The (001) oriented epitaxial BaTiO3 thin films were grown on La0.7Sr0.3MnO3, SrRuO3, and Nb-SrTiO3 layers, respectively, and the inhomogeneous compressive strain is suggested to be the main origin of their upward self-polarization. The upward self-polarization exists in the thin BaTiO3 films with strong inhomogeneous compressive strain, while it disappears in thick BaTiO3 films due to strain relaxation. Since the upward self-polarization is unchangeable when the p-type La0.7Sr0.3MnO3 was replaced by the n-type SrRuO3 and Nb-SrTiO3, the depletion region and the polar discontinuity at interface are excluded to be their origins. Similarly, the density gradient of negative charged cation vacancies is not the main origin because it would introduce downward self-polarization.

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