Phase Change Materials for RF Microsystems
Vanadium Dioxide (VO2) is a phase-change material (PCM) that exhibits a solid-to-solid crystalline structure transition at 68 °C. The electrical conductivity of VO2 is that of a dielectric (or insulator) below the transition temperature and that of a conductor above it. This is caused by the transition between monoclinic dielectric phase and tetragonal rutile metallic phase. VO2 is particularly conducive to development of non-linear devices because of its relatively low transition temperature, as compared to other PCMs, e.g. germanium telluride (GeTe).
We have previously used VO2 in frequency-selective surface (also known as FSS or metamaterial) THz filters, at operating frequency band of 500 GHz. Currently, we are developing thin films of VO2 for use in micro-bolometer imagers/sensors as well as use in millimeter wave (30-300 GHz) phase shifters and switches.
Thin film deposition technique is being researched in our group using reactive DC magnetron sputtering and atomic layer deposition (ALD) to fabricate films on various substrates and films. One example of the unique feature of our fabricated films is shown in Figure 1. Here, one can observe a high resistivity contrast between the temperatures above and below the transition point. Our films exhibit low hysteresis of 4 °C and very large, non-linear change in resistivity of nearly 1×105Ω•cm.
Currently, we are studying how to improve the VO2 for use in silicon-based devices. Successful results on this front will have implications towards monolithically integrating VO2 devices with traditional integrated circuits (IC). Crystalline sapphire (Al2O3) makes an excellent basis for high quality VO2 thin films, but due to its extremely low etch rate, it is impractical for many devices. To deposit VO2 on a silicon substrate, we need a buffer layer to electrically isolate the VO2 from the semiconductor behavior of the silicon. The present challenge we are making strides in is depositing and processing alumina (Al2O3) thin films to best approximate the results from our VO2 on bulk sapphire.
1) V. Sanphuang, N. Ghalichechian, N. Nahar, J. Volakis, “Equivalent Circuit for VO2 Phase Change Material Film in Reconfigurable Frequency Selective Surfaces,” Applied Physics Letters, vol. 107, p.253106, 2015. http://dx.doi.org/10.1063/1.4938468 (Download PDF)
2) V. Sanphuang; N. Ghalichechian; N. K. Nahar; J. L. Volakis, "Reconfigurable THz Filters Using Phase-Change Material and Integrated Heater," IEEE Transactions on Terahertz Science and Technology, 2016. http://dx.doi.org/10.1109/TTHZ.2016.2560175 (Download PDF)