- Email: [email protected]
Hopping and trap controlled conduction in Cr-doped SrTiO3 thin films
Solid-State Electronics 75 (2012) 43–47
Contents lists available at SciVerse ScienceDirect
Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
Hopping and trap controlled conduction in Cr-doped SrTiO3 thin films Bach Thang Phan a,b, Taekjib Choi c, A. Romanenko a, Jaichan Lee a,⇑ a
School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea Faculty of Materials Science and Laboratory of Advanced Materials, University of Science, Vietnam National University, HoChiMinh, Viet Nam c Hybrid Materials Research Center and Faculty/Institute of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul 143-747, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 13 January 2012 Received in revised form 9 May 2012 Accepted 13 May 2012 Available online 14 June 2012 The review of this paper was arranged by Prof. S. Cristoloveanu
a b s t r a c t This study examined the electrical conduction of Cr-doped SrTiO3 thin films in a metal (Pt)–insulator– metal (La0.5Sr0.5CoO3) structure. Two DC transport mechanisms, variable range hopping and the trap-controlled space-charge-limited current conduction, were found to be responsible for the conduction behavior. Resistance switching mechanism involved the trapping/detrapping of injected carriers at the weakly localized states. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: SrTiO3 thin films Resistive switching Electrical transport Variable range hopping Space charge limited conduction
1. Introduction Recently, transition-metal oxides with a perovskite structures (Cr-doped SrZrO3, Cr-doped SrTiO3) [1–4] and perovskite manganite structure (Pr0.7Ca0.3MnO3, La0.7Sr0.3MnO3) [5–8] have been proposed as candidate materials for resistive random access memorie (ReRAM). Various mechanisms have been suggested for resistance switching including carrier trapping/detrapping process [5], interface effects [3,6,7], metallic paths, and migration of oxygen ions [4,8]. In those mechanisms, defects play important roles since distribution and fundamental nature of defects strongly influence the electrical properties of materials, where disordered states are most likely to occur and have a random space and energy distributions. Since Anderson suggested the localization of carriers due to multiple scattering from the disordered defects [9], disordered states have been considered to induce weak localization (Anderson localization). Mott then reported that typical electrical conduction in disordered materials with weakly localized states followed the variable range hopping conduction model [10]. In a highly disordered or distorted state, electron–phonon interaction further increases the degree of localization. As a result, small polarons form and charge transport follows polaron conduction [11,12]. Small polaron formation in SrTiO3 is typically accompanied by ⇑ Corresponding author. Tel.: +82 31 290 7397; fax: +82 31 290 7410. E-mail address: [email protected] (J. Lee). 0038-1101/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.sse.2012.05.007
electron localization and has been investigated by far-infrared reflectively and luminescence studies [12–14]. In highly oxygen deficient Cr-doped SrTiO3d thin films, small polarons formed and their conductivity was reduced due to strong electron–phonon interaction with the large polaron coupling constant a 28 [15]. This led to the suppression of resistance switching. However, the mechanism of electrical conduction in stoichiometric Cr-doped SrTiO3 films is not yet clearly understood despite of the occurrence of resistance switching. This study reports in detail the electrical conduction and associated resistance switching mechanisms of stoichiometric Cr-doped SrTiO3 films.
2. Experiments A 60 nm-thick conducting oxide La0.5Sr0.5CoO3 (LSCO) layer was deposited as a bottom electrode on a single crystalline (1 0 0) oriented SrTiO3 substrate, followed by the deposition of a 60 nm thick 0.2% Cr-doped SrTiO3 (Cr-STO3) layer. The LSCO and Cr-STO3 films were made by pulsed laser deposition (PLD). During deposition, the substrate was kept at 650 °C under an oxygen pressure of 101 Torr and then cooled under an oxygen pressure of 400 Torr. The 100 nm-thick Pt top electrodes were defined by a lift-off process using photolithography and sputtering. Current–voltage (I–V) measurements were carried out using a Keithley 2400 source meter and a low temperature probe station in the temperature
44
B.T. Phan et al. / Solid-State Electronics 75 (2012) 43–47
range from 100 to 420 K. The voltage profile for the I–V measurement was 0 V ? Vmax ? 0 V ? +Vmax ? 0 V. Vmax was 10 V. The positive direction of the bias voltage corresponds to a positive bias applied to the Pt top electrode.
3. Results and discussion Fig. 1 shows the I–V characteristics of the Pt/Cr-STO3/LSCO structure in the voltage control mode at various temperatures. The initial (as-prepared) state was a high resistance state (HRS). The HRS was changed to a low resistance state (LRS) by applying a negative bias (0 ? 10 V) to the Pt top electrode, whereas HRS was not changed by a positive bias (0 ? 10 V). The resistance switching occurred without forming process often known to be necessary for the switching of several oxides. The LRS was progressively changed to the HRS only by a voltage sweep in the positive voltage region (0 ? +10 V), which is essentially a bipolar character. The I–V characteristics did not change even after repeated stress cycles. The temperature dependence of I–V characteristics was investigated to clarify the leakage current mechanism. The I–V characteristics followed neither Schottky conduction nor Poole–Frenkel conduction. Therefore, the I–V characteristics were examined in terms of trap-controlled space-charge-limited conduction (SCLC) [16,17] and variable range hopping conduction (VRH) [10,18–20]. Fig. 2a presents the I–V characteristics of the negative sweep (0 ? 10 V). The I–V characteristics of the HRS followed three distinct voltage regimes corresponding to three different slopes of the current. This leakage current behavior is well described by trap-controlled space-charge-limited conduction, in which Ohmic behavior at low voltages (0 ? 1.2 V) is followed by trap-filled limit region (1.2 V ? 3 V) with I / Vm dependence (m = 11–25) and trap-free space-charge-limited conduction (Child’s law, I / V2) above 3 V. A trap-filled limit voltage (VTFL = 1.2 V) was obtained, which corresponds to the onset of a rapid increase in current (i.e., resistance switching to LRS) and is expressed as VTFL = 8qNtd2/ 9K1e0 (d is the film thickness and Nt is the trap density) [17]. Using K1 = n2 = 5.76 [21], Nt was estimated to be 1.2 1017 cm3 at 200 K. The power law dependence of the current on the voltage in the trap-filled limit region indicates an exponential trap distribution (I / Vm, m = l + 1 with l = Tt/T, Tt is the characteristic temperature related to the trap distribution) [22], where the slope m typically decreases with temperature. The slope m obtained in the trap-filled limit region decreased with increasing the temperature from 100 K (m 25) to 330 K (m 11), indicating that the Crdoped SrTiO3 films has localized states with an exponential trap distribution. It is expected that the localized state close to conduction band edge (i.e., shallow trap) is heavily populated and its density decreases exponentially away from the conduction band edge. Injected
Fig. 1. Current vs. voltage (I–V) hysteretic curve of the Pt/Cr-STO/LSCO structure at various temperatures.
Fig. 2. (a) Current vs. voltage characteristic at various temperatures and (b) temperature dependence of current in a sweep from 0 to 10 V.
carriers would then influence the filling of the localized states through quasi-Fermi level positioning and further conducting (or resistance) state. I–V characteristics were examined in the low negative voltage region (0 ? 2 V) to find whether the carrier injection affects states’ occupancies. Fig. 2b shows the linear dependence of ln(I) vs. (T1/4) below 1.25 V that suggests VRH conduction, typical of disordered materials and expressed by the following equation:
IðTÞ / exp½ðT 0 =TÞ1=4 ; 3
ð1Þ
a , C = 24/p, a is the inverse length on which the where T 0 ¼ C T kB NðE T FÞ amplitude of the wave function falls down, kB is the Boltzmann constant, T0 is an indirect measurement of the density of localized states participating in hopping conduction and N(EF) is the energy density of localized sites at the quasi-Fermi level. For VRH conduction, a material requires many empty sites for hopping. The available hopping carrier density inside the film and the injected carrier density are not sufficient to fill the empty trap sites fully. Therefore, conduction behavior follows trap-controlled SCL conduction as well as VRH conduction. However, at VTFL = 1.2 V, all the empty sites are filled by injected carriers and available hopping carriers, leading to the rapid increase of current with increasing the negative voltage, as shown in Fig. 2a. Accordingly, the conduction mechanism changes from VRH conduction to trap-filled limit conduction and resistance switches from HRS to LRS. At further increase in the negative bias voltage above 3 V, the current followed a square law with its temperature independence, characteristic of Child’s law. The hopping carrier density Nhopping was estimated at the threshold voltage in VRH conduction, VTFL = 1.2 V in order to understand the transition from VRH to trap-filled limit conduction. N(EF) estimated from the temperature dependence of VRH conduction was
B.T. Phan et al. / Solid-State Electronics 75 (2012) 43–47
45
2.1 1019 eV1cm3. Since all localized sites were fully filled at VTFL, hopping carrier density was estimated using Nhopping 2kBTN(EF) [19]. Nhopping was 7.2 1017 cm3 at 200 K. Nhopping and Nt at VTFL are comparable (1017 cm3), which is clear evidence of the transition from VRH to trap-filled limit conduction being accompanied by resistance switching from HRS to LRS. The I–V characteristics of the LRS were also examined in the successive sweep (10 V ? 0), and fitted well to trap-free SCL conduction above 3 V (I V2) with the temperature independence (Fig. 3a). VRH conduction with a linear relationship of ln(I) vs. T1/4 was observed below 3 V (Fig. 3b). In this sweep, most localized states or trap sites remained occupied under carrier injection although carrier injection was reduced at low voltages. This caused LRS to persist as long as voltage remained negative. This was verified by repeated sweeps between 0 and 10 V once the LRS was obtained. In addition, a previous impedance spectroscopy study also suggested trap-controlled space-charge-limited conduction [23]. The ac conductivity study of the HRS in negative bias showed that Cr-doped SrTiO3 films exhibited dielectric relaxation and the ac conductivity obeyed a square-law dependence at high frequencies [24]. The dielectric relaxation and the quadratic dependence of the conductivity on frequency is an indication of bulk-limited ac conductivity in which carriers hop between localized states [22,25,26]. The activation energies obtained from both dielectric relaxation and electrical conduction were similar and small (0.2 eV) [24], suggesting that charge carrier hopping occurs between weakly localized states and causes the trap controlled conduction. This is consistent with the current analysis of dc conduction. The localized states in SrTiO3-based perovskite oxides may be induced by cation vacancies, oxygen vacancies, and a random distribution of dopant ions [13,18]. The effect of Cr concentration between 0.2% and 3.7 wt%) was investigated: it did not play a significant role in the conduction and resis-
a , C = 81/16p. The transition from temperaturewhere E0 ¼ C E eNðE E FÞ to electric field dependence of current only appeared above the threshold field Et = akBT/2e. According to VRH conduction, charge carriers hop to distant empty states of similar energies more easily than to nearby ones of much higher energy. In the LRS, empty localized states of similar energy levels are not readily available, which initiates hopping to the nearby empty states of different energy levels under an electric field. Therefore, the transition from a temperature to field dependence of the current occurs above the threshold electric field, which is seen in Fig. 4.
Fig. 3. (a) Current vs. voltage characteristic at various temperatures and (b) temperature dependence of current in a sweep from 10 to 0 V.
Fig. 4. (a) Temperature dependence and (b) electric field dependence of current in a sweep from 0 to 10 V. Arrows indicate the first step down of current at each temperature. Voltage of the first jumping progressively decreases with temperature.
tance switching behaviors. As discussed in the introduction, no resistance switching was observed as long as oxygen deficient layer was used [3,15]. Then the Cr-doped SrTiO3 films were slowly cooled at a highly oxidizing atmosphere (PO2 = 400 Torr) to avoid oxygen vacancy formation and make oxygen stoichiometry, which excludes the role of oxygen vacancies on the weakly localized states. The calculation of defect formation energies by density functional theory showed that the formation energies of cation vacancies and antisites (SrTi) were lower than those of other native defects such as TiSn antisite and cation interstitials by more than 2 eV, suggesting that the localized states might be induced by other native defects, such as Sr/Ti vacancies and SrTi antisite. The LRS changed to a HRS in the subsequent positive bias sweep from 0 to 10 V. At these positive biases, the I–V characteristics are also described by VRH conduction in terms of temperature and field dependences. The current had a linear dependence on temperature (T1/4) below +1 V (Fig. 4a), while the linear dependence of current on electric field (E1/4) only appeared at voltages above +1 V (Fig. 4b) with the following relationship:
IðEÞ / exp½ðE0 =EÞ1=4 ;
ð2Þ
4
46
B.T. Phan et al. / Solid-State Electronics 75 (2012) 43–47
Fig. 5. Current vs. voltage (I–V) curve at various temperatures in the sweep from 0 to 10 V, showing the halt of detrapping.
In the 0–10 V sweep, the Cr-doped SrTiO3 films showed gradual change of resistance from LRS to HRS, accompanied by successive step changes in current, as shown in Figs. 4b and 5. The resistance change occurs through slow detrapping from weakly localized states under limited carrier injection at the positive bias, lowering the quasi-Fermi level in the distribution of localized states. Detrapping is the escape of electrons from localized states with different potential barriers. As shown in Fig. 5, at low positive biases, weakly localized states were still filled and initial detrapping may occur in the weakly localized states with low potential barriers, resulting in the increase of current. As the localized states with low potential barriers are emptied, the current steps down (1st jumping). With increasing bias voltage, the second detrapping from weakly localized states with higher potential barriers could occur, leading to a further increase of current until these localized states are emptied. At the exhaustion of the second detrapping, the current steps down
again (2nd jumping). As temperature increases, thermal activation assists early halt of detrapping, represented by the progressive decrease in voltage of the 1st jumping with temperature shown in Fig. 4b. Fig. 5 (or Fig. 1 at the positively biased state) shows the corresponding successive step down in current. Such jumps in the current were not observed in the 10–0 V sweep once the HRS was reached from the preceding sweep (0–10 V), as shown in Fig. 1. The density of empty localized states increases with detrapping until most of the trapped carriers are released, resulting in the HRS. Once HRS is reached, no change in resistance state will occur at any positive bias, as there is no way to fill the localized states by carrier injection. The current density in the HRS during the 10–0 V sweep also followed VRH conduction behavior in terms of the temperature (ln(I) T1/4) and electric field (ln(I) E1/4), as shown in Fig. 6. Although VRH conduction was observed at both low and high resistance states, the threshold field was significantly different for each state: 1 V and 5 V for the LRS and HRS, respectively. The HRS has a sufficiently high density of empty localized state to make hopping between empty localized states of similar energies readily available when compared with the case of the LRS. This extends the voltage region in which current follows the temperature dependence (ln(I) T1/4) and is manifested in the much larger threshold field. It is worth mentioning that the carrier injection from the electrode into the Cr-doped SrTiO3 layer is required for resistance switching. If carrier injection is limited, resistance switching does not occur. No resistance switching occurred when the interface between the top Pt electrode and the Cr-doped SrTiO3 layer was modified by either replacing the Cr-doped SrTiO3 layer with an oxygen-deficient layer (Cr-STO3d) or inserting an ultra-thin (five unit cell thick) oxygen-deficient STO layer at the interface. Small polaron formation in the reduced Cr-SrTiO3d films, especially in the interfacial region, was responsible for the suppressed charge injection [3,15]. 4. Conclusions The electrical conduction of a Pt/Cr-STO/LSCO structure was investigated. Two dc electrical conduction mechanisms were found: variable range hopping and trap-controlled space-chargelimited current conduction. Variable range hopping conduction dominated the entire positive bias region and at small negative biases, while trap-controlled space-charge-limited conduction occurred in the more negative bias region. Resistance switching between these two conductions was discussed through charge injection and trapping/detrapping at the weakly localized states. Acknowledgments This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (20090092809) and National Foundation of Science and Technology Development of Vietnam (NAFOSTED – 103.99-2010.12). References
Fig. 6. (a) Temperature dependence and (b) electric field dependence of current in a sweep from 10 to 0 V.
[1] Rossel C, Meijer GI, Bremaud D, Widmer D. J. Appl. Phys. 2001;90:2892. [2] Meijer G, Staub IU, Janousch M, Johnson SL, Delley B, Neisius T. Phys. Rev. B 2005;72:155102. [3] Phan BT, Lee J. Appl. Phys. Lett. 2008;93:222906. [4] Janousch M, Meijer GI, Staub U, Delley B, Karg SF, Andreasson BP. Adv. Mater. 2007;19:2232. [5] Odagawa A, Sato H, Inoue IH, Akoh H, Kawasaki H, Tokura MY, et al. Phys. Rev. B 2004;70:224403. [6] Sawa A, Fujii T, Kawasaki M, Tokura Y. Appl. Phys. Lett. 2006;88:232112. [7] Quintero M, Levy P, Leyva AG, Rozenberg MJ. Phys. Rev. Lett. 2007;98:116601. [8] Nian YB, Strozier J, Wu NJ, Chen X, Ignatiev A. Phys. Rev. Lett. 2007;98:146403. [9] Anderson PW. Phys. Rev. 1958;109:1492.
B.T. Phan et al. / Solid-State Electronics 75 (2012) 43–47 [10] [11] [12] [13] [14] [15] [16] [17]
Mott NF. J. Non-Cryst. Solid. 1968;1:1. Anderson PW. Phys. Rev. Lett. 1975;34:953. Leonelli R, Berbner JL. Phys. Rev. B 1986;33:8649. Ma JY, Bi CZ, Fang X, Zhao HY, Kamran M, Qiu XG. Phys. C 2007;463–465:107. Hasegawa T, Shirai M, Tanaka K. J. Lumin. 2000;87–89:1217. Phan BT, Lee J. Appl. Phys. Lett. 2009;94:232102. Lampert MA, Mark P. Current injection in solids. NY: Academic Press; 1970. Wang SY, Cheng BL, Wang C, Dai SY, Lu HB, Zhou YL, et al. Appl. Phys. Lett. 2004;84:4116. [18] Karg S, Meijer GI, Widmer D, Bednorz JG. Appl. Phys. Lett. 2006;89:072106.
47
[19] J.J.V. Hapert. Ph.D. dissertation. University of Utrecht; 2002. [20] Mott NF, Davis EA. Electronic processes in non-crystalline materials. Oxford, Clarendon; 1979. [21] Peng CJ, Hu H, Krupanidhi SB. Appl. Phys. Lett. 1993;63:1038. [22] Simmons JG. J. Phys. D: Appl. Phys. 1971;4:613. [23] Phan BT, Choi T, Lee J. Integr. Ferroelectr. 2008;96:146. [24] Phan BT, Kim NC, Lee J. J. Korean Phys. Soc 2009;54:873. [25] Argall F, Jonscher AK. Thin Solid Films 1968;2:185. [26] Funke K. Prog. Solid State Chem. 1993;22:111.
Contents lists available at SciVerse ScienceDirect
Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
Hopping and trap controlled conduction in Cr-doped SrTiO3 thin films Bach Thang Phan a,b, Taekjib Choi c, A. Romanenko a, Jaichan Lee a,⇑ a
School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea Faculty of Materials Science and Laboratory of Advanced Materials, University of Science, Vietnam National University, HoChiMinh, Viet Nam c Hybrid Materials Research Center and Faculty/Institute of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul 143-747, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 13 January 2012 Received in revised form 9 May 2012 Accepted 13 May 2012 Available online 14 June 2012 The review of this paper was arranged by Prof. S. Cristoloveanu
a b s t r a c t This study examined the electrical conduction of Cr-doped SrTiO3 thin films in a metal (Pt)–insulator– metal (La0.5Sr0.5CoO3) structure. Two DC transport mechanisms, variable range hopping and the trap-controlled space-charge-limited current conduction, were found to be responsible for the conduction behavior. Resistance switching mechanism involved the trapping/detrapping of injected carriers at the weakly localized states. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: SrTiO3 thin films Resistive switching Electrical transport Variable range hopping Space charge limited conduction
1. Introduction Recently, transition-metal oxides with a perovskite structures (Cr-doped SrZrO3, Cr-doped SrTiO3) [1–4] and perovskite manganite structure (Pr0.7Ca0.3MnO3, La0.7Sr0.3MnO3) [5–8] have been proposed as candidate materials for resistive random access memorie (ReRAM). Various mechanisms have been suggested for resistance switching including carrier trapping/detrapping process [5], interface effects [3,6,7], metallic paths, and migration of oxygen ions [4,8]. In those mechanisms, defects play important roles since distribution and fundamental nature of defects strongly influence the electrical properties of materials, where disordered states are most likely to occur and have a random space and energy distributions. Since Anderson suggested the localization of carriers due to multiple scattering from the disordered defects [9], disordered states have been considered to induce weak localization (Anderson localization). Mott then reported that typical electrical conduction in disordered materials with weakly localized states followed the variable range hopping conduction model [10]. In a highly disordered or distorted state, electron–phonon interaction further increases the degree of localization. As a result, small polarons form and charge transport follows polaron conduction [11,12]. Small polaron formation in SrTiO3 is typically accompanied by ⇑ Corresponding author. Tel.: +82 31 290 7397; fax: +82 31 290 7410. E-mail address: [email protected] (J. Lee). 0038-1101/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.sse.2012.05.007
electron localization and has been investigated by far-infrared reflectively and luminescence studies [12–14]. In highly oxygen deficient Cr-doped SrTiO3d thin films, small polarons formed and their conductivity was reduced due to strong electron–phonon interaction with the large polaron coupling constant a 28 [15]. This led to the suppression of resistance switching. However, the mechanism of electrical conduction in stoichiometric Cr-doped SrTiO3 films is not yet clearly understood despite of the occurrence of resistance switching. This study reports in detail the electrical conduction and associated resistance switching mechanisms of stoichiometric Cr-doped SrTiO3 films.
2. Experiments A 60 nm-thick conducting oxide La0.5Sr0.5CoO3 (LSCO) layer was deposited as a bottom electrode on a single crystalline (1 0 0) oriented SrTiO3 substrate, followed by the deposition of a 60 nm thick 0.2% Cr-doped SrTiO3 (Cr-STO3) layer. The LSCO and Cr-STO3 films were made by pulsed laser deposition (PLD). During deposition, the substrate was kept at 650 °C under an oxygen pressure of 101 Torr and then cooled under an oxygen pressure of 400 Torr. The 100 nm-thick Pt top electrodes were defined by a lift-off process using photolithography and sputtering. Current–voltage (I–V) measurements were carried out using a Keithley 2400 source meter and a low temperature probe station in the temperature
44
B.T. Phan et al. / Solid-State Electronics 75 (2012) 43–47
range from 100 to 420 K. The voltage profile for the I–V measurement was 0 V ? Vmax ? 0 V ? +Vmax ? 0 V. Vmax was 10 V. The positive direction of the bias voltage corresponds to a positive bias applied to the Pt top electrode.
3. Results and discussion Fig. 1 shows the I–V characteristics of the Pt/Cr-STO3/LSCO structure in the voltage control mode at various temperatures. The initial (as-prepared) state was a high resistance state (HRS). The HRS was changed to a low resistance state (LRS) by applying a negative bias (0 ? 10 V) to the Pt top electrode, whereas HRS was not changed by a positive bias (0 ? 10 V). The resistance switching occurred without forming process often known to be necessary for the switching of several oxides. The LRS was progressively changed to the HRS only by a voltage sweep in the positive voltage region (0 ? +10 V), which is essentially a bipolar character. The I–V characteristics did not change even after repeated stress cycles. The temperature dependence of I–V characteristics was investigated to clarify the leakage current mechanism. The I–V characteristics followed neither Schottky conduction nor Poole–Frenkel conduction. Therefore, the I–V characteristics were examined in terms of trap-controlled space-charge-limited conduction (SCLC) [16,17] and variable range hopping conduction (VRH) [10,18–20]. Fig. 2a presents the I–V characteristics of the negative sweep (0 ? 10 V). The I–V characteristics of the HRS followed three distinct voltage regimes corresponding to three different slopes of the current. This leakage current behavior is well described by trap-controlled space-charge-limited conduction, in which Ohmic behavior at low voltages (0 ? 1.2 V) is followed by trap-filled limit region (1.2 V ? 3 V) with I / Vm dependence (m = 11–25) and trap-free space-charge-limited conduction (Child’s law, I / V2) above 3 V. A trap-filled limit voltage (VTFL = 1.2 V) was obtained, which corresponds to the onset of a rapid increase in current (i.e., resistance switching to LRS) and is expressed as VTFL = 8qNtd2/ 9K1e0 (d is the film thickness and Nt is the trap density) [17]. Using K1 = n2 = 5.76 [21], Nt was estimated to be 1.2 1017 cm3 at 200 K. The power law dependence of the current on the voltage in the trap-filled limit region indicates an exponential trap distribution (I / Vm, m = l + 1 with l = Tt/T, Tt is the characteristic temperature related to the trap distribution) [22], where the slope m typically decreases with temperature. The slope m obtained in the trap-filled limit region decreased with increasing the temperature from 100 K (m 25) to 330 K (m 11), indicating that the Crdoped SrTiO3 films has localized states with an exponential trap distribution. It is expected that the localized state close to conduction band edge (i.e., shallow trap) is heavily populated and its density decreases exponentially away from the conduction band edge. Injected
Fig. 1. Current vs. voltage (I–V) hysteretic curve of the Pt/Cr-STO/LSCO structure at various temperatures.
Fig. 2. (a) Current vs. voltage characteristic at various temperatures and (b) temperature dependence of current in a sweep from 0 to 10 V.
carriers would then influence the filling of the localized states through quasi-Fermi level positioning and further conducting (or resistance) state. I–V characteristics were examined in the low negative voltage region (0 ? 2 V) to find whether the carrier injection affects states’ occupancies. Fig. 2b shows the linear dependence of ln(I) vs. (T1/4) below 1.25 V that suggests VRH conduction, typical of disordered materials and expressed by the following equation:
IðTÞ / exp½ðT 0 =TÞ1=4 ; 3
ð1Þ
a , C = 24/p, a is the inverse length on which the where T 0 ¼ C T kB NðE T FÞ amplitude of the wave function falls down, kB is the Boltzmann constant, T0 is an indirect measurement of the density of localized states participating in hopping conduction and N(EF) is the energy density of localized sites at the quasi-Fermi level. For VRH conduction, a material requires many empty sites for hopping. The available hopping carrier density inside the film and the injected carrier density are not sufficient to fill the empty trap sites fully. Therefore, conduction behavior follows trap-controlled SCL conduction as well as VRH conduction. However, at VTFL = 1.2 V, all the empty sites are filled by injected carriers and available hopping carriers, leading to the rapid increase of current with increasing the negative voltage, as shown in Fig. 2a. Accordingly, the conduction mechanism changes from VRH conduction to trap-filled limit conduction and resistance switches from HRS to LRS. At further increase in the negative bias voltage above 3 V, the current followed a square law with its temperature independence, characteristic of Child’s law. The hopping carrier density Nhopping was estimated at the threshold voltage in VRH conduction, VTFL = 1.2 V in order to understand the transition from VRH to trap-filled limit conduction. N(EF) estimated from the temperature dependence of VRH conduction was
B.T. Phan et al. / Solid-State Electronics 75 (2012) 43–47
45
2.1 1019 eV1cm3. Since all localized sites were fully filled at VTFL, hopping carrier density was estimated using Nhopping 2kBTN(EF) [19]. Nhopping was 7.2 1017 cm3 at 200 K. Nhopping and Nt at VTFL are comparable (1017 cm3), which is clear evidence of the transition from VRH to trap-filled limit conduction being accompanied by resistance switching from HRS to LRS. The I–V characteristics of the LRS were also examined in the successive sweep (10 V ? 0), and fitted well to trap-free SCL conduction above 3 V (I V2) with the temperature independence (Fig. 3a). VRH conduction with a linear relationship of ln(I) vs. T1/4 was observed below 3 V (Fig. 3b). In this sweep, most localized states or trap sites remained occupied under carrier injection although carrier injection was reduced at low voltages. This caused LRS to persist as long as voltage remained negative. This was verified by repeated sweeps between 0 and 10 V once the LRS was obtained. In addition, a previous impedance spectroscopy study also suggested trap-controlled space-charge-limited conduction [23]. The ac conductivity study of the HRS in negative bias showed that Cr-doped SrTiO3 films exhibited dielectric relaxation and the ac conductivity obeyed a square-law dependence at high frequencies [24]. The dielectric relaxation and the quadratic dependence of the conductivity on frequency is an indication of bulk-limited ac conductivity in which carriers hop between localized states [22,25,26]. The activation energies obtained from both dielectric relaxation and electrical conduction were similar and small (0.2 eV) [24], suggesting that charge carrier hopping occurs between weakly localized states and causes the trap controlled conduction. This is consistent with the current analysis of dc conduction. The localized states in SrTiO3-based perovskite oxides may be induced by cation vacancies, oxygen vacancies, and a random distribution of dopant ions [13,18]. The effect of Cr concentration between 0.2% and 3.7 wt%) was investigated: it did not play a significant role in the conduction and resis-
a , C = 81/16p. The transition from temperaturewhere E0 ¼ C E eNðE E FÞ to electric field dependence of current only appeared above the threshold field Et = akBT/2e. According to VRH conduction, charge carriers hop to distant empty states of similar energies more easily than to nearby ones of much higher energy. In the LRS, empty localized states of similar energy levels are not readily available, which initiates hopping to the nearby empty states of different energy levels under an electric field. Therefore, the transition from a temperature to field dependence of the current occurs above the threshold electric field, which is seen in Fig. 4.
Fig. 3. (a) Current vs. voltage characteristic at various temperatures and (b) temperature dependence of current in a sweep from 10 to 0 V.
Fig. 4. (a) Temperature dependence and (b) electric field dependence of current in a sweep from 0 to 10 V. Arrows indicate the first step down of current at each temperature. Voltage of the first jumping progressively decreases with temperature.
tance switching behaviors. As discussed in the introduction, no resistance switching was observed as long as oxygen deficient layer was used [3,15]. Then the Cr-doped SrTiO3 films were slowly cooled at a highly oxidizing atmosphere (PO2 = 400 Torr) to avoid oxygen vacancy formation and make oxygen stoichiometry, which excludes the role of oxygen vacancies on the weakly localized states. The calculation of defect formation energies by density functional theory showed that the formation energies of cation vacancies and antisites (SrTi) were lower than those of other native defects such as TiSn antisite and cation interstitials by more than 2 eV, suggesting that the localized states might be induced by other native defects, such as Sr/Ti vacancies and SrTi antisite. The LRS changed to a HRS in the subsequent positive bias sweep from 0 to 10 V. At these positive biases, the I–V characteristics are also described by VRH conduction in terms of temperature and field dependences. The current had a linear dependence on temperature (T1/4) below +1 V (Fig. 4a), while the linear dependence of current on electric field (E1/4) only appeared at voltages above +1 V (Fig. 4b) with the following relationship:
IðEÞ / exp½ðE0 =EÞ1=4 ;
ð2Þ
4
46
B.T. Phan et al. / Solid-State Electronics 75 (2012) 43–47
Fig. 5. Current vs. voltage (I–V) curve at various temperatures in the sweep from 0 to 10 V, showing the halt of detrapping.
In the 0–10 V sweep, the Cr-doped SrTiO3 films showed gradual change of resistance from LRS to HRS, accompanied by successive step changes in current, as shown in Figs. 4b and 5. The resistance change occurs through slow detrapping from weakly localized states under limited carrier injection at the positive bias, lowering the quasi-Fermi level in the distribution of localized states. Detrapping is the escape of electrons from localized states with different potential barriers. As shown in Fig. 5, at low positive biases, weakly localized states were still filled and initial detrapping may occur in the weakly localized states with low potential barriers, resulting in the increase of current. As the localized states with low potential barriers are emptied, the current steps down (1st jumping). With increasing bias voltage, the second detrapping from weakly localized states with higher potential barriers could occur, leading to a further increase of current until these localized states are emptied. At the exhaustion of the second detrapping, the current steps down
again (2nd jumping). As temperature increases, thermal activation assists early halt of detrapping, represented by the progressive decrease in voltage of the 1st jumping with temperature shown in Fig. 4b. Fig. 5 (or Fig. 1 at the positively biased state) shows the corresponding successive step down in current. Such jumps in the current were not observed in the 10–0 V sweep once the HRS was reached from the preceding sweep (0–10 V), as shown in Fig. 1. The density of empty localized states increases with detrapping until most of the trapped carriers are released, resulting in the HRS. Once HRS is reached, no change in resistance state will occur at any positive bias, as there is no way to fill the localized states by carrier injection. The current density in the HRS during the 10–0 V sweep also followed VRH conduction behavior in terms of the temperature (ln(I) T1/4) and electric field (ln(I) E1/4), as shown in Fig. 6. Although VRH conduction was observed at both low and high resistance states, the threshold field was significantly different for each state: 1 V and 5 V for the LRS and HRS, respectively. The HRS has a sufficiently high density of empty localized state to make hopping between empty localized states of similar energies readily available when compared with the case of the LRS. This extends the voltage region in which current follows the temperature dependence (ln(I) T1/4) and is manifested in the much larger threshold field. It is worth mentioning that the carrier injection from the electrode into the Cr-doped SrTiO3 layer is required for resistance switching. If carrier injection is limited, resistance switching does not occur. No resistance switching occurred when the interface between the top Pt electrode and the Cr-doped SrTiO3 layer was modified by either replacing the Cr-doped SrTiO3 layer with an oxygen-deficient layer (Cr-STO3d) or inserting an ultra-thin (five unit cell thick) oxygen-deficient STO layer at the interface. Small polaron formation in the reduced Cr-SrTiO3d films, especially in the interfacial region, was responsible for the suppressed charge injection [3,15]. 4. Conclusions The electrical conduction of a Pt/Cr-STO/LSCO structure was investigated. Two dc electrical conduction mechanisms were found: variable range hopping and trap-controlled space-chargelimited current conduction. Variable range hopping conduction dominated the entire positive bias region and at small negative biases, while trap-controlled space-charge-limited conduction occurred in the more negative bias region. Resistance switching between these two conductions was discussed through charge injection and trapping/detrapping at the weakly localized states. Acknowledgments This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (20090092809) and National Foundation of Science and Technology Development of Vietnam (NAFOSTED – 103.99-2010.12). References
Fig. 6. (a) Temperature dependence and (b) electric field dependence of current in a sweep from 10 to 0 V.
[1] Rossel C, Meijer GI, Bremaud D, Widmer D. J. Appl. Phys. 2001;90:2892. [2] Meijer G, Staub IU, Janousch M, Johnson SL, Delley B, Neisius T. Phys. Rev. B 2005;72:155102. [3] Phan BT, Lee J. Appl. Phys. Lett. 2008;93:222906. [4] Janousch M, Meijer GI, Staub U, Delley B, Karg SF, Andreasson BP. Adv. Mater. 2007;19:2232. [5] Odagawa A, Sato H, Inoue IH, Akoh H, Kawasaki H, Tokura MY, et al. Phys. Rev. B 2004;70:224403. [6] Sawa A, Fujii T, Kawasaki M, Tokura Y. Appl. Phys. Lett. 2006;88:232112. [7] Quintero M, Levy P, Leyva AG, Rozenberg MJ. Phys. Rev. Lett. 2007;98:116601. [8] Nian YB, Strozier J, Wu NJ, Chen X, Ignatiev A. Phys. Rev. Lett. 2007;98:146403. [9] Anderson PW. Phys. Rev. 1958;109:1492.
B.T. Phan et al. / Solid-State Electronics 75 (2012) 43–47 [10] [11] [12] [13] [14] [15] [16] [17]
Mott NF. J. Non-Cryst. Solid. 1968;1:1. Anderson PW. Phys. Rev. Lett. 1975;34:953. Leonelli R, Berbner JL. Phys. Rev. B 1986;33:8649. Ma JY, Bi CZ, Fang X, Zhao HY, Kamran M, Qiu XG. Phys. C 2007;463–465:107. Hasegawa T, Shirai M, Tanaka K. J. Lumin. 2000;87–89:1217. Phan BT, Lee J. Appl. Phys. Lett. 2009;94:232102. Lampert MA, Mark P. Current injection in solids. NY: Academic Press; 1970. Wang SY, Cheng BL, Wang C, Dai SY, Lu HB, Zhou YL, et al. Appl. Phys. Lett. 2004;84:4116. [18] Karg S, Meijer GI, Widmer D, Bednorz JG. Appl. Phys. Lett. 2006;89:072106.
47
[19] J.J.V. Hapert. Ph.D. dissertation. University of Utrecht; 2002. [20] Mott NF, Davis EA. Electronic processes in non-crystalline materials. Oxford, Clarendon; 1979. [21] Peng CJ, Hu H, Krupanidhi SB. Appl. Phys. Lett. 1993;63:1038. [22] Simmons JG. J. Phys. D: Appl. Phys. 1971;4:613. [23] Phan BT, Choi T, Lee J. Integr. Ferroelectr. 2008;96:146. [24] Phan BT, Kim NC, Lee J. J. Korean Phys. Soc 2009;54:873. [25] Argall F, Jonscher AK. Thin Solid Films 1968;2:185. [26] Funke K. Prog. Solid State Chem. 1993;22:111.