The epitaxial growth of two-dimensional oxide layers on metal surfaces is examined in view of the 1949 van der Merwe proposition that epitaxy requires a pseudomorphic monolayer. It is argued that this limitation is relaxed in the 2-D case and that ordered oxide phases can grow out of a variety of interface scenarios, ranging from pseudomorphic to incommensurate. Prototypical examples of binary and ternary oxides supported on noble metal surfaces are presented, and the structural peculiarities of 2-D oxide phases are emphasized. The usually strong coupling at the oxide–metal interface leads to the stabilization of novel structure concepts that are not encountered in the, respective, bulk phases. The structural flexibility of 2-D lattices is discussed, and their ability to accommodate strain in generating novel 2-D oxide phases is emphasized. In the case of weakly coupled systems, it is reported that more subtle interactions at the interface can create periodic nanoscale morphologies and particular growth patterns in subsequent layers.

Pulsed laser deposition is one of the most flexible growth methods for high-quality epitaxial multifunctional thin films and short-period superlattices. The following examples of current research interest demonstrate the state-of-the art: First, it is shown that the magnetoelectric performance of multiferroic BiFeO3–BaTiO3 (001)-oriented superlattices depends on the crystalline coherence of the different layers at the interfaces. Second, it is exemplified that dielectric-plasmonic superlattices built from the electrically insulating oxide MgO and the metallically conducting nitride TiN are promising metamaterials with hyperbolic dispersion. As a third example, it is demonstrated that LaNiO3- and LaMnO3-based superlattices with (001)-, (011)-, and (111)-out-of-plane orientation and controlled single layer thickness from 2 to 15 atomic monolayers show metal-insulator transitions and tunable gaps, in partial agreement with density functional theory calculations. Underlined by these examples, it is shown that the precise control of an epitaxially coherent, or two-dimensional layer-by-layer growth, named after Jan van der Merwe, is a prerequisite to achieve the desired functionality of oxide–oxide and oxide–nitride superlattices.

The interaction between interface dislocation networks and a single lattice Volterra-type dislocation is analyzed by superposition using anisotropic elastic theory in dissimilar materials. The general and nontrivial field solutions for the displacements and stresses are derived by applying the Stroh sextic formalism and Fourier transform in heterogeneous bimaterials. The present approach therefore enables the calculation of the elastic interaction forces for the glide and climb components with different elastic constants and unequal partitioning of elastic fields between adjacent crystals neighboring a semicoherent heterophase interface. Two-dimensional application examples to the pure misfit Au/Cu interface are evaluated, where the infinitely long straight lattice dislocation, parallel to the interface dislocations, is embedded in Au. The repulsive and attractive interaction forces between these two types of (intrinsic and extrinsic) defects are investigated and discussed, for which the results provide a novel basis for examining multiple large-scale dislocation interactions in anisotropic interface-dominated materials with accurate mechanical boundary conditions.

The growth behavior of epitaxial transition metal oxides with the perovskite structure often shows discrepancies with models established for semiconductor and metal films. The reason is rooted in the versatility of such octahedral framework structures to accommodate the interfacial dissimilarity and the participation of strongly coupled electron and lattice degrees of freedom in strain relaxation mechanisms. Here, we revisit the behavior of the prototypic La0.7Sr0.3MnO3 manganite under specific growth conditions, enabling the isolation of pure octahedral tilting and misfit dislocation mechanisms in the same material. Analysis of the observed behavior provides insights into the competition between octahedral tilting and classical relaxation mechanisms by misfit dislocations or domain formation, and the effect of additional contributions to dissimilarity such as symmetry mismatch and polar discontinuities. Moreover, given the intimate association between misfit relaxation and self-organization mechanisms, opportunities and limitations of the observed behavior in the generation of novel bottom-up functional nanostructures is also addressed.

The kinetic relaxation pathways for strained heteroepitaxial films are mapped using a process simulator that integrates experimental and model descriptions of the energetic and kinetic parameters that define the nucleation, propagation, and interaction of strain relieving dislocations. This paper focuses on Ge x Si1−x /Si(100), but the methodologies described should be extendible to other systems. The kinetic pathways for strain evolution are plotted for film growth as functions of the primary kinetic parameters: growth temperature, growth rate, and initial lattice mismatch, generating relaxation surfaces for parameter pairs. Sensitivity analyses are presented of how deviations from mean parameters disperse the resultant relaxation surfaces. Finally, multi-parameter “fingerprinting” of the dislocation array is shown to illustrate how fundamental kinetic mechanisms—particularly dislocation nucleation mechanisms—define the final dislocation array. The overarching goal is to establish a robust framework for predicting, interrogating, and optimizing strain relaxation pathways and underlying mechanisms, for misfit dislocations in strained heteroepitaxial films.

Excited by the great success of metal halide perovskites in the optoelectronic and electro-optic fields and the interesting emerging physics (Rashba splitting, quantum anomalous hall effect) of layered metal halides, metal halides have recently been attracting significant attentions from both research and industrial communities. It is shown that most progresses have been made when these materials are obtained at reduced dimensions. Among several growth methods, vapor phase epitaxy has been demonstrated with a universal control on morphology, phase, and composition. We thus believe that a thorough understanding on the physical properties and on the growth of general metal halide compounds at reduced dimensions would be very beneficial in the study of recent perovskites and layered metal halide materials. This review covers the physical properties of most studied metal halides and summarizes the vapor phase epitaxial growth knowledge collected in the past century. We hope that this comprehensive review could be helpful in designing new physical properties and in planning growth parameters for emerging metal halide crystals.

Integration of photonic devices on silicon (Si) substrates is a key method in enabling large scale manufacturing of Si-based photonic–electronic circuits for next generation systems with high performance, small form factor, low power consumption, and low cost. Germanium (Ge) is a promising material due to its pseudo-direct bandgap and its compatibility with Si-CMOS processing. In this article, we present our recent progress on achieving high quality germanium-on-silicon (Ge/Si) materials. Subsequently, the performance of various functional devices such as photodetectors, lasers, waveguides, and sensors that are fabricated on the Ge/Si platform are discussed. Some possible future works such as the incorporation of tin (Sn) into Ge will be proposed. Finally, some applications based on a fully monolithic integrated photonic–electronic chip on an Si platform will be highlighted at the end of this article.

The Ph.D. work of Jan H. van der Merwe in 1949 established a new paradigm for the understanding of dislocation dynamics in restricted volumes. This led to a comprehensive understanding of plasticity, or strain relaxation, in the context of strained-layer semiconductor structures. However, this understanding was largely overlooked in the context of traditional metallurgy and micromechanics. We identify four reasons for this, the apparent need for an unstrained substrate in van der Merwe’s theory, the supposed inapplicability to strain gradients, the supposed inapplicability to the Hall–Petch effect (dependence of strength on the inverse square root of grain size), and an emphasis on understanding strain hardening rather than the yield point. Addressing these four points in particular, here it is shown how the insights of van der Merwe and of the earlier work by Lawrence Bragg lead to a coherent and unified view of the size-effect phenomena ranging from the Hall–Petch effect to the strain-gradient plasticity theory.

Self-assembled oxide-based vertically aligned nanocomposite (VAN) thin films have aroused tremendous research interest in the past decade. The interest arises from the range of unique nanostructured films which can form and the multifunctionality arising from these forms. Hence, a large number of oxide VAN systems have been demonstrated and explored for enhancing specific physical properties, such as strain-enhanced ferroelectricity, tunable magnetotransport, and novel electrical/ionic transport properties. The epitaxial growth of the nanocomposite thin films and the coupling at the heterogeneous interfaces are critical considerations for future device applications. In this review, the advantages of strain coupling along vertical interfaces and film-substrate interfaces in nanocomposite films over conventional single phase films are discussed. Specifically, a unique strain compensation model enabling the epitaxial growth of two-phase nanocomposites having large lattice mismatch with substrates is proposed. Out-of-plane strain coupling between the two phases is also discussed in terms of designing strain states for desired functionalities.

The nucleation and growth of Al on 7 × 7 and $\sqrt 3 \times \sqrt 3$R30 Al reconstructed Si(111) that result in strain-free Al overgrown films grown with an atomically abrupt metamorphic interface are compared. The reconstructed surfaces and abrupt strain relaxations are verified using reflection high-energy electron diffraction. The topography of evolution is examined with atomic force microscopy. The growth of Al on both the surfaces exhibits 3D island growth, but the island evolution of growth is dramatically different. On the 7 × 7 surface, mounds formed are uniformly distributed across the substrate, and growth appears to proceed uniformly. Alternatively, on the $\sqrt 3 \times \sqrt 3$R30 surface, Al atoms exhibit a clear preference to form mounds near the step edges. During Al growth, mounds increase in size and number, expanding out from step edges until they cover the whole substrate. Consistent expression of a mounded nucleation and growth mode imparts a physical limitation to the achievable surface roughness that may impact the ultimate performance of layered devices such as Josephson junctions that are critical components of superconducting quantum circuits.

We analyze the microscopic origins of subgap photoexcitations of individual gallium nitride (GaN) triangular cross-section nanowires (NWs), which are highly photoactive over a broadband spectral range. Using confocal hyperspectral photoluminescence (PL) microscopy, mid-gap states on the NWs were excited using subgap illumination, resulting in two distinct PL spectra corresponding to the polar (0001) and the semipolar $\left( {\bar 1101} \right)$ / $\left( {1\bar 101} \right)$ surfaces. Emission spectra are well represented by Gaussian functions with fitted centers of 1.99 ± 0.01 eV and 2.26 ± 0.01 eV, respectively. PL collected from the end facets exhibits interference fringes and a relative blue shift. Furthermore, the PL spectrum shifts strongly to the blue when the excitation intensity is increased. These observations are consistent with a qualitative model in which the PL results from excitation into a broad manifold of surface-associated states which are rapidly populated at a high excitation intensity and can couple to etalon modes via longitudinal photon emission.

Zinc oxide (ZnO) layers and nanowires were grown by chemical vapor deposition (CVD) using methane (CH4) as reducing agent. Compared to conventional CVD processes, which commonly use graphite powder to reduce the ZnO powder source material, this low-cost method allows an improved controllability of the growth processes. Specifically, the consumption of the source material–a commercially available ZnO powder–can be controlled in a very precise way by varying the flow of the reducing CH4 or the re-oxidizing O2. Using this parameter, the growth can be switched between ZnO layers and nanostructures. High-quality ZnO layers have been grown on gallium nitride (GaN) substrates and on c-plane sapphire with an intermediate aluminum nitride (AlN) nucleation layer. By adjusting the growth conditions accordingly, ZnO nanowires were also grown with this method catalyst-free using a- and c-plane sapphire with ZnO nucleation layer as a substrate. The optical properties of the nanowires were investigated by low-temperature photoluminescence (PL) and compared to samples grown by conventional carbo-thermal CVD.

Isolated single quantum dots (QDs) enable the investigation of quantum-optics phenomena for the application of quantum information technologies. In this work, ultralow-density InAs QDs are grown by combining droplet etching epitaxy and the conventional epitaxy growth mode. An extreme low density of QDs (∼106 cm−2) is realized by creating low-density self-assembled nanoholes with the high temperature droplet etching epitaxy technique and then nanohole-filling. The preferred nucleation of QDs in nanoholes has been explained by a theoretical model. Atomic force microscopy and the photoluminescence technique are used to investigate the morphological and optical properties of the QD samples. By varying In coverages, the size of InAs QDs can be controlled. Moreover, with a thin GaAs cap layer, the position of QDs remains visible on the sample surface. Such a low density and surface signature of QDs make this growth method promising for single QD investigation and single dot device fabrication.