Electronic excitation induced controlled modifications of semiconductor-to-metal transition in epitaxial VO 2 thin films
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hul Singhal Inter University Accelerator Center, New Delhi 110067, India
Jagdish Narayan Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695
Devesh K. Avasthi Inter University Accelerator Center, New Delhi 110067, India (Received 12 September 2011; accepted 26 October 2011)
We report controlled modifications in the semiconductor-to-metal transition characteristics of VO2 single-crystal thin films induced by swift heavy ion (SHI) irradiation with varying ion fluences. At very high energies of ions (200 MeV Au), the electronic stopping (;2009 eV/Å) dominates over nuclear stopping (;16 eV/Å). Under these extreme electronic excitation conditions caused by electronic stopping and the passage of SHIs through the entire thickness of the film, creation of certain unique type of defects and disordered regions occurs. X-ray diffraction, Raman spectroscopy, infrared transmission spectroscopy, x-ray photoelectron spectroscopy (XPS), and electrical measurements were performed to investigate the characteristics and role of these defects on structural, optical, and electrical properties of VO2 thin films. XPS and electrical resistivity measurements suggest that the ion irradiation induces localized defect states that appear to correlate well with the creation of disordered regions in the VO2 thin films. The high-energy heavy-ion irradiation changes the transition characteristics drastically from a first-order to a second-order transition (electronic—Mott type). The low-temperature conductance data for these ion-irradiated films fit well with the quasiamorphous model for resistivity of VO2, where ion irradiation is believed to create mid-bandgap defect states.
I. INTRODUCTION
Vanadium oxide (VO2) exhibits a sharp first-order semiconductor-to-metal transition (SMT) at 68 °C, where the crystal structure changes from monoclinic to tetragonal (metal) at high temperatures. In VO2 single crystal, the typical SMT characteristics involve more than four orders of magnitude change in resistivity and a drastic drop in far infrared (IR) transmittance.1,2 Since the phase transformation involves crystal distortions, it is envisaged that repeated thermal cycles across the transition temperature can lead to cracking of bulk VO2 crystals making them unsuitable for practical device applications. On the other hand, thin films are able to withstand these distortions; therefore, practical devices needing repeated thermal cycling can be fabricated using only thin films. However, fabrication of high-quality VO2 thin films with desirable a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.392 J. Mater. Res., Vol. 26, No. 23, Dec 14, 2011
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properties requires a good control over epitaxial growth procedure, as the SMT characteristics of VO2 thin films are known to be a strong function of epitaxial characteristics, defects, microstructure, and stoichiometry.3,4 Recently, high-quality VO2 thin films have been grow
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