Single Crystalline Thin Films Research Articles

Large-area epitaxial growth of MoSe2 via an incandescent molybdenum source

We have developed an incandescent Mo source to fabricate large-area single-crystalline MoSe2 thin films. The as-grown MoSe2 thin films were characterized using transmission electron microscopy, energy dispersive x-ray spectroscopy, atomic force microscopy, Raman spectroscopy, photoluminescence (PL), reflection high energy electron diffraction (RHEED) and angular resolved photoemission spectroscopy (ARPES). A new Raman characteristic peak at 1591 cm−1 was identified. Results from Raman spectroscopy, PL, RHEED and ARPES studies consistently reveal that large-area single crystalline mono-layer of MoSe2 could be achieved by this technique. This technique enjoys several advantages over conventional approaches and could be extended to the growth of other two-dimensional layered materials containing a low-vapor-pressure element.

Normal-state gap in the parent cuprate Pr2CuO4±δ

We present a tunneling study on single crystalline parent cuprate thin films, i.e. a series of Pr2CuO4+/-{\delta}(PCO) with tunable superconducting transition temperature. The zero-bias anomaly of differential conductance, well reported in the normal state of R2-xCexCuO4 (R = Pr, Nd, La) and named as normal state gap (NSG), is observed in the Ce-free samples. This NSG behaves quite robust against the magnetic field up to 16 T, but fades away with increasing the temperature. Most importantly, we find that the magnitude of the NSG becomes larger with increasing point-contact junction resistance on the superconducting films, which is further enhanced in the non-superconducting samples of more oxygen disorders. The origination of NSG can be understood in the framework of Altshuler-Aronov-Lee (AAL) theory, where the disorder-induced electron-electron interactions suppress the density of states and thereby result in a soft Coulomb gap.

High‐Temperature Ionic Epitaxy of Halide Perovskite Thin Film and the Hidden Carrier Dynamics

High-temperature vapor phase epitaxy (VPE) has been proved ubiquitously powerful in enabling high-performance electro-optic devices in III-V semiconductor field. A typical example is the successful growth of p-type GaN by VPE for blue light-emitting diodes. VPE excels as it controls film defects such as point/interface defects and grain boundary, thanks to its high-temperature processing condition and controllable deposition rate. For the first time, single-crystalline high-temperature VPE halide perovskite thin film has been demonstrated-a unique platform on unveiling previously uncovered carrier dynamics in inorganic halide perovskites. Toward wafer-scale epitaxial and grain boundary-free film is grown with alkali halides as substrates. It is shown the metal alkali halides could be used as universal substrates for VPE growth of perovskite due to their similar material chemistry and lattice constant. With VPE, hot photoluminescence and nanosecond photo-Dember effect are revealed in inorganic halide perovskite. These two phenomena suggest that inorganic halide perovskite could be as compelling as its organic-inorganic counterpart regarding optoelectronic properties and help explain the long carrier lifetime in halide perovskite. The findings suggest a new avenue on developing high-quality large-scale single-crystalline halide perovskite films requiring precise control of defects and morphology.

Molecular beam epitaxy of Nd2PdO4 thin films

We synthesized high quality, single crystalline thin films of a layered, complex 4d transition metal oxide (Nd2PdO4) by reactive molecular beam epitaxy, in order to determine the electronic structures associated with square-planar coordinated palladium ions. The single crystalline thin films of Nd2PdO4 show resistivity values of 100 mΩcm at 300 K and this value is independent of annealing procedures.

Epitaxial Lift-Off of Centimeter-Scaled Spinel Ferrite Oxide Thin Films for Flexible Electronics.

Mechanical flexibility of electronic devices has attracted much attention from research due to the great demand in practical applications and rich commercial value. Integration of functional oxide materials in flexible polymer materials has proven an effective way to achieve flexibility of functional electronic devices. However, the chemical and mechanical incompatibilities at the interfaces of dissimilar materials make it still a big challenge to synthesize high-quality single-crystalline oxide thin film directly on flexible polymer substrates. This study reports an improved method that is employed to successfully transfer a centimeter-scaled single-crystalline LiFe5 O8 thin film on polyimide substrate. Structural characterizations show that the transferred films have essentially no difference in comparison with the as-grown films with respect to the microstructure. In particular, the transferred LiFe5 O8 films exhibit excellent magnetic properties under various mechanical bending statuses and show excellent fatigue properties during the bending cycle tests. These results demonstrate that the improved transfer method provides an effective way to compose single-crystalline functional oxide thin films onto flexible substrates for applications in flexible and wearable electronics.

Fabrication of pseudo single crystalline thin films composed of polydiacetylene nanofibers and their optical properties

Highly-orientated thin films of polydiacetylene nanofibers were successfully fabricated as a pseudo single crystalline thin film through convective assembly. The third-order nonlinear optical susceptibility of highly-orientated film with an order parameter of 0.87 was five times as high as that of randomly orientated film owing to high orientation and dense packing.

Epitaxial CaTi5O11 and TiO2-B Thin Films for High Rate Lithium-Ion Batteries

The bronze polymorph of titanium dioxide (TiO2-B) is interesting for many applications including high rate energy storage, solar cells, photocatalysis, thermoelectrics and sensing, owing to its uniquely layered structure and highly asymmetric unit cell. Although known to have advantages over anatase or rutile, high quality bronze phase TiO2-B specimens that demonstrate good electrochemical properties thus far have exclusively been nano-structured powders prepared by hydrothermal methods. We have recently discovered that Ca can stabilize the bronze structure, forming a variant phase CaTi5O11, which has then been successfully synthesized in epitaxial single-crystalline thin films by pulsed laser deposition (PLD), a completely waterless process. Due to the near-perfect lattice match, the CaTi5O11 film can be used as a template layer to grow high quality, water-free TiO2-B films on top, which facilitates the synthesis and application of both materials on a wide variety of substrates, including SrTiO3, Nb:SrTiO3, LaAlO3, LSAT and SrTiO3 buffered Si. Lithium ion transport in the bronze structure is highly anisotropic. By utilizing substrates with a different orientation to align the more open channels with out-of-plane directions, extremely high rates of lithium ion transport, up to 600C (1C=335 mA g-1), with extraordinary structural stability has been achieved. In a battery half-cell using metallic lithium as counter electrode, the orientation-engineered CaTi5O11 film discharged to 155 mA h g-1 at a rate of 60C, corresponding to a time of 60 s to fully discharge the capacity, at the 100th cycle, delivering specific power of ~20 kW kg-1. Post-mortem examinations by x-ray diffraction (XRD) and transmission electron microscopy (TEM) confirmed that both the TiO2-B and CaTi5O11 structures were essentially unchanged after aggressively cycling for more than 60 days. The film microstructure and interfacial atomic structure were characterized by atomic resolution transmission electron microscopy. In addition, we have employed novel in situ TEM to study the Li-intercalation of the films. Revealed by TEM of electrochemical lithiation in TiO2-B, many defects were induced by strain relaxation upon Li-induced TiO2-B lattice expansion. Depending on Li intercalation direction in the crystal structure, either high-symmetry structural transformation or plain shears was generated. These results provide the basic knowledge needed to realize and utilize TiO2-B single crystals, while also supporting theoretical studies with determinate experimental data.

Effect of dislocation absorption by surfaces on strain hardening of single crystalline thin films

In the paper, by taking advantage of a strain gradient crystal plasticity theory with consideration of dislocation absorption by surfaces, plastic behaviors of thin films with two active slip systems under constrained shear is analytically studied. It is found that the critical loads for the onset of dislocations absorption by surfaces for the two slip systems are size dependent and are greatly affected by the latent hardening in the grain interior, and dislocations absorption by surfaces can significantly change the distributions of the plastic deformation and dislocation density and hence the strain-hardening behaviors.

Growth of Bi2Se3 topological insulator thin film on Ge(1 1 1) substrate

Atomically smooth, single crystalline Bi2Se3 thin films were grown on a Ge(111) substrate using molecular beam epitaxy. Crystallinities of both the surface and the bulk as well as the stoichiometry of the grown film were characterized by using low-energy electron diffraction, scanning tunneling microscopy, X-ray diffraction, and photoelectron spectroscopies. Hexagonal atomic structures, quintuple layer steps observed in STM images confirmed that the Bi2Se3 film with a (0001) surface was grown. Diffraction peak positions as well as the chemical composition determined from the core-level photoelectron spectra coincide well with those expected for the Bi2Se3 crystal. The surface state with a Dirac cone was observed in the valence photoelectron spectra, which also support that a high-quality Bi2Se3 film was grown on the Ge(111) substrate. The interface between Ge(111) and Bi2Se3(0001) is expected to be abrupt due to the small lattice between them.

The room temperature deposition of high-quality epitaxial yttrium iron garnet thin film via RF sputtering

Abstract Yttrium iron garnet (YIG) has great potential in the application of microwave devices and spintronics. However, the extremely high temperature for the growth of single crystalline YIG thin film on garnet substrate has being a barrier hindering its massive production and wide application, especially for some integrated devices which cannot bear high temperature for a long time. In this work, epitaxial single crystalline YIG thin films were successfully obtained by deposition on (111)-oriented gadolinium gallium garnet (GGG) at room temperature via RF sputtering followed by a short-time post-annealing treatment. Their structural and physical properties were carefully studied and compared with that of YIG/GGG(111) thin films deposited at high temperatures. The relatively large saturated magnetization (∼120 emu/cc) and a comparable linewidth (∼55 Oe) of those room-temperature-deposited YIG thin films can be achieved. This provides a viable alternative for growing YIG thin films on GGG or other garnet substrates.

Formation of a magnetite/hematite epitaxial bilayer generated with low energy ion bombardment

We have used a low-energy ion bombardment to fabricate an epitaxial single-crystalline magnetite/hematite bilayer grown on Au(111). This non-conventional fabrication method involves the transformation of the upper layers of a single-crystalline hematite thin film to single-crystalline magnetite, a process driven by the preferential sputtering of oxygen atoms and favoured by the good structural matching of both phases. We show the reversibility of the transformation between hematite and magnetite, always keeping the epitaxial and single-crystalline character of the films. The magnetic characterization of the bilayer grown using this method shows that the magnetic response is mainly determined by the magnetite thin film, exhibiting a high coercivity.

Towards a Better Understanding of the Anomalous Hall Effect

Recent experimental efforts to identify the intrinsic and extrinsic contributions in the anomalous Hall effect are reviewed. Benefited from the experimental control of artificial impurity density in single crystalline magnetic thin films, a comprehensive physical picture of the anomalous Hall effect involving multiple competing scattering processes has been established. Some new insights into the microscopic mechanisms of the anomalous Hall effect are discussed.

General Space-Confined On-Substrate Fabrication of Thickness-Adjustable Hybrid Perovskite Single-Crystalline Thin Films.

Organic-inorganic hybrid perovskite single-crystalline thin films (SCTFs) are promising for enhancing photoelectric device performance due to high carrier mobility, long diffusion length, and carrier lifetime. However, bulk perovskite single crystals available today are not suitable for practical device application due to the unfavorable thickness. Herein, we report a facile space-confined solution-processed strategy to on-substrate grow various hybrid perovskite SCTFs in a size of submillimeter with adjustable thicknesses from nano- to micrometers. These SCTFs exhibit photoelectric properties comparable to bulk single crystals with low defect density and good air stability. The clear thickness-dependent colors allow fast visual selection of SCTFs with a suitable thickness for specific device application. The present substrate-independent growth of perovskite SCTFs opens up opportunities for on-chip fabrication of diverse high-performance devices.

Large-Area Dry Transfer of Single-Crystalline Epitaxial Bismuth Thin Films.

We report the first direct dry transfer of a single-crystalline thin film grown by molecular beam epitaxy. A double cantilever beam fracture technique was used to transfer epitaxial bismuth thin films grown on silicon (111) to silicon strips coated with epoxy. The transferred bismuth films retained electrical, optical, and structural properties comparable to the as-grown epitaxial films. Additionally, we isolated the bismuth thin films on freestanding flexible cured-epoxy post-transfer. The adhesion energy at the bismuth/silicon interface was measured to be ∼1 J/m2, comparable to that of exfoliated and wet transferred graphene. This low adhesion energy and ease of transfer is unexpected for an epitaxially grown film and may enable the study of bismuth's unique electronic and spintronic properties on arbitrary substrates. Moreover, this method suggests a route to integrate other group-V epitaxial films (i.e., phosphorus) with arbitrary substrates, as well as potentially to isolate bismuthene, the atomic thin-film limit of bismuth.

Stable growth of large-area single crystalline thin films from an organic semiconductor/polymer blend solution for high-mobility organic field-effect transistors

We developed an effective and steady solution-processing technique for a small molecule–type semiconductor, C10–DNBDT–NW, by adding an amorphous PMMA polymer to produce stable growth of a two-dimensional large-area single-crystalline thin film by effective phase separation at a crucially faster processing speed compared to the case without the addition of a polymer. By using this solution-processing technique, it is noteworthy that the single-crystalline films of C10–DNBDT–NW/PMMA exhibit the highest and average mobilities of 17 and 10.6 cm2/Vs, respectively. Furthermore, we also show the limitations of two-dimensional continuous growth of a single-crystalline film in terms of the solution technique.

Science and technology of cuprate-based high temperature superconductor thin films, heterostructures and superlattices—the first 30 years (Review Article)

During the three decades after the discovery of superconductivity at high temperatures in copper oxides, intense research activities generated a tremendous progress in both, mastering the scientific challenges underpinning the understanding of the properties of these chemically and structurally complex materials as well as achieving a mature technology in preparing single phase bulk specimens—including single crystals—and epitaxially grown single crystalline thin films. This review covers in addition to more basic physics oriented developments mainly technological aspects of complex oxide thin film deposition as an enabling technology to explore the physics of these materials. It consists of two parts: after a brief introduction to the materials development prior to the discovery of superconducting copper oxides, a description of the relevant properties of copper oxide superconductors with focus on YBa2Cu3O7−δ is given, followed by the coverage of essentials of complex oxide thin film deposition technology with the copper oxides at its core. Here, the major physical vapor deposition technologies (evaporation and oxide molecular beam technology, sputtering and pulsed laser deposition) are described followed by an overview of substrate requirements to deposit high quality thin films. Opportunities by choosing special substrates with unique properties far beyond the usual mechanical support for a film are introduced with examples aside from usual lattice mismatch induced strain effects. One is the continuous modification of the strain state by poling ferroelectric oxide substrates linked to a piezoelectric effect, the other is the nanoscale tailoring of substrate step-and-terrace structures resulting in a controllable generation of planar defects in complex oxides, thus contributing to the physics of flux-line pinning in cuprate superconductors. In the second part of this review, first some highlights of single layer thin film research are given such as to tailor thin film orientation, generating well defined antiphase boundaries in YBa2Cu3O7−δ thin films as flux-line pinning centers as well as contributions to understand fluctuation conductivity in relation to the pseudogap state. In the last section new developments in high Tc cuprate based heterostructures and superlattices are reviewed with a special focus on the opportunities offered by interface-induced electronic interactions.

(Invited) Unseeded Growth of Poly-Crystalline Ge with (111) Surface Orientation on Insulator by Pulsed Green Laser Annealing

Thin-film transistors (TFTs) with a high channel mobility for both electrons and holes are required for the three-dimension large scale integrations or system-on-panels. Lower temperature process is more favorable for TFT fabrication because the application of plastic substrate to system-on-panels is expected to add the flexibility function. Germanium (Ge) has higher hole and electron mobilities than Si, suggesting that large-grain poly-crystalline or single-crystalline Ge thin films lead to high channel mobility TFTs. This paper describes unseeded growth of poly-crystalline Ge with well-oriented large-size grains on insulator from amorphous Ge by scanning pulsed green laser annealing. In experimental, a 50-nm-thick amorphous Ge layer was deposited on a Si substrate with a 50-nm-thick SiO2 film. The Ge layer was patterned to narrow stripes of 2-µm-width. Then a capping layer of 650-nm-thick SiO2 was deposited. For this processed template, a scanning pulse green laser annealing was performed. Q-switch Nd:YAG laser beam with the following properties was used for annealing; a Gaussian line beam of 70 x 1400 µm2 at a laser wavelength of 532 nm, a pulse frequency of 7.5 kHz, a scanning velocity of 150 µm/s, and energy densities from 0.5 to 0.7 J/cm2. The Ge stripes were crystallized by a single scanning, where the beam was scanned perpendicular to the longitudinal direction of the beam. The Ge stripes crystallized at the optimum energy of 0.56 J/cm2 by the scanning pulsed laser annealing was characterized by electron back scattering diffraction patterns (EBSD). The EBSD mapping of normal direction to the sample surface indicates that the crystallized Ge film has (111) surface orientation in the 600 µm-long stripe. The (111) Ge crystal begins within only 2 µm after the starting point of the laser scanning. It was observed that the crystal orientation was recovered to (111) even if the prior part of crystalized Ge film was rotated from (111) surface. In the mapping of rolling direction, which corresponds to the laser scanning direction, two types of grains are dominant. One shows (101) and the other shows (112) for the laser scanning direction. From the normal direction and rolling direction images of EBSD, the Ge stripes were crystallized with the grains of 2 µm wide and ~10 µm long. The maximum grain length of 20 µm was also observed. Raman scattering was measured to evaluate the crystalline quality of the (111) surface-oriented Ge stripes. The full-width at half-maximum of 2.35 cm-1 shows good crystal quality of the film, which is smaller than the values of 3.2-3.3 cm-1 for the single crystalline Ge film obtained by lateral liquid-phase epitaxy from a Si seed. For laser fluence dependence, Ge stripes were not crystallized when the fluence was smaller than 0.56 J/cm2, and Ge stripes show non-oriented crystal with a long grain boundary in the center of stripe width when the fluence was larger than 0.67 J/cm2. Simulation analyses of thermal diffusion reveal the crystallization process. The pulse laser irradiation heats the Ge stripe and melts it. Simultaneously the underlying Si in the outside region of the Ge stripe patterns absorbs the laser energy and is also heated up. During the cooling period after each pulse a temperature gradient between the edge and the center is produced in the molten Ge stripe, because of suppression of cooling at the edge. Then the temperature at the center of the Ge stripe drops below the solid point first rather than the center. Crystal growth occurs from the center to the edges, resulting that random nucleation at the edge is suppressed and crystal growth proceeds with controlled orientation in the Ge stripe. This crystallization method is promising to realize high-mobility (111) Ge TFTs and easily extended to the glass or plastic substrate by making amorphous Si layer, which works as a laser absorbing layer, and SiO2, which works as a heat insulating layer, on the substrate.

The spin Hall effect in single-crystalline gold thin films**Project supported by the National Basic Research Program of China (Grant Nos. 2015CB921400 and 2011CB921802) and the National Natural Science Foundation of China (Grant Nos. 11374057, 11434003, and 11421404).

The spin Hall effect has been investigated in 10-nm-thick epitaxial Au (001) single crystal films via H-pattern devices, whose minimum characteristic dimension is about 40 nm. By improving the film quality and optimizing the in-plane geometry parameters of the devices, we explicitly extract the spin Hall effect contribution from the ballistic and bypass contribution which were previously reported to be dominating the non-local voltage. Furthermore, we calculate a lower limit of the spin Hall angle of 0.08 at room temperature. Our results indicate that the giant spin Hall effect in Au thin films is dominated not by the interior defects scattering, but by the surface scattering. Besides, our results also provide an additional experimental method to determine the magnitude of spin Hall angle unambiguously.

Soft X-ray absorption spectroscopy investigations of Bi6FeCoTi3O18 and LaBi5FeCoTi3O18 epitaxial thin films

High-quality single-crystalline Bi6FeCoTi3O18 and LaBi5FeCoTi3O18 thin films were prepared by pulsed laser deposition. X-ray diffraction characterizations indicate a more disordered lattice structure of the LaBi5FeCoTi3O18 film. The magnetic measurement results demonstrated significantly enhanced ferromagnetism in the LaBi5FeCoTi3O18 film. The modulation of oxidation and hybridization states caused by substituting Bi with La was studied using the soft X-ray absorption spectroscopy. The spectroscopic results revealed the reduced concentration of oxygen vacancies and the more distorted lattice structure in the LaBi5FeCoTi3O18 film, which explained the enhanced ferromagnetism.

Oxidation of Single Crystalline Ti2AlN Thin Films between 300 and 900 °C: A Perspective from Surface Analysis

High temperature oxidation of 300 nm single crystalline Ti2AlN MAX phase thin film deposited on MgO(111) substrate between 300 and 900 °C has been investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and mass spectrometry. As shown by XRD, Ti2AlN remained structurally stable up to 700 °C, before it began to react with MgO substrate and ambient O2 to form MgTi2O5 and MgAl2O4 at 900 °C. However, as revealed by XPS, oxidation of Ti2AlN occurred at room temperature from its surface by forming TiO2, TiNxOy and Al2O3 with surface enrichment of Al. This initial oxidation continued up to 300 °C, until Ti and Al in the surface layer (∼7.1 nm thick) have been completely oxidized into TiO2 and Al2O3 at 500 °C, where Al in the subsurface preferentially diffused to the edges of the terraces and agglomerated into Al2O3 islands. At 700 °C and above, surface of Ti2AlN lost its characteristic hexagonal terrace morphology by transforming into round islands as a res...