Combined local mode spectroscopy and ab initio modeling are used to demonst
rate for the first time that oxygen atoms in thermal double donors (TDD) in
Si are in close proximity. The observed vibrational modes in O-16,O-18, an
d mixed isotopic samples are consistent with a model involving [110] aligne
d oxygen chains made up of an insulating core lying between electrically ac
tive ends. The model also explains the minute spin density observed on oxyg
en in TDD+ as well as the piezospectroscopic tensors of the donors. The ana
logy between the thermal donors and quantum dots is emphasized.