Roland E. Allen

Surface defects and Fermi‐level pinning in InP

Deep levels for native substitutional defects at the (110) surface of InP are predicted, and the Schottky barrier height data are interpreted as follows: (i) The pinning levels 0.4 to 0.5 eV below the conduction band edge for n‐InP and 0.75 to 0.8 eV above the valence band edge for p‐InP are assigned to an antisite defect. (ii) The pinning level that lies within 0.1 eV of the conduction band edge is assigned to a shallow donor level of a P vacancy.

Unified theory of point–defect electronic states, core excitons, and intrinsic electronic states at semiconductor surfaces

A simple unified theory has been developed for predicting three different types of electronic states at semiconductor surfaces: (i) point–defect states, (ii) core excitons, and (iii) intrinsic surface states. The predictions are in agreement with experiment for each class of states. The theory indicates that Fermi level pinning at III–V semiconductor interfaces is usually caused by surface antisite defects.

Detailed dynamics of a complex photochemical reaction: Cis–trans photoisomerization of stilbene

Detailed simulations are reported for the dynamics of electrons and nuclei during the cis to trans photoisomerization of stilbene. Our method, which employs a semiclassical description of both the nuclear motion and the radiation field, is described in the text. After excitation of electrons from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) by a femtosecond-scale laser pulse, two principal avoided crossings are observed between the HOMO and LUMO levels, each of which leads to substantial depopulation of the LUMO. Based on our results and those of other groups, we propose that the first such HOMO–LUMO coupling can lead to the formation of 4a,4b-dihydrophenanthrene (DHP). The second coupling, on the other hand, leads to the formation of trans-stilbene. It is found that pyramidalization of the two carbon atoms of the vinyl group is involved significantly in both couplings, and that rotation of the two phenyl rings, together with their interaction, plays an i...

Semiclassical electron-radiation-ion dynamics (SERID) and cis-trans photoisomerization of butadiene

Abstract Detailed simulations are reported for the dynamics of electrons and nuclei during the cis—trans isomerization of butadiene following a femtosecond-scale laser pulse. Our technique, semiclassical electron-radiationion dynamics (SERID), is fully described in the text. The one-electron Hamiltonian and ion—ion interactions employed in the present work are density-functional based. Following excitation of electrons by the laser pulse, all three C—C bonds in the butadiene molecule become longer, as electrons are promoted from the HOMO to both the LUMO and LUMO+1 levels. In the excited electronic state, the molecule rotates about all of its three C—C bonds. There are then non-adiabatic events near avoided crossings, with electronic transitions to the ground state via creation of vibrational excitations. The molecule continues to twist around the central bond and one of the terminal C—C bonds, until the trans-conformation is achieved. Various features in the behaviour of the vibrational modes can also be observed, including initial excitations due to the laser pulse, further excitations at avoided crossings and the redistribution of vibrational energy between modes.

Surface Thermodynamic Functions for Noble‐Gas Crystals

Surface thermodynamic quantities have been calculated as functions of temperature between 0°K and the melting temperature for the (111), (100), and (110) surfaces of the noble‐gas solids Ne, Ar, Kr, and Xe. The method of calculation differs from the methods used in previous treatments of surface thermodynamic functions in that the atomic vibrations are taken into account, and the vibrational frequencies are properly calculated rather than obtained from a Debye model or an Einstein model. The quantities calculated are the static surface energy, vibrational surface energy, surface entropy, vibrational surface free energy, and surface specific heat. In addition, a surface frequency‐distribution function f8(ω) has been calculated; f8(ω) is positive at low frequencies, because of the presence of surface modes of vibration, and negative at higher frequencies. This behavior of f8(ω) produces a narrow peak in the graph of the surface specific heat as a function of temperature.

Bound and resonant (110) surface electronic states for GaAs, GaP and GaSb

Abstract Dispersion curves E( k ) are predicted for the surface electronic bound states and resonances at the relaxed (110) surfaces of GaAs, GaP, and GaSb. GaAs and GaSb are predicted to have no surface states within the fundamental band gap, but GaP is predicted to have unoccupied surface states with energies extending below the conduction band edge, in agreement with experiment. The predicted dispersion curves are in good agreement with the angle-resolved photoemission measurements for GaAs. Using the analytic Greens function, effective-Hamiltonian technique, we are able to locate some new resonant structure.

Comparative Studies of the trans-cis Photoisomerizations of Azobenzene and a Bridged Azobenzene

Using density-functional-based molecular dynamics simulations, we have performed comparative studies of the trans-cis isomerizations of azobenzene and bridged azobenzene (B-Ab) 5,6-dihydrodibenzo[c,g][1,2]diazocine induced by nπ* electronic excitation. The quantum yields found in our calculations, 45% for the bridged azobenzene versus 25% for azobenzene, are consistent with the experiment. Both isomerization processes involve two steps: (1) Starting from the trans structure, each molecule moves on its S(1) excited-state potential energy surface, via rotation around the NN bond, to an avoided crossing near the S(1)/S(0) conical intersection, where de-excitation occurs. (2) Subsequently, in the electronic ground state, there is further rotation around the NN bond, accompanied by twisting of the phenyl rings around their CN bonds, until the cis geometry is achieved. Because of its lower symmetry and smaller initial CNNC dihedral angle, the bridged azobenzene has a much shorter lifetime for the S(1) excited state, about 30 fs, as compared to about 400 fs for azobenzene. However, we find that the complete isomerizations have approximately the same time scales. Although the bridging feature in trans-B-Ab does not hinder rotation around the NN bond in step 1, it makes twisting of the two phenyl rings around the CN bonds much slower in step 2.

Dangling bonds and Schottky barriers

We review theoretical interpretations of Schottky barriers and Fermi‐level pinning, which result when metals and other chemical species are deposited on semiconductor surfaces. Experiments indicate that these two phenomena are closely connected, so a theory of Schottky barriers must also explain Fermi‐level pinning for submonolayer coverages of both metallic and nonmetallic species. Proposed mechanisms include the following: (a) Intrinsic surface states. For GaAs and several other materials, there are no intrinsic surface states within the band gap; GaP, e.g., does have surface states in the gap, but they are not at the correct energy to explain Schottky barrier formation. (b)Metal‐induced gap states. These states, which require a thick metal overlayer, cannot explain Fermi‐level pinning at submonolayer metallic coverages. They also cannot explain why a single semiconductor (n‐type InP) exhibits two distinct Schottky barrier heights. Furthermore, they cannot explain why the Schottky barrier persists when ...

Another important coordinate in the photoisomerization of cis-stilbene

Detailed simulations are reported for the coupled dynamics of electrons and nuclei in the isomerization reaction of cis-stilbene stimulated by laser excitation. The results demonstrate that, in addition to the traditional vinyl and vinyl– phenyl torsions, the HCCH torsional coordinate of the vinyl group also makes a significant contribution to the HOMO and LUMO couplings that yield nonradiative electronic transitions to the ground state. Moreover, variations in the CCH angles of the vinyl group in the vicinity of the HOMO–LUMO couplings indicate that pyramidalization of the vinyl-group carbon atoms is strongly involved in these molecular orbital couplings. 2003 Published by Elsevier B.V.

Si/Transition-metal Schottky barriers: Fermi-level pinning by Si dangling bonds at interfacial vacancies

Abstract The Schottky barriers formed by deposition of transition metals on Si surfaces are explained in terms of Fermi-level pinning by Si dangling bonds that are sheltered by vacancies at Si/transition-metal-silicide interfaces.