The effects of nanocavity and photonic crystal in InGaN/GaN nanorod LED arrays

Jiao et al. Nanoscale Research Letters (2016) 11:340 DOI 10.1186/s11671-016-1548-9 NANO EXPRESS Open Access The effects of nanocavity and photonic crystal in InGaN/GaN nanorod LED arrays Qianqian Jiao1 , Zhizhong Chen1*, Yulong Feng1, Shunfeng Li2, Shengxiang Jiang1, Junze Li1, Yifan Chen1, Tongjun Yu1, Xiangning Kang1, Bo Shen1 and Guoyi Zhang1,2 Abstract InGaN/GaN nanorod light-emitting diode (LED) arrays were fabricated using nanoimprint and reactive ion etching. The diameters of the nanorods range from 120 to 300 nm. The integral photoluminescence (PL) intensity for 120 nm nanorod LED array is enhanced as 13 times compared to that of the planar one. In angular-resolved PL (ARPL) measurements, there are some strong lobes as resonant regime appeared in the far-field radiation patterns of small size nanorod array, in which the PL spectra are sharp and intense. The PL lifetime for resonant regime is 0. 088 ns, which is 40 % lower than that of non-resonant regime for 120 nm nanorod LED array. At last, three dimension finite difference time domain (FDTD) simulation is performed. The effects of guided modes coupling in nanocavity and extraction by photonic crystals are explored. Keywords: GaN, Light emitting diode, Nanocavity, Photonic crystal, Guided modes Background Nanoscale light-emitting devices have attracted much attention for their potential applications in biotechnology [1, 2], communication [3] and solid state lighting [4, 5]. Compared to planar light-emitting diodes (LEDs), nanorod LEDs show high performances with higher internal quantum efficiency (IQE), higher light extraction efficiency (LEE) and optimal directionality [4–20]. The improvement of IQE is reasonable for nanorod LEDs, because of the strain relaxation [6–9] and extra in-plane excitonic confinement [7, 10] in InGaN active layer. Moreover, the nanocavity effect is confirmed to enhance the spontaneous emission (SpE) rate in well-ordered nanostructures [11, 12]. Nevertheless, the emission intensity of nanorod array is improved by an order of magnitude or more, which is mainly due to the reduction of modified guided modes [13–15]. Kuo et al [13] reported an ultrahigh extraction efficiency of 79 % at λ = 460 nm for a 100-nm diameter nanorod LED without packaging. Three key mechanisms are suggested for the high efficiency: guided modes reduction, embedded quantum wells (QWs) and ultra-efficient out-coupling of fundamental * Correspondence: 1 State Key Laboratory for Artificial Microstructure and Mesoscopic Physics, School of Physics, Peking University, Haidian, Beijing, China Full list of author information is available at the end of the article modes. Furthermore, the radiation patterns can be controlled when the guided modes are modified by nanostructures [8, 13, 16–18]. However, the mechanism of light emitting from nanorod array, including light extraction from the nanocavity and light diffraction by their array, is not clear yet. No vertical index confinement makes the mode distribution more complex [19]. Although thermal dissipation and defects/surface states should be dealt with for practical applications [8, 20], the optical modes in nanorod LEDs array are worthy of being manipulated exactly and carefully. The luminescence lifetimes of nanorod LEDs have been reported by some groups [7, 8, 10–12, 21, 22]. The radiative recombination rate is enhanced when the size of nanorod decreases [14, 21]. It is due to the reduction of the quantum confined Stark effect (QCSE) caused by the strain relaxation in InGaN QWs. However, photoluminescence (PL) decay time for nanorod LEDs may be much longer than that of the planar one [7, 8, 10, 22]. The causes include long exciton diffusion length [7], deep localization in the band-tail [10] and surface localization [22], and so on. It is well known that the SpE is inhibited in photonic bandgap (PBG) [23]. The propagation modes in the photonic crystal (PhC) do not alter SpE significantly [19]. On the other hand, the strong coupling of quantized photon modes with quantized excitations in © 2016 The Author(s). Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Jiao et al. Nanoscale Research Letters (2016) 11:340 confined photon structure will enhance SpE rate about two orders [11, 12]. The blue stimulated emission has been obtained by coupling a specific mode with InGaN active layer in nanocavity [24]. However, the contribution of optical modes is seldom distinguished from SpE rate in nanorod LED [25]. The dry etching damage control is also important for nanorod LED. High temperature annealing or wet etching can be used to recover or remove the several tens nm defective layer [16, 20, 26]. Furthermore, the wet etching is suitable for fabricating straight, smooth and well faceted nanorods [20]. Due to the difference in surface potential between n-type and p-type GaN, “flashlight” shaped nanorod can be easily obtained by specific wet etching. The “flashlight” shaped nanorod shows a bottleneck in the region of MQWs layer and n-GaN layer, which would reduce guided modes and enhance the light emitting out of the sidewall. Then, the light into the bottom layer would be reduced. In this work, the “flashlight” shaped nanorod arrays with top diameters from 120 to 300 nm were fabricated by nanoimprint, induced coupled plasma (ICP) etching and KOH wet etching. Temperature-dependent PL (TDPL), time-resolved PL (TRPL), and angular-resolved PL (ARPL) spectra were measured to study on the effects of nanocavity and PhC on the light emission in the nanorod LEDs array. Near field and far-field characteristics were calculated by three dimensional finite-difference time domain (3D-FDTD) solution. It were used to analyze the modes coupling and light extraction in the nanorod LEDs arrays. Methods The LED epilayer structure was grown on a c-plane sapphire substrate by metal organic chemical vapor deposition (MOCVD). It mainly consisted of a 2-μm undoped GaN layer, a 2.5-μm n-GaN layer, ten pairs of InGaN/ GaN (2.2 nm/13 nm) multiple quantum wells (MQWs) with dominant wavelength at about 445 nm, and a 130 nm-thick p-GaN layer. To fabricate nanorods, a 200-nm SiO2 mask layer was first deposited on the LED epilayer by plasma-enhanced chemical vapor deposition (PECVD). Secondly, a 230 nm-thick resist was spin-coated on the SiO2 mask layer. Thirdly, the pattern of triangular nanodisks array with 380-nm diameter and 525-nm pitch were transferred to the resist layer on wafer using nanoimprint lithography (NIL) by an Obducat Eitre® 3 instrument. Next, the residual resist was removed by O2 plasma. And then, the exposed SiO2 layer was pattern (...truncated)