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 <i>p-(i)-n</i>
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 <i>p-i-n</i> 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<sup>2</sup>, 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.
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/In<sub>x</sub>Ga<sub>1-x</sub>N/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.
We report on fabrications and characteristics of high performance ZnO nanorod nanodevices including Schottky diodes, metal-oxide-semiconductor field-effect transistors (MOSFETs), metal-semiconductor field-effect transistors (MESFETs) and logic gate devices. Electrical characteristics of several ZnO nanorod MOSFETs are compared in this proceeding. In particular, after coating polymer thin films on ZnO nanorod surfaces, the nanorod MOSFETs exhibited much improved field effect transistor characteristics including field effect electron mobility as high as 3000 cm<sup>2</sup>/Vs. In addition, ZnO nanorod Schottky diodes and MESFETs were fabricated using Au/ZnO Schottky contacts without any specific oxide etching process. These devices have been used for realization of ZnO nanorod logic gates.
We report on catalyst-free growth of ZnO nanorods and their nano-scale electrical and optical device applications. Catalyst-free metalorganic vapor-phase epitaxy (MOVPE) enables fabrication of size-controlled high purity ZnO single crystal nanorods. Various high quality nanorod heterostructures and quantum structures based on ZnO nanorods were also prepared using the MOVPE method and characterized using scanning electron microscopy, transmission electron microscopy, and optical spectroscopy. From the photoluminescence spectra of ZnO/Zn<sub>0.8</sub>Mg<sub>0.2</sub>O nanorod multi-quantum-well structures, in particular, we observed a systematic blue-shift in their PL peak position due to quantum confinement effect of carriers in nanorod quantum structures. For ZnO/ZnMgO coaxial nanorod heterostructures, photoluminescence intensity was significantly increased presumably due to surface passivation and carrier confinement. In addition to the growth and characterizations of ZnO nanorods and their quantum structures, we fabricated nanoscale electronic devices based on ZnO nanorods. We report on fabrication and device characteristics of metal-oxidesemiconductor field effect transistors (MOSFETs), Schottky diodes, and metal-semiconductor field effect transistors (MESFETs) as examples of the nanodevices. In addition, electroluminescent devices were fabricated using vertically aligned ZnO nanorods grown p-type GaN substrates, exhibiting strong visible electroluminescence.
We report on photoluminescent properties of ultrafine ZnO nanorods and ZnO/Zn<sub>0.8</sub>Mg<sub>0.2</sub>O nanorod quantum-well structures. The catalyst-free metalorganic chemical vapor deposition (MOCVD) technique enables control of ZnO nanorod diameters in the range of 5 to 150 nm. From the PL spectra of ultrafine ZnO nanorods with a mean diameter smaller than 10 nm, a systematic blue-shift in their PL peak position was observed by decreasing their diameter, presumably due to the quantum confinement effect along the radial direction in ZnO nanorods. In addition, we obtained time-integrated and time-resolved PL spectra of ZnO/Zn<sub>0.8</sub>Mg<sub>0.2</sub>O nanorod single-quantum-well structures (SQWs) in the temperature range of 10 K to 300 K. The nanorod SQWs also showed a PL blue-shift and the energy shift was dependent on ZnO well layer width. The PL peak position shift originates from the quantum confinement effect of carriers in nanorod quantum structures. Furthermore, we investigated spatially-resolved PL spectra of individual nanorod SQWs using scanning near-field optical microscopy.