formation of zno thinfilms consisting of nano-prisms and nano-rods with a high

Formation of ZnO thin films consisting of nano-prisms and nano-rods with a high aspect ratio by a hydrothermal technique at 60  C Ye Bin Kwon a , Seung Wook Shin b , Hyun-Ki Lee a , Jeong Yong Lee b , Jong-Ha Moon a , Jin Hyeok Kim a , * a Department of Materials Science and Engineering, Chonnam National University, 300 Yongbong e Dong, Puk-Gu, Gwangju 500-757, Republic of Korea b Photonics Technology Research Institute, Department of Materials Science and Engineering, KAIST, Daejeon 305-701, Republic of Korea article info Article history: Received 6 September 2010 Received in revised form 22 November 2010 Accepted 26 November 2010 Available online 4 December 2010 Keywords: ZnO thin film Nano structure Hydrothermal technique Low growth temperature abstract ZnO thin films that consist of elongated nano-prisms and nano-rods were successfully grown on 100 nm- thick ZnO seeded glass substrates by hydrothermal synthesis at 60  C and pH 10.3 in an aqueous solution containing Zn(NO 3 )$6H 2 O, Al(NO 3 ) 3 $9H 2 O, Na 3 -citrate and NH 4 OH. The effect of Al(NO 3 )$6H 2 O and Na 3 -citrate, as surfactant chemicals, on the structural, morphological, optical and electrical properties of ZnO thin films were investigated. X-ray diffraction results showed that all the deposited films were grown as a polycrystalline wurtzite hexagonal phase with a c-axis preferred, out-of-plane orientation and without unwanted second phase. ZnO thin films deposited without any surfactant chemicals or deposited only with Al(NO 3 )$6H 2 O consist of elongated needle shaped nano-rods with a very rough surface morphology. On the other hand, ZnO thin films prepared using Na 3 -citrate as a surfactant chemical consist of hexagonal nano-prisms with a very smooth surface morphology. The thickness of the ZnO thin films with the very smooth surface morphology was increased remarkably using both Na 3 -citrate and Al(NO 3 )$6H 2 O as surfactant chemicals, in which ZnO thin films consisted of el ongated hexagonal nano-prisms. These results show that relatively thick ZnO thin films with a good surface morphology can be grown easily by the appropriate use of surface modifying chemicals, such as Na 3 -citrate and Al(NO 3 )$6H 2 O. The photoluminescence results showed strong defect-related emission peak centered near 545 nm in the rough surfaced ZnO thin film grown without any surfactant chemicals and strong band-edge peak centered near 368 nm in the smooth surfaced ZnO thin film grown using both Na 3 -citrate and Al(NO 3 )$6H 2 O as the surfactant chemicals. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Zinc oxide (ZnO) is considered the most promising candidate for a range of applications including room temperature ultraviolet lasers transparent conducting electrodes, light emitting diodes, sensors and surface-acoustic wave devices owing to its unique properties such as wide band gap (E g ¼ 3.37 eV), large exciton binding energy (60 meV) and strong-chemical stability compared to other materials [1e5]. In particular, the device performance of nano-scale ZnO crystals with various morphologies, such as nano- rod, belt, wire and tubes are superior to those of the bulk-scale due to the quantum-sized effects [6,7]. Nano-scale ZnO thin films can be synthesized by several methods, such as metal organic chemical vapor deposition [8], pulsed laser deposition [9], solvothermal [10] and hydrothermal technique [11e14]. Among these, the hydro- thermal technique is one of the most attractive for industrial use because industrial processes generally require rapid, low process costs, which are the main advantages of the hydrothermal tech- nique [15]. On the other hand, the performance of ZnO thin film devices with a continuous, dense and smooth surface microstructure is superior to that of ZnO thin film devices with discontinuous, defective and rough surface microstructures [16,17]. As a result, it is desired to grow high quality ZnO thin films by controlling the microstructure with varying process parameters. Na 3 -citrate is used widely to change the morphology of hydro- thermally grown ZnO thin films grown from a needle structure to a hexagonal prism structure due to easily adjusting the surfactant ions for Zn 2þ [17]. Our previous reports [18,19] also showed that the morphology of ZnO thin films can be controlled by introducing chemical species in the reaction solution, in which the surfactant ions inhibit needle shape growth. Although ZnO thin films with a smooth and dense microstructure were obtained using Na 3 -citrate as a surfactant chemical, the growth rate of ZnO thin films in the c-axis direction was poor, causing an increase in the growth processing time. Therefore, it required a new approach to increase * Corresponding author. Tel.: þ82 62 530 1709; fax: þ82 62 530 1699. E-mail address: jinhyeok@chonnam.ac.kr (J.H. Kim). Contents lists available at ScienceDirect Current Applied Physics journal homepage: www.elsevier.com/locate/cap 1567-1739/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2010.11.086 Current Applied Physics 11 (2011) S197eS201 the growth rate of ZnO crystal in the c-axis direction with keeping hexagonal prism shape rather than needle likes shape. The purpose of this study is to grow dense and smooth ZnO thin films with high growth rate. We report that it is possible to grow a ZnO thin film that consists of ZnO nano-scale hexagonal prisms with a high growth rate by using both Na 3 -citrate and Al(NO 3 )$ 6H 2 O as surfactant chemicals in the hydrothermal synthesis at 60  C. 2. Experimental details Zinc nitrate hexahydrate (Zn(NO 3 )$6H 2 O) was used to grow ZnO crystals, and aluminum nitrate hexahydrate (Al(NO 3 )$6H 2 O) and tri-sodium citrate (Na 3 C 6 H 5 O 7 ) were used as surfactant chemicals to control the shape of the ZnO crystals. Four different solutions, defined as solutions A, B, C and D in Table 1, were used to grow the ZnO thin films. Solution A was formulated with 0.03 M of Zn(NO 3 )$ 6H 2 O dissolved in 80 ml of deionized (DI) water. Solution B was formulated with 0.03 M of Zn(NO 3 )$6H 2 O and 4 mM of Na 3 C 6 H 5 O 7 dissolved in 80 ml of DI water. Solution C was formulated with 0.03 M of Zn(NO 3 )$6H 2 O and 5 mM of Al(NO 3 )$6H 2 O dissolved in 80 ml of DI water. Solution D was formulated with 0.03 M of Zn (NO 3 )$6H 2 O, 5 mM of Al(NO 3 )$6H 2 O and 4 mM of Na 3 C 6 H 5 O 7 dis- solved in 80 ml of DI water. 100 nm-thick ZnO seeded substrates, which were deposited by RF magnetron sputtering at room temperature, were cleaned sequentially in acetone and isopropyl alcohol. The substrates were then fixed to a Teflon insert with the ZnO seed layer side facing down within a fluoropolymer digestion vessel (Savillex, MN, USA), which was placed into an oven at 60  C for 6 h. The lid to the vessel was sealed to prevent evaporation. At the end of the period, the vessel was cooled to room temperature to remove the specimens and the specimens were rinsed in DI water. The surface and cross-sectional morphologies of deposited thin films were examined by field emission scanning electron micros- copy (FE-SEM, JSM-6710F, JEOL, Tokyo, Japan). The crystallographic information was obtained by X-ray diffraction (XRD, X’pert PRO, Philips, and Eindhoven, Netherlands). The microstructure and crystallographic orientation of the deposited thin films were analyzed by transmission electron microscopy (TEM, JEM 2000EX, JEOL, Tokyo, Japan). The optical properties of the deposited thin films were determined by UVevis spectroscopy (Cary 100, Varian, Mulgrave, Australia) at room temperature. The photoluminescence (PL) properties of the deposited thin films were characterized by room temperature (RT) PL (APD, SH-4, USA). The electrical char- acteristics of the deposited thin films were characterized by Hall Effect measurements (Van der Paw configuration, USA). 3. Results and discussion Fig. 1 shows XRD patterns of the ZnO thin films grown in solu- tions A, B, C and D at 60  C for 6 h. All the films showed only (000l) reflection family of ZnO without any other peaks corresponding to other planes (JCPDS No. 89-1397 (ZnO)), indicating that all the films had a polycrystalline hexagonal wurtzite structure with a c-axis preferred, out-of-plane orientation. The samples grown in solutions A and D showed a relatively strong (0002) peak intensity compared to samples grown in solutions B and C. The full width at half maximum (FWHM) values of the ZnO (0002) peak for the deposited thin film grown in solution C are smaller than those grown in solutions A and B. The smallest FWHM value of 0.32  for the ZnO (0002) peak was observed in the sample grown in solution D, indicating the best c-axis preferred, out-of-plane orientation. Fig. 2(a,b,c,d) shows tilted-view FE-SEM images of ZnO thin films grown on the ZnO seeded glass substrates in solutions A, B, C and D, respectively. The inset images show the corresponding plan- view surface morphology of the ZnO thin films. ZnO thin films consisting of needle shaped nano-rods were observed in the samples grown in the solution without Na 3 C 6 H 5 O 7 as the surfactant chemical (Fig. 2(a,c)). The surface morphology of ZnO thin films was rough and discontinuous and the lengths of the ZnO nano-rods were w1500 nm (Fig. 2(a)) and w500 nm (Fig. 2(c)). On the contrary, ZnO thin films consisting of nano-prisms were observed in the samples grown in the solutions containing Na 3 C 6 H 5 O 7 (Fig. 2 (b,d)). A smooth and dense surface morphology was observed in those samples and the lengths of the ZnO nano-prisms were w200 nm (Fig. 2(b)) and w1000 nm (Fig. 2(d)). Noticeable information was obtained from the aforementioned XRD and FE-SEM results. ZnO thin films with a dense and contin- uous microstructure and reduced growth rate could be synthesized in the sample grown in solution B by introducing Na 3 C 6 H 5 O 7 as the surfactant chemical. However, a dramatic decrease in the thickness of the ZnO film, which was typically observed in the ZnO growth process using Na 3 C 6 H 5 O 7 as the only surfactant chemical [18],was not observed in the sample grown in solution D, which contained both Na 3 C 6 H 5 O 7 and Al(NO 3 )$6H 2 O as the surfactant chemicals. These characteristics were attributed to the surfactant ions. It is well known that the oppositely charged ions produce positively charged Zn (0001) and negatively charged O (000l) surfaces, resulting in a normal dipole moment and spontaneous polarization along the c-axis as well as a variance in surface energy [15]. However, ZnO thin film grown using surfactant specifically absor- bed on Zn-terminated (0001) planes of ZnO to slow down the Table 1 Experimental conditions to prepare different solution of specimens. Solution A Solution B Solution C Solution D Zn(NO 3 )$6H 2 OO O O O Al(NO 3 )$6H 2 OX X O O Na 3 -citrate X O X O Fig. 1. XRD patterns of the ZnO thin film grown on ZnO seeded glass substrate in different solutions at pH 10.3 for 6 h. Y.B. Kwon et al. / Current Applied Physics 11 (2011) S197eS201S198 growth rate in the [0001] direction. Especially, the morphology of ZnO thin film using solution B showed nano-prism structure, indicating that the Na 3 -citrate acted the fully remove for electrical charges in surface [18]. For ZnO thin film grown in solution C, the thickness of film was thin compared to that grown in solution A. This behavior was attributed to the Al(NO 3 )$6H 2 O as the surfactant chemical. The Na 3 C 6 H 5 O 7 is more dominant than Al(NO 3 )$6H 2 O (Fig. 2(b,c)). However, the morphology of ZnO thin film grown in solution D was high aspect ratio nano-prism with a smooth surface as compared to these grown in solution B and C. This phenomenon might be explained by considering the role of Al 3þ ions during the hydrothermal process. There were three more additive ions of Na þ , C 6 H 5 O 7 3À and Al 3þ in solution D compared to solution A, which had no surfactant chemicals. It is well known that C 6 H 5 O 7 3À ions inhibit the growth of ZnO crystals in the c-axis direction, which results in the formation of hexagonal prism-shaped ZnO rods with a low aspect ratio. It seems that Al 3þ ions suppress the role of C 6 H 5 O 7 3À ions by reducing the concentration of C 6 H 5 O 7 3À ions through the possible formation of an AlC 6 H 5 O 7 compound in solution. There- fore, ZnO thin film grown in solution D consists of elongated nano- prisms with a smooth surface. Fig. 3 shows a cross-sectional bright-field TEM image of the ZnO thin film grown in solution D. The inset is the corresponding selected area electron diffraction (SAED) pattern obtained at the interface. The film consisted of dense nano-scale elongated ZnO crystals (w20 nm in width) grown from the ZnO seeded layer. The interfaces between the ZnO thin film, ZnO seeded layer and substrate were very sharp without any indication of an interfacial reaction or any formation of amorphous compounds. The SAED pattern showed that the film was c-axis oriented in the substrate normal direction with a slight out-of-plane variation. This TEM result is consistent with the XRD and SEM results shown in Figs. 1 and 2. Fig. 4 shows a plot of ( a h y ) 2 as a function of the photon energy of the ZnO thin films grown on the ZnO seeded glass substrates in the four different solutions. The band gap energy was derived by Fig. 2. The tilted-view FE-SEM images of ZnO thin film grown on a ZnO seeded glass substrate in solutions A (a), B (b), C (c), and D (d), respectively. Fig. 3. The cross-sectional bright-field TEM image of the ZnO thin films grown on ZnO seeded glass substrates in solution D at pH 10.3 for 6 h. Y.B. Kwon et al. / Current Applied Physics 11 (2011) S197eS201 S199 extrapolating the straight-line portion of the ( a h y ) 2 vs h y plot to a zero absorption coefficient value. The optical band gap energies of the thin films grown in different solutions A, B, C and D were 3.25 eV, 3.2 eV, 3.3 eV and 3.33 eV, respectively. The abortion edges of the thin films grown in solutions A, C and D were approximately 365 nm except for the films grown in solution B (w355 nm). The wider band gap energy of ZnO thin film grown in C and D than those grown in solution A and B were attributed to the Al(NO 3 )$ 6H 2 O, indicating that the Al 3þ ions substitute for Zn 2þ ions in the hexagonal ZnO crystal structure. The substituted Al 3þ ions can be generated excess electron and finally band gap energy is wider. Fig. 5 shows the RT PL spectra of ZnO films grown in four different solutions on the ZnO seeded glass substrates. Two emis- sion peaks were observed in ultraviolet (UV, w377 nm) and visible (Vis, w560 nm) regions in all samples. The UV emission was attributed to recombination between electrons in the conduction band and holes in the valence band and visible emission is related to the defects in the ZnO [20]. The UV emission of ZnO thin films grown in the solutions containing Na 3 C 6 H 5 O 7 (solutions B and D) shifted towards a lower wavelength compared to those grown in the solutions without Na 3 C 6 H 5 O 7 (solutions A and C). The intensi- ties of visible emission of the ZnO thin films with Na 3 C 6 H 5 O 7 (solutions B and D) were lower than those of without Na 3 C 6 H 5 O 7 (solution A and C) indicating low defect densities in the thin films with Na 3 C 6 H 5 O 7 (solution B and D) [18]. Fig. 6 shows the electrical resistivity, carrier concentration and mobility of the ZnO thin film grown in solutions A and D. The ZnO thin film grown in solution D showed a better resistivity, carrier concentration and mobility of 2.15 Â 10 À1 U cm, 4.17 Â 10 18 cm À3 and 6.98 cm 2 V À1 S À1 , respectively, compared to those of the ZnO thin film grown in solution A, i.e., 8.06 Â 10 0 U cm, 1.23 Â 10 18 cm À3 , and 6.35 cm 2 V À1 S À1 . These improved PL and electrical properties of ZnO thin films grown in solution D compared to those grown in solution A was attributed to the Al(NO 3 ) 3 $9H 2 O and Na 3 -citrate as surfactant chemicals. These chemicals resulted in different morphology, crystallinity and microstructure of the deposited thin films (Figs. 1 and 2). The Al(NO 3 ) 3 $9H 2 O acted impurity of ZnO structure as well as surfactant chemicals. From FE-SEM images of ZnO thin films grown in solutions C and D, the Al(NO 3 ) 3 $9H 2 O was kept the continuous film with smooth morphology. The continuous ZnO thin films grown in solution D showed better crystallinity and a smoother morphology than those grown in solution A with discontinuous morphology. The surface morphology and crystal- linity of the ZnO thin films were reported to affect their PL and electrical properties. Many defects in ZnO thin films act as free 3.0 3.1 3.2 3.3 3.4 3.5 (αhυ) 2 (arb.unit) hυ (eV) Solution A Solution B Solution C Solution D Fig. 4. The plot of ( a h y ) 2 vs photon energy of the ZnO thin film grown on ZnO seeded glass substrate in different solutions. 300 400 500 600 700 800 362 368 377 Wavelength (nm) Intensity (arb. unit) Solution C Solution D Solution A Solution B Fig. 5. RT PL spectra of the ZnO thin film grown on ZnO seeded glass substrate in different solutions. 10 -1 10 0 10 1 Solution D Solution A 5.0x10 18 1.0x10 19 2 4 6 8 10 Resistivity ( cm) Carrier concentration (cm -3 ) Mobility ( cm 2 V -1 s -1 ) Resistivity Carrier concentration Mobility Fig. 6. The electrical resistivity, carrier concentration and mobility of the ZnO thin film grown on ZnO seeded glass substrate in solutions A and D. Y.B. Kwon et al. / Current Applied Physics 11 (2011) S197eS201S200 electron trap centers, which reduce the charge carrier concentra- tion and show a high intensity of deep-level emission results from defect-related emission in the thin film indicating the deterioration of characteristics. In addition, the small grain size and empty space reduces the mean free path of the free electron, leading to an increase in film resistivity. In addition, the impurity of ZnO struc- ture, which released the free electron, resulted in the improved carrier concentration of ZnO thin film grown in solution D compared that grown in solution A (Fig. 6). 4. Conclusion It was possible to grow dense and smooth ZnO thin films with an improved growth rate using Na 3 C 6 H 5 O 7 and Al(NO 3 )$6H 2 Oas surfactant chemicals during hydrothermal synthesis at 60  C. The ZnO thin films consisted of elongated hexagonal nano-prisms with a dense microstructure and smooth surface morphology. The films showed improved photoluminescence and electrical properties. Acknowledgements This work was supported partially by Energy R&D program (2008-N-PV08-P-08) under the Korea Ministry of Knowledge Economy (MKE) and partially by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST). (No. 2010-0007691) References [1] C. Klingshirn, Phys. Status Solid B 71 (1975) 547. [2] J.J. Chen, F. Zeng, D.M. Li, J.B. Niu, F. Pan, Thin Solid Films 485 (2005) 257. [3] M. Tadatsugu, Semicond. Sci. Technol. 20 (2005) S35. [4] T. Gao, T.H. Wang, J. Appl. Phys. A 80 (2005) 1451. [5] G.Q. Ding, M.J. Zheng, W.L. Xu, W.Z. Shen, Nanotechnol 16 (2005) 1285. [6] Y.G. Chul, W. Chunrui, P.W. Ii, Semicond. Sci. Technol. 20 (2005) S22. [7] Fan Zhiyong, L.G. Jia, J. Nanosci. Nanotechnol. 5 (2005) 1561. [8] J. Dai, H. Liu, W. Fang, L. wang, Y. Pu, Y. Cen, F. Jiang, J. Cryst. Growth 283 (2005) 93. [9] R.P. Wang, H. Muto, Y. Yamada, T. Kusumori, Thin Solid Films 411 (2002) 69. [10] D. Wang, C. Song, J. Phys. Chem. B 109 (2005) 12697. [11] H. Zhang, D. Yang, S. Li, X. Ma, Y. Ji, J. Xu, D. Que, Mater. Lett. 59 (2005) 1696. [12] L. Vayssieres, K. Keis, S.E. Lindquist, A. Harfeldt, J. Phys. Chem. B 105 (2001) 3350. [13] T.L. Sounart, J. Liu, J.A. Voigt, J.W.P. Hsu, E.D. Spoerke, Z. Tian, Y.B. Jiang, Adv. Funct. Mater. 16 (2006) 335. [14] S. Hirano, K. Masuya, M. Kuwabara, J. Phys. Chem. B 108 (2004) 4576. [15] J.H. Kim, D. Andeen, F.F. Lange, Adv. Mater. 18 (2006) 2453. [16] C.H. Ahn, D.K. Seo, C.H. Woo, H.K. Cho, Phys. B 404 (2009) 4835. [17] S.H. Cho, J.W. Jang, S.H. Jung, B.R. Lee, E.G. Oh, K.H. Lee, Langmuir 25 (2009) 3825. [18] J.A. Kim, I.K. Kim, T.W. Kim, J.H. Moon, J.H. Kim, J. Nanosci. Nanotechnol. 8 (2008) 5485. [19] J.H. Kim, E.M. Kim, D. Andeen, D. Thomson, S.P. Denbaars, F.F. Lange, Adv. Funct. Mater. 17 (2007) 463. [20] F. Wen, W. Li, J.H. Moon, J.H. Kim, Solid State Commun. 135 (2005) 34. Y.B. Kwon et al. / Current Applied Physics 11 (2011) S197eS201 S201 . Formation of ZnO thin films consisting of nano-prisms and nano-rods with a high aspect ratio by a hydrothermal technique at 60  C Ye Bin Kwon a , Seung Wook Shin b , Hyun-Ki Lee a , Jeong. sharp without any indication of an interfacial reaction or any formation of amorphous compounds. The SAED pattern showed that the film was c-axis oriented in the substrate normal direction with. orientation and without unwanted second phase. ZnO thin films deposited without any surfactant chemicals or deposited only with Al(NO 3 )$6H 2 O consist of elongated needle shaped nano-rods with a