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• Un- and Ni-doped ZnO thin films are synthesized and characterized. • Experimental and theoretical characterizations demonstrate stabilization of a small p-Schottky barrier upon Ni doping. • Reduction in p-Schottky barrier is mainly due to amelioration in the surface uniformity and modulization in the relative alignment of band edges at the interfaces. Large contact resistance at metal-substrate/ZnO heterostructure interfaces prevents achieving highly efficient device performance. Herein, we present a systematic study on the effect of Ni-doping in the reduction of the Schottky contact barrier at metal-substrate/ZnO heterostructure. To this end, Ni-doped zinc oxide (Ni:ZnO) thin films were deposited on glass substrate by a spray technique with different Ni-doping concentrations. X-ray Diffraction and Atomic Force Microscopy (AFM) measurements showed that Ni-doping enhances the surface uniformity as compared to the undoped-ZnO films and significantly decreases the roughness (RMS) from 35 to 17 nm. Conductive Atomic Force Microscopy (C-AFM) with a Bruker's platinum coated probe (Pt-Ir) tip results in the stabilization of a p-type Schottky contact with a small height barrier of ~0.4 eV, which is among the smallest values reported in literature for ZnO thin films. Our first principle calculations, which are based on the relative alignment of the band edges of the components, also confirm the reduction in the Schottky barrier height by Ni-doping in line with the experimental tendency. Both experimental and theoretical results provide a robust evidence of the potential of stabilization of a small p-type Schottky contact at metallic-substrate/ZnO interface through a Ni-doping. [ABSTRACT FROM AUTHOR]