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We report a remarkable photoinduced relaxation process and its dependence of thickness and temperature in mixed-phase BiFeO3 films grown on (001) LaAlO3 substrates. When the films are illuminated by the light above the bandgap, their resistances are reduced with the increase of temperature. The photoinduced change of resistance reaches to the maximum of about 2.17 × 105% at 300 K. It is noted that the relaxation processes of the resistance are significantly different between T-like phase and T-R mixed phase due to structural strain, symmetry breaking and built-in electric field at the phase boundaries. These results provide more insights into intrinsic mechanisms of mixed-phase multiferroic materials and potential applications in all-oxide photoelectric devices.
© 2016 Optical Society of America
1. Introduction
Multifunctional optoelectronic properties in transition metal oxides have attracted great attentions due to coupling effects between the electronic, spin, and structural degrees of freedoms and response characteristics [1–6]. Especially, multiferroics with the ferromagnetic and ferroelectric coexistence within one material have become the focus of much research owing to potential applications in memory devices, sensors and so on [7–10]. Meanwhile, the light, as a contactless external field, can interact with the materials, yielding some emergent physical phenomena, such as the photosensitive properties, photoferroelectrics [11,12], the tip-enhanced photovoltages at the nanoscale or the domain structures [13–21]. Furthermore, this allows us to detect the magnetic-electric effect with an additional degree of freedom and opens up an innovation research of multifunctional effect for next generation of optoelectronic devices. On the other hand, complex oxides with the peroviskite structure provide a novel platform for studying these effects. In these oxides, BiFeO3 (BFO) is the most extensively studied multiferroic material, showing both the ferroelectric and magnetic ordering above the room temperature due to the high ferroelectric Curie temperature (1100 K) and G-type antiferromagnetic Neel temperature (640 K). In addition, the BFO has outstanding photoresponsive properties, such as photostrictive properties under the illumination of visible light [22, 23], the photovoltaic effect [24–28], the photoconductivity [29, 30], the spatially resolved photodetection [31] and the giant ultrafast photoinduced shear strain [32]. Recently, the strain-driven phase boundary with its evolution has been observed in BFO films, which has the potential applications for lead-free system in the probe-based data storages and actuators [33, 34]. The epitaxial BFO films (pseudocubic lattice parameter of about 3.96 Å) sustain a large lattice misfit of about 4.4% when deposited on LaAlO3 (LAO) (001) (3.78 Å) substrates. Thereby, the crystal structure of BFO films induced by the strain exhibits a transformation into a mixed-phase state of rhombohedrally (R) and tetragonally (T) distorted monoclinics [35–37]. The mixed-phase BFO films have shown intriguing properties, many of which are different from the nonstrained BFO phase, such as the large piezo/ferroelectric response [38–40], the enhancement of ferroelectric retention [41], and the non-zero magnetic moment [42, 43]. Considering the significant changes in general physical properties, we can presume a deeper insight into the variations of photoinduced characteristics and the mechanisms in the mixed-phase BFO films by tracking the carrier dynamics of a relaxation process in the conduction. Here, we show a photoinduced relaxation process with its dependence of thickness and temperature in the mixed-phase BFO films grown on LAO substrates. In particular, we observe a change of about 3 orders of magnitude in the transient photoinduced effect, offering a promising opportunity in fabricating optoelectronic devices. These results contribute to fundamental understandings and future practical optical applications of multiferroic materials.
2. Experimental details
The pristine BFO films with different thicknesses of 20, 33, 47, 75, and 104 nm were deposited on (001)-oriented LAO substrates via a pulsed laser depostion method with a KrF excimer laser. During deposition, the BFO films were kept at the temperature of 680 °C and the oxygen pressure of 100 mTorr. We use similar conditions as those described in Ref [44]. The structures and the surface morphology of these films were analyzed by X-ray diffractometer (XRD) using Ni filtered Cu kα radiation operated (RigakuD/max-2400, the wavelength is 0.15432 nm) and an Asylum Research MFP-3DTM atomic force microscope (AFM), respecticely. The thickness of BFO films was measured using the SpecEI-2000-VIS ellipsometer. The transmittances of BFO films were measured by the U-3010UV-VIS spectrometer. The resistances of films were measured using a low-noise probe station and a current (DC) voltage source (6487 Keithley multimeter) at the temperature range from 80 to 300 K. The samples were illuminated by the light resources with the wavelengths of 473 nm (1.4 to 11.4 mW/mm2), 365 nm, 405 nm, 532 nm and 650 nm. Before the light illumination, we kept the samples in darkness for 24 h in order to accurately get the resistance.
3. Results and discussions
Figure 1 (a) presents the XRD patterns of BFO films with different thicknesses grown on (001) LAO substrates. It can be seen that BFO films are exclusively characterized by the (001) orientation without obvious impurity peaks. For the 20 nm film, only the diffraction peaks from the T-like phase are observed. The c-axis cell parameter of BFO film calculated from the (002) peak is about 4.627 Å, which is much larger than the bulk value of 3.960 Å. The strong peak of T-like phase suggests that T-like phase with small monoclinic distortion can be stabilized by a large lattice misfit. As the thickness approaches to 47 nm, the R-like phase of BFO appears, indicating the coexistence of T-like and R-like phases. With a further increase of the thickness, the intensity of the (002) peak corresponding to R-like phase increases sharply. Moreover, there is a systematic shift in the (002) peak position of the R-like phase toward lower angles, resulting in a gradual increase of the out-of-plane lattice parameter. As shown in Fig. 1(b), the out-of-plane lattice constants obtained from the x-ray diffraction increase with the increase of thickness, showing the lattices gradually relax by the formation of R-phase. Further evidences of the mixed phase could be confirmed by the changes in the surface morphology with increasing the thickness. In the mixed phase BFO films, the change of lattice constant is not only dependent on the lattice strain (or stress relaxation), but, more importantly, is also determined by the phase transition. As the thickness is increased, the pure T-like phase is changed into the mixed phase with the R-stripe-phase embedded into the T-phase matrix. To our best knowledge, the lattice constant of T-phase BFO film is 3.69 Å, while the R-phase one is 3.96 Å. It indicates that the out-of plane lattice of BFO film is more likely to be under a tensile strain instead of a compressive strain as the phase of BFO is changed from the pure T-phase into the T-R phase resulting from the increase of thickness. Eventually, this induces the increase of lattice constant by the formation of R-phase [45].
Fig. 1 (a) XRD patterns of BFO films with different thicknesses grown on (001) LAO substrates. (b) Out-of-plane lattice parameter as a function of thickness.
Figure 2 shows typical atomic force microscopy (AFM) topography images of the films with 2 µm × 2 µm scanning size, indicating a strong thickness-dependent surface morphology of the BFO films. There are two types of topography images: flat-like and stripe-like features. The flat-like structure is caused by T-like phase and the stripe-like structure originates from coexistence of T and R phase, which is consistent with other work [29, 45]. The stripe-like morphologies are observed in the BFO films once the thickness is larger than 47 nm. Combined with these results, we speculate that the value of the critical thickness is about 47 nm and the BFO films have the pure T-like phase structure below the value. For thicker films, the lattice strain is gradually relaxed, leading to the formation of the R-like phase. To our best knowledge, there are no literatures reporting the critical thickness of BFO deposited on LAO substrates. However, Z. H. Chen et al [45] have reported that the critical thickness is 50 nm for BFO films deposited on LSAO substrates, which is similar to our critical thickness of 47 nm.
Fig. 2 AFM images of BFO films on LAO substrates illustrating the thickness-dependence of the morphology. (a) 20 nm; (b) 33 nm; (c) 47 nm; (d) 75 nm; (e) 104 nm. Each image has the square size of 2 um × 2 um.
The results of AFM suggest the controllable transition of T and R phase depending on the thickness of BFO films. Due to strain effects induced by the distinguished thickness, BFO films suffer the transition from the pure T phase to the T-R mixed-phase. In these T-R mixed-phase films, the morphological phase boundary with the stripe shape emerges in the mixed-phase areas. The T-phase BFO is a metastable phase that can be stabilized through the strain. It is noteworthy that the ordering domain structure in the mixed-phase films would exhibit intriguing electric or optical responses due to the spatial symmetry of morphological phase boundary. First of all, we analyze the optical properties that can be affected by the ordering domain structure in the mixed-phase systems. Thus, a plot (αhν)2 against photon energy (hv) is shown in Fig. 3, in which the adsorption coefficient α is derived from -ln(T)/d, where d and T represent the thickness and the transmittance of BFO films at the same wavelength, respectively. It can be seen that the adsorption coefficients at a fixed value of hv are decreased as the films get thicker, indicating that the formation of mixed-phase suppresses the adsorption. The bandgap (Eg) can be obtained from a linear extrapolation of (αhν)2 versus hν plot to zero. The bandgap of BFO films as a function of the thickness is illustrated in the inset of Fig. 3. The bandgaps are increased from 2.416 eV to 2.529 eV with the thickness of BFO films increasing from 20 nm to 104 nm. This suggests that strain leads to different responses of the optical absorption in the mixed-phase BFO films. The thicker films have more R-phases and thereby exhibit larger values of bandgap. Additionally, it should be mentioned that the measured optical band gaps of BFO is lower than that previously reported [24, 46] (2.8 eV), which can be attributed to the larger in-plane lattice constants of BFO films that weaken the energy splitting of Fe 3d [24, 47–49]. We can see that the optical band gap has increased to 2.529 eV with c = 4.666 Å, which is very close to that previously reported [50–52].
Fig. 3 The variation of (ahν)2 with the photon energy for the BFO films grown on LAO substrates. Inset shows the bandgap of BFO films as a function of thickness.
Mixed phases in BFO films have the ability to be reversibly switched into T-phases by applying external fields, thus inducing a significant piezoelectric response due to the phase boundary motion [53]. In our considerations, the phase-mixture of BFO films induced by the strain would also lead to the photoelectric response because of the strong correlations between the phase boundary and local photoinduced electronic conduction. To emphasis this point, we investigate the photoinduced effect of BFO films illuminated by the above-bandgap blue light with the wavelength of 473 nm (2.62 eV) and the intensity of 11.4 mW/mm2. As shown in Fig. 4(a), the films are exposed to the light for 20 s, and then recovered for 60 s in darkness at room temperature. The resistances of films under the illumination are immediately reduced to lower values and then recover quickly to the original values when the light is switched off. The on/off state has perfect repetition when the films are illuminated again. As shown in Fig. 4(a), the thickness plays a crucial role in affecting the photoinduced conductive properties and the larger photoinduced change of resistance arises in the thicker films under the illumination. The conductive feature is related to both the microscopic electronic state and the morphological phase mixture. As mentioned above, the mixed-phase exhibits the stripe shape with the form of periodical domain patterns embedded into the T-phase matrix. Phase boundaries within the length scale of several unit cells are created during the phase transition between R and T phases. Generally, the strain gradients at the phase boundaries could change the octahedral tilting and the Fe-O-Fe bonding angles, which are responsible for the modulation of orbital overlap between Fe 3d and O 2p orbitals [35]. Thus, they determine the conductivity of BFO films under the illumination. More R-phases possess more phase boundaries and generally exhibit the smaller resistance [35]. Meanwhile, the strain gradients could produce exotic dipole-charged domain walls. The dipolar charge induces a larger built-in electric field that effectively separates photoinduced electron-hole pairs at the phase boundaries, yielding the enhanced photocurrent [54]. In addition, more localized electrons are maintained in the periodical domain of mixed phases. The successive ordering domains could open a new channel of charge transfer, allowing localized electrons to jump from the valence band of O2p and Fe3d to the conduction band of Bi6p. Therefore, the resistances of films under the illumination decrease with the increase of thickness. The photoexcited carrier relaxation dynamics can be described by the recovery process of photoinduced effect [55, 56]. Due to the electron-phonon coupling, the high-energy photogenerated carriers accumulated in the conduction band of Bi6p release their energy through the emission of longitudinal-optical phonons as the light is off. Then, the energy of the photoexcited carriers is rapidly reduced to the minimum in the conduction band. Additionally, the energy exchange between the lattice and spin system affects the relaxation process. Thus, the resistance decay curves can be fitted by the Kohlrausch expression in terms of stretched exponential function [57, 58],
where R0 represents the initial resistance without the light illumination, A is a prefactor standing for the difference of resistance between the on and off conditions, τ is the time constant of relaxation and β is the relaxation order (0 << 1). Figure 4 (b) shows an example of the fitting curve of resistance of film with the thickness of 104 nm at room temperature using Eq. (1). The fitting curve is in good agreement with the experimental values. Figure 4(c) shows the τ and β obtained from the fitted curves as a function of the thickness of BFO films. The τ increases with the increase of thickness, while the β exhibits a reverse trend. The T-like BFO films always relax quickly in comparison with the R-like ones due to the structural strain and symmetry broken structures of T-like phase [50]. In addition, the large built-in electric field [54] at the boundaries of two phases easily separates photoinduced electron-hole pairs, hindering the recombination. Thus, the films with the mixed phases have the longer relaxation time. The relaxation order of β decreases with the phase transition from T-like phase to mixed phase due to the enhanced structure symmetry of mixed phase.Fig. 4 (a) The resistance of BFO films illuminated at room temperature with a 473 nm light at the intensity of 11.4 mW/mm2 as a function of time. (b) The resistance vs. time fitting curve of film with the thickness of 104 nm at room temperature using Eq. (1). (c) Time constants (τ) and the relaxation order (β) of BFO films at room temperature as a function of thickness when the light is off.
To further investigate the effects of temperature and the wavelength on the photoresponse, the mixed-phase film with the critical thickness of 47 nm is selected. Firstly, we perform the current-voltage behavior for the BFO film under the illumination, as shown in Fig. 5. Inset is the measuring schematic diagram of film. The straight line suggests that the interface is the Ohmic-type contact, which is identical to that of other measured BFO films. The film presents a lower dark current of about 0.2 pA with the voltage ranging from −10 V to 10 V and the current is increased drastically under the illumination. The photocurrent is about 3 orders of magnitude higher than the dark current at the same bias, suggesting the strong sensitivity to the light for the mixed-phase BFO film. The resistance of film as a function of time at different temperatures and the temperature-dependent resistance of the film with and without the illumination are shown in Figs. 6(a) and 6(b), respectively. After being illuminated by the light, the resistances of BFO film nonlinearly decrease with the increasing temperature from 1.9 × 1011 Ω at 80 K to 1.4 × 1010 Ω at 300 K. The temperature dependence of the photoinduced relative change in the resistance is shown in the inset of Fig. 6(b). Here, the photoinduced relative change in the resistance is defined as, where RD and RL represent the resistance of BFO film in darkness and under the illumination, respectively. The values of ΔR/RL increase with the increase of temperature and reach to the maximum of about 2.17 × 105% at 300 K. Based on the broken structural symmetry [50], the BFO film undergoes the phase transition from the T-phase to the T-R mixed phase with the increase of temperature, which would yield the enhanced photoconductive response due to the increasing fraction of R phases. It should be mentioned that the result is completely different from that in SrTiO3 [59], which shows a decrease in the photoinduced relative change with the temperature owing to the thermal fluctuation. According to the report of S. R. Basu et al. [30], we regard that the BFO thin films can be considered as an n-type semiconductor because of the oxygen vacancies. Actually, we adopted the similar preparation conditions in the experiments. Thus, the BFO films exhibit the n-type conduction. The specific conduction mechanism can be explained using the energy band diagram shown in Figs. 6(c) and 6(d), where Ec, Ev, Efn, Et1, and Et2 denote the conduction band, the valence band, the quasi Fermi level, the shallower subband level, the deeper subband level, respectively [29]. When the BFO film is illuminated by the light with the photon energy larger than the bandgap, nonequilibrium free carriers generated from the valence band would be trapped at the subband levels at lower temperatures. With the gradual increase of temperature, the Efn shifts towards the valence band (Fig. 6(d)), meanwhile the fraction of R phase increases in the BFO film. The large built-in electric field at the boundaries of two phases contributes to the separation of photoinduced electron-hole pairs, which results in a decrease of the resistance with an increase in the temperature. When the temperature is raised up to 140 K, the Efn begins to cross over the shallower subband level Et1, leading to the significant release of trapped carriers in the subband level to the conduction band (Fig. 6(d)). Subsequently, the carriers attain the thermal equilibrium and accordingly the resistance of BFO films shows a decreased jump at 140 K, as shown in Fig. 6(b). As mentioned above, the resistance recovery is related with the photoexcited carrier relaxation dynamics. Therefore, we make a fit to the resistance relaxation at different temperatures using Eq. (1). The fittings are in well accordance with the experimental data. We further study the mechanism of recovery process through investigations on the temperature dependence of the τ and β obtained from the fittings. As shown in Fig. 6(e), the values of τ and β are strongly related to the temperature. τ shows an increase, while β decreases with the increase of temperature. With the increase of temperature, the quasi Fermi level Efn is decreased, reducing the recombination center and suppressing the recombination process. Hence, it yields a longer lifetime of the photoexcited recovery process. Additionally, it is further supported that the BFO film possesses more R-phases at higher temperatures, resulting in the longer lifetime of excited electrons because of the built-in electric field at the phase boundaries. Furthermore, we fit the data using the Arrhenius equation [60] given as,, where ΔE and kb are the activation energy for detrapping the photogenerated carriers and Boltzmann constant, respectively. Figure 6(f) shows the plot of ln(τ) against (1/T). The red solid lines are the fitting curves, which show a good agreement with the experimental data. It is clear that there are two obvious temperature regions of activation. At low temperatures (T < 140 K), the calculated value of ΔE1 is about 4.97 meV. Nevertheless, the ΔE2 is estimated as 11.02 meV at higher temperatures. The activation energy (ΔE2) of the deeper subband level is 2.2 times higher than the activation energy (ΔE1) of the shallower subband level. The difference of the activation energy can be attributed to the different depths of the localization at the subband of BFO film at different temperatures.
Fig. 5 The current-voltage behavior for the BFO film of 47 nm in darkness and under the illumination. The measuring schematic diagram of film is displayed in the inset.
Fig. 6 (a) The resistance of the 47 nm BFO film at different temperatures as a function of time with the light (11.4 mW/mm2) at a time period of 80 s each. (b) The resistance of the film versus the temperature in the dark and under the illumination, inset is the temperature dependence of the photoinduced relative change in the resistance. (c) and (d) The energy band diagram of the BFO film. (c) The film is under illumination at low temperatures, and the red and blue dots show electron and holes, respectively; (d) Efn moves toward the middle of the band gap at higher temperatures. (e) Time constants τ and the relaxation order β of the film as a function of temperature when the light is off. (f) Logarithm of the time constant τ as a function of the reciprocal temperature. The red solid lines are the fitting curves.
As shown in Fig. 7, we also investigate the photoresponse of the film illuminated by the 473 nm light at different light intensities (1.4, 2.8, 5.7 and 11.4 mW/mm2) and different wavelengths (650 nm, 532 nm, 473 nm, 405 nm and 365 nm) at room temperature. The resistances are decreased at the higher light intensities and lower wavelengths. In contrast, the below-band-gap excitation using the 532 nm and 650 nm light yields no observable change in the resistance because their photon energies are lower than the bandgap, suggesting an important role of optoelectronic excitation. The photoinduced relative changes in the resistance of film as a function of light intensities and wavelengths are shown in Fig. 8. The values of ΔR/RL are nonlinearly related with the light intensity and increase obviously when the light intensity is higher than 5.7 mW/mm2. The values of ΔR/RL of BFO film sharply are enhanced with the decrease of wavelength. Results indicate that the high energy of external optical field induces more remarkable photoresponses in mixed-phase BFO films. More experimental and theoretical researches are expected to further reveal the underlying mechanisms.
Fig. 7 The ON/OFF state of the 47 nm BFO film at different light intensities as a function of time at a time period of 80 s each. (b) The ON/OFF state of 47 nm BFO film illuminated by the light with different wavelengths as a function of time at a period of 80 s each.
Fig. 8 The photoinduced relative changes in the resistance of BFO film as a function of the light intensities and wavelengths.
4. Conclusion
In summary, we have investigated the optical properties and photoresponses of BFO films with the phase mixture. The photocurrent is about 3 orders of magnitude higher than the dark current at the same bias. The resistance recovery process agrees well with the exponential function. Our work shows that the photoresponse and relaxation process of BFO films under the illumination can be tuned through the thickness, the temperature, and even the photon energy of light. These results favor further understandings of multifunctional materials and future practical applications in all-oxide optoelectronic devices.
Acknowledgments
This work is supported by the National Natural Science Foundation of China (Nos. 51202195, 51572222 and 51172183), and the Graduate Starting Seed Fund of Northwestern Polytechnical University (No. Z2015154).
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