Carrier density modulation in a germanium heterostructure by ferroelectric switching.

Patrick Ponath,Agham B Posadas,Sergei V Kalinin,Toshihiro Aoki,Martha R Mccartney,Yuan Ren,David J Smith,Kurt Fredrickson,Alexander A Demkov,M Baris Okatan,Keji Lai,Rama K Vasudevan,S Jesse,Xiaoyu Wu

doi:10.1038/ncomms7067

Abstract

The development of non-volatile logic through direct coupling of spontaneous ferroelectric polarization with semiconductor charge carriers is nontrivial, with many issues, including epitaxial ferroelectric growth, demonstration of ferroelectric switching and measurable semiconductor modulation. Here we report a true ferroelectric field effect-carrier density modulation in an underlying Ge(001) substrate by switching of the ferroelectric polarization in epitaxial c-axis-oriented BaTiO3 grown by molecular beam epitaxy. Using the density functional theory, we demonstrate that switching of BaTiO3 polarization results in a large electric potential change in Ge. Aberration-corrected electron microscopy confirms BaTiO3 tetragonality and the absence of any low-permittivity interlayer at the interface with Ge. The non-volatile, switchable nature of the single-domain out-of-plane ferroelectric polarization of BaTiO3 is confirmed using piezoelectric force microscopy. The effect of the polarization switching on the conductivity of the underlying Ge is measured using microwave impedance microscopy, clearly demonstrating a ferroelectric field effect.

Highlights

The development of non-volatile logic through direct coupling of spontaneous ferroelectric polarization with semiconductor charge carriers is nontrivial, with many issues, including epitaxial ferroelectric growth, demonstration of ferroelectric switching and measurable semiconductor modulation
To have a substantial effect on the semiconductor charge carrier density, the ferroelectric polarization charge must be as close to the transistor channel as possible
BTO tends to grow with its long tetragonal axis (c axis), which is the direction of ferroelectric polarization, in the film plane

Summary

Introduction

The development of non-volatile logic through direct coupling of spontaneous ferroelectric polarization with semiconductor charge carriers is nontrivial, with many issues, including epitaxial ferroelectric growth, demonstration of ferroelectric switching and measurable semiconductor modulation. The conceptual simplicity of the FeFET architecture, combined with the large surface polarization charge density of a typical ferroelectric (B20 mC cm À 2), an order of magnitude larger than what dielectrics can typically sustain, contribute to the obvious attraction of having a ferroelectric gate in a field-effect transistor This seemingly straightforward approach has not yet worked because of the fundamental challenge of combining a ferroelectric oxide directly with a semiconductor without any interfacial reaction. By inserting an ultrathin STO layer between BTO and Ge, one can impose compressive in-plane strain on BTO that can overcome tensile stress caused by the thermal expansion mismatch and thereby achieve BTO on Ge with out-of-plane polarization[5]; a similar approach was recently used to grow BTO on GaAs. Ferroelectric switching of c-axisoriented BTO on STO-buffered GaAs was demonstrated by Huang et al.[23] using preferentially oriented films with columnar crystallites, and by Contreras-Guerrero et al.[24] using flat epitaxial films

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Results

Conclusion