Lattice-mismatched Films Research Articles

Effect of substrate-induced lattice strain on the electrochemical properties of pulsed laser deposited nickel oxide thin film

The storage of renewable energy is an important step toward the global effort to combat air contamination and climate change. In this work, the influence of substrate-induced strain on the electrocatalytic properties of nickel oxide (NiO) films toward the hydrogen evolution reaction (HER) is studied. Using pulsed laser deposition, NiO thin films were deposited on strontium titanate, lanthanum aluminate, and sapphire substrates to examine how the substrate–film lattice mismatch influences the electrochemical properties. It was observed that the electrocatalytic activities of the NiO thin films exhibited strong sensitivity to strain; the NiO film with the smallest strain recorded the lowest overpotential for the HER. The NiO films were further explored to estimate the charge storage capacity and surface roughness. This work shows the use of simple thin-film synthesis as a way to evaluate the strain effect in electrocatalysis.

Magnetic-force microscopy of thin Bi:IG films for thermomagnetic recording

Thermomagnetic recording on thin films is currently one of the most promising methods for copying records with high density and weak magnetization. These films can be characterized by a couple of parameters very suitable for recording reconfigurable magnetic atomic traps whose geometry is visible. Different types of materials are investigated to be the most effective for thermomagnetic recording propose. In this paper authors showed possibility and features of thermomagnetic recording on Bi:IG thin films. High coercivity thin films with low Curie temperature and different film-lattice mismatch parameters were synthesized. Thermomagnetic recording of magnetic disc was performed. Features of magnetic records were investigated and influence of film-lattice mismatch parameters on meta-stable domain structure that defines the thermomagnetic recording resolution was showed. Magnetic-force microscopy was used for topography and magnetic structure characterization of thin films with different film-lattice mismatch parameters. Some features of films morphology were investigated using atomic-force microscopy with sharp AFM probe.

Thin-film growth and structural characterization of a novel layered iridate Ba7Ir3O13+δ

Iridates have attracted immense interest since their strong spin–orbit coupling (SOC) can lead to rich exotic phenomena such as a Jeff = ½ Mott insulating state. Here we report a novel iridate discovered in our efforts which aimed to synthesize Ba2IrO4 thin films. Through systematic transmission and scanning transmission electron microscopy studies, we have shown this new compound possesses a layered orthorhombic structure with the composition of Ba7Ir3O13+δ (BIO). This material is an insulator with an optical band gap of ∼1.3 eV. Furthermore, we found that the thin films of this material can be grown on differently orientated perovskite substrates or MgO substrates. Although all these films maintain an identical crystallographic orientation, i.e. its c-axis perpendicular to the substrate surface, they form various domain structures dependent on the substrate. When (001)-oriented LaAlO3 and (111) oriented SrTiO3 perovskites are used as substrates, the domains show 12 fold and 6 fold symmetry respectively, and the domain orientations are highly coherent and the domain-walls are atomically sharp. However, the films on the (110) oriented MgO substrates feature much less coherent domain walls and thread dislocations occur at the domain boundaries. These findings not only reveal a new playground for the study of the novel SOC physics of iridates, but also provide a route to tailor the domain wall structure via epitaxial lattice mismatch in films.

Perspective: Fundamentals of coalescence-related dislocations, applied to selective-area growth and other epitaxial films

Although selective area growth (SAG) and coalesced SAG (cSAG) have been utilized extensively for many years to moderate the material quality of lattice-mismatched films, the geometrical factors controlling dislocations in coalesced films are difficult to visualize, and some confusion regarding the topology of dislocations for cSAG still persists. In this paper, we describe the topology of dislocations during island coalescence for cSAG, framed in terms of fundamental dislocation properties and virtual dislocations, which are used as a helpful visualization tool. We also show how the results and methods are generally applicable to coalescence of any other epitaxial film.

Surface acoustic waves in strain-engineered K0.7Na0.3NbO3 thin films

Epitaxial K0.7Na0.3NbO3 thin films are grown via metal-organic chemical vapor deposition on (110)-oriented TbScO3. The films are strained due to the substrate–film lattice mismatch and therefore exhibit a strong and anisotropic modification of all its ferroelectric properties. The compressive in-plane strain leads to a reduction of the ferroelectric transition temperature from approximately 700 K for unstrained K0.7Na0.3NbO3 to 324 K and 330 K with maximum permittivities of 10 270 and 13 695 for the main crystallographic directions [001]TSO and [11¯0]TSO, respectively. Moreover, the quite thin films (approx. 30 nm thick) exhibit very large piezoelectric properties. For instance, surface acoustic waves with intensities of up to 4.7 dB are recorded for wave propagation along the [11¯0]TSO direction. The signal is smaller (up to 1.3 dB) along [001]TSO, whilst for the intermediate direction [11¯2]TSO, the signal seems to vanish (<0.1 dB). The results indicate that the choice of material, (K,Na)NbO3, in combination with strain-engineering via epitaxial growth onto lattice-mismatched substrates represents a promising way to optimize ferroelectric materials for piezoelectric thin-film applications.

Anomalous lattice deformation in GaN/SiC(0001) measured by high-speed in situ synchrotron X-ray diffraction

We report an anomalous lattice deformation of GaN layers grown on SiC(0001) by molecular beam epitaxy. The evolution of the lattice parameters during the growth of the GaN layers was measured by in situ synchrotron X-ray diffraction. The lattice parameters in the directions parallel and normal to the surface showed significant deviation from the elastic strains expected for lattice-mismatched films on substrates up to a thickness of 10 nm. The observed lattice deformation was well explained by the incorporation of hydrostatic strains due to point defects. The results indicate that the control of point defects in the initial stage of growth is important for fabricating GaN-based optoelectronic devices.

Analytical Bond-Order Potential for the Cd–Te–Se Ternary System

CdTe/CdSe core/shell structured quantum dots do not suffer from the defects typically seen in lattice-mismatched films and can therefore lead to improved solid-state lighting devices as compared to the multilayered structures (e.g., InxGa1–xN/GaN). To achieve these devices, however, the quantum dots must be optimized with respect to the structural details at an atomistic level. Molecular dynamics simulations are effective for exploring nano structures at a resolution unattainable by experimental techniques. To enable accurate molecular dynamics simulations of CdTe/CdSe core/shell structures, we have developed a full Cd–Te–Se ternary bond-order potential based on the analytical formalisms derived from quantum mechanical theories by Pettifor et al. A variety of elemental and compound configurations (with coordination varying from 1 to 12) including small clusters, bulk lattices, defects, and surfaces are explicitly considered during potential parametrization. More importantly, enormous iterations are performed to strictly ensure that our potential can simulate the correct crystalline growth of the ground-state structures for Cd, Te, and Se elements as well as CdTe, CdSe, and CdTe1–xSex compounds during molecular dynamics vapor deposition simulations. Extensive test simulation results clearly indicate that our new Cd–Te–Se potential has unique advantages over the existing literature potential involving Cd, Te, and Se elements.

Pulsed laser deposition of La1−xSrxMnO3: thin-film properties and spintronic applications

Materials engineering on the nanoscale by precise control of growth parameters can lead to many unusual and fascinating physical properties. The development of pulsed laser deposition (PLD) 25 years ago has enabled atomistic control of thin films and interfaces and as such it has contributed significantly to advances in fundamental material science. One application area is the research field of spintronics, which requires optimized nanomaterials for the generation and transport of spin-polarized carriers. The mixed-valence manganite La1−xSrxMnO3 (LSMO) is an interesting material for spintronics due to its intrinsic magnetoresistance properties, electric-field tunable metal–insulator transitions, and half-metallic band structure. Studies on LSMO thin-film growth by PLD show that the deposition temperature, oxygen pressure, laser fluence, strain due to substrate–film lattice mismatch and post-deposition annealing conditions significantly influence the magnetic and electrical transport properties of LSMO. For spintronic structures, robust ferromagnetic exchange interactions and metallic conductivity are desirable properties. In this paper, we review the physics of LSMO thin films and the important role that PLD played in advancing the field of LSMO-based spintronics. Some specific application areas including magnetic tunnel junctions, multiferroic tunnel junctions and organic spintronic devices are highlighted, and the advantages, drawbacks and opportunities of PLD-grown LSMO for next-generation spintronic devices are discussed.

Growth Aspects of Thin-Film Composite Heterostructures of Oxide Multicomponent Perovskites for Electronics

We review, based on our results, the problems and solutions for the growth of thin films and composite heterostructures emphasizing the general growth aspects and principles vs specifics for each material or heterostructure. The materials used in our examples are Bi2Sr2Ca2Cu3O10, Bi2Sr2CaCu2O8, YBa2Cu3O7, (Sr, Ca)CuO2, (Ba, Ca)CuO2, and Bi4Ti3O12. The growth method was metal organic chemical vapor deposition (MOCVD). The presented thin films or heterostructures have c- and non-c-axis orientations. We discuss the implications of the film–substrate lattice relationships, paying attention to film–substrate lattice mismatch anisotropy and to film–film lattice mismatch, which has a significant influence on the quality of the non-c-axis heterostructures. We also present growth control through the use of vicinal substrates and two-temperature (template) and interrupted growth routes allowing significant quality improvements or optimization. Other key aspects of the growth mechanism, that is, roughness, morphology, and interdiffusion, are addressed. It is concluded that the requirements for the growth of non-c-axis heterostructures are more severe than those for the c-axis ones.

Stress relaxation scenario for buried quantum dots

We report on theoretical and experimental studies of general critical conditions for the onset of relaxation in stressed quantum dots (QDs) embedded into a crystalline matrix. The fingerprint of the relaxation scenario is the formation of specific satellite dislocation loops. The scenario is applicable to both the QDs nucleated and ripened in the bulk and the QDs formed on the surface and then buried by the overgrowth. The results demonstrate that the critical relaxation radius for buried QDs is larger than the critical thickness for stressed lattice-mismatched films or surface islands.

A model of threading dislocation density in strain-relaxed Ge and GaAs epitaxial films on Si (100)

Strain relaxation in large lattice-mismatched epitaxial films, such as Ge and III-V materials on Si, introduces high threading dislocation densities (TDDs). A thermodynamic model of TDD dependence on film thickness is developed. According to this model, the quasiequilibrium TDD of a given strain-relaxed film scales down with the inverse square of its thickness. The quasiequilibrium TDDs in both Ge and GaAs films follow this model consistently. Our model predicts the lowest possible TDD of a large lattice-mismatched film on Si (100), which is determined by the dislocation glide activation energy and the film thickness.

Influence of misfit stress on the structure and magnetoresistive properties of magnetron sputtered La 0.7Sr 0.3MnO 3 thin films on LaAlO 3(1 0 0) substrate

La 0.7Sr 0.3MnO 3 (LSMO) films of variable thickness were deposited by magnetron sputtering on a LaAlO 3(1 0 0) substrate. The substrate–film lattice mismatch causes changes in the microstructure and magnetoresistance. The film structure is characterized by XRD. The substrate–film lattice mismatch is compensated by large stress in the LSMO layer at interface. With increasing film thickness this stress relaxes and the upper film layer exhibits bulk value lattice parameters. It was shown that deposition conditions also influence film stress. Due to variable strain in the thin LSMO films, different magnetoresistive behaviour has been found. The sample with the lowest thickness of 10 nm showed semiconducting behaviour. Epitaxial LSMO films of thickness 20 nm< d<215 nm exhibit a high Curie temperature up to T C∼365 K and colossal magnetoresistance behaviour. A highly stressed film with a metal–insulator transition at ∼160 K showed both high negative magnetoresistance at T=150 K and marked high positive magnetoresistance at T<50 K.

Phase transitions of self-polarized PZT thin films

AbstractOptical properties and thermal properties of self-polarized thin PZT films were determined by variable angle spectral ellipsometry and by the AC hot strip method, respectively. Analyzing the temperature dependencies of the optical gap and the specific heat, evidence of two not as yet observed phase transitions in the ferroelectric was provided. The origin of these phase transitions was attributed to film stress caused by substrate/PZT thin film lattice mismatch and to the presence of a negative space charge layer in the PZT film at the bottom electrode/PZT interface.

Effect of substrate–film lattice mismatch in La0.7Ba0.3MnO3−δ thin films for transport properties

In this study, we explore the use of La0.7Ba0.3MnO3−δ (LBMO) for use in uncooled infrared bolometers. To obtain a high temperature coefficient of resistance (TCR) (defined as 1/R dR/dT) at near-room temperature, thin films were deposited on SrTiO3 (100), LaAlO3 (100) and MgO (100) substrates by a laser ablation method. The substrate–film lattice mismatch between these substrates and LBMO films caused misfit stress which changed the surface structure of the films and influenced their temperature dependence of resistivity. The microstructures of films were characterized by x-ray diffraction and atomic force microscope. As a result, the LBMO film deposited on SrTiO3 (the LBMO/STO film) had crystallinity like LBMO bulk and showed high orientation of the substrate. And its temperature dependence of resistivity agreed with LBMO bulk. Near 300 K the LBMO/STO film showed high TCR values, about 4%/K. In addition, the TCR values of the LBMO/STO films were over 2%/K over a wide temperature range centered near 300 K. These results suggest that LBMO/STO films are possible for use as an uncooled infrared bolometer material.

Kinetics of strain relaxation in semiconductor films grown on borosilicate glass-bonded substrates

Changes in the strain relaxation of semiconductor films due to growth on a thin epitaxial template bonded via a borosilicate glass to a mechanical handle wafer have been observed. Akinetic analysis of the mechanical decoupling between the film/template heterostructure and the handle wafer is developed in order to estimate and evaluate the contribution of glass viscous deformation to the observed strain relaxation. The thickness and elastic constants of the template and film, the thickness and viscosity of the bonding media, and the lateral dimension of the bonded structure are included in this model. Based on this model, the viscous flow of the glass is unlikely to have played a role in previous observations of changes in film strain relaxation due to growth on glass-bonded substrates. The calculations are used in conjunction with simple expressions for the temperature and composition dependence of borosilicate glass viscosity to result in design guidelines for substrate structures and annealing schedules in which the viscous flow of a borosilicate glass will contribute to the strain relaxation of a lattice-mismatched film.

Influence of misfit stress on the magnetoresistive properties of La0.7Ca0.3MnO3−δ thin films

For this study La0.7Ca0.3MnO3−δ (LCMO) films of various thicknesses were deposited on SrTiO3(100), LaAlO3(100), and MgO(100) substrates. The substrate–film lattice mismatch causes changes in the microstructure of the films and can therefore influence their magnetoresistive behavior. The structure of the films was characterized by x-ray diffraction and high-resolution transmission electron microscopy. Magnetic and electric properties were determined using standard techniques. Samples grown on SrTiO3 show a strongly distorted growth. A large number of defects are incorporated in the initial LCMO layers. Beginning at a thickness of approximately 500 nm films on these substrates show strong deviations in electric behavior and develop macroscopic cracks. In the case of LaAlO3 substrates the films show only a few lattice defects. The substrate–film lattice mismatch is compensated by large stresses in the LCMO films. The best samples with the least amount of defects are grown on MgO substrates even though the substrate–film mismatch is largest in this case. The misfit is completely compensated in a narrow region near the substrate interface. From there on the LCMO grows with only few lattice defects and little strain. These films exhibit electric and magnetic properties similar to bulk materials. The results demonstrate that the colossal magnetoresistance properties of thin films are significantly influenced by strain caused by the substrate. They behave different than bulk samples under hydrostatic pressure. These aspects must be considered when interpreting the properties of such films and comparing the results of individual experiments.

Infared spectroscopic, x-ray, and nanoscale characterization of strontium titanate thin films

Attenuated total reflectance (ATR) measurements were performed using Fourier transform infrared (FTIR) spectroscopy in the ATR mode with a thallium iodobromide (KRS-5) crystal to measure the frequencies of the ν3 and ν4 phonon absorption bands in thin strontium titanate films deposited on single-crystal yttriumbarium copper oxide (YBCO), lanthanum aluminate, magnesium oxide, and strontium titanate substrates. The KRS-5 crystal enabled FTIR-ATR measurements to be made at frequencies above 400 cm−1. Atomic force microscopy (AFM) and X-ray diffraction (XRD) measurements were also made to further characterize the films. The measurements were repeated on single-crystal specimens of strontium titanate and the substrates for comparison. Softening in the frequency of the ν4 transverse optical phonon in the lattice-mismatched films below the established value of 544 cm−1 is indicative of the highly textured, polycrystalline ceramic nature of the films and is consistent with the XRD and AFM results.

New material for photoemission electron source: semiconductor alloy InGaAsP grown on GaAs substrate

Wide-bandgap epitaxial In x Ga 1− x As y P 1− y layers grown on GaAs substrates were investigated as a material for photoemission electron sources for the first time. By variation of x and y, both thick (of about one μm) lattice-matched unstrained layers and thin (of about 0.2 μm) lattice-mismatched strained layers with direct energy gap from 1.78 to 1.92 eV were grown. The layers were chemically treated in a glove-box, loaded into UHV via a loading chamber without exposure to air, heat cleaned, and activated to the state of negative electron affinity (NEA). For InGaAsP it was possible to reach the NEA-state by an activation with cesium only, without oxygen. The estimated e-folding lifetime of InGaAsP photocathodes operating at DC current of 400 μA was above one thousand hours in the regime with automatical additional cesiations and above one hundred hours without additional cesiations. Lifetime limitation factors are discussed. Strain-induced splitting of the valence band up to 32 meV was observed in the lattice-mismatched films; this observation gives prospects for an increase of spin polarization beyond the 50% limit.

Heteroepitaxy of GexSi1−x on porous Si substrates

Molecular-beam epitaxial growth of GexSi1−x on 〈100〉 porous Si substrate was studied using Rutherford backscattering spectrometry, transmission electron microscopy, and defect etching. This study was stimulated by the theoretically predicted possibility of stress field reduction in a lattice-mismatched film grown on a patterned substrate. The experimental results show predominantly 60° dislocations with long misfit segments, and no reduction of dislocation density is observed by the use of porous Si substrates. The nature of interconnected growth area of the porous Si substrate is believed to be the reason for the ineffectiveness of the stress reduction.

The initial growth stages of III–V lattice-mismatched films grown by liquid phase epitaxy

Abstract It is shown that the liquid phase must be supersaturated (supercooled) in quasi-equilibrium with the substrate if the resulting film and the seed are lattice mismatched; otherwise the substrate dissolves. The following characteristics of the film formation are observed: (1) etch-back of the seed and the growth of centers occur simultaneously, and as a result the composition of the centres varies continuously; (2) the centres of a new phase or even a continuous film may grow at constant temperature if the mismatch is large.