Transient current in a-Si:H-based MIS photosensors
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1066-A18-06
Transient current in a-Si:H-based MIS photosensors Miguel Fernandes1, Yuriy Vygranenko1, Manuela Vieira1, Gregory Heiler2, Timothy Tredwell2, and Arokia Nathan3 1 DEETC, ISEL, Rua Conselheiro Emidio Navarro, Lisbon, 1959-007, Portugal 2 Carestream Health Inc., 1049 Ridge Road West, Rochester, NY, NY 14615 3 London Centre for Nanotechnology, UCL, London, WC1H 0AH, United Kingdom ABSTRACT In this work we analyze the transient current in the metal/a-SiNx/a-Si:H/n+/ITO structures under different biasing conditions and temperatures. The dark current decay was measured within an interval of 1 second in the temperature range from 294 to 353K. It was found that when the bias pulse amplitude is kept constant, the transient current strongly depends upon the offset voltage of the bias pulse. This result is in good agreement with device modeling performed using ATLAS. The detailed analysis shows that the transient dark current originates from traps in the i-layer bulk and traps at the semiconductor-insulator interface. Under optimized biasing conditions and elevated temperatures the bulk current component becomes dominant INTRODUCTION The hydrogenated amorphous silicon (a-Si:H) PIN photodiode and the MIS photoelectric converter are two alternative sensing elements used in indirect-conversion flat panel X-ray detectors [1], [2]. The major advantage of the MIS structure over PIN is fact that this device has the same layer sequence as the a-Si:H TFT switch and therefore, they can be fabricated simultaneously resulting in an effective reduction in the lithography mask count [3]. Since the blocking p-layer is replaced by the insulator in the MIS structure, problems related with cross contamination are avoided leading to an increase on the device yield. The main disadvantage of the MIS structure is that the noise level may be high because of the transient dark current. The transient dark current originates from traps at the semiconductor-insulator interface and a-Si:H bulk trap states [4], [5]. The results reported in our previous work show that the noise component associated with the transient dark current can be largely eliminated by adjusting the biasing conditions [6]. In this work we analyze the transient current in a-Si:H-based MIS structures under different biasing conditions and temperatures.
EXPERIMENTAL PROCEDURE The samples used in this work are metal(Mo)/a-SiNx (100 nm)/a-Si:H (1 µm)/n+ (20 nm)/ITO (65 nm) structures on the glass substrate. Semiconductor and dielectric layers were deposited by plasma enhanced chemical vapor deposition (PECVD) at 300ºC. Magnetron sputtering was used for the metal- and ITO films. The photosensitive area of the segmented structures is 1×1 mm2. Further details on the device design, fabrication, and characterization under illumination including spectral response measurements can be found elsewhere [6].
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