Bound Exciton States Research Articles

Bound-exciton states of multiple-quantum-well structures in external fields.

The bound states of excitons of all quantum numbers are calculated for different lattice sizes of the multiple-quantum-well structure GaAs/${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Al}}_{\mathrm{x}}$As in an external field. The Coulomb interactions in the same well and different wells are included in the Kronig-Penney model. The results show that the single-well model calculation is not accurate as the size becomes smaller. A detailed comparison of earlier works is given for the field dependence of the binding energy of various states.

Optical nonlinearity of bound single and multiexcitons in CdS

By means of two-beam experiments, the transmission properties of highly excited CdS crystals of different thicknesses (6 μm to 1 mm) have been investigated in the energy regime of the bound single and multiexcitons. Induced transmission was observed in the whole considered region for lower pump intensities. However, nonlinearly growing absorption counteracts this increase for rising excitation densities. Excitation spectroscopy of select points of the transmission spectrum and the temperature dependence of the observed phenomena show that bound single and multiexcitons as well as charge transfer processes play an important role in determining these properties of CdS samples. However, intraband transitions of free electrons and two step-processes of free single and biexciton's generation via bound exciton states have also to be discussed.

Photoluminescence and infrared absorption studies of double acceptors in germanium

Recent studies of double acceptors using near-, mid-, and far-infrared spectroscopy are reviewed, and new results are presented in each of these areas. Bound excitons and bound multiexciton complexes associated with these centers are investigated using photoluminescence and near-infrared absorption spectroscopy, and also measurements of the far-infrared absorption due to transitions between bound exciton states. Many bound exciton excited states are observed, and are well explained by a pseudo-donor model. The earlier predictions that the double acceptor ground states were split by hole-hole coupling, which were based upon near-infrared results and the temperature dependence of the mid-infrared neutral acceptor absorption, are verified by detailed uniaxial stress measurements of Ge:Be and Ge:Zn.

Exciton binding energy in a quantum well with inclusion of valence-band coupling and nonparabolicity.

Binding energies of excitons associated with several transitions between different subbands in undoped quantum wells are calculated for different well widths using a novel perturbational approach. The degeneracy of the valence band, the nonparabolicity of the conduction band, and the matching of the wave function at the interfaces are taken into account. The zeroth-order wave function is taken to be the product of the envelope functions in the z direction (perpendicular to the layers) for the electron and the hole and a purely two-dimensional (2D) exciton wave function. The difference between the 2D and 3D interaction between the electron and the hole is included in a variational-perturbational approach. The effect of the valence-band degeneracy on the properties of the holes is described with the use of the 4\ifmmode\times\else\texttimes\fi{}4 Luttinger Hamiltonian. We include the off-diagonal elements of this matrix in perturbation theory up to second order. This involves summations over the bound exciton states and integrals over the continuum states, which are found to be important. These off-diagonal elements in some cases modify the results considerably and improve the agreement with recent experimental results.

Resonant light scattering at localized electronic states: Evidence for silver clusters in silver halides

In AgCl and AgBr at low temperature an elastic light scattering process is discovered below the indirect exciton absorption edges. Considering the energy-, polarization- and time-dependence, we attribute this process to resonance fluorescence of electronic excitations of small non-metallic Ag-clusters embedded in the crystals. The strong reduction in cw intensity and decay time near and above the exciton edge is interpreted to be due to resonant energy transfer between the electronic states of the clusters and both intrinsic and impurity bound exciton states. The importance of the method to study small concentrations of metal clusters with high sensitivity is pointed out.

Effect of coupling between heavy and light holes on the binding energy of an exciton in a quantum well

The binding energy of excitons belonging to the different subbands in a quantum well has been calculated for different well widths and barrier heights. The degeneracy of the valence band, the non-parabolicity of the conduction band and the matching of the wave function at the interfaces are taken into account. The zeroth order wave function is taken to be the product of the envelope functions in the z-direction (perpendicular to the layers) for the electron and the hole and a purely two-dimensional (2D) exciton wave function. The difference between the 2D and 3D interaction between the electron and the hole is included in a variational-perturbational approach. The effect of the valence band degeneracy on the properties of the holes is described with the use of the 4 × 4 Luttinger Hamiltonian. Most previous authors have only considered the diagonal elements of this matrix, which are included in our zeroth order Hamiltonian. We include the off-diagonal elements in perturbation theory up to second order. This involves summations over bound exciton states and integrals over the continuum states, which are found to be important. These off-diagonal elements in some cases modify the results considerably and improve the agreement with recent experimental results.

An ODMR investigation of the (Cu-Li)Iand (Cu-Li)IIIcomplex defects in GaP

Co-doping of GaP with Cu and Li with a two-step diffusion procedure gives rise to several neutral associates involving both Cu and Li, as revealed by bound-exciton spectra in luminescence at low temperature. ODMR studies of two such associates are reported, the (Cu-Li)I centre with the lowest bound exciton lines at 2.3062 eV (triplet) and 2.3072 eV (singlet), and the (Cu-L:i)III centre which has four electronic bound-exciton lines in luminescence the lowest being a triplet a 2.2419 eV. In both cases the ODMR spectra show clear triplet resonances associated with the respective lowest-energy bound-exciton states. For (Cu-Li)I an orthorhombic distortion of a basically trigonal axial centre is deduced from the angular dependence of ODMR data, supporting an identification of the defect as Lii-CuGa-Lii where one Liei is bond-centred and the other is slightly distorted away from a tetrahedral interstitial site. For (Cu-Li)III the symmetry of this defect is very close to trigonal, suggesting an identification as Li1-CuGa-Cui where the Cui essentially remains in a tetrahedral interstitial position.

Erratum: Chain model of Si (111)2 x 1 surface: Optical properties and surface-state excitons

The optical properties of the chain model of Si(111)2\ifmmode\times\else\texttimes\fi{}1 are computed both within a single-particle picture and with the inclusion of excitonic and local-field effects. The presence of bound exciton states depends on the magnitude of the screening of electrons in surface states. For weak screening the singlet exciton is strongly bound (~ 0.3 eV) and gives rise to a narrow Lorentzian peak which dominates the absorption spectrum. For strong screening, bound excitons are not present. The resulting absorption line shape is asymmetrical, in qualitative agreement with experiment, but almost twice as broad. Polarization-dependent reflectance measurements are suggested as an important test for the various models.

Magneto-optical study of Li and Na acceptor bound excitons in CdTe: Fine structure and cubic crystal-field effect.

We present a high-resolution optical study of Li and Na shallow-acceptor bound excitons in CdTe. Results of absorption and Zeeman-effect experiments on the photoluminescence are discussed in terms of the j-j coupling scheme in cubic symmetry. It is shown that (i) the low-lying bound-exciton states are derived from the J=2 two-hole state, and (ii) the cubic crystal-field effect is dominant over the electron-hole interaction. The magnetic parameter values are the same as those previously determined for the Cu and Ag deeper-acceptor bound excitons: ${g}_{e}$=-1.77 for the electron, K=+0.61, and L=-0.04 for the hole.

Tightly bound exciton in AgBr:Ir 3+

Luminescence spectral and kinetic measurements lead to a model in which an exciton bound to a highly relaxed excited-state configuration of Ir 3+ in silver bromide undergoes radiative recombination. In addition to the previously reported infrared emission bands observed at ≈0.98 and ≈1.6 μm, a new highly structured band is seen at ≈2.5 μm. The luminescence decay lifetimes of these emissions are ≈0.3 ± 0.05, 1.8 ± 0.1, and 0.35 ± 0.05 msec, respectively, at <−10 K. The luminescence rise kinetics and decay kinetics have been studied as a function of temperature and excitation wavelength. An energy-level scheme for the Ir 3+ AgBr system is presented. It is believed that a bound-exciton state is a precursor to the trapped-electron species detected by EPR spectroscopy. A model for the low-temperature photochemistry of Ir 3+ in AgBr is given.

The far infrared absorption spectra of bound excitons in silicon

The first application of Fourier transform infrared spectroscopy to the measurement of the odd-parity excited states of a bound exciton is described. These results have reversed the prevailing model for this particular center, and allowed the even-parity excited states to be identified as well. All of these excited states can be put into excellent correspondence with acceptor states, providing the best existing confirmation of the pseudo-donor/pseudo-acceptor model of isoelectronic bound exciton states. Possible refinements of the technique are discussed, along with some suggested areas for further studies.

Determination of electron effective mass, bandgap energy and excited impurity states by photoluminescence experiments

The paper reviews refined photoluminescence experiments. The first part deals with magnetoluminescence yielding the conduction band electron effective mass via free-to-bound transitions from different Landau levels or alternatively from the Zeeman splitting of the shallow donor 2p state. The latter method relies essentially on selective luminescence excitation at excited donor bound exciton states. The concept of deliberate choice of excitation energy is the focal point of the second part, where excitation spectroscopy on donor-acceptor pair transitions is outlined in detail as a powerful technique to derive excited impurity states and identify impurities in moderately doped semiconductors. Selective pair luminescence as a method complementary to excitation spectroscopy is described briefly and finally it is shown how the bandgap energy can be determined with high accuracy from excitation spectra.

Disclosure of defects in YAG crystals by the thermoluminescence method

The topograms revealing the anisotropic distribution of defects in the volume of monocrystalline YAG samples have been obtained by the thermoluminescence (TL) technique. It has also been shown that the anisotropic distribution of the lattice defects affects strongly the shape of the TL curves. The greatest changes in the TL intensity were observed in the areas of the samples distributed symmetrically every 120°. It was noted that the selective distribution of the TL intensity is caused mainly by the presence of the (211) facets as well as growth striations formed during the growth process. The groups of lines observed in the TL spectrum have been ascribed to the Tb3+ ions, excited owing to the radiationless energy transfer from the bound exciton states (BES).

Chain model of Si(111)2×1 surface: Optical properties and surface-state excitons

The optical properties of the chain model of Si(111)2\ifmmode\times\else\texttimes\fi{}1 are computed both within a single-particle picture and with the inclusion of excitonic and local-field effects. The presence of bound exciton states depends on the magnitude of the screening of electrons in surface states. For weak screening the singlet exciton is strongly bound (~ 0.3 eV) and gives rise to a narrow Lorentzian peak which dominates the absorption spectrum. For strong screening, bound excitons are not present. The resulting absorption line shape is asymmetrical, in qualitative agreement with experiment, but almost twice as broad. Polarization-dependent reflectance measurements are suggested as an important test for the various models.

Thermoluminescence of YAG:Nd crystals coloured with x-rays

The spectral distribution of the thermoluminescence (TL) of YAG:Nd crystals coloured by x-ray irradiation at room temperature (RT) and at 80 K has been investigated. The spectral distribution of TL in the uv-, visible and near ir ranges shows that the energy transfer by bound exciton states (BES) to the RE3+ ions (Nd3+ and Tb3+) decreases with increasing temperature. The TL spectrum in the uv range is ascribed to the hole defect centers. Diminution of the energy transferred by the BES to the Nd3+ and Tb3+ ions is the direct cause for the occurrence of the group of lines ascribed to the Nd3+ ions in TL observed at LT, whereas at RT and higher only the groups of Tb lines are observed.

Stress effects on excitons bound to neutral acceptors in InP

We present stress-dependent photoluminescence measurements on excitons bound to neutral acceptors in InP. We analyze the experimental results in two theoretical schemes: a $j\ensuremath{-}j$ coupling scheme and a crystal-field scheme. We find that the triplet structure of the (${A}^{0}X$) complex results from hole-hole coupling in the cubic crystal field. We show that the three bound-exciton states correspond to ${\ensuremath{\Gamma}}_{8},{\ensuremath{\Gamma}}_{7,8}, \mathrm{and} {\ensuremath{\Gamma}}_{6}$ in order of increasing energies, the electron-hole exchange energy being found to be vanishingly small.

Zeeman spectroscopy of donor bound exciton states in ZnTe

We have performed a series of Zeeman experiments on the lines corresponding to the recombination of the different states of the exciton bound to the neutral donor in ZnTe. This definitely proves that the lowest of these states has the expected Γ 8 symmetry of the valence band, while it displays a cubic anisotropy much larger than the one measured on neutral acceptor. The higher excited states do not show the large orbital magnetism expected for p-like states. This result is however consistent with a description of the exciton states in terms of configurations Experiences d'effet Zeeman sur les raies correspondant a la recombinaison des differents etats de l'exciton piege sur le donneur neutre dans ZnTe. Preuve definitive que l'etat fondamental possede la symetrie attendue de la bande de valence (Γ 8 ). Mise en evidence d'une anisotropie cubique superieure a celle mesuree dans le cas d'un accepteur neutre. Absence de magnetisme orbital pour les etats de symetrie p dans les etats plus excites

Forbidden luminescence and resonance Raman scattering of bound exciton states in CdS

AbstractA series of six energy levels is found at energy distances 2.5, 3.8, 4.7, 6.1, 7.0, and 8.3 meV on the high energy side of the neutral‐acceptor bound exciton I1 in CdS. They appear in the emission and absorption spectra of some freshly grown virgin crystals and of some crystals damaged by laser‐irradiation. Resonance Raman scattering (RRS) establishes the new levels as intermediate states in every high quality crystal. These levels are interpreted as excited states of the bound exciton I1. The direct radiative recombination is forbidden due to parity selection rules. These selection rules are removed in symmetry disturbed crystals, at high excitation intensities, or by recombination with phonon participation. In consequence they are detected as forbidden luminescence, as LO phonon replica, or as 1‐LO‐ and 2‐LO‐Raman lines. The 2‐LO scattering cross section of the energy region of bound excitons in CdS is determined.

Resonant exciton trapping at impurity states in silver bromide

Excitation spectra near the indirect exciton edge of AgBr at 1.8K are reported for several luminescence lines from weakly localized excitons. Excitation below the exciton absorption threshold reveals several excited bound exciton states the energetic positions of which are determined. For excitation above the threshold, strong energy dependent structure is observed. It is interpreted in terms of resonant trapping of free excitons in both ground and excited bound exciton states associated with emission of LO(Γ), long wavelength acoustic and intervalley TA(X) and LA(X) phonons as well as combinations and overtones of these. From measurements in doped crystals two bound exciton systems are found to be correlated with Cd 2+ and Ca 2+, respectively.

Luminescence decays of N-bound excitons inGsAs1−xPx

We report photoluminescence-lifetime studies of nitrogen-bound excitons in $\mathrm{Gs}{\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x}$:N at low excitation power for $0.22&lt;x\ensuremath{\le}1.0$. Temperature was varied from $T=1.8 \mathrm{to} 200$ K. A time resolution of 400 psec was obtained with the use of time-correlated single-photon counting. At low temperatures, a short (10 nsec) nonexponential decay and a longer matrix-element-determined exponential decay are observed. Results prove that earlier 77-K phase-shift measurements gave information regarding only the nonexponential component. We report here on the true exponential luminescence lifetime. Luminescence-lifetime quenching as the temperature is raised from 1.8 to 40 K demonstrates that both $A(J=1)$ and $B(J=2)$ bound-exciton states exist in alloys with minimal strain mixing. Radiative lifetimes of both exciton spin states are obtained across the alloy for the first time and are compared with earlier theories. Both lifetimes show a bandstructure enhancement of 2 orders of magnitude, with $A$-state radiative lifetimes varying from about 100 nsec in GaP to 1 nsec for $x=0.23$. When the bound exciton is near resonance with the direct edge (for $x\ensuremath{\cong}0.22$), the radiative lifetime approaches but does not fall below the direct-edge luminescence lifetime, thus showing that the recombination rate does not diverge with resonance. For $T40$ K, two distinct thermal processes occur, one of which (hole release) increases the luminescence lifetime, while the other (electron detrapping) decreases the lifetime and quenches the luminescence intensity. Observation of hole release confirms the Hopfield-Thomas-Lynch model of N isoelectronic traps, and proves for the first time that isolated N binds a bare electron.