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October 6, 2005

Epitaxial Tilting of GaN Grown on Vicinal Surfaces of Sapphire

X.R. Huang¹, J. Bai¹, M. Dudley¹, R.D. Dupuis², and U. Chowdhury^2
1Department of Materials Science and Engineering, Stony Brook University, Stony Brook, NY; ²School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA

Using the synchrotron Laue method and high-resolution x-ray diffraction, we have revealed the epitaxial tilting effect of gallium nitride (GaN) films grown on vicinal (0001) surfaces of sapphire, and their relationship to the offcut angles and the substrate surface steps. This effect is a consequence of the large out-of-plane lattice mismatch between GaN and sapphire, and can be explained by an extended Nagai theory. The large lattice tilts and their formation mechanism indicate that the substrate surface morphology is an important factor that affects the epitaxy process and the crystalline quality of GaN films grown via vicinal-surface epitaxy.

Authors (from left): Xian-Rong Huang, Michael Dudley, and Jie Bai.

Although the growth of III-nitrides on sapphire (Al₂O₃) has been extensively investigated, little attention has been paid on the large c lattice mismatch (Δc/c 20% for GaN/sapphire), particularly the case of vicinal surface epitaxy (VSE). We have recently demonstrated that GaN grown on a vicinal sapphire surface is generally tilted from the substrate (known as a Nagai tilt). For 1-bilayer steps of sapphire, shown in Figure 1(a), the tilt α is related to the offcut angle θ by the relationship tanα = -(Δc/c)tanθ 0.2tanθ (<0). For step bunches, however, this principle may change. In Figure 1(c), due to 3c 2.5c_e, the GaN lattices can be tilted in two directions with equal probabilities, tanα ±0.2 tanθ, leading to α 0 on average. For 4-bilayer steps, the GaN lattice is tilted toward the offcut direction by tanα = [(4c 3c_e)/(4c)]tanθ 0.1 tanθ (>0). Similarly, the tilt for 5-bilayer steps is tanθ = [(5c 4c_e)/(5c)]tanθ 0.04 tanθ (>0).

Figure 1. Epilayer tilts induced by steps of various heights in VSE of GaN on sapphire. ns - substrate surface normal, nc - offcut direction.

The above principles have been explicitly verified by synchrotron Laue patterns and high-resolution x-ray diffraction performed on vicinal GaN/sapphire. GaN films 4 m thick were grown using low-pressure metal organic chemical vapor deposition (MOCVD) in a temperature range of 1020-1080°C. Figure 2 shows the back-reflection Laue patterns of three samples, where the displacement of the 0001 spot from O (white arrows) is caused by the offcut, and the displacement of the GaN 0001 spot from that of sapphire reflects the epilayer tilt, which is always parallel to the offcut direction n_c. In Figure 2(a), the measured tilt (negative) is in perfect agreement with the tilting model in Figure 1(a) and 1(b), indicating that on the small-offcut substrate the steps are dominantly 1- or 2-bilayer steps. For the 6.29°-off sample, the tilt shows that steps of various types coexist on the substrate surface, with 1- or 2-bilayer steps contributing to negative tilts and the other types reducing this tendency. The positive tilt of the 10.6°-off sample shows that 4- or 5-bilayers steps outweigh the other steps.

Figure 2. Laser-guided back-reflection synchrotron Laue patterns of vicinal GaN/Al2O3 samples showing the epitaxial tilts. The film-to-sample distance is 30 cm. Inset: a side view of the diffraction geometry.

The tilting effect reveals a series of important mechanisms of GaN epitaxy that have been widely ignored in the past, including: 1) the sapphire surface morphology can greatly affect the epitaxy process and the crystalline quality of GaN films; 2) drastic step bunching and faceting of sapphire can occur during nitridation; 3) mismatch strains are dominantly relaxed by pure basal plane dislocations without out-of-plane Burgers vector components; and 4) the epilayer tilt-related shear stresses are believed to significantly affect the growth kinetics and strain relaxation. In our latest work, we also found the tilting effect and the related mechanisms in GaN/SiC systems [X.R. Huang et al., PRL 95, 086101 (2005)], where, compared to flat-surface epitaxy, VSE shows explicit advantages for relaxing strains and improving epilayer quality.

BEAMLINE
X19C

FUNDING
Office of Naval Research
U.S. Department of Energy - Office of Basic Energy Sciences

PUBLICATION
X. R. Huang, J. Bai, M. Dudley, R. D. Dupuis, and U. Chowdhury, Epitaxial tilting of GaN grown on vicinal surfaces of sapphire, Applied Physics Letter 86, 211916 1-3 (2005).

FOR MORE INFORMATION
Prof. Michael Dudley
Department of Materials Science and Engineering
Stony Brook University
Stony Brook, NY
Email: mdudley@notes.cc.sunysb.edu