c-texture versus a-texture low pressure metalorganic chemical vapor deposition ZnO films: Lower resistivity despite smaller grain size

Recently, it has been shown that it is possible to tune the morphology of zinc oxide films deposited by low-pressure metalorganic chemical vapor deposition (LP-MOCVD) while preserving good electrical conductivity. Here a closer look is taken at films deposited under two different deposition conditions; one leading to LP-MOCVD a-texture (i.e., with the a-axis perpendicular to the substrate), the other resulting in c-texture (i.e., with the c-axis perpendicular to the substrate), with the aim of correlating their structural and electrical characteristics. We introduce the concept of a “selection layer” to indicate the initial region of growth that precedes the establishment of a clear preferential crystallographic film orientation. With a strong preferential c-texture of initial nucleation the selection layer for c-texture films is minimal (< 50 nm), while for a-texture it extends for about 0.25 μm of film thickness. The non-intentionally doped c-textured material has an electrical resistivity lower by an order of magnitude than the a-textured one, due to a higher carrier concentration and higher carrier mobility. Electrical transport measurements indicate that grain boundaries are the main limitation to conductivity for both film textures; in which case it is unexpected that the c-textured films show higher carrier mobility despite having smaller grains (i.e., greater grain boundary density). This inconsistency is explained by referring to their thinner selection layer, and lower activation energy for inter-grain transport as determined by temperature-dependent Hall measurements.