Raman scattering study on Ga1–x Mnx As prepared by Mn ions implantation,deposition and post‐annealing

Raman scattering measurements have been performed in Ga_1–x Mn_x As crystals prepared by Mn ions implantation, deposition, and post‐annealing. The Raman spectrum measured from the implanted surface of the sample shows some weak phonon modes in addition to GaAs‐like phonon modes, where the GaAs‐like LO and TO phonons are found to be shifted by approximately 4 and 2 cm^‐1, respectively, in the lower frequency direction compared to those observed from the unimplanted surface of the sample. The weak vibrational modes observed are assigned to hausmannite Mn₃O₄ like. The coupled LO‐phonon plasmon mode (CLOPM), and defects and As related vibrational modes caused by Mn ions implantation, deposition, and post‐annealing are also observed. The compositional dependence of GaAs‐like LO phonon frequency is developed for strained and unstrained conditions and then using the observed LO_GaAs peak, the Mn composition is evaluated to be 0.034. Furthermore, by analyzing the intensity of CLOPM and unscreened LO_GaAs phonon mode, the hole density is evaluated to be 1.84×10¹⁸ cm^‐3. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Raman scattering characterization of Mn composition and strain in Ga1–x Mnx Sb/GaSb epitaxial layers

Raman scattering (RS) experiments have been performed for simultaneous determination of Mn composition and strain in Ga_1–x Mn_x Sb thin films grown on GaSb substrate by liquid phase epitaxy technique. The Raman spectra obtained from various Ga_1–x Mn_x Sb samples show only GaSb‐like phonon modes whose frequency positions are found to have Mn compositional dependence. With the combination of epilayer strain model, RS and energy dispersive x‐ray (EDX) experiments, the compositional dependence of GaSb‐like LO phonon frequency is proposed both in strained and unstrained conditions. The proposed relationships are used to evaluate Mn composition and strain from the Ga_1–x Mn_x Sb samples. The results obtained from the RS data are found to be in good agreement with those determined independently by the EDX analysis. Furthermore, the frequency positions of MnSb‐like phonon modes are suggested by reduced‐mass model. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photoluminescence of Si or Ge nanocrystallites embedded in silicon oxide

This paper presents the results of photoluminescence, its temperature dependence and Raman scattering investigations on magnetron co-sputtered silicon oxide films with (or without) embedded Si (or Ge) nanocrystallites. It is shown the oxide related defect origin of the visible PL centers peaked at 1.7, 2.06 and 2.30 eV. The infrared PL band centered at 1.44–1.58 eV in Si–SiO_x, system has been analyzed within a quantum confinement PL model. Comparative PL investigation of Ge–SiO_x system has confirmed that high energy visible PL bands (1.60–1.70 and 2.30 eV) are connected with oxide related defects in SiO_x. The PL band in the spectral range of 0.75–0.85 eV in Ge–SiO_x system is attributed to exciton recombination inside of Ge NCs.     

Raman amplification in nanoparticles doped glasses

Resonant Raman effects are studied for CdS_xSe_1−x nanoparticles in a silicate glass matrix in order to explore new possibilities of Raman amplification for telecommunication fibers. Nanoparticles with diameters ranging from less than 2 nm up to 6 nm are excited with the 458 nm, 488 nm and 633 nm laser lines corresponding both to resonance and off-resonance conditions. Due to confinement effects the resonance conditions are achieved at a given frequency by varying the size of the nanoparticles. In resonance no enhancement is observed for the silicate glass matrix but the (1LO-CdS), (2LO-CdS), (1LO CdSe + 1 LO CdS) modes are strongly enhanced showing the possibility of resonant Raman amplification of the discrete frequencies of the LO modes and of their overtones.     

Spectroscopic investigation of strain induced by compositional variation in bulk InxGa1–xAs crystal grown by MCZM method

Raman scattering (RS), photoluminescence (PL) and energy dispersive X‐ray (EDX) experiments have been carried out to investigate residual strain and hence to understand breakage issue in bulk In_x Ga_1–x As crystal grown by multi component zone melting (MCZM) method. It is found from a comparison that there is a large discrepancy among the RS, PL and EDX results due to the strain induced by compositional variation in the crystal. The strain induced changes in TO_GaAs and PL peak positions are found to be 4.04 cm^–1 and 0.097 eV, respectively, for the variation of composition from 0.06 to 0.29 from the seed‐end to the tail‐end of the crystal. By assuming a simple one‐dimensional strain distribution, the strain value corresponding to 4.04 cm^–1/ 0.097 eV can be obtained of the order of 10^–2, which is large enough for understanding the breakage issue in the crystal investigated here. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Physical aging effect on the boson peak and heterogeneous nanostructure of a silicate glass

The effect of physical aging on a silicate glass has been investigated by low-frequency Raman scattering. It was observed that the low-frequency side of the excess of Raman scattering, or boson peak, due to harmonic vibration modes decreases in intensity with the thermal annealing (aging) at a temperature lower than the glass transition temperature (T_g), after quenching (rejuvenation) from a temperature higher than T_g. Moreover, it was found that the lowering of the very low-frequency scattering mainly due to anharmonic modes becomes more pronounced when the aging temperature is decreased. These observations are interpreted in the frame of the energy landscape, and by considering the model of the glass heterogeneous cohesion at the nanometric scale.     

Raman scattering in amorphous films of In1−xSex alloys

Raman scattering (RS) in amorphous films of In_1−xSe_x with 0.67±x?0.38 has been studied in backscattering geometry with the use of a microscope. Recorded RS spectra are revealing a mixed vibrational density-of-states and molecular character. The spectra spread from the Rayleigh line up to 200-250 cm⁻¹. The bands superimposed on the continua are related to zone center modes of the relevant crystal counterpart, Se-Se or In-In vibrations. The RS spectra suggest the structure of the In_1−xSe_x alloys to be the continuous random network built up of In centered tetrahedral clusters with In and Se atoms at the corners. The structure of the Se-rich alloys is similar to 4-2 networks with dominant InSe_4/2 clusters and two-fold coordination of Se bridging atoms. That of the In-rich alloys is expected to resemble 4-3 network with rather strong involvement of In atoms at corner of the In-centered tetrahedral clusters and Se atoms being linked to three In ones.     

Grain boundary defects induced room temperature ferromagnetism in V doped ZnO thin films

Vanadium (V) doped ZnO thin films (Zn_1‐xV_x O, where x = 0, 0.05, 0.10) have been grown on sapphire substrates by RF magnetron sputtering to realize room temperature ferromagnetism (RTFM). The grown films have been subjected to X‐ray diffraction (XRD), resonant Raman scattering, photoluminescence (PL) and vibrating sample magnetometer (VSM) measurements to investigate their structural, optical and magnetic properties, respectively. The full width at half maximum of XRD and Raman scattering peaks increases with V ion concentration indicates that the V ions have been substituted on Zn²⁺ ions in the ZnO matrix. The increase in oxygen vacancies with V concentration is evidenced by PL measurements. Rutherford backscattering spectrometry analysis confirms the presence of the V ions in the films. The room temperature VSM measurements reveal the signature of ferromagnetism in V doped ZnO thin films. It has been observed that the grain boundary defects, i.e., oxygen vacancies play a crucial role in inducing RTFM in V doped ZnO films. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

High intense UV-luminescence of nanocrystalline ZnO thin films prepared by thermal oxidation of ZnS thin films

High quality zinc oxide (ZnO) films were obtained by thermal oxidation of high quality ZnS films. The ZnS films were deposited on a Si substrate by a low-pressure metalorganic chemical vapor deposition technique. X-ray diffraction spectra indicate that high quality ZnO films possessing a polycrystalline hexagonal wurtzite structure with preferred orientation of (0 0 2) were obtained. A fourth order LO Raman scattering was observed in the films. In photoluminescence (PL) measurements, a strong PL with a full-width at half-maximum of 10 nm around 380 nm was obtained for the samples annealed at 900°C at room temperature. The maximum PL intensity ratio of the UV emission to the deep-level emission is 28 at room temperature, providing evidence of the high quality of the nanocrystalline ZnO films.     

Structural peculiarities, and electrical and optical properties of 70TeO2·30PbCl2 glasses doped with Pr, prepared in Pt or Au crucibles

The influence of crucibles (Au or Pt) on the structure, electrical, dielectric and optical properties of 70TeO₂·30PbCl₂ glasses doped with Pr³⁺ added as a metal, chloride, or oxide, in concentrations of 500–1500 wt-ppm, is reported. The dc conductivity of ‘pure’ glasses prepared in Au crucibles is two orders of magnitude higher than that of those prepared in Pt crucibles. Upon doping, the dc conductivity of glasses prepared in Pt and Au crucibles increases or decreases, respectively. The static relative permittivity is equal to 33 ± 2. In the range of 640–700 nm, six photoluminescence (PL) peaks were observed, at 641.5, 647.1, 652.4, 660.8, 662.9, and 664.5 nm. In the range of 200–1200 cm⁻¹, seven Raman scattering (RS) peaks were observed at 184, 217, 321, 468, 654, 735 cm⁻¹, and a small peak at 650 cm⁻¹. Both spectra were deconvoluted using symmetrical Gaussian functions. Relative intensities of PL and RS bands depend on the concentration and chemical form of Pr³⁺ and on the material of the crucible. However, positions of these bands are independent of these conditions.     

Effect of pressure on the structural stability of iron phosphate glass: Role of trace water

Raman and infrared spectroscopic measurements were carried out on 40 mol% Fe₂O₃:60 mol% P₂O₅ iron phosphate glass samples to study the effect of increasing external pressure on the local structure of both anhydrous and water trapped glasses. In-situ high‐pressure Raman measurements, done up to 24.2 GPa on anhydrous samples, revealed hardening of the phonon modes accompanied by a loss of Raman intensity and ultimately leading to complete smearing of modes at very high pressures. These changes were found to be reversible upon quenching the sample back to ambient pressure. Raman and infrared spectroscopy of pressure quenched IPG samples containing trace amounts of water have revealed definite signatures of devitrification at room temperature.     

Optical and structural proprieties of nc-Si:H prepared by argon diluted silane PECVD

Using argon as a diluent of Silane, hydrogenated amorphous and nanorocrystalline silicon films Si:H were prepared by radio-frequency (13.56 MHz) plasma enhanced chemical vapor deposition (rf-PECVD). The deposition rate and crystallinity varying with the deposition pressure and rf power, were systematically studied. Structural analysis (Raman scattering spectroscopy and X-ray diffraction), combined with optical measurements spectroscopy were used to characterize the films. The argon dilution of silane for all samples studied was 95% by volume, and the substrate temperature was 200 °C. The deposition pressure was varied from 400 mTorr to 1400 mTorr and varying rf power from 50 to 250 W. The structural evolution studies, shows that beyond 200 W of rf power, an amorphous-nanocrystalline transition was observed, with an increase in crystalline fraction by increasing rf power and working pressure. The films were grown at high deposition rates. The deposition rates of the films near the amorphous-nanocrystalline phase transition region were found in the range 6–10 Å/s. A correlation between structural and optical properties has been found and discussed.     

Size dependent photoluminescence of SiC nanocrystals

The paper presents the results of porous SiC study using photoluminescence and scanning electronic microscopy. It is shown that the intensity of defect-related PL bands (2.08, 2.27, 2.44 and 2.63 eV) increases monotonically with the rise of PSiC thickness from 2.1 up to 12.0 μm. These luminescence centers are assigned to surface defects which appear at the PSiC etching process. Photoluminescence intensity stimulation for surface defects is attributed to rise of defect concentrations with increasing of porous layer thickness and to realization of the hot carrier ballistic mechanism at surface defect excitation. Intensity enhancement for exciton-related PL bands (2.79, 2.98 and 3.26 eV ) is attributed to increasing the exciton recombination rate as result of exciton weak confinement in big size SiC NCs of different polytypes (6H–PSiC with inclusions of 15R- and 4H–PSiC).     

Raman Scattering Studies on the Thin Graded Band Gap AlGaAs Hetero-Epitaxial Layer

Thin graded hetero-epitaxial AlGaAs layer has been grown from the undersaturated Liquid Phase Epitaxial (LPE) technique. The grown layers have been characterized using Laser Raman scattering studies. The peak position and intensity ratio of GaAs and AlAs like LO phonon frequencies have been measured and compared with conventional LPE grown AlGaAs epitaxial layer. The behaviour of GaAs and AlAs like LO phonons has been found to vary with the aluminum composition in the grown layer. Raman peak positions have been observed to shift to lower wavenumber in GaAs like LO phonon and higher wavenumber side of AlAs like LO phonon. Aluminum free features have been noticed in IEE grown AlGaAs (x > 0.8) hetero epitaxial layers.     

Temperature dependence of photoluminescence from ordered GaInP2 epitaxial layers

The temperature behavior of the integrated intensity of photoluminescence (PL) emission from ordered GaInP₂ epitaxial layer was measured at temperatures of 10 ‐ 300 K. Within this temperature range the PL emission is dominated by band‐to‐band radiative recombination. The PL intensity temperature dependence has two regions: at low temperatures it quenches rapidly as the temperature increases, and above 100 K it reduces slowly. This temperature behavior is compared with that of disordered GaInP₂ layer. The specter of the PL emission of the disordered layer has two peaks, which are identified as due to donor‐accepter (D‐A) and band‐to‐band recombination. The PL intensity quenching of these spectral bands is very different: With increasing temperature, the D‐A peak intensity remains almost unchanged at low temperatures and then decreases at a higher rate. The intensity of the band‐to‐band recombination peak decays gradually, having a higher rate at low temperatures than at higher temperatures. Comparing these temperature dependencies of these PL peaks of ordered and disordered alloys and the temperature behavior of their full width at half maximum (FWHM), we conclude that the different morphology of these alloys causes their different temperature behavior. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and characterization of potassium, rubidium, and cesium thiogermanate glasses

Glass-forming regions were investigated for the binary xM₂S + (1 − x)GeS₂ (M=K, Rb, Cs) systems. Glasses were prepared from 0?x?0.20 mole fraction alkali sulfide using a novel preparation route involving the decomposition of the alkali hydrosulfides in situ. At higher alkali concentrations near x=0.33, the glass-forming regions are limited by the readily formed adamantane-like M₄Ge₄S₁₀ crystals. Structural characterization of the glasses and polycrystals for x?0.33 were performed using Raman scattering and IR absorption. Terminal Ge-S⁻ vibrational modes, observed between 473 and 479 cm⁻¹, increased in intensity and decreased in frequency with increasing alkali modifier content. Glass transition temperatures decreased with increasing alkali modifier, ranging from 250 to 215 °C. Corresponding crystallization onset temperatures were between 340 and 385 °C. DC conductivity values of the glasses ranged from 10⁻¹⁰ to 10⁻⁷ (Ω cm)⁻¹ with activation energies between 0.54 and 0.93 eV for the temperature range of ∼100-250 °C. Higher ionic conductivities were observed with increasing alkali concentration and decreasing alkali radii. Additionally, an increase in the activation energy was observed above the glass transition temperature.     

Quasielastic Light Scattering from Orientational Fluctuations in a Cholesteric Liquid Crystal

Abstract

Examination of the Rayleigh line of a lyotropic cholesteric polypeptide liquid crystal by the techniques of quasielastic light scattering spectroscopy reveals the presence of two new purely dissipative low frequency light scattering modes of comparable intensity. Angular and electric field dependences of the deconvoluted spectra are consistent with the identification of the mechanism as due to twist and viscous-splay normal modes recently predicted theoretically. Analysis of the narrow mode on this basis yields a value of γ₁, the twist viscosity coefficient of the medium. The broader mode results in an estimate of the value of the ratio of the splay to bend elastic moduli, K ₁₁/K ₃₃ ～ 10^?5, which is shown to be in accord weith mean field theoretical calculations for a system of such highly elongated macromolecules.     

XRD,TEM and Raman scattering studies of PbTiO3 nanopowders

Lead titanate nanopowders were fabricated by mechanochemical synthesis. The obtained materials were characterized by X‐ray diffraction, high resolution electron microscopy and Raman scattering. The X‐ray diffraction showed the perovskite structure of the powder. It was found that: (1) the powder consists of loosely packed grains with a broad distribution of sizes between a few nm and 45 nm, (2) the grains of sizes larger than about 30 nm exhibit well developed crystalline structure, (3) the Raman lines of PbTiO₃ nanopowder exhibit a conspicuous broadening in comparison to Raman lines of the bulk material, and 4) in the spectrum of nanopowder the lines related to A₁(1TO), E(2TO) and A₁(3TO) modes are shifted into lower frequencies and their spectral linewidth increase and there is no significant change in frequency shift for the remaining modes. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Vibrational Properties of LiGaO2 (I). Experimental Results

Infrared reflectivity spectra of LiGaO₂ are measured at room temperature in the wave-number range from 200 to 4000 cm¹ for the polarization directions E ∥ a and E ∥ c. The parameters of 10 B₁ modes and 4 A₁ modes are determined by a dispersion analysis of the spectra. The results obtained are compared with previous Raman scattering measurements.     

Raman and infrared spectra on silica gel evolving toward glass

Infrared and Raman spectra of gels obtained from specially prepared solutions of Si(OC₂H₅)₄ (TEOS) which have been thermally treated in the 40–800°C temperature range, are reported and discussed with reference to the spectra of fused quartz.The results show that the gel to glass transformation is an hydrolytic polycondensation process, which takes plase gradually and is practically completed in the samples treated at 800°C.Particularly revealing is the behaviour of the bands due to OH (associated and/or unassociated H-bonded) stretching modes, including those of water, which are weakened by increasing temperature; the behaviour of the band due to the SiOH stretching mode; the behaviour of the network bands, some of which become more intense with increasing temperature, due to asymmetric and symmetric stretching and the bending SiOSi modes.In the Raman spectra of gels heated at different temperatures two peaks also appear at ≈600 and ≈490 cm^?1, which are also present in the Raman spectrum of fused quartz, they are related to network defects of the glass structure, for which no definite interpretation has yet been given in the literature.