Single Crystalline Films Research Articles

Flexible dual-mode surface acoustic wave strain sensor based on crystalline LiNbO3 thin film

This work presents the development of flexible dual-mode surface acoustic wave (SAW) sensor based on single crystalline thin film lithium niobate (TF-LN). Numerical modeling is conducted to investigate the SAW propagation and the effects on strain sensitivity. The dependence of strain sensitivity on angles between the applied strain and SAW propagation direction is analyzed numerically and experimentally, showing that the maximum strain sensitivity is at 45° rather than longitudinal direction. 128° Y-cut TF-LN (~50 µm), obtained by micromachining technique, is utilized as the piezoelectric substrate to fabricate the SAW strain sensors with dual-mode, namely Rayleigh mode and thickness shear mode. The sensor has excellent flexibility and demonstrates remarkable capability for an ultra-wide range strain measurement up to ±3000 . Temperature effects on resonant frequency and strain sensitivity are investigated in the range of 25 °C–100 °C, and similar temperature characteristics are observed for the dual modes. A method of beat frequency between the dual modes is introduced which is able to eliminate the temperature effect on strain sensing, an on-chip temperature influence removing capability. All the results clearly show that this sensor exhibits great potential for applications in flexible electronics and microsystems.

Grain Growth and Solid-State Dewetting of Bi-Crystal Ni-Fe Thin Films on Sapphire

We studied the solid-state dewetting behavior of thin Ni80Fe20 films deposited on basal plane oriented sapphire substrate and annealed in the range of temperatures of 1023-1323 K. All studied films exhibited strong texture and maze bicrystal microstructure, with only two grains misoriented by 60° around the common axis present in the film. The morphology of partially dewetted films changed from the one typical for polycrystalline thin films to the one typical for single crystalline heteroepitaxial films with increasing temperatures and annealing times. This change of dewetting behavior was associated with the fast grain growth in the films. The films of pure Ni of identical thickness, annealed under identical conditions exhibited significantly slower grain growth and lower thermal stability. Both the high-resolution X-ray diffraction and the cross-sectional high-resolution transmission electron microscopy observations revealed the phase separation of the Ni80Fe20 films into two parallel layers of the face-centered cubic (adjacent to the substrate) and hexagonal close-packed (on the top of the film) phases of similar compositions. Our density functional theory (DFT) calculations indicated that this phase separation is driven by the decrease of the film surface and interface energy, leading to the thermodynamically equilibrium thickness of the metastable hexagonal close-packed phase. This phase exhibits higher surface anisotropy than its stable face-centered cubic counterpart and is instrumental in accelerating the grain growth in the film via suppression of grain boundary grooving.

Growth, Single-Crystalline Rutile TiO2 Nanorod Thin Film By Hydrothermal Technique

A facile, hydrothermal technique was developed to growth, single-crystalline rutile TiO2 nanorod thin films on transparent conductive fluorine-doped tin oxide (FTO) substrates. In this study, the deferent periods(6, 9, 12, 15, and 18 hour) were selective with constant temperature at 160 oC to growth this films and growth in deferent temperature (130, 150,170,190, and 210 oC) with constant period of deposition at 6h. SEM image showed all the films were nanorodand the EDX measurement was given the portion of element that contribution in rutile TiO2 nanorod thin films. The diameter, length, and density of the nanorods could be varied by changingthe growth parameters. The XRD to determine the structure properties, and optical properties were measured of these rutile TiO2 nanorod thin films and obtained the absorbance and energy gap.

Direct van der Waals Epitaxy of Crack-Free AlN Thin Film on Epitaxial WS₂.

Van der Waals epitaxy (vdWE) has drawn continuous attention, as it is unlimited by lattice-mismatch between epitaxial layers and substrates. Previous reports on the vdWE of III-nitride thin film were mainly based on two-dimensional (2D) materials by plasma pretreatment or pre-doping of other hexagonal materials. However, it is still a huge challenge for single-crystalline thin film on 2D materials without any other extra treatment or interlayer. Here, we grew high-quality single-crystalline AlN thin film on sapphire substrate with an intrinsic WS2 overlayer (WS2/sapphire) by metal-organic chemical vapor deposition, which had surface roughness and defect density similar to that grown on conventional sapphire substrates. Moreover, an AlGaN-based deep ultraviolet light emitting diode structure on WS2/sapphire was demonstrated. The electroluminescence (EL) performance exhibited strong emissions with a single peak at 283 nm. The wavelength of the single peak only showed a faint peak-position shift with increasing current to 80 mA, which further indicated the high quality and low stress of the AlN thin film. This work provides a promising solution for further deep-ultraviolet (DUV) light emitting electrodes (LEDs) development on 2D materials, as well as other unconventional substrates.

Terahertz-field-induced carrier generation in Bi1−xSbx Dirac electron systems

Terahertz-field-induced carrier generation processes were investigated in Dirac electron systems, single-crystalline bismuth antimony alloy thin films ($\mathrm{B}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{S}{\mathrm{b}}_{x}$; $0\ensuremath{\le}x\ensuremath{\le}0.16$). This investigation was performed by precisely tuning, via the substituent ratio $x$, the band structure of the films from that associated with a semimetal to that characteristic of a narrow-gap semiconductor. Terahertz-field-induced absorption was clearly observed within a few picoseconds after the terahertz pump-pulse illumination of $\mathrm{B}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{S}{\mathrm{b}}_{x}$ semimetal and semiconductor samples. The field-strength dependence of the induced absorption was compared with the calculated Zener tunneling probability in the Dirac-like band dispersion. Through this comparison, the mechanism of the induced absorption was attributed to the carrier generation via the terahertz-field-induced Zener tunneling. The tunneling occurred in subpicosecond timescales even at room temperature, demonstrating that $\mathrm{B}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{S}{\mathrm{b}}_{x}$ thin films are promising for future high-speed electronics and the investigation of universal ultrafast tunneling dynamics.

Grazing Incidence X-Ray Diffraction using Synchrotron Light at SLRI

This work demonstrates the capability of the grazing incidence X-ray diffraction technique using synchrotron light. The measurement system is set up at the BL7.2W:MX beamline of the Synchrotron Light Research Institute (SLRI). The beamline utilizes hard X-rays from a 6.5-Tesla Superconducting Wavelength Shifter. The photon energy can be chosen between 7 to 18 keV with a photon flux of more than 1010 photons/sec at 100mA stored electron beam. The X-ray beam size can be reduced down to 20 microns, allowing XRD measurements in grazing geometry, thus crystal structures of very thin films with a thickness of tens nanometers can be identified. The diffraction patterns are recorded with a 2D CCD detector, allowing more diffraction spots of single crystalline films to be recorded.

Metalorganic vapor phase epitaxy of large size CdTe grains on mica through chemical and van der Waals interactions

High quality heteroepitaxy of CdTe is challenging due to lattice mismatches of CdTe with many substrates. Herein, we demonstrate the epitaxial growth of single crystalline CdTe films, in multilevel island format, on mica using metalorganic chemical vapor deposition, regardless of large in-plane lattice mismatch (\ensuremath{\sim}13%) between CdTe(111) and mica(001). X-ray and electron diffractions suggest that CdTe is epitaxially aligned with mica: out-of-plane $\mathrm{CdTe}(111)\text{//mica}(001)$ and in-plane CdTe $[\overline{1}2\overline{1}]$//mica $[010]$. Full-width-at-half-maximum (FWHM) of x-ray rocking curve and FWHM of x-ray azimuthal in-plane angular dispersion of CdTe are shown to be $0.{11}^{\ensuremath{\circ}}$ and $0.{38}^{\ensuremath{\circ}}$, respectively, better than most CdTe films reported. Electron backscattering diffraction shows that CdTe grains are tens of \ensuremath{\mu}m and, if twin boundaries are excluded, in excess of 250 \ensuremath{\mu}m in size. In contrast to the belief that overlayer growth on mica is purely through van der Waals interaction, our first-principle calculations uncover that van der Waals interaction only contributes to 20% of the total interfacial energy, and 80% of the interfacial energy comes from chemical interaction. We believe such a strong chemical interaction is accountable for the high-quality epitaxy. The demonstration of epitaxial growth of high-quality semiconductor on mica with a large lattice mismatch creates opportunities for flexible optoelectronic devices.

Epitaxial growth of Pd1−xFex films on MgO single-crystal substrate

In the paper we report on growth conditions, morphology, crystallographic structure and magnetic anisotropy of 20 nm thick, palladium-rich Pd1−xFex (0.01 < x < 0.1) alloy films grown on (001)-oriented MgO single-crystal substrate. Molecular beam deposition at ultra-high vacuum conditions has been utilized along with the three-step procedure to achieve the epitaxy conditions for the synthesized films. The scanning electron microscopy and atomic force microscopy have shown flat and smooth morphology of the films studied in a wide range of lateral scales. Insitu low energy electron diffraction and exsitu X-ray diffraction measurements confirmed that our Pd1−xFex alloy films are epitaxial. Ferromagnetic resonance investigations have shown the in-plane four-fold magnetocrystalline anisotropy characteristic for single-crystalline films with the cubic structure. The whole set of our measurements testifies the epitaxial cube-on-cube growth and excellent magnetic homogeneity of Pd1−xFex films on MgO substrate obtained within the three-step deposition process.

Magnetic and Structural Properties of L10 Mn–Ga–Al Epitaxially Grown Thin Films of Island Structure

Magnetic and Structural Properties of L1₀ Mn–Ga–Al Epitaxially Grown Thin Films of Island Structure

Intrinsic and defect-related luminescence of YAlO3 and LuAlO3 single crystals and films

The work is dedicated to investigation of the luminescent properties of YAlO3 (YAP) and Lu0.95Y0.05AlO3 (LuYAP) crystals, grown from the melt by different crystallization methods. The intrinsic luminescent properties of YAP and LuYAP crystals were compared also with behavior of single crystalline films of these perovskites, grown using the liquid phase epitaxy method from the melt-solution based on the PbOB2O3 flux. The significant differences in the luminescent properties of YAP and LuYAP crystals were observed. These differences are explained by the various concentrations of the YAl and LuAl antisite defects (ADs) and oxygen vacancies formed during high-temperature crystallization of YAP and LuYAP crystals from the melt at the temperatures above 2000 °C. The ADs and oxygen vacancies form in YAP and LuYAP crystals the different types of emission centers in the UV and visible ranges. The luminescence of defect centers in the 4.1–6.2 eV range are related to the emission of exciton localized around and bound with the ADs as analogues of cations isoelectronic impurities. The emission centers of YAP and LuYAP crystals in the 2.75–4.1 eV range are formed by F+ and F centers. Apart from the intrinsic radiative transitions of F+ and F centers, the luminescence of excitons localized around F and F+ related centers in YAP and LuAP crystals is found as well. The emission bands of YAP and LuAP crystals in the visible range probably are caused by the aggregates of F+ and F centers.

Superconductivity in nano- and micro-patterned high quality single crystalline boron-doped diamond films

We demonstrate nano- and micro-size patterning processes for high quality single crystal superconducting boron-doped diamond films with low damage using selective microwave plasma chemical vapour deposition and selective oxygen plasma etching. The offset critical temperature Tc(offset) of the micro strip is 10.2 K even if the strip width is 1 μm. However, we show that the critical temperature at zero resistivity Tc(zero) of several narrow strips is decreased owing to the unexpected defects induced by polishing damage and natural nanoscale scratch injury. These defects will induce a two-step superconducting transition. The probability of this tendency increases with decreasing the strip width. We also reveal that the critical current density Jc increases with decreasing the strip width, especially below 30 μm. By using a transport measurement, the maximum Jc is estimated to be 917,000 A/cm2 at 2.0 K with a strip width of 2 μm, which is the highest value reported for a superconducting diamond film. The upper critical field is estimated to be 11.5 T by the WHH approximation and 9.3 T by BCS fitting. Such high values mean superconducting diamond wiring can be used under a high magnetic field. We also attempt to fabricate nano-patterning and reveal that the transition temperature suddenly decreases below 400 nm, and superconductivity is not observed below 300 nm. This work contributes to the future development of superconducting nano- and micro-electro mechanical systems by exploiting the excellent properties of robustness, processability, high transition temperature, critical current density, and critical field associated with diamond.

Efficient ion-slicing of InP thin film for Si-based hetero-integration

Integration of high quality single crystalline InP thin film on Si substrate has potential applications in Si-based photonics and high-speed electronics. In this work, the exfoliation of a 634 nm crystalline InP layer from the bulk substrate was achieved by sequential implantation of He ions and H ions at room temperature. It was found that the sequence of He and H ion implantations has a decisive influence on the InP surface blistering and exfoliation, which only occur in the InP pre-implanted with He ions. The exfoliation efficiency first increases and then decreases as a function of H ion implantation fluence. A kinetics analysis of the thermally activated blistering process suggests that the sequential implantation of He and H ions can reduce the InP thin film splitting thermal budget dramatically. Finally, a high quality 2 inch InP-on-Si(100) hetero-integration wafer was fabricated by He and H ion sequential implantation at room temperature in combination with direct wafer bonding.

Polarity governs atomic interaction through two-dimensional materials.

The transparency of two-dimensional (2D) materials to intermolecular interactions of crystalline materials has been an unresolved topic. Here we report that remote atomic interaction through 2D materials is governed by the binding nature, that is, the polarity of atomic bonds, both in the underlying substrates and in 2D material interlayers. Although the potential field from covalent-bonded materials is screened by a monolayer of graphene, that from ionic-bonded materials is strong enough to penetrate through a few layers of graphene. Such field penetration is substantially attenuated by 2D hexagonal boron nitride, which itself has polarization in its atomic bonds. Based on the control of transparency, modulated by the nature of materials as well as interlayer thickness, various types of single-crystalline materials across the periodic table can be epitaxially grown on 2D material-coated substrates. The epitaxial films can subsequently be released as free-standing membranes, which provides unique opportunities for the heterointegration of arbitrary single-crystalline thin films in functional applications.

Surface alteration evidence for a mechanism of anoxic dissolution of UO2

A secondary phase has been observed to nucleate on the surface of UO2 in a solution with uranium concentration values of ∼10−9 mol/l. The UO2 was in the form of a 100 nm single crystalline film of UO2 epitaxially deposited on the (0 0 1) surface of a single crystalline silicon substrate. An extended (140 days) dissolution experiment with UO2 in contact with a solution in deoxygenated, deionised water, under an Ar atmosphere (∼0.1 O2 ppm) at ambient temperature (∼25 °C) suggests that uranium dioxide should dissolve and precipitate while remaining in the U4+ oxidation state to enable nucleation of a low solubility secondary phase. A mechanism for the anoxic dissolution of UO2 in deionised water is proposed that involves U4+ dissolution at defect sites that subsequently nucleate and precipitate in a less defective form.

Anisotropic Carrier Transport in CH3NH3PbI3 Single Crystal Field-Effect Transistor

By constructing vertical-structured field-effect transistors with CH3NH3PbI3 single-crystalline films of different orientations, anisotropic carrier transport in CH3NH3PbI3 single crystal was observed. Compared with the [100] and [112] orientation, carriers show lower mobility along the [001] orientation. The anisotropy of carrier transport is attributed to the migration of $\mathrm {CH}_{3}\mathrm {NH}_{3}^{+}$ under electric field.

Unconventional Relation between Charge Transport and Photocurrent via Boosting Small Polaron Hopping for Photoelectrochemical Water Splitting

Doping in semiconductor photoelectrodes controls defect formation and carrier transport that critically determine the device performance. Here we report an unconventional carrier transport relation that is tuned by extrinsic molybdenum (Mo) doping in BiVO4 photoanodes. Using the single-crystalline thin film approach, we identify that Mo doping significantly condenses the optimization regime between carrier transport and photon collection. For Mo-doped BiVO4 films, an unprecedentedly thin layer (50 nm), less than one-third of the pristine BiVO4 thickness, delivers larger photocurrents by overcoming the charge transport limitation, representing a regime not covered in conventional models. We provide direct evidence that Mo doping improves electron transport by boosting not only the donor density but also the electron mobility in the form of a small polaron, with the latter applying substantial impact on the photoelectrochemical performance. Density functional theory calculations reveal that fully ionized Mo...

Crystal growth and metal-insulator transition in two-dimensional layered rare-earth palladates

We synthesized high-quality, single-crystalline thin films of layered rare-earth (R) palladates R${}_{2}{\mathrm{PdO}}_{4}$ ($R$ = La, Nd, and Sm) and Ce substituted ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{PdO}}_{4}$ by reactive molecular beam epitaxy. For ${\mathrm{La}}_{2}{\mathrm{PdO}}_{4}$, ${\mathrm{Nd}}_{2}{\mathrm{PdO}}_{4}$, and ${\mathrm{Sm}}_{2}{\mathrm{PdO}}_{4}$, we find that the electronic conduction is independent of R elements. Doping charge carriers into the ${\mathrm{PdO}}_{2}$ planes of ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{PdO}}_{4}$ increases the electronic conduction and this effect is enhanced by vacuum annealing. The enhanced electronic conduction originates not solely from the doped charge carriers but is superimposed by Pd vacancies. X-ray photoelectron spectroscopy combined with inductively coupled plasma mass spectrometry revealed Pd deficiencies in ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{PdO}}_{4}$ and these defects play a crucial role for the electronic conduction. We observe a monotonic enhancement of the electronic conduction in ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{PdO}}_{4}$ thin films induced by Ce substitution and vacuum annealing. The estimated charge carrier concentration necessary for metallic conduction ($x\ensuremath{\approx}0.45$) at $T=0$ K is far beyond the experimentally accessible solubility limit.

Novel All-Solid-State Composite Scintillators Based on the Epitaxial Structures of LuAG Garnet Doped With Pr, Sc, and Ce Ions

This paper presents our achievement in the development of new types of advanced composite scintillators (CSs) based on the single crystalline films (SCFs) of Lu3Al5O12 (LuAG) garnet, doped with Pr3+ and Sc3+ ions, and substrate from single crystals (SCs) of Ce3+-doped LuAG using the technology of liquid phase epitaxy. We show the possibility of the simultaneous registration of $\alpha$ -particles and $\gamma$ -quanta by means of separation of the decay kinetics of SCF and crystal parts of such CSs. Namely, the significant differences in the scintillation decay kinetics of LuAG:Pr SCF/LuAG:Ce SC and LuAG:Sc SCF/LuAG:Ce SC CSs under excitation by $\alpha$ -particles of 241Am (5.5 MeV) source and $\gamma$ -quanta of 137Cs (662 keV) source are observed. Thus, such types of CSs can be successfully applied for separation of the signals coming from their film and crystal parts at the registration of the mixed radiation fluxes of $\alpha$ -particles and $\gamma$ -quanta.

Scintillation Characteristics of GAGG:Ce Single-Crystalline Films Grown by Liquid Phase Epitaxy

The Gd3(Al,Ga)5O12:Ce single-crystalline films were grown by a liquid phase epitaxy (LPE) technique from BaO-B2O3-BaF2 flux. The scintillation characteristics were investigated and compared to the bulk Gd3(Al2.7Ga2.3)O12:Ce single crystal (SC) grown by the Czochralski technique. The light yield (LY) and energy resolution were measured using an R6231 photomultiplier. At 5.5-MeV $\alpha$ -particles, the LY value of 7100 photons/MeV obtained for Gd3Al2.62Ga2.38O12:Ce-LPE sample is lower than that of 9310 photons/MeV for the Gd3(Al2.7Ga2.3)O12:Ce-SC sample, whereas an energy resolution of the LPE sample is better (5.9% versus 6.6%). The ratio of LY value under excitation with $\alpha$ - and $\gamma$ -rays ( $\alpha /\gamma$ ratio) was also determined. Faster scintillation decay time and lower slow-component content were obtained for LPE samples with respect to Gd3(Al2.7Ga2.3)O12:Ce-SC one.

Generalized Thermo-Elastodynamics for Polycrystalline Metallic Thin Films in Response to Ultrafast Laser Heating

A generalized thermo-elastodynamic model applicable to investigating single- and polycrystalline metallic films for their coupled thermal-mechanical responses to ultrafast laser heating is presented. Built upon the two-temperature model, the governing field equations feature hyperbolic electron and lattice energy transports that admit finite propagation speed and incorporate a term of energy dissipation to the generation of thermally induced mechanical disturbance. Electron energy transport in polycrystalline thin film is investigated in-depth by considering temperature dependence and size effect due to film surface and grain boundary scattering. Numerical results generated using the generalized one-dimensional model for a single-crystalline gold film are favorably examined against published experimental data for validation. Several averaged grain diameters are considered to explore the coupled thermal-mechanical responses in polycrystalline gold films of different thicknesses to better understand the dynamics and size effect on energy transport subject to pulsed femtosecond laser.