1. Introduction

Due to strong couplings between charge, spin, orbit, and lattice in manganites with a perovskite-type structure, a variety of energetically competing electronic phases with different electric and magnetic properties coexist, leading to rich exotic effects. External stimuli from magnetic field, electric field, light illumination, temperature, pressure, and so on can break the subtle phase balance, resulting in the significant changes in the electric and magnetic properties [1–18]. One of the most famous phenomena is the so-called colossal magnetoresistance (CMR), which is viewed as a magnetic-field induced insulator-to-metal (IM) transition. In this sense, these external driving forces can be used as the tuning parameters to modulate the properties of the manganites, which is of special interest from the application viewpoint. However, many effective stimuli work at low temperature, which restricts its application. Much effort has been concentrated on obtaining effective properties change using relatively small external stimuli at room temperature, such as room-temperature magnetoresistance effect [19, 20].

Recently researches have proved that the tilted manganite film (the epitaxial orientation of the film is tilted to the film surface normal due to the miscut substate) shows picosecond photoresponses for the laser pulse at room temperature [21]. Due to the tilted structure, photoinduced nonequilibrium carriers can be separated spontaneously in the diffusion process without any applied bias leading to a transient lateral photovoltage [21]. Such transient photovoltaic characteristics enable us to develop ultrafast optoelectronic devices work at or above room temperature, such as photodetectors and photoswitches. To improve photovoltaic sensitivity of manganite-based photodetectors and also to meet the demand for useful photoelectronic devices operated at and above room temperature, researches on the external stimulus effect at room temperature should be done to improve their performance. Previously we presented the voltage-tunable transient photoresponse properties in a tilted La_0.4Ca_0.6MnO₃ film [22]. The applied external electric field can improve or depress the sensitivity of transient photoresponse, but at the same time it also increased the lifetime of nonequilibrium carrier, leading to an increase of response time or even worse—the breakdown current will break the devices. In this paper, we reported the enhancement of transient photovoltaic responses at room temperature by pulse current stimuli in a tilted manganite film. By applying a pulsed current stimulus before laser irradiation, we observed that the photovoltaic signal displays notable enhancement. Nucleation of metallic filamentary paths induced by the pulse current play an important role in the enhancement effect.

2. Experimental details

A La_2/3Ca_1/3MnO₃ (LCMO) thin film was deposited on a miscut MgO substrate by facing-target sputtering technique from stoichiometry targets [22]. The miscut substrate means that the substrates were cut with a tilt angle α with respect to the [001] direction (Fig. 1(a) ). During deposition, the substrate temperature was being kept at 680 °C and oxygen partial pressure was ~30 mTorr. The film thickness of ~100 nm was controlled by sputtering time and deposition rate. After deposition, the vacuum chamber was immediately filled with 1 atm oxygen gas to improve the oxygen stoichiometry. Subsequently, we switched off the substrate heater power and let the sample cool to room temperature naturally. Cross-sectional high-resolution transmission electron microscopy (JEOL 2100) was adopted to characterize the structure of the as-deposited sample [21]. The film exhibited good [101]-oriented epitaxial growth and uniform in-plane orientation.

 

Fig. 1 Schematic illustrations of the sample, electrode structure and the measurement circuit. (a) Diagram of the sample showing the epitaxial growth relationship of [001]MgO//[101]LCMO. α is the tilt angle. (b) Schematic diagram of the electrodes configuration and the measurement circuit.

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Before transport experiments we cut the LCMO/MgO sample into 4 mm × 2.5 mm in-plane dimensions. Figure 1 shows a schematic illustration of the samples, electrode structure and the circuit of the measurement used in our experiments. Owing to the epitaxial growth, the LCMO [101] axis as well as the MgO [001] axis is tilted to the surface normal n at an angle of 10°. Rectangular colloidal silver electrodes with the size of 0.75 mm × 2.5 mm were painted on the surface of LCMO film, shown in Fig. 1(b), and an ohm contact was confirmed by the linear current-voltage characteristics. A single-pole double-throw switch (SPDT) was used to complete the conversion between current stimuli and photovoltage measurement. We adopted a Keithley 2400 source meter as the stimulating current I_S source. Pulsed current stimuli (current value of I_S; pulse width t_S) were directly applied to the film along the tilt direction (the projection of the [101] axis to the film surface) (SPDT at position “1”). As shown in Fig. 1(b) after a relaxation time τ SPDT was changed to position “2” to record a photovoltaic signal between two silver electrodes on the surface of LCMO film by a bandwidth sampling digital oscilloscope terminated into 50 Ω under a 248 nm KrF excimer pulsed laser irradiation (a pulse width 20 ns duration, energy density of 0.2 mJmm⁻² and a repetition rate of 1 Hz). An aperture with 2.5 × 2.5 mm² in area on the surface of LCMO film was chosen to keep the laser away from irradiating electrodes.

3. Results and discussion

A transient open-circuit photovoltaic signal V⁰ of the LCMO film was recorded under the irradiation of a 248 nm laser pulse without any bias current application as shown in Fig. 2 . When we applied a current pulse (I_S = 20 mA; t_S = 2 s) to the film and measured the transient photovoltaic signals two seconds after the current-stimulated process, we found that the signal is largely enhanced by the pulse current stimulus, and the top peak photovoltage V_PT and the down peak photovoltage -V_PD increased rapidly from 38.8 mV and 6.4 mV to 56.0 mV and 16.0 mV for τ = 2 s, respectively. Such photoresponse enhancement due to the current pulse is reproducible strongly and depends on the relaxation time τ after the current stimulus process. With increasing of τ, the peak photovoltage undergoes a consecutive drop and reduces to 42.0 mV and 8.0 mV for V_PT and -V_PD at τ = 15 s, respectively. When we measured photovoltaic signals 50 seconds (τ = 50 s,) after the current stimulus, the enhancement of the photoresponse signal disappeared. It is obvious that not only the positive photovoltage but also the negative photovoltage in the waveform of the photoresponse were enhanced by the current pulse stimulus. This overall amplification is different from other signal gain mechanisms such as the applied bias [22]. In the present measurement, it is worth note that the full width at half maximum (FWHM) of the photovoltaic signal is ~20 ns which is limited by the laser pulse. For this reason, both the rise and fall times of the waveform show invariability to the current stimulus.

 

Fig. 2 The open-circuit photovoltaic signals under a 248 nm pulse laser irradiation after different relaxation time τ following a current stimulus (I_S = 20 mA; t_S = 2 s). The inset shows the corresponding top peak photovoltage V_PT and down peak photovoltage –V_PD as a function of relaxation time τ. The dash lines are guides to eyes.

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The transient photovoltaic response is sensitive to the current pulse stimulating time. Figure 3 presented the photovoltaic enhancement with the increasing of t_S (I_S = 20 mA; τ = 2 s). V_PT and –V_PD exhibit a rapid growth to 62.0 m V and 19.6 mV, respectively, as t_S increasing to 3 s. And then, further increase in t_S (from 3 to 7 s) produces a slight variation of V_PT and –V_PD. The rate of change in photovoltage, defined as ∆V_P/V⁰_P = (V_P-V⁰_P)/V⁰_P, can reach to ~50% and ~240% for V_PT and –V_PD using a current stimulus, respectively. In addition the influences on the photovoltage of different stimulating current I_S (t_S = 2 s; τ = 2 s) were analyzed and summarized in Fig. 4 . V_PT and -V_pD increase from 38.8 mV and 6.4 mV to 56.0 mV at 20 mA following a slight variation and 25.2 mV as I_S increases from 0 to 40 mA. The ∆V_P/V⁰_P = (V_P-V⁰_P)/V⁰_P reached to ~50% and ~300% for V_PT and –V_PD using a 40 mA current stimulus, respectively. These results suggested that the current-pulse-induced photovoltaic enhancement can be tuned not only by the stimulating time but also the stimulating current value.

 

Fig. 3 The open-circuit photovoltaic signals under a 248 nm pulse laser irradiation after relaxation time τ of 2 s following a current stimulus I_S of 20 mA with different current stimulating time t_S. The inset shows the rate of change in photovoltage, defined as ∆V_P/V⁰_P, as a function of current stimulation time t_S. The dash lines are guides to eyes.

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Fig. 4 The open-circuit photovoltaic signals under a 248 nm pulse laser irradiation after relaxation time τ of 2 s following different current stimulus I_S with a current stimulating time t_S = 2 s. The inset shows the rate of change in photovoltage, defined as ∆V_P/V⁰_P, as a function of the stimulating current I_S. The dash lines are guides to eyes.

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As we know, when the direct current passes through a resistance, it will generate Joule heat leading to the increase of temperature. With the increase of stimulating current or stimulating time the temperature of the film increases and accelerates the recombination of photoinduced nonequilibrium carriers. From Fig. 3, we can see that the peak voltage shows the saturation at t_S>2s. Our experiments based on the temperature dependence of photovoltaic effects in tilted LCMO films also showed that the peak photovoltage kept stable in the temperature range of 300 K–673 K [23]. Thus, we cannot simply attribute the enhancement of transient photodetective effect to the increasing temperature due to pulsed current stimuli. There should be another mechanism working in current stimulation which resulted in the enhancement of transient photovoltages.

In perovskite manganites magnetic-field can largely depress the resistance with IM transition near Curie temperature T_C. Except for magnetic field, there exist several other external stimuli causing a transition from an insulating state to a metallic state, which includes pressure, optical stimuli and electric field stimuli [6–10]. Y. Moritomo reported that application of pressure induces a metallic phase from the low-temperature side in the charge ordered insulating phase [6]. A. Asamitsu presented current switching of resistive states in magnetoresistive manganites [8]. K. Miyano found photoinduced IM transition in a perovskite manganite [9]. Electric-pulse-induced reversible resistance change in magnetoresistive films was also reported [20]. A nucleation of metallic filamentary paths was proposed at low temperature in some of the above cases.

Here, the current-pulse-stimulated enhancement effect of transient photovoltaic responses occurs in the paramagnetic insulating state of the LCMO at room temperature, and filamentary high conductivity paths should be responsible for the present results. The paths are volatile with respect to removal of the pulsed current, and occur in the paramagnetic insulating state. Ferromagnetic clusters associated with magnetic polarons can exist above T_C [24]. Within the present work, the applied big current pulse could change the shape of such ferromagnetic clusters, and arrange them directionally from an expected random state to an organized state. An improved percolation path for charge carriers through the conductive clusters could be realized resulting in filamentary paths of increased conductivity. In the transient photovoltaic process, the higher mobilities of electron and hole in metallic filamentary paths will result in a higher photoresponses. However, the paths are volatile with respect to removal of the pulsed current and disappear in a short time due to intense thermal motion above room temperature.

4. Conclusion

In conclusion, we reported current-pulse-induced enhancement of transient photodetective effect in a LCMO thin film on a miscut MgO substrate. By applying a pulsed current stimulus before pulse laser irradiation, we observed a significant enhancement of more than 50% in photovoltaic signals responded to the laser pulse. Such photoresponse enhancement by the current pulse is volatile and reproducible. This understanding of the current-stimulated enhancement of transient photovoltaic properties in manganite films should open a route for devising manganite-based photodetectors working at and above room temperature.