High-pressure study of optical transitions in strained ${\mathrm{In}}_{0.2}$${\mathrm{Ga}}_{0.8}$As/GaAs multiple quantum wells

We have measured low-temperature photoluminescence (PL) and absorption spectra of ${\mathrm{In}}_{0.2}$${\mathrm{Ga}}_{0.8}$As/GaAs multiple quantum wells (MQW’s) under hydrostatic pressures up to 8 GPa. In PL, only a single peak is observed below 4.9 GPa, corresponding to the n=1 heavy-hole (HH) exciton in the ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As wells. Above 4.9 GPa, new PL lines related to X-like conduction band states appear. They are assigned to the type-II transition from the ${\mathit{X}}_{\mathit{Z}}$ states in GaAs to the HH subband of the ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As wells and to the zero-phonon line and LO-phonon replica of the type-I transition involving the ${\mathit{X}}_{\mathit{XY}}$ valleys of the wells. In addition to absorption peaks corresponding to direct exciton transitions in the wells, a new strong absorption feature is apparent in spectra for pressures between 4.5 and 5.5 GPa. This absorption is attributed to the pseudodirect transition between the HH subband and the ${\mathit{X}}_{\mathit{Z}}$ state of the wells. This gives clear evidence for an enhanced strength of indirect optical transitions due to the breakdown of translational invariance in MQW structures. From experimental level splittings we determine the valence band offset and the shear deformation potential for X states in the ${\mathrm{In}}_{0.2}$${\mathrm{Ga}}_{0.8}$As layer. © 1996 The American Physical Society.