Excitation Intensity Dependence Research Articles

Relaxation dynamics of excitons in J-aggregates revealing a two-component Davydov splitting

The J-aggregates of the thiacarbocyanine dye THIATS revealing a clear-cut two-component Davydov splitting have been investigated by different optical methods. The presence of two molecules per unit cell was proved both by polarised fluorescence and linear dichroism in the absorption of aligned aggregates in a rotating cell. The fluorescence excitation spectrum of the aggregates and their quantum yield (QY) were measured across the entire exciton band. The QY was found to be 0.1 and 0.4 for excitation via the upper and lower Davydov components respectively. The radiative lifetime of emission of the J-band was estimated to be about 15 ns combining fluorescence decay data with the fluorescence quantum yield. The excitation intensity dependence of the fluorescence decays suggests the occurrence of exciton–exciton annihilation (EEA). EEA occurs even at an excitation photon fluxes below 10 10 photons/(pulse cm 2) corresponding to less than one absorbed photon per 10 6 monomer units.

Auger Effect Seen in the Porous Silicon Fast Luminescent Band

AbstractTime resolved photoluminescence (PL) measurements are performed on oxidized and fresh porous silicon at room temperature. Comparing the evolution of the nanosecond time delayed PL in both cases, a new feature of the PL spectra is identified: the fast-red band, present as well in fresh or aged samples. The nonlinear excitation intensity dependence of this component is described by a simple model where, the Auger effect inside isolated silicon nanocrystallites plays the dominant role.

The third subband population in modulation-doped InGaAs/InAlAs heterostructures

We have observed the population of the third (n=3) two-dimensional electron subband of InGaAs/InAlAs modulation-doped structures with very dense sheet carrier density by means of Fourier transform photoluminescence (PL). Three well-resolved PL peaks centered at 0.737, 0.908, and 0.980 eV are observed, which are attributed to the recombination transitions from the lowest three electron subbands to the n=1 heavy-hole subband. The subband separations clearly exhibit the features of the stepped quantum well with triangle and square potential, consistent with numerical calculation. Thanks to the presence of the Fermi cutoff, the population ratio of these three subbands can be estimated. Temperature and excitation intensity dependence of the quantum well luminescence intensity is also analyzed.

Photoluminescence study of ZnSe single crystals melt-grown with B 2O 3 encapsulant

Some properties of ZnSe single crystals grown from melt using a B 2O 3 encapsulant were investigated. B 2O 3 encapsulation prevents the crystal from contamination of impurities during growth and improves the crystallinity, specifically by subsequent annealing at 1390°C. However, it is revealed that the concentration of boron in the ZnSe crystal is increased by using B 2O 3 encapsulant. At the same time, the photoluminescence spectrum of the annealed sample shows characteristic emission peaks at 2.733 and 2.752 eV at 16 K by 16 mW cm −2 excitation of 325 nm emission line from a He-Cd laser. From both the temperature dependence and the excitation intensity dependence of the peak energy, these emissions are identified as a donor-acceptor pair and a conduction-band-to-acceptor emission, respectively. A shallow acceptor level is located at about 70 meV above the valence band. From the facts that the crystal contains 10 17–10 18 atoms cm −3 of oxygen as an impurity and the formation of this level correlates to the annealing process, isoelectronic oxygen or oxygen associated complex is believed to be a possible origin of the shallow level.

Photoluminescence study of the dynamical properties of GaAs sawtooth superlattices

The dynamic properties of a sawtooth superlattice (δ-doped nipi) were examined by photoluminescence (PL) spectroscopic techniques. The structure was grown on a semi-insulating GaAs substrate by metalorganic vapor phase epitaxy, using C and Si δ-doping. The excitation intensity dependence of the sample’s PL was measured over six decades which produced a shift of 200 meV in the peak of the PL photon energy. The dynamic properties of the sawtooth superlattice were probed using time resolved PL and carrier lifetime measurements. Time resolved PL was measured over 6 orders of magnitude in delay time. The luminescence wavelength from the sawtooth superlattice sample was found to shift to low energies over time after pulsed excitation, indicating the temporal evolution of the band edges. A new and sensitive technique for measuring radiative recombination lifetimes at low excitation intensities was developed. Therefore δ-doped sawtooth superlattices are shown to have a tunable band gap as well as an intensity tunable carrier lifetime.

Photoluminescence properties of cubic GaN grown on GaAs(100) substrates by metalorganic vapor phase epitaxy

Photoluminescence (PL) measurements were performed on cubic GaN films grown on GaAs(100) substrates by metalorganic vapor phase epitaxy. The cubic GaN films show a high optical quality that enables us to study the PL spectra in detail. From temperature and excitation intensity dependence, the emission lines at 3.274 and at 3.178 eV were assigned to the excitonic transition and the donor–acceptor pair transition, respectively. We also observed two additional emission lines at 3.088 and 3.056 eV. An excitonic emission at 3.216 eV with full width half maximum value as small as 73 meV was observed at 300 K.

Optical spectroscopy of MBE grown [formula omitted] quantum wells at various acceptor doping levels

An experimental study of the optical properties of acceptor-center doped quantum wells (QWs) is presented. We have studied the effects of acceptor doping at concentration levels varying from 10 16 up to 10 19 cm −3 using steady-state photoluminescence (PL) and photoluminescence excitation (PLE). The quenching of excitons has been investigated by comparing the results obtained for n-and p-type bulk and QW structures doped both in the well and in the barrier. At a doping concentration higher than 5 × 10 16 cm −3, an additional excitonic component is observed on the low energy side of the principal neutral acceptor bound exciton (BE). The second feature is associated with BEs formed by excitons bound at interacting acceptors. Additional information is obtained from temperature, excitation intensity dependence and time resolved measurements. The effect of hydrogen passivation of the same samples is also studied.

Room temperature intrinsic optical transition in GaN epilayers: The band-to-band versus excitonic transitions

The mechanism of room-temperature (RT) intrinsic optical transition in high quality and purity GaN epilayer grown by metalorganic chemical vapor deposition (MOCVD) has been investigated. Our results show that the band-to-band instead of excitonic transition is the dominant transition in MOCVD grown GaN epilayer at RT. This conclusion is supported by the observation of the excitation intensity dependence of the photoluminescence emission peak position and by a model calculation. The band-to-band transition energy at RT at the limit of low carrier concentration has been determined to be 3.429 eV. Since the band-to-band transition is the dominant optical transition at RT, it thus suggests that the electron-hole plasma is most likely responsible for gain in GaN blue lasers similar to the case in other III–V semiconductor lasers.

Luminescence determination of donor concentration in n-type 6H-SiC

The low temperature donor-acceptor pair photoluminescence spectra of n-type 6H-SiC has been investigated, both theoretically and experimentally. The excitation intensity dependence of the maximum position and width of the no-phonon line has been observed down to the lowest limit of experimental excitation intensity. We show that the majority impurity (donor) concentration can be obtained from the line-shape of the experimental spectra. The value of the donor concentration found in this way is in good agreement with the result of independent electrical measurements.

Influence of photon reabsorption on the transfer efficiency of output intensity in semiconductor microcavities

Photon reabsorption effect on the transfer efficiency /spl beta//sub t/ of half-wavelength semiconductor microcavities was investigated by examining the excitation intensity dependence of the output light intensity. It is shown that /spl beta//sub t/ increases under intense excitation, and approaches to over-all spontaneous emission coupling coefficient /spl beta//sub 0/, as a result of the elimination of photon reabsorption. The results clearly demonstrate that the photon reabsorption is the predominant mechanism of the suppression of /spl beta//sub t/ at weak excitation, especially in the case of half-wavelength high-Q cavities.

Cooperative Emission inπ-Conjugated Polymer Thin Films

Picosecond dynamics of exciton emission and absorption have been studied in neat thin films of a variety of poly (phenylene vinylene) derivatives. We found that the stimulated emission band of 120 nm width and $\ensuremath{\sim}1\mathrm{ns}$ duration, which is observed at low exciton density $n$, collapses at $ng{10}^{17}{\mathrm{cm}}^{\ensuremath{-}3}$ into a much narrower band of 7 nm width and lifetime $\ensuremath{\tau}\ensuremath{\ll}10\mathrm{ps}$. Based on its excitation intensity dependence, polarization, lifetime, illuminated area, and film thickness dependencies, we assign this narrow band to superfluorescence rather than to amplified spontaneous emission.

High Quality ZnSe and Zn(Se,S) Produced by Low Temperature Physical Vapour Transport

High quality, large (25 mm in diameter) crystals of ZnSe and Zn(Se,S) were grown by low-temperature physical vapour transport method and characterized by various techniques, i.e. energy dispersive X-ray fluorescence spectrometry, double-crystal X-ray diffraction rocking curve FWHM, reflectivity measurements in the region of free exciton, electrical measurements and photoluminescence measurements for both low and high excitation densities. The measurement of the photoluminescence related to the neutral-donor-bound exciton revealed superlinear log(photoluminescence intensity) versus log(excitation intensity) dependence, which indicates stimulated emission phenomena.

Temperature and excitation intensity dependencies of the photoluminescence spectra of GaAs/(AlGa)As disordered superlattices.

The temperature and excitation intensity dependencies of the photoluminescence (PL) spectra of GaAs/Al0.3Ga0.7As superlattices (SL's) having randomly distributed well widths are studied. Our results indicate that the electronic properties of disordered SL's are similar to those of other disordered semiconductors: at low energies the electronic states are essentially localized, while at higher energies they are extended. Further, the PL spectra feature a disorder-induced fine structure, and they shift to the red with T simply following the reduction of the band gap. The PL efficiency shows a weaker decrease with increasing T than the ordered SL. The dependence of the PL spectra on the excitation intensity shows an anomalous behavior of the disorder-induced fine-structure recombination lines.

Size-dependent luminescence of GaAs quantum wires on vicinal GaAs (110) surfaces with giant steps formed by MBE

Two size-dependent photoluminescence peaks are observed for GaAs quantum wires naturally formed on vicinal GaAs(1 1 0) surfaces with giant steps by molecular beam epitaxy. These two peaks are found to have different activation energies from the temperature dependence of these peaks. Monochromatic CL images shows the formation of two group of quantum wires with different size. Excitation intensity dependence of relative intensities of these peaks indicates no thermalization between these two group of wires.

Luminescence kinetics of iodine-doped silver bromide crystals: concentration and excitation intensity dependences

The decay kinetics of iodine bound excitons in AgBr:I crystals have been studied. Luminescence kinetics, quenching and changes in kinetics are observed to depend on the iodine concentration and the excitation intensity. These observation are discussed in terms of donor - donor and donor - acceptor interactions. The probabilities of energy migration via iodide ions and probability of luminescence quenching are estimated.

Magnetoluminescence properties of Hg1−xCdxTe epitaxial layers and superlattice structures grown by metalorganic molecular beam epitaxy

We present a study of the electro-optical properties ofHg1- xCdxTe epitaxial layers and Hg1-x CdxTe/CdTe (0.28 < x < 0.30) superlattice structures by x-ray diffraction, lateral transport and photo- and magneto-luminescence measurements. Systematic studies of the excitation intensity and magnetic field dependence of the photoluminescence revealed direct evidence of an excitonic contribution to the observed luminescence in Hg1- xCdxTe epitaxial layers. Similar investigations of the superlattice structures indicated that excitonic corrections were required to adequately fit the luminescence data. Optical gains of 80 cm−1 were obtained for an excitation intensity of 100 kW/cm2 indicating suitable electro-optical properties for making efficient mid-infrared laser diodes.

Low-temperature photoluminescence of disordered thin-layer GaAs/AlAs superlattices: Experiment.

Low-temperature photoluminescence spectra for various disordered thin-layer (GaAs${)}_{\mathit{m}}$(AlAs${)}_{\mathit{n}}$ superlattices have been experimentally studied. The samples were grown by molecular-beam expitaxy (MBE) growth, and disorder was introduced by varying the thickness of GaAs and AlAs layers in the growth direction in various specific but randomly generated sequences. The photoluminescence from disordered superlattice samples shows a strong disorder sequence dependence. Disordered (GaAs${)}_{\mathit{m}}$(AlAs${)}_{\mathit{n}}$ superlattices with m,n distributed in a range of 1--6 shows mainly a sharp peak at the high-energy side and a broad peak at the low-energy side. According to the study of the temperature, excitation-intensity, and disorder-sequence dependence of the photoluminescence, the sharp peak is assigned to bound-exciton recombination and the broad peak to the recombination of electron-hole pairs localized by a disorder potential induced by layer thickness disorder. Samples with m,n larger than 6 exhibit multiple peaks of similar recombination mechanism. In addition, the results demonstrate that the luminescence intensity of disordered superlattices at T=10 K can be increased by up to two orders of magnitude over that of ordered short-period superlattices. \textcopyright{} 1996 The American Physical Society.

Nitrogen acceptors confined in CdZnTe quantum well structures

Nitrogen acceptors confined in a CdZnTe single quantum well, grown by molecular beam epitaxy, are investigated by different optical methods. The transitions related to nitrogen acceptors confined in the well are observed in the doping range between 1017 and 1018/cm3. The temperature and excitation intensity dependence of the nitrogen-related transitions in photoluminescence spectra indicates that these transitions correspond to the nitrogen acceptor bound excitons and to free electron to neutral nitrogen acceptor recombination. The binding energy of nitrogen acceptors confined in an 130-Å-wide Cd0.96Zn0.04Te/Cd0.86Zn0.12Te structure is deduced to be 51.7±0.5 meV from this study.

Semiconductor Nanocrystals: Exciton Quantum Mechanics, Single Nanocrsytal Luminescence, and Metastable High Pressure Phases

AbstractWe review three areas where significant progress has recently occurred in our understanding of semiconductor nanocrystals. The first two involve luminescence properties of single and ensembles of Cadmium Selenide (CdSe) nanocrystallites (Quantum Dots) between 10 and 50 Å in radius. The size, magnetic field, and temporal dependence of emission from ensembles of nanocrystallites at cryogenic temperatures uncovers the fundamental mechanism of radiative recombination in these nanocrystals. Effective mass models that take into account the electron-hole exchange interaction can quantitatively account for observed luminescence Stokes shifts. Furthermore, the magnetic field dependence of luminescence lifetimes and longitudinal-optical (LO) phonon ratios demonstrate that the exciton ground state in these nanocrystals is optically passive (“dark exciton”) with spin projection ±2. Picosecond time resolved measurements probe exciton relaxation into this level. Recent results on the spectroscopy of single CdSe nanocrystals at room temperature are also presented. Remarkably, emission from a single CdSe nanocrystal under C.W illumination is observed to turn on and off discretely (fluorescence intermittency) on a ∼0.5s timescale. The excitation intensity dependence, and the influence of a passivating high band gap shell of Zinc Sulfide (ZnS) encapsulating the CdSe nanocrystal on the on/off times, suggest that this phenomenon is caused by photoionization. Finally, the third area originates in diamond anvil studies of the solid-solid phase transitions of nanocrystals under pressure. These studies show that a single nucleation event occurs per nanocrystal, and that as a consequence the nanocrystals change shape. The kinetic activation barrier increases with increasing size. Under suitable conditions nanocrystals in dense, six-coordinate high pressure phases may be metastable at STP.

Excitation Intensity and Temperature Dependent Photoluminescence Behavior of Silicon Nanoparticles

AbstractThe luminescence properties of silicon nanoparticles have been studied as a function of the excitation light intensity, the temporal Nature of the excitation source, and of sample temperature. The excitation intensity dependence of the luminescence was found to depend strongly on the temporal Nature of the excitation source. Under high intensity excitation from a pulsed 355 nm source, the photoluminescence (PL) intensity saturates and the peak PL wavelength shifts to the blue at room temperature. This behavior persists at reduced temperature. In contrast, under high intensity excitation using a cw 488 nm source at room temperature, the PL intensity saturates but does not shift in wavelength. At reduced temperatures, there is no saturation of luminescence intensity with high intensity cw excitation. These differences indicate that photogenerated carrier recombination occurs via different pathways depending on the temporal profile of the excitation, with cw excited samples following the expected Auger pathway while pulsed samples exhibit a state filling mechanism. Auger models for the pulsed behavior are found to be inconsistent with the experimental data. The temperature dependence of the PL from a pulsed excited sample for a constant excitation intensity was also monitored. The variation of the peak emission wavelength was found to be similar in magnitude to that observed for amorphous silicon, suggesting that structural disorder may play a role in the luminescence. The change in emission intensity was fairly weak, indicating enhanced carrier confinement, as would be expected in a quantum confined system.