Single-crystalline GaN films of ≤ 110 µm in thickness have been fabricated on hydride vapor phase-grown (0001) GaN substrates by employing a relatively low-pressure (≤170 MPa) ammonothermal growth method with NH4Cl as mineralizer. Metallic Ga and polycrystalline GaN has been the chosen precursor. An average growth speed on the (0001) face of the substrate of ≥5 µm/day was observed when using metallic Ga and about 7 µm/day for GaN. The maximum growth speed of 27.5 µm/day was achieved for the film grown at a supersaturation from a combined Ga/GaN precursor. The surface morphology is not affected by the nature of the precursor.
A relatively large sample of gallium nitride (GaN) was grown as a single crystal using the hydride vapor phase epitaxy (HVPE) process. The thermal diffusivity of the single crystal has been measured using a vertical-type laser flash method. The thermal expansion was measured using a dilatometer in order to estimate the thermal diffusivity with sufficient reliability. The effect of sample thickness and temperature on thermal diffusivity was evaluated. The specific heat capacity of GaN was also measured by using a differential scanning calorimeter. The thermal properties of single-crystal GaN have been compared with the measured thermal properties of single-crystal silicon carbide (SiC). The thermal conductivity of single-crystal GaN at room temperature is found to be 253 AE 8:8% W/mK, which is approximately 60% of the value obtained for SiC. The excellent thermal property that is obtained in this study clearly indicates that GaN crystals are one of the promising materials for use in high-power-switching devices.
NH 4 F is demonstrated to be a promising mineralizer for the acidic ammonothermal crystal growth of GaN. In comparison with other acidic mineralizers such as NH 4 Cl, NH 4 Br, and NH 4 I, NH 4 F behaves distinctively different. First, NH 4 F affords a negative temperature gradient for crystal growth of GaN in supercritical NH 3 at a temperature range from 550 to 650 °C. Second, it enables GaN crystal growth in polar (c plane), semipolar, and nonpolar directions (a plane and m plane). Third, NH 4 F remarkably increases both the growth rate and quality of the GaN crystal. With the aid of NH 4 F, self-nucleation of GaN and bulk growth of hexagonal GaN crystals from the self-nucleated seed have been realized.
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