TECHNOLOGY LEDs
Accelerating adoption of GaN substrates for
LED manufacture
Recycling native GaN substrates via chemical lift-off promises cost-competitive manufacture of high-performance vertical LEDs
BY DAVID ROGERS FROM NANOVATION
COMMERCIALISATION of the GaN LED can be traced back to the development of p-type doping of this wide bandgap semiconductor in the early 1990s. Since then, the performance of this device has improved exponentially, enabling it to progress from use in the backlighting of mobile screens to providing a source for solid-state lighting. However, although LED lighting is now commonplace, its cost-performance profi le has a long way to go untill the incumbent vacuum-tube- based lighting technologies will cease to dominate.
One of today’s key bottlenecks is the requirement to use a ‘non-native’ substrate. Currently about 95 percent of GaN-based LEDs are grown hetero- epitaxially on c-sapphire, because the ideal ‘native’ GaN substrates, which are limited in availability, are prohibitively expensive. A 2-inch substrate, for example, retails for thousands of dollars, and is primarily used for laser manufacturing.
Sapphire is far from the ideal foundation for the GaN LED. Two of its biggest downsides are that its lattice constants and its thermal expansion coeffi cient are markedly different from those of GaN. This creates strain in the epilayers, which leads to the generation of point defects and dislocations that limit the potential light output.
The insulating properties of sapphire are another impediment to the manufacture of high-performance LEDs. The high electrical resistivity of sapphire imposes a non-ideal lateral LED device architecture with top-contacts and a confi ned lateral current fl ow. This results in current crowding and localized thermal hot spots, which are detrimental to the effi ciency, lifetime and maximum brightness of the device.
Compounding these issues is the thermally insulating nature of sapphire, which dramatically restricts heat dissipation. In turn, this further limits effi ciency, lifetime and brightness.
Alternative platforms To combat these issues, producers of
chips for high-end LED applications transfer the GaN-based epilayer to a substrate with superior electrical and thermal conductivity. Often this is accomplished with a laser-lift-off and wafer bonding process to remove the LED from the sapphire and transfer it to an alternative substrate. The laser lift-off process involves fi ring a short wavelength laser beam − typically the 248 nm emission line from a KrF excimer − through the sapphire substrate. Light is absorbed in the fi rst 100 nm of GaN, which decomposes to liberate the LED from the sapphire.
After bonding the LED to an electrically and thermally conductive substrate, this device can exhibit much better heat dissipation and vertical LEDs (VLEDs)
Figure 1. Current crowding is far more severe in a lateral LED (a) than its vertical cousin (b), due to the insulating substrate.
June 2014
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