The Electrical Transport Characteristics Research Of Ferroelectric Tunnel Junctions

Ferroelectric tunnel junction is a kind of heterojunction, which consist of an ultrathin ferroelectric thin film sandwiched between two electrodes and have quantum tunneling and resistance switching characteristics, so-called tunneling electroresistance effect. The high resistance state and low resistance state can stand for logic "0" and "1" respectively and therefore FTJs can be used to next-generation non-volatile random-access memories. We studied the tunneling electroresistance effect of ferroelectric tunnel junction from both theoretical and experimental. The research contents and results are as follows:(1) We studied the single ferroelectric layer tunnel junction and ferroelectric tunnel junction with quantum well structures theoretically. The results show that:compared with the traditional single ferroelectric layer tunnel junction, the ferroelectric tunnel junction with a single quantum well structure can enhance the TER value by at least an order of magnitude and compared with the single quantum well structure ferroelectric tunnel junction the ferroelectric tunnel junction with a multi-well structure can also enhance the TER value on the base of it by an order of magnitude. The enhanced TER ratio of the multi-well structure is related to the proper asymmetry of the potential energy profile of tunneling barrier and the enhanced resonant tunneling strength.(2) The effects of different pressure, substrate temperature and annealing temperature during the film prepared process on the surface morphology, structure and electrical properties of the SrRuO3bottom electrodes were systematically studied. The optimum deposition pressure, substrate temperature and annealing temperature were determined separately. The results show that:the deposition pressure and the substrate temperature have great influence on the surface morphology and the conductivity of SrRuO3thin film and we can only get condensed SrRuO3thin film with good conductivity at appropriate atmospheric pressure and substrate temperature; The substrate temperature and the annealing temperature have certain influence on the preferred orientation degree of film. As it is easy for SrRuO3to form the (121) preferred orientation, so it is beneficial for a lower substrate temperature and a higher annealing temperature to increase (020) and (040) orientation degree. Finally, we got SrRuO3films with good quality at air pressure of1Pa, substrate temperature of500℃and annealing temperature of650℃.(3) The effects of different pressure, substrate temperature and sputtering power during the film prepared process on the surface morphology, structure and electrical properties of the BaTiO3films were systematically studied. The optimum deposition pressure, substrate temperature and sputtering power are determined separately. The results show that:the sputtering pressure and substrate temperature have great impact on the preferred orientation of the BaTiO3film, and we got BaTiO3films with good quality at sputtering pressure of1Pa, substrate temperature of650℃and sputtering power of50W.(4) We prepared the traditional single ferroelectric layer tunnel junction and the ferroelectric tunnel junction with quantum-well structures under the above optimal conditions, and found apparent resistance switching characteristics on either of them. When we fix the ferroelectric layer thickness, the ferroelectric tunnel junction with optimal quantum-well structures can enhance the TER at least an order of magnitude. The improved on off ratio of the quantum-well structure is due to the combined action of barrier asymmetry and the resonant tunneling effect.