Over the last two decades, III-V compound semiconductor nanowires (NWs) have been the focus of extensive research efforts. III-V NWs are particularly attractive for a wide variety of next-generation nanoelectronics and optoelectronics applications, in part due to their proven potential for unprecedented freedom in bandgap engineering, monolithic heteroepitaxial integration, and materials cost reduction. Of further interest is the technologically relevant subset of laterally-grown or planar epitaxial NWs, which hold the promise for nanoscale transistor scaling beyond the limits of the current Si-based roadmap. However, to date, in-situ doping dynamics and the effects of non-steady state dopant incorporation on the morphology and structure of planar III-V NWs has remained relatively unexplored, partially based on the shortcomings of conventional dopant profilometry techniques. Using planar GaAs NWs containing multiple, laterally isolated p-n junctions, we show that microwave impedance microscopy coupled with atomic force microscopy (MIM-AFM) has the capability of non-destructively spatially mapping charge carrier density profiles with nanoscale resolution. Particular attention is given to a novel observation of cyclical Zn impurity incorporation enhancement during vapor-liquid-solid (VLS) growth of p-type NW segments, simultaneously associated with the formation of a laterally twinned planar NW crystal structure. Our results are validated through correlation of MIM-AFM data with near-field infrared spectroscopy (NFIR) measurements, which reveals chemical information with ~ 10 nm spatial resolution. A theoretical model of the nanoprobe-NW interaction for both scan probe techniques is presented, which allows for the qualitative correlation of the collected data to doping signatures. These techniques, coupled with helium ion microscopy (HIM) and conventional transmission electron microscopy (TEM), allow us to validate the impact of the doping process on the structural topology of the NWs. The above microscopy techniques are pivotal to developing a new understanding of the growth dynamics of lateral VLS multi-junction GaAs NWs, and are likewise applicable for the direct measurement of composition and dopant distribution profiles in a wide variety of inorganic nanostructures.
University of Illinois - Urbana Champaign, Fredrick Seitz Materials Research Laboratory
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