Metallic and insulating interfaces of amorphous SrTiO3-based oxide heterostructures (1212.6901v1)

Abstract: The conductance confined at the interface of complex oxide heterostructures provides new opportunities to explore nanoelectronic as well as nanoionic devices. Herein we show that metallic interfaces can be realized in SrTiO3-based heterostructures with various insulating overlayers of amorphous LaAlO3, SrTiO3 and yttria-stabilized zirconia films. On the other hand, samples of amorphous La7/8Sr1/8MnO3 films on SrTiO3 substrates remain insulating. The interfacial conductivity results from the formation of oxygen vacancies near the interface, suggesting that the redox reactions on the surface of SrTiO3 substrates play an important role.

• The paper demonstrates that oxygen vacancy formation, triggered by reactive plasma species during film growth, is key to metallic interfacial conductivity.
• It shows that low oxygen pressure and critical film thickness during pulsed laser deposition are crucial for inducing conductive behavior.
• XPS analysis reveals increased Ti³⁺ concentration with film thickness, confirming defect-mediated conduction at the SrTiO₃ interface.

Overview of Interfacial Conductivity in Amorphous SrTiO$_3$-Based Oxide Heterostructures

This paper presents a comprehensive paper into the interfacial conductivity phenomena observed in heterostructures composed of strontium titanate (SrTiO$_3$ or STO) substrates and amorphous overlayers. These heterostructures, with their potential application in oxide electronics and thermoelectric materials, are analyzed for their conductive properties which arise due to complex interactions at the STO interface.

Key Findings

1. Interfacial Conductivity from Oxygen Vacancies: Metallic conducting interfaces are consistently observed in STO-based heterostructures with amorphous overlayers of LaAlO$_3$ (LAO), STO itself, and yttria-stabilized zirconia (YSZ). The paper attributes this conductivity primarily to the formation of oxygen vacancies at the interface of the STO substrate, a phenomenon induced by the reactive species involved in film growth. Conversely, an amorphous lanthanum strontium manganite (LSMO) overlayer results in an insulating interface, indicating the absence of redox activity necessary for vacancy formation.
2. The Role of Growth Conditions: Oxygen pressure during the pulsed laser deposition (PLD) is a critical parameter influencing the conductivity. A low-pressure environment (P$_\text{O2}$ ≤1×10$^-2$ mbar) facilitates the creation of oxygen vacancies due to the high kinetic energy of plasma species, enabling interfacial conductivity. In contrast, a higher pressure scenario results in negligible conduction, likely due to reduced reactive plasma species.
3. Thickness-Dependent Transition: A notable metal-insulator transition is observed, dictated by the critical thickness of the amorphous films. For LAO/STO, STO/STO, and YSZ/STO heterostructures, conductive behavior commences beyond specific thickness thresholds, reinforcing the notion of a minimum volume of reactive plasma species required for sustaining conductivity.
4. XPS Analysis and Ti$^3+$ Presence: X-ray photoelectron spectroscopy (XPS) reveals the presence of Ti$^3+$ in samples with interfacial conductivity, indicating defect formation in the STO substrate. The Ti$^3+$ concentration increases with the deposited film's thickness, aligning with enhanced oxygen vacancy formation rather than simple cation intermixing.

Implications and Future Directions

The findings suggest that the chemical interactions, particularly redox reactions at the interface, extend the design parameters for achieving tunable electronic properties in oxide heterostructures. The paper underscores the importance of controlled plasma conditions during deposition to modulate these properties, thus bearing significant implications for the fabrication of next-generation nanoelectronic devices.

This work opens avenues for more precise control over the chemical environment during PLD, such as manipulating pressure and plasma dynamics, to further tailor interfacial properties. Future inquiries may focus on extending these observations to crystalline overlayers and exploring additional oxide compositions and substrate orientations. Additionally, further elucidation of the role of specific atomic species in promoting or inhibiting vacancy formation could enhance the predictability and efficacy of these heterostructures in practical applications.