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Figure 5-15. Cross-sectional SEM micrographs of InN for 60 min growth at 530, 550, and 570 oC, and N/In = 50,000 with LT-GaN buffer.

Table 5-3. Optimum growth temperature of InN on LT-GaN and LT-InN buffer layers on various substrates.

In summary, the optimum growth temperature of InN was near 550 oC for LT-GaN and 530 oC for the LT-InN buffer layer on Al2O3 (0001), and 530 oC for both GaN/Al2O3

(0001) and Si (111). The growth of InN was not chemical reaction-limited but mass transport-limited. The results for the optimized growth temperature are summarized in Table 5-3.

5.1.5.2. Influence of Substrate Nitridation

The nitridation of Al2O3 is an important step to enhance the quality of InN film. For Al2O3 (0001), it has been reported that the nitridation treatment results in the formation of an AlN or amorphous AlOxN1-x layer on the Al2O3 substrate, which was

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confirmed by several scientists with XPS and EDS [Bry92a, Yam94b, Uch96, Pan99, Tsu99]. The results presented below are based on growth of InN on sapphire prepared with the AlOxN1-x layer by the procedure previously described.

The first run compared the InN quality with and without using nitridation of the sapphire on an InN/LT-InN (TLT-InN = 450 oC)/Al2O3 (0001) substrate with the conditions N/In = 50,000 and TLT-InN = 450 oC. The result of a XRD θ-2θ scan showed that the InN (0002) is single crystal using nitridation (T = 850 oC in NH3 for 15 min), while without the nitridation, both InN (0002) and InN (10-11) were evident (Fig. 5.16). For InN/LTInN GaN/Al2O3 (0001) at N/In = 50,000, T = 530 oC, and TLT-InN = 450, 500 oC

Figure 5-16. X-ray Diffraction (XRD) θ-2θ scan for InN/LT-InN (TLT-InN = 450 oC) on Al2O3 (0001) at N/In = 50,000, and TLT-InN = 450 oC without and with nitridation.

The presence of the nitrided Al2O3 layer is believed to promote wetting of the successive InN over layer and therefore to improve the film quality. The nitridation of the Al2O3 surface significantly improves the crystalline quality of InN as a result of the formation of AlN or AlOxN1-x layer through the reaction of nitrogen at the Al2O3 surface as discussed above. The marked improvement of InN film quality by the formation of this layer reduces the mismatch between the sapphire and III nitride layer. For example, AlN

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has the same lattice structure as InN, and the lattice mismatch is reduced from 25.7 % for InN/Al2O3 to 13.7 % for InN/AlN.

Another set of runs were made using LT-InN GaN/Al2O3 (0001) with and without nitridation of the GaN buffer (nitridation performed at 850 oC for 15 min in NH3). The growth of InN was performed at T = 530 oC, while two buffer layer temperatures were compared; TLT-InN = 450 and 500 oC at N/In = 50,000. The subsequent XRD patterns shown in Fig. 5.17 reveal that nitridation again had an impact on the crystallinity. In contrast to previous results, the nitridation of sapphire produced polycrystalline InN with the InN (10-11) peak increasing in intensity and the peak of InN (0002) broader.

Figure 5-17. X-ray Diffraction (XRD) θ-2θ scan for InN/LT-InN GaN/Al2O3 (0001) at

N/In = 50,000, T = 530 oC, and TLT-InN = 450, 500 oC with and without nitridation.

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A final set of runs was performed with Si (111) with and without nitridation at T = 850 oC for 15 min in NH3 ambient. For this study, the growth of InN was performed 530 oC using a TLT-InN = 450 or 500 oC, and N/In = 50,000. Although good results were note expected, for completeness this experiment was performed. The film with the nitridation treatment showed an InN (0002) peak while the one without nitridation did not show this peak (see Fig. 5.18). It is known that the nitridation of Si gives amorphous SiNx formation, which was reported to severely degrade the film quality [Yan02c]. However, the result of this study indicates that the nitridation process improved the structural properties of InN.

Figure 5-18. X-ray Diffraction (XRD) θ-2θ scan for (a) InN/LT-InN on Si (111), at N/In

= 50,000, T = 530 oC, and TLT-InN = 450, 500 oC with the nitridation and without the nitridation.

During the nitridation treatment in this system, the SiOxN formation on Si substrate was thought to exist on the surface of Si substrate as shown by the analysis using ESCA by a previous student in the group [Mas01]. SiOxN formation on Si (111), instead of SiNx formation, is believed to lead to the growth of the single crystalline InN (Fig. 5.19)

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[Mas01]. To understand the initial stages of InN growth on Si, NH3 treatment on bare silicon was studied. Figures 5.19 shows ESCA spectra of the Si 2p3 peak for bare silicon treated with and without NH3 at T = 850 oC. The bulk Si-Si bonds have a binding energy at 100.1 eV. This peak shows a doublet splitting of the 2p subshell into 2p3/2 and 2p1/2 bands with an intensity ratio of 1:2. Emission of an electron with up or down spin from a p-type orbital creates a photoelectron with two possible energy levels. These two emission levels separated by 0.6 eV create an asymmetry in the overall Si peak. Strained Si-Si bonds near the Si/SiOx interface are distorted from their typical tetragonal bonding configuration. This compressive distortion results in a silicon bonding peak shifted to 102.2 eV. A strong peak observed at 104.1 eV was assigned to the Si-O bond. For samples treated in ammonia, a strong peak from the Si-N bond was observed at 103.3 eV. By integrating the area of each peak, an estimation of the bonding configuration of the surface atoms is possible. Table 5-4 showed that nitrogen incorporates into the SiOx film for samples annealed in NH3 [Mas01].

Figure 5-19. Electron Spectroscopy of Chemical Analysis (ESCA) spectra of Si 2p3 peak for Si annealed at 850 oC in 1.0 slm N2 (a) with 100% NH3 at 1.0 slm (b) without NH3.