Hanay Kamimura

Synthesis and electrical characterization of Zn3P2 nanowires

Single crystalline Zn3P2 nanowires were synthesized on Si substrates via vapour phase deposition catalysed by In–Au seeds. Single nanowire devices were fabricated and the metal–Zn3P2 nanowire contacts were studied using a model based on two Schottky barriers as a function of temperature. As far we know, these are the first reported values of Schottky barriers of Ti/Zn3P2 nanowire contacts. The obtained values showed no significant dependence on the temperature, indicating that the defects at the nanowires surfaces did not affect the device characteristics. We found evidence of an acceptor level at 49 meV, also indicating that the dominant transport mechanism is the thermal activation of carriers as is found in the bulk Zn3P2. It seems that the p-type behaviour is independent of the dimensionality of the Zn3P2 samples and primarily associated with the phosphorous interstitial atoms.

Oxygen-induced metal-insulator-transition on single crystalline metal oxide wires

In this work we report on the transition from metal to insulator conduction of individual single crystalline In2 O3 wires induced by different oxygen concentration during their growth. The transport measurements revealed that the metallic conduction was mainly governed by the acoustic phonon scattering and the insulating character was addressed by the variable range hopping mechanism, which in turn can be considered as a first evidence of the occurrence of an Anderson-like metal-insulator-transition (MIT). The experimental data provided the critical carrier density to be 8×1018 cm-3 corresponding to a critical impurities spacing of 2.5 nm, which was found to be in agreement with previous reported data on polycrystalline indium oxide samples and with our recent finding on In2 O3 semiconducting samples. The approach presented here can be used to grow other metal oxide systems in which oxygen vacancies play a fundamental role for the electron transport features.

Measuring the mobility of single crystalline wires and its dependence on temperature and carrier density

Kinetic transport parameters are fundamental for the development of electronic nanodevices. We present new results for the temperature dependence of mobility and carrier density in single crystalline In(2)O(3) samples and the method of extraction of these parameters which can be extended to similar systems. The data were obtained using a conventional Hall geometry and were quantitatively described by the semiconductor transport theory characterizing the electron transport as being controlled by the variable range hopping mechanism. A comprehensive analysis is provided showing the contribution of ionized impurities (low temperatures) and acoustic phonon (high temperatures) scattering mechanisms to the electron mobility. The approach presented here avoids common errors in kinetic parameter extraction from field effect data, serving as a versatile platform for direct investigation of any nanoscale electronic materials.

CARACTERIZAÇÃO DE NANOFIOS DE GERMÂNIO SINTETIZADOS UTILIZANDO COBRE COMO CATALISADOR

This paper presents reliable synthesis of germanium nanowires by the vapor-liquid-solid method using copper as an alternative catalyst to gold, the most commonly used metal. The morphological study showed long range single-crystalline germanium nanowires with diamond structure and diameters ranging from 20 nm to 80 nm and lengths in tenths of a micrometer, which was much larger than the cross-section. In an investigation of electronic properties, devices built from a thin film of nanowires were studied and the semiconductor behavior of the samples confirmed, with variable range hopping identified as the main transport mechanism. Additionally, metal-nanowire contact behavior - ohmic or with Schottky barrier - was found to depend on the metal used in the device. Therefore, the possibility of controlling current-voltage behavior combined with the good crystalline quality of the nanowires renders these nanostructures an attractive option for future electronic and optoelectronic applications.

Optical and transport properties correlation driven by amorphous/crystalline disorder in InP nanowires

Indium phosphide nanowires with a single crystalline zinc-blend core and polycrystalline/amorphous shell were grown from a reliable route without the use of hazardous precursors. The nanowires are composed by a crystalline core covered by a polycrystalline shell, presenting typical lengths larger than 10 μm and diameters of 80-90 nm. Raman spectra taken from as-grown nanowires exhibited asymmetric line shapes with broadening towards higher wave numbers which can be attributed to phonon localization effects. It was found that optical phonons in the nanowires are localized in regions with average size of 3 nm, which seems to have the same order of magnitude of grain sizes in the polycrystalline shell. Regardless of the fact that the nanowires exhibit a crystalline core, any considerable degree of disorder can lead to a localized behaviour of carriers. In consequence, the variable range hopping was observed as the main transport instead of the usual thermal excitation mechanisms. Furthermore the hopping length was ten times smaller than nanowire cross-sections, confirming that the nanostructures do behave as a 3D system. Accordingly, the V-shape observed in PL spectra clearly demonstrates a very strong influence of the potential fluctuations on the exciton optical recombination. Such fluctuations can still be observed at low temperature regime, confirming that the amorphous/polycrystalline shell of the nanowires affects the exciton recombination in every laser power regime tested.

Optoelectronic characteristics of single InP nanowire grown from solid source

InP nanowires were synthesized on quartz substrates via vapour phase deposition using gold seeds and solid source. The properties of the InP nanowires were investigated by x-ray diffraction , scanning electronic microscopy (FEG-SEM) and high resolution transmission electron microscopy demonstrating micrometric lengths and diameter distribution ranging from 40 to 100 nm with crystalline core. Photo-current on-off characteristics were obtained from single nanowire devices, presenting a delay of a few seconds in the current decay when the illumination is turned off. This time is much larger than some usual microseconds decays and may be caused by localized states whose presence was confirmed by thermally stimulated current TSC measurements.

Synthesis and properties of crystalline InP nanowires with amorphous sheath grown from solid source

The nanowires were synthesized by the vapor-liquid-solid (VLS) mechanism on quartz substrates in a tube furnace, using gold seeds as catalysts. The nanowires were examined by XRD, SEM and HRTEM analysis showing InP nanowires with a single-crystalline core covered by an amorphous sheath with diameters ranging from 40 - 100 nm and lengths of tens of micrometers. Single nanowire devices were fabricated by conventional photolithography techniques, using Ti metallic contacts. I-V curves were obtained under dark conditions and under focused illumination. Aiming the illustration of the samples for optoelectronic devices, photoconductivity measurements were conducted, submitting the samples to repeated dark-light cycles under 1 V bias. The presented delay in photo-response can be caused by the presence of carrier trap states originated from the self-organized growth mechanism or from the crystalline/amorphous interface.