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Silicon nanowires, nanopillars and quantum dots: Fabrication and characterization
KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
2005 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Semiconductor nanotechnology is today a very well studied subject, and demonstrations of possible applications and concepts are abundant. However, well-controlled mass-fabrication on the nanoscale is still a great challenge, and the lack of nanofabrication methods that provide the combination of required fabrication precision and high throughput, limits the large-scale use of nanodevices. This work aims in resolving some of the issues related to nanostructure fabrication, and deals with development of nanofabrication processes, the use of size-reduction for reaching true nanoscale dimensions (20 nm or below), and finally the optical and electrical characterization to understand the physics of the more successful structures and devices in this work. Due to its widespread use in microelectronics, silicon was the material of choice throughout this work.

Initially, a fabrication process based on electron beam lithography (EBL) was designed, allowing controlled fabrication of devices of dimensions down to 30 nm, although, generally, initial device dimensions were above 70 nm, allowing the flexible but low-throughput EBL, to be replaced by state-of-the-art optical lithography in the case of industrialization of the process. A few main processes were developed throughout the course of this work, which were capable of defining silicon nanopillar and nano-wall arrays from bulk silicon, and silicon nanowire devices from silicon-on-insulator (SOI) material.

Secondly, size-reduction, as a means of providing access to few-nanometer dimensions not available by current lithography techniques was investigated. An additional goal of the size-reduction studies was to find self-limiting mechanisms in the process, that would limit the impact of variations in the size and other imperfections of the initial structures. Thermal oxidation was investigated mainly for self-limited size-reduction of silicon nanopillars, resulting in well-defined quantum dot arrays of few-nm dimensions. Electrochemical etching was employed to size-reduce both silicon nanopillars and silicon nanowires down into the 10-nm regime. This being a novel application, a more thorough study of electrochemical etching of low-dimensional and thin-layer structures was performed as well as development of a micro-electrochemical cell, enabling electrochemical etching of fabricated nanowire devices with improved control.

Finally, the combination of nanofabrication and size-reduction resulted in two successful device structures: Sparse and spatially well-controlled single silicon quantum dot arrays, and electrically connected size-reduced silicon nanowires. The quantum dot arrays were investigated through photoluminescence spectroscopy demonstrating for the first time atomic-like photoemission from single silicon quantum dots. The silicon nanowire devices were electrically characterized. The current transport through the device was determined to be through inversion layer electrons with surface states of the nanowire surfaces greatly affecting the conductance of the nanowire. A model was also proposed, capable of relating physical and electrical properties of the nanowires, as well as demonstrating the considerable influence of charged surface states on the nanowire conductance.

Place, publisher, year, edition, pages
Stockholm: KTH , 2005. , ix, 98 p.
Series
Trita-FTE, ISSN 0284-0545 ; 2005:4
Keyword [en]
Silicon nanowires, silicon nanopillars, silicon quantum dots, size-reduction, thermal oxidation, electrochemical etching of silicon, photoluminescence
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:kth:diva-420OAI: oai:DiVA.org:kth-420DiVA: diva2:11200
Public defence
2005-09-27, C1, KTH-Electrum, Isafjordsgatan 26, Kista, 10:15
Opponent
Supervisors
Note
QC 20101101Available from: 2005-09-15 Created: 2005-09-15 Last updated: 2010-11-01Bibliographically approved
List of papers
1. Laser assisted electrochemical etching of silicon: simulations and experiment
Open this publication in new window or tab >>Laser assisted electrochemical etching of silicon: simulations and experiment
2001 (English)In: Pits and Pores II: Formation Properties and Significance for Advanced Materials, Pennington, NJ: Electrochemical Society , 2001, 500-508 p.Chapter in book (Other academic)
Place, publisher, year, edition, pages
Pennington, NJ: Electrochemical Society, 2001
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-6469 (URN)1-56677-292-3 (ISBN)
Note
QC 20101101Available from: 2005-09-15 Created: 2005-09-15 Last updated: 2010-11-01Bibliographically approved
2. Size reduction of silicon nanopillars by photo-electrochemical etching
Open this publication in new window or tab >>Size reduction of silicon nanopillars by photo-electrochemical etching
2001 (English)In: Materials Research Society Symposium Proceedings, ISSN 0272-9172, E-ISSN 1946-4274, Vol. 638, F851-F855 p.Article in journal (Refereed) Published
Abstract [en]

Silicon nanopillars, formed by electron beam lithography, were electrochemically etched to provide controlled size reduction. The smallest dimensions achieved were pillars of 15 nm in diameter, restricted mainly by the scanning electron microscope used for characterization. The etch rate was mainly determined by the photogeneration of carriers, by the HF concentration and by the applied voltage bias. The applied bias also controlled the resulting shape of the pillars such that a high bias resulted in etching of the pillar top whereas a negative bias caused etching only at the pillar base. For 0 V, a relatively conform etching of the pillar was observed. We discuss these phenomena in terms of electropolishing or pore formation effects on a local scale.

Keyword
Characterization, Charge carriers, Electric potential, Electrochemistry, Electrolytic polishing, Electron beam lithography, Etching, Scanning electron microscopy, Silicon, Photo-electrochemical etching, Nanostructured materials
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-25817 (URN)
Note
QC 20101101Available from: 2010-11-01 Created: 2010-11-01 Last updated: 2017-12-12Bibliographically approved
3. Three-dimensionally controlled size-reduction of silicon nanopillars by photoelectrochemical etching
Open this publication in new window or tab >>Three-dimensionally controlled size-reduction of silicon nanopillars by photoelectrochemical etching
2001 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 78, no 20, 3118-3120 p.Article in journal (Refereed) Published
Abstract [en]

Silicon nanopillars, fabricated by electron beam lithography and reactive ion etching, were size-reduced using laser-assisted electrochemical etching in a dilute hydrofluoric acid solution. The progressing size reduction was followed by scanning electron microscopy down to final diameters of similar to 15 nm. By varying the voltage bias, it was found that etching could be directed primarily at the pillar top (7 V) or at the pillar base (-0.6 V) whereas in an intermediate regime, conformal etching could be obtained. From the rate of volume change during etching, it was concluded that holes, participating in the dissolution reaction, were primarily generated within the pillar volume. The corresponding effective dissolution valence was similar to5-9, indicating substantial recombination losses within the pillar.

Keyword
N-TYPE SILICON, POROUS SILICON, FABRICATION
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-6471 (URN)000168559600040 ()
Note
QC 20101101Available from: 2005-09-15 Created: 2005-09-15 Last updated: 2017-12-14Bibliographically approved
4. Photoluminescence spectroscopy of single silicon quantum dots
Open this publication in new window or tab >>Photoluminescence spectroscopy of single silicon quantum dots
2002 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 80, no 6, 1070-1072 p.Article in journal (Refereed) Published
Abstract [en]

Photoluminescence (PL) from single silicon quantum dots have been recorded and spectrally resolved at room temperature. The Si nanocrystals (NCs) were fabricated using electron-beam lithography and reactive ion etching resulting in Si nanopillars that were subsequently oxidized to produce luminescent silicon cores. The NCs are organized in a regular matrix which enables repeated observation of a specific single NC. By reflection and PL imaging, the emission is shown to originate from the Si nanopillars. The single-NC PL spectrum has a single band with a width of similar to130 meV. The emission is polarized in arbitrary directions suggestive of geometrical differences in the shape of the nanocrystals. The quantum efficiency of the PL has been found to reach as much as 35% for some nanocrystals. Our experiments support the quantum-confinement model for the PL emission of Si nanocrystals and elucidate the critical role of defect passivation.

Keyword
POROUS SILICON, LUMINESCENCE, NANOCRYSTALS
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-6472 (URN)10.1063/1.1448400 (DOI)000173612900056 ()
Note
QC 20101101Available from: 2005-09-15 Created: 2005-09-15 Last updated: 2017-12-14Bibliographically approved
5. Silicon nanofabrication by electron beam lithography and laser-assisted electrochemical size-reduction
Open this publication in new window or tab >>Silicon nanofabrication by electron beam lithography and laser-assisted electrochemical size-reduction
2002 (English)In: Microelectronic Engineering, ISSN 0167-9317, E-ISSN 1873-5568, Vol. 61-62, 563-568 p.Article in journal (Refereed) Published
Abstract [en]

Laser-assisted electrochemical size-reduction has been carried out on silicon nanostructures produced by electron beam lithography and reactive ion etching. We demonstrate the ability to reduce nanopillars down to 10 nm diameter while preserving shape, but also the possibility of preferential etching of different parts of the pillar by varying the applied bias voltage. Furthermore, the origin of the carriers responsible for the etching is discussed, and we note the presence of a 'dark' etching mechanism working in parallel with the normal dissolution reaction. Finally, the etching of shallow Si dots on a Si surface shows further localization of etching, with a different etching reaction taking place in the vicinity of the structures as opposed to the planar surface, far from the structures.

Keyword
electrochemical etching, laser-assisted etching, size-reduction, silicon nanofabrication, nanostructures
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-6473 (URN)10.1016/S0167-9317(02)00532-4 (DOI)000176594700077 ()
Note
QC 20101101Available from: 2005-09-15 Created: 2005-09-15 Last updated: 2017-12-14Bibliographically approved
6. Size-reduced silicon nanowires: Fabrication and electrical characterization
Open this publication in new window or tab >>Size-reduced silicon nanowires: Fabrication and electrical characterization
2005 (English)In: Materials science & engineering. C, biomimetic materials, sensors and systems, ISSN 0928-4931, E-ISSN 1873-0191, Vol. 25, no 5-8, 733-737 p.Article in journal (Refereed) Published
Abstract [en]

Silicon nanowires of diameters down to 100 nm and typical lengths of 1-3 μm have been fabricated in silicon-on-insulator material by electron beam lithography and plasma etching. They were subsequently size-reduced by photoelectrochemical etching resulting in wire widths down to 10 nm. To enable accurate control of the photoelectrochemical size-reduction, a micro-electrochemical cell was developed, enabling single nanowires to be exposed to the etching solution while being illuminated by a laser or a lamp. The arrangement allows contact leads to be extended to metal contact pads located outside the cell, which can be connected by probes, allowing in situ electrical characterization of a nanowire during etching. In this paper, we describe the experimental setup, the fabrication method and show examples of achieved wire widths together with some preliminary results from the electrical characterization.

Keyword
Electrochemical etching of silicon, Electron beam lithography, Hydrofluoric acid, Micro-electrochemical cell, Silicon nanowires, Size-reduction
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-6474 (URN)10.1016/j.msec.2005.06.036 (DOI)000233784800038 ()2-s2.0-27744488489 (Scopus ID)
Note
QC 20100929. Uppdaterad från In press till Published (20100929).Available from: 2005-09-15 Created: 2005-09-15 Last updated: 2017-12-14Bibliographically approved
7. Narrow luminescence linewidth of a silicon quantum dot
Open this publication in new window or tab >>Narrow luminescence linewidth of a silicon quantum dot
2005 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 94, no 8, 087405 (1)-087405 (4) p.Article in journal (Refereed) Published
Abstract [en]

Single-dot luminescence spectroscopy was used to study the emission linewidth of individual silicon nanocrystals from low temperatures up to room temperature. The results show a continuous line narrowing towards lower temperatures with a linewidth as sharp as 2 meV at 35 K. This value, clearly below the thermal broadening at this temperature, proves the atomiclike emission from silicon quantum dots subject to quantum confinement. The low temperature measurements further reveal a similar to6 meV replica, whose origin is discussed. In addition, an similar to60 meV TO-phonon replica was detected, which is only present in a fraction of the dots.

Place, publisher, year, edition, pages
American Physical Society, 2005
Keyword
electron-phonon interactions, photoluminescence spectroscopy, semiconductor nanocrystals, porous silicon, confinement
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-6697 (URN)10.1103/PhysRevLett.94.087405 (DOI)000227386000066 ()2-s2.0-18144403210 (Scopus ID)
Note
QC 20100922Available from: 2012-01-11 Created: 2006-12-29 Last updated: 2017-12-14Bibliographically approved
8. Controlled Fabrication of Silicon Nanowires by Electron Beam Lithography and Electrochemical Size Reduction
Open this publication in new window or tab >>Controlled Fabrication of Silicon Nanowires by Electron Beam Lithography and Electrochemical Size Reduction
2005 (English)In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 5, no 2, 275-280 p.Article in journal (Refereed) Published
Abstract [en]

We demonstrate that electrochemical size reduction can be used for precisely controlled fabrication of silicon nanowires of widths approaching the 10 nm regime. The scheme can, in principle, be applied to wires defined by optical lithography but is here demonstrated for wires of similar to100-200 nm width, defined by electron beam lithography. As for electrochemical etching of bulk silicon, the etching can be tuned both to the pore formation regime as well as to electropolishing. By in-situ optical and electrical characterization, the process can be halted at a certain nanowire width. Further electrical characterization shows a conductance decreasing faster than dimensional scaling would predict. As an explanation, we propose that charged surface states play a more pronounced role as the nanowire cross-sectional dimensions decrease.

Keyword
Electric conductivity; Electrochemistry; Electrolytic polishing; Electron beam lithography; Etching; Morphology; Photolithography; Silicon; Charged surface states; Electrochemical etching; Electrochemical size reduction; Silicon nanowires
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-6476 (URN)10.1021/nl0481573 (DOI)000227100500015 ()2-s2.0-14644409706 (Scopus ID)
Note
QC 20100719Available from: 2005-09-15 Created: 2005-09-15 Last updated: 2017-12-14Bibliographically approved
9. Electrical Properties of Silicon Nanowire Devices
Open this publication in new window or tab >>Electrical Properties of Silicon Nanowire Devices
(English)Manuscript (Other academic)
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-6477 (URN)
Note
QC 20101101Available from: 2005-09-15 Created: 2005-09-15 Last updated: 2010-11-01Bibliographically approved

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