Optically modulated resistive switching in BiFeO3 thin film

Optically modulated resistive switching in BiFeO3 thin film solidi status physica Phys. Status Solidi A, 1–6 (2016) / DOI 10.1002/pssa.201533035 a www.pss-a.com applications and materials science Kashinath Bogle*,1, Ranjana Narwade1, Ambadas Phatangare2, Shailendra Dahiwale2, Megha Mahabole1, 1 and Rajendra Khairnar 1 2 School of Physical Sciences, Swami Ramanand Teerth Marathwada University, Nanded 431606, India Department of Physics, Savitribai Phule Pune University, Pune 411007, India Received 28 November 2015, revised 22 January 2016, accepted 3 February 2016 Published online 1 March 2016 Keywords BiFeO3, nonvolatile memory, perovskites, photovoltaics, resistive switching, thin films * Corresponding author: e-mail: [email protected], Phone: 02462 229559, Fax: 02462229245 Exploiting the photosensitive property of BiFeO3 thin films, we demonstrated a resistive switching memory cell having low Vset voltage (þ2.0 V), an ultrahigh ON/OFF ratio of 107 and a good retention time of more than 106 s. Synthesis conditions were optimized during a sol–gel-assisted spin-coating method to get phase-pure BiFeO3 films on Al substrate, at room temperature. Current–voltage analysis revealed that during optical illumination, photon-induced charge carriers migrate towards their respective electrodes along grain boundaries under an externally applied field, which initiate a substantial shift in the normal Vset of þ10.4 V to a lower voltage (þ2.0 V). The Poole–Frenkel emission at the metal/BiFeO3 interface is proposed and the role of electronic reconstruction at the interface is further investigated. Thus the write process in BiFeO3-based resistive-switching devices can be modulated in a controlled manner, which has the potential for integrating current resistive switching (memristive) memory device technology towards exciting optomemristive device technology. ß 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction The concept of altering the resistance of metal oxide thin film under external electrical stress opened a new remarkable research area attractive for nextgeneration nonvolatile memories with scenarios including very high-density integration and multistate logic implementation [1–4]. Among the nonvolatile memories considered till now, the resistive random access memory (ReRAM), employ reversible resistive switching (RS) behavior, is increasingly important due to its simple structure, long retention time, small size, and fast switching speed [5, 6]. The simplicity of the geometrical structures (metal/meal oxide/metal) makes the resistance switching extremely attractive and promising. Many transition-metal oxide (TMO) materials possess the RS behavior and among them perovskite materials also show excellent RS [5, 7]. Recently, this phenomenon has also been found in BiFeO3 (BFO) films [8–10]. Usually, BFO is an extensively studied multiferroic material, which exhibits ferroelectric and ferromagnetic behaviors simultaneously [11]. As to this, BFO has been widely studied since it exhibits a high Curie temperature (Tc  1100 K), a high N
eel temperature (TN  643 K) and a large remnant polarization over 90 mC cm2. In addition, photoresponse is also one of the most viable properties of BFO and has gained significance over recent years [8, 12]. Numerous reports show photoresistance and photovoltaic effect in BFO heterostructures and correlated them with the bandgap (2.2 eV) of BFO [8], that is within the visible light range, may be due to the energy band bending at domain walls [13]. The output photoresponse, which may be a photocurrent or photovoltage, is a function of the product of the remnant polarization from a previously applied polarization voltage and the incident light intensity [14, 15]. Owing to its remarkable properties it attracted much attention for multifunctional applications. In this work, we investigate the influence of optical illumination on SET voltage (Vset) during RS in polycrystalline BFO thin films. It is found that the current–voltage (I–V) curves of BFO films before and after exposure to an optical pulse show a large shift in the Vset, which may be due to the photoinduced charge carriers. 2 Experimental The BFO thin films were fabricated by a sol–gel-assisted spin-coating method on 99.99% pure aluminum (Al) substrate of thickness 0.1 mm procured from Alfa Aesar. BFO gel was synthesized by using bismuth(III) ß 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim solidi status physica pss a 2 K. Bogle et al.: Optically modulated resistive switching in BiFeO3 nitrate pentahydrate (99.99%) and iron(III) nitrate nonahydrate (99 þ %). In a typical synthesis of BFO gel 0.05 mol of Bi(NO3)3  5H2O and Fe(NO3)3  9H2O were added successively to 12 ml of ethylene glycol. The mixture was stirred at 80 8C, after which a transparent gel was recovered upon evaporation of the excess ethylene glycol. The resultant gel was coated on Al substrate using a spincoating technique at a speed of 2000 rpm for 15 s and after each coating the film was dried. This process was repeated 20 times to get a uniform coating on Al substrate. Samples were then preheated to 120 8C to remove excess hydrocarbons/NOx impurities, followed by annealing at 350 8C for 3 h to obtain BFO phase. In a separate experiment, a BFO film was coated on a glass slide by the above-mentioned procedure and utilized for UV–Vis spectroscopy. Scanning electron microscopy along with energy-dispersive X-ray spectroscopy (SEM, EDS) was used for analysis of the surface morphology and purity of the BFO film. In order to measure the electrical properties of the BFO films, a Cu top electrode was mounted onto the BFO films by a mechanical manipulator. The I–V characteristics of a Cu/BFO/Al sandwich structures were studied using a standard voltage source and a current meter with the Al bottom electrodes being grounded. 3 Results The setup required to study the RS and optical modulation in RS behavior of the Cu/BFO/Al structure is shown in Fig. 1a. This consists of a BFO thin film synthesized on aluminum (Al) substrate (used as a bottom electrode) and a Cu top electrode. The Cu/BFO/Al structure was held under a 25-W tungsten lamp and slit assembly. The distance between the slit and Cu/BFO region was 10 cm, such that the energy density was 20 mW cm2, as shown in Fig. 1a. The BFO thin films used in this device was synthesized a via sol–gel-assisted spin-coating technique with a thickness of 200 nm. Prior to study the optical modulation effect in the Cu/BFO/Al structure, RS behavior of the Cu/BFO/Al structure was monitored in the dark and is shown in Fig. 1b, while a positive bias was applied to the top Cu electrode (the effective contact area is limited to 50 mm2) and negative to the bottom Al electrode. The bias voltage was swept as 0 V!11 V!0 V!3 V!0 V. As the bias voltage increased from 0 to þ11 V, the output current of the Cu/BFO/Al structure increased abruptly at þ10.4 V and reached the set current compliance value (30 mA), which is defined as the forming or Vset. This abrupt transition switches an initial highresistance state (HRS) of the structure to a low-resistance state (LRS). The film sustains the LRS until the applied voltage decreases to þ0.2 V, after which a negative differential resistance appears till 0 V. Now, on returning to HRS from LRS, setting the current compliance to a higher value (100 mA) and sweeping the applied voltage from 0 to negative bias, the output current was increased linearly till 0.7 V and then decreased with subsequent increase in the negative voltage. This indicates switching back to the HRS and the voltage is referred as the RESET voltage (VReset), as shown in Fig. 1b. For repeated set operations subsequent to the first RESET, the switching voltage from HRS to LRS and LRS to HRS is roughly the same as in the first sweep. The overall resistive switching observed in the Cu/BFO/Al structure is bipolar since the switching sequence is dependent on the polarity of the bias voltage (Fig. 1b). Although two-state resistive switching is observed in the Cu/BFO/Al structure, the Vset of þ10.4 V is reasonably high for device application. In order to reduce Vset, I–V measurements were carried out under optical illumination by taking advantage of the photovoltaic effect in BFO. The Cu/BFO/Al structure was kept under the assembly, as shown in Fig. 1a. Figure 1 (a) Schematic of the setup used to study the optical modulation in the Cu/BFO/Al structure. (b) I–V curves measured on a Cu/BFO/Al structure; with current compliance of 30 mA with positive biasing and 100 mA with negative biasing. (c) Optical response during I–V measurement after illuminating the Cu/BFO junction at an operating voltage of þ2 V. ß 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-a.com Original Paper Phys. Status Solidi A (2016) Prior to the optical switching measurement, a resistiveswitching measurement was carried out on the structure and it resembles the I–V curve shown in Fig. 1b. Later, a fixed forward voltage þ0.5 V was applied to the structure and an optical pulse of width 10 ms was fired at the structure around Cu/BFO region. No abrupt increase in output current was seen. Similar results were obtained for forward voltages of þ1.0 V and þ1.5 V, whereas, an abrupt increase in the current was observed at þ2.0 V and the output current subsequently reaches the current compliance, as shown in Fig. 1c. A similar pattern was observed for higher forward voltages up to þ 9 V (Fig. 2a). As the applied voltage decreased a negative differential resistance appears starting from þ0.2 V till 0 V. The reverse-bias measurement was carried out in a similar manner as discussed above for Fig. 1b. This revealed that the photocarriers generated in BFO thin film under an external optical stimulus trigger conduction process that eventually shifts the Vset lower. Figure 2b represents the endurance properties of the Cu/BFO/Al structure under optical pulse-induced ON and normal OFF conditions for several cycles. The ON state is 3 achieved by applying a positive voltage of þ2.0 V under optical illumination (20 mW cm2) and the OFF state is achieved through erasing the ON state by applying a negative voltage pulse of –1.0 V. Both these states were read at þ0.1 V. The estimated ON/OFF ratio for Cu/BFO/Al structure is very high 107 and remain constant (std. deviation 2.19%) with increasing number of cycles. This indicates that the Cu/BFO/Al structure has a very stable ON/OFF ratio. After the device was switched ON or OFF, no electrical power was required to maintain the resistance in that state. The retention performance of the structure, at room temperature, is shown in Fig. 2c. A retention time of 105 min has been demonstrated for both resistance states. During the first 4 h, after a writing pulse, the current in the ON state drops by 16%. In the next 80 h, it drops again by 3% and later remains stable for the next 80 h (it is also stable for 3 months, data not shown here), whereas, a stable OFF state current was observed till 160 h. This reveals that a Cu/BFO/Al structure shows good retention characteristics within 105 min, as represented in Fig. 2c with a stable current during ON (after the first 4 h of a Figure 2 (a) Optical switching of the Cu/BFO/Al structure in the dark and after firing light pulses at various operating voltages in the range from þ2 to þ9 V. A current compliance of 30 mA was applied during forward biasing of sample, (b) retention property of the Cu/ BFO/Al structure measured at read voltage þ0.2 V for 104 min, (c) plot of ON/OFF ratio obtained on a Cu/BFO/Al structure, and (d) linear fittings of the I–V curves obtained on a Cu/BFO/Al structure: Ohmic behavior and SE conduction at LRS; P–F emission at HRS. www.pss-a.com ß 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim solidi status physica pss a 4 K. Bogle et al.: Optically modulated resistive switching in BiFeO3 writing pulse) and OFF states indicates the stable state of memory devices. To understand resistive switching, it is required to distinguish the conduction mechanism in the Cu/BFO/Al structure via considering three leakage mechanisms as follows: space charge-limited conduction (SCLC), Schottky emission (SE), and Poole–Frenkel (P–F) emission [16]. The linear fitting obtained in forward and reverse biasing on Cu/BFO/Al is shown in Fig. 2d. By comparing the fitted and expected values of the dielectric constant, the conduction mechanism of the BFO thin film in the four different voltage ranges, labeled regions 1–4 in Fig. 2d, can be deduced. In the voltage region 1 from 0 to þ11 V, the BFO thin film reveals PF conduction, as the estimated dielectric constant is 54, which matches with an earlier reported value of 54 [17]. In the voltage region 2 from þ11 to 0 V, the BFO thin film exhibits PF conduction till þ0.4 V and later follows Ohmic behavior, which is suggested by the slope 1 obtained by linear fitting. In the negative sweeping down, from 0 to –3 V, (region 3) it also shows prominent Ohmic conduction and sweeping up from 3 to 0 V (region 4) shows that the BFO thin film reveals SE conduction, as the estimated dielectric constant agrees with the value of 54. In the BFO thin film, the conduction process is dominated by the existence of a larger resistance, which is an intrinsic property of a ferroelectric material. In addition, the fabrication process also introduces oxygen vacancies (OV) as an intrinsic defect along the grain boundaries of polycrystalline BFO. As discussed and shown above in Fig. 2d, the dominant conduction mechanism is PF in HRS. The observed transient current is lower in HRS than in LRS, revealing that electron hopping is more difficult in HRS than in LRS. It is well known that the electron hopping in BFO occurs between Fe2þ and Fe3þ, thus more Fe2þ ions can cause a larger current [18]. This is consistent with theoretical predictions of the introduction of interband states upon transformation of Fe3þ to Fe2þ in the lattice. The defect chemistry in BFO can be expressed using Kr€ oger–Vink notation [19]. The presence of Fe2þ is also a prerequisite for charge compensation of the OV, i.e., more Fe2þ ions also imply more OV. When a positively charged OV attracts an electron, the Fe3þ in the vicinity of this OV is reduced to Fe2þ. Therefore, the OV act as electron trapping centers in the BFO thin film [20]. In LRS, more Fe2þ are present, which means more trapping centers are occupied by electrons, favoring electron hopping. During reverse biasing, the electrons are blocked at BFO/Al contact, such that far fewer electrons can be injected into the BFO thin film from the Cu top electrode. The depletion region extends as the reverse bias increases. The trapped electrons inside the BFO thin film are released and emitted into the bottom Al electrode, and iron ions are reduced from Fe2þ to Fe3þ, which destroys the electron hopping paths. The stack is therefore turned to HRS. It is obvious that BFO is ferroelectric. Therefore, we could not completely exclude the effect of polarization on the observed resistive switching. Choi et al. reported that the direction of bulk electric polarization can influence the diode behavior in a single crystal BFO thin film [8]. However, in our case, the relationship between the polarization and the transport property is difficult to investigate due to the large number of grain boundaries present in polycrystalline BFO thin films, which may dominate the transport. In addition, the lower voltage (Vset) of þ2.0 V for switching is not large enough to switch the polarization sufficiently; therefore, the effect of ferroelectric polarization does not dominate the resistive switching in our polycrystalline BFO thin films. Based on the above discussion, a simple model is proposed to explain the observed resistance-switching behaviors, as shown in Fig. 3. The distribution of OV in grain boundaries is different from that in grain interiors due to the defect structure. A fresh polycrystalline BFO film is most likely to be in an HRS considering the large grainboundary resistances. When a high positive bias voltage is applied, OV or trapping sites redistribute under the high electrical field and conduction paths might be formed across grain boundaries, as shown in Fig. 3a and b, thus the sample switches to the LRS. After a certain negative bias voltage is applied, the redistributed OV or trapping sites are recovered, which slowly degrades the conductive channels (Fig. 3c) Figure 3 Schematic diagram of details of filament-formation process along the grain boundaries of polycrystalline BFO thin film sandwiched between Cu (top) and Al (bottom) electrodes; (a) represents filament growth process during forward bias from 0 to þ10 V, (b) represents full filament formation at þ10.4 V, (c) filament decay occurs during reverse bias from 0 to 3 V and (d) complete filament rupture occurred at the bottom electrode. ß 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-a.com Original Paper Phys. Status Solidi A (2016) 5 Figure 4 Energy band diagram of the Cu/BFO/Al structure (a) before switching, and (b) photocarrier production under optical illumination and their separation in applied small forward voltage of þ2.0 V. and at higher negative voltages it then destroys completely (Fig. 3d), then the sample returns to the HRS. Based on the obtained results and theoretical analysis of the resistive-switching characteristics of the Cu/BFO/Al structure and modulation in switching characteristics due to photovoltaic effect, a band diagram is constructed as shown in Fig. 4. The output current in BFO was attributed to the presence of oxygen vacancies and due to which the BFO thin films can be thought of as an n-type semiconductor (the naturally produced oxygen vacancies that act as donor impurities in the bandgap) [19]. Normally, the rectifying behavior in semiconductors appears due to the existence of an energy barrier at the interface. The ideal Schottky barrier at a metal/semiconductor interface can be determined by the difference of the metal work function and the semiconductor electron affinity. The work function and bandgap of BFO is 3.3 and 2.2 eV, whereas, the work function of Cu and Al is about 4.3 and 4.1 eV, respectively. When BFO and Cu or Al are joined, some of the electrons in the BFO move spontaneously into the Cu or Al due to the higher Fermi level of BFO than that of Cu or Al, and leave behind positive charges in BFO. Then, depletion regions are formed by the positively charged regions in BFO near the bottom and top electrodes set up barrier heights at both interfaces (Fig. 4a) that are sufficiently large to inhibit charge transfer from the metals into the BFO conduction band, so whether the applied voltage is positive or negative, the BFO film always starts in HRS. Under normal (dark) conditions, at lower applied positive voltage (þ2.0 V) a very low current flows through the structure, which may be due to trapping some charge at the interface, leading to a “bending” of the band diagram, as shown in Fig. 4b. The significant shift (of 8.4 V) in the Vset observed under light illumination on a Cu/BFO/Al structure is evidenced from Figs. 1c and 2a, which is related with the bandgap (Eg) of BFO of about 2.1 eV (Supporting Information, online at: www.pss-a.com). The photon of wavelength 590 nm can stimulate the electrode-hole pairs in a BFO thin film. In general, the photogenerated charge carriers are localized and tightly www.pss-a.com bound, therefore, such charge carriers in the bulk of the BFO are expected to quickly recombine, resulting in no net photoeffect. When the light is incident at the domain wall, the significantly higher applied electric field enables a more efficient separation of these carriers, creating a net imbalance in charge carriers near the domain walls and resulting in the formation of a conducting track. These photogenerated carriers drift towards the respective electrode under an applied voltage via domain boundaries, which (have higher conductivities and) act as conducting paths for current flow in the Cu/BFO/Al structure. This eventually switches HRS of the structure to LRS. 4 Conclusions The resistive-switching properties of polycrystalline BFO thin film prepared by a sol–gel-assisted spin-coating method was investigated. The BFO films were found to show bipolar resistive-switching behavior. Based on the linear theoretical fitting of the experiment I–V data the conduction mechanisms are concluded to be Poole– Frenkel during switching from HRS to LRS and it stays in the Ohmic conduction region throughout LRS. A mechanism is also proposed to explain the transition among LRS and HRS. Charge traps composed of oxygen vacancies along the grain boundaries are considered to play a key role in forming the conducting paths. The microscopy and spectroscopy results reveal a uniform or crack-free BFO film surface with an optical bandgap of 2.1 eV. The resistive-switching experiment done under optical illumination of the Cu/BFO/Al structure reveals a substantial shift (of approximately þ8.4 V) in the Vset due to photogenerated charge carries, which has never been observed before. Our result suggests a new approach for decreasing Vset in BFO thin film using a photovoltaic effect in BFO for nonvolatile memory technology as well as for energy-related applications. This major finding may be applicable for future resistive-switching memory devices that may be known as optomemristive devices. Supporting Information Additional supporting information may be found in the online version of this article at the publisher’s web-site. ß 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim solidi status physica pss a 6 K. Bogle et al.: Optically modulated resistive switching in BiFeO3 Acknowledgments KAB would like to acknowledge the financial support partly from the UGC-BSR project (No. F. 20-9 (10)/2012(BSR)) sanctioned to him and the UGC XII plan grants to SRTMUN. The authors acknowledge Dr. P. B. Vidyasagar (ViceChancellor, SRTMUN) and Dr. A. L. Chaudhri for constant support and discussions. References [1] T. W. Hickmott, J. Appl. Phys. 33, 2669 (1962). [2] G. Dearnaley, A. M. Stoneham and D. V. Morgan, Rep. Prog. Phys. 33, 1129 (1970). [3] A. Beck, J. G. Bednorz, J. G. Gerber, C. Rossel and D. Widmer, Appl. Phys. Lett. 77, 139 (2000). [4] D. B. Strukov, G. S. Snider, D. R. Stewart and R. S. Williams, Nature 453, 80 (2008). [5] A. Sawa, Mater. Today 11, 28 (2008). [6] R. Waser, R. 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