DOI: 10.1007/s00339-004-3128-6 Appl. Phys. A 80, 1005–1010 (2005) Materials Science & Processing Applied Physics A m. hohage ✉ l.d. sun p. zeppenfeld Reflectance difference spectroscopy – a powerful tool to study adsorption and growth Institut für Experimentalphysik, Johannes Kepler Universität Linz, 4040 Linz, Austria Received: 18 April 2004/Accepted: 22 October 2004 Published online: 23 February 2005 • © Springer-Verlag 2005 ABSTRACT Changes in the optical anisotropy of the Cu(110) surface due to adsorption and growth have been studied by re- flectance difference spectroscopy (RDS). The optical anisotropy signal at 2.13 eV was found to be extremely sensitive to small quantities of CO, while the signal at 4.3 eV turned out to be par- ticularly sensitive to the transition from a (3 × 1) to a (2 × 1) CO superstructure. In the case of the oxygen covered Cu(110), we have successfully monitored and analyzed the transition between two oxygen induced Cu(110) reconstructions: The c(6 × 2)O and the (2 × 1)O phases have clearly distinguish- able RDS features, and the transition between them leads to a gradual transformation of the related RDS spectra. For the sys- tem Co on Cu(110)(2 × 1)O characteristic features of a CoCuO alloy phase have been identified, which allow to monitor the crucial stages of the Co thin film growth. The alloying and de-alloying are easily recognized with RDS. PACS 78.40.-q; 68.35.-p; 68.43.-h; 68.55.-a; 78.66.Bz; 78.68.+m 1 Introduction Typically, optical methods are more suited to study bulk properties, rather than to investigate surface phenomena. Only experimentally and theoretically more complicated non- linear techniques like second harmonic generation (SHG) and sum frequency generation (SFG) appeared to close this gap in applying optical methods. However, RDS – a differential optical technique based on simple linear optics – becomes surface sensitive when an isotropic bulk is terminated by an anisotropic surface. While RDS has already been widely applied to the analysis and process control of semiconduc- tors [1–6], only recently its scope was expanded to metal surfaces [7–11, 13, 14]. RDS measures the difference of the reflectivity for light linearly polarized along two orthogonal directions (x , y), normalized to the average reflectivity: Δr r = 2 r x − r y r x + r y Since RDS uses the reflection of light at normal incidence, the vectors of polarization are – in contrast to regular ellipsom- etry – always within the surface plane. In the case of cubic ✉ Fax: +43-732-2468-8509, E-mail: Michael.Hohage@jku.at crystals, RDS becomes a surface sensitive method since the bulk itself is electronically isotropic. The signal is exclusively generated by a symmetry breaking surface. The RDS signal of a cubic crystal terminated by an anisotropic surface may be contributed either by surface state transitions or by surface modified bulk transitions. In case of adsorbate layers or films grown on an anisotropic crystal surface, new specific features may be introduced in the optical anisotropy. In the following, we will show that the RDS features may be used as sensors for different modifications at metal surfaces and interfaces. 2 Experimental setup Our RDS system of the Aspnes type [15] uses light generated by a Xe-lamp with energies ranging from 1.5 to 5.5 eV. After the light is linearly polarized by a prism- polarizer, it passes a strain-free window (in case of in situ UHV experiments). The polarization direction is chosen in a way to maximize the anisotropy signal: for samples with two-fold symmetry the polarization direction is adjusted to be 45 ◦ off the primary axes of the surface, resulting in an equal projection of the polarization vector on these primary axes. After reflection from the surface, the difference of the reflec- tivity of both polarization directions leads to a rotation of the polarization vector and to a relative phase shift (ellipticity) in the reflected beam. This change in the polarization state is analyzed spectroscopically with a lock-in technique using a photoelastic modulator, analyser, diffraction grating and photomultiplier for detection. This procedure allows one to simultaneously determine the amplitude |Δr/r | and the phase shift ΔΘ and, hence, the real and imaginary part of the com- plex reflectance difference Δr/r . All optical components are placed outside the vacuum. The light is transmitted through a strain-free window into the UHV chamber, where the ex- periments discussed in this paper have been performed. The base pressure in the chamber is below 10 −10 mbar. To conduct complementary measurements the chamber is equipped with facilities for auger electron spectroscopy (AES), low energy electron diffraction (LEED), scanning tunneling microscopy (STM) and quadrupole mass spectroscopy (QMS). A high quality Cu(110) sample was mounted on a manipu- lator connected to a continuous flow He-cryostat. The sample temperature can be varied between 10 K and 1000 K and pre- cisely set or ramped to any intermediate temperature by means