Formation of a conducting LaAlO3/SrTiO3 interface studied by low-energy electron reflection during growth

The two-dimensional electron gas occurring between the band insulators ${\mathrm{SrTiO}}_{3}$ and ${\mathrm{LaAlO}}_{3}$ continues to attract considerable interest, due to the possibility of dynamic control over the carrier density and due to ensuing phenomena such as magnetism and superconductivity. The formation of this conducting interface is sensitive to the growth conditions, but despite numerous investigations there are still questions about the details of the physics involved. In particular, not much is known about the electronic structure of the growing ${\mathrm{LaAlO}}_{3}$ layer at the growth temperature (around $800{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$) in oxygen (pressure around $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ mbar), since analysis techniques at these conditions are not readily available. We developed a pulsed laser deposition system inside a low-energy electron microscope in order to study this issue. The setup allows for layer-by-layer growth control and in situ measurements of the angle-dependent electron reflection intensity, which can be used as a fingerprint of the electronic structure of the surface layers during growth. By using different substrate terminations and growth conditions we observe two families of reflectivity maps, which we can connect either to samples with an ${\mathrm{AlO}}_{2}$-rich surface and a conducting interface or to samples with a LaO-rich surface and an insulating interface. Our observations emphasize that substrate termination and stoichiometry determine the electronic structure of the growing layer, and thereby the conductance of the interface.