Negatively charged excitons and photoluminescence in asymmetric quantum wells - art. no. 085305

We study photoluminescence (PL) of charged excitons (X-) in narrow asymmetr ic quantum wells in high magnetic fields B. The binding of all X- states st rongly depends on the separation delta of electron and holt: layers. The mo st sensitive is the "bright" singlet, whose binding energy decreases quickl y 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 d epends on delta. and decreases from 35 T in a symmetric 10 nm GaAs well to 16 T for delta = 0.5 nm. Since the critical values of delta at which differ ent X- states unbind are surprisingly small compared to the well width, the observation of strongly bound X- states in an experimental PL spectrum imp lies virtually no layer displacement in the sample. This casts doubt on the interpretation of FL, spectra of heterojunctions in terms of X- recombinat ion.