Influence of Isovalent Doping on the Processes of Thermal Donors Formation in Silicon

Abstract

The doping of CZ silicon with the isovalent impurity germanium is known to lead to a decrease in the rate of thermal donor (TD) formation during heat treatment at 450–650 °C [1,2]. This effect is still not completely understood. Some authors suggested it was due internal elastic stresses caused by the Ge [1]. These stresses may increase the activation energy for oxygen diffusion during TD formation, though some authors have noted the enhanced oxygen diffusion in silicon doped with germanium [3]. In references 4 and 5 the influence of Ge doping was explained by a decrease in the diffusion coefficient of oxygen atoms and by the delayed decay of the supersaturated solid solution of oxygen. The change in the rate of TD formation in Si(Ge) was explained in reference 6 by the decrease of the capture radius at defect nucleation sites. All these models may be correct but they cannot explain the concentration dependence of Ge in Si(Ge) on the rate. The decrease of TD generation is only obsevered for Ge concentrations higher than (2-3)xlO19 cm-3 . However, even at lower concentrations than this, considerable elastic stresses in Si(Ge) are found [7]. That leads, for example, to a nonuniform broadening of the IR absorption resonances and EPR lines, without affecting the TD formation rate.