Dopant Incorporation and Doping Efficiency in a-Si:H and a-Ge:H
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DOPANT INCORPORATION AND DOPING EFFICIENCY IN a-Si:H AND a-Ge:H MARTIN STUTZMANN Xerox Palo Alto Research Center, Palo Alto, CA 94304 and Max-Planck-Institut fUr Festkbrperforschung, Heisenbergstrasse 1, D-7000 Stuttgart 80, Federal Republic of Germany ABSTRACT Doping of hydrogenated amorphous silicon and germanium with boron, phosphorus, and arsenic is investigated. The incorporation of the dopants from the gas phase into the solid film is found to differ strongly for the various dopant-host systems. The doping efficiency is calculated from measurements of the density of dangling bond defects and of shallow band-tail states as a function of the doping level. A common square root dependence of the efficiency on dopant gas concentration is obtained. INTRODUCTION The possibility to dope hydrogenated amorphous silicon and germanium (a-Si:H and a-Ge:H) substitutionally with group III and V atoms such as B, P, or As [1] has provided us with the means to taylor the electronic properties of these materials as required for an increasing number of potential applications. However, as it is also too often the case for other phenomena in amorphous semiconductors, our knowledge about the doping process is mostly empirical, with very little insight into the underlying microscopic picture. Two of the key issues that need to be explained by any serious model for substitutional doping of a-Si:H and a-Ge:H are the peculiar dependence of the doping efficiency on the doping level and the doping induced increase of the dangling bond density, especially in a-Si:H [2]. In the following, I will address these questions by a comparison of the dopant incorporation and the doping efficiency in five different dopant-host systems. RESULTS AND DISCUSSION Samples were deposited by rf glow-discharge under standard conditions. In Fig. 1 the concentration of dopants in the solid a-Si:H and a-Ge:H films is shown as a function of the volume concentration of the dopant gases in the rf plasma. It is evident that the incorporation efficiency for the different dopant gases in a-Si:H and a-Ge:H varies by as much as two orders of magnitude, especially at low gas phase concentrations. In addition, the incorporation efficiency tends to decrease with increasing gas phase concentration. This leads to deviations from a simple linear relationship between the solid and gas phase dopant concentrations, notably in the two cases of arsenic in a-Si:H and phosphorus in a-Ge:H, where the experimental results indicate a square root rather than a linear dependence (slopes s = 0.6 and 0.5 for As and P, respectively, see Fig. 1). The reason for this behavior is not known at present, but there is some experimental evidence that deviations from linearity typically occur for deposition processes operating under low energy density conditions [31. Then, the growth of the amorphous film and the incorporation of the dopants is likely to be determined by ratelimiting chemical reactions in the gas phase and at the growing surface. This being the case, it is quite conceivable that th
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