Showing papers on "Photoluminescence published in 1998"
TL;DR: In this article, a wide gap II-VI semiconductor alloy, MgxZn1−xO, was proposed for the fabrication of heteroepitaxial ultraviolet light emitting devices based on ZnO.
Abstract: We propose a widegap II–VI semiconductor alloy, MgxZn1−xO, for the fabrication of heteroepitaxial ultraviolet light emitting devices based on ZnO. The c-axis oriented MgxZn1−xO films were epitaxially grown by pulsed laser deposition on ZnO epitaxial films and sapphire (0001) substrates using ceramic targets. Solid solution films were prepared with Mg content up to x=0.33, achieving a band gap of 3.99 eV at room temperature. MgO impurity phase segregated at x⩾0.36. Lattice constants of MgxZn1−xO films changed slightly (∼1%), increasing in a axis and decreasing in c-axis direction with increasing x. These films showed ultraviolet photoluminescence at energies from 3.36 (x=0) to 3.87 eV (x=0.33) at 4.2 K.
1,441 citations
TL;DR: In this paper, the optical properties of ZnO crystals using a variety of optical techniques were compared and it was concluded that the room temperature band gap is 3.3 eV and that the other values are attributable to a valence band-donor transition at ∼3.15
Abstract: Three different values (3.1, 3.2, and 3.3 eV) have been reported for the optical band gap of zinc oxide single crystals at room temperature. By comparing the optical properties of ZnO crystals using a variety of optical techniques it is concluded that the room temperature band gap is 3.3 eV and that the other values are attributable to a valence band-donor transition at ∼3.15 eV that can dominate the optical absorption when the bulk of a single crystal is probed.
1,388 citations
TL;DR: In this paper, a two dimensional nucleation during the initial growth was observed by in situ reflection high energy electron diffraction, followed by a morphology transition to 3D nucleation by using molecular beam epitaxy.
Abstract: ZnO single crystal thin films were grown on c-plane sapphire using oxygen microwave plasma assisted molecular beam epitaxy. Atomically flat oxygen-terminated substrate surfaces were obtained by pre-growth cleaning procedures involving an oxygen plasma treatment. A two dimensional nucleation during the initial growth which is followed by a morphology transition to three dimensional nucleation was observed by in situ reflection high energy electron diffraction. X-ray diffraction (XRD) and photoluminescence investigations suggest that the ZnO epilayer consists of a high quality layer on top of a transition layer containing a high density of defects in the interfacial region. A full width at half maximum (FWHM) of 0.005° is obtained for the ZnO(0002) diffraction peak in an XRD rocking curve, while a broad tail extending from the peak can also be observed. The photoluminescence spectra exhibit dominant bound exciton emission with a FWHM of 3 meV at low temperatures and free exciton emission combined with a ver...
1,261 citations
TL;DR: In this paper, a spontaneous emission rate enhancement by a factor of up to 5 was selectively observed for the QB's which are on resonance with one-cavity mode. But the effect of the random spatial and spectral distributions of the QBs was not considered.
Abstract: Semiconductor quantum boxes (QB's) are well suited to cavity quantum electrodynamic experiments in the solid state because of their sharp emission. We study by time-resolved photoluminescence InAs QB's placed in the core of small-volume and high-finesse GaAs/AlAs pillar microresonators. A spontaneous emission rate enhancement by a factor of up to 5 is selectively observed for the QB's which are on resonance with one-cavity mode. We explain its magnitude by considering the Purcell figure of merit of the micropillars and the effect of the random spatial and spectral distributions of the QB's.
1,016 citations
TL;DR: In this paper, the emission spectrum of high quality ZnO epilayers is studied from room temperature up to 550 K. At room temperature and low excitation power, a single emission peak is observed which may be identified with the free exciton from its peak energy and dependence on temperature.
Abstract: The emission spectrum of high quality ZnO epilayers is studied from room temperature up to 550 K. At room temperature and low excitation power a single emission peak is observed which may be identified with the free exciton from its peak energy and dependence on temperature. However, when excitation intensities exceed 400 kW cm−2 a sharp peak emerges at lower energy which we attribute to exciton-exciton scattering. At higher excitation intensities (>800 kW cm−2) a second stimulated emission peak emerges at even lower energies: we attribute this peak to be stimulated emission of an electron hole plasma. Similar features are observed for all temperatures up to 550 K.
741 citations
TL;DR: In this article, the photoluminescence efficiency of CdTe nanocrystals strongly depends on the pH value of the colloidal solution, and the maximum quantum yield at room temperature is approximately 18% when brought to 4.5 by using thioglycolic acid.
Abstract: CdTe nanoclusters were prepared in aqueous solution by the reaction between Cd2+ and NaHTe in the presence of thioglycolic acid. Under reflux, the clusters start to crystallize and show a narrow band emission. The photoluminescence efficiency of CdTe nanocrystals strongly depends on the pH value of the colloidal solution. The maximum quantum yield at room temperature is approximately 18% when the pH value of the CdTe solution is brought to 4.5 by using thioglycolic acid. The optical spectroscopy studies imply that the pH-dependent behavior of the CdTe nanocrystals' fluorescence is caused by structural changes on the surface rather than the size of the nanocrystals. Systematic absorption and fluorescence studies on dialyzed samples suggest that in the acidic range a shell of cadmium thiol complexes is formed around the CdTe core. Thus, the fluorescence quantum yield is enhanced dramatically when the solution is made acidic. In contrast, such a shell can also be produced in the alkaline range, but only afte...
694 citations
TL;DR: In this article, a model for the donor VXO and acceptor (VO, VGa) microstructure is proposed, where donors would be assembled in shallow clusters responsible for delocalized electron behavior with minority acceptors between.
670 citations
TL;DR: In this article, temperature-dependent time-integrated and time-resolved photoluminescence (PL) studies of InGaN/GaN multiple quantum wells (MQWs) grown by metalorganic chemical vapor deposition were performed.
Abstract: We report temperature-dependent time-integrated and time-resolved photoluminescence (PL) studies of InGaN/GaN multiple quantum wells (MQWs) grown by metalorganic chemical vapor deposition. We observed anomalous emission behavior, specifically an S-shaped (decrease–increase–decrease) temperature dependence of the peak energy (Ep) for InGaN-related PL with increasing temperature: Ep redshifts in the temperature range of 10–70 K, blueshifts for 70–150 K, and redshifts again for 150–300 K with increasing temperature. In addition, when Ep redshifts, the spectral width is observed to narrow, while when Ep blueshifts, it broadens. From a study of the integrated PL intensity as a function of temperature, it is found that thermionic emission of photocarriers out of local potential minima into higher energy states within the wells is the dominant mechanism leading to the thermal quenching of the InGaN-related PL. We demonstrate that the temperature-induced S-shaped PL shift is caused by a change in the carrier dyna...
648 citations
TL;DR: In this article, the performance of these heterostructure devices is influenced by the thickness of the dot layer, and they show that the electroluminescence signal is almost exclusively generated within the inorganic layer, with a very weak contribution from the PPV layer at higher applied voltage.
Abstract: Electroluminescence (EL) and photoluminescence (PL) from heterostructure thin films made of organic poly (phenylene vinylene), PPV, and inorganic semiconductor CdSe nanocrystals are investigated. In these devices, the organic PPV structure is built next to an indium tin oxide anode, using the technique of molecular layer-by-layer sequential adsorption, and serves primarily as the hole transport layer. The inorganic layer, adjacent to an Al electrode, is made of spin cast CdSe nanocrystals, passivated with either organic groups or with a wider band gap semiconductor, e.g., ZnS in the present case. We find that the electroluminescence signal is almost exclusively generated within the inorganic layer, with a very weak contribution from the PPV layer at higher applied voltage. The performance of these heterostructure devices is influenced by the thickness of the dot layer. Lifetime tests reveal promising stability, with devices operating continuously over 50–100 h. Values of the external quantum efficiency, η...
527 citations
TL;DR: In this article, the authors observed visible light emission from nanosize gold clusters and found that relatively intense photoluminescence occurs only when the size of the metal nanocluster is sufficiently small (<5 nm.
Abstract: We have observed visible light emission from nanosize gold clusters. Liquid chromatographic analysis of the metal clusters shows that relatively intense photoluminescence occurs only when the size of the metal nanocluster is sufficiently small (<5 nm). The emission is strongly Stokes shifted and is assigned to radiative recombination of Fermi level electrons and sp- or d-band holes. The electron and/or hole states are perturbed by surface states, as indicated by the dependence of the emission spectrum on the nature of the cluster surface. Finally, we found that large, nonemitting gold clusters can also be made luminescent by partial dissolution using KCN.
512 citations
TL;DR: In this paper, a doublet pattern observed in the long wavelength region is attributed to the coexistence of the two crystalline forms in ZnS particles, while the visible luminescent radiation at 590.1 nm is due to Mn impurities.
Abstract: ZnS nanoparticles were prepared by chemical precipitation of Zn2+ with sulfur ions in aqueous solution. The ultraviolet-excited samples reveal detailed structure in the luminescence spectra. A doublet pattern observed in the long wavelength region is attributed to the coexistence of the two crystalline forms in ZnS particles. The visible luminescent radiation at 590.1 nm is due to Mn impurities. The dominant emission band at short wavelengths exhibits a quadruple fine structure with peaks located at 416.1, 423.9, 430.1, and 437.8 nm which are identified with optical transitions arising from vacancy and interstitial sites for both Zn and S atoms.
TL;DR: In this paper, the emission mechanisms of strained InxGa1−xN quantum wells (QWs) were shown to vary depending on the well thickness, L, and x. The absorption edge was modulated by the quantum confined Stark effect and quantum confined Franz-Keldysh effect (QCFK) for the wells, in which, for the first approximation, the product of the piezoelectric field, FPZ and L exceed the valence band discontinuity, ΔEV.
Abstract: The emission mechanisms of strained InxGa1−xN quantum wells (QWs) were shown to vary depending on the well thickness, L, and x. The absorption edge was modulated by the quantum confined Stark effect and quantum confined Franz-Keldysh effect (QCFK) for the wells, in which, for the first approximation, the product of the piezoelectric field, FPZ, and L exceed the valence band discontinuity, ΔEV. In this case, holes are confined in the triangular potential well formed at one side of the well producing the apparent Stokes-like shift. Under the condition that FPZ×L exceeds the conduction band discontinuity ΔEC, the electron-hole pair is confined at opposite sides of the well. The QCFK further modulated the emission energy for the wells with L greater than the three dimensional free exciton Bohr radius aB. On the other hand, effective in-plane localization of carriers in quantum disk size potential minima, which are produced by nonrandom alloy compositional fluctuation enhanced by the large bowing parameter and...
TL;DR: An unusual luminescent inorganic oxide, Sr2CeO4, was identified by parallel screening techniques from within a combinatorial library of more than 25,000 members prepared by automated thin-film synthesis, and its structure reveals one-dimensional chains of edge-sharing CeO6 octahedra.
Abstract: An unusual luminescent inorganic oxide, Sr2CeO4, was identified by parallel screening techniques from within a combinatorial library of more than 25,000 members prepared by automated thin-film synthesis. A bulk sample of single-phase Sr2CeO4 was prepared, and its structure, determined from powder x-ray diffraction data, reveals one-dimensional chains of edge-sharing CeO6 octahedra, with two terminal oxygen atoms per cerium center, that are isolated from one another by Sr2+ cations. The emission maximum at 485 nanometers appears blue-white and has a quantum yield of 0.48 +/- 0.02. The excited-state lifetime, electron spin resonance, magnetic susceptibility, and structural data all suggest that luminescence originates from a ligand-to-metal Ce4+ charge transfer.
TL;DR: In this article, a study of the elastic exciton-exciton Coulomb scattering in a semiconductor quantum well is presented, including the interexciton exchange of carriers and the spin degrees of freedom.
Abstract: A study of the elastic exciton-exciton Coulomb scattering in a semiconductor quantum well is presented, including the interexciton exchange of carriers and the spin degrees of freedom. The theoretical results show that electron-electron and hole-hole exchanges are the dominant mechanisms of interaction, while the classical direct term is negligible. The density-dependent homogeneous linewidth is calculated within the Born approximation and good agreement with the existing experimental data is obtained. Owing to the interexciton exchange of carriers, collisions lead to spin relaxation as actually observed in recent time-resolved photoluminescence experiments. [S0163-1829(98)06736-8].
TL;DR: In this paper, the blue Mg induced 2.8 eV photoluminescence (PL) band in metalorganic chemical vapor deposition grown GaN has been studied in a large number of samples with varying Mg content.
Abstract: The blue Mg induced 2.8 eV photoluminescence (PL) band in metalorganic chemical vapor deposition grown GaN has been studied in a large number of samples with varying Mg content. It emerges near a Mg concentration of 1×1019 cm−3 and at higher concentrations dominates the room temperature PL spectrum. The excitation power dependence of the 2.8 eV band provides convincing evidence for its donor–acceptor (D–A) pair recombination character. It is suggested that the acceptor A is isolated MgGa while the spatially separated, deep donor (430 meV) D is attributed to a nearest-neighbor associate of a MgGa acceptor with a nitrogen vacancy, formed by self-compensation.
ETSI1
TL;DR: In this article, the effect of the III/V ratio and substrate temperature on the growth of GaN and A1N films on Si(1 1 1) substrates by molecular beam epitaxy, where active nitrogen was generated by a radio frequency plasma source.
TL;DR: In this article, photoluminescence at low temperature is studied for a CdTe-based microcavity tuned to resonance with a quantum well exciton, and two distinct stimulation effects are observed with increasing excitation.
Abstract: Photoluminescence at low temperature is studied for a CdTe-based microcavity tuned to resonance with a quantum well exciton. Two distinct stimulation effects are observed with increasing excitation. The first one is associated with the lower polariton state in the strong exciton-photon coupling regime. This effect, whose physical origin has not yet been identified, could be favored by the higher stability of exciton in CdTe. The second stimulation, obtained for much higher excitation, can be assigned to the electron-hole plasma in the weak coupling regime.
01 Jan 1998
TL;DR: Theories of luminescence as discussed by the authors have been studied in a wide range of applications, e.g. Bioluminenscence and Chemiluminescence, as well as electroluminescenters.
Abstract: Theories of Luminescence. Experimental techniques. Photoluminescence. Cathodoluminescence. Ionoluminescence. Electroluminescence. Thermoluminescence. Lyoluminescence. Sonluminescence. Mechanoluminescence. Bioluminenscence and Chemiluminescence. Index.
TL;DR: In this article, the spectral photocurrent response of GaInNAs lattice-matched to GaAs with a bandgap of 1 eV has been measured in an electrochemical cell.
TL;DR: In this paper, the influence of free carrier scavengers (CH3OH and O2) on the photophysics at solid/liquid interfaces was explored to explore the correlation between surface chemistry and the nature of preexisting or photogenerated defect centers (e.g., F-type and V-type color centers).
Abstract: Powdered zirconia and colloidal zirconia aqueous sols have been examined by diffuse reflectance and absorption spectroscopies and by photoluminescence methods in solid/gas and solid/liquid systems. The former system was examined following high-temperature treatment in vacuo and under reducing and oxidizing atmospheres. Studies of the influence of H2 and O2 on the photophysics of microparticles (powder) and nanoparticles (colloidal sols) of zirconia at solid/gas interfaces and the effects of free carrier scavengers (CH3OH and O2) on the photophysics at solid/liquid interfaces were undertaken to explore the correlation between surface chemistry and the nature of preexisting or photogenerated defect centers (e.g., F-type and V-type color centers). ZrO2 is an insulating, direct wide-gap metal oxide with an optical band gap of ∼5.0 eV; another optical transition occurs at 5.85 eV. The optical behavior depends on whether zirconia is preirradiated in the intrinsic (hν > 5.0 eV) or extrinsic (hν < 5.0 eV) absorpt...
TL;DR: In this paper, the size of the Si nanocrystals is limited by the thickness of the a-Si layer, the shape is nearly spherical, and the orientation is random.
Abstract: Nanocrystalline-silicon superlattices are produced by controlled recrystallization of amorphous-Si/SiO2 multilayers. The recrystallization is performed by a two-step procedure: rapid thermal annealing at 600–1000 °C, and furnace annealing at 1050 °C. Transmission electron microscopy, Raman scattering, x-ray and electron diffraction, and photoluminescence spectroscopy show an ordered structure with Si nanocrystals confined between SiO2 layers. The size of the Si nanocrystals is limited by the thickness of the a-Si layer, the shape is nearly spherical, and the orientation is random. The luminescence from the nc-Si superlattices is demonstrated and studied.
TL;DR: In this article, light emission from different systems of silicon nanocrystals does behave as expected for indirect-band-gap quantum dots, and the ratio of no-phonon transitions to TA and TO phonon-assisted processes over a wide range of confinement energies is determined.
Abstract: We show that light emission from different systems of silicon nanocrystals does behave as expected for indirect-band-gap quantum dots. Photoluminescence excited on the low energy part of the distribution of Si nanocrystals exhibits a set of narrow peaks associated with Si TA and TO momentum-conserving phonon-assisted optical transitions. These spectra allow us to determine the ratio of no-phonon transitions to TA and TO phonon-assisted processes over a wide range of confinement energies. The ratio between these recombination channels changes by 2 orders of magnitude with increasing confinement energy. For confinement energies above 0.7 eV the radiative transitions are governed by no-phonon quasidirect processes.
TL;DR: In this paper, the authors reported far-infrared photoconductivity in self-organized InAs/GaAs quantum dots grown by molecular beam epitaxy using a Fourier transform infrared spectrometer.
Abstract: We report far-infrared photoconductivity in self-organized InAs/GaAs quantum dots grown by molecular beam epitaxy. Through use of a Fourier transform infrared spectrometer, a photoconductivity signal peaked at 17 μm is observed from a n–i–n detector structure with doped InAs quantum dots in the intrinsic region. Comparison of photoluminescence and band-to-band photocurrent absorption spectra suggests the far-infrared response is due to intersubband transitions in the quantum dots.
TL;DR: In this article, the optical properties of GaN quantum dots with the wurtzite structure result from a balance between confinement and piezoelectric effects, and the photoluminescence peak is centered at 2.95 eV, nearly 0.5 eV below the bulk GaN band gap.
Abstract: It is shown that the optical properties of GaN quantum dots with the wurtzite structure result from a balance between confinement and piezoelectric effects. In ``large'' quantum dots with an average height and diameter of 4.1 and 17 nm, respectively, the photoluminescence peak is centered at 2.95 eV, nearly 0.5 eV below the bulk GaN band gap. We attribute this enormous redshift to a giant 5.5 MV/cm piezoelectric field present in the dots, in agreement with theoretical calculations.
TL;DR: In this article, it was shown that annealing these samples in flowing O2 at 1000 °C for times up to 30 min results in oxidation of the Si nanocrystals, first close to the SiO2 surface and later at greater depths.
Abstract: Si nanocrystals (diameter 2–5 nm) were formed by 35 keV Si + implantation at a fluence of 6 × 1016 Si/cm2 into a 100 nm thick thermally grown SiO2 film on Si (100), followed by thermal annealing at 1100 °C for 10 min. The nanocrystals show a broad photoluminescence spectrum, peaking at 880 nm, attributed to the recombination of quantum confined excitons. Rutherford backscattering spectrometry and transmission electron microscopy show that annealing these samples in flowing O2 at 1000 °C for times up to 30 min results in oxidation of the Si nanocrystals, first close to the SiO2 film surface and later at greater depths. Upon oxidation for 30 min the photoluminescence peak wavelength blueshifts by more than 200 nm. This blueshift is attributed to a quantum size effect in which a reduction of the average nanocrystal size leads to emission at shorter wavelengths. The room temperature luminescence lifetime measured at 700 nm increases from 12 µs for the unoxidized film to 43 µs for the film that was oxidized for 29 min.
TL;DR: Spontaneous, stimulated and laser emission spectra of ZnO epitaxial layers, grown by plasma-assisted molecular beam epitaxy, are presented in this paper, where samples are found to exhibit high-intensity near band-edge emissions at room temperature.
TL;DR: In this article, SiO2 films containing Si nanocrystals (nc-Si) and Er were prepared and their photoluminescence (PL) properties were studied, which could be assigned to the electron-hole recombination in nc-Si and the intra-4f transition in Er3+, respectively.
Abstract: SiO2 films containing Si nanocrystals (nc-Si) and Er were prepared and their photoluminescence (PL) properties were studied. The samples exhibited PL peaks at 0.8 and 1.54 μm, which could be assigned to the electron-hole recombination in nc-Si and the intra-4f transition in Er3+, respectively. Correlation between the intensities of the two PL peaks was studied as functions of the size of nc-Si, Er concentration, excitation power and excitation wavelength. It was found that the 1.54 μm PL of Er3+ is strongly enhanced by incorporating nc-Si in films. Furthermore, the intensity of the 1.54 μm peak was found to depend strongly on the size of the incorporated nc-Si.
TL;DR: In this article, the authors investigated the emission mechanisms of a device-quality quantum well (QW) structure and bulk three dimensional (3D) InGaN materials grown on sapphire substrates without any epitaxial lateral overgrown GaN base layers.
Abstract: Emission mechanisms of a device-quality quantum well (QW) structure and bulk three dimensional (3D) InGaN materials grown on sapphire substrates without any epitaxial lateral overgrown GaN base layers were investigated. The InxGa1−xN layers showed various degrees of in-plane spatial potential (band gap) inhomogeneity, which is due to a compositional fluctuation or a few monolayers thickness fluctuation. The degree of fluctuation changed remarkably around a nominal InN molar fraction x=0.2, which changes to nearly 0.08–0.1 for the strained InxGa1−xN. This potential fluctuation induces localized energy states both in the QW and 3D InGaN, showing a large Stokes-like shift. The spontaneous emission from undoped InGaN single QW light-emitting diodes (LEDs), undoped 3D LEDs, and multiple QW (MQW) laser diode (LD) wafers was assigned as being due to the recombination of excitons localized at the potential minima, whose lateral size was determined by cathodoluminescence mapping to vary from less than 60 to 300 nm...
TL;DR: In this article, the emission spectrum consists of two narrow green lines at 537 and 558 nm and a broad peak at light blue wavelengths (480-510 nm) and the narrow lines have been identified as Er transitions from the 2H11/2 and 4S3/2 levels to the 4I15/2 ground state.
Abstract: Visible light emission has been obtained from Er-doped GaN thin films. The GaN was grown by molecular beam epitaxy on sapphire substrates using solid sources (for Ga, Al, and Er) and a plasma gas source for N2. Above GaN band-gap photoexcitation resulted in strong green emission. The emission spectrum consists of two narrow green lines at 537 and 558 nm and a broad peak at light blue wavelengths (480–510 nm). The narrow lines have been identified as Er transitions from the 2H11/2 and 4S3/2 levels to the 4I15/2 ground state. The intensity of the 558 nm emission decreases with increasing temperature, while the intensity of the 537 nm line actually peaks at ∼300 K. This effect is explained based on the thermalization of electrons between the two closely spaced energy levels.