Stefan K. Estreicher

Hydrogen-related defects in crystalline semiconductors a theorist's perspective

Abstract Hydrogen is a common impurity in all semiconductors. Although it is sometimes deliberately introduced, hydrogen often penetrates into the crystal during device processing. It interacts with broken or weak covalent bonds, such as those found at extended and localized defect centers. The main results of these covalent interactions are shifts of energy levels out of (or into) the gap and new optical activity (infrared absorption and Raman scattering). The shifts in energy levels lead to the passivation (or activation) of the electrical activity of various centers. Hydrogen can also interact with the perfect crystal and with itself, sometimes leading to the formation of extended structures known as platelets. Finally, H also acts as a catalyst, dramatically enhancing the diffusivity of interstitial oxygen in Si. The consequences of these interactions are substantial changes in the electrical and optical properties of the crystal, and in the lifetime of charge carriers. The thermal stability of the complexes containing hydrogen varies from room temperature up to several hundreds of degrees Celsius, and the diffusion of H is trap-limited up to rather high temperatures. Hydrogen normally exists in more than one configuration and charge state in semiconductors. A range of experimental and theoretical techniques have been used to investigate the rich properties of hydrogen in semiconductors, and several extensive reviews focusing mostly on the experimental side of these issues have been published in the past five years. The present review focuses mostly on the theoretical work performed in this field. However, the most recent experimental results are also discussed, and the current understanding of hydrogen interactions in semiconductors summarized.

Theory of defects in semiconductors

1. Defect Theroy: An Armchair History.- 2. Supercell Methods for Defect Calculations.- 3. Marker-Method Calculations for Electrical Levels Using Gaussian-orbital Basis-sets.- 4. Dynamical Matrices and Free Energies.- 5. The Calculation of Free Energies in Semiconductors: Defects, Transitions and Phase Diagrams.- 6. Quantum Monte Carlo Techniques and Defects in Semiconductors.- 7. Quasiparticle Calculations for Point Defects at Semiconductor Surfaces.- 8. Multiscale Modelling of Defects in Semiconductors: A Novel Molecular Dynamics Scheme.- 9. Empirical Molecular Dynamics: Possibilities, Requirements, and Limitations.- 10. Defects in Amorphous Semiconductors: Amorphous Silicon.- 11. Light-induced Effects in Amorphous and Glassy Solids.

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.

Localized molecular orbitals and electronic structure of buckminsterfullerene

Abstract The method of PRDDO is employed to calculate the optimized geometry, energy of formation, ionization potential, bond orders, and localized molecular orbitals for Buckminsterfullerene (C 60 ).

Stable and metastable states of C60H: buckminsterfullerene monohydride

Abstract Approximate ab initio Hartree—Fock and first-principles density functional calculations of potential energy surfaces and electronic structures of C 60 H show that the stable state has H attached to one C atom, outside the buckyball. This C atom is displaced radially outward and is close to being sp 3 hybridized. The unpaired electron of C 60 H is delocalized. The calculated Fermi contact density at the proton is in good agreement with recent low-temperature μSR data. A metastable configuration has atomic H at the center of the ball (H@C 60 ). Once trapped there, H must overcome a large barrier to go through the surface of C 60 . Other configurations we considered include H attached to one C atom but inside the buckyball, and H bridging one of the two inequivalent CC bonds. The barrier for diffusion of H from outside to the center of C 60 have also been calculated. The results are compared to recent muon spin rotation studies in solid C 60 and to the states of hydrogen in other forms of carbon.

PRELIMINARY CALCULATIONS CONFIRMING THAT ANOMALOUS MUONIUM IN DIAMOND AND SILICON IS BOND-CENTERED INTERSTITIAL MUONIUM

The model of anomalous muonium as bond-centered interstitial muonium has been examined by approximate ab-initio Hartree-Fock calculations in diamond and silicon and found to be in excellent agreement with experiment.

Ab initio local vibrational modes of light impurities in silicon

We have developed a formulation of density-functional perturbation theory for the calculation of vibrational frequencies in molecules and solids, which uses numerical atomic orbitals as a basis set for the electronic states. The (harmonic) dynamical matrix is extracted directly from the first-order change in the density matrix with respect to infinitesimal atomic displacements from the equilibrium configuration. We have applied this method to study the vibrational properties of a number of hydrogen-related complexes and light impurities in silicon. The diagonalization of the dynamical matrix provides the vibrational modes and frequencies, including the local vibrational modes (LVMs) associated with the defects. In addition to tests on simple molecules, results for interstitial hydrogen, hydrogen dimers, vacancy-hydrogen and self-interstitial-hydrogen complexes, the boron-hydrogen pair, substitutional C, and several O-related defects in

Hydrogen-defect interactions in Si

c\ensuremath{-}\mathrm{Si},

Thermal phonons and defects in semiconductors: The physical reason why defects reduce heat flow, and how to control it

are presented. The average error relative to experiment for the \ensuremath{\sim}60 predicted LVMs is about 2% with most highly harmonic modes being extremely close and the more anharmonic ones within 5\char21{}6 % of the measured values.

Structure and Dynamics of Point Defects in Crystalline Silicon

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 *.