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 $\delta$ of electron and hole layers. The most sensitive is the “bright” singlet, whose binding energy decreases quickly with increasing $\delta$ 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 $\delta ,$ and decreases from 35 T in a symmetric 10 nm GaAs well to 16 T for $\delta =0.5\mathrm{}\hspace{0.5em}\mathrm{nm}.$ Since the critical values of $\delta$ 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.

• Received 11 September 2000

DOI:https://doi.org/10.1103/PhysRevB.63.085305