The influence of mixed phases on optical properties of HfO 2 thin films prepared by thermal oxidation
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ium dioxide (HfO2) thin films were synthesized on silicon and quartz substrates by thermal oxidation of metallic hafnium films in oxygen. The crystalline structure and optical properties of the HfO2 films were systematically investigated using x-ray diffraction, ultraviolet (UV)-Raman, and UV-visible spectrophotometer techniques. All the films thermally oxidized at 450 to 800 °C were mostly monoclinic. Interestingly, cubic phase coexisted with monoclinic phase in the films thermally oxidized at 500 to 600 °C. The corresponding optical band gap (Eg) varied from 5.92 to 6.08 eV for the films with a different phase ratio (cubic to monoclinic one) ranging between 0 and 1:3. These results imply that the mixed phase could have a certain effect on the increase of the Eg of HfO2 films. I. INTRODUCTION
Because of their outstanding chemical stability, electrical and mechanical properties, high-dielectric constant, and wide band gap, hafnium dioxide (HfO2) materials have been considered as one of the most important materials with a wide range of potential scientific and technological applications, such as electronics,1,2 magnetoelectronics,3,4 optoelectronics,5 and metal oxide semiconductor devices.6,7 Moreover, HfO2 is prospective material for optical filters, high-reflectivity mirrors, highpower lasers, and other optoelectric devices8–11 because of their high refractive index, low absorption from nearultraviolet (UV) to mid-infrared (IR), and high threshold for laser damage. It is well known that the optical property of HfO2 is one of the most important prerequisites for the preparation of HfO2 films with desired functions. Generally, the optical and electric properties of HfO2 films are highly dependent on the surface and interface structures, morphologies, and chemical stoichiometric. These aspects can usually be controlled by the preparation techniques, growth conditions, and post treatments.3–5,8–11 Recently, the optical properties of HfO2 films grown by various methods have been extensively studied.5,8–12 Much effort has been made to investigate the optical properties of HfO2 films with different structures.11,12 HfO2 has been known to possess several crystalline phases, such as monoclinic, tetragonal, and cubic ones.12,13 However, few studies have been focused on the optical properties of HfO2 films with mixed phases. Therefore, to explore the ability to tailor the a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2010.61 50
J. Mater. Res., Vol. 26, No. 1, Jan 14, 2011
http://journals.cambridge.org
Downloaded: 14 Mar 2015
properties so as to optimize performance requires a detailed understanding of the relationship between the optical properties and mixed phase. This work was, therefore, performed on thermally oxidized HfO2 thin films with mixed phase (monoclinic and cubic) to understand the influence of mixed phase on the optical properties. Our experimental results reveal that the mixed phase has a certain effect on the increase of the optical band gap (Eg) of HfO2 films. II. EXPERIMENTAL PRO
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