Optical properties of a-HfO 2 thin films Jebreel M. Khoshman a , Martin E. Kordesch b, ⁎ a Department of Physics, Al-Hussein Bin Talal University, Ma'an, Jordan b Department of Physics and Astronomy, Ohio University, Athens, Ohio, United States Received 27 June 2006; accepted in revised form 3 August 2006 Available online 15 September 2006 Abstract Amorphous hafnium oxide (a-HfO 2 ) thin films were grown on silicon and quartz substrates by RF reactive magnetron sputtering at temperature b 52 °C. X-ray diffraction revealed that the thin films grown on the substrates are amorphous. The optical constants of a-HfO 2 films were obtained by analysis of the measured ellipsometric spectra in the wavelength range 200–1400 nm, using the Cauchy–Urbach and Sellmeier models. Refractive indices and extinction coefficients of the films were determined to be in the range 1.86–2.15 and 0.07–2.6 × 10 - 5 , respectively. The absorption coefficients, α, of a-HfO 2 has been determined by spectroscopic ellipsometry and spectrophotometric methods over the energy range 0.88–6.2 eV. Analysis of α shows the bandgap energy of the films to be 5.68±0.09 eV. Measurement of the polarized optical properties reveals a high transmissivity (80%–97%) and low reflectivity (b 15%) in the visible and near infrared regions at angles of incidence between 10° to 80°. © 2006 Elsevier B.V. All rights reserved. PACS: 78.20.-e; 78.20.Ci Keywords: Amorphous; Hafnium oxide; RF sputtering; Ellipsometry; Optical constants 1. Introduction Hafnium oxide (HfO 2 ) is a promising dielectric material for future metal-oxide-semiconductor field effect transistor tech- nologies. This is because of its hardness, high melting point, thermal and chemical stability, and high dielectric constant. Also, HfO 2 is very resistive to impurity diffusion and inter- mixing at the interface because of its high density (9.68 gm/ cm 3 ) [1]. Due to its hardness and transparent spectral range from the infrared to the ultraviolet, HfO 2 is one of the most commonly used high index coating materials for optical components [2]. However, it can also be used as an insulator in electronic and optoelectronic devices. Growing the thin films for these applications must be usually carried out with accurate thickness control while the layers themselves have to be very thin to avoid direct tunneling and still maintain the capacitance of a much thinner SiO 2 film [1,3]. Although the optical properties of HfO 2 thin films have been studied at a variety of wavelengths, recent research mainly focuses on its crystalline structure [4–6], and there is no sys- tematic experimental and theoretical investigation of amorphous HfO 2 films. These amorphous materials, for example, find po- tential application in flexible thin film capacitors, fiber optic waveguides for communication networks, computer-memory elements, and optical coatings on polymer substrates [7,8]. Accurate knowledge of the refractive indices and absorption coefficients of amorphous materials is indispensable for un- derstanding the electronic structure of amorphous solids as well as for the design and analysis of various optoelectronic devices. In this work, we investigated the optical properties of (100 nm–750 nm) thick a-HfO 2 films over the wavelength range 200–1400 nm using variable angle spectroscopic ellipsometry (SE) and spectrophotometric (SP) techniques. These films were deposited on Si (111) and quartz substrates by RF magnetron sputtering at a temperature T b 52 °C. X-ray diffraction analysis was performed to confirm the amorphous nature of the studied films. The optical constants of the a-HfO 2 were derived by analysis of the measured ellipsometric spectra through the Cauchy–Urbach model. The absorption coefficient, α, of the studied films was obtained by the analysis of the Surface & Coatings Technology 201 (2006) 3530 – 3535 www.elsevier.com/locate/surfcoat ⁎ Corresponding author. Permanent address: Department of Physics, 251B Clippinger Labs, Ohio University, Athens, OH 45701, USA. Tel.: +1 740593 1703; fax: +1 740 593 0433. E-mail address: kordesch@phy.ohiou.edu (M.E. Kordesch). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.08.074