금속 갈륨과 암모니아의 직접반응에 의한 GaN 후막성장과 특성 연구

안상현1, 양승현2, 이상현2, 남기석1,2

전북대학교 반도체과학기술학과1, 화학공학부2

High Temperature Vapor Phase Growth and Characterization of GaN

by the Direct Reaction of Ga and NH3

1Sang Hyun Ahn, 2Seung Hyun Yang, 2Sang Hyun Lee and 1,2Kee Suk Nahm

1Department of Semiconductor Science & Technology

2School of Chemical Engineering & Technology and

Chonbuk National University, Chonju 561-756, Republic of Korea

1. Introduction

Group Ⅲ-nitrides, such as GaN, InN and AlN are wide direct band-gap semiconductors because of their excellent physical properties, including high mechanical, thermal and chemical stability, and high thermal conductivities. Blue/green light-emitting diodes (LEDs) have already been fabricated using GaN/InGaN single quantum-well (SQW) structures[1].

Owing to the lack of GaN structures, sapphire has been mostly commonly used as a substrate for the growth of GaN epilayers although it has large lattice mismatch(13 %) with GaN[2]. Thick GaN is a promising substrate for the growth of defect-free GaN epilayers. The growth of thick GaN has been attempted by using various method, such as hydride vapor-phase epitaxy (HVPE)[3], sublimation of amorphous GaN powders[4], and the direct reaction of metallic Ga with NH3 using high-temperature vapor-phase epitaxy (HTVPE)[5].

In this work, we have grown thick GaN films on sapphire by the direct reaction of gallium and ammonia in a conventional RF-inductively heated chemical vapor deposition (CVD) reactor. GaN films were grown at various growth conditions. The structure and the optical properties of the films were characterized and correlated with each other to suggest the origin of the yellow luminescence.

2. Experiment

Thick GaN films were grown on (0001) sapphire using RF inductively heated CVD reactor originally in our previous report[6]. Metallic Ga (0.1g) was placed in a hole dug on the lower part of the graphite susceptor. The sapphire substrate was mounted vertically to the Ga source at the distance of 7 mm in a cylindrical graphite crucible consisting of two parts, and injected NH3 into the gap between the Ga source and the substrate. The temperature of the susceptor was measured using an optical pyrometer and calibrated with a Pt/Ru thermocouple. Metallic Ga (0.1g) was placed in a hole of the graphite susceptor and □-sapphire substrate was used the size of 10×10mm2.

The reactor was evacuated for 30 min. For cleaning the sapphire surface with hydrogen was carried out for 10min at 540 ℃. The thick GaN films were grown for 60 min at 760 Torr, varying the temperature at the range of 1070 ℃ ∼ 1270 ℃ and supplying with NH3 of 150 ∼ 300 sccm. The distance between the Ga source and sapphire substrate at the intervals of 7 mm.

X-ray diffraction (XRD) and Raman spectroscopy were employed to investigate the structural properties of thick GaN films. The surface and cross-sectional morphologies were examined using a scanning electron microscope (SEM). The optical properties of the thick GaN were determined using photoluminescence (PL) excited by using the 325-nm line of a He-Cd laser.

3. Results and discussion

Shown in Fig. 1 are the XRD spectra for thick GaN grown as a function of growth temperature (1070 ∼ 1270 ℃) with a 200 sccm NH3 gas flow. The figure reveals that the growth of (100), (0002), (101), (102), (100) and (0004) plane is included in the GaN films. Sharp GaN (0002) and (0004) peaks are observed at 2□=34.4°and 72.75°respectively in the XRD spectra for the GaN films grown at 1070 and 1170 ℃. The intensity of the GaN(0002) peak was much higher than that of GaN(0004) at both temperatures. For the GaN films grown at 1070 ℃, a strong GaN(101) peak appears at 2□ = 36.7°, but the GaN(100) peak does appear. However, weak (100), (101), and (102) peaks are observed from the GaN films grown at 1170 ℃. The growth of the GaN (110) plane occurs in the GaN films grown at all the temperatures. This implies that the GaN films grown on sapphire substrates mostly have a wurtzite structure, but the structure of the GaN films turns into polycrystalline phase with various crystal orientations as the growth temperature increases. This demonstrate that the growth temperature has a significant effect on the crystal orientation of GaN films grown.

Many researchers have observed the inclusion of various crystal planes in hexagonal GaN films, depending on the growth condition. Roman and Adriassen observed that the XRD signal of the (100) plane appeared during the growth of GaN films at low temperatures and pressures. Hashimoto et al. detected the XRD signal of the (100) plane from GaN films grown on 3C-SiC/Si substrate[16].

Our experimental results showed that at 1070 ℃, primarily GaN (101) was grown while at 1170 ℃, the 101) plane to go, but conditions were unfavorable for the growth of the (101) plane

Figure 2 shows SEM photographs for the GaN thick films grown at 1070 and 1170 ℃. For the GaN films grown at 1070 ℃, the SEM photograph shows that the GaN crystals are in the shape of a triangle and that the surfaces of the films had very rough V-shaped features, resulting in the observation of pinhole formation on the surface. The XRD data of Fig. 2(a) show that the GaN films predominantly grew along the c-plane, mainly mixed with the (101) plane, at 1070 ℃. It has been reported that the growth of hexagonal GaN along the [101] direction formed a V-shaped surface[17]. For the case of the GaN(0001) films containing the (101) plane, the crystal structure had needle-like habits with (101) pyramidal faces, as shown in Fig. 2(a).

Meanwhile, Fig. 2(b) shows that the GaN crystal grows with plate-like habits with (100), (101), and (102) pyramidal faces at 1170 ℃. Figure 2(b) revealed that small amount of the (100), (101), and (100) planes were included in the GaN films grown at 1170 ℃ although the films grew primary along the c-plane. The plate-like crystals were characterized by a hexagonal shape with a flat-top surface, as shown in Fig. 2(b). The results of SEM shown in Fig. 2 for the GaN thick films are consistent with the XRD data given in Fig. 1.

Figure 3 shows PL spectra measured at room temperature for the GaN thick films. The GaN films grown at 1070 and 1170 ℃ emitted strong blue spectrum, the yellow emission being significantly suppressed. The FWHM values of the band-edge blue peak observed from the GaN films grown at 1070 and 1170 ℃ were 156 and 111 meV with peak energies of 3.394 and 3.404 eV, respectively. In addition, the peak intensity of the yellow emission for GaN grown at 1170 ℃ is strongest than that from the GaN grown at 1070 ℃. When these results are compared with the XRD observations, c-plane GaN is seen to exhibit strong blue emission whereas GaN with other orientations emits yellow light. This clearly shows that yellow emission was closely related to the growth direction of GaN thick film.

4. Conclusion

GaN films were grown on (0001) sapphire substrates by using direct reaction of gaseous gallium and ammonia. The GaN films grown at 1070 ℃ including the (101) plane, which appeared at 2□=36.7°in the XRD pattern. Growth of the (100) and (101) planes was detected in the GaN films grown at 1070 ℃. The SEM observation showed that the GaN films containing the (101) plane had very rough V-shape surface due to the growth of GaN crystals in the shape of a triangle. The inclusion of the (100), (101), and (102) planes in the GaN films resulted inplate-like crystals with hexagonal shapes and flat-top surfaces. The crystal and the optical qualities of the GaN films improved with increasing NH3 flow rate. The intensity of the yellow emission increased with increasing (100) peak intensity. The growth rate of the GaN thick films increased with the growth temperature, but decreased at 1270 ℃ due to the decomposition of GaN into elementary Ga and N. The growth rate of the GaN films increased with the NH3 flow rate.

5. References

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5) S. Fischer, C. Wetzel, W. L. Hansen, and E. D. Bourret-Courchesne, Appl. Phys.      Lett., 69(18), 2716 (1996).

6) K. C. Kim, H. W. Shim, E.-K. Suh, H. J. Lee, and K. S. Nahm, J. Korean         Phys. Soc., 32(4), 558 (1998).

7) T. Hashimoto, O. Imafuji, M. Ishida, Y. Terakoshi, T. Sugino, A. Yoshikawa,

K. Itho, and J. Shirafuji, J. Cryst. Growth, 169, 185 (1996).

8)

Fig. 1. XRD spectra for GaN films grown for 60 min with 200 sccm NH3 at (a) 1070 ℃, (b) 1170 ℃, and (c) 1270 ℃.

Fig. 2. Cross-sectional and surface SEM images for thick GaN grown under the conditions of Fig. 2(a) and (b).

Z. Liliental-Weber, Y. Chen, S. Ruvimov, and J. Washburn, Phys. Rev. Lett. 79,      2835  (1997).

Fig. 3. PL spectra measured at room temperature for GaN films grown for 60 min         with 200 sccm NH3 and at (a) 1070 ℃, (b) 1170 ℃, and (c) 1270 ℃.