CC BY
9Scientific Notes of Taurida National V.I. Vemadsky University
Series : Physics and Mathematics Sciences. Volume 27 (66). 2014. No. 2. P. 79-85
UDK 537.226
MODIFICATION OF THE ELECTRICAL PROPERTIES OF LiNbO3 SINGLE CRYSTALS BY ANNEALING IN SATURATED WATER VAPOR Pritulenko A. S.1, Yatsenko A. V.1, Sugak D. Yu.23, Solskii I. M.3
1Taurida National V. I. Vernadsky University, 4 Vernadsky Ave., Simferopol 295007, Crimea, Russia
2Lviv Polytechnic National University, Lviv, Ukraine
3Scientific Research Company "Carat", Lviv, Ukraine
E-mail: I.ab2@ crimea. edu
Electrical and optical properties of the LiNbO3 single crystals which were annealed in saturated H2O and D2O vapor were investigated. It is found that the activation energy of the electrical conductivity for these samples is close this value of LN samples, reduced in hydrogen atmosphere. It is shown too, that the annealing of LiNbO3 in saturated H2O vapor also leads to a strong increasing of the optical absorption of the samples in visible area. The nature of this phenomenon is discussed. Keywords: LiNbO3, annealing, electrical conductivity.
PACS: 78.20. ± e, 72.80. ± r
INTRODUCTION
Ferroelectric lithium niobate (LN) LiNbO3 is technologically important material with wide applications in nonlinear optics and electro-acoustic devices. It is well-known that the high temperature annealing in reducing environments containing hydrogen strongly influenced the electrical and optical properties of LN crystals [1-6]. It is established, that heating LN wafers in benzoic acid vapor causes formation of protons enriched surface layers with few ^m thickness in them [4]. At the same time the investigation of interrelated changes of the electrical properties and IR optical spectra for LN treated in H2O vapor or H2, do not provide definitive answers on the entry of H+ into LN structure. For example in [2, 3] was not observed any changes of electrical and optical properties of LN annealed in H2O vapor flow.
On the other hand, changes of electrical properties and increasing of absorption in OH- and OD- bands after annealing in ampoules with H2O vapor at P = 10.. .30 bar were observed in [2, 3] earlier. But comparison of the changes of LN electrical properties their optical properties in visible and IR range after annealing in H2O, D2O vapor and H2 atmosphere is not performed in a single study. The aim of the present paper is to obtain the additional information through the investigation of electrical and optical properties of LN crystals, annealed in H2, H2O and D2O saturated vapor.
1. EXPERIMENTAL DETAILS
For our experiments we use the samples which were cut from the same boules of congruent LN grown at SRC "Carat" [7]. Samples №1 and №2 were annealed at 600 °C for 1 h in the H2 atmosphere in separate ampoules. Sample №3 was annealed at 500°C for
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5 h in saturated H2O vapor and samples №4-5 were annealed at 500°C for 5 h in saturated D2O vapor. All samples were carefully polished to obtain a good optical quality. Annealing was held in ampoules at a pressure P « 1 bar.
The measuring of the optical absorption coefficient a in a visible part of spectra were realized at room temperature for three fixed wavelength (A=625, 525 and 465 nm) when the wave vector k is coincide to the polar axis z of a crystal by using a simple home-made device. Optical absorption coefficient were calculate according to Beer-Lambert law:
a = d 1 • ln(ljl), where Io and I are the intensity of incident light and transmitted light and d is a sample thickness.
Electrical properties of the samples were studied with a specially designed device, which can realize the measurements of electric impedance (frequency range 10-3...105 Hz) and dc conductivity of the crystals [8].
2. RESULTS AND DISCUSSION
First of all the measurements of optical absorption of all annealed samples were realized and these data are presented in the Table. Optical measurements revealed no significant differences in the values of the absorption coefficients of the crystals which were annealed in saturated H2O and D2O vapor. Moreover the obtained values of optical absorption for the crystals, treated in pure hydrogen and in H2O (or D2O) vapor are perfectly comparable - taking into account the difference of treatment temperature.
Table 1
Optical absorption coefficients and activation energy of electric conductivity for all
annealed samples
sample «625 HM, cm-1 «525 HM, Cm-1 a465 HM, cm-1 Ea, eV
№1 5.6 ± 0.1 5.8 ± 0.1 5.8 ± 0.1 0.67 ± 0.01
№2 5.9 ± 0.1 6.3 ± 0.1 6.5 ± 0.1 0.68 ± 0.01
№3 1.55 ± 0.05 1.82 ± 0.05 1.95 ± 0.05 0.70 ± 0.01
№4 1.91 ± 0.05 2.30 ± 0.05 2.42 ± 0.05 0.70 ± 0.01
№5 2,26± 0.05 2,68± 0.05 2,68± 0.05 0.70 ± 0.01
Temperature dependences of the specific electrical conductivity (a) of three investigated samples along z axis in a temperature range 294.370 K obtained by two-terminal method and usual dc technique are presented in Fig. 1. They are fully described by simple Arrhenius law with the similar activation energy Ea (indicates in a Table 1). Additional experiments were preformed using three-terminal method for excluding the surface conductivity and these results are in good agreement with those, shown in Fig. 1.
These results are unexpected, because predicted increasing of OH- or OD- groups volume concentration in the annealed samples must lead to the increasing of the ionic contribution to electrical conductivity of the samples and the respective activation energy must be equal to 1,00.1.12 eV. [1, 6]. Increasing of OH- volume concentration in the crystal can not strongly affect its electrical conductivity at room temperature. But specific
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electrical conductivity of non-annealed LN crystals at room temperature is equal to (10-15...10-16) (Q-cm)"1 and this value is by some orders smaller that for reduced LN crystals. Moreover, in accordance with the data obtained earlier [9] conductivity of LN annealed in H2O vapor flow is close to the value for non-annealed sample.
2,8 3,0 3,2 3,4
1000/7; K-1
Fig. 1. Temperature dependence of the specific electric conductivity of annealed LN samples: (1) - sample №2; (2) - sample №3; (3) - sample №4.
Measuring the electrical conductivity of the sample №4 showed the presence of small quantities of conductivity anisotropy along the polar and non-polar axis (as in the case of the sample annealed in a hydrogen atmosphere). Since all investigated samples, annealed in ampoules are characterized by the similar value of activation energy, one can conclude that the nature of the main contribution to their electrical conductivity is identical.
It was demonstrates earlier [10] that non-controlled heating of LN samples, which were early reduced in hydrogen, up to 430...450 K leads to significant decreasing of sample electric conductivity along polar axis (more then twice at T = 300 K). This effect was explained as a result of the possible diffusion of atmospheric oxygen into a near-surface layers of reduced sample. It may lead to the destruction of bipolarons in these layers and a sharp increasing of these layers resistance.
For the testing of this assumption we investigate the temperature dependence of electrical conductivity of the sample №4 by impedance spectroscopy - before and after additional heat treatment in dry air at 573 K during 3 h. It is estimated that Nyquist diagrams of the sample №4 before additional heat treatment can be described by practically ideal semicircle in a temperature range (300...390) K. It is a serious argument to believe the uniformity of electric properties of the sample.
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In accordance to our prediction, the additional heat treatment in dry air at 573 K strongly influenced the electrical properties of LN sample №4. Typical Nyquist diagram obtained in this series of experiments is shown in Fig. 2. Nevertheless, the optical absorption coefficient after additional heating was not change in the limits of experimental errors.
Re(Z)
Fig. 2. Nyquist diagram of the sample №4 after additional heating in dry air, obtained at T = 356 K. Equivalent electrical circuit of the sample is shown in inset.
The data which are illustrate by Fig. 2 demonstrates that equivalent electrical circuit of the crystal can be presented as a consecutive connection of two parallel RC chains, one of which simulate the crystal's internal volume and described by capacity Cb and resistance Rb and the second one simulate the capacity Cs and resistance Rs of near-surface layers. Respective circuit is shown in the inset of Fig. 2.
Due to analysis of impedance spectra according to this simple model, we determined the temperature dependences of electric conductivities Sb and Ss in the temperature range (300...450) K. These data illustrates by Fig. 3. It is concluded too, that the ratio of capacities Cs/Cb is temperature independent in a limits of experimental errors and is equal to (50 ± 7).
In a process of investigation of LN crystals reduced in hydrogen it was estimate, that this annealing do not affects the components of the tensor of dielectric permittivity en and 833 in analyzed here temperature range [10]. So we can try to evaluate the depth of the near-surface layers in which diffusion of atmospheric oxygen results in a destroying of the majority of bipolarons, which are responsible for the electric conductivity of "black" LN crystals.
According to the equivalent circuit of the crystal, which is illustrates by Fig. 2, the total depth of electrically modified near-surface layers (di) approximately will be equal to
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C
d1 =--—dc
1 C- + Cb c
where dc is a thickness of the crystal. In a result we obtain that di = (69 ± 7) ^m. It is clear that the varying of additional heat treatment conditions gives a possibility to investigate the dynamics of the oxygen diffusion into a "black" LN crystal.
1000/T7, K-1
Fig. 3. Temperature dependences of Sb (open circles) and Ss (dash circles) for the sample №4 after additional heating in dry air. Mean values of activation energy are pointed too.
CONCLUSIONS
So the annealing of LN crystals in saturated H2O or D2O vapor (at negligible oxygen partial pressure) and a pure H2 probably affects the electrical properties by the same way. The annealing in these media leads to the oxygen loss in the crystal and forming of oxygen vacancies and bipolarons.
Taking into account the data obtained by us, we can conclude that the annealing of congruently grown LN crystals in saturated pure H2O or D2O vapor in ampoules at 500 °C and P «1 bar leads to changes of LN optical properties in visible range as well as electrophysical properties similar as those that occur after reducing treatment in H2 atmosphere. Thus it can be assumed that the main reason for the increasing of the LN conductivity after annealing in ampoules with H2O or D2O is formed bipolarons.
It has been established, that the additional heat treatment in dry air of the "black" LN samples leads to the electric modification of its near-surface layers. Investigations of the
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oxygen diffusion, which is responsible to this phenomenon, will be the goal of the next experiments.
References
1. W. Bollman, H.-J. Stöhr, Phys. Stat. Sol. (a) 39, 477 (1977).
2. R. Gonzalez, Y. Chen, K. L. Tsang, G. P. Summers, Appl. Phys. Let. 41, 739 (1982).
3. R. Pareja, R. González, M. A. Pedrosa, Phys. Stat. Sol. (a) 82, 179 (1984).
4. J. M. Cabrera, J. Olivares, M. Carrascosa, J. Rams, R. Müller, E. Diéguez, Advances in Physics 45, 349 (1996).
5. P. Bordui, D. Jundt, E. Standifer, R. Norwood, R. Sawin, J. Galipeau, J. Appl. Phys. 85, 3766 (1999).
6. T. Volk, M. Wöhlecke, Lithium Niobate. Defects, photorefraction and ferroelectric switching (SpringerVerlag, Berlin, 2008) 284 p.
7. I. M. Solskii, D. Yu. Sugak, M. M. Vakiv, Acta Physica Polonica A 124, 314 (2013).
8. S. V. Yevdokimov, A. S. Pritulenko, A. A. Sapiga, A. V. Yatsenko, Scientific Notes of Taurida National V. I. Vernadsky University. Ser. Physics and Mathematics Sciences 24(63), No. 2, 187 (2011).
9. D. Yu. Sugak, I. M. Solskii, I. I. Syvorotka, M. M. Vakiv, New Technologies 35, 19 (2012).
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Притуленко А. C. Модифжащя електричних властивостей монокристалiв LiNbO3 ввдпалюванням у насиченш воднш napi / А. С. Притуленко, О. В. Яценко, Д. Ю. Сугак, В. М. Сольский // Вчеш записки Тавршського нацюнального ушверситету ÍMern В. I. Вернадського. Серш : Фiзико-математичнi науки. - 2014. - Т. 27 (66), № 2. - С. 79-85.
Дослщжено електричш та оптичш властивосп монокристалiв LiNbO3, що пройшли вдаалювання у насиченш rnpi H2O та D2O. Встановлено, що енергш активацп електрично! провщносп в таких зразках е близькою до значення, що спостершаетъся для зразкв НЛ, що пройшли вщпалювання у водт. Також встановлено, що вдаалювання у насиченш водянш парi призводить до сильного збiльшення оптично! густини зразка у видимш област! Обговорюеться природа цих ефектiв. KnwHoei слова: LiNbO3, ,вщпалювання,, електричнi властивосп.
Притуленко А. C. Модификация электрических свойств монокристаллов LiNbO3 отжигом в насыщенных парах воды / А. С. Притуленко, А. В. Яценко, Д. Ю. Сугак, И. М. Сольский //
Ученые записки Таврического национального университета имени В. И. Вернадского. Серия : Физико-математические науки. - 2014. - Т. 27 (66), № 2. - С. 79-85.
Исследованы электрические и оптические свойства монокристаллов LiNbO3, прошедших отжиг в насыщенных парах H2O и D2O. Обнаружено, что энергия активации электрической проводимости в таких образцах близка к значению, наблюдаемому для образцов НЛ, восстановленных в водороде. Также обнаружено, что отжиг в насыщенных парах воды приводит к сильному увеличению оптической плотности образца в видимой области. Обсуждается природа этих эффектов. Ключевые слова: LiNbO3, отжиг, электрические свойства.
Список литературы
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3. Pareja R. Study of thermochemically reduced and electron-irradiated LiNb03 single crystals by positron annihilation and optical absorption measurements / R. Pareja, R. González, M. A. Pedrosa / Phys. Stat. Sol. (a). - 1984. - Vol. 82. - P. 179-183.
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4. Hydrogen in lithium niobate / J. M. Cabrera, J. Olivares, M. Carrascosa, et al. / Advances in Physics. -1996. - Vol. 45. - P. 349-392.
5. Chemically reduced lithium niobate single crystals: processing, properties and improved surface acoustic wave device fabrication and performance / P. Bordui, D. Jundt, E. Standifer, et al. / J. Appl. Phys. - 1999. - Vol. 85 - P. 3766-3769.
6. Volk T. Lithium Niobate. Defects, photorefraction and ferroelectric switching / T. Volk, M. Wohlecke / Springer-Verlag, Berlin, 2008. - 284 p.
7. Solskii I. M. Growing large size complex oxide single crystals by Czochralski technique for electronic devices / I. M. Solskii, D. Yu. Sugak, M. M. Vakiv / Acta Physica Polonica A. - 2013. - Vol. 124. -P. 314-320.
8. Device for low and ultra low frequency impedance investigation in dielectric materials / S. V. Yevdokimov, A. S. Pritulenko, A. A. Sapiga, A. V. Yatsenko // Scientific Notes of Taurida National V. I. Vernadsky University. Series: Physics and Mathematics Sciences. - 2011. - Vol. 24(63), No 2. - P. 187.
9. Influence of thermo-chemical treatment on the lithium niobate single crystal optical properties / D. Yu. Sugak, I. M. Solskii, I. I. Syvorotka, M. M. Vakiv / New Technologies. - 2012. - Vol. 35. -P. 19-26.
10. The peculiarities of the electrical conductivity of LiNbO3 crystals, reduced in hydrogen / A. V. Yatsenko, A. S. Pritulenko, S. V. Yevdokimov, et al. / Solid State Phenomena. - 2013. -Vol. 200. - P. 193-198.
Received 07 September 2014.
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