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Next: Conclusions Up: Methodology & results Previous: Substitutional Au and Pt-

TM-Hn complexes


Recent local vibrational-mode (LVM) spectroscopy studies [#!evans-97!#] have revealed the existence of a gold-hydrogen defect, containing a single hydrogen atom in a C3v symmetry. Two local vibrational modes have been identified, at 1813.3 and 1827.1 cm-1, associated with two different charge states of the AuH1 complex. These bands shifted by 502.4 and 507.7 cm-1 when hydrogen was replaced by deuterium, demonstrating unequivocally the involvement of hydrogen in the defect. Samples containing an admixture of H2 and D2 exhibit no additional vibrational bands. Despite these facts, it was impossible to identify the atom to which the H atom is bonded, along the $\mathopen{<}111 \mathclose{>}$ axis of the centre. However, recent studies indicate that the AuH1 might have a lower symmetry of C1h [#!evans-97!#].

Table I. Hydrogen stretch modes for AuH1 complexes.
Observed isotope shifts are given in parentheses. Values in cm-1.

(AuH1)q q = 0 -1 -2 (AuD1)q q = 0 -1 -2
Expt. [#!evans-97!#] 1813.3 (1827.1) & 1310.9 (1319.4)
BC 2419.0 2500.8 2454.3 BC 1726.2 1783.5 1748.9
AB outC1h 1947.4 1890.5 1893.2 AB outC1h 1401.1 1359.6 1361.4
AB inC1h 2014.5 1980.0 2118.0 AB inC1h 1427.3 1402.8 1500.2

To study the structural and vibrational properties of the AuH1 defect, three structures were used, in three different charge states. These were: a) BC (with the H atom sitting bond-centred between the TM impurity and a Si atom; b) ABout (the H atom at the anti-bonding site to one of the Si neighbours) and c) ABin (H is inside the vacancy along the [111] direction. A lower symmetry ABout configuration was also considered, in which the bond between two of the vacancy Si neighbours was slightly reconstructed. The BC and ABin structures of the AuH1 complex were found to be less stable than the ABout by 0.47 and 0.23 eV, respectively; the C3v and C1h ABout structures where found to be degenerate in energy. The Si-H length was 1.52 Å for both C3v and C1h. For ABin, the Au-H length was 1.60 Å. The calculated stretch LVM frequencies are given in Table I. In all the structures, the off-site displacement of the TM impurity atom is rather small. For ABin, the Au moves off-site by 0.06 Å along $[\,\overline{1} \, \overline{1}\,
\overline{1}\,]$ and 0.02 Å along the [ 111 ] for AB outC1h. The energy barriers for re-orientation between symmetric configurations were, 0.41 eV for AB outC1h, and 0.23 eV for ABin. These barriers are probably reduced by tunnelling.

The calculated quasi-harmonic local-vibrational modes (LVMs) for (PtH1)- were 1830.0 and 1728.1 cm-1, for AB outC1h and ABin, respectively. Again, as in the case of AuH1, the closeness between LVM does not identify the structure responsible for the vibrational band at 1897.2 cm-1 seen in hydrogenated Pt-doped Si and attributed to a (PtH1) defect [#!uftring-95!#].

We now investigate (PtH2)- and (AuH2)0. The structure of (PtH2)- is consistent with experiment and previous modelling. The AB (C2v) configuration was found to be more stable than the competing BC by 0.40 eV for (PtH2)- and 0.53 eV for (AuH2)0. The Si-H lengths were 1.54 and 1.52 Å, for the Pt-H and Au-H related defects, respectively. For (PtH2)-, the splitting between the calculated A1 (1763.1) and B1 (1756.2) modes was $\sim$ 7 cm-1, in fair agreement with the observed value of 19.4 cm-1 [#!uftring-95!#]. As expected, due to a shorter Si-H length, the stretch mode frequencies for the AB AuH2 defect were 151.0 and 156.4 cm-1 higher than the Pt analogue.


next up previous
Next: Conclusions Up: Methodology & results Previous: Substitutional Au and Pt-
Antonio Resende
1998-06-12