LiNbO3 Layer Research Articles

Lithium Transport through Nanosized Amorphous Silicon Layers

Lithium migration in nanostructured electrode materials is important for an understanding and improvement of high energy density lithium batteries. An approach to measure lithium transport through nanometer thin layers of relevant electrochemical materials is presented using amorphous silicon as a model system. A multilayer consisting of a repetition of five [(6)LiNbO3(15 nm)/Si (10 nm)/(nat)LiNbO3 (15 nm)/Si (10 nm)] units is used for analysis, where LiNbO3 is a Li tracer reservoir. It is shown that the change of the relative (6)Li/(7)Li isotope fraction in the LiNbO3 layers by lithium diffusion through the nanosized silicon layers can be monitored nondestructively by neutron reflectometry. The results can be used to calculate transport parameters.

Loss Reduction of Longitudinal-Type Leaky Surface Acoustic Wave by Reverse Proton Exchange

The longitudinal-type leaky surface acoustic wave (LLSAW) is one of the SAW modes that is advantageous for application to high-frequency SAW devices. However, the LLSAW has huge inherent attenuation. In this study, to reduce the attenuation of the LLSAW, we proposed a layered structure of air, a bulk LiNbO3 (LN) layer, and an elastically softened LN substrate. When the layered structure was applied to X–36°Y LN with a large coupling factor K2 for the LLSAW, the calculated attenuation for the metallized surface decreased at a certain depth of the bulk layer. To realize such a layered structure, a reverse proton exchange (RPE) process was applied to X–36°Y LN, and the propagation and resonance properties of the LLSAW were evaluated. A “good region” existed on the RPE wafer, in which the value of K2 for the Rayleigh wave was recovered to approximately 80% of that of the virgin wafer. In comparison with the virgin wafer, the propagation loss for the free surface of the good region on the RPE wafer was decreased threefold, the insertion loss was decreased drastically, and the resonance properties were improved markedly.

Efficient terahertz generation by optical rectification in Si-LiNbO3-air-metal sandwich structure with variable air gap

A record high optical-to-terahertz conversion efficiency of 0.25% was realized with femtosecond laser pulses propagated in a planar Si-LiNbO3-air-metal structure. Terahertz spectrum tuning was demonstrated by adjusting an air gap between the LiNbO3 layer and the metal plate. The influence of optical pulse chirp on the efficiency of terahertz generation was investigated.

High-capacity thin film lithium batteries with sulfide solid electrolytes

Thin film batteries with amorphous sulfide-based solid electrolytes were investigated. The interfacial resistance of the battery has been drastically reduced by inserting a very thin LiNbO3 layer between the LiCoO2 cathode and solid electrolyte. The cycle performance of the battery is affected by the composition of the Li2S–P2S5 solid electrolyte. From XPS analysis, it is suggested that the electrochemical stability of the solid electrolyte changes as the composition of the solid electrolyte changes. Thin film batteries have shown both high capacity and good cycle performance with a solid electrolyte of suitable composition and silicon thin layer at the anode interface. Finally, a capacity of 400μAhcm−2 at 500 cycles has been achieved with a 9.1μm-thick cathode.

Room temperature direct bonding of LiNbO3 crystal layers and its application to high-voltage optical sensing

LiNbO3 is a crystal widely used in photonics and acoustics, for example in electro-optic modulation, nonlinear optical frequency conversion, electric field sensing and surface acoustic wave filtering. It often needs to be combined with other materials and used in thin layers to achieve the adequate device performance. In this paper, we investigate direct bonding of LiNbO3 crystals with other dielectric materials, such as Si and fused silica (SiO2), and we show that specific surface chemical cleaning, together with Ar or O2 plasma activation, can be used to increase the surface free energy and achieve effective bonding at room temperature. The resulting hybrid material bonding is very strong, making the dicing and grinding of LiNbO3 layers as thin as 15 µm possible. To demonstrate the application potentials of the proposed bonding technique, we have fabricated and characterized a high-voltage field sensor with high sensitivity in a domain inverted and bonded LiNbO3 waveguide substrate.

Relationship between the Crystallinity and Magnetic Properties of Cr<sub>2</sub>O<sub>3</sub>/LiNbO<sub>3</sub>/Cr<sub>2</sub>O<sub>3</sub> Multi-Layer Materials

The magnetic properties of an artificial multiferroic material, a Cr2O3/LiNbO3/Cr2O3 hetero structure with various-thickness LiNbO3 layers, were investigated. All samples showed ferromagnetism due to the existence of oxygen deficiency or an excess of the Cr2O3 layer in the LNO/Cr2O3 interface. The ferromagnetism of the samples was affected by the crystal orientation of the LiNbO3 layer, and seemed to have a major impact on the magneto-electric behavior of this hetero system.

Study of Er+ ion-implanted lithium niobate structure after an annealing procedure by RBS and RBS/channelling

Abstract Erbium-doped lithium niobate (Er:LiNbO3) is a prospective photonics component, operating at λ = 1.5 μm, which could be used as an optical amplifier or waveguide laser. We have focused on the structure of Er:LiNbO3 layers created by 330 keV erbium ion implantation (fluences 1.0 × 1015, 2.5 × 1015 and 1.0 × 1016 cm−2 1) in the X, Z and two various Y crystallographic cuts of LiNbO3. Five hours annealing at 350 °C was applied to recrystallize the as-implanted layer and to avoid clustering of Er. Depth distribution of implanted Er has been measured by Rutherford Backscattering Spectroscopy (RBS) using 2 MeV He+ ions. Defects distribution and structural changes have been described using the RBS/channelling method. Data obtained made it possible to reveal the relations between the crystallographic orientation of the implanted crystal and the behaviour during the restoration process. The deepest modified layer has been observed in the perpendicular Y cut, which also exhibits the lowest reconstruction after annealing. The shallowest depth of modification and good recovery after annealing were observed in the Z cut of LiNbO3. Since Er-depth profiles changed significantly in the perpendicular Y cut, we suppose that the crystal structure recovery inhibits Er mobility in the crystalline structure.

Si–LiNbO3–air–metal structure for concentrated terahertz emission from ultrashort laser pulses

A planar Si–LiNbO3–air–metal structure is proposed as a further development of the highly efficient optical-to-terahertz conversion scheme in sandwich structures that was recently demonstrated. The new structure allows one to collect the terahertz emission into one spatial direction and to control its spectrum by varying an air gap between the metal substrate and the LiNbO3 layer. While the overall increase in the terahertz generation can reach a factor of 2, the spectral density in the interesting for practical application interval 0.5–1.5 THz can be increased by a factor of 3.5–4.

Large electromechanical coupling factor film bulk acoustic resonator with X-cut LiNbO3 layer transfer

As layer transfer techniques have been notably improved in the past years, lithium niobate (LiNbO3) appears as a candidate for the next generation of ultrawide band radio frequency (rf) filters. Depending on the crystalline orientation, LiNbO3 can achieve electromechanical coupling factors Kt2 more than six times larger than those of sputtered aluminum nitride films. In this letter, a process based on direct bonding, grinding, polishing, and deep reactive ion etching is proposed to fabricate a single crystal LiNbO3 film bulk acoustic resonator. From the fabricated test vehicles, Kt2 of 43% is measured confirming the values predicted by theoretical computations.

Far-infrared spectroscopy of Au-implanted lithium niobate (LiNbO3)

The optical phonons and the behavior of dielectric functions of ion-implanted LiNbO3 were studied using far-infrared spectroscopy. First, single crystal LiNbO3 specimens were implanted with −75keV Au ions at two different levels of ion fluence (1×1016 and 1×1017cm−2), and then annealed at 800°C for 1h. Then, before and after annealing, the optical spectra were measured in two frequency ranges: (1) reflectance far-infrared spectra in the range from 40 to 700cm−1 and (2) transmission ultraviolet, visible and near-infrared spectra in the range from 250 to 2000nm. The far-infrared spectra were analyzed by fitting with a harmonic oscillation model of the dielectric function to determine the three oscillation parameters of each optical phonon-mode, namely, mode frequency, oscillator strength and damping constant. The behavior of these oscillation parameters and the dielectric function revealed that the ion implantation caused a phase transition, probably amorphization, in the implanted layer, and that the post-annealing enabled the implanted layers to completely “recover” their dielectric function to that of the unimplanted crystalline LiNbO3 substrate, despite the presence of embedded Au clusters in the layer as seen in the visible spectra. This “recovery” result indicates that the metal-embedded LiNbO3 layer should show a dielectric response to an electric field in high frequency equivalent to the response by an unimplanted structure.

Er+ medium energy ion implantation into lithium niobate

Erbium-doped lithium niobate (Er:LiNbO3) is a prospective photonics component, operating at 1.5μm, which could find its use chiefly as an optical amplifier or waveguide laser. In this study, we have focused on the properties of the optically active Er:LiNbO3 layers, which are fabricated by medium energy ion implantation under various experimental conditions. Erbium ions were implanted at energies of 330 and 500keV with fluences of 1.0×1015, 2.5×1015 and 1.0×1016cm−2 into LiNbO3 single-crystalline cuts of various orientations. The as-implanted samples were annealed in air at 350°C for 5h. The depth distribution and diffusion profiles of the implanted Er were measured by Rutherford Backscattering Spectroscopy (RBS) using 2MeV He+ ions. The projected range RP and projected range straggling ΔRP were calculated employing the SRIM code. The damage distribution and structural changes were described using the RBS/channelling method. Changes of the lithium concentration depth distribution were studied by Neutron Depth Profiling (NDP). The photoluminescence spectra of the samples were measured to determine whether the emission was in the desired region of 1.5μm. The obtained data made it possible to reveal the relations between the structural changes of erbium-implanted lithium niobate and its luminescence properties important for photonics applications.

Analysis of Quasi-Periodic Structural Defects in H:LiNbO3 Layers

Quasi-periodic structural defects on X-cut lithium niobate proton exchange waveguides surface were investigated. The structural defects are indicated as parallel bulge strips protruding above the surface up to 70 nm. It was found that under certain conditions proton exchange in pure benzoic and succinic acids leads to the formation of lamellar phase inclusions with crystal lattice deformation from 9.6 · 10 −3 to 22.0 · 10 −3 in waveguide layer, which induce the arising of the surface structural defects. It was proved that interstitial protons play a dominant role in a formation of these phase inclusions.

Influence of different substrates on the ionic conduction in LiCoO2/LiNbO3 thin‐film bi‐layers

AbstractLiNbO3 thin films, deposited by e‐gun evaporation, show lithium deficiency, which is cured by “Li doping”. The “Li doping” of the films was achieved by preparing a structure of Li‐Nb‐O/Li/Li‐Nb‐O, which after annealing forms a homogenized LiNbO3 layer because of diffusion of Li in the two Li‐Nb‐O layers. The LiCoO2/LiNbO3 bi‐layers were prepared either on Stainless Steel/TiN or on Al2O3/Co/Pt substrates/ohmic‐contacts by depositing first either the cathode LiCoO2 or the electrolyte LiNbO3. The Nyquist plots of the AC impedance measurements of all structures showed that the interfaces prepared on Stainless‐Steel/TiN consisted of two semicircles. The structures deposited on Al2O3/Co/Pt showed a third semicircle, which is probably due to the roughness of the substrate. It is important that the ionic properties of the bi‐layers with the cathode material deposited first, a usual structure in a microbattery, are improved compared to the other structures. The quality of the LiNbO3 layer depends very much on the substrate. It can be evaluated from Arrhenius plots that the activation energy of this layer is considerably lower when the whole structure is deposited on Stainless Steel/TiN. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photonic Crystals - Optical Structures for Advanced Technology

This paper presents experimental results in the field of photonic crystals investigation. There are presented our results of experimental investigation of one-dimensional photonic crystals appearing in nature, two-dimensional photonic crystals we formed in LiNbO3 and thin photoresist layers and results of our investigation of chromatic dispersion of photonic crystal fibers as well as the influence of fiber bending on their transmittance.

Proton Sites Occupation Sequence in Crystal Lattice of H x Li1−x NbO3 Monocrystal Layers

The infrared OH absorption band has been studied by Fourier spectroscopy for proton exchanged layers of HxLi 1−xNbO3 and nominally pure congruent LiNbO3. Frequencies of OH band components were associated with a set of structurally nonequivalent proton positions in oxygen plane perpendicular to polar c axis. The model was developed to describe the sequence of proton sites occupation in oxygen plane of HxLi 1−xNbO3 crystal lattice with proton concentration in the range from x < 0.01 to x ≈ 0.63. It has been found that nominally pure LiNbO3 and α-phase layer of HxLi 1−xNbO3 have hydrogen bonds with semi-uniform vibration frequency distribution.

Strain-induced effects in H: Ti: LiNbO3 single-crystal layers

The parameters of proton-exchange waveguide H: LiNbO3 layers and doubly doped H: Ti: LiNbO3 layers were comparatively analyzed by x-ray diffractometry and optical mode spectroscopy. It was established that the change in the lattice parameter in hybrid H: Ti: LiNbO3 layers is significantly smaller than that in proton-exchange H: LiNbO3 layers formed under the same proton exchange conditions. The behavior of the optical and structural parameters of H: Ti: LiNbO3 layers was studied in a wide hydrogen concentration range.

Precipitation of lamellar phases in H: LiNbO3 layers

Structural defects in a surface layer of the proton-exchanged waveguide layer formed on a substrate prepared from an X-cut lithium niobate single crystal are revealed and investigated using scanning electron microscopy and optical interference profilometry in combination with selective etching. It is established that these structural defects, which are oriented along the [0 15 1] crystallographic direction and matched to the matrix (the β phase of the HxLi1 − xNbO3 solid solution) through a network of misfit dislocations, consist of one of the HxLi1 − xNbO3 phases precipitated in the form of lamellar regions ∼100 nm thick. Precipitation of lamellar phases precedes the destruction of the surface of the proton-exchanged layer. The presence of residual internal stresses in the LiNbO3 initial crystal favors the formation of the lamellar phases.

High-frequency surface acoustic wave devices based on LiNbO3∕diamond multilayered structure

This research is focused on the development of a new technology for the fabrication of high-frequency surface acoustic wave (SAW) filters based on diamond and lithium niobate (LiNbO3) materials. LiNbO3 films are in situ deposited at 490 °C on polycrystalline diamond substrates by radio-frequency magnetron sputtering; a multistep growth process is proposed to deposit oriented LiNbO3 films with a smooth surface. Conventional interdigital transducers are fabricated on top of the piezoelectric LiNbO3 layer by standard optical lithography. We report experimental results for a SAW bandpass filter operating at a frequency above 2.30 GHz. We have observed that Rayleigh SAW modes are excited within this structure, with extremely high phase velocities (up to 12000m∕s). This latter result illustrates the greatest advantage of using diamond as an acoustic substrate. It allows the fabrication of SAW devices operating in the gigahertz frequency range using standard optical lithography.

Quenching effects on crystallographic and optical properties of H : LiNbO3 layers

The formation and decay of high temperature phases in HxLi1−xNbO3 waveguide layers have been studied by means of waveguide mode spectroscopy and x-ray rocking curve measuring. The hydrogen doped layers with x ∼ 0.47 in (001) LiNbO3 were created by proton exchange reaction in melt benzoic acid. The high temperature phases were frozen by quenching from T = 200°C. It has been shown that these phases possess a 9% higher refractive index and 16% lower deformation in reference to the characteristic of HxLi1−xNbO3 phase equilibrium at room conditions. The relaxation of the high temperature phase to the equilibrium one has been traced at T = 100°C.

M-line spectroscopy for optical analysis of thick LiNbO3 layers grown on sapphire substrates by radio-frequency multistep sputtering

Thick films of lithium niobate (LiNbO3) have been epitaxially grown on sapphire substrates by multistep radio-frequency magnetron sputtering. We have investigated the optical properties of the deposited layers by the m-line spectroscopy technique. The relationship between the film-substrate interface, the surface morphology of the LiNbO3 layer, and the optical measurements has been carefully established. Since this optical technique is sensitive to material defects, a detectable degradation of the optical data is observed when the microstructure of the LiNbO3 film changes. In particular, the angular width of the excited optical guided modes (Δθ) varies with the film thickness. This agrees perfectly with the evolution of the surface roughness as a function of the film thickness. We report a surface roughness rms of 2 nm corresponding to an angular width Δθ of 0.1° for the TE0 mode. The accuracy and the sensitivity of this technique can be advantageously compared to the classical tool of atomic force microscopy.