Chung-ho Woo

Schwoebel-Ehrlich barrier: from two to three dimensions

The Schwoebel-Ehrlich barrier—the additional barrier for an adatom to diffuse down a surface step—dictates the growth modes of thin films. The conventional concept of this barrier is two dimensional (2D), with the surface step being one monolayer. We propose the concept of a three-dimensional (3D) Schwoebel-Ehrlich barrier, and identify the 2D to 3D transition, taking aluminum as a prototype and using the molecular statics method. Our results show that: (1) substantial differences exist between the 2D and 3D barriers; (2) the transition completes in four monolayers; and (3) there is a major disparity in the 3D barriers between two facets; further, alteration of this disparity using surfactants can lead to the dominance of surface facet against thermodynamics.

Curie temperature and critical thickness of ferroelectric thin films

The dynamic Ginzburg–Landau theory is applied to establish the critical conditions that control the transition between the paraelectric and ferroelectric states. Analytic expressions of the para-ferroelectric transition temperatures in a thin film under various electromechanical surface conditions are derived via a linear stability analysis of the evolutionary trajectory of the system for both first- and second-order transitions. Explicit expressions are then derived for the critical thickness, below which the thin film is paraelectric for all temperatures. For first-order transitions, the difference between the superheating and supercooling transition temperatures is found to be insensitive to the film thickness and surface boundary conditions. From these expressions, the relative importance on ferroelectricity in thin films due to applied mechanical constraints on the transformation strain and the depolarizing effect of surface charges is discussed and compared with experimental data.

Dislocation nucleation in the initial stage during nanoindentation

The microstructure origin of the elastic–plastic response of a Cu substrate during nanoindentation is studied using molecular dynamics simulation. The elastic response is found to deviate from the Hertzian solution observed experimentally. The departure can be traced to the small tip radius used in the simulation. Further penetration sees the development of an inhomogeneous microstructure. Even at the same strain rate, different parts of the contact surface deform via different mechanisms: some elastically, some via the dislocation bow-out and some via the nucleation and growth of Shockley partials that sometimes interact to form stair-rod locks. The resultant effect produces the observed quasi-elastic behaviour on the load–displacement curve, characterized by interspersed minor yields. The present computer simulation shows in some detail the corresponding dislocation structure development. The stair-rod lock formation is found to provide a more satisfactory explanation to the experimentally observed time-delayed occurrence of pop-in below the spontaneous pop-in load.

Development of 〈110〉 texture in copper thin films

Apart from the scientific interest, texture development in copper thin films is of crucial importance to their applications as interconnects or corrosion resistant coating. We report here a dominant 〈110〉 texture of copper thin films—preferred for oxidation-resistant applications—deposited by direct current magnetron sputtering. Scanning electron microscopy shows that the copper films go through a transition from 〈111〉 columns to 〈110〉 hillocks as the deposition proceeds. Cross-sectional transmission electron microscopy (TEM) indicates that the 〈110〉 grains nucleate at boundaries of 〈111〉 grains. Further, we have proposed a stress-driven nucleation and growth model of 〈110〉 grains based on the x-ray diffraction characterization and the TEM observations.

Surface tension and size effect in ferroelectric nanotubes

Using a thermodynamic model, linear stability analysis is performed on the associated dynamic Ginsburg–Landau equation for a ferroelectric nanotube. Analytic expressions of the transition temperature and Curie–Weiss relation are derived. The ferroelectric properties of the tube are found to generally depend on its dimensions through effects of the surface tension and near-surface eigenstrain relaxation. Our results also show that the transition temperature and polarization are both enhanced due to the dominance of the effective radial pressures induced by the surface tension, resulting in a remnant polarization and coercive field that may become larger than the bulk values.

Simulation of interface dislocations effect on polarization distribution of ferroelectric thin films

Effects of interfacial dislocations on the properties of ferroelectric thin films are investigated, using the dynamic Ginzburg–Landau equation. Our results confirm the existence of a dead layer near the film/substrate interface. Due to the combined effects of the dislocations and the near-surface eigenstrain relaxation, the ferroelectric properties of about one-third of the film volume suffers.

Curie-Weiss law in thin-film ferroelectrics

The stationary self-polarization field of a thin film in an open circuit is analytically solved for temperatures near the para-/ferroelectric transformation within the Ginzburg-Landau theory. For second-order ferroelectrics, or first-order ferroelectrics with a sufficiently large elastic self-energy of the transformation strain, the solution is real and stable, from which the corresponding electric susceptibility of the film can be derived. A Curie-Weiss-type relation of the permittivity is obtained for both the supercritical and subcritical temperature regimes near the transition. In the paraelectric state, the Curie parameter of the thin film is found to be independent of its thickness, whereas in the ferroelectric state, its magnitude decreases rapidly with decreasing film thickness.

A self-consistent model for polycrystals undergoing simultaneous irradiation and thermal creep

Abstract A self-consistent model is presented that describes the creep response of a polycrystalline aggregate consisting of grains which exhibit a single crystal deformation law expressed as a sum of stress powers. A number of limitations of the self-consistent formulation are discussed and an approximate scheme is proposed for solving the mixed creep problem. In particular the model is applied to simulate a material that deforms simultaneously under neutron irradiation and thermal creep. In this case the single crystal creep rate has two components, one due to irradiation creep, which depends linearly on stress, and another, due to thermal creep, that depends nonlinearly on stress. Using the approximate scheme mentioned above and scanning the stress space, it is shown that, at the macroscopic level, the coupling between thermal creep and irradiation creep is significant in a range of applied stress which is a function of the irradiation and thermal creep parameters. In general the coupling of both mecha...

Vanishing critical thickness in asymmetric ferroelectric tunnel junctions : first principle simulations

The stability of the remnant polarization in the ferroelectric barrier layer is a prerequisite to applications involving ferroelectric tunnel junctions (FTJs) or capacitors. One of the most important issues in the pursuit of further developments in this area is to overcome the limitations due to the critical thickness, below which the ferroelectric polarization disappears. In this paper we report first-principle density-functional calculations of the charge distribution and polarization in an asymmetric FTJ (A-FTJ), i.e., one with dissimilar electrodes. We found that a significant and stable polarization can be retained down to thicknesses as small as 0.8 nm (two unit-cells) in a BaTiO3 thin film between Pt and SrRuO3 electrodes, quite unlike the case of symmetric FTJs. We trace this surprising result to the large electric field produced by the charge transfer between the electrodes caused by their different electronic environments, which acts against the depolarization field and enhances the ferroelectri...

Critical thickness for dislocation generation during ferroelectric transition in thin film on a compliant substrate

The formation energy of misfit dislocations in a ferroelectric thin film grown on compliant substrate is calculated based on the Landau-Devonshire formalism and Timosheko’s method for thermal stresses. The critical thickness is shown to change significantly according to the polarization in the film, leading to serious concerns, particularly for thick substrates, in the device design stage.