Deposition of F-doped ZnO transparent thin films using ZnF2-doped ZnO target under different sputtering substrate temperatures
© Wang et al.; licensee Springer. 2014
Received: 22 October 2013
Accepted: 14 February 2014
Published: 26 February 2014
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.
Transparent conducting oxide (TCO) thin films based on zinc oxide (ZnO) are promising for applications in various optoelectronic devices. In spite of extensive studies on preparation, characterization, and effect of doping on the properties of ZnO, certain effects of either some dopant or preparation procedures are still remaining unclear. However, ZnO-based thin films present a lot of advantages such as low material cost, non-toxicity, and high chemical stability under the hydrogen plasma as compared to tin-doped indium oxide (ITO) . For that, transparent conducting ZnO thin films have already been extensively used in solar cells, light-emitting diodes, and liquid crystal displays as a substitute for ITO [2, 3]. Much more interest has been given to TCOs based on ZnO such as undoped ZnO thin films , Al-doped ZnO (AZO) thin films , and Ga-doped ZnO (GZO) thin films  due to their stability under hydrogen plasma which makes them a potential candidate for solar cells' technology based on thin-film silicon. Fluorine, the ionic radius (0.136 nm) of which is similar to that of oxygen (0.132 nm), may be an adequate anion doping candidate due to lower lattice distortion compared with Al, Ga, and In, but comparatively few studies on fluorine-doped ZnO (F-doped ZnO) can be found in the past researches .
Many different physical and chemical deposition methods were used to investigate the properties of the F-doped ZnO thin films. For example, the F-doped ZnO thin films were deposited on Corning glass by radio-frequency (RF) magnetron sputtering of pure ZnO target in CF4-containing gas mixtures. The fluorine content in F-doped ZnO thin films increased with increasing CF4 concentration in sputter gas . Treharne attempted to achieve substitutional doping of ZnO with F by using trifluoromethane (CHF3) as the partial pressures of Ar-H2-CHF3, where the constant ppH2 of 5% was used and the partial pressures of CHF3 were changed in the range of 0% to 7% . The F-doped ZnO thin films could also be deposited on glass slide substrates using d.c. reactive magnetron sputter at room temperature, with the substrate holder at the floating potential using an Ar/O2(/F2) gas mixture by using the pure metallic Zn as target . Anandhi et al. used the SnCl2 · 2H2O (0.1 M) and Zn(CH3COO)2 · 2H2O (0.2 M) as the host precursors and NH4F as the dopant precursor for the deposition of the F-doped SnO2 layer on the F-doped ZnO (FZO) layer to get the bi-layer thin films by using a simplified spray pyrolysis technique . Rozati et al. used 0.4 M solution containing zinc acetate dehydrated as the host solution, which was dissolved in a mixture of double-distilled water, methanol (3:7 volume proportion), and acetic acid. They used the ammonium fluoride as the doped starting solution, with a fixed [F]/[Zn] ratio of 2 at.%; the F-doped ZnO thin films were also deposited by using a spray pyrolysis technique .
As we know, if the fluorine-based gases are used during the sputtering process, the environmental pollution problem and the problem of the chamber being etched are unavoidable. When the chemical deposition methods are used to deposit F-doped ZnO thin films on the substrate with large area, surface roughness and thickness uniformity are two important problems needed to overcome. In the past, Cao et al. prepared the highly transparent and conducting F-doped ZnO thin films on glass substrates by pulsed laser deposition using a sintered ZnO target containing 1 at.% zinc fluoride (ZnF2) as a function of oxygen pressure ranging from 0.01 to 0.5 Pa . Ku et al. deposited the F-doped ZnO thin films at room temperature by using ZnO target with different ZnF2 contents. The fluorine content which increased almost linearly with increasing ZnF2 content in sputter target was expectable . As physical vapor deposition method is used to deposit the TCO thin films, the substrate temperature will have large effect on their physical and electrical characteristics. Nevertheless, the two articles ([13, 14]) do not investigate the effect of substrate temperature on the characteristics of the F-doped ZnO thin films.
Despite wide usage of magnetron sputtering technique in the fabrication of TCO thin films, only few studies have been reported on using the F-doped ZnO targets to deposit the FZO thin films by using sputtering technique. In this study, 1.5 wt% ZnF2 was added into ZnO powder to prepare the FZO target. The first important topic is that the FZO thin films were deposited by reactive RF magnetron sputtering on glass substrate by changing the substrate temperature. The effects of different substrate temperatures on the physical and electrical properties of the FZO thin films, including the crystallinity, surface and cross-sectional morphologies, carrier mobility, carrier concentration, resistivity, optical transmission spectrum, and optical band gap (Eg) were all well investigated. Because the substrate temperature is increased from room temperature to 300°C, the second important topic is that the secondary ion mass spectrometry (SIMS) analysis is used to find the effect of substrate temperature on the fluorine content in the FZO thin films.
Results and discussion
As Figure 2 shows, as substrate temperatures were RT, 100°C, 200°C, and 300°C, the full width at half maximum (FWHM) values for the (002) peak of the FZO thin films were 0.320°, 0.294°, 0.280°, and 0.268°, respectively. The increase of relative diffraction intensity and the decrease of FWHM value of (002) peak with raising substrate temperature suggest that FZO thin films deposited at higher temperature have the better crystalline structure and the defects in the FZO thin films decrease with raising substrate temperature. This is because as higher substrate temperature is used to deposit the FZO thin films, the FZO particles or molecules can have larger active energy for the crystallization. For that, the number of thin films' defects decreases and the crystallization of the FZO thin films is improved; then, the FWHM value decreases.
When sputtering method is used to deposit the FZO thin films on a glass substrate, the FZO molecules are changed to plasma by the bombardment of Ar. Many defects are believed to be formed during the deposition process, which will inhibit electron movement. The crystallinity of the FZO thin films can be improved by many processes, the increase in the substrate temperature is believed the simplest process. However, as ZnF2 is added into ZnO as dopants and the FZO thin films are deposited at different substrate temperatures, three factors are believed to cause the variations of electrical properties of the FZO thin films. First, the higher substrate temperature provides more energy and thus enhances the mobility of deposition particles or molecules, which can improve crystallization and decrease the number of defects in the FZO thin films; the XRD pattern shown in Figure 2 has proven this result. Second, as the substrate temperature increases, the density of the FZO thin films increases and the barriers inhibiting electron transportation decrease; the SEM morphologies shown in Figure 3 has proven this result. Third, as we know, oxygen vacancies are formed during the deposition processes of the ZnO-based thin films, which will form the intrinsic n-type semiconductors. Too many oxygen vacancies may lead to an increase in the defect and scattering centers of the ZnO-based thin films, which will catch the electron and result in the decreases of carrier concentration and mobility. As the ZnF2 is added into ZnO as dopants, the fluoride will occupy the sites of ionic oxygen, and the problem in the decreases of carrier concentration and mobility can be improved. Figure 4 proves that even if FZO is deposited at 300°C, the fluoride can still be found in the FZO thin films.
where n is a constant, n = 1/2 is the allowed direct transition and n = 2 is the allowed indirect transition, hν is the photon energy, and Eg is the optical band gap. Figure 7b shows the typical (αhν)2 versus hν plots of the FZO thin films as deposited at various substrate temperatures. The n value is around 1/2, and the linear dependence of (αhv)2 on hν indicates that the GZO thin films are direct transition type semiconductor. Figure 7b shows that as the substrate temperature increased from RT to 300°C, the Eg value increased from 3.606 to 3.643 eV.
Equation 4 shows the important relationship between the degenerated semiconductor and the carrier concentration n e , where m e is approximately equal to 0.28 m0, and m0 is the mass of the free electron . In this study, the ne value calculated from Equation 4 is around 0.692, which is close to the theoretical value of 0.667. When the wavelength is equal to 300 nm, the visible light absorbed by the thin films is due to a quantum phenomenon called band edge absorption. Burstein indicated that an increase of the Fermi level in the conduction band of a degenerated semiconductor leads to the energy band widening effect . For that, the Burstein-Moss shift of the absorption edge to the shorter wavelength region is due to the increase in carrier concentration.
FZO thin films had been successfully prepared by the RF magnetron sputtering under different substrate temperatures. A minimum resistivity of 5.27 × 10−4 Ω cm, with a maximum carrier concentration of 5.00 × 1020 cm−3 and a maximum Hall mobility of 23.72 cm2/V s, was obtained for the FZO thin films prepared at the substrate temperature of 300°C. The deposited FZO thin films had the stable resistivity values, because as the FZO thin films were measured after 25 days, only 0.4% increase in the thin films' resistivity was observed. The FZO thin films were uniform, and the average optical transmittance ratio in the entire visible wavelength region was higher than 90%, independent on the substrate temperature. As the substrate temperature increased from RT to 300°C, the Eg value increased from 3.606 to 3.643 eV, which indicated that the blue-shift effect really happened in the FZO thin films.
The authors acknowledge the financial support from NSC 101-2221-E-005-065, NSC 102-2622-E-390-002-CC3, and NSC 102-2221-E-390-027.
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