Characterization of LiTaO3 thin films fabricated by sol-gel technique

Lithium tantalite (LiTaO₃) thin films have been fabricated by sol-gel technique and crystallized by RTA process. The effect of heating temperature on the structural properties of LiTaO₃ is investigated. The thin films are characterized by means of X-ray diffraction, Raman spectroscopy and thermal analysis (DCA/DTA). After the optimization of the growth parameters of LiTaO₃ prepared by sol-gel processing, the pyroelectric property of LiTaO₃ thin films deposited on Si (1 0 0) substrates have been also investigated.     

Hyper frequency modeling of resonated systems based on piezoelectric LiTaO3 thin layers

In this work, we discuss the piezoelectric activity of lithium tantalite (LiTaO₃) thin layers and to more understand this phenomenon we have developed a model for our LiTaO₃ resonators based on mason model and simulated the hyper frequency behavior. Our LiTaO₃ resonators are made from three layers staked on silicon substrates. The aluminum thin film constitutes the external electrode, the platinum forms the internal electrode and the lithium tantalite constitutes the piezoelectric layer. Each element of these layers is represented by an arrangement of impedances. The simulation shows the reflection coefficient, ρ, as a function of the frequency. We observe a resonant frequency that decreases with the increase of the thickness of the piezoelectric LiTaO₃ layers. A slight variation of this resonant frequency is obtained when comparing it with that of the uncharged piezoelectric device, which is due to the different layers loading the system. Over oscillations superposing to the envelope are observed and found to be related to the propagation of the acoustic wave in the silicon substrate. From these over oscillations one can see that this system can be used as an efficient method to calculate the thickness of any substrate.     

Characteristics of THz-wave radiation using a monolithic grating coupler on a LiNbO3 crystal

Wide tunable terahertz (THz) wave generation was successfully demonstrated utilizing a grating coupler fabricated on the surface of a LiNbO₃ crystal which was pumped by a Q-switched Nd:YAG laser. In this paper, we report the detailed characteristics of the oscillation and the radiation including tunability, spatial and temporal coherency, directivity, and efficiency. Oscillation using a LiTaO₃ crystal was also performed, in which experimental phasematching condition values agreed well with the calculated one.     

混合结构声表面波滤波器的设计

为了提高矩形系数,该文针对混合结构的声表面波滤波器的设计展开研究。建立仿真模型,采用纵向耦合结构级联单个谐振器或多个谐振器,并在36°Y-X LiTaO₃和41°Y-X LiNbO₃压电材料上研制出两种声表面波滤波器。测试结果表明,滤波器的矩形系数分别为1.7和2,测试与仿真结果基本吻合。     

Solution Processing Route to Multifunctional Titania Thin Films: Highly Conductive and Photcatalytically Active Nb:TiO2

This paper reports the synthesis of highly conductive niobium doped titanium dioxide (Nb:TiO₂) films from the decomposition of Ti(OEt)₄ with dopant quantities of Nb(OEt)₅ by aerosol‐assisted chemical vapor deposition (AACVD). Doping Nb into the Ti sites results in n‐type conductivity, as determined by Hall effect measurements. The doped films display significantly improved electrical properties compared to pristine TiO₂ films. For 5 at.% Nb in the films, the charge carrier concentration was 2 × 10²¹ cm^?3 with a mobility of 2 cm² V^–1 s^–1 . The corresponding sheet resistance is as low as 6.5 Ω sq^–1 making the films suitable candidates for transparent conducting oxide (TCO) materials. This is, to the best of our knowledge, the lowest reported sheet resistance for Nb:TiO₂ films synthesized by vapour deposition. The doped films are also blue in colour, with the intensity dependent on the Nb concentration in the films. A combination of synchrotron, laboratory and theoretical techniques confirmed niobium doping into the anatase TiO₂ lattice. Computational methods also confirmed experimental results of both delocalized (Ti⁴⁺) and localized polaronic states (Ti³⁺) states. Additionally, the doped films also functioned as photocatalysts. Thus, Nb:TiO₂ combines four functional properties (photocatalysis, electrical conductivity, optical transparency and blue colouration) within the same layer, making it a promising alternative to conventional TCO materials.     

Interfacial Reaction Between Nb Foil and n-Type PbTe Thermoelectric Materials During Thermoelectric Contact Fabrication

PbTe is a high-conversion-efficiency thermoelectric (TE) material that is commonly used in space exploration applications. Integration of PbTe in TE devices has a significant impact on the conversion efficiency and reliability of TE devices. Hence, our effort focuses on developing novel approaches for bonding metallic contacts to PbTe to improve device performance and reliability. In this study, pure Nb foil was directly bonded to PbTe-based TE materials to fabricate the hot-side contacts of TE elements using a rapid hot-press. The materials were sintered at 700°C under pressure of 40 MPa for various holding times. We found that a reaction layer of needle-like Nb₃Te₄ mixed with Pb forms at the interface of the Nb/PbTe joints and that Pb is distributed in the gaps of the Nb₃Te₄ grains. We analyze the resulting microstructure and finally calculate the time exponent of the growth kinetics of the Nb₃Te₄ layer. Fracture surface analysis showed that the Nb/PbTe joint fractures at the interface between Nb and Nb₃Te₄ and within the PbTe matrix, indicating that the bonding between Nb and Nb₃Te₄ is weak.     

改进型3IDT-LCR结构的低损耗小矩形系数SAW滤波器

通过理论仿真,该文研制了一种改进型的3IDT-LCR结构,并基于该结构在42°Y-X LiTaO₃压电基片上研制出一种低损耗、小矩形系数的SAW滤波器。滤波器标称频率为253.75 MHz,实测插入损耗1.05 dB,低端带外抑制接近80 dB,高端带外抑制接近65 dB,-1 dB带宽10.52 MHz,-3 dB带宽12.54 MHz,-40 dB带宽16.68 MHz,矩形系数1.33。实测结果表明,该改进型3IDT-LCR结构的SAW滤波器具备低插入损耗、小矩形系数和高带外抑制的特点。     

铌酸锂热释电性能的研究

由材料参数测试和器件响应率测试证实,LiNbO3晶体的热释电响应率优值Mv为一般公认值的二倍。而且当温度升高时,Mv缓慢降低,并不出现某些文献所预示的明显增加。着重研究了LiNbO3探测器在高功率密度连续激光辐照下的输出特性。研究结果表明,在相同条件下,LiNbO3探测器的输出开始偏离线性所对应的最大功率密度远高于LiTaO3。仔细测定了LiNbO3晶体的固有介质损耗,并据此估计LiNbO3器件探测率的极限值。分析表明,就制备背景限探测率的器件的潜力而言,LiNbO3不低于TGS。     

Equivalent doping profile transformation: a new analytical method to predict breakdown voltage of the composite pn junction

This paper describes a new analytical method called the equivalent doping profile of a composite p–n junction, that is, a simplified diffused junction with a linear gradient G _L on the diffused side and a constant concentration N _sub on the substrate. The analytical results for various combinations of the substrate doping concentration and the diffused side gradient levels agree well with the numerical analysis, showing the validity of the method presented here.     

Far Infrared Properties of Electro-Optic Crystals Measured by THz Time-Domain Spectroscopy

In this article we report an experimental investigation of the far-infrared properties of several nonlinear crystals, LiNbO₃, LiTaO₃, ZnTe and CdTe. Using Terahertz Time-Domain Spectroscopy (THz-TDS) we have measured the complex frequency response, i.e. both index of refraction and absorption up to 3 THz (100 cm^?1) for the electro-optic crystals at room temperature. The single Lorentzian oscillator model is used to describe the aquired data. Additional resonance features are observed, especially in the II-VI compounds.     

Neural network based modeling of diffusion process for high-speed avalanche photodiodes fabrication

This paper presents the modeling methodology of Zn diffusion process utilized for high-speed avalanche photodiode fabrication using neural networks. The modeling scheme can characterize the effects of diffusion process conditions on the performance metrics of diffusion process. Three different InP-based test structures with different doping concentrations in diffused layer are explored. Three input factors (sealing pressure, amount of Zn₃P₂ source per volume, and doping concentration of diffused layer) are examined with respect to the two performance metrics (diffusion-rate and Zn doping concentration) by means of D-optimal design experiment. Diffusion rate and Zn doping concentration in diffused layer are characterized by a response model generated by training feed-forward error back-propagation neural networks. It is observed that the neural network based process models developed here exhibit good agreement with experimental results.     

Real-time measurements on a four stage RSFQ-counter with Nb–Al2O3–Nb Josephson junctions

RSFQ-toggle-flipflops with a SFQ-trigger circuit a Josephson transmission line at the input and a SFQ/dc-circuit at the output of each stage are implemented in the Nb–Al₂O₃–Nb Josephson junction technology on a single chip having coplanar wave guides at input and output. The counter is tested successfully at 4.2 K via coplanar/coaxial transitions using a bit pattern generator and a digital oscilloscope at room temperature up to f_I≈2 GHz pulse repetition frequency at the input. The highest test frequency f_I is limited by the available pattern generator.     

Large-scale modelling of the phase transitions in KTa1−xNbxO3 perovskite solid solutions

We use quantum chemical method of the intermediate neglect of the differential overlap (INDO) based on the Hartree–Fock formalism for a large-scale modelling of the atomic and electronic structure of KNb_xTa_1−xO₃ (KTN) perovskite solid solutions. Results for periodic defect models (large unit cell) of 40 and 320 atoms are compared with 135-atom INDO cluster calculations. Periodic Nb impurities in KTaO₃ reveal clear off-center displacement beginning with the smallest calculated concentrations, so does an isolated Nb impurity in a cluster INDO calculation. The magnitude of Nb off-center displacement is close to the X-ray-absorption fine-structure observation (0.27 a.u.). In contrast, Ta impurities in KNbO₃ always remain on-center, due to higher ionicity of Ta as compared to Nb atom. Using the calculated energy gain due to the off-center displacements of Nb atoms for several concentrations of Nb in KTN, we construct the non-empirical Ginzburg–Landau-type functional for the excess energy. The coefficients of this functional are concentration dependent. This dependence allows to define the type of the concentration-induced phase transition in KTaO₃ alloyed by Nb.     

Nb-Mediated Grain Growth and Grain-Boundary Engineering in Mg3Sb2-Based Thermoelectric Materials

The poor carrier mobility of polycrystalline Mg₃Sb₂ at low temperatures strongly degrades the thermoelectric performance. Ionized impurities are initially thought to dominate charge carrier scattering at low temperatures. Accordingly, the increased electrical conductivity by replacing Mg with metals such as Nb is also attributed to reduced ionized impurity scattering. Recent experimental and theoretical studies challenge this view and favor the grain boundary (GB) scattering mechanism. A reduction of GB scattering improves the low-temperature performance of Mg₃(Sb, Bi)₂ alloys. However, it is still elusive how these metal additions reduce the GB resistivity. In this study, Nb-free and Nb-added Mg₃Sb₂ are studied through diffraction, X-ray absorption spectroscopy, solid-state nuclear magnetic resonance spectroscopy, and atom probe tomography. It is shown that Nb does not enter the Mg₃Sb₂ matrix and remains in the metallic state. Besides, Nb diffuses along the GB forming a wetting layer, which modifies the interfacial energy and accelerates grain growth. The GB resistivity appears to be reduced by Nb-enrichment, as evidenced by modeling the electrical transport properties. This study not only confirms the GB scattering in Mg₃Sb₂ but also reveals the hitherto hidden role of metallic additives on enhancing grain growth and reducing the GB resistivity.     

Design of conduction band structure of TiO2 electrode using Nb doping for highly efficient dye‐sensitized solar cells

A barrier layer of undoped TiO₂ was deposited on the Nb‐doped TiO₂ electrode to suppress the recombination at the Nb‐doped TiO₂/dye–electrolyte interface for highly efficient dye‐sensitized solar cells (DSCs). The Nb content in TiO₂ was varied in a range of 0.7–3.5 mol% to modify the TiO₂ energy‐band structure. Nb‐doped TiO₂/dye interfaces were characterized by a combination of ultraviolet photoemission spectroscopy and optical absorption spectroscopy measurements, allowing the determination of the conduction band minimum (CBM) of the TiO₂ electrode and the lowest unoccupied molecular orbital of the N719 dye. The lowering of TiO₂ CBM by Nb doping induced the increase in short‐circuit current of DSCs. However, open‐circuit voltage and fill factor are decreased, and this result was ascribed to the enhanced recombination at the Nb‐doped TiO₂/dye–electrolyte interface. The effect of doping on charge transport in DSCs was analyzed using electrochemical impedance spectroscopy. We have shown that by introducing of TiO₂ barrier layer, the Nb doping content, which results in DSC highest efficiency, can be increased because of the suppression of the dopant‐induced recombination. The energy conversion efficiency of the solar cells increased from 7.8% to 9.0% when undoped TiO₂ electrode is replaced with electrode doped with 2.7 mol% of Nb because of the improvement of the electron injection and collection efficiencies. The correlation between the electronic structure of the TiO₂ electrode, charge transfer characteristics, and photovoltaic parameters of DSCs is discussed. Copyright © 2012 John Wiley & Sons, Ltd.     

Phosphorus diffusion in gallium arsenide

The formation of monocrystalline GaAs_1?xP_x by the solid state diffusion of phosphorus into monocrystalline GaAs at a phosphorus pressure of 35 atm has been confirmed by X-rays and reflectivity measurements. Phosphorus distribution in GaAs specimens diffused in the temperature range 800–1100°C has been determined by reflectance studies. The diffusion coefficient and the activation energy computed from the known phosphorus distributions in the diffused specimens are found to depend on the content of phosphorus in the diffused region. The variation of the depth of GaAs_1?xP_x-GaAs junction with time (at a fixed temperature, 1000°C) and temperature (for a fixed time, 24.5 hr) has been studied.     

基于层次级联模型的声表面波COM参数提取方法

该文研究了基于层次级联模型的耦合模(COM)参数提取方法。通过仿真42°Y-X LiTaO₃衬底的有限长声表面波(SAW)单端谐振器,提取COM参数及建立COM参数数据库,研制了一款1 200 MHz的三换能器混合结构的SAW滤波器。结果表明,实际制作器件响应和仿真幅频响应符合性好,验证了该文提取COM参数方法的可行性,可用于指导SAW滤波器设计。     

Parameters of metal one-electron transistors based on various materials

Limiting parameters (operating temperature and cutoff frequency) and current-voltage characteristics of one-electron transistors based on various metal compounds (Al/AlO_x/Al, Al/SiO₂/Al, Au/Al₂O₃/Au, Nb/Al₂O₃/Nb, Ti/TiO_x/Ti, Cr/Cr₂O₃/Cr, and Nb/NbO_x/Nb) were theoretically studied. Practical recommendations for the choice of materials and structure sizes were formulated. The characteristics were calculated using a SET-NANODEV software package based on the effect of one-electron tunneling and developed for structure simulation according to a technique for estimating the limiting parameters and a two-dimensional numerical model of the metal one-electron transistor.     

Planar optical waveguides prepared in GIL49 and BK7 glass substrates by K+-Na+ ion exchange with the electric field assistance

This report focuses on research into waveguides prepared by K⁺-Na⁺ ion exchange with the help of an electric field, and the subsequent comparison with waveguides prepared by pure thermal ion exchange. The goals of this work were to determine the characteristics of and to address the technological problems associated with waveguides prepared in two types of highly pure optical glass: special soda-lime silica GIL49 glass produced from pure raw materials and commercially prepared borosilicate BK7 glass. An appropriate chemical mixture, KNO₃:Ca(NO₃)₂ in the molar ratio of 41:59, was used as the source of potassium ions. Experiments were conducted at temperatures between 250 and 410°C, and electric field values between 0 and 150 V/mm. The number of modes, depth, profile, and the change in refractive index (Δn) were measured for samples from each type of glass under various technological conditions. All of these parameters can be controlled accurately and repeatedly by the electric field. These experiments have also shown that a particular advantage of these types of pure glass is the low waveguide optical losses (0.1 to 0.2 dB/cm) attainable.     

Nb-doped Ga2O3 as charge-trapping layer for nonvolatile memory applications

The charge-trapping properties of Nb-doped Ga₂O₃ are investigated by using an Al/Al₂O₃/GaNbO/SiO₂/Si structure. Compared with the memory capacitor with pure Ga₂O₃, the one with lightly Nb-doped Ga₂O₃ shows better charge-trapping characteristics, including larger memory window (5.5 V at ± 6 V sweeping voltage), higher programming and erasing speeds due to its higher trapping efficiency resulted from increased trap density induced by the Nb doping. The sample with heavily Nb-doped Ga₂O₃ also shows improvements compared with the one with pure Ga₂O₃, but not as large as those for the lightly Nb-doped sample due to defects generated by excessive Nb doping. Erase saturation phenomenon is observed in all the samples, and can be suppressed by hole traps created by the Nb doping.