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Stabilized amorphous selenium (a-Se) is one of the x-ray photoconductors that is currently used in recently developed direct conversion flat panel x-ray image detectors. We have studied the recombination of free holes with trapped electrons in stabilized a-Se. Electrons were deeply trapped in a-Se by carrying out repetitive electron time-of-flight (TOF) transient photoconductivity experiments. By using conventional and interrupted field hole time-of-flight (IFTOF) transient photoconductivity techniques in a TOF, IFTOF, TOF sequence, we were able to develop a technique that allows the measurement of the capture coefficient between free holes and trapped electrons. We find that the capture process of holes by trapped electrons closely follows the Langevin recombination mechanism.

Although amorphous selenium (a-Se) has a long and successful history of application in optical and X-ray imaging, some of its fundamental properties are still puzzling. In particularly, the mechanism of carrier recombination following x-ray excitation and electric field and temperature dependences of the electron-hole pair creation energy (Wehp) remain unclear. Using the combination of X-ray photocurrent and pulse height spectroscopy measurements, we measure Wehp in a wide range of temperatures (218–320 K) and electric fields (10–100 V/µm) and show that the conventional columnar recombination model which assumes Langevin recombination within a column (a primary electron track) fails to explain experimental results in a wide range of electric fields and temperatures. The reason for the failure of the conventional model is revealed in this work, and the theory of the columnar recombination is modified to include the saturation of the recombination rate at high electric field in order to account for the expe...

Theoretical reasoning suggests that direct conversion digital x-ray detectors based upon photoconductive amorphous-selenium (a-Se) could attain very high values of the MTF (modulation transfer function) at spatial frequencies well beyond 20 cycles mm(-1). One of the fundamental factors affecting resolution loss, particularly at x-ray energies just above the K-edge of selenium (12.66 keV), is the K-fluorescence reabsorption mechanism, wherein energy can be deposited in the detector at locations laterally displaced from the initial x-ray interaction site. This paper compares measured MTF changes above and below the Se K-edge of a CCD based a-Se x-ray detector with theoretical expectations.A prototype 25 μm sampling pitch (Nyquist frequency = 20 cycles mm(-1), 200 μm thick a-Se layer based x-ray detector, utilizing a specialized CCD readout device (200 × 400 area array), was used to make edge images with monochromatic x-rays above and below the K-edge of Se. A vacuum double crystal monochromator, exposed to polychromatic x-rays from a synchrotron, formed the monochromatic x-ray source. The monochromaticity of the x-rays was 99% or better. The presampling MTF was determined using the slanted edge method. The theory modeling the MTF performance of the detector includes the basic x-ray interaction physics in the a-Se layer as well as effects related to the operation of the CCD and charge trapping at a blocking layer present at the CCD/a-Se interface.The MTF performance of the prototype a-Se CCD was reduced from the theoretical value prescribed by the basic Se x-ray interaction physics, principally by the presence of a blocking layer. Nevertheless, the K-fluorescence reduction in the MTF was observed, approximately as predicted by theory. For the CCD prototype detector, at five cycles mm(-1), there was a 14% reduction of the MTF, from a value of 0.7 below the K-edge of Se, to 0.6 just above the K-edge.The MTF of an a-Se x-ray detector has been measured using monochromatic x-rays above and below the K-edge of selenium. The MTF is poorer above the K-edge by an amount consistent with theoretical expectations.

The attenuation of ultrasonic waves at 117 kHz was investigated as a means of measuring the water content of a highly porous ceramic medium in order to evaluate indirectly the water suction of soils. A porous ceramic disc, with a porosity of about 66%, was prepared whose volumetric water content was related to matric suction. Two PZT transducers were attached to opposite ends of the porous ceramic disc to monitor the attenuation of ultrasonic waves. The output voltage from the receiving transducer was correlated to the water content of the ceramic and hence the matric suction. The present ultrasonic technique was suitable for matric suctions above ∼50 kPa. Although the output voltage versus water content relationship was non-linear it could, nevertheless, be represented with reasonable accuracy by a logarithmic-parabolic calibration curve which was independent of the excitation voltage. The main disadvantage of the technique, however, is that the calibration curve depends on the salt content of the water in the ceramic. On the other hand, when the ultrasonic method is used in conjunction with other techniques it provides a further means of characterizing matric suction.

In conventional time-of-flight (TOF) transient photoconductivily experiments, a high resistivity semiconductor device is biased and thin sheet of charge carriers is photoinjected at one end of the sample by a short duration light pulse. The drift of the injected charge sheet across the device under the action of the internal field gives rise to a photocurrent which is readily measured in the external circuit. The interrupted field TOF (IFTOF) technique is a powerful tool for studying the interaction of the photoinjected carriers with traps and recombination centres in the material by momentarily stopping the charge sheet anywhere in the sample. Although the IFTOF technique has a number of distinct advantages for studying charge trapping and release processes in high resistivity solids, it has generally not been adopted as a convenient experiment since the sample capacitance results in large displacement currents at switching. An experimental apparatus is described that eliminates displacement current effe...

The decay of the surface potential on a charged insulating film, which is inhomogeneous across its thickness, is theoretically analysed by considering the injection of surface charges into the bulk and their subsequent transport through the film towards the grounded substrate. The present analysis models xerographic discharge that involves unipolar transport. It is assumed that during transport the carriers interact with a monoenergetic set of traps from which there may be release. A second-order partial differential equation is derived for the electric displacement D(x,t) which describes the time evolution of D at any point in the sample. This partial differential equation is numerically solved for the xerographic time-of-flight (XTOF) condition which requires the injection of a small fraction of the surface charges into the film by pulsed photoinjection. The sample is assumed to be composed of a double-layer photoreceptor type device where the first layer has a lower drift mobility than the second layer. The drift mobility, trapping and release times as well as the dielectric constant vary across the interface from one layer to the other. The numerical calculations provide insight into the interpretation of XTOF signals obtained from double-layer devices and demonstrate that the main peak in the photocurrent is due to prolonged injection into the second layer as a result of dispersion in the first layer.

Results are presented on the evaluation of thermal fatigue in three fiber reinforced polymer composites, using ultrasonic techniques and scanning electron microscopy. The composites examined were (a) continuous carbon fibers in a vinylester matrix (b) continuous aramid fibers in a vinylester matrix and (c) randomly oriented aramid fibers in a polyphenylene matrix. Specimens of these composites were subjected to thermal fatigue by thermal cycling from −25°C to 75°C. Changes in ultrasonic attenuation and velocity were monitored during thermal cycling, and scanning electron microscopy was used to qualitatively evaluate any damage. It was observed that ultrasonic attenuation is sensitive to thermal fatigue, increasing with increasing number of thermal cycles. SEM evaluations showed that the primary damage due to thermal fatigue is due to fiber-matrix debonding.

The effects of charge carrier trapping (i.e., incomplete charge collection) on the detective quantum efficiency (DQE) of a photoconductive detector are studied by using a cascaded linear system model. The model includes signal and noise propagations in the following stages: (1) x-ray attenuation, (2) conversion gain, (3) charge collection, and (4) the addition of electronic noise. We examine and discuss the DQE of an a-Se fluoroscopic detector as a function of charge transport parameters and x-ray exposure. The theoretical model shows a very good agreement with experimental DQE versus exposure characteristics.

The conductivity σ of amorphous In2O3−x depends sensitively on the oxygen concentration; σ increases with lower oxygen concentration (increasing x) and reversibly decreases as oxygen is added. Illumination with ultraviolet light releases oxygen and exposure to air reoxidizes causing σ to vary by many orders of magnitude. At a sufficiently large value of x, the material undergoes an insulator-metal transition. We measured the spectrum of conductance fluctuations in thin films of amorphous In2O3−x as a function of oxygen concentration. Between 5 Hz and 10 kHz, the spectra fit well to a 1/fα power law. For small x, the magnitude of the noise is unusually large, and α is near 1.0. As oxygen is removed the noise at first increases and then decreases by a factor of 500 as the material enters the metallic state. At the point where the noise begins to decrease, α jumps to about 1.12.

A generalized expression for charge carrier transport and absorption-limited sensitivity of x-ray photoconductors is derived by analytically solving the continuity equation for both holes and electrons considering the drift of electrons and holes in the presence of deep traps. The normalized sensitivity equation is applied to stabilized a-Se and HgI2. In the latter case, the sensitivity model is fitted to published data to determine the electron and hole ranges (6.4×10−6 cm2/V and 7×10−8 cm2/V, respectively) in screen-printed polycrystalline HgI2.

We report on the excess, low-frequency noise in pin-like amorphous selenium alloy structures that are used in direct-conversion x-ray imaging detectors. These are the first measurements of the noise power density spectrum in these structures under reverse bias. Of the two samples measured, one has a noise spectrum that fits well to a 1=fα power law with α near one. The 1=f noise is not atypical except for a nonlinear dependence on d.c. current at fields above 5 Vµm. The variance in correlated double sampling measurements of the noise signal is calculated and related to the 1=f noise spectrum. The other sample has a white noise spectrum down to 10−2 Hz. The white noise of the second sample is larger than the 1=f noise and is likely masking the 1=f noise over the measured frequency range. The origin of the white noise is not known.

Digital imaging systems for medical applications use amorphous silicon thin-film transistor (TFT) technology due to its ability to be manufactured over large areas. However, TFT technology is far inferior to crystalline silicon CMOS technology in terms of the speed, stability, noise susceptibility, and feature size. This work investigates the feasibility of integrating an imaging array fabricated in CMOS technology with an a-Se detector. The design of a CMOS passive pixel sensor (PPS) array is presented, in addition to how an 8×8 PPS array is integrated with the 75 micron thick stabilized amorphous selenium detector. A non-linear increase in the dark current of 200 pA, 500 pA and 2 nA is observed with 0.27, 0.67 and 1.33 V/micron electric field respectively, which shows a successful integration of selenium layer with the CMOS array. Results also show that the integrated Selenium-CMOS PPS array has good responsivity to optical light and X-rays, leaving the door open for further research on implementing CMOS imaging architectures going forward. Demonstrating that the PPS chips using CMOS technology can use a-Se as a detector is thus the first step in a promising path of research, which should yield substantial and exciting results for the field. Though area may still prove challenging, larger CMOS wafers can be manufactured and tiled to allow for a large enough size for certain diagnostic imaging applications and potentially even large area applications like digital mammography.

We have investigated the dark current, optical TOF (time of flight) properties, and the X-ray response of amorphousselenium (a-Se)/crystalline-silicon (c-Si) heterostructures for application in digital radiography. The structures have been studied to determine if an x-ray generated electron signal, created in an a-Se layer, could be directly transferred to a c-Si based readout device such as a back-thinned CCD (charge coupled device). A simple first order band-theory of the structure indicates that x-ray generated electrons should transfer from the a-Se to the c-Si, while hole transfer from p-doped c-Si to the a-Se should be blocked, permitting a low dark signal as required. The structures we have tested have a thin metal bias electrode on the x-ray facing side of the a-Se which is deposited on the c-Si substrate. The heterostructures made with pure a-Se deposited on epitaxial p-doped (5×10 14 cm -3 ) c-Si exhibited very low dark current of 15 pA cm -2 at a negative bias field of 10 V μm -1 applied to the a-Se. The optical TOF (time of flight) measurements show that the applied bias drops almost entirely across the a-Se layer and that the a-Se hole and electron mobilities are within the range of commonly accepted values. The x-ray signal measurements demonstrate the structure has the expected x-ray quantum efficiency. We have made a back-thinned CCD coated with a-Se and although most areas of the device show a poor x-ray response, it does contain small regions which do work properly with the expected x-ray sensitivity. Improved understanding of the a-Se/c-Si interface and preparation methods should lead to properly functioning devices.

There is a renewed interest in the use of amorphous Se based alloys in x-ray imaging applications. The present work reports the dependence of the x-ray sensitivity of Cl doped a-Se: 0.2% As detectors on the x-ray exposure, applied field, energy of the incident x-ray photons, and thickness of the detector, which are technologically important parameters. Further, the dependence of x-ray energy required for free electron-hole-pair creation on the applied field and x-ray photon energy is examined. The present set of measurements complement the dependence of x-ray sensitivity on charge transport parameters (charge-carrier ranges) reported in the 14th ICAS.

A method and a convenient inexpensive apparatus are described for carrying out thermoacoustic analysis (TAA) of materials from room temperature to about 350 degrees C by monitoring the changes in the ultrasonic velocity in the sample as a function of temperature with the heating rate as a user-defined variable parameter up to 10 degrees C min-1. The thermoacoustimetric apparatus described is readily constructed from commercially available components with minimum workshop machining. The sample is loaded by sandwiching it between two identical glass buffer rods, one of which is stationary and the other vertically movable, without the need to attach transducers and thermocouples directly to the sample. The ultrasonic velocity is obtained from the time difference between the through signal at the receiving end and the echo at the transmitter end. The apparatus was used to measure the glass transition temperatures of polycarbonate, polyphenylenesulphide in matrix form as part of an aramid fibre-PPS matrix composite, poly(methyl methacrylate) and near stoichiometric As2Se3 glass from the break point of the semilogarithmic plot of the ultrasonic velocity against the temperature as in the case of thermomicrohardness measurements. The measurements, where possible, have been compared with those obtained from conventional differential scanning calorimetry and thermomicrohardness analysis.

Measurements have been carried out on a p+ n type Ge avalanche photodiode (typical guard ring structure with channel stoppers) to obtain the excess noise factor, F; multiplication, M; and the dark current, Id over a wide range of temperatures (143–323 K). The multiplication initially increases with the illumination wavelength and peaks in the 1550 nm region because of more holes being injected into the avalanche region as the absorption depth becomes much longer than the width of the p+ region. The excess noise factor, F, at À = 1550 nm was typically slightly less than multiplication, M, and showed very little dependence on temperature. The effective ratio of electron to hole ionization coefficients was approximately 0.7–0.8, the dark current activation energy close to room temperature was about Eg, indicating that the dark current is controlled by the diffusion of minority carriers. Below ~ 232 K, dark current activation energy is about Eg/2, indicative of carrier generation in the depletion region. M varied approximately as T−n, with n depending strongly on the reverse bias. In the low-frequency range below ~ 200 Hz, the noise current followed a 1/f behavior.

Amorphous Se–Te–P photoreceptor films of various compositions were deposited on to Al substrates by conventional vacuum evaporation techniques. Both xerographic and charge transport measurements have been carried out on the films to evaluate their possible suitability for photoreceptor applications. In xerographic measurements, the photoreceptor films were corona charged to a voltage (V0) and then photodischarged by illumination under light of preselected wavelength (λ) and constant intensity (I). Xerographic photosensitivity (S) was obtained from the photoinduced discharge characteristic via the empirical definition S = (V0 – V1/2)/V0t1/2I where V1/2 = V0/2 is half the initial voltage and t1/2 is the time taken to photo-discharge the surface potential to V1/2. The photosensitivity was studied as a function of illumination wavelength and material composition. Charge transport measurements were carried out using a time-of-flight transient photoconductivity technique to determine the electron and hole mobility – lifetime (μτ) products in the films. The results indicate that Se–Te–P alloys have poor xerographic photosensitivity in the short wavelength region (e.g., λ ~ 500 nm) owing to poor electron and hole transport, but in the long wavelength region (e.g., λ ~ 800 nm) the photosensitivity is considerably higher than the Se–Te system.

Results are presented on the evaluation of thermally fatigued E-glass composite specimens using ultrasonic techniques. The changes of the acoustic velocity and the attenuation coefficient in the ultrasonic range have been examined as a function of accumulated damage in various thermally fatigued composites. Three types of E-glass fiber reinforced composites have been used in this study having the following fiberglass weaves: (a) randomly oriented short fiber, (b) uniaxial fiber mat with continuous fiber bundles, (c) bidirectional continuous glass fiber mat. Representative specimens were subjected to thermal fatigue by thermal cycling between 25°C and 75°C. The structural integrity of these specimens was evaluated by determining the flexural strength after a number of thermal cycles via three point bending tests. It was observed that both the acoustic velocity and the flexural strength decreased whereas the ultrasonic attenuation increased with the number of thermal cycles which have been attributed to an accumulation of defects in the material and especially to debonding at the fiber-matrix interface. It has been observed that the use of ultrasonics may be very appropriate in evaluating composites in service since a good correlation has been noted between changes in the flexural strength, acoustic velocity and the attenuation coefficient in thermally fatigued composites.

The Kanazawa-Batra formulation of deep-trapping kinetics in high-resistivity semiconductors is reformulated to obtain a general differential equation that describes the instantaneous electric field E(x, t) in a solid in which charge transport and trapping occurs in the presence of discrete levels of deep traps. The model developed herein specifically takes into account that, as deep trapping proceeds, the concentration of empty traps decreases due to trap filling. The latter effect was ignored in previous models. Furthermore, the possibility of release from deep traps is also included. A second-order nonlinear partial differential equation is obtained for E(x, t) which fully describes the deep trapping kinetics in a high-resistivity solid. The numerical solution of this differential equation for a photo-induced discharge (PID) process, with a field-dependent photo-injection mechanism, gives the instantaneous and the final electric fields in the sample and hence the time evolution of the surface voltage and the residual surface potential as a function of charge transport and trapping parameters. The relationship between the xerographic residual potential and the mobility-lifetime product (charge-carrier range) is examined and discussed in terms of charge transport and trapping parameters in high-resistivity solids, and a-Si:H is considered as an example. The present formulation of deep-trapping kinetics puts a limitation on the interpretation of xerographic residual-voltage experiments and moreover emphasizes the need for other collaborating experiments for a meaningful evaluation of trap parameters.

Amorphous selenium (a-Se) has been widely used as a direct conversion X-ray detection material. Vertical structures are employed in most cases, where >200 μm thick a-Se photoconductor layer is inserted between top and bottom electrodes. In this paper, we design a lateral metal-semiconductor-metal (MSM) structure in which a relatively thin layer of a-Se (~ 8 μm) is coated on top of two lateral electrodes. The simulation results indicate that dark current of such a structure stays extremely low level and external quantum efficiency (EQE) reaches over 30% with wavelengths ranging from 320 to 680 nm. We further fabricate the lateral MSM photoconductor by a two-mask photolithography process. The fabricated photoconductor exhibits a dark current below 40 fA under electric fields ranging from 6 V/μm to 9 V/μm, a responsivity up to 0.06 A/W, a measured EQE of 18% towards a short wavelength of 468 nm, and a high photoresponse speed at 500 Hz with a rise time of 250 μs, fall time of 350 μs, and time constant of 250 μs, respectively. Furthermore, an architecture of indirect conversion X-ray imager is proposed with the use of such a lateral MSM structure and a blue-emitting scintillator material atop.

The photoluminescence (PL) emission band corresponding to 4F3/2 to 4I15/2 transition has been revealed using a double lock-in technique and quadrature frequency resolved spectroscopy (QFRS) at room temperature in Nd3+ in a GaLaS glass. The band is centred around 1840 nm and has a lifetime ~77µs, which seems to be the same for all four radiative transitions 4F3/2 to 4IJ (J = 9/2, 11/2, 13/2 and 15/2). The branching ratios beta J of the four above transitions have been estimated from the PL data and agree well with the results of Judd-Ofelt (JO) analysis. In addition to the 1840 nm band, we have observed another PL band appearing around 1580 nm and having a dominant lifetime ~3.8 µs. This band may be identified with the radiative transition from the doublet (4F5/2, 2H9/2) to the 4I15/2 manifold. The short lifetime ~3.8 µs may be caused by effective multiphonon relaxation from the doublet to the 4F3/2 manifold.

Commercially available flat panel, direct conversion, digital X-ray image detectors use a photoconductive layer of stabilized amorphous selenium (a-Se) sandwiched between two electrodes to convert incident X-ray radiation into stored electrical charge. These detectors use thin n -like and p -like blocking layers to trap charge carriers injected from the contacts and thus reduce the dark current by limiting the rate of carrier injection. The effect of each of these blocking layers is determined by measuring the dark current in 200 μm thick mammographic detector-like samples of a-Se with three different structures: a single intrinsic layer and n-i and i-p double layers. Dark current is measured as a function of time over 20,000 s and as a function of applied electric field over the range of −10 V/μm to +10 V/μm. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

ICOOPMA 2008 is the third in the ICOOPMA series, an International Conference on Optical, Optoelectronic and Photonic Materials and Applications (also known as The International Conference on Optical and Optoelectronic Properties of Materials and Applications), originally sponsored by Springer, that was held for the first time in Darwin, Australia, in July 2006. The ICOOPMA series arose from a need for such a conference for those researchers who sought a truly international conference that covered a wide range of materials and applications in optics, optoelectronics and photonics. One of the goals is to provide discussions between re-searchers working on different classes of materials that have similar applications; or have been character-ized by similar techniques. The conference has a large number of invited speakers to allow such cross-fertilization between researchers working on different classes of materials. The conference also seeks papers in interesting or novel applications, or papers that enhance material properties for applications. The In-ternational and Local Organizing Committees have the responsibility of ensuring an in-depth scientific coverage with invited and contributed papers from various countries and in various disciplines; and en-suring an enjoyable scientific program. The Second ICOOPMA was held in London, England, 29 July–3 August 2007, and had over 250 participants, and five plenary lectures. The Third ICOOPMA was held in Edmonton, Canada, 20–25 July 2008, and again had nearly 250 participants and had seven plenary talks. TRLabs in Edmonton was responsible for organizing and man-aging the conference and there were a number of ma-jor sponsors without whom this conference would not have had the success it deserves. We thank TRLabs for their organization, and our sponsors for their gra-cious contributions and commitment to the ICOOPMA. There were three parallel oral sessions from Monday to Thursday, and a poster session on Tuesday to cover all the 300 papers. The current Pro-ceedings represent a collection of papers that have been presented at the conference, and have been fully refereed for inclusion in the proceedings. With the agreement of the journal's Chief Editor and the authors, a selection of papers have been transferred to physica status solidi (a) 206(5) (May 2009). The papers in both issues cover not only topics on fundamental concepts, including various optical properties (luminescence being the most popular) of a wide class of materials, e.g. organics, glasses, semi-conductors, nanocrystals, quantum wells, wires and dots etc., but also a number of interesting applications from photonics modulators to X-ray detectors. There were four best poster prizes that were given during the conference to J. E. Cunningham and Edmund Lin-field et al. (University of Leeds, UK) on “Terahertz imaging and spectroscopy using on-chip systems”; Shabnam Homampour et al. (McMaster University, Canada) on “Thermal tuning of planar Bragg gratings in silicon-on-insulator rib waveguides”; Mamiko Kujiraoka et al. (National Institute of Information and Commu-nications Technology, Japan) on “Ensemble effect on Rabi oscillations of exciton in quantum dots”; Osamu Kojima et al. (Kobe University, Japan) on “Lengthening of photoluminescence decay time owing to expansion of electron envelope functions in stacked quantum dots”. Timely publication of conference papers in an easily accessible journal is one of the attractions in presenting one's latest work at conferences. It allows the work to appear relatively quickly after it has been discussed at the conference. The papers in the proceedings were therefore prepared in publica-tion-ready form. However, it is also important to en-sure that proper refereeing has been done before a paper has been accepted. We specifically allowed pa-per submissions to continue well after the conference date to ensure that the papers included discussions during the conference. We thank the many referees who have unselfishly dedicated their valuable time in reviewing some ninety-five manuscripts and, in many cases, reviewing the revised versions as well. We are grateful to the editorial team at Wiley-VCH, in par-ticular Stefan Hildebrandt in Berlin, for their help in getting the proceedings out as soon as possible once the refereeing of the papers were complete (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have studied the effects of adding small amounts of Mg on the electrical and thermal properties of a-Se films. The addition of Mg in the ppm range has resulted in significant changes in the charge transport and stability. The doped samples have shown to be excellent n -like layers which have electron ranges much greater than hole ranges, μeτe ≫ μhτh. The n -like layers block the injection of holes from the positive electrode which results in a lower dark current in a-Se based X-ray detectors. The Differential scanning calorimetry studies have shown that a-Se doped with lower concentrations of Mg is more stable than samples with higher concentrations. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have investigated the effects of dopants from Group IIA on the electrical and thermal properties of amorphous selenium (a-Se). We have found that additions in the parts per million range of Mg, Ca, and Ba can alter significantly both the electronic and the thermal properties of a-Se. Vacuum evaporated films from a-Se doped with a Group IIA element have demonstrated excellent n-like transport (electron ranges being much longer than the hole ranges). Differential scanning calorimetry (DSC) studies of the thermal stability of doped glasses and films have shown that these additives encourage crystallization.

The glass transition and crystallization temperatures (T g , T c ), heat capacity, thermal stability and glass uniformity of GeSGa, GeSeGa, Ge(SeTe)Ga chalcogenide glasses doped with Er3+ by the addition of Er2S3 have been investigated by conventional differential scanning calorimetry (DSC) and Temperature-Modulated DSC (TMDSC). While some of the glasses have two crystallization peaks, these glasses were nonetheless optically actively and uniform. Essential optical properties have been evaluated, such as the photoluminescence (PL) intensity and lifetime as a function of the glass composition. We present typical results to emphasize some of the important characteristics of these systems and discuss trends within a glass system; and also highlight differences between glass systems.

Recently, a new class of rare-earth (RE) doped fluorochlorozirconate (FCZ) glass-ceramics has been developed and studied with the goal of using these materials in indirect conversion x-ray imaging applications. We report selected thermal and x-ray induced luminescence (XL) properties of rare-earth (EuF2 and SmF3) doped fluorochlorozirconate glasses and glass-ceramics, and highlight some of their interesting properties. Sm-doped FCZ glasses exhibit XL even in the unannealed state, while annealing at a temperature of 250°C in a reducing atmosphere (5% H2 and 95% Ar) significantly enhances the XL efficiency.

Changes in the electrical properties of pure a‐Se and Cl‐doped a‐Se:0.35% As electroradiographic layers upon exposure to x‐ray radiation has been examined using xerographic techniques based on the first residual potential V R1 and the cycled‐up residual potential V R∞ Hole lifetime τ as determined from the Warter expression V R1=L 2/2μτ has been found to decrease sharply with the x‐ray dose, tending to saturate at higher dose levels in both pure a‐Se and Cl‐doped a‐Se:0.35%As. The hole liftime following x‐ray exposure recovers toward its equilibrium value at the anneal temperature. The recovery time is dependent on the dose and is accelerated appreciably by temperature, indicative of a thermally activated process. Cycled‐up xerographic residual‐potential measurements have indicated that the deep‐hole‐trap population increases with exposure to x rays. Comparison of cycled‐up saturated residual potential and first‐cycle residual‐potential measurements have shown that as a result of x‐ray exposure, the apparent capture coefficient increases, which can only be accounted for by introducing two x‐ray‐induced effects: (a) generation of deep hole traps and (b) formation of an equal number of trapped electrons and holes in the bulk due to irradiation. Recombination of holes with the trapped electrons in the bulk causes the apparent increase in the capture coefficient. A xerographic spectroscopy technique, based on the analysis of the rate of discharge of the saturated potential after the end of xerographic cycling, was used to map out the energy distribution of the occupied density of states in the mobility gap. It was found that the energy location of the peak of the occupied density of localized states remained unaffected by the exposure to x rays, but the peak value of the density of states increased with the exposure. The x‐ray induced trap generation is thus simply an enhancement of the integrated concentration of inherent structural defects in the films located at their characteristic energies: 0.87 eV in pure a‐Se and 0.90 eV in Cl‐doped a‐Se:0.35%As. The present results are consistent with the x‐ray‐sensitivity predictions of Kalade, Montrimas, and Rakauskas [Phys. Status Solidi A 25, 629 (1974)].

Charge-carrier drift-mobility-lifetime (μτ) product, the range of the carrier, is one of the most important electronic properties of a semiconductor material for device applications. The determination of μτ is, therefore, of fundamental importance in the characterization of amorphous semiconductors and has been the key issue in a number of recent papers. This paper describes two experimental techniques for μτ product measurements and their application to a-Se and Cl-doped Se:0.35%As electro-radiographic films. Xerographic measurements involve corona charging the surface of an amorphous semiconductor film to a voltage Vo. The film is then exposed to a highly absorbed step illumination at the end of which the residual potential, VR1, on the surface is measured. VR1/Vo is then related to μτ. In the interrupted field time-of-flight (IFTOF) measurement technique, during the flight of the photoinjected holes through the specimen, the applied field is removed at time T1 for an interruption duration of ti and then reapplied and the remaining holes extracted. The fractional recovered photocurrent at the end of the interruption time, ti was found to follow the deep-trapping kinetics described by i(T1 + ti)/i(T1) = exp (−ti/τ) where τ is the charge-carrier lifetime. By interrupting the time-of-flight photocurrent at different locations, the technique allows for the determination of the trapping time τ as a function of location across the specimen and hence the examination of sample inhomogeneities. Xerographic residual potential and IFTOF experiments carried out on a range of a-Se and Cl-doped a-Se:0.35%As electroradiographic films show that there is a good correlation between the trapping time determined from xerographic measurements and that determined from IFTOF. The results are discussed in terms of carrier-trapping processes in amorphous semiconductors. The capture cross section of deep hole traps in a-Se and Cl-doped a-Se:0.35%As is determined from the measurement of the hole-mobility-lifetime product and the saturated cycled-up residual potential. Application of the ballistic and diffusional models of Street (Philos. Mag. B, 49, L15 (1984)) indicate that the hole-capture radius in a-Se is 2–3 Å (1 Å = 10−1 m) and that the chemical modification of a-Se by combinational doping with 0.35%As and 10–20 ppm Cl does not affect the basic capture process. The two experimental techniques described represent the most meaningful methods of charge transport and trapping characterization in amorphous semiconductors.

Interrupted field time-of-flight (IFTOF) transient photoconductivity measurements have been performed in conjunction with first residual and cycled-up xerographic measurements on pure a-Se and chlorinated a-Se:7.4%Te and a-Se:0.2–0.3%As electrophotographic films. The hole lifetime has been measured as a function of Cl doping and location in the sample. Cycled-up xerographic measurements were used in conjunction with IFTOF experiments to estimate the capture coefficient C t and diffusional capture radius r c .

The authors describe a floating bridge technique and a high-voltage apparatus for carrying out interrupted-field time-of-flight (IFTOF) transient photoconductivity measurements with applied voltages up to 1 kV. Switching transient voltages appearing as a result of the removal and reapplication of the applied bias voltage, which otherwise would obliterate the normal TOF photocurrent signal, have been reduced to an undetectable level by using a floating Schering bridge network with two nulling capacitors. The bias voltage is applied from a single triggerable floating high-voltage switch which utilizes n-channel TMOS transistors and can deliver up to 1 kV in a switching time of approximately 150 ns. By switching the gain of an amplifier during the interruption time, even very small recovered photocurrent signals can be measured with respect to the pre-interrupted signal. The technique has been successfully applied to measure both hole and electron trapping lifetimes in a-Se:0.5%As electroradiographic photoreceptors, and to determine the range of the charge carriers. The distinct advantage of the present apparatus, as well as its shortcomings, over the complementary bias and single nulling capacitor method, which was described previously, are also addressed.

It is shown that the dark decay of the electrostatic surface potential on a corona-charged a-Se:Te alloy photoreceptor occurs via electric field-enhanced xerographic depletion discharge (FEXDD) in which Poole-Frenkel-assisted thermal emission of holes from deep mobility-gap states and their subsequent sweep out generates a negative bulk space charge. The theoretical model development is applied to explain the observed experimental dark discharge data over a wide range of charging (initial) voltages. It is shown that although the time required for the surface potential to decay to its half value t/sub 1/2/ initially increases with the charging voltage V/sub 0/ at the highest charging voltage, t/sub 1/2/ actually decreases with V/sub 0/. Results obtained from cycled-up xerographic experiments on single and double-layer photoreceptors are also reviewed and discussed in conjunction with transient photoconductivity experiments. >

The present work reports measurements on the x-ray sensitivity of Cl doped a-Se:0.1–0.3% As x-ray receptor films as a function of electric field, film thickness, and electron and hole schubwegs.

Differential scanning calorimetry (DSC) was used to determine the specific heat of lentil seed. Accurate measurement with this method requires a good seal condition of the sample pan and relatively thin sample specimen. The value of specific heat of lentil seeds ranged from 0.81 to 2.2 kJ/kgK. Specific heat increased quadratically with moisture content over the range from 2.1% to 25.8% (w.b.) and linearly with temperature varying from 10° C to 80° C. The classical compartment model was modified to accommodate the variation of specific heat with temperature and moisture content. KEYWORDS. Specific heat. Physical properties. Differential scanning calorimetry. Lentils.

The well-known deep trapping kinetics model of Kanazawa and Batra for photoinduced discharge which relates the residual potential, V r , to the range of the charge carriers (μτ) has been reformulated by specifically including the effect of rate of trapping as being proportional to the instantaneous unoccupied density of traps. A partial differential equation is derived that describes the space and time evolution of the electric field within the material. By numerically solving this differential equation and integrating the electric field, the residual potential, V r , has been related to the charge carrier range and the deep trap capture coefficient. The theory is compared with experimental results from xerographic residual potential measurements to show that trap saturation effects can play an important role in the interpretation of V r .

Combined techniques based on first-cycle xerographic residual potential, cycled-up saturated xerographic residual potential, and interrupted field time-offlight (IFTOF) measurements are used to determine the hole-drift-mobility-lifetime product μτ deep trapping time,τ, integrated deep-trap concentration, N p trap capture coefficient and trap capture radius in a range of a-Se films. Very good agreement is observed between the μτ products determined via xerographic and IFTOF methods. Ballistic and diffusional deep-trapping models applied to this prototype amorphous semiconductor indicate that the capture radius is 2-3 [Abreve], indicative of neutral trapping centres with an integrated density of about 1013cm−3

Thermal and mechanical properties of photoreceptor-type amorphous selenium (a-Se) films have been studied via differential scanning calorimetry (DSC), thermomechanical analysis (TMA) as well as thermomicrohardness analysis (T mu HA). The a-Se films were vacuum deposited by thermal evaporation onto heated Al substrates and identically aged. The experiments were carried out over a temperature range encompassing the glass transformation to examine the nature of physical processes controlling the thermal and mechanical properties. In DSC experiments, the glass transformation kinetics was investigated via both heating and cooling schedules. By applying the present thermoanalytical methods-Ritland and Kissinger analyses-it proved possible to identify a typical Vogel-Tammann-Fulcher ( approximately exp(A/(T-T0))) type of response in the structural retardation times controlling the relaxation of the enthalpy. It was found that over a common temperature range, the temperature dependence of the mean relaxation time tau (T), whether determined from heating or cooling schedules, followed the viscosity-temperature data for a-Se suggesting that tau (T) approximately eta (T). The temperature dependence of the microhardness, Hv(T), on a semilogarithmic plot evinced a sharp decrease at a temperature Tg in the glass transformation region. The origin of this sharp decrease in the microhardness has been attributed to a structural relaxation process which has a mean retardation time proportional to the viscosity as in the enthalpy relaxation process. In the case of TMA however, it was found that the shift in the softening temperature with the heating rate indicated an apparent activation energy less than that for the viscosity. Two possible models are discussed for the interpretation of the TMA softening temperature which are based on the relaxation of the elastic modulus and an elastic deformation to viscous flow transition. By comparing the results from the present thermal and mechanical studies with the reported viscosity-temperature data, it is concluded that the rate of relaxation of enthalpy and microhardness of a-Se in the glass transformation region is inversely proportional to the melt viscosity which can be adequately described over a temperature range above approximately 30 degrees C by a Vogel-Tammann-Fulcher expression.

Recently there has been an emphasis on the importance of using cooling scans in DSC experiments in studying the glass transformation kinetics of glasses. The physical interpretation of the apparent activation energy from DSC heating scans has been questioned as not being meaningful. The present paper reports glass transition temperature (Tg) measurements derived from Differential Scanning Calorimetry (DSC) experiments on bulk and film amorphous selenium samples subjected to heating, at constant rate r, and cooling, at constant rate q, scans. Film samples were prepared by thermal evaporation techniques in vacuum. It is shown that for both bulk and film forms of a–Se, within experimental errors, log(r/T2gm) vs 1/Tgm plot where Tgm is the peak glass transformation temperature, and log(r) vs 1/Tgh plot, where Tgh is the glass transition onset temperature from DSC heating scans, are parallel to the log(q) vs 1/Tgc plot where Tgc is the glass transition temperature from cooling scans. Within the Hutchinson and Kovacs formulation of the glass transformation phenomenon, the results imply that the structural contribution to the mean retardation time, τ, is negligible in comparison with the temperature dependent part. The mean structural relaxation time for both bulk and film forms was found to exhibit a typical Vogel-Tammann-Fulcher type of temperature dependence. Furthermore, the structural relaxation rate was observed to be inversely proportional to the viscosity, η(T), implying that the mean structural retardation time is proportional to the viscosity, τ ∼ η. The results also confirm that the earlier studies of glass transformation kinetics in a–Se utilizing only DSC heating scans remain meaningful.

Time‐of‐flight (TOF) hole drift mobility (μd) measurements have been carried out on vacuum deposited and identically aged a‐Se photoreceptor type films over a temperature range encompassing the glass transformation region to study the nature of mobility controlling shallow traps in this elemental chalcogenide semiconductor. Differential scanning calorimetry (DSC) experiments using both heating and cooling scans have also been carried out on the same films to correlate the enthalpy relaxation phenomenon with the TOF drift mobility dependence on temperature. By considering the heating rate dependence of the minimum peak in the μd vs T behavior and the heating and cooling rate dependence of the DSC glass transition temperature, it is shown that the mean retardation times, τμ and τH, associated with the relaxation of the shallow traps and the enthalpy, respectively, have similar temperature dependencies, essentially Vogel–Tammann–Fulcher type, with negligible structural contribution. Correlation is also estab...

We have investigated X-ray induced radioluminescence (XL) properties of Sm-doped fluorochlorozirconate (FCZ) glasses and glass-ceramics. The FCZ glass is a modified ZBLAN glass which shows a very high optical transmission over a wide spectral range. The glass matrix includes Sm3+-doped nanocrystals of BaCl2 after heat-treatment at temperatures above 250 °C. The glass-ceramic emits red light under UV and X-ray exposure. Since conventional Si-based photodetectors, e.g., CCDs, have the highest quantum efficiency to red light in general, the Sm-doped FCZ glass-ceramic plate can be a good candidate as a scintillator material for indirect radiation detection. Moreover, a very broad emission is present in the glass-ceramic around 300–500 nm, which is attributed to a self-trapped exciton (STE) emissions. The temperature dependence of X-ray induced luminescence and photoluminescence are very similar. The XL light yield is linearly proportional to the X-ray exposure rate for rates higher than 20 mR/s. For low exposure rates, emissions by Sm2+ are more sensitive than others, leading to a nonlinear response.

Secondary optical processes are becoming more and more important in health and environmental applications. Ultraviolet produced from secondary emission or scintillation can damage DNA by direct photoexcitation or by the creation of reactive oxygen species. X-ray Excited Optical Luminescence (XEOL) and Time Resolved XEOL (TRXEOL) results for the fast emitter, CaF2:ZnO, that have been treated by heating in air and in vacuum, show that the scintillation from the Self Trapped Exciton (STE) emission of CaF2 at 282 nm is dominated by a slow process (>100 ns). A faster but weaker 10 ns component is also present. The ZnO and CaF2 show independent emission. The ZnO bandgap emission at 390 nm has dominant lifetimes of less than 1 ns.

Starting with the optical properties, this chapter covers a wide range of topics associated with the photoexcitation of solids. Lattice absorption and reflection, free carrier and band to band absorptions are summarized in sections 2–4. The excitonic absorption in the crystalline and amorphous solids is reviewed in sections 5 and 6, respectively, where the newly developed concept of the effective mass of charge carriers in amorphous solids is also introduced. Excitonic processes in quantum wells are described in the section 7, where rates of various processes are presented. Finally section 8 reviews the recent advances in the study of photoluminescence in amorphous semiconductors.

We have measured the dichroism in amorphous As2Se3 induced by a polarized beam of near band-gap light (632.8 nm). Most glassy material is initially isotropic and homogeneous. When As2Se3 absorbs an intense pump-beam of polarized light, the absorption coefficient for light polarized in the same direction as the pump-beam is less than for the perpendicular polarization, i.e. the sample becomes anisotropic (dichroic). Induced dichroism is potentially useful in various photonic devices including optical switches, optical memory, and polarizers. In our experiments, the total induced dichroism amounts to 0.24% after 10 minutes of illumination by polarized laser light (632.8 nm) with an intensity of 130 mW/cm2. The induced dichroism is reversible by rotating the polarization of the pump light by 90deg. A total change of 0.48% is achieved between the two orthogonal states. We find that the time dependence of the induced dichroism fits a stretched exponential function

A model for studying the effects of large signals on charge collection efficiency in amorphous selenium based X-ray image detectors is described by considering bimolecular recombination between drifting charge carriers and space charge effects. The continuity equations for both holes and electrons, and the Poisson's equation across the photoconductor for a short step X-ray exposure are simultaneously solved by the finite difference method. The numerical results are compared with the Monte Carlo simulation results. It is found that the recombination plays practically no role on charge collection up to the total carrier generation rate q0 of 1019 EHPs/m2-s at the applied electric field of 10 V/mum. At large values of q0 , the charge collection efficiency gradually decreases with increasing q0 and approaches almost zero at q0 larger than 1024 EHPs/m2-s. The effect of recombination on charge collection increases with decreasing applied electric field and increasing the normalized absorption depth (absorption depth per unit thickness)

Thick film resistors (TFRs) are currently the only materials in microelectronic use whose conductivity can be tailored through composite mixtures of conductive phases within a glassy matrix [1, 2], to span resistivities in the wide range of 10y3 – 103 U cm, giving sheet resistances of 1 U=u to 1 MU=u. Although DuPont-Birox 1400-series or similar ruthenium based TFRs are widely used in modern microelectronic circuits, there is limited data on their intrinsic electrical transport properties such as the charge carrier concentration, drift mobility, etc. TFRs have generally been found to be relatively noisy in the audio frequency range and up to ,105 Hz above which the 1= f noise becomes buried in the white noise. Although the noise characteristics of TFRs have been studied (e.g. [3, 4]), most of the results published to date have not been correlated to, or used to infer, their electrical properties such as carrier concentration, drift mobility, momentum relaxation time, etc. Given the lack of data on the latter electrical transport parameters we have therefore tried to extract these electrical properties from the measured noise spectra and conductivity. We have measured the excess noise (or 1= f noise) in a number of well-defined TFRs made from inks of the Birox 1400 series in an effort to evaluate, at least as an order of magnitude, the free carrier concentration (nc), the drift mobility (μ), mean free path (λ) diffusion coefficient (D), Debye screening length (L), etc. Three values of nominal thick film sheet resistance were chosen — 1, 10 and 100 kU=u which were used to span the resistors range used in hybrid microelectronic circuits. Table I gives the basic geometries and data for some of the samples investigated. The resistors were produced by the standard method specified by the manufacturer of the Du-Pont resistive inks series Birox-1400 [5] and then trimmed by laser to their final values. The Birox 1400 series of TFR inks is based on pyrochlore (Bi2Ru2O7) embedded in lead containing glass. The sheet resistance is decreased by increasing the ratio of pyrochlore in the composition. The noise measurement system and procedures used have been described in detail previously [6]. All measurements were carried out in an electromagnetic (EM), shielded cell and a PC-586 hardware and software environment was used to process the noise spectra obtained from the low-noise amplifiers [6] in real time. Variable bias current was obtained from a series connection of the TFR with a higher value wirewound resistor with a very low noise, i.e. C ˆ 1:5 3 10y16, where C is the relative noise index (a unitless coefficient) defined in Equation 1 below. The smallest detectable spectral noise current density was 10y14 A Hzy1=2 in the frequency region of 1–105 Hz. Ohmic I–V behaviour was observed for all the resistors measured throughout the current range used, i.e. 1–100 μA. Noise spectra were acquired many times for each resistor to ensure reproducibility. The measured excess noise power spectral density Si was found to be well-described by the usual 1= f behaviour