江风益院士团队大幅提升InGaN基橙-红光LED发光性能

InGaN薄膜因其宽带隙可调的优点,在可见光领域内拥有广阔的应用前景,用于micro-LED全彩显示是其中最有潜力的应用之一,未来的智能手机、手表、虚拟现实眼镜等小尺寸显示屏都将受益于micro-LED技术。

目前micro-LED技术正面临两大挑战,首先是大家熟知的实现巨量转移技术非常困难,另一个就是缺乏高效可靠的红光micro-LED芯片。目前的红光LED是由AlGaInP材料制成,在正常芯片尺寸下,其效率高达60%以上。然而,当芯片尺寸缩小到微米量级时,其效率会急剧降低到1%以下。此外,AlGaInP材料较差的力学性能给巨量转移增加了新的困难,因为巨量转移要求材料具有良好的机械强度,以避免在芯片抓取和放置过程中出现开裂。

InGaN材料在具有较好机械稳定性和较短空穴扩散长度的同时,又与InGaN基绿光、蓝光micro-LED兼容,是红光micro-LED的较佳选择。然而,InGaN基红光量子阱存在严重的铟偏析问题,这将导致红光量子阱中的非辐射复合增加,从而引起效率降低。在过去20年的研究中,InGaN基红光LED功率转换效率不足2.5%。铟偏析问题严重阻碍了InGaN基红光LED的发展。因此,如何解决铟偏析问题是获得高效InGaN基红光LED的关键。

近日,南昌大学的江风益院士课题组在Photonics Research 2020年第8卷第11期上(Shengnan Zhang, Jianli Zhang, Jiangdong Gao, et al. Efficient emission of InGaN-based light-emitting diodes: toward orange and red[J]. Photonics Research, 2020, 8(11): 11001671)展示了他们最新研制的高光效InGaN基橙-红光LED结果。

(a) 高光效橙光LED外延材料结构示意图与(b) 其断面TEM测试结果

此项工作基于硅衬底氮化镓技术,引入了铟镓氮红光量子阱与黄光量子阱交替生长方法,并结合V形坑技术,从而大幅缓解了红光量子阱中高In组分偏析问题。再依据V形p-n结和量子阱带隙工程大幅提升了红光量子阱中的辐射复合速率。

使用该技术成功制备了一系列高效的InGaN基橙-红光LED。当发光波长分别为594、608和621 nm时,其功率转换效率分别为30.1% 、24.0%以及 16.8%,光效相较于以往报道的相同波段InGaN基LED结果整体提高了约十倍。

研究人员认为,该项技术在未来还有较大的进步空间,同时该团队的实验结果也证明了InGaN材料在制作显示应用的红光像素芯片上将有巨大潜力和美好前景。

Efficient emission of InGaN-based light-emitting diodes: toward orange and red

InGaN-based materials have broad application prospects in visible light range due to its tunable wide band-gap. Micro size light emitting diode (micro-LED) display is one of the most promising applications. Future smart phones, watches, virtual reality glasses, and other small screens will benefit from micro-LED technology.

The micro-LED technology faces two great challenges, one challenge is known as massive transfer, and the other challenge is the lacking of efficient and reliable red micro-LEDs. The current red LEDs are made of AlGaInP materials with high efficiency above 60% in normal chip size. However, when the chip size is reduced to micro meter, its efficiency decreases dramatically to less than 1%. The drop of efficiency can be attributed to the strong surface recombination associated with long diffusion length of holes in AlGaInP materials. Besides, the poor mechanical property of AlGaInP materials adds new difficulty to the mass transfer process, which requires good mechanical strength to avoid chip cracking during the pick and place process.

The InGaN material has better mechanical stability and shorter hole diffusion length, which is a good candidate for red micro-LED, and it also compatible with current InGaN green and blue micro-LEDs. However, the InGaN-based red LEDs suffer from indium segregation problem, which will lead to an increase in non-radiative recombination in the red quantum wells (QWs) and decrease in efficiency. In the past two decades, the power conversion efficiency of InGaN-based red LEDs was reported to be less than 2.5%. The problem of indium segregation has seriously hindered the development of InGaN-based red LEDs. Therefore, how to solve the problem of indium segregation is the key to obtain efficient InGaN based red LEDs.

The research group led by Academician Prof. Fengyi Jiang from Nanchang University demonstrated their latest research result on efficient InGaN-based orange and red LEDs, which is now published in Photonics Research, Volume 8, No. 11, 2020 (Shengnan Zhang, Jianli Zhang, Jiangdong Gao, et al. Efficient emission of InGaN-based light-emitting diodes: toward orange and red[J]. Photonics Research, 2020, 8(11): 11001671).

Schematic diagram of the epi-structure (a) and cross-sectional TEM test results (b) of efficient orange LED

In this work, based on GaN/Si technology, by introducing alternative red and yellow QWs, and combining the technology of V-pits, the problem of high indium content segregation in the red QWs is greatly alleviated. And with the help of V-shaped p-n junctions and QWs band gap engineering, the radiative recombination in the red QWs is greatly enhanced. A series of efficient InGaN-based orange and red LEDs were successfully fabricated with power conversion efficiency of 30.1% at wavelength of 594 nm, 24.0% at wavelength of 608 nm, and 16.8% at wavelength of 621 nm. These values are about ten times higher than those of the previous reports on InGaN-based orange and red LEDs.

The researchers believe that this technology will have large advancement in near future. The results demonstrate the great potential and beautiful perspective of InGaN materials to be used as red pixel for full-color micro-LED display.