Monolithically integrated white light LEDs on (11–22) semi-polar GaN templates

www.nature.com/scientificreports OPEN Received: 30 July 2018 Accepted: 30 November 2018 Published: xx xx xxxx Monolithically integrated white light LEDs on (11–22) semi-polar GaN templates N. Poyiatzis, M. Athanasiou, J. Bai, Y. Gong & T. Wang Carrier transport issues in a (11–22) semi-polar GaN based white light emitting diode (consisting of yellow and blue emissions) have been investigated by detailed simulations, demonstrating that the growth order of yellow and blue InGaN quantum wells plays a critically important role in achieving white emission. The growth order needs to be yellow InGaN quantum wells first and then a blue InGaN quantum well after the growth of n-type GaN. The fundamental reason is due to the poor hole concentration distribution across the whole InGaN quantum well region. In order to effectively capture holes in both the yellow InGaN quantum wells and the blue InGaN quantum well, a thin GaN spacer has been introduced prior to the blue InGaN quantum well. The detailed simulations of the band diagram and the hole concentration distribution across the yellow and the blue quantum wells have been conducted, showing that the thin GaN spacer can effectively balance the hole concentration between the yellow and the blue InGaN quantum wells, eventually determining their relative intensity between the yellow and the blue emissions. Based on this simulation, we have demonstrated a monolithically multi-colour LED grown on our high quality semi-polar (11–22) GaN templates. General illumination is one of the major sources for electricity demand globally. Due to global warming and the impending energy crisis, it is crucially important to develop energy-saving solid-state lighting (SSL). White light-emitting diodes (LEDs), which are primarily based on III-nitride semiconductor LEDs, are expected to ultimately replace incandescent bulbs and fluorescent tubes for a host of outdoor and indoor lighting applications due to the advantages of low power consumption and long lifetime1–3. The demand driven by energy saving has made the development of III-nitride based optoelectronics emerge as one of the fastest growing semiconductor areas over the last two decades. So far, the “blue LED + yellow phosphor” approach is maintaining its strong lead for the fabrication of white LEDs4,5. The performance of such white LEDs has almost approached its limit, but is still far from the requirements described in the US road map for developing SSL6. Furthermore, the phosphor-converted approach suffers from numerous drawbacks, such as down-conversion losses, optical losses due to backscattering, heat related effects and the degradation of yellow phosphor as a result of its long-term exposure7–9. In order to address these great challenges, a number of approaches have been proposed, such as monolithically integrated hybrid III-nitride/colloidal quantum dots10,11; hybrid III-nitride/organic conjugated polymers12–15. Although these white LEDs are still based on a down-conversion approach, they demonstrate a unique non-radiative energy transfer effect, which cannot be achieved by the “blue LED + phosphor” white LEDs mentioned above. One of the most direct routes for the fabrication of monolithic white LEDs is to utilize InGaN quantum wells with different emission wavelengths, where these emissions with different wavelengths can be obtained by controlling either InGaN quantum well (QW) thickness or indium content in InGaN16–20. In this approach, a combination of either blue/green/red (RGB) emissions or blue/yellow emissions is required. In principle, this approach not only is cost-effective but also matches the current growth and fabrication techniques for III-nitride optoelectronics. However, two major challenges need to be addressed before the potential of this approach can be possibly achieved. The first challenge is to obtain long wavelengths such as green and yellow emission with high performance. Current III-nitride LEDs are grown on c-plane substrates. The polar orientation poses strain-induced piezoelectric fields due to the lattice-mismatch between InGaN and GaN, which is the so-called quantum-confined Stark effect (QCSE). As a result, internal quantum efficiency is reduced, and drops significantly further when InGaN quantum wells move towards longer wavelengths such as the green or yellow spectral region (where higher Department of Electronic and Electrical Engineering, University of Sheffield, Mappin Street, Sheffield, S1 3JD, United Kingdom. Correspondence and requests for materials should be addressed to T.W. (email: ) Scientific Reports | (2019) 9:1383 | https://doi.org/10.1038/s41598-018-37008-5 1 www.nature.com/scientificreports/ Figure 1. Schematics of the structures of Sample A and Sample B. indium content is required, leading to an enhancement in QCSE), thus forming the well-known “Green-Yellow gap” phenomenon. Furthermore, the c-plane GaN also leads to fundamental limitations in incorporating indium into GaN21,22. The second issue is due to the complicated carrier transport in InGaN QWs with different indium composition as a result of much lower hole mobility and hole concentration than those of electrons, potentially leading to severe non-uniform carrier distribution across all the InGaN QWs involved. This complexity is further enhanced by InGaN structures grown on c-plane GaN due to piezo-electrical fields induced polarisation. This issue becomes even more complicated with increasing indium content as a result of an enhancement in piezo-electrical fields induced polarisation. So far, there is no systematic study addressing this issue. Growth of III-nitrides along a semi-polar direction, in particular the (11–22) orientation, would be a promising solution to achieve long wavelength emissions, as this orientation is expected to lead to not only significantly reduced piezoelectric polarization fields but also enhanced indium incorporation efficiency in InGaN23,24. Furthermore, an increasing demand on Li-Fi applications requires a white LED with an ultra-fast response. Current blue LEDs on c-plane substrates suffer from a long carrier recombination lifetime as a result of QCSE, typically on a scale of a few to 10 nanoseconds for blue emission and ~100 nanoseconds for green emission25. Phosphors generally exhibit even a longer response time, typically on a microsecond scale. In contrast, semi-polar (in particular (11–22)) InGaN quantum wells exhibit a much shorter carrier recombination lifetime, typically hundreds of picoseconds for blue emission26. Therefore, semi-polar phosphor-free LEDs are ideal for Li-Fi applications. Recently, Sizov et al.27 observed severe non-uniform carrier distribution among the InGaN multiple quantum wells (MQWs) of a laser diode grown on a c-plane substrate, leading to an increase in threshold current when the number of InGaN MQWs is above 2, while they did not observe this phenomenon on the LDs grown on semi-polar substrates. This fact (...truncated)