Research progress in electrophotography in the U.S.S.R. and Eastern Europe

A review of the latest investigations carried out in Bulgaria, East Germany, Poland, and the U.S.S.R. is presented. The discharge mechanisms both in dark and under illumination are being analyzed for two types of electrophotographic layers--with a binder and without it. In both cases, the discharge mechanism does not only depend on the layer type, its structure and technology, but also on the conditions of charging and discharging. It is shown that the photo-discharge mechanisms of selenium electrophotographic layers and of zinc oxide layers with binder differ in principle. The characteristics of As-Se electrophotographic layers and electrophotographic process with an effective quantum yield exceeding unity have been considered. Some works investigating the prospects of application of organic photoconductors in electrophotography have been reviewed. Information on the electrophotographic copiers and electrophotography for medical purposes is also provided.     

Defects in Hybrid Perovskites: The Secret of Efficient Charge Transport

The interaction of free carriers with defects and some critical defect properties are still unclear in methylammonium lead halide perovskites (MHPs). Here, a multi-method approach is used to quantify and characterize defects in single crystal MAPbI₃, giving a cross-checked overview of their properties. Time of flight current waveform spectroscopy reveals the interaction of carriers with five shallow and deep defects. Photo-Hall and thermoelectric effect spectroscopy assess the defect density, cross-section, and relative (to the valence band) energy. The detailed reconstruction of free carrier relaxation through Monte Carlo simulation allows for quantifying the lifetime, mobility, and diffusion length of holes and electrons separately. Here, it is demonstrated that the dominant part of defects releases free carriers after trapping; this happens without non-radiative recombination with consequent positive effects on the photoconversion and charge transport properties. On the other hand, shallow traps decrease drift mobility sensibly. The results are the key for the optimization of the charge transport properties and defects in MHP and contribute to the research aiming to improve perovskite stability. This study paves the way for doping and defect control, enhancing the scalability of perovskite devices with large diffusion lengths and lifetimes.     

Highly Uniform,Electroforming‐Free,and Self‐Rectifying Resistive Memory in the Pt/Ta2O5/HfO2‐x/TiN Structure

The development of a resistance switching (RS) memory cell that contains rectification functionality in itself, highly reproducible RS performance, and electroforming‐free characteristics is an impending task for the development of resistance switching random access memory. In this work, a two‐layered dielectric structure consisting of HfO₂ and Ta₂O₅ layers, which are in contact with the TiN and Pt electrode, is presented for achieving these tasks simultaneously in one sample configuration. The HfO₂ layer works as the resistance switching layer by trapping or detrapping of electronic carriers, whereas the Ta₂O₅ layer remains intact during the whole switching cycle, which provides the rectification. With the optimized structure and operation conditions for the given materials, excellent RS uniformity, electroforming‐free, and self‐rectifying functionality could be simultaneously achieved from the Pt/Ta₂O₅/HfO₂/TiN structure.     

Gain in electrophotography—I: Phototransistor configuration

The potential of the phototransistor or "hook" structure for realizing gain in the electrophotographic process is considered. By considering the phototransistor to be an emission-limited device, it can be shown that gain in excess of unity can only be produced in the external circuit. The charge necessary to produce the image is accumulated on an external capacitor whose size limits the gain to the ratio between the external and device capacitances. The predictions of this analysis are supported by experiments using a commercial phototransistor in simple circuits that simulate electrophotographic plates.     

Trapping and recombination processes via deep level T3 in semi-insulating gallium arsenide

Trapping and recombination of free carriers by deep level T₃ has been studied. Occupancy of the level by electrons and dynamics of its filling and emptying as a function of illumination with monoenergetic photons in 0.69–1.55 eV range has been monitored by the thermally stimulated currents method. We have found that level T₃ behaves more like a recombination center than like an ordinary electron trap. Besides trapping free electrons from conduction band, this trap can also communicate with valence band, trapping holes. The capture cross section for trapping a hole is estimated to be comparable or even larger than the capture cross section for trapping an electron. However, in many experimental conditions free electrons are generated more abundantly than free holes, and free carrier mobility and thermal velocity are both much higher for electrons than for holes. Therefore, electron trapping often prevails, so that this frequently detected defect, has been up to now most often perceived as a deep electron trap.     

Electrophotography—A review

This review is concerned primarily with the electrophotographic systems currently in use, i.e., the electrodeless stored energy systems. First, the history of reproduction is traced from printing, through wet- and dry-chemical photography, to electrophotographic systems. The electrodeless system is defined and described in terms of the processing. Then, the light sensitive materials and processing are examined, and the characteristics of the material comprising the electrophotographic layer are related to the operational steps of charging and exposure. The principal development materials, dry powder, and liquid toners, are described on an electrochemical basis, and the toning mechanisms are reviewed. Finally, the principal applications and future trends of electrophotographic systems are outlined.     

Cu掺杂对ZnO氧化物电子结构与电输运性能的影响

采用平面波超软赝势密度泛函理论计算的方法研究了p型Cu掺杂的纤锌矿结构氧化物ZnO的电子结构,在此基础上分析了其电输运性能。计算结果表明,Cu掺杂ZnO氧化物具有0.6eV的直接带隙,且为p型半导体,在导带和价带中都出现了由Cu电子能级形成的能带,体系费米能级附近的能带主要由Cup态、Cud态和Op态电子构成,且他们之间存在着强相互作用。电输运性能分析结果表明,Cu掺杂的ZnO氧化物价带中的载流子有效质量较大,导带中的载流子有效质量较小;其载流子输运主要由Cup态、Cud态、Op态电子完成,且需要载流子（空穴和电子）跃迁的能隙宽度较未掺杂的ZnO氧化物减小。     

InGaAs/GaAs单量子阱的瞬态光荧光特性(英文)

We used the time-resolved equipment consisting of a streak camera and normal photoluminescence set-up to measure the transcient behavior of the PL from the InGaAs/GaAs single quantum well sample, and thus obtained both the temporal resolution and the spectral resolution of the photoluminescence. A rapid decay of PL signal from the GaAs layer and the slow PL decay from the InGaAs well have beed found. From the experimetal results, the trapping efficiency of the carriers by the well is estimated to be about 80%.     

The exciton photoluminescence and vertical transport of photoinduced carriers in CdSe/CdMgSe superlattices

The photoluminescence and photoluminescence excitation spectra, phonon-related Raman scattering, and vertical transport of photoinduced carriers and excitons along the growth direction in type I lowstrained CdSe/CdMgSe superlattices, which are grown on InAs substrates using molecular-beam epitaxy, are studied for the first time. The studies are carried out at various temperatures and excitation intensities. The vertical transport is studied by a purely optical method involving an enlarged quantum well built in into the superlattice. This well serves as a sink for the excitons and charge carriers tunneled through the superlattice. At 2–150 K, the carriers are preferentially transported by free excitons. However, in superlattices with periods of 5.9 and 7.3 nm, this transport is not of the Bloch type. A comparison of the calculated energies of the band-to-band transitions in the superlattices with the experimental data yields the relative magnitude of the valence-band offset in the range 0.4–0.5. The Raman spectra indicate that the behavior of optical phonons in CdMgSe is bimodal.     

The camel diode as photodetector with high internal gain

The camel diode is a three layer majority carrier device with an interior layer sufficiently thin so that it is depleted of carriers at all values of bias voltage. The current flow is controlled by a potential barrier in the bulk of the semiconductor, the height of which can be controlled by free carriers trapped in the potential minimum. They will be generated optically within the space charge layer and the adjacent diffusion region. Basic theoretical considerations and first experimental results, which exhibit a gain of values of up to 500, are presented.     

Trapping of charge carriers into InAs/AlAs quantum dots at liquid-helium temperature

The problem of how the probability of trapping of charge carriers into quantum dots via the wetting layer influences the steady-state and time-dependent luminescence of the wetting layer and quantum dots excited via the matrix is analyzed in the context of some simple models. It is shown that the increase in the integrated steady-state luminescence intensity of quantum dots with increasing area fraction occupied by the quantum dots in the structure is indicative of the suppression of trapping of charge carriers from the wetting layer into the quantum dots. The same conclusion follows from the independent decays of the time-dependent luminescence signals from the wetting layer and quantum dots. The processes of trapping of charge carriers into the InAs quantum dots in the AlAs matrix at 5 K are studied experimentally by exploring the steady-state and time-dependent photoluminescence. A series of structures with different densities of quantum dots has been grown by molecular-beam epitaxy on a semi-insulating GaAs (001) substrate. It is found that the integrated photoluminescence intensity of quantum dots almost linearly increases with increasing area occupied with the quantum dots in the structure. It is also found that, after pulsed excitation, the photoluminescence intensity of the wetting layer decays more slowly than the photoluminescence intensity of the quantum dots. According to the analysis, these experimental observations suggest that trapping of excitons from the wetting layer into the InAs/AlAs quantum dots at 5 K is suppressed.     

Effects of DC power on the properties of DLC coatings on the OPC drum

Diamond-like carbon (DLC) films were deposited on the organic photoconductor (OPC) drum of the laser printer using the dc-remote plasma enhancement chemical vapor deposition (RPECVD) method. Only methane (CH₄) was used as a source gas for DLC coating. The transmittance of the DLC coatings is over 90% and the hardness of the DLC film (130 nm thick) coated OPC drum is higher than 1,000 H_v. Effects of dc-power on the physical properties of the DLC film and the electrophotographic properties of the DLC coated OPC drum were investigated. The hardness and the deposition rate of the DLC film deposited on the OPC drum increased but the transmittance and the surface roughness nearly did not change with increasing the dc power of the dc-RPECVD. The residual potential, the acceptance voltage, and the dark decay rate significantly increase but the photodischarge rate decreases with increasing the dc power of the RPECVD. The optimum dc power is 8.6 kW (950 V, 6A). The friction coefficient of the OPC drum is also lowered by DLC coating. In addition, it was found that the DLC deposition rate could be doubled by imposing magnetic fields around the cathode.     

Nonvolatile Floating‐Gate Memories Based on Stacked Black Phosphorus–Boron Nitride–MoS2 Heterostructures

Research on van der Waals heterostructures based on stacked 2D atomic crystals is intense due to their prominent properties and potential applications for flexible transparent electronics and optoelectronics. Here, nonvolatile memory devices based on floating‐gate field‐effect transistors that are stacked with 2D materials are reported, where few‐layer black phosphorus acts as channel layer, hexagonal boron nitride as tunnel barrier layer, and MoS₂ as charge trapping layer. Because of the ambipolar behavior of black phosphorus, electrons and holes can be stored in the MoS₂ charge trapping layer. The heterostructures exhibit remarkable erase/program ratio and endurance performance, and can be developed for high‐performance type‐switching memories and reconfigurable inverter logic circuits, indicating that it is promising for application in memory devices completely based on 2D atomic crystals.     

Electrophotographic process embedded in direct binary search

A novel halftoning approach that has embedded in it a model for the electrophotographic process is presented. Models for the laser beam, exposure of the organic photoconductor, and the resulting absorptance on the paper are embedded into the direct binary search (DBS) halftoning algorithm. The algorithm is applicable to any arbitrary pixel modulation scheme and is also highly portable between different electrophotographic print engines. Computational issues are addressed to make the approach viable. Results show good exploitation of pixel modulation and improvement over DBS with no printer model throughout most of the tone scale.     

Air Effect on the Ideality of p‐Type Organic Field‐Effect Transistors: A Double‐Edged Sword

Organic field‐effect transistors (OFETs) often deviate from ideal behaviors in air, which masks their intrinsic properties and thus significantly impedes their practical applications. A key issue of how the presence of air affects the ideality of OFETs has not yet been fully understood. It is revealed that air atmosphere may exert a double‐edged sword effect on the active semiconductor layer when determining the ideality of OFETs fabricated from p‐type crystalline organic semiconductors. Upon exposing the as‐fabricated device to air, water and oxygen mainly function as efficient p‐type dopants for the active layer in the contact regions, enhancing charge carrier injection and consequently improving device ideality. Nevertheless, as the exposure time increases, the trapping centers for the injected minority charge carriers appear in the channel region, leading to degradation of device ideality. Inspired by the double‐edged sword behavior of air, a near‐ideal OFET is achieved by ingeniously utilizing the doping/positive effect and eliminating the trapping/negative effect. The effect of air on the ideality of p‐type OFETs is clarified, which not only illuminates some common observations of OFETs in air but also offers useful guidance for the construction of high‐performance ideal OFETs.     

High-speed 1.3 μm GaInAs detectors fabricated on GaAs substrates

High-speed interdigitated metal-semiconductor-metal detectors have been fabricated on non-lattice-matched, semi-insulating, GaAs substrates using two GaInAs layers of differing indium concentrations to accommodate most of the lattice mismatch by interface misfit dislocations. Bandwidths as high as 3 GHz were measured with none of the detrimental low-frequency gain usually observed in this type of device. This is attributed to the inhibition of the surface trapping of photoinduced carriers by a graded pseudomorphic layer at the surface     

The transient response of insulated-gate field-effect transistors

The equations describing the current flow and charge distribution in insulated-gate field-effect transistors are developed. These include Ohm's law, the continuity equation and the trapping equation when it is assumed that only one trapping level is dominant. The boundary conditions at the source, at the drain and at the gate insulator-semiconductor surface are also included. The result is a set of non-linear partial differential equations, whose solution is obtained by numerical methods.

The important physical properties of the gate insulator-semiconductor surface are: the initial lifetime of free carriers, the lifetime of a carrier in a trap, the density of trap sites, the initial ratio of trapped carriers to trap sites and the carrier mobility. The important geometrical properties of the device are the gate insulator thickness and the channel length. Solutions for various values of these parameters are given.     

Carrier lifetime investigation in 4H–SiC grown by CVD and sublimation epitaxy

Depth-resolved carrier lifetime measurements were performed in low-doped epitaxial layers of 4H silicon carbide samples. The technique used was a pump-and-probe technique where carriers are excited by an above-bandgap laser pulse and probed by free carrier absorption. Results from chemical vapour deposition samples show that lifetimes as high as 2 μs may be observed in the mid-region of 40 μm thick epilayers. For epilayers grown by the sublimation method decay transients were characterised by a fast (few nanoseconds) initial recombination, tentatively assigned to the ‘true’ lifetime, whereas a slow tail of several hundred microsecond decay time was assigned to trapping centres. From the saturation of this level at increased pumping we could derive the trapping concentration and their depth distribution peaking at the epilayer/substrate interface.     

Transport of hot carriers in semiconductor quantized inversion layers

The transport of warm and hot carriers in quantized inversion layers has recently become of considerable interest, due in part to the quasi-two-dimensional nature of the carrier system and to the multitude of subbands present. Generally, the number of carriers in the inversion layer is sufficiently large that carrier-carrier scattering maintains a quasi-Maxwellian for the isotropic part of the distribution function, but the inter-subband interactions are sufficiently weak that each subband possesses a separate electron temperature. The treatment of carrier transport can be naturally separated into two regimes. In the first, the carriers are hot. In this regime, the transport can be found from energy and momentum balance equations and the transport differs little from a classical three-dimensional model, except in the field region in which inter-subband transfer of carriers is important. In this field range, subtle changes in the velocity-field curve are observed and significant effects are found in the microwave conductivity at frequencies on the order of the inter-subband repopulation rate. In the warm electron regime, however, for low and moderate electric fields, the degenerate nature of the carrier distribution function must be considered. Although the electron temperature concept remains valid in this regime, the agreement between theory and experiment is not good and the lack of this agreement makes it difficult to assess the physical processes occurring. The situation is complicated at low temperatures where many of the scattering mechanisms are not fully understood and the carrier densities and transport can show activation behavior. This lack of understanding is especially true in warm carrier magneto-transport. For this reason, care must be exercised in evaluating the role played by the electric field. In this paper, these various regimes are discussed and compared to the available experimental data.