UC Irvine

The International Roadmap for Semiconductor Devices calls for transistors that will extend into the low-dimensional regime. As electronic devices are shrunk to increasingly lowered dimensions, the restricted geometry and extremely low carrier density results in an extraordinary sensitivity to interfaces, defects and disorder. This sensitivity is a significant barrier to their use in future technologies as it sufficiently alters their electronic properties so that their transport physics is no longer well understood. In this work, electronic scattering due to contacting electrodes, supporting substrates, defects and disorder is investigated in the case of field effect transistor (FET) devices composed of one-dimensional, single-walled carbon nanotubes (SWNTs). In order to investigate the various mechanisms of electronic scattering in SWNT FETs, scanning probe microscopy (SPM) techniques are developed in order to spatially resolve and distinguish each scattering mechanism. This dissertation gives a brief introduction to low-dimensional conductors in Chapter 1 and outlines basic experimental methods for characterizing SWNT FETs in Chapter 2. Next, the development of SPM techniques are described in Chapters 3-4, 6 and their application to understanding transport in SWNTs is described in Chapters 5-7.

The SPM techniques described in this dissertation are based upon Kelvin probe force microscopy (KPFM) and scanning gate microscopy (SGM). Chapter 3 describes modifications to an existing atomic force microscope (AFM) electronics to increase the imaging speed of KPFM in order to extend it into a transport spectroscopy. Next, Chapter 4 describes a new KPFM technique, Parameterized-KPFM, that was developed to quantitatively measure the voltage gradients associated with electronic scattering in SWNT FETs. Finally, Chapter 6 describes extending the SGM technique into a spectroscopy called scanning gate spectroscopy (SGS) in order to investigate a SWNT's local sensitivity to electric fields. Both the KPFM and SGS techniques were used to investigate FETs with pristine SWNTs, and those with SWNTs containing a single defect site.

In the first study, described in Chapter 5, KPFM was used to measure voltage gradients associated with diffusive electronic scattering along pristine SWNTs. The KPFM technique allowed for an accurate separation of SWNT resistance from the contact resistance that was greatly improved compared to previous work. The voltage gradients directly determined the inelastic mean free path (lambda) as a function of applied bias. At a temperature of 185 K, lambda was found to decrease from nearly 1 micron at low bias to 100 nm at high bias. Fitting the lambda to established transport models determined the relative roles of substrate induced, surface polar-phonon scattering and SWNT optical phonon scattering. The optical phonon mean free path for spontaneous emission was found to be 62 +/- 20 nm at 300 K, significantly longer than observed in previous experimental studies and in much better agreement with theoretical predictions.

In another study, described in Chapter 7, KPFM was used to investigate SWNTs with defects. Single, covalent type defects were incorporated into pristine SWNTs by the method of electrochemical point-functionalization. Electronic scattering at defect sites was characterized by a combination of two-terminal transport spectroscopy and KPFM over a wide range of temperature and bias. Both transport measurements and KPFM were used to independently confirm high-resistance depletion regions that can extend to 1.0 micron in width surrounding a single defect. Fitting transport measurements to models demonstrated that conductance through such wide depletion regions occurs via a modified, 1D version of Poole-Frenkel emission. The Poole-Frenkel model suggests that conduction through the depletion region is mediated by at least one or more localized defect trap states. The Poole-Frenkel trapping potential was found to be relatively weak with effective barrier heights measured to be 10-40 meV.

Finally, Chapter 6 discusses the use of SGS to investigate similar types of covalent defects. Point-functionalization of SWNTs in water, sulfuric acid, or hydrochloric acid produced single defect sites with different chemistries. Using SGS, the defect's sensitivity to electric fields was measured and used to extract the energy-dependent transmission function. The three defect chemistries were distinguished by a spectroscopic fingerprint found in each transmission function.