We develop a thermal-elastic stress
model using the finite element method to predict three-dimensional
anisotropic stress in AlN single crystals homoepitaxially grown by
the physical vapor transport process; we also perform numerical experiments
for a 1-in. AlN crystal surrounded by different cone-tube designs
and grown along various orientations. The influences of the cone-tube
shape and the growth orientation on the stresses inside the AlN crystal
are investigated in detail. The simulation results show that the von
Mises stress exceeds 1.11 GPa under all specified growth conditions,
while the anisotropy is negligible. The resolved shear stresses are
strongly dependent on the thermal gradient inside the growing crystal
and the growth orientation. Strong anisotropy of the resolved shear
stress is observed upon tilting of the growth orientation. The resolved
shear stress along {0001}⟨112̅0⟩ primary slip
system reveals that the c-axis growing crystal is
under tensile stress along all three primary slip directions. Nevertheless,
an inversion of the resolved shear stress from tensile to compressive
along the −a
3 slip direction is
observed when changing the growth orientation. The total resolved
shear stress shows 6-fold symmetry, reflection symmetry and 2-fold
symmetry along [001], [10√3], and [100] growth orientations,
respectively.
Crack-free bulk AlN single crystals up to 60 mm in diameter are successfully grown for the first time using a series of proprietary techniques by the physical vapor transport method. The single crystals are sliced into on-axis (AE0.2 ) wafers and then lapped/polished following common wafering standards. The obtained wafers are characterized by Raman spectroscopy and high-resolution X-ray diffraction (HRXRD). The Raman spectra show an E 2 (high) full width at half maximum (FWHM) of 2.85-2.87 cm À1 . The symmetric and asymmetric HRXRD rocking curves show FWHMs of 172-288 and 103-242 arcsec, respectively. The optical transmission spectra reveal that the entire wafers exhibit excellent ultraviolet (UV) transparency with absorption coefficients of 14-21 cm À1 in the UV range 4. 43-4.77 eV (260-280 nm). The average etch pit density (EPD) determined by preferential chemical etching is about 2.3 Â 10 5 cm À2 . The major impurities determined by evolved gas analysis and glow discharge mass spectrometry are carbon at 7.4 Â 10 18 cm À3 (45 ppmw), oxygen at 1.2 Â 10 19 cm À3 (100 ppmw), and silicon at 6.8 Â 10 17 cm À3 (9.7 ppmw). The usable area of the 60 mm wafers exceeds 98%.
Crack-free and parasitic-free Al-polar aluminum nitride (AlN) single crystals up to Φ56 mm were iteratively grown by the homoepitaxial physical vapor transport method. The detailed iterative growth processes from the spontaneous AlN boules to Φ56 mm single crystals were presented. Our growth experiments revealed that stable growth of large Al-polar crystals with good structural quality and UV transparency was possible using a pure tungsten setup. For each iteration, the initial expansion angle, which was dominated by the radial temperature gradient (ΔT r ), reached 50−60°under our specific growth system and growth conditions and then gradually decreased to 10−20°during the growth process. For all as-grown crystals, mirror-like facets with a step-flow growth mode could be observed. However, the {101̅ 0} side planes were strongly suppressed when using larger AlN seeds. Material characterization showed that the full width at half maximum of symmetric and asymmetric highresolution X-ray diffraction rocking curves was 84−144 arcsec and 45−70 arcsec, respectively. The average etch pit density evaluated by preferential chemical etching was approximately 8.5 × 10 4 cm −2 . The optical transmission spectra revealed that the entire wafer exhibited excellent ultraviolet (UV) transparency, with absorption coefficients of 19−29 cm −1 in the UV range of 4.43−4.77 eV (260−280 nm).
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