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By incorporating a 1550 nm laser diode, bidirectional laser triggering was investigated in a two-terminal planar device based on vanadium dioxide (VO2) thin film grown by sol-gel method. A specific bias voltage range enabling the bidirectional laser triggering was experimentally found from the current-voltage characteristics of the VO2-based device, which was measured in a current-controlled mode. At a bias voltage selected within the range, 10 mA bidirectional triggering was implemented with a maximum amplitude switching ratio of ~68.2, and the transient responses of light-triggered currents were also analyzed.
©2014 Optical Society of America
1. Introduction
A vanadium dioxide (VO2) thin film has a reversible phase transition (PT) between an insulating state and a metallic state, which can be induced by temperature [1], pressure [2], light [3], and so on. The unique PT property of the VO2 thin film makes it a fascinating material to realize novel electrical and optical devices [4–6]. When a VO2 thin film forms a two-terminal device, electric field applied to the device can cause this PT [4], and a negative differential resistance (NDR) [5,7], originated from the discontinuity of the PT, is observed in the VO2-based device, resulting in not an exponential increase but an abrupt jump of electrical current. In particular, through the interaction with external light illuminating the VO2 device, this highly nonlinear current-voltage (I-V) behavior can be modulated temporarily [8,9] or permanently [10]. By incorporating this photo-induced modulation of the I-V behavior of the device, field-induced electrical oscillation in VO2 can be generated or extinguished, and its frequency can be externally controlled [11]. In addition, bistable electrical switching, i.e., bistable device resistances, can also be realized at a single illumination (probe) power [12,13]. In our previous studies, photo-assisted electrical gating was implemented in a two-terminal planar VO2 device, and its threshold voltage, after which a current jump occurred triggering the PT of VO2, could be shifted by controlling the optical power of the infrared laser illuminating the device [6,8,9]. However, the maximum on-state current of the device was at most ~3.9 mA resulting in an amplitude switching ratio between on- and off-state currents of less than 4 [6]. In particular, the relationship between the maximum on-state current and the bias voltage applied to the device has not been clarified. It was restrictive to control the temporal duration of the on-state current, during which the laser illuminated the device [8,9]. For VO2 devices to be considered for the practical application of optically-gated switches like light-triggered thyristors, lots of device parameters including the switching ratio, the off-state current, and the on-state current should be significantly improved, and the duration of the switched state should be flexibly chosen. In the optical triggering of VO2 devices, efforts for the enhancement of these parameters have been relatively deficient while most studies focused on the modulation of the optical properties of VO2 thin films, including refractive index, transmittance, etc [14–19], and the laser triggering study for exploring the feasibility of VO2-based oxide devices as light-triggered switches was not reported yet. In this paper, we investigate bidirectional laser triggering in a two-terminal planar device based on VO2 thin film fabricated by sol-gel method, using a ~1550 nm laser diode (LD). Here, the bidirectional triggering means that the forward or reverse PT of VO2 is triggered in accordance with the switched state (on- or off-state) of the LD. A specific bias voltage range enabling the bidirectional triggering was obtained from I-V characteristics of the device, measured in a current-controlled mode (I-mode) instead of a voltage-controlled mode. It was also observed that the I-mode I-V behavior defined the maximum on-state current in the optical triggering of the device. Common field-effect transistors are triggered on or off, i.e., bidirectionally triggered through the gate control [20], and the triggering laser can play a role of optically gating the VO2 device when the bias voltage exists in the bidirectional triggering region (to be shown later). When the bias voltage belongs to the unidirectional triggering region (to be shown later), the VO2 device shows the switching behavior of light-triggered thyristors. The bidirectional laser triggering can be regarded as optically-controlled threshold switching realized by photo-assisted NDR reduction of VO2. In particular, 10 mA bidirectional laser triggering with a maximum amplitude switching ratio of ~68.2 was demonstrated in the VO2 device that was biased at a DC voltage chosen within the above bias voltage range. This high switching contrast could be obtained by increasing the on-state current (after triggering the phase transition of VO2) with the use of VO2 devices with parallel conducting layers. The transient responses of light-triggered currents were also analyzed.
2. Experiments and discussion
2.1 Device preparation and experimental setup
Figure 1 shows the experimental setup for bidirectional laser triggering in a two-terminal planar VO2 device. An inset figure in the right upper corner shows the plane-view optical microscope image of the fabricated device. VO2 thin films prepared for planar devices were grown on sapphire (Al2O3) substrates by sol-gel method [21]. The average thickness of grown films was measured as ~100 nm. By incorporating photolithographic method, Au/Ni electrodes were formed on an isolated VO2 film [22], which was etched by ion beam-assisted milling technique, for preparing a two-terminal planar device. L and W indicate an electrode separation and a width of one of VO2 patches, which are 10 and 5 μm, respectively, and VO2 patches are separated by 5 μm. Here, the VO2 devices with total channel widths of 30 μm ( = 6 × W) and 40 μm ( = 8 × W), designated as Device I and Device II, respectively, were selected for the experiment. The cross-sectional-view schematic diagram of the fabricated device is shown in the lower left side of Fig. 1. In the laser illumination and monitoring section, the optical output of an LD (Thorlabs S3FC1550) is introduced into an optical fiber amplifier (Luxpert LXI-2000). Typical output power and side-mode suppression ratio of the LD are ~1.5 mW and > 40 dB, respectively, and its output power stability is ± 0.1 dB for 24 hours. The output optical power of the amplifier can be adjusted by using an optical attenuator with a dynamic range of 60 dB. The attenuator output is separated into two light components via a 3 dB optical fiber coupler [23]. One component enters a fiber-pigtailed focuser to focus an incident beam on the VO2 film, and the other component an optical spectrum analyzer (Yokogawa AQ6370) for spectral monitoring. The beam emerging from the focuser is launched into the film at 30° incidence. The minimum spot diameter and working distance of the focuser were ~18 µm and ~10 mm, respectively, and its position was set using an xyz translation stage for the surface spot diameter to be ~100 µm. The optical density at the film surface was ~127.3 W/cm2 at an input optical power of 10 mW. For the measurement of the I-V property of the device, a parameter analyzer (HP 4156C) and a micro-probe station containing micro-manipulators were utilized. Transient responses of electrical currents were measured in a closed-loop circuit constructed by serially connecting a resistor of resistance RE, the parameter analyzer as a voltage source for a DC bias VS, and the device, by monitoring the voltage across RE with a digital multimeter (Keithley 2000).
Fig. 1 Experimental setup for bidirectional laser triggering in planar VO2 device. The inset figure in the right upper corner shows the optical microscope image of the fabricated device with a total channel width of 30 μm ( = 6 × W).
2.2 Current-voltage behavior of device and bidirectional laser triggering regime
Figure 2(a) shows the I-V characteristics of Device I, measured in I-mode with the laser switched off or on, indicated as blue square or red circular symbols, respectively. The current compliance was set as 10 mA corresponding to a current density of ~333.33 kA/cm2. The inset shows the output optical spectrum of the fiber focuser, measured by an optical spectrum analyzer with a resolution bandwidth of 0.05 nm, when the laser is switched on. The output power of the illumination laser was ~11.60 dBm (~14.45 mW) at ~1549.99 nm, and its signal to noise ratio was ~56.69 dB. The upper and lower threshold voltages were measured as (VTh1,off, VTh2,off) = (~10.20 V, ~3.83 V) and (VTh1,on, VTh2,on) = (~2.51 V, ~1.89 V) with the LD switched off and on, respectively. Because the LD emits a Gaussian beam, a tolerance limit with respect to the spatial deviation of the beam spot is wider in the transverse direction of the VO2 patches, compared with the longitudinal direction. In laser excitation of VO2, portion of VO2 metallic grains increases with the illumination power, resulting in the decrease of the threshold voltage [11]. If the position and the power of the aligned beam are fixed, therefore, it is expected that there is no significant difference in the threshold voltage although the beam profile is changed from a Gaussian distribution into a uniform distribution. Bidirectional laser triggering can be achieved at a specific VS range designated as Region I (VTh1,on < VS < VTh2,off), and unidirectional triggering, in which the reverse PT of VO2 does not occur even when switching the laser off, happens at a range designated as Region II (VTh2,off < VS < VTh1,off).
Fig. 2 (a) I-V characteristics of Device I, measured in I-mode with laser switched off (blue square symbols) or on (red circular symbols). (b) Optical gating characteristics of Device I at increasing VS from 0 to 5 V (with laser randomly switched on or off). The inset in Fig. 2(b) indicates the current switching behavior of Device I when the laser is repeatedly switched on or off on a VS range of 3.40−3.45 V.
This can be confirmed from Fig. 2(b), which shows optically-controlled switching characteristics of Device I under VS increasing from 0 to 5 V with the laser randomly switched on or off four times per each state. During a VS range of 2.40−3.30 V, the first three bidirectional triggering operations are obtained, and at VS > 3.30 V, the device current that is dropping with the turn off of the laser rises again due to increasing VS, that is, unidirectional triggering operation is partially observed at the fourth triggering operation. It can be inferred from this optical gating (triggering) result that the bidirectional triggering regime and, in particular, the maximum on-state current of the optically-triggered device are directly determined by the I-V behavior of the device, measured in I-mode with the laser switched on or off. The inset of Fig. 2(b) shows the current switching behavior of Device I, obtained when repeatedly switching the laser on or off on a much narrower VS range of 3.40−3.45 V. As can be seen from this inset, at the above VS (selected within Region I), stable bidirectional triggering operation between ~0.161 and 10 mA is obtained by switching the illumination laser on or off. In addition, it is found from Fig. 2(b) that effective bidirectional triggering region is somewhat smaller than Region I due to laser heating effect. Because the I-V curves in Fig. 2(a) have been measured a few seconds after switching the CW laser on or off, VTh1,on and VTh2,off of the device that experiences the instantaneous change of the illumination power, as shown in the inset of Fig. 2(b), are slightly larger and smaller than lower and upper limits (~2.51 and ~3.83 V) of Region I, respectively.
2.3 Transient responses of laser-triggered device
Figure 3 shows the transient responses of laser-triggered devices (Device I and Device II) for a variety of on-state temporal durations. The compliance current was also set as 10 mA for all transient responses. Figures 3(a) and 3(b) show the transient responses of Device I laser-triggered for relatively shorter (5, 10, and 20 s) and longer (30, 60, and 120 s) on-state durations, respectively, in the closed-loop circuit with RE = 100 Ω and VS = 3.45 V. In Fig. 3(a), the bidirectional triggering operation was performed five times for each on-state duration. The off-state duration, i.e., the illumination-free interval between adjacent two triggering operations, was set to be the same as the corresponding on-state duration, except for the interval between two triggering operations with different on-state durations. In Fig. 3(b), six bidirectional triggering operations were sequentially performed for on-state durations of 30, 30, 60, 60, 120, and 30 s, and off-state durations, unlike the case in Fig. 3(a), were irregularly selected among 5, 10, and 20 s. It is observed from measured responses that stable bidirectional switching between ~0.161 and 10 mA can be obtained at VS = 3.45 V for a variety of on- or off-state durations. The average amplitude switching ratio was evaluated as ~62.0. In order to examine the dimension effect of the device on the bidirectional triggering, Device II with an increased channel width was employed for the experiments. Similarly, Figs. 3(c) and 3(d) show the transient responses of Device II laser-triggered for relatively shorter (5, 10, and 20 s) and longer (30, 60, and 120 s) on-state durations, respectively, in the closed loop circuit with RE = 10 Ω and VS = 2.76 V. The inset of Fig. 3(d) shows I-V characteristics of Device II, measured in I-mode with the laser switched off (blue square symbols) or on (red circular symbols). From the inset, the bidirectional laser triggering region (Region I) of Device II can be determined as 1.93 V < VS < 3.20 V. In comparison with Figs. 3(a) and 3(b), the number and the on-state duration of the bidirectional triggering operation were quite similar, except for a few points. In Fig. 3(c), the triggering sequence is opposite to the case of Fig. 3(a), and the off-state duration is 10 s, instead of 20 s, for the on-state duration of 20 s. In Fig. 3(d), on-state durations are replaced with 30 and 5 s in the third and sixth triggering operations, respectively. The off-state duration is 5 s, except for one interval (30 s) between the second and third triggering operations. It is observed from measured responses that stable bidirectional switching can be obtained at VS = 2.76 V for a variety of on- or off-state durations. The average amplitude switching ratio was evaluated as ~68.2.
Fig. 3 Transient responses of laser-triggered device: Device I for relatively (a) short and (b) long on-state durations and Device II for relatively (c) short and (d) long on-state durations. In the closed-loop circuit, Devices I and II were biased at VS = 3.45 and 2.76 V with RE = 100 and 10 Ω, respectively. The inset of Fig. 3(d) shows I-V characteristics of Device II, measured in I-mode with the laser switched off (blue square symbols) or on (red circular symbols).
A higher amplitude switching ratio obtained in Device II is attributed to a smaller off-state current that results from a smaller bias voltage VS determined mainly by VTh1,on. This implies that the increased channel width allowing photo excitation on lager area of VO2 causes the decrease of VTh1,on. Decreased resistances of the external resistor (RE) and the device in the closed loop circuit deteriorate the amplitude switching ratio because they increase the off-state current. Although both the external resistance and the device resistance are decreased in the transient responses shown in Figs. 3(c) and 3(d), the amplitude switching ratio is higher than that obtained in Figs. 3(a) and 3(b). This means that smaller VS and VTh1,on induced by laser excitation on larger VO2 channel dominantly affects the off-state current, and the amplitude switching ratio obtained in Device II can be further improved through the adjustment of the external resistance. Regarding the effect of the electrode separation L on the device performance, if the electrode separation L of the device is reduced, the device resistance decreases resulting in the increase of the off-state current in the laser bidirectional triggering. In particular, the bidirectional triggering region becomes narrow shrinking a bias voltage margin for the laser bidirectional triggering because the decrease of the upper threshold voltage (VTh1) is accompanied by the decrease of the lower threshold voltage (VTh2) [24]. Thus, the L reduction may increase the off-state current and decrease the bias voltage margin. On the contrary, the L increase may lower the off-state current and widen the bias voltage margin. But the excess increase of the electrode separation will limit the maximum on-state current due to the large device resistance.
In particular, the rising time was measured as ~192 ms regardless of the on-state duration, which was the minimum data acquisition interval in our experimental setup. The actual rising time is expected to be faster than the measured one. The falling time, measured as ~320 ms, was longer than the rising time, but it also showed little dependence on the on-state duration. Slower falling time attributes to laser heating effect and increased thermal conductivity in the metallic state of VO2. The laser-induced PT is mediated initially by photo-induced PT at lower illumination power, but the thermally-induced PT is beginning to dominate over the photo-excitation at higher illumination power more than 30 mW [13]. Laser-induced temperature increase makes most of the VO2 film be occupied with metallic VO2 grains and decreases the threshold voltage of the VO2 device [25] reducing the bidirectional triggering region. This thermal reduction of the bidirectional triggering region facilitates the unidirectional triggering but hinders the bidirectional triggering resulting in the deterioration of the switching contrast, caused by the increase of the off-state current. Auxiliary thermoelectric coolers attached to the device may improve the switching times and the switching contrast. Here we focused on the on-state durations which are on the order of ten seconds to examine laser-induced heating effect, coupled with photo-induced PT. Previous works on optical switches based on thermal switching of VO2 reported a switching time of 1−3 ms [14,15]. If another data acquisition instrument with a high sampling rate is utilized for shorter on-state durations, it is expected for the actual switching time to be on the order of milliseconds. Therefore, further study should include the response time of the bidirectional laser triggering when the on- or off-state duration is on the order of milliseconds or microseconds because it has been reported that the voltage-triggered PT has an ultrafast switching time of ~2 ns [26].
3. Conclusion
Bidirectional laser triggering was investigated in a two-terminal planar VO2 device using a ~1550 nm LD. The specific bias voltage range for the stable implementation of bidirectional laser triggering was determined from the I-mode I-V property of the device. 10 mA bidirectional laser triggering with a maximum amplitude switching ratio of ~68.2 was implemented in the VO2 device adequately biased at a selected DC voltage, and the transient responses of light-triggered currents were also analyzed. The high amplitude switching ratio coupled with bidirectional triggering capability could lead to further interest in the use of light-triggered oxide PT devices for future power electronics and electrical systems [27].
Acknowledgments
This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Science, ICT and future Planning(2013R1A2A2A01068390).
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