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Current Status of Very Large Sapphire Crystal Growth for Optical Applications

Sapphire is an ideal visible-MWIR window due to its excellent optical and mechanical properties and its availability in large sizes up to 340-mm diameter boules. Anticipated applications for new, high performance optical systems call for even larger, 450-750 mm diameter, windows. The present effort has focused on producing 500-mm diameter sapphire boules using the Heat Exchanger Method. Three experimental growth runs demonstrated the feasibility of producing 500-mm diameter sapphire boules. Completely crack- free boules have not been grown, but large size sapphire pieces up to 400 mm by 280 mm have yielded from these experimental runs.

A-plane GaN View the MathML source epilayers have been grown on r-plane View the MathML source sapphire by MOCVD, and investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), and atomic force microscopy (AFM). This particular orientation is non-polar, as opposed to the c-direction, and avoids polarization charge and the associated screening charge, and the consequent band bending. Our results showed that low pressure, low ammonia flow rate (namely low V/III ratio), high-temperature conditions lead to fully coalesced and relatively high crystalline-quality a-plane GaN films. Both low-temperature GaN buffer and high-temperature AlN buffer were used for a-plane GaN growth on r-plane sapphire, and produced similar crystalline quality and surface morphology. Surface morphological evolution during early stages of a-plane GaN growth revealed behavior different from that of c-plane GaN growth. The possible reasons for striped features and surface undulations of a-plane GaN are discussed.


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