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Nb : SrTiO3 heterojunction
Solid State Communications 151 (2011) 465–469
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Studies on the resistance switching properties of the La0.5 Ca0.5 MnO3 /Nb:SrTiO3 heterojunction Wang Jian a , Yu Qingxuan a,b,∗ , Yao Yiping a , Zhang Xintao a , Yuan Kai a , Li Xiaoguang a a
Hefei National Laboratory for Physical Sciences at Microscale, Department of Physics, University of Science and Technology of China, Hefei 230026, PR China
b
State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, PR China
article
info
Article history: Received 10 October 2010 Received in revised form 2 December 2010 Accepted 29 December 2010 by P. Chaddah Available online 8 January 2011 Keywords: A. Manganite B. Magnetron sputtering C. Heterojunctions D. Resistance switching
abstract In this study, we have investigated the resistance switching behavior of the La0.5 Ca0.5 MnO3 /Nb:SrTiO3 heterojunction. The junction shows a negative resistance switching ratio (ER) below 140 K. When 220 K > T ≥ 140 K, the ER goes from negative to positive with increasing bias voltage. When T > 220 K, the junction shows a positive ER. This variation from a negative to a positive value indicates that the ER is determined primarily by two phenomena: (a) the negative ER value can be attributed to a disruption of the charge-ordered insulating domains in La0.5 Ca0.5 MnO3 under large electric fields, and (b) the positive ER value at high temperatures is due to the modulation of the interface barrier width driven by the electrochemical migration of oxygen vacancies. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction The perovskite manganite-based heterojunctions have attracted significant scientific and technological attention due to their rich variety of physical phenomena and widespread applications in spintronics [1–8]. These heterojunctions exhibit magnetically tunable transport properties, rectifying characteristics, photovoltaic features, and other interesting behavior. In the last few years, a new physical phenomenon known as the resistance switching effect, which is induced by an electric current or electric field, has become an interesting topic of study for next-generation non-volatile memory [9–16]. This effect is observed in various transition metal oxides including perovskite manganites such as Pr1−x Cax MnO3 [9,10] and La1−x Cax MnO3 [11,12], niobium-doped strontium titanates [13,14] (Nb:STO) and binary metal oxides such as NiO [15], and TiO2 [16]. Most of the research groups investigating the resistance switching effect have focused on Schottky heterostructures composed of a metal and transition metal oxide. However, a few recent reports have shown that heterostructures that are composed of two oxides can also exhibit good switching behavior [17]. Most oxides exhibit various physical properties. Therefore, a heterojunction composed of two oxide materials may
∗ Corresponding author at: State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, PR China. Tel.: +86 551 3603943; fax: +86 551 3603943. E-mail address: [email protected] (Q. Yu). 0038-1098/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2010.12.035
exhibit many interesting phenomena. In addition, as oxygen vacancies can exist on both sides of the heterostructure, the switching effect could be controlled through the tuning of the oxygen content of the two sides. In this regard, it is interesting to study the switching behavior of the La1−x Cax MnO3 /Nb:SrTiO3 (LCMO/SNTO) heterojunction. Furthermore, researchers have also reported an electric currentdependent resistance switching phenomenon in thin films of manganite [18,19], which is due to the high current density inducing a transition from the charge ordered insulating (COI) phase to the ferromagnetic (FM) phase. The coexistence of the FM and charge order anti-ferromagnetic (COAFM) phases has been observed over a wide temperature range in the prototypical phase-separated manganite La0.5 Ca0.5 MnO3 [20,21], and the current–voltage (I–V ) and capacitance–voltage (C –V ) characteristics of the La0.5 Ca0.5 MnO3 /Nb:SrTiO3 heterojunctions have been studied extensively [22]. Although some interesting characteristics related to phase separation have been observed, the relationship between the charge order and the resistance switching behavior of the La0.5 Ca0.5 MnO3 /Nb:SrTiO3 heterojunction is not completely clear. Therefore, studies on the current-correlated characteristics of phase-coexistent manganite-based junctions are required. In this work, the hysteretic I–V characteristics of the heterojunction composed of La0.5 Ca0.5 MnO3 (LCMO) and 0.5 wt% Nb:SrTiO3 (SNTO) were observed. The junction showed a positive resistance switching ratio (ER) at temperatures above 220 K and a negative ER at temperatures below 220 K. The ER values increased rapidly with increasing temperature and exhibited a strong dependence
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the film was approximately 150 nm. X-ray diffraction analysis revealed good epitaxy of our films. The I–V characteristics of the heterojunction were measured using an Agilent E5270 I–V parametric measurement system through the two-probe method to avoid the current distribution effect in the junction. The C –V data were collected using an Agilent 4294A precision impedance analyzer at 1 kHz. The electrical property of LCMO/ STO film was measured using a superconducting quantum interference device magnetometer (SQUID), and the resistance–temperature (R–T ) curve of LCMO/STO film was also measured using the four- probe method. 3. Results and discussion Fig. 1. (Color version available online) The I–V curves of the LCMO/SNTO heterojunction at temperatures of 100, 140, 180, 220, 260, and 300 K. The inset shows a schematic illustration of the electrode setting.
on the bias voltage. The crossover of ER from the negative to the positive range in the La0.5 Ca0.5 MnO3 /Nb:SrTiO3 heterojunction indicates the presence of the LCMO tunable features therein and the strong effect of manganite electron correlations on the interface properties of the junction. To obtain more information about the interface properties of the junction, we also investigated the C –V characteristics of the heterojunction. So as to describe the physical characteristic of LCMO film, we collected the data of resistivity and magnetization of LCMO/STO film. The role of the interface in the ER phenomenon and the origin of the different polarities of the ER have been discussed. 2. Experiment A stoichiometric mixture of La2 O3 , CaCO3 and MnO2 was used for the target, and a polycrystalline-target of La0.5 Ca0.5 MnO3 (LCMO) was synthesized by the conventional solid-state reaction method. The La0.5 Ca0.5 MnO3 films were respectively deposited on a (001) 0.5 wt% Nb:SrTiO3 substrate, and on a (001) SrTiO3 (STO) substrate using the magnetron sputtering technique. The substrate temperature was kept at 750 °C. The Ar/O2 mixture gas with a ratio of 1:1 was introduced into the chamber with the sputtering pressure maintained at 4 Pa. The thickness of
Fig. 1 shows the I–V characteristics of the heterojunction from 100 to 300 K, which exhibits good rectifying properties. The positive bias (i.e., forward bias) is defined as the voltage at which the current flows from the LCMO film to the SNTO substrate. We investigated the electric field-induced resistance switching behavior in the LCMO/SNTO heterojunction, and the I–V sweeps of the samples at different temperature ranges are displayed in Fig. 2. The voltage was scanned as follows: 0 → Vmax → 0 → Vmax → 0 in steps of 0.02 V. The I–V curves showed obvious hysteresis patterns throughout the temperature range studied, and the switching direction showed strong temperature dependence. To describe the switching effect, we defined ER as follows: ER = (Rdown –Rup )/Rup . At room temperature, the current was larger during the up-scan process, i.e., going from 0 V to +Vmax , corresponding to the low-resistance state and smaller during the down-scan process, i.e., going from +Vmax to 0 V, corresponding to the highresistance state. This behavior indicates that the junction resistance increases when the bias voltage is applied and the value of ER is positive. At temperatures between 140 and 220 K, the ER is clearly negative at a low bias voltage, but crosses over to the positive range at a high bias voltage. At temperatures below 140 K, the ER is negative throughout the region of bias voltage applied. The temperature dependence of the ER values at various bias voltages is shown in Fig. 3(a). The figure clearly shows that the ER increases rapidly with temperature at both values of the bias voltage. The ER-T curves exhibit a critical temperature at which the ER crosses over to the positive range. The critical temperature is 140 K for a
Fig. 2. (Color version available online) The I–V sweep for the junction with the positive bias voltage measured at temperatures of 100, 140, 220, and 300 K. The insets show an enlargement of the I–V sweep at the low bias voltage value.
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Fig. 3. (Color version available online) (a) The temperature dependence of Rjun at various positive and negative voltages; (b) the temperature dependence of ER calculated at 0.9 V and 2.0 V, respectively.
Fig. 4. (Color version available online) The C –V characteristics of the heterojunction in the 0 → Vmax up-scanning process and the Vmax → 0 down-scanning process, respectively, at different temperatures. The C –V curves were measured at a frequency of 1 kHz.
bias voltage of 2.0 V and 240 K for a bias voltage of 0.9 V respectively. The different polarities and the crossing over of the ER imply that the complex physical features of LCMO played an important role in the resistance switching phenomenon. Calculated from the I–V data, the junction resistance (defined as Rjun = V /I) under different bias voltages is shown in Fig. 3(b). The magnitude of Rjun is greater than 104 , which indicates an insulating behavior. Due to the low resistance of LCMO film and SNTO substrate, the resistance Rjun mainly originates from the interface layer. Therefore, the property of the depletion layer controls the transport process, and the change in the depletion layer induces the changes in the junction resistance. Rjun is observed to decrease with increasing temperature between 100 and 300 K, which can be attributed to the thinning of the depletion layer at high temperatures through thermionic emission of carriers, leading to a low junction resistance. The capacitance characteristics of the LCMO/SNTO heterojunction were also measured. As shown in Fig. 4, the junction also exhibited hysteresis in the C –V characteristics, which implies that the voltage variation altered the junction capacitance by influencing the status of the depletion. The C –V properties measured at high bias voltages are unreliable due to the large associated tunnel current; hence, we present the data only in the low bias regions. The nearly linear relationship between the capacitance and the voltage can be observed throughout the temperature range studied. The capacitance of the junction can be expressed as C = ε0 εr S /d, where ε0 is the dielectric constant
of vacuum, εr is the dielectric constant of the junction, S is the effective junction area, and d is the width of the dielectric layer between the two materials. Above 220 K, the capacitance after up-scanning of the voltage is greater than the capacitance following the down-scanning of the voltage. According to the relationship C = ε0 εr S /d, the thickness of the depletion layer under an electric field must be greater than its initial thickness, which corresponds to the positive ER value observed at high temperatures in the I–V sweeps. The opposite case is observed at temperatures below 220 K as expected; the capacitance is smaller after the down-scanning process, which corresponds to the negative ER seen in the I–V sweep. The relationship between the temperature and the zero-bias junction capacitance, which is calculated from the C –V curves at the zero-bias voltage (C-T) as shown in Fig. 5, shows that the direct proportionality of the capacitance with temperature corresponds to the inverse proportionality of Rjun with temperature, and the width of the depletion layer decreases with temperature. This result clearly reveals that the depletion layer is sensitive to the external electric voltage, and the switching phenomenon is strongly dependent on the modulation of the depletion layer. As we know, in the n-type semiconductor SNTO, oxygen vacancies can be considered as effective donors; therefore, a decrease in the number of vacancies may widen the depletion layer and increase the junction resistance [23]. On the other hand, oxygen vacancies act as acceptor scavengers in the p-type semiconductor, LCMO. Therefore, a reduction in the number of oxygen vacancies may narrow the depletion
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Fig. 5. (Color version available online) The zero-bias capacitance as a function of temperature.
Fig. 6. (Color version available online) (a) The temperature dependence of the resistance of LCMO/STO, and d ln ρ/dT −1 vs temperature curve. (b) The temperature dependence of the magnetization of LCMO/STO film.
layer and consequently decrease the junction resistance [24]. Due to the high growth temperature, a large number of oxygen vacancies exist in the two oxide materials which composed the heterojunction, which will migrate along the direction of positive current flow driven by the applied electric voltage. When the positive bias reaches a value high enough, the vacancies in the LCMO film are driven to the interface and accumulate in the LCMO part near the interface, thereby widening the depletion layer. Meanwhile, the vacancies that initially accumulated in the SNTO part near the interface are forced away from the interface layer and extracted into the inside of the SNTO, which further widens the depletion layer. The electron tunneling process becomes more difficult due to the widened depletion layer, which consequently prevents the charge tunnel. Therefore, the junction resistance becomes larger after positive bias scanning at high temperatures and the ER becomes positive. The discussion above revealed that the physical properties of LCMO played important role in the heterojunction, thus we measured the electrical and magnetic properties of LCMO/STO. As shown in Fig. 6(a) the resistance increasing with temperature decreasing, exhibits insulating behavior, which is consistent with a previous report [25]. At low temperature, the resistance increasing rapidly implies the localization of conduction electrons, indicating that the transition of charge order emerges. According
to Ref. [26], the CO transition temperature TCO can be calculated from d ln ρ/dT −1 ; as seen in Fig. 6(a) this temperature of our sample is about 250 K, which was reported previously as the CO temperature TCO of La0.5 Ca0.5 MnO3 film [27]. Thus we considered that the grown film is actually La0.5 Ca0.5 MnO3 . Fig. 6(b) shows temperature dependence of magnetization, it can be seen that the LCMO film is ferromagnetic at low temperature. Tc is about 230 K which is consistent with bulk half-doped LCMO. So we can conclude that the FM region and CO region coexist in the LCMO film. At low temperatures, we must account for the influence of the migration ability of oxygen vacancies on the switching behavior and the phase-separated characteristics of the LCMO part. Perovskite manganite shows widely existing phase-separated characteristics that have been observed through various techniques. Manganite is a well known half-metal that exhibits the following three properties: (a) the electrons in the FM phase are fully spinpolarized, (b) the spins of all conducting electrons are parallel to the localized spin of the region (according to the Hund’s rule coupling), and (c) the CO regions are anti-ferromagnetic with insulating characteristics. Electrons at a low current density can only flow through the FM regions. As the current density increases with the applied voltage and reaches a critical value, the electrons are injected into the COI regions. Parts of the COI regions are then modified into FM regions because the higher density current from the spin-polarized electrons can force the anti-ferromagnetically ordered localized electron spins in the COI to align parallel to the conducting electrons. Therefore, the resistance decreases as the FM regions enlarge. The junction is expected to be inhomogeneous due to the coexistence of FM and CO in the interface, and the effective junction area is determined by the heterostructure between the FM regions and the SNTO. The effective junction area becomes larger under high applied voltage, leading to a decline in the junction resistance. According to the relation C = ε0 εr S /d, the capacitance becomes larger; this can explain the negative value of ER at 100 K well. From the oxygen diffusion constant expressed by D = D0 exp(−E /kT ) [24], the migration ability of oxygen at low temperature is nearly negligible. Based on semiconductor theory, the external electric field is mostly applied to the depletion layer, and a wider depletion layer at low temperature corresponds to a lower electric field. Thus, the width of the interface barrier is not significantly affected by the external bias voltage and the junction only exhibits a negative ER value below 140 K. At temperatures between 140 and 220 K, the migration ability of the oxygen vacancies shows an exponential increase with increasing temperature according to D = D0 exp(−E /kT ), leading to the emergence of the positive ER value. However, the effect of the negative ER value also exists simultaneously, and the two competing actions lead to the ER crossover effect. The transport at small bias voltages reflects the situation of the interface layer, i.e., the negative ER correspond to an increase in the effective junction area and the capacitance increases after voltage is applied, as seen in the C –V curves. Above 220 K, the phase separation disappears [28] along with the corresponding negative ER, and only a positive ER value can be seen. The widening of the depletion layer under an electric voltage leads to a decrease in the capacitance. The width of the depletion layer decreases with increasing temperature, and the electric field applied on the depletion layer increases. Consequently, the migration ability of oxygen increases with increasing temperature, and the positive ER increases rapidly with increasing temperature. 4. Conclusion The external applied electric field induced I–V and C –V characteristics of the La0.5 Ca0.5 MnO3 /Nb:SrTiO3 heterojunction have been studied in this work. The ER effect was observed in
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the heterojunction, and the value and polarity of the ER showed a strong dependence on the temperature and bias voltage. The C –V measurements demonstrated that the modulation of the depletion width played a key role in the ER behavior. The negative ER at low temperatures was attributed to the melting of the COI phase, and the positive ER at high temperatures could be attributed to the electrochemical migration of the oxygen vacancies. The ER crossover effect appeared as a result of the competition between the two types of electro-resistance. Acknowledgements The work is supported by NSFC (No. 50972139) and National Basic Research Program of China (Grant No. 2009CB929502) and CAS. References [1] A. Sawa, A. Yamamoto, H. Yamada, T. Fujii, M. Kawasaki, J. Matsuno, Y. Tokura, Appl. Phys. Lett. 90 (2007) 252102. [2] J.R. Sun, S.Y. Zhang, B.G. Shen, H.K. Wong, Appl. Phys. Lett. 86 (2005) 053503. [3] Y.W. Xie, J.R. Sun, D.J. Wang, S. Liang, W.M. Lü, B.G. Shen, Appl. Phys. Lett. 90 (2007) 192903. [4] T.F. Zhou, G. Li, N.Y. Wang, B.M. Wang, X.G. Li, Appl. Phys. Lett. 88 (2006) 232508. [5] J. Qiu, H.B. Lu, K.J. Jin, M. He, J. Xing, Physica B 400 (2007) 66. [6] K. Zhao, K.J. Jin, H.B. Lu, Y.H. Huang, Q.L. Zhou, M. He, Z.H. Chen, Y.L. Zhou, G.Z. Yang, Appl. Phys. Lett. 88 (2006) 141914. [7] T. Susaki, N. Nakagawa, H.Y. Hwang, Phys. Rev. B 75 (2007) 104409.
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Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Studies on the resistance switching properties of the La0.5 Ca0.5 MnO3 /Nb:SrTiO3 heterojunction Wang Jian a , Yu Qingxuan a,b,∗ , Yao Yiping a , Zhang Xintao a , Yuan Kai a , Li Xiaoguang a a
Hefei National Laboratory for Physical Sciences at Microscale, Department of Physics, University of Science and Technology of China, Hefei 230026, PR China
b
State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, PR China
article
info
Article history: Received 10 October 2010 Received in revised form 2 December 2010 Accepted 29 December 2010 by P. Chaddah Available online 8 January 2011 Keywords: A. Manganite B. Magnetron sputtering C. Heterojunctions D. Resistance switching
abstract In this study, we have investigated the resistance switching behavior of the La0.5 Ca0.5 MnO3 /Nb:SrTiO3 heterojunction. The junction shows a negative resistance switching ratio (ER) below 140 K. When 220 K > T ≥ 140 K, the ER goes from negative to positive with increasing bias voltage. When T > 220 K, the junction shows a positive ER. This variation from a negative to a positive value indicates that the ER is determined primarily by two phenomena: (a) the negative ER value can be attributed to a disruption of the charge-ordered insulating domains in La0.5 Ca0.5 MnO3 under large electric fields, and (b) the positive ER value at high temperatures is due to the modulation of the interface barrier width driven by the electrochemical migration of oxygen vacancies. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction The perovskite manganite-based heterojunctions have attracted significant scientific and technological attention due to their rich variety of physical phenomena and widespread applications in spintronics [1–8]. These heterojunctions exhibit magnetically tunable transport properties, rectifying characteristics, photovoltaic features, and other interesting behavior. In the last few years, a new physical phenomenon known as the resistance switching effect, which is induced by an electric current or electric field, has become an interesting topic of study for next-generation non-volatile memory [9–16]. This effect is observed in various transition metal oxides including perovskite manganites such as Pr1−x Cax MnO3 [9,10] and La1−x Cax MnO3 [11,12], niobium-doped strontium titanates [13,14] (Nb:STO) and binary metal oxides such as NiO [15], and TiO2 [16]. Most of the research groups investigating the resistance switching effect have focused on Schottky heterostructures composed of a metal and transition metal oxide. However, a few recent reports have shown that heterostructures that are composed of two oxides can also exhibit good switching behavior [17]. Most oxides exhibit various physical properties. Therefore, a heterojunction composed of two oxide materials may
∗ Corresponding author at: State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, PR China. Tel.: +86 551 3603943; fax: +86 551 3603943. E-mail address: [email protected] (Q. Yu). 0038-1098/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2010.12.035
exhibit many interesting phenomena. In addition, as oxygen vacancies can exist on both sides of the heterostructure, the switching effect could be controlled through the tuning of the oxygen content of the two sides. In this regard, it is interesting to study the switching behavior of the La1−x Cax MnO3 /Nb:SrTiO3 (LCMO/SNTO) heterojunction. Furthermore, researchers have also reported an electric currentdependent resistance switching phenomenon in thin films of manganite [18,19], which is due to the high current density inducing a transition from the charge ordered insulating (COI) phase to the ferromagnetic (FM) phase. The coexistence of the FM and charge order anti-ferromagnetic (COAFM) phases has been observed over a wide temperature range in the prototypical phase-separated manganite La0.5 Ca0.5 MnO3 [20,21], and the current–voltage (I–V ) and capacitance–voltage (C –V ) characteristics of the La0.5 Ca0.5 MnO3 /Nb:SrTiO3 heterojunctions have been studied extensively [22]. Although some interesting characteristics related to phase separation have been observed, the relationship between the charge order and the resistance switching behavior of the La0.5 Ca0.5 MnO3 /Nb:SrTiO3 heterojunction is not completely clear. Therefore, studies on the current-correlated characteristics of phase-coexistent manganite-based junctions are required. In this work, the hysteretic I–V characteristics of the heterojunction composed of La0.5 Ca0.5 MnO3 (LCMO) and 0.5 wt% Nb:SrTiO3 (SNTO) were observed. The junction showed a positive resistance switching ratio (ER) at temperatures above 220 K and a negative ER at temperatures below 220 K. The ER values increased rapidly with increasing temperature and exhibited a strong dependence
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the film was approximately 150 nm. X-ray diffraction analysis revealed good epitaxy of our films. The I–V characteristics of the heterojunction were measured using an Agilent E5270 I–V parametric measurement system through the two-probe method to avoid the current distribution effect in the junction. The C –V data were collected using an Agilent 4294A precision impedance analyzer at 1 kHz. The electrical property of LCMO/ STO film was measured using a superconducting quantum interference device magnetometer (SQUID), and the resistance–temperature (R–T ) curve of LCMO/STO film was also measured using the four- probe method. 3. Results and discussion Fig. 1. (Color version available online) The I–V curves of the LCMO/SNTO heterojunction at temperatures of 100, 140, 180, 220, 260, and 300 K. The inset shows a schematic illustration of the electrode setting.
on the bias voltage. The crossover of ER from the negative to the positive range in the La0.5 Ca0.5 MnO3 /Nb:SrTiO3 heterojunction indicates the presence of the LCMO tunable features therein and the strong effect of manganite electron correlations on the interface properties of the junction. To obtain more information about the interface properties of the junction, we also investigated the C –V characteristics of the heterojunction. So as to describe the physical characteristic of LCMO film, we collected the data of resistivity and magnetization of LCMO/STO film. The role of the interface in the ER phenomenon and the origin of the different polarities of the ER have been discussed. 2. Experiment A stoichiometric mixture of La2 O3 , CaCO3 and MnO2 was used for the target, and a polycrystalline-target of La0.5 Ca0.5 MnO3 (LCMO) was synthesized by the conventional solid-state reaction method. The La0.5 Ca0.5 MnO3 films were respectively deposited on a (001) 0.5 wt% Nb:SrTiO3 substrate, and on a (001) SrTiO3 (STO) substrate using the magnetron sputtering technique. The substrate temperature was kept at 750 °C. The Ar/O2 mixture gas with a ratio of 1:1 was introduced into the chamber with the sputtering pressure maintained at 4 Pa. The thickness of
Fig. 1 shows the I–V characteristics of the heterojunction from 100 to 300 K, which exhibits good rectifying properties. The positive bias (i.e., forward bias) is defined as the voltage at which the current flows from the LCMO film to the SNTO substrate. We investigated the electric field-induced resistance switching behavior in the LCMO/SNTO heterojunction, and the I–V sweeps of the samples at different temperature ranges are displayed in Fig. 2. The voltage was scanned as follows: 0 → Vmax → 0 → Vmax → 0 in steps of 0.02 V. The I–V curves showed obvious hysteresis patterns throughout the temperature range studied, and the switching direction showed strong temperature dependence. To describe the switching effect, we defined ER as follows: ER = (Rdown –Rup )/Rup . At room temperature, the current was larger during the up-scan process, i.e., going from 0 V to +Vmax , corresponding to the low-resistance state and smaller during the down-scan process, i.e., going from +Vmax to 0 V, corresponding to the highresistance state. This behavior indicates that the junction resistance increases when the bias voltage is applied and the value of ER is positive. At temperatures between 140 and 220 K, the ER is clearly negative at a low bias voltage, but crosses over to the positive range at a high bias voltage. At temperatures below 140 K, the ER is negative throughout the region of bias voltage applied. The temperature dependence of the ER values at various bias voltages is shown in Fig. 3(a). The figure clearly shows that the ER increases rapidly with temperature at both values of the bias voltage. The ER-T curves exhibit a critical temperature at which the ER crosses over to the positive range. The critical temperature is 140 K for a
Fig. 2. (Color version available online) The I–V sweep for the junction with the positive bias voltage measured at temperatures of 100, 140, 220, and 300 K. The insets show an enlargement of the I–V sweep at the low bias voltage value.
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Fig. 3. (Color version available online) (a) The temperature dependence of Rjun at various positive and negative voltages; (b) the temperature dependence of ER calculated at 0.9 V and 2.0 V, respectively.
Fig. 4. (Color version available online) The C –V characteristics of the heterojunction in the 0 → Vmax up-scanning process and the Vmax → 0 down-scanning process, respectively, at different temperatures. The C –V curves were measured at a frequency of 1 kHz.
bias voltage of 2.0 V and 240 K for a bias voltage of 0.9 V respectively. The different polarities and the crossing over of the ER imply that the complex physical features of LCMO played an important role in the resistance switching phenomenon. Calculated from the I–V data, the junction resistance (defined as Rjun = V /I) under different bias voltages is shown in Fig. 3(b). The magnitude of Rjun is greater than 104 , which indicates an insulating behavior. Due to the low resistance of LCMO film and SNTO substrate, the resistance Rjun mainly originates from the interface layer. Therefore, the property of the depletion layer controls the transport process, and the change in the depletion layer induces the changes in the junction resistance. Rjun is observed to decrease with increasing temperature between 100 and 300 K, which can be attributed to the thinning of the depletion layer at high temperatures through thermionic emission of carriers, leading to a low junction resistance. The capacitance characteristics of the LCMO/SNTO heterojunction were also measured. As shown in Fig. 4, the junction also exhibited hysteresis in the C –V characteristics, which implies that the voltage variation altered the junction capacitance by influencing the status of the depletion. The C –V properties measured at high bias voltages are unreliable due to the large associated tunnel current; hence, we present the data only in the low bias regions. The nearly linear relationship between the capacitance and the voltage can be observed throughout the temperature range studied. The capacitance of the junction can be expressed as C = ε0 εr S /d, where ε0 is the dielectric constant
of vacuum, εr is the dielectric constant of the junction, S is the effective junction area, and d is the width of the dielectric layer between the two materials. Above 220 K, the capacitance after up-scanning of the voltage is greater than the capacitance following the down-scanning of the voltage. According to the relationship C = ε0 εr S /d, the thickness of the depletion layer under an electric field must be greater than its initial thickness, which corresponds to the positive ER value observed at high temperatures in the I–V sweeps. The opposite case is observed at temperatures below 220 K as expected; the capacitance is smaller after the down-scanning process, which corresponds to the negative ER seen in the I–V sweep. The relationship between the temperature and the zero-bias junction capacitance, which is calculated from the C –V curves at the zero-bias voltage (C-T) as shown in Fig. 5, shows that the direct proportionality of the capacitance with temperature corresponds to the inverse proportionality of Rjun with temperature, and the width of the depletion layer decreases with temperature. This result clearly reveals that the depletion layer is sensitive to the external electric voltage, and the switching phenomenon is strongly dependent on the modulation of the depletion layer. As we know, in the n-type semiconductor SNTO, oxygen vacancies can be considered as effective donors; therefore, a decrease in the number of vacancies may widen the depletion layer and increase the junction resistance [23]. On the other hand, oxygen vacancies act as acceptor scavengers in the p-type semiconductor, LCMO. Therefore, a reduction in the number of oxygen vacancies may narrow the depletion
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Fig. 5. (Color version available online) The zero-bias capacitance as a function of temperature.
Fig. 6. (Color version available online) (a) The temperature dependence of the resistance of LCMO/STO, and d ln ρ/dT −1 vs temperature curve. (b) The temperature dependence of the magnetization of LCMO/STO film.
layer and consequently decrease the junction resistance [24]. Due to the high growth temperature, a large number of oxygen vacancies exist in the two oxide materials which composed the heterojunction, which will migrate along the direction of positive current flow driven by the applied electric voltage. When the positive bias reaches a value high enough, the vacancies in the LCMO film are driven to the interface and accumulate in the LCMO part near the interface, thereby widening the depletion layer. Meanwhile, the vacancies that initially accumulated in the SNTO part near the interface are forced away from the interface layer and extracted into the inside of the SNTO, which further widens the depletion layer. The electron tunneling process becomes more difficult due to the widened depletion layer, which consequently prevents the charge tunnel. Therefore, the junction resistance becomes larger after positive bias scanning at high temperatures and the ER becomes positive. The discussion above revealed that the physical properties of LCMO played important role in the heterojunction, thus we measured the electrical and magnetic properties of LCMO/STO. As shown in Fig. 6(a) the resistance increasing with temperature decreasing, exhibits insulating behavior, which is consistent with a previous report [25]. At low temperature, the resistance increasing rapidly implies the localization of conduction electrons, indicating that the transition of charge order emerges. According
to Ref. [26], the CO transition temperature TCO can be calculated from d ln ρ/dT −1 ; as seen in Fig. 6(a) this temperature of our sample is about 250 K, which was reported previously as the CO temperature TCO of La0.5 Ca0.5 MnO3 film [27]. Thus we considered that the grown film is actually La0.5 Ca0.5 MnO3 . Fig. 6(b) shows temperature dependence of magnetization, it can be seen that the LCMO film is ferromagnetic at low temperature. Tc is about 230 K which is consistent with bulk half-doped LCMO. So we can conclude that the FM region and CO region coexist in the LCMO film. At low temperatures, we must account for the influence of the migration ability of oxygen vacancies on the switching behavior and the phase-separated characteristics of the LCMO part. Perovskite manganite shows widely existing phase-separated characteristics that have been observed through various techniques. Manganite is a well known half-metal that exhibits the following three properties: (a) the electrons in the FM phase are fully spinpolarized, (b) the spins of all conducting electrons are parallel to the localized spin of the region (according to the Hund’s rule coupling), and (c) the CO regions are anti-ferromagnetic with insulating characteristics. Electrons at a low current density can only flow through the FM regions. As the current density increases with the applied voltage and reaches a critical value, the electrons are injected into the COI regions. Parts of the COI regions are then modified into FM regions because the higher density current from the spin-polarized electrons can force the anti-ferromagnetically ordered localized electron spins in the COI to align parallel to the conducting electrons. Therefore, the resistance decreases as the FM regions enlarge. The junction is expected to be inhomogeneous due to the coexistence of FM and CO in the interface, and the effective junction area is determined by the heterostructure between the FM regions and the SNTO. The effective junction area becomes larger under high applied voltage, leading to a decline in the junction resistance. According to the relation C = ε0 εr S /d, the capacitance becomes larger; this can explain the negative value of ER at 100 K well. From the oxygen diffusion constant expressed by D = D0 exp(−E /kT ) [24], the migration ability of oxygen at low temperature is nearly negligible. Based on semiconductor theory, the external electric field is mostly applied to the depletion layer, and a wider depletion layer at low temperature corresponds to a lower electric field. Thus, the width of the interface barrier is not significantly affected by the external bias voltage and the junction only exhibits a negative ER value below 140 K. At temperatures between 140 and 220 K, the migration ability of the oxygen vacancies shows an exponential increase with increasing temperature according to D = D0 exp(−E /kT ), leading to the emergence of the positive ER value. However, the effect of the negative ER value also exists simultaneously, and the two competing actions lead to the ER crossover effect. The transport at small bias voltages reflects the situation of the interface layer, i.e., the negative ER correspond to an increase in the effective junction area and the capacitance increases after voltage is applied, as seen in the C –V curves. Above 220 K, the phase separation disappears [28] along with the corresponding negative ER, and only a positive ER value can be seen. The widening of the depletion layer under an electric voltage leads to a decrease in the capacitance. The width of the depletion layer decreases with increasing temperature, and the electric field applied on the depletion layer increases. Consequently, the migration ability of oxygen increases with increasing temperature, and the positive ER increases rapidly with increasing temperature. 4. Conclusion The external applied electric field induced I–V and C –V characteristics of the La0.5 Ca0.5 MnO3 /Nb:SrTiO3 heterojunction have been studied in this work. The ER effect was observed in
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the heterojunction, and the value and polarity of the ER showed a strong dependence on the temperature and bias voltage. The C –V measurements demonstrated that the modulation of the depletion width played a key role in the ER behavior. The negative ER at low temperatures was attributed to the melting of the COI phase, and the positive ER at high temperatures could be attributed to the electrochemical migration of the oxygen vacancies. The ER crossover effect appeared as a result of the competition between the two types of electro-resistance. Acknowledgements The work is supported by NSFC (No. 50972139) and National Basic Research Program of China (Grant No. 2009CB929502) and CAS. References [1] A. Sawa, A. Yamamoto, H. Yamada, T. Fujii, M. Kawasaki, J. Matsuno, Y. Tokura, Appl. Phys. Lett. 90 (2007) 252102. [2] J.R. Sun, S.Y. Zhang, B.G. Shen, H.K. Wong, Appl. Phys. Lett. 86 (2005) 053503. [3] Y.W. Xie, J.R. Sun, D.J. Wang, S. Liang, W.M. Lü, B.G. Shen, Appl. Phys. Lett. 90 (2007) 192903. [4] T.F. Zhou, G. Li, N.Y. Wang, B.M. Wang, X.G. Li, Appl. Phys. Lett. 88 (2006) 232508. [5] J. Qiu, H.B. Lu, K.J. Jin, M. He, J. Xing, Physica B 400 (2007) 66. [6] K. Zhao, K.J. Jin, H.B. Lu, Y.H. Huang, Q.L. Zhou, M. He, Z.H. Chen, Y.L. Zhou, G.Z. Yang, Appl. Phys. Lett. 88 (2006) 141914. [7] T. Susaki, N. Nakagawa, H.Y. Hwang, Phys. Rev. B 75 (2007) 104409.
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