Preferential orientation of fluorine-doped SnO2 thin films: The effects of growth temperature

Polycrystalline fluorine-doped SnO₂ thin films with a given thickness of about 250 nm have been grown by ultrasonic spray pyrolysis with a growth temperature in the range of 360–480 °C. A texture transition from 〈1 0 1〉 to 〈1 0 0〉 and 〈3 0 1〉 crystallographic orientations has experimentally been found by X-ray diffraction measurements as growth temperature is raised, revealing that a process of abnormal grain growth has occurred. The texture effects have been investigated within a thermodynamic approach considering that grain growth is driven by the minimization of total free energy. The anisotropic character of the physical quantities and the effects of growth temperature have been shown both on the surface energy per unit volume through its dependence on the oxygen chemical potential and on the strain energy density through its variation with the elastic strain and biaxial modulus. Importantly, it is demonstrated by thermodynamic simulations that the oxygen chemical potential increases with growth temperature in the spray pyrolysis conditions, showing that the atmosphere is less and less reducing. For low growth temperature, it is revealed that the 〈1 0 1〉 preferred orientation is due to surface energy minimization since the (1 0 1) reduced surfaces have a surface energy lower than the (1 0 0) reduced surfaces. In contrast, as growth temperature is raised, the 〈1 0 0〉 crystallographic orientation becomes predominant owing to strain energy minimization. A texture map is finally determined, revealing the expected texture as a function of elastic strain and oxygen chemical potential.     

Correlation between electrical properties and point defects in NiO thin films

The wide band-gap semiconductor NiO has p-type characteristics and is an alternative to p-ZnO, due to conduction mechanisms coming from the Ni vacancies and O interstitials. The correlation between the electrical properties and point defects was studied with NiO thin films grown by sputtering at various temperatures and in pure Ar and O₂ atmospheres. The p-type characteristics of the NiO thin films were double-checked by Hall-effect and Seebeck coefficient measurements. Below 300 °C, the electrical resistivity of NiO films grown in an O₂ atmosphere was lower than those grown in an Ar atmosphere due to the suppressed point defects. On the other hand, the electrical resistivity over 300 °C became semi-insulating due to the relatively stoichiometric NiO. The optical transmittance of the NiO film deposited in an O₂ atmosphere and 600 °C was around 80% in the visible region. Finally, the p-NiO/n-ZnO heterojunction diodes showed a well-rectifying current-voltage curve.     

Circular-difference effects in second-harmonic generation from thin films

Chiral molecules have received much attention in nonlinear optics because they are inherently noncentrosymmetric. It has been shown that second-harmonic generation (SHG) from chiral, isotropic surfaces and thin films is sensitive to the handedness of circularly-polarized fundamental light. These circular-difference (CD)-effects in SHG are used to probe the chirality of a sample. However, an achiral, anisotropic surface can also give rise to CD-effects in SHG. This can be explained by the fact that for some orientations of the sample, the total geometry becomes chiral. We investigated a chiral, anisotropic Langmuir–Blodgett film. For such a sample both chirality and anisotropy give rise to CD-effects in SHG. Our goal was to extract the optical activity effects due to chirality. Our technique is based on the fact that CD-effects originating from anisotropy will critically depend on the azimuthal rotation angle of the sample in the set-up whereas the CD-effects coming from chirality will remain constant.     

Influences of vacancy defects on thermal conductivities of Ge thin films

The effects of vacancy defects on the thermal conductivity of Ge thin films were investigated by employing molecular dynamics(MD) simulations and theoretical analysis based on the Boltzmann equation.Both the MD and theoretical results show that the lattice thermal conductivity dramatically decreases with the increasing of vacancy concentration at 400 and 500 K.In addition,the dependence of vacancy concentration on the thermal conductivity of Ge thin films becomes less sensitive as the temperature increases.Theoretical results also confirm that the major part of the lattice thermal conductivity reduction is associated with the point-defect scattering and phonon-phonon scattering processes.     

The effects of vacuum annealing on the structure of VO2 thin films

Noncrystalline VO_x thin films were deposited onto p-doped Si (100) substrates at 400 °C using magnetron sputtering. By vacuum annealing, we obtained polycrystalline VO₂ thin films with two different structures under a variety of annealing conditions. With the annealing temperature increasing and the annealing time developing, structures of the films underwent the following transformation: amorphous structure→metastable VO₂ (B)→VO₂ (B) + VO₂ (M). Vacuum annealing is useful of acquiring VO₂ thin films with high surface quality, but too high annealing temperature (500 °C) and too long time (15 h) are harmful, which make the surface degenerate.     

Size effects in the fatigue behavior of thin Ag films

Thin film dimensions and microstructure affect the microscopic processes responsible for fatigue. This work focuses on the characterization of such mechanisms and the resulting fatigue behavior. The fatigue behavior of 0.2–1.5 μm thick, Ag films on SiO₂ was investigated. The films were tested using cantilever microbeam deflection with respect to the influence of loading conditions. Extrusions similar to those observed in bulk material were found at the Ag film surfaces after cyclic loading. Voids observed beneath the extrusions, close to the film-substrate interface, contributed significantly to fatigue failure. Fatigue damage was observed to occur predominantly in (100)-oriented grains. Thinner films were more fatigue resistant and contained fewer, smaller extrusions than thicker films.     

Investigation of the structural defects in GaN thin films grown by organometallic vapor phase epitaxy

GaN thin films were prepared on {0001} planes of sapphire substrates by organometallic vapor phase epitaxy (OMVPE) techniques. The crystall orgaphic relation between the film and the substrate as well as the structural features of the defects in the film were investigated by transmission electron microscopy (TEM). Epitaxial relationship was observed in the GaN/sapphire heterostructure preparedin this investigation; (0001) GaN//(0001) sapphire; sapphire. Dislocations of Burgers vector were observed in the film; the propagation behavior of the dislocations exhibits a slip system is operative in the film. Inversion domain boundary (IDB) facets lying parallel to and planes were observed; the type of anti-site bonds (Ga-Ga, N-N) is altemate along these IDB planes, keeping the simple stoichiometry of the compound.     

Laser annealing of flash-evaporated CuInSe2 thin films

In this paper, the impact of laser annealing on the structural, electrical, and optical properties of CuInSe₂ (CIS) thin films has been investigated. The films were deposited using a modified flash evaporation system onto glass substrates. Structural analysis using x-ray diffraction (XRD) showed that the films have a strong preferred growth direction in the 〈112〉 plane. After laser annealing with a diffused beam of 20 ns width, the structure was relaxed and an increase in the intensity of 〈112〉 diffraction line was observed. A gas-microphone-type, high-resolution, near-infared (IR) photoacoustic spectrometer was used for the analysis of nonradiative defect states in as-grown and laser-annealed CIS thin film samples at room temperature. The absorption coefficient has been derived from photoacoustic spectra to establish activation energies for several defect-related energy levels. The calculated intrinsic defect ionization energies were also compared with the existing data available in the literature. The changes in the optical properties of the films have been explained in terms of the variations in the structural characteristics within the material. This paper was presented at the fourth International Surface Engineering Congress and Exposition held August 1–3, 2005 in St. Paul, MN.     

The pulsed-laser deposition of superconducting thin films

High-critical-temperature (high-T_c) superconductors have generated tremendous interest for the fabrication of superconducting thin films in advanced microelectronic applications. Superconducting thin films operating at liquid nitrogen temperatures offer great possibilities for faster, more sensitive and more precise electronic devices. The pulsed-laser deposition technique is an excellent method of fabricating high-T_c superconducting thin films with exceptional microstructural and property control, and its unique features have made it quite popular. This article discusses the nature of laser-solid-plasma interactions during the deposition process and processing aspects which result in films exhibiting excellent superconducting properties.     

Thermal stability of Ti3SiC2 thin films

The thermal stability of Ti₃SiC₂(0 0 0 1) thin films is studied by in situ X-ray diffraction analysis during vacuum furnace annealing in combination with X-ray photoelectron spectroscopy, transmission electron microscopy and scanning transmission electron microscopy with energy dispersive X-ray analysis. The films are found to be stable during annealing at temperatures up to ∼1000 °C for 25 h. Annealing at 1100–1200 °C results in the rapid decomposition of Ti₃SiC₂ by Si out-diffusion along the basal planes via domain boundaries to the free surface with subsequent evaporation. As a consequence, the material shrinks by the relaxation of the Ti₃C₂ slabs and, it is proposed, by an in-diffusion of O into the empty Si-mirror planes. The phase transformation process is followed by the detwinning of the as-relaxed Ti₃C₂ slabs into (1 1 1)-oriented TiC_0.67 layers, which begin recrystallizing at 1300 °C. Ab initio calculations are provided supporting the presented decomposition mechanisms.     

Preparation of CulnSe2 thin films by paste coating

Precursor pastes were obtained by milling Cu-In alloys and Se powders.CuInSe2 thin films were successfully prepared by precursor layers,which were coated using these pastes,and were annealed in a H2 atmosphere.The pastes were tested by laser particle diameter analyzer,simultaneous thermogravimetric and differential thermal analysis instruments (TG-DTA),and X-ray diffractometry (XRD).Selenized films were characterized by XRD,scanning electron microscopy (SEM),and energy dispersive spectroscopy (EDS).The results indicate that chalcopyrite CuInSe2 is formed at 180℃ and the crystallinity of this phase is improved as the temperature rises.All the CuInSe2 thin films,which were annealed at various temperatures,exhibit the preferred orientation along the (112) plane.The compression of precursor layers before selenization step is one oftbe most essential factors for the preparation of perfect CulnSe2 thin films.     

The oxidation of titanium thin films in phosphoric medium

The properties of TiO₂ thin film grown under potentiostatic mode are investigated by the linear voltammetry, photo-electrochemical technique and capacitance measurements. The anodic film thickens up to ～10 nm by diffusion-controlled process and the growth obeys to inverse and direct logarithmic laws respectively in acidic and alkaline solutions. The effect of the anion on the stability of the films is discussed. The film grown at pH 2 has the best corrosion resistance, as evidenced from the electro-kinetic parameters. The electrochemical impedance spectroscopy (EIS) reveals the contribution of the bulk effect. The experimental data are fitted by shifting the centre of the semi-circle down the real axis and interpreted in terms of constant phase element (CPE). The Mott-Schottky plots exhibit a linear behavior, characteristic of n type conductivity and the flat band potential follows nearly a Nerstian slope (?0.056 V pH^?1), the H₃O⁺ specie is identified as potential determining specie.     

Defect structures in TaSi2 thin films produced by co-sputtering

Phase transformation and defect structures in thin-film TaSi₂ produced by co-sputtering have been investigated as a function of annealing temperature by transmission electron microscopy. Crystallization of amorphous TaSi₂ thin films occurs at 400 °C without forming any metastable phases. Most of C40 TaSi₂ crystallites contain planar faults parallel to (0001) basal planes. Convergent-beam electron diffraction (CBED) indicates that these planar faults are not simple stacking faults but are twin boundaries bounded by two adjacent enantiomorphically-related twin domains; i.e., domains belonging to the space groups P6₂22 (right-handed) and P6₄22 (left-handed) having the identical crystal orientation arrange alternatively separated by twin boundaries parallel to (0001). The formation of these enantiomorphically-related domains in TaSi₂ thin films with the hexagonal C40 structures is discussed in comparison with the formation of twin-related domains in MoSi₂ (the tetragonal C11_b structure) and TiSi₂ (the orthorhombic C54 structure) thin films. A new CBED method is proposed for identification of enantiomorphically-related crystals, in which asymmetry in the intensity of Bijvoet pairs of FOLZ disks in an experimental symmetrical zone-axis CBED pattern is compared with that in a computer simulated CBED pattern.     

Preparation of CuInSe2 thin films by paste coating

Precursor pastes were obtained by milling Cu-In alloys and Se powders. CuInSe2 thin films were successfully prepared by precursor layers, which were coated using these pastes, and were annealed in a H2 atmosphere. The pastes were tested by laser particle diameter analyzer, si- multaneous thermogravimetric and differential thermal analysis instruments (TG-DTA), and X-ray diffractometry (XRD). Selenized films were characterized by XRD, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The results indicate that chal- copyrite CuInSe2 is formed at 180°C and the crystallinity of this phase is improved as the temperature rises. All the CuInSe2 thin films, which were annealed at various temperatures, exhibit the preferred orientation along the (112) plane. The compression of precursor layers before selenization step is one of the most essential factors for the preparation of perfect CuInSe2 thin films.     

Cu-doping effects on the physical properties of cadmium sulfide thin films

Thin films of CdS were fabricated by close spaced sublimation technique under vacuum of ∼10⁻⁵ mbar at a source temperature of 550 °C for various periods of time. These as-deposited thin films were immersed into Cu (NO₃)₂ solution at 80 ± 5 °C for variety of time to ensure Cu doping. The structural, surface, optical and electrical analyses were completed with the help of X-Ray Diffraction, Scanning Electron Microscope with Energy Dispersive X-Ray, UV-VIS-NIR Spectrophotometer and Hall Measurement System respectively. The transmittance of as-deposited sample is reduced from 80% to 30% with increasing copper immersion time. The mobility increased from 6.67 × 10¹ to 1.15 × 10³ cm²/Vs due to the change in the carrier concentration.     

Modeling the effects of impurities on microstructure formation in nanocrystalline nickel thin films

The Atomic Kinetic Lattice Monte Carlo method was used to model the effects of a clustering impurity species on the deposition of nanocrystalline nickel films onto (001) copper. The deposition model was partly parameterized by physical inputs from atomistic calculations of copper-nickel alloys. The grain size and shape is shown to depend on the concentration of impurities in the deposited film. Low concentrations yield elongated columnar grains, while higher concentrations produce much smaller and more equiaxed grains. For more information, contact Corbett C. Battaile, Sandia National Laboratories, P.O. Box 5800, MS 1411, Albuquerque, NM 87111, USA; (505) 844-7039; fax (505) 844-9781; e-mail ccbatta@sandia.gov.     

The formation of thin oxide films on metals

It is shown that one may not expect simple rate laws between the limiting case (i) of transport of ions across a thin oxide film by virtue of an electrical field due to electrical charges of opposite sign at the metal/oxide and the oxide/gas interface and the limiting case (ii) of diffusion of ions and electrons across a thick oxide layer in accord with classical diffusion equations.