Influence of atomic termination on the LaAlO${}_3$/SrTiO${}_3$ interfacial polar rearrangement

We report the results of second-harmonic optical spectroscopy of LaAlO${}_3$ films grown on SrTiO${}_3$ (001) substrates having either TiO${}_2$ or SrO atomic terminations. The atomic termination is found to have a large effect on the observed spectra, with a strong dependence on the $\omega$-$2\omega$ light polarizations. These results are analyzed through a model based on symmetry-controlled selection rules and atomic orbital overlaps. The analysis shows that, as suspected from transport measurements, the SrO termination prevents the formation of a mobile interfacial electron gas. Yet, by explicitly probing and identifying the electronic states at the O${}^{2-}$–Ti${}^{3+}$ band gap we now obtain unique and direct evidence that this is caused by the absence of any detectable charge injection in the interfacial region rather than by carrier localization.

Figure 1

RHEED oscillations for the growth of five monolayers of LaAlO${}_3$ onto SrO-terminated substrate. To obtain the latter a monolayer of SrO is grown on TiO${}_2$-terminated SrTiO${}_3$ substrate prior to the LaAlO${}_3$ deposition. Panels (b) and (c) show the final RHEED patterns after SrO and LaAlO${}_3$ deposition, respectively. The RHEED intensity is measured at the specular spot for the SrO layer, while it is measured on the (0 1) diffraction spot for the LaAlO${}_3$ film, thus explaining the phase shift. RHEED oscillations for LaAlO${}_3$ layer growth on TiO${}_2$-terminated surfaces can be seen in Ref. 17.

Figure 2

Comparison of SHG spectra from LAO/STO heterostructures with five unit cells of LAO and different interfacial atomic terminations. The labels $sp$ (a), $ds$ (b), and $pp$ (c) refer to different $\omega$-$2\omega$ polarization combinations (see text). The spectra for the SrO-terminated substrate (red symbols) are multiplied by a factor 4 for better visibility. The colored areas divide the plot into two energy ranges corresponding to the identified electronic transitions: the blue area is dominated by $A_1\to E,E\to B_2$ transitions, while the purple area is guided by $E\to E$ transitions, as shown in Fig. 3.

Figure 3

The symmetry-allowed O(2$p$)-Ti(3$d$) 2$\omega$ transitions for all three symmetry-allowed ${\chi }_{ijk}$ components within the $4mm$ symmetry group are shown. According to the selection rules the allowed transitions for ${\chi }_{zzz}$ (contributing to the $pp$ signal together with the other $\chi$ elements) and ${\chi }_{zxx}$ ($sp$ signal) are the same, whereas two different transitions are allowed for ${\chi }_{xxz}$ ($ds$ signal).

Figure 4

A schematic picture of the two possible STO terminations. The oxygen 2$p$ orbitals are shown in dark red and pink, where the dark-red highlights the direction maximizing the overlap with the Ti atom. In the case of the SrO-terminated STO 2$p_z$ orbitals point towards the Ti atoms beneath, thus favoring the orbital overlap.