Growth of Self-Seeded Aluminum Nitride by Sublimation-Recondensation and Substrate Preparation
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Growth of Self-Seeded Aluminum Nitride by Sublimation-Recondensation and Substrate Preparation J. Carlos Rojo,* Glen A. Slack,* Kenneth Morgan,* Leo J. Schowalter,*,** and Michael Dudley*** * Crystal IS, Inc., Latham, NY 12110 U.S.A. ** on sabbatical from Rensselaer Polytechnic Institute, Troy, NY 12180, U.S.A. *** SUNY, Stony Brook, NY 11794 U.S.A.
ABSTRACT Bulk aluminum nitride boules have been grown at driving rates of 0.9mm/h by the self-seeded sublimation-recondensation technique. Up to 15mm diameter substrates cut from those boules present large single crystal grains that have been analyzed using different techniques. X-ray double crystal diffraction shows a full-width-at-half-maximum of around 100 arcsec and X-ray topography reveals extensive areas with a density of dislocations less than 104 cm-2. These substrates have been prepared by chemical mechanical polishing techniques to obtain a surface roughness of 1.4-1.6nm. INTRODUCTION The lack of methods to produce large bulk GaN single crystal substrates for the III-nitride epitaxy industry has triggered a worldwide search for alternative substrates. Out of the possible alternative substrates, aluminum nitride (AlN) has attracted much interest due to its excellent properties. AlN posses the same hexagonal (2H) crystalline structure as that of gallium nitride (GaN) with a lattice mismatch of approximately 2.4%. The chemical compatibility is excellent and the thermal expansion mismatch between GaN and AlN is almost negligible for temperatures ranging from 1000oC to room temperature. In addition to that, the high thermal conductivity of AlN (3.2 W/cm-K [1]) is ideal for high power application where the heat generated by the operating devices needs to be efficiently dissipated. While AlN is a suitable substrate in the fabrication of GaN blue/UV laser diodes, it could be the perfect candidate for high power microwave devices and UV detectors, due to its high resistivity and UV transparency. Among all the techniques used to grow AlN single crystals, the sublimation-recondensation method, first developed by Slack and McNelly [2,3], has been demonstrated to be the most promising method for obtaining large bulk AlN boules. Even though high quality AlN crystals with full-width-at-half-maximum (FWHM) of less than 40 arcsec have been reported before [4], those crystals were mainly platelets or needles and, therefore, too small to allow device fabrication. In addition, the temperature used to grow those perfect platelets (< 2000 oC) would be inadequate to achieve large growth rates for bulk crystal growth. To the best of our knowledge, we report here the largest bulk AlN crystal grain grown by the sublimationrecondensation technique. Prior to epitaxy, achieving submicron roughness during surface preparation is an issue of main concern in the fabrication of commercial substrates for epitaxy. We also present results on the preparation of AlN substrates using chemical-mechanical polishing (CMP) techniques. G1.10.1
SELF-SEEDED GROWTH AND SUBSTRATE PREPARATION For our study,
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