D. Dobrovolskas

Confocal spectroscopy of InGaN LED structures

Photoluminescence of InGaN structures for green light-emitting diodes (LEDs) with multiple quantum wells as an active medium was studied with spatial and spectral resolution using confocal microscopy. Bright spots of ~200 nm diameter were observed. Emission from these bright areas was up to 8 times more intense than from the rest of the sample surface and the band peak position in these areas was blueshifted with respect to the band position in the background surface of lower photoluminescence intensity. The data on emission properties in bright and dark areas and the dependence of these properties on the excitation power density were interpreted by assuming inhomogeneous distribution of defects acting as nonradiative recombination centres.

Optical and structural properties of BGaN layers grown on different substrates

Growth of BGaN epitaxial layers by metalorganic chemical vapor deposition (MOCVD) using triethylboron (TEB) as a boron source was studied on 6H-SiC substrate and on GaN and AlN templates on sapphire. X-ray diffraction, atomic force microscopy and photoluminescence spectroscopy were exploited to characterize the structural quality, surface morphology, luminescence efficiency, and boron content. Silicon carbide was shown to be slightly superior to AlN/sapphire and considerably better than GaN/sapphire as the most favorable substrate to incorporate a possibly higher boron content. Increasing TEB flow rate at correspondingly optimized growth temperature and V/III ratio enabled us to achieve the boron content of up to 5.5%, though at the expense of structural quality. We showed that the band gap bowing parameter is similar for the epilayers deposited on all the three templates/substrates under study and is approximately equal to 4 eV, substantially lower than reported before.

Influence of quantum-confined Stark effect on optical properties within trench defects in InGaN quantum wells with different indium content

The trench defects in InGaN/GaN multiple quantum well structures are studied using confocal photoluminescence (PL) spectroscopy and atomic force microscopy. A strong blueshift (up to ∼280 meV) and an intensity increase (by up to a factor of 700) of the emission are demonstrated for regions enclosed by trench loops. The influence of the difference in the well width inside and outside the trench loops observed by transmission electron microscopy, the compositional pulling effect, the strain relaxation inside the loop, and corresponding reduction in the built-in field on the PL band peak position and intensity were estimated. The competition of these effects is mainly governed by the width of the quantum wells in the structure. It is shown that the PL band blueshift observed within the trench defect loops in the InGaN structures with wide quantum wells is mainly caused by the reduction in efficiency of the quantum-confined Stark effect due to strain relaxation.

A systematic study of light extraction efficiency enhancement depended on sapphire flipside surface patterning by femtosecond laser

We report on the investigation of light extraction efficiency enhancement at the flip-side surface of sapphire patterned by using femtosecond laser pulse irradiation. The enhancement was measured by means of fluorescence and confocal microscopies. Theoretical simulation (FDTD) gave the results. The variation of patterning geometry provided by different gap and pit size, by triangle and square array configuration revealed compactness of the pit network as an optimum condition for the enhancement. Maximum enhancement over 40% was obtained for the triangle array configuration. The appearance of ripples at the bottom of the pit corresponded to the enhancement. A deeper insight about consequences to radiative effects influenced by the ripples is presented.

Confocal Microscopy of Luminescence Inhomogeneity in LGSO:Ce Scintillator Crystal

Confocal microscopy was exploited to study spatial distribution of luminescence characteristics in Lu2SiO5:Ce (LSO:Ce) and mixed Lu2xGd2-2xSiO5:Ce (LGSO:Ce) crystals. Spatial distributions of spectrally-integrated photoluminescence (PL) intensity, PL band peak and spectral center of mass positions, and band width were determined. In both crystals, the luminescence spectra at 405 nm excitation are dominated by one broad band peaked at ~ 510 nm, while a long-wavelength shoulder is also observed in LGSO:Ce. Spatial inhomogeneities of the order of 1-3 micron in diameter are observed for the band position in LGSO:Ce. A comparative analysis of the spectra shows that the inhomogeneity is caused by spatial distribution in the spectral component at 500-700 nm, which may be attributed to Ce3+ located in CeO6 polyhedra (Ce2), or, more probably, to some defects in the crystal. Our results indicate that the changes in luminescence spectrum shape reflect Lu/Gd ratio fluctuations on a micrometer scale in the bulk of the mixed LGSO:Ce crystals.

Nonradiative and Radiative Recombination in CdS Polycrystalline Structures

Properties of polycrystalline CdS layers, employed in formation of the CdS-Cu2S heterostructures, have been studied by combining contactless techniques of the time and spectrally resolved photoluminescence (TR-PL) spectroscopy and microwave-probed photoconductivity (MW-PC) transients. The confocal microscopy has been employed to correlate the homogeneity of photoluminescence and grain size in CdS layers. Three types of samples with crystallite grain size of <1 μm (the I-type) and of 2–10 μm of homogeneous (II-type) and inhomogeneous (III-type) grain distribution have been separated. The simultaneous record of MW-PC and TR-PL responses ensures the same sampling area on the layer under investigation, as both (MW-PC and TR-PL) signals are generated by the same UV laser excitation beam. Two PL bands peaked at 500 and 700 nm were revealed. It has been demonstrated that photoluminescence intensity strongly depends on the properties of the polycrystalline 15–26 μm thick CdS layers with equilibrium carrier density of about  cm−3, which serve as the substrates to form CdS-Cu2S junctions. The different carrier decay components were ascribed to different microareas with characteristic MW-PC and PL decay lifetimes of 2–10 ns, ascribed to microcrystallites with PL instantaneous decay lifetimes of 40–200 ns, and MW-PC decay lifetimes in the range of 100–1000 μs attributed to the inter-crystallite areas of CdS polycrystalline material.

InGaN/GaN MQW Photoluminescence Enhancement by Localized Surface Plasmon Resonance on Isolated Ag Nanoparticles

Spectroscopic study of photoluminescence (PL) enhancement due to the coupling of the light emitters in InGaN/GaN multiple quantum wells (MQWs) with the localized surface plasmon (LSP) resonance on silver (Ag) nanoparticles (NPs) is performed using the confocal microscopy and scanning near-field optical microscopy (SNOM) techniques. The paper is focused on revealing the emission enhancement due to coupling with a single metal nanoparticle. The enhancement is confirmed by time-resolved study of differential transmission (DT). The enhancement suppression caused by potential fluctuations due to the variations of indium content and quantum well (QW) width is also studied. A strong photoexcitation intensity dependence of the emission enhancement due to spectral runaway of the MQW emission from the resonance as carrier density increases is observed both in spatially integrated spectra and in the vicinity of a single nanoparticle.

Spatially resolved study of InGaN photoluminescence enhancement by single Ag nanoparticles

Spatially resolved photoluminescence (PL) of an InGaN/GaN multiple quantum well (QW) structure covered by Ag nanoparticles (NPs) is studied using confocal microscopy. Up to fourfold enhancement of PL intensity due to the coupling of the QWs with localized surface plasmons (LSPs) induced on single Ag NPs is demonstrated. The enhancement is accompanied by a redshift of the PL spectral peak position towards the LSP resonance wavelength. The correlation between PL intensity and spectral position indicates that the enhancement is stronger in the sample areas emitting at wavelengths that are a better match with the LSP resonance wavelength. The enhancement of light emission and extraction facilitated by photon coupling with LSP in a single Ag NP is demonstrated.

Correlation between structure and photoluminescence properties in InGaN epilayers with thicknesses below and above critical thickness

The layer strain and its relaxation effects on the photoluminescence (PL) of InGaN layers are studied using confocal microscopy. The relaxation imposed structural changes are studied by X-ray diffraction (XRD) reciprocal space mapping and atomic force microscopy. Initial layer relaxation generated misfit dislocations were observed by confocal microscopy as intersecting parallel lines of lower PL intensity. The splitting of the PL spectrum into several PL bands indicated an onset of changes in the layer structure, which were confirmed by XRD measurements. The PL bands were attributed to two sub-layers of the sample: A relaxed upper sub-layer and a strained sub-layer underneath. Bright spots, approximately 250 nm in diameter, were observed on the background of the inhomogeneous PL intensity distribution due to fluctuations of In content. The bright spots correspond to column-like structures with relaxed lattice, In content as in the initial strained layer, and lower density of nonradiative recombination cen...

Influence of defects and indium distribution on emission properties of thick In-rich InGaN layers grown by the DERI technique

We report on the spatial variation of optical properties in thick, In-rich InGaN layers, grown by a novel droplet elimination by radical beam irradiation (DERI) technique. The increase of layer thickness causes layer relaxation and results in double-peaked photoluminescence spectra. Spatially resolved measurements show that the defects in the strained sub-layer are distributed inhomogeneously. An increase in the layer thickness results in faster nonradiative recombination due to increasing density of nonradiative recombination centers, as evidenced by time-resolved free carrier absorption, and facilitates larger indium incorporation in the upper part of the layer.