  The two-dimensional electron gas occurring between the band insulators
SrTiO$_3$ and LaAlO$_3$ continues to attract considerable interest, due to the
possibility of dynamic control over the carrier density, and the 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 LaAlO$_3$ layer at the growth temperature (around 800
$^o$C) in oxygen (pressure around $5\times 10^{-5}$ 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 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.
