Fluorine-doped ZnO Thin Films Research Articles

Near room-temperature synthesis of transparent conducting fluorine-doped ZnO thin films as window layer for solar cells

Highly transparent and conducting fluorine-doped ZnO (FZO) thin films were developed by 13.56 MHz RF magnetron sputtering near room-temperature (∼45 °C) on glass substrates. The effects of sputtering gas pressures in the range (10–40) mTorr were studied on the morphological, electronic, and optical characteristics of the FZO thin films. At a plasma pressure of 20 mTorr, the optimum FZO films were obtained with a wide optical band gap Eg ∼3.28 eV and high electrical conductivity of ∼102 S cm−1 via minimization of strain in the network containing 1.2 at.% F. A structurally favored c-axis orientated F-doped ZnO network has been achieved with an average grain size of ∼29 nm, high electrical mobility, μ ∼13 cm2 V−1 s−1 and carrier concentration, ne ∼1020 cm−3 that provided an excellent conduction channel for the stacked-layer devices. Using the optimized FZO films as a transparent conducting (TCO) window layer of the nc-Si solar cell in ‘Glass/FZO/p-nc-SiOX:H (∼20 nm)/i-nc-Si (∼2000 nm)/n-a-Si:H (∼30 nm)/Ag/Al’ configuration, a photovoltaic conversion efficiency (η) of 5.68% with short-circuit current density, JSC ∼13.36 mA cm−2, fill factor, FF ∼58.4%, and open-circuit voltage, VOC ∼727.5 mV was obtained.

Fabrication of spray derived nanostructured n-ZnO/p-Si heterojunction diode and investigation of its response to dark and light

Abstract Fluorine doped ZnO thin films were grown by chemical spray pyrolysis technique of zinc acetate and ammonium fluoride, and the effect of fluorine content on structural, optical and electrical properties were evaluated. The structural, morphological, optical properties of ZnO films were investigated by XRD (X-ray diffraction), AFM (Atomic force microscopy), SEM (Scanning electron microscop) and UV–Vis spectroscopy, respectively. According to results, it was observed that all films had polycrystalline texture with hexagonal wurtzite crystal structure and film surface were made up of nano-scale grains, varied by fluorine content. Optical properties showed that optical band gap energy of ZnO changed from 3.28 to 3.24 eV with F content. Shrinkage effect was assessed as the cause in the variation of optical band gap values. Finally, current-voltage (I-V) analysis was performed in Au/ZnO:F/p-Si device in dark and light conditions and certain diode parameters such as ideality factor, barrier height and series resistance were calculated and discussed in detail.

Effect of Atomic Layer Deposition Temperature on the Growth Orientation, Morphology, and Electrical, Optical, and Band-Structural Properties of ZnO and Fluorine-Doped ZnO Thin Films

The deposition temperature has a significant effect on the growth and physicochemical properties of ZnO thin films. However, changes within a low temperature range have not yet been fully investigated. In this study, ZnO and fluorine-doped (F-doped) ZnO (ZnO:F) thin films were synthesized on glass substrates by atomic layer deposition, and the effect of deposition temperature (80–160 °C) on the crystallization behavior and electrical, optical, and band-structural properties of the thin films were analyzed. During deposition, a constant fluorine concentration was maintained in the anionic pulse gas by employing a 200:1 (v/v) mixing ratio of deionized water to hydrofluoric acid. We found that c-axis growth was preferred with ZnO thin films, while a-axis growth was preferred for ZnO:F thin films. An enhancement in the carrier concentration was also observed in both thin films with increase in the deposition temperature. In addition, the optical transmittance of ZnO:F thin films was slightly higher than that ...

Effect of electron beam on structural, linear and nonlinear properties of nanostructured Fluorine doped ZnO thin films

Electron beam induced effects on Fluorine doped ZnO thin films (FZO) grown by chemical spray pyrolysis deposition technique were studied. The samples were exposed to 8MeV electron beam at different dose rate ranging from 1kGy to 4kGy. All films exhibit a polycrystalline nature which shows an increase in crystallanity with irradiation dosages. The electron beam irradiation effectively controls the films surface morphology and its linear optical characteristics. Z-Scan technique was employed to evaluate the sign and magnitude of nonlinear refractive index and nonlinear absorption coefficient using a continuous wave laser at 632.8nm as light source. Enhancement in the third order nonlinear optical properties was were noted due to electron beam irradiation. Tailoring the physical and NLO properties by electron beam, the FZO thin films becomes a promising candidate for various optoelectronic applications such as phase change memory devices, optical pulse compression, optical switching and laser pulse narrowing.

Optical, Electrical, and UV Photoresponse Properties of Fluorine-Doped ZnO Thin Films Grown on Flexible Mica Substrates

Fluorine-doped ZnO (FZO) thin films have several potential applications, for instance, in low-cost optoelectronic devices; understanding how their optical, electrical, and photoresponse properties depend on and can be controlled via the synthesis conditions is essential for application of these systems. In this study, FZO thin films with different annealing temperatures were grown on muscovite mica substrates via sol–gel spin-coating. In photoluminescence measurements, a strong peak in the ultraviolet (UV) region and a broad peak in the visible region were observed for all films, being strongly dependent on the annealing temperature. The transmittance of the annealed films was slightly higher than that of as-grown film, and the absorption edges in the transmittance spectra red-shifted with increasing annealing temperature. The optical bandgap and Urbach energy of the films were calculated from the absorption coefficient values, using the Tauc and Urbach relations, respectively. Finally, the electrical (i.e., resistivity and carrier concentration) and photoresponse properties of the films were investigated to assess their applicability for use in FZO-based UV detectors.

Interaction of H and F atoms—Origin of the high conductive stability of hydrogen-incorporated F-doped ZnO thin films

We intentionally incorporated H into fluorine-doped ZnO thin films (FZO) by plasma treatment. Upon treatment, both mobility and electron concentration have an observable increase, especially the mobility reaches up to 30.1cm2V−1s−1 (3 times higher than the untreated films). H distributes in the FZO thin films uniformly via plasma treatment. The treated FZO thin films showed good conductive stability at 500°C and saved for 6months, until now. Origin of high conductive stability was explained by first principle calculation. The results predicted that hydrogen atoms in interstitial sites next to FO are attracted by the incorporated fluorine atoms and this configuration has lower formation energy than the hydrogen in oxygen vacancy configuration. Thus, we owed the highly stable conductivity to the hydrogen as interstitial as well as hydrogen in oxygen vacancy.

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%.

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.

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.

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.

Fluorine doped ZnO thin films by RF magnetron sputtering

Growth of ZnO films by reactive RF sputtering in ambients of Ar-H 2 and Ar-H 2-CHF 3 has been investigated as a function of the reactive gas partial pressure (ppx%), substrate temperature and sputtering power. Use of H 2 reduces the resistivity of ZnO by a factor of 10 4, with doping through use of CHF 3 giving a further factor of 10 decrease. The lowest resistivity (2.9 × 10 − 3 Ω cm) was achieved using ppH 2 = 5%, ppCHF 3 = 4% in a total pressure of 5 mTorr made up with Ar. An optical transmittance of > 80% in the visible range was achieved for all doped films. Analysis of Burstein–Moss shifts in measured indirect and direct band-gap values for doped films yield values for the electron and hole effective masses of 0.5 m 0 and 1.2 m 0 respectively, according to a parabolic band model.

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%.

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%.

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.

On the effect of acetic acid on physical properties of chemically sprayed fluorine-doped ZnO thin films

Fluorine-doped zinc oxide (ZnO:F) thin films onto sodocalcic glass substrates, starting from a highly concentrated starting solution (0.4 M) containing zinc acetate and hydrofluoric acid diluted in a mixture of deionized water, acetic acid , and methanol, using the chemical spray deposition technique, were deposited and characterized. The effect of the acetic acid content in the starting solution, and the substrate temperature on the electrical resistivity, structure, morphology and optical characteristics was studied. The samples were polycrystalline in nature, but as the acetic acid content in the starting solution increases, the preferential orientation shows a switching from (002) to (101). For a predetermined deposition temperature, as the acetic acid content increases, the film resistivity values show an increase. The minimum resistivity in the order of 6 × 10 − 3 Ω cm was found for the films deposited with the lowest acetic acid content used. The surface morphology varies from agglomerated grains to rod-like shaped grains as a function of the acetic acid content.

Structural, optical and cathodoluminescence characteristics of sprayed undoped and fluorine-doped ZnO thin films

Undoped ZnO and F-doped ZnFxO(1−x) (x = 5 at%) films prepared by the spray pyrolysis (SP) technique have been studied using cathodoluminescence imaging and spectroscopy, x-ray diffraction and spectrophotometry. These films, deposited at the optimal substrate temperature (Ts = 450 °C), present a hexagonal close packed structure. The doping of this material with fluorine keeps their optical absorption threshold unchanged. At room temperature, the cathodoluminescence (CL) spectra of the undoped and the doped ZnO films exhibit the common near-ultra-violet (UV) band gap peak at λ = 382 nm but they differ as regards their visible emissions. The undoped ZnO emits an intensive blue-green light (λ = 520 nm) and a red emission (λ = 672 nm). The presence of fluorine gives rise to a new light emission corresponding to λ = 454 nm and the disappearance of the blue-green emission. The CL imaging of undoped ZnO films shows that the luminescence is located at defined sites giving rise to a grain-like structure inherent to the sample surface morphology. The presence of fluorine leads to a more homogeneous re-partition of the emitting centres.

Electrical and optical properties of fluorine-doped ZnO thin films prepared by spray pyrolysis

Undoped and fluorine-doped ZnO thin films deposited by spray pyrolysis onto soda-lime glass substrates were electrically and optically characterized. Resistivities as low as 1×10 −1 Ω cm, Hall mobility as high as 10 cm 2/V per s and effective carrier concentration as high as 4×10 19 cm −3 have been obtained. Relative fluorine concentration was determined by the resonant nuclear reaction method. Electron concentrations due to the fluorine incorporation as an effective electrically active donor is always lower than fluorine contents on the films and in the starting solution. Average optical transmisttance on the whole visible range as high as 92% (substrate losses not included) have been obtained for the best conductive films. The refractive index of layers was found to increase with fluorine doping and substrate temperature. The fluorine incorporation, at ZnO thin films, results in a band gap widening effect.