Electronic states at polar/nonpolar interfaces grown on SrTiO 3 studied by optical second harmonic generation

Optical second harmonic generation (SHG) spectra of LaAlO₃/SrTiO₃, LaGaO₃/SrTiO₃, and NdGaO₃/SrTiO₃ interfaces with SrTiO₃ substrates have been measured up to 4.2 eV as a function of film thickness and temperature. This spectral range is characterized by two-photon transitions from the valence to conduction bands of SrTiO₃ which are classified with a model based on symmetry, selection rules, and atomic orbital overlaps. This model is further confirmed in spectral measurements as a function of film thickness, material overlayer, and temperature. SHG enhancement at low temperature indicates an increase of the interfacial polarity with decreasing temperature. This confirms the relation between SHG and the spatial translation of Ti ions which are more prone to move at lower temperatures because of the quantum-paraelectric nature of SrTiO₃. We finally show that SHG spectroscopy captures structural details of the interfaces. In particular, we find evidence for proximity effects such as LaAlO₃-induced distortions of the TiO₆ octahedra or bond buckling. We also observe a correlation between SHG and the in-plane lattice mismatch between the SrTiO₃ substrate and the LaAlO₃, LaGaO₃, and NdGaO₃ overlayers.