Investigation of the effects of thickness on the metal-insulator transition in vanadium dioxide nanocrystals, and development of a novel vanadium dioxide mott field-effect transistor
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Vanadium dioxide (VO2) is a material that has attracted a lot of attention for its prospective potential to be utilized in the eld of electrical and ultrafast optical switching in one hand, and for the fundamental physics that can be revealed through studying this strongly correlated material on the other hand. One of the most attractive qualities of VO2 is the metal-insulator transition (MIT) which takes place slightly above room temperature in this material. Controlling such phase transition through external stimuli would open unprecedented avenues of electrical and optical applications. However, thin VO2 nanocrystal are required to overcome the limitation imposed thought the Thomas-Fermi screening length which limits the changes and the control that external electrical stimuli would have on any crystal that exceeds this length. The screening length in VO2 is known to be no more than 6 nm. Here, we avoided the use of epitaxial and sputtered lms for the complications in such materials that arise from the stress due to lattice mismatch and the interdi usion with substrates in epitaxial lms, and the polycrystalline nature of sputtered lms. In this work, vapor-phase grown VO2 nanocrystals are used instead. One reason behind this is that unlike epitaxial lms vapor-phase grown VO2 nanocrystals can be released out of the growth substrate and transferred in order to eliminate the stress induced on the crystals due to adhesion to the substrate. The main shortcoming of this type of crystals, which is addressed thoroughly in this study, is that vapor-phase grown VO2 nanocrystals are produced with dimensions no less than 30 nm due to the lack of thickness control in physical vapor deposition technique. Mainly in this study, a systematic method to mill down vapor-phase grown VO2 nanocrystals to sub-5 nm thicknesses is developed. Ar-ion milling is utilized to achieve this goal. Photoresist protection and shadowing methods are introduced and used to reveal the etch rate of VO2 nanocrystals which is found to be equal to 3.3 0:3 nm/min using ion-gun energy of 1 KeV with medium monatomic ux. Our results show some surface damage caused by the Ar-ions bombardment that is limited maximum to the top 5.6 nm of the surface of the etched crystals. This damage and related changes in the electrical properties in the milled crystals are completely eliminated by short duration treatment in a 37% hydrochloric acid (HCl(aq)) solution of these crystals. The results presented here in this regards show complete recovery of the relative order of changing in resistance that accompanies the MIT of treated etched crystals when compared to their pristine form. The last part of this study is dedicated to the investigation of implementing mill down vapor-phase grown VO2 nanocrystals in possible prospective applications. Mainly, the use of these crystals in constructing Mott-Field E ect Transistors (Mott-FETs) is investigated. Further investigation are yet to be done in this regards in order to draw a nal conclusion in the possibility of using VO2 nanocrystals in reliable Mott-FETs. However, the results presented here along with the suggestions related to the fabrication of vapor-phase grown VO2 nanocrystals based three-terminal devices are of a vital importance in setting directions for future works.
Strongly correlated materials
Argon-ion beam milling