PHOTOCURRENT SPECTROSCOPY OF A (001)-ORIENTED AND A (111)-ORIENTED GAAS/AL0.33GA0.67AS QUANTUM-WELL STRUCTURE

Photocurrent (PC) spectroscopy was used to study exciton oscillator strengths in a (001)- and a (111)-oriented GaAs/Al0.33Ga0.67As quantum-well structure (QWS) as a function of the electric field. In the (111) quantum well, an extremely large oscillator strength of the e1-hh2 forbidden-transition exciton was observed that was larger than that of the e1-lh1 exciton, even at low electric fields. [The notation en-hh(lh)m represents a transition between the nth electron and the mth heavy- (light-) hole subband.] The exciton oscillator strength ratios estimated from the PC spectra were compared with those calculated theoretically with the exciton reduced-mass ratio used as a parameter. The comparison revealed that, whereas the reduced masses of the e1-hh1 and e1-hh2 heavy-hole exciton are lighter than that of the e1-lh1 light-hole exciton in the (001) QWS, reflecting the mass reversal effect in the (001) QWS, it is not so in the (111) QWS; especially, the reduced mass of the e1-hh2 exciton was revealed to be as much as 1.5 times larger than that of the e1-lh1 exciton in the (111) QWS. This difference between the (001) and the (111) QWS is discussed, taking valence-band coupling into account; it is attributed to a difference in order between the lh1 and the hh2 subband, since a strong repulsion between the two subbands is expected to give either a large or a negative in-plane mass to the upper subband of the two. © 1990 The American Physical Society.