D. J. Leopold

Nuclear magnetic resonance studies of amorphous deuterated silicon nitride thin films

Abstract Amorphous hydrogenated/deuterated silicon nitride (a-SiN:H,D) thin films were studied by deuteron magnetic resonance (DMR). DMR spectra show two quadrupolar broadened Pake-type doublets with splittings of 65.4 kHz for SiD, and 154 kHz for ND bonds; the ratio of these DMR doublet cusp-splittings ( Δ ND Δ SiD = 2.35 ) is, to within experimental error, equal to the ratio of the squares of the respective bond-stretching frequencies (( ω ND ω SiD ) 2 = 2.36 ), as determined by analysis of Fourier transform infrared spectra. The center frequencies of the SiD and ND DMR doublets are not coincident, but rather are shifted by ∼ 2 kHz, indicating that the SiD and ND groups are interacting, and must therefore be near-neighbors in the amorphous network. This explanation consistent with an interpretation of the shifts in bond-stretching frequencies of IR modes that occur when Si(H)D and N(H)D modes are found in the same film.

Interstitial molecular hydrogen and deuterium in PECVD and HW amorphous silicon

Abstract For many years it has been known that high quality plasma-enhanced chemical vapor deposition (PECVD) hydrogenated amorphous silicon contains small quantities (⩽100 ppm) of molecular hydrogen in microvoids. More recently it has been demonstrated that there exist major (10–40%) molecular hydrogen populations singly trapped as interstitials in the amorphous equivalents of tetragonal T sites of the a-Si network. Comparative NMR studies have shown that a substantial fractional molecular hydrogen population residing in T sites correlates well with high photovoltaic quality as measured by films mobility–lifetime product ημτ . Now our current NMR measurements have turned to 1 H– 29 Si double resonance experiments in hot wire amorphous silicon films (HW a-Si:H). Our HW a-Si:H films have been provided by Schropp and Branz and the PECVD films by Paul. The HW films show several resolved proton NMR components at 80 and 300 K. These components include a narrow line arising from molecular hydrogen resident in T-like sites and perhaps also in voids. A second broader line reflects clusters of Si-bonded H. A third superbroad (∼80 kHz FWHM) line also is present as is a diamagnetically shifted feature.

Interstitial Trapped Hydrogen Molecules in Pecvdamorphous Silicon

New NMR measurements show that interstitial T site-trapped molecular hydrogen can amount to more than one third of the contained hydrogen in high quality PECVD amorphous silicon. Microvoid-contained dense molecular hydrogen is negligible in these good films. Experiments on a sequence of hydrogenated and/or deuterated a-Si films have characterized individually-trapped molecular HD and D 2 in films deposited from SiD 4 , and from SiH 4 +D 2 . The T site-trapped molecular hydrogen fraction observed here is larger than previously reported because of recent efforts to measure very slowly relaxing molecular components and the employment of radiofrequency pulse sequences to detect ortho-D 2 with nuclear spin I =2. The population of interstitially trapped molecular hydrogen increases with increasing photovoltaic quality over a range of an order of magnitude in photoresponse product ημτ. Above 200 K, hopping transport of molecular hydrogen among the amorphous equivalent of interstitial T sites occurs with an activation energy near 50 meV.

Deuterium in crystalline and amorphous silicon

The authors report deuteron magnetic resonance (DMR) measurements on aged deuterium-implanted single crystal n-type silicon and comparisons with amorphous silicon spectra. The sample film was prepared six years ago by deuteration from a-D{sub 2} plasma and evaluated by a variety of experimental methods. Deuterium has been evolving with time and the present DMR signal shows a smaller deuteron population. A doublet from Si-D configurations along (111) has decreased more than have central molecular DMR components, which include 47 and 12 kHz FWHM gaussians. Transient DMR magnetization recoveries indicate spin lattice relaxation to para-D{sub 2} relaxation centers.

Annealing of weakly bound deuterium in a-Si:D,H☆

Abstract A plasma-deposited sample of a-Si:D,H was incrementally annealed in a series of 30′ heatings at temperatures between 250 and 600°C. Subsequent deuteron magnetic resonance measurements resolve changes in various 2D components. A weakly bound deuterium fraction vanishes after a 350°C anneal. Reduced, but detectable, strongly bound deuterium and molecular deuterium signals persist after progressive anneals to 600°C. The magnetic resonance line shapes of the weakly bound deuterium and the strongly bound deuterium show that they reside in very different local environments. Several characteristics of these environments can be deduced.

Correlation of Film Quality and Hydrogen NMR Resonance Shifts in a-Si:D,H; a-Ge:D,H; and a-SiGe:D,H

A systematic study of both proton and deuteron NMR in hydrogenated amorphous silicon films has revealed significant resonance shifts among various resolved components. The shifts include both paramagnetic and diamagnetic displacements of resolved spectral features from trapped molecular hydrogens. The shifts depend on film quality and deposition conditions. Some of the shifts vary as 1/T and reflect Curie susceptibilities characteristic of local regions of differing dangling bond densities. Spectra from relatively immobile hydrogen molecules trapped in nanovoids are shifted diamagnetically and broadened as temperature decreases. Hydrogens tightly bound to Si do not show similar changes and thus are more remote from dangling bonds and other magnetic defects. Similar spectrally-resol ved shifts have been observed in a-Ge and a-SiGe films and are correlated with film photovoltaic quality as measured by mobility-lifetime products.