Controllable growth of wafer-scale PdS and PdS_(2) nanofilms via chemical vapor deposition combined with an electron beam evaporation technique

Palladium(Pd)-based sulfides have triggered extensive interest due to their unique properties and potential applications in the fields of electronics and optoelectronics.However,the synthesis of large-scale uniform PdS and PdS_(2)nanofilms(NFs)remains an enormous challenge.In this work,2-inch wafer-scale PdS and PdS_(2) NFs with excellent stability can be controllably prepared via chemical vapor deposition combined with electron beam evaporation technique.The thickness of the pre-deposited Pd film and the sulfurization temperature are critical for the precise synthesis of PdS and PdS_(2) NFs.A corresponding growth mechanism has been proposed based on our experimental results and Gibbs free energy calculations.The electrical transport properties of PdS and PdS_(2) NFs were explored by conductive atomic force microscopy.Our findings have achieved the controllable growth of PdS and PdS_(2) NFs,which may provide a pathway to facilitate PdS and PdS_(2) based applications for next-generation high performance optoelectronic devices.

Fabrication, microstructure and properties of electron beam-physical vapor deposited TiAl sheet and TiAl/Nb laminated composites

The TiAl-based alloys sheet with 150 mm×100 mm×0.4 mm and the TiAl/Nb laminated composites with 150 mm×100 mm×0.2 mm were fabricated by using electron beam-physical vapor deposition(EB-PVD) method, respectively. The microstructure and properties of the sheet were investigated by AFM, SEM and EDS. The results show that the TiAl based alloys sheet has a good surface quality, and its microstructure is columnar crystal. The component of the alloys indicates a regular and periodical gradient change which leads to the spontaneous delamination along the normal direction of substrate. In the TiAl/Nb laminated composites alternating overlaid by TiAl of 24 layers and Nb of 23 layers, the interface of each layer evenly distributed throughout the cross-section is transparent, and the interlayer spacing is about 8μm. The component of TiAl layers also changes regularly along the normal direction of substrate, but no delamination phenomenon is found. The TiAl/Nb laminated composites have better ductility than the TiAl-based alloys sheet.

Observation of positive and small electron affinity of Si-doped AlN films grown by metalorganic chemical vapor deposition on n-type 6H–SiC

We have investigated the electron affinity of Si-doped AlN films（N_（Si）= 1.0 × 10^（18）–1.0 × 10_（19）cm^（-3）） with thicknesses of 50, 200, and 400 nm, synthesized by metalorganic chemical vapor deposition（MOCVD） under low pressure on the ntype（001）6H–SiC substrates. The positive and small electron affinity of AlN films was observed through the ultraviolet photoelectron spectroscopy（UPS） analysis, where an increase in electron affinity appears with the thickness of AlN films increasing, i.e., 0.36 eV for the 50-nm-thick one, 0.58 eV for the 200-nm-thick one, and 0.97 e V for the 400-nm-thick one.Accompanying the x-ray photoelectron spectroscopy（XPS） analysis on the surface contaminations, it suggests that the difference of electron affinity between our three samples may result from the discrepancy of surface impurity contaminations.

1.0μm gate-length InP-based InGaAs high electron mobility transistors by mental organic chemical vapor deposition

InGaAs high electron mobility transistors(HEMTs)on InP substrate with very good device performance have been grown by mental organic chemical vapor deposition(MOCVD).Room temperature Hall mobilities of the 2-DEG are measured to be over 8700cm2/V-s with sheet carrier densities larger than 4.6×10 12cm-2.Transistors with 1.0μm gate length exhibits transconductance up to 842mS/mm.Excellent depletion-mode operation,with a threshold voltage of-0.3V and IDSS of 673mA/mm,is realized.The non-alloyed ohmic contact special resistance is as low as 1.66×10-8Ω/cm2,which is so far the lowest ohmic contact special resistance.The unity current gain cut off frequency(fT)and the maximum oscillation frequency(fmax)are 42.7 and 61.3 GHz,respectively.These results are very encouraging toward manufacturing InP-based HEMT by MOCVD.

Monte Carlo Simulation of Electron Velocity Distribution and Gas Phase Process in Electron-Assisted Chemical Vapor Deposition

The gas phase process of diamond film deposition from CH4/H2 gas mixture by electron-assisted chemical vapor deposition is simulated by the Monte-Carlo method. The electron velocity distribution under different E/P (the ratio of the electric field to gas pressure) is obtained, and the velocity profile is asymmetric. The variation of the number density of CH3 and H with different CH4 concentrations and gas pressure is investigated, and the optimal experimental parameters are obtained: the gas pressure is in the range of 2.5 kPa - 15 kPa and the CH4 concentration is in the range of 0.5% - 1%. The energy carried by the fragment CH3 as the function of the experiment parameters is investigated to explain the diamond growth at low temperature. These results will be helpful to the selection of optimum experimental conditions for high quality diamond films deposition in EACVD and the modeling of plasma chemical vapor deposition.

Preparation and mechanical properties of NiCoCrAlY/NiCr laminates by electron beam physical vapor deposition

Laminates with alternating layers of NiCoCrAlY and NiCr were fabricated by using electron beam physical vapor deposition (EB-PVD) method. The influence of the substrate temperature on morphology of the laminates was investigated. The results show that in order to produce NiCoCrAlY/NiCr laminates with lower porosity, higher substrate temperature is required. The mechanical properties of the as-deposited samples and heat-treated samples were examined using tensile tests. The stress-strain curve of the as-deposited laminate shows a typical characteristic of multilayered materials and the fracture behavior is improved by annealing the samples at high temperatures. The tensile strength of the samples annealed at 760℃ is 658.4MPa, and the elongation reaches 6.2%.

Fabrication of InAlGaN/GaN High Electron Mobility Transistors on Sapphire Substrates by Pulsed Metal Organic Chemical Vapor Deposition

Nearly lattice-matched InAIGaN/GaN heterostructure is grown on sapphire substrates by pulsed metal organic chemical vapor deposition and excellent high electron mobility transistors are fabricated on this heterostructure. The electron mobility is 1668.08cm2/V.s together with a high two-dimensional-electron-gas density of 1.43 × 10^13 cm-2 for the InAlCaN/CaN heterostructure of 2Onto InAlCaN quaternary barrier. High electron mobility transistors with gate dimensions of 1 × 50 μm2 and 4μm source-drain distance exhibit the maximum drain current of 763.91 mA/mm, the maximum extrinsic transconductance of 163.13 mS/mm, and current gain and maximum oscillation cutoff frequencies of 11 GHz and 21 GHz, respectively.

Preparation of Ti-Al alloy sheet by electron beam physical vapor deposition

Ti-Al thin sheet with dimension of 450 mm×450 mm×0.2 mm was prepared by electron beam physical vapor deposition(EB-PVD) technology. The surface and cross-section pattern of as-deposited sample were studied by SEM and AFM,and then the composition and phase were analysed by XRD and EPMA. Finally,the effect on deposit by re-evaporation of Al was explored by calculating the ratio of re-evaporating capacity with depositing capacity of Al on the substrate. The results indicate that the evaporation process with Nb addition into the molten pool makes it earlier to reach the steady-state. The existing equiaxed crystal and columnar crystal along the cross-sectional may be caused by the transformation latent heat released during the transition course of atoms from gaseous state to solid state. The effect on deposit by re-evaporation of Al can be neglected because the re-evaporating capacity of Al is far below that of the depositing capacity.

Characterization of atomic-layer MoS_2 synthesized using a hot filament chemical vapor deposition method

Atomic-layer MoS_2 ultrathin films are synthesized using a hot filament chemical vapor deposition method. A combination of atomic force microscopy（AFM）, x-ray diffraction（XRD）, high-resolution transition electron microscopy（HRTEM）, photoluminescence（PL）, and x-ray photoelectron spectroscopy（XPS） characterization methods is applied to investigate the crystal structures, valence states, and compositions of the ultrathin film areas. The nucleation particles show irregular morphology, while for a larger size somewhere, the films are granular and the grains have a triangle shape. The films grow in a preferred orientation（002）. The HRTEM images present the graphene-like structure of stacked layers with low density of stacking fault, and the interlayer distance of plane is measured to be about 0.63 nm. It shows a clear quasihoneycomb-like structure and 6-fold coordination symmetry. Room-temperature PL spectra for the atomic layer MoS_2 under the condition of right and left circular light show that for both cases, the A1 and B1 direct excitonic transitions can be observed. In the meantime, valley polarization resolved PL spectra are obtained. XPS measurements provide high-purity samples aside from some contaminations from the air, and confirm the presence of pure MoS_2. The stoichiometric mole ratio of S/Mo is about 2.0–2.1, suggesting that sulfur is abundant rather than deficient in the atomic layer MoS_2 under our experimental conditions.

Effect of nitrogen on deposition and field emission properties of boron-doped micro-and nano-crystalline diamond films

In this paper,we report the effect of nitrogen on the deposition and properties of boron doped diamond films synthesized by hot filament chemical vapor deposition.The diamond films consisting of micro-grains(nano-grains) were realized with low(high) boron source flow rate during the growth processes.The transition of micro-grains to nano-grains is speculated to be strongly(weekly) related with the boron(nitrogen) flow rate.The grain size and Raman spectral feature vary insignificantly as a function of the nitrogen introduction at a certain boron flow rate.The variation of electron field emission characteristics dependent on nitrogen is different between microcrystalline and nanocrystalline boron doped diamond samples,which are related to the combined phase composition,boron doping level and texture structure.There is an optimum nitrogen proportion to improve the field emission properties of the boron-doped films.

Numerical modeling for electron-beam evaporated process of magnetic alloy Fe-6.5%Si

Based on Langmuir equation and thermodynamic properties of iron-silicon binary alloy, a mathematical model about the process of electron-beam evaporated binary alloy Fe-6.5%Si was established. Variation of the composition of molten pool, vapor and deposit with time, length of transient time and the composition of molten pool, deposit under the steady condition were presented according to the numerical model. The experimental results on the composition of deposit were compared to the data calculated through the model. The results show that the model is applicable, after evaporating for about 50min, the compositions of the deposit are equal to those of the ingot.

Electronic tattoos based on large-area Mo2C grown by chemical vapor deposition for electrophysiology

Tattoo electronics has attracted intensive interest in recent years due to its comfortable wearing and imperceivable sensing,and has been broadly applied in wearable healthcare and human-machine interface.However,the tattoo electrodes are mostly composed of metal films and conductive polymers.Two-dimensional(2D)materials,which are superior in conductivity and stability,are barely studied for electronic tattoos.Herein,we reported a novel electronic tattoo based on large-area Mo_(2)C film grown by chemical vapor deposition(CVD),and applied it to accurately and imperceivably acquire on-body electrophysiological signals and interface with robotics.High-quality Mo_(2)C film was obtained via optimizing the distribution of gas flow during CVD growth.According to the finite element simulation(FES),bottom surface of Cu foil covers more stable gas flow than the top surface,thus leading to more uniform Mo_(2)C film.The resulting Mo_(2)C film was transferred onto tattoo paper,showing a total thickness of~3μm,sheet resistance of 60-150Ω/sq,and skin-electrode impedance of~5×10^(5)Ω.Such thin Mo_(2)C electronic tattoo(MCET in short)can form conformal contact with skin and accurately record electrophysiological signals,including electromyography(EMG),electrocardiogram(ECG),and electrooculogram(EOG).These body signals collected by MCET can not only reflect the health status but also be transformed to control the robotics for human-machine interface.

多元稀土掺杂YSZ热障涂层的热物理和热循环性能研究

4.5wt.%Gd_(2)O_(3)-5.5wt.%Yb_(2)O_(3)-10.5wt.%Y_(2)O_(3)-79.5wt.%ZrO_(2)(GdYbYSZ)稀土复合氧化物陶瓷是一类适用于更高温度下潜在应用的新型热障涂层(TBCs)材料。采用高温固相合成法制备了GdYbYSZ陶瓷粉体和陶瓷块材,在1 100℃和1 300℃煅烧不同时间后GdYbYSZ陶瓷粉末无相变,具有非常优异的高温相稳定性。在1 200℃时,GdYbYSZ陶瓷块材的平均热扩散系数和平均热导率分别比同等温度下YSZ陶瓷块材降低了2.1%和5.1%。采用电子束物理气相沉积(EB-PVD)工艺在单晶合金(Ni,Pt)Al粘结层表面制备了GdYbYSZ新型热障涂层。沉积态GdYbYSZ陶瓷涂层的主相结构为立方相,有少量游离态Y_(2)O_(3)和ZrO_(2)共存,其Y和Zr元素的相对含量均比靶材中的高,而Gd和Yb元素含量相当。经1 100℃长期冷热交替循环后,GdYbYSZ陶瓷层表面出现大量规则分布的“泥巴状”微观裂纹,陶瓷层内滋长的横向裂纹已经扩展到陶瓷层与TGO层的界面处,并引起该界面退化分离。陶瓷涂层的剥落位置主要出现在TGO层上下两个临域的界面处。TGO层严重的褶皱、波动起伏、扭曲交联、凸面尖端应力积聚和快速松弛是引起GdYbYSZ/(Ni,Pt)Al热障涂层体系层间界面分离和剥落失效的关键性因素。

Thickness and component distributions of yttrium-titanium alloy films in electron-beam physical vapor deposition

Thickness and component distributions of large-area thin films are an issue of in-ternational concern in the field of material processing. The present work employs experiments and direct simulation Monte Carlo (DSMC) method to investigate three-dimensional low-density, non-equilibrium jets of yttrium and titanium vapor atoms in an electron-beams physical vapor deposition (EBPVD) system furnished with two or three electron-beams, and obtains their deposition thickness and component distributions onto 4-inch and 6-inch mono-crystal silicon wafers. The DSMC results are found in excellent agreement with our measurements, such as evaporation rates of yttrium and titanium measured in-situ by quartz crystal reso-nators, deposited film thickness distribution measured by Rutherford backscat-tering spectrometer (RBS) and surface profilometer and deposited film molar ratio distribution measured by RBS and inductively coupled plasma atomic emission spectrometer (ICP-AES). This can be taken as an indication that a combination of DSMC method with elaborate measurements may be satisfactory for predicting and designing accurately the transport process of EBPVD at the atomic level.

DC Characteristics of Lattice-Matched InAlN/AlN/GaN High Electron Mobility Transistors

Lattice-matched InAlN/AlN/GaN high electron mobility transistors (HEMTs) grown on sapphire substrate by using low-pressure metallorganic chemical vapor deposition were prepared, and the comprehensive DC characteristics were implemented by Keithley 4200 Semiconductor Characterization System. The experimental results indicated that a maximum drain current over 400 mA/mm and a peak external transconductance of 215 mS/mm can be achieved in the initial HEMTs. However, after the devices endured a 10-h thermal aging in furnace under nitrogen condition at 300 ℃, the maximum reduction of saturation drain current and external transconductance at high gate-source voltage and drain-source voltage were 30% and 35%, respectively. Additionally, an increased drain-source leakage current was observed at three-terminal off-state. It was inferred that the degradation was mainly related to electron-trapping defects in the InAlN barrier layer.

Electron Emission of Graphene-Diamond Hybrid Films Using Paraffin Wax as Diamond Seeding Source

We present a scalable, reproducible and economic process for the fabrication of diamond and diamond-graphene hybrid films using paraffin wax as a seeding source for diamond. The films were characterized using Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron energy loss spectroscopy (EELS). Raman spectra show the characteristic band of diamond at 1332 cm-1 and the D, G, and 2D bands of graphene at 1360, 1582 and 2709 cm-1, respectively. Electron microscopy confirms the microcrystalline nature of the diamond films with crystal size in the range of 0.5 μm to 1.0 μm, and the hybrid film consists of microcrystalline diamond attached to thin, semi-transparent graphene flakes. The graphene-diamond hybrid films exhibit a turn-on field of about 3.6 V/μm with a prolonged current stability of at least 135 h.

基于复合势垒的AlGaN/GaN异质结材料的制备与性能研究

针对高线性氮化镓微波功率器件研制需求,设计并外延生长了复合势垒的Al_(0.26)Ga_(0.74)N/GaN/Al_(0.20)Ga_(0.80)N/GaN异质结构材料,通过理论计算和电容-电压(C-V)测试表明复合势垒材料存在两层二维电子气沟道。生长的复合势垒材料二维电子气迁移率达到1510 cm^(2)·V^(-1)·s^(-1),面密度达到9.7×10^(12)cm^(-2)。得益于双沟道效应,基于复合势垒材料研制的器件跨导存在两个峰,使得跨导明显展宽,达到3.0 V,是常规材料的1.5倍。复合势垒结构器件的跨导一阶导数与二阶导数具有更加优异的特性,表明其具有更高的谐波抑制能力,显示复合势垒AlGaN/GaN异质结构在高线性应用上的优势。

集成式石英微机械陀螺质量块制作工艺研究

为了满足集成式陀螺芯片对高精度质量块的要求,该文采用电子束镀膜(EBPVD)工艺代替传统的电镀工艺,在石英微机械陀螺芯片音叉端部制作了厚约2μm的质量块,并对质量块厚度均匀性、对准精度一致性及膜层附着力进行了研究。首先,通过调整镀膜高度和角度以满足厚度精度要求;其次,为了保证对准精度,设计了掩膜夹具和位置调整夹具;最后,通过实验确定了最佳镀膜参数。此外,对制作的质量块进行了激光修调实验,经过修调后,陀螺机械耦合误差信号的幅值从初始的301.0 mV降至17.6 mV。

Chemical vapor deposition growth and transport properties of MoS_(2)-2H thin layers using molybdenum and sulfur as precursors

This paper introduces a feasible process to achieve the molybdenum disulfide atomic layers using chemical vapor deposition(CVD) method,with molybdenum thin film and solid sulfur as precursors.And some improvements were made to reduce the amount of metastable MoS_(2)-3 R.The morphology of the acquired MoS_(2) layers,existing as triangular flakes or large-area continuous films,can be controlled by adjusting the synthesis time and reacting temperature.The characterization results show that the monolayer MoS_(2) flakes reveal a(002)-oriented growth on SiO_(2)/Si substrates,and its crystalline domain size is approximately 30 μm,and the thickness is 0.65 nm.Since the synthesis of MoS_(2)-3 R is restrained,the electronic transport properties of MoS_(2) with different layers were investigated,revealing that those properties equal with those of MoS_(2) samples prepared by exfoliation methods.

Nonlinear electronic and ultrafast optical signatures in chemical vapor-deposited ultrathin PtS_(2) ribbons

The emerging two-dimensional(2D)platinum disulfide(PtS_(2))has driven increasing attentions due to its high electron mobility,good air-stability,and strong interlayer interaction which leads to a widely tunable electronic structure.However,a detailed study on its covalent-like layer-dependent properties remains infant.Herein,we demonstrate the successful production of ultrathin 1T-PtS_(2) ribbons with thickness centralized almost at monolayer 1L-4L and large domain size up to 210 μm on Au foils using chemical vapor deposition(CVD)technique,which enables macro-and microscopic study of its extraordinary layer-dependent features with precise control of the number of layers.Using electron energy loss spectroscopy(EELS)and optical pump-probe spectroscopy(OPPS),we reveal that both the electron and ultrafast optical absorption signals of the as-grown 2D PtS_(2) show strong nonlinear layer-dependent responses which manifest discriminated transition in 1L-4L PtS_(2) ribbons.The layer-dependent nonlinear response of 2D PtS_(2) can be well interpreted in the frame of calculated electron and phonon structures.These achievements offer a platform for successfully fabricating large-sized ultrathin 2D PtS_(2) and facilitating our knowledge about its electronic and optoelectronic properties.