K. Morita

Energy dependence of the ion-induced sputtering yields of monatomic solids

Abstract The ion-induced sputtering yields of monatomic solids are presented graphically for various ion-target combinations as a function of the energy of normally incident ions. Each graph shows the available experimental data points for an ion-target combination and the energy dependence of the sputtering yield calculated from an empirical formula whose parameters were determined by the best fit to available data. The method of calculating the sputtering yields for any ion-target combination based upon the empirical formula is also described.

Carbonization of polyimide film “Kapton”

Abstract Polyimide film “Kapton” C22H10O5N2 has been known to give well-oriented graphite film by high temperature heat treatment. Its carbonization behavior was followed as a function of carbonization temperature up to 1000°C. Carbon films without any appreciable deformation and cracks were obtained by carbonization up to 1000°C in between two alumina plates, though it showed large weight loss up to 40 wt% and also large shrinkage up to 23%. Carbonization proceeded in two steps; in the first step, an abrupt release of oxygen associated with remarkable weight loss and shrinkage occurred over a rather narrow temperature range of 550–650°C and in the second, nitrogen and the remaining oxygen were released gradually above 700°C. In the second carbonization step, a rapid increase of electrical conductivity, reaching about 65 S/cm after 1000°C and growth of carbon layers were observed.

A semiempirical formula for the energy dependence of the sputtering yield

Abstract We have derived a simple empirical formula for sputtering yields of monatomic metals and semiconductors, taking the threshold effect for knock-on of atoms into account in the Sigmund theory. It is found that this semi-empirical formula fits to the energy dependence of the sputtering yields over a wide range of ion-target combinations. The best fit values of the parameters for the formula are given as a function of the ratio of the mass of target atoms and the mass of incident ions.

EFFECT OF SEGREGATION ON PREFERRED SPUTTERING OF ALLOYS

The effects of segregation on the preferred sputtering of alloy surfaces have been analyzed. The top layer of the surface is regarded as behaving differently from the layers beneath the second layer: segregation and dissolution rate constants between the top and second layers are included in the kinetic equations explicitly. It is shown that the fundamental kinetic equations are of a form similar to that developed by Ho, Webb, Carter and Collins, even though the coefficients of the equations are different. The time-dependent solution of the kinetic equations shows that the segregation rate constant plays an important role in the build-up of the steady state. Similarly to the previous result, the altered layer is shown to have a thickness of the diffusion constant divided by the velocity of the recession of the surface. The steady-state compositions at the top and second layers relative to the bulk composition were found to be a function of three parameters : the surface enrichment factor, the diffusion constant divided by the velocity, and the ratio of the yields of atomic removal from the top layer, a correction factor being needed in a particular boundary condition. The existing experimental data on Cu-Ni, Cu-Au and some other alloys are surveyed critically in terms of the results of the theoretical calculation.

Dynamic measurements of depth profiles of hydrogen implanted into graphite at elevated temperatures

Abstract Time variations of depth profiles of hydrogen retained in graphite during and after implantation of 3 keV H 2 + ions have been studied from dynamic measurements by means of elastic recoil detection at temperatures between 300 and 1000 K and at ion fluxes of 4 × 10 13 , 2 × 10 14 and 4 × 10 14 / cm 2 s . It is shown that the maximum steady state concentration of hydrogens is about 6 × 10 22 / cm 3 at room temperature and is by a factor of 1.7 higher than the maximum static concentration after implantation. It is also shown that the depth profiles have the maximum at the projected range of implanted ions, the width of the profile broadens gradually inwards as the fluence and the temperature increase and the shapes of steady state profiles are almost the same as those of static profiles. By analytically solving mass balance equations for hydrogens, in which diffusion, trapping, ion-induced detrapping and recession of the surface are taken into account, the experimental steady state depth profiles are demonstrated to be well reproduced. The diffusion constants of hydrogen are determined as a function of temperature by fitting the calculated to the experimental depth profiles.

Dechanneling of MeV Protons from Axial and Planar Channels of Germanium Crystal

Energy spectra of protons backscattered from the surface of germanium single crystals are measured. The dechanneling rate of channeled protons into random direction and the stopping power ratio of the aligned to the random beam are obtained .The stopping power ratios for 1.5 MeV protons in the , , and directions are 0.31±0.04, 0.40±0.06, and 0.43±0.06 respectively. The dechanneling rate of 1.5 MeV protons at the axial channel in which atoms in the row are uniformly spaced is proportional to the reverse of the square of the critical angle. The theoretical values of the dechanneling rate are calculated using the diffusion model and compared with the experimental valules. The experimental dechanneling rate in the direction is considerably lower than the theoretical value.

Suppression of sputtering of nickel by coverage with self-sustaining thin segregated carbon layers

Abstract Sputtering of two-layered films composed of nickel (~5000 A) and nickel carbide (~1500 A) at 600° C by 5 keV Ar + bombardment on the nickel side has been studied using Rutherford backscattering of 1.3 MeV H + ions. It is found that the removal rate of nickel atoms from specimens is dependent on ion current density and that the removal rate of nickel atoms is very much smaller than that of carbon atoms when the ion current density is low. During ion bombardments at a low current density carbon segregation by a thickness of nearly two monolayers is observed at the nickel surface. Thus suppression of the removal rate of nickel atoms is ascribed to coverage of the nickel surface with segregated carbon atoms which are continuously supplied by diffusion through the nickel film from the carbide layer.

Backscattering coefficients of H, D, and He ions from solids

Abstract Experimental data on the number-backscattering coefficient R N , the energy-backscattering coefficient R E , and the mean fractional energy r E of backscattered particles are tabulated for H, D, and He ions normally incident on elemental solids. References through 1981 are covered. The dependence of R N and R E on incident energy is shown graphically for energies from about 10 eV to 100 keV by plotting the experimental data and the empirical formulas of Tabata et al. Graphs are provided for 36 elemental targets of atomic numbers from 6 to 92.

Effects of foreign metals on the thermal stability of noble metal adsorbates at the Si(111) surface

Abstract Effects of monolayer foreign metals on the thermal stability of noble metal adsorbates (Au, Ag, and Cu) at the Si(111) surface have been studied by means of LEED-AES-RBS techniques. It is found that the thermal stability of Au atoms at the Si(111) surface does not change due to deposition of both Ag and Cu atoms and subsequent annealing. On the other hand, it is found that the thermal stability of Ag at the Si(111) surface decreases due to coexistence of Au atoms on the post annealing but is not influenced by coexistence of Cu atoms. Moreover, it is shown that Cu atoms deposited on the Si(111)-√3 × √3-Ag and √3 × √3-Au surfaces become very unstable on the post annealing.

Thermal stability of two-dimensional atomic structures of Au-Ag adsorbates on Si(111) surfaces

Isochronal annealing of Au–Ag binary adsorbates at the Si(111) surface at temperatures from 200 to 430 °C has been studied by means of low energy electron diffraction, Auger electron spectroscopy, and Rutherford backscattering spectroscopy techniques. It was found that a √3×√3−(Au,Ag) structure was formed by annealing for 15 min at 200 °C Ag atoms on the Si(111)–5×1‐Au surface at Au coverages less than 0.60 ML, and that the √3×√3 structure was kept until Ag decreases from 0.70 ML down to 0.18 ML due to the annealing, while the initial Au coverage was kept. Furthermore, when the Ag atoms, which are more than 0.60 ML, on the Si(111)–√3×√3‐Au surface were annealed at 200 °C, a new structure of 2√3×2√3‐(Au,Ag) was observed. Then, the √21 ×√21R(±10.89 °)‐(Au,Ag) structure was formed by prolonged isochronal annealing and was kept until Ag decreases down to 0.12 ML, while the initial Au coverage was kept. From analysis of the decay curves of Ag coverage, it was determined that the activation energies for Ag atom...