Structure of Amorphous Silicon and Silicon Hydrides

Abstract

but interior ones do not.The full 0 2 -C potential gives results qualitatively similar to those calculated with use of the spherically symmetric expression except that the 0 2 -C anisotropics cause the 0 2 molecules to lie flat on the substrate and the triangular structure is distorted by the anisotropy. Some results for this case are given in parentheses in Table I. For AT^6, T 0^4 K; and for N = l, T Q^6 K. In the plastic crystallite phase, T 0^T <T M9 the 0 2 molecules relax from the distorted configurations to 2D structures constructed by connecting equilateral triangles. This is caused by the 0 2 rotations averaging out the anisotropics responsible for the distortion. There is no indication of registry.To summarize our work on infinite 0 2 monolayers, it is significant that the nearest-neighbor separation agrees to within 3% of experiment and is almost exactly the same for those calculations on bulk ]8-0 2 that used the English-Venables potential. 6 This, coupled with our determination of an equilateral-triangular lattice and 0 2 orientations perpendicular to the substrate, supports the view that 2D monolayers have the same character as the densest, packed plane of bulk 0 2 . Depending on the strength of the 0 2 -C anisotropy, a possible new low-density state at p-1.29 is also predicted, where the 0 2 lay flat on the substrate. 10 In the fractional-monolayer p«l region, our 2D-cluster model gives structures, nearestneighbor distances, orientations, melting temperatures, etc., that are consistent with experi-Amorphous silicon (a-Si) and hydrogenated a -Si have received considerable attention very recently. 1 This has been motivated by recent interest ment. 1 " 4 ' 10 Particularly unusual are the well-defined orientational order-disorder transitions into a plastic crystallite phase. Our results indicate that monolayer 0 2 will gasify at T-55 K. which agrees qualitatively and quantitatively with our predicted low-density "laying flat'' state. This contradicts earlier experiments (Ref. 1). There is also new evidence that the 6-phase melting line approaches p = 0 at T^25Kas we predict; see Figo 1 0 in the general subject of amorphous materials as well as by the discovery that incorporation of hydrogen changes the conductivity of a-Si by sever-Neutron scattering measurements have been made on pure, hydrogenated, and deuterated samples of amorphous silicon (a-Si) in the wave-vector range 0.007-8.75 A" 1 . Small-angle data indicate structures in the samples of average radius of gyration as large as 270 A. Large-angle data show that for the concentrations we have measured (14%), the structure of a-Si is not altered by the incorporation of large amounts of H or D. The silicon-hydrogen and silicon-deuterium partial structure factors have also been obtained.

Microstructural analysis of nanoporous paracrystalline atomistic models of amorphous silicon

“…Fig. 2 illustrates this fact and shows two experimental structure factors obtained for two a-Si specimens of different origins [9,25], where only the sputtered a-Si shows the intense small-angle scattering.…”

“…2. For comparison, this figure shows the S(k) obtained experimentally on ion-implanted a-Si by Laaziri et al [25] as well as that derived from neutron diffraction measurements on sputtered a-Si by Postol et al [9]. The characterization of the intense small-angle scattering (k < 1 Å À1 ) in our amorphous models has been performed by means of the Guinier plot ln[S(k)] versus k 2 [26].…”

“…Structural inhomogeneities such as voids are commonly believed to result in the density of a-Si being lower than its corresponding crystalline phase. The characteristics of voids in a-Si have been inferred from intense small-angle scattering of neutrons [9,10], electrons [11] and X-rays [12,13]. These experiments measured a static structure factor displaying a rapid increase for wave vector less than 1 Å À1 .…”

“…The experimental data are from Laaziri et al[24] (continuous line) and Postol et al[9] (dots). Traces are vertically offset by two for clarity.…”

Microstructure of amorphous-silicon-based solar cell materials by small-angle x-ray scattering. Annual subcontract report, 6 April 1994--5 April 1995

“…Measurements up to x d . 6 have been done with a-Sil,,Ge,:H alloys [15,16,26]. The observed density deficits compared to c-Si (2.329 g/cm3), pure a-Si (2.287 g/cm3) [40], c-Sil,,Ge, alloys [41], diamond (3.52 g/cd), graphite (2.26 g/cm3), or Sic (3.22 g/cm3) can be helpful in interpretation of the SAXS and in determining the average effect of H alloying.…”