Wechselwirkungen von Gold und Versetzungen in Silizium

Abstract

The knowledge about interactions of different defects in crystalline sillicon are of major importance for the optimisation of the performance of solar cells based on multicrystalline silicon. Due to their recombination activity these defects degrate the lifetime of minority charge carriers and limit the efficiency compared to solar cells based on monocrystalline silicon whose fabrication, however, is more expensive. Apart from modified recombination properties there are other aspects of defect interaction: The solubility and diffusion properties of metallic impurities modified by defect interactions and binding energies to extended defects are important parameters for defect engineering processes during device processing. Properties of gold in dislocated silicon have been investigated by Deep Level Transient Spectroscopy (DLTS). It was found that the deep levels of substitutional gold introduce lines into the spectrum of dislocated silicon which are well-known from the spectra of undislocated silicon. The properties of these levels are almost unchanged except for the reduced capture rate of the acceptor state of the substitutional dissolved gold. This finding is explained by the assumption that the capture of electrons to a significant fraction of these atoms is affected by the electrostatic potential of charged dislocations which has formed in a cylinder symmetrical shape around them. The charges along the dislocations resulted from a deep acceptor state which is attributed to gold atoms bounded to the core of dislocations. By measuring the densities of free charge carriers using capacitance-voltage characteristics the existence of this deep acceptor state could be revealed. The relation of this state to gold atoms bounded to the dislocation cores has been substantiated by an accumulation of gold atoms at dislocations. This accumulation has been shown using the data of the work by Rodriguez and co-workers[1] who measured the total gold concentration in dislocated silicon. Based on these data a binding energy of 1.72(2)eV of gold atoms to the dislocation core is calculated. Using two models [2,3] which describe two limiting cases of possible interactions of the gold atoms along the dislocations, it is concluded that this deep acceptor state has a bandlike character.[1] A. Rodriuez, H. Bracht, I. Yonenaga, J. Appl. Phys. 95, 7841 (2004) [2] W. Read, Phil. Mag. 45, 1119 (1954) [3] H. Veth, M. Lanoo, Phil. Mag. B 50, 93 (1984)