RESEARCH REVIEW Laser-lift off for ultraviolet LEDs
A superlattice built by pairing InGaN with GaN allows laser-lift off of an ultraviolet LED from its light-absorbing, native substrate
THANKS TO A NOVEL laser-lift off technique, a team from Meijo University in Japan has fabricated an ultraviolet LED that combines high-crystal quality with low levels of substrate absorption. To produce this device, the researchers began by forming a 380 nm LED on a native GaN substrate. By subsequently removing this foundation, which absorbs emission, output increased by 70 percent. GaN is not the conventional substrate for ultraviolet LEDs emitting around 380 nm. Sapphire is far more common, because a well-established technique exists for separating this foundation from the device layers.
This widely used approach begins by directing the emission from a high- energy pulsed ultraviolet laser, such as a 248 nm KrF pulsed excimer laser, through the substrate’s backside. Strong light absorption occurs in the GaN next to the sapphire, with the wide bandgap material decomposing to induce the formation of gallium droplets. It is then easy to remove the device from its substrate. Note that any roughening of the GaN that occurs during this step may be beneficial, because it increases light extraction from the chip.
Sapphire substrates are not ideal for making ultraviolet LEDs, because they are not perfectly lattice-matched to GaN, so there is a high level of defects in the epilayer, impairing the internal quantum efficiency. Switching to a native substrate improves efficiency, but laser lift-off is harder, because the absorption properties of the substrate are essentially the same as the initial GaN device layers.
To overcome this lack of contrast in absorption, the team from Meijo University inserted an InGaN-GaN superlattice underneath the device structure. During growth of subsequent layers at 1040 °C, this composite decomposes to form a layer of indium droplets, which absorb visible and infrared radiation due to surface plasmon resonance. The researchers exploited this resonance, directing the emission from a 532 nm laser at the epiwafer, with
Substrate removal is possible by using a superlattice with eight pairs of Ga0.85
In0.15
N and GaN. This
composite converts to a layer with indium droplets after the growth of additional material at 1040 °C. Light extraction is increased by etching the surface with hot potassium hydroxide.
absorption of light in the indium droplet layer leading to separation of the device from the substrate (see Figure 1). This approach is very promising, because it could allow re-use of GaN substrates in ultraviolet LED fabrication, trimming production costs associated with this technique.
Ultraviolet LEDs were formed by taking a native substrate with a threading dislocation density of less than 106
1 μm-thick layer of n-type GaN; a 2 μm-thick, n-type layer of Al0.03
In0.15 In0.03 In0.05 N and GaN; another Ga0.97 N and
GaN; a multi-quantum well with 6 nm-thick layers of Ga0.95
Ga0.87
500 μm by 600 μm and were mounted on sub-mounts with gold bumps. Gaps between the chip and the sub- mount were filled with an epoxy resin. Cathodoluminescense revealed that inserting the superlattice beneath the device increased threading dislocation density from 3 x 106
cm-2 to 5 x 107
cm-2
. cm-2 ,
and depositing on this a: 1 μm-thick layer of n-type GaN; a superlattice with eight pairs of Ga0.85
N;
a ten-period superlattice with 2 nm-thick alternating layers of Ga0.97
N and
15 nm layers of GaN; a p-type, 20 nm-thick electron-blocking layer made from Al0.13
N; and a 120 nm-thick,
p-type GaN contact layer. The team also produced a control, which differed due to removal of the first superlattice and a 1 μm-thick layer of n-type GaN. Flip-chip LEDs were formed from these epiwafers. Devices had dimensions of
70
www.compoundsemiconductor.net October 2014 Copyright Compound Semiconductor
“We believe that the epilayer can be of lower defect density after optimization of the indium droplet layer,” claims Daisuke Iida from Meijo University.
Measurements of light output revealed that once the LED had been separated from a 200 μm-thick GaN substrate, it had a 70 percent higher output at a 50 mA. “We didn’t optimize the growth condition and structure for our LEDs,” admits Iida, which explains why they produced a inferior performance to devices emitting at similar wavelengths made by other groups. One of the aims for the team is to now see how many times it is possible to re- use the GaN substrate during ultra-violet LED fabrication.
D. Iida et. al. App. Phys. Lett. 105 072101 (2014)
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