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Localization effects in ternary nitride semiconductors
KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

InGaN based blue and near-ultraviolet light emitting diodes and laser diodes have been successfully commercialized for many applications such as general lighting, display backlighting and high density optical storage devices. Despite having a comparably high defect density, these devices are known for their efficient operation, which is attributed to localization in potential fluctuations preventing carriers from reaching the centers of nonradiative recombination. Nitride research is currently headed towards improving deep ultraviolet AlGaN and green InGaN emitters with higher Al and In molar fractions. The efficiency of these devices trails behind the blue counterparts as the carrier localization does not seem to aid in supressing nonradiative losses. In addition, the operation of ternary nitride heterostructure based devices is further complicated by the presence of large built-in electric fields. Although the problem can be ameliorated by growing structures in nonpolar or semipolar directions, the step from research to production still awaits.

In this thesis, carrier dynamics and localization effects have been studied in three different nitride ternary compounds: AlGaN epitaxial layers and quantum wells with high Al content, nonpolar m-plane InGaN/GaN quantum wells and lattice matched AlInN/GaN heterostructures. The experimental methods of this work mainly consist of spectroscopy techniques such as time-resolved photoluminescence and differential transmission pump-probe measurements as well as spatial photoluminescence mapping by means of scanning near-field microscopy.

The comparison of luminescence and differential transmission measurements has allowed estimating the localization depth in AlGaN quantum wells. Additionally, it has been demonstrated that the polarization degree of luminescence from m-InGaN quantum wells decreases as carriers diffuse to localization centers.What is more, dual-scale localization potential has been evidenced by near-field measurements in both AlGaN and m-InGaN. Larger scale potential fluctuation have been observed directly and the depth of nanoscopic localization has been estimated theoretically from the recorded linewidth of the near-field spectra. Lastly, efficient carrier transport has been observed through AlInN layer despite large alloy inhomogeneities evidenced by broad luminescence spectra and the huge Stokes shift. Inhomogeneous luminescence from the underlying GaN layer has been linked to the fluctuations of the built-in electric field at the AlInN/GaN interface.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , xii, 70 p.
Series
Trita-ICT/MAP AVH, ISSN 1653-7610 ; 2012:18
Keyword [en]
AlGaN, InGaN, AlInN, LEDs, near-field microscopy, carrier dynamics, alloy fluctuations, carrier localization, built-in electric field, nonpolar planes, polarized luminescence
National Category
Condensed Matter Physics Atom and Molecular Physics and Optics
Identifiers
URN: urn:nbn:se:kth:diva-104290ISBN: 978-91-7501-530-9 (print)OAI: oai:DiVA.org:kth-104290DiVA: diva2:563700
Public defence
2012-11-19, sal C2, KTH-Electrum, Isafjordsgatan 26, Kista, 10:30 (English)
Opponent
Supervisors
Funder
Swedish Research Council
Note

QC 20121101

Available from: 2012-11-01 Created: 2012-10-31 Last updated: 2012-11-01Bibliographically approved
List of papers
1. Dynamics of carrier recombination and localization in AlGaN quantum wells studied by time-resolved transmission spectroscopy
Open this publication in new window or tab >>Dynamics of carrier recombination and localization in AlGaN quantum wells studied by time-resolved transmission spectroscopy
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2009 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 95, no 9Article in journal (Refereed) Published
Abstract [en]

Time-resolved transmission and photoluminescence measurements were performed on Al0.35Ga0.65N/Al0.49Ga0.51N quantum well structures with different well widths. Comparison of transmission and luminescence data shows that dynamics of electrons and holes excited into extended quantum well states are governed by nonradiative recombination. For excitation into potential minima formed by band gap fluctuations, localization of electrons was observed. Excitation energy dependence of the pump-probe transient shape allows estimating localization potential, which is about 80 meV independently of the well width, and is probably caused by fluctuations of AlN molar fraction.

Keyword
relaxation
Identifiers
urn:nbn:se:kth:diva-18738 (URN)10.1063/1.3222972 (DOI)000269625800022 ()2-s2.0-69949157239 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2012-11-01Bibliographically approved
2. Photoluminescence dynamics of AlGaN quantum wells with built-in electric fields and localized states
Open this publication in new window or tab >>Photoluminescence dynamics of AlGaN quantum wells with built-in electric fields and localized states
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2010 (English)In: Physica Status Solidi (A) Applications and Materials, ISSN 1862-6300, Vol. 207, no 2, 423-427 p.Article in journal (Refereed) Published
Abstract [en]

We report on photoluminescence (PL) studies of AlGaN/AlGaN multiple quantum well (MQW) structures with well widths spanning from 1.65 to 5.0 nm under various excitations and temperatures. The samples were fabricated by migration-enhanced metal-organic chemical vapor deposition technique (MEMOCVD (R)). Screening of the built-in electric field by photogenerated carriers reduced quantum-confined Stark effect (QCSE). This is confirmed by solving the Poisson and Schrodinger equations for AlGaN-based quantum wells (QWs) under study. Analysis of the PL internal quantum efficiency under different excitations in MQWs with different widths shows strong influence of the carrier localization on the radiative properties of MQWs.

Keyword
LIGHT-EMITTING-DIODES, AL(X)GA1-XN ALLOYS, DEPENDENCE
National Category
Electrical Engineering, Electronic Engineering, Information Engineering Physical Sciences
Identifiers
urn:nbn:se:kth:diva-28689 (URN)10.1002/pssa.200925227 (DOI)000275148400032 ()2-s2.0-76949104460 (Scopus ID)
Note
QC 20110121Available from: 2011-01-21 Created: 2011-01-19 Last updated: 2012-11-01Bibliographically approved
3. Dynamics of polarized photoluminescence in m-plane InGaN/GaN quantum wells
Open this publication in new window or tab >>Dynamics of polarized photoluminescence in m-plane InGaN/GaN quantum wells
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2010 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 108, no 2, 023101- p.Article in journal (Refereed) Published
Abstract [en]

Spectrally-, polarization-, and time-resolved photoluminescence (PL) experiments have been performed on 2.5 nm thick m-plane single InGaN quantum wells. It has been found that PL decay is mainly determined by nonradiative recombination through several types of recombination centers, while PL rise is largely affected by exciton transfer into localization minima. Prolonged PL rise times and time-dependent spectral shift were used to study exciton transfer into the localization centers. CharacteristiC time of the exciton transfer is 80-100 ps at lower temperatures and about 50 ps at room temperature, which corresponds to the exciton diffusion length of 200-500 nm. Degree of PL linear polarization was found to decrease at a similar rate. Decreased PL polarization for the localized excitons suggests that the localization centers are related to areas with relaxed strain. (C) 2010 American Institute of Physics. [doi:10.1063/1.3460278]

National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-26889 (URN)10.1063/1.3460278 (DOI)000280909900001 ()2-s2.0-77955822740 (Scopus ID)
Note

QC 20101130

Available from: 2010-11-30 Created: 2010-11-29 Last updated: 2012-11-01Bibliographically approved
4. Carrier localization in m-plane InGaN/GaN quantum wells probed by scanning near field optical spectroscopy
Open this publication in new window or tab >>Carrier localization in m-plane InGaN/GaN quantum wells probed by scanning near field optical spectroscopy
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2010 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 97, no 15, 151106- p.Article in journal (Refereed) Published
Abstract [en]

Scanning near field optical microscopy (SNOM) was applied to study the carrier localization in single InGaN/GaN quantum well structures grown on nonpolar m-plane GaN substrates. Dual localization potential consisting of hundreds of nanometers- to micrometer-size areas as well as smaller localization centers were identified from the SNOM scans and near field photoluminescence spectral widths. The localization areas were found to align along the [0001] direction, which was attributed to partial strain relaxation at the monolayer steps. (C) 2010 American Institute of Physics. [doi:10.1063/1.3502482]

Keyword
EMISSION, PHONON, GAN
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-26283 (URN)10.1063/1.3502482 (DOI)000283216900006 ()2-s2.0-77958092390 (Scopus ID)
Funder
Swedish Research Council
Note
QC 20110112Available from: 2011-01-12 Created: 2010-11-21 Last updated: 2012-11-01Bibliographically approved
5. Localization potentials in AlGaN epitaxial films studied by scanning near-field optical spectroscopy
Open this publication in new window or tab >>Localization potentials in AlGaN epitaxial films studied by scanning near-field optical spectroscopy
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2011 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 109, no 11Article in journal (Refereed) Published
Abstract [en]

Scanning near-field photoluminescence spectroscopy has been applied to evaluate bandgap fluctuations in epitaxial AlGaN films with the AlN molar fraction varying from 0.30 to 0.50. A dual localization pattern has been observed. The potential of the small-scale (<100 nm) localization, evaluated from the width of the photoluminescence spectra, is between 0 and 51 meV and increases with increased Al content. These potential variations have been assigned to small-scale compositional fluctuations occurring due to stress variations, dislocations, and formation of Al-rich grains during growth. Larger area potential variations of 25-40 meV, most clearly observed in the lower Al-content samples, have been attributed to Ga-rich regions close to grain boundaries or atomic layer steps. The density, size, and bandgap energy of these domains were found to be composition dependent. The lower bandgap domains were found to be strongly correlated with the regions with efficient nonradiative recombination.

National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-36870 (URN)10.1063/1.3594239 (DOI)000292214700037 ()2-s2.0-79959480315 (Scopus ID)
Note
QC 20110721Available from: 2011-07-21 Created: 2011-07-18 Last updated: 2017-12-08Bibliographically approved
6. Near- and far-field optical characterization of InGaN photonic crystal light emitting diodes
Open this publication in new window or tab >>Near- and far-field optical characterization of InGaN photonic crystal light emitting diodes
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2012 (English)In: Physica Status Solidi. C, Current topics in solid state physics, ISSN 1610-1634, E-ISSN 1610-1642, Vol. 9, no 7, 1664-1666 p.Article in journal (Refereed) Published
Abstract [en]

Light extraction from InGaN photonic crystal (PhC) light emitting diodes (LEDs) has been studied by scanning near-field optical microscopy and time-resolved photoluminescence. The observed patterns of the near-field intensity are complex and do not correspond to the physical structure of the photonic crystal. Comparison with conventional LEDs without the PhC showed that PhC not only improves the light extraction, but also makes the LED emission more homogeneous.

Keyword
photonic crystal, LED, SNOM, InGaN
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-100801 (URN)10.1002/pssc.201100571 (DOI)000306479300040 ()2-s2.0-84863994477 (Scopus ID)
Conference
16th International Semiconducting and Insulating Materials Conference (SIMC-XVI), JUN 19-23, 2011, Stockholm, Sweden
Note
QC 20120817Available from: 2012-08-17 Created: 2012-08-17 Last updated: 2017-12-07Bibliographically approved
7. Photoexcited carrier dynamics in AlInN/GaN heterostructures
Open this publication in new window or tab >>Photoexcited carrier dynamics in AlInN/GaN heterostructures
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2012 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 100, no 24, 242104- p.Article in journal (Refereed) Published
Abstract [en]

Photoexcited carrier dynamics and localization potentials in Al0.86In0.14N/GaN heterostructures have been examined by time-resolved and scanning near-field photoluminescence (PL) spectroscopy. The large GaN and AlInN PL intensity difference, and the short AlInN PL decay and GaN PL rise times indicate efficient photoexcited hole transfer from AlInN to GaN via sub-band-gap states. These states are attributed to extended defects and In clusters. Near-field PL scans show that diameter of the localization sites and the distance between them are below 100 nm. Spatial variations of the GaN PL wavelength have been assigned to the electric field inhomogeneities at the heterostructure interface.

Keyword
Recombination, Gan
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-98936 (URN)10.1063/1.4729033 (DOI)000305269200033 ()2-s2.0-84863330168 (Scopus ID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note

QC 20120711

Available from: 2012-07-11 Created: 2012-07-05 Last updated: 2017-12-07Bibliographically approved

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