1 November 1999 Ž . Optics Communications 170 1999 181–185 www.elsevier.comrlocateroptcom Infrared reflectivity and dielectric permeability of ultra-thin Cu and Al films F.A. Pudonin a , R. Villagomez b, ) , O. Keller c a LebedeÕ Physical Institute, RAS, 117924 Moscow Leninskii pr.53, Russia b Centro de InÕestigacion Cientıfica y de Educacion Superior en Ensenada Baja California, Apartado Postal 2732, CP 22800, ´ ´ ´ Ensenada Baja California, Mexico c Institute of Physics, Aalborg UniÕersity, Pontoppidanstræde 103, DK-9220 Aalborg Øst, Denmark Received 2 June 1999; accepted 18 August 1999 Abstract Ž . Ž . In this report we present an experimental investigation of the reflectivity R and the dielectric permeability ´ for Cu and Al ultra-thin films ranging in thickness from a few monolayers to 12 nm at infrared and visible wavelengths. The metal Ž . films were prepared by RF-sputtering on SiO glass and Si substrates. IR reflectivity was measured at 9.2 mm, while ´ 2 Ž . Ž . was measured with the help of laser ellipsometer at a wavelength of 632.8 nm. Two types of oscillations on Rd and ´ d were discovered for two thickness regions determined by the critical thickness value d U . Oscillations at d -d U with periods Ž . Ž . Ž . U near 0.3 nm for Al and Cu films were observed on Rd and ´ d due to quantum sized effects QSEs . At d )d Ž . Ž . Ž . thickness between 6–12 nm we discover a new type of strong oscillation of Rd and ´ d with an oscillating period of ;0.2 nm. For thickness larger than 12 nm all the oscillations tend to disappear and R and ´ behave almost as their volume values. A possible explanation for the appearance of these two kinds of oscillations is based on the introduction of the critical film thickness d U . q 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Quantum wells; Ultra-thin films; Thin films 1. Introduction Ž . Quantum sized effect QSE in ultra-thin metallic Cu and Al films is clearly experimentally observed by linear optical reflection studies, both in visible Ž . Ž . ellipsometry and IR p-polarized reflection mea- surements. Very few experimental results in this ) Ž Corresponding author. Postal address: CICESE Research . Center, P.O. Box 434944, San Diego, CA 92143-4944, USA. Tel: q 52-6175-0552; fax: q 52-6175-0553; e-mail: rvillago@cicese.mx field have been reported until now. Preparation of Ž suitable thin metal film samples smooth, continuous . with required thickness and roughness is an ex- tremely difficult task and it is very advisable to explore the microscopic structure of ultra-thin metal- lic films and its growth mechanisms above and below d U by using, for instance, atomic force micro- scopic studies. On the other hand, the investigation of the optical response from metallic thin films is an important task in opto- and microelectronics. There is specific interest for research on the optical proper- Ž . ties on two dimensional 2D and quasi-2D metallic 0030-4018r99r$ - see front matter q 1999 Published by Elsevier Science B.V. All rights reserved. Ž . PII: S0030-4018 99 00467-8