Papers by Keyword: Excitons

Abstract: The centers of bismuth (Bi) in silicon are being scrutinized as the defect qubits for mostly developed integrated electronics including its photonic component and we have applied the positron annihilation lifetime spectroscopy (PALS) to gain deeper insight into symmetry of the Bi impurity center whose configuration was modified by 15 MeV proton irradiation. It was revealed that hyperfine (hf) and super-hyperfine (shf) interactions of the nuclear and electron spin systems of the bismuth impurity center, ²⁰⁹Bi (J = 9/2), with the regular ²⁹Si (J = 1/2) atoms of silicon delay the essentially local event of emitting of a couple of annihilation gamma–quanta from within the crystal cell which comprises Bi impurity atom (J is the nuclear spin). This phenomenon is observed under increasing occupancy of Bi donor ground and excited states, in contrast to a profoundly enriched ²⁸Si (J = 0) material (so-called “semiconductor vacuum”) where content of ²⁹Si (J = 1/2) isotope was suppressed up to the value of ≈ 50 ppm. The many-body exciton-like states comprising a polyelectronic exciton {e⁻e⁺e⁻h} at Bi donor center are suggested for interpreting the data. The proton irradiation leads to acquiring by Bi impurity atom of an open volume ( V_op ) which is splitted in [V_op – Bi] complex. This defect possessing of D_3d symmetry dominates in the irradiated material. Being thermally stable up to ≈ 370 °C, [V_op – Bi] complex is annealed at ~ 470 – 500 °C. These data agree well with the results of ab intio cluster calculations performed on the basis of LDA-KKR formalism for exploring both the energy gain and symmetry of Bi–vacancy complex.

Abstract: Photoluminescence (PL), ultraviolet-visible (UV-VIS) absorption spectroscopy and spectroscopic ellipsometry (SE) are combining to study the optical transitions and electronic structures of Cu doped ZnO films with Cu concentration of 0, 2 and 8 at. %. By increasing Cu concentration, PL spectra show the enhancement of optical transition through the significantly increase of the green emission. Correspondingly, the in-plane UV visible absorption spectra show the redshift of first interband transition as the evidence of ZnO film become less insulating. More interestingly, extracted optical properties such as n and k clearly shows the mid-gap optical states, interband transitions and excitons. The excitonic peak decrease by increasing Cu concentration accompanied by a new broad structure assigned as a mid-gap state located under the conduction band area. Excitonic states are screened as decreasing its intensities by increasing Cu doping. Our results are beneficial to define interband transition and explain the mid-gap state in the oxide based semiconductor.

Abstract: Light-emitting diodes (LEDs) based on single crystal SiGe with the Ge content of 5.2% were fabricated using a planar technology. Their electroluminescence (EL) parameters were studied over a wide range of measured currents (up to 11 A) and temperatures (80 - 300) K. The integrated EL intensity at a fixed current increased approximately two times with temperature increasing from 80 to 200 K and changed insignificantly in the temperature range of 200 – 300 K. The analysis of the EL spectra shows that the recombination involving excitons is the dominant mechanism of radiative recombination at both no-phonon and phonon-assisted transitions in SiGe LEDs not only at low temperature but at room temperature, too. The linear dependence of the integrated EL intensity on the current and the exponential decay of the integrated EL intensity confirm this conclusion. The room temperature internal quantum efficiency of EL in the region of band-to-band transitions is estimated to be 0.5%. A sublinear current dependence of the integrated EL intensity and a fast decay of the integrated EL intensity after the diode turn-off were observed at room temperature and currents > 2.5 A. The effect is associated with the appearance of an additional (Auger) mechanism of non-radiative recombination parallel to Shockley-Read-Hall recombination.

Abstract: Dispersion laws and states (i.e. probability of finding) of Frankel excitons in ultra-thin molecular films are found using a well-known method of Green’s functions. Space boundaries and changes of energetic parameters on boundaries are considered as perturbations. The cubic crystalline system with complex cell consisted of two molecules (a and b), i.e. bimolecular film, was analyzed in harmonic approximation, and then compared with the results obtained for simple cubic cell systems (i.e. monomolecular film). In both cases the energy spectra show sharp discrete levels, although the energy spectra of bimolecular films split into two zones with discrete levels. Probability of finding exciton in the mono- or bimolecular ultra-thin films is significantly influenced by the perturbation and the values of on-site energies of molecules a and b. Obtained conditions of the existence of localized exciton states at boundaries are of special interest.

Abstract: The linear and nonlinear optical properties of pure 3,3'-bis(3-carboxypropyl)-5,5',6,6'- tetrachloro-1,1'-dioctylbenzimidacarbocyanine (C₈O₃) aggregates as well as of C₈O₃ aggregates in the presence of PVA are studied at room temperature. The obtained results reveal that absorption spectra of both aggregates are governed by four absorption bands. Three of these represent transitions having dipole moments oriented parallel to the axis of the aggregate and one oriented perpendicular to it. The two lower energy parallel transitions are found to be weakly coupled which suggests that they represent two excitons located on different walls of the double-wall cylinder. In contrast, the lowest energy parallel transition and the transition having a transition dipole moment oriented perpendicular to the aggregate axis share the same ground state which reveals that they belong to the same exciton band. The manner of modification of the linear absorption with addition of PVA as well as the similarity of the pump-probe spectra of the two investigated aggregates suggests that this exciton band originates from the inner wall of the double-wall cylinder.