First Principles Study of Size Effect in BaTiO 3 Ultrathin Films
- PDF / 216,138 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 44 Downloads / 300 Views
CC5.10.1
First Principles Study of Size Effect in BaTiO3 Ultrathin Films Bo-Kuai Lai, Igor Kornev, Laurent Bellaiche and Greg Salamo Physics Department, University of Arkansas, Fayetteville, Arkansas 72701 ABSTRACT Properties and phase transition behaviors of ferroelectric thin films that are different from that of their bulk form is usually referred to as size effect. A first-principles-based scheme is used to investigate the effects of four important factors contributing to the size effects in epitaxial (001) BaTiO3 ultrathin films: misfit strain, existence of surface, film thickness, and electrical boundary conditions. INTRODUCTION Ferroelectric thin films have attracted a lot of research attention because of their potential in applications such as memories [1] and radio-frequency devices [2]. To incorporate ferroelectric thin films into current semiconductor processing technology, one has to choose suitable substrates, electrodes, buffer layers, and a thermal processing procedure to deposit highquality films ranging from several micrometers down to several monolayers. The use of substrates and thermal processing inevitably introduces strains into films, which can significantly affect their properties. For instance, compared to their bulk counterpart, higher paraelectric-toferroelectric transition temperatures have been reported for epitaxial thin films under tensile [3] and compressive strains [4]. In fact, many other factors (e.g., thickness, electrical boundary conditions, surface termination, interface roughness, and charge transfer at the free surface and interface) [5-7] have also been associated with the decrease in dimensionality when going from bulk to thin films. The properties of real thin films are a combined result of these factors and the effect of each factor is usually very difficult to be evaluated from experimental results. Many theoretical studies have been performed to evaluate the size effect. The effect of misfit strains on single-domain and two-domain (001) BaTiO3 thin films have been studied by Pertsev et al. [8,9] using a phenomenological method. Phase transition behaviors that are different from bulk were observed and five crystallographic phases, shown in Table 1, were obtained in their temperature-misfit strain phase diagrams. However, it was pointed out by Dieguez et al. [10] that different sets of parameters were used in Refs. 8 and 9, which result in different low temperature phase behaviors. They thus decided to use ab initio approaches to avoid such dependency on experimentally deduced parameters. However, the result obtained by the ab initio method [10] is actually for bulk because of the use of periodic bulk supercells; while the phenomenological method can only apply to thin films thicker than 100 nm. Therefore, the theoretical schemes of these works are not suitable for study two important factors associated to ultrathin films: the existence of surfaces and film thickness. Both works also assume ideal shortcircuit (SC) conditions. Under SC conditions, there is no internal dep
Data Loading...
