Crystallographic Characteristics and p-Type to n-Type Transition in Epitaxial NiO Thin Film

We were able to systematically control crystallographic characteristics and electrical properties of nickel oxide epitaxial thin films integrated with cubic yttria-stabilized zirconia (c-YSZ)-buffered silicon(001) substrates. The NiO epilayers were grown under several oxygen partial pressures by pulsed laser deposition. The out-of-plane orientation of the NiO layers showed an interesting behavior where it changed from < 111 > at lower pressures (7 x 10(-6) Torr) to < 100 > at higher pressures (5 x 10(-2) Torr). This observation was attributed to the nature of surface termination and templating effect of the c-YSZ{100} platform at different pressures. With the use of theta-2 theta and phi scans of X-ray diffraction, the epitaxial alignment across the NiO/c-YSZ interface was determined to be {111}(NiO)parallel to vertical bar{100}(c-YSZ) and < 110 >(NiO)parallel to < 100 >(c-YSZ) for the heterostructure grown under a low pressure and {100}(NiO)parallel to{100}(c-YSZ) and < 100 >(NiO)parallel to < 100 >(c-YSZ) for the heterostructure grown under a high oxygen pressure. Our high-resolution TEM studies revealed formation of atomically sharp interfaces with no evidence of interfacial reaction and confirmed the established epitaxial relationships. n-Type electrical conductivity was observed in the NiO epilayers grown under lower pressures, which was turned into p-type in the films deposited under higher oxygen partial pressures. Besides, the electrical resistivity increased with the growth pressure. These observations were correlated to the nature of point defects in the NiO crystal. The formation of oxygen vacancies and metallic nickel at lower pressures and excess oxygen and trivalent nickel at higher pressures was revealed by XPS. We established a structure-property correlation in NiO/c-YSZ{100}/Si{100} thin film epitaxial heterostructures with special emphasis on the stoichiometry and crystallographic characteristics.