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Raman spectroscopy study of ZnO-based ceramic films fabricated by novel sol–gel process
Materials Science and Engineering B 97 (2003) 111 /116 www.elsevier.com/locate/mseb
Raman spectroscopy study of ZnO-based ceramic films fabricated by novel sol gel process /
Yanqiu Huang a,*, Meidong Liu b, Zhen Li a, Yike Zeng b, Shaobo Liu b a
b
Faculty of Materials Science and Chemical Engineering, China University of Geosciencses, Wuhan 430074, People’s Republic of China Department of Electronic Science and Technology, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China Received 5 April 2002; accepted 23 September 2002
Abstract The ZnO-based ceramic films doped with different dopants were prepared by a novel sol /gel process. The phase composition of the films was determined via X-ray diffraction analysis. The influence of the dopants on the residual stress, carrier concentration and the secondary phases was studied by means of Raman spectroscopy. Raman spectra show that the E2 phonon frequency shifts 3 /6 cm 1 to lower wavenumbers, whereas the A1(LO) mode shifts 3.2 /6.1 cm 1 to higher wavenumbers when the films were doped with Bi2O3, Sb2O3, MnO, Cr2O3, Y2O3 and Al2O3, indicating that both the tensile residual stress and the free carrier concentration were increased with doping. The larger stress is considered to originate from the lattice distortion, which was caused by the substitution of the doping ions for Zn2 , and the lattice mismatch between the ZnO crystals and the interfacial phases. The secondary phases were affected markedly by both Y2O3 and Al2O3. The intensity and the position of Raman bands of Zn7Sb2O12 and ZnCr2O4 phases changed obviously. The films showed remarkable nonlinear voltage /current characteristics, but the nonlinear coefficient of the films decreased evidently as the addition of Y2O3 or Al2O3. # 2002 Elsevier Science B.V. All rights reserved. Keywords: ZnO-based ceramic film; Sol /gel process; Raman spectroscopy; Residual stress; Electrical properties
1. Introduction ZnO based ceramic films fabricated by a novel sol /gel process have excellent nonlinear current /voltage (I /V ) characteristics [1], and have potential applications for low-voltage varistors, which are used to protect LSI or VLSI against abnormal voltage surges [2 /6]. To improve the electrical characteristics of the films, several additives of metal oxide, such as Bi2O3, Sb2O3, Co2O3, MnO and Cr2O3, are added to ZnO. The former studies of ZnO varistors have indicated that these additives play a different role in enhancing the non-ohmic property of ZnO [7 /9]. They result in the formation of the secondary phases in ZnO, influence the microstructure of the materials, and improve the electrical properties of the varistors. However, it is still not very clear how these additives influence the properties of ZnO.
* Corresponding author
Raman scattering can give information about the crystal structure on the scale of a few lattice constants. Any distortion of the lattice, excursion of the component, crystal defect and phase transformation could be shown in Raman bands. Therefore, Raman spectroscopic technique is one of the useful methods to gain insight into the microscopic structural effects of materials. In this work, we use Raman spectroscopy to examine the influence of the dopants on ZnO crystals and the secondary phases, and discuss the electrical properties of the films with different dopants.
2. Experimental ZnO ceramic films were deposited on Au/Si substrates by a novel sol /gel process. The details of the process have been described by the authors in an early work [1]. Reagent-grade Zn(CH3COO)2 ×/ 2H2O, Bi(NO3)3 ×/ 2H2O, C6H7O3Sb, Mn(CH3COO)2 ×/ 2H2O, Cr(NO3)3 ×/ 9H2O, Al(NO)3 ×/ 9H2O and Y2O3 were used. The
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samples of undoped ZnO films, ZnO-based ceramic films doped with 0.5 mol% Bi2O3, 1 mol% Sb2O3, 0.5 mol% MnO, 0.5 mol% Cr2O3, and that doped with 0.25 mol% Y2O3 or 0.02 mol% Al2O3 on the base of addition of Bi2O3, Sb2O3, MnO and Cr2O3 were prepared. The precursors were fabricated by dispersing the ZnO nanopowders throughout into the sols. And the films were prepared on the Au/Si substrates by repeated spin coating at room temperature. After each deposition, the films were heated in air at 400 8C for 10 min. And the films with the thickness of about 2 mm were obtained after 20 layers had been deposited and annealed at 750 8C for 2 h. Raman spectra of the films were obtained by means of Renishaw System RM-1000. Raman spectra were excited with the 514.5 nm line of an Ar laser at an incident power of 20 mW and obtained in the range 100 /2000 cm 1. The crystal phases of ZnO ceramic films were measured by X-ray diffractometer with CuKa radiation. And the electrical properties were surveyed with transistor characteristics testing instrument.
3. Results and discussion 3.1. X-ray diffraction of the films Fig. 1 show the X-ray diffraction patterns of the ZnO ceramic films doped with different dopants. In all the samples, Bi2O3 were found to be b-Bi2O3, which remained unchanged by the addition of other dopants such as Sb2O3, MnO and Cr2O3. Other phases of Bi2O3, i.e. a-Bi2O3, d-Bi2O3 and g-Bi2O3, were not observed in the diffraction patterns of all samples. When the samples were doped with Sb2O3 and Cr2O3, Zn2Sb3Bi3O14 pyrochlore phase, Zn7Sb2O12 spinel phase and ZnCr2O4 were observed (Fig. 1b,c). 3.2. Raman spectroscopy of ZnO ceramic films 3.2.1. Raman modes of ZnO ZnO crystallizes in the hexagonal wurtzite structure, which belongs to the space group C46v. The group theory predicts eight sets of phonon modes: 2E2, 2A1, 2E1 and 2B1. Among them, the 2B1 modes are not Raman active [10]. In different scattering geometry, different phonon modes can be detected by Raman spectroscopy. For example, in the Z(XX )Z geometry, i.e. incident and scattered polarization parallel, both the E2 and A1(LO) mode can be detected, whereas in the Z(XY )Z geometry, i.e. incident and scattered polarization perpendicular, only the E2 mode is observed [11]. In Fig. 2(b) E2(high) mode at 437 cm 1 is the only Raman peak being observed, for the ZnO crystals grew at preferred orientation with the c -axis perpendicular to the substrate surface when the thin films were prepared by
Fig. 1. X-ray diffraction patterns of ZnO ceramic films doped with (a) Bi2O3, (b) Bi2O3 and Sb2O3, (c) Bi2O3, Sb2O3, MnO and Cr2O3.
conventional sol /gel method. In Fig. 2(a) E2(high), A1(LO) and A1(TO) phonon mode at 435, 577 and 380 cm 1, respectively, were observed clearly, for the ZnO crystals did not grow at preferred orientation when the films were prepared by a novel sol /gel process. The band of 330 cm 1 in Fig. 2(a) is the second order Raman spectrum arising from zone-boundary phonons 2-E2 (M) of ZnO [12]. 3.2.2. The shift of E2(high) mode and the residual stress of the films To the wurtzite structure crystals, stress induced in the crystals affects the E2 phonon frequency obviously, and from which the information on stress can be extracted [11]. An increase in the E2 phonon frequency is ascribed to compressive stress, whereas a decrease in the E2 phonon frequency is ascribed to tensile stress. Therefore, the high Raman scattering of the E2 phonon is of great advantage for studies of residual stress within ZnO crystals. It is generally agreed that the stress arises from the mismatch of thermal expansion coefficients of the films and the substrates, or the lattice mismatch and distor-
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Fig. 2. Raman spectroscopy of undoped ZnO thin film prepared by (a) novel sol /gel process, and (b) traditional sol /gel process.
tion. And doping is considered to be the main factor that would cause the lattice distortion of the crystals, for it is usually different from the atomic radii of different elements. In order to understand the influence of the dopants on the crystals of ZnO, the ZnO ceramic films doped with different metal oxides were studied via Raman spectroscopy. Fig. 2 shows the Raman spectra of the undoped ZnO films prepared by a novel sol /gel process and conventional sol /gel method. In Fig. 2(b), the position of the E2(high) mode of the ZnO thin film, the thickness is about 300 nm, prepared by conventional sol /gel process is observed at 347 cm 1, which is almost the same position as the ZnO standard. It indicates that the thin film is almost free of stress. But in Fig. 2(a) the position of the E2(high) mode of the undoped ZnO film, the thickness is about 2 mm, prepared by a novel sol /gel process is observed at 435.3 cm 1. A Raman shift of 1.7 cm 1 had taken place. This indicates that a tensile stress remained in the film. It could be considered that this stress was caused by the mismatch of thermal expansion coefficients of the films and the substrates. Fig. 3 shows the Raman band of the ZnO ceramic film doped with Bi2O3. The position of the E2(high) mode of ZnO shifts to 433.7 cm 1. In comparison with the Raman spectrum in Fig. 2(a), the shift of the E2(high) mode increases 1.6 cm 1. It indicates that the addition
of Bi2O3 strengthens the tensile stress of the ZnO crystals. Theoretically, it is difficult to substitute Zn2 by Bi3 in ZnO crystal, for the radius of Bi3 is much larger than that of Zn2. The former study results indicate that Bi2O3 segregates mainly at ZnO /ZnO grain boundaries as Bi-rich phases during sintering in the ZnO /Bi2O3 system [13]. And in X-ray diffraction of the films doped with Bi2O3, b-Bi2O3 is observed. This implies that a Bi-rich liquid phase formed at the temperature of B/750 8C in sol /gel process. The liquid phase penetrated into the boundaries between ZnO grains and enhanced the grain growth and the densification of the film. This process would strengthen the interfacial stress of the ZnO grains, which may be the main cause of the shift of E2 mode in the Raman spectrum. Nevertheless, if there is any affection to the microstructure of ZnO crystal by Bi3, the possibility could not be excluded, for solid solubility of Bi2O3 in ZnO is less than 0.06 mol% [14]. Fig. 4 shows the Raman spectrum of ZnO ceramic film doped with Sb2O3. It can be seen that the E2(high) mode of ZnO split into two peaks. One of the peaks is 433.7 cm 1, which shifts obviously from 437 cm 1 mode, and another is 437 cm 1, which is the same position of the E2(high) mode when the stress is free. These characteristics imply that the crystal structure of ZnO was influenced by Sb3 to a certain extent.
Fig. 3. Raman spectroscopy of (Bi2O3)-doped ZnO ceramic film.
Fig. 4. Raman spectroscopy of (Sb2O3)-doped ZnO ceramic film.
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According to the former studies [8], Sb2O3 would react with ZnO to form ZnSb2O6 at 700 /800 8C and then form spinel (Zn7Sb2O12) at /800 8C. In the X-ray diffraction analysis of the films, spinel phase was observed. Since the content of Sb2O3 is a little, the reaction between Sb2O3 and ZnO would probably take place on the surface of ZnO grains. When the grain surfaces of ZnO transformed to the spinel phase, the vicinal lattice may be distorted, and the inner lattice remains unchanged. The distortion of the surface lattice caused the stress on the surface of the ZnO grains. Therefore, the peak of 433.7 cm 1 could be considered originating from the distortion of the surface lattice of ZnO, and the peak of 437 cm 1 could be considered originating from the normal lattice of ZnO crystals. Fig. 5 shows the Raman spectrum of ZnO-based ceramic film doped with Bi2O3, Sb2O3, MnO and Cr2O3. A larger peak shift from 437 to 431 cm 1 of the E2(high) mode of ZnO is observed. The shift of 6 cm 1 towards lower wavenumbers indicates that larger tensile stress remained in the ZnO crystals when more additives were added to the ceramic film. It implies that ZnO crystals were affected obviously by the dopants. The ion radii of manganese and chromium are similar to the radius of Zn2, and some of the Zn2 in the lattice may probably be substituted by Mn2, Mn4 or Cr3 when the ZnO grains grow during sintering. The substitution would cause the distortion of the lattice of ZnO. This kind of lattice distortion of ZnO would cause the stress increase. Therefore, besides the surface lattice distortion of the ZnO crystals caused by the formation of the secondary phases between ZnO /ZnO grains, the distortion of the lattice caused by the substitution of the dopant ions such as Mn2, Mn4 and Cr3 for Zn2 is considered to be the main origin of the large shift of the E2 mode of ZnO. Figs. 6 and 7 show the Raman spectra of ZnO based ceramic films doped with 0.25 mol% Y2O3 and 0.02 mol% Al2O3, respectively, on the base of addition of Bi2O3, Sb2O3, MnO and Cr2O3. The positions of the E2(high) mode in both Raman spectra are the same at
Fig. 5. Raman spectrum of ZnO ceramic film doped with Bi2O3, Sb2O3, MnO and Cr2O3.
Fig. 6. Raman spectrum of ZnO ceramic film doped with Bi2O3, Sb2O3, MnO, Cr2O3 and Y2O3.
Fig. 7. Raman spectrum of ZnO ceramic film doped with Bi2O3, Sb2O3, MnO, Cr2O3 and Al2O3.
433.7 cm 1, the E2 frequency shift is 3.3 cm 1. In comparison with Fig. 4, the shift of E2 mode decreases obviously, indicating that the stress of the ZnO crystals had relaxed to a certain extent with the addition of Y2O3 and Al2O3.
3.2.3. The shift of A1(LO) mode and the free carrier concentration in ZnO crystals The A1(LO) Raman mode is sensitive to changes in the free carrier concentration, because in the polar semiconductor collective excitation of free carrier interact with the A1(LO) longitudinal-optical phonons. In the Raman study of GaN, the A1(LO) mode shifts to higher wavenumbers, broadens and weakens in intensity with increasing carrier concentration [11]. Similar characteristics can be seen in the Raman spectra of the ZnObased ceramic films. Fig. 8 shows the Raman spectra of the ZnO-based films doped with different additives. The position of the A1(LO) mode of undoped ZnO film is at 576.7 cm 1, and shifts to 582.8 cm 1 when the dopants of Bi2O3, Sb2O3, MnO and Cr2O3 were added. The A1(LO) mode shifts to 581.5 and 579.9 cm 1 when Y2O3 and Al2O3 were added, respectively, on the base of addition of Bi2O3, Sb2O3, MnO and Cr2O3. These characteristics indicate that the free carrier concentra-
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Fig. 8. Raman spectra of the ZnO-based ceramic films showing the A1(LO) mode shift with the dopants: (a) undoped ZnO film; (b) doped with Bi2O3, Sb2O3, MnO and Cr2O3; (c) doped with Bi2O3, Sb2O3, MnO, Cr2O3 and Y2O3; (d) doped with Bi2O3, Sb2O3, MnO, Cr2O3 and Al2O3.
tion in the ZnO-based ceramic films increased when the films were doped with donors.
3.2.4. Raman bands of the secondary phases in the ZnObased ceramic films The bands of the secondary phases such as b-Bi2O3, Zn2Sb3Bi3O14, Zn7Sb2O12 and ZnCr2O4 were observed obviously in the Raman spectra. The position of the Raman peak of b-Bi2O3 is at 329 cm 1 when the films were doped with Bi2O3 only (Fig. 3), and shifts to 327 cm 1 when the films were doped with Bi2O3, Sb2O3, MnO and Cr2O3 (Fig. 5). The position of the Raman peak of b-Bi2O3 remains at 327 cm 1 when 0.25 mol% Y2O3 was added on the base of addition of Bi2O3, Sb2O3, MnO and Cr2O3 (Fig. 6), but shifts back to 329 cm 1 when 0.02 mol% Al2O3 was added together with Bi2O3, Sb2O3, MnO and Cr2O3 (Fig. 7). These characteristics imply that b-Bi2O3 phase was affected by the additives to some degree. In Fig. 5, the peak of 709 cm 1 is ascribed to Zn7Sb2O12, and the peaks of 529 and 613 cm 1 are ascribed to Zn2Sb3Bi3O14. Compared with Fig. 3, the peak of 709 cm 1 shows strong in intensity, which indicates that Zn7Sb2O12 grows well. In the system of ZnO /Bi2O3 /Sb2O3, since Bi2O3 acts as a liquid phase during sintering, the dopants would probably be redistributed uniformly between ZnO particles. Bi2O3 reacted with Sb2O3 and ZnO to form Zn2Sb3Bi3O14 and then transformed to Zn7Sb2O12. Therefore, the distribution of Zn7Sb2O12 phase is uniform in the system. Corresponding to the strong intensity of the Raman band of Zn7Sb2O12, the Raman bands of Zn2Sb3Bi3O14 at 529 and 613 cm 1 are weak. These characteristics imply that spinel phase has the preponderance over pyrochlore phase.
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The intensity of the peak of 709 cm 1 was affected obviously by the kind of dopants. When 0.25 mol% Y2O3 was added, the intensity of the band of spinel not only became very weak but also split into two peaks (Fig. 6). It seems that the formation of the spinel phase was affected evidently by Y2O3. The same characteristics can be seen in the Raman spectra of ZnO based ceramic film when 0.02 mol% Al2O3 was added (Fig. 7). In addition to weakening in intensity, a shift of 3 cm 1 of the spinel phase towards higher wavenumbers is observed in Fig. 7, indicating the spinel crystals were distorted to some extent when Al2O3 was added. The peak of 825 cm 1 in Fig. 5 is ascribed to ZnCr2O4. The intensity and the frequency of ZnCr2O4 are also affected by dopants. Strong intensity with a shift of 1 cm 1 towards higher wavenumber is observed when Y2O3 was added on the base of the addition of Bi2O3, Sb2O3, MnO and Cr2O3 (Fig. 6). The intensity weakens and the position shifts to 828 cm 1 as seen in Fig. 7 when Al2O3 was used to substitute for Y2O3. In Figs. 5 /7, the broad features between 1050 and 1150 cm 1 are assigned to the two-phonon modes (2 LO) characteristic of this II /IV semiconductor [15]. 3.3. Electrical properties of ZnO ceramic films The films were used as film varistors when the upper electrodes were made, and the electrical properties were measured. The results were shown in Table 1. Sample Z1 is the ZnO-based film doped with Bi2O3, Sb2O3, MnO and Cr2O3, sample Z2 is the film doped with 0.25 mol% Y2O3 on the base of addition of Bi2O3, Sb2O3, MnO and Cr2O3, and sample Z3 is the film doped with 0.02 mol% Al2O3 on the base of addition of Bi2O3, Sb2O3, MnO and Cr2O3. The nonlinear voltage shifted to higher voltage and the nonlinear coefficient and leakage current decreased obviously when Y2O3 was added. Lower nonlinear coefficient and higher leakage current together with lower nonlinear voltage were obtained when Al2O3 was added. These characteristics are considered to be the results of the influence of the dopants on the crystals of ZnO, the secondary phases, such as Zn7Sb2O12 phase and ZnCr2O4 phase, and the carrier concentration as observed in the Raman spectra (Figs. 5 /8).
4. Conclusion ZnO-based ceramic films doped with different dopants were prepared by a novel sol /gel process and studied by Raman spectroscopy and X-ray diffraction analysis. In Raman spectra, the E2 phonon frequency and the A1(LO) mode shift 3 /6 cm 1 to lower wavenumbers and 3.2 /6.1 cm 1 to higher wavenumbers, respectively, when the films were doped with Bi2O3,
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Table 1 The electrical characteristics of ZnO-based ceramic film varistors Samples
Nonlinear voltage (V)
Nonlinear coefficient (a )
Leakage current (mA)
Z1 Z2 Z3
5 8 4
20 11 7
58 27 92
Sb2O3, MnO, Cr2O3, Y2O3 and Al2O3, indicating that the tensile residual stress and the free carrier concentration were increased with doping. The larger stress originated from the lattice distortion, which was caused by the substitution of the doping ions for Zn2, and the lattice mismatch between the ZnO crystals and the interfacial phases. The intensity and the position of Raman bands of Zn7Sb2O12 and ZnCr2O4 phases changed obviously with the addition of Y2O3 or Al2O3. The films showed remarkable nonlinear V /I characteristics, and the nonlinear coefficient (a ) decreased evidently as the addition of Y2O3 or Al2O3. The change of the electrical properties is considered to be the results of the influence on the crystal structure of ZnO, the secondary phases and the carrier concentration by the dopants.
Acknowledgements This work was primarily supported by the Laboratory of Quantitative Prediction and Exploration Assessment for Mineral Resources, Ministry of Land and Resources, China. The authors thank Mr. Mouchun He for the Raman scattering measurement.
References [1] Y.Q. Huang, M.D. Liu, Y.K. Zeng, C.R. Li, D.L. Xia, S.B. Liu, Mater. Sci. Eng. B86 (2001) 232 /236. [2] S. Bernik, P. Zupancic, D. Kolar, J. Eur. Ceram. Soc. 19 (6 /7) (1999) 709 /713. [3] M.W. Barsoum, A. Elkind, F.A. Selim, J. Am. Ceram. Soc. 79 (4) (1996) 962 /966. [4] A.-J. Yen, Y.-S. Lee, T.-Y. Tseng, J. Am. Ceram. Soc. 77 (11) (1994) 3006 /3011. [5] N. Horio, M. Hiramatsu, M. Nawata, K. Imaeda, T. Torii, Vacuum 51 (4) (1998) 719 /722. [6] R. Jia, F.Q. Qu, G.M. Wu, S.G. Song, M.D. Tao, Gongneng Cailiao (J. functional Mater.) 30 (6) (1999) 636 /638. [7] T.K. Gupta, A.C. Miller, J. Mater. Res. 3 (4) (1988) 745 /754. [8] R. Metz, H. Delalu, J.R. Vignalou, N. Achard, M. Elkhatib, Mater. Chem. Phys. 63 (2) (2000) 157 /162. [9] C.-H. Lu, N. Chyi, H.-W. Wong, W.-J. Hwang, Mater. Chem. Phys. 62 (2) (2000) 164 /168. [10] Z.C. Zhang, B.B. Huang, Y.Q. Yu, D.L. Cui, Mater. Sci. Eng. B86 (2001) 109 /112. [11] M. Kubal, Surf. Interface Anal. 31 (2001) 987 /999. [12] X.Q. Wang, S.R. Yang, J.Z. Wang, M.T. Li, X.Y. Jiang, G.T. Du, X. Liu, R.P.H. Chang, J. Cryst. Growth 226 (2001) 123 /129. [13] J. Kim, T. Kimura, T. Yamaguchi, Am. Ceram. Soc. 72 (8) (1989) 1541 /1544. [14] J. Luo, Y.-M. Chiang, J. Eur. Ceram. Soc. 19 (1999) 697 /701. [15] G.J. Exarhos, A. Rose, C.F. Windisch, Jr, Thin Solid Films 308 / 309 (1997) 56 /62.
Raman spectroscopy study of ZnO-based ceramic films fabricated by novel sol gel process /
Yanqiu Huang a,*, Meidong Liu b, Zhen Li a, Yike Zeng b, Shaobo Liu b a
b
Faculty of Materials Science and Chemical Engineering, China University of Geosciencses, Wuhan 430074, People’s Republic of China Department of Electronic Science and Technology, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China Received 5 April 2002; accepted 23 September 2002
Abstract The ZnO-based ceramic films doped with different dopants were prepared by a novel sol /gel process. The phase composition of the films was determined via X-ray diffraction analysis. The influence of the dopants on the residual stress, carrier concentration and the secondary phases was studied by means of Raman spectroscopy. Raman spectra show that the E2 phonon frequency shifts 3 /6 cm 1 to lower wavenumbers, whereas the A1(LO) mode shifts 3.2 /6.1 cm 1 to higher wavenumbers when the films were doped with Bi2O3, Sb2O3, MnO, Cr2O3, Y2O3 and Al2O3, indicating that both the tensile residual stress and the free carrier concentration were increased with doping. The larger stress is considered to originate from the lattice distortion, which was caused by the substitution of the doping ions for Zn2 , and the lattice mismatch between the ZnO crystals and the interfacial phases. The secondary phases were affected markedly by both Y2O3 and Al2O3. The intensity and the position of Raman bands of Zn7Sb2O12 and ZnCr2O4 phases changed obviously. The films showed remarkable nonlinear voltage /current characteristics, but the nonlinear coefficient of the films decreased evidently as the addition of Y2O3 or Al2O3. # 2002 Elsevier Science B.V. All rights reserved. Keywords: ZnO-based ceramic film; Sol /gel process; Raman spectroscopy; Residual stress; Electrical properties
1. Introduction ZnO based ceramic films fabricated by a novel sol /gel process have excellent nonlinear current /voltage (I /V ) characteristics [1], and have potential applications for low-voltage varistors, which are used to protect LSI or VLSI against abnormal voltage surges [2 /6]. To improve the electrical characteristics of the films, several additives of metal oxide, such as Bi2O3, Sb2O3, Co2O3, MnO and Cr2O3, are added to ZnO. The former studies of ZnO varistors have indicated that these additives play a different role in enhancing the non-ohmic property of ZnO [7 /9]. They result in the formation of the secondary phases in ZnO, influence the microstructure of the materials, and improve the electrical properties of the varistors. However, it is still not very clear how these additives influence the properties of ZnO.
* Corresponding author
Raman scattering can give information about the crystal structure on the scale of a few lattice constants. Any distortion of the lattice, excursion of the component, crystal defect and phase transformation could be shown in Raman bands. Therefore, Raman spectroscopic technique is one of the useful methods to gain insight into the microscopic structural effects of materials. In this work, we use Raman spectroscopy to examine the influence of the dopants on ZnO crystals and the secondary phases, and discuss the electrical properties of the films with different dopants.
2. Experimental ZnO ceramic films were deposited on Au/Si substrates by a novel sol /gel process. The details of the process have been described by the authors in an early work [1]. Reagent-grade Zn(CH3COO)2 ×/ 2H2O, Bi(NO3)3 ×/ 2H2O, C6H7O3Sb, Mn(CH3COO)2 ×/ 2H2O, Cr(NO3)3 ×/ 9H2O, Al(NO)3 ×/ 9H2O and Y2O3 were used. The
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samples of undoped ZnO films, ZnO-based ceramic films doped with 0.5 mol% Bi2O3, 1 mol% Sb2O3, 0.5 mol% MnO, 0.5 mol% Cr2O3, and that doped with 0.25 mol% Y2O3 or 0.02 mol% Al2O3 on the base of addition of Bi2O3, Sb2O3, MnO and Cr2O3 were prepared. The precursors were fabricated by dispersing the ZnO nanopowders throughout into the sols. And the films were prepared on the Au/Si substrates by repeated spin coating at room temperature. After each deposition, the films were heated in air at 400 8C for 10 min. And the films with the thickness of about 2 mm were obtained after 20 layers had been deposited and annealed at 750 8C for 2 h. Raman spectra of the films were obtained by means of Renishaw System RM-1000. Raman spectra were excited with the 514.5 nm line of an Ar laser at an incident power of 20 mW and obtained in the range 100 /2000 cm 1. The crystal phases of ZnO ceramic films were measured by X-ray diffractometer with CuKa radiation. And the electrical properties were surveyed with transistor characteristics testing instrument.
3. Results and discussion 3.1. X-ray diffraction of the films Fig. 1 show the X-ray diffraction patterns of the ZnO ceramic films doped with different dopants. In all the samples, Bi2O3 were found to be b-Bi2O3, which remained unchanged by the addition of other dopants such as Sb2O3, MnO and Cr2O3. Other phases of Bi2O3, i.e. a-Bi2O3, d-Bi2O3 and g-Bi2O3, were not observed in the diffraction patterns of all samples. When the samples were doped with Sb2O3 and Cr2O3, Zn2Sb3Bi3O14 pyrochlore phase, Zn7Sb2O12 spinel phase and ZnCr2O4 were observed (Fig. 1b,c). 3.2. Raman spectroscopy of ZnO ceramic films 3.2.1. Raman modes of ZnO ZnO crystallizes in the hexagonal wurtzite structure, which belongs to the space group C46v. The group theory predicts eight sets of phonon modes: 2E2, 2A1, 2E1 and 2B1. Among them, the 2B1 modes are not Raman active [10]. In different scattering geometry, different phonon modes can be detected by Raman spectroscopy. For example, in the Z(XX )Z geometry, i.e. incident and scattered polarization parallel, both the E2 and A1(LO) mode can be detected, whereas in the Z(XY )Z geometry, i.e. incident and scattered polarization perpendicular, only the E2 mode is observed [11]. In Fig. 2(b) E2(high) mode at 437 cm 1 is the only Raman peak being observed, for the ZnO crystals grew at preferred orientation with the c -axis perpendicular to the substrate surface when the thin films were prepared by
Fig. 1. X-ray diffraction patterns of ZnO ceramic films doped with (a) Bi2O3, (b) Bi2O3 and Sb2O3, (c) Bi2O3, Sb2O3, MnO and Cr2O3.
conventional sol /gel method. In Fig. 2(a) E2(high), A1(LO) and A1(TO) phonon mode at 435, 577 and 380 cm 1, respectively, were observed clearly, for the ZnO crystals did not grow at preferred orientation when the films were prepared by a novel sol /gel process. The band of 330 cm 1 in Fig. 2(a) is the second order Raman spectrum arising from zone-boundary phonons 2-E2 (M) of ZnO [12]. 3.2.2. The shift of E2(high) mode and the residual stress of the films To the wurtzite structure crystals, stress induced in the crystals affects the E2 phonon frequency obviously, and from which the information on stress can be extracted [11]. An increase in the E2 phonon frequency is ascribed to compressive stress, whereas a decrease in the E2 phonon frequency is ascribed to tensile stress. Therefore, the high Raman scattering of the E2 phonon is of great advantage for studies of residual stress within ZnO crystals. It is generally agreed that the stress arises from the mismatch of thermal expansion coefficients of the films and the substrates, or the lattice mismatch and distor-
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Fig. 2. Raman spectroscopy of undoped ZnO thin film prepared by (a) novel sol /gel process, and (b) traditional sol /gel process.
tion. And doping is considered to be the main factor that would cause the lattice distortion of the crystals, for it is usually different from the atomic radii of different elements. In order to understand the influence of the dopants on the crystals of ZnO, the ZnO ceramic films doped with different metal oxides were studied via Raman spectroscopy. Fig. 2 shows the Raman spectra of the undoped ZnO films prepared by a novel sol /gel process and conventional sol /gel method. In Fig. 2(b), the position of the E2(high) mode of the ZnO thin film, the thickness is about 300 nm, prepared by conventional sol /gel process is observed at 347 cm 1, which is almost the same position as the ZnO standard. It indicates that the thin film is almost free of stress. But in Fig. 2(a) the position of the E2(high) mode of the undoped ZnO film, the thickness is about 2 mm, prepared by a novel sol /gel process is observed at 435.3 cm 1. A Raman shift of 1.7 cm 1 had taken place. This indicates that a tensile stress remained in the film. It could be considered that this stress was caused by the mismatch of thermal expansion coefficients of the films and the substrates. Fig. 3 shows the Raman band of the ZnO ceramic film doped with Bi2O3. The position of the E2(high) mode of ZnO shifts to 433.7 cm 1. In comparison with the Raman spectrum in Fig. 2(a), the shift of the E2(high) mode increases 1.6 cm 1. It indicates that the addition
of Bi2O3 strengthens the tensile stress of the ZnO crystals. Theoretically, it is difficult to substitute Zn2 by Bi3 in ZnO crystal, for the radius of Bi3 is much larger than that of Zn2. The former study results indicate that Bi2O3 segregates mainly at ZnO /ZnO grain boundaries as Bi-rich phases during sintering in the ZnO /Bi2O3 system [13]. And in X-ray diffraction of the films doped with Bi2O3, b-Bi2O3 is observed. This implies that a Bi-rich liquid phase formed at the temperature of B/750 8C in sol /gel process. The liquid phase penetrated into the boundaries between ZnO grains and enhanced the grain growth and the densification of the film. This process would strengthen the interfacial stress of the ZnO grains, which may be the main cause of the shift of E2 mode in the Raman spectrum. Nevertheless, if there is any affection to the microstructure of ZnO crystal by Bi3, the possibility could not be excluded, for solid solubility of Bi2O3 in ZnO is less than 0.06 mol% [14]. Fig. 4 shows the Raman spectrum of ZnO ceramic film doped with Sb2O3. It can be seen that the E2(high) mode of ZnO split into two peaks. One of the peaks is 433.7 cm 1, which shifts obviously from 437 cm 1 mode, and another is 437 cm 1, which is the same position of the E2(high) mode when the stress is free. These characteristics imply that the crystal structure of ZnO was influenced by Sb3 to a certain extent.
Fig. 3. Raman spectroscopy of (Bi2O3)-doped ZnO ceramic film.
Fig. 4. Raman spectroscopy of (Sb2O3)-doped ZnO ceramic film.
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According to the former studies [8], Sb2O3 would react with ZnO to form ZnSb2O6 at 700 /800 8C and then form spinel (Zn7Sb2O12) at /800 8C. In the X-ray diffraction analysis of the films, spinel phase was observed. Since the content of Sb2O3 is a little, the reaction between Sb2O3 and ZnO would probably take place on the surface of ZnO grains. When the grain surfaces of ZnO transformed to the spinel phase, the vicinal lattice may be distorted, and the inner lattice remains unchanged. The distortion of the surface lattice caused the stress on the surface of the ZnO grains. Therefore, the peak of 433.7 cm 1 could be considered originating from the distortion of the surface lattice of ZnO, and the peak of 437 cm 1 could be considered originating from the normal lattice of ZnO crystals. Fig. 5 shows the Raman spectrum of ZnO-based ceramic film doped with Bi2O3, Sb2O3, MnO and Cr2O3. A larger peak shift from 437 to 431 cm 1 of the E2(high) mode of ZnO is observed. The shift of 6 cm 1 towards lower wavenumbers indicates that larger tensile stress remained in the ZnO crystals when more additives were added to the ceramic film. It implies that ZnO crystals were affected obviously by the dopants. The ion radii of manganese and chromium are similar to the radius of Zn2, and some of the Zn2 in the lattice may probably be substituted by Mn2, Mn4 or Cr3 when the ZnO grains grow during sintering. The substitution would cause the distortion of the lattice of ZnO. This kind of lattice distortion of ZnO would cause the stress increase. Therefore, besides the surface lattice distortion of the ZnO crystals caused by the formation of the secondary phases between ZnO /ZnO grains, the distortion of the lattice caused by the substitution of the dopant ions such as Mn2, Mn4 and Cr3 for Zn2 is considered to be the main origin of the large shift of the E2 mode of ZnO. Figs. 6 and 7 show the Raman spectra of ZnO based ceramic films doped with 0.25 mol% Y2O3 and 0.02 mol% Al2O3, respectively, on the base of addition of Bi2O3, Sb2O3, MnO and Cr2O3. The positions of the E2(high) mode in both Raman spectra are the same at
Fig. 5. Raman spectrum of ZnO ceramic film doped with Bi2O3, Sb2O3, MnO and Cr2O3.
Fig. 6. Raman spectrum of ZnO ceramic film doped with Bi2O3, Sb2O3, MnO, Cr2O3 and Y2O3.
Fig. 7. Raman spectrum of ZnO ceramic film doped with Bi2O3, Sb2O3, MnO, Cr2O3 and Al2O3.
433.7 cm 1, the E2 frequency shift is 3.3 cm 1. In comparison with Fig. 4, the shift of E2 mode decreases obviously, indicating that the stress of the ZnO crystals had relaxed to a certain extent with the addition of Y2O3 and Al2O3.
3.2.3. The shift of A1(LO) mode and the free carrier concentration in ZnO crystals The A1(LO) Raman mode is sensitive to changes in the free carrier concentration, because in the polar semiconductor collective excitation of free carrier interact with the A1(LO) longitudinal-optical phonons. In the Raman study of GaN, the A1(LO) mode shifts to higher wavenumbers, broadens and weakens in intensity with increasing carrier concentration [11]. Similar characteristics can be seen in the Raman spectra of the ZnObased ceramic films. Fig. 8 shows the Raman spectra of the ZnO-based films doped with different additives. The position of the A1(LO) mode of undoped ZnO film is at 576.7 cm 1, and shifts to 582.8 cm 1 when the dopants of Bi2O3, Sb2O3, MnO and Cr2O3 were added. The A1(LO) mode shifts to 581.5 and 579.9 cm 1 when Y2O3 and Al2O3 were added, respectively, on the base of addition of Bi2O3, Sb2O3, MnO and Cr2O3. These characteristics indicate that the free carrier concentra-
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Fig. 8. Raman spectra of the ZnO-based ceramic films showing the A1(LO) mode shift with the dopants: (a) undoped ZnO film; (b) doped with Bi2O3, Sb2O3, MnO and Cr2O3; (c) doped with Bi2O3, Sb2O3, MnO, Cr2O3 and Y2O3; (d) doped with Bi2O3, Sb2O3, MnO, Cr2O3 and Al2O3.
tion in the ZnO-based ceramic films increased when the films were doped with donors.
3.2.4. Raman bands of the secondary phases in the ZnObased ceramic films The bands of the secondary phases such as b-Bi2O3, Zn2Sb3Bi3O14, Zn7Sb2O12 and ZnCr2O4 were observed obviously in the Raman spectra. The position of the Raman peak of b-Bi2O3 is at 329 cm 1 when the films were doped with Bi2O3 only (Fig. 3), and shifts to 327 cm 1 when the films were doped with Bi2O3, Sb2O3, MnO and Cr2O3 (Fig. 5). The position of the Raman peak of b-Bi2O3 remains at 327 cm 1 when 0.25 mol% Y2O3 was added on the base of addition of Bi2O3, Sb2O3, MnO and Cr2O3 (Fig. 6), but shifts back to 329 cm 1 when 0.02 mol% Al2O3 was added together with Bi2O3, Sb2O3, MnO and Cr2O3 (Fig. 7). These characteristics imply that b-Bi2O3 phase was affected by the additives to some degree. In Fig. 5, the peak of 709 cm 1 is ascribed to Zn7Sb2O12, and the peaks of 529 and 613 cm 1 are ascribed to Zn2Sb3Bi3O14. Compared with Fig. 3, the peak of 709 cm 1 shows strong in intensity, which indicates that Zn7Sb2O12 grows well. In the system of ZnO /Bi2O3 /Sb2O3, since Bi2O3 acts as a liquid phase during sintering, the dopants would probably be redistributed uniformly between ZnO particles. Bi2O3 reacted with Sb2O3 and ZnO to form Zn2Sb3Bi3O14 and then transformed to Zn7Sb2O12. Therefore, the distribution of Zn7Sb2O12 phase is uniform in the system. Corresponding to the strong intensity of the Raman band of Zn7Sb2O12, the Raman bands of Zn2Sb3Bi3O14 at 529 and 613 cm 1 are weak. These characteristics imply that spinel phase has the preponderance over pyrochlore phase.
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The intensity of the peak of 709 cm 1 was affected obviously by the kind of dopants. When 0.25 mol% Y2O3 was added, the intensity of the band of spinel not only became very weak but also split into two peaks (Fig. 6). It seems that the formation of the spinel phase was affected evidently by Y2O3. The same characteristics can be seen in the Raman spectra of ZnO based ceramic film when 0.02 mol% Al2O3 was added (Fig. 7). In addition to weakening in intensity, a shift of 3 cm 1 of the spinel phase towards higher wavenumbers is observed in Fig. 7, indicating the spinel crystals were distorted to some extent when Al2O3 was added. The peak of 825 cm 1 in Fig. 5 is ascribed to ZnCr2O4. The intensity and the frequency of ZnCr2O4 are also affected by dopants. Strong intensity with a shift of 1 cm 1 towards higher wavenumber is observed when Y2O3 was added on the base of the addition of Bi2O3, Sb2O3, MnO and Cr2O3 (Fig. 6). The intensity weakens and the position shifts to 828 cm 1 as seen in Fig. 7 when Al2O3 was used to substitute for Y2O3. In Figs. 5 /7, the broad features between 1050 and 1150 cm 1 are assigned to the two-phonon modes (2 LO) characteristic of this II /IV semiconductor [15]. 3.3. Electrical properties of ZnO ceramic films The films were used as film varistors when the upper electrodes were made, and the electrical properties were measured. The results were shown in Table 1. Sample Z1 is the ZnO-based film doped with Bi2O3, Sb2O3, MnO and Cr2O3, sample Z2 is the film doped with 0.25 mol% Y2O3 on the base of addition of Bi2O3, Sb2O3, MnO and Cr2O3, and sample Z3 is the film doped with 0.02 mol% Al2O3 on the base of addition of Bi2O3, Sb2O3, MnO and Cr2O3. The nonlinear voltage shifted to higher voltage and the nonlinear coefficient and leakage current decreased obviously when Y2O3 was added. Lower nonlinear coefficient and higher leakage current together with lower nonlinear voltage were obtained when Al2O3 was added. These characteristics are considered to be the results of the influence of the dopants on the crystals of ZnO, the secondary phases, such as Zn7Sb2O12 phase and ZnCr2O4 phase, and the carrier concentration as observed in the Raman spectra (Figs. 5 /8).
4. Conclusion ZnO-based ceramic films doped with different dopants were prepared by a novel sol /gel process and studied by Raman spectroscopy and X-ray diffraction analysis. In Raman spectra, the E2 phonon frequency and the A1(LO) mode shift 3 /6 cm 1 to lower wavenumbers and 3.2 /6.1 cm 1 to higher wavenumbers, respectively, when the films were doped with Bi2O3,
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Table 1 The electrical characteristics of ZnO-based ceramic film varistors Samples
Nonlinear voltage (V)
Nonlinear coefficient (a )
Leakage current (mA)
Z1 Z2 Z3
5 8 4
20 11 7
58 27 92
Sb2O3, MnO, Cr2O3, Y2O3 and Al2O3, indicating that the tensile residual stress and the free carrier concentration were increased with doping. The larger stress originated from the lattice distortion, which was caused by the substitution of the doping ions for Zn2, and the lattice mismatch between the ZnO crystals and the interfacial phases. The intensity and the position of Raman bands of Zn7Sb2O12 and ZnCr2O4 phases changed obviously with the addition of Y2O3 or Al2O3. The films showed remarkable nonlinear V /I characteristics, and the nonlinear coefficient (a ) decreased evidently as the addition of Y2O3 or Al2O3. The change of the electrical properties is considered to be the results of the influence on the crystal structure of ZnO, the secondary phases and the carrier concentration by the dopants.
Acknowledgements This work was primarily supported by the Laboratory of Quantitative Prediction and Exploration Assessment for Mineral Resources, Ministry of Land and Resources, China. The authors thank Mr. Mouchun He for the Raman scattering measurement.
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