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Indium Doping Effect on Structural, Optical and Electrical Properties of Sprayed ZnO Thin Films [Sensors & Transducers (Canada)](Sensors & Transducers (Canada) Via Acquire Media NewsEdge) Abstract: In the current work, indium doped zinc oxide thin films were deposited by spray pyrolysis technique on glass substrate at 350 °C. The effect of the preparation conditions on the structural, morphological, optical and electrical properties of the films has been studied. The molar ratio of indium in the spray solution was varied from 0 to 5 at %. All the deposited films are polycrystalline with a (002) preferential orientation at low indium concentration. X-ray diffraction technique shows that the quality of the films was deteriorated when increasing indium concentration. Scanning Electron Microscopy and Atomic Force microscopy were performed to examine the surface morphology of the films. The deposited films showed an average optical transmittance around 85 % in the visible region; meanwhile the band gap value was varied between 3.13 and 3.25 eV. Hall Effect measurements revealed that the indium doping induces an increase in the electron concentrations, making the films heavily n type. A lowest resistivity (0.3 ohmcm), compared to that of the undoped ZnO (66 ohmcm), is obtained for the film doped with 3 % of indium. Copyright © 2014 IFSA Publishing, S.L. Keywords: ZnO, TCO, Spray Pyrolysis, Thin films, Characterization. 1. Introduction Transparent conducting oxides (TCO) such as L12O3, ZnO and Sn02, have been widely studied [1-5]. Particularly, thin doped indium oxide (ITO) is the most widely used in solar cells due to its high transparency (90 % at 550 run) associated to a low resistivity (2 1 O'4 Qcm) [6-7]. Nonetheless, since indium is a rare and expensive element, pure and doped zinc oxide films are actively investigated as an alternate candidate. Moreover, these compounds are non toxic, inexpensive, abundant and offer noticeably high chemical, mechanical and thermal stabilities [8]. For that reasons, ZnO is a promising material for several technological applications such as solar cells [9-10], chemical sensors [11-12], electroluminescence displays [13], heat mirrors [14] and Schottky diodes [15]. ZnO is an n type wide bandgap semi-conductor (Eg=3.2 eV), its electrical conductivity is mainly due to intrinsic defects (interstitial zinc atoms, oxygen vacancies) and could be enhanced by adding group III dopants (Al, B, In, Ga) or Group VII (F) [16]. The type of dopant has to be chosen taking into account the fact that the size of impurity ionic radius has to be similar to that of the substituted ion, in order to avoid lattice distortions. The efficiency of the dopant element is related to its electro-negativity and its ionic radius. The deposition of high quality ZnO thin films is reported using a wide variety of techniques, such as sputtering [17], chemical vapor deposition [18], spray pyrolysis [19], sol-gel [20] and electrochemical deposition [21]. Among these techniques, chemical spray technique is an excellent method for depositing thin oxides films as in the case of ZnO material. In this technique, a starting solution, containing the Zn and dopant precursors, is sprayed by means of a nozzle, assisted by a carrier gas, over a heated substrate. When the fine droplets arrive at the substrate surface, the chemical reaction occurs and leads to a new solid compound. The quality and the physical properties of the films depend strongly on many deposition parameters, such as substrate temperature, molar concentration of the starting solution, spray rate, type and rate of carrier gas and geometric characteristics of the spray system. In this work, we have studied the effect of zinc chloride and indium molar concentrations in the starting solution on the structural, optical and electrical properties of the deposited thin films. 2. Experimental Details Zinc oxide thin films are deposited on glass substrate using chemical spray method, which is effective for the large area thin films deposition. The films are prepared with 5102 molar concentrations of pure zinc chloride dissolved in distilled water; then Indium chloride is added to the solution as a dopant with different concentrations (varying from 0.5 to 5 %). Dry air was used as a carrier gas and the spray rate of the solution was 2.6 ml/min. The nozzle was located approximately 40 cm above the substrate. The temperature of the substrate was maintained at 350 ± 10 °C and monitored by a chrome 1-alumel thermocouple close to the substrates. The structural characterization is obtained by mean of an X-ray diffractometer (XRD) with Cu-Ka radiation. Meanwhile, the surface morphology and topography are investigated using both the Scanning Electron Microscopy (SEM) and a Digital Instrument Dimension 3100 atomic force microscopy (AFM), operating in tapping mode. All AFM images were collected using the same silicon tips (320 kHz resonance frequency, 40 N/m stiffness, radius <10 nm), with scan speed of 1 Hz and scan sizes of 2 pm x 2 pm. A Lambda 900 UVYVISYNIR Spectrophotometer is used to determine the optical transmittance properties of the samples. The electrical resistivity, mobility and carrier concentration are determined by Hall Effect measurements using Van Der Paw configuration at room temperature in air. 3. Results and Discussions 3.1. Structural Properties The X-ray diffraction patterns of undoped and indium doped ZnO thin films (IZO) are assembled in Fig. 1. ZnO and IZO films exhibit a strong (002) peak, which indicates that they have a preferential growth orientation along the c-axis perpendicular to the substrate surface, while other orientations like (100), (101) and (102) are also seen with lower intensities. The position of the peaks fits well with the hexagonal wurtzite-type structure of ZnO [22]. On the other hand, it is observed that with the increase in indium doping percentage, the spectra present a noise signal in the baseline, as a consequence of the poor crystallinity of the films, which is due to the fact that the indium incorporation in film network inhibits the growth of crystal grains [23]. Another effect has been observed on the intensity of the peaks corresponding to (101), (102) and (100) planes increases. Similar behavior has been reported by other works [24-25]. It was shown that the intensity of the peak corresponding to (002) plane had a strong dependence on oxygen content in the sample [26]. Hence one could state that due to indium incorporation oxygen concentration was not varying much in the sample. However, some authors reported that the crystallinity degraded at higher indium doping [27] and the preferential orientation are changed at higher doping from (002) to (101) and (100) [28]. It can be deduced that the indium incorporation into our compounds is at the origin of the enhancement of (101) and (100) orientations. No new phases were observed, even when the concentration of In was increased to 5 %, indicating that incorporation of indium neither could change wurtzite structure of ZnO nor resulted in the formation of fri203. The calculated Texture co-efficient [29] (TC) for (002) orientation at different doping concentrations is listed in Table 1. The highest TC value is obtained for the undoped ZnO film. Nevertheless, as the doping concentration is increased, the TC value goes down indicating that the doping process influences strongly the degree of crystallinity of the deposited thin films. The mean crystallites size is calculated for the (002) diffraction peak using Debyes Scherrer formula [30]. Table 1 shows a decrease of the calculated mean grain size from 49 to 23 nm, when increasing the dopant concentration. This is due to the increase of nucléation centers density in the doped films leading to the formation of smaller crystallites [31]. Another effect of increasing In content is the lattice spacing (d) between the planes corresponding to (002) has increased slightly, due to the difference in radii of the Zn2+ and In3+ ions (0.074 and 0.08 nm respectively). The Lattice constant a and c are found to narrowly increase with the In doping. It is reported that indium atoms takes interstitial sites rather than replacing zinc sites thereby deforming the lattice [32]. Moreover it was known that the increase in lattice parameter c could be related to microstrains produced by the residual stress in the film [33]. 3.2. Morphological Properties The deposited films exhibit different morphological aspects according to the (In/Zn) ration. In fact, Fig. 2 showcases micrographs of ZnO:In thin films doped with different atomic [In]/[Zn] contents. It is observed that the surface morphology is clearly modified when changing the In doping ratio. The sample doped with 0.5 at. % (Fig. 2b) presents a surface formed by well-defined grains, with a hexagonal-geometry. Meanwhile, when doping concentration is raised from 0 to 5 at. % the form of the grains (Fig. 2c) involves towards an irregular triangular pyramid with a tetrahedralmorphology. The same behavior has been reported by many authors [34-35]. The observed evolution in the grains form is also accompanied by a structural change which consists in an enhancement of the (001) peak intensity in comparison with (002) peak. This is in good agreement with XRD patterns and SEM observations. Fig. 3 shows the AFM micrographs of ZnO doped with different Indium concentrations. These micrographs illustrate that the substrates are entirely covered with grains of different sizes and reveals that the film deposited at lower dopant concentrations has well defined grains, however, high dopant concentration lead to a more uniform and smoother surface. The Values of root mean square roughness (RMS) and mean roughness (Ra) are given in Table 1. 3.3. Optical Properties Fig. 4 shows the optical transmittance spectra for wave lengths varied from 300 nm to 1000 nm for Zni-xInxO films. These films have a high transmittance (over 85 %) for all samples at 1=550 nm. Usually, the optical absorption at absorption edge corresponds to the transmission from valence band to conduction band, while the absorption in the visible region corresponds to some localized energy states in the band gap. For allowed electronic transition in materials, the absorption coefficient a is given by [36]. ahv = b(hv -Eg)p, (1) where p have discrete values like 1/2, 3/2, 2, or more depending on whether the transition is direct or indirect, and allowed or forbidden. In the direct and allowed cases p =1/2 whereas for direct but forbidden cases it is 3/2. But for the indirect and allowed cases p =2 and for the forbidden cases it will be 3 or more. Eg is the optical band gap, b is a constant, a is the optical absorption coefficient and hv is the photon energy. In our case, the optical transition is direct transition that forms the crystalline semiconductors. There is no doubt that the direct transition across the band gap is feasible between the valence and the conduction band edges in k space. The total energy and momentum of the electronphoton system must be conserved, in this transition [37]. The direct band gap values were obtained by an extrapolation of the linear portion in a plot of (hva)2 against hv (see Fig. 5). The band gap values of Zni. xInxO for x=0, x=0.005, x=0.01, x=0.03 and x=0.05 thin films were found to be 3.25, 3.25, 3.27, 3.17 and 3.13 eV, respectively. It has been found that the optical band gap demonstrated two behaviors while doping. At lower doping (x < 0.01), it was seen that the band gap values increased. In Zinc Oxide structure, Zn2+ has two valence electrons and O2' takes these two electrons from Zn2+ for making ionic bond, thus, in the crystal system there is no free electron and hole. However, In3+ gives three electrons and O2'takes two electrons, so one electron can become free in the system and occurs carrier gradient in the crystal structures as named MossBurstein effect [38] which is an effective mechanism for understanding the optical band gap treatment of degenerate semiconductors such as ZnO. The optical band gap is related with the transition of the electrons from valance band to Fermi energy levels in the conduction band for degenerate semiconductors. The increase in the Eg should be also caused by increasing carrier concentration with In incorporation. According to Moss-Burstein effect optical band values should increase with In doping, but Eg value decreased at higher doping (x > 0.03). That could be attributed to the increase of defects, impurities or completely not substitution of dopant with host atoms or interstitial. Since the ionic radius of In is larger than that of Zn, the In introduction into the film is followed by the increase of lattice parameter. As a consequence, disorder creation happen which gives rise to the localized states near the conduction band in the energy band gap [29] and cause a decrease of the carrier concentration. In fact, the Eg values in this case were due to the electronic transition from the filled valence states to energy levels of defects instead of the electronic transition from the filled valence band to the empty conduction band as usual. In addition, the optical band gap decrease can be due to the influence of various factors such as grain size, carrier concentration, or by deviation from the stoichiometry and lattice strain as it is suggested by other authors [39-42]. This result is in good agreement with electrical characteristics and XRD analysis. 3.4. Electrical Properties Table 2 shows the effect of the doping concentration on the electrical resistivity (p), the hall mobility (p) and the carrier density (/z) of ZnO:In thin films. It was observed that all the films exhibit n type conductivity, which is attributed to a deviation from stoichiometry, due to the oxygen vacancies and interstitial zinc and/or indium atoms. Both vacancies and interstitial atoms act as donors. At room temperature, resistivity values decrease from 66 to 0.3 (Q.cm) as the indium doping percentage increases from 0 to 3 at. % and then increases. Two orders of magnitude lower than that of undoped samples. The lowest resistivity value was obtained for the films doped with 3 % of In. It is shown that indium content is a parameter which affects the electrical properties of these films. Similar behavior has been reported by many authors in In doped ZnO thin film [27, 43-44]. An initial decrease in the resistivity is due to an increase in the free-electron concentration with indium incorporation in the ZnO film. In other words, it can be attributed to the optimal incorporation of indium atoms into the lattice, increasing the donor concentration and contributing to a decrease of the resistivity [45]. However, higher doping content (3 at. %) will hinder the growth in both c-axis and in-plane direction [46]. As a result, the grain will not fully coalesced and create more voids and pits. The film has rough surface with high defect densities, which lead to the decrease of the carrier concentrations as more carriers will be trapped in the defect states and grain boundaries. So there is no more decrease in resistivity. It should be noted that the values of electrical resistivity obtained in the present study are higher than the values reported by K. C. Aw, et al. for Influence of radio frequency sputtering power towards the properties of indium zinc oxide semiconducting films [47]. 4. Conclusions Transparent and conductive indium-doped ZnO thin films are prepared and characterized. The analysis of their structural, morphological and optical properties is performed. The ZnO:In films presented a polycrystalline structure, with a (002) preferred orientation for all the samples. Surface morphology of the films studied showed a uniform surface and variation of roughness with doping. The optical transmittance for all samples were above 85 % at X= 550 run. The adsorption band-edge is found to increase at lower doping while at higher doping it shifts towards lower values. Hall effects measurements show that the resistivity decrease upon x=0.03 of In doping. While the carrier mobility increases. ZnO thin film doped with 3 % indium, presents the lowest resistivity 0.3 Qcm associated to a high transmittance ~85 %. 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Mzerd 1 University Mohammed V-Agdal, Faculty of Sciences, Physics Department, LPM, B. P. 1014, Rabat, Morocco 2 University Mohammed V-Agdal, Faculty of Sciences, Physics Department, LCS, B. P. 1014, Rabat, Morocco 3 University Chouaib Doukkali, Faculty of Sciences, LCCM, Chemistry Department, B. P. 24000, El Jadida, Morocco * E-mail: [email protected] Received: 12 May 2014 /Accepted: 29 August 2014 /Published: 30 September 2014 (c) 2014 IFSA Publishing, S.L. |