Piezoelectricity of ordered (Sc x Ga 1-x N) alloys from first principles

First-principles calculations are performed to compute the e 33 piezoelectric coefficients of GaN, ScN and (Sc x Ga 1-x N) alloys exhibiting an alternation of hexagonal GaN, with hexagonal ScN along the c-axis. For Sc compositions larger than 50%, each atom has nearly five nearest neighbors (i.e., the ground state exhibits a phase that is five-fold coordinated). On the other hand, Sc-deficient (Sc, Ga) N alloys adopt a ground-state that is four-fold coordinated). The magnitude of e 33 in the Sc-deficient ideally ordered (Sc 0.25 Ga 0.75 N) is found to be larger than the magnitude of the corresponding e 33 coefficients resulting from the compositional weighted average over the hexagonal (h-ScN) and the wurtzite (w-GaN) parent compounds. On the other hand, the e 33 coefficients of the Sc-rich ordered (Sc 0.75 Ga 0.25 N) is found to be negligibly small. In addition, e 33 piezoelectric coefficients in ordered (Sc 0.5 Ga 0.5 N) exhibit quite large magnitudes, due to the nonpolar to polar transition occurs at Sc composition x=0.5, and thus can bridge the corresponding coefficients of (Ga, In) N and ferroelectric alloys. The microscopic origins for this huge enhancement in the piezoelectric behavior in Sc-low and Sc-intermediate alloys and the role of each atom are revealed and discussed. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 2006