Self-Diffusion in Amorphous Silicon by Local Bond Rearrangements

Abstract

Experiments on self-diffusion in amorphous silicon (Si) were performed at temperatures between 460 to 600° C. The amorphous structure was prepared by Si ion implantation of single crystalline Si isotope multilayers epitaxially grown on a silicon-on-insulator wafer. The Si isotope profiles before and after annealing were determined by means of secondary ion mass spectrometry. Isothermal diffusion experiments reveal that structural relaxation does not cause any significant intermixing of the isotope interfaces whereas self-diffusion is significant before the structure recrystallizes. The temperature dependence of self-diffusion is described by an Arrhenius law with an activation enthalpy Q=(2.70±0.11) eV and preexponential factor D_{0}=(5.5_{-3.7}^{+11.1})×10^{-2} cm^{2} s^{-1}. Remarkably, Q equals the activation enthalpy of hydrogen diffusion in amorphous Si, the migration of bond defects determining boron diffusion, and the activation enthalpy of solid phase epitaxial recrystallization reported in the literature. This close agreement provides strong evidence that self-diffusion is mediated by local bond rearrangements rather than by the migration of extended defects as suggested by Strauß et al. (Phys. Rev. Lett. 116, 025901 (2016)PRLTAO0031-900710.1103/PhysRevLett.116.025901).

Atomic and electronic structures of a vacancy in amorphous silicon

“…The most probable value for the formation energy of a vacancy in a-Si ranges from 2.1 eV to 2.7 eV. Assuming a low migration energy, as is the case for a vacancy in c-Si, these values compare fairly well with recent self-diffusion measurements in a-Si 25 . Another interesting aspect of the distribution is its continuity from slightly above 0 eV to almost 4 eV.…”

Evidence for Glass Behavior in Amorphous Carbon

“…This resemblance is not unreasonable given that liquid carbon fundamentally involves breaking of bonds. This correspondence between activation energy and bond strength is also found in amorphous silicon where experiments [31] show that the self-diffusion constant also follows an Arrhenius relation. The measured value of E a is 2.7 eV, of similar order to the dissociation energy of a silicon σ-bond (2.3 eV [30]).…”

Growth‐Mode‐Modulated Defects in Non‐Hydrogenated a‐Si Thin Films for Photothermal Conversion

“…[ 24–27 ] Built on the advantage of structure disorder favorable for band‐edge absorption, nanostructured a‐Si is a good candidate for cost‐effective SAC on account of the efficient absorption in the solar spectrum range and the structural stability in air below 500 °C. [ 28,29 ] As has been reported, the multilayered SAC with a‐Si solar absorbing layer achieved the α sol of 0.83 and ε 873 K of 0.14 ( α sol / ε 873 K = 5.93) prepared by chemical vapor deposition (CVD), [ 30,31 ] and improved spectral selectivity from 13.7 to 46.5 can be realized after air‐annealing at 400 °C when deposited by magnetron sputtering. [ 32 ] Nevertheless, the application prospect of nanostructured a‐Si for SAC is also determined by the photoexcited behavior after electronic transition.…”