Gintautas Tamulaitis - Academia.edu (original) (raw)

Papers by Gintautas Tamulaitis

Journal of Physics D: Applied Physics, 2016

Dynamics of radiative and nonradiative recombination of non-equilibrium carriers is investigated ... more Dynamics of radiative and nonradiative recombination of non-equilibrium carriers is investigated in thick AlGaN epitaxial layers with Al content ranging from 0.11 to 0.71. The internal quantum efficiency (IQE) in the epilayers was obtained using two approaches: either estimated from PL measurements or calculated using the recombination coefficients of a simple ABC model, retrieved by fitting the kinetics of light induced transient gratings (LITG). At photoexcited carrier densities below ~1019 cm−3, both approaches provided similar IQE values indicating that the simple ABC model is applicable to analyze carrier recombination at such carrier densities. The increase in IQE at higher carrier densities slowed down for the values extracted from PL considerably faster than for those obtained from LITG transients. This discrepancy is explained in terms of the mixed nature of the rate coefficient B caused by the onset of the density-activated nonradiative recombination at high carrier densities.

Journal of Electronic Materials, 2015

The interplay between nonradiative recombination and carrier localization in AlxGa1−xN epilayers ... more The interplay between nonradiative recombination and carrier localization in AlxGa1−xN epilayers (0.11 < x < 0.78) was studied using the photoluminescence (PL) and light-induced transient grating (LITG) techniques. The carrier localization conditions were estimated from the temperature dependence of the PL band peak position. The room-temperature carrier lifetimes estimated by LITG were limited by the competition between the carrier localization and nonradiative recombination. Strong carrier localization was shown to be insufficient to achieve high internal quantum efficiency in AlGaN epilayers.

Applied Physics Letters, 2008

... To make the study more thorough, we examine a set of QW structures with different QW ... and ... more ... To make the study more thorough, we examine a set of QW structures with different QW ... and are the QW and barrier widths, and is the composition-dependent static dielectric constant. ... For thesurface, the Fermi level pinning at about below the conduction band has been reported ...

Y3� xMg2AlSi2O12:Ce 3þ x (YMASG:Ce) phosphors were synthesized by sol–gel combustion technique at... more Y3� xMg2AlSi2O12:Ce 3þ x (YMASG:Ce) phosphors were synthesized by sol–gel combustion technique at different temperatures from 1400 to 1550 C. Samples with x = 0.015, 0.03, 0.045, and 0.06 were fabricated and characterized using powder X-ray diffraction (XRD), photoluminescence spectroscopy, and fluorescence lifetime measurements in frequency domain. XRD patterns confirmed single-phase garnet crystal structure for all the samples independently of their substitutional level and annealing temperature. In respect to Y3Al5O12:Ce 3+ (YAG:Ce) phosphor, which was synthesized for comparison by a different sol– gel procedure, the photoluminescence band of these garnets is red shifted, indicating a prospective for application of this novel phosphor in warm-white light emitting diodes (LEDs). The luminescence decays bi-exponentially. The main component has a characteristic decay time decreasing from 72 to of 50 ns with increasing sintering temperature and cerium content, while � 2% of the excit...

Optical Materials, 2015

We investigated the influence of crystallization conditions on the luminescent properties of Ce 3... more We investigated the influence of crystallization conditions on the luminescent properties of Ce 3+-ions doped stoichiometric glass BaO-2SiO 2 at the different stages of its transformation from amorphous phase to glass ceramics. The samples were investigated by confocal luminescence spectroscopy. It was found that the luminescent properties of the cerium-doped glass and the glassceramics composite consisting of BaSi 2 O 5 crystallites and residual glass are quite similar, though the luminescence band in the composites are slightly redshifted and narrower.

Journal of Physics D: Applied Physics, 2005

Based on perspectives of the development of semiconductor materials systems for high-power light-... more Based on perspectives of the development of semiconductor materials systems for high-power light-emitting diodes (LEDs), an illumination facility for greenhouse plant cultivation was designed with the dominating 640 nm photosynthetically active component delivered by AlGaInP LEDs and supplementary components from AlGaN (photothropic action, 455 nm) and AlGaAs (photosynthetic 660 nm and photomorphogenetic 735 nm) LEDs. Photosynthesis intensity, photosynthetic productivity and growth morphology as well as chlorophyll and phytohormone concentrations were investigated in radish and lettuce grown in phytotron chambers under the LED-based illuminators and under high-pressure sodium (HPS) lamps with an equivalent photon flux density. Advantages of the high-power LED-based illuminators over conventional HPS lamps, applicability of AlGaInP LEDs for photosynthesis and control of plant growth by circadian manipulation of a relatively weak far-red component were demonstrated.

Current Applied Physics, 2016

Nonradiative recombination rate and diffusivity of nonequilibrium carriers were modified by inten... more Nonradiative recombination rate and diffusivity of nonequilibrium carriers were modified by intense laser pulses in AlGaN epilayers with Al content ranging from 16 to 71%. The epilayers were examined before and after the photomodification using light-induced transient grating and photoluminescence spectroscopy techniques. The photomodification resulted in (i) enhancement of the nonradiative recombination rate and (ii) large changes of the diffusion coefficient of the nonequilibrium carriers, without imposing any macroscopic structural damage to the epilayers. The photomodification effect on the recombination rate was stronger in the layers with higher Al content indicating the involvement of the Al atoms in this process. The carrier diffusivity exhibited a rapid initial increase as a consequence of the photomodification followed by a slow decline, as the photomodification duration was increased. The enhancement of the diffusion coefficient of up to 2.4 times was accompanied by 13% decrease in the carrier lifetime.

AIP Conference Proceedings, 2005

AlGaN epilayers grown by conventional metal organic chemical vapor deposition (MOCVD) and by migr... more AlGaN epilayers grown by conventional metal organic chemical vapor deposition (MOCVD) and by migration enhanced MOCVD (MEMOCVD™) with different aluminum content have been comparatively studied by photoluminescence (PL) and light-induced transient grating (LITG) techniques. The carrier diffusion length is shown to be close to the average distance between two first-neighbor dislocations (determined using etch pit technique). The carrier lifetime and PL intensity in epilayers containing the same amount of Al are shown to be inversely proportional to the dislocation density. Increase in Al content at constant dislocation density results in decrease of the PL intensity due to decreasing rate of radiative recombination. Enhancement of PL intensity by introduction of defect-filtering AlN/AlGaN superlattices on sapphire substrates is demonstrated.

Applied Physics Letters, 1999

ABSTRACT

Physics of Fast Processes in Scintillators, 2020

This chapter describes the fastest processes during the propagation of electronic excitations thr... more This chapter describes the fastest processes during the propagation of electronic excitations through the media, Cherenkov radiation, intraband luminescence, cross-luminescence, elastic polarization, and their relaxation. The spectral features and time profiles of these types of emission are discussed.

Lithuanian Journal of Physics, 2008

Radiation Measurements, 2010

Influence of irradiation on the light yield of PWO (lead tungstate, PbWO 4) scintillation crystal... more Influence of irradiation on the light yield of PWO (lead tungstate, PbWO 4) scintillation crystals was studied in the temperature range from À25 C to þ60 C. Light output and optical transmittance were simultaneously measured as a function of time under irradiation in PWO single crystals grown in first and second recrystallization cycle of raw material, doped with different lanthanides and annealed in different conditions. Increased sensitivity to irradiation and slower recovery of the initial light yield were observed at decreased temperatures. The model of tungsten oxide clusters in a regular PWO lattice is used to interpret the experimental results, and dynamics of the light yield under irradiation at different temperatures are qualitatively explained using rate equations describing composition changes in the clusters.

Radiation Measurements, 2004

Technology for the mass production of high-quality PbWO4 (PWO) scintillating crystals is describe... more Technology for the mass production of high-quality PbWO4 (PWO) scintillating crystals is described. Scintillators produced from PWO crystals are intented for the ALICE CERN heavy ion experiment. Light yield, emission and decay time spectra as well as optical transmission of about 3600 crystals (dimensions 22 × 22 × 180 mm 3) were measured. Beam-test results of the ALICE PHOS prototype obtained with such PWO crystals are presented.

physica status solidi (a), 2005

AlGaN epilayers with different aluminum molar fractions have been comparatively studied by photol... more AlGaN epilayers with different aluminum molar fractions have been comparatively studied by photoluminescence (PL) and light-induced transient grating (LITG) techniques. The carrier diffusion length determined by LITG is shown to be close to the average distance between two first-neighbor dislocations, which was roughly estimated by using the dislocation density obtained by etch pit technique. For epilayers containing the same amount of Al, the carrier lifetime and PL intensity are inversely proportional to the density of the dislocations, which limit the carrier lifetime by serving as nonradiative recombination centers. For AlGaN epilayers with similar lifetimes but with different Al contents, the PL intensity decreases with increasing Al fraction, which is explained by the Al content dependence of the radiative recombination rate. It is demonstrated that introduction of AlN/AlGaN superlattices on sapphire substrates by using Migration Enhanced Metal Organic Chemical Vapor Deposition (MEMOCVDTM) enables growth of AlGaN with an increased nonequilibrium carrier lifetime and enhanced photoluminescence intensity.

The band gap of III-nitride semiconductors cover a wide range from 0.77 eV (band gap of InN) to 6... more The band gap of III-nitride semiconductors cover a wide range from 0.77 eV (band gap of InN) to 6.2 eV (AlN). Thus, light-emitting diodes emitting from infrared to deep into ultraviolet can be fabricated using ternary III-nitrides InGaN, AlGaN, and AlInN with appropriate composition. However, growing the compounds with any desirable composition often encounters substantial difficulties due to phase separation, structural quality of the epilayers, impurities and extended defects, etc. The spatial inhomogeneity of emission properties in III-nitride epilayers and quantum well structures provides an informative insight into carrier migration, localization, and recombination and is important for development of light-emitting devices. In this paper, we introduce the techniques for luminescence study with spatial resolution (microphotoluminescence, confocal microscopy, scanning near field optical microscopy and cathodoluminescence), discuss material properties leading to emission inhomogen...

A series of red-emitting phosphores containing CaWO 4 : RE (RE = Tb 3+ , Eu 3+ , Sm 3+ ) was prep... more A series of red-emitting phosphores containing CaWO 4 : RE (RE = Tb 3+ , Eu 3+ , Sm 3+ ) was prepared by an aqueous sol–gel method using tungsten (VI) oxide, WO 3 , calcium nitrate tetrahidrate, Ca(NO 3 ) 2 · 4H 2 O, terbium oxide, Tb 4 O 7 , europium oxide, Eu 2 O 3 , and samarium oxide, Sm 2 O 3 as starting materials. Synthesized ceramics were characterized using infrared spectroscopy (IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and photoluminescence (PL) measurements. The obtained results clearly show that only CaWO 4 : Eu phosphor can be effectively excited under photoexcitation at 280, 360 and 460 nm and exhibits red emission in the vicinity of 620 nm.

Communications in Physics, 2017

In this study, we investigate transient phenomena in synthetic diamonds obtained by High Pressu... more In this study, we investigate transient phenomena in synthetic diamonds obtained by High Pressure High Temperature and Chemical Vapor Deposition methods. Study was aimed at searching for inorganic crystalline media combining ionizing radiation detecting properties and non-linear absorption of ultra-short laser pulses. The nonlinear pump-and-probe optical absorption technique with of 140 fs laser pulses was used to study the effects.

We report on the application of flashing amber (596 nm) light-emitting diodes (LEDs), supplementa... more We report on the application of flashing amber (596 nm) light-emitting diodes (LEDs), supplemental to high pressure sodium lamps, for the cultivation of green sprouts, such as wheatgrass, barley grass, and leafy radish. The flashing light was found to significantly affect metabolism, thus conditioning the nutritional quality of the sprouts. In particular, it causes stressful conditions for the plants and within a short growth period can promote the synthesis of antioxidative compounds, such as vitamin C, phenolic compounds and carotenoids. However, the flashing amber light effect is dependent on the plant species.

physica status solidi (a), 1999

We discuss structural, optical and electrical properties of Al x In y Ga 1± ±x± ±y N/GaN heterost... more We discuss structural, optical and electrical properties of Al x In y Ga 1± ±x± ±y N/GaN heterostructures grown on sapphire and 6H-SiC substrates. The incorporation of In reduces the lattice mismatch in a much stronger way than the energy gap discontinuity. An In to Al ratio close to 1 : 5 should result in nearly strain-free heterostructures. The incorporation of In and resulting changes in the built-in strain in AlInGaN/GaN heterostructures strongly affect the transport properties of the two-dimensional (2D) electron gas at the heterointerface. Using the incorporation of In in order to increase the Al molar fraction preserving the same value of strain should allow us to greatly enhance the sheet carrier density at the AlGaInN/GaN heterointerface. Further work on improving In incorporation techniques is needed in order to meet these expectations.

Dilute bismide semiconductor compound GaBiAs is a promising material for ultrafast infrared light... more Dilute bismide semiconductor compound GaBiAs is a promising material for ultrafast infrared light emitters and detectors for optical fiber systems and terahertz applications. Introducing a small percentage of bismuth into gallium arsenide significantly reduces the band gap of the compound. However, growth of high quality GaBiAs is still a challenge. GaBiAs epitaxial layers with Bi content of up to 10% can be grown at low temperatures (~240-380°C). Post-growth rapid thermal annealing at high temperatures (~600-700°C) improves the crystalline structure of GaBiAs layers and, as a result, nonradiative recombination rate decreases [1]. In this work, the PL properties of GaBiAs epitaxial layers and GaBiAs/GaAs quantum wells were investigated with spatial resolution. The GaBiAs structures used in this study were grown by molecular beam epitaxy (MBE) technique on GaAs substrates at the Center for Physical Sciences and Technology, Vilnius, Lithuania. Four low-temperature grown samples were i...