In our increasingly connected world, there is an explosion of demand for new display technologies which are the window through which we interface to the external world. Existing displays include direct-view displays which typically use liquid crystal (LCD) or Organic LED (OLED) technologies, and projection technologies such as liquid crystal on silicon (LCOS) and DLP. These technologies all suffer from relatively low "wallplug" energy efficiency, of the order of 5% or less, leading to shorter-than desired battery life on mobile devices. In addition the brightness of these technologies is often inadequate for the applications which are emerging – giving poor outdoors readability for smartphones, smartwatches, tablets and laptops, and poor contrast in sunshine for augmented reality and heads-up-display projectors.
LEDs have very high energy efficiency and brightness. They are therefore being looked at as potential alternatives to today's low energy efficient and low-brightness technologies. However, when 2D LEDs are pixelized to create high-resolution displays, they encounter many issues such as loss of efficiency with decreasing pixel size, lack of in-pixel redundancy making difficult their use in monolithic displays, the need to use pick-and-place technologies which are still low-yielding, color variations with temperature due to the use of different materials to make the RGB colors, and generally low manufacturability.
Aledia's technology avoids these issues and is ideally suited to provide mobile displays offering higher energy efficiency than incumbent technologies and vastly higher brightness, both for direct view and projection applications, at a cost compatible with that required for adoption in consumer electronics.
Aledia is the first company to grow high-density, coaxial gallium nitride (GaN) microwires directly onto large-diameter 200mm silicon wafers (extendable to 300mm wafers) using low-cost processes that are fully compatible with the back end of line of today's semiconductor foundries. When processed these nanowires become LEDs.
Each nanowire is a fully independent, isolated and passivated emitter. There are many nanowires in each pixel. Therefore, even if a single nanowire malfunctions there is in-pixel redundancy such that the pixel will still work when individual nanowires fail. This allows making monolithic megapixel displays. By contrast with 2D LEDs when individual microLED chips fail the pixel fails and needs to be replaced. 2D microLEDs therefore tend to require the picking and placing of millions individual LED subpixels to create a display, and makes it difficult to envision high-resolution displays as these would require handling and high-yield interconnection of very small chips. By contrast, nanowire displays can be made of single-chip, monolithic "solid silicon" megapixel emitters.
Since each nanowire is independent of the others, as resolution is increased there are fewer nanowires in each pixel but the efficiency of any given nanowire remains the same. This avoids the drop in efficiency seen with 2D LEDs as the pixel size is reduced, a reduction thought to come from non-radiative recombination which increasingly starts occurring at the edges of the 2D chips when the radiative layers (quantum wells) become exposed through processing. With nanowires by contrast the quantum wells structure is already passivated during epitaxy.
Brightness can be as high or higher than with traditional 2D LEDs, as nanowires offer a high surface area for light emission and as, being near-perfect crystals, they can be driven at higher current density than that tolerated by 2D LED chips. Finally, the low defect density of nanowires and the fact they they are on high-conductivity Silicon leads to better thermal conductivity for the LED structure which in turn facilitates high-brightness operation.
Finally, GaN nanowires can be made to emit in all wavelengths, from blue to red, giving rise to native-RGB chips. All colors are thus made with the same material. This avoids the need experienced in 2D LEDs to use different materials to make the red, green and blue emitters which, as the materials behave differently under temperature changes, leads to color change with temperature. This also avoids the need to use color-converters which absorb a significant fraction of the emitted light.
Aledia's 3D nanowire-on-Silicon LED technology enables the next generation of displays, that can be brighter, can give longer battery life, can have more stable colors, and can be mass manufactured on existing foundries at very competitive cost points.
2D (Planar) LEDs:
3D (Microwire) LEDs: