Arrowhead Research Corporation announced today that its majority-owned subsidiary, Aonex Technologies, and partner, Sandia National Laboratories, have demonstrated the growth of gallium nitride structures on proprietary A-Sapph™ substrates. Gallium nitride and its alloys (III-nitride materials) are the building blocks of devices such as light emitting diodes (LEDs) and high frequency power amplifiers (PAs). According to Strategies Unlimited, the market for III-nitride-based LEDs will surpass $4B in 2006 and expected to grow to $10B in five years. Aonex’s substrate product line, which includes A-Sapph™, provides a pathway to dramatically reducing the manufacturing cost of III-nitride LEDs by enabling production on larger diameter substrates, simplifying fabrication of more efficient vertical LEDs, and increasing process uniformity and yield.
A-Sapph™ substrates are comprised of a thin layer of single crystal sapphire (< 500 nm) that is bonded to a polycrystalline aluminum nitride support substrate. The resulting substrate has a coefficient of thermal expansion (CTE) that is nearly identical to GaN yet offers an industry standard sapphire growth surface (available in both c- and r-plane sapphire orientations) suitable for MOCVD and HVPE growth environments. Unlike conventional sapphire substrates, the close CTE-match between the A-SapphTM and GaN reduces substrate bowing following growth of III-nitride devices. This reduction in wafer bow could result in higher yield post-growth device processing, especially for HEMT or other device structures where submicron features are desired. In addition to reduced substrate bow, A-Sapph™ substrates also offer substantially higher thermal conductivity than bulk sapphire, which may significantly improve growth uniformity through improved substrate temperature uniformity. Taken together, these improved thermal characteristics could enable the scaling of GaN production to larger wafer diameters and corresponding lower device fabrication costs. Aonex has also designed A-Sapph™ substrates to enable vertical LED fabrication without laser lift-off.
In the collaboration, Sandia National Laboratories grew GaN on A-SapphTM and sapphire reference substrates using MOCVD. Sandia researchers then characterized the GaN using an x-ray diffraction (XRD) defect estimation technique developed at Sandia. The XRD analysis indicates GaN grown on A-SapphTM has a defect density within 60% of that on the bulk reference. Ongoing efforts at Aonex to optimize the growth surface of the A-SapphTM 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-SapphTM substrates had reduced residual strain relative to GaN grown on conventional sapphire substrates. This reduced residual strain is a result of growing on a substrate that is CTE-matched to GaN, thus verifying that A-SapphTM substrates have the potential to address the processing challenges currently encountered with GaN growth on conventional sapphire substrates. Sandia also grew 430 nm InGaN multi-quantum well (MQW) structures on the substrates. The InGaN MQWs grown on A-SapphTM and the bulk sapphire reference exhibited comparable photoluminescence intensity indicating that III-nitride materials grown on A-SapphTM substrates have sufficient optoelectronic performance to fabricate LED devices. Efforts are currently underway to fabricate functional LEDs on A-SapphTM substrates for additional characterization.
In addition to A-Sapph™, Aonex also offers A-GaN™ substrates which are comprised of thin layers of single-crystal GaN that are bonded to a polycrystalline aluminum nitride support wafer. A-GaN™ substrates offer a lower cost alternative to bulk GaN wafers for devices such as laser diodes (LDs) and LEDs.