Layer Of Amorphous Selenium Research Articles

A novel flat-panel imager for medical x-rays

The last decade has seen rapid development and clinical adoption of active-matrix flat-panel imagers (AMFPIs) for diagnostic x-ray imaging. AMFPIs provide better image quality than traditional screen films and computed radiography,1, 2 but further improvement is desirable, especially in spatial resolution and low-dose performance. Existing AMFPIs use a two-dimensional array of thin-film transistors to read out a charge image generated by an x-ray image sensor. In ‘direct conversion’ this sensor is an x-ray photoconductor, while in ‘indirect conversion’ the sensor is a scintillator coupled with discrete photodiodes. These methods have two main limitations. First, in low-dose applications such as fluoroscopy (real-time x-ray imaging), electronic noise degrades the imaging performance behind dense tissues. Second, the smallest pixel size currently used for digital mammography, 70μm, may not be adequate in some cases. For example, characterizing the shape of microcalcifications has been shown to be compromised with pixel size del = 100μm, while del = 50μm can preserve the needed information.3 To improve the detector low-dose performance with high resolution, we are investigating a new, indirect-conversion flatpanel imager with avalanche gain and a field-emitter array (FEA), which is referred to as SAPHIRE (scintillator avalanche photoconductor with high-resolution emitter readout). The concept of SAPHIRE is shown in Figure 1. It consists of a needle-structured cesium iodide (CsI) scintillator, optically coupled (for example, through fiber optics) to a uniform thin layer of amorphous selenium (a-Se) photoconductor, with thickness dSe ∼ 4 − 25μm. The selenium layer is operated in avalanchemultiplication mode, and is called HARP (high avalanche rushing amorphous photoconductor).4 Figure 1. Cross-sectional schematic of the SAPHIRE detector (thickness not to scale). Light generated by x-rays striking the cesium iodide (CsI) scintillator passes through the transparent indium tin oxide (ITO) electrode to generate electrons and holes (charge carriers) in the amorphous selenium layer, designated HARP. A high voltage (HV) causes avalanche multiplication as the holes traverse the HARP layer, and the amplified signal is detected using an electron beam emitted by a field emitter array (FEA).

Influence of light on structure of amorphous selenium layers

After exposure of amorphous selenium islands to luminous ux of 1¢10 6 lx, they do not become crystalline, but remain amorphous. However, light stimulates polymerization of molecules in those islands. For this reason, heating of a layer that is exposed to light does not cause a decrease of the mean thickness of the layer, but heating of the layer in the dark causes a decrease of its mean thickness. Therefore, the mean thickness of the illuminated layer is 3 to 4 times greater than the mean thickness of the layer in the dark. In addition, the fraction of the substrate covered by the islands is 1.5 to 2 times larger in the exposed layer than in the unexposed layer. However, illumination of a continuous layer speeds up its transformation into an island-type layer. Therefore, in order to obtain an extremely thin continuous layer of amorphous selenium, during growth of the layer it must be exposed to light only until it becomes continuous. Starting from that moment, the light must be switched off.

Photovoltaic devices based on palladium(II) meso-tetra(benzo-15-crown-5)porphyrinate

The properties of photovoltaic cells containing an MEH-PPV composition with palladium(II) meso-tetra(benzo-15-crown-5)porphyrinate as an electron-conduction layer and copper(II) phthalocyanine as a hole transport layer were studied. The influence of the environment and a top-electrode metal on the operation efficiency was also examined. It was found that oxygen at the fabrication steps affects in different manners the formation of open-circuit voltage and the performance of the device. A certain correlation between the properties of electrodes and the photoelectromotive force was established. The highest conversion efficiency of 0.02 electrons per incident photon was obtained for samples that additionally contained an amorphous selenium layer applied over the aforementioned conducting layers.

Kinetics of the photostructural changes in a-Se films

The kinetics of the photodarkening effect has been studied experimentally for amorphous selenium (a-Se) layers at room temperature and at an elevated temperature (35°C) close to the glass transition. By switching an intense pumping light on and off with a period of 100s, we have studied the kinetics of both the buildup of photodarkening and its relaxation (recovery). It was found that at 35°C, only a reversible component of photodarkening has been observed. This result has been interpreted within the framework of a phenomenological model assuming that photodarkening is caused by light-induced transitions of structural units from their ground states into metastable states. Our estimate for the energy barrier EB between these states obtained for the photodarkening process (EB∼0.8eV) coincides with that obtained from the analysis of the relaxation process. At room temperature, an irreversible component of photodarkening has been observed along with the reversible one. The energy barrier responsible for the relaxation of the reversible component at room temperature appears the same as at 35°C. This suggests that the energy barrier identified represents a fundamental feature of the photoinduced structural metastability in amorphous selenium.

Drift mobility of excess charge carriers in molecularly doped polymers upon photo-and radiation-induced charge generation

Comparative experiments on molecularly doped (with aromatic hydrazone) polycarbonate were performed to measure the drift mobility of excess charge carriers using carrier generation by exposure to light and ionizing radiation. Measurements showed practically complete coincidence of both mobility values and signal shapes. Compared were the results of measurements on polymer specimens with various thicknesses supported on substrates with different surface roughness, as well as for samples with or without an injecting amorphous-selenium layer. Dispersive carrier transport was observed in all cases. However, current transients sometimes displayed a plateau, depending on the sample preparation conditions. As the magnitude of generated (injected to the polymer layer) charge increased, a current spike appeared by the end of the transit time, thus indicating the emergence of local space-charge-limited current.

TH-C-I-611-08: A New Concept of Indirect Flat-Panel Detector with Avalanche Gain: SAPHIRE (Scitillator Avalanche Photoconductor with High Resolution Emitter Readout)

Purpose: To investigate the feasibility of a new concept of indirect flat-panel detector with avalanche gain and field emitter array (FEA) readout to improve the imaging performance at low dose with high resolution. Method and Materials: The proposed detector is made by optically coupling a structured scintillator (CsI) to a uniform layer of avalanche amorphous selenium (a-Se) photoconductor called HARP (High Avalanche Rushing amorphous Photoconductor). The HARP layer absorbs the visible photons emitted from the scintillator and generates electron-hole pairs. These carriers undergo avalanche multiplication under a sufficiently high electric field ESe and form an amplified charge image. A two-dimensional FEA, which is essentially a flat electron source, is proposed as a new method to read out the charge image. The proposed detector is called SAPHIRE (Scintillator Avalanche Photoconductor with HIgh Resolution Emitter readout). Experimental and theoretical methods were developed to study the factors affecting the x-ray response and spatial resolution of SAPHIRE. Results: The avalanche gain in the HARP layer depends on both its thickness and ESe. The avalanche gain for an 8-μm-thick HARP layer is 46 under an ESe of 110V/μm. The gain can be turned off in high dose applications (e.g. radiography) by decreasing the ESe. Our electron optics simulation of SAPHIRE shows the dominant source of blur is the lateral spread of the electron beams from the FEA, which depends on the geometry and bias potentials of different electrodes in the detector. With proper electron optics design a pixel size of 50 microns is achievable. Conclusion: The avalanche gain of SAPHIRE can provide x-ray quantum noise limited images down to a single x-ray photon. The gain is programmable by changing ESe, which ensures wide dynamic range for SAPHIRE. The FEA can potentially provide a smaller pixel size and higher readout speed than existing flat-panel detectors.

Phase composition of amorphous Se island layers

Results of investigations of island amorphous selenium layers evaporated upon a carbon film are presented. The layers were heated at various temperatures. In all cases, selenium formed islands of almost perfectly hemispherical shape with various diameters. Such well-defined shape of the islands made it possible to calculate total mass of the islands. Experimental data indicates that heating of the layer causes an increase of the mass of islands up to 7 times in comparison with their mass prior to thermal processing. This is possible only if the islands grow by accepting atoms of the same kind adsorbed on the substrate. It follows that a part of the layer mass is accumulated in the gas adsorbed on the substrate. This part depends on the layer temperature, substrate material, evaporation rate, etc. Investigation of distribution of island diameters immediately after layer evaporation and during thermal processing of the layer allowed determination of conditions that are needed in order to be able to estimate defect density in the substrate from the measured density of the islands.

Photosensitive media based on amorphous selenium layers

Photosensitive media based on amorphous selenium layers

Defect levels and charge carrier photogeneration in amorphous selenium layers

Time-of-flight post-transit data that suggested positions in the band gap at ∼Ev+0.4 eV and ∼Ec−0.55 eV for the thermally accessible levels of the intrinsic negative-U centers of a-Se have been re-examined. The introduction into the Onsager formalism of a distribution of initial carrier separations in the photogeneration process, together with random internal field fluctuations, and of the ensuing processes of bimolecular recombination and deep trapping lead to a good description of experimentally observed photogeneration efficiency in a-Se.

Time of Flight of Drifting Electrons and Holes in Stabilized a-Se Film

AbstractLarge area, flat panel detectors are being investigated for digital radiogrpahy and fluoroscopy. Theses detectors employ an x-ray conversion layer of photoconductor to detect x-rays. The amorphous selenium layer that is currently being studied for its use as an x-ray photoconductor is not pure a-Se but rather a-Se doped with 0.2-0.5% As and 10-30 ppm Cl, also known as stabilized a-Se. The suitability of the stabilized a-Se is largely determined by its charge on generating, transporting and trapping properties.In this paper, a conventional time-of-flight measurement was carried out to analyze the transport properties of charge carriers. A laser beam with pulse duration of 5 ns and wavelength of 350 nm was illuminated on the surface of the stabilized a-Se with thickness of 400 μm. The photo response signals of the hole and electron were measured at the applied electric field of 10 V/μm as a function of time. The measured transit times of the hole and electron were about 229.17μs and about 8.73μs at 10 V/μm, respectively. The measured mobility indicated a slight dependence with respect to the applied electric field with a range of 4-10 V/μm. The experimental results showed that the measured mobility of the hole and electron was 0.04584 cm2V-1s-1 and 0.00174 cm2V-1s-1 at the electric field of 10 V/μm.

Experimental Evaluation of a-Se Flat-Panel X-ray Detector for Digital Radiography

AbstractNowadays, large area, flat panel solid state detectors are being investigated for digital radiography. In this paper, development and evaluation of a selenium-based flat-panel digital xray detector are described. The prototype detector has a pixel pitch of 139μm and a total active imaging area of 7″×8.5″, giving a total of 1.9 million pixel. This detector include a x-ray imaging layer of amorphous selenium as a photoconductor which is evaporated in vacuum state on a TFT flat panel, to make signals in proportion to incident x-ray. The film thickness was about 500μm. To evaluate the imaging performance of the digital radiography (DR) system developed in our group, sensitivity, linearity of the response of exposure, the modulation transfer function(MTF) and detective quantum efficiency(DQE) of detector was measured. The measured sensitivity was 4.16×106 ehp/pixel mR at the bias field of 10 V/μm: The beam condition was 41.9 KeV. Measured MTF at 2.5 lp/mm was 52%, and the DQE at 1.5 lp/mm was 75%.

Ergebnisse einer Langzeituntersuchung zum Fatigue-Effekt in der digitalen Selenradiographie (THORAVISION)

Amorphous selenium is the detector material for x-ray in the digital selenium radiography (DSR). The surface charge pattern caused by an interaction with the selenium layer on the selenium surface is detected by probe arrays without contacting the drum.The processing of these signals by a system of fast post-processing offers the well known advantages of the digital technology. The sensitivity to x-ray may vary depending on the quality of the selenium layer and the dose level. This is called memory/fatigue effect. The read out of the films may be influenced by this effects.Two selenium drums from different charges used each one for a year showed different fatigue effects from 50 seconds up to 3 minutes respectively duration using the same test conditions (150kVp, 31 mAs). An exponential graph of first order is the correlat for the relaxation of the amorphous selenium layer. The characteristic parameters of this function vary between the selenium layers, therefore one can conclude that the fatigue effect is depending on the composition of the selenium layer. Clinical relevance of the digital selenium radiography using THORA VISION is discussed in this paper.

Precursor modification for preparation of CIS films by selenization technique

CuInSe 2 films have been prepared using the selenization technique. Preparation of the precursor as well as selenization were carried out by the vacuum evaporation technique. The sequence of copper and Indium layer deposition during precursor preparation affects the morphological and structural properties of precursor which directly have effects on the properties of selenized CIS films. A thin layer of amorphous selenium at the substrate/film interface has been used to improve the adherence of the film. The effect of the Se under-layer has been studied on the layers of copper, indium, CuIn precursors and CIS films, using structural, morphological and optical properties. The surface morphology of a single layer of copper and indium, with and without the Selenium under-layer, are quite different and drastically affect the properties of the precursor and selenized films. The Se under-layer does not take part in the chemical reaction of CIS formation during the selenization process. The modified CIS films are uniform, single phase, polycrystalline, chalcopyrite with (1 1 2) preferred orientation showing an energy band gap of 0.99 eV and an absorption coefficient of ∼10 5 cm −1, and have good adherence to the substrate for the scotch tape test.

Digital radiology using active matrix readout of amorphous selenium: detectors with high voltage protection.

A flat-panel x-ray imaging detector is being investigated for digital radiography and fluoroscopy. The detector uses a layer of amorphous selenium (a-Se) to convert x rays to a charge image, which is then electronically read out with a two-dimensional array of thin film transistors (TFTs). In order to sensitize the a-Se layer to x rays, a high voltage (of the order of several thousand volts) is applied to its top surface. The TFTs, which are at the bottom surface of the a-Se layer, are not subjected to any high voltage under normal radiological operational conditions since the pixel potential is < 10 V. However under a fault condition where these two events occur simultaneously: (1) suspended detector scan; and (2) an x-ray exposure more than ten times higher than normal, the voltage on the TFTs could rise to a damaging value. This paper describes a method for protecting the TFTs from high voltage damage under this fault condition. It employs a dual-gate TFT structure, one gate is for scanning control and the other is connected to the pixel electrode for high voltage protection. Before the pixel potential reaches a damaging value, the protection gate turns on the TFT automatically and drains excess charge away from the pixel thus providing a safe pixel saturation potential. In this paper, the characteristic curves of dual-gate TFTs are studied both theoretically and experimentally. The pixel x-ray response for imaging detectors with high voltage protection are predicted, and it is shown that with practical TFT designs the detector can provide a safe pixel saturation potential as well as satisfy the dynamic range required for diagnostic x-ray imaging applications.

Current advances and future trends in X-ray digital detectors for medical applications

Despite the progress made in imaging modalities based on nonionizing radiation such as ultrasound and magnetic resonance imaging, X-ray is still the primary imaging method in most radiology departments. In recent years, digital electronics and manufacturing methods have led to many approaches for the design and construction of X-ray detectors. The ideal medical X-ray system is a digital, flat-panel able to perform all current radiological modalities at reduced dose. It would, immediately after the patient's X-ray exposure, provide a high quality radiograph on a video monitor. It would also be usable for real-time imaging (e.g., digital cine loops and fluoroscopy). The physical form would be similar to a film/screen cassette (except for the addition of an umbilical cord) which could easily fit into the Bucky tray of existing X-ray rooms. There are two main approaches both based on active matrix technology developed for lap-top displays. The first, called the direct method employs a photoconductive layer of amorphous selenium (a-Se) to detect X-rays. The charge image formed on the surface of the a-Se is digitally read out in situ using a two-dimensional array of thin-film transistors (TFT's), i.e., an active matrix. The second flat panel method is called the indirect method and uses an active matrix to readout a phosphor layer. Phosphors give off light on interaction with X-rays and this is sensed by photo diodes on the active matrix. The relative practicality of the direct and indirect active matrix readout systems is an area of current investigation.

Digital radiology using active matrix readout of amorphous selenium: Theoretical analysis of detective quantum efficiency

A flat-panel x-ray imaging detector using a layer of amorphous selenium (a-Se) for direct conversion of x rays (to charge) and an active matrix for self-scanned readout is being investigated for digital radiology. A theoretical analysis of the spatial frequency dependent detective quantum efficiency (DQE(f)) of the self-scanned a-Se detector is performed based on a model of signal and noise propagation in a cascaded imaging system. Because of the high intrinsic resolution of a-Se and the pixelated active matrix readout method, such detectors are inherently undersampled and aliasing is present. The presampling modulation transfer function (MTF) and aliased noise power spectrum (NPS) of the detector were used in the analysis of DQE(f). It is proven that the aliased NPS for the self-scanned a-Se detectors is white. Since the shape of DQE(f) is determined by the ratio of MTF squared and the NPS, the shape of DQE(f) follows the square of the presampling MTF of the detector as a result of the white NPS. The analysis also shows that DQE(0) is proportional to the pixel fill factor, i.e., the fraction of each pixel area used for image charge collection. The DQE analysis is applied to detector parameters for three x-ray imaging applications: mammography, chest radiography, and fluoroscopy. The effects of pixel fill factor, imaging geometry (i.e., incident angle of x rays), and various sources of electronic noise on the detector DQE(f) are discussed. Strategies for maximizing detector DQE for each x-ray imaging application are proposed.

Digital radiology using active matrix readout of amorphous selenium: construction and evaluation of a prototype real-time detector.

The goal of the present work is to develop a large area, flat-panel solid-state detector for both digital radiography and fluoroscopy. The proposed detector employs a photoconductive layer of amorphous selenium (a-Se) to convert x rays into charge. The charge image formed by the a-Se layer is electronically read out in situ using a two dimensional array of thin film transistors (TFTs), or active matrix. Since the active matrix readout is capable of producing x-ray images in real-time, it can potentially be applied in both radiography and fluoroscopy. In this paper, the imaging performance of this concept is investigated using a prototype x-ray imaging detector. The designs for the active matrix, the peripheral electronic circuits, and the image acquisition system are described. Measurements of x-ray imaging properties of the prototype detector, i.e., x-ray sensitivity, presampling modulation transfer function (MTF), and noise power spectrum (NPS), were performed, and from which the spatial frequency dependent detective quantum efficiency (DQE) of the prototype was derived. The experimental results are in agreement with the results of our theoretical analysis. The factors affecting the imaging performance and methods of improvement in the future are discussed.

Flat-panel digital radiology with amorphous selenium and active-matrix readout.

Future imaging techniques will be capable of high image quality, fast acquisition, compactness, and versatile operation. A flat-panel imager that is expected to achieve these goals is under development. It consists of a thin layer of amorphous selenium that converts x rays directly to an electric charge and a thin-film electronic circuit, or active matrix, to read out the electronic signal directly to a computer host. The advantages of amorphous selenium include high resolution and low noise without loss of signal strength. The advantages of the active matrix are real-time readout, flexible design parameters, and compactness of the readout structures. A prototype of this imager has been built and operated, and initial images (of an x-ray test bar pattern and a hand phantom) have been acquired. Although the prototype was built to test scientific principles and many possibilities for optimization remain, the images already possess the quality necessary for many radiographic procedures. A large range of current and new applications exist for this imager.

Characteristics of dual-gate thin-film transistors for applications in digital radiology

A large-area flat-panel detector for digital radiology is being developed. The detector uses an array of dual-gate thin-film transistors (TFTs) to read out X-ray-generated charge produced in an amorphous selenium (a-Se) layer. The TFTs use CdSe as the semiconductor and use the bottom gate for row selection. The top gate can be divided into a "deliberate" gate, covering most of the channel length, and smaller "parasitic" gates that consist of (i) overlap of source or drain metal over the top-gate oxide, and (ii) gap regions in the metal that are covered only by the a-Se. In this paper we present the properties of dual-gate TFTs and examine the effect of both the deliberate and parasitic gates on the detector operation. Various options for controlling the top-gate potential are analyzed and discussed.

Metal overlayers on the MBE-grown ZnSe(001) surface

N-type ZnSe thin films have been grown by MBE on GaAs (001) surfaces and capped with an amorphous selenium layer. The Se cap was thermally desorbed under ultrahigh vacuum to recover the (2 × 1) and c(2 × 2) reconstructed surfaces. Selected metal contact formation was monitored using core and valence level photoelectron emission spectroscopy by in-situ exposure of the surface to heated sources of Au, Ag and Pb. In each case, lineshape analysis of emission spectra indicated a low level of interfacial mixing and provided an insight into the metal layer growth model. Both Au and Ag were found to grow in closely spaced islands of approximately equal height. The morphology of Au and Ag layers was confirmed by cross-sectional transmission electron microscopy. Monitoring of core and valence level emission peak positions allowed the determination of the metal-n-ZnSe Schottky barrier height, for a sufficiently thick metallic layer. Measurements on this wide-gap semiconductor, even at 300 K, were influenced by the presence of a surface photovoltage, which could be identified and subtracted for a fully-formed metallic layer. The n-type ZnSe Schottky barrier heights inferred from the relative Fermi level shifts ( Φ BN( Au) = 1.74 eV , Φ BN( Ag) = 1.47 eV and Φ BN( Pb) = 1.25 eV ), were found to scale with the metal work function for these three unreactive interfaces.