Structural and optical properties of ultrathin films

¹ Beijing National Laboratory for Condensed Matter Physics, Institute of Physics Chinese Academy of Sciences - P.O. Box 603, Beijing 100080, PRC
² Department of Physics, Capital Normal University - Beijing 100037, PRC

Corresponding author: zhchen@aphy.iphy.ac.cn

Received: 27 June 2005
Accepted: 2 November 2005

Abstract

Using laser molecular beam epitaxy technique, ultrathin films of 7, 12, 25, 750 monolayer (ML) thicknesses were obtained on substrates. The effect of reducing the thickness of a film on its structural and optical properties was investigated by Raman scattering measurement, X-ray diffraction technique, and optical transmittance spectra. In situ reflection high-energy electron diffraction and X-ray diffraction observations have revealed that the films are epitaxially grown on substrates from the initial stage without any other phase formation. Even in the sample of 7, Raman scattering can reveal the structural information of the film. The ultrathin films have a highly c-axis–oriented tetragonal phase, and the formation of the perovskite phase is found in films as thin as 7. An increase of the energy gap and the blue shifts of Raman modes are observed with reduction of the film thickness and the possible origin was analyzed.