Core Excitons In Semiconductors Research Articles

Electronic structure of cationic core excitons in II–VI semiconductors

The electronic structure of cationic core excitons in ZnS, ZnSe, ZnTe, and CdTe was analyzed by means of self-consistent-field multiple scattering Xα molecular cluster calculations. The exciton level introduced in the gap by the core hole is found to be weakly localized with a charge distribution similar to that of an isocoric single donor level. Core exciton binding and formation energies and also core ionization energies were calculated. The binding energies of the core excitons in ZnS are found to be almost independent of the depth of the core level involved. This behavior seems to be a general property of core excitons in semiconductors. Trends for core exciton binding energies, core level shifts and charge distributions are analyzed in the ZnX (X = S, Se, Te) series and also in CdTe.

Effects of dynamical screening on resonances at inner-shell thresholds in semiconductors

A theory of core excitons in semiconductors is formulated, taking into account the frequency dependence of the dielectric matrix which screens the electron-hole attraction. The present approach combines standard many-body techniques (which reduce the Bethe-Salpeter equation for the two-particle Green's function to an effective eigenvalue problem) with elements drawn from Fano's formalism for discrete states interacting with continuum channels. The positions and the widths of core-exciton resonances are affected by dynamical screening, which increases the binding energy above its value for static screening and decreases its Auger width below its value for a core hole. The latter effect is peculiar to a dynamical theory and has recently been confirmed experimentally.

Core excitons in semiconductors with a decaying core hole

It is shown that deviations from static electronic screening, in addition to contributing an increase of the core-exciton binding energy EB, produce also a reduction of the core-exciton spectral width Γ as compared with the corresponding spectral width γ of the core hole alone. The occurence of this effect is interpreted as a fingerprint of dynamical screening since it cannot be obtained within the framework of a static theory. Numerical estimates for the Si 2p transition give about 70 meV for the difference γ-Γ, in qualitative agreement with experimental data.

Dynamical Shift and Broadening of Core Excitons in Semiconductors

A non-Hermitian eigenvalue equation is proposed to determine binding energies and widths of core excitons in semiconductors, taking into account the time dependence of screening effects through the dielectric matrix ${\ensuremath{\epsilon}}^{\ensuremath{-}1}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{r}}, {\stackrel{\ensuremath{\rightarrow}}{\mathrm{r}}}^{\ensuremath{'}};\ensuremath{\omega})$. Deviations from static screening contribute both an increase of the binding energy and a narrowing of the Auger width. Numerical estimates of both effects for the Si $2p$ transition give qualitative agreement with experimental data when the exciton size is reduced by band-structure effects.

Fine structure and temperature dependence of shallow core excitons in insulators and semiconductors

The excitonic spectra associated with the shallow core levels K(+)3p, Ga3d, and In4d in potassium halide and III-V semiconductor single crystals were studied by high resolution reflectivity (DeltaE < 10 meV) as a function of temperature down to 20 K. Analysis of the fine structure, transition energies, line profiles, halfwidths, and their temperature dependence yields information on the exciton (electron)-phonon interaction and on the influence of lattice expansion. Strong electron-phonon coupling dominates in the K(+)3p excitons as it does in F centers, whereas weak coupling describes the core excitons in semiconductors. The contributions of initial and final states are discussed. Core surface excitons in semiconductors show a significantly smaller temperature coefficient than the volume excitons.

Binding energies of core excitons in semiconductors

Excitons from core levels in semiconductors are treated by the effective-mass approximation. The problem of the appropriate electron-hole interaction is considered and the electron polaron model which treats valence electrons as quantum oscillators is adapted in order to include dynamical effects. An effective Hamiltonian is obtained, which contains electron and hole polarization self-energies and an electronhole interaction, all terms depending on variational parameters. Because of the large-hole self-energy the best choice of the variational parameters corresponds to free-electron and hole self-energies and to a Coulomb interaction screened by the static dielectric constant. Consideration of dispersive effects preserves the above results, but modifies the self-energies and introduces dispersive screening. The above results are unable to explain the large increase in binding energies of core excitons with respect to valence excitons. It is argued that this effect can be explained by nonlocal screening contributions due to off-diagonal terms in the dielectric matrix coupled to the interference of Bloch functions from equivalent minima at the hole position.

Theory of core excitons in semiconductors

A theoretical description of core excitons in semiconductors is given. The treatment is similar to that of isocoric impurities reported earlier. It is shown that the core exciton in silicon is valley—orbit split, unlike the indirect exciton formed during the excitation of valence-band electrons in that material. Numerical results are reported and general conclusions are drawn.

Theory of core excitons in semiconductors : S. T. Pantelides. Solid-St. Commun.16, 217 (1975)

Theory of core excitons in semiconductors : S. T. Pantelides. Solid-St. Commun.16, 217 (1975)