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Published April 15, 2012 | Published
Journal Article Open

Effect of doping on polarization profiles and switching in semiconducting ferroelectric thin films


This paper proposes a theory to describe the polarization and switching behavior of ferroelectrics that are also wide-gap semiconductors. The salient feature of our theory is that it does not make any a priori assumption about either the space charge distribution or the polarization profile. The theory is used to study a metal-ferroelectric-metal capacitor configuration, where the ferroelectric is n-type doped. The main result of our work is a phase diagram as a function of doping level and thickness that shows different phases, namely, films with polarization profiles that resemble that of undoped classical ferroelectrics, paraelectric, and a new head-to-tail domain structure. We have identified a critical doping level, which depends on the energy barrier in the Landau energy and the built-in potential, which is decided by the electronic structures of both the film and the electrodes. When the doping level is below this critical value, the behavior of the films is almost classical. We see a depleted region, which extends through the film when the film thickness is very small, but is confined to two boundary layers near the electrodes for large film thickness. When the doping level is higher than the critical value, the behavior is classical for only very thin films. Thicker films at this doping level are forced into a tail-to-tail configuration with three depletion layers, lose their ferroelectricity, and may thus be described as nonlinear dielectric or paraelectric. For films which are doped below the critical level, we show that the field required for switching starts out at the classical coercive field for very thin films, but gradually decreases.

Additional Information

© 2012 American Institute of Physics. Received 13 June 2011; accepted 10 March 2012; published online 19 April 2012. It is our pleasure to acknowledge Jim Scott (Cambridge) for bringing this problem to our attention and many delightful discussions. This work started when V.B.S. and K.B. were visiting the Isaac Newton Institute, Cambridge and we are grateful for their hospitality. We gratefully acknowledge the support of the National Science Foundation (K.B. and Y.X.: MRSEC.FNP-1.2-NSF.CSEM) and the Army Research Office (K.B. and Y.X.: DAAD19-01-1-0517 and W911NF0810269; V.B.S.: W911NF-11-1-0171).

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