Fluorine-doped Zinc Oxide Thin Films Research Articles

Structural, optical and electrical properties of F doped ZnO films: Experimental and theoretical study

In this paper, we theoretically and experimentally investigated the structural, optical and electronic properties of Fluorine-doped zinc oxide (FZO) with different concentrations of the doping element. The undoped and F doped ZnO thin films have been elaborated on glass substrate by spray pyrolysis technique. X-ray diffraction (XRD) and photoluminescence measurement are used to characterize the structural and optical properties of ZnFxO1-x films with x = 0.0, 0.02, 0.05, 0.07. The results showed that all the elaborate thin films crystallize in the hexagonal structure of the wurtzite type with a preferential orientation of the crystallites along the direction (002) along the c axis perpendicular to the surface of the substrate. All FZO thin films had relatively higher average transmittances, approximately 80–90%, in the visible region as compared to the undoped ZnO thin films. The electronic band structure, total and partial density of states of FZO were investigated studied by Density Functional Theory to examine the change in the band gap of ZnO with F doping. The results of this study indicate that with the increase of the fluorine concentration in the ZnO matrix, the band gap energy decreases which is in good agreement with the experimental results.

A Rapid Deposition of Fluorine Doped Zinc Oxide Using the Atmospheric Pressure Chemical Vapour Deposition Method

Fluorine-doped zinc oxide (FZO) (ZnO:F) thin films were manufactured by atmospheric pressure chemical vapor deposition (APCVD) on glass substrates using zinc acetate dihydrate [C4H6O4Zn·2H2O, ZnAc] and ammonium fluoride (NH4F) as the source of fluorine with deposition duration of only 120 s for each sample. The effects of different amounts of fluorine as the dopant on the structural, electrical and optical properties of FZO thin films were investigated. The results show a polycrystalline structure at higher temperatures compared to amorphous structure at lower temperatures. The x-ray diffraction patterns of the polycrystalline films were identified as a hexagonal wurtzite structure of zinc oxide (ZnO) with the (002) preferred orientation. Also, the sheet resistance decreased from 17.8 MΩ/□ to 28.9 KΩ/□ for temperatures 325°C to 450°C, respectively. In order to further decrease the sheet resistance of the undoped ZnO thin films, fluorine was added using NH4F as the precursor, and again a drastic change in sheet resistance of only 17.7 Ω/□ was obtained. Based on the field emission scanning electron microscopy images, the fluorine concentration in CVD source is an important factor affecting the grain size and modifies electrical parameters. Ultraviolet–visible measurements revealed reduction of transparency of the layers with increasing fluorine as the dopant.

Influence of RF power on physical properties of ZnO:ZnF2 thin films by RF magnetron sputtering

Fluorine-doped zinc oxide (FZO) thin films were prepared on glass substrates at room temperature by radio frequency (RF) magnetron sputtering with a ceramic ZnO target containing 1.5wt% zinc fluoride (ZnF2). This study investigates the influences of RF powers of sputtering and H2 plasma treatment on properties of FZO films. For as-deposited films, all films had a highly (002) preferential c-axis orientation and film crystallinity was improved with increasing deposition power. The film resistivity decreased and the average optical transmittance in the visible range increased with increasing deposition power. The lowest resistivity of 9.29×10−4Ω-cm and the average transmittance above 90% were obtained at the power of 150W. For plasma treated films, the crystal structure had no significant change but the film resistivity further decreased to 7.92×10−4Ω-cm and the optical bandgap increased to 3.725eV. The calculated figures of merit exhibited that the film deposited with a high deposition power and a low H2 plasma power possessed the optimized optoelectronic properties. The H2 plasma treated FZO thin films have potential to be applied as transparent conducting electrodes.

A simple approach to the fabrication of fluorine-doped zinc oxide thin films by atomic layer deposition at low temperatures and an investigation into the growth mode

A simple low-temperature fabrication of fluorine-doped ZnO (ZnO:F) thin films by atomic layer deposition (ALD) was investigated and the growth mode of the films was analyzed. A novel method for fluorine doping into a ZnO matrix was successfully developed that uses ALD with a home-made fluorine source at a low deposition temperature of 140 °C, which is low enough for films to be applied to a plastic substrate. The fluorine doping concentration was controlled from 0 at.% to 1.2 at.% by manipulating the pulse sequence ratio of the oxygen source (deionized water) and the home-made fluorine source. Structural and morphological properties were investigated by X-ray diffraction and field emission scanning electron microscopy. The growth mode of the films was studied by grazing-incidence wide-angle X-ray diffraction. The grain growth orientation was found to change significantly as the fluorine concentration was increased due to the characteristics of fluorine doping in the oxygen sites of ZnO. This phenomenon could be explained by a perturbation and passivation effect resulting from the fluorine doping mechanism, with the fluorine anions filling oxygen-related defect sites in the ZnO lattice. A photoluminescence study confirmed the diminishment of defect sites and showed that, due to the Burstein–Moss effect, the optical transmittance in the visible region increased significantly with an increase in the fluorine doping concentration from 80.87% to 83.02%. The lowest resistivity was 1.876 × 10−3 Ω cm for ZnO thin films doped with 1.0 at.% fluorine when the carrier concentration and mobility were 1.375 × 1020 cm−3 and 24.20 cm2 V−1 s−1, respectively.

Effect of solvent volume on the physical properties of sprayed fluorine‐doped zinc oxide thin films

AbstractFluorine‐doped zinc oxide (FZO) films were deposited from aged starting solutions having different solvent volumes (10, 20, 30, 40 and 50 ml) using a simplified spray pyrolysis technique. The electrical studies showed that the resistivity is minimum (4.68 × 10–2 Ω cm) for lowest solvent volume (10 ml) and the value progressively increased with the increase in the solvent volume indicating that the fluorine incorporation increases with solvent volume. The systematic study clearly showed that the spray flux rate plays a crucial role in determining the electrical, optical, surface and structural properties of the FZO films. The optical transparency is found to be gradually increased (from 85 to 95%) as the volume of the solvent increases. The scanning electron microscope images depicted that the decrease in the spray flux rate caused an enhancement in the grain size. The X‐ray diffraction profile clearly showed that all the films have preferential orientation along the (002) plane with hexagonal wurtzite structure. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Chemical spray pyrolysis deposited fluorine-doped zinc oxide thin films: Effect of acetic acid content in the starting solution on the physical properties

Fluorine-doped zinc oxide [ZnO:F] thin solid films have been deposited on soda lime glass substrates using the chemical spray technique. The effect of the acetic acid content in the starting solution on the composition, morphology, and photoluminescent properties of the ZnO:F thin films was studied. The fluorine content in the samples increases as the acetic acid content in the solutions increases. Morphology of the ZnO:F thin films changes drastically with the variation of acetic acid content. The photoluminescence spectra revealed the presence of high defect content in the films, nevertheless similar characteristic emissions, irrespective of the acetic acid content in the starting solution were presented.

Electrical, structural and optical properties of fluorine-doped zinc oxide thin films: Effect of the solution aging time

In this paper ageing effects of the solution used to prepare fluorine-doped ZnO films by the spray pyrolysis technique were investigated, concerning its role on the structure, the electrical and optical properties of films produced. The data reveal that the sheet resistance of the ZnO:F thin film decreases with the age of the solution used, reaching a minimum of 24 Ω/□, after 15 days. On the other hand the optical transmittance increases for films deposited using 6 days aging solution, decreasing afterwards as the aging time increases, being the optical transmittance in the visible range below 55%, for films deposited from solutions 36 days in age. The X-ray diffraction spectra show that the aged films are polycrystalline in nature with a [100] predominant orientation. The data also show that the intensity of (100) peak increases as the time of solution age increases, which is related to an improvement of the film crystallinity.

Fluorine-doped zinc oxide transparent and conducting electrode by chemical spray synthesis

Highly transparent and conducting fluorine-doped zinc oxide thin films, consisting of spherical nanometer-sized grains, were synthesized onto soda-lime glass substrate by using a chemical spray unit. The effect of fluorine doping concentration in starting solution was investigated. Both doped and undoped films were preferentially oriented along [002] direction. Electrical resistivity decreases from 5.7 x 10 -2 to 8.6 x 10 -3 ohm-cm after 1 at.% fluorine doping, and increases for higher doping concentration. However, surface morphology of films obtained at 3 at.% fluorine doping appeared smooth and uniform. A shift of the edge of the optical transmission in the ultraviolet region as a result of fluorine doping was obtained.

Second harmonic generation of fluorine-doped zinc oxide thin films grown on soda-lime glass substrates by a chemical spray technique

Second harmonic generation (SHG) studies of fluorine-doped zinc oxide (ZnO:F) thin films deposited on soda-lime glass substrates from an aged solution in conjunction with zinc pentanedionate, using the chemical spray deposition technique were carried out. The χ31(2) and χ33(2) independent tensorial components of the quadratic nonlinear optical susceptibility of the ZnO:F thin films were evaluated. Scanning electron microscopy and X-ray diffraction investigations revealed a homogeneous distribution of nanoparticles of similar size and morphology for various samples deposited at different substrate temperatures (ranging from 400 to 525°C). The SHG-technique revealed a clear dependence of the nonlinear optical response with the deposition temperature. Typical optical transmittance and photoluminescence (PL) studies were also performed, from which a bandgap (Eg) of 3.3eV was evaluated in films deposited under optimal conditions of conductivity and transmittance.

Composition and morphological characteristics of chemically sprayed fluorine-doped zinc oxide thin films deposited on Si(1 0 0)

Fluorine-doped zinc oxide thin films (ZnO:F) were deposited on Si(1 0 0) substrates by the chemical spray technique (CST) from an aged-solution. The effect of the substrate temperature on the morphology and composition of the ZnO:F thin films was studied. The films were polycrystalline, with a preferential growth along the ZnO (0 0 2) plane, irrespective of the deposition temperature. The average crystal size within the films was ca. 35 nm and the morphology of the surface was found to be dependent on the substrate temperature. At low substrate temperatures irregular-shaped grains were observed, whereas at higher temperatures uniform flat grains were obtained. Elemental analysis showed that the composition of the films is close to stoichiometric ZnO and that samples contain quite a low fluorine concentration, which decreases as a function of the deposition temperature.

Influence of the molar concentration and substrate temperature on fluorine-doped zinc oxide thin films chemically sprayed

The effect of both the molar concentration of the starting solution and the substrate temperature on the electrical, morphological, structural and optical properties of chemically sprayed fluorine-doped zinc oxide (ZnO:F) thin films deposited on glass substrates is analyzed in this work. All the starting solutions employed were aged for 10 days before the deposition. The results show that as the molar concentration increases, a decrease in the electrical resistivity values is obtained, reaching the minimum resistivity in films ZnO:F deposited from a 0.4 M solution at 500 °C. A further increase in the molar concentration leads to a very slight increase in the resistivity. On the other hand, as the substrate temperature is increased, the resistivity decreases and a tendency towards to minimum value is evidenced; taking the molar concentration as parameter, minimum values are reached at 500 °C. The obtaining of ZnO:F thin films, with a resistivity as low as 7.8×10−3 Ω cm (sheet resistance of 130 Ω/□ and film thickness of 600 nm) measured in as-deposited films is reported here for the first time. The concurrent effect of the high molar concentration of the starting solution, the substrate temperature values used, and the ageing of the starting solution, which might cause polymerization of the zinc ions with the fluorine species, enhance the electrical properties. The structure of the films is polycrystalline, with a (002) preferential growth. Molar concentration rules the surface morphology as at low concentration an hexagonal and porous structure is developed changing to a uniform compact and small grain size surface in the films deposited with the high molar concentrations.

Physical properties of ZnO:F obtained from a fresh and aged solution of zinc acetate and zinc acetylacetonate

Abstract Fluorine-doped zinc oxide thin films, ZnO:F, were deposited by the spray pyrolysis technique on sodocalcic glass substrates. Two different zinc precursors were used separately, namely, zinc acetate and zinc pentanedionate. The effect of the zinc precursor type, the aging of the starting solution, the substrate temperature and a vacuum-annealing treatment on the electrical, morphological, structural and optical properties was studied, in order to obtain conductive and transparent ZnO:F thin films. The resistivity values of ZnO:F thin films deposited from aged solutions were lower than those films obtained from fresh solutions. The lowest resistivity values of as-grown films deposited at 500 °C, using a two-day aged starting solution of zinc acetate and zinc pentanedionate, were 1.4×10 −2 and 1.8×10 −2 Ω cm, respectively. After a vacuum annealing treatment performed at 400 °C for 30 min a decrease in the resistivity was obtained, reaching a minimum value of 6.5×10 −3 Ω cm for films deposited from an aged solution of zinc acetate. The films were polycrystalline, with a (0 0 2) preferential growth orientation in all the cases. Micrographs obtained by SEM show a uniform surface covers by rounded grains. No evident change in the surface morphology was observed with the different precursors used. The transmittance of films in the visible region was higher than 80%.

Chemically Sprayed Fluorine-Doped Zinc Oxide Thin Films Deposited from Zn(C5H7O2)2: Effect of the Molarity and Substrate Temperature on the Physical Properties

Fluorine-doped zinc oxide thin films, ZnO : F, were deposited by the chemical spray pyrolysis technique using zinc acetylacetonate, Zn(C5H7O2)2, and ammonium fluoride as precursors. The molar concentration in the starting solution and the substrate temperature, Ts, were varied in order to obtain transparent and conductive films. For molarities higher than 0.3 M, growth rate values on the order of 3.5 nm/s were obtained. As the molarity increases, the resistivity ϱ decreases up to a minimum value in the order of 10—2 Ωcm for films deposited from a 0.4 M starting solution and Ts = 475 °C; for higher molarities an increase in ϱ is observed. The preferential growth direction is affected by the molarity and Ts, showing a switching of the (101) planes obtained for the lowest molarities (0.1 and 0.2 M) and Ts < 475 °C to the (002) preferential growth for molarities higher than 0.1 M and all the substrate temperatures used. The mobility and the carrier concentration were varied from 1 to 3 cm2/Vs and 4 × 1020 to 5 × 1020 cm—3, respectively. From scanning electron micrographs, small grain size is evidenced, which can give rise to a high density of trapping states in the grain boundaries. Films were uniform, pinholes-free, with a transmittance in the visible in the order of 85%.