Microstructural Effects in the Thermochromic Behavior of VO 2 /Al/Si Thin Film Heterostructures
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Microstructural Effects in the Thermochromic Behavior of VO2/Al/Si Thin Film Heterostructures K. Dovidenko, S. Beasor, A. Topol, H. Efstathiadis, S. Oktyabrsky, S. Shokhor1, S. Naar1 and A.E. Kaloyeros UAlbany Institute for Materials and Center for Advanced Thin Film Technology, the University at Albany-SUNY, Albany, New York, 12222, U.S.A. 1 CopyTele, Inc., Melville, New York 11747, U.S.A. ABSTRACT We present the results of detailed chemical and structural characterization of the VO2/Al/Si(100) thin film heterostructures fabricated by chemical vapor deposition and physical vapor deposition methods. The presence of the thermochromic behavior in the VO2 layer around 680C and its correlation with the layer composition and structure are discussed. The study compares the structural and chemical properties of the two types of thin VO2 layers: one which exhibits a metal-insulator phase transition around 68oC, and another which fails to undergo the temperature-induced transition. The study of interfaces was carried out by transmission electron microscopy, and showed the existence of an aluminum oxide interfacial layer between VO2 and Al for the samples with the metal-insulator phase transition. The smooth VO2/Al interface and the presence of more than one VO2 monoclinic polytypes were observed for the CVD films exhibiting no phase transition. INTRODUCTION Vanadium dioxide is a promising candidate material for use in various electronic and optoelectronic applications that are based on temperature-induced metal-insulator transitions. When heated above a threshold temperature of 68oC, vanadium dioxide experiences a reversible transition from an insulator to a metallic phase. The corresponding increase in electrical conductivity is quite significant, possibly as high as 5 orders of magnitude [1]. This rise is also accompanied by a change in its index of refraction (∆n), with ∆n exhibiting values of 0.28, 0.32, and 0.54 for incident wavelengths in, respectively, the blue, green and red spectral regions, respectively [2]. As a result, the thermochromic behavior of vanadium oxide has generated great interest for applications in a number of optoelectronic technologies. However, the existence of a wide range of vanadium oxide crystalline phases, including V4O, V2O, VO, V2O3, and V3O5 [3, 4] results in significant technological challenges since only one particular phase, namely VO2, does exhibit the desired phase transition. In the case of VO2, the insulator-to-metal transition is accompanied by a structural transformation from the monoclinic VO2 to the tetragonal VO2 phase as the temperature increases above 68oC. In order to fully examine critical material and process challenges pertaining to the reliable and reproducible development of the target VO2 system, VO2/Al thin film heterostructures were fabricated on Si(100) by chemical vapor deposition (CVD) and physical vapor deposition (PVD). Detailed chemical and structural characterization of the resulting heterostructures was carried out using Rutherford Backscattering (RBS), Auger
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