Transparent Conductive Thin Films Research Articles

Growth and electro-optical properties of Ga-doped ZnO films prepared by aerosol assisted chemical vapour deposition

Abstract Transparent conductive Ga-doped ZnO thin films were deposited onto glass substrates by a low-cost aerosol assisted chemical vapour deposition technique and the effect of gallium content on the ZnO film growth behaviour and opto-electronic properties was systematically investigated. It is found that, upon increasing Ga addition, the ZnO film crystallinity exhibits a continuous reduction in quality associated with the preferential orientation transformed from (002) to (102). The (002) oriented samples had a microstructure of parallel columnar grains while the (102) oriented coating was thickened by overlapping particles. The ZnO:Ga coatings exhibit high carrier concentration (up to 4.1 × 1020 cm− 3) but low carrier mobility (up to 0.8 cm2 V− 1 s− 1), resulting in a minimum resistivity value of 2.3 × 10− 2 Ω cm. The inferior carrier mobility performance could result from a profound ionized and neutral impurity scattering effect. Good visible transmittance (≈ 70–80%) is observed in these ZnO:Ga films and samples with higher carrier density present better infrared reflection performance (up to 37.2% at 2500 nm).

English

English

Effect of Multiple Frequency H<sub>2</sub>/Ar Plasma Treatment on the Optical, Electrical, and Structural Properties of AZO Films

Transparent conductive Al-doped zinc oxide thin films for film solar cells were prepared on Si and quartz substrates by radio frequency magnetron sputtering at room temperature. Then, these films were treated by high-density H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Ar plasma, which was capacitively coupled plasma combined with inductively coupled plasma modes. The density of hydrogen atoms was characterized by optical emission spectroscopy equipped in the system. The structural, electrical, and optical properties of the films were investigated in terms of spectrophotometry, Hall effect measurements, X-ray diffraction, scanning electron microscope, and atomic force microscopy. The effects of low-frequency power on the optical, electrical, and structural properties were discussed.

Characterization of Optical and Electrical Properties of Transparent Conductive Boron-Doped Diamond thin Films Grown on Fused Silica

Abstract A conductive boron-doped diamond (BDD) grown on a fused silica/quartz has been investigated. Diamond thin films were deposited by the microwave plasma enhanced chemical vapor deposition (MW PECVD). The main parameters of the BDD synthesis, i.e. the methane admixture and the substrate temperature were investigated in detail. Preliminary studies of optical properties were performed to qualify an optimal CVD synthesis and film parameters for optical sensing applications. The SEM micro-images showed the homogenous, continuous and polycrystalline surface morphology; the mean grain size was within the range of 100-250 nm. The fabricated conductive boron-doped diamond thin films displayed the resistivity below 500 mOhm cm-1 and the transmittance over 50% in the VIS-NIR wavelength range. The studies of optical constants were performed using the spectroscopic ellipsometry for the wavelength range between 260 and 820 nm. A detailed error analysis of the ellipsometric system and optical modelling estimation has been provided. The refractive index values at the 550 nm wavelength were high and varied between 2.24 and 2.35 depending on the percentage content of methane and the temperature of deposition.

H2 plasma effect toward AZO/Mo/AZO transparent conductive film

The hydrogen plasma towards transparent conductive oxide thin films, triple-layered AZO/Mo/AZO (OMO) structure, is studied in the paper. The OMO films are deposited on glass substrates by a co-sputtering process at room temperature. The DC power towards Mo target is constant at 40 W for 30 s and the RF power towards AZO target is 130 W for 6, 8, 10, 13 and 15 min, respectively. The best thin film was obtained at the condition of Mo deposition for 30 s and AZO deposition for 10 min. It shows the figure of merit (FOM) of 2.23 × 10−5 and sheet resistance of 517.5 Ω/□ on glass substrate, the average transmittance is about 64 %. The FOM value of the OMO film can be improved by the treatment of H2 plasma at 150 W for 10 min. The improvement is due to the increase of carrier concentration and results in the decrease of the sheet resistance of the film.

Investigation on optoelectronic performances of Al, N codoped ZnO: First-principles method

To analyze the electronic structure and optical properties of (Al, N) codoped ZnO, We investigated the parameters such as band structure, density of states (DOS), charge population, dielectric constant, absorption and reflection spectra of pure ZnO, N-doped, Al–N codoped and Al–2N codoped ZnO by using first-principles based on density functional theory (DFT). The results demonstrated that the cases of N doping and Al–2N codoping performance p-type, and Al–2N codoping can obtain a high optical quality and more stable p-type ZnO. Compared with other conditions, the real and imaginary part of dielectric function of Al–2N codoped ZnO have a significant increase and move to the lower energy direction; in the ultraviolet and visible region, the absorption spectrum increases greatly, and the red shift can be observed; in the infrared region, the reflectivity spectrum of Al–2N codoped ZnO is about 4 times that of pure ZnO or Al–N codoped ZnO. The results may supply a certain theoretical reference for study of ZnO-based transparent conductive thin films.

Effect of thermal annealing on the performance of WO3–Ag–WO3 transparent conductive film

As a candidate for transparent electrodes, oxide–metal–oxide tri-layer films have attracted a lot of interest by virtue of their low cost and decent performance. However, their thermal stability needs to be considered before device integration. Here, we report a thermal annealing effect on the performance of WO3–Ag–WO3 (WAW) transparent conductive thin film prepared by thermal evaporation. We find that the sheet resistance of the as-prepared WAW film gradually increases from 6.55 to 21.4Ω/sq when the annealing temperature reaches 400°C. With a low annealing temperature (below 200°C), the luminous transmittance of the WAW film slightly increases but decreases rapidly when the annealing temperature exceeds 200°C. The maximum figure of merit (11×10−3Ω−1) was obtained at the annealing temperature of 100°C. Above the annealing temperature of 300°C, the film shows significant transmittance and conductivity degradation, which can be attributed to the decrease of intrinsic dielectric constant of WO3 layers and the reduction of connectivity between Ag islands upon high temperature annealing, respectively. Annealing at a temperature of 500°C leads to severe destruction of the tri-layer structure. In addition, we believe that the localized surface plasmonic absorption of annealing-generated Ag nanoparticles results in a valley centered at 410nm on the transmittance spectrum of the 500°C annealed WAW film.

Morphological differences in transparent conductive indium-doped zinc oxide thin films deposited by ultrasonic spray pyrolysis

In-doped ZnO thin films were deposited on glass substrates by an ultrasonic spray pyrolysis technique, using indium chloride (InCl3) as a dopant and zinc acetate solution as a precursor. Increasing the [at.% In]/[at.% Zn] ratio changed the crystal orientations of thin films, from the (100) preferred orientation in the undoped, to the (101) and (001) preferred orientations in the In-doped ZnO thin films with 4at.% and 6–8at.%, respectively. Undoped ZnO thin film shows relatively smooth surface whereas In-doped ZnO thin films with 4at.% and 6–8at.% show surface features of pyramidal forms and hexagonal columns, respectively. X-ray diffraction patterns of the In-doped ZnO thin films with [at.% In]/[at.% Zn] ratios of 6–8% presented an additional peak located at 2-theta of 32.95°, which possibly suggested that a metastable Zn7In2O10 phase was present with the ZnO phase. ZnO thin films doped with 2at.% In resulted in a sheet resistance of ~645Ω/sq, the lowest value among thin films with [at.% In]/[at.% Zn] ratio in a range of 0–8%. The precursor molarity was changed between 0.05M and 0.20M at an [at.% In]/[at.% Zn] ratio of 2%. Increasing the precursor molarity in a range of 0.10M–0.20M resulted in In-doped ZnO thin films with the (100) preferred orientation. An In-doped ZnO thin film deposited by 0.20M precursor showed a sheet resistance of 25Ω/sq, and an optical transmission of 75% at 550nm wavelength. The optical band gap estimated from the transmission result was 3.292eV.

Effects of parameters on the performance of amorphous IGZO thin films prepared by RF magnetron sputtering

Amorphous indium-gallium-zinc oxide (IGZO) transparent conductive thin films are prepared on glass substrates by radio frequency (RF) magnetron sputtering. The effects of seven factors, which are substrate temperature, sputtering atmosphere, working pressure, sputtering power, annealing temperature, negative bias voltage and sputtering time, on Hall mobility, transmittance and surface roughness are studied through orthogonal experiments. The results show that the effects of working pressure, substrate temperature and sputtering atmosphere on performance of films are the most prominent. According to the experimental results and discussion, relatively reasonable process parameters are obtained, which are working pressure of 0.35 Pa, substrate temperature of 200 °C, sputtering atmosphere of Ar, sputtering power of 125 W, sputtering time of 30 min, negative bias voltage of 0 V and annealing temperature of 300 °C.

The crystalline structure, conductivity and optical properties of Co-doped ZnO thin films

Transparent conductive Co-doped ZnO thin films were deposited by ultrasonic spray technique. Conditions of preparation have been optimized to get good quality. A set of cobalt (Co)-doped ZnO (between 0 and 3wt%) thin films were grown on glass substrate at 350°C. The thin films were annealed at 500°C for improvement of the physical properties. Nanocrystalline films with hexagonal wurtzite structure and a strong (002) preferred orientation were obtained. The maximum value of grain size G=63.99nm is attained with undoped ZnO film. The optical transmissions spectra showed that both the undoped and doped ZnO films have transparency within the visible wavelength region. The band gap energy decreased after doping from 3.367 to 3.319eV when Co concentration increased from 0 to 2wt% with slight increase of electrical conductivity of the films from 7.71 to 8.33(Ωcm)−1. The best estimated structure, optical and electrical results are achieved in Co-doped ZnO film with 2wt%.

Correlation in calculations of electrical conductivity with and without undoped in Al, Co and In doped ZnO thin film

The transparent conductive pure and doped zinc oxide thin films with aluminum, cobalt and indium were deposited by ultrasonic spray technique on glass substrate at 350°C. This paper is to present a new approach to the description of correlation between electrical conductivity and crystallite size with dopant concentration of Al, Co and In. The correlation between electrical and structural properties with dopant concentration suggests that the electrical conductivity of the films is predominantly estimated by the crystallite size and the concentration of Al, Co and In. The measurement in the electrical conductivity of doped films with correlation is equal to the experimental value of without undoped ZnO thin films, here the error is limited to zero %. The minimum error value was estimated in the cobalt and indium doped ZnO thin films. The correlation between the electrical conductivity and the crystallite size with the doping concentration was investigated.

Metal Decorated Carbon-Based Materials for Transparent Conductive Thin Films

A facile approach to decorate carbon-based materials, such as graphene oxides (GOx) and carbon nanotubes (CNTs), with metal nanoparticles was developed to enhance the electrical conductivity of coated thin films. CNTs were first surface-functionalized with hydroxyl, carbonyl, and carboxyl functional groups using a simple and green method without strong acid. The as-prepared CNTs were then dispersed in solvents and coated on PET or glass substrates to form conductive thin films. Copper or silver nanoparticles were then decorated on the functionalized CNT films using formaldehyde as a reduction agent. The microstructures of metallized thin films were characterized by FESEM, XRD, and UV measurements. The electrical and mechanical properties of the composite thin films were measured and compared with regular CNT films. The results showed that electrical conductivity of CNT thin films were greatly enhanced after the metallization process. This simple method can be also applied to two-dimensional nanostructured GOx materials to provide thin films with both good transparency and conductivity.

Improving the optoelectronic properties of gallium ZnO transparent conductive thin films through titanium doping

The optoelectronic characteristics of gallium-doped ZnO ternary alloy transparent conductive oxide were improved through titanium (Ti) doping, and showed enhanced optoelectronic properties, and crystallite quality by radio frequency magnetron sputtering. The room-temperature photoluminescence measurement showed improved near band edge emission for ZnO peaks, and suppressed deep defect level emission at the green light band. The study results show that the lowest thin film resistivity was 4.95×10−4Ω-cm, with a mobility and carrier concentration of 4.8cm2/V-s and 2.64×1021cm−3, respectively. The superior carrier concentration of Ti-doped GZO alloys (>1021cm−3) with high figure of merit (35.3×10−3Ω−1), demonstrating the pronounced contribution made by Ti doping, and is highly suitable for the optoelectronic device applications.

Effect of band gap energy on the electrical conductivity in doped ZnO thin film

The transparent conductive pure and doped zinc oxide thin films with aluminum, cobalt and indium were deposited by ultrasonic spray technique on glass substrate at 350 °C. This paper is to present a new approach to the description of correlation between electrical conductivity and optical gap energy with dopants' concentration of Al, Co and In. The correlation between the electrical and optical properties with doping level suggests that the electrical conductivity of the films is predominantly estimated by the band gap energy and the concentrations of Al, Co and In. The measurement in the electrical conductivity of doped films with correlation is equal to the experimental value, the error of this correlation is smaller than 13%. The minimum error value was estimated in the cobalt-doped ZnO thin films. This result indicates that such Co-doped ZnO thin films are chemically purer and have far fewer defects and less disorder owing to an almost complete chemical decomposition.

The in situ growth of silver nanowires on multi-walled carbon nanotubes and their application in transparent conductive thin films

A novel composite was constructed by the in situ growth of silver nanowires (AgNW) on multi-walled carbon nanotubes (MWCNT). Firstly, thiol groups were covalently bonded onto the surface of MWCNT by the esterification of thioglycolic acid with hydroxylated MWCNT. Reduced platinum (Pt) nanoparticles were then attached to the MWCNT via the thiol groups. In the presence of poly(vinyl pyrrolidone), Pt nanoparticles served as seeds for generating AgNW of uniform diameter. Finally, a MWCNT/AgNW composite with a conductive network structure was fabricated. The structure and morphology of the composite were measured by a variety of methods, including UV-visible extinction spectrometry, energy-dispersive spectrometry, X-ray diffractometry, Raman spectrometry and electron microscopy. An electrically conductive thin film of 80% transparency with a sheet resistance of 47.8 Ω sq−1 was prepared by coating the composite onto a polyethylene terephthalate film substrate.

Transparent conductive thin films of Cd2SnO4 obtained by the sol–gel technique and their use in a solar cell made with CdTe

In this work Cd2SnO4 polycrystalline thin films with cubic phase were deposited on glass substrates by the sol–gel technique starting from the mixture of two simple precursor solutions of CdO and SnO2, obtained at room temperature. Both solutions were prepared using metallic salts, cadmium acetate and stannous chloride. The Sn atomic concentration percentages (X) in the precursor solution with respect to Cd (1−X) were 16, 25, 29, 33, 36 and 40. The films were sintered at 550°C in air and, after that, exposed to two different annealing treatments, 96/4N2/H2 gas mixture and vacuum, at 350°C and 550°C, respectively. X-ray diffraction patterns of the films without annealing showed three types of films: i) for X<29at%, the films were constituted of CdO+Cd2SnO4 crystals, ii) for X=29at% and X=33at%, Cd2SnO4 polycrystalline thin films were obtained, and iii) for X>33at%, Cd2SnO4+ CdSnO3 crystals were formed. Cd2SnO4 films maintained the spinel-type crystalline structure of the Cd2SnO4 after the annealing processes. On the other hand, Electron Dispersion Spectroscopy measurements revealed that Cd2SnO4 films obtained at X=29at%, without annealing, had a Cd/Sn ratio nearest to the one of the Cd2SnO4 compounds. With and without annealing Cd2SnO4 films showed high optical transmission ~85% in the 500nm<λ<1500nm range. A minimum resistivity value was around 2×10−3Ωcm, which was obtained for Cd2SnO4 films (X=33at%) annealed in vacuum at 550°C, and a direct band gap energy value of 3.55eV. As a test of the quality of these films as transparent electrodes, Au/Cu2Te/CdTe/CdS/Cd2SnO4/glass solar cells were obtained with average efficiency of 10.7%.

Extracting the effective mass of electrons in transparent conductive oxide thin films using Seebeck coefficient

A method is proposed that combines Seebeck coefficient and carrier concentration to determine the electron effective mass of transparent conductive oxide (TCO) thin films. Experiments were conducted to test the validity of this approach on the transparent conductive Ga-doped ZnO thin films deposited by magnetron sputtering. An evident agreement of the calculated electron effective mass of the films is observed between the proposed approach and the previous studies. Besides, the optical carrier concentration and mobility derived from the calculated electron effective mass and spectroscopic ellipsometry using a complex dielectric function are consistent with those from direct Hall-effect measurement. The agreements suggest that Seebeck coefficient can serve as an alternative tool for extracting the effective mass of electrons in TCO films.

Quality improvement of high-performance transparent conductive Ti-doped GaZnO thin film

Ti-doped GaZnO transparent conductive oxide films were deposited at various substrate temperatures by radio-frequency magnetron sputtering to examine the effects of crystalline quality on thin-film optoelectronic properties. The results show a low resistivity of 2.97×10−4Ω-cm and a high optical transmittance of 94.15% (wavelength region of 400–800nm) in 300-nm-thick Ti-doped GaZnO thin films grown at the substrate temperature of 300°C, with a post-annealing temperature of 400°C. To evaluate the performance of Ti-doped GaZnO transparent conductive oxide films, this study employed the figure of merit (ΦTC), expressed as ΦTC=T10/RS (T: transmittance RS: sheet resistance), and calculated as 55.2×10−3Ω−1, which is suitable for high-performance optoelectronic device applications.

Effect of doped boron on the properties of ZnO thin films prepared by sol-gel spin coating

Transparent conductive boron-doped ZnO thin films were prepared by sol-gel spin coating method. The effect of doped boron concentration on the properties of the films was systematically discussed. The films were characterized by X-ray diffraction, atomic force microscopy, spectrophotometry, and Hall effect measurement system. All the doped and undoped ZnO films were of a single hexagonal structure, and showed a preferred orientation of (002). The particle size and surface roughness of the films decreased with increased doped boron concentration. All the films exhibited an average transmittance of approximate 90% in visible-light region and an energy gap of about 3.3 eV. The maximum carrier concentration, the highest carrier mobility and the lowest resistivity were observed at a doped boron concentration of 0.5%(molar fraction). Based on these results, we suggested that the saturation concentration of doped boron in ZnO film is 0.5%(molar fraction).

InOx thin films deposited by plasma assisted evaporation: Application in light shutters

An integration of undoped InOx and commercial ITO thin films into laboratory assembled light shutter devices is made. Accordingly, undoped transparent conductive InOx thin films, about 100 nm thick, are deposited by radiofrequency plasma enhanced reactive thermal evaporation (rf-PERTE) of indium teardrops with no intentional heating of the glass substrates. The process of deposition occurs at very low deposition rates (0.1–0.3 nm/s) to establish an optimized reaction between the oxygen plasma and the metal vapor. These films show the following main characteristics: transparency of 87% (wavelength, λ = 632.8 nm) and sheet resistance of 52 Ω/sq; while on commercial ITO films the transparency was of 92% and sheet resistance of 83 Ω/sq. The InOx thin film surface characterized by AFM shows a uniform grain texture with a root mean square surface roughness of Rq∼2.276 nm. In contrast, commercial ITO topography is characterized by two regions: one smoother with Rq∼0.973 nm and one with big grains (Rq∼3.617 nm). For the shutters assembled using commercial ITO, the light transmission coefficient (Tr) reaches the highest value (Trmax) of 89% and the lowest (Trmin) of 1.3% [13], while for the InOx shutters these values are 80.1% and 3.2%, respectively. Regarding the electric field required to achieve 90% of the maximum transmission in the ON state (Eon), the one presented by the devices assembled with commercial ITO coated glasses is 2.41 V/μm while the one presented by the devices assembled with InOx coated glasses is smaller, 1.77 V/μm. These results corroborate the device quality that depends on the base materials and fabrication process used.