Optical spectroscopy of negative-U centers (original) (raw)

Electron localization in the insulating state: Application to crystalline semiconductors

Physical Review B, 2001

We measure electron localization in different materials by means of a "localization tensor", based on Berry phases and related quantities. We analyze its properties, and we actually compute such tensor from first principles for several tetrahedrally coordinated semiconductors. We discuss the trends in our calculated quantity, and we relate our findings to recent work by other authors. We also address the "hermaphrodite orbitals", which are localized (Wannier-like) in a given direction, and delocalized (Bloch-like) in the two orthogonal directions: our tensor is related to the optimal localization of these orbitals. We also prove numerically that the decay of the optimally localized hermaphrodite orbitals is exponential.

Electronic states at junctions of molecular semiconductors

Journal of Physics and Chemistry of Solids, 2008

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Optical spectroscopy of single quantum dots at tunable positive, neutral, and negative charge states

2001

We report on the observation of photoluminescence from positive, neutral and negative charge states of single semiconductor quantum dots. For this purpose we designed a structure enabling optical injection of a controlled unequal number of negative electrons and positive holes into an isolated InGaAs quantum dot embedded in a GaAs matrix. Thereby, we optically produced the charge states-3,-2,-1, 0, +1 and +2. The injected carriers form confined collective 'artificial atoms and molecules' states in the quantum dot. We resolve spectrally and temporally the photoluminescence from an optically excited quantum dot and use it to identify collective states, which contain charge of one type, coupled to few charges of the other type. These states can be viewed as the artificial analog of charged atoms such as H − , H −2 , H −3 , and charged molecules such as H + 2 and H +2 3. Unlike higher dimensionality systems, where negative or positive charging always results in reduction of the emission energy due to electron-hole pair recombination, in our dots, negative charging reduces the emission energy, relative to the charge-neutral case, while positive charging increases it. Pseudopotential model calculations reveal that the enhanced spatial localization of the hole-wavefunction, relative to that of the electron in these dots, is the reason for this effect.

Deep Level Spectroscopy In Semiconductors. Some Problems Of Interpretation

Lithuanian Journal of Physics

In the work we discuss a problem of a choice of correct model for the theoretical calculations of deep level spectra for experimental data analysis. The realistic model defines the reliability of determined parameters of centres. We have undertaken attempt to compare various approximations developing popular Lucowsky model, under the framework of δ-potential of the centre. Is shown, that the smooth exponential edge of photoionization spectra of centres can with the same success be determined both by the influence of an electron-phonon interaction, and by the influence of the effects of Coulomb interaction between a centre and optically released carriers. Both approximations give essentially different parameters of centres with the description of the same spectra. The necessity of the additional experimental analysis, for example, with attraction of temperature changes of spectra is discussed.

Characterization of Deep Levels Spectra in Semiconductors

Lithuanian Journal of Physics

A problem of a choice of correct model for the calculation and analysis of the spectra of deep levels is discussed. The realistic model determines reliability of the centers parameters defined from a data analysis. Here we have undertaken attempt to compare various approximations developing popular Lucowsky model, based on the delta-potential approximation for the core of the centers. Is shown, that the smooth exponential edge of photoionization spectra of centers can with the same success be determined by the influence both of an electron-phonon interaction, and of a Coulomb interaction between a centre and optically released carriers. Both approximations give essentially different parameters of centers for the description of the same their spectrum. The necessity of the additional experimental analysis, for example, with use of temperature changes of spectra is discussed.

Spectroscopy of Single Semiconductor Quantum Dots at Negative, Neutral, and Positive Charge States

physica status solidi (a), 2002

We study optically single self-assembled quantum dots embedded within the wide quantum well of a mixed type quantum structure. We compare the steady state and pulsed photoluminescence spectra of these dots to those of previously studied "regular" dots. We unambiguously identify experimentally emission from various discrete charge state of the dots. We provide means for optically tune the charge state of the dot, both negatively and positively. Our observations are used to accurately determine the asymmetry between the quantum dots' confined electron and hole envelope wavefunctions.

Donor and acceptor-like electronic states in a one-dimensional semiconductor

Surface Science, 2006

Donor and acceptor-like electronic states were observed near the conduction and valence band, respectively, in a semiconducting single-wall carbon nanotube using scanning tunneling microscopy and spectroscopy. They are observed to be, spatially, at the same location and spread within a few nm. Their physical origin is suggested as locally formed excessive and deficient number of electrons per a closed carbon network.

Experimental self-energy corrections for various semiconductors determined by electron spectroscopy

Physical Review B, 1988

This paper determines the energy dependence of experimental self-energy corrections, h(E), for local-density-functional-theory eigenvalues due to many-body interactions in Si, Ge, GaAs, and ZnSe. The self-energy corrections are determined by comparing shifts between the theoretical local-density approximation and experimental densities of state obtained with use of x-ray photoemission and bremsstrahlung isochromat spectroscopy (inverse photoemission). 8(E) is nominally independent of energy in the conduction band for these semiconductors but exhibits a distinct energy dependence within the valence band. The results compare well with recent quasiparticle calculations in general, although the observed energy dependence is not predicted by theory in the case of Ge. The self-energy corrections are strongly dependent on the localization of the state. Many-body effects due to plasmon effects are also investigated with use of core-level data. The inelastic background due to electron-hole pair production is roughly a factor of 10 lower for inverse photoemission compared with photoemission; a result which suggests that some many-body effects may be intrinsic. Intriguing conduction-band results for states up to 600 eV above the Fermi level are presented. ' This error accounts for a major part of the error when the LDA is applied to atoms. ' ' " Unfortunately the selfinteraction correction does not improve the agreement of the band structure and does not alter the energy of the unoccupied states. ' '" Furthermore, calculations of many electron atoms have indicated that the problem is not due to the locality assumption in LDA but rather to density-functional theory itself. ' ' Sham and Schliiter have proposed that the band-gap problem of LDA may be due to a discontinuity in the exchange-correlation potential at the band gap. ' This discontinuity could account for the band-gap errors but would not necessarily improve theoretical bandwidth or dispersion curves. The alternate approach is to calculate the self-energy 37 4618

Band-Resolved Double Photoemission Spectroscopy on Correlated Valence Electron Pairs in Metals

Physical review letters, 2017

Correlated valence electrons in Ag and Cu are investigated using double photoemission spectroscopy driven by a high-order harmonic light source. Electron pairs consisting of two d electrons as well as pairs with one sp and one d electron are resolved in the two-dimensional energy spectrum. Surprisingly, the intensity ratio of sp-d to d-d pairs from Ag is 3 times higher than in the self-convoluted density of states. Our results directly show the band-resolved configurations of electron pairs in solids and emphasize a band-dependent picture for electron correlation even in these paradigmatic metals.

One-photon two-electron transitions at surfaces

Solid-State Photoemission and Related Methods, 2003

We observed the emission of correlated electron pairs from the valence band of solids following the absorption of single photons in the vacuum ultraviolet range. Applying a time-of-flight technique, we measured the energy distributions of correlated electron pairs emitted from clean Cu(001) and Ni(001) crystals. Significant differences between both metals were found. For the interpretation we suggest a single step two-electron-photoionization process and a competing two step mechanism involving a single photoionization and a subsequent electron-electron collision in the valence band.