Thin Film Optical Switching Materials
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J.F. DE NATALE Rockwell Science Center, 1049 Camino Dos Rios, Thousand Oaks, CA 91358
ABSTRACT The oxides of vanadium, V0 2 and V203, are attractive materials for optical switching applications due to their large excursions in both electrical and IR optical properties. These materials can be prepared in thin film form by a number of deposition techniques, making them compatible with optical and electrical device applications. The performance of these films is a sensitive function of microstructure and processing conditions. The effects of these variables are discussed.
1.
BACKGROUND
Many transition metal oxides exhibit abrupt changes in physical properties with temperature. The most extensively studied of these are the oxides of vanadium, most notably vanadium dioxide, V0 2 , and vanadium sesquioxide, V 20 3 .These materials undergo first-order crystallographic phase transitions from low-temperature semiconducting states to high temperature metallic states. The interest in V02 and V203 derives primarily from the extremely large, discontinuous changes in optical and electrical properties associated with their phase transitions. Indeed, changes in electrical resistivity as large as 105 have been reported in V02 single crystals over a temperature interval of less than 1 K [1,2]. This large discontinuity in physical properties makes these materials extremely attractive for a wide variety of electrical and optical switching applications, such as passive thermochromic "smart" windows for solar energy control [3-5], IR modulators and spatial filters [6], and microwave switching devices [7]. The semiconductor-to-metal transition in V0 2 occurs at a nominal temperature of 67'C, with the material transforming from a low-temperature monoclinic structure to a high-temperature tetragonal form isostructural with the rutile phase of TiO2 . This transition is of shear type and requires no long-range atomic transport. Rather, the monoclinic and tetragonal structures are closely related, with the transition involving a relatively minor lattice distortion. In the lowtemperature monoclinic structure, the vanadium cations are alternately paired along the am axis and displaced slightly off-axis [8], Fig. 1. This gives rise to alternate internuclear separations of 0.316 nm and 0.262 nm. Above the transition temperature, the vanadium cations shift slightly to become equidistant along the rutile c-axis with separation 0.287 nm. This higher symmetry atomic ordering results in overlapping d-bands, eliminating the band gap and providing metallic conduction [2,8]. This transformation from a higher symmetry (P4 2/mnm) to lower symmetry (P2 1/c) state upon cooling leads to the formation of significant domain structure in the low-temperature monoclinic state [9-11]. The small atomic displacements and nondiffusional mechanism associated with the phase transformation make it inherently rapid. Theoretical estimates of the atomic rearrangement times less than 10 fsec have been reported [12], and experimental measurements support values less than
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