Technical Insight
Tiny LEDs produce intense, directional emission
Multiple markets beckon for substrate-emitting, miniature LEDs
Tiny LEDs built into the substrate can feature parabolic mirrors that increase light extraction and enable directed emission from the device.
A research team led by engineers at Tyndall National Institute, Ireland, has fabricated tiny, blue-emitting LEDs featuring parabolic mirrors.
The collaboration, which involves InfiniLED of Cork, Ireland, and the University of Cambridge, claims that its novel LEDs - which emit an intense, relatively narrow beam of light - are suitable for a wide range of applications. They include sub-millimetre-scale projectors for spatially controlled stimulation of cells, optical fibre systems for the delivery of light to remote locations, and forms of LED-based data communication using plastic optical fibre.
Lead author of the paper detailing this class of LED, Brian Corbett from Tyndall National Institute, believes that this tiny, parabolically shaped device is unique. "It solves many problems with getting light out of LEDs, but with the added advantages of pixellation, low divergence, low crosstalk and high bandwidth."
He and his co-workers from Tyndall reported a similar device based on GaAs in 2008, when they were trying to increase light extraction from an LED.
"We were inspired by classical optics and parabolic reflectors," explains Corbett, who admits that approximations have to be made when it comes to real devices. "We have a truncated parabola and the light source is planar."
Fabrication of the team's devices begins with MOCVD growth of LED structures on free-standing, n-doped GaN substrates. These consist of a 2 µm-thick, silicon-doped layer; five compressively strained 1.5 nm-thick InGaN quantum wells separated by 9.6 nm-thick GaN barriers; and a 99 nm-thick magnesium-doped layer that included a heavily doped contact layer.
Devices were fabricated using a 8 µm-diameter, partially reflective palladium discs that also provided p-type contacts. Etching with an inductively coupled plasma to a depth of 6.3 µm formed individual mesas with 20 µm diameters.
Controlled resist erosion during the etching process produced the desired mesa shape resulted from, and the addition of a SiO2 film enabled passivation of the sidewalls. A bonding metal provided a contact to either individual LEDs or clusters of 14, and after the substrate was thinned to 100 µm, an 80 µm thick layer of SiO2 was added to form an anti-reflection coating.
The substrate for these devices, GaN, is much more expensive than sapphire, the common platform for making LEDs. However, Corbett believes that this additional cost is acceptable, because savings can be made elsewhere: "Packaging costs are generally the greatest. Our LED is not only optically efficient, but also thermally efficient, thus reducing the need to deal with those aspects, so you will save considerably when you consider the full application." And Corbett expects GaN substrate costs to fall, due to the on going development of this material.
A single LED emitting at 470 nm produced four times the power of an unshaped contact when the light was collected with a lens with a numerical aperture of 0.5 (collection angles of ±30¡). Driven at 10 mA, which corresponds to a current density of 20 kA cm-2, the LED produces 0.25 mW. It can be driven at higher current densities, but droops kicks in, reducing light emission efficiency. According to the team, it may be possible to combat this by adding a current-blocking layer to the device.
The team have also studied the characteristics of a cluster of 14 of their LEDs. They produce a leakage current of just 5 nA at 2.1 V, suggesting that there are no weaknesses related to surface recombination or shunt leakage.
Data transmission capabilities of their microLEDs have also been investigated. Measurements revealed that an LED with its output collected by a lens and coupled into a plastic optical fibre had a small signal bandwidth of 500 MHz, while studies of a cluster of 14 devices sending data through several centimetres of free space showed that it was possible to transmit signals at error-free data rates up to 500 Mbit/s.
Corbett explained that the team is now using these LEDs to excite neural cells within the EU-funded Optoneuro project. "With our partners there, we are making arrays of CMOS driven addressable elements. We plan to extend the size of the array." He expects that many new applications will emerge for this class of LED, due to the precision associated with this light source.
P. Maasant et al. Appl. Phys. Express 6 022102 (2013)
