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Synthesis and conductive properties of Ga-doped ZnO nanosheets by the hydrothermal method
Materials Letters 97 (2013) 34–36
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis and conductive properties of Ga-doped ZnO nanosheets by the hydrothermal method Jing Guo, Ji Zheng n, Xinzhao Song, Kun Sun Department of Materials Science and Engineering, Tianjin University, Tianjin 300072, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 November 2012 Accepted 13 January 2013 Available online 23 January 2013
Ga doped ZnO (GZO) nanosheets were synthesized with good conductive performance by the hydrothermal method. The structural characteristics of the GZO were measured by X-ray diffraction (XRD). It was revealed that the GZO nanosheets showed good crystallinity with the wurtzite structure and a decrease in grain size with the increase of Ga content. The microstructure of the GZO was studied by scanning electrical microscope (SEM) and the average diameter was 42 nm. The existence of Ga was examined by X-ray photoelectron spectra (XPS), indicating Ga atom entered into the ZnO lattice. The electrical resistivity of GZO significantly decreased with the increase of Ga content, and achieved the minimum when Ga content was 2.0. The resistivity of GZO nanosheet materials increased as the doped Ga concentration was increased. In this study, the optimum Ga doping concentration was 2.0%, and the corresponding lowest resistivity was 2.3 104 O cm. & 2013 Elsevier B.V. All rights reserved.
Keywords: Ga-doped ZnO Nanosheets Hydrothermal method Resistivity Nanoparticles Semiconductors
1. Introduction Zinc oxide (ZnO) is one of the most promising semiconducting materials for electronic applications on account of its wide direct band gap (3.37 eV) at room temperature [1]. As a kind of conductive filler, ZnO can be a good candidate for antistatic materials due to its low electrical resistivity. Conductive fillers have been used to enhance the conductivity of polymers, and hence the polymer composites show good antistatic property for its special structure and electrical properties [2]. However, in order to be applied as conductive fillers, the electrical conductivity of ZnO still need to be improved. It is well known that highly conductive n-type ZnO have been achieved by replacing Zn2 þ ions by other ions (acting as efficient donors) such as In3 þ , Al3 þ and Ga3 þ [3–5]. Among the metal dopants, Ga seems to be the best chemical element due to the fact that atomic radius of Ga3 þ (0.062 nm) is similar to Zn2 þ (0.074 nm) [5]. The ZnO nanoparticles can be prepared by various methods, such as co-precipitation [6], chemical vapor synthesis [3] and sol–gel [7]. The hydrothermal route is a simple low-temperature method for wet chemistry and has been employed to fabricate ZnO in previous works [8–10]. And this method can get product directly that have high crystallinity and excellent conductive property, with no need for sintering. However, few reports had been published that Ga-doped ZnO nanoparticles prepared by the hydrothermal method was used as antistatic filler. In this work, the GZO nanosheet conductive fillers with good antistatic
n
Corresponding author. Tel.: þ86 13821217929; fax: þ86 02227404724. E-mail addresses: [email protected], [email protected] (J. Zheng).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.01.055
property was synthesized using a simple hydrothermal method, and the crystalline structure, morphology as well as conductivity of the GZO nanosheets were investigated.
2. Experimental All chemicals (analytical grade reagents) were purchased from Tianjin Jiangtian Chemicals Co. Ltd. and were used as received without further purification. The GZO nanoparticles were prepared by the hydrothermal method and the molar ratio of Ga:Zn varied from 0:100, 1:100 to 3:100. In a typical experiment, 7.44 g Zn(NO3)2 6 H2O and proper amount of Ga(NO3)3 9H2O were dissolved in 30 ml de-ionized water and stirring to complete dissolution. Subsequently, 30 ml of sodium hydroxide (4.0 mol L 1) aqueous solution was slowly dripped into the former mixture to obtain precursor. After 30 min of stirring, the precursor was transferred into a 100 ml Teflon autoclave, then the autoclave was heated at 160 1C for 3 h. After the reaction finished, the precipitate was separated and washed with de-ionized water and absolute ethanol several times to remove the ions possibly remaining in the final products. Finally, the resulting products were dried in oven at 75 1C for 15 h. The crystalline structure of the GZO was investigated by a Rigaku D/max 2500v/pc X-ray diffractometer with Cu Ka radiation (l ¼0.15418 nm). Scanning electrons microscopy (SEM) examinations were performed employing a Hatchi S-4800. Samples for SEM analysis were prepared by spreading a drop of as-prepared products dilute dispersion in absolute ethyl alcohol and then dried in air. The measurement of the electrical resistivity of each sample were dry pressed using an oil press under 2 MPa, and was measured
J. Guo et al. / Materials Letters 97 (2013) 34–36
by an ohmmeter (UT60D, China). The volume resistivity was calculated using the following equation: r ¼ RðS=hÞ where r was the volume resistivity, R was the tested value of resistance, S was the effective area of the measuring electrode, and h was the thickness of the samples.
3. Results and discussion Structure and morphology of samples: The XRD patterns of GZO nanoparticles with different concentration of Ga were depicted in
Fig. 1. X-ray diffraction patterns of the Ga-doped ZnO samples with different Ga/Zn molar ratio: (a) 0; (b) 1.0% and (c) 2.0%.
35
Fig. 1. All the peaks were completely matched with JCPDS card of ZnO (JCPDS 36-1451). No impurity peaks except ZnO were observed, indicating that gallium ions entered into the crystal lattice of ZnO to substitute for zinc ions. Therefore, it can be concluded that GZO nanoparticles were single phase with a hexagonal wurtzite structure. Moreover, the diffraction peaks intensity reduced and the diffraction peak width increased with an increase in the concentration of Ga dopant, meaning a lattice deformation and a decrease in the crystalline size owing to the introduction of Ga atoms. The average crystalline size of ZnO and GZO (2.0%) calculated by the Scherrer equation were around 65 nm and 42 nm respectively. This is also supported by the following SEM images. The SEM images of the four samples were shown in Fig. 2a–d. It can be seen that most of the particles displayed the sheet-like morphologies and the sheets had random orientations. The surface of ZnO was rather smooth in Fig. 2a, however, after doping with Ga, the surface were rough in Fig. 2b–d. As the above XRD results, the particle size of ZnO was decreased by the increase of Ga doping content. The mean particle size decreased from 65 nm to 31 nm by the addition of 3.0% Ga in pure ZnO. This phenomenon was also existed in other systems, such as Al doped ZnO or Sb doped SnO2 [3,11]. Moreover, GZO nanosheets were observed consisting of very thin irregular sheets with lateral dimension of 100–400 nm, as shown in Fig. 2b–d. The results showed that the amount of sodium hydroxide had a major impact on the morphology of ZnO. According to the reaction of Zn2 þ þ2OH -ZnO þH2O, the concentration of OH seriously influenced the nucleation rate of ZnO. The system had a high concentration of OH , and the chemical potential was high, so the ZnO crystal would grow into nano-rod structure; On the contrary, because of lower driving force and chemical potential, it produced nanosheets. The crystal growth was affected by doping
Fig. 2. SEM images of the Ga-doped ZnO samples with different Ga/Zn molar ratio: (a) 0; (b) 1.0%; (c) 2.0% and (d) 3.0%.
36
J. Guo et al. / Materials Letters 97 (2013) 34–36
concentration. With increasing Ga doping concentration, lattice deformation and uniform phase increased, which made the particle surface rough. X-ray photoelectron spectrum of samples: The XPS spectrums of the GZO (1.0% and 2.0%) were shown in Fig. 3a and b. It was used to determine the chemical bonds such as Ga–O and Zn–O of GZO. From Fig. 3a and b, the GZO was shown Zn and O peaks, which located at 1042 eV, 1019 eV and 528 eV, corresponding to the electrostatic of Zn 2p1, Zn 2p2 and O 1s, [12]. The GZO (1.0% and 2.0%) were shown the position of Ga 3d and Ga 3p peaks were found to be at 18 eV and 103 eV, respectively [12]. Moreover, in Fig. 3b, the GZO (2.0%) were shown Ga 2p1 and Ga 2p3, locating at 1143 eV and 1116 eV, respectively. These peak positions agreed with the reference values, which indicated that Ga exists as Ga3 þ and Zn exists as Zn2 þ . From the above XRD analyses, the absence of gallium oxide suggested that Ga3 þ come into the crystal lattice to substitute for zinc ions, and Ga3 þ was effective donor. Each Ga3 þ released one free electron, and more free electrons changed the energy band structure of GZO, so the resistivity was reduced. Resistivity of samples: The influence of Ga doping concentration on the electrical resistivity of GZO nanoparticles was shown in Fig. 4. It was found that the electrical resistivity of GZO
nanoparticles began to decrease tremendously from 9.9 107 O cm to 5.9 106 O cm with Ga content from 0% to 1%, and then, gradually dropped down to 2.0% Ga, where the resistivity reached a minimal value of 2.3 104 O cm. When Ga doping concentration continued to increase to 3.0%, the resistivity of GZO increased to 4.1 106 O cm. The electrical resistivity of GZO depended on the carrier concentration and mobility. When the Ga doping concentration was lower than 2.0%, the Ga ion could substitute for Zn ion because ionic radius of Zn was bigger than that of Ga. Therefore, an excessive electron was released to the conduction band when each Ga ion entered into the lattice, causing the increase of the carrier concentration and mobility with the rise of the Ga doping concentration. When the Ga doping concentration exceeded 2.0%, the carrier concentration was saturated and the lattice would produce disorders and impurities as a result of Ga ions getting into the interstitial sites [13]. The above defects made impurities start to dominate and excess Ga segregated at the grain boundary, leading to an increase in the resistivity with the reduction of the carrier mobility. In this study, the optimum Ga doping content was 2.0%, and the lowest electrical resistivity was 2.3 104 O cm. 4. Conclusions In summary, synthesis of GZO nanosheets had been studied by the hydrothermal method. The XRD measurements showed that GZO had good crystallinity with the wurtzite structure and a decrease in grain size with the increase of Ga content. The results from the SEM measurements revealed that the GZO doped with 2.0 mol% Ga had a mean particle size of about 42 nm with nanosheet morphology. The EDS and XPS analysis indicated that gallium is present as Ga3 þ that entered into the ZnO lattice, therefore, it can act as an effective donor. The electrical resistivity of GZO significantly decreased with the increase of Ga content, and obtained the lowest value of 2.3 104 O cm when Ga content was 2.0%. If Ga concentration continued to increase, the resistivity decreased instead. So, in this study, the optimum Ga doping concentration was 2.0%. This method was easy to operate and suitable for industrial production of GZO nanoparticles as conductive filler.
Fig. 3. XPS spectrum of GZO with different Ga doping concentration: (a) 1.0% and (b) 2.0%.
References [1] Li PJ, Liao ZM, Zhang XZ, Zhang XJ, Zhu HC, Gao JY, et al. Nano Lett 2009;9:2513–8. [2] Zhou Z, Peng W, Ke S, Deng H. J Mater Proc Tech 1999;90:415. [3] Hartner S, Ali M, Schulz C, Winterer M, Wiggers H. Nanotechnology 2009;20:445701. [4] Lee CH, Lim KS, Song JS. Solar Energy Mater Solar Cells 1996;43:37–43. [5] Khranovskyy V, Grossner U, Lazorenko V, Lashkarev G, Svensson BG, Yakimova R. Superlattices Microstruct 2007;42:379–86. [6] Du SF, Tian YJ, Liu HD, Liu J, Chen YFJ. Am Ceram Soc 2006;89:2440. [7] Chen KJ, Fang TH, Hung FY, Ji LW, Chang SJ, Young SJ, et al. Appl Surf Sci 2008;254:5791. [8] Cimitan S, Albonetti S, Forni L, Peri F, Lazzari DJ. Colloid Interface Sci 2009;32:73. [9] Choy JH, Jang ES, Won JH, Chung JH, Jang DJ, Kim YW. Appl Phys Lett 2004;84:287. [10] Hou HW, Xiong YJ, Xie Y, Li Q, Zhang JY, Tian XB. J Solid State Chem 2004;177:176. [11] Zhang J, Gao R, Inorg L. Chem Commun 2004;7(9):1. [12] Moulder JF, Sticks WF, Sobol PE, Bomben KD. Handbook of X-ray photoelectron spectroscopy. USA: Physical Electronics Inc.; 1995. [13] Joseph B, Manoj PK, Vaidyan VK. Ceram Int 2006;32:487.
Fig.4. Electric resistivity of the Ga-doped ZnO samples with different Ga/Zn molar ratio.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis and conductive properties of Ga-doped ZnO nanosheets by the hydrothermal method Jing Guo, Ji Zheng n, Xinzhao Song, Kun Sun Department of Materials Science and Engineering, Tianjin University, Tianjin 300072, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 November 2012 Accepted 13 January 2013 Available online 23 January 2013
Ga doped ZnO (GZO) nanosheets were synthesized with good conductive performance by the hydrothermal method. The structural characteristics of the GZO were measured by X-ray diffraction (XRD). It was revealed that the GZO nanosheets showed good crystallinity with the wurtzite structure and a decrease in grain size with the increase of Ga content. The microstructure of the GZO was studied by scanning electrical microscope (SEM) and the average diameter was 42 nm. The existence of Ga was examined by X-ray photoelectron spectra (XPS), indicating Ga atom entered into the ZnO lattice. The electrical resistivity of GZO significantly decreased with the increase of Ga content, and achieved the minimum when Ga content was 2.0. The resistivity of GZO nanosheet materials increased as the doped Ga concentration was increased. In this study, the optimum Ga doping concentration was 2.0%, and the corresponding lowest resistivity was 2.3 104 O cm. & 2013 Elsevier B.V. All rights reserved.
Keywords: Ga-doped ZnO Nanosheets Hydrothermal method Resistivity Nanoparticles Semiconductors
1. Introduction Zinc oxide (ZnO) is one of the most promising semiconducting materials for electronic applications on account of its wide direct band gap (3.37 eV) at room temperature [1]. As a kind of conductive filler, ZnO can be a good candidate for antistatic materials due to its low electrical resistivity. Conductive fillers have been used to enhance the conductivity of polymers, and hence the polymer composites show good antistatic property for its special structure and electrical properties [2]. However, in order to be applied as conductive fillers, the electrical conductivity of ZnO still need to be improved. It is well known that highly conductive n-type ZnO have been achieved by replacing Zn2 þ ions by other ions (acting as efficient donors) such as In3 þ , Al3 þ and Ga3 þ [3–5]. Among the metal dopants, Ga seems to be the best chemical element due to the fact that atomic radius of Ga3 þ (0.062 nm) is similar to Zn2 þ (0.074 nm) [5]. The ZnO nanoparticles can be prepared by various methods, such as co-precipitation [6], chemical vapor synthesis [3] and sol–gel [7]. The hydrothermal route is a simple low-temperature method for wet chemistry and has been employed to fabricate ZnO in previous works [8–10]. And this method can get product directly that have high crystallinity and excellent conductive property, with no need for sintering. However, few reports had been published that Ga-doped ZnO nanoparticles prepared by the hydrothermal method was used as antistatic filler. In this work, the GZO nanosheet conductive fillers with good antistatic
n
Corresponding author. Tel.: þ86 13821217929; fax: þ86 02227404724. E-mail addresses: [email protected], [email protected] (J. Zheng).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.01.055
property was synthesized using a simple hydrothermal method, and the crystalline structure, morphology as well as conductivity of the GZO nanosheets were investigated.
2. Experimental All chemicals (analytical grade reagents) were purchased from Tianjin Jiangtian Chemicals Co. Ltd. and were used as received without further purification. The GZO nanoparticles were prepared by the hydrothermal method and the molar ratio of Ga:Zn varied from 0:100, 1:100 to 3:100. In a typical experiment, 7.44 g Zn(NO3)2 6 H2O and proper amount of Ga(NO3)3 9H2O were dissolved in 30 ml de-ionized water and stirring to complete dissolution. Subsequently, 30 ml of sodium hydroxide (4.0 mol L 1) aqueous solution was slowly dripped into the former mixture to obtain precursor. After 30 min of stirring, the precursor was transferred into a 100 ml Teflon autoclave, then the autoclave was heated at 160 1C for 3 h. After the reaction finished, the precipitate was separated and washed with de-ionized water and absolute ethanol several times to remove the ions possibly remaining in the final products. Finally, the resulting products were dried in oven at 75 1C for 15 h. The crystalline structure of the GZO was investigated by a Rigaku D/max 2500v/pc X-ray diffractometer with Cu Ka radiation (l ¼0.15418 nm). Scanning electrons microscopy (SEM) examinations were performed employing a Hatchi S-4800. Samples for SEM analysis were prepared by spreading a drop of as-prepared products dilute dispersion in absolute ethyl alcohol and then dried in air. The measurement of the electrical resistivity of each sample were dry pressed using an oil press under 2 MPa, and was measured
J. Guo et al. / Materials Letters 97 (2013) 34–36
by an ohmmeter (UT60D, China). The volume resistivity was calculated using the following equation: r ¼ RðS=hÞ where r was the volume resistivity, R was the tested value of resistance, S was the effective area of the measuring electrode, and h was the thickness of the samples.
3. Results and discussion Structure and morphology of samples: The XRD patterns of GZO nanoparticles with different concentration of Ga were depicted in
Fig. 1. X-ray diffraction patterns of the Ga-doped ZnO samples with different Ga/Zn molar ratio: (a) 0; (b) 1.0% and (c) 2.0%.
35
Fig. 1. All the peaks were completely matched with JCPDS card of ZnO (JCPDS 36-1451). No impurity peaks except ZnO were observed, indicating that gallium ions entered into the crystal lattice of ZnO to substitute for zinc ions. Therefore, it can be concluded that GZO nanoparticles were single phase with a hexagonal wurtzite structure. Moreover, the diffraction peaks intensity reduced and the diffraction peak width increased with an increase in the concentration of Ga dopant, meaning a lattice deformation and a decrease in the crystalline size owing to the introduction of Ga atoms. The average crystalline size of ZnO and GZO (2.0%) calculated by the Scherrer equation were around 65 nm and 42 nm respectively. This is also supported by the following SEM images. The SEM images of the four samples were shown in Fig. 2a–d. It can be seen that most of the particles displayed the sheet-like morphologies and the sheets had random orientations. The surface of ZnO was rather smooth in Fig. 2a, however, after doping with Ga, the surface were rough in Fig. 2b–d. As the above XRD results, the particle size of ZnO was decreased by the increase of Ga doping content. The mean particle size decreased from 65 nm to 31 nm by the addition of 3.0% Ga in pure ZnO. This phenomenon was also existed in other systems, such as Al doped ZnO or Sb doped SnO2 [3,11]. Moreover, GZO nanosheets were observed consisting of very thin irregular sheets with lateral dimension of 100–400 nm, as shown in Fig. 2b–d. The results showed that the amount of sodium hydroxide had a major impact on the morphology of ZnO. According to the reaction of Zn2 þ þ2OH -ZnO þH2O, the concentration of OH seriously influenced the nucleation rate of ZnO. The system had a high concentration of OH , and the chemical potential was high, so the ZnO crystal would grow into nano-rod structure; On the contrary, because of lower driving force and chemical potential, it produced nanosheets. The crystal growth was affected by doping
Fig. 2. SEM images of the Ga-doped ZnO samples with different Ga/Zn molar ratio: (a) 0; (b) 1.0%; (c) 2.0% and (d) 3.0%.
36
J. Guo et al. / Materials Letters 97 (2013) 34–36
concentration. With increasing Ga doping concentration, lattice deformation and uniform phase increased, which made the particle surface rough. X-ray photoelectron spectrum of samples: The XPS spectrums of the GZO (1.0% and 2.0%) were shown in Fig. 3a and b. It was used to determine the chemical bonds such as Ga–O and Zn–O of GZO. From Fig. 3a and b, the GZO was shown Zn and O peaks, which located at 1042 eV, 1019 eV and 528 eV, corresponding to the electrostatic of Zn 2p1, Zn 2p2 and O 1s, [12]. The GZO (1.0% and 2.0%) were shown the position of Ga 3d and Ga 3p peaks were found to be at 18 eV and 103 eV, respectively [12]. Moreover, in Fig. 3b, the GZO (2.0%) were shown Ga 2p1 and Ga 2p3, locating at 1143 eV and 1116 eV, respectively. These peak positions agreed with the reference values, which indicated that Ga exists as Ga3 þ and Zn exists as Zn2 þ . From the above XRD analyses, the absence of gallium oxide suggested that Ga3 þ come into the crystal lattice to substitute for zinc ions, and Ga3 þ was effective donor. Each Ga3 þ released one free electron, and more free electrons changed the energy band structure of GZO, so the resistivity was reduced. Resistivity of samples: The influence of Ga doping concentration on the electrical resistivity of GZO nanoparticles was shown in Fig. 4. It was found that the electrical resistivity of GZO
nanoparticles began to decrease tremendously from 9.9 107 O cm to 5.9 106 O cm with Ga content from 0% to 1%, and then, gradually dropped down to 2.0% Ga, where the resistivity reached a minimal value of 2.3 104 O cm. When Ga doping concentration continued to increase to 3.0%, the resistivity of GZO increased to 4.1 106 O cm. The electrical resistivity of GZO depended on the carrier concentration and mobility. When the Ga doping concentration was lower than 2.0%, the Ga ion could substitute for Zn ion because ionic radius of Zn was bigger than that of Ga. Therefore, an excessive electron was released to the conduction band when each Ga ion entered into the lattice, causing the increase of the carrier concentration and mobility with the rise of the Ga doping concentration. When the Ga doping concentration exceeded 2.0%, the carrier concentration was saturated and the lattice would produce disorders and impurities as a result of Ga ions getting into the interstitial sites [13]. The above defects made impurities start to dominate and excess Ga segregated at the grain boundary, leading to an increase in the resistivity with the reduction of the carrier mobility. In this study, the optimum Ga doping content was 2.0%, and the lowest electrical resistivity was 2.3 104 O cm. 4. Conclusions In summary, synthesis of GZO nanosheets had been studied by the hydrothermal method. The XRD measurements showed that GZO had good crystallinity with the wurtzite structure and a decrease in grain size with the increase of Ga content. The results from the SEM measurements revealed that the GZO doped with 2.0 mol% Ga had a mean particle size of about 42 nm with nanosheet morphology. The EDS and XPS analysis indicated that gallium is present as Ga3 þ that entered into the ZnO lattice, therefore, it can act as an effective donor. The electrical resistivity of GZO significantly decreased with the increase of Ga content, and obtained the lowest value of 2.3 104 O cm when Ga content was 2.0%. If Ga concentration continued to increase, the resistivity decreased instead. So, in this study, the optimum Ga doping concentration was 2.0%. This method was easy to operate and suitable for industrial production of GZO nanoparticles as conductive filler.
Fig. 3. XPS spectrum of GZO with different Ga doping concentration: (a) 1.0% and (b) 2.0%.
References [1] Li PJ, Liao ZM, Zhang XZ, Zhang XJ, Zhu HC, Gao JY, et al. Nano Lett 2009;9:2513–8. [2] Zhou Z, Peng W, Ke S, Deng H. J Mater Proc Tech 1999;90:415. [3] Hartner S, Ali M, Schulz C, Winterer M, Wiggers H. Nanotechnology 2009;20:445701. [4] Lee CH, Lim KS, Song JS. Solar Energy Mater Solar Cells 1996;43:37–43. [5] Khranovskyy V, Grossner U, Lazorenko V, Lashkarev G, Svensson BG, Yakimova R. Superlattices Microstruct 2007;42:379–86. [6] Du SF, Tian YJ, Liu HD, Liu J, Chen YFJ. Am Ceram Soc 2006;89:2440. [7] Chen KJ, Fang TH, Hung FY, Ji LW, Chang SJ, Young SJ, et al. Appl Surf Sci 2008;254:5791. [8] Cimitan S, Albonetti S, Forni L, Peri F, Lazzari DJ. Colloid Interface Sci 2009;32:73. [9] Choy JH, Jang ES, Won JH, Chung JH, Jang DJ, Kim YW. Appl Phys Lett 2004;84:287. [10] Hou HW, Xiong YJ, Xie Y, Li Q, Zhang JY, Tian XB. J Solid State Chem 2004;177:176. [11] Zhang J, Gao R, Inorg L. Chem Commun 2004;7(9):1. [12] Moulder JF, Sticks WF, Sobol PE, Bomben KD. Handbook of X-ray photoelectron spectroscopy. USA: Physical Electronics Inc.; 1995. [13] Joseph B, Manoj PK, Vaidyan VK. Ceram Int 2006;32:487.
Fig.4. Electric resistivity of the Ga-doped ZnO samples with different Ga/Zn molar ratio.