Study of 3-MeV electron irradiation damage in amorphous silicon with TRMC
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Study of 3-MeV electron irradiation damage in amorphous silicon with TRMC A. Klaver1, J.M. Warman2, M.P. de Haas2, J.W. Metselaar1, R.A.C.M.M. van Swaaij1 1 Delft University of Technology, DIMES-ECTM, P.O. Box 5053, 2600 GB, DELFT, The Netherlands. 2 Delft University of Technology, Interfaculty Reactor Institute, Radiation Chemistry Department, Mekelweg 15, 2629 JB Delft, The Netherlands ABSTRACT The effects of 3-MeV electron irradiation on a-Si:H have been studied using Time-Resolved Microwave Conductivity (TRMC). A Van der Graaff electron accelerator is used to generate the probe-beam pulses for the TRMC experiment as well as for the in-situ irradiation of the samples for the degradation of the material. Using several probe-beam pulse doses, TRMC transients were obtained on samples that have been subjected to various radiation fluences. These transients were later analyzed using a simple model based on the Shockley-Read-Hall capture and emission processes. Using these simulations we deduce a relationship between the radiation fluence and the defect density in the material. INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) solar cells can be interesting for space applications, mainly because of its high radiation tolerance, favourable annealing properties [1], and low cost. In order to predict the end-of-life efficiency of an a-Si:H solar cell in a space environment, quantitative information on the defect generation in a-Si:H material due to electron irradiation is needed. Various measurement techniques have been used to study the degradation of a-Si:H due to electron irradiation, in particular the constant photocurrent method, and electron-spin resonance [2,3].Time-Resolved Microwave Conductivity (TRMC) is a valuable addition to these techniques to study degradation by electron-beam irradiation of a-Si:H, as will be described below. With TRMC the change in the electrical conductivity of a material is monitored as a function of time by measuring the change in microwave absorption in the material following a high-energy electron beam probe pulse [4]. A high-energy pulse is favourable because this provides a uniform electron-hole (e-h) generation rate across the whole layer. The conductivity change depends on the density of the free excess charge carriers generated by the probe-beam pulse and their mobility. The decay rate of the conductivity change provides information on recombination processes of the excess charge carriers in the material. The degradation of the a-Si:H is performed with the same electron beam as used for the probe-beam pulse. This has the great advantage that the degradation can be carried out in-situ in the analysis setup. Another advantage of TRMC is that the measurements are contactless. In this paper we will first briefly describe the TRMC technique. Then we show the model used to obtain the defect density of states from a given TRMC transient. Finally, we present the results derived from the TRMC measurements, on as-deposited and electron-irradiated a-Si:H layers. We aim to find a
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