On the quantum efficiency of InGaN light emitting diodes: Effects of active layer design, electron cooler, and electron blocking layer



Efficiency and efficiency retention in InGaN LEDs has recently received considerable attention. In this realm, we investigated internal quantum efficiency (IQE) and relative external quantum efficiency (EQE) of c-plane InGaN LEDs designed for emission at ∼420 nm from the active region which contains multiple quantum wells (MQWs) of different barrier height (either In0.01Ga0.99N or In0.06Ga0.94N barriers) and thickness (3 and 12 nm) as well as a 9-nm double heterostructure (DH). Pulsed electroluminescence (EL) and laser excitation power-dependent measurements indicated that both the relative EQE and the IQE were enhanced due to the incorporated two-layer InGaN stair-case electron injector (SEI) with indium mole fraction steps of 4 and 8% as compared to the conventional AlGaN electron blocking layer (EBL). Furthermore, the lowered In0.06Ga0.94N interwell barriers (LB) instead of the traditional In0.01Ga0.99N barriers improved the EQE and the IQE of MQW LEDs. Specifically, the MQW LEDs with the 6-period 2-nm In0.2Ga0.8N quantum well and 3-nm In0.06Ga0.94N barrier structure showed 6% higher IQE at an injected carrier density of 6 × 1018 cm−3 and 35% higher EQE as compared to that of the same structure with a higher In0.01Ga0.99N barrier. The DH LEDs showed 30% higher EQEs compared to MQW LEDs, albeit at a relatively higher injection current density of 150 A/cm2. The relatively low EQE in the DH LEDs at low injection levels is attributed to spatial separation of electrons and holes due to confinement in the interfacial triangular well and thus the associated decrease in radiative efficiency and possible increase in nonradiative recombination due to degradation of material quality with increasing InGaN layer thickness.