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.
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.
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."