S.L. Cheng

A study of the synthesis, characterization, and kinetics of vertical silicon nanowire arrays on (001)Si substrates

Large-area, vertically aligned silicon nanowire (SiNW) arrays have been successfully synthesized in an aqueous solution containing AgNO 3 and HF on (001)Si substrates. The as-synthesized SiNWs were determined to be perfectly single crystalline with the axis of the wire parallel to the [001] direction. The typical widths of the SiNWs were in the range of 30-200 nm. The lengths of SiNWs could be tuned from several to tens of micrometers by adjusting the synthesis temperature and time. We measured the formation rate at different reaction temperatures. The activation energy for linear growth of the SiNWs, as obtained from an Arrhenius plot, was found to be about 0.36 eV. In addition, the Si substrates with highly oriented SiNW arrays were found to exhibit significant hydrophobic properties. The water contact angles of the HF-treated SiNW arrays were measured to be about 120-148°, much greater than that with a flat silicon wafer surface (∼85 to 88°).

Fabrication, characterization, and kinetic study of vertical single-crystalline CuO nanowires on Si substrates

We report here on the first study of the growth kinetics of high-yield, vertical CuO nanowires on silicon substrates produced by the process of thermal oxidation. The length of the CuO nanowires could be tuned from several to tens of micrometers by adjusting the oxidation temperature and time. The grown CuO nanowires were determined to be single-crystalline with different axial crystallographic orientations. After a series of scanning electron microscopy examinations, the average length of CuO nanowires produced at each temperature was found to follow a parabolic relationship with the oxidation time. The parabolic growth rate at different oxidation temperatures was measured. The activation energy for the growth of CuO nanowires calculated from an Arrhenius plot was found to be about 174.2 kJ/mole. In addition, the current-voltage characterization indicated that the sample with high-density CuO nanowires exhibited ohmic behavior, and its resistance was found to significantly decrease with increasing environmental temperature. The result can be attributed to an increase in the number of carriers at higher temperatures.

Characterization and Growth Kinetics of Electroless Pure Nickel Thin Films on Si(001) Substrates

We report the results on the growth behaviors of electroless pure nickel thin films on Si(001) substrates from a systematic investigation. Continuous pure Ni films with a polycrystalline structure were successfully plated on the surfaces of Si substrates using the hydrazine-modified electroless Ni deposition processes. The thicknesses of the pure Ni films can be controlled by tuning the electroless plating temperature and time. The deposition rate of the electroless pure Ni films increased exponentially with the plating temperatures. The activation energy of the electroless pure Ni deposition on Si(001) substrate for samples plated at 55-75°C could be calculated from an Arrhenius plot, and it was about 0.95 eV/atom.

Uniform SiGe/Si quantum well nanorod and nanodot arrays fabricated using nanosphere lithography

This study fabricates the optically active uniform SiGe/Si multiple quantum well (MQW) nanorod and nanodot arrays from the Si0.4Ge0.6/Si MQWs using nanosphere lithography (NSL) combined with the reactive ion etching (RIE) process. Compared to the as-grown sample, we observe an obvious blueshift in photoluminescence (PL) spectra for the SiGe/Si MQW nanorod and nanodot arrays, which can be attributed to the transition of PL emission from the upper multiple quantum dot-like SiGe layers to the lower MQWs. A possible mechanism associated with carrier localization is also proposed for the PL enhancement. In addition, the SiGe/Si MQW nanorod arrays are shown to exhibit excellent antireflective characteristics over a wide wavelength range. These results indicate that SiGe/Si MQW nanorod arrays fabricated using NSL combined with RIE would be potentially useful as an optoelectronic material operating in the telecommunication range.

Large-area Co-silicide nanodot arrays produced by colloidal nanosphere lithography and thermal annealing.

We report here the successful fabrication of large-area size-tunable periodic arrays of cobalt and Co-silicide nanodots on silicon substrates by employing the colloidal nanosphere lithography (NSL) technique and heat treatments. The growth of low-resistivity epitaxial CoSi(2) was found to be more favorable for the samples with smaller Co nanodot sizes. The sizes of the epitaxial CoSi(2) nanodots can be tuned from 50 to 100 nm by varying the diameter of the colloidal spheres and annealing temperatures. The epitaxial CoSi(2) nanodots were found to grow with an epitaxial orientation with respect to the (001)Si substrates: [001]CoSi(2)//[001]Si and (200)CoSi(2)//(400)Si. From the results of planview HRTEM, XTEM, and SAED analysis, the epitaxial CoSi(2) nanodots were identified to be inverse pyramids in shape, and the average sizes of the faceted silicide nanodots were measured to decrease with annealing temperature. The observed results present the exciting prospect that with appropriate controls, the colloidal NSL technique promises to facilitate the growth of a variety of well-ordered silicide nanodots with selected shape, size, and periodicity.