p-(001)NiO/n-(0001)ZnO heterojunction devices grown by pulsed laser deposition technique

NiO/ZnO heterostructures are grown on c-sapphire substrates using the pulsed laser deposition technique. X-ray diffraction (XRD) study shows that the ZnO layer epitaxially grows along the [0001]-direction on the (0001)sapphire surface, as expected, while the epitaxial NiO film is found to be deposited along the [001]-direction on the (0001)ZnO surface. Moreover, the presence of three (001)NiO domains laterally rotated by 30° with respect to each other has also been confirmed by XRD and precision electron diffraction techniques in NiO layers. The study reveals the continuous nature of the NiO film, which also possesses a very smooth surface morphology. In a sharp contrast, ZnO films are found to grow along the [0001]-direction when deposited on (111)NiO layers. These films show columnar morphology. (001)NiO/(0001)ZnO layers exhibit the rectifying current–voltage characteristics that suggests the existence of p–n junctions in these devices. However, the behavior could not be observed in (0001)ZnO/(111)NiO heterojunctions. The reason could be the columnar morphology of the ZnO layer. Such a morphology can facilitate the propagation of metal ions from the contact pads to the underlying NiO layer and suppress the p–n junction effect.