Photolithographic fabrication of gated self-aligned parallel electron beam emitters with a single-stranded carbon nanotube

In this paper we report on the development of a photolithographic process to fabricate a gated-emitter array with single-stranded carbon nanotubes (CNTs) self-aligned to the center of the emitter gate using plasma-enhanced chemical vapor deposition (PECVD). Si tips are formed on a silicon wafer by anisotropic etching of Si using SiO(2) as a mask. Deposition of a SiO(2) insulating layer and Cr-W electrode layers creates protrusions above the Si tips. This wafer is polished, and the Cr-W on the tips is removed. Etching of the SiO(2) using hydrofluoric acid is performed to expose the gated Si tip. Incorporation of a novel diffusion process produces single-stranded CNTs by depositing a thin Ni layer on the Si tips and thermally diffusing the Ni layer to yield a catalyst particle for single-stranded CNT growth. The large surface to volume ratio at the apex of the Si tip allows a Ni particle to remain to act as a catalyst to grow a single-stranded CNT for fabricating the CNT based emitter structure. Diffusion of the Ni is carried out in situ during the heating phase of the PECVD CNT growth process at 600?°C. The diameters of the observed CNTs are on the order of 20?nm. The field emission characteristics of the gated field emitters are evaluated. The measured turn-on voltage of the gated emitter is 5?V.     

Synthesis and characterization of amorphous silicon oxide nanowires embedded with Ni nanoparticles

The growth of silicon oxide nanowires (SiOxNWs) was obtained by thermal process of nickel (Ni) nanoparticles (NPs) deposited on silicon (Si) wafer in mixed gases of nitrogen (N₂) and hydrogen (H₂). TEM analysis showed that SiOxNWs had diameters ranging from 100 to 200 nm with lengths extending up to a few μm and their structure was amorphous. SiOxNWs were grown by the reaction between Ni NPs and Si wafer and Ni NPs acted as catalysts. Ni silicides (NixSi) were also formed inside the wafer by Ni diffusion into Si wafer.     

石英微晶天平技术在原子层沉积碳化镍薄膜过程中的应用研究

采用等离子体增强原子层沉积技术分别以Ni (amd)₂、Ni (dad)₂为镍、碳前驱体,氢等离子体为还原剂成功沉积了碳化镍薄膜。两个沉积过程中,碳化镍薄膜厚度都随着反应循环次数的增加而线性增加。利用石英微晶天平技术对碳化镍薄膜的沉积过程进行了原位在线测量。初步提出碳化镍薄膜的沉积机理,其中由等离子体作用产生的氢原子对沉积过程有重要作用。一方面,在等离子体中原子氢与Ni (amd)₂或Ni (dad)₂发生化学反应,生成Ni、挥发性的N-叔丁基乙酰胺盐和碳氢化合物,这些镍在镍表面上有吸附和分解的趋势。此外,原子氢对吸附的碳氢化合物脱氢生成碳化镍和大量碳物种具有促进作用。另一方面,预计它还会刻蚀碳化镍表面的无定形碳和石墨,或者将碳化镍分解成具有催化活性的金属镍。     

Carbon nanotubes synthesis in microwave plasma torch at atmospheric pressure

Well aligned multi-walled carbon nanotubes were synthesized at atmospheric pressure using a microwave plasma torch on silicon substrates with silicon oxide buffer layer and catalyst overlayer in the mixture of argon, hydrogen and methane. Iron or nickel was used as catalysts. The optimum substrate temperature for the deposition on Si/SiO₂/Fe substrates was about 970 K. In this case SEM micrographs of the deposits revealed a presence of vertically aligned nanotubes with the diameters around 15 nm. TEM micrographs showed a presence of amorphous carbon particles in the samples and some defects in the wall structure of the produced nanotubes. In Raman spectra two peaks at 1332 and 1584 cm⁻¹ were observed. The CNTs were also synthesized on the substrates without SiO₂ buffer layer but their quality was lower. The synthesis with Ni instead of Fe catalyst required lower temperature and the alignment of the nanotubes was worse. The deposition process was monitored by optical emission spectroscopy. Atomic lines of hydrogen and argon, an emission of CN due to a presence of nitrogen impurities from atmosphere, a weak molecular band of CH and strong C₂ emission were detected in the spectra.     

One-step Ge/Si epitaxial growth

Fabricating a low-cost virtual germanium (Ge) template by epitaxial growth of Ge films on silicon wafer with a Ge(x)Si(1-x) (0 < x < 1) graded buffer layer was demonstrated through a facile chemical vapor deposition method in one step by decomposing a hazardousless GeO(2) powder under hydrogen atmosphere without ultra-high vacuum condition and then depositing in a low-temperature region. X-ray diffraction analysis shows that the Ge film with an epitaxial relationship is along the in-plane direction of Si. The successful growth of epitaxial Ge films on Si substrate demonstrates the feasibility of integrating various functional devices on the Ge/Si substrates.     

Silicide formation with nickel and platinum double layers on silicon

Diffusion effects and silicide formation in double layers of electron-gun-evaporated thin films of nickel and platinum on 〈100〉 and 〈111〉 silicon substrates were studied by megaelectronvolt backscattering spectrometry, transmission electron microscopy and glancing angle X-ray diffraction as a function of heat treatment (200–900 °C) for both sequences of thin films. It was found for the Si/Ni/Pt(Si/Pt/Ni) system that Ni₂Si(Pt₂Si) starts growing first. When all the nickel (platinum) has been consumed by this compound growth, platinum (nickel) diffuses through the Ni₂Si(Pt₂Si) layer and accumulates at the SiNi₂Si(SiPt₂Si) interface. This platinum (nickel) diffusion seems to be a grain boundary diffusion.For 〈100〉 Si/Ni/Pt samples with thin platinum layers it has been shown that platinum acts as a marker for the moving species in the transition from Ni₂Si to NiSi. For thick platinum layers it was observed that similar processes occur, leading to essentially a four-layered silicide where the layers are alternately rich in nickel and rich in platinum (450 °C, 20 min). In the silicide for the 〈100〉 Si/Pt/Ni system the distribution of nickel and platinum is approximately the reverse of the asdeposited distribution (about 450 °C, 20 min). In the further evolution of the profiles the elemental distribution becomes smooth and flat for both sequences of the layers (750 °C, 20 min). We suggest the existence of a ternary of the type SiNi_1?xPt_x.     

Atomic layer deposition of SiO2 thin films using tetrakis(ethylamino)silane and ozone

We examined the atomic layer deposition (ALD) of silicon dioxide thin films on a silicon wafer by alternating exposures to tetrakis(ethylamino)silane [Si(NHC2H5)4] and O3. The growth kinetics of silicon oxide films was examined at substrate temperatures ranging from 325 to 514 degrees C. The deposition was governed by a self-limiting surface reaction, and the growth rate at 478 degrees C was saturated at 0.17 nm/cycle for Si(NHC2H5)4 exposures of 2 x 10(6) L (1 L = 10(-6) Torr x s). The films deposited at 365-404 degrees C exhibited a higher deposition rate of 0.20-0.21 nm/cycle. However, they contained impurities, such as carbon and nitrogen, and showed poor film qualities. The concentration of impurities decreased with increasing substrate temperature. It was found that the films deposited in the high-temperature regime (478-514 degrees C) showed excellent physical and electrical properties equivalent to those of LPCVD films.     

Time and temperature dependence of multi-walled carbon nanotube growth on Inconel 600

The growth kinetics of vertically aligned multi-walled carbon nanotubes (MWNTs) on conductive substrates is investigated by synthesizing MWNTs at different growth temperatures and measuring ex situ the length of the nanotubes as a function of growth duration. A typical 'root growth' mechanism (saturation of MWNT lengths with increasing growth duration) was observed. The value of the activation energy (E(a)≈136 ± 5?kJ?mol(-1)) of the MWNT growth reaction is suggestive of carbon diffusion through the bulk of the iron catalyst particle versus diffusion on its surface. These findings will help in optimizing MWNT growth on conductive substrates for various applications.     

Root growth of multi-wall carbon nanotubes by MPCVD

This work examines the relationships among the growth and interlayer reactions of carbon nanotubes (CNTs) to develop an effective process for controlling the nanostructure, orientation and characteristics of CNTs. Vertically oriented CNTs were successfully synthesized by microwave plasma chemical vapor deposition (MPCVD) with CH₄/H₂ as source gases. Additionally, the Ti and SiO₂ barrier layers and the Co catalyst were used in an experiment on the growth of CNTs on the Si wafer. Then, the SiO₂ barrier layer was deposited by low-pressure chemical vapor deposition (LPCVD). The Ti barrier layer and Co catalyst films were deposited on the Si wafer by physical vapor deposition (PVD). The deposited nanostructures were characterized by scanning and transmission electron microscopy, the results of which reveal that the deposited MWCNTs were grown under the influence of a catalyst on Si substrates with or without a barrier layer, by MPCVD. Vertically grown, dense MWCNTs attached to a catalytic film demonstrate that various MWCNTs penetrated the root particles. The diameter of the root particles, of approximately in the order of 100 nm, is larger than those of the tube, 10–15 nm. The well-known model of the growth of CNTs includes base- and tip-root growth. The interaction between the catalytic film and the supporting barrier layer is suggested to determine whether the catalytic particles are driven up or pinned down on the substrate during the growth.     

Multi-walled carbon nanotube growth on oxidized silicon substrates using cyclohexane precursor by chemical vapor deposition

Randomly oriented multi-walled nanotubes (MWNTs) are grown by a thermal chemical vapor deposition (CVD) process from cyclohexane precursor on a 20% copper-80% nickel (Cu-Ni) catalyst on oxidized silicon substrates. This combination of precursor and catalyst, to our knowledge, has been employed for the first time to demonstrate growth of multi-walled carbon nanotubes. The effects of annealing, gas ambient and catalyst layer thickness on the morphology of the grown carbon layers are discussed. The low resistivity values of the MWNTs grown on oxidized silicon substrates are attractive for their potential use in photonic devices and display applications.     

预置镍化学气相沉积法制备定向碳纳米管(英文)

采用化学气相沉积法(CVD),在溅射了镍薄膜的硅基底上制备了定向碳纳米管薄膜。对镍薄膜的氨气预处理过程及其机理进行了研究。结果发现预处理后的岛状区域随着薄膜厚度的增加而增加,纳米粒子区域的变化则与之相反。对5nm的镍薄膜进行预处理能获得细化和均匀分布的纳米粒子,有利于定向碳纳米管的生长。碳纳米管的生长过程及其细微结构与温度有很大关系。碳源的分解、碳原子在催化剂内部的扩散以及催化剂粒子的团聚三者之间的竞争决定了碳纳米管的生长情况。本文分析了碳纳米管的顶部生长模式及该模式下催化剂粒子的形态变化。     

Nickel diffusion and silicide island formation on silicon (111)

Crystalline nickel disilicide islands have been observed on the Si(111) surface by atomic force microscopy (AFM). The nickel disilicide islands coalesce following a high temperature anneal (≈1260K). The islands differ from those formed at lower temperature in both shape and orientation. To explain the differences, we discuss kinetically limited growth accompanying phase and surface segregation of Ni from the bulk silicon wafer, and condensation of a Ni-rich NiSi_2−x liquid phase at the surface. Condensation from the liquid phase to NiSi₂ is concluded to be responsible for the structure of the crystallites. High temperature growth conditions lead preferentially to A-type (non-twinned) silicide structures.     

Au/Ni bilayers on n-type silicon: Metallurgical and electrical behaviour

Diffusion effects during the formation of silicides in the Ni-Au-Si system were investigated by means of ⁴He⁺ MeV Rutherford backscattering spectrometry, Auger electron spectroscopy coupled with Ar⁺ ion sputtering and X-ray diffraction as a function of the heat treatment temperature (280–350°C) and time (10–1000 min). Schottky barrier heights were used to identify the type of metal present at the silicon surface. Au/Ni/Si and Ni/Au/Si structures were prepared by electron gun deposition of thin gold and nickel films onto n-type Si〈111〉 single crystals. After thermal treatment only Ni₂Si and NiSi compounds were observed and their formation follows the phase order confirmed by previous investigations on the Ni/Si system, with a growth controlled by a lattice diffusion process. In the Ni/Au/Si〈111〉 structure the diffusion of the silicon through the gold film was detected during the formation of nickel silicide and the kinetics of growth of Ni₂Si and NiSi were similar to those studied in the Ni/Si〈100〉 system. A diffusion of gold towards the Si-NiSi interface was observed during the growth of NiSi in the Au/Ni/Si〈111〉 structure. The Schottky barrier height measurements confirm these findings.     

Integration of coplanar (Ba,Sr)TiO3 microwave phase shifters onto Si wafers using TiO2 buffer layers.

In this paper, a Ba0.6Sr0.4TiO3 (BST) tunable phase shifter with TiO2 films as microwave buffer layer between BST and silicon (Si) substrates is presented. The TiO2 buffer layer is grown by atomic layer deposition (ALD) onto Si substrate followed by pulsed laser deposition (PLD) of BST thin films onto the TiO2 buffer layer. The phase shifter fabricated on BST films grown on TiO2/Si substrate shows a good figure of merit (FOM) of 75.4 degrees/dB by exhibiting improved tunablity while retaining an appropriate dielectric Q as compared to 55.1 degrees/dB of BST/MgO structure. The TiO2 buffer layer grown by ALD enables successful integration of BST-based microwave tunable devices with high resistive Si wafer.     

Antireflective hydrophobic si subwavelength structures using thermally dewetted Ni/SiO2 nanomask patterns

We report the disordered silicon (Si) subwavelength structures (SWSs), which are fabricated with the use of inductively coupled plasma (ICP) etching in SiCl4 gas using nickel/silicon dioxide (Ni/SiO2) nanopattens as the etch mask, on Si substrates by varying the etching parameters for broadband antireflective and self-cleaning surfaces. For the fabricated Si SWSs, the antireflection characteristics are experimentally investigated and a theoretical analysis is made based on the rigorous coupled-wave analysis method. The desirable dot-like Ni nanoparticles on SiO2/Si substrates are formed by the thermal dewetting process of Ni films at 900 degrees C. The truncated cone shaped Si SWS with a high average height of 790 +/- 23 nm, which is fabricated by ICP etching with 5 sccm SiCl4 at 50 W RF power with additional 200 W ICP power under 10 mTorr process pressure, exhibits a low average reflectance of approximately 5% over a wide wavelength range of 450-1050 nm. The water contact angle of 110 degrees is obtained, indicating a hydrophobic surface. The calculated reflectance results are also reasonably consistent with the experimental data.     

Biocompatibility of multi-walled carbon nanotubes grown on titanium and silicon surfaces

Cell adhesion and cell viability of aligned multi-walled carbon nanotube (MWCNT) films were verified using Fibroblast L929 mouse cells. The MWCNTs were produced by a microwave plasma chemical vapor deposition (2.45 GHz) on silicon (Si), with a nickel catalyst, and titanium (Ti), with an iron catalyst. MTT assay and cellular adhesion were used for biocompatibility tests (ISO 10993-5). The results show very high cell viability and many layers of cells adhered on the surface formed by the nanotube tips at films grown on silicon surfaces. The MWCNT grown on Ti surfaces presented lower cell viability and a reduced number of cells on the surface formed by the nanotube tips. The different behavior is most probably related to excess iron contamination present in the case of titanium substrate, while nickel catalyst is probably enclosed by the nanotubes.     

辅助气体对RF-PECVD制备碳纳米管薄膜形貌的影响

采用射频等离子体增强化学气相沉积(RF-PECVD)技术,以Ni为催化剂,经600℃裂解C2H2在Si基底上制备出定向碳纳米管薄膜。采用扫描电子显微镜(SEM)表征了刻蚀后Ni颗粒与沉积的碳纳米管薄膜的形貌。研究了辅助气体对等离子体预处理催化剂与碳纳米管生长的影响。结果表明:辅助气体(H2与N2)流量比对催化剂颗粒尺寸、分布以及碳纳米管生长有显著影响;合适的气体流量比有利于减少碳纳米管薄膜的杂质颗粒,促进其定向生长。预处理过程中气体流量比H2:N2=20:5时,预处理后催化剂Ni颗粒分布密度大、粒径小且分布范围窄,适合碳纳米管均匀着床;沉积生长碳纳米管薄膜时,H2:N2=20:15可得到纯度高、定向性好的碳纳米管。     

Effect of interface on mid-infrared photothermal response of MoS2 thin film grown by pulsed laser deposition

This study reports on the mid-infrared (mid-IR) photothermal response of multilayer MoS2 thin films grown on crystalline (p-type silicon and c-axisoriented single crystal sapphire) and amorphous (Si/SiO2 and Si/SiN) substrates by pulsed laser deposition (PLD).The photothermal response of the MoS2 films is measured as the changes in the resistance of the MoS2 films when irradiated with a mid-IR (7 to 8.2 μm) source.We show that enhancing the temperature coefficient of resistance (TCR) of the MoS2 thin films is possible by controlling the film-substrate interface through a proper choice of substrate and growth conditions.The thin films grown by PLD are characterized using X-ray diffraction,Raman,atomic force microscopy,X-ray photoelectron microscopy,and transmission electron microscopy.The high-resolution transmission electron microscopy (HRTEM) images show that the MoS2 films grow on sapphire substrates in a layer-by-layer manner with misfit dislocations.The layer growth morphology is disrupted when the films are grown on substrates with a diamond cubic structure (e.g.,silicon) because of twin growth formation.The growth morphology on amorphous substrates,such as Si/SiO2 or Si/SiN,is very different.The PLD-grown MoS2 films on silicon show higher TCR (-2.9％ K-1 at 296 K),higher mid-IR sensitivity (△R/R =5.2％),and higher responsivity (8.7 V·W-1) compared to both the PLD-grown films on other substrates and the mechanically exfoliated MoS2 flakes transferred to different substrates.     

Diffusion of nickel in silicon below 475 °C

Nickel films were deposited on (100) and (111) surfaces of single-crystal silicon and were then annealed. The conditions under which the nickel is deposited determine whether or not an NiSi compound forms on annealing. It is postulated that defects are necessary for the formation of an NiSi compound at annealing temperatures below at least 475 °C, although the presence of defects may not necessarily cause the formation of a silicide. For substrate temperatures below 70 °C, defects are created during the vapor deposition of nickel on silicon. These defects always result in the formation of nickel silicide when the sample is annealed at higher temperatures. When nickel is deposited on defect-free silicon at temperatures of about 250 °C no defects are generated and, although interdiffusion of nickel and silicon occurs, silicide formation does not take place upon subsequent annealing below 475 °C. The activation energies for the diffusion of nickel into (100) silicon and (111) silicon were determined.     

High throughput nanofabrication of silicon nanowire and carbon nanotube tips on AFM probes by stencil-deposited catalysts

A new and versatile technique for the wafer scale nanofabrication of silicon nanowire (SiNW) and multiwalled carbon nanotube (MWNT) tips on atomic force microscope (AFM) probes is presented. Catalyst material for the SiNW and MWNT growth was deposited on prefabricated AFM probes using aligned wafer scale nanostencil lithography. Individual vertical SiNWs were grown epitaxially by a catalytic vapor-liquid-solid (VLS) process and MWNTs were grown by a plasma-enhanced chemical vapor (PECVD) process on the AFM probes. The AFM probes were tested for imaging micrometers-deep trenches, where they demonstrated a significantly better performance than commercial high aspect ratio tips. Our method demonstrates a reliable and cost-efficient route toward wafer scale manufacturing of SiNW and MWNT AFM probes.