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