Hydrogen sensing properties of Pd-doped SnO2 sputtered films with columnar nanostructures

Pd-doped SnO₂ sputtered films with columnar nanostructures were deposited using reactive magnetron sputtering at the substrate temperature of 300 °C and the discharge gas pressures of 1.5, 12, and 24 Pa. Structural characterization by means of X-ray diffraction and scanning electron microscopy shows that the films composed of columnar nanograins have a tetragonal SnO₂ structure. The films become porous as the discharge gas pressure increases. Gas sensing measurements demonstrate that the films show reversible response to H₂ gas. The sensitivity increases as the discharge gas pressure increases, and the operating temperature at which the sensitivity shows a maximum is lowered. The highest sensitivity defined by (R_a − R_g) / R_g, where R_a and R_g are the resistances before and after exposure to H₂, 84.3 is obtained for the Pd-doped film deposited at 24 Pa and 300 °C upon exposure to 1000 ppm H₂ gas at the operating temperature of 200 °C. The improved gas sensing properties were attributed to the porosity of columnar nanostructures and catalytic activities of Pd doping.     

Comparison of the optical and structural properties of nickel oxide-based thin films obtained by chemical bath and sputtering

Nickel oxide thin films were prepared using chemical bath deposition and reactive magnetron dc-sputtering. Through the chemical route, Ni(OH)₂ films were deposited with a nano-porous structure providing large specific surface area. Subsequent annealing at 300 °C transformed the films into NiO. These films showed high absorption in the visible range and low crystallinity due to Ni vacancies. Annealing at higher temperatures removes Ni vacancies improving transmittance and crystallinity. Sputtered films were obtained in Ar + O₂ and Ar + H₂ + O₂ atmospheres at different flux ratios. During deposition in the former atmosphere, substrate temperature was 300 °C producing dense polycrystalline films with excellent optical properties. In the hydrogen containing atmosphere, the substrate was at room temperature and polycrystalline films with a dark-yellowish color and expanded lattice were obtained.     

Preparation of nanocrystalline nickel oxide thin films by sol–gel process for hydrogen sensor applications

Preparation of nanocrystalline NiO thin films by sol–gel method and their hydrogen (H₂) sensing properties were investigated. The thin films of NiO were successfully deposited on the glass and SiO₂/Si substrate by a sol–gel coating method. The films were characterized for crystallinity, electrical properties, surface topography and optical properties as a function of calcination temperature and substrate material. It was found that the films produced by this method were polycrystalline and phase pure NiO. The H₂ gas sensitivity of these films was studied as a function of H₂ concentration and calcination temperature. The results indicated that the sol–gel derived NiO films could be used for the fabrication of H₂ gas sensors to monitor low concentration of H₂ in air quantitatively at low temperature range (< 200 °C).     

Atomic layer deposition of iron(III) oxide on zirconia nanoparticles in a fluidized bed reactor using ferrocene and oxygen

Conformal films of amorphous iron(III) oxide and α-Fe₂O₃ have been coated on zirconia nanoparticles (26 nm) in a fluidized bed reactor by atomic layer deposition. Ferrocene and oxygen were alternately dosed into the reactor at temperatures between 367 °C and 534 °C. Self-limiting chemistry was observed via in situ mass spectrometry, and by means of induced coupled plasma-atomic emission spectroscopy analysis. Film conformality and uniformity were verified by high resolution transmission electron microscopy, and the growth rate was determined to be 0.15 Å per cycle. Energy dispersive spectroscopy, X-ray diffractometry, and X-ray photoelectron spectroscopy were utilized as a means to determine film composition at each deposition temperature. Over all of the deposition temperatures investigated, films were deposited as amorphous iron(III) oxide. However, after heat treatment at 850 °C in air and N₂ atmospheres, α-Fe₂O₃ was the predominant species.     

Nanoporous TiO2 thin film based conductometric H2 sensor

Nanoporous titanium dioxide (TiO₂) based conductometric sensors have been fabricated and their sensitivity to hydrogen (H₂) gas has been investigated. A filtered cathodic vacuum arc (FCVA) system was used to deposit ultra-smooth Ti thin films on a transducer having patterned inter-digital gold electrodes (IDTs). Nanoporous TiO₂ films were obtained by anodization of the titanium (Ti) thin films using a neutral 0.5% (wt) NH₄F in ethylene glycol solution at 5 V for 1 h. After anodization, the films were annealed at 600 °C for 8 h to convert the remaining Ti into TiO₂. The scanning electron microscopy (SEM) images revealed that the average diameters of the nanopores are in the range of 20 to 25 nm. The sensor was exposed to different concentrations of H₂ in synthetic air at operating temperatures between 100 °C and 300 °C. The sensor responded with a highest sensitivity of 1.24 to 1% of H₂ gas at 225 °C.     

Influence of effective surface area on gas sensing properties of WO3 sputtered thin films

WO₃ thin films having different effective surface areas were deposited under various discharge gas pressures at room temperature by using reactive magnetron sputtering. The microstructure of WO₃ thin films was investigated by X-ray diffraction, scanning electron microscopy, and by the measurement of physical adsorption isotherms. The effective surface area and pore volume of WO₃ thin films increase with increasing discharge gas pressure from 0.4 to 12 Pa. Gas sensors based on WO₃ thin films show reversible response to NO₂ gas and H₂ gas at an operating temperature of 50-300 °C. The peak sensitivity is found at 200 °C for NO₂ gas and the peak sensitivity appears at 300 °C for H₂ gas. For both kinds of detected gases, the sensor sensitivity increases linearly with an increase of effective surface area of WO₃ thin films. The results demonstrate the importance of achieving high effective surface area on improving the gas sensing performance.     

Wide optical bandgap p-type μc-Si:Ox:H prepared by Cat-CVD and comparisons to p-type μc-Si:H

Oxygen-impurity boron-doped hydrogenated microcrystalline silicon (p-μc-Si:O_x:H) films have been deposited using catalytic chemical vapor deposition (Cat-CVD). Pure silane (SiH₄), hydrogen (H₂), oxygen (O₂), and diluted diborane (B₂H₆) gases were used. The tungsten catalyst temperature (T_fil) was varied from 1900 to 2100 °C and films were deposited on glass substrates at temperatures of 100 to 300 °C. Different catalyst-to-substrate distances were employed and single- or double-coiled filaments were used. In addition to p-μc-Si:O_x:H deposition, we have also deposited conventional p-type microcrystalline silicon (p-μc-Si:H) in order to compare their electrical and optical properties to p-μc-Si:O_x:H.     

Thermally-induced optical property changes of sputtered PdOx films

Optical property changes of reactively sputtered palladium oxide (PdO_x) thin films during heating have been investigated by light transmission measurements and in situ Raman spectroscopy, combined with X-ray fluorescence and thermogravimetry analysis (TGA). The composition ratio and refractive index of as-deposited PdO_x films varies depending on the oxygen gas-flow ratio during sputtering deposition. The transmitted light intensity at wavelengths of 405 and 635 nm measured during heating up to 1000 °C in air exhibits a sharp leap at 832 °C due to the thermal decomposition of PdO, and shows a gradual increase around 200-300 °C. The decomposition process of PdO is also measured by TGA as a significant weight loss of the sample. In situ Raman spectra of sputtered PdO_x film obtained during heating up to 600 °C in air demonstrate that the crystallization of PdO starts above 200 °C and progresses during the whole annealing process.     

Ferroelectric properties of Bi3.4Dy0.6Ti3O12 thin films crystallized in N2

Bi_3.4Dy_0.6Ti₃O₁₂ (BDT) ferroelectric thin films were deposited on Pt/Ti/SiO2/Si substrates by chemical solution deposition (CSD) and annealed in an N₂ environment after pre-annealing in air at 400 °C. The effect of crystallization temperature on the structural and electrical properties of the BDT films was studied. The BDT films annealed in N₂ in the temperature range of 600 °C to 750 °C were crystallized well and the average grain size increased with increasing crystallization temperature, while the remanent polarization of the films is not a monotonic function of the crystallization temperature. The BDT films crystallized at 650 °C have the largest remanent polarization value of 2P_r = 39.4 μC/cm², and a fatigue-free characteristic.     

Properties of atomic layer deposited HfO2 thin films

The growth, composition and morphology of HfO₂ films that have been deposited by atomic layer deposition (ALD) are examined in this article. The films are deposited using two different ALD chemistries: i) tetrakis ethylmethyl amino hafnium and H₂O at 250° and ii) tetrakis dimethyl amino hafnium and H₂O at 275 °C. The growth rates are 1.2 Å/cycle and 1.0 Å/cycle respectively. The main impurities detected both by X-ray Photoelectron Spectroscopy and Fourier transform infrared spectroscopy (FTIR) are bonded carbon (~ 3 at.%) and both bulk and terminal OH species that are partially desorbed after high temperature inert anneals up to 900 °C. Atomic Force Microscopy reveals increasing surface roughness as a function of increasing film thickness. X-ray diffraction shows that the morphology of the as-deposited films is thickness dependent; films with thickness around 30 nm for both processes are amorphous while ~ 70 nm films show the existence of crystallites. These results are correlated with FTIR measurements in the far IR region where the HfO₂ peaks are found to provide an easy and reliable technique for the determination of the crystallinity of relatively thick HfO₂ films. The index of refraction for all films is very close to that for bulk crystalline HfO₂.     

Deposition of WNxCy thin films for diffusion barrier application using the dimethylhydrazido (2) tungsten complex (CH3CN)Cl4W(NNMe2)

Tungsten nitride carbide (WN_xC_y) thin films were deposited by chemical vapor deposition using the dimethylhydrazido (2⁻) tungsten complex (CH₃CN)Cl₄W(NNMe₂) (1) in benzonitrile with H₂ as a co-reactant in the temperature range 300 to 700 °C. Films were characterized using X-ray diffraction (XRD), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy and four-point probe to determine film crystallinity, composition, atomic bonding, and electrical resistivity, respectively. The lowest temperature at which growth was observed from 1 was 300 °C. For deposition between 300 and 650 °C, AES measurements indicated the presence of W, C, N, and O in the deposited film. The films deposited below 550 °C were amorphous, while those deposited at and above 550 °C were nano-crystalline (average grain size < 70 Å). The films exhibited their lowest resistivity of 840 µΩ-cm for deposition at 300 °C. WN_xC_y films were tested for diffusion barrier quality by sputter coating the film with Cu, annealing the Cu/WN_xC_y/Si stack in vacuum, and performing AES depth profile and XRD measurement to detect evidence of copper diffusion. Films deposited at 350 and 400 °C (50 and 60 nm thickness, respectively) were able to prevent bulk Cu transport after vacuum annealing at 500 °C for 30 min.     

New transparent conductive films: FTO coated ITO

New transparent conductive films, fluorine doped tin oxide (FTO) films coated on indium-tin-oxide (ITO) films, were developed. These transparent conductive films were prepared by the spray pyrolysis deposition method at a substrate temperature of 350 °C in ITO and 400 °C in FTO. For ITO deposition, an ethanol solution of indium(III) chloride, InCl₃·4H₂O, and tin(II) chloride, SnCl₂·2H₂O [Sn/(In+Sn), 5 at.%] was sprayed on a Corning #7059 glass substrate (100×100×1.1 mm³). After the deposition, FTO films were consecutively deposited for protecting oxidation of ITO films. FTO deposition was carried out by an ethanol solution of tin(IV) chloride, SnCl₄·5H₂O within the saturated water solution of NH₄F. These new transparent conductive films achieved the lowest resistivity of 1.4×10⁻⁴ Ω cm and the optical transmittance of more than 80% in the visible range of the spectrum. The electrical resistance of these new transparent conductive films increased by less than 10% even when exposed to high temperatures of 300-600 °C for 1 h in the air.     

Influence of thickness on H2 gas sensor properties in polycrystalline SnO x films prepared by ion-beam sputtering

Amorphous SnO _x films were deposited on sintered alumina substrates by ion-beam sputtering. They were annealed at 500° C for 2 h in air and polycrystalline films with thickness varying from about 1 to 700 nm were prepared. Film-sensor properties against 0.47% H₂ gas were measured as a function of thickness and the operating temperature for 150 to 350° C. The film thickness exhibiting a sensitivity maximum increased gradually with temperature. The optimum thickness shifted from 7 nm at 150° C to 175 nm at 350° C. Highly sensitive films lay in a narrow thickness range of 60 to 180 nm and films thinner or thicker than this were relatively insensitive at 300 and 350°C. A model was proposed to interpret the sensitivity behaviour in terms of thickness and grain-boundary effect.     

ZnO thin films prepared by a single step sol-gel process

ZnO thin films were prepared on fused silica from a single spin-coating deposition of a sol-gel prepared with anhydrous zinc acetate [Zn(C₂H₃O₂)₂], monoethanolamine [H₂NC₂H₄OH ] and isopropanol. Crystallization annealing was performed over the range 500 to 650 °C. X-ray analysis showed that thin films were preferentially orientated along the [002] c-axis direction of the crystal. The films had a transparency of greater than 85% in the visible region for sol-gels with a zinc content of up to 0.7 M and exhibited absorption edges at ∼ 378 nm. The optical band-gap energy was evaluated to be 3.298-3.306 eV. Photoluminescence showed a strong emission centered at ca. 380 nm along with a broad yellow-orange emission centered at ca. 610 nm. Single step sol-gel thin film deposition in the film thickness range from 80 nm to 350 nm was demonstrated. The effect of sol-gel zinc concentration, film thickness and crystallization temperature on film microstructure, morphology and optical transparency is detailed. A process window for single spin coating deposition of c-axis oriented ZnO discussed.     

Stability and effect of annealing on the optical properties of plasma-deposited Ta2O5 and Nb2O5 films

Tantalum and niobium oxide optical thin films were prepared at room temperature by plasma-enhanced chemical vapor deposition using tantalum and niobium pentaethoxide (M(OC₂H₅)₅) precursors. We studied the evolution of their optical and microstructural properties as a result of annealing over a broad temperature range from room temperature up to 900 °C. The as-deposited films were amorphous; their refractive index, n, and extinction coefficient, k, at 550 nm were n = 2.13 and k < 10^− 4 for Ta₂O₅, and n = 2.24 and k < 10^− 4 for Nb₂O₅. The films contained a small amount of residual carbon (∼ 2-6 at.%) bonded mostly to oxygen. During annealing, the onset of crystallization was observed at approximately T_C1 = 650 °C for Ta₂O₅ and at T_C1 = 450 °C for Nb₂O₅. Upon annealing close to T₁ (300 °C for Nb₂O₅ and 400 °C for Ta₂O₅), n at 550 nm decreased by less than 1%. This was correlated with the decrease of carbon content, as suggested by Fourier transform infrared spectroscopy, elastic recoil detection and static secondary ion mass spectroscopy (SIMS) results. During annealing, we observed phase transition from the δ- (hexagonal) phase to the L- (orthorhombic) phase between 800 °C and 900 °C for Ta₂O₅, and between 600 °C and 700 °C for Nb₂O₅. The structural changes were also marked by silicon diffusion from the substrate into the oxide layer at annealing temperatures above 500 °C for Ta₂O₅ and above 400 °C for Nb₂O₅. As a consequence of oxygen, silicon and metal interdiffusion, the interface between the Si substrate and the metal oxide (Ta₂O₅ or Nb₂O₅) is characterized by its broadening, well documented by spectroscopic ellipsometry and SIMS data.     

Properties of nanocrystalline zinc oxide thin films prepared by thermal decomposition of electrodeposited zinc peroxide

Zinc peroxide thin films were electrodeposited from aqueous solution at room temperature using H₂O₂ as the oxidation agent. Nanocrystalline zinc oxide thin films were then obtained from thermal decomposition of zinc peroxide thin films. The grain sizes of ZnO through thermal decomposition of ZnO₂ at 200 °C, 300 °C and 400 °C were estimated from the peak width of ZnO(110) obtained from X-ray diffraction and were 6.3 nm, 9.1 nm and 12.9 nm, respectively. The optical properties of zinc oxide thin films have been studied. The photoluminescence results indicate that ZnO thin films have low Stokes blue shift (about 110 meV) and low oxygen vacancies.     

Pulsed laser deposition of crystalline ZrC thin films

ZrC thin films were grown on Si substrates by the pulsed laser deposition (PLD) technique under various conditions. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), X-ray diffraction and reflectivity, spectroscopic ellipsometry, and four point probe measurements were used to characterize the properties of the deposited films. It has been found that crystalline films could be grown only by using laser fluences higher than 5 J/cm² and substrate temperatures in excess of 500 °C. For a fluence of 10 J/cm² and a substrate temperature of 700 °C, cubic ZrC films (a = 0.469 nm) exhibiting a (200)-texture were deposited under vacuum or low pressure C₂H₂ atmosphere. These films were smooth, with surface roughness values below 1.0 nm and mass densities around the tabulated value of 6.7 g/cm³. AES depth profiling investigations showed oxygen contamination around 7% in the bulk region. Despite the relatively high levels of oxygen contamination, the deposited ZrC films were very conductive. The use of a low C₂H₂ pressure atmosphere during deposition had a small beneficial effect on crystallinity and stoichiometry of the films.     

Fabrication of TiO2-based transparent conducting oxide on glass and polyimide substrates

We report on preparation and properties of anatase Nb-doped TiO₂ transparent conducting oxide films on glass and polyimide substrates. Amorphous Ti_0.96Nb_0.04O₂ films were deposited at room temperature by using sputtering, and were then crystallized through annealing under reducing atmosphere. Use of a seed layer substantially improved the crystallinity and resistivity (ρ) of the films. We attained ρ = 9.2 × 10^− 4 Ω cm and transmittance of ~ 70% in the visible region on glass by annealing at 300 °C in vacuum. The minimum ρ of 7.0 × 10^− 4 Ω cm was obtained by 400 °C annealing in pure H₂.     

Effects of deposition temperature on structure and properties of mo-doped indium oxide by radio frequency magnetron sputtering

Transparent conductive oxide (TCO) thin films of Mo-doped In₂O₃ (IMO) were prepared on glass substrates by radio frequency magnetron sputtering from the 2 wt% Mo-doped In₂O₃ ceramic target. The depositions were carried out under an oxygen-argon atmosphere by varying the deposition temperature from 200 °C to 350 °C. The crystal structure and thickness of IMO thin films were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The effects of deposition temperature on the electrical and optical transmittance properties of IMO thin films were investigated by four-point probe Hall system and UV-VIS-NIR spectrophotometer separately. The optimum deposited IMO thin films were obtained with resistivity of 6.9 × 10⁻⁴ Ω cm and carrier mobility 45 cm²v⁻¹s⁻¹ at 350 °C. The average optical transmittance of IMO films on glass substrates are over 80% in the near-infrared region.     

Synthesis,characterization and gas sensing performance of SnO<Subscript>2</Subscript> thin films prepared by spray pyrolysis

In this work, SnO₂ thin films were deposited onto alumina substrates at 350°C by spray pyrolysis technique. The films were studied after annealing in air at temperatures 550°C, 750°C and 950°C for 30 min. The films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and optical absorption spectroscopy technique. The grain size was observed to increase with the increase in annealing temperature. Absorbance spectra were taken to examine the optical properties and bandgap energy was observed to decrease with the increase in annealing temperature. These films were tested in various gases at different operating temperatures ranging from 50–450°C. The film showed maximum sensitivity to H ₂S gas. The H₂S sensing properties of the SnO₂ films were investigated with different annealing temperatures and H ₂S gas concentrations. It was found that the annealing temperature significantly affects the sensitivity of the SnO₂ to the H ₂S. The sensitivity was found to be maximum for the film annealed at temperature 950°C at an operating temperature of 100°C. The quick response and fast recovery are the main features of this film. The effect of annealing temperature on the optical, structural, morphological and gas sensing properties of the films were studied and discussed.