| Project/Area Number |
06804021
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| Research Category |
Grant-in-Aid for General Scientific Research (C)
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| Allocation Type | Single-year Grants |
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| Research Field |
物理学一般
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| Research Institution | University of Tokyo |
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Principal Investigator |
MIYACHI Takashi Institute for Nuclear Study, Dr.University of Tokyo, 原子核研究所, 助手 (20013401)
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| Project Period (FY) |
1994 – 1995
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| Project Status |
Completed (Fiscal Year 1995)
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| Budget Amount *help |
¥1,800,000 (Direct Cost: ¥1,800,000)
Fiscal Year 1995: ¥800,000 (Direct Cost: ¥800,000)
Fiscal Year 1994: ¥1,000,000 (Direct Cost: ¥1,000,000)
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| Keywords | radiation detector / stability of radiation detector / silicon-metal interface / quantum states at the surface / unstable quantum states |
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| Research Abstract |
We studied the instability of radiation detectors using infrared light. Then the photocurrent (q) is obtained as a function of bias voltage (v) and aging time (t). The time dependence is expressed as the time derivative (dq/dt). The voltage dependence (dq/dv) is proportional to the conductance of the detector. In previous experients, we found some discrete levels in dq/dv curves of Si (Li) -type detectors. The levels were unstable and shifted toward the low voltage side with a time constant of several days. This project was aimed at studying the physical mechanism for the unstable states. The discrete levels were not caused by bulk effects but surface effects. Hence we made an apparatus with which the sample was irradiated by 1150nm light and its surface was maintained not to be disturbed. As the results, we found two distinct detector states ; dq/dt>0 and dq/dt<0. We denote the former as progressive (P-) phase and the later regressive (R-) phase. The detector state is P-phase whennever the surface is formed. At P-phase, the amplitude of dq/dt decreases with a time constant of -10 days. This means that aging corresponds to inactivating the surface states with this time constant. Based on this behavior, the slow surface states which are partially unfilled take part in the instability process. Consequently, we observe discrete levels which reflect existing unfilled states through which electric charge communicates between the bulk of the detector and the electrode. In R-phase, the surface states are filled to an allowable maximum energy and the change in the detector sensitivity results from accumulation of an oxide layr. Therefore, the discrete levels disappear at this phase. The speed of the sensitivity change is much slower than that of P-phase. It is remarkable that aging is promoted when the bias voltage is applied in increasing direction.
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