Oxygen transport in single-crystal LaAlO3 substrates

The rates of oxygen incorporation into, and oxygen diffusion in, cubic, nominally undoped ${\mathrm{LaAlO}}_{3}$ were investigated by means of isotope exchange experiments and subsequent time-of-flight secondary ion mass spectrometry analysis. $^{18}\mathrm{O}$ diffusion profiles were introduced into single-crystal samples by annealing in isotope-enriched molecular oxygen ($^{18}\mathrm{O}_{2}$) at various temperatures in the range $873\phantom{\rule{4.pt}{0ex}}\text{K}\ensuremath{\le}T\ensuremath{\le}1173\phantom{\rule{4.pt}{0ex}}\text{K}$ at an oxygen activity of $a{\mathrm{O}}_{2}=0.2$ or at various oxygen activities $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\ensuremath{\le}a{\mathrm{O}}_{2}\ensuremath{\le}1$ at $T=983\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. All measured isotope profiles consisted of a sharp drop at the surface, attributed to hindered diffusion through a depletion space-charge zone, followed by a shallow decrease, attributed to faster diffusion in a uniform bulk phase. A numerical description of an entire isotope profile with a single solution of the diffusion equation yielded the tracer diffusion coefficient ${D}^{*}$, the surface exchange coefficient ${k}^{*}$, and the surface space-charge potential ${\mathrm{\ensuremath{\Phi}}}_{0}$. The relatively high activation enthalpy of oxygen diffusion of $\mathrm{\ensuremath{\Delta}}{H}_{{D}^{*}}=(0.90\ifmmode\pm\else\textpm\fi{}0.06)\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ was attributed to oxygen vacancies interacting with cation vacancies. The activation enthalpy for oxygen surface exchange was surprisingly low ($\mathrm{\ensuremath{\Delta}}{H}_{{k}^{*}}<3\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$) in view of the large band gap of ${\mathrm{LaAlO}}_{3}$ ($\ensuremath{\approx}\phantom{\rule{0.28em}{0ex}}5.6\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$).