Saint-Laurent, Canada
Saint-Laurent, Canada

Time filter

Source Type

Tousignant O.,ANRAD Corporation | Karim K.S.,University of Waterloo | Rowlands J.A.,Thunder Bay Regional Research Institute
Applied Physics Letters | Year: 2012

We demonstrate a high granularity multi-well solid-state detector with the unipolar time-differential property. Results show an improvement in the temporal pulse response by more than two orders-of-magnitude using amorphous selenium as the photoconductive film. The significance of the results presented here is the ability to reach the intrinsic physical limit for detector pulse speed by transitioning from the slow transit-time-limited response which depends on the bulk carrier transport mechanism, to the ultrafast dispersion-limited response which depends on the spatial spreading of the collected carrier packet. © 2012 American Institute of Physics.


Kasap S.,University of Saskatchewan | Frey J.B.,University of Saskatchewan | Belev G.,University of Saskatchewan | Tousignant O.,Anrad Corporation | And 10 more authors.
Sensors | Year: 2011

In the last ten to fifteen years there has been much research in using amorphous and polycrystalline semiconductors as x-ray photoconductors in various x-ray image sensor applications, most notably in flat panel x-ray imagers (FPXIs). We first outline the essential requirements for an ideal large area photoconductor for use in a FPXI, and discuss how some of the current amorphous and polycrystalline semiconductors fulfill these requirements. At present, only stabilized amorphous selenium (doped and alloyed a-Se) has been commercialized, and FPXIs based on a-Se are particularly suitable for mammography, operating at the ideal limit of high detective quantum efficiency (DQE). Further, these FPXIs can also be used in real-time, and have already been used in such applications as tomosynthesis. We discuss some of the important attributes of amorphous and polycrystalline x-ray photoconductors such as their large area deposition ability, charge collection efficiency, x-ray sensitivity, DQE, modulation transfer function (MTF) and the importance of the dark current. We show the importance of charge trapping in limiting not only the sensitivity but also the resolution of these detectors. Limitations on the maximum acceptable dark current and the corresponding charge collection efficiency jointly impose a practical constraint that many photoconductors fail to satisfy. We discuss the case of a-Se in which the dark current was brought down by three orders of magnitude by the use of special blocking layers to satisfy the dark current constraint. There are also a number of polycrystalline photoconductors, HgI2 and PbO being good examples, that show potential for commercialization in the same way that multilayer stabilized a-Se x-ray photoconductors were developed for commercial applications. We highlight the unique nature of avalanche multiplication in a-Se and how it has led to the development of the commercial HARP video-tube. An all solid state version of the HARP has been recently demonstrated with excellent avalanche gains; the latter is expected to lead to a number of novel imaging device applications that would be quantum noise limited. While passive pixel sensors use one TFT (thin film transistor) as a switch at the pixel, active pixel sensors (APSs) have two or more transistors and provide gain at the pixel level. The advantages of APS based x-ray imagers are also discussed with examples. © 2011 by the authors; licensee MDPI, Basel, Switzerland.


Goldan A.H.,State University of New York at Stony Brook | Rowlands J.A.,Thunder Bay Regional Research Institute | Tousignant O.,ANRAD Corporation | Karim K.S.,University of Waterloo
Journal of Applied Physics | Year: 2013

The use of high resistivity amorphous solids as photodetectors, especially amorphous selenium, is currently of great interest because they are readily produced over large area at substantially lower cost compared to grown crystalline solids. However, amorphous solids have been ruled out as viable radiation detection media for high frame-rate applications, such as single-photon-counting imaging, because of low carrier mobilities, transit-time-limited photoresponse, and consequently, poor time resolution. To circumvent the problem of poor charge transport in amorphous solids, we propose unipolar time-differential charge sensing by establishing a strong near-field effect using an electrostatic shield within the material. For the first time, we have fabricated a true Frisch grid inside a solid-state detector by evaporating amorphous selenium over photolithographically prepared multi-well substrates. The fabricated devices are characterized with optical, x-ray, and gamma-ray impulse-like excitations. Results prove the proposed unipolar time-differential property and show that time resolution in non-dispersive amorphous solids can be improved substantially to reach the theoretical limit set by spatial spreading of the collected Gaussian carrier cloud. © 2013 AIP Publishing LLC.


Kabir M.Z.,Concordia University at Montréal | Chowdhury L.,University of Toronto | DeCrescenzo G.,University of Toronto | Tousignant O.,ANRAD Corporation | And 3 more authors.
Medical Physics | Year: 2010

Purpose: A numerical model and the experimental methods to study the x-ray exposure dependent change in the modulation transfer function (MTF) of amorphous selenium (a-Se) based active matrix flat panel imagers (AMFPIs) are described. The physical mechanisms responsible for the x-ray exposure dependent change in MTF are also investigated. Methods: A numerical model for describing the x-ray exposure dependent MTF of a-Se based AMFPIs has been developed. The x-ray sensitivity and MTF of an a-Se AMFPI have been measured as a function of exposure. The instantaneous electric field and free and trapped carrier distributions in the photoconductor layer are obtained by numerically solving the Poisson's equation, continuity equations, and trapping rate equations using the backward Euler finite difference method. From the trapped carrier distributions, a method for calculating the MTF due to incomplete charge collection is proposed. Results: The model developed in this work and the experimental data show a reasonably good agreement. The model is able to simultaneously predict the dependence of the sensitivity and MTF on accumulated exposure at different applied fields and bias polarities, with the same charge transport parameters that are typical of the particular a-Se photoconductive layer that is used in these AMFPIs. Under negative bias, the MTF actually improves with the accumulated x-ray exposure while the sensitivity decreases. The MTF enhancement with exposure decreases with increasing applied field. Conclusions: The most prevalent processes that control the MTF under negative bias are the recombination of drifting holes with previously trapped electrons (electrons remain in deep traps due to their long release times compared with the time scale of the experiments) and the deep trapping of drifting holes and electrons. © 2010 American Association of Physicists in Medicine.


Goldan A.H.,University of Waterloo | Tousignant O.,ANRAD Corporation | Laperrire L.,ANRAD Corporation | Karim K.S.,University of Waterloo
Applied Physics Letters | Year: 2010

Memory effects in direct detection solid-state photoconductors are attributed to interrupted charge transport by traps in the bulk and result in persistent photocurrent lag and ghosting. The identified sources for image lag following the cessation of x-ray exposure are the inhomogeneous electric field's spatial distribution and the detrapping of the bulk space charge. This work shows that the latter is the dominant mechanism for the persistent photocurrent lag in stabilized n-i-p amorphous selenium photoconductors and proposes unipolar charge-sensing detector design for reducing image lag and improving the temporal performance of direct conversion x-ray imagers. © 2010 American Institute of Physics.


Izadi M.H.,University of Waterloo | Tousignant O.,ANRAD Corporation | Mokam M.F.,ANRAD Corporation | Karim K.S.,University of Waterloo
IEEE Transactions on Electron Devices | Year: 2010

Active pixel sensor (APS) circuits are an alternate to passive pixel sensor (PPS) circuits, which, while common in CMOS technology, have yet to be incorporated into commercial amorphous silicon (a-Si) large-area imagers. A proof-of-concept 64 × 64 APS array for low-exposure medical X-ray imaging is fabricated in a-Si technology and mated with an amorphous selenium photoconductor. Modulation transfer function (MTF) response and transient response for the APS imager indicate significant charge trapping at the top insulator/a-Se interface. MTF response indicates an effective fill factor of 94.5% for a geometric fill factor of 57% at an electric field strength of 10 V/μ m. Signal-to-noise ratio (SNR) performance from the prototype imager is comparable to a state-of-the-art commercially available a-Si PPS X-ray imager for X-ray exposures down to 1.5 μR using an RQA5 standard fluoroscopic characterization beam. Pixel design and fabrication process improvements are suggested to improve the SNR performance of the APS imager below 1.5 μR. © 2010 IEEE.


Allec N.,University of Waterloo | Abbaszadeh S.,University of Waterloo | Fleck A.,Grand River Regional Cancer Center | Tousignant O.,Anrad Corporation | Karim K.S.,University of Waterloo
IEEE Transactions on Nuclear Science | Year: 2012

Dual-layer, or stacked, detectors reduce motion artifacts in combined X-ray images, such as k-edge images, by acquiring low-and high-energy signals simultaneously. In this work we constructed a prototype single pixel dual-layer detector using amorphous selenium (a-Se) as the conversion material based on the same technology used for commercial large area flat panel imagers. A cascaded detector model was used to model the detector and for comparison with the experimental measurements. The detector was demonstrated to obtain contrast-enhanced mammography signals using an iodinated contrast agent. The experimentally obtained contrast was compared with the model and good agreement was found demonstrating the feasibility of the dual-layer technology. For comparison purposes, a single-layer single pixel detector capable of k-edge imaging but prone to motion artifacts (acquiring low-and high-energy signals sequentially) was also studied. © 2012 IEEE.


Mahmood S.A.,Concordia University at Montréal | Kabir M.Z.,Concordia University at Montréal | Tousignant O.,Anrad Corporation | Greenspan J.,Anrad Corporation | Mokam M.F.,Anrad Corporation
Progress in Biomedical Optics and Imaging - Proceedings of SPIE | Year: 2010

The ghosting recovery techniques and mechanisms in multilayer selenium X-ray detector structures for mammography are experimentally and theoretically investigated. The experiments have been carried out under low positive applied electric field (∼l-2V/μm) since a very little ghost can be seen under normal operating applied electric field (10V/μm). A ghost removal technique is investigated by reversing the bias polarity during the natural recovery process. The theoretical model considers accumulated trapped charges and their effects (trap filling, recombination, electric field profile, and electric field dependent electron-hole pair creation), the carrier transport in the blocking layers, and the effects of charge injection from the metal contacts. We consider carrier trapping in both charged and neutral defect states. It has been assumed that the X-ray induced deep trap centers are neutral defects. The time dependent carrier detrapping and structural relaxation (recovery of meta-stable trap centers) are also considered. The sensitivity in a rested sample is recovered mainly by the carrier detrapping, the recombination of the injected carriers with the existing trapped carriers, and the relaxation of the X-ray induced deep trap centers. A faster sensitivity recovery is found by reversing the bias during the natural recovery process. During reverse bias huge number of holes are injected from the metal and recombine with the trapped electrons. This results in faster sensitivity recovery. The electric fields at the metal contacts increase with time at the beginning of the natural ghosting recovery process which leads to the initial increase of the dark current. Later the electric fields at the metal contacts decrease and hence the dark current decays over time during the natural recovery process. The theoretical model shows a very good agreement with the experimental results. © 2010 SPIE.


Mahmood S.A.,Concordia University at Montréal | Kabir M.Z.,Concordia University at Montréal | Tousignant O.,Anrad Corporation | Greenspan J.,Anrad Corporation
IEEE Transactions on Nuclear Science | Year: 2012

The ghosting recovery mechanisms in multilayer selenium X-ray detector structures for mammography are experimentally and theoretically investigated. The experiments have been carried out under low positive applied electric field (∼ 1 V/μm). A ghost removal technique is investigated by reversing the bias polarity during the natural recovery process. The theoretical model considers accumulated trapped charges and their effects (trap filling, recombination, detrapping, structural relaxation and electric field dependent electron-hole pair creation), and effects of charge injection from the metal contacts. Carrier trapping in both charged and neutral defect states has been considered in the model. It has been found that the X-ray induced deep trap centers are charged defects. A faster sensitivity recovery is found by reversing the bias during the natural recovery process. During the reverse bias, a huge number of carriers are injected from the metal contacts, and fill the existing trap centers. This results in an abrupt recovery of the relative sensitivity. However, the relative sensitivity slightly decreases with time after this abrupt recovery due to the release of the trapped electrons as well as the long recovery time of the induced trap centers. The theoretical model shows a very good agreement with the experimental results. © 2012 IEEE.


Frey J.B.,University of Saskatchewan | Belev G.,University of Saskatchewan | Tousignant O.,Anrad Corporation | Mani H.,Anrad Corporation | And 2 more authors.
Journal of Applied Physics | Year: 2012

We report on experimental results which show that the dark current in n-i-p structured, amorphous selenium films is independent of i-layer thickness in samples with consistently thick blocking layers. We have observed, however, a strong dependence on the n-layer thickness and positive contact metal chosen. These results indicate that the dominant source of the dark current is carrier injection from the contacts and any contribution from carriers thermally generated in the bulk of the photoconductive layer is negligible. This conclusion is supported by a description of the dark current transients at different applied fields by a model which assumes only carrier emission over a Schottky barrier. This model also predicts that while hole injection is initially dominant, some time after the application of the bias, electron injection may become the dominant source of dark current. © 2012 American Institute of Physics.

Loading Anrad Corporation collaborators
Loading Anrad Corporation collaborators