The ion energy determines the initial penetration depth of the particle and the vacancy concentration varies as a function of the implantation fluence.247 The higher the ion energy, the lower the fluence and the temperature required to create material damage beyond vacancies and vacancy clusters. For example, very small blisters were observed for 8 keV ions at RT and a fluence of 4 x 1021 He+-m~2 and above.247 In contrast, for low energies (<30 eV), temperatures >1300 K and fluences of about 1026 He+ m~2 and more are necessary to form bubbles and surface holes.149,248 The reason for the lack of blistering at low temperature and low ion energy is assumed to be the trapping of He-ions at defects/vacancies in the very near-surface range. With increasing temperature, the defects and He-atoms debond and the He-atoms diffuse toward the bulk, agglomerate, and result in blistering.249 Similarly, with increasing energy, the penetration depth of the He-atoms increases from nm for eV-ions up to 1.7 pm for 1.3 MeV He-ions and the probability of blister formation correspondingly rises. In both cases, whether the process is driven by He-diffusion or high penetration depth, blister­ing and exfoliation are expected to occur when the amount of He locally reaches 4at.% and 20-40 at.%, respectively.250

4.17.4.4.1.1 Influence of temperature

Vacancy mobility is dependent on temperature and starts at 523-573 k.251,252 As the mobility of vacancies and the formation of thermal vacancies are driving forces for the formation of bubbles, holes, and blis­ters, an increase in temperature increases the size and decreases the density of material damage.149,253 How­ever, it is not only the temperature during ion irradi­ation but also the annealing temperature during experiments such as thermal desorption measure­ments, which can influence the damage characteristics.247 The formation of holes and porous structures observed after thermal treat — ments,254 particularly for temperatures above the material-dependent recrystallization temperature, is related to the movement of vacancies, accelerating the expansion and coalescence of He bubbles, their migration to the surface,253 and subsequently the release of He. The latter is also a function of temper­ature, showing several release peaks between 400 and 1600 K related to different trapping sites247 and determines the amount of He retained as a func­tion of the incident fluence.242,247,255 However,

helium retention may be mitigated by cyclic He-implantation and high temperature heating, for example, flash heating to 2000 °C, because He flows away before critical amounts accumulate and form complex He-vacancy clusters with higher binding

250

energy.