Electrical and optical studies on carbon nanotube arrays

Single-walled carbon nanotubes (SWNTs) have been studied as a prominent class of high performance electronic materials for next generation electronics. Their geometry dependent electronic structure, ballistic transport and low power dissipation due to quasi one dimensional transport, and their capability of carrying high current densities are some of the main reasons for the optimistic expectations on SWNTs. However, device applications of individual SWNTs have been hindered by uncontrolled variations in characteristics and lack of scalable methods to integrate SWNTs into electronic devices. One relatively new direction in SWNT electronics, which avoids these issues, is using arrays of SWNTs, where the ensemble average may provide uniformity from device to device, and this new breed of electronic material can be integrated into electronic devices in a scalable fashion. This dissertation describes (1) methods for characterization of SWNT arrays, (2) how the electrical transport in these two-dimensional arrays depend on length scales and spatial anisotropy, (3) the interaction of aligned SWNTs with the underlying substrate, and (4) methods for scalable integration of SWNT arrays into electronic devices. The electrical characterization of SWNT arrays have been realized by polymer electrolyte-gated SWNT thin film transistors (TFTs). Polymer electrolyte-gating addresses many technical difficulties inherent to electrical characterization by gating through oxide-dielectrics. Having shown polymer electrolyte-gating can be successfully applied on SWNT arrays, we have studied the length scaling dependence of electrical transport in SWNT arrays. Ultrathin films formed by sub-monolayer surface coverage of SWNT arrays are very interesting systems in terms of the physics of two-dimensional electronic transport. We have observed that they behave qualitatively different than the classical conducting films, which obey the Ohm’s law. The resistance of an ultrathin film of SWNT arrays is indeed non-linear with the length of the film, across which the transport occurs. More interestingly, a transition between conducting and insulating states is observed at a critical surface coverage, which is called percolation limit. The surface coverage of conducting SWNTs can be manipulated by turning on and off the semiconductors in the SWNT array, leading to the operation principle of SWNT TFTs. The percolation limit depends also on the length and the spatial orientation of SWNTs. We have also observed that the percolation limit increases abruptly for aligned arrays of SWNTs, which are grown on single crystal quartz substrates. In this dissertation, we also compare our experimental results with a two-dimensional stick network model, which gives a good qualitative picture of the electrical transport in SWNT arrays in terms of surface coverage, length scaling, and spatial orientation, and briefly discuss the validity of this model. However, the electronic properties of SWNT arrays are not only determined by geometrical arguments. The contact resistances at the nanotube-nanotube and nanotube-electrode (bulk metal) interfaces, and interactions with the local chemical groups and the underlying substrates are among other issues related to the electronic transport in SWNT arrays. Different aspects of these factors have been studied in detail by many groups. In fact, I have also included a brief discussion about electron injection onto semiconducting SWNTs by polymer dopants. On the other hand, we have compared the substrate-SWNT interactions for isotropic (in two dimensions) arrays of SWNTs grown on Si/SiO2 substrates and horizontally (on substrate) aligned arrays of SWNTs grown on single crystal quartz substrates. The anisotropic interactions associated with the quartz lattice between quartz and SWNTs that allow near perfect horizontal alignment on substrate along a particular crystallographic direction is examined by Raman spectroscopy, and shown to lead to uniaxial compressive strain in as-grown SWNTs on single crystal quartz. This is the first experimental demonstration of the hard-to-achieve uniaxial compression of SWNTs. Temperature dependence of Raman G-band spectra along the length of individual nanotubes reveals that the compressive strain is non-uniform and can be larger than 1% locally at room temperature. Effects of device fabrication steps on the non-uniform strain are also examined and implications on electrical performance are discussed. Based on our findings, there are discussions about device performances and designs included in this dissertation. The channel length dependences of device mobilities and on/off ratios are included for SWNT TFTs. Time response of polymer-electrolyte gated SWNT TFTs has been measured to be ~300 Hz, and a proof-of-concept logic inverter has been fabricated by using polymer electrolyte gated SWNT TFTs for macroelectronic applications. Finally, I dedicated a chapter on scalable device designs based on aligned arrays of SWNTs, including a design for SWNT memory devices.

Atomic resolution studies of oxide superlattices and ultrathin films

The ability to grow ultrathin films layer-by-layer with well-defined epitaxial relationships has allowed research groups worldwide to grow a range of artificial films and superlattices, first for semiconductors, and now with oxides. In the oxides thin film research community, there have been concerted efforts recently to develop a number of epitaxial oxide systems grown on single crystal oxide substrates that display a wide variety of novel interfacial functionality, such as enhanced ferromagnetic ordering, increased charge carrier density, increased optical absorption, etc, at interfaces. The magnitude of these novel properties is dependent upon the structure of thin films, especially interface sharpness, intermixing, defects, and strain, layering sequence in the case of superlattices and the density of interfaces relative to the film thicknesses. To understand the relationship between the interfacial thin film oxide atomic structure and its properties, atomic scale characterization is required. Transmission electron microscopy (TEM) offers the ability to study interfaces of films at high resolution. Scanning transmission electron microscopy (STEM) allows for real space imaging of materials with directly interpretable atomic number contrast. Electron energy loss spectroscopy (EELS), together with STEM, can probe the local chemical composition as well as local electronic states of transition metals and oxygen. Both techniques have been significantly improved by aberration correctors, which reduce the probe size to 1 Å, or less. Aberration correctors have thus made it possible to resolve individual atomic columns, and possibly probe the electronic structure at atomic scales. Separately, using electron probe forming lenses, structural information such as the crystal structure, strain, lattice mismatches, and superlattice ordering can be measured by nanoarea electron diffraction (NED). The combination of STEM, EELS, and NED techniques allows us to gain a fundamental understanding of the properties of oxide superlattices and ultrathin films and their relationship with the corresponding atomic and electronic structure. In this dissertation, I use the aforementioned electron microscopy techniques to investigate several oxide superlattice and ultrathin film systems. The major findings are summarized below. These results were obtained with stringent specimen preparation methods that I developed for high resolution studies, which are described in Chapter 2. The essential materials background and description of electron microscopy techniques are given in Chapter 1 and 2. In a LaMnO3-SrMnO3 superlattice, we demonstrate the interface of LaMnO3-SrMnO3 is sharper than the SrMnO3-LaMnO3 interface. Extra spectral weights in EELS are confined to the sharp interface, whereas at the rougher interface, the extra states are either not present or are not confined to the interface. Both the structural and electronic asymmetries correspond to asymmetric magnetic ordering at low temperature. In a short period LaMnO3-SrTiO3 superlattice for optical applications, we discovered a modified band structure in SrTiO3 ultrathin films relative to thick films and a SrTiO3 substrate, due to charge leakage from LaMnO3 in SrTiO3. This was measured by chemical shifts of the Ti L and O K edges using atomic scale EELS. The interfacial sharpness of LaAlO3 films grown on SrTiO3 was investigated by the STEM/EELS technique together with electron diffraction. This interface, when prepared under specific conditions, is conductive with high carrier mobility. Several suggestions for the conductive interface have been proposed, including a polar catastrophe model, where a large built-in electric field in LaAlO3 films results in electron charge transfer into the SrTiO3 substrate. Other suggested possibilities include oxygen vacancies at the interface and/or oxygen vacancies in the substrate. The abruptness of the interface as well as extent of intermixing has not been thoroughly investigated at high resolution, even though this can strongly influence the electrical transport properties. We found clear evidence for cation intermixing through the LaAlO3-SrTiO3 interface with high spatial resolution EELS and STEM, which contributes to the conduction at the interface. We also found structural defects, such as misfit dislocations, which leads to increased intermixing over coherent interfaces.

Optical properties of semiconductor quantum wires

This thesis presents theoretical investigations of the sub band structure and optical properties of semiconductor quantum wires. For the subband structure, we employ multiband effective-mass theory and the effective bond-orbital model both of which fully account for the band mixing and material anisotropy. We also treat the structure geometry in detail taking account of such effects as the compositional grading across material interfaces. Based on the subband structure, we calculate optical properties of quantum-wire structures. A recuring theme is the cross-over from one- to ~wo-dimensional behavior in these structures. This complicated behavior procludes the application of simple theoretical models to obtain the electronic structure. In particular, we calculate laser properties of quantum wires grown in V-grooves and find enhanced performance compared with quantum-well lasers. We also investigate optical anisotropy in quantum-wire arrays and propose an electro-optic device based on such structures.

Functional surface chemistry of carbon-based nanostructures

The recently discovered abilities to synthesize single-walled carbon nanotubes and prepare single layer graphene have spurred interest in these sp2-bonded carbon nanostructures. In particular, studies of their potential use in electronic devices are many as silicon integrated circuits are encountering processing limitations, quantum effects, and thermal management issues due to rapid device scaling. Nanotube and graphene implementation in devices does come with significant hurdles itself. Among these issues are the ability to dope these materials and understanding what influences defects have on expected properties. Because these nanostructures are entirely all-surface, with every atom exposed to ambient, introduction of defects and doping by chemical means is expected to be an effective route for addressing these issues. Raman spectroscopy has been a proven characterization method for understanding vibrational and even electronic structure of graphene, nanotubes, and graphite, especially when combined with electrical measurements, due to a wealth of information contained in each spectrum. In Chapter 1, a discussion of the electronic structure of graphene is presented. This outlines the foundation for all sp2-bonded carbon electronic properties and is easily extended to carbon nanotubes. Motivation for why these materials are of interest is readily gained. Chapter 2 presents various synthesis/preparation methods for both nanotubes and graphene, discusses fabrication techniques for making devices, and describes characterization methods such as electrical measurements as well as static and time-resolved Raman spectroscopy. Chapter 3 outlines changes in the Raman spectra of individual metallic single-walled carbon nantoubes (SWNTs) upon sidewall covalent bond formation. It is observed that the initial degree of disorder has a strong influence on covalent sidewall functionalization which has implications on developing electronically selective covalent chemistries and assessing their selectivity in separating metallic and semiconducting SWNTs. Chapter 4 describes how optical phonon population extinction lifetime is affected by covalent functionalization and doping and includes discussions on static Raman linewidths. Increasing defect concentration is shown to decrease G-band phonon population lifetime and increase G-band linewidth. Doping only increases G-band linewidth, leaving non-equilibrium population decay rate unaffected. Phonon mediated electron scattering is especially strong in nanotubes making optical phonon decay of interest for device applications. Optical phonon decay also has implications on device thermal management. Chapter 5 treats doping of graphene showing ambient air can lead to inadvertent Fermi level shifts which exemplifies the sensitivity that sp2-bonded carbon nanostructures have to chemical doping through sidewall adsorption. Removal of this doping allows for an investigation of electron-phonon coupling dependence on temperature, also of interest for devices operating above room temperature. Finally, in Chapter 6, utilizing the information obtained in previous chapters, single carbon nanotube diodes are fabricated and characterized. Electrical performance shows these diodes are nearly ideal and photovoltaic response yields 1.4 nA and 205 mV of short circuit current and open circuit voltage from a single nanotube device. A summary and discussion of future directions in Chapter 7 concludes my work.