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Optimization of InGaN quantum well interfaces for fast interwell carrier transport and low nonradiative recombination
KTH, School of Engineering Sciences (SCI), Applied Physics, Photonics.ORCID iD: 0000-0001-8496-9668
Univ Calif Santa Barbara, Mat Dept, Santa Barbara, CA 93106 USA..
Univ Calif Santa Barbara, Mat Dept, Santa Barbara, CA 93106 USA..
Univ Calif Santa Barbara, Mat Dept, Santa Barbara, CA 93106 USA..
Show others and affiliations
2022 (English)In: Gallium Nitride Materials and Devices XVII / [ed] Fujioka, H Morkoc, H Schwarz, UT, SPIE-Intl Soc Optical Eng , 2022, Vol. 12001, article id 1200104Conference paper, Published paper (Refereed)
Abstract [en]

Efficient high-power operation of light emitting diodes based on InGaN quantum wells (QWs) requires rapid interwell hole transport and low nonradiative recombination. The transport rate can be increased by replacing GaN barriers with that of InGaN. Introduction of InGaN barriers, however, increases the rate of the nonradiative recombination. In this work, we have attempted to reduce the negative impact of the nonradiative recombination by introducing thin GaN or AlGaN interlayers at the QW/barrier interfaces. The interlayers, indeed, reduce the nonradiative recombination rate and increase the internal quantum efficiency by about 10%. Furthermore, the interlayers do not substantially slow down the interwell hole transport; for 0.5 nm Al0.10Ga0.90N interlayers the transport rate has even been found to increase. Another positive feature of the interlayers is narrowing of the QW PL linewidth, which is attributed to smoother QW interfaces and reduced fluctuations of the QW width.

Place, publisher, year, edition, pages
SPIE-Intl Soc Optical Eng , 2022. Vol. 12001, article id 1200104
Series
Proceedings of SPIE, ISSN 0277-786X
Keywords [en]
InGaN, quantum wells, LED, carrier transport, nonradiative recombination, internal quantum efficiency
National Category
Atom and Molecular Physics and Optics
Identifiers
URN: urn:nbn:se:kth:diva-316432DOI: 10.1117/12.2608695ISI: 000836321400003Scopus ID: 2-s2.0-85131224841OAI: oai:DiVA.org:kth-316432DiVA, id: diva2:1688204
Conference
Conference on Gallium Nitride Materials and Devices XVII at SPIE OPTO Conference, JAN 22-FEB 28, 2022, ELECTR NETWORK
Note

Part of proceedings: ISBN 978-1-5106-4874-6, ISBN 978-1-5106-4873-9

QC 20220818

Available from: 2022-08-18 Created: 2022-08-18 Last updated: 2023-01-26Bibliographically approved
In thesis
1. Carrier dynamics in blue and green InGaN LED structures
Open this publication in new window or tab >>Carrier dynamics in blue and green InGaN LED structures
2022 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis focuses on effects that are critical to achieving high internal quantum efficiency (IQE) in GaN-based light-emitting diodes (LEDs) that emit in a broad spectral range, from violet to green-yellow. These effects include interwell carrier transport in multiple quantum well (QW) structures, lateral transport in the QW plane, and radiative and nonradiative recombination. 

The investigation is conducted with the time-integrated and time-resolved near- and far-field photoluminescence (PL) spectroscopy. Measurements are performed on polar single and multiple InxGa1-xN QW structures of different alloy compositions, which are supplemented with a self-consistent solution of one-dimensional Schrödinger and Poisson equations and an evaluation of the carrier density dynamics. 

Interwell carrier transport is studied to determine the conditions required for a uniform interwell carrier distribution in an LED active region. Such a distribution would decrease the detrimental impact of the nonradiative Auger recombination and increase the IQE. Since the hole transport is the bottleneck for this process, ambipolar interwell transport, determined by the slower holes, is studied. Standard time-resolved PL measurements are performed on multiple QW structures with a different number of In0.12Ga0.88N QWs and different barrier parameters in terms of thickness and material. Photoexcited carrier transport over the multiple QW structure is monitored by measuring PL rise times from a deeper detector QW. Such measurements make it possible to distinguish the interwell transport mechanism at high temperatures (e.g., thermionic emission - ns range) and low temperatures (e.g., ballistic - sub-ps range). In standard InGaN/GaN structures, the interwell hole transport is found to be inefficient. Studies of transport and IQE in structures with InGaN barriers of different compositions, as well as thin GaN or AlGaN interlayers between the QWs and barriers, allowed the design of structures with fast, efficient interwell transport and high IQE. These measurements are performed for blue LED structures; however, the conclusions could be extended to QWs emitting at longer wavelengths, for which the issue of the nonuniform interwell carrier distribution is even more severe. 

Studies of the carrier recombination and IQE are performed on single QWs with a focus on long wavelength (green, green-yellow) emitting structures, in which the IQE is much smaller than for the violet and blue-emitting wells. Radiative and nonradiative carrier recombination times are determined at different temperatures, revealing a record-high IQE of ∼60% in the green-yellow QWs. 

Since nonradiative recombination is often assigned to extended defects, near-field spectroscopy is applied to study the impact of V-defects related to dislocations in polar GaN-based structures. The parameters of PL spectra, as well as radiative and nonradiative recombination times, show large spatial variations. The increased nonradiative recombination related to the dislocations is revealed only in their immediate vicinity, suggesting that their impact on the IQE and device performance, contrary to common belief, should be small.

Abstract [sv]

Avhandlingen fokuserar på effekter som är avgörande för att uppnå hög intern kvanteffektivitet (IQE) i GaN-baserade lysdioder (LED) som emitterar inom ett brett spektralområde, från violett till gröngult. Dessa effekter inkluderar laddningsbärartransport mellan brunnar i flera kvantbrunnar (QW)-strukturer, lateral transport i QW-planet och strålnings- och icke-strålningsrekombination. 

Undersökningen gjordes genom tidsintegrerad och tidsupplöst fotoluminescensspektroskopi (PL) i när- och fjärrfält. Mätningar utfördes på polära enkla och multipla InxGa1-xN QW-strukturer bestående av olika legeringssammansättningar. De kompletterades med självkonsistent lösning av endimensionella Schrödinger- och Poisson-ekvationer och utvärdering av bärardensitetsdynamiken. 

Laddningsbärartransporten mellan brunnar studerades för att bestämma förhållanden som krävs för en enhetlig bärarfördelning mellan brunnar i en aktiv LED-region. Sådan distribution skulle minska den skadliga effekten av den icke-strålande Auger-rekombinationen och öka IQE. Eftersom håltransporten är flaskhalsen under denna process, studerades ambipolär transport mellan brunnar, utifrån de långsammare hålen. Standard tidsupplösta PL-mätningar utfördes på flera QW-strukturer med olika antal In0,12Ga0,88N QW:er och olika barriärparametrar (tjocklek, material). Fotoexciterad bärartransport över den multipla QW-strukturen övervakades genom att mäta PL-stigningstider från en djupare detektor-QW. Sådana mätningar gjorde det möjligt att särskilja transportmekanismen mellan brunnar vid höga temperaturer (termionisk emission, ns-intervall) och låga temperaturer (ballistiskt, sub-ps-intervall). Det konstaterades att håltransporten mellan brunnar i standard InGaN/GaN-strukturer var ineffektiv. Studier av transport och IQE i strukturer med InGaN-barriärer av olika sammansättning, och tunna GaN- eller AlGaN-mellanskikt mellan QW:erna och barriärer möjliggjorde design av strukturer med snabb och effektiv transport mellan brunnar och hög IQE. Dessa mätningar utfördes för blå LED-strukturer; dock skulle slutsatserna kunna utvidgas till QW:er som emitterar vid längre våglängder och för vilka problemet med olikformig bärarfördelningen mellan brunnar är ännu större.

Studier av laddningsbärarrekombinationen och IQE utfördes på enstaka kvantbrunnar med fokus på emitterande strukturer med lång våglängd (grön, grön-gul), där IQE är mycket mindre än för de violett och blåemitterande brunnarna. Strålande och icke-strålande bärarekombinationstider bestämdes vid olika temperaturer, vilket avslöjade en rekordhög IQE på ∼60 % i de gröngula QW:erna. 

Eftersom den icke-strålande rekombinationen ofta tillskrivs utökade defekter, användes närfältsspektroskopi för att studera effekten av V-defekter relaterade till dislokationer i polära GaN-baserade strukturer. Parametrar för PL-spektra, samt strålande och icke-strålande rekombinationstider visade stora rumsliga variationer. Den ökade ickestrålande rekombinationen relaterad till dislokationerna avslöjades endast i deras omedelbara närhet, vilket tyder på att deras inverkan på IQE och enhetens prestanda, i motsats till vad man tror, borde vara liten.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2022. p. 95
Series
TRITA-SCI-FOU ; 2022:56
Keywords
InGaN quantum wells, interwell transport, near-field scanning optical microscopy, internal quantum efficiency, V-defects, InGaN-kvantbrunnar, transport mellan brunnar, optisk mikroskopi med närafältsskanning, intern kvanteffektivitet, V-defekter
National Category
Physical Sciences
Research subject
Physics; Physics, Optics and Photonics
Identifiers
urn:nbn:se:kth:diva-320563 (URN)978-91-8040-411-2 (ISBN)
Public defence
2022-11-25, (Room 4205), Hannes Alfvéns väg 12, Alba Nova, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 218-04783Swedish Energy Agency, 45390-1
Available from: 2022-10-28 Created: 2022-10-25 Last updated: 2022-10-31Bibliographically approved

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