Peter A. Fedders

The ring-hexavacany in silicon: A stable and inactive defect

Molecular dynamics simulations as well as ab initio and near ab initio Hartree-Fock calculations in crystalline silicon predict that the configuration of the hexavacancy that has a hexagonal ring missing from the crystal is remarkably stable. The energetics imply that it does form and is more likely to grow than to dissociate during heat treatments. Further, the energy eigenvalues and the charge distribution imply that it has no electrical or optical activity. However, it is a large void in the crystal and could be an efficient gettering center and a precursor of extended defects.

Defects, doping, and conduction mechanisms in nitrogen-doped tetrahedral amorphous carbon

First principles methods are used to study N doping of diamondlike amorphous carbon. A structural model containing 216 atoms is introduced, whose properties are in agreement with the available experimental data. The topological and electronic properties for different N doping concentrations are investigated. We find that N occurring in tetrahedral sites or chains of an even number of π bonded sites results in an increase of the Fermi energy, while N incorporation in strained network sites induces structural changes that lead to an increase in the sp2 fraction of the material. The prevalent conduction mechanisms are identified and discussed. While the Fermi energy increases upon N doping, the localization of the conduction-band-tail states limits extended state conduction. These results are compared to the recent experimental reports on N doping of ta-C and we find that the nondoping threefold N incorporation (N30) is energetically most likely, which explains the low doping efficiency seen in experiments.

Spin Susceptibilities of Organic Systems on a Narrow‐Band Model

A narrow‐band model is used to explain the temperature dependence of the spin susceptibilities of organic free‐radical solids. It is shown that for temperatures in excess of the bandwidth the spin susceptibility is half the expected Curie susceptibility, while at low temperatures exponentially vanishing susceptibilities are obtained through the splitting of the bands due to the asymmetry of the crystal structure. A consideration of the effects of nuclear motion on the band structure leads to a “Curie–Weiss” law behavior at high temperatures, with a quite different interpretation of the Weiss constant. The results of the calculations are compared with available experimental data for DPPH (diphenylpicrylhydrazil), PAC (picrylaminocarbazyl), and D(NO2)2 [bis(p‐nitrophenyl)picrylhydrazyl]. Very good agreement with experiment is obtained. Further applications of the narrow‐band model are discussed.

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.

Hydrogen-defect interactions in Si

Abstract The interactions between hydrogen and intrinsic defects in silicon are studied using ab-initio (tight-binding) molecular-dynamics simulations in supercells and ab-initio Hartree-Fock in clusters. The configurations, electronic structures, and binding energies of H bound to small vacancy aggregates are calculated. The vacancy (V) and the self-interstitial (I)—both rapid diffusers in Si—efficiently dissociate interstitial H 2 molecules. At low temperatures, this results in the formation of {V, H, H} or {I, H, H} complexes. At high temperatures, one or both H’s may be released as interstitials. Preliminary calculations show that H 2 * result from the reaction {I, H, H}+V→H 2 *.

Molecular-dynamics studies of self-interstitial aggregates in Si

The interactions between neutral self-interstitials in silicon are studied using ab initio tight-binding molecular-dynamics simulations in periodic supercells containing 64 up to 216 Si atoms. A number of configurations with three or more self-interstitials are found, and the lowest-energy ones are discussed. The binding energies of In relative to In−1+I show that the first ‘magic number’ (particularly stable aggregate) is I3. The potential energy surfaces for aggregates of three or more Is have several local minima, leading to a range of metastable configurations.

Theory of electromechanically induced acoustic phonon echoes

A theory of electromechanically induced acoustic phonon echoes is presented. The predicted echo pulse height and width are found to depend on the exciting pulse widths and frequencies, in agreement with experimental observations. Interesting new experiments with much larger predicted echo effects are suggested.

Electronic structure of dangling bonds in amorphous silicon studied via a density-matrix functional method

A structural model of hydrogenated amorphous silicon containing an isolated dangling bond is used to investigate the effects of electron interactions on the electronic level splittings, localization of charge and spin, and fluctuations in charge and spin. These properties are calculated with a recently developed density-matrix correlation-energy functional applied to a generalized Anderson Hamiltonian, consisting of tight-binding one-electron terms parametrizing hydrogenated amorphous silicon plus a local interaction term. The energy level splittings approach an asymptotic value for large values of the electron-interaction parameter U, and for physically relevant values of U are in the range 0.3--0.5 eV. The electron spin is highly localized on the central orbital of the dangling bond while the charge is spread over a larger region surrounding the dangling bond site. These results are consistent with known experimental data and previous density-functional calculations. The spin fluctuations are quite different from those obtained with unrestricted Hartree-Fock theory.

Electron scattering in semiconductor alloys

Suitability of the Born approximation and the Boltzmann equation is demonstrated for the scattering of free‐carrier electrons by random‐alloy atomic potentials in semiconductor alloys. Composition dependences of alloy‐scattering potential strengths are hypothesized and electron scattering rates are derived. ‘‘Order parameters’’ are derived from scattering theory and compared to those derived previously from statistical and thermodynamic arguments by Warren and Cowley. The treatment is generalized to include ternary, quaternary, and lattice‐matched alloys which, in general, show more complicated order‐parameter dependencies than the previously known x(1−x) dependence for ternary zincblende alloys. Electron‐momentum relaxation‐rate expressions are given, including nonparabolic Kane bands and admixed wave functions appropriate to small energy‐gap semiconductors. Electron drift mobility, as determined by alloy scattering, is derived in the effective‐mass limit which shows that any short‐range alloy potential ...

Strain scattering of electrons in piezoelectric semiconductors

Static strains in piezoelectric semiconductors give rise to an electric field or potential which can have an effect on the electrical properties of the material. We have calculated the electric potential due to the strain field arising from a random distribution of point defects. This potential contributes a term to the mobility that is proportioned to T1/2 and a Hall factor of 1.10. Crude estimates of strain strengths indicate that this scattering mechanism may contribute significantly to the mobility of electrically rather pure III‐V semiconductors below room temperature when neutral impurity concentrations are greater than 1018 cm−3. The mechanism may also constitute a dominant one in the mobility of some III‐V alloys at fairly low temperatures. The existence of strain induced electric potentials also provides at least a possible mechanism whereby different donors can have different line shapes as measured in photoconductivity experiments.