Shyama Rath

Alloy effects on the Raman spectra of Si1−xGex and calibration protocols for alloy compositions based on polarization measurements

The Raman spectra of bulk and epitaxial Si1−xGex alloys (0 < x < 1) were studied with the aim of establishing a metrology for the determination of alloy compositions from the first-order Si–Si, Ge–Ge and Si–Ge optical phonon Raman modes. However, the lineshapes and intensities of these modes are distorted by various subsidiary features including (i) a broad quasi-amorphous feature on the low-energy side of the Ge–Ge mode and (ii) peaks in the range of 340–500 cm−1, very close to the main modes and believed to arise from various types of Si–Ge clusters. Standard curve-fitting procedures cannot cope with such background features and fail to give reliable values for the intensities of the main lines. We demonstrate that the background features can be minimized by protocols based on polarization measurements. One of these methods that reveals less distorted Ge–Ge and Si–Si modes involves subtraction of the first-order-forbidden spectrum from the allowed spectrum in a parallel polarization configuration. The other involves measurements in a cross-polarization geometry in which the background is suppressed. These protocols allow better estimates of the integrated phonon intensities for a wide range of alloy compositions by usual curve-fitting, and improvements in the ratios of the IGe–Ge/ISi–Si integrated intensities by factors as large as 8 were obtained for Si-rich samples. The simpler procedure for calibration based on phonon frequency positions, sufficient for bulk alloys, may be incorrect for heterostructures due to the additional strain dependence of the frequencies superimposed on the alloy compositional dependence. Therefore, a protocol based on the integrated phonon intensities is applicable and performs better in both strained-layer devices and bulk alloys.

Direct determination of the generalized Stokes parameters from the usual Stokes parameters

We report an experimental method to determine the generalized Stokes parameters for a pair of points in the cross section of an electromagnetic beam, e.g., an expanded laser beam, with the help of a Youngs interferometer and a set of polarizers and quarter-wave plates. The method is investigated theoretically using the electromagnetic spectral interference law. The generalized Stokes parameters, owing to their two-point nature, determine the behavior of the single-point polarization properties of the electromagnetic beam at a field point. The present method offers a unique means to determine the two-point parameters (correlation functions) by measuring the usual Stokes parameters (intensities) and the contrast parameters (visibilities) of the beam. The method might be applicable to determine the polarization dependent changes in various optical measurements.

Quantum confinement effects in silicon nanocrystals produced by laser-induced etching and cw laser annealing

A comparative study of quantum confinement of electrons and phonons in silicon nanocrystals produced by laser-induced etching on a silicon (Si) substrate and continuous wave (cw) laser-induced crystallization in a-Si:H film on a quartz substrate is presented here. Raman line shapes of optical phonons confined in Si nanocrystals were utilized using a phenomenological phonon-confinement model. It is appropriately modified by incorporating a Gaussian distribution of sizes for two-dimensional (columnar) and three-dimensional (spherical) confinement parameters for laser-induced etching and cw laser-induced crystallization processes, respectively. The calculated mean crystallite sizes were in consonance with those calculated from the bond-polarizability model. Confinement effects were found to be more prominent in Si nanocrystals prepared by laser-induced crystallization in comparison to laser-etched Si. Photoluminescence spectra of both the samples were also utilized to study the dimensionality aspect of nanocrystals.

Characterization of thick epitaxial GaAs layers for X-ray detection

Abstract We have studied the current–voltage and capacitance–voltage characteristics of p/i/n structures made on non-intentionally doped epitaxial GaAs layers grown by the chemical reaction method. Deep level transient spectroscopy demonstrates that these layers contain a low defect concentration. X-ray photoconductivity shows that the diffusion length is large. The homogeneity of the properties of these layers, which has been evaluated over large area (cm 2 ), is confirmed by photoluminescence mapping.

Observation of the Fresnel and Arago laws using the Mach-Zehnder interferometer

An experimental study is conducted to determine the effect of polarization on the interference of light waves. By using the temporal coherence property of light in a Mach-Zehnder interferometer, we verified the four important Fresnel and Arago laws for linearly polarized and circularly polarized light. This experiment provides a simple method for undergraduates to study the phenomena of interference and polarization.

Optical properties of tetragonal germanium nanocrystals deposited by the cluster-beam evaporation technique: New light emitting material for future

The germanium (Ge) nanocrystals were deposited on substrates whose temperature was kept at room or liquid nitrogen (LN2) temperature by the cluster-beam evaporation technique. The deposited films are found to consist of the tetragonal crystalline structure rather than the diamond structure of bulk Ge. Such a phase-transition has been theoretically predicted for sizes smaller than 4 nm, which agrees with the size measured by the transmission electron microscopy (TEM). The tetragonal Ge is expected to have a direct band gap of 1·47 eV. Furthermore, the Ge film deposited at LN2 temperature exhibits unique properties, such as photo-oxidation and blue-light emission. The Ge-nanocrystal films deposited by the cluster-beam evaporation technique are attractive materials for application to light emitting devices in future.

Temperature dependence of the fundamental band gap parameters in cadmium-rich ZnxCd1-xSe using photoluminescence spectroscopy

Thin films of ternary ZnxCd1-xSe were deposited on GaAs (100) substrate using metalorganic-chemical-vapour-deposition (MOCVD) technique. Temperature dependence of the nearband-edge emission from these Cd-rich ZnxCd1-xSe (forx = 0025, 0.045) films has been studied using photoluminescence spectroscopy. Relevant parameters that describe temperature variation of the energy and broadening of the fundamental band gap have been evaluated using various models including the two-oscillator model, the Bose-Einstein model and the Varshni model. While all these models suffice to explain spectra at higher temperatures, the two-oscillator model not only explains low temperature spectra adequately but also provides additional information concerning phonon dispersion in these materials.

Polarization-dependent Raman spectroscopic protocols for calibration of the alloy composition and strain in bulk and thin-film Si1-xGex

Polarization-dependent Raman spectroscopic methods are presented for extracting the integrated intensities of the first-order Si-Si, Si-Ge and Ge-Ge modes in Si1-x Gex . In unpolarized spectra, these modes are distorted by a broad background causing anomalies in their apparent intensities, as derived by standard curve-fitting techniques, which can amount to 300%. The background is strongly polarization dependent. However, the methods described here are successful in minimizing the background in the Raman spectrum and, as a result, the relative intensities measured in the parallel and crossed polarizations differ by less than about 20%. Consequently, the protocols could be developed into process-control techniques for rapid determination of alloy composition (x ) and strain in Si1-x Gex . Additional weak features are revealed when the Raman scattering geometry is altered from a configuration where the first-order optical phonons are allowed to one in which they are forbidden.

Study of growth process of germanium nanocrystals using a grazing incidence x-ray diffraction method

The growth process of germanium nanocrystals in a SiOx matrix was studied by an x-ray diffraction (XRD) technique using the grazing incidence of x rays. The conclusion based on the XRD analysis of the Ge-doped glass is in good agreement with that from Raman spectroscopy and x-ray photoelectron spectroscopy. There are two stages of crystallization: the first stage is nucleation of the Ge nanocrystals at 600 °C, and the second stage is growth of the nanocrystals at 800 °C. During the second stage, the decomposition of GeOx into Ge and O in the Ge-doped glass sample results in fast growth of Ge nanocrystals. This is the first report that studied the growth of Ge nanocrystals in an SiOx matrix by XRD.

A spectroscopic ellispometric study of the tunability of the optical constants and thickness of GeOx films with swift heavy ions

Sub-stoichiometric GeOx films were fabricated by electron-beam evaporation method. The films were irradiated with 100 MeV Ag7+ ions at fluences between 1 × 1012 and 1 × 1014 ions-cm−2. Spectroscopic ellipsometric measurements were performed in air at room temperature. The values of the layer thickness and refractive index were extracted from ellipsometry using a multilayer analysis and the Tauc Lorentz model. The refractive index (at 633 nm) of the as-deposited GeOx film was estimated to be 1.860 and decreased to 1.823 for films irradiated at an ion fluence of 1 × 1014 ions-cm−2. The thickness of the films also decreased after irradiation and is due to a sputtering induced by the ion beam. The change in the refractive index with ion fluence is attributed to a stoichiometric change and structural transformation represented by GeOx→ Ge + GeOy (y > x) occurring due to a thermal spike induced by ion irradiation. Swift heavy ions thus provide a scope for modulating the refractive index of GeOx films. The thick...