Crystallinity of the mixed phase silicon thin films by Raman spectroscopy

Raman spectra of the mixed phase silicon films were studied for a sample with transition from amorphous to fully microcrystalline structure using four excitation wavelengths (325, 514.5, 632.8 and 785 nm). Factor analysis showed the presence of two and only two spectrally independent components in the spectra within the range from 250 to 750 cm^?1 for all four excitation wavelengths. The 785 nm excitation was found optimal for crystallinity evaluation and by comparison with surface crystallinity obtained by atomic force microscopy, we have estimated the ratio of integrated Raman cross-sections of microcrystalline and amorphous silicon at this wavelength as y = 0.88 ± 0.05.     

Characterization of mixed phase silicon by Raman spectroscopy

We have examined the common methods for determination of the crystallinity of mixed phase silicon thin films from the TO–LO phonon band in Raman spectra. Spectra are decomposed into contributions of amorphous and crystalline phase and empirical formulas are used to obtain crystallinity either from the integral intensities (peak areas) or from magnitudes (peak maxima). Crystallinity values obtained from Raman spectra excited by Ar⁺ laser green line (514.5 nm) for a special sample with a profile of structure from amorphous to fully microcrystalline were compared with surface crystallinity obtained independently from atomic force microscopy (AFM). Analysis of the Raman collection depth in material composed of grains with absorption depth 1000 nm in an amorphous matrix (absorption depth 100 nm), was used to explain reasons for systematic difference between surface and Raman crystallinities. Recommendations are given for obtaining consistent results.     

Growth and characterization of lead-free Ba(1−x)CaxTi(1−y)ZryO3 single crystal

A lead-free Ba_(1?x)Ca_xTi_(1?y)Zr_yO₃ (BCZT) single crystal (x=0.08, y=0.26) was grown by the Czochralski (CZ) method in a mixed flux of TiO₂ and ZrO₂. The composition of as-grown BCZT was analyzed by electron probe micro-analysis. The structure, dielectric properties and phase transition were investigated at different temperatures. The X-ray diffraction results confirmed that the structure of the as-grown BCZT crystal was cubic both at 25 °C and 500 °C. The temperature dependence of the dielectric constant and Raman spectra characterization revealed that there was a phase transition from cubic to tetragonal, which happened between 200 K and 250 K. With increasing frequency, the Curie temperature shifted towards high temperature.     

Quantitative Raman spectroscopy: Principles and application to potassium silicate glasses

A mathematical approach was developed to interpret Raman spectra of binary silicate glasses and melts without the necessity of external calibration, e.g., from NMR spectroscopy. The developed approach is based on Principal Component Analysis (PCA), linear combinations of partial Raman spectra and a linear optimization technique. In order to apply and to test this approach, we developed an experimental method to collect a large number of Raman spectra efficiently. We applied the quantification and the experimental approaches to investigate potassium silicate glasses with compositions from 17.4 to 38 mol% K₂O. The equilibrium constant for the reaction 2Q³ ⇄ Q² + Q⁴ was found log K₃ = −2.37 ± 0.07, in excellent agreement with NMR studies for the same glasses.     

Quantitative Raman spectroscopy: High-temperature speciation of potassium silicate melts

In situ, high-temperature Raman spectroscopy was used to study the Qⁿ speciation in binary potassium silicate melts. Over 300 Raman spectra in the compositional range from 20 to 38 mol% K₂O were collected at temperatures between 800 and 1200 K. Quantitative information on the relative abundances of species in melts was obtained from the Raman spectra through a quantification procedure that does not require any a priori assumptions about the line shapes or external calibration of the Raman scattering efficiencies for the various Qⁿ species. The ΔH⁰ associated with the speciation reaction 2Q³ = Q⁴ + Q² was found to be 33.1 ± 7.3 kJ/mol.     

Microstructural analysis of Ga2S3–2MCl (M = K, Rb, Cs) glasses using Raman scattering

Raman scattering spectra of Ga₂S₃–2MCl (M = K, Rb, Cs) glasses have been conducted at room temperature. Based on the analysis of the local co-ordination surroundings of Cs⁺ ions, the similarities and differences of Raman spectra for the glass Ga₂S₃–2CsCl and the bridged molecular GaCl₃ were explained successfully. Through considering the effect of M⁺ ions on mixed anion units [GaS_4?xCl_x] and bridged units [Ga₂S_6?xCl_x] and the corresponding microstructural model, the Raman spectral evolution of the Ga₂S₃–2MCl (M = K, Rb, Cs) glasses was reasonably elucidated.     

Optical phonons and Raman scattering in ternary II–VI spheroidal nanocrystals embedded in a glass matrix

Theoretical and experimental studies of the spatial phonon confinement in ternary CdS_xSe_1−x nanocrystals embedded in a glass matrix formed by the composites (40)SiO₂₋(30)Na₂CO₃–(29)B₂O₃–(1)Al₂O₃ (mol%) + [(2)CdO + (2)S + (2)Se] (wt%) were carried out. From the analysis of the surface phonon modes, the theoretical procedure has allowed the determination of the geometrical characteristics of the nanocrystals. The calculated frequencies were compared with the experimental values obtained from the Raman spectra of CdS_xSe_1−x nanocrystals grown under different thermal treatments. A good correlation between the experimental and calculated CdS-like and CdSe-like surface optical modes was observed. The Raman selection rules and their connection with the nature of the surface optical phonons is discussed in order to use Raman spectroscopy as a probe to determine the composition x and the geometrical shape of the semiconductor nanocrystals.     

Preparation of mixed arsenic/antimony chalcogenide glasses and some optical and thermo-physical properties

The preparation of mixed glasses of As₂S_3−xSe_x (x = 0–3) and (1 − y) · As₂S₃y · Sb₂S₃ (y = 0–1) has been carried out by an in situ pouring technique. X-ray diffraction (XRD) was used to confirm the glassy nature of the materials and monitor devitrification. Visible-IR transmission, photoluminescence, refractive index and micro-Raman were measured as a function of composition. Microhardness (MH) and thermal expansion coefficient (TEC) were also measured. Raman peaks in As₂S₃ and As₂Se₃ were observed around 338 cm⁻¹ and 230 cm⁻¹, respectively in this first composition series in which S was replaced by Se. When As was replaced by Sb, in the case of second composition series, the As₂S₃ related Raman peak became broader and shifted to lower wave number, reflecting some structural change/devitrification. MH increased (1.31–1.50 GPa) with Se and Sb content while the TEC was found to decrease (2.5–1.4 × 10⁻⁵/K). The progressive increase in the content of either Se or Sb in As₂S₃ is anticipated to modify bond lengths and bond angles. The combined effect of these structural modifications would change the local structure of the glass forming a more rigid glass network thereby increasing the hardness and decreasing TEC.     

Optical band gap and Raman spectra in some (Bi2O3)x(WO3)y(TeO2)100−x−y and (PbO)x(WO3)y(TeO2)100−x−y glasses

The glasses representing (Bi₂O₃)_x(WO₃)_y(TeO₂)_100?x?y and (PbO)_x(WO₃)_y(TeO₂)_100?x?y systems were prepared. The dilatometric glass-transition temperatures of examined glass samples were found in the region 383–434 °C, the coefficient of thermal expansion varied from 12 to 16 ppm/°C and the density ranged from 6.30₂ to 6.80₈ g/cm³. From the optical transmission measurements of thin glassy bulk samples prepared by a glass blowing, the optical gap values were found in the narrow region 3.21–3.36 eV. For the temperature interval 300–480 K, the values of the temperature coefficient of the optical band gap varied from 3.7 × 10^?4 to 5.24 × 10^?4 eV/K. It is suggested that Raman feature observed at around 350 cm^?1 can be assigned to an overlap of Raman bands attributed to WO₆ corner shared octahedra and to the following three atomic linkages: Bi–O–Te, Pb–O–Te and W–O–Te.     

The preparation and structural studies in the (80 − x)TeO2–20ZnO–(x)Er2O3 glass system

Er³⁺-doped tellurite glasses of the (80 ? x)TeO₂–20ZnO–(x)Er₂O₃ system (0.5 mol% ? x ? 2.5 mol%) have successfully been made by melt-quenching technique and their structure has been investigated by means of DTA and Raman spectroscopy. The DTA results show the thermal parameters; such as the glass transition temperature (T_g) and crystallization temperature (T_c) were determined. It is found that this system provides a stable and wide glass formation range in which the glass stability around 99–140 °C may be obtained. The Raman spectroscopy used the structural studies in the glass system. Two Raman shift peaks were observed around 640–670 cm^?1 and 720–740 cm^?1, which correspond to the stretching vibration mode of TeO₄ tbp and TeO₃ tp, respectively. It is found that the spectral shift in Raman spectra is depending on the Er₂O₃ content. This evolution is an indication of the changes in the basic unit of the glass structure.     

Electrooptical Kerr phenomenon and Raman spectroscopy of one lithium–niobium–silicate glass-forming system

Raman spectra and electrooptical Kerr coefficients of glasses belonging to one lithium–niobate–silicate glass-forming system xNb₂O₅ · (66 ? x)SiO₂ · 19Li₂O · 11K₂O · 2B₂O₃ · 2CdO are studied. It has been found that these glasses demonstrate a record value of electrooptical Kerr coefficient; the glass with x = 35 showed electrooptical Kerr coefficient equal to 266 × 10^?16 m/V². Using Raman spectroscopy combined with the concept of Constant Stoichiometric Groupings, a correlation of electrooptical Kerr coefficients of these glasses with the content of Li₂O · Nb₂O₅ (or 2LiNbO₃) groupings has been demonstrated. The hypothesis that electrooptical Kerr sensitivity of glasses is related to the ordered regions with composition and symmetry corresponding to some of known electrooptical crystals has been verified. These regions, which the authors called ‘Crystal Motifs’, are identified with the groupings found in studying Raman spectra of the glasses.     

Dielectric low-k composite films based on PMMA,PVC and methylsiloxane-silica: Synthesis,characterization and electrical properties

This report describes the preparation of low-k inorganic–organic hybrid dielectric films, based on a polymethylmethacrylate–polyvinylchloride (PMMA–PVC) blend and a silica powder functionalized on the surface with methylsiloxane groups (m-SiO₂). By dispersing m-SiO₂ into a [(PMMA)_x(PVC)_y] 50/50 (x/y) wt% polymer blend, six [(PMMA)_x(PVC)_y]/(m-SiO₂)_z hybrid inorganic–organic materials were obtained, with z ranging from 0 to 38.3 wt% and x = y = (100 − z)/2. The transparent, homogeneous, crack-free films were obtained by a solvent casting process from a THF solution. The morphology, thermal stability and transitions of hybrid materials were studied by environmental scanning electron microscopy (ESEM), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). ESEM revealed that hybrid dielectric films are very homogeneous materials. The electrical response of the dielectric films was studied by detailed broadband dielectric spectroscopy (BDS). BDS measurements were performed at frequencies of 40 Hz to 10 MHz and a temperature range of 0–130°C. In these temperature and frequency ranges the proposed materials have a dielectric constant of <3.5 and a tan δ of <0.05. BDS also revealed molecular relaxation events in [(PMMA)_x(PVC)_y]/(m-SiO₂)_z materials as a function of temperature and sample composition. Results showed that these films with z in the range 25–35 wt% are very promising low-k dielectrics for applications in organic thin film transistor (OTFT) devices.     

Structural investigations of sodium diborate glasses containing PbO,Bi2O3 and TeO2: Elastic property measurements and spectroscopic studies

Pseudo-binary sodium borate glasses containing (1 − y)Na₂B₄O₇–yM_aO_b (where M_aO_b = PbO, Bi₂O₃ and TeO₂) (y = 0.25, 0.5, 0.67 and 0.79) have been investigated. Sound velocities (longitudinal and shear) have been measured at 10 MHz frequency using quartz transducers. Density increases with increase of y and the molar volume decreases. Sound velocities also decrease with increasing y till y ≈ 0.66 above which it increases slightly. Steeper decrease in velocities has been observed in TeO₂ containing glasses. Elastic moduli, Poisson’s ratio and Debye temperature have been calculated. Glass transition temperatures have also been determined and it decreases with increase of y. T_g also exhibits a dependence on the cationic charge in M_aO_b. Infrared spectra of the glasses reveal that the strong network consisting of diborate units is affected only by PbO and only very marginally by Bi₂O₃ and TeO₂. Only glasses with high concentrations of Bi₂O₃ and TeO₂ reveal the presence of mixed bridges such as Bi–O–B and Te–O–B. Consistent with the IR spectral observations, the N₄ values of ¹¹B MAS-NMR remain close to the ideal value of 0.5 of the diborate composition in most of the glasses. A structural model based on the observation that the diborate network is unaffected by Bi₂O₃ and TeO₂ where as PbO opens up and breaks the diborate units is shown to be consistent with all of the experimental observations including mechanical properties.     

Octahedral fluoride glasses: Raman spectra and structure of niobium pentafluoride

Raman spectroscopy is used to characterize the NbF₅ phases in the temperature range 80–500 K. A new clear glass is formed by quenching the melt to liquid nitrogen temperatures having a glass transition at ～206 K and crystallization at ～233 K. For all phases including the melt, the glass, the supercooled liquid, the crystalline solid and the gas, the Raman spectra show a rather common high frequency band at ～760 cm^?1 which is attributed to the Nb–F terminal frequency of partially bridged ‘NbF₆’ octahedra. Based on the systematics of the Raman spectra for all phases and the literature physicochemical data a model is proposed for the glass and the liquid phases where ‘NbF₆’ octahedral bridged in cis and/or trans configurations form a variety of cyclic and/or chain structures which intermix building up the overall structure. At exceptionally low energies (<11 cm^?1) a rather weak in intensity Boson peak is observed in the glass which shifts to even lower energies with increasing temperature. Librational and/or tortional motions of the bridged octahedra participating in the glass structure are possible candidates for the origin of this peak.     

Influence of structural disorder on the dynamics of mobile oxygen ions in Dy2(Ti1−yZry)2O7

We report on the influence of structural disorder on the oxide-ion conductivity of Dy₂(Ti_1−yZr_y)₂O₇ (y = 0.55 and 0.90). A significant disorder is induced by mechanical milling synthesis of the samples, and, depending on the Zr/Ti, a partial and progressive structural ordering can be achieved by subsequent annealing at temperatures between 800 and 1500 °C. Ionic conductivity is relatively high for both compositions (up to 10⁻⁴ S/cm at 900 K), and the activation energies for dc conductivity (in the range 1.02–1.32 eV) are found to be larger in samples with more structural disorder. This result is quantitatively explained, by using Ngai’s coupling model, in terms of the enhancement of interactions between mobile oxygen vacancies in a more disordered structure.     

Effect of Li2O on the heterogeneous structure of equi-molar lead borate glasses

Different glass samples in the Li₂O, PbO and B₂O₃ system have been prepared by melt quenching method. These glasses were classified in two groups such as 0.5 B₂O₃, (0.5 ? x) PbO, xLi₂O and (0.5 + y) B₂O₃; (0.25 ? y)PbO; 0.25Li₂O. The IR spectra almost show broad bands in the frequency range (800–1050) cm^?1 and (1100–1500) cm^?1, together with different weak bands over the range of investigation (2400–3000) cm^?1. The deconvolution analyses of these IR spectra reveals presence of multi structure arrangements from BO₄ and BO₃ groups, such as penta, tri, and diborates grouping together with meta; ortho borates as well as PbO_n groups. Partial replacement of PbO by Li₂O causes decrease in microhardness, a change which is attributed to the decrease in the concentration of ortho borate groups as is it revealed from the bands area analysis. The IR analysis shows also that the total concentration of (meta + ortho) borates is nearly constant while their individual concentrations is proportional depending on the relative concentration of PbO and Li₂O.     

Microstructures of high-growth-rate (up to 8.3 nm/s) microcrystalline silicon photovoltaic layers and their influence on the photovoltaic performance of thin-film solar cells

Microstructures of microcrystalline silicon (μc-Si) deposited at a high-growth-rate have been investigated in order to apply to the photovoltaic i-layer. μc-Si films were prepared by very-high-frequency (100 MHz) plasma-enhanced chemical vapor deposition at 180 °C. High growth rates of 3.3–8.3 nm/s have been achieved utilizing high deposition pressures up to 24 Torr and large input powers. Applying μc-Si to n–i–p junction solar cells, as the optimum result in this experimental series, a conversion efficiency of 6.30% (J_SC: 22.1 mA/cm², V_OC: 0.470 V, and FF: 60.7%) has been achieved employing the i-layer deposited at 8.1 nm/s. Raman scattering and X-ray diffraction measurements revealed the crystalline volume fraction of around 50% with the (2 2 0) crystallographic preferential orientation, respectively. The cross-sectional transmission electron microscope image shows densely columnar structure grown directly on the underlying n-layer. These structural features are basically in good agreement those of low-growth-rate μc-Si used for a high efficiency solar cell as previously reported, implying advantages of the use of high pressures with regard to providing the photovoltaic i-layers. Finally, the implication is discussed from the photovoltaic performance as a function of the crystalline volume fraction of i-layer, and current problems in improving the photovoltaic performance are extracted.     

Studies on crystal growth,optical and electrical characterization of pure and Dy-doped bismuth silicate single crystals

(0.5 mol%) Dysprosium (Dy) doped bismuth silicon oxide (BSO) single crystals were grown by the Czochralski technique under air atmosphere. Detailed analysis of Dy-doped BSO with pure BSO has been studied through optical analysis. The absorption edges of pure and Dy-doped BSO crystals are found to be 405 nm and 415 nm, respectively. The shift in the absorption edge is contributed to the defect centers created in the crystal with Dy-doping. The shifts observed in the Raman spectra on doping Dy are found to be lower, when compared with the pure BSO crystal. This effect can be correlated to the lattice distortion induced by the Dy doping. The oxide formation and intrinsic defects in the BSO crystal have been identified by photoluminescence analysis. Dielectric measurements reveal that higher permeability value in the BSO sample is due to the presence of charged defects, which can be related to the space charge polarization. There is a slight decrease in dielectric constant on doping with Dy. The piezoelectric value explains the defects formed in the crystal. On poling, d₃₃ value of BSO and Dy-doped BSO are 32 pC/N and 40 pC/N, respectively.     

Structure–property correlations in lead borate and borosilicate glasses doped with aluminum oxide

Glass samples from four systems: xPbO–(100?x)B₂O₃ (x = 30, 40, 50 and 60 mol%), 50PbO–yAl₂O₃–(50?y)B₂O₃ (y = 2, 4, 6, 8 mol%), 50PbO–ySiO₂–(50?y)B₂O₃ (y = 5, 10, 20, 30 mol%) and 50PbO–5SiO₂–yAl₂O₃–(45?y)B₂O₃ (y = 2, 4, 6, 8 mol%) were prepared by a melt-quench technique. Characterization of these systems was carried out using density measurements, UV–visible spectroscopy, differential scanning calorimetry (DSC), and ¹¹B and ²⁷Al magic-angle spinning (MAS) solid-state nuclear magnetic resonance (NMR). Our studies reveal an increase in glass density with increasing lead(II) oxide concentration in pure lead borates and also with addition of silica into 50PbO–50B₂O₃ glass. ¹¹B MAS NMR measurements determine that the fraction of tetrahedral borons (N₄) reaches a maximum for the glass containing 50 mol% of PbO in the PbO–B₂O₃ glass series and that N₄ is sharply reduced upon adding small amounts of Al₂O₃ into lead borate and lead borosilicate systems. ²⁷Al MAS NMR experiments performed on glasses doped with aluminum oxide show that the Al³⁺ are tetra-, penta- and hexa-coordinated with oxygen, even without any excess concentration of Al³⁺ over charge-balancing Pb²⁺ cations. ^[5]Al and ^[6]Al concentrations are found to have unusually high values of up to 30%. The results of UV–visible absorption spectroscopy, DSC and density measurements support the conclusions drawn from the NMR studies, providing a consistent picture of structure–property relations in these glass systems.     

Compositional dependence of optical parameters in Se–Bi–Te–Ag thin films

Compositional dependence of optical parameters in thermally evaporated amorphous Se_80.5Bi_1.5Te_18 ? yAg_y (for y = 0, 1.0, 1.5 and 2.0 at.%) quaternary thin films has been studied using well established Swanepoel method. The optical properties like, refractive index (n), extinction coefficient (k), absorption coefficient (α) and optical band gap (E_g) have been determined from the transmission spectra in the spectral range from 500 to 2500 nm. The optical band gap (E_g) has been estimated by using Tauc's extrapolation method and is found to increase with an increase in the Ag concentration. Present study shows that the refractive index, extinction coefficient and optical band gap increase with the increasing Ag content which is in agreement with the earlier studies. While the increase in the refractive index with Ag content over the entire spectral range can be attributed to the increased polarizability of larger Ag atomic radius (153 pm) compared to the Te atomic radius (135 pm), the increase in the optical band gap with increasing Ag concentration is correlated to an increase in the cohesive energy and decrease in the electronegativity of the films under study. The dielectric constant and optical conductivity (σ) of the thin films under study are also found to increase with the Ag concentration.