Jinkyoung Yoo

Nanostructures provide novel opportunities of studying epitaxy in nano/mesoscale and on nonplanar substrates. Epitaxial growth of silicon (Si) on the surfaces of Si nanowires along radial direction is a promising way to prepare radial p-(i)-n junction in nanoscale for optoelectronic devices. Comprehensive studies of Si radial epitaxy in micro/nanoscale reveal that morphological evolution and size-dependent radial shell growth rate for undoped and doped Si radial shells. Single crystalline Si radial p-i-n junction wire arrays were utilized to fabricate photovoltaic (PV) devices. The PV devices exhibited the photoconversion efficiency of 10%, the short-circuit current density of 39 mA/cm(2), and the open-circuit voltage of 0.52 V, respectively.

Ultrafast optical microscopy (UOM) combines a typical optical microscope and femtosecond (fs) lasers that produce high intensity, ultrashort pulses at high repetition rates over a broad wavelength range. This enables us new types of imaging modalities, including scanning optical pump-probe microscopy, which varies the pump and probe positions relatively on the sample and ultrafast optical wide field microscopy, which is capable of rapidly acquiring wide field images at different time delays, that is measurable nearly any sample in a non-contact manner with high spatial and temporal resolution simultaneously. We directly tracked carriers in space and time throughout a NW by varying the focused position of a strong optical "pump" pulse along the Si core-shell nanowires (NWs) axis while probing the resulting changes in carrier density with a weaker "probe" pulse at one end of the NW. The resulting time-dependent dynamics reveals the influence of oxide layer encapsulation on surface state passivation in core-shell NWs, as well as the presence of strong acoustic phonon oscillations, observed here for the first time in single NWs. Time-resolved wide field images of the photoinduced changes in transmission for a patterned semiconductor thin film and a single silicon nanowire after optical excitation are also captured in real time using a two dimensional smart pixel array detector. Our experiments enable us to extract several fundamental parameters in these samples, including the diffusion current, surface recombination velocity, diffusion coefficients, and diffusion velocities, without the influence of contacts.

An understanding of non-equilibrium carrier dynamics in silicon (Si) nanowires (NWs) and NW heterostructures is very important due to their many nanophotonic and nanoelectronics applications. Here, we describe the first measurements of ultrafast carrier dynamics and diffusion in single heterostructured Si nanowires, obtained using ultrafast optical microscopy. By isolating individual nanowires, we avoid complications resulting from the broad size and alignment distribution in nanowire ensembles, allowing us to directly probe ultrafast carrier dynamics in these quasi-one-dimensional systems. Spatially-resolved pump-probe spectroscopy demonstrates the influence of surface-mediated mechanisms on carrier dynamics in a single NW, while polarization-resolved femtosecond pump-probe spectroscopy reveals a clear anisotropy in carrier lifetimes measured parallel and perpendicular to the NW axis, due to density-dependent Auger recombination. Furthermore, separating the pump and probe spots along the NW axis enabled us to track space and time dependent carrier diffusion in radial and axial NW heterostructures. These results enable us to reveal the influence of radial and axial interfaces on carrier dynamics and charge transport in these quasi-one-dimensional nanosystems, which can then be used to tailor carrier relaxation in a single nanowire heterostructure for a given application.

This proceeding summarizes the materials preparation of position-controlled ZnO-based nanorod heterostructures and fabrication of vertically-aligned wide band gap semiconductor nanorod light-emitting devices. Especially the fabrication of GaN/InxGa1-xN/GaN/ZnO nanorod heterostructured visible-light-emitter arrays on sapphire and Si substrates, representing important progress in the field of nanoheteroepitaxy and photonic devices in nanoscale, are reported. Particularly, position-controlled vertical nanostructure arrays make those possible to prepare high-quality material systems without stress or strain accumulation and to fabricate high-performance light-emitting devices (LEDs) with a three-dimensional device configuration. Our method based on nanoheteroepitaxy and position-controlled nanodevice integration for fabricating GaN-based micro-LED arrays constitutes a promising strategy for resolving the issues of conventional GaN LEDs and fabricating high-performance LEDs on various substrates for potential optoelectronic integrated circuits and solid-state lighting applications.