Experimental Work

There were no reports of isotopic effects on the LVMs of the NNO defect even though this information is of the greatest importance in deducing its structure. Therefore concurrently with our theoretical investigation, experimental IR studies were performed by Berg Rasmussen et al [177]. Float-zone n-type Si samples were implanted at room temperature with various combinations of ¹⁶O, ¹⁷O, ¹⁴N and ¹⁵N, with a concentration of 6.5$\times 10^{19}$ cm^-3 for both O and N. All samples were annealed under flowing N₂ ambient for an hour in the temperature range from 500 to 650$^\circ$C. Infrared absorption measurements (shown in Figure 7.2) were carried out at 77 K. The LVMs due to the NNO defect are given in Table 7.1.

  

Figure: Absorbance spectrum observed after annealing at 650$^\circ$C for 1 hour, of samples with various combinations of ¹⁴N, ¹⁵N, ¹⁶O and ¹⁷O. In all cases the total N and O doses are equal. Data obtained by Berg Rasmussen et al [103]

The ¹⁴N frequencies agree with the earlier studies [174,175,178], allowing for their shift ($\approx$ 4 cm^-1) between low and room temperature. All the modes shift with ¹⁵N showing that at least one N atom is involved in the defect. Unexpectedly, the peak in the 1030 cm^-1 LVM is shifted by around 3 cm^-1 with ¹⁵N and this suggests that this mode is almost entirely due to Si-O stretch. This lies between the LVMs for O_i (1136 cm^-1[179]) and O_s (836 cm^-1[77]), suggesting that O_i bridges a dilated Si bond near the N-pair. The modes at 999 and 805 cm^-1 are slightly above those of an isolated N-pair (967 and 770 cm^-1) and show a similar shift with ¹⁵N (25 and 19 cm^-1 compared to 26 and 17 cm^-1 for the N-pair), suggesting that the pair remains intact but with compressed bonds due to the nearby O_i. No additional LVMs are observed in the mixed implantation, implying that the N atoms are no longer dynamically coupled but vibrate independently of each other. They must also be inequivalent atoms as they possess distinct LVMs. This implies that O lies closer to one N atom than the other.