Light Ion Bombardment Research Articles

Effect of irradiation temperature and ion flux on electrical isolation of GaN

We study the evolution of sheet resistance of n-type GaN epilayers irradiated with MeV 1H and 12C ions. Results show that both implantation temperature (varied from 77 up to 423 K) and ion beam flux affect the process of electrical isolation in the case of irradiation with 12C ions. This behavior is consistent with significant dynamic annealing occurring in GaN during MeV light-ion bombardment, which suggests a scenario where the centers responsible for electrical isolation are defect clusters or anti-site-related defects. Dynamic annealing causes simple ion-beam-generated Frenkel pairs to annihilate (or cluster) during irradiation at liquid nitrogen temperature and above. These beam-flux and irradiation-temperature effects are not observed during bombardment with lighter 1H ions, which produce very dilute collision cascades. A qualitative model is proposed to explain temperature and flux effects in GaN in the MeV light-ion bombardment regime used for electrical isolation.

Nanocavity formation processes in MgO( [formula omitted]) by light ion (D, He, Li) and heavy ion (Kr, Cu, Au) implantation

In studies on the controlled growth of metallic precipitates in MgO it is attempted to use nanometer size cavities as precursors for formation of metallic precipitates. In MgO nanocavities can easily be generated by light gas ion bombardment at room temperature with typically 30 keV ion energy to a dose of 10 16 cm −2, followed by annealing to 1300 K. It has been shown earlier by transmission electron microscopy (TEM) that the cavities (thickness 2–3 nm and length/width 5–10 nm) have a perfectly rectangular shape bounded by {1 0 0} faces. The majority of the gas has been released at this temperature and the cavities are stable until annealing at 1500 K. The depth location of the cavities and the implanted ions is monitored by positron beam analysis, neutron depth profiling, RBS/channeling and energy dispersive spectroscopy. The presence of metallic nanoprecipitates is detected by optical absorption measurements and by high-resolution XTEM. Surprisingly, all the metallic implants induce, in addition to metallic precipitates in a band at the mean ion range, small rectangular and cubic nanocavities. These are most clearly observed at a depth shallower than the precipitate band. In the case of gold the cavities are produced in close proximity to the crystal surface. The results indicate that in MgO vacancy clustering dominates over Frenkel-pair recombination. Results of molecular dynamics calculations will be used to discuss the observed defect recovery and clustering processes in MgO.

Effect of ion species on the accumulation of ion-beam damage inGaN

Wurtzite GaN epilayers bombarded with a wide range of ion species (10 keV ${}^{1}\mathrm{H},$ $40\mathrm{}\mathrm{keV}$ ${}^{12}\mathrm{C},$ 50 keV ${}^{16}\mathrm{O},$ 600 keV ${}^{28}\mathrm{Si},$ 130 keV ${}^{63}\mathrm{Cu},$ 200 keV ${}^{107}\mathrm{Ag},$ 300 keV ${}^{197}\mathrm{Au},$ and 500 keV ${}^{209}\mathrm{Bi})$ are studied by a combination of Rutherford backscattering/channeling (RBS/C) spectrometry and cross-sectional transmission electron microscopy. Results show that strong dynamic annealing processes lead to a complex dependence of the damage-buildup behavior in GaN on ion species. For room-temperature bombardment with different ion species, bulk disorder, as measured by RBS/C, saturates at some level that is below the random level, and amorphization proceeds layer-by-layer from the GaN surface with increasing ion dose. The saturation level of bulk disorder depends on implant conditions and is much higher for light-ion bombardment than for the heavy-ion irradiation regime. In the case of light ions, when ion doses needed to observe significant lattice disorder in GaN are large $(\ensuremath{\gtrsim}{10}^{16}{\mathrm{cm}}^{\ensuremath{-}2}),$ chemical effects of implanted species dominate. Such implanted atoms appear to stabilize an amorphous phase in GaN and/or to act as effective traps for ion-beam-generated mobile point defects and enhance damage buildup. In particular, the presence of a large conce ntration of carbon in GaN strongly enhances the accumulation of implantation-produced disorder. For heavier ions, where chemical effects of implanted species seem to be negligible, an increase in the density of collision cascades strongly increases the level of implantation-produced lattice disorder in the bulk as well as the rate of layer-by-layer amorphization proceeding from the surface. Such an increase in stable damage and the rate of planar amorphization is attributed to (i) an increase in the defect clustering efficiency with increasing density of ion-beam-generated defects and/or (ii) a superlinear dependence of ion-beam-generated defects, which survive cascade quenching, on the density of collision cascades. Physical mechanisms responsible for such a superlinear dependence of ion-beam-generated defects on collision cascade density are considered. Mechanisms of surface and bulk amorphization in GaN are also discussed.

The effects of ion mass, energy, dose, flux and irradiation temperature on implantation disorder in GaN

The effects of implant conditions on the damage build-up in wurtzite GaN films are studied by Rutherford backscattering/channeling spectrometry and transmission electron microscopy. The results point to substantial dynamic annealing of radiation defects even during heavy ion ( 197 Au ) bombardment at liquid nitrogen (LN 2) temperature. A marked similarity between damage build-up for light ( 12 C ) and heavy ( 197 Au ) ion bombardment regimes at LN 2 temperature is observed. However, the damage build-up behavior during room temperature (RT) bombardment with 12 C or 197 Au ions is different. Implantation temperature not only affects the gross level of implantation disorder but also controls the general behavior of damage build-up during light or heavy ion bombardment. For all implant conditions of this study, a band of planar defects nucleates and grows in the bulk with increasing ion dose. An increase in the energy of Au ions from 100 to 2000 keV does not change the main features of damage build-up at LN 2 temperature. The results also show that implantation disorder in GaN depends on beam flux.

Local Modification of the Optical Constants of Polymeric Films by Ion Implantation

An experiment on the modification of the optical constants of polymethyl-methacrylate and polystyrene films by light-ion bombardment is presented. It is shown that ion implantation is an efficient technique to modify the optical properties of polymers and that the values of their refractive index can be predicted from changes in the molecular refraction.

Cathodoluminescence depth profiling of ion-implanted GaN

Cathodoluminescence (CL) spectroscopy shows that even relatively low-dose keV light-ion bombardment (corresponding to the generation of ∼5×1019 vacancies/cm3) of wurtzite GaN results in a dramatic quenching of visible CL emission. Postimplantation annealing at temperatures up to 1050 °C generally causes a partial recovery of measured CL intensities. However, CL depth profiles indicate that, in most cases, such a recovery results from CL emission from virgin GaN, beyond the implanted layer due to a reduction in the extent of light absorption within the implanted layer. In this case, CL emission from the implanted layer remains completely quenched even after such an annealing. These results show that an understanding of the effects of ion bombardment and postimplantation annealing on luminescence generation and light absorption is required for a correct interpretation of luminescence spectra of GaN optically doped by keV ion implantation.

Damage buildup in GaN under ion bombardment

The damage buildup until amorphization in wurtzite GaN films under keV light ${(}^{12}\mathrm{C})$ and heavy ${(}^{197}\mathrm{Au})$ ion bombardment at room and liquid nitrogen $({\mathrm{LN}}_{2})$ temperatures is studied by Rutherford backscattering/channeling (RBS/C) spectrometry and transmission electron microscopy (TEM). The effect of beam flux on implantation damage in GaN is reported. A marked similarity between damage buildup for light and heavy ion bombardment regimes is observed. The results point to substantial dynamic annealing of irradiation defects even during heavy ion bombardment at ${\mathrm{LN}}_{2}$ temperature. Amorphization starts from the GaN surface with increasing ion dose for both ${\mathrm{LN}}_{2}$ and room-temperature bombardment with light or heavy ions. A strong surface defect peak, seen by RBS/C, arises from an amorphous layer at the GaN surface, as indicated by TEM. The origin of such an amorphous layer is attributed to the trapping of mobile point defects by the GaN surface, as suggested by the flux behavior. However, in the samples implanted with light ions to low doses $(1\ifmmode\times\else\texttimes\fi{}{10}^{15}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}),$ no amorphous layer on the GaN surface is revealed by TEM. Damage buildup is highly sigmodal for ${\mathrm{LN}}_{2}$ temperature irradiation with light or heavy ions. Formation of planar defects in the crystal bulk is assumed to provide a ``nucleation site'' for amorphization with increasing ion dose during irradiation at ${\mathrm{LN}}_{2}$ temperature. For room-temperature bombardment with heavy ions, the damage in the GaN bulk region saturates at a level lower than that of the amorphous phase, as measured by RBS/C, and amorphization proceeds from the GaN surface with increasing ion dose. For such a saturation regime at room temperature, implantation damage in the bulk consists of point-defect clusters and planar defects which are parallel to the basal plane of the GaN film. Various defect interaction processes in GaN during ion bombardment are proposed to explain the observed, somewhat unexpected behavior of disorder buildup.

Strong surface disorder and loss of N produced by ion bombardment of GaN

The damage buildup in wurtzite GaN films under light (12C) and heavy (197Au) ion bombardment at temperatures from −196 to 550 °C is studied by Rutherford backscattering/channeling spectrometry. A strong surface peak of lattice disorder in addition to the expected damage peak in the region of the maximum of nuclear energy loss has been observed for all implant conditions of this study. Capping of GaN with SiOx and SixNy layers prior to implantation somewhat reduces but does not eliminate surface disordering. This suggests that nitrogen loss is not the main reason for the observed enhanced surface disorder, but, rather, the GaN surface acts as a strong sink for migrating point defects. However, pronounced loss of N during ion bombardment is observed for high dose implantation when the near-surface region is amorphized. Moreover, after amorphization, annealing at temperatures above about 400 °C leads to complete decomposition of the near-surface layer.

Ion beam induced amorphization in α quartz

The amorphization of α quartz induced by light and medium mass ion bombardment has been investigated by means of Rutherford backscattering spectrometry in channeling geometry, resonant nuclear reaction analysis and mechanical surface profiling. The single crystalline samples were irradiated at 77 K with H+, N+, Ne+ and Na+ ions at energies of 10–100 keV and fluences up to 5×1016 ions/cm2. Disordering of α quartz by ion bombardment was found to occur by three-dimensional nucleation and growth of defect agglomerates and, possibly, small, spatially separated amorphous zones in the still crystalline surrounding. The nucleation rate scales with the energy density FD in elastic collisions. Above a critical energy density EC=1.92(7)eV/at deposited by nuclear collisions (corresponding to a displacement rate of 0.04 dpa) a coherent amorphous layer forms at the depth of maximum energy deposition, which then grows towards the surface and into larger depths during continued ion bombardment. Mechanical surface profiling reveals large compressive stresses (∼1.5 GPa), built up at low fluences, which are released at higher fluences after the amorphous layer extends to the surface. The stress release is accompanied by a significant reduction of the atomic density by about 19%.

Diffusion in amorphous metallic alloys – experiments, molecular-dynamics simulations, interpretation

After a brief description of the experimental technique used in the authors' diffusion studies of metallic glasses, a representative selection of their self-diffusivity data referring to both the unrelaxed and the relaxed glassy state is presented. On the basis of these observations a model of the diffusion mechanisms operating in metallic glasses is developed and considerably refined with the aid of what has been learnt on diffusive atomic-displacement mechanisms in Fe-Zr glasses from molecular-dynamics simulations. Finally, the strength of these ideas is demonstrated by straightforwardly explaining the extraordinary diffusion-enhancement behaviour of metallic glass under light-ion bombardment and by elucidating the diffusion mechanisms in supercooled molten alloys just above the glass transition temperature.

Roughening instability and ion-induced viscous relaxation of SiO2 surfaces

We characterize the development of nanometer scale topography (roughness) on SiO2 surfaces as a result of low energy, off-normal ion bombardment, using in situ energy dispersive x-ray reflectivity and atomic force microscopy. Surfaces roughen during sputtering by heavy ions (Xe), with roughness increasing approximately linearly with ion fluence up to 1017 cm−2. A highly coherent ripple structure with wavelength of 30 nm and oriented with the wave vector parallel to the direction of incidence is observed after Xe sputtering at 1 keV. Lower frequency, random texture is also observed. Subsequent light ion (H, He) bombardment smoothens preroughened surfaces. The smoothing kinetics are first order with ion fluence and strongly dependent on ion energy in the range 0.2–1 eV. We present a linear model to account for the experimental observations which includes roughening both by random stochastic processes and by development of a periodic surface instability due to sputter yield variations with surface curvature which leads to ripple development. Smoothing occurs via ion bombardment induced viscous flow and surface diffusion. From the smoothing kinetics with H and He irradiation we measure the radiation enhanced viscosity of SiO2 and find values on the order of 1–20×1012 N s m−2. The viscous relaxation per ion scales as the square root of the ion induced displacements in the film over the range of the ion penetration, suggesting short-lived defects with a bimolecular annihilation mechanism. The surface instability mechanism accounts for the ripple formation, while inclusion of stochastic roughening produces the random texture and reproduces the observed linear roughening kinetics and the magnitude of the overall roughness.

Monte Carlo study of isolated defect production in ion-induced collision cascades

Abstract Monte Carlo simulation is used to study defect production during ion bombardment. The systems of 300 keV Ni+ → Ni and 1 MeV H+ → Ni are studied as representatives of heavy-ion and light-ion bombardment, respectively. The number of isolated defects—vacancies and interstitials—is determined which have no recombination partner within a distance of one recombination radius r rec. Our simulations predict a fraction of freely-migrating defects of 1.5% for 300 keV Ni+ bombardment, and 20% for 1 MeV H+ bombardment, if a common value of r rec ≌ 10 Å is employed. This is in good agreement with measured data.

Statistics of light-ion-induced kinetic electron emission: The sum of Poisson distribution.

A model on the statistics of ion-induced kinetic electron emission is presented. The model is primarily designed to describe light-ion bombardment at energies not too far above the threshold for kinetic-electron emission. We use an invariant-embedding approach, which includes angular scattering and stopping of the ion, electron excitation, and elastic and inelastic scattering. A particularly simple picture evolves when the ion travels on a straight line in the vicinity of the surface: Then the emission statistics is simply a sum of two Poisson distributions, where the ion reflection coefficient determines the weight of the Poisson terms. Comparison to experiment is favorable. Extensions with respect to ion trajectory, energy dependence, and recoil contribution are discussed.

Sputtering of solids under light-ion bombardment

An analytical theory of sputtering of amorphous solids by light-ion bombardment is presented. It is based on the following two mechanisms: sputtering due to incident ions directly, and sputtering due to backscattered ions, which includes the effects caused by knocked-off atoms. Calculated sputter yields are compared with Monte Carlo simulation data.

Development of the fluorescent materials test chamber

Alabama A&M university and Nichols Research Corporation have recently completed construction and preliminary testing of the fluorescent materials test chamber (FMTC) with its associated vacuum subsystem at the Alabama A&M University Pelletron accelerator laboratory. The FMTC will provide physicists and material scientists with the capability of exposing a variety of samples in a controlled temperature environment to light ion bombardment. Efforts have also been completed toward the design for a low cost beam pulser subsystem for the FMTC. This equipment will allow the measurement of the fluorescent prompt decay time for a wide variety of phosphors. Current design and equipment capabilities allow for prompt fluorescence decay time measurements down to 1 μs. A vacuum tube based grid leak bias Hartley LC oscillator will be used to generate beam pulses with approximately a 2.2% duty cycle at electric field deflection frequencies of 1 and 100 kHz. This technique provides low-cost post-acceleration beam pulsing when compared to more traditional pulsed ion source arrangements.

Ion-activated interface adsorption of oxygen during silver deposition on silicon

Abstract The process of ion-activated oxygen adsorption on silicon has been investigated using an experimental concept with simultaneous deposition of silver films. Auger electron spectroscopy in combination with sputter depth profiling is subsequently performed to determine the amount of oxygen adsorbed at the AgSi interface. Noble gas ions ( 4 He + , 20 Ne + and 40 Ar + )with energies between 50 and 175 keV were used, and it was found that for substrate temperatures of 300–700 K the oxygen adsorption depends strongly on ion mass, ion energy and ion flux density. For flux densities of 5 × 10 11 cm −2 s −1 or less, adsorption dominates and, in particular, for light-ion bombardment the majority of adsorbed oxygen atoms form chemical bonds with the silicon surface atoms (SiO). However, for heavy ions, physical sputtering starts to compete and limits the effective rate of adsorption. At sufficiently high ion fluxes the adsorption starts to decrease, and for all ions and energies used in this work it is found that, if the electronic energy deposition density exceeds a critical value of about 1.2 × 10 21 eV cm −2 s −1 , dissociation of the SiO bonds prevails with a corresponding loss in the adsorbed oxygen quantity.

Light-ion bombarded p-type In0.53Ga0.47As photoconductive detectors

The effect of H+ and B+ bombardment on the dark current and the impulse response speed of p-type InGaAs photoconductive detectors was studied. The dark current of the detectors decreased with light ion bombardment before the material turned n-type. The lowest dark current of H+ bombarded detectors was 10 μA at 6 V bias. The impulse response speed improved with H+ bombardment but did not change with B+ bombardment, as long as the material remained p-type. However the decay time of the detector response increased when the material turned n-type. The photoresponsivity in H+ bombarded detectors at 1.3 μm was 1 A/W, the bandwidth was 1.8 GHz, and the dark noise power into a 50 Ω load was −110 dBm with a source-drain bias of 6 V. The test structures had 5 μm contact spacing.

L-subshell ionization of rare earth elements by light ion bombardment

Thin targets of La, Pr, Nd, Eu, Tb, Ho and Er were bombarded by 1H, 2H, 3He and 4He projectiles. The production cross sections for the Lalpha 1,2, L beta 1,3,4,6, L beta 2,7,9,10,15, L gamma 1,5, Lgamma 2,3, L gamma 4,4 and Ll X-ray transitions have been extracted and corresponding ionization cross sections for L1-, L2- and L3-subshells have been calculated. The ratios of the experimental data to the predictions of the ECPSSR and the SCA theories are presented as a function of the corrected reduced projectile velocity parameter xi Lir. The Coulomb deflection of the projectile by the target nucleus predicted by these two approaches is compared with the experimental data. Possible reasons for the observed disagreement between experiment and theory are discussed.

Irradiation-enhanced and -induced mass transport

Irradiation can be used both to enhance diffusion, that is, to increase the rate at which equilibrium is attained, as well as to induce nonequilibrium changes. The main factors influencing whether irradiation will drive a material toward or away from equilibrium are the initial specimen microstructure and geometry, the irradiation temperature, and the primary recoil spectrum. This paper summarizes known effects of irradiation temperature and primary recoil spectrum on mass transport during irradiation. In comparison to either electron or heavy-ion irradiation, it is concluded that relatively low-energy, light-ion bombardment at intermediate temperatures offers the greatest potential to enhance the rate at which equilibrium is attained. The greatest departures from equilibrium can be expected from irradiation with similar particles at very low temperatures.

M-subshell ionization of U by light-ion bombardment

M X-rays of U were produced by proton, deuteron and alpha-particle bombardment in the energy range of 0.20–1.00 MeV/u. N 6,7 → M 5 (M α ), N 6 → M 4 (M β ), N 5 → M 3 (M γ ), N 4 → M 2 and N 2 → M 1 line yields were obtained from a least-squares fit to the spectra and used to convert M X-ray production into M-subshell ionization cross sections. The uncertainty induced by the atomic parameters (X-ray fluorescence yields, Coster-Kronig and radiative transition rates) used in the conversion is discussed. The subshell ionization cross sections are then compared to PWBA values corrected for Coulomb deflection and energy loss according to Brandt and Lapicki, to the semiclassical theoretical values of Kocbach and to relativistic PWBA results, corrected for Coulomb and binding effects, of Chen et al. Intrashell transitions induced by the projectile and multiple ionization are suggested as causes of disagreement between theory and experiment, especially for alpha-particles. It is concluded that theory must go beyond the simple picture of the first-order perturbation approximation to explain M-subshell results and that care must be taken in the choice of wave functions.