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Table 5-8. Optimum growth condition of InN for Al2O3 (0001), GaN/Al2O3 (0001), Si (111).

The mirror-like surface of InN was obtained using a LT-InN buffer (RMS roughness = 4.2 nm). The best quality single crystalline InN grown on GaN/Al2O3 (0001) showed that the FWHM of XRC is 1039 arcsec with LT-InN buffer layer. The growth of InN was found to be the transport-limited under most conditions. The growth rate decreased with N/In ratio due to the increased amount of H2 produced by thermal decomposition of NH3. The band gap energy of InN films on Al2O3 (0001) is 0.84 eV. The carrier concentration of 7.0 ×1018 cm3 at T = 266 K and 4.5 ×1017 cm3 at T = 41 K and the mobility of 623 cm2/Vs at T = 266 K and 9288 cm2/Vs at T = 45 K were obtained on InN grown on Si (111).

5.1.6. Indium Nitride (InN) Droplet Formation

Indium droplets normally occurred during the growth of InN at low N/In ratio. In this part, indium droplet formation was studied in more detail through XRD, Scanning Electron Microscopy (SEM), Energy Dispersive Microscopy (EDS), and Auger Electron Spectroscopy (AES) (AES Perkin-Elmer PHI 660 Scanning Auger Multiprobe). For this study, InN was grown on Al2O3 (0001) and Si (111) at T = 550 oC, TLT-GaN = 560 oC, and N/In = 3000 to 50,000 with LT-GaN buffer layer.

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At N/In = 3000, a high density of indium droplets formed at the surface. As the N/In ratio increases from 3000 to 50,000, the indium droplet density decreases and finally disappears. From N/In = 20,000, the size of the indium droplet is reduced significantly. Finally, at N/In = 50,000, indium droplets completely disappeared (Fig. 5.39). These results show that an increase of the N/In ratio effectively reduces and then eliminates indium droplet formation.

At N/In = 6000, the indium and InN phases were characterized by EDS (Fig. 5.39). For the indium phase, the strong intensity of indium peak was obtained from thick indium precipitates. The weak Al peak was also obtained from Al2O3 substrate due to the depth penetration of EDS to ~1 µm. For the InN phase, a weaker peak intensity was obtained compared to that of indium phase and a stronger Al peak was obtained from the Al2O3 substrate and thin InN film (Fig. 5.40).

Figure 5-39. Scanning Electron Microscopy (SEM) and EDS for the surface of InN/LTGaN on Al2O3 (0001) at N/In = 3000, 6000, 9000, 20,000, 30,000, and 50,000.

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The relation between the number of indium droplets per unit area and N/In ratio was studied in the range of N/In = 3000 to 30,000. The percent of the indium droplets with small size was increased with the N/In ratio (Fig. 5.40 and Fig. 5.41). The number density of indium droplets increases again from N/In = 9000 to 20,000 because the big indium droplets break into many small indium droplets (Fig. 5.39). The density of indium droplets continued to be decreased as the N/In ratio changed from 20,000 to 30,000. These results indicated that the indium droplets formation can be reduced by increasing the N/In ratio.

Figure 5-40. Number density of indium droplets vs. N/In ratio depending on different N/In, when InN was grown on Al2O3 (0001) at T = 550 oC and N/In = 3000, 6000, 9000.

Figure 5-41. Percent (%) vs. indium droplet size depending on different N/In, when InN was grown on Al2O3 (0001) at T = 550 oC and N/In = 3000, 6000, 9000.

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The possibility that the residual indium can be etched in the diluted HCl solution (15 %), was studied using XRD and AES. For this study, the growth of InN was performed on Si and Al2O3 substrates at N/In = 3000, and T = 450, 550, 650, and 750 oC. The results of XRD θ-2θ scans of InN grown on Al2O3 (0001) and Si (111) showed that after HCl etching all peaks of indium droplets disappeared on both substrates (Fig. 5.42 to Fig. 5.45). Because the InN (10-11) peak is at 33.1o and In (101) peak at 32.9o, these two peaks overlapped before the HCl wet etching.

Figure 5-42. X-ray Diffraction (XRD) θ-2θ scans for InN/LT-GaN on Al2O3 (0001) at N/In = 3000, T = 450, 550, 650, and 750 oC before HCl wet etching.

Figure 5-43. X-ray Diffraction (XRD) θ-2θ scans for InN/LT-GaN on Al2O3 (0001) at N/In = 3000, T = 450, 550, 650, and 750 oC after HCl wet etching.

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Figure 5-44. X-ray Diffraction (XRD) θ-2θ scan for InN/LT-GaN on Si (111) at N/In = 3000, T = 450, 550, 650, and 750 oC before HCl wet.

Figure 5-45. X-ray Diffraction (XRD) θ-2θ scan for InN/LT-GaN on Si (111) at N/In = 3000, T = 450, 550, 650, and 750 oC after HCl wet etching.

AES characterization also showed that the indium droplets could be removed in

HCl solution (Fig. 5.46). Before the HCl wet etching, only the peak due to indium

droplets was detected with the indium atomic concentration 37.2 % and the peak of

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nitrogen was not detected. After the HCl wet etching, the indium atomic concentration was decreased from 37.2 to 18 % and the peaks of In and N were detected together, which came from the InN film obtained after the HCl wet etching.

Figure 5-46. Characterization result by AES for In droplets formed during the growth of InN on Al2O3 (0001) with LT-GaN buffer layer before the HCl wet etching after the HCl wet etching at N/In = 3000.

The indium atomic concentration obtained from InN is 18 % and the nitrogen atomic concentration obtained from InN film and LT-GaN buffer layer was 29.1 %.

Through the characterization results of XRD, SEM, EDS and AES, it is concluded that the increase of N/In ratio can reduce indium droplet formation effectively during the growth of InN.