keV Si Research Articles

Highly 28Si enriched silicon by localised focused ion beam implantation

Solid-state spin qubits within silicon crystals at mK temperatures show great promise in the realisation of a fully scalable quantum computation platform. Qubit coherence times are limited in natural silicon owing to coupling to the 29Si isotope which has a non-zero nuclear spin. This work presents a method for the depletion of 29Si in localised volumes of natural silicon wafers by irradiation using a 45 keV 28Si focused ion beam with fluences above 1 × 1019 ions cm−2. Nanoscale secondary ion mass spectrometry analysis of the irradiated volumes shows residual 29Si concentration down to 2.3 ± 0.7 ppm and with residual C and O comparable to the background concentration in the unimplanted wafer. After annealing, transmission electron microscopy lattice images confirm the solid phase epitaxial re-crystallization of the as-implanted amorphous enriched volume extending over 200 nm in depth.

Atomic-scale revealing defects in ion irradiated 4H-SiC

Here we identified atomic-scale defects in ion irradiated 4H-SiC, by integrated differential phase contrast technique equipped on aberration-corrected transmission electron microscopy and Raman scattering spectrometer. In 21 MeV Ni+ ions irradiated 4H-SiC, in which electronic energy loss is dominant almost at the whole penetration depth, Si- and C-related vacancies and interstitials are successfully imaged. In 900 keV Si+ ions irradiated 4H-SiC, however, the electronic energy loss plays the key role near the surface, and the nuclear and electronic energy loss are comparable at the depth of the peak damaged zone. As a result, 900 keV Si+ ions produce more point defects, atomic-scale in-plane and out-of-plane lattice distortions, and plenty of dislocations at the depth of damage peak. Meantime, these imaged point defects may provide direct detailed information for the vibration modes of defects in the Raman spectrum of the damaged 4H-SiC. This study illustrates the compelling intrinsic nature of point defects induced by energetic ions, providing important fundamental information for nuclear and electronic applications.

Isotopically Enriched Layers for Quantum Computers Formed by 28Si Implantation and Layer Exchange.

28Si enrichment is crucial for production of group IV semiconductor-based quantum computers. Cryogenically cooled, monocrystalline 28Si is a spin-free, vacuum-like environment where qubits are protected from sources of decoherence that cause loss of quantum information. Currently, 28Si enrichment techniques rely on deposition of centrifuged SiF4 gas, the source of which is not widely available, or bespoke ion implantation methods. Previously, conventional ion implantation into naturalSi substrates has produced heavily oxidized 28Si layers. Here we report on a novel enrichment process involving ion implantation of 28Si into Al films deposited on native-oxide free Si substrates followed by layer exchange crystallization. We measured continuous, oxygen-free epitaxial 28Si enriched to 99.7%. Increases in isotopic enrichment are possible, and improvements in crystal quality, aluminum content, and thickness uniformity are required before the process can be considered viable. TRIDYN models, used to model 30 keV 28Si implants into Al to understand the observed post-implant layers and to investigate the implanted layer exchange process window over different energy and vacuum conditions, showed that the implanted layer exchange process is insensitive to implantation energy and would increase in efficiency with oxygen concentrations in the implanter end-station by reducing sputtering. Required implant fluences are an order of magnitude lower than those required for enrichment by direct 28Si implants into Si and can be chosen to control the final thickness of the enriched layer. We show that implanted layer exchange could potentially produce quantum grade 28Si using conventional semiconductor foundry equipment within production-worthy time scales.

Effect of low-energy ion bombardment on structural and optical properties of gold functionalized hBN nanosheets

This work reports the effect of low-energy ion (30 KeV Si−1) bombardment on the structural and optical properties of gold-functionalized hexagonal boron nitride (Au-hBN) nanosheets. FE-SEM and XRD results indicate the successful synthesis of highly crystalline Au-hBN nanosheets. The grain size modulation and strain variation in Au-hBN nanosheets with increasing ion fluence clearly indicate defect-induced structural modifications. The optical properties of Au-hBN are investigated by Raman and UV–vis absorption spectroscopy. A systematic drop in the band gap and gradual rise in the Urbach energy of Au-hBN nanosheets with increasing fluence is observed. In addition, the FTIR results reveal the growth of cBN, eBN and wBN phases after irradiation. The theoretical simulations are done to study the defect formation and estimate the damage yields. The experimental and computational results establish the efficacy of the low-energy ion bombardment approach in systematically altering the structural and optical behavior of Au-hBN nanosheets.

Investigation of 500 keV Si ion induced surface structuring of Ni and Ti for enhancement of field emission properties

The present paper reports the investigation of surface morphology, elemental composition, phase changes and field emission properties of Si ion irradiated nickel (Ni) and titanium (Ti). The Ni and Ti targets have been irradiated with 500 keV Si ions generated by Pelletron accelerator at various fluences ranging from 6.9 × 1013 to 77.1 × 1013 ions/cm2. Stopping range of ions in matter analysis revealed higher values of electronic stopping and sputtering yield for Ni as compared with Ti. For both irradiated metals, electronic energy loss dominant over the nuclear stopping. The growth of induced surface structures have been analysed by using field emission scanning electron microscopy (FESEM) analysis. In case of Ni, as the ion fluence increases from 6.9 × 1013 to 65.8 × 1013 ions/cm2, the formation of spherical particulates, agglomers and sputtering is observed. Although in the case of Ti, with the increase of Si ion fluence from 11.6 × 1013 to 77.1 × 1013 ions/cm2, the formation of irregular‐shaped particulates along with crater and sputtered channels is observed. X‐ray diffraction (XRD) analysis shows that no new phase is identified. However, a significant increase in peak intensity is observed with increasing ion fluence. The variation in crystallite size and dislocation line density is also observed as a function of Si ion fluence. Fourier transform infrared spectroscopy analysis shows that no bands are formed after the Si ion irradiation. Field emission properties of ion‐structured Ni and Ti are well correlated with the growth of surface structures observed by SEM and dislocation line density evaluated by XRD analysis.

Radiation-induced defect accumulation and annealing in Si-implanted gallium oxide

Defect accumulation and annealing phenomena in Si-implanted monoclinic gallium oxide (β-Ga2O3) wafers, having (2¯01), (010), and (001) orientations, were studied by Rutherford backscattering spectrometry in channeling mode (RBS/c), x-ray diffraction (XRD), and (scanning) transmission electron microscopy [(S)TEM]. Initially, the samples with different surface orientations were implanted with 300 keV 28Si+-ions, applying fluences in the range of 1 × 1014–2 × 1016 Si/cm2, unveiling interesting disorder accumulation kinetics. In particular, the RBS/c, XRD, and (S)TEM combined data suggested that the radiation disorder buildup in Si-implanted β-Ga2O3 is accompanied by significant strain accumulation, assisting crystalline-to-crystalline phase transitions instead of amorphization. Selected samples having (2¯01) orientation were subjected to isochronal (30 min) anneals in the range of 300–1300 °C in air. Systematic RBS/c and XRD characterization of these samples suggested complex structural transformations, which occurred as a function of the fluence and the temperature. Moreover, a detailed (S)TEM analysis of the sample implanted with 2 × 1016 Si/cm2 and annealed at 1100 °C was enhanced by applying dispersive x-ray and electron energy-loss spectroscopies. The analysis revealed silicon agglomerations in the form of silicon dioxide particles. Signal from silicon was also detected outside of the agglomerates, likely occurring as substitutional Si on Ga sites.

Raman scattering investigations on disorder and recovery induced by low and high energy ion irradiation on 3C-SiC

The cubic polytype of SiC (3C-SiC) epilayer grown on Si has been irradiated with 200 keV Si+ ions for various ion doses (0.025 dpa- 0.6 dpa) followed by 14 MeV Si+ ion irradiation (ion fluences: 1 × 1014 –1 × 1015 ions/cm2) at room temperature to study the recovery of damage caused by low energy ions. The disorder was estimated using Raman Scattering results and it is explained by direct impact model. From the total disorder and chemical disorder values, the three stages of damage evolution is clearly observed upon 200 keV Si+ ion irradiation. Upon 14 MeV Si+ ion irradiation on 3C-SiC, the sample has not amorphized even upto the ion fluence of 1 × 1015 ions/cm2. The present experimental results show that when the damage(caused by low energy ion) is less (<0.05dpa), the recovery of damage occurred upon high energy Si+ ion irradiation and it is mainly due to ionization induced annealing of defects.

Growth of low resistive nickel mono-silicide phase under low energy Si ion irradiation at room temperature

Nickel mono-silicide (NiSi) is considered as a promising material for developing low resistance contacts in complementary metal-oxide-semiconductor technology. In the present work, we report the effect of Ni film thickness on orientation control of low resistive Nickel mono-silicide (NiSi) phase formation using low energy ion irradiation at room temperature. In order to study the effect of Ni film thickness on NiSi phase formation, the Ni films of thicknesses 30 nm and 60 nm were deposited on Si (111) substrate using thermal evaporation technique in a high vacuum chamber. The as prepared Ni/Si samples were then irradiated via 120 keV Si ions with different fluences of 7 × 1014, 1 × 1015, 3 × 1015 and 7 × 1015 ions-cm−2 at room temperature. The x-ray diffraction and transmission electron microscopy measurements clearly confirm the formation of mono-silicide phase. The composition of the NiSi phase is determined by Rutherford backscattering spectrometry measurements. The crystallinity of NiSi phase has been observed to be better for 60 nm Ni film as compared to the 30 nm Ni film on Si substrate. Most of the NiSi crystallites are found to be oriented in (103) lattice plane. The resistivity and sheet resistance of the NiSi/Si films are found to be very low. The role of composition of Ni and Si in NiSi phase on resistivity and sheet resistance of the films has been investigated carefully. The detailed mechanisms behind the above observations have been discussed in the paper.

A study of the formation of isotopically pure 28Si layers for quantum computers using conventional ion implantation

We have investigated the use of conventional ion implantation to fabricate enriched 28Si layers for use in quantum computers. The final compositions of samples enriched using ultra-low energy (ULE) (800 eV and 2 keV) and low energy (20 keV) 28Si implants of varying fluences (1 × 1016–3.8 × 1017 cm−2) using two different implanters were measured using channelled Rutherford Backscattering Spectroscopy (RBS). The dynamic, binary collision approximation program TRIDYN was used to model the implantation profiles to guide the analysis of the RBS spectra. It was found that ULE implants achieved high 28Si enrichment levels but were heavily contaminated with oxygen due to poor vacuum in the implanter wafer end station. It was shown that oxidation could be reduced by using an accelerator with an end station with better vacuum and increasing the implant energy to 20 keV. However, TRIDYN simulations predict that the best 28Si enrichment levels that could be achieved under these conditions would saturate at ∼99.2% due to self-sputtering. We modelled a range of conditions with TRIDYN and so recommend low energies (<3 keV), ultra-high vacuum implantation for high 28Si enrichment (>99.9%) with the lowest possible fluences (∼5–10 × 1017 cm−2).

Silicon-ion implantation induced doping and nanoporosity in molecular beam epitaxy grown GaSb epitaxial films

This paper reports two specific aspects of Si implantation in the molecular beam epitaxy grown p-type GaSb epilayer, namely, the evolution of nanoporosity and doping characteristics. The implantation is done with 100 keV Si ions at four different fluences, i.e., 5 × 1013, 1 × 1014, 5 × 1014, and 1 × 1015 ions/cm2. A field-emission scanning electron microscope (in-plane and cross-sectional view) shows a smooth nanoporous GaSb with a pore diameter of 10–20 nm for the higher fluences. The thickness of the porous film is enhanced from 235 nm (fluence: 1 × 1014 ions/cm2) to 515 nm (fluence: 1 × 1015 ions/ cm2). Development of smooth uniform porous GaSb using Si ion implantation is discussed from the viewpoint of displacement per atom and distribution of vacancies estimated through Stopping and Range of Ions in Matter simulation tool. Hall measurement shows conversion to n-type behavior after implantation and a variation in the sheet carrier concentration from 2.22 × 1014 to 8.52 × 1014 cm−2. There is a drastic change in the mobility with the onset of void formation by implantation.

Synergistic effects assessment between nuclear damage and electronic energy dissipation in LiTaO3 under heavy ion irradiation using optical waveguides properties and the irradiation angle of incidence

High energy heavy ion irradiation has been performed in LiTaO3 in a broad set of experimental conditions to study the synergistic effects in the damage generation between the nuclear and the electronic energy loss mechanisms. Optical waveguides have been fabricated in LiTaO3, using 20–25 MeV F and 40 MeV Si ions, and their refractive index profiles were used as a very accurate method for the in-depth damage profile determination and its correlation with the energy loss curves. In-situ optical reflectance of low energy irradiations (500 keV F and 300 keV Si ions) has been performed to estimate the nuclear damage kinetics of the buried regions of the high energy irradiations that are not optically accessible with the optical waveguide characterization in the fluence regime when the electronic damage is intense. The angle of incidence has been varied to enhance the damage and further ascertain the existence of a synergistic mechanism.

Damage effects in 6H-SiC single crystals by Si&H dual ion irradiation: A combined Raman and XRD study

6H-SiC single crystals were implanted with Si and H single and dual ion beams at room temperature to study synergistic effects. 600 keV Si ions with a fluence of 5 × 1014 cm−2 and 75 keV H ions with fluences of 1 × 1015 cm−2 and 5 × 1015 cm−2 were used. The damage states and strain build-up in the as-implanted samples were characterized by Raman spectroscopy combined with high-resolution X-ray diffraction (HRXRD). Irradiation sequences of dual ions influence the damage states significantly. Si + H successively irradiations result in larger damage level, while H + Si successively irradiations give a higher strain level. These results indicate that the damage effects are highly related to irradiating ion species, ion fluences, and irradiation sequences.

Ohmic contact formation to GaN by Si+ implantation doping: Retarding layer, implantation fluence, encapsulation, and activation annealing temperature studies

The aspects of Si+ implantation for low resistivity ohmic contact formation to gallium nitride (GaN) with moderate annealing temperatures for dopant activation were studied: encapsulation layer and temperature for activation annealing, and implantation fluence. The uniqueness of our approach was to implant GaN through a SiO2 retarding layer, which shifted the dopant distribution, allowing us to obtain a quasi-uniform Si profile with just one implantation. The retarding layer was extensively studied both theoretically and experimentally and its applicability for the formation of good ohmic contacts was presented. We characterized GaN after Si doping by electrical and structural methods, the latter supported by depth profiles of point and extended defects, calculated by McChasy simulation code based on measured Rutherford Backscattering/channeling spectra. Implantation with 250 keV Si+ to the effective fluence of 1.1×1016 cm−2 allowed us to obtain the best electrical parameters, sheet resistances RSH ≲ 50 Ω/sq. and specific contact resistance RC ≲ 0.1 Ω⋅mm and ρC ≈ 2×10−7 Ω⋅cm2, although there were still defects present.

Electronic stopping in molecular dynamics simulations of cascades in 3C–SiC

We investigate the effect of the electronic stopping power on defect production due to ion irradiation of cubic silicon carbide using molecular dynamics simulations. We simulate 20 keV and 30 keV Si and C ions, with and without the electronic energy loss. The results show that the electronic stopping effects are more profound in the case of C irradiation, where the ratio of the electronic energy loss Se to the nuclear energy loss Sn is much larger compared to the ratio for Si ions. These findings indicate that this ratio plays a role in the effect of the electronic stopping on ion irradiation.

Micromechanical properties and microstructural evolution of Amosic-3 SiC/SiC composites irradiated by silicon ions

In this work, Amosic-3 SiC/SiC composites were irradiated to 10 dpa and 115 dpa with 300 keV Si ions at 300 °C. To evaluate its irradiation behaviour and investigate the underlying mechanism, nanoindentation, AFM, Raman and electron microscopy were utilized. Nanoindentation showed that although micromechanical properties declined after irradiation, hardness and Young’s modulus were maintained better under 115 dpa. AFM manifested differential swelling among PyC interface, fiber and matrix and SEM showed irradiation-induced partial interface debonding, which are both more obvious under 115 dpa. TEM revealed the generation and proliferation of amorphous regions, which is according with the decline and broadening of peaks in Raman spectra. The material was almost completely amorphous after irradiated to 10 dpa while recrystallization occurred under 115 dpa. All results mentioned above contribute to the decline of hardness and Young’s modulus and may explain why the micromechanical degradation was more significant under 10 dpa.

Investigations of irradiation effects in crystalline and amorphous SiC

The effects of irradiation on 3C-silicon carbide (SiC) and amorphous SiC (a-SiC) are investigated using both in situ transmission electron microscopy (TEM) and complementary molecular dynamics (MD) simulations. The single ion strikes identified in the in situ TEM irradiation experiments, utilizing a 1.7 MeV Au3+ ion beam with nanosecond resolution, are contrasted to MD simulation results of the defect cascades produced by 10–100 keV Si primary knock-on atoms (PKAs). The MD simulations also investigated defect structures that could possibly be responsible for the observed strain fields produced by single ion strikes in the TEM ion beam irradiation experiments. Both MD simulations and in situ TEM experiments show evidence of radiation damage in 3C-SiC but none in a-SiC. Selected area electron diffraction patterns, based on the results of MD simulations and in situ TEM irradiation experiments, show no evidence of structural changes in either 3C-SiC or a-SiC.

Irradiation effects on Amosic-3 silicon carbide composites by Si ions implantation

SiCf/SiC composites were irradiated to over 100 dpa with 300 keV Si ions at 300 ℃. Here, electron microscopy and Raman spectroscopy were utilized to study the microstructural evolution. The Raman spectra of fiber and matrix showed that the crystal structure was seriously damaged. TEM images revealed that the fiber underwent grain nucleation and growth in lower fluence region, accompanied by an amorphous layer near the damage peak area. Also, the matrix went through recrystallization, and the columnar grains turned into equiaxial ones. Moreover, stacking faults and massive amorphous islands were observed in high resolution TEM images. Following irradiation at 300 ℃, the matrix swelled, but the fiber and interphase shrunken along the axis. And, more remarkably, the hardness of fiber and matrix decreased to different extents, a result that was explained by the generation of amorphous islands and breakdown of covalent bonds, and recrystallization might be responsible for this.

Computer modeling of single-layer nanocluster formation in a thin SiO2 layer buried in Si by ion mixing and thermal phase decomposition

A single sheet of Si nanoclusters with an average diameter of about 2 nm has been formed in a 30 nm Si/7 nm SiO2/Si layer stack by 50 and 60 keV Si+ ion-beam mixing at room temperature and fluences between 8.5 ⋅ 1015 and 2.6 ⋅ 1016 ions/cm2 and by subsequent thermal annealing at a temperature above 1000 °C. Computer modeling of the process is accomplished by TRIDYN dynamic ballistic simulation of ion mixing and subsequent lattice kinetic Monte Carlo simulation of the phase decomposition of substoichiometric silicon oxide into Si nanoclusters in a SiO2 matrix. The simulation algorithms are briefly described with special emphasis on the choice of governing parameters for the present system. In comparison to the experimental results, it is concluded that the predicted ion mixing profiles overestimate the interface broadening. This discrepancy is attributed to the neglect of chemical driving forces in connection with thermal-spike induced diffusion, which tends to reconstitute the Si/SiO2 interfaces. With a corresponding correction and a suitable number of Monte Carlo steps, the experimentally obtained areal densities and average diameters of the nanoclusters are successfully reproduced.

Strain engineering 4H-SiC with ion beams

Single crystals of 4H-SiC irradiated with 900 keV Si and 21 MeV Ni ions separately and sequentially were studied by Rutherford backscattering spectrometry in channeling geometry, single crystal X-ray diffraction, and Raman scattering. SiC irradiated with 900 keV Si ions to a fluence of 6.3 × 1014 ions/cm2 experiences 7.3% strain over the depth of 650 nm. Strain relaxation from ionization-induced annealing was directly observed due to subsequent irradiation with 21 MeV Ni ions to a fluence of 2 × 1014 ions/cm2. These competitive processes suggest the use of ion irradiation to create a specific strain state in 4H-SiC, particularly in films.

Pure hydrogen references synthesized by low energy ion implantation into amorphous silicon

We developed a hydrogen standard by ion implantation in amorphous silicon, produced by implantation of 170 keV 28Si with 2·1015 at./cm2 in a silicon wafer (1 0 0) to obtain an amorphous depth of a few hundred of nanometers. The tilt and twist angles of the target were fixed to avoid channeling and the sample was cooled during the implantation. The amorphous silicon was then irradiated successively with hydrogen at 3.0 and 1.5 keV with doses 1.0 and 0.7·1017 at./cm2 respectively. The characteristics of the standards (retained dose, depth profile and stability under irradiation) were investigated by elastic recoil detection analysis (ERDA) and resonant nuclear reaction analysis (RNRA) with a 1H(15N,αγ)12C reaction at 6.385 MeV. These properties were compared with another previously developed standard (hydrogen implantation in monocrystalline silicon (1 0 0)). Results showed that, for the same implanted dose, the retained dose is about 55% larger in amorphous silicon. Furthermore, we showed these new standards are stable under beam irradiation for low current (10 nA/mm2) and could be used as IBA reference. Stability for higher currents (100 nA/mm2) showed a 10% release after an integrated charge about 2 mC. We showed that this release which occurs at the beginning of irradiation is due to desorption of the H-contamination layer on the surface. Implanted hydrogen remains stable under irradiation. We conclude that those standards are suitable target to study nuclear reactions of astrophysical interest, namely the 1H(13C,γ)14N reaction.