Coherent Epitaxial Layers Research Articles

Giant Critical Thickness in Highly Conducting Epitaxial SrMoO3Electrodes Investigated by Lift‐Off Membranes

AbstractWithin the huge perovskite materials family, thin films of highly conducting materials such as SrMoO3, SrNbO3, and SrVO3are candidates for low‐loss bottom electrodes in epitaxial all‐oxide devices, in particular for high‐frequency applications. Recently, the fully coherent growth of more than 5 µm thick SrMoO3electrodes in a varactor device prototype epitaxial heterostructure has been reported. This result raises the question of the strain mechanism in such anomalously thick coherent epitaxial layers. Here, this question is addressed by comparing the lattice constants of coherently strained layers and their free‐standing membranes. SrMoO3is mainly elastically strained within the heterostructure and fully relaxed after removal of the substrate. These results indicate a giant critical thickness, making highly conducting perovskites even more outstanding materials for high‐frequency applications that require electrode thicknesses beyond the skin depth. The described technology of lifting off thick SrMoO3membranes joins the emerging field of freestanding oxide layer technology, opening unexplored avenues for single crystal investigations, novel perovskite nanostructures, and wafer transfer of functional oxides, walking in the footsteps of recent developments in van der Waals epitaxial heterostructures.

Growth mechanism of Al2O3 on surface of ZrO2 inclusions: experimental and first principles investigation

Zr has been widely used in high-quality steel due to its strong deoxidizing capacity and ability of improve performance of steel. ZrO2–Al2O3 inclusions with core-shell structure were observed in this study. The structure, electronic properties, and energy changes of ZrO2 and ZrO2–Al2O3 systems were studied based on first principles calculation. It indicates that ZrO2 can be used as the heterogeneous nucleation core of Al2O3. Al atoms were preferentially adsorbed on the bridge site of ZrO2 (001) and then form the ZrO2–Al2O3 interface. The template effect of ZrO2 was nine adsorption atomic layers, among which layers 1–5 was a coherent epitaxial layer, layers 5–9 was the transition layer, and the atomic ratio was 3:2. The growth mechanism of Al2O3 on ZrO2 surface was discussed. The results help understand the formation of inclusion at atomic scale and find ways to control the characteristics of inclusion in steel with Zr treatment.

First-principles study on growth mechanism of TiN on MgO (1 0 0) and (1 1 0) surfaces

Substrate nucleation is one of the important modes of heterogeneous nucleation. The substrate nucleation occurs with the absorbed atoms transforming atomic layers phase of the nucleated phase on the substrate in the actual production process. For example, TiN grows layer by layer on MgO substrate during solidification of steel, which is an important way for MgO inclusions to play the role of oxide metallurgy to induce inclusions or precipitated phase. In this work, the first-principles calculation is employed to study the growth mechanism of TiN on MgO (100) and (110) surfaces. Study on atomic binding energy and charge density difference have shown that the effecting of MgO substrate template effect can be divided into two parts: coherent epitaxial layer and transition layer. For MgO(100)/TiN(100) and MgO(110)/TiN(110) systems, the critical size includes four and five atomic layers(TiN), respectively. The interfacial energy and work of adhesion of MgO(100)/TiN(100) system are both smaller than those of MgO(110)/TiN(110) system in the evolution process of TiN nucleation layer by layer on MgO substrate. The interfacial and surface properties of MgO/TiN systems are both related to MgO substrate, especially at the beginning of nucleation. The relation between electron work function or outermost atoms adsorption energy and charge distribution is also discussed simply.

Growth mechanism of MnS/Fe on TiN surface: First principle investigation

It is an important way for TiN inclusions to play the role of oxide metallurgy to induce inclusions or ferrite heterogeneous nucleation, while MnS and Fe precipitate on TiN are two typical growth modes. Understanding the formation process of TiN–MnS/Fe composite interface is extremely important for the control of TiN containing composite inclusions in steel and the effectiveness of catalyst. Based on the thermodynamic characteristics of TiN induced precipitation of MnS and Fe in steel, the bond structure, charge and energy of TiN crystal structure, TiN (111) N terminal surface structure and TiN–MnS/Fe interface structure are calculated by the first principle method. The growth mechanism of MnS/Fe on TiN surface at atomic scale is analyzed. The results show that TiN is a semi-metallic crystal with a band gap of 3.0 eV; Mn and Fe atoms will be adsorbed on the TiN (111) N terminal surface in the form of face-center cubic (FCC) stacking; MnS and Fe will grow on the TiN surface to form an interface system, and the influence distances of TiN template effect are 12.587 Å and 10.588 Å, respectively. The growth part (MnS and Fe) affected by TiN can be divided into two layers: coherent epitaxial layer (CEL) and transition layer (TL), in which the CEL and TL of TiN–MnS interface are both four layers, while the two and four layers of TiN–Fe interface.

Phase transformations and coercivity in rare-earth permanent magnets

The hypothetic Sm–Co and Nd–Fe phase diagrams are compared. The phase transformations, which may cause boundaries modification of the SmCo 5 and Nd 2Fe 14B phases, are considered. The phase transformation-induced coercivity mechanism in both Sm–Co and Nd–Fe–B sintered magnets are suggested. It is supposed that formation of coherent epitaxial layers at the surface of grains of the principal phase decreases the number of defect sites for nucleation of reversal magnetization domains and increases the coercive force.

Surface melting of nanoscopic epitaxial films

By introducing finite size surface and interfacial excess quantities, interactions between interfaces are shown to modify the usual surface premelting phenomenon. It is the case of surface melting of a thin solid film s deposited on a planar solid substrate S. More precisely to the usual wetting condition of the solid s by its own melt l, necessary for premelting (wetting factor Φ<0), is adjoined a new quantity Γ describing the interactions of the l/s interface with the s/S interface. When Γ>0 this interface attraction boosts the premelting so that a two stage boosted surface premelting is foreseen: a continuous premelting, up to roughly half the deposited film, is followed by an abrupt first order premelting. When Γ<0 these interfaces repell each other so that premelting is refrained and the film remains partly solid above the bulk melting point (overheating) what is called astride melting. Elastic stress modifies both types of melting curves. Bulk and surface stresses have to be distinguished. For coherent epitaxial layers the natural misfit determining the strain and the elastic energy density (independent of the thickness of the solid) only shifts the melting curves to lower temperature, up to thicknesses where strain relief happens. Surface stress, as a finite size surface excess quantity, modifies the wetting factor Φ and the coefficient Γ, therefore the wetting properties and thus the melting curves are slightly modified. For perfect glissile epitaxies things are more complex since bulk strain and elastic energy density (now induced by surface stress) varies with the film thickness. The melting curves are thus distorted on their initial part (either in the sense of assisted or refrained premelting) depending upon the set of interfacial stresses. Lastly there is a z-inhomogeneity of stress due to the interactions between the bulk of the various material layers. This leads to measurable strain gradients in the film but only distorts the final part of the melting curve. Some of these theoretical results have been experimentally illustrated in the Γ<0 case where then useful interfacial data, adhesion energies and interfacial stress data have been collected but the Γ>0 case remains fully open to future exploration.

Characterisation of semiconductor structures by high resolution X-ray diffraction

AbstractA simulation program based on the Takagi–Taupin equations from the dynamical theory of X-ray diffraction has been developed. The novelty of the program lies mainly in that it includes the elastic distortion of coherent epitaxial layers by employing a model based on the elasticity theory of materials. The simulation model is fairly flexible, allowing the calculation of any symmetric or asymmetric reflection, any substrate orientation, graded and relaxed layers, and instrumental effects. Some simulation results for different materials and multilayered structures are presented. A preliminary study of the influence of beam dispersion and detection noise has also been carried out.MST/3251

In Situ Observations of Misfit Dislocations in Lattice-Mismatched Epitaxial Semiconductor Heterostructures

This article describes the application of transmission electron microscopy (TEM) to real-time, in situ dynamic observations of dislocations in strained epitaxial semiconductor heterostructures. Such experiments allow us to directly observe the formation, motion, and interaction of mis-fit dislocations. Preliminary extension of this work to the in situ measurement of the electrical properties of misfit dislocations will also be described.The Fundamental Scientific IssueIt is well established that it is possible to grow a thin, coherent epitaxial layer on a substrate with a slightly different lattice parameter, as illustrated in Figure la. This concept is known as strained layer epitaxy. In the fields of semiconductor physics and device design, strained layer epitaxy offers many exciting new opportunities (see Reference 1 for a review). A coherently strained structure, however, will store an enormous elastic strain energy density in the epitaxial layer, due to the distortion of interatomic bonds. Therefore, as the epitaxial layer increases in thickness during growth, it will become increasingly energetically favorable to relax this strain energy. A number of relaxation routes exist: (1) roughening of the epitaxial layer surface (see, for example, Reference 2); (2) interdiffusion of the layers (this will generally only be significant at temperatures which are a large fraction of the layer melting temperatures (e.g., Reference 3)); and (3) introduction of a dislocation network into the substrate/epilayer interface, which as shown schematically in Figure lb, will allow the epitaxial layer to relax toward its bulk lattice parameter. This dislocation mechanism is the most prevalent strain relaxation mechanism at typical crystal growth and processing temperatures, and we concentrate on this mechanism in our experimental studies.

Determination of a rigid shift across the al(001)-gaas(001) interface

Abstract A recent generalized Bloch wave treatment for propagating a fast-electron wavefunction through an arbitrary number of coherent epitaxial layers is used to describe propagation through a bicrystal. The effects of a rigid shift ΔR across the interface on excitation amplitudes of Bloch waves in the lower crystal is discussed and shown to be a function of top-crystal thickness and orientation. This model is used to interpret electron diffraction data from an Al(001)/GaAs(001) bicrystal to determine the rigid shift ΔR across the interface. This shift is inferred from contrast associated with bright-field imaging and convergent-beam electron diffraction (CBED). Symmetry is inferred from CBED, and measurements of 220 systematic row rocking curves form regions between misfit dislocation enable the rigid shift to be determined from two possible configurations. Large-angle CBED patterns also enable unambiguous distinction between two models for the interface. Results are consistent with ΔR = ¼<100>GaAs, in agreement with previous observations. This method of analysis should be generally applicable to a wide range of systems such as InP/In1-xGaxAs multiple quantum wells and Si/GexSi1-x multilayers.

On the Critical Layer Thickness of Strained-Layer Heteroepitaxial CoSi2 Films on 〈111〉Si

ABSTRACTThe technique of Double Crystal X-Ray Diffractometry (DXD) and ion beam channeling are applied to investigate, as a function of thickness, the average perpendicular strain and crystal quality of CoSi2 layers grown by MBE on 〈111〉Si. The results show that thin layers (from 20 to 30 nm) are partially relaxed but with a strain greater than that expected for a free CoSi2 lattice. For layers thicker than 30nm the magnitude of the CoSi2 strain incrgases to 1.7%, somewhat less than the maximum magnitude strain expected for coherent growth (2.1%). For layers thicker than 50 nm, the perpendicular strain relaxes very slowly, with the strain at 225 nm only 5% less than that at 50nm. It was concluded that a coherent epitaxial layer does not form initially and the relaxation of the strained layers is not consistent with a planar growth mechanism of the CoSi2 epilayers. Therefore the concept of a critical thickness, below which the epilayers are strained and above which the epilayers are relaxed, cannot be applied to our CoSi2/Si system.

Energy of a dislocation near an epitaxial interface

The energy of a straight mixed dislocation in a coherent epitaxial layer at a uniform distance t from the interface is calculated. It is shown that, in addition to terms associated with coherency strain and the dislocation energy, there is a term associated with the interaction of the coherency and dislocation strain fields which makes the total energy decrease as the dislocation approaches the interface, but this term is negligible when the dislocation is nearer the epilayer surface than the interface. The result is used to modify a calculation of Jesser and Matthews of the critical thickness of an epitaxial layer at which the introduction of misfit dislocations becomes energetically favourable. The present work reduces the calculated critical thickness by 30–50%.