Effect of a built-in electric field in asymmetric ferroelectric tunnel junctions

The contribution of a built-in electric field to ferroelectric phase transition in asymmetric ferroelectric tunnel junctions is studied using a multiscale thermodynamic model. It is demonstrated in detail that there exists a critical thickness at which an unusual ferroelectric-polar nonferroelectric phase transition occurs in asymmetric ferroelectric tunnel junctions. In the polar nonferroelectric phase, there is only one nonswitchable polarization which is caused by the competition between the depolarizing field and the built-in field, and closurelike domains are proposed to form so as to minimize the system energy. The transition temperature is found to decrease monotonically as the ferroelectric barrier thickness is decreased and the reduction becomes more significant for the thinner ferroelectric layers. As a matter of fact, the built-in electric field not only results in smearing of the phase transition, but also forces the transition to take place at a reduced temperature. Such findings may impose a fundamental limit on the work temperature and thus should be further taken into account in the future ferroelectric-tunnel-junction-type or ferroelectric-capacitor-type devices.