Performance characteristics of non-polar and semipolar InGaN QW LEDs
External quantum efficiencies and emission wavelength
Within the past decade the efficiency and spectral range of non- and semipolar LEDs has increased rapidly. First, non- and semipolar InGaN QW LEDs were demonstrated on heteroepitaxially-grown defect-reduced (1120) GaN on a-plane sapphire substrates (Chakraborty et al., 2004) as well as on (1011) and (10l3) oriented GaN grown on (100) and (110) spinel (MgA^O-Q substrates (Chakraborty et al., 2005a). However, due to the relatively large threading dislocation densities and basal-plane stacking faults in these heteroepitaxially grown layers, the light output of the LEDs just barely exceeded one mW, and the external quantum efficiencies were a fraction of 1% (Chakraborty et al., 2005a). The performance of non- and semipolar LED improved significantly with the development of low-defect-density bulk GaN substrates cut along non- and semipolar orientations. First, blue- and violet-emitting InGaN QW LEDs were demonstrated on low-defect-density (1010) m-plane GaN single crystals by Okamoto et al. as well as Chakraborty et al. with EQE of 3.1 % and 1.1 %, respectively (Okamoto et al., 2006; Chakraborty et al., 2006a). Simultaneously, first blue, green, and even amber InGaN quantum-well LEDs were realized on (1122) semipolar bulk GaN substrates with EQEs of 4.0%, 4.9%, and 1.6%, respectively (Funato et al., 2006). After these initial breakthroughs, the efficiency of non- and semipolar LEDs increased significantly within a short period of time. Schmidt et al. (Schwarz and Kneissl, 2007) reported on high-efficiency non-polar InGaN QW violet LEDs on low-defect-density bulk m-plane GaN substrates with EQEs of 38.9 %, and high-efficiency semipolar blue LEDs on free-standing (1011) bulk GaN substrates have been demonstrated by Zhong et al. (2007) with EQEs of 29%. Shortly thereafter, Sato et al. (2007) reported high-power green InGaN multiple-quantum-well (MQW) light-emitting diodes grown on low extended-defect-density semipolar (1122) bulk GaN substrate with peak external quantum efficiencies of more than 12 %. More recently, high-power and high- efficiency blue InGaN LEDs have also been demonstrated by Koslow et al. on semipolar (3033) GaN substrates with peak EQEs of 26.5% (Koslow et al., 2010), and Pan et al. on semipolar (2021) GaN with maximum external quantum efficiencies of 50.1 % (Koslow et al., 2010).
Figure 8.15 shows examples of InGaN MQW and SQW LED heterostructure on semipolar (10П) and (2021) GaN substrate, respectively. The active region of the LED on (1011) GaN substrate is comprised of six periods of 3-nm thick InGaN quantum wells separated by 20-nm wide undoped GaN barriers. The MQW stack is covered by a 10-nm thick nominally undoped Alo.15Gao.85N electron blocking layer followed by a 200-nm thick Mg-dopded GaN contact layer. Although the design of the device heterostructure appears quite straightforward and is very similar to c-plane LEDs, there are some remarkable differences. Kim et al. (2007a), as well as Zhong et al. (2007), demonstrate high-efficiency non- and semipolar blue LEDs on m-plane and (10П) GaN with a nominally undoped Al0.15Ga0.85N electron blocking layers (EBL). This design detail is very different from c-plane LED, where the functionality of the EBL within an LED heterostructure depends on the ability to incorporate p-type AlGaN:Mg electron blocking layers. Another interesting detail is the utilization of 12-nm thick InGaN single-quantum-well active region in semipolar LEDs, as described e.g. by Pan et al. (2012), and also shown in Fig. 8.15b). This design is a direct result of the reduced polarization fields in semipolar InGaN/GaN heterostructures, which, due to a significantly reduced piezoelectric field, enables the use of thick InGaN QWs without detrimental effect on the electron-hole envelope wavefunction overlap. As a consequence of the larger QW thickness and the shorter radiative recombination lifetimes, the carrier densities for a fixed current density are also reduced, which could reduce higher-order carrier-density effects such as Auger-type recombination. This should result in much smaller efficiency droop at increasing
Fig. 8.15. Schematic of a typical InGaN multiple-quantum-well (MQW) LED heterostructure grown on (a) semipolar (1011) GaN substrate (Zhong et al., 2007), and (b) a InGaN single-quantum-well (SQW) LEDs on (2021) GaN (Pan et al., 2012).
current densities, as was also by observed (Pan et al., 2012), where the EQE dropped from 50.1 % at 100 A/cm2 to 41.2 % at a current density of 400 A/cm2.