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Localization effects in ternary nitride semiconductors
KTH, Skolan för informations- och kommunikationsteknik (ICT), Fotonik och optik, Optik.
2012 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
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.

Ort, förlag, år, upplaga, sidor
Stockholm: KTH Royal Institute of Technology, 2012. , s. xii, 70
Serie
Trita-ICT/MAP AVH, ISSN 1653-7610 ; 2012:18
Nyckelord [en]
AlGaN, InGaN, AlInN, LEDs, near-field microscopy, carrier dynamics, alloy fluctuations, carrier localization, built-in electric field, nonpolar planes, polarized luminescence
Nationell ämneskategori
Den kondenserade materiens fysik Atom- och molekylfysik och optik
Identifikatorer
URN: urn:nbn:se:kth:diva-104290ISBN: 978-91-7501-530-9 (tryckt)OAI: oai:DiVA.org:kth-104290DiVA, id: diva2:563700
Disputation
2012-11-19, sal C2, KTH-Electrum, Isafjordsgatan 26, Kista, 10:30 (Engelska)
Opponent
Handledare
Forskningsfinansiär
Vetenskapsrådet
Anmärkning

QC 20121101

Tillgänglig från: 2012-11-01 Skapad: 2012-10-31 Senast uppdaterad: 2012-11-01Bibliografiskt granskad
Delarbeten
1. Dynamics of carrier recombination and localization in AlGaN quantum wells studied by time-resolved transmission spectroscopy
Öppna denna publikation i ny flik eller fönster >>Dynamics of carrier recombination and localization in AlGaN quantum wells studied by time-resolved transmission spectroscopy
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2009 (Engelska)Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 95, nr 9Artikel i tidskrift (Refereegranskat) 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.

Nyckelord
relaxation
Identifikatorer
urn:nbn:se:kth:diva-18738 (URN)10.1063/1.3222972 (DOI)000269625800022 ()2-s2.0-69949157239 (Scopus ID)
Anmärkning
QC 20100525Tillgänglig från: 2010-08-05 Skapad: 2010-08-05 Senast uppdaterad: 2017-12-12Bibliografiskt granskad
2. Photoluminescence dynamics of AlGaN quantum wells with built-in electric fields and localized states
Öppna denna publikation i ny flik eller fönster >>Photoluminescence dynamics of AlGaN quantum wells with built-in electric fields and localized states
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2010 (Engelska)Ingår i: Physica Status Solidi (A) Applications and Materials, ISSN 1862-6300, Vol. 207, nr 2, s. 423-427Artikel i tidskrift (Refereegranskat) 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.

Nyckelord
LIGHT-EMITTING-DIODES, AL(X)GA1-XN ALLOYS, DEPENDENCE
Nationell ämneskategori
Elektroteknik och elektronik Fysik
Identifikatorer
urn:nbn:se:kth:diva-28689 (URN)10.1002/pssa.200925227 (DOI)000275148400032 ()2-s2.0-76949104460 (Scopus ID)
Anmärkning
QC 20110121Tillgänglig från: 2011-01-21 Skapad: 2011-01-19 Senast uppdaterad: 2012-11-01Bibliografiskt granskad
3. Dynamics of polarized photoluminescence in m-plane InGaN/GaN quantum wells
Öppna denna publikation i ny flik eller fönster >>Dynamics of polarized photoluminescence in m-plane InGaN/GaN quantum wells
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2010 (Engelska)Ingår i: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 108, nr 2, s. 023101-Artikel i tidskrift (Refereegranskat) 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]

Nationell ämneskategori
Atom- och molekylfysik och optik
Identifikatorer
urn:nbn:se:kth:diva-26889 (URN)10.1063/1.3460278 (DOI)000280909900001 ()2-s2.0-77955822740 (Scopus ID)
Anmärkning

QC 20101130

Tillgänglig från: 2010-11-30 Skapad: 2010-11-29 Senast uppdaterad: 2017-12-12Bibliografiskt granskad
4. Carrier localization in m-plane InGaN/GaN quantum wells probed by scanning near field optical spectroscopy
Öppna denna publikation i ny flik eller fönster >>Carrier localization in m-plane InGaN/GaN quantum wells probed by scanning near field optical spectroscopy
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2010 (Engelska)Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 97, nr 15, s. 151106-Artikel i tidskrift (Refereegranskat) 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]

Nyckelord
EMISSION, PHONON, GAN
Nationell ämneskategori
Elektroteknik och elektronik
Identifikatorer
urn:nbn:se:kth:diva-26283 (URN)10.1063/1.3502482 (DOI)000283216900006 ()2-s2.0-77958092390 (Scopus ID)
Forskningsfinansiär
Vetenskapsrådet
Anmärkning
QC 20110112Tillgänglig från: 2011-01-12 Skapad: 2010-11-21 Senast uppdaterad: 2017-12-11Bibliografiskt granskad
5. Localization potentials in AlGaN epitaxial films studied by scanning near-field optical spectroscopy
Öppna denna publikation i ny flik eller fönster >>Localization potentials in AlGaN epitaxial films studied by scanning near-field optical spectroscopy
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2011 (Engelska)Ingår i: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 109, nr 11Artikel i tidskrift (Refereegranskat) 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.

Nationell ämneskategori
Atom- och molekylfysik och optik
Identifikatorer
urn:nbn:se:kth:diva-36870 (URN)10.1063/1.3594239 (DOI)000292214700037 ()2-s2.0-79959480315 (Scopus ID)
Anmärkning
QC 20110721Tillgänglig från: 2011-07-21 Skapad: 2011-07-18 Senast uppdaterad: 2017-12-08Bibliografiskt granskad
6. Near- and far-field optical characterization of InGaN photonic crystal light emitting diodes
Öppna denna publikation i ny flik eller fönster >>Near- and far-field optical characterization of InGaN photonic crystal light emitting diodes
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2012 (Engelska)Ingår i: Physica Status Solidi. C, Current topics in solid state physics, ISSN 1610-1634, E-ISSN 1610-1642, Vol. 9, nr 7, s. 1664-1666Artikel i tidskrift (Refereegranskat) 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.

Nyckelord
photonic crystal, LED, SNOM, InGaN
Nationell ämneskategori
Atom- och molekylfysik och optik
Identifikatorer
urn:nbn:se:kth:diva-100801 (URN)10.1002/pssc.201100571 (DOI)000306479300040 ()2-s2.0-84863994477 (Scopus ID)
Konferens
16th International Semiconducting and Insulating Materials Conference (SIMC-XVI), JUN 19-23, 2011, Stockholm, Sweden
Anmärkning
QC 20120817Tillgänglig från: 2012-08-17 Skapad: 2012-08-17 Senast uppdaterad: 2017-12-07Bibliografiskt granskad
7. Photoexcited carrier dynamics in AlInN/GaN heterostructures
Öppna denna publikation i ny flik eller fönster >>Photoexcited carrier dynamics in AlInN/GaN heterostructures
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2012 (Engelska)Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 100, nr 24, s. 242104-Artikel i tidskrift (Refereegranskat) 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.

Nyckelord
Recombination, Gan
Nationell ämneskategori
Atom- och molekylfysik och optik
Identifikatorer
urn:nbn:se:kth:diva-98936 (URN)10.1063/1.4729033 (DOI)000305269200033 ()2-s2.0-84863330168 (Scopus ID)
Forskningsfinansiär
VetenskapsrådetKnut och Alice Wallenbergs Stiftelse
Anmärkning

QC 20120711

Tillgänglig från: 2012-07-11 Skapad: 2012-07-05 Senast uppdaterad: 2017-12-07Bibliografiskt granskad

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