IOP PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS J. Phys. D: Appl. Phys. 40 (2007) 5256–5265 doi:10.1088/0022-3727/40/17/037 Optical properties and structure of HfO 2 thin films grown by high pressure reactive sputtering F L Mart´ ınez 1 , M Toledano-Luque 2 , J J Gand´ ıa 3 ,JC´ arabe 3 , W Bohne 4 ,JR¨ ohrich 4 , E Strub 4 and I M ´ artil 2 1 Departamento de Electr ´ onica y Tecnolog´ ıa de Computadoras, Universidad Polit´ ecnica de Cartagena, Campus Universitario Muralla del Mar, E-30202 Cartagena, Spain 2 Departamento de F´ ısica Aplicada III, Universidad Complutense de Madrid, E-28040 Madrid, Spain 3 Departamento de Energ´ ıa, CIEMAT, Avda. Complutense 22, E-28040 Madrid, Spain 4 Hahn-Meitner-Institut Berlin, Abteilung SF4, D-14109 Berlin, Germany Received 27 April 2007, in final form 20 June 2007 Published 16 August 2007 Online at stacks.iop.org/JPhysD/40/5256 Abstract Thin films of hafnium oxide (HfO 2 ) have been grown by high pressure reactive sputtering on transparent quartz substrates (UV-grade silica) and silicon wafers. Deposition conditions were adjusted to obtain polycrystalline as well as amorphous films. Optical properties of the films deposited on the silica substrates were investigated by transmittance and reflectance spectroscopy in the ultraviolet, visible and near infrared range. A numerical analysis method that takes into account the different surface roughness of the polycrystalline and amorphous films was applied to calculate the optical constants (refractive index and absorption coefficient). Amorphous films were found to have a higher refractive index and a lower transparency than polycrystalline films. This is attributed to a higher density of the amorphous samples, which was confirmed by atomic density measurements performed by heavy-ion elastic recoil detection analysis. The absorption coefficient gave an excellent fit to the Tauc law (indirect gap), which allowed a band gap value of 5.54 eV to be obtained. The structure of the films (amorphous or polycrystalline) was found to have no significant influence on the nature of the band gap. The Tauc plots also give information about the structure of the films, because the slope of the plot (the Tauc parameter) is related to the degree of order in the bond network. The amorphous samples had a larger value of the Tauc parameter, i.e. more order than the polycrystalline samples. This is indicative of a uniform bond network with percolation of the bond chains, in contrast to the randomly oriented polycrystalline grains separated by grain boundaries. 1. Introduction The electrical properties of hafnium oxide (HfO 2 ) and other metallic oxides with high dielectric constants are being currently investigated with great interest, especially due to the promising possibilities that these materials have found for replacing silicon oxide at the heart of metal–oxide– semiconductor (MOS) transistors [1, 2]. Over the past years plenty of work has been done with several dielectric materials such as silicon nitrides [3–6] and oxynitrides [7–9], while nitridation of the oxide layer has been used by the semiconductor industry to reduce tunnelling currents through the gate dielectric [10]. However, at the present scale of integration (65 nm process generation), the oxynitride has to be so thin (less than 1.7 nm) that tunnelling currents impose severe limitations to the performance of the transistors [10]. A dielectric with higher permittivity than silicon oxynitride could be made thicker and still have the same capacitance as an 0022-3727/07/175256+10$30.00 © 2007 IOP Publishing Ltd Printed in the UK 5256