D. L. Williamson

Metastable phase formation and enhanced diffusion in f.c.c. alloys under high dose, high flux nitrogen implantation at high and low ion energies

The use of elevated target temperatures near 400 °C during high flux ion implantation of N2+ at energies ranging from 60 keV to 0.4 keV leads to a metastable, f.c.c. high nitrogen solid solution phase induced in austenitic (f.c.c.) Cr-containing stainless steels. This phase has not been produced in an f.c.c. Ni-Fe alloy containing no Cr. Penetration depths of the N are significantly larger than expected on the basis of known diffusion coefficients of N in Cr-containing stainless steels or in pure f.c.c.-Fe. X-ray diffraction data suggest unusual differences in N penetration and concentration depending on the f.c.c. grain orientation. Consideration is given to possible residual stresses induced by N expansion of the lattice and to the anisotropic elastic constants for austenitic stainless steels. The amount of N in interstitial solid solution approaches 40 at.% under the lower energy, higher flux conditions, but only in the (200) crystallographic planes parallel to the surface. The N-expanded f.c.c. phase with the highest amount of N is found by conversion electron Mossbauer spectroscopy to be magnetic in AISI 304 and 310 stainless steels. A comparison of our results with those from related methods such as conventional plasma ion nitriding and pulsed plasma ion implantation is made to demonstrate that the observed metastable solid solution phase is often produced by other methods provided that appropriate temperatures and processing times are used. The similar penetration depths of N in the f.c.c. Cr-containing stainless steels for the various methods are consistent with thermal diffusion and metastable solubility that are enhanced by the Cr. The metastability is associated with the low Cr mobility below about 450 °C and the strong N-Cr bond. Evidence for vacancy-enhanced diffusion is not found and we see at present no advantage to using ion energies higher than about 2 keV at these elevated temperatures for producing surfaces with optimized tribological behavior based on the extremely high strength, metastable, N solid solution phase.

Structural, defect, and device behavior of hydrogenated amorphous Si near and above the onset of microcrystallinity

High-hydrogen-diluted films of hydrogenated amorphous Si (a-Si:H) 0.5 μm in thickness and optimized for solar cell efficiency and stability, are found to be partially microcrystalline (μc) if deposited directly on stainless steel (SS) substrates but are fully amorphous if a thin n layer of a-Si:H or μc-Si:H is first deposited on the SS. In these latter cases, partial microcrystallinity develops as the films are grown thicker (1.5–2.5 μm) and this is accompanied by sharp drops in solar cell open circuit voltage. For the fully amorphous films, x-ray diffraction (XRD) shows improved medium-range order compared to undiluted films and this correlates with better light stability. Capacitance profiling shows a decrease in deep defect density as growth proceeds further from the substrate, consistent with the XRD evidence of improved order for thicker films.

Phase and composition depth distribution analyses of low energy, high flux N implanted stainless steel

The phase and composition depth distributions of a low‐energy (0.7 keV), high‐flux (2.5 mA/cm2) N implanted fcc AISI 304 stainless steel held at 400 °C have been investigated by step‐wise Ar+ beam sputter removal in conjunction with conversion electron Mossbauer spectroscopy and x‐ray diffraction (XRD). A metastable, fcc, high‐N phase (γN), with both magnetic and paramagnetic characteristics, was found to be distributed in the N implanted layer generated by the low‐energy, elevated temperature, implantation conditions. The magnetic γN was found to be ferromagnetic and was distributed in the highest N concentration region of the implanted layer (the top 0.5 μm) while the paramagnetic γN becomes predominant below 0.5 μm, where the N content is only slightly lower. The ferromagnetic state is linked to large lattice expansions due to high N contents (∼30 at.%) as determined by XRD and electron microprobe. The relatively uniform XRD N distribution to a depth of ∼1 μm suggests a sensitive dependence of the magn...

A comparative study of beam ion implantation, plasma ion implantation and nitriding of AISI 304 stainless steel

Abstract This paper presents the results of a comparative study using beam ion implantation (BII), plasma ion implantation (PII), ion nitriding and gas nitriding of AISI 304 stainless steel. We have demonstrated that under controlled conditions (the same treatment times of 30 and 60 min, and the same treatment temperature of 400 °C), the microstructures produced by all four techniques are similar, being mainly the formation of nitrogen in solid solution (γ N phase). However, the concentrations of nitrogen and the detectable depths of the nitrogen-enriched layers are significantly different, depending on the process. Both BII and PII produce thick nitrogen-enriched layers (greater than 1 μm) at high concentrations (20–30 at.%) compared with either ion nitriding or gas nitriding (layers less than 1 μm thick with low nitrogen concentrations). As a result, the load-bearing capacity after either BII or PII is much greater than after either ion or gas nitriding. It has also been found that high current density implantation is crucial for the formation of the thick N-enriched layers.

Hydrogen dilution profiling for hydrogenated microcrystallinesilicon solar cells

The structural properties of hydrogenated microcrystalline silicon solar cells are investigated using Raman, x-ray diffraction, and atomic force microscopy. The experimental results showed a significant increase of microcrystalline volume fraction and grain size with increasing film thickness. The correlation between the cell performance and the microstructure suggests that the increase of grain size and microcrystalline volume fraction with thickness is the main reason for the deterioration of cell performance as the intrinsic layer thickness increases. By varying the hydrogen dilution in the gas mixture during deposition, microstructure evolution has been controlled and cell performance significantly improved.

The nitrogen transport in austenitic stainless steel at moderate temperatures

A model for the transport of nitrogen in austenitic stainless steel at temperatures around 400 °C is presented and discussed. The model considers the diffusion of nitrogen under the influence of trapping and detrapping at trap sites formed by local chromium. Nitrogen depth profiles simulated on the basis of the model with diffusion and detrapping activation energies of 1.1 and 1.45 eV, respectively, are in good agreement with experimental nitrogen profiles.

Microvoids in amorphous Si1−xCx:H alloys studied by small‐angle x‐ray scattering

The microstructure of hydrogenated amorphous silicon‐carbon alloys has been analyzed by small‐angle x‐ray scattering, infrared absorption, and density measurements. Decreasing density with C incorporation is due to microvoids about 0.6 nm in average radius, which are either approximately spherical in shape or randomly oriented nonspheres. The microvoid number density increases from about 5×1019/cm3 for a‐Si:H to about 4×1020/cm3 for a‐Si0.7 C0.3 :H. The CH3 species probably causes the enhanced microvoid formation in these alloys. A large fraction of the microvoid surfaces is not hydrogenated.

Effect of microvoids on initial and light‐degraded efficiencies of hydrogenated amorphous silicon alloy solar cells

Using a combination of infrared absorption and small‐angle x‐ray scattering on hydrogenated amorphous silicon alloy films and efficiency measurements of solar cells with intrinsic layers prepared under nominally identical conditions to those for the deposition of the films, we observe a correlation between microstructure in the films and solar cell performance. With increasing microvoid density, both the initial and light‐degraded performance of solar cells are found to deteriorate.

Structural characterization of zinc stannate thin films

Polycrystalline thin films of Zn2SnO4 were deposited by rf magnetron sputtering onto glass substrates. Films were characterized by θ–2θ x-ray diffraction and by 119Sn conversion electron Mossbauer spectroscopy. The films were randomly oriented in a cubic spinel structure. Comparison of x-ray diffraction peak intensities with structure-factor-calculated peak intensities confirmed that the films were in an inverse spinel configuration. Mossbauer studies detected two distinct Sn4+ octahedral sites. These distinct sites may be induced by distortions in the lattice associated with equally distinct Zn2+ octahedral sites. A model is suggested to explain that the relatively low electron mobility of Zn2SnO4 may be associated with disorder on the cation octahedral sites. This may disrupt transport between edge-sharing d10s0 electronically configured cations.

solid solution strengthening of stainless steel surface layers by rapid, high-dose, elevated temperature nitrogen ion implantation

Abstract Extremely strong and wear-resistant surfaces of AISI 304 and 310 stainless steels have been produced by rapid, high-dose nitrogen ion implantation at elevated temperatures. X-ray diffraction, conversion electron Mossbauer spectroscopy, and Auger electron spectroscopy reveal that this behavior is due to thick layers (> 1 μm) of austenite which contain as much as 20 at% N in solid solution after implantation of 2 × 1018 N atoms/cm2 into samples held near 400°C. Much larger retained doses are obtained at the elevated temperatures.