Release of nitrogen from SiOxNy films during RBS measurement

We have found that nitrogen atoms are released very rapidly from ultrathin SiO_xN_y films (2.6 nm) during RBS measurement with 500 keV He⁺ ions. The release behavior strongly depends on the preparation technique of the SiO_xN_y films. There is no release from the film prepared by thermal nitridation of SiO₂, while 80% of the nitrogen atoms are released from the film prepared by plasma nitridation at a fluence of 1×10¹⁶ cm⁻². The release cross-section for plasma SiO_xN_y films is of the order of 10⁻¹⁶ cm². This large cross-section cannot be explained by a simple recoil mechanism. The nitrogen release is also observed under irradiation with 5–10 keV electrons though the cross-section is of the order of 10⁻¹⁹ cm². These findings suggest that the observed nitrogen release is an electronic excitation induced process.     

Density measurement of amorphous SixGe1 − x alloys

The atomic density of amorphous Si_xGe_{1 − x} alloys (x = 1, 0.85, 0.67, 0.50, 0.20 and 0) has been measured. Mono-crystalline Si_xGe_1−x layers Were implanted with 1.50–2.75 MeV Si²⁺ and Ge²⁺ ions to produce the amorphous material. Using surface profilometry and RBS/channeling, it was found that amorphous alloys are less dense than the crystalline alloys, and that Vegard's law underestimates the a-Si_xGe_1−x density.     

Characterization of Si/GexSi1 − x heterojunction bipolar transistors formed by Ge ion implantation in Si

Epitaxial Si/Ge_xSi_{1 − x} heterojunctions were formed by high dose Ge ion implantation in Si followed by rapid thermal annealing at 1000°C for 10 s. This technique was adopted to fabricate Si/Ge_xSi_{1 − x} heterojunction n-p-n bipolar transistors (HBT) using a self-aligned, double polycrystalline silicon process commonly used for fast Si bipolar transistors. The devices are characterized by a 60 nm wide neutral base with a Ge concentration peak of ≈ 7 at.% at the base-collector junction. Good static and dynamic electrical characteristics are demonstrated and discussed.     

Electrical and optical properties of Co ion implanted a-Si1 − xCx:H alloys

Low resistivity a-Si_1 − xC_x:H alloy films have been formed by high dose Co⁺ ion implantation. The influence of the carbon content of the films on the resistivity has been studied and the lowest values, of the order of 10 Ω/Sq, have been observed for the carbon free films. Even lower resistivities, a further reduction of up to 50%, have resulted from annealing at temperatures up to 500°C. Changes in the optical and structural properties of the implanted a-Si_1 − xC_x:H films have been studied by means of IR and Raman spectroscopy. Results show that the implantation produces considerable structural and chemical modifications. The formation of, and the transition to, a possible CoSi₂ phase has been observed by examining the IR and Raman spectra as a function of implant dose.     

2 MeV Si ion implantation damage in relaxed Si1−xGex

The damage produced by implanting, at room temperature, 3 μm thick relaxed Si_1−xGe_x layers with 2 MeV Si⁺ ions has been measured as a function of Ge content (x = 0.04, 0.13, 0.24 or 0.36) and Si dose in the dose range 10¹⁰–10¹⁵ cm⁻². The accumulation of damage with increasing dose has been studied as a function of Ge content by Rutherford Backscattering Spectrometry, Optical Reflectivity Depth Profiling and Transmission Electron Microscopy and an increased damage efficiency in Si_1−xGe_x with increasing x is observed. The characteristics of implantation-induced defects have been investigated by Electron Paramagnetic Resonance. The results are discussed in the context of a model of the damage process in SiGe.     

Rapid thermal annealing of arsenic implanted relaxed Si1−xGex

The electrical activity and redistribution during rapid thermal annealing (RTA) of high concentrations of As implanted into epitaxially grown, relaxed Si_1−xGe_x for x≤0.5 have been studied as a function of composition x and RTA parameters. At a given RTA temperature the maximum carrier concentration decreases and the redistribution increases with increasing x. Maximum carrier concentrations and junction depths as a function of composition and RTA parameters are given.     

RBS investigations of buffer layers between YBa2Cu3O7−x and silicon

The deposition of high-quality high-T_c superconducting films on silicon wafers for future hybrid electronic devices is strongly hampered by the interdiffusion between films and substrate. This effect degrades the superconducting properties seriously and is a strong function of temperature. Since high processing temperatures are inevitable for good films, suitable buffer layers are needed to reduce the interdiffusion. We have investigated the combinations ZrO₂/Si(100), BaF₂/Si(100), and noble-metal/TiN/Si(100) at temperatures up to 780°C in oxidizing ambient. YBa₂Cu₃O_7−x films have been deposited onto the buffer layers by laser ablation. Thereafter the interfaces have been analyzed by Rutherford backscattering. So far only ZrO₂ has demonstrated sufficient stability to serve as a buffer layer for the laser-ablated YBa₂Cu₃O_7−x films. All other combinations suffer from interdiffusion or oxidation.     

Determination of the Li/Ni ratio in LixNi1 − xO thin films by PIXE-PIGE analysis

Li/Ni ratios in Li_xNi₁ − xO films were determined by simultaneous use of the PIXE (particle-induced X-ray emission) and PIGE (proton-induced gamma-ray emission) techniques using the 3.5 MeV proton beam provided by the CISE tandem Van de Graaff accelerator. The general features of the experimental setup are presented and discussed along with the results on the dependence of Li/Ni ratios on the e-beam evaporation and thermal treatments. Preliminary results obtained by XRD (X-ray diffraction) are also discussed. The combined use of PIXE and PIGE proves to be a unique opportunity for nondestructive measurement in thin films and, through the correlation with structural data, it allows a careful optimization of the deposition process and thermal treatment parameters to be obtained.     

Diffusion and strain relaxation in Si/Si1−xGex/Si structures studied with Rutherford backscattering spectrometry

The thermal stability of strained Si/Si_1−xGe_x/Si structures grown by molecular beam epitaxy was investigated by resistive heating and in situ Rutherford backscattering spectrometry. Ge profiles obtained from a 50 nm Si_1−xGe_x layer on a Si(100) substrate capped with 50 nm Si were evaluated for different Ge concentrations after sequential heating periods at a particular temperature between 850 and 1010° C. The diffusion coefficients, calculated from the increase in signal in the tail of the Ge profile, proved to be comparable to the value for Ge in bulk Si. A more pronounced decrease of the signal at the center of the Ge profile indicated a faster diffusion within the SiGe layer which was confirmed by analysis of the FWHM of the Ge profile. Ion channeling measurements were used to characterize tetragonal strain in the buried SiGe layers. Angular scans through the 111 direction were interpreted with Monte Carlo channeling calculations and used to study strain relaxation in dislocation-free and partially relaxed layers.     

A comparative EPR study of ion implantation induced damage in Si, Si1−xGex (x ≠ 0) and SiC

Electron Paramagnetic Resonance (EPR) measurements have been made to investigate the build up of damage in silicon in relaxed crystalline Si_1−xGe_x (x = 0.04, 0.13, 0.24, 0.36) and in 6H-SiC as a result of increasing the ion dose from low levels (10¹² cm⁻²) up to values (10¹⁵ cm⁻²) sufficient to produce an amorphous layer. Si, Si_1−xGe_x (x ≠ 0) and SiC were implanted at room temperature with 1.5 MeV Si, 2 MeV Si and 0.2 MeV Ge ions respectively. A comparison is made between the ways in which the type and population of paramagnetic defects depend on ion dose for each material.     

Electrical conductivity and non-stoichiometry in the (U,Gd)O2±x system

The isothermal electrical conductivity and oxygen potential of the (U,Gd)O_2±x solid solution were measured in various oxygen partial pressure regions at 1200 °C and 1300 °C. The electrical conductivity gradually decreased with decreasing oxygen partial pressure even in the hypo-stoichiometric region. These findings were in contrast to the implication of a hypo-stoichiometry where the electrical conductivity is increased through the formation of oxygen vacancies. The (U_1−yGd_y)O_2−y/2 was defined as a new stoichiometric composition to determine the relationship between the deviation of the oxygen composition from stoichiometry and oxygen partial pressure. The dependence of the new oxygen deviation, z in (U_1−yGd_y)O_2−y/2+z, on the oxygen partial pressure corresponds to the dependence of the electrical conductivity, and thus a consistent defect structure model can be deduced from both the dependence curves. It suggests that the defect type is oxygen interstitial even below the oxygen composition of 2.     

Analysis of Cu and O in high-Tc superconductors by 7–9 MeV protons

The use of proton-induced γ-ray emission for the simultaneous analysis of Cu and O in high-T_c superconductors is demonstrated. Utilizing 7–9 MeV protons, the ratio of O relative to Cu can be determined reliably to a few percent accuracy in homogeneous bulk samples and films thicker than 130 μm using standard bulk samples of O and Cu. Results of the present method are compared with those of the analysis of YBa₂Cu₃O_7xdone by the Rutherford scattering of particles and the non-Rutherford scattering through the ¹⁶O(p,p₀)¹⁶O reaction.     

Chemical thermodynamic representation of AmO2−x

The AmO_2−x solid solution data set for the dependence of the oxygen potential on the composition, x, and temperature was retrieved from the literature and represented by a thermodynamic model. The data set was analysed by least-squares using equations derived from the classical thermodynamic theory for the solid solution of a solute in a solvent. Two representations of the AmO_2−x data were used, namely the Am_5/4O₂–AmO₂ and AmO_3/2–AmO₂ solid solution. No significant difference was found between the two, and the Am_5/4O₂–AmO₂ solution was preferred on the basis of the phase diagram. From the results the Gibbs energy of formation of Am_5/4O₂ has been derived.     

Carbon clustering in Si1−xCx formed by ion implantation

Silicon-carbon alloys were formed by multiple energy implantation of C⁺ ions in silicon and in Silicon on Sapphire (SOS). The ion fluence ranged between 5 × 10¹⁶ − 3 × 10¹⁷ ions/cm² and the energy between 10–30 keV in order to obtain constant carbon concentration into a depth of 100 nm. The carbon atomic fraction (x) was in the range 0.22–0.59 as tested by Rutherford backscattering spectrometry (RBS). Thermal annealing of the implanted films induced a transition from amorphous to a polycrystalline structure at temperatures above 850°C as detected by Infrared spectrometry (IR) in the wavenumber range 600–900 cm⁻¹. The optical energy gap and the intensity of the infrared signal after annealing at 1000°C depended on the film composition: they both increased linearly with carbon concentration reaching a maximum at the stoichiometric composition (x = 0.5). At higher carbon concentration the IR intensity saturated and the optical energy gap decreased from the maximum value of 2.2 to 1.8 eV. The behaviour at the high carbon content has been related to the formation of graphitic clusters as detected by Raman spectroscopy.     

Magnetic ordering in superconducting YBa2(Cu1−xFex)3Oy oxide

Magnetic susceptibility measurement and the zero-field muon-spin-relaxation experiment have been made for the YBa₂(Cu_1−xFe_x)₃O_y system. The susceptibility has been measured in the field strong enough for the magnetic flux to penetrate the superconductor and the resultant temperature dependence has demonstrated the field cooled effect and a cusp at low temperatures below T_c. The cusp has been observed when the high field has been applied. The dynamical depolarization rate of the muon obtained by the muon-spin-relaxation experiment has shown the maximum at the same temperature as the cusp. The results suggest that the spin glass freezing of localized magnetic moment takes place at the temperature of the cusp, which is consistent with the previous Mössbauer effect experiments. The magnetic phase diagram for the system has been obtained.     

A mechanism for the ease of slip in UO2+x

The microstructure of plastically deformed hyperstoichiometric uranium dioxide crystals has been reexamined by transmission electron microscopy. Although the U₄O₉ phase is present with its usual superlattice structure, dislocations which thread interfaces between it and the adjacent UO₂ phase exhibit uniform contrast, (i.e., no separation into superdislocations is resolved). Dislocations lying totally within the UO₂ phase are decorated by “shadows” of gray contrast, which can be removed by heating in the microscope atmosphere. These have been identified as “atmospheres” of U₄O₉. A geometrical argument, based on the coordinates of anion interstitials published by Willis, is used to explain how these “atmospheres” might account for the observed incidence of extensive cross-slip in plastically deforming UO_2+x.     

Sputtering yield of Ti1−xBx films by 2 keV deuterium bombardment

We deposited titanium borides (Ti_1−xB_x; 0.40 < x < 0.77) by the co-sputter coating method and measured their sputtering yield by 2 keV deuterium ion bombardment as a function of their chemical composition at room temperature. The total sputtering yield is found to increase with increase of the boron content in Ti_1−xB_x. The total sputtering yield of stoichiometric TiB₂ is estimated to be 2.8 × 10⁻², about the same as those reported previously. Concerning the partial sputtering yield, that of the titanium does not depend on the chemical composition, but that of the boron increases with increase of the boron content. These experimental results could be explained by assuming that the partial sputtering yield is proportional to the spatial concentration of each atom in the Ti_1−xB_x matrix.     

Range distribution of fluorine in F-implanted LiNbO3

Depth profiles of fluorine in ¹⁹F⁺implanted LiNbO³ have been accurately measured using the ₁₉F(p, γ)¹⁶O resonant nuclear reaction at E_R = 872.1 keV, with Γ = 4.2 keV. A proper convolution calculation method was used to extract the true distribution of fluorine from the experimental excitation yield curves. The experimental range distribution parameters, R_p and ΔR_p, were compared with those obtained by Monte Carlo simulation using a computer code developed recently in this group and with results obtained using the TRIM90 code. It shows that the experimental R_p values agree with the Monte Carlo simulation values very well, while the experimental ΔR_p values are larger than those obtained from the simulations. The simulation with our computer code improves the agreement between the experimental and calculated range straggling, ΔR_p.     

Delayed neutron study using ENDF/B-VI basic nuclear data

The basic nuclear data of the latest releases of ENDF/B-VI were used in preliminary calculations with the CINDER'90 nuclide inventory code to simulate the activity of fission delayed-neutron precursors. Total delayed-neutron production was obtained at times during and following pulse (0.1-ms) and equilibrium (4-hr) fission histories for each of the sixty fission systems having fission-product yields in ENDF/B-VI. The equilibrium studies — at unit fission rate for constant fission periods sufficiently long that all precursors reached saturation inventories — yielded the value for each system. Delayed-neutron production rates at 54 decay times t, extending to 500 s following a fission pulse, were fit using the STEPIT code to the pulse function R(t) = ∑a_iλ_ie^−λ_it. Results following equilibrium irradiations were fit to the equilibrium function R(∞, t) = ∑a_iλ_ie^−λ_it. It was observed that functions from fits to pulse results did not well represent equilibrium results at long cooling times. Similarly, functions fit to the equilibrium results did not well represent pulse results at short cooling times.

A comprehensive series of CINDER'90 calculations was then made for irradiation times T of 0.1 ms, 1 s, 10 s, 100 s, 1000 s, and 4 hours; results were obtained at 60 decay times t extending to 800 s following irradiation. Comprehensive calculations were made using both the 1989 Pn data of England and Brady and the new Pn data of Pfeiffer, Kratz and Möller described elsewhere in this issue. The body of results for each system was included in fits to obtain the neutron production rate R(T, t) = ∑a_ie^−λ_it(1 − e^−λ_iT) for each system. Fits were made for the traditional sum of six exponentials, with all variables free to vary; additional fits were made for a sum of eight exponentials with decay constants set to values suggested by Piksaikin. The resulting pulse functions R(t), defined by the a_i and λ_i thus obtained, accurately represent calculated delayed-neutron production when integrated with any irradiation history.

The pulse functions thus produced and other published pulse functions fit to past measurements and calculations are compared numerically at several times after fission. Reactivity effects of all functions from measurements and calculations for each of the sixty systems are indicated by the asymptotic periods following positive 10¢– 50¢ reactivity steps simulated in point-reactor kinetics calculations using the AIREK-10 code.     

Atomic layer deposition of copper thin film and feasibility of deposition on inner walls of waveguides

Copper thin films were deposited by plasma-enhanced atomic layer deposition at low temperature, using copper(I)-N,N′-di-sec-butylacetamidinate as a precursor and hydrogen as a reductive gas. The influence of temperature, plasma power, mode of plasma, and pulse time, on the deposition rate of copper thin film, the purity of the film and the step coverage were studied.The feasibility of copper film deposition on the inner wall of a carbon fibre reinforced plastic waveguide with high aspect ratio was also studied. The morphology and composition of the thin film were studied by atomic force microscopy and x-ray photoelectron spectroscopy,respectively. The square resistance of the thin film was also tested by a four-probe technique. On the basis of on-line diagnosis, a growth mechanism of copper thin film was put forward, and it was considered that surface functional group played an important role in the process of nucleation and in determining the properties of thin films. A high density of plasma and high free-radical content were helpful for the deposition of copper thin films.