Tuesday, August 22, 2017

Gallium Nitride ‘Tangoes’ with Silicon to Overcome Nature’s Material Limitations

Gallium nitride (GaN) is a material that is used for radio and satellite communications in civil and military applications and in solid-state lighting such as LED bulbs. Researchers are also exploring GaN for use in high power applications such as power grids and electric vehicles. The market for GaN power devices is expected to reach $2.6 billion dollars by 2022. However, GaN is not an earth abundant material and only recently, small diameter GaN substrates have started to become available. Researchers have been growing GaN on foreign substrates for almost 5 decades, but the quality of the grown materials is compromised, especially on the standard microelectronics substrate, silicon (Si), which is over 1000 times cheaper than GaN substrates. The origin of the problem is a classical one: high quality material deposition is usually carried out near 1,000 degrees Celsius, but when dissimilar materials are cooled down to room temperature, their contraction can be disproportionate, resulting in the formation of cracks and material failure. This is exactly what happens when GaN is grown on Si. And because the crack severity depends on the thickness of the layers, the thickest pure and semiconductive GaN layer that can be grown on Si is 4.5 micrometers thick — too thin to provide good use of GaN for high power (kilovolt-scale) applications which require much thicker layers (10 microns or more). 

Scanning electron microscopy image of 
crack-free GaN on Si (19 μm thick at center).
Now researchers at the Integrated Electronics and Biointerfaces Group at UC San Diego led by electrical engineering professor Shadi Dayeh have solved this classical problem of thermal mismatches in the growth of dissimilar materials. In an article published on Aug. 21 in Advanced Materials, they combined fundamental crystal properties of GaN and geometrical effects to deflect strain from the crystal planes that usually crack under stress to the surface facets that can freely expand and contract in response to stress. By doing so, they were able to grow crack-free 19-micron-thick layers of GaN on Si — thicker than what’s needed for high-power applications. In the resulting structures, both GaN and Si had exposed surfaces to enable them to move, twist or “tango” together without cracking despite their thermal mismatch. 

Electrical engineering professor Shadi Dayeh (left) and 
Ph.D. graduate student Atsunori Tanaka (right) 
near the GaN MOCVD facility in the Qualcomm Institute 
at UC San Diego.
Thick layers also allowed the crystal defects — threading dislocations — to reduce from commonly achieved 108 – 109 per centimeter squared on Si to 107. And with the high material quality, Dayeh and his team demonstrated the first vertical GaN switches on Si. “This is the result of nearly four years of diligent efforts by graduate student Atsunori Tanaka, who learned and quickly excelled in the GaN metal organic chemical vapor deposition here at UC San Diego,” said Dayeh. “Our graduate students go through a full cycle of rigorous training in all aspects in electronic materials and devices and are prepared to tackle the greatest challenges in this area. A group of very talented students including Atsunori Tanaka, Woojin Choi, who fabricated the vertical switches, and Renjie Chen, who did the electron microscopy, have teamed up to complete the research,” Dayeh continued. Based on this work, Dayeh received funding in July from the National Science Foundation to realize a monolithically integrated GaN power converter on Si.

The growth, device fabrication and characterization were performed at UC San Diego and the electron microscopy was performed at the Center for Integrated Nanotechnologies (CINT), a Department of Energy Office of Basic Science user facility that provides access to top-of-the-line equipment under a user proposal system.

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