THz lasing of shallow donors in stressed silicon crystal

Results of experimental and theoretical study of terahertz stimulated emission from optically excited group-V donors (phosphor P, antimony Sb, arsenic As, bismuth Bi) in uniaxially stressed and liquid helium cooled silicon crystal are summarized and discussed. It is shown that compressive force of 1-1.5 kbar for P, Sb and of 2-3 kbar for As, Bi applied along {100} crystallographic orientations results in the remarkable enhancement of the laser gain as well as THz emission efficiency and laser threshold intensity is decreased by the order of magnitude or even more. For As and Bi donors it is accompanied by a switching of the emission line because of the upper laser state change. The effect of uniaxial stress on donor lasing originates from energy shift of the conduction band valleys of silicon which split donor states changing their eigen-values and eigen-functions. According to the calculations of phonon-assisted relaxation rates appropriate stress-induced donor modification increases the lifetime and makes pump efficiency of the upper laser states better. Thus THz laser performance of donors in silicon can be substantially improved by host crystal deformation.