World Journal Of Engineering
VERTICAL GROWTH OF METALLIC BORIDE NANOWIRES AND
THEIR FIELD EMISSION PROPERTIES
Robert M. Jacobberger, Joseph R. Brewer, Chin Li Cheung (ccheung2@unl.edu)
Department of Chemistry, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, USA.
work [5]. (100) silicon (Si) wafers and (100) textured
polycrystalline zirconium nitride (ZrN) coated onto Si
wafers by ion beam assisted deposition were used as
substrates. These LaB6 structures were characterized
with X-ray diffractometry (XRD), scanning electron
microscopy (SEM), transmission electron microscopy
(TEM), energy-dispersive X-ray spectroscopy, and atom
probe tomography. Detailed results and analysis of this
characterization are presented elsewhere [5].
Introduction
Cold field emission (CFE) devices rely on the ballistic
transport of electrons under an electric field through a
vacuum, allowing them to operate in environments with
high levels of radiation. Solid-state devices, which rely
on the drift of electrons through a solid, cannot function
in such conditions. CFE devices also offer high current
densities, operate at high frequencies, and have relatively
low power consumption [1]. CFE is usually modeled by
the Fowler-Nordheim (F-N) equation [2]:
Device Fabrication and Characterization
LaB6 nanowire arrays grown on Si or ZrN substrates
were used as the cathode materials for the CFE devices.
The device architecture is shown in Fig. 1. Samarium
(100 nm) and tantalum (100 nm) were sputtered onto the
LaB6 as electrical contacts. Stainless steel anodes were
separated
from
the
LaB6
cathodes
with
polytetrafluoroethylene (PTFE) spacers at a distance of
75 or 125 μm. These devices were evaluated in a
vacuum chamber at a pressure of 7 x10-7 Torr. Voltage
sweeps of 0 to 1100 V were applied to the steel anode
and emission current was measured.
where J is the current density, β is the field enhancement
factor, φ is the work function, V is the applied voltage, d
is the cathode-to-anode distance, V/d is the applied
electric field, and A and B are constants. The F-N
equation predicts that under a constant applied electric
field, a greater flux of electrons is emitted from materials
with low work function and high field enhancement
factor. This field enhancement factor is strongly
dependent on the geometry of the emitters and the
arrangement of these emitters in the array [3]. For
example, one-dimensional materials, such as nanowires
and nanocones, have increased field enhancement factors
due to the converging of electric field lines at their tips.
Lanthanum hexaboride (LaB6) has been extensively
studied as the cathode material in CFE devices because
of its low work function (2.5 eV), low resistivity (5
μΩ·cm), high melting point (2500°C), and high chemical
stability [4]. Nanoscale LaB6 structures with high aspect
ratio have the ability to increase the current density and
durability and decrease the power consumption of CFE
devices. Here, we report our study of the use of LaB6
nanowire arrays as the cathode in CFE devices and the
effects of the nanostructure alignment and cathode-toanode distance on the performance of these devices.
Fig. 1 Architecture of the CFE devices incorporating
nanostructured LaB6 cathodes.
Results and Discussion
LaB6 nanowires synthesized via CVD were single
crystalline and had diameters of about 50 nm and lengths
of several microns, as indicated by SEM and TEM data.
The high aspect ratio of these nanowires resulted in field
enhancement factors as high as ≈1400, making these
one-dimensional structures ideal emitters. Fig. 2.b and
2.d provide the XRD data of LaB6 emitters grown on
Experimental
Synthesis and Characterization
LaB6 nanowires were grown via a palladium-catalyzed
chemical vapor deposition (CVD) method as in previous
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World Journal Of Engineering
ZrN and Si substrates, respectively. The relative
intensity of the (100) LaB6 peak indicates that nanowires
synthesized on ZrN more preferentially grow
perpendicular to the substrate in the [100] direction than
those on Si substrates. Fig. 2a and 2c show SEM images
of LaB6 nanowire arrays grown on ZrN and Si. This
highly oriented alignment of LaB6 on ZrN is attributed to
the possible close cube-on-cube lattice matching
relationship between [100] ZrN and [100] LaB6.
improved performance of CFE devices on ZrN
substrates is attributed to the highly perpendicular
alignment of the LaB6 emitters to the ZrN substrate. This
alignment possibly reduces the electric field screening
between individual nanowires in the array, which
increases the enhanced field at the nanowires tips.
The CFE device performance was found to be strongly
dependent on the cathode-to-anode distance. The turn-on
field and field enhancement factor for emitter arrays
grown on Si with a cathode-to-anode distance of 125 μm
are 2.1 V/μm and 1394, respectively. The turn-on
voltage and field enhancement factor are significantly
improved with an increase in the cathode-to-anode
distance. This dependence of CFE device performance
on such a parameter is surprising because the F-N
equation predicts that the current density is only
dependent on the applied electric field. The F-N equation
assumes that the emitters are a planar surface, which is a
gross approximation. The inadequacy of the F-N
equation is probably due to the shortcoming of the F-N
model to account for the electric field screening effects
between individual nanowires in the emitter array. Our
CFE device data suggests that electric field screening is
increased at small cathode-to-anode distances.
Fig. 1. SEM and XRD data of LaB6 nanowires
grown on (a & b) ZrN and (c & d) Si substrates.
Conclusion
The performance of CFE devices made with LaB6
nanowires as the cathode material was investigated.
Aligned LaB6 nanowire emitter arrays on substrates that
promote growth in the [100] direction decreased the
turn-on field and increased the field enhancement factor.
Increasing the cathode-to-anode distance reduced
screening effects and further enhanced the CFE device
performance. The ability to control the geometry and
alignment of LaB6 nanowires makes it possible to realize
durable CFE devices with low power consumption.
Fig. 2 Plot of current density vs. applied field for CFE
devices with LaB6 nanowire arrays on different
substrates and cathode-to-anode distances, d.
Acknowledgement
Financial support for this work was provided by the
NASA Nebraska Space Grant Fellowship program.
CFE emitter arrays of LaB6 nanowires grown on ZrN
and Si substrates were characterized at cathode-to-anode
distances of 75 and 125 μm. The current density vs.
applied electric field plots are given in Fig. 3. For CFE
devices with 75 μm cathode-to-anode distance, the turnon field for nanowire arrays grown on ZrN and Si are 2.3
and 3.1 V/μm, respectively. The turn-on field is defined
as the applied field at which the plot of ln(J/(V/d)2) vs.
1/(V/d) becomes linear (approximately 0.1 μA/cm2). The
field enhancement factors are 1254 and 515 for the
emitter arrays grown on ZrN and Si, respectively. This
References
1. Xu, N. S. and Huq, S. E. New cold cathode materials and
applications. Mat. Sci. Eng., 48 (2005) 47-179.
2. Stern, T. E., Gossling, B. S., and Fowler, R. H. Further
studies in the emission of electrons from cold metals. Proc. R.
Soc. Lond. A-Conta., 124 (1929) 699–723.
3. Hawkes, P. W. “Advances in Electronics and Electron
Physics” Academic Press, MO. (1992).
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4. Zhang, H., Tang, J., Zhang, Q., Zhao, G., Yang, G., Zhang,
J., Zhou, O., Qin, L. C. Field emission of electrons from single
LaB6 nanowires. Adv. Mater., 18 (2006) 87-91.
5. Brewer, J. R., Jacobberger, R. M., Diercks, D. R., and
Cheung, C. L. Rare earth hexaboride nanowires: general
synthetic design and analysis using atom probe tomography.
Chem. Mater., ASAP.
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