Direct bandgap optical transitions in Si nanocrystals

JETP Letters - The effect of quantum confinement on the direct bandgap of spherical Si nanocrystals has been modelled theoretically. We conclude that the energy of the direct bandgap at the...     

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.     

Theoretical modeling of excitation and de-excitation processes of Er in SiO2 with Si nanocrystals

We construct the theory of carriers confined in Si quantum dots with finite energy barriers for electrons and holes in the framework of the multiband effective mass theory. We apply this theory for theoretical modeling of the excitation of erbium inside and outside of Si nanocrystals in SiO₂ matrix due to the Auger process induced by the recombination of a confined electron-hole pair as well as the intraband transitions of “hot” confined carriers. Auger de-excitation processes of the Er³⁺ ion leading to the quenching of erbium luminescence are discussed as well.     

Effect of a magnetic field on thermally activated tunneling ionization of impurity centers in semiconductors

An expression for the probability of thermally activated tunneling ionization in an electric field in the presence of a magnetic field is obtained. It is shown that the logarithm of the ionization probability is proportional to the squared electric field, and the coefficient of proportionality decreases with increasing magnetic field. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 10, 763–767 (25 November 1998)     

Effective Hamiltonian of silicene in the presence of electric and magnetic fields

An effective Hamiltonian of silicene in the neighborhood of Dirac points in the presence of electric and magnetic fields perpendicular to the plane of the film is constructed on the basis of symmetry analysis. Numerical coefficients of various terms in the Hamiltonian are obtained by the tight binding method in the basis sp ³ d ⁵ s* with regard to the interaction with one nearest neighbor. This method was developed in the previous paper [1] in the case of a sublattice displacement of 0.44 Å, which corresponds to the theoretical value of displacement obtained from first principles for a free film of silicene. The effect of the displacement of sublattices on the orientation of spin and pseudospin in silicene is analyzed. The Hamiltonian obtained allows one to consider spin and electron transport for charge carriers with energy less than 0.5 eV. The orbital motion of electrons in an external magnetic field perpendicular to the film is analyzed in detail.     

Resonant acceptor states in uniaxially strained semiconductors

A new approach to calculating the parameters of resonant states is proposed, which also makes it possible to determine the probabilities of the resonant scattering and capture probabilities at the resonant state. This approach is based on the application of the method of configuration interaction, which was proposed for the first time by Fano for an analysis of field ionization of the helium atom. Following Fano, use is made of two different Hamiltonians of the initial approximation for the states of continuum and the initial local state. Following Dirac, the wave functions are constructed in the same way as in the general theory of scattering. A detailed analysis and specific calculations are made for resonant acceptor states in uniaxially strained germanium under a pressure directed along the [001] and [111] axes.     

Nanosecond dynamics of the near-infrared photoluminescence of Er-doped SiO2 sensitized with Si nanocrystals

We report on an observation of a fast 1.5 microm photoluminescence band from Er3+ ions embedded in an SiO2 matrix doped with Si nanocrystals, which appears and decays within the first microsecond after the laser excitation pulse. We argue that the fast excitation and quenching are facilitated by Auger processes related to transitions of confined electrons or holes between the space-quantized levels of Si nanocrystals dispersed in SiO2. We show that a great part--about 50%--of all Er dopants is involved in these fast processes and contributes to the submicrosecond emission.     

Resonant acceptor states and terahertz stimulated emission of uniaxially strained germanium

The stimulated emission spectrum of uniaxially strained p-Ge is presented. The energy spectrum of the states of a shallow acceptor in Ge under uniaxial compression is calculated. The threshold pressure at which the acceptor state split off from the ground state becomes resonant is found. The pressure dependence of the width of this resonant level is calculated. The stimulated emission lines are identified. In particular, it is shown that the principal emission peak corresponds to the transition of holes from the resonant 1s (1s _r) state to the local p _±1 state. The probabilities of optical transitions are calculated. A mechanism of population inversion due to the intense resonant scattering of hot holes with an energy corresponding to the position of the 1s _r level is proposed. Zh. éksp. Teor. Fiz. 115, 89–100 (January 1999)     

Erbium electroluminescence excitation in amorphous hydrogenated silicon under thermally stimulated deep-center tunneling ionization

A study of the electroluminescence of erbium-doped, amorphous hydrogenated silicon, a-Si:H 〈Er〉, is reported. It has been found that the electroluminescence intensity at the wavelength λ=1.54 μm corresponding to the ⁴ I _13/2→⁴ I _15/2 intra-4f shell transition in Er passes through a maximum near room temperature. The unusual temperature and field dependences of the electroluminescence indicate electric-field induced multi-phonon tunneling emission of electrons from deep centers. The electroluminescence of Er³⁺ ions is due to their becoming excited as conduction-band electrons are captured by neutral dangling bonds (D ⁰ centers), which form when erbium is incorporated into the amorphous matrix. This Auger process transforms the center from its neutral state, D ⁰, to a negatively charged state, D ⁻, and the energy released in the capture is transferred by Coulomb interaction into the erbium-ion 4f shell. The steady-state current through the electroluminescent structure is supported by the reverse process of multi-phonon tunneling-electron emission from the D ⁻ center to the conduction band. The proposed theoretical model is in a good agreement with experimental data. Fiz. Tverd. Tela (St. Petersburg) 41, 210–217 (February 1999)     

Energy transfer between silicon nanocrystals

It is shown that the energy migration between silicon nanocrystals embedded into a silicon dioxide host is caused by the “nonresonant” dipole-dipole interaction. This process is efficient only for a part of small nanocrystals among the whole ensemble of nanocrystals. The nonresonant dipole-dipole energy transfer has such a feature as the emission of two optical phonons at each step of the process. The time of the excitation transfer has been experimentally determined for nanocrystals 1.5 nm in size existing in the ensemble of nanocrystals with a density of 10¹⁸ cm⁻³ and the size distribution with a standard deviation of 20%. A value of 30 μs obtained for this time is in good agreement with the performed theoretical estimation.