Atomistic prediction on the configuration- and temperature-dependent dielectric constant of Be0.25Mg0.75O superlattice as a high-κ dielectric layer

Abstract

The solid solution of Be_xMg_1−xO is examined as a candidate for high-κ dielectric materials by considering the dielectric constant, bandgap, and phase stability at the same time. Using ab initio calculations including phonon calculations, the subtle interrelation between atomic structure and electrical properties is elucidated. Due to the different stable phases between BeO (wurtzite structure) and MgO (rock salt structure), Be and Mg atoms have a distinctive preference on the site occupation, leading to various unexpected configurations. Notably, the instability of Be atoms located at the octahedral sites in the rock salt structure Be_xMg_1−xO (x < 0.5) triggers the movement of Be atoms toward the tetrahedral-like sites. It results in the modified rock salt structure Be_xMg_1−xO: shortened Be–O bonds at the tetrahedral-like sites in the rock salt structure composed of octahedral Mg-O bonds. The modified rock salt structure Be_xMg_1−xO has a high bandgap over 7.3 eV irrespective of the composition and atomic configuration. In contrast, the energetic stability and dielectric constant highly depend on the atomic configuration, where a configuration with longer apical Be–O bond length tends to show lower energetic stability and higher dielectric constant. From this key finding, superlattice structures in Be_0.25Mg_0.75O are proposed as a suitable high-κ material providing the opportunities to systematically control the dielectric constant by the design of the atomic arrangement. Further examination reveals that the proposed superlattice structures are stable, and their high-κ values slightly increase as temperature increases.