| 1. |
- Chen, Shula
(författare)
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Excitonic Effects and Energy Upconversion in Bulk and Nanostructured ZnO
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Zinc Oxide (ZnO), a II-VI wurtzite semiconductor, has been drawing enormous research interest for decades as an electronic material for numerous applications. It has a wide and direct band gap of 3.37eV and a large exciton binding energy of 60 meV that leads to intense free exciton (FX) emission at room temperature. As a result, ZnO is currently considered among the key materials for UV light emitting devices with tailored dimensionality and solid state white lighting. Full exploration of ZnO in various applications requires detailed knowledge of its fundamental and materialrelated properties, which remains incomplete. The research work summarized in this thesis addresses a selection of open issues on optical properties of ZnO based on (but not limited to) detailed time-resolved photoluminescence (PL) and magneto-optical studies of various excitonic transitions as specified below.Papers 1 and 2 analyze recombination dynamics of FX and donor bound excitons (DX) in bulk and tetrapod ZnO with the aim to evaluate contributions of radiative and nonradiative carrier recombination processes in the total carrier lifetime. We show that changes in relative contributions of these processes in “bulk” and near-surface areas are responsible for bi-exponential exciton decays typically observed in these materials. The radiative FX lifetime is found to be relatively long, i.e. >1 ns at 77 K and >14 ns at room temperature. In the case of DX, the radiative lifetime depends on exciton localization. Radiative recombination is concluded to dominate the exciton dynamics in “bulk regions” of high-quality materials. It leads to appearance of a slow component in the decays of no-phonon (NP) FX and DX lines, which also determines the dynamics of the longitudinal optical (LO) phonon-assisted and two-electronsatellite DX transitions. On the other hand, the fast component of the exciton decays is argued to be a result of surface recombination.Paper 3 evaluates exciton-phonon coupling in bulk and tetrapod ZnO. It is found that, in contrast to bulk ZnO, the NP FX emission in ZnO tetrapods is weak as compared with the LO phonon assisted transitions. We show that the observed high intensity of the FX-1LO emission does not reflect enhanced exciton-phonon coupling in nanostructured ZnO. Instead, it is a result of stronger suppression of the NP FX emission in faceted regions of the tetrapods as revealed from spatially resolved cathodoluminescence (CL) studies. This is attributed to enhanced re-absorption due to multiple internal reflections, which become especially pronounced in the vicinity of the FX resonance.Effects of exciton-photon coupling on light propagation through the ZnO media are studied in Papers 4 and 5. By employing the time-of-flight spectroscopy, in Paper 4 we demonstrate that the group velocity of laser pulses propagating through bulk ZnO can be slowed down to as low as 2044 km/s when photon energies approach the optical absorption edge of the material. The magnitude of this decrease can be manipulated by changing light polarization. In Paper 5 we show that the observed slow-down is caused by the formation of free exciton-polaritons and is determined by their dispersion. On the other hand, contributions of DX polaritons become important only in the proximity to their corresponding resonances.Excitonic effects can also be utilized to investigate fundamental properties and defect formation in ZnO. In Paper 6, we employ DX to study magneto-optical properties of the B valence band (B-VB) states as well as dynamics of inter-VB energy relaxation. We show that PL decays of the emissions involving the B-VB holes are faster than that of their counterparts involving the A-VB holes, which is interpreted as being due to energy relaxation of the holes assisted by acoustic phonons. Values of effective Landé g factors for the B-VB holes are also accurately determined. In paper 7, we uncover the origin of a new class of bound exciton lines detected within the nearband-edge region. Based on their magnetic behavior we show that these lines do not stem from DXs bound to either ionized or neutral donors but instead arise from an exciton bound to an isoelectronic center with a hole-attractive local potential.In Paper 8, DX emissions are used to monitor energy upconversion in bulk and nanorod ZnO. Based on excitation power dependent PL measurements performed with different energies of excitation photons, the physical processes responsible for the upconversion are assigned to two-photon-absorption (TPA) via virtual states and twostep TPA (TS-TPA) via real states. In the former case the observed threshold energy for the TPA process is larger than half of that for one-photon absorption across the bandgap, which can be explained by the different selection rules between the involved optical transitions. It is also concluded that the TS-TPA process occurs via a defect/impurity with an energy level lying within 1.14-1.56 eV from one of the band edges, likely a zinc vacancy.
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| 2. |
- Yao, Nannan, 1992-
(författare)
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Fill factor of organic solar cells and applications of dilute donor devices
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Organic solar cells (OSCs) have attracted great attention due to their low cost, flexibility and solution-processibility. In recent years, the development of nonfullerene acceptors (NFAs) has truly promoted the efficiency of OSCs up to 19%, implying high potential for commercial applications. However, the stateof- the-art OSCs still lag behind the Shockley-Queisser limit, besides the intrinsic losses, understanding the extrinsic losses during charge generation, transport and extraction in devices is necessary.The short-circuit current (JSC) and open-circuit voltage (VOC) can be simultaneously optimized in OSCs by tuning the energy levels of NFAs. However, less attention has been paid to the fill factor (FF), a crucial parameter for device efficiency. The FF reflects how the output photocurrent changes for a solar cell with a load from zero to infinity, indicating the charge extraction capability. In this thesis, the roles of energy offset, electric field, disorder and morphology on charge carrier dynamics as well as how these factors influence FF and energy loss are introduced. It is observed that fast and field-insensitive charge extraction is essential for high FF, which can be enabled by balanced transport and reduced bimolecular recombination. Additionally, the correlation between FF and voltage loss are studied based on four NFA systems with different highest occupied molecular orbital (HOMO) offsets. Larger HOMO offset could suppress hole back transfer from donor to acceptor and then lead to a larger FF, but it also induces more voltage loss.The morphology of the active layer governs the charge dynamics and device performance. A comparative study based on all-polymer solar cells processed from chlorobenzene (CB) and o-Xylene has been performed. Film formation process and morphology characteristics demonstrate that CB-cast films exhibit better donor/acceptor miscibility and relatively ordered structure, yielding good device performance. Contrary, in o-Xylene cast devices, electron trapping leads to a smaller FF and more non-radiative recombination.The state-of-the-art OSCs usually require comparable donor/acceptor contents in bulk-heterojunctions. Herein, NFA’s contribution to hole transport is investigated in dilute donor solar cells (10 wt% PM6:Y6). Comparable hole mobilities of PM6 diluted in Y6 and insulators (PS &PMMA) indicate that the hole transport in dilute donor solar cells is still mainly via PM6 phases, although pristine Y6 can support ambipolar transport. Furthermore, impressive performance of the dilute donor solar cells motivate us to explore semitransparent OSCs for building-integrated photovoltaics (BIPV). Decent photovoltaic performance and acceptable visible transparency have been realized in dilute donor solar cells by decreasing visible-absorption and increasing near-infrared absorption.
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| 3. |
- Zhang, Huotian
(författare)
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Loss Mechanisms In Non-Fullerene Organic Solar Cells
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Photovoltaics are one of the most important sustainable energy sources in the 21st century. Among photovoltaics, organic solar cells (OSCs) offer many advantages such as ease of processing, lightweight, the potential for flexibility, and tunable properties. Its peculiar nature and complexity present a fascinating charm, attracting many researchers. Thanks to researchers' efforts, the power conversion efficiency (PCE) of OSCs has been boosted from 1% to 19% during the last three decades. Despite the exciting PCE, some problems remain to be solved, for example, the large voltage loss and long-term stability. The aim of this thesis is to understand the fundamental physics of the state-of-the-art OSCs, especially the loss mechanism. Ultimately, properly understanding the mechanisms will sever as the basis of OSCs further improvements and commercialization. This work focuses on the loss mechanisms of OSCs, particularly the open-circuit voltage and the fill factor. The beginning of this thesis introduces basic concepts regarding semiconductors physics and donor-acceptor OSCs. This part explains how a photon is used to generate electricity and the fundamentals of organic electronics. Subsequently, the detailed balance in a solar cell is reviewed, which is the basis of voltage loss analysis. In this part, we see how the input, recombination, and output form a balance. Then, the way to determine the voltage loss is shown, and the latest understandings in reducing the loss are reviewed. The fill factor, as a measure of the quality of a solar cell, is a complex parameter, especially in OSCs.The latter part of this thesis starts from the photophysical processes in an OSC, and then relates intrinsic parameters to the fill factor. The figure of merits has been employed to express the fill factor analytically. In the end, experimental methods and basic principles for the previous analysis are introduced, including Fourier transform infrared spectroscopy, the external quantum efficiency of photovoltaics (EQEPV), spectrograph for electroluminescence or photoluminescence, transient absorption, and time-delayed collection field. Overall, the thesis combined thermal dynamics and charge dynamics to analyze voltage losses and fill factor losses. The author hopes this work can contribute to a better understanding of the loss mechanisms OSCs.
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| 4. |
- Beyer, Jan, 1980-
(författare)
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Spin Properties in InAs/GaAs Quantum Dot based Nanostructures
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Semiconductor quantum dots (QDs) are a promising building block of future spin-functional devices for applications in spintronics and quantum information processing. Essential to the realization of such devices is our ability to create a desired spin orientation of charge carriers (electrons and holes), typically via injection of spin polarized carriers from other parts of the QD structures. In this thesis, the optical orientation technique has been used to characterize spin generation, relaxation and detection in self-assembled single and multi-QD structures in the InAs/GaAs system prepared by modern molecular beam epitaxy technique.Optical generation of spin-oriented carriers in the wetting layer (WL) and GaAs barrier was carried out via circularly polarized excitation of uncorrelated electron-hole pairs from band-to-band transitions or via resonant excitation of correlated electron-hole pairs, i.e. excitons. It was shown that the generation and injection of uncorrelated electron-hole pairs is advantageous for spin-preserving injection into the QDs. The lower spin injection efficiency of excitons was attributed to an enhanced spin relaxation caused by the mutual electron-hole Coulomb exchange interaction. This correlation affects the spin injection efficiency up to elevated temperatures of around 150 K.Optical orientation at the energy of the WL light-hole (lh) exciton (XL) is accompanied by simultaneous excitation from the heavy-hole (hh) valence band at high ~k-vectors. Quantum interference of the two excitation pathways in the spectral vicinity of the XL energy resulted in occurrence of an asymmetric absorption peak, a Fano resonance. Complete quenching of spin generation efficiency at the resonance was observed and attributed to enhanced spin scattering between the hh and lh valence bands in conjunction with the Coulomb exchange interaction in the XL. This mechanism remains effective up to temperatures exceeding 100 K.In longitudinal magnetic fields up to 2 T, the spin detection efficiency in the QD ensemble was observed to increase by a factor of up to 2.5 in the investigated structures. This is due to the suppression of two spin depolarization mechanisms of the QD electron: the hyperfine interaction with the randomly oriented nuclear spins and the anisotropic exchange interaction with the hole. At higher magnetic fields, when these spin depolarization processes are quenched, only anisotropic QD structures (such as double QDs, aligned along a specific crystallographic axis) still exhibit a rather strong field dependence of the QD electron spin polarization under non-resonant excitation. Here, an increased spin relaxation in the spin injector, i.e. the WL or GaAs barrier, is suggested to lead to more efficient thermalization of the spins to the lower Zeeman-split spin state before capture to the QD.Finally, the influence of elevated temperatures on the spin properties of the QD structures was studied. The temperature dependence of dynamic nuclear polarization (DNP) of the host lattice atoms in the QDs and its effect on the QD electron spin relaxation and dephasing were investigated for temperatures up to 85 K. An increase in DNP efficiency with temperature was found, accompanied by a decrease in the extent of spin dephasing. Both effects are attributed to an accelerating electron spin relaxation, suggested to be due to phonon-assisted electronnuclear spin flip-flops driven by the hyperfine interaction. At even higher temperatures, reaching up to room temperature, a surprising, sharp rise in the QD polarization degree has been found. Experiments in a transverse magnetic field showed a rather constant QD spin lifetime, which could be governed by the spin dephasing time T*2. The observed rising in QD spin polarization degree could be likely attributed to a combined effect of shortening of trion lifetime and increasing spin injection efficiency from the WL. The latter may be caused by thermal activation of non-radiative carrier relaxation channels.
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| 5. |
- Chen, Shula
(författare)
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Excitonic effects in ZnO
- 2012
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Zinc Oxide (ZnO) is an extensively researched II-VI wide bandgap semiconductor material. As a promising material for future optoelectronic and spintronic applications, it continues to attract enormous amount of interest. Though over the past decades extensive experimental and theoretical work has been done to deepen the understanding of its fundamental material properties, there are still controversial and unexplored areas. The research work summarized in this thesis was aimed at clarifying and exploring some of these areas, as will be introduced below. One of attractive properties of ZnO is a very large binding energy of free excitons (FX), which makes excitonic effects of particular importance in this material. The excitons couple with other elementary excitations inside the material such as longitudinal optical (LO) phonons or photons. The former leads to the intense LO phonon-assisted radiative transitions, while the latter causes formation of the exciton-polariton. The exciton-phonon coupling was suggested to be enhanced in ZnO-based nano- and microstructures. This conclusion was based on the prevalence in these structures at room temperature of LO phonon-assisted FX transitions, which is in contrast with bulk ZnO photoluminescence (PL) where the no-phonon (NP) FX emission dominates. The exact mechanism for this effect, however, was not clear. In paper 1, we have clarified these issues by employing PL and cathodoluminescence (CL) measurements performed for bulk ZnO material and ZnO tetrapods. From spatially resolved CL studies, we have shown that the suppression of the NP FX emission strongly depends on structural morphology of the ZnO tetrapods and becomes most significant within areas with faceted surfaces. The effect is interpreted using a model based on re-absorption due to multiple internal reflections in the vicinity of the FX resonance. As to the exciton-photon coupling, it usually leads to formation of mixed or coupled states of excitons and photons known as exciton-polaritons. The exciton-polariton formation has been demonstrated to lead to slow-down of light in several semiconductor materials such as CdZnTe, GaN, etc. Due to the strong exciton-photon coupling in ZnO, the polariton formation may also affect light velocity in this medium. To explore this effect, we have performed timeof-flight measurement using pulsed laser light. Our studies that are summarized in paper 2 have shown that the group velocity of light in bulk ZnO could be decreased down to 2044km/s and the magnitude of this decrease depends on light polarization. The main physical mechanism responsible for this effect was singled out as being due to the formation of free exciton-polaritons that propagate coherently via ballistic transport. Based on the experimentally determined spectral dependence of the polariton group velocity, the polariton dispersion was also determined. Excitonic effects in ZnO could also be utilized to investigate fundamental properties of ZnO. For example, previous magneto-optical studies of donor bound excitons allowed to establish ordering of valence band (VB) states and also provided consistent information on the sign and g-factor of holes from the upper A-valence subband. On the other hand, properties of the higher lying B-VB subband were not fully understood. To clarify this issue, we have performed time-resolved and magneto-PL studies for the so-called I6 B and I7 B excitonic transitions which involved a hole from the B-VB subband as summarized in paper 3. From the magneto-PL measurements, values of effective g-factors for conduction band electrons and B valence band holes were determined as ge =1.91, gh =1.79 and gh =0, respectively.
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| 6. |
- Dagnelund, Daniel, 1980-
(författare)
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Magneto-optical studies of dilute nitrides and II-VI diluted magnetic semiconductor quantum structures
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis work aims at a better understanding of magneto-optical properties of dilute nitrides and II-VI diluted magnetic semiconductor quantum structures. The thesis is divided into two parts. The first part gives an introduction of the research fields, together with a brief summary of the scientific results included in the thesis. The second part consists of seven scientific articles that present the main findings of the thesis work. Below is a short summary of the thesis.Dilute nitrides have been of great scientific interest since their development in the early 1990s, because of their unusual fundamental physical properties as well as their potential for device applications. Incorporation of a small amount of N in conventional Ga(In)As or Ga(In)P semiconductors leads to dramatic modifications in both electronic and optical properties of the materials. This makes the dilute nitrides ideally suited for novel optoelectronic devices such as light emitting devices for fiber-optic communications, highly efficient visible light emitting devices, multi-junction solar cells, etc. In addition, diluted nitrides open a window for combining Si-based electronics with III-V compounds-based optoelectronics on Si wafers, promising for novel optoelectronic integrated circuits. Full exploration and optimization of this new material system in device applications requires a detailed understanding of their physical properties.Papers I and II report detailed studies of effects of post-growth rapid thermal annealing (RTA) and growth conditions (i.e. presence of N ions, N2 flow, growth temperature and In alloying) on the formation of grown-in defects in Ga(In)NP. High N2 flow and bombardment of impinging N ions on grown sample surface is found to facilitate formation of defects, such as Ga interstitial (Gai) related defects, revealed by optically detected magnetic resonance (ODMR). These defects act as competing carrier recombination centers, which efficiently decrease photoluminescence (PL) intensity. Incorporation of a small amount of In (e.g. 5.1%) in GaNP seems to play a minor role in the formation of the defects. In GaInNP with 45% of In, on the other hand, the defects were found to be abundant. Effect of RTA on the defects is found to depend on initial configurations of Gai related defects formed during the growth.In Paper III, the first identification of an interfacial defect at a heterojunction between two semiconductors (i.e. GaP/GaNP) is presented. The interface nature of the defect is clearly manifested by the observation of ODMR lines originating from only two out of four equivalent <111> orientations. Based on its resolved hyperfine interaction between an unpaired electronic spin (S=1/2) and a nuclear spin (I=1/2), the defect is concluded to involve a P atom at its core with a defect/impurity partner along a <111> direction. Defect formation is shown to be facilitated by N ion bombardment.In Paper IV, the effects of post-growth hydrogenation on the efficiency of the nonradiative (NR) recombination centers in GaNP are studied. Based on the ODMR results, incorporation of H is found to increase the efficiency of the NR recombination via defects such as Ga interstitials.In Paper V, we report on our results from a systematic study of layered structures containing an InGaNAs/GaAs quantum well, by the optically detected cyclotron resonance (ODCR) technique. By monitoring PL emissions from various layers, the predominant ODCR peak is shown to be related to electrons in GaAs/AlAs superlattices. This demonstrates the role of the SL as an escape route for the carriers confined within the InGaNAs/GaAs single quantum well.The last two papers are within a relatively new field of spintronics which utilizes not only the charge (as in conventional electronics) but also the quantum mechanical property of spin of the electron. Spintronics offers a pathway towards integration of information storage, processing and communications into a single technology. Spintronics also promises advantages over conventional charge-based electronics since spin can be manipulated on a much shorter time scale and at lower cost of energy. Success of semiconductor-based spintronics relies on our ability to inject spin polarized electrons or holes into semiconductors, spin transport with minimum loss and reliable spin detection.In Papers VI and VII, we study the efficiency and mechanism for carrier/exciton and spin injection from a diluted magnetic semiconductor (DMS) ZnMnSe quantum well into nonmagnetic CdSe quantum dots (QD’s) by means of spin-polarized magneto PL combined with tunable laser spectroscopy. By means of a detailed rate equation analysis presented in Paper VI, the injected spin polarization is deduced to be about 32%, decreasing from 100% before the injection. The observed spin loss is shown to occur during the spin injection process. In Paper VII, we present evidence that energy transfer is the dominant mechanism for carrier/exciton injection from the DMS to the QD’s. This is based on the fact that carrier/exciton injection efficiency is independent of the width of the ZnSe tunneling barrier inserted between the DMS and QD’s. In sharp contrast, spin injection efficiency is found to be largely suppressed in the structures with wide barriers, pointing towards increasing spin loss.
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| 7. |
- Filippov, Stanislav
(författare)
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Micro-photoluminescence and micro-Raman spectroscopy of novel semiconductor nanostructures
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Low-dimensional semiconductor structures, such as one-dimensional nanowires (NWs) and zerodimensional quantum dots (QDs), are materials with novel fundamental physical properties and a great potential for a wide range of nanoscale device applications. Here, especially promising are direct bandgap II-VI and III-V compounds and related alloys with a broad selection of compositions and band structures. For examples, NWs based on dilute nitride alloys, i.e. GaNAs and GaNP, provide both an optical active medium and well-shaped cavity and, therefore, can be used in a variety of advanced optoelectronic devices including intermediate band solar cells and efficient light-emitters. Self-assembled InAs QDs formed in the GaAs matrix are proposed as building blocks for entangled photon sources for quantum cryptography and quantum information processing as well as for spin light emitting devices. ZnO NWs can be utilized in a variety of applications including efficient UV lasers and gas sensors. In order to fully explore advantages of nanostructured materials, their electronic properties and lattice structure need to be comprehensively characterized and fully understood, which is not yet achieved in the case of aforementioned material systems. The research work presented this thesis addresses a selection of open issues via comprehensive optical characterization of individual nanostructures using micro-Raman ( -Raman) and micro-photoluminescence ( -PL) spectroscopies.In paper 1 we study polarization properties of individual GaNP and GaP/GaNP core/shell NWs using polarization resolved μ-PL spectroscopy. Near band-edge emission in these structures is found to be strongly polarized (up to 60% at 150K) in the orthogonal direction to the NW axis, in spite of their zinc blende (ZB) structure. This polarization response, which is unusual for ZB NWs, is attributed to the local strain in the vicinity of the N-related centers participating in the radiative recombination and to their preferential alignment along the growth direction, presumably caused by the presence of planar defects. Our findings therefore show that defect engineering via alloying with nitrogen provides an additional degree of freedom to control the polarization anisotropy of III-V nanowires, advantageous for their applications as a nanoscale source of polarized light.Structural and optical properties of novel coaxial GaAs/Ga(N)As NWs grown on Si substrates, were evaluated in papers 2-4. In paper 2 we show by using -Raman spectroscopy that, though nitrogen incorporation shortens a phonon correlation length, the GaNAs shell with [N]<0.6% has a low degree of alloy disorder and weak residual strain. Additionally, Raman scattering by the GaAs-like and GaNlike phonons is found to be enhanced when the excitation energy approaches the E+ transition energy. This effect was attributed the involvement of intermediate states that were created by N-related clusters in proximity to the E+ subband. Recombination processes in these structures were studied in paper 3 by means of μ-PL, μ-PL excitation (μ-PLE), and time-resolved PL spectroscopies. At low temperatures, the alloy disorder is found to localize photo-excited carriers leading to predominance of localized exciton (LE) transitions in the PL spectra. Some of the local fluctuations in N composition are suggested to create three-dimensional confining potentials equivalent to that for QDs, based on the observation of sharp PL lines within the LE contour. In paper 4 we show that the formation of these QD-like confinement potentials is somewhat facilitated in spatial regions of the NWs with a high density of structural defects, based on correlative spatially-resolved structural and optical studies. It is also concluded the principal axis of these QD-like local potentials is mainly oriented along the growth direction and emit light that is linearly polarized in the direction orthogonal to the NW axis. At room temperature, the PL emission is found to be dominated by recombination of free carriers/excitons and their lifetime is governed by non-radiative recombination via surface states. The surface recombination is found to become less severe upon N incorporation due to N-induced modification of the surface states, possibly due to partial surface nitridation. All these findings suggest that the GaNAs/GaAs hetero-structures with the onedimensional geometry are promising for fabrication of novel optoelectronic devices on foreign substrates (e.g. Si).Fine-structure splitting (FSS) of excitons in semiconductor nanostructures has significant implications in photon entanglement, relevant to quantum information technology and spintronics. In paper 5 we study FSS in various laterally-arranged single quantum molecular structures (QMSs), including double QDs (DQDs), quantum rings (QRs), and QD-clusters (QCs), by means of polarization resolved μ-PL spectroscopy. It is found that FSS strongly depends on the geometric arrangements of the QMSs, which can effectively tune the degree of asymmetry in the lateral confinement potential of the excitons and can reduce FSS even in a strained QD system to a limit similar to strain-free QDs.Fabrication of nanostructured ZnO-based devices involves, as a compulsory step, deposition of thin metallic layers. In paper 6 we investigate impact of metallization by Ni on structural quality of ZnO NWs by means of Raman spectroscopy. We show that Ni coating of ZnO NWs causes passivation of surface states responsible for the enhanced intensity of the A1(LO) in the bare ZnO NWs. From the resonant Raman studies, strong enhancement of the multiline Raman signal involving A1(LO) in the ZnO/Ni NWs is revealed and is attributed to the combined effects of the Fröhlich interaction and plasmonic coupling. The latter effect is also suggested to allow detection of carbon-related species absorbed at the surface of a single ZnO/Ni NW, promising for utilizing such structures as efficient nano-sized gas sensors.
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| 8. |
- Huang, Yuqing, 1990-
(författare)
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Spin generation and detection in low-dimensional semiconductors
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Semiconductor spintronics and opto-spintronics have intrigued intense attention as they promise great advance of contemporary semiconductor information technology with integrated spin functionalities. Over the last few decades, the development of growth techniques and discovery of topological band structures have led to the explosion of a wide range of low-dimensional semiconductor materials, many of which have superior properties compared to their bulk ancestors. The limited dimension of materials imposes constraint on the motion of charge carriers and causes spin interactions of various forms, which have profound influence on the spin properties that are important for various spintronic and/or opto-spintronic applications.In this context, semiconductor quantum dot (QD) structures (QDS) and 3D topological insulator (TI) have emerged as promising material systems that exhibit distinct spin properties: In QDS, carriers are restricted in all three dimensions. The 3D confinement quenches the spin-orbit interaction (SOI) mediated spin depolarization/dephasing processes and, as a consequence, leads to a prolonged spin relaxation time, which can be used for non-volatile storage or quantum bits in quantum information technology; Whereas, the surface state of 3D TI, on the contrary, has the electronic structure that is dominated by SOI such that the orientation of the electron spin is tied to its momentum. The strong SOI limits the spin relaxation time but can be utilized to generate spin polarized current that is free from backscattering. This thesis work focuses on these two prototypical materials to provide an in-depth understanding of the spin phenomena as well as to tailor their spin properties such that novel spintronic and/or opto-spintronic devices can be built on.To employ QDS for storage of spin information, first and foremost is to be able to generate and detect spin polarization effectively and efficiently. For this purpose, we have carefully inspected both the spin injection and spin detection processes in various QDS. In this thesis work, spin polarized carriers or excitons are generated via optical orientation that converts the angular momentum of the absorbed photons to the photo-generated carriers or excitons. The as-generated spin polarized carriers/excitons then need to relax their energy before getting injected to the QDS. We have found that the spin injection process is influenced by the interactions with phonons (Paper 1) and disordered environment associated with the injection path (Paper 2). In the former case, we show that the longitudinal optical (LO) phonon contributes to accelerated relaxation of the carrier/exciton energy to the QDS ground state, which preserves the spin polarization. By engineering the energy of the QDS, we can take advantage of such LO-phonon assisted process and can avoid the spin injection loss due to the commonly observed phonon bottleneck effect. In the latter case, we discover that the surrounding media of the QDS is generally disordered, distributed by potential fluctuations caused by alloying or strain randomness. Exciton injection via such localized potential undergoes spin relaxation caused by an anisotropic exchange interaction (AEI), which leads to appreciable spin injection loss at low temperatures.The AEI is also found to be responsible for the low spin detection efficiency observed in the undoped QDS reported in Paper 3. The AEI causes mixing and splitting of exciton spin states, which leads to not only a low PL polarization degree of the QDS but also a serious issue in generation of entangled photon pairs utilizing QDS. We show that the aforementioned spin injection (Paper 2) and spin detection (Paper 3) loss associated with the AEI can be effectively tuned in the QDS by the arrangement of the constituting QDs. The effect originates from the modification of the strain and shape anisotropy both inside and outside the QDS due to the collective interaction with the neighboring QDs, which introduces a new degree of freedom in electronic-structure engineering of the QDS.In the doped QDS, we have found that the spin detection efficiency can additionally be affected by the exciton charge states and a hyperfine interaction (HFI) with the nuclear-spin bath. In Paper 4, we discover a dynamic charging process that the charged states of an InGaAs QD ensemble are altered with different excitation power densities and excitation photon energies. The charging effect leads to an anomalous spectral dependence of PL polarization such that the copolarized emission can be dynamically converted to the counter-polarized one. This finding thus calls for caution in the correlation between the optical and spin polarization in QDS with a complex charging environment. The effect of the HFI depends on the condition of nuclear spin polarization. In QDS with an unpolarized nuclear-spin bath, the HFI is a primary electron/exciton spin depolarization/dephasing source in QDS at low temperatures. In Paper 5, we show that the ensemble spin dephasing time of QDs at a cryogenic temperature correlates with the averaged size of QDs. The behavior can be accounted for by electron spin dephasing in a fluctuating nuclear field, which is experimentally verified for the first time. The results thus highlight the important role of the HFI in the electron spin dephasing in the QDs. On the other hand, finite nuclear spin polarization can be achieved through the dynamic nuclear polarization (DNP) process that transfers the angular momentum from the spin-polarized electron to nuclei. DNP is recognized to be important for spintronics and quantum information in nuclear spin-rich nanostructures. This is not only because of its role in suppressing the aforementioned electron spin dephasing, but also because it is behind the idea of exploring the long coherent nuclear spin as a quantum computing qubit. In Paper 6, we have investigated the effect of DNP in a series of QDS, where the strength and orientation of the nuclear field resulted from the DNP are identified and measured. We find that the DNP is built along a tilted axis that deviates from the commonly observed orientation along the QD growth axis and the nuclear field develops a substantial transverse component. This anomalous behavior of the DNP is found to arise from the nuclear quadrupole interaction with an oblique principal axis. The resulting tilting nuclear field can further lead to dephasing and depolarization of the electron spin that has previously been overlooked. The results uncover the detrimental effect rooted in the complex electrostatic environment of the nuclei inside the QDS and call for special care of the strain and alloying engineering of the nanostructures.In the case of 3D TI, we aim at providing both the experimental and theoretical understanding of the surface spin photocurrent as well as innovations in future opto-spintronic applications utilizing the semiconductor-TI interface. As has been shown earlier, the circular polarized excitation light creates a spin photocurrent that is resistant to moderate scattering. In Paper 7, we present detailed studies of the dependence of the spin photocurrent on the incident angle of the excitation light in a prototypical 3D TI, Bi2Te3. We point out that the spin photocurrent, as a result of spin-selective optical transitions, is associated with both the in-plane and out-of-plane spin texture of the topological surface states. We focus on the contribution of the out-of-plane spin texture, which is less explored, and demonstrate, for the first time, spin injection from a conventional semiconductor, GaAs, to a 3D TI. In favor of this hybrid system, we show that the spin photocurrent contributed by the spin injection exceeds that from the TI alone and the magnitude and direction of the current can be controlled by applying a transverse magnetic field. In Paper 8, we give a tight-binding description of the microscopic origin of the spin photocurrent in Bi2Te3, where we have provided theoretical calculations of the spin photocurrent as a function of the excitation incidence angle, Fermi energy and different scattering potentials. The results explain the observation of the out-of-plane spin texture contribution reported in Paper 7, which should have been forbidden by symmetry, and provide a pathway for opto-spintronic applications based on a TI-semiconductor hybrid system.
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| 9. |
- Puttisong, Yuttapoom, 1984-
(författare)
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Room-temperature defect-engineered spin functionalities in Ga(In)NAs alloys
- 2014
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Semiconductor spintronics is one of the most interesting research fields that exploits both charge and spin properties for future photonics and electronic devices. Among many challenges of using spin in semiconductors, efficient generation of electron spin polarization at room temperature (RT) remains difficult. Recently, a new approach using defect-mediated spin filtering effect, employing -interstitial defects in Ga(In)NAs alloys, has been shown to turn the material into an efficient spin-polarized source capable of generating >40% conduction electron spin polarization at RT without an application of external fields. In order to fully explore the defectengineered spin functionalities, a better understanding and control of the spin filtering effects is required. This thesis work thus aims to advance our understanding, in terms of both physical and material insights, of the recently discovered spin filtering defects in Ga(In)NAs alloys. We have focused on the important issues of optimization and applications of the spin filtering effects.To improve spin filtering efficiency, important material and defect parameters must be addressed. Therefore, in Papers I–III formation of the defects in Ga(In)NAs alloys has been examined under different growth and post-growth treatment conditions, as well as in different structures. We found that the defects were the dominant and important nonradiative recombination centers in Ga(In)NAs epilayers and GaNAs/GaAs multiple quantum wells, independent of growth conditions and post-growth annealing. However, by varying growth and post-growth conditions, up to four configurations of the defects, exhibiting different hyperfine interaction (HFI) strengths between defect electron and nuclear (e-n) spins, have been found. This difference was attributed to different interstitial sites and/or complexes of . Further studiesfocused on the effect of post-growth hydrogen (H) irradiation on the spin filtering effect. Beside the roles of H passivation of N resulting in bandgap reopening of the alloys, H treatment was shown to lead to complete quenching of the spin filtering effect, accompanied by strong suppression in the concentrations of the defects. We concluded that the observed effect was due to the passivation of the defects by H, most probably due to the formation of H- complexes.Optimizing spin filtering efficiency also requires detailed knowledge of spin interactions at the defect centers. This issue was addressed in Papers IV and V. From both experimental and theoretical studies, we were able to conclude that the HFI between e-n spins at the defects led to e-n spin mixing, which degraded spin filtering efficiency at zero field. Moreover, we have identified the microscopic origin of electron spin relaxation (T1) at the defect centers, that is, hyperfine-induced e-n spin cross-relaxation. Our finding thus provided a guideline to improve spin filtering efficiency by selectively incorporating the defects with weak HFI by optimizing growth and post-growth treatment conditions, or by searching for new spin filtering defect centers containing zero nuclear spin.The implementation of the defect-engineered spin filtering effect has been addressed in Papers VI–VIII. First, we experimentally demonstrated for the first time at RT an efficient electron spin amplifier employing the defects in Ga(In)NAs alloys, capable of amplifying a weak spin signal up to 27 times with a high cut-off frequency of 1 GHz. We further showed that the defectmediated spin amplification effect could turn the GaNAs alloy into an efficient RT optical spin detector. This enabled us to reliably conduct in-depth spin injection studies across a semiconductor heterointerface at RT. We found a strong reduction of electron spin polarization after optical spin injection from a GaAs layer into an adjacent GaNAs layer. This observation was attributed to severe spin loss across the heterointerface due to structural inversion asymmetry and probably also interfacial point defects.Finally, we went beyond the generation of strongly polarized electron spins. In Paper IX we focused on an interesting aspect of using strongly polarized electron spins to induce strong nuclear spin polarization at RT, relevant to solid-state quantum computation using a defect nuclear spin of long spin memory as a quantum bit (qubit). By combining the spin filtering effect and the HFI, we obtained a sizeable nuclear spin polarization of ~15% at RT that could be sensed by conduction electrons. This demonstrated the feasibility of controlling defect nuclear spins via conduction electrons even at RT, the first case ever being demonstrated in a semiconductor.
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| 10. |
- Puttisong, Yuttapoom
(författare)
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Spin-dependent recombination in Ga(In)NAs alloys
- 2012
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The abilities to control and manipulate electron spin, especially in semiconductors, lead to many interesting proposals for spin-functional devices in future spintronics and quantum information technology. A key requirement for the success of these proposals is that the spin functionality should be operational at room temperature (RT), which remains as a great challenge. Very recently, spin-dependent recombination (SDR) via paramagnetic defects that dominate in carrier recombination, i.e Gai - interstitial defects in Ga(In)NAs alloys, has been shown to turn the material into a highly efficient defectengineered spin filter operating at RT and without requiring an external applied field. This finding has demonstrated the great potential of such a spin filter as an efficient spin source, which is capable of generating up to 90% electron spin polarization at RT. Essential to realization of this attractive application in spintronics is a fundamental understanding of this alloy system and their related spin filtering defects. Therefore, factors controlling this spin filter must be studied, understood and optimized. In this licentiate thesis, we aim at optimization of the spin filtering effect in Ga(In)NAs alloys and the related quantum structures by studying influence of material fabrication techniques, post growth treatments and material structures. In paper I, we employed the optically detected magnetic resonance (ODMR) technique to study formation of Ga interstitial-related defects in Ga(In)NAs alloys. We showed that these spin-filtering defects are common grown-in defects in these alloys, independent of the employed fabrication techniques and post-growth annealing treatment. The defect formation was suggested to be thermodynamically favorable in the presence of nitrogen, possibly because of local strain compensation. In paper II, we further investigated the role of post-growth hydrogen treatment in the spin filtering effect in GaNAs epilayers and GaNAs/GaAs multiple quantum wells (QWs). From optical orientation studies, we found that the hydrogen treatment has led to nearly complete quenching of the spin filtering effect. Together with a detailed ODMR study and a rate equation analysis, the observed effect of hydrogen was attributed to hydrogen passivation of the spin filtering defects, likely by formation of complexes between the Gai-interstitial defects and hydrogen. This finding also ruled out the possibility of hydrogen as a part of the spin filtering defects in the as-grown materials, though hydrogen is known to be commonly present during the growth process. In Paper III, we examined the effectiveness of the spin filtering effect in the GaNAs/GaAs QWs as a function of QW width. Even with rather narrow QW widths of 3-9 nm, the spin filtering effect was shown to be efficient. It was further revealed that the spin filtering effect is more efficient in the wider QWs. From studies of transient behavior of photoluminescence and ODMR, it was concluded that this was mainly due to an increase in the sheet concentration of the spin filtering defects.
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| 11. |
- Filippov, Stanislav
(författare)
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Optical properties of novel semiconductor nanostructures
- 2014
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Semiconductor nanostructures, such as one-dimensional nanowires (NWs) and zerodimensional quantum dots (QDs), have recently gained increasing interest due to their unique physical properties that are found attractive for a wide variety of applications ranging from gas sensing and spintronics to optoelectronics and photonics. Here, especially promising are nanostructures based on compound semiconductors, including ZnO, GaNP and GaAs/InAs. For examples, ZnO NWs are used for gas sensing. They also serve as an active material in UV light sources, owing to its wide band gap combined with a large exciton binding energy. GaNP NWs are a novel material system that allows realization of efficient amber lightemitting diodes and novel intermediate-band solar cells with an anticipated high efficiency. InAs QDs formed in the GaAs matrix are efficient emitters of near IR light and can be utilized in future spin-functional devices for applications in spintronics and quantum information processing. The realization of the full potential of semiconductor nanostructures requires detailed knowledge of their electronic and structural properties which is far from being complete at the present stage of research. In this thesis we address some of these important issues using optical characterization techniques, such as micro-Raman and microphotoluminescence (μ-PL) spectroscopies.In paper I we use Raman spectroscopy to investigate effects of metallization by nickel on electronic and structural properties of ZnO/Ni core/shell NWs. We show that coating ZnO NWs with Ni shells causes passivation of surface states whereas subsequent annealing leads to formation of new defects, evident from appearance of the corresponding local vibrational modes. Ni coating is also found to strongly enhance the multiline Raman signal involving A1(LO) phonon scattering, based on the performed resonant Raman studies. This is attributed to an enhanced Fröhlich interaction at the ZnO/Ni heterointerface combined with coupling of the scattered light with local surface plasmons excited in the Ni shell. The latter effect is also suggested to allow detection of carbon-related species absorbed at the surface of a single ZnO/Ni NW, promising for utilizing such structures as efficient nano-sized gas sensors.In paper II we study polarization properties of GaNP nanowires and related axial structures. By employing polarization resolved μ-PL spectroscopy performed on a single NW, we show that alloying with nitrogen allows one to achieve strong orthogonal polarization of light emission even in zinc-blende nanowires of various diameters and that the polarization anisotropy can be retained up to room temperature. This polarization response, which is unusual for zinc blende NWs, is attributed to the local strain in the vicinity of the N-related centers participating in the radiative recombination and to the preferential alignment of their principal axis along the growth direction. Our findings therefore show that defect engineering via alloying with nitrogen provides an additional degree of freedom to control the polarization anisotropy of III-V nanowires, advantageous for their applications as nanoscale emitters of polarized light.In paper III we investigate exciton fine-structure splitting (FSS) in self-organized InGaAs/GaAs nanostructures including laterally-aligned double quantum dots (DQDs), quantum-dot clusters (QCs) and quantum rings (QRs), by employing polarization resolved μ-PL spectroscopy. We find a clear trend in FSS between the studied nanostructures depending on their geometric arrangements, from a large FSS in the DQDs to a smaller FSS in the QCs and QRs with an overall higher geometric symmetry. This trend is accompanied by a corresponding difference in the polarization directions of the excitonic emissions between these nanostructures, namely, the bright-exciton lines are linearly polarized along or perpendicular to a specific crystallographic axis in the DQDs structure that also defines the alignment of the two QDs, whereas in the QCs and QRs the polarization directions are randomly oriented. We attribute these trends to the interplay between intrinsic effects, such as a statistic shape deviation, atomistic randomness and strain-induced piezoelectricity. Our work demonstrates that FSS can be effectively controlled by geometric engineering of the nanostructures, capable of reducing FSS to the limit similar to strain-free QDs and thus providing a new pathway in fabricating high-symmetry quantum emitters desirable for realizing photon entanglement and spintronic devices based on such nanostructures.
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| 12. |
- Jansson, Mattias, 1989-
(författare)
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Magnetooptical properties of dilute nitride nanowires
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Nanostructured III-V semiconductors have emerged as one of the most promising materials systems for future optoelectronic applications. While planar III-V compounds are already at the center of the ongoing lighting revolution, where older light sources are replaced by modern white light LEDs, fabricating such materials in novel architectures, such as nanowires and quantum dots, creates new possibilities for optoelectronic applications. Not only do nanoscale structures allow the optically active III-V materials to be integrated with silicon microelectronics, but they also give rise to new fascinating properties inherent to the nanoscale.One of the key parameters considered when selecting materials for applications in light-emitting and photovoltaic devices is the band gap energy. While alloying of conventional III-V materials provides a certain degree of band gap tunability, a significantly enhanced possibility of band gap engineering is offered by so-called dilute nitrides, where incorporation of a small percentage of nitrogen into III-V compounds causes a dramatic down-shift of the conduction band edge. In addition, nitrogen-induced splitting of the conduction band in dilute nitrides can be utilized in intermediate band solar cells, belonging to the next generation of photovoltaic devices.For any material to be viable for optoelectronic applications, detailed knowledge of the electronic structure of the material, as well as a good understanding of carrier recombination processes is vital. For example, alloying may not only cause changes in the electronic structure but can also induce disorder. Disorder-induced potential fluctuations may alter charge carrier and exciton dynamics, and may even induce quantum confinement. Moreover, various defects in the material may introduce detrimental non-radiative (NR) states in the band gap deteriorating radiative efficiency. It is evident that, due to their different growth mechanisms, such properties could be markedly different in nanowires as compared to their planar counterparts. In this thesis, I aim to describe the electronic structure of dilute nitride nanowires, and its effects on the optical properties. Firstly, we investigate the electronic structure, and the structural and optical properties of novel GaNAsP nanowires, with a particular focus on the dominant recombination channels in the material. Secondly, we show how short-range fluctuations in the nitrogen content lead to the formation of quantum dots in dilute nitride nanowires, and investigate their electronic structure. Finally, we investigate the combined charge carrier and exciton dynamics of the quantum dots and effects of defects in their surroundings.Before considering individual sources of NR recombination, it is instructive to investigate the overall effects of nitrogen incorporation on the structural properties of the nanowires. In Paper I, we show that nitrogen incorporation up to 0.16\% in Ga(N)AsP nanowires does not affect the overall structural quality of the material, nor does nitrogen degrade the good compositional uniformity of the nanowires. It is evident from our studies, however, that nitrogen incorporation has a strong and complex effect on recombination processes. We first show that nitrogen incorporation in GaNAsP nanowires reduces the NR recombination at room temperature as compared to the nitrogen-free nanowires (Paper I). This is in stark contrast to dilute nitride epilayers, where nitrogen incorporation enhances NR recombination. The reason for this difference is that in nanowires the surface recombination, rather than recombination via point defects, is the dominant NR recombination mechanism. We suggest that the nitrogen-induced suppression of the NR surface recombination in the nanowires is due to nitridation of the nanowire surface.Another NR recombination channel common in III-V nanowires is caused by the presence of structural defects, such as rotational twin planes and stacking faults. Interestingly, while nitrogen incorporation does not appear to affect the density of such structural defects, increasing nitrogen incorporation reduces the NR recombination via the structural defects (Paper II). This is explained by competing trapping of excited carriers/excitons to the localized states characteristic to dilute nitrides, and at nitrogen-induced NR defects. This effect is, however, only present at cryogenic temperatures, while at room temperature the NR recombination via the structural defects is not the dominant recombination channel.Importance of point defects in carrier recombination is highlighted in Paper III. Using the optically detected magnetic resonance technique, we show that gallium vacancies (VGa) that are formed within the nanowire volume act as efficient NR recombination centers, degrading optical efficiency of the Ga(N)AsP-based nanowires. Interestingly, while the defect formation is promoted by nitrogen incorporation, it is also readily present in ternary GaAsP nanowires. This contrasts with previous studies on planar structures, where VGa was not formed in the absence of nitrogen, unless subjected to irradiation by high-energy particles or heavy n-type doping. This, again, highlights how the defect formation is strikingly different in nanowires as compared to planar structures, likely due to the different growth processes.Potential fluctuations in the conduction band, caused by non-uniformity of the nitrogen incorporation, is characteristic to dilute nitrides and is known to cause exciton/carrier localization. We find that in dilute nitride nanowires, such fluctuations at the short range cause three-dimensional quantum confinement of excitons, resulting in optically active quantum dots with spectrally ultranarrow and highly polarized emission lines (Paper IV). A careful investigation of such quantum dots reveals that their properties are strongly dependent on the host material (Papers V, VI). While the principal quantization axis of the quantum dots formed in the ternary GaNAs nanowires is preferably oriented along the nanowire axis (Paper V), it switches to the direction perpendicular to the nanowire axis in the quaternary GaNAsP nanowires (Paper VI). Another aspect illustrating the influence of the host material on the quantum-dot properties is the electronic character of the captured hole. In both alloys, we show coexistence of quantum dots where the captured holes are of either a pure heavy-hole character or a mixed light-hole and heavy-hole character. In the GaNAs quantum dots, the main cause of the light- and heavy-hole splitting is uniaxial tensile strain induced by a combination of lattice mismatch with the nanowire core and local alloy fluctuations (Paper V). In the GaNAsP quantum dots, however, we suggest that the main mechanism for the light- and heavy-hole splitting is local fluctuations in the P/As ratio (Paper VI).Using time correlation single-photon counting, we show that the quantum dots in these dilute nitride nanowires behave as single photon emitters (Paper VI), confirming the three-dimensional quantum confinement of the emitters. Finally, since the quantum dots are formed by fluctuations mainly in the conduction band, only electrons are preferentially captured in the 0D confinement potential, whereas holes are expected to be mainly localized through the Coulomb interaction once an electron is captured by the quantum dot. In Paper VII, we investigate this rather peculiar capture mechanism, which we show to lead to unipolar, negative charging of the quantum dot. Moreover, we demonstrate that carrier capture by some quantum dots is strongly affected by the presence of defects in their local surroundings, which further alters the charge state of the quantum dot, where formation of the negatively charged exciton is promoted at the expense of its neutral counterpart. This underlines that the local surroundings of the quantum dots may greatly affect their properties and illustrates a possible way to exploit the defects for charge engineering of the quantum dots.In summary, in this thesis work, we identify several important non-radiative recombination processes in dilute nitride nanowires that can undermine the potential of these novel nanostructures for future optoelectronic applications. The gained knowledge could be found useful for designing strategies to mitigate these harmful processes, thereby improving the efficiency of future light-emitting and photovoltaic devices based on these nanowires. Furthermore, we uncover a set of optically bright quantum dot single-photon emitters embedded in the dilute nitride nanowires, and reveal their unusual electronic structure with strikingly different confinement potentials between electrons and holes. Our findings open a new pathway for charge engineering of the quantum dots in nanowires, attractive for applications in e.g. quantum computation and optical switching.
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| 13. |
- Ji, Fuxiang, 1991-, et al.
(författare)
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Remarkable Thermochromism in the Double Perovskite Cs2NaFeCl6
- 2023
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Ingår i: Advanced Optical Materials. - : Wiley-Blackwell. - 2162-7568 .- 2195-1071.
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Tidskriftsartikel (refereegranskat)abstract
- Lead-free halide double perovskites (HDPs) have emerged as a new generation of thermochromic materials. However, further materials development and mechanistic understanding are required. Here, a highly stable HDP Cs2NaFeCl6 single crystal is synthesized, and its remarkable and fully reversible thermochromism with a wide color variation from light-yellow to black over a temperature range of 10 to 423 K is investigated. First-principles, density functional theory (DFT)-based calculations indicate that the thermochromism in Cs2NaFeCl6 is an effect of electron–phonon coupling. The temperature sensitivity of the bandgap in Cs2NaFeCl6 is up to 2.52 meVK−1 based on the Varshni equation, which is significantly higher than that of lead halide perovskites and many conventional group-IV, III–V semiconductors. Meanwhile, this material shows excellent environmental, thermal, and thermochromic cycle stability. This work provides valuable insights into HDPs' thermochromism and sheds new light on developing efficient thermochromic materials.
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| 14. |
- Ji, Fuxiang, 1991-, et al.
(författare)
-
Remarkable Thermochromism in the Double Perovskite Cs2NaFeCl6
- 2024
-
Ingår i: Advanced Optical Materials. - : John Wiley & Sons. - 2162-7568 .- 2195-1071. ; 12:8
-
Tidskriftsartikel (refereegranskat)abstract
- Lead-free halide double perovskites (HDPs) have emerged as a new generation of thermochromic materials. However, further materials development and mechanistic understanding are required. Here, a highly stable HDP Cs2NaFeCl6 single crystal is synthesized, and its remarkable and fully reversible thermochromism with a wide color variation from light-yellow to black over a temperature range of 10 to 423 K is investigated. First-principles, density functional theory (DFT)-based calculations indicate that the thermochromism in Cs2NaFeCl6 is an effect of electron-phonon coupling. The temperature sensitivity of the bandgap in Cs2NaFeCl6 is up to 2.52 meVK(-1) based on the Varshni equation, which is significantly higher than that of lead halide perovskites and many conventional group-IV, III-V semiconductors. Meanwhile, this material shows excellent environmental, thermal, and thermochromic cycle stability. This work provides valuable insights into HDPs' thermochromism and sheds new light on developing efficient thermochromic materials.
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| 15. |
- Ji, Fuxiang, et al.
(författare)
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Amine Gas-Induced Reversible Optical Bleaching of Bismuth-Based Lead-Free Perovskite Thin Films
- 2024
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Ingår i: Advanced Science. - : Wiley-VCH Verlagsgesellschaft. - 2198-3844. ; 11:4
-
Tidskriftsartikel (refereegranskat)abstract
- Reversible optical property changes in lead-free perovskites have recently received great interest due to their potential applications in smart windows, sensors, data encryption, and various on-demand devices. However, it is challenging to achieve remarkable color changes in their thin films. Here, methylamine gas (CH3NH2, MA0) induced switchable optical bleaching of bismuth (Bi)-based perovskite films is demonstrated for the first time. By exposure to an MA0 atmosphere, the color of Cs2AgBiBr6 (CABB) films changes from yellow to transparent, and the color of Cs3Bi2I9 (CBI) films changes from dark red to transparent. More interestingly, the underlying reason is found to be the interactions between MA0 and Bi3+ with the formation of an amorphous liquefied transparent intermediate phase, which is different from that of lead-based perovskite systems. Moreover, the generality of this approach is demonstrated with other amine gases, including ethylamine (C2H5NH2, EA0) and butylamine (CH3(CH2)3NH2, BA0), and another compound, Cs3Sb2I9, by observing a similar reversible optical bleaching phenomenon. The potential for the application of CABB and CBI films in switchable smart windows is investigated. This study provides valuable insights into the interactions between amine gases and lead-free perovskites, opening up new possibilities for high-efficiency optoelectronic and stimuli-responsive applications of these emerging Bi-based materials.
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| 16. |
- Ji, Fuxiang, et al.
(författare)
-
Amine Gas‐Induced Reversible Optical Bleaching of Bismuth‐Based Lead‐Free Perovskite Thin Films
- 2023
-
Ingår i: Advanced Science. - : WILEY. - 2198-3844. ; 11:4
-
Tidskriftsartikel (refereegranskat)abstract
- Reversible optical property changes in lead-free perovskites have recently received great interest due to their potential applications in smart windows, sensors, data encryption, and various on-demand devices. However, it is challenging to achieve remarkable color changes in their thin films. Here, methylamine gas (CH3NH2, MA0) induced switchable optical bleaching of bismuth (Bi)-based perovskite films is demonstrated for the first time. By exposure to an MA0 atmosphere, the color of Cs2AgBiBr6 (CABB) films changes from yellow to transparent, and the color of Cs3Bi2I9 (CBI) films changes from dark red to transparent. More interestingly, the underlying reason is found to be the interactions between MA0 and Bi3+ with the formation of an amorphous liquefied transparent intermediate phase, which is different from that of lead-based perovskite systems. Moreover, the generality of this approach is demonstrated with other amine gases, including ethylamine (C2H5NH2, EA0) and butylamine (CH3(CH2)3NH2, BA0), and another compound, Cs3Sb2I9, by observing a similar reversible optical bleaching phenomenon. The potential for the application of CABB and CBI films in switchable smart windows is investigated. This study provides valuable insights into the interactions between amine gases and lead-free perovskites, opening up new possibilities for high-efficiency optoelectronic and stimuli-responsive applications of these emerging Bi-based materials.
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| 17. |
- Mopoung, Kunpot, et al.
(författare)
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Understanding Antiferromagnetic Coupling in Lead-Free Halide Double Perovskite Semiconductors
- 2024
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Ingår i: The Journal of Physical Chemistry C. - : AMER CHEMICAL SOC. - 1932-7447 .- 1932-7455. ; 128:12, s. 5313-5320
-
Tidskriftsartikel (refereegranskat)abstract
- Solution-processable semiconductors with antiferromagnetic (AFM) order are attractive for future spintronics and information storage technology. Halide perovskites containing magnetic ions have emerged as multifunctional materials, demonstrating a cross-link between structural, optical, electrical, and magnetic properties. However, stable optoelectronic halide perovskites that are antiferromagnetic remain sparse, and the critical design rules to optimize magnetic coupling still must be developed. Here, we combine the complementary magnetometry and electron-spin-resonance experiments, together with first-principles calculations to study the antiferromagnetic coupling in stable Cs-2(Ag:Na)FeCl6 bulk semiconductor alloys grown by the hydrothermal method. We show the importance of nonmagnetic monovalence ions at the B-I site (Na/Ag) in facilitating the superexchange interaction via orbital hybridization, offering the tunability of the Curie-Weiss parameters between -27 and -210 K, with a potential to promote magnetic frustration via alloying the nonmagnetic B-I site (Ag:Na ratio). Combining our experimental evidence with first-principles calculations, we draw a cohesive picture of the material design for B-site-ordered antiferromagnetic halide double perovskites.
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| 18. |
- Xu, Kai, et al.
(författare)
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On the Origin of Seebeck Coefficient Inversion in Highly Doped Conducting Polymers
- 2022
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Ingår i: Advanced Functional Materials. - : Wiley-V C H Verlag GMBH. - 1616-301X .- 1616-3028. ; 32:20
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Tidskriftsartikel (refereegranskat)abstract
- A common way of determining the majority charge carriers of pristine and doped semiconducting polymers is to measure the sign of the Seebeck coefficient. However, a polarity change of the Seebeck coefficient has recently been observed to occur in highly doped polymers. Here, it is shown that the Seebeck coefficient inversion is the result of the density of states filling and opening of a hard Coulomb gap around the Fermi energy at high doping levels. Electrochemical n-doping is used to induce high carrier density (>1 charge/monomer) in the model system poly(benzimidazobenzophenanthroline) (BBL). By combining conductivity and Seebeck coefficient measurements with in situ electron paramagnetic resonance, UV-vis-NIR, Raman spectroelectrochemistry, density functional theory calculations, and kinetic Monte Carlo simulations, the formation of multiply charged species and the opening of a hard Coulomb gap in the density of states, which is responsible for the Seebeck coefficient inversion and drop in electrical conductivity, are uncovered. The findings provide a simple picture that clarifies the roles of energetic disorder and Coulomb interactions in highly doped polymers and have implications for the molecular design of next-generation conjugated polymers.
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