Electron Spin Resonance Study of the Dangling Bond in a-Si:H and Porous Silicon
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ELECTRON SPIN RESONANCE STUDY OF THE DANGLING BOND IN a-Si:H AND POROUS SILICON T.J. MC MAHON* AND Y.XIAO* ** *NREL, Golden, CO 80401 USA, * **Univ. of Colorado, Boulder, CO 80309. ABSTRACT We compare the electron spin resonance (ESR) signal of the dangling bond in porous silicon films, produced by electrochemical etching, to the ESR signal from hydrogenated amorphous Si (a-Si:H). The anisotropy of the ESR signal in porous Si showed g values varying as for the Pb Si/Si02 interface dangling bond. The g value varies from gll - 2.0020 to gL - 2.0080 with an inhomogeneously broadened line width increasing from 1.8 to 3.8 G. A porous Si ESR powder line, with superhyperfine and strain broadening intrinsic to porous Si, is compared to the a-Si:H dangling bond line. The result is more inhomogeneous broadening of line widths parallel and perpendicular to the dangling bond axis in a-Si:H, and less anisotropy in goj- gL. No evidence was seen for light-induced metastability on a H-passivated porous Si film. INTRODUCTION AND BACKGROUND We began this study to see if the dangling bond found on the surfaces of the pores of porous Si, which is very much like the well defined, thoroughly characterized Pb defect 11, compares to the ESR signal of the dangling bond found in a-Si:H. Brodsky and Title [21 originally proposed that dangling bonds found in a-Si:H were a defect residing on surfaces of microclusters or microvoids, because of similarities to the ESR signal observed for the dangling
bond on crystal Si surfaces. Later, this model for the defect was abandoned because it was found that (111) surfaces of Si were nearly free of dangling bonds and could not adequately explain the spin densities found in the unpassivated films made at that time. [31 Since that time, the dangling bond in a-Si:H has been viewed as an isolated, topological bulk defect. This defect occurs at sites where it becomes more favorable to form a dangling bond than to increase the stress in local bonds required by four-fold coordination. Now, more precise ESR studies exist for a-Si:H. [4-61 For example, Biegelson and Stutzmann [41 use superhyperfine studies in 29Si-enriched films to conclude that "a strong wave function overlap exists with back-bonded nearest-neighbor nuclei." Because the dangling bond wave function does sense the local bonds, it was thought that the number of bonds involved in the defect could be determined. Stathis 151 attempted to resolve the superhyperfine lines on the shoulders of a standard a-Si:H film with 4.7% 29Si, but no conclusion about the number of nearest neighbors involved in the defect could be drawn. On a similar sample, anisotropic line broadening was used to fit the main Zeeman line and hyperfine satellites with no attempt to improve the fit in the superhyperfine shoulder region. 161 Their g-value fitting to the a-Si:H varied from gil - 2.0040 to gL - 2.0080 with an inhomogeneously broadened line width increasing from 3.8 to 5.0 G. Mat. Res. Soc. Symp. Proc. Vol. 297. ý 1993 Materials Research Society
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