Single Crystalline Thin Films Research Articles

Large-scale organic single-crystal thin films and transistor arrays via the evaporation-controlled fluidic channel method.

We report a facile and versatile approach for fabricating large-area organic thin film transistor (OTFTs) arrays via a fluidic channel method. Evaporation-controlled fluidic channel-containing organic semiconductors easily produce large-area organic single-crystalline thin films in a quite uniform manner. The unidirectional movement of the meniscus and the subsequent film growth via solvent evaporation inside the fluidic channel correspond to the simulation based on the finite element method. Utilizing this fluidic channel method, we fabricated high-performance 6,13-bis(triisopropylsilylethynyl)pentacene OTFT arrays with average and maximal mobilities of 0.71 and 2.18 cm(2) V(-1) s(-1), respectively, while exhibiting current on:off ratios of >1 × 10(6). We claim that this scalable fluidic channel method offers a competitive way to fabricate high-performance and large-area organic semiconductor devices for a variety of applications.

Single-crystalline silver film grown on Si (100) substrate by using electron-gun evaporation and thermal treatment

The authors demonstrate a simple method to fabricate ultrasmooth single-crystalline silver (Ag) films with high reflectivity and low plasmonic damping. The single-crystalline Ag thin film on the clean Si (100) substrate is first deposited by electron-gun evaporator and then treated by rapid thermal annealing (RTA) to improve its quality. The crystal structure and surface morphology are characterized by x-ray diffraction, transmission electron microscopy, and atomic force microscopy. Optical constants of the prepared films are extracted by fitting the measured reflectivity spectra with the Drude model. These results show that the Ag film with 340 °C RTA has the best film quality, including small surface roughness of 0.46 nm, a sharp x-ray diffraction peak with FWHM of 0.3°, and lowest damping in the visible and near-infrared wavelength regime. Therefore, our method is not only cost-effective but also useful for fabricating metal-based plasmonic and nanophotonic devices.

Ultrafast atomic layer-by-layer oxygen vacancy-exchange diffusion in double-perovskite LnBaCo2O5.5+δ thin films.

Surface exchange and oxygen vacancy diffusion dynamics were studied in double-perovskites LnBaCo2O5.5+δ (LnBCO) single-crystalline thin films (Ln = Er, Pr; −0.5 < δ < 0.5) by carefully monitoring the resistance changes under a switching flow of oxidizing gas (O2) and reducing gas (H2) in the temperature range of 250 ~ 800°C. A giant resistance change ΔR by three to four orders of magnitude in less than 0.1 s was found with a fast oscillation behavior in the resistance change rates in the ΔR vs. t plots, suggesting that the oxygen vacancy exchange diffusion with oxygen/hydrogen atoms in the LnBCO thin films is taking the layer by layer oxygen-vacancy-exchange mechanism. The first principles density functional theory calculations indicate that hydrogen atoms are present in LnBCO as bound to oxygen forming O-H bonds. This unprecedented oscillation phenomenon provides the first direct experimental evidence of the layer by layer oxygen vacancy exchange diffusion mechanism.

A microwave phase shifter based on a planar ferrite-ferroelectric thin-film structure

Microwave phase shifters employing slot transmission lines based on thin ferroelectric films of barium strontium titanate and thin single-crystalline films of yttrium iron garnet ferrite have been experimentally studied for the first time. The phase shifters admit double electronic control based upon the phenomenon of hybridization of the electromagnetic wave propagating in a slot delay line on the ferroelectric film and the spin wave propagating in the ferrite film. At a bias voltage of 150 V applied to electrodes of the slot lines with 50- and 150-μm-wide slots, the phase shift amounted to 53° and 26°, respectively.

Molecularly Smooth Single-Crystalline Films of Thiophene–Phenylene Co-Oligomers Grown at the Gas–Liquid Interface

Single crystals of thiophene–phenelyne co-oligomers (TPCOs) have previously shown their potential for organic optoelectronics. Here we report on solution growth of large-area thin single-crystalline films of TPCOs at the gas–liquid interface by using solvent–antisolvent crystallization, isothermal slow solvent evaporation, and isochoric cooling. The studied co-oligomers contain identical conjugated core (5,5′-diphyenyl-2,2′-bithiophene) and different terminal substituents, fluorine, trimethylsilyl, or trifluoromethyl. The fabricated films are molecularly smooth over areas larger than 10 × 10 μm2, which is of high importance for organic field-effect devices. The low-defect structure of the TPCO crystals is suggested from the monoexponential kinetics of the PL decay measured in a wide dynamic range (up to four decades) and from low crystal mosaicity assessed by microfocus X-ray diffraction. The TPCO crystal structure is solved using a combination of X-ray and electron diffraction. The terminal substituents affect the crystal structure of TPCOs, bringing about the formation of a noncentrosymmetric crystal lattice with a crystal symmetry Cc for the bulkiest trimethylsilyl terminal groups, which is unusual for linear conjugated oligomers. Comparing the different crystal growth techniques, it is concluded that the solvent–antisolvent crystallization is the most robust for fabrication of single-crystalline TPCOs films. The possible nucleation and crystallization mechanisms operating at the gas–solution interface are discussed.

Epitaxial growth of single-crystalline Ni46Co4Mn37In13 thin film and investigation of its magnetoresistance

Single-crystalline thin film of Ni46Co4Mn37In13 alloy grown on MgO(001) was prepared by Pulsed Laser Deposition (PLD) method. The epitaxial growth process was monitored by in situ Reflection High Energy Electron Diffraction (RHEED). Structure measurements reveal that the single-crystalline Ni46Co4Mn37In13 film could be stabilized on MgO(001) as a face-centered-cubic (fcc) structure. From the evolution of RHEED, it can be deduced from the patterns that Volmer-Weber growth mechanism (3-D) dominates at the initial stage. Then, it becomes layer-by-layer growth mechanism (2-D) with the increase of the film thickness. Lastly, growth mechanism converts back to 3-D when the film is thick enough. Both electrical resistance and magnetoresistance (MR) were measured at various temperatures using Physical Property Measurement System (PPMS). The electrical resistance measurement indicates that the film sample does not have martensitic transformation in the measurement temperature range. However, with the temperature increasing, the film sample exhibits a transition from metallic to semiconductor-like properties. Moreover, a small negative magnetoresistance was observed at different temperature, which can be explained by the spin-dependent scattering of the conduction electrons.

Exploration of Electronic Functionalities in Metal Oxides by Combinatorial Lattice Engineering

Abstract A review is given for our originally-developed thin film technology, “combinatorial lattice engineering.” This is extended from a conventional pulsed laser deposition, where the surface of a sintered oxide tablet (target) is irradiated with focused laser pulses and the evaporated precursors are condensed on a heated substrate to grow single crystalline thin films. By this method, it has become possible to concurrently synthesize many oxide thin films on a substrate at different locations (segments) with such diversities as composition, growth conditions, and the sequence of heterostructures. By inserting a masking system between the target and substrate and computer controlling the mask motion as well as target switching, one can integrate many oxide thin film segments on the substrate. Starting from the background of and demand for this technology as well as the advancement of the key ingredient technology, i.e., atomically regulated epitaxial growth of oxide thin films, the concept and examples of this combinatorial lattice engineering are introduced highlighting the representative demonstration on magnetic, photonic, and electronic functionalities, especially emphasizing the benefit of this combinatorial technology. In addition, further progress of device physics triggered by the combinatorial discoveries are introduced, for example, electric field control of magnetism, bright ultraviolet light emitting devices, and quantum effects in oxide semiconductors.

Motion, collision and annihilation of polarization vortex pair in single crystalline BaTiO3 thin film

Domain evolution of a single crystalline BaTiO3 thin film, initially possessing two vortex-antivortex pairs, placed under compressive displacement loading with a constant strain rate, is simulated using a molecular dynamics method based on the shell model. The evolution details, including the relative motion and collision between the vortices and antivortices and their annihilation, are carefully observed, and both the movement velocity and the equilibrium time after annihilation are estimated. When the vortex-antivortex pairs annihilate, the polarization configuration evolves into a 180° domain structure. These distinctive domain evolution characteristics could open up opportunities for designing ferroelectric nanodevices.

Atomistic Simulation on Indented Defects in Silicon

Silicon is known as one of the widely used materials in electronic fields for its excellent semiconductive characteristics. However, these characteristics are vulnerable to internal defects, which randomly exist in any materials. In the present study, defects in single crystalline silicon thin film were investigated by atomistic simulation of nano-indentation at zero temperature. The Tersoff potential and the spherical indenter were applied to the model of silicon. The symmetric axis parameter method is novelly proposed to identify defects in the diamond cubic structure. Under the nanoindentation condition, the ring slip appears close to the indentation region on the free surface and propagates along with [110]/(111). The dislocation is initiated closely to the ring slip and emitted on the (111) plane by the dissociation into two partial dislocations. It was found that the symmetric axis parameter method successfully separated the perfect dislocations, the partial dislocations and the stacking fault from perfect structure, i.e., diamond cubic structure, even though it was not able to distinguish between glide set and shuffle set dislocations.

Flux Pinning Mechanism in Single-Crystalline MgB2Thin Films

We investigate the flux pinning mechanism for single-crystalline MgB2 thin films showing a peculiar flux pinning force behavior. The vortex–vortex distance is considered in this study through an analysis of the critical current density (\(J_{\text{c}}\)) and the flux pinning force density (\(F_{\text{p}}\)) over a wide range of temperatures from 5 to 35 K. Two unusual kinds of peaks, a near zero-field peak like a hump and another peak with a broad shape, are observed in the field dependences of the \(F_{\text{p}}\). The second peak with a broad shape begins to be depressed at temperatures above 25 K while the first sharp peak is only detected at temperatures above 30 K. In the single-crystalline MgB2 thin films weak collective pinning is dominant at all temperatures, and the pinning mechanism shows a crossover from δTc-pinning to δl-pinning with increasing magnetic field. We describe these unique features by considering the relation between the inter-vortex distances and the vortex–vortex interaction energy.

Correlation between structure and semiconductor-to-metal transition characteristics of VO2/TiO2/sapphire thin film heterostructures

This study focuses on the role of strain and thin film epitaxy on the semiconductor-to-metal transition (SMT) characteristics of single crystalline VO2 thin films. The VO2/TiO2 heterostructures of controlled orientations were epitaxially grown on m-cut, r-cut and c-cut sapphire substrates. Detailed structural investigations were performed using high-resolution X-ray diffraction (2θ–θ and φ scans) and high-resolution transmission electron microscopy techniques to correlate SMT properties with microstructural characteristics. Monoclinic (M1) VO2 thin films with (100), (001) and (2¯01) out-of-plane orientations were grown on TiO2(101)/r-sapphire, TiO2(100)/c-sapphire and TiO2(001)/m-sapphire platforms, respectively. The in-plane alignments across the interfaces were established to be [010](100)VO2||[010](101)TiO2, [100](001)VO2||[001](100)TiO2 and [010](2¯01)VO2‖[010](001)TiO2 for r-sapphire, c-sapphire and m-sapphire substrates, respectively. We were able to tune the SMT temperature of VO2 epilayers from ∼313K to 354K (bulk Tc≈340K). The SMT characteristics were interpreted based upon the residual strain in the VO2 lattice, particularly along the c-axis of tetragonal VO2. This research introduces the VO2-based single crystalline heterostructures as a potential candidate for a wide range of applications where different transition temperatures are required.

Ferroelectric ground state and polarization-switching path of orthorhombic YMnO3with coexistingE-type and cycloidal spin phases

Orthorhombic YMO${}_{3}$ ($o$-YMO) is currently being intensively investigated since a single-crystalline thin film of $o$-YMO recently showed a pronounced degree of magnetoelectric coupling. According to first-principles calculations, the ferroelectric ground state is represented by two degenerate $E$-type collinear spin configurations with the computed polarization $P$ of \ensuremath{\sim}1.4 \ensuremath{\mu}C/cm${}^{2}$. The $bc$-cycloidal spin phase is identified as the spin configuration that corresponds to the transition state in the $+P\ensuremath{\leftrightarrow}\ensuremath{-}P$ polarization switching. The remarkable coexistence of the $E$-type and cycloidal spin phases below \ensuremath{\sim}40 K is attributed to a small value of the Kohn-Sham energy difference between these two phases, 2.2 meV per formula unit. A modulated spin structure, which is characterized by the tilting of the Mn-spin vectors to the $a$-axis direction of Pbnm setting, is proposed to account for the observed strong magnetic-field-dependent polarization in $o$-YMO.

Dynamics and instability of solid-state dewetting

Abstract Spontaneous dewetting of solid thin films proceeds by edge retraction of film edges and/or by heterogeneous void growth. Classical 1D and 2D continuous models of the evolution of a dewetting film, based on surface diffusion mechanisms, predict that in the long-time limit dewetting obeys universal scaling laws. In this paper, we review 1D and 2D predictions and recent experimental results. For this purpose, using Si(001)/SiO2 and Ge(001)/SiO2 single-crystalline thin films in different geometries, we have been able to compare theoretical predictions to experimental results obtained by combining in situ LEEM and ex situ AFM measurements. For dewetting from film edges, experimental results partially differ from continuous models predictions. More precisely, because of the crystallographic anisotropy: (i) the facetted edges remain stable during dewetting (they simply recede at constant shape) while poorly or un-facetted edges are unstable (they recede by finger formation); (ii) rim formation, induced by mass-conservation condition, proceeds in a layer-by-layer mode and is limited by 2D nucleation properties on the top of the rim; (iii) the island generation mechanism differs from the mass shedding behaviour predicted by 1D models. For dewetting mechanisms involving void growth, different behaviours are reported and discussed. For thin Si(001)/SiO2 films, the corners of the opening square-shaped voids lead to a local destabilisation of the growing voids. For thin Ge(001)/SiO2 films, the side of the voids invariably turns instable and forms tip dendrites whose branch density depends on the temperature and the initial film thickness. Finally, ultra-thin films, more sensitive to local fluctuations, dewet in a fractal geometry.

Epitaxial growth of superlattice YbGaO3(ZnO)5 and InGaO3(ZnO)5 films by the combination of sputtering and reactive solid phase epitaxy

This article describes the epitaxial growth of superlattice YbGaO3(ZnO)5 (YGZO) and InGaO3(ZnO)5 (IGZO) thin films, which feature nanoscale multilayered structures, on (111) plane yttria-stabilized zirconia (YSZ) substrates through a combination of sputtering and reactive solid phase epitaxy processes. Our fabrication process involved thin ZnO epilayers deposited through sputtering onto the YSZ substrates, and then YbGaO3(ZnO)5 or InGaO3(ZnO)5 thin films deposited at room temperature on ZnO epi-layers, followed by high-temperature annealing (1200°C) of the bilayer structures. To suppress vaporization of ZnO from the films, high-temperature annealing was performed in a box made of ceramic ZnO. We used X-ray diffraction and transmission electron microscopy (TEM) to analyze the thin films annealed under various conditions. The microstructural evolution in the film formed during reactive solid phase epitaxy process was explored by TEM observations. Single-crystalline YGZO and IGZO thin films without varying composition could be synthesized through a suitable annealing process in the ceramic ZnO box.

Growth and Alignment of Thin Film Organic Single Crystals from Dewetting Patterns

Studying and understanding the conditions under which organic semiconductors can be engineered to form aligned single crystals in thin films is of primary importance owing to their unique orientation-dependent optoelectronic properties. Efforts to reach this goal by self-assembly from solution-processed films have been rewarded only with limited success. In this article we present a new method to grow single crystalline thin films via solvent annealing. We identify solvate crystal growth in combination with a specific film dewetting morphology as key to successful fabrication of single crystals. Furthermore, these 2D single crystals can align on chemically patterned substrates to minimize their interfacial energy. We explore in situ the conditions for crystal formation and alignment.

On the role of Fe in the growth of single crystalline heteroepitaxial Au thin films on sapphire

Thin Au–Fe bilayers were deposited on c-plane sapphire (α-Al2O3) substrates at room temperature employing the electron beam deposition method. The layers were found to be single crystalline (i.e. the grain size was much larger than the film thickness), with a [111] and [110] texture for Au and Fe, respectively, and strong heteroepitaxy to the substrate. Au films deposited on sapphire and Au–Fe bilayers deposited on amorphous SiO2 were polycrystalline and exhibited random in-plane orientation of the grains. The effects of Fe and the Fe–sapphire interface on the microstructure of the Au film were investigated and discussed in terms of the orientation relationships, in-plane strain, interface energy and adhesion. The microstructures of annealed and as-deposited films were very similar, indicating that as-deposited films are close to thermodynamic equilibrium in terms of the orientation relationship with the substrate. This is uncommon for non-equilibrium thin film deposition processes, which usually result in a high density of defects in the as-deposited films.

Controlled epitaxial integration of polar ZnO(0001) with Si(001)

We have grown ZnO(0001) single-crystalline thin films on Si(001) using cubic yttria-stabilized-zirconia (c-YSZ) buffer and analyzed details of epitaxy, twins, and interfaces. In-plane epitaxial relationship between ZnO and c-YSZ showed an interesting dependence on growth temperature where it changed from (0001)[2¯110]ZnO||(001)[110]c-YSZ to (0001)[2¯110]ZnO||(001)[100]c-YSZ as the temperature increased from 500 to 750 °C. At temperatures in between, a combination of these epitaxial relationships was observed. We found that the epitaxial relationships are determined by the surface termination characteristics of c-YSZ across the ZnO/c-YSZ interface. The crystallographic characteristics of c-ZnO/c-YSZ/Si(001) heterostructures can be precisely tuned to address the needs of next-generation solid-state devices.

Low temperature crystallization of amorphous silicon by gold nanoparticle

• Gold nanoparticle production. • Deposition of amorphous silicon by e-beam evaporation method. • Annealing of amorphous silicon for crystallization by gold nanoparticles. Single crystalline Si thin film fabricated on glass substrate by a process called Solid Phase Crystallization (SPC) is highly desirable for the development of high efficiency and low cost thin film solar cells. However, the use of ordinary soda lime glass requires process temperatures higher than 600 °C. Crystallization of Si film at around this temperature takes place in extremely long time exceeding 20 h in most cases. In order to reduce this long process time, new crystallization techniques such as Metal Induced Crystallization (MIC) using thin metal films as a catalyst layer is attracting much attention. Instead of using continuous metal films, the use of metal nanoparticles offers some advantages. In this work, gold thin films were deposited on aluminum doped zinc oxide (AZO) coated glass and then annealed for nanoparticle formation. Amorphous silicon was then deposited by e-beam evaporation onto metal nanoparticles. Silicon films were annealed for crystallization at different temperatures between 500 °C and 600 °C. We showed that the crystallization occurs at lower temperatures and with higher rates with the inclusion of gold nanoparticles (AuNP). Raman and XRD results indicate that the crystallization starts at temperatures as low as 500 °C and an annealing at 600 °C for a short process time provides sufficiently good crystallinity.

Peculiarly strong room-temperature ferromagnetism from low Mn-doping in ZnO grown by molecular beam epitaxy

Strong room-temperature ferromagnetism is demonstrated in single crystalline Mn-doped ZnO thin films grown by molecular beam epitaxy. Very low Mn doping concentration is investigated, and the measured magnetic moment is much larger than what is expected for an isolated ion based on Hund's rules. The ferromagnetic behavior evolves with Mn concentration. Both magnetic anisotropy and anomalous Hall effect confirm the intrinsic nature of ferromagnetism. While the Mn dopant plays a crucial role, another entity in the system is needed to explain the observed large magnetic moments.

Wet chemical deposition of single crystalline epitaxial manganite thin films with atomically flat surface

We report the wet chemical deposition of single crystalline epitaxial thin films of the colossal magneto-resistive manganite La0.67Sr0.33MnO3 on the lattice-matched (001)-face of a La0.3Sr0.7Al0.65Ta0.35O3 substrate. Topographic images of these films taken with a scanning tunneling microscope show atomically flat terraces separated by steps of monatomic height. The resistivity of these films shows an insulator-metal transition at 310K, nearly coincident with the Curie temperature of 340K, found from magnetization measurements. The films show a magnetoresistance of 7% at 300K and 1.2T. Their saturation magnetization value at low temperatures is consistent with that of the bulk.