Understanding polarity compensation across polar LaAlO$_3$ films

Dipole screening mechanisms for polar crystals can manifest in a variety of ways depending on bandgaps, surface energies and environmental conditions. Here we study the polarity compensation process in LaAlO$_{3}$ thin film grown on the two different surface terminations of [001] SrTiO$_{3}$ crystals (SrO and TiO$_{2})$. An electron gas that appears at the interface between LaAlO$_{3}$ and TiO$_{2}$-SrTiO$_{3}$ surface (n-type) potentially screens the LaAlO$_{3}$ polarity, while the interface between LaAlO$_{3}$ and SrO-SrTiO$_{3}$ (p-type) remains insulating. To understand this asymmetry, we probed the LaAlO$_{3}$ surface using a variety of element-specific probes and observe a change in the LaAlO$_{3}$ stacking structure in the p-type geometry. Tunneling measurements reveal remnants of a built-in field reflective of LaAlO$_{3}$'s intrinsic polarity across the n-type structure, but no such signatures are detected for the p-type structure. When combined with density functional theory simulations, the results suggest that while free charge screens the LaAlO$_{3}$ dipole in the n-type geometry, a change in LaAlO$_{3}$'s structure during growth nullifies the dipole in the p-type geometry. In essence, SrTiO$_{3}$ surface layers drastically affect LaAlO$_{3}$ polarity compensation and in turn the electronic properties.