Exciton Binding Research Articles

Study of the photoluminescence emission line at 3.33 eV in ZnO films

We study properties of the line at 3.33 eV observed in photoluminescence (PL) emission spectra of various ZnO films prepared using pulsed laser deposition method. The influence of deposition parameters, such as oxygen pressure, laser fluence, post-annealing, and electric field exposure on intensity of this luminescence band has been investigated. The recombination characteristics are probed by temperature and excitation dependent PL spectroscopy. The obtained experimental data suggest that the 3.33 eV luminescence line in ZnO depends strongly on surface band bending and originates from recombination of bound excitons (BEs) complex located near the surface and grain boundaries. The anomalously small thermal activation energy of BE in comparison with the localization energy is explained by decreasing of the interface barrier. Possible nature of defects that bind free excitons and cause the 3.33 eV emission line in ZnO is proposed.

Fine-structure splitting in large-pitch pyramidal quantum dots

Site-controlled pyramidal quantum dots (PQDs) are high-symmetry structures grown on (111)B-oriented substrates by metal--organic vapor phase epitaxy. Given the relatively straightforward technique used for their growth and the high degree of dot uniformity achievable, they are natural candidates for the realization of reliable single and entangled photon sources. Nevertheless, even though (111)-oriented surfaces show, in terms of lattice symmetry, ideal characteristics for the production of entangled photon pairs, in PQDs the fine-structure splitting (FSS) can vary significantly depending on the dot composition and growth conditions. Here, we present the results of a study of the FSS in six samples of different composition. We investigate correlations among FSS and other parameters, such as biexciton binding energy and exciton lifetime, and extract information on how to manipulate the dot to reproducibly obtain a nearly zero FSS. To gain more detailed insight into the electronic properties of (111)-oriented In${}_{x}$Ga${}_{1\ensuremath{-}x}$As PQDs, we also perform calculations based on an eight-band k\ifmmode\cdot\else\textperiodcentered\fi{}p model. These calculations are in good qualitative agreement with the experimental outcomes and outline the importance of piezoelectric potentials and of the carrier confining potential for predicting the optoelectronic properties.

Effects of fermion flavor on exciton condensation in double-layer systems

We use fermionic path integral quantum Monte Carlo to study the effects of fermion flavor on the physical properties of dipolar exciton condensates in double layer systems. We find that by including spin in the system weakens the effective interlayer interaction strength, yet this has very little effect on the Kosterlitz-Thouless transition temperature. We further find that, to obtain the correct description of screening, it is necessary to account for correlation in both the interlayer and intralayer interactions. We show that while the excitonic binding cannot completely surpress screening by additional fermion flavors, their screening effectiveness is reduced leading to a much higher transition temperatures than predicted with large-N analysis.

Effect of annealing and hydrogen plasma treatment on the luminescence of hydrothermally grown bulk ZnO

The optical properties of hydrothermally grown bulk ZnO is investigated by low temperature photoluminescence (PL) spectroscopy. The effects of annealing in an argon atmosphere, as well as the influence of hydrogen plasma exposure, on the PL of as-grown material are studied. The 11K PL spectrum of the as-grown ZnO shows different excitonic lines in the NBE region: several bound exciton lines are clearly visible. The origin of these bound excitons is discussed, as well as the influence of annealing on these lines. A shift in the deep level emission (DLE) is also observed with an increase in annealing temperature: a red shift is detected when the annealing temperature is increased up to 650°C, while a subsequent blue shift is observed upon an increase in the annealing temperature. The involvement of Li and Cu in this phenomenon will be discussed and the effect of hydrogen on this DLE will also be studied.

Bound exciton photoluminescence from ion‑implanted phosphorus in thin silicon layers

We report the observation of clear bound exciton (BE) emission from ion-implanted phosphorus. Shallow implantation and high-temperature annealing successfully introduce active donors into thin silicon layers. The BE emission at a wavelength of 1079 nm shows that a part of the implanted donors are definitely activated and isolated from each other. However, photoluminescence and electron spin resonance studies find a cluster state of the activated donors. The BE emission is suppressed by this cluster state rather than the nonradiative processes caused by ion implantation. Our results provide important information about ion implantation for doping quantum devices with phosphorus quantum bits.

Enhancement of the Ultraviolet Luminescence Intensity from Cd-Doped ZnO Films Caused by Exciton Binding

ZnO films doped with the cadmium (0.4–0.6%) were grown on crystalline sapphire c-Al2O3 substrates applying radiofrequency magnetron sputtering at the temperature of 400 ◦C in Ar–O2 atmosphere. The as-grown films were investigated in detail using X-ray diffraction, X-ray photoelectron spectroscopy, and cathodoluminescence spectra. The X-ray diffraction analysis revealed that the films possess a hexagonal wurtzite-type structure with the dominant crystallite orientation along the c axis. It was found that the small concentration of the cadmium significantly enhances the ultraviolet emission associated with excitonic transitions. We suggest that this enhancement effect mainly results from appearance of the cadmium isoelectronic traps, which may bind an exciton, thereby increasing the probability of radiation recombination. The effect of Cd isoelectronic impurity on structural and luminescent properties of ZnO films is discussed.

New Results on the Bound Exciton Luminescence in Germanium

Photoluminescence from excitons bound to shallow donors or acceptors was studied in Al-, As-, B-, Ga- and P-doped Ge. Excitons bound to Al and B acceptors were identified for the first time. The dissociation energy of the excitons satisfies Haynes rule and changes with a factor of 0.1 linearly with the ionization energy of the dopants.

Slowdown of light due to exciton-polariton propagation in ZnO

By employing time-of-flight spectroscopy, the group velocity of light propagating through bulk ZnO is demonstrated to dramatically decrease down to 2044 km/s when photon energy approaches the absorption edge of the material. The magnitude of this decrease is found to depend on light polarization. It is concluded that even though the slowdown is observed in the vicinity of donor bound exciton (BX) resonances, the effect is chiefly governed by dispersion of free exciton (FX) polaritons that propagate coherently via ballistic transport. Based on the experimentally determined spectral dependence of the polariton group velocity, the polariton dispersion is accurately determined.

Photoluminescence spectroscopy of defects in ZnO nano/microwires

Photoluminescence spectroscopy is used to study defects found in single ZnO nano/microwires at 90 K. The defect, acting as binding site for bound exciton (BX) transition, is represented by BF, the fractional intensity of the BX peak in the whole near-band edge ultraviolet (UV) luminescence. The concentration of defects as origins of the visible emissions is proportional to the intensity fraction DF, i.e., the intensity fraction of visible emissions in the sum total of all UV and visible luminescences. By comparing BF and DF, it is concluded that the two defects are not correlated to each other. The former kind of defect is considered to be related to the blueshift of the near-band edge peak as the radius of the nano/microwires decreases at room temperature.

Förster Coupling in Nanoparticle Excitonic Circuits

Exciton transport in semiconductor nanoparticles underlies recent experiments on electrically controlled nanostructures and proposals for new artificial light-harvesting systems. In this work, we develop a novel method for the numerical evaluation of the Forster matrix element, based on a three-dimensional real space grid and the self-consistent solution of the mesoscopic exciton in a macroscopic dielectric environment. This method enables the study of the role of the nanoparticle shape, spatially varying dielectric environments, and externally applied electric fields. Depending on the orientation of the transition dipole, the Forster coupling is shown to be either increased or decreased as a function of the nanoparticle shape and of the properties of the dielectric environment. In the presence of an electric field, we investigate the relation between excitonic binding and confinement effects. We also study a type II core-shell quantum dot where electron and hole are spatially separated due to a particular configuration of the bandstructure.

Indirect optical absorption and origin of the emission from β-FeSi2 nanoparticles: Bound exciton (0.809 eV) and band to acceptor impurity (0.795 eV) transitions

We investigated the optical absorption of the fundamental band edge and the origin of the emission from β-FeSi2 nanoparticles synthesized by ion-beam-induced epitaxial crystallization of Fe+ implanted SiO2/Si(100) followed by thermal annealing. From micro-Raman scattering and transmission electron microscopy measurements it was possible to attest the formation of strained β-FeSi2 nanoparticles and its structural quality. The optical absorption near the fundamental gap edge of β-FeSi2 nanoparticles evaluated by spectroscopic ellipsometry showed a step structure characteristic of an indirect fundamental gap material. Photoluminescence spectroscopy measurements at each synthesis stage revealed complex emissions in the 0.7–0.9 eV spectral region, with different intensities and morphologies strongly dependent on thermal treatment temperature. Spectral deconvolution into four transition lines at 0.795, 0.809, 0.851, and 0.873 eV was performed. We concluded that the emission at 0.795 eV may be related to a radiative direct transition from the direct conduction band to an acceptor level and that the emission at 0.809 eV derives from a recombination of an indirect bound exciton to this acceptor level of β-FeSi2. Emissions 0.851 and 0.873 eV were confirmed to be typical dislocation-related photoluminescence centers in Si. From the energy balance we determined the fundamental indirect and direct band gap energies to be 0.856 and 0.867 eV, respectively. An illustrative energy band diagram derived from a proposed model to explain the possible transition processes involved is presented.

Hyperfine Structure and Nuclear Hyperpolarization Observed in the Bound Exciton Luminescence of Bi Donors in Natural Si

As the deepest group-V donor in Si, Bi has by far the largest hyperfine interaction and also a large I = 9/2 nuclear spin. At zero field this splits the donor ground state into states having total spin 5 and 4, which are fully resolved in the photoluminescence spectrum of Bi donor bound excitons. Under a magnetic field, the 60 expected allowed transitions cannot be individually resolved, but the effects of the nuclear spin distribution, -9/2 < or = I(z) < or = 9/2, are clearly observed. A strong hyperpolarization of the nuclear spin towards I(z) = -9/2 is observed to result from the nonresonant optical excitation. This is very similar to the recently reported optical hyperpolarization of P donors observed by EPR at higher magnetic fields. We introduce a new model to explain this effect, and predict that it may be very fast.

Extended excitons and compact heliumlike biexcitons in type-II quantum dots

We have used magnetophotoluminescence measurements to establish that InP/GaAs quantum dots have a type-II (staggered) band alignment. The average excitonic Bohr radius and the binding energy are estimated to be 15 nm and 1.5 meV, respectively. When compared to bulk InP, the excitonic binding is weaker due to the repulsive (type-II) potential at the heterointerface. The measurements are extended to over almost 6 orders of magnitude of laser excitation powers and to magnetic fields of up to 50 T. It is shown that the excitation power can be used to tune the average hole occupancy of the quantum dots and hence the strength of the electron-hole binding. The diamagnetic shift coefficient is observed to drastically reduce as the quantum dot ensemble makes a gradual transition from a regime where the emission is from (hydrogenlike) two-particle excitonic states to a regime where emission from (heliumlike) four-particle biexcitonic states also becomes significant.

Defect properties of ZnO nanopowders and their modifications induced by remote plasma treatments

Photoluminescence (PL) and positron lifetime (LT) measurements were used on several commercial ZnO nanopowders. We observed that sample-to-sample differences in the quality of the powders overshadow any observation of probable size effects. However, the average LT for all nanocrystals is longer than in a bulk sample, consistent with the hypothesis of crystals with surface and subsurface layers rich in defects. Temperature-dependent PL spectra from the ZnO nanopowders were analyzed in detail for the bound-exciton (BEx) range and the numerical fits of the peak positions yielded activation energies that suggested different channels of recombination for the BEx. Also, fits for the full width at half maximum (FWHM) show nonlinear behavior, indicating contribution from surface phonons. We, also, used remote nitrogen and hydrogen plasma treatment on the ZnO nanosystems to manipulate their surface and subsurface defect states. We demonstrated that those plasma species induce a variety of changes in the deep defect visible emission as well as in the BEx luminescence, most likely associated with the surface/subsurface states.

Photoluminescence study of single ZnO nanostructures: Size effect

Spatially resolved photoluminescence (PL) investigations were carried out on ZnO single nanowires, tetrapods, and nanocrystals. The fractional intensity for bound exciton (BX) transitions was shown to be correlated with the size in all these ZnO nanostructures. This size dependency is attributed to the inhomogeneous density distribution of the defects as binding sites for BX in the ZnO nanostructures, in good agreement with a simple model calculation.

Luminescence enhancement of ZnO nanoparticles on metal surface

We have studied luminescence enhancement of zinc oxide (ZnO) nanoparticles with the average size of 30 nm on several metal surfaces at low temperatures. Bandedge luminescence originated from bound exciton (BE) annihilation is observed at 3.360 eV, and strongly depends on the kind and surface roughness of metal. The luminescence intensity is about 10 times larger for Ag surface than that for quartz surface. Furthermore, the luminescence increases remarkably when the roughness of Ag surface is almost the same as the particle size. The intensity ratio of the fast decay component to the slow one decreases for Ag surface compared with quartz. These results suggest that the luminescence enhancement is partially attributed to suppressing of the nonradiative recombination process in ZnO nanoparticles on metal surface.

The effects of crystallographic orientation and strain on the properties of excitonic emissionfrom wurtzite InGaN/GaN quantum wells

We have examined in detail crystal orientation effects on the properties of excitonic emission fromwurtzite InxGa1−xN/GaN quantum wells (QWs) with piezoelectric polarization using exciton binding andtransition energy calculations based on a single-band effective-mass theory. Weshow numerical results for the bandgaps, effective heavy-hole masses, piezoelectricpolarizations and fields, exciton wavefunctions, exciton binding and transitionenergies and radiative lifetimes of excitonic emission as a function of the QWcrystallographic growth planes. Band-edge and effective-mass parameters for a continuumof GaN crystallographic orientations, on which InGaN/GaN QWs are grown, wereobtained from In-composition- and strain-dependent calculation for wurtzite InxGa1−xN, using the Hamiltonian in appropriate {hkil} representations. We have performed calculations for a continuum of technologically relevantQW growth planes oriented at various angles θ relative to the (0001) c-plane. The excitonic ground- and first-excited-state energies and wavefunctions were calculatedusing an effective potential method. A strong reduction of average in-plane heavy-holeeffective mass and normal to the plane piezoelectric polarization and field is observed asθ varies fromθ = 0° (i.e. thec-axisdirection) to θ = 49.5°, where the piezoelectric polarization and electric field reverse their orientationwith respect to the plane of the QW. The decrease of the electric field in theInGaN/GaN QW growth direction leads to an increased exciton transitionenergy and oscillator strength, which results in the increase of the excitonbinding energy and decrease of the excitonic radiative lifetime. For anglesθ>49.5° only small variationson the order of ∼10% in the exciton binding and transition energies and excitonic radiative lifetime are observed for narrowIn0.12Ga0.88N/GaN QWs that havewidths less than ∼ 3.5 nm. The average in-plane heavy-hole effective mass reaches its minimum forθ = 90°,i.e. m-plane growth. These results indicate that InGaN/GaN QW structuresgrown on non-(0001)-oriented planes in a wide variety of angles49.5°≤θ≤90° can be used for optimized operation of optoelectronic devices.

Optical properties of nearly stacking-fault-free m-plane GaN homoepitaxial films grown by metal organic vapor phase epitaxy on low defect density freestanding GaN substrates

Prospective optical properties were demonstrated for nearly stacking fault-free m-plane GaN homoepitaxial films grown by metal organic vapor phase epitaxy on the m-plane freestanding GaN substrates. Values of full width at half maximum of x-ray rocking curves were close to the substrate values being 31arcsec for the (101¯0) diffraction with ⟨0001⟩ azimuth and 48arcsec for the (101¯2) diffraction. Threading dislocation densities were lower than 5×106cm−2. The film surfaces exhibited atomically flat morphology with well-aligned monolayer steps. Low-temperature photoluminescence (PL) spectra exhibited polarization-dependent well-resolved bound and free exciton emission lines, and a characteristic π(k⊥c,E∥c)-polarized PL line was also observed. Room-temperature effective PL lifetime of the free exciton peak increased with increasing supply ratio of ammonia to trimethylgallium, and a record long value for m-plane GaN (268ps) was obtained.

Resonant Light Delay in GaN with Ballistic and Diffusive Propagation

We report on a strong delay in light propagation through bulk GaN, detected by time-of-flight spectroscopy. The delay increases resonantly as the photon energy approaches the energy of a neutral-donor bound exciton (BX), resulting in a velocity of light as low as 2100 km/s. In the close vicinity of the BX resonance, the transmitted light contains both ballistic and diffusive components. This phenomenon is quantitatively explained in terms of optical dispersion in a medium where resonant light scattering by the BX resonance takes place in addition to the polariton propagation.

Insights into Excitons Confined to Nanoscale Systems: Electron–Hole Interaction, Binding Energy, and Photodissociation

The characteristics of nanoscale excitons--the primary excited states of nanoscale systems like conjugated polymers, molecular aggregates, carbon nanotubes, and nanocrystalline quantum dots--are examined through exploration of model systems. On the basis of a valence bond-type model, an intuition is developed for understanding and comparing nanoscale systems. In particular, electron-hole interactions are examined in detail, showing how and why they affect spectroscopy and properties such as binding energy. The relationship between the bound exciton states and the nanoscale analogue of free carriers (charge-transfer exciton states) is developed. It is shown why the electron and hole act as independent particles in this manifold of states. The outlook for the field is discussed on the basis of the picture developed in the paper, with an emphasis on exciton binding and photodissociation.