Transparent, high refractive index oxides: Control of the nanostructure of titanium hafnium oxide alloys by variation of
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Transparent, high refractive index oxides: Control of the nanostructure of titanium hafnium oxide alloys by variation of the ion energy during reactive magnetron sputtering deposition Juan J. Díaz León1,2, Matthew P. Garrett1,2, David M. Fryauf1,2, Junce Zhang1,2, Kate J. Norris1,2, Sharka M. Prokes3, Nobuhiko P. Kobayashi1,2 1 Baskin School of Engineering, University of California Santa Cruz Santa Cruz, California, U.S.A. 2 Nanostructured Energy Conversion Technology and Research (NECTAR) Advanced Studies Laboratories, University of California Santa Cruz and NASA Ames Research Center, Moffett Field, California, U.S.A. 3 Code 6786, Naval Research Laboratory, Washington, DC (USA) ABSTRACT A range of optical and optoelectronic applications would benefit from high refractive index (n), dense and transparent films that guide, concentrate and couple light. However, materials with high n usually have a high optical extinction coefficient ( ) which keeps these materials from being suitable for optical components that require long optical paths. We studied titanium hafnium oxide alloy films to obtain high refractive index (n>2) with minimum optical extinction coefficients ( < 10 ) over the visible and near IR spectrum (380-930 nm). Titanium hafnium oxide alloys were deposited using pulsed DC reactive magnetron sputtering with and without RF substrate bias on silicon dioxide. For a given deposition condition intended for a specific titanium/hafnium molar fraction ratio, the ion energy of deposition species was explicitly controlled by varying the RF substrate bias. Spectroscopic ellipsometry, transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS) and atomic force microscopy (AFM) were used to characterize the films. It appears that applying RF substrate bias reduces the nanocrystalline size, changes the surface morphology and increases the refractive index while maintaining comparable titanium/hafnium cation molar fraction. Precise control of the nanostructure of ternary metal oxides can alter their macroscopic properties, resulting in improved optical films. INTRODUCTION The addition of cations to binary oxides to form ternary and quaternary metal oxides provides flexibility in designing material properties, providing opportunies to optimize material properties exclusive to each other in binaries such as transparency, electrical conductivity or corrosion resistance 1–3. A large library of ternary metal oxides has been studied for different applications. For example, aluminum zinc oxide (AZO) and indium tin oxide (ITO) are used as transparent conductive oxides 1,4. Aluminum titanium oxide films have been studied as novel transparent corrosion barriers 2. Titanium hafnium oxide alloys have been studied as gate insulators due to their high dielectric constant, low leakage current and good thermal stability 5–8. Titanium hafnium oxide alloys are also interesting because of their large bandgap and low absorption in the visible spectrum 9. In our previous study, we compared a number of titanium hafnium
