Time-resolved cathodoluminescence study of carrier relaxation, transfer, collection, and filling in coupled In_xGa_(1-x)N/GaN multiple and single quantum wells

摘要

We have examined in detail the optical properties and carrier capture dynamics of coupled In_xGa_(1-x)N/GaN multiple and single quantum well (MQW and SQW) structures that possess various numbers of QWs in the confinement region adjacent to a SQW. The aim is to study the influence of the structure of an InGaN MQW confinement region on carrier transfer and collection into a coupled SQW. By applying in a complementary way temperature- and excitation-dependent cathodoluminescence (CL) spectroscopy and time-resolved CL measurements, we have analyzed the carrier dynamics and state filling in the SQW and the adjacent MQW. We solved self-consistently the nonlinear Poisson-Schrodinger equation for wurtzite materials including strain, deformation potentials, and piezoelectric field of our In_xGa_(1-x)N/GaN single and multiple QW structures to obtain the excitation-dependent eigenstates that are used to calculate band filling, excitonic lifetimes, and exciton binding energies. We show that it is possible to treat a coupled In_xGa_(1-x)N single and multiple QW system in a way that allows for a determination of the quasi-Fermi levels, carrier densities in separated QWs, luminescence efficiencies, thermal activation energies for carrier transfer, and carrier capture and recombination rates. We demonstrate in this unique method an improved determination of the piezoelectric field and In composition x by a non-contact optical means alone. The results demonstrate an enhanced luminescence efficiency and yet decreased carrier capture rate by the SQW as the number of QWs increases in the adjacent MQW confinement region.