Precipitates and defects in silicon co-implanted with helium and oxygen

Nano-bubbles or voids introduced by He implantation before the oxygen implantation collect oxygen and increase the oxygen content in the sample. Furthermore, nano-bubbles or voids can trap Si interstitials to decrease the dislocations at the edge of precipitates. The density and shape of precipitates formed in the initial stage of the separation-by-implanted-oxygen process are related to the size and density of He-induced vacancy-type defects (nano-bubbles and voids). A high density of nano-bubbles is more efficient in gettering than that of a low density of voids.     

Defect- and strain-enhanced cavity formation and Au precipitation at nano-crystalline ZrO2/SiO2/Si interfaces

Defect- and strain-enhanced cavity formation and Au precipitation at the interfaces of a nano-crystalline ZrO₂/SiO₂/Si multilayer structure resulting from 2 MeV Au⁺ irradiation at temperatures of 160 and 400 K have been studied. Under irradiation, loss of oxygen is observed, and the nano-crystalline grains in the ZrO₂ layer increase in size. In addition, small cavities are observed at the ZrO₂/SiO₂ interface with the morphology of the cavities being dependent on the damage state of the underlying Si lattice. Elongated cavities are formed when crystallinity is still retained in the heavily-damaged Si substrate; however, the morphology of the cavities becomes spherical when the substrate is amorphized. With further irradiation, the cavities appear to become stabilized and begin to act as gettering sites for the Au. As the cavities become fully saturated with Au, the ZrO₂/SiO₂ interface then acts as a gettering site for the Au. Analysis of the results suggests that oxygen diffusion along the grain boundaries contributes to the growth of cavities and that oxygen within the cavities may affect the gettering of Au. Mechanisms of defect- and strain-enhanced cavity formation and Au precipitation at the interfaces will be discussed with focus on oxygen diffusion and vacancy accumulation, the role of the lattice strain on the morphology of the cavities, and the effect of the binding free energy of the cavities on the Au precipitation.     

Strong segregation gettering of transition metals by implantation-formed cavities and boron-silicide precipitates in silicon

We have mechanistically and quantitatively characterized the binding of transition-metal impurities in Si to cavities formed by He implantation and to B---Si precipitates resulting from B implantation. Both sinks are inferred to act by the segregation of metal atoms to pre-existing low-energy sites, namely surface chemisorption sites in the case of cavities and bulk solution sites in the case of the B---Si phase. These gettering processes exhibit large binding energies, and they are predicted to remain active for arbitrarily small initial impurity concentrations as a result of the segregation mechanisms. Both appear promising for gettering in Si devices.     

Spatial distribution of defects in ion-implanted and annealed Si: The RP/2 effect

Gettering of metal impurities in ion-implanted Si occurs midway between the surface and the projected ion range, R_P, after annealing at temperatures in the range of 700–1000°C and vanishes at higher temperatures. This phenomenon, called the R_P/2 effect, seems to be a common feature of ion-implanted and annealed Si. The gettering ability of the damage at R_P/2 is commensurate with or may exceed that of the damage at R_P. The defects around R_P/2 acting as gettering sites have not yet been identified by other analysis techniques. They are formed after ion implantation in the process of defect evolution during annealing and, probably, consist of small complexes of intrinsic defects (vacancies or/and self-interstitials).     

Gettering of copper impurity in silicon by aluminum precipitates and cavities

Al precipitates as well as cavities (or open-volume defects) are known for their ability to getter impurities within Si. In order to compare their relative gettering strength we produced both Al precipitates and cavities at different depths within one Si wafer. This was done by H+ and Al+ implantation with different energies and subsequent annealing process, resulting in Al-Si alloy and cavities at depth of 300 nm and 800 nm, respectively. Cu was then implanted with an energy of 70 keV to a fluence of 1 X 1014 / cm". The Cu implanted samples were annealed at temperature from 700C to 1200C. It was found that Cu impurities were gettered primarily by the precipitated Al layer rather than by cavities at the temperature of 700~1000C, while gettering of Cu occured in both regions at the temperature of 1200C. The secondary ion mass spectrometry and transmission electron microscopy analyses were used to reveal the interaction between Cu impurities and defects at different trap sites.     

Structure and tribological performance of helium-implanted layer on Ti6Al4V alloy by plasma-based ion implantation

The present paper concentrates on tribological performance of Ti6Al4V alloy treated by helium plasma-based ion implantation with a voltage of −30 kV and a dose range of 1, 3, 6 and 9 × 10¹⁷ He/cm². X-ray photoelectron spectroscopy (XPS), Transmission electron microscopy (TEM) and Atomic force microscopy (AFM) were used to characterize composition, structure and surface morphology, respectively. The variation of hardness with indenting depth was measured and tribological performance was evaluated. The uniform cavities with a diameter of several nanometers are formed in the helium-implanted layer on Ti6Al4V alloy. Helium implantation enhances the ingress of O, C and N and produces TiO₂, Al₂O₃, TiC, TiN in the near surface layer on their removal from the vacuum and exposure to normal atmospheric condition. In the near surface layer, the hardness of implanted samples increases remarkably comparing with the untreated sample, and the maximum peak increasing factor is up to 2.9 for the sample implanted with 3 × 10¹⁷ He/cm². A decrease in surface roughness, resulting from the leveling effect of sputtering and re-deposition during implantation, has also been observed. Comparing with the untreated sample, implanted samples have a good wear resistance property. And the maximum increase in wear resistance reaches over seven times that of the untreated one for the sample implanted with 3 × 10¹⁷ He/cm². The wear mechanism of implanted samples is abrasive-dominated.     

Effect of oxygen on ion-beam induced synthesis of SiC in silicon

The properties of Si-structures with a buried silicon carbide (SiC) layer created by high-dose carbon implantation into Cz–Si or Fz–Si wafers followed by high-temperature annealing were studied by Raman and infrared spectroscopy. The effect of additional oxygen implantation on the peculiarities of SiC layer formation was also studied. It was shown that under the same implantation and post-implantation annealing conditions the buried SiC layer is more effectively formed in Cz–Si or in Si (Cz-or Fz-) subjected to additional oxygen implantation. So we can conclude that oxygen in silicon promotes the SiC layer formation due to SiO_x precipitate creation and accommodation of the crystal volume in the region where SiC phase is formed. Carbon segregation and amorphous carbon film formation on SiC grain boundaries were revealed.     

Formation of cavities in GaAs and InGaAs

The ion implantation of He is examined as a means to form thermally stable cavities in GaAs. Room-temperature implantation of 2–10 × 10¹⁶ He/cm² at 40 or 50 keV forms bubbles, but subsequent annealing at 250°C or above leads to exfoliation of the implanted surface layer. The exfoliation appears related to the agglomeration of bubbles on dislocations at the back of the layer; evidence suggests these may be misfit dislocations formed to relieve compressive stress in the implanted layer. Implantation of He at 150°C produces similar results, whereas the He diffuses out of GaAs without forming cavities during implantation at 300°C. However, implantations of immobile Ar followed by He at 400°C produce extended defects with bubbles in the implanted layer; the He can be degassed by subsequent annealing at 400°C to produce 1.5–3.5 nm cavities that are stable at this temperature. The same treatment applied to an In_0.10Ga_0.90As/GaAs heterostructure produces larger cavities preferentially located on dislocations at the interface, with only slight reduction in strain of the epitaxial layer. The microstructures of both GaAs and the heterostructure clearly demonstrate an attractive interaction between bubbles or cavities and dislocations.     

Void nucleation, growth, and coalescence in irradiated metals

A novel computational treatment of dense, stiff, coupled reaction rate equations is introduced to study the nucleation, growth, and possible coalescence of cavities during neutron irradiation of metals. Radiation damage is modeled by the creation of Frenkel pair defects and helium impurity atoms. A multi-dimensional cluster size distribution function allows independent evolution of the vacancy and helium content of cavities, distinguishing voids and bubbles. A model with sessile cavities and no cluster–cluster coalescence can result in a bimodal final cavity size distribution with coexistence of small, high-pressure bubbles and large, low-pressure voids. A model that includes unhindered cavity diffusion and coalescence ultimately removes the small helium bubbles from the system, leaving only large voids. The terminal void density is also reduced and the incubation period and terminal swelling rate can be greatly altered by cavity coalescence. Temperature-dependent trapping of voids/bubbles by precipitates and alterations in void surface diffusion from adsorbed impurities and internal gas pressure may give rise to intermediate swelling behavior through their effects on cavity mobility and coalescence.     

On the modeling of the high-temperature embrittlement of metals containing helium

Models for premature intergranular fracture of metals in creep tests during He injection and after room temperature He implantation are critically assessed.The creep rupture lifetime and its dependences upon stress, temperature, He generation rate, He concentration and microstructure of the metal are determined by the intrinsic properties of the lifetime-controlling mechanism as well as by the He flux to and the resulting bubble density on the grain boundaries. The He flux on the other hand is controlled by the He generation rate and the bubble density developing within the grains.On the basis of these considerations the differences in the stress and temperature dependences of the lifetimes observed in creep tests during He implantation and after room temperature He implantation are attributed to differences in the bubble nucleation and growth kinetics of the two cases. For creep tests during He injection the approximate agreement between predicted and observed trends in the time to rupture indicate gas driven stable bubble growth up to the stability limit as the life time controlling mechanism. A set of special creep tests to check this indication is suggested. For creep tests after room temperature implantation the lifetime controlling mechanism is less clear because of secondary nucleation and coarsening processes. It could be bubble nucleation or creep controlled bubble growth on grain boundaries. For both types of creep tests the possibilities and limits of solid precipitates for reducing the He embrittlement is discussed.     

Cavity formation during single- and dual-ion irradiation in a 9Cr-1Mo ferritic alloy

The dose and temperature dependence of cavity formation in a 9Cr-1Mo ferritic alloy irradiated simultaneously with Ni ⁺ and He⁺ has been studied with TEM. Comparisons are made with parallel experiments on Ni⁺-irradiated material that was preinjected with He. For dual-ion irradiation, both intergranular and intragranular cavities formed at all temperatures (450–600°C) and doses (5–25 dpa) investigated. The size of the intergranular cavities increased with increasing temperature, while the size of intragranular cavities decreased. In preinjected samples, cavities formed only at the lowest irradiation temperature (450°C). For 450°C single-ion irradiation and for 450 and 500°C dual-ion irradiation, there was a correlation between subgrain size and maximum cavity size, suggesting that the boundaries of the small (typically ~ 0.5 μm) subgrains act as the primary point-defect sink.     

Formation of complex Al–N–C layer in aluminium by successive carbon and nitrogen implantation

The results of Auger electron spectroscopy and transmission electron microscopy of the surface layer of aluminium after successive implantation by carbon and nitrogen ions are presented in this work. The energy of implanted ions is 40 keV. The implantation dose varies in the range (3.3–6.5) × 10¹⁷ ions/cm². The findings show that successive implantation leads to the formation of two main layers in aluminium. The first layer is AlNC_x (0 < x < 0.5) layer with violated hcp. AlN structure, where carbon atoms form bonds with nitrogen atoms. The second layer contains disoriented Al₄C₃ precipitates and carbon atoms migrated from the first layer. The mechanism of migration is discussed.     

Defects remaining in MeV-ion-implanted and annealed Si away from the peak of the nuclear energy deposition profile

Damage has been observed in MeV-ion-implanted Si away from the maximum of the nuclear energy deposition profile, mainly around the half of the projected ion range, R_P/2. Cu gettering has been used for the detection of irradiation defects which are formed during annealing at temperatures between 700°C and 1000°C. This damage is primarily created by the implanted ions on their trajectory and consists of intrinsic defects remaining so small that they have not yet been resolved. These defects undergo a defect evolution during annealing which results in a decrease of the width of the damage layer with increasing temperature and prolonged time of the annealing.     

Controlling the density distribution of SiC nanocrystals for the ion beam synthesis of buried SiC layers in silicon

The depth distribution of SiC nanocrystals formed during high-dose implantation of carbon ions into silicon at conditions suitable for the ion beam synthesis of buried SiC layers in silicon is studied in this paper. For implantation temperatures of 400–600°C and dose rates of 10¹² − 10¹³ C⁺/cm²s, SiC precipitates in crystalline silicon are observed to be of approximately equal size, independent of the depth position beneath the surface. Ballistic destruction of small precipitates and difficulties in precipitate growth are thought to be responsible for the observed narrow size distribution. The destruction of precipitates may lead to the simultaneous release of a superthreshold concentration of carbon atoms resulting in a carbon-induced amorphization of the silicon host lattice. The local reduction of the number density of SiC nanocrystals involved with this amorphization can be used to tailor discontinuous depth distributions of oriented SiC precipitates providing ideal starting conditions for the synthesis of well-defined single-crystalline SiC layers in silicon.     

Vacancy-type defects near Al surface studied by slow positron annihilation spectroscopy before and after He implantation

The vacancy-type defects He_nV_m near Al surface before and after He⁺ implantation and their evolutions with annealing temperatures and aging time have been investigated by mono-energy slow positron annihilation spectroscopy (SPAS) with S parameters. The results show that many vacancies are produced during the sample preparation process, which can be re-occupied by Al atoms during annealing, Al⁺ and MeV He⁺ implantation. S parameters denote the concentration and size of He_nV_m clusters induced by He⁺ implantation in Al. The higher fluence of He implanted, the larger S parameters will be, indicating more He_nV_m clusters produced. S parameters decrease with the increase of annealing temperatures until the fastest change temperature, and then an opposite or minor change occurs depending on the fluence of He implanted in Al, showing that the concentration and size of He_nV_m clusters will vary with the annealing temperatures. Aged at RT for some time, the concentration and mean size of He_nV_m clusters in Al will get smaller and larger, respectively, resulting in the decrease of S parameters with the aging time. In conclusions, the evolution of vacancy-type defects He_nV_m near Al surface after He⁺ implantation depends on the annealing temperatures, He concentration and aging time.     

Operation of TCSU with Internal Flux-Conserving Rings

The Translation, Confinement, and Sustainment Upgrade (TCSU) device is a facility to form and sustain a field-reversed configuration (FRC) in quasi-steady state using rotating magnetic fields (RMF). Recent campaigns include Ti gettering, the installation of a set of internal flux rings, and RMF frequency scans. The Ti gettering campaign was successful, reduced impurities, and reduced deuterium recycling from the walls allowing density control and hotter FRCs [J.A. Grossnickle et al., Phys. Plasmas 17, 032506 (2010)]. Internal flux rings have been installed to provide a uniform flux surface and minimize plasma-wall contact. Results from the internal flux ring operation and an additional Ti gettering campaign are reported. RMF frequencies of 123 kHz and 170 kHz have been investigated and initial results are reported.     

Electrically active defects in BF2+ implanted and germanium preamorphized silicon

Ultra-shallow p⁺-n junctions have been formed using 15 keV/10¹⁵ cm⁻² BF₂⁺ implantation into both Ge⁺-preamorphized and crystalline 〈1 0 0〉 silicon substrates. Rapid thermal annealing (RTA) for 15 s at 950°C was used for dopant electrical activation and implantation damage gettering. The electrically active defects present in these samples were characterized using Deep Level Transient Spectroscopy (DLTS) and isothermal transient capacitance (ΔC(t, T)). Two electron traps were detected in the upper half of the band gap at, respectively, E_c - 0.20 eV and E_c - 0.45 eV. They are shown to be related to Ge⁺ implantation-induced damage. On the other hand, BF₂⁺ implantation along with RTA give rise to a depth distributed energy continuum which lies within the forbidden gap between E_c - 0.13 eV and E_c - 0.36 eV. From isothermal transient capacitance (ΔC(t, T)), reliable damage concentration profiles were derived. They revealed that preamorphization induces not only defects in the regrown silicon layer but also a relatively high concentration of electrically active defects as deep as 3.5 μm into the bulk.     

Dual beam irradiation of nanostructured FeCrAl oxide dispersion strengthened steel

Nanostructured ferritic oxide dispersion strengthened (ODS) alloy is an ideal candidate for fission/fusion power plant materials, particularly in the use of a first-wall and blanket structure of a next generation reactor. These steels usually contain a high density of Y-Ti-O and Y-Al-O nanoparticles, high dislocation densities and fine grains. The material contains nanoparticles with an average diameter of 21 nm and was treated by several cold rolling procedures, which modify the dislocation density. Structural analysis with HRTEM shows that the chemical composition of the initial Y₂O₃ oxide is modified to perovskite YAlO₃ (YAP) and Y₂Al₅O₁₂ garnet (YAG). Irradiation of these alloys was performed with a dual beam irradiation of 2.5 MeV Fe⁺/31 dpa and 350 keV He⁺/18 appm/dpa. Irradiation causes atomic displacements resulting in vacancy and self-interstitial lattice defects and dislocation loops. Extended SRIM calculations for ODS steel indicate a clear spatial separation between the excess vacancy distribution close to the surface and the excess interstitials in deeper layers of the material surface. The helium atoms are supposed to accumulate mainly in the vacancies. Additionally to structural changes, the effect of the irradiation generated defects on the mechanical properties of the ODS is investigated by nanoindentation. A clear hardness increase in the irradiated area is observed, which reaches a maximum at a close surface region. This feature is attributed to synergistic effects between the displacement damage and He implantation resulting in He filled vacancies. Fine He cavities with diameters of a few nanometers were identified in TEM images.     

Surface exfoliation and defect structures in Si induced by 160 keV He and 110 keV H ion implantation

Cz n-type Si(100) wafers were implanted at room temperature with 160 keV He ions at a fluence of 5 × 10¹⁶/cm² and 110 keV H ions at a fluence of 1 × 10¹⁶/cm², singly or in combination. Surface phenomena and defect microstructures have been studied by various techniques, including scanning electron microscopy (SEM), atomic force microscopy (AFM) and cross-sectional transmission electron microscopy (XTEM). Surface exfoliation and flaking phenomena were only observed on silicon by successive implantation of He and H ions after subsequent annealing at temperatures above 400 °C. The surface phenomena show strong dependence on the thermal budget. At annealing temperatures ranging from 500 to 700 °C, craters with size of about 10 μm were produced throughout the silicon surface. As increasing temperature to 800 °C, most of the implanted layer was sheared, leaving structures like islands on the surface. AFM observations have demonstrated that the implanted layer is mainly transfered at the depth around 960 nm, which is quite consistent with the range of the ions. XTEM observations have revealed that the additional low fluence H ion implantation could significantly influence thermal growth of He-cavities, which gives rise to a monolayer of cavities surrounded by a large amount of dislocations and strain. The surface exfoliation effects have been tentatively interpreted in combination of AFM and XTEM results.     

Study of the trapping of Co---Fe impurities at the internal wall of nanosized Si voids

The trapping of transition metal impurities to cavities in c-Si recently attracted much interest, stemming from the possibility to use the process for proximity gettering. Mössbauer spectroscopy is employed on ion-implanted ⁵⁷Co/⁵⁷Fe to elucidate the nature of the site of the impurity atom at the internal surface of the voids. We observe a trapping effect upon thermal treatment, hampering normal silicide formation. Also a pre-existing silicide phase can be partially dissolved in favour of cavity trapping. The binding energy for cavity trapping is found to be lower than for silicide formation.