Technical Insight
Slashing defects in GaN-on-sapphire films
Serpentine mask paves the way to ultra-high-quality GaN epilayers
An international collaboration has produced incredibly high-quality GaN on sapphire with a fabrication process involving a single epitaxial growth step.
“We expect that the devices benefiting the most from this technology are the ones requiring very high performance,” says Tahong Xie from the University of California Los Angeles. “These devices include lasers, power transistors pushing the limit for power handling, photodetectors requiring extremely low dark current, and LEDs for solid-state lighting,”
Xie’s group, working with researchers at Peking University, has produced GaN films with a dislocation density of just 7 x 105cm-2by using a serpentine mask to block threading dislocations.
His initial idea for using this form of mask structure dates back to 2001 and draws on popular epitaxial lateral overgrowth approaches and dislocation necking. However, it overcomes the deficiencies associated with these technologies.
If device manufacturers were to employ this serpentine mask technology, they could trim production costs, thanks to a simplified epitaxial process. What’s more, chipmakers would benefit from a four-fold reduction in the size of highly defective regions, such as coalescence fronts, compared to GaN material formed by conventional epitaxial layer overgrowth techniques.
Fabrication of GaN films by the process pioneered by Xie and his co-workers begins with CVD deposition of a 100 nm-thick layer of SiNx on (0001) sapphire. A set of [1120]-orientated stripes are then formed with standard photolithographic processes, before 200 nm-thick films of SiO2and SiNxare deposited onto this mask.
Figure 1. The serpentine mask is formed by a combination of Sinx and SiO2 film growth, photolithography and etching.
Another set of stripes in SiNxis then formed, separated by window areas and offset from the lower openings (see Figure 1). Finally, etching with hydrofluoric acid exposes the sapphire in the lower window. To form high-quality nitride layers, engineers deposit a 25 nm-thick GaN nucleation layer on this serpentine mask at 530 °C, followed by epitaxial growth at 1040 °C (see Figure 2).
Figure 2. As more GaN is deposited, material quality improves, because this structure can block threading dislocations.
GaN islands formed at the edge of this mask that are probably riddled with gallium vacancies. In the centre of the mask material quality is far higher, thanks to complete coalescence of GaN to create a film with a mirror-like surface.
Here the width of X-ray diffraction peaks is narrower than it is for GaN grown on sapphire, defect densities are lower by several orders of magnitude and photoluminescence intensity is 16 times higher. The US-China collaboration is now partnering with engineers at National Cheng Kung University, Taiwan, to grow lasers and LEDs on these low-defect-density substrates.
L. Li et al. Appl. Phys. Express 5 051001 (2012)
