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For electrical devices based on vanadium dioxide thin film, various methods have been implemented on the electrical gating of the devices. In this paper, a photo-assisted electrical gating in a two-terminal device is demonstrated based on vanadium dioxide thin film, instead of a three-terminal device with a gate terminal, by illuminating infrared light directly onto the film. Based on the light-induced phase transition, the threshold voltage of the device, in which an abrupt current jump take places, was theoretically anticipated to be controlled (electrically gated) by adjusting the light intensity. Finally, the prediction was experimentally verified.
©2007 Optical Society of America
1. Introduction
Owing to potential applications [1–5] in ultrafast optical switches, passive optical devices, and bistable optoelectronic devices, much attention has been paid to the phase transition (PT) between insulating and metallic states in vanadium dioxide (VO2) over the past decades. It has been reported that the PT in a VO2 thin film can be induced by temperature [6,7], pressure [8,9], light [10,11], as well as by an electric field including a current injection [12,13]. When VO2 thin films are fabricated as two- or three-terminal electrical devices, they show strong nonlinear current-voltage (I–V) behaviors [12,13] that can include an abrupt current jump that takes place at a specific threshold voltage. These nonlinear I–V characteristics that are similar to those of Zener diodes can be usefully applied to electrical switching devices such as thyristors [14]. Particularly, in the practical application of these switching devices, switching or tuning of the threshold voltage, i.e., the gate control of the threshold voltage (electrical gating), is an essential function required in general switching devices. A variety of approaches such as those incorporating a three-terminal scheme [12] and a thermal control [12,13] have been made for the implementation of the electrical gating in VO2 thin film devices. In the case of the three-terminal scheme [12], however, the voltage applied to the gate terminal is several tens of volts, which can deteriorate the reliability and durability of the switching devices and give rise to considerable switching power loss; moreover, for the thermal control [12,13], the switching speed is essentially restricted to the time of the heat dissipation.
In this paper, a photo-assisted electrical gating in a two-terminal device based on a VO2 thin film, instead of a three-terminal device, is demonstrated by illuminating infrared light with a wavelength of ~1555 nm directly onto the film. In the two-terminal devices, the photo-assisted electrical gating takes the role of the electrical gating of the gate terminal found in three-terminal devices. In this optical gating scheme, the device structure is simple because a gate oxide and electrode are not necessary and the light intensity irradiated onto the device is less than 20 W/cm2, which is not large enough to degrade the reliability and durability of the device. In particular, the switching speed of the threshold voltage of the device in the optical gating can be much faster than that of the thermal control because it depends on the modulation speed of the illuminated light. Based on the light-induced PT [10,11], the threshold voltage of the device is theoretically anticipated to decrease as the light intensity increases with the help of the photo-induced hole excitation [12,15,16]. Through an experimental demonstration, it is verified in this study that the threshold voltage can be controlled by the adjustment of the light intensity, i.e., the photo-assisted electrical gating in VO2 thin film devices is embodied by the direct illumination of the infrared light onto the device.
2. Experimental setups
Figure 1 shows a cross-section and plane-view of the two-terminal device based on the VO2 thin film with Ni electrodes. As shown in Fig. 1, VO2 thin films were deposited onto an Al2O3 substrate by pulsed laser deposition using a KrF excimer laser at 248 nm [12]. The thickness of the VO2 film was ~100 nm. Through an X-ray diffraction analysis, it was found that the VO2 film had a monoclinic VO2 structure in its insulating state (the high resistance state). For the device fabrication, the film was selectively etched by the RF ion milling technique in order to isolate the current channel. Using the lift-off method and the RF magnetron sputtering technique, Ni electrodes were patterned and formed on the etched film. In the fabricated devices, the dimensions of the devices (L×W) were 30×3 µm2 (Device I) and 30×10 µm2 (Device II), where L and W are the length and width of the exposed film, respectively. The figure in upper right corner of Fig. 1 shows a zoomed snapshot of Device I.
Figure 2 shows a schematic diagram of the experimental setup in which the infrared light with a wavelength of ~1555 nm is illuminated onto the film for the implementation of the photo-assisted electrical gating in the fabricated devices. In Fig. 2, the output light of a distributed feedback laser diode enters an erbium-doped fiber amplifier to increase its intensity, and the amplified light passes through an optical switch and fiber pigtailed in-line power meter sequentially. The optical switch (Thorlabs OSW12-1310-SM) has a crosstalk of - 75 dB, an insertion loss <0.7 dB, and a switching speed <1 ms. Through an optical fiber adapter, the optical fiber from the power meter is connected with a tapered lensed optical fiber, whose spot diameter at the beam waist, working distance to the beam waist, and tapered angle are ~3.5 µm, ~21 µm, and 70°, respectively. In the experiments, the output beam from the lensed optical fiber was launched into the film with an incident angle of 30° and was manipulated until its spot diameter was ~930 µm.
Fig. 2. Schematic diagram of experimental setup used to implement the photo-assisted electrical gating in the fabricated two-terminal devices.
3. Experimental results and discussion
Fig. 3. (a) Optical spectra of the light coming out of the optical switch, measured (b) photo-assisted electrical gating operation and (c) linearity curves of Device I, and measured (d) photo-assisted electrical gating operation and (e) linearity curves of Device II.
As mentioned earlier, when the light intensity increases, the threshold voltage of the device is expected to decrease because the photo-induced hole excitation assists the VO2 devices to reach faster the metallic state (the low resistance state) compared with the case of no light illumination [10–12,15,16]. For the verification of the above prediction, the photo-assisted electrical gating operation was experimentally investigated with respect to the fabricated VO2 devices (Device I and II). Figure 3(a) shows the optical spectra of the light coming out of the optical switch with respect to various optical intensities (-30~20 dBm) as measured by an optical spectrum analyzer (Yokogawa AQ6370) with a resolution bandwidth of 0.1 nm. The center wavelength and signal-to-amplified spontaneous emission ratio of the optical output from the optical switch were measured to be 1554.6 nm and at least more than 30 dB, respectively. The optical density of the light beam projected onto the film was ~14.7 W/cm2 at an input optical intensity of 100 mW.
Figure 3(b) shows the change of the threshold voltage of the Device I with respect to various optical intensities as measured with a parameter analyzer (Agilent B1500A) in a voltage-controlled mode. To protect the device from the high current flow, the device current was limited to the current compliance. In Device I, an abrupt current jump, a typical I–V characteristic of VO2 devices, was observed at a specific threshold voltage of ~12 V with no input light (current compliance: 2 mA). As shown in the figure, the threshold voltage of Device I decreases, i.e., is tuned toward 0 V, as the light intensity increases. The tuning displacement was measured as greater than 4.5 V with respect to the optical intensity variation of 100 mW. This implies that the threshold voltage of Device I can be controlled by the adjustment of the light intensity, explaining why this switching operation is designated as photo-assisted electrical gating compared to the gate control of a source-drain current in a common field-effect transistor. Figure 3(c) shows the linearity curves of the measured threshold voltage and detuned voltage with respect to optical intensity in square and triangle symbols, respectively. The detuned voltage was evaluated with the threshold voltage at zero input optical intensity as the reference voltage. The tuning sensitivity of the threshold voltage with respect to optical intensity in Device I was measured as ~43.7 V/W. The standard deviation of the linear fit was measured as 1.282. The rms deviations of the measured threshold voltages at various optical intensities were measured as less than 0.09 V, but were above 0.09 V when the optical intensity exceeded 100 mW. Thus they may be affected by heat generated from the illumination.
Figure 3(d) shows the photo-assisted electrical gating operation of Device II with respect to various optical intensities (current compliance: 1 mA). As can be seen from the figure, the threshold voltage of Device II is also tuned toward 0 V as the light intensity increases. The tuning displacement was measured to be greater than 9 V with respect to the optical intensity variation of 100 mW. Figure 3(e) shows the linearity curves of the measured threshold voltage and detuned voltage with respect to optical intensity in square and triangle symbols, respectively. The tuning sensitivity of the threshold voltage and the standard deviation of the linear fit in Device II were measured as ~95.8 V/W and 0.199, respectively. The rms deviations of the measured threshold voltages at various optical intensities were determined to be less than 0.07 V, but were greater than 0.07 V when the optical intensity exceeded 90 mW; they also appear to have been affected by light-induced heat.
In order to investigate the switching speed of the threshold voltage, i.e., the gating speed, in the VO2 devices, a test electrical circuit was constructed that utilized a standard resistor (RS) connected in series with the VO2 device and DC voltage source (VS), as shown in Fig. 4(a). When VS is set at a fixed value that is slightly less than the threshold voltage of the VO2 device, the device remains in its high resistance state. If the device is illuminated by the infrared light, however, its threshold voltage becomes less than VS; then, it changes into its low resistance state. In the test circuit, the voltage across the standard resistance (VR) is defined as VS minus the device voltage, and VR has a complementary relationship with the device voltage. For the investigation of the gating speed, the transient electrical responses of VR were observed with VS and RS fixed at 11 V and 2 kΩ, respectively, when the optical switch was turned on and off with the laser diode continuously operated. The optical switch was driven in a burst mode of the standard function generator by applying an electrical pulse with a pulse width of 10 ms to the external control terminal of the optical switch and used the “sync” signal of the function generator as the triggering source of the oscilloscope. The wavelength of the light was 1554.6 nm and light intensity coming out of the optical switch was 50 mW. When the optical switch was turned on and off with Device I employed in the circuit, the measured transient responses of VR were plotted, as shown in Fig. 4(b) and 4(c), respectively. The rising and falling times were measured to be ~1.0 ms and ~1.2 ms, respectively, which were comparable to the switching speed of the optical switch. A falling time slightly longer than a rising time is considered to be affected by the heat energy conveyed from the light illumination until the optical switch is turned off. A faster gating speed is potentially available by employing a commercial RF intensity modulator or acousto-optic modulator. In a viewpoint of the electrical circuit, however, an RC time constant, which is calculated as ~0.2 µs based on the measured capacitance of the VO2 device (~100 pF) and the resistance of RS (2 kΩ), may restrict the maximum gating speed in the test circuit. Particularly, for the examination of the light-induced film damage that may occur from local heating by the infrared light, the reproducibility of the photo-assisted electrical gating was investigated in Device II. In the examination, an input light with an intensity of 50 mW was illuminated during the ON state and was blocked during the OFF state by the optical switch. The reproducibility test was carried out more than 1000 times, and no fluctuation of the threshold voltage exceeding the rms deviation could be observed during and after the examination.
Fig. 4. (a) The test electrical circuit used to investigate the gating speed in the VO2 devices and transient electrical responses of VR when the optical switch is turned (b) on and (c) off in Device I.
4. Conclusion
In this paper, a photo-assisted electrical gating operation was experimentally investigated with respect to fabricated two-terminal devices based on VO2 thin films by the direct illumination of the infrared light onto the film of the device. Through an experimental demonstration, a theoretical prediction verified that the threshold voltage could be tuned by the adjustment of the light intensity. In Device I and Device II, the tuning sensitivities were measured to be ~43.7 V/W and ~95.8 V/W, respectively. The rising and falling times were measured to be ~1.0 ms and ~1.2 ms, respectively, which were comparable to the switching speed of the optical switch. This optical gating technique can be beneficially applied to direct light triggered thyristors [16], which have been installed in many power stations due to their high reliability, durability, and availability.
Acknowledgments
The present research was financially supported by the “High Risk High Return Project” of ETRI.
References and links
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