G.N. Ayre

Growth of Single-Walled Carbon Nanotubes Using Germanium Nanocrystals Formed by Implantation

This paper presents a complementary metal oxide semiconductor compatible method for the chemical vapor deposition of singlewalled carbon nanotubes. The method uses Ge implantation into a SiO2 layer to create Ge nanocrystals, which are then used to produce SWNTs. The results of atomic force microscopy and scanning electron microscopy analyses indicate that Ge implantation provides good control of particle size and delivers a well-controlled SWNT growth process. The SWNT area density of 4.1 +- 1.2 um in length/um2 obtained from the Ge nanocrystals is comparable to that obtained from metal-catalyst-based methods used to fabricate SWNT field-effect transistors. A carbon implantation after Ge nanocrystal formation significantly enhances the process operating window for the growth of the SWNTs and increases the area density.

On the mechanism of carbon nanotube formation: the role of the catalyst.

This work examines the recent developments in non-traditional catalyst-assisted chemical vapour deposition of carbon nanotubes (CNTs) with a view to determining the essential role of the catalyst in nanotube growth. A brief overview of the techniques reliant on the structural reorganization of carbon to form CNTs is provided. Additionally, CNT synthesis methods based upon ceramic, noble metal, and semiconducting nanoparticle catalysts are presented. Experimental evidence is provided for CNT growth using noble metal and semiconducting nanoparticle catalysts. A model for CNT growth consistent with the experimental results is proposed, in which the structural reorganization of carbon to form CNTs is paramount.

Electrical transport properties of isolated carbon nanotube/Si heterojunction Schottky diodes

A detailed study of the electrical transport properties of Pd contacted carbon nanotube (CNT)/Si heterojunctions is presented. The CNT with a diameter ranging from 1.2 to 2.0 nm on n-type Si substrates showed rectifying behavior with the ideality factor of 1.1–2.2 and turn on voltage of 0.05–0.34 V. The current-voltage characteristics of the CNT/n+-Si diodes were investigated in the temperature range from 50 to 300 K. The transition from thermionic emission to tunneling process was seen in the forward current around 150 K and the Schottky barrier height at Pd/CNT interface is estimated to be 0.3–0.5 eV.

CMOS compatible synthesis of carbon nanotubes

Traditionally, carbon nanotube (CNT) growth involves the use of transition metal nanoparticles as a catalyst. However, the integration of CNT synthesis based on metal catalysts with CMOS technology is very problematic due to the detrimental effect of transition metals on silicon device performance. Transition metals, such as Ni or Fe, create deep level defects in the silicon band gap and result in unwanted trap states [1, 2]. Other drawbacks include the high propensity of silicon-metal inter-diffusion, leading to the formation of silicides. In order to reap the benefits of silicon very-large-integration (VLSI) technology, an alternate approach is required. This work reports metal free-growth of carbon nanotubes, with a process compatible with current silicon VLSI technology, using chemical vapour deposition of CNTs on germanium nanoparticles. Various approaches to germanium catalyst preparation, based upon standard CMOS processes, are compared in terms of density of growth and quality of synthesized nanotubes. These approaches include thermal treatment of silicon-germanium islands [3] and germanium Stranski-Krastanow quantum dots, germanium colloidal nanoparticles and germanium nanoparticles formed by ion implantation. Scanning electron microscopy measurements indicate that a good density of growth is achievable using this methodology. Raman measurements have identified the synthesized nanotubes as single walled and, in terms of graphitisation and structure, of a high quality. Extensive atomic force microscopy characterisation of the catalyst has been undertaken in order to ascertain the influence of morphology on the ability of germanium to catalyse CNT growth. Experimental evidence has shown that this technique offers a commercially scalable method of reliably growing metal-free CNTs for various applications, while opening the prospect of merging CNT devices with silicon electronics.

Metal-Catalyst-Free Growth of Silica Nanowires and Carbon Nanotubes Using Ge Nanostructures

The use of Ge nanostructures is investigated for the metal-catalyst-free growth of silica nanowires and carbon nanotubes (CNTs). Silica nanowires with diameters of 10-50 nm and lengths of ? 1 ?m were grown from SiGe islands, Ge dots, and Ge nanoparticles. High-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectroscopy (EDS) reveal that the nanowires grow from oxide nanoparticles on the sample surface. We propose that the growth mechanism is thermal diffusion of oxide through the GeO2 nanostructures. CNTs with diameters 0.6-2.5 nm and lengths of less than a few ?m were similarly grown by chemical vapor deposition from different types of Ge nanostructures. Raman measurements show the presence of radial breathing mode peaks and the absence of the disorder induced D-band, indicating single walled CNTs with a low defect density. HRTEM images reveal that the CNTs also grow from oxide nanoparticles, comprising a mixture of GeO2 and SiO2.

Metal catalyst-free growth of carbon nanotubes and their application in field effect transitors

The metal-catalyst-free growth of carbon nanotubes (CNTs) using chemical vapor deposition and the application in field-effect transistors (FETs) is demonstrated. The CNT growth process used a 3-nm-thick Ge layer on SiO2 that was subsequently annealed to produce Ge nanoparticles. Raman measurements show the presence of radial breathing mode peaks and the absence of the disorder induced D-band, indicating single walled CNTs with a low defect density. The synthesized CNTs are used to fabricate CNTFETs and the best device has a state-of-the-art on/off current ratio of 3×108 and a steep sub-threshold slope of 110 mV/dec.

Growth of Carbon Nanotubes on HfO2 towards Highly Sensitive Nano-Sensors

Carbon nanotube (CNT) growth on HfO2 is reported for the first time. The process uses a combination of Ge and Fe nanoparticles and achieves an increase in CNT density from 0.15 to 6.2 µm length/µm2 compared with Fe nanoparticles alone. The synthesized CNTs are assessed by the fabrication of back-gate CNT field-effect transistors with Al source/drain contacts for nano-sensor applications. The devices exhibit excellent p-type behavior with an Ion/Ioff ratio of 105 and a steep sub-threshold slope of 130 mV/dec.

Chemical Vapour Deposition of CNTs Using Structural Nanoparticle Catalysts

This work examines the recent developments in non-traditional CCVD of CNTs with a view to determine the essential role of the catalyst in nanotube growth. A brief overview of the techniques reliant on the structural reorganization of carbon to form CNTs is provided. An in-depth analysis of CNT synthesis based upon ceramic, noble metal, and semiconducting nanoparticle catalysts is presented. Various approaches to germanium catalyst preparation are compared in terms of growth density and quality of synthesized nanotubes. Scanning electron microscopy measurements indicate that a technologically relevant density is achievable using non conventional catalysts. Raman measurements have identified the synthesized nanotubes as single walled and, in terms of graphitization and structure, of a high quality. Extensive atomic force microscopy characterisation of the catalyst has been undertaken in order to ascertain the influence of morphology on the ability of the catalyst to yield CNT growth. A model for CNT growth consistent with the experimental results is proposed.

Fe/Ge catalyzed carbon nanotube growth on HfO 2 for nano-sensor applications

A carbon nanotube (CNT) growth process on HfO 2 is reported for the first time for application in nano-sensors. The process uses a combination of Ge nanoparticles and ferric nitrate dispersion and achieves an increase in CNT density from 0.15 to 6.2 µm length/µm2 compared with the use of ferric nitrate dispersion alone. The growth process is validated by the fabrication of back-gate CNT field-effect transistors (CNTFETs) using Al source/drain (S/D) contacts and a H 2 anneal at 400 °C. The transistors exhibit p-FET behavior with an I on /I off ratio of 105 and a steep sub-threshold slope of 130 mV/dec. These results are rather surprising, as earlier research in the literature on CNTFETs with Al S/D electrodes showed n-FET behavior. The p-FET behavior is shown to be due to the H 2 anneal, which we ascribe to the smaller electron affinity of hydrogenised CNTs. Measurements of the temperature dependence of the drain current show low Schottky barrier height Al S/D contacts after a H 2 anneal, which tends to confirm this explanation.

Fe/Ge Catalyzed Carbon Nanotube Growth on HfO 2 for Nano-Sensor Applications

A carbon nanotube (CNT) growth process on HfO2 is reported for the first time for application in nano-sensors. The process uses a combination of Ge nanoparticles and ferric nitrate dispersion and achieves an increase in CNT density from 0.15 to 6.2 µm length/µm2 compared with the use of ferric nitrate dispersion alone. The growth process is validated by the fabrication of back-gate CNT field-effect transistors (CNTFETs) using Al source/drain (S/D) contacts and a H2 anneal at 400 °C. The transistors exhibit p-FET behavior with an Ion/Ioff ratio of 105 and a steep sub-threshold slope of 130 mV/dec. These results are rather surprising, as earlier research in the literature on CNTFETs with Al S/D electrodes showed n-FET behavior. The p-FET behavior is shown to be due to the H2 anneal, which we ascribe to the smaller electron affinity of hydrogenised CNTs. Measurements of the temperature dependence of the drain current show low Schottky barrier height Al S/D contacts after a H2 anneal, which tends to confirm this explanation.