Influence of patterned sapphire substrates with different symmetry on the light output power of InGaN-based LEDs
© You et al.; licensee Springer. 2014
Received: 30 June 2014
Accepted: 24 October 2014
Published: 3 November 2014
This paper aims to investigate the light output power (LOP) of InGaN-based light-emitting diodes (LEDs) grown on patterned sapphire substrates (PSSs) with different symmetry. The GaN epitaxial layers grown on the hexagonal lattice arrangement PSS (HLAPSS) have a lower compressive strain than the ones grown on the square lattice arrangement PSS (SLAPSS). The quantum-confined Stark effect (QCSE) is also affected by the residual compressive strain. Based on the experimentally measured data and the ray tracing simulation results, the InGaN-based LED with the HLAPSS has a higher LOP than the one with the SLAPSS due to the weaker QCSE within multiple-quantum wells (MQWs).
In recent years, InGaN-based light-emitting diodes (LEDs) are widely used for applications in the backlight of flat-panel displays and solid-state lighting. In order to compete with conventional lighting sources and realize the ultimate lamp, the external quantum efficiency (EQE) of InGaN-based LEDs must be enhanced. The EQE of InGaN-based LEDs is determined by internal quantum efficiency (IQE) and light extraction efficiency (LEE). Many significant methods have been proposed in the literatures for improving the EQE of InGaN-based LEDs, such as patterned sapphire substrates (PSSs)[2–6], epitaxial lateral overgrowth (ELOG)[7, 8], surface structure[9–11], semi/non-polar quantum wells (QWs)[12–15], and so on. However, among these technologies, the PSS method has attracted considerable attention because of its ability to improve both IQE and LEE. While implementing the InGaN-based LEDs on PSSs with various periodic patterns, structural parameters of PSSs should be taken into consideration cautiously. The previous published articles have shown that the light output power (LOP) of InGaN-based LEDs is dependent on the configuration of these parameters including spacing, slanted angle, shape, height, and density. Nevertheless, studies concerning the effect of PSSs with different symmetry on the LOP of InGaN-based LEDs were still limited.
In this paper, the effect between the LOP and the InGaN-based LEDs grown on PSSs with different symmetry is investigated in detail through simulation and measurements. The different symmetry of PSS used for experimentation include square lattice arrangement (SLA) and hexagonal lattice arrangement (HLA).
After the cleaning process, the InGaN-based LED samples were grown on the PSSs with Taiyo Nippon Sanso SR2000 (Taiyo Nippon Sanso, Dalian, China) atmospheric pressure metal organic chemical vapor deposition (AP-MOCVD) under a three-flow gas injection. Prior to the growth, substrates were thermally baked at 1,180°C in hydrogen gas to remove surface contamination. The InGaN-based LED structures were initially grown on the PSSs, and their structure consists of a 25-nm-thick low-temperature GaN nucleation layer, a 2.5-μm-thick unintentionally doped GaN buffer layer (grown at 1,180°C), and a 3-μm-thick n-GaN layer, using SiH4 as the n-type dopant. Then, five pairs of InGaN/GaN multiple-quantum wells (MQWs) having a 2.9-nm-thick InGaN well and an 11-nm-thick GaN barrier (grown at 800°C and 850°C, respectively) were deposited, followed by a 20-nm-thick p-AlGaN electron-blocking layer and a 120-nm-thick p-GaN layer, using Cp2Mg as a p-type dopant. The InGaN-based LEDs with a conventional sapphire substrate (CSS), SLAPSS, and HLAPSS were grown under the same growth condition.
Results and discussion
In this paper, the superiority of InGaN-based LEDs on the HLAPSS is demonstrated. It relaxes compressive strain in the GaN epitaxial layer more than the ones on the SLAPSS. With the relaxation in compressive strain, the reduction of QCSE is observed due to the lower lattice mismatch in the InGaN/GaN MQWs. As a result, the mitigation of the efficiency droop for the InGaN-based LED on the HLAPSS occurs. Furthermore, the LEE of InGaN-based LEDs on SLAPSS and HLAPSS appeared similar from the ray tracing simulation. In comparison to the InGaN-based LED grown on the CSS at an injection current of 20 mA, the increased LOP value of the samples grown on the SLAPSS and HLAPSS is reported to be 60% and 82%, respectively.
This study was supported by the National Science Council of Taiwan, ROC (project no. NSC 99-2221-E-182-040) and in part by Sino-American Silicon Products Incorporated.
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