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Effects of forming gas annealing on LiNbO3 single crystals
Physica B 406 (2011) 683–686
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Effects of forming gas annealing on LiNbO3 single crystals Feng Chen a, Wan Ping Chen a,b,n, Yu Wang a, Yong Ming Hu a, Zhen Jiang Shen b, Helen Lai Wah Chan a a b
Department of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University, Hong Kong, China Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education and School of Physics and Technology, Wuhan University, Wuhan 430072, China
a r t i c l e in f o
abstract
Article history: Received 8 August 2010 Received in revised form 28 October 2010 Accepted 24 November 2010
Z-cut congruent LiNbO3 single crystals were annealed in 95%N2 +5%H2 at high temperatures. X-ray diffraction showed that 2y of (0 0 0 6) peak is obviously reduced by 0.61 and 1.01 for the samples annealed at 600 and 900 1C, respectively. A new peak appears at the high-energy side of O 1s spectrum in X-ray photoelelectron spectroscopy (XPS) analyses, and the leakage current is greatly increased. It is proposed that hydrogen is incorporated in LiNbO3 single crystals through forming gas annealing at temperatures up to 900 1C and exists in LiNbO3 as a proton bound to an oxygen ion through O–H bond with its electron donated. & 2010 Elsevier B.V. All rights reserved.
Keywords: LiNbO3 Hydrogen Annealing
1. Introduction Ferroelectric lithium niobate (LiNbO3) single crystals possess relatively large electro-optic and nonlinear optical coefficients and are widely applied in many optical devices. In recent years, their applications have been extended to surface acoustic wave (SAW) devices and piezoelectric devices. In contrast to polycrystalline materials, whose properties can be changed considerably through grain size control and grain boundary engineering, LiNbO3 single crystals are mainly affected by the point defects in them. There have been extensive investigations on the defect subsystem in LiNbO3 single crystals and now their properties can be tailored differently for various applications [1,2]. Among the point defects in LiNbO3 single crystals, hydrogen has been recognized as one of the most common and most important. H2O vapor in atmospheres is a source of hydrogen in LiNbO3 at high temperatures and hydrogen is present in most as-grown LiNbO3 single crystals. More hydrogen can be introduced into LiNbO3 through lithium–proton exchange in hot acids (typically, benzoic acid) and this process is currently adopted for LiNbO3 single crystals applied as optical waveguides [3]. To date, however, it is still quite controversial whether hydrogen can be introduced into LiNbO3 single crystals through annealing in hydrogen-containing atmosphere. Some early studies indicated that oxygen vacancies are the dominant defects under reducing conditions at high temperatures [4,5], while some other researchers proposed that anti-site defects Nb and lithium Li vacancies VuLi are the dominant defects [6,7]. Recently, another in situ optical spectroscopy study suggested that when annealing in n Corresponding author at: School of Physics and Technology, Wuhan University, Wuhan 430072, China. Tel.: + 86 27 6875 2060; fax: + 86 27 6875 2569. E-mail address: [email protected] (W.P. Chen).
0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.11.085
95%Ar+5%H2 atmosphere, hydrogen is incorporated into LiNbO3 single crystals starting from 450 1C while oxygen is removed from the crystals above 600 1C [8]. Through crystal structure and X-ray photoelelectron spectroscopy (XPS) analyses, we will present in this paper some clear evidence that hydrogen can be incorporated in LiNbO3 single crystals through annealing in hydrogen-containing atmospheres at high temperatures up to 900 1C. Hydrogen incorporation during annealing in hydrogen-containing atmospheres may be another important source of hydrogen in LiNbO3 single crystals.
2. Experimental Z-cut polished congruent LiNbO3 single crystals of 10 5 0.5 mm3 were purchased from KMT Corporation (Hefei, China). Forming gas annealing was conducted in a tube furnace with flowing 95%N2 + 5%H2. An X-ray diffractometer (BRUKER axs D8 ADVANCE) with CuKa radiation was used for crystal structure analyses. Fourier-transform infrared (FTIR) absorption spectra were obtained using a Perkin-Elmer Spectrum 100 FTIR Spectrometer to measure transmittance. XPS analyses were performed at room temperature on a PHI Quantera SXMX with a spherical capacitance analyzer. AlKa X-ray was used as the excitation source operating at 26.5 W. Electron binding energies were calibrated against the reference peak of C 1s (285.0 eV). I–V characteristics of the samples were recorded through a Keithley 6517A electrometer/ high resistance meter.
3. Results and discussion The as-received single crystals are colorless and transparent. Fig. 1 shows the FTIR transmission spectrum obtained for an
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F. Chen et al. / Physica B 406 (2011) 683–686
as-received single crystal (Sample A). There is an absorption peak at 3490 cm 1, which is the characteristic absorption peak of the stretch mode of O–H bond. Hence hydrogen is present in the asreceived crystals and exists as a proton attached to an oxygen ion through an O–H bond, which can be expressed as ðOHÞO [9]. This is the standard configuration of hydrogen as a shallow donor in oxides [10]. An obvious colouration of crystals occurs after forming gas annealing at temperatures above 600 1C and the extent of colouration increases with increasing annealing temperature. For example, a crystal annealed in the forming gas at 600 1C for 2 h became gray but was still transparent (Sample B) while a crystal annealed at 900 1C for 1 h was black and opaque (Sample C). The FTIR transmission spectrum obtained for Sample B is also shown in Fig. 1. It is very similar to that of Sample A, except that the transmittance is smaller due to the colouration. Fig. 2 shows the X-ray diffraction patterns taken for Samples A–C. As a Z-cut hexagonal single crystal (JCPDS file 85-2456), a very strong peak, namely the (0 0 0 6) peak, is observed for Sample A. A very strong peak is observed for Sample B too, but its 2y is smaller than that of Sample A by 0.61. Sample B is still a single crystal but its lattice constant c is obviously increased so its (0 0 0 6) peak is
50
Transmittance (%)
40
30
20 Sample A Sample B
10
0 500
considerably shifted to the left. This lattice expansion is very unusual considering existing point defects models for annealing LiNbO3 single crystals in hydrogen-containing atmosphere. For example, Sugak et al. [8] proposed that forming gas annealing at temperatures above 600 1C leads to the removal of oxygen from LiNbO3 single crystals and hence some lattice shrinkage should be observed, which is contrary to our result. On the other hand, this obvious crystal change can be well explained in terms of hydrogen incorporation. When hydrogen is incorporated and exists as a proton forming an O–H bond with an oxygen ion, the formation of O–H bond will interfere with the original Opp–Nbdp interaction in NbO6 octahedra of LiNbO3 and relax the ferroelectric distortion of NbO6 octahedra, resulting considerable changes in crystal structure [11]. These hydrogen-induced crystal changes are well known for oxides with hydrogen insertion HxMOn [12]. It is quite surprising that the strong peak of Sample C is shifted more to the left. Its 2y is smaller than that of Sample A by 1.01. It appears that more hydrogen has been incorporated in Sample C. So our results indicate that hydrogen is incorporated into LiNbO3 single crystals through forming gas annealing up to 900 1C; the higher the annealing temperature, the more the hydrogen incorporated. It is through a diffusion process that hydrogen enters LiNbO3 single crystals. As Sample C had been annealed for a relatively short period of time (1 h), the diffusion depth of hydrogen should be limited; hence a relatively weak peak can be seen in its diffraction pattern, which is obviously the (0 0 0 6) peak of the original lattice beneath a surface layer with hydrogen incorporated. It seems that the concentration of hydrogen exhibits a very narrow distribution along the thickness direction in the surface layer and then sharply drops down to zero across the interface between the surface layer and the inner part of the crystal. In this way two separate peaks, rather than partial overlapping of the peaks, are observed. First-principles calculations have shown that interstitial hydrogen is a shallow donor in LiNbO3 [13]. The incorporation of hydrogen into LiNbO3 through forming gas annealing can thus be expressed as H2 þ 2O O -2ðOHÞO þ2eu
1000
1500
2000
2500
3000
3500
4000
Wavenumber (cm-1) Fig. 1. FTIR transmittance spectra of an as-received LiNbO3 single crystal, Sample A, and an LiNbO3 single crystal annealed in 95%N2 + 5%H2 at 600 1C for 2 h, Sample B.
Some studies suggested that the electron from Eq. (1) is trapped at a niobium site to form Nb4 + and leads to electronic conduction via hopping between Nb5 + and Nb4 + [6,7]. In–Ga electrodes, 2 mm in diameter, have been coated on the centers of the two major surfaces of Samples A–C and their I–V curves were measured between the electrodes, which are shown in Fig. 3. The leakage current of Sample A cannot be detected in this measurement, and
Sample A Sample B Sample C
10 -6 10 -7
Current (A)
Intensity (arb. unit)
ð1Þ
Sample A Sample B Sample C
10 -8 10 -9 10 -10 10 -11 10 -12
30
35
40 2 Theta (degree)
45
50
Fig. 2. X-ray diffraction patterns taken of the surface of Samples A–C. Sample C—LiNbO3 single crystal annealed in 95%N2 + 5%H2 at 900 1C for 1 h.
-50 -40 -30 -20 -10 0 10 Voltage (V)
20
30
40
50
Fig. 3. I–V curves measured between 2 mm diameter In–Ga electrodes on the centers of the two major surfaces of Samples A–C.
Intensity (arb. unit)
Sample A
Intensity (arb. unit)
Intensity (arb. unit)
Intensity (arb. unit)
F. Chen et al. / Physica B 406 (2011) 683–686
Sample C
214
212
210
208
206
204
685
Sample A
Sample C
538 536 534 532 530 528 526
Binding Energy (eV)
Binding Energy (eV)
Fig. 4. XPS spectra of Samples A and C: (a) Nb 3d spectra and (b) O 1s spectra.
the leakage current of Sample B is very small, while the leakage current of Sample C is 104 times larger than that of Sample B. The increase in leakage current of Samples B and C can be well explained by electron formation according to Eq. (1). It is worth noting, however, that the leakage current is still relatively low so the electron concentration should be relatively small, especially in Sample B. Fig. 4 presents some XPS spectra obtained with Samples A and C. For Sample A, the binding energy of O 1s is 530.0 eV, the binding energies of Nb3d5/2 and Nb3d3/2 are 206.5 and 209.2 eV, respectively, and the intensity ratio of Nb3d5/2 to Nb3d3/2 is about 3:2. All these values reveal that the valence states of O and Nb in Sample A are 2 and + 5, respectively [14]. It can be seen that the spectrum of O 1s is a little asymmetric, with a much sharper decrease at the lowbinding-energy side, which should be ascribed to the presence of some OH groups and is in agreement with the FTIR analyses [15]. For Sample C, the most remarkable change is that another peak appears at the high-binding-energy side of O 1s spectrum, which clearly shows that the number of OH groups on the surface of Sample C has been considerably increased. This result further proves that hydrogen is incorporated to LiNbO3 single crystals at temperatures up to 900 1C. The binding energies of O 1s and Nb 3d spectra are shifted a little to the high-energy side for Sample C. Such increases in binding energy of O 1s and Nb 3d were also observed in lithium-charged Nb2O5 and hydrogen-charged Nb2O5 [16,17]. Obviously for Nb2O5 and LiNbO3, the chemical environment of O2 and Nb5 + is similarly influenced when hydrogen or lithium is introduced. It should be pointed out that the shape of the Nb 3d spectrum remains unchanged, which is quite unusual as Nb5 + has been expected to be partially reduced to Nb4 + in Sample C. For example, due to the formation of some Nb4 + , the Nb 3d spectrum of Nb2O5 is markedly distorted in shape and is much broader on hydrogen incorporation [17]. As a matter of fact, Nb4 + EPR signal has also been found to be absent in reduced LiNbO3 and the complex defect structure associated with Nb4 + in LiNbO3 has been proposed responsible for the absence [18]. Here we would like to propose another tentative explanation. While hydrogen is confined to the surface layer through O–H bond, the electron from Eq. (1) is able to reach the inner part of LiNbO3 and its overall concentration
is relatively small as indicated by the leakage current. So the concentration of Nb4 + in the surface is small and cannot be detected in the XPS analyses.
4. Conclusions The lattice constant c of a surface layer and the content of OH groups on the surface are dramatically increased, and the leakage current can be increased by several orders of magnitude when Z-cut congruent LiNbO3 single crystals are annealed in 95%N2 +5%H2 at temperatures up to 900 1C. These evidences strongly suggest that hydrogen is incorporated in LiNbO3 single crystals through forming gas annealing at temperatures as high as 900 1C and exists in LiNbO3 as a proton bound to an oxygen ion to form an OH group with its electron donated. This study has revealed some clear evidence for hydrogen incorporation, which may be the key to studying the defects subsystem in LiNbO3 single crystals formed through annealing in hydrogencontaining atmosphere.
Acknowledgements This work has been supported by the Centre for Smart Materials of The Hong Kong Polytechnic University and the National Natural Science Foundation of China under Grant no. 50772077. References [1] V. Gopalan, T. Mitchell, Y. Furukawa, K. Kitamura, Appl. Phys. Lett. 72 (1998) 1981. [2] E.M. de Miguel-Sanz, M. Carrascosa, L. Arizmendi, Phys. Rev. B 65 (2002) 165101. [3] J.M. Cabrera, J. Olivares, M. Carrascosa, J. Rams, R. Muller, E. Dieguez, Adv. Phys. 45 (1996) 349. [4] G. Bergmann, Solid State Commun. 6 (1968) 77. [5] P.J. Jorgensen, R.W. Bartlett, J. Phys. Chem. Solids 30 (1969) 2639. [6] R. Courths, P. Steiner, H. Hochst, S. Hufner, J. Appl. Phys. 21 (1980) 345. [7] D.M. Smyth, Ferroelectrics 50 (1983) 93. [8] D. Sugak, Y. Zhydachevskii, Y. Sugak, O. Buryy, S. Ubizskii, I. Solskii, M. Schrader, K.D. Becker, J. Phys.: Condens. Matter 19 (2007) 086211.
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F. Chen et al. / Physica B 406 (2011) 683–686
[9] H. Iwahara, Proton Conductors, in: P. Colomban (Ed.), Cambridge University Press, New York, 1992, p. 133. [10] C. Kilic, A. Zunger, Appl. Phys. Lett. 81 (2002) 73. [11] P.G. Dickens, S. Crouch-Baker, M.T. Weller, Solid State Ion. 18-19 (1986) 89. [12] W.P. Chen, X.F. Zhu, J. Shen, J.Q. Sun, J. Shi, X.G. Qiu, Y. Wang, H.L.W. Chan, J. Mater. Sci. 42 (2007) 2524. [13] H.H. Nahm, C.H. Park, Appl. Phys. Lett. 78 (2001) 3812.
[14] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer, New York, 1992. [15] N. Kaufherr, D.J. Eichorst, D.A. Payne, J. Vac. Sci. Technol. A 14 (1996) 299. [16] N. Ozer, D.G. Chen, C.M. Lampert, Thin Solid Films 277 (1996) 162. [17] W.P. Chen, K.F. He, Y. Wang, Y.M. Hu, J.L. Cao, H.L.W. Chan, Jpn. J. Appl. Phys. 49 (2010) 051103. [18] K.L. Sweeney, L.E. Halliburton, Appl. Phys. Lett. 43 (1983) 336.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Effects of forming gas annealing on LiNbO3 single crystals Feng Chen a, Wan Ping Chen a,b,n, Yu Wang a, Yong Ming Hu a, Zhen Jiang Shen b, Helen Lai Wah Chan a a b
Department of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University, Hong Kong, China Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education and School of Physics and Technology, Wuhan University, Wuhan 430072, China
a r t i c l e in f o
abstract
Article history: Received 8 August 2010 Received in revised form 28 October 2010 Accepted 24 November 2010
Z-cut congruent LiNbO3 single crystals were annealed in 95%N2 +5%H2 at high temperatures. X-ray diffraction showed that 2y of (0 0 0 6) peak is obviously reduced by 0.61 and 1.01 for the samples annealed at 600 and 900 1C, respectively. A new peak appears at the high-energy side of O 1s spectrum in X-ray photoelelectron spectroscopy (XPS) analyses, and the leakage current is greatly increased. It is proposed that hydrogen is incorporated in LiNbO3 single crystals through forming gas annealing at temperatures up to 900 1C and exists in LiNbO3 as a proton bound to an oxygen ion through O–H bond with its electron donated. & 2010 Elsevier B.V. All rights reserved.
Keywords: LiNbO3 Hydrogen Annealing
1. Introduction Ferroelectric lithium niobate (LiNbO3) single crystals possess relatively large electro-optic and nonlinear optical coefficients and are widely applied in many optical devices. In recent years, their applications have been extended to surface acoustic wave (SAW) devices and piezoelectric devices. In contrast to polycrystalline materials, whose properties can be changed considerably through grain size control and grain boundary engineering, LiNbO3 single crystals are mainly affected by the point defects in them. There have been extensive investigations on the defect subsystem in LiNbO3 single crystals and now their properties can be tailored differently for various applications [1,2]. Among the point defects in LiNbO3 single crystals, hydrogen has been recognized as one of the most common and most important. H2O vapor in atmospheres is a source of hydrogen in LiNbO3 at high temperatures and hydrogen is present in most as-grown LiNbO3 single crystals. More hydrogen can be introduced into LiNbO3 through lithium–proton exchange in hot acids (typically, benzoic acid) and this process is currently adopted for LiNbO3 single crystals applied as optical waveguides [3]. To date, however, it is still quite controversial whether hydrogen can be introduced into LiNbO3 single crystals through annealing in hydrogen-containing atmosphere. Some early studies indicated that oxygen vacancies are the dominant defects under reducing conditions at high temperatures [4,5], while some other researchers proposed that anti-site defects Nb and lithium Li vacancies VuLi are the dominant defects [6,7]. Recently, another in situ optical spectroscopy study suggested that when annealing in n Corresponding author at: School of Physics and Technology, Wuhan University, Wuhan 430072, China. Tel.: + 86 27 6875 2060; fax: + 86 27 6875 2569. E-mail address: [email protected] (W.P. Chen).
0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.11.085
95%Ar+5%H2 atmosphere, hydrogen is incorporated into LiNbO3 single crystals starting from 450 1C while oxygen is removed from the crystals above 600 1C [8]. Through crystal structure and X-ray photoelelectron spectroscopy (XPS) analyses, we will present in this paper some clear evidence that hydrogen can be incorporated in LiNbO3 single crystals through annealing in hydrogen-containing atmospheres at high temperatures up to 900 1C. Hydrogen incorporation during annealing in hydrogen-containing atmospheres may be another important source of hydrogen in LiNbO3 single crystals.
2. Experimental Z-cut polished congruent LiNbO3 single crystals of 10 5 0.5 mm3 were purchased from KMT Corporation (Hefei, China). Forming gas annealing was conducted in a tube furnace with flowing 95%N2 + 5%H2. An X-ray diffractometer (BRUKER axs D8 ADVANCE) with CuKa radiation was used for crystal structure analyses. Fourier-transform infrared (FTIR) absorption spectra were obtained using a Perkin-Elmer Spectrum 100 FTIR Spectrometer to measure transmittance. XPS analyses were performed at room temperature on a PHI Quantera SXMX with a spherical capacitance analyzer. AlKa X-ray was used as the excitation source operating at 26.5 W. Electron binding energies were calibrated against the reference peak of C 1s (285.0 eV). I–V characteristics of the samples were recorded through a Keithley 6517A electrometer/ high resistance meter.
3. Results and discussion The as-received single crystals are colorless and transparent. Fig. 1 shows the FTIR transmission spectrum obtained for an
684
F. Chen et al. / Physica B 406 (2011) 683–686
as-received single crystal (Sample A). There is an absorption peak at 3490 cm 1, which is the characteristic absorption peak of the stretch mode of O–H bond. Hence hydrogen is present in the asreceived crystals and exists as a proton attached to an oxygen ion through an O–H bond, which can be expressed as ðOHÞO [9]. This is the standard configuration of hydrogen as a shallow donor in oxides [10]. An obvious colouration of crystals occurs after forming gas annealing at temperatures above 600 1C and the extent of colouration increases with increasing annealing temperature. For example, a crystal annealed in the forming gas at 600 1C for 2 h became gray but was still transparent (Sample B) while a crystal annealed at 900 1C for 1 h was black and opaque (Sample C). The FTIR transmission spectrum obtained for Sample B is also shown in Fig. 1. It is very similar to that of Sample A, except that the transmittance is smaller due to the colouration. Fig. 2 shows the X-ray diffraction patterns taken for Samples A–C. As a Z-cut hexagonal single crystal (JCPDS file 85-2456), a very strong peak, namely the (0 0 0 6) peak, is observed for Sample A. A very strong peak is observed for Sample B too, but its 2y is smaller than that of Sample A by 0.61. Sample B is still a single crystal but its lattice constant c is obviously increased so its (0 0 0 6) peak is
50
Transmittance (%)
40
30
20 Sample A Sample B
10
0 500
considerably shifted to the left. This lattice expansion is very unusual considering existing point defects models for annealing LiNbO3 single crystals in hydrogen-containing atmosphere. For example, Sugak et al. [8] proposed that forming gas annealing at temperatures above 600 1C leads to the removal of oxygen from LiNbO3 single crystals and hence some lattice shrinkage should be observed, which is contrary to our result. On the other hand, this obvious crystal change can be well explained in terms of hydrogen incorporation. When hydrogen is incorporated and exists as a proton forming an O–H bond with an oxygen ion, the formation of O–H bond will interfere with the original Opp–Nbdp interaction in NbO6 octahedra of LiNbO3 and relax the ferroelectric distortion of NbO6 octahedra, resulting considerable changes in crystal structure [11]. These hydrogen-induced crystal changes are well known for oxides with hydrogen insertion HxMOn [12]. It is quite surprising that the strong peak of Sample C is shifted more to the left. Its 2y is smaller than that of Sample A by 1.01. It appears that more hydrogen has been incorporated in Sample C. So our results indicate that hydrogen is incorporated into LiNbO3 single crystals through forming gas annealing up to 900 1C; the higher the annealing temperature, the more the hydrogen incorporated. It is through a diffusion process that hydrogen enters LiNbO3 single crystals. As Sample C had been annealed for a relatively short period of time (1 h), the diffusion depth of hydrogen should be limited; hence a relatively weak peak can be seen in its diffraction pattern, which is obviously the (0 0 0 6) peak of the original lattice beneath a surface layer with hydrogen incorporated. It seems that the concentration of hydrogen exhibits a very narrow distribution along the thickness direction in the surface layer and then sharply drops down to zero across the interface between the surface layer and the inner part of the crystal. In this way two separate peaks, rather than partial overlapping of the peaks, are observed. First-principles calculations have shown that interstitial hydrogen is a shallow donor in LiNbO3 [13]. The incorporation of hydrogen into LiNbO3 through forming gas annealing can thus be expressed as H2 þ 2O O -2ðOHÞO þ2eu
1000
1500
2000
2500
3000
3500
4000
Wavenumber (cm-1) Fig. 1. FTIR transmittance spectra of an as-received LiNbO3 single crystal, Sample A, and an LiNbO3 single crystal annealed in 95%N2 + 5%H2 at 600 1C for 2 h, Sample B.
Some studies suggested that the electron from Eq. (1) is trapped at a niobium site to form Nb4 + and leads to electronic conduction via hopping between Nb5 + and Nb4 + [6,7]. In–Ga electrodes, 2 mm in diameter, have been coated on the centers of the two major surfaces of Samples A–C and their I–V curves were measured between the electrodes, which are shown in Fig. 3. The leakage current of Sample A cannot be detected in this measurement, and
Sample A Sample B Sample C
10 -6 10 -7
Current (A)
Intensity (arb. unit)
ð1Þ
Sample A Sample B Sample C
10 -8 10 -9 10 -10 10 -11 10 -12
30
35
40 2 Theta (degree)
45
50
Fig. 2. X-ray diffraction patterns taken of the surface of Samples A–C. Sample C—LiNbO3 single crystal annealed in 95%N2 + 5%H2 at 900 1C for 1 h.
-50 -40 -30 -20 -10 0 10 Voltage (V)
20
30
40
50
Fig. 3. I–V curves measured between 2 mm diameter In–Ga electrodes on the centers of the two major surfaces of Samples A–C.
Intensity (arb. unit)
Sample A
Intensity (arb. unit)
Intensity (arb. unit)
Intensity (arb. unit)
F. Chen et al. / Physica B 406 (2011) 683–686
Sample C
214
212
210
208
206
204
685
Sample A
Sample C
538 536 534 532 530 528 526
Binding Energy (eV)
Binding Energy (eV)
Fig. 4. XPS spectra of Samples A and C: (a) Nb 3d spectra and (b) O 1s spectra.
the leakage current of Sample B is very small, while the leakage current of Sample C is 104 times larger than that of Sample B. The increase in leakage current of Samples B and C can be well explained by electron formation according to Eq. (1). It is worth noting, however, that the leakage current is still relatively low so the electron concentration should be relatively small, especially in Sample B. Fig. 4 presents some XPS spectra obtained with Samples A and C. For Sample A, the binding energy of O 1s is 530.0 eV, the binding energies of Nb3d5/2 and Nb3d3/2 are 206.5 and 209.2 eV, respectively, and the intensity ratio of Nb3d5/2 to Nb3d3/2 is about 3:2. All these values reveal that the valence states of O and Nb in Sample A are 2 and + 5, respectively [14]. It can be seen that the spectrum of O 1s is a little asymmetric, with a much sharper decrease at the lowbinding-energy side, which should be ascribed to the presence of some OH groups and is in agreement with the FTIR analyses [15]. For Sample C, the most remarkable change is that another peak appears at the high-binding-energy side of O 1s spectrum, which clearly shows that the number of OH groups on the surface of Sample C has been considerably increased. This result further proves that hydrogen is incorporated to LiNbO3 single crystals at temperatures up to 900 1C. The binding energies of O 1s and Nb 3d spectra are shifted a little to the high-energy side for Sample C. Such increases in binding energy of O 1s and Nb 3d were also observed in lithium-charged Nb2O5 and hydrogen-charged Nb2O5 [16,17]. Obviously for Nb2O5 and LiNbO3, the chemical environment of O2 and Nb5 + is similarly influenced when hydrogen or lithium is introduced. It should be pointed out that the shape of the Nb 3d spectrum remains unchanged, which is quite unusual as Nb5 + has been expected to be partially reduced to Nb4 + in Sample C. For example, due to the formation of some Nb4 + , the Nb 3d spectrum of Nb2O5 is markedly distorted in shape and is much broader on hydrogen incorporation [17]. As a matter of fact, Nb4 + EPR signal has also been found to be absent in reduced LiNbO3 and the complex defect structure associated with Nb4 + in LiNbO3 has been proposed responsible for the absence [18]. Here we would like to propose another tentative explanation. While hydrogen is confined to the surface layer through O–H bond, the electron from Eq. (1) is able to reach the inner part of LiNbO3 and its overall concentration
is relatively small as indicated by the leakage current. So the concentration of Nb4 + in the surface is small and cannot be detected in the XPS analyses.
4. Conclusions The lattice constant c of a surface layer and the content of OH groups on the surface are dramatically increased, and the leakage current can be increased by several orders of magnitude when Z-cut congruent LiNbO3 single crystals are annealed in 95%N2 +5%H2 at temperatures up to 900 1C. These evidences strongly suggest that hydrogen is incorporated in LiNbO3 single crystals through forming gas annealing at temperatures as high as 900 1C and exists in LiNbO3 as a proton bound to an oxygen ion to form an OH group with its electron donated. This study has revealed some clear evidence for hydrogen incorporation, which may be the key to studying the defects subsystem in LiNbO3 single crystals formed through annealing in hydrogencontaining atmosphere.
Acknowledgements This work has been supported by the Centre for Smart Materials of The Hong Kong Polytechnic University and the National Natural Science Foundation of China under Grant no. 50772077. References [1] V. Gopalan, T. Mitchell, Y. Furukawa, K. Kitamura, Appl. Phys. Lett. 72 (1998) 1981. [2] E.M. de Miguel-Sanz, M. Carrascosa, L. Arizmendi, Phys. Rev. B 65 (2002) 165101. [3] J.M. Cabrera, J. Olivares, M. Carrascosa, J. Rams, R. Muller, E. Dieguez, Adv. Phys. 45 (1996) 349. [4] G. Bergmann, Solid State Commun. 6 (1968) 77. [5] P.J. Jorgensen, R.W. Bartlett, J. Phys. Chem. Solids 30 (1969) 2639. [6] R. Courths, P. Steiner, H. Hochst, S. Hufner, J. Appl. Phys. 21 (1980) 345. [7] D.M. Smyth, Ferroelectrics 50 (1983) 93. [8] D. Sugak, Y. Zhydachevskii, Y. Sugak, O. Buryy, S. Ubizskii, I. Solskii, M. Schrader, K.D. Becker, J. Phys.: Condens. Matter 19 (2007) 086211.
686
F. Chen et al. / Physica B 406 (2011) 683–686
[9] H. Iwahara, Proton Conductors, in: P. Colomban (Ed.), Cambridge University Press, New York, 1992, p. 133. [10] C. Kilic, A. Zunger, Appl. Phys. Lett. 81 (2002) 73. [11] P.G. Dickens, S. Crouch-Baker, M.T. Weller, Solid State Ion. 18-19 (1986) 89. [12] W.P. Chen, X.F. Zhu, J. Shen, J.Q. Sun, J. Shi, X.G. Qiu, Y. Wang, H.L.W. Chan, J. Mater. Sci. 42 (2007) 2524. [13] H.H. Nahm, C.H. Park, Appl. Phys. Lett. 78 (2001) 3812.
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