Improving the conductance of ZnO thin film doping with Ti by using a cathodic vacuum arc deposition process

The Ti-doped ZnO films compared to un-doped ZnO films were deposited onto Corning XG glass substrates by using a cathodic vacuum arc deposition process in a mixture of oxygen and argon gases. The structural, electrical and optical properties of un-doped and Ti-doped ZnO films have been investigated. When the Ti target power is about 750 W, the incorporation of titanium atoms into zinc oxide films is obviously effective. Additionally, the resistivity of un-doped ZnO films is high and reduces to a value of 3.48 × 10⁻³ Ω-cm when Ti is incorporated. The Ti doped in the ZnO films gave rise to the improvement of the conductivity of the films obviously. The Ti-doped ZnO films have > 85% transmittance in a range of 400-700 nm.     

Rapid thermal annealed Al-doped ZnO film for a UV detector

A low-resistive Al-doped ZnO (AZO) film was achieved by rapid thermal annealing. A co-sputtering method was used in the initial growth of AZO films and a rapid annealing process was performed on the as-deposited AZO film under N₂ atmosphere for 3 min. An as-deposited AZO film had an optical transmittance of 84.78% at 550 nm and a resistivity of 7.8 × 10^− 3 Ω cm. A rapid annealing process significantly improved the optical transmittance and electrical resistivity of the AZO film to 99.67% and 1 × 10^− 3 Ω cm, respectively. The structural changes of the AZO films were investigated by X-ray diffraction and transmission electron microscopy. The high quality AZO film was used to fabricate a metal-semiconductor-metal (MSM) structure for a UV detector. The MSM device provided a stable current of 25 μA at a bias of 2 V in a dark condition. Under UV illumination, the MSM device was highly responsive to UV light uniformly and repeatedly, and it enhanced the current by 80% at 45 μA. This rapid thermal annealing process may provide a useful method to fabricate quality AZO films for photoelectric applications with a low thermal budget.     

Influences of post-annealing on structural and electrical properties of Bi1.5Zn1.0Nb1.5O7 thin films prepared by pulsed laser deposition

Bi_1.5Zn_1.0Nb_1.5O₇ (BZN) thin films were prepared on Pt/TiO₂/SiO₂/Si(100) substrates at 650 °C under an oxygen pressure of 10 Pa by using pulsed laser deposition process. The crystallinity, microstructure and electrical properties of BZN thin films were investigated to verify the influences of post-annealing thermal process on them. The X-ray diffractometer (XRD) results indicate that all Bi_1.5Zn_1.0Nb_1.5O₇ thin films without post-annealing process or with post-annealing in situ vacuum chamber and in oxygen ambient exhibit a cubic pyrochlore structure. The improved crystallinity of BZN thin films through post-annealing was confirmed by XRD and scanning electron microscope (SEM) analysis. Dielectric constant and loss tangent of the as-deposited BZN thin films are 160 and 0.002 at 10 kHz, respectively. After annealing, dielectric properties of thin films are significantly improved. Dielectric constant and loss tangent of the in situ annealed films are 181 and 0.0005 at 10 kHz, respectively. But the films post-annealed in O₂ oven show the largest dielectric constant of 202 and the lowest loss tangent of 0.0002, which may attribute to the increase in grain size and the elimination of oxygen vacancies. Compared with the as-deposited BZN thin films, the post-annealed films also show the larger dielectric tunability and the lower leakage current density.     

Physical and sensing properties of ZnO:F:Al thin films deposited by sol-gel

Fluorine and aluminum-doped zinc oxide thin films, ZnO:F:Al, were prepared on soda-lime glass substrates by the sol-gel method and repeated dip-coating. The effect of the solution ageing and film thickness on the physical characteristics of the films was studied. Two ageing times, namely, two and seven days, and three different thicknesses, in the order of 220, 330, and 520 nm, were the main variables used in this work. As-deposited ZnO:F:Al films showed a high electrical resistivity, however after a vacuum thermal treatment, it was registered a significant decrease. Structural, optical, and morphological characterizations were carried out in vacuum-annealed films. The X-ray diffraction (XRD) patterns revealed that both as-deposited and vacuum-annealed ZnO:F:Al thin films were polycrystalline with a hexagonal wurtzite-type structure with a well-defined (002) diffraction peak, irrespective of the ageing time of the starting solution. The (002) peak shows a proportional increase with the thickness magnitude. An average crystallite size of about 20 nm was estimated using the well-known Scherrer's formula. From the surface morphological study it was observed that the grain size is almost independent of the ageing time of the starting solution, and the film thickness. Films presented an average optical transmittance in the visible range (400-700 nm) in the order of 90%, as well as a band gap of 3.3 eV. The gas-sensing properties of ZnO:F:Al thin films in an atmosphere containing different concentrations of carbon monoxide, and at different operation temperatures were probed. The highest sensitivity registered was of the order of 93%.     

Thermoelectric properties of nanostructured Al-substituted ZnO thin films

ZnO:Al nano-polycrystalline thin films were deposited by radio-frequency magnetron sputtering on glass substrates. The analysis of the morphology reveals well-connected whiskers with a preferred c-axis orientation perpendicular to the substrate and a dense columnar grain structure. The as-deposited films exhibited a low electrical resistivity of 1 × 10^− 3 Ω cm. Annealing in air produces an increase of the resistivity by more than three orders of magnitude and an increase in the absolute value of the Seebeck coefficient proportional to the resistivity. Annealing of the as-deposited sample in reducing Ar/H₂ atmosphere leads to a decrease in both the resistivity and the absolute value of the Seebeck coefficient. The change in the electrical transport properties is caused by the absorption and desorption of oxygen. Both resistivity and Seebeck coefficient recover to their initial values during annealing of the air-treated sample in reducing Ar/H₂ atmosphere, indicating a reversible process. The analysis by transmission electron microscopy after annealing reveals a stable columnar grain structure with an increase of the grain size. The increase in grain size is larger when the sample is annealed in reducing rather than in oxidising atmosphere. In summary, the reducing Ar/H₂ atmosphere was found to be advantageous for the thermoelectric properties resulting in a maximum power factor of 0.3 mW/K²m at 800 K.     

The effect of annealing on Al-doped ZnO films deposited by RF magnetron sputtering method for transparent electrodes

In this study, transparent conducting Al-doped zinc oxide (AZO) films with a thickness of 150 nm were prepared on Corning glass substrates by the RF magnetron sputtering with using a ZnO:Al (Al₂O₃: 2 wt.%) target at room temperature. This study investigated the effects of the post-annealing temperature and the annealing ambient on the structural, electrical and optical properties of the AZO films. The films were annealed at temperatures ranging from 300 to 500 °C in steps of 100 °C by using rapid thermal annealing equipment in oxygen. The thicknesses of the films were observed by field emission scanning electron microscopy (FE-SEM); their grain size was calculated from the X-ray diffraction (XRD) spectra using the Scherrer equation. XRD measurements showed the AZO films to be crystallized with strong (002) orientation as substrate temperature increases over 300 °C. Their electrical properties were investigated by using the Hall measurement and their transmittance was measured by UV-vis spectrometry. The AZO film annealed at the 500 °C in oxygen showed an electrical resistivity of 2.24 × 10^− 3 Ω cm and a very high transmittance of 93.5% which were decreased about one order and increased about 9.4%, respectively, compared with as-deposited AZO film.     

Characterization of zinc-tin-oxide films deposited by thermal co-evaporation

The structural, electrical, and optical properties of zinc-tin-oxide (ZTO) films with different compositions prepared by thermal co-evaporation using ZnO and SnO₂ sources were investigated. The as-deposited films were amorphous but opaque. The lowest resistivity, ~ 8 × 10^− 5 Ω cm, was obtained for ZTO with 33 at.% Sn. Upon post-annealing in air, a sharp increase in transparency was observed between 350 and 550 °C, accompanying with a marked decrease in conductivity. This was attributed to re-oxidation of partially reduced oxides, leading to a lower density of oxygen vacancies. Our study showed that conductive and transparent ZTO films with low Sn content may be prepared by co-evaporation deposition, and suitable for use in devices as transparent electrodes.     

Filtered vacuum arc deposition of transparent conducting Al-doped ZnO films

Transparent conducting ZnO:Al and ZnO films of 380-800 nm thickness were deposited on glass substrates by filtered vacuum arc deposition (FVAD), using a cylindrical Zn cathode doped with 5-6 at.% Al or a pure Zn cathode in oxygen background gas with pressure P = 0.4-0.93 Pa. The crystalline structure, composition and electrical and optical properties of the films were studied as functions of P. The films were stored under ambient air conditions and the variation of their resistance as function of storage time was monitored over a period of several months.The Al concentration in the film was found to be 0.006-0.008 at.%, i.e., a few orders of magnitude lower than that in the cathode material. However, this low Al content influenced the film resistivity, ρ, and its stability. The resistivity of as-deposited ZnO:Al films, ρ = (6-8) × 10^− 3 Ω cm, was independent of P and lower by a factor of 2 in comparison to that of the ZnO films deposited by the same FVAD system. The ρ of ZnO films 60 days after deposition increased by a factor of ∼ 7 with respect to as-deposited films. The ZnO:Al films deposited with P = 0.47-0.6 Pa were more stable, their ρ first slowly increased during the storage time (1.1-1.4 times with respect to as-deposited films), and then stabilized after 30-45 days.     

The properties of transparent semiconductor Zn1 − xTixO thin films prepared by the sol-gel method

Transparent semiconductor thin films of Zn_1 − xTi_xO (0 ≦ x ≦ 0.12) were deposited on alkali-free glass substrates by the sol-gel method. The effects of Ti addition on the crystallization, microstructure, optical properties and resistivity of ZnO thin films were investigated. The as-coated films were preheated at 300 °C, and then annealed at 500 °C in air ambiance. X-ray diffraction results showed all polycrystalline Zn_1 − xTi_xO thin films with preferred orientation along the (002) plane. Ti incorporated within the ZnO thin films not only decreased surface roughness but also increased optical transmittance and electrical resistivity. In the present study, the Zn_0.88Ti_0.12O film exhibited the best properties, namely an average transmittance of 91.0% (an increase of ~ 12% over the pure ZnO film) and an RMS roughness value of 1.04 nm.     

ZnO thin films prepared by pulsed laser deposition

Zinc oxide (ZnO) thin films were deposited on soda lime glass substrates by pulsed laser deposition (PLD) in an oxygen-reactive atmosphere. The structural, optical, and electrical properties of the as-prepared thin films were studied in dependence of substrate temperature and oxygen pressure. High quality polycrystalline ZnO films with hexagonal wurtzite structure were deposited at substrate temperatures of 100 and 300 °C. The RMS roughness of the deposited oxide films was found to be in the range 2-9 nm and was only slightly dependent on substrate temperature and oxygen pressure. Electrical measurements indicated a decrease of film resistivity with the increase of substrate temperature and the decrease of oxygen pressure. The ZnO films exhibited high transmittance of 90% and their energy band gap and thickness were in the range 3.26-3.30 eV and 256-627 nm, respectively.     

Effect of annealing on the electrical, optical and structural properties of cadmium stannate thin films prepared by spray pyrolysis technique

Polycrystalline thin films of cadmium stannate (Cd₂SnO₄) were deposited by spray pyrolysis method on the Corning substrates at substrate temperature of 525 °C. Further, the films were annealed at 600 °C in vacuum for 30 min. These films were characterized for their structural, electrical and optical properties. The experimental results showed that the post-deposition annealing in vacuum has a significant influence on the properties of the films. The average grain size of the film was increased from 27.3 to 35.0 nm on heat treatment. The average optical transmittance in the visible region (500-850 nm) is decreased from 81.4% to 73.4% after annealing in vacuum. The minimum resistivity achieved in the present study for the vacuum annealed films is the lowest among the reported values for the Cd₂SnO₄ thin films prepared by spray pyrolysis method.     

Structural and luminescence correlation of annealed Er-ZnO/Si thin films deposited by AACVD process

Er doped ZnO thin films have been synthesized from zinc acetates dihydrate (C₄H₆O₄Zn·2H₂O) and Erbium tris (2,2,6,6-tetramethyl-3,5-heptadionate) (Er(TMHD)₃) by aerosol assisted chemical vapor deposition AACVD atmospheric pressure technique. Films were deposited in the temperature range of [370–500 °C] on Si (1 1 1) substrate. Nano-disk shaped grains were grown on the top of the film surface. The morphology of the as-deposited films was found to be dependent on the substrate temperature. After annealing in air atmosphere, XRD patterns revealed a highly oriented c-axis Er:ZnO films with hexagonal wurtzite structure without any second phase. Under 488 nm excitation, the intra 4f–4f green emission (²H_11/2, ⁴S_3/2 → ⁴I_15/2 transitions) gradually increased with increasing annealing temperature. Also, the local structure of Er changes to a pseudo-octahedral structure with C_4v symmetry. The ZnO film with 2.504 at.% Er³⁺ doping has the best crystalline structure and the best resolved PL spectra. Using 325 nm excitation, all the samples showed an ultraviolet emission centered at 380 nm originating from a near band EDGE emission and a broad band green emission centered at 520 nm from deep levels. The optical response was correlated with crystallinity of the synthesized thin films.     

Characteristics of n-Cd0.9 Zn0.1S/p-CdTe heterojunctions

CdTe thin films were prepared by thermal evaporation under a vacuum of 10⁻⁶ Torr and with a deposition rate of about 60 nm/min. X-ray diffraction studies of the as-deposited films revealed polycrystalline films with cubic structure. The effect of heat treatment with or without CdCl₂ enhances the grain size and improves the crystallinity of the films. Moreover, the activation energy decreases upon heat treatment with or without CdCl₂ for CdTe thin films. The optical spectra of CdTe films show interference oscillations indicating the good optical quality of these films. The calculated energy gap decreases with or without CdCl₂ treatments. The current-voltage and capacitance-voltage characteristics for dark and illuminated three junction cells are measured. By analysing these measurements the different junction parameters are obtained and the effect of CdCl₂ treatment on the performance of the heterojunctions is investigated.     

Effect of high temperature post-annealing of La0.7Sr0.3MnO3 films deposited by radio frequency magnetron sputtering on SiO2/Si substrates heated at low temperature

La_0.7Sr_0.3MnO₃ thin films were deposited on SiO₂/Si substrates by RF magnetron sputtering under different oxygen gas flow rates with a sputtering power of 100 W. During deposition, the substrate was heated at 623 K. To investigate post-annealing effects, the as-deposited La_0.7Sr_0.3MnO₃ thin films were thermal-treated at 973 K for 1 h. The effects of oxygen gas flow rate and post-annealing treatment on the physical properties of the films were systematically studied. X-ray diffraction results show that the growth orientation and crystallinity of the films were greatly affected by the oxygen gas flow rate and substrate heating during deposition. The sheet resistance of the films gradually decreased with increasing oxygen gas flow rate, while the post-annealed films showed the opposite behavior. The temperature coefficient of resistance at 300 K of La_0.7Sr_0.3MnO₃ thin films deposited at an oxygen gas flow rate of 40 sccm decreased from − 2.40%/K to − 1.73%/K after post annealing. The crystalline state of the La_0.7Sr_0.3MnO₃ thin films also affected its electrical properties.     

Influence of hydrogen introduction on structure and properties of ZnO thin films during sputtering and post-annealing

ZnO thin films were prepared in Ar and Ar + H₂ atmospheres by rf magnetron sputtering, and then they were annealed in vacuum and Ar + H₂ atmosphere, respectively. The structure and optical-electrical properties of the films were investigated by X-ray diffraction, transmittance spectra, and resistivity measurement, and their dependences on deposition atmosphere, annealing treatment, and aging were studied. The results showed that adding H₂ in deposition atmosphere improved the crystallinity of the films, decreased lattice constant, increased band gap, decreased the resistivity by the order of 10⁴ Ω cm, but exhibited poor conductive stability with aging. After Ar + H₂ and vacuum annealing, crystallinity of the films deposited in Ar and Ar + H₂ was further improved; their resistivity was decreased by the order of 10⁵ and 10¹ Ω cm, respectively, and exhibited high conductive stability with aging. We suggest that the formed main defect is V_O and H_i when H₂ is introduced during deposition, which decreases the resistivity but cannot improve the conductive stability; hydrogen would remove negatively charged oxygen species near grain boundaries during Ar + H₂ annealing to decrease the resistivity, and grain boundaries are passivated by formation of a number of V_O-H complex (H_O) to improve the conductive stability at the same time. Under vacuum annealing, the hydrogen that is introduced non-intentionally from deposition chamber maybe plays an important role; it exists as H_O in the films to improve the conductive stability of the films.     

Tungsten-doped ZnO transparent conducting films deposited by direct current magnetron sputtering

Highly conducting and transparent thin films of tungsten-doped ZnO (ZnO:W) were prepared on glass substrates by direct current (DC) magnetron sputtering at low temperature. The effect of film thickness on the structural, electrical and optical properties of ZnO:W films was investigated. All the deposited films are polycrystalline with a hexagonal structure and have a preferred orientation along the c-axis perpendicular to the substrate. The electrical resistivity first decreases with film thickness, and then increases with further increase in film thickness. The lowest resistivity achieved was 6.97 × 10⁻⁴ Ω cm for a thickness of 332 nm with a Hall mobility of 6.7 cm² V⁻¹ s⁻¹ and a carrier concentration of 1.35 × 10²¹ cm⁻³. However, the average transmittance of the films does not change much with an increase in film thickness, and all the deposited films show a high transmittance of approximately 90% in the visible range.     

Fabrication of a n-ZnO/p-Si heterojunction diode by ultra-high vacuum magnetron sputtering

Heterojunction diodes of n-type ZnO were fabricated on a p-type Si(100) substrate using an ultra-high vacuum radio frequency magnetron sputtering method at room temperature. A short-time post-annealing process was performed to prevent inter-diffusion of Zn, dopants, and Si atoms. The post-annealing process at 600 °C enhanced the crystallinity of ZnO films and produced a high forward to reverse current ratio of the heterojunction diode with a barrier height of approximately 0.336 eV. A thin SiO_x layer at the interface of the ZnO film and Si substrate appeared distinctly at the 600 °C annealing, however the post-annealing at 700 °C showed an a-(Zn_2xSi_1 − xO₂) structure caused by diffusion of silicon into the ZnO film. In the n-ZnO/p-Si sample annealed at 700 °C, a rapid change in the barrier height was considered due to the effect of the dopant segregation from the substrate and deformation of the a-SiO_x structure.     

Effect of thermal annealing treatment on structural, electrical and optical properties of transparent sol-gel ZnO thin films

Effect of thermal annealing in different ambients on the structural, electrical and optical properties of the sol-gel derived ZnO thin films are studied. XRD results show that the annealed ZnO films with wurtzite structure are randomly oriented. Crystallite size, carrier concentration, resistivity and mobility are found to be dependent on the annealing temperature. The change in carrier concentration is discussed with respect to the removal of adsorbed oxygen from the grain boundaries. The highest carrier concentration and lowest resistivity are 8 × 10¹⁸ cm⁻³ and 2.25 × 10⁻¹ Ω cm, respectively, for the film annealed at 500 °C in vacuum. The annealed films are highly transparent with average transmission exceeding 80% in the wavelength region of 400-800 nm. In all three ambients, the optical band gap value does not change much below 500 °C temperature while above this temperature band gap value decreases for nitrogen and air and increases for vacuum.     

Annealing effect on the structural, mechanical and electrical properties of titanium-doped diamond-like carbon films

Titanium-doped diamond-like carbon (Ti-doped DLC) films with a Ti content of 1.1 at.% were synthesized on a Si substrate by a process that involves filtered cathodic vacuum arc (FCVA) and metal vapor vacuum arc (MeVVA) systems. The effect of annealing temperature on the microstructure, surface roughness, hardness and electrical resistivity of the resulting films was evaluated in this study. The Raman spectra revealed that the degree of graphitization of the Ti-doped DLC thin films was increased from 25 to 600 °C and the microstructure of the films is converted to a nano-crystalline graphite structure. The resulting films maintain a smooth surface after the annealing process. The hardness of the Ti-doped DLC films increases as the annealing temperature increases up to 400 °C because the induced defects and the inter-atomic bonds are repaired after the annealing process. But the hardness decreases at the higher temperature due to the increase of number and size of the nano-crystalline graphitic domains. Since the degree of graphitization of the thin films increases, the electrical resistivity of the Ti-doped DLC thin films decreases from 0.038 to 0.006 Ω cm.     

Effects of the annealing treatment on electrical and optical properties of ZnO transparent conduction films by ultrasonic spraying pyrolysis

Effects of the annealing treatment on properties of ZnO thin films prepared on silica glass substrates by the ultrasonic spraying pyrolysis process were studied. Zinc acetate dihydrate and methanol were used as a starting material and a solvent, respectively. For ZnO thin films untreated with annealing, the preferred grain growth along the (0 0 2) plane was observed. The electrical resistivity and the direct band gap values of these films decreased with increasing the deposit temperature. By applying the annealing treatment in a reducing atmosphere, while the degree of the preferred (0 0 2) orientation of films decreased, the electrical conductivity of films was improved. When compared with the resistivity values of films without the annealing treatment, the values of films annealed in the reducing atmosphere were decreased by about two orders of magnitude. The lowest resistivity value was 1.62×10⁻¹ Ω cm, which was obtained in the film annealed at 500 °C in nitrogen with 5% hydrogen. The optical transmittances of the films were higher than 80% regardless of the application of the annealing treatment in a reducing atmosphere. The direct band gap values of films annealed in a reducing atmosphere were approximately 3.27 eV.