THE EFFECT OF GAMMA-X MIXING ON THE DIRECT EXCITONIC PHOTOLUMINESCENCE IN GAAS ALGAAS QUANTUM-WELLS/

We studied the pressure shift of direct PL lines in two different GaAs /AlGaAs quantum-well samples in the diamond anvil cell and in the gas cell. We found that up to the Gamma-X crossover pressure the shift of the lines agrees with the effective-mass calculation and exhibits smal l dependence on the well parameters. When the X minima in the barriers become lower than the Gamma level in the well, the pressure coefficie nt of the direct line is lowered. We attribute the Gamma-X mixing to t wo factors: (i) boundary conditions at the interface and (ii) electron -optical phonon interaction. We believe that the first factor is more important than the second because the Gamma-X resonant effects are muc h stronger in quantum wells than in the bulk. Our results imply that t he pressure shift of the PL lines in quantum wells should not be fitte d with a single linear or quadratic dependence below and above the cro ssover.