Quantum Computing, page-50

I should have highlighted "SOI: for it much better heat dissipation see here-:
Enriched 28Si has a thermal conductivity up to ten times higher than natural Si.136
With regard to spin-photon interfaces, isotopically pure 28Si offers two major advantages over natural silicon.

Will they use it in today's substrate manufacture?
I believe it would be logical for them to do so, because the better the heat dissipation the more transistors you can add to a given area, it would be costlier, but only small amounts would be required for today's SOI chip manufacture I believe?
That is the reason why they use SOI, less silicon required.



https://aip.scitation.org/doi/10.1063/5.0049372

1. Isotopically pure Si and SOI

In order to realize a large spin coherence time, it is important to eliminate the magnetic noise from neighboring crystal lattice defects as much as possible. It is therefore fortunate that silicon can be grown with a high degree of isotopic purity. Natural silicon is comprised of three stable isotopes: 28Si, 29Si, and 30Si with natural isotopic abundances of 92.22%, 4.69%, and 3.09%, respectively.136 Besides their different masses, these three isotopes also differ in their nuclear spin: 28Si and 30Si are spin-free, while 29Si has a nuclear spin of 12
.137 As a result, the physical properties of a Si crystal can differ greatly depending on its isotopic composition.

Enriched 28Si has a thermal conductivity up to ten times higher than natural Si.136 With regard to spin-photon interfaces, isotopically pure 28Si offers two major advantages over natural silicon.