Transition from two-dimensional electron-hole to geminate-exciton photoluminescence in GaAs Alx Ga1-x As heterostructures under a high in-plane magnetic field

We studied the evolution of the photoluminescence (PL) spectra in modulation-doped GaAs-based heterostructures (single quantum wells and heterojunctions) at TL =1.2 K under a high magnetic field B (up to 33 T), which was applied parallel to the two-dimensional electron gas (2DEG) layer. Under low in-plane fields, B <7 T, the radiative recombination of the photoexcited hole with the 2DEG gives rise to a broad PL band that shifts quadratically with B. This band transforms into a narrow PL line whose peak energy E shifts linearly with B in the range of 10-33 T. The slope of the linear E (B) dependence was measured as αex = 0.77±0.02 meV T in all the studied structures. The same linear slope is also measured in the PL spectra of bulk, undoped GaAs under high B. We thus attribute the sharp PL line observed in the doped heterostructures to magnetoexcitons that are photogenerated outside the 2DEG layer by a geminate formation process. The slope of the magnetoexciton energy dependence on B is compared with that measured for unbound-electron-hole Landau level transitions under a perpendicular B. The ratio of the measured slopes, αex/alpha;0 0.8, is found to be equal to the ratio of the reduced excitonic mass to the reduced cyclotron mass of GaAs.