RESEARCH REVIEW Azzurro improves electroluminescence
uniformity of large-diameter substrates On-wafer testing of LEDs on 150 mm wafers reveals high uniformity in terms of electroluminescence, photoluminescence and forward voltage
GERMAN EPIWAFER VENDOR Azzurro Semiconductors claims to have raised the bar for the electroluminescence (EL) uniformity of LEDs fabricated on large-diameter substrates. GaN-on- silicon LEDs formed on 150 mm (111) silicon have a 3.9 percent EL brightness uniformity at a drive current of 350 mA.
“The high uniformity confirms the validity of the GaN-on-silicon technology on large-diameter substrates for next- generation LED manufacturing,” writes the team in its paper.
GaN-on-silicon technology could help to reduce the cost-per-lumen of solid-state lighting. Today, most LED epiwafers are grown on 2-inch and 4-inch sapphire, and making a transition to a larger diameter platform is hampered by the low availability and high cost of the substrates.
According to Azzurro, depositing nitride films on large diameter sapphire is also hampered by higher growth non- uniformity and lower processing yield. The German firm argues that this makes it very challenging for GaN-on-sapphire LEDs to meet the cost roadmap for solid- state lighting.
Switching to silicon trims costs associated with both the substrate and the production of the LED, because wafers can be processed in depreciated silicon lines.
The big challenges associated with the growth of GaN-on-silicon are the 17 percent lattice mismatch between the two materials, and the 46 percent mismatch in thermal expansion coefficient. The latter difference can cause wafers to bow, and even crack, when they cool down after growth.
Azzurro addresses mismatch with a proprietary buffer technology that is claimed to also lead to a significantly reduced dislocation density in the LED’s active region. With this technology, wafer
sizes. By collecting emission in an integrating sphere, engineers determined standard deviations of 0.6 percent (2.8 nm) and 0.8 percent (3.7 nm) for the 150 mm and 200 mm wafers, respectively.
Widespread uptake of LED lighting hinges on a substantial fall in its costs. Replacing sapphire or SiC with a larger, cheaper silicon substrate will help to make LED lamps more affordable
bow can be below ± 20 µm across a 200 mm wafer. This level of flatness ensures that the wafers would not crack during processing and handling in silicon lines, and processing-induced non- uniformities would be at an acceptable level.
In this latest research effort, engineers formed LED epiwafers on 150 mm and 200 mm silicon (111) using MOCVD. They deposited their proprietary, strain-engineered GaN-on-silicon buffer structure, and followed this with a 2 µm- thick, silicon-doped GaN layer, an active region, and a magnesium-doped contact.
To enable on-wafer EL testing, 400 nm-thick ITO contacts with dimensions of 1 mm by 1mm were deposited on the p-type GaN surface and annealed in a mixture of oxygen and nitrogen gas to increase transparency. 150 µm-diameter bond pads of platinum and aluminium were deposited on ITO to enable electrical on-wafer testing. EL measurement times were not excessive, thanks to a simple test structure. This contained light-absorbing silicon, the top surface had not been roughened to improve light extraction, and there was no mesa etching for device isolation.
On-wafer EL testing revealed excellent wavelength uniformity for both wafer
October 2013
www.compoundsemiconductor.net 61
According to Azzurro, correlating a photoluminescence intensity map with an EL map is conventionally difficult. “This is due to poorer carrier transport through electrical injection when compared with direct photo-excitation in the active region,” says the team in its paper. However, Azzurro’s engineers measured the photoluminescence by pumping the device with a 375 nm laser, and found a strong correlation between these values and those for EL. Meanwhile, measurements of the uniformity of forward voltage reveal a 1.3 percent standard deviation for a 150 mm wafer.
This combination of high uniformities for photoluminescence, EL and forward voltage has led the team at Azzurro to claim that these wafers can enable reduced binning, higher manufacturing yield, and cost savings resulting from the need to test from fewer bins.
Current-voltage characteristics have also been studied, with two leakage mechanisms uncovered at different bias levels. At a bias of less than -12 V, the engineers determined an activation energy for conduction leakage of 200 meV, which is consistent with a variable range hopping model. When biasing at higher reverse voltages, Frenkel-Poole conduction takes over – this is the flow of minority carriers through the space charge region, hopping between randomly distributed traps.
Understanding of the origin of these traps is needed to improve the leakage characteristics of GaN-on-silicon LEDs.
A. Pinos et. al. Appl. Phys. Express 6 095502 (2013)
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