Smashing Ions Into Silicon Crystal Structures

Crystalline structures can develop defects of many different varieties. Atoms of the crystal can be substituted for larger or smaller atoms; a vacancy can occur when an atom is removed completely and a gap remains in its place; or an interstitial atom can be found inbetween the normal crystal structure. When those last two defects combine the crystal develops what is known as a Frenkel pair, where there is a gap in the structure because the atom should fill it has become interstitial. This implies that the defect was formed after the lattice was fully developed and is most often caused by radiation damage in the crystal…

Crystalline silicon is the structure that is most commonly studied and yet how the Frenkel defects form and then move within the crystal is still not fully understood. It is generally believed to be due to a process called dynamic annealing where all the defects are formed during the irradiation phase. In order to see what effect temperature had on this process samples of silicon were struck with ions of argon (Ar) that had been accelerated to energies of 500 keV that could produce multiple mobile defects through a series of collisions. The defects formed had their behaviour monitored over a temperature range of −20°C to 140 °C. It was revealed that a drastic difference between the dynamic annealing below 60 °C and that which occurs above 60 °C. This implies that the defect formation itself changes across this temperature bound but neither electron microscopy nor Raman scattering spectroscopy could reveal a difference in the defects formed. The large success here is that the ion bombardment technique proved an effective way of gathering information about the processes in the crystals. This method will hopefully prove effective in the future for analysing other crystalline structures beyond silicon in more detail.

Paper Links: The role of Frenkel defect diffusion in dynamic annealing in ion-irradiated Si

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