Electronic charges and electric potential at LaAlO${}_3$/SrTiO${}_3$ interfaces studied by core-level photoemission spectroscopy

We studied LaAlO${}_3$/SrTiO${}_3$ interfaces for varying LaAlO${}_3$ thickness by core-level photoemission spectroscopy. In Ti $2p$ spectra for conducting “$n$-type” interfaces, Ti${}^{3+}$ signals appeared, which were absent for insulating “$p$-type” interfaces. The intensity of the Ti${}^{3+}$ signals was significantly higher than that expected for the transport carrier densities, indicating that part of the carriers at the interfaces are localized. The Ti${}^{3+}$ signals increased with LaAlO${}_3$ thickness, but started well below the critical thickness of 4 unit cells for metallic transport. Core-level shifts with LaAlO${}_3$ thickness were much smaller than predicted by the polar catastrophe model. We attribute these observations to surface defects and/or adsorbates providing charges to the interface even below the critical thickness.

Figure 1

Schematic view of LaAlO${}_3$ thin films grown on SrTiO${}_3$ (001) substrates and their photoemission measurement geometry. Panels (a) and (b) represent the two types of samples assuming bulklike charge assignments. (a) LaAlO${}_3$ is deposited on a TiO${}_2$-terminated SrTiO${}_3$ substrate forming the “$n$-type” interface. (b) LaAlO${}_3$ is deposited on a SrO-terminated SrTiO${}_3$ substrate forming the “$p$-type” interface.

Figure 2

Sheet conductivity of the $n$-type LaAlO${}_3$/SrTiO${}_3$ (001) samples with varying LaAlO${}_3$-layer thickness.

Figure 3

La, Al, and Sr core-level spectra of the LaAlO${}_3$($x$ uc)/SrTiO${}_3$ (001) samples with varying LaAlO${}_3$-layer thickness $x$. (a) LaAlO${}_3$ layer grown on the TiO${}_2$-terminated SrTiO${}_3$ substrates forming an $n$-type interface. (b) LaAlO${}_3$ layer grown on the SrO-terminated SrTiO${}_3$ substrates forming a $p$-type interface. All the spectra have been normalized to the area of the Sr $3d$ core-level spectrum and the Al $2p$ core-level peak positions have been aligned.

Figure 4

Ti $2p$ core-level spectra of LaAlO${}_3$($x$ uc)/SrTiO${}_3$ (001) samples with varying LaAlO${}_3$-layer thickness $x$ plotted on the logarithmic scale. The solid curves show the spectrum of SrTiO${}_3$. (a) LaAlO${}_3$ layers grown on the TiO${}_2$-terminated SrTiO${}_3$ substrates forming $n$-type interfaces. (b) LaAlO${}_3$ layers grown on the SrO-terminated SrTiO${}_3$ substrates forming $p$-type interfaces.

Figure 5

Ti${}^{3+}$ intensity ratio of the Ti $2p$ core-level spectra as a function of $x$. The dashed curve is a guide to the eye. Inset: Ti $2p$ spectrum of the $x=6$ sample and its fitted curves. Two solid curves show those with different parameters yielding the range of error bars. The dashed curve shows the Ti${}^{4+}$ spectrum.

Figure 6

Shifts of core levels for LaAlO${}_3$($x$ uc) thin films grown on the SrTiO${}_3$ substrates. (a) Relative shifts within the LaAlO${}_3$ layers. (b) Relative shifts within the SrTiO${}_3$ substrates. (c) Relative shifts between the LaAlO${}_3$ and SrTiO${}_3$ layers. (d), (e) Schematic diagram of the Madelung potential in the LaAlO${}_3$/SrTiO${}_3$ samples with the $n$-type and $p$-type interfaces. The dashed and solid curves represent the electrostatic potential with and without electronic reconstruction, respectively.