N-polar InGaN/GaN nanowires: overcoming the efficiency cliff of red-emitting micro-LEDs
Figure 1: Schematic illustration of N-polar InGaN/GaN nanowire LED heterostructures grown GaN on sapphire substrate (left) ; SEM images of N-polar InGaN/GaN nanowire arrays, showing site-controlled epitaxy and high uniformity in low (bottom right) and high (top right) magnification.
High efficiency, high brightness, and robust micro or sub-microscale light emitting diodes (LEDs) are essential components of emerging virtual/augmented reality devices and systems as well as future ultrahigh resolution mobile displays. Realization of such ultra-small LEDs can also allow large scale integration of electronic and optoelectronic devices on the same chip.
However, scaling down the device size while maintaining high efficiency has proved extremely challenging, especially for deep visible emissions. The resulting efficiency cliff, i.e., a drastic reduction of the device efficiency with reducing dimensions, is mainly due to damage caused by the fabrication process and material limitations for conventional quantum well LEDs. Such a critical challenge is now addressed by a team of researchers led by Prof. Zetian Mi at the University of Michigan, Ann Arbor.
The main material systems in use for deep visible LEDs are phosphides and nitrides. The phosphide material system can give efficiency values of 50-70% for larger (>100 µm) device sizes, but efficiency drops drastically for smaller sizes. The resulting efficiency cliff is primarily limited by material properties like large carrier diffusion lengths, poor charge carrier confinement and large surface recombination velocities.
The nitride system has better material parameters but one unresolved issue is the very large lattice mismatch between InN and GaN. To achieve red emission, very high In composition is required in InGaN. This can cause defects and dislocations which contribute to undesirable non-radiative recombination of charge carriers.
Another challenge comes from conventional fabrication methods wherein a planar structure is etched to make devices of smaller size. This damages the surface and sidewalls of the device. With decreasing size, the proportion of charge carriers lost to the surface defects becomes significant and leads to severe efficiency cliff.
Recently, Prof. Mi's group at the University of Michigan has found that a possible solution to obtaining high efficiency red LEDs is to epitaxially grow InGaN based nanostructures. Since this is a bottom-up approach, the surface damage can be minimized. Moreover, nanowires have a larger surface-volume ratio which can drastically lower the number of defects due to strain relaxation. The research results are published in Photonics Research, Volume 10, No. 4, 2022 (A. Pandey, Y. Malhotra, P. Wang, K. Sun, X. Liu, Z. Mi. N-polar InGaN/GaN nanowires: overcoming the efficiency cliff of red-emitting micro-LEDs[J]. Photonics Research, 2022, 10(4): 04001107).
In this work, they have realized submicron scale LEDs emitting in the red spectral range using nanostructures, and further reported an efficiency value significantly higher than any other comparable device in this size and wavelength range.
To develop such small scale red LEDs, a GaN on sapphire template is utilized. A mask was then deposited onto this substrate and a pattern of nanoholes was transferred onto it. Molecular beam epitaxy was utilized to grow very uniform arrays of high-quality nanowire crystals. In ideal conditions, the wires selectively grow through the patterned nanoholes.
The device structure is a standard p-i-n diode wherein the intrinsic region is made of InGaN and is responsible for the emission. Each nanowire is essentially a single LED. By optimizing the growth conditions, the emission wavelength of these devices can be controllably tuned. The accompanying figure shows a schematic of the device and a scanning electron microscopy (SEM) image of the wires after crystal growth. The nanowire diameter is in the range of just 200-300 nm.
Following the crystal growth, the device fabrication was done. An insulating layer was deposited to provide electrical isolation and surface passivation. Device windows of lateral sizes ranging from 0.75-1 µm were opened on the insulator. The smallest devices contain just a few nanowires.
After opening the device windows, n and p metal contacts were deposited and the device was ready for electrical probing. The devices showed excellent rectifying behavior with negligible reverse bias leakage. The 750 nm devices, some of the smallest red emitting LEDs ever reported, showed a peak external quantum efficiency of 1.2% with emission at 620 nm.
This is the first demonstration of a sub-micron scale LED emitting red light which can overcome the efficiency cliff of conventional top-down etched quantum well micro-LEDs. This has been made possible by the bottom-up approach to grow dislocation-free nanowires.
By optimizing the design parameters of nanowire patterns and controlling growth conditions, wavelength tuning progressively from yellow to orange to red has also been demonstrated. Moreover, nanowire micro and nanoscale LEDs offer significant advantages of monolithic full-color emission without any transfer, highly directional emission, and very narrow spectral linewidths, compared to conventional quantum well LEDs.
"This exciting development is made possible by the unique N-polar InGaN nanowire heterostructures, which offer several critical advantages compared to conventional Ga-polar materials and devices," Mi said. "With this demonstration, now there is a clear path to achieve very high efficiency, monolithically integrated micro, or sub-micron scale full color LEDs for many emerging applications."
利用纳米结构实现高效率红光LED
图1 蓝宝石基GaN衬底上生长的N极性InGaN/GaN纳米线LED异质结构示意图（左）；N极性InGaN/GaN纳米线阵列的SEM图像，在低（右下）和高（右上）放大倍数下都展现了位置可控的外延和良好均匀性。
高效率、高亮度和可靠的微米或亚微米级发光二极管（LED）是新兴虚拟/增强现实设备和系统以及未来超高分辨率移动显示器的关键组件。这种超小型LED的实现还可以使电子和光电子器件大规模集成在同一芯片上。
然而，事实证明，在保持高效率的同时缩小器件尺寸极具挑战性，尤其是对于长波可见光发射。由于传统量子阱LED的制造工艺造成的损坏和材料限制，器件效率随着尺寸的减小急剧降低，即导致效率陡崖。密歇根大学Zetian Mi教授带领的研究团队现在解决了这个关键挑战。
用于长波可见光LED的主要材料是磷化物和氮化物。对于大尺寸(> 100 μm) 器件，磷化物材料可实现的效率达50%~70%，但对于小尺寸器件其效率则急剧下降。该效率陡崖主要受到材料特性的影响，比如较大的载流子扩散长度、较弱的电荷载流子限制和较大的表面复合速率。
氮化物具有更好的材料参数，但目前尚未解决的问题是InN和GaN之间存在非常大的晶格失配。要实现红光发射，InGaN需要非常高的In组分，这会产生缺陷和位错，从而导致电荷载流子的非辐射复合。
另一个挑战来自制造更小尺寸器件的传统方法，即蚀刻平面结构，这会损坏器件的表面和侧壁。随着尺寸的减小，相当一部分电荷载流子流失到表面缺陷，导致严重的效率陡崖。
近日，密歇根大学Zetian Mi教授领衔的研究团队发现生长InGaN基的纳米结构是获得高效率红光LED的一个可能的方案。由于这是一种自下而上的方法，表面损伤可以最小化。此外，纳米线具有更大的比表面积，可以大幅减少因应变弛豫而产生的缺陷数量。相关研究成果发表于Photonics Research2022年第4期（A. Pandey, Y. Malhotra, P. Wang, K. Sun, X. Liu, Z. Mi. N-polar InGaN/GaN nanowires: overcoming the efficiency cliff of red-emitting micro-LEDs[J]. Photonics Research, 2022, 10(4): 04001107）。
该团队利用纳米结构实现了红色光谱范围的亚微米级LED，其报道的效率显著高于任何其他类似尺寸和波长范围的器件。
为了开发这样的小型红光LED，他们采用了蓝宝石基GaN基板，然后在该衬底上沉积掩膜，并将纳米孔图案转移到掩膜，接着用分子束外延生长非常均匀的高质量纳米线晶体阵列。在理想的条件下，纳米线会选择性地通过纳米孔生长。
该器件结构是一个标准的p-i-n二极管，其中本征区由InGaN构成，并发射红光。每根纳米线本质上都是一个LED。通过优化生长条件，可以控制这些器件的发光波长。图1为该器件的示意图和晶体生长后的纳米线扫描电子显微镜（SEM）图像。纳米线的直径范围仅为200~300 nm。
晶体生长之后是器件制备。首先沉积一个绝缘层进行电气隔离和表面钝化，然后在绝缘层上打开横向尺寸为0.75~1 µm的器件窗口，最小的器件只有几根纳米线。打开器件窗口后，沉积n型和p型金属接触，器件即为电学测量就绪。这些器件展现出良好的整流性能，其反向偏压下漏电流极小可忽略。
这些750 nm大小的器件是迄今报道的最小的红光LED，展现了1.2%的外量子效率峰值和620 nm的红光发光波长。这个是首个亚微米级红光LED的示范，通过自下而上的方法生长无位错纳米线克服了传统自上而下蚀刻的量子阱微型LED的效率悬崖。
通过优化纳米线图案的设计参数和控制生长条件，可以实现波长从黄色到橙色再到红色的渐进调节。此外，与传统量子阱LED相比，纳米线微米级和纳米级 LED 具有单片全彩色发光、无需转移、高方向性发射和非常窄的光谱线宽的的显著优点。
“这一令人兴奋的进展是通过独特的N极性InGaN纳米线异质结构实现的，与传统的Ga极性材料和器件相比，它提供了几个关键优势。”Mi教授认为，“通过这个示范，现在就有了一条清晰的道路去实现面向多种新兴应用的非常高效、单片集成的微米或亚微米级全彩LED。”