Electronic Structure and Band Offsets of High-Dielectric-Constant Gate Oxides
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Electronic Structure and Band Offsets of High-DielectricConstant Gate Oxides John Robertson
(MgO, SrO, BaO)4 plus some rare-earth oxides (Gd2O3, Pr2O3). The oxides already known to the semiconductor industry, but notable by their absence as viable candidates, are TiO2 and titanates like (Ba, Sr)TiO3 and Ta2O5. Work has tended to focus on the transition-metal oxides and their silicates and aluminates. This article focuses on some materials physics and chemistry aspects of high- oxides—their bonding, electronic structure, the nature of their dielectric constant, their band alignments or offsets with Si, and, finally, their defect and interface properties. The need for the new oxide to form a good interface means that polycrystalline oxides are not allowed in direct contact with Si. The oxide must either be epitaxial and lattice-matched to Si or be amorphous. The epitaxial version is possible, but it is uneconomical, so it is desirable that the new oxide be amorphous and remain amorphous under processing conditions.
Bonding and Electronic Structure Abstract Identifying candidate materials to replace SiO2 as the gate dielectric for complementary metal oxide semiconductor (CMOS) applications is a difficult task. Proper assessment of the critical materials requirements is essential, and it is important to devise an approach to predict materials properties without having to make many unnecessary measurements on high- materials. Such an approach helps to eliminate unlikely candidates and focus on the most promising ones. Clearly, this type of modeling approach requires an understanding of several physical and chemical characteristics, including the bonding and electronic structure, band alignment with Si, and the nature of the dielectric constant and interface properties. We present a critical assessment of some existing methods and models of materials properties, as well as a comparison of the present modeling approach with some experimentally determined values.
There are considerable differences between the behavior of the new oxides and SiO2. This is largely because of their greater degrees of ionic bonding. Atomic coordinations and bonding determine not just whether a solid can be amorphous or not, but also its electronic structure, because the electronic structure depends fundamentally on coordination, not on the presence of crystalline order. Figure 1 compares the atomic coordinations in various oxides. In SiO2 , Si forms four directional covalent bonds to oxygen
Keywords: band offset, dielectric constant, electronic structure, high- dielectric materials, modeling.
Introduction The decrease of device dimensions has led to the need for oxides with a high dielectric constant () to replace silicon dioxide as a gate dielectric in complementary metal oxide semiconductor (CMOS) devices. This is because silicon dioxide layers thinner than about 1.6 nm have a leakage current of 1 A/cm2 due to direct tunneling through the oxide. The alternative is to use thicker layers of a “new” high- dielectric, with the same equ
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