Fluorine-doped ZnO Research Articles

Effect of fluorine doping on the NO2-sensing properties of ZnO thin films

Fluorine-doped ZnO (FZO) thin films were fabricated on glass substrates using a chemical spray pyrolysis method for NO2 gas detection. The physical properties of the films were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and ultraviolet–visible (UV–Vis) spectrometry as a function of fluorine concentration (2 and 5at%). The films exhibited a hexagonal wurtzite structure, and the inclusion of fluorine did not significantly influence the morphology of the films. Optical measurements showed that the band gap energies of the films were affected by the fluorine concentration. Furthermore, the NO2 gas-sensing properties of pristine and FZO thin films were investigated in the concentration range of 100–1000ppb. Fluorine doping in ZnO was found to improve the NO2-sensing capability at the least. All gas sensor measurements showed an optimal sensor response at a temperature of 200°C. In addition, the 2% FZO was found to be more suitable candidate for detecting NO2 gas.

Enhancement of optical and electrical properties of SILAR deposited ZnO thin films through fluorine doping and vacuum annealing for photovoltaic applications

Undoped and fluorine doped ZnO thin films were deposited onto glass substrates using successive ionic layer adsorption and reaction (SILAR) technique and then annealed at 350°C in vacuum ambience. The F doping level was varied from 0 to 15 at% in steps of 5 at%. The XRD analysis showed that all the films are polycrystalline with hexagonal wurtzite structure and preferentially oriented along the (002) plane. Crystallite sizes were found to increase when 5 at% of F is doped and then decreased with further doping. It was seen from the SEM images that the doping causes remarkable changes in the surface morphology and the annealing treatment results in well-defined grains with an improvement in the grain size irrespective of doping level. All the films exhibit good transparency (>70%) after vacuum annealing. Electrical resistivity of the film was found to be minimum (1.32×10−3Ωcm) when the fluorine doping level was 5 at%.

Simultaneous Enhancement of Transparent and Antibacterial Properties of ZnO Films by Suitable F Doping

Undoped and fluorine doped ZnO (ZnO:F) thin films were deposited onto glass substrates by employing a simplified spray pyrolysis technique using a perfume atomizer. The fluorine doping level in the starting solution was varied from 0 to 20 at.% in steps of 5 at.% and the effects of fluorine doping level on the structural, optical, surface morphological properties along with their antibacterial activity against Escherichia coli (E. coli) and Bacillus subtilis (B. subtilis) bacteria were studied and reported. The X-ray diffraction studies revealed that despite the orientation of crystallites was strongly influenced by fluorine doping, the preferential orientation was retained in favor of (002) plane even at the highest doping level. The crystallite size decreased as the F doping level was increased and the reason for this variation has been explained on the basis of Zener pinning effect. The average optical transparency in the visible range was about 80%–90% for doped films and the optical band gap was found to be slightly increased with the F doping level. The antibacterial efficiency of the F doped ZnO films was found to be enhanced as the F doping level was increased. The antibacterial efficiency was more pronounced in the case of B. subtilis (Gram-positive) when compared with E. coli (Gram-negative).

Optimizing the optical properties of fluorine-doped ZnO thin films deposited by sol-gel spin-coating

The optical properties of sol-gel spin-coated ZnO and F-doped ZnO (FZO) thin films were investigated using ultraviolet-visible and temperature-dependent photoluminescence spectroscopies. The FZO films showed on average of > 80% optical transmittance over the entire visible region, and the transmittance spectra and the optical bandgaps were blueshifted owing to the Burstein-Moss effect. The 4 at% FZO film showed the lowest Urbach energy and, thus, the best crystallinity of all the films. Changes in the free exciton, neutral-donor-bound exciton (D0X), donor-acceptor pair, and second-order longitudinal optical phonon replica peaks were represented from the temperature-dependent photoluminescence spectra. The strength of the exciton-phonon coupling was investigated using the Bose-Einstein approximation to fit the experimental data, and the thermal activation energies of the as-deposited ZnO and 4 at% FZO thin films were determined using Arrhenius plots for the integrated PL intensities of the D0X peak.

Deposition of F-doped ZnO transparent thin films using ZnF2-doped ZnO target under different sputtering substrate temperatures

Highly transparent and conducting fluorine-doped ZnO (FZO) thin films were deposited onto glass substrates by radio-frequency (RF) magnetron sputtering, using 1.5 wt% zinc fluoride (ZnF2)-doped ZnO as sputtering target. Structural, electrical, and optical properties of the FZO thin films were investigated as a function of substrate temperature ranging from room temperature (RT) to 300°C. The cross-sectional scanning electron microscopy (SEM) observation and X-ray diffraction analyses showed that the FZO thin films were of polycrystalline nature with a preferential growth along (002) plane perpendicular to the surface of the glass substrate. Secondary ion mass spectrometry (SIMS) analyses of the FZO thin films showed that there was incorporation of F atoms in the FZO thin films, even if the substrate temperature was 300°C. Finally, the effect of substrate temperature on the transmittance ratio, optical energy gap, Hall mobility, carrier concentration, and resistivity of the FZO thin films was also investigated.

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.

The Structural, Optical and Electrical Properties of Spray Deposited Fluorine Doped ZnO Thin Films

ABSTRACTHighly transparent and conducting Fluorine doped zinc oxide thin films were deposited using spray pyrolysis method on glass substrates held at 450 °C. The X-ray diffraction study revealed that as the dopant concentration increases in ZnO films, the intensity of the preferential orientation of (002) reflection decreased and (101) was found to increase up to 5 at. % F. The crystallite size was varied from 40 to 50 nm with dopant concentration. The optical band gap of the un-doped films was 3.30 eV and it increased to 3.34 eV for 3 at. % F. The refractive index of the films was increased from 2.05 to 2.18 with the increase of dopant concentration from 0 to 5 at. %. The scanning electron microscopy results depicted that the microstructure of ZnO: F films highly influenced by the fluorine doping. After annealing the films in hydrogen atmosphere, the resistivity of the films decreased as increase the dopant concentration and it is 4×10−3 Ω cm for 3at. % F beyond which it increased. The mobility of the charge carriers was 14 cm2/ V sec and the carrier concentration was 7.8×1019 cm3 obtained for the films doped with 3 at. % of fluorine concentration in the starting solution.

Synthesis of fluorine doped zinc oxide by reactive magnetron sputtering

ZnO and fluorine doped ZnO (FZO) thin films were prepared by d.c. reactive magnetron sputtering using a zinc target in an Ar/O2(/F2) mixture. In a first attempt ZnO films were synthesized in order to optimize the matrix properties in terms of crystalline properties and transparency. The parameters studied were the d.c. power (Pdc), the total pressure (PTot) and the O2 content in the discharge (%O2). The highest grain size of ∼25nm is obtained for Pdc=70W, PTot=30mtorr and %O2=7.5%. F2 was then introduced in the discharge. The influence of the presence of fluorine on the crystallographic, chemical, electrical and optical properties of the deposited films were evaluated. Our X-ray photoelectron spectroscopy and X-ray diffraction (XRD) data suggest that only a certain part of the measured fluorine atoms substitute for oxygen atoms in the ZnO structure. The rest of fluorine could be adsorbed as F2 on the grain boundaries or located in interstices of the ZnO structure. XRD data reveal a decrease in the crystallite size with an increase in the fluorine content. Above a fluorine concentration of ∼2% the FZO films become amorphous. The electrical properties have been investigated by Hall effect measurements. The optimal synthesis conditions (∼2% of fluorine in the film) were a charge carrier density of ∼1020cm−3, an electrical resistivity of 10−2Ωcm, and a charge mobility of 4cm2Vs−1. Finally, all deposited FZO films had >80% transmission in the visible range.

First-Principles Calculations for Electronic Structures of Flourine-Doped ZnO

The formation energy and electronic structures of Fluorine-doped ZnO have been calculated by the first principles. The calculated results show that Fi, which is easier to form under O-rich conditions, is helpful to achieve p-type ZnO. The main contribution of accept level are the hybridization between F(2p)，O (2p) and Zn(3d) states which are near the top of the valence band.

Luminescence and electrical properties of solution-processed ZnO thin films by adding fluorides and annealing atmosphere

To develop an efficient channel layer for thin film transistors (TFTs), understanding the defect-related luminescence and electrical property is crucial for solution-processed ZnO thin films. Film growth with the fluorides addition, especially using LiF, followed by the oxygen ambient post-annealing leads to decreased defect-related emission as well as enhanced switching property. The saturation mobility and current on/off ratio are 0.31 cm 2 V −1 s −1 and 1.04 × 10 3. Consequently, we can visualize an optimized process condition and characterization method for solution-processed TFT based on the fluorine-doped ZnO film channel layer by considering the overall emission behavior.

Fluorine-doped ZnO thin films deposited by spray pyrolysis technique

Fluorine doped ZnO thin films (FZO) are prepared onto glass substrates at 350 °C by the chemical spray pyrolysis technique. X-ray diffraction spectra show a polycrystalline of ZnO (wurtzite structure) where the amount of fluorine doping affects to preferential orientation (002 plane along c -axis) and does not vary the lattice parameters. Therefore, F introduction in lattice is by the substitution of O -2 ions by F -1 ions. Any variation is observed in transmittance and reflectance measurements in 400–2000 nm wavelength range when samples present F dopant; they have transmittance around 80% in the near infrared and visible zones. The FZO films are degenerate and exhibit n -type electrical conductivity. The best resistivity and mobility are 7.6 × 10 -3 Ω cm and 3.77 cm 2 V -1 s -1 respectively. The calculated values of the mean free path are very small compared to the grain sizes calculated using XRD measurements. Therefore, we suggest that ionized impurity and/or neutral impurity scattering are the dominant scattering mechanisms in these films.

Highly transparent and conducting fluorine-doped ZnO thin films prepared by pulsed laser deposition

Highly transparent and conducting fluorine-doped ZnO (FZO) thin films were deposited on glass substrates by pulsed laser deposition. Structural, electrical, and optical properties of the films were investigated as a function of oxygen pressure ranging from 0.01 to 0.5 Pa. All the films had a highly preferential c-axis orientation. The films obtained were dense and very smooth with a typical columnar structure. A minimum resistivity of 4.83×10 −4 Ω cm, with a carrier concentration of 5.43×10 20 cm −3 and a Hall mobility of 23.8 cm 2 V −1 s −1, was obtained for FZO film prepared at the optimal oxygen pressure of 0.1 Pa. The average optical transmittance in the entire visible wavelength region was higher than 90%.

Study of the properties of undoped and fluorine doped zinc oxide nanoparticles

ZnO and ZnO:F nanoparticles were successfully prepared by precipitation in the presence of oxalic acid. This procedure yields ZnO and ZnO:F nano-arrangements of different lengths consisting of chains of quasi-spherical nanoparticles of ∼ 30 nm in size. Only the wurtzite ZnO structure was detected for both undoped and fluorine doped ZnO nanoparticles. By absorption and photoluminescence measurements, a diminishing of singly ionized oxygen vacancies was detected when fluorine is present in the nanoparticles; while a blue shift of the exciton absorption was observed. Additionally, by NIR reflectance measurements, the creation of free carriers by the fluorine doping of ZnO nanoparticles was observed.

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 cm 2 V − 1 s − 1 , the highest carrier concentration of 3.78 × 10 20 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.

Formation of F-doped ZnO transparent conductive films by sputtering of ZnF 2

Fluorine-doped ZnO transparent conductive thin films were successfully deposited on glass substrate by radio frequency magnetron sputtering of ZnF 2. The effects of rapid thermal annealing in vacuum on the optical and electrical properties of fluorine-doped ZnO thin films have been investigated. X-ray diffraction spectra indicate that no fluorine compounds, such as ZnF 2, except ZnO were observed. The specimen annealed at 500 °C has the lowest resistivity of 6.65 × 10 − 4 Ω cm, the highest carrier concentration of 1.95 × 10 21 cm − 3 , and the highest energy band gap of 3.46 eV. The average transmittance in the visible region of the F-doped ZnO thin films as-deposited and annealed is over 90%.

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.

Structural, optical and electrical properties of F-doped ZnO nanorod semiconductor thin films deposited by sol–gel process

Structural, optical and electrical properties of fluorine-doped ZnO nanostructure semiconductor thin films prepared by sol–gel spin coating method have been investigated. The thin films have polycrystalline structure with a preferential growth along the ZnO (0 0 2) plane. The grain size for the films was found to be in the range of 24–35 nm. Scanning electron microscopy (SEM) images clearly revealed that the 10% F-doped ZnO film was composed of nanorods. Transmittance spectra of the films indicate that the films have high transparency. The optical band gap and Urbach energy of the F-doped ZnO films vary with fluorine doping. The refractive index dispersion curve of 20% F-doped ZnO film obeys the single-oscillator model. The dispersion parameters, E o and E d were found to be 6.104 and 12.045 eV, respectively. For 10% F-doped ZnO film, temperature-dependent conductivity studies revealed that the conduction mechanism is changed from the thermally activated conductivity to the grain boundary scattering with increase in temperature.

Properties of fluorine doped ZnO thin films deposited by magnetron sputtering

Fluorine doped ZnO (FZO) films were deposited on Corning glass by radio frequency (rf) magnetron sputtering of pure ZnO target in CF 4 containing gas mixtures, and the compositional, electrical, optical, and structural properties of the as-grown films as well as the vacuum-annealed films were investigated. The fluorine content in FZO films increased with increasing CF 4 content in sputter gas. FZO films deposited at elevated temperature of 150 °C had considerably lower fluorine content and showed a poorer electrical properties than the films deposited at room temperature. Despite high fluorine contents in the films, for all the FZO films, the carrier concentration remained below 2×10 20 cm −3, leading to fairly low doping efficiency level. Vacuum-annealing of the FZO films deposited at room temperature resulted in substantial increase of Hall mobilities, reaching as high as 43 cm 2/Vs. This was attributed partly to the removing of oxygen vacancies and/or the forming chemical bonds with interstitial zinc atoms by fluorine interstitials and partly to the passivation effect of excess fluorine atoms by filling in the dangling bonds at the grain boundaries. For all the films with thickness of around 300 nm, the optical transmissions in visible were higher than 80%, and increased with increasing fluorine content up to 85% for the film with highest fluorine content.

Effect of fluorine doping on the properties of ZnO films deposited by radio frequency magnetron sputtering

ZnO films with varying fluorine content were prepared on Corning glass by radio frequency magnetron sputtering of ZnO target containing ZnF2 at room temperature, and the compositional, electrical, optical, and structural properties of the as-grown films together with the vacuum-annealed films were investigated. The fluorine content in the fluorine doped ZnO (FZO) films increased almost linearly with increasing ZnF2 content in sputter target, and the highest atomic concentration was 7.3%. Vacuum-annealing caused a slight reduction of fluorine content in the films. The resistivity of the as-grown FZO films deposited showed a typical valley-like behavior with respect to the fluorine content in film, i.e. having minimum resistivity at intermediate fluorine content. Despite high fluorine content in the FZO films, the carrier concentration remained below 1.2 × 1020 cm−3, leading to very low doping efficiency level. Upon vacuum-annealing, the resistivity of FZO films decreased substantially due to increase in both the carrier concentration and the Hall mobility. From the structural analysis made by X-ray diffraction study, it was shown that addition of small amount of fluorine enhanced the crystallinity of FZO films with (002) preferred orientation, and that large amount of fluorine addition yielded disruption of preferred orientation. It was also shown that doping of fluorine rendered a beneficial effect in reducing the absorption loss of ZnO films in visible range, thereby substantially enhancing the figure of merit.

Synthesis of ethyl zinc fluoroalkoxide complexes and the crystal structure of [EtZn(py){μ-OCH(CF 3) 2}] 2

Fluorinated alkoxide complexes of zinc were synthesized for possible use as precursors to fluorine-doped ZnO films. Diethyl zinc reacted with fluorinated alcohols to form [EtZn(OR f)] n (R f = CH(CF 3) 2, CMe 2CF 3, CMe(CF 3) 2) compounds. The [EtZn(OR f)] n compounds reacted with excess pyridine to yield the pyridine adducts [EtZn(py)(μ-OR f)] 2. The X-ray structure of [EtZn(py){μ-OCH(CF 3) 2}] 2 showed that it has virtual C i symmetry.