Long-wavelength Luminescence Research Articles

Long-wavelength room-temperature luminescence from InAs/GaAs quantum dots with an optimized GaAsSbN capping layer

An extensive study on molecular beam epitaxy growth conditions of quaternary GaAsSbN as a capping layer (CL) for InAs/GaAs quantum dots (QD) was carried out. In particular, CL thickness, growth temperature, and growth rate were optimized. Problems related to the simultaneous presence of Sb and N, responsible for a significant degradation of photoluminescence (PL), are thereby solved allowing the achievement of room-temperature (RT) emission. A particularly strong improvement on the PL is obtained when the growth rate of the CL is increased. This is likely due to an improvement in the structural quality of the quaternary alloy that resulted from reduced strain and composition inhomogeneities. Nevertheless, a significant reduction of Sb and N incorporation was found when the growth rate was increased. Indeed, the incorporation of N is intrinsically limited to a maximum value of approximately 1.6% when the growth rate is at 2.0 ML s−1. Therefore, achieving RT emission and extending it somewhat beyond 1.3 μm were possible by means of a compromise among the growth conditions. This opens the possibility of exploiting the versatility on band structure engineering offered by this QD-CL structure in devices working at RT.PACS81.15.Hi (molecular beam epitaxy); 78.55.Cr (III-V semiconductors); 73.21.La (quantum dots)

Supramolecular Luminescent Gold(I)−Copper(I) Complexes: Self-Assembly of the AuxCuyClusters inside the [Au3(diphosphine)3]3+Triangles

The reactions between diphosphino-alkynyl gold complexes (PhC2Au)PPh2(C6H4)(n)PPh2(AuC2Ph) (n = 1, 2, 3) with Cu(+) lead to formation of the heterometallic aggregates, the composition of which may be described by a general formula [{Au(x)Cu(y)(C2Ph)2x}Au3{PPh2(C6H4)(n)PPh2}3](3+(y-x)) (n = 1, 2, 3; x = (n + 1)(n + 2)/2; y = n(n + 1)). These compounds display very similar structural patterns and consist of the [Au(x)Cu(y)(C2Ph)2x](y-x) alkynyl clusters "wrapped" in the [Au3(diphosphine)3](3+) triangles. The complex for n = 1 was characterized crystallographically and spectrally, the larger ones (n = 2, 3) were investigated in detail by NMR spectroscopy. Their luminescence behavior has been studied, and a remarkably efficient emission with a maximum quantum yield of 0.92 (n = 1) has been detected. Photophysical experiments demonstrate that an increase of the size of the aggregates leads to a decrease in photostability and photoefficiency. Computational studies have been performed to provide additional insight into the structural and electronic properties of these supramolecular complexes. The theoretical results obtained are in good agreement with the experimental data, supporting the proposed structural motif. These studies also suggest that the observed efficient long-wavelength luminescence originates from metal-centered transitions within the heterometallic Au-Cu core.

Luminescence of synthetic rutile implanted with cobalt ions

Synthetic rutile plates have been implanted with high-energy cobalt ions. Unusually long-wavelength luminescence (at 820 nm) of Ti VI 3+ ions in rutile, whose intensity increases upon cobalt implantation, has been revealed. Analysis of the luminescence spectra and the luminescence excitation spectra made it possible to refine the electron energy level diagram of rutile and determine the energy levels of Ti3+ impurity ions occupying vacant octahedra with the C2h symmetry in the mineral structure.

The reasons the scintillation efficiency decrease of CsI(Tl) crystals exposed by the high-dosed radiation

The role of radiation defects in the light yield degradation of CsI(Tl) scintillation detectors has been investigated. The spectra and decay kinetic study of the luminescence of CsI(Tl) crystals coloured by γ -radiation ( Co 60 , 10 6 Gy ) allowed to conclude that two types of the centres induced by irradiation absorb the luminescence of Tl + centres responsible for the scintillations. The reabsorption of the Tl + emission leads to the appearance of the long-wavelength luminescence caused by the colour centres of the first type. The luminescence of the centres of the second type is quenched at room temperature. These centres are not only deteriorating the efficiency of the activator luminescence, but also functioning as quenching centres to which the non-radiative energy transfer from Tl + centres is realized.

Oxygen-Induced Modification of Dislocation Luminescence Centers in Silicon

Prospects for using the long-wavelength dislocation luminescence line D1 in silicon-based light-emitting diodes are considered. The standard spectral position of this line at 807 meV, rather than being canonic, depends on the morphology of the dislocation structure and the impurity environment of an individual dislocation. Data on the spectral distribution of luminescence intensity in the region of the D1 line have been analyzed in terms of the concentration of interstitial oxygen in a sample, plastic deformation parameters, and thermal treatment. The results obtained suggest that oxygen exerts a dominant effect on the spectral position of line D1 and luminescence intensity in its vicinity. It is shown that the probable structure of recombination centers can be described in terms of the donor-acceptor pair model, in which oxygen complexes serve as donors and the acceptors are structural defects in the dislocation core.

Long-wavelength luminescence from GaSb quantum dots grown on GaAs substrates

Self-assembled GaSb quantum dots (QDs) with a photoluminescence wavelength longer than 1.3 μm were successfully grown by suppressing the replacement of As and Sb on the surface of the GaSb QDs. This result means that GaSb can thus join InAs or GaInAs as a suitable material for QD lasers for optical communications.

Long wavelength infrared negative luminescent devices with strong Auger suppression

We have investigated the negative luminescent properties of HgCdTe photodiodes having peak emission at a wavelength of 7.6 μm, as a function of temperature and applied bias. Under reverse bias at room temperature, intrinsic Auger processes are almost fully suppressed. The internal negative luminescence efficiency of ≈85% is nearly independent of temperature in the 240–300 K range, and at 296 K corresponds to an apparent temperature reduction of 43 K. The small currents needed to drive these diodes (as little as 0.7 A/cm2 at 296 K), together with their long-wavelength luminescence, make them suitable for a range of device applications.

Luminescence of microcrystals and solutions of 8-azagona-1,3,5(10),13-tetraene-12,17-dione

We present the results of the investigation of delayed luminescence of 2,3-methoxy-8-azagona-1,3,5(10),13-tetraene-12,17-dione (8,9-dimethoxy-1,2,3,5,6,10b,11,12-octahydrocyclopentane[5,6]pyrido[2,1-a]isoquinoline-1,12dione) in solid solutions. Dual delayed luminescence with maxima in the region of 400 and 500 nm depending on the excitation wavelength has been revealed. It is shown that the observed delayed luminescence is a phosphorescence of individual molecules of the substance (short-wavelength luminescence) and molecular pairs (long-wavelength luminescence) resulting from the dipole–dipole (Coulomb) interaction of strongly polarized molecules. The conclusion has been drawn that the spectral features observed for the solutions of 8-azasteroids are due to both individual molecules and their aggregates.

Luminescence of fluorohafnate glasses

The effect of reduction–oxidation (redox) conditions on the luminescent properties of Ce-doped fluorohafnate glasses was studied using UV–VUV (synchrotron radiation) and X-ray (X-ray tube) excitation. In several cases luminescence not related to Ce 3+ dopant was observed. In glasses doped with oxides as well as in ones melted under reducing atmosphere a broad band peaking at 340–460 nm arised. The formation of Hf 3+ ions and fluorine vacancies as the precursors of the observed long-wavelength luminescence band are discussed.

Luminescence Properties of Terbium-, Cerium-, or Europium-Doped α-Sialon Materials

New interesting luminescent α-sialon (M(m/val+)val+ Si12-(m+n) Al(m+n)OnN(16−n)) (M=Ca, Y) materials doped with Ce, Tb, or Eu have been prepared and their luminescence properties studied. These show that Tb and Ce are in the 3+ and Eu in the 2+ state. Low-energy 4f↔5d transitions are observed as compared to the luminescence of these ions doped in oxidic host-lattices. This is partially explained by the nitrogen-rich coordination of the rare-earth ion and partially by the narrow size of the lattice site. The latter gives rise to a strong crystal-field splitting of the 5d band and a rather large Stokes shift for Ce3+ and Eu2+ (6500–7500 and 7000–8000 cm−1, respectively). For (Y,Tb)-α-sialon the Tb3+ 4f→5d excitation band (∼260 nm) is in the low-energy host-lattice absorption band (⩽290 nm), giving rise to a strong absorption for 254-nm excitation, but a low quantum efficiency. The latter is due to photoionization processes or selective excitation of Tb3+ at the defect-rich surface, resulting in radiationless transitions. Ce- and Eu-doped Ca–α-sialon show bright long-wavelength luminescence (maxima at 515–540 and 560–580 nm for Ce and Eu, respectively) with a high quantum efficiency and high absorption for 365- and 254-nm excitation. The Eu2+ emission intensity and absorption increases for increasing m, which is explained by the Eu2+ richer α-sialon composition. The position of the Eu2α emission does not shift with changing composition of the host-lattice (m, n values), indicating that the local coordination of the Eu2+ ion is hardly dependent on the matrix composition.

Long-wavelength HgCdTe negative luminescent devices

We have investigated the negative luminescent properties of a HgCdTe device, fabricated from a 1 mm diameter array of photodiodes having peak emission at a wavelength of 8.5 μm. This long-wavelength luminescence is of sufficient efficiency and area to be useful in device applications.

Long-wavelength luminescence from In0.5Ga0.5As/GaAs quantum dots grown by migration enhanced epitaxy

We report on a systematic study of In0.5Ga0.5As quantum dots grown by the migration enhanced epitaxy technique. A maximum room-temperature luminescence wavelength of 1.37 μm has been achieved. We observe the presence of two emission maxima, which we attribute to the formation and interaction of two different dot-size distributions. Photoluminescence data analysis shows the formation of large quantum dots with good size uniformity that emit at 1.3 μm and longer due to the enhanced atomic migration lengths.

Optical spectra of silicon nanostructures from the random-matrix model

We apply the method of random matrices to a calculation of the optical properties of porous silicon and silicon nanopowders, analyzing the effects of disorder on the visible luminescence properties. The maximum of the luminescence peal; displays a blueshift, which adds up to the effects induced by other mechanisms, such as quantum confinement or surface passivation, and agrees with the experimental findings, The long-wavelength luminescence edge, instead, is redshifted by the presence of disorder, thus partially compensating for the gap-opening effect induced by the other mechanisms.

The structure of high-temperature blue luminescence centers in zinc selenide and mechanisms of this luminescence

The characteristic features of temperature quenching of the intensity of the edge luminescence bands of n-ZnSe crystals annealed in different media (vacuum, Zn, Se) are investigated a wide temperature range. A change in the mechanisms of high-temperature exciton luminescence in the short-wavelength region of the spectrum (443 nm) with increase in temperature of the crystal is observed. It is shown that the nature of temperature quenching of the long-wavelength edge luminescence band (458 nm) is evidence of dissociation of associative luminescence centers with increase in the sample temperature.

The effect of copper doping of n-ZnSe crystals on the structure of luminescence centers of long-wavelength luminescence

The photoluminescence and luminescence excitation spectra of copper-doped n-ZnSe crystals were investigated in the temperature range 77–400 K during diffusion of the copper into the crystal from the liquid, gas, and and solid phases. It is shown that the structure of the associative centers responsible for emission in the red and green regions of the spectrum depends on the method of copper doping. It is established that during the interphase interaction on the (ZnSe crystal)-(copper film) boundary, zinc atoms dissolve in the copper film and the copper atoms diffuse into the zinc selenide crystal.

Exchange interactions of excitons localized at opposite interfaces in type-II GaAs/AlAs superlattices: Optically detected magnetic resonance and level anticrossing

Two types of excitons, localized at opposite interfaces and characterized by different magnitudes of the exchange interactions at the same radiation energies, are simultaneously in type-II GaAs/AlAs superlattices. It is shown that the additional long-wavelength luminescence line in superlattices grown with growth interruptions after the GaAs layers is due to the recombination of an exciton localized at an inverted interface in regions where the quantum-well width is increased by one monolayer.

Comprehensive Model of the Photophysics of N-Phenylnaphthalimides: The Role of Solvent and Rotational Relaxation

Absorption and fluorescence spectra, fluorescence decay times, and quantum yields of fluorescence and triplet state formation have been determined for N-phenyl and substituted N-phenyl derivatives of 1,2-, 2,3-, and 1,8-naphthalimides, using stationary irradiation and laser flash excitation methods. The effects of substituents on the N-phenyl group on solvent polarity and viscosity have been studied. A short-wavelength (SW) fluorescence, similar to the luminescence emitted by the N-alkyl derivatives, and/or a considerably red shifted long-wavelength (LW) luminescence are observed, and the ratio of the SW and LW fluorescence components is found to depend on substitution and on solvent properties. A striking characteristic of the N-phenylnaphthalimides (in contrast to the N-alkyl derivatives) is the very efficient internal conversion which results in short fluorescence decay times and in low fluorescence and triplet yields. On the basis of the experimental results, it is suggested that solvent and geometric...

Influence of GaAs(001) substrate misorientation towards {111} on the optical properties of InxGa1−xAs/GaAs

Local variations in the optical properties of thick In0.13Ga0.87As films grown on GaAs(001) substrates misoriented toward {111} planes have been studied with polarized and spectrally-resolved cathodoluminescence (CL) imaging. The degree of anisotropic relaxation and density of dark line defects (DLDs) in CL was found to depend on the choice of the substrate miscut orientation. An enhanced anisotropy in DLD density and strain relaxation was found for a misorientation towards (111)A relative to that for a misorientation towards (111)B. Local variations and spatial correlations in polarization anisotropy, band-gap energy shifts, luminescence efficiency, and defect-induced long-wavelength luminescence were examined.

Precision determination of long-wavelength cleavage luminescence energy and derivation of minimum surface state gap on clean cleaved Si surfaces

The luminescence from silicon cleaved in high vacuum has been measured with precision in the wavelength range previously interpreted as corresponding to recombination of cleavage-excited carriers across the indirect surface state gap. The nature of the emission, including the role of defects is discussed. The radiation energy is sharply defined as 0.26 ± 0.01 eV.

Precision determination of long-wavelength cleavage luminescence energy and derivation of minimum surface state gap on clean cleaved Si surfaces

The luminescence from silicon cleaved in high vacuum has been measured with precision in the wavelength range previously interpreted as corresponding to recombination of cleavage-excited carriers across the indirect surface state gap. The nature of the emission, including the role of defects is discussed. The radiation energy is sharply defined as 0.26±0.01 eV.