Three-dimensional graphoepitaxial growth of oxide films by pulsed laser deposition

Metaloxide thin films were deposited on tilted-axes NdGaO3 substrates (TAS NGO) by pulsed laser deposition. A specific growth mode resulting in an inclination between crystallographic planes of the top layer and that of the bottom layer was commonly observed. A simple geometrical growth model, taking into account faceting of the surface of the bottom layer, explains the observed dependencies well. The matching of the top and the bottom layer is essentially three dimensional, with graphoepitaxial matching in the substrate plane. The three-dimensional graphoepitaxial (3DGE) growth mechanism seems to be quite common for deposition on TAS with tilt angles more than 5 degrees. It was observed for eight of ten studied combinations of materials, including multilayer heterostructures, for four different deposition techniques, and on substrates with different predeposition treatment. The 3DGE growth was observed both with increase and decrease of the top layer tilt angle compared to the tilt angle of the bottom layer. Two different 3DGE dependencies can be distinguished in the high-angle range (>15 degrees): with a tendency towards standard growth above some threshold angle, and retaining 3DGE behavior until a tilt angle of 45 degrees is reached, either by the top or by the bottom layer. In a simplified way the difference may be attributed to two different formation mechanisms: the first one generates the additional tilt when the growing grain overgrows another grain, seeded on the next step on the substrate surface, while for the second mechanism the inclination is formed when the grain is seeded over the step. The first type is better described by a tangent angular dependence, it is observed usually when a compressive strain is induced in the top layer. The second type follows a sine dependence, and is usually seen for a tensile-strained top layer.