Heteroepitaxial Thin Films Research Articles

Self-assembling growth of (Ba,Sr)TiO3 epitaxial thin films

Ferroelectric thin films of SrTiO3, BaTiO3, and (Ba,Sr)TiO3 were fabricated epitaxially on SrTiO3(100) and Si(111) single crystal substrates in an ultrahigh vacuum (10−5 Pa) by laser molecular beam epitaxy. The lowest crystallization temperatures of homoepitaxial thin films of SrTiO3 and heteroepitaxial thin films of BaTiO3 and (Ba,Sr)TiO3 on SrTiO3(100) substrates obtained were as low as 280, 330, and 340 °C, respectively. With the growth rate lower than 0.17 Å/s, it was discovered that the thickness of each layer of BTO film was an integral of the BTO unit cell height. When the growth rate was decreased from 0.17 to 0.05 Å/s, the thickness of each layer varied from nine unit cells to one unit cell. It is very important to construct a nanometer structure of inorganic ABO3 perovskite oxides by this kind of self-assembly.

Three-dimensional simulations of self-assembly of hut-shaped Si–Ge quantum dots

This article presents the results of three-dimensional modeling of heteroepitaxial thin film growth with the objective of understanding recent experiments on the early stages of quantum dot formation in SiGe/Si systems. We use a continuum model, based on the underlying physics of crystallographic surface steps, to study the growth of quantum dots, their spatial ordering and coarsening behavior. Using appropriate parameters, obtained from atomistic calculations, the (100) orientation is found to be unstable under compressive strains. The surface energy now develops a minimum at an orientation that may be interpreted as the (105) facet observed in SiGe/Si systems. This form of the surface energy allows for the growth of quantum dots without any barrier to nucleation—dots are seen to start off via a surface instability as shallow stepped mounds, which steepen continuously to reach their low energy orientations. During the very initial stages of growth, mounds are seen to grow in a dense array with several of them impinging on each other and subsequently coalescing to form larger mounds. This behavior occurs due to the competition between surface energy which seeks to minimize the free-energy by the formation of islands with side-walls at the strain stabilized orientations and repulsive elastic interactions between such closely spaced islands. Using simple analytical calculations, we show the existence of a critical island size for this coalescence behavior. A key result of our analysis is the inverse scaling of this critical size with the misfit strain in the film. While energetic analyses may be used to obtain useful insights, the growth of quantum dots is essentially a nonequilibrium process and requires a fundamental understanding of the kinetics. Numerical studies show that the growth kinetics has a profound effect on surface morphology: arrays of well-separated islands or, alternatively, intersecting ridges are obtained in different kinetic regimes. We also study an alternative model of a stable but nonfacet (100) orientation and point out the inconsistencies of this assumption.

High-mobility InSb thin films on GaAs (0 0 1) substrate grown by the two-step growth process

InSb heteroepitaxial thin films were grown on GaAs (0 0 1) substrate by the temperature variation two-step growth process. X-ray rocking curve widths and electrical properties of the InSb films with different thickness were investigated. The best value of a 2-μm thick InSb film had an X-ray rocking curve width of 277 arcsec and room-temperature Hall mobilities, the carrier concentrations obtained were 55,000 cm 2/V s and 1.5×10 16/cm 3, respectively. The enhanced mobilities and reduced carrier concentration of InSb films suggested a low defect density due to confinement of the dislocation in the InSb/GaAs interface.

Hydrothermal Synthesis of Undoped and Mn-Doped ZnGa2O4 Powders and Thin Films

Undoped and manganese-doped zinc gallate powders and thin films have been synthesized via the hydrothermal route from aqueous solutions at a pH between 9.5 and 12.5. The chemical composition and the crystal size of the synthesized powder have been studied as a function of pH, temperature, and the composition of the precursor solution, such as the type of hydroxide used and the zinc, gallium, and lithium content. Specimens have been analyzed by x-ray diffraction, scanning and transmission electron microscopy, energy-dispersive x-ray spectroscopy, thermogravimetry, and inductively coupled plasma atomic emission spectroscopy. It is shown that crystallite size and chemical composition strongly depend on the solubility of the gallium species within the solution, which appears to determine the number of ZnGa2O4 nuclei per unit volume. Crystals of a few micrometers in size can be synthesized from solutions where Ga is very soluble (e.g., pH ≥ 11), whereas when Ga is not very soluble, the synthesis product is nanocrystalline. Thermogravimetric studies indicate that a considerable amount of OH− is incorporated at the oxygen sites (every 5th to 10th site). Hydrothermal growth of heteroepitaxial zinc gallate thin films has been carried out on (111) MgAl2O4 and (00.1) LiNbO3 substrates. Heteroepitaxy is based on a coincident pattern of oxygen sites in the two structures; that is, (111)ZGO ∥ (111)MAO and (111)ZGO ∥ (00.6)LNO for the out-of plane orientation and (1¯10)ZGO ∥ (1¯10)MAO and (1¯10)ZGO ∥ (1¯10)LNO for the in-plane orientation, respectively. The thin films cover the substrate completely and consist of oriented grains of up to several hundred nanometers in size. In general, the films are twinned with two variants (in separate grains) that are related by a 60° rotation around [111]. This twinning can only be suppressed at high pH and high temperature and only on the MgAl2O4 substrates.

Analysis of the substrate influence on the ordering of epitaxial molecular layers: The special case of point-on-line coincidence

The physical structure of organic-inorganic heteroepitaxial thin films is usually governed by a fine balance between weak molecule-molecule interactions and a weakly laterally varying molecule-substrate interaction potential. Therefore, in order to investigate the energetics of such a layer system one has to consider large molecular domains. So far, layer potential calculations for large domains of organic thin films on crystalline substrates were difficult to perform concerning the computational effort which stems from the vast number of atoms which have to be included. Here, we present a technique which enables the calculation of the molecule-substrate interaction potential for large molecular domains by utilizing potential energy grid files. This technique allows the investigation of the substrate influence in systems prepared by organic molecular beam epitaxy (OMBE), like 3,4,9,10-perylenetetracarboxylicdianhydride on highly oriented pyrolytic graphite. For this system the so-called point-on-line coincidence was proposed, a growth mode which has been controversially discussed in literature. Furthermore, we are able to provide evidence for a general energetic advantage of such point-on-line coincident domain orientations over arbitrarily oriented domains which substantiates that energetically favorable lattice structures in OMBE systems are not restricted to commensurate unit cells or coincident super cells.

Raman Spectroscopy of Bulk and Thin-Layer (Ba,Sr)TiO3 Ferroelectrics

A comparative Raman study of (Ba,Sr)TiO3 bulk ceramics, sol-gel derived polycrystalline thin film, and heteroepitaxial thin film deposited by rf sputtering onto (001) MgO substrate is reported. It is found that phase transition sequence in the heteroepitaxial thin film is markedly different from those observed in the bulk ceramics and polycrystalline thin film.

Mechanism for Heteroepitaxial Growth of Transparent P-Type Semiconductor: LaCuOS by Reactive Solid-Phase Epitaxy

A unique epitaxial growth method, reactive solid-phase epitaxy, was used to fabricate heteroepitaxial thin films of LaCuOS, a transparent p-type semiconductor with layered structure. The epitaxial growth mechanism is examined through microscopic observations. A thin metallic Cu layer deposited between the amorphous LaCuOS (a-LaCuOS) and the yttria-stabilized zirconia (YSZ) single-crystal substrate before thermally annealing is a key to realizing the epitaxial LaCuOS films in this process. To grow the epitaxial films, it is critical for the Cu layer to have a discontinuous structure with triple junctions among the Cu, a-LaCuOS layer, and the YSZ substrate. The Cu layer is needed to create seed grains for the epitaxial LaCuOS at the junctions. The resulting seed grains work as an epitaxy template for subsequent epitaxial growth from the substrate to the film's top surface, and high temperatures such as 1000 °C are necessary to completely convert the a-LaCuOS layer to an epitaxial layer.

Phase field microelasticity modeling of surface instability of heteroepitaxial thin films

A development of the nonlinear surface instability of heteroepitaxial films during annealing process has been studied by using the phase field microelasticity model. The developed model considers the diffusional surface profile evolution of nonlinear grooved and coarsened regime, which is driven by the strain energy relaxation and is not constrained by any ad hoc assumptions concerning possible surface morphology. The method is based on the numerical solution of the exact elastic equilibrium equations. The simulation has shown that an initial small amplitude random roughening of a flat film develops to form a quasi-periodical array of islands under a certain balance between the strain and surface energies. This occurs through the competitive growth of amplitudes with different wavelengths. The surface morphology simultaneously grows and coarsens. The fully coarsened morphology exhibits a dominant wavelength that is significantly greater than the one that follows from the linear analysis of initial instability. The equilibrium morphology corresponding to a periodic array of islands is reached independent of the initial fluctuations of the surface profile.

Recent developments and applications of invariant integrals

Although invariant integrals (path independent integrals) have been used extensively in the 20th century, mainly in the calculation of dominant parameters that govern the initiation and propagation of both linear and nonlinear cracks, new applications are increasingly being identified. This article presents developments and applications of the invariant integrals in recent years, focusing on four major application areas: i) fracture mechanics of functional materials (eg, piezoelectric ceramics and ferromagnets), which exhibit features different from those found in purely mechanical problems due to the coupling of electric, magnetic, thermal, and mechanical quantities; ii) damage mechanics of multiple interacting cracks, and new damage measures; iii) domain integrals, two-state integrals, and their applications in determining the dominant parameters of 3D cracks and in clarifying the role of higher order singular terms in the Williams eigenfunction expansions; and iv) nano-structures (eg, stress driven surface evolution in a heteroepitaxial thin film). In writing this review article, we have been able to draw upon a large number of published works on invariant integrals over the last three decades, and yet it is impossible to cover the whole subject in the limited space available. Consequently, the main aim of the article is to summarize the major developments and applications in the four important areas mentioned above. Still, 261 references are reviewed in the article.

Optical and electrical properties of epitaxial (Mg,Cd) xZn 1− xO, ZnO, and ZnO:(Ga,Al) thin films on c-plane sapphire grown by pulsed laser deposition

A consistent set of epitaxial, n-type conducting ZnO thin films, nominally undoped, doped with Ga or Al, or alloyed with Mg or Cd, was grown by pulsed laser deposition (PLD) on single-crystalline c-plane sapphire (0 0 0 1) substrates, and characterized by Hall measurement, and UV/VIS optical transmission spectroscopy. The optical band gap of undoped ZnO films at nearly 3.28 eV was shifted by alloying with Mg up to 4.5 eV and by alloying with Cd down to 3.18 eV, dependent on the alloy composition. In addition, highly doped ZnO:Al films show a blue-shifted optical absorption edge due to filling of electronic states in the conduction band. The Hall transport data of the PLD (Mg,Zn,Cd)O:(Ga,Al) thin films span a carrier concentration range of six orders of magnitude from 3 × 10 14 to 3 × 10 20 cm −3, which corresponds to a resistivity from 5 × 10 −4 to 3 × 10 3 Ω cm. Structurally optimized, nominally undoped ZnO films grown with ZnO nucleation and top layer reached an electron mobility of 155 cm 2/V s (300 K), which is among the largest values reported for heteroepitaxial ZnO thin films so far. Finally, we succeeded in combining the low resistivity of ZnO:Ga and the band gap shift of MgZnO in MgZnO:Ga thin films. This results demonstrate the unique tunability of the optical and electrical properties of the ZnO-based wide-band gap material for future electronic devices.

Phase field microelasticity modeling of dislocation dynamics near free surface and in heteroepitaxial thin films

Dislocation dynamics near a free surface and in heteroepitaxial thin films are simulated using an extended version of the nanoscale Phase Field Microelasticity model of dislocations [Acta Mater. 49 (2001) 1847]. The model automatically takes into account the effect of image forces on dislocation motions. In particular, the operations of Frank–Read sources in epitaxial films grown on infinitely thick and relatively thin substrates are investigated. The simulation reveals different misfit dislocation behaviors at the interface. Its implication on the interface susceptibility to crack nucleation is discussed.

Phase transitions in (Ba0.7Sr0.3)TiO3/(001)MgO thin film studied by Raman scattering

Polarized Raman spectra of heteroepitaxial (Ba0.7Sr0.3)TiO3 thin film deposited on the (001)MgO substrate have been studied in the broad temperature range from 30 to 1300 K. Our results show that the ferroelectric–paraelectric phase transition is markedly shifted to higher temperatures with respect to the bulk ceramics of the same composition. Because of the two-dimensional compressive stresses imposed on the film by the substrate, the symmetry of the paraelectric phase is found to be tetragonal well above the Curie temperature. Near the deposition temperature (1173 K), the thermoelastic compressive stresses vanish and Raman response suggests nearly cubic symmetry. Raman measurements below room temperature show clearly that the phase transition to the rhombohedral phase is completely suppressed and only one phase transition from the tetragonal to the monoclinic phase is revealed at ∼150 K.

Annealing of the radiation damage in Mg-implanted GaN thin films: Temperature development of lattice parameters and stresses

Heteroepitaxial GaN thin films implanted with Mg ions with a concentration of 1.3 × 10 19 cm −3 are analyzed using in-situ X-ray diffraction in the temperature range of 20–700 °C. The temperature dependence of unstressed lattice parameters and stresses in the implanted films is evaluated and compared with the results from a virgin GaN thin film. The measurements indicate that the annealing of the radiation damage in the implanted GaN is accompanied by a unique temperature hysteresis of the GaN structural characteristics and the main part of the radiation damage is removed during heating in the temperature range of 100–300 °C. The temperature of 1620 °C is extrapolated as an important annealing limit in order to significantly decrease implantation-induced disorder in the films.

Reduced critical thickness for relaxing heteroepitaxial films on compliant substrates

It is argued that heteroepitaxial thin films (layers) grown on a compliant substrate are not able to relax their strain elastically by large-area slip across a “weak” layer. Instead, the Matthews model of plastic relaxation is modified by supposing that the interfacial misfit dislocations relax their strain field or even disappear into the weak layer. Consequently, the moving film-threading dislocations experience a reduced drag force. Therefore, the critical film thickness is lowered, in contrast to the enhanced thickness predicted by current theories. A quantitative estimate is given which depends on the nature of the weak layer. Implications include a larger free slip path and potentially a lower density of film-threading dislocations.

Molecular dynamics simulations of dislocation instability in a stress gradient

We present simulation studies of a morphological instability arising in dislocation dynamics. When an initially straight dislocation line is driven by a stress whose magnitude grows along the direction of dislocation motion, vibrational modes of the dislocation line with wavelength above a threshold value become linearly unstable. Molecular dynamics simulation studies of screw dislocations in Al demonstrate the onset of the instability during dislocation pair annihilation and annihilation at a crack tip. The wavelength of the unstable vibrational mode observed in each case agrees with the predictions of theoretical analysis. We discuss the role of temperature in nucleating the instability and speculate about how instabilities in dislocation motion could affect the density of threading dislocations during growth of heteroepitaxial thin films.

Off-axis 고주파 마그네트론 스퍼터링법을 이용한 이종에피텍셜 ZnO 박막 성장

Off-axis 고주파 마그네트론 스퍼터링법을 이용한 이종에피텍셜 ZnO 박막 성장

Theory of strain relaxation in heteroepitaxial systems

We introduce a general approach to calculating the morphological consequences of coherent strain relaxation in heteroepitaxial thin films based on lattice statics using linear elasticity. The substrate and film are described by a simple cubic lattice of atoms with localized interactions. The boundary conditions at concave and convex corners that appear as a result of this construction, those along straight interfacial segments, and the governing equations are obtained from a variational calculation applied to a discretized form of the total elastic energy. The continuum limit of the equations and the boundary conditions along straight boundaries reproduces standard results of elasticity theory, but the boundary conditions at corners have no such analog. Our method enables us to calculate quantities such as the local strain energy density for any surface morphology once the lattice misfit and the elastic constants of the constituent materials are specified. The methodology is illustrated by examining the strain, displacement, and energies of one-dimensional strained vicinal surfaces. We discuss the effects of epilayer thickness on the energy of various step configurations and suggest that coupling between surface and substrate steps should affect the equilibration of the surface toward the bunched state.

Growth of iron films on MgO(100) substrates at 560-600 K

The growth mechanism of heteroepitaxial thin iron films, deposited on MgO(100) substrates at 560-600 K, was investigated by means of tunnelling electron microscopy and spectroscopy. Scanning tunnelling microscopy (STM) analysis of the surface indicates the formation of long flat (about 10-20 nm) terraces for thicknesses greater than 3-4 nm. The fine structure of those samples contains sets of interpenetrating screw and helical dislocations. The onset of the formation of islands on these terraces and also their sizes were a result of stress relaxation in these films and this was supported by the Monte Carlo simulations. The resulting critical number of island layers needed to build up the gain in energy in a relaxed volume (compensating the misfit losses on its boundary) perfectly correlates with the observed STM pattern. At even larger thicknesses (greater than 9-10 nm) the surface becomes uniformly flat with a large number of mosaic voids. The differential conductivity spectra show the distinct surface peak of iron, positioned between the values for the iron crystal surface and the atomic limit. Manipulation of iron nanoparticles from the tip to the surface and back is demonstrated.

Preparation of epitaxial Pb(Mg1/3Ta2/3)O3 thin film by chemical solution deposition

Pb(Mg1/3Ta2/3)O3 (PMT) powders and heteroepitaxial thin films on (001) SrTiO3 (STO) substrates were prepared by the chemical solution deposition method. The phase development in PMT powders and thin films was investigated by x-ray diffraction, and the microstructure of PMT thin films was studied by transmission electron microscopy. Phase transformation from pyrochlore to perovskite in the PMT powder samples occurred in the temperature range of 700–800 °C, and perovskite phase was mainly observed in samples annealed above 800 °C. However, perovskite phase in PMT thin films on STO substrates was formed mainly in samples annealed as low as 750 °C/1 h, and it shows an epitaxial orientation relationship of [100](001)PMT∥[100](001)STO.

Hydrothermal epitaxy of KtaO3 thin films

Heteroepitaxial KTaO3 thin films were synthesized by the hydrothermal method below 175 °C in highly alkaline aqueous KOH solutions. KTaO3 films synthesized in 7 M KOH solutions contained a defect pyrochlore phase that had (400) and (111) out-of-plane orientations. The (400) oriented pyrochlore islands were epitaxially related to the (100) SrTiO3 substrate while the (111) oriented pyrochlore islands had four in-plane variants. Pyrochlore-free KTaO3 films could be synthesized in a 15 M KOH solution. Pyrochlore-free films were also grown at 7 M KOH by delaying the introduction of the (100) SrTiO3 substrate into the synthesis solution until the conditions favoring the formation of the pyrochlore phase had dissipated.