| 1. |
- Geijselaers, Irene, et al.
(författare)
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Atomically sharp, crystal phase defined GaAs quantum dots
- 2021
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 119:26
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Crystal phase defined heterostructures, or polytype heterostructures, are atomically sharp with no intermixing, which makes them ideal contenders for a wide number of applications. Although polytype quantum dots have shown promising results as single photon sources, a high degree of control on the dimensions and number of polytype quantum dots is necessary before any application can be developed.In this work we show results from optical characterization of highly controlled wz-zb GaAs quantum dots with sharp photoluminescence signal and a strong indication of 0D density of states. One band effective mass calculations show good agreement with the measured data. Radially confined nanowires with a single wz-zb GaAs interface also show sharp photoluminescence signal and a 0D density of states. This indicates the existence of quantum dot like states in the triangular wells formed at the wz-zb GaAs interface. These results show the potential of polytype quantum dots for physics and optics applications.
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| 2. |
- Geijselaers, Irene, et al.
(författare)
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Two-dimensional electron gas at wurtzite–zinc-blende InP interfaces induced by modulation doping
- 2020
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 116
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Tidskriftsartikel (refereegranskat)abstract
- The quality, such as long-range correlation and mobility, of a two-dimensional electron gas (2DEG) is limited by, among other factors, interface roughness, which is inherent to the use of compositional heterostructures. Polytypic heterostructures have atomically sharp interfaces and minimal strain, decreasing the interface roughness, which may increase the mobility and long-range correlation of 2DEGs. In this work, we show the formation of a 2DEG at the wurtzite–zinc blende interface in partially n-type-doped InP nanowires using power-dependent photoluminescence. We additionally determined the wurtzite–zinc blende InP valence band offset to be 35 meV
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| 3. |
- Jash, Asmita, et al.
(författare)
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Excitonic Dynamics at the Type-II Polytype Interface of InP Platelets
- 2023
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Ingår i: ACS Photonics. - 2330-4022. ; 10:9, s. 3143-3148
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Tidskriftsartikel (refereegranskat)abstract
- Indirect excitons are the focus of intense research due to the opportunity of studying degenerate quantum gases and liquids in an excitonic system. To realize such systems, it is highly advantageous to have as little scattering as possible. A polytype type-II interface is formed between wurtzite and zincblende InP due to the band alignment. Electrons gather on the zincblende and holes on the wurtzite side of the interface. Therefore, electrons and holes that are spatially separated by the interface form indirect excitons with aligned dipoles. This polytype type-II interface is perfectly flat, which limits scattering. Here we report that repulsive interaction between the indirect excitons is the driving force behind the long-range transport of indirect excitons along the interface at high exciton densities. This is indicative of less scattering than in conventional type-II heterostructures. The spatial separation of the charge carriers across the interface leads to a low recombination rate of the indirect excitons since the overlap of the electron-hole wavefunction at the interface is small. Emission from the long-lived indirect excitons can be detected even after 40 μs. Our studies have been performed by using spatially and temporally resolved photoluminescence at the low temperature.
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| 4. |
- Jash, Asmita, et al.
(författare)
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Time-resolved photoluminescence studies of single interface wurtzite/zincblende heterostructured InP nanowires
- 2022
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 120:11
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Tidskriftsartikel (refereegranskat)abstract
- The interface between wurtzite and zinc blende InP has been identified as type-II, where electrons gather on the zinc blende side and holes on the wurtzite side of the interface. The photoluminescence resulting from recombination across the interface is expected to be long-lived and to exhibit non-exponential decay of emission intensity after pulsed excitation. We verify this prediction using time-resolved photoluminescence spectroscopy on nanowires containing a single heterostructure between a single segment of wurtzite and zinc blende. We find that a significant intensity of type-II emission remains even more than 30 ns after excitation. The decay of the emission intensity is also non-exponential and considerably longer than the exponential decay of the wurtzite InP segment (260 ps). Our results are consistent with the expected photoluminescence characteristics of a type-II interface between the two polytypes. We also find that the lifetime becomes shorter if we create an electron gas at the interface by n-type doping the entire wurtzite segment of the nanowire. This is expected since there are many electrons that a given hole can recombine with, in contrast to the undoped case.
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| 5. |
- Khalilian, Maryam, et al.
(författare)
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Dislocation-Free and Atomically Flat GaN Hexagonal Microprisms for Device Applications
- 2020
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Ingår i: Small. - : Wiley-VCH Verlag. - 1613-6810 .- 1613-6829. ; 16:30
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Tidskriftsartikel (refereegranskat)abstract
- III-nitrides are considered the material of choice for light-emitting diodes (LEDs) and lasers in the visible to ultraviolet spectral range. The development is hampered by lattice and thermal mismatch between the nitride layers and the growth substrate leading to high dislocation densities. In order to overcome the issue, efforts have gone into selected area growth of nanowires (NWs), using their small footprint in the substrate to grow virtually dislocation-free material. Their geometry is defined by six tall side-facets and a pointed tip which limits the design of optoelectronic devices. Growth of dislocation-free and atomically smooth 3D hexagonal GaN micro-prisms with a flat, micrometer-sized top-surface is presented. These self-forming structures are suitable for optical devices such as low-loss optical cavities for high-efficiency LEDs. The structures are made by annealing GaN NWs with a thick radial shell, reforming them into hexagonal flat-top prisms with six equivalents either m- or s-facets depending on the initial heights of the top pyramid and m-facets of the NWs. This shape is kinetically controlled and the reformation can be explained with a phenomenological model based on Wulff construction that have been developed. It is expected that the results will inspire further research into micron-sized III-nitride-based devices. © 2020 The Authors.
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| 6. |
- Lantz, Victor, et al.
(författare)
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Deep learning for inverse problems in quantum mechanics
- 2021
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Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 121:9
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Tidskriftsartikel (refereegranskat)abstract
- Inverse problems are important in quantum mechanics and involve such questions as finding which potential give a certain spectrum or which arrangement of atoms give certain properties to a molecule or solid. Inverse problems are typically very hard to solve and tend to be very compute intense. We here show that neural networks can easily solve inverse problems in quantum mechanics. It is known that a neural network can compute the spectrum of a given potential, a result which we reproduce. We find that the (much harder) inverse problem of computing the correct potential that gives a prescribed spectrum is equally easy for a neural network. We extend previous work where neural networks were used to find the electronic many-particle density given a potential by considering the inverse problem. That is, we show that neural networks can compute the potential that gives a prescribed many-electron density.
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| 7. |
- Liu, Tian Xiang, et al.
(författare)
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Coarse-grained tight-binding models
- 2022
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Ingår i: Journal of Physics Condensed Matter. - 0953-8984. ; 34:12
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Tidskriftsartikel (refereegranskat)abstract
- Calculating the electronic structure of systems involving very different length scales presents a challenge. Empirical atomistic descriptions such as pseudopotentials or tight-binding models allow one to calculate the effects of atomic placements, but the computational burden increases rapidly with the size of the system, limiting the ability to treat weakly bound extended electronic states. Here we propose a new method to connect atomistic and quasi-continuous models, thus speeding up tight-binding calculations for large systems. We divide a structure into blocks consisting of several unit cells which we diagonalize individually. We then construct a tight-binding Hamiltonian for the full structure using a truncated basis for the blocks, ignoring states having large energy eigenvalues and retaining states with energies close to the band edge energies. A numerical test using a GaAs/AlAs quantum well shows the computation time can be decreased to less than 5% of the full calculation with errors of less than 1%. We give data for the trade-offs between computing time and loss of accuracy. We also tested calculations of the density of states for a GaAs/AlAs quantum well and find a ten times speedup without much loss in accuracy.
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| 8. |
- Liu, Tian Xiang, et al.
(författare)
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Speeding up tight binding calculations using zone-folding methods
- 2022
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Ingår i: Computational Materials Science. - : Elsevier BV. - 0927-0256. ; 211
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Tidskriftsartikel (refereegranskat)abstract
- Tight binding models are widely used in large scale electronic structure calculations of nanostructures. Their atomistic nature makes them flexible, but also means the computational cost increases rapidly with system size. The large number of calculations required to design nanostructures makes computational efficiency desirable. We have developed a method to increase computational speed while retaining most of its accuracy. The method is based on the use of supercells and zone folding combined with a truncation of the Hamiltonians to only include states close to the band-edges. We apply the method to model the band edge energies of a GaAs/AlAs quantum well grown along the [110]-directions with 3D and 2D periodic boundary conditions as well as the density of states and dielectric function of the quantum well. We typically find a speed-up of ten times with only a small loss of accuracy of the calculation result.
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| 9. |
- Vainorius, Neimantas, et al.
(författare)
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Non-resonant Raman scattering of wurtzite GaAs and InP nanowires
- 2020
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Ingår i: Optics Express. - 1094-4087. ; 28:8, s. 11016-11022
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Tidskriftsartikel (refereegranskat)abstract
- It is now possible to synthesize the wurtzite crystal phase of most III-V semiconductors in the form of nanowires. This sparks interest for fundamental research and adds extra degrees of freedom for designing novel devices. However, the understanding of many properties, such as phonon dispersion, of these wurtzite semiconductors is not yet complete, despite the extensive number of studies published. The E2 L and E2 H phonon modes exist in the wurtzite crystal phase only (not in zinc blende) where the E2 H mode has been already experimentally observed in Ga and In arsenides and phosphides, while the E2 L mode has been observed in GaP, but not in GaAs or InP. In order to determine the energy of E2 L in wurtzite GaAs and InP, we performed Raman scattering measurements on wurtzite GaAs and InP nanowires. We found clear evidence of the E2 L phonon mode at 64 cm−1 and 54 cm−1, respectively. Polarization-dependent experiments revealed similar selection rules for both the E2 L and the E2 H phonon modes (as expected) where the intensity peaked with excitation and detection polarization being perpendicular to the [0001] crystallographic direction. We further find that the splitting between the E1(TO) and A1(TO) modes is around 2 cm−1 in wurtzite GaAs and below 1 cm−1 in wurtzite InP. We believe these results will be useful for a better understanding of phonons in wurtzite crystal phase of III-V semiconductors as well as for testing and improving phonon dispersion calculations.
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