Temperature dependent preferential sputtering in CoSi2 and NbSi2

Sputtering of CoSi₂ and NbSi₂ has been carried out by Xe ion bombardment at room temperature, as well as at elevated temperatures putting these systems in their radiation-enhanced diffusion regimes. The range of the Xe ions (at 200–260 keV) was appreciably less than the thickness of the silicides. The samples were analyzed by 2 MeV He⁺ backscattering spectrometry, x-ray diffraction and optical microscopy. The ratio of the sputtering yield of Si to that of the metal (i.e., Co or Nb) always exceeds the stoichiometric ratio 21, leading to Si depleted surface layers. The amount of the sputtered species increases almost linearly with dose until intermixing of the silicide with the underlying Si becomes appreciable. This happens at lower doses in the radiation-enhanced diffusion regime than at room temperature. Irradiation of CoSi₂ samples at high temperature leads to a broadening of the implanted Xe profile compared to the room temperature profile. No such phenomenon has been found in NbSi₂. The effect of Xe broadening on the sputtering yields is discussed.     

The emission of neutral clusters in sputtering

The mass, angle, and energy resolved emission of neutral clusters in sputtering was studied for a variety of metals and semiconductors. The main phenomena and results are the following: (i) Cluster emission from a series of transition metals reveals a prominent contribution of clusters to the total flux of ejected particles but there is no simple scaling of cluster intensities with the average sputtering yields. With increasing number of constituents, relative intensities of neutral clusters decrease much faster than those of secondary-ion clusters. (ii) The relative intensities of clusters emitted from amorphous and crystalline semiconductors are identical, but the energy spectra of Ge _n -clusters (n = 1–4) sputtered from Ge (111) peak at a slightly higher energy (1 eV) as compared to spectra taken from amorphous Ge. The intensities of all Ge _n -clusters exhibit the same dependence on emission angle; this holds for both the amorphous and crystalline Ge-sample. (iii) The flux of neutral monomers, dimers, and trimers sputtered from Cu(111), Ni(111), and Ag(111) crystals shows a pronouncedly anisotropic emission along the 110 lattice directions which is ascribed to a momentum alignment in the anisotropic part of the collision cascade. Energy spectra taken along 110 peak at higher energies than those obtained from a random emission angle.     

Studies of CdTe surfaces with secondary ion mass spectrometry,rutherford backscattering and ellipsometry

The composition and the stability of chemically etched, mechanically polished and oxidized surfaces of single crystals of cadmium-telluride were studied by secondary ion mass spectroscopy (SIMS), Rutherford backscattering (RBS) and ellipsometry. CdTe surfaces etched using a solution of bromine in methanol were found to be enriched in cadmium but a film, identified to be an oxide of tellurium, was observed to grow on it at room temperature and in air. The thickness of this film increased over long periods of timet linearly versus lnt. Mechanically polished samples and also chemically etched surfaces which were oxidized in a solution of hydrogen peroxide in amoniac were found to be stable.     

On direction-invariance of fractal dimension on a surface

It is shown that fractal dimensions along line transects of a stationary, stochastic surface are all identical to one another, save that in one special direction the dimension may be less than in all others. We discuss the implications of this result for practical measurement of surface roughness and for interpretation of fractal dimension. It is argued that the result provides significant new motivation for employing fractal dimension as a canonical measure of surface roughness. Nevertheless, in view of the virtual direction-invariance of fractal dimension, measurement of scale assumes new significance. This matter is addressed.     

Energy deposition,reflection and sputtering in hyperthermal rare-gas→Cu bombardment

Using molecular-dynamics simulation, we study the scattering and penetration of normally incident hyperthermal (5–400 eV) Ne, Ar, and Xe atoms off a Cu crystal. We find that between 80% and 98% of the incident energy is deposited in the solid; the fraction depends primarily on the projectile mass, and — for not too low energies — only slightly on the bombarding energy. At low energy, the major part of the non-deposited energy is carried away by the reflected projectile. At energies above the sputter threshold, an increasingly important contribution of between 2% and 6% of the incident energy is carried away by sputtered particles. These results compare well with experiment. Electronic inelastic losses show only little influence on this behaviour. We demonstrate that the inclusion of a realistic attractive interaction between the projectile and the target atoms influences the energy deposition considerably at energies below around 100 eV.     

Analysis of temperature and enhanced diffusion effects in sputtering of CrSi2

Previously reported experimental results on sputtering and enhanced diffusion processes in CrSi₂ during 100 keVXe⁺ bombardment at different temperatures have been quantitatively analyzed.The framework for the analysis is a simple theoretical model in which the Si atoms are considered mobile in a matrix of Cr atoms whose density is assumed constant and diffusivity is considered zero everywhere. Erosion velocity of the matrix (due to the sputtering of Cr atoms), sputtering and enhanced diffusion processes of Si atoms are taken into account in the mathematical model.In our analysis we show that the time evolution of the total number of sputtered atoms in binary solids cannot yield an unambigous conclusion as to the existence of preferential sputtering.Further, it is found that in the case of CrSi₂ the preferential sputtering of Si atoms depends on the suicide temperature.     

Analysis of solids by secondary ion and sputtered neutral mass spectrometry

A mass spectrometer is described, which allows the analysis of sputtered neutral and charged particles as well as of residual gas composition. This combined SIMS, SNMS, and RGA instrument consists of a scanning primary ion beam column, an electron impact ionizer, an electrostatic energy filter and an rf quadrupole mass analyzer.Various examples of surface and bulk analysis are presented which demonstrate the beneficial complementary features of these techniques. These are, in particular: a substantial reduction of the matrix effect and fewer complications with samples of low electrical conductivity in SNMS, and the possibility of measuring the depth distribution of gases included in small cavities in the solid in the SNMS/RGA mode. SIMS, on the other hand, allows in many cases higher detection sensitivities.EURATOM Association     

The influence of the coating metals with various work function on the photoluminescence of a GaN-based blue LED wafer

A GaN light emitting diodes (LED) wafer was coated with Pt, Au, Al and In. The photoluminescence (PL) excited from wafer back shows that Pt and Au coating can quench the PL while In and Al can increase the PL intensity by eight times and make the wavelength red shift. When connected to the Pt with a wire, indium can also quench the PL. The potential difference between any two kinds of the metals was measured and the PL intensity from each coated area showed a remarkable pertinence to the coating metal potentials. A built-in potential barrier model is proposed to explain results.     

Silicide formation by furnace annealing of thin Si films on large-grained Ni substrates

Si films with a thickness of approximately 250 nm have been electron-beam evaporated on thick, large-grained Ni substrates (grain size a few mm to 1 cm in diameter). An in situ sputter cleaning procedure has been used to clean the Ni surface before the Si deposition. Thermal annealings have been performed in a vacuum furnace. Ni₂Si is the first phase that grows at temperatures between 240 °C and 300 °C as a laterally uniform interfacial layer with a diffusion-controlled kinetics. The layer thicknessx follows the growth lawx ²=kt, withk=k ₀ exp(-E _a k _B T), wherek ₀=6.3 × 10^–4cm 2/s andE _a=(1-1±0.1) eV. Because of the virtually infinite supply of Ni, annealing at 800 °C for 130min yields a Ni-based solid solution as the final phase. The results are compared with those reported in the literature on suicide formation by the reaction of a thin Ni film on Si substrates, as well as with those for interfacial phase formation in Ni/Zr bilayers.     

Calculation of geometrical changes in cylindrical copper hollow cathodes due to sputtering

To fine the reasons for the distortion of a cylindrical hollow cathode into a row of hollow spheres, the system of diffusion equations for the three most important types of particles in the hollow cathode discharge is solved, taking into consideration the essential source terms. In qualitative agreement with the experiments, the results show inhomogeneities of the cathode erosion at the cathode edges and at the boundary between hollow cathode discharge and normal glow discharge areas.     

Thermal Conductivity of Carbon Nanotubes Embedded in Solids

A carbon-nanotube-atom fixed and activated scheme of non-equilibrium molecular dynamics simulations is put forward to extract the thermal conductivity of carbon nanotubes （CNTs） embedded in solid argon. Though a 6.5% volume fraction of CNTs increases the composite thermal conductivity to about twice as much as that of the pure basal material, the thermal conductivity of CNTs embedded in solids is found to be decreased by 1/8-1/5 with reference to that of pure ones. The decrease of the intrinsic thermal conductivity of the solid-embedded CNTs and the thermal interface resistance are demonstrated to be responsible for the results.     

The effect of target texture and topography on the spatial distribution of material sputtered from tungsten

The spatial distributions of sputtered particles have been investigated both experimentally and by computer simulation using the TRIM.SP code for 30 keV argon-ion bombardment of tungsten in a wide range of primary-ion incidence angles (0°–80°). Two sets of the targets were used. One of them was prepared from fine-grained polycrystalline ingot, another one from rolled sheet W. It was found that the experimental results for these targets were different. For rolled tungsten a typical Wehner-spot picture, although smeared, is observed. For fine-grained tungsten the sputtered particle spatial distributions are practically cupola-shaped. Some differences between the experimental results and computer-simulation data can be attributed to the effect of surface topography either initial or developed during the ion bombardment.     

Temperature-dependent sputtering of metals and insulators

The temperature dependence of the sputter yield and the energy spectrum of sputtered atoms have been investigated on the basis of a standard model for thermal spikes. A high-temperature and a low-temperature regime have been identified in the temperature spectrum making up the evaporation yield. The high-temperature component of the yield as well as the associated energy spectrum are only very weakly dependent on ambient target temperature. The relative variation is the less pronounced the higher the spike temperature. The low-temperature component is associated with the long-time behavior of the spike, and measurable evaporation takes place over time intervals where spikes overlap. The importance of time constants for macroscopic heat transport is pointed out. The results are shown to provide a framework within which experimental results on the temperature dependence of the sputter yield of metals can be explained. The results are also consistent with measured temperature dependences in the sputter yield of insulators.On leave from Instytut Fizyki, Uniwersytet Jagiellonski, PL-30-059 Krakow, Poland     

Collision cascade evolution in media with energy independent scattering: spatial,energy, and angular distribution

The evolution of an ion induced collision cascade in a solid medium is studied by means of a DPl-approximation to the linear transport equation. Infinite medium and half space geometries are considered. Special attention is given to the effect of the anisotropy of the energy independent scattering cross section. We present results on the spatial distribution of particles moving at different energies, and the energy and angle distribution at the target surface. The spatial distributions are found to obey simple scaling laws; the energy and angular distributions are independent of the form of the scattering cross section, unless it is very strongly forward peaked.     

Ground Band and Excited Band of Spin-1 BEC in Cigar Shaped Laser Trap

The wavefunctions that conserve the total spin are constructed for the fully condensed states and the states with one particle excited. A set of equations are deduced for the spatial longitudinal wavefunctions and the chemical potentials. These equations are solved numerically for ^23Na and ^87Rb condensates. The deformed trap shows significant effects on the spectrum. This implies that the spin effect of the spinor BEC are more easily detected in an optical trap of larger aspect ratio.     

Effect of Quantizing Magnetic Field on Cyclotron Energy and Cyclotron Effective Mass in Size Quantized Films with Non-Parabolic Energy Band

The Fermi energy, cyclotron energy and cyclotron effective mass of degenerate electron gas in a sizeoquantized semiconductor thin film with non-parabolic energy bands are studied. The influences of quantizing magnetic field on these quantities in two-band approximation of the Kane model are investigated. It is shown that the Fermi energy oscillates in a magnetic field. The period and positions of these oscillations are found as a function of film thickness and concentration of electrons. Cyclotron energy and cyclotron effective mass are investigated as a function of film thickness in detail The results obtained here are compared with experimental data on GaAs quantum wells.     

Adsorption and catalysis: The effect of confinement on chemical reactions

Confinement within porous materials can affect chemical reactions through a host of different effects, including changes in the thermodynamic state of the system due to interactions with the pore walls, selective adsorption, geometrical constraints that affect the reaction mechanism, electronic perturbation due to the substrate, etc. In this work, we present an overview of some of our recent research on some of these effects, on chemical equilibrium, kinetic rates and reaction mechanisms. We also discuss our current and future directions for research in this area.     

Sputtering of multicomponent materials

The nonlinear balance equation, describing the evolution of concentration profiles for a multicomponent target under the action of ion bombardment as a function of the depth inside the target and the incident ion fluence, was solved numerically. The quantities of interest have been calculated with reference to a binary system showing that the balance equation is consistent with the correct stoichiometry for the sputtering yield ratio in the stationary state.     

Energy spectra of atoms sputtered by KeV light-ion bombardment

A calculation has been made for the sputtering of heavy targets by KeV light-ion bombardment. The calculation is based both on linear transport theory and on the assumption that only primary recoiling atoms are candidates for the sputtering process. The energy spectra calculation predicts the experimentally observed peak shift towards lower velocities than those obtained by standard linear cascade theory.     

Influence of surface roughness on adsorbate mobility

Field-emission fluctuations of the coadsorption system W(112)Ni-K were investigated by the cross-correlation technique in two perpendicular crystallographic directions at various temperatures. In- and cross-channel movements show a considerable reduction of _max which is the time delay corresponding to the cross-correlation function maximum with increasing Ni coverage at a constant potassium probe concentration. _max, its activation energy and the signal velocity are discussed in terms of the well-known W(112)Ni surface structure.