What is it about?
An important challenge in the gigascale silicon-device industry is the management of oxygen-related defects as allies (e.g. for “impurity gettering”) in the device-making process. However, nucleation and growth of oxygen clusters from ~10-20 ppma of dissolved interstitial oxygen Oi is a complex process, predicated on thermal history in the 600 deg C range, and involving electrically active thermal donors, lattice vacancies early on], a variety of precipitate “shape changes” e.g. from unstrained mono- layer plates to (111) octahedra to platelets on (100), and expansion-related silicon self-interstitial dislocation loops and stacking faults as more and more oxygen comes out of solution. The population of oxygen clusters with fewer than 10^5 oxygen atoms, often associated with thermal history during crystal growth, is of special importance. However it has been resistant to quantitative characterization because of unstrained-configurations and surface O-intrusions in that size range.
Because silicon’s diamond lattice naturally cleaves on (111), the preference for strained platelet formation on (100) is a mystery. This unexpected break in symmetry might be explained by anecdotal evidence that unstrained monolayer oxygen-molecule or “ninja-plates” naturally form on (100) because of lattice potential considerations, but are very hard to find given their lack of strain. At some point these “decloak” to form strained octahedra. As size increases and surface energies become less important, these octahedral clusters return to the original (100) plate for 2D growth above say 10-nm in size. What TEM images of the strained octahedral won’t reveal are the broken bonds left by the Oi as the puddle drains to form the octahedral defect. The paper also shows a three stage model for the ninja defect uncloaking in plot form.
Why is it important?
Oxygen in Czochralski silicon plays an important role in strengthening wafers, and in "gettering" electrically active impurities during device processing to improve yields. But mysteries remain, especially concerning the earliest stages of oxygen precipitate nucleation and growth from the interstitial state.