Atomistic Modeling of an MFM ferroelectric capacitor made of HfO 2 :Si
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.333
Atomistic Modeling of an MFM ferroelectric capacitor made of HfO2:Si P. Blaise Univ. Grenoble Alpes, CEA, LETI, 38000 Grenoble, France
ABSTRACT:
Using ab initio simulation, we study a ferroelectric layer of a few nanometers made of hafnia (HfO2) under the influence of Si doping with TiN electrodes. We evaluate the orthorhombic phase of Pca21 symmetry, its ferroelectric switching and the incidence of doping with silicon. We show that the ferroelectric switching can involve a 90° characteristic angle with corresponding activation energy which is lowered by a factor three due to Si doping at 3% at. A full MFM (Metal-Ferroelectric-Metal) model is derived in order to simulate finite-size effects. This model is compatible with a reversal of a polar HfO2:Si with a (111) preferential orientation. Validity and usefulness of such a model are discussed for ferroelectric devices optimization.
INTRODUCTION: Ferroelectric behavior of doped-HfO2, discovered for microelectronics applications [1], has recently attracted a lot of attention for several types of ultra-scaled devices: Ferroelectric Field-Effect Transistor FE-FET [2], Ferroelectric Tunnel Junction FTJ [3] and Negative Capacitance NC-FET [4]. Each requires the basic understanding of ferroelectricity at a small scale with a various amount of dopants. Ferroelectric behavior is detected for thin films below the thickness of 10-40 nm, with a few percents of dopants, like Si, Al, La, Gd, or with more than 50% of Zr, (in substitution of Hf atoms). For example, among recent experimental findings [5], obtained a ferroelectric behavior with a typical remnant polarization 2·Pr of 15 C/cm2 and a coercive field Ec in between 1.5 to 2 MV/cm. The ferroelectric layer is deposited by ALD, doped by Si ion implantation (1.2% at.), for 10 nm of dielectric and with TiN electrodes. Using XRD and HRTEM microscopy, a mixture of monoclinic and orthorhombic phases with a
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pronounced orthorhombic peak is revealed with a grain orientation linked to (111) plane, with a grain height equal to the thickness of the thin film. Here, we propose to model such a ferroelectric using ab initio simulation, in density functional theory. First, we compute several phases and their energetics. Then we discuss how a bulk switching mechanism with a characteristic angle of 90° can reverse the polarization direction. By using an activation energy evaluation technique, we put in evidence a ferroelectric monodomain switching with a saddle point of quasi-tetragonal symmetry, strongly influenced by doping. This bulk switching is extended to a layer of a few nm, in contact with TiN, allowing to study electrostatic at equilibrium. We focus our attention on a thin slab of HfO2:Si (3% at.) with a thickness o
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