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Dynamic properties of excitons in ZnO/AlGaN/GaN hybrid nanostructures
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.ORCID iD: 0000-0002-2597-3322
Department of Electrical Engineering and Computer Science, Nagoya University, Japan.
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.ORCID iD: 0000-0002-9840-7364
2015 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 5, no 7889, 1-5 p.Article in journal (Refereed) Published
Abstract [en]

Hybrid samples based on ZnO colloidal nanocrystals (NCs) deposited on AlGaN/GaN quantum well (QW) structures with different top barrier thickness d = 3, 6 and 9 nm are studied by time-resolved photoluminescence. Thermal behavior of the QW exciton lifetime in the hybrids and in the bare QW structures has been compared and it has been found that the QW exciton recombination rate increases in the hybrid having d = 3 nm and decreases in the hybrid with d = 6 nm, while no change has been observed for the structure with d = 9 nm. It is suggested that non-radiative resonance energy transfer from the QW excitons to the ZnO NCs and a variation of the surface potential can both influence the QW exciton lifetime in the hybrids.

Place, publisher, year, edition, pages
Nature Publishing Group, 2015. Vol. 5, no 7889, 1-5 p.
Keyword [en]
Ultrafast photonics; Two-dimensional materials
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:liu:diva-113739DOI: 10.1038/srep07889ISI: 000348028300008PubMedID: 25601650OAI: oai:DiVA.org:liu-113739DiVA: diva2:784408
Note

Article

Available from: 2015-01-29 Created: 2015-01-29 Last updated: 2017-12-05Bibliographically approved
In thesis
1. Optical and Structural Characterization of GaN Based Hybrid Structures and Nanorods
Open this publication in new window or tab >>Optical and Structural Characterization of GaN Based Hybrid Structures and Nanorods
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

GaN belongs to the group III nitrides and is today the material of choice for efficient blue light emission, enabling solid state white lighting by combining red, blue and green light emitting diodes (LED) or by having a blue LED illuminating a phosphor. By combining GaN quantum well (QW) structures with colloids, nanoparticles or polyfluorene films, LEDs may be fabricate at lower cost. Such hybrid structures are promising for future micro-light sources in full-color displays, sensors and imaging systems. In this work, hybrid structures based on an MOCVD grown GaN QW sandwiched between two layers of AlGaN have been studied. On top of the structure, colloidal ZnO nano-crystals were deposited by spin-coating. Time-resolved photoluminescence was used to investigate the QW exciton dynamics in these hybrids depending on the cap layer thickness. From comparison of the recombination rate in the bare QW structure and the hybrid, the efficiency of the non-radiative resonant energy transfer between the QW and the nano-crystals could be obtained.

Bulk GaN of large area is difficult to synthesize. Thus, due to lack of native substrates, GaN-based structures are grown on SiC or sapphire, which results in high threading dislocation density in the active layer of the device. Fabricating GaN nanorods (NR) can be a way to produce GaN with lower defect density since threading dislocations are annihilated toward the NR wall during growth. Here, GaN(0001) NRs grown on Si(111) substrates by magnetron sputter epitaxy using a liquid Ga target have been investigated. Sputter deposition has the advantage of being easy to scale up for depositions on large surfaces. It is also possible to deposit at lower temperatures, which allows the use of substrates with lower decomposition temperature. In the second paper of this thesis, optical and structural properties of sputtered GaN NRs have been studied.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2015. 45 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1703
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-113741 (URN)10.3384/lic.diva-113741 (DOI)978-91-7519-141-6 (ISBN)
Presentation
2015-02-27, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 14:30 (Swedish)
Opponent
Supervisors
Funder
Swedish Energy Agency
Available from: 2015-01-29 Created: 2015-01-29 Last updated: 2015-09-22Bibliographically approved
2. Study of GaN Based Nanostructures and Hybrids
Open this publication in new window or tab >>Study of GaN Based Nanostructures and Hybrids
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

GaN and its alloys with Al and In belong to the group III nitride semiconductors and are today the materials of choice for efficient white light emitting diodes (LEDs) enabling energy saving solid state lighting. Currently, there is a great interest in the development of novel inexpensive techniques to fabricate hybrid LEDs combining high quality III-N quantum well (QW) structures with inexpensive colloidal nanoparticles or conjugated polymers. Such hybrid devices are promising for future micro-light sources in full-color displays, sensors and imaging systems. Organics can be engineered to emit at different wavelengths or even white light based on functional groups or by blend of several polymers. This is especially important for the green region, where there is still a lack of efficient LEDs. Besides optoelectronics, other applications such as biochemical sensors or systems for water splitting can be realized using GaN-based nanostructures. Despite a significant progress in the field, there is still a need in fundamental understanding of many problems and phenomena in III-nitride based nanostructures and hybrids to fully utilize material properties on demand of specific applications.

In this thesis, hybrid structures based on AlGaN/GaN QWs and colloidal ZnO nano-crystals have been fabricated for down conversion of the QW emission utilizing non-radiative (Förster) resonant energy transfer. Time-resolved photoluminescence (TRPL) was used to investigate the QW exciton dynamics depending on the cap layer thickness in the bare QW and in the hybrid samples. Although the surface potential influences the exciton dynamics, the maximum pumping efficiency assuming a non-radiative energy transfer mechanism was estimated to be ~40 % at 60 K in the structure with thin cap layer of 3 nm.

Since bulk GaN of large area is difficult to synthesize, there is a lack of native substrates. Thus, GaN-based structures are usually grown on SiC or sapphire, which results in high threading dislocation density in the active layer of the device and can be the reason of efficiency droop in GaN based LED structures. Fabricating GaN nanorods (NR) can be a way to produce GaN with lower defect density since threading dislocations can be annihilated toward the NR wall during growth. Here, GaN(0001) NRs grown on Si(111) substrates by magnetron sputtering using a liquid Ga target have been investigated. A high quality of NRs have been confirmed by transmission electron microscopy (TEM) and TRPL. Two strong near band gap emission lines at ~3.42 eV and ~3.47 eV related to basal plane stacking faults (SF) and donor-bound exciton (DBE), respectively, have been observed at low temperatures. TRPL properties of the SF PL line suggest that SFs form a regular structure similar to a multiple QWs, which was confirmed by TEM. The SF related PL measured at 5 K for a single NR has a significantly different polarization response compared to the GaN exciton line and is much stronger polarized (> 40 %) in the direction perpendicular to the NR growth axis.

Hybrids fabricated using GaN NRs and the green emitting polyfluorene (F8BT) have been studied using micro-TRPL. In contrast to the DBE emission, the recombination time of the SF-related emission was observed to decrease, which might be due to the Förster resonance energy transfer mechanism.

Compared to chemical vapor deposition, sputtering allows synthesis at much lower temperatures. Here, sputtering was employed to grow InAlN/GaN heterostructures with an indium content targeted to ~18 %, which is lattice matched to GaN. This means that near strain-free GaN films can be synthesized. It was found that using a lower temperature (~25 C) while depositing the top InAlN results in an improved interface quality compared to deposition at 700 C. In latter case, regions of quaternary alloy of InAlGaN forming structural micro-defects have been observed at the top InAlN/GaN interface in addition to optically active flower-like defect formations. 

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2016. 57 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1788
Keyword
Sputtering, Forster, Semiconductor, Nitride, GaN, Hybrid, Nanorods, Nanostructure, Thin film, Förstoffning, Sputtring, Förster, Halvledare, Nitrid, GaN, Hybrider, Nanostavar, Nanostrukturer, tunnfilmer
National Category
Materials Engineering
Identifiers
urn:nbn:se:liu:diva-132267 (URN)10.3384/diss.diva-132267 (DOI)9789176856925 (ISBN)
Public defence
2016-11-25, Planck, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2016-10-25 Created: 2016-10-24 Last updated: 2016-11-11Bibliographically approved

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