Tunable Interface Non-linear Electron Transport in Semiconductor Nanowire Heterostructure and Its Application in Optoelectronics

Understanding the effects of finite size and dimensionality on the interaction of light with nanoscale semiconductor heterostructure is central to identifying and exploiting novel modes in optoelectronic devices. In type-I heterostructured core-shell GaAs/AlxGa1-xAs nanowires, the real space transfer (RST) of photogenerated hot electrons across the interface from the GaAs core to the AlxGa1-xAs shell forms the basis of a new family of optoelectronic devices by a carefully designed and optimized nanofabrication process. Due to the large mobility difference, we observed negative differential resistance (NDR) on single nanowire devices. External modulation of the transfer rates, manifested as a large tunability of the voltage onset of NDR, is achieved using three different modes: electrostatic gating, incident photon flux, and photon energy. In this dissertation, the physics of coupling of external control to transfer rate was investigated. The combined influences of geometric confinement, heterojunction shape and carrier scattering on hot-electron transfer is discussed. Temperature-dependent transport study under monochromatic tunable laser illumination reveals an ultrafast carrier dynamics related to RST of excess carriers, which provides an insight into hot carrier cooling. Device element showing adjustable phase shift and frequency doubling of ac modulation is demonstrated. For a full understanding, Carrier transport properties are probed through electron beam induced current, which is capable of imaging sub-surface feature in excess carrier transport. Along with simulation of injected electron trajectories, selective probing of core and shell by tuning electron beam energies reveals axial and bias dependent transport along parallel channels. The drift and diffusion component of the excess carrier current is deconvoluted from a coupled decay length, from which lower than bulk shell electron mobility is extracted. A precise knowledge of band edge discontinuities at heterostructure interface is inevitable to understand electronic and optoelectronic properties where real space transfer is involved. We used a combined photocurrent and photoluminescence spectroscopic study, in GaAs/AlxGa1-xAs and GaAs/AlAs core-shell nanowire systems, to construct a band diagram of an individual heterostructure nanowire with high spectral resolution, enabling quantification of conduction band offsets. The thesis outlines a promising novel device paradigm with a well-studied carrier transport physics at heterostructure nanowires, which can be further extended to a broad range of semiconductor material systems.Ph.D., Materials Science and Engineering -- Drexel University, 201