We fabricate and experimentally characterize metal-semiconductor-metal (MSM) photodetectors with CNT film
Schottky electrodes on <i>n</i>-type and p-type silicon substrates. We extract a Schottky barrier height of ~0.45 eV and ~0.51
eV for CNT films on <i>n</i>-type and p-type Si respectively. The extracted barrier height corresponds to a CNT film
workfunction of 4.5-4.7 eV, which is within the range of the previously reported workfunction values for individual
CNTs. Furthermore, we find that while at temperatures above 240°K thermionic emission is the dominant transport
mechanism, at lower temperatures tunneling begins to dominate. We also characterize the photoresponse of the CNT
film-Si MSM photodetector by illuminating the samples with a 633 nm HeNe laser. We observe that while the
photocurrent of the CNT film MSM devices is similar to that of the Ti/Au control samples at high biases, their lower
dark current results in a higher photo-to-dark current ratio relative to the control devices. We explain these observations
by comparing the two interfaces. This work opens up the possibility of integrating CNT films as transparent and
conductive Schottky electrodes in conventional semiconductor electronic and optoelectronic devices.
We present the scaling of percolation resistivity in nanotube films as a function of nanotube and device
parameters both experimentally and using simulations. We first characterize the resistivity of these films down to 200
nm lateral dimensions by fabricating standard four-point-probe structures. We find that the film resistivity starts to
increase at device widths below 20 microns, and exhibits an inverse power law dependence on width below a critical
width of 2 microns. We then use quasi-3D Monte Carlo simulations to model and fit these experimental results. In
addition to fitting the experimental data, we also study the effect of four parameters, namely nanotube density, length,
alignment, and measurement direction on resistivity and its scaling with device width. We explain these simulation
results by simple physical and geometrical arguments. Nanoscale study of percolation transport mechanisms in
nanotube films is essential for understanding and characterizing their performance in nanosensing device applications.
Proc. SPIE. 6464, MEMS/MOEMS Components and Their Applications IV
KEYWORDS: Oxides, Ions, Silicon, Chemical vapor deposition, Atomic force microscopy, Scanning electron microscopy, Transmission electron microscopy, Raman spectroscopy, Single walled carbon nanotubes, Carbon nanotubes
Transmission electron microscopy (TEM) and micro-Raman spectroscopy are key techniques in the structural
characterization of carbon nanotubes. For device applications, carbon nanotubes are typically grown by chemical vapor
deposition (CVD) on silicon substrates. However, TEM requires very thin samples, which are electron transparent.
Therefore, for TEM analysis, CVD grown nanotubes are typically deposited on commercial TEM grids by post-processing.
This procedure has two problems: It can damage the nanotubes, and it does not work reliably if the nanotube
density is too low. The ability to do TEM directly on as-grown nanotubes lying on the silicon substrate would solve these
two problems. In this talk, for this purpose, we fabricate micromachined TEM grids from silicon substrates.
Subsequently, we grow nanotubes on these micromachined TEM grids by CVD, and characterize the nanotubes by TEM,
micro-Raman spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM). We show that
these substrates provide a low cost, mass producible, efficient, and reliable platform for direct TEM, Raman, AFM, and
SEM analysis of as-grown nanotubes or other nanomaterials on the same substrate, eliminating the need for any post-processing
after CVD growth.
We present experimental evidence that iron ions implanted into silicon/silicon dioxide substrates form nanoscale islands during subsequent annealing, which act as catalyst for nanotube chemical vapor deposition (CVD) growth. We have implanted Fe+ ions into thermally grown SiO<sub>2</sub> layers on silicon substrates with an energy of 60 keV and a dose of 10<sup>15</sup> cm<sup>-2</sup>. Using this implanted catalyst, we have then grown carbon nanotubes by CVD at 900°C with methane as the precursor gas. We have characterized the catalyst islands and the grown carbon nanotubes by Atomic Force Microscopy (AFM) and Raman spectroscopy. The diameters of carbon nanotubes we have grown from ion implanted catalyst in this work are much smaller than those reported previously. The presence of small diameter nanotubes implies single-walled nanotube (SWNT) growth. The height distribution of the catalyst islands correlates very well with the diameter distribution of nanotubes. This is consistent with previous work which has found evidence that nanotube diameter depends strongly on the size of the catalyst particles.
Since ion-implantation can be easily masked by lithography, this technique of nucleating nanotube growth opens up the possibility of controlling the origin of nanotubes at the nanometer scale over high aspect ratio topography. This technique also has the advantage that it can easily be integrated with silicon processing, and scaled to larger substrates.