Karolis Kazlauskas

Double-Scaled Potential Profile in a Group-III Nitride Alloy Revealed by Monte Carlo Simulation of Exciton Hopping

The temperature dependences of the peak position and width of the photoluminescence band in Al0.1In0.01Ga0.89N layers were explained by Monte Carlo simulation of exciton localization and hopping. The introduction of a doubled-scaled potential profile due to inhomogeneous distribution of indium allowed obtaining a good quantitative fit of the experimental data. Hopping of excitons was assumed to occur through localized states distributed on a 16 meV energy scale within the In-rich clusters with the average energy in these clusters dispersed on a larger (42 meV) scale.

Optical Bandgap Formation in AlInGaN Alloys

We report on the spectral dynamics of the reflectivity, site-selectively excited photoluminescence, photoluminescence excitation, and time-resolved luminescence in quaternary AlInGaN epitaxial layers grown on GaN templates. The incorporation of a few percents of In into AlGaN causes significant smoothening of the band-bottom potential profile in AlInGaN layers owing to improved crystal quality. An abrupt optical bandgap indicates that a nearly lattice-matched AlInGaN/GaN heterostructure with large energy band offsets can be grown for high-efficiency light-emitting devices.

Impact of intramolecular twisting and exciton migration on emission efficiency of multifunctional fluorene-benzothiadiazole-carbazole compounds

Novel donor-acceptor compounds consisting of singly bonded fluorene (Fl), benzothiadiazole (BT), and carbazole (Cz) functional units in the same molecule were investigated. Analysis of the optical spectra and fluorescence transients of the compounds revealed the domination of intramolecular charge transfer (ICT) states with high fluorescence quantum yield (72%-85%). A similar Cz-Fl-Cz compound exhibiting 100% fluorescence quantum yield and no ICT character was also studied as a reference to reveal the impact of electron-accepting BT groups. Thorough examination of the optical properties of the compounds in different media, i.e., dilute solution and polymer matrix, indicated their twisted conformations due to steric hindrance in the ground state and flattened geometry in the excited state for both reference and ICT compounds. Remarkable fluorescence efficiency losses (amounting to 70%) observed upon casting the molecular solutions into neat films were determined to originate from the low-fluorescent twisted conformers and migration-facilitated exciton quenching. The majority of emission efficiency losses (over 70%) were caused by the twisted conformers, whereas only less than 30% by exciton-migration-induced nonradiative deactivation.

Exciton and carrier motion in quaternary AlInGaN

Temperature and excitation power dependences of the photoluminescence Stokes shift and bandwidth were studied in quaternary AlInGaN epilayers as a function of indium content. At low excitation power, gradual incorporation of indium into AlGaN is shown to result in S- and W-shaped temperature dependences of the band peak position and bandwidth, respectively. At high excitation power, the S- and W-behavior disappears; however, increased indium molar fraction boosts the redshift of the luminescence band at high temperatures. Our results indicate that the incorporation of indium into AlGaN has a noticeable impact on the alloy transport properties. At low temperatures and low excitation power, the indium incorporation facilitates hopping of localized excitons, whereas at high temperatures and high excitation power, it sustains free motion of delocalized carriers that results in the band-gap renormalization via screening.

Exciton hopping and nonradiative decay in AlGaN epilayers

Monte Carlo simulation of phonon-assisted localized exciton hopping has been employed to describe the photoluminescence linewidth variation with temperature and to reveal band potential profile of ternary AlGaN epilayers with different carrier lifetimes. The lifetimes of 30 and 190 ps were experimentally determined in the layers with AlN buffers grown by conventional metal-organic chemical vapor deposition (MOCVD) and by migration-enhanced MOCVD (MEMOCVD™), respectively. The potential profile in AlGaN is shown to consist of double-scaled fluctuations. Exciton hopping in Al0.26Ga0.74N occurs within the random potential fluctuations (on the scale σ≈19meV) in isolated low-potential regions with the average localization energy dispersed on the scale Γ≈19meV. Such a pattern of band potential profile was found to be independent on the growth technique used for the deposition of their AlN buffer layers. This implies that the large difference in carrier lifetimes estimated in the AlGaN epilayers with the same Al ...

The Role of Triplet Exciton Diffusion in Light-Upconverting Polymer Glasses

Light upconversion (UC) via triplet-triplet annihilation (TTA) by using noncoherent photoexcitation at subsolar irradiance power densities is extremely attractive, particularly for enhanced solar energy harvesting. Unfortunately, practical TTA-UC application is hampered by low UC efficiency of upconverting polymer glasses, which is commonly attributed to poor exciton diffusion of the triplet excitons across emitter molecules. The present study addresses this issue by systematically evaluating triplet exciton diffusion coefficients and diffusion lengths (LD) in a UC model system based on platinum-octaethylporphyrin-sensitized poly(methyl methacrylate)/diphenylanthracene (emitter) films as a function of emitter concentration (15-40 wt %). For this evaluation time-resolved photoluminescence bulk-quenching technique followed by Stern-Volmer-type quenching analysis of experimental data was employed. The key finding is that although increasing emitter concentration in the disordered PMMA/DPA/PtOEP films improves triplet exciton diffusion, and thus LD, this does not result in enhanced UC quantum yield. Conversely, improved LD accompanied by the accelerated decay of UC intensity on millisecond time scale degrades TTA-UC performance at high emitter loadings (>25 wt %) and suggests that diffusion-enhanced nonradiative decay of triplet excitons is the major limiting factor.

Stimulated emission in AlGaN/AlGaN quantum wells with different Al content

Stimulated emission (SE) is studied in AlGaN/AlGaN multiple quantum wells (MQWs) with different Al content grown on sapphire substrate. The spectra of spontaneous and stimulated emission and their transformations with increasing temperature as well as stimulated emission thresholds were measured in the temperature range from 8 to 300 K. Phonon-assisted band broadening in low-Al-content MQWs and double-scaled potential profile in high-Al-content MQWs were observed in the samples and linked with carrier localization conditions. The temperature dependence of the stimulated emission threshold was similar in the samples where the stimulated transitions occur between extended states and in the samples where the transitions occur in localized states. The stimulated emission threshold depends predominantly on the density of nonradiative recombination centers.

Radiation-defect-dependent photoconductivity transients and photoluminescence in semi-insulating GaN

Effect of radiation defects on the photoconductivity transients and photoluminescence (PL) spectra have been examined in semi-insulating GaN epitaxial layers grown on bulk n-GaN∕sapphire substrates. Manifestation of defects induced by 10keV x-ray irradiation with the dose of 600Mrad and 100keV neutrons with the fluence of 5×1014cm−2 have been revealed through steady-state and pulsed PL as well as through contact photoconductivity and microwave absorption transients. Synchronous decrease of the PL intensity of yellow, blue, and ultraviolet bands peaked at 2.19, 2.85, and 3.42eV, respectively, with density of radiation-induced defects is observed. The decrease of the PL intensity is accompanied by an increase of asymptotic decay lifetime, which is due to excess-carrier multi-trapping. The decay fits the stretched exponent approximation exp[−(t∕τ)α] with the different factors α in as-grown material (α≈0.7) and irradiated samples (α≈0.3).

Stimulated emission due to localized and delocalized carriers in Al0.35Ga0.65N/Al0.49Ga0.51N quantum wells

The effect of carrier localization on stimulated emission (SE) in Al0.35Ga0.65N/Al0.49Ga0.51N quantum wells (QWs) on sapphire substrate was studied under photoexcitation in the edge emission configuration in the temperature range from 8 K to 300 K. The band potential profile responsible for carrier localization was modulated by the variation of QW width and monitored using fitting the experimental temperature dependence of the spontaneous luminescence band width to that obtained by the Monte Carlo simulation of exciton hopping. A faster increase of SE threshold with increasing temperature was observed in narrow QWs and was attributed to deeper carrier localization due to the modulation of quantum confinement energy by well width fluctuations. Meanwhile, delocalized carriers were shown to contribute to the filling of states at the mobility edge, where SE occurs. These results imply that the deep ultraviolet AlGaN/AlGaN laser structures can be optimized in terms of carrier localization effect through the selection of appropriate QW width.The effect of carrier localization on stimulated emission (SE) in Al0.35Ga0.65N/Al0.49Ga0.51N quantum wells (QWs) on sapphire substrate was studied under photoexcitation in the edge emission configuration in the temperature range from 8 K to 300 K. The band potential profile responsible for carrier localization was modulated by the variation of QW width and monitored using fitting the experimental temperature dependence of the spontaneous luminescence band width to that obtained by the Monte Carlo simulation of exciton hopping. A faster increase of SE threshold with increasing temperature was observed in narrow QWs and was attributed to deeper carrier localization due to the modulation of quantum confinement energy by well width fluctuations. Meanwhile, delocalized carriers were shown to contribute to the filling of states at the mobility edge, where SE occurs. These results imply that the deep ultraviolet AlGaN/AlGaN laser structures can be optimized in terms of carrier localization effect through the se...

Fluorene- and benzofluorene-cored oligomers as low threshold and high gain amplifying media

Deliberate control of intermolecular interactions in fluorene- and benzofluorene-cored oligomers was attempted via introduction of different-length alkyl moieties to attain high emission amplification and low amplified spontaneous emission (ASE) threshold at high oligomer concentrations. Containing fluorenyl peripheral groups decorated with different-length alkyl moieties, the oligomers were found to express weak concentration quenching of emission, yet excellent carrier drift mobilities (close to 10−2 cm2/V/s) in the amorphous films. Owing to the larger radiative decay rates (>1.0 × 109 s−1) and smaller concentration quenching, fluorene-cored oligomers exhibited down to one order of magnitude lower ASE thresholds at higher concentrations as compared to those of benzofluorene counterparts. The lowest threshold (300 W/cm2) obtained for the fluorene-cored oligomers at the concentration of 50 wt % in polymer matrix is among the lowest reported for solution-processed amorphous films in ambient conditions, wha...