Search

A photoluminescence (PL) method to characterize phase transition processes of Li+/Er3+:BaTiO3 (BLET) ferroelectric ceramics by infrared fluorescence intensities has been proposed. The fluorescence intensities are applied to detect the phase transition of BLET ceramics in multiple phase transition regions. It proves that the change in fluorescence intensities at the phase transition is due to a variation in the radiative transition probability of the 4I13/2 level. A physical mechanism is elucidated considering the relationship between fluorescence properties and the phase structure. Our research benefits an in-depth understanding of the relationship between the PL characteristics and the phase structure of ferroelectric materials and further develops the application potential of PL methods in ferroelectric phase transition detection. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) of Eu3+ ions in single layers of ZnO and ZnMgO and in short-period ZnO/ZnMgO and ZnO/MgO superlattices (SLs) grown by molecular beam epitaxy was investigated, and the results have been presented. Single ZnO and ZnMgO layers were grown on GaN/sapphire substrates, whereas the SLs on c-oriented crystalline ZnO. All samples were implanted with 300 keV Eu ions, and then rapid thermal annealing was performed at 800 °C for 5 min in an oxygen atmosphere to activate the intra-4f-shell PL of Eu3+ ions. After annealing, the red emission due to the 5D0 → 7F2 radiative transitions of Eu3+ was observed at excitation above the energy gap. The results clearly show that Mg is essential for obtaining the high 4f–4f PL intensity of Eu3+ ions, as in ZnMgO layers and short period SLs, the red emission is several times more intense than in ZnO. The PL excitation spectra were examined to confirm the energy transfer from the bandgap to the Eu3+ ions. Clear evidence is presented that in the studied short period SLs, the excitation of the Eu3+ PL occurs via generation of excitons in the barrier layers. [ABSTRACT FROM AUTHOR]

We report electroluminescence originating from L-valley transitions in n-type Ge/Si0.15Ge0.85 quantum cascade structures centered at 3.4 and 4.9 THz with a line broadening of Δ f / f ≈ 0.2. Three strain-compensated heterostructures, grown on a Si substrate by ultrahigh vacuum chemical vapor deposition, have been investigated. The design is based on a single quantum well active region employing a vertical optical transition, and the observed spectral features are well described by non-equilibrium Green's function calculations. The presence of two peaks highlights a suboptimal injection in the upper state of the radiative transition. Comparison of the electroluminescence spectra with a similar GaAs/AlGaAs structure yields one order of magnitude lower emission efficiency. [ABSTRACT FROM AUTHOR]

In recent years, semiconductor quantum dots have demonstrated their potential to reach the goal of being an ideal source of single and entangled photon pairs. Exciting reports of near unity entanglement fidelity, close to unity photon indistinguishability, and high collection efficiency in nanophotonic structures have been demonstrated by several distinct groups, showing unequivocally the maturity of this technology. To achieve the required complexity and scalability in realistic quantum photonic implementations, two-photon interference of photons from multi-sources must be reached. While high indistinguishability values have been observed for photons generated from the same source within a relatively short time separation, achieving similar visibility for larger time separation or in multi-source experiments still requires intensive efforts. In fact, the coupling to the particular mesoscopic environment of charge carriers confined in the quantum dot leads to decoherence processes, which limit the quantum interference effects to a short time window. Here, we discuss the progress in studying the dynamics of this decoherence, which crucially depends on the evolution of line broadening in high-quality self-assembled InGaAs quantum dots. Characterization of line broadening mechanisms is the first fundamental step to be able to counteract them. Optimization of the growth and active and passive control of the radiative transitions are crucial for the technological readiness of non-classical light sources based on semiconductor platforms. [ABSTRACT FROM AUTHOR]

AlGaN-based multiple-quantum-well (MQW) heterostructures were irradiated with a pulsed electron beam. Excitation with a beam energy of 12 keV and a beam current of 4:4mA produced cathodoluminescense at λ= 246 nm with a measured peak output power of >200 mW. The emission is dominated by radiative recombination from the MQW up to the maximum tested excitation power density of 1MW/cm2, as evidenced by unity slope in a double-logarithmic plot of the light output power vs. excitation power density. Monte Carlo simulations of the depth distribution of deposited energy for different beam energies produced good agreement with the measured peak output power vs. beam energy for an assumed carrier diffusion length of ~200 nm. [ABSTRACT FROM AUTHOR]

Semipolar GaN crystal stripes larger than 100 μm with dislocation densities below 5×106 cm-2 are achieved using a low cost fabrication process. An original sapphire patterning procedure is proposed, enabling selective growth of semipolar oriented GaN stripes while confining the defects to specific areas. Radiative and non-radiative crystalline defects are investigated by cathodoluminescence and can be correlated to the development of crystal microstructure during the growth process. A dislocation reduction mechanism, supported by transmission electron microscopy, is proposed. This method represents a step forward toward low-cost quasi-bulk semipolar GaN epitaxial platforms with an excellent structural quality which will allow for even more efficient III-nitride based devices. [ABSTRACT FROM AUTHOR]

Index-near-zero materials can be used for effective photon management. They help to restrict the angle of acceptance, resulting in greatly enhanced light trapping limit. In addition, these materials also decrease the radiative recombination, leading to enhanced open circuit voltage and energy efficiency in direct bandgap solar cells. [ABSTRACT FROM AUTHOR]

Recent defect calculations suggest that the open circuit voltage of CuGaSe2 solar cells can be limited by deep intrinsic electron traps by GaCu antisites and their complexes with Cu-vacancies. To gain experimental evidence, two radiative defect transitions at 1.10 eV and 1.24 eV are characterized by steady-state photoluminescence on epitaxial-grown CuGaSe2 thin films. Cu-rich samples are studied, since they show highest crystal quality, exciton luminescence, and no potential fluctuations. Variations of the laser intensity and temperature dependent measurements suggest that emission occurs from two deep donor-like levels into the same shallow acceptor. At 10K, power-law exponents of 1 (low excitation regime) and 1/2 (high excitation regime) are observed identically for both transitions. The theory and a fitting function for the double power law is derived. It is concluded that the acceptor becomes saturated by excess carriers which changes the exponent of all transitions. Activation energies determined from the temperature quenching depend on the excitation level and show unexpected values of 600meV and higher. The thermal activation of non-radiative processes can explain the distortion of the ionization energies. Both the deep levels play a major role as radiative and non-radiative recombination centers for electrons and can be detrimental for photovoltaic applications. [ABSTRACT FROM AUTHOR]

We report on the experimental observation of intrinsic photoluminescence from semiconductors in the terahertz spectral range. The intrinsic terahertz photoluminescence is caused by intraexciton radiative transitions excited during the process of the binding of nonequilibrium electrons and holes into free excitons. The experiments were carried out on high-purity Si crystals at helium temperatures and under interband photoexcitation intensities not exceeding 1W/cm². [ABSTRACT FROM AUTHOR]

We demonstrated the lasing action and remarkable reduction in long radiative lifetimes of type-II GaSb/GaAs quantum dots using a circular photonic-crystal nano-cavity with high Purcell factors. The associated enhancement in carrier recombination was surprisingly high and could even surpass type-I counterparts in similar conditions. These phenomena reveal that the type-II sample exhibited extremely low nonradiative recombination so that weak radiative transitions were more dominant than expected. The results indicate that type-II nanostructures may be advantageous for applications which require controllable radiative transitions but low nonradiative depletions. [ABSTRACT FROM AUTHOR]

We investigated the mechanisms of radiative recombination in a CH3NH3PbBr3 hybrid perovskite material using low-temperature, power-dependent (77 K), and temperature-dependent photoluminescence (PL) measurements. Two bound-excitonic radiative transitions related to grain size inhomogeneity were identified. Both transitions led to PL spectra broadening as a result of concurrent blue and red shifts of these excitonic peaks. The red-shifted bound-excitonic peak dominated at high PL excitation led to a true-green wavelength of 553 nm for CH3NH3PbBr3 powders that are encapsulated in polydimethylsiloxane. Amplified spontaneous emission was eventually achieved for an excitation threshold energy of approximately 350 μJ/cm². Our results provide a platform for potential extension towards a true-green light-emitting device for solid-state lighting and display applications. [ABSTRACT FROM AUTHOR]

Very high polarization degree of 0.98, considerably larger than theoretical predictions, has been measured in (2021) In0.24Ga0.76N/GaN quantum well by low temperature photoluminescence. With increasing temperature, the polarization degree decreases due to thermal population of the excited valence band level. This effect suggests an accurate method to determine the interlevel energy, which, for the studied well, is 32?meV. Time-resolved photoluminescence measurements set radiative recombination times between 2 and 12 ns for temperatures from 3 to 300 K. Nonradiative recombination was found to be slow, over 2?ns at 300 K, taking place via traps with activation energy of 0.19 eV. [ABSTRACT FROM AUTHOR]

InGaN/GaN light-emitting diodes (LEDs) with graded-thickness quantum barriers (GTQB) are designed and grown by metal-organic chemical-vapor deposition. The proposed GTQB structure, in which the barrier thickness decreases from the n-GaN to p-GaN side, was found to lead to an improved uniformity in the hole distribution and thus, radiative recombination rates across the active region. Consequently, the efficiency droop was reduced to 28.4% at a current density of 70 A/cm2, which is much smaller than that of the conventional equal-thickness quantum barriers (ETQB) LED, which is 48.3%. Moreover, the light output power was enhanced from 770 mW for the ETQB LEDs to 870 mW for the GTQB LEDs at 70 A/cm2. [ABSTRACT FROM AUTHOR]

A hybrid Monte Carlo-density matrix transport simulator is used to analyze the transport properties of resonant-phonon type terahertz quantum cascade lasers. By comparing calculated and experimental results, the importance of stimulated emission to the interpretation of experimental data is highlighted, particularly for devices based on diagonal radiative transitions. Finally, we discuss the absence of mode competition effects on transport. [ABSTRACT FROM AUTHOR]

The influences of defects and surface roughness on the indirect bandgap radiative transition of Ge were studied. Bulk Ge has 15 times the integrated intensity of photoluminescence of Ge-on-Si. However, for Ge-on-Si sample, the direct transition related photoluminescence intensity is higher than the indirect transition related one. We affirm that the defects in the Ge-on-Si are responsible for the weak indirect transition and relatively strong direct transition. The scattering of electrons by roughness at Ge/oxide interface can provide extra momentum of the indirect band transition of Ge, and thus enhance the indirect radiative transition. [ABSTRACT FROM AUTHOR]

Low-temperature photoluminescence is studied in detail in GaN nanocolumns (NCs) grown by plasma-assisted molecular beam epitaxy under various conditions (substrate temperature and impinging Ga/N flux ratio). The relative intensities of the different emission lines, in particular those related to structural defects, appear to be correlated with the growth conditions, and clearly linked to the NC sample morphology. We demonstrate, in particular, that all lines comprised between 3.10 and 3.42 eV rapidly lose intensity when the growth conditions are such that the NC coalescence is reduced. The well-known line around 3.45 eV, characteristic of GaN NC samples, shows, however, a behavior that is exactly the opposite of the other lines, namely, for growth conditions leading to reduced NC coalescence, this line tends to become more prominent, thus proving to be intrinsic to individual GaN NCs. [ABSTRACT FROM AUTHOR]

Both direct and indirect transitions of photoluminescence and electroluminescence are observed in a Ge n+p diode. The relative intensity of direct radiative recombination with respect to indirect radiative recombination increases with the increase in the optical pumping power, injection current density, and temperature. The increase in electron population in the direct valley is responsible for the enhancement. The spectra can be fitted by the combination of direct and indirect transition models. The direct radiative transition rate is ∼1600 times of the indirect transition, estimated by electroluminescence and photoluminescence spectra near room temperature. [ABSTRACT FROM AUTHOR]

Gold doped ruby glasses are classical examples of metal-glass nanocomposites that have been investigated for their striking optical properties. For the example of weakly Au doped lithium-borate glasses these materials are shown to exhibit a transition from ionic to electronic conduction within the same sample. It is achieved via appropriate heat treatment, presumably by introducing polarons formed from the trapping of electrons at partially ionized gold atoms. This unique electrical response is expected to introduce new functionality to this class of nanocomposites. [ABSTRACT FROM AUTHOR]

The radiative transitions in P- and B-doped Si nanocrystals are investigated by means of first-principle calculations. Using a three-level model, we show that the radiative lifetimes and oscillator strengths of the transitions between the conduction and the impurity bands, as well as the transitions between the impurity and the valence bands are strongly affected by the impurity position. On the other hand, the direct conduction-to-valence band decay is practically unchanged due to the presence of the impurity. In addition, the emission intensity of P(B)-doped nanocrystals with impurities positioned in the surface (anywhere) is higher (lower) than for pure nanocrystals. [ABSTRACT FROM AUTHOR]

The authors have employed variable temperature photoluminescence (PL) and time-resolved PL spectroscopy to probe the exciton recombination in high density and vertically aligned ZnO nanorods grown on p-type GaN/sapphire template. The low-temperature PL characterizes the dominant near-band-edge excitonic emissions from such nanorod arrays. At 4.3 K, a PL decay time of 432 ps reveals improved crystalline quality. The PL decay time shows irregular behavior due to different types of excitonic transitions dominating the PL spectra at different temperatures and a competitive effect of radiative recombination and nonradiative relaxation processes. [ABSTRACT FROM AUTHOR]

Nanodiamond (ND) is surrounded by layers of graphite on its surface. This unique structure feature creates unusual fluorescence spectra, which can be used as an indicator to monitor its surface modification. Meanwhile, the impurity, nitroso (C[Single_Bond]N==O) inside the ND can be photolyzed by two-photon absorption, releasing NO to facilitate the formation of a sp3 diamond structure in the core of ND and transforming it into a sp2 graphite structure. Such a conformational transition enlarges the size of ND from 8 to 90 nm, resulting in a popcornlike structure. This transition reaction may be useful as nanoknives in biomedical application. [ABSTRACT FROM AUTHOR]

The authors generated high-order harmonics by using the two-component laser-ablation plumes, which were irradiated by a femtosecond laser pulse. The harmonic spectra from double-target schemes comprised those obtained from separate targets. For indium/chromium plumes, the enhancement of two harmonics (13th and 29th orders) was obtained at the wavelengths of 61.15 and 27.41 nm, respectively. The conversion efficiencies of these two harmonics were estimated to be 10-5. The authors attribute the enhancement of 13th and 29th harmonics to the multiphoton resonances with the strong radiative transitions of ions in indium and chromium plumes, respectively. [ABSTRACT FROM AUTHOR]

Intersubband transitions in GaN/AlN short period superlattices prepared by molecular beam epitaxy were investigated using the optical absorption technique. The peak position wavelengths of these transitions are found to span the spectral range of 1.35–2.90 μm for samples cut into 45° waveguides with GaN quantum well thicknesses ranging between 1.70 and 2.41 nm. The Fermi energy levels are estimated from the carrier concentrations, which were measured using an electrochemical capacitance-voltage profiler. The well widths were inferred from comparing the measured peak position energy of the intersubband transitions and the bound state energy levels calculated using the transfer matrix method. [ABSTRACT FROM AUTHOR]

We have measured the photoluminescence (PL) lifetime of a freestanding GaN film using one-photon and two-photon excitations to demonstrate the dramatic difference in exciton recombination dynamics at the surface and in the bulk. An ultralong exciton PL lifetime of 17.2 ns at 295 K is observed from a GaN freestanding film using two-photon excitation, whereas less than 100 ps lifetime is observed for one-photon excitation, suggesting that nonradiative processes from surface defects account for the short PL lifetime measured. A monotonic increase in two-photon excited PL lifetime with increasing temperature and the linear dependence of the exciton lifetime with emission wavelength show good agreement with the theoretical predictions, indicating that radiative recombination dominates for bulk excited state relaxation processes. [ABSTRACT FROM AUTHOR]

We have performed time-resolved resonant pump-probe experiment to study the dynamic response of InAs/InP quantum dot transitions. A 72-stacked InAs/InP quantum dot layer sample is grown on (311)B substrate. Photoluminescence at high excitation power reveals ground and excited transitions. Carrier radiative lifetimes and differential transmission are determined under strong excitation powers. The variation of measured carrier radiative lifetimes with increasing excitation powers is attributed to the exciton and biexciton lifetimes difference. The implications of such a difference on differential transmission are discussed, and finally exciton and biexciton lifetimes are measured to be about 1720 and 530 ps, respectively. [ABSTRACT FROM AUTHOR]

Using quantum-mechanical analysis, a strain-balanced stack of coupled GaN/AlGaN quantum wells has been engineered for bandwidth-optimized all-optical switching at low switching powers. Intersubband transitions between three conduction subbands provide the basis for the large, fast, nonlinear optical response. Optimized performance for a given symbol rate is obtained by engineering the response time and nonlinear phase shift. [ABSTRACT FROM AUTHOR]

We demonstrate electroluminescence (EL) with an external efficiency of more than 0.1% at room temperature from glide dislocations in silicon. The key to this achievement is a considerable reduction of nonradiative carrier recombination at dislocations due to impurities and core defects by impurity gettering and hydrogen passivation, respectively, which is shown by means of deep-level transient spectroscopy. Time-resolved EL measurements reveal a response time below 1.8 μs, which is much faster, compared to the band-to-band luminescence of bulk silicon. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We studied time-resolved photoluminescence PL over a temporal range 10[SUP-6]- 10[SUP3]s in high-purity freestanding GaN templates. Red, yellow, green, blue, and shallow donor-acceptor emission bands can be resolved in the PL spectrum. Observation of luminescence long after the excitation is switched off is a striking feature of our study. The persistent PL observed for all above bands, except for the green band, is primarily attributed to the donor-acceptor-pair-type recombination. An unusually slow, nonexponential decay of radiative transitions from the conduction band to the shallow acceptor was also observed, pointing to some additional mechanism for the persistent PL. Possible role of the surface states in this effect is discussed. [ABSTRACT FROM AUTHOR]

Temperature dependent photoreflectance (PR) and photoluminescence experiments of the InAs/GaAs quantum dot (QD) structures were performed. At 20 K, effective band-gap transitions due to the InAs QDs, wetting layers, and GaAs buffer and cap layers were identified. Transition energies of the ground state and four excited states with nearly equal interlevel spacings (75–80 meV) were observed. The linewidth of the ground-state transition decreased as the temperature increased from 20 K to 100 K while the linewidth became broader at temperatures above 100 K. Energy features of the PR spectra originating from QDs and relating to the in-plane parabolic potentials were discussed. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

By capping InAs quantum dots (QDs) with a thin intermediate layer of InAlAs instead of GaAs, the radiative transition wavelengths are redshifted. Surface morphology studies confirm that the redshift is due to a better preserved QD height as compared with capping by GaAs only. In contrast, the energy levels are blueshifted when using AlGaAs instead of GaAs as the barrier material. In both cases, the energy separation between the ground and the first-excited state increases significantly. Combining these approaches, we demonstrate InAs QDs with a record transition energy separation of 108 meV and ground-state emission at 1.3 µm. [ABSTRACT FROM AUTHOR]

Intersubband transport and electroluminescence has been investigated in a double-quantum-well structure based on an intrawell (vertical) THz radiative transition. Magnetotunneling spectroscopy was used to determine subband energies, including the minimum energy separation (∼1.7 meV) between two anticrossed levels. The presence of this anticrossing indicates that in this structure, electron removal from the lower radiative state should be modeled by coherent resonant tunneling rather than incoherent sequential tunneling. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A quantum mechanical formalism to calculate the effect of compositional disorder on the linewidth of localized radiative transitions in semiconducting alloys is developed. A variational-statistical approach is used to calculate the line broadening. The general theory is applied to the specific case of excitonic transitions in which the exciton is in the ground state. The linewidth as a function of alloy composition is derived and the results thus obtained are compared with those of previous theories. For illustration, the calculations are done for GaAlAs and the results are compared with the available low-temperature photoluminescence data. [ABSTRACT FROM AUTHOR]

Hydrogenlike oxygen and carbon lines from transitions (Δn=1) between levels of high principal quantum number n, with wavelengths above 2000 Å have been observed. Observations of such transitions were possible due to charge-exchange processes during neutral beam injection of hydrogen atoms into the Princeton Large Torus tokamak. The lines are O VIII 2976 Å (8-7 transitions), C VI 3434 Å (7-6), and C VI 5291 Å (8-7). Application of these lines for ion temperature measurements and initial observations of neutral beam vertical distributions in the plasma are presented. [ABSTRACT FROM AUTHOR]

Observes phononless radiative recombination across the indirect gap in type-II germanium quantum wells modulated by self-organized Stranki-Krastanow dots. Significance of the three-dimensional confinement of electrons in an electronic dot; Details on the weak oscillator strength of phononless recombination; Indication on low-lying broad band features.

Assesses the process of barrier state electron lasing transitions in indium gallium arsenide/indium gallium arsenide phosphide multiple quantum lasers. Effects of carrier density on self-localization of electrons; Responses of lasing transitions to temperature; Impact of negative perturbation on transitions responses.

Describes the normal incidence infrared (IR) photodetectors using intersubband transitions in gallium antimonide L-valley quantum wells. Achievement of a strong IR absorption and good photodetector response; Presentation of the orientation dependence of the absorption; Influence of the electric field induced Stark shift on the responsivity peak.

By synchronously measuring the electroluminescence (EL) of Pt/NiO/Pt unipolar resistive switching (RS) devices during switching, we have nondestructively observed the dynamic evolution of conducting filaments (CFs) across the device. We demonstrate that both the random distribution and the subtle change in the chemical composition of CFs contribute to fluctuation of switching parameters. The EL emission during RS is attributed to the radiative transition through oxygen vacancy related defect levels of NiO rather than from the thermal radiation. Our findings offer a method to investigate the RS mechanism which further can be utilized to improve the stability and scalability of RS devices. [ABSTRACT FROM AUTHOR]

Titanium silicide films were grown under roughing vacuum on single crystal silicon wafers using high power quartz-halogen tungsten lamps to provide the thermal driving force. After processing for various time intervals, ranging from 5 to 25 s, the samples were characterized by measuring their sheet resistance. Major phases were detected with x-ray diffractometry. Elemental composition and film thickness were measured using Rutherford backscattering and Auger electron spectroscopy. These techniques consistently indicated that silicide formation was completed after 10-12 seconds processing time. The quality of these films and its potential usefulness are evidenced by their low resistivity of 21 μΩ cm. [ABSTRACT FROM AUTHOR]

A study of ns-laser ablation is presented, which focuses on the transient behavior of the physical processes that act in and above a copper sample. A dimensionless multiphase collisional radiative model describes the interplay between the ablation, collisional, and radiative mechanisms. Calculations are done for a 6 ns-Nd:YAG laser pulse operating at 532 nm and fluences up to 15 J/cm2. Temporal intensity profiles as well as transmissivities are in good agreement with experimental results. It is found that volumetric ablation mechanisms and photo-processes both play an essential role in the onset of ns-laser induced breakdown. [ABSTRACT FROM AUTHOR]

We propose and substantiate the concept of terahertz (THz) laser enabled by the resonant electron radiative transitions between graphene layers (GLs) in double-GL structures. We estimate the THz gain for TM-mode exhibiting very low Drude absorption in GLs and show that the gain can exceed the losses in metal-metal waveguides at the low end of the THz range. The spectrum of the emitted photons can be tuned by the applied voltage. A weak temperature dependence of the THz gain promotes an effective operation at room temperature. [ABSTRACT FROM AUTHOR]

Recombination processes in GaP/GaNP core/shell nanowires (NWs) grown on a Si substrate by molecular beam epitaxy are examined using a variety of optical characterization techniques, including cw- and time-resolved photoluminescence and optically detected magnetic resonance (ODMR). Superior optical quality of the structures is demonstrated based on the observation of intense emission from a single NW at room temperature. This emission is shown to originate from radiative transitions within N-related localized states. From ODMR, growth of GaP/GaNP NWs is also found to facilitate formation of complex defects containing a P atom at its core that act as centers of competing non-radiative recombination. [ABSTRACT FROM AUTHOR]