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5.1.5. Indium Nitride (InN) Growth and Optimization

5.1.5.1. Influence of Growth Temperature

Among all the factors, the growth temperature has the largest impact on the epitaxial film quality in terms of the growth habit (single crystalline or polycrystalline) and the structural quality. The growth habit was determined by XRD θ-2θ scan (XRD Philips APD 3720) and the structural quality was judged by evaluating the FWHM of X- ray Rocking Curve (XRC) (Philips MRD X'Pert System).

For Al2O3 (0001), a low temperature GaN buffer layer (LT-GaN) grown at TLT-GaN = 560 oC, was employed after the nitridation at 850 oC for 15 min. The TMI flow rate was fixed at 0.26 sccm, NH3 at 800 sccm, and the N/In ratio was 3000. In this growth condition, indium droplets (metal) formed on the surface in addition to InN (0002) as shown in the XRD spectrum in Fig. 5.5.

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

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With the reference to the peak position of indium droplets that solidified upon cooling, peaks are anticipated at 36.3° corresponding to In (002), 39.2° In (110), 54.5° In (112), 67.0° In (103), and 69.1° In (202). As shown in Figure 5-5, it is clear that In formed.

Indium droplet formation was found to occur when the N/In ratio was low, as will be confirmed by the results of subsequent the study in which the N/In ratio was varied. Wet etching was used to remove the indium with a 15 % HCl solution.

It is noted that the InN (10-11) peak at 33.1o is close to the In (101) reflection at 32.9o, thus the two peaks are likely to overlap. After the indium droplets were removed by etching with HCl, the underlying InN film was characterized again by taking a XRD

θ-2θ scan.

For Al2O3 (0001), single crystal InN (0002) film was obtained and the peak intensity of InN (0002) was the greatest at T = 550 oC. However, InN (10-11) occurred at T = 650 oC, which indicates that the growth direction of InN depends on the growth temperature.

A very broad and low intensity peak for InN was observed when growth was at T = 450 oC, indicative of poor quality InN. No growth of InN was observed at T = 750 oC (Fig. 5.6). It is generally accepted that InN can not be grown at a growth temperature above 700 oC due to the InN thermal decomposition, nor below a growth temperature of 400 oC due to the low decomposition efficiency of NH3. From these results, it was found that the optimum growth condition is 550 oC.

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Figure 5-6. X-ray Diffraction (XRD) θ-2θ scan for InN/LT-GaN on (a) Al2O3 (0001) at N/In = 3000, T = 450, 550, 650, and 750 oC. Pure In was removed by etching with HCl.

Growth Temperature The growth temperature effect was studied over a more limited set of temperatures: 530, 550, and 570 oC to fine tune the optimized growth temperature. A relatively small flow rate of TMI in the range 13 to 44 sccm, and high flow rate of NH3

(1600 sccm), gave a high N/In ratio of 50,000 to prevent the indium droplet formation during the growth. As expected, single crystalline InN was grown without indium droplet formation at this growth condition.

For Al2O3 (0001), the intensity of the peak of the InN (0002) reflection was lower at growth temperatures 530 and 570 oC while the InN (10-11) reflection was evident at T = 570 oC. Therefore, it is appears that the optimum growth temperature of InN film is in the vicinity of 550 oC on Al2O3 (0001) with a LT-GaN buffer layer.

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Figure 5-7. X-ray Diffraction (XRD) θ-2θ scan for InN/LT-GaN on Al2O3 (0001) at N/In = 50,000, T = 530, 550, and 570 oC.

The growth temperature of LT-InN buffer layer was also examined in the range 500 to 550 oC. The result is an optimum growth temperature for the InN/LT-InN of 530 oC. Polycrystalline InN with the appearance of both InN (10-11) and InN (0002) reflections occurred at 500 and 550 oC, while a single reflection, InN (0002), was the strongest at 530 oC (see Fig. 5.8).

Figure 5-8. X-ray Diffraction (XRD) θ-2θ scan for InN/LT-InN on Al2O3 (0001) at N/In = 50,000 and T = 500, 530, and 550 oC.

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The FWHM of the XRC (X-ray rocking curve) for InN on Al2O3 (0001) at N/In =

50,000 and T = 530 oC was 4860 arcsec (see Fig. 5.9).

Figure 5-9. Full Width Half Maximum (FWHM) of XRC for InN/LT-InN on Al2O3 (0001) at N/In = 50,000 and T = 500, 530, and 550 oC.

A similar set of runs (T = 500, 530, and 550 oC, and N/In = 50,000 with LT-InN

buffer layer) was conducted using GaN/ Al2O3 (0001) substrates. Again polycrystalline

InN was observed at the 500 and 550 oC growth temperatures, while single crystalline

InN (0002) appeared at 530 oC (see Fig. 5.10).

Figure 5-10. X-ray Diffraction (XRD) θ-2θ scan for InN/LT-InN on GaN/Al2O3 (0001) at N/In = 50,000, T = 500, 530, and 550 oC.

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The FWHM of the XRCs for the InN film grown at T = 530 and 550 oC are shown in Fig. 5.11, with the best crystallinity obtained at T = 530 oC with a FWHM of the XRC of 1039 arcsec. Therefore, the optimum growth temperature of InN/LT-InN is near 530 oC.

Figure 5-11. Full Width Half Maximum (FWHM) of XRC for InN/LT-InN on GaN/Al2O3 (0001) at N/In = 50,000, T = 500, 530, and 550 oC.

Finally for the third substrate examined, Si (111), InN was grown at T = 500, 530, 550, and 570 oC, and N/In = 50,000 with two buffer layers: LT-GaN and LT-InN. A weak InN (0002) reflection appeared in the films grown at all growth temperatures (see Fig. 5.12 and 5.13). With the LT-GaN buffer layer, the strongest peak of single crystalline InN (0002) occurred at T = 530 oC, while polycrystalline InN (InN (10-11) and InN (0002) reflections) occurred at T = 550 oC (see Fig. 5. 12). For a LT-InN buffer layer on Si, a strong single peak of was recorded, InN (0002), also at T = 530 oC (Fig. 5. 13). The peak intensity of InN grown on Si substrate was smaller compared to those grown on either Al2O3 or GaN/Al2O3. This is believed to be due to difficulty in wetting InN on Si. From these results, the optimum growth temperature is thought to be 530 oC on Si (111) with both LT-GaN and LT-InN.

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Figure 5-12. X-ray Diffraction (XRD) θ-2θ scan for InN/LT-GaN on Si (111), at N/In = 50,000, T = 500, 530, 550, and 570 oC.

Figure 5-13. X-ray Diffraction (XRD) θ-2θ scan for InN/LTInN on Si (111), at N/In = 50,000, T = 500, 530, 550, and 570 oC.

Growth Rate Studies The growth rate was determined as a function of the N/In ratio on Al2O3 (0001). In these studies InN was grown for 1 hr at selected growth temperatures in the range 450 to 650 oC (N/In = 3000) and 500 to 550 oC (N/In = 50,000) to determine if the growth is chemical reaction-limited or transport-limited. The thickness was measured

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on cross-sections by SEM (FEG-SEM JEOL JSM 6335F) and the results are displayed in Figures 5.14 and 5.15. As shown in Fig. 5.14, the temperature dependence changed depending on the kind of substrate, presumably because of different texture of the InN. Figure 5- 14 (a) and (b) show that the growth rate remained unchanged in the range 550 to 650 oC at N/In = 3000 and in the range 530 to 570 oC at N/In = 50,000 for Al2O3

(0001), which is expected for a mass transfer limited growth condition. For N/In = 3000 run, the low growth rate (T = 450 oC) was a result of the lower efficiency of NH3 decomposition at the relatively low temperature (Fig. 5.14 (a)). For the N/In = 50,000 condition on Al2O3 (0001) the growth rate remains unchanged in the range 530 to 570 oC (Fig. 5.14 (b)), and thus mass transport-limited. The same results is seen for GaN/Al2O3

(0001) and Si (111) (Fig. 5. 14 (c) and (d)). Furthermore the growth rate is independent of the substrate for the last 3 conditions. When the growth rate was compared between the N/In ratios of 3000 and 50,000, the rate at N/In = 3000 is higher than that at N/In = 50,000 because of the increased flow rate of TMI, the limiting reagent, from 0.03 to 0.26 sccm.

Figure 5-14. Growth rate of InN on various substrates (a) InN/LT-GaN on Al2O3 (0001) at N/In = 3000, (b) for InN/LT-GaN on Al2O3 (0001) at N/In = 50,000, (c) InN/LT-InN on GaN/Al2O3 (0001) at N/In = 50,000, and (d) InN/LT-InN on Si (111) at N/In = 50,000