The 25th International Conference on Amorphous and Nano-crystalline Semiconductors
August 18–23, 2013 Toronto, Ontario Canada
Chalcogenides: Photoinduced Changes and Devices II
Chair: Julia Berashevich, Lakehead University
Investigation of Electrical Conduction in Polyimide/Amorphous Selenium Films under High Electric Fields
Electrical and Computer Engineering, University of Waterloo, Ontario N2L 3G1, Canada
Polyimide (PI) has been used as a hole-blocking layer to mitigate injection of holes from positively biased electrode into the bulk of amorphous selenium (a-Se) in a-Se based X-ray detectors. It was also shown that PI layer reduces the strain between the a-Se and substrate layer leading to better reliability in the form of reduced recrystallization. At low electric fields (≤ 10 V/μm), PI contributes to charge buildup at PI/a-Se interface and lowers the reliability of the detector by reducing the field inside a-Se layer. However, no significant charge buildup was reported at higher electric fields. The PI layer uses a simple fabrication process that can be easily integrated into current large area digital imager manufacturing processes and it is a promising material for the development of high conversion gain a-Se detectors. It is therefore desirable to understand the current conduction mechanism in PI/a-Se detectors operated under high electric fields.
In this paper, the electrical conduction current in PI/a-Se films is studied by measuring the current from ITO glass/PI/a-Se/Au fabricated device structures operated at high fields (up to approximately 100 V/μm). Different PI thicknesses (0.8 to 2.5 μm) with the same thickness of a-Se (15 μm) were fabricated and the dark and photo current transient behavior of each device under different biasing voltages was tested. From the results of these measurements the effect of PI thicknesses on dark and photo performance of the PI/a-Se detector is investigated. The contribution of electron and hole injection in the dark current behavior of the device is discussed under different electric fields. The current-voltage characteristics of ITO glass/PI/Au structures were also measured. It was found that the electrons are the major carrier responsible for conduction within PI layer. In the presence of light, the conductivity within the a-Se layer increases and leads to an increase in the voltage drop across the PI layer and improves conduction of electrons within the PI layer.
Keywords: detector, amorphous selenium, high electric field, large area electronics, polyimide
Amorphous Selenium Mamographic Detector Modulation Transfer Function Energy Dependence Measured with Monochromatic X-rays at the Canadian Light Source
1. Departmennt of Electrical and Computer Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK, S7N 5A9, Canada
2. Canadian Light Source Inc., University of Saskatchewan, 44 Innovation Boulevard, Saskatoon, SK, S7N 2V3, Canada
3. Sunnybrook and Women's College Health Sciences Centre, University of Toronto, Toronto, M4N 3M5, Canada
4. Analogic Canada, 4950 rue Lévy, Saint-Laurent, Québec, H4R 2P1, Canada
It has been theoretically reasoned that the major mechanism of spatial resolution loss in the direct conversion x-ray image detectors based on a-Se photoconductor is due to the K-fluorescence reabsorption mechanism. The loss of resolution has been modeled for energies above and below the Se K-edge (12.66 keV) and the corresponding drop in the MTF (modulation transfer function) have been predicted theoretically. However, precise experimental data about the drop in the MTF is difficult to measure because most widely accessible x-ray sources are polychromatic.
In this work we report experimental results describing the dependence of the MTF of commercial mammographic detector (LMAM, ANRAD Corp.) on the energy of the incident x-ray photons in the range 11–40 keV. The measurements were done at BMIT 05B1-1 beamline at Canadian Light Source with nearly monochromatic x-rays originating from double crystal Si (220) monochromator.
We have found that the value of MTF at 5 cycles/mm drops with about 16% when the Se K-edge is crossed. As the energy of the x-ray increases above 12.7 keV the MTF at 5 cycles/mm improves and reaches a maximum in the region 20–30 keV. However, that partly recovered value of MTF at 5 cycles/mm is about 5% lower compared to the values measured with x-rays with energies below the Se K-edge.
Keywords: amourphous selenium, x-ray detector, MTF, Se K-edge
Measured Electron-hole Pair Creation Energy in Amorphous Selenium (a-Se) at High Electric Fields
1. Physics Department, Lakehead University, Thunder Bay, Ontario
2. Thunder Bay Regional Research Institute, Thunder Bay, Ontario
Amorphous selenium (a-Se) is a photoconducting material with the set of unique properties. A-Se unique properties made it the only commercially viable x-ray to charge transducer used in radiation medical imaging detectors. One of the most important properties of any photoconductor used in radiation detectors is electron-hole pair creation energy (W+/–). Previously it was shown that W+/– value in a-Se does not follow general rule so called Klein rule for semiconductors. Moreover it was shown that in the ranges of electric fields from 5–30 V/um the value of W+/– changes with the increase of electric field. However the previous attempts to measure electron-hole pair creation energy for high electric fields was not possible due to the complications with a-Se stable operation in the range of higher electric fields (40–75 V/μm) to the a-Se structure.
In this work we used modified HARP structure with pixel electrodes to measure W+/– value in the high electric field range. We measured accurately the value of W+/– for electric fields up to avalanche threshold of 75 V/μm.
Keywords: amorphous selenium (a-Se), detectors, electron-hole pair creation energy, photoconductor
Simplification of Amorphous Selenium based Photovoltaic Device Fabrication through Aerosol Deposition Method and Electrochemical Doping
1. Graduate School of Arts and Sciences, International Chrisitian University (ICU), 3-10-2 Osawa, Mitaka, Tokyo 181-8585, Japan
2. Department of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117 576, Singapore
3. Nanotube Research Centre, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan
4. Department of Engineering, Civil Engineering Division, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, UK
5. Deptartment of Electrical Engineering, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
6. Deptartment of Chemical Engineering, Graduate School of Engineering, Kyoto University, Katsura A4 Saikyouku, Kyoto 615-8510, Japan
7. Department of Engineering, Electrical Engineering Division, University of Cambridge, 9 JJ Thomson Avenue Cambridge CB3 0FA, UK
Amorphous selenium (a-Se) devices has been revisited from the 1980s due to the development of the HARP (High-gain Avalanche Rushing amorphous Photoconductor) and is pioneered through many branches of photon detection such as X-ray and UV detectors and even photovoltaic devices. Despite many researches about photovoltaics that absorbs non-visible wavelength light, the popularity is still low mainly due to its cost and efficiency. Thus, this paper will explore alternative ways to simplify the fabrication of a-Se films for lower cost, along with methods that has a potential to increase the concentration of photo excited carriers for higher efficiency.
An a-Se film was fabricated through aerosol deposition method and its crystal structure was compared through Raman spectroscopy, with films fabricated by the conventional thermal evaporation process. Aerosol deposited a-Se film exhibited Raman shift of 250 cm–1 that corresponds to the amorphous peak, along with the 235 cm–1 peak that corresponds to trigonal (crystalline) peak where the former became more apparent as the deposition pressure was increased. Through scanning electron microscope observation, it was found that a-Se was found only within a limited area under the nozzle whereas the conventional evaporator deposited a wider area. Deposition time and the preparation time were roughly the same with the exception of the pressure reduction process, which is time consuming yet necessary for vacuum evaporation.
Also, chlorine and sodium were electrochemically doped. The composition was characterized by time-of-flight secondary ion mass spectroscopy (ToF-SIMS), and its carrier concentration was estimated through photoelectric measurements. Successful impurity doping were confirmed towards more than half the depth of the film where approximately one order of magnitude larger carrier concentration was estimated.
Potential advantages and challenges of these new methods will be introduced and discussed.
Keywords: amorphous selenium, aerosol deposition, electrochemical doping, structure sensitivity, carrier concentration