GaN Stacked Carbon Film LED Grow Light

Leave it to the Empire of the Sun to conquer Solid State Lighting, if this ever makes it into the LED Grow Light market the rays of the rising sun will be upon us all.

A joint research group announced that they developed a new technique to form gallium nitride (GaN) based LED on a flexible substrate.

The group was led by Hiroshi Fujioka a professor at Institute of Industrial Science at the University of Tokyo and the Kanagawa Academy of Science and Technology (KAST). The technique employs a kind of physical vapor deposition (PVD) process, which is suitable for the production of large size LEDs in large quantities.

"With this technique, it is highly possible to form, for example, a 1m square GaN LED that emits light from the entire surface at a low cost," Fujioka said.

The group has only confirmed emission by "photoexcitation" with the irradiation of UV light on a 2cm square so far, but it is planning to conduct the experiment of emission by electrical excitation, as in the ordinary LEDs, in a few months.

Fujioka and others used an "organic polymer sintered graphite sheet (PGS)" for the substrate, instead of a sapphire substrate, which is commonly used in GaN LEDs. PGS is a thin graphite sheet obtained by sintering a sheet of plastic film in an anoxic environment at a high temperature of 3,000°C.

The C atoms are bonded together in a hexagonal shape in a planar manner, and this planar structure is vertically stacked to form layers. The graphite sheet is 25-100μm thick. "The surface is flat at the atomic level," Fujioka said.

The group formed crystals of GaN and aluminum nitride (AlN) on PGS by using a proprietary PVD process developed by Fujioka and others and found that the crystals thus grown have a superior quality free from defects.

"We conducted X-ray analysis and examined the photoexcitation emission spectrum," Fujioka said. "The crystal quality was on par with or even higher than that of commercially available GaN LEDs."

The wavelength corresponding to the emission inherent to GaN (3.3eV) is approximately 365nm. But an ordinary GaN has some emission peaks at wavelengths longer than that due to the crystal defects. GaN crystals formed by the latest technology do not have such undesired emission peaks.

The PVD process used in the formation of GaN film is called "pulse excitation deposition." It is Fujioka's proprietary technique developed based on a sputtering method used in the production of LCD panels, etc. According to this technique, metal Ga is sublimed by pulse plasma and then subjected to a reaction with nitrogen. The process temperature is reportedly 600-800°C when forming a GaN film.

"The technique is applicable to the formation of large films," Fujioka said. "It doesn't take a long time for a manufacturer to make large LEDs."

N atoms in GaN bonded to C atoms in PGS

According to the group, the key factor that made the formation of high quality GaN crystals on PGS is that the lattice constant of carbon (C) atoms constituting PGS and that of the hexagonal nitrogen (N) atoms in the nitride coincide with each other. Moreover, the C-C bonding in PGS tends to attract and bond N atoms.

"First, N atoms are fixed on PGS and then GaN crystals are formed on it," Fujioka said.

PGS is "widely used in heat radiation sheets of personal computers, etc because it has a thermal conductivity four times as high as that of copper (Cu) and they are much cheaper than sapphire substrates" (Fujioka). Because it is flexible and thin and has a high heat resistance, large bendable emission sheets as thin as cloth can be produced at a low cost, he said.

Gallium (Ga), known as an expensive material, "may only be required in a small amount even for a large film if the sheet is less than 1μm thick," he added.

The details of the latest technology will be presented by Fujioka and others at 55th Spring Meeting 2008 sponsored by the Japan Society of Applied Physics, which runs from March 27-30, 2008.