Strain And Doping Control Of The Leakage Current Of The Ferroelectric Thin Films

The leakage current of the thin ferroelectric film with a nano scale thicknessplays important roles for the material performance, fatigue, aging, and so on. Thus itprevents the practical development process of the thin ferroelectric film and itsdevices. Therefore, we have to find the right way to reduce the leakage current of thethin ferroelectric film. Stress is inevitable between the substrate and the electrodeduring the thin ferroelectric film preparation process. it has a great influence on thepreparation and performance of the thin ferroelectric film. Obviously, it is importantto manipulate the leakage current by suitable stress. In addition, oxygen vacanciesexist in the oxide material. It will affect the leakage current of the thin ferroelectricfilm. The doping is an effective method to improve the performance of theferroelectric film. Accordingly, In order to find the method of suppressing the leakagecurrent of ferroelectric thin films and improving their performance of thinferroelectric films and ferroelectric memory, we investigate the strain effect anddoping effect on the leakage current of the thin ferroelectric films.1. Manipulating the leakage current of the ferroelectric thin films by strain:Combining nonequilibrium Green’s functions and first principles quantum transportcalculations within density functional theory, we have investigated the effect ofbiaxial strain on the leakage current of BaTiO₃ferroelectric thin films. The resultsshow that the compressive strain can effectively reduce the leakage current offerroelectric thin films. Especially, the leakage current will be reduced by nearly10times for BTO films with compressive strain of4%compared with strain free BTOfilms. By calculating the transmission coefficient and the density of states, we foundthat the transmission probability of ferroelectric tunnel junctions with compressivestrain is smaller than that with tensile strain. Moreover, we find that the valence bandsshift to the lower energy zone while the conduction bands move towards the highenergy zone, which leads to enlarge the energy band gap and thereby reduce theleakage current. Our study suggested a suitable way to reduce the ferroelectric thinfilm leakage current and improve the performance of ferroelectric thin films and theirrelated ferroelectric memory.2. The doping effect on the leakage current of the ferroelectric thin films inducedby oxygen vacancies: We firstly investigated the the leakage current induced by oxygen vacancies in PTO films. Then, we investigated the doping PbTiO₃thinferroelectric films having oxygen vacancies. We discussed that doping effect on theleakage current induced by oxygen vacancies in PTO films. It is found that, theleakage current increased for PTO films with doping Fe and Al while the leakagecurrent is effectively reduced when doping Cu thereby improving the retentionperformance of the ferroelectric thin film. Moreover, we found that the oxygenvacancies is pinned by Cu atoms due to the fact that the formation energy of oxygenvacancies can be remarkly reduced. So Cu doping in PTO not only induce the leakagecurrent but also improved the fatigue resistance of the ferroelectric thin film inducedby oxygen vacancies. These conclusions have important theoretical significance andapplication value to improve the performance of ferroelectric thin films and theirferroelectric memory.