Investigation of the Nonthermal Mechanism of Efficiency Rolloff in InGaN Light-Emitting Diodes

We present a comparative study on the optical characteristics of InGaN-based multiple quantum well light-emitting diodes (LEDs) with peak emission ranging from green to ultraviolet (UV) over a wide injection range. It is found that by pulsing the LEDs with a duty cycle that is below 1%, thermally induced peak red shift and efficiency reduction are largely eliminated. The current dependence of both the quantum efficiency and peak shift appears to be a strong function of the indium content in the active region. The quantum efficiencies of the blue and green LEDs peak at very low currents and dramatically decrease at high currents, whereas the UV LED has a nearly constant quantum efficiency under high injection conditions. In contrast to the minimal current- induced energy shift in the UV LED, a monotonic blue shift of the peak energy, which has a total amount of ~110 meV-1 kA/cm2, is seen for the green LED. These results offer a strong support for the argument that the current overflow from localized states is the major nonthermal mechanism underlying the efficiency rolloff in InGaN-based visible LEDs.