Ever since their discovery in 1991, carbon nanotubes are of great interest to the scientific community due to their outstanding optical, mechanical and electrical properties. Considering their impressive properties, as for instance the high current carrying capability and the possibility of ballistic charge transport, carbon nanotubes are a desired channel material in field-effect transistors, especially with respect to high frequency communication electronics. Thus, many scientific studies on CNT-based field-effect transistors have been published so far. But despite the successful verification of excellent individual electric key values, corresponding experiments are mostly performed under synthetic conditions (considering e.g. temperature or gas atmosphere), which are not realizable during realistic application scenarios. Furthermore, technologically relevant factors like homogeneity, reproducibility and yield of functioning devices are often subordinated to the achievement of a single electric record value. Hence, this work focuses on the development of a fabrication technology for carbon nanotube field-effect transistors, that takes those factors into account. Thereby, this work expands the state of the art by introduction and statistical assessment of two cleaning processes: a) wet chemical removal of surfactant residues (sodium dodecylsulfate) from CNTs, integrated using the dielectrophoretic approach, by investigation and comparison of four procedures (de-ionized water, HNO3, oDCB, Ethanol); b) the reduction of process-related substrate contaminations by application of an oxygen plasma. Beyond that, the passivation of the final, working devices is developed further, as their typical definition as diffusion barrier is expanded by the reduction of parasitic capacitances in the transistor. In this context, two so far barely considered materials, hydrogen silsesquioxane and Xdi-dcs, a polymer mixture of poly(vinylphenol) and polymethylsilsesquioxane, are investigated and assessed. The novelty of the Xdi-dcs mixture causes the necessity of fundamental considerations on controllable etching procedures and resulting adaptions of the technological fabrication sequence.:Bibliographic description 3
List of abbreviations 10
List of symbols 10
1 Introduction 13
2 Basics of carbon nanotubes 15
2.1 Structural fundamentals 15
2.1.1 Hybridization of carbon 15
2.1.2 Structure of carbon nanotubes 17
2.2 Electronic properties 19
2.2.1 Band structure of graphene 19
2.2.2 Band structure of carbon nanotubes 20
2.2.3 Electronic transport in CNTs 22
2.3 Procedures for CNT integration 23
2.3.1 Growth by chemical vapor deposition 24
2.3.2 Transfer techniques 24
2.3.3 Dispersion-related integration procedures 25
2.4 Interaction of CNT and surfactant 28
3 Basics of CNT field-effect transistors 31
3.1 Principle of operation of conventional FETs 31
3.2 Distinctive features of CNT-based FETs 32
3.2.1 Metal - semiconductor contact 33
3.2.2 Linearity 38
3.3 Performance determining factors 41
3.3.1 Device architecture 41
3.3.2 Contact geometry 46
3.3.3 Other transistor dimensions 48
3.3.4 CNT-related characteristics 49
3.4 Hysteresis in transfer characteristics 51
3.4.1 Definition of hysteresis 51
3.4.2 Origins of hysteresis 52
3.4.3 Appearance of hysteresis 53
3.5 Passivation 56
3.5.1 Requirements 56
3.5.2 Importance of pre-treatments and process conditions 57
3.5.3 Overview of established passivation materials 58
4 Experimental work 63
4.1 Transistor design 63
4.2 Technology flow 66
4.3 Experimental procedures 71
4.3.1 Procedures for dissolution of SDS 71
4.3.2 Plasma treatment against surface contaminations 72
4.3.3 Evaluation of diffusion barriers 72
4.4 Instrumentation and characterization 74
4.4.1 Dielectrophoresis instrumentation 74
4.4.2 Topographical Characterization 74
4.4.3 Chemical characterization 75
4.4.4 Electrical characterization 76
5 Reduction of hysteresis 77
5.1 Removal of surfactant molecules from CNTs 77
5.1.1 Influence on molecule and CNT chemistry 78
5.1.2 Effect on transistor performance 80
5.2 Plasma-assisted removal of substrate contaminations 87
5.2.1 Influence on substrate surface 88
5.2.2 Effect on transistor performance 92
6 Passivation 97
6.1 Protection against environmental effects 97
6.1.1 Alterability of unpassivated CNT-FETs 98
6.1.2 Effects of O2 exclusion by dense passivation 99
6.1.3 Intentional doping using Y2O3 101
6.2 Passivation considering electrostatic aspects 106
6.2.1 Integration of Xdi-dcs as novel passivation 107
6.2.2 Comparison of two spin-coated dielectrics 111
6.3 Potential of double-layer approaches 113
6.3.1 Evaluation of the gas barrier performance 113
6.3.2 Influence on the transistor behavior 116
7 Summary and Outlook 121
Danksagung 127
Appendix 129
Bibliography 137
List of figures 156
List of tables 161
Selbstständigkeitserklärung 163
8 Thesen 165
9 Curriculum vitae 169 / Bereits seit ihrer Entdeckung 1991 sind Kohlenstoffnanoröhren, aufgrund ihrer herausragenden optischen, mechanischen und elektrischen Eigenschaften, für die wissenschaftliche Community von großem Interesse. Ihre Verwendung als Kanalmaterial in Feld-Effekt Transistoren ist in Anbetracht ihrer außergewöhnlichen Eigenschaften, wie z. B. die hohe Stromtragfähigkeit, sowie die Möglichkeit des ballistischen Transports von Ladungsträgern besonders für die hochfrequente Kommunikationselektronik erstrebenswert. Dementsprechend viele wissenschaftliche Arbeiten befassen sich mit der Erforschung von auf Kohlenstoffnanoröhren basierenden Transistoren. Doch trotz des erfolgreichen Nachweises ausgezeichneter Werte für viele individuelle elektrische Kenngrößen, finden entsprechenden Experimente zumeist unter anwendungsfernen Bedingungen bezüglich Temperatur bzw. Gasatmosphäre statt. Darüber hinaus werden dem Erreichen eines elektrischen Rekordwertes oft technologisch relevante Größen wie Homogenität, Reproduzierbarkeit und Ausbeute an funktionsfähigen Bauteilen untergeordnet. Der Fokus dieser Arbeit liegt daher auf der Erarbeitung einer Technologie zur Herstellung Kohlenstoffnanoröhrenbasierter Feld-Effekt Transistoren, unter Berücksichtigung dieser Aspekte. Dabei erweitert diese Arbeit den Stand der Technik durch die Einführung und statistische Beurteilung zweier Reinigungsprozesse: a) der nasschemischen Beseitigung von Tensidresten (Natriumdodecylsulfat) an mittels Dielektrophorese integrierten CNTs, wobei insgesamt vier Prozeduren (de-ionisiertes Wasser, HNO3, oDCB, Ethanol) betrachtet und miteinander verglichen wurden; b) der Beseitigung von prozessbedingten Substratkontaminationen durch ein Sauerstoffplasma. Darüber hinaus wird die Passivierung der funktionsfähigen Bauelemente weiterentwickelt, indem ihre typische Definition als Diffusionsbarriere um den Aspekt der Verringerung parasitärer Kapazitäten im Transistor erweitert wird. In diesem Zusammenhang werden mit Wasserstoff-Silsesquioxane und Xdi-dcs, einem Polymergemisch aus Poly(vinylphenol) und Polymethylsilsesquioxane, zwei bislang wenig beachtete Materialien, untersucht und bewertet. Die Neuheit des Xdi-dcs Gemisches macht dabei fundamentale Untersuchungen zur Strukturierbarkeit und entsprechende technologische Anpassungen im Gesamtablauf nötig.:Bibliographic description 3
List of abbreviations 10
List of symbols 10
1 Introduction 13
2 Basics of carbon nanotubes 15
2.1 Structural fundamentals 15
2.1.1 Hybridization of carbon 15
2.1.2 Structure of carbon nanotubes 17
2.2 Electronic properties 19
2.2.1 Band structure of graphene 19
2.2.2 Band structure of carbon nanotubes 20
2.2.3 Electronic transport in CNTs 22
2.3 Procedures for CNT integration 23
2.3.1 Growth by chemical vapor deposition 24
2.3.2 Transfer techniques 24
2.3.3 Dispersion-related integration procedures 25
2.4 Interaction of CNT and surfactant 28
3 Basics of CNT field-effect transistors 31
3.1 Principle of operation of conventional FETs 31
3.2 Distinctive features of CNT-based FETs 32
3.2.1 Metal - semiconductor contact 33
3.2.2 Linearity 38
3.3 Performance determining factors 41
3.3.1 Device architecture 41
3.3.2 Contact geometry 46
3.3.3 Other transistor dimensions 48
3.3.4 CNT-related characteristics 49
3.4 Hysteresis in transfer characteristics 51
3.4.1 Definition of hysteresis 51
3.4.2 Origins of hysteresis 52
3.4.3 Appearance of hysteresis 53
3.5 Passivation 56
3.5.1 Requirements 56
3.5.2 Importance of pre-treatments and process conditions 57
3.5.3 Overview of established passivation materials 58
4 Experimental work 63
4.1 Transistor design 63
4.2 Technology flow 66
4.3 Experimental procedures 71
4.3.1 Procedures for dissolution of SDS 71
4.3.2 Plasma treatment against surface contaminations 72
4.3.3 Evaluation of diffusion barriers 72
4.4 Instrumentation and characterization 74
4.4.1 Dielectrophoresis instrumentation 74
4.4.2 Topographical Characterization 74
4.4.3 Chemical characterization 75
4.4.4 Electrical characterization 76
5 Reduction of hysteresis 77
5.1 Removal of surfactant molecules from CNTs 77
5.1.1 Influence on molecule and CNT chemistry 78
5.1.2 Effect on transistor performance 80
5.2 Plasma-assisted removal of substrate contaminations 87
5.2.1 Influence on substrate surface 88
5.2.2 Effect on transistor performance 92
6 Passivation 97
6.1 Protection against environmental effects 97
6.1.1 Alterability of unpassivated CNT-FETs 98
6.1.2 Effects of O2 exclusion by dense passivation 99
6.1.3 Intentional doping using Y2O3 101
6.2 Passivation considering electrostatic aspects 106
6.2.1 Integration of Xdi-dcs as novel passivation 107
6.2.2 Comparison of two spin-coated dielectrics 111
6.3 Potential of double-layer approaches 113
6.3.1 Evaluation of the gas barrier performance 113
6.3.2 Influence on the transistor behavior 116
7 Summary and Outlook 121
Danksagung 127
Appendix 129
Bibliography 137
List of figures 156
List of tables 161
Selbstständigkeitserklärung 163
8 Thesen 165
9 Curriculum vitae 169
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:72449 |
| Date | 16 October 2020 |
| Creators | Tittmann-Otto, Jana |
| Contributors | Schulz, Stefan E., Tegenkamp, Christoph, Hermann, Sascha, Technische Universität Chemnitz |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
| Relation | 10.1063/1.4944835 |
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