Epitaxial Growth of III-Nitride on Nano-Patterned Substrate for Light-Emitting Diode

Abstract

Group III-nitride has been regarded as one of the most promising material for optoelectronic device applications such as light-emitting diode (LED) and laser diode (LD) over past few decades. In order to realize highly efficient and reliable optoelectronic devices, high quality III-nitride epitaxial layers are required. A major problem in the epitaxial growth of III-nitride is that the use of native substrates is still limited due to lack of commercially available substrates. Therefore, III-nitride epitaxial layers are grown on foreign substrates such as sapphire and Si. However, large lattice mismatch and thermal mismatch between the III-nitride epitaxial layers and the substrates lead to several problems such as high-density dislocations, low light extraction efficiency (LEE), and residual film stress, thus hinder the realization of highly efficient III-nitride optoelectronic devices. Therefore, to obtain high quality III-nitride epitaxial layers that are less defective, less strained, and more effective to enhance the LEE is very important for various III-nitride LED applications. In this study, nano-patterned substrates have been proposed to obtain the high quality III-nitride layers for important epitaxial structures in the III-nitride LED applications such as GaN on Si substrate, AlN on sapphire substrate, and GaN on sapphire substrate. The epitaxial growth of III-nitrides on the nano-patterned substrates was investigated by metal-organic chemical vapor deposition. Firstly, for the case of GaN on Si substrate, nanoheteroepitaxy (NHE) of GaN on the AlN/Si(111) nanorod structure was investigated. Silica nanosphere lithography was employed to fabricate the periodic hexagonal nanorod array with a narrow gap of 30 nm between the nanorods. Fully coalesced GaN film was obtained over the nanorod structure and its threading dislocation density (TDD) was found to decrease down to half, compared to that of GaN grown on the planar AlN/Si(111) substrate. Transmission electron microscopy (TEM) revealed that threading dislocation (TD) bending and TD termination by stacking faults occurred near the interface between GaN and AlN/Si(111) nanorods, contributing to the TDD reduction. Moreover, the 70% relaxation of the tensile stress of the NHE GaN was confirmed by Raman and PL measurements compared to GaN on the planar AlN/Si(111) substrate. These results suggested that NHE on the AlN/Si(111) nanorods fabricated by nanosphere lithography is a promising technique to obtain continuous GaN layers with the improved crystalline quality and the reduced residual stress. Secondly, a nano-patterned AlN/sapphire substrate was developed to improve the performance of deep ultraviolet (DUV) LEDs, for the case of AlN on sapphire substrate. We demonstrated AlGaN-based DUV LEDs with periodic air-voids-incorporated nanoscale patterns enabled by nanosphere lithography and epitaxial lateral overgrowth (ELO). The nanoscale ELO on the nano-patterned substrate improved the crystal quality of overgrown epitaxial layers at relatively low growth temperature of 1050 oC and at small coalescence thickness. The air voids formed in the AlN epitaxial layer effectively relaxed the tensile stress during growth, so that crack-free DUV LED epitaxial layers were obtained on 4-in. sapphire substrate. In addition, the periodically embedded air-void nanostructure enhanced the LEE of DUV LEDs by breaking the total internal reflection that is particularly severe for the predominant anisotropic emission in AlGaN-based DUV LEDs. The light output power of the DUV LEDs on the nano-patterned substrate was enhanced by 67% at an injection current of 20 mA compared to that of the reference DUV LEDs. We attribute such a remarkable enhancement to the formation of embedded periodic air voids which cause simultaneous improvements in the crystal quality of epitaxial layers by ELO and LEE enabled by breaking the predominant in-plane guided propagation of DUV photons. Lastly, we proposed the ELO of GaN using the nano-cavity patterned sapphire substrate (NCPSS), which has hexagonally non-close-packed nano-cavity patterns on the sapphire substrates, to grow high quality GaN on sapphire substrate. The fabrication of the NCPSS was enabled by polystyrene coating followed by deposition of alumina and thermal annealing. The coalescence of GaN on the NCPSS was achieved by the formation of relatively large GaN islands and enhanced ELO of the GaN islands over several nano-cavity patterns. The TDD was significantly reduced from 2.4×10^8 /cm^2 to 6.9×10^7 /cm^2 by using the NCPSS. Dislocation behaviors that contribute to the reduction of TDD of the GaN layer were observed by TEM. Raman spectroscopy revealed that the compressive stress in the GaN layer was reduced by 21% due to the embedded nano-cavities. In addition, the diffuse reflectance of GaN on the NCPSS was enhanced by 54% ~ 62%, which is attributed to the increased probability of light extraction through effective light scattering by nano-cavities.