Structural and Electronic Reconstructions at the LaAlO₃/SrTiO₃ Interface

Abstract

The physics of transition metal oxide heterostructures is a subject of great interest due to the observations of new interface electronic and magnetic states. [ 1 ] The two-dimensional electron gas (2DEG) discovered at the interface between insulating LaAlO 3 (LAO) thin fi lms and bulk SrTiO 3 (STO) crystals stands as a model and has boosted the great expectations placed in oxide electronics. [ 2 , 3 ] Recently, it has been found that also the (001) SrTiO 3 surfaces, obtained by cleaving SrTiO 3 in ultra-high vacuum (UHV) [ 4 , 5 ] or by annealing in reducing conditions, [ 6 ] exhibit two-dimensional electronic states, independent of the bulk doping. From these studies it emerges that, despite possible different doping mechanisms, a similar band splitting characterizes the electronic properties of vacuum/STO and LAO/STO [ 7 ] interfaces.As matter of fact, while electron doping is believed to occur by creation of oxygen vacancies in STO surfaces, [ 5 , 6 , 8 ] in the case of LAO/STO interfaces the primary origin of the charge carriers is attributed to an electrostatic instability of the system. In particular, a scenario of electronic reconstruction has been proposed, [ 9 , 10 ] where a Zener breakdown takes place above a LAO thickness of 3 unit cells (uc), as experimentally observed. [ 11 , 12 ] According to this picture, once the electron transfer occurs, carriers spontaneously occupy bands composed by Ti-3d orbitals, starting with d xy -symmetry bands located within the fi rst adjacent SrTiO 3 layers. [ 13 ] Moreover, according to DFT calculation on LAO/STO bilayers, [ 14 ] the distribution of electrons accumulated within the fi rst STO layers gives rise to an electric fi eld, which in turn induces a rumpling of the SrO and TiO 2 interface atomic planes within the same region hosting the 2DEG.Thus, a polar instability scenario predicts for LAO/STO heterostructures structural and electronic modifi cations of the fi rst SrTiO 3 unit cells occurring simultaneously with the appearance of a 2D-metal. However, not all the experimental results are fully consistent with this picture. For example, second harmonic generation experiments suggested that polar distortions in STO appear below the LAO threshold thickness of 4-uc, i.e., before the realization of 2D electron system. [ 15 ] More recently, a rumpling of the STO interface layers was observed also in p-type LAO/STO insulating interfaces, as shown by grazing incidence X-ray diffraction (GIXD). [ 16 ] To clarify these issues, we have used a combination of advanced X-ray synchrotron-based spectroscopic and structural measurements to study the evolution of the electronic properties and ionic positions at LAO/STO interfaces as a function of the LAO thickness ( Figure 1 ). We fi nd that an orbital reconstruction occurs also in insulating 2-uc LAO/STO, before the appearance of the metallic state. Partial or complete inversion of the 3d-t 2g energy levels in insulating samples is correlated to partial or complete coverage of the STO surface by an ordered polar layer....

Electronic Reconstruction at the Interface Between Band Insulating Oxides: The LaAlO3/SrTiO3 System

“…In particular, for each La(Sr) or Al(Ti) exchange, the interface will get both donor and acceptor levels (thus electron and holes) which are expected to be annihilated being located essentially at the same place. Note that cation intermixing were observed by high-resolution transmission electron microscopy (HRTEM) [54] and by grazing incidence surface diffraction (GXID) by several groups on samples grown in different laboratories [37,55]. As matter of fact the degree of La/Sr and Al/Ti substitution was found dependent on the growth conditions, the sample thickness, and varied from sample to sample.…”

“…These results were confirmed in ref. [37], where it was shown that below 4 uc, i.e. in nominally 2 uc LAO/STO films, the LAO layers are strongly rumpled, indicating the presence of an internal electric field, and the unit cells are expanded in a direction perpendicular to the interface.…”

“…For example, stoichiometric growth is preferred at high oxygen pressure since the kinetic energy of the cations is lower and the differences for light (Al) and heavy (La) species reduce. As a consequence, deviation from the ideal La/Al ratio in PLD grown LAO/STO films is common [37]. Moreover, a systematic molecular beam epitaxy study showed that conducting interfaces are created only in the case of La deficient samples.…”

“…Similar arguments hold for oxygen vacancies in the STO layers, which can be present even in oxygen annealed LAO/STO interfaces to some degree (see ref. [37] and Fig. 9), but it does not compensate the charge polarity, thus leaving the polar instability problem unsolved.…”

“…However, while according to this scenario the STO interface layers should be essentially flat (no rumpling) before the critical thickness, we showed in ref. [37] the opposite behaviour, suggesting the presence of an electric field, localised charges, and consequent lattice distortions of the strongly polarisable STO layers before the appearance of a mobile electron system [ Fig. 8].…”