Abstract
The photoluminescence (PL) spectrum of modulation-doped $\mathrm{GaAs}∕\mathrm{AlGaAs}$ quantum wells and heterojunctions (HJ) is studied under a magnetic field $({B}_{\ensuremath{\Vert}})$ applied parallel to the two-dimensional electron gas (2DEG) layer. The effect of ${B}_{\ensuremath{\Vert}}$ strongly depends on the electron-hole separation, and we revealed remarkable ${B}_{\ensuremath{\Vert}}$-induced modifications of the PL spectra in both types of heterostructures. A model considering the direct optical transitions between the conduction and valence subbands that are shifted in $k$-space under ${B}_{\ensuremath{\Vert}}$, accounts qualitatively for the observed spectral modifications. In the HJs, the 2DEG-hole PL intensity is strongly enhanced relatively to the bulk exciton PL with increasing ${B}_{\ensuremath{\Vert}}$. This means that the distance between the photoholes and the 2DEG decreases with increasing ${B}_{\ensuremath{\Vert}}$, and thus free holes are responsible for the 2DEG-hole PL.
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