Morphology and electronic structure of nanostructured carbon films embedding transition metal nanoparticles

Inclusions of metals in the growth process of carbon cluster assembled materials (ns-C) induce modifications in the structural and electronic properties of the material. A novel pulsed microplasma cluster source (PMCS) is able to deliver highly intense, collimated and stable beams suitable for producing bulk quantities of cluster-assembled nanocomposite films. Loading of metal nanoparticles into carbon cluster based films is obtained either by mixing a gas phase metallorganic compound with the carrier gas (He) before entering into the source (for example molybdenum (V) isopropoxide), or by using a double component sputtering target (metal (Ti, Ni)/graphite). The study of film morphology on nanometer scale, carried out by transmission electron microscopy (TEM), reveals the dispersion in a ns-C matrix of metallic particles and, in the case of molybdenum containing films, also of carbide particles. Spatially resolved ultraviolet photoemission spectroscopy confirms the segregation of metal particles and exhibits evident anisotropy in the Mo:ns-C films, mainly ascribable to the formation of carbide nanoparticles.     

Pulsed-laser deposition of MgB2 and B thin films

Thin films of the novel superconductor MgB₂ were deposited from an Mg-enriched MgB₂ target or by alternating ablation from Mg and B targets, depositing multilayers. The superconducting films were achieved in situ by a two-step process: deposition at low temperatures ranging from room temperature to 200 °C and subsequently heating to 600 °C. The color of the plasma originating from Mg or Mg-enriched MgB₂ targets during the deposition is an indicator of the constituents of the plasma and can be used to adjust the plasma parameters like pressure and energy density. The films showed a reduced critical temperature (T_c) compared to the bulk value (39 K), which is attributed to the small grain sizes and the relatively high base pressure of the system (10^-7 mbar) causing impurities (oxygen, carbon...). To investigate B oxidation and to determine the suitable deposition conditions for B, films made by pulsed-laser deposition (PLD) from B target were analyzed by XPS. The films are very sensitive to the ambient gas purity and the base pressure. We anticipate an improvement of T_c and the crystallinity of MgB₂ thin films by using PLD in high vacuum and with a high purity Ar and H₂ gas mixture. PACS 74.70.Ad     

Doping and photoelectric properties of C60 films prepared by ionized cluster beam deposition

60 films by means of ionized cluster beam (ICB) deposition. X-ray diffraction (XRD) measurement showed the C₆₀ films to be polycrystalline. The films show negative resistance–temperature coefficients, and their room-temperature resistivity is greater than 10² Ω cm. The films were implanted with 80-keV phosphorus, BBr₃, Ar, and He ions, under doses ranging up to 10¹⁶ cm^-2. The resistivity of the implanted films decreases with increasing doses. n-type electrical conduction was observed for phosphorus-implanted C₆₀ films. The interaction of impinging ions with C₆₀ clusters was found to force the C₆₀ molecules to disintegrate and the films to amorphize. p-type conduction was observed for the C₆₀ films doped with aluminum by simultaneously sputtering aluminum during deposition. C₆₀/Si structures show heterojunction characteristics that can be influenced by light illumination. The photoelectric properties of the films were found to be improved by doping with aluminum. Received: 12 January 1998/Accepted: 24 March 1998     

Structural and electrical properties of CdSexTe1−x thin films

The ternary semiconductors compounds are found to be very useful in the fabrication of thin film devices. This paper reports the preparation of CdSe_xTe_1?x films (1 ? x ? 0) in the thickness range 1000–3000 Å by vacuum evaporation technique onto glass and mica substrates held at temperatures, 303 to 623 K in a vacuum better than 5 × 10^?6 torr. The films were characterised by determining their composition and structure. The structure of the films, examined using XRD and TEM techniques, was found to be cubic (zincblende) in the entire composition range. The electrical resistivity and Hall mobilities have been determined as a function of film composition and deposition temperature.     

Pulsed laser deposition of high temperature protonic films

Pulsed laser deposition has been used to fabricate nanostructured BaCe_0.85Y_0.15O_3−δ films. Protonic conduction of the fabricated BaCe_0.85Y_0.15O_3−δ films was compared to the sintered BaCe_0.85Y_0.15O_3−δ. Sintered samples and laser targets were prepared by sintering BaCe_0.85Y_0.15O_3−δ powders derived by solid state synthesis. Films 1 to 8 μm thick were deposited by KrF excimer laser on porous Al₂O₃ substrates. Thin films were fabricated at deposition temperatures of 700 to 950 °C at O₂ pressures up to 200 mTorr using laser pulse energy densities of 1.4–3 J/cm². Fabricated films were characterized by X-ray diffraction, electron microscopy and electrical impedance spectroscopy. Single phase BaCe_0.85Y_0.15O_3−δ films with a columnar growth morphology are observed with preferred crystal growth along the [100] or [001] direction. Results indicate [100] growth dependence upon laser pulse energy. Electrical conductivity of bulk samples produced by solid state sintering and thin film samples were measured over a temperature range of 100 to 900 °C. Electrical conduction behavior was dependent upon film deposition temperature. Maximum conductivity occurs at deposition temperature of 900 °C; the electrical conductivity exceeds the sintered specimen. All other deposited films exhibit a lower electrical conductivity than the sintered specimen. Activation energy for electrical conduction showed dependence upon deposition temperature, it varied from 115 to 54 kJ. Film microstructure was attributed to the difference in electrical conductivity of the BaCe_0.85Y_0.15O_3−δ films.     

Effect of Al doping on structural,optical and electrical properties of SnO2 thin films synthesized by pulsed laser deposition

Aluminium-doped (Al = 0–5?wt.%) SnO₂ thin films with low-electrical resistivity and high optical transparency have been successfully synthesized by pulsed laser deposition technique at 500 °C. Structural, optical and electrical properties of the as-deposited and post-annealed thin films were investigated. X-ray diffraction patterns suggest that the films transform from crystalline to amorphous state with increasing aluminium content. The root mean square (R_q) surface roughness parameter, determined by atomic force microscopy decreases upon annealing of the as-deposited film. While resistivity of the film is the lowest (9.49 × 10^?4 Ω-cm) at a critical doping level of 1?wt.% Al, optical transparency is the highest (nearly 90%) in the as-deposited condition. Temperature dependence of the electrical resistivity suggests that the Mott’s variable range hopping process is the dominant carrier transport mechanism in the lower temperature range (40–135 K) for all the films whereas, thermally activated band conduction mechanism seems to account for conduction in the higher temperature region (200–300 K).     

Correlation of sp and sp fraction of carbon with electrical, optical and nano-mechanical properties of argon-diluted diamond-like carbon films

In the present work the correlation of electrical, optical and nano-mechanical properties of argon-diluted diamond-like carbon (Ar-DLC) thin films with sp³ and sp² fractions of carbon have been explored. These Ar-DLC thin films have been deposited, under varying C₂H₂ gas pressures from 25 to 75 mTorr, by radio frequency-plasma enhanced chemical vapor deposition technique. X-ray photoelectron spectroscopy studies are performed to estimate the sp³ and sp² fractions of carbon by deconvoluting C 1s core level spectra. Various electrical, optical and nano-mechanical parameters such as conductivity, I-V characteristics, optical band gap, stress, hardness, elastic modulus, plastic resistance parameter, elastic recovery and plastic deformation energy have been estimated and then correlated with calculated sp³ and sp² fractions of carbon and sp³/sp² ratios. Observed tremendous electrical, optical and nano-mechanical properties in Ar-DLC films deposited under high base pressure conditions made it a cost effective material for not only hard and protective coating applications but also for electronic and optoelectronic applications.     

Comparison and semiconductor properties of nitrogen doped carbon thin films grown by different techniques

Amorphous carbon nitride (a-CN_x) thin films have been synthesised by three different deposition techniques in an Ar/N₂ gas mixture and have been deposited by varying the percentage of nitrogen gas in the mixture (i.e. the N₂/Ar + N₂ ratio) from 0 to 10%. The variation of the electrical conductivity and the gap values of the deposited films versus the N₂/Ar + N₂ ratio were investigated in relation with their local microstructure. Film composition was analysed using Raman spectroscopy and optical transmission experiments. The observed variation of electrical conductivity and optical properties are attributed to the changes in the atomic bonding structures, which were induced by N incorporation, increasing both the sp² carbon content and their relative disorder. The low N content samples seem to be an interesting material to produce films with interesting properties for optoelectronic applications considering the facility to control the gas composition as a key parameter.     

The study of electronic conduction in amorphous thin films of Al-In<Subscript>2</Subscript>O<Subscript>3</Subscript>-Al structure deposited by thermal evaporation

A discussion of electronic conduction in amorphous thin films of Al-In₂O₃-Al structure is presented. Particular attention is given to the question of film thickness, substrate temperature during deposition and post-deposition annealing, since these conditions are known to have a profound effect on the structure and electrical properties of the films. The effects of temperature on the V-I characteristics and effects of frequency on conductivity and capacitance of the Al-In₂O₃-Al structure are also reported. Activation energies for conduction processes are estimated and the results are discussed in terms of the hopping model. The conduction at higher temperature is seemingly a contact-limited, i.e. Schottky type process, so a transition from hopping to free-band conduction takes place. The capacitance decreases with the rise of frequency and the lowering of temperature. The values of dielectric constants are estimated and the results are discussed in terms of Schottky type of conduction. The increase in conductivity with the increase in temperature during measurements of electrical properties, film thickness, substrate temperature and post deposition annealing is reported and results are discussed in terms of current theory.     

Current voltage analysis and band diagram of Ti/TiO2 nanotubes Schottky junction

We report variable temperature resistivity measurements and mechanisms related to electrical conduction in 200 keV Ni²⁺ ion implanted ZnO thin films deposited by vapor phase transport. The dc electrical resistivity versus temperature curves show that all polycrystalline ZnO films are semiconducting in nature. In the room temperature range they exhibit band conduction and conduction due to thermionic emission of electrons from grain boundaries present in the polycrystalline films. In the low temperature range, nearest neighbor hopping (NNH) and variable range hopping (VRH) conduction are observed. The detailed conduction mechanism of these films and the effects of grain boundary (GB) barriers on the electrical conduction process are discussed. An attempt is made to correlate electrical conduction behavior and previously observed room temperature ferromagnetism of these films.     

Preparation and characterization of boron-incorporated amorphous carbon films from a natural source of camphoric carbon as a precursor material

The boron-incorporated amorphous carbon (a-C:B) film has been prepared by pulsed laser deposition (PLD) in high vacuum at room temperature using natural source of camphoric carbon (CC) as a precursor material. The effects of various B weight percentages in the target (Bwt.%) on the properties of a-C:B films have been investigated using standard measurement techniques and discussed. The optical band gap (E_g) is almost unchanged up to 10 Bwt.%, decreased a little, and with decrease of electrical resistivity (ρ) with higher Bwt.%, we considered that the variation of E_g and electrical properties can be related to interstitial doping of B in carbon films through modifications of C-B bonding configurations by rearranging B atoms and the B incorporation induced by doping, which are responsible for the decrease in ρ. The decrease of ρ is considered not due to the graphitization caused by the increase of sp²-bonded carbon. This is further confirmed by the variation of surface morphology (AFM), Raman and FT-IR as the structural and bonding properties of these films was unchanged with B incorporation up to 16 Bwt.%.     

Structural manipulation of ultrathin organic films on metal surfaces: the case of decane thiol/Au(111)

3 (CH₂)₉]SH) have been adsorbed on Au(111) single crystals both via vacuum deposition and from an ethanolic solution. The epitaxial structure of the ultrathin organic films has been identified at room temperature via low-energy electron diffraction to be c(4×2)R30° for the solution grown film and rectangular c(23×) for the vacuum deposited film. These structures correspond to molecules adsorbed on the surface with their carbon chains flat on the surface (vacuum deposited) and nearly perpendicular (solution grown). It is demonstrated that this orientation can be changed reversibly in vacuum via either annealing the films or exposing them to additional gas. Received: 7 February 1997/Accepted: 27 May 1997     

Improvement of corrosion resistance of carbon steel using chemical vapor deposition from the mixture of Mo(CO)6 and Cr(CO)6 with an ArF-excimer laser

The corrosion resistance of carbon steel has been improved by the deposition from the mixture of Mo(CO)₆ and Cr(CO)₆ as well as from each carbonyl alone with an ArF-excimer laser (193 nm). The corrosion resistance attained by coating with the films from the mixture is higher than from Mo(CO)₆ alone, while lower than from Cr(CO)₆ alone. While the corrosion resistance increases with beam intensity monotonically over the range 4–25 MW cm^–2 for the deposition from Cr(CO)₆ alone, it tends to decrease slightly above 15 MW cm^–2 for the deposition from Mo(CO)₆ alone and from the mixture. SEM (Scanning Electron Microscope) photographs show that the films from each carbonyl and their mixture consist of small grains that are more densely packed at higher beam intensities. The comparison of the film thickness evaluated from sputtering time to remove the films with that from direct observation with SEM suggests that the density of the film increases with beam intensity. In the films deposited from the mixture, molybdenum is preferentially incorporated from the gas phase. Present name: Department of Quantum Engineering and Systems Science     

Phosphorus cluster production by laser ablation

Neutral and charged phosphorus clusters of a wide size range have been produced by pulsed laser ablation (PLA) in vacuum at 532, 337, and 193 nm ablating wavelengths and investigated by time-of-flight mass spectrometry. The neutral P_n clusters are even-numbered with local abundance maxima at n=10 and 14, while the cationic and anionic clusters are preferentially odd-numbered with P₇⁺, P₂₁⁺, and P₁₇^- being the most abundant ions. The dominance of the magic clusters is more pronounced at 337-nm ablation that is explained by efficient direct ejection of their building blocks under these conditions. Nanocrystalline phosphorus films have been produced by PLA in ambient helium gas. PACS 52.38.MF; 61.46.+w; 79.20.Ds; 81.07.B; 81.16.Mk     

Evaluation of hydrogen chemisorption in nanostructured carbon films by near edge X-ray absorption spectroscopy

We have developed a method for the quantitative evaluation of the chemisorbed fraction of hydrogen in nanostructured carbon films using Near Edge X-ray Absorption Spectroscopy (NEXAFS). In the carbon K-edge spectrum the peak related to carbon bonded to hydrogen is assumed to be correlated with the amount of hydrogen bonded to carbon. This assumption is supported by a comparative analysis of gas-phase hydrocarbons obtained via Electron Energy Loss Spectroscopy (EELS). We applied the method to nanostructured carbon (ns-C) films synthesized by supersonic cluster beam deposition. The evaluated quantity of chemisorbed hydrogen in different samples exposed to molecular hydrogen (pressure of 0.12 MPa, for 3 hours at room temperature) is ∼1.5 wt.%.     

离子束增强沉积制备p型氧化锌薄膜及其机理研究

采用离子束增强沉积方法在Si和SiO₂/Si衬底上制备In-N共掺杂ZnO薄膜(INZO)，溅射靶是用ZnO和2 atm% In₂O₃粉体均匀混合并压制而成，在氩离子溅射ZnO靶的同时，氮、氩混合离子束垂直注入沉积的薄膜.实验结果显示INZO薄膜具有(002)的择优取向，并且为p型导电，电阻率最低为0.9Ωcm.薄膜在氮气、氧气气氛下退火，对薄膜的结构和电学特性与成膜和退火条件的关系进行了分析. 关键词： 氧化锌薄膜 p型掺杂 离子束增强沉积     

Capacitance humidity sensor with carbon nitride detecting element

The humidity sensitive properties of carbon nitride (CN_x) films deposited by two methods, inductively coupled plasma chemical vapour deposition utilizing transport reactions and pulsed laser deposition combined with an rf discharge, have been investigated. For this purpose capacitance humidity sensors with a CN_x detecting element have been fabricated and tested. Fast and significant responses toward moisture are registered by the changes of the electrical parameters. The CN_x films sensing mechanism has been discussed. The results obtained show unambiguously that CN_x films appear to be a promising candidate as a humidity sensitive element in up-to-date electronic noses. Received: 6 December 1999 / Accepted: 7 January 2000 / Published online: 24 March 2000     

Preparation and characterization of molybdenum diselenide thin films

MoSe₂ layers, synthesized by annealing a molybdenum foil under selenium pressure, have been investigated by scanning electron microscopy, X-ray analysis, electron spectroscopy (XPS) and electrical resistance measurements. It has been found that stoichiometric layers are obtained after appropriate processing. The films crystallize in the hexagonal structure. The crystallites develop preferentially along the c axis. The binding energies deduced from the XPS lines were found to be in good agreement with those of the reference powder. The electrical resistance is governed by hopping conduction in the low temperature range (80–250 K) and by grain boundary scattering mechanisms at higher temperatures.     

Effect of air humidity on the microstructure and properties of large-area YBa2Cu3O7−x films by TFA-MOD process

Large-area YBa₂Cu₃O_7?x (YBCO) films had been successfully fabricated on LaAlO₃ (LAO) substrate by metalorganic deposition using trifluoroacetates (TFA-MOD) and the effect of air humidity on the structure and properties of YBCO films was systematically investigated. The precursor coating process was performed in the air humidity range of 70–40%. According to the X-ray diffraction (XRD) and scanning electron microscopy (SEM) images, the films prepared at high air humidity showed poor electrical performance due to large pores and impurity phases. In contrast, dense, homogeneous and epitaxially grown YBCO films prepared at low air humidity with the critical current densities of 3.8–5.0 MA cm^?2 at 77 K had been obtained.     

Growth and post-annealing effect on the properties of the new sulfosalt SnSb2S4 thin films

In this work, the effect of the thermal annealing atmosphere on the structural, optical and electrical properties of the sulfosalt SnSb₂S₄ films obtained by thermal vacuum evaporation was studied. The samples are annealed at different atmospheres in the temperature range 50−275 °C for 1 h. It is observed that SnSb₂S₄ films exhibit a dramatic change in their electrical properties at transition temperatures of about 150 °C, 170 °C and 200 °C after an annealing process under air, nitrogen and vacuum atmospheres, respectively. The electrical resistivity measurements suggest that obtained films show ‘semiconducting’ behavior with resistivities between 10 and 100 Ω cm; the annealed films present rather lower resistivities between 10⁻² and 10⁻³ Ω cm and exhibit obvious p⁺-type semiconductor behavior with a dominant crystalline component.