Photocurrent spectroscopy of a (001)- and a (111)-oriented GaAs/Al0.33Ga0.67As quantum-well structure

Abstract

Photocurrent (PC) spectroscopy was used to study exciton oscillator strengths in a (001)- and a (111)-oriented GaAs/${\mathrm{Al}}_{0.33}$ ${\mathrm{Ga}}_{0.67}$As quantum-well structure (QWS) as a function of the electric field. In the (111) quantum well, an extremely large oscillator strength of the ${\mathit{e}}_1$-${\mathrm{hh}}_2$ forbidden-transition exciton was observed that was larger than that of the ${\mathit{e}}_1$-${\mathrm{lh}}_1$ exciton, even at low electric fields. [The notation ${\mathit{e}}_{\mathit{n}}$-hh(lh${)}_{\mathit{m}}$ 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 ${\mathit{e}}_1$-${\mathrm{hh}}_1$ and ${\mathit{e}}_1$-${\mathrm{hh}}_2$ heavy-hole exciton are lighter than that of the ${\mathit{e}}_1$-${\mathrm{lh}}_1$ 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 ${\mathit{e}}_1$-${\mathrm{hh}}_2$ exciton was revealed to be as much as 1.5 times larger than that of the ${\mathit{e}}_1$-${\mathrm{lh}}_1$ 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 ${\mathrm{lh}}_1$ and the ${\mathrm{hh}}_2$ 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.