Transport imaging for the study of nanowires and related nanostructures
Ang, Goon Hwee
Haegel, Nancy M.
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The goals of this thesis are to demonstrate the operation of a near-field scanning microscope (NSOM) inside a scanning electron microscope (SEM) to collect spatially resolved luminescence and to image transport on nano-scale structures, particularly nanowires. The SEM is used to generate localized charge and the NSOM is used to observe the motion of the excess charge due to diffusion and/or drift via its recombination emission. This will allow direct determination of transport parameters, such as minority carrier mobility and lifetime, that are key to the performance of LEDs, lasers and bipolar devices. For nano-structures such as nanowires, device sizes are commonly less than 100 nm. The resolution in a standard optical microscope is diffraction limited and hence resolution of luminescence from individual devices requires collection of light in the near field limit. An atomic force microscope (AFM/NSOM) has been installed in the SEM to allow for simultaneous near-field optical collection with an electron beam for charge generation. The work in this thesis has observed CL luminescence from ZnO nanowires using normal SEM-OM and anaylzed the spectra. In addition, the work has demonstrated successful AFM/NSOM operation within the SEM. Light is collected from both GaAs heterostructures and single ZnO nanowires and control experiments have been performed. Challenges for transport imaging in SEM have been identified Finally, a suggestion for further work is to specifically image charge motion along a single wire, including electric field mapping in these nanoscale, low dimensional structures. These nanoscale wires are poised to revolutionize solid state devices in the near future and direct measurements of key electronic parameters will be required.
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