3C – also know as ‘cubic’ – silicon carbide (SiC) showed early promise for power semiconductors, but fabrication difficulties meant it was dropped in favour of hexagonal (4H) crystal SiC, which has a higher bandgap and is more temperature dependent.
By pre-trenching wafers to form stress-relieving gaps, Coventry-based Anvil Semiconductors has found a way to make 3C SiC – and is experimenting with Schottky diodes and mosfets made from it.
SiC can be used as a substrate for growing LED-grade GaN (as Cree does), and right from the start Anvil has been proposing making 3C GaN on 3C SiC for LEDs.
Using Anvil techniques, LED guru Professor Colin Humphreys of the University of Cambridge has grown 3C GaN on 3C-SiC-on-Si wafers by MOCVD (scroll down for 3C GaN LEDs).
“The properties of cubic GaN have been explored before, but the challenges of growing this thermodynamically unstable crystal structure have limited its development,” said Humphreys. “The quality of Anvil’s cubic SiC-on-Si substrates and our experience of developing conventional GaN LED structures on large wafers have enabled a breakthrough in material quality.”
Humphreys’ lab was also the source of the GaN-on-Si technology which (via spin-out CamGaN) underpins Plessey’s lighting-grade LED fab in Plymouth. At the moment it grows conventional blue LEDs, using hexagonal GaN-on-Si, on wafers up to 150mm (6inch).
Now all three organisations have teamed up with funded by Innovate UK (from the £14m Energy Catalyst Programme) to grow 3C GaN-on-SiC-on-Si LEDs.
“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,” said Plessey. “This is particularly true for green LEDs where the internal electric fields are stronger and are believed to cause a rapid reduction in efficiency at green wavelengths.”
Anvil makes 3C-SiC on 100mm silicon wafers. “The process is readily migrated onto 150mm wafers, and potentially beyond, without modification and is therefore suitable for large, industrial-scale applications,” said Plessey. “The availability of cubic GaN from a readily commercialisable process on large diameter silicon wafers is as a key enabler for increasing the efficiency of green LEDs.”
“This will, for the first time, deliver green LED devices with efficiency approaching that in blue and red LEDs,” said Humphreys.