Word out of the Xerox Palo Alto Research Center (Palo Alto, CA) is that researchers have successfully grown simple polycrystalline-nitride semiconductor light-emitting diodes (LEDs) on quartz substrates. In recent experiments, scientist David Bour and colleagues obtained electroluminescence at 430 nm from a nitride LED with a spectral width of 38 nm and emission efficiency approximately two orders of magnitude lower than for single-crystal LEDs grown on sapphire substrates.1 Because such polycrystalline materials can be deposited on much larger substrates than conventional single-crystal LED materials, the Xerox research team believes the new LED configuration may have the potential to take large-area displays to a new level of efficiency.
According to Xerox, nitride LEDs offer unusually high efficiency compared to other compound semiconductor LEDs because of the high-density dislocations running through the materialdefects that apparently do not influence carrier flow and recombination in devices fabricated from crystalline material. "If these defects are truly so inert," say the researchers, "efficient LED operation may also be possible from even more highly dislocated material; for example, randomly oriented polycrystals grown on glass substrates."
The scientists grew a polycrystalline nitride LED structure on quartz using an organometallic-vapor-phase-epitaxy growth sequence identical to that used to produce single-crystal films on sapphire substrates. The first step in the process deposited an amorphous gallium nitride (GaN) buffer layer roughly 30 nm thick at 550°C and then solid-phase crystallized it by increasing the temperature to 1050°C in an ammonia/hydrogen ambient environment. The final result of the growth process was a simple polycrystalline LED structure comprised of a 4-µm n-type gallium nitride/silicon (GaN:Si) layer, a 30-Å indium gallium nitride single quantum well, and a 0.2-µm p-type gallium nitride/magnesium layer (see figure).
To produce crude LEDs from the material, the researchers evaporated titanium/gold p-contact metal, patterning it into dots roughly 500 µm in size on 1-mm centers, and argon-ion milling the surrounding material. Etch depth was 1 µm, which is similar to depths required to expose the underlying n-type material with epitaxial LEDs grown on sapphire. No n metal was deposited, and the researchers made the n-type contact required for LED operation by touching a metal probe tip to the GaN:Si surface.
Bour and colleagues report several differences between single-crystal LED material grown on sapphire substrates and the polycrystalline nitride semiconductor LED structures grown on quartz. One example is the rough surface of the polycrystalline material compared to the specular surface obtained for the single-crystal materials, which precluded the use of ion milling to uniformly remove the p-n junction. In addition, typical crystal size is in the range of a few microns (roughly on the same order as the film thickness), and there are several flat-topped crystals with hexagonal cross sections. The researchers believe this latter characteristic suggests a tendency toward growth in a c orientation.
Paula Noaker Powell
REFERENCE
- D. P. Bour et al., App. Phys. Lett. 76, 2182 (April 17, 2000).