Al in the di-y-lid core

**Table 9.7:** Vibrational modes (cm^-1) of the NL10(Al) proposed structure, a `di-y-lid' with centrally substituted Al. Also included is the dipole moment squared for the ¹⁶O higher modes, which is proportional to absorption intensity.

3cLocal Vibrational Modes (cm^-1)	Dipole moment squared
¹⁶O	¹⁷O	¹⁸O	for ¹⁶ O

1127.0	23.9	45.8	0.093
1062.1	18.4	34.9	0.308
973.4	18.9	34.2	0.022
945.5	17.6	35.2	0.219
933.8	18.7	35.7	0.271
917.3	18.3	35.2	0.143
849.2	17.6	33.6	0.035
827.7	14.4	27.7	0.019
652.1	2.5	4.9
643.1	9.1	17.4
630.3	0.2	0.4
591.0	1.3	2.8

Several of these thermal donor structures feature a core Si atom surrounded by oxygen atoms, notably the di-y-lid model. In this structure the core Si would be a prime candidate for substitution by Al. For full background description of the NL10(Al) defect, see Chapter 8.

**Figure 9.15:** The top Kohn-Sham eigenvalues for the NL10(Al) Al/di-y-lid model, Al_sO_4i. Defect is in the +1 charge state. Black boxes denote filled state, white boxes for empty states. The eigenvalues have been arbitrarily scaled to the experimental band gap of 1.16eV.
$\begin{figure} \begin{center} \ \psfig {file=oxygen/thermal/diags/levels.Al.eps,width=8cm} \end{center}\end{figure}$

**Figure 9.16:** The proposed NL10(Al) structure, consisting of the di-y-lid structure with the core Si atom replaced by Al (cross-hatched atom). All bond lengths in Å, black dots mark ideal Si lattice sites.
$\begin{figure} \begin{center} \ \psfig {file=oxygen/thermal/diags/Al/Al.eps,width=14cm} \end{center}\end{figure}$

When we relaxed this structure we found it to be stable in a single positive charge state, where it possessed a (now depopulated) level very close to the conduction band, in agreement with experiment. It also has a filled singlet quite close to the valence band (see Figure 9.15).

It possesses the correct symmetry of approximately C_2v (there are small out of plane deviations similar to the di-y-lid discussed above). In addition it has a single Al atom on the C₂ axis and no oxygen on this axis, in agreement with ENDOR studies. The relaxed structure is shown in Figure 9.16. There are several variants on the NL10(Al) structure observed [10] which are explainable in terms of Al subsituting for different lattice Si atoms; in the di-y-lid structure there are other Si atoms neighbouring more than one oxygen atom, all of which would be sites suitable for Al attack. Isotopic ENDOR studies led to the conclusion that the local structures around oxygen must be identical for both NL8 and NL10 [10], which is the case in these di-y-lid based models.

Unfortunately there is no unambiguous vibrational data to compare to, so we cannot use this to help strengthen the assignment[265]. However an absorption at 992 cm^-1 has been observed in Al-doped Cz-Si which has an integrated absorption intensity linearly proportional to the Al content [239]. This lies mid-way between the absorptions of TD2 and TD3 (988, 999 cm^-1 respectively), suggesting that this could be a perturbed form of these early thermal donors. If the Al_sO_4i complex described here forms from dimer aggregation at Al_s, then we might expect to first observe the formation of Al_sO_2i, which could be the source of this absorption. The calculated vibrational modes of Al_sO_4i are given in Table 9.7 along with the square of their dipole moments.

Substitution for the core Si in the 5O model is also stable, although the structure exhibits slightly more distortion. However this cannot be a model for NL10(Al) since it has O on the C₂ axis, in contravention of ENDOR observation[10].

Thus the di-y-lid with Al substituted for the core Si seems a sensible candidate for the NL10(Al), which fits all of the available ENDOR structural restrictions, and possesses a single shallow gap state at around the right value.

Al in the di-y-lid core - NL10(Al)?