Carrier relaxation and recombination in GaAs/AlGaAs quantum heterostructures and nanostructures probed with time-resolved cathodoluminescence

Abstract

We have examined the kinetics of carrier relaxation in GaAs/AlGaAs quantum wells (QWs), quantum wires (QWRs), and quantum boxes (QBs) with time-resolved cathodoluminescence (CL). In the cases of QWRs and QBs, the nanostructures were grown via a size-reducing growth approach on pre-patterned GaAs(001) substrates composed of stripes and mesas, respectively. The growth involved deposition of multiple GaAs/AlGaAs layers in order to establish both structural and optical markers which facilitated the identification of important features in transmission electron microscopy (TEM) and CL experiments. In TEM measurements, the lateral dimensions of the top-most GaAs layers in typical stripe and mesa structures comprising the QWRs and QBs delineate GaAs regions expected to exhibit 2D and 3D quantum confinement effects, respectively. Time-delayed CL spectra of all three structures reveal that the initial capture of carriers in the active regions occurs on a time scale less than the temporal resolution of the CL system, ∼100 ps, during the onset of luminescence. Hot carriers, as a result of re-emission out of thin QWs surrounding the QWRs and QBs, exhibit diffusive transport followed by relaxation into laterally confined regions which exhibit confined states of lower energy. This thermalization gives rise to a relatively slow onset and decay of luminescence attributed to the lowest energy optical transitions. By comparing time-resolved CL transients in these three structures, we find that the average luminescence onset and initial-decay rates both decrease as the dimensionality of the system reduces from 2D to 0D. These results demonstrate that the rate of carrier relaxation, including the re-emission and diffusive transport of carriers, will depend on details of the total surrounding structure which comprises the excitation region.