TM-H_n 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 C_3v 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 C_1h [#!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) AB_out (the H atom at the anti-bonding site to one of the Si neighbours) and c) AB_in (H is inside the vacancy along the [111] direction. A lower symmetry AB_out configuration was also considered, in which the bond between two of the vacancy Si neighbours was slightly reconstructed. The BC and AB_in structures of the AuH₁ complex were found to be less stable than the AB_out by 0.47 and 0.23 eV, respectively; the C_3v and C_1h AB_out structures where found to be degenerate in energy. The Si-H length was 1.52 Å for both C_3v and C_1h. For AB_in, 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 AB_in, 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 _out^C_1h. The energy barriers for re-orientation between symmetric configurations were, 0.41 eV for AB _out^C_1h, and 0.23 eV for AB_in. 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 _out^C_1h and AB_in, 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 (C_2v) 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.