Industry-research cooperation to bridging the “green gap”

EFFICIENCY issues in LEDs, particularly green LEDs, are squarely in the cross hair, as Plessey, Anvil Semiconductors and the University of Cambridge work to fabricate high efficiency LEDs in cubic GaN.


Cubic GaN has the potential to overcome the problems caused in conventional LEDs by the strong internal electric fields which impair carrier recombination and contribute to efficiency droop.


This issue is particularly serious in green LEDs where the internal electric fields are stronger and are believed to cause a rapid reduction in efficiency at green wavelengths known as "the green gap". This in turn increases the cost of LED lighting and cuts down the efficiency of LED applications that require the use of green LEDs, such as projectors.


The increase the efficiency will depend on the availability of cubic GaN from a readily commercialisable process on large diameter silicon wafers.


The collaboration is partly funded by Innovate UK under the Energy Catalyst Programme. It follows on from work by Anvil Semiconductors and the Cambridge Centre for GaN at the University of Cambridge where they successfully grew cubic GaN on 3C-SiC on silicon wafers by MOCVD.


The underlying 3C-SiC layers were produced by Anvil using its patented stress relief IP that enables growth of device quality silicon carbide on 100mm diameter silicon wafers.  This process can be applied to 150mm diameter wafers and potentially beyond without modification, allowing large, industrial-scale applications.


Plessey has started to commercialise LEDs produced in conventional (Hexagonal) GaN grown 150mm silicon wafers using IP originally developed at The University of Cambridge.  Anvil's high quality 3C-SiC on Silicon technology, which is being developed for SiC power devices, provides an effective substrate, to allow single phase cubic GaN epitaxy growth and provides a process which is compatible with Plessey's GaN on Si device technology.


While researchers have certainly explored Cubic GaN before, the crystal structure is thermodynamically unstable, thus limiting its development. The high quality of Anvil's cubic SiC on Si substrates and the researchers' experience of developing conventional GaN LED structures on large area wafers have, however, enabled a breakthrough in material quality.