Instability, Intermixing and Electronic Structure at the Epitaxial LaAlO3/SrTiO3(001) Heterojunction (1006.1378v1)

Abstract: The question of stability against diffusional mixing at the prototypical LaAlO3/SrTiO3(001) interface is explored using a multi-faceted experimental and theoretical approach. We combine analytical methods with a range of sensitivities to elemental concentrations and spatial separations to investigate interfaces grown using on-axis pulsed laser deposition. We also employ computational modeling based on the density function theory as well as classical force fields to explore the energetic stability of a wide variety of intermixed atomic configurations relative to the idealized, atomically abrupt model. Statistical analysis of the calculated energies for the various configurations is used to elucidate the relative thermodynamic stability of intermixed and abrupt configurations. We find that on both experimental and theoretical fronts, the tendency toward intermixing is very strong. We have also measured and calculated key electronic properties such as the presence of electric fields and the value of the valence band discontinuity at the interface. We find no measurable electric field in either the LaAlO3 or SrTiO3, and that the valence band offset is near zero, partitioning the band discontinuity almost entirely to the conduction band edge. Moreover, we find that it is not possible to account for these electronic properties theoretically without including extensive intermixing in our physical model of the interface. The atomic configurations which give the greatest electrostatic stability are those that eliminate the interface dipole by intermixing, calling into question the conventional explanation for conductivity at this interface - electronic reconstruction. Rather, evidence is presented for La indiffusion and doping of the SrTiO3 below the interface as being the cause of the observed conductivity.

• The paper demonstrates that significant cation intermixing occurs over several unit cells, with La diffusing into STO and Sr/Ti infiltrating LAO.
• The paper shows that energetically favorable cation exchanges (Al↔Ti and La↔Sr) reduce interfacial dipole moments, challenging abrupt interface models.
• The paper finds that the near-zero valence band offset, observed experimentally and via DFT, shifts the focus from pure electronic reconstruction to interdiffusion-driven conductivity.

Analysis of Intermixing and Electronic Structure in LaAlO₃/SrTiO₃ (001) Interfaces

The paper presents an exhaustive investigation of intermixing phenomena and electronic properties at the LaAlO₃/SrTiO₃ (001) (LAO/STO) interface, employing a combination of experimental techniques and computational models. The paper aims to elucidate the mechanism driving interfacial conductivity and challenge previously held notions assuming abrupt interfaces and purely electronic reconstruction mechanisms.

Key Insights and Numerical Results

The rigorous analytical approach integrates multiple experimental methodologies, including RBS, ToF-SIMS, HAADF STEM/EELS, ARXPS, and MEIS, coupled with first-principles DFT and classical modeling techniques. This combined strategy reveals significant intermixing at the interface, contesting the assumption of sharp junctions generally used in theoretical frameworks. Critical findings include:

• Extent of Intermixing: The analysis consistently indicates cation intermixing across several unit cells, where La diffuses into the STO layer and Sr, Ti spread into LAO. MEIS results, for instance, detect La up to 1 nm into the STO, with significant Sr and Ti presence observed within the LAO film.
• Thermodynamic Preference for Intermixing: DFT calculations corroborate the experimental results, with Al↔Ti and La↔Sr site exchanges proving energetically favorable, effectively eliminating or reducing net dipole moments at the interface, which destabilize the ideal abrupt one.
• Electronic Structure and Valence Band Offset (VBO): The valence band offset (VBO) is measured as near-zero or slightly positive, aligning with or being slightly above theoretical predictions once intermixing is accounted for. This challenges the straightforward electronic reconstruction narrative, shifting focus towards ion interdiffusion-driven conductivity.

Implications

The implications of these results are multifaceted. From a theoretical standpoint, it underscores the necessity for models considering intermixing as intrinsic to many oxide interfaces, fundamentally altering the electronic landscape compared to predictions based on abrupt interfaces. Practically, understanding these interactions better informs the growth processes and engineering of interfaces requisite for devices, potentially offering pathways to modulate conductivity more precisely by controlling interdiffusion.

Future Prospects

Further investigations could refine the understanding of local atomic arrangements and electronic phenomena through enhanced computational power and more sophisticated imaging techniques. Exploration beyond thermodynamic conditions—into kinetic factors affecting intermixing during growth and how they might be harnessed or altered—would provide additional leverage over interface properties. Additionally, the development and utilization of growth techniques, such as molecular beam epitaxy (MBE), that might better minimize kinetic intermixing influences, could open avenues for fine-tuning interface characteristics with high precision.

Overall, this investigation paves the way for a deeper comprehension of complex oxide interfaces and highlights the critical role of interfacial layer compositional dynamics in framing their electronic characteristics. As understanding improves, the design landscape for multifunctional oxide heterostructures broadens, suggesting new opportunities in the nanoelectronics sphere built atop oxide interfaces.