**1. Introduction**

Single crystal sapphire with corundum structure is a widely used substrate for film deposition and epitaxial growth in many technological fields, such as in optoelectronics for the growth of AlN, GaN, and InN nitride materials. This material exhibits high melting point (2050°C), extremely high chemical stability even at high temperatures, and transparency in the

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ultraviolet (UV) region, making it a suitable substrate especially for the growth of AlN, which requires high temperature above 1200°C due to the high viscosity of Al atoms. Sapphire also exhibits a hardness of 9 in the Mohs scale, compared to 10 for diamond. On the other hand, AlN is a promising material for UV and deep‐UV light emitters and power electronic devices because of its wide bandgap energy (6.05 eV), good stability at elevated temperature, high thermal conductivity (3.4 W cm‐1 K‐1) and high electric breakdown field (11.7 × 106 V cm‐1). Although the native bulk AlN or GaN substrates are already available for homoepitaxial growths, the utilization of sapphire as the substrate material for heteroepitaxial growth of AlN, GaN, InN, and other emerging materials is expected for the years to come, owing to its mature growth technology, availability of large size wafer, and cost advantage [1–2]. In fact, the advances in heteroepitaxial growths have already successfully demonstrated deep‐UV light‐emitting diodes (LEDs) and photo‐pumped AlGaN multi‐quantum well lasers [3–7]. However, the radiative emission efficiencies of deep‐UV light emitters are still low, prompting for further reduction of dislocations that act as nonradiative recombination centers [3–6].

The heteroepitaxial growth of AlN on sapphire substrate induces several types of dislocations that are driven by their lattice mismatch and difference in crystal structure. With lattice mismatch, a pseudomorphic growth initially occurs, followed by misfit dislocations after exceeding the critical thickness for plastic relaxation. A 30° rotation of AlN epilayer with respect to sapphire substrate in the basal (0001) plane occurs [8]. However, the development of various growth methods has improved the epitaxial quality of AlN in recent years. These growth methods include alternating supply of source precursors (e.g., modified migration‐ enhanced epitaxy (MEE)) [9–15], direct and high‐temperature growth [16–18], substrate pretreatment (e.g., nitridation) [19, 20], two‐step low‐temperature (LT) AlN buffer layer (BL) and high‐temperature (HT) growth [15, 21–23], multiple‐step V/III growth [18, 24], precursor preflow [25], and so on. However, despite the improvement in the surface morphology and structural quality of AlN epilayer, the existence of in‐plane rotation domain as exhibited by small‐angle grain boundary (SAGB) is still observed, regardless of growth method employed [16, 19–21, 25, 26]. This kind of defect must be eliminated as it can have a negative impact in the optical as well as electrical properties of the devices by acting as barriers for transport or carrier sinks. Small‐angle grain boundary is one type of special grain boundary which results when the two crystals have only a slight misorientation relative to each another. Moreover, this kind of special grain can be characterized as pure low‐angle tilt boundary or pure low‐ angle twist boundary, where the former is composed of an array of parallel edge dislocations, while the latter is characterized as the slight rotation of crystals about a common axis which is normal to the plane of the boundary.
