Electric Field Enhancement of Dark Current Generation in Detectors
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Electric Field Enhancement of Dark Current Generation in Detectors James P. Lavine Display and Components, Image Sensor Solutions Eastman Kodak Company, 1999 Lake Avenue Rochester, NY 14650-2008, U.S.A. ABSTRACT The performance of detectors and sensors is degraded by dark current generation, which is due to defects and impurities in the materials. Electric fields enhance the generation from the resulting deep levels. When the electric field is in the mid-105 V/cm range, the present work finds enhancements of the order of 100 or more for iron and gold in silicon. The activation energy of the generation rate as a function of temperature is seen to decrease when the electric field increases. Many detectors have pixels that form a charge packet before the detectors are read out. Since the presence of charge decreases the electric field, the electric field enhancement varies with time. This is modeled for iron in silicon with an illustrative charge versus electric field relation. The resulting activation energy is found to be barely affected. INTRODUCTION The performance of semiconductor detectors and sensors is degraded by dark current generation, which is due to defects and impurities in the materials. The present emphasis is on generation caused by isolated deep levels or traps. If the deep level can be identified, its source may be determined. In many cases, this permits a reduction in the relevant impurity and an improved detector. Electric fields enhance the generation from deep levels through the PooleFrenkel effect, phonon-assisted tunneling, and band-to-band tunneling [1]. The latter mechanism occurs at very high fields [2,3] and is not considered further. The other mechanisms are treated within the framework of the Shockley-Read-Hall (SRH) [4,5] approach to recombination and generation. Phonon-assisted tunneling is also referred to as trap-assisted or thermally assisted tunneling and has been treated within a variety of formalisms. A popular approach is based on the work of Vincent et al. [4] and involves an energy integral that contains the tunneling probability of electrons with energy. A series of approximations to, and generalizations of, the integral have been developed and are widely used [1,4,5,6]. A more detailed approach uses quantum mechanics to treat the electron’s interactions with phonons [7,8,9,10], and approximations are available [8,11]. The present work puts the integrals of [4] into the SRH theory of generation and recombination as detailed in [12]. The model is outlined and evaluated in the second section for a fixed electric field strength. Many detectors collect charge packets in pixels before they are read out, and the accumulating charge reduces the electric field. The third section explores this process and shows the effect on the thermal activation energy of the deep level [13]. The final
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section contains a comparison of the present calculations with that of Schenk [8], and includes conclusions, brief comments on data versus theory, and suggestions for future work. NUM
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