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DBR boosts the brightness of vertical GaN-on-silicon LEDs

Brighter, cheaper LEDs are promised by inserting a mirror between the LED and its silicon substrate

The researchers from Sun Yat-sen University fabricate LED chips by depositing an epitaxial structure on silicon (a), dry etching to expose n-GaN and form through-holes (b) and depositing metals, which fill the holes and provide electrodes (c). Emission intensity is fairly uniform at an injection current of 100 mA (d). 

Researchersfrom Sun Yat-sen University, China, have developed a novel GaN-on-silicon LED that combines high light extraction with a low operating voltage.

High values for light extraction are known to result from the insertion of a reflecting stucture known as a distributed Bragg reflector between the LED and the silicon substrate.

However, up until now, the addition of this reflector mirror paid a big penalty: A high operating voltage. The team from China have addressed this by inserting holes through the entire structure, which are subsequently filled with metal.

This revolutionary design will be of interest to the solid-state lighting industry, which views the GaN-on-silicon LED as a promising devices for reducing the cost-per-lumen. Silicon is a low-cost substrate, and the processing of GaN-on-silicon wafers in depreciated 200 mm silicon lines could lead to a significant reduction in LED production costs.

GaN-on-silicon LED fabrication traditionally involves removal of the silicon LED, which is light absorbing, and the transfer of the stack of nitride epilayers to a new carrier. According to the team from China, however, performing out this task is tricky, and it results in high yields and low costs.

Baijung Zhang from Sun Yat-sen University told Compound Semiconductorthat one of the reasons behind these weaknesses is that GaN-based LEDs grown on silicon have a bowed surface that is induced by lattice and thermal mismatch. “This affects the wafer-bonding process, resulting in low yields.”

In 2012, Zhang and co-workers reported the production of vertical LED structures transferred from a 2-inch silicon substrate to copper.

“Although the yield was increased by using the mature electroplating technique, the substrate transfer technique needed some additional processing steps in comparison with the normal LED fabrication process.” These included chemical wet etching, sub-mount protecting and metal reflector forming, which resulted in a complex, costly fabrication process.

To form the team’s latest LEDs, which avoid any silicon lift-off processes, silicon (111) substrates are chemically cleaned and loaded into an MOCVD tool.

Growth begins with deposition of aluminium for 5s, to prevent the formation of SiN, followed by the addition of a 100 nm-thick AlN seeding layer that prevents melt-back etching caused by Ga-Si alloys. 

Engineers then grow: a 660 nm-thick graded AlGaN buffer; a distributed Bragg reflector, which is formed from five pairs of 56 nm-thick AlN and 48 nm-thick GaN; a 300 nm-thick cap; and an LED comprising 800 nm of n-type GaN, a six period multiple quantum well, and 240 nm of p-type GaN.

Plasma etching forms 100 mm-wide holes in the LEDs, which are filled with metal during the fabrication of the n-type and p-type wire bonding pads. This addition addressed the resistance induced by the AlN/GaN distributed Bragg reflector and the high resistivity of the AlGaN buffer and AlN seeding layer.

Driven at 20 mA, the LED made in this manner had an operating voltage of 3.96 V and produced 2.1 W at 350 mA − which is 24 percent more power than that of a conventional LED.

However, the operating voltage and light output of the vertical chip are inferior to those of an equivalent GaN-on-silicon LED with a lateral conduction path. 

Zhang blames the relatively poor output compared to the lateral device on the contact resistances between the n-type electrode and silicon, and the high resistance of the silicon substrate.

“We will optimise the process conditions to improve the ohmic contact,” says Zhang.

According to hom, the team will also try to increase the reflectivity of the distributed Bragg reflector by adding more pairs of AlN and GaN, while trying to improve the interfaces between these materials.

Ref: Y. Yang et. al.
Appl. Phys. Express 7042102 (2014)
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