Effect of substrate mis-orientation on GaN thin films grown by MOCVD under different carrier gas condition

phys. stat. sol. (c) 2, No. 7, 2170–2173 (2005) / DOI 10.1002/pssc.200461582

© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Effect of substrate mis-orientation on GaN thin films grown by MOCVD under different carrier gas condition

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 16 July 2004, revised 25 August 2004, accepted 27 January 2005 Published online 17 March 2005

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

We have studied the effect of a slight mis-orientation angle on surface and crystal quality of GaN thin films grown under different carrier gas conditions. Two types of carrier gas conditions were applied to the growth. One was pure H2 and the other was mixed N2/H2. As the result, we found dependence of surface and crystal quality of GaN thin films on the substrate mis-orientation angle, and they indicated almost the same tendency under both growth conditions. Therefore, it was confirmed that mis-orientation angle of sapphire substrate was one of the most critical factors for GaN thin films. Then, the effect of the addi-tional N2 into the conventional H2 carrier gas was studied, and we found that the conversion of carrier gas from the conventional H2 to N2/H2 mixture was effective against degradation of GaN crystallinity at any mis-orientation angle. Considering that the crystal quality of GaN thin films became insensitive to mis-orientation angle as the condition became more suitable for GaN growth, the optimal substrate mis-orientation angle was consequently decided to be approximately 0.15° from the morphological aspect.


1 Introduction GaN-based devices have been successfully fabricated by metal-organic chemical vapor deposition (MOCVD). A slightly mis-oriented c-plane sapphire had been widely used as a substrate since it was reported that the smoothest surface of GaN films was obtained on approximately 0.17° mis-oriented sapphire substrate [1]. We reported not only advantages of using mis-oriented sapphire substrate but also the effect of mis-orientation angle on crystal quality which was different from that on surface morphology [2]. The best result was obtained at 0.15° from the morphological aspect, and 1.00° from the crystalline aspect. This raised a question which angle was more suitable for GaN growth. In this work, we tried to improve the GaN crystallinity by the converting of carrier gas from the con-ventional H2 to N2/H2 mixture [3, 4], and then carried out further expanded study on mis-orientation angle of c-plane sapphire substrate to confirm the effects on surface and crystal quality of GaN thin films and to lead the conclusive determination of optimal mis-orientaion angle. Furthermore, the effect of addi-tional N2 gas into the conventional H2 gas was discussed briefly.

2 Experimental We conducted the growth of undoped-GaN films using an EMCORE D-125 multi-wafer rotating disc low-pressure MOCVD system. The growth conditions were the same as the previous work [2] except for the carrier gas. The LT-GaN layer was grown under conventional H2 carrier gas condition. After the growth of LT-GaN layer, N2 gas started to be added into the carrier gas as well as the temperature started being ramped up. Prior to the epitaxial growth, the mis-orientation angle of all the substrates used for this study was measured by x-ray diffraction (XRD) with high accuracy. Using our

* Corresponding author: e-mail: [email protected], Phone: +81 480 33 7720, Fax: +81 480 33 7745

phys. stat. sol. (c) 2, No. 7 (2005) / www.pss-c.com 2171


MOCVD system, we can grow GaN thin films on three pieces of sapphire substrate simultaneously. Therefore, four sets of growth experiment were taken for this study, as shown in Table 1. Atomic force microscopy (AFM) and X-ray rocking curves (XRCs) for both symmetrical and asym-metrical reflections were applied to evaluate the surface and crystal quality of GaN thin films, respectively.

3 Results and discussion First, we observed the morphological change of GaN surface concerned with the mis-orientation angle and N2 gas addition by AFM. The macro area AFM images (400 µm x 400 µm) of GaN thin films with or without N2 addition are shown in Fig.1. The surface of GaN film was evolved gradually from the spherical grain structure, flattest surface, to needle-like structure as mis-orientation angle increased. This tendency was quite the same in spite of the different carrier gas conditions.

The micro area AFM images (5 µm x 5 µm) of the samples are shown in Fig. 2. When the mis-orientation angle was small (0.00°) the direction of the micro-step was not uniform but formed sharp

Table 1 Mis-orientation angle of the sap-

phire substrate.

Mis-orientation angle (°)

Set-1 0.00 0.20 0.60

Set-2 0.06 0.15 0.25

Set-3 0.30 0.40 0.60

Set-4 0.15 0.80 1.00

H2+N2

(a) 0.00° (b) 0.15° (c) 0.20° (d) 0.60°

H2

(e) 0.00° (f) 0.15° (g) 0.19° (h) 0.64°

Fig. 1 Macro AFM images of GaN films on mis-oriented substrate with (a–d) and without (e–h) N2 addition.

H2+N2

(a) 0.00° (b) 0.15° (c) 0.20° (d) 0.60°

H2

(e) 0.00° (f) 0.15° (g) 0.19° (h) 0.64°

Fig. 2 Micro AFM images of GaN films on mis-oriented substrate with (a–d) and without (e–h) N2 addition.

2172 Seong-Woo Kim et al.: Effect of substrate mis-orientation on GaN thin films


curvatures. However, the direction of the micro-step inclined toward the mis-orientation direction as the angle increased. Both tendency were almost the same in spite of the different carrier gas conditions, suggesting that the mis-orientation angle helps the step-flow mode of GaN growth [2]. Using N2/H2 mixed carrier gas, the termination of surface micro-step by threading dislocations (TDs) decreased as compared with using pure H2 carrier gas. The step-flow structure was also observed more clearly at any mis-orientation angle. Hence the N2 addition seemed to improve the GaN crystal quality. Figure 3 shows the strong dependence of GaN surface quality on mis-orientation angle, indicating that the mis-orientation angle is one of the most critical factors for the surface morphology of GaN thin films. Consequently, the optimal mis-orientaiton angle for surface quality of GaN thin films was confirmed approximately 0.15° in spite of the different growth conditions [1, 2]. By the carrier gas conversion from

pure H2 to N2/H2 mixture, the surface roughness increased at any mis-orientation angle. This might be caused by suppression of N desorption from GaN surface by the N2 gas [5]. Then, the XRC measurement for both symmetrical GaN (002) and asymmetrical GaN (201) reflections was applied to evaluate the crystal quality of GaN thin films. In Fig. 4, the FWHMs of XRC for GaN (002) and GaN (201) reflections were plotted in conformity with the same manner of Ref. [2]. Both reflections were improved as mis-orientation angle increased in spite of the different carrier gas conditions. Thus, we found the dependence of GaN crystallinity on mis-orientation angle. But the dependence of crystal quality on mis-orientaiton angle under the condition with N2/H2 mixed carrier gas was weaker than that with pure H2 carrier gas. Here, we point out that the crystal quality of GaN films with N2 gas addition was better than that without N2 gas addition at any mis-orientation angle. As the XRC-FWHM was correlated to the TD densities [6, 7], it

was suggested that TD densities decreased by N2 addition into carrier gas. From the XRD results, it is found that the determination of the crystal quality was associated with the mis-orientation angle of the substrate. However, the crystallinity of GaN films seemed to be insensitive to mis-orientation angle as the condition became more suitable for GaN growth. Thus, we think the optimization of growth conditions is more important for crystal quality than that of mis-orientation angle, which is a contrast to the importance of mis-orientation angle for the morphological aspect.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.00 0.20 0.40 0.60 0.80 1.00

RM

S (

nm

) 5µ

mx5µ

m


0

5

10

15

20

0.00 0.20 0.40 0.60 0.80 1.00

RM

S (

nm

) 4

00µ

mx400µ

m


Fig. 3 GaN surface roughness of macro (a) and micro

(b) area as a function of mis-orientation angles.

�: H2

�: N2+H2

�: H2

�: N2+H2

(a)

(b)

phys. stat. sol. (c) 2, No. 7 (2005) / www.pss-c.com 2173


4 Conclusion We have investigated the effects of slight mis-orientation angle of vicinal c-plane sapphire substrate on surface morphology and crystal quality of GaN thin films grown by MOCVD under different carrier gas conditions. It was confirmed that the mis-orientation angle of sapphire substrate was one of the most critical factors for the surface morphology of GaN thin films. We also found the crystalline dependence on the mis-orientation angle. However, the crystal quality became insensitive to mis-orientation angle when we applied the more suitable growth condition which was achieved by using N2/H2 mixed carrier gas. Consequently, the suitable mis-orientation angle was decided to be approximately 0.15° from the morphological aspect.

References

[1] T. Yuasa, Y. Ueta, Y. Tsuda, A. Ogawa, M. Taneya, and K. Takao, Jpn. J. Appl. Phys. 38, L703 (1999).

[2] S-W. Kim, H. Aida and T. Suzuki, phys. stat. sol. (2005), to be published.

[3] S. Yamaguchi, M. Kariya, M. Kosaki, Y. Yukawa, S. Nitta, H. Amano, and I. Akasaki, J. Appl. Phys. 89, 7820

(2001).

[4] V. Wagner, O. Parillaud, H. J. Bühlmann, M. Ilegems, S. Gradecak, P. Stadelmann, T. Riemann, and J. Christen.

J. Appl. Phys. 92, 1307 (2002).

[5] Y. Kobayashi and N. Kobayashi, J. Cryst. Growth 189/190, 301 (1998).

[6] T. Metzger, R. Höpler, E. Born, O. Ambacher, M. Stutzmann, R. Stömmer, M. Schuster, H. Göbel,

S.Christiansen, M. Albrecht, and H. P. Strunk, Philos. Mag. A 77, 1013 (1998).

[7] H. Heinke, V. Kirchner, S. Einfeldt, and D. Hommel, Appl. Phys. Lett. 77, 2145 (2000).

200

300

400

500

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00

0

1000

2000

3000

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00



(20

1)

FW

HM

(ar

csec

) (0

02

) F

WH

M (

arcs

ec)

�: H2

�: N2+H2

�: H2

�: N2+H2

Fig. 4 Changes of the XRC-FWHM as a function of mis-orientation angles.

Documents

Effect of substrate mis-orientation on GaN thin films grown by MOCVD under different carrier gas condition