Diamagnetic Susceptibility of Micron Thick a-Si:H Films Measured via Proton NMR: A Probe of Structural Disorder
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Diamagnetic Susceptibility of Micron Thick a-Si:H Films Measured via Proton NMR: A Probe of Structural Disorder J. Baugh1, D. X. Han1, A. Kleinhammes1, Q. Wang2, and Y. Wu1* 1 Department of Physics and Astronomy, University of North Carolina, Chapel Hill, NC 27599-3255 2 National Renewable Energy Laboratory, 1617 Cole Blvd. Golden, CO 80401
ABSTRACT Magnetic susceptibility is predicted to be sensitive to structural disorder in the group IV elemental semiconductors. A proton nuclear magnetic resonance (NMR) technique is shown to precisely measure bulk magnetic susceptibilities (χ) of hot-wire (HW) CVD a-Si:H films asdeposited on quartz substrates. Differences in χ correlate with growth rate, but not with hydrogen content or deposition temperature. Qualitative agreement is found between smaller diamagnetic susceptibility (less average disorder) and desirable electronic properties measured by photo- and dark conductivity on the same samples. Comparing films with similar hydrogen distribution and content (as evidenced by the NMR spectra) allows the unambiguous identification of the importance of overall Si network structural disorder in the metastability of a-Si:H.
INTRODUCTION Thin film hydrogenated amorphous silicon (a-Si:H) presents a fascinating material for the study of how four-fold coordinated networks form and how alloying can significantly influence such structures. Much is known about the Si (and H) microstructure at the range of nearest neighbors [1], as well as long-range structure (i.e. observable by transmission electron microscopy) [2], whereas very little is known about structural order over all the scales in between, e.g. intermediate range order (IRO). While device quality films exhibit very little differences in short and long-range order, it is suspected that variously prepared films may possess different characteristic IRO [3], possibly related to observed differences in optoelectronic properties and stabilities against light-induced degradation. Such variations in IRO are likely related to observed differences in H microstructure: comparing H-diluted and standard plasma-enhanced (PE) CVD films, we note that x-ray diffraction (XRD) finds an increase of average order identified with an intermediate scale in the H-diluted films [3], whereas proton NMR finds less of the clustered Si-H component relative to the dilute Si-H, as well as lower H contents in the H-diluted films [4]. The more favorable H microstructure of the H-diluted a-Si:H thus correlates with a backbone Si structure that is modified to some degree, as well as the much improved stability against the Staebler-Wronski effect (SWE) exhibited by H-diluted films [5]. The low H contents of optimal HWCVD films coupled with analysis of proton NMR data strongly suggests that the Si network in these films is highly inhomogeneous, with large volume fractions having nearly no H content [6]. These H-poor regions must have markedly different microstructure, and possibly different IRO, than the H-containing regions. It has been suggested that improved
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