Andrei Schliwa - Academia.edu (original) (raw)
Papers by Andrei Schliwa
Physical Review B
Two-photon transition rates are investigated in resonance to the ground state in wurtzite GaN/AlN... more Two-photon transition rates are investigated in resonance to the ground state in wurtzite GaN/AlN quantum dots. The ground state transition is two-photon allowed because of the electronhole separation inherent to polar wurtzite III-nitride heterostructures. We show that this built-in parity breaking mechanism can allow deterministic triggering of single-photon emission via coherent two-photon excitation. Radiative lifetimes obtained for single-photon relaxation are in good agreement with available time-resolved micro-photoluminescence experiments, indicating the reliability of the employed computational framework based on 8-band k •p-wavefunctions. Two-photon singly-induced emission is explored in terms of possible cavity and non-degeneracy enhancement of two-photon processes.
arXiv: Mesoscale and Nanoscale Physics, 2019
We investigate room temperature lasing of terahertz quantum cascade lasers using quantum dot chai... more We investigate room temperature lasing of terahertz quantum cascade lasers using quantum dot chains as active material. Bandstructure calculations for such extended systems of coupled quantum dots are made possible by a novel "linear combination of atomic orbitals"-like approximation, based on single quantum dot wavefunctions. Our results demonstrate that terahertz quantum cascade lasers greatly benefit from intrinsic properties of quantum dots, such as reduced phonon coupling and in-plane scattering, enabling room temperature lasing with significantly reduced threshold current densities.
Contents Part 1. Electronic Structure Calculations Chapter 1. Introduction Chapter 2. Method of c... more Contents Part 1. Electronic Structure Calculations Chapter 1. Introduction Chapter 2. Method of calculation 2.1. Calculation of strain 2.2. Piezoelectricity and the reduction of lateral symmetry 2.3. Single Particle States 2.4. Many-Particle States 2.5. Optical Properties Part 2. InGaAs/GaAs Quantum Dots Chapter 3. Impact of Size, Shape and Composition on Piezoelectric Effects and Single-Particle States 3.1. The Investigated structures: Variation of size, shape and compostion 3.2. The Impact of the piezoelectric field 3.3. The vertical and lateral aspect ratio 3.4. Varying composition profiles 3.5. Conclusions Chapter 4. Few-particle Energies versus Geometry and Composition 4.1. Interrelation of QD-structure, strain and piezoelectricity, and Coulomb interaction 4.2. The Impact of QD size (series A and H) 4.3. The aspect ratio 4.4. Different composition profiles 4.5. Correlation vs. QD size, shape and particle type 4.6. Conclusions Chapter 5. Multimodal QD-size distribution: Theory and Experiment 5.1. Sample growth 5.2. Determination of QD-morphology and the spectrum of excited states 5.3. Predicted absorption spectra of truncated pyramidal InAs/GaAs QDs 5.4. Single-dot spectra obtained from cathodoluminescence spectroscopy 5.5. Results and Interpretation 5.6. Conclusion Chapter 6. Stacked quantum dots 6.1. Energetics of QD stacks 6.2. Role of strain and piezoelectricity 6.3. Strength of electronic coupling in pairs of identical QDs 6.4. Small perturbations of the size homogeneity 6.5. Asymmetric QD molecules: Coupling of different electronic shells 6.6. Tailoring the TE-TM ratio in semiconductor optical amplifiers 6.7. Conclusions 6 CONTENTS Part 3. Other Material Systems Chapter 7. Electronic and optical properties of InAs/InP quantum dots on InP(100) and InP(311)B substrates 7.1. Choice of model QDs 7.2. Absorption spectra for InAs/InP QDs 7.3. Impact of substrate orientation on the QDs optical properties 7.4. Conclusions Chapter 8. Inverted GaAs/Al x Ga 1−x As Quantum Dots 8.1. Choice of the model QDs 8.2. Influence of interface intermixing on the optical properties 8.3. External magnetic fields 8.4. Discussion 8.
Physical Review B, 2021
We investigate the emission directionality of electronic intraband (intersubband) transitions in ... more We investigate the emission directionality of electronic intraband (intersubband) transitions in stacked coupled quantum dots. Using a well-established eight-band k • p method, we demonstrate that the minor contributions from the valence band mixing into the conduction band govern the polarization and emission directionality of electronic p-to-s-type intraband transitions. Despite that the contribution from the central-cell part to the momentum matrix element is dominant, we find, the contribution from the matrix elements among the envelope functions cannot be neglected. With the help of an artificial cuboidal quantum dot, we show that the vertical emission from intraband transitions can be tuned via the emitter's vertical aspect ratio. Subsequently, we show that these results can be transferred to more realistic geometries of quantum dots and quantum dot stacks and demonstrate that the vertical emission can be enhanced from 23 % to 46 % by increasing the emitter's vertical aspect ratio to the isotropic case with a vertical aspect ratio of 1.0. Therefore, a stacking of a few quantum dotss (∼4 for the investigated structures) is already sufficient to improve the vertical radiation significantly. Additionally, we discuss the impact of the number of stacked QDs as well as the effect of the interdot coupling strength on the radiation properties.
Physical Review B, 2021
We investigate room temperature lasing of terahertz quantum cascade lasers using quantum dot chai... more We investigate room temperature lasing of terahertz quantum cascade lasers using quantum dot chains as active material suitable for wireless communication and imaging technologies. Bandstructure calculations for such extended systems of coupled quantum dots are made possible by a novel 'linear combination of quantum dot orbitals'-method, based on single quantum dot wavefunctions. Our results demonstrate strong vertical-emission of coupled quantum dots, reduced phonon coupling and in-plane scattering, enabling room-temperature lasing with significantly reduced threshold current densities.
Optical Materials Express, 2019
Spectrally-tunable quantum-light sources are key elements for the realization of long-distance qu... more Spectrally-tunable quantum-light sources are key elements for the realization of long-distance quantum communication. A deterministically fabricated single-photon source with a photonextraction efficiency of = (20 ± 2) % and a tuning range of Δ = 2.5 meV is presented here. The device consists of a single pre-selected quantum dot monolithically integrated into a microlens which is bonded onto a piezoelectric actuator via thermocompression goldbonding. The thin gold layer simultaneously acts as a backside mirror for the quantum dot emission, which is efficiently extracted from the device by an optimized lens structure patterned via 3D in-situ electron-beam lithography. The single-photon nature of the emission is proven by photon-autocorrelation measurements with (2) (= 0) = 0.04 ± 0.02. The combination of deterministic fabrication, spectral-tunability and high broadband photon-extraction efficiency makes the microlens single-photon source an interesting building block towards the realization of quantum repeaters networks.
Physical Review B, 2005
Charged ͑X + , X − , XX + ͒ and neutral ͑X , XX͒ exciton complexes in single InAs/ GaAs quantum d... more Charged ͑X + , X − , XX + ͒ and neutral ͑X , XX͒ exciton complexes in single InAs/ GaAs quantum dots ͑QDs͒ are investigated by cathodoluminescence spectroscopy. The relative spectral positions of the few-particle transition energies compared to the X transition are shown to be strongly correlated to the QD size. Starting from an unprecedented detailed knowledge about the size, shape, and composition of the investigated quantum dots these energies are calculated using an eight-band k • p theory for the single-particle states and the configuration interaction method for the few-particle states. The observed strong variation of the few-particle energy positions is found to originate from a depletion of the number of excited states in the QDs when they become smaller. Then the degree of correlation is reduced. From a detailed comparison of the numerical results with the experimental data we identify the number of hole states bound in the QD to be the key parameter for size and sign variations of the relative few-particle energies.
Physical Review Letters, 2016
We report excited state emission from p-states at excitation fluences well below ground state sat... more We report excited state emission from p-states at excitation fluences well below ground state saturation in CdSe nanoplatelets. Size dependent exciton ground state-excited state energies and dynamics are determined by three independent methods, time-resolved photoluminescence (PL), time-integrated PL and Hartree renormalized k•p calculations-all in very good agreement. The ground state-excited state energy spacing strongly increases with the lateral platelet quantization. Our results suggest that the PL decay of CdSe platelets is governed by an LO-phonon bottleneck, related to the reported low exciton phonon coupling in CdSe platelets and only observable due to the very large oscillator strength and energy spacing of both states.
Physical Review B, 2015
In this study, we analyze the interaction between excitons and longitudinal-optical phonons (LO p... more In this study, we analyze the interaction between excitons and longitudinal-optical phonons (LO phonons) in single, wurtzite GaN quantum dots (QDs) by means of micro-photoluminescence (μPL) spectroscopy. We report on Stokes-shifted emission lines measured for hundreds of single QDs. A decrease of the Huang-Rhys factor (∼0.5-0.01) is observed with increasing QD emission energy that can be modeled in an adiabatic approximation applying two-particle eight-band k • p wave functions. In order to obtain the QD dimensions and shape needed for these calculations, we conduct a scanning transmission electron microscope (STEM) analysis, not only focusing on the QD dimensions, but also on alloying effects. The QD height is identified as the most detrimental parameter for the exciton-LO-phonon interaction in strongly polar QD systems based on nitrides. Additionally, we extract the LO-phonon energy for a significant number of individual QDs from our μPL data set scaling in-between the bulk values for the QD (GaN) and the matrix material (AlN). Such a large variation of the LO-phonon energy cannot be explained by the alloying effects attested by our STEM analysis. Hence, the exciton-LO-phonon interaction resides in a volume that encloses the QD in the growth direction and a fraction of the matrix material depending on the QD height. We approximate this exciton-LO-phonon interaction volume by a sphere with a constant diameter of 2.6 ± 0.2 nm.
Physical Review B, 2001
The effects of a thin gallium-rich In x Ga 1Ϫx As cap layer on the electronic properties of self-... more The effects of a thin gallium-rich In x Ga 1Ϫx As cap layer on the electronic properties of self-organized InAs quantum dots ͑QD's͒ are investigated both experimentally and theoretically. Increasing the indium concentration of the cap layer allows tuning the ground state transition to lower energies maintaining strong quantization of the electronic states. Strain-driven partial decomposition of the In x Ga 1Ϫx As cap layer increases the effective QD size during growth and the altered barrier composition leads to a partial strain relaxation within the capped InAs QD's. Strain engineering the structural properties of the QD's as well as the actual confining potential offers a pathway to control the electronic properties, e.g., to shift the emission wavelength of lasers based on self-organized InAs QD's to the infrared.
2006 International Conference on Transparent Optical Networks, 2006
Chip gain of 26 dB upon amplification of 1.3 picosecond pulses at frequencies up to 80 GHz is dem... more Chip gain of 26 dB upon amplification of 1.3 picosecond pulses at frequencies up to 80 GHz is demonstrated for temperature stable QD-SOAs. Pulse broadening in the below 200 fs range is observed. The results agree with our predictions on decoupling of gain and index of refraction in Q-based amplifiers. Modelling of wavefunctions in stacked QD-layers based on 8-band k.p-theory indicates the potential of obtaining in the future polarisation insensitive amplification
AIP Conference Proceedings, 2007
Electrons and holes captured in self-assembled quantum dots (QDs) are subject to symmetry breakin... more Electrons and holes captured in self-assembled quantum dots (QDs) are subject to symmetry breaking that cannot be represented in with continuum material representations. Atomistic calculations reveal symmetry lowering due to effects of strain and piezo-electric fields. These effects are fundamentally based on the crystal topology in the quantum dots. This work studies these two competing effects and demonstrates the fine structure splitting that has been demonstrated experimentally can be attributed to the underlying atomistic structure of the quantum dots.
2007 2nd IEEE International Conference on Nano/Micro Engineered and Molecular Systems, 2007
Self-assembled quantum dots (QDs) realized by Stranski-Krastanov growth mode often demonstrate po... more Self-assembled quantum dots (QDs) realized by Stranski-Krastanov growth mode often demonstrate polarized transitions in quantized electron (hole) states and non-degeneracy in the first exicted state in various spectroscopic analyses. These observations confirm the presence of certain symmetry breaking/lowering mechanisms in these low dimensional QDs which cannot be represented accurately with continuum material representations. Atomistic calculations indeed reveal optical polarization anisotropy and non-degeneracy in the excited states due to underlying crystalline symmetry, atomistic long-range strain and piezoelectric fields. This work presents the use of the comprehensive atomistic simulation package NEMO 3-D in the study of competing effects in realistically-sized pyramidal InAs/GaAs quantum dots.
Physical Review B, 2006
We present an eight-band k•p-model for the calculation of the electronic structure of wurtzite se... more We present an eight-band k•p-model for the calculation of the electronic structure of wurtzite semiconductor quantum dots (QDs) and its application to indium gallium nitride (InxGa1−xN) QDs formed by composition fluctuations in InxGa1−xN layers. The eight-band k•p-model accounts for strain effects, piezoelectric and pyroelectricity, spin-orbit and crystal field splitting. Exciton binding energies are calculated using the self-consistent Hartree method. Using this model, we studied the electronic properties of InxGa1−xN QDs and their dependence on structural properties, i.e., their chemical composition, height, and lateral diameter. We found a dominant influence of the built-in piezoelectric and pyroelectric fields, causing a spatial separation of the bound electron and hole states and a redshift of the exciton transition energies. The single-particle energies as well as the exciton energies depend heavily on the composition and geometry of the QDs.
Physical Review B, 2011
Coupling of acoustic and optical phonons to excitons in single InGaAs/GaAs quantum dots is invest... more Coupling of acoustic and optical phonons to excitons in single InGaAs/GaAs quantum dots is investigated in detail experimentally and theoretically as a function of temperature. For the theoretical description of the luminescence spectrum, including acoustic and optical phonon scattering, we used the exactly solvable independent boson model. Surprisingly, only GaAs bulk-type longitudinal-optical (LO) phonons are detected in experiment. A quantitatively correct theoretical description of the optical-phonon replica is obtained by including a limited lifetime of the phonons and the dispersion of the LO phonon energy. Similarly, a numerically correct description of the acoustic phonon wings is again based on GaAs bulk material parameters for the phonon dispersion and deformation coupling. In addition, the line shape of the calculated spectra agrees with experiment only when realistic wave functions (e.g., based on eight-band k•p theory) are used for the electron-phonon coupling matrix elements. Gaussian wave functions describing the ground state of a harmonic oscillator fail to describe high-energy tails. Thus, fundamental insights of importance for the correct prediction of properties of nonclassical light sources, based on semiconductor nanostructures, are obtained.
Physical Review B, 2009
ABSTRACT Energies of exciton, biexciton, and charged excitons for a large variety of realistic qu... more ABSTRACT Energies of exciton, biexciton, and charged excitons for a large variety of realistic quantum dots are calculated using the configuration-interaction model in conjunction with eight-band kṡp theory. The interrelation between quantum dot (QD) geometry and composition, the resulting shape and position of electron and hole wave functions, the direct Coulomb energies, and the changes introduced by correlation effects are analyzed in detail. The QD size, the base shape—being either circular, square, or rhombohedral—and the vertical and lateral aspect ratio are varied. Different average compositions and composition profiles, such as the “trumpet shape,” or an isotropic In gradient resulting from postgrowth annealing processes are studied. The resulting spectroscopic signatures turn out to be very sensitive to all these structural and chemical parameters. We analyze their interrelation to address the band-structure inversion problem, gaining information on the QD morphology from its spectroscopic signature. The results are compared to available experimental data.
Physical Review B, 2011
We demonstrate detection of many-particle hole states in InAs/GaAs quantum dots with single charg... more We demonstrate detection of many-particle hole states in InAs/GaAs quantum dots with single charge resolution up to a temperature of 75 K. Capacitance-voltage measurements as well as time-resolved current measurements in an adjacent two-dimensional hole gas are used to determine the emission and capture time constants from 4 K up to 130 K. A transition from pure tunneling to thermally assisted tunneling is observed with increasing temperature. An equivalent circuit model gives access to the energy level splittings of the many-particle hole states and explains the broadening of the peaks at higher temperatures.
Physical Review B, 2007
Magneto-optics of unstrained GaAs/ Al x Ga 1−x As quantum dots are investigated theoretically in ... more Magneto-optics of unstrained GaAs/ Al x Ga 1−x As quantum dots are investigated theoretically in the presence of an external magnetic field. Single-particle states, exciton binding energies, and the exciton diamagnetic shift are calculated with a confinement potential based on atomically resolved scanning tunneling microscopy pictures. The degree of interface intermixing is treated as a variable. The electronic structure of the dot in the presence of a magnetic field is calculated using eight-band k • p theory including a magnetic field. We find that varying interface roughness sensitively affects the interband but hardly the intraband energies. For magnetic fields applied both in the growth direction and perpendicular to it ͑for B ഛ 50 T͒, we find good agreement between our predicted exciton diamagnetic shift and recent experimental magnetophotoluminescence data ͓N. Schildermans et al., Phys. Rev. B 72, 115312 ͑2005͔͒. The inherent coupling of valence and conduction bands taken into account in the eight-band k • p model explains well the observed experimental results.
physica status solidi (b), 2006
A comprehensive study of the exchange interaction between charge carriers in self-organized InAs/... more A comprehensive study of the exchange interaction between charge carriers in self-organized InAs/GaAs quantum dots is presented. Single quantum-dot cathodoluminescence spectra of quantum dots of different sizes are analyzed. Special attention is paid to the energetic structure of the charged excited exciton (hot trion). A varying degree of intermixing within the hot trion states leads to varying degrees of polarization of the corresponding emission lines. The emission characteristics change from circularly polarized for small quantum dots to elliptically polarized for large quantum dots. The findings are explained by a change of magnitude of the anisotropic exchange interaction and compared to the related effect of fine-structure splitting in the neutral exciton and biexciton emission.
physica status solidi (b), 2001
The electronic and optical properties of vertical pairs of capped, structurally identical InAs py... more The electronic and optical properties of vertical pairs of capped, structurally identical InAs pyramidal quantum dots with {101} facets in GaAs are investigated theoretically, For distances smaller than 17 ML a strong distance dependent quantum coupling between the two dots is predicted for electrons, leading to a term splitting between the ground and the first excited state, Holes are asymmetrically affected by the strain and the piezoelectric potential which, together, prevent a term splitting like in the electron case. The excited hole states vary in their character, dependent on the spacer thickness. Consequently, exciton absorption spectra significantly depend on the vertical dot distance, regarding the energetic range, order, and shape of peaks, and the polarization anisotropies.
Physical Review B
Two-photon transition rates are investigated in resonance to the ground state in wurtzite GaN/AlN... more Two-photon transition rates are investigated in resonance to the ground state in wurtzite GaN/AlN quantum dots. The ground state transition is two-photon allowed because of the electronhole separation inherent to polar wurtzite III-nitride heterostructures. We show that this built-in parity breaking mechanism can allow deterministic triggering of single-photon emission via coherent two-photon excitation. Radiative lifetimes obtained for single-photon relaxation are in good agreement with available time-resolved micro-photoluminescence experiments, indicating the reliability of the employed computational framework based on 8-band k •p-wavefunctions. Two-photon singly-induced emission is explored in terms of possible cavity and non-degeneracy enhancement of two-photon processes.
arXiv: Mesoscale and Nanoscale Physics, 2019
We investigate room temperature lasing of terahertz quantum cascade lasers using quantum dot chai... more We investigate room temperature lasing of terahertz quantum cascade lasers using quantum dot chains as active material. Bandstructure calculations for such extended systems of coupled quantum dots are made possible by a novel "linear combination of atomic orbitals"-like approximation, based on single quantum dot wavefunctions. Our results demonstrate that terahertz quantum cascade lasers greatly benefit from intrinsic properties of quantum dots, such as reduced phonon coupling and in-plane scattering, enabling room temperature lasing with significantly reduced threshold current densities.
Contents Part 1. Electronic Structure Calculations Chapter 1. Introduction Chapter 2. Method of c... more Contents Part 1. Electronic Structure Calculations Chapter 1. Introduction Chapter 2. Method of calculation 2.1. Calculation of strain 2.2. Piezoelectricity and the reduction of lateral symmetry 2.3. Single Particle States 2.4. Many-Particle States 2.5. Optical Properties Part 2. InGaAs/GaAs Quantum Dots Chapter 3. Impact of Size, Shape and Composition on Piezoelectric Effects and Single-Particle States 3.1. The Investigated structures: Variation of size, shape and compostion 3.2. The Impact of the piezoelectric field 3.3. The vertical and lateral aspect ratio 3.4. Varying composition profiles 3.5. Conclusions Chapter 4. Few-particle Energies versus Geometry and Composition 4.1. Interrelation of QD-structure, strain and piezoelectricity, and Coulomb interaction 4.2. The Impact of QD size (series A and H) 4.3. The aspect ratio 4.4. Different composition profiles 4.5. Correlation vs. QD size, shape and particle type 4.6. Conclusions Chapter 5. Multimodal QD-size distribution: Theory and Experiment 5.1. Sample growth 5.2. Determination of QD-morphology and the spectrum of excited states 5.3. Predicted absorption spectra of truncated pyramidal InAs/GaAs QDs 5.4. Single-dot spectra obtained from cathodoluminescence spectroscopy 5.5. Results and Interpretation 5.6. Conclusion Chapter 6. Stacked quantum dots 6.1. Energetics of QD stacks 6.2. Role of strain and piezoelectricity 6.3. Strength of electronic coupling in pairs of identical QDs 6.4. Small perturbations of the size homogeneity 6.5. Asymmetric QD molecules: Coupling of different electronic shells 6.6. Tailoring the TE-TM ratio in semiconductor optical amplifiers 6.7. Conclusions 6 CONTENTS Part 3. Other Material Systems Chapter 7. Electronic and optical properties of InAs/InP quantum dots on InP(100) and InP(311)B substrates 7.1. Choice of model QDs 7.2. Absorption spectra for InAs/InP QDs 7.3. Impact of substrate orientation on the QDs optical properties 7.4. Conclusions Chapter 8. Inverted GaAs/Al x Ga 1−x As Quantum Dots 8.1. Choice of the model QDs 8.2. Influence of interface intermixing on the optical properties 8.3. External magnetic fields 8.4. Discussion 8.
Physical Review B, 2021
We investigate the emission directionality of electronic intraband (intersubband) transitions in ... more We investigate the emission directionality of electronic intraband (intersubband) transitions in stacked coupled quantum dots. Using a well-established eight-band k • p method, we demonstrate that the minor contributions from the valence band mixing into the conduction band govern the polarization and emission directionality of electronic p-to-s-type intraband transitions. Despite that the contribution from the central-cell part to the momentum matrix element is dominant, we find, the contribution from the matrix elements among the envelope functions cannot be neglected. With the help of an artificial cuboidal quantum dot, we show that the vertical emission from intraband transitions can be tuned via the emitter's vertical aspect ratio. Subsequently, we show that these results can be transferred to more realistic geometries of quantum dots and quantum dot stacks and demonstrate that the vertical emission can be enhanced from 23 % to 46 % by increasing the emitter's vertical aspect ratio to the isotropic case with a vertical aspect ratio of 1.0. Therefore, a stacking of a few quantum dotss (∼4 for the investigated structures) is already sufficient to improve the vertical radiation significantly. Additionally, we discuss the impact of the number of stacked QDs as well as the effect of the interdot coupling strength on the radiation properties.
Physical Review B, 2021
We investigate room temperature lasing of terahertz quantum cascade lasers using quantum dot chai... more We investigate room temperature lasing of terahertz quantum cascade lasers using quantum dot chains as active material suitable for wireless communication and imaging technologies. Bandstructure calculations for such extended systems of coupled quantum dots are made possible by a novel 'linear combination of quantum dot orbitals'-method, based on single quantum dot wavefunctions. Our results demonstrate strong vertical-emission of coupled quantum dots, reduced phonon coupling and in-plane scattering, enabling room-temperature lasing with significantly reduced threshold current densities.
Optical Materials Express, 2019
Spectrally-tunable quantum-light sources are key elements for the realization of long-distance qu... more Spectrally-tunable quantum-light sources are key elements for the realization of long-distance quantum communication. A deterministically fabricated single-photon source with a photonextraction efficiency of = (20 ± 2) % and a tuning range of Δ = 2.5 meV is presented here. The device consists of a single pre-selected quantum dot monolithically integrated into a microlens which is bonded onto a piezoelectric actuator via thermocompression goldbonding. The thin gold layer simultaneously acts as a backside mirror for the quantum dot emission, which is efficiently extracted from the device by an optimized lens structure patterned via 3D in-situ electron-beam lithography. The single-photon nature of the emission is proven by photon-autocorrelation measurements with (2) (= 0) = 0.04 ± 0.02. The combination of deterministic fabrication, spectral-tunability and high broadband photon-extraction efficiency makes the microlens single-photon source an interesting building block towards the realization of quantum repeaters networks.
Physical Review B, 2005
Charged ͑X + , X − , XX + ͒ and neutral ͑X , XX͒ exciton complexes in single InAs/ GaAs quantum d... more Charged ͑X + , X − , XX + ͒ and neutral ͑X , XX͒ exciton complexes in single InAs/ GaAs quantum dots ͑QDs͒ are investigated by cathodoluminescence spectroscopy. The relative spectral positions of the few-particle transition energies compared to the X transition are shown to be strongly correlated to the QD size. Starting from an unprecedented detailed knowledge about the size, shape, and composition of the investigated quantum dots these energies are calculated using an eight-band k • p theory for the single-particle states and the configuration interaction method for the few-particle states. The observed strong variation of the few-particle energy positions is found to originate from a depletion of the number of excited states in the QDs when they become smaller. Then the degree of correlation is reduced. From a detailed comparison of the numerical results with the experimental data we identify the number of hole states bound in the QD to be the key parameter for size and sign variations of the relative few-particle energies.
Physical Review Letters, 2016
We report excited state emission from p-states at excitation fluences well below ground state sat... more We report excited state emission from p-states at excitation fluences well below ground state saturation in CdSe nanoplatelets. Size dependent exciton ground state-excited state energies and dynamics are determined by three independent methods, time-resolved photoluminescence (PL), time-integrated PL and Hartree renormalized k•p calculations-all in very good agreement. The ground state-excited state energy spacing strongly increases with the lateral platelet quantization. Our results suggest that the PL decay of CdSe platelets is governed by an LO-phonon bottleneck, related to the reported low exciton phonon coupling in CdSe platelets and only observable due to the very large oscillator strength and energy spacing of both states.
Physical Review B, 2015
In this study, we analyze the interaction between excitons and longitudinal-optical phonons (LO p... more In this study, we analyze the interaction between excitons and longitudinal-optical phonons (LO phonons) in single, wurtzite GaN quantum dots (QDs) by means of micro-photoluminescence (μPL) spectroscopy. We report on Stokes-shifted emission lines measured for hundreds of single QDs. A decrease of the Huang-Rhys factor (∼0.5-0.01) is observed with increasing QD emission energy that can be modeled in an adiabatic approximation applying two-particle eight-band k • p wave functions. In order to obtain the QD dimensions and shape needed for these calculations, we conduct a scanning transmission electron microscope (STEM) analysis, not only focusing on the QD dimensions, but also on alloying effects. The QD height is identified as the most detrimental parameter for the exciton-LO-phonon interaction in strongly polar QD systems based on nitrides. Additionally, we extract the LO-phonon energy for a significant number of individual QDs from our μPL data set scaling in-between the bulk values for the QD (GaN) and the matrix material (AlN). Such a large variation of the LO-phonon energy cannot be explained by the alloying effects attested by our STEM analysis. Hence, the exciton-LO-phonon interaction resides in a volume that encloses the QD in the growth direction and a fraction of the matrix material depending on the QD height. We approximate this exciton-LO-phonon interaction volume by a sphere with a constant diameter of 2.6 ± 0.2 nm.
Physical Review B, 2001
The effects of a thin gallium-rich In x Ga 1Ϫx As cap layer on the electronic properties of self-... more The effects of a thin gallium-rich In x Ga 1Ϫx As cap layer on the electronic properties of self-organized InAs quantum dots ͑QD's͒ are investigated both experimentally and theoretically. Increasing the indium concentration of the cap layer allows tuning the ground state transition to lower energies maintaining strong quantization of the electronic states. Strain-driven partial decomposition of the In x Ga 1Ϫx As cap layer increases the effective QD size during growth and the altered barrier composition leads to a partial strain relaxation within the capped InAs QD's. Strain engineering the structural properties of the QD's as well as the actual confining potential offers a pathway to control the electronic properties, e.g., to shift the emission wavelength of lasers based on self-organized InAs QD's to the infrared.
2006 International Conference on Transparent Optical Networks, 2006
Chip gain of 26 dB upon amplification of 1.3 picosecond pulses at frequencies up to 80 GHz is dem... more Chip gain of 26 dB upon amplification of 1.3 picosecond pulses at frequencies up to 80 GHz is demonstrated for temperature stable QD-SOAs. Pulse broadening in the below 200 fs range is observed. The results agree with our predictions on decoupling of gain and index of refraction in Q-based amplifiers. Modelling of wavefunctions in stacked QD-layers based on 8-band k.p-theory indicates the potential of obtaining in the future polarisation insensitive amplification
AIP Conference Proceedings, 2007
Electrons and holes captured in self-assembled quantum dots (QDs) are subject to symmetry breakin... more Electrons and holes captured in self-assembled quantum dots (QDs) are subject to symmetry breaking that cannot be represented in with continuum material representations. Atomistic calculations reveal symmetry lowering due to effects of strain and piezo-electric fields. These effects are fundamentally based on the crystal topology in the quantum dots. This work studies these two competing effects and demonstrates the fine structure splitting that has been demonstrated experimentally can be attributed to the underlying atomistic structure of the quantum dots.
2007 2nd IEEE International Conference on Nano/Micro Engineered and Molecular Systems, 2007
Self-assembled quantum dots (QDs) realized by Stranski-Krastanov growth mode often demonstrate po... more Self-assembled quantum dots (QDs) realized by Stranski-Krastanov growth mode often demonstrate polarized transitions in quantized electron (hole) states and non-degeneracy in the first exicted state in various spectroscopic analyses. These observations confirm the presence of certain symmetry breaking/lowering mechanisms in these low dimensional QDs which cannot be represented accurately with continuum material representations. Atomistic calculations indeed reveal optical polarization anisotropy and non-degeneracy in the excited states due to underlying crystalline symmetry, atomistic long-range strain and piezoelectric fields. This work presents the use of the comprehensive atomistic simulation package NEMO 3-D in the study of competing effects in realistically-sized pyramidal InAs/GaAs quantum dots.
Physical Review B, 2006
We present an eight-band k•p-model for the calculation of the electronic structure of wurtzite se... more We present an eight-band k•p-model for the calculation of the electronic structure of wurtzite semiconductor quantum dots (QDs) and its application to indium gallium nitride (InxGa1−xN) QDs formed by composition fluctuations in InxGa1−xN layers. The eight-band k•p-model accounts for strain effects, piezoelectric and pyroelectricity, spin-orbit and crystal field splitting. Exciton binding energies are calculated using the self-consistent Hartree method. Using this model, we studied the electronic properties of InxGa1−xN QDs and their dependence on structural properties, i.e., their chemical composition, height, and lateral diameter. We found a dominant influence of the built-in piezoelectric and pyroelectric fields, causing a spatial separation of the bound electron and hole states and a redshift of the exciton transition energies. The single-particle energies as well as the exciton energies depend heavily on the composition and geometry of the QDs.
Physical Review B, 2011
Coupling of acoustic and optical phonons to excitons in single InGaAs/GaAs quantum dots is invest... more Coupling of acoustic and optical phonons to excitons in single InGaAs/GaAs quantum dots is investigated in detail experimentally and theoretically as a function of temperature. For the theoretical description of the luminescence spectrum, including acoustic and optical phonon scattering, we used the exactly solvable independent boson model. Surprisingly, only GaAs bulk-type longitudinal-optical (LO) phonons are detected in experiment. A quantitatively correct theoretical description of the optical-phonon replica is obtained by including a limited lifetime of the phonons and the dispersion of the LO phonon energy. Similarly, a numerically correct description of the acoustic phonon wings is again based on GaAs bulk material parameters for the phonon dispersion and deformation coupling. In addition, the line shape of the calculated spectra agrees with experiment only when realistic wave functions (e.g., based on eight-band k•p theory) are used for the electron-phonon coupling matrix elements. Gaussian wave functions describing the ground state of a harmonic oscillator fail to describe high-energy tails. Thus, fundamental insights of importance for the correct prediction of properties of nonclassical light sources, based on semiconductor nanostructures, are obtained.
Physical Review B, 2009
ABSTRACT Energies of exciton, biexciton, and charged excitons for a large variety of realistic qu... more ABSTRACT Energies of exciton, biexciton, and charged excitons for a large variety of realistic quantum dots are calculated using the configuration-interaction model in conjunction with eight-band kṡp theory. The interrelation between quantum dot (QD) geometry and composition, the resulting shape and position of electron and hole wave functions, the direct Coulomb energies, and the changes introduced by correlation effects are analyzed in detail. The QD size, the base shape—being either circular, square, or rhombohedral—and the vertical and lateral aspect ratio are varied. Different average compositions and composition profiles, such as the “trumpet shape,” or an isotropic In gradient resulting from postgrowth annealing processes are studied. The resulting spectroscopic signatures turn out to be very sensitive to all these structural and chemical parameters. We analyze their interrelation to address the band-structure inversion problem, gaining information on the QD morphology from its spectroscopic signature. The results are compared to available experimental data.
Physical Review B, 2011
We demonstrate detection of many-particle hole states in InAs/GaAs quantum dots with single charg... more We demonstrate detection of many-particle hole states in InAs/GaAs quantum dots with single charge resolution up to a temperature of 75 K. Capacitance-voltage measurements as well as time-resolved current measurements in an adjacent two-dimensional hole gas are used to determine the emission and capture time constants from 4 K up to 130 K. A transition from pure tunneling to thermally assisted tunneling is observed with increasing temperature. An equivalent circuit model gives access to the energy level splittings of the many-particle hole states and explains the broadening of the peaks at higher temperatures.
Physical Review B, 2007
Magneto-optics of unstrained GaAs/ Al x Ga 1−x As quantum dots are investigated theoretically in ... more Magneto-optics of unstrained GaAs/ Al x Ga 1−x As quantum dots are investigated theoretically in the presence of an external magnetic field. Single-particle states, exciton binding energies, and the exciton diamagnetic shift are calculated with a confinement potential based on atomically resolved scanning tunneling microscopy pictures. The degree of interface intermixing is treated as a variable. The electronic structure of the dot in the presence of a magnetic field is calculated using eight-band k • p theory including a magnetic field. We find that varying interface roughness sensitively affects the interband but hardly the intraband energies. For magnetic fields applied both in the growth direction and perpendicular to it ͑for B ഛ 50 T͒, we find good agreement between our predicted exciton diamagnetic shift and recent experimental magnetophotoluminescence data ͓N. Schildermans et al., Phys. Rev. B 72, 115312 ͑2005͔͒. The inherent coupling of valence and conduction bands taken into account in the eight-band k • p model explains well the observed experimental results.
physica status solidi (b), 2006
A comprehensive study of the exchange interaction between charge carriers in self-organized InAs/... more A comprehensive study of the exchange interaction between charge carriers in self-organized InAs/GaAs quantum dots is presented. Single quantum-dot cathodoluminescence spectra of quantum dots of different sizes are analyzed. Special attention is paid to the energetic structure of the charged excited exciton (hot trion). A varying degree of intermixing within the hot trion states leads to varying degrees of polarization of the corresponding emission lines. The emission characteristics change from circularly polarized for small quantum dots to elliptically polarized for large quantum dots. The findings are explained by a change of magnitude of the anisotropic exchange interaction and compared to the related effect of fine-structure splitting in the neutral exciton and biexciton emission.
physica status solidi (b), 2001
The electronic and optical properties of vertical pairs of capped, structurally identical InAs py... more The electronic and optical properties of vertical pairs of capped, structurally identical InAs pyramidal quantum dots with {101} facets in GaAs are investigated theoretically, For distances smaller than 17 ML a strong distance dependent quantum coupling between the two dots is predicted for electrons, leading to a term splitting between the ground and the first excited state, Holes are asymmetrically affected by the strain and the piezoelectric potential which, together, prevent a term splitting like in the electron case. The excited hole states vary in their character, dependent on the spacer thickness. Consequently, exciton absorption spectra significantly depend on the vertical dot distance, regarding the energetic range, order, and shape of peaks, and the polarization anisotropies.