M. H. Brodsky

We have studied the number and nature of the silicon-hydrogen bonds in amorphous silicon films prepared in plasmas either of silane or of hydrogen and argon. The films from silane glow discharges have qualitatively different Raman and infrared spectra which depend on deposition parameters such as substrate temperature and silane gas pressure. Three main groups of spectral bands are seen associated with the Si-H bonds: the Si-H bond stretch bands, the bands due to relative bending of two or three Si-H bonds with a common silicon atom, and the "wagging" bands of Si-H bonds with respect to the Si matrix. These bands are split in a way suggestive of the presence of SiH, Si${\mathrm{H}}_{2}$, and Si${\mathrm{H}}_{3}$ complexes: the bond-bending bands are absent when only SiH bonds are present. All three types of complexes are identified in films deposited from glow discharges of silane at pressures \ensuremath{\sim} 1 Torr and room temperature. Higher substrate temperatures and/or lower pressures reduce the Si${\mathrm{H}}_{2}$ and Si${\mathrm{H}}_{3}$ concentrations: films deposited at 250\ifmmode^\circ\else\textdegree\fi{}C and 0.1 Torr contain only SiH groups. From the strength of the corresponding absorption bands, H concentrations as high as 35 to 52 atomic percent are estimated. Films sputtered at 200\ifmmode^\circ\else\textdegree\fi{}C in a 10% ${\mathrm{H}}_{2}$-90% Ar mixture contain all three groupings observed in the silane-derived samples. Deuterated sputtered films are used to confirm the analysis. The first- and second-order Raman scattering spectra of the Si-Si bonds in pure and hydrogenated $a\ensuremath{-}\mathrm{S}\mathrm{i}$ are also discussed. The scattering efficiency of $a\ensuremath{-}\mathrm{S}\mathrm{i}$ is found to be as much as 10 times that of crystal Si. The depolarization ratio of the $a\ensuremath{-}\mathrm{S}\mathrm{i}$ Raman spectrum has been remeasured. Finally, a picture is presented of when it is appropriate to refer to heavily hydrogenated $a\ensuremath{-}\mathrm{S}\mathrm{i}$ as still being a material describable by $a\ensuremath{-}\mathrm{S}\mathrm{i}$ network models.

We report measurements on the x-ray diffraction, electron spin resonance (ESR), optical absorption, and electrical conductivity of amorphous Si films. The x-ray diffraction results show there is no long-range ordering of the atoms in amorphous Si independent of the sample's thermal history, while all of the other properties show strong dependences on annealing. The ESR results indicate a large number of microscopic surfaces distributed throughout the amorphous bulk, and lead us to interpret the optical and electrical properties in terms of "building blocks" with linear dimensions between 10 and 15 \AA{}.

The $g$ values, line shapes, and linewidths of the ESR signals from within the bulk of amorphous silicon, germanium, and silicon carbide are found to be similar to those of the electron states observed in the surface regions of the corresponding crystalline forms. Discussion is given in terms of a microcrystalline model.

Raman scattering has been studied in the amorphous form of Si and several related, tetrahedrally bonded semiconductors (Ge, GaAs, GaP, InSb). All vibrational modes of the material can take part in the scattering process, and the Raman spectrum is a measure of the density of vibrational states. The amorphous phases are found to have vibrational spectra very similar to the corresponding crystals, reflecting the similarity in short-range order of the two phases.

Calculations of the vibrational density of states and the Raman and infrared spectra have been performed for random-network, microcrystalline, and polymorph structures of Si and Ge. The polymorphs considered include Si III, Ge III, and two clathrate structures. The calculations are based on simple semiempirical forms for interatomic interactions and Raman and infrared activities. The results for some representations of the random network compare favorably with experimental measurements on amorphous Si and Ge. The apparent similarity of the vibrational densities of states of amorphous Si and Ge to those of diamond cubic Si and Ge is explained by a study of the form of the density of states for nearest-neighbor central forces. There is an interesting relationship to a simple tight-binding theory of the electronic density of states in this limit. The variations of infrared and Raman activities in different parts of the spectrum are discussed. Numerical calculations for energy-loss spectra in neutron scattering are also presented. A simple model explains the oscillation of intensity between the high- and low-frequency parts of the spectrum as the scattering vector increases.