Optical gap in carbon nitride films

In this paper we study the effect of introducing nitrogen into different carbon networks. Two kinds of carbon nitride films were deposited: (a) Using a DC-magnetron sputtering system sp2 bonded carbon nitride (a-CN) films were deposited and (b) Using a combination of filtered cathodic vacuum arc and a low-pressure N2 plasma source, N was introduced into sp3 carbon networks (ta-C), leading to the formation of a more dense CN film named ta-CN. For ta-CN films we found that the optical gap initially decreases as the N content and the sp2 fraction rises, but above a certain N quantity there is a level-off of the value, and the gap then remains constant despite further increases in the fraction and clustering of the sp2 phase. However, for a-CN films the optical gap increases with the nitrogen content. These two different trends are not easily explained using the same framework as that for carbon films, in which any decrease in the band gap is associated to an increase in the sp2 fraction or its clustering. Here we discuss the conditions that lead to high optical gap in sp2-bonded carbon nitride samples, which are clearly not associated to the presence of any crystalline super-hard phase. We also compared other differences in properties observed between the two films, such as deposition rate, infrared and Raman spectra. © 2003 Elsevier Science B.V. All rights reserved.

Deposition and characterisation of ultralow-stress ZnO thin films for application in FBAR-based gravimetric biosensors

Zinc oxide (ZnO) thin films were deposited at high rates ( > 50 nm min-1) using a unique technique known as high target utilisation sputtering (HiTUS). The films obtained possess good crystallographic orientation, low surface roughness, very low stress and excellent piezoelectric properties. We have utilised the films to develop highly sensitive biosensors based on thickness longitudinal mode (TLM) thin film bulk acoustic resonators (FBARs). The FBARs have the fundamental TLM at a frequency near 1.5 GHz and quality factor Q higher than 1,000, which is one of the largest values ever reported for ZnO-based FBARs. Bovine Serum Albumin (BSA) solutions with different concentrations were placed on the top of different sets of identical FBARs and their responses to mass-loading from physically adsorbed protein coatings were investigated. These resonators demonstrated a high sensitivity and thus have a great potential as gravimetric sensors for biomedical applications. © 2011 Inderscience Enterprises Ltd.

Low temperature (< 100 °c) deposited P-type cuprous oxide thin films: Importance of controlled oxygen and deposition energy

With the emergence of transparent electronics, there has been considerable advancement in n-type transparent semiconducting oxide (TSO) materials, such as ZnO, InGaZnO, and InSnO. Comparatively, the availability of p-type TSO materials is more scarce and the available materials are less mature. The development of p-type semiconductors is one of the key technologies needed to push transparent electronics and systems to the next frontier, particularly for implementing p-n junctions for solar cells and p-type transistors for complementary logic/circuits applications. Cuprous oxide (Cu2O) is one of the most promising candidates for p-type TSO materials. This paper reports the deposition of Cu2O thin films without substrate heating using a high deposition rate reactive sputtering technique, called high target utilisation sputtering (HiTUS). This technique allows independent control of the remote plasma density and the ion energy, thus providing finer control of the film properties and microstructure as well as reducing film stress. The effect of deposition parameters, including oxygen flow rate, plasma power and target power, on the properties of Cu2O films are reported. It is known from previously published work that the formation of pure Cu2O film is often difficult, due to the more ready formation or co-formation of cupric oxide (CuO). From our investigation, we established two key concurrent criteria needed for attaining Cu2O thin films (as opposed to CuO or mixed phase CuO/Cu2O films). First, the oxygen flow rate must be kept low to avoid over-oxidation of Cu2O to CuO and to ensure a non-oxidised/non-poisoned metallic copper target in the reactive sputtering environment. Secondly, the energy of the sputtered copper species must be kept low as higher reaction energy tends to favour the formation of CuO. The unique design of the HiTUS system enables the provision of a high density of low energy sputtered copper radicals/ions, and when combined with a controlled amount of oxygen, can produce good quality p-type transparent Cu2O films with electrical resistivity ranging from 102 to 104 Ω-cm, hole mobility of 1-10 cm2/V-s, and optical band-gap of 2.0-2.6 eV. These material properties make this low temperature deposited HiTUS Cu 2O film suitable for fabrication of p-type metal oxide thin film transistors. Furthermore, the capability to deposit Cu2O films with low film stress at low temperatures on plastic substrates renders this approach favourable for fabrication of flexible p-n junction solar cells. © 2011 Elsevier B.V. All rights reserved.

High-density remote plasma sputtering of high-dielectric-constant amorphous hafnium oxide films

Hafnium oxide (HfOx) is a high dielectric constant (k) oxide which has been identified as being suitable for use as the gate dielectric in thin film transistors (TFTs). Amorphous materials are preferred for a gate dielectric, but it has been an ongoing challenge to produce amorphous HfOx while maintaining a high dielectric constant. A technique called high target utilization sputtering (HiTUS) is demonstrated to be capable of depositing high-k amorphous HfOx thin films at room temperature. The plasma is generated in a remote chamber, allowing higher rate deposition of films with minimal ion damage. Compared to a conventional sputtering system, the HiTUS technique allows finer control of the thin film microstructure. Using a conventional reactive rf magnetron sputtering technique, monoclinic nanocrystalline HfOx thin films have been deposited at a rate of ∼1.6nmmin-1 at room temperature, with a resistivity of 1013Ωcm, a breakdown strength of 3.5MVcm-1 and a dielectric constant of ∼18.2. By comparison, using the HiTUS process, amorphous HfOx (x=2.1) thin films which appear to have a cubic-like short-range order have been deposited at a high deposition rate of ∼25nmmin-1 with a high resistivity of 1014Ωcm, a breakdown strength of 3MVcm-1 and a high dielectric constant of ∼30. Two key conditions must be satisfied in the HiTUS system for high-k HfOx to be produced. Firstly, the correct oxygen flow rate is required for a given sputtering rate from the metallic target. Secondly, there must be an absence of energetic oxygen ion bombardment to maintain an amorphous microstructure and a high flux of medium energy species emitted from the metallic sputtering target to induce a cubic-like short range order. This HfOx is very attractive as a dielectric material for large-area electronic applications on flexible substrates. A remote plasma sputtering process (high target utilization sputtering, HiTUS) has been used to deposit amorphous hafnium oxide with a very high dielectric constant (∼30). X-ray diffraction shows that this material has a microstructure in which the atoms have a cubic-like short-range order, whereas radio frequency (rf) magnetron sputtering produced a monoclinic polycrystalline microstructure. This is correlated to the difference in the energetics of remote plasma and rf magnetron sputtering processes. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Near-field characterization of chemical vapor deposition graphene in the microwave regime

Near-field measurements were performed at X-band frequencies for graphene on copper microstrip transmission lines. An improvement in radiation of 0.88 dB at 10.2 GHz is exhibited from the monolayer graphene antenna which has dc sheet resistivity of 985 Ω/sq. Emission characteristics were validated via ab initio simulations and compared to empirical findings of geometrically comparable copper patches. This study contributes to the current knowledge of the electronic properties of graphene. © 2013 AIP Publishing LLC.

Low-temperature growth of carbon nanotubes by plasma-enhanced chemical vapor deposition

Vertically aligned carbon nanotubes were grown at temperatures as low as 120degreesC by plasma-enhanced chemical vapor deposition. A systematic study of the temperature dependence of the growth rate and the structure of the as-grown nanotubes is presented using a C2H2/NH3 system and nickel as the catalyst. The activation energy for the growth rate was found to be 0.23 eV, much less than for thermal chemical vapor deposition (1.2-1.5 eV). This suggests growth occurs by surface diffusion of carbon on nickel. The result could allow direct growth of nanotubes onto low-temperature substrates like plastics, and facilitate the integration in sensitive nanoelectronic devices. (C) 2003 American Institute of Physics.

The role of the catalytic particle in the growth of carbon nanotubes by plasma enhanced chemical vapor deposition

Vertically aligned carbon nanotubes were synthesized by plasma enhanced chemical vapor deposition using nickel as a metal catalyst. High resolution transmission electron microscopy analysis of the particle found at the tip of the tubes reveals the presence of a metastable carbide Ni3C. Since the carbide is found to decompose upon annealing at 600 degreesC, we suggest that Ni3C is formed after the growth is stopped due to the rapid cooling of the Ni-C interstitial solid solution. A detailed description of the tip growth mechanism is given, that accounts for the composite structure of the tube walls. The shape and size of the catalytic particle determine the concentration gradient that drives the diffusion of C atoms across and though the metal. (C) 2004 American Institute of Physics.