POLARIZATION-DEPENDENT ELECTROABSORPTION IN LATTICE-MATCHED AND STRAINED QUANTUM WELL STRUCTURES

Abstract

The polarization dependence of the refractive index variation (?n/n) and the absorption change (??) in lattice-matched and strained quantum well (QW) structures due to an electric field applied perpendicular to the well layers is studied. The Stark shifts in the energy levels and the wave functions are calculated using a perturbation method that includes all confined states in the QW. For an incident light propagating parallel to the plane of the QW, the TE and TM polarization dependence of ?n/n and ?? are obtained by calculating the dipole moments formed between electrons and holes (heavy and light) based on the density matrix theory. The results show that the polarization dependence of the field-effect spectral response is significant in thin QW's. This polarization dependence can be reduced or enhanced if the QW structure is placed under a biaxial tensile or compressive strain respectively. This is because the electroabsorption of the TE and TM polarizations are contributed mainly by the heavy-hole and light-hole transitions respectively, and the ground-state energy levels of the heavy and light holes are separated in a QW due to their difference in mass. By applying strain, the fourfold degenerate valence band at the r point is split into two doubly degenerate heavy and light-hole components. The ordering in energy of these two decoupled bands depends on whether the strain is compressive or tensile. The electroabsorption of strained and unstrained QW structures grown by molecular beam epitaxy is measured by photocurrent spectroscopy at room temperature and 77 K. It is observed that the electroabsorption in narrow wells is more sensitive to polarization than in large wells in GaAs/AlGaAs QW structures. The application of a tensile strain by the appropriate selection of the Ga mole fraction and the well size in GaInAs/AlInAs QW structures grown on InP substrates reduces the difference between the heavy-hole and light-hole transition energies due to quantization. On the other hand, compressive strains in GaInAs/ AlInAs and InGaAs/GaAs QW structures further enhance the spacing between the heavy-hole and light-hole energy subbands.