HIGH-PRESSURE STUDY OF OPTICAL-TRANSITIONS IN STRAINED IN0.2GA0.8AS GAAS MULTIPLE-QUANTUM WELLS/

We have measured low-temperature photoluminescence (PL) and absorption spectra of In0.2Ga0.8As/GaAs multiple quantum wells (MQW's) under hyd rostatic 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 th e InxGa1-xAs wells. Above 4.9 GPa, new PL lines related to X-like cond uction band states appear. They are assigned to the type-II transition from the X(Z) states in GaAs to the HH subband of the InxGa1-xAs well s and to the zero-phonon line and LO-phonon replica of the type-I tran sition involving the X(XY) valleys of the wells. In addition to absorp tion peaks corresponding to direct exciton transitions in the wells, a new strong absorption feature is apparent in spectra for pressures be tween 4.5 and 5.5 GPa. This absorption is attributed to the pseudodire ct transition between the HH subband and the X, state of the wells. Th is gives clear evidence for an enhanced strength of indirect optical t ransitions due to the breakdown of translational invariance in MQW str uctures. From experimental level splittings we determine the valence b and offset and the shear deformation potential for X states in the In0 .2Ga0.8As layer.