**2. Origin of small‐angle grain boundary (SAGB)**

The SAGB in AlN grown on (0001) sapphire substrate is considered to originate from the substrate's surface structure. As in any heteroepitaxial growth, the surface structure influences the growth mode. For example, the appearance of a defect structure on the substrate surface (e.g., protrusion) could possibly lead to spiral growth. It is therefore necessary to keep the surface free from any defects as possible. However, as‐received sapphire substrates are not free from any surface defects even after undergoing polishing treatment. This includes scratches on the surface, as shown in **Figure 1(a)**. Hence, thermal cleaning under H2 ambient is performed prior to AlN growth either at the same AlN growth temperature or slightly above it. After thermal cleaning, the substrate's surface transformed into parallel step‐ and‐terrace structure as shown in **Figure 1(b)**. The estimated step height from the line scan is about 0.21 nm.

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

The SAGB in AlN grown on (0001) sapphire substrate is considered to originate from the substrate's surface structure. As in any heteroepitaxial growth, the surface structure influences the growth mode. For example, the appearance of a defect structure on the substrate surface (e.g., protrusion) could possibly lead to spiral growth. It is therefore

is normal to the plane of the boundary.

44 Study of Grain Boundary Character

**2. Origin of small‐angle grain boundary (SAGB)**

**Figure 1.** AFM surface morphology of (a) as‐received and (b) after thermally cleaned sapphire substrate under H2 am‐ bient (Ph.D. Thesis, R.G. Banal, Kyoto University).

The crystal lattice of sapphire (α‐Al2O3) is formed by Al3+ and O2‐ ions. In Al2O3 corundum structure, O2‐ ions are shifted slightly from the idealized hexagonal close‐packed positions within the (0001) basal plane due to the empty octahedral sites (note that only two out of every three octahedral sites are occupied by Al3+ cations) as shown in **Figure 2(a)** [27, 28]. This results in the formation of two distorted oxygen hexagonal layers as also indicated in **Figure 2(a)** appearing alternately along the [0001] direction with monolayer (ML) periodicity (**Figure 2(b)**). The two distorted oxygen hexagonal layers are labeled as *A* and *B* stacking. By adapting the Thompson's notation [29], the sense of rotation of the distorted hexagon for each oxygen layer can be determined (**Figure 2(c)**). Hence, the successive oxygen layers with *AB* stacking (one ML step) create an opposing rotation, either inwardly or outwardly (**Fig‐ ure 2(d)**), while the *AA*(*BB*) oxygen stacking (two ML step) would have the same rotation direction either clockwise or counter‐clockwise [26, 29]. On the other hand, the coulombic repulsion between Al3+ cation causes each to move slightly toward the adjacent unoccupied octahedral site along the [0001] direction (perpendicular to the (0001) basal plane). This results in the formation of slightly puckered layer of Al in the basal plane, where it follows a face‐centered cubic‐type *abc* stacking. Taking into account the periodic spacing of both the cation and anion layers, the structure repeats itself after six oxygen layers and six double layers of Al3+ cation (= 0.1299 nm) [28]. Therefore, the step height between the *A* and *B* oxygen stacking is equal to 0.217 nm as the monolayer step (**Figure 2(b)**). Moreover, no such opposing in‐plane rotational geometry can be deduced from the successive Al hexagon layers in contrast to that of the distorted oxygen hexagons, suggesting that the origin of SAGB comes from the oxygen‐terminated surface of sapphire substrate.

**Figure 2.** (a) Schematic of distorted oxygen hexagon layers in *A* and *B* stacking. (b) Side view of the *A* and *B* oxygen stacking showing the one monolayer step height. (c) Adaptation of the Thompson's notation to identify the rotation of the distorted oxygen hexagons. (d) Identification of rotation of distorted hexagons from *A* and *B* oxygen stacking layer.

To confirm this hypothesis, the AlN epilayer was grown directly on thermally cleaned sapphire substrate, which is having a monolayer step‐and‐terrace structure. The AlN growth temperature was 1285°C and the AlN thickness was about 0.9 μm. The atomic force micro‐ scopy (AFM) surface morphology of AlN (**Figure 3(a)**) indicates step‐and‐terrace structure which replicates the surface of thermally cleaned sapphire substrate. The x‐ray diffraction

**Figure 3.** (a) AFM surface morphology of AlN grown directly on thermally annealed sapphire substrate (*T*g = 1285°C). (b) (10‐12) XRD *φ*‐scan of AlN showing twin peaks which correspond to two AlN grains rotated in the in‐plane. The (11‐23) phi‐scan of the sapphire substrate is also shown.

(XRD) *φ*‐scan measurement of the AlN (10‐12) asymmetric plane observed two peaks, which is attributed to the two periodic grains in AlN, as shown in **Figure 3(b)**. Moreover, because the XRD *φ*‐scan did not show any double domain structure from the sapphire substrate, the periodic domain is only observed in the AlN epilayer. In the previous study using similar growth method, cross‐sectional transmission electron microscope (TEM) analysis confirmed the existence of two grains as indicated by their periodic bright and dark contrast [26]. The plan‐view high‐resolution TEM observation further shows an array of edge dislocations along the grain boundary between the two AlN grains [26]. Hence, the monolayer step *ABAB* oxygen stacking is likely the origin of the small‐angle grain boundary. Therefore, the surface structure of the sapphire substrate must be prevented from having such structure or modified in order to effectively eliminate the SAGB.

Several techniques have been introduced to eliminate the rotation domain. These include pre‐ nitrogen radical treatment of the nitrided sapphire substrate [19] and post‐annealing after AlN growth [20]. However, these techniques not only entail an additional process but also obtain unsatisfactory results. Another technique is by thermal annealing in the air of sapphire substrate [26]. With the proper annealing temperature and off‐cut angle, a substrate surface with *AA*(*BB*) stacking and two monolayer step height can be achieved through step bunching. On the other hand, while the TMA preflow [25] and the LT‐AlN BL with pre‐nitridation [21] seem promising in situ methods to eliminate the small‐angle grain boundary, the influence of substrate's surface structure on its formation/elimination is not yet investigated in detail. Hence, the fragmentary understanding of the influence of surface structure is also evident after the substrate is being subjected to thermal cleaning prior to AlN growth [11, 14, 16, 18, 21, 24] or the lack of it [10, 13, 15, 22, 25]. Therefore, in this chapter, we study the influence of surface structure of sapphire substrate on the formation/elimination of SAGB to improve the quality of AlN epilayer. Then, we introduce the low‐temperature (LT) AlN buffer layer technique, with emphasis on its proper timing, to eliminate the SAGB.
