Temperature Dependent Defect Density Calculated from Activated Conductivity of a-Si:H
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TEMPERATURE DEPENDENT DEFECT DENSITY CALCULATED FROM ACTIVATED CONDUCTIVITY OF a-Si:H T. DRU0SEDAU*, V. KIRBS**, H. FIEDLER** *FB Physik, Universitit Kaiserslautern, Postfach 3049, W-6750 Kaiserslautern, (on leave from: Inst. Exp. Physik der TU "Otto von Guericke", PSF 124, 0-3010 Magdeburg), Fed. Rep. of Germany "**Inst. Exp. Physik der TU "Otto von Guericke", PSF 124, 0-3010 Magdeburg), Fed. Rep. of Germany ABSTRACT Thermally activated conductivity of a-Si:H at a slow cooling rate of 0.3 K/min is connected with temperature dependent changes of the mobility gap states. By means of the Fermi-level shift calculated from these data and a density of states model it is possible to determine this dependence. The results mainly reveal a decrease of the defect state density by about a tenth between 375 K and 400 K. INTRODUCTION The phenomena of the thermally activated dc-conductivity a(T) of a-Si:H, for example (a) the variations of the conductivity prefactor ao and activation energy EA both correlated by the Meyer-Neldel rule or (b) kinks in the Arrhenius plots, were the subject of great interest over the last decade. A complete discussion on this topic is given by Overhof and Thomas [11. In 1981, Overhof and Beyer showed that the statistical shift of the Fermi level EF(T) is of important influence on the activated conductivity of a-Si:H. Note that the EF(T) shift results from the temperature dependence of the Fermi-Dirac distributions function 1 fF-1 + exp (E-EF)/kT
(1)
and an asymmetric shape of a temperature independent density of states (DOS). Irsigler et al. [3] found strong alterations of ao and EA caused by a stepwise annealing of light induced defects. This experiment was successfully simulated by means of the statistical shift and a systematic variation of the DOS [41. In 1986 Smith et al. gave first experimental evidence for a thermal equilibrium defect density [5]. They assumed a monotonic increase of defect density with temperature. From rate equation considerations of the metastable defect density Redfield derived a non-monotonic temperature dependence of the defect density under light irradiation with a minimum in the range of between 400 and 450 K [6]. Thermally induced metastable effects on the conductivity were investigated during the last years with growing interest (see e.g. [7] and refs. cited therein). Recently it has been shown [8] that stepwise decrease of the annealing temperature causes variations of ao and EA which are well described by the Meyer-Neldel rule. The behaviour of the conductivity during the isothermal phases could only be explained by changes of the DOS which do not depend monotonically on temperature. We suggested the use of slow cooling rates to observe these changes during the measurement. Now we have to answer the question whether the calculation of a temperature dependent defect density from conductivity data is possible. THE MODEL Starting from the conductivity data u(T) one is able to calculate the EF (T) shift with respect to the mobility edges Ec and E, for n- and p-condu
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