Changes in the structural and electrical properties of vacuum post-annealed tungsten- and titanium-doped indium oxide films deposited by radio frequency magnetron sputtering

Tungsten- and titanium-doped indium oxide (IWO and ITiO) films were deposited at room temperature by radio frequency (RF) magnetron sputtering, and vacuum post-annealing was used to improve the electron mobility. With increasing deposition power, the as deposited films showed an increasingly crystalline nature. Compared with ITiO films, IWO films showed crystallinity at lower RF power. IWO films are partially crystallized at 10 W deposition power and become nearly fully crystalline at 20 W. ITiO films are fully crystalline only at 75 W. For this reason, film thickness has a greater impact on the electrical properties of IWO films than ITiO films. Vacuum post-annealing is more effective in improving electron mobility for amorphous than for (partially) crystalline IWO and ITiO films. Changes in the electrical properties of ITiO films can be better controlled as a function of annealing temperature than those of IWO films. Finally, post annealed 308 nm-thick IWO and 325 nm-thick ITiO films have approximately 80% transmittance in visible and near infrared wavelengths (up to 1100 nm), while their sheet resistances decrease to 9.3 and 10 Ω/□, and their electron mobilities are 51 cm²V^− 1 s^− 1 and 50 cm²V^− 1 s^− 1, respectively, making them suitable for use as Transparent Conductive Oxide layers of low bandgap solar cells.     

Ultrathin Al-doped transparent conducting zinc oxide films fabricated by pulsed laser deposition

Al-doped transparent conducting zinc oxide (AZO) films, approximately 20-110 nm-thick, were deposited on glass substrates at substrate temperatures between 200 and 300 °C by pulsed laser deposition (PLD) using an ArF excimer laser (λ = 193 nm). When fabricated at a substrate temperature of 260 °C, a 40-nm-thick AZO film showed a low resistivity of 2.61 × 10^− 4 Ω·cm, carrier concentration of 8.64 × 10²⁰ cm^− 3, and Hall mobility of 27.7 cm²/V·s. Furthermore, for an ultrathin 20-nm-thick film, a resistivity of 3.91 × 10^− 4 Ω·cm, carrier concentration of 7.14 × 10²⁰ cm^− 3, and Hall mobility of 22.4 cm²/V·s were obtained. X-ray diffraction (XRD) spectra, obtained by the θ-2θ method, of the AZO films grown at a substrate temperature of 260 °C showed that the diffraction peak of the ZnO (0002) plane increased as the film thickness increased from 20 to 110 nm. The full-width-at-half-maximum (FWHM) values were 0.5500°, 0.3845°, and 0.2979° for film thicknesses of 20, 40, and 110 nm, respectively. For these films, the values of the average transmittance in visible light wavelengths (400-700 nm) were 95.1%, 94.2%, and 96.6%, respectively. Field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM) observations showed that even the 20-nm-thick films did not show island structures. In addition, exfoliated areas or vacant and void spaces were not observed for any of the films.     

On the annealing temperature, penetration depth of oxygen and film thickness on the DC and AC electrical properties and nano-structure of Ti thin films

Ti films of different thickness ranging from 12.3 to 246.2 nm were deposited, using resistive heat method and post-annealed at different temperatures with a flow of 5 cm³ s⁻¹ oxygen. The nano-structures of the films were obtained using X-ray diffraction (XRD) and atomic force microscopy (AFM). The results showed an initial reduction of the grain size at 373 K annealing temperature and increase of the grain size at higher temperatures. The cause of this was due to the reaction of oxygen with Ti atoms which breaks up the Ti grains and hence needle-like features form. The enhancement of activation processes at higher temperatures results in larger grains. The analysis of XRD in conjunction with AFM images showed that those films containing (004) line of anatase phase and sub-oxide phases of titanium oxide also show two types of grains in the AFM images. The resistivity of the film increased with annealing temperature, which is due to competition between increased diffusion rate and the increased reaction rate of oxygen with Ti atoms. The Hall coefficient R_H and the mobility μ decreased with increasing film thickness at all annealing temperatures, while R_H increases and μ decreases with increasing the annealing temperature. The carrier concentration increased with film thickness and decreased with annealing temperature. The impedance spectroscopy showed that all films have a pure RC behaviour, where the magnitude of R depends on the annealing temperature and film thickness. The apparent activation energies E_a, obtained from three different methods, namely σ, R_H and grain size showed good agreement within 0.30-0.46 eV for the range of film thickness examined in this work. It was found that films with thickness less than 70 nm can be recognized as Ti-oxide films while thicker films are only surface-oxidised Ti films.     

Combinatorial growth and analysis of Ti-In-Zn-O films deposited by radio‐frequency and direct-current magnetron co-sputtering system

The compositional dependence of co-sputtered Ti-In-Zn-O film properties was investigated by means of a combinatorial technique. The X-ray diffraction result showed that the amorphous Ti-In-Zn-O films were fabricated regardless of the Ti contents [Ti / (Ti + In + Zn), at.%] of 4.5-34.4 at.%. The surface of amorphous Ti-In-Zn-O film is quite smooth. The obtained surface roughness (R_RMS) values ranged from 0.5 nm to 1.7 nm. The superior resistivity of 3.8 × 10^− 4 Ω cm and the transmittance of 92% (at 550 nm) was obtained for the Ti-In-Zn-O film with the elemental composition ratio of 18.6/68.5/12.9 at.% [Ti/In/Zn, at.%]. The indium quantity actually could be reduced to as high as ~ 15 at.% compared to that of commercial indium tin oxide or indium zinc oxide having similar resistivity value of ~ 10^− 4 Ω cm. Overall, the amorphous Ti-In-Zn-O films may serve as a viable, low-cost alternative for flexible transparent conducting electrode applications.     

Deterioration and recovery in the resistivity of Al-doped ZnO films prepared by the plasma sputtering

A hot-cathode plasma sputtering technique was used for fabricating the highly transparent and conducting aluminum-doped zinc oxide (AZO) films on glass substrates from a disk-shaped AZO (Al₂O₃: 2 wt.%) target. Under particular conditions where the target voltage was V_T = −200 V and the plasma excitation pressure was P_S = 1.5 × 10⁻³ Torr, the lowest resistivity of 4.2 × 10⁻⁴ Ω cm was obtained at 400 nm, and this was associated with a carrier density of 8.7 × 10²⁰ cm⁻³ and a Hall mobility of 17 cm²/V s. From the annealing experiment of the AZO films in the oxygen and nitrogen gases of the atmospheric pressure it was revealed that both the oxygen vacancies and the grain boundaries in the polycrystalline AZO film played an important role in the electrical properties of the film.     

Zinc cadmium oxide thin film transistors fabricated at room temperature

Zinc cadmium oxide (ZnCdO) transparent thin film transistors (TFTs) have been fabricated with a back-gate structure using highly p-type Si (001) substrate. For the active channel, 30 nm, 50 nm, and 100 nm thick ZnCdO thin films were grown by pulsed laser deposition. The ZnCdO thin films were wurtzite hexagonal structure with preferred growth along the (002) direction. All the samples were found to be highly transparent with an average transmission of about 80%~ in the visible range. We have investigated the change of the performance of ZnCdO TFTs as the thickness of the active layer is increased. The carrier concentration of ZnCdO thin films has been confirmed to be increased from 10¹⁶ to 10¹⁹ cm⁻³ as the film thickness increased from 30 to 100 nm. Base on this result, the ZnCdO TFTs show a thickness-dependent performance which is ascribed to the carrier concentration in the active layer. The ZnCdO TFT with 30 nm active layer showed good off-current characteristic of below ~ 10¹¹, threshold voltage of 4.69 V, a subthreshold swing of 4.2 V/decade, mobility of 0.17 cm²/V s, and on-to-off current ratios of 3.37 × 10⁴.     

Structural and optical properties of Ti-doped ZnO thin films prepared by the cathodic vacuum arc technique with different annealing processes

Highly transparent Ti-doped ZnO thin films were prepared on glass substrates at a deposition rate of approximately 33 nm/min using the cathodic vacuum arc technique with a Zn target power of 550 W and a Ti target power of 750 W, respectively. X-ray diffraction measurements have shown that the Ti-doped ZnO thin film with a vacuum post-annealing condition is c-axis oriented but an amorphous phase at the other post-annealing atmosphere and as-deposited condition. Transmittance measurements show that the best optical quality of the Ti-doped ZnO thin films occurred at a post-annealing atmosphere of N₂/H₂ mixed gases. Additionally, the optical transmittance of all films has been found more than 85% in a range of 500-700 nm. The lowest electrical resistivity was 3.48 × 10⁻³ Ω cm, obtained on as-deposited films. However, the post-annealing condition greatly increased the resistivity.     

Fabrication of transparent conductive films with a sandwich structure composed of ITO/Cu/ITO

New transparent conductive films that had a sandwich structure composed of ITO/Cu/ITO multilayer films were prepared by a conventional RF and DC magnetron sputtering process on a polycarbonate substrate without intentional substrate heating. The thickness of each layer in the ITO/Cu/ITO films was kept constant at 50 nm/5 nm/45 nm. The optoelectrical and structural properties of the films were compared with conventional ITO single-layer films and ITO/Cu/ITO multilayered films. Although both films had identical thickness, 100 nm, the ITO/Cu/ITO films showed a lower resistivity, 3.5 × 10⁻⁴ Ω cm. In optical transmittance measurements, however, the ITO single-layer films showed a higher transmittance of 74% in the wavelength range of 300-800 nm. XRD spectra showed that both the ITO and ITO/Cu/ITO films were amorphous. The figure of merit, φ_TC, reached a maximum of 5.2 × 10⁻⁴ Ω⁻¹ for the ITO/Cu/ITO films, which was higher than the φ_TC of the ITO films (1.6 × 10⁻⁴ Ω⁻¹). The φ_TC results suggested that ITO/Cu/ITO films had better optoelectrical properties than conventional ITO single-layer films.     

Characterization of ultra-thin TiO2 films grown on Mo(112)

Ultra-thin TiO₂ films were grown on a Mo(112) substrate by stepwise vapor depositing of Ti onto the sample surface followed by oxidation at 850 K. X-ray photoelectron spectroscopy showed that the Ti 2p peak position shifts from lower to higher binding energy with an increase in the Ti coverage from sub- to multilayer. The Ti 2p peak of a TiO₂ film with more than a monolayer coverage can be resolved into two peaks, one at 458.1 eV corresponding to the first layer, where Ti atoms bind to the substrate Mo atoms through Ti-O-Mo linkages, and a second feature at 458.8 eV corresponding to multilayer TiO₂ where the Ti atoms are connected via Ti-O-Ti linkages. Based on these assignments, the single Ti 2p_3/2 peak at 455.75 eV observed for the Mo(112)-(8 × 2)-TiO_x monolayer film can be assigned to Ti³⁺, consistent with our previous results obtained with high-resolution electron energy loss spectroscopy.     

Titanium diffusion in gold thin films

In the present study, diffusion phenomena in titanium/gold (Ti/Au) thin films occurring at temperatures ranging between 200 and 400 °C are investigated.The motivation is twofold: the first objective is to characterize Ti diffusion into Au layer as an effect of different heat-treatments. The second goal is to prove that the implementation of a thin titanium nitride (TiN) layer between Ti and Au can remarkably reduce Ti diffusion.It is observed that Ti atoms can fully diffuse through polycrystalline Au thin films (260 nm thick) already at temperatures as a low as 250 °C. Starting from secondary ion mass spectroscopy data, the overall diffusion activation energy ΔE = 0.66 eV and the corresponding pre-exponential factor D₀ = 5 × 10^− 11 cm²/s are determined. As for the grain boundary diffusivity, both the activation energy range 0.54 < ΔE_gb < 0.66 eV and the pre-exponential factor s₀D_gb0 = 1.14 × 10^− 8 cm²/s are obtained. Finally, it is observed that the insertion of a thin TiN layer (40 nm) between gold and titanium acts as an effective diffusion barrier up to 400 °C.     

Microstructure and tribological performance of CNx–TiNx composite films prepared by pulsed laser deposition

CN_x–TiN_x composite films were prepared on high-speed steel (HSS) substrate by pulsed KrF excimer laser co-deposition process with graphite/Ti combined targets and a substrate temperature of 200 °C. The composition, morphology and microstructure of the films were characterized by energy dispersive X-ray spectrum (EDS), scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM). The adhesion and tribological performance of the films were investigated using a conventional scratch tester and a ball-on-disk tribometer, respectively. In the graphite/Ti range of 0.5–2.0 of the target, TiN_x, a-CN_x and metallic Ti phase were found in the composite films. The TiN_x disappeared in the films at a high graphite/Ti ratio of the target. With increasing the graphite/Ti ratio of the target, the adhesion to substrate of the composite films deteriorated from 46 N to 26 N, and the friction coefficient decreased from 0.23 to 0.17. The composite film deposited at the graphite/Ti ratio of 1.0 showed a low friction coefficient, good adhesion and wear rate of 3.2 × 10⁻⁷ mm³/Nm in humid air.     

Electrical properties of quaternary HfAlTiO thin films grown by atomic layer deposition

Quaternary alloyed HfAlTiO thin (~ 4-5 nm) films in the wide range of Ti content have been grown on Si substrates by Atomic Layer Deposition technique, and the effect of both the film composition and the interfacial reactions on the electrical properties of HfAlTiO films is investigated. It is shown that depending on the Ti content, the permittivity and the leakage current density I_leak in HfAlTiO films vary in the range k = 18 ÷ 28 and 0.01-2.4 A cm^− 2, respectively. The incorporation of ultra thin SiN interlayer in Al/HfAlTiO/SiN/Si stack gives rise to the sharp (× 10³) decrease of the I_leak ~ 6 · 10^− 5 A/cm² at the expense of the rather low capacitance equivalent thickness ~ 0.9 nm.     

Surface textured molybdenum doped zinc oxide thin films prepared for thin film solar cells using pulsed direct current magnetron sputtering

In this study, we examined the effect of etching on the electrical properties, transmittance, and scattering of visible light in molybdenum doped zinc oxide, ZnO:Mo (MZO) thin films prepared by pulsed direct current magnetron sputtering. We used two different etching solutions - KOH and HCl - to alter the surface texture of the MZO thin film so that it could trap light. The experimental results showed that an MZO film with a minimum resistivity of about 8.9 × 10^− 4 Ω cm and visible light transitivity of greater than 80% can be obtained without heating at a Mo content of 1.77 wt.%, sputtering power of 100 W, working pressure of 0.4 Pa, pulsed frequency of 10 kHz, and film thickness of 500 nm. To consider the effect of resistivity and optical diffuse transmittance, we performed etching of an 800 nm thick MZO thin film with 0.5 wt.% HCl for 3-6 s at 300 K. Consequently, we obtained a resistivity of 1.74-2.75 × 10^− 3 Ω cm, total transmittance at visible light of 67%-73%, diffuse transmittance at visible light of 25.1%-28.4%, haze value of 0.34-0.42, and thin film surface crater diameters of 220-350 nm.     

Effect of density and thickness on H2-gas sensing property of sputtered SnO2 films

Undoped and Pd-doped SnO₂ films were deposited under various conditions for the investigation of the effect of Pd doping, porosity, and thickness on their H₂ gas sensing properties. The temperature of the substrate and the pressure of the discharge gas were varied. All films formed were composed of columns with thicknesses between 20 and 30 nm. The film density decreased as the discharge gas pressure increased and the substrate temperature decreased. It showed values between 4.2×10³ and 7.0×10³ kg/m³ depending on the deposition condition. Low film density and Pd doping resulted in high sensitivity and fast response. The largest sensitivity was observed for a Pd-doped film with a low density of 4.7×10³ kg/m³ and a thickness of 20 nm.     

Effect of Ag doping on the performance of ZnO thin film transistor

Ag-doped zinc oxide (SZO) thin film transistors (TFTs) have been fabricated using a back-gate structure on thermally oxidized and heavily doped p-Si (100) substrate. The SZO thin films were deposited via pulsed laser deposition (PLD) from a 1, 3, and 5 wt.% Ag-doped ZnO (1SZO, 3SZO, and 5SZO, respectively) target using a KrF excimer laser (λ, 248 nm) at oxygen pressure of 350 mTorr. The deposition carried out at both room-temperature (RT) and 200 °C. The SZO thin films had polycrystalline phase with preferred growth direction of (002) as well as a wurtzite hexagonal structure. Compare with ZnO thin films, the SZO thin films were characterized by confirming the shift of (002) peak to investigate the substitution of Ag dopants for Zn sites. The as-grown SZO TFTs deposited at RT and 200 °C showed insulator characteristics. However the SZO TFTs annealed at 500 °C showed good n-type TFT performance because Ag was diffused from Zn lattice site and bound themselves at the high temperature, and it caused generation of electron carriers. The post-annealed 5SZO TFT deposited at 500 °C exhibited a threshold voltage (V_th) of 11.5 V, a subthreshold swing (SS) of 2.59 V/decade, an acceptable mobility (μ_SAT) of 0.874 cm²/V s, and on-to-off current ratios (I_on/off) of 1.44 × 10⁸.     

Metal-semiconductor transition in Nb-doped ZnO thin films prepared by pulsed laser deposition

The structural, electrical and optical properties of Nb-doped ZnO films were investigated with different Nb contents (0, 0.15, 0.31, 0.46, 0.62, and 0.94 at.%) in this article. The film with 0.46 at.% Nb content showed the lowest resistivity of 8.95 × 10^− 4 Ω cm and high transmittance about 80% with high c-axis orientation. The undoped ZnO film showed a semiconducting behavior. And Nb-doped ZnO films showed a metal-semiconductor transition (MST), which was connected with localization of degenerate electrons. The films showed metallic conductivity at temperatures closer to the ambient temperature and semiconducting behavior at lower temperatures. It was noted that the NZO films with much lower Nb concentration of 0.15 at.% presented MST compared with other transparent conducting oxides films.     

Application of plasma polymerized siloxane films for the corrosion protection of titanium alloy

Organosilicon film and SiO_x-like film are deposited on titanium alloy (Ti6Al4V) surfaces by atmospheric pressure (~ 10⁵ Pa) dielectric barrier discharge to improve its corrosion resistance in Hanks solution. Hexamethyldisiloxane (HMDSO) is used to be the chemical precursor. The organosilicon film deposited in Ar/HMDSO system has high growth rate (75 nm/min) and low surface roughness (3 nm), while the SiO_x-like film deposited in Ar/O₂/HMDSO system has lower growth rate (35 nm/min) and slightly higher surface roughness (9 nm). The potentiodynamic polarization tests show that both the two siloxane films coated Ti6Al4V samples have more positive corrosion potential and one order of magnitude lower corrosion current density than the substrate, indicating the corrosion resistance of Ti6Al4V can be improved by depositing siloxane film on its surface. In particular, as the surface is more compact and cross-linked, the SiO_x-like film has better corrosion resistance than the organosilicon film.     

Titanium-doped indium oxide films prepared by d.c. magnetron sputtering using ceramic target

Polycrystalline thin films of Ti-doped indium oxide (indium–titanium-oxide, ITiO) were prepared by d.c. magnetron sputtering and their electrical and optical properties were investigated. Doping of Ti was effective in improvement of the electroconductivity of the indium oxide: the electrical resistivity of 1.7 × 10⁻³ Ω cm of non-doping decreased to minimum value of 1.8 × 10⁻⁴ Ω cm at 2.4 at.% Ti-doping when the films were deposited at 300 °C. The polycrystalline ITiO films of 0.8–1.6 at. % Ti-doping showed the high Hall mobilitiy (82–90 cm² V⁻¹ s⁻¹) and the relatively low carrier density (2.4–3.5 × 10²⁰ cm⁻³) resulting in characteristics of both low resistivity (2.1–3.0 × 10⁻⁴ Ω cm) and high transmittance in the near-infrared region (over 80% at 1550 nm), which cannot be shown in the conventional Sn-doped indium oxide (ITO) films.     

Ohmic contacts and n-type doping on TixCr2 − xO3 films and the temperature dependence of their transport properties

A procedure to dope n-type Cr_2 − xTi_xO₃ thin films is proposed. Besides doping the material, at the same time the method forms ohmic contacts on Ti_xCr_2 − xO₃ films. It consists on the deposition of 10 nm Ti and 50 nm Au, followed by thermal annealing at 1000 °C for 20 min in N₂ atmosphere. Ohmic contacts were formed on three samples with different composition: x = 0.17, 0.41 and 1.07 in a van der Pauw geometry for Hall effect measurements. These measurements are done between 35 K and 373 K. All samples showed n-type nature, with a charge carrier density (n) on the order of 10²⁰ cm^− 3, decreasing as x increased. As a function of temperature, n shows a minimum around 150 K, while the mobilities have an almost constant value of 11, 28 and 7 cm²V^− 1 s^− 1 for x = 0.17, 0.41 and 1.07, respectively.     

Effect of tungsten on the electrochromic behaviour of sol-gel dip coated molybdenum oxide thin films

The paper describes the results obtained on the performance of Mo oxide and mixed W/Mo oxide thin films for possible electrochromic applications. Mo and W/Mo oxide films were deposited on conductive (FTO) glass substrates using sol-gel dip coating method. The films were annealed at 250 °C for 30 min. The structure and morphology of Mo and W/Mo oxide films were examined using XRD, SEM and EDS. XRD results indicate the amorphous nature of the Mo and W/Mo oxide films annealed for 30 min. The CV measurements revealed that the films prepared with 10 wt.% of tungsten exhibit maximum anodic/cathodic diffusion coefficient of 24.99/12.71 × 10⁻¹¹ cm²/s. The same film exhibits a maximum transmittance variation (ΔT%) of 83.4% at 630 nm and 81.06% at 550 nm with the optical density of 1.00 and 1.13 respectively.