Spectroscopic ellipsometry studies of reactively sputtered nitrogen-rich GaAsN films

Nitrogen-rich GaAsN thin films have been deposited at 500 °C by reactive rf sputtering of GaAs target in argon-nitrogen atmosphere. The arsenic content of the films was varied by changing the nitrogen percentage in the sputtering atmosphere and the As/Ga ratio in the films was estimated by X-ray fluorescence measurements. Spectroscopic ellipsometry measurements have been carried out on these films and the measured ellispometric spectra were fitted with theoretical spectra generated by using suitable model sample structures. From the best fit parameters of the dispersion model, band-gap values and variation of refractive index and extinction coefficient as a function of wavelength have been obtained for films deposited with different percentages of nitrogen in the sputtering atmosphere. The films deposited with 12% to 100% nitrogen in the sputtering atmosphere, which are of hexagonal GaN, exhibit GaN-like optical properties, though effects due to excess arsenic in amorphous phase are seen in the films deposited with less than 40% nitrogen. The films deposited with 5% to 12% nitrogen in sputtering atmosphere are dominantly polycrystalline GaAs_xN_1−x (x ≈ 0.01 to 0.08) and exhibit variations in optical parameters, which are consistent with their structure and composition. The films deposited with less than 5% nitrogen in sputtering atmosphere are arsenic-rich and amorphous.     

Photovoltaic properties of sputtered silicon films

The photovoltaic properties of several 1 μm thick films of rf-sputtered amorphous Si (a-Si) sandwiched between lower Al electrodes and upper semitransparent Mo or Al electrodes have been investigated. After fabrication, the Al–Si–Al samples were annealed in vacuum at temperatures between 100 and 450°C. The spectral variation of the photoresponse in the wavelength region between 300 nm and 2 μm was measured. The results indicate that the optical gap of a-Si is 1.5 eV and the Schottky barrier height at the Al–a-Si interface is 0.75 eV. the current-voltage characteristics of these Schottky-type devices were investigated under illumination. Photovoltaic energy conversion efficiencies of up to 0.03% were observed for the 100 mW/cm² white light of a tungsten-halogen lamp.     

Deep-level spectroscopy of reactively sputtered a-Si:H films

We have performed light-induced modulation of absorption spectroscopy, thermally stimulated current spectroscopy, capacitance measurements as a function of temperature and frequence, and electron spin resonance experiments on reactively sputtered a-Si:H films. We find evidence for seven different localized gap states in a range of ±0.4 eV around the midgap region. The possible origin of these states is discussed.     

Characterization of reactive DC magnetron sputtered TiAlN thin films

Thin films of about 1μm Titanium Aluminum Nitride (TiAlN) were deposited onto mild steel substrates by reactive direct current (DC) magnetron sputtering using a target consisting of equal segments of titanium and aluminum. X‐ray diffraction (XRD) analysis showed that the TiAlN phase had preferred orientations along 111 and 200 with the face‐centered cubic structure. Scanning Electron Microscope (SEM) and Atomic Force Microscope (AFM) analyses indicated that the films were uniform and compact. Photoluminescence (PL) spectra reveal that TiAlN thin films are of good optical quality. Laser Raman studies revealed the presence of characteristic peaks of TiAlN at 312.5, 675, and 1187.5 cm^–1. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photo-induced phenomena in sputtered GeO2 films

Macroscopic and microscopic changes in sputtered GeO₂ films induced by band-gap light from an ArF excimer laser have been studied. When irradiated at 1 atm, the film thickness increases, surface-roughness increases, refractive-index decreases, hygroscopic enhancements, and Ge-O-Ge distance increases. Irradiations in vacuum make these changes smaller or undetectable. These photo-induced changes are discussed from phenomenological and structural viewpoints, and compared with characteristics in GeO₂-SiO₂ and GeS₂ films.     

Optical characterization of sputtered amorphous aluminum nitride thin films by spectroscopic ellipsometry

We have measured and analyzed the optical constants and polarized optical properties of amorphous aluminum nitride (a-AlN) thin films deposited by RF reactive magnetron sputtering onto silicon(1 1 1) and glass substrates. The optical constants were obtained by analysis of the measured ellipsometric spectra in the wavelength range 300–1400 nm, using the Cauchy–Urbach model. Refractive indices and extinction coefficients of the films were determined to be in the range 1.80–2.11 and 8.6 × 10⁻³–1.5 × 10⁻⁵, respectively. Analysis of the absorption coefficient, in the wavelength range 200–1400 nm, shows the bandgap of a-AlN thin films to be 5.84 ± 0.05 eV. From the angle dependence of the p-polarized reflectivity we deduce a Brewster angle of 61° and a principal angle of 64°. Measurement of the polarized optical properties reveals a high transmissivity and very low absorptivity for a-AlN films in the visible and near infrared regions. X-ray diffraction analysis confirmed the amorphous nature of the films under study.     

Characteristics of sputtered zinc-oxide films prepared by UBM sputtering for thin film transistors

In this work, we describe the characteristics of zinc-oxide (ZnO) thin films synthesized on Si (001) and Corning (7059) glass substrates by unbalanced magnetron (UBM) sputtering under various ZnO film thicknesses and RF powers. We investigated the characteristics of ZnO films' structural, optical, and electrical properties with various film thicknesses and RF powers for the active channel in a thin film transistor (TFT). We used the UBM sputtering system for high rate deposition of ZnO films. The ZnO film surface was rough and the grain size of the film increased with increasing RF power and film thickness. The electrical properties improved with the increase of RF power and film thickness. These results can be explained by the improvement of the crystallinity in the ZnO film related to the grain size increase with increasing RF power and ZnO film thickness.     

Structural analysis of anodic alumina films

Structures of amorphous anodic alumina films have been investigated by the X-ray radial distrubution function and the correlation method. The amorphous anodic alumina films are considered to contain AlO₅ coordination polyhedra as well as AlO₄ tetrahedra and AlO₆ octahedra. Two types of conformation are taken into account by the correlation method; one (model A) is the conformation in which the above coordination polyhedra are arranged in a similar way to that found in the crystalline modifications of γ-Al₂O₃ within a short range of r < 4.5 Å and the other (model B) is the conformation in which coordination polyhedra are randomly arranged. The final model formed by mixing these two conformations (model A and B) in 1 : 1 ratio describes quantitatively the observed scattering intensity curves of the amorphous anodic alumina films.     

Low-temperature growth of nanocrystalline silicon films prepared by RF magnetron sputtering: Structural and optical studies

In order to contribute to the understanding of the optoelectronics properties of hydrogenated nanocrystalline silicon films, a detailed study has been conducted. Structural analysis (infrared absorption and Raman scattering spectroscopy), combined with optical measurements spectroscopy (optical transmission, photothermal deflection spectroscopy and photoconductivity) were used to characterize the films. The samples were elaborated by radio-frequency magnetron sputtering of crystalline silicon target, under a hydrogen (70%) and Argon (30%) gas mixture, at three different total pressures (2, 3 and 4 Pa) and varying substrate temperature (100, 150 and 200 °C). The results clearly indicate that the films deposited at 2 Pa are amorphous, while for 3 and 4 Pa nanocrystalline structures are observed. These results are discussed in the framework of the existing models.     

Characteristics of sputtered amorphous carbon films prepared by a closed-field unbalanced magnetron sputtering method

We discuss the tribological performance of sputtered amorphous carbon (a-C) films deposited by closed-field unbalanced magnetron (CFUBM) sputtering with a graphite target using a mixture of helium (He) and argon (Ar) as sputtering gases. We investigated the effects of the graphite target power density on the micro-structural and physical properties. In the Raman spectra, the G-peak position moved to the higher wavenumbers. The I_D/I_G ratio increased with the increase of target power density in the fixed DC bias voltage. This was the result of the structural change in the a-C film that resulted with the increase in sp² bonding fraction. Also, the maximum hardness of the a-C film was 23 GPa, the friction coefficient was 0.1, and the critical load was 25.9 N on the Si wafer. In addition, the compressive residual stress of the film increased a little with increasing target power density. Consequently, the various properties of a-C films, with an increase of the target power density, were associated with the increase of cross-linked sp² bonding fraction and the cluster size. The tribological properties of a-C film showed clear dependence on the energy of ion bombardment with the increase of plasma density during film growth.     

Investigation of DTS effect on r.f. magnetron sputtered ZnO thin films

The aim of this study depends on understanding the effect of target‐to‐substrate distance (D_TS) on ZnO thin films deposited by r.f. magnetron sputtering on to glass substrates at room temperature conditions. The D_TS was changed from 35 mm to 65 mm with steps of 5 mm at 165 W and 0.2 Pa. The deposition rate of the films were ranged from 76 Ǻ / min to 146 Ǻ / min, while 10^‐3 Ω.cm was obtained as the resistivity value with the help of four point probe technique. The structural investigations were carried out by using both the x‐ray diffraction (XRD) and high resolution transmission electron microscopy. According to XRD observations, the films were (002) oriented. Surface behaviour of the ZnO films was examined with atomic force microscopy and scanning electron microscopy. The root mean square (RMS) values were varied from 4.6 nm to 22.8 nm. Also, optical properties were obtained from UV–visible spectrophotometer and the transmittances as around 80 %. At 45 mm D_TS value, the minimum resistivity measured as 9 × 10^{− 4} Ω.cm with 76 Ǻ / min deposition rate. The RMS was obtained as 4.9 nm and transmission was measured as 85.30 %, while band gap was 3.45 eV.     

Heat treatment induced structural and optical properties of rf magnetron sputtered tantalum oxide films

Rf magnetron sputtering technique was employed for preparation of tantalum oxide films on quartz and crystalline silicon (111) substrates held at room temperature by sputtering of tantalum in an oxygen partial pressure of 1x10^‐4 mbar. The films were annealed in air for an hour in the temperature range 573 – 993 K. The effect of annealing on the chemical binding configuration, structure and optical absorption of tantalum oxide films was systematically studied. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of hydrogen on the intrinsic stress in ion beam sputtered amorphous silicon films

The results of measurements of the intrinsic stress in hydrogenated amorphous silicon (a-Si:H) films deposited by ion beam sputtering are presented. The data show that the total intrinsic stress is composed of growth stress and hydrogen induced stress. At high hydrogen concentration (> 24%), an abrupt decrease in stress is observed which correlates with a microstructural change as shown by SEM and spectrosoopic ellipsometry measurements. The change in microstructure is accompanied by a modification in the IR transmission spectrum at 2000 cm⁻¹ indicating a change in hydrogen bonding configuration or in local environment near SiH bonds. A model is proposed which reconciles the observation of both tensile and compressive stress in unhydrogenated silicon reported in the literature.     

Optical constants of DC magnetron sputtered titanium dioxide thin films measured by spectroscopic ellipsometry

Optical constants of DC magnetron sputtered TiO₂ thin film have been determined by Spectroscopic Ellipsometry in the photon energy range 1.2 to 5.5 eV at room temperature. The measured dielectric‐function spectra reveal distinct structures at energies of the E1, E1 + Δ1 and E2 critical points are due to interband transitions. The root mean square roughness of the magnetron sputtered TiO₂ thin films evaluated by ex‐situ atomic force microscopy is 5.8 nm. The Dielectric constant values were found to be substantially lower than those for the bulk TiO₂. The dielectric related Optical constants, such as the refractive index, extinction coefficient, absorption coefficient and normal incidence of reflectivity determined from the spectroscopic ellipsometry data are presented and analyzed. The optical constants of the films were also determined using the optical transmittance measurements and the results were discussed. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural relaxation of Al-W amorphous thin films

The pronounced variation of the electrical resistivity of the amorphous Al-W thin films observed during initial heating above room temperature was examined. Both isochronal and isothermal treatments were performed in order to investigate the effects of the film composition, substrate material, and substrate temperature, on the magnitude of the relaxation phenomena. Regarding the isochronal heating, it was observed that the relaxation effects decreased with an increase of the heating rate, and decreased with the aluminum content in the film. The Al₇₈W₂₂ amorphous thin films were subjected to isothermal annealing for 6 h at a temperature of 515 °C. The effects of the substrate material (alumina ceramic, glass and sapphire), and the deposition temperature (LNT, RT, 200 and 400 °C) were examined. The relaxation decreased in a sequence of: alumina ceramic-glass-sapphire substrates, as well as with an increase of the substrate temperature. An assumed dominant role of the aluminum in the effects observed, was tested by the corresponding investigation of Al-Ti and Cu-Ti amorphous thin films.     

Structural studies on fullerenes

Amongst the other physical properties, the structure of fullerene solids, and the structural transformations with temperature, pressure and doping have all evoked considerable interest and during the last two years tremendous progress has been made in this area. This paper provides a brief review of the studies on the structural properties of pristine and doped fullerenes. The results of our own investigations are also presented.     

Defect and stress relaxation in HVPE-GaN films using high temperature reactively sputtered AlN buffer

The influence of high temperature buffer layers on the structural characteristics of GaN grown by hydride vapour phase epitaxy on sapphire was investigated. Strain relaxation as well as mismatch-induced defect reduction in thick GaN layers grown on AlN buffer was microscopically identified using cathodoluminescence and micro-Raman spectroscopy in cross-section of the films. The results were correlated with photoluminescence and Hall-effect data of layers with different thicknesses. These relaxation processes were suggested to account for the specific defect distribution in the buffers revealed by high-resolution X-ray diffraction and transmission electron microscopy.     

Electrical properties of r.f. sputtered thin films of CuInSe2(I)

Thin films of CuInSe₂ are deposited by r.f. sputtering onto glass substrates at different r.f. voltages and substrate temperatures using pressed and powder targets. The films are polycrystalline and p-type conducting. The temperature dependence of the conductivity and the chemical composition of the films are measured as a function of the deposition conditions.     

Structural phase transitions in heteroepitaxial ferroelectric films

The changes in the structural—deformational characteristics occurring during the phase transitions in the crystal lattice of epitaxially grown thin (Ba,Sr)TiO₃ films on the (001) cleavage of MgO crystals have been studied. It is shown that microdeformations along the surface normal of the wall of a c-domain film increase with the temperature and attain maximum values in the vicinity of the phase transition. No decrease in the dimensions of the coherent-scattering regions was observed. The comparison of the deformation of the “average” lattice and microdeformations led to the assumption that the transition from the paraelectric to the ferroelectric phase is accompanied by the dislocation-induced formation of a dipole-glass-type intermediate phase at the temperatures exceeding that of the phase transition.