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29 November 2006


Sandia demonstrates reduced-bowing GaN growth on Aonex's sapphire-on-AlN substrates

Sandia National Laboratories has demonstrated the growth of gallium nitride (GaN) structures on the A-Sapph substrates of Aonex Technologies Inc, a majority-owned subsidiary of Arrowhead Research Corp of Pasadena, CA, USA. Aonex claims that its substrates provides a way to reduce the manufacturing cost of III-nitride LEDs dramatically by enabling production on larger-diameter substrates, simplifying fabrication of more efficient vertical LEDs, and increasing process uniformity and yield.

The company's founder and chief scientific officer Harry Atwater is the Howard Hughes Professor of Applied Physics and Materials Science at Pasadena-based California Institute of Technology (Caltech). Aonex has an exclusive license to a suite of intellectual property developed at Caltech. Its A-Sapph substrates consist of a thin layer of single-crystal sapphire (less than 500nm thick) bonded to a polycrystalline aluminum nitride support substrate. The resulting substrate has a coefficient of thermal expansion
(CTE) that is nearly identical to GaN, claims Aonex, yet offers an industry-standard sapphire growth surface (available in both c- and r-plane sapphire orientations) suitable for growth of epilayers by MOCVD or HVPE.

Unlike conventional sapphire substrates, the close CTE match between the A-Sapph and GaN reduces substrate bowing following III-nitride device growth, Aenox claims. This could result in higher-yield post-growth device
processing, especially for high-electron-mobility transistors (HEMTs) or other device structures where submicron features are desired. In addition, the A-Sapp substrates also offer substantially higher thermal conductivity than bulk sapphire, which may significantly improve growth uniformity through improved substrate temperature uniformity, says Aenox. Together, these improved thermal characteristics could enable the scaling of GaN production to larger wafer diameters and corresponding lower device
fabrication costs. Aonex says it has also designed A-Sapph substrates to enable vertical LED fabrication without laser lift-off.

Sandia National Laboratories grew GaN on both A-Sapph and bulk sapphire reference substrates using MOCVD. It then characterized the GaN using an x-ray diffraction (XRD) defect estimation technique developed at Sandia. The XRD analysis indicates that GaN grown on A-Sapph has a defect density within 60% of that grown on the bulk reference. Ongoing efforts at Aonex to optimize the growth surface of the A-Sapph substrate are focused on achieving GaN quality that meets or exceeds GaN grown on conventional
sapphire substrates.

XRD also showed that the GaN grown on A-Sapph substrates had reduced residual strain relative to GaN grown on conventional sapphire substrates. This is a result of growing on a substrate that is CTE-matched to GaN,
verifying that the substrates have the potential to address the processing challenges currently encountered with GaN growth on conventional sapphire.

Sandia also grew 430nm InGaN multi-quantum well (MQW) structures on both substrates. Comparable photoluminescence intensity indicates that III-nitride materials on A-Sapph substrates have sufficient optoelectronic performance to fabricate LED devices, says Aenox. Efforts are currently underway to fabricate functional LEDs for additional characterization.

In addition to A-Sapph, Aonex also offers A-GaN substrates, which consist of thin layers of single-crystal GaN bonded to a polycrystalline AlN support wafer. The substrates offer a lower-cost alternative to bulk GaN wafers for devices such as laser diodes and LEDs, claims Aenox.

Aonex is currently sampling its substrates to select device and wafer manufacturers.