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Effects of oxygen partial pressure on film growth and electrical properties of undoped ZnO films with thickness below 100 nm
Surface & Coatings Technology 201 (2006) 4000 – 4003 www.elsevier.com/locate/surfcoat
Effects of oxygen partial pressure on film growth and electrical properties of undoped ZnO films with thickness below 100 nm S. Kishimoto ⁎, T. Yamada, K. Ikeda, H. Makino, T. Yamamoto Materials Design Center, Research Institute, Kochi University of Technology, Tosayamada-cho, Kami city, Kochi 782-8502, Japan Available online 20 October 2006
Abstract The dependences of electrical and structural properties on film thickness below 100 nm have been studied on polycrystalline undoped zinc oxide (ZnO) thin films on glass substrates at 200 °C prepared by plasma-assisted electron-beam deposition. From Hall effect measurements, we find that resistivity decreases from 0.47 to 0.02 Ω cm with increasing film thickness, whereas carrier concentration remains almost constant, 1.65– 2.0 × 1019 cm− 3, Hall mobility increases from 1.7 to 16.7 cm2/Vs with increasing film thickness. From both high-resolution out-of-plane and inplane X-ray diffraction (XRD) data, we find substantial changes in the lattice parameters with increasing film thickness below 40 nm; a reduction in the lattice parameter of the a-axis and an increase in the lattice parameter of the c-axis. Williamson–Hall analysis reveals an increase in in-plane grain size with increasing film thickness. This indicates that the dominant scattering mechanism that determines electrical properties is a boundary scattering mechanism. © 2006 Published by Elsevier B.V. Keywords: Zinc oxide; Undoped zinc oxide; Electron beam; Electrical property; Thin film
1. Introduction Very recently, ZnO has been extensively studied for various applications such as inexpensive transparent conducting electrodes, sensors, solar cells, and thin-film transistors. For these applications, the high optical transmission and low resistivity of ZnO film have great potential. While polycrystalline ZnO thin films are commonly used in these conventional applications, there has been a growing interest in obtaining them with a thickness below 100 nm on various substrates. In our previous work [1], in order to clarify the factors that determine electrical properties of undoped ZnO films prepared by rf-plasma-assisted electron-beam deposition, we have investigated the dependences of electrical and structural properties on film thickness, ranging widely from 23 to 316 nm. Hall effect measurements showed that whereas Hall mobility increased with increasing film thickness of up to almost 130 nm, it remains almost constant with further increasing film thickness
⁎ Corresponding author. E-mail address: [email protected] (S. Kishimoto). 0257-8972/$ - see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2006.08.009
(N 130 nm). In-plane X-ray diffraction (XRD) data show that inplane grain size increases with increasing film thickness for undoped ZnO with a film thickness below 130 nm. On the other hand, it changes little for ZnO films with larger thicknesses (N 130 nm). These results led us to the conclusion that the dominant scattering mechanism that determines electrical properties is a boundary scattering mechanism. It is important to control the properties of undoped ZnO films with thicknesses below 100 nm from a practical viewpoint. The purpose of this study is to investigate the factors that determine the dependences of electrical properties on film thickness for the ZnO thin films described above. 2. Experimental Undoped ZnO thin films were deposited on glass substrates by plasma-assisted electron-beam deposition. The apparatus used in this study was the same one reported previously [1]. Film thickness was changed from 5.8 to 101 nm. The ZnO films were deposited on alkali-free glass substrates with a size of 4 in. (Nippon Electric Glass OA-10), and growth temperature was maintained at 200 °C. Undoped ZnO pellets (purity = 99.9%,
S. Kishimoto et al. / Surface & Coatings Technology 201 (2006) 4000–4003
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Fig. 2. Dependence of lattice constant of c-axis on film thickness estimated from (002) diffraction peak of out-of-plane XRD pattern.
at an angle of 0.35°. We investigated the dependence of the degree of c-axis orientation on the distance from the surface by changing the incident angle for in-plane XRD measurements. XRD line broadening analysis, using the Williamson–Hall method [2], was also performed to determine the retained microstrain, and to estimate crystallite size. 3. Results and discussions Fig. 1 shows the dependences of the electrical properties on film thickness for undoped ZnO films with a thickness below 100 nm at the different O2 gas flow rates. The properties of the two samples exhibit similar behaviors: (1) resistivity was found to decrease with increasing film thickness; (2) Hall mobility for the ZnO films increases with increasing film thickness, whereas Fig. 1. Dependences of electrical properties on film thickness for undoped ZnO films with thickness below 100 nm; (a) resistivity, (b) Hall mobility, and (c) carrier concentration of undoped ZnO films measured by RT Hall measurement.
Sumitomo Metal Mining I-01236) were used as source material. rf power was maintained at 20 W. The oxygen (O2) gas flow rate was fixed at 5 sccm or 30 sccm, causing a minimum resistivity at a fixed film thickness [1]. Argon gas was not passed. The pressures in the growth chamber for O2 gas flow rates of 5 and 30 sccm were 0.1 and 0.3 Pa, respectively. Films with various thicknesses were obtained by varying deposition time. Hall effect measurements at room temperature were carried out in the van der Pauw configuration (Accent HL5500PC). The thickness of the films was determined with a surface profile measurement system (Alfa-Step IQ). The crystalline structure was characterized by high-resolution XRD (RIGAKU ATX-G system) analysis using CuKα radiation (λ = 0.15422 nm). The orientation of the ZnO films was examined by out-of-plane and in-plane XRD measurement. For in-plane XRD measurement, a parallel monochromatic X-ray beam was incident on the sample
Fig. 3. Dependence of lattice constant of a-axis on film thickness estimated from in-plane XRD (100) peak pattern.
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S. Kishimoto et al. / Surface & Coatings Technology 201 (2006) 4000–4003
the carrier concentration remains almost constant. We find that the Hall mobility of the ZnO film grown under the O2 gas flow rate of 30 sccm is larger than that of the film grown under the O2 gas flow rate of 5 sccm. The difference in Hall mobility between the two samples may result from the difference in film structure and/or crystallite size. This issue will be discussed below. First, we will discuss the film structure and crystallite size described above on the basis of the analysis of data obtained by XRD measurements. Figs. 2 and 3 show the dependences of the lattice constants of the a- and c-axes on film thickness estimated from the in-plane XRD (100) and (002) peak patterns, respectively. From these figures, we find that the lattice constants of the two axes show an abrupt change with increasing film thickness of up to 40 nm and slowly vary with further increasing thickness. We note that the volume of a unit cell is maintained at each film thickness. For undoped ZnO thin films with a small thickness, the crystal lattice stretches along the c-axis relative to the bulk crystal (a = 0.32501,
Fig. 4. Columnar-grain size and remaining microstrain estimated from (100) diffraction peak of in-plane measurements using Williamson–Hall method.
Fig. 5. Dependence of FWHM of rocking curve on film thickness.
c = 0.52071 [3], units: nm). With thick films, the two lattice parameters reach closely those of the bulk ZnO crystal. Fig. 4(a) and (b) show the dependences of columnar-grain size and remaining microstrain on film thickness estimated from the (100) diffraction peak of in-plane measurements using the Williamson–Hall method [2], respectively. From Fig. 4(a), we find that average columnar grain size increases with increasing film thickness. By considering the small grain size for undoped ZnO with a small film thickness and little changes in carrier concentration, the decrease in the lattice constant of the a-axis described above may be due to tensile stress caused by surface tension. In Fig. 4(b), the remaining microstrain for ZnO grown at the O2 gas flow rate of 5 sccm is larger than that under the O2 gas flow of 30 sccm. Thus, it is possible that Hall mobility is influenced by remaining microstrain, as discussed below. From these above findings, one may say that the grain boundary scattering mechanism substantially reduces Hall mobility for undoped ZnO with a very small film thickness. The model concerning the scattering mechanism leads us to consider the possibility that the grain size of ZnO films at the O2 gas flow rate of 30 sccm is larger than that of ZnO films at the O2 gas flow rate of 5 sccm. However, this consideration is refuted by the estimation of the grain sizes for the two samples obtained by the Williamson–Hall method (see Fig. 4(a)). In addition, Fig. 5, showing the dependence of the full width at half maximum (FWHM) of the (002) X-ray rocking curve on film thickness, indicates little difference in the orientation for the two samples under consideration. It is interesting to study what causes the difference in Hall mobility between ZnO films at O2 gas flow rates of 5 (sample A) and 30 sccm (sample B). From the Hall effect measurements, the two samples had the same total density of intrinsic defects including oxygen vacancies (ρ(VO)) and Zn interstitials (ρ(Zni)). It is likely that the ρ(Zni) of sample A is larger than that of sample B, considering high vapor pressure of Zn atoms and difference in O2 gas flow rates. It follows that the ρ(VO) of sample A is smaller than that of sample B. We note that the radius of Zn atoms is considerably larger than that of oxygen atoms. This may explain not only the data shown in Fig. 4(b) but also the issue to be resolved. In order to fully explain the observed phenomena, further defect studies are needed.
S. Kishimoto et al. / Surface & Coatings Technology 201 (2006) 4000–4003
4003
4. Conclusions
Acknowledgements
The dependences of electrical and structural properties on film thickness for polycrystalline undoped ZnO films with a thickness below 100 nm were studied. The undoped ZnO films were deposited on glass substrates at 200 °C by the plasmaassisted electron-beam method. From Hall effect measurement and out-of-plane and in-plane XRD data, we find that carrier concentration changes little for ZnO films with small thicknesses below 40 nm, whereas Hall mobility strongly depends on grain size. In order to realize low resistivity in the polycrystalline undoped ZnO film with very small thickness, large grain size with high orientation and control of the ratio of Zn to O are required.
S.K. wishes to thank Dr. Katsuhiko Inaba (RIGAKU Corporation) for assistance with the XRD measurements. The financial support from the collaboration of Regional Entities for the Advancement of Technological Excellence of the Japan Science and Technology Agency is gratefully acknowledged. References [1] S. Kishimoto, T. Yamamoto, Y. Nakagawa, K. Ikeda, H. Makino, T. Yamada, Superlattices Microstruct. 39 (2006) 306. [2] G.K. Williamson, W.H. Hall, Acta Metall. 1 (1953) 22. [3] E. Kisi, M.M. Elcombe, Acta Crystallogr., C Cryst. Struct. Commun. 45 (1989) 1867.
Effects of oxygen partial pressure on film growth and electrical properties of undoped ZnO films with thickness below 100 nm S. Kishimoto ⁎, T. Yamada, K. Ikeda, H. Makino, T. Yamamoto Materials Design Center, Research Institute, Kochi University of Technology, Tosayamada-cho, Kami city, Kochi 782-8502, Japan Available online 20 October 2006
Abstract The dependences of electrical and structural properties on film thickness below 100 nm have been studied on polycrystalline undoped zinc oxide (ZnO) thin films on glass substrates at 200 °C prepared by plasma-assisted electron-beam deposition. From Hall effect measurements, we find that resistivity decreases from 0.47 to 0.02 Ω cm with increasing film thickness, whereas carrier concentration remains almost constant, 1.65– 2.0 × 1019 cm− 3, Hall mobility increases from 1.7 to 16.7 cm2/Vs with increasing film thickness. From both high-resolution out-of-plane and inplane X-ray diffraction (XRD) data, we find substantial changes in the lattice parameters with increasing film thickness below 40 nm; a reduction in the lattice parameter of the a-axis and an increase in the lattice parameter of the c-axis. Williamson–Hall analysis reveals an increase in in-plane grain size with increasing film thickness. This indicates that the dominant scattering mechanism that determines electrical properties is a boundary scattering mechanism. © 2006 Published by Elsevier B.V. Keywords: Zinc oxide; Undoped zinc oxide; Electron beam; Electrical property; Thin film
1. Introduction Very recently, ZnO has been extensively studied for various applications such as inexpensive transparent conducting electrodes, sensors, solar cells, and thin-film transistors. For these applications, the high optical transmission and low resistivity of ZnO film have great potential. While polycrystalline ZnO thin films are commonly used in these conventional applications, there has been a growing interest in obtaining them with a thickness below 100 nm on various substrates. In our previous work [1], in order to clarify the factors that determine electrical properties of undoped ZnO films prepared by rf-plasma-assisted electron-beam deposition, we have investigated the dependences of electrical and structural properties on film thickness, ranging widely from 23 to 316 nm. Hall effect measurements showed that whereas Hall mobility increased with increasing film thickness of up to almost 130 nm, it remains almost constant with further increasing film thickness
⁎ Corresponding author. E-mail address: [email protected] (S. Kishimoto). 0257-8972/$ - see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2006.08.009
(N 130 nm). In-plane X-ray diffraction (XRD) data show that inplane grain size increases with increasing film thickness for undoped ZnO with a film thickness below 130 nm. On the other hand, it changes little for ZnO films with larger thicknesses (N 130 nm). These results led us to the conclusion that the dominant scattering mechanism that determines electrical properties is a boundary scattering mechanism. It is important to control the properties of undoped ZnO films with thicknesses below 100 nm from a practical viewpoint. The purpose of this study is to investigate the factors that determine the dependences of electrical properties on film thickness for the ZnO thin films described above. 2. Experimental Undoped ZnO thin films were deposited on glass substrates by plasma-assisted electron-beam deposition. The apparatus used in this study was the same one reported previously [1]. Film thickness was changed from 5.8 to 101 nm. The ZnO films were deposited on alkali-free glass substrates with a size of 4 in. (Nippon Electric Glass OA-10), and growth temperature was maintained at 200 °C. Undoped ZnO pellets (purity = 99.9%,
S. Kishimoto et al. / Surface & Coatings Technology 201 (2006) 4000–4003
4001
Fig. 2. Dependence of lattice constant of c-axis on film thickness estimated from (002) diffraction peak of out-of-plane XRD pattern.
at an angle of 0.35°. We investigated the dependence of the degree of c-axis orientation on the distance from the surface by changing the incident angle for in-plane XRD measurements. XRD line broadening analysis, using the Williamson–Hall method [2], was also performed to determine the retained microstrain, and to estimate crystallite size. 3. Results and discussions Fig. 1 shows the dependences of the electrical properties on film thickness for undoped ZnO films with a thickness below 100 nm at the different O2 gas flow rates. The properties of the two samples exhibit similar behaviors: (1) resistivity was found to decrease with increasing film thickness; (2) Hall mobility for the ZnO films increases with increasing film thickness, whereas Fig. 1. Dependences of electrical properties on film thickness for undoped ZnO films with thickness below 100 nm; (a) resistivity, (b) Hall mobility, and (c) carrier concentration of undoped ZnO films measured by RT Hall measurement.
Sumitomo Metal Mining I-01236) were used as source material. rf power was maintained at 20 W. The oxygen (O2) gas flow rate was fixed at 5 sccm or 30 sccm, causing a minimum resistivity at a fixed film thickness [1]. Argon gas was not passed. The pressures in the growth chamber for O2 gas flow rates of 5 and 30 sccm were 0.1 and 0.3 Pa, respectively. Films with various thicknesses were obtained by varying deposition time. Hall effect measurements at room temperature were carried out in the van der Pauw configuration (Accent HL5500PC). The thickness of the films was determined with a surface profile measurement system (Alfa-Step IQ). The crystalline structure was characterized by high-resolution XRD (RIGAKU ATX-G system) analysis using CuKα radiation (λ = 0.15422 nm). The orientation of the ZnO films was examined by out-of-plane and in-plane XRD measurement. For in-plane XRD measurement, a parallel monochromatic X-ray beam was incident on the sample
Fig. 3. Dependence of lattice constant of a-axis on film thickness estimated from in-plane XRD (100) peak pattern.
4002
S. Kishimoto et al. / Surface & Coatings Technology 201 (2006) 4000–4003
the carrier concentration remains almost constant. We find that the Hall mobility of the ZnO film grown under the O2 gas flow rate of 30 sccm is larger than that of the film grown under the O2 gas flow rate of 5 sccm. The difference in Hall mobility between the two samples may result from the difference in film structure and/or crystallite size. This issue will be discussed below. First, we will discuss the film structure and crystallite size described above on the basis of the analysis of data obtained by XRD measurements. Figs. 2 and 3 show the dependences of the lattice constants of the a- and c-axes on film thickness estimated from the in-plane XRD (100) and (002) peak patterns, respectively. From these figures, we find that the lattice constants of the two axes show an abrupt change with increasing film thickness of up to 40 nm and slowly vary with further increasing thickness. We note that the volume of a unit cell is maintained at each film thickness. For undoped ZnO thin films with a small thickness, the crystal lattice stretches along the c-axis relative to the bulk crystal (a = 0.32501,
Fig. 4. Columnar-grain size and remaining microstrain estimated from (100) diffraction peak of in-plane measurements using Williamson–Hall method.
Fig. 5. Dependence of FWHM of rocking curve on film thickness.
c = 0.52071 [3], units: nm). With thick films, the two lattice parameters reach closely those of the bulk ZnO crystal. Fig. 4(a) and (b) show the dependences of columnar-grain size and remaining microstrain on film thickness estimated from the (100) diffraction peak of in-plane measurements using the Williamson–Hall method [2], respectively. From Fig. 4(a), we find that average columnar grain size increases with increasing film thickness. By considering the small grain size for undoped ZnO with a small film thickness and little changes in carrier concentration, the decrease in the lattice constant of the a-axis described above may be due to tensile stress caused by surface tension. In Fig. 4(b), the remaining microstrain for ZnO grown at the O2 gas flow rate of 5 sccm is larger than that under the O2 gas flow of 30 sccm. Thus, it is possible that Hall mobility is influenced by remaining microstrain, as discussed below. From these above findings, one may say that the grain boundary scattering mechanism substantially reduces Hall mobility for undoped ZnO with a very small film thickness. The model concerning the scattering mechanism leads us to consider the possibility that the grain size of ZnO films at the O2 gas flow rate of 30 sccm is larger than that of ZnO films at the O2 gas flow rate of 5 sccm. However, this consideration is refuted by the estimation of the grain sizes for the two samples obtained by the Williamson–Hall method (see Fig. 4(a)). In addition, Fig. 5, showing the dependence of the full width at half maximum (FWHM) of the (002) X-ray rocking curve on film thickness, indicates little difference in the orientation for the two samples under consideration. It is interesting to study what causes the difference in Hall mobility between ZnO films at O2 gas flow rates of 5 (sample A) and 30 sccm (sample B). From the Hall effect measurements, the two samples had the same total density of intrinsic defects including oxygen vacancies (ρ(VO)) and Zn interstitials (ρ(Zni)). It is likely that the ρ(Zni) of sample A is larger than that of sample B, considering high vapor pressure of Zn atoms and difference in O2 gas flow rates. It follows that the ρ(VO) of sample A is smaller than that of sample B. We note that the radius of Zn atoms is considerably larger than that of oxygen atoms. This may explain not only the data shown in Fig. 4(b) but also the issue to be resolved. In order to fully explain the observed phenomena, further defect studies are needed.
S. Kishimoto et al. / Surface & Coatings Technology 201 (2006) 4000–4003
4003
4. Conclusions
Acknowledgements
The dependences of electrical and structural properties on film thickness for polycrystalline undoped ZnO films with a thickness below 100 nm were studied. The undoped ZnO films were deposited on glass substrates at 200 °C by the plasmaassisted electron-beam method. From Hall effect measurement and out-of-plane and in-plane XRD data, we find that carrier concentration changes little for ZnO films with small thicknesses below 40 nm, whereas Hall mobility strongly depends on grain size. In order to realize low resistivity in the polycrystalline undoped ZnO film with very small thickness, large grain size with high orientation and control of the ratio of Zn to O are required.
S.K. wishes to thank Dr. Katsuhiko Inaba (RIGAKU Corporation) for assistance with the XRD measurements. The financial support from the collaboration of Regional Entities for the Advancement of Technological Excellence of the Japan Science and Technology Agency is gratefully acknowledged. References [1] S. Kishimoto, T. Yamamoto, Y. Nakagawa, K. Ikeda, H. Makino, T. Yamada, Superlattices Microstruct. 39 (2006) 306. [2] G.K. Williamson, W.H. Hall, Acta Metall. 1 (1953) 22. [3] E. Kisi, M.M. Elcombe, Acta Crystallogr., C Cryst. Struct. Commun. 45 (1989) 1867.