This paper reviews the present knowledge on tantalum pentoxide (Ta 2 O 5 ) thin films and their applications in the field of microelectronics and integrated microtechnologies. Different methods used to produce tantalum oxide layers are described, emphazing elaboration mechanisms and key parameters for each technique. We also review recent advances in the deposition of Ta 2 O 5 in the particular field of microelectronics where high quality layers are required from the structural and electrical points of view. The physical, structural, optical, chemical and electrical properties of tantalum oxide thin films on semiconductors are then presented and essential film parameters, such as optical index, film density or dielectric permittivity, are discussed. After a reminder of the basic mechanisms that control the bulk electrical conduction in insulating films, we carefully examine the origin of leakage currents in Ta 2 O 5 and present the state-of-the-art concerning the insulating behaviour of tantalum oxide layers. Finally, applications of tantalum oxide thin films are presented in the last part of this paper. We show how Ta 2 O 5 has been employed as an antireflection coating, insulating layer, gate oxide, corrosion resistant material, and sensitive layer in a wide variety of components, circuits and sensors.

Tantalum pentoxide (Ta2O5) thin films for advanced dielectric applications

Three-dimensional highly porous TiO2-4%SiO2-1%TeO2/Al2O3/TiO2 composite nanostructures (φ30-120 nm) directly fixed on glass substrates were fabricated by anodization of a superimposed Al/Ti layer sputter-deposited on glass and a sol-gel process. The porous composite nanostructures exhibited enhanced photocatalytic performances in decomposing acetaldehyde gas under UV illumination, which can be mainly ascribed to the combination of their large surface areas (7750-14770 m(2)/m(2)), high porosities (34.2-45.6%), and transparency. Specially, the composite nanostructure with ∼φ120 nm pores calcined at 500 °C showed the highest photocatalytic activity that is 6-10 times higher than commercial P-25 TiO2 under the experimental conditions.

Highly Porous (TiO2-SiO2-TeO2)/Al2O3/TiO2 Composite Nanostructures on Glass with Enhanced Photocatalysis Fabricated by Anodization and Sol-Gel Process.

In the field of photovoltaics, semiconductors of the III-V group such as GaAs and InP have been considered as the most efficient absorber materials due to their direct energy band gap and high mobility. In these compounds, arsenic and phosphorus are highly toxic and expensive. In this work we present systematic preparation of low cost SnS thin films and characterize these films to test their suitability for photovoltaic applications. We have observed that the films (with thickness ≅0.5μm) grown at the substrate temperature of 275°C exhibit a low resistive single SnS phase and have a direct optical band gap of 1.35eV with an absorption coefficient of ∼105cm−1. SnS films could be alternative semiconductor materials as absorbers for the fabrication of photovoltaic devices.

Microstructure dependent physical properties of evaporated tin sulfide films

Tin sulphide films were grown at different substrate temperatures by a thermal co-evaporation technique. The crystallinity of the films was evaluated from X-ray diffraction studies. Single-phase SnS films showed a strong (040) orientation with an orthorhombic crystal structure and a grain size of 0.12 mu m. The films showed an electrical resistivity of 6.1 Omega cm with an activation energy of 0.26 eV. These films exhibited an optical band gap of 1.37 eV and had a high optical absorption coefficient (> 10(4) cm(-1)) above the band-gap energy. The results obtained were analysed to evaluate the potentiality of the co-evaporated SnS films as an absorber layer in solar photovoltaic devices.

Synthesis and characterisation of co-evaporated tin sulphide thin films

Transparent semiconducting thin films of titanium oxide (TiO2) were deposited on glass substrates by the sol–gel method and spin-coating technique. The physical properties of the prepared films were studied as a function of the number of spun-cast layers. The microstructure and surface morphology of the TiO2 films were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM), with respect to the film thickness. The XRD analysis reveals that the films are polycrystalline with an anatase crystal structure and a preferred grain orientation in the (101) direction. The morphological properties were investigated by AFM, which shows a porous morphology structure for the films. The optical properties of the films were characterized by UV–Visible spectrophotometry, which shows that the films are highly transparent in the visible region and their transparency is slightly influenced by the film thickness, with an average value above 80 %. The dependence of the refractive index (n), extinction coefficient (k), and absorption coefficient (α) of the films on the wavelength was investigated. A shift in the optical band gap energy of the films from 3.75 to 3.54 eV, as a function of the film thickness, has been observed.

Structural and optical properties of TiO2 thin films prepared by spin coating

Photocatalytic TiO2 thin-films deposited by a pulsed laser deposition technique

Electrical properties of evaporated polycrystalline Ge thin-films

A new off-axis pulsed laser deposition method using an aperture plate is proposed for the purpose of depositing high surface-quality Ta2O5 thin films without large fragments and/or droplets. The angular distribution of droplets and growth species emitted from the target is examined. Then, the oxygen pressure and laser energy dependencies of the number of droplets and the film thickness are also examined. The results indicate that the aperture plate plays an important role of limiting the path of the traveling droplets and of capturing the droplets traveling toward the off-axis substrate. On the other hand, the film deposition rate is not slow even when using the plate because the species pass through the aperture hole and reach the substrate by scattering. Using a 5 mm aperture plate, a good surface quality Ta2O5 film is obtained under the optimum condition of 200–300 mTorr oxygen pressure.

Narumi Inoue

Papers

Photocatalytic TiO2 thin-films deposited by a pulsed laser deposition technique

Electrical properties of evaporated polycrystalline Ge thin-films

A new pulsed laser deposition method using an aperture plate

Annealing effect of vacuum evaporated InSb thin films

Off-axis excimer laser deposition of Ta 2 O 5 thin films