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MOLECULAR HYDROGEN IN AMORPHOUS SILICON D. J. Leopolda,b, P. A. Feddersa, and R. E. Norberga Washington University, Department of Physics, St. Louis, MO 63130 b Center for Molecular Electronics, University of Missouri-St. Louis, MO 63121

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ABSTRACT Proton NMR studies on hydrogen in numerous plasma deposited a-Si:H thin film samples over many years have established that there are primarily two dipolar broadened lines, representing different distinct degrees of proton clustering involving most of the contained hydrogen. In addition, both of these configurations were thought to be hydrogen tightly bonded to silicon. However, deuteron magnetic resonance established long ago that a substantial fraction of the hydrogen (deuterium) population was a weakly bonded species, very different than the silicon bonded hydrogen typically seen in infrared measurements. Deuteron magnetic resonance can easily distinguish bond strength because deuterons have spin 1, an electric quadrupole moment, and are sensitive to electric field gradients. The much weaker deuteron dipolar coupling results in NMR line shapes and widths that are determined by distributions of electric field gradients and angular averages about the applied magnetic field direction. The primary features of the deuteron line shape are a sharp powder-pattern Pake doublet attributed to the tightly silicon bonded species and a broad central Gaussian component originally interpreted as a weakly bound component, but now recognized as molecular deuterium and HD trapped in the amorphous equivalent of T-sites. This latter result is confirmed by spin-echo double resonance experiments using 1H and 29Si nuclides, which reveal a very appreciable population of molecular hydrogen in T-like sites. NMR line shape subtractions also indicate that the majority of this species resides in the less-clustered hydrogen phase. Also a comparison of the T-site molecular hydrogen fraction with measured photoresponse, combined with light-induced changes in the DMR line shape, indicate that this species is in regions of a-Si:H that control the optoelectronic properties of the material.

INTRODUCTION Nuclear magnetic resonance (NMR) studies on hydrogen in plasma-deposited a-Si:H thin films established long ago that there are primarily two distinct lines consisting roughly of a 25 kHz FWHM broad Gaussian component and 4 kHz FWHM narrow Lorentzian component [1]. Protons have spin ½ and interact with each other via the magnetic dipolar interaction, which decreases in strength as the inverse proton separation distance cubed. The observed wide and narrow dipolar-broadened NMR lines were thus originally interpreted to be entirely silicon-bonded protons in more- and less-clustered hydrogen configurations respectively. Also in the early years, proton NMR spin-lattice relaxation times measured as a function of temperature exhibited a minimum near 40 K. This minimum was originally attributed to hydrogen associated disorder modes [2], but was later shown to be controlled by proton spin diffusion to a very dilut