Essential to the understanding of key fundamental properties in the newly discovered iron-based superconductors as well as critically assessing their potential for applications and devices is their fabrication into very high quality thin films. Thin films of the Ba(Fe1-xCox)2As2 phase – discovered by Sefat et al.  and commonly referred to as “Ba-122” – are particularly interesting since this compound appears to be resilient against oxidation and degradation due to water vapor , it is considerably easier to deposit compared to other iron-based superconductors, and it has a relatively high critical temperature of 25.5 K. The search for optimal thin film growth parameters is currently the subject of a number of international research groups. [2 – 11]. However, difficulties overcoming the poor metal/oxide bond at the interface of many substrates has necessitated the need for significant optimization of the deposition parameters [9,10] as well as the use of various intermediate layers  to produce well-textured films. In spite of these efforts, nearly all of these films contain an unintentional amorphous or iron-containing layer at the interface. While the nature of this interface is not yet fully understood, the disruption of local crystallographic ordering associated with it precludes the use of these films for interface-sensitive applications such as multilayers or heterostructures where coherent and chemically inert phase boundaries are required.
A detailed investigation into the nature of this interface on both STO and LSAT substrates reveals that it is characterized by the presence of an interdiffusion layer. Ba diffuses approximately 2 nm into both substrates and is mass-balanced by the previously mentioned amorphous layer of unknown composition (figure 1). This amorphous layer is rich in iron and, on LSAT, even contains significant amounts of textured iron . The nature of this interface is the first topic of this contribution.
Additionally, the current transport properties of these thin films are observed to be strongly dependent on the type and concentration of defects present. An overview of the defects observed in this system constitutes the second topic of this contribution.
Finally, it has been shown that, by using a metallic iron buffer layer, a highly coherent and chemically inert interfacial bond can take place [12-14]. The effect of such an interface on the defect landscape and, subsequently, the macroscopic superconducting properties of the Ba-122 thin films will be discussed in detail.
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