BY JYH-CHIA CHEN, MILTON YEH AND HUSSEIN S. EL-GHOROURY FROM OSTENDO TECHNOLOGY INC.
There are fundamental flaws with both of the leading approaches for producing LED-based white light sources and full-colour displays. If white light is formed from the colour mixing of separate red, green and blue LEDs that are packaged together as one device, the resulting unit must combine red LEDs made from AlGaInP with GaN-based green and blue emitters – and pairing these different types of LED requires special efforts, due to substantial differences in electrical characteristics and material properties. With the main alternative, there are also major downsides. In this case, a blue LED is coated with a yellow phosphor, and white light results from colour mixing. Efficiency is then compromised, due to cascade losses that occur during the blue-to-yellow colour conversion of the photons.
In addition to these drawbacks, both of these approaches are held back by basic issues related to the source, including a high packaging cost, low efficiency, a large device size, low yield and poor reliability. What’s needed, but not commercially available, is a single device that emits all the three primary colours of light.
Progress in this direction has been made by several teams, including Korean electronics giant Samsung. It has produced GaN-based, pyramid-shaped LEDs that emit multiple wavelengths simultaneously to create a white-light source. However, these devices cannot emit all three primary colours independently, so while they are suitable for lighting applications, they can’t be used for displays.
An alternative approach that is pursued by Zetian Mi’s group at McGill University is to produce monolithic red-green-blue LEDs from nanowire structures. Using a three-step selective-area growth technique, these researchers form three sets of nanowire structures on three different regions. Several wafer preparations are used before and after each selective growth. With this approach, each display pixel is capable of producing the three primary colours, which are emitted from three areas, or sub-pixels. This architecture compromises the pixel dimension of the display, and ultimately its resolution.
Unlike conventional III-nitride LEDs, we use a different quantum well section for each primary colour. Between these sections are specially-designed intermediate carrier blocking layers, made from AlGaN layers with different thicknesses and dopant concentrations (see Figure 1 for a cross-sectional view of our monolithic, three-colour InGaN-based LED).
We have found that the intermediate carrier blocking layers play a key role in controlling the transport and distribution of the carriers across the active region. Careful selection of the thickness, composition and dopant concentration of each of the intermediate carrier blocking layers enables control of the transport of electrons and holes under various current injection conditions. Thanks to this, electrons and holes can be directed into quantum wells that lead to emission at specific dominate wavelengths.
Figure 1. Ostendo has broken new ground with its monolithic, three-colour, multi-layer InGaN-based LEDs.
Our monolithic LEDs are grown by MOCVD, using a close-coupled shower-head reactor, on c-plane (0001) sapphire substrates. They comprise: a 2 mm-thick, undoped GaN layer; a 3 mm-thick, highly silicon-doped n-type GaN layer; an InGaN compliance layer, inserted for quality improvement purposes; blue InGaN/GaN multi-quantum wells emitting at 460 nm, which contain three 3 nm-thick In0.15Ga0.85N wells separated by 10 nm-thick InGaN barriers; a green-emitting section producing 530 nm emission from three 3 nm-thick In0.25Ga0.75N quantum wells separated by 10 nm-thick InGaN barriers; a region producing red, 650 nm emission from a single 3 nm-thick In0.35Ga0.65N well sandwiched between 10 nm-thick GaN barriers; a 20 nm-thick, magnesium-doped p-type Al0.15Ga0.85N electron blocking layer; and a 200 nm-thick p-GaN contact layer.
To control carrier distribution and improve the material quality in the multi-quantum well active region, devices incorporate a variety of AlGaN-based alloy layers. Growth temperatures are optimised for each emitting section, with blue, green and red wells formed at about 864 °C, 829 °C and 809 °C, respectively.
Using a photoluminescence mapping tool – a Nanometric RPM 2000 equipped with a 405 nm laser producing up to 150 mW – we have studied our epiwafer’s emission characteristics. Results reveal that all three primary colours appear at the same time (see Figure 2).
In addition, we have investigated electroluminescence, collecting data from multiple positions on each epiwafer. This has been undertaken with an in-house ‘Quick Tester’ attached to an integrating sphere. Using indium-ball contacts, typically 0.8 mm in diameter, we have obtained normalized electroluminescence spectra for devices under different injection currents (see Figure 3). In contrast to the photoluminescence results, only one predominant emission peak appears at one particular current. As the injection current is cranked up from 15 mA to 200 mA and then on to 400 mA, emission shifts from red (~650 nm) to green (~530 nm) and finally blue (~460 nm).
Figure 2. Photo-luminescence from a tri-color LED wafer that features active regions for red, green and blue emission.
The changes in colour are actually more subtle than this. Emission appears red at around 5 mA, and with increasing current changes to amber, then yellow, green, cyan and finally blue (see Figure (a)-(f)). The red emission has the lowest efficiency, due to the highest indium content in the quantum wells. Note that this finding is consistent with that of other researchers.
We have plotted the light emission gamut of our LEDs on CIE chromaticity diagrams (see Figure 5). Plots shows that the colour emission trajectory of our LEDs with increasing injection current fully covers the standard red-green-blue colour gamut.
As well as providing the three primary colours, our LEDs can produce many other colours through colour mixing. This includes different tones of white, such as cool, neutral and warm white light. These are obtained by using different combinations of the intensity and width of driving current pulses, such as pulsed intensity and width modulation. With this technique, it is also possible to produce white LEDs with a high colour-rendering index.
Our technology is of great interest to LED and display manufacturers, because it can create micro-LEDs that produce: a far higher luminous efficiency than organic LEDs; and thanks to their self-illumination, eliminate backlight demands associated with LCDs. Displays made from micro-LEDs are also more robust than those incorporating organic LED and LCD technologies, making them better-suited to harsh outdoor applications. And micro-LEDs can be housed on flexible substrates, making them excellent candidates for wearable devices.
One group that has made significant strides with displays employing micro-LED arrays is that of Kei May Lau and co-workers from Hong Kong University of Science and Technology. This team recently demonstrated a micro-display with a resolution of 1700 pixels-per-inch that features micro-LED arrays integrated on silicon CMOS substrates. This micro-display has the potential to serve many applications, such as active driving of pico-projectors, augmented reality, head-up displays in cars, and headsets for gaming. However, the work by this group has been limited to blue light of differing intensity. Turning to our LEDs, and using them in combination with the bonding-on-silicon technology, would enable full-colour displays, without adding any complexity to device fabrication.
Bigger, flexible displays
As well as being a key building block in micro-displays, the micro-LED has emerged as an important component for future flexible lighting and displays. For examples, John Rogers’ group at the University of Illinois, Urbana-Champaign, has shown that this class of LED can be attached to plastic substrates using micro-transfer printing. This technique has tremendous potential for the production of large-scale, thin and flexible lighting.
One of the merits of printing is that it can slash manufacturing costs by eliminating individual device packaging. What’s more, the small, distributed micro-LEDs can be passively cooled, due to their small size, enabling operation at higher current densities. And last, but by no means least, quantum efficiency can increase through better cooling and superior light extraction, due to the greater surface area and surface scattering. Other techniques that have been actively pursued for the realization of large-area lighting and displays include pick-and-place, ink-jet printing, self-assembly and screen printing of micro-LEDs. So far, the majority of these demonstrations have involved monochromatic light, or white-emission from blue LEDs and a yellow phosphor. Full-colour displays require a more complicated process to precisely place red, green and blue LEDs within the same display. Manufacturing costs will rise, and resolution could be sacrificed. However, if our approach to making full-colour LEDs is adopted, it will avoid additional manufacturing steps and not compromise resolution.
Figure 3. Normalized electroluminescence spectra of devices under different injection currents.
Figure 4.(a)-(f). Full colour emission under different injection currents.
Figure 5. The light emission gamut (CIE chromaticity diagrams) from full-colour LED structures.
In short, we believe that our novel, InGaN-based LED technology that incorporates intermediate blocking layers will deliver a significant reduction in the fabrication complexity of full-colour micro-displays. In addition, it will allow large-scale flexible displays to be realized that can compete favourably with existing organic LED technology. We are now developing our technology for displays, which will also be a game changer for next-generation lighting, due to its efficiency, compactness, simplicity and robustness.