Thin Solid Films 429 (2003) 135–143 0040-6090/03/$ - see front matter  2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0040-6090(03)00068-3 Correlation between structure, stress and optical properties in direct current sputtered molybdenum oxide films S.H. Mohamed *, O. Kappertz , J.M. Ngaruiya , T.P. Leervad Pedersen , R. Drese , M. Wuttig a,b, a a a a a,c I. Physikalisches Institut der RWTH Aachen, D-52056 Aachen, Germany a Physics Department, Faculty of Science, South Valley University, Sohag, Egypt b ISG3, Forschungszentrum, Julich, D-52428 Julich, Germany c ¨ ¨ Received 17 January 2003; received in revised form 17 January 2003; accepted 25 January 2003 Abstract MoO films were prepared by reactive DC magnetron sputtering in an atmosphere of argon plus oxygen in an effort to x determine the influence of the oxygen partial pressure on structural and optical properties of the films, as well as the resulting stress. Sputtering was carried out on glass and Si(100) substrates held at room temperature. X-Ray diffraction revealed the amorphous nature for most of the as-deposited films. Rutherford backscattering spectroscopy showed an almost linear increase in oxygen concentration at low oxygen partial pressure, while a small amount of excess oxygen was found at higher oxygen partial pressure. Optical spectroscopy and spectroscopic ellipsometry revealed that films, which were slightly substoichiometric, still showed polaron absorption of approximately 1.4 eV. For higher oxygen partial pressure, fully transparent films were deposited, which showed a slight increase in optical bandgap with increasing oxygen partial pressure, while the refractive index simultaneously decreased. This is in line with the slight decrease in film density observed in this regime by X-ray reflectometry. Possibly in this regime films possess an increasing amount of voids, since a moderate tensile stress is observed for these samples. On the other hand, a very high compressive stress is observed for low oxygen partial pressure.  2003 Elsevier Science B.V. All rights reserved. Keywords: DC sputtering; MoO ; Structural properties; Optical properties x 1. Introduction Metal oxide films show a wide variety of optical, electrical and magnetic properties that make them prime candidates for the exploration of structure–property relationships in solids and provide ample opportunities for technological applications. Molybdenum oxides are very interesting in this regard, since they show a wide range of stoichiometry with rather different and inter- esting behaviour, including chromogenic and catalytic properties. This leads to a wide range of applications, which include electrochromic display devices w1x, UV photographs w2x, optical disk memories w3x, lithium batteries w4x and diffusion barriers w5x. In addition, catalysts of MoO have been used for a variety of partial 3 oxidations, including the oxidation of methanol and ethanol w6x, propylene and butane w7x. Furthermore, *Corresponding author. Tel.: q49-16-20130364; fax: q49-24- 18022331. E-mail address: abo_95@yahoo.com (S.H. Mohamed). molybdenum trioxide has appreciable sensing capability for CO in the temperature range of 300–500 8C w8x. This wide range of interesting applications has led to numerous efforts to study the different properties of molybdenum oxides prepared using various deposition methods. The chromogenic properties (electro-, photo- and thermochromic) have been studied w1–3,9–12x. These studies used different preparation techniques, including RF sputtering w1,2x, thermal evaporation w9– 11x and electron beam evaporation w11,12x. Kolawa et al. w5x have studied reactively sputtered Mo O films 1yx x as diffusion barriers. They stated that all as-deposited Mo O films are electrically conducting except for 1yx x Mo O , which corresponds to the insulating MoO 25 75 3 phase. Dong et al. w13x have explored the structural differences of amorphous, crystalline and nano-crystal- line MoO aerogels synthesised through the sol–gel 3 process, in which heat treatment enables the production of these different forms of the same nominal material. The optical constants of thin polycrystalline MoO films 3