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Optical properties of novel semiconductor nanostructures
Linköping University, Department of Physics, Chemistry and Biology, Functional Electronic Materials. Linköping University, The Institute of Technology.
2014 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2014. , 34 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1691
National Category
Physical Sciences Nano Technology
Identifiers
URN: urn:nbn:se:liu:diva-112355ISBN: 978-91-7519-185-0 (print)OAI: oai:DiVA.org:liu-112355DiVA: diva2:765605
Presentation
2014-12-17, Jordan-Fermi, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 09:30 (English)
Opponent
Supervisors
Note

The series name Linköping Studies in Science and Technology Licentiate Thesis is incorrect. The correct series name is Linköping Studies in Science and Technology Thesis.

Available from: 2014-11-24 Created: 2014-11-24 Last updated: 2017-03-27Bibliographically approved
List of papers
1. Effects of Ni-coating on ZnO nanowires: A Raman scattering study
Open this publication in new window or tab >>Effects of Ni-coating on ZnO nanowires: A Raman scattering study
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2013 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 113, no 21, 214302-1-214302-6 p.Article in journal (Refereed) Published
Abstract [en]

Structural properties of ZnO/Ni core/shell nanowires (NWs) are studied in detail by means of Raman spectroscopy. It is shown that formation of the Ni shell leads to passivation of surface states responsible for the observed enhanced intensity of the A1(LO) Raman mode of the bare ZnO NWs. It also causes appearance of 490 cm−1 and 710 cm−1 modes that are attributed to local vibrational modes of a defect/impurity (or defects/impurities). This defect is concluded to be preferably formed in annealed ZnO/Ni NWs and is unlikely to contain a Ni atom, as the same Raman modes were also reported for the Ni-free ZnO nanostructures. From our resonant Raman studies, we also show that the ZnO/Ni core/shell NWs exhibit an enhanced Raman signal with a multiline structure involving A1(LO). This observation is attributed to combined effects of an enhanced Fröhlich interaction at the ZnO/Ni heterointerface and coupling of the scattered light with local surface plasmons excited in the Ni shell. The plasmonic effect is also suggested to allow detection of carbon-related species absorbed at the surface of a single ZnO/Ni NW, promising for applications of such structures as efficient nano-sized gas sensors.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2013
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-93878 (URN)10.1063/1.4807912 (DOI)
Available from: 2013-06-11 Created: 2013-06-11 Last updated: 2017-12-06Bibliographically approved
2. Origin of strong photoluminescence polarization in GaNP nanowires
Open this publication in new window or tab >>Origin of strong photoluminescence polarization in GaNP nanowires
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2014 (English)In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 14, no 9, 5264-5269 p.Article in journal (Refereed) Published
Abstract [en]

The III-V semiconductor nanowires (NWs) have a great potential for applications in a variety of future electronic and photonic devices with enhanced functionality. In this work, we employ polarization resolved micro-photoluminescence (µ-PL) spectroscopy to study polarization properties of light emissions from individual GaNP and GaP/GaNP core/shell nanowires (NWs) with average diameters ranging between 100 and 350 nm. We show that the near-band-edge emission, which originates from the GaNP regions of the NWs, is strongly polarized (up to 60 % at 150 K) in the direction perpendicular to the NW axis. The polarization anisotropy can be retained up to room temperature. This polarization behavior, which is unusual for zinc blende NWs, is attributed to local strain in the vicinity of the N-related centers participating in the radiative recombination and to 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 tailor the polarization anisotropy of III-V nanowires, advantageous for their applications as nanoscale emitters of polarized light.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2014
Keyword
Nanowire; photoluminescence; polarization
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-109932 (URN)10.1021/nl502281p (DOI)000341544500053 ()25162940 (PubMedID)
Available from: 2014-08-28 Created: 2014-08-28 Last updated: 2017-12-05Bibliographically approved
3. Control of exciton fine-structure splitting in geometrically engineered self-assembled InAs/GaAs quantum molecular structures
Open this publication in new window or tab >>Control of exciton fine-structure splitting in geometrically engineered self-assembled InAs/GaAs quantum molecular structures
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Fine-structure splitting (FSS) of excitons in semiconductor nanostructures is a key parameter that has significant implications in photon entanglement and polarization conversion between electron spins and photons, relevant to quantum information technology and spintronics. Here, we investigate exciton FSS in self-organized InAs/GaAs quantum molecular structures (QMSs) including laterally-aligned double quantum dots (DQDs), quantum-dot clusters (QCs) and quantum rings (QRs), by employing polarization-resolved micro-photoluminescence spectroscopy. We find a clear trend in FSS between the studied QMSs 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 optical polarization directions of the excitons between these QMSs, namely, the bright-exciton lines are linearly polarized preferably along or perpendicular to the [11̅0] crystallographic axis in the DQDs that also defines the alignment of the two constituting QDs, whereas in the QCs and QRs the polarization directions are randomly oriented. We attribute the observed trends in the FSS to a significant reduction of the anisotropic strain field in the high symmetry QCRs and QCs as compared with the low-symmetry  DQDs. Our work demonstrates that FSS can be effectively controlled by geometric engineering of the QMSs, capable of reducing FSS even in a strained QD system to a limit similar to strain-free QDs. This approach provides a new pathway in obtaining high-symmetry quantum emitters desirable for realizing photon entanglement and spintronic devices based on such nanostructures, without special requirements for lattice-matched materials combinations, specific substrate orientations and nanolithography.

National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-112353 (URN)
Available from: 2014-11-24 Created: 2014-11-24 Last updated: 2017-03-27Bibliographically approved

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