Journal of Vacuum Science & Technology B, Vol.14, No.4, 2922-2935, 1996
Effect of Interface Defect Formation on Carrier Diffusion and Luminescence in In0.2Ga0.8As/AlxGa1-xAs Quantum-Wells
We have examined the influence of strain relaxation on the excitonic recombination and diffusion in In0.2Ga0.8As/AlxGa1-xAs quantum-well (QW) samples designed for high-electron-mobility transistors, using spectrally and spatially resolved polarized cathodoluminescence (CL). Six molecular-beam epitaxial grown samples, with varying channel thicknesses ranging from 75 to 300 Angstrom, were examined at various temperatures between 87 and 300 K. An increase in misfit dislocation density occurred with increasing channel thicknesses and resulted in changes in the dark line defect (DLD) density, polarization anisotropy, QW excitonic luminescence energy, and luminescence activation energy, as observed in CL. The influence of misfit dislocations on the ambipolar diffusion of excess carriers in a direction parallel to the dislocation line, in varying proximity to the DLDs, was examined with a CL-based diffusion experiment. The temperature dependence of the CL imaging was examined, enabling a study of the spatial variation of the activation energies associated with thermal quenching of the GaAs/Al0.25Ga0.75As multiple QW and In0.2Ga0.8As QW luminescence. The CL intensity exhibits an Arrenhius-type dependence on temperature and is controlled by thermally activated nonradiative recombination. The activation energies for both the In0.2Ga0.8As QW and Al0.25Ga0.75As MQW luminescence are found to vary spatially in close proximity to the misfit dislocations. We have utilized a new approach to obtain 2D images of the activation energies. The influence of the strain relaxation on the polarization and energy of the In0.2Ga0.8As QW excitonic luminescence was examined with linearly polarized CL and CL wavelength imaging. A strain-induced modification of the luminescence energy and an increase in the polarization anisotropy was measured near DLDs. Thus, we find that certain DLDs exhibit significant polarization and energy variations in their optical properties, in addition to their more familiar nonradiative behavior.