Shiva Abbaszadeh

Measurement of UV from a Microplasma by a Microfabricated Amorphous Selenium Detector

We spectrally demonstrate for the first time that an amorphous selenium metal-semiconductor-metal detector can be used for the measurement of ultraviolet photons (200-400 nm) generated from a portable battery-operated microplasma that is used as a light source. An advantage of this low-cost detector is that the device structure allows photons to strike the light-sensitive layer directly rather than through electrodes or blocking layers. Another advantage is that despite operation at high electric fields of up to 43 V/μm, the dark current of the detector at room temperature is 3 pA/mm2. Therefore, detector cooling is not required, and this facilitates portability for measurements on-site (i.e., in the field and away from a laboratory). Spectral response was monitored using a scanning monochromator, and it was compared with that obtained by a portable spectrometer fitted with a linear charge-coupled device detector. To demonstrate detector responsivity, emission signals with an appreciable signal-to-noise ratio were obtained by introducing nanogram amounts of the sample into the microplasma.

Gas Sensitive Porous Silver-Rutile High-Temperature Schottky Diode on Thermally Oxidized Titanium

The fabrication of porous silver-TiO2 gas sensitive Schottky diodes by utilizing thermally grown oxide layer on titanium substrates is described. The junction was formed by the partial sintering of silver particles on the oxidized metal substrate. The connection between titanium metal and its native oxide is ohmic and the substrate performs as a stable back-contact to the oxide semiconductor. For the characterization of the device, electrical contacts were made by connecting silver wires to the titanium substrate and the silver aggregate. Operating at elevated temperatures, the device behaved as a Schottky diode of high junction energy barrier in clean air, while in highly reducing atmospheres the barrier height dropped to zero and the junction was characterized as an ohmic contact. This reversible transition afforded a broad dynamic range and high sensitivity for chemical detection: Operating at 300degC, the reverse current of the diode increased by six orders of magnitude in response to the presence of 1 w-% 1-butanol vapor in the surrounding air, and a single diode was able to detect hydrogen in the 50 ppm to 8% concentration range. The observed electronic features of the device were described through a model constructed based on the work function of silver varying from 4.3 to ~ 6.5 eV with oxygen adsorption.

Low Dark-Current Lateral Amorphous-Selenium Metal–Semiconductor–Metal Photodetector

We report a lateral amorphous-selenium (a-Se) metal-semiconductor-metal detector with a blocking contact. The blocking contact, a polyimide layer, is shown to significantly reduce the dark current even at high applied biases that result in high photo-current-to-dark-current ratios, thus leading to wide dynamic range and high signal-to-noise ratio. The use of the polyimide blocking contact prevents the injection of both holes and electrons and improves considerably upon the high dark current of previously reported lateral a-Se detectors. The proposed detector demonstrates the feasibility of low-cost lateral a-Se devices for indirect conversion digital X-ray imaging applications such as chest radiography, fluoroscopy, and computed tomography.

Low Dark Current Amorphous Silicon Metal-Semiconductor-Metal Photodetector for Digital Imaging Applications

A novel lateral amorphous silicon (a-Si) metal-semiconductor-metal photodetector architecture is proposed using an organic blocking layer. Fabricated devices exhibit low dark-current, high dynamic range, and a measured external quantum efficiency of 65%, which represents a considerable improvement over previously reported designs. The higher performance is enabled by the introduction of a thin organic blocking layer and subsequently operating at high electric-fields. Unlike industry standard p-i-n photodiodes, our high performance lateral photosensor does not require doped p+/n+ layers. Thus, the reported device is compatible with current and previous generation a-Si thin film transistor display fabrication process making it promising for low-cost optical touch panel or diagnostic medical imaging applications.

Enhanced Detection Efficiency of Direct Conversion X-ray Detector Using Polyimide as Hole-Blocking Layer

In this article we demonstrate the performance of a direct conversion amorphous selenium (a-Se) X-ray detector using biphenyldisnhydride/1,4 phenylenediamine (BPDA/PPD) polyimide (PI) as a hole-blocking layer. The use of a PI layer with a-Se allows detector operation at high electric fields (≥10 V/μm) while maintaining low dark current, without deterioration of transient performance. The hole mobility of the PI/a-Se device is measured by the time-of-flight method at different electric fields to investigate the effect of the PI layer on detector performance. It was found that hole mobility as high as 0.75 cm2/Vs is achievable by increasing the electric field in the PI/a-Se device structure. Avalanche multiplication is also shown to be achievable when using PI as a blocking layer. Increasing the electric field within a-Se reduces the X-ray ionization energy, increases hole mobility, and improves the dynamic range and sensitivity of the detector.

Enhanced Dark Current Suppression of Amorphous Selenium Detector With Use of IGZO Hole Blocking Layer

We examined the potential application of indium gallium zinc oxide (IGZO) as a hole-blocking layer for an amorphous selenium (a-Se)-based detector to reduce the dark current and improve the sensitivity of the detector. By employing a thin layer of IGZO (375 nm), the dark current of an a-Se detector remains below 1 pA/mm2 up to electric fields as high as 60 V/μm. The measured dark current at different electric fields is comparable to the thermal generation currents in a-Se, thus demonstrating the good hole-blocking properties of IGZO. The detectors photo response was characterized using a blue light-emitting diode at different electric fields. A factor of three improvement in external quantum efficiency was observed by increasing the electric field of the detector from 10 to 50 V/μm.

Amorphous selenium direct detection CMOS digital x-ray imager with 25 micron pixel pitch

We have developed a high resolution amorphous selenium (a-Se) direct detection imager using a large-area compatible back-end fabrication process on top of a CMOS active pixel sensor having 25 micron pixel pitch. Integration of a-Se with CMOS technology requires overcoming CMOS/a-Se interfacial strain, which initiates nucleation of crystalline selenium and results in high detector dark currents. A CMOS-compatible polyimide buffer layer was used to planarize the backplane and provide a low stress and thermally stable surface for a-Se. The buffer layer inhibits crystallization and provides detector stability that is not only a performance factor but also critical for favorable long term cost-benefit considerations in the application of CMOS digital x-ray imagers in medical practice. The detector structure is comprised of a polyimide (PI) buffer layer, the a-Se layer, and a gold (Au) top electrode. The PI layer is applied by spin-coating and is patterned using dry etching to open the backplane bond pads for wire bonding. Thermal evaporation is used to deposit the a-Se and Au layers, and the detector is operated in hole collection mode (i.e. a positive bias on the Au top electrode). High resolution a-Se diagnostic systems typically use 70 to 100 μm pixel pitch and have a pre-sampling modulation transfer function (MTF) that is significantly limited by the pixel aperture. Our results confirm that, for a densely integrated 25 μm pixel pitch CMOS array, the MTF approaches the fundamental material limit, i.e. where the MTF begins to be limited by the a-Se material properties and not the pixel aperture. Preliminary images demonstrating high spatial resolution have been obtained from a frst prototype imager.

Integration of an amorphous silicon passive pixel sensor array with a lateral amorphous selenium detector for large area indirect conversion x-ray imaging applications

Previously, we reported on a single-pixel detector based on a lateral a-Se metal-semiconductor-metal structure, intended for indirect conversion X-ray imaging. This work is the continuous effort leading to the first prototype of an indirect conversion X-ray imaging sensor array utilizing lateral amorphous selenium. To replace a structurally-sophisticated vertical multilayer amorphous silicon photodiode, a lateral a-Se MSM photodetector is employed which can be easily integrated with an amorphous silicon thin film transistor passive pixel sensor array. In this work, both 2×2 macro-pixel and 32×32 micro-pixel arrays were fabricated and tested along with discussion of the results.

First acquisition of data from a prototype 3-D position sensitive CZT PET system

This study presents the first system characterization results of a novel 1 mm resolution small animal PET system based on cadmium zinc telluride (CZT). The 3-D position-sensitive dual-CZT detector module and readout electronics previously developed in our lab were scaled up to complete significant portions of the final PET system. This sub-system was configured as two opposing detection panels containing a total of twelve 40 mm × 40 mm × 5 mm monolithic CZT crystals for proof of concept. System-level characterization studies, including optimizing the trigger threshold of each channels comparators, were performed to bring the system to an acceptable operating point. 68Ge and 137Cs radioactive isotopes were used to characterize the anode energy resolution of all channels in the system. The mean measured global 511 keV photopeak energy resolution over 240 anode channels was found to be 8.03 ± 0.96% FWHM.

Characterization of Low Dark-Current Lateral Amorphous-Selenium Metal-Semiconductor-Metal Photodetectors

We report a lateral amorphous-selenium (a-Se) metal-semiconductor-metal photodetector with a blocking contact. The blocking contact, a polyimide layer, is shown to significantly reduce the dark current even at high applied biases that result in high photo-to-dark-current ratios, thus leading to wide dynamic range and high signal-to-noise ratio. The use of the polyimide blocking contact prevents the injection of both holes and electrons and improves considerably upon the dark current of previously reported lateral a-Se detectors. The presence of charge trapping at the polyimide/a-Se interface is found to be negligible through the use of pulsed light experiments. The effects of electrode spacing and electrode width on device performance are investigated through experiment and simulation for device optimization. From the devices that are fabricated, it is found that the dark current is strongly dependent on the comb fingers density while the same trend is not observed for the photocurrent. It is found that the device with 10- μm electrode spacing and 10-μm electrode width has the best performance in terms of photo and dark current. This paper demonstrates the promise of low-cost lateral a-Se devices for use in indirect conversion large area digital medical X-ray imaging applications, such as chest radiography, real-time fluoroscopy, and cone beam computed tomography.