Charge Collection Processes in a-Si:H p-i-n Detectors
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CHARGE COLLECTION PROCESSES IN a-Si:H p-i-n DETECTORS T. POCHET LETI (CEA-Technologies Avancees), DEIN/SPE, Centre d'Etudes Nucleaires de Saclay 91191 Gif-sur-Yvette Cedex, France. ABSTRACT Charge induced as a function of time in a-Si:H p-i-n detectors by protons having energies ranging between 5 and 12 MeV have been studied. A multiple trapping transport model is developed in order to explain the non saturation of the collected charge with bias and evidence of a bias dependent recombination process is shown. The dependence of the total induced charge (the detector efficiency) with bias can be expressed by an Onsager function with a thermalization distance of 34
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1. INTRODUCTION a-Si:H detectors have been developed and studied over the last few years [1-7]. Progress realized in this field renders this material very attractive for some specific applications in medical radiology (X Ray detection [5,7]) and nuclear detection [1,6]. Nevertheless, a study of the signal formation in a-Si:H detectors is needed to obtain a better insight on how the charges are generated and move inside an a-Si:H detector. More specifically the observed absence of saturation of the charges induced by the passage of an ionizing particle (see Figure 3) with the bias voltage is a major problem to be resolved. After a brief description of the experimental set-up in section 2, we present in section 3 the transport model developed in order to study the signal formation in an a-Si:Hp-i-n detector. Finally in section 4, the experimental results are compared to the model and the best fitting transport parameters are deduced. 2. EXPERIMENTAL SET-UP p-i-n diodes having thicknesses ranging from 4 to 6 pm have been tailored to sustain high reverse voltages (equivalent to 70 V/pm) [8]. Experiments were conducted to record the time response on a 2 ps time scale of these diodes, for various polarizations, as a function of the proton energies (5 to 12 MeV). Figure 1 shows the experimental set-up used. It includes a DC power supply (Ortec 428) and a Fast Filter Amplifier (Ortec 579) which amplifies the time response before it is recorded in a sampling oscilloscope Tektronix 7D20. The signal is self-triggered via a Timing Single Channel Analyser (Ortec 420). Noise contribution is reduced by averaging 64 time responses
Mat. Res. Soc. Symp. Proc. Vol. 302. @1993 Materials Research Society
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before the signal is recorded. The ultimate time resolution obtained with this set-up is about 20 nanoseconds which is good enough to study the drifting of the holes in a-Si:H. An energetic particle passing through a thin layer of a-Si:Hcreates a sharp column of charges around its track. 12 MeV and 5 MeV protons lose 7.1 keV/pm and 15.5 keV/ 1 im, respectively along their tracks. The exact number of e-h pairs generated by this energy loss is unknown since the pair creation energy E, has not yet been accurately evaluated in a-Si:H despite preliminary measurements leading to 3.4 eV < ep < 4.4 eV [1,9], compatible with the Ep value of cristalline silicon (E = 3.62 eV)
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