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.     

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.     

Annealing effect on properties of transparent and conducting ZnO thin films

This work presents the effect of postdeposition annealing on the structural, electrical and optical properties of undoped ZnO (zinc oxide) thin films, prepared by radio-frequency sputtering method. Two samples, 0.17 and 0.32 µm-thick, were annealed in vacuum from room temperature to 350 °C while another 0.32 µm-thick sample was annealed in air at 300 °C for 1 h. X-ray diffraction analysis revealed that all the films had a c-axis orientation of the wurtzite structure normal to the substrate. Electrical measurements showed that the resistivity of samples annealed in vacuum decreased gradually with the increase of annealing temperature. For the 0.32 µm-thick sample, the gradual decrease of the resistivity was essentially due to a gradual increase in the mobility. On the other hand, the resistivity of the sample annealed in air increased strongly. The average transmission within the visible wavelength region for all films was higher than 80%. The band gap of samples annealed in vacuum increased whereas the band gap of the one annealed in air decreased. The main changes observed in all samples of this study were explained in terms of the effect of oxygen chemisorption and microstructural 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.     

Structural, electrical and optical studies of SILAR deposited cadmium oxide thin films: Annealing effect

Successive ionic layer adsorption and reaction (SILAR) method has been successfully employed for the deposition of cadmium oxide (CdO) thin films. The films were annealed at 623 K for 2 h in an air and changes in the structural, electrical and optical properties were studied. From the X-ray diffraction patterns, it was found that after annealing, H₂O vapors from as-deposited Cd(O₂)_0.88(OH)_0.24 were removed and pure cubic cadmium oxide was obtained. The as-deposited film consists of nanocrystalline grains of average diameter about 20-30 nm with uniform coverage of the substrate surface, whereas for the annealed film randomly oriented morphology with slight increase in the crystallite size has been observed. The electrical resistivity showed the semiconducting nature with room temperature electrical resistivity decreased from 10⁻² to 10⁻³ Ω cm after annealing. The decrease in the band gap energy from 3.3 to 2.7 eV was observed after the annealing.     

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.     

Preparation and characterization of indium zinc oxide thin films by electron beam evaporation technique

In this work, the preparation of In₂O₃-ZnO thin films by electron beam evaporation technique on glass substrates is reported. Optical and electrical properties of these films were investigated. The effect of dopant amount and annealing temperature on the optical and electrical properties of In₂O₃-ZnO thin films was also studied. Different amount of ZnO was used as dopant and the films were annealed at different temperature. The results showed that the most crystalline, transparent and uniform films with lowest resistivity were obtained using 25 wt% of ZnO annealed at 500 °C.     

Dependence of the electrical and optical properties of sputter-deposited ZnO:Ga films on the annealing temperature,time, and atmosphere

Effects of annealing process parameters such as annealing temperature, time, and atmosphere on the electrical resistivity and transmittance properties of Ga-doped ZnO (ZnO:Ga) thin films deposited on glass by rf magnetron sputtering were investigated. The electrical resistivity of a ZnO:Ga thin film is effectively decreased with increasing annealing temperature and time in a reducing atmosphere such as N₂ + 5%H₂. This is attributed to passivation of grain boundaries and zinc ions by hydrogen atoms resulting in increases in carrier concentration and mobility. Also the resistivity of 4.9 × 10⁻⁴Ω cm was obtained by annealing at 200°C for 15 h in the same atmosphere, which is not bad for a transparent conductor for solar cell applications. However, annealing at a temperature higher than 400°C is less effective. The lowest resistivity of 2.3 × 10⁻⁴Ω cm was obtained by annealing at 400°C for 1 h in an N₂ + 5%H₂ atmosphere. The optical transmittance of the ZnO:Ga film is improved by annealing regardless of the annealing atmosphere. Annealing in N₂ + 5%H₂ atmosphere widens the optical band gap, while annealing in an O₂ atmosphere makes the band gap narrower, which can be explained as a blue shift phenomenon.     

Influence of the annealing in nitrogen atmosphere on the XRD, EDX, SEM and electrical properties of chemical bath deposited CdSe thin films

The influence of annealing in nitrogen atmosphere on the structure, optical and electrical properties of cadmium selenide (CdSe) thin films deposited by chemical bath deposition (CBD) onto glass substrates was studied. The samples were annealed in nitrogen atmosphere at various temperatures. A transition from metastable nanocrystalline cubic to stable polycrystalline hexagonal phase has been observed after annealing. The as-deposited CdSe thin films grow in the nanocrystalline cubic phase with optical band gap 1.93 eV. The electrical resistivity of the thin films has been measured in order of 10⁶ Ω cm. The activation energy of the samples has been found to be 0.26–0.19 eV at low temperature region, and 0.36–0.56 eV at high temperature region. It was also found that the activation energy and the resistivity of the films decrease with the increasing annealing temperature.     

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.     

Structure, luminescence and electrical properties of ZnO thin films annealed in H2 and H2O ambient: A comparative study

Undoped ZnO films were grown on a c-plane sapphire by plasma-assisted molecular-beam epitaxy technique, and subsequently annealed at 200-500 °C with steps of 100 °C in water vapour and hydrogen ambient, respectively. It is found that the c-axis lattice constant of the ZnO films annealed in hydrogen or water vapour at 200 °C increases sharply, thereafter decreases slowly with increasing annealing temperature ranging from 300 °C to 500 °C. The stress in the as-grown ZnO films was more easily relaxed in water vapour than in hydrogen ambient. Interestingly, the controversial luminescence band at 3.310 eV, which is often observed in photoluminescence (PL) spectra of the ZnO films doped by p-type dopants, was observed in the PL spectra of the annealed undoped ZnO films and the PL intensity increases with increasing annealing temperature, indicating that the 3.310 eV band is not related to p-type doping of ZnO films. The electron concentration of the ZnO films increases sharply with increasing annealing temperature when annealed in hydrogen ambient but decreases slowly when annealed in water vapour. The mechanisms of the effects of annealing ambient on the properties of the ZnO films are discussed.     

Effect of heat treatment on characteristics of nanocrystalline ZnO films by electron beam evaporation

Zinc oxide thin films have been grown on glass substrate at room temperature by electron beam evaporation and then were annealed in annealing pressure 600 mbar at different temperatures ranging from 250 to 550 °C for 30 min. Electrical, optical and structural properties of thin films such as electrical resistivity, optical transmittance, band gap and grain size have been obtained as a function of annealing temperature. X-ray diffraction has shown that the maximum intensity peak corresponds to the (002) predominant orientation for ZnO films annealed at various temperatures. The full width at half maximum, decreases after annealing treatment which proves the crystal quality improvement. Scanning electron microscopy images show that the grain size becomes larger by increasing annealing temperature and this result agrees with the X-ray diffraction analysis.     

Fluorine-doped ZnO transparent conducting thin films prepared by radio frequency magnetron sputtering

Fluorine-doped ZnO transparent conducting thin films were prepared by radio frequency magnetron sputtering at 150 °C on glass substrate. Thermal annealing in vacuum was used to improve the optical and electrical properties of the films. X-ray patterns indicated that (002) preferential growth was observed. The grain size of F-doped ZnO thin films calculated from the full-width at half-maximum of the (002) diffraction lines is in the range of 18-24 nm. The average transmittance in visible region is over 90% for all specimens. The specimen annealed at 400 °C has the lowest resistivity of 1.86 × 10^− 3 Ω cm, the highest mobility of 8.9 cm² V^− 1 s^− 1, the highest carrier concentration of 3.78 × 10²⁰ cm^− 3, and the highest energy band gap of 3.40 eV. The resistivity of F-doped ZnO thin films increases gradually to 4.58 × 10^− 3 Ω cm after annealed at 400 °C for 4 h. The variation of the resistivity is slight.     

The influence of annealing temperature on the structural, electrical and optical properties of ion beam sputtered CuInSe2 thin films

CuInSe₂ (CIS) thin films were prepared by ion beam sputtering deposition of copper layer, indium layer and selenium layer on BK7 glass substrates followed by annealing at different temperatures for 1 h in the same vacuum chamber. The influence of annealing temperature (100-400 °C) on the structural, optical and electrical properties of CIS thin films was investigated. X-ray diffraction (XRD) analysis revealed that CIS thin films exhibit chalcopyrite phase and preferential (112) orientation when the annealing temperature is over 300 °C. Both XRD and Raman show that the crystalline quality of CIS thin film and the grain size increase with increasing annealing temperature. The reduction of the stoichiometry deviation during the deposition of CIS thin films is achieved and the elemental composition of Cu, In and Se in the sample annealed at 400 °C is very near to the stoichiometric ratio of 1:1:2. This sample also has an optical energy band gap of about 1.05 eV, a high absorption coefficient of 10⁵ cm⁻¹ and a resistivity of about 0.01 Ω cm.     

Deposition and annealing effect on lanthanum sulfide thin films by spray pyrolysis

Lanthanum sulfide thin films were prepared on glass substrates from aqueous medium using spray pyrolysis technique. The effect of preparative parameters such as substrate temperature and solution concentration on the films was studied. The lanthanum sulfide films were annealed in air at 300 °C for 2 h. The films were characterized by X-ray diffraction (XRD), optical microscopy, optical absorption, electrical resistivity and thermo-emf measurement techniques. The XRD studies revealed that the as deposited films are amorphous, while annealed films are polycrystalline. The optical band gap of the as deposited film is decreased from 2.5 to 2.2 eV after annealing due to improvement in crystallinity. The electrical resistivity is of the order of 10⁴-10⁵ Ω cm and showed semiconducting behaviour. Thermo-emf measurement revealed that the conductivity of lanthanum sulfide is p-type.     

Preparation of aluminum doped zinc oxide films and the study of their microstructure, electrical and optical properties

Aluminum doped zinc oxide (AZO) polycrystalline thin films were prepared by sol-gel dip-coating process on optical glass substrates. Zinc acetate solutions of 0.5 M in isopropanol stabilized by diethanolamine and doped with a concentrated solution of aluminum nitrate in ethanol were used. The content of aluminum in the sol was varied from 1 to 3 at.%. Crystalline ZnO thin films were obtained following an annealing process at temperatures between 300 °C and 500 °C for 1 h. The coatings have been characterized by X-ray diffraction, UV-Visible spectrophotometry, scanning electron microscopy, and electrical resistance measurement. The ZnO:Al thin films are transparent (∼ 90%) in near ultraviolet and visible regions. With the annealing temperature increasing from 300 °C to 500 °C, the film was oriented more preferentially along the (0 0 2) direction, the grain size of the film increased, the transmittance also became higher and the electrical resistivity decreased. The X-ray diffraction analysis revealed single-phase ZnO hexagonal wurtzite structure. The best conductors were obtained for the AZO films containing 1 at.% of Al, annealed at 500 °C, 780 nm film thickness.     

Effects of post-deposition heat treatment on the microstructure and properties of Al-doped ZnO thin films prepared by aqueous phase deposition

In this study, transparent conducting aluminum-doped ZnO thin films (AZO) were deposited on glass substrates by a water-based liquid phase deposition method. The results show that by employing a two-step post-deposition heat treatment, the preferential orientation of ZnO (002) appeared as soon as the polycrystalline films were formed. Under a reducing atmosphere, the crystallinity of the films was effectively improved. Furthermore, the reducing atmosphere was also beneficial for the removal of the residual stress of the prepared films and the c-axis lattice constant was less stretched as compared to those under an inert atmosphere at identical T_p. Both the atomic force micrograph and scanning electron micrograph clearly exhibited that the heat treatment induced considerable grain growth. The X-ray photoelectron spectrum revealed that the heat treatment atmosphere had little impact on the bonding state of zinc and that the reducing atmosphere was favorable for the non-stoichiometric alumina, which in turn, resulted in more oxygen vacancies and led to improvement in electrical conductivity. The ratio of chemisorbed oxygen declined substantially when applying the reducing atmosphere. Accordingly, hydrogen was helpful for the reduction of chemisorbed oxygen onto AZO films. Generally, the electrical resistivity declined linearly with T_p. A minimum resistivity of 9.90 × 10^− 3 Ω·cm was obtained with a doping concentration of Al/Zn = 2.25 at.% at T_p = 700 °C. The largest mean free path of the carriers was 1.2 Å, which was much smaller than the observed grain sizes of the AZO films. Accordingly, the grain boundary scattering was not the detrimental scattering mechanism. In contrast, the scattering within the grains was responsible for the low mobility. An increase in optical transparency with the heat treatment temperature was observed due to the compact and smooth topography with larger grains, among which, less porous structures were formed at elevated temperature.     

Annealing induced reorientation of crystallites in Sn doped ZnO films

Tin doped ZnO thin films were prepared by employing a simplified spray pyrolysis technique using a perfume atomizer and subsequently annealed under different temperatures from 350 °C to 500 °C in steps of 50 °C. The structural, optical, electrical, photoluminescence and surface morphological properties of the as-deposited films were studied and compared with that of the annealed films. The X-ray diffraction studies showed that as-deposited film exhibits preferential orientation along the (0 0 2) plane and it changes in favour of (1 0 0) plane after annealing. The increase in crystallite size due to annealing is explained on the basis of Ostwald ripening effect. It is found that the optical transmittance and band gap increases with increase in annealing temperature. A slight decrease in resistivity caused by annealing is discussed in correlation with annealing induced defect modifications and surface morphology.     

X-ray absorption spectroscopy study of thermally annealed Cu–Al–O thin films

Cu–Al–O thin films are deposited on (0001) sapphire substrates by radio-frequency sputtering using an Al–Cu mosaic target. The Cu/Al atomic ratio of as-deposited Cu–Al–O films is measured to be 1.1. After deposition, the Cu–Al–O films are annealed at 600, 800, and 1000 °C, respectively, for 1 h in a N₂ atmosphere. The film crystal structure, electronic structure, valence band, and electrical properties are studied. The as-deposited films are amorphous and films annealed at 600 °C contain the crystallized CuO phase; the structure becomes crystallized CuAlO₂ after annealing at 800 °C and 1000 °C. The 800 °C annealed film grows along the (00l) plane. The crystallization decreases with the growth of the (012) and (018) planes for films annealed at 1000 °C. The resistivity values of the 800 °C and 1000 °C annealed films were measured as 1.07 Ω-cm and 864.01 Ω-cm, respectively. The lower resistivity of the 800 °C annealed film is attributed to preferred (00l) growth orientation and a reduction of the energy band gap.     

Effect of substrate temperature and annealing treatment on the electrical and optical properties of silver-based multilayer coating electrodes

Multilayer coatings consisting of thin silver layers sandwiched between layers of transparent conducting metal oxides are investigated from the view point of low-resistance electrodes for use in flat panel displays, solar cells, etc. ZnO/Ag/ZnO multilayer films were prepared on glass substrates by simultaneous RF magnetron sputtering of ZnO and dc magnetron sputtering of Ag. Optimization of the deposition conditions of both ZnO layers and metallic layers were performed for better electrical and optical properties. The structural, electrical and optical properties of the films (deposited at room temperature, different substrate temperature and annealed at different conditions) were characterized with various techniques. We could not produce high-quality transparent conductive electrodes simply by annealing at various temperatures. However, improved electrical properties and a considerable shift in the transmittance curves was observed after heat treatment. The experimental results show that the electrical resistivity of as-grown films can be decreased to 10^− 5 Ω cm level with post-annealing at 400 °C for 2 h in vacuum atmosphere. After heat treatment, the sheet resistance was reduced as much as 20% which was due to the increased grain size of Ag film. The samples heat treated at 200-400 °C under vacuum or nitrogen atmosphere showed the best electrical properties. The key to the superior electrical and optical properties of the multilayer is the optimization of growth conditions of the silver layer by careful control of the oxide properties and the use of appropriate annealing temperature and atmosphere.