Fast Photoluminescence Research Articles

Photoluminescence of colloidal CdSe/ZnS quantum dots under oxygen atmosphere

The effects of oxygen versus vacuum ambients on colloidal CdSe/ZnS quantum dots (QDs) were studied using both continuous and time-resolved photoluminescence (PL) measurements. The PL intensities were found to be an order of magnitude higher in an oxygen atmosphere, which is explained by the passivation of surface defects by oxygen absorption. The decay of PL intensities can be best fitted by a biexponential function with lifetimes of approximately 1 ns for the fast decay and approximately 10 ns for the slow decay. Based on the emission-energy dependence of carrier lifetimes and of the amplitude ratio of the fast-decay component to the slow-decay component, we suggest that the fast and slow PL decay of colloidal CdSe/ZnS QDs is caused by the recombination of delocalized carriers in the internal core states and the localized carriers in the surface states, respectively.

Recombination dynamics of luminescence in colloidal CdSe/ZnS quantum dots

Recombination dynamics of photoluminescence (PL) in colloidal CdSe/ZnSquantum dots (QDs) were studied using time-resolved PL measurements. ThePL intensity shows a biexponential decay at 9 K, consisting of a fast component(∼1 ns) and a slowcomponent (∼6.3 ns). Based on the emission-energy and temperature dependence of carrier lifetimes, wesuggest that the fast and slow PL decay of colloidal CdSe/ZnS QDs originates fromrecombination of the delocalized carriers in the internal core states and the localizedcarriers at the interface, respectively.

Spectroscopy and carrier dynamics in CdSe self-assembled quantum dots embedded in ZnxCdyMg1−x−ySe

Time-resolved and steady-state photoluminescence, reflectivity, and absorption experiments were performed on CdSe quantum dots in ZnxCdyMg1−x−ySe barriers. Studies of the capture times of the photoexcited carriers into the quantum dots and of electron-hole recombination times inside the dots were performed. Photoluminescence rise time yielded capture times from 20 ps to 30 ps. All samples exhibit fast and slow photoluminescence decays, consistent with observing two independent but energetically overlapping decays. The faster relaxation times for the sample emitting in the blue range is 90 ps, whereas for the two samples emitting in the green it is 345 ps and 480 ps. The slower relaxation times for the sample emitting in blue is 310 ps, whereas for the samples emitting in green is 7.5 ns. These results are explained on the basis of the structural differences among the quantum-dot samples.

Electronic excitations and intrinsic defects in nanostructural Al 2 O 3

Research results on cathodo-and photoluminescence with time-resolution of Al2O3 nanostructural powders at excitation with strong electron beams as well as synchrotron radiation pulses have been presented. Nanopowders Al2O3 before and after irradiation by Fe+ ions have been found to be characterized by an identical phase composition (γ-phase/δ-phase ratio equals 1), an average grain size ∼17 nm. Nanopowders Al2O3 have been determined to have fast photoluminescence (PL) (band at 3.2 eV), whose decay kinetics is described by two exponential stages (τ1=0.5 ns and τ2=5.5 ns). In the fast photoluminescence excitation spectrum three bands 5.24, 6.13, 7.44 eV were distinguished. Two alternative models of centres according to which luminescence 3.2 eV is related to the radiative relaxation of P− centres or is due to the formation of the surface F+s-type centers have been proposed. On the basis of analysis of spectral-luminescence properties of nanostructural Al2O3 a slow luminescence of nanopowders at 3.5 eV is assumed to be due to a radiative annihilation of excitons localized near structural defects. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electronic Excitations and Defects in Nanostructural Al[sub 2]O[sub 3

A time-resolved cathodo-and photoluminescence study of nanostructural modifications of Al2O3 (powders and ceramics) excited by heavy-current electron beams, as well as by pulsed synchrotron radiation, is reported. It was found that Al2O3 nanopowders probed before and after Fe+ ion irradiation have the same phase composition (the γ-phase/δ-phase ratio is equal to 1), an average grain size equal to ∼17 nm, and practically the same set of broad cathodoluminescence (CL) bands peaking at 2.4, 3.2, and 3.8 eV. It was established that Al2O3 nanopowders exhibit fast photoluminescence (PL) (a band at 3.2 eV), whose decay kinetics is described by two exponential stages (τ1 = 0.5 ns, τ2 = 5.5 ns). Three bands, at 5.24, 6.13, and 7.44 eV, were isolated in the excitation spectrum of the fast PL. Two alternate models of PL centers were considered, according to which the 3.2-eV luminescence either originates from radiative relaxation of the P− centers (anion-cation vacancy pairs) or is due to the formation of surface analogs of the F+ center (FS+-type centers). In addition to the fast luminescence, nano-Al2O3 was found to produce slow luminescence in the form of a broad band peaking at 3.5 eV. The excitation spectrum of the 3.5-eV luminescence obtained at T = 13 K exhibits two doublet bands with maxima at 7.8 and 8.3 eV. An analysis of the luminescent properties of nanostructural and single-crystal Al2O3 suggests that the slow luminescence of nanopowders at 3.5 eV is due to radiative annihilation of excitons localized near structural defects.

Two-Dimensional and Multi-Experimental Modeling of Polycrystalline Cu(In,Ga)Se2 Solar Cells

AbstractWe use two-dimensional simulations to explore if charged columnar grain boundaries (GBs) in Cu(In,Ga)Se2 (CIGS) solar cells increase device performance. Although the simulations confirm that charged GBs can increase photocurrent by forming minority-carrier collection channels, this generally occurs at the expense of overall efficiency. Furthermore, improvements in photocurrent require significant GB minority collection beyond a diffusion length of the space-charge region. This collection from deep within the base can be detected by quantum efficiency (QE) spectra, electron-beam-induced current (EBIC) experiments, near-field scanning optical microscopy (NSOM), and fast photoluminescence decay. Simulations of all these experiments indicate that GB charge sufficient to significantly increase photocurrent collection is generally inconsistent with actual observations.

Photoluminescence decay in the ps time regime and structural properties of pulsed-laser deposited GaN

In this work we focus on the study of the fast photoluminescence decay in the ps time regime in comparison with structural properties of GaN films deposited by a cyclic pulsed laser deposition technique. Structural film properties are obtained from X-ray analysis and atomic force microscopy. Steady-state photoluminescence performed at 13K shows the typical donor-bound excitonic transition (D0X) at 3.47eV and a line near 3.42eV which can be related to structural defects. Fast fluorescence decays were obtained after excitation at 287nm (4.32eV) with 5ps pulses from a frequency-doubled cavity-dumped dye laser. At room temperature the decay of the blue fluorescence at 367nm showed decay times between 4 and 40 ps, whereas the yellow contribution at 530nm could best be described by a multi-exponential fit with decay times of several ns. The results are compatible with published exciton decay times in epitaxial MOCVD-grown GaN films.

Analysis of Photoluminescence Efficiency of Annealed GaInNAs Quantum Well Grown by Solid Source Molecular Beam Epitaxy

ABSTRACTThe carrier dynamics of a 7.5 nm GaInNAs quantum well (QW) are studied by photoluminescence (PL) at a low temperature regime of 4 K to 150 K. The PL emission efficiency of the QW is initially evaluated to examine the recombination mechanisms in the QW. A dual-activation-energy model is later found to fit the integrated PL intensity vs. temperature curve better than a single-activation-energy model. The two states that correspond to the above activation energies could have resulted in a much faster PL intensity quenching in the GaInNAs QW as compared to that of a reference GaInAs QW. One of the states is identified as a localized state that traps carriers at a low temperature range of less than ∼100 K. The other state has a larger quenching effect at temperatures higher than 100 K and this state is not studied in this paper. By fitting the original PL spectra with two Gaussian functions, the temperature dependent PL integrated intensity of both Gaussian functions was also studied to further characterize the GaInNAs QW. The analysis gives evidence of the localization behaviour in this QW.

Structural and electrooptical characteristics of quantum dots emitting at 1.3 μm on gallium arsenide

We present a comprehensive study of the structural and emission properties of self-assembled InAs quantum dots emitting at 1.3 /spl mu/m. The dots are grown by molecular beam epitaxy on gallium arsenide substrates. Room-temperature emission at 1.3 /spl mu/m is obtained by embedding the dots in an InGaAs layer. Depending on the growth structure, dot densities of 1-6/spl times/10/sup 10/ cm/sup -2/ are obtained. High dot densities are associated with large inhomogeneous broadenings, while narrow photoluminescence (PL) linewidths are obtained in low-density samples. From time-resolved PL experiments, a long carrier lifetime of /spl ap/1.8 ns is measured at room temperature, which confirms the excellent structural quality. A fast PL rise (/spl tau//sub rise/=10/spl plusmn/2 ps) is observed at all temperatures, indicating the potential for high-speed modulation. High-efficiency light-emitting diodes (LEDs) based on these dots are demonstrated, with external quantum efficiency of 1% at room temperature. This corresponds to an estimated 13% radiative efficiency. Electroluminescence spectra under high injection allow us to determine the transition energies of excited states in the dots and bidimensional states in the adjacent InGaAs quantum well.

Optical properties of Si +-ion implanted sol–gel derived SiO 2 films

Optical properties of commonly investigated Si +-implanted SiO 2 layers (thin SiO 2 films prepared by thermal growth on c-Si substrate) are compared with those of non-conventional SiO 2 layers fabricated by sol–gel process. The sol–gel films, deposited on different substrates, were implanted and annealed in a similar way as the thermal SiO 2 films on c-Si. Striking differences between these two types of samples were found. The conventional Si +-implanted SiO 2/c-Si layers contain Si nanocrystals and their photoluminescence (PL) properties were found very similar to porous silicon (PL in the red/NIR spectral regions, PL decay time of the order of 10 μs, PL temperature dependence well described by the exciton singlet–triplet splitting model). On the other hand, the Si +-implanted sol–gel SiO 2 films exhibit room temperature PL spectra peaked in the blue–green region, PL decay is considerably faster (of the order of 1 ns) and also PL temperature dependence differs substantially from the samples of the first type. Possible influence of different substrates (silica, c-Si) is also investigated and it is shown that the observed PL is an inherent property of the implanted sol–gel SiO 2 layers. The slow red PL, in agreement with other authors, is ascribed to radiative recombination of excitons in Si nanocrystals, while the fast blue PL characteristic of ion-implanted sol–gel derived SiO 2 films is obviously of defect origin.

Time-resolved Photoluminescence and Carrier Dynamics in Vertically-coupled Self-assembled Quantum Dots

The relaxation mechanisms of an array of 10 vertically coupled layers of InGaAs/AlGaAs quantum dots were studied by time-resolved photoluminescence. Both resonant and non-resonant excitation were employed and the photoluminescence (PL) intensity in the non-resonant case is a factor of 200 larger than the intensity with resonant excitation. The results obtained in the non-resonant pumping experiment were analysed with a rate equation model. It was found that the PL decay time increases rapidly with the wavelength of detection. Large carrier capture cross-sections [(2.5±0.9)×10-5 cm3/s] were deduced, resulting in a capture time of 1 ps for a carrier concentration of 4×1016 cm-3. A very fast PL risetime was observed with resonant pumping, ruling out a phonon bottleneck effect in these samples. The decay times at a given wavelength are always shorter for resonant than for non-resonant excitation and their difference increases rapidly with wavelength. This is interpreted in terms of a state filling effect for the non-resonant case.

The origin of the fast blue photoluminescence from spark processed silicon

We show that the fast blue (maximum at about 3.0–3.2 eV) photoluminescence from spark processed silicon originates from tungsten doping when the sample is processed with a tungsten counter-electrode. A similar effect is found also for molybdenum and chromium. If a silicon tip is used as counter-electrode, the photoluminescence is about a factor of 30 weaker and red shifted with a maximum at about 2.2–2.3 eV. Because of the limited sensitivity of the RBS technique used, transition metal impurities, such as Zn, are suspected but cannot be definitively proven as the radiative centers in this case.

Fast photoluminescence decay and optical nonlinearity of a [formula omitted] multiple quantum well around an excitonic peak

Fast photoluminescence decay in a ZnCdSe ZnSe multiple quantum well (MQW) has been measured at room temperature (RT) and 77 K. The decay times are typically 70 ps at RT and 160 ps at 77 K, which are much shorter than those of III-V semiconductors. The decay time is changed by the doping of the MQW layers, which could be related to the increase in traps. Strong laser-light irradiation also results in shorter decay times, indicating degradation induced by the excitation. Absorptive and non-absorptive nonlinear optical properties of the II-VI MQW have also been measured to compare them with those of III-V semiconductors.

Possible Mechanism of the 30–100 Picoseconds Fast and Efficient Photoluminescence From nc-Si/a-SiO2Doped with Transition Metals

AbstractThe nc-Si/a-SiO2composite thin films doped with tungsten show very fast and efficient photoluminescence (PL). In order to obtain insight into the PL mechanism we have performed a comparative study with other metals. The results lend support to the suggested mechanism which includes the photogeneration of charge carriers due to efficient absorption of the excitation UV light in the silicon nanocrystals followed by energy transfer to the Wn+radiative center from which the light emission occurs.

Photoluminescence Study of C70 Film Manufactured by Physical Jet Deposition

We have fabricated C70 film by physical jet deposition (PJD) technique and measured the absorption and photoluminescence (PL) spectra of this kind C70 film. In comparison with the deposited C70 film in high vacuum, the absorption and integrated PL spectra are similar, but there is a big difference in the time-resolved PL. The C70 film made by PJD shows a very fast PL decay of less than 20 ps which should result from the fast relaxation process of C70 molecule in the excited state S1 of this new kind film.

Picosecond time-resolved photoluminescence characterization of a-SiC:H films prepared by electron cyclotron resonance plasma

Microwave excited electron cyclotron resonance (ECR) plasmas were used to produce five a-SiC:H films of varying relative carbon content. This was achieved by adjusting the relative hydrogen gas/organic liquid source flow rates and resulted in optical gaps between 2.3eV and 3.3eV. A time-correlated single photon counting technique incorporating a picosecond laser was used to acquire the fast photoluminescence decay emitted at several wavelengths for each sample. A lifetime distribution analysis of the photoluminescence decay reveals the presence of up to four distinct components. These resolved decay times range from 80ps to over 20ns. Each component decay has its own emission spectrum. The blue component has the fastest and the red the slowest decay. We present the variation of photoluminescence lifetime distributions with deposition parameters. We also present continuous wave (cw) and absorption photoluminescence spectra.

Photoluminescence and x-ray photoelectron spectroscopy measurements of InP surface treated by acid and base solutions

A very strong effect up to three orders of magnitude of chemical treatment on photoluminescence (PL) intensity from InP surface is demonstrated by in situ and ex situ measurements. It is found that InP treatment by acid solutions gives generally high PL intensity, in contrast to the base solutions which give low PL amplitude. Very detrimental effect of H2O2 in H2O and/or in presence of bases is demonstrated. Similar results are obtained in situ, in liquid ambiance and ex situ, immediately after treatment. A reduction of the PL signal is always observed after low-temperature (150 °C) annealing in vacuum (10−6 Torr). Although HF, HNO3, and H2SO4 give similar PL intensity immediately after treatment, a great difference between these treatments becomes apparent after annealing. The amount and nature of oxide layers left by various treatments on In P surface were evaluated by x-ray photoelectron spectroscopy. The average oxide thickness and composition do not seem to be directly correlated with fast surface state density and PL intensity. Our results suggest that InP surface treatment by oxidizing acid solution may be the most promising for device application.