Molecular volume and polarizability in the amorphous dielectric Zr 0.2 Sn 0.2 Ti 0.6 O 2
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Molecular volume and polarizability in the amorphous dielectric Zr0.2 Sn0.2Ti0.6O2 S. C. Barron and R. B. van Dover Department of Materials Science and Engineering Cornell University, Ithaca NY 14853, USA ABSTRACT The dielectric constant of a material is strongly dependent on both the polarizability and the density of the material through the Clausius Mossotti equation. While the atomic polarizability depends on the stoichiometry of the constituent atoms, the molecular polarizability is a function of the atoms’ short range bonding structure and hence can be strongly dependent on processing conditions. Since the density of the material also depends on the thermally activated diffusivity of atoms during processing, varying the processing temperatures has an effect on both the molecular polarizability and the density. The dielectric constant of Zr0.2 Sn0.2Ti0.6O2 is shown to be a strong function of the substrate temperature during sputter deposition with the highest value ε~55 at 200 °C and lower values at both higher and lower process temperatures. We have investigated the bonding structure and density of the oxide dielectric deposited at a variety of substrate temperatures in order to elucidate the relative effects of each. INTRODUCTION As new semiconductor systems are developed, the need for new dielectric materials becomes prevalent. The benchmark dielectric for the past thirty years has been silicon dioxide for its ease of processing in standard silicon technology. Novel semiconductor systems, such as carbon nanotubes and other organic based materials, are promising in niche applications, however, and have different processing and property requirements than the standard silicon technologies. There is an opportunity, therefore, to develop new dielectric materials whose properties can be exploited by these new semiconductors. Regardless of the semiconductor chosen, a key parameter for the dielectric material is the maximum areal charge density, CVbr/A = εoεrEbr, i.e. the charge that can be supported at the breakdown field (Ebr) of the dielectric. This figure of merit (FOM) physically corresponds to the charge that can be gated into the channel region of a transistor. In 1998, van Dover et al. used a continuous composition spread technique to search for complex amorphous oxides which optimize this FOM [1]. They found that the amorphous form of Zr0.15Sn0.3Ti0.55O2-δ has a dielectric constant of 62.5 and a breakdown field of 4.4 MV/cm. The dielectric could therefore gate up to 24.3 µC/cm2 into a semiconductor; this represents a six-fold improvement over the maximum possible performance of SiO2 and three-fold over some of the other high performance dielectrics under investigation such as ZrO2 and HfO2 [2]. Van Dover et al. used a reactive rf sputter deposition technique to deposit the material using both on- and off-axis configurations [1,3]. They found that the material deposited on a substrate heated to 200 °C gave the optimal properties and a dielectric constant of 62.5 at 10 kHz. Lu et al. [4] prepare
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