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.     

Relaxation processes in disordered heterogeneous dielectrics: Statistical mixtures

It is found that the average relaxation frequency of dielectric losses (conductivity) of statistical mixtures significantly (in some cases by several orders of magnitude) exceeds the permittivity relaxation frequency. It is also shown that the permittivity and conductivity undergo a percolation increase at different concentrations of the components corresponding to the dielectric-conductor and conductor-dielectric transitions, respectively.     

The localization of electrons in amorphous semiconductors: A twenty-first century perspective

This review covers those consequences of the localization of electronic states that appear to be universal features embracing all amorphous semiconducting materials where the electron–lattice interaction can be neglected, such as hydrogenated amorphous silicon. Several experimental measurements of these features are described. The role of strong electron–lattice interactions in some amorphous semiconducting systems, such as many chalcogenide glasses, is also discussed. In these systems, the electron–lattice interaction is so strong that it more than offsets the coulomb repulsion needed to put two electrons in the same energy state. Some experimental consequences of these so-called negative-U_eff systems are described. In addition, some universal features of metastable excitations for systems with both weak and strong electron–lattice interactions are discussed in the light of some recent experimental results.     

III-V compound semiconductors: Growth and structures

The semiconductors formed from group 13 metals and from group 15 anions, referred to as the III-V semiconductors, have found use in a broad range of technologies. Their versatility arises from the wide range of optical and electronic properties accessed through the formation of multi-component alloys. These alloys can be synthesized using the epitaxial growth techniques for devices consisting of several-to-hundreds of highly controlled individual layers monolithically formed into a nearly defect-free structure. This ability to design and fabricate such detailed structures, whose dimensions can be at the nanometer scale, has been driven by an understanding of the crystal growth and materials technology. The paper introduces key features of these materials, their materials science and crystal growth.     

Elliptical birefringence: New interpretation and experiments on BSO crystals

An elliptical birefringence can be decomposed in two successive operators using equivalent rotatory power and birefringence which are different from the physical ones. These equivalent operators are determined here, with application to the Bi₁₂SiO_2O crystal. Important results are derived for the electro-optic effect in thick samples.     

Absorption currents and steady currents in polymer dielectrics

It is shown that published experimental data on absorption currents in polymers (mainly polyethylene terephthalate) are consistent with the Walden model of injection, followed by build up of trapped space charge. The published results do not agree with the tunnelling process, dipolar relaxation or electrode polarisation. Detailed analysis indicates that the injection efficiency rises sharply with field, and that the observed current-time plots arise from the change in field at the injecting electrode, rather than from a modification of the barrier shape.It follows that the knee in the current-time plot does not reflect a transit time, but is a measure of t₀, a complex function of field and injection mechanism. It also follows that the steady-state currents are controlled by the transport mechanism in the bulk.     

A model of photostructural changes in chalcogenide vitreous semiconductors: 1. Theoretical considerations

Light-induced processes in chalcogenide vitreous semiconductors are considered in the framework of a configurational model of two stable states (ground and metastable) of atomic units with optical and thermal transitions between these states. Probabilities of these transitions as functions of photon energy and temperature are calculated. an expression for the metastable state population kinetics is given.     

Advances in gel growth: A review

The enormous literature on crystal growth in gels have been categorised into four different methods which have been reviewed and dealt with here. The various variants and modifications in the growth equipment and growth procedure have been critically examined. Several problems for future development of the gel technique have been suggested at occasional places in this review.     

Conducting filaments and switching phenomena in chalcogenide semiconductors

The sizes of the conducting filaments formed after switching operations in chalcogenide semiconductors have been measured as functions of input power in the on-state and specimen thickness using a scanning electron microscope. The experimental results show that the threshold switching involves double injection from both metallic contacts in the sandwich structure, and the combination of the electrothermal and the electronic processes. The conducting filament responsible for the switching consists of two permanent portions, one started from the anode and the other from the cathode, which have undergone a permanent change in material composition and structure after even one switching operation; and one temporary portion between the ends of the two permanent portions, which has not undergone any change in material composition or structure after many switching operations. The size of each portion depends strongly on the current level in the on-state, at which the filament is formed.     

Electronic-thermal switching and memory in chalcogenide glassy semiconductors

A constant value of the activation energy of direct current conductivity in chalcogenide glassy semiconductors is a strong evidence of the existence of negative-U centers in these materials. Multiphonon tunnel ionization of negative-U centers in strong electric fields is the cause of current-voltage characteristic nonlinearity. Taking into account the Joule heating allows to obtain electronic-thermal instability and then form an S-shaped current-voltage characteristic. The model described in this paper is in good agreement with the available experimental data on chalcogenide glassy semiconductors.     

Internal residual stresses in glass-ceramics: A review

Internal residual stresses arise in glass-ceramics upon cooling down from the crystallization temperature. These stresses are due to the thermal expansion and the elastic mismatch between the crystalline and glassy phases. Therefore, the mechanical properties of glass-ceramics are likely to depend not only on their composition and microstructure but also on the type (tension or compression) and magnitude of these residual stresses. In this work, we critically review the most commonly used theoretical models concerning residual stresses in glass-ceramics and glass-matrix composites, taking into consideration the effects of crystallized volume fraction, crystal shape and thermal expansion anisotropy. We also discuss most of the reported measurements of residual stresses in these dual-phase materials using different techniques, such as X-ray diffraction, nuclear magnetic resonance, Raman and fluorescence spectroscopy, and indentation. The available models and experimental results regarding spontaneous microcracking due to residual stresses are also discussed. Finally, guidelines for future work are suggested.     

Diffuse-dynamic multiparameter diffractometry: A review

The results reported at the Conference on Application of X-Rays, Synchrotron Radiation, Neutrons, and Electrons in Nano-, Bio-, Information-, and Cognitive Technologies (RSNE-NBIC 2009) are briefly reviewed. This review is based on a cycle of studies [1–6] where a new method for studying the structure of real crystals—diffuse-dynamic multiparameter diffractometry (DDMD)—was proposed and substantiated.     

A note on the temperature dependence of the electrical conductivity in amorphous semiconductors

The conductivity in many amorphous materials obeys the intrinsic semiconductor relation σ = σ₀ exp( − E_σ/ kT). This note discusses the effects that various assumptions about the temperature dependence of carrier mobility have on the relationship between the experimentally-derived value of σ₀ and the carrier mobility at a given temperature.     

Ferroelectricity and antiferroelectricity in CuCrO2-type multiferroic semiconductors

Multiferroic semiconductors of the CuCrO₂ type are considered. These materials, in the presence of free charge carriers, allow for the existence of opposite domains (in the ferroelectric FE _d phase), along with ordinary 180° domains (in the FE phase). The magnetization phase transition in a chiral multiferroic, allowing for piezoelectric effects in an antiferromagnet with a layered triangular structure which result in an incommensurate (helicoidal) spin structure, is phenomenologically described. The behavior of the ferroelectric polarization in the considered phases is characterized. The antiferroelectric is considered a system of parallel layers alternating in chirality sign with oppositely directed polarization vectors. The possibility of antiferroelectric phase transition due to the specific features of the dipole-dipole interaction is discussed.     

Force constant scaling and interatomic potential in tetrahedral semiconductors

The force constants of tetrahedral semiconductors derived from lattice vibration data are analysed in terms of a generalized Morse potential and of a potential with power laws of the interatomic distance for the repulsive and attractive term. It is found that the Morse potential is an appropriate model for describing the dependence of the force constants on the bond length in the elemental, A^IIIB^V, A^IIB^VI and A^IB^IIIC semiconductors.