energy of the PL band and its shape depended on the size distribution, including bandgap enlargement due to quantum confinement. Combining the dot sample PL with an established dependence of emission efficiency on dot diameter, it was possible to derive a dot size distribution and compare it with results obtained independently from atomic force microscopy.

Ge nanocrystals have been obtained from the dewetting process during thermal annealing of anamorphous Ge layer deposited by molecular beam epitaxy on a thin SiO 2 layer on Si(001). The Genanocrystals were then capped with a thin layer of amorphous Si. The mean nanocrystal size – 2.5–60nm – depends on the initial Ge layer thickness. Low-temperature photoluminescence (PL) measurementswere performed to investigate quantum confinement effects on the Ge nanocrystal energy gap and defectstates. For the present range of particle sizes, the nanoparticle PL emission appeared as a wide near-infraredband near 900 meV although a weak confined band was also observed for the smallest nanoparticles. Furtherthermal annealing of the samples increased the interband recombination by nearly two orders of magnitude.Crown Copyright c 2008 Published by Elsevier Ltd. All rights reserved. Keywords: Photoluminescence; Nanocrystals; Germanium; Silicon dioxide; Silicon 1. IntroductionAlthough achieving efficient light emission from group IV semiconducting materials, asubject of intense research activity, has proven elusive, carrier localization methods have led tosignificantly enhanced optical emission from indirect gap materials and at higher temperatures.For example, room-temperature visible photoluminescence (PL) has been observed from porousSi [1] and strong infrared PL from various island or quantum dot systems [2–9]. A particular

Infrared vibrational spectroscopy in an attenuated total reflection (ATR) geometry has been employed to investigate the presence of organic thin layers on Si-wafer surfaces. The phenomena have been simulated to show there can be a field enhancement with the presented single-reflection ATR (SR-ATR) approach which is substantially larger than for conventional ATR or specular reflection. In SR-ATR, a discontinuity of the field normal to the film contributes a field enhancement in the lower index thin film causing a two order of magnitude increase in sensitivity. SR-ATR was employed to characterize a single monolayer of undecylenic acid self-assembled on Si(1 1 1) and to investigate a two monolayer system obtained by adding a monolayer of bovine serum albumin protein.

Photoluminescence (PL) measurements were performed in Si/Si1−xGex nanostructures with a single Si0.92Ge0.08 nanometer-thick layer incorporated into Si/Si0.6Ge0.4 cluster multilayers. Under pulsed laser excitation, the PL decay associated with the Si0.92Ge0.08 nano-layer is found to be nearly a 1000 times faster compared to that in Si/Si0.6Ge0.4 cluster multilayers. A model considering Si/SiGe hetero-interface composition and explaining the fast and slow time-dependent recombination rates is proposed.

We review recent advances in the growth of Si1−xGex islands and Ge dots on (001) Si. We first discuss the evolution of the island morphology with Si1−xGex coverage and the effect of growth parameters or post-growth annealing on the shape of the islands and dots. We outline some of the structural and optical properties of Si1−xGex islands and assess progress in the determination of their composition and strain distribution. Finally, we discuss various approaches currently being investigated to engineer Si1−xGex quantum dots and in particular to control their size, density, and spatial distribution. For example, we show how C pre-deposition on Si (001) can influence the nucleation and growth of Ge islands.

The transverse (TO) and longitudinal (LO) optical phonons in AlAs, GaP, GaAs, InP, InAs and InSb have been measured at room temperature by infrared spectroscopy using an oblique incidence reflectance method. The spectra obtained were then fitted using a novel approach to determine the TO and LO phonon frequencies and damping. The results obtained are found to be more precise than in earlier reflectivity measurements using near-normal angles of incidence and provide information on the damping of both phonons. Apart from the GaAs LO mode, the observed damping parameters are found to be quite different from those predicted by theory. From these results the Lowndes condition governing the relative magnitudes of the TO and LO phonon line widths is found to be violated for all these zincblende semiconductors.

The structural phase transitions occurring at the first heating of as-grown Na2SO4(V) are so peculiar that there have been many controversial reports about them, especially with regard to the possible existence of several intermediate phases (II–IV) between the room-temperature (V) and high-temperature (I) phases. From this first detailed study of polarized Raman spectra based on oriented single crystals, we have obtained new results regarding the existence and nature of intermediate phases between phases V and I. We normally found that single-crystal Na2SO4(V) transforms directly to phase I at 242 °C without any occurrence of an intermediate phase (type-1 transition), but occasionally it transforms to a mixed phase of II and III near 234 °C, and then the phase II regions transform to phase I near 240 °C, and finally the phase III regions transform to phase I near 250 °C (type-2 transition). These results were confirmed from single-crystal ionic conductivity measurements and thermal calorimetry studies. For polycrystalline powder, however, both of these two types of transition sequences occur simultaneously, resulting in a complex and sluggish behaviour with temperature change. Hence it is now evident for powdered samples why many authors have hitherto reported so many diverse and contradictory results regarding the structural transitions of phase V.

The present work deals with anodization processes of p-type InP(100) in different halogenic acids. The morphology of the attack depends strongly on the electrochemical conditions and the type of anion present in the electrolyte. Only electropolishing was observed in HCl while the formation of a porous oxide layer was obtained in HF. In HBr, however, pore growth into the bulk material can be achieved.

The present work deals with anodization processes of n-type InSb(100). Preferential etching of InSb can be electrochemically initiated in HCl, HBr and HF solutions. Except for etch features also the formation of porous layers can be observed. The resulting features were characterized by SEM and AES measurements. Due to the narrow bandgap of the material the results of the anodization process are neither sensitive to illumination of the n-type material nor to the doping level. The morphology of the attack depends strongly on the electrochemical conditions and the type of halogen acid present in the electrolyte. In HCl and HBr a black porous layer can be formed that is likely to consist to a certain extent of an antimony-oxo-chloride or antimony-oxo-bromide. In HF, however, polarization under a wide range of electrochemical conditions leads to a uniform etching of the InSb surface.

This paper describes photoluminescence (PL) quenching of hydrogen-terminated and chemically derivatized porous silicon (PSi) nanostructures by a green ruthenium cluster (I). Chemisorption of freshly prepared PSi surfaces in a hexane solution of the Ru cluster for several days at room temperature led to a complete quenching of the PSi PL. The only visible PL was due to the original PL of the cluster. When the PSi surface functionalized with undecylenic acid was immersed in the same hexane solution of (I), the PSi PL was completely quenched and accompanied with a shift to a lower energy of the cluster PL. This shift was assigned to the formation of an ester linkage resulting from the nucleophilic attack of the PO anion of the cluster on the terminal acid functional group.

We have observed self-organization in Si nanocrystals fabricated by high temperature annealing of nanometer-thick layers of amorphous Si confined between thin SiO 2 layers. Silicon nanoparticles larger than ∼8 nm spontaneously form brick-shaped crystallites oriented along the [1 1 1] direction while smaller nanocrystals adopt a more spherical geometry. The observed ordering and self-organization are important steps towards reproducible maskless Si nanofabrication and the consequent construction of Si/SiO 2 quantum devices.

Detailed Raman measurements were performed on the optical phonons in the disordered alloy Ga1−xAlxAs at room temperature. The results obtained are compared with earlier measurements and expressions are given for the Al concentration dependence of the AlAs-like and GaAs-like longitudinal and transverse optical modes. Copyright © 2002 John Wiley & Sons, Ltd.

The role of quantum confinement (QC) in Si and Ge nanostructures (NSs) including quantum dots, quantum wires, and quantum wells is assessed under a wide variety of fabrication methods in terms of both their structural and optical properties. Structural properties include interface states, defect states in a matrix material, and stress, all of which alter the electronic states and hence the measured optical properties. We demonstrate how variations in the fabrication method lead to differences in the NS properties, where the most relevant parameters for each type of fabrication method are highlighted. Si embedded in, or layered between, SiO2, and the role of the sub-oxide interface states embodies much of the discussion. Other matrix materials include Si3N4 and Al2O3. Si NSs exhibit a complicated optical spectrum, because the coupling between the interface states and the confined carriers manifests with varying magnitude depending on the dimension of confinement. Ge NSs do not produce well-defined luminescence due to confined carriers, because of the strong influence from oxygen vacancy defect states. Variations in Si and Ge NS properties are considered in terms of different theoretical models of QC (effective mass approximation, tight binding method, and pseudopotential method). For each theoretical model, we discuss the treatment of the relevant experimental parameters.

Experimental data are reported for the temperature and polarization dependence of the one-magnon Raman light scattering in the rutile-structure antiferromagnet CoF₂ (Neel temperature TN = 38 K). The low-lying excitons are also investigated at low temperatures and comparisons made with results from earlier Raman, infrared, and neutron scattering work. A detailed analysis of the one-magnon Stokes and anti-Stokes Raman spectra is presented resulting in comprehensive data for the temperature variation up to TN of the one-magnon frequency, line width, and integrated intensity. A theory of the one-magnon scattering and other magnetic transitions is constructed based mainly on a spin S = 3/2 exchange model, extending a simpler effective S = 1/2 approach. The excitation energies and spectral intensities over a broad range of temperatures are obtained using a Green's function equation of motion method that allows for a careful treatment of the single-ion anisotropy. Overall the S = 3/2 theory compares well with the experimental data.

We have produced a novel form of amorphous silicon (a-Si) using ultra-high-vacuum molecular beam epitaxy. By depositing silicon atoms onto a fused quartz substrate at ∼100°C, we have obtained a silicon based material that lacks the characteristic periodicity of crystalline silicon but nevertheless has 98% of its density. Infrared and secondary ion mass spectroscopic measurements demonstrate that there are only small amounts of hydrogen (0.1 at.%) in this clean form of a-Si, which contrasts dramatically with the case of conventional a-Si. The optical properties of this new form of a-Si are compared with those of conventional a-Si, and conclusions are drawn regarding the amount of disorder.

The thermal reaction of ethyl undecylenate with a hydrogen-terminated porous silicon surface takes place at 85 °C to yield an organic monolayer covalently attached to the surface through Si−C bonds. The presence of traces of water in ethyl undecylenate induces a partial oxidation of the surface and leads to a surface that is composed of alkylated and oxidized regions. A PSi surface with a comparable chemical composition was prepared by the direct reaction of an electrochemically anodized PSi surface (in 1 M H2SO4) with anhydrous ethyl undecylenate. The physical and chemical properties of the functionalized surfaces have been characterized using photoluminescence, transmission infrared, and X-ray photoelectron spectroscopies. The derivatized surfaces proved to be very stable in boiling CCl4 and water, and against corrosion when exposed to 100% humidity in air.

We have produced a novel form of amorphous silicon (a-Si) using ultra-high-vacuum molecular beam epitaxy (MBE). From measurements of the specular reflectance spectrum at near normal incidence and the regular transmittance spectrum at normal incidence we have determined the spectral dependence of the refractive index, the extinction coefficient, the optical absorption coefficient, and the real and complex components of the dielectric function. These optical dispersion relationships are contrasted with those corresponding to other forms of a-Si.

Raman spectroscopy has been used to assess the concentration dependence of optical phonons in In1−xGaxAs epilayers grown by chemical beam epitaxy on InP(100). The alloy composition was varied from x=0.325 to x=0.55 to cover the technologically important and physically interesting range where the strain changes sign at x=0.468 from negative to positive. The Raman spectra were curve resolved to reveal the GaAs-like longitudinal optic (LO), disorder induced, InAs-like LO, and InAs-like transverse optic phonons. An examination of the concentration dependence of the phonon frequencies showed that the GaAs-like LO mode varied as ω(cm−1)=252.77+58.643x−50.108x2 for 0.325⩽x⩽0.55. A comparison of these results with previous infrared and Raman work on In1−xGaxAs has revealed that the concentration dependence of the optical phonon frequencies in the unstrained system is still not accurately known.

This paper describes photoluminescence (PL) stabilization through chemical modification of freshly prepared porous silicon (PSi) surfaces. As-anodized PSi surfaces react with 1-alkenes, non-conjugated dienes and aldehydes at elevated temperatures to form organic monolayers covalently bonded to the surface. This thermal route is very general and tolerant of different functional groups. We have characterized these organic monolayers using diffuse reflectance infrared Fourier-transform (DRIFT), Auger and Raman spectroscopies. The PL intensity and peak energy of the as-anodized PSi is not affected by the chemical functionalization. Aging these derivatized PSi samples in ambient air has no effect on the PL. In fact, it is completely preserved even when they are steam treated for six weeks at 70 °C and 100% humidity. This treatment completely destroys the structural integrity of H-terminated PSi.

A nanocrystalline silicon superlattice (nc-Si SLs) is a structure consisting of Si nanocrystal layers separated by nanometer-thick SiO2. A long range order in the nc-Si SL is obtained along the direction of growth by periodically alternating layers of Si nanocrystals and SiO2. A number of characterization techniques such as transmission electron microscopy (TEM) and atomic force microscopy (AFM), Auger elemental microanalysis. X-ray diffraction and X-ray small angle reflection have proved that the nc-Si SL exhibits a very narrow nanocrystal size distribution (less than 5% in average) and very abrupt and flat nc-Si/SiO2 interfaces with a roughness of

Brillouin spectroscopy of porous Si $(\ensuremath{\pi}$-Si) formed on p-type (100) Si has revealed two acoustic-phonon peaks associated with the surface and film modes of the thick porous slab. They exhibit relatively large peak widths that are attributed to fractons. Longitudinal wave guiding is enhanced in high porosity $\ensuremath{\pi}$-Si due to its very low sound velocities compared with Si. Unlike opaque semiconductors where ripple scattering is dominant, the elasto-optic contribution to the scattering mechanism is found to be significant in $\ensuremath{\pi}$-Si. Both phonon velocities show appreciable negative dispersion due to a porosity gradient.

The fabrication of Si-based light emitting materials is a very active area of research. We have pursued two avenues of research that have provided interesting prospects for practical applications. 1. Bright quantum-confined luminescence can be obtained from Si/SiO2 superlattices. Placing them in an optical microcavity results in a pronounced modulation of the photoluminescence (PL) intensity with emission wavelength. The superlattice PL efficiency can be considerably enhanced and its peak wavelength and bandwidth modified selectively. 2. For porous semiconductors, a method that allows the writing of visible light emitting patterns of arbitrary shape down to the nanometer scale is described. The process relies on selective electrochemistry for pore formation at surface defects created by focused ion beam implantation of Si.

The optical properties of porous GaAs formed electrochemically on n- and p-type GaAs in HCl electrolyte are reported. The porous structure comprises GaAs crystallites ranging in size from micrometers to nanometers and under certain chemical conditions other transparent crystallites of As 2 O 3 and Ga 2 O 3 form. Photoluminescence (PL) measurements at 295 K reveal an “infrared” PL at ∼840 nm and a “green” PL at ∼ 540 nm, which could easily be seen by the naked eye in some samples. The infrared and green PL peak wavelength and intensity varied from sample to sample consistent with an assignment to quantum confinement effects in GaAs micro- and nanocrystallites, respectively.

Pore formation on n-type GaAs(100) under anodic polarization in I M HCI has been studied. Focused ion beam implantation of Si 2+ into GaAs was used to write defined surface damage/implant patterns into the substrate. These implant sites represent initiation sites for pore growth and show pore formation at potentials significantly cathodic to the intact surface. Hence, pore growth within the patterns can be achieved selectively if anodic polarization is kept below the pore formation potential of the unimplanted surface. The results show that both the polarization potential and the implantation dose strongly influence the morphology and the photoluminescence behavior of the porous structures. Monte Carlo simulations of the implantation process revealed that the morphology of the etch process and its kinetics are both strongly connected to the implant/damage profile. Furthermore, it is demonstrated that green-light-emitting porous GaAs lift-off layers can be produced.

Bright quantum confined luminescence due to band-to-band recombination can be obtained from Si/SiO2 superlattices. Placing them in a one-dimensional optical microcavity results in a pronounced modulation of the photoluminescence (PL) intensity with emission wavelength, as a consequence of the standing wave set up between the substrate and top interfaces. For a Si substrate, absorption of light reduces the PL efficiency, but for an Al-coated glass substrate the PL intensity is twice that of a quartz substrate case. The addition of a broad-band high reflector to the superlattice surface results in enhanced narrow-band emission. These results show that a suitably designed planar microcavity can not only considerably increase the external efficiency of luminescence in Si/SiO2 superlattices but can also be used to decrease the bandwidth and selectively tune the peak wavelength.

a-Si/insulator multilayers have been deposited on (0 0 1) Si by electron gun Si evaporation and periodic electron cyclotron resonance plasma oxidation or nitridation. Exposure to an O or N plasma resulted in the formation of a thin SiO 2 and SiN x layer whose thickness was self-limited and controlled by process parameters. For thin-layer (∼2 nm) Si/SiO 2 and Si/SiN x multilayers no visible photoluminescence (PL) was observed in most samples, although all exhibited weak “blue” PL. For the nitride multilayers, annealing at 750°C or 850°C induced visible PL that varied in peak energy with Si layer thickness. Depth profiling of a-Si caps on thin insulating layers revealed no detectable contamination for the SiN x layers, but substantial O contamination for the SiO 2 films.

Although NiO is often considered the classic example of an antiferromagnetic insulator, recent investigations have revealed unexplained features of the magnon spectrum. The present study of the temperature and polarization behavior of first-order magnetic Raman scattering reveals that the polarization selection rules are not described by the generally accepted antisymmetric scattering tensor. The inclusion of quadratic magneto-optic coupling terms can explain the symmetry of the scattering tensor, but does not lead to results consistent with the accepted [112] spin alignment direction. {copyright} {ital 1998} {ital The American Physical Society}

Inelastic light scattering from first- and second-order longitudinal optic phonons using infrared excitation near the direct gap (1.6 eV) of CdTe quantum wells in CdTe/Cd0.9Mn0.1Te superlattices grown on (001) Cd0.95Zn0.05Te has revealed higher lying confined electronic states. The energies of the observed excitations compare well with predictions of a three-band Kronig–Penney model including the effects of strain and the nonparabolicity of the conduction band.

Diamond-like carbon films were deposited by excimer laser ablation of graphite on Zr-2.5Nb pressure tube material at substrate temperatures of 25 and 200 °C. Characterizations of the films by nanoindentation, Raman spectroscopy, and Fourier transform infrared reflection spectroscopy show that the films have a very high hardness, are transparent to infrared light, and exhibit Raman spectra typical of diamond-like carbon films. The films also exhibit a high refractive index in the infrared spectrum. Increasing the substrate temperature from 25 to 200 °C results in a 25% increase in the deposition rate, but produces lower mechanical properties for the DLC films. The high refractive index was attributed to a lack of incorporation of hydrogen into the films and the lower mechanical properties of the films deposited on the heated substrate were explained by an increase in the sp 2 sp 3 bonds ratio.

The inelastic light scattering spectra of acoustic modes have been measured for a series of CdTe Cd 1−x Mn x Te superlattices with x = 0.1. Analysis of the Brillouin to folded acoustic mode intensities yields a photoelastic coupling coefficient ratio of P alloy P CdTe = 0.18 for excitation at 1.648 eV (752.5 nm). The results indicate that the superlattice photoelastic modulation contribution dominates that of the acoustic modulation in the Raman scattering mechanism.

Although the Al x Ga (1 − x ) As alloy system has been extensively investigated, there are still considerable uncertainties in measuring the value of x . Here a new Al x Ga (1 − x ) As calibration structure, grown by molecular beam epitaxy, has been used to establish unambiguous alloy compositions. Such ‘standard’ Al x Ga (1 − x As layers were measured by high-resolution X-ray diffraction, photoluminescence, and Raman spectroscopy to determine the compositional variations of the measured physical parameters. The formulae provided give a reliable calibration base for other characterization tools. A detailed analysis of the X-ray results shows that Vegard's law does not hold for the variation of the Al x Ga (1 − x ) As lattice constant with x .

Although the AlxGa1−xAs alloy system has been extensively investigated, there are still considerable uncertainties in measuring the value of x. Here a new AlxGa1−xAs calibration structure, grown by molecular beam epitaxy, has been used to establish unambiguous alloy compositions. Such “standard’’ AlxGa1−xAs layers were measured by high-resolution x-ray diffraction, photoluminescence, and Raman spectroscopy to determine the compositional variations of the measured physical parameters. The phenomenological equations derived from these measurements can now be used to establish the Al content of unknown alloys with confidence. In addition, the results show that Vegard’s law does not hold for the variation of the AlxGa1−xAs lattice constant with x. The small quadratic term has very important implications for a correct analysis of x-ray results.

The Rashba spin-orbit interaction of the two-dimensional electron gas with high mobility in the inversion layer of p-type Hg0.77Cd 0.23Te is investigated by magnetotransport measurements. Both the Rashba spin splitting and Rashba coefficient are extracted by analysis of the weak anti-localization effect using the Golub model. It is found that both the splitting and coefficient increase with increasing electron density (∼3.0-6.0 × 1015 m-2), i.e., with the gate voltage. A self-consistent Schrodinger-Poisson calculation is performed and suggests that the nonlinear Rashba effect caused by the weakening of interband coupling, especially at high electron density, dominates this system. © 2013 American Institute of Physics.

The magneotransport properties of a high carrier concentration and high mobility 20-nm thick In0.53Ga0.47As/In 0.52Al0.48As quantum well (QW) are investigated by tilt angle dependent Shubnikov-de Haas oscillations and by weak antilocalization (WAL) in an in-plane magnetic field. The effective g-factor g* and zero field spin splitting Δ 0 are extracted from tilt angle dependent beating pattern. We found that g* shows a dramatic reduction with increasing carrier density due to the increased effective band gap. Furthermore, an anomalously rapid suppression of the WAL effect with increasing in-plane magnetic field B|| is observed. This reveals that the total dephasing rate is not solely contributed by Zeeman splitting. The microroughness scattering in the QW is proposed to be another factor to cause the dephasing and thus responsible for this effect. © 2013 American Institute of Physics.

Several multiquantum wells of InP/Ga x In1−x As y P1−y grown by chemical‐beam epitaxy have been studied by high‐resolution x‐ray diffraction, low‐temperature photoluminescence, and Raman scattering to characterize interfacial layers between the barriers and the wells. These interfacial layers are created during the initial stage of growth of the quaternary material as a result of the longer transient for the saturation of the group‐III elements flux. The combination of x‐ray diffraction and photoluminescence allows a precise determination of the interfacial layer thickness and composition grading and shows that interface roughness is of the order of 1 monolayer.Raman scattering confirms these results and is used to determine values of the sound velocity and of the index of refraction in the quaternary alloy material.

Resonant Raman spectroscopy of modulation-doped GaAs/AlGaAs multiple quantum well dots and wires is reported. Deep etching with a SiCl 4 reactive ion etching process achieved an excellent aspect ratio (>10:1) and low surface damage for dots and wires of sizes in the range 60–250 nm. A rich spectrum of single particle excitations was observed at Raman shifts in the range 1–35 meV for both dots and wires. Sharp resonances were found for the Raman intensities. The electronic scattering in wires exhibits distinct polarization properties in agreement with theoretical predictions and the spin density excitation energies are in reasonable agreement with Hartree approximation calculations. The dispersion of the intrasubband plasmon collective mode in 60 nm wires has been determined. The excitations in dots show a systematic shift to higher energy with decreasing dot diameter consistent with increased confinement. Magneto-Raman scattering from dot samples was also investigated at magnetic fields up to 12 T and the excitation spectra show level splitting, level crossing and mode softening with increasing magnetic field.