Dipolar biexcitons in lateral traps in Si/SiGe/Si heterostructures

Si/Si$_{1-x}$Ge$_x$ heterostructures with long-range lateral potential fluctuations that appear in the capping Si layer near the SiGe/Si heterointerface due to the presence of relaxed areas in the SiGe layer have been studied. Analysis of the low-temperature photoluminescence spectra indicates that, upon the photoexcitation of the structure, the accumulation of nonequilibrium charge carriers, formation of dipolar excitons, and their recombination take place in long-range lateral traps formed by these fluctuations. It is found that, at temperatures $T<$ 10 K, a new narrow line appears with an increase in the excitation level at the blue tail of the broad photoluminescence band of dipolar excitons localized by short-range potential fluctuations. At temperatures $T\approx$ 2 K, this line is dominant in the spectra even at the lowest excitation levels. It is shown that, at moderate excitation levels, this line is caused by the recombination of free dipolar biexcitons in long-range traps. At high excitation levels, the width of the new line increases by more than a factor of 2 compared to that at lower excitation levels, and under these conditions this line is associated with the recombination of dipolar electron-hole plasma in long-range traps.