R. E. Norberg

Non-bonded hydrogen in a-Si:H

Abstract We report deuteron and proton nuclear magnetic resonance (NMR) measurements on high quality plasma enhanced chemical vapor deposition amorphous silicon films containing from 5 to 15 at.% hydrogen (H and/or D). Our results show that from 2% to 40% of the hydrogen in these samples is not atomically bonded by the Si–H chemical bond. We present conclusive evidence that in highest quality films nearly all of this non-bonded hydrogen is present as isolated hydrogen molecules. These molecules are located in centers of atomic dimensions, perhaps in the analogue of T-sites in crystalline Si. In high quality films these molecules are not the small population of densely packed molecules in the occasional microvoids. The photoelectronic quality of the films (as measured by the ημτ photoresponse product) increases as the fraction of non-bonded hydrogen increases. Our results are consistent with IR spectral measures of hydrogen. Our experiments also suggest that the non-bonded hydrogen is in the vicinity of light induced defects. Finally by comparing successive annealings and other studies it appears that the molecular hydrogen can be identified with the less-clustered proton narrow NMR line.

Nuclear magnetic resonance studies of hydrogen in amorphous silicon

Proton and deuteron NMR in hydrogenated amorphous silicon yield quantitative measures of species-specific structural configurations and their dynamics. Populations of silicon-bonded and molecular hydrogens correlate with photovoltaic quality, doping, illumination/dark anneal sequences, and with infrared and other characterizations. High quality films contain substantial populations of nanovoid-trapped molecular hydrogen.

Connections between photovoltaic quality and the structure of deuterated amorphous Si and Ge films

Hydrogen arrangements and rearrangements have been examined in a series of photoillumination and 150°C dark anneal sequences. Deuteron nuclear magnetic resonance (DMR) distinguishes among a variety of hydrogen configurations in a series of plasma-deposited a-Si:D,H and a-Ge:D,H films. These populations and their spin-lattice relaxation correlate with the photovoltaic quality in a series of differently-prepared films. Films with a higher fraction of tightly bound H (or D) are of better photovoltaic quality. We have detected reversible light-induced DMR changes including a resonance shift and changes in a quadrupolar doublet resonance line shape. We hope to determine whether the light-induced changes reflect hydrogen rearrangements or changes in the Si (Ge) environment.

Hydrogen Populations in PECVD a-Si:H,D

Proton NMR and deuteron NMR (DMR) have been used to measure hydrogen populations in a series of PECVD a-Si:H,D films. The sharp DMR doublet from Si-D is fitted and subtracted out. The residual spectra then show specific signatures for molecular D{sub 2} and HD. The fitting procedures yield quantitative measures of Si-bonded and molecular species. A particular comparison is made between a pair of films prepared as the powered and unpowered electrodes in the same plasma deposition. Both silicon-bonded and molecular populations are significantly different in the two films and correlate with photoresponse products {eta}{mu}{tau} and with IR as well as other materials characterizations.

Molecular Hydrogen in Amorphous Silicon

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, 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 1 H and 29 Si 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.

Stability of the number of silicon-hydrogen bonds upon photoillumination of undoped amorphous hydrogenated silicon

Abstract Deuteron magnetic resonance measurements on high quality a-Si:H,D films reveal three principal features: a sharp 66 kHz quadrupolar Pake doublet corresponding to silicon-bonded D passivating dangling bonds, a broad central line which has been shown to arise primarily from relatively isolated D2 and HD molecules in nanovoids, and a small narrow central line arising from mobile molecular D2 and HD in greater than 3 A microvoids. The results of measurements on high quality plasma-deposited partially deuterated amorphous silicon cyclically exposed to a 2 h 150°C dark anneal and a 36 h photoillumination at 0.5 W/cm2 with a 400–700 nm xenon are lamp are described. The spectra show that, following an initial spin-lattice relaxation-related transient, the intensities of all three components of the deuterium may vary by less than 1% upon illumination or dark anneal. The broad central line, primarily from isolated molecules, shows a small diamagnetic shift upon illumination. The shift becomes strongly paramagnetic at short magnetization recovery times following radio frequency saturation. The SiD doublet and the small narrow central line do not shift appreciably. This absence strongly suggests that Staebler-Wronski magnetic changes are remote from the environments of most of the silicon bonded hydrogen.

Deuteron Magnetic Resonance in Some Amorphous Semiconductors

Deuteron magnetic resonance (DMR) measurements have been made on plasma-deposited samples of a-Ge: D, H; a-Si:D,F; and a-SiGe:D,F. The experiments were performed at 30.7 MHz with a Bruker CXP 200 spectrometer and a probe constructed to permit operation at high rf power down to temperatures near 4 K. The 50 to 100 mg samples comprise less than 1 mm flakes in Kel-F containers of 5 mm diameter.