Visible emission from a dense biexciton gas in SiGe/Si quantum wells under external anisotropic strain

The effect of external anisotropic strain on the infrared and visible luminescence spectra of SiGe/Si quantum well heterostructures at liquid helium temperatures is investigated for the first time. It is shown that, at a temperature of $T=5$ K, the stretching of the SiGe layer along the $[100]$ direction leads to an increase in the relative intensity of the visible luminescence by a factor of $7/3 \simeq 2.3$. This effect is absent when the sample is stretched along the $[110]$ direction. These observations are explained by considering “bright” and “dark” biexciton states involved in multiparticle recombination. At a temperature of $2$ K, the relative intensity of the visible luminescence increases upon stretching by a factor of $3.4$–$3.9$, which may indicate either the splitting of the ground states of biexcitons with different electron configurations or the deviation of their distribution function from the Boltzmann law.