Single Crystalline Thin Films Research Articles

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.

(Invited) Electrocatalysts for Fuel Cell Cathodes

The studies of the oxygen reduction reaction on well-defined surfaces have guided the approach to design of the real world nanostructured catalysts. Insights from single crystalline systems and thin film surfaces have yielded atomically precise information about surface structure and alloying with other metals in the quest to improve their intrinsic catalytic activity. These experimental efforts are also producing information necessary for the design of catalysts with enhancing stability. These accomplishments were achieved through electrochemical in situ ICP-MS detection of parts per trillion levels of metal dissolution. Atomic level insight, has guided the chemical synthesis of well-defined nanoscale surfaces and creation of novel architectures that are associated with advanced catalytic performance. The progress in platinum-transition metal alloy nanocatalysts will be in focus of this lecture, with the goal of improving the activity and durability of the oxygen reduction electrocatalyst in a proton exchange membrane fuel cell. Atomic level control of the positioning of elements within alloy nanocrystals enables the design of shapes and surfaces that mimic the key descriptors discovered through research on well-defined surfaces. The meticulous characterization of nanocatalysts using extremely high-purity conditions and cutting-edge techniques provides greater understanding of their performance and degradation pathways. This knowledge allows better implementation of electrocatalysts in membrane electrode assemblies (MEA) for demonstration of expected performance in a fuel cell vehicle. An MEA is still viewed as a complex system with multiple interfaces. The catalyst, its support, water, ionomer, and gaseous reactants create challenging environment to control and engineer at the level that would be capable to address all technical targets that are necessary for wide deployment of this technology. The transition from high-purity liquid electrolytes into MEA still needs to be fully addressed in order to boost fuel cell stack performance. More recent research directions, such as platinum group metal (PGM) free catalysts will also be discussed. The main focus is being placed on replacement of platinum by more abundant cost effective constituents that could significantly lower the total cost of fuel cell. Typical PGM-free catalysts are consisted of transition metal sites coordinated by nitrogen dopants within a carbon matrix. While their intrinsic activity requires improvement, stability is equally critical in order to implement this class of materials in real fuel cells. Once again, well-defined, high-precision characterization and synthesis are proving vital to development of new generation of cathode catalysts. The overall research strategy presented here exhibits the merits of a continuous pipeline from fundamental to applied research in the development of fuel cell technology.

Magnetotransport properties of perovskite EuNbO3 single-crystalline thin films

We investigated the magnetic and transport properties of perovskite EuNbO3 single-crystalline thin films deposited by pulsed laser deposition. The EuNbO3 thin films showed metallic transport properties and ferromagnetism with a Curie temperature (TC) of ∼6 K. The carrier concentration and the mobility of the EuNbO3 thin films were nearly independent of temperature, suggesting that the excess oxygen in the films behaves as a scattering center. The sign of magnetoresistance changed around TC, possibly due to competition between the weak anti-localization effect and the magnetic coupling between Eu2+ 4f localized spins and Nb4+ 4d itinerant electrons.

Growth and transport properties of topological insulator Bi2Se3 thin film on a ferromagnetic insulating substrate**Project supported by the National Key R&D Program of China (Grant Nos. 2016YFA0300904 and 2016YFA0202301), the National Natural Science Foundation of China (Grant Nos. 11334011, 11674366, 11674368, and 11761141013), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant Nos. XDB07010200 and XDPB06).

Exchange coupling between topological insulator and ferromagnetic insulator through proximity effect is strongly attractive for both fundamental physics and technological applications. Here we report a comprehensive investigation on the growth behaviors of prototype topological insulator Bi2Se3 thin film on a single-crystalline LaCoO3 thin film on SrTiO3 substrate, which is a strain-induced ferromagnetic insulator. Different from the growth on other substrates, the Bi2Se3 films with highest quality on LaCoO3 favor a relatively low substrate temperature during growth. As a result, an inverse dependence of carrier mobility with the substrate temperature is found. Moreover, the magnetoresistance and coherence length of weak antilocalization also have a similar inverse dependence with the substrate temperature, as revealed by the magnetotransport measurements. Our experiments elucidate the special behaviors in Bi2Se3/LaCoO3 heterostructures, which provide a good platform for exploring related novel quantum phenomena, and are inspiring for device applications.

Solution-crystallized n-type organic thin-film transistors: An impact of branched alkyl chain on high electron mobility and thermal durability

Abstract Single-crystal organic thin-film field-effect transistors (OTFTs) have been actively studied because they are regarded as an ideal platform for charge carrier transport phenomena in small-molecule organic materials. Moreover, recently developed solution-based techniques to grow single-crystalline thin films have attracted industrial attention beyond fundamental physics. In this work, one of the solution-based techniques, namely, the edgecasting method, was employed to study single-crystal OTFTs of n-type organic semiconductors, dimerized naphtho[2,3-b]thiophene diimides, with linear and branched alkyl chains. A high electron mobility of >1 cm2 V−1 s−1 and an excellent thermal durability were observed in the branched alkyl-chain derivative, while the OTFT based on linear alkyl-substituted one showed clear degradation of mobility after thermal annealing. Therefore, this work will demonstrate a potential availability of branched alkyl chains for improving carrier mobility and thermal durability with large-scalable printing methods.

Observation of superconductivity in structure-selected Ti2O3 thin films

The search for new superconductors capable of carrying loss-free current has been a research theme in condensed matter physics for the past decade. Among superconducting compounds, titanates have not been pursued as much as Cu2+ (3d9) (cuprate) and Fe2+ (3d6) (pnictide) compounds. Particularly, Ti3+-based compounds or electron systems with a special 3d1 filling are thought to be promising candidates as high-TC superconductors, but there has been no report on such pure Ti3+-based superconducting titanates. With the advent of thin-film growth technology, stabilizing new structural phases in single-crystalline thin films is a promising strategy to realize physical properties that are absent in the bulk counterparts. Herein, we report the discovery of unexpected superconductivity in orthorhombic-structured thin films of Ti2O3, a 3d1 electron system, which is in strong contrast to the conventional semiconducting corundum-structured Ti2O3. This is the first report of superconductivity in a titanate with a pure 3d1 electron configuration. Superconductivity at 8 K was observed in the orthorhombic Ti2O3 films. Leveraging the strong structure-property correlation in transition-metal oxides, our discovery introduces a previously unrecognized route for inducing emergent superconductivity in a newly stabilized polymorph phase in epitaxial thin films.

Bulk and Thin Film Synthesis of Compositionally Variant Entropy-stabilized Oxides.

Here, we present a procedure for the synthesis of bulk and thin film multicomponent (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O (Co variant) and (Mg0.25(1-x)Co0.25(1-x)Ni0.25(1-x)CuxZn0.25(1-x))O (Cu variant) entropy-stabilized oxides. Phase pure and chemically homogeneous (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O (x = 0.20, 0.27, 0.33) and (Mg0.25(1-x)Co0.25(1-x)Ni0.25(1-x)CuxZn0.25(1-x))O (x = 0.11, 0.27) ceramic pellets are synthesized and used in the deposition of ultra-high quality, phase pure, single crystalline thin films of the target stoichiometry. A detailed methodology for the deposition of smooth, chemically homogeneous, entropy-stabilized oxide thin films by pulsed laser deposition on (001)-oriented MgO substrates is described. The phase and crystallinity of bulk and thin film materials are confirmed using X-ray diffraction. Composition and chemical homogeneity are confirmed by X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy. The surface topography of thin films is measured with scanning probe microscopy. The synthesis of high quality, single crystalline, entropy-stabilized oxide thin films enables the study of interface, size, strain, and disorder effects on the properties in this new class of highly disordered oxide materials.

Single-crystalline thin-film growth via solution-mediated polymorphic transformation of octahexyl-substituted phthalocyanine and its optical anisotropy

Abstract The fabrication of a single-crystalline thin film of non-peripheral octahexyl-phthalocyanine (C6PcH2) was realized by performing a tetrahydrofuran (THF) vapor treatment on a the spin-coated film. The solution-mediated polymorphic transformation of C6PcH2 was promoted in the THF vapor, and a single-crystalline thin film with α-phase crystal grew in the thin film when setting an appropriate temperature. The thin film exhibited optical anisotropy, such as a high dichroic ratio, and the color of the transmitted light varied from green to transparent depending on the polarization direction. The molecular packing structure on the substrate was determined by X-ray diffraction and polarized absorption measurements.

(Invited) Toward High Frequency and High Power Flexible Electronics

Radio frequency (RF) devices have been widely used for commercial wireless communication and military electronics. With the rapid development of wireless communication and radar technology, RF transistors for higher frequency and higher power applications are in demand. Meanwhile, the emerging market of wearable devices and internet of things (IoT) require electronic systems including RF parts to be in flexible or bendable form. Conventional semiconductors for flexible electronics like oxide semiconductors, and amorphous/poly Si and organic semiconductors cannot meet the requirement of wireless communication at the GHz range due to their low electron mobility (0.1 ~ 10 m2/V∙s). To fabricate high frequency flexible transistors, single crystal semiconductor nanomembranes have attracted great attention due to their high electron mobility (>100 m2/V∙s). Our group has reported GHz thin film Si MOSFET with an fT of 1.9 GHz and a maximum oscillation frequency (fmax ) of 3.1 GHz by transferring Si nanomembrane from a silicon-on-insulator wafer to a flexible plastic substrate. To further improve the performance of flexible RF Si MOSFETs, different device structures have been investigated. A strained Si nanomembrane channel has been demonstrated to enhance electron mobility. Flexible Si MOSFET with strained channel has shown an fT of 5.1 GHz and fmax of 15.1 GHz. Gate trench Si MOSFETs using nanoimprint technology has been successfully fabricated. By reducing the gate trench to 100 nm, the transistor shows an fT of 5 GHz and fmax of 38 GHz. Besides Si, we have also fabricated GaAs-based thin film heterojunction bipolar transistors (HBTs) for flexible RF electronics due to the high electron mobility of GaAs. We have successfully demonstrated thin film GaAs HBTs on biodegradable cellulose nanofibril (CNF) substrates by the transfer printing technique. HBTs on CNF substrate show an fT of 37.5 GHz and fmax of 6.9 GHz, facilitating use in GHz range applications. The reported flexible RF transistors based on Si and GaAs thin film have shown the capability of working at the GHz range. However, due to a small bandgap and low breakdown field, they are not suitable for high power applications at high frequency. GaN has been rapidly developed for high-frequency and high-power applications due to its large bandgap, high breakdown field, high electron mobility, and high electron saturation velocity of two dimensional electron gas (2DEG). Several flexible AlGaN/GaN HEMTs have been fabricated by the transfer printing AlGaN/GaN thin film to flexible substrate. Our group have reported flexible RF AlGaN/GaN HEMT on PET substrate with an fT of 60 GHz and fmax of 115 GHz. A layer of 3.5 µm intrinsic single crystalline GaN thin film was used as the heat spreading layer for the HEMT due to large thermal conductivity of single crystalline GaN. The HEMT can successfully work under 0.5 W, which indicates the potential use of AlGaN/GaN HEMTs in high-frequency and high-power flexible electronics. We further combined flexible RF AlGaN/GaN HEMTs with environmental friendly green flexible substrates to fabricate flexible green RF electronics for high-power applications. We successfully fabricated a 500 µm × 500 µm AlGaN/GaN HEMT on biodegradable CNF substrate using the transfer printing technique. The HEMT can achieve an fT and fmax of 40 GHz and 79 GHz, respectively, and work under 0.3 W on CNF substrate. To further exploit the application of flexible green RF AlGaN/GaN HEMTs, we fabricated a flexible RF power amplifier based on the HEMT. The RF power amplifier was fabricated by combining the flexible HEMT and impedance matching network consisting of a spiral inductor and metal-insulator-metal (MIM) capacitor on a temporary Si substrate with a PMMA sacrificial layer. After dissolving the PMMA sacrificial layer, the entire RF power amplifier (~10 µm thick) was transfer printed on a CNF substrate. The RF power amplifier on CNF substrate shows a small signal gain of ~6 dB from 5 GHz to 6 GHz. The RF power performance measurement shows it can deliver an RF power of ~10 mW with peak power added efficiency (PAE) of 4% at 5.5 GHz. The recent progress of flexible RF AlGaN/GaN HEMTs and RF power amplifiers will significantly advance the development of flexible electronics for high-frequency and high-power applications.

Consequences of phase separation on magnetotransport in dc magnetron sputtered Sm0.50Sr0.50MnO3 thin films on LSAT substrate

Single crystalline thin films of Sm0.50Sr0.50MnO3 (SSMO) (thickness ∼200 nm) were prepared on LSAT (100) single crystal substrates by dc magnetron sputtering. The X-ray diffraction θ‒2θ and ω-2θ scan reveals that these films (i) have very good crystallinity, (ii) are oriented along out-of-plane c-direction, and (iii) are under small tensile strain. Temperature and magnetic field dependent electrical transport of these films shows (i) sharp paramagnetic insulator (PMI) to ferromagnetic metal (FMM) transition, (ii) large enhancement in TIM when subjected to magnetic field, (iii) hysteretic variation in resistivity with magnetic field, and (iv) huge occurrence of magnetoresistance near PMI-FMM transition. The magnetic state in the FMM (T < TC) regime resembles cluster glass, which is formed by the presence of charge ordered-antiferromagnetic clusters in the ferromagnetic matrix. The results shown in the present study could be regarded as the consequence of strong nature of phase separation in these films due to competing FMM and AFM-COI phases in these films which is known to be the generic feature of low bandwidth manganites.

Evolutionary selection growth of two-dimensional materials on polycrystalline substrates

There is a demand for the manufacture of two-dimensional (2D) materials with high-quality single crystals of large size. Usually, epitaxial growth is considered the method of choice 1 in preparing single-crystalline thin films, but it requires single-crystal substrates for deposition. Here we present a different approach and report the synthesis of single-crystal-like monolayer graphene films on polycrystalline substrates. The technological realization of the proposed method resembles the Czochralski process and is based on the evolutionary selection 2 approach, which is now realized in 2D geometry. The method relies on 'self-selection' of the fastest-growing domain orientation, which eventually overwhelms the slower-growing domains and yields a single-crystal continuous 2D film. Here we have used it to synthesize foot-long graphene films at rates up to 2.5 cm h-1 that possess the quality of a single crystal. We anticipate that the proposed approach could be readily adopted for the synthesis of other 2D materials and heterostructures.

Tunable critical temperature for superconductivity in FeSe thin films by pulsed laser deposition

Stabilized FeSe thin films in ambient pressure with tunable superconducting critical temperature would be a promising candidate for superconducting electronic devices. By carefully controlling the depositions on twelve kinds of substrates using a pulsed laser deposition technique single crystalline FeSe thin films were fabricated. The high quality of the thin films was confirmed by X-ray diffraction with a full width at half maximum of 0.515° in the rocking curve and clear four-fold symmetry in φ-scan. The films have a maximum Tc ~ 15 K on the CaF2 substrate and were stable in ambient conditions air for more than half a year. Slightly tuning the stoichiometry of the FeSe targets, the superconducting critical temperature becomes adjustable below 15 K with quite narrow transition width less than 2 K. These FeSe thin films deposited on different substrates are optimized respectively. The Tc of these optimized films show a relation with the out-of-plane (c-axis) lattice parameter of the FeSe films.

Strain-induced high-temperature perovskite ferromagnetic insulator

Ferromagnetic insulators are required for many new magnetic devices, such as dissipationless quantum-spintronic devices, magnetic tunneling junctions, etc. Ferromagnetic insulators with a high Curie temperature and a high-symmetry structure are critical integration with common single-crystalline oxide films or substrates. So far, the commonly used ferromagnetic insulators mostly possess low-symmetry structures associated with a poor growth quality and widespread properties. The few known high-symmetry materials either have extremely low Curie temperatures (≤16 K), or require chemical doping of an otherwise antiferromagnetic matrix. Here we present compelling evidence that the LaCoO3 single-crystalline thin film under tensile strain is a rare undoped perovskite ferromagnetic insulator with a remarkably high TC of up to 90 K. Both experiments and first-principles calculations demonstrate tensile-strain-induced ferromagnetism which does not exist in bulk LaCoO3 The ferromagnetism is strongest within a nearly stoichiometric structure, disappearing when the Co2+ defect concentration reaches about 10%. Significant impact of the research includes demonstration of a strain-induced high-temperature ferromagnetic insulator, successful elevation of the transition over the liquid-nitrogen temperature, and high potential for integration into large-area device fabrication processes.

Flexible surface acoustic wave strain sensor based on single crystalline LiNbO3 thin film

A flexible surface acoustic wave (SAW) strain sensor in the frequency range of 162–325 MHz was developed based on a single crystalline LiNbO3 thin film with dual resonance modes, namely, the Rayleigh mode and the thickness shear mode (TSM). This SAW sensor could handle a wide strain range up to ±3500 με owing to its excellent flexibility, which is nearly six times the detecting range of bulk piezoelectric substrate based SAW strain sensors. The sensor exhibited a high sensitivity of 193 Hz/με with a maximum hysteresis less than 1.5%. The temperature coefficients of frequency, for Rayleigh and TSM modes, were −85 and −59 ppm/°C, respectively. No visible deterioration was observed after cyclic bending for hundreds of times, showing its desirable stability and reliability. By utilizing the dual modes, the strain sensor with a self-temperature calibrated capability can be achieved. The results demonstrate that the sensor is an excellent candidate for strain sensing.

Evolution of Intrinsic and Magnetic Field-Induced Magnetic Anisotropies in Strongly Phase-Separated Manganite Thin Films

Single crystalline thin film (∼ 100 nm) of La0.18Pr0.40Ca0.42MnO3 is grown on (001) oriented LaAlO3 substrates and the evolution of anisotropy associated with the supercooling behaviour of the magnetic liquid is studied as a function of temperature and magnetic field. The angle-dependent magnetization measurements ascertain that the easy magnetic axis lies in the plane of the film while the hard axis is along the plane normal. The ratio of the easy and hard axes magnetizations (M|| and M⊥) measured at 10 K, viz., M||/M⊥ (10 K) = 2.6, confirms the strongly anisotropic nature of the film. The easy axis ferromagnetic (FM) transition temperature (TC) is smaller than that along the hard axis. The giant hysteresis in FCC–FCW M–T, which manifests the magnetic liquid behaviour of the strongly phase-separated manganites is appreciably narrowed along the hard axis. The strain glass state is also less dominant along the easy axis. The analysis of the M–H data brings out that the easy axis remanence (Mr||) shows a nonlinear temperature dependence, while the one corresponding to the hard axis (Mr⊥) shows a nearly linear behaviour. The coercivity (HC) follows a law of the type \(H_{\mathrm {C}}\left (T \right )\mathrm {=}H_{\mathrm {C}}\mathrm {(0)(1-}{\text {AT}}^{b}\mathrm {)}\). The value of exponent b ∼ 0.5 for single-domain particles, but in the present case, the best fit to the experimental data yields the exponent b ≈ 0.25 for both HC|| and HC⊥. The smaller value of the exponent is attributed to the non-canonical nature of the ferromagnetic state and the associated strongly phase-separated nature of the LPCMO thin film.

High-Performance Single-Crystalline Perovskite Thin-Film Photodetector.

The best performing modern optoelectronic devices rely on single-crystalline thin-film (SC-TF) semiconductors grown epitaxially. The emerging halide perovskites, which can be synthesized via low-cost solution-based methods, have achieved substantial success in various optoelectronic devices including solar cells, lasers, light-emitting diodes, and photodetectors. However, to date, the performance of these perovskite devices based on polycrystalline thin-film active layers lags behind the epitaxially grown semiconductor devices. Here, a photodetector based on SC-TF perovskite active layer is reported with a record performance of a 50 million gain, 70 GHz gain-bandwidth product, and a 100-photon level detection limit at 180 Hz modulation bandwidth, which as far as we know are the highest values among all the reported perovskite photodetectors. The superior performance of the device originates from replacing polycrystalline thin film by a thickness-optimized SC-TF with much higher mobility and longer recombination time. The results indicate that high-performance perovskite devices based on SC-TF may become competitive in modern optoelectronics.

Research progress of multiferroicity in Bi-layered oxide single-crystalline thin films

Room temperature multiferroics with a single phase is very rare, and magnetic elements doped Bi-layered Aurivillius oxides are an important family of room temperature single phase multiferroics. However, due to the lack of single crystalline samples, the multiferroic related researches of these materials are mostly based on polycrystalline bulk or thin film samples. And the multiferroic characterizations are performed mostly by using the bulk type of samples. Therefore the studies of the origin and mechanism of the multiferroicity of these materials are extremely difficult. Recently, multiple magnetic elements doped singlecrystalline thin films have been successfully prepared, which makes it possible to study the physics mechanism of the Bi-layered Aurivillius oxides of multiferroicity. The current study shows that most of the single-crystalline thin films exhibit in-plane orientated spontaneous ferroelectric polarization and very weak room temperature magnetism. Moreover, at low temperatures the single-crystalline films exhibit a second magnetic transition. The resonant inelastic X-ray scattering experiments indicate that the doped structure exhibits a changed crystal field split, which may enhance the weak ferromagnetism through Dzyaloshinskii-Moriya interaction. On the other hand, the polarized neutron reflectivity experiments reveal that the single-crystalline thin film possesses much weaker room temperature magnetism than the bulk sample, which indicates that the origin of the magnetism and the magnetoelectric coupling in the single-crystalline samples are different from those in the polycrystalline samples. The current study of the multiferroicity in the single-crystalline Bi-layered Aurivillius thin film opens the road to designing better multiferroic systems of the Aurivillius materials.

A universal top-down approach toward thickness-controllable perovskite single-crystalline thin films

A facile and general top-down strategy to fabricate hybrid perovskite single-crystalline film from its bulk single crystal is developed.

Solution-processed high performance organic thin film transistors enabled by roll-to-roll slot die coating technique

Abstract In this study, we achieved high performance organic small molecule transistors by using roll-to-roll slot die coating technique and polymer dielectric layers. By carefully controlling the coating conditions, highly ordered single crystalline thin films could be achieved. Hence, the organic thin film transistor (OTFT) device exhibited an average mobility of 4.2 cm2V−1s−1 and maximum mobility of 6.5 cm2V−1s−1 based on polystyrene (PS) dielectric layer in the ambient air condition, which is among the best values for TIPS-pentacene based devices. The thin film morphology, crystallinity, and optical properties were studied in details. A complementary inverter based on p-type TIPS-pentacene and n-type 6,13-Dicyano pentacene-2,3:9,10-bis(dicarboximide) (PDI-CN) with a high voltage gain of 54 was achieved. Finally, the flexible devices based on roll-to-roll slot die coated PS dielectric and TIPS-pentacene were achieved with a charge carrier mobility of 1.2 cm2V−1s−1.

Surface modifications of crystal-ion-sliced LiNbO3 thin films by low energy ion irradiations

Single crystalline 128°Y-cut LiNbO3 thin films with a thickness of 670nm are fabricated onto Si substrates by means of crystal ion slicing (CIS) technique, adhesive wafer bonding using BCB as the medium layer to alleviate the large thermal coefficient mismatch between LiNbO3 and Si, and the X-ray diffraction pattern indicates the exfoliated thin films have good crystalline quality. The LiNbO3 thin films are modified by low energy Ar+ irradiation, and the surface roughness of the films is decreased from 8.7nm to 3.4nm. The sputtering of the Ar+ irradiation is studied by scanning electron microscope, atomic force microscope and X-ray photoelectron spectroscopy, and the results show that an amorphous layer exists at the surface of the exfoliated film, which can be quickly removed by Ar+ irradiation. A two-stage etching mechanism by Ar+ irradiation is demonstrated, which not only establishes a new non-contact surface polishing method for the CIS-fabricated single crystalline thin films, but also is potentially useful to remove the residue damage layer produced during the CIS process.