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Archive for October, 2014

Nakamura Co-Wins Nobel for Blue LEDs

Tuesday, October 7th, 2014

The Nobel Prize in Physics 2014 was awarded jointly to Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura “for the invention of efficient blue light-emitting diodes which has enabled bright and energy-saving white light sources”. In the late 1980s red and green LEDs had been around for decades, but despite large programs in both academia and industry there had been almost no R&D progress in blue LEDs (this editor did process R&D in an LED fab in that era). Then Akasaki and Amano at the University of Nagoya showed work on improved p-doping in GaN due to electron irradiance, leading to p-n junctions to make diodes.

Structure of a blue LED with a InGaN/AlGaN double heterojunction [Source: S. Nakamura, T. Mukai & M. Senoh, Appl. Phys. Lett. 64, 1687 (1994)]. Structure of a blue LED with a InGaN/AlGaN double heterojunction (Source: S. Nakamura, T. Mukai & M. Senoh, Appl. Phys. Lett. 64, V1687, 1994).From 1989 to 1994, Shuji Nakamura worked at Nichia Chemicals in Tokushima, Japan where he led a small team of co-workers to achieve a quantum efficiency of 2.7% using a double heterojunction InGaN/AlGaN (see Figure). With these important first steps, the path was cleared towards the development of efficient blue LEDs and solid-state white lighting. Nakamura-sensei is now a Professor of Physics at the University of California, Santa Barbara, and co-founder of Soora Corp. where GaN-on-GaN technology is used to increase efficiency through the elimination of the buffer-layers needed with saphhire substrates. The “Tales of Nakamura” article at IEEE Spectrum provides an excellent summary of this extraordinary man’s life story, including the US$600M payout from Nichia that was reduced to US$8M by a higher court.
Incandescent light bulbs lit the 20th century; the 21st century will be lit by LED lamps with high lm/W efficiency. The most recent record is just over 300 lm/W, which can be compared to 16 for regular light bulbs and close to 70 for fluorescent lamps. As about one fourth of world electricity consumption is used for lighting purposes, the LEDs contribute to saving the Earth’s resources.
Shine on!
—E.K.

IBM Shows Graphene as Epi Template

Thursday, October 2nd, 2014

Last month in Nature Communications (doi:10.1038/ncomms5836) IBM researchers Jeehwan Kim, et al. published “Principle of direct van der Waals epitaxy of single-crystalline films on epitaxial graphene.” They show the ability to grow sheets of graphene on the surface of 100mm-diameter SiC wafers, the further abilitity to grow epitaxial single-crystalline films such as 2.5-μm-thick GaN on the graphene, the even greater ability to then transfer the grown GaN film to any arbitrary substrate, and the complete proof-of-manufacturing-concept of using this to make blue LEDs.

(Source: IBM) (Source: IBM)

The figure above shows the basic process flow. The graphenized-SiC wafer can be re-used to grow additional transferrable epi layers. This could certainly lead to competition for the Leti/Soitec/ST “SmartCut” approach to layer-transfer using hydrogen implants into epi layers.
No mention is made of the kinetics of growing 100mm-diameter sheets of single-crystalline GaN on graphene. Supplemental information in the online article mentions 1 hour at 1250°C to cover the full wafer, but the thickness grown in that time is not mentioned. From first principles of materials engineering, they must either:

A) Go slow at first to avoid independent islands growing to form a multicrystalline layer, or
B) Initially grow a multicrystalline layer and then zone anneal (perhaps using a scanned laser) to transform it into a single-crystal.
In either case, we would expect that after just a few single-crystalline atomic layers had been either slowly grown or annealed, that a 2nd much-higher speed epi process would be used to grow the remain microns of material. More details can be seen in the EETimes write up.
—E.K.