eV Ion Beam Research Articles

Convergent ion beam alteration of 2D materials and metal-2D interfaces

Tailoring the properties of two-dimensional (2D) crystals is important for both understanding the material behavior and exploring new functionality. Here we demonstrate the alteration of MoS2 and metal-MoS2 interfaces using a convergent ion beam. Different beam energies, from 60 eV to 600 eV, are shown to have distinct effects on the optical and electrical properties of MoS2. Defects and deformations created across different layers were investigated, revealing an unanticipated improvement in the Raman peak intensity of multilayer MoS2 when exposed to a 60 eV Ar+ ion beam, and attenuation of the MoS2 Raman peaks with a 200 eV ion beam. Using cross-sectional scanning transmission electron microscopy (STEM), alteration of the crystal structure after a 600 eV ion beam bombardment was observed, including generated defects and voids in the crystal. We show that the 60 eV ion beam yields improvement in the metal-MoS2 interface by decreasing the contact resistance from 17.5 kΩ · µm to 6 kΩ · µm at a carrier concentration of n2D = 5.4 × 1012 cm−2. These results advance the use of low-energy ion beams to modify 2D materials and interfaces for tuning and improving performance in applications of sensors, transistors, optoelectronics, and so forth.

Temperature decrease and multiple acceleration of structural and phase transformations in metastable metals and alloys under cascade-forming irradiation. Part 1 – General questions and theory

Classical radiation physics describes well a number of known phenomena observed under irradiation of metals and alloys (radiation embrittlement, swelling, radiation creep), based on relatively slow processes of thermo- and radiation-enhanced diffusion. Mechanisms based on the description of the defect migration processes can not, however, explain the “low- dose effect” under neutron irradiation and the low-dose “long-range effect” under irradiation with accelerated ions E ∼ (104 – k×105) eV (1<k≲3). The paper is devoted to a brief review of the model that takes into account the nanoscale dynamic effects during cascade-forming irradiation. We are talking about explosive energy release in the regions of dense cascades of atomic displacements (thermal spikes) emitting powerful post-cascaded solitary waves, which can initiate structural-and-phase transformations in metastable media, theoretically, at unlimited distances. The distances at which the effect of accelerated (104 – k×105) eV ion beams is observed (in the continuous irradiation mode) are sometimes more than a few tens/hundreds of micrometers (at projected ion ranges of less than 1 μm) and, as recent studies have shown, can reach 1-10 millimeters. These effects are considered on the basis of experimental research data of more than ten different systems. The foundations of the theory of undamped propagation of plane and spherical waves in metastable media are presented. It is noted that the most probable energy of recoil atoms generated by reactor neutrons and fission fragments also belong to the above energy range, which indicates the need to take into account the nanoscale dynamic effects, regardless of the type of the cascade-forming irradiation.

Nanoscale dynamic and long-range effects under cascade-forming irradiation

Classical radiation physics describes well a number of known phenomena in irradiating metals and alloys (radiation embrittlement, swelling, radiation creep) on the basis of relatively slow processes of thermo- and radiation-enhanced diffusion. Mechanisms based on the description of the defect migration processes cannot, however, explain the “small-dose effect” under neutron as well as low-dose “long-range effect” under ~(104–k · 105) eV (1 ≤ k ≲ 3) ion irradiation.11Relatively cheap and simple accelerators (sources) of ions of the indicated energy range, generating high-current ion beams of large cross section, are increasingly used to modify the properties of materials. They can easily be included in the technological processes, in contrast to high-energy accelerating equipment. This paper takes a brief look at the model taking into account the explosive energy release in the regions of the dense cascades of atomic displacements (formation of thermal spikes) and the emission of powerful post-cascade solitary waves that are able initiate structural-and-phase transformations at their front in metastable media, theoretically, at unlimited distances. In practice observed distances of accelerated (104–k · 105) eV ion beams action (in continuous mode) are sometimes more than a few tens/hundreds of micrometers (at Rp < 1 μm; Rp is the projected ion range) and up to 1–10 mm as it was shown in the last experiments. It should be noted that the energies of recoil atoms generated by reactor neutrons and fission fragments also partly belong to the above-mentioned energy range, but the consideration of post-cascade effects for these types of irradiation is beyond the scope of the survey.

Changes of electronic properties of p-GaN(0 0 0 1) surface after low-energy N+-ion bombardment

The p-GaN(0 0 0 1) crystal with a relatively low acceptor concentration of 5 × 1016 cm−3 is used in these studies, which are carried out in situ under ultrahigh vacuum (UHV) by ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED). The p-GaN(0 0 0 1)-(1 × 1) surface is achieved by thermal cleaning. N+-ion bombardment by a 200 eV ion beam changes the surface stoichiometry, enriches it with nitrogen, and disorders it. Such modified surface layer inverts its semiconducting character from p- into n-type. The electron affinity for the already cleaned p-GaN surface and that just after bombardment shows a shift from 2.2 eV to 3.2 eV, as well as an increase of band bending at the vacuum/surface interface from 1.4 eV to 2.5 eV. Proper post-bombardment heating of the sample restores the initial atomic order of the modified layer, leaving its n-type semiconducting character unchanged. The results of the measurements are discussed based on two types of surface states concepts.

The enhanced low resistance contacts and boosted mobility in two-dimensional p-type WSe2 transistors through Ar+ ion-beam generated surface defects

We intentionally generated surface defects in WSe2 using a low energy argon (Ar+) ion-beam. We were unable to detect any changes in lattice structure through Raman spectroscopy as expected through simulation. Meanwhile, atomic force microscopy showed roughened surfaces with a high density of large protruding spots. Defect-activated Photoluminescence (PL) revealed a binding energy reduction of the W 4f core level indicating significant amounts of defect generation within the bandgap of WSe2 even at the lowest studied 300 eV ion-beam energy. The intensity ratio increase of direct PL peak demonstrated the decoupling of surface layers, which behave like consecutive defective monolayers. Electrical measurements after post-irradiation showed p-type ohmic contacts regardless of the ion-beam energy. The resulting ohmic contact contributed to an increased on/off current ratio, mobility enhancement of around 350 cm2V-1s-1 from a few cm2V-1s-1 in pristine devices and electron conduction suppression. Further increased ion-beam energy over 700 eV resulted in a high shift of threshold voltage and diminished subthreshold slope due to increased surface roughness and boosted interface scattering. The origin of the ohmic contact behavior in p-type WSe2 is expected to be from chalcogen vacancy defects of a certain size which pins the Fermi level near the valence band minimum. An optimized ion-beam irradiation process could provide solutions for fabricating ohmic contacts to transition metal dichalcogenides.

Precise Damage Observation in Ion-Beam Etched MTJ

Patterning damage at the sidewall in a magnetic tunnel junction (MTJ) was observed precisely using a rectangular MTJ where deterioration in crystallinity is easier to identify than in the case of a dot-shaped conventional MTJ. A 200–500 nm-square rectangular MTJ was patterned by a 200 eV ion beam (IB). Cross-sectional transmission electron microscopy was used for damage observation. A bright-field image showed that crystallinity deteriorated to a depth of $\sim 1.3$ nm in the MgO-barrier layer. A Fourier transform mapping image and a dark-field transmission electron microscopy image indicated the existence of an amorphous region at the patterning edge in the MgO layer. IB etching is one of the strong candidates for magnetic random access memory (MRAM) fabrication. However, a typical IB etching energy, e.g., 200 eV, introduces a damage depth of several monolayers at the patterned surface. Since nearly damage-free-patterned surface would be needed for high-density MRAM with nanoscale MTJs of $\sim 10$ nm in diameter, IB etching with much lower energy would be necessary for fabrication.

Optimization of ion-atomic beam source for deposition of GaN ultrathin films.

We describe the optimization and application of an ion-atomic beam source for ion-beam-assisted deposition of ultrathin films in ultrahigh vacuum. The device combines an effusion cell and electron-impact ion beam source to produce ultra-low energy (20-200 eV) ion beams and thermal atomic beams simultaneously. The source was equipped with a focusing system of electrostatic electrodes increasing the maximum nitrogen ion current density in the beam of a diameter of ≈15 mm by one order of magnitude (j ≈ 1000 nA/cm(2)). Hence, a successful growth of GaN ultrathin films on Si(111) 7 × 7 substrate surfaces at reasonable times and temperatures significantly lower (RT, 300 °C) than in conventional metalorganic chemical vapor deposition technologies (≈1000 °C) was achieved. The chemical composition of these films was characterized in situ by X-ray Photoelectron Spectroscopy and morphology ex situ using Scanning Electron Microscopy. It has been shown that the morphology of GaN layers strongly depends on the relative Ga-N bond concentration in the layers.

Characteristics of low-energy ion beams extracted from a wire electrode geometry

Beams of argon ions with energies less than 50 eV were extracted from an ion source through a wire electrode extractor geometry. A retarding potential energy analyzer (RPEA) was constructed in order to characterize the extracted ion beams. The single aperture RPEA was used to determine the ion energy distribution function, the mean ion energy and the ion beam energy spread. The multi-cusp hot cathode ion source was capable of producing a low electron temperature gas discharge to form quiescent plasmas from which ion beam energy as low as 5 eV was realized. At 50 V extraction potential and 0.1 A discharge current, the ion beam current density was around 0.37 mA/cm(2) with an energy spread of 3.6 V or 6.5% of the mean ion energy. The maximum ion beam current density extracted from the source was 0.57 mA/cm(2) for a 50 eV ion beam and 1.78 mA/cm(2) for a 100 eV ion beam.

Effect of ion-beam etching damage in Fe–Co tapered main pole

Effect of ion-beam etching damage in Fe–Co films was estimated and magnetic calculation of the planar head using tapered main pole (MP) including the etching damage was performed. The etching test on the Fe–Co films, whose thickness corresponds to half of the trackwidth for several Tbits/in.2, showed decrease in saturation magnetic flux density (Bs) and increase in coercivity with a 250 eV ion beam. The magnetic calculation showed relatively large decrease in head field and head field gradient as Bs of the damaged region decreases from 2.4 T. Damage control in the MP fabrication is one of the important issues for Tbits/in.2-era write heads.

Low-energy Ion Beam Biotechnology at Chiang Mai University

AbstractMeV-ion beam has long been applied to biology research and applications for many decades as highly energetic ions are undoubtedly able to interact directly with biology molecules to cause changes in biology. However, low-energy ion beam at tens of keV and even lower has also been found to have significant biological effects on living materials. The finding has led to applications of ion-beam induced mutation and gene transfer. From the theoretical point of view, the low-energy ion beam effects on biology are difficult to understand since the ion range is so short that the ions can hardly directly interact with the key biological molecules for the changes. This talk introduces interesting aspects of low-energy ion beam biology, including basis of ion beam biotechnology and recent developments achieved in Chiang Mai University in relevant applications such as mutation and gene transfer and investigations on mechanisms involved in the low-energy ion interaction with biological matter such as eV-keV ion beam bombardments of naked DNA and the cell envelopes.

Nano-graphite deposits on multi-walled carbon nanotubes induced by low energy ion beam irradiation in a methane and hydrogen mixture

Nano-graphite (NG) deposits were formed on multi-walled carbon nanotubes (MWCNTs) lying on a single-crystal-silicon plane by irradiating them with an 80 eV ion beam perpendicular to the plane. The ion beam was produced using a methane and hydrogen mixture (1: 5) at 700 degrees C. Electron microscopy indicates that there is an angle in the range 45 degrees-90 degrees between the (0002) planes of the formed NG particles and the axis of the MWCNTs. The MWCNTs retained their inner hollow Structure. The formation of the NGs can be ascribed to the high temperature decomposition and deposition of methane. and the observation of specific angles (45 degrees-90 degrees) between the (0002) planes of the NGs and the MWCNT axis may be attributed to the selective etching, or removal, by the hydrogen ions of NG nuclei With 0 degrees-45 degrees between their (0002) planes and the MWCNT axis.

Fiber textures of titanium nitride and hafnium nitride thin films deposited by off-normal incidence magnetron sputtering

We studied the development of crystallographic texture in titanium nitride (TiN) and hafnium nitride (HfN) films deposited by off-normal incidence reactive magnetron sputtering at room temperature. Texture measurements were performed by x-ray pole figure analysis of the (111) and (200) diffraction peaks. For a deposition angle of 40° from substrate normal, we obtained TiN biaxial textures for a range of deposition conditions using radio frequency (rf) sputtering. Typically, we find that the ⟨111⟩ orientation is close to the substrate normal and the ⟨100⟩ orientation is close to the direction of the deposition source, showing substantial in-plane alignment. We also introduced a 150 eV ion beam at 55° with respect to substrate normal during rf sputtering of TiN. Ion beam enhancement caused TiN to align its out-of-plane texture along ⟨100⟩ orientation. In this case, (200) planes are slightly tilted with respect to the substrate normal away from the ion beam source, and (111) planes are tilted 50° toward the ion beam source. For comparison, we found that HfN deposited at 40° without ion bombardment has a strong ⟨100⟩ orientation parallel to the substrate normal. These results are consistent with momentum transfer among adatoms and ions followed by an increase in surface diffusion of the adatoms on (200) surfaces. The type of fiber texture results from a competition among texture mechanisms related to surface mobilities of adatoms, geometrical, and directional effects.

Self-assembly of germanium islands under pulsed irradiation by a low-energy ion beam during heteroepitaxy of Ge/Si(100) structures

The effect of pulsed irradiation by a low-energy (50-250 eV) ion beam with a pulse duration of 0.5 s on the nucleation and growth of three-dimensional germanium islands during molecular-beam heteroepitaxy of Ge/Si(100) structures is investigated experimentally. It is revealed that, at specific values of the integrated ion flux (less than 10 12 cm -2 ), pulsed ion irradiation leads to an increase in the density of islands and a decrease in their mean size and size dispersion as compared to those obtained in the case of heteroepitaxy without ion irra- diation. The observed phenomena are explained in the framework of the proposed model based on the concept of a change in the diffusion mobility of adatoms due to the instantaneous generation of interstitial atoms and vacancies under pulsed ion irradiation. It is assumed that the vacancies and interstitial atoms give rise to an addi- tional surface strain responsible for the change in the binding energy of the adatoms. Under certain conditions, these processes bring about the formation of centers of preferential nucleation of three-dimensional islands at the places where the ions impinge on the surface. The model accounts for the possibility of annihilating vacan- cies and interstitial atoms on the surface of the growing layer. It is demonstrated that the results obtained from the Monte Carlo calculations based on the proposed model are in good agreement with the experimental data.

Stress evolution to steady state in ion bombardment of silicon

Low temperature ion bombardment of initially crystalline, defect-free silicon with 700 eV ion beam energy creates a highly-damaged stressed layer a few nanometers thick on the surface. An apparent steady state in structure is achieved at a fluence of 2 × 10 14–3 × 10 14 ions/cm 2. In this work, the stresses are computed using the interatomic force definition of stress. The stress evolution is studied as a function of argon implantation into the target. Stress per implanted argon atom is observed to reach a nearly constant value between 20 MPa and 25 MPa at a fluence of 1.2 × 10 14 ions/cm 2.

Homeotropic Alignment Effect for Nematic Liquid Crystal on the SiOxThin Film Layer by New Ion Beam Exposure

We studied homeotropic alignment effect for a nematic liquid crystal (NLC) on the <TEX>$SiO_{x}$</TEX> thin film irradiated by the new ion beam method. <TEX>$SiO_{x}$</TEX> thin films were deposited by plasma enhanced chemical vapor deposition (PECVD) and were treated by the DuoPIGatron ion source. A uniform liquid crystal alignment effect was achieved over 2100 eV ion beam energy. Tilt angle were about <TEX>$90^{\circ}$</TEX> and were not affected by various ion beam energy.

Nuclear polarization measurement of H/D atoms extracted from a storage cell with a Lamb-shift polarimeter

The occupation numbers of the individual hyperfine substates in a beam of hydrogen or deuterium atoms are measured with a Lamb-shift polarimeter. Therefore, the nuclear polarization of an atomic beam can be determined with high precision of about 0.5 % within 20 s. The measurements are carried out with the atomic beam source for the internal polarized storage-cell gas target of ANKE ( I ∼ 7 × 10 16 atoms/s). The polarization of a slow (500–2000 eV) ion beam with currents higher than 1 nA can be measured as well. When the intensity of the atomic beam is reduced by a factor of 100, the Lamb-shift polarimeter still provides a precise measurement with an error of about 2% within 30 s. Thus, it is thought possible to determine the nuclear polarization of the atomic gas inside the storage cell, when a sample of these atoms effuse into the Lamb-shift polarimeter through a sample tube. First, results obtained with a simple Teflon cell, a new ionizer with an internal getter pump, a newly designed Wien filter and a larger photomultiplier are presented. Planned studies to optimize the polarization measurement are discussed as well.

Modification of Hg1−x CdxTe properties by low-energy ions

We present an overview concerning the modification of properties of HgCdTe solid solutions and related Hg-containing materials under surface treatment with low-energy (60–2000 eV) ion beams. The conditions for conductivity-type conversion in p-material, dose, and time dependences of the depth of the conversion layer are analyzed. The modification of electrical properties of n-type material subjected to ion-beam treatment is discussed. The suggested mechanisms of conductivity-type conversion under low-energy ion treatment of HgCdTe doped with vacancies or acceptor impurities are regarded. Properties of p-n junctions produced by this technique are reviewed, and electrical and photoelectric parameters of HgCdTe IR photodetectors fabricated by low-energy ion treatment are analyzed. Several examples of novel device structures developed with the use of the method are presented.

Improved Crystalline Quality of GaN by Substrate Ion Beam Pretreatment

The use of a buffer layer and nitridation are the most common methods employed to reduce defects in GaN epilayers grown on a sapphire substrate by metal organic chemical vapor deposition (MOCVD). Ion beam pretreatment of a substrate was carried out for further reduction of strain and dislocation in the GaN epilayer. The ion dose was set at 1×1016 cm-2; the ion energies were 800 eV and 55 keV. The 800 eV ion beam was a reactive (N2+) ion beam (RIB) and the 55 keV N+ ion beam was from a normal ion implanter (N+ implantation). Both ion beam pretreatments resulted in the formation of a disordered amorphous phase on the substrate surface. Obviously, the thickness of the amorphous phase increased with the ion beam energy. However, the phase formed by the 800 eV ion beam was AlON, whereas the phase formed by the 55 keV ion beam was AlN. A Raman shift clearly showed that the strain in the GaN epilayer was decreased by the ion beam pretreatment of the sapphire substrate. Defect density in the GaN was reduced up to 50% by the ion beam pretreatment. The present results show that sapphire (0001) substrate ion beam pretreatment can be used to improve the properties of GaN epilayers grown by MOCVD.

Focused ion beam patterned Hall bars and Ohmic columns embedded in molecular-beam-epitaxial-grown GaAs/AlGaAs

Focused ion beam lithography combined with molecular-beam-epitaxial growth can be a useful tool for the formation of real-time patterned, embedded structures. For this purpose, sub-50 eV ion beams are essential to minimize ion induced damage and to ensure vertical localization of the deposited ions. The simultaneous patterning of a beam of dopant ions during wafer growth allows the realization of three-dimensional structures with doping profiles otherwise unattainable through conventional methods. This article reports on the successful fabrication of focused ion beam patterned Hall bars in epitaxially grown bulk GaAs and GaAs/AlGaAs heterostructures. The bulk Si2+ doped sample achieved a 77 K mobility of 4000 cm2 V−1 s−1 for a carrier concentration of 3.4×1017 cm−3 while the heterostructure showed a 1.5 K mobility of 1.8×105 cm2 V−1 s−1 at a carrier density of 5.5×1011 cm−2. It is also demonstrated that in situ device patterning reduces the number of required ex situ processing steps while maintaining the high quality of molecular-beam epitaxially grown material. Furthermore, the development of in-grown ohmic contact columns to buried structures is presented. A proposed three dimensionally integrated circuit and future applications of this new technology are also discussed.

Epitaxial growth of β-SiC on Si (100) by low energy ion beam deposition

Epitaxial β-silicon carbide (SiC) films on mirror-polished (100) Si substrates were deposited using low energy (150 eV) ion beam modification at different ion doses (1×10 17/cm 2, 1×10 18/cm 2, 1×10 19/cm 2). The Si substrates were exposed to a broad ion beam of hydrocarbon, argon, and hydrogen ions, generated in a Kaufman ion source. These ions reacted with the Si substrates and formed the epitaxial β-SiC films. Amorphous carbon films were generated on top of β-SiC films. Some voids were found with transmission electron microscopy (TEM) at the interface between the Si substrate and the β-SiC film. By studying the ion dose dependence of the quality and thickness of the SiC and amorphous carbon films, we suggest that the formation of the epitaxial β-SiC was driven by thermal diffusion of Si atoms in SiC and preferential etching of non-epitaxial SiC crystals by H ions.