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.     

Volume relaxation of a-Se studied by mercury dilatometry

Mercury dilatometry was used to study volume relaxation of amorphous selenium (a-Se). Influence of experimental setting and reactivity of a-Se with mercury was studied using two differently constructed dilatometers (dissimilar sensitivity of sample volume change and dissimilar values of characteristic time constants of the dilatometers). Three types of volume–time–temperature experiments were made. Temperature of glass transition and thermal expansion coefficients in glassy and liquid state were determined from non-isothermal experiments. Volume relaxation was studied mainly on the basis of isothermal experiments. It was found that classical isothermal relaxation experiments are strongly influenced by the loss of information at the experiment beginning due to the proceeding relaxation. On the other hand, more complex thermal history experiments seem to be less dependent on these procedural parameters and were considered as most suitable for studying structural relaxation. The reaction of a-Se and mercury was found not to be influencing the results of volume relaxation measurements.     

Glass transition temperature dependence on heating rate and on ageing for amorphous selenium films

The study, by differential thermal analysis (DTA) of the glass transition of thermally evaporated amorphous selenium, allows the effect of ageing on structural properties to be observed. An enthalpic diagram is used to explain the shift of the glass transition region, with heating rate, and also with ageing. These shifts are directly dependent on structural relaxation in amorphous selenium. The model used, to express the change of the structural time relaxation with temperature, and with the departure of thermodynamic equilibrium leads to a slightly different expression for the dependence of T_g on heating rate than that previously published (by Moynihan et al.).The activation enthalpies of the studied phenomena compared with other results, seem to show that structural relaxation in a-Se principally involves bond breaking and reformation of chains.     

Investigation of charge carrier lifetime in high-resistivity semiconductor layers by the method of small charge photocurrent

A method of determination of charge carrier lifetime from experimental data of small charge current transient under constant electric field strength is discussed. On the basis of experimental results on transient of small charge photocurrent of holes and electrons which were obtained at different electric field strengths, an analysis of charge carrier lifetimes in a-Se layer has been performed. For interpretation of experimental results the approximation, according to which photogenerated electrons or holes can be trapped in levels of three types, has been used. We found that the process of localized charge carrier release is influenced by the electric field strength and that in a-Se layer, in the vicinity of the substrate, there is a narrow region where the density of localized states exceeds its mean in a layer.     

Charge transport in chlorine doped amorphous Se:Te xerographic photoreceptor films

Time-of-flight drift mobility experiments were carried out on a-Se_1?xTe_x (x = 0-0.1) with chlorine as an additive up to 70 atomic parts per million to investigate the charge transport mechanism in these xerographically important photoreceptor films. Hole drift mobility-temperature data indicate that hole transport in a-Se:Te alloys is controlled by a relatively discrete set of Te-introduced shallow traps (probably Te₁^? centres) at ～ 0.43 eV above E_v whose concentration increases nearly linearly with Te addition. Chlorine doping generates an additional set of traps (probably Cl₀^? centres) around the same energy as Te-induced traps in a similar fashion to the effect of Cl addition to a-Se. Electron drift mobility-temperature data for a-Se:Te alloys containing no Cl can also be interpreted by assuming Te-introduced electron traps at ～ 0.49 below E_c. There was no electron transport observable in Cl-doped a-Se:Te alloys.     

Acoustic study of low-energy activation processes in amorphous Se,Ge and SeGe compounds

The study of the elastic properties of amorphous selenium over a wide range of frequency (30 kHz−100 MHz) reveal a single internal friction peak below the glass transition temperature.This one involves low activation energies ranging down to very small values and is reduced when the chains are crosslinked. In amorphous germanium, there is no evidence for the existence of activation processes exhibiting very small values. We suggest that the underlying microscopic mechanisms responsible for the peak in a-Se are closely related to the polymeric structure and so should exist in any amorphous polymers. We also suggest that the numerous other peaks existing in most polymers are not intrinsically related to structure in chains.     

Photodarkening of amorphous selenium

Various parameters associated with photodarkening of amorphous selenium have been measured at 100 K. The spectral response of photodarkening shows a step at the absorption edge indicating that electron-hole pair creation is a basic feature of the process. It is shown that the slope of the Urbach edge decreases during photodarkening, and it is found that the change in the optical joint-density-of-states peaks in the region of photon energy between 2.0 and 2.5 eV. A film of a-Se subjected to ion bombardment at 100 K does not then show photodarkening, and this and the other observations are discussed in terms of a model of local change in atomic arrangement.     

Medium range order in amorphous AsSe systems

The effects of photon and As on the crystallization of a-Se were investigated and the structure factors of a-As_xSe_1-x (0≤×≤0.36) systems were measured by neutron diffraction. We obtained the information about the medium range order (MRO) in these systems. The length of the MRO in these systems is 10～15A.     

Enthalpic structural relaxation in Te-Se glassy system

Differential scanning calorimetry (DSC) measurements were used to study enthalpic structural relaxation in the Te_ySe_(1−y) glassy system. The examined compositions were y = 0.1, 0.2 and 0.3. Single set of Tool-Narayanaswamy-Moynihan parameters was obtained from the curve-fitting procedure for each studied glass. A comparison with our previous measurements on a-Se was made and the development of particular TNM parameters with increasing Te content was discussed in terms of changes in molecular structure of the material. The curve-fitting results were further verified by several independent non-fitting methods.     

Xerographic spectroscopy of gap states in Se-rich amorphous semiconductors review

One of the most important parameters which determine the performance of many modern devices based on amorphous semiconductors is the drift mobility-lifetime product, μτ. There has been much interest in determination of charge-carrier ranges in amorphous semiconductors by various measurement techniques. Although the mobility, μ, can be measured by the conventional time-of-flight transient photoconductivity technique, the determination of the lifetime, τ, is often complicated by both experimental and theoretical limitations. The present article provides an overview of xerographic measurements as a tool for studying the electrical properties of amorphous semiconductors. First, details of the experimental set-up are discussed. Thereafter, the analysis and interpretation of dark discharge, the first cycle residual potential, cycled-up saturated residual potential are considered. It is shown that from such measurements the charge-carrier lifetime, τ, the range of the carriers, μτ, and the integrated concentration of deep traps in the mobility gap can be readily and accurately determined. Xerographic measurements on Se-rich amorphous photoconductors have indicated the presence of relatively narrow distribution of deep hole traps with integrated density of about 10¹³ cm^? 3. These states are located at ~ 0.85 eV from the valence band. A good correlation was observed between residual potential and the hole range, in agreement with the simple Warter expression. The capture radius is estimated to be r_c = 2–3 Å. Since r_c for pure a-Se and a-As_xSe_1 ? x is comparable to the Se–Se interatomic bond length in a-Se, it can be suggested that deep hole trapping centers in these chalcogenide semiconductors are neutral-looking defects, possibly of intimate valence-alternation pair (IVAP) in nature. The absence of any electron spin resonance signal (ESR) at room temperature seemed to be a strong argument in favor of this suggestion. Finally, photoinduced effects on xerographic parameters are discussed. It has been shown that photoexcitation of a-As_xSe_1 ? x amorphous films with band-gap light alters deep hole and electron states. During room-temperature annealing photosensitized states relax to equilibrium. Recovery process becomes slower with increasing As content. Qualitative explanation of the observed behavior may be based on associating the deep states with C₃⁺ and C₁^?intimate-valence-alternation–pair (IVAP) centers.     

多铁性材料的应变调控

多铁性材料是一种同时具有铁电、铁弹、铁磁等两种或者两种以上“铁性”的材料,可以通过多种序参量的耦合产生新的效应,在电子信息、传感、存储、无线网络等领域具备广阔的应用前景。当前在室温下具有强磁电耦合效应的多铁性材料仍然是学者们研究的重点,但基于多铁材料的器件还没有实现应用。应变工程是一种可以有效影响多铁材料物理性质的调控手段,通过晶格与电子、自旋、轨道等的相互作用来影响材料的电、磁、光、声等物理特性,因此通过应变调控多铁性薄膜结构和性能,受到了研究人员的广泛关注。本文通过调研多铁性材料中应变工程的研究,总结了应变调控手段及其对材料物理性能的影响,期望为多铁性材料的研究和发展提供研究思路。     

Infrared spectra of amorphous thallium-selenium alloys

Amorphous thallium-selenium alloys (a-Tl_xSe_1?x) have been studied by Fourier transform infrared spectroscopy within the composition range 0 ? x ? 0.53. Differential scanning calorimetry measurements were used to check the selenium content and the homogeneity of the samples. For the stoichiometric composition a-TlSe there is strong evidence of short-range order similar to that found in the crystalline phase, c-TlSe. The spectra of both amorphous and crystalline TlSe display features associated with bond-bending and bond-stretching modes of distorted tetrahedra at ～97 cm^?1 and ～163 cm^?1, reflecting the similarity of tetrahedral coordination within the [(TlSe₂)^?1]n chains. At the Se-rich side with x ? 0.24, a superposition of typical infrared bands of a-TlSe and amorphous selenium (a-Se) is observed. The results are discussed in terms of a structural model for a-Tl_xSe_1?x.     

Effect of isothermal annealing and visible-light illumination on the AC-impedance behavior of undoped selenium thin films

The effect of post-deposition isothermal annealing (30 °C ? T_A ? 70 °C) and visible-light illumination on the complex AC-impedance of undoped selenium thin films deposited at the substrate temperatures T_S = 30, 50, 70 °C has been studied in the frequency range 0.2–12 kHz. The AC-impedance of amorphous selenium (a-Se) films (T_S, T_A < 50 °C) was mainly capacitive, with no loss peaks being observed in their Z″(ω)–ω curves, irrespective of illumination. This behavior was ascribed to a dominant charge-carrier trapping effect of bulk/surface charged defects usually present in a-Se. On the other hand, the measured Z″(ω)–Z′(ω) diagrams of illuminated polycrystalline Se samples (50 °C ? T_S, T_A ? 70 °C) exhibited almost full semicircles, whereas their Z″(ω)–ω curves revealed prominent loss peaks at well-defined frequencies. As the annealing temperature or light intensity is increased the loci of the points determined by intersections of these semicircles with the Z′-axis at the low-frequency side shift greatly towards the origin, while the loss-peak positions shift to higher frequencies. These experimental findings were explained in terms of a significant increase in electrical conductivity of selenium films due to thermally-induced crystallization at temperatures beyond glass-transformation region of undoped selenium and to creation of electron–hole pairs by visible-light illumination.     

Structural and optical properties of TiO2:SnO2 thin films prepared by sol gel method

ABSTRACT

TiO₂:SnO₂ thin films were deposited on glass substrates, by using sol gel spin coating method with different ratio (3%, 5% and 7%) at 3200 rpm, to study their effect on different properties of TiO₂: SnO₂ thin films. The structural and optical properties of films have studied for different ratio. These deposited films have been characterized by various methods such as X-Ray Diffraction (XRD), Ultra Visible spectroscopy. The (XRD) can be used to identify crystal structure of as deposited films. The Transmission spectra have shown the transparent and opaque parts in the visible and UV wavelengths.     

A computational study of elastic properties of disordered systems with voids

We present a computational study of elastic properties of disordered systems with voids. The influence of hydrogen and voids on the elastic properties has been investigated by means of atomistic simulations using empirical potentials for hydrogenated amorphous silicon. The elastic constants have been obtained from the fluctuations of the simulation cell in Monte Carlo simulations at constant pressure and temperature. Our results indicate that the softening of the elastic constants of a-Si:H observed experimentally upon increasing hydrogen content cannot be explained by a weakening of the network induced by the reduced coordination but should be attributed instead to the formation of voids. A simple relation between Young’s modulus and the density is presented.     

Photo-crystallization in a-Se imaging targets: Raman studies of competing effects

Photo-induced crystallization of a-Se is investigated by Raman spectroscopy as a function of temperature (250–340 K) and exposure time in thin-film structures used as targets in high-gain avalanche rushing photoconductor (HARP) video cameras. The Stokes-to-Antistokes ratio is monitored to obtain the local temperature T_loc at the laser spot; fluxes (632 nm) of 17 and 10 W/cm² are used. We find a rich temperature behavior that reflects the competition of changes in viscosity and strain, and defines four distinct regimes. No photo-crystallization is seen for T_loc below 260 K, nor in a 15 K range around T_g ～ 310 K. For T_loc in the regime 260–302 K the initial rate of crystal growth after onset of photo-crystallization is temperature independent, whereas for T_loc > 318 K the growth rate is thermally enhanced. Our results are in qualitative accord with a theory by Stephens treating the effects of local strain on the secondary growth of crystalline nuclei in a-Se. We conclude that the observed growth rate between 260 and 302 K is driven by local strain, and that relaxation of this strain near T_g suppresses crystal growth until thermally assisted processes accelerate the photo-crystallization at higher temperatures.     

Detailed study of surface and interface properties of μc-Si films

The properties of surfaces and interfaces of microcrystalline films deposited by radio-frequency plasma enhanced chemical vapor deposition (RF-PECVD) were studied by spectroscopic ellipsometry. The effect of the low-power reactive ion etching (RIE) on the properties of the films was investigated. The surface properties could be effectively improved using RIE to eliminate of the top porous part of the films, without deterioration of bulk layer properties. Ellipsometric measurements from both film and substrate sides were used for the study of the interface properties of the various samples deposited on fused silica substrates. We show that the crystalline fraction that determined from modeling of ellipsometric spectra measured from the film side could be overestimated.     

Electrooptical Properties of Triglycine Selenate

Materials having noticeable electrooptical properties can be used in different mechanisms for modulation and scanning of light. In this connection a detailed study of the electrooptical effect (EOE) nature and the detection of new materials having electrooptical properties are of great interest. In spite of the fact that the electrooptical properties of triglycine sulphate (TGS) were the subject of research in a number of papers, EOE in isomorphic compounds of TGS has not been studied up till now. The present paper deals with the investigation of the electrooptical effect features in triglycine selenate (TGSe) which represent one of TGS crystallochemical analogues. EOE in TGSe was observed at different directions of the applied electric field: along the ferroelectric axis and perpendicular to it — along OZ. Both longitudinal and transverse electrooptical effects occur in both cases. The features of the temperature variation of longitudinal EOE in TGSe are also considered in this paper.     

Dielectric properties of natural rubber/chitosan blends: Effects of blend ratio and compatibilization

The electrical properties of blends from natural rubber and chitosan have been studied in the frequency range of 10²-10⁶ Hz. Blending of chitosan with a non-polar polymer such as natural rubber is expected to improve the dielectric performance of chitosan, by reducing the polarity. This paper presents the results of dielectric properties of natural rubber/chitosan blends. Special attention has been paid to analyze the effect of blend composition, vulcanization of the major phase and the influence of compatibilization using maleic anhydride on the dielectric properties of natural rubber/chitosan blends. The temperature dependence of dielectric properties on the blends has also been studied.