High fluence He irradiation of materials using Helium Ion Microscopy

Gregor Hlawaceka
Institute for Ion Beam Physics and Materials Research, Helmholtz—Zentrum Dresden—Rossendorf, 01328 Dresden,
Germany

Abstract:

I will present some recent results on the high fluence irradiation of metals using gas field ion source (GFIS) based
helium ion microscope (HIM)1
.
High entropy alloys (HEAs) are a relatively new class of metal alloys composed of several principal elements, usually
at (near) equiatomic ratios. Here, our goal is to understand how such a multicomponent alloy behaves under irradiation.
The FeCoCrNiV HEA exhibits both a face-centred cubic (fcc) and a body-centred tetragonal (bct) phase, thus allowing
us to specifically study the influence of crystalline structure at very similar chemical composition. We irradiated both
phases with a focussed He beam provided by a HIM at temperatures between room temperature and 500 ∘C. The
irradiation fluence was varied between 6×1017 ions cm−2
to 1×1020 ions cm−2
. High-resolution images of the irradiated
areas were taken with the same HIM. Selected irradiated areas were additionally studied by transmission electron
microscopy (TEM) in combination with energy dispersive X-ray spectroscopy (EDXS). Under irradiation, pores start
to be generated in the material with pore sizes differing significantly between the two phases. At higher fluences and
above a critical temperature, a tendril structure forms in both phases. We found that the critical temperature depends
on the phase and is lower for fcc. TEM images reveal that the tendrils span the whole depth of the irradiated area, and
are accompanied by bubbles of various sizes. Scanning TEM-based EDXS of these structures indicates a He-induced
change in composition.
In the second part I want to present an intriguing observation shedding light on the fundamental processes related
to interstitial diffusion during irradiation. I will show how epitaxial growth of tin extrusions on tin-oxide-covered tin
spheres can be induced and simultaneously observed by implanting helium using a HIM2
. Calculations of collision
cascades based on the binary collision approximation (BCA) and 3D-lattice-kinetic Monte Carlo (3D-lkMC) simulations
show that the implanted helium will occupy vacancy sites, leading to a tin interstitial excess. Sputtering and phase
separation of the tin oxide skin, which is impermeable for tin atoms, create holes and will allow the epitaxial overgrowth
to start. Simultaneously, helium accumulates inside the irradiated spheres. Fitting the simulations to the experimentally
observed morphology allows us to estimate the tin to tin-oxide interface energy to be 1.98 J m−2
.
Both approach have in common that they employ spatially resolved irradiation and in-situ observation of defect
diffusion-driven effects to improve the understanding of the formation mechanism of ion induced structures.
Financial support by the COST Action CA19140 is acknowledged. http://www.fit4nano.eu/

Bio:

Gregor Hlawaceka received his PhD in Materials Science in 2007 from the Institute for Physics at the Monta-
nuniversität Leoben in Austria. The topic of his thesis was Molecular growth mechanisms

in para-sexiphenyl thin film deposition. After a 1.5 year PostDoc at the same institution,
in 2009 he moved to the University of Twente in the Netherlands where he started to
perform Helium Ion Microscopy (HIM). Being one of the first users of this new method
he was involved in the design of a new UHV vacuum chamber for this instrument
and the development of ionoluminescence for the characterization of defects created by
the highly focused ion beam. In 2014, he moved to the Helmholtz Zentrum Dresden
Rossendorf and since then is in charge of the Orion NanoFAB HIM and the development
of high resolution ion beam analysis methods for HIM such as RBS and TOF-SIMS.
In addition, HIM has successfully been used for the analysis or the spatially resolved
modification of a number of material systems. Since 2019 he is the group leader of the
Ion Induced Nanostructures group at the Ion Beam Center and responsible for the low
energy ion activities at the institute.
Gregor Hlawacek received the AT&S award of the Austrian Physical Society for the
best PhD thesis in solid state physics in 2008. He is co-organizers of several HIM and
FIB related meetings, conferences and sessions at large international conferences (AVS,
M&M, EU-F-N, HeFIB, . . . ) and organized the HeFIB meeting in 2018. He received
funding through the European Commissions H2020 program (IONS4SET, npSCOPE),

the Leverhulme Trust (picoFIB), the Luxembourg national research fund (STHIM), M-
era.net/SAB (GoFIB) and the DFG. Recently, he has successfully applied and been

granted a COST Action on Focused Ion Technology for Nanomaterials connecting more
than 80 groups in Europe.
Gregor Hlawacek has published over 90 papers in peer reviewed journals with 2000
citations. He also published 9 book chapters and is editor on the first full book on
Helium Ion Microscopy released in 2016. He currently supervises two PhD students and
has successfully promoted three more in the past.