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
axis of the
centre. However, recent studies indicate that the AuH1 might have a
lower symmetry of C1h [#!evans-97!#].
(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
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
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.