Green Gap LEDs

Under DST (India)-Japan Society for the Promotion of Science (JSPS) collaborative project, a green gap broadband emitter has been developed to achieve a broad emission in the yellow-green region for GaN-InGaN LEDs, using single InGaN/AlN nano-disk, sandwiched between n-GaN and p-GaN epitaxial layers grown by MOCVD. Self-assembled InGaN/AlN structure was grown on a single-sided polished c-plane (0001) sapphire substrate. Initially, a 30 nm low-temperature GaN nucleation layer was deposited at 550°C. The temperature was ramped up to 1180°C for growing 4 μm thick Sidoped GaN cladding layer followed by ramp down of temperature to 805°C for growing nano-disks in active layers consisting of AlN and InGaN. During the growth, there was no temperature swing between InGaN and AlN. The growth temperature of InGaN was same as that of blue LED (450 nm) using InGaN/GaN MQWs, but barrier GaN was replaced by AlN. Nanodisks were formed due to crack formation in the first AlN layer. The succeeding InGaN/AlN layers supported branching of nanodisks from the initial cracks of the AlN layer. The structure was capped with a 120 nm thick Mgdoped GaN layer grown at 1020°C. The schematic cross section of the device is depicted in Fig. 1. LEDs were fabricated with indium contact on both p-and n-GaN layers. The electro-luminescence (EL) spectrum showed a peak emission of 558 nm as shown in Fig. 2. The broad spectrum is due to the fluctuation of indium composition at the interface of InGaN and AlN. Fig. 3 shows light emission from the green LED. It is learnt that this is the first indigenous research effort.

Fig. 1: Schematic cross-section of the nanodisk green LED

Fig. 2: Electro-luminiscence (EL) spectrum of green LED

Fig. 3: Light emission from green LED