Ion Beam Current Density Research Articles

Highly Efficient Nitrogen Doping Into GaAs Using Low-Energy Nitrogen Molecular Ions

AbstractLow-energy N2+ molecular-ions were irradiated during the epitaxial growth of GaAs. Ion acceleration energy and ion beam current density were varied in the range of 30-200 eV and 3-37 nA/cm2, respectively. GaAs growth rate was kept constant at 1µm/ h and the thickness of N-doped GaAs layer was about 1 µm. N concentration was obtained by using secondary ion mass spectroscopy. Strong N-related emissions were observed in the low-temperature photoluminescence spectra, which indicates that N atom is efficiently substituted at As site and is optically active as an isoelectronic impurity

Synthesis of titanium dioxide films by ion beam enhanced deposition

Titanium dioxide films were prepared by ion beam enhanced deposition (IBED), where the films were synthesized by depositing titanium atoms and simultaneously bombarding with Ne+, Ar+ and Xe+ ion beams respectively at an energy of 40 keV in an O2 environment. Rutherford backscattering spectroscopy (RBS), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) and Raman spectroscopy were used to characterize the deposited films. It was found that the OTi ratio of films deposited is less than 2 : 1. XPS analysis shows that Ti2+, Ti3+ and Ti4+ chemical states exist on the titanium oxide films. The films deposited at different ion beam bombardment are principally crystalline TiO2 with a rutile structure. The films deposited with Ne+ bombardment exhibit a (1 0 0) preferred orientation. Films deposited with Xe+ bombardment exhibit a high (1 0 0) orientation and increase the (1 0 0) orientation with decreasing ion beam current density. The lattice parameter of the deposited films is larger than that of bulk materials. The grain size of the films is between 20 and 60 nm and decreases with increasing mass of bombarding ions.

Zirconium oxide formation and surface hardening behavior by nitrogen implantation under oxygen atmosphere in Zircaloy-4

Zircaloy-4 has been implanted with nitrogen at 120 keV and various total ion doses under an oxygen partial pressure of 1.5×10 −5 to 4×10 −5 torr. The substrate temperature was varied from 300 °C to 600 °C by controlling the ion-beam current density. It was observed from Auger electron spectroscopic analysis that oxygen penetration was three to five times deeper than nitrogen penetration depth for all implantation conditions at elevated temperatures above 300 °C. In addition, a composition ratio of oxygen to zirconium of approximately 2:1 was reached in a layer thickness range 0.3 to 0.5 μm with increasing total ion dose up to 5×10 17 ions cm −2 below 500 °C. Another noted phenomenon in nitrogen implantation under oxygen atmosphere was deep penetration of oxygen to >20 μm at 500 °C and 600 °C. The corresponding ion current density was above 70 μm cm −2. However, deep penetration of oxygen decreased the oxygen concentration to about 10 at.% over the whole penetration depth from the surface. Therefore ZrN as well as ZrO 2 formation was observed by X-ray diffraction below 500 °C while ZrO 2 formation was almost suppressed owing to low oxygen concentration above 500 °C. Surface hardness was highly enhanced at all implantation conditions. The hardness range of implanted Zircaloy-4 was 500 to 1800 HK (0.01 N) depending on implantation conditions, which corresponds to a two- to sixfold increase compared with the hardness (300 HK) of unimplanted Zircaloy-4. The improvement of hardness was found to be closely associated with ZrN as well as ZrO 2 formation and depended on total ion dose, temperature and O 2 partial pressure.

Roles of ion irradiation for crystalline growth and internal stresses in nickel films onto silicon substrates prepared by the ion beam and vapor deposition method

The roles of ion irradiation for crystalline growth and internal stresses in Ni films prepared by the ion beam and vapor deposition method were studied. Ni films were prepared on Si〈100〉 wafers by evaporation of Ni metal and simultaneous irradiation with inert gas ions, Ne, Ar, Kr, and Xe. The energies of inert gas ions were changed in the range of 0.5–10.0 keV. Transport ratios of vaporized Ni atoms to inert gas ions to substrates were kept at 15. Ion beam current densities and ion irradiation directions were fixed at 40 μA/cm2 and perpendicular to the substrate surface, respectively. From the x-ray analyses, crystallinities and preferred orientation were changed by ion irradiation conditions. On the other hand, internal stresses were also changed from compressive to tensile depending on ion energies and ion species. It is understood that the variations of crystalline growth and internal stresses in Ni films were caused by the difference of nuclear and electronic energy transfer abilities of irradiating ions.

Optical characterization of low-energy nitrogen-ion doped GaAs

Optical characterization of nitrogen (N)-doped GaAs was performed by low-temperature photoluminescence (PL) measurements. N doping in GaAs was successfully made by irradiating low-energy (100 eV) N + ion beam during the growth of GaAs using the combined ion beam and molecular beam epitaxy (CIBMBE) system. For the sample with N + ion beam current density ( I N) of 3 nA/cm 2, which correspond to N concentration ([N]) of ∼5 × 10 17 cm −3, two sharp PL emissions were observed at 1.508 eV ( X 1) and 1.495 eV ( X 2). For the sample with I N of 75 nA/cm 2 ([N] ≈ 1 × 10 19 cm −3), several novel PL emissions tentatively labeled by Y j ( j = 1, …, 8) were observed in the energy range of 1.491 – 1.449 eV by furnace annealing at 750°C for 20 min. Photoluminescence excitation measurements indicated that X 1 is correlated with the PL emission at 1.514 eV. Since the energy separation between individual Y j ( j = 1, …, 8) and X 2 is almost linear to r k −3, where r k is the distance between nitrogen-nitrogen (NN) pairs, we ascribed X 2 and Y j as the radiative transition of excitons bound to isolated N and that to NN pairs, respectively.

Ion energy spread and current measurements of the rf-driven multicusp ion source

Axial energy spread and useful beam current of positive ion beams have been carried out using a radio frequency (rf)-driven multicusp ion source. Operating the source with a 13.56 MHz induction discharge, the axial energy spread is found to be approximately 3.2 eV. The extractable beam current of the rf-driven source is found to be comparable to that of filament-discharge sources. With a 0.6 mm diameter extraction aperture, a positive hydrogen ion beam current density of 80 mA/cm2 can be obtained at a rf input power of 2.5 kW. The expected source lifetime is much longer than that of filament discharges.

Investigation of nonthermal particle effects on ionization dynamics in high current density ion beam transport experiments

Light ion inertial fusion experiments require the presence of a moderate density background gas in the transport region to provide charge and current neutralization for a high current density ion beam. In this article, we investigate the effects of nonthermal particles such as beam ions or non-Maxwellian electron distributions on the ionization dynamics of the background gas. In particular, we focus on the case of Li beams being transported in an argon gas. Nonthermal particles as well as thermal electrons are included in time-dependent collisional-radiative calculations to determine time-dependent atomic level populations and charge state distributions in a beam-produced plasma. We also briefly discuss the effects of beam ions and energetic electrons on the visible and vacuum ultraviolet (VUV) spectral regions. It is found that the mean charge state of the gas, and hence the electron density, is significantly increased by collisions with energetic particles. This higher ionization significantly impacts the VUV spectral region, where numerous resonance lines occur. On the other hand, the visible spectrum tends to be less affected because the closely spaced excited states are populated by lower energy thermal electrons.

Influence of argon ion bombardment on the oxidation of nickel surfaces

The effect of argon ion bombardment on the oxidation of nickel films was studied using X-ray photoelectron spectroscopy (XPS). In the absence of any ion beams, exposure of nickel surfaces to oxygen leads to the moderately rapid formation of a thin (3–5 monolayers thick) nickel oxide overlayer. At room temperature the oxygen uptake stops once this limit is reached, but at higher temperatures the slow growth of a thicker oxide is seen. The NiO growth kinetics can be phenomenologically described by a diffusion coefficient for either oxygen or (more likely) nickel ions through the forming oxide film of the order of 2 × 10 −18cm 2 s −1 at 625 K. The simultaneous impingement of argon ions on the surface during oxygen exposures was found to enhance the oxidation process, and ion beam current densities as low as 0.01 μA cm −2 were found to be sufficient to induce nickel oxidation past the 3–5 ML limit at room temperature. The oxidation rate was found to be roughly proportional to both the ion flux and the square of the oxygen pressure, suggesting that the Ar + bombardment oxidation enhancement may be due to an increase in diffusivity through the NiO surface film induced by local heating around the ion impact areas. The build-up of an NiO film during this Ar +- ion oxygen treatment was also found to slow down at higher temperatures, presumably because of the combined effect of a higher probability for desorption of molecular oxygen from the surface and a higher atomic-oxygen mobility into the bulk. The oxide films prepared at low temperatures appear to be quite disordered, and display an extra feature in the Ni 2p XPS spectra around 853.2 eV which could be assigned to partially reduced nickel, Ni x+, x < 2. Annealing of those films to temperatures above 400 K leads to the possible ordering of the surface and to the disappearance of the signal for the Ni x+ species in XPS, and further heating above 600 K leads to the diffusion of oxygen atoms into the bulk and to the partial reduction of the surface nickel to its metallic state.

Low energy nitrogen ion doping into GaAs using combined ion-beam and molecular-beam epitaxy method

Low energy nitrogen (N) ions were irradiated during the epitaxial growth of GaAs using combined ion beam and molecular beam epitaxy (CIBMBE) method as a function of N + ion acceleration energy ( E a) and N + ion beam current density ( I N). E a was varied from 70 to 170 eV I N from 900 pA/cm 2 to 75 nA/cm 2. GaAs growth rate was fixed to 1 μm/h. In 2 K photoluminescence (PL) spectra of the samples with I N = 3 nA/cm 2 and E a = 70–100 eV, two sharp emissions at 1.508 eV ( X 1) and 1.495 eV ( X 2), which have been attributed to the emissions of excitons bound to isolated N atoms, and another one at 1.443 eV ( X 5) were observed. These results show that nitrogen (N) atom in GaAs becomes optically active as an isoelectronic impurity at least in as-grown condition. For N + ion-irradiated samples with rather high I N, e.g., with I N = 75 nA/cm 2 and E a = 100 eV, a broad emission together with multiple sharp ones were observed after furnace annealing at 750°C which were ascribed to emissions of excitons bound to nitrogen-nitrogen (NN) pairs.

Cu films deposited by a partially ionized beam (PIB)

A partially ionized beam (PIB) source for formation of Cu thin films is characterized by measuring ion beam current density at various acceleration voltages ( V a) and ionization potentials ( V I). Increase of ion beam current density is intimately related with the variation of electron emission current in ionization of vaporized Cu particles. In X-ray diffraction (XRD) spectra of deposited Cu films on Si (100) substrate, preferred orientations along the 〈111〉 and 〈200〉 axis only can be found and relative intensity ratio of R = I〈111〉/ I〈200〉 is increased as V a is raised. At V a = 4 kV, R reaches the value of 29.4 which indicates that the deposited films is in a sence epitaxially grown. After annealing at 500 °C for 30 min, XRD specta of the films do not show any difference in peak intensity and preferred orientations as before. In consequence, the Cu films deposited by PIB have thermally stable structure. Resistivity of the deposited Cu films is reduced as acceleration voltage is increased and is 2.14 μΩ cm at V a = 3 kV, and the value is a little reduced to a value of 2.07 μΩ cm after annealing. Adhesion of the Cu/Si system is examined by a scratch test and critical load to cause adhesion failure is increased to about 17 newton at V a = 3 kV, which is four times higher than that of V a = 0 kV.

C+-energy-dependent residual ion damage in GaAs:C grown by the low-energy ion-beam doping method

The effects of residual ion damage in low-energy (30 eV–30 keV) C+-doped GaAs were investigated with regard to the electrical and optical activation of C as a function of C+ ion acceleration energy EC+. Systematic variation of EC+ demonstrated that, in the energy range of EC+&amp;lt;170 eV, the net hole concentration (|NA−ND|) slightly increases as EC+ increases and the highest |NA−ND| was obtained at EC+=170 eV under the constant C+ ion-beam current density. For EC+≳170 eV, an increase in EC+ gave rise to an abrupt decrease of |NA−ND| down to two orders of magnitude smaller than that obtained at EC+=170 eV. In low-temperature (2 K) photoluminescence spectra for as-grown samples with EC+=240 and 350 eV, a novel emission ascribable to residual ion damage was observed instead of an essential acceptor–acceptor emission of [g−g]β. However, subsequent annealing at 850 °C made this novel emission disappear and the proper [g−g]β emission was merely observed. An activation process observed for EC+=5, 10, and 30 keV samples was very similar to that by high-energy ion implantation, indicating low activation rate of 10%–15%. Minority-carrier lifetime measurements using scanning tunneling microscope stimulated time-resolved luminescence demonstrated the presence of residual ion damage in as-grown samples at EC+=240 and 350 eV and annealed ones at EC+=5, 10, and 30 keV while no ion damage was observed in as-grown sample at EC+=30 eV. The incorporation and activation behaviors of C atoms that take the form of low-energy ions were found to be considerably affected by changes in ion–surface interaction with increasing EC+ and by the presence of residual ion damage in the layer.

A simple model of radiation swelling of silicon

The radiation swelling of silicon is explained as a diffusion-like process where the flux of interstitials out of the plane is defined by the gradient of the concentration of interstitials and the gradient of mechanical stresses in the ion-implanted region of the solid. This model was applied to describe the dynamics and the main regularities (dependence of strain on the ion flux density, ion energy, substrate temperature) of ion implanted silicon (Ni +, E = 40–160 keV, j = 5–180 μA cm −2). It is demonstrated that suppression of radiation swelling at high temperatures of the substrate or high ion beam current density can be explained by the annihilation of radiation defects.

Space-charge compensation of highly charged ion beam from laser ion source

The problem of matching an ion beam delivered by a high-intensity ion source with an accelerator is considered. The experimental results of highly charged ion beam transport with space-charge compensation by electrons are presented. A tungsten thermionic cathode is used as a source of electrons for beam compensation. An increase of ion beam current density by a factor of 25 is obtained as a result of space-charge compensation at a distance of 3 m from the extraction system. The process of ion beam space-charge compensation, requirements for a source of electrons, and the influence of recombination losses in a spacecharge-compensated ion beam are discussed.

Improvement of the homogeneity of optical thin films by ion-assisted deposition

Abstract The inhomogeneity of thin films of ZrO2 and TiO2 was reduced by ion beam bombardment with mixed argon and oxygen as a working gas during deposition. The inhomogeneities of the films were calculated from transmission monitoring curves. The optimum ion beam voltage and ion beam current density are 900 V and 12 μA cm−2 for TiO2 film.

Amorphous Layer Formation on Nickel Alloy Surface by Intense Pulsed Ion Beam Irradiation

The formation of an amorphous layer due to irradiation of intense pulsed ion beams (PIBs) has been studied experimentally. A mixed carbon and fluorine PIB with energy of 180 keV, ion beam current density of 180 A/cm2 and pulse duration of 30 ns is irradiated on a Ni65Cr15P16B4 alloy, resulting in the formation of an amorphous layer on the substrate surface to a depth of 0.66 µ m which is consistent with the thermal diffusion length. Repeated irradiations of PIB lead to the formation of an amorphous phase at depths exceeding 0.66 µ m from the surface. PIB irradiation leads to rapid heating of the substrate surface to its boiling point (2732° C), and the cooling rate from the melting point to the glass transition point is estimated to be 2.2×109° C/s which is larger than the critical cooling rate for amorphous layer formation of nickel alloys.

Development of a D+ source for International Fusion Materials Irradiation Facility

A volume source, based on the high efficiency ion source (HIEFS), is being developed for D+ production in steady state operation. The source will be optimized for the extraction of atomic deuterium ions with high current densities. It was found that a maximum deuterium ion beam current density of 210 mA/cm2 can be achieved with a D+ fraction above 90%. At an extraction voltage of 35 kV and with an aperture radius of 4 mm, the source delivers 61 mA D+. After a description of the source and the experimental setup, the results of detailed studies of the beam composition in dependence of the plasma parameters are presented.

Improvement of the homogeneity of optical thin films by ion-assisted deposition

Abstract The inhomogeneity of thin films of ZrO2 and TiO2 was reduced by ion beam bombardment with mixed argon and oxygen as a working gas during deposition. The inhomogeneities of the films were calculated from transmission monitoring curves. The optimum ion beam voltage and ion beam current density are 900 V and 12 μA cm−2 for TiO2 film.

Uniform beam profiles of 5 cm convex gridded ion beam source

A Kaufman-type 5 cm convex gridded ion-beam source is characterized in terms of angle-resolved ion-beam current density and beam uniformity at various discharge currents, electromagnet currents, and acceleration potentials.

Effect of residual gas on Cu film deposition by partially ionized beam

Cu films were deposited on Si(100) substrate by partially ionized beam in a pressure of 10−6 Torr. Ion beam current densities were measured with and without Cu evaporation, respectively, in order to evaluate an effect of residual gas on the Cu deposition process. The ion beam current density measured without Cu evaporation, which is mainly due to ionized residual gas, was linearly increased with acceleration voltage, and the dependence of the ion beam current density on vacuum pressure was significantly decreased with 10−5–10−7 Torr. Total ion beam current density measured during Cu deposition, effects of ionized Cu particles and residual gas, was increased with acceleration voltage, and the magnitude of the total ion beam current density is nearly the same as the ion beam current density due to ionized residual gas. Growth rate and preferred orientation were increased with acceleration voltage, and grain size of the deposited films was in a range of 1000 Å, which was not dependent on acceleration voltage. Root-mean-square roughness of deposited films was reduced from 138 to 18 Å with a change in acceleration voltage from 0 to 3 kV. These changes of the film’s features could be attributed to the bombardment effect of ionized residual gas.

Three-dimensional, tight focusing of intense pulsed light-ion beam by spherical plasma focus diode

A new type of ion-beam diode, self-magnetically insulated, spherical plasma focus diode (SPFD), was developed. With the SPFD, three-dimensional focusing of an intense pulsed light-ion beam was obtained. Experiments and simulations were carried out to study the behavior of the SPFD. In the experiments, diagnostic results of the Rutherford scattering pinhole camera and the shadow-box showed that the ion beam was focused into a small cylindrical area with ∽0.5 mm in diameter and ∽2.5 mm in length. The average ion-beam current density at the anode surface was found to be ∽2 kA/cm2. In the simulations, it was observed that most of the diode gap is well insulated by the self-magnetic field induced by the diode current. The electron sheath in the diode gap significantly enhances the ion flow from the anode. As a result, the ion current density is several times higher than the single-species space-charge limited value.