Resistive switching and synaptic simulation behaviors of epitaxial ZnO/Nb-doped SrTiO3 heterojunction controlled by the substrate orientation

Abstract

Zinc oxide (ZnO) films were grown epitaxially on Nb-doped SrTiO3 (NSTO) substrates with different orientations using laser pulse deposition to create the (112¯0)ZnO/(100)NSTO, (0002)ZnO/(110)NSTO and (0002)ZnO/(111)NSTO heterojunctions. Epitaxial ZnO films on the (100) NSTO had two orthorgonal domains, resembling the fourfold symmetry of the (100) NSTO substrate. This good matching between the grown ZnO film and NSTO substrate resulted in a lower growth rate, smoother and denser surface, and a higher density of interface defect states. The interface state density was approximately 4.4 × 1012 eV−1cm−2, which was notably higher than that of the other two heterojunctions. Electrical transport studies revealed that ZnO/(100)NSTO heterojunction demonstrated bipolar resistive switching and negative differential resistance (NDR) due to a higher density of donor interface states from contributions of oxygen vacancies, while both the ZnO/(110)NSTO and ZnO/(111)NSTO heterojunction exhibited typical rectifying behavior. The preliminary investigation into synaptic behaviors suggested that (112¯0) ZnO/(100)NSTO heterojunction holds significant potential in brain-inspired neuromorphic computing systems.