Comments on the paper of S. Chaudhuri et al., entitled “Amorphous to crystalline transition of selenium thin films of different thicknesses”

The crystallization kinetics of a 33SrF₂16MgF₂16YF₃35AlF₃ glass for infrared transmission have been determined by calorimetry over the 735 K and 755 K temperature range. In this temperature range, time dependence and activation energy for crystallization are observed to be t^1.50±0.05 and (136±5) kcal/mol, respectively.     

The photo-crystallization of amorphous selenium thin films

The photocrystallization of amorphous selenium under the influence of light or electrons used to produce hole-electron pairs has been studied. Illumination increases the growth rate of crystallites and modifies their morphology. Conversely, electron irradiation alters the structure of the amorphous material and induces a decrease of the nucleation rate. An explanation is proposed, which takes into account recent publications on the ‘band structure’ and the nature of bonds in amorphous selenium.     

Amorphous to crystalline transition and optoelectronic properties of nanocrystalline indium tin oxide (ITO) films sputtered with high rf power at room temperature

ITO thin films were deposited on quartz substrates by the rf sputtering technique using various rf power keeping the substrates at room temperature. The influence of rf power on the structural, electrical, optical and morphological properties was studied by varying the rf power in the range 50–350 W. X-ray diffraction results show an amorphous – crystalline transition with nano grains. At a power of 250 W, the ITO film showed preferential orientation along (4 0 0) peak. It is observed from the optical transmission studies that the optical band gap increased from 3.57 to 3.69 eV when the rf power was increased from 50 to 250 W. The resistivity value is minimum and grain size is maximum for the ITO film deposited at 250 W. The X-ray photoelectron spectroscopy (XPS), Energy dispersive X-ray (EDX) and Atomic force microscopy AFM results confirm that the ITO films are stoichiometric and the surface contained nano-sized grains distributed uniformly all over the surface. It can be concluded that the ITO film deposited at room temperature with 250 W rf power, can provide the required optical and electrical properties useful for developing optoelectronic devices at lower temperatures.     

Low temperature plasma deposition of silicon thin films: From amorphous to crystalline

We report on the epitaxial growth of crystalline silicon films on (100) oriented crystalline silicon substrates by standard plasma enhanced chemical vapor deposition at 175 °C. Such unexpected epitaxial growth is discussed in the context of deposition processes of silicon thin films, based on silicon radicals and nanocrystals. Our results are supported by previous studies on plasma synthesis of silicon nanocrystals and point toward silicon nanocrystals being the most plausible building blocks for such epitaxial growth. The results lay the basis of a new approach for the obtaining of crystalline silicon thin films and open the path for transferring those epitaxial layers from c-Si wafers to low cost foreign substrates.     

Electrical properties of amorphous and crystalline InSb and InAs thin films

Amorphous films of indium antimonide (0.02–0.26 μm) and indium arsenide (0.03–0.3 μm) were formed on goldseal glass, freshly cleaved mica and NaCl substrates by using a “flash evaporation” technique. The post-deposition heat treatment was carried out on these films when the amorphous → crystalline transformation was observed. The transformation was characterized by a sudden and large fall in the resistance of the film at a particular temperature depending on the thickness. This transformation was confirmed by transmission electron micrographs and diffraction patterns obtained on the films before and after heat treatment. The transformation temperatures lie between 495–525 K for indium antimonide and 550–575 K for indium aresenide, for the thickness range involved in our investigations. The electrical conductivity measurements showed a temperature dependent activation in the high temperature region and hopping conduction in the low temperature region (Mott's theory). The activation energies, at different temperatures for various thicknesses were calculated and presented. While no Hall mobility could be observed in as-deposited films, very low mobilities were observed in annealed thick films (t > 2000 Å). Thermoelectric power for InSb films was found to vary from 0.075–0.17 mV/K for films of thickness ranging from 1000–2300 Å, whereas for InAs films, its value varied from 0.09–0.27 mV/K for the thickness range, 1250–2500 Å. These measurements indicated the conductivity to be n-type and supported the hopping conduction mechanism observed in low temperature conductivity measurements.     

Photoelectrical properties of crystalline titanium dioxide thin films after thermo‐annealing

This paper reports the photoelectrical properties of sol gel derived titanium dioxide (TiO₂) thin films annealed at different temperatures (425‐900°C). The structure of the as‐grown film was found to be amorphous and it transforms to crystalline upon annealing. The trap levels are studied by thermally stimulated current (TSC) measurements. A single trap level with activation energy of 1.5 eV was identified. The steady state and transient photocurrent was measured and the results are discussed on the basis of structural transformation. The photocurrent was found to be maximum for the films annealed at 425°C and further it decreases with annealing at higher temperatures. The photoconduction parameters such as carrier lifetime, lifetime decay constant and photosensitivity were calculated and the results are discussed as a function of annealing temperature. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Glass transition temperature dependence on heating rate and on ageing for amorphous selenium films

The study, by differential thermal analysis (DTA) of the glass transition of thermally evaporated amorphous selenium, allows the effect of ageing on structural properties to be observed. An enthalpic diagram is used to explain the shift of the glass transition region, with heating rate, and also with ageing. These shifts are directly dependent on structural relaxation in amorphous selenium. The model used, to express the change of the structural time relaxation with temperature, and with the departure of thermodynamic equilibrium leads to a slightly different expression for the dependence of T_g on heating rate than that previously published (by Moynihan et al.).The activation enthalpies of the studied phenomena compared with other results, seem to show that structural relaxation in a-Se principally involves bond breaking and reformation of chains.     

Free-standing crystalline CaCO3 films composed of three-dimensional microstructures with different morphologies

Crystalline vaterite CaCO₃ self-standing films composed of three-dimensional microstructures were synthesized by a simple gas diffuse method with the assistance of poly(vinyl alcohol) (PVA). The microstructures with different morphology (sphere-like, rose-like, and pumpkin-like) were self-assembled by three-dimensional oriented attachment of faceted micro-platelets, and the films were all stable in air for more than one year. The concentrations of Ca²⁺ ions and PVA all played important roles in the formation of vaterite crystal phase. In addition, the introduction of salt (NaCl, NaNO₃) would reduce the size of the micro-platelets significantly. A possible mechanism for the formation of the self-standing films is put forward.     

Effect of isothermal annealing and visible-light illumination on the AC-impedance behavior of undoped selenium thin films

The effect of post-deposition isothermal annealing (30 °C ? T_A ? 70 °C) and visible-light illumination on the complex AC-impedance of undoped selenium thin films deposited at the substrate temperatures T_S = 30, 50, 70 °C has been studied in the frequency range 0.2–12 kHz. The AC-impedance of amorphous selenium (a-Se) films (T_S, T_A < 50 °C) was mainly capacitive, with no loss peaks being observed in their Z″(ω)–ω curves, irrespective of illumination. This behavior was ascribed to a dominant charge-carrier trapping effect of bulk/surface charged defects usually present in a-Se. On the other hand, the measured Z″(ω)–Z′(ω) diagrams of illuminated polycrystalline Se samples (50 °C ? T_S, T_A ? 70 °C) exhibited almost full semicircles, whereas their Z″(ω)–ω curves revealed prominent loss peaks at well-defined frequencies. As the annealing temperature or light intensity is increased the loci of the points determined by intersections of these semicircles with the Z′-axis at the low-frequency side shift greatly towards the origin, while the loss-peak positions shift to higher frequencies. These experimental findings were explained in terms of a significant increase in electrical conductivity of selenium films due to thermally-induced crystallization at temperatures beyond glass-transformation region of undoped selenium and to creation of electron–hole pairs by visible-light illumination.     

High-resolution XRD study of stress-modulated YBCO films with various thicknesses

High-quality epitaxial YBa₂Cu₃O_7−δ (YBCO) superconducting films with thicknesses between 0.2 and 2 μm were fabricated on (0 0 l) LaAlO₃ with direct-current sputtering method. The influence of film thickness on the structure and texture was investigated by X-ray diffraction conventional θ–2θ scan and high-resolution reciprocal space mapping (HR-RSM). The films grew with strictly c-axis epitaxial, and no a-axis-oriented growth was observed up to a thickness of 2 μm. Lattice parameters of the YBCO films with different thicknesses were extracted from symmetry and asymmetry HR-RSMs. The X-ray lattice parameter method was used to determine the residual stress in YBCO films by measuring the a-, b-, c-axis strains, respectively. The results showed that YBCO films within thinner than 1 μm were under compressive stress, which was relieved increasing of film thickness. However, beyond 1 μm in thickness, YBCO films exhibited a tensile stress. Based on the experimental analysis, the variety of residual stresses in the films is mainly attributed to oxygen vacancies with thickness of YBCO film increasing.     

Photoluminescence in silicon rich oxide thin films under different thermal treatments

Photoluminescence (PL) was studied in silicon rich oxide (with the atomic percentage ranges of Si from 35% to 75%) thin film samples, fabricated by the plasma assisted CVD technique. A broad PL peak, blue-shifted from the bulk silicon band edge of ～1.1 eV, was observed. In one typical sample, the PL peak intensity shows a non-monotonic temperature dependence. This non-monotonic dependence was also observed in previous work by others and attributed to an energy splitting between the excitonic singlet and triplet levels in silicon nanocrystals, a consequence of quantum confinement effect. Finally, in more than 20 samples under different thermal treatments (with the annealing temperature range from 800 °C to 1100 °C), the wavelength of PL peak was observed to be pinned between ～900 and ～1000 nm, independent of thermal budget. This pinning effect, we believe, is probably due to the formation of oxygen-related interface states.     

Determination of size distribution and the activation energy of growth of the crystalline particle in TiNi thin films

Ti-49 at.% Ni thin films prepared by magnetron sputtering were annealed at 733-793 K for several time periods. Using small angle X-ray scattering, we have investigated the growth behavior of the crystalline particles in the TiNi films. We have obtained the size distribution of the samples annealed at 753 and 793 K for several time periods. The activation energy of growth of the crystalline particle was obtained.     

A solvothermal route to tellurium based thin films

Tellurium thin film was prepared via a solvothermal route on glass substrates modified by exchaning the positive charge with high charge. It was characterized by SEM and XRD. The possible growth of tellurium thin film was proposed. It is expected that this method would be extended to other functional thin films. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structure and optical properties of amorphous silicon oxide thin films with different porosities

Amorphous silicon oxide thin films were prepared by evaporation of a silicon oxide powder. Samples were prepared under ultrahigh vacuum, under a flow of hydrogen ions or under a molecular hydrogen atmosphere. Two others sets of samples were prepared using deuterium instead of hydrogen. These five groups of samples were then annealed to different temperatures up to 950 °C and were exposed to the ambient air. The samples present different densities and microstructures. The sample prepared under ultrahigh vacuum is dense, hydrogen free and OH-bond free. Samples prepared under atomic hydrogen and deuterium flows contain Si–H and Si–D bonds, respectively, and are OH-bond free. The sample prepared under a molecular hydrogen atmosphere is very similar to that prepared under a molecular deuterium atmosphere. Both samples are porous and contain Si–H bonds and OH-groups coming from the exposure to the air. All the samples show visible photoluminescence attributed to isolated silicon clusters. The photoluminescence intensity increases with thermal annealing post-treatments up to an optimal annealing temperature. This maximum value is equal to 650 °C for the unhydrogenated sample and the sample prepared under an atomic hydrogen flow and to 800 °C for the sample prepared under a molecular hydrogen atmosphere. This difference is correlated to the different microstructures of the samples. Moreover the strongest photoluminescence intensity is obtained for the porous sample.     

Structure of anthracene thin films

Structure of anthracene thin films as dependent on the temperature of the substrate, thickness and condensation rate is studied. It was found that at temperatures t_s ≦ 55°C condensation is governed by a vapour crystal mechanism while at temperatures t_s > 55°C it follows as vapour fluid one. Depending on the condensation rate the crystals take pyramidal forms, those of dendrites and spherolites. At temperatures t_s < 55°C the films are mainly oriented with its plane (001) parallel to the substrate.     

Amorphous tin–iron oxide thin films with 3D reticular porous morphology for lithium‐ion batteries

Porous reticular tin–iron oxide composite have been prepared by electrostatic spray deposition method for lithium‐ion batteries. The structure of depsitied electrodes are investigated by X‐ray Diffraction (XRD), which shows that the film is amorphous. Electrochemical characterization by galvanostatic charge‐discharge tests demonstrate that the conversion reactions in these electrodes enable initial discharge capacities of about 1551 mAh/g for tin–iron oxide thin film electrode, and the reversible capacity stayed in the range of 769–601 mAh/g during the successive 30 cycles. The high porosity of the 3D reticular structure can provide more reaction sites on the electrode surface and accommodate volume variation of Sn particles during Li alloying–dealloying. Such a composite film are expected to be applied as attractive anodes for lithium‐ion batteries.     

High intense UV-luminescence of nanocrystalline ZnO thin films prepared by thermal oxidation of ZnS thin films

High quality zinc oxide (ZnO) films were obtained by thermal oxidation of high quality ZnS films. The ZnS films were deposited on a Si substrate by a low-pressure metalorganic chemical vapor deposition technique. X-ray diffraction spectra indicate that high quality ZnO films possessing a polycrystalline hexagonal wurtzite structure with preferred orientation of (0 0 2) were obtained. A fourth order LO Raman scattering was observed in the films. In photoluminescence (PL) measurements, a strong PL with a full-width at half-maximum of 10 nm around 380 nm was obtained for the samples annealed at 900°C at room temperature. The maximum PL intensity ratio of the UV emission to the deep-level emission is 28 at room temperature, providing evidence of the high quality of the nanocrystalline ZnO films.     

Electrical properties of a-Ge-Se-In thin films

Steady state and transient photoconductivity has been measured on Ge₂₀Se_80−xIn_x (x = 0, 5, 10, 15, 20) vacuum evaporated thin films. Study of temperature dependent dark conductivity σ_d and photoconductivity σ_ph measurements in the temperature range 303-375 K, shows that the conduction in this glass is through an activated process having single activation energy. The activation energy value of photoconduction is smaller in comparison to activation energy in dark. The photosensitivity shows a maximum value at 10 at.% of In concentration. This is attributed to the decrease in the density of defect states of Ge-Se alloy with increase of In content. The results of intensity dependent steady state photoconductivity σ_ph follow a power law with intensity (F), i.e. σ_ph α F^γ where the value of power γ lies between 0.5 and 1.0, suggesting bimolecular recombination. Rise and decay of photocurrent for different concentration of In shows that photocurrent rises monotonically to the steady state value and the decay of photocurrent is also very fast. An attempt has been made to explain the results on the basis of defects and density of states.     

Properties of oriented TTF-TCNQ thin films

Vapour deposited thin films of the organic molecular complex tetrathiafulvaliniumtetracyanoquinodimethane (TTF-TCNQ) are investigated by electrical and electronmicroscopical methods. Morphology studies by means of electron micrographs show, that the thin films up to thicknesses of 300 nm consist of small crystals of a size of 1.2 · 0.2 μm² (deposited on cleaved (100) faces of NaCl) or 15 · 2 μm² (on glass substrate). Dependend on deposition conditions and on material of the substrate thin films are produced with strong or statistic orientation of this crystals. Strong oriented thin films exhibit conductivities up to σ = 65 ω⁻¹ cm⁻¹ and activation energies of W_A = 0.02 eV. The found dependences of the conductivity on electrical field strength. temperature, and size of microcrystals are explained by a linear hopping model.