Photon Factory Activity Report 2004 #22 Part B (2005)
Materials Science

1C, 2C/2002S2-002

Thickness dependence of SrRuO3 epitaxial thin films studied by in-situ
photoemission spectroscopy
D. Toyota*1, M.Takizawa2, I. Ohkubo1, H. Kumigashira1, M. Oshima1,T. Ohnishi3, M. Lippmaa3, A. Fujimori2, K. Ono4, M. Kawasaki5,and H. Koinuma6
1

Department of Applied Chemistry, The Univ. of Tokyo, Tokyo 113-8656, Japan
Department of Physics and Department of Complexity Science and Engineering, The Univ. ofTokyo, Kashiwa 277-8561, Japan3Institute for Solid State Physics, The Univ. of Tokyo, Kashiwa 277-8581, Japan
4
Institute of Materials Structure Science, KEK, Tsukuba 305-0801, Japan
5
Institute for Materials Research, Tohoku Univ., Sendai 980-8577, Japan
6Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan
2Introduction
A metallic oxide SrRuO3 (SRO) with high chemical
stability has a great potential for future oxides electronic
device applications such as an electrode in theheteroepitaxial structures consisting of a variety ofperovskite-based ferroelectric, ferromagnetic, andsuperconducting films [1-3]. However, the electronic
and magnetic properties of the oxide films are very
sensitive to the film thickness and heterointerface
structure. For designing devices with desired properties,
it is critically important to understand the thickness
dependent physical properties of the films as well as tocontrol the valence band discontinuity at heterointerfaces.
In this study, we report the thickness dependence of insitu photoemission (PES) spectra of SRO layers depositedon SrTiO3 (STO) substrates.

evolved into the Ru-4d sharp peak just at EF above 15 ML,which is responsible for itinerant ferromagnetism in SRO.
These results suggest that a layer of SRO more than 15
ML in thickness is necessary to obtain intrinsic propertiesof SRO.

Experimental
SRO thin films were fabricated by a laser molecularbeam epitaxy on TiO2-terminated STO substrates. A
Nd:YAG laser was used for ablation in its frequencytripled mode (λ= 355 nm) at a repetition rate of 1 Hz.
During deposition, the substrate temperature was kept at-3750 °C and the oxygen pressure was 1 x 10 Torr. The
nominal thicknesses of the films range from 1 to 100 ML.
After thin film growth, in-situ PES measurements were
performed at the beam line BL-2C. The energy resolution
is set to 150 meV at the photon energy of 600 eV.
Results and Discussion
Figure 1 shows the PES spectra of SRO thin films withvarying the nominal film thickness. The PES spectra
show remarkable and systematic changes as a function ofSRO film thickness; (1) the PES spectra for the film
thickness of 1 – 4 ML clearly exhibit the existence of anenergy gap at the Fermi level (EF), while the Ru-4d
derived in-gap states emerge at EF above 5 ML, indicating
the occurrence of metal-insulator transition at film
thickness of 4 – 5 ML. (2) The in-gap states graduallygrow with increasing film thickness and finally haveFig.1: PES spectrum of SRO thin films
References[1] J. F. Scott , Ferroelectric Memories (Springer-Verlay,Berlin) (2000).
[2] S. C. Gausepohl et al., Appl. Phys. Lett. 67, 1313
(1995).
[3] K. S. Takahashi et al., Phys. Rev. B 67, 094413(2003).
* dtoyota@sr.t.u-tokyo.ac.jp

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