Bonding and Epitaxial Relationships at High-K Oxide:Si interfaces
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Bonding and Epitaxial Relationships at High-K Oxide:Si interfaces J Robertson* and P W Peacock Engineering Department, Cambridge University, Cambridge CB2 1PZ, UK * [email protected] Abstract The bonding at interfaces of Si(100) with SrTiO3, LaAlO3 and HfO2 are considered using simple electron counting models. Introduction The decrease of dimensions of CMOS transistors has led to the need for alternative, high dielectric constant (κ) oxides to replace SiO2 as gate dielectric [1,2]. The prototype high K oxides are HfO2 and its silicate. These oxides are usually amorphous. Capacitors and devices using high K oxides presently suffer from low performance due partly to bulk and interfacial defects in the oxides. The interface of the oxide and the poly Si gate electrode can also possess defects which appear to pin the interface potential. To define an interface defect, we must first define an ideally bonded interface. We do this here by electron counting [3,4] using epitaxial interface models of cubic oxides. The ideas can be generalised to interfaces of amorphous or lower symmetry oxides. SrTiO3 interfaces
Fig. 1. (001)Si:SrTiO3 interface viewed in the (110) plane of Si and (100) plane of SrTiO3. (a) Ideal Si(100) surface. (b) dimerized, (c) add 0.5 ML Sr, (d) build up STO lattice. At the (100)Si:SrTiO3 interface, the (001)SrTiO3 lattice is matched to (001)Si if the SrTiO3 lattice rotated by 450 so that the[100]SrTiO3 direction lies parallel to[110]Si [5]. The
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ideal Si(100) surface shown in Fig 1(a) has two Si dangling bonds (DBs) per surface atom. One of these per atom can be removed by a 2x1 dimer reconstruction of the surface. This leaves one DB per surface Si. The DBs are half filled orbitals with one electron per state, and they would form a state in the band gap of Si, and give a metallic interface. It is important for an electrically useful interface that any interface states are removed from the Si gap [6-8]. The SrTiO3 lattice consists of non-polar (100) atomic planes of SrO and TiO2. Each plane has a satisfied valence or closed shell. A SrTiO3 surface terminated by either plane is non-polar and has no states deep in its band gap. If we place one of these non-polar SrO or TiO2 faces onto the dimerized (100) Si surface, we do not get an insulating interface. The oxide side of the interface is closed shell, but the Si DBs still remain on the Si side. The way to remove these DB states is to passivate the Si surface with a half monolayer (ML) of Sr. The Sr is divalent and electropositive. It can transfer its two valence electrons one to each of the Si DBs to make Si anions. This fills the DB states. More importantly, if the interaction is right, the Sr s orbital will repel the Si DB states into the Si valence band, so that the states no longer lie in the Si band gap. Then the Si surface is fully passivated. This is the Sr-terminated interface. It is then possible to build up a SrTiO3 crystal on top of this surface by adding alternate SrO or a TiO2 planes, Fig 1(d).
Fig. 2 . (a) Dimerized Si(100).
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