Investigation of Li-induced structural disorder and phase transition in ZnO by Raman spectroscopy

Solid State Communications 137 (2006) 78–81 www.elsevier.com/locate/ssc

Investigation of Li-induced structural disorder and phase transition in ZnO by Raman spectroscopy Y.L. Du a,*, Y. Deng b, M.S. Zhang b b

a Department of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China National Laboratory of Solid State Microstructures and Center for Materials Analysis, Nanjing University, Nanjing 210093, China

Received 20 August 2005; received in revised form 9 October 2005; accepted 11 October 2005 by R. Merlin Available online 27 October 2005

Abstract Zn0.8Li0.2O ceramics with wurtzite structure have been fabricated by a solid reaction of ZnO and Li2CO3. The effects of substitutional Li atoms on the crystal structure and structural phase transition of ZnO are studied by Raman spectroscopy. The enhancement of E1(LO) mode in Zn0.8Li0.2O ceramics reveals the occurrence of Li-induced structural disorder. Temperature dependent Raman spectra strongly indicate that a structural phase transition occurs at about 448 K in Zn0.8Li0.2O ceramics. q 2005 Elsevier Ltd. All rights reserved. PACS: 78.30.Fs; 77.80.Bh Keywords: A. Semiconductors; D. Phase transitions; D. Phonons

1. Introduction ZnO is a well-known wide band-gap semiconductor material of great interest for fundamental research as well as for applications in gas sensors, varistors, transducers and in the fields of luminescence and nonlinear optics [1–4]. ZnO crystallizes in the wurtzite structure, and no phase transition has been reported in pure ZnO at atmospheric pressure. However, dielectric anomaly, ferroelectric D–E hysteresis loop and specific heat anomaly have recently been discovered in Lidoped ZnO thin films and ceramics, which strongly suggest that ferroelectric activity exists in Li-doped ZnO [5–8]. Similar ferroelectric behavior was earlier found in Pb1KxGexTe [9] and Cd1KxZnxTe [10], where the large difference in ionic radii between the dopant atom and the host atom is believed to play a key role in the appearance of ferroelectricity. In the case of Li˚, doped ZnO, the ionic radii of Zn2C and LiC are 0.74 and 0.6 A respectively. Because of the atomic-size mismatch, Li ions can occupy off-centered positions, forming electric dipoles locally, and thereby leading to ferroelectric behavior [5–8]. Besides the

* Corresponding author. Tel.: C86 25 84301625; fax: C86 25 84315159. E-mail address: [email protected] (Y.L. Du).

0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.10.002

studies of ferroelectric properties [11,12], much work has been done to clear the ferroelectric phase transition mechanism in Li-doped ZnO. For example, Raman spectroscopy studies of Li-doped ZnO single crystal and ceramics had been carried out previously [13,14]. Only a slight cusp like anomaly of peak position, without drastic phonon anomaly, was observed at about 300 K, which indicated that the ferroelectric phase transition in Li-doped ZnO was most likely caused by the electronic origin. Recently, an ellipsometric investigation of phase transitions in Zn0.7Li0.3O films did not confirm the ferroelectric phase transition. However, strong changes of ellipsometric parameters at 743 K and change of band gap temperature behavior at 573 K were found [15,16]. The ellipsometric investigation strongly revealed the existence of high temperature phase transitions in Li-doped ZnO. These results indicate that structural phase transitions in Zn1KxLixO systems may be more complex than previously thought. In this communication, Raman spectroscopy, an effective tool to study the structural properties and phase transitions in oxide materials, was used to investigate the vibration properties of undoped and Li-doped ZnO ceramics to get insight into the effects of substitutional Li atoms on the crystal structure and structural phase transitions of ZnO. The existence of Liinduced structural disorder and a high temperature phase transition at about 448 K was strongly supported by the present work.

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2. Experimental

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The crystal structure of the as-prepared Zn0.8Li0.2O ceramics was checked by X-ray diffraction (XRD) on a Rigaku D/MAX-RA diffractometer with Cu Ka as the incident radiation. The curve (a) in Fig. 1 shows XRD pattern of Zn0.8Li0.2O ceramics. For comparison, the curve (b) in the figure gives the standard XRD pattern of pure ZnO. All the diffraction peaks of the as-prepared Zn0.8Li0.2O ceramics are assigned to typical wurtzite structure of ZnO. As shown in the figure, no extra diffraction lines are observed, which shows that the obtained Li-doped ZnO ceramics has single phase with wurtzite structure at room temperature. The Raman spectra of Zn0.8Li0.2O and ZnO ceramics at 300 K are illustrated in Fig. 2. According to group theory, the Raman active modes in ZnO are A1CE1C2E2, where A1 and E1 are polar and split into TO and LO phonons with different frequencies [17,18]. By comparison with the assignments of pure ZnO, Raman peaks at 99, 381, and 407 cmK1 of Zn0.8Li0.2O can be assigned to vibration modes of E2 (low), A1(TO), E1(TO) mode, respectively. The peak at 438 cmK1 is

(b) ZnO

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562 584

438 332 381 407

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3. Results and discussion

(a) Zn0.8Li0.2O

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Li-doped ZnO ceramics with nominal composition Zn0.8Li0.2O were synthesized by solid reaction of ZnO and Li2CO3. The stoichiometric amounts of ZnO (A.R.) and Li2CO3 (A.R.) were first weighed, and then combined by mixing in a planetary ball mill for 1 h to assure the homogeneity. Finally, the homogenized mixture was heated in air at 1073 K for 72 h. Raman spectra were measured with a JY LabRAM HR800 spectrometer using 488 nm exciting light at a power of 10 mW under backscattering conditions. The resolution was about 1 cmK1. For measuring the temperature dependence of the Raman spectra, the samples were mounted on a THMS 600 stage equipped with a platinum resistor sensor controlled by a TMS 94 temperature controller.

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2θ (deg) Fig. 1. XRD pattern of (a) Zn0.8Li0.2O ceramics; (b) ZnO ceramics.

Fig. 2. Raman spectrum of (a) Zn0.8Li0.2O ceramics; (b) ZnO ceramics at 300 K.

the high frequency E2 mode characteristic of the wurtzite structure. The 332 cmK1 mode is attributed to the second order Raman process, which is twice that of the low-frequency E2 phonon branch at the Brillouin-zone edge point (166 cmK1) [19]. As can be seen in the figure, the Raman spectrum of Zn0.8Li0.2O differs very little from the corresponding spectrum of pure ZnO in the range of 50–500 cmK1, which indicates that the as-prepared Li-doped ZnO ceramics are of the wurtzite structure. Such a conclusion is quite consistent with the result of XRD analysis. The noteworthy differences that occur between the spectra of pure and Li-doped ZnO are the dramatic changes of the E1(LO) mode at 584 cmK1. As seen in Fig. 2, after Li doping, this mode becomes stronger and much broader. E1(LO) mode is known to be related to the defects such as oxygen vacancy, interstitial Zn in ZnO [20,21]. Li doping produces large amounts of defects in ZnO, which obviously leads to the enhancement of the intensity of E1(LO) mode. The Li-induced defects cut the long-range lattice ordering, which causes the relaxation of Raman selection rules. As a result, phonons whose frequencies are close to G point participate in the Raman spectrum, which makes the E1(LO) mode becomes broader. Thus, the changes of E1(LO) mode strongly suggests the existence of Li-induced local disorder in Zn0.8Li0.2O ceramics. It is noted that a low frequency shoulder appears near 562 cmK1. This peak is also observed in ZnO thin films and hydrogen-implanted ZnO single crystals and can be ascribed to oxygen vacancy [22,23]. Consequently, its appearance also reveals the structural disorder induced by Li doping in Zn0.8Li0.2O ceramics. The temperature dependence of the high frequency E2 mode is displayed in Fig. 3(a) and (b). As shown in the figures, the frequency of E2 mode downshifts from 438 cmK1 at 300 K to 433 cmK1 at 573 K and shows a linear decreasing, which is similar to pure ZnO. It is also observed that the Raman intensity of E2 mode exhibits an obvious exponential decreasing with temperature. It decreases rapidly in the range

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E2

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573 K 548 K 523 K 498 K 448 K 423 K 398 K 348 K 323 K

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ZnO 438

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Temperature, K

Fig. 3. (a) Temperature dependence of E2 mode of Zn0.8Li0.2O ceramics; (b) Raman intensity and frequency shift of E2 mode of Zn0.8Li0.2O ceramics as a function of temperature. The solid line and dotted line represent fits to the linear equation and exponential equation, respectively.

300–448 K. And with further increasing temperature, it becomes to decrease slightly. The inflection temperature 448 K may be related to structural changes at this temperature in Li-doped ZnO. The temperature dependence of Raman spectra of Zn0.8Li0.2O and ZnO ceramics in the range of 250–420 cmK1 are shown in Fig. 4(a) and (b), respectively. As can be seen, the 332 cmK1 mode of Zn0.8Li0.2O is observed in the entire temperature range. Its intensity is reduced with increasing temperature. However, the intensity of the 332 cmK1 mode of pure ZnO has no obvious changes in the same temperature range. The E1(TO) mode of pure ZnO and Li-doped ZnO exhibits very similar temperature dependence. They can be detected at 300 K and becomes too weak to be observed with increasing the temperature. Special attentions are paid to the A1(TO) mode at 381 cmK1. The intensity of A1(TO) mode of Zn0.8Li0.2O weakens as the temperature increases and disappears completely when the temperature is above 448 K. However, as shown in Fig. 4(b), this mode is observed in the entire temperature range in pure ZnO. Thus, the different

E1(TO)

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Raman shift (cm ) Fig. 4. Temperature dependence of Raman spectra of (a) Zn0.8Li0.2O; and (b) ZnO ceramics in the range of 250–430 cmK1.

temperature dependence of A1(TO) mode must be due to the Li-doping effect. Fig. 5 shows the temperature dependent Raman spectra of Zn0.8Li0.2O ceramics in the range of 450– 650 cmK1. As the temperature increases, the intensity of the E1(LO) mode is reduced. The intensity of the peak at 562 cmK1 is gradually decreasing with increasing temperature. When the temperature is above 423 K, it can not be detected. As mentioned above, the E1(LO) mode and the mode at 562 cmK1 are strongly related to the structural disorder such as oxygen vacancy, interstitial Zn in ZnO [20–23]. Thus, the temperature dependence of these two modes indicates that the Li-doped ZnO ceramics become much order with increasing temperature. Based on the changes of A1(TO) mode, E1(LO) mode and the 562 cmK1 mode, we can not find any evidence of ferroelectric phase transition at about 343 K described in the previous reports [5–8], but we can conclude that a new Liinduced structural phase transition occurs at about 448 K in Zn0.8Li0.2O ceramics. Our conclusion is supported by the A. Deyneka’s latest ellipsometric investigation of high temperature phase transition in Li-doped ZnO thin films [15,16]. They found band gap and surface roughness jumps at 573 and 723 K in the Zn0.5Li0.5O thin films that is a strong evidence of structural changes at these temperatures. The difference of

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Acknowledgements

Intensity (arb. units)

573 K 548 K 523 K 498 K 473 K 448 K 423 K 398 K 373 K

348 K 323 K 300 K

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Raman shift (cm ) Fig. 5. Temperature dependence of Raman spectra of Zn0.8Li0.2O ceramics in the range of 450–650 cmK1. The dotted curves are a Lorentzian distribution fitting.

the phase transition temperature between A. Deyneka’s results and the present work may be due to the different Li concentration. More detailed studies are being carried out to elucidate the Li concentration effects on the structural phase transition in Li-doped ZnO ceramics.

4. Conclusion In conclusion, Zn0.8Li0.2O ceramics with wurtzite structure have been fabricated by a solid reaction of ZnO and Li2CO3. The effects of substitutional Li atoms on the crystal structure and structural phase transition of Zn0.8Li0.2O ceramics are studied by Raman spectroscopy. Structural disorder is induced in ZnO by Li doping, which is supported by the enhancement of E1(LO) mode. The temperature dependence of Raman spectra strongly indicates that a new structural phase transition occurs at about 448 K in Zn0.8Li0.2O ceramics.

This work is partially supported by the Progress Foundation of Scientific Research of Nanjing University of Science and Technology under XKF05055, and the Young Scholar Foundation of Nanjing University of Science and Technology under Njust200404. References [1] D.P. Norton, Y.W. Heo, M.P. Ivill, K. Ip, S.J. Pearton, M.F. Chisholm, T. Steiner, Mater. Today 7 (6) (2004) 34. [2] Y.L. Du, M.S. Zhang, Y. Deng, Q. Chen, Z. Yin, W.C. Chen, Chin. Phys. Lett. 19 (2002) 372. [3] H. Cao, J.Y. Wu, H.C. Ong, J.Y. Dai, R.P.H. Chang, Appl. Phys. Lett. 73 (1998) 572. [4] X. Tang, A. Clauzonnier, H.I. Campbell, K.A. Prior, B.C. Cavenett, Appl. Phys. Lett. 84 (2004) 3043. [5] X.S. Wang, Z.C. Wu, J.F. Webb, Z.G. Liu, Appl. Phys. A 77 (2003) 561. [6] H.Q. Ni, Y.F. Lu, Z.Y. Liu, et al., Appl. Phys. Lett. 79 (2001) 812. [7] M. Joseph, H. Tabata, T. Kawai, Appl. Phys. Lett. 74 (1999) 2534. [8] A. Onodera, N. Tamaki, Y. Kawamura, Jpn. J. Appl. Phys. 35 (1996) 5160. [9] Q.T. Islam, B.A. Bunker, Phys. Rev. Lett. 59 (1987) 2701. [10] R. Weil, R. Nkum, E. Muranevich, L. Benguigui, Phys. Rev. Lett. 62 (1989) 2744. [11] T. Nagata, T. Shimura, Y. Nakano, A. Ashida, N. Fujimura, T. Ito, Jpn. J. Appl. Phys. 40 (2001) 5615. [12] A. Onodera, N. Tamaki, K. Jin, H. Yamashita, Jpn. J. Appl. Phys. 36 (1997) 6008. [13] E. Islam, A. Sakai, A. Onodera, J. Phys. Soc. Jpn. 70 (2) (2001) 576. [14] E. Islam, A. Sakai, A. Onodera, J. Phys. Soc. Jpn. 71 (6) (2002) 1594. [15] A. Deyneka, G. Suchaneck, Z. Hubicka, L. Jastrabik, G. Gerlach, Ferroelectrics 298 (2004) 55. [16] A. Deyneka, Z. Hubicka, M. Cada, G. Suchaneck, M. Savinov, L. Jastrabik, G. Gerlach, Integr. Ferroelectr. 63 (2004) 209. [17] T.C. Damen, S.P.S. Porto, B. Tell, Phys. Rev. 142 (1966) 570. [18] J.M. Calleja, M. Cardona, Phys. Rev. B 16 (1977) 3753. [19] Z.Q. Chen, A. Kawasuso, Y. Xu, H. Naramoto, X.L. Yuan, T. Sekiguchi, R. Suzuki, T. Ohdaira, J. Appl. Phys. 97 (2005) 013528. [20] X.J. Zhang, H.L. Ma, Q.P. Wang, J. Ma, F.J. Zong, H.D. Xiao, F. Ji, Chin. Phys. Lett. 22 (2005) 995. [21] J.N. Zeng, J.K. Low, Z.M. Ren, T. Liew, Y.F. Lu, Appl. Surf. Sci. 197– 198 (2002) 362. [22] K. Mcguire, Z.W. Pan, Z.L. Wang, D. Milkie, J. Mene´ndez, A.M. Raoa, J. Nanosci. Nanotech. 2 (2002) 1. [23] Z.Q. Chen, A. Kawasuso, Y. Xu, H. Naramoto, X.L. Yuan, T. Sekiguchi, R. Suzuki, T. Ohdaira, Phys. Rev. B 71 (2005) 115213.