Zi-Kui Liu

In situ epitaxial MgB2 thin films for superconducting electronics

The newly discovered 39-K superconductor MgB21 holds great promise for superconducting electronics. Like the conventional superconductor Nb, MgB2 is a phonon-mediated superconductor2, with a relatively long coherence length3. These properties make the prospect of fabricating reproducible uniform Josephson junctions, the fundamental element of superconducting circuits, much more favourable for MgB2 than for high-temperature superconductors. The higher transition temperature and larger energy gap4,5 of MgB2 promise higher operating temperatures and potentially higher speeds than Nb-based integrated circuits. However, success in MgB2 Josephson junctions has been limited because of the lack of an adequate thin-film technology6,7. Because a superconducting integrated circuit uses a multilayer of superconducting, insulating and resistive films, an in situ process in which MgB2 is formed directly on the substrate is desirable. Here we show that this can be achieved by hybrid physical–chemical vapour deposition. The epitaxially grown MgB2 films show a high transition temperature and low resistivity, comparable to the best bulk samples, and their surfaces are smooth. This advance removes a major barrier for superconducting electronics using MgB2.

Effect of substrate constraint on the stability and evolution of ferroelectric domain structures in thin films

The stability and evolution of ferroelectric domain structures in thin films are studied. Elastic solutions are derived for both elastically anisotropic and isotropic thin films with arbitrary domain structures, subject to the mixed stress-free and constraint boundary conditions. These solutions are employed in a three-dimensional phase-field model to investigate simultaneously the effect of substrate constraint and temperature on the volume fractions of domain variants, domain-wall orientations, surface topology, domain shapes, and their temporal evolution for a cubic-to-tetragonal ferroelectric phase transition. A specific example of a [001] orientated film heteroepitaxially grown on a [001] cubic substrate is considered. It is shown that the shapes of a-domains with tetragonal axes parallel to the film surface are significantly different from those of c-domains with tetragonal axes perpendicular to the film surface. For the substrate constraints and temperatures under which both a- and c-domains coexist, both types of a-domains are present with their tetragonal axes perpendicular to each other, and the domain wall orientations deviate from the 45 orientation generally assumed in thermodynamic analyses. It is demonstrated that a substrate constraint results in sequential nucleation and growth of different tetragonal domains during a ferroelectric phase transition.

Optical band gap of BiFeO3 grown by molecular-beam epitaxy

BiFeO3 thin films have been deposited on (001) SrTiO3 substrates by adsorption-controlled reactive molecular-beam epitaxy. For a given bismuth overpressure and oxygen activity, single-phase BiFeO3 films can be grown over a range of deposition temperatures in accordance with thermodynamic calculations. Four-circle x-ray diffraction reveals phase-pure, epitaxial films with ω rocking curve full width at half maximum values as narrow as 29arcsec (0.008°). Multiple-angle spectroscopic ellipsometry reveals a direct optical band gap at 2.74eV for stoichiometric as well as 5% bismuth-deficient single-phase BiFeO3 films.

Phase-field model of domain structures in ferroelectric thin films

A phase-field model for predicting the coherent microstructure evolution in constrained thin films is developed. It employs an analytical elastic solution derived for a constrained film with arbitrary eigenstrain distributions. The domain structure evolution during a cubic→tetragonal proper ferroelectric phase transition is studied. It is shown that the model is able to simultaneously predict the effects of substrate constraint and temperature on the volume fractions of domain variants, domain-wall orientations, domain shapes, and their temporal evolution.

Thermodynamics of the Mg–B system: Implications for the deposition of MgB2 thin films

We have studied the thermodynamics of the Mg–B system with the calculation of phase diagrams modeling technique using a computerized optimization procedure. Temperature–composition, pressure–composition, and pressure–temperature phase diagrams under different conditions are obtained. The results provide helpful insights into appropriate processing conditions for thin films of the superconducting phase, MgB2, including the identification of the pressure–temperature region for adsorption-controlled growth. Due to the high volatility of Mg, MgB2 is thermodynamically stable only under fairly high-Mg overpressures for likely growth temperatures. This places severe temperature constraints on deposition techniques employing high-vacuum conditions.

Effect of electrical boundary conditions on ferroelectric domain structures in thin films

The domain structures in a ferroelectric thin film are studied using a phase-field model. A cubic-to-tetragonal ferroelectric phase transition in lead titanate thin film is considered. Both elastic interactions and electrostatic interactions are taken into account. The focus is on the effect of electrical boundary conditions on the domain morphologies and volume fractions. It is shown that different electric boundary conditions may have a significant effect on the domain structures.

First-principles elastic constants of α- and θ-Al2O3

Using an efficient strain-stress method, the first-principles elastic constants cij’s of α-Al2O3 and θ-Al2O3 have been predicted within the local density approximation and the generalized gradient approximation. It is indicated that more accurate calculations of cij’s can be accomplished by the local density approximation. The predicted cij’s of θ-Al2O3 provide helpful guidance for future measurements, especially the predicted negative c15. The present results make the stress estimation in thermally grown oxides containing of α- and θ-Al2O3 possible, which in turn provide helpful insights for preventing the failure of thermal barrier coatings on components in gas-turbine engines.

A first-principles approach to finite temperature elastic constants

A first-principles approach to calculating the elastic stiffness coefficients at finite temperatures was proposed. It is based on the assumption that the temperature dependence of elastic stiffness coefficients mainly results from volume change as a function of temperature; it combines the first-principles calculations of elastic constants at 0 K and the first-principles phonon theory of thermal expansion. Its applications to elastic constants of Al, Cu, Ni, Mo, Ta, NiAl, and Ni₃Al from 0 K up to their respective melting points show excellent agreement between the predicted values and existing experimental measurements.

Superconducting MgB2 thin films on silicon carbide substrates by hybrid physical–chemical vapor deposition

We have used two polytypes of silicon carbide single crystals, 4H-SiC and 6H-SiC, as the substrates for MgB2 thin films grown by hybrid physical-chemical vapor deposition (HPCVD). The c-cut surface of both polytypes has a hexagonal lattice that matches closely with that of MgB2. Thermodynamic calculations indicate that SiC is chemically stable under the in situ deposition conditions for MgB2 using HPCVD. The MgB2 films on both polytypes show high-quality epitaxy with a Rutherford backscattering channeling yield of 12%. They have Tc above 40 K, low resistivities, high residual resistivity ratios, and high critical current densities. The results demonstrate that SiC is an ideal substrate for MgB2 thin films.

Linking phase-field model to CALPHAD: application to precipitate shape evolution in Ni-base alloys

Abstract A three-dimensional phase-field model is proposed with the thermodynamic and kinetic parameters directly extracted from existing databases using the CALPHAD method. We modelled the γ′ precipitate microstructure evolution in a Ni-base alloy, particularly a single precipitate morphology at different sizes using independently assessed thermodynamic, kinetic and structural parameters.