ABSTRACT
Single wire p(+)-n(+) radial junction nanowire solar cell devices were fabricated by low pressure chemical vapor deposition of n(+) silicon shell layers on p(+) silicon nanowires synthesized by vapor-liquid-solid growth. The n(+)-shell layers were deposited at two growth temperatures (650 °C and 950 °C) to study the impact of shell crystallinity on the device properties. The n-type Si shell layers deposited at 650 °C were polycrystalline and resulted in diodes that were not rectifying. A pre-coating anneal at 950 °C in H2 improved the structural quality of the shell layers and yielded diodes with a dark saturation current density of 3 × 10(-5) A cm(-2). Deposition of the n-type Si shell layer at 950 °C resulted in epitaxial growth on the nanowire core, which lowered the dark saturation current density to 3 × 10(-7) A cm(-2) and increased the solar energy conversion efficiency. Temperature-dependent current-voltage measurements demonstrated that the 950 °C coated devices were abrupt junction p(+)-n(+) diodes with band-to-band tunneling at high reverse-bias voltage, while multi-step tunneling degraded the performance of devices fabricated with a 950 °C anneal and 650 °C coating. The higher trap density of the 950 °C annealed 650 °C coated devices is believed to arise from the polycrystalline nature of the shell layer coating, which results in an increased density of dangling bonds at the p(+)-n(+) junction interface.
ABSTRACT
The microstructure of an annealed alloy with a Zr8Ni21 composition was studied by both scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The presence of three phases, Zr8Ni21, Zr2Ni7, and Zr7Ni10, was confirmed by SEM/X-ray energy dispersive spectroscopy compositional mapping and TEM electron diffraction. Distribution of the phases and their morphology can be linked to a multi-phase structure formed by a sequence of reactions: (1) L â Zr2Ni7 + L'; (2) peritectic Zr2Ni7 + L' â Zr2Ni7 + Zr8Ni21 + L"; (3) eutectic L" â Zr8Ni21 + Zr7Ni10. The effect of annealing at 960 °C, which was intended to convert a cast structure into a single-phase Zr8Ni21 structure, was only moderate and the resulting alloy was still multi-phased. TEM and crystallographic analysis of the Zr2Ni7 phase show a high density of planar (001) defects that were explained as low-energy boundaries between rotational variants and stacking faults. The crystallographic features arise from the pseudo-hexagonal structure of Zr2Ni7. This highly defective Zr2Ni7 phase was identified as the source of the broad X-ray diffraction peaks at around 38.4° and 44.6° when a Cu-K was used as the radiation source.
ABSTRACT
A nanowire array of metal-organic complex copper-tetracyanoquinodimethane (CuTCNQ) was obtained by depositing a layer of copper in the bottom of anodic alumina template channels during a vapor-induced reaction method. SEM observation showed that the channel diameters of anodic alumina membranes prepared under 40 V and 200 V are about 60 nm and 200 nm, respectively, and CuTCNQ nanowire arrays were synthesized in these channels. Nanodevice prototypes with electrical switching characteristics based on a CuTCNQ nanowire array were fabricated, whose reproducible electrical switching and memory effects were observed. The on-off ratio for switching reaches 10(4). The potential applications in information storage devices are also discussed.
ABSTRACT
Vertically aligned ZnO nanowires were synthesized on the p(+) silicon chip by modifying the CVD process with a vapor trapping design. Scanning electron microscopy was used to investigate the morphology of as-obtained nanowires. X-ray diffraction showed that the obtained nanowires were ZnO crystalline. The rectifying characteristics of the p-n heterojunction composed of ZnO nanowires and a p(+) silicon chip were observed. The positive turn-on voltage was 0.5V and the reverse saturation current was 0.01mA. These vertically aligned ZnO nanowires showed a low field emission threshold of 4V/microm at a current density of 0.1microA/cm(2). The dependence of emission current density on the electric field followed Fowler-Nordheim relationship.
