Extreme Ultraviolet Transmission of Crystalline and Amorphous Silicon

Abstract

The small polarization dependence of the fundamental Much attention has recently been given to the comparison of electronic-state densities in amorphous and crystalline solids. 1 ' 2 In this paper we present very high-resolution absorption spectra near the L m edge (photon energy 99.8 eV) of both amorphous and crystalline silicon. The observed spectrum for crystalline silicon is such that banddensity maxima, and even one or two critical points, can be identified upon comparison with band theory. The spectrum of the amorphous material is dramatically different in that the peaks and critical structure associated with the band details are not seen.Transmission measurements on thin films of silicon were carried out using the synchrotron continuum from the 240-MeV electron storage ring at the University of Wisconsin Physical Science Laboratory. 3 Earlier results were reported on both evaporated layers and a very small etched single crystal of silicon. 4 ' 5 The present results on evaporated films differ, in that both amorphous and crystalline samples of large area have been characterized, and signal to noise has been greatly improved. The 2-m grazing-incidence spectrometer 6 was repositioned so that its Rowland circle was in a plane perpendicular to the orbital plane of the storage ring. A marked increase in counting rate was thus achieved probably mainly as a result of improved polarization (E perpendicular rather than parallel to the plane of incidence) upon reflection direct edge in Ge reported by Fischer (Ref. 24) results from exceeding the low-field limit, and does not occur at higher thresholds where the lifetime broadening is much larger. 34 This simple picture, which neglects hybridization, Coulomb effects, degeneracy, etc., has been found to give quite accurate polarization dependences for lower energy thresholds in GaAs and InP. See Ref. 28. at the focusing mirror and grating. Scans could now be taken at highest resolution (10-/xni slitwidth) with little or no noise so as to reveal minute singularities or changes in slope.In the present investigation the samples were evaporated in high vacuum (10" 6 Torr) using a Varian 6-kW electron gun. The source-to-substrate distance was about 20 cm and the deposition rate 5 A/sec. The amorphous films were deposited onto NaCl substrates at room temperature, and samples were then floated off on water and picked up on screens. Electron-diffraction analysis of these unbacked films yielded a diffuse ring structure which was used to define a film in the amorphous state. 7 Crystalline silicon films were prepared by vacuum deposition onto fused silica substrates at an intermediate temperature, less than 500°C, followed by an anneal for several minutes at temperatures up to 1000°C. Recrystallization took place somewhere between 600 and 1000°C depending upon conditions such as film thickness, time of anneal, etc. After floating off on hydrofluoric acid, the unbacked films were analyzed in transmission by electron diffraction. Sharp rings characteristic of the {ill}, {220}, ...

Optical Properties of Nanostructured Silicon

“…The theoretically calculated radius of the core exciton at the L 2,3 edge in bulk Si is $1.6 nm [93], which agrees well with measured X-ray absorption photon yield [89,94]. The radius of the elemental Si core of the NCs studied here is, at d/2 ¼ 1.1 AE 0.2 nm [71], substantially smaller than the core exciton radius, hence X-ray absorption in the silicon core at threshold should favor an excitonic final state more tightly bound than the corresponding bulk exciton.…”

“…A broadening of absorption onset was observed by Eisebitt et al [87,88] in XAS from porous Si, and attributed to the crystallite size distribution in their samples. However, in the spectra shown in Figure 18, which are from a sample with a welldefined and narrow diameter distribution [71], a clear 0.61 eV spin-orbit splitting [89] is observed in the PPY from the NCs (magnified region of Figure 18, curve b), while such splitting may be washed out by the size inhomogeneity in the porous Si crystallites [87,88]. Single-dot luminescence spectroscopy has been used to study the emission line width of individual Si NCs and these studies have confirmed that oxidized NCs exhibit discrete energy levels rather than continuous bands [90].…”

“…Although a sharp rise in X-ray absorption is observed in bulk silicon, a shallow onset has been theoretically predicted [89,91], because the absorption threshold should be related to the CB minima s-like electronic states due to dipole selection rules. The steep rise in absorption in bulk Si at the L 2,3 threshold, which is much steeper than the predicted CB DOS, has been attributed to excitonic effects in core-electron transitions to the CB, with bound core excitons being of Wannier type [92].…”

Excitonic Effects in Core-Excitation Spectra of Semiconductors

“…The best illustration of the problem is provided by the Si L 2,3 edge, namely, transitions from the 2p core level (which is split into 2p 1͞2 and 2p 3͞2 levels) that the spectrum is a superposition of two virtually identical spectra separated by a small energy). When Brown and Rustgi [14] first reported the x-ray absorption (XAS) spectrum in 1972, they compared it with the calculated total conduction-band density of states (DOS) and noted that the latter could not account for the observations. The DOS exhibits a slow rise to a broad peak ϳ2 eV above threshold, whereas the observed spectrum exhibits a very steep rise and then a relative flattening.…”

“…Thick curve: experiment (Ref. [14]); solid curve: full calculation including electron-hole interactions with a self-consistent hole orbital; dashed curve: calculation using the Z 1 1 model; dotted curve: calculation of excitation spectrum without electron-hole interactions; dash-dotted curve: effective-mass (parabolic band) spectrum without electron-hole interactions; long-dashed curve: effective-mass spectrum with hydrogenic excitonic effects as first done by Altarelli and Dexter (Ref. [4]).…”