Search

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.

Photoluminescence (PL) spectroscopy has been used to study the incorporation of C in several samples consisting of strained Si 1 − x − y Ge x C y epilayers lattice matched to Si (001). To obtain the total C concentration, these samples were characterized by both SIMS and Auger emission spectroscopy, and X-ray diffraction data was analyzed to obtain the substitutional C concentration. The difference of the total and substitutional C concentrations, i.e., the non-substitutional carbon fraction, was found to be directly correlated with specific spectral lines in both the room-temperature Raman and low-temperature PL spectra.

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.

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 luminescence behaviour of n-type gallium phosphide (1 0 0) was studied during electrochemical pore etching in HF and HBr electrolytes. Strong visible oscillating light emission is observed that is correlated with current oscillations during the electrochemical etching process. The intensity of the electroluminescence strongly depends on the etchant and there is an increase of two orders of magnitude in intensity by using a HBr instead of HF electrolyte. Therefore, we conclude direct coupling between electron transfer at the semiconductor–electrolyte interface and the light emission mechanism.

We report the growth of Si1−yCy and Si1−x−yGexCy alloys on Si(001) by electron cyclotron resonance plasma-assisted Si molecular beam epitaxy using an argon/methane gas mixture. Various Si/Si1−yCy and Si/Si1−x−yGexCy multilayers have been grown and characterized principally by X-ray diffraction and Raman spectroscopy. The influence of growth parameters and electron cyclotron resonance plasma source operating conditions on the C substitutional incorporation was studied. Under optimum growth conditions the structures show good structural properties and sharp interfaces with carbon being essentially substitutionally incorporated up to concentrations of ∼1%. No significant carbon incorporation was measured in films grown under a high methane partial pressure without plasma excitation. Si1−x−yGexCy layers grown with this technique exhibit the strain compensation and enhanced thermal stability expected for these ternary alloys. Carbon pre-deposition of Si through surface exposure to the argon/methane plasma is shown to act as an antisurfactant on the growth of Ge islands by suppressing the formation of a Ge wetting layer on the surface.

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.

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.

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.

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

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.

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.

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 .

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.

Light-scattering from magnons in Bi-doped yttrium iron garnet (YIG) is found to have a lineshape that depends strongly on the excitation wavelength. At excitation wavelengths in the blue region, a subsidiary peak in the spectrum occurs in addition to the main one-magnon peak. A theory is presented to account for this lineshape behavior in terms of density-of-states effects and the kinematics of light scattering in an absorptive semi-infinite magnetic medium. Information on the pinning parameter for this sample is thereby obtained.

The optical properties of porous Si films produced by electrochemical dissolution of Si are reviewed. From measurements of the optical absorption spectra an inverse relationship between the optical gap energy and the average nanoparticle size has been obtained demonstrating the quantum confinement of electron-hole pairs in Si nanocrystallites. There is also good agreement with theoretical predictions for Si quantum dots. A number of different sources of photoluminescence in anodized Si have been tentatively identified, and at least the red emission appears to be principally due to confinement in Si nanocrystallites.

Strained layer superlattices of InSb/In1−xAlxSb with 0

Detailed Raman scattering studies of InSb/In1 − xAlxSb (0.15 < x < 0.5) strained-layer superlattices grown by magnetron sputter epitaxy on (001)InSb substrates are reported for the first time. A number of folded acoustic phonons appear in the Raman spectra and their frequencies are accurately predicted by Rytov's theory of acoustic vibrations in layered media. The longitudinal optic phonons in the In1 − xAlxSb layers exhibit two-mode behavior and their shift due to the intralayer strain is discussed. Resonant Raman scattering was used to probe the electronic structure in the region of the E1 and E1 + Δ1 optical gaps. The two sets of peaks observed in the plots of the Raman cross section vs photon energy are shown to originate from the independent electronic transitions in the alternating layers. Estimates of the strain and confinement effects were made and these agree with the observed differences from bulk material.

The high electron mobility and low effective mass associated with InSb would suggest superior performance of devices based on systems such as InSb/In1−xAlxSb, particularly in high speed and non-linear optoelectronic applications. However, difficulties encountered in the growth of antimony/aluminum alloys by mature techniques such as MBE and MOVPE have resulted in a slower development of these systems as compared to those based on GaAs. In this study we report on the growth of InSb/In1−xAlxSb superlattices using the technique of magnetron sputter epitaxy (MSE). High purity solid targets of InSb, Sb and Al were used to deposit 20 period superlattices with periods ranging from 76 to 154 A and with x values of 0

We report on a structural study of molecular-beam epitaxy grown Si/Si 1- x Ge x superlattices by grazing incidence X-ray reflectometry and Raman spectroscopy. X-ray reflectometry proved to be very sensitive to superlattice stacking anomalies and modeling of the reflectivity profiles has allowed estimation of the period length fluctuations and the sharpness of the interfaces. The results of simulations are consistent with thickness fluctuations of the order of 1–2% and symmetrical interfaces with a roughness of about monolayers. Perfection of the superlattices was also assessed by high resolution (∼0.1 cm -1 ) Raman spectroscopy of folded longitudinal acoustic phonons. In the best samples the recorded spectrum exhibited a rich low-frequency phonon structure with a well-resolved Brillouin peak and sharp-folded acoustic phonons extending up to 90 cm -1 . Measurement of the dispersion relation of the folded phonons allowed accurate determination of the superlattice parameters. The line shapes and widths of the folded phonon peaks are also sensitive to thickness variations, as is demonstrated in a study of a superlattice which contains intentionally built-in thickness inhomogeneities.

High-resolution Raman spectroscopy of thick layer Si/Si 1− x Ge x superlattices has revealed the folded acoustic modes expected at frequencies below the Brillouin line, which is near 5 cm −1 in these structures. However, the measurements unexpectedly revealed much more-complicated fine structure, which is thought to arise from structural irregularities.

The Raman spectra of high-temperature phases I and II of Na2SO4 crystals are reported. The phase I spectra indicate considerable disorder of sulfate tetrahedra even at 520 K, which is well below the melting temperature of 1157 K. Although the phase II spectra are complicated by multidomain effects, they show that the crystal c axis is common to phases I, II and III and that the as-yet unknown crystal structure of phase II is more closely related to the phase I hexagonal structure than the phase III orthorhombic structure.

Recently there has been a great deal of interest in the structural, vibrational, and electronic properties of nonperiodic superlattices.1 This work has been stimulated by the discovery of quasicrystals2 and the realization that 1-D analogs of quasicrystals could be created artificially in multilayer systems.3 By far the majority of the work in these systems has concentrated on quasiperiodic Fibonacci superlattices.4 The Fibonacci structure is a particular case of a class of quasiperiodic structures defined by the recursion relation5 $$ {{S}_{j}} = {{({{S}_{{j - 1}}})}^{n}}{{S}_{{j - 2}}} $$ (1) By defining the basic building blocks S1 and S2 in terms of layers of different materials and thicknesses we have attached a basis to the quasiperiodic lattice. Table 1 illustrates how the recursion relation (1) is used to build up the first 5 generations in terms of S1 and S2.

The Raman spectrum of Na 2 SO 4 in its metastable low temperature phase (space group D 2 h 17 ) has been measured at room temperature. All but two of the 18 Raman-active vibrational modes have been identified and assigned to symmetry species. The Raman spectrum of the metastable (phase III) form of Na 2 SO 4 is very different from the corresponding spectrum of the stable (phase V) form consistent with large differences in their respective orthorhombic structures. The b axis of phase V coincides with the c axis of phase III and the a axis is likely to be common to both phases.

Measurements of the optical and structural characteristics of various porous Si samples have been correlated. Although the photoluminescence peak wavelength shows no correlation with the Si nanoparticle size, the optical absorption edge exhibits a strong inverse correlation that is in excellent agreement with theoretical predictions for the optical gap in Si spheres or quantum dots. This constitutes direct evidence for quantum confinement effects in porous Si.

The five Raman-active k = 0 phonons have been measured at low temperatures and for a range of x in the disordered lattice CsMg1-xCoxCl3. While the E2ga, E2gb, E2gc and A1g modes at 55.0, 132.0, 189.0 and 255.0 cm−1 for x = 0 exhibit normal one-mode behaviour, the intensity of the E1g phonon at 127.5 cm−1 has a most unusual concentration dependence which requires a new theory.

The 10 and 300 K Raman spectra of CdBr2 and CdCl2 have been investigated at high sensitivity. While some of the new lines are of second-order origin, others are associated with local modes of halogen impurities. Lines at 107 and 107 cm−1 in CdBr2 are assigned to the Eu(TO) and A2u(TO) lattice modes respectively. The corresponding IR-active modes are not evident in the CdCl2 spectrum; it is suggested that they appear for CdBr2 as a result of disorder.

On increasing the temperature, several new bands appear at higher energy from their parent vibrational modes in the Raman spectrum of the molecular complex Cr 2 Cl 3− 9 . The parent and new bands have intensities that follow the thermal population factors of various 4 A 2g 4 A 2g Cr 3+ pair states. This behaviour is attributed to exchange striction which produces a change in vibrational energy with excited state for those normal modes with large net axial Cr 3+ − Cr 3+ displacements.

A Raman scattering investigation of magnetic US3 has been made from 7 to 300 K. Comparison of room temperature spectra with those of the non magnetic isostructural sulfide HfS3 allowed the assignment of most of the lines to = 0 optical phonons. Drastic changes take place in the 10−150 cm−1 range when lowering the temperature down to 7 K : four equally spaced lines appear at 54, 72.5, 91 and 109.5 cm−1. Three of them broaden significantly with increasing temperature and disappear near 50 K, at which previous measurements indicate a maximum in the magnetic susceptibility and suggest a magnetic phase transition. The stronger fourth line is still observed at 100 K and merges into a phonon line at higher temperature. These four lines are attributed to electronic transitions within the 5f2 configuration of U4+. Their temperature dependences appear to involve a spin-dependent scattering mechanism and are consistent with antiferromagnetic ordering.

We report the growth of various Si/Si 1− x Ge x and Si/Ge heterostructures including thin quasi-periodic strained-layer super-lattices and ultra-thin layers of Ge n (1 ≤ n ≤ 128) and multilayers of Si m Ge n ( m , n ≤ 6). The structural properties of these films were investigated by X-ray diffraction, Raman scattering spectroscopy and transmission electron microscopy. The Raman spectroscopy of the ultra-thin structures showed unusual results. It is shown both experimentally and theoretically that the phonons in these systems cannot be treated with the usual boundary conditions. The presence of the substrate, the multilayer structure and the explicit boundary conditions on the surface need to be considered to account for the observed peak structure and intensities. The three mode behavior for the optical phonons in the Si-Ge systems is discussed for the case of thin Ge films on Si.

Light scattering from magnons in CsCoBr 3 has been measured for temperatures well below the upper antiferromagnetic 3D ordering temperature of T l N = 28 K . These experiments reveal multiple magnon features of energies in the range 90 to 170 cm -1 similar to those found in CsCoCl 3 but previously unobserved for the bromide. Prominent features in the spectrum and their temperature dependence are described in terms of a recent theory by Shiba. Other, weaker features are explained by a simple extension of the theory to include fluctuations. A new band is observed at 178 cm -1 , whose intensity drops sharply prior to the lower 3D ordering transition at T l N = 10 K . This band is assigned to magnon-phonon combination scattering.

Results of Raman scattering experiments on (a) periodic superlattices made up of GaAs/In x Ga 1−x As layers with high indium concentrations, (b) GaAs/Ga 1−x Al x As Fibonacci superlattices, are presented. We discuss the observed peak positions and intensities using the continuum theory of acoustic wave propagation in layered media and the photo-elastic coupling model.

The physical properties of both single GexSi1−x epilayers and GexSi1−x/Si strained layer superlattices (SLS's) grown by MBE on Si(100) substrates have been investigated. Raman scattering studies of Si/GexSi1−x superlattices of period d = dSi + d GexSi1−x, with 17 ⩽ d ⩽ 65 nm, 10 ⩽ dSi ⩽ 45 nm, 4 ⩽ dGexSi1−x ⩽ 20 nm and 0.2 ⩽ × ⩽ 0.5, revealed a series of low-frequency Raman lines associated with zone-folded acoustic phonons. The frequency shifts of optic phonons in SLS's compared with bulk alloys were used to reveal the strain distribution and composition of GexSi1−x epilayers. Specific phonon features are observed in the alloy spectra that are attributed to Si-Ge ordering. Crystalline perfection and interface abruptness were assessed using cross-sectional transmission electron microscopy (TEM), secondary ion mass spectroscopy (SIMS) and Rutherford backscattering spectrometry (RBS). In addition, the compositional uniformity, the strain tensor and superlattice periodicity were evaluated using X-ray diffraction.

The Raman spectrum of Na2SO4 in its stable low-temperature phase (space group D242h) has been measured at temperatures of 10 and 295 K. Spectral features have been observed that were not reported in the earlier study of Montero. All of the 21 Raman-active vibrational modes have been identified and assigned to symmetry species. The division into lattice and sulfate internal modes is well obeyed in Na2SO4. Other weaker features are assigned variously to isotope modes and Fermi resonance modes, and the coupling between two internal modes is analyzed. The Raman spectrum indicates that there is no phase change in Na2SO4 (V) between 10 and 295 K.

Evidence in transmission electron microscopy and Raman scattering supports the claim by Ourmazd and Bean that the unit cell in Si 1− x Ge x grown by molecular beam epitaxy is doubled in a 〈111〉 direction by weak long-range order of Si and Ge atoms.