Integration of VO2 on Silicon for thermotronic applications

International audiencePresent day information processing is mainly the result of the development of the microelectronics industry where information is processed via electrical currents and voltages. Another technologically important realm of energy transport is heat. Industrial waste heat potential in EU is estimated about 300TWh/year, which is freely available yet largely remains either unharvested. Unlike its electronic counterparts, thermal information processing (thermotronics) via radiative heat flow through thermal analogs of transistors remains a nascent technology. In order to achieve this, we use a phase change material, Vanadium dioxide (VO2), and exploit its non-linear change of optical properties as a function of its temperature. VO2 undergoes a structural phase transformation (SPT) at approximately 70°C from monoclinic structure at room temperature to rutile. The two crystallographic structures have large variation in their complex refractive index in the mid-IR frequency range. In order to keep with the current trends of microelectronic industry, it is imperative to integrate VO2 on Si. However, the higher lattice mismatch and formation of oxides and silicates at the interface between VO2 and crystalline Si degrade the quality and functionality of VO2 film. Additionally, VO2 (M1) is a challenging material to integrate into patterned heterostructures because it can exist not only as multiple polymorphs (A, B, M1) but the high temperature depositions can lead to formation of various oxidation states phases that are present in the V-O system (VnO2n-1, VnO2n+1). This work was conducted to study the growth of VO2 on silicon with hafnium zirconium oxide (HZO) buffer layer. VO2 and HZO are deposited by magnetron sputtering of V2O5 and Hf0.5Zr0.5O2 ceramic targets. We studied structure-property relationship, specifically electrical and optical properties as a function of temperature across the Tc. Structural and compositional characterization are carried out using X-ray diffraction and X-ray Photoemission spectroscopy respectively, optical responses are studied under spectroscopic ellipsometry and electrical characterizations are performed using four-point probe method. The results provide insights into the influence of substrates, deposition/growth parameters, crystal structure of HZO buffer layer on the structural and physical properties of interfaces and film morphology which subsequently affect the electrical and thermal bistability of VO2. The preliminary findings mentioned here are being utilized to improve the thermal bistability, thus allowing us to improve the reproducibility in operational modes (switching, memory, logical operations, etc.) of thermotronics devices