Low-energy Ar Ion Irradiation Research Articles

Toward a systematic discovery of artificial functional magnetic materials

Although ferromagnets are found in all kinds of technological applications, only few substances are known to be intrinsically ferromagnetic at room temperature. In the past twenty years, a plethora of new artificial ferromagnetic materials have been found by introducing defects into non-magnetic host materials. In contrast to the intrinsic ferromagnetic materials, they offer an outstanding degree of material engineering freedom, provided one finds a type of defect to functionalize every possible host material to add magnetism to its intrinsic properties. Still, one controversial question remains: Are these materials really technologically relevant ferromagnets? To answer this question, in this work the emergence of a ferromagnetic phase upon ion irradiation is systematically investigated both theoretically and experimentally. Quantitative predictions are validated against experimental data from the literature of SiC hosts irradiated with high energy Ne ions and own experiments on low energy Ar ion irradiation of TiO$_2$ hosts. In the high energy regime, a bulk magnetic phase emerges, which is limited by host lattice amorphization, whereas at low ion energies an ultrathin magnetic layer forms at the surface and evolves into full magnetic percolation. Lowering the ion energy, the magnetic layer thickness reduces down to a bilayer, where a perpendicular magnetic anisotropy appears due to magnetic surface states.

Influence of porosity and pore size on sputtering of nanoporous structures by low-energy Ar ions: Molecular dynamics study

In this paper we have carried out molecular dynamics simulation of the low-energy Ar ion irradiation of nanoporous homogeneous material with different porosity and pore sizes. Our results demonstrate that in a model with small pores (Rpore = 0.8 nm) and relatively low (22%) porosity, the pores at near-surface layers collapsed due to the ion bombardment, whereas in a model with larger pores (Rpore = 2.8 nm) and higher (44%) porosity no significant structural changes occurred under the same irradiation conditions. To study thermal stability of porous structures and to reveal the effects of both the pore radius and the porosity on pore collapsing, our nanoporous structures were subjected to gradual heating. The simulation results demonstrate distinct mechanisms of structural changes in the nanoporous materials depending on the value of the excess surface energy per unit volume.

Temporal evolution on SiO2 surface under low energy Ar+-ion bombardment: roles of sputtering, mass redistribution, and shadowing

Self-organized pattern evolution on SiO2 surface under low energy Ar-ion irradiation has been investigated extensively at varied ion energies, angles of ion incidence, and ion flux. Our investigations reveal an instability on SiO2 surface in an angular window of 40° ̶ 70° and for a comprehensive range of Ar-ion energies (200–1000 eV). Different topographical features, viz. ripples, mounds, and elongated nanostructures evolve on the surface, depending upon the angle of incidence and ion fluence. The results are compiled in the form of a parametric phase diagram (ion energy versus angle of incidence) which summarizes the pattern formation on SiO2 surface. To understand the evolution of observed patterns, we have carried out theoretical estimation, taking into account the synergetic roles of ion induced curvature-dependent sputter erosion and prompt atomic redistribution. It is shown that irradiation-induced mass redistribution of target atoms plays a crucial role in determining the critical angle of ion incidence for pattern formation on SiO2 under the present experimental conditions, whereas the contribution of curvature-dependent sputtering needs to be considered to understand the existence of the angular window of pattern formation. In addition, ion-beam shadowing by surface features are shown to play a dominant role in the formation of mounds and elongated structures at higher ion fluences.

Transformation of multiwalled carbon nanotubes to amorphous carbon nanorods under ion irradiation

Amorphous carbon nanorods were synthesized from hollow multiwalled carbon nanotubes (MWCNTs) by low-energy Ar ion irradiation at room temperature. The obtained nanorods were investigated by Raman spectroscopy, transmission electron microscopy (TEM), soft X-ray photoelectron spectroscopy (XPS), and soft X-ray absorption spectroscopy (XAS). It was found that the diameter of the MWCNTs significantly increased with increasing the fluence. Finally, the original hollow structure and the graphite (002) TEM diffraction spots of MWCNTs completely disappeared and a broadening of Raman spectra occurred, indicating the amorphization of MWCNTs. The increase in the diameter by the irradiation can be explained by the bending and the randomization of the broken carbon hexagonal networks, and the accumulation of knocked-on atoms. The XPS and XAS measurements also support the formation of amorphous carbon nanorods.

Synthesis, characterization and effect of low energy Ar ion irradiation on gadolinium oxide nanoparticles

In this work, we report on the surfactant assisted synthesis of gadolinium oxide (Gd 2O 3) nanoparticles and their characterization through various microscopic and spectroscopic tools. Exhibiting a monoclinic phase, the nanoscale Gd 2O 3 particles are believed to be comprising of crystallites with an average size of ∼3.2 nm, as revealed from the X-ray diffraction analysis. The transmission electron microscopy has predicted a particle size of ∼9 nm and an interplanar spacing of ∼0.28 nm. Fourier transform infrared spectroscopy studies show that Gd–O inplane vibrations at 536.8 and 413.3 cm −1 were more prominent for 80-keV Ar-ion irradiated Gd 2O 3 nanosystem than unirradiated system. The photoluminescence (PL) spectra of irradiated specimen have revealed an improvement in the symmetry factor owing to significant enhancement of surface-trap emission, compared to the band-edge counterpart. Irradiation induced creation of point defects (oxygen vacancies) were predicted both from PL and electron paramagnetic resonance (EPR) studies. Further, the Raman spectra of the irradiated sample have exhibited notable vibrational features along with the evolution of a new peak at ∼202 cm −1. This can be ascribed to an additional Raman active vibrational response owing to considerable modification of the nanostructure surface as a result of ion bombardment. Probing nanoscale defects through prime spectroscopy tools would find a new avenue for precise tuning of physical properties with generation and annihilation of defects.

Formation of single electron transistors in single-walled carbon nanotubes with low energy Ar ion irradiation technique

The single electron transistor (SET) fabrication process on the individual single-walled carbon nanotubes (SWCNTs) using low energy Ar ion irradiation technique has been demonstrated. The individual SWCNT was partly irradiated through the protection resist with the two opening windows, whose width was 50 nm, fabricated by electron beam lithography. The irradiated segments had higher resistance and worked as barriers. The SWCNT-SET successfully operated up to 100 K.

Behavior of carbon readsorbed on tungsten during low energy Ar ion irradiation at elevated temperatures

A study of the behavior of carbon sputtered and readsorbed after scattering collisions with particles of surrounding gas on the tungsten surface affected by Ar ion irradiation with the flux equal to 2 × 10 16 cm −2 s −1 extracted from plasma under 300 V negative bias voltage in the temperature range 370–870 K was performed. The dependence of the W sample weight change on the working gas pressure in the range 0.1–10 Pa was registered and the information was deduced about prevailing sputtering-redeposition processes. The depth profiles of carbon at the tungsten surface were measured. We found that carbon distribution profiles in tungsten depend on the C redeposition rate for fixed ion irradiation parameters. Three regimes have been distinguished: (i) at working gas pressure equal to 5 Pa and more, the C redeposition rate prevails the sample surface erosion rate and the W surface is covered by continuous amorphous carbon film (the C film growth regime), (ii) at working gas pressure equal to about 1 Pa, the C redepostion rate is approximately equal to the erosion rate and the W surface is partially covered by redeposited carbon, and (iii) at working gas pressure less than 0.2 Pa, the erosion rate prevails the C redeposition rate (the W surface erosion regime). In the regime of balanced redeposition and erosion deep C penetration depth into nanocrystalline W was registered. It is suggested that under simultaneous C adsorption and ion irradiation at elevated temperature C adatoms are driven from the W surface into grain boundaries and into the bulk by the difference in chemical potentials between the activated W surface and grain boundaries. As the W surface is covered by amorphous C film, the grain boundaries are blocked and the efficiency of carbon transport decreases.

Effects of carbon readsorption on tungsten under high-flux, low-energy Ar ion irradiation at elevated temperature

The erosion kinetics under irradiation by Ar ions extracted from plasma by 300 V negative bias voltage of 2 μm-thick W film contaminated by redeposited carbon atoms was investigated in dependence on the Ar-working gas pressure in the range 0.2–10 Pa at 410 K. The erosion and deposition parameters were deduced from the dependence of the sample's weight change on irradiation time. Two regimes were distinguished which contributed differently to the carbon transport efficiency from the surface towards the bulk, namely, the weight-decrease regime, when sputtering prevailed redeposition, and the weight-increase regime, when redeposition prevailed sputtering. Carbon distribution profiles measured by the SIMS technique showed that carbon was efficiently transported into the W film when its surface was not covered by a continuous amorphous C layer. The C transport efficiency decreased when W was covered by a continuous amorphous C film. These results were qualitatively explained by dynamic mixing of atoms on the surface. It was assumed that surface chemical potential increased under irradiation and that C adatoms were driven into grain boundaries of nanocrystalline W film. The continuous amorphous C film on tungsten blocked the access of activated C atoms to the grain boundaries.

Influence of ion irradiation effects on the hydriding behavior of nanocrystalline Mg–Ni films

Mg–Ni films were grown on a silicon substrate using two magnetron sputter deposition sources and simultaneous Ar ion irradiation. X-ray diffraction microstructure and phase composition, EDX elemental composition and atomic force microscopy surface topography analysis showed that under low-energy Ar ion irradiation (bias voltages from 0 to −120 V), the Mg 2Ni phase was dominant and on the contrary with the increase of ion energy (bias voltages from −120 to −200 V), the MgNi 2 phase appeared. The Mg content changed from 63 at% down to 42 at% in films grown under bias voltages of 0 and −200 V, respectively. During hydrogenation at 8 bar, 270 °C for 3 h, films with a dominant phase of Mg 2Ni were transformed into Mg 2NiH 4. Hydrogen in MgNi 2 films was mainly in interstitials and tended to form bubbles.

Influence of Low Energy Ar Ion Irradiation Treatment on Surface Properties of Deposited Metal Thin Films

Influence of Low Energy Ar Ion Irradiation Treatment on Surface Properties of Deposited Metal Thin Films

2406 低エネルギーイオン照射による金属薄膜の機械特性の改善(J22-2 材料特性計測とマイクロ機構,J22 マイクロメカトロニクス,2005年度年次大会)

A method of improving mechanical properties of Nickel (Ni) thin films was proposed in this study. Ni thin films were electroplated on the Silicon (Si) substrate. The film with which some kinds of film thickness differs was formed, and it investigated about the difference in the influence of low energy Ar ion irradiation. Irradiation conditions were Ar ion acceleration voltage 30 V, ion current density 0.15mA/cm^2 and irradiation time 10 minutes. Young modulus of Ni thin film which was electroplating time is 5 min were increased by the irradiation, but Ni thin film which was electroplating time is 20 min is not increased. It is found that the irradiation effect changes with crystalline state of film before the irradiation.

Influence of Low Energy Ar Ion Irradiation Treatment on Mechanical Properties of Deposited Metal Thin Films.

A method of improving mechanical properties of Al thin films by low energy ion irradiation treatment after the deposition was proposed in this study. The surface of the Al thin films deposited on the glass substrate was irradiated with low energy Ar ions for 30 minutes, ion acceleration voltage 50 V and 100 V, ion current density was changed from 0.15 to 0.4 mA/cm2. Young modulus and hardness of Al thin film were increased, spacing of lattice planes d was decreased and peak intensity of Al (111) was increased after the irradiation. It is found that the improvement of mechanical properties of the Al films and increasing of peak intensity of Al (111) is related. Moreover it turns out that the irradiation effect of the ion acceleration voltage 50 V compared with 100 V is more remarkable. The proposed treatment method is expected to improve surface mechanical properties of metal thin films deposited by various formation techniques.

Non-equilibrium impurity redistribution in Si

A low-energy Ar ion irradiation of a boron-doped silicon sample results in the formation and subsequent propagation of an n–p junction from the front surface into the bulk, to the depth comparable to the sample thickness. The effect is attributed to the generation of self-interstitials at the irradiated surface, and their long-range diffusion into the sample. The in-diffused self-interstitials lead to a loss of boron acceptors, either by the kick-out reaction or by acceleration of oxygen transport (and formation of boron–oxygen complexes).

Kinetics of vacancy diffusion on Si(111) surfaces studied by scanning reflection electron microscopy.

The kinetics of vacancy diffusion on Si(111) surfaces is studied by using scanning reflection electron microscopy (SREM). Two types of layer-by-layer etching are observed during low-energy Ar ion irradiation (500 eV) at elevated substrate temperatures. One is step retreat, which is a reversal of step-flow growth, and the other is two-dimensional vacancy island nucleation. These results show that vacancies created by low-energy ion impact diffuse on the surfaces, and are annihilated at the step edges. The vacancy diffusion kinetics on the surfaces are examined by using a SREM technique. An activation energy of 3.0\ifmmode\pm\else\textpm\fi{}0.2 eV is obtained from the vacancy diffusion length estimated from the width of the denuded zone, which is created on both sides of the atomic step by thermal heating after vacancy introduction by ion irradiation at room substrate temperature. These results indicate that vacancy diffusion kinetics is dominated by monovacancy formation and diffusion. These processes require thermal excitation to overcome the potential barrier for surface diffusion of adatoms, and to overcome the barrier for lateral binding energy to release adatoms from the step edges. \textcopyright{} 1996 The American Physical Society.

Vacancy Diffusion Kinetics on Si(111) and (001) Surfaces Studied by Scanning Reflection Electron Microscopy

AbstractThe kinetics of vacancy diffusion on Si(111) and (001) surfaces are studied by using scanning reflection electron microscopy (SREM). Two types of layer-by-layer etching (reversal of step-flow growth and two-dimensional vacancy island nucleation) are observed during lowenergy Ar ion irradiation (500 eV) at elevated substrate temperatures. This means that vacancies created by low-energy ion impact diffuse on the surfaces, and are annihilated at the step edges. Although isotropic vacancy diffusion is observed on Si(111), anisotropic vacancy diffusion along the dimer rows and preferential vacancy annihilation at the SB steps are observed on Si(001). This anisotropic vacancy diffusion results in single-domain formation. The diffusion length of vacancies is estimated from the width of the denuded zones, which are formed on both sides of the atomic steps by thermal heating after the introduction of vacancies at room temperature. The activation energy of 3.0±0.2 eV obtained for Si(111) corresponds to the potential barrier both for surface adatom diffusion and for lateral binding energy to release adatoms from the step edges. For Si(001) surfaces, the activation energy obtained for vacancy diffusion along the dimer rows is 2.3±0.2 eV. The vacancy diffusion model mediated by dimer vacancy complexes, rather than by single-dimer vacancies, best accounts for our experimental results.

Fabrication and Magnetotransport of One-Dimensional Lateral Surface Superlattice Fabricated by Low-Energy Ion Irradiation

A new fabrication process of a lateral surface superlattice has been reported. A low-energy Ar ion irradiation has been used to fabricate a one-dimensional lateral surface superlattice. The decrease of carrier concentration and mobility has been investigated to avoid the serious damage caused by ion bombardment. The mobility decreases significantly with ion dose rather than carrier concentration and its scattering mechanisms are discussed. In addition, a Schottky gate on top of the superlattice is used to change the Fermi energy of two-dimensional electron gas and the anisotropic threshold characteristics are observed. Weiss oscillations can be modified by applying positive bias voltage to the gate.

Sputtering Angular Distributions of Individual Elements for Low-Energy Ar Ion Irradiation of PtCu Alloy

The dynamic Monte Carlo program including a bombardment-induced Gibbsian segregation process was used to calculate angular distributions of individual elements from sputtered Cu0.5Pt0.5 alloy in the Ar ion energy range between 0.2 and 1.5ke V. Calculated results show that for each investigated incident energy, the angular distribution of the Pt element is more forward-pointed than that of the Cu one; the reason may be that different percentage of sputtered atoms (37 similar 45% of Pt and 12 similar 19% of Cu) come from beneath the topmost layer.

GaAs surface low-dose irradiation pretreatment related reduction of Schottky barrier height. Real or effective?

Experimental data by Isothermal Capacitance Transient Spectroscopy (ICTS) have been applied to the investigation of correlation between the surface state density at the Si N- nGaAs interface and gallium arsenide surface irradiation pretreatment. The samples were pretreated by low-energy Ar ion-irradiation prior to deposition of the silicon nitride film. Upon irradiation, the shift of the density maximum of surface states from the familiar value E = 0.72 eV towards the conduction band has been detected. For initial samples the Fermi level pinning position was found to lie at E c − 0.68 eV, while it moved to value E c − 0.51 eV after irradiation. Hence, as revealed by ICTS examination, ion-irradiation pretreatment of the GaAs surface prior to contact deposition results in real, no merely effective reduction of the barrier height. The phenomenon was observed to hold both for MIS structures and Shottky contacts.