We propose a series of new compositions of AlN-based piezoelectric material based on the results of first-principles calculation. The composition is expressed by ð y ; 1Ày Þ x Al 1%x N, where the elements α and β are selected to maintain the charge neutrality of the host AlN. We found that the selection of Mg 2+ for α and Hf 4+ and Zr 4+ for β with y = 0.5 show good chemical stability with much better piezoelectric properties than pure AlN. Our results indicate broad compositional freedom for improving the piezoelectric properties of AlN by using the co-doping technique.

Native defects and oxygen and hydrogen-related defect complexes in CdTe: Density functional calculations J. Appl. Phys.With a specific focus on defects formed from H atoms and oxygen vacancies ͑V O ͒ in perovskite-type oxide, stability and stable valence states of complex defects formation processes within BaTiO 3 are studied by using first-principle density-functional theory calculations. In our findings, H atoms diffuse as protons ͑H + ͒ into interstitial sites in BaTiO 3 , whereas these atoms when trapped at centers of V O sites convert to negatively ionized states ͑H − ͒. We also find that H atom trapping at V O sites occurs only in n-type carrier-rich environments without carrier compensation of V O 2+ and H + . If carrier electrons are compensated, H + ions are excluded from V O 2+ sites due to the repulsive Coulomb potential existing between the H + ion and the positively charged V O 2+ site. Difficulties in the calculation of the diffusion-energy diagram for H atoms, involving essential changes in the stable valence states during the diffusion process, are discussed and a practical solution is presented.

Carrier-electron-induced phase transition from tetragonal to cubic phases in BaTiO 3 is studied using firstprinciples calculation. Our results show that the disappearance of the ferroelectric phase is an intrinsic effect resulting from carrier electron doping in BaTiO 3 . We further clarify that the lattice disorder induced by donor dopants such as oxygen vacancies and substitutionally doped Nb 5+ at Ti 4+ sites accelerates the disappearance of the tetragonal phase in BaTiO 3 .