Epitaxial relationships sapphire

Table 7.4 summarizes the out-of-plane and in-plane epitaxial relationships of ZnO films and sapphire substrates, the c-axis and a-axis lattice constants of ZnO, the ZnO full peak widths at half maximum (FWHM) of 20-to and uj scans, and the tilt of the ZnO structure along surface normal [47]. Because of the low intensity of the asymmetric (10l4) reflection, the a-lattice constant has larger uncertainty compared to the c-axis lattice constant. The epitaxial relationships correspond to the results of Ohtomo (see Table 1 in [22]). 

Growth on (1120) A-plane sapphire yields an epitaxial relationship of (0001)g,n//(1 120)ai2O3 with inplane orientations of [1120]g.n//[0001]Ai2O3 and [1120]g,n//[1120]ai2O3 [2,4], This results in approximately a 30% lattice mismatch for both in-plane orientations by comparing the oxygen sublattice distance with the GaN lattice. 

Surface orientation of the sapphire substrate used for most growth of GaN and its alloys is the c-plane, i.e. (0001), but growth on other orientations such as the a-plane (11-20) and r-plane (1-102) is also conducted. The epitaxial relationship is summarised in TABLE 4 [23], There is a discussion on the relationship on the (01-12) surface in [23], The relationships on the (0001) surface are schematically illustrated in FIGURE 3 [24,25],... 

FIGURE 3 Schematic illustration of epitaxial relationship between GaN and sapphire (0001) (a) (b) shows the top view, GaN (closed circles) and sapphire (open circles) representing the rotation of the GaN basal plane by 30°. 

Erom a crystallographic point of view, (1120) a-plane SiC is a better substrate for epitaxial growth of a-plane GaN than the r-plane sapphire. First, the layer and substrate have identical crystal orientation and the epitaxial relationships... 

In short, MgO on sapphire is initially of wurtzitic nature for 1 nm or so, which gives way to the rocksalt symmetry as deposition continues. If ZnO is grown on wurtzitic MgO, O polarity ZnO layer is obtained. On the contrary, if the rocksalt phase of MgO is allowed to form, ZnO grown on it is of Zn polarity [97]. The in-plane epitaxial relationship between ZnO and sapphire in the latter case is [2il0]2 o//... 

Figure 2.35 Epitaxial relationships for o-plane ZnO grown on R-plane sapphire. The relative orientation of ZnO on sapphire is shown in the boxed area as well.

The nature of the termination layer, i.e, either N or Ga, is expected to be dependent on polarity matching considerations between the substrate and film. Recent ab initio total energy calculations by Capaz, et al. (17) have shown that the lowest energy interfaces are expected to be those on which cations(anions) of the substrate bind to anions(cations) of the film. Hence, the final polarity of the film is strongly dependent on the polarity of the substrate. Therefore, the nature of the atomic termination layers on sapphire surfaces is important to the understanding of the effects of substrate structure on the epitaxial relationship and overlayer lattice structure of thin film deposition. However, the exact termination elements of the... 

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