Aae Alquin Stevens

Time-resolved cavity ring-down spectroscopic study of the gas phase and surface loss rates of Si and SiH3 plasma radicals

Abstract Time-resolved cavity ring-down spectroscopy (CRDS) has been applied to determine gas phase and surface loss rates of Si and SiH 3 radicals during plasma deposition of hydrogenated amorphous silicon. This has been done by monitoring the temporal decay of the radicals densities as initiated by a minor periodic modulation applied to a remote SiH 4 plasma. From pressure dependence, it is shown that Si is reactive with SiH 4 [(1.4±0.6)×10 −16 m −3 s −1 reaction rate constant], while SiH 3 is unreactive in the gas phase. A surface reaction probability β of 0.9 β ⩽1 and β =0.30±0.05 has been obtained for Si and SiH 3 , respectively.

Surface reactions of hydrocarbon radicals: suppression of the re-deposition in fusion experiments via a divertor liner

The formation of re-deposited layers consisting of hydrocarbon compounds is of major concern for the development of next-step devices, because the hydrogen bound in these layers contributes to tritium retention in a future fusion reactor. This film formation might be controlled by using a liner in the divertor pump duct to trap or transform neutral growth precursors before they deposit in remote areas of the vacuum vessel. For the understanding of film formation in such a liner the knowledge of sticking coefficients of the growth precursors is mandatory. Therefore, experiments based on the cavity technique were performed to measure surface loss probabilities of hydrocarbon radicals. In addition sticking coefficients of methyl radicals were measured directly applying well-characterized quantified radical beams. To simulate film formation in a liner, a test experiment was performed consisting of a stainless steel tube, which was exposed to a low-temperature plasma from acetylene. The variation of the film thickness along the inner surface of this tube after plasma exposure can be described by a flux comprising of different neutral hydrocarbon radical species using the previously determined sticking coefficients. Suggestions for the control of the formation of re-deposited layers are made and a possible design of such a divertor liner is discussed.

Surface roughness in XeF2 etching of a-Si/c-Si(100)

Single wavelength ellipsometry and atomic force microscopy (AFM) have been applied in a well-calibrated beam-etching experiment to characterize the dynamics of surface roughening induced by chemical etching of a ∼12nm amorphous silicon (a-Si) top layer and the underlying crystalline silicon (c-Si) bulk. In both the initial and final phase of etching, where either only a-Si or only c-Si is exposed to the XeF2 flux, we observe a similar evolution of the surface roughness as a function of the XeF2 dose proportional to D(XeF2)β with β≈0.2. In the transition region from the pure amorphous to the pure crystalline silicon layer, we observe a strong anomalous increase of the surface roughness proportional to D(XeF2)β with β≈1.5. Not only the growth rate of the roughness increases sharply in this phase, also the surface morphology temporarily changes to a structure that suggests a cusplike shape. Both features suggest that the remaining a-Si patches on the surface act effectively as a capping layer which causes th...

Magnetization and magnetic anisotropy of BaFe12 − xTixO19 hexaferrites

Abstract Saturation magnetization and magnetic anisotropy data of BaFe 12 − x Ti x O 19 single crystals are presented in relation to the crystal chemistry of these compounds. For x = 0.2 an enhancement of the anisotropy is observed, whereas for higher Ti-concentrations the temperature coefficient of the magnetization decreases substantially.

Molecular dynamics simulations of Ar + bombardment of Si with comparison to experiment

The authors present molecular dynamics (MD) simulations of energetic Ar+ ions (20–200eV) interacting with initially crystalline silicon, with quantitative comparison to experiment. Ar+ bombardment creates a damaged or amorphous region at the surface, which reaches a steady-state thickness that is a function of the impacting ion energy. Real-time spectroscopic ellipsometry data of the same phenomenon match the MD simulation well, as do analogous SRIM simulations. They define positional order parameters that detect a sharp interface between the amorphous and crystalline regions. They discuss the formation of this interesting feature in the simulation, and show that it provides insight into some assumptions made in the analysis of experimental data obtained by interface-sensitive surface spectroscopy techniques.

Amorphous silicon layer characteristics during 70-2000 eV Ar+-ion bombardment of Si(100)

Spectroscopic ellipsometry (SE) has been applied to characterize the damaged, amorphous silicon (a-Si) layer created by Ar+-ion bombardment in the ion energy range of 70–2000eV impinging at 45° angle of incidence on Si(100). The dielectric functions of a-Si during ion bombardment have been determined using the Tauc-Lorentz model for the dielectric functions ϵ1 and ϵ2. The dielectric functions resemble literature reports on a-Si-like dielectric functions. The a-Si layer thickness under ion bombardment conditions reaches values from ≈17A at 70eV up to ≈95A at 2000eV. These values compare reasonably well with SRIM and molecular dynamics simulations. The surface roughness, as determined with SE, is typically 5–15A during ion bombardment, with a minimum roughness at Eion=250eV. The creation of the amorphous silicon top layer upon 70eV Ar+-ion bombardment with an ion flux of 0.07MLs−1 has been resolved using real-time spectroscopic ellipsometry. The creation of the amorphous layer shows a double exponential ion...

Circular dichroism in second harmonic generation from oxidized Si (001)

Circular dichroism in second harmonic generation (SHG) is often related to molecules and materials with chiral structures. In this letter, we report circular dichroism in SHG from anisotropic achiral oxidized Si (001) at room temperature. The dichroism value depends on the azimuthal angles of the crystal axes. Due to the simple nonlinear susceptibility elements involved, we were able to attribute the dichroism in SHG to interference between particular terms of the bulk electric quadrupole and surface dipole contributions. The presence of a phase shift between the bulk and surface SHG is required to observe circular dichroism.

Roughening during XeF2 etching of Si(100) through interface layers: H:Si(100) and a-Si∕Si(100)

Real-time spectroscopic ellipsometry has been applied in situ in an Ar+∕XeF2 beam-etching experiment to study the roughening of Si(100) etched by XeF2 at room temperature. The role of initial surface conditions has been examined. For the etching of hydrogen-terminated (H:)Si(100), the roughness evolution as a function of XeF2 dose can be characterized by an initially fast roughening phase followed by a slower, final roughening phase. Similar behavior is observed when etching through an amorphous silicon (a-Si) layer on top of crystalline Si(100) bulk as obtained by sputter cleaning of Si(100) substrates. These observations can be explained as follows. Both H termination and a-Si lead to patch formation on the surface where etching is impeded and hence, high aspect-ratio etch pits develop. The quantitative differences in roughening can then be attributed to the duration and timing of the influence of the H-terminated and a-Si patches on the etch process until H-bonded Si surface atoms or a-Si are totally r...

Surface roughness in XeF2 etching of a-Si/c-Si(100). [Erratum to document cited in CA143:121009]

On page 134, Figure 16 is incorrect; the correct version of the figure is given. Abstract not available.