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Electrical and Optical Properties of ZAO Thin Films Prepared by Sol-Gel Dip-Coating Method Chang-Bin Shen1ˈ2, Hai-Feng Li2, Jian-Feng Li2Yu-Zhi Xue21,Liaoning Key Materials Laboratory for Railway Dalian Jiaotong UniversityDalian,China 2, School of materials Science and Engineering Dalian Jiaotong UniversityDalian,ChinaAbstract:Firstly, undoped ZnO thin films were prepared on glass substrates by Sol-Gel dip-coating (by assembled diminutive dip-coating device),and radio-frequency magnetron sputtering respectively. X-ray diffraction(XR D). Scanning Electron Microscopy(SEM), Spectrophotometer, Surface profilometry were used to characterize their properties. The results indicated:on the glass substrates, at the same annealed temperature, the films prepared by radio-frequency magnetron sputtering had more excellent crystallization quality. Then, Al-doped (ZnO:Al(ZAO)) thin films were solely prepared on glass substrates by Sol-Gel dip-coating method(by assembled diminutive dip-coating device), Scanning Electron Microscopy(SEM), UV-Visible spectrophotometry(UV-Vis), digital four-point probe electrical resistance measurement were used to characterize their surface morphologies, transmission property, film thickness, resistance etc. The results indicated: The morphologies of ZAO thin films were more compact when the Al3+ dopant concentration was 1.0at%; with increasing annealing temperature, the average grain size of the film increased, the resistance decreased. Under the Al3+ dopant concentration of 1.5at%, annealing temperature of 5500C and annealing timeof 2h , the average transmissivity of the film exceeded 70% within the visible region and resistance was 5.9×10-2ȍ.cm.Key words- Sol-gel dip-coating, Radio-frequency magnetron sputtering, ZnO/ZAO thin Film,Resistance, transmissivityI.INTRODUCTIONZinc oxide is a sort of n-type semiconductor that possesses piezoelectricity and photoelectricity characteristics, having applications in shock-resistance, sound insulation, etc [1,2].I n particular,Al-doped zinc oxide(ZAO) films exhibit high transparency, low resistivity and are suitable for fabricating transparent electrodes for efficient solar cell windows, gas sensors and ultrasonic oscillators[3-7]. They are alternative materials to tin oxide and indium oxide films, the tin oxide and indium oxide films have some internal problems such as high cost, low stability to H2 plasma and toxicity. Whereas, ZnO film has a lot of advantages, such as low cost, non-toxicity and high stability in H2 plasma atmosphere with good electrical and optical properties.The thin films have been prepared by a variety of techniques such as sol-gel method[8-12], metal organic chemical vapor deposition(MOCVD)[13], pulsed laser deposition[14], RF magnetron sputtering [15], spray pyrolysis [1], etc. Among them, the sol-gel method has distinct advantages over the other techniques due to excellent compositional control, homogeneityon the molecular level.n the present communication, firstly,undoped ZnO thin films were prepared on glass substrates by Sol-Gel dip-coating method(by assembled diminutive dip-coating device),and RF magnetron sputtering respectively.Then Al-doped ZnO thin films with high degree of c-axis orientations were obtained on glass substrates by sol-gel dip-coating method.Based on X-ray diffraction(XRD) data,Scanning electron microscopy (SEM),UV-Visible spectrophotometry(UV-Vis), digital four-point probe resistance measurement, the effect of the fabrication methods on the roughness and crystalliazation degrees of the undoped films,the doping effect of aluminum ion,optical and electrical properties of the ZAO films were observed and discussed.II.EXPERIMENT PROCEDURE2.1.Apparatus The phase evaluation of the films was examined by high resolution X-ray diffraction ( XRD,XRD-6000, Shimadzu,Japan) by using Cu KĮ1 radiation (Ȝ=0.15406nm) in ș-2ș geometry, the surface morphologies of samples were evaluated from scanning electron microscopy (SEM,JSM-6360LV,Jeol,Japan). The transmissivity in the visible-near infrared spectra region(Ȝ=300-900nm) was obtained by UV-Visible spectrophotometer(UV-Vis,U-3310,Hitachi,Japan),the transmissivity was automatically calibrated against a bare glass substrate as a reference sample,the roughness of the films were studied by means of a surface profilometry(DEKTAK 6M,VEECO,USA), the resistance was measured from contact-type DC 2 electrode-4 terminal appliance(GKD11X1A, Midwest, China), the sol-gel dip-coating experiment was carried out on a diminutivedip-coating device (TL-1) assembled by the affiliated research group of the authors, the schematic drawing was plotted in Fig.1.The radio-frequency magnetron sputtering was carriedout on a RF magnetron sputter assembled by State Key Laboratory of Materials Modification by Laser,Ion and Electron Beams ,Dalian University of Technology,DUT.978-1-4244-4412-0/09/$25.00 ©2009 IEEE2.2. Chemicals and materials I n the RF magnetron sputtering and Sol-Gel dip-coating method, the bare glass slices were chosen as the substrates, they were firstly cleaned by ultrasonic with a sequence of ethanol, acetone and isopropyl alcohol for 10min, rinsed in deionized water, and subsequently dried before the deposition.n the RF magnetron sputtering, a twin-counter 99.99% purity zinc target with 52mm diameter and 3mm thickness was used. The distance between the substrates and the target was 40mm. Before the deposition, the chamber of the coating system was pumped to the background pressure of 5.0×10-3Pa, and the Zn target was pre-sputtered in an argon atmosphere during 10min to eliminate the surface pollutions. After that, a mixture gas of Ar (99.999% purity) and O 2 (99.995% purity) was introduced as discharge and reaction gas to deposit ZnO films with a O 2-to-Ar mass ratio of 1:2, and the total mixture pressure (working pressure) was kept at 0.6Pa, while the sputtering power was 500W, the frequency was 45KHz. The substrate was kept at room temperature without heating in the fabrication process. After the deposition, the films were annealed for 2h in the atmosphere at 5000C.1.Power2.Engine3.Reducer4. Receptacle5.Glass Substrate6.Clamp7.PulleyFig.1. Schematic drawing of diminutive dip-coatingdevice TL-1I n the Sol-Gel dip-coating method, zinc acetate dihydrate(Zn(CH 3COO)2.2H 2O) was chosen as precursor, aluminumchloride hexahydrate (AlCl 3.6H 2O) was chosen as the starting materials,I sopropanol and monoethanola- mine (MEA) were used as a solvent and stabilizer, respectively. Aluminum chloride hexahydrate was first dissolved in ethanol, and the concentration of Al 3+ was 0.2mol/L (solution A). Zinc acetate dihydrate was dissolved in a mixture of isopropanol and MEA solution, and the precursor solution have been mixed thoroughly by a magnetic stirrer. The molar ratio of MEA to zinc acetate was maintained at 1:1 and the concentration of zinc acetate was 0.75mol/L (solution B). The molar ratio of dopant in the solution, [Al/Zn], was varied between 1% and 3% by dripping different volumes solution A to solution B. The glass substrate was dipped in the solution by using the dip-coating device TL-1 illustrated by Fig.1. Then the films were put into a furnace and dried at 3000C for 10 minutes. this procedure was repeated 10 times for the purpose of needed films thickness, then annealing for 2h at 5000C.III. RESULTS AND DISCUSSIONS3.1.Undoped ZnO thin films characteristics comparsion between those prepared by sol-g el dipping coating and those by RF magnetron sputtering3.1.1. XRD results Fig.2. was the XRD patterns of the undoped ZnO thin films, Fig.2(a) was that of the sample prepared by sol-gel,Fig.2(b) was that of the sample prepared by magnetron sputtering. In Fig.2(a),there were three peaks corresponding to (100),(002) and (101) respectively. From Fig.2(b), it showed the formation of wurtzite ZnO structure with dominant peak at 34.44o or so corresponding to (002) and indicated the polycrystalline nature of films with a preferred c-axis orientation[17]. Meanwhile, the increase of intensity of (002) was an indication of a larger amount of volume of crystallites oriented along (002) plane and clearly indicated that the preparation by RF magnetron sputtering improved the crystalline nature of deposited ZnO film.303234363840200400600800100012001400(101)(100)(002)I n t e n s i t y (a .u .)2θ(d eg )3032343638400.05.0x1031.0x1041.5x1042.0x1042.5x1043.0x104(002)I n t e n s i t y (a .u .)2θ(deg)Fig.2. (a) sample prepared by sol-gel;(b)sample preparedby RF magnetron sputtering3.1.2. Transmissivity results Fig.3. was the relationship between the wavelengh and the transmissivity for both the samples. From Fig.3, at the visible –near infrared region, it was observed that the average transmissions of both the films were higher than 80% and were highly transparent in the visible region. For the ZnO film prepared by Sol-Gel, at the vicinity of Ȝ=375nm, a sharp fundamental absorption edge could be showed. When the normal wavelength was less than 375nm, the transmissivity was negligible because of the absorbtion of the ZnO film. When it exceeded 375nm, the transmissivity was abruptly up to 70% or so, which approached the absorption band edge(Ȝ0). According to Eq.1, the optical band gap of ZnO film was approximately evaluated as 3.31eV . Ȝ0˙1.24/E g (eV) (ȝm) (1)Herein , Ȝ0 was the absorption band edge, E g was the optical band gap. For the ZnO film prepared by RF magnetron sputtering, a sharp fundamental absorption edge could be observed at the vicinity of Ȝ=390nm,by the same means, the optical band gap of ZnO film was approximately evaluated as 3.18eV . the optical band gap obtained by sol-gel was more consistent with the published results[17],namely 3.3eV , than that by RF magnetron sputtering. When Ȝ> 850nm, the transmissivities of both the samples decreased rapidly, but in the visible region, the average transmissivity of the sample by sol-gel was higher than that by RF magnetron sputtering. This would be attributed to thickness difference, the higher the thickness of the film, the lower the transmissivity due to the reduction in the scattering at the grain boundaries. By DEKTAK 6M surface profilometry, the film thickness for sol-gel was 321nm, for RF magnetron sputtering, 1130nm.-20020406080100T (%)W a vele ng th (n m )Fig.3. The transmissivity of ZnO thin films prepared bySol-Gel and magnetron sputtering3.2. the ZAO thin films characteristics by sol-g el dipping coating3.2.1. Structural properties with different aluminum ion concentrations I n order to investigate the effect of aluminum ion concentration on properties of Al-doped ZnO thin films, different concentrations of the dopant (1.0, 1.5 and 2.0 at.% respectively) were used. The XRD patterns of three films at different dopant concentrations were shown in Fig.4. It indicated that the films were polycrystalline but with a preferential orientation along the c-axis. With the increase of dopant concentration, the crystallinity of films was deteriorated, which may be due to the formation of stresses by the different ion sizes between zinc and the aluminum ion.2θ(d e g .)Fig.4 XRD patterns of ZAO thin films with different Al-dopant concentrationThe 1.0 at.% aluminum-doped ZnO thin film had the highest (002) diffraction intensity. Moreover, the peak intensity of these films decreased with increasing dopant concentration. To calculate the average gain size of the samples, the Scherrerequation[18] as follows was employed:D=k Ȝ/(Bcos θ) (2)Where D is the crystallite size; k , a fixed number of 0.94; Ȝ,the X-ray wavelength of 1.54 Å; θ, the Bragg’s angle in degree and B, the full width at half maximum of the (002) plane. Among them, the calculated average crystallite size of 1.0at.% aluminum-doped thin films was about 33nm. 3.2.2. Electrical properties Being a transparent conducting film, the electrical properties are an important aspect of the performance of the ZAO thin films. The effects of doping aluminum ion concentration (a), annealing temperatures (b) on the resistance of the doped ZAO thin films were presented in Fig.5.The resistance variation of Al-doped ZnO thin films with different dopant concentration was displayed in Fig.5 (a). The resistance first decreased with increased aluminum ion concentration. A minimum resistance of 5.9×10-2ȍcm was obtained at a dopant concentration of 1.5 at.%, then the resistance of doped films increased with increasing dopant concentration, it may be due to a decrease of the mobility of the carriers caused by the segregation of the dopant at the grain boundary . When a little amount of Al was introduced into the film, Al was ionized into Al 3+ and replaced the position of Zn 2+. Thus one free electron was obtained from one zinc atom replacement. Therefore, at first, the carrier concentration increased with Al concentration up to 1.5 at.%. At higher Al concentrations, the carrier concentration decreased ,because the increasing of the dopant atoms may form some kinds of neutral defects and these neutralized Al atoms didn’t release free electrons. The amount of electrically-active Al in the film decreased when the concentration of the dopant was high, i.e. more Al atoms were neutralized at higher Al concentration.3.2.4. Optical properties As a transparent conductive thin film, besides the electrical properties, the optical properties were also an important aspect of the ZAO thin films. The effect of Al dopant on optical properties of ZnO films had been studied. The transmission spectra of the samples with various dopant concentrations were plotted in Fig.6. All of the films exhibited a transmission of more than 75% in the visible region with a sharp fundamental absorption edge. Moreover, the transmissivity of doped film with 1.0 at% Al were near 90% for wavelengths over 400nm, and was higher than that of the doped film with 2.0 at% and 3.0 at.%. This may be due to the fact that the film with 1.0 at.% dopant presented more voids than the films with 2.0 mol% and 3.0 mol% dopant, which may lead to a decrease in optical scattering.Fig.6 (b) indicated that, when the layer number of the ZAO films increased from 2 to 6, the transmissivity of the ZAO films decreased gradually.IV.CONCLUSIONS(1) On the same glass substrates, at the same annealed temperature, the films prepared by radio-frequency magnetron sputtering had more excellent crystallization quality, Both of the transmissivities of ZnO films were over 80% when the wavelength was at the range from 390 to 850 nm, The fundamental absorption edge of ZnO thin film prepared by Sol-Gel method was about 375nm,and that of ZnO thin film prepared by RF magnetron sputtering was about 390 nm.(2) Al-doped zinc oxide (ZAO) transparent thin films were prepared by sol-gel dip-coating method. The 1.0 at.% aluminum-doped ZnO thin film had the highest (002) diffraction peak intensity and largest crystallite size of 33 nm. When the annealing temperature rose from 3500C to 4500C, the films were oriented more preferentially along the (002) direction, the grain size of the films increased from 8nm to 30nm, and the resistivity decreased. Films doped with 1.0 at% Al had stronger orientation along the (002) direction, higher transmissivity than those of the other doped films, but the films doped with 1.5at.% Al had the lowest resistance of 5.9×10-2ȍcm when the annealing temperature was 5500C for 1h.5678910111213R e s i s t a n c e ( 10 -2Ω.c m )A l io n c o n c e n tra tio n (a t.%)5678910R e s i s t a n c e (10 -2Ω.c m )A n n e a lin g te m p e r a tu r e (0C )Fig.5. 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