Electronic Structure and Spontaneous Polarization in Sc_xAl_yGa_(1-x-y)N Alloys Lattice-Matched to GaN: A First-Principles Study

摘要

We performed first-principles calculations of the spontaneous polarization and electronic band structures in Sc_xAl_yGa_(1-x-y)N alloys assuming their growth on freestanding GaN. We found an apparent deviation from the Vegard's law of the lattice constants of the Sc_xAl_yGa_(1-x-y)N alloys lattice-matched to GaN. It was supposed that this deviation comes from the different bonding properties of IIIB and MIA nitrides, resulting in different crystal structures, such as hexagonal and wurtzite structures. As was reported in our previous report on Yc_xAl_yGa_(1-x-y)N [K. Shimada et al.: J. Appl. Phys. 110 (2011) 074114], we also found that in Sc_xAl_yGa_(1-x-y)N alloys, the superlattice-like structure of Sc atoms reduced the magnitude of spontaneous polarization [K. Shimada et al.: Semicond. Sci. Technol. 27 (2012) 105014]. The magnitude of the spontaneous polarization of Sc_xAl_yGa_(1-x-y)N is larger than that of Yc_xAl_yGa_(1-x-y)N in a wide mole fraction range of Ga. We found the nonlinearity and dependence of the atomic arrangement of Sc in the alloys. The band-gap energies at r have the same characteristics as the spontaneous polarization. The band-gap energies of Sc_xAl_yGa_(1-x-y)N are also larger than those of Yc_xAl_yGa_(1-x-y)N in the wide mole fraction range of Ga. The band structures of Sc_xAl_yGa_(1-x-y)N have a direct gap at r and form a flat band around the valence band top originating from the hybridization of the Sc 3d and N 2p electrons.