Abstract
The photovoltaic properties of miscut La2/3Ca1/3MnO3 films were systematically investigated with different bias voltage () and load resistance (). The photoresponse depended strongly on and and was improved greatly by increasing and . The maximum photovolvaic sensitivity reached 62.5 mV/mJ. The present results suggested the promising potential of increasing and in high-sensitivity detectors.
1. Introduction
Hydrocarbon resource is important and strategic for the national modernization, defense, and security. Present concept of Digital Oilfield suggests that an optical detector in oil well can operate at high temperature upto ~500°C and high pressure upto ~100 MPa with high-speed response. Recently there have been active studies of the photoresponse characteristics in the manganite thin films which can work in a harsh environment (features such as thermal instability and high pressure). Technological interest has centered on bolometers [1], while more basic issues have involved quasiparticle generation and carrier relation times [2–6]. Ultrafast photovoltaic effect has been observed in manganite oxide with a picosecond response time, which was due to a combination mechanism of photoinduced carriers and Seebeck effect [7–9].
In this work, to improve the photosensitivity of manganite films and meet the needs of oil and gas optical engineering, we focused on load-resistance- (-) and bias-voltage-(-) tunable lateral photovoltages of La2/3Ca1/3MnO3 (LCMO) films grown on miscut LaSrAlO4 (LSAO) substrates with 10° tilted to direction of LSAO. The laser-induced voltage (LIV) depended strongly on and . Under an irradiation of 248 nm ultraviolet laser, when is changed from 30 to −30 V, the LIV peak sensitivity can be tuned from −10.8 to 12.5 mV/mJ and from −52.1 to 62.5 mV/mJ at and 72 Ω, respectively.
2. Experimental
A LCMO (120 nm) thin film was deposited by facing-target sputtering technique on the LSAO substrate cut along the (001) surface with an intentional 10° vicinal angle toward the direction [10, 11]. The substrate temperature was kept at 680°C with the oxygen pressure of 30 mTorr during deposition. After the deposition, the vacuum chamber was immediately backfilled with 1 atm oxygen. The LCMO film was then cooled to room temperature with the substrate heater power cutoff.
Figure 1 shows the schematic circuit of the photoresponse measurement. Before the measurement, the sample was carefully cleaned using alcohol and acetone. Two colloidal silver electrodes of 1 mm × 2.5 mm area were prepared on LCMO surface. Compex 50 excimer-pulsed laser was used as the light source, operating at a wavelength of 248 nm with 20 ns duration at a repetition rate of 1 Hz. The on-sample laser pulse energy is 2.88 mJ. The waveforms were recorded by a sampling oscilloscope with an input impedance of 1 MΩ. All the measurements were carried out at room temperature.
3. Results and Discussion
Figure 2 shows a typical voltage transient of LCMO film under the 248 nm laser irradiation without bias (). The rise time (RT) and full width at half-maximum (FWHM) are independent of and 7 ns and 13 ns, respectively. As reviewed in the left inset of Figure 2, the peak voltage signal has a linear relationship with the from 10 to 72 Ω.
The laser-induced voltage waveform is plotted in Figures 3(a) and 3(b) as a function of time, and the photovoltage peak value increases monotonously from −0.15 to 0.18 V and from −0.031 to 0.036 V with from 30 to −30 V at and 10 Ω. Figure 3(c) reviews as a function of applied bias voltage , which depended linearly on and showed no saturation for selected . In addition, is also very sensitive to the load resistance and shows a higher value for a larger . As shown in Figure 3(d), the10–90% rise time RT of the photovoltaic signals nearly keeps constant with varying , while the response speed is faster for the lower bias and the RT difference between and 30 V is about 10 ns.
(a)
(b)
(c)
(d)
Figure 4 summarizes the spatial distribution of the as a function of and . shows a higher value for a larger while a lower value for a smaller and increases with increasing load resistance . The result indicates the potential possibility to improve the photovoltage sensitivity by introducing the load resistance and applying bias voltage.
Since the photon energy of 248 nm wavelength (4.86 eV) is above the bandgap of LCMO (~1.2 eV), electron-hole pairs are generated in the LCMO film. In our case, the laser we used is a 248 nm KrF excite laser beam in duration of 20 ns, so the amount of laser-induced carriers should be comparable with or even much larger than that of the majority carriers in the LCMO; on the other hand, there exists no built-in field which exists in the p-n junction to separate holes and electrons. Therefore, both the electrons and holes play an important role in the photovoltaic. Under the external bias, the carriers are sped up and the photoresponse is enhanced.
From the basic laws of circuit networks, the readout voltage from the oscilloscope can be calculated from , where is the voltage signal generated in the sample and is the sample impedance. In our case, the load resistance is small and . Thus, can be presented as as shown in Figures 2 and 4.
4. Conclusions
In summary, the bias and load resistance-dependent photovoltaic effects in LCMO thin film grown on miscut LSAO were studied systematically. With the increase in and , the peak photovoltage signals increase monotonically and a maximum of 0.18 V was achieved at , V. The experimental results showed that increasing and is an effective method for improving the photovoltage sensitivity in manganite, suggesting the potential for optoelectronic detection applications.
Acknowledgments
This work has been supported by NCET, NSFC, RFDP, and Direct Grant from the Research Grants Council of the Hong Kong Special Administrative Region (Grant no. C001-2060295).