Electronic excitations lie at the origin of most of the commonly measured spectra. However, the first-principles computation of excited states requires a larger effort than ground-state calculations, which can be very efficiently carried out within density-functional theory. On the other hand, two theoretical and computational tools have come to prominence for the description of electronic excitations. One of them, many-body perturbation theory, is based on a set of Green’s-function equations, starting with a one-electron propagator and considering the electron-hole Green’s function for the response. Key ingredients are the electron’s self-energy S and the electron-hole interaction. A good approximation for S is obtained with Hedin’s GW approach, using density-functional theory as a zero-order solution. First-principles GW calculations for real systems have been successfully carried out since the 1980s. Similarly, the electron-hole interaction is well described by the Bethe-Salpeter equation, via a functional derivative of S. An alternative approach to calculating electronic excitations is the time-dependent density-functional theory (TDDFT), which offers the important practical advantage of a dependence on density rather than on multivariable Green’s functions. This approach leads to a screening equation similar to the Bethe-Salpeter one, but with a two-point, rather than a four-point, interaction kernel. At present, the simple adiabatic local-density approximation has given promising results for finite systems, but has significant deficiencies in the description of absorption spectra in solids, leading to wrong excitation energies, the absence of bound excitonic states, and appreciable distortions of the spectral line shapes. The search for improved TDDFT potentials and kernels is hence a subject of increasing interest. It can be addressed within the framework of many-body perturbation theory: in fact, both the Green’s functions and the TDDFT approaches profit from mutual insight. This review compares the theoretical and practical aspects of the two approaches and their specific numerical implementations, and presents an overview of accomplishments and work in progress.

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Electronic excitations: density-functional versus many-body Green's-function approaches

A new reversible photoelectronic effect is reported for amorphous Si produced by glow discharge of SiH4. Long exposure to light decreases both the photoconductivity and the dark conductivity, the latter by nearly four orders of magnitude. Annealing above 150 °C reverses the process. A model involving optically induced changes in gap states is proposed. The results have strong implications for both the physical nature of the material and for its applications in thin‐film solar cells, as well as the reproducibility of measurements on discharge‐produced Si.

Reversible conductivity changes in discharge‐produced amorphous Si

Acknowledgements 1. Introduction 2. Electromagnetic wave propagation 3. The absorption of light 4. Specular reflection 5. Single particle scattering: perfect spheres 6. Single particle scattering: irregular particles 7. Propagation in a nonuniform medium: the equation of radiative transfer 8. The bidirectional reflectance of a semi-infinite medium 9. The opposition effect 10. A miscellany of bidirectional reflectances and related quantities 11. Integrated reflectances and planetary photometry 12. Photometric effects of large scale roughness 13. Polarization 14. Reflectance spectroscopy 15. Thermal emission and emittance spectroscopy 16. Simultaneous transport of energy by radiation and conduction Appendix A. A brief review of vector calculus Appendix B. Functions of a complex variable Appendix C. The wave equation in spherical coordinates Appendix D. Fraunhoffer diffraction by a circular hole Appendix E. Table of symbols Bibliography Index.

Theory of Reflectance and Emittance Spectroscopy

Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (e) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.

A review of Ga2O3 materials, processing, and devices

It is shown that the ordinary semiclassical theory of the absorption of light by exciton states is not completely satisfactory (in contrast to the case of absorption due to interband transitions). A more complete theory is developed. It is shown that excitons are approximate bosons, and, in interaction with the electromagnetic field, the exciton field plays the role of the classical polarization field. The eigenstates of the system of crystal and radiation field are mixtures of photons and excitons. The ordinary one-quantum optical lifetime of an excitation is infinite. Absorption occurs only when "three-body" processes are introduced. The theory includes "local field" effects, leading to the Lorentz local field correction when it is applicable. A Smakula equation for the oscillator strength in terms of the integrated absorption constant is derived.

a Quantum-Mechanical Theory of the Contribution of Excitons to the Complex Dielectric Constant of Crystals.

Physics of color centers

Oxygen vacancy model for the E1′ center in SiO2

The oxygen vacancy in \ensuremath{\alpha}-quartz has been studied in various charge states using the molecular-orbital techniques modified intermediate neglect of differential overlap (MINDO/3) and its open-shell version (MOPN). The solid containing the defect is simulated by a cluster of 30 to 40 atoms, including hydrogen terminators. In the case of the neutral O vacancy the neighboring silicon atoms are predicted to relax significantly, forming a strong Si---Si bond which is comparable to that in bulk Si. We find that the positively charged (+1) oxygen vacancy relaxes asymmetrically, consistent with most aspects of published electron paramagnetic resonance (EPR) spectroscopy data on the ${E}_{1}^{\mathcal{'}}$ center. Our calculations support the model of Feigl et al., as opposed to the divacancy model of Jani et al. which had been proposed in response to controversies over the two weak hyperfine interactions of the ${E}_{1}^{\mathcal{'}}$ center. Our calculations, furthermore, reveal that the long-bond-side silicon which, in the model of Feigl et al., relaxes into the plane of its three backbonded oxygens (we call this configuration the planar ${E}_{1}^{\mathcal{'}}$ configuration), in fact relaxes through the plane of the oxygens, emerging on the other side of the vacancy in a puckered configuration. The latter is calculated to be more stable by about 0.3 eV than the planar configuration, and it agrees even better with EPR data. Our study also shows that the O vacancy introduces two levels (0/+ and +/2+), and possibly a third (-/0) into the band gap of silicon dioxide.

Oxygen vacancy and the E 1 ’ center in crystalline SiO 2

It is pointed out that the Einstein relationship between spontaneous emission probability and induced absorption probability for radiative transitions is not generally valid for impurity centers in condensed media. An alternative approach to the problem is available by which estimates of the possible discrepancies can be made. Several mechanisms are discussed, particularly the Jahn-Teller effect, which depend on the existence of the phonon field and the lattice relaxation following absorption of a photon. In some simple cases, such as the $F$ center in KCl and LiF, the decay time predicted from the integrated absorption cross section may differ by an order of magnitude from that observed. In other simple and cubic systems, where the atomic symmetries are appropriate, the discrepancy may be even larger. In noncubic crystals similar discrepancies may be expected.

Relation between Absorption and Emission Probabilities in Luminescent Centers in Ionic Solids

A linear combination of localized orbitals-molecular orbital (LCLO-MO) cluster method is applied to calculate the electronic structure of ${{E}_{1}}^{\ensuremath{'}}$ centers in the $\ensuremath{\alpha}$-quartz structure of Si${\mathrm{O}}_{2}$ An ${\mathrm{O}}^{\ensuremath{-}}$ vacancy model, implying a net charge of $|e|$, is used This corresponds to the ${F}^{+}$ center in simpler oxides Calculations on the intrinsic ${{E}_{1}}^{\ensuremath{'}}$ center predict a highly asymmetric relaxation of the two silicons adjacent to the ${\mathrm{O}}^{\ensuremath{-}}$ vacancy Detailed calculations of hyperfine parameters are carried out, and reasonable agreement with experiment is obtained The asymmetric relaxation of the two silicons is an example of a pseudo-Jahn-Teller effect; parameters associated with this description are obtained and appear to be reasonable Calculations on the extrinsic ${{E}_{1}}^{\ensuremath{'}}$ (Ge) center in Si${\mathrm{O}}_{2}$ also give satisfactory results

W. Beall Fowler

Papers

Physics of color centers

Oxygen vacancy model for the E1′ center in SiO2

Oxygen vacancy and the E 1 ’ center in crystalline SiO 2

Relation between Absorption and Emission Probabilities in Luminescent Centers in Ionic Solids

Electronic structure of E 1 ′ centers in Si O 2