Single Crystal Thin Films Research Articles

Altering heating area assisted space confined method for growth of large scale and high quality MAPbBr3 single crystal thin films

Large-scale, high-quality MAPbBr3 SCFs with high carrier lifetime and carrier mobility are grown by the altering heating area assisted space confined method.

In-situ Growth of Perovskite Single Crystal Thin Films on Transport Layers with High Area to Thickness Ratio and Low Trap Density Simultaneously

In-situ Growth of Perovskite Single Crystal Thin Films on Transport Layers with High Area to Thickness Ratio and Low Trap Density Simultaneously

Toward Epitaxial Growth of Misorientation-Free Graphene on Cu(111) Foils.

The epitaxial growth of single-crystal thin films relies on the availability of a single-crystal substrate and a strong interaction between epilayer and substrate. Previous studies have reported the roles of the substrate (e.g., symmetry and lattice constant) in determining the orientations of chemical vapor deposition (CVD)-grown graphene, and Cu(111) is considered as the most promising substrate for epitaxial growth of graphene single crystals. However, the roles of gas-phase reactants and graphene-substrate interaction in determining the graphene orientation are still unclear. Here, we find that trace amounts of oxygen is capable of enhancing the interaction between graphene edges and Cu(111) substrate and, therefore, eliminating the misoriented graphene domains in the nucleation stage. A modified anomalous grain growth method is developed to improve the size of the as-obtained Cu(111) single crystal, relying on strongly textured polycrystalline Cu foils. The batch-to-batch production of A3-size (∼0.42 × 0.3 m2) single-crystal graphene films is achieved on Cu(111) foils relying on a self-designed pilot-scale CVD system. The as-grown graphene exhibits ultrahigh carrier mobilities of 68 000 cm2 V-1 s-1 at room temperature and 210 000 cm2 V-1 s-1 at 2.2 K. The findings and strategies provided in our work would accelerate the mass production of high-quality misorientation-free graphene films.

High-quality and single-crystal ZnSnO3 thin films: Fabrication and properties

LiNbO3-type ZnSnO3 single crystal thin films are deposited on LiTaO3(0001) substrates by pulsed laser deposition, and then an annealing process is carried out at a higher temperature. The 700°C-deposited film with a full width at half-maximum of ∼316 arcsec for the ZnSnO3(0006)-plane X-ray rocking curve has the highest crystal quality, whereas the 800°C-annealed film is partially decomposed. The epitaxial relationship between the highest quality film and the substrate is identified as ZnSnO3(0001) || LiTaO3(0001) with ZnSnO3[21‾1‾0] || LiTaO3<21‾1‾0 >. The hexagonal structure of the ZnSnO3 thin film shows slightly shrunken a-lattice and c-lattice constants. The average transmittance of the sample prepared at 700 °C in the visible light range is ∼79%, and the optical band gap of the film is about 3.90 eV. Due to high transmittance, wide bandgap and high crystal quality, the ZnSnO3 thin film should have wide application potential in optoelectronic devices.

Low-loss erbium-incorporated rare-earth oxide waveguides on Si with bound states in the continuum and the large optical signal enhancement in them

Integrated optical amplifiers are essential building blocks for emerging applications of large-scale and complex silicon photonic integrated circuits. Single-crystal erbium-incorporated rare-earth oxide thin films epitaxially grown on silicon substrate could be a promising material candidate. Based on the mechanism of bound states in the continuum, we have developed a low-loss waveguide platform for such materials. The waveguides are formed by patterning a silicon nitride cap layer grown on a rare-earth oxide thin film on silicon-on-insulator substrate. Low propagation loss of 4.70 dB/cm has been obtained in waveguides with silicon nitride/rare-earth oxide/silicon layer stack thickness of 72 nm/90 nm/300 nm, width of 1.08 µm, at a wavelength of 1445.8 nm. Optical signal enhancement in these waveguides upon optical pumping has been observed in a wide wavelength range of 1510 to 1560 nm, with a large peak enhancement of ∼16 dB/cm around 1536 nm. Our results represent an important step towards the realization of high-performance monolithic-integrated optical amplifiers on Si.

Integration Technology for Wafer-Level LiNbO3 Single-Crystal Thin Film on Silicon by Polyimide Adhesive Bonding and Chemical Mechanical Polishing.

An integration technology for wafer-level LiNbO3 single-crystal thin film on Si has been achieved. The optimized spin-coating speed of PI (polyimide) adhesive is 3500 rad/min. According to Fourier infrared analysis of the chemical state of the film baked under different conditions, a high-quality PI film that can be used for wafer-level bonding is obtained. A high bonding strength of 11.38 MPa is obtained by a tensile machine. The bonding interface is uniform, completed and non-porous. After the PI adhesive bonding process, the LiNbO3 single-crystal was lapped by chemical mechanical polishing. The thickness of the 100 mm diameter LiNbO3 can be decreased from 500 to 10 μm without generating serious cracks. A defect-free and tight bonding interface was confirmed by scanning electron microscopy. X-ray diffraction results show that the prepared LiNbO3 single-crystal thin film has a highly crystalline quality. Heterogeneous integration of LiNbO3 single-crystal thin film on Si is of great significance to the fabrication of MEMS devices for in-situ measurement of space-sensing signals.

Vertical nanoporous GaN substrates for photonic engineering: Lu2O3:Eu single crystal thin films as an example

To improve the efficiency of light extraction, the vertical-oriented nanoporous (NP) GaN film as scattering medium as well as light-coupling component was prepared by electrochemical (EC) etching followed by annealing process. The Lu2O3:Eu film as fluorescence medium was deposited on the NP-GaN substrate via pulse laser deposition (PLD) method, which has out-of-plane epitaxial relationship of Lu2O3 (222) || GaN (0002). By changing etching and annealing conditions, the average pore diameter, pore density and pore sidewall roughness of NP-GaN are controlled. The experiment shows that the photoluminescence (PL) intensities and quantum efficiencies of epitaxial films are related to the pore density and sidewall roughness of NP-GaN substrates. Due to the scattering effect of nanopores and light-coupling inside Lu2O3:Eu films on the NP-GaN substrates, the PL intensities were enhanced 13–96% compared with that of the films on as-grown GaN substrates, although the crystal quality of the epitaxial film becomes worse with the increase of porosity of the substrates.

Epitaxial growth technique for single-crystalline PbTiO3 thin film on Si substrate using an HfO2 buffer layer

We have succeeded in obtaining thin-film single crystals of PbTiO3 (PT) piezoelectrics on a Si single crystal with a few defects in the film at the extremely low temperature of 450 °C, which was considered difficult in the past. We confirmed that the obtained thin film demonstrates dielectric characteristics (lattice constant: a = 0.3904 nm, c = 0.4159 nm; relative dielectric constant: 120), along with an extremely high residual polarization value (Pr = 85 μC cm−2). A martensitic transformation in the HfO2 buffer layer plays an important role in the c-domain formation of the epitaxial PT film by an elastic lattice matching. This discovery will demonstrate a new function of HfO2 film and indicates that the system can be applied in a broad range of scenarios seeking to obtain single crystals for various functional oxide layer thin films on Si wafer substrates.

Tuning magnetic and optical properties through strain in epitaxial LaCrO3 thin films

We report on the effect of epitaxial strain on magnetic and optical properties of perovskite LaCrO3 (LCO) single crystal thin films. Epitaxial LCO thin films are grown by pulsed laser deposition on proper choice of substrates to impose different strain states. A combined experimental and theoretical approach is used to demonstrate the direct correlation between lattice-strain and functional properties. The magnetization results show that the lattice anisotropy plays a critical role in controlling the magnetic behavior of LCO films. The strain induced tetragonality in the film lattice strongly affects the optical transitions and charge transfer gap in LCO. This study opens possibilities to tailor the functional properties of LCO and related materials by strain engineering in epitaxial growth.

Epitaxial lift-off of flexible single-crystal magnetite thin films with tunable magnetic performances by mechanical deformation

Integrating magnetic oxide thin films without restriction of substrates is crucial for their applications in electronic and spintronic devices. Here the water-soluble Sr3Al2O6 (SAO) layer is employed as the sacrificial layer to transfer the millimeter-size single-crystal Fe3O4 films from SrTiO3 (STO) to silicon and flexible PET substrates. Structural characterizations indicate that the transferred films have the same epitaxial quality with the as-grown films. The hysteresis loops measurements of flexible Fe3O4/PET samples show a deceasing magnetic anisotropy under both of convex and concave deformations. Such tunable magnetic anisotropy is confirmed by fitting the angular dependence of ferromagnetic resonance (FMR) fields. These results provide a damage-free way to fabricate single-crystal spinel ferrites films on the selected substrates for flexible spintronic devices.

Thin-film lithium niobate ridge-waveguide grating couplers integrated with anti-reflection slots interface

Based on the X-cut single crystal lithium niobate thin film (TFLN), the deeply etched ridge-waveguide tapered grating couplers (GCs) embedded with additional modified slots, had been firstly theoretically and experimentally demonstrated. The entire device structures were simultaneously defined in the chip, which were patterned by a single step of electron-beam lithography, and subsequent monolithically manufactured by dry-etching process. For the fabricated GCs when operated at the telecom bands for transverse-electric (TE) polarization, the minimum insertion loss (IL) of −6.6 dB/coupler was measured at 1541 nm in the common GCs interfacing with positive scattering angle fiber, while for the backscattering design of GCs coupled with negative scattering angle fiber, a relatively much lower loss of −5.4 dB/coupler was even obtained at 1567 nm owing to the anti-reflection slots interface between optical waveguide and gratings, the evaluated 3-dB optical bandwidth was greater than 80 nm, and there was a good agreement between the modeling and experimental results. Moreover, the improved integrated TFLN GCs structure, which not only could be used for effective light-coupling between negative angle tilted fiber and waveguide gratings, but also could be simultaneously applied for the bidirectional transmission of optical beams. And this work might boost the developments of large scale photonic integrated components.

Oriented Halide Perovskite Nanostructures and Thin Films for Optoelectronics.

Oriented semiconductor nanostructures and thin films exhibit many advantageous properties, such as directional exciton transport, efficient charge transfer and separation, and optical anisotropy, and hence these nanostructures are highly promising for use in optoelectronics and photonics. The controlled growth of these structures can facilitate device integration to improve optoelectronic performance and benefit in-depth fundamental studies of the physical properties of these materials. Halide perovskites have emerged as a new family of promising and cost-effective semiconductor materials for next-generation high-power conversion efficiency photovoltaics and for versatile high-performance optoelectronics, such as light-emitting diodes, lasers, photodetectors, and high-energy radiation imaging and detectors. In this Review, we summarize the advances in the fabrication of halide perovskite nanostructures and thin films with controlled dimensionality and crystallographic orientation, along with their applications and performance characteristics in optoelectronics. We examine the growth methods, mechanisms, and fabrication strategies for several technologically relevant structures, including nanowires, nanoplates, nanostructure arrays, single-crystal thin films, and highly oriented thin films. We highlight and discuss the advantageous photophysical properties and remarkable performance characteristics of oriented nanostructures and thin films for optoelectronics. Finally, we survey the remaining challenges and provide a perspective regarding the opportunities for further progress in this field.

Young's modulus and corresponding orientation in β-Ga2O3 thin films resolved by nanomechanical resonators

We report on the nondestructive measurement of Young's modulus of thin-film single crystal beta gallium oxide (β-Ga2O3) out of its nanoscale mechanical structures by measuring their fundamental mode resonance frequencies. From the measurements, we extract the Young's modulus in the (100) plane, EY,(100) = 261.4 ± 20.6 GPa, for β-Ga2O3 nanoflakes synthesized by low-pressure chemical vapor deposition (LPCVD), and in the [010] direction, EY,[010] = 245.8 ± 9.2 GPa, for β-Ga2O3 nanobelts mechanically cleaved from bulk β-Ga2O3 crystal grown by the edge-defined film-fed growth (EFG) method. The Young's moduli extracted directly on nanomechanical resonant device platforms are comparable to theoretical values from first-principle calculations and experimentally extracted values from bulk crystal. This study yields important quantitative nanomechanical properties of β-Ga2O3 crystals and helps pave the way for further engineering of β-Ga2O3 micro/nanoelectromechanical systems (M/NEMS) and transducers.

In-Situ Process and Simulation of High-Performance Piezoelectric-on-Silicon Substrate for SAW Sensor

This paper studied the manufacturing process of Piezoelectric-on-Silicon (POS) substrate which integrates 128° Y–X Lithium niobate thin film and silicon wafer using Smart-Cut technology. The blistering and exfoliation processes of the He as-implanted LN crystal under different annealing temperatures are observed by the in-situ method. Unlike the conventional polishing process, the stripping mechanism of the Lithium niobate thin film is changed by controlling annealing temperature, which can improve the surface morphology of the peeling lithium niobate thin film. We prepared the 128° Y–X POS substrate with high single-crystal Lithium niobate thin film and surface roughness of 3.91 nm through Benzocyclobutene bonding. After simulating the surface acoustic wave (SAW) characteristics of the POS substrate, the results demonstrate that the Benzocyclobutene layer not only performs as a bonding layer but also can couple more vibrations into the LN thin film. The electromechanical coupling coefficient of the POS substrate is up to 7.59% in the Rayleigh mode when hLN/λ is 0.3 and hBCB/λ is 0.1. Therefore, as a high-performance substrate material, the POS substrate has proved to be an efficient method to miniaturize and integrate the SAW sensor.

Perovskite (PEA)2Pb(I1-xBrx)4 single crystal thin films for improving optoelectronic performances

Single crystal perovskites have become advanced materials in many applications due to their excellent optoelectronic properties and stability. However, bulk single crystal with unsatisfactory thickness would limit its application in flexible optoelectronic devices. Here, large-area two-dimensional perovskite (PEA) 2 Pb(I 1-x Br x ) 4 (x = 0–1) single crystal thin films (SCTFs) were prepared with a space-confined solution temperature-lowering crystallization method. Through rational control of halide compositions, we succeeded in growing (PEA) 2 Pb(I 1-x Br x ) 4 SCTFs with x increased from 0 to 1, which achieved band gap fine-tuning from 2.28 to 2.95 eV, corresponding to the edge of the absorption spectrum adjusted from green to blue. Using the excellent (PEA) 2 Pb(I 1-x Br x ) 4 SCTFs properties, a series of photodetectors were designed. The photodetector based on the I-rich composition (x = 0.1) exhibits optimum photosensitivity ( I light / I dark = 2 × 10 4 ) at wavelength of 473 nm (0.52 mW cm −2 ), while the photodetector based on the Br-rich component (x = 0.8) has the highest I light / I dark ratio (~1.6 × 10 4 ) under 405 nm (0.52 mW cm −2 ), showing the selectivity in light detection. At the same time, those devices show extremely low dark current, high specific detectivity (~10 13 Jones) and excellent stability. • Perovskite (PEA) 2 Pb(I 1-x Br x ) 4 (x = 0–1) SCTFs are prepared with a space-confined solution crystallization method. • Uniform Br doping in (PEA) 2 Pb(I 1-x Br x ) 4 (x = 0–1) SCTFs is evaluated by EDS analysis and elemental mapping. • Perovskite (PEA) 2 Pb(I 1-x Br x ) 4 (x = 0–1) SCTFs achieve band gap fine-tuning from 2.28 to 2.95 eV by Br doping. • Perovskite (PEA) 2 Pb(I 1-x Br x ) 4 SCTFs photodetectors show the selectivity in light detection under different wavelength.

Impacts of Mode Mixity on Controlled Spalling of (100)-Oriented Germanium

Controlled spalling is a technology to prepare single-crystal thin films of semiconductors by fracture with a subsurface crack propagating nearly parallel to the substrate surface. Practical applications require uniform thickness and a smooth surface across the whole film. Both wafer-scale and patterned-stressor-defined small-area spalling of germanium substrates are conducted experimentally and numerically. River line features are observed on spalled surfaces close to lateral edges of the spall, regardless of the spall direction and the size of the spalled area. Three-dimensional finite element method modeling shows the river lines are caused by mixed mode I + III loading near the lateral edges of spall and predicts a spall depth variation near the lateral edges of spall due to mixed mode I + II loading. The absolute range of river lines increases with lateral size of spall, while the relative range of river lines decreases, consistent with variations in mode mixity.

Size-Controlled Polarization Retention and Wall Current in Lithium Niobate Single-Crystal Memories.

Highly conductive domain walls in insulating ferroelectric LiNbO3 (LNO) single-crystal thin films with atomic smoothness are attractive for use in high-density integration of the ferroelectric domain wall random access memory (DWRAM) because of their excellent reliability and high read currents. However, downscaling of the memory size to the nanoscale could cause poor polarization retention. Understanding the size-dependent electrical performance of a memory cell is therefore crucial. In this work, highly insulating X-cut LNO thin films were bonded to SiO2/Si wafers and lateral mesa-like cells were fabricated on the film surfaces, where contact occurred with two-sided electrodes along the polar z-axis. Under application of an in-plane electric field above a coercive field (Ec), the domain within each memory cell was switched to be antiparallel to the unswitched referencing domain at the bottom; this resulted in the formation of a conducting domain wall, which enables the nondestructive readout strategy of the DWRAM. The cell, which has a lateral length (l) above a critical size (l0) of 105 nm, is found to be a mixture of two phases across the cell area. The inner area of the cell suffers from poor polarization retention because Ec = 150 kV/cm, as demonstrated by in-plane piezoresponse force microscopy imaging. In comparison, the outer periphery domains, which have lengths of 70 nm (∼l0/2), show good retention but require a much higher Ec of 785 kV/cm. The relevant physics is discussed as phase reconstruction occurs after release of the in-plane compressive strain near the outer regions; the results show good agreement with those of one-dimensional thermodynamic calculations and phase-field simulations. The measured current-voltage curves demonstrated a sudden enhancement of the wall current across the cell when l < l0, thus implying higher readout wall currents and better retention for the DWRAM at higher storage densities.

A flexible high quality-factor bulk acoustic resonator enabled with transferred single-crystal piezoelectric thin film for sensing applications

This paper proposes a flexible lateral-field-excited (LFE) film bulk acoustic resonator (FBAR) using transferred single-crystalline LiTaO3 piezoelectric thin films. Coplanar electrodes are adopted to excite the acoustic wave and the flexible LFE FBAR exhibits three wave modes: thickness-shear mode (TSM), thickness-longitudinal mode (TLM), and the second harmonic of TSM. Because of the adoption of high-quality transferred single-crystal piezoelectric thin films, a record high quality factor (Q) of 1837 is obtained for the TSM with a resonance frequency of 2.510 GHz. The TLM (4.700 GHz) and the second harmonic of TSM (7.490 GHz) also exhibit a high Q of 1516 and 928, respectively. The Qs are among the highest reported values for flexible piezoelectric MEMS resonators, and comparable to those of the silicon-based FBARs. The temperature sensitivities of the three wave modes are in the range of −58 to −67 ppm/℃ over the temperature range of 20–90 ℃. The electrical performance remains almost unchanged under a minimum bending radius of 3 mm. The flexible LFE FBARs exhibit high mechanical flexibility and a remarkable figure of merit. The excellent performance would enable application scenarios including low-loss, high-selectivity filters for future flexible front-end modules in the Internet of Things (IoT), and flexible temperature sensors that can be integrated into wearable healthcare and biochemical systems.

Design and analysis of piezoelectric micromachined ultrasonic transducer using high coupling PMN-PT single crystal thin film for ultrasound imaging

Due to its higher frequency, improved impedance matching, and easy formation of array, piezoelectric micromachined ultrasonic transducer (PMUT) has drawn much attentions for application in ultrasound imaging. In this paper, PMUT based on [011] poled 0.70Pb(Mg1/3Nb2/3)O3–0.30PbTiO3 (PMN-0.30PT) single crystal thin film was proposed and modeled by performing finite element method. Influences of different configurations of top electrode shape and bottom cavity shape on PMUT performance were studied. The investigation results show that the resonant frequency of PMN-PT-based PMUT with circular top electrode and square bottom cavity is up to 27.155 MHz. The effective electromechanical coupling coefficient ( of PMUT was up to 2.32%, which was 127% larger than that of lead zirconate titanate-based PMUT. The static transmitting sensitivity was three times enhancement compared to PMUT with square top electrode and square bottom cavity. Furthermore, acoustic field characteristics of PMUT and PMUT arrays based on PMN-PT in water were investigated. The relative pulse-echo sensitivity level of 2 × 2 PMUT array was −35 dB with the reflector at 200 μm. The obtained results demonstrated that the proposed PMN-PT-based PMUT can achieve high frequency and large as well as considerable sensitivity, which is promising for application in high resolution ultrasound imaging.

Distinct Carrier Transport Properties Across Horizontally vs Vertically Oriented Heterostructures of 2D/3D Perovskites.

Two-dimensional-on-three-dimensional (2D/3D) halide perovskite heterostructures have been extensively utilized in optoelectronic devices. However, the labile nature of halide perovskites makes it difficult to form such heterostructures with well-defined compositions, orientations, and interfaces, which inhibits understanding of the carrier transfer properties across these heterostructures. Here, we report solution growth of both horizontally and vertically aligned 2D perovskite (PEA)2PbBr4 (PEA = phenylethylammonium) microplates onto 3D CsPbBr3 single crystal thin films, with well-defined heterojunctions. Time-resolved photoluminescence (TRPL) transients of the heterostructures exhibit the monomolecular and bimolecular dynamics expected from exciton annihilation, dissociation, and recombination, as well as evidence for carrier transfer in these heterostructures. Two kinetic models based on Type-I and Type-II band alignments at the interface of horizontal 2D/3D heterostructures are applied to reveal a shift in balance between carrier transfer and recombination: Type-I band alignment better describes the behaviors of heterostructures with thin 2D perovskite microplates but Type-II band alignment better describes those with thick 2D microplates (>150 nm). TRPL of vertically aligned 2D microplates is dominated by directly excited PL and is independent of the height above the 3D film. Electrical measurements reveal current rectification behaviors in both heterostructures with vertical heterostructures showing better electrical transport. As the first systematic study on comparing models of 2D/3D perovskite heterostructures with controlled orientations and compositions, this work provides insights on the charge transfer mechanisms in these perovskite heterostructures and guidelines for designing better optoelectronic devices.