Realising Red GaN-based MicroLEDs With Europium-doping
The novel red luminescence provided by doping GaN with europium ions enables the monolithic integration of nitride-based full-colour LEDs for ultrahigh-resolution microLED displays
BY SHUHEI ICHIKAWA, DOLF TIMMERMAN, JUN TATEBAYASHI AND YASUFUMI FUJIWARA FROM OSAKA UNIVERSITY
WE ARE ON THE CUSP of a new era, known as ‘Smart society', that will see cyber space and the physical space interactively fused. Critical to this is the introduction of a small-size, high-resolution display, which is an essential ingredient in virtual-reality, augmented-reality and mixed-reality glasses.
One candidate for producing these tiny high-resolution displays is the microLED. Screens formed from a vast array of these miniature LEDs have many virtues, including a high efficiency, high contrast and high stability. Fabrication of this class of display draws together devices emitting each of the three primary colours. A very common combination is the use of InGaN/GaN-based LEDs for the blue and green and AlInGaP/GaAs-based LEDs for the red. These devices are brought together to form pixels using a pick-and-place technique. However, there is a problem: when the dimensions of the LEDs are reduced, the efficiency of the red variant plummets, due to an increase in surface recombination associated with the hike in the surface-to-volume ratio.
Figure 1. Europium-doped GaN LED arrays exhibit bright red luminescence.
Several solutions are being explored for reducing the dimensions of the red LED while maintaining its efficiency. One option, receiving much attention, is to use GaN-based LEDs emitting in the blue or ultraviolet to pump red-emitting quantum dots or phosphors. However, while this approach also offers monolithic integration, it suffers from insufficient stability and an inadequate colour-conversion efficiency. What's more, with this particular technology, there is a need to increase the pixel contrast ratio and enhance the colour purity by suppressing crosstalk between various light emitters. Unfortunately, when black light-blocking partitions are applied between all the subpixels to accomplish this, the effective area of the pixels diminishes, impairing the number of pixels-per-inch (PPI).
Switching to InGaN nanocolumns avoids this issue. It's an architecture that effectively relaxes material strain, even for the high-indium contents that are necessary for red light emission. But it is challenging to produce nanocolumn structures with a high colour purity - they are held back by large variations in emission wavelength, due to the ensemble nature of these structures and the sensitivity of individual nanocolumns to size fluctuations. Another drawback is the need for electron-beam lithography, prior to epitaxial growth, to precisely control the nanocolumn size and ultimately ensure the desired emission wavelength. Note that for this type of microLED, and also for that based on quantum dots, fabrication requires a lateral integration method.
There is much merit in taking a very different approach, based on the vertical-stacking integration of tri-colour LEDs. Realising this with a single epitaxial sequence eradicates several steps in the microLED production process, such as the mask-patterning prior to growth and the addition of black partitions. What's more, with this technique the areal density of the microLEDs is only limited by lithography and flip-chip accuracy, so it is possible to integrate these emitters with a PPI beyond 5,000.
Figure 2. Eu3+ ions doped in GaN under current injection deliver a very sharp emission peak, crucial for a high colour purity.
A pioneer of this approach is the group led by the Nobel-prize winning physicist Hirsohi Amano. Recently his team demonstrated the monolithic integration of tri-colour microLEDs using InGaN quantum wells. These devices, with a 100 µm by 100 µm active area, are composed of independent subpixels, which emit in the blue, green and yellow-orange range and have a colour saturation of over 90 percent. However, there are drawbacks: one concern is that the longer wavelength emission is much broader; and another is that its peak position shifts to shorter wavelengths as the injected current increases, due to a reduction in the quantum-confined Stark effect.
Our team offers a way to overcome this issue, drawing on our previous success from more than a decade ago. Back in 2009 we invented a novel red LED, formed by adopting europium-doped GaN. In the intervening years we have continued to refine this device, enabling its output power to steadily increase to the milliwatt level. In the remainder of this feature we will detail this triumph, and how we have used vertically-stacked integration to combine it with conventional blue/green LEDs to create a key technology for the next-generation of microLED displays.
Our approach fulfils the requirement for the vertical-stacking of tri-colour LEDs, which is the monolithic integration of all three colours. We needed to develop and alternative to the more obvious approach of just using InGaN quantum wells, because although blue and green LEDs based on this have been successfully commercialized, a red cousin is lacking. The route to producing red emission with this type of LED is to either increase the indium-content or the thickness of the InGaN quantum well, or to use a combination of the two. But this strategy is precarious: it leads to an increase in piezo-electric polarization fields, and the low miscibility of indium in GaN hampers efforts to realise a sufficient indium content in the well while maintaining sufficient crystal quality. We are able to sidestep these issues by manipulating the emission through the doping of GaN with an intrinsic red emitter, the rare-earth element europium Eu3+.
The Eu3+ ion is no stranger to display applications, having been widely deployed as a luminescent centre in red-emitting phosphors in cathode ray tubes and plasma display panels. In general, rare-earth ions, including Eu3+, are characterized by partially filled 4f shells, which are localized inside completely filled 5s and 5p shells. This localization shields the electrons in the 4f shell from the surrounding environment, allowing rare-earth ions to maintain their atom-like properties and exhibit sharp luminescence bands associated with radiative transitions within the 4f manifold. These transitions are virtually insensitive to changes in current injection and temperature. When Eu3+ ions are incorporated into GaN, they take the place of the Ga3+ metallic cations and occupy sites with reduced symmetry.
Adding Eu3+ ions alters the light-emitting mechanism. Conventional band-to-band transitions are replaced by the transfer of energy from injected carriers in the GaN host material to Eu3+ ions, which are promoted to an excited state that leads to the emission of red light. This energy transfer is strongly dependent on the material properties, and can be enhanced by changing the material fabrication processes, or by intentionally introducing other dopants, which can act as energy hubs.
Eu3+ ions exhibit a sharp luminescence around 620 nm, due to intra-4f shell transitions from the 5D0 to 7F2 states (see Figure 2). This attribute is most welcome, helping to create red-emitting devices with a high colour purity and robust emission wavelength stability. Over the last decade and more, we have developed expertise to epitaxially grow high-quality GaN materials that are doped with Eu3+ ions and are optimised to deliver excellent performance in LED structures. The output power of these devices has exponentially increased over time, and has recently exceeded 1 mW at 20 mA, a respectable figure for mass production (see Figure 3).