Negatively charged excitons and photoluminescence in asymmetric quantum wells

We study photoluminescence (PL) of charged excitons (X^-) in narrow asymmetric quantum wells in high magnetic fields B. The binding of all X^- states strongly depends on the separation δ of electron and hole layers. The most sensitive is the ``bright'' singlet, whose binding energy decreases quickly with increasing δ even at relatively small B. As a result, the value of B at which the singlet-triplet crossing occurs in the X^- spectrum also depends on δ, and decreases from 35 T in a symmetric 10 nm GaAs well to 16 T for δ=0.5 nm. Since the critical values of δ at which different X^- states unbind are surprisingly small compared to the well width, the observation of strongly bound X^- states in an experimental PL spectrum implies virtually no layer displacement in the sample. This casts doubt on the interpretation of PL spectra of heterojunctions in terms of X^- recombination.