Effect of illumination on hydrogenated amorphous silicon thin films doped with chalcogens

Hydrogenated amorphous silicon thin films doped with chalcogens (Se or S) were prepared by the decomposition of silane (SiH₄) and H₂Se/H₂S gas mixtures in an RF plasma glow discharge on 7059 corning glass at a substrate temperature 230 °C. The illumination measurements were performed on these samples as a function of doping concentration, temperature and optical density. The activation energy varied with doping concentration and is higher in Se-doped than S-doped a-Si:H thin films due to a low defect density. From intensity versus photoconductivity data, it is observed that the addition of Se and S changes the recombination mechanism from monomolecular at low doping concentration films to bimolecular at higher doping levels. The photosensitivity (σ_ph/σ_d) of a-Si, Se:H thin films decreases as the gas ratio H₂Se/SiH₄ increased from 10^?4 to 10^?1, while the photosensitivity of a-Si, S:H thin films increases as the gas ratio H₂S/SiH₄ increased from 6.8 × 10^?7 to 1.0×10^?4.     

Structural change of laser-irradiated Ge2Sb2Te5 films studied by electrical property measurement

Sheet resistance of laser-irradiated Ge₂Sb₂Te₅ thin films prepared by magnetron sputtering was measured by the four-point probe method. With increasing laser power the sheet resistance undergoes an abrupt drop from 10⁷ to 10³ Ω/□ at about 580 mW. The abrupt drop in resistance is due to the structural change from amorphous to crystalline state as revealed by X-ray diffraction (XRD) study of the samples around the abrupt change point. Crystallized dots were also formed in the amorphous Ge₂Sb₂Te₅ films by focused short pulse laser-irradiated, the resistivities at the crystallized dots and the non-crystallized area are 3.375 × 10^?3 and 2.725 Ω m, sheet resistance is 3.37 × 10⁴ and 2.725 × 10⁷ Ω/□ respectively, deduced from the I–V curves that is obtained by conductive atomic force microscope (C-AFM).     

Large grain μc-Si:H films deposited at low temperature: Growth process and electronic properties

We have studied structural and electronic properties of μc-Si:H films deposited from SiH₄ + H₂ and SiH₄ + H₂ + Ar gas mixtures. The use of Ar containing gas mixtures for depositions allows us to increase deposition rate by a factor of two and to obtain films with an important fraction of large grains in comparison with SiH₄ + H₂ gas mixtures. Electronic properties of fully crystallized films become more intrinsic with the increase of large grain fraction. Deposition of highly p- and n-doped μc-Si:H layers from the dopant/SiH₄ + H₂ gas mixture at a temperature of 175 °C is possible without any remarkable changes in crystallinity in comparison with undoped films deposited with the same discharge conditions.     

Nanocrystalline silicon TFT process using silane diluted in argon–hydrogen mixtures

We have investigated the effect of Ar dilution on the deposition process of intrinsic nc-Si:H (hydrogenated nanocrystalline silicon) thin films used as active layers of top-gate TFTs, in order to improve the TFTs performances. The nc-Si:H films were deposited by plasma enhanced chemical vapor deposition (PECVD) at low temperature (165 °C) and the related TFTs were fabricated with a maximum process temperature of 200 °C. During the nc-Si:H films deposition, the SiH₄ fraction and the total flow of the diluting gases Ar + H₂ mixture was kept constant, H₂ being substituted by Ar. We have pointed out the active role played by the metastable states of excited Ar atoms in both the dissociation of SiH₄ and H₂ by quenching reactions in the plasma. The role of the atomic hydrogen during the film deposition seems to be promoted by the addition of argon into the discharge, leading to an increase of the deposition rate by a factor of about three and an enhancement of the crystalline quality of the nc-Si:H films. This effect is maximized when the Ar fraction in the Ar + H₂ gases mixture reaches 50%. The evolution with Ar addition of the carriers mobility of the related TFTs is closely connected to the evolution of the crystalline fraction of the intrinsic nc-Si:H film. Mobilities values as high as 8 cm² V^?1 s^?1 are obtained at the Ar fraction of 50%. For higher Ar fractions, the fall of the mobility comes with a degradation of the I_D–V_G transfer characteristics of the processed TFTs due to a degradation of the nc-Si:H films quality. OES measurements show that the evolution of the H_α intensity is closely connected to the evolution of the deposition rate, intrinsic films crystalline fraction and TFTs mobility, providing an interesting tool to monitor the TFTs performances.     

Thermodynamic analysis of gas phase chemistry in hot wire chemical vapor deposition of a-Si:H and μc-Si:H

Gas phase reactions amongst filament-generated radicals play a crucial role in growth and properties of films deposited by hot wire chemical vapor deposition (HWCVD) technology. Gas phase species of interest are SiH₄, H₂, Si, H, SiH₃, SiH₂ and SiH. Partial pressures of these species for different sets of deposition conditions have been determined from the standard Gibbs free energy data. Equilibrium concentrations of the film forming precursors have been determined. The effect of the various process parameters on the equilibrium concentration of the precursors has been studied. H, Si and SiH are found to be the dominant species in gas phase above a filament temperature of 2300 K. However SiH₃ and SiH₂ concentration peaks are between 1900 and 2300 K, of the filament temperature.     

Electrical and optical properties of titanium nitride coatings prepared by atmospheric pressure chemical vapor deposition

In order to examine the possibility for TiN coatings to be low-E, TiN coatings were deposited on the glass substrates by atmospheric pressure chemical vapor deposition using titanium tetrachloride (TiCl₄) and ammonia (NH₃) as precursors. X-ray diffraction, sheet resistance measurement, optical transmittance spectroscopy and infrared reflectance spectroscopy were carried out to determine the relationships between the preparation parameters and the microstructure, electrical and optical properties of the coatings. The results showed that the concentration of crystals increased with increasing the substrate temperature and the flow of TiCl₄, resulting in a decrease of the electrical resistivity. The optical transmittance of TiN thin films was strongly dependent on the gas flow and the substrate temperature. Under optimum conditions, continuous polycrystalline TiN coatings with FCC structure were obtained with an electrical conductivity around 34.5 Ω/□, an optical transmittance around 50% in the visible range, and an infrared reflectance higher than 50% above 3000 nm. This indicates that TiN coated glasses may be possible candidates for high IR reflectance windows.     

Self-limiting nature in atomic-layer epitaxy of rutile thin films from TiCl4 and H2O on sapphire (0 0 1) substrates

The atomic-layer epitaxy of rutile thin films on sapphire (0 0 1) substrates was studied in the controlled growth of titanium oxide films by sequential surface chemical reactions using sequentially fast pressurized titanium tetrachloride (TiCl₄) and water (H₂O) vapor pulses. Optical constants and thicknesses of these rutile films were investigated in terms of vapor pressure using a variable-angle spectroscopic ellipsometer. As a result, the self-limiting nature in the atomic-layer epitaxy of rutile thin films was demonstrated clearly under various conditions of dosing reactant vapors, where growth rates were almost constant at approximately 0.077 nm/cycle (0.77 nm/min) and refractive indices were also constant at 2.59.     

Electrodeposited and selenized CIGS thin films for solar cells

Cu(In,Ga)Se₂ polycrystalline thin films were deposited adopting the potentiostatic electrochemical method on Mo/soda lime glass substrate. All the as-deposited Cu(In,Ga)Se₂ thin films were annealed in a selenium atmosphere at 550 °C for 1 h to improve the film crystalline properties. The selenized CIGS thin films were characterized by energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and scanning electron microscopy (SEM).The results indicate Cu(In,Ga)Se₂ thin films have single chalcopyrite structure and the grain size varies from 0.8 to 2.5 μm.     

Dynamics of low temperature PECVD growth of microcrystalline silicon thin films: Impact of substrate surface treatments

Microcrystalline silicon (μc-Si) films have been deposited on polyimide, Corning glass and c-Si(0 0 1) by rf plasma-enhanced chemical vapour deposition (PECVD) using both SiF₄–H₂ and SiH₄–H₂ plasmas. The effect of substrate pre-treatment using SiF₄–He and H₂ plasmas on the nucleation of crystallites is investigated. Real-time laser reflectance interferometry monitoring (LRI) revealed the existence of a ‘crystalline seeding time’ that strongly impacts on the crystallite nucleation, on the structural quality of the substrate/μc-Si interface and on film microstructure. It is found that SiF₄–He pre-treatment of substrates is effective in suppressing porous and amorphous interface layer at the early nucleation stage of crystallites, resulting in direct deposition of μc-Si films also on polyimide at the temperature of 120 °C.     

Toward the fast deposition of highly crystallized microcrystalline silicon films with low defect density for Si thin-film solar cells

In this study, employing a high-density, low-temperature SiH₄–H₂ mixture microwave plasma, we investigate the influence of source gas supply configuration on deposition rate and structural properties of microcrystalline silicon (μc-Si) films, and demonstrate the plasma parameters for fast deposition of highly crystallized μc-Si films with low defect density. A fast deposition rate of 65 Å/s has been achieved for a SiH₄ concentration of 67% diluted in H₂ with a high Raman crystallinity of X_c > 65% and a low defect density of (1–2) × 10¹⁶ cm⁻³ by adjusting source gas supply configuration and plasma conditions. A sufficient supply of deposition precursors, such as SiH₃, as well as atomic hydrogen H on film growing surface is effective for the high-rate synthesis of highly crystallized μc-Si films, for the reduction in defect density, and for the improvement in film homogeneity and compactability. A preliminary result of p–i–n structure μc-Si thin-film solar cells using the resulting μc-Si films as an intrinsic absorption layer is presented.     

Thin film membrane based on a-SiGe: B and MEMS technology for application in cochlear implants

In this work is presented the fabrication of a thin film membrane as a bio-transducer for aural assistance detection, therefore it will operate at low pressure. The resonant membrane was deposited by PECVD technique at low temperature of deposition T = 270 °C, using SiH₄, GeH₄, and Boron gases. The membrane was suspended on a micromachined crystalline silicon frame obtained by wet chemical etching. The a-SiGe:B film presented a resistivity of 2.46 × 10³ (Ω-cm), resistance of 20.8 kΩ. Using these experimental data we succeeded in designing a simple structure for sensing low pressure variations. The output voltage of the sensor was measured for a range of pressure from 0 to 3000 Pa and at bias voltage of 10 V.     

Fabrication and characterization of transparent conductive ZnO:Al thin films prepared by direct current magnetron sputtering with highly conductive ZnO(ZnAl2O4) ceramic target

Using a highly conductive ZnO(ZnAl₂O₄) ceramic target, c-axis-oriented transparent conductive ZnO:Al₂O₃ (ZAO) thin films were prepared on glass sheet substrates by direct current planar magnetron sputtering. The structural, electrical and optical properties of the films (deposited at different temperatures and annealed at 400°C in vacuum) were characterized with several techniques. The experimental results show that the electrical resistivity of films deposited at 320°C is 2.67×10⁻⁴ Ω cm and can be further reduced to as low as 1.5×10⁻⁴ Ω cm by annealing at 400°C for 2 h in a vacuum pressure of 10⁻⁵ Torr. ZAO thin films deposited at room temperature have flaky crystallites with an average grain size of ∼100 nm; however those deposited at 320°C have tetrahedron grains with an average grain size of ∼150 nm. By increasing the deposition temperature or the post-deposition vacuum annealing, the carrier concentration of ZAO thin films increases, and the absorption edge in the transmission spectra shifts toward the shorter wavelength side (blue shift).     

Deposition of microcrystalline silicon films at ultrafast rate by using a new microwave induced plasma source

Synthesis of microcrystalline silicon (μc-Si) film at an ultrafast deposition rate over 100 nm/s is achieved from SiH₄ + He by using a high density microwave plasma source even without employing H₂ dilution and substrate heating techniques. Systematic deposition studies show that high SiH₄ flow rate and working pressure increase film deposition rate while high He flow rate decreases the rate. On the other hand, crystallinity of deposited Si film decreases with increasing SiH₄ or He flow rate and working pressure. Enhancements of gas phase and surface reactions during film deposition process are responsible for the achievement of high deposition rate and high film crystallinity.     

The effect of the underlying substrate on crystallization kinetics of TiNi thin films

The crystallization kinetics of amorphous thin TiNi films deposited on SiO₂ (or NaCl)/Al foils substrates were investigated. A dramatic acceleration of the crystallization rate was observed for amorphous attached-substrate films. The acceleration originated from the presence of the thin film/middle-wafer interface which served as a two-dimensional nucleus for the growth of the crystalline phase. In the process of non-isothermal annealing by DSC, apparent activation energies for two kinds of underlying thin TiNi films were determined to be 352.96 and 403.69 kJ/mol, respectively, which was lower than those free-standing films studied in previous works. For the process of isothermal annealing, the crystallization kinetics parameters had remarked drop, reflected from the lower Avrami exponent n (the range of 1.35–2.11) and shorter incubation time τ (the range of 0.1–0.4 min) between 758 and 775 K.     

Structural study of undoped and (Mn, In)-doped SnO2 thin films grown by RF sputtering

Undoped and 5%(Mn, In)-doped SnO₂ thin films were deposited on Si(1 0 0) and Al₂O₃ (R-cut) by RF magnetron sputtering at different deposition power, sputtering gas mixture and substrate temperature. X-ray reflectivity was used to determine the films thickness (10–130 nm) and roughness (～1 nm). The combination of X-ray diffraction and Mössbauer techniques evidenced the presence of Sn⁴⁺ in an amorphous environment, for as-grown films obtained at low power and temperature, and the formation of crystalline SnO₂ for annealed films. As the deposition power, substrate temperature or O₂ proportion are increased, SnO₂ nanocrystals are formed. Epitaxial SnO₂ films are obtained on Al₂O₃ at 550 °C. The amorphous films are quite uniform but a more columnar growth is detected for increasing deposition power. No secondary phases or segregation of dopants were detected.     

Effect of tellurium deposition rate on the properties of Cu–In–Te based thin films and solar cells

To investigate the effects of tellurium (Te) deposition rate on the properties of Cu–In–Te based thin films (Cu/In=0.30–0.31), the films were grown on both bare and Mo-coated soda-lime glass substrates at 200 °C by co-evaporation using a molecular beam epitaxy system. The microstructural properties were examined by means of scanning electron microscopy and X-ray diffraction. The crystalline quality of the films was improved with increase in the deposition rate of Te, and exhibited a single CuIn₃Te₅ phase with a highly preferred (1 1 2) orientation. Te-deficient film (Te/(Cu+In)=1.07) grown with a low Te deposition rate showed a narrow bandgap of 0.99 eV at room temperature. The solar cell performance was affected by the deposition rate of Te. The best solar cell fabricated using CuIn₃Te₅ thin films grown with the highest deposition rate of Te (2.6 nm/s) yielded a total area (0.50 cm²) efficiency of 4.4% (V_oc=309 mV, J_sc=28.0 mA/cm², and FF=0.509) without light soaking.     

Structure and non-linear optical properties of BaO–TiO2–SiO2 glass containing Ba2TiSi2O8 crystal

The BaO–TiO₂–SiO₂ crystallized glass containing Ba₂TiSi₂O₈ crystal phase was examined using X-ray diffraction (XRD) measurement and Maker fringe technique. It has been found that the crystallized glasses with a composition of close to the stoichiometric Ba₂TiSi₂O₈ showed bulk crystallization, whereas other crystallized glasses with non-stoichiometric composition of Ba₂TiSi₂O₈ showed surface crystallization. The parameter ΔT, the difference in temperature between the crystallization onset and the glass transition temperature, strongly affects the crystallization behaviors of the present BTS glasses. The 30BaO–20TiO₂–50SiO₂ transparent surface crystallized glass with strong c-oriented Ba₂TiSi₂O₈ phase showed its second-order non-linear optical constant, d₃₃ = 6.1 pm/V.     

Morphology and crystal structure of A1-doped TiO2 nanoparticles synthesized by vapor phase oxidation of titanium tetrachloride

Al-doped titanium dioxide nanoparticles with precisely controlled characteristics were synthesized in an aerosol reactor between 900 °C and 1500 °C by vapor-phase oxidation of titanium tetrachloride. The effect of process variables (reactor temperature, initial TiCl₄ concentration, residence time and feeding temperature of oxygen) on particle morphology and phase characteristics was investigated using TEM, XRD, EDS, ICP and XPS, etc. The average particle size increased with decreasing oxygen feeding temperature and increasing reaction temperature, residence time and TiCl₄ concentration. The presence of aluminum during gas phase reaction increased the rate of phase transformation from anatase to rutile and altered the particle morphology from polyhedral to irregular crystals. TiO₂ and Al₂O₃ co-precipitated during particle formation which lead to the aluminum solid solution in titania. α-Al₂O₃ and Al₂TiO₅ were observed at AlCl₃/TiCl₄ ratios higher than 1.1 and reactor temperatures in excess of 1400 °C. The rutile content, which increased with increasing Al/Ti ratio and residence time, was at a maximum at about 1200 °C and decreased at both lower and higher reactor temperatures.     

Crystallization of bismuth titanate and bismuth silicate grown as thin films by atomic layer deposition

Bismuth silicate and bismuth titanate thin films were deposited by atomic layer deposition (ALD). A novel approach with pulsing of two Bi-precursors was studied to control the Si/Bi atomic ratio in bismuth silicate thin films. The crystallization of compounds formed in the Bi₂O₃–SiO₂ and Bi₂O₃–TiO₂ systems was investigated. Control of the stoichiometry of Bi–Si–O thin films was studied when deposited on Si(1 0 0) and crystallization was studied for films on sapphire and MgO-, ZrO₂- and YSZ-buffered Si(1 0 0). The Bi–Ti–O thin films were deposited on Si(1 0 0) substrate. Both Bi–Si–O and Bi–Ti–O thin films were amorphous after deposition. Highly a-axis oriented Bi₂SiO₅ thin films were obtained when the Bi–Si–O thin films deposited on MgO-buffered Si(1 0 0) were annealed at 800 °C in nitrogen. The full-width half-maximum values for 200 peak were also studied. An excess of bismuth was found to improve the crystallization of Bi–Ti–O thin films and the best crystallinity was observed with Ti/Bi atomic ratio of 0.28 for films annealed at nitrogen at 1000 °C. Roughness of the thin films as well as the concentration depth distribution were also examined.     

Morphology and dispersion state of Ba2TiSi2O8 nanocrystals in transparent glass-ceramics and their nanoindentation behavior

The morphology and dispersion state of Ba₂TiSi₂O₈ (BTS) nanocrystals in transparent glass-ceramics (composition: 40BaO–20TiO₂–40SiO₂) were examined from high resolution transmission electron microscope observations, and their nano-scale deformation behavior was examined using Berkovich nanoindentation technique (standard-type and continuous stiffness measurement (CSM)-type). In the early stage of crystallization, BTS crystalline layers with a thickness of ~ 120 nm were formed at the surface and ellipsoid-shaped crystallites with a diameter of 100–200 nm were dispersed in the glass matrix. In the late stage, BTS crystals with a diameter of 200–400 nm were formed densely, but a glassy phase was present between BTS crystals. The Young's modulus evaluated from CSM-type nanoindentation measurements for a deformation scale of about 100 nm shows the values of 98 GPa for the glass and 110 GPa for the glass-ceramics with nanocrystals. It was suggested that CSM-type measurements are very sensitive in the nano-scaled homogeneity in the heat-treated samples.