Structural disorder induced in hydrogenated amorphous silicon by light soaking

Abstract

We show, using variable coherence transmission electron microscopy, that light soaking of amorphous hydrogenated silicon thin films leads to structural changes. We speculate that the structural changes are associated with instability in the as-deposited material. We suggest that improved immunity to Staebler-Wronski degradation could be achieved by a less-ordered material which is closer to the ideal continuous random network. © 1998 American Institute of Physics. ͓S0003-6951͑98͒02947-7͔The light-induced creation of dangling bond defects in hydrogenated amorphous silicon ͑a-Si:H͒, called the Staebler-Wronski effect ͑SWE͒, 1-3 has proven an obstacle to the widespread technological application of this material. Although the Staebler-Wronski effect saturates with the creation of about 10 17 /cm 3 , there is growing indirect evidence that a much larger number of atoms are affected by light soaking. [4][5][6] Here, we report that there is a significant structural change in hydrogenated amorphous silicon after light soaking, which we have observed using variable coherence electron microscopy. Our experiments show no such structural change in hydrogen-free silicon. In the light of our previous work on pure amorphous Ge and Si, 7,8 we interpret this result as evidence for a structural instability in the as-deposited a-Si:H structure, which has more medium-range order than the continuous random network. We propose that the Staebler-Wronski effect is intimately related to this structural instability.Electron fluctuation microscopy techniques have high sensitivity to such subtle structural changes. Diffraction, including small angle scattering, and other single-scattering techniques such as extended x-ray absorption fine structure ͑EXAFS͒ reveal correlations between pairs of atoms only, as embodied in the radial distribution function ͑RDF͒, which is not sensitive to subtle differences between network structures. 7 We have shown that higher-order atomic correlation functions ͑three and four body͒, which are measured by variable coherence microscopy, 9 are sensitive to such differences. 7 Specifically, variable coherence electron microscopy can detect correlations at a characteristic length scale of 10-20 Å that fall short of the perfect ordering necessary for strong diffraction. Raman scattering and x-ray absorption near-edge structure may also provide useful information about medium-range order, but the interpretation of these techniques is complicated by the need to know the internal properties of the atoms in order to calculate the vibrational spectrum or electronic density of states. Because variable coherence electron microscopy uses high energy electrons as a probe, the atoms' positions are their most important attribute, supplemented by the well-known atomic scattering factor.For this experiment amorphous hydrogenated silicon films of varied composition were deposited by reactive magnetron sputtering of a silicon target in argon and hydrogen. This method permits independent control of hydrogen content, and produces m...

Structure and physical properties of paracrystalline atomistic models of amorphous silicon

“…16 Fluctuation microscopy on thin films of a-Si and a-Ge indicate that both materials have significant ordering on a length scale of ϳ15 Å, 14 and that this order decays on thermal annealing 17 ͑for a-Ge͒ and on exposure to light 18 ͑for hydrogenated amorphous silicon͒. In contrast to the CRN model, our recently proposed paracrystalline ͑PC͒ model of amorphous semiconductors exhibits MRO that is in qualitative agreement with the fluctuation microscopy data.…”

Fluctuation Transmission Electron Microscopy: Detecting Nanoscale Order in Disordered Structures

“…[25] The MRO in hydrogenated amorphous silicon are also changed by light-soaking. [26] In disordered graphite, FTEM has detected the signature of C 60 in the matrix. [27,28] In amorphous diamondlike carbon, diamond-like clustering increases by annealing up to 600 8C and graphitic clustering appears at higher temperatures.…”

Depth profile study on Raman spectra of high-energy-electron-irradiated hydrogenated amorphous silicon films

“…The degradation in the properties of a-Si:H films, called Staebler-Wronski effect (S-W effect) [4] , has been an obstacle to the wide spread applications of this material. It is reported that light soaking or high-energy-particle irradiation made the amorphous network of a-Si:H films to be reorganized, leading to a change in the structural order of the films [5][6][7][8][9] . The local distortions of amorphous network due to higher density defects in the irradiated a-Si:H films result in a change of structural order [9] and a macroscopic volume expansion [10] .…”

“…It is reported that light soaking or high-energy-particle irradiation made the amorphous network of a-Si:H films to be reorganized, leading to a change in the structural order of the films [5][6][7][8][9] . The local distortions of amorphous network due to higher density defects in the irradiated a-Si:H films result in a change of structural order [9] and a macroscopic volume expansion [10] . Chini et al [11] have reported the formation of 50-260 nm a-Si:H layers on the high-energy-particle-irradiated crystalline silicon surfaces by means of transmission electron microscopy, despite the fact that the penetration depth of high energy particles in crystalline silicon was far beyond the formed amorphous-layer thickness.…”