Anisotropic In-Plane Strains in Non-Polar Al1-xGaxN (120) Films (x<0.2)
Tetsuya Akasaka, Yasuyuki Kobayashi, and Makoto Kasu
Materials Science Laboratory
Nitride semiconductor films with non-polar faces, such as (100) and (1 20) faces, are attracting much attention, because emission efficiency of quantum wells using the non-polar faces is higher than those using polar faces, such as (0001) face, by the elimination of piezoelectric polarization fields . On the other hand, SiC has been widely used as the substrate for heteroepitaxy of nitride semiconductors, because of its high electrical and thermal conductivities and the smaller lattice mismatches (〜1% for AlN). However, there is anisotropy in the lattice mismatches between nonpolar nitride semiconductors and SiC substrates and the lattice matching of the heteroepitaxy is complicated. For example, the a- and c-axes of AlN are 1.05% longer and 1.13% shorter than those of SiC, respectively. In this study, we have grown Al1-xGaxN (1 20) thin films (x<0.2) on SiC (1 20) substrates and evaluated the lattice constants, in-plane strains and crystallinity of the films.
The pseudomorphic growth (lattice-matched with substrate) is essential for heteroepitaxy for reducing the densities of dislocations and stacking faults. Figure 1 shows dependence of in-plane strains in Al1-xGaxN (1 20) thin films on the Ga composition. The in-plane strains along the [1 00] and  directions, εxx and εzz, are on the pseudomorphic lines at the Ga compositions lower than 0.06, which means that the in-plane lattice constants of the films are almost the same as those of SiC. This pseudomorphic growth is successfully achieved by balance between compressive stress along [1 00] and tensile one along  . Dependence of full width at half maximum (FWHM) of (1 20) X-ray rocking curves (XRC) is plotted as a function of the Ga composition in Fig. 2. The tilt angle toward , tilt , which is related to the density of stacking faults parallel to the (0001) plane, decreases by increasing the Ga composition. This is because the in-plane strain along the  direction, εzz, decreases by increasing the Ga composition (see Fig. 1).
The pseudomorphic growth of Al1-xGaxN (1 20) thin films (x<0.06) are achieved in this study. In addition, our findings can provide a basic guideline for the design of nonpolar light-emitting devices, because both the band structures and the polarization of the emitted lights can be modified by anisotropic strains.
 P. Waltereit et al., Nature 406 (2000) 865.
 T. Akasaka, Y. Kobayashi, and M. Kasu, Appl. Phys. Lett. 93 (2008) 161908.
Fig. 1. In-plane strains along the  direction, εxx (●) and along the  direction, εzz (▲) plotted as a function of the Ga composition.
Fig. 2. Tilt angle along the  direction plotted as a function of the Ga composition.
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