Effects of bias on cathodoluminescence in ZnCdSe quantum well light emitting diodes

Abstract

Bias voltages applied to Zn0.24Cd0.76Se quantum well light emitting diodes (QW-LEDs) affect both the intensity and wavelength of room temperature cathodoluminescence (CL). These effects have been studied experimentally and theoretically to advance understanding of the CL and optoelectronic behavior of these devices. QW CL intensity and photon energy are increased by forward bias, and they are decreased by reverse bias, with an exponential dependence of CL intensity on bias voltage from −1 to +1 V and little dependence from 1.5 to 2.5 V. The p-n junction current and electroluminescence increase rapidly for forward bias greater than 2.34 V, the calculated built-in potential. The bias dependence of QW CL intensity is little affected when electron beam currents change by ∼300 times, from 0.1 to 29 nA with 10 kV beam voltage and ∼1 μm2 irradiated area. The QW CL intensity increases sublinearly with beam current. Small hysteresis effects are seen in bias-dependent CL intensity for low beam currents. The effects of bias voltage on CL intensity and photon energy have been modeled, including bias dependence of carrier transport, QW energy levels, wave functions, overlap integrals, internal electric fields, exciton ionization, and rates of carrier capture in and escape from the QW. For the QW-LED and experimental conditions used in this study, the bias dependence of CL intensity at room temperature results mainly from electric field dependence of exciton ionization and of electron and hole captures in the QW, and the bias dependence of CL photon energy results from field-dependent shifts in QW energy levels of electrons and holes.