Phosphor-Free Apple-White LEDs with Embedded Indium-Rich Nanostructures Grown on Strain Relaxed Nano-epitaxy GaN

C. B. Soh 0 1 W. Liu 0 1 A. M. Yong 0 1 S. J. Chua 0 1 S. Y. Chow 0 1 S. Tripathy 0 1 R. J. N. Tan 0 1 0 S. J. Chua Singapore-MIT Alliance, National University of Singapore, 4 Engineering Drive 3 , Singapore 117576, Singapore 1 C. B. Soh (&) W. Liu A. M. Yong S. J. Chua S. Y. Chow S. Tripathy R. J. N. Tan Institute of Materials Research and Engineering , A 2 STAR (Agency for Science, Technology and Research), 3 Research Link , Singapore 117602, Singapore Phosphor-free apple-white light emitting diodes have been fabricated using a dual stacked InGaN/ GaN multiple quantum wells comprising of a lower set of long wavelength emitting indium-rich nanostructures incorporated in multiple quantum wells with an upper set of cyan-green emitting multiple quantum wells. The lightemitting diodes were grown on nano-epitaxially lateral overgrown GaN template formed by regrowth of GaN over SiO2 film patterned with an anodic aluminum oxide mask with holes of 125 nm diameter and a period of 250 nm. The growth of InGaN/GaN multiple quantum wells on these stress relaxed low defect density templates improves the internal quantum efficiency by 15% for the cyan-green multiple quantum wells. Higher emission intensity with redshift in the PL peak emission wavelength is obtained for the indium-rich nanostructures incorporated in multiple quantum wells. The quantum wells grown on the nanoepitaxially lateral overgrown GaN has a weaker piezoelectric field and hence shows a minimal peak shift with application of higher injection current. An enhancement of external quantum efficiency is achieved for the apple-white light emitting diodes grown on the nano-epitaxially lateral overgrown GaN template based on the light -output power - measurement. The improvement in light extraction efficiency, gextraction, was found to be 34% for the cyan-green emission peak and 15% from the broad long wavelength emission with optimized lattice period. Quantum dots III-Nitride semiconductor High efficient group-III nitride-based light emitting diodes (LEDs) have been intensively developed in recent years for various applications such as street and traffic lights, back lighting and for headlights of automobiles. Solid-state lighting would replace conventional light bulbs and will change the way we light the world [1, 2]. InGaN/GaN, multiple quantum wells (MQWs) are often employed as the active layers due to their relatively high recombination efficiency and blue to green III-Nitride LEDs are commercially available. However, III-Nitride LEDs faced severe constraint when we attempt to incorporate high indium content in the materials due to large lattice mismatch (11%) between InN and GaN [3]. This leads to spinodal decomposition once InN content reaches a critical limit (*30%) [4]. The conventional white III-Nitride LEDs generated from phosphor coating have the disadvantages of having poor color rendering index, low yield issues in production and reduced thermal stability [5]. Pioneering work to generate phosphor-free white light was done by Damilano et al. [6] and Yamada et al. [7] by having InxGa1-x N/GaN quantum wells (QWs) of different indium compositions. Recent work by Huang et al. made use of prestrained InGaN well layer to generate white light [8] while Funato et al. has demonstrated a polychromatic emission (inclusive of white) from LEDs using microstructured multifaceted quantum wells [9]. Nanostructures, especially III-As-based quantum dots are a subject of wide variety of studies ranging from fundamental physics, quantum electrodynamics [10] to quantum information and computing [11, 12]. InGaAs/GaAs, quantum dots system with high uniformity has been generated on AlGaAs, which demonstrated narrow linewidth and improvement in excitons confinement [13]. III-Nitride quantum dots have also been applied by Chua et al. [14], not to generate narrow linewidth but to have a broad emission spectrum covering from 400 to 700 nm to mimic white (daylight) emission. However, there is still a need to improve on the light extraction efficiency due to total internal reflection, limiting the amount of light which can escape from the LEDs surface [15]. Various patterning techniques have been employed to enhance light extraction from LEDs, which includes surface roughening [16], geometric modification [17] and use of photonic crystals [18, 19]. Nanostructures and photonic crystals have been created by deposition into patterned substrates produced by conventional lithographic approaches such as e-beam lithography, interference lithography or X-ray lithography [20, 21]. These lithography approaches enable precise control of the spacing and dimensions of the nanostructures but the techniques are limited by high cost and low throughput. On the other hand, patterning based on selforganization technique such as the self-ordered aluminum oxide template enables the fabrication of arrays of nanostructures over a large area [21]. Earlier work has reported on the advantages of using nano-air bridge GaN template, which includes threading dislocation reduction and strain relaxation in subsequent InGaN/GaN MQWs or InN quantum dots grown [22, 23]. In this paper, we demonstrate the growth of InGaN/GaN LEDs incorporating indium-rich InGaN nanostructures on the nano-epitaxy lateral overgrown GaN template. Photoluminescence show enhanced peak intensity and higher activation energy for the multiple quantum wells grown on the nano-epitaxy lateral overgrown GaN template. Structural analysis of samples with embedded nanostructures and conventional InGaN/GaN well was carried out by scanning electron microscope (SEM). Preparation of the Nano-ELO GaN Template To generate the nano-epitaxially lateral overgrown (ELO) air bridge GaN template, a layer of 1.0 lm thick aluminum is first e-beam deposited on 200 nm SiO2 coated undoped GaN grown on sapphire substrate. The alumina mask was formed by a two-step anodization process carried out in 0.3M oxalic acid at 2 C with an injection voltage of 60 V. After the first anodization step, the sample was immersed in H2CrO4 acid at 60 C for 25 min to remove the oxide, leaving behind self-ordered pattern of pits for a subsequent anodization step in oxalic acid. A regular array of nanopores is generated on the alumina template as shown in Fig. 1a after the pores are enlarged in 5wt % of H3PO4, which serve as chemical etchant for alumina. Inductively coupled plasma (ICP) etching in CHF3 ambient is done to SiO2 and voids Fig. 1 Cross-section and plane view SEM images of GaN undergoing anodization to generate nano-ELO GaN structures. a SEM image of the anodized alumina oxide on GaN coated with a SiO2 film b SEM image after FIB to expose the transfer of the self-ordered patterned site on SiO2 film with subsequent growth of GaN pillars from these nanopores on SiO2 c re-growth of a thin buffer GaN on the nanopores SiO2 film d Lateral overgrowth to generate strain relaxed GaN template for subsequent growth of the multiple qu (...truncated)