Elsevier

Chemical Physics Letters

Volume 400, Issues 1–3, 11 December 2004, Pages 179-184
Chemical Physics Letters

Atomic-silicon cryptates in siloxanic networks

https://doi.org/10.1016/j.cplett.2004.10.099Get rights and content

Abstract

The existence of atomic-silicon cryptates in siloxanic networks has been studied theoretically via high level quantum mechanical calculations. Modeling with model molecules the candidate sites to host atomic silicon, we found that metastable adducts can be formed only in regions where the siloxanic network is not subjected to steric constraints; stationary states are instead impossible in highly reticulated siloxanic networks. This analysis suggests that the atomic silicon injected into the oxide during thermal oxidation of silicon with O2 may be trapped as a metastable adduct at the oxide surface.

Introduction

Because of its relevance in integrated-circuit processing, the thin film (in the length scale in the interval 1–10 nm) produced by high temperature (T  800 °C) oxidation in O2 of single crystalline (1 0 0) silicon is certainly one of the most studied materials. However, though the Si–SiO2interface and the ‘bulk’ SiO2 have been the matter of extended experimental and theoretical investigations (see, for instance, the review of [1] or [2], [3], [4], [5]), scarce attention has been posed to the SiO2surface. This fact is somewhat astonishing, especially because it plays a fundamental role on the performances of some devices (erasing of flash memories) and it also manifests anomalous reactivities (compared to that of the oxide underneath) toward O2[6] and H2[7].

A theory of silicon oxidation by O2 has recently been proposed by Kageshima and co-workers [8], [9] to account for these anomalies. In that theory, the tensile stress accumulated at the silicon–SiO2 interface (due to the fact that 6.6 × 1022 cm−3 – the atomic density of SiO2– must be accommodated in a region previously hosting 5.0 × 1022 cm−3 – the atomic density of silicon) is relieved by injection of silicon atoms not only into silicon (the injection of self-interstitials is a well known phenomenon, responsible for oxidation-enhanced diffusion of dopants and for the growth of extrinsic stacking faults) but also into the growing SiO2. In view of its expected high reactivity, atomic silicon was assumed to react with dissolved oxygen and to evaporate as SiO (for thin films) or transform into SiO2 (for thick films). The isotope-exchange experiments of Gusev et al. [6] and the hydrogen-desorption experiments of Baumvol et al. [7], suggest however the presence of Si(n) (silicon in an oxidation state n with n < 4) at the SiO2 surface. Actually, though the redox reaction, in which atomic silicon cleaves two siloxanic bridges,Si(0)+2(-O)3Si(4)OSi(4)(O-)3[(-O)3Si(3)]2Si(2)[OSi(4)(O-)3]2,is certainly an exothermic process [expectedly by ca. 2Eb(Si–Si)  6 eV, where Eb(Si–Si) is the Si–Si bond energy], the possibility that silicon remains in the silica in an atomic form (or as cluster of a few atoms) is not manifestly absurd, especially in view of the high activation energy required to cleave the Si–O bridge forming the silica network. That reaction (1) may indeed require a high activation energy is strongly supported by the fact that interfacial reactionSi(s)+SiO2(s)2SiO(g)in the absence of oxygen does really occur, but only at temperature higher than 1000 °C.

An octet configuration may be achieved by donation of lone pairs from two oxygen atoms of two oxo bridges between silicon, the resulting Lewis formula of the center beingwhere an unterminated bond implies bonding to silicon (or to hydrogen, at the surface of hydroxylated silica).

To test this hypothesis, we have thus performed density functional calculations, using model molecules to mimic the oxide.

Section snippets

Choice of the model molecules

The thin (in the nanometer length scale) oxide grown via thermal oxidation in O2 of hydrogen-terminated silicon is highly heterogeneous because of the existence of several regions with different compositions: an interfacial region, with thickness of a few ångstroms in which the stoichiometry changes swiftly from Si to SiO2 (through all intermediate oxidation states Si(n), n = 1, 2, 3); a ‘bulk’ region, whose stoichiometry is very close to that of SiO2; and a superficial region expectedly

Results

Fig. 2, Fig. 3, Fig. 4 show the most stable adducts resulting from the reaction of atomic silicon with molecules shown in Fig. 1. The optimized geometrical parameters (bond lengths in Å and angles in °) and the corresponding electronic states are also given. The symbols have the following meaning:2N+SiNforN=1,2,3N+SiNforN>3andO1SiOnN+SiNforN>3andO2SiOn-1N+SiNforN>3andO3SiOn-2

We plan to publish the detailed results of our calculations in a separate Letter. For the present

Conclusions

The existence of atomic silicon in siloxanic networks has been theoretically investigated through density functional calculations. Modeling with model molecules the candidate sites to host atomic silicon, we found that metastable adducts can be formed only in regions where the siloxanic network is not subjected to steric constraints. The triplet state of the adducts is always more stable than the singlet one and it invariably contains a Si atom with a neutral charge which retains most of its

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