The photoluminescence (PL) spectrum of modulation-doped GaAs/AlGaAs quantum wells (MDQW) and heterojunctions (HJ) is studied under a magnetic field (B ) applied parallel to the two-dimensional electron gas (2DEG) layer. The effect of B strongly depends on the electron-hole separation (d eh ), and we revealed remarkable B -induced modifications of the PL spectra in both types of heterostructures. A model considering the direct optical transitions between the conduction and valence subband that are shifted in k-space under B , accounts qualitatively for the observed spectral modifications. In the HJs, the PL intensity of the bulk excitons is strongly reduced relatively to that of the 2DEG with increasing B . This means that the distance between the photoholes and the 2DEG decreases with increased B , and that free holes are responsible for the hole-2DEG PL.

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A comprehensive experimental study of the photoluminescence (PL) spectral evolution under a magnetic field (B ^ 25 T) applied perpendicularly to a high-mobility two-dimensional electron gas (2DEG), is performed on modulation-doped GaAs/AlGaAs heterojunctions at TL=0.3 K. The abrupt transfer of the free exciton to hole-2DEG PL occurring at integer and fractional filling factors is analyzed in a phenomenological model, wherein free excitons photogenerated in the GaAs layer dissociate into a 2D electron and 3D hole near the 2D-electron channel. Such magnetic field induced exciton-(2De-h) transitions are able to explain the remark able strong PL anomalies in single hetrojunctions as compared to those observed in modulation-doped quantum wells.

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