First-principles study on the oxygen vacancy in semiconducting oxides

Abstract

diffusion, defect level and association of VO. First with respect to the poorly understand experimental phenomena, we first investigated the migration of VO in metal-oxides such as NiO, including the effect of grain boundary in ploy-crystal. We found that the lattice imperfection such as the grain boundary could be the favorite path for VO due to the lower migration barrier than bulk. Further, we also estimated the electomigration of VO in various metal-oxides under the external electric field. We found that the lowered migration barrier though the migration path of VO by external field can be negligible, and that the faster migration of VO under electric field can be explained by Joule-heating effect to increase the local temperature around VO site, giving rise to the increasing the jumping rate of atoms toward near vacant site. Second, relating to the defect nature of VO, we estimate the nature of VO in TCOs such as In2O3 and SnO2 in order to identify whether the VO can contribute to the n-type conductivity of both In2O3 and SnO2 or not. From the analysis of defect level and charge transition level of VO, we found that for In2O3, the nature of VO is shallow while it is deep in SnO¬2. Even for In2O3, however, the ionization energy is still large to release two electrons as free carrier at room temperature, indicating that the VO alone in In2O3 cannot be origin of the intrinsic conductivity.

Semiconducting oxides has been widely attractive due to its potential for alternative materials to silicon which has revealed the limitation in performance of devices consisted of it as the size of devices is decreased. Furthermore, since the enormous probability of oxides to apply to various devices due to variety properties from many compounds, semiconducting oxides will be the most promising technologies for leading the next generation of semiconductor-devices such as TFT and display panel. Interestingly, the material properties of oxides, especially electrical properties, critically depend on the presence of lattice imperfections such as the grain boundary and point defects. In particular, the oxygen vacancy (VO), which constitutes a most fundamental point defect in oxides, plays crucial roles in determining the performance of various electronic devices that include the oxide materials. For instance, for the operation of resistance random access memory (ReRAM) with resistance switching phenomenon under the external field, it is widely accepted that the electromigration of the charged VO is crucial for the formation and rupture of the local conducting path in insulating oxides. For transparent conducting oxides (TCOs) with wide band-gap, likewise, the n-type conductivity even in undoped sample is exhibited and has been revealed the strong dependence on oxygen partial pressure, indicating that VO may play an important role in the conductivity of TCOs. Since the importance of the VO in the material properties of oxides has been revealed, it is strongly required that the roles of VO in oxides are sufficiently understood. However, because the nature of VO be defined in atomic level, the study on the role of VO needs to be performed in point of atomic scale view. Unfortunately, it does not allow us to identify the nature of VO due to the limitation of experimental research such as the resolution and the difficulty of direct measurement. In this thesis, we carried out First-principles study on the VO in oxides to approach to the full-understand about the role of VO in oxides with respect to the three kind of point