Growth and small-polaron conduction of hole-dopedLaTiO3+δ∕2andNdTiO3+δ∕2thin films

Abstract

$\mathrm{Re}\mathrm{Ti}{\mathrm{O}}_{3+\ensuremath{\delta}∕2}$ ($\mathrm{Re}=\mathrm{La}$ and Nd) thin films have been deposited on $(100)\mathrm{La}\mathrm{Al}{\mathrm{O}}_{3}$ substrates using the pulsed laser deposition technique. The oxygen content in the formula can be tuned to some extent by carefully adjusting the oxygen pressure during film deposition. When grown at higher oxygen pressures, the films tend to be in their fully oxidized insulating form ${\mathrm{Re}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ $(\ensuremath{\delta}=1,{\mathrm{Ti}}^{4+})$ with a twinned monoclinic structure of ${[01\overline{2}]}_{m}$ orientations. When grown in a background vacuum around $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\phantom{\rule{0.3em}{0ex}}\mathrm{Pa}$, the films are lightly hole doped $(\ensuremath{\delta}\ensuremath{\sim}0,\ensuremath{\sim}{\mathrm{Ti}}^{3+})$ and retain the near-cubic perovskite structure with the ${[001]}_{c}$ axis perpendicular to the film surface, though high-density defects may exist. These two phases coexist in the films grown at intermediate oxygen pressures. For the vacuum-grown near-cubic films, the antiferromagnetic ordering dies in the lanthanum titanate due to oxygen doping but still survives in the neodymium titanate owing to the higher electron correlation strength in the latter. The transport behavior in the $\ensuremath{\sim}{\mathrm{Ti}}^{3+}$ Mott insulating films can be described well by the small-polaron hopping or variable range hopping model. In the hole-doped metallic films, a temperature-dependent carrier density and Hall mobility were observed, especially at low temperatures. The temperature dependence of the resistance can be almost perfectly fitted by a small-polaron coherent conduction model $[{R}_{s}(T)={R}_{s}(0)+C{\ensuremath{\omega}}_{\ensuremath{\alpha}}∕{\mathrm{sinh}}^{2}({T}_{\ensuremath{\omega}}∕T)]$.