3.8. The behavior of Ti at the interface to Si
We observed that when a Pr silicate film containing a high concentration of rechargeable interface states (e.g., with every atom in ten or in hundred interfacial atoms being a source of such a state) is covered with a Ti layer and subsequently annealed even in a relatively low temperature of 200°C, most of the interfacial states disappear. The mechanism by which titanium passivates the interface states is not immediately obvious.
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TiSi and H-saturated Si dangling bond at the interface between Si(001) and SiO2 |
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Electron states of TiSi and of unsaturated Si dangling bond |
Electrically active interface states at the interface between Si(001) and a SiO2 of a good electrical quality are due to Si dangling bonds. How can a Ti atom passivate such a defect? A Si dangling bond has an odd number of electrons, hence a Ti atom cannot passivate it directly as it has an even number of electrons. Indeed, a Ti atom (green) substituting an interfacial Si atom with a dangling bond (n the left-hand side of the model, such a red Si atom, but with the dangling bond saturated by the black H atom, is visible) has similar electrically active states in the gap as a Si dangling bond. In the diagram, electron transition states introduced by such a substitutional Ti atom (top) are compared to the states introduced by an unsaturated Si dangling bond (bottom). The pink areas correspond to the valence and conduction bands of the Si substrate.
Therefore, Ti atom cannot passivate the dangling bond by substitution; neither a passivation by attachment (as done by H) works. One can imagine that Ti acts indirectly, through a strain field which induces recombination of dangling bond pairs. Still, this means a significant re-arrangement of the atomic structure, requiring a long-range reconstruction of the oxidized Si and for that reason it may be difficult to realize, given that the passivation occurs already at 200°C.
We propose therefore that the electrically active defects passivated by Ti are "excess" defects (excess in comparison to the typical dangling bond states) associated with atomic-scale variations of Si density at the interface, such as dimers protruding from a locally flat Si(001)/SiO2 interface. Titanium dissolves these defects remotely, first by adsorbing losely bonded interfacial oxygen, and second by expelling silicon from the silicate film and causing regrowth of a more perfect interface.
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Si dimer at the interface between Si(001) and SiO2 |
Let us consider the interface dimer defect. In the model of this structure, the dimer atoms are the two semi-transparent pink and red atoms. The purple atom of the dimer disturbs the coordination of the purple Si atoms and the greenish O atoms; the properly coordinated Si atoms are red and the properly coordinated O atoms are yellow. We expect such defects at "interstitial" sites in the oxide network or at edges of small Si islands. Due to the over-coordination, electrically active states appear in the gap. This type of compression-induced defects is plausible since the oxidation of Si occurs in a constrained manner, under a silicate film. In contrast to that, the SiO2 film growing on the silicon substrate during a classical thermal oxidation process is stress-free (substantial stress exists only in the immediate vicinity to the interface). Since the volume occupied by a Si atom is about twice as large in SiO2 as in Si, oxidation leads to the appearance of compressive stress that is relaxed not only through changes in the topology of bonds, but also through ejection of Si atoms into the substrate as well as into the oxide, where the ejected atom becomes finally oxidized (if the oxidation is not complete, a positive fixed charge (SiNcO) may be formed). Naturally, the ejection into the oxide happens more easily when the oxide is stress-relaxed and can accommodate the additional compressive stress associated with the presence of the new atom. When the growth of the oxide is mechanically constrained, as is in the case of the growth of the interfacial layer or during low-temperature oxidation of Si, less Si atoms are expected to be injected into the oxide, the Si density at the interface will thus be higher, and compression-type defects should become more abundant. Indeed, it has been observed experimentally that low-temperature oxidation of Si produces interfacial defects which are apparently not due to regular Si dangling bonds.
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Regrown SiO2/Si(001) interface |
The presence of titanium helps to reduce the density of such defects. Ti enters the silicate from the metallic Ti film, drains O atoms from the defected interfacial sites (Ti oxidizes easily) and expels Si atoms from the silicate (Pr silicate and Ti oxide do not mix easily). This results in Si regrowth in previously compressed areas; in the ideal case, the electrically active states are removed completely, as indicated in the figure. Semi-transparent atoms in this figure are now the original atoms of the Si dimer and the Si atoms expelled from the silicate. No long-range reconstruction of the oxide is needed for this process to take place. The only requirement is that the temperature is high enough for Si and O transport between the Ti-containing silicate and the interface.
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Interstitial Ti below SiO2/Si(001) interface |
Finally, we find that when the Ti atom comes so close to the interface with Si that it is able to substitute the Si atom in the dangling bond site, then it will not stop there, but it will advance into the substrate, where it acquires an interstitial position. The Ti atom (green) does not stop at the dangling-bond site but advances into the substrate. The resulting sub-surface complex consists of the Ti interstitial atom and the Si atom with the dangling bond. (Note that the model is viewed from the direction rotated by 90° with respect to those of the previous images.) A seed of a metallic Ti silicide inclusion is formed in this way.
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Stability of substitutional and interstitial Ti at the interface |
This formation of Ti silicate seeds is energetically favorable for all Fermi levels within the Si band gap. Such inclusions are highly undesirable as they would electrically damage the transistor.