Optical characterization of hafnium oxide thin films for heat mirrors M. Ramzan a,n , A.M. Rana a , E. Ahmed a , M.F. Wasiq a , A.S. Bhatti b , M. Hafeez b , A. Ali b , M.Y. Nadeem a a Department of Physics, Bahauddin Zakariya University, Multan 60800, Pakistan b Center for Micro and Nano Devices (CMND), Department of Physics, COMSATS Institute of Information Technology (CIIT), Islamabad 44000, Pakistan article info Keywords: Optical properties Annealing X-ray diffraction Atomic force microscopy Insulator-metal-insulator (IMI) structure abstract HfO 2 thin films (80 nm) fabricated at various substrate temperatures (25–120 1C) using an electron beam evaporation technique and thermally annealed at 500 1C were character- ized through X-ray diffraction, atomic force microscopy and UV–vis–NIR spectroscopy. X-ray diffraction results illustrate that as-deposited HfO 2 films are amorphous, which transform to polycrystalline (monoclinic) structure on annealing. Films reveal various morphologies and crystallite orientations, which seem to be responsible for variations in surface roughness (5–12 nm), mean crystallite size (5.2–6.3 nm), refractive index (1.43– 1.79), extinction coefficient (0.066–0.103) etc. In addition, better reflectivity (5–10%) of annealed HfO 2 films in near infrared (NIR) region has been improved by inserting a metallic Ag layer to form insulator-metal-insulator structure useful for heat mirror applications. Optical characterization of such HfO 2 (10 nm)/Ag(5 nm)/HfO 2 (10 nm) struc- ture illustrates minimum transmittance ( 35%) in the visible region and maximum reflectance ( 90%) in the visible (vis) and near infrared regions. & 2015 Elsevier Ltd. All rights reserved. 1. Introduction HfO 2 is preferred for optical devices such as optical filters, high-reflectivity mirrors, high power lasers and other optoe- lectronic devices because of its desirable properties such as high refractive index, high laser damage threshold and high transparency in the visible and ultraviolet (UV) spectral ranges [1–5]. Because of its outstanding chemical stability, electrical and mechanical properties, high-dielectric constant and wide band gap, hafnium dioxide (HfO 2 ) has been considered as one of the most important materials with a wide range of potential scientific and technological applications in electro- nics [6, 7], magneto-electronics [8, 9], optoelectronics [10] and metal oxide semiconductor devices [11]. Being a shiny, silvery, ductile and corrosion-resistant metal, oxides and other com- pounds of hafnium can be utilized in gate insulators in the 45 nm generation of integrated circuits as recommended by Intel, IBM and others [12]. In general, such physical, optical and electric properties of HfO 2 films are highly dependent on film surfaces, interface structures, morphologies and chemical stoichiometry. These aspects can usually be controlled by the preparation techniques, growth conditions and post treat- ments [3, 6–10]. So that with suitable coating materials, dielectric films having properties such as: high transparency, suitable refractive index, good adhesion, low stress, reasonable hardness, good chemical stability and environmental behavior, can be produced. In addition, multilayer films with a vast variety of optical filtering properties can be fabricated. Such multilayer films are used as spectrically selective filters which reflect the long wavelength infrared (IR) radiations (due to the properties of metal layer sandwiched between them) and transmit the most visible and NIR part of the spectrum. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/mssp Materials Science in Semiconductor Processing http://dx.doi.org/10.1016/j.mssp.2014.12.079 1369-8001/& 2015 Elsevier Ltd. All rights reserved. n Corresponding author. E-mail address: mrkhawar81@yahoo.com (M. Ramzan). Materials Science in Semiconductor Processing 32 (2015) 22–30