Thin Film Metal-Oxides by Shriram Ramanathan

Author:Shriram Ramanathan
Language: eng
Format: epub
Publisher: Springer US, Boston, MA

In parallel with the advances in the growth techniques, recent progress in the physical understanding of perovskite interfaces have raised the field into a new era. Two such examples can be found in structures with metallic interfaces induced between insulating perovskites. First is the microscopic characterization of the LaTiO3 ∕ SrTiO3 superlattice which demonstrated that the charge penetration in a delta-doping heterostructure is confined to a significantly smaller region compared with those of conventional semiconductors [9]. The second is the discovery of interface dependent electrical conductivity at the LaAlO3 ∕ SrTiO3(100) interface [10]. Both of these results suggest a novel concept of charge reconstruction at interfaces owing to the variable valence degree of freedom in the perovskites. Triggered by these findings, a variety of experimental and theoretical studies have been made to explore charge, spin, and orbital reconstructions at oxide interfaces [11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24]. These studies have expanded from the original interface between oxide insulators to interfaces involving ferroelectric, magnetic or superconducting materials.

Meanwhile, these new concepts developed from the capability to tailor artificial structures with atomic scale control have given an ideal platform to investigate heterostructures which can be directly related to device applications using perovskites. Metal–semiconductor interfaces are among the most fundamental interfaces in electronic devices and require understanding of various important physical concepts such as band bending in semiconductors, band alignments, Schottky barrier heights (SBHs), and the formation of interface states. It is not clear, however, whether the existing concepts of Schottky junctions in conventional semiconductors can be applied directly to perovskites. For example, the multiple valence accessibility and the strong interaction between electrons, spins, and the lattice are not accounted for in the existing Schottky junction concepts. Therefore, expanding the existing knowledge of interface semiconductor physics to incorporate the complexity found in the perovskites and exploring phenomena unique to these oxides are vital steps toward establishing a platform for designing electronic devices.

In this context, one of the initial motivations to study complex oxide Schottky junctions in detail was as part of structures such as superconducting FETs or superconducting base transistors. After the discovery of superconducting cuprates and their sensitivity to the hole concentration, efforts to modulate their properties by field effect were carried out in metal–insulator–superconductor (MIS) structures (Fig. 5.3). Here, the superconductor and the insulator were typically YBa2Cu3O6 + δ and SrTiO3, respectively [25, 26, 27]. The leakage properties of the SrTiO3 gate-insulator were investigated by capacitance–voltage (C − V ) and current–voltage (I − V ) techniques, which revealed the formation of dead layers and reduced relative permittivity at interfaces. However, the main focus was on the field effect on the cuprates and less attention was paid to the current processes across the MIS structure.

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