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Light-induced transverse voltage effects in c-axis inclined BiCuSeO thin films have been investigated by using different lasers with the wavelength ranging from ultraviolet (UV) to near infrared (NIR). Open-circuit voltage signals are all clearly observed when the film surface is irradiated by these lasers. Especially, a very large voltage signal with a magnitude exceeding several volts is obtained under the irradiation of the UV pulsed laser, and the voltage polarity of this signal is reversed when the sample is irradiated from the substrate side. A possible mechanism based on the transverse thermoelectric effect is proposed to explain the above experiment results. This work demonstrates the potential application of BiCuSeO for broad-band photodetectors, especially for UV pulsed light detection.
© 2016 Optical Society of America
1. Introduction
The light-induced transverse voltage (LITV) effect in c-axis inclined thin films and single crystals has been extensively studied in the past decades due to its great potential applications in photodetectors [1–16]. Most previous works on LITV effects have been carried out on thin films of high temperature superconductor (high-Tc) copper oxides and colossal magneto resistance (CMR) manganites [1–5]. Recently, several research groups reported the observation of giant LITV effects in thermoelectric (TE) layered cobaltites [6–13], which exhibited much larger open-circuit voltage signals than those of high-Tc or CMR materials and therefore paved the way to the practical applications of LITV effects in the field of light detection. The origin of the giant LITV effect in these cobaltites was suggested to be associated with the large anisotropy of the Seebeck coefficient due to their layered crystal structure [6–13].
BiCuSeO, a newly-discovered oxide TE material, has recently attracted great attention because of its excellent TE performance in the moderate temperature range [17–19]. This material has similar layered crystal structure to that of cobaltites. However, so far there have been no reports on the LITV effect in BiCuSeO due to the difficulty in growing thin film samples due to the high volatility of both Bi and Se elements. In the present work we reported on the fabrication of high quality c-axis inclined BiCuSeO thin films and the investigation of LITV effect in the films. Open-circuit voltage signals were all clearly detected when the film surface was irradiated by both pulsed and continuous-wave (CW) lasers with different wavelengths. Especially, a very large voltage signal with a magnitude exceeding several volts was obtained under the irradiation of the 308 nm pulsed laser.
2. Experiments
BiCuSeO thin films were grown on 10° miscut (001) oriented single crystal LaAlO3 substrates via a 308 nm pulsed laser ablation of the corresponding ceramic target under an atmosphere of high purity argon. Details about the synthesis of the ceramic target were described in [20]. During the film growth, the laser energy density on the target was about 1.5 J/cm2, the repetition rate of the laser was 5 Hz, the distance between the target and the substrate was about 50 mm, the argon pressure was about 0.1 Pa and the substrate temperature was about 350 °C, respectively. It should be mentioned here that since the layered BiCuSeO contains unstable elements in oxygen, preparation of the samples is always in a non-oxidative environment, which is different from most normal oxide thin films.
The crystal structure of the films was measured using a Bruker AXS D8 advanced x-ray diffractometer with Cu Kα radiation. The elemental composition and the microstructure of the films were analyzed by using the energy-dispersive x-ray spectroscopy (EDS) and the field-emission transmission electron microscopy (TEM, Tecnai G2 F20), respectively. For the measurements of LITV effects, two indium electrodes separated by 8 mm were deposited on the film surface along the x-axis direction, as shown in Fig. 1(a). Good Ohmic contacts between the electrodes and the film were confirmed by a linear current-voltage (I-V) curve shown in Fig. 1(b). Different lasers with the wavelength ranging from UV to NIR were used as the light sources, and the corresponding light-induced voltage signals were recorded by using a digital oscilloscope terminated into 1 MΩ (Agilent DSO9254A) and a 2700 Keithley source meter, respectively.
Fig. 1 (a) Schematic illustration of the LITV effect measurements, where a, b, c and x, z represent the crystal axis and the spatial axis, respectively. (b) I-V curve of the two In electrodes on the BiCuSeO film.
3. Results and discussion
Figure 2(a) shows the XRD θ-2θ pattern of a c-axis inclined BiCuSeO thin film grown on the 10° miscut double-polished LaAlO3 substrate measured in a “Locked-Coupled” mode. The inset is the sketch map of the XRD θ-2θ measurement for the inclined samples. Here, the offset angle ω is set as 10° to satisfy the Bragg diffraction condition. Apart from substrate peaks, all peaks in the figure can be indexed to the (00l) diffractions of BiCuSeO, indicating that the film is single phase and the c-axis of the film is tilted by about 10° away from its surface normal. The EDS analysis of the film, as seen in Fig. 2(b), reveals that the ion ratio of Bi:Cu:Se in the film is about 1.00:1.06:1.03, which is very close to that of the nominal composition of this material. Figure 2(c) is the high-resolution TEM (HRTEM) image of the inclined BiCuSeO film on LaAlO3 substrate. Well-ordered layered structures of BiCuSeO stacked along the c axis can be clearly seen in the image, and the ab-plane of the film is inclined about 10° with respect to the sample surface, which agrees well with the XRD result. It needs to be mentioned here that we have performed the EDS analysis on different parts of the BiCuSeO thin film between two indium electrodes and the experimental results indicate that the film is very uniform. Moreover, the XRD and TEM/EDS results shown in Fig. 2 also do not depend on the position between two electrodes for the BiCuSeO thin film.
Fig. 2 (a) XRD θ-2θ scan, (b) EDS and (c) HRTEM of the BiCuSeO thin film grown on 10° miscut LaAlO3 (001) single crystal substrate. The inset of Fig. 2(a) is the sketch map of the XRD θ-2θ measurement in a “Locked-Coupled” mode and the offset angle ω is set as 10°.
Figure 3(a) displays the typical LITV signals of a c-axis inclined BiCuSeO film under the irradiation of the 308 nm pulsed laser. Here, the film thickness is about 180 nm, the laser energy density on the film is about 0.5 mJ/mm2 and the laser spot on the film is about 5 mm × 2 mm. A large open-circuit voltage signal with a peak value (Vp) of about 4.4 V can be detected when the film is irradiated from the film/front side. When the film is irradiated from the substrate/back side, the voltage polarity of the signal is reversed and the corresponding Vp value of the reversed signal is reduced by about 50% as compared to the front side irradiation. Based on the above experimental results, we suggest that the LITV signals observed in the present c-axis inclined BiCuSeO films very likely originate from the transverse thermoelectric (TE) effect, which is also known as the off-diagonal thermoelectric (ODTE) effect and emerges uniquely in inclined structures with anisotropic Seebeck coefficient [1,2,5–16]. According to the transverse TE theory, an open-circuit voltage signal parallel to the sample surface, Vx, will be generated if applying a temperature difference ΔTz normal to the surface of the c-axis inclined sample, which can be expressed by the following equation [1]:
Here, l is the laser spot diameter, d is the thickness of the sample, α is the inclination angle of the ab-plane with respect to the sample surface, ΔS = Sab−Sc is the difference in Seebeck coefficient between the ab-plane and the c-axis of the sample. In this work, when the BiCuSeO film surface is irradiated by the laser, a ΔTz between the film surface and its bottom is created, leading to a lateral voltage signal along the film surface because of the transverse TE effect. When the film is irradiated from the substrate side, the value of ΔTz is opposite as compared to the film side irradiation, so the polarity of the induced voltage will be reversed. The reduced Vp for this reversed voltage signal is mostly due to the transmission loss of the laser energy associated with two surfaces of the LaAlO3 substrate [13]. That is, when the film is irradiated by the laser from the substrate side, the incident laser light will be partially reflected by the surface of the LaAlO3 substrate as well as the interface between the substrate and the film. Therefore, only part of incident laser light can transmit through the substrate and then be absorbed by the film, resulting in a small temperature difference ΔTz and thus a small Vp. Figure 3(b) shows the variation of Vp with the laser energy E on the film. Vp increases linearly with E when E is below the destruction limit of the film. Such behavior is also a signature of the transverse TE effect since the temperature difference ΔTz in Eq. (1) is directly proportional to the amount of absorbed laser energy of the film [8,9]. From the Vp-E curve, a sensitivity R (R = Vp/E) of about 0.9 V/mJ is obtained for the LITV signals of the present BiCuSeO thin films, which is even higher than that reported for the Bi-based layered cobaltites [11–13]. We also investigate the dependence of Vp on the laser spot position on the film, as presented in Fig. 3(c). Here, the laser is scanned along the x-axis direction between two electrodes and the spot is adjusted into a circle shape with diameter of 0.8 mm. Figure 3(c) clearly shows that both the magnitude and the polarity of the induced voltage signals are all almost independent on the irradiation position, excluding the possibility that the observed voltage signals originate from the photo-Dember effect.Fig. 3 (a) LITV signals of a c-axis inclined BiCuSeO film irradiated by a 308 nm pulsed laser from the film side (the top curve) and substrate side (the bottom curve). (b) Dependence of the peak value of the induced voltage, Vp, on the on-sample laser energy. (c) Vp as a function of incident laser spot position on the film surface along x direction. (d) Voltage responses of the inclined BiCuSeO film to a point-like thermal heater with temperature varying from 200 to 400 °C. The inset shows the voltage waveforms for heating the sample from film side and substrate side, respectively.
To further confirm the thermal origin of the light-induced voltage in inclined BiCuSeO thin films, we record the voltage response by using a point-like thermal heater as a heating source. The temperature of the heater can be adjusted by changing the input electric power. Figure 3(d) is the voltage responses of the 10° inclined BiCuSeO thin film to the thermal heater. Here, the heater is located at the center position between two electrodes and the distance between the heater and the sample surface is about 2 mm. Open-circuit voltage signals are also clearly observed when the film surface is heated by the heater, and the magnitude of the voltage signal increases with increasing input electric power of the heater. In addition, as shown in the inset, similar polarity reversal occurs when we heat the sample from the substrate/back side. These results further demonstrate that the observed open-circuit voltage signals in the c-axis inclined BiCuSeO thin film is associated with a TE effect.
Finally, we investigated the LITV effect in the c-axis inclined BiCuSeO films by using CW lasers with visible (Vis, 532 nm) and near-infrared (NIR, 980 nm) output. In order to uniformly irradiate the film surface between two electrodes, the laser spot is adjusted to 8 mm × 1 mm by an optical grating. Figure 4 displays the voltage responses of the BiCuSeO thin film to the irradiation of Vis and NIR CW lasers. The voltage signals exhibit good switching characteristics with the lasers turning on or off, and the magnitude of the voltage signal induced by the Vis irradiation is obviously larger than that induced by the NIR irradiation. As the incident laser power P increases, as shown in the inset of Fig. 4, the Vp value of both voltage signals increases linearly. It is known that the steady-state ΔTz induced by the CW laser irradiation in the transverse TE effect is given by the following equation [9]
Whereγ is the absorption coefficient of the film to the incident light, κ and d are thermal conductivity and thickness of the film, respectively. Equation (2) reveals that ΔTz, i.e. Vx in Eq. (1), will increase with increasing γ. Therefore, the larger Vp induced by the Vis laser is due to the stronger light absorption of BiCuSeO film for the Vis irradiation [21].Fig. 4 LITV signals of the inclined BiCuSeO film under the CW laser illuminations of 532 and 980 nm with the power of 50 mW. The inset shows the variation of Vp with the laser power on the film surface.
4. Conclusion
In conclusion, high quality c-axis inclined BiCuSeO single crystalline thin films were prepared by pulsed laser deposition technique and the LITV effect of the films was studied. Open-circuit voltage signals were all clearly detected when the film surface was irradiated by UV pulsed laser, Vis or NIR CW laser as well as a thermal heater. Especially, a very large voltage signal with a magnitude exceeding several volts was obtained under the irradiation of the 308 nm UV pulsed laser, and the polarity of this voltage signal was found to be reversed if the film surface was irradiated from the substrate side. All experimental results indicate that the laser-induced voltage observed in the inclined BiCuSeO thin films mainly originates from the transverse TE effect. This work demonstrates that BiCuSeO, a newly-discovered TE material which has great potential application in TE devices, might find its new application in photoelectric devices, especially for pulsed UV light detection.
Acknowledgments
This project was supported by the National Natural Science Foundation of China (No. 51372064), the Nature Science Foundation for Distinguished Young Scholars of Hebei Province, China (No. 2013201249), Science and Technology Research Projects of Colleges and Universities in Hebei Province (No. QN20131040), and Science and Technology Research and Development Foundation of Baoding (No. 15ZG047).
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