The Development of Novel Nuclear Magnetic Resonance Techniques for the Study of Solids, Thin Films and Surfaces with Particular Application to Amorphous Semiconducting Silicon-Hydrogen Films

Citation

Reimer, Jeffrey Allen (1981) The Development of Novel Nuclear Magnetic Resonance Techniques for the Study of Solids, Thin Films and Surfaces with Particular Application to Amorphous Semiconducting Silicon-Hydrogen Films. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/c2h9-0m19. https://resolver.caltech.edu/CaltechTHESIS:03062018-123026017

Abstract

Part 1

Proton magnetic resonance data are presented for twenty different plasma-deposited amorphous silicon-hydrogen films. The two phase compositional inhomogeneity observed in these films is found to be independent of film thickness down to less than 1μ. Models for various structural configurations show that these films contain heavily monohydride clustered regions such as divacancies and voids, as well as (SiH₂)_n and SiH₃ local bonding configurations. The presence of the divacancies in films showing predominantly monohydride vibrational modes provides some insight into the controversy over the assignment of the 2090 cm^-1 vibrational mode. The films also contain regions in which monohydride groups are distributed at random. Based on changes in a film whose proton NMR lineshapes are metastable as deposited, a model based on strain relief is proposed for film development which explains the ubiquitous presence of the two phase inhomogeneity. Examination of the changes in proton NMR data as a function of deposition conditions furnishes new insight on the role SiH₂ and SiH⁺_x groups have in models for the gas phase reactions involved in the developing films. Finally, p- or n-type doping is found to increase the the hydrogen content of the films, and, under heavy p-type doping with diborane, boron clustering may occur within the films. Proton NMR lineshapes are also presented as a function of annealing temperature up to 650°C. The data indicate that hydrogen diffuses internally before major evolution occurs, that transfer of hydrogen occurs from a heavily clustered phase to a dilute phase coincident with evolution and that evolution occurs initially from the heavily clustered phase. Internal hydrogen diffusion is found to be concomitant with the reduction in paramagnetic center density.

Silicon-29 and hydrogen magic angle sample spinning experiments on amorphous silicon-hydrogen films (involving cross-polarization and homonuclear multiple pulse techniques respectively) fail to yield quantitative determinations of local silicon-hydrogen bonding environments. However, the ²⁹Si data are qualitatively consistent with infra-red assignments of (SiH₂)_n groups. Furthermore, the lack of significant line narrowing for the ²⁹Si spectra upon magic angle sample spinning shows that there are large chemical shift dispersions, indicative of the disorder in the amorphous lattice.

Proton spin-lattice relaxation data are presented for several plasma deposited amorphous silicon-hydrogen films when (i), homonuclear dipolar interactions are suppressed, (ii), deuterium is isotopically substituted for hydrogen, and (iii), films are annealed. These data are consistent with a model in which proton nuclei are relaxed by hydrogen-containing disorder modes. Analysis of these data shows that the density of disorder modes is ~30% higher in the low hydrogen density domain and that more than one hydrogen nucleus is associated with each disorder mode. The behavior of T₁ upon annealing indicates that a small fraction of unpaired spins or "dangling bonds" may be associated with the disorder modes. These results suggest that the role of hydrogen in amorphous silicon is more complex than passivation of "dangling bond" intrinsic defects.

Finally, proton magnetic resonance data are presented for the hydrogen alloys of plasma-deposited amorphous boron, carbon, silicon carbide and silicon nitride. Linewidth and lineshape analysis leads to the conclusion that hydrogen nuclei are clustered in a-Si/C:H, a-C:H, and a-Si/N:H. Both a-Si/C:H and a-C:H data show hydrogen exists in two phases. Modeling of linewidths in a-Si/C:H indicates that the two phases are heavily hydrogenated carbon clusters imbedded in a weakly hydrogenated a-Si lattice. In addition, evidence is presented for the presence of motionally narrowed hydrogen spectra in a-Si/N:H, a-B:H, and a-C:H. It is suggested that the hydrogen nuclei giving rise to these spectra are associated with disorder modes.

Part 2

The production of heteronuclear dipolar modulated chemical shift spectra of specific protons within polycrystalline solids is described by consideration of explicit double resonance pulse schemes which remove both heteronuclear and homonuclear dipolar interactions. These spectra furnish an accurate method of determining bond distances in local environments of complicated chemical systems. When used in conjunction with selectively observed chemical shift spectra, these schemes present a means for characterization of both geometrical and electronic properties in the solid state. To demonstrate the utility of these experiments, selective proton observed NMR results are presented for the carbonyl protons of adsorbed formic acid on ammonium-Y zeolite and the hydrogen bonded proton in polycrystalline acetanilide. Chemical shift information for the adsorbed formic acid shows a chemisorbed species consistent with formate ions. The data for the amide bond in polycrystalline acetanilide show a hydrogen bond with a chemical shift anisotropy of 17.7 ppm and a N-H distance of 1.05 ± 0.01 Å.

Thesis Files