Figures

Figure 1a

SEM and AFM images of as-grown and LT-buffer GaN for an annealing temperature of initial buffer at 550° (RMS: 2.4 nm).

Figure 1b

SEM and AFM images of as-grown and LT-buffer GaN for an annealing temperature of 950° (RMS: 7.1 nm).

Figure 1c

SEM and AFM images of as-grown and LT-buffer GaN for an annealing temperature of 1020° (RMS: 22 nm).

Figure 1d

SEM and AFM images of as-grown and LT-buffer GaN for an annealing temperature of 1050° (RMS: 24 nm)

Figure 2

HRXRD diffraction data for as-grown and LT-buffer GaN at different annealing temperatures: initial buffer at 550°, 950°, 1020°, and 1050°

Figure 3

PL spectra at 10 K for as-grown and LT-buffer GaN at different annealing temperatures: initial buffer at 550°, 950°, 1020°, and 1050°

Figure 4

Sheet resistance and electron mobility of undoped GaN films grown at various temperatures: 950°, 980°, 1000°, 1020°, and 1050°

Figure 5a

SEM photographs of undoped GaN films grown at 950°.

Figure 5b

SEM photographs of undoped GaN films grown at 980°.

Figure 5c

SEM photographs of undoped GaN films grown at 1000°.

Figure 5d

SEM photographs of undoped GaN films grown at 1020°.

Figure 5e

SEM photographs of undoped GaN films grown at 1050°.

Figure 6

Proposed two-step growth procedure. Inset shows SEM photographs of semi-insulating GaN grown based on two-step growth.

Figure 7a

SEM and AFM images of a sample grown at 1020 ° for 3 min based on typical one-step growth..

Figure 7b

SEM and AFM images of a sample grown at 950 ° for 1 min and at increasing temperature up to 1020 ° for 2 min based on proposed two-step growth.

Figure 8a

Cross-sectional TEM images of typical one-step sample under 0002 two-beam.

Figure 8b

Cross-sectional TEM images of special two-step sample under 0002 two-beam.

Figure 9

PL spectra for samples grown based on one-step growth and two-step growth for 3 min. Inset shows PL spectra for both samples grown for 40 min.

Figure 10a

Frequency response characteristics of fabricated GaN SAW filters with wavelength of 40 µm for undoped GaN film grown at 1020°.

Figure 10b

Frequency response characteristics of fabricated GaN SAW filters with wavelength of 40 µm for semi-insulating GaN film grown based on two-step growth.

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