Influence of substrate temperature on the chemical and microstructural properties of MO-CVD ZrTiO<Subscript>4</Subscript> thin films

In the last few years, intensive research activity has been focused on the development of suitable synthesis methods for high-permittivity materials, used for the realization of next-generation microdevices able to fulfil the previsions of the Technology Roadmap of Semiconductors. The use of high-permittivity materials can overcome the difficulties concerning the production of SiO₂-based ultra-thin dielectrics, such as the generation of pinholes and the non-uniformity of the film, which may result in a malfunction in high-density systems. Recently, zirconium titanate thin films were discovered to have very interesting dielectric properties, which suggests a use for them in microwave integrated systems, such as receivers or DRAMs, since they are monophasic, have little dissipation and show a good thermal stability and a high value for the dielectric constant, independent of frequency in the range from kilohertz to a few gigahertz. Real application is possible only in strict connection with the development of a suitable preparation method which allows production with controlled and reproducible characteristics. In this work, the synthesis and characterization of Zr_xTi_1-xO₄ (ZT) thin films grown via MO-CVD is described, studying the influence of growth parameters on their structural, chemical and physical properties. Received: 17 June 2002 / Accepted: 24 June 2002 / Published online: 4 November 2002 RID="*" ID="*"Corresponding author. Fax: +39-06/9067-2445, E-mail: Pad@mlib.cnr.it     

Local properties of the electronic structure of cubic SrTiO3, BaTiO3 and PbTiO3 crystals, analysed using Wannier-type atomic functions

The electronic structure of the cubic perovskites SrTiO₃, BaTiO₃ and PbTiO₃ is calculated by Hartree-Fock and density functional theory methods. Wannier-type atomic orbitals (WTAOs) are obtained from symmetrized combinations of Bloch states of some occupied and vacant bands by a variational method. Population analysis, based on the WTAOs, attributes a mixed ionic-covalent type of bonding with partly covalent Ti-O bonds to the perovskites under study. The atomic charges thus calculated are then compared to those obtained by the traditional Mulliken population analysis.     

Conductive nanodots on the surface of irradiated CaF2

CaF₂(111) single crystal surfaces have been irradiated with fast heavy ions under oblique angles resulting in chains of nanosized hillocks. In order to characterize these nanodots with respect to their conductivity we have applied non‐contact atomic force microscopy using a magnetic tip. Measurements in ultra high vacuum as well as under ambient conditions reveal a clearly enhanced electromagnetic interaction between the magnetic tip and the nanodots. The dissipated energy per cycle is comparable to the value found for metals, indicating that the interaction of the ion with the target material leads to the creation of metallic Ca nanodots on the surface. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Non-Contact to Contact Transition： Direct Measurements of Interaction Forces between a Solid Probe and a Planar Air-Water Interface

The interaction force between a solid probe and a planar air-water interface is measured by using an atomic force microscope. It is demonstrated that during the approach of the probe to the air-water interface, the force curves decline all the time due to the van der Waals attraction and induces a stable profile of water surface raised. When the tip approaches very close to the water surface, force curves jump suddenly, reflecting the complex behaviour of the unstable water surface. With a theoretical analysis we conclude that before the tip touches water surface, two water profiles appear, one stable and the other unstable. Then, with further approaching, the tip touches water surface and the non-contact to contact transition occurs.     

The influence of substrate temperature variation on tungsten oxide thin film growth in an HFCVD system

Tungsten oxide (WO₃) thin films were deposited by a modified hot filament chemical vapor deposition (HFCVD) technique using Si (1 0 0) substrates. The substrate temperature was varied from room temperature to 430 °C at an interval of 100 °C. The influence of the substrate temperature on the structural and optical properties of the WO₃ films was studied. X-ray diffraction and Raman spectra show that as substrate temperature increases the film tends to crystallize from the amorphous state and the surface roughness decreases sharply after 230 °C as confirmed from AFM image analysis. Also from the X-ray analysis it is evident that the substrate orientation plays a key role in growth. There is a sharp peak for samples on Si substrate due to texturing. The film thickness also decreases as substrate temperature increases. UV-vis spectra show that as substrate temperature increases the film property changes from metallic to insulating behavior due to changing stoichiometry, which was confirmed by XPS analysis.     

Compositional contrast in AlxGa1−xN/GaN heterostructures using scanning spreading resistance microscopy

Scanning spreading resistance microscopy has found extensive use as a dopant-profiling technique for silicon-based devices, and to a lesser extent for some III-V materials. Here we demonstrate its efficacy for wide bandgap nitrides and, in particular, show that it may be used to differentiate between layers of different Al-content in an Al_xGa_1−xN/GaN heterostructure. A monotonic increase in resistance signal with increasing Al-content is demonstrated, under optimal imaging conditions. The variation in measured resistance with applied bias is shown to be dependent on the aluminium content, and this is discussed, along with other issues, in the context of potential quantification of unknown samples. The procedure for forming an optimal image is different from that for silicon, in terms of contact forces and applied biases.     

Synthesis and structural determination of the new oxide fluoride BaFeO2F

Low temperature fluorination of BaFeO_3−x using poly(vinylidene fluoride) leads to the formation of the oxide fluoride BaFeO₂F. Mössbauer spectroscopy shows that this phase exhibits magnetic ordering at room temperature due to interactions between the Fe³⁺ ions, with an ordering temperature of 645 (±5) K. Neutron diffraction studies show that the phase has cubic symmetry and confirm the presence of magnetic ordering (G-type antiferromagnetic) at room temperature.     

Nano- and micro-scale patterning of Si (1 0 0) under keV ion irradiation

Evolution of Si (1 0 0) surface under 100 keV Ar⁺ ion irradiation at oblique incidence has been studied. The dynamics of surface erosion by ion beam is investigated using detailed analysis of atomic force microscopy (AFM) measurements. During an early stage of sputtering, formation of almost uniformly distributed nano-dots occurs on Si surface. However, the late stage morphology is characterized by self-organization of surface into a regular ripple pattern. Existing theories of ripple formation have been invoked to provide an insight into surface rippling.     

Substrate temperature influence on the properties of nanostructured ZnO transparent ultrathin films grown by PLD

Zinc oxide films of 40 nm thickness have been deposited on glass substrates by pulsed laser deposition using an excimer XeCl laser (308 nm) at different substrate temperatures ranging from room temperature to 650 °C. Surface investigations carried out by using atomic force microscopy have shown a strong influence of temperature on the films surface topography. UV-VIS transmittance measurements have shown that our ZnO films are highly transparent in the visible wavelength region, having an average transmittance of ∼90%. The optical band gap of the films was found to be 3.26 eV, which is lower than the theoretical value of 3.37 eV. Besides the normal absorption edge related to the transition between the valence and the conduction band, an additional absorption band was also recorded in the wavelength region around 364 nm (∼3.4 eV). This additional absorption band may be due to excitonic, impurity, and/or quantum size effects. Photoreduction/oxidation in ozone of the ZnO films lead to larger conductivity changes for higher deposition temperature. In conclusion, the ozone sensing characteristics as well as the optical properties of the ZnO thin films deposited by pulsed laser deposition are strongly influenced by the substrate temperature during growth. The sensitivity of the films towards ozone might be enhanced significantly by the control of the films deposition parameters and surface characteristics.     

Microstructure and nanohardness of the diluted magnetic semiconducting Cd1−xMnxS nano-crystalline films

Cd_1−xMn_xS nano-crystalline films (0 ≤ x ≤ 0.5) were formed on glass substrates by thermal evaporation technique at room temperature (300 K). AFM studies showed that all the films were in nano-crystalline form with the grain size varying in the range between 36 and 58 nm and exhibited hexagonal structure of the host material. The lattice parameters varied linearly with composition, following Vegard's law in the entire composition range. The nanohardness and Young's modulus decreased sharply with ‘Mn’ content upto x = 0.3 and increased with high Mn content.