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Fabrication and characterization of single luminescing quantum dots from 1D silicon nanostructures
KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Silicon as a mono-crystalline bulk semiconductor is today the predominant material in many integrated electronic and photovoltaic applications. This has not been the case in lighting technology, since due to its indirect bandgap nature bulk silicon is an inherently poor light emitter.With the discovery of efficient light emission from silicon nanostructures, great new interest arose and research in this area increased dramatically.However, despite more than two decades of research on silicon nanocrystals and nanowires, not all aspects of their light emission mechanisms and optical properties are well understood, yet.There is great potential for a range of applications, such as light conversion (phosphor substitute), emission (LEDs) and harvesting (solar cells), but for efficient implementation the underlying mechanisms have to be unveiled and understood.Investigation of single quantum emitters enable proper understanding and modeling of the nature and correlation of different optical, electrical and geometric properties.In large numbers, such sets of experiments ensure statistical significance. These two objectives can best be met when a large number of luminescing nanostructures are placed in a pattern that can easily be navigated with different measurement methods.This thesis presents a method for the (optional) simultaneous fabrication of luminescent zero- and one-dimensional silicon nanostructuresand deals with their structural and optical characterization.Nanometer-sized silicon walls are defined by electron beam lithography and plasma etching. Subsequent oxidation in the self-limiting regime reduces the size of the silicon core unevenly and passivates it with a thermal oxide layer.Depending on the oxidation time, nanowires, quantum dots or a mixture of both types of structures can be created.While electron microscopy yields structural information, different photoluminescence measurements, such as time-integrated and time-resolved imaging, spectral imaging, lifetime measurements and absorption and emission polarization measurements, are used to gain knowledge about optical properties and light emission mechanisms in single silicon nanocrystals.The fabrication method used in this thesis yields a large number of spatially separated luminescing quantum dots randomly distributed along a line, or a slightly smaller number that can be placed at well-defined coordinates. Single dot measurements can be performed even with an optical microscope and the pattern, in which the nanostructures are arranged, enables the experimenter to easily find the same individual dot in different measurements.Spectral measurements on the single dot level reveal information about processes that are involved in the photoluminescence of silicon nanoparticles and yield proof for the atomic-like quantized nature of energy levels in the conduction and valence band, as evidenced by narrow luminescence lines (~500 µeV) at low temperature. Analysis of the blinking sheds light on the charging mechanisms of oxide-capped Si-QDs and, by exposing exponential on- and off-time distributions instead of the frequently observed power law distributions, argues in favor of the absence of statistical aging. Experiments probing the emission intensity as a function of excitation power suggest that saturation is not achieved. Both absorption and emission of silicon nanocrystals contained in a one-dimensional silicon dioxide matrix are polarized to a high degree. Many of the results obtained in this work seem to strengthen the arguments that oxide-capped silicon quantum dots have universal properties, independently of the fabrication method, and that the greatest differences between individual nanocrystals are indeed caused by individual factors like local environment, shape and size (among others).

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , xv, 60 p.
Series
Trita-ICT/MAP AVH, ISSN 1653-7610 ; 2012:15
Keyword [en]
silicon, quantum dot, nanocrystal, nanowire, nanostructure, photoluminescence
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-102524ISBN: 978-91-7501-486-9 (print)OAI: oai:DiVA.org:kth-102524DiVA: diva2:555258
Public defence
2012-10-12, Sal E, Forum KTH-ICT, Isafjordsgatan 39, Kista, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council
Note

QC 20120920

Available from: 2012-09-20 Created: 2012-09-19 Last updated: 2012-09-20Bibliographically approved
List of papers
1. Controlled fabrication of individual silicon quantum rods yielding high intensity, polarized light emission
Open this publication in new window or tab >>Controlled fabrication of individual silicon quantum rods yielding high intensity, polarized light emission
2009 (English)In: Nanotechnology, ISSN 0957-4484, E-ISSN 1361-6528, Vol. 20, no 50, 1-5 p.Article in journal (Refereed) Published
Abstract [en]

Elongated silicon quantum dots (also referred to as rods) were fabricated using a lithographic process which reliably yields sufficient numbers of emitters. These quantum rods are perfectly aligned and the vast majority are spatially separated well enough to enable single-dot spectroscopy. Not only do they exhibit extraordinarily high linear polarization with respect to both absorption and emission, but the silicon rods also appear to luminesce much more brightly than their spherical counterparts. Significantly increased quantum efficiency and almost unity degree of linear polarization render these quantum rods perfect candidates for numerous applications.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2009
Keyword
porous si, photoluminescence, nanocrystals, anisotropy, spectroscopy, nanowires, oxidation
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-18995 (URN)10.1088/0957-4484/20/50/505301 (DOI)000272166200006 ()
Note
QC 20100525Available from: 2012-03-21 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
2. Fabricating single silicon quantum rods for repeatable single dot photoluminescence measurements
Open this publication in new window or tab >>Fabricating single silicon quantum rods for repeatable single dot photoluminescence measurements
2011 (English)In: Physica Status Solidi A-applications and materials science, ISSN 1862-6319, Vol. 208, no 3, 631-634 p.Article in journal (Refereed) Published
Abstract [en]

A fabrication method for a matrix pattern of laterally separated silicon quantum rods was developed, consisting of a three-step recipe utilizing electron beam lithography (EBL), reactive ion etching (RIE), and oxidation. Photoluminescence (PL) measurements -images, spectra, and blinking-verified that the presented method results in a high number of luminescing single silicon quantum rods in well defined positions on the sample. These are suitable for single dot spectroscopy and repeatable measurements, even using different measurement methods and instruments. [GRAPHICS] Colorized scanning electron microscope images of undulating silicon nanowalls for controlled single quantum rod fabrication.

Place, publisher, year, edition, pages
Malden: Wiley-VCH Verlagsgesellschaft, 2011
Keyword
nanocrystal, quantum dot, quantum rod, silicon, single dot spectroscopy
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-32127 (URN)10.1002/pssa.201000556 (DOI)000288177600030 ()2-s2.0-79952526662 (Scopus ID)
Note
QC 20110407Available from: 2012-03-21 Created: 2011-04-07 Last updated: 2015-10-01Bibliographically approved
3. Transition fromsilicon nanowires to isolated quantum dots: Optical and structural evolution
Open this publication in new window or tab >>Transition fromsilicon nanowires to isolated quantum dots: Optical and structural evolution
Show others...
2009 (English)Manuscript (preprint) (Other (popular science, discussion, etc.))
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-102556 (URN)
Note

QS 2012

Available from: 2012-09-20 Created: 2012-09-20 Last updated: 2012-09-20Bibliographically approved
4. Coexistence of 1D and Quasi-0D Photoluminescence from Single Silicon Nanowires
Open this publication in new window or tab >>Coexistence of 1D and Quasi-0D Photoluminescence from Single Silicon Nanowires
2011 (English)In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 11, no 7, 3003-3009 p.Article in journal (Refereed) Published
Abstract [en]

Single silicon nanowires (Si-NWs) prepared by electron-beam lithography and reactive-ion etching are investigated by imaging optical spectroscopy under variable temperatures and laser pumping intensities. Spectral images of individual Si-NWs reveal a large variability of photoluminescence (PL) along a single Si-NW. The weaker broad emission band asymmetrically extended to the high-energy side is interpreted to be due to recombination of quasi-free 1D excitons while the brighter localized emission features (with significantly variable peak position, width, and shape) are due to localization of electron hole pairs in surface protrusions acting like quasi-0D centers or quantum dots (QDs). Correlated PL and scanning electron microscopy images indicate that the efficiently emitting QDs are located at the Si-NW interface with completely oxidized neck of the initial Si wall. Theoretical fitting of the delocalized PL emission band explains its broad asymmetrical band to be due to the Gaussian size distribution of the Si-NW diameter and reveals also the presence of recombination from the Si-NW excited state which can facilitate a fast capture of excitons into QD centers.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2011
Keyword
Nanowire, quantum dot, silicon, luminescence, exciton
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-37270 (URN)10.1021/nl201610g (DOI)000292849400075 ()2-s2.0-79960239636 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20110809

Available from: 2012-03-21 Created: 2011-08-08 Last updated: 2017-06-09Bibliographically approved
5. Blinking Statistics of Silicon Quantum Dots
Open this publication in new window or tab >>Blinking Statistics of Silicon Quantum Dots
2011 (English)In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 11, no 12, 5574-5580 p.Article in journal (Refereed) Published
Abstract [en]

The blinking statistics of numerous single silicon quantum dots fabricated by electron-beam lithography, plasma etching, and oxidation have been analyzed. Purely exponential on- and off-time distributions were found consistent with the absence of statistical aging. This is in contrast to blinking reports in the literature where power-law distributions prevail as well as observations of statistical aging in nanocrystal ensembles. A linear increase of the switching frequency with excitation power density indicates a domination of single-photon absorption processes, possibly through a direct transfer of charges to trap states without the need for a bimolecular Auger mechanism. Photoluminescence saturation with increasing excitation is not observed; however, there is a threshold in excitation (coinciding with a mean occupation of one exciton per nanocrystal) where a change from linear to square-root increase occurs. Finally, the statistics of blinking of single quantum dots in terms of average on-time, blinking frequency and blinking amplitude reveal large variations (several orders) without any significant correlation demonstrating the individual microscopic character of each quantum dot.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2011
Keyword
Blinking, intermittency, silicon, quantum dot, nanocrystal, photoluminescence, statistical aging, bleaching
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-58819 (URN)10.1021/nl203618h (DOI)000297950200081 ()2-s2.0-83655164324 (Scopus ID)
Note
QC 20120109Available from: 2012-03-21 Created: 2012-01-09 Last updated: 2017-12-08Bibliographically approved
6. Polarization of photoluminescence excitation and emission spectra of silicon nanorods within single Si/SiO2 nanowires
Open this publication in new window or tab >>Polarization of photoluminescence excitation and emission spectra of silicon nanorods within single Si/SiO2 nanowires
2011 (English)In: Physica Status Solidi. C, Current topics in solid state physics, ISSN 1610-1634, E-ISSN 1610-1642, Vol. 8, no 3, 1017-1020 p.Article in journal (Refereed) Published
Abstract [en]

Polarization properties of individual silicon nanowires are studied using an optical micro-spectroscopy setup equipped with a Fresnel rhomb to rotate the polarization of the exciting laser and the analyzer to characterize the polarization of emitted photoluminescence. The Si nanowire samples are prepared by electron-beam lithography, plasma etching and oxidation. The fabricated wires are embedded in SiO2 and oriented parallel to the Si substrate. Due to the fluctuating wire diameter (around 5 nm) the very long wires (several tens of μm) are effectively divided into an array of quantum rods (prolate ellipsoids). These structures have strong photoluminescence under UV-blue excitation at room temperature. The degree of photoluminescence linear polarization of both excitation and emission is very high, between 0.9-1, and reveals relatively low fluctuations at different spots of the wires. Experimental results are compared with available theoretical models leading to the conclusion that the high polarization degree is mostly due to surface charges (dielectric confinement) with smaller contribution of quantum confinement effects.

Place, publisher, year, edition, pages
Wiley-VCH Verlagsgesellschaft, 2011
Keyword
Luminescence, Nanorod, Nanowire, Polarization, Silicon
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-49809 (URN)10.1002/pssc.201000398 (DOI)000301537100087 ()2-s2.0-79952675785 (Scopus ID)
Note
QC 20111130Available from: 2012-03-21 Created: 2011-11-29 Last updated: 2017-12-08Bibliographically approved
7. Exciton lifetime measurementson single silicon quantum dots: explanation of stretched exponentialdecay
Open this publication in new window or tab >>Exciton lifetime measurementson single silicon quantum dots: explanation of stretched exponentialdecay
2011 (English)Manuscript (preprint) (Other academic)
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-102562 (URN)
Note

QS 2012

Available from: 2012-09-20 Created: 2012-09-20 Last updated: 2012-09-20Bibliographically approved

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