Epitaxial Growth Of Thin Films Research Articles

Effect of nitrogen vacancies on the growth, dislocation structure, and decomposition of single crystal epitaxial (Ti1-xAlx)Ny thin films

The effect of varying nitrogen vacancies on the growth, microstructure, spinodal decomposition and hardness values of predominantly single crystal cubic phase c-(Ti1-xAlx)Ny films was investigated. Epitaxial c-(Ti1−xAlx)Ny films with y = 0.67, 0.79, and 0.92 were grown on MgO(001) and MgO(111) substrates by magnetron sputter deposition. High N vacancy c-(Ti1−xAlx)N0.67 films deposited on MgO(111) contained coherently oriented w-(0001) structures while segregated conical structures were observed on the films grown on MgO(001). High resolution STEM images revealed that the N-deficient growth conditions induced segregation with small compositional fluctuations that increase with the number of N vacancies. Similarly, strain map analysis of the epitaxial c-(Ti1−xAlx)Ny (001) and (111) films show fluctuations in strain concentration that scales with the number of N vacancies and increases during annealing. The spinodal decomposition coarsening rate of the epitaxial c-(Ti1−xAlx)Ny films was observed to increase with decreasing N vacancies. Nanoindentation showed decreasing trends in hardness of the as-deposited films as the N vacancies increase. Isothermal post-anneal at 1100 °C in vacuum for 120 min revealed a continuation in the increase in hardness for the film with the largest number of N vacancies (y = 0.67) while the hardness decreased for the films with y = 0.79 and 0.92. These results suggest that nitrogen-deficient depositions of c-(Ti1-xAlx)Ny films help to promote a self-organized phase segregation, while higher N vacancies generally increase the coherency strain which delays the coarsening process and can influence the hardness at high temperatures.

Nano-interface engineering in all-solid-state lithium metal batteries: Tailoring exposed crystal facets of epitaxially grown LiNi0.5Mn1.5O4 films

Understanding the solid electrolyte/cathode nano-interfacial kinetics is critical in designing advanced all-solid-state lithium metal batteries with improved performance and stability. However, the correlation between crystallographic features of cathodes and the solid nano-interface behaviour remains controversial due to the difficulty in eliminating the impact of other factors. Here, we systematically investigated the effect of exposed crystal facets of LiNi 0.5 Mn 1.5 O 4 on the solid nano-interface using the substrate orientation-dependent epitaxial growth of thin films as a model study. (100), (110), and (111)-oriented Pt/MgO substrates were used to make selective high-quality epitaxial LiNi 0.5 Mn 1.5 O 4 films with different {100}/{111}-exposed facet ratios. The atomic arrangement of the exposed facets was found to affect the electrochemical performance. Loosely packed {100} facets and densely packed {111} facets were beneficial for lithium ion diffusion and cycle stability, respectively. In particular, stable {111} facets effectively suppressed the dissolution and diffusion of transition metals at the solid nano-interface during charge-discharge, enabling a 99.6% retention after 100 cycles. In addition, this model study reveals that an amorphous cathode surface layer and a twin boundary inside the cathode are crystallographic origins that hinder the electrochemical performance of batteries. These findings suggest that crystallographic modifications of cathodes can be a key to improving the solid nano-interface. • Precise epitaxial growth creates different exposed crystal facets of LiNi 0.5 Mn 1.5 O 4. • Loosely packed {100} facets induces high capacity via fast lithium ion diffusion. • {111} facets blocks mutual diffusion across the nano-interface, enhancing stability. • Thin film model reveals crystallographic origins of the solid interface degradation.

Ferromagnetic Rh2Mn5Bi4 thin film alloy epitaxially grown on MgO(001)

Epitaxial growth of ferromagnetic Rh2Mn5Bi4 thin films from the family of X2Mn5Bi4 (X = Cu, Ni, Pd and Rh) compounds was pursued employing magnetron sputtering on MgO(001) substrates. Composition, structure and microstructure of prepared films were studied using a wide range of diffraction, microscopy and spectroscopy techniques, while Superconducting quantum interference device and Magneto-Optical Kerr Effect were used for magnetic and magneto-optical characterizations. The huge difference between Rh and Bi in their physicochemical properties - melting points, vapor pressures and deposition yields - manifested in a substantial influence of growth temperature on resulting properties of prepared films. Rh2Mn5Bi4 alloy with characteristic microstructure and favorable ferromagnetic and magneto-optical response was obtained after growth at 400 °C. The arrangement of Mn atoms within the structure should give rise to the magnetic moment in a similar fashion as in Heusler alloys. Curie temperature of these films was found to be 270 K. The films appear to be promising for magnetic or spintronic applications.

Improved Structural Properties in Homogeneously Doped Sm0.4Ce0.6O2-δ Epitaxial Thin Films: High Doping Effect on the Electronic Bands.

The study of ionic materials on nanometer scale is of great relevance for efficient miniaturized devices for energy applications. The epitaxial growth of thin films can be a valid route to tune the properties of the materials and thus obtain new degrees of freedom in materials design. High crystal quality SmxCe1-xO2-δ films are here reported at a high doping level up to x = 0.4, thanks to the good lattice matching with the (110) oriented NdGaO3 substrate. X-ray diffraction and transmission electron microscopy demonstrate the ordered structural quality and absence of Sm segregation at the macroscopic and atomic level, respectively. Therefore, in epitaxial thin films, the homogeneous doping can be obtained even with the high dopant content not always approachable in bulk form, getting even an improvement of the structural properties. In situ spectroscopic measurements by X-ray photoemission and X-ray absorption show the O 2p band shift toward the Fermi level, which can favor the oxygen exchange and vacancy formation on the surface when the Sm doping is increased to x = 0.4. X-ray absorption spectroscopy also confirms the absence of ordered oxygen vacancy clusters and further reveals that the 5d eg and t2g states are well separated by the crystal field in the undistorted local structure even in the case of a high doping level up to x = 0.4.

Realization of closed-loop optimization of epitaxial titanium nitride thin-film growth via machine learning

Closed-loop optimization of epitaxial titanium nitride (TiN) thin-film growth was accomplished using metal-organic molecular beam epitaxy (MO-MBE) combined with a Bayesian machine-learning technique and reduced the required number of thin-film growth experiments. Epitaxial TiN thin films grown under the process conditions optimized by the Bayesian approach exhibited abrupt metal–superconductor transitions above 5 K, demonstrating a new approach to the efficient development of less-studied materials, such as transition metal nitrides. The combination of the thin-film growth technique and Bayesian approach is expected to pave the way toward accelerating the development of the automated operation of thin-film growth apparatuses.

AgGaTe2 – The thermoelectric and solar cell material: Structure, electronic, optical, elastic and vibrational features

Silver gallium telluride AgGaTe2 is a promising and one of few multifunctional energy convertor crystal for processing solar energy or waste heat into electricity (by the thermoelectric, photovoltaic and recently intensively studied thermophotovoltaic systems). Its electronic structure, optical, elastic and vibrational properties were consistently and systematically studied in the framework of the first principles calculations. It should be emphasized that the results of our investigations were verified by comparison with the collected available experimental and vestigial theoretical data. The main spectral features of the electronic band structure and origin of the electronic levels affecting the physical and chemical properties are analyzed in detail. Spectral dependencies of both real ε1 and imaginary ε2 components of dielectric function ε(ω) have been calculated showing a good agreement with spectral ellipsometry measurements. The anisotropy of optical and elastic properties, which is important for possible applications, has been highlighted by means of the 2D and 3D graphs. Phonon dispersion curves and corresponding density of states are studied in the frame of linear response approach based on the density functional perturbation theory. Unified approach focused on the analysis of a wide-range physical properties of anisotropic crystals not only results in good agreement with the experimental data, but also allows to explain the observed optical effects at the microscopic level. Therefore, the presented results should be very useful for future possible improvements in the desired properties of the base material both by changing the chemical composition and/or introducing stresses by epitaxial growth of thin films.

Low Thermal Budget Heteroepitaxial Gallium Oxide Thin Films Enabled by Atomic Layer Deposition.

This work explores the applicability of atomic layer deposition (ALD) in producing highly oriented crystalline gallium oxide films on foreign substrates at low thermal budgets. The effects of substrate, deposition temperature, and annealing process on formation of crystalline gallium oxide are discussed. The Ga2O3 films exhibited a strong preferred orientation on the c-plane sapphire substrate. The onset of formation of crystalline gallium oxide is determined, at which only two sets of planes, i.e., α-Ga2O3 (006) and β-Ga2O3 (4̅02), are present parallel to the surface. More specifically, this work reports, for the first time, that epitaxial gallium oxide films on sapphire start to form at deposition temperatures ≥ 190 °C by using an optimized plasma-enhanced ALD process such that α-Ga2O3 (006)∥α-Al2O3 (006) and β-Ga2O3 (2̅01)∥α-Al2O3 (006). Both α-Ga2O3 (006) and β-Ga2O3 (2̅01) planes are polar planes (i.e., consisting of only one type of atom, either Ga or O) and, therefore, favorable to form by ALD at such low deposition temperatures. Ellipsometry and van der Pauw measurements confirmed that the crystalline films have optical and electrical properties close to bulk gallium oxide. The film grown at 277 °C was determined to have superior properties among as-deposited films. Using TEM to locate α-Ga2O3 and β-Ga2O3 domains in the as-deposited crystalline films, we proposed a short annealing scheme to limit the development of α-Ga2O3 domains in the film and produce pure β-Ga2O3 films via an energy-efficient process. A pure β-Ga2O3 phase on sapphire with β-Ga2O3 (2̅01)∥α-Al2O3 (006) was successfully achieved by using the proposed process at the low annealing temperature of 550 °C preceded by the low deposition temperature of 190 °C. The results of this work enable epitaxial growth of gallium oxide thin films, with superior material properties offered by ALD, not only with potential applications as a high-performance material in reducing global energy consumption but also with an energy-efficient fabrication process.

Adaptive Bayesian optimization for epitaxial growth of Si thin films under various constraints

We applied a Bayesian optimization (BO) to optimize the epitaxial growth process of Si thin films. The BO enables us to effectively explore the optimal film growth conditions considering several experimental parameters and their interactions. In this way we reduced the total number of experiments required in the optimization. The epitaxial growth rate was maximized while five quality parameters were maintained within an acceptable range. Additionally, we considered two practical issues: eliminating equipment errors and the time cost of the quality parameter evaluation. To overcome these issues, we adaptively conducted BO with different constraints for different situations. As a result of these optimizations, the crystal growth rate was increased to be approximately twice as high as that under standard conditions, while satisfying the five quality parameter conditions.

Epitaxial Thin Film Growth of Europium Dihydride

This paper reports the epitaxial growth of EuH2 thin films with an ω-scan full width at half-maximum of 0.07°, the smallest value for metal hydride thin films reported so far. The thin films were d...

Rock salt structure GdO epitaxial thin film with a high ferromagnetic Curie temperature

GdO with unusual valence of Gd2+ (4f75d1) has been known as the only gas phase so far. In this study, we report on the growth of solid phase rock salt structure GdO epitaxial thin films. The GdO thin film was an n-type semiconductor with the bandgap of 0.1 eV. The ferromagnetic Curie temperature (TC) was as high as 276 K, which was much higher than that of archetypal ferromagnetic rare earth monoxide EuO (TC = 69 K). TC increased slightly with electron doping similar to non-stoichiometric EuO1−δ, while decreased significantly with La doping probably caused by the magnetic dilution effect due to substitution of Gd with divalent La.

Tetragonal and uniaxial strains in pristine and doped half-Heusler AuMnSn alloy

Using the first-principles density functional theory calculations, we examined the effect of tetragonal strain (t-strain) and uniaxial strain (u-strain) on the electronic and magnetic properties of pristine and Sb-doped half-Heusler AuMnSn alloy. Our result shows that ferromagnetic half-Heusler AuMnSn (AMS) exhibits half metallic ferromagnetism (HMF) only from −8% to −10% u-strain. On the other hand, the Sb-doped half-Heusler AuMnSn (AMSS) exhibits HMF from −2% to 4% t-strain and from −4% to 2% u-strain while the width of the minority spin band gap remains unaltered. We also observed changes in the magnetic moments as the applied strain changes from compressive to tensile strain. Our result suggests the possibility of epitaxial growth of thin-film of half-metallic AMSS on a semiconductor substrate, even with a small lattice mismatch.

Frontispiece: Flexible and Epitaxial Metal Oxide Thin Film Growth by Photoreaction Processing for Electrical and Optical Applications

Frontispiece: Flexible and Epitaxial Metal Oxide Thin Film Growth by Photoreaction Processing for Electrical and Optical Applications

Comparative studies on optoelectronic properties of epitaxial MgxCr2-xO3 and AlxCr2-xO3 (x = 0, 0.1, 0.2 and 0.3) thin films deposited on sapphire substrates

In this report, we have investigated the comparative studies on structural, optical, and electrical properties of pristine Cr2O3 (PCO), magnesium (Mg) substituted Cr2O3 (MCO), and aluminium (Al) substituted Cr2O3 (ACO) thin films. The epitaxial pristine Cr2O3, MgxCr2-xO3, and AlxCr2-xO3 (x = 0.1, 0.2, and 0.3) thin films having a thickness of ∼74 ± 2 nm were deposited on (0001) sapphire substrates using pulsed laser deposition (PLD) technique. X-ray diffraction (XRD) results showed that all thin films are single crystalline with (006) orientation, and exhibited rhombohedral structure with space group R3−c. And crystal structure of Cr2O3 thin films remains unaffected with Mg and Al substitution. Also, ϕ-scan diffraction patterns confirmed the epitaxial growth of as-deposited thin films. Atomic force microscopy (AFM) illustrated the smooth morphology with extremely low roughness of thin films. Optical studies demonstrated that PCO thin film is highly transparent in the visible region with an optical band gap of 3.64 eV. However, optical band gap energy found to be decreasing with Mg content, while on the other hand, blue shift has been observed with Al substitution into Cr2O3 thin films. Room temperature electrical measurements revealed that PCO is insulating in nature. But a reduction in resistivity value has been observed from 126.19 (PCO) to 2.52 Ω-cm at x = 0.2 of Mg content (MCO) while retaining the p-type character. On the contrary, the substitution of Al into Cr2O3 thin films leads to an insulating sample. This work can lead to the development of a new class of p-type transparent conducting c-axis oriented MCO thin films as a potential candidate for optoelectronic devices-based applications.

Structural and Optical Properties of Phase-Pure UO2, α-U3O8, and α-UO3 Epitaxial Thin Films Grown by Pulsed Laser Deposition.

Fundamental understanding of the electronic, chemical, and structural properties of uranium oxides requires the synthesis of high-crystalline-quality epitaxial films of different polymorphs of one material or different phases with various oxygen valence states. We report the growth of single-phase epitaxial UO2, α-U3O8, and α-UO3 thin films using pulsed laser deposition. Both oxygen partial pressure and substrate temperature play critical roles in determining the crystal structure of the uranium oxide films. X-ray diffraction and Raman spectroscopy demonstrate that the films are single phase with excellent crystallinity and epitaxially grown on a variety of substrates. Chemical valance states and optical properties of epitaxial uranium oxide films are studied by X-ray photoelectron spectroscopy and UV-vis spectroscopy, which further confirm the high-quality stoichiometric phase-pure uranium oxide thin films. Epitaxial UO2 films show a direct band gap of 2.61 eV, while epitaxial α-U3O8 and α-UO3 films exhibit indirect band gaps of 1.89 and 2.26 eV, respectively. The ability to grow high-quality epitaxy actinide oxide thin films and to access their different phases and polymorphous will have significant benefits to the future applications in nuclear science and technology.

Diffusion of F atoms from fluoride substrates promotes the epitaxial growth of metal fluorides

We report the growth of YF3 epitaxial thin films by magnetron sputtering using MgF2(100) substrates as a fluorine source instead of fluorination gases. We find that the substrate selection based on the relative film/substrate Gibbs free energies of formation can drive F-ion diffusion from the fluoride substrates into the Y thin films. Due to the high diffusivity of F ions in YF3, this method can grow ∼40 nm thick epitaxial thin films with uniform F distribution. These results indicate that fluoride substrates can serve as the F-ion source, providing a novel synthetic route for growing metal-fluoride thin films.

Flexible and Epitaxial Metal Oxide Thin Film Growth by Photoreaction Processing for Electrical and Optical Applications.

This review summarizes the use of photoreactions that replace conventional heating processes for growing oxide thin films from chemical solutions. In particular, this review outlines key variables in photoreactions that affect epitaxial and polycrystalline thin film growth, including precursor materials, laser wavelength, laser fluence, and carbon. In addition, the features of the photoreaction process that can be controlled at a low temperature by oxygen non-stoichiometry are examined. Likewise, functions that are neither achieved by developing a gradient structure nor controlled by a thermal equilibrium reaction are detailed. Two new concepts are presented, known as photoreaction of nanoparticles (PRNP) and photoreaction of a hybrid solutions (PRHS), in which crystal nuclei are pre-dispersed in a metal-organic compound film. This method has successfully produced flexible phosphor films used as resistor or thermistor electronic components. Finally, thin film growth using different light sources such as flash lamps and femtosecond lasers (fs) is explored.

Energy stable higher-order linear ETD multi-step methods for gradient flows: application to thin film epitaxy

We discuss how to combine exponential time differencing technique with multi-step method to develop higher order in time linear numerical scheme that are energy stable for certain gradient flows with the aid of a generalized viscous damping term. As an example, a stabilized third order in time accurate linear exponential time differencing (ETD) scheme for the epitaxial thin film growth model without slope selection is proposed and analyzed. An artificial stabilizing term $$A\tau ^3\frac{\partial \Delta ^3 u}{\partial t}$$ is added to ensure energy stability, with ETD-based multi-step approximations and Fourier pseudo-spectral method applied in the time integral and spatial discretization of the evolution equation, respectively. Long-time energy stability and an $$\ell ^{\infty }(0,T; \ell ^2)$$ error analysis are provided, based on the energy method. In addition, a few numerical experiments are presented to demonstrate the energy decay and convergence rate.

Lower bound of blow-up time to a fourth order parabolic equation modeling epitaxial thin film growth

In this paper, we considered a class of fourth order equation modeling epitaxial thin film growth. We establish a blow up result for the initial and boundary value problem. Furthermore, the lower bound of the blow up time and the blow up rate are derived.

Evolution of structure and electrical properties of epitaxial BiFeO3 thin films through solution and annealing atmosphere

Abstract Solution derived high-quality epitaxial BiFeO3 (BFO) thin films, with robust spontaneous polarization, are still desired because of their advantages. Herein, epitaxial BFO thin films and SrRuO3 bottom electrodes are prepared on LaAlO3 substrates via chemical solution deposition (CSD) technique. The influences of precursor solution on the microstructure and electrical properties are investigated by using nitrate salt and acetate salt system. The XRD and P-E hysteresis loops results demonstrate that the BFO thin film prepared by acetate salt system as a relatively green CSD route shows good epitaxial characteristic and ferroelectric performances. In addition, the effect of the annealing atmosphere on the microstructure and electrical properties are studied for the acetate salt derived BFO thin films. The microstructure and defects chemistry are analyzed by SEM and XPS, the BFO thin film annealed in air shows better comprehensive ferroelectric performances with the lowest leakage current and enhanced ferroelectric polarization due to the improved microstructure and decreased defect concentrations. The results here thus provide an instructive approach to optimize the growth of solution-derived epitaxial BFO-based thin films.

Evolution of structural and magnetic properties of multifunctional bismuth iron garnets upon Ca and Y doping

A multiscale approach is reported to understand the influence of the Ca and Y co-substitution in Bi-rich iron garnet thin films. Recently, it has been demonstrated that site-selective co-substitution of ${\mathrm{Ca}}^{2+}$ and ${\mathrm{Y}}^{3+}$ in bismuth iron garnet (BIG) leads to either $n$- or $p$-type semiconductor behavior. However, the evolution of the structural and magnetic properties of bismuth iron garnet upon doping is still unknown. In the (Ca,Y):BIG, thin films grown by pulsed laser deposition onto ${\mathrm{Gd}}_{3}{\mathrm{Ga}}_{5}{\mathrm{O}}_{12}$ substrates, structural investigations confirm the epitaxial growth of doped thin films with high crystallinity and a complete strain relaxation from 20 nm above the film/substrate interface. While x-ray diffraction only evidences a single-phase garnet, the presence of secondary phase nanocrystallites, that are absent in pure BIG grown in similar conditions, is observed by aberration-corrected scanning transmission electron microscopy. These nanocrystallites form in between garnet grains as textured and poorly crystallized hematite. Despite the presence of nanoneedles, Ca and Y co-substitution preserves the giant Faraday rotation of pure BIG and maintains the Curie temperature above 590 K. Only minor energy shifts (80 meV) or small intensity changes (10%) of the Faraday rotation are observed upon doping or annealing and are mainly related to band-gap evolution and cell volume change. Furthermore, the easy magnetization axis rotates towards in the out-of-plane direction upon doping which is also promising for potential applications in spintronics.