Impact of ballistic electron transport on efficiency of InGaN based LEDs

Abstract

InGaN light emitting diodes (LEDs), which have become key components of the lighting technology owing to their improved power conversion efficiencies and brightness, still suffer from efficiency degradation at high injection levels. Experiments showing sizeable impact of the barrier height provided by an electron blocking layer (EBL) or the electron cooling layer prior to electron injection into the active region strongly suggest that the electron overflow resulting from ballistic and quasi-ballistic transport is the major cause of efficiency loss with increasing injection. Our previous report using a first order simple overflow model based on hot electrons and constant LO phonon scattering rates describes well the experimental observations of electron spillover and the associated efficiency degradation in both nonpolar <i>m</i>-plane and polar <i>c</i>-plane LEDs with different barrier height EBLs and electron injection layers. LEDs without EBLs show three to five times lower efficiencies than those with Al_0.15Ga_0.85N EBLs due to significant electron overflow to the <i>p</i>-type region in the former. For effective means of thermalization in the active region within their residence time and possibly longitudinal optical phonon lifetime, the electrons were cooled prior to their injection via a staircase electron injector, i.e. an InGaN staircase structure with step-wise increased In composition. The investigated <i>m</i>-plane and <i>c</i>-plane LEDs with incorporation of staircase electron injector show comparable electroluminescence performance regardless of the status of EBL. This paper discusses hot electron effects on efficiency loss, means to cool the electrons prior to injection.