Measurement and Modeling of the Local Photoresponse in Nanostructured Semiconductor Devices

This thesis describes the experimental characterization and modeling of nanostructured semiconductor devices including one-dimensional nanowire heterostructures and van der Waals heterostructures incorporating two-dimensional materials. Logic and optoelectronic devices made from low-dimensional and van der Waals materials have properties that are distinct from and/or superior to that of heterojunctions made from conventional materials. One of the primary goals of this thesis is to understand how intrinsic materials properties and the device geometry combined to determine the electrical characteristics and photoresponse of heterojunction devices. Spatially resolved measurements of photocurrent and photovoltage are compared with electromagnetic and charge transport modeling to identify the origin and limitations of the performance of devices as solar cells and photodetectors. Mechanistic understanding is provided for how the sub-wavelength geometry and spatial variations in the composition of GaN/InGaN core-shell nanowire arrays leads to spatial variations in carrier generation and collection that impact the external quantum efficiency of the solar cell. Origins of photocurrent are identified in devices with geometry-induced heterojunctions arising from abrupt discontinuities in physical thickness due to the thickness-dependent band structure of MoS2. The gate tunable rectification and photovoltage of hybrid organic-inorganic pentacene-MoS2 heterojunction devices was also investigated with a combination scanning photocurrent microscopy and finite element modeling. The intrinsic materials parameters that control the anti-ambipolar transport characteristic are identified through the modeling, and the impact of the lateral geometry of device components photovoltage and transfer curves is determined. The geometry and recombination properties that govern the origin and time dependence of the photoresponse of a vertical Gr/CNT/MoS2/Ti/Au device were identified. Characterization of photoresponse with local scanning probes combined with modeling advance fundamental understanding of mechanisms dominating charge transport and photoresponse in nanostructured semiconductor devices and of how the device geometry, band alignments, and other material properties influence the optoelectronic performance. The application of this knowledge can inform the design of next-generation devices.