Characterizations of a-Se based photodetectors using X-ray photoelectron spectroscopy and Raman spectroscopy

The ‘PLUMBICON’ was one of the first successful imaging tubes using amorphous selenium (a-Se) and many followed. Significant properties of a-Se based imaging tubes have been rediscovered through the invention of the ‘HARP (high-gain avalanche rushing amorphous photoconductor)’, but its operational mechanism and the physics, however are yet poorly described. Previously, we have fabricated photodetectors using nitrogen (N)-doped diamond as a cold cathode and a-Se as a photoconducting target, which successfully responded to light illumination, The device performance,in this case, deteriorates after continuous use largely due to the degradation of a-Se. In this paper, a-Se and amorphous arsenic selenide (a-As₂Se₃) films have been deposited Stoichiometry has been determined by XPS (X-ray photoelectron spectroscopy) followed by Raman spectroscopy characterization. We have found that even an extremely weak incident laser power causes sample degradation during signal accumulation. We speculate that either the incident laser itself and/or the temperature rise due to illumination causes the phase transition in a-Se films. In addition, when As is added into the film, the phase transition leading the degradation is hardly observed, implying that As affects the formation of crystalline Se making chemical bonds in the crystallographic network stronger.     

Chemical modification of the electrical behavior of a-Se and its alloys-xerographic studies

The extent to which the photoelectronic properties of amorphous chalcogenides can be varied by chemical modification remains a key issue in the study of this technologically important class of semiconductors. a-Se is a prototypical system whose elemental nature and ambipolarity make it particularly attractive for such a study. In the following, the effect of the equilibrium doping of a-Se with halogens and alkalis will be summarized. Doping effects on thermal generation and deep trapping have been examined using xerographic techniques. The effect of dopants on magnetic properties has been measured by electron spin resonance (esr). The experimental results are compared to predictions of recently proposed defect models.     

Computational and Raman studies of phospho-tellurite glasses as ultrabroad Raman gain media

Molecular orbital calculations of two phospho-tellurite model clusters were performed to clarify the origins of the Raman bands in the Stokes region of over 1000 cm^− 1 in phospho-tellurite glasses. The Raman bands could be attributed to two components of 900-1050 cm^− 1 of symmetrical stretching vibrations of PO₄ units and 1050-1200 cm^− 1 of anti-symmetrical stretching vibrations of PO₄ units. It was also clarified that the top of the valence band of phospho-tellurite glasses consists of the lone pair electrons in a TeO_4 + 1 unit and the bottom of the conduction band of the glass consists of the antibonding hybrids of Te 5p and O 2p orbitals in the equatorial plane of a TeO₄ unit.We have developed new phospho-tellurite glasses which have the Raman gain peak of 30 times as large as silica glass or the Raman gain bandwidth of more than 1200 cm^− 1.     

Raman spectroscopic studies of aluminium fluoroberyllate glasses

Aluminium fluoroberyllate glasses obtained in the vitreous domain from the quaternary system BaF₂CaF₂AlF₃BeF₂ have been studied by Raman spectroscopy. The observed bands have been assigned by comparison with those of cryolite molten aluminofluorides, and of various fluoroberyllates in the crystalline and molten state. The following species have been identified: (AlF₄)^? tetrahedral groups, (AlF₆)^?3 octahedral groups polymerized in chains, and a few polymerized species of beryllium fluoride and probably Be₂F₇^?3 dimers.     

Volume and enthalpy relaxation of a-Se in the glass transition region

Volume and enthalpy relaxation studies of amorphous Se have been performed in the glass transition region by mercury dilatometry and differential scanning calorimetry. For simple temperature jump experiments, as well as for more complex thermal history the volume and enthalpy relaxation data can be described by a single set of kinetic parameters for Tool-Naraynaswamy-Moynihan (TNM) model [Δh¹/R = 42.8 kK, ln(A_TNM/s) = ?133]. Slightly different non-linearity and non-exponentiality parameter were found for volume [x = 0.42, β = 0.58] and enthalpy [x = 0.52, β = 0.65] relaxation data. Similar results were obtained also for Adam-Gibbs-Scherer (AGS) model. The activation energy of viscous flow in the glass transition range is identical with the effective activation energy for relaxation process. A self-consistent data evaluation shows that at moderate departure from equilibrium, volume and enthalpy in amorphous selenium relax in the same way as expressed by TNM and AGS models. Both volume and enthalpy change can be interpreted within the same fictive temperature concept.     

Raman and luminescence studies of alkali borate tungstate glasses

Raman and luminescence spectroscopy were used to determine the structure of alkali borate tungstate glasses: M₂O(B₂O₃)₂·xWO₃, M = Li or Na (0 < x < 1). Raman scattering results showed the dominant tungstate species in these photochromic glasses to be tetrahedral WO₄⁼. At high concentrations of WO₃, WO₃·H₂O, and W₂O₇⁼ are also present. Luminescence measurements provided evidence for an octahedral WO₃ structure not identified by the Raman results. The results also revealed a possible change in the structure of the glasses similar to that observed in alkali borate glasses and associated with the “borate anomaly”. In addition, preliminary measurements are reported on the variation of the band gap, density, index of refraction, and the elastic coefficient C₁₁ determined by Brillouin scattering with composition.     

X-ray and Raman studies of single crystals of CsSbF6

Crystals of CsSbF₆ belong to the rhombohedral space groupR¯3-C _3i ² witha=7.904(1)andc=8.261(1) Å,V=446.95 ų,Z=3,D _c=4.11 gcm^–3. The antimony atom is surrounded by six fluorine atoms in a nearly perfect octahedral configuration with Sb-F 1.875(9) Å, while 12 fluorine atoms surround the cesium atom with closest contact 3.116 Å. Polarized Raman spectra of single crystals of CsSbF₆ have been obtained, and it is shown that these results can be interpreted in terms of a unimolecular rhombohedral structure. The small distortion from an octahedral arrangement for the SbF ₆ ^– group is clearly reflected in the spectra. The Raman results are in better agreement with the space groupR¯3m-D _3d ⁵ than withR¯3-C _3i ² , but this conclusion must be regarded with caution since the two features in the vibrational spectra of CsSbF₆ which can be used to distinguish between the two structures are weak and ill-defined.     

Raman scattering and electron spin resonance studies of fluorozirconate glasses

Electron spin resonance and Raman spectra are reported for fluorozirconate glasses which contain transition metal impurities. From the observed g values at 2.0 and a broad signal from 1 to 10, it is concluded that the dominant contribution to the ESR signal is due to Fe³⁺ in sites with rhombic distortion. The Raman spectra are shown to be superposed by luminescence emissions for paramagnetic impurity concentrations as low as ∼10¹⁶ spin/mol of ZrF₄. However, the amplitude and frequency of the dominant polarized Raman mode near 580 cm⁻¹ are observed to be relatively independent of impurities.     

Enthalpy and volume relaxation of PMMA, PC, and a-Se: evaluation of aging bulk moduli

Enthalpy, h, and volume, v, relaxation of poly(methyl methacrylate), PMMA, and polycarbonate, PC, were measured using differential scanning calorimetry and mercury-in-glass dilatometry. For PMMA both temperature down-jumps and up-jumps were studied. Attention was paid to the influence of cooling/heating rates on aging process with the result that it is related to the corresponding shift of glass transition temperature, T_g. From the time dependence of enthalpy, h, and volume, v, the aging modulus was defined as K_a = (∂h/∂v)_T, and was found to be ca. 2 GPa. This corresponds roughly to the inverse value of the compressibility, κ_l, in the vicinity of T_g. The results of the measurements on amorphous selenium supported the notion that K_a assumes values close to 1/κ_l. The measured K_a values were about 5 GPa, in fair agreement with compressibility data extracted from pvT measurements.     

Raman investigation of the drawing effects on Ge-doped fibers

We have investigated the Raman activity of various germanosilicate fibers and their associated preforms. Our data indicate an enhancement in small rings' (3-member rings) concentration in the silica-based matrix of the fibers during the drawing process. The generation of such rings appears compatible with an increase of the sample density and fictive temperature. The data regarding the drawing effects on the fiber stress appear less clear, and it is possible to suggest that in some cases the drawing could lower the tensile stress. Finally we have also provided evidence that changing the drawing conditions within the usual range of application leads to no significant changes of the matrix structure as can be observed using Raman spectroscopy.     

Raman spectroscopic studies of gadolinium trichloride as a function of temperature

Laser Raman spectrometry is being used in our laboratory to identify the crystal structures of lanthanide and actinide compounds. The phonon Raman spectrum of an ionic, crystalline sample is characteristic of the particular crystal structure(s) exhibited by the sample. GdCl₃ exhibits two crystal structures, UCl₃-type hexagonal and PuBr₃-type orthorhombic. In the literature it is proposed that the low-temperature form is the PuBr₃-type orthorhombic. Results of experiments in our laboratory suggest that the low-temperature form is the UCl₃-type hexagonal. Interconversion between these two forms can be accomplished with temperature. In the present work, laser Raman spectrometry was used to monitor crystal structure changes in GdCl₃ as a function of temperature and to determine the temperature at which the orthorhombic-to-hexagonal phase transformation occurs.     

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)     

Solid-state studies. Part XXIV: Raman spectral consequences of disorder in the structure of phase II of ammonium nitrate

The Raman spectra of a wide variety of isotopomeric polycrystalline samples of phase II of ammonium nitrate are explained on a model in which the disorder within the lattice is incorporated within factor-group correlations. This model also provides an explanation for the single crystal Raman data.     

β-Irradiation effects in Gd-doped borosilicate glasses studied by EPR and Raman spectroscopies

Modifications of Gd-environment in aluminoborosilicate glasses under irradiation have been studied by Raman and Electron Paramagnetic Resonance (EPR) spectroscopies. No detectable effects of Gd₂O₃ addition on the structural changes under irradiation previously analysed in pure aluminoborosilicate glasses are observed by Raman investigations. On the contrary, the EPR spectra of the studied samples reveal an increase of the content of Gd³⁺ diluted in network former position relative to Gd³⁺ diluted in network modifiers one under ionizing radiation. The evolution of the EPR lines appears to result from the reduction processes of Gd³⁺ into Gd²⁺ observed both for isolated gadolinium ions and for gadolinium contained in magnetic clusters.     

Raman study of fluorine effects on silica with embedded SnO2 nanoparticles

Silica samples with embedded SnO₂ nanoparticles have been produced by sol–gel method with a synthesis modified by the introduction of fluorinated silicon precursors. The structural features of silica network and SnO₂ nanophase have been mapped inside the samples by means of confocal micro-Raman scattering spectroscopy. The results show that the detection of residual fluorine into the matrix as Si–F groups is accompanied by network dehydration, by intensity depletion of the D₂ peak assigned to three-member rings of coordinated tetrahedra, and by SnO₂ nanoparticles with much larger sizes than in fluorine-free material.     

Sugar bioprotective effects on thermal denaturation of lysozyme: Insights from Raman scattering experiments and molecular dynamics simulation

The bioprotective properties of trehalose, sucrose and maltose on lysozyme protein are studied using Raman spectroscopy and molecular dynamics simulation. The temperature dependencies of the α-helical unfolding processes are determined in presence of different sugars and at various concentrations using the amide I band. The energies of stabilization and acting forces are discussed. The problem of the threshold sugar concentration needed to protect proteins is addressed. The results point out that the three sugars exhibit very similar bioprotective behaviors in the investigated temperature and concentration ranges and suggest that the difference between their respective effects arise mostly from the different denaturated states of lysozyme in the three sugar solutions. Indeed, trehalose gives rise to a significantly more important shift of the denaturation temperature T_m and of the Gibbs net free energy Δ(ΔG_ND) of stabilization of lysozyme with respect to the two other sugars.     

Probing the local structural changes across the amorphous to crystalline transition in iron phosphate glass: Positron annihilation and micro Raman studies

Iron phosphate glasses have been investigated using positron lifetime and micro Raman spectroscopy to study the crystallization kinetics and defect evolution across the glass and crystallization transitions. Positron lifetime parameters show distinct changes at temperatures corresponding to both the transitions. These changes have been understood as the signature of the evolution of open volume defects and their clustering across the amorphous to crystalline interfaces. Micro Raman measurements on annealed samples have identified the crystalline phases as Fe₃(P₂O₇)₂ and FePO₄. Ab-initio positron lifetime computations reveal that the positron probes the Fe/P sites in defect free crystals. The nature of open volume defects produced upon annealing is identified by computing positron lifetimes for various possible defect clusters. A combination of both experimental and theoretical techniques has provided insights in understanding the evolution of defects across the phase transition in these glasses.     

Effect of silver doping on the structure and phase separation of sulfur-rich As–S glasses: Raman and SEM studies

We report on the structural details and microphase separation of the bulk glasses Ag_x·(As₃₃S₆₇)_100-x for 0x25. Glass–glass phase separation occurs over a wide range of Ag content, i.e. 4x20. An off-resonant polarized Raman spectroscopic study has been carried out to elucidate structural aspects at the short- and medium-range structural order of the glasses. Analysis of Raman spectra revealed quantitative changes of the sulfur-rich microenvironments that reduce upon adding Ag. Scanning electron microscopy combined with X-rays microanalysis have been utilized to examine the type and extent of phase separation, and to provide quantitative details on the atomic concentrations in the Ag-poor and Ag-rich phases. It has been shown that at 7 at.% Ag the Ag-rich phase percolates through the structure; this effect can be associated with an ionic-to-superionic behavior of these glasses in accordance with similar studies on the stoichiometric arsenic sulfide glass; although the phase separation observed in the present glasses is qualitatively different.