Silicon Nanowire based Reconfigurable Field Effect Transistor (SiNW RFET) presents a solution to increase the system functionality beyond the limits of classical CMOS scaling in More-than-Moore era of semiconductor technology. They are not only spatially reconfigurable, i.e., the source and the drain can be interchangeable in design, but in such devices one can also control the primary charge carrier by controlling the voltage in the control gate. The two key morphological factors controlling reconfigurability are the structure and composition of the Schottky junctions, which serve as the location for Program and Control gates and radial strain induced by the self-limiting oxidation, which influences the carrier mobility resulting in symmetric p and n characteristic curves of an RFET. Despite its potential, in-depth nanoscale studies on the structural and compositional characterization of the key features controlling the reconfigurability are limited and thereby presents as a novel area of research.
In this study, the composition and morphology of the Schottky junction and the radial strain profile due to self-limiting oxidation were studied using advanced imaging and sample preparation techniques like Transmission Electron Microscope (TEM) and Scanning Electron Microscope (SEM) imaging alongside with precise sample preparation methods like Focused Ion Beam (FIB) liftout techniques.
For analysis of radial strain in nanowires that underwent self-limiting oxidation, a TEM lamella was taken of a cross-section of the NW. The lamella was kept at 200 nm thickness to preserve the strain state of the nanowire cross-section.
It was observed that nanowires undergoing such oxidation have an omega (Ω) shaped oxide shell where the shell was discontinued at the spot where the nanowire was touching the substrate. Fast Fourier transform of the high-resolution image of such a NW crossection was used to calculate the strain profile.
The strain is also found to be not radially uniform for such Ω shaped oxide shells. The strain profile shows a local maxima near the nanowire base where it touches the substrate then a minima approximately at the geometric center followed by the maximum strain at the area adjacent to the oxide shell thereby showing a sinusoidal profile. Theoretical simulations performed by Dr. Tim Baldauf further verified the nature of the sinusoidal strain that was observed experimentally. Similar simulations were done for different omega shell shapes, which yielded strain plots of similar sinusoidal strain plots, with the local maxima depending on the level of encapsulation of the NW by the shell.
In the characterization of the Schottky junction, a TEM lamella was taken along the longitudinal direction of a nanowire, which was silicidized from both ends, similar to ones used in SiNW RFET devices. High resolution TEM micrographs and EDX (Energy dispersive X-Ray Spectroscopy) in the TEM along the Schottky junction showed a Ni rich phase and pure Si on either side of the junction. This participating phase was identified as NiSi2. However, the transition between the phases shows a gradient and in-situ experiments were designed to verify the sharpness of the junction.
In in-situ silicidation experiments, Si nanowires with a thin native oxide shell were distributed on an electron transparent surface and were partially covered with Ni islands by shadow sputtering. The whole setup was then heated in a heating stage of a TEM and the Ni was allowed to disperse within the Si nanowires forming NiSi2. HRTEM (High Resolution TEM), EDX and EELS (Electron Energy Loss Spectroscopy) studies were performed on the silicidized samples for further ex-situ analysis.
During the in-situ experiment, it was observed that Ni-phase interface is atomistically sharp and seldom progresses perpendicularly to the nanowire’s direction but through the closed packed planes of the NW. The interface velocity at different temperatures was used to calculate the activation energy of the silicidation process. The value of the activation energy indicates the Ni undergoing volume diffusion through the Ni-rich phase. The velocity of the interface was observed to be much higher in nanowires with smaller diameters than those with higher diameters, further proving the hypothesis.
During the in-situ experiments, in around 10% of nanowires that underwent complete silicidation and held isothermally, the crystalline silicide phase was observed to partially or fully diffuse out of the nanowire core, leaving only a thin shell of Silicon oxide forming ultra-thin walled SiO2 nanotubes (NT).
The onset and the time required for completion of the process varies in the nanowires depending on size of the nanowire, the distance and contact to the nearest Ni islands and presence of defects such as kinks and twists within the nanowire.
In order to study the dynamics of the process, the velocity of the receding front was calculated for nanowires of two different diameters. They are found to be identical, indicating the volume flow rate of the process is directly proportional to the cross-sectional area. The voids were formed by the reduced diffusivity of Ni in Ni2Si phase in comparison to phases with lower percent of Ni. This indicates that the reason behind the phenomenon is coalition of Kirkendall voids and thus dependent on volume diffusion.
From this study, it can be concluded that the extent of self-limiting oxidation and shape of the shell can influence the radial strain state. This can be used to manipulate the strain to tailor the electron and hole transfer characteristics within the RFET. A variety of factors including temperature, time, orientation and radius of the nanowires has been studied with respect to silicidation of a SiNW. The calculated activation energy can be used for precise process control over the location and morphology of Schottky junction. Although not directly related to SiNW RFET devices, the self-assembly of ultra-thin-walled SiO2 NT is a novel research area in itself, the findings of which can be applied in to design novel electronics and sensors.:TABLE OF CONTENTS
Preface
List of Abbreviations
CHAPTER 1: Introduction and Motivation
1.1 Definition and History
1.2 Synthesis Routes
1.3 Properties and Applications
1.4 Nanoscale Electronics and Role of Si Nws
1.4.1
1.4.2 SiNW Reconfigurable Field Effect Transistor
1.5 Introduction to The Topic of The Thesis
1.6 Outline of The Thesis
CHAPTER 2: Physical Basics and Previous Research: A Short Summary
2.1 Strain Measurement and Effects of Strain on on Nanoelectronics
2.1.1 Strain Analysis in Planar CMOS Structures
2.2 Silicidation and Schottky Junction
2.2.1 In-situ Silicidation
2.2.2 Silicon oxide nanotubes
CHAPTER 3: Background of Instruments and Experimental Set-up
3.1 Scanning Electron Microscope
3.2 Transmission Electron Microscope
3.2.1 Imaging Techniques
3.2.2 TEM sample preparation
3.3 Focused Ion Beam
CHAPTER 4: Strain in Nanowire
4.1 Goal of This Study
4.2 Strain in SiNW RFET Devices
4.3 Strain Analysis in SiNW Cross-section
4.3.1 Sample Preparation
4.3.2 Experimental Process
4.3.3 Results and Discussion
4.4 Conclusions
CHAPTER 5: Schottky Junction
5.1 Crystallographic Data on Nickel Silicides
5.2 Formation of Silicides in 2-D Structures
5.2.1 Sample History
5.2.2 Sample Preparation
5.2.3 Results and Discussion
5.3 Formation of Silicides in 1-D Structures: Schottky Junction in NWs
5.3.1 Sample History
5.3.2 Sample Preparation
5.3.3 Results and Discussion
5.3.4 Shortcomings of The Lift-out Technique
5.4 In-situ Silicidation
5.4.1 Motivation
5.4.2 Sample Preparation
5.4.3 Experimental Procedure
5.4.4 Results and Discussions
5.4.5 Shortcoming of The Experiment
5.5 Self-assembling SiO2 Nanotubes
5.5.1 Sample Preparation
5.5.2 Experimental Process
5.5.3 Results and Discussion .
5.5.4 Post In-situ Experiment TEM Analysis
5.5.5 Conclusions
CHAPTER 6: Conclusions and Outlook
6.1 Strain Analysis
6.2 Schottky Junction Studies
Bibliography
Acknowledgements
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:75531 |
| Date | 26 July 2021 |
| Creators | Bukovsky, Sayanti |
| Contributors | Mikolajick, Thomas, Cuniberti, Gianaurelio, Braun, Hans-Georg, Technische Universität Dresden, Center for Advancing Electronics Dresden |
| 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 |
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