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1.	Hsiao, Ching-Lien, et al. (author) Composition tunable Al1-xInxN nanorod arrays grown by ultra-high-vacuum magnetron sputter epitaxy 2011 Other publication (other academic/artistic)abstract Self-assembled ternary Al1-xInxN nanorod arrays with variable In concentration, 0.10 ≤ x ≤ 0.32 have been realized onto c-plane sapphire substrates by ultra-high-vacuum magnetron sputter epitaxy with Ti0.21Zr0.79N or VN seed layers assistance. The formation of nanorods was very sensitive to the applied seed layer. Without proper seed layer assistance a continuous Al1-xInxN film was grown. The nanorods exhibit hexagonal crosssections with preferential growth along the c axis. A coaxial rod structure with higher In concentration in the core was observed by (scanning) transmission electron microscopy in combination with low-loss electron energy loss spectroscopy and energy dispersive xray spectroscopy. 5 K cathodoluminescence spectroscopy of Al0.86In0.14N nanorods revealed band edge emission at ~5.46 eV, which was accompanied by a strong defectrelated emission at ~ 3.38 eV.
2.	Hsiao, Ching-Lien, et al. (author) Spontaneous Formation of AlInN Core–Shell Nanorod Arrays by Ultrahigh-Vacuum Magnetron Sputter Epitaxy 2011 In: Applied Physics Express. - : Japan Society of Applied Physics. - 1882-0786. ; 4:115002 Journal article (peer-reviewed)abstract The spontaneous formation of AlInN core–shell nanorod arrays with variable In concentration has been realized by ultrahigh-vacuum magnetron sputter epitaxy with Ti0.21Zr0.79N or VN seed layer assistance. The nanorods exhibit hexagonal cross sections with preferential growth along the c-axis. A core–shell rod structure with a higher In concentration in the core was observed by (scanning) transmission electron microscopy in combination with low-loss electron energy loss spectroscopy and energy dispersive X-ray spectroscopy. 5 K cathodoluminescence spectroscopy of Al0.86In0.14N nanorods revealed band edge emission at ∼5.46 eV, which was accompanied by a strong defect-related emission at ∼3.38 eV
3.	Amloy, Supaluck, et al. (author) Excitons and biexcitons in InGaN quantum dot like localization centers Other publication (other academic/artistic)abstract Indium segregation in a narrow InGaN single quantum well creates quantum dot (QD) like exciton localization centers. Cross section transmission electron microscopy reveals varying shapes and lateral sizes in the range ~1-5 nm of the QD-like features, while scanning near field optical microscopy demonstrates a highly inhomogeneous spatial distribution of optically active individual localization centers. Microphotoluminescence spectroscopy confirms the spectrally inhomogeneous distribution of localization centers, in which the exciton and the biexciton related emissions from single centers of varying geometry could be identified by means of excitation power dependencies. Interestingly, the biexciton binding energy (Ebxx) was found to vary from center to center, between 3 to -22 meV, in correlation with the exciton emission energy. Negative binding energies justify the three-dimensional quantum confinement, which confirms QD-like properties of the localization centers.! The observed energy correlation is proposed to be understood as variations of the lateral extension of the confinement potential, which would yield smaller values of Ebxx for reduced lateral extension and higher exciton emission energy. The proposed relation between lateral extension and Ebxx is further supported by the exciton and the biexciton recombination lifetimes of a single QD, which suggest a lateral extension of merely ~3 nm for a QD with strongly negative Ebxx = -15.5 meV.
4.	Amloy, Supaluck, et al. (author) Excitons and biexcitons in InGaN quantum dot like localization centers 2014 In: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 25:49, s. 495702- Journal article (peer-reviewed)abstract Indium segregation in a narrow InGaN single quantum well creates quantum dot (QD) like exciton localization centers. Cross-section transmission electron microscopy reveals varying shapes and lateral sizes in the range ∼1–5 nm of the QD-like features, while scanning near field optical microscopy demonstrates a highly inhomogeneous spatial distribution of optically active individual localization centers. Microphotoluminescence spectroscopy confirms the spectrally inhomogeneous distribution of localization centers, in which the exciton and the biexciton related emissions from single centers of varying geometry could be identified by means of excitation power dependencies. Interestingly, the biexciton binding energy (Ebxx) was found to vary from center to center, between 3 to −22 meV, in correlation with the exciton emission energy. Negative binding energies are only justified by a three-dimensional quantum confinement, which confirms QD-like properties of the localization centers. The observed energy correlation is proposed to be understood as variations of the lateral extension of the confinement potential, which would yield smaller values of Ebxx for reduced lateral extension and higher exciton emission energy. The proposed relation between lateral extension and Ebxx is further supported by the exciton and the biexciton recombination lifetimes of a single QD, which suggest a lateral extension of merely ∼3 nm for a QD with strongly negative Ebxx = −15.5 meV.
5.	Chen, Yen-Ting, et al. (author) Nucleation of single GaN nanorods with diameters smaller than 35 nm by molecular beam epitaxy 2013 In: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 103:20, s. 203108- Journal article (peer-reviewed)abstract Nucleation mechanism of catalyst-free GaN nanorod grown on Si(111) is investigated by the fabrication of uniform and narrow (andlt; 35 nm) nanorods without a pre-defined mask by molecular beam epitaxy. Direct evidences show that the nucleation of GaN nanorods stems from the sidewall of the underlying islands down to the Si(111) substrate, different from commonly reported ones on top of the island directly. Accordingly, the growth and density control of the nanorods is exploited by a "narrow-pass" approach that only narrow nanorod can be grown. The optimal size of surrounding non-nucleation area around single nanorod is estimated as 88 nm.
6.	Hsu, Chih-Wei, et al. (author) Controlled Growth of GaN Pyramidal template hosting InGaN Quantum Dots Other publication (other academic/artistic)abstract The emission properties of InGaN grown on hexagonal GaN pyramids with various pitch distances (PD) are studied. Emissions associated with InGaN quantum wells (QWs) and InGaN quantum dots (QDs) can be identified. The emission energies of InGaN QWs and QDs shift toward opposite directions with increasing PD; red-shift for QWs and blue-shift for QDs. Based on the source supply mechanism in a selective area growth process, the formation of InGaN QDs on GaN pyramids is believed to be a combined effect of Stranski-Krastanow growth mode and spinodal decomposition taking place at the microscopic (0001) surfaces on GaN pyramids.
7.	Larsson, Mats, et al. (author) Magnetic field effects on optical and transport properties in InAs/GaAs quantum dots 2006 In: Physical Review B. - 1098-0121. ; 74:24 Journal article (peer-reviewed)abstract A photoluminescence study of self-assembled InAs/GaAs quantum dots under the influence of magnetic fields perpendicular and parallel to the dot layer is presented. At low temperatures, the magnetic field perpendicular to the dot layer alters the in-plane transport properties due to localization of carriers in wetting layer (WL) potential fluctuations. Decreased transport in the WL results in a reduced capture into the quantum dots and consequently a weakened dot-related emission. The effect of the magnetic field exhibits a considerable dot density dependence, which confirms the correlation to the in-plane transport properties. An interesting effect is observed at temperatures above approximately 100 K, for which magnetic fields, both perpendicular and parallel to the dot layer, induced an increment of the quantum dot photoluminescence. This effect is ascribed to the magnetic confinement of the exciton wave function, which increases the probability for carrier capture and localization in the dot, but affects also the radiative recombination with a reduced radiative lifetime in the dots under magnetic compression.
8.	Lenz, Annika, et al. (author) ZnO Nanoparticles Functionalized with Organic Acids: An Experimental and Quantum-Chemical Study 2009 In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 113:40, s. 17332-17341 Journal article (peer-reviewed)abstract Electrochemical synthesis and physical characterization of ZnO nanoparticles functionalized with four different organic acids, three aromatic (benzoic, nicotinic, and trans-cinnamic acid) and one nonaromatic (formic acid), are reported. The functionalized nanoparticles have been characterized by X-ray powder diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, UV−vis, and photoluminescence spectroscopy. The adsorption of the organic acids at ZnO nanoparticles was further analyzed and interpreted using quantum-chemical density-functional theory computations. Successful functionalization of the nanoparticles was confirmed experimentally by the measured splitting of the carboxylic group stretching vibrations as well as by the N(1s) and C(1s) peaks from XPS. From a comparison between computed and experimental IR spectra, a bridging mode adsorption geometry was inferred. PL spectra exhibited a remarkably stronger near band edge emission for nanoparticles functionalized with formic acid as compared to the larger aromatic acids. From the quantum-chemical computations, this was interpreted to be due to the absence of aromatic adsorbate or surface states in the band gap of ZnO, caused by the formation of a complete monolayer of HCOOH. In the UV−vis spectra, strong charge-transfer transitions were observed.
9.	Lundskog, Anders, et al. (author) InGaN quantum dot formation mechanism on hexagonal GaN/InGaN/GaN pyramids 2012 In: Nanotechnology. - : Institute of Physics (IOP). - 0957-4484 .- 1361-6528. ; 23:30, s. 305708- Journal article (peer-reviewed)abstract Growing InGaN quantum dots (QDs) at the apex of hexagonal GaN pyramids is an elegant approach to achieve a deterministic positioning of QDs. Despite similar synthesis procedures by metal–organic chemical vapor deposition, the optical properties of the QDs reported in the literature vary drastically. The QDs tend to exhibit either narrow or broad emission lines in the micro-photoluminescence spectra. By coupled microstructural and optical investigations, the QDs giving rise to narrow emission lines were concluded to nucleate in association with a (0001) facet at the apex of the GaN pyramid.
10.	Lundskog, Anders, et al. (author) Unexpected behavior of InGaN quantum dot emission energy located at apices of hexagonal GaN pyramids Other publication (other academic/artistic)abstract InGaN quantum dots (QDs) have been grown at the apices of hexagonal GaN pyramids. The pyramids were selectively grown on a (0001) oriented GaN template through circular apertures in a SiN mask positioned in square arrays. The emission of the InGaN QDs was shifted towards higher energies when the center-to-center distance of the pyramids was increased, while the emission from InGaN quantum wells located on the {1101} facets of the pyramids was energetically shifted towards lower energies. No energy shift was observed for (0001) truncated pyramids with truncation diameters larger than 100 nm.
11.	Adnane, Bouchaib (author) Optical characterization of Silicon-based self-assembled nanostructures 2010 Doctoral thesis (other academic/artistic)abstract This PhD thesis summarizes the work carried on the optical characterizations of some Si-based self-assembled nanostructures, particularly SiGe/Si quantum dots (QDs) and nanocrystalline (nc)-Si embedded in mesoporous silica (MS) using photoconductivity (PC), photoluminescence (PL), and photoluminescence excitation (PLE) measurements. The spectroscopic studies of SiGe/Si QDs grown on Si by molecular beam epitaxy revealed for the first time well-resolved PLE resonances. When correlated with numerical analysis, these resonances were directly related to the co-existence of spatially direct (inside the SiGe dot) and indirect (across the Si/Ge interface) recombination processes involving different dot populations selected by the monitored detection energy for PLE acquisition. The characteristics of these two transitions were further studied in detail by PLE (in some case implemented together with selective PL) on various samples, which contained either only one Ge dot layer or multiple Gedot/Si stacks, grown at substrate temperatures ranging from 430 to 580 °C; especially the temperature- and excitation power-dependence of the excitation properties. The results illustrated that the electronic structure of SiGe dots are influenced by size, Ge composition, as well as strain connected, and sometimes a mixed effect. Another attempt of the project was the fabrication of lateral transport mid-infrared photodetectors based on multiple Ge-dot/Si stacked structures. A broadband photoresponsivity of the processed multi-finger detectors was estimated to be about 90 mA/W over 3-15 μm range at 20 K, and the peaked photoresponse was measured at ~10 μm. The origin of the measured photocurrent, as elucidated by photoluminescence and photoluminescence excitation spectroscopies, was related to intersubband absorption of normal incidence infrared radiation corresponding to energies between the ground states of the heavy hole and the light hole in the valence band of the SiGe/Si QDs, and subsequent charge transfer to the Ge 2D wetting layer acting as a conduction channel. The absence of photocurrent in the energy range expected for a transition from the ground state to the first excited state of the heavy hole indicated that the holes in the SiGe dots behave essentially as 2D in character rather than a truly 3D confinement, where the transitions between heavy holes states are not allowed for TE polarized radiation (normal incidence). Finally, Si(or Ge) nanocrystals embedded in mesoporous silica samples prepared by spincoating and atomic layer chemical vapor deposition were optically investigated by means of PL with various excitation powers, together with several attempts using different post rapid thermal annealing processes. The shape and energy position of the PL spectra of the nc-Si embedded in MS samples and a reference MS template without nc incorporation were rather similar, but the luminescence was much more intense for those embedded with nanocrystals. This implies that the emission mechanism for MS samples with or without nc-Si could be the same, i.e., the light emission was governed by the surface properties of silica. The semiconductor nanocrystals played a role by sensitizing the luminescence emission through generating more photo-excited carriers. These carriers were then trapped in the defect state e.g. the interfacial oxygen defect sites and subsequently recombine to increase the PL intensity.
12.	Adnane, Bouchaib, et al. (author) Origin of photoresponse at 8-14 μm in stacks of self-assembled SiGe/Si quantum dots 2009 In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. Journal article (other academic/artistic)abstract A normal incidence photodetector operating at 8-14 μm is demonstrated using p-type δ-doped SiGe dot multilayer structures grown by molecular beam epitaxy on Si(001) substrates. Based on the experimental results of photoluminescence and photoluminescence excitation spectroscopies together with numerical analysis, the origin of the measured photocurrent was attributed to intersubband optical transitions between the heavy hole and light hole states of the valence band of the self-assembled SiGe dots and subsequent lateral transport of photo-excited carriers in the conduction channels formed by Ge wetting layers.
13.	Adnane, Bouchaib, et al. (author) Photoluminescence excitation spectroscopy of self-assembled SiGe/Si quantum dots 2009 Other publication (other academic/artistic)abstract Photoluminescence excitation (PLE) experiments are reported for various self-assembled SiGe/Si dot samples grown on Si(001) by molecular beam epitaxy at substrate temperatures ranging from 430 to 580 C. Two excitation peaks were observed, and the characteristics of the involved optical transitions were studied in detail by PLE (in one case implemented together with selective photoluminescence, SPL) on different samples containing either only one SiGe dot layer or multiple SiGe-dot/Si stacks. The temperature- and power-dependence of the excitation properties together with the results of six-band k.p calculations support the assignment of the observed PLE peaks to spatially direct and indirect transitions collected from two different SiGe dot populations.
14.	Adnane, Bouchaib, et al. (author) Photoluminescence study of nanocrystalline-Si(Ge) embedded in mesoporous silica 2009 In: Solid-State Electronics. - : Elsevier BV. - 0038-1101 .- 1879-2405. ; 53:8, s. 862-864 Journal article (peer-reviewed)abstract Photoluminescence (PL) properties of mesoporous silica (MS) samples incorporated with Si or Ge nanocrystals (nc) have been investigated with various excitation powers and post-RTA processes. The analysis of experimental results revealed a superlinear intensity dependence (m = 1.7) in the MS reference sample without nanocrystals, while a sublinear behavior (m = 0.8) is observed for the nc-Si in MS. It thus suggests the same recombination responsible for the luminescence at similar to 2.75 eV for both samples, but different kinetic limitations for the carrier transfer processes. Si nanocrystals play in this case an important role in generating more photo-excited carriers, enhancing the PL intensity.
15.	Adnane, Bouchaib, et al. (author) Spatially direct and indirect transitions of self-assembled SiGe/Si quantum dots studied by photoluminescence excitation spectroscopy 2010 In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 96:18, s. 181107- Journal article (peer-reviewed)abstract Well-resolved photoluminescence excitation (PLE) spectra are reported for selfassembled SiGe dots grown on Si(100) by molecular beam epitaxy. The observation of two excitation resonance peaks is attributed to two different excitation/de-excitation routes of interband optical transitions connected to the spatially direct and indirect recombination processes. It is concluded that two dot populations are addressed by each monitored luminescence energy for the PLE acquisition.
16.	Agekyan, V F, et al. (author) Effect of a magnetic field on energy transfer of band states to the Mn2+ 3d shell in the CdMgTe matrix with ultrathin CdMnTe layers 2010 In: PHYSICS OF THE SOLID STATE. - 1063-7834. ; 52:1, s. 27-31 Journal article (peer-reviewed)abstract The effect of external magnetic fields on two radiative (band-to-band and on-site) recombination channels in II-VI dilute magnetic semiconductors and related nanostructures has been considered. The 3d on-site emission of manganese ions in CdMgTe matrices containing periodic inclusions of CdMnTe narrow-band-gap layers with thicknesses of 0.5, 1.5, and 3.0 monolayers has been investigated in magnetic fields of up to 6 T. It has been shown that, in a magnetic field, luminescence of manganese ions weakens because of the decrease in the rate of spin-dependent excitation transfer from band states to the Mn2+ 3d shell. The maximum suppression of 3d luminescence has been observed in the matrix with a CdMnTe layer 3.0 monolayers thick. This indicates that the main factor responsible for the energy transfer is the internal field near the CdMnTe layers, which determines the magnetic splitting and spin polarization of band states.
17.	Agekyan, V F, et al. (author) Magnetoluminescence of CdTe/MnTe/CdMgTe heterostructures with ultrathin MnTe layers 2011 In: Semiconductors (Woodbury, N.Y.). - : MAIK Nauka/Interperiodica (and#1052;and#1040;and#1048;and#1050; and#1053;and#1072;and#1091;and#1082;and#1072;/and#1048;and#1085;and#1090;and#1077;and#1088;and#1087;and#1077;and#1088;and#1080;and#1086;and#1076;and#1080;and#1082;and#1072;). - 1063-7826 .- 1090-6479. ; 45:10, s. 1301-1305 Journal article (peer-reviewed)abstract CdTe/MnTe/CdMgTe quantum-well structures with one or two monolayers of MnTe inserted at CdTe/CdMgTe interfaces were fabricated. The spectra of the excitonic luminescence from CdTe quantum wells and their variation with temperature indicate that introduction of ultrathin MnTe layers improves the interface quality. The effect of a magnetic field in the Faraday configuration on the spectral position of the exciton-emission peaks indicates that frustration of magnetic moments in one-monolayer MnTe insertions is weaker than in two-monolayer insertions. The effect of a magnetic field on the exciton localization can be explained in terms of the exciton wave-function shrinkage and obstruction of the photoexcited charge-carrier motion in the quantum well.
18.	Aleksandrov, I.A., et al. (author) Linearly polarized photoluminescence from an ensemble of wurtzite GaN/AlN quantum dots 2010 In: JETP Letters. - : Springer Science Business Media. - 0021-3640 .- 1090-6487. ; 91:9, s. 452-454 Journal article (peer-reviewed)abstract Microphotoluminescence from GaN/AlN quantum dots grown by molecular beam epitaxy on sapphire substrates along the (0001) axis has been studied. To produce quantum dots of different average sizes and densities, the nominal amount of deposited GaN has been varied from 1 to 4 ML. The density of the quantum dots was about 10(11) cm(-2), which corresponded to about 10(3) quantum dots excited in the experiments. The photo-luminescence from the quantum dots was linearly polarized and the maximum polarization degree (15%) has been observed for the sample with the lowest amount of deposited GaN. The photoluminescence intensity from this sample under continuous laser excitation decreased by more than two orders of magnitude for about 30 min and then stabilized. The photoluminescence intensity from other samples under continuous excitation remained constant. We suggest that a rather high polarization degree is caused by anisotropy in the strain and shape of the quantum dots formed near the dislocations, which also act as the centers of nonradiative recombination.
19.	Amloy, Supaluck, et al. (author) Dynamic characteristics of the exciton and the biexciton in a single InGaN quantum dot 2012 In: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 101:6 Journal article (peer-reviewed)abstract The dynamics of the exciton and the biexciton related emission from a single InGaN quantum dot (QD) have been measured by time-resolved microphotoluminescence spectroscopy. An exciton-biexciton pair of the same QD was identified by the combination of power dependence and polarization-resolved spectroscopy. Moreover, the spectral temperature evolution was utilized in order to distinguish the biexciton from a trion. Both the exciton and the biexciton related emission reveal mono-exponential decays corresponding to time constants of similar to 900 and similar to 500 ps, respectively. The obtained lifetime ratio of similar to 1.8 indicates that the QD is small, with a size comparable to the exciton Bohr radius.
20.	Amloy, Supaluck, et al. (author) III-nitride based quantum dots for photon emission with controlled polarization switching Other publication (other academic/artistic)abstract Computational studies based on 6 band k⋅p!theory are employed on lens-shaped III-nitride quantum dots (QDs) with focus on the polarization properties of the optical interband transitions. The results predict pronounced linear polarization of the ground-state related transitions for asymmetric QDs of a material with small split-off energy. It is demonstrated that a moderate externally applied electric field can be used to induce a linear polarization and to control its direction. InN is found to be the most efficient choice for dynamic polarization switching controlled by an electric field, with potential for polarization control on a photon-by-photon level.
21.	Amloy, Supaluck, et al. (author) On the polarized emission from exciton complexes in GaN quantum dots 2012 In: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 100:021901 Journal article (peer-reviewed)abstract The optical linear polarization properties of exciton complexes in asymmetric Stranski-Krastanov grown GaN quantum dots have been investigated experimentally and theoretically. It is demonstrated that the polarization angle and the polarization degree can be conveniently employed to associate emission lines in the recorded photoluminescence spectra to a specific dot. The experimental results are in agreement with configuration interaction computations, which predict similar polarization degrees for the exciton and the biexciton (within 10%) in typical GaN quantum dots. The theory further predicts that the polarization degree can provide information about the charge state of the dot.
22.	Amloy, Supaluck, et al. (author) Polarization-resolved fine-structure splitting of zero-dimensional InxGa1-xN excitons 2011 In: PHYSICAL REVIEW B. - : American Physical Society. - 1098-0121. ; 83:20, s. 201307- Journal article (peer-reviewed)abstract The fine-structure splitting of quantum confined InxGa1-x Nexcitons is investigated using polarization-sensitive photoluminescence spectroscopy. The majority of the studied emission lines exhibits mutually orthogonal fine-structure components split by 100-340 mu eV, as measured from the cleaved edge of the sample. The exciton and the biexciton reveal identical magnitudes but reversed sign of the energy splitting.
23.	Amloy, Supaluck, 1980- (author) Polarization-resolved photoluminescence spectroscopy of III-nitride quantum dots 2013 Doctoral thesis (other academic/artistic)abstract In this thesis, results from studies on (In)GaN quantum dots (QDs) are presented, including investigations of the structural, optical and electronic properties. The experimental studies were performed on GaN and InGaN QDs grown by molecular beam epitaxy, taking advantage of the Stranki-Krastanov growth mode for the GaN QD samples and the composition segregation for the InGaN QD samples.Optical spectroscopy of the (In)GaN QDs was performed with a combination of different experimental techniques, e.g. stationary microphotoluminescence (μPL) and timeresolved μPL. The μPL spectroscopy is suitable for studies of single QDs due to the wellfocused excitation laser spot, and it typically does not require any special sample preparation. The powerful combination of power and polarization dependences was used to distinguish the exciton and the biexciton emissions from other emission lines in the recorded spectra.The QDs could be observed with random in-plane anisotropy, as determined by the strong linear polarization for single QDs but with different angular orientation from dot to dot. Additionally, these experimental results are in good agreement with the computational results revealing a similar degree of polarization for the exciton and the biexciton emissions. Further, the theory predicts that the discrepancy of the polarization degree is larger between the positive and negative trions in comparison with the exciton and the biexciton. Based on this result, polarization resolved spectroscopy is proposed as a simple tool for the identification of trions and their charge states.The fine-structure splitting (FSS) and the biexciton binding energy (Ebxx) are essential QD parameters of relevance for the possible generation of quantum entangled photon pairs in a cascade recombination of the biexciton. In general, the Coulomb interaction between the negatively charged electron and the positively charged hole lifts the fourfold degeneracy of the electron and hole pair ground state, forming a set of zero-dimensional exciton states of unequal energies. This Coulomb-induced splitting, referred to as the FSS, results in an electronic fine structure, which is strongly dependent on the symmetry of the exciton wave function. The FSS was in this work resolved and investigated for excitons in InGaN QDs, using polarization-sensitive μPL spectroscopy employed on the cleaved-edge of the samples. As expected, the FSS is found to exhibit identical magnitudes, but with reversed sign for the exciton and the biexciton. For quantum information applications, a vanishing FSS is required, since otherwise the emissions of the polarization-entangled photon pairs in the cascade biexciton recombination will be prohibited.The biexcitons are found to exhibit both positive and negative binding energies for the investigated QDs. Since a negative binding energy indicates a repulsive Coulomb interaction, such biexcitons (or exciton complexes) cannot exist in structures of higher dimensionality. On the other hand, a biexciton with a negative binding energy can be found in QDs, since the exciton complexes still remain bound due to their three dimensional confinement. Moreover, the biexciton binding energy depends on the dot size, which implies that a careful size control of dots could enable manipulation of the biexciton binding energy. A large Ebxx value enables better and cheaper spectral filtering, in order to purify the single photon emission, while a proposed time reordering scheme relies on zero Ebxx for the generation of entangled photons.The dynamics of the exciton and the biexciton emissions from InGaN QD were measured by means of time-resolved μPL. The lifetimes of the exciton related emissions are demonstrated to depend on the dot size. Both the exciton and the biexciton emissions reveal mono-exponential decays, with a biexciton lifetime, which is about two times shorter than the exciton lifetime. This implies that the QD is small, with a size comparable to the exciton Bohr radius. The photon generation rates can be manipulated by controlling the QDs size, which in turn can be utilized for generation of single- and entangled-photons on demand, with a potential for applications in e.g. quantum information.The polarization of the emitted single photons can be manipulated by using a polarizer, but to the prize of photon loss and reduced emission intensity. Alternative methods to control the polarization of the emission light are a manipulation of the dot symmetry statically by its shape or dynamically by an externally applied electric field. Predictions based on performed calculations show that in materials with a small spin-orbit split-off energy (ΔSO), like the III-nitride materials, the polarization degree of the emission is more sensitive to dot asymmetry than in materials with a large value for ΔSO, e.g. the III-arsenide materials. Moreover, for an electric field applied in the 1͞10 and the 11͞2 directions of the zinc-blende lens-shaped QDs grown on the (111) plane, the polarization degree of InN QDs is found to be significantly more, by a factor of ~50 times, sensitive to the electric field than for GaN QDs. This work demonstrates that especially the InN based QD, are suitable for manipulation of the polarization by the direct control of the dot symmetry or by externally applied electric fields.
24.	Amloy, Supaluck, et al. (author) Polarized emission from single GaN quantum dots grown by molecular beam epitaxy 2011 In: AIP Conference Proceedings. - : AIP. - 1551-7616 .- 0094-243X. - 9780735410022 ; 1399, s. 541-542 Conference paper (peer-reviewed)abstract Polarization resolved microphotoluminescence measurements of single MBE-grown GaN/Al(Ga)N quantum dots (QDs) have been performed. The exciton and biexciton peaks with full width at half maximum as narrow as
25.	Amloy, Supaluck, et al. (author) Size dependent biexciton binding energies in GaN quantum dots 2011 In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 99:25 Journal article (peer-reviewed)abstract Single GaN/Al(Ga)N quantum dots (QDs) have been investigated by means of microphotoluminescence. Emission spectra related to excitons and biexcitons have been identified by excitation power dependence and polarization resolved spectroscopy. All investigated dots exhibit a strong degree of linear polarization (similar to 90%). The biexciton binding energy scales with the dot size. However, both positive and negative binding energies are found for the studied QDs. These results imply that careful size control of III-Nitride QDs would enable the emission of correlated photons with identical frequencies from the cascade recombination of the biexciton, with potential applications in the area of quantum information processing.
26.	Andersson, J. Y., et al. (author) Quantum structure based infrared detector research and development within Acreo's centre of excellence IMAGIC 2010 In: Infrared physics & technology. - : Elsevier BV. - 1350-4495 .- 1879-0275. ; 53:4, s. 227-230 Journal article (peer-reviewed)abstract Acreo has a long tradition of working with quantum structure based infrared (IR) detectors and arrays. This includes QWIP (quantum well infrared photodetector), QDIP (quantum dot infrared photodetector), and InAs/GaInSb based photon detectors of different structure and composition. It also covers R&D on uncooled microbolometers. The integrated thermistor material of such detectors is advantageously based on quantum structures that are optimised for high temperature coefficient and low noise. Especially the SiGe material system is preferred due to the compatibility with silicon technology. The R&D work on IR detectors is a prominent part of Acreo's centre of excellence "IMAGIC" on imaging detectors and systems for non-visible wavelengths. IMAGIC is a collaboration between Acreo, several industry partners and universities like the Royal Institute of Technology (KTH) and Linkoping University. (C) 2010 Elsevier B.V. All rights reserved.
27.	Arnaudov, B., et al. (author) Radiative recombination mechanism in highly modulation doped GaN/AlGaN multiple quantum wells 2006 In: Physica Status Solidi. C, Current topics in solid state physics. - 1610-1634 .- 1610-1642. ; 6, s. 1888-1891 Journal article (peer-reviewed)
28.	Bergman, Peder, 1961-, et al. (author) Radiative recombination in InGaN/GaN multiple quantum well 2000 In: ICSCRM 99,1999. ; , s. 1571- Conference paper (peer-reviewed)
29.	Chen, Ruei-San, et al. (author) Electronic transport properties in aluminum indium nitride nanorods grown by magnetron sputter epitaxy 2013 In: Applied Surface Science. - : Elsevier. - 0169-4332 .- 1873-5584. ; 285, s. 625-628 Journal article (peer-reviewed)abstract The electronic transport properties of the wide-bandgap aluminum indium nitride (AlInN) nanorods (NRs) grown by ultrahigh-vacuum magnetron sputter epitaxy (MSE) have been studied. The conductivities of the ternary compound nanostructure locates at the value of 15 Q-1 cm -1, which is respectively one and two orders of magnitude lower than the binary GaN and InN counterparts grown by chemical vapor deposition (CVD). The very shallow donor level/band with the activation energy at 11 + 2 meV was obtained by the temperature-dependent measurement. In addition, the photoconductivity has also been investigated. The photoconductive (PC) gain of the NRs device can reach near 2400 under a low bias at 0.1 V and the light intensity at 100W m-2 for ultraviolet response in vacuum. The power-insensitive gain and ambience-dependent photocurrent are also observed, which is attributed to the probable surfacecontrolled PC mechanism in this ternary nitride nanostructure.
30.	Chen, Yen-Ting, et al. (author) Determination of critical diameters for intrinsic carrier diffusion-length of GaN nanorods with cryo-scanning near-field optical microscopy 2016 In: Scientific Reports. - : NATURE PUBLISHING GROUP. - 2045-2322. ; 6:21482, s. 1-7 Journal article (peer-reviewed)abstract Direct measurements of carrier diffusion in GaN nanorods with a designed InGaN/GaN layer-in-a-wire structure by scanning near-field optical microscopy (SNOM) were performed at liquid-helium temperatures of 10 K. Without an applied voltage, intrinsic diffusion lengths of photo-excited carriers were measured as the diameters of the nanorods differ from 50 to 800 nm. The critical diameter of nanorods for carrier diffusion is concluded as 170 nm with a statistical approach. Photoluminescence spectra were acquired for different positions of the SNOM tip on the nanorod, corresponding to the origins of the well-defined luminescence peaks, each being related to recombination-centers. The phenomenon originated from surface oxide by direct comparison of two nanorods with similar diameters in a single map has been observed and investigated.
31.	Donchev, V., et al. (author) Enhancement of the photoluminescence intensity of a single InAs/GaAs quantum dot by separate generation of electrons and holes 2006 In: Physics of the solid state. - 1063-7834 .- 1090-6460. ; 48:10, s. 1993-1999 Journal article (peer-reviewed)abstract It is demonstrated that the microphotoluminescence (µPL) spectrum of a single InAs/GaAs self-assembled quantum dot (QD) undergoes considerable changes when the primary laser excitation is complemented with an additional infrared laser. The primary laser, tuned slightly below the GaAs band gap, provides electron-hole pairs in the wetting layer (WL), as well as excess free electrons from ionized shallow acceptors in the GaAs barriers. An additional IR laser with a fixed energy well below the QD ground state transition generates excess free holes from deep levels in GaAs. The excess electron and hole will experience diffusion separately, due to the time separation between the two events of their generation, to eventually become captured into the QD. Although the generation rates of excess carries are much lower than that of the electron-hole pair generation in the WL, they considerably influence the QD emission at low temperatures. The integrated PL intensity increases by several times as compared to single-laser excitation, and the QD exciton spectrum is redistributed in favor of a more neutral charge configuration. The dependence of the observed phenomenon on the powers of the two lasers and the temperature has been studied and is consistent with the model proposed. The concept of dual excitation could be successfully applied to different low-dimensional semiconductor structures in order to manipulate their charge state and emission intensity. © Nauka/Interperiodica 2006.
32.	Donchev, V., et al. (author) Temperature Study of the Photoluminescence of a Single InAs/GaAs Quantum Dot 2004 In: Physica Status Solidi. C, Current topics in solid state physics. - 1610-1634 .- 1610-1642. ; 3, s. 608- Journal article (peer-reviewed)abstract Presented at: The 8th Conference on Optics of Excitons in Confined Systems, Lecce, Italy, February 3 - 4, 2003
33.	Dufåker, Daniel, et al. (author) Evidence of nonadiabatic exciton-phonon interaction probed by second-order LO-phonon replicas of single quantum dots 2013 In: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 87:8 Journal article (peer-reviewed)abstract In this experimental study of single InGaAs/GaAs quantum dots (QDs) the photoluminescence intensity of the second order LO-phonon replica of the excitonic interband recombination was measured along with the intensities of the first and zeroth orders. The results show that the intensity of the second-order replica is three to four times stronger than expected from the adiabatic Huang-Rhys theory, indicating that the neglected nonadiabaticity plays an important role for the understanding of the exciton-phonon coupling in QDs.
34.	Dufåker, Daniel, et al. (author) Exciton-phonon coupling in single quantum dots with different barriers 2011 In: Applied Physics Letters. - : American Institute of Physics. - 0003-6951 .- 1077-3118. ; 98:25, s. 251911- Journal article (peer-reviewed)abstract The coupling between longitudinal-optical (LO) phonons and neutral excitons in two different kinds of InGaAs pyramidal quantum dots embedded in either AlGaAs or GaAs barriers is experimentally examined. We find a slightly weaker exciton-LO-phonon coupling and increased linewidth of the phonon replicas for the quantum dots with GaAs barriers compared to the ones with AlGaAs barriers. These results, combined with the fact that the LO-phonon energy of the exciton is the same for both kinds of dots, are taken as evidence that the excitons mainly couple to LO-phonons within the QDs.
35.	Dufåker, Daniel, 1971- (author) Few particle effects in pyramidal quantum dots - a spectroscopic study 2013 Doctoral thesis (other academic/artistic)abstract In this thesis two very similar processes have been studied, both involving excitations of particles during recombination of exciton complexes in quantum dots, reducing the energy of the emitted photon. Different exciton complexes are defined according to the number of electrons and holes in the quantum dot upon recombination. The neutral exciton complexes with one electron and one hole (X) and two electrons and two holes (2X) respectively are referred to as the exciton and the biexciton. Accordingly the charged exciton complexes consisting of two electrons and one hole (X−) and one electron and two holes (X+), respectively, are referred to as negatively and positively charged excitons, respectively. Whenever another particle is excited during the recombination of one electron-hole pair within these complexes, the result is a weak satellite peak, spectrally redshifted with respect to the main emission peak related to the exciton complex.In the first part of this thesis, described in papers 1 - 3, the first and second order exciton-LO-phonon interaction is studied with weak satellite peaks, redshifted by the LOphonon energy (ћωLO or 2ћωLO), as the signature, referred to as phonon replicas. The intensity ratio between the first order replicas and the corresponding main emission were determined from the obtained micro-photoluminescence spectra. It was found that this ratio was significantly weaker for the positively charged exciton X+ compared to the neutral exciton, X, and the negatively charged exciton, X−. This experimentally obtained result was further supported by computations. Interestingly, the computations revealed that despite that X+ displays the weakest phonon replica among the investigated complexes, it possesses the strongest Fröhlich coupling to phonons in the lattice before recombination. The spectral broadening of the phonon replicas compared to the main emission is also discussed. The origin of the exciton-LO-phonon coupling is concluded to be from within the quantum dot (QD) itself, based on a comparison between quantum dots with different barriers. In addition, the measured intensity of the second order LO-phonon replica was approximately three times stronger than predictions made with the adiabatic Huang-Rhys theory but much weaker than the two orders in magnitude enhancement that was predicted when non adiabatic effects was included.Symmetrical QDs are a requirement for achieving entangled photon emission, desired for applications within quantum cryptography. In the fourth paper we relate the emission pattern of the doubly positively charged exciton X2+ to the symmetry of the QDs. In particular the splitting between the two low-energy components was found to be a measure of the asymmetry of the QDs. The emission pattern of the doubly charged exciton may then be used as a post-growth uninvasive selection tool were high-symmetry QDs could reliably be selected.In the last paper an additional weak redshifted satellite peak in the recombination spectra is studied. The intensity of this weak satellite peak is correlated to the peak intensity of the positively charged exciton, X+, main emission peak. In addition to this photoluminescence excitation experiments and computations further support our interpretation that the satellite peak is related to the shake-up of the ground state hole in the QD that is not involved in the optical recombination. This hole is excited by Coulomb interaction to an excited state yielding a photon energy that has been reduced with the difference between the ground state and the excited state of the spectator hole.
36.	Dufåker, Daniel, et al. (author) Phonon Coupling of Exciton Complexes in Single InGaAs/AlGaAs Quantum Dots 2010 Conference paper (peer-reviewed)
37.	Dufåker, Daniel, et al. (author) Phonon replicas of charged and neutral exciton complexes in single quantum dots 2010 In: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 82:20 Journal article (peer-reviewed)abstract The longitudinal-optical (LO)-phonon coupling is experimentally examined by the optical decay of various charged and neutral exciton species in single quantum dots, and the related Huang-Rhys parameters are extracted. A positive trion exhibits significantly weaker LO-phonon replicas in the photoluminescence spectrum than the neutral and negatively charged species. Model computations show that the strength of the replicas is determined by the Coulomb interactions between electrons and holes, which modify the localization of the envelope wave functions and the net charge distribution.
38.	Dufåker, Daniel, et al. (author) Quantum dot asymmetry and the nature of excited hole states probed by the doubly positively charged exciton X2 2013 In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 88:4 Journal article (peer-reviewed)abstract In this experimental and theoretical study, it was found that the emission pattern of the doubly positively charged exciton complex X2+ strongly depends on the nature of the involved excited hole states as well as the quantum dot symmetry. The two-hole system in the final state of the X2+ recombination for the investigated high-symmetry pyramidal InGaAs quantum dots does not exhibit the singlet-tripletlike arrangement previously observed for the two-electron counterpart. Instead, the final states exhibit two true doublets, in accordance with group-theoretical predictions. Asymmetry is manifested in the photoluminescence spectra of X2+ by a significant splitting of one doublet, which is a spectral feature exhibited to some degree by all of the measured quantum dots. The analysis demonstrates that an external magnetic field elevates the symmetry of the quantum dots. This work highlights the exciton complex X2+ as a very sensitive probe of the quantum dot shape as well as the nature of the involved quantum states. Thus, its spectral features are very suitable for an efficient uninvasive postgrowth symmetry characterization of quantum dots.
39.	Dufåker, Daniel, 1971- (author) Spectroscopy studies of few particle effects in pyramidal quantum dots 2011 Licentiate thesis (other academic/artistic)abstract In this thesis work two very similar processes have been studied both involving excitations of particles during recombination of exciton complexes in quantum dots, reducing the energy of the emitted photon. Different exciton complexes are defined according to the number of electrons and holes in the quantum dot upon recombination. The neutral exciton complexes with one electron and one hole (X–) and two electrons and two (X+) holes respectively are referred to as the exciton andthe biexciton. Accordingly the charged exciton complexes consisting of two electrons and one hole (X–) and one electron and two holes (X+), respectively, are referred to as negatively- and positively charged excitons, respectively. Whenever another particle is excited during the recombination of one electron-hole pair within these complexes, the result is a weak satellite peak, spectrally redshifted with respect to the main emission peaks related to the exciton complex.In the first part of this thesis work, described in the first two papers, the exciton-LO-phonon interaction is studied with a weak redshifted satellite peak as the signature, referred to as a phonon replica. The intensity ratio between the replicas and the corresponding main emission were determined from the obtained microphotoluminescencespectra. It was found that this ratio was significantly weaker for the positively charged exciton X+ compared to the neutral exciton, X, and the negatively charged exciton, X–. This experimentally obtained result was further supported by computations. Interestingly, the computations revealed that despite that X+ displays the weakest phonon replica among the investigated complexes, it possesses the strongest Fröhlich coupling to phonons in the lattice before recombination. In addition, the spectral broadening of the phonon replicas compared to the main emission is discussed. Also, the origin of the exciton-LO-phonon coupling is concluded to be from the QD itself, based on a comparison between quantum dots with different barriers.In the last paper an additional weak redshifted satellite peak in the recombination spectra is studied. The intensity of this weak satellite peak is correlated to the peak intensity of the positively charged exciton, X+, main emission peak. In addition to this photoluminescence excitation experiments, magnetic field measurement and calculations further support our interpretation that the satellite peak is related to the shake-up of the ground-state hole in the QD that is not involved in the optical recombination. This hole is thus excited by Coulomb interaction to an excited state yielding a photon energy reduced with the difference between the ground-state and the excited state of the spectator hole.
40.	Dupertuis, M A, et al. (author) Symmetries and the Polarized Optical Spectra of Exciton Complexes in Quantum Dots 2011 In: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 107:12, s. 127403- Journal article (peer-reviewed)abstract A systematic and simple theoretical approach is proposed to analyze true degeneracies and polarized decay patterns of exciton complexes in semiconductor quantum dots. The results provide reliable spectral signatures for efficient symmetry characterization, and predict original features for low C(2 nu) and high C(3 nu) symmetries. Excellent agreement with single quantum dot spectroscopy of real pyramidal InGaAs/AlGaAs quantum dots grown along [111] is demonstrated. The high sensitivity of biexciton quantum states to exact high symmetry can be turned into an efficient uninvasive postgrowth selection procedure for quantum entanglement applications.
41.	Duteil, Fabrice (author) MBE growth of Si-based heterostructures for optoelectronic applications 2000 Licentiate thesis (other academic/artistic)abstract Molecular beam epitaxy (MBE) is a powerful technique used to grow semiconductor crystalline Jayers at low (200°C - 900 °C) temperature. In this study we report on the growth of silicon-based materials used for their electronic and optical properties. Parts of the work have involved alloys of silicon (Si) with germanium (Ge) or carbon (C). One possibility to have infrared light emission from silicon-based material is to use doping with the rare-earth erbium ions (Er3+) combined with oxygen (O), and the incorporation behaviors of these atoms were studied for two different doping source combinations. The influence on the crystalline quality of the doping concentrations and the substrate temperature were extensively investigated and e.g. it has been shown that very high concentrations of Er (1x1020 cm-3) and O (5x1020 cm-3) can be incorporated at 430°C into a Si crystal without extended defects. The material properties were principally characterized by electroluminescence (EL), and cross-sectional transmission electron microscopy (XTEM).
42.	Elfving, Anders, et al. (author) Efficient near infrared Si/Ge quantum dot photodetector based on a heterojunction bipolar transistor 2003 In: Material Research Society Symposium Proceedings. Conference paper (peer-reviewed)
43.	Eriksson, Martin O., et al. (author) Recombination processes in Mg doped wurtzite InN films with p- and n-type conductivity 2019 In: AIP Advances. - : AMER INST PHYSICS. - 2158-3226. ; 9:1 Journal article (peer-reviewed)abstract Obtaining high quality, wurtzite InN films with p-type conductivity is a challenge, and there is limited information about the photoluminescence (PL) characteristics of such films. In this study, we present a comprehensive PL study and discuss in detail the recombination processes in Mg-doped InN films with varying Mg concentrations. We find that at low Mg-doping of 1x10(18) cm(-3), which yields p-type conductivity, the PL in InN is spatially inhomogeneous. The latter is suggested to be associated with the presence of n-type pockets, displaying photoluminescence at 0.73 eV involving electrons at the Fermi edge above the conduction band edge. Increasing the Mg concentration to 2.9x10(19) cm(-3) in p-type InN yields strong and spatially uniform photoluminescence at 0.62 eV and 0.68 eV visible all the way to room temperature, indicating homogeneous p-type conductivity. An acceptor binding energy of 64 meV is determined for the Mg acceptor. Further increase of the Mg concentration to 1.8x10(20) cm(-3) leads to switching conductivity back to n-type. The PL spectra in this highly doped sample reveal only the emission related to the Mg acceptor (at 0.61 eV). In the low-energy tail of the emission, the multiple peaks observed at 0.54 - 0.58 eV are suggested to originate from recombination of carriers localized at stacking faults. (C) 2019 Author(s).
44.	Eriksson, Martin. O., et al. (author) The Dynamics of Charged and Neutral Excitons in an InGaN Quantum Dot on a GaN Pyramid Other publication (other academic/artistic)abstract The neutral (X0) and negatively charged excitons (X-) in an InGaN QD on a GaN pyramid is studied by the timeintegrated micro-photoluminescence (μPL) and time-resolved micro-photoluminescence (TRμPL) microcopies. Both X0 and X- exhibit mono-exponential decay curves with fitted lifetimes of 310 and 140 ps, respectively. Neither energy shifts nor changes in the life times X0 and X- with increasing excitation power were observed, indicating the QD is small and free from the quantum confine Stark effect. The TRμPL is not only a powerful technique for studying the dynamics of exciton in QDXs, but also for the identification of exciton complexes in QDs.
45.	Eriksson, Martin (author) Photoluminescence Characteristics of III-Nitride Quantum Dots and Films 2017 Doctoral thesis (other academic/artistic)abstract III-Nitride semiconductors are very promising in both electronics and optical devices. The ability of the III-Nitride semiconductors as light emitters to span the electromagnetic spectrum from deep ultraviolet light, through the entire visible region, and into the infrared part of the spectrum, is a very important feature, making this material very important in the field of light emitting devices. In fact, the blue emission from Indium Gallium Nitride (InGaN), which was awarded the 2014 Nobel Prize in Physics, is the basis of the common and important white light emitting diode (LED).Quantum dots (QDs) have properties that make them very interesting for light emitting devices for a range of different applications, such as the possibility of increasing device efficiency. The spectrally well-defined emission from QDs also allows accurate color reproduction and high-performance communication devices. The small size of QDs, combined with selective area growth allows for an improved display resolution. By control of the polarization direction of QDs, they can be used in more efficient displays as well as in traditional communication devices. The possibility of sending out entangled photon pairs is another QD property of importance for quantum key distribution used for secure communication.QDs can hold different exciton complexes, such as the neutral single exciton, consisting of one electron and one hole, and the biexciton, consisting of two excitons. The integrated PL intensity of the biexciton exhibits a quadratic dependence with respect to the excitation power, as compared to the linear power dependence of the neutral single exciton. The lifetime of the neutral exciton is 880 ps, whereas the biexciton, consisting of twice the number of charge carriers and lacks a dark state, has a considerably shorter lifetime of only 500 ps. The ratio of the lifetimes is an indication that the size of the QD is in the order of the exciton Bohr radius of the InGaN crystal making up these QDs in the InGaN QW.A large part of the studies of this thesis has been focused on InGaN QDs on top of hexagonal Gallium Nitride (GaN) pyramids, selectively grown by Metal Organic Chemical Vapor Deposition (MOCVD). On top of the GaN pyramids, an InGaN layer and a GaN capping layer were grown. From structural and optical investigations, InGaN QDs have been characterized as growing on (0001) facets on truncated GaN pyramids. These QDs exhibit both narrow photoluminescence linewidths and are linearly polarized in directions following the symmetry of the pyramids.In this work, the neutral single exciton, and the more rare negatively charged exciton, have been investigated. At low excitation power, the integrated intensity of the PL peak of the neutral exciton increases linearly with the excitation power. The negatively charged exciton, on the other hand, exhibits a quadratic power dependence, just like that of the biexciton. Upon increasing the temperature, the power dependence of the negatively charged exciton changes to linear, just like the neutral exciton. This change in power dependence is explained in terms of electrons in potential traps close to the QD escaping by thermal excitation, leading to a surplus of electrons in the vicinity of the QD. Consequently, only a single exciton needs to be created by photoexcitation in order to form a negatively charged exciton, while the extra electron is supplied to the QD by thermal excitation.Upon a close inspection of the PL of the neutral exciton, a splitting of the peak of just below 0.4 meV is revealed. There is an observed competition in the integrated intensity between these two peaks, similar to that between an exciton and a biexciton. The high energy peak of this split exciton emission is explained in terms of a remotely charged exciton. This exciton state consists of a neutral single exciton in the QD with an extra electron or hole in close vicinity of the QD, which screens the built-in field in the QD.The InGaN QDs are very small; estimated to be on the order of the exciton Bohr radius of the InGaN crystal, or even smaller. The lifetimes of the neutral exciton and the negatively charged exciton are approximately 320 ps and 130 ps, respectively. The ratio of the lifetimes supports the claim of the QD size being on the order of the exciton Bohr radius or smaller, as is further supported by power dependence results. Under the assumption of a spherical QD, theoretical calculations predict an emission energy shift of 0.7 meV, for a peak at 3.09 eV, due to the built-in field for a QD with a diameter of 1.3 nm, in agreement with the experimental observations.Studying the InGaN QD PL from neutral and charged excitons at elevated temperatures (4 K to 166 K) has revealed that the QDs are surrounded by potential fluctuations that trap charge carriers with an energy of around 20 meV, to be compared with the exciton trapping energy in the QDs of approximately 50 meV. The confinement of electrons close to the QD is predicted to be smaller than for holes, which accounts for the negative charge of the charged exciton, and for the higher probability of capturing free electrons. We have estimated the lifetimes of free electrons and holes in the GaN barrier to be 45 ps and 60 ps, in consistence with excitons forming quickly in the barrier upon photoexcitation and that free electrons and holes get trapped quickly in local potential traps close to the QDs. This analysis also indicates that there is a probability of 35 % to have an electron in the QD between the photoexcitation pulses, in agreement with a lower than quadratic power dependence of the negatively charged exciton.InN is an attractive material due to its infrared emission, for applications such as light emitters for communication purposes, but it is more difficult to grow with high quality and low doping concentration as compared to GaN. QDs with a higher In-composition or even pure InN is an interesting prospect as being a route towards increased quantum confinement and room temperature device operation. For all optical devices, p-type doping is needed. Even nominally undoped InN samples tend to be heavily n-type doped, causing problems to make pn-junctions as needed for LEDs. In our work, we present Mg-doped p-type InN films, which when further increasing the Mg-concentration revert to n-type conductivity. We have focused on the effect of the Mg-doping on the light emission properties of these films. The low Mg doped InN film is inhomogeneous and is observed to contain areas with n-type conductivity, so called n-type pockets in the otherwise p-type InN film. A higher concentration of Mg results in a higher crystalline quality and the disappearance of the n-type pockets. The high crystalline quality has enabled us to determine the binding energy of the Mg dopants to 64 meV. Upon further increase of the Mg concentration, the film reverts to ntype conductivity. The highly Mg doped sample also exhibits a red-shifted emission with features that are interpreted as originating from Zinc-Blende inclusions in the Wurtzite InN crystal, acting as quantum wells. The Mg doping is an important factor in controlling the conductivity of InN, as well as its light emission properties, and ultimately construct InN-based devices.In summary, in this thesis, both pyramidal InGaN QDs and InGaN QDs in a QW have been investigated. Novel discoveries of exciton complexes in these QD systems have been reported. Knowledge has also been gained about the challenging material InN, including a study of the effect of the Mg-doping concentration on the semiconductor crystalline quality and its light emission properties. The outcome of this thesis enriches the knowledge of the III-Nitride semiconductor community, with the long-term objective to improve the device performance of III-Nitride based light emitting devices.
46.	Esmaeili, M, et al. (author) Experimental and theoretical investigations of optical properties of GaN/AlGaN MQW nanostructures. Impact of built-in polarization fields 2009 In: OPTO-ELECTRONICS REVIEW. - : Polish Academy of Sciences Chancellery. - 1230-3402 .- 1896-3757. ; 17:4, s. 293-299 Journal article (peer-reviewed)abstract We report the results from detailed optical spectroscopy from MOCVD grown GaN/AlGaN multiple quantum wells (MQWs), as opposed to most previous studies where MBE was employed by means of photoluminescence (PL) technique. In this paper we will present theoretical and experimental results demonstrating how polarization induced electric fields and bound interface charges in GaN/AlGaN MQWs affect the emission peak energy, PL line shape, as well as the emission line width. Theoretically estimated fields in this work are consistent with experimental data. Transition energy of the heavy hole and electron ground state Ee-hh in GaN/AlGaN MQWs were calculated and it is found that it stays in good agreement with the experimental data.
47.	Esmaeili, M., et al. (author) Optical properties of GaN/AlGaN QW nanostructures with different well and barrier widths 2007 In: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 19:35 Journal article (peer-reviewed)abstract Optical properties of wurtzite AlGaN/GaN quantum well (QW) structures grown by molecular-beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD) on c-plane sapphire substrates have been investigated by means of photoluminescence (PL) and time-resolved photoluminescence (TRPL) at low temperature. The PL spectra exhibit a blue-shifted emission of GaN/AlGaN QW nanostructures by decreasing the barrier width, in contrast to the arsenide system (Pabla A S et al 1993 Appl. Phys. Lett. 63 752). This behavior is attributed to a redistribution across the samples of the huge built-in electric field (several hundreds of kV cm(-1)) induced by the polarization difference between wells and barriers. The trend of the barrier width dependence of the internal polarization field is reproduced by using simple electrostatic arguments. In addition, the effect of well width variation on the optical transition and decay time of GaN multiple quantum wells (MQWs) have been investigated, and it has been shown that the screening of the piezoelectric field and the electron-hole separation are strongly dependent on the well thickness and have a profound effect on the optical properties of the GaN/AlGaN MQWs. The time-resolved PL spectra of 3 nm well MQWs reveal that the spectral peak position shifts toward lower energies as the decay time increases and becomes red-shifted at longer decay times.
48.	Esmaeili, M., et al. (author) Photoluminescence study of MOCVD-grown GaN/AlGaN MQW nanostructures : Influence of Al composition and Si doping 2007 In: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 18:2 Journal article (peer-reviewed)abstract A detailed study of low-temperature photoluminescence (PL) in GaN/AlGaN multiple quantum well (MQW) nanostructures has been reported. We have investigated the effect of Si doping and Al content on PL spectra and PL decay time of these structures. The temperature dependence of radiative as well as non-radiative lifetimes have been evaluated between 2K and room temperature for different Si doping. We found that radiative recombination at higher temperatures even up to RT is stronger in the doped sample, compared to the undoped one. Hole localization in GaN/AlGaN MQWs with different compositions of Al is demonstrated via PL transient decay times and LO phonon coupling. It is found that there is an increasing of the decay time at the PL peak emission with increasing Al composition. For the undoped sample, a non-exponential PL decay behaviour at 2K is attributed to localized exciton recombination. A slight upshift in QWs PL peak with increasing Al composition is observed, which is counteracted by the expected rise of the internal QW electric field with increasing Al. The localization energies have been evaluated by studying the variation of the QW emission versus temperature and we found out that the localization energy increases with increasing Al composition. © IOP Publishing Ltd.
49.	Gholami, M., et al. (author) Alternation of band gap and localization of excitons in InGaNAs nanostructures with low nitrogen content 2008 In: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 19:31 Journal article (peer-reviewed)abstract Continuous wave photoluminescence (cw PL) spectroscopy has been used to study the optical properties of a set of InGaNAs epilayers and single quantum wells with nitrogen concentration less than a few per cent at different temperatures and different excitation powers. We found that nitrogen has a critical role on the emission light of InGaNAs nanostructures and the recombination mechanism. The incorporation of a few per cent of nitrogen leads to shrinkage of the InGaNAs band gap. The physical origin of such band gap reduction has been investigated both experimentally and theoretically by using a band anticrossing model. We have found that localization of excitons that have been caused by incorporation of a few per cent of nitrogen in these structures is the main explanation of such anomalous behavior observed in the low-temperature photoluminescence spectra of these nanostructures. The localization energies of carriers have been evaluated by studying the variation of the quantum well (QW) emission versus temperature, and it was found that the localization energy increases with increasing nitrogen composition. Our data also show that, with increasing excitation intensity, the PL peak position moves to higher energies (blue shift) due to the filling of localized states and capture centers for excitons by photo-generated carriers.
50.	Gholami, M., et al. (author) Evaluation of optical quality and properties of Ga0.64In0.36N0.006As0.994 lattice matched to GaAs by using photoluminescence spectroscopy 2009 In: Opto-Electronics Review. - : Polish Academy of Sciences Chancellery. - 1230-3402 .- 1896-3757. ; 17:3, s. 260-264 Journal article (peer-reviewed)abstract We have investigated optical properties of Ga0.64In0.36N0.006As0.994/GaAs single quantum-well structures using photoluminescence technique. We have found that nitrogen creates potential fluctuations in the InGaNAs structures, so it is the cause of trap centres in these structures and leads to localized excitons recombination dynamics. The near-band edge PL at 2 K exhibited a blueshift with an increase in excitation intensity of a sample but there is not such a shift in the PL peak position energy of same sample at 150 K. It has been found that PL spectra have a large full width at half maximum (FWHM) value at 2 K. These results are discussed in terms of carrier localization. Additionally, our results suggest decreasing PL integrated intensity in this structure, possibly due to non-radiative recombination. It has been shown that thermal annealing reduces the local strain created by nitrogen. By annealing process, a blue shifted emission can be observed.

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