LiNbO3 Single-crystal Substrate Research Articles

Ultrasound: A new strategy for artificial synapses modulation

AbstractDue to its non‐invasive nature, ultrasound has been widely used for neuromodulation in biological systems, where its application influences the synaptic weights and the process of neurotransmitter delivery. However, such modulation has not been emulated in physical devices. Memristors are ideal electrical components for artificial synapses, but up till now they are hardly reported to respond to ultrasound signals. Here we design and fabricate a HfOx‐based memristor on 64°Y‐X LiNbO3 single crystal substrate, and successfully realize artificial synapses modulation by shear‐horizontal surface acoustic wave (SH‐SAW). It is a prominent short‐term resistance modulation, where ultrasound has been shown to cause resistance drop for various resistance states, which could fully recover after the ultrasound is shut off. The physical mechanism illustrates that ultrasound induced polarization potential in the HfOx dielectric layer acts on the Schottky barrier, leading to the resistance drop. The emulation of neuron firing frequency modulation through ultrasound signals is demonstrated. Moreover, the joint application of ultrasound and electric voltage yields fruitful functionalities, such as the enhancement of resistance window and synaptic plasticity through ultrasound application. All these promising results provide a new strategy for artificial synapses modulation, and also further advance neuromorphic devices toward system applications.image

Characterization of LiNbO3 Single-Crystal Substrates Irradiated with Electrons

Electron irradiation effects on lithium niobate (LiNbO3) single crystals have been investigated by photoacoustic spectroscopy (PAS), Fourier transform infrared (FT-IR) spectroscopy, and Raman scattering spectroscopy. The photoacoustic signal intensity at 750 nm increase with increasing electron electron fluence. The OH- absorption spectra in the IR region are used to examine the defect structure of LiNbO3. The absorption spectra are almost the same at electron fluencies below 5 ×1017/cm2. We found that the influence of the electron irradiation was different between the mirror-polished side and the as-sliced side, as determined from the Raman spectra. There was no change in the Raman spectra on the mirror-polished LiNbO3 surface side. The control of the composition is most important to avoid the effects of electron irradiation.

Preparation of zinc-containing thin coatings on LiNbO3 substrates

Zinc oxide thin films are prepared on LiNbO3 single-crystal substrates with the preliminary deposition of a buffer layer consisting of zinc niobate of the composition ZnNb2O6 with an orthorhombic crystal lattice or the composition Zn3Nb2O8 with a monoclinic crystal lattice. The phase composition, the thickness, and the quality of the films are characterized using X-ray powder diffraction and electron microscopy.

Surface Steps Observed in a Dry Annealed z-Cut LiNbO3 Substrate

Surface reformation is observed in an OH-reduced z-cut LiNbO3 single-crystal substrate after annealing at 1000° C in a dry oxygen atmosphere. Atomic force microscopic observation reveals a step-like texture with step height of 0.7 nm height, corresponding to half of the z-axial length of LiNbO3.

薄膜初期成長過程に及ぼす金属シード層の効果

Ag and Cu films were deposited by vacuum evaporation on a LiNbO3 single-crystal substrate on which surface acoustic waves (SAW) at 45 MHz were propagating. Prior to the deposition, ultra-thin Ti, Cr, Cu, and Ag films were fabricated as seed layers. The coalescence processes in an earlier stage of the film growth were investigated by measuring the SAW amplitudes during deposition. The critical thickness, at which the film structure changes from islandlike to continuous, was found to decrease with increasing seed thickness, except in the case of Cu films deposited on Ag seed layers. Ag films deposited on ultra-thin seed layers had topologically smooth surfaces in comparison with films directly deposited on the substrate. These results suggest that islands of the seed layer capture the adatoms diffused over the substrate surface and become the growth centers for the subsequent film deposition.

弾性表面波で励振された基板上に作製した金属薄膜の初期成長過程

Co and Fe ultra-thin films of various thicknesses coated with 50-Å-thick Cu film were prepared by vacuum evaporation on an LiNbO3 single-crystal substrate excited by surface acoustic waves (SAW) at 45 MHz. In order to clarify the detailed correlation between the SAW attenuation and the microstructure of the metallic film in the initial growth stage, the magnetization curves and megnetoresistance of Cu-coated ultra-thin films were investigated as functions of Co or Fe thickness, d, in connection with the SAW propagation properties during deposition. With increasing d, the behavior of the magnetization curves changed from superparamagnetic to ferromagnetic, and the magnetoresistance, which is similar to that of Co-Cu granular alloy films, vanished at d=dp, where the SAW attenuation reaches a maximum. From these results, it was concluded that the film thickness dp corresponds to the critical thickness at which the film structure changes from island-like to continuous.

弾性表面波で励振された基板上への金属薄膜の作成とその構造制御

Pure Ag, Cu, Ni, and Co films were prepared by vacuum evaporation on a LiNbO3 single-crystal substrate excited by surface acoustic waves (SAW) at 45 MHz under various amplitude conditions. The surface morphology of the films and the SAW propagation properties during deposition were investigated in order to determine the influence of SAW excitation on the structural changes in the films. In the Ag and Cu films, the surface roughness Ra and the thickness dp at which the SAW attenuation reached a maximum changed drastically as a function of the SAW amplitude, in contrast to the case for the Ni and Co films. Taking into account that dp corresponds to the thickness at which the film structure changes from island to continuous, it was considered that SAW influences the initial stage of film growth and that a large SAW amplitude enhances the agglomeration of islands. By in situ monitoring of the propagation properties of SAW, microscopic structural changes were also found in the Ag and Cu films during the initial growth stage.