Core Exciton Research Articles

Electron spectroscopy on boron nitride thin films: Comparison of near-surface to bulk electronic properties

Combining spectroscopic methods probing both occupied as well as unoccupied electronic states, the surface electronic structure of ex situ prepared boron-nitride films is analyzed and compared to experimental and theoretical bulk-electronic properties taken from the literature. X-ray photoelectron spectroscopy is applied to probe the core-level and valence-band electronic states, electron-energy-loss spectroscopy in the reflection geometry to investigate conduction band states as well as excitations like plasmons and core excitons. For films with hexagonal structure, the results from the near-surface region are found to reflect both the ground state and the many-body properties of the bulk material. Cubic boron nitride films in all cases exhibit a hexagonal-like top layer with a thickness of about 0.9 nm. Low-energy ion bombardment at room temperature is found to significantly increase the amount of disorder in both types of films, leading to the transformation of the cubic phase into a hexagonal-like material.

An Electron-Ion Coincidence Spectroscopy Study of Ion Desorption Induced by Core-Electron Transitions of Surfaces

Ion desorption induced by core-electron transitions is studied for various surfaces by electron-ion coincidence spectroscopy combined with synchrotron radiation. In a study of F+ desorption from Si(100) terminated by fluorine using photoelectron photoion coincidence spectroscopy, the site-selective F+ desorption is directly verified; that is, F+ desorption is induced by Si 2p photoionizations at the ≡SiF, =SiF2, and –SiF3 sites. An Auger electron photoion coincidence study of a CaF2(111) film epitaxially grown on a Si(111) surface also presents direct evidence of F+ desorption, in this case induced by F 1s surface core exciton. These investigations demonstrate that electron-ion coincidence spectroscopy combined with synchrotron radiation is a powerful tool for studying ion desorption induced by core-electron excitations.

Impurity core–valence luminescence in Rb 1− xCs xCl mixed crystals

Core–valence luminescence, caused by radiative recombination of holes of the upper core band with valence electrons, has been studied in Rb 1− x Cs x Cl ( x=0.02–1) crystals using X-ray and VUV synchrotron excitation in the temperature range from 7 to 300 K. The luminescence excitation spectra exhibit thresholds of 14.2 and 14.5 eV at 300 and 7 K, respectively, and clearly resolved structures in the range of creation of 5pCs and 4pRb core excitons. Light yields of the mixed crystals turn out to be larger than that of pure CsCl under excitation by 15.1 eV photons, i.e. at the maximum of the excitation spectrum. It is revealed that the low energy core excitons (5pCs, j= 3 2 →Γ 12 ) can also be involved in excitation of core-valence luminescence at high temperature. Creation of [CsCl 8] clusters in heavy-doped crystals ( x>0.36) is suggested. The clusters are responsible for the additional emission band (5.1 eV) in Rb 1− x Cs x Cl.

Peculiarities of excitation of Ce-emission in core region of chlorine perovskites

Excitation spectra of Ce-doped KCaCl3, RbCaCl3, CsSrCl3 and CsCaCl3 crystals were studied at 300 and 9K in the 4–35eV spectral range using synchrotron radiation. Peculiarities of the excitation spectra in the region of the uppermost core are considered from the view point of feasibility of the energy transfer from the core excitons and holes to Ce-centers. The possible channels for excitation of the activator luminescence are assumed for the CsCaCl3 and CsSrCl3 crystals exhibiting core-valence luminescence (CVL) as follows: (i) the reabsorption of the CVL by Ce-centers; (ii) the creation of the valence holes on account of radiative decay of core excitations via CVL and (iii) the migration of the core holes. All these schemes are distinctly impossible for the CVL-free KCaCl3 and RbCaCl3 crystals where core excitations decay just through the non radiative Auger-process.

Electronic and atomic structures of Si–C–N thin film by X-ray-absorption spectroscopy

Abstract This study measures X-ray absorption spectra of crystalline (c)-Si–C–N thin film at the C and Si K-edge using the sample drain current mode, and at the N K-edge using the fluorescent mode. A resonance peak resembling the C 1s core exciton in CVD-diamond/Si is observed, and a broad feature occurring in the energy range between ∼290 and 300 eV can be assigned to the antibonding C 2p–Si 3sp hybridized states and the C 2p–N 2sp hybridized states as well. Analysis of the N K-edge near edge absorption spectra reveals a similar feature in c-Si–C–N and a-Si3N4, suggesting that nitrogen atoms generally have similar local environment in these two materials. Moreover, results obtained from Si K-edge absorption spectra for c-Si–C–N demonstrate a proportional combination of local Si–N and Si–C bonds, associated with the local tetrahedral C–Si–N3 as well as the long-range ordered atomic structure around Si atoms.

Resonant behavior of satellite photoelectrons in the Ni 3p and 3s region at the Ni 2p excitation of K 2Ni(CN) 4

Resonant Ni 3p and 3s photoelectron spectra were measured at the Ni 2p edge for anhydrous K 2Ni(CN) 4 complex. A number of satellite series, which are mostly forbidden in the non-resonant spectrum, become strongly observable via the resonant excitations. Each satellite lowers its kinetic energy for the higher resonance energy, showing a one-to-one correspondence with the Ni 2p 3/2 and 2p 1/2 resonantly excited states. This behavior manifests a one-electron feature in the core excitons, where electrons are bound by the singly and doubly charged states in the intermediate and final states, respectively, and is characteristic of molecular systems in solids such as Ni(CN) 4 2−.

Site-Specific Excitation in Free Krypton Clusters

Site-specific excitation in Kr clusters is investigated by high resolution inner-shell excitation in combination with model calculations, which are based on a core exciton model. Partial cation and total electron yield spectra of variable size Kr clusters are reported for the $\mathrm{Kr}3d$ excitation regime (90--96 eV) using synchrotron radiation. A cluster size-dependent spectral evolution is observed, corresponding to the transition from low lying cluster Rydberg states into surface and bulk exciton states. The results indicate that individual sites in perfectly and imperfectly shaped clusters are clearly distinguished.

Effect of confined electrons on the centre-of-mass quantization states of the exciton in wide modulation-doped CdTe/CdZnTe quantum wells

Optical spectra of wide quantum wells show lines associated with the quantization of the exciton centre-of-mass motion. In wide (400−600 Å) CdTe/CdZnTe quantum wells containing electrons we observe that photon absorption creates not only the fundamental negatively charged exciton X − but also excited states of X − in which the core exciton in the X − structure is size quantized. The absorption spectra show a charged exciton line X −(N) below each quantized (neutral) exciton line X(N).

Luminescence of cation excitons in and crystals in a wide excitation VUV region

Host luminescence excitation spectra and reflection spectra of and crystals at low temperatures have been measured in the energy region of 4-30 eV using synchrotron radiation. High-efficiency host luminescence is connected to radiative decay of self-trapped cation excitons. It is shown that also in the region of excitation multiplication, e.g. if the excitation energy is larger than twice the band gap energy , and in the relaxation process of core excitons the radiative self-trapped cation excitons are formed. However, the recombination of hole and self-trapped electrons (band-band excitation) does not give rise to host luminescence.

Raman vs Rayleigh scattering in the soft-x-ray region

A sharp and intense structure of the core exciton is observed in total photon yield spectra of a cubic BN crystal at 192 eV, below the transition from the boron 1$s$ level to the conduction-band edge. We have discovered, however, that the Raman intensity for leaving behind the valence exciton does not show any enhancement even when the frequency of the incident field crosses the core-exciton level. This comes from the fact that relative magnitudes among the nonlinear channels depend on the relevant frequency. We analyzed the Raman spectra taking into account the strong coupling between the core exciton and radiation fields, i.e., the polariton effect, Rayleigh scattering, and the Auger process.

Manifestation of the 3 p K +-core excitons in impurity core-valence luminescence of K 1− xCs xCl mixed crystals

A specific impurity emission caused by radiative recombination of valence electrons with the holes, originating from the highest-lying impurity core states is studied in K 1− x Cs x Cl mixed crystals. The emission excitation spectra show a clear anticorrelation with the reflection spectra of KCl in the region of 3 p K-core excitons. We propose that the observed effect originates from reflection losses of the excitation energy and/or luminescence quenching caused by formation of 3 p K-core excitons. The influence of Auger decay of the outermost core excitations is briefly discussed.

Effects of the Core Hole Potential on the B 1s Emission in Hexagonal BN

We have investigated the anisotropy of the spectra and the polarization degree of the B 1 s emission in hexagonal boron nitride using synchrotron radiation as a light source. Spectator emission appears when the B 1 s hole is radiatively filled with the valence electron while the core exciton state being occupied. It has been found that π emission is absent or very weak in the spectator emission, while both π and σ components are present in the normal emission. This result has been explained in terms of the Coulombic effect of the core hole on the localized B 2 p z valence orbital.

Photoemission study of GaSb(111) and Ge adsorbed on GaSb(111) surfaces

Clean - surfaces, prepared by cycles of ion bombardment and annealing, and GaSb(111)A with various Ge coverages were investigated using low-energy electron diffraction and different modes of photoelectron spectroscopy with synchrotron radiation. Valence band energy distribution curves, and constant-initial-state, and constant-final-state spectra obtained for the clean surface exhibit strong resonances at 19.2 and 19.7 eV which are associated with surface Ga and core excitons. The amplitudes of the resonances decrease monotonically with increasing adsorbate layer thickness. At a Ge coverage of 1.9 Å the resonances disappear, and concomitant changes in the adsorbate and substrate core-level spectra are observed. The data provide detailed information about the chemical bonding between adsorbate and substrate atoms, and indicate that the growth mode is primarily layer by layer but is not epitaxial.

Phonon Relaxation and Resonant X-Ray Emission Spectrum in 1s Core Exciton State of Diamond

The phonon relaxation dynamics in the creation of the C Is core exciton in diamond is investigated theoretically by calculating the X-ray absorption and resonant X-ray emission spectra. The C Is core exciton state in diamond strongly couples with the lattice local phonon mode. As a result of this, the exited C atom relaxes to a new equilibrium position lying in the [111] direction. Due to the relatively short lifetime of the core excited state, this off-center displacement is exhibited as the hot luminescence in the resonant X-ray emission spectrum. Spectral features experimentally obtained by Ma et al. are qualitatively well reproduced by the simple vibronic cluster model.

Core Excitonic Effects in the NEXAFS Spectra of Condensed Polycyclic Aromatic Hydrocarbons

NEXAFS spectra of three condensed aromatic hydrocarbons, chrysene, perylene, and coronene, were measured and analyzed in particular emphasis on the effect of core excitonic effect as the origin of modification of the observed spectra from density of unoccupied states (DOUS). The comparison of the observed spectra with reported inverse photoemission spectra and calculated DOUS showed poor correspondence, indicating large deviation between the NEXAFS spectra and DOUS. On the other hand, the simulation of the spectra by ab inito MO calculations, using the hole potential method, reproduced the experimental results well. The analysis of the results of MO calculations revealed that core excitonic effect significantly depends on the site of the excited atom and the character of the unoccupied orbital, making the observed spectra much different from the DOUS.

Lattice relaxation and resonant X-ray emission of C 1s core exciton in diamond

The lattice relaxation dynamics in the creation of the C 1 s core exciton in diamond is investigated theoretically by calculating the X-ray absorption and resonant X-ray emission spectra. The excited C atom relaxes to a new equilibrium position lying in the [111] direction as a result of the strong exciton-phonon coupling. Due to the relatively short lifetime of the core excited state, this off-center displacement is exhibited as the hot luminescence in the resonant X-ray emission spectrum. Spectral features experimentally obtained by Ma et al. are qualitatively well reproduced.

Observation of resonant Raman scattering at the Si L2,3 core exciton.

The resonant soft-x-ray emission spectra at the Si ${\mathit{L}}_{2,3}$ core exciton have been measured at 18 K by using synchrotron radiation. As an excitation light source, a very high-resolution monochromator with the undulator was used. The Raman scattering whose energy shifts in proportion to the excitation energy is found below the ${\mathit{L}}_{2,3}$ absorption edge. The intensity of the Raman scattering increases rapidly as the excitation energy comes close to the ${\mathit{L}}_{2,3}$ core exciton. It is found that the Raman scattering resonates at the ${\mathit{L}}_{2,3}$ core exciton which is made around the ${\mathit{X}}_{1}$ conduction minimum. The elementary excitation of the Raman scattering is the valence exciton that is a transition from ${\mathit{X}}_{1}$ or ${\mathit{X}}_{4}$ points in the valence band to the ${\mathit{X}}_{1}$ conduction minimum. The ${\mathit{X}}_{4}$ valence exciton has a characteristic excitation-energy dependence, so that it becomes broad and weak as the resonant Raman process goes to the ordinary Raman process. \textcopyright{} 1996 The American Physical Society.

Decay of core excitons and STE formed at high density in BaF 2 and CsCl single crystals

It was aimed to study the effect of high-density excitation on the core excitons and STE's using ion iradiation. Both exciton luminescence present the shortening of the decay times dependent on the density of the excitation. Models of competition of the recombination of free electrons ejected with core holes and exchange interaction between STEs are proposed for the core excitons and STE, respectively.

Surface core exciton of alkali chloride film on Si(100)

Surface core exciton has been observed in the partial-photoelectron-yield spectra in the Cl-L 2,3 region of alkali chloride (KCl and NaCl) film deposited on the Si(100) surface. Compared to the bulk core exciton peaks, the surface core exciton peaks were shifted by 0.7 eV towards lower photon energy. This may be to the change in Madelung potential at the surface. The photoelectron spectra was measured with various photon energies near the Cl-L 2,3 edge for NaCl/Si(100), and it was found that the Auger spectra drastically change in shape at eht excitation energies corresponding to the bulk and surface core excitons.

Non-radiative decay channels of B 1 s core exciton in h-BN

The non-radiative decay channels following the resonant photoexcitation of B 1s to π ∗ of h-BN have been investigated using synchrotron radiation-excited photoelectron spectroscopy. Based on the comparison of the x ray absorption (XAS) spectrum to the constant initial state (CIS) spectrum with the initial state at the upper valence band, we have obtained a value of 0.66±0.08 for the probability for direct recombination of the exciton. This value is much higher than previously reported values for the probability for direct recombination of the Al 2 p, Na 2 p and Li 1 s excitons.