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 removed from the surface. Surface area increase due to the roughening can therefore be held responsible for observed trends and differences in etch rates, reaction layer thickness, and composition as a function of etch time.

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.

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.

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 ≈17Å at 70eV up to ≈95Å at 2000eV. These values compare reasonably well with SRIM and molecular dynamics simulations. The surface roughness, as determined with SE, is typically 5–15Å 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-dose dependence: a fast, initial period of a-Si creation, with 1∕e constant Δτ1=2ML, and a slower period, Δτ2=9ML, until the matrix is fully amorphous after ∼30ML of Ar+ dosing. Relaxation of the a-Si top layer has been observed after the ions are switched off and has been analyzed with a stretched-exponential decay as a function of time, which is characteristic for a defect-controlled relaxation in the bulk a-Si layer. The corresponding time constant τ is found to be ∼360s, which is typically observed for self-annealing in amorphous silicon materials.

Spectroscopic and real time optical second-harmonic generation (SHG) has been applied to gain insight into the surface and interface processes during low-energy $(70--1000\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ ${\mathrm{Ar}}^{+}$-ion bombardment of $H$ terminated Si(100). The ${\mathrm{Ar}}^{+}$-ion bombardment of the crystalline silicon $(c\text{\ensuremath{-}}\mathrm{Si})$, which creates a layer of amorphous silicon $(a\text{\ensuremath{-}}\mathrm{Si})$, has been studied in the SH photon energy range of $2.7--3.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The time-resolved SHG signal has been observed to increase with an order of magnitude upon ion bombardment. Spectroscopic SHG during ion bombardment and after subsequent $\mathrm{Xe}{\mathrm{F}}_{2}$ dosing indicates that the SHG signal has both a contribution generated at the buried interface between the $a\text{\ensuremath{-}}\mathrm{Si}$ and the $c\text{\ensuremath{-}}\mathrm{Si}$ and an additional contribution originating from the $a\text{\ensuremath{-}}\mathrm{Si}$ surface. By separating these contributions using a critical point model it has been shown that the SHG spectra consist of a sharp resonance at $3.36\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ with a linewidth of $0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ at the buried $a\text{\ensuremath{-}}\mathrm{Si}∕c\text{\ensuremath{-}}\mathrm{Si}$ interface and a much broader resonance at a resonance energy of $3.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ with a linewidth of $0.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ at the $a\text{\ensuremath{-}}\mathrm{Si}$ surface. The former resonance is identified to originate from ${E}_{0}^{\ensuremath{'}}∕{E}_{1}$ transitions between bulk electronic states in the $c\text{\ensuremath{-}}\mathrm{Si}$ that are modified due to the vicinity of the interface, while the latter resonance is caused by transitions related to Si-Si bonds in the surface region of the $a\text{\ensuremath{-}}\mathrm{Si}$. The time-resolved dynamics of the SHG signal can help in understanding the mechanism of ion-beam and plasma etching of silicon.

Spectroscopic second harmonic generation (SHG) has been applied to study thin layers of amorphous silicon in the second harmonic photon energy range of 2.7 - 3.5 eV. The layers were synthesized by hot-wire CVD of hydrogenated amorphous silicon (a-Si:H) and by Ar+ ion bombardment of crystalline silicon (c-Si). For a-Si:H a broad feature has been observed in the SHG spectrum. It is discussed that the SHG signal originates from strained Si-Si bonds in the surface or interface region of the a-Si:H film. For H-terminated Si(100) both in spectroscopic and real-time experiments the SHG signal increases by an order of magnitude upon bombardment with 70 eV Ar+ ions. We argue that the SHG signal from the amorphized Si layer is generated mainly at the buried interface with c-Si, while an additional contribution seems to originate from the amorphous Si surface region. From the combination of spectroscopic and real-time SHG studies insight into the role of strained bonds in the growth and etching processes of silicon can be gained. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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 the growth of deep trenches in the c-Si. The ellipsometry data on the roughness are corroborated by the AFM results, by equating the thickness of the rough layer to 6σ, with σ the root-mean-square variation of the AFM’s distribution function of height differences. In the AFM data, the anomalous behavior is reflected in a too small value of σ which again suggests narrow and deep surface features that cannot be tracked by the AFM tip. The final phase morphology is characterized by an effective increase in surface area by a factor of two, as derived from a simple bilayer model of the reaction layer, using the experimental etch rate as input. We obtain a local reaction layer thickness of 1.5 monolayer consistent with the 1.7ML value of Lo et al. [Lo et al., Phys. Rev. B 47, 648 (1993)] that is also independent of surface roughness.

Time-resolved cavity ring-down spectroscopy (CRDS) has been applied to determine gas phase and surface loss rates of Si and SiH3 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 SiH4 plasma. From pressure dependence, it is shown that Si is reactive with SiH4[(1.4±0.6)×10−16m−3s−1 reaction rate constant], while SiH3 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 SiH3, respectively.

Saturation magnetization and magnetic anisotropy data of BaFe12 − xTixO19 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.