High-gain Avalanche Rushing Photoconductor Research Articles

Observation of two-level defect system in amorphous Se superlattices

Amorphous Se is a well-known photoconductor from its early applications in xerography and ultra-sensitive photodetectors like the High-gain Avalanche Rushing Photoconductor (HARP) device. The established way of fabricating the photoconductor for the HARP is rotational thermal evaporation using multilayers of Se and As2Se3. However, the electronic effects of multilayering have not yet been clarified. In this report, we investigated the multilayer structure as a superlattice of Se and As2Se3 fabricated using rotational evaporation and show that the superlattice structure results in the uniformization of the defect levels in the base materials. We found four energy levels associated with defects in As2Se3 and three levels in amorphous Se. In comparison, the superlattice structure of the two materials shows two clear energy levels at EC,Se − 0.533 eV and EV,Se + 0.269 eV. The resulting two occupied energy levels explain the photoelectronic and transport properties observed in multilayer amorphous Se. This result “reinvents” the multilayer structure as a material with observed quantum effects, which significantly improves the material performance in photodetection.

Toward Scintillator High-Gain Avalanche Rushing Photoconductor Active Matrix Flat Panel Imager (SHARP-AMFPI): Initial fabrication and characterization.

We present the first prototype Scintillator High-Gain Avalanche Rushing Photoconductor Active Matrix Flat Panel Imager (SHARP-AMFPI). This detector includes a layer of avalanche amorphous Selenium (a-Se) (HARP) as the photoconductor in an indirect detector to amplify the signal and reduce the effects of electronic noise to obtain quantum noise-limited images for low-dose applications. It is the first time avalanche a-Se has been used in a solid-state imaging device and poses as a possible solution to eliminate the effects of electronic noise, which is crucial for low-dose imaging performance of AMFPI. We successfully deposited a solid-state HARP structure onto a 24 × 30 cm2 array of thin-film transistors (TFT array) with a pixel pitch of 85 μm. The HARP layer consists of 16 μm of a-Se with a hole-blocking and electron-blocking layer to prevent charge injection from the high-voltage bias and pixel electrodes, respectively. An electric field (ESe ) up to 105 V μm-1 was applied across the a-Se layer without breakdown. A 150 μm thick-structured CsI:Tl scintillator was used to form SHARP-AMFPI. The x-ray imaging performance is characterized using a 30 kVp Mo/Mo beam. We evaluate the spatial resolution, noise power, and detective quantum efficiency at zero frequency of the system with and without avalanche gain. The results are analyzed using cascaded linear system model (CLSM). An avalanche gain of 76 ± 5 was measured at ESe = 105 V μm-1 . We demonstrate that avalanche gain can amplify the signal to overcome electronic noise. As avalanche gain is increased, image quality improves for a constant (0.76 mR) exposure until electronic noise is overcome. Our system is currently limited by poor optical transparency of our high-voltage electrode and long integrating time which results in dark current noise. These two effects cause high-spatial frequency noise to dominate imaging performance. We demonstrate the feasibility of a solid-state HARP x-ray imager and have fabricated the largest active area HARP sensor to date. Procedures to reduce secondary quantum and dark noise are outlined. Future work will improve optical coupling and charge transport which will allow for frequency DQE and temporal metrics to be obtained.

MO‐AB‐BRA‐07: Low Dose Imaging with Avalanche Amorphous Selenium Flat Panel Imager

Purpose:We present the first active matrix flat panel imager (AMFPI) capable of producing x‐ray quantum noise limited images at low doses by overcoming the electronic noise through signal amplification by photoconductive avalanche gain (gav). The indirect detector fabricated uses an optical sensing layer of amorphous selenium (a‐Se) known as High‐Gain Avalanche Rushing Photoconductor (HARP). The detector design is called Scintillator HARP (SHARP)‐AMFPI. This is the first image sensor to utilize solid‐state HARP technology.Methods:The detector's electronic readout is a 24 × 30 cm2 array of thin film transistors (TFT) with a pixel pitch of 85 µm. The HARP structure consists of a 15 µm layer of a‐Se isolated from the high voltage (HV) and signal electrode by a 2 µm thick hole blocking layer and electron blocking layer, respectively, to reduce dark current. A 150 µm thick structured CsI scintillator with reflective backing and a fiber optic faceplate (FOP) was coupled to the semi‐transparent HV bias electrode of the HARP structure. Images were acquired using a 30 kVp Mo/Mo spectrum typically used in mammography.Results:Optical sensitivity measurements demonstrate that gav = 76 ± 5 can be achieved over the entire active area of the detector. At a constant dose to the detector of 6.67 µGy, image quality increases with gav until the effective electronic noise is negligible. Quantum noise limited images can be obtained with doses as low as 0.18 µGy.Conclusion:We demonstrate the feasibility of utilizing avalanche gain to overcome electronic noise. The indirect detector fabricated is the first solid‐state imaging sensor to use HARP, and the largest active area HARP sensor to date. Our future work is to improve charge transport within the HARP structure and utilize a transparent HV electrode.

Charge transport model in solid-state avalanche amorphous selenium and defect suppression design

Avalanche amorphous selenium (a-Se) in a layer of High Gain Avalanche Rushing Photoconductor (HARP) is being investigated for its use in large area medical imagers. Avalanche multiplication of photogenerated charge requires electric fields greater than 70 V μm−1. For a-Se to withstand this high electric field, blocking layers are used to prevent the injection of charge carriers from the electrodes. Blocking layers must have a high injection barrier and deep trapping states to reduce the electric field at the interface. In the presence of a defect in the blocking layer, a distributed resistive layer (DRL) must be included into the structure to build up space charge and reduce the electric field in a-Se and the defect. A numerical charge transport model has been developed to optimize the properties of blocking layers used in various HARP structures. The model shows the incorporation of a DRL functionality into the p-layer can reduce dark current at a point defect by two orders of magnitude by reducing the field in a-Se to the avalanche threshold. Hole mobility in a DRL of ∼10−8 cm2 V−1 s−1 at 100 V μm−1 as demonstrated by the model can be achieved experimentally by varying the hole mobility of p-type organic or inorganic semiconductors through doping, e.g., using Poly(9-vinylcarbozole) doped with 1%–3% (by weight) of poly(3-hexylthiopene).

Development of solid-state avalanche amorphous selenium for medical imaging.

Active matrix flat panel imagers (AMFPI) have limited performance in low dose applications due to the electronic noise of the thin film transistor (TFT) array. A uniform layer of avalanche amorphous selenium (a-Se) called high gain avalanche rushing photoconductor (HARP) allows for signal amplification prior to readout from the TFT array, largely eliminating the effects of the electronic noise. The authors report preliminary avalanche gain measurements from the first HARP structure developed for direct deposition onto a TFT array. The HARP structure is fabricated on a glass substrate in the form of p-i-n, i.e., the electron blocking layer (p) followed by an intrinsic (i) a-Se layer and finally the hole blocking layer (n). All deposition procedures are scalable to large area detectors. Integrated charge is measured from pulsed optical excitation incident on the top electrode (as would in an indirect AMFPI) under continuous high voltage bias. Avalanche gain measurements were obtained from samples fabricated simultaneously at different locations in the evaporator to evaluate performance uniformity across large area. An avalanche gain of up to 80 was obtained, which showed field dependence consistent with previous measurements from n-i-p HARP structures established for vacuum tubes. Measurements from multiple samples demonstrate the spatial uniformity of performance using large area deposition methods. Finally, the results were highly reproducible during the time course of the entire study. We present promising avalanche gain measurement results from a novel HARP structure that can be deposited onto a TFT array. This is a crucial step toward the practical feasibility of AMFPI with avalanche gain, enabling quantum noise limited performance down to a single x-ray photon per pixel.

WE‐E‐18A‐01: Large Area Avalanche Amorphous Selenium Sensors for Low Dose X‐Ray Imaging

Purpose:A large area indirect flat panel imager (FPI) with avalanche gain is being developed to achieve x‐ray quantum noise limited low dose imaging. It uses a thin optical sensing layer of amorphous selenium (a‐Se), known as High‐Gain Avalanche Rushing Photoconductor (HARP), to detect optical photons generated from a high resolution x‐ray scintillator. We will report initial results in the fabrication of a solid‐state HARP structure suitable for a large area FPI. Our objective is to establish the blocking layer structures and defect suppression mechanisms that provide stable and uniform avalanche gain.Methods:Samples were fabricated as follows: (1) ITO signal electrode. (2) Electron blocking layer. (3) A 15 micron layer of intrinsic a‐Se. (4) Transparent hole blocking layer. (5) Multiple semitransparent bias electrodes to investigate avalanche gain uniformity over a large area. The sample was exposed to 50ps optical excitation pulses through the bias electrode. Transient time of flight (TOF) and integrated charge was measured. A charge transport simulation was developed to investigate the effects of varying blocking layer charge carrier mobility on defect suppression, avalanche gain and temporal performance.Results:Avalanche gain of ∼200 was achieved experimentally with our multi‐layer HARP samples. Simulations using the experimental sensor structure produced the same magnitude of gain as a function of electric field. The simulation predicted that the high dark current at a point defect can be reduced by two orders of magnitude by blocking layer optimization which can prevent irreversible damage while normal operation remained unaffected.Conclusion:We presented the first solid state HARP structure directly scalable to a large area FPI. We have shown reproducible and uniform avalanche gain of 200. By reducing mobility of the blocking layers we can suppress defects and maintain stable avalanche. Future work will optimize the blocking layers to prevent lag and ghosting.

(Invited) Flat-Panel X-ray Image Sensor Using Thin Film Transistors and Field Emitter Arrays

Active matrix flat panel imagers (AMFPI) have been commercialized for a wide range of x-ray imaging applications. They have demonstrated superior imaging performance except in fluoroscopy, where the electronic noise degrades the image quality, making it inferior to x-ray imaging intensifier (XRII). In this paper we will present two FPI approaches under investigation to overcome the electronic noise limitation. They use an amorphous selenium layer with programmable avalanche gain to detect light generated from an x-ray scintillator upon x-ray absorption. Two charge readout methods are being investigated: a thin-film transistor (TFT) array; and a field emitter array (FEA). The amorphous selenium (a-Se) avalanche photoconductor is called HARP (high-gain avalanche rushing photoconductor). The avalanche gain of HARP depends on both a-Se thickness and applied electric field ESe. At ESe of > 80 V/μm, the avalanche gain can enhance the signal at low dose (e.g. fluoroscopy) and make the detector x-ray quantum noise limited down to a single x-ray photon. At high exposure (e.g. radiography), the avalanche gain can be turned off by decreasing ESe to < 70 V/μm, thus ensuring a wide dynamic range without burdening the readout electronics. The potential x-ray imaging performance of both FPI approaches, especially the aspect of programmable gain to ensure wide dynamic range and x-ray quantum noise limited performance at the lowest exposure in fluoroscopy, have been investigated.

Electroded avalanche amorphous selenium (a-Se) photosensor

Although avalanche amorphous selenium (a-Se) is a very promising photoconductor for a variety of imaging applications, it is currently restricted to applications with electron beam readout in vacuum pick-up tube called a High-gain Avalanche Rushing Photoconductor (HARP). The electron beam readout is compatible with high definition television (HDTV) applications, but for use in solid-state medical imaging devices it should be replaced by an electronic readout with a two-dimensional array of metal pixel electrodes. However, due to the high electric field required for avalanche multiplication, it is a technological challenge to avoid possible dielectric breakdown at the edges, where electric field experiences local enhancement. It has been shown recently that this problem can be overcome by the use of a Resistive Interface Layer (RIL) deposited between a-Se and the metal electrode, however, at that time, at a sacrifice in transport properties.Here we show that optimization of RIL deposition technique allows for electroded avalanche a-Se with transport properties and time performance previously not achievable with any other a-Se structures. We have demonstrated this by detailed analysis of transport properties performed by Time-of-Flight (TOF) technique. Our results showed that a stable gain of 200 is reached at 104 V/μm for a 15-μm thick a-Se layer, which is the maximum theoretical gain for this thickness. We conclude that RIL is an enabling technology for practical implementation of solid-state avalanche a-Se image sensors.

A solid‐state amorphous selenium avalanche technology for low photon flux imaging applications

The feasibility of a practical solid-state technology for low photon flux imaging applications was investigated. The technology is based on an amorphous selenium photoreceptor with a voltage-controlled avalanche multiplication gain. If this photoreceptor can provide sufficient internal gain, it will be useful for an extensive range of diagnostic imaging systems. The avalanche photoreceptor under investigation is referred to as HARP-DRL. This is a novel concept in which a high-gain avalanche rushing photoconductor (HARP) is integrated with a distributed resistance layer (DRL) and sandwiched between two electrodes. The avalanche gain and leakage current characteristics of this photoreceptor were measured. HARP-DRL has been found to sustain very high electric field strengths without electrical breakdown. It has shown avalanche multiplication gains as high as 10(4) and a very low leakage current (< or = 20 pA/mm2). This is the first experimental demonstration of a solid-state amorphous photoreceptor which provides sufficient internal avalanche gain for photon counting and photon starved imaging applications.

Direct‐conversion flat‐panel imager with avalanche gain: Feasibility investigation for HARP‐AMFPI

The authors are investigating the concept of a direct-conversion flat-panel imager with avalanche gain for low-dose x-ray imaging. It consists of an amorphous selenium (a-Se) photoconductor partitioned into a thick drift region for x-ray-to-charge conversion and a relatively thin region called high-gain avalanche rushing photoconductor (HARP) in which the charge undergoes avalanche multiplication. An active matrix of thin film transistors is used to read out the electronic image. The authors call the proposed imager HARP active matrix flat panel imager (HARP-AMFPI). The key advantages of HARP-AMFPI are its high spatial resolution, owing to the direct-conversion a-Se layer, and its programmable avalanche gain, which can be enabled during low dose fluoroscopy to overcome electronic noise and disabled during high dose radiography to prevent saturation of the detector elements. This article investigates key design considerations for HARP-AMFPI. The effects of electronic noise on the imaging performance of HARP-AMFPI were modeled theoretically and system parameters were optimized for radiography and fluoroscopy. The following imager properties were determined as a function of avalanche gain: (1) the spatial frequency dependent detective quantum efficiency; (2) fill factor; (3) dynamic range and linearity; and (4) gain nonuniformities resulting from electric field strength nonuniformities. The authors results showed that avalanche gains of 5 and 20 enable x-ray quantum noise limited performance throughout the entire exposure range in radiography and fluoroscopy, respectively. It was shown that HARP-AMFPI can provide the required gain while maintaining a 100% effective fill factor and a piecewise dynamic range over five orders of magnitude (10(-7)-10(-2) R/frame). The authors have also shown that imaging performance is not significantly affected by the following: electric field strength nonuniformities, avalanche noise for x-ray energies above 1 keV and direct interaction of x rays in the gain region. Thus, HARP-AMFPI is a promising flat-panel imager structure that enables high-resolution fully quantum noise limited x-ray imaging over a wide exposure range.

Photo-crystallization in a-Se imaging targets: Raman studies of competing effects

Photo-induced crystallization of a-Se is investigated by Raman spectroscopy as a function of temperature (250–340 K) and exposure time in thin-film structures used as targets in high-gain avalanche rushing photoconductor (HARP) video cameras. The Stokes-to-Antistokes ratio is monitored to obtain the local temperature T loc at the laser spot; fluxes (632 nm) of 17 and 10 W/cm 2 are used. We find a rich temperature behavior that reflects the competition of changes in viscosity and strain, and defines four distinct regimes. No photo-crystallization is seen for T loc below 260 K, nor in a 15 K range around T g ∼ 310 K. For T loc in the regime 260–302 K the initial rate of crystal growth after onset of photo-crystallization is temperature independent, whereas for T loc > 318 K the growth rate is thermally enhanced. Our results are in qualitative accord with a theory by Stephens treating the effects of local strain on the secondary growth of crystalline nuclei in a-Se. We conclude that the observed growth rate between 260 and 302 K is driven by local strain, and that relaxation of this strain near T g suppresses crystal growth until thermally assisted processes accelerate the photo-crystallization at higher temperatures.

The effect of temperature on photoinduced metastability in avalanche a-Se layers

The effect of temperature on the initiation of light-induced defects in a-Se targets of high-gain avalanche rushing photoconductor (HARP) camera tubes was studied. At room temperature, avalanche a-Se was found to be subject to both reversible and irreversible photodarkening. At 35°C, the irreversible photodarkening component was completely absent. This result is attributed to the instantaneous annealing of light-induced metastable coordination defects at elevated temperatures. A model for the formation of permanent crystalline areas at low temperatures via transition states associated with light-induced metastable defects is proposed.

Indirect flat‐panel detector with avalanche gain: Fundamental feasibility investigation for SHARP‐AMFPI (scintillator HARP active matrix flat panel imager)

An indirect flat-panel imager (FPI) with avalanche gain is being investigated for low-dose x-ray imaging. It is made by optically coupling a structured x-ray scintillator CsI(Tl) to an amorphous selenium (a-Se) avalanche photoconductor called HARP (high-gain avalanche rushing photoconductor). The final electronic image is read out using an active matrix array of thin film transistors (TFT). We call the proposed detector SHARP-AMFPI (scintillator HARP active matrix flat panel imager). The advantage of the SHARP-AMFPI is its programmable gain, which can be turned on during low dose fluoroscopy to overcome electronic noise, and turned off during high dose radiography to avoid pixel saturation. The purpose of this paper is to investigate the important design considerations for SHARP-AMFPI such as avalanche gain, which depends on both the thickness d(Se) and the applied electric field E(Se) of the HARP layer. To determine the optimal design parameter and operational conditions for HARP, we measured the E(Se) dependence of both avalanche gain and optical quantum efficiency of an 8 microm HARP layer. The results were used in a physical model of HARP as well as a linear cascaded model of the FPI to determine the following x-ray imaging properties in both the avalanche and nonavalanche modes as a function of E(Se): (1) total gain (which is the product of avalanche gain and optical quantum efficiency); (2) linearity; (3) dynamic range; (4) gain nonuniformity resulting from thickness nonuniformity; and (5) effects of direct x-ray interaction in HARP. Our results showed that a HARP layer thickness of 8 microm can provide adequate avalanche gain and sufficient dynamic range for x-ray imaging applications to permit quantum limited operation over the range of exposures needed for radiography and fluoroscopy.