Insight into the performance of multi-color InGaN/GaN nanorod light emitting diodes

www.nature.com/scientificreports OPEN Received: 29 January 2018 Accepted: 20 April 2018 Published: xx xx xxxx Insight into the performance of multi-color InGaN/GaN nanorod light emitting diodes Y. Robin1,2, S. Y. Bae1,3, T. V. Shubina4, M. Pristovsek 1, E. A. Evropeitsev4, D. A. Kirilenko4, V. Yu. Davydov4, A. N. Smirnov4, A. A. Toropov4, V. N. Jmerik4, M. Kushimoto5, S. Nitta1, S. V. Ivanov4 & H. Amano1,2 We report on the thorough investigation of light emitting diodes (LEDs) made of core-shell nanorods (NRs) with InGaN/GaN quantum wells (QWs) in the outer shell, which are grown on patterned substrates by metal-organic vapor phase epitaxy. The multi-bands emission of the LEDs covers nearly the whole visible region, including UV, blue, green, and orange ranges. The intensity of each emission is strongly dependent on the current density, however the LEDs demonstrate a rather low color saturation. Based on transmission electron microscopy data and comparing them with electroluminescence and photoluminescence spectra measured at different excitation powers and temperatures, we could identify the spatial origination of each of the emission bands. We show that their wavelengths and intensities are governed by different thicknesses of the QWs grown on different crystal facets of the NRs as well as corresponding polarization-induced electric fields. Also the InGaN incorporation strongly varies along the NRs, increasing at their tips and corners, which provides the red shift of emission. With increasing the current, the different QW regions are activated successively from the NR tips to the side-walls, resulting in different LED colors. Our findings can be used as a guideline to design effectively emitting multi-color NR-LEDs. Currently, practicable electronics and photonics are based on 2D planar materials, while recently emerging novel 3D materials would be highly desired for the development of future nanoscale systems. For instance, semiconductor nano-crystals are expected to find various applications in, photonics and bioengineering1. Due to their relative growth simplicity and size reproducibility compared, e.g., to self-organized quantum dot (QD) arrays, nanorods (NRs) are promising building blocks for nanophotonics. Moreover, NRs present advantageous features for the fabrication of challenging 3D structures. Their low footprint on substrates allows a heteroepitaxy on highly lattice- and thermal-mismatched substrates2. Due to the small size of NRs, the strain accumulated in the material during the earliest stage of the growth is released at the sidewalls. Besides, the annihilation and bending of dislocations toward the sidewalls allow the growth of nanocrystals with few or without extended defects. As a result, each NR owns facets of different polarity of high crystalline quality3. These facets are very interesting as they possess different surface energy and offer different growth kinetics4. Especially, for III-nitride materials which exhibit a hexagonal lattice, different polarization induced fields are inherent for each facet. Therefore, in the case of NR-LEDs, the quantum wells (QWs) grown at each facet are necessarily affected by different local quantum confinement Stark effect (QCSE). Indeed, several group reported multi-emission for such kind of devices5–8. Monolithic integration of RGB nitride-based displays is probably the most important application of the NR-based devices. So far, different growth approaches have been investigated. For instance, catalyst-assisted growth9, self-assembled growth10,11, selective area epitaxy with12 or without pulsing the precursors13 have been reported. The device processing of such structures is rather complicate. The common strategy consists in planarization of the device after growth by filling the gaps in the NRs array with an insulating polymer14 or directly during the p-GaN deposition by coalescing the rods15–17. All of these approaches usually result in more or less leaking devices with insufficient color controllability. The lack of color purity is an additional problem related 1 Institute of Materials and Systems for Sustainability (IMaSS), Nagoya University, Nagoya, Japan. 2Center for Integrated Research of Future Electronics (CIRFE), Nagoya University, Nagoya, Japan. 3Korea Institute of Ceramic Engineering and Technology, Jinju, South Korea. 4Ioffe Institute, 194021, St. Petersburg, Russia. 5Department of Electrical Engineering and Computer Science, Nagoya University, Nagoya, Japan. Correspondence and requests for materials should be addressed to Y.R. (email: ) SCIENTIFIC RePorts | (2018) 8:7311 | DOI:10.1038/s41598-018-25473-x 1 www.nature.com/scientificreports/ to the sum of QW emissions coming simultaneously from the different facets. Local inhomogeneity of the shell structure is another deleterious reason which contributes to desaturation of the colors18–20. In order to achieve high quality monolithic RGB devices, a deeper insight into the core-shell layer structure and its impact on the optical properties is required. Several reports already presented thorough investigations of the nanoscale properties of the rods19,21, while many other were mainly focused on the devices performances and applications22–25. However, it is difficult to directly compare the results of such different publications as NRs are often obtained using different growth techniques and exhibit different core-shell structures and crystalline qualities. In addition, due to the lack (or complexity) of characterization techniques at such low scale, it is not rare to find contradictory reports. Therefore, complex exhaustive studies, even if each method is not novel, are important to establish the solid foundation of the NRs technology and to exclude the misleading conclusions from so dispersed data. In this paper, we demonstrate the successful growth of well-ordered NR arrays and analyze the performance of obtained NR-based LEDs by comparative structural and optical studies done with high spatial resolutions. With detailed data consistently collected from the growth of materials to the device processing and analysis, we draw a critical overview of NR-based LEDs challenge for RGB display application. Experimental Section The NRs were grown by pulsed selective area epitaxy (SAE) in an EpiQuest showerhead MOCVD reactor. At first, a 30 nm-thick SiO2 dielectric mask was deposited by reactive sputtering on a commercial n-GaN template (n = 4 × 1018 cm−3) on silicon (111). An array of 460 nm-wide holes was then opened by a combination of nanoimprint lithography and plasma etching. The NR growth was performed at about 1000 °C under H2. The 4.2 Pa trimethyl-gallium (TMGa) and 1.3 × 104 Pa ammonia (NH3) were used as precursors and injected alternatively in the growth chamber for 5 and 15 s, respectively. A purge time of 1 s was introduced between each pulse to promote the vertical growth26. The pulse sequence cycle was repeated 200 times. Although we (...truncated)