Maocai Wei*ab,
Meifeng Liub,
Lun Yangb,
Xiang Lib,
Yunlong Xieb,
Xiuzhang Wang*b,
Zijiong Lia,
Yuling Sua,
Zhongqiang Huc and
Jun-Ming Liubd
aSchool of Physics and Electronic Engineering, Zhengzhou University of Light Industry, Zhengzhou 450002, China
bInstitute for Advanced Materials, Hubei Normal University, Huangshi 435002, China
cElectronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Xian Jiaotong University, Xian 710049, China
dLaboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China
First published on 22nd April 2020
A single device with extensive new functionality is highly attractive for the increasing demands for complex and multifunctional optoelectronics. Multi-field coupling has been drawing considerable attention because it leads to materials that can be simultaneously operated under several external stimuli (e.g. magnetic field, electric field, electric current, light, strain, etc.), which allows each unit to store multiple bits of information and thus enhance the memory density. In this work, we report an electro–opto–mechano-driven reversible multi-state memory device based on photocurrent in Bi0.9Eu0.1FeO3 (BEFO)/La0.67Sr0.33MnO3 (LSMO)/0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) heterostructures. It is found that the short-circuit current density (Jsc) can be switched by the variation of the potential barrier height and depletion region width at the Pt/BEFO interface modulated by light illumination, external strain, and ferroelectric polarization reversal. This work opens up pathways toward the emergence of novel device design features with dynamic control for developing high-performance electric–optical–mechanism integrated devices based on the BiFeO3-based heterostructures.
BiFeO3 (BFO) is a multiferroic materials with robust ferroelectric and magnetic orders above room temperature15–17 and a narrow optical bandgap (∼2.5 eV)18,19 than traditional ferroelectric photo-voltaic materials (such as LiNbO3,20 BaTiO3,21 (Pb,Zr)TiO3,22 with quite large bandgap Eg > 3.0 eV). It offers considerable light absorption capability and endows with novel physics and new degrees of freedom for multifunctional devices. However, many efforts have been devoted to enhancing the photovoltaic (PV) of BFO,18,23–28 but unfortunately the power conversion efficiency is still poor (<0.2%) compared with other types of solar cells.28 Thus, until recently ferroelectric photovoltaic (FE-PV) effect is still in its infancy rather than any real application. Alternatively, not just limited in the power conversion efficiency, the signal of photovoltaic effect can allow for information transfer and storage applications.29,30 A unique characteristic of FE-PV devices is that the photocurrent direction can be switched by reversing the spontaneous polarization direction of a FE material with electric field.18 This feature is compatible and of central importance for the development of on-chip optical communication technology.
More significantly, since Choi et al. first reported the switchable ferroelectric diode and visible light-induced photovoltaic effects in BFO single crystals,23 the coupling of ferroelectric polarization with optical properties in photo-active ferroelectrics has received a renewed attention, triggered notably by low bandgap ferroelectrics and original photovoltaic effects.31–34 Interestingly, experimental and theoretical investigations have evidenced that the bandgap and polarization of a perovskite-type ferroelectric thin film can be tuned by substrate-induced strain.31–34 Fu et al. realized a maximum bandgap shift of 0.7 eV by growing BFO films on SrTiO3 substrates with various in-plane compressive strain.33 Sando et al. reported that BFO films deposited on different substrates can accommodate different kinds of strains from compressive one to tensile one.34 However, these strain control schemes cannot rule out the effects of other extrinsic factors (oxygen non-stoichiometry, defects, dead layer, disorder, etc.) on the photovoltaic effects.
Alternatively, ferroelectric Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) single crystals are well known for their ultrahigh piezoelectric coefficient, large electromechanical coupling, and excellent ferroelectric and pyroelectric responses.35 Numerous experimental works have demonstrated that the in-plane strain of oxide films epitaxially grown on (1 − x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (0.28 ∼ x ∼ 0.33) ferroelectric substrates can be in situ and dynamically manipulated by applying an electric field to the PMN-PT substrates, and thus the aforementioned various influences can be avoided to a great degree.10–12,35–39
Along this line, an emergent scheme is to fabricate multifunctional devices that can be achieved by combining a series of monofunctional components into a tight space.40 However, as monofunctional devices mature and approach their fundamental limits, a pivotal question now is how to develop technologies enabled with increasing functionalities and reduced product size. A better and highly concerned scheme is to integrate multiple functionalities into one material for multi-parametric detection. If it applies, the device structure would be significantly simplified, benefiting to the fabrication complexity reduction, integration level boosting, and possible application expansion.
So far, a comprehensive understanding of the combined electric-field, light, and mechanical control of FE-PV and the resultant functionalities, both in situ and dynamically, are still limited for BFO thin films. Moreover, there is an urgent demand for optoelectronics devices with a multifunctional integration and minimal size, which can satisfy the demand for fast, high density, high energy efficiency data processing/storage. There is no doubt that a systematic investigation of multi-field tuned physical properties of BFO thin films is highly expected. In this work, we report an electro–opto–mechano-driven reversible and multi-state memory device, based on photocurrent effect in Bi0.9Eu0.1FeO3 (BEFO)/La0.67Sr0.33MnO3 (LSMO)/PMN-PT heterostructures. The main property to be targeted for such a multi-field tuning is the photo-induced electronic conduction. For a high-performance consideration, we focus on BEFO thin films rather than BFO simply due to the fact that a slight Eu substitution at the Bi site would benefit to the ionic defect and leakage suppression.
Our comprehensive investigation suggests that the photo-current Jsc of our heterostructures can be effectively switched by modulating the potential barrier height and depletion layer width at the Pt/BEFO interface, by means of various stimuli such as light illumination, strain, and ferroelectric polarization. More importantly, the device can produce photocurrent that allows us to read out the state in a self-powered manner. This discovery provides the means for engineering tunable single memory unit but enabling multi-functionalities such as switching between the volatile and nonvolatile modes by electric–optical–mechanical combination mode. This mode and associated functionalities open the door to novel multifunctional storage devices.
Herein, we propose a memory micro-array based on the Pt/BEFO/LSMO/PMN-PT heterostructures to demonstrate our idea. In this prototype, “0” state and “1” state represents the Pup state and Pdown state of the BEFO film layer, respectively. The writing of “0” or “1” state in each cell can be easily realized by reversing the spontaneous polarization direction of the BEFO film with electric field. During reading the data, an incident light is irradiated to the whole micro-array, meanwhile, the induced Jsc is directly detected for each cell at a zero-bias. Moreover, different polarization state produces different Jsc, the data state in each cell is thus read out. This data storage mode can be called as “electrical writing and optical reading”, which consumes fewer power than conventional modes. We can also develop a logical switching device that a unit cell with Pup (or Pdown) state, its logic state will be alternately switched to ON or OFF state when the incident light is on or off, then be read out by detecting the corresponding Jsc. What's more, this kind of electronic device has much room to improvement when the light was turned on and off with and without a certain strain applied to the unit cell, which could get more reliable, larger/smaller, and switchable Jsc.
The structure of the as-prepared BEFO/LSMO/PMN-PT heterostructure was characterized by X-ray diffraction (XRD, Bruker D8 Advance). The ferroelectric domain structures were probed by the piezoresponse force microscopy (PFM, Bruker Multimode 8) using the Pt-coated Si cantilevers. For the strain control, we applied electric field across the piezoelectric PMN-PT substrate and induced the in-planar lattice distortion (strain) which was transferred into the BEFO thin film above the LSMO bottom electrode. A voltage source was employed to supply an electric field across the PMN-PT substrate through the LSMO bottom electrode and the In back-electrode on the back side of PMN-PT substrate (as shown in Fig. 1(a)). It is noted that the positive bias was applied to the In electrode and the ground was applied to the LSMO electrode in our measurement.
For the transport behaviors of the as-prepared BEFO thin film, the current–voltage (J–V) curve was measured across the BEFO film using the Keithley 4200 characterization system with the voltage sweeping mode. Here, the top Pt electrode with a 0.5 mm × 0.5 mm square shadow mask was deposited on the BEFO film surface using the magnetron sputtering technique. The positive (negative) voltage was defined as the positive (negative) bias applied to the top Pt electrodes.
For the light illumination, a diode laser with a wavelength of 405 nm (hv = 3.06 eV) and power density of 200 mW cm−2 was used as the illumination source. The measurement setup schematic is shown in Fig. 1(a), where photo-current Jsc was obtained using the following mode. First, the sample was poled by a voltage pulse VP of 100 ms in width in the dark or light illumination condition. Second, the sample was submitted to a low voltage sweeping from −0.5 V to 0.5 V during which the J–V curve was obtained, noting that a voltage of ±0.5 V is insufficient to change the initial polarization state. Third, the value of Jsc was extracted from the J–V curve at V = 0. Certainly, Jsc can be also directly probed in the illumination ON/OFF testing (Jsc = 0 in the OFF case).
A further discussion on the strain state in the BEFO thin film begins by looking at the lattice mismatch of 1.58% between the film and substrate. The BEFO film has smaller OP lattice constant than that of bulk BFO (∼2.80 Å),41 revealing that the BEFO film is subjected to an OP compressive strain (∼−0.11%). Similarly, the OP lattice constant of the LSMO film is smaller than that of bulk LSMO (∼2.736 Å),42 revealing that the LSMO film is subjected to an OP compressive strain (∼−0.44%). According to the Poisson equation, the OP strain can be deduced as εOP = −2νεIP/(1 − ν), where εOP and εIP are the OP and in-plane (IP) strains and ν is the Poisson's ratio, given the condition of constant unit cell volume. The Poisson's ratios for the BEFO and LSMO films are ∼0.34 (ref. 43) and ∼0.37 (ref. 44), respectively. Thus, the estimated IP tensile strains for the BEFO and LSMO films are ∼0.10% and ∼0.37%, respectively. These data suggest that the lattice strain in the PMN-PT substrate could largely be coherently transferred into the BEFO film.
The PFM phase imaging also clearly identified that the self-polarization of the fresh state is oriented upwards, which can be seen from the indistinct colour contrast between the ±10 V switching zone and the non-switching zone. This preference is believed to originate from the lattice mismatch induced residual stress in the thin film. It is noted that the fresh BEFO film was in the OP compressive state, leaving the positive and negative polarization charges to be created at the Pt/BEFO and BEFO/LSMO interfaces, as evidenced by earlier reports.45
First, at zero bias, no open-circuit voltage Voc and no short-circuit current Jsc can be observed in the dark, as shown in Fig. 3(a). However, the BEFO film under the light illumination shows significant photovoltaic responses with Voc = −0.7 mV and Jsc = 24.4 μA cm−2, suggesting that the built-in electric field can be affected by the illumination due to the photo-generated charges. Such an effect shows good retention, supported by the modulated photocurrent Jsc as a function of time t by the light ON/OFF operation with an interval of 20 s, as shown in Fig. 3(b).
Second, to investigate the polarization-modulated photovoltaic effect, the BEFO layer in the heterostructure was pre-poled using a pulse of VP = 10 V and −10 V (pulse width 100 ms) so that the BEFO film had the down- and up-polarization states, respectively, noting that the PMN-PT substrate remained to be unpoled. Here, the positive (negative) voltage is defined as the positive (negative) bias applied to the top Pt electrodes, and the positively and negatively poled states represent the up- and down-states, respectively. As shown in Fig. 3(a), the Voc and Jsc values for the up-state increase up to −0.9 mV (by −28%) and 82.8 μA cm−2 (by 2390%), respectively. For the down-state with remnant polarization Pr−, the measured Voc and Jsc changed their signs and became 0.6 mV (by 186%) and −70.1 μA cm−2 (by −3870%), respectively.
Third, the measured Jsc shows the opposite responses for the thin film in the up-state and down-state, as more directly seen in Fig. 3(c), by turning the light ON/OFF with an interval of 20 s, respectively. Here, it is possible to determine the polarization direction (stored information) by sensing the value of Jsc, and this process is non-destructive. Subsequently, we performed a series of measurements in which different pulse voltages (VP) to pole the BEFO thin film were set and the measured Jsc–VP hysteresis loop is plotted in Fig. 3(d). This loop exhibits hysteretic behaviors similar to the P–V loop shown in Fig. 2(d), indicating that the ferroelectric polarization of the BEFO thin film plays a key role in the switchable photovoltaic effect. The asymmetric Jsc–VP curves also suggest that the photovoltaic response was dominated by the polarization modulated interfacial barriers rather than the bulk photovoltaic effect (BPE),46 noting that the bulk effect dominated effect would produce a symmetric Jsc–VP curve.
As a result of the substrate piezoelectric strain, the photovoltaic properties can be modified due to the variation of strain state inside the BEFO film. Fig. 4(b) shows the measured J–V curves of the BEFO film in the dark and under light illumination (λ = 405 nm), given that the PMN-PT substrate was in the unpoled and poled states respectively for a comparison. In this experiment, the BEFO film was in the unpoled state with remnant polarization Pr0 in this measurement. Obviously, the J–V curve of the fresh BEFO film does not exactly pass through the origin in the dark, as shown in the inset of Fig. 3(a), indicating a built-in electric field in the heterostructure. Under the light illumination, the fresh BEFO film showed significant photo-voltaic response, also an indirect evidence for the built-in electric field.48 The measured Jsc ∼ −1.6 μA cm−2 and Voc ∼ 0.1 mV were obtained, respectively, and the sign changes of Voc and Jsc suggest that the built-in electric field can be affected by the illumination due to the photo-generated charges.30,49 The observed zero-bias photocurrent density was −1.6 μA cm−2, which is superior to most active ferroelectric oxide BiFeO3 (∼0.4 μA cm−2).50 Moreover, the M − H loops at different poling states have shown (Fig. S1†) that the magnetization of the heterostructure can be manipulated by the electric field induced piezo-strain, which could be used as evidence for magnetoelectric coupling effect at room temperature. However, we may need further research to understand this effect in depth.
Now we come to see the consequence of applying an electric field of +10 kV cm−1 to the PMN-PT substrate. The measured Voc and Jsc significantly increased up to 1.8 mV and −7.2 μA cm−2, resulting in an enhancement by 1700% and 350%, respectively. These effects are much larger than earlier reported data for Fe3O4/PMN-0.29 PT (011) heterostructure where the modulation magnitude for magnetization is only <3% at 300 K and that for resistance is also only <3% at 300 K.51 Certainly, it is highly desirable to achieve a sufficiently large modulation of the concerned properties in a reversible and nonvolatile manner at room temperature, and thus the present work represents a substantial step to practical applications.
To check the stability and reliability of this piezoelectric strain induced effect, we measured Jsc as a function of time by turning the light ON and OFF, given that the PMN-PT substrate was under zero electric field and non-zero field E = +10 kV cm−1. The results are plotted in Fig. 4(c). It is seen that the measured Jsc under E = +10 kV cm−1 was much larger than that under E = 0. The magnitude of Jsc under E = +10 kV cm−1 is even more stable than the case under E = 0, upon turning the light on and off. These effects can be further demonstrated by turning ON/OFF the electric field applied to the PMN-PT substrate between E = +10 kV cm−1 and E = 0, given the on-state of the light illumination. As seen in Fig. 4(d), the sequence of E = +10 kV cm−1 and E = 0 did make the response of Jsc sharply and repeatedly. All these results demonstrate that the piezoelectric strain from the substrate can be used to modulate the photovoltaic effects in the present heterostructure.
However, it should be mentioned that the difference in Jsc among the four light illumination off states is technically indistinguishable, and therefore usually the four states are treated as one high-resistance state (Jsc ∼ 0). In consequence, we can access the five states rather than eight states: Jsc(↑, 0, on), Jsc(↓, 0, on), Jsc(↑, 1, on), Jsc(↓, 1, on), Jsc(off). Fig. 5 presents one set of measured data for multi-field control.
From Fig. 5(a and b), it is clearly shown that the five states can be easily accessed by simply switching on/off the light-illustration, the electric field applied to the PMN-PT substrate, and the ↑↓ ferroelectric polarization states. For a quantitative estimation of the photocurrent switching performance, one may define the equivalence-of-merit (EOM) as:
(1) |
First, it is interesting to note that the EOM↑ and EOM↓ can be as large as ∼1130% and 1200%, respectively, representing the largest values so far reported in literature. Second, it is further demonstrated that the photocurrent switching on/off can be safely and reliably realized by either switching on/off the electric field applied to the PMN-PT substrate (piezoelectric switching on/off), or switching the ferroelectric polarization of the BEFO thin film between Pr+ and Pr−, or even switching both simultaneously. Whether with or without the application of E = +10 kV cm−1 to the PMN-PT, the signs of Jsc are always positive for Pr+ and negative for Pr−, respectively (as shown in Fig. 5 (a and b)). Moreover, when applying an electrical field of E = +10 kV cm−1 to the PMN-PT, the value of Jsc are reduced for Pr+ and enhanced for Pr−, respectively. Just looking at the data in Fig. 5 indicates that the light turn-on can make the magnitude of Jsc to rapidly rise up to 0.124 mA cm−2 and −0.088 mA cm−2 for the ↑ and ↓ states respectively, from the black state with Jsc ∼ 8.0 μA cm−2. Furthermore, sharp response to the on/off switching of the substrate piezoelectric strain is also clearly identified.
Obviously, once the piezoelectric strain is applied, the negative photocurrent (for Pr−) will be enhanced, the positive photocurrent (for Pr+) will be reduced. The underlying mechanism is dominated by the enhanced (or reduced) the barrier of the Pt/BEFO film by the piezoelectric potential, and this enhanced (or reduced) the barrier does suppress (or enhanced) the photovoltaic effect of the heterostructure. Such a stable and distinct electric-field- and light-driven ferroelectric photovoltaic effect in the heterostructures may provide a pathway toward multistate memory and electro-optical devices.
To understand this behavior, it is noted that Fu et al. systematically investigated the relationship between the bandgap (Eg) and the in-plane stress (σxx) for the BFO film, and it was found that the Eg as a function of σxx can be described by the following relation:33
Eg = 2.71 + 0.67σxx, | (2) |
From eqn (2), the electric-field-induced shift in bandgap ΔEg can be written as:
ΔEg(E) = 0.067[σxx(E) − σxx(0)], | (3) |
σxx(E) = σxx(0) + YdeffE, | (4) |
Y = c12 − c11(c11 + c12)/2c12, | (5) |
(6) |
To further understand the electro- and mechano-driven photovoltaic effect, we sketch a schematic of the multi-field coupling and interfacial band diagrams in Fig. 6. In order to determine the energy band alignment of BEFO, we carried out ultraviolet photoelectron spectroscopy (UPS) measurement (the results are shown in Fig. S2†). The band-gap and electron affinity of BFO are 2.8 eV and 3.39 eV respectively.53 The work function of LSMO and Pt are 4.7 eV and 5.3 eV.54 Thus, a Schottky barrier can be developed at the Pt/BEFO interface, while the BEFO/LSMO interfacial band is nearly flat. On the other hand, due to lattice mismatch-induced residual stress, the fresh BEFO film is in the compressive strain state. This results in the positive and negative polarization charges created at the Pt/BEFO and BEFO/LSMO barriers. It is the reason why the self-polarization of the fresh BEFO film was oriented upwards.
The piezoelectric potential can be generated, which can modulate effectively the carrier transport and subsequently the energy band at the interface. The so-called piezo-phototronic effect, which causes the interfacial band bending via piezoelectric strain, can also modulate the interfacial carrier transport behavior. Under the light illumination, the photo-generated carriers are excited from the BEFO layer, and then separated by the net built-in electric field (Ebi) which is the summation of the built-in field (Ein) based on the Pt/BEFO Schottky barrier, the piezoelectric field Epiez induced by piezoelectric polarization charges, and the depolarization field Edp caused by ferroelectric polarization.
It is well known that the ferroelectric polarization plays a dominant role in the transport of the metal/ferroelectrics/metal structure. For the BEFO with the downward polarization, the depletion region at the Pt/BEFO interface becomes wider and the band bending goes up. The direction of Ebi is upward aligned, generating a negative photocurrent and a positive photovoltage. On the other hand, for the BEFO film with the upward polarization, due to the positive polarization charges, the depletion region at the Pt/BEFO interface becomes narrower and the band bending goes down. The direction of Ebi is downward aligned, generating a positive photocurrent and negative photovoltage.
If the polarization is reversed, the direction of built-in field in the Pt/BEFO interface can be reversed. Therefore, the signs of Jsc and Voc are opposite for the two different polarization states in the Pt/BEFO/LSMO/PMN-PT heterostructure (as shown in Fig. 3(b)). Moreover, the higher Schottky barrier at the Pt/BEFO interface provides a larger built-in field as a driving force to separate the photo-generated electron–hole pairs more efficiently. Therefore, the larger magnitudes of Voc and Jsc in the Pt/BEFO/LSMO/PMN-PT heterostructure with the upward polarization were observed experimentally.
The modulation of the photocurrent by strain can be also discussed following this scheme. If a strain is applied, a certain amount of negative or positive piezo-charges will gather at the Pt/BEFO and BEFO/LSMO barriers, respectively. These piezo-charges can adjust the height and width of the Pt/BEFO barriers. When the direction of Epiez is parallel to Edp, the transport of photon-generated holes to the bottom of the BEFO layer will be enhanced. If the direction of Epiez is anti-parallel to Edp, the mobility of photon-generated holes would be suppressed.
Meanwhile, if the BEFO thin film is submitted to a compressive strain, the reduced Eg in BEFO will result in the release of more electrons from oxygen vacancies to the conduction band under the UV light illumination, leading to the enhanced carrier density. Conversely, if the BEFO film is in the tensile strain state, the enlarged Eg in BEFO reduces the carrier density. Given that tensile and compressive strains result in opposite electric field in a piezoelectric material, as negative and positive piezo-potential is applied to the BEFO film. Consequently, a permanent and controllable internal electric field can be introduced to reversibly modulate the height of potential barrier and width of depletion region at the Pt/BEFO interface by piezo-phototronic effect and ferroelectric polarization effect. Such an electric–optical–mechano driven photocurrent in a single device with diverse functionalities may provide a pathway toward multi-state memory and electro-optical devices.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra00725k |
This journal is © The Royal Society of Chemistry 2020 |