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Figure 2-19. Perspective views of zincblende GaN along various directions: (a) [100] (1×1×1 unit); (b) [110] (2×2×2 units); (c) [111] (2×2×2 units).

AlN normally has the wurtzite structure, although epitaxial layers of zincblende structure AlN have been made [Oku98a, Oku98b]. Wurtzite AlN has the space group of

P63mc (no. 186) as same as wurtzite GaN. The (0001) surfaces of AlN are polar, which has an important effect on its etching, bulk crystal growth and GaN epitaxy. AlN has the properties such as high thermal conductivity, low thermal expansion coefficient, high electrical resistivity, good dielectric properties, and excellent oxidation resistance.

2.5.4 Other Substrates

GaAs has the same structure as zincblende GaN. GaAs is less stable than SiC or sapphire. Above 800 °C its decomposition rate to liquid gallium and arsenic vapor is considerable. GaAs has the large lattice mismatch of 37.4 % for InN film.

Zinc oxide (ZnO) has a wurtzite structure and its stacking order match with lattice constants closely matched to GaN (a=3.249 Å, c= 5.205 Å). The small lattice mismatch of 8.8 % for InN makes ZnO attractive substrate for InN growth. Lithium gallate (LiGaO2) also has the small lattice mismatch of 11 % for InN film. Therefore, LiGaO2 is another candidate for the suitable substrate for InN growth.

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2.5.5 Buffer Layer

There is no lattice matched substrate available for InN so far. For example, the InN has a lattice mismatch of 25 % with sapphire, 8% with Si (111), 37.4 % with GaAs, and 11 % with GaN. High quality single crystalline InN is very difficult to be obtained because of these problems.

The two-step growth method or growth using buffer layer has now become a standard method for the heteroepitaxial growth of thin films. This method is commonly used to alleviate lattice mismatch and thermal expansion coefficient difference the substrate and epilayer. In this method, a thin buffer layer is grown at a low temperature in the first step. The main epilayer is grown in the second step at a high temperature. The buffer layer provides the high density of nucleation centers and promotes the lateral growth of the main epilayer. The two-step growth of InN is not well studied, especially in the MOVPE growth.

There are very few studies about the MOVPE growth of InN using buffer layer such as GaN, AlN, and InN. There is no significant report that use of low temperature InN buffer layers in the growth InN gives improvement. Pan et al. studied two-step growth of InN using conventional MOVPE [Pan99]. Based on their findings, they concluded that the two-step growth is not adequate for InN, which may correlate to the unstable nature of the InN film. Guo et al. reported that if a single crystalline InN film is heated above 550 oC in a N2 flow, the surface undergoes a considerable change, owing to the decomposition and desorption of nitrogen [Guo93].