Hafnia surface and high-k gate stacks
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1155-C11-02
Hafnia surface and high-k gate stacks X. Luo1, Alexander A. Demkov1, O. Sharia1, G. Bersuker2, 1 2
Department of Physics, The University of Texas at Austin, Austin, Texas 78712, USA SEMATECH, Austin, Texas 78741, USA
ABSTRACT Hafnium dioxide that belongs to a class of metal oxides with a high dielectric constant or high-k dielectrics has been recently introduced as a gate dielectric in field effect transistors. We report a theoretical study of structural and electronic properties of hafnia surface, and the electronic structure and band alignment at hafnia interfaces with metals, oxides and semiconductors that are crucial in gate stack engineering. INTRODUCTION As scaling of the complementary metal oxide semiconductor (CMOS) technology takes us below 45 nm many new materials, traditionally not associated with the semiconductor process, are being introduced into manufacturing. Notably, several transition metal (TM) oxides, or more generally dielectrics with a high dielectric constant or high-k dielectrics, are being considered for the gate stack applications instead of SiO2. The gate stack is a multilayer structure in place of the metal oxide semiconductor capacitor. Its capacitance controls the transistor saturation current and has been traditionally maintained by reducing its thickness in accord with the gate length reduction (the so-called scaling at the heart of Moor’s law). However, after reaching the oxide thickness of 12 Å the scaling has stopped due to the prohibitively large gate leakage current caused by direct tunneling across the gate oxide. Thus a new dielectric with a larger dielectric constant had to be introduced [1]. In the current generation of field-effect transistors (FETs) hafnia-based dielectric films started to replace silica as a gate dielectric [1,2]. Hafnia (HfO2) has been selected due to its high thermodynamic stability on Si [3,4], sufficiently wide band gap, and high dielectric constant (k=20-25) [5]. Bulk hafnia crystallizes in three polymorphs; at room temperature and ambient pressure it is monoclinic, and transforms first to a tetragonal form and then to a fluorite-type cubic form at elevated temperatures. Thin hafnia films are typically amorphous as deposited, and polycrystalline after a post-deposition anneal, typically they are mixed phase [6] with mainly monoclinic ( 1 11) , (001) and (111) textures [7,8,9]. The structure-property relation of thin hafnia films is not completely clear. In recent years the bulk properties of hafnia, such as structure, electronic spectrum and dielectric constant have been studied theoretically [5]. However, thin hafnia films behave differently from bulk hafnia. In particular, phase transitions occur at much lower temperature. In addition, the surface of hafnia may play a role in the gate dielectric film thermodynamics due to film’s high surface area to volume ratio. For example, a low surface energy phase may be stabilized below a critical thickness. Thus a systematic study of hafnia surface including monoclinic and tetragonal phases for
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