Load-Resistance-and Voltage-Tunable Photovoltaic Effect in TiltingManganite Films

The photovoltaic properties of miscut La2/3Ca1/3MnO3 films were systematically investigated with different bias voltage (Vb) and load resistance (RS). The photoresponse depended strongly on Vb and RS and was improved greatly by increasing Vb and RS. The maximum photovolvaic sensitivity reached 62.5 mV/mJ. The present results suggested the promising potential of increasing Vb and RS in high-sensitivity detectors.


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][3][4][5][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][8][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-(R S -) and bias-voltage-(V b -) tunable lateral photovoltages of La 2/3 Ca 1/3 MnO 3 (LCMO) films grown on miscut LaSrAlO 4 (LSAO) substrates with 10 • tilted to [001] direction of LSAO.The laser-induced voltage (LIV) depended strongly on V b and R S .Under an irradiation of 248 nm ultraviolet laser, when V b 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 R S = 10 and 72 Ω, respectively.

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 [010] 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.

Results and Discussion
Figure 2 shows a typical voltage transient of LCMO film under the 248 nm laser irradiation without bias (V b = 0 V).The rise time (RT) and full width at half-maximum (FWHM) are independent of R S and 7 ns and 13 ns, respectively.As reviewed in the left inset of Figure 2, the peak voltage signal V P has a linear relationship with the R S 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 V P increases monotonously from −0.15 to 0.18 V and from −0.031 to 0.036 V with V b from 30 to −30 V at R S = 72 and 10 Ω. Figure 3(c) reviews V P as a function of applied bias voltage V b , which depended linearly on V b and showed no saturation for selected R S .In addition, V P is also very sensitive to the load resistance R S and shows a higher value for a larger R S .As shown in Figure 3(d), the10-90% rise time RT of the photovoltaic signals nearly keeps constant with varying R S , while the response speed is faster for the lower bias and the RT difference between V b = 0 and 30 V is about 10 ns.

Figure 4 Figure 4 :
Figure 4: Three-dimensional plot for V P as a function of R S and V b .