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PR Highlights(Vol.8, Iss.6): 一招大幅提升深紫外LED中横磁模式光

2020-07-30

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一招大幅提升深紫外LED中横磁模式光

深紫外LED(UVC-LED)主要利用200~280 nm波段的光对微生物(细菌、病毒、芽孢等病原体)造成辐射损伤,破坏并改变DNA结构,使微生物当即死亡或不能繁殖后代,达到杀菌消毒的目的。基于UVC-LED的消毒产品凭借安全、环保、稳定性高、寿命长、维护成本低、无化学残留等特性,在空气、物表和水处理上得到了更多的创新应用。

然而,UVC-LED当下急需解决外量子效率和大电流下输出光功率密度的问题。除了外延材料质量差、电注入效率低之外,导致其外量子效率低的主要因素还在于TM模式(E//C)深紫外光的提取效率很低。TM模式的UVC光被限制在外延层中,在其中发生全反射,难以出射。而已知的提升可见光LED出光效率的方法对TM模式光收效甚微。

西安交通大学的李虞锋博士、云峰教授课题组在Photonics Research 2020年第6期的论文中提出了一种简单易行的大幅提升UVC-LED中TM模式光的方法(Yufeng Li, Chenyu Wang, Ye Zhang, et al. Analysis of TM/TE mode enhancement and droop reduction by a nanoporous n-AlGaN underlayer in a 290 nm UV-LED[J]. Photonics Research, 2020, 8(6): 06000806)。

论文深入讨论了该方法对TE和TM的影响机制和提升程度。借助该课题组之前在GaN电化学刻蚀的经验,研究人员在确保导电特性不受影响的前提下,把更难刻蚀的高Al组分的n-AlGaN转换成了周期性的纳米多孔光栅层。纳米多孔层的随机散射极大地降低了p-GaN层和有源区的全反射和对发射光子的再吸收,从而促进了TM模光子的出射效率,使得顶部和侧面发光强度分别提高了1.57倍和22倍。

此外,纳米多孔光栅层还弛豫了压缩应变产生的应力,降低了量子限制斯塔克效应(QCSE),从而抑制了载流子的非辐射复合。电致发光显示,具有纳米多孔光栅层的UVC-LED的光输出功率提高了36.5%。此外,UVC-LED芯片的效率“droop”得到大幅缓解:100 mA电流驱动下,EQE相比其最大值只下降了36%,而参考样品的EQE下降了61%。

具有纳米多孔光栅层的UVC-LED外延结构

云峰教授认为,该方法简单、可靠,广泛适用于商用深紫外外延芯片结构,非常有望大幅提升UVC-LED的外量子效率。此外,该方法有望最终替代传统深紫外光源,为公共卫生安全事业提供最终的解决方案。

Analysis of TM/TE mode enhancement and droop reduction by a nanoporous n-AlGaN underlayer in a 290 nm UV-LED

UVC-LEDs with light emission from 200-280 nm were mainly used to introduce germicidal radiation to microorganisms (bacteria, viruses, spores and other pathogens). It damages and changes the structure of DNA (deoxyribonucleic acid), so that the microorganisms die or loss the reproduction capability immediately, and achieve the purpose of sterilization and disinfection. UVC-LED has a great market potential with a wide range of applications in air sterilization, surface disinfection and water treatment, because of its stability and safety, environmental friendliness, long lifetime, low maintenance cost and zero chemical residue. However, the biggest challenge for the UVC-LED is its low external quantum efficiency and insufficient output optical power density under high current. In addition to the poor quality of the Al-rich UVC epitaxial material and the low injection efficiency of the carrier, the main reason for the low quantum efficiency is due to the low extraction efficiency of TM mode (E//C) photons. TM mode UVC photons are confined in the epitaxial layer and suffer from the total internal reflection and re-absorption. Previous methods known to improve the light extraction of visible LEDs have little effect on TM mode light in the UVC-LED.

The research group led by Dr. Yufeng Li and Prof. Feng Yun from Xi'an Jiaotong University put forward a simple and easy method to improve the TM mode light emission of a 290 nm UVC-LED. The related results are published in Photonics Research, Vol. 8, Issue 6, 2020 (Yufeng Li, Chenyu Wang, Ye Zhang, et al. Analysis of TM/TE mode enhancement and droop reduction by a nanoporous n-AlGaN underlayer in a 290 nm UV-LED[J]. Photonics Research, 2020, 8(6): 06000806). With the help of their previous experience in electro-chemical etching of GaN material, the researchers converted the Al-rich n-AlGaN, which is more difficult to be etched, into a periodic nanoporous grating layer, while maintaining its conductivity. The random photon scattering by the nanoporous layer greatly reduces the total internal reflection and re-absorption of the TM mode photons by the p-GaN layer and the active region. As a result, it promotes the light extraction of TM mode photons, and increases the top and side PL emission intensity by 1.57 and 22 times, respectively. The nanoporous grating layer also relaxes the stress produced by compressive strain, reduces the quantum-confined Stark effect (QCSE), and inhibits the non-radiative recombination. The output power of the UVC-LED with nanoporous grating layer is increased by 36.5%. The EQE droop at 100 mA was 61% for the reference sample and 36% for the nanoporous sample.

Epitaxial structure of the scribed via holes and nanoporous grating layer.

Prof. Feng Yun of Xi'an Jiaotong University considers it a simple and reliable approach to greatly improve the external quantum efficiency, which can be widely applied in commercial UVC-LED chips. Eventually, UVC-LEDs with much improved efficiency can replace the traditional deep ultraviolet light source, and provide final solutions for the public health and safety.