An-Jen Cheng

Thermal chemical vapor deposition growth of zinc oxide nanostructures for dye-sensitized solar cell fabrication

Dye-sensitized solar cells (DSSCs) were fabricated using a quasialigned one-dimensional zinc oxide (ZnO) nanostructure. The ZnO nanostructures were grown on indium tin oxide (ITO) coated glass substrate via a thermal chemical vapor deposition (CVD). It has been considered that thermal CVD is not suitable for the growth of ZnO nanostructure on ITO/glass due to the high processing temperature. However, we have demonstrated that a densely populated ZnO nanostructure can be prepared on ITO/glass substrate by a double-source double-tube CVD process. The power conversion efficiency of our device is 0.6%.

Effects of postgrowth annealing treatment on the photoluminescence of zinc oxide nanorods

Postgrowth annealing was carried out to investigate the photoluminescence of zinc oxide (ZnO) nanorods synthesized using a thermal chemical vapor deposition method. The observed change in photoluminescence after the annealing processes strongly suggests that positively charged impurity ions or interstitial Zn ions are the recombination centers for green luminescence observed in the present sample. A model based on the interplay between the band bending at the surface and the migration of positively charged impurity ions or Zn ions was proposed, which satisfactorily explains the observed photoluminescence.

Raman analysis of longitudinal optical phonon-plasmon coupled modes of aligned ZnO nanorods

The electronic properties of vertically aligned ZnO nanorods have been investigated using micro-Raman spectroscopy. The concentration and mobility of the charge carriers were determined via Raman line shape analysis using longitudinal-optical-phonon-plasmon coupled mode. The local laser heating and the stress effects have been considered when analyzing the Raman spectra. The mobility and carrier concentration of the aligned ZnO nanorods are 84.8cm2∕Vs and 3.8×1017cm−3, respectively. As a comparison, the mobility and carrier concentration of the undoped bulk ZnO were also obtained from the Raman line shape analysis. The mobility of the aligned ZnO nanorods is about 20% lower than that of the undoped bulk ZnO, which can be attributed to enhanced surface scattering due to the reduction in dimension.

Fabrication and device characteristics of Schottky-type bulk GaN-based “visible-blind” ultraviolet photodetectors

The authors present the fabrication and characterization of vertical-geometry Schottky-type ultraviolet (UV) photodetectors based on a bulk n-GaN substrate. By using low temperature rapid thermal annealing of the semitransparent Schottky contacts (nickel with 7% vanadium), they obtained an ultralow dark current of 0.56pA at −10V reverse bias. A responsivity of ∼0.09A∕W at zero bias was measured for wavelength shorter than the absorption edge of GaN, and it was found to be independent of the incident power in the range measured (50mW∕m2–2.2kW∕m2). The devices are visible blind, with an UV/visible contrast of over six orders of magnitude. An open-circuit voltage of 0.3V was also obtained under a broadband UV illumination.

Visible Luminescence from ZnO Nanostructures

Room temperature photoluminescence (PL) spectra from zinc oxide (ZnO) nanostructures were studied. ZnO samples were produced via thermal chemical vapor deposition (thermal-CVD) and a variety of ZnO nanostructures were synthesized by adjusting the oxygen content during the growth process. All samples exhibit a sharp and strong ultra-violet near-band-edge (NBE) emission at about 3.18 eV. The visible emission from the samples deposited under an oxygen-deficient condition were dominated by blue-green band emission at 2.34 eV. The intensity of the blue-green band was greatly reduced (so-called green band free) for the ZnO deposited at the center of the wafer while strong violet-blue emission bands and broad bands at yellow-orange-red range were collected from the ZnO grown along the edge of the wafer. We believe that the spatial inhomogeniety was caused by turbulent gas flow in the reaction chamber, which resulted in different local oxygen concentration. Origin of visible luminescence from ZnO nanostructures will be discussed and a model to explain the observed visible luminescence process will be presented.

Nano-ZnO cold cathodes for controlling plasma switches

Electron beam generated by a cold cathode coated with ZnO nanoneedles and other forms of nanomaterials was used to turn on a pseudospark plasma switch for pulsed power applications. A pseudospark switch filled with gases between 10 and 60 mtorr is designed and tested. Electron field emission current generated by the ZnO oxide coated cold cathode was sufficient to turn on a high voltage plasma switch.

CVD diamond for cold cathode applications

Microcrystalline and nanocrystalline diamond coatings grown by microwave plasma enhanced chemical vapor deposition (CVD) techniques are examined for their properties for applications as cold cathodes. Low-field electron field emission and high-yield secondary electron emission are promising properties of diamond and for applications to power modulators