Charge-trapping Metastability in p-Type Hydrogenated Amorphous Silicon: Meyer-Neldel Rule and Entropy Change
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Charge-trapping Metastability in p-Type Hydrogenated Amorphous Silicon: Meyer-Neldel Rule and Entropy Change Richard S. Crandall National Renewable Energy Laboratory, Golden, Colorado 80401 ABSTRACT Measurements of capacitance transients due to charge carrier emission from metastable defects in p-type hydrogenated amorphous silicon (a-Si:H) p/n junction structures, show that both holes and electrons can be metastably trapped in the p layer. At 350 K electrons and holes are emitted at the same rate defining the isokinetic temperature (Tiso) for the Meyer-Neldel rule. The enthalpy changes for hole and electron emission are 0.94 and 0.51 eV, respectively. The entropy changes for hole and electron emission are 31 and 16 Boltzmann constant, respectively. Charge emission rates are measured, for the first time, above and below Tiso. Below Tiso electrons are emitted faster than holes and above Tiso the reverse is true. These relative changes in emission rates are a direct consequence of the large entropy changes in the defect reactions. Tiso, within experimental error is the same as the H glass equilibration temperature (360 K) measured by others in p-type a-Si:H, suggesting that the defect reactions measured here are involved in H glass equilibration. Activation and deactivation of B atoms with accompanying charge emission likely govern both processes. INTRODUCTION Previous measurements of metastable defect annealing kinetics in doped hydrogenated amorphous silicon (a-Si:H) showed that both majority and minority carriers can be metastably trapped in either boron or phosphorous-doped films [1].These results were obtained using junction capacitance to measure charge carrier emission times on p/n or n/p structures. By plotting the characteristic emission time for each charge versus reciprocal temperature (an Arrhenius plot), straight lines are obtained and from their slope the activation energy for defect annealing is determined in boron doped a-Si:H. These lines are not parallel and their extrapolation to lower temperature suggested that they would intersect at about 360 K. This intersection at a finite temperature suggested that this metastable system obeyed the MeyerNeldel rule (MNR) [2].The intersection defines the isokinetic temperature (Tiso)); nevertheless from the limited experimental data it is not at all clear that the characteristic emission times will follow the straight line extrapolations all the way down to 360 K. Furthermore it is a question of deep scientific interest whether the emission times will continue to follow the same straight lines on the Arrhenius plot below Tiso. In the present study I show for the first time that this is indeed the case. The measurements determine both the enthalpy and entropy changes for the two reactions. In addition I suggest how these defect reactions control dopant equilibration [3]. EXPERIMENTAL DETAILS The a-Si:H junction devices used in this study are fabricated using plasma-assisted chemical-vapor deposition (PECVD) of silane and doping gases. Trimethylboron (TMB) and ph
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