| Because of having quantum tunneling effect and resistance reversal effect, ultra-thin ferroelectric film is suitable for fabricating memory device, which is widely applied in the fields of military, satellite communication, automotive design, and so on. The research on ultra-thin ferroelectric film is the hot topic in the field of memory device. In this paper, ultra-thin BaTiO3(BT) ferroelectric films were deposited by magnetron sputtering, and then microstructures and electrical properties of BT ferroelectric films were studied. Finally, tunneling mechanism was discussed. The experimental results are as follows:1. The effects of TiO2-X layer on the quality of Pt bottom electrode were investigated. The morphology of BT film revealed that there were many bright dots on the film surface without TiO2-x due to the rough surface of oxidized metal Ti layer and the main ingredient of the bright dots was metal Pt through the energy dispersive x-ray spectroscopy analysis. When TiO2-x was used to substitute Ti as transit layer between substrate and electrode, bright dots were disappeared on the surface of the Pt electrode. So the flat surface of Pt electrode is benefit of depositing ultra-thin ferroelectric film.2. The ultra-thin BT films were deposited by magnetron sputtering. The optimized sputtering parameters were investigated using orthogonal design method. Under the experiment condition of the volume ratio of oxygen and argon of1:3, working pressure of4Pa, substrate temperature of630℃and annealing temperature of730℃, the remanent polarization value of ultra-thin BT film is found to be maximum by using direct analysis method. The effects of sputtering parameters on electrical properties of films were discussed. Experimental result revealed that the working pressure can be optimized further. Finally, the film deposited at3Pa is optimal with respect to microstructure and electrical properties. So the optimized sputtering parameters for ultra-thin BT thin films were obtained:substrate temperature was630℃; the volume ratio of oxygen and argon was1:3; working pressure was3Pa; annealing temperature was730℃.3. The microstructure for ultra-thin BT ferroelectric films with different thickness was studied. The experimental results of XRD crystalline phase analysis revealed that all the ultra-thin films were (111) oriented growth with perovskite structure. The calculative results of Scherrer formula indicated that different thickness thin films showed the similar grain sizes of～46nm. The SEM image revealed very clear and well-contacted interface between ultra-thin BT film and Pt and very uniform for film thickness. The results of ferroelectric hysteresis loop revealed that the remanent polarization and coercive field reduced sharply with the decrease of thickness of ultra-thin BT ferroelectric films. The results of current-voltage characteristic curves revealed that unipolar resistance switching phenomenon was ahead of bipolar resistance switching phenomenon.4. Equivalent circuits were introduced to explain the current-voltage characteristic curves of Pt/BT/Pt tunnel junctions and Pt/BT/SrTiO3:Nb tunnel junction. The electron tunneling processes of different tunnel junctions were studied. Different linear fittings of current-voltage characteristic curves were used to study the physics mechanisms of unipolar resistance switching and bipolar resistance switching. The results revealed that there were three tunneling mechanisms for Pt/BT/Pt tunnel junctions, however, two tunneling mechanisms for Pt/BT/SrTiO3:Nb tunnel junction. Space charge limited current (SCLC) was non-existent in Pt/BT/SrTiO3:Nb tunnel junction. The other two tunneling mechanisms were Schottky emission and Poole-Frenkel(P-F) emission. |
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