Technical Insight
Cree licenses "pendeo" patent for low-defect-density GaN growth (Nitride News)
North Carolina State University (NCSU) has granted Cree an exclusive license for patented technology that allows the growth of low-defect-density GaN layers. Cree has licensed US patent no. 6,265,289, issued to NCSU on July 24, 2001, which describes methods of fabricating GaN layers and device structures using lateral growth from sidewalls into trenches. The technique, known as pendeo-epitaxy, reduces the defect density of GaN grown on sapphire or SiC substrates. During conventional growth, lattice mismatch causes dislocations that propagate in the vertical growth direction, resulting in GaN with dislocation densities of around 108109/cm2. In pendeo-epitaxy, trenches are etched in a GaN layer and growth conditions are adjusted so that subsequent GaN growth occurs from the trench sidewalls (see Compound Semiconductor March 1999, p16). Since the dislocations do not propagate significantly in the lateral direction, defect densities are typically several orders of magnitude lower, which is important to the performance and longevity of certain devices, particularly lasers. The same effect is achieved using lateral epitaxial overgrowth (also known as LEO or ELOG), which requires the use of an SiO2 or other mask. Nitronex uses alternative technique Cree is not the only company using the pendeo-epitaxy approach. Nitronex (Raleigh, NC), which is developing GaN-on-Si devices for RF and optoelectronic applications, has trademarked the term "Pendeo" to include its entire process from epitaxial growth to device fabrication. Nitronex has a number of patents in this area, including several licensed from NCSU. A search of US patents unearths at least five documents covering pendeo-epitaxy and lateral growth recipes for GaN layers. All are assigned to NCSU, and some include authors who now work at Nitronex. While Nitronex has not revealed which patents it has licensed from NCSU, the company says there is no intellectual property overlap with Cree's technology. "There are many ways to grow lateral epitaxial layers of GaN," says Warren Weeks, who heads up Nitronex's optoelectronics division. In fact, Nitronex has filed patents on its own proprietary approach. Low-defect-density lateral growth As well as including lateral growth of GaN outwards from trench sidewalls, the patent licensed by Cree describes how further reductions in defect density can be achieved if masks with windows are subsequently deposited. The GaN layer grows upwards through these mask openings and then also grows laterally, forming an overgrowth layer of low-defect-density GaN material. Alternatively, further growth in the vertical direction can produce a plurality of trenches, which are again used to grow lateral layers of GaN. Devices can then be grown on these surfaces. "The issuance of this patent significantly extends Cree's portfolio of technology, critical in the development of high performance GaN-based devices," said president and CEO, Chuck Swoboda. "Growing low-defect layers of GaN is essential to the realization of long-lifetime GaN-based laser diodes and other high-performance devices. We believe this patent strengthens our intellectual property position immensely, since it covers use of the patented process on any substrate."
