Structural, optical, and electrical properties of epitaxial titanium oxide thin films on LaAlO_3 substrate

摘要

Titanium oxide thin films were prepared by pulsed-laser deposition on LaAlO_3 single crystal substrate at 700 ℃. Pure anatase films are obtained at high oxygen pressure (10~(-1) mbar), while the rutile phase is evidenced at low oxygen pressure (10~(-5) mbar) despite a large oxygen deficiency (O/Ti=1.75). From asymmetric x-ray diffraction measurements, the in-plane epitaxial relationships beOtween the substrate and the titanium oxide phases are highlighted. Optical constants (refractive index n and extinction coefficient k) were deduced from ellipsometric measurements. The optical band gap energies of the anatase and rutile films are found to be 3.4 and 3.3 eV, respectively. Since the nearly stoichiometric anatase films are resistive (>10~3 H cm), the large oxygen deficiency in rutile films leads to noticeable increase in the conductivity due to the Ti~(3+) species, which supply electrons in the conduction band. At low temperature (T<200 K) the resistivity of rutile films versus temperature may be explained by a variable range hopping mechanism based on both two or three dimensional electron transfer between the Ti~(3+) and Ti~(4+) species.