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RESISTIVITY CHANGE IN SrTiO3 UPON
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IRRADIATION AND ITS APPLICATION IN LIGHT
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SENSING
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Running head: Resistivity Change in SrTiO3
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Sanit Suwanwong1, Jirapat Kullapapinyokul1, Tanachat
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Eknapakul1, Pornpana Buaphet1, Rungrueang Pattanakul2, and
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Worawat Meevasana1, 3,*
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School of Physics, Institute of Science, Suranaree University of Technology,
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Nakhon Ratchasima, 30000, Thailand.
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Synchrotron Light Research Institute, Nakhon Ratchasima, 30000, Thailand.
NANOTEC-SUT Center of Excellence on Advanced Functional Nanomaterials,
Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand.
*
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Corresponding author. E-mail: worawat@g.sut.ac.th
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Abstract
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Typical materials used for light sensing (e.g. CdSe, CdS) usually have small
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band gaps and hence they are opaque under visible light. To look for
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materials which have properties different from these opaque ones, we are
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interested in transparent oxide materials. In our recent study, we found that
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the insulating surfaces of strontium titanate (SrTiO3) crystals in vacuum
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condition could become conducting upon synchrotron radiation. In this
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article, we then choose to study the resistivity change of SrTiO3 at ambient
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pressure under various types of irradiation. By using sputtering technique
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for making gold electrodes on SrTiO3 samples, we found that the resistance
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of insulating SrTiO3 single crystals could decrease dramatically upon
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exposing to violet-to-ultraviolet (UV) light sources. The light sources which
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the samples have good response are deuterium lamp and UV light emitting
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diode (LED) where the ratio change was found to be up to 3 orders of
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magnitude. The response times of the resistivity change when the light
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sources were on and off were found to be clearly different (e.g. 12 ms (on)
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versus 480 ms (off)); however, these response times can be tuned externally
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by further modifying the circuit. These results indicate that SrTiO3 has the
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potential usage as a light sensor which is transparent and responsive in
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violet-to-ultraviolet range.
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Keywords: strontium titanate (SrTiO3), light sensing, irradiation effect,
oxygen vacancy
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Introduction
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Today’s electronic devices rely largely on the semiconducting properties of
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materials. While the progress of new development using conventional
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semiconductors (e.g. silicon) nearly reaches its limit, new materials (e.g.
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graphene) have been studied to overcome the problem (Geim and Novoselov,
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2007). Besides graphene, two-dimensional electron gases (2DEG) at oxide
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interfaces (Takagi and Hwang, 2010; Mannhart and Schlom, 2010) also show
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much potential for electronic devices with functionalities beyond what
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conventional semiconductors can offer. The seminally-discovered 2DEGs at the
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interfaces between SrTiO3 and LaAlO3 show a number of appealing properties
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including, a high electron mobility (Ohtomo and Hwang, 2004), superconductivity
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(Reyren et al., 2007) and large magnetoresistance (Brinkman et al., 2007). Indeed,
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metal-insulator switching devices in nanoscale have already been demonstrated
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(Cen et al., 2008). From our previous studies, we also found that the charge
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densities of 2DEGs at oxide surfaces, e.g. SrTiO3 (Meevasana et al., 2011) and
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KTaO3 (King et al., 2012), can respond on external stimuli, like irradiations. We
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also show that the dielectric constant of CaCu3Ti4O12 could be enhanced upon
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excitation of violet laser (Masingboon et al., 2013), suggesting some application
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in novel oxide optoelectronics.
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In this article, we are interested in light-sensing application, using
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perovskite oxide SrTiO3. Since most of common light sensors use small-band-gap
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semiconductors, e.g. CdS and CdSe with band gaps of 2.42 and 1.73 eV
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respectively (Mohanchandra and Uchil, 1998; Antohe et al., 2003), they are
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opaque. On the other hand, since SrTiO3 with atomic structure shown in Figure
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1(a) has wider band gap of 3.3 eV (Benthem et al., 2001), it is transparent (see
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Figure 1(b)) making it possible to be used as a component in transparent
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electronics. Importantly, the wide-band-gap SrTiO3 could respond better to UV
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light comparing to conventional semiconductors and hence this could be useful in
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civil and military applications with the need of ultraviolet detectors (Xing et al.,
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2007). While the SrTiO3-based UV detector has been shown possible, here, we
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have investigated further in the response to various types of available light
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sources, including sunlight. The response time periods of on-off switching are
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measured and discussed; furthermore, we also show that this response time can be
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modified by externally modifying the circuit.
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Materials and Methods
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Sample preparation and resistance measurement
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SrTiO3 samples measured in the work (Crystal Base Co., Japan) are single
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crystals with (100) crystal orientation and 550.5 mm3 in dimension. The
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samples are first cleaned with KI/I2 (KI: I2: H2O = 10 g: 2.5 g: 100 ml), and then
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rinsed with water, acetone and ethanol. We then make gold electrode patterns on
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the sample surfaces by using DC sputtering coater. As shown in Fig. 1(c)-1(d) the
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SrTiO3 surface is covered by gold films except in the middle part whose width is
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around 147 m.
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This middle area of SrTiO3 surface, which is initially insulating without
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any excitation, will later be exposed to various types of irradiation, including
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violet laser (405 nm), green laser (530 nm), red laser (650 nm), ultraviolet light
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emitting diode (LED), deuterium lamp (Ocean Optics DH-2000), halogen lamp
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(Ocean Optics DH-2000), and sunlight. To observe the change at the SrTiO3
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surface after being irradiated, we measure the surface resistance by Agilent source
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meter (model: B2901A). The setup diagram is shown in the inset of Fig. 2(a) and
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Fig. 2(d). For sunlight, we also measure the change under both normal and
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focusing conditions. To focus the sunlight, we use magnifying glass with 14 cm in
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diameter; the size of partially focused spot is around 0.8 cm in diameter.
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Results and Discussion
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As shown in Fig. 2(a), we observe a pronounced change in surface
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resistance when the sample is exposed to violet laser with intensity around 708.53
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W/m2. The original surface resistance is around 20 G and then decreases quickly
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to around 200 M after turning on the violet laser. After turning off the violet
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laser, the resistance increases back close to the original value but at noticeably
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slower rate. The zoom-in periods during turning on and off the laser are shown in
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Fig. 2(b) and 2(c). By defining the response time to be the period which resistance
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changes half way to the saturate value, the response times after turning on and off
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are around 11.94 ms and 480 ms respectively.
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This resistance drop, when light is on, could be caused by several effects,
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including photoconductivity (Jiang and Hasegawa, 1999), photoelectric effect
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(Sorokin et al., 2007) and the creation of oxygen vacancies at surface (Meevasana
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et al., 2011; King et al., 2012). For the photoconductivity and photoelectric effect,
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the resistance should recover quickly after turning off the light. However, for the
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oxygen vacancy effect, it could last much longer after turning off the light; the
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recovery rate could last very long in order of hours, especially under vacuum
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condition (Meevasana et al., 2011). In our case, the recovery takes very long in
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order of 480 ms from the observed response time (see Fig. 2(c)). This suggests
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that the oxygen vacancy effect has the major role in this change; however, we
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should note that we believe that photoconductivity and photoelectric effect could
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still affect the change but in the lesser degree.
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Technically, some application which needs the response times between on
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and off to be in the same order could suffer from this much difference we observe
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here (e.g. 11.94 ms versus 480 ms in Fig. 2(b) and 2(c)). Therefore, we investigate
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further and found that indeed, this difference can be adjusted by external tuning.
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As shown in Fig. 2(d)-2(f), after we connect a 118 M resistor in parallel to the
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SrTiO3 sample (see inset of Fig. 2(d)), the response times, when turning on and
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off, change to 14.35 ms and 24.78 ms in Fig. 2(e) and 2(f), respectively. However,
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there is a trade-off effect; although the response time for turning off is reduced
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significantly, the effective changes in resistance also reduced largely and the
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turning-on response time also increases slightly.
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Besides the irradiation from violet laser, we also irradiate the SrTiO3
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samples by using other various light sources, including red laser, green laser, UV
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LED, deuterium lamp, halogen lamp and sunlight. The intensity spectra of these
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light sources as a function of wavelength are shown in Figure 3(a). And the
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responses in resistance after turning on and off the light sources are shown in
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Figure 3(b); note that each change in resistance along y-axis is normalized by the
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overall intensity of each light source. The changes in resistance per overall
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intensity are high when using deuterium lamp and UV LED. This suggests that the
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resistance change of SrTiO3 is more sensitive in UV range. Deuterium lamp has a
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large spectral weight of wavelengths below 387 nm, i.e. photon energy higher
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than band gap of 3.2 eV. For the violet laser, the wavelength is around 405 nm or
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3.06 eV (just below the band gap); we can see that the change per intensity is
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relatively much lower comparing to the two light sources above; this corresponds
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quite well with the onset of spectral response of SrTiO3 (Xing et al., 2007).
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We also investigate further to see the resistance change under different
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intensity of sunlight as shown in Figure 4. In the measurement, we use a
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magnifying glass to focus the sunlight and hence increase the intensity to around
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4400.87 W/m2 while the unfocused intensity is 915.6 W/m2. We observe that the
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resistance change (see Fig. 4) can largely depend on the intensity. When there is
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no light, the resistance is around 23 GΩ. And, when there is sunlight, the
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resistance drops to 6.23 GΩ under indoor light, 505.42 MΩ under unfocused light,
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and 10 MΩ under focused light. This pronounced change and its variation
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suggests that the SrTiO3 detector can also be used for detecting the intensity of
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sunlight in quantitative level. The mapping of resistance change versus intensity
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variation will be investigated further. While the samples here are single crystals,
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in the future we are also interested in other titanium oxides prepared by various
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methods, e.g. WO3 film (Paipitak et al., 2012) and BaTiO3 film (Hodak et al.,
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2010).
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Conclusions
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In this article, we show that the surface resistance of transparent SrTiO3
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material could reduce largely under irradiation from various light sources,
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especially deuterium lamp and UV LED. The intrinsic response times, when the
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light is on and off, can be different around one order of magnitude but this
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difference can be adjusted externally. We also show that the resistance change can
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also largely depend on the sunlight intensity. These results promise the light-
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sensing application of this transparent SrTiO3 material.
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Acknowledgements
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This work was supported by the Suranaree University of Technology (OROG)
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and Office of Higher Education Commissions under NRU.
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Figure 1. SrTiO3 single crystal. (a) atomic structure of SrTiO3 sample (b) Transparency
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of SrTiO3 single crystal (c-d) pattern of gold electrodes on the sample surface
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by using DC sputtering coater.
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Figure 2. (a) The resistance before and after violet laser exposure on SrTiO3. The inset
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shows measurement setup in series. (b-c) The corresponding zoom-in periods
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when violet laser is on and off. (d) The resistance before and after violet laser
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exposure on SrTiO3 with 180 M resistor added in parallel (see inset). (e-f)
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The corresponding zoom-in periods when violet laser is on and off.
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Figure 3. (a) The intensity spectra of various light sources, including deuterium lamp,
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UV LED, halogen lamp, violet laser, sunlight, green laser, and red laser as a
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function of wavelength (the overall intensity for each light source is adjusted
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to fit in the graph). (b) The responses in resistance after turning on and off the
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light sources; note that each change in resistance is normalized by the overall
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intensity of each light source.
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Figure 4. The resistance before and after sunlight exposure on SrTiO3, using indoor,
unfocused sunlight and focused sunlight.
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