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Self-Assembled and Selective-Area Growth of Group III-Nitride Semiconductor Nanorods by Magnetron Sputter Epitaxy
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.ORCID iD: 0000-0002-9796-8995
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The III-nitride semiconductor family includes gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN), and the related ternary and quaternary alloys. The research interest on this group of materials is sparked by the direct bandgaps, and excellent physical and chemical properties. Moreover, the ternary alloys (InGaN, InAlN and AlGaN) present the advantage of bandgap tuning, giving access to the whole visible spectrum, from near infrared into deep ultraviolet wavelengths. The intrinsic properties of III-nitride materials can be combined with characteristic features of nanodimension and geometry in nanorod structures. Moreover, nanorods offer the advantage of avoiding problems arising from the lack of native substrates with film/substrate lattice and thermal expansion mismatch.

The growth and characterization of group III-nitride semiconductor nanorods, namely InAlN and GaN nanorods, is presented in this Thesis. All the nanostructures were grown by employing direct-current reactive magnetron sputter epitaxy. The results include the growth and study of both self-assembled and site-controlled grown nanorods.

InxAl1−xN self-assembled, core-shell nanorods on Si(111) substrates were demonstrated. A comprehensive study of temperature effect upon the morphology and composition of the nanorods was realized. The radial nanorod heterostructure consists of In-rich cores surrounded by Al-rich shells with different thicknesses. The spontaneous formation of core-shell nanorods is suggested to originate from phase separation due to spinodal decomposition. As the growth temperature increases, In desorption is favored, resulting in thicker Al-rich shells and larger nanorod diameters. Moreover, the in-plane crystallographic relationship of the nanorods and substrate was modified from a fiber-textured to an epitaxial growth by removing the native SiOx layer from the substrate.

Self-assembled growth of GaN nanorods on cost-effective substrates offers a cheaper alternative and simplifies device processing. Successful growth of high-quality GaN (exhibiting strong bandedge emission and high crystalline quality) on conductive templates/substrates such as Si, SiC, TiN/Si, ZrB2/Si, ZrB2/SiC, Mo, and Ti is supported by the possibility to be used as electrodes when integrated in optoelectronic devices. The influence of growth temperature upon the resulting size and optical properties of the nanorods was investigated. By applying a kinetic model, average diffusion length was calculated in correlation with growth temperature in order to explain the nanorods’ morphology evolution.

The self-assembled growth leads to random nucleation, resulting in nanorods with large varieties of diameters, heights and densities within a single growth run. This translates into non-uniform properties and complicates device processing. These problems can be circumvented by employing selective-area growth. Pre-patterned substrates by nanosphere lithography resulted in GaN nanorods with controlled length, diameter, shape, and density. Well-faceted caxis oriented GaN nanorods were grown directly onto the native SiOx layer inside opening areas exhibiting strong bandedge emission at room-temperature and single-mode lasing. The time-dependent growth series helped define a comprehensive growth mechanism from the initial thin wetting layer formed inside the openings, to the well-defined, uniform, hexagonal nanorods resulted from the coalescence of multiple initial nuclei.

Although nanosphere lithography is a fast and cheap patterning method, it does not offer the control on the size, position or density. The growth parameters were transferred onto focused ion beam lithography - patterned substrates which offers more control on the design. Focused ion beam lithography optimization included tailoring of the milling current (2-50 pA) and milling time (5-50 s). The patterning process optimisation enabled the minimization of mask and substrate damage, the key to attain uniform, welldefined, single, and straight nanorods. Destruction of the mask results in selective-area growth failure, while damage of the substrate surface promotes inclined nanorods grown into the openings, owning to random oriented nucleation. At lower growth temperatures (950 °C) nanostructures resulted from the coalescence of multiple, tilted, and irregular nanorods are observed. The tilting of the nanorods is reduced when increasing the growth temperature to 980 °C resulting in single and straight nanorods. The partial pressure of the Ar/N2 working gas was also varied for achieving selectivity and single nanorods, and study the growth behaviour. By increasing the amount of Ar in the working gas from 0 to 50%, we observe a transition of the target from a nitridized to metallic-state, affecting the sputtering conditions of the GaN nanorods. The change in the sputtering and deposition conditions influences the growth selectivity, coalescence, and growth rates. By balancing these effects, the selective growth of faceted, single nanorods was achieved.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2018. , p. 47
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1935
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:liu:diva-147647DOI: 10.3384/diss.diva-147647ISBN: 9789176853085 (print)OAI: oai:DiVA.org:liu-147647DiVA, id: diva2:1203277
Public defence
2018-06-08, Schrödinger, E324, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2018-05-03 Created: 2018-05-03 Last updated: 2018-05-03Bibliographically approved
List of papers
1. Structural and compositional evolutions of InxAl1-xN core-shell nanorods grown on Si(111) substrates by reactive magnetron sputter epitaxy
Open this publication in new window or tab >>Structural and compositional evolutions of InxAl1-xN core-shell nanorods grown on Si(111) substrates by reactive magnetron sputter epitaxy
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2015 (English)In: Nanotechnology, ISSN 0957-4484, E-ISSN 1361-6528, Vol. 26, no 21, p. 215602-Article in journal (Refereed) Published
Abstract [en]

Catalystless growth of InxAl1-xN core-shell nanorods have been realized by reactive magnetron sputter epitaxy onto Si(111) substrates. The samples were characterized by scanning electron microscopy, x-ray diffraction, scanning transmission electron microscopy, and energy dispersive x-ray spectroscopy. The composition and morphology of InxAl1-xN nanorods are found to be strongly influenced by the growth temperature. At lower temperatures, the grown materials form well-separated and uniform core-shell nanorods with high In-content cores, while a deposition at higher temperature leads to the formation of an Al-rich InxAl1-xN film with vertical domains of low In-content as a result of merging Al-rich shells. The thickness and In content of the cores (domains) increase with decreasing growth temperature. The growth of the InxAl1-xN is traced to the initial stage, showing that the formation of the core-shell nanostructures starts very close to the interface. Phase separation due to spinodal decomposition is suggested as the origin of the resultant structures. Moreover, the in-plane crystallographic relationship of the nanorods and substrate was modified from a fiber textured to an epitaxial growth with an epitaxial relationship of InxAl1-xN[0001]//Si[111] and InxAl1-xN[11 (2) over bar0]//Si[1 (1) over bar0] by removing the native SiOx layer from the substrate.

Place, publisher, year, edition, pages
IOP Publishing: Hybrid Open Access, 2015
Keywords
InAlN; core-shell; nanorods; sputtering; MSE; STEM; EDX
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-118979 (URN)10.1088/0957-4484/26/21/215602 (DOI)000354598800010 ()25944838 (PubMedID)
Note

Funding Agencies|Swedish Research Council (VR) [621-2012-4420]; Swedish Governmental Agency for Innovation Systems (VINNOVA) under the VINNMER international qualification program; Knut and Alice Wallenberg Foundation

Available from: 2015-06-08 Created: 2015-06-05 Last updated: 2018-05-03
2. Magnetron Sputter Epitaxy of High-Quality GaN Nanorods on Functional and Cost-Effective Templates/Substrates
Open this publication in new window or tab >>Magnetron Sputter Epitaxy of High-Quality GaN Nanorods on Functional and Cost-Effective Templates/Substrates
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2017 (English)In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 10, no 9, article id 1322Article in journal (Refereed) Published
Abstract [en]

We demonstrate the versatility of magnetron sputter epitaxy by achieving high-quality GaN nanorods on different substrate/template combinations, specifically Si, SiC, TiN/Si, ZrB2/Si, ZrB2/SiC, Mo, and Ti. Growth temperature was optimized on Si, TiN/Si, and ZrB2/Si, resulting in increased nanorod aspect ratio with temperature. All nanorods exhibit high purity and quality, proved by the strong bandedge emission recorded with cathodoluminescence spectroscopy at room temperature as well as transmission electron microscopy. These substrates/templates are affordable compared to many conventional substrates, and the direct deposition onto them eliminates cumbersome post-processing steps in device fabrication. Thus, magnetron sputter epitaxy offers an attractive alternative for simple and affordable fabrication in optoelectronic device technology.

Place, publisher, year, edition, pages
Basel, Switzerland: MDPI AG, 2017
Keywords
GaN, nanorods, Si, SiC, Ti, Mo, TiN and ZrB2 templates, magnetron sputtering, epitaxy
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-141597 (URN)10.3390/en10091322 (DOI)000411225200078 ()2-s2.0-85029362447 (Scopus ID)
Note

Funding agencies: Swedish Research Council (VR) [621-2012-4420, 621-2013-5360, 2016-04412]; Swedish Governmental Agency for Innovation Systems (VINNOVA) under the VINNMER international qualification program; Swedish Foundation for Strategic Research (SSF) through the Resea

Available from: 2017-10-02 Created: 2017-10-02 Last updated: 2018-05-03Bibliographically approved
3. Selective-area growth of single-crystal wurtzite GaN nanorods on SiOx/Si(001) substrates by reactive magnetron sputter epitaxy exhibiting single-mode lasing
Open this publication in new window or tab >>Selective-area growth of single-crystal wurtzite GaN nanorods on SiOx/Si(001) substrates by reactive magnetron sputter epitaxy exhibiting single-mode lasing
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2017 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 7, article id 12701Article in journal (Refereed) Published
Abstract [en]

Selective-area growth (SAG) of single-crystal wurtzite GaN nanorods (NRs) directly onto Si(001) substrates with un-etched native SiOx amorphous layer, assisted by a patterning TiNx mask fabricated by nanosphere lithography (NSL), has been realized by reactive magnetron sputter epitaxy (MSE). The GaN NRs were grown vertically to the substrate surface with the growth direction along c-axis in the well-defined nano-opening areas. A 5-step structural and morphological evolution of the SAG NRs observed at different sputtering times depicts a comprehensive growth model, listed in sequence as: formation of a polycrystalline wetting layer, predominating c-axis oriented nucleation, coarsening and coalescence of multi-islands, single NR evolution, and finally quasi-equilibrium crystal shape formation. Room-temperature cathodoluminescence spectroscopy shows a strong GaN bandedge emission with a uniform luminescence across the NRs, indicating that the SAG NRs are grown with high quality and purity. In addition, single-longitudinal-mode lasing, attributed to well-faceted NR geometry forming a Fabry-Perot cavity, was achieved by optical pumping, paving a way for fabricating high-performance laser optoelectronics using MSE.

Place, publisher, year, edition, pages
NATURE PUBLISHING GROUP, 2017
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-142426 (URN)10.1038/s41598-017-12702-y (DOI)000412358000007 ()28983102 (PubMedID)
Note

Funding Agencies|Swedish Research Council (VR) [621-2012-4420, 621-2013- 5360]; Swedish Governmental Agency for Innovation Systems (VINNOVA) under the VINNMER international qualification program; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [SFO-Mat-LiU 2009-00971]; Swedish Foundation for Strategic Research (SSF) through the Research Infrastructure Fellow [RIF 14-0074]; Knut and Alice Wallenberg Foundation; Myfab National Access program at MC2 NanoFabrication Laboratory, Chalmers University of Technology; Ministry of Science and Technology (MOST) of Taiwan [MOST 105-2112-M-006-004-MY3]

Available from: 2017-10-31 Created: 2017-10-31 Last updated: 2018-05-03
4. Site-controlled growth of GaN nanorod arrays by magnetron sputter epitaxy
Open this publication in new window or tab >>Site-controlled growth of GaN nanorod arrays by magnetron sputter epitaxy
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2018 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 660, p. 950-955Article in journal (Refereed) Published
Abstract [en]

Catalyst-free GaN nanorod regular arrays have been realized by reactive magnetron sputter epitaxy. Two nanolithographic methods, nanosphere lithography (NSL) and focused ion beam lithography (FIBL), were applied to pattern Si substrates with TiNx masks. The growth temperature was optimized for achieving selectivity and well-faceted nanorods grown onto the NSL-patterned substrates. With increasing temperature from 875 to 985 °C, we observe different growth behaviors and associate them with selective insensitive, diffusion-dominated, and desorption-dominated zones. To further achieve site-specific and diameter control, these growth parameters were transferred onto FIBL-patterned substrates. Further investigation into the FIBL process through tailoring of milling current and time in combination with varying nanorod growth temperature, suggests that minimization of mask and substrate damage is the key to attain uniform, well-defined, single, and straight nanorods. Destruction of the mask results in selective area growth failure, while damage of the substrate surface promotes inclined nanorods grown into the openings, owning to random oriented nucleation.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Gallium nitride, Magnetron sputter epitaxy, Selective-area growth, Nanorods, Lithography, Focused ion beam, Nanosphere
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-147648 (URN)10.1016/j.tsf.2018.01.050 (DOI)000441177500138 ()2-s2.0-85041572645 (Scopus ID)
Note

Funding agencies: Swedish Research Council (VR) [621-2012-4420, 621-2013-5360, 2016-04412]; Swedish Governmental Agency for Innovation Systems (VINNOVA) under the VINNMER international qualification program; Swedish Foundation for Strategic Research (SSF) [RIF 14-0074]; Sw

Available from: 2018-05-03 Created: 2018-05-03 Last updated: 2018-09-03Bibliographically approved

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