The fabrication of nickel silicide ohmic contacts to n-type 6H-silicon carbide

This paper reports that the nickel silicide ohmic contacts to n-type 6H-SiC have been fabricated. Transfer length method test patterns with NiSi/SiC and NiSi硅化镍;欧姆触点;n型碳化硅;制造;能带;带隙Project supported by the National Basic Research Program of China (Grant No~2002CB311904), the National Defense Basic Research Program of China (Grant No~51327010101) and the National Natural Science Foundation of China (Grant No~60376001).2006-09-192006-10-30This paper reports that the nickel silicide ohmic contacts to n-type 6H-SiC have been fabricated. Transfer length method test patterns with NiSi/SiC and NiSi2/SiC structure axe formed on N-wells created by N^＋ ion implantation into Si-faced p-type 6H-SiC epilayer respectively. NiSi and NiSi2 films are prepared by annealing the Ni and Si films separately deposited. A two-step annealing technology is performed for decreasing of oxidation problems occurred during high temperature processes. The specific contact resistance Pc of NiSi contact to n-type 6H-SiC as low as 1.78× 10^-6Ωcm^2 is achieved after a two-step annealing at 350 ℃for 20 min and 950℃ for 3 min in N2. And 3.84×10-6Ωcm^2 for NiSi2 contact is achieved. The result for sheet resistance Rsh of the N＋ implanted layers is about 1210Ω/□. X-ray diffraction analysis shows the formation of nickel silicide phases at the metal/n-SiC interface after thermal annealing. The surfaces of the nickel silicide after thermal annealing are analysed by scanning electron microscope.     

Creating room temperature Ohmic contacts to 4H-SiC: studied by specific contact resistance measurements and X-ray photoelectron spectroscopy

The use of a silicon interface pre-treatment to produce low resistance Ohmic nickel contacts to 4H-SiC, circumventing the need for contact post annealing, is reported. The effects of two different SiC pre-metal deposition surface preparation techniques: RCA cleaning (control sample) and a silicon interlayer pre-treatment (SIP), are discussed. Electrical characterization of contacts on treated surfaces, using circular transfer length measurements (CTLM), revealed that contacts to RCA cleaned samples were Schottky in nature, unless annealed at temperatures greater than 700 °C. In contrast, contacts formed on SIP SiC surfaces exhibited Ohmic behaviour directly after fabrication, without the need for post metallisation annealing. Average contact resistances as low as 1.3E−05 Ω cm² have been recorded for SIP samples. This fabrication process has distinct technological advantages compared to standard techniques for forming Ohmic contacts to SiC. To consolidate our findings the chemical and electrical nature of the SIP nickel-SiC interface, as it was sequentially formed and annealed, was examined using X-ray photoelectron spectroscopy (XPS). Based on these results, a model is proposed to explain the as-deposited Ohmic contact nature of the SIP sample.     

Position-selective growth of vertically aligned carbon nanotubes for application of electronic-measuring nanoprobes

Position-selective growth of carbon nanotubes (CNTs) and vertically aligned CNTs (VACNTs) on patterned metal electrodes have been prepared by thermal chemical vapor deposition (TCVD) and DC plasma enhanced chemical vapor deposition (PECVD). We propose newly a position-controlling method of CNTs by controlling not only a position of Ni as catalysts but also the morphology of Mo as underlayers for the catalysts. The position-selective growth of CNTs was achieved at the edges of the patterned metal by TCVD. The morphologies of the Mo underlayer at the selected area were rough and porous. No CNTs grew on smooth Mo surfaces. The minimum width of selectively grown CNTs, ca. 2.6 μm, was approximately one-eightieth of the patterned metal, 200 μm. VACNTs were synthesized by a PECVD method, however, the VACNTs grew up all over the patterned metal. The Ni catalysts formed into fine particles on rough surfaces of the Mo underlayer. Then the selective growth was achieved by Ni fine particles formed only at the edges of the metal pattern. The results of PECVD suggest that the plasma promoted the Ni catalysts to become fine particles on smooth surfaces of Mo. Conclusively a position-controlling method of CNTs was demonstrated in the optimum conditions of the TCVD.     

Deposition of Ni<Subscript> 13</Subscript> and Cu<Subscript> 13</Subscript> clusters on Ni(111) and Cu(111) surfaces

The soft deposition of Ni₁₃ and Cu₁₃ clusters on Ni(111) and Cu(111) surfaces is studied by means of constant-energy molecular-dynamics simulations. The atomic interactions are described by the Embedded Atom Method. It is shown that the shape of the nickel clusters deposited on Cu(111) surfaces remains rather intact, while the copper clusters impacting on Ni(111) surfaces collapse forming double and triple layered products. Furthermore, it is found that for an impact energy of 0.5 eV/atom the structures of all investigated clusters show the lowest similarity to the original structures, except for the case of nickel clusters deposited on a Cu(111) surface. Finally, it is demonstrated that when cluster and substrate are of different materials, it is possible to control whether the deposition results in largely intact clusters on the substrate or in a spreading of the clusters. This separation into hard and soft clusters can be related to the relative cohesive energy of the crystalline materials.     

The characterization of nanostructured CVD Ni powders using transmission electron microscopy

Studies of Ni powders using a state-of-the-art transmission electron microscope with a field-emission electron source show that this instrument can be an invaluable tool for studying the morphology and character of nanostructured Ni powder manufactured by a commercial nickel carbonyl vapor deposition process. This study has revealed the presence of surface coatings, including NiO and turbostratic graphite on some powders, which can degrade the electrical conductivity of the powder. Due to the high curvature of hexagonal carbon (graphene) planes, and subsequent high density of defects, the turbostratic graphitic nanocarbon may exhibit new properties.     

Growth and characterization of β-SiC films obtained by fs laser ablation

We achieved the growth of cubic silicon carbide (SiC) films on (1 0 0)Si substrates by pulsed laser deposition (PLD) at moderate temperatures such as 750 °C, from a SiC target in vacuum. The as-deposited films are morphologically and structurally characterized by scanning electron microscopy (SEM), conventional and high-resolution transmission electron microscopy (TEM/HRTEM). The morphology of deposited films is dominated by columns nucleated from a thin nanostructured beta silicon carbide (β-SiC) interface layer. The combined effects of columnar growth, tilted facets of the emerging columns and the presence of particulates on the film surface, lead to a rather rough surface of the films.     

Characterization of thin Zn-Ni alloy coatings electrodeposited on low carbon steel

The characteristics of initial layer formation in alkaline bath for Zn-Ni (12-15%) alloy electrodeposition on low carbon steel plates are detected in a nanometric thickness range by electron probe microanalysis (EPMA), with both bulk sample and thin film on substrate correction procedure, glow discharge optical emission spectroscopy (GDOES) and gracing incidence X-ray diffraction (GIXRD). The Zn-Ni coatings were elaborated using either intensiostatic or potentiostatic mode. A preferential deposition of Ni, in the initial thin layer, is detected by these analyses; according to EPMA and GDOES measurements, a layer rich in nickel at the interface substrate/deposit is observed (90 wt.% Ni) and approved by GIXRD; the thin layer of Ni formed in the first moments of electrolysis greatly inhibits the Zn deposition. The initial layer depends upon the relative ease of hydrogen and metal discharge and on the different substrate surfaces involved. The electrodeposition of zinc-nickel alloys in the first stage is a normal phenomenon of codeposition, whereby nickel - the more noble metal - is deposited preferentially.     

Structural and electrochemical characterization of Ni nanostructure films on steels with brush plating and sputter deposition

Layers of nickel were coated on low carbon steel substrates applying both brush plating and DC magnetron sputtering techniques. X-ray diffraction analysis showed a preferential orientation along (1 1 1) for both sputter deposited and brush plated Ni nanostructure coatings. The sputtered Ni film showed better crystallinity as observed from XRD compared to brush plated Ni film on steel on account of the favorable conditions for grain growth in sputtering. SEM analysis indicated that the coatings are very regular without pores, with columnar structure for the sputter deposited Ni coatings. AFM was also applied for surface topography examination. Microhardness value was found to be higher for sputtered Ni film. Corrosion performance of these nanostructured Ni coatings were evaluated using electrochemical techniques and observed that the corrosion resistance of brush plated Ni film sample was significantly higher than that of the sputtered Ni film.     

自催化沉积非晶态复合材料的组成与晶化特性研究

用自催化沉积非晶态(Ni,Cu)_100-xP_x+SiC复合材料,研究了复合材料的组成与晶化特性,结果表明,当x>14或者SiC体积分数小于12.8%时,复合材料即保持非晶态;时效温度达到603—618K,复合材料逐渐向晶态过渡,晶化相为fc c Ni和Ni_yP_z,稳定组织为(Ni,Cu)P+Ni₃P+SiC.较非晶态复合材料,晶态材料具有更高的力学性能. 关键词：     

纳米NiAl-Ni和Ni3Al-Ni合金的低温电阻特性

为了深入理解纳米Al-Ni合金低温下的电子输运过程，使用自主研发的电磁感应加热-自悬浮定向流法制备出Al，Ni和Al-Ni纳米合金粉末，并采用真空热压设备将纳米粉末压制成纳米晶块体，利用自主搭建的低温热电测量系统研究了Al-Ni纳米合金的电阻率随温度(8~300 K)的变化规律。研究结果表明:Al-Ni纳米合金由于形成有序晶相而仍然与Al，Ni纳米晶一样，电阻率随温度的降低而降低。纳米Ni₃Al-Ni和NiAl-Ni在居里温度点附近出现了电阻率随温度变化的极大值点，因为单质Ni的影响，Ni₃Al-Ni的居里温度比粗晶Ni₃Al提高了20 K。由于磁子-电子散射作用和声子-电子散射作用，纳米Ni₃Al-Ni，NiAl-Ni和Ni的电阻率在低温下(8~40 K)与温度呈T2和T4关系。     

Highly reproducible ideal SiC Schottky rectifiers: effects of surface preparation and thermal annealing on the Ni/6H-SiC barrier height

In this work, the effects of surface preparation and thermal annealing on the Ni/6H-SiC Schottky barrier height were studied by monitoring the forward I–V characteristics of Schottky diodes. The ideality factor n and the barrier height _B were found to be strongly dependent on the impurity species present at the metal/SiC interface. The physical mechanism which rules the Schottky barrier formation is discussed by considering the nature of the impurities left from the different surface preparation methods prior to metal deposition. In contrast, nickel silicide/SiC rectifiers (Ni₂Si/6H-SiC), formed by thermal reaction of Ni/6H-SiC above 600 °C, have an almost ideal I–V curve, independent of the surface preparation. Further improvement in the barrier height distribution can be obtained by increasing the annealing temperature to 950 °C. This behaviour is discussed in terms of the silicide phases and the consumption of a SiC layer during the thermal reaction. PACS 73.30.+y; 81.65.Cf; 81.05.Je     

Electronic Structure of Nanostructured Nickel Oxide Using Ni 2p XPS Analysis

We examine the electronic structure of nanostructured nickel oxide with an average particle size of 4–5nm using Ni 2p X-ray photoelectron spectrum. The most striking features of the spectrum are the Ni 2p main line broadening and an increase in the relative intensity of the 1.5eV satellite. We explain the observations as due to an enhancement in the non-local screening process on the basis of Ni₇O₃₆ cluster model by taking into account the large surface area to volume ratio and high Ni²⁺ vacancy concentration in nanostructured nickel oxide. The important contribution of an enhanced Ni 3d–O 2p hybridization and the origin of shake-up peaks above the 7eV satellite is also discussed. The study underlines the importance of factors such as the actual local environment of the core hole site, defect density and distribution, possible structural transitions, etc. in determining the Ni 2p core level X-ray photoelectron spectrum of nanostructured nickel oxide and shows that the actual synthesis routes and the thermal history greatly influence the electronic structure of nanostructured nickel oxide.     

Electronic properties of SiC surfaces and interfaces: some fundamental and technological aspects

The wide band gap semiconductor silicon carbide (SiC) is the first-choice material for power electronic devices operating at high voltages, high temperatures, and high switching frequencies. Due to their importance for crystal growth, processing, and device fabrication, the electronic properties of SiC surfaces and interfaces to other materials such as metals and dielectrics are of particular interest. Unreconstructed, H-terminated SiC surfaces which are passivated in a chemical as well as an electronic sense are obtained in a thermal hydrogenation process. It is demonstrated that deposition of Al₂O₃ on H-terminated SiC(0001) leads to an interface which is lower in defects than the thermally grown SiO₂/SiC interface. Furthermore, starting from hydrogenated SiC{0001} surfaces it is possible to prepare unreconstructed (1×1) surfaces with one dangling bond per unit cell. These surfaces show indications for strong electron correlation effects. PACS 68.47.Fg; 73.20.At; 79.60.Bm; 68.35.Bs; 68.35.Dv     

SiC在高温高压下溶于金属触媒中形成金刚石的原子过程

从原子间相互作用及运动出发，阐明ＳｉＣ在高温高压下溶于金属触媒中形成金刚石的原子过程，并在此基础上进而阐明Ｎｉ７０Ｍｎ２５Ｃｏ５是促使ＳｉＣ形成金刚石的高效率触媒合金。日本文部省的资助     

Friction and wear properties of the co-deposited Ni-SiC nanocomposite coating

Ni-SiC nanocomposite coatings were produced by electrodeposition from a nickel sulfate bath containing SiC nanoparticles with an average particle size of 30 nm. The characteristics of the coatings were assessed by scanning electron microscopy and microhardness test. The friction and wear performance of Ni-SiC nanocomposite coatings and Ni film were comparatively investigated sliding against Si₃N₄ ceramic balls under non-lubricated conditions. The results indicated that compared to Ni film, Ni-SiC nanocomposite coating exhibited enhanced microhardness and wear resistance. The effect of SiC nanoparticles on the friction and wear resistance is discussed in detail.     

Annealed Ni/Ti/SiC structure analysed by SIMS and GDMS

Depth profile analysis was performed using two mass spectrometry analytical techniques applying argon ion sputtering—secondary ion mass spectrometry (SIMS) and glow discharge mass spectrometry (GDMS). 150 nm Ni/20 nm Ti/4H-SiC structure was prepared by evaporation coating with titanium and nickel. The structure was analysed as prepared and annealed in 600°C in dry N₂. Obtained results allow monitoring several processes present during annealing of Ni/Ti/SiC structures. These are nickel diffusion and its reaction with SiC leading to formation of Ni₂Si, precipitation of carbon and segregation of titanium. Results are also used for comparison of the two analytical techniques used. The techniques base on different ionisation mechanisms. In SIMS, secondary ions are formed at the bombarded surface during ion sputtering process in ultra-high vacuum environment, while in GDMS, ionisation occurs in glow discharge above the eroded surface in low vacuum.     

Control of the shell structure of ZnO–ZnS core-shell structure

Nanostructured ZnO–ZnS core-shell powders were synthesized through a solution method using a thioacetamide (TAA) solution in deionized water. ZnO powder and TAA solution were employed to supply zinc and sulfur ions to form the ZnO–ZnS core-shell structures. The structure of the ZnS shell was strongly affected by the mole concentration of the TAA, and the structural properties were characterized by X-ray diffraction and high-resolution transmission electron microscopy. The optical properties of the nanostructured powders were also compared with those of pure ZnO powder. The ultraviolet (UV) emission was greatly enhanced compared to when pure ZnO powder was used in the nanostructured powder synthesized using the 0.5 M TAA solution, while the UV emission of that with the 0.05 M TAA solution was reduced. The green emission of the nanostructured powders was reduced compared to when the pure ZnO powder was used. The mechanism of the structural changes in the core-shell structures is proposed here and its effect on the luminescent properties is discussed.     

Effect of pulse electrodeposition parameters on the properties of Ni/nano-SiC composites

Pure nickel and nickel matrix composite deposits containing nano-SiC particles were produced under both direct and pulse current conditions from an additive-free nickel Watts’ type bath. It has been proved that composite electrodeposits prepared under pulse plating conditions exhibited higher incorporation percentages than those obtained under direct plating conditions, especially at low duty cycles. The study of the textural perfection of the deposits revealed that the presence of nano-particles led to the worsening of the quality of the observed [1 0 0] preferred orientation. Composites with high concentration of embedded particles exhibited a mixed crystal orientation through [1 0 0] and [2 1 1] axes. The embedding SiC nano-particles in the metallic matrix by an intra-crystalline mechanism resulted in the production of composite deposits with smaller crystallite sizes and more structural defects than those of pure Ni deposits. A dispersion-hardening effect was revealed for composite coatings independently from applied current conditions. Pulse electrodeposition significantly improved the hardness of the Ni/SiC composite deposits, mainly at low duty cycle and frequency of imposed current pulses.     

Synthesis of carbon nanotubes directly over TEM grids aiming the study of nucleation and growth mechanisms

A novel approach to produce electron-transparent multi-layer membranes over TEM grids for transmission electron microscopy analysis is presented. The membranes have been used to grow and analyze carbon nanostructured materials in a thermal-chemical vapor deposition process using Ni and Cu as catalysts and silicon oxide thin films as support layers, at temperatures as high as 900 °C. It is important that conditions of the synthesis using the electron-transparent multi-layer membrane system are similar to those for a conventional chemical vapor deposition process, where oxidized silicon wafers are employed. In particular, the thickness of the silicon oxide and the catalyst layers are the same, and similar carbon tubular structures were grown using both substrates. The use of membranes was crucial especially for the study of the nucleation mechanism for carbon nanotubes. These electron-transparent multi-layer membranes are relatively easy to obtain and they can be used for transmission electron microscopy studies of high-temperature synthesis of different nanostructured materials.     

Modifying effects of polyethylene glycols and sodium dodecyl sulfate on synthesis of Ni nanocrystals in 1,2-propanediol

Morphology-controlled synthesis of nickel (Ni) nanocrystals has been carried out from nickel acetate tetrahydrate with 1,2-propanediol as both solvent and reductant in the presence of modifiers. The as-prepared nanostructured Ni samples have been characterized by powder X-ray diffraction (XRD), transmission electron micrographs (TEM), selected area electron diffraction (SAED) and Fourier transform infrared (FTIR). The presence of modifiers plays an important role in morphology-controlled synthesis of Ni nanocrystals. The modifying and stabilizing effects of single modifiers such as polyethylene glycols (PEGs) and sodium dodecyl sulfate (SDS), and their composites have been investigated.