Fabrication and Characterization of 2-inch diameter AlN Single-Crystal Wafers cut From Bulk Crystals
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0955-I03-08
Fabrication and Characterization of 2-inch diameter AlN Single-Crystal Wafers cut From Bulk Crystals Robert T. Bondokov, Kenneth E. Morgan, Glen A. Slack, and Leo J. Schowalter Crystal IS, Inc, 70 Cohoes Avenue, Green Island, NY, 12183
ABSTRACT Aluminum nitride (AlN) boules larger than 2 inches in diameter were grown by the sublimation-recondensation technique. X-ray rocking curves and Laue diffraction were used to characterize the crystallinity and orientation of the boules, and 2” dia. substrates were sliced with typical thickness of ~500 µm. The wafers were then polished in order to meet the common standards for wafer thickness and flatness. The on-axis Al-polar surface was finished with a proprietary chemical-mechanical process and showed RMS roughness of 0.5 nm or less as measured by atomic force microscopy (5x5 µm area). Currently, the substrates have some polycrystalline regions that are highly textured but about 85% of the total area is single crystal and usable. The dislocation density in the crystalline regions of the substrate was measured by preferential chemical etching and then determining the resulting etch pit density (EPD). The etching technique involves potassium hydroxide and has been qualified through correlation with x-ray topography measurements of the dislocations. Measured EPD vary from a maximum of 3x104 cm-2 to nearly zero. Other structural defects such as low angle grain boundaries, prismatic slip bands, inversion domains, have also been observed. The rare appearance of these defects will be discussed even though their role in the epitaxial growth of GaN and AlGaN is yet to be clarified.
INTRODUCTION The III-nitride semiconductors are expected to play a significant role in next generation electronic and photonic devices. However, the industry needs further improvement of material quality to improve device performance and reliability. Most studies suggest that dislocation density and epitaxial layer crystallinity should be significantly improved. One of the reasons for high dislocation densities in GaN and AlGaN epitaxial layers is the use of foreign substrates such as sapphire or silicon carbide. The lattice and thermal expansion mismatch between these two substrates and GaN is far from ideal. High quality, native-nitride substrates have been shown to improve the crystallinity and reduce the dislocation densities in device layers and are greatly desired by the nitride device community. “Quasi-bulk” substrates of GaN and AlN are being developed where the initial nucleation is on a foreign substrate such as sapphire, GaAs or Si. However, those substrates still have relatively high defect densities (on average, greater than 106 cm-2) which seriously impact the yield and reliability of LED and RF power amplifiers. Native AlN substrates offer excellent lattice and thermal expansion match with AlGaN compounds and dislocation densities in the order of 103 cm-2. These substrates have been available for the past several years in limited
amounts, generally in pieces of 25 mm diamete
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