Patterned Sapphire Creates Semi-polar GaN
Researchers from Yamaguchi University, Japan, have produced semi-polar (1122) GaN on a maskless r-plane patterned sapphire surface. Their advance could help to spur the development of green-emitting devices that are grown on the semi-polar planes of GaN. These faces enable the fabrication of epistructures that are not hampered by strong internal electric fields, and they facilitate the growth of InGaN layers with a high enough indium content for green emission.
This Japanese team is not the first to grow semi-polar GaN on patterned, foreign substrates. Silicon has also been used, but this has the downside of absorbing some of the light that is generated by the device.
Fabrication of a series of semi-polar GaN films began with the creation of micron-sized grooves in sapphire via a combination of photolithography and fluorine-based inductively coupled plasma reactive-ion etching. A 30 nm thick GaN buffer was then deposited onto this substrate by MOCVD at 4600C, followed by the growth of GaN at temperatures ranging from 9000C to 10000C. Lower deposition temperatures created an undesirable mixture of (1122) and (112 0) GaN, but growth at 1000 0C produced (1122) only. Cathodoluminscence measurements revealed a threading dislocation of more than 3 x 108 cm-2 in an n-doped layer.
The researchers have produced LEDs on this material with the processes that were initially developed for device fabrication on c-plane sapphire. “However, the performances were inferior to that on csapphire," says Narihito Okada.
They now want to understand why the performances of LEDs grown on the semipolar and non-polar planes are inferior to those grown on the c-plane. However, the primary goal of this research team is to grow high-quality GaN layers with defect density below 108 cm-2.
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