phys. stat. sol. (c) 1, No. 10, 2483–2486 (2004) / DOI 10.1002/pssc.200405028

© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The effect of a slight mis-orientation angle of c-plane sapphire substrate on surface and crystal quality of MOCVD grown GaN thin films

Seong-Woo Kim1, Hideo Aida2, and Toshimasa Suzuki*, 1 1 Nippon Institute of Technology, 4-1 Gakuendai, Miyashiro, Saitama, 345-8501, Japan 2 NAMIKI Precision Jewel Co. Ltd., 3-8-22 Shinden, Adachi-ku, Tokyo, 123-8511, Japan

Received 29 June 2004, accepted 5 July 2004 Published online 16 August 2004

PACS 61.10.Nz, 68.37.Ps, 68.55.Jk, 81.15.Gh

The effect of a slight mis-orientation of c-plane sapphire substrate on the surface morphology and crystal quality of GaN thin films grown by MOCVD has been investigated . The mis-orientation angle of vicinal c-plane sapphire substrate was changed within the range of 0.00 (zero) – 1.00 (one) degree, and the expe-rimental results were compared with those on just angle (zero degree) c-plane sapphire substrate. The sur-face morphology and crystal quality were found to be very sensitive to mis-orientation angle. Consequent-ly, the mis-orientation angle was optimized to be 0.15°.

© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction GaN and the related compounds appear to be of considerable scientific and technologi-cal importance for many applications including blue lasers, light-emitting diodes (LEDs), and high-power, high-frequency field-effect transistors (FETs) [1, 2]. GaN-based devices have been fabricated by using metal-organic chemical vapor deposition (MOCVD). The slightly mis-oriented c-plane sapphire has been widely used as substrate after it was reported that the slight mis-orientation of the substrate markedly affected the surface morphology of the GaN within the range of 0.03 to 0.25 degree and the smoothest surface were obtained at approximately 0.17° [3]. However, to our knowledge, there are no reports dealing with the relationship between surface morphology and crystal quality of GaN thin films as a function of the mis-orientation angle of the sapphire substrate. In this study, we investigated the effect of a slight mis-orientation angle of vicinal c-plane sapphire substrate on surface and crystal quality of GaN thin films grown by MOCVD. The mis-orientation angle of was changed within the range of 0.00 (zero) – 1.00 (one) degree, and the experimental results were compared with those on just angle (zero degree) c-plane sapphire substrate. 2 Experimental The growth of 2 µm thick undoped-GaN films was performed at 1035 °C in an EM-CORE D-125 multi-wafer rotating disc low-pressure MOCVD system. Thirty nm thick GaN layers grown at 510 °C were used as nucleation layers (NLs). The reactor pressure was fixed at 200 Torr throughout the study. The GaN films were grown on both just (zero degree) and vicinal c-plane sapphire substrates. Prior to GaN growth experiment, mis-orientation angle of the substrate was measured by XRD. The measured mis-orientation angles of the substrates were 0.00°, 0.09°, 0.13°, 0.15°, 0.19°, 0.29°, 0.64°, and 1.00° toward (10–10) direction of c-plane of single crystal sapphire. The inclination angle of c-plane of single crystal sapphire toward (11–20) direction (toward orientation flat of sapphire substrate) was within 0.01° for all the substrates used in this study. Using our MOCVD system, we can

* Corresponding author: e-mail: tsuzuki@nit.ac.jp, Phone: +81 480 33 7720, Fax: +81 480 33 7745

2484 S.-W. Kim et al.: The effect of mis-orientation angle of sapphire substrate on GaN thin films

© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

grow GaN thin films on 3 pieces of sapphire substrate simultaneously. Therefore, 3 sets of growth ex-periment were taken for this study, as shown in Table 1. All the growth conditions were the same. AFM and X-ray rocking curve (XRC) for both symmetrical and asymmetrical reflections were applied to evaluate the surface and crystal quality of GaN thin films.

Table 1 The mis-orientation angle of the sapphire substrate measured by XRD.

Mis-orientations angles (degree)

Set-1 0.00 0.09 0.15 Set-2 0.15 0.29 1.00 Set-3 0.13 0.19 0.64

0.00° 0.13° 0.15°

0.19° 0.29° 0.64°

0.09°

Fig. 1 Macro area AFM images of the GaN surface grown on sapphire c-plane substrates with different mis-orientation angle. (400 µm × 400 µm)

Fig. 2 Micro area AFM images of the GaN surfaces grown on sapphire c-plane substrates with different mis-orientation angle. (2 µm × 2 µm)

0.00° 0.09° 0.13° 0.15°

0.19° 0.29° 0.64° 1.00° <1010> sapphire

1.00°

phys. stat. sol. (c) 1, No. 10 (2004) / www.pss-c.com 2485

© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

3 Results and discussion Figure 1 shows the macro area AFM images (400 µm × 400 µm) of GaN thin films grown on mis-oriented sapphire substrates. The spherical grain structure was observed on zero degree c-plane sapphire substrate. The grain size was around 10–30 µm in diameter. The spherical grain structure disappeared gradually as mis-orientaiton angle increased, and the flattest surface was obtained on 0.15° mis-oriented substrate. Beyond this mis-orientation angle the surface morphology was changed to needle-like structure. The micro area AFM images (2 µm × 2 µm) of the samples are shown in Fig. 2. When the mis-orientation angle was small (0.00°, 0.09°) the direction of the micro-step was not uniform but formed sharp curveture. However, the direction of the micro-step inclined toward the mis-orientation direction as the angle increased. The terrace width of the micro-step decreased as mis-orientation angle increased from 0.00° to 0.29°, but it was almost constant over 0.29°. From the macro and micro AFM observation, it was suggested that the spherical grain structure and ununiform micro-step having sharp curvature were caused by dominated spiral growth mode. The growth mode shifted from spiral-dominated to step-flow with increasing mis-orientation angle from 0.00° to 0.29°. This shift led to the improvement of micro-step uniformity and decrement of terrace width of the micro-step, resulting in the flat macro structure. However, the excessive step-flow growth mode seemed to appear when the mis-orientation angle was over 0.29°. This mode in micro region assosiated with the formation of needle-like structure in macro area. Figure 3 shows surface roughness measured by both macro and micro area AFM. The smoothest surface was obtained on 0.15° mis-oriented substrate. The surface roughness in both macro and micro area on just angle (zero degree) or on 1.00° mis-oriented substrate was several times higher than that on 0.15° mis-oriented one. This indicates that the surface morphology was easily affected by mis-orientation angle of the substrate. The optimal mis-orientaiton angle for surface quality of GaN thin films was found to be 0.15°. X-ray rocking curve (XRC) measurement for both symmetrical GaN (002) and asymmetrical GaN (201) reflections was applied to evaluate the crystal quality of GaN thin films. We empirically understand that crystal quality of GaN thin films differs from run to run in spite of same growth conditions. The variation of crystal quality between sets measured by XRC was about 30arcsec and 80arcsec for (002) and (201) reflections, respectively. Considering these variations, the FWHM of XRC for GaN (002) and GaN (201) reflections was plotted as shown in Fig. 4. Both of them were drastically improved as mis-orientation angle increased from 0.00 to 0.10 degree, but the improvement of crystal quality became more slightly as mis-orientation angle increased above 0.10° mis-orientation. The FWHM of the symmetrical GaN (002) and asymmetrical GaN (201) reflections was correlated to the mixed and edge threading dislocations (TDs) densities, respectively [4, 5]. Therefore, it was ascertained that TDs tended to decrease as mis-orientation angle of

0

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RM

S (

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400 µ

m)

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m ×

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m)

Fig. 3 RMS of GaN grown on sapphire c-plane substrates with different mis-orientation angle. Squares represent the average values in observation area of 400 µm square, and triangles represent those of 2 µm square.

2486 S.-W. Kim et al.: The effect of mis-orientation angle of sapphire substrate on GaN thin films

© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

the substrate increased [6]. The improvement in TD densities seemed to be caused by mis-orientation which can relieve the lattice mismatch between GaN and sapphire substrate [7] and the growth mode shift from spiral-dominated to step-flow. Comparing Fig. 4 with Fig. 3, it was found that the effect of mis-orientation angle of substrate on GaN crystal quality was different from that on GaN surface morphology. As mentioned above, the optimal mis-orientation angle for surface roughness of GaN thin films was 0.15°. On the other hand, the crystal quality improved with increasing mis-orientation angle, and the narrowest diffraction peak was obtained on 1.00° mis-oriented substrate by XRD in this study. In the greater mis-orientation angle region, the role of promoted step-flow growth of GaN thin films on surface mor-phology may be different from that on crystal quality. The excess of mis-orientation of substrate may prevent crystal quality from degrading due to low TDs density. However, the surface roughness degraded by excess of step-flow growth. Further intensive works are necessary to understand these phenomena. Here we point out that the optimal mis-orientation angle should be 0.15° in view of its great influence on surface quality of GaN thin films grown by MOCVD.

4 Conclusion We have investigated the effects of slight mis-orientation angle of vicinal c-plane sap-phire substrate on surface morphology and crystal quality of GaN thin films grown by MOCVD. The smoothest surface was obtained on 0.15° mis-oriented substrate. On the other hand, the crystal quality was slightly improved as mis-orientation angle increased. The surface/interface flatness of GaN films is one of the most important factors to realize highly efficient optical/electrical devices. The mis-orientation angle was optimized to be 0.15° in view of its great influence on surface quality of GaN thin films grown by MOCVD.

References

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Phys. 86, 6059 (1999). [7] J. M. Lee, Mater. Res. Soc. Symp. Proc. 67, 27 (1986).

Fig. 4 FWHM of XRC for GaN (002) and GaN (201) reflections plotted as a function of the mis-orientation angle of substrate.

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HM

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Mis-orientatiom angle (°)

Mis-orientatiom angle (°)