Evolution of the properties of helium nanobubbles during in situ annealing probed by spectrum imaging in the transmission electron microscope

Abstract

The evolution of nanometric helium bubbles in silicon has been investigated using spatially resolved electron energy-loss spectroscopy during in situ annealing in the transmission electron microscope. This approach allows the simultaneous determination of both the morphology and the helium density in the bubbles at each step of the annealing. Structural modification and helium emission from bubbles of various diameters in the range 7.5 to 20 nm and various aspect ratios of 1.1 to 1.9 have been studied. We clearly show that helium emission takes place at temperatures where bubble migration had hardly started. At higher temperatures, the migration (and coalescence) of voids is clearly revealed. For helium density lower than 150 He nm −3 , the Cerofolini's model taking into account the thermodynamical properties of an ultradense fluid reproduces well the helium emission from the bubbles, leading to an activation energy of 1.8 eV. When bubbles exhibit a higher initial helium density, the Cerofolini's model fails to reproduce the helium emission kinetics. We ascribe this to the fact that helium may be in the solid phase and we propose a tentative model to take into account the properties of the solid.

Atomistic simulations of a helium bubble in silicon carbide

“…Furthermore, recent investigations in our group based on spatially resolved electron energy-loss spectroscopy in transmission electron microscopy experiments revealed that very high helium densities, ranging from 120 to 200 He nm −3 , can be obtained for bubbles in silicon [34,35]. Such values largely exceed predictions coming from the well known Laplace-Young relation.…”

On the Possibility of Helium Adsorption in Nitrogen Doped Graphitic Materials

“…Adsorption materials, which are another option for storage and transportation of He gas, share a similar problem: the polarizability of He is so low that there is little He adsorption at all. On the one hand, this weakness of interaction of He with various metals/materials is exploited because of its inertness and low solubility in various materials, reference states can be marked [20][21][22][23][24][25] . On the other hand, He adsorption materials would therefore enable new technologies and are hence in high demand.…”

The density and pressure of helium nano-bubbles encapsulated in silicon

“…The first method for attempting to determine the densities relies on the result that the lowest allowed electronic excitation is always shifted to energies greater than that of the free atom. This raises the possibility that the density could be deduced by measuring [15][16][17] just this energy shift. However, the density can only be reliably deduced from such shift measurements if the functional dependence of the shift on density has already been independently established and validated for the particular matrix material enclosing the bubble.…”