Tag Archives: heterojunction

Photoelectric measure of atomically thin stacks

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A team led by researchers at the University of Warwick have discovered a breakthrough in how to measure the electronic structures of stacked 2D semiconductors using the photoelectric (PE) effect. Materials scientists around the world have been investigating various heterostructures to create different 2D materials, and stacking different combinations of 2D materials creates new materials with new properties.

The new PE method measures the electronic properties of each layer in a stack, allowing researchers to establish the optimal structure for the fastest, most efficient transfer of electrical energy. “It is extremely exciting to be able to see, for the first time, how interactions between atomically thin layers change their electronic structure,” says Neil Wilson, who helped to develop the method. Wilson is from the physics department at the University of Warwick.

Wilson formulated the technique in collaboration with colleagues at the University of Warwick, University of Cambridge, University of Washington, and the Elettra Light Source in Italy. The team reported their findings in Science Advances (DOI: 10.1126/sciadv.1601832).

—E.K.

Nakamura Co-Wins Nobel for Blue LEDs

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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.