Nanoscale modification of silicon and germanium surfaces exposed to low-energy helium plasma

Complex surface nanostructures were observed in germanium and silicon samples exposed to low energy (24 or 36eV ion kinetic energy) helium plasma. Pyramidal growth is observed in germanium across the temperature range studied (185°C to 336°C), while signifcant modifcation in silicon was only observed at 630°C. Nano-wire growth was observed in both germanium and silicon, and appears to be linked to the strength of the electric feld, which in turn determines the implantation energy of the helium ions. Nanostructure formation is proposed to be driven by surface adatom migration which is strongly infuenced by an Ehrlich-Schwoebel-type surface instability. The role of helium in this model is to drive germanium interstitial formation by ejecting germanium atoms from lattice sites, leading to germanium interstitial difusion towards the sample surface and subsequent adatom and surface nanostructure formation

Fluorine negative ion density measurement in a dual frequency capacitive plasma etch reactor by cavity ring-down spectroscopy

F⁻ negative ions were detected by direct observation of the weak photodetachmentabsorption continuum below 364.5nm by cavity ring-down spectroscopy. The negative ions were generated in a modified industrial dielectricplasmaetch reactor, with 2+27MHz dual frequency capacitive excitation in Ar∕CF₄∕O₂ and Ar∕C₄F₈∕O₂ gas mixtures. The F⁻ signal was superimposed on an unidentified absorption continuum, which was diminished by O₂ addition. The F⁻ densities were in the range of (0.5–3)×10¹¹cm⁻³, and were not significantly different for single (27MHz) or dual (2+27MHz) frequency excitation, not confirming recent modeling predictions.The authors wish to thank Lam Research Corporation for donation of equipment and financial support

Effect of W self-implantation and He plasma exposure on early-stage defect and bubble formation in tungsten

To determine the effect of pre-existing defects on helium-vacancy cluster nucleation and growth, tungsten samples were self-implanted with 1 MeV tungsten ions at varying fluences to induce radiation damage, then subsequently exposed to helium plasma in the MAGPIE linear plasma device. Positron annihilation lifetime spectroscopy was performed both immediately after self-implantation, and again after plasma exposure. After self-implantation vacancies clusters were not observed near the sample surface (<30 nm). At greater depths (30-150 nm) vacancy clusters formed, and were found to increase in size with increasing W-ion fluence. After helium plasma exposure in the MAGPIE linear plasma device at ∼300 K with a fluence of 1023 He-m-2, deep (30-150 nm) vacancy clusters showed similar positron lifetimes, while shallow (<30 nm) clusters were not observed. The intensity of positron lifetime signals fell for most samples after plasma exposure, indicating that defects were filling with helium. The absence of shallow clusters indicates that helium requires pre-existing defects in order to drive vacancy cluster growth at 300 K. Further samples that had not been pre-damaged with W-ions were also exposed to helium plasma in MAGPIE across fluences from 1 × 1022 to 1.2 × 1024 He-m-2. Samples exposed to fluences up to 1 × 1023 He-m-2 showed no signs of damage. Fluences of 5 × 1023 He-m-2 and higher showed significant helium-cluster formation within the first 30 nm, with positron lifetimes in the vicinity 0.5-0.6 ns. The sample temperature was significantly higher for these higher fluence exposures (∼400 K) due to plasma heating. This higher temperature likely enhanced bubble formation by significantly increasing the rate interstitial helium clusters generate vacancies, which is we suspect is the rate-limiting step for helium-vacancy cluster/bubble nucleation in the absence of pre-existing defects.The authors are grateful to the technical assistance within the Australian Plasma Fusion Research Facility that is partly funded by the Australian Government under the Super Science Initiative, financed from the Education Investment Fund. GISAXS research was undertaken on the SAXS/WAXS beamline at the Australian Synchrotron, part of ANSTO. We gratefully acknowledge beamline scientists at the Australian Synchrotron for their assistance. PK acknowledges the Australian Research Council for financial support

Effect of temperature and incident ion energy on nanostructure formation on silicon exposed to helium plasma

Helium plasma can be used to deliver low‐energy (<100 eV) helium ions to stimulate the growth of nanostructures on silicon surfaces. This can produce a wide range of surface features including nanoscale roughening, nanowires and porous structures. In this study, nanostructure sizes varied from ∼10 to over 100 nm in diameter. The effect of these structures on surface reflectivity for photovoltaic and photocatalytic applications is also investigated. Broadband suppression of photoreflectivity is achieved across the 300-1,200 nm wavelength range studied for silicon exposed to helium plasma at 600°C, with an average reflectivity of 3.2% and 2.9% for incident helium ion energies of 42 and 62 eV, respectively.Japan Society for the Promotion ofScience, Grant/Award Numbers:17KK0132, 19H01874; AustralianResearch Council, Grant/Award Number:DP20010283

Mechanical properties of tungsten following rhenium ion and helium plasma exposure

Mechanical properties of Tungsten (W) samples irradiated with 2 MeV Rhenium (Re) ions and helium (He) plasma were investigated using nanoindentation. It was found that there was an increase in hardness for all samples following separate irradiation with both Re ion and He plasma. A slight increase in hardness was obtained for combined exposures. A comparable increase in hardness was observed for a pure He plasma with a sample temperature of 473 K and 1273 K. Optical interferometry was employed to compare surface modification of the samples. Grazing incidence small angle x-ray scattering confirmed He nano-bubble formation of approximately 1 nm diameter in the higher temperature sample, which was not observed with samples at the lower temperaturesPK, CC and JB acknowledge support from the Future Fellowship Scheme of the Australian Research Council (FT120100289, FT100100825 and FT130101355). This research has also been supported by the Science and Industry Endowment Fund grant (PS034)

Effect of rhenium irradiations on the mechanical properties of tungsten for nuclear fusion applications

As-received and annealed tungsten samples were irradiated at a temperature of 400 °C with Re and W ions to peak concentrations of 1600 appm (atomic parts per million) and damage levels of 40 dpa (displacements per atom). Mechanical properties were investigated using nanoindentation, and the orientation and depth dependence of irradiation damage was investigated using Electron Back Scatter Diffraction (EBSD). Following irradiation there was a 13% increase in hardness in the as received sheet and a 23% increase in the annealed material for both tungsten and rhenium irradiation. The difference between the tungsten and rhenium irradiated samples was negligible, suggesting that for the concentrations and damage levels employed, the presence of rhenium does not have a significant effect on the hardening mechanism. Energy dependent EBSD of annealed samples provided information about the depth distribution of the radiation damage in individual tungsten grains and confirmed that the radiation damage is orientation dependant

At the Edge Plasma-Surface Science for Future Fusion Reactors

Concerns over energy security and climate change are driving the development of novel sustainable energy sources. Fusion, the process that powers the Sun, has great potential to provide clean industrial-scale baseload electrical power, with negligible CO2 emissions and produce little long-term waste. Results from the 500 MW international magnetic confinement experiment ITER will determine the future of fusion energy as a viable alternative source of clean energy. The science and technology of materials under extreme heat loads, and in particular plasma-surface interactions, are critical to the success of plasma fusion sources such as ITER [1] and the ultimate viability of generating fusion power under steady state conditions

Ion flux dependence of atomic hydrogen loss probabilities on tungsten and carbon surfaces

Concerns over fuel inventory and consequently radiation safety in a fusion device necessitates a better understanding of complex plasma-material interactions. Pulsed induced fluorescence is employed to determine the loss probability of atomic hydrogen o

Atomic and molecular hydrogen gas temperatures in a low-pressure helicon plasma

Neutral gas temperatures in hydrogen plasmas are important for experimental and modelling efforts in fusion technology, plasma processing, and surface modification applications. To provide values relevant to these application areas, neutral gas temperatures were measured in a low pressure (<10 mTorr) radiofrequency helicon discharge using spectroscopic techniques. The atomic and molecular species were not found to be in thermal equilibrium with the atomic temperature being mostly larger then the molecular temperature. In low power operation (<1 kW), the molecular hydrogen temperature was observed to be linearly proportional to the pressure while the atomic hydrogen temperature was inversely proportional. Both temperatures were observed to rise linearly with input power. For high power operation (5 20 kW), the molecular temperature was found to rise with both power and pressure up to a maximum of approximately 1200 K. Spatially resolved measurements near a graphite target demonstrated localised cooling near the sample surface. The temporal evolution of the molecular gas temperature during a high power 1.1 ms plasma pulse was also investigated and found to vary considerably as a function of pressure