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. These are based on the material design and science of semiconductor stacks in the late 1980’s and 1990’s.
Light-emitting diodes (LEDs) are semiconductor-based devices converting an electric current into light with a well-defined wavelength (color), whose development started in the 1950’s. While research in materials and designs resulted in the availability of various wavelengths, efficient blue LEDs remained for long a missing brick. A promising material was GaN, a semiconductor with a large and direct bandgap of 3.4eV, in principle suitable to produce even ultraviolet light. However, the inability to grow GaN crystals of decent quality and to p-dope GaN prevented the fabrication of efficient enough LEDs. At the beginning of the 80’s, research on GaN was quite limited and applications to LEDs seemed hopeless.
In this context, the three awardees made several key contributions in a ten-year period (1986-1996). First, Akasaki and Amano at Nagoya University developed the growth of improved quality GaN layers on sapphire substrates using the newly-developed epitaxy technique of Metalorganic Vapour Phase Epitaxy (MOVPE). They then demonstrated efficient p-doping of GaN by activating the acceptors under an electron flux. Nakamura, working for the Nichia company, showed that a simple thermal treatment was enough to enable p-doping. Both groups contributed to the improvement of AlGaN/GaN/InGaN thin film heterostructures and efficient blue LEDs were demonstrated in 1994, while the first articles on GaN-based lasers were published in 1996
Blue light being the most energetic in the rainbow, it can be converted to any other color (through the use of phosphors). Thus the availability of intense blue LEDs allowed the development of white LEDs. While reaching only a few percent at the time of discovery, today’s electric-to-light conversion efficiency is close to 50% for the best LEDs. This is way better than incandescent bulbs and even fluo-compact lights. These are thus progressively being replaced by LEDs in displays and lighting for the sake of decreasing our energy footprint.
Epitaxy and physics of heterostructures based on GaN has been for the past 20 years a key expertise of the research team Nanophysics and Semiconductors (NPSC), joint between Institut NEEL and CEA/INAC. The team both works on fundamental properties of nitride semiconductor heterostructures (quantum dots, nanowires, and superlattices grown by MBE and MOCVD), and on possible applications such as ultraviolet LEDs and lasers, THz emitters and detectors, nanowire-based visible LEDs, photovoltaic devices.