Time-resolved Photoluminescence Studies Research Articles

X-ray studies and time-resolved photoluminescence on optically pumped antimonide-based midinfrared type-II lasers

We report on high-resolution X-ray diffraction and time-resolved photoluminescence (TR-PL) studies of antimonide-based midinfrared (MIR) type-II laser samples. A structural characterization taking into account asymmetrical strain, layer tilting, and relaxation enables an accurate determination of the average lattice constant of the active region and the composition of the cladding layers. By designing the antimonide-to-arsenide interfaces, we achieve exact lattice matching of the active region to the substrate. Non-radiative recombination processes are investigated with time-resolved photoluminescence. The samples are also characterized under optically pumped laser operation. By an examination of the time-integrated and time-resolved amplified spontaneous emission (TR-ASE), we investigate the modal gain and gain dynamics. The variable stripe length method is combined with the TR-PL approach. Compared to the time-integrated gain spectra the spectral dependence of the maximum and minimum time-resolved gain shows a broad plateau. The full width half maximum (FWHM) of the TR-ASE pulse is 5.5±0.5 ps. Thus, short pulses in this range should be achievable upon laser operation. The active regions of the laser structures investigated here are promising subunits of type-II quantum cascade lasers.

Influence of internal fields on radiative and nonradiative processes in AlN/GaN superlattices

We have investigated the effect of the internal electric fields on radiative and nonradiative processes in a series of AlN/GaN superlattices. The results of measurements involving direct detection of phonons emitted as a result of nonradiative recombination and carrier energy relaxation are compared with time resolved photoluminescence studies. Using these complementary techniques we have observed directly the effect of free carrier screening on the radiative and nonradiative recombination processes in the superlattice samples. We find that at high excitation power, photoinjected carriers screen the strong internal fields, resulting in an enhanced radiative recombination. As the carriers recombine, the descreening effect results in an increase in recombination via nonradiative processes, which we observe directly as a delayed phonon signal in the time of flight measurements.

Time resolved photoluminescence study of Si modulation doped GaN/Al0.07Ga0.93N multiple quantum wells

AbstractThe effects of the Si doping level on the recombination dynamics and carrier (exciton) localization in modulation doped GaN/Al0.07Ga0.93N multiple‐quantum‐well (MQW) structures were studied by means of photoluminescence (PL) and time‐resolved PL measurements. All samples with different doping levels show a QW emission which is blue shifted with respect to the 3.48 eV PL peak from the GaN buffer layer. The decay time at the peak position remains nearly constant in the range of 320–420 ps at 2 K for all doping levels. For the undoped and low‐doped samples (3 × 1018 cm−3), which have less free electrons in the QWs, a non‐exponential PL decay behaviour at 2 K is attributed to localized exciton recombination. The more highly doped samples (5 × 1018 cm−3 to 1020 cm−3) show almost exponential decay curves at 2 K, suggesting the recombination of free electrons and localized holes. This localization effect appears even at high electron concentrations to cancel the expected lowering of the radiative lifetime with doping at 2 K; such a lowering is clearly observed at elevated temperatures for the highly doped samples, however. The internal polarization‐induced fields of the medium and highly‐doped samples are partly screened by the electrons originating from the doping in the barriers. Only the PL peak of the undoped and low‐doped samples shows a redshift with time delay, related to the photogenerated carriers. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Tunnelling of spin-polarized holes in asymmetric double quantum well structures

We report on time-resolved photoluminescence studies of tunnelling in a system of asymmetric double quantum wells (QWs). We employ a structure with a semimagnetic QW to align carrier spins and with excess concentration of electrons to provide a sensitive tool for studying the heavy hole dynamics. We prove the existence of hole tunnelling and demonstrate an efficient spin transfer in the process. A semi-empirical model describes qualitatively the experimental findings.

Time resolved photoluminescence studies of Zn–Se–Te nanostructures with sub‐monolayer quantities of Te grown by molecular beam epitaxy

AbstractWe investigate triple‐delta‐doped ZnSe:Te samples grown with different Te/Zn flux ratios using time‐resolved photoluminescence (TRPL). We show that the properties of the TRPL of both samples are consistent with the presence of quantum islands with a type‐II band alignmrnt. Moreover, from the comparison of the PL, we show that higher Te/Zn flux ratio during the growth leads to the formation of larger quantum islands. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Internal electric field effect on luminescence properties of ZnO/(Mg,Zn)O quantum wells

Spectroscopic studies have been conducted for wurtzite ZnO/Mg 0.27Zn 0.73O multiple quantum wells. Internal electric fields at interfaces are found to have an influence on the photoluminescence (PL) properties for well width ( L w) greater than 4.2 nm . These experimentally observed features are characteristic of the quantum-confined Stark effects; the magnitude of the electric field due to spontaneous and piezoelectric polarizations and the depth of the triangle-shaped potential wells are the monotonically increasing functions of Mg concentration and the L w, respectively. Results of the time-resolved PL study are also presented.

Transient linear dichroism in InAs/GaAs self‐assembled quantum dots

We have shown that a transient linear dichroism can be photo-induced in self-assembled InAs/GaAs QDs along the [110] and [] crystallographic axes. The transient signal shows characteristic times of 42 ps and 1.1 ns at 1.35 eV. We have also performed time-resolved photoluminescence studies which lead to a lifetime between 700 ps and 1.4 ns, depending on the emission energy. By comparison of these results, we conclude that the spin relaxation time is seven times longer than the lifetime. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Photophysics of Fac-Tris(2-Phenylpyridine) Iridium(III) Cored Electroluminescent Dendrimers in Solution and Films

We present time-resolved photoluminescence (PL) studies of novel first- and second-generation electrophosphorescent fac-tris(2-phenylpyridine) iridium(III) [Ir(ppy)3] cored dendrimers and compare them with neat films of molecular Ir(ppy)3. A PL quantum yield of ∼0.8 is observed in blends of the dendrimers with 4,4‘-bis(N-carbazolyl)biphenyl (CBP) at room temperature, and the natural radiative lifetime of the emissive state (1.5 μs) is observed to be the same for dendrimers and molecular Ir(ppy)3. Quenching of the PL occurs in neat films, because of an energy transfer to less-emissive sites, which have ∼10 times lower oscillator strength. The PL quenching rate in spin-coated films of the first- and second-generation dendrimers is slower by a factor of 11 and 20, respectively, as compared to neat Ir(ppy)3 films prepared by evaporation. Dendrimer films showed a much smoother surface than Ir(ppy)3 films, which is consistent with more extensive aggregation of molecular Ir(ppy)3 than dendrimers.

Time-Resolved Photoluminescence Study on Energy Transfer from Alq3 (tris(8-hydroxyquinoline)aluminum) to Red-Emissive Tetraphenylchlorin

Energy transfer and concentration quenching for a red-emitting electroluminescence system, tetraphenylchlorin (TPC)-doped tris(8-hydroxyquinoline)aluminum (Alq3) at various concentrations, were studied using a time-resolved photoluminescence (PL) technique. The PL decay of Alq3 in the presence of TPC was observed to be non-exponential in nature due to the Förster-type energy transfer from Alq3 to TPC. The PL decay of TPC became faster with the increase in TPC concentration due to the concentration quenching of TPC emission.

Photoluminescence studies of Si-doped AlN epilayers

Si-doped AlN epilayers were grown by metalorganic chemical vapor deposition on sapphire substrates. Deep ultraviolet picosecond time-resolved photoluminescence (PL) spectroscopy has been employed to study the optical transitions in the grown epilayers. The donor bound exciton (or I2) transition was found to be the dominant recombination line in Si-doped AlN epilayers at 10 K and its emission intensity decreases with increasing Si dopant concentration. Doping-induced PL emission linewidth broadening and band-gap renormalization effects have also been observed. Time-resolved PL studies revealed a linear decrease of PL decay lifetime with increasing Si dopant concentration, which was believed to be a direct consequence of the doping-enhanced nonradiative recombination rates.

Time-resolved photoluminescence studies of CdTe solar cells

We show that time-resolved photoluminescence measurements of completed polycrystalline CdTe solar cells provide a measure of recombination near the CdTe/CdS metallurgical interface that is strongly correlated to the open-circuit voltage in spite of complex carrier dynamics in the junction region. Oxygen in the growth ambient during close-spaced sublimation generally reduces this recombination rate; grain size does not have a strong effect.

Exciton kinetics and luminescence in disordered In x Ga 1− x P/GaAs quantum wells

Continuous wave and time-resolved photoluminescence studies as a function of temperature have been performed on disordered InxGa1−xP/GaAs quantum wells. Simulations of stationary and transient spectra, by using a two-exciton kinetic model, allow the observation of a red shift of the effective mobility edge when temperature increases. From the temperature dependence of the radiative lifetime it can be also deduced that independent localization of electrons and holes seems to be the most likely mechanism for exciton localization in our samples. On the other hand, values ranging from 0.8 to 5.6 ps have been obtained for the radiative lifetime at k‖ = 0.

Time-Resolved Photoluminescence Studies of AlInGaN Alloys

We study the two samples of AlInGaN, i.e., 1-µm GaN grown at1030°C on the buffer and followed by a 0.6-µm-thick epilayerof AlInGaN under the low pressure of 76 Torr and the AlInGaN layerdeposited directly on the buffer layer without the high-temperature GaNlayer, by temperature-dependent photoluminescence (PL) spectroscopy andpicosecond time-resolved photoluminescence (TRPL) spectroscopy. TheTRPL signals of both the samples were fitted well as a stretchedexponential decay at all temperatures, indicating significantdisorder in the material. We attribute the disorder to nanoscalequantum dots or discs of high indium concentration. Temperaturedependence of dispersive exponent β shows that the stretchedexponential decay of the two samples comes from different mechanisms.The different depths of the localization potential account for thedifference, which is illustrated by the results of temperaturedependence of radiative recombination lifetime and PL peak energy.

Time-resolved photoluminescence study of a ZnO thin film grown on a (1 0 0) silicon substrate

By employing picosecond time-resolved photoluminescence, we investigated the temporal behavior of the near band-gap exciton emission located at ∼3.264 eV of a ZnO epilayer deposited onto a (1 0 0) silicon substrate by plasma-assisted MOCVD. The emission exhibits a biexponential decay behavior composed of an initial fast component (30–50 ps) followed by a second slower component (100–400 ps). The extracted time constant for the capture of free-excitons at the band-tail states is in the order of 30 ps. The results strongly suggest that the decay of the exciton population is governed by the initial fast decay due to the capture of excitons and trapping of carriers by deep centers at defects and/or impurities, and the measured slower decay component is due to the radiative recombination of free- or localized-excitons.

Spectral and time-resolved photoluminescence studies of Eu-doped GaN

We report on spectral and time-resolved photoluminescence (PL) studies performed on Eu-doped GaN prepared by solid-source molecular-beam epitaxy. Using above-gap excitation, the integrated PL intensity of the main Eu3+ line at 622.3 nm (5D0→7F2 transition) decreased by nearly 90% between 14 K and room temperature. Using below-gap excitation, the integrated intensity of this line decreased by only ∼50% for the same temperature range. In addition, the Eu3+ PL spectrum and decay dynamics changed significantly compared to above-gap excitation. These results suggest the existence of different Eu3+ centers with distinct optical properties. Photoluminescence excitation measurements revealed resonant intra-4f absorption lines of Eu3+ ions, as well as a broad excitation band centered at ∼400 nm. This broad excitation band overlaps higher lying intra-4f Eu3+ energy levels, providing an efficient pathway for carrier-mediated excitation of Eu3+ ions in GaN.

Deep ultraviolet picosecond time-resolved photoluminescence studies of AlN epilayers

AlN epilayers with high optical qualities have been obtained by metalorganic chemical vapor deposition on sapphire substrates. Deep UV picosecond time-resolved photoluminescence (PL) spectroscopy has been employed to study the optical transitions in AlN epilayers. Two PL emission lines associated with the donor bound exciton (D0X, or I2) and free exciton (FX) transitions have been observed, from which the binding energy of the donor bound excitons in AlN epilayers was determined to be around 16 meV. Time-resolved PL measurements revealed that the recombination lifetimes of the I2 and free exciton transitions in AlN epilayers were around 80 and 50 ps, respectively. The temperature dependencies of the free exciton radiative decay lifetime and emission intensity were investigated, from which a value of about 80 meV for the free exciton binding energy in AlN epilayer was deduced. This value is believed to be the largest free exciton binding energy ever reported in semiconductors, implying excitons in AlN are an extremely robust system that would survive well above room temperature. This together with other well-known physical properties of AlN may considerably expand future prospects for the application of III-nitride materials.

Time-resolved photoluminescence study of excitons in thin PTCDA films at various temperatures

In the present work excitons in thin 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) films are studied by means of time-resolved photoluminescence (TRPL) in the temperature range from 10 to 300K. The obtained temperature dependence of different recombination channels, arising from an indirect minimum of the Frenkel exciton dispersion, different types of self-trapped excitons and relaxed excited monomer states, are compared to the corresponding PL recombination channels for single crystals. The Frenkel excitons (FEs), the slow PL channel, the excimer decay times, and the PL intensities reveal a similar temperature dependence like in the single crystals, while the contribution of the charge-transfer exciton (CTE) is twice as large in films.

Time-dependence of erbium(III) tris(8-hydroxyquinolate) near-infrared photoluminescence: implications for organic light-emitting diode efficiency

We report the first time-resolved photoluminescence studies of erbium tris(8-hydroxyquinolate), [ErQ 3], as a powder, a thin film blend with polycarbonate, an evaporated film and in DMSO- d 6 solution. The 4 I 13/2→ 4 I 15/2 transition at 1.5 μm displays biexponential kinetics with photoluminescence lifetimes in the region of 0.2 μs for the powder, 0.4 μs for the thin film blend, 0.9 μs for the evaporated film, and 2 μs in solution. These observed lifetimes are much shorter than the natural radiative lifetimes of this Er 3+ transition, implying photoluminescence quantum yields that range from 0.002 to 0.03%. This has significant consequences for the device efficiencies achievable by near IR-emitting organic light-emitting diodes (OLEDs) containing [ErQ 3] as the emissive layer.

Quantum Dot-Organic Oligomer Nanostructures: Electronic Excitation Migration and Optical Memory Design

AbstractEnergy, in the form of an electronic excitation, can be directed within an inorganicorganic composite of semiconductor quantum dots and organic oligomers by manipulating the structural conformations of the organic component or the size of the inorganic component. Continuous-wave and time-resolved photoluminescence studies indicate that weak electromagnetic resonant coupling between discrete intra-chain and inter-chain excitations of the oligomer and quantum dot excitations can be used to produce a potentially useful optical display material. Thin film blends demonstrate a thermally-induced luminescence-detected chainmelting phenomenon that has the potential for writable optical memory.

Time Resolved Photoluminescence of Si-doped High Al Mole Fraction AlGaN Epilayers Grown by Plasma-Enhanced Molecular Beam Epitaxy

ABSTRACTWe report on detailed temperature dependent, time-resolved photoluminescence (TRPL) studies of Si-doped AlGaN epilayers. In these samples, the Al concentration varies from 25% to 66%. The samples were found to exhibit metallic-like temperature-independent conductivity. The deep level “yellow” emission, whose presence would indicate the existence of a large number of defects associated with growth, Si incorporation, and/or alloy formation, is absent. In addition to emission corresponding to the donor-bound exciton, the PL spectrum exhibits features associated with transitions involving localized carriers. This assignment of the emission mechanisms is based on the activation energies extracted from the temperature dependent photoluminescence (PL) quenching. Specifically, at room temperature the PL spectrum is dominated by transitions involving localized states. The localization energy varied from sample to sample and was observed to be between 115 meV to 200 meV. The PL intensity decay in the lower Al mole fraction epilayers exhibits a slow component associated with the presence of donor-bound excitons.