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Controlled growth of hexagonal GaN pyramids and InGaN QDs
Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Gallium-nitride (GaN) and its related alloys are direct band gap semiconductors, with a wide variety of applications. The white light emitting diode (LED) is of particular importance as it is expected to replace energy inefficient light bulb and hazardous incandescent lamps used today. However, today’s planar hetero epitaxial grown LEDs  structures contain an unavoidable number of dislocations, which serves as non-radiative recombination centers. The dislocations harm the luminous efficiency of the LEDs and generate additional heat. Pseudomorphically grown quantum dots (QDs) are expected to be dislocation free thus the injected carriers captured by the QDs essentially recombine radiatively since the dislocations remain outside the QD. Furthermore the continuous character of the density of states in bulk materials is redistributed when the size of the dot is reduced within the Bohr radius of the material. Fully discret energy levels are eventually reached, which offers additional control of the optical properties. The Coulomb interaction between the confined carriers also has influence on the emission energy of the recombining carriers, which opens up the possibility of manufacturing novel light sources such as the single photon emitter. Single photon emitters are essential building blocks for quantum cryptography and teleportation applications.

The main contribution of the present work is the investigation of growth and characterization of sitecontrolled indium-gallium-nitride QDs embedded in GaN matrixes. The goal has been to demonstrate the ability to grow site-controlled InGaN QDs at the apex of hexagonal GaN pyramids in a controlled way using hot-wall metal organic chemical vapor deposition (MOCVD). Strong emphasis was set on the controlled growth of InGaN QDs. For example the growth of a single InGaN QD located at the apex of hexagonal GaN pyramids with tunable emission energy, the QD emission energy impact on the mask design, and a novel approach for the growth of InGaN QDs with polarization deterministic photon vectors were reported. The thesis is mainly based on experimental investigations by secondary electron microscope (SEM), micro photo-luminescence (μPL), and scanning transition electron microscopy ((S)TEM) characterization techniques.

In Paper 1 and 2, we present the growth of symmetric GaN hexagonal pyramids which served as template for the InGaN QDs grown. In paper 1, it was concluded that the selective area growth (SAG) of hexagonal GaN pyramids by MOCVD through symmetric openings in a SIN mask roughly can be divided in two regimes where either the pyramid expands laterally or not. When the pyramid expanded laterally the resulting pyramid apex became (0001) truncated even after prolonged growth times. Lateral expansion also had major impact on the pyramid-to-pyramid uniformity. In paper 2, the MOCVD process parameter impact on the pyramid morphology was investigated. By tuning the growth temperature, the ammonia, and TMGa-flows a self limited pyramid structure with only {1101} facets visible was achieved. The presence of the {1101}, {1102}, and {1100} facets were discussed from surface stabilities under various growth conditions.

Paper 3 and 4 concern the growth of InGaN QDs located at the apex of hexagonal GaN pyramids. In paper 3, we showed that it is possible to grow single QDs at the apex of hexagonal pyramids with emission line widths in the Ångström range. The QD emission energy was demonstrated to be tunable by the growth temperature. Basic spectroscopy data is also presented on a single QD in paper 3. In paper 4, the growth mechanisms of the QDs presented in paper 3 are presented. We concluded that (0001) truncated GaN pyramid base initiated the growth of InGaN QDs which gave rise to narrow luminescence peaks in the μPL spectra.

In paper 5, the QD emission energy impact of the mask design was investigated. To our big surprise the QD emission energy increased with increasing pyramid pitch while the emission energy of the InGaN quantum wells located on the {1101} facets of the pyramids energetically shifted towards lower energies. The energy shift at the apex was found to be associated with the (0001) truncation diameter of the underlying GaN pyramid since no energy shift was observed for (0001) truncated pyramids with truncation diameters larger than 100 nm.

In paper 6, the symmetry of the GaN pyramids were intentionally broken through the introduction of elongated openings in the SiN mask (symmetric openings was used in the previous five papers). The emission polarization vectors of the subsequently grown InGaN QDs were deterministically linked to the in-plane orientation of the pyramid it was nucleated upon, implying that the QDs inhibit an inplane anisotropy directly inherited from the pyramid template.

Finally, paper 7 describes a hot-wall MOCVD reactor improvement by inserting insulating pyrolytic boron-nitride (PBN) stripes in the growth chamber. By doing this, we have completely eliminated the arcing problem between different susceptor parts. As a consequence, the reactor gained run-to-run reproducibility. Growth of state of the art advanced aluminum-gallium-nitride high electron mobility transistor structures on a 100 mm wafer with electron mobility above 2000 Vs/cm2 was demonstrated by the improved process.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2012. , 53 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1464
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-79326ISBN: 978-91-7519-842-2 (print)OAI: oai:DiVA.org:liu-79326DiVA: diva2:540394
Public defence
2012-09-20, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2012-07-10 Created: 2012-07-10 Last updated: 2012-12-04Bibliographically approved
List of papers
1. Controlled growth of hexagonal GaN pyramids by hot-wall MOCVD
Open this publication in new window or tab >>Controlled growth of hexagonal GaN pyramids by hot-wall MOCVD
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2013 (English)In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 363, 287-293 p.Article in journal (Refereed) Published
Abstract [en]

Hexagonal GaN pyramids have been fabricated by hot-wall metal organic chemical vapor deposition (hot-wall MOCVD) and the growth evolution have been studied. It was concluded that the pyramid growth can be divided into two regimes separated by the adsorption kinetics of the {1101} surfaces of the pyramids. In the adsorption regime, the pyramids grow simultaneously in the <1101> and [0001] -directions. In the zero-adsorption regime the pyramids grow only in the [0001] direction. Thus the pyramid growth ceases when the (0001) facet growth has been terminated. Large arrays consisting of highly uniform pyramids with apex radii of 3 nm or less were achieved in the zeroadsorption regime. The growth-regime type was concluded to have a large impact on the uniformity degradation of the pyramids, and their optical properties. The impacts of threading dislocations which enter the pyramid from underneath are also discussed.

Keyword
A3. Hot wall epitaxy A3. Metalorganic vapor phase epitaxy A3. Selective epitaxy B1. Nitrides
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-79317 (URN)10.1016/j.jcrysgro.2012.11.014 (DOI)000313205400047 ()
Available from: 2012-07-10 Created: 2012-07-10 Last updated: 2017-12-07Bibliographically approved
2. Morphology control of hot-wall MOCVD selective area -grown hexagonal GaN pyramids
Open this publication in new window or tab >>Morphology control of hot-wall MOCVD selective area -grown hexagonal GaN pyramids
2012 (English)In: Crystal Growth & Design, ISSN 1528-7483, E-ISSN 1528-7505, Vol. 12, no 11, 5491-5496 p.Article in journal (Refereed) Published
Abstract [en]

Morphological variations of gallium polar (0001) oriented hexagonal GaN pyramids grown by hotwall metal organic chemical vapor deposition under various growth conditions are investigated. The stability of the semi-polar {1102} and non-polar {1100} facets are particularly discussed. The presence of the {1102} facets near the apex of the pyramid was found to be controllable by tuning the absolute flow rate of ammonia during the growth. Vertical non-polar {1100} facets appeared ingallium rich-conditions which automatically were created when the growth time was prolonged beyond pyramid completion. The result was attributed to a gallium passivation of the {1100} surface.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-79320 (URN)10.1021/cg301064p (DOI)000311240100041 ()
Available from: 2012-07-10 Created: 2012-07-10 Last updated: 2017-12-07Bibliographically approved
3. Single Excitons in InGaN Quantum Dots on GaN Pyramid Arrays
Open this publication in new window or tab >>Single Excitons in InGaN Quantum Dots on GaN Pyramid Arrays
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2011 (English)In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 11, no 6, 2415-2418 p.Article in journal (Refereed) Published
Abstract [en]

Fabrication of single InGaN quantum dots (QDs) on top of GaN micropyramids is reported. The formation of single QDs is evidenced by showing single sub-millielectronvolt emission lines in microphotoluminescence (mu PL) spectra. Tunable QD emission energy by varying the growth temperature of the InGaN layers is also demonstrated. From mu PL, it is evident that the QDs are located in the apexes of the pyramids. The fact that the emission lines of the QDs are linear polarized in a preferred direction implies that the apexes induce unidirected anisotropic fields to the QDs. The single emission lines remain unchanged with increasing the excitation power and/or crystal temperature. An in-plane elongated QD forming a shallow potential with an equal number of trapped electrons and holes is proposed to explain the absence of other exciton complexes.

Place, publisher, year, edition, pages
American Chemical Society, 2011
Keyword
InGaN, quantum dots, pyramid, exciton, photoluminescence
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-69169 (URN)10.1021/nl200810v (DOI)000291322600038 ()
Available from: 2011-06-17 Created: 2011-06-17 Last updated: 2017-12-11Bibliographically approved
4. InGaN quantum dot formation mechanism on hexagonal GaN/InGaN/GaN pyramids
Open this publication in new window or tab >>InGaN quantum dot formation mechanism on hexagonal GaN/InGaN/GaN pyramids
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2012 (English)In: Nanotechnology, ISSN 0957-4484, E-ISSN 1361-6528, Vol. 23, no 30, 305708- p.Article in journal (Refereed) Published
Abstract [en]

Growing InGaN quantum dots (QDs) at the apex of hexagonal GaN pyramids is an elegant approach to achieve a deterministic positioning of QDs. Despite similar synthesis procedures by metal–organic chemical vapor deposition, the optical properties of the QDs reported in the literature vary drastically. The QDs tend to exhibit either narrow or broad emission lines in the micro-photoluminescence spectra. By coupled microstructural and optical investigations, the QDs giving rise to narrow emission lines were concluded to nucleate in association with a (0001) facet at the apex of the GaN pyramid.

Place, publisher, year, edition, pages
Institute of Physics (IOP), 2012
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:liu:diva-79321 (URN)10.1088/0957-4484/23/30/305708 (DOI)000306333500030 ()
Available from: 2012-07-10 Created: 2012-07-10 Last updated: 2017-12-07Bibliographically approved
5. Unexpected behavior of InGaN quantum dot emission energy located at apices of hexagonal GaN pyramids
Open this publication in new window or tab >>Unexpected behavior of InGaN quantum dot emission energy located at apices of hexagonal GaN pyramids
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

InGaN quantum dots (QDs) have been grown at the apices of hexagonal GaN pyramids. The pyramids were selectively grown on a (0001) oriented GaN template through circular apertures in a SiN mask positioned in square arrays. The emission of the InGaN QDs was shifted towards higher energies when the center-to-center distance of the pyramids was increased, while the emission from InGaN quantum wells located on the {1101} facets of the pyramids was energetically shifted towards lower energies. No energy shift was observed for (0001) truncated pyramids with truncation diameters larger than 100 nm.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-79322 (URN)
Available from: 2012-07-10 Created: 2012-07-10 Last updated: 2016-08-31Bibliographically approved
6. Polarization-controlled photon emission from site-controlled InGaN quantum dots
Open this publication in new window or tab >>Polarization-controlled photon emission from site-controlled InGaN quantum dots
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The optical polarization properties of hot-wall MOCVD grown of InGaN quantum dots (QDs) located at the apex of elongated hexagonal GaN pyramids are presented. The QDs showed spectrally narrow and strongly linearly polarized emission lines with average polarization ratios above 0.8 in the microphoto-luminescence spectra. By a comprehensive statistical analysis including more than 1000 InGaN QDs it was concluded that the polarization direction of the QDs follows the spatial elongation of the underlying GaN pyramids when elongated in the <2110> directions.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-79323 (URN)
Available from: 2012-07-10 Created: 2012-07-10 Last updated: 2015-01-23Bibliographically approved
7. Improved hot-wall MOCVD growth of highly uniform AlGaN/GaN/HEMT structures
Open this publication in new window or tab >>Improved hot-wall MOCVD growth of highly uniform AlGaN/GaN/HEMT structures
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2009 (English)In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 311, no 10, 3007-3010 p.Article in journal (Refereed) Published
Abstract [en]

The inherent advantages of the hot-wall metal organic chemical vapor deposition (MOCVD) reactor (low temperature gradients, less bowing of the wafer during growth, efficient precursor cracking) compared to a cold-wall reactor make it easier to obtain uniform growth. However, arcing may occur in the growth chamber during growth, which deteriorates the properties of the grown material. By inserting insulating pyrolytic BN (PBN) stripes in the growth chamber we have completely eliminated this problem. Using this novel approach we have grown highly uniform, advanced high electron mobility transistor (HEMT) structures on 4 semi-insulating (SI) SiC substrates with gas-foil rotation of the substrate. The nonuniformities of sheet resistance and epilayer thickness are typically less than 3% over the wafer. The room temperature hall mobility of the 2DEG is well above 2000 cm(2)/V s and the sheet resistance about 270 Omega/sqr.

Keyword
Metalorganic chemical vapor deposition, Nitrides, High electron mobility transistors
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-19663 (URN)10.1016/j.jcrysgro.2009.01.045 (DOI)
Note
Original Publication: Urban Forsberg, Anders Lundskog, A Kakanakova-Georgieva, Rafal Ciechonski and Erik Janzén, Improved hot-wall MOCVD growth of highly uniform AlGaN/GaN/HEMT structures, 2009, JOURNAL OF CRYSTAL GROWTH, (311), 10, 3007-3010. http://dx.doi.org/10.1016/j.jcrysgro.2009.01.045 Copyright: Elsevier Science B.V., Amsterdam. http://www.elsevier.com/Available from: 2009-08-17 Created: 2009-07-10 Last updated: 2017-12-13Bibliographically approved

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