Fig. 1. (Color online) Surface topography of the AlN crystal grown at a seed surface temperature of 2200 °C. (a) Photograph. (b) Differential interference microscope. (c) SEM image.

Fig. 2. (Color online) Surface topography of the AlN crystal grown at an increasing seed surface temperature of 2240 °C. (a) Photograph of the AlN crystal. (b) Differential interference microscope image in the center of the crystal. (c) Differential interference microscope image of terraces at the rim. (d) The AlN ingot after 20 h growth.

Fig. 3. (Color online) The AlN crystal grown at optimized growth temperature of 2260 °C. Photograph of (a) the AlN crystal, (b) the AlN crystal grown after 24 h, and (c) the AlN ingot grown after 72 h.

Fig. 4. (Color online) Crystallization properties of the AlN wafer. (a) 9 point Raman spectrums of the AlN crystal. (b) Mapping of 657 cm^–1 Raman shifts. (c) HRXRD pattern of the AlN crystal with ω–2θ mode. (d) Rocking curve of (002) planes with Omega Rel scan mode. (e) Rocking curve of (102) planes with Omega Rel scan mode. (f) SEM image of etch pits.

Fig. 5. (Color online) (a) AlN polished wafer. (b) Surface roughness of the AlN polished wafer @ 10 × 10 μm². (c) Schematic diagram of the epitaxial structure. (d) AFM of Al_xGa_1–xN epilayer. (e) HRXRD of Al_xGa_1–xN epilayer with ω–2θ scan mode. (f) High-resolution XRD reciprocal space mapping (RSM) of the (105) plane.

Table 1. FWHMs of (002) and (102) reflection peaks in the substrate and epitaxial layer.