Effects of Defect on Ferroelectric Stability in PbTiO3 Thin Films

ABSTRACTEffects of defect on ferroelectric stability in PbTiO3 (PTO) thin films have been investigated using molecular dynamics with first-principles effective Hamiltonian. In this paper, oxygen vacancy (Vo) has been considered to study the hysteresis loop, spontaneous polarization as a function of film thickness. Vo has been modeled as a charged point defect. Density functional theory (DFT) calculations are performed to determine the Vo-induced localized fields (both mechanical and electrical) and the calculated DFT results are used as inputs to molecular dynamics simulations in a large system. The strain field induced by the Vo is calculated by DFT calculations and fitted by the continuum strain modeling, and the electrostatic field is given by the superposition of the Vo-induced field and the external field. Vo significantly reduces the spontaneous polarization and increases the critical thickness.

Defect Formation in La2Ni(M)O4+δ (M= Co, Cu): Modelling and Coulometric Titration Study

The p(O2)-T- diagrams of La2Ni1-xMxO4+ (M=Co and Cu, x= 0-0.20), determined by the coulometric titration technique at 923-1223 K in the oxygen partial pressure range 10-4 to 0.6 atm, can be adequately described by equilibrium processes of oxygen intercalation into the rock-salt type layers and hole localization on B-site cations forming 3+ oxidation states. For the hole activity, a non-ideal solution model taking into account the repulsion of p-type electronic charge carriers can be used. The electrostatic repulsion excludes occupation of nearest neighboring sites and leads to splitting of the energy levels for more distant sites. The affinity of Ni and Cu cations with respect to the hole localization is similar and cannot be statistically separated analyzing the oxygen nonstoichiometry data only. On the contrary, cobalt cations tend to remain in the trivalent state and Co3+ should be treated as a separate type of charged point defect. Oxygen vacancies formed in the perovskite-like layers due to intrinsic Frenkel disorder have no essential effect on the oxygen thermodynamics. As expected, the thermodynamic functions governing the intercalation-related processes are independent of defect concentrations.

Modeling Fermi Level Effects in Atomistic Simulations

In this work, variations in electron potential are incorporated into a Kinetic Lattice Monte Carlo (KLMC) simulator and applied to dopant diffusion in silicon. To account for the effect of dopants, the charge redistribution induced by an external point charge immersed in an electron (hole) sea is solved numerically using the quantum perturbation method. The local carrier concentrations are then determined by summing contributions from all ionized dopant atoms and charged point defects, from which the Fermi level of the system is derived by the Boltzmann equation. KLMC simulations with incorporated Fermi level effects are demonstrated for charged point defect concentration as a function of Fermi level, coupled diffusion phenomenon and field effect on doping fluctuations.

Boron Diffusion in Si1−x Gex Strained Layers

ABSTRACTBoron diffusion in Sia7 Gea3 strained layers has been studied as a function of annealing temperature and is compared to boron diffusion in unstrained Si epitaxial layers. A lower diffusivity was observed in Sia7 Gea3 strained layers as compared to the Si control samples. Using the well-accepted model for boron diffusion via charged point defects in Si, the intrinsic diffusivities of the boron atoms in the Sia7 Gea3 layers were derived and an activation energy of 4.4 eV was estimated for the diffusion process.A new approach to describe the diffusion process is also presented here. In this model, we relate the lower diffusivity to a corresponding change in the charged point defect equilibrium concentration caused by a change in band-gap due to the strain and the existence of foreign atoms in a Si matrix.

Mechanisms of Doping-Enhanced Superlattice Disordering and of Gallium Self-Diffusion in GaAs

ABSTRACTRecently available Ga-Al interdiffusion results in GaAs/AlAs superlattices allow to conclude that Ga self-diffusion in GaAs is carried by triply-negatively charged Ga vacancies under intrinsic and n-doping conditions. The mechanism of the Si enhanced superlattice disordering is the Fermi-level effect which increases the concentrations of the charged point defect species. For the effect of the p-dopants Be and Zn, the Fermi-level effect has to be considered together with dopant diffusion induced Ga self-interstitial supersaturation or undersaturation. Self-diffusion of Ga in GaAs under heavy p-doping conditions is governed by positively charged Ga self-interstitials.