Exploring nanoscale characterization of low dimensional electronic materials

The advent of Atomic Force Microscopy (AFM) has allowed researchers to probe materials on the atomic scale with relative simplicity. For the study of nanoscale materials, structure is very important and often has a large impact on the materials intrinsic properties. The conventional form of Atomic Force Microscopy was developed to study material structure in the form of surface topography measurements. Since then there has been many advances which have taken advantage of the ability to detect small forces using an AFM tip along with surface topology. A driving motivation in the scan probe microscopy field is the ability to spatially correlate properties of electronic materials such as charge density, conductivity, and doping distribution with nanoscale structure. Nanoscale characterization has become increasingly relevant as device features continue to shrink according to Moore’s Law leading to the advent of next generation electronic materials such as semiconducting nanowires and Carbon nanotubes. The primary issue with measuring nanoscale materials properties is that the tip-sample coulomb forces and quantum effects that provide insight into the material’s properties are very difficult to detect. A semiconducting Nanowire (NW) typically less than 500nm in diameter, is a quasi 1-dimensional structure with feature sizes approaching the diffraction limit of light rendering conventional optical spectroscopy ineffective; hence scan probe techniques are the most promising for characterization. Carbon Nanotubes, typically 1.0nm – 3.0nm in diameter, are 1-dimensional structures that are particularly difficult to characterize due to their infinitesimal sample volume. So far there has been very limited success electrically characterizing CNTs at the individual nanotube scale. Despite the challenges associated with nanomaterial characterization there have been successes at characterizing the electrical and chemical composition in parallel with morphology using capacitance sensitive AFM techniques. In this study I will describe and present data from AFM techniques with the ability to characterize semiconducting nanowires and carbon nanotubes. In chapter 1, there is a review of several variations of capacitive AFM used to measure electrical properties and chemical properties of nanomaterials, some of which require specific sample preparation making them incompatible with nanotube and nanowire characterization. Next, in chapter 2, is an introduction to Microwave Impedance Microscopy (MIM), a novel nondestructive scan probe technique we offer as a viable alternative for low dimensional electronic material characterization. The goal of Chapter 3 is to demonstrate the ability to measure the quantum capacitance of individual CNTs using MIM illustrating it’s capability to measure nanoscale electrical phenomena. In chapter 4, MIM-AFM is used to provide insight into the structurally correlated doping dynamics of laterally grown GaAs nanowires. Finally, in chapter 5, a new scan probe technique called Near Field Infrared Microscopy (NFIR) is shown to be a complimentary characterization technique to MIM by probing the dopant distribution in GaAs nanowires. Many of the observations made using MIM-AFM and NFIR have never been seen before and could potentially have a high impact on nanowire device fabrication and characterization

Characterization of nanoscale electronic materials using novel methods for scan probe microscopy

Transistors have been improved to achieve higher performance by substantially scaling down the physical size of the devices. Currently, high performance Silicon based transistors have been shrunk to the nanoscale. To further improve the performance of transistors researchers are exploring the use of novel semiconductors with unique nanoscale morphologies. To create processes to utilize the properties of new materials, there has been significant effort to better understand how these material’s electrical properties effect transistors in real devices. The primary challenge associated with electronic material characterization for process optimization is the difficulty of mapping electrical properties with a resolution high enough to spatially resolve nanoscale phenomena. In this thesis we will explore several scan probe based microscopy techniques capable of mapping changes in electronic properties with sub-diffraction spatial resolution. Using novel methods for scan probe based microscopy, we combined electrical and morphology mapping to reveal structural driven electrical properties to provide insight into growth physics and electrical transport. We used novel methods for Electric Force Microscopy, Near Field Infrared Microscopy, and Microwave Impedance Microscopy (MIM) to map non-uniform doping and the free carrier distribution in the bulk Gallium Arsenide nanowires. Our results revealed cyclical doping inhomogeneity in regions with morphological defects; we used that information to create a physical model to predicts the impurity distribution along the nanowire. This enables us to better understand the physics behind in situ doping during the growth process. In addition, we used of Microwave Impedance Microscopy to qualitatively characterize carbon nanotube (CNT) electrical properties. Using novel methods to maximize the signal and sensitivity of the microwave reflectivity response to the carbon nanotubes, we were able to spatially map and distinctly identify the electronic character of individual carbon nanotubes in an array with 50nm resolution. Our results provide that MIM can be used to distinguish semiconducting, semi-metallic, and metallic carbon nanotubes by detecting their quantum capacitance, which is directly related to the density of states. We also explored Carbon Nanotube heterojunctions and metal-semiconductor interfaces; we believe that our results are direct evidence of electron-electron screening in 1-dimensional semiconductors. Finally, we introduce a novel, intermittent-contact, approach to Microwave Impedance Microscopy that uses the native water layer, which exists on surfaces in ambient humidity conditions, to further improve sensitivity and resolution. In addition, this approach doesn’t require any special sample preparation making 100% clean, which is preferred in an industrial laboratory setting. Our results prove that both tapping mode Atomic Force Microscopy and force curve mapping can be used with MIM to electronically characterize carbon nanotube arrays at the nanoscale. The Fast Force Curve mapping variant of MIM shows the most promise for acquiring accurate, high resolution, maps of CNT electronic character without altering the sample. It is worth noting that high resolution mapping of the electrical character of individual carbon nanotubes in a large array has never been achieved before this work