Effect of O_2 content on optical and electrical properties of Si/NiO:Na pn heterojunction

In the present study,we have fabricated a heterojunction Si/NiO:Na diode by magnetron sputtering method.The X-ray diffraction(XRD)results show that only one diffraction peak appears for NiO:Na films on Si substrates,which is corresponding to(111)NiO:Na preferred orientation and indicates that NiO:Na belongs to cubic structure.When O2/Ar+O2ratio changes from 0%to 30%,there is no obvious change about(111)diffraction peak position,but the(111)peak intensity has been improved greatly,which means that the introduction of oxygen is benefit for the NiO:Na film crystallization.When further increasing O2/Ar+O2ratio to 60%,the(111)peak shifts to left,indicating that c axis distance increases.This phenomenon can be explained that Ni depletion or oxygen vacancies may be complemented through introducing oxygen atmosphere.The(111)peak reversely shifts to larger diffraction angle when sputtering NiO:Na film in pure oxygen atmosphere,which may result from the excessive oxygen defects.From I-V curves,the electrical properties show that when changing O2/Ar+O2ratio from 0%to 30%,Si/NiO:Na heterojunction shows the best rectifying property,where its Vocis 4.9Vand the leakage current appears until the negative voltage reaches-7V.It may be explained by that the crystallization of NiO:Na thin films has been improved and their defects decrease.When further increasing O2/Ar+O2ratio,both the crystallization qualities and rectifying properties of NiO:Na thin films are weakened.Considering the results of XRD,atomic force microscope(AFM),UV and I-Vcharacteristics,it can be concluded that the partial introduction of oxygen atmosphere is beneficial to the crystal structure and reduces the crystal defect,causing the improvement of the electrical properties of Si/NiO:Na pn junctions.