industry interview
Silicon-based LEDs leap
from lab to fab Silicon offers a large, low-cost platform for making nitride LEDs, but realizing high-quality epitaxy is tough due to the stress between the two materials. However, it is possible to produce the crack-free, low-defect-density films demanded by high-power LEDs by turning to a patterned substrate and a multi-layer buffer, says Lattice Power Corporation.
L
EDs are widely used in displays, automobiles, handsets, notebook backlighting, TV backlighting,
and general lighting. The light-emitting films - InAlGaP for red LEDs, and InAlGaN for their blue and green cousins - need to be grown on a carrier substrate.
There is no good substrate to match the GaN material system, in terms of lattice constant and thermal expansion, but it is possible to grow high-quality films on sapphire, SiC and silicon.
Almost all GaN-based LEDs are fabricated on sapphire substrates, and Cree is the notable exception, using SiC substrates instead.
Massive adoption of solid-state lighting requires further advances in large-scale, low-cost manufacturing. Silicon- based GaN LEDs have been attracting researchers in universities and industry for many years, due to their promise of large-scale production and compatibility with the IC manufacturing platform. In comparison, sapphire and SiC substrates are much smaller, and they can’t be processed through silicon lines.
The biggest roadblock for manufacturing high- performance GaN-on-silicon LEDs is the material stress that results from a combination of lattice mismatch and thermal expansion mismatch.
But it is possible to use special epistructures, novels substrate designs and sophisticated growth techniques to make GaN-on-silicon structures that lead to high- performance, high-reliability LEDs. At Lattice Power, which is based in Nanchang, China, we have done exactly that, and demonstrated the promise of this approach for
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www.compoundsemiconductor.net June 2010
Figure 1: Cracking in GaN film grown on silicon substrate
making high-performance, lighting-class LEDs.
The substrate used for GaN growth is (111) silicon. Its lattice-constant mismatch to GaN at room temperature leads to a tensile strain of +17%, but it is +40% tensile- strained following pseudo-morphological growth at around 1000 degrees Celcius, due to the thermal expansion coefficient mismatch. This results in more than a 2 µm lattice constant mismatch for a 1mm die. The upshot is that cracking occurs, sometime massively.
However, more often it causes the wafer to bow during growth, yielding wafer non-uniformity and poor device performance. In comparison, when GaN films are grown on SiC substrates, the strain caused by lattice constant offsets that caused by thermal mismatch.
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