Improved Switching Response of VO 2 Devices Deposited on Silicon Nitride Membranes
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Improved Switching Response of VO2 Devices Deposited on Silicon Nitride Membranes Yan Wang1, 2 and John F. Muth1 1 Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, North Carolina 27695, USA 2 Department of Physics, North Carolina State University, Raleigh, North Carolina 27695, USA
ABSTRACT VO2 films were deposited on sapphire, ITO glass and 200 nm thick silicon nitride membranes by Pulsed Laser Deposition (PLD). The electrical and optical properties have been investigated. Joule heating devices fabricated on silicon nitride membranes switches from semiconductor phase to metal phase by applying a constant voltage across two metal contacts. Compared to the devices fabricated on the normal substrates, such as sapphire, silicon or glasses, the switching speed of the devices on membrane is an order of magnitude faster. Decreasing the area and thickness of VO2 on top of thinner membranes allows kHz bandwidth to be achieved. INTRODUCTION VO2 is well known for its semiconductor to metal transition at 68 ºC, which is associated with an abrupt change of resistance and a decrease in infrared wavelength transmittance [1-4]. These properties make VO2 an interesting material for novel electronic and optical device applications, such as optical switches, reconfigurable antennas and smart window [5]. In this work, highly oriented crystalline VO2 films were deposited on different substrates by Pulsed Laser Deposition (PLD). The results show the transition temperature, the width of the hysteresis loop, and the amplitude of the transition depend strongly on the substrates, which affect the strain on VO2 films. The VO2 thin films on sapphire show large amplitude transition (4 orders) and narrow hysteresis, about 8 oC. The transition temperature of heating and cooling are 70 oC and 62 oC respectively. VO2 thin films were also deposited on top of SiNx membrane to construct VO2 devices. On the 200 nm thick SiNx membrane, the VO2 films show 3 orders resistance change and a 15 oC wide hysteresis loop during the semiconductor to metal transition. The device switches from semiconductor phase to metal phase by applying a constant voltage across two metal contacts. The transition is caused by joule heating from the current flowing through the VO2 thin films. A 300 ȝm × 300 ȝm area/100 nm thick VO2 device takes about 1000 ȝs to reach the fully “on” state. The transition time of the devices can be controlled by changing the area and the thickness of VO2 thin films. EXPERIMENT Pulsed Laser Deposition (PLD) was used to deposit the VO2 films on different substrates. A KrF excimer laser at a wavelength of 248 nm was focused on a homemade V2O5 target. The repetition rate and energy level of laser pulses were kept at 10 Hz and 200 mJ during deposition. The base pressure of the deposition chamber was pumped to 10-7 Torr. The depositions were performed in an ambient of 20 mTorr oxygen and the substrate temperature was maintained at 800 °C. After the deposition, the samples were cooled down at the deposition o
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