(a) XRD overview scan of a 50 nm thick Fe-doped film grown under deposition conditions I. The film structure has merged completely with the substrate structure, leaving only the single crystal substrate peaks visible. (b) A magnified look at the (002) region of the film shown in (a) and a separate XRD scan of a bare substrate show no discernable structural difference.
(a) Tapping mode AFM surface scan of a 50 nm thick Fe-doped film grown under deposition conditions I. The surface is very smooth and shows step terraces with a height of one atomic layer. (b) Conductive AFM surface and (c) conductivity scan. The strongly shrunk scan size as well as the reduced lateral resolution of the conductive AFM contact mode scan lead to a different perception of the topography in part (b) as opposed to part (a). The conductivity is inhomogeneous and has a pattern comparable to that of single crystals (Ref. 7). (d) The corresponding TEM data show a flat, 2D layer-by-layer growth.
(a) Topography of a Nb-doped thin film grown under deposition conditions II of Table I. (b) The conductive AFM topography and (c) conductivity scan were recorded with a smaller scan range, focusing onto a single terrace. (d) Overview topography, (e) magnified topography, and (f) conductivity of a Fe-doped thin film grown under identical conditions (set II of Table I). Neither morphology nor local conductivity are significantly altered by the change in dopant type.
(a) XRD overview scan of a 200 nm thick Nb-doped film grown under deposition conditions III. (001), (002), and (003) peaks are visible for both, the thin film and substrate, corresponding to lattice constants of 3.943 Å in case of the film and 3.903 Å in case of the substrate. (b) A magnified look at the (002) region showing the difference in peak position and lattice constant for film and substrate. The respective rocking curve for the film peak is shown in the inset.
(a) Topography and (b) current image of a Nb-doped film grown under deposition conditions III. The surface consists of small, circular hills with a conductive center and nonconducting boundaries. (c) This correlation is visualized by two line scans taken at the same position within topography and current image. [(d) and (e)] The defect structure was checked by cross-sectional TEM and exhibits a dense distribution of defect-rich nanoclusters.
(a) Topography and (b) current image of a Nb-doped film grown under an increased deposition pressure (deposition conditions IV). The surface consists of large islands with conducting boundaries. (c) The correlation between topography and conductivity is again visualized by two line scans taken. [(d) and (e)] The defect structure was checked by cross-sectional TEM. The film contains large growth islands, extending from the substrate to the surface, with defect-rich boundaries. These growth islands can be identified with the surface islands.
[(a) and (b)] Reversible resistive switching in a Nb-doped sample grown in a coherent, 3D way. The conductive AFM current images show an array of conducting units which can be switched between two different resistance states. [(c) and (d)] Reversible resistive switching in a Nb-doped sample grown in an island growth mode. The array of conducting rings corresponds to the film’s defect structure and can be switched between two different resistance states.
Parameters used for PLD growth of Nb-doped and Fe-doped thin films.
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