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Study of GaN Based Nanostructures and Hybrids
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.ORCID iD: 0000-0001-7038-1222
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 [en]
Sputtering, Forster, Semiconductor, Nitride, GaN, Hybrid, Nanorods, Nanostructure, Thin film
Keyword [sv]
Förstoffning, Sputtring, Förster, Halvledare, Nitrid, GaN, Hybrider, Nanostavar, Nanostrukturer, tunnfilmer
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:liu:diva-132267DOI: 10.3384/diss.diva-132267ISBN: 9789176856925 (Print)OAI: oai:DiVA.org:liu-132267DiVA: diva2:1039614
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
List of papers
1. Dynamic properties of excitons in ZnO/AlGaN/GaN hybrid nanostructures
Open this publication in new window or tab >>Dynamic properties of excitons in ZnO/AlGaN/GaN hybrid nanostructures
2015 (English)In: Scientific Reports, 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
Keyword
Ultrafast photonics; Two-dimensional materials
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-113739 (URN)10.1038/srep07889 (DOI)000348028300008 ()25601650 (PubMedID)
Note

Article

Available from: 2015-01-29 Created: 2015-01-29 Last updated: 2016-11-10Bibliographically approved
2. Time-resolved photoluminescence properties of hybrids based on inorganic AlGaN/GaN quantum wells and colloidal ZnO nanocrystals
Open this publication in new window or tab >>Time-resolved photoluminescence properties of hybrids based on inorganic AlGaN/GaN quantum wells and colloidal ZnO nanocrystals
2015 (English)In: Superlattices and Microstructures, ISSN 0749-6036, E-ISSN 1096-3677, Vol. 87, 38-41 p.Article in journal (Refereed) Published
Abstract [en]

Dynamic properties are studied for the AlGaN/GaN quantum well (QW) structures with and without the coating of colloidal ZnO nanocrystals (NCs). The QW exciton recombination rate was reduced in such hybrids compared to the bare QW structure only in the sample with the thinnest cap layer of 3 nm. Assuming that one of the recombination mechanisms in this hybrid is non-radiative resonant energy transfer (NRET) between the QW and the energy acceptor material i.e. ZnO NCs, the maximum pumping efficiency was estimated to be similar to 42% at 60 K. The NRET effect is, however, vanished after several months despite that the hybrid structures are composed of chemically stable components. (C) 2015 Published by Elsevier Ltd.

Place, publisher, year, edition, pages
ACADEMIC PRESS LTD- ELSEVIER SCIENCE LTD, 2015
Keyword
Quantum well structures; Energy transfer
National Category
Physical Sciences Chemical Sciences
Identifiers
urn:nbn:se:liu:diva-123818 (URN)10.1016/j.spmi.2015.07.017 (DOI)000366876100007 ()
Note

Funding Agencies|Swedish Research Council (VR); Carl Trygger Foundation; Angpanneforeningens Foundation for Research and Development

Available from: 2016-01-11 Created: 2016-01-11 Last updated: 2016-11-10
3. Stacking fault related luminescence in GaN nanorods
Open this publication in new window or tab >>Stacking fault related luminescence in GaN nanorods
Show others...
2015 (English)In: Nanotechnology, ISSN 0957-4484, E-ISSN 1361-6528Article in journal (Refereed) Published
Abstract [en]

Optical and structural properties are presented for GaN nanorods grown in the [0001]direction on Si(111) substrates by direct-current reactive magnetron sputter epitaxy.Transmission electron microscopy reveals clusters of dense stacking faults (SFs) regularlydistributed along the c-axis. A strong emission at ~3.42 eV associated with basal plane SFsdemonstrates thermal stability up to room temperatures together with a relatively shortrecombination time suggesting carrier localization in the system similar to multiple quantumwells.

Place, publisher, year, edition, pages
Institute of Physics (IOP), 2015
Keyword
GaN nanorods, stacking faults, time-resolved photoluminescence, recombination time, multiple quantum wells, sputtering
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-113740 (URN)10.1088/0957-4484/26/35/355203 (DOI)000360947200008 ()
Note

Funding: Swedish Research Council (VR); SFF; Angpanneforeningen

Available from: 2015-01-29 Created: 2015-01-29 Last updated: 2016-11-10Bibliographically approved

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