I. INTRODUCTION
Currently, smart materials are the subject of increasing attention for the scientific community because of their ability to change their properties (resistivity, dielectric, magnetic, optical constants, etc.) under external stimuli, which makes them potential attractive candidates for a broad range of applications where tunable devices and systems are needed [Reference Gevorgian1]. Among these materials, the vanadium dioxide (VO2) is an interesting system to be studied, since it presents a reversible, temperature-induced metal–insulator transition (MIT) at 68°C, transition that induces major changes in its electrical and optical properties [Reference Morin2, Reference Stefanovich, Pergament and Stefanovich3]. Thus, at room temperature, VO2 has the properties of a semiconductor with monoclinic crystal structure but above its transition temperature of 68°C, the material behaves like a metal, with a rutile tetragonal structure. Consequently, this electronic and structural transition is accompanied by a significant change in the electrical resistivity of the material (three to five orders of magnitude) and modifies consequently its optical and tribological properties [Reference Morin2, Reference Stefanovich, Pergament and Stefanovich3]. More interesting, besides the thermal-induced transition [Reference Morin2, Reference Kim4], the MIT of VO2 thin films can be triggered on faster timescales (picoseconds to nanoseconds) using different stimuli: electron injection (voltage- or current-induced MIT) [Reference Stefanovich, Pergament and Stefanovich3, Reference Kim4], optically (photon absorption) [Reference Cavalleri5], or under the effect of an external stress [Reference Kikuzuki and Lippmaa6]. The electrically triggered MIT-inducing resistivity change of VO2-based two-terminal (2 T) devices was already used to demonstrate electrical switches from DC to microwave [Reference Dumas-Bouchiat, Champeaux, Catherinot, Crunteanu and Blondy7], and THz frequencies [Reference Choi8], integrated reconfigurable filters in the microwave domain [Reference Dumas-Bouchiat, Champeaux, Catherinot, Givernaud, Crunteanu and Blondy9, Reference Bouyge10] or microwave power limiters [Reference Givernaud11].
Recently, several reports [Reference Kim4, Reference Sakai12] highlighted the non-linear I–V characteristic of electrical switches based on VO2 thin layers and the occurrence of periodic signals (self-oscillations) maintained across 2 T devices. The phenomenon is initiated by applying a continuous voltage to the 2 T device (carrier injection).
In this paper, we demonstrate the appearance of similar self-oscillations across VO2-based 2 T devices in the case of continuous or pulsed current injection in the devices. We will show that this current activation scheme generating self-oscillations across the device has several advantages compared to the voltage-activated oscillations and we will try to establish a physical model explaining its appearance in the non-linear device. We will also compare the properties and the conditions for self-oscillation generation in the VO2-based 2 T devices for both current- and voltage-activated phenomena.
II. EXPERIMENTAL
The 2 T VO2-based electrical devices were fabricated in a similar way as presented in [Reference Dumas-Bouchiat, Champeaux, Catherinot, Crunteanu and Blondy7, Reference Crunteanu13]. Briefly, VO2 thin films were deposited on C-type sapphire substrates using reactive laser ablation of a pure vanadium target (99.95%) in an oxygen atmosphere (the experimental conditions are detailed elsewhere [Reference Dumas-Bouchiat, Champeaux, Catherinot, Givernaud, Crunteanu and Blondy9]). The obtained films, with thicknesses up to 200 nm, are crystalline and show a resistivity change of four to five orders of magnitude during the thermally initiated MIT [Reference Givernaud11]. The 2 T electrical switches were manufactured in clean room environment [Reference Dumas-Bouchiat, Champeaux, Catherinot, Givernaud, Crunteanu and Blondy9, Reference Givernaud11] using conventional micro-fabrication techniques. We prepared rectangular patterns of VO2 (defined by optical lithography and wet etching) on which we deposit a pair of metallic electrodes (Cr/Au bi-layer) separated by different distances. The distance between the metallic electrodes defines the length of the VO2 device [Reference Crunteanu13]. We investigated devices with different lengths from 5 µm up to 350 µm.
The current–voltage (I–V) characteristics of the obtained 2 T devices is recorded by introducing the device into a simple electrical circuit (insert in Fig. 1(a)), containing a series resistance, R S (typical 1 kΩ), and a source meter (Keithley 2612A) operating in both voltage and current modes. The source provides power to the circuit and allows measuring the circuit current and the voltage across its terminals. For studying the appearance of the oscillation regime, we measured the voltages across all the circuit elements (VO2 device and series resistance) using a broad bandwidth four-channel oscilloscope (Tektronix DPO7254): the first channel was used to measure the voltage across the VO2 device, while the second one monitors the current flowing in the circuit (through the voltage drop in the series resistor).
Fig. 1. (a) I–V characteristic in voltage- and current-mode of a device incorporating a VO2 pattern (25-µm-long and 18-µm-wide) within insert, the test circuit; (b) zoom on the I–V characteristic of the current mode.
III. RESULTS AND DISCUSSION
A) Non-linear I–V characteristic of VO2-based devices
A typical I–V characteristic of a 2 T VO2-based switch (VO2 pattern of 25 µm in length and 18 µm in width) is shown in Fig. 1(a) for both voltage and current modes. The MIT electrically induced MIT transition is clearly visible for both activation modes as an abrupt change in the device's resistance, marked by a sudden increase of current in the circuit for V th ~ 12 V in the voltage mode (indicated by an arrow, red curve) or by a sudden decrease of the switch voltage in the current mode, for injected currents of I th ~ 4 mA. In the voltage-activated MIT mode, the transition presents an important hysteresis loop (when sweeping back the voltage), associated probably with thermal effects developed in the device due to “jumping” toward high current values after the MIT. This hysteretic behavior becomes practically inexistent during the current-activated MIT (almost perfect superposition of the two upward and downward current-sweeping curves, see Fig. 1(a)). For both types of electrical activation, one can identify three main regions in the I–V characteristic, as indicated in Fig. 1(b) for the current-activated transition: the first region (A), of low currents (between 0 and ~4 mA for the investigated device), corresponding to a highly resistive, semi-conducting state for the VO2 layers, the second, highly non-linear region (B) related to the MIT transition of the material and, finally, the third zone (C) corresponding to a low resistance state of the VO2, when the material changes to its metallic state (strong currents, above 15 mA). As indicated in Fig. 1(b), the non-linear region (B) (in the current mode) shows two sub-regions presenting negative differential resistance (NDR1 and NDR2). It was suggested that the emergence of these NDR regions is the result of a progressive apparition of the MIT in the VO2 material, in a percolative manner (gradual growth of metallic VO2 nano-domains within the VO2 semiconductor matrix and co-existence between the semi-conductor and metallic domains in the VO2 films) [Reference Kikuzuki and Lippmaa6, Reference Chang14, Reference Rozen, Lopez, Haglund and Feldman15]. Similar 2 T VO2 circuits integrated on coplanar waveguides were previously extensively studied in the RF/microwave domains [Reference Dumas-Bouchiat, Champeaux, Catherinot, Crunteanu and Blondy7, Reference Dumas-Bouchiat, Champeaux, Catherinot, Givernaud, Crunteanu and Blondy9–Reference Givernaud11] and besides their employment as broadband, high-speed microwave switches, they show potential to sustain RF powers levels up to several watts, depending on their geometry and type of implementation (shunt or series configurations) [Reference Givernaud11].
B) Current-induced self-oscillations in the VO2 switches
It is well known that the presence of an NDR region in a component I–V characteristic is one of the prerequisite properties for obtaining current/voltage oscillations [Reference Kishida, Ito, Nakamura, Takaishi and Yamashita16, Reference Mori, Bando, Kawamoto, Terasaki, Takimiya and Otsubo17]. To verify this and for initiating the oscillating phenomenon in the current mode, we applied in the electrical circuit (insert in Fig. 1(a)) square-type current signals (2-ms long) with different amplitudes that corresponds to the different regions (A, B-NDR1 and B-NDR2 and C) in the I–V characteristic of the 25 µm × 18 µm VO2 device.
As illustrated in Fig. 2, we observed the apparition of the current-induced self-oscillations in the VO2 material only for current amplitudes corresponding to the first NDR region (NDR1). Thus, the origin of these oscillations is directly related to the presence of the NDR region in the I–V characteristic of the device operated in the current mode.
Fig. 2. Voltage pulses observed across of VO2 pattern and series resistance during the injection of current pulses of 2 ms with different amplitudes in the device of Fig. 1.
Figure 3 shows the typical shape of the voltage oscillation over a period for both the voltage across the VO2 device (red curve) and the voltage across the series resistance (blue curve) (for a 350-µm-long and 50-µm-wide device, heated to 50°C). The shape of the VO2 voltage curve suggests behavior similar to a relaxation oscillator: a capacitance (the VO2 material, seen as a percolative mixture of metallic and isolating nano-domains) is charging until the electrical field across it reaches a critical value (zone (E) in Fig. 3). For certain materials, this critical field may cause the destruction of the material because it corresponds to the maximum electrical field that the material can handle (disruptive or electrical breakdown field). In the case of VO2, this threshold electrical field will trigger the MIT transition and the material changes abruptly to a low-resistance metallic state, leading to the formation of a highly conductive path between the two metallic electrodes (zone (F) in Fig. 3). The charges accumulated on the electrodes of the device are rapidly released in the circuit and a current pulse is created (corresponding to the voltage pulse on the series resistor, blue curve in Fig. 3). The electric field across the VO2 switch drops sharply to values corresponding to the VO2's isolating state and the material recovers its original semiconductor state and, if the conditions for the onset of oscillating behavior are satisfied, a new cycle of oscillations begins. During one period of oscillation (zones (E) and (F) in Fig. 3), the shape of the VO2 voltage-charging curve (before the initiation of the MIT) can be fitted (equivalent to several constants) with a well-known expression derived from an RC circuit analysis:
where V ap is the overall delivered voltage by the source meter and τ = RC T is the time constant of the circuit, as the product of the overall resistance in the circuit (R S + R VO2) and of its overall capacitance, C T.
Fig. 3. Evolution of the voltage across the pattern of VO2 (round symbols) and across the series resistance (squared symbols) during a period of oscillation for a device incorporating a VO2 pattern of 350-µm-long and 50-µm-wide, heated to 50°C.
The equivalent circuit of the VO2 2 T switch (in the electrical diagram in the inset in Fig. 1(a)) can be represented as a variable capacitor, C VO2, in parallel with a variable resistor, R VO2, both of which are functions of the voltage across the device (but also of the ambient temperature as for the thermally triggered MIT [Reference Morin2–Reference Kim4, Reference Dumas-Bouchiat, Champeaux, Catherinot, Crunteanu and Blondy7]). Most likely, the shape of the VO2 voltage-charging curve during oscillations in the zone (E) in Fig. 3 is governed by the overall capacitance (C T) of the electrical circuit depicted in the inset in Fig. 1(a). This overall capacitance includes, besides the variable capacitance C VO2, also a small contribution from parasitic capacitances coming from connecting cables. To verify the influence of external capacitances in the circuit, we included, in parallel to the VO2 switch, additional capacitors with precise capacitance values (C P). Preliminary results (experiments are still underway) show that, in these cases, the self-oscillation frequencies decrease with increase in the added capacitances values. The phenomenon can be explained by an increase in the overall capacitance of the circuit (C P and C VO2 will add, since in parallel) and, consequently, of the time constant in the circuit (τ = RC T), which involves a decrease in the oscillating frequencies.
The self-oscillation frequency appearing in the NDR1 varies between 1 kHz and 1 MHz, depending primarily not only on the size of the VO2 pattern but also on excitation or external environmental parameters (temperature) as will be shown in Section III(D).
Regarding the second NDR region (NDR2) of the device I–V characteristic, for the excitation circuit we used, we were not able to identify the onset of electrical oscillations. A possible explanation is that this NDR2 is not intrinsic to the VO2 material as is the NDR1, but rather is circuit dependent. When increasing the current along the I–V characteristic, at the end of the NDR1, the component has a negative resistance that largely compensates that of the series resistance, R S. Further increasing the current will decrease (in absolute value) this negative resistance until the point where R S will become larger (at the end of NDR2). After this point, the component will enter an ohmic regime (metallic region in Fig. 1(b)). However, it is possible to initiate oscillations also in this NDR2 zone, by integrating in the excitation circuit additional elements (R, C, and L components).
C) Voltage-induced self-oscillations in the VO2 devices
In the voltage-excitation mode, the oscillatory phenomenon seems more difficult to initiate although the oscillating mechanism is the same as for the current mode. As already demonstrated [Reference Kim4, Reference Sakai12], the appearance of the oscillating phenomenon is conditioned by specific ranges of excitation voltage and series resistance in the circuit, as shown in Fig. 4. In agreement with previous results [Reference Kim4, Reference Sakai12], we obtained a specific “window” in the applied voltage series resistance diagram for which the onset of the self-oscillating phenomenon is possible. The properties and the shape of the oscillations are similar to those obtained for the current-activation mode. We note that the oscillations do not occur for low series resistances, R S, but only for specific values. In fact, the couple “applied voltage”–“series resistance” defines a load-line in the circuit which, if properly chosen, will intercept the I–V characteristics at current values corresponding to the NDR1 and thus, initiating the self-oscillating mechanism.
Fig. 4. Evolution of the voltage range where oscillations are observed as a function of the series resistance on a device with a VO2 pattern 25 × 18 µm2.
D) Self-oscillation's amplitude and frequency dependence on excitation parameters
We investigated the influence of the excitation amplitude (for both current- or voltage-activation modes) on the amplitude and frequency of the self-oscillating phenomenon appearing across the VO2-based devices. The two graphs in Fig. 5 show that for both activation modes, an increase in the excitation current or voltage (in the limits imposed by the NDR1) results in an increase in the oscillation frequencies and slightly reduces their amplitudes. In both cases, these variations can be explained by the percolative mechanism responsible for the onset of self-oscillations: if the applied voltage or current increases, the proportion of metallic domains within the insulating material increases causing a decrease in the device resistivity and possibly in its overall capacitance. Since the oscillating phenomenon is governed by an RC-type time constant (the rising part of the VO2 voltage oscillations), the frequency will decrease with decrease in the time constant τ = RC.
Fig. 5. (a) Self-oscillations observed during the application of DC currents of different values in the NDR1 of the I–V characteristic of a device with a VO2 pattern of 350 × 50 µm2 and (b) self-oscillations observed for different voltage values for a VO2 pattern of 25 × 18 µm2.
Another important parameter affecting the oscillations properties is the device temperature. Increasing the temperature of the VO2 device will result in an increase in the oscillation frequencies and in a more marked decrease of their amplitudes for both current- and voltage-induced oscillations (Figs 6(a) and 6(b), respectively). This behavior can be simply explained by a decrease of the initial device resistivity with temperature (the initial τ = RC time constant decreasing consequently with the temperature). It is also worth noting that above 68°C (temperature transition of the VO2 material), no oscillating phenomenon occurs, since the material is totally transformed to its metallic state and its I–V characteristic is typical of a resistance (ohmic type).
Fig. 6. (a) Self-oscillations observed in a device with a VO2 pattern of 350 × 50 µm2 for different temperatures in the case of a DC current (3 mA) and (b) in the case of a voltage of 35 V on a VO2 pattern of 25 × 18 µm2.
The impact of the series resistances in the excitation circuit on the characteristics of the oscillations was also investigated, for both excitation modes. In the case of current-induced self-oscillations R S has little impact on the oscillation frequencies and affects moderately the oscillations amplitudes (Fig. 7(a)): the frequencies are slightly increased and the amplitude tends to increase as R S grows stronger. Instead, for the voltage-induced self-oscillations mode, the increase in the values of the series resistance will slightly reduce the oscillations amplitudes but will strongly affect the frequencies of oscillations, decreasing them. It is relatively difficult to explain this phenomenon for the voltage-activated mode, but intuitively, a modification of the R S (within the limits imposed by the oscillation window shown in Fig. 4) will affect the way the load-line (applied voltage-series resistance) excites the NDR region of the device.
Fig. 7. (a) Self-oscillations observed in a device incorporating a VO2 pattern of 350 × 50 µm2 for different values of resistance during the application of a DC current of and (b) in the case of a voltage of 35 V on a VO2 pattern of 25 × 18 µm2.
IV. CONCLUSIONS
In conclusion, we demonstrated electrical self-oscillations generation in simple 2 T VO2-based devices using their NDR properties, in both current- or voltage-activated modes. The physical mechanism explaining the onset of the oscillating phenomenon can be explained by the dynamic percolative occurrence of metallic and isolating nano-domains within the VO2 material during excitation which makes the device behave like a charging capacitor. We demonstrated that, for both excitation modes, the oscillation amplitude and frequencies can be controlled by the values of the continuous excitation signal, the circuit parameters, the geometrical parameters of the devices, and by external parameters such as temperature. Work is in progress for expanding the oscillation frequencies of the presented devices toward higher values (in the RF/microwave frequency domains) by coupling them with external RLC (resistance-inductance-capacitance) resonators or designing in-plane metal-insulator-metal (MIM)-type devices. The results presented here can provide simple, innovative solution for applications in the oxide electronics field: on-chip, highly integrated inverters and on-chip oscillators, a.c. signal generators for integrated nano-devices, extremely sensitive pressure/position and temperature sensors, etc.
ACKNOWLEDGEMENT
This work was supported by the ANR France under project “Admos-VO2”, ANR 07-JCJC-0047.
Jonathan Leroy received the Master's degree in electronics from the University of Limoges, Limoges, France, in 2010. He is currently working toward obtaining his Ph.D. at the University of Limoges. Since 2010, he is with the XLIM Research Institute, CNRS/University of Limoges. His research interests include the study of smart materials (mainly VO2) and their applications in RF/microwave and THz devices.
Aurelian Crunteanu received the Phys. Eng. Degree in optics and optical technologies, Master's degree, and Ph.D. degree in physics from the University of Bucharest, Bucharest, Romania, in 1995, 1996, and 2000, respectively, and the Ph.D. degree in material sciences from the Claude Bernard University, Lyon 1, France, in 2001. From 2001 to 2003, as a Post-Doctoral Fellow with the Institute of Imaging and Applied Optics, Swiss Federal Institute of Technology, Lausanne, Switzerland, his research was oriented to the fabrication and characterization of micro- and nanostructures in laser host materials, and laser-assisted thin-film deposition. Since 2003, he is a Researcher with the Centre National de la Recherche Scientifique (CNRS), XLIM Research Institute, University of Limoges, France. His current research activities are focused on the development of new materials for microelectronics and optics, RF-MEMS reliability, and optical switching using MEMS technology.
Julien Givernaud received the Bachelor degree in material science from the Engineering National School of Limoges (ENSIL), France, in 2006, and he is currently working for obtaining his Ph.D. at the University of Limoges. Since 2007, he is with the XLIM Research Institute, CNRS/University of Limoges. His research interests include the integration of smart materials (mainly VO2) in RF microwave devices.
Jean-Christophe Orlianges received the Ph.D. degree in material sciences from the University of Limoges, Limoges, France, in 2003. From 2008 to 2009, he was with the “Centre National de la Recherche Scientifique” (CNRS), as a Research Engineer with XLIM Laboratory. He is currently an Assistant Professor with the Faculty of Science, University of Limoges. He conducts research with the “Sciences des Procédés Céramiques et de Traitements de Surface” (SPCTS) Laboratory, Unité Mixte de Recherche (UMR) 6638, CNRS/University of Limoges. His main research interests include pulsed-laser thin-films deposition techniques, nanostructured materials, development, and integration of new materials in electronic and optic devices.
Corinne Champeaux received the Ph.D. degree in electrical engineering from the University of Limoges, Limoges, France, in 1992. Since 1992, she has been an Assistant Professor with the Faculty of Science, University of Limoges. She currently conducts research with the Sciences des Procédés Céramiques et de Traitements de Surface (SPCTS) Laboratory, Unité Mixte de Recherche (UMR) 6638, Centre National de la Recherche Scientifique (CNRS), University of Limoges. Her main research interests are laser–matter interactions and pulsed-laser thin-films deposition techniques. She is involved with the development and fabrication of MEMS components through the elaboration of new materials and fabrication processes.
Pierre Blondy (M'00) received the Ph.D. and Habilitation degrees from the University of Limoges, Limoges, France, in 1998 and 2003, respectively. From 1998 to 2006, he was with the Centre National de la Recherche Scientifique (CNRS), as a Research Engineer with XLIM Laboratory, where he began research on RF-MEMS technology and applications to microwave circuits. He is currently a Professor at the University of Limoges, leading a research group on RF-MEMS. He was a visiting researcher at the University of Michigan, Ann Arbor, USA in 1997 and at the University of California at San Diego, La Jolla, USA in 2006 and 2008. Dr Blondy was an Associate Editor for the IEEE Microwave and Wireless Components Letters in 2006. He is a member of the IEEE International Microwave Conference Technical Program Committee since 2003.
