照明、通信“一肩挑”:无荧光粉的单芯片白光LED

可见光通信是一种通过可见光来进行无线高速数据传输的方式。这种数据是通过调制光源发出的光的强度来传输的。首先由光电二极管器件接收信号,然后将数据转换成终端用户可读、可随时使用的形式。普遍认为,可见光通信系统及其扩展到完全的网络化、双向多用户无线系统(称为LiFi)将在5G及以上的连接中发挥关键作用,尤其是在室内环境中。

相比传统的通信技术,可见光通信展现出了许多突出优势:1、在无线频谱资源日趋紧张的形势下,可见光波段尚属空白频谱,无需授权即可使用。同时也不存在电磁干扰问题,可在无线电屏蔽的场所使用;2、可见光通信技术是通过改变光的强弱来传递信息,光在高速的变化下不会使人眼感到不适,且无电磁波辐射伤害;3、LED具有高速调制特性,因此十分契合可见光通信技术,在原有的照明、显示功能上,还能附加通信的作用。LED的普及也给可见光通信的应用带来了便利。

虽然已有很多基于LED的可见光通信的相关报道,但其中多数均采用蓝光光源,这限制了实际应用。由于LED照明的广泛普及,白光光源才是理想的可见光通信光源。因此,研发同时具有高调制带宽和照明性能的白光光源至关重要。

目前,面向通用照明的白光LED广泛采用蓝色LED激发黄色荧光粉的方式。但是,这种荧光粉转换型的白光LED的带宽只有几个MHz ,主要受以下因素限制:荧光粉的荧光寿命长且存在缓慢的斯托克斯转移过程;大面积芯片尺寸的白光LED存在电阻电容时间延迟;来自固有压电极化的InGaN / GaN量子阱的量子限制斯塔克效应。此外,荧光粉还具有发射光谱宽、缺少红色光谱成分、粒径大(约10 μm)、颜色转换和荧光粉效率droop等缺点。

为了解决上述问题,研究人员采用了多种手段:1、采用新型的颜色转换材料,例如共轭聚合物,量子点,碳点等。但是它们的稳定性很差,并且颜色转换材料的荧光寿命仍然比LED芯片的载流子复合寿命更长,这导致颜色转换材料始终限制白光LED的带宽。2、采用Micro-LED以减少RC时间延迟并释放极化场。但是,芯片太小不适用于照明应用。同时,用于可见光通信的Micro-LED的都是在相当高的电流密度下(超过kA/cm2)运行的,严重效率droop和散热问题使其不适合实际应用。3、采用外延生长手段。外延生长在半极性、非极性平面上是可行的,但生长过程复杂,需要进一步提高材料质量。4、其他结构。诸如谐振腔LED和光子晶体LED等结构也有报道,其设计和制造也很复杂。红色,绿色和蓝色LED芯片可共同获得更高的调制带宽,同时调制的难度也会变得更加复杂。

近日,中南大学汪炼成教授课题组和中国科学院半导体所伊晓燕研究员课题组在Photonics Research 2020年第7期上(Rongqiao Wan, Xiang Gao, Liancheng Wang, et al. Phosphor-free single chip GaN-based white light emitting diodes with a moderate color rendering index and significantly enhanced communications bandwidth[J]. Photonics Research, 2020, 8(7): 07001110)展示了在低电流密度下,同时具有较高调制带宽和适合显色指数的无荧光粉单芯片白光LED。本实验材料为扬州中科半导体照明公司李盼盼提供,特别感谢。

实验中采用自组装InGaN 量子点结构的无荧光粉宽光谱单芯片白光LED,其具有可调的相关色温(从1600 K至6000 K),最大显色指数为75且在72 A/cm2的低电流密度下带宽可达150 MHz。与传统的InGaN / GaN量子阱结构相比,自组装量子点可以显著减小内建的压电极化场,从而减轻了量子限制斯塔克效应。同时,这种无荧光粉结构可以彻底摆脱荧光粉对白光LED带宽的限制作用。 此外,在InGaN / GaN量子点中,受到了准三维约束的载流子抑制了由局域化效应引起的非辐射复合。因此,基于InGaN 量子点的单芯片白光LED在满足照明和快速可见光通信应用方面显示出潜力。

这项工作展示了单芯片白光LED在低电流密度下具有的高调制带宽和适合的照明性能及其在可见光通信中的应用。未来的工作重点是进一步优化InGaN量子点的生长条件来改善发射光谱的组成。此外,还需进一步提升发光效率,以提高信噪比,降低误码率。

(a) InGaN量子点基单芯片白光LED中载流子的复合机制示意图; (b) 单芯片白光LED的带宽随电流密度的变化; (c) 单芯片白光LED在72 A/cm2 (90 mA)时的EL光谱,以及对应的实物图.

Lighting and communication based on phosphor-free single chip white light emitting diodes

Visible light communication (VLC) is a wireless method that enables high-speed transmission of data with visible light. This data is transmitted by modulating the intensity of light given off by a light source. The signal is received by a photodiode device that transforms the data into forms that are readable and readily-consumed by end users. It is widely expected that VLC systems and their extension to fully networked, bi-directional multiuser wireless systems referred to as LiFi will play a key part in 5G-and-beyond connectivity, especially for indoor environments.

Compared with traditional communication technology, VLC shows many outstanding advantages: 1. In the form of increasingly tight wireless spectrum resources, the visible light band is still a blank spectrum and can be used without authorization. At the same time, there is no electromagnetic interference problem, and it can be used in radio-shielded places; 2. VLC technology transmits information by changing the intensity of light. Light will not cause discomfort to human eyes under high-speed changes, and there is no harm of electromagnetic wave radiation; 3. Light emitting diode (LED) has high-speed modulation characteristics, so it is very suitable for VLC technology, in the original lighting and display functions, it can also add communication function. The popularity of LED will also bring convenience to the application of VLC. Although there have been many related reports on LED-based VLC, most of them use blue light sources, which will be very limited for practical applications. Due to the widespread popularity of LED lighting, the white light source is the ideal light source for VLC. Therefore, it is important to develop a white light source with high modulation bandwidth and lighting performance.

At present, the widely adopted approach for lighting-oriented white LEDs (WLEDs) is to utilize the blue LED to excite the yellow phosphors (YAG:Ce). However, the overall bandwidth of above mentioned phosphor converted WLED is only few MHz, majorly limited by the following factors: long lifetime of YAG: Ce phosphor and slow stokes transfer process; resistance-capacitance (RC) time delay of the broad area WLEDs; quantum-confined Stark effect (QCSE) of the InGaN/GaN quantum wells (QWs) from intrinsic piezoelectric polarization filed. Besides, YAG:Ce phosphor exhibits some disadvantages of wide emission spectrum, absent of red spectrum component, large particles size (~10 μm), color conversion and phosphors efficiency droop.

Some approaches have proposed to overcome the above-mentioned limits. Novel color conversion materials are developed, such as conjure polymer, quantum dots (QDs), carbon dots. Although some novel color conversion material are proposed to replace phosphors, their stability is poor and the fluorescence lifetime of color conversion material is still longer than the carrier recombination lifetime of LED chips, resulting in the fact that color conversion material always limit the bandwidth of WLEDs. Micro-LED is usually used to reduce the RC time delay and release the polarization field. However, too small chip is not suitable for lighting applications. Especially what should be noted is that the reports on VLC oriented Micro-LEDs are operated at substantially high current density, normally over kA/cm2, which is obviously not suitable for real application due to severe efficiency droop of Micro-LEDs and heat dissipation issue. Epitaxial grow on semi-polar, non-polar plane is feasible yet the growth process is complicate and needs to improve the material quality further. Structures such as resonant cavity LEDs (RC LEDs), and photonic crystal LEDs (PhC LEDs) have been studied and the design and fabrication is also complicate. Red, green and blue tri-color LED chips together can achieve higher modulation bandwidth, yet the modulation is complex for communication.

The research group led by Prof. Liancheng Wang from Central South University and Prof. Xiaoyan Yi from Institute of Semiconductors, Chinese Academy of Sciences demonstrated a phosphor-free single chip WLED with high modulation bandwidth and moderate color rendering index (CRI) at low current density. The research results are published in Photonics Research, Vol. 8, Issue 7, 2020 (Rongqiao Wan, Xiang Gao, Liancheng Wang, et al. Phosphor-free single chip GaN-based white light emitting diodes with a moderate color rendering index and significantly enhanced communications bandwidth[J]. Photonics Research, 2020, 8(7): 07001110). The material was provided by Panpan Li, from Yangzhou Zhongke Semiconductor Lighting Company.

The phosphor-free broadband spectrum single chip WLED by employing self-assembled InGaN QDs structure, which exhibits tunable correlated color temperature (CCT, from 1600 K to 6000 K), a maximum CRI of 75 and large -3 dB modulation bandwidth of 150 MHz at low current density of 72 A/cm2. Compared to traditional InGaN/GaN QWs structures, the self-assembled QDs can significantly reduce the built-in piezoelectric polarization field, leading to an alleviation of QCSE. This phosphor-free structure can completely get rid of the restriction of phosphor on the bandwidth of WLED Furthermore, the quasi-three-dimensional confinement of carriers in the InGaN/GaN QDs inhibits non-radiative recombination due to localization effect. Consequently, the InGaN QDs based single chip WLEDs shows potential in meeting both lighting and fast VLC applications.

This work presented the high modulation bandwidth and moderate lighting performance of the single chip WLEDs at low current density. Moreover, the application of these single chip WLEDs in VLC has also been demonstrated. Future work will focus on further optimizing the growth conditions of InGaN QDs to optimize the composition of the emission spectrum. In addition, the luminous efficiency also needs to be improved, in order to improve the signal-to-noise ratio and reduce the bit error rate.

(a) Schematic illustration the carrier recombination mechanism of InGaN QDs based single chip WLED; (b) The bandwidth of single chip WLED versus injection current density; (c) EL spectra of the single chip WLED at 72 A/cm2 (90 mA), the corresponding EL image in the inset.