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Electro-optic effect in ZnO:Mn thin films
Journal of Alloys and Compounds 371 (2004) 157–159
Electro-optic effect in ZnO:Mn thin films T. Nagata, A. Ashida∗ , N. Fujimura, T. Ito Department of Applied Materials Science, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan Received 13 October 2002; received in revised form 26 March 2003; accepted 15 June 2003
Abstract The electro-optic effect of ZnO epitaxial films was studied. Since the electro-optic effect in ZnO is inferior to that of ferroelectric materials, an increase in the electro-optic effect was expected in ZnO having ferroelectricity, due to the increase in the dielectric constant. Although Li and Mg doped ZnO film exhibited electro-optic response, the mobile Li ions influenced the polarization behaviour. To eliminate the electro-optic effect induced by other space charge effects, the birefringence shift measurement was performed. The measurement with ac electric field at various frequencies could abstract the electro-optic effect of Mn-doped ZnO thin films induced only by dipolar polarizability. © 2003 Elsevier B.V. All rights reserved. PACS: 78.20.JQ Keywords: Ferroelectrics; Semiconductors; Laser processing; Nonlinear optics
1. Introduction We have proposed the application of ZnO:X (X = Li, Mg, Mn, Ni, Al etc.) films for monolithic optical integrated circuits (OICs) [1]. Since ZnO films can be grown as the insulating or metallic film and possesses an electro-optic or magneto-optic property, monolithic integration without hetero-interfaces will be possible. Although it was reported that ZnO bulk crystal shows an electro-optic effect [2], the effect is inferior to those of ferroelectric materials such as Pb(Zr, Ti)O3 (rc = 30–50 pm/V) [3,4] Recently, it was reported that ZnO:Li bulk crystals exhibited a ferroelectric property [5]. The use of ZnO having ferroelectricity is believed to be one of the solutions for enhancing the electro-optic effect. We demonstrated that the ZnO films doped with Li and Mg exhibited electro-optic behaviour under a dc electric field [6]. However, the existence of the mobile Li ions influenced the polarization behaviors. To distinguish the effect of such space charges from the polarization behaviours, we have attempted to measure the electro-optic effect with an ac bias voltage.
∗ Corresponding author. Tel.: +81-722-54-9332; fax: +81-722-54-9327. E-mail address: [email protected] (A. Ashida).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.06.016
The paper reports the birefringence shift measurement performed for ZnO and ZnO:Mn films as the first step for evaluating the electro-optic effect of ZnO:Li film. Detailed studies on the dielectric property and frequency dependence of the electro-optic effect reveal the effect of space charges on the electro-optic response of ZnO films.
2. Experimental Thick ZnO films of 1.0 m were deposited by a pulsed laser deposition system on a 0.2 m thick Pt bottom electrode layer. The Pt layer for the bottom electrode was prepared by RF sputtering on (0 0 0 1) sapphire. A 0.02 m thick Au film with the optical transmission factor of about 50% was used for the top electrode. The deposition conditions of the ZnO films were as follows: the energy density of the laser 1.5 J/cm2 , the wavelength 248 nm (KrF), the substrate temperature 600 ◦ C, and oxygen partial pressure during deposition of 1.3 × 10−2 Pa. The dielectric permittivity and the dissipation factor as a function of frequency (C–f) were measured using an LCR meter (HP4284A). Leakage current properties were measured using a picoampere meter with a voltage source (HP4140B). The electro-optic properties of the films were evaluated by measuring the birefringence induced by the electric
158
T. Nagata et al. / Journal of Alloys and Compounds 371 (2004) 157–159
Fig. 2. n of 5 at.% Mn-doped ZnO film as a function of electric field at (a) 100 Hz, (b) 1 kHz and (c) 10 kHz. Fig. 1. A schema of the electro-optic measurement system.
field [6]. The schematic view of the measurement system is shown in Fig. 1. The beam of the He–Ne laser (λ = 632.8 nm) was linearly polarized and focused on the sample. The reflected light passed through a quarter-wave plate. A polarizing beam splitter oriented at 45◦ to the incident light polarization was used to split the beam into two orthogonally polarized beams for differential detection. The small polarization shift δ of reflected light was then determined from the difference between the signals generated in the detectors. The sample was electrically biased with ac square-wave voltages from 10 Hz to 100 kHz using a programmable pulse generator. A lock-in amplifier was used to detect the difference in the voltages. The ordinary refractive index n0 (=1.95) was also measured using a spectroscopic ellipsometer(Photal FTM-7700W). The birefringence shift n was obtained using the equation [7], n =
λδ , 2πn0 d
(1)
where is the wavelength of the laser, ␦ is the measured polarization shift and d is the total optical path length.
3. Results and discussion All the ZnO films fabricated in this work were characterized as (0 0 0 1) oriented epitaxial layers without 30◦ rotated domains by reflection of high-energy electron diffraction and X-ray diffraction. Since 0 0 0 1 direction of ZnO is the polarization axis, the bias voltage was applied perpendicular to the film surface. Although the birefringence shift measurement was performed for non-doped ZnO film, no significant electro-optic response was observed regardless of the frequency of applied voltage. To understand the poor electro-optic response of the film, the I–V property was measured. The leakage current density at 10 kV/cm was 3.27 A/cm2 . Since the leakage current density is very high, it is considered that the applied electric field did not work for polarization. To improve the leakage property, Mn doping was performed. The leakage current density at 10 kV/cm of 5 at.% Mn-doped ZnO significantly decreased to 5.21 × 10−7 A/cm2 , which is an eighth order of
magnitude smaller than that of non-doped ZnO. Using this resistive film, the birefringence shift was measured. Fig. 2 shows the change in the refractive index of Mn-doped ZnO film as a function of frequency of applied electric field of 100 Hz (a), 1 kHz (b) and 10 kHz (c). With increases in the applied electric field, n linearly increases. Although linear electro-optic responses were observed, there is a strong frequency dependence. This result suggests that the electro-optic response includes the effect of slow polarization such as rearrangement of space charge. Therefore, the frequency dependence of n was measured more carefully. The solid circle in Fig. 3 shows the change in n against the frequency of applied ac bias voltage at 2 V. n increases with increasing frequency until about 500 Hz, and then decreases, which reveals some frequency dispersions were included in this measurement. Therefore, the dielectric properties were measured. The frequency dependence of the capacitance and the dissipation factor at the amplitude voltage of 10 mV did not show any dielectric dispersion. Although the ac signal amplitude used in the normal C–f measurement is less than 10 mV, 2 V of the ac electric field is applied for the electro-optic effect measurement. Hence, ac signal amplitude dependence of the C–f was measured at the applied electric fields of 2 V. The open circle in Fig. 3 shows dielectric loss at the amplitude voltage of 2 V. The dielectric dispersion is clearly observed below 10 kHz. This dielectric dispersion observed under the high applied voltage is considered to originate from the space charge effect. Therefore, the rearrangement of the space charge effect,
Fig. 3. Dependence of n (䊉) and tan␦ (䊊) of 5 at.% Mn-doped ZnO film on the frequency of applied electric field.
T. Nagata et al. / Journal of Alloys and Compounds 371 (2004) 157–159
such as oxygen deficienies, interfacial polarization and the fluctuation valency of Mn ion also seems to be responsible for the frequency dependence of the electro-optic effect. The electro-optic response at a measurement frequency above 10 kHz is considered to be originated from only dipolar polarizability. The linear electro-optic coefficient of ZnO:Mn film was calculated to be 0.5 pm/V. Although the value is smaller than that of the single-crystal ZnO (2.6 pm/V [2]), we have succeeded in measuring the electro-optic coefficient of ZnO thin film for the first time. Since Li doped ZnO films still have a leakage problem, we have not observed an improvement of ZnO electro-optic effect by Li doping.
4. Conclusion To investigate the electro-optic effect in ZnO films, birefringence shift measurements were performed. Regarding unndoped ZnO, the large leakage current should be responsible for low electro-optic response. Mn doping to ZnO significantly improved the leakage current problem and the sample exhibited a linear electro-optic response. By investigating frequency dependence of the dielectric properties and the electro-optic effect, it was revealed that the rearrangement of the space charge affects the electro-optic response
159
at the frequency below 10 kHz. The linear electro-optic coefficient of ZnO:Mn film which originated in only dipolar polarizability was confirmed to be 0.5 pm/V.
Acknowledgements This work was funded by the Sasakawa Scientific Research Grant from The Japan Science Society.
References [1] N. Fujimura, T. Shimura, T. Wakano, A. Ashida, T. Ito, Mater. Res. Soc. Symp. Proc. 574 (1999) 317. [2] I.P. Kaminow, An Introduction to Electrooptic Devices, Academic Press, New York, 1974. [3] J.R. Busch, S.D. Ramamurthi, S.L. Swartz, V.L. Wood, Electron Lett. 28 (1992) 1591. [4] K. Nashimoto, S. Nakamura, H. Moriyama, Jpn. J. Appl. Phys. 34 (1995) 5091. [5] A. Onodera, N. Tamaki, Y. Kawamura, T. Sawada, H. Tamashita, Jpn. J. Appl. Phys. 35 (1996) 5160. [6] T. Nagata, Y. Nakano, A. Ashida, N. Fujimura, T. Ito, Jpn. J. Appl. Phys. 40 (2001) 5615. [7] D.H. Reitze, E. Haton, R. Ramesh, S. Etemad, D.E. Leaird, T. Sands, Z. Karim, A.R. Tanguay Jr., Appl. Phys. Lett. 63 (1993) 596.
Electro-optic effect in ZnO:Mn thin films T. Nagata, A. Ashida∗ , N. Fujimura, T. Ito Department of Applied Materials Science, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan Received 13 October 2002; received in revised form 26 March 2003; accepted 15 June 2003
Abstract The electro-optic effect of ZnO epitaxial films was studied. Since the electro-optic effect in ZnO is inferior to that of ferroelectric materials, an increase in the electro-optic effect was expected in ZnO having ferroelectricity, due to the increase in the dielectric constant. Although Li and Mg doped ZnO film exhibited electro-optic response, the mobile Li ions influenced the polarization behaviour. To eliminate the electro-optic effect induced by other space charge effects, the birefringence shift measurement was performed. The measurement with ac electric field at various frequencies could abstract the electro-optic effect of Mn-doped ZnO thin films induced only by dipolar polarizability. © 2003 Elsevier B.V. All rights reserved. PACS: 78.20.JQ Keywords: Ferroelectrics; Semiconductors; Laser processing; Nonlinear optics
1. Introduction We have proposed the application of ZnO:X (X = Li, Mg, Mn, Ni, Al etc.) films for monolithic optical integrated circuits (OICs) [1]. Since ZnO films can be grown as the insulating or metallic film and possesses an electro-optic or magneto-optic property, monolithic integration without hetero-interfaces will be possible. Although it was reported that ZnO bulk crystal shows an electro-optic effect [2], the effect is inferior to those of ferroelectric materials such as Pb(Zr, Ti)O3 (rc = 30–50 pm/V) [3,4] Recently, it was reported that ZnO:Li bulk crystals exhibited a ferroelectric property [5]. The use of ZnO having ferroelectricity is believed to be one of the solutions for enhancing the electro-optic effect. We demonstrated that the ZnO films doped with Li and Mg exhibited electro-optic behaviour under a dc electric field [6]. However, the existence of the mobile Li ions influenced the polarization behaviors. To distinguish the effect of such space charges from the polarization behaviours, we have attempted to measure the electro-optic effect with an ac bias voltage.
∗ Corresponding author. Tel.: +81-722-54-9332; fax: +81-722-54-9327. E-mail address: [email protected] (A. Ashida).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.06.016
The paper reports the birefringence shift measurement performed for ZnO and ZnO:Mn films as the first step for evaluating the electro-optic effect of ZnO:Li film. Detailed studies on the dielectric property and frequency dependence of the electro-optic effect reveal the effect of space charges on the electro-optic response of ZnO films.
2. Experimental Thick ZnO films of 1.0 m were deposited by a pulsed laser deposition system on a 0.2 m thick Pt bottom electrode layer. The Pt layer for the bottom electrode was prepared by RF sputtering on (0 0 0 1) sapphire. A 0.02 m thick Au film with the optical transmission factor of about 50% was used for the top electrode. The deposition conditions of the ZnO films were as follows: the energy density of the laser 1.5 J/cm2 , the wavelength 248 nm (KrF), the substrate temperature 600 ◦ C, and oxygen partial pressure during deposition of 1.3 × 10−2 Pa. The dielectric permittivity and the dissipation factor as a function of frequency (C–f) were measured using an LCR meter (HP4284A). Leakage current properties were measured using a picoampere meter with a voltage source (HP4140B). The electro-optic properties of the films were evaluated by measuring the birefringence induced by the electric
158
T. Nagata et al. / Journal of Alloys and Compounds 371 (2004) 157–159
Fig. 2. n of 5 at.% Mn-doped ZnO film as a function of electric field at (a) 100 Hz, (b) 1 kHz and (c) 10 kHz. Fig. 1. A schema of the electro-optic measurement system.
field [6]. The schematic view of the measurement system is shown in Fig. 1. The beam of the He–Ne laser (λ = 632.8 nm) was linearly polarized and focused on the sample. The reflected light passed through a quarter-wave plate. A polarizing beam splitter oriented at 45◦ to the incident light polarization was used to split the beam into two orthogonally polarized beams for differential detection. The small polarization shift δ of reflected light was then determined from the difference between the signals generated in the detectors. The sample was electrically biased with ac square-wave voltages from 10 Hz to 100 kHz using a programmable pulse generator. A lock-in amplifier was used to detect the difference in the voltages. The ordinary refractive index n0 (=1.95) was also measured using a spectroscopic ellipsometer(Photal FTM-7700W). The birefringence shift n was obtained using the equation [7], n =
λδ , 2πn0 d
(1)
where is the wavelength of the laser, ␦ is the measured polarization shift and d is the total optical path length.
3. Results and discussion All the ZnO films fabricated in this work were characterized as (0 0 0 1) oriented epitaxial layers without 30◦ rotated domains by reflection of high-energy electron diffraction and X-ray diffraction. Since 0 0 0 1 direction of ZnO is the polarization axis, the bias voltage was applied perpendicular to the film surface. Although the birefringence shift measurement was performed for non-doped ZnO film, no significant electro-optic response was observed regardless of the frequency of applied voltage. To understand the poor electro-optic response of the film, the I–V property was measured. The leakage current density at 10 kV/cm was 3.27 A/cm2 . Since the leakage current density is very high, it is considered that the applied electric field did not work for polarization. To improve the leakage property, Mn doping was performed. The leakage current density at 10 kV/cm of 5 at.% Mn-doped ZnO significantly decreased to 5.21 × 10−7 A/cm2 , which is an eighth order of
magnitude smaller than that of non-doped ZnO. Using this resistive film, the birefringence shift was measured. Fig. 2 shows the change in the refractive index of Mn-doped ZnO film as a function of frequency of applied electric field of 100 Hz (a), 1 kHz (b) and 10 kHz (c). With increases in the applied electric field, n linearly increases. Although linear electro-optic responses were observed, there is a strong frequency dependence. This result suggests that the electro-optic response includes the effect of slow polarization such as rearrangement of space charge. Therefore, the frequency dependence of n was measured more carefully. The solid circle in Fig. 3 shows the change in n against the frequency of applied ac bias voltage at 2 V. n increases with increasing frequency until about 500 Hz, and then decreases, which reveals some frequency dispersions were included in this measurement. Therefore, the dielectric properties were measured. The frequency dependence of the capacitance and the dissipation factor at the amplitude voltage of 10 mV did not show any dielectric dispersion. Although the ac signal amplitude used in the normal C–f measurement is less than 10 mV, 2 V of the ac electric field is applied for the electro-optic effect measurement. Hence, ac signal amplitude dependence of the C–f was measured at the applied electric fields of 2 V. The open circle in Fig. 3 shows dielectric loss at the amplitude voltage of 2 V. The dielectric dispersion is clearly observed below 10 kHz. This dielectric dispersion observed under the high applied voltage is considered to originate from the space charge effect. Therefore, the rearrangement of the space charge effect,
Fig. 3. Dependence of n (䊉) and tan␦ (䊊) of 5 at.% Mn-doped ZnO film on the frequency of applied electric field.
T. Nagata et al. / Journal of Alloys and Compounds 371 (2004) 157–159
such as oxygen deficienies, interfacial polarization and the fluctuation valency of Mn ion also seems to be responsible for the frequency dependence of the electro-optic effect. The electro-optic response at a measurement frequency above 10 kHz is considered to be originated from only dipolar polarizability. The linear electro-optic coefficient of ZnO:Mn film was calculated to be 0.5 pm/V. Although the value is smaller than that of the single-crystal ZnO (2.6 pm/V [2]), we have succeeded in measuring the electro-optic coefficient of ZnO thin film for the first time. Since Li doped ZnO films still have a leakage problem, we have not observed an improvement of ZnO electro-optic effect by Li doping.
4. Conclusion To investigate the electro-optic effect in ZnO films, birefringence shift measurements were performed. Regarding unndoped ZnO, the large leakage current should be responsible for low electro-optic response. Mn doping to ZnO significantly improved the leakage current problem and the sample exhibited a linear electro-optic response. By investigating frequency dependence of the dielectric properties and the electro-optic effect, it was revealed that the rearrangement of the space charge affects the electro-optic response
159
at the frequency below 10 kHz. The linear electro-optic coefficient of ZnO:Mn film which originated in only dipolar polarizability was confirmed to be 0.5 pm/V.
Acknowledgements This work was funded by the Sasakawa Scientific Research Grant from The Japan Science Society.
References [1] N. Fujimura, T. Shimura, T. Wakano, A. Ashida, T. Ito, Mater. Res. Soc. Symp. Proc. 574 (1999) 317. [2] I.P. Kaminow, An Introduction to Electrooptic Devices, Academic Press, New York, 1974. [3] J.R. Busch, S.D. Ramamurthi, S.L. Swartz, V.L. Wood, Electron Lett. 28 (1992) 1591. [4] K. Nashimoto, S. Nakamura, H. Moriyama, Jpn. J. Appl. Phys. 34 (1995) 5091. [5] A. Onodera, N. Tamaki, Y. Kawamura, T. Sawada, H. Tamashita, Jpn. J. Appl. Phys. 35 (1996) 5160. [6] T. Nagata, Y. Nakano, A. Ashida, N. Fujimura, T. Ito, Jpn. J. Appl. Phys. 40 (2001) 5615. [7] D.H. Reitze, E. Haton, R. Ramesh, S. Etemad, D.E. Leaird, T. Sands, Z. Karim, A.R. Tanguay Jr., Appl. Phys. Lett. 63 (1993) 596.