Optical and electrical properties of aluminum doped zinc oxide thin films at various doping concentrations

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1 Journal of the Ceramic Society of Japan 7 [] Paper Optical and electrical properties of aluminum doped zinc oxide thin films at various doping concentrations Mohamad Hafiz MAMAT, Mohamad Zainizan SAHDAN, Suhaidah AMIZAM, * Hartini Ahmad RAFAIE, * Zuraida KHUSAIMI * and Mohamad RUSOP * Solar Cell Laboratory, Faculty of Electrical Engineering, Universiti Teknologi MARA, Shah Alam, Selangor, Malaysia * NANO-SciTech Centre, Institute of Science, Universiti Teknologi MARA, Shah Alam, Selangor, Malaysia Aluminum (Al) doped zinc oxide (ZnO) thin films have been prepared using sol gel spin-coating method at various doping concentrations. The thin films were characterized using UV-Vis-NIR spectrophotometer and current-voltage (I V) measurement system for optical and electrical properties respectively. The results show all films exhibit low absorbance in visible and near infrared (NIR) region. The calculated Urbach energy indicated the defects in the thin films increase with doping concentrations. The electrical properties of Al doped ZnO thin films improved with Al doping as measured through I V measurement system The Ceramic Society of Japan. All rights reserved. Key-words : Al doped ZnO, Sol gel spin-coating method, Doping concentrations, Optical properties, Electrical properties [Received May 2, 2009; Accepted July 6, 2009]. Introduction Zinc oxide (ZnO) is a transparent semiconductor material which has wide band gap energy of about 3.3 ev (300 K) and has large exciton energy of 60 mev. ) It is non-toxic, readily available and cheap material with high thermal stability and harsh environment resistance. Due to its properties, ZnO has been used in a wide range of applications such as dye-sensitized solar cell, light emitting diode and sensors. 2) 5) Naturally ZnO is n-type conductivity semiconductor material due to native defects such as Zn interstitials and O vacancies. 6) The non-stoichiometry of intrinsic ZnO could produce low resistance characteristics by its native defects, but the properties are unstable. Since the performances of the thin film based electronic and optoelectronic devices are greatly dependent on the thin film properties, it is very important to degenerate stable wide band gap semiconductors with low specific resistance and high transparency in the visible wavelength range. One way to improve n-type conductivity of ZnO thin films properties is by doping process. The doping process could be achieved using group-iii elements such as aluminum (Al), boron (B), gallium (Ga) and indium (In). The doped ZnO produce highly conductive ZnO thin films with high transmittance in the visible range which are very useful in electronic devices fabrication. In this paper, we report the properties of Al doped ZnO thin film prepared at different Al doping concentrations using sol gel spin-coating method. The study is important to indentify the effect of Al doping on the ZnO thin films properties towards the electronic and optoelectronic devices application. 2. Experimental The solutions were prepared by dissolving zinc acetate, (Zn(CH 3CO 2) 2 2H 2O) and aluminum nitrate (Al(NO 3) 3 9H 2O) in 2-methoxyethanol solution with the presence of monoethanolamine (MEA) as a stabilizer. The molar ratio between zinc acetate and MEA was fixed at.0. The quantity of aluminum nitrate were controlled to achieved 0.0 at.% (undoped), 0.2 at.%, 0.4 at.%, 0.6 at.%, 0.8 at.%,.0 at.%, 2.0 at.% and 3.0 at.% doping concentration in the ZnO solutions. The solutions were stirred at 80 C for 3 h before aged at room temperature for 24 h. The deposition process by spin-coating technique was conducted at the speed of 3000 rpm for 60 s. The glass substrate was spin while coating with 0 drops of the prepared solution. After the process, the coated substrate was dried at 50 C for 0 min using furnace to evaporate the solvent and remove organic residual. The spin-coating deposition and drying procedures were repeated 0 times to increase the film thickness. The deposition process was conducted for all prepared solutions to obtain thin films at different Al doping concentrations. The thin films were then annealed at 500 C for h in atmospheric ambient for decomposition and oxidation of the precursors. The optical and electrical properties of the thin films were investigated using UV-Vis-NIR spectrophotometer (Cary 5000) and 2 probes current-voltage (I V) measurement system (Advantest R6243), respectively. The platinum (Pt) metal contacts or electrodes for I V measurement were deposited using sputter coater (EMITECH K550X) at deposition current of 50 ma and deposition time of 4 min. Surface morphologies of the thin films were investigated using field emission electron microscope (FESEM, JEOL Ltd., JSM5600F). The crystallinity properties were studied using X-ray diffractometer (XRD, Rigaku Co., D/MAX 2000). 3. Results and discussion The absorption coefficient, of undoped and Al doped ZnO thin films as a function of the wavelength at different doping concentration is shown in Fig.. The absorption coefficient was obtained through Lambert s law which is indicated by following relation Eq. (): Corresponding author: M. H. Mamat; hafiz_030@yahoo. com 2009 The Ceramic Society of Japan α = ln t T () 263

2 JCS-Japan Mamat et al.: Optical and electrical properties of aluminum doped zinc oxide thin films at various doping concentrations Table. Optical Band Gap Energy, Urbach Energy, Electron Concentration and Porosity of ZnO thin Films at Different Doping Concentrations Fig.. Absorption coefficient, α of Al doped ZnO thin films at various doping concentrations calculated from optical transmittance data. where t is the thin film thickness and T is the transmittance spectrum of thin film. The transmittance spectra of ZnO thin films were measured using UV-Vis-NIR spectrophotometer. The result shows all films exhibit very low absorption in the visible and near infra-red (NIR) range ( nm) but exhibit high absorption in the ultra-violet (UV) range. The properties indicate transparency characteristic of ZnO thin films in the visible and NIR region. High absorbance characteristic in the UV range are due to its wide band gap properties. The optical band gap energy was calculated using transmittance data and Tauc s plot to give the values as indicates in Table. The data shows that undoped and Al doped ZnO thin films have optical band gap energy which is in the range of UV light band energy. When photon energy which is equal or higher than optical band gap energy is supplied, the electron will have adequate energy to jump from the valence band to conduction band. The situation explained the strong absorbance properties of undoped and Al doped ZnO thin films in the UV region where the photon energy is used for electron excitation from valence band to conduction band. The studies of carrier concentration using UV-Vis-NIR spectrophotometer measurement data are not new which have been reported in the literatures. 7),8) The Burstein-Moss effect explained the broadening of band gap energy with the increasing of carrier concentration. 9) It is due to the donor electron occupying the states at the bottom of the conduction band since the Pauli principle prevent donor electron from being doubly occupied. In that case, measured optical band gap energy is equal to the sum of intrinsic band gap energy plus energy of state occupied by the donor electron at the conduction band which caused band gap widening. The electron concentration could be calculated by the shift of band gap energy as shown in Eq. (2): 3N = 2 3 (2) h 8 m * π where ΔE BM is the energy band gap broadening, h is the Planck constant, m* is the effective mass of the electron and N is the carrier concentration. Calculated carrier concentrations with doping concentrations are shown in Table. The data indicated that carrier concentrations increased with Al doping concentrations which promoted the optical band gap energy. The porosity of the film is determined by Lorentz Lorentz equation as have been discussed in the literatures. 0),) It is ΔE BM 2 Doping Concentration (at.%) shown by following Eq. (3): 2 n f 2 n Porosity = f 2 (3) 2 n s 2 ns 2 where n f is the refractive index of the ZnO thin films and n s is the refractive index of ZnO skeleton. The value of n s is widely accepted as 2. 2) The n f is calculated using following Eqs. (4) and (5): where Optical Band Gap Energy, E g (ev) Urbach Energy, E 0 (ev) Electron Concentration, N (cm 3 ) Porosity (%) 0.0 (Undoped) nf = N + ( N S 2 ) N ( ) 2 2s s + N = (5) Tm 2 where s is the refractive index of the glass substrate (.52) and T m is the envelope function of the transmittance maxima. 3) The T m value is obtained by averaging transmittance data at the transparent region or where a value is closed to 0 which in our study to be in nm region. 4) The calculated porosity for all films are summarized in Table. The result suggested the tendency of the porosity increment with doping concentration. The structural disorder in the material could be characterized by the slope of Urbach tail or Urbach energy. The value could also be used to evaluate defect concentration in ZnO thin films. The Urbach law stated absorption coefficient α (λ) near the band edges is an exponential dependence on photon energy, hv as given by following Eq. (6): 4) α = α0 exp hv (6) E 0 where E 0 is Urbach energy and α 0 is a constant. Hence, the Urbach energy could be determined through the plot of ln [α(λ)] versus photon energy, hv where the value of the Urbach energy is equal to the reciprocal gradient of the linear potion of the plot. The urbach energy has the dimension of energy which is related to the width of the localized states in the band gap. It is considered as a free parameter which indicates the effect of all possible defects. The defects in the ZnO give local electric fields that affect band tailing. 5) Band tailing also occurs due to donor levels broadening into impurity bands which merge with the conduction band. 6) The Urbach energy obtained from Fig. 2 shows (4) 264

3 Journal of the Ceramic Society of Japan 7 [] JCS-Japan increment pattern with doping concentrations indicating that more defects are generated at higher doping concentration as shown in Fig. 3. Zn and Al have a different ionic radius (rzn 2+ = nm and ral 3+ = nm) which affects on lattice parameters. 7) The more Al 3+ is introduced, the more influence will be on ZnO lattice structure by the formation of stress. When doping concentration increase, due to the stress formation as the result of ionic radius difference between Zn 2+ and Al 3+, lattice distortion level in the thin films increase. The increment of the distortion level will lead to energy level extending from band edge into the forbidden gap resulting in the increment of Urbach energy with doping concentrations. The doping process also increases electron concentration which could induce band tailing in the forbidden gap. With increment of carrier concentrations, the possibility of electron electron and electronimpurity scattering occurrence increase which then increases the Urbach energy of the thin film at higher doping concentration. In order to look into the electrical properties of Al doped ZnO thin films, I V characteristics have been studied using 2 probes system at room temperature under room illumination. Figure 4 presents the I V curve of Al doped ZnO thin films at different doping concentrations at applied voltage from 0 to 0 V. The I V curves show that all films exhibit Ohmic behavior which indicate Pt gives Ohmic contact with the thin films. The current intensity of ZnO thin film doped at at.% Al is the highest, indicating the best electrical properties of the thin film, while undoped ZnO thin film shows the lowest current intensity reflecting poor properties of its electrical conductance behavior compared to doped films. The conductivity of the thin films as the function of Al doping concentrations is shown in Fig. 5. The result shows the conductivity of thin films increases as doping concentration increases from 0.0 at.% to.0 at.%. However, when the doping concentration is further increased to 2.0 at.% and 3.0 at.%, the conductivity plot shows reduction pattern. The result suggested that doping concentration at.0 at.% is the optimum doping concentration parameter which gives the highest conductivity of the thin films. The increment of the thin films conduction behavior at Al concentration from 0.0 at.% to.0 at.% is due to effective substitution of Al atoms in Zn sites of the ZnO structure. The substitution produces free electron which increased the carrier concentration in the thin films. As more dopant is introduced, more free electrons will be generated and the higher will be carrier concentration in the thin film. The same condition is believed to occur for thin films doped with 2.0 at.% and 3.0 at.% Al, where substitutional doping increased the carrier concentration of the thin films as indicated from increment of band gap energy with doping concentration. The reduction pattern of thin films conductivity at doping concentration above.0 at.% might be due to the decrement of electron mobility as a result of ZnO structure deterioration. 8) The structure deterioration occurred when excessive Al produce defects in the ZnO thin film such as segregation of Al 2O 3 at the grain boundaries. Besides that, due to different ionic radius between Al 3+ and Zn 2+, the lattice par- Fig. 2. ln (α) vs hv graph for Urbach energy estimation of undoped and Al doped ZnO thin films. Fig. 4. I V curves of ZnO thin films at different Al doping concentrations using Pt as metal contacts by 2 probes measurement. Urbach energy of Al doped ZnO thin films with doping concen- Fig. 3. trations. Fig. 5. Electrical conductivity of ZnO thin films at different Al doping concentrations. 265

4 JCS-Japan Mamat et al.: Optical and electrical properties of aluminum doped zinc oxide thin films at various doping concentrations Fig. 6. Surface morphology of ZnO thin films at (a) 0.0 at.%(undoped), (b) at.%, (c) 2 at.% and (d) 3 at.% Al doping concentration deposited using sol gel spin-coating method. ameter will be affected heavily as the concentration of Al doping increased which disturbed the ZnO structural properties. As have been explained by Urbach energy calculation, the structural disorder concentration in thin film becomes higher with doping concentration. This results in decrement of thin films electrical conductance behavior. FESEM images of Al doped ZnO thin films at doping concentration of 0.0 at.% (undoped),.0 at.%., 2.0 at.%. and 3.0 at.% are shown in Fig. 6. The FESEM images reveal all films are uniformly deposited with nanoscale ZnO particles. It could be observed from the FESEM images that particle size tends to reduce with the increment of doping concentration. The average Al doped ZnO particles size estimated from FESEM images to be 56 nm, 39 nm, 36 nm and 20 nm for undoped,.0 at. %., 2.0 at. %. and 3.0 at. % Al doped ZnO thin films, respectively. For Al doped ZnO thin films at doping concentration of 2.0 at. %. and 3.0 at. %, there are crack areas presented which might be caused by lattice strain due to difference in ionic radius between Zn 2+ and Al 3+. XRD patterns of undoped ZnO and Al doped ZnO thin films are shown in Fig. 7. The XRD pattern indicates all films exhibited polycrystalline structure that belongs to ZnO hexagonal wurtzite type (JCPDS #36 45). It is observed that all XRD peaks were influenced by doping process where the peaks intensity reduced with doping concentrations. The peaks at doping concentration of 2.0 at.% and 3.0 at.% were more reduced to indicate higher doping concentration heavily affects ZnO thin films structural properties. The result suggested that the crystallinity of ZnO thin films decreased with dopant concentrations. The aggravation of ZnO crystalinity with doping concentration might be from the formation of stress by the difference in ion size between Zn and Al. 7) The segregation of dopants in grain boundaries at higher dopant concentration is also believed to Fig. 7. XRD patterns of ZnO thin films at different Al doping concentrations. deteriorate ZnO crystallinity properties. 9) At higher doping concentration, due to nuclear charge of Al 3+ is larger than Zn 2+, extrinsic Al 3+ will captured more oxygen in competition of Zn 2+ which decrease crystallinity and hexagonal structure of the thin films. 20) 4. Conclusion ZnO thin films at different doping concentration were successfully prepared using sol gel spin-coating method. The optical and electrical properties of the thin films have been investigated. The absorption coefficient spectra indicate all films transparent in the visible and NIR region and exhibit high UV absorption properties. The optical band gap energy and Urbach energy found to be increased with doping concentration. The conductivity of thin films increased with doping concentration up to.0 266

5 Journal of the Ceramic Society of Japan 7 [] JCS-Japan at.% but show reduction tendency at 2.0 at.% and 3.0 at.%. FESEM images reveal reduction of ZnO particles size with doping concentrations. XRD investigation suggested that crystallinity properties of the thin films were aggravated at higher doping concentration. Acknowledgement The author would like to thank Universiti Teknologi MARA (UiTM) Malaysia and Jabatan Perkhidmatan Awam (JPA) Malaysia for the financial support. References ) J.-P. Kim, S.-A Lee, J. S. Bae, S.-K. Park, U.-C. Choi and C.-R. Cho, Thin Solid Films, 56, 5223 (2008). 2) D. C. Kim, W. S. Han, B. H. Kong, H. K. Cho and C. H. Hong, Physica B, , 386 (2007). 3) T.-J. Hsueh, C.-L. Hsu, S.-J. Chang, P.-W. Guo, J.-H. Hsieh and I.-C. Chen, Scripta Mater., 57, 53 (2007). 4) J. B. K. Law and J. T. L. Thong, Nanotechnology, 9, (2008). 5) Z. Bai, C. Xie, M. Hu, S. Zhang and D. Zeng, Mater. Sci. Engi. B, 49, 2 (2008). 6) H. S. Kang, G. H. Kim, S. H. Lim, H. W. Chang, J. H. Kim and S. Y. Lee, Thin Solid Films, 56, 347 (2008). 7) S. Bandyopadhyay, G. K. Paul and S. K. Sen, Solar Energy Materials & Solar Cells, 7, 06 (2002). 8) T. A. Vijayan, R. Chandramohan, S. Valanarasu, J. Thirumalai and S. P. Subramanian, J. Mater. Sci., 43, 780 (2008). 9) E. Burstein, Phys. Rev., 93, 632 (954). 0) C. J. Brinker and G. W. Scherer, Sol Gel Science: The Physics and Chemistry of Sol Gel Processing, Academic Press, New York (975) pp. 87. ) J. H. Yim, J. B. Kim, H. D. Jeong, Y. Y. Lyu, S. K. Mah, J. H. Lee, K. H. Lee, S. Chang, L. S. Pu, Y. F. Hu, J. N. Sun and D. W. Gidley, Mat. Res. Soc. Symp. Proc., 766, E8.0. (2003). 2) G. Wypych, Handbook of Fillers, 2nd ed., Chem. Tech Publishing, Canada (999) pp ) R. Tricker, Optoelectronics and Fiber Optic Technology, Newnes, Woburn (2002) pp ) F. Urbach, Phys. Reiew., 92, 324 (953). 5) J. Mass, P. Bhattacharya and R. S. Katiyar, Mater. Sci. Engi. B, 03, 4 (2003). 6) S. R. Dhariwal, V. N. Cijha and G. P. Srivastava, IEEE Trans. Electron Devices, ED-32[], 47 (985). 7) J.-H. Lee and B.-O. Park, Mater. Sci. Engi. B, 06, (2004). 8) P. Sagar, M. Kumar and R. M. Mehra, Mater. Sci.-Poland, 23[3], 690 (2005). 9) Z. B. Ayadi, L. E. Mir, K. Djessas and S. Alaya, Mater. Sci. Engi. C, 28, 65 (2008). 20) L. J. Li, H. Deng, L. P. Dai, J. J. Chen, Q. L. Yuan and Y. Li, Mater. Res. Bull., 43, 46 (2008). 267