Barrier layer

Abstract: The morphology of porous anodic oxide films formed on aluminium in phosphoric acid electrolytes at constant current density or voltage, and under changing electrical or electrolytic conditions, has been studied quantitatively by electron microscopy. Replicas from film sections and from both film interfaces have been prepared, as well as transmission micrographs of thin films, produced under accurately defined conditions. During formation at constant current density, pore initiation occurs by the merging of locally thickening oxide regions, which seem related to the substructure of the substrate, and the consequent concentration of current into the residual thin areas. The pores grow in diameter and change in number until the steady-state morphology is established. The film barrier layer thickness has been measured directly for the first time. The steady-state barrier-layer thickness, cell diameter and pore diameter are all observed to be directly proportional to the formation voltage. It becomes evident that the barrier-layer thickness, decided largely by an equilibrium established between oxide formation in the barrier-layer and field-assisted dissolution (probably thermally enhanced) at the pore bases, determines the cell and pore sizes by a simple geometrical mechanism. Anion incorporation into the film and its hydrogen-bonded structure play secondary roles to these factors in determining the actual film morphology, although not its subsequent properties. A consequence of the mechanism is that, at constant current density, relatively non-aggressive electrolytes give thicker barrier layers, larger cells and larger pores next to the barrier layer than aggressive media, although subsequent pore widening at the outer surface of the film by simple chemical dissolution is more severe in aggressive electrolytes.

Abstract: We investigate the electronic properties of ultrathin hexagonal boron nitride (h-BN) crystalline layers with different conducting materials (graphite, graphene, and gold) on either side of the barrier layer. The tunnel current depends exponentially on the number of h-BN atomic layers, down to a monolayer thickness. Conductive atomic force microscopy scans across h-BN terraces of different thickness reveal a high level of uniformity in the tunnel current. Our results demonstrate that atomically thin h-BN acts as a defect-free dielectric with a high breakdown field. It offers great potential for applications in tunnel devices and in field-effect transistors with a high carrier density in the conducting channel.

Abstract: We review the features of amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs), as well as circuit operation based on these TFTs. We also report a novel TFT structure which improves environmental stability of the TFT operation by taking full advantage of the a-IGZO properties, where a conventional PECVD a-SiNX:H films serve not only as an effective barrier layer but also as a hydrogen source to form the coplanar source and drain.