Cathodoluminescence properties of silicon nanocrystallites embedded in silicon oxide thin films

Cathodoluminescence (CL) spectra for the Si nanocrystallites embedded in a matrix of silicon oxide films are measured at room temperature. The CL spectra consist of two principal bands whose peak energies are in a near-infrared (NIR) region (<1.6 eV) and in a blue region (2.6 eV), respectively. The spectral feature of the NIR CL band is similar to the corresponding PL spectra. The strong correlation between the presence of Si nanocrystallites and the formation of the NIR CL band are found as well as the PL spectrum. The peak energy of the blue CL band is slightly lower than that of the luminescence band originating from oxygen vacancies (≡Si–Si≡) in SiO₂. Therefore, the blue CL band is considered to come from Si_n clusters with n3 in the oxide matrix. Under irradiation of electron beams, degradation of the intensity is observed for both the CL bands but the decay characteristics are different.     

Modification of porous silicon in ultrahigh vacuum and contribution of graphite nanocrystallites to photoluminescence

A replacement of the adsorbate in porous silicon is carried out in ultra-high vacuum. The photoluminescence line is shifted and quenched as the products of anodization of silicon — silicon hydrides and atomic and molecular hydrogen — undergo thermal decomposition and desorption. Adsorption of molecular chlorine restores the 560 nm photoluminescence band, which we identified as radiation from graphite nanoparticles. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 2, 106–111 (25 January 1996)     

Size evolution and photoluminescence of silicon nanocrystallites in evaporated SiOx thin films upon thermal processing

Silicon suboxide thin films have been fabricated by physical vapor deposition of silicon monoxide in vacuum at controlled oxygen partial pressure. These films undergo a phase separation into silicon- and oxygen-enriched regions upon thermal processing. At temperatures around 900 °C, the onset of Si nanocrystallite formation is observed, regardless of film stoichiometry. With increasing initial oxygen content of the films, the mean size of created nanocrystallites decreases whereas the corresponding photoluminescence emission blueshifts. The photoluminescence intensity increases with increasing annealing temperature up to 1050 °C. Upon resonant excitation at low temperatures, the photoluminescence exhibits phonon replica signature. Therefore, the emission may be attributed to excitonic recombination in the nanocrystallites. Received: 3 July 2001 / Accepted: 6 August 2001 / Published online: 17 October 2001     

Excitons in Si nanocrystals

The states of electron-hole pairs in spherical silicon nanocrystals are theoretically studied using the “multiband” effective-mass approximation in the limit of an infinitely high potential barrier at the boundary. The degeneracy of the states at the top of the valence band is taken into account in the spherical approximation, and the ellipsoidal character of the electronic spectrum in the conduction band is allowed for. Coulomb interaction-induced corrections to the energy of an electron-hole pair are found.     

Excitons in organic semiconductors

In this article, excitonic effects in organic semiconductors investigated within the framework of many-body perturbation theory are reviewed. As an example for this technologically relevant class of materials the oligoacene series is studied. The electron–hole interaction is included by solving the Bethe–Salpeter equation for the electron–hole Green's function. This approach allows for the evaluation of the exciton binding energies, which are of major interest concerning the application in organic opto-electronic devices. We start the discussion with a comparison of the Kohn–Sham band structure with recent angular resolved photo-emission data. Starting from this one-electron band structure we focus on the impact of the electron–hole interactions on the optical properties by solving the Bethe–Salpeter equation. We demonstrate the dependence of the exciton binding energy on the molecular size and emphasize the effect of the intermolecular interaction on the exciton binding energies by means of pressure investigations. To cite this article: P. Puschnig, C. Ambrosch-Draxl, C. R. Physique 10 (2009).     

Excitons in the polyenes

The excited electronic states of an infinite, all-trans, polyene are calculated using a Wannier function expansion. The relationship between these states and the excited electronic states of the finite polyenes, as found in molecular orbital configuration interaction studies, is explored.     

Excitons in quantum wires

This paper deals with excitons in quantum wires. We first study these excitons as the limit of excitons in D dimensions when . In order to do it, we have had to find a new resolution of the hydrogen atom Schrödinger equation: besides the fact that the usual resolution found in textbooks is not valid for D exactly equal to 1, it is, surprisingly enough, inconsistent since it relies on two hypergeometric functions which are not independent for the parameters of physical interest! In a second part, we write down the exact potential felt by the exciton relative motion along the wire in terms of the wire confinement. This allows a quite precise determination of the effective Coulomb potential for this 1D motion, which is of crucial importance to obtain a meaningfull finite value for the exciton ground state energy. In a last part, we study the dependence of the exciton energies on the wire area and anisotropy. While the quantitative results are here given for cylindrica l and rectangular wires with infinite barriers, we show how they can easily be extended to any particular wire shape and barrier height.Received: 5 August 2002, Published online: 23 July 2003PACS: 71.35.-y Excitons and related phenomena - 73.21.Hb Quantum wires     

Excitons in GaAs quantum wells

This paper attempts to summarize some of the salient properties of excitons in GaAs quantum wells and in doing so it will emphasize work at AT&T Bell Labs with which the authors have been associated. Although the text relies heavily on published material, an effort has been made to stress new material, and where feasible, unpublished aspects, e.g., figures, related to earlier work. Topics discussed on the quasi-2D excitons in GaAs quantum well include: their inherent tendency for intrinsic free-exciton emission, exciton binding energies, bound and localized excitons including biexcitons and excitons bound to neutral impurities, effects of n- and p-type modulation and antimodulation doping, and the developments leading to a proposed set of quantum well parameters that results in acceptable fits to the observed exciton transitions for GaAs quantum wells with both square and parabolic potential profiles.     

Excitons in hybrid organic-inorganic nanostructures

In two-dimensional heterostructures made of semiconductor and organic layers, when resonance between the Wannier and Frenkel excitons is realized, the dipole-dipole interaction coupling them leads to novel effects. First, we discuss the pronounced nonlinear optical properties of the hybrid Frenkel-Wannier excitons appearing when the energy splitting of the excitonic spectrum is large compared to the exciton linewidths (the case of strong resonant coupling). Next, we consider the case of weak resonant coupling for which the Förster mechanism of energy transfer from an inorganic quantum well to an organic overlayer is of great interest: the electrical pumping of excitons in the semiconductor quantum well could be employed to turn on efficiently the organic material luminescence.     

Excitons in ZnO Quantum Wells

Physics of the Solid State - Reflectance and photoluminescence spectra of the ZnO/Zn0.78Mg0.22O structures with ZnO quantum wells and thick ZnO and Zn0.78Mg0.22O layers have been thoroughly...     

Excitons in coupled quantum dots

Properties of excitons in vertically coupled GaAs/AlGaAs quantum dots were investigated using the variational method within the envelope function and effective mass approximations. It was found that when the thickness of the spacer layer becomes less than about one exciton Bohr radius, both the exciton binding energy and the fundamental optical transition energy are reduced compared to those in isolated quantum dots. This is a result of increased space extension of exciton due to the penetration of carrier wave functions into the spacer layer and corresponding reduction in confinement energy which dominates over the Coulomb interaction between the electron and the hole.     

Excitons in tubular molecular aggregates

We present a brief overview of recent work on the optical properties of molecular aggregates with a tubular (cylindrical) shape. The exciton states responsible for these properties can be distinguished with regard to a transverse wave number, which directly relates to optical selection rules and polarization direction of the associated absorption line. We discuss two types of analytical solutions for the exciton wave functions and the associated absorption and dichroism spectra.     

Excitons in quantum—dot quantum—well nanoparticles

A variational calculation is presented for the ground-state properties of excitons confined in spherical core-shell quantum-dot quantum-well (QDQW) nanoparticles. The relationship between the exciton states and structure parameters of QDQW nanoparticles is investigated, in which both the heavy-hole and the light-hole exciton states are considered. The results show that the confinement energies of the electron and hole states and the exciton binding energies depend sensitively on the well width and core radius of the QDQW structure. A detailed comparison between the heavy-hole and light-hole exciton states is given. Excellent agreement is found between experimental results and our calculated 1s_e－1s_h transition energies.     

Excitons and electron-hole plasma in InSe

The spontaneous emission from either thin or thick samples of layered indirect-gap InSe semiconductor under dye laser excitation has been investigated either above or below the critical Mott density. The observed spectra consist of five bands in the near infrared region; four of them are localized in the energy range between the direct and the indirect gap and one only lies below the indirect energy gap. These bands can be associated with cooperative indirect and direct excitonic transitions and with electronhole plasma recombinations.     

Excitons in strong coupling polyatomic crystals

In this paper, the effective Hamiltonian of the exciton in strong-coupling polyatomic crystals is obtained by the method of a linear combination operator and a simple unitary transformation. The self-trapping energy of the exciton could be written as a series in ^-1, the first term being proportion to , the coupling constant. The selftrapping energy contains an extra contribution due to crossed terms between the different phonon branches.