Field emission from molybdenum carbide

Article abstract of DOI:10.1063/1.126415

The thermal stability and the resiliency of molybdenum carbide field-emission tips deposited at room temperature by electrophoresis have been studied. The field emission from Mo₂C films deposited on Mo tips does not change after being heated to 800°C while exposed to 360 L of air, although MoO₂, MoO₃, and possibly MoO, are present in the films. The field-emission thresholds agree with photoelectric work functions determined from photoelectron spectroscopy measurements of similarly grown flat samples. These films are found to exist in three distinct phases as a function of temperature after formation by room-temperature electrophoresis. From room temperature to 500°C, MoO₃ is the dominant oxide, from 500 to 775°C, MoC₂ is the dominant oxide, and above 825°C both oxides have virtually disappeared.

Full text of DOI:10.1063/1.126415

Field emission from molybdenum carbide
Ambrosio A. Rouse, John B. Bernhard, Edward D. Sosa, and David E. Golden
Citation: Applied Physics Letters 76, 2583 (2000); doi: 10.1063/1.126415
View online: http://dx.doi.org/10.1063/1.126415
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/76/18?ver=pdfcov
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APPLIED PHYSICS LETTERS
VOLUME 76, NUMBER 18
1 MAY 2000
Field emission from molybdenum carbide
Ambrosio A. Rouse, John B. Bernhard, Edward D. Sosa, and David E. Goldena)
Department of Physics and Material Science, University of North Texas, Denton, Texas 76203
͑Received 23 November 1999; accepted for publication 13 March 2000͒
The thermal stability and the resiliency of molybdenum carbide field-emission tips deposited at
room temperature by electrophoresis have been studied. The field emission from Mo2C films
deposited on Mo tips does not change after being heated to 800 °C while exposed to 360 L of air,
although MoO2, MoO3, and possibly MoO, are present in the films. The field-emission thresholds
agree with photoelectric work functions determined from photoelectron spectroscopy measurements
of similarly grown flat samples. These films are found to exist in three distinct phases as a function
of temperature after formation by room-temperature electrophoresis. From room temperature to
500 °C, MoO3 is the dominant oxide, from 500 to 775 °C, MoC2 is the dominant oxide, and above
825 °C both oxides have virtually disappeared. © 2000 American Institute of Physics.
͓S0003-6951͑00͒04218-2͔
Cold high-brightness electron sources have been made
using molybdenum field-emission arrays ͑FEAs͒ based on
micron-sized field emitters deposited using the Spindt depo-
sition process.1 Molybdenum has been the emitter material of
choice for FEAs because it has good thermal, mechanical,
and electrical properties and easy to achieve high-aspect
ratios.2 However, when molybdenum emitters are exposed to
oxygen, they easily form insulating molybdenum oxides that
degrade the emission.3 In fact, when a Mo FEA was exposed
to 10 L of O2, the work function of the FEA increased by 0.3
eV and when exposed to 100 L of O2, its work function
increased by 0.6 eV. This corresponded to a decrease in
emission current of 17% for 10 L exposure and 50% for 100
L at the 60 V operating voltage.3
determined from FE energy distribution measurements and
compared to ultraviolet photoelectron spectroscopy ͑UPS͒
measurements using similarly coated flat samples. These
samples also have been studied using x-ray photoelectron
spectroscopy ͑XPS͒ and x-ray diffraction ͑XRD͒ as a func-
tion of annealing.
FE tips were fabricated from 0.02-in.-thick molybdenum
wire that was electrochemically etched in a 2 mol % KOH
solution using a ϩ10 V bias voltage on the wire and then
rinsed in distilled water. Molybdenum carbide powder
͑99.5%͒ of 3–4 ␮m average size was mixed in an aqueous
solution of 10% ethanol and used to deposit molybdenum
carbide via electrophoresis8 on the tips and molybdenum
foils at room temperature by applying a ϩ240 V bias to the
sample for 10 min.9
Molybdenum carbide has a density, hardness, melting
point, coefficient of thermal expansion, and conductivity
similar to that of molybdenum,4 and could also be used to
make Spindt FEAs. Molybdenum carbide has a work func-
tion of 3.8 eV,5 which is considerably lower than that of
molybdenum with a work function of 4.6 eV,6,7 but it has not
been investigated as a potential field emitter material.
In the present work, field emission ͑FE͒ from molybde-
num carbide-coated field-emission tips has been studied in a
single gated-diode configuration before and after exposure to
air at 10Ϫ7 Torr and temperatures as high as 800 °C. Fowler–
Nordheim FE current–voltage characteristics and simulta-
neous FE energy distributions ͑FEEDs͒ were obtained from
tips mounted in a gated-diode configuration in the analysis
chamber of a VG-ESCALAB system. In a Fowler–
Nordheim plot, the natural log of the ratio of current to volt-
age squared (I/V2) is plotted versus 1/V. The slope of the
resulting straight line yields the product of the work function
of the surface to the 3/2 power times the ‘‘tip-shape param-
eter.’’ Since the work function may be directly measured
with a FEED, a straight-line current–voltage characteristic
insures that the tip-shape function did not change during the
FEED measurement.
Flat samples were processed in a chamber attached to a
VG ESCALAB II system over a temperature range from 26
to 900 °C in 1 h intervals at an average base pressure of 1
ϫ10Ϫ7 T of air. Additional samples were annealed at 412,
525, and 800 °C in 1 h intervals at an average base pressure
of 10Ϫ7 T of air. All samples were characterized with XPS
before and after each annealing stage and with scanning elec-
tron microscope ͑SEM͒ micrographs taken with a JEOL
JMS-T300 before processing and analysis, and after analysis.
Low-energy ͑4.49, 5.06, and 5.64 eV͒ UPS data with a Xe
UV source and 2 nm bandpass monochromator also were
obtained. UPS and XPS data were analyzed with the PEAKFIT
program using Gaussian profiles.10 XRD was obtained with a
Siemens D500 diffractometer using Cu K␣1 radiation.
The XRD result for Mo2C deposited on a flat Mo sheet is
given in Fig. 1 and corresponds to hexagonal Mo2C.11 The
three major Mo2C peaks found in Mo2C powder are clearly
seen. In addition, the intense Mo ͑200͒ peak drops by a fac-
tor of 2.5 after Mo2C deposition, indicating good Mo2C cov-
erage. A slight shift of the diffraction lines toward lower 2␪
angles between the deposited samples and the Mo2C powder
was seen, indicating lattice expansion. This is likely due to
oxygen substitution for carbon, as previously reported.12
This stress was relieved with increasing annealing tempera-
ture. Finally, the MoO3 ͑002͒, ͑200͒, and ͑101͒ planes are
Molybdenum carbide-coated tip FE thresholds have been
a͒
Electronic mail: golden@unt.edu
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0003-6951/2000/76(18)/2583/3/$17.00 2583 © 2000 American Institute of Physics
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2584
Appl. Phys. Lett., Vol. 76, No. 18, 1 May 2000
Rouse et al.
TABLE II. Mo d5/2 /Mo d3/2 ratio with annealing time.
Temperature ͑°C͒
Time ͑h͒
412
525
800
1
2
3
1.5
1.3
1.4
1.4
0.5
0.5
0.5
0.5
1.3
1.5
1.5
1.4
Average
Mo.15,16 There are no peaks due to adsorbed CO above
775 °C.
In the first temperature regime, CO, MoO, MoO2,
MoO3, and Mo2C are present, with MoO3 the dominant mo-
lybdenum oxide. In the second phase, CO, MoO2, MoO3,
and Mo2C are present, with MoO2 the dominant oxide and in
the last phase, MoO2, MoO3 are only present in trace
amounts. To further understand how the transitions between
the regions where MoO3, MoO2, and Mo2C dominate de-
pend on time and temperature, the Mo 3d5/2 /Mo 3d3/2 ratio
was studied as a function of time in the three regions. As
may be seen in Table II, this ratio did not change over a 3 h
interval, indicating that the transitions are temperature de-
pendent and not time dependent. The sample annealed at
412 °C for 3 h contained Moϩ6, indicating MoO3 as the
dominant oxide at that temperature. The sample annealed at
525 °C for 3 h showed Moϩ4, indicating MoO2 as the domi-
nant oxide at that temperature. The sample annealed at
800 °C for 3 h, showed Mo2C was dominant over both ox-
ides at that temperature.
FIG. 1. XRD for a Mo2C film.
present in the XRD results, although this cannot be seen in
Fig. 1. The flat samples were annealed in the processing
chamber over a temperature range from 26 to 900 °C in 1 h
intervals at an average base pressure of 1ϫ10Ϫ7 T to inves-
tigate the surface constituents of the Mo2C films. These
samples were characterized with XPS before and after each
annealing stage and with SEM micrographs before and after
processing. The XPS spectra contained the Mo 3d and Mo
3p doublets, and the C 1s and O 1s peaks. The Mo 3d
doublet was used to indicate the presence of MoO3 and
MoO2 by stoichiometry. Table I has the peak height ratios of
the O 1s/C 1s, O 1s/Mo 3d5/2 , C 1s/Mo 3d5/2 , and the Mo
3d5/2 /Mo 3d3/2 peaks during annealing at different tempera-
tures. As can be seen by reference to the O 1s/Mo 3d5/2 and
O 1s/C 1s ratios, the O content initially increases and then
decreases with increasing temperature. However, both the C
1s/Mo 3d5/2 and Mo 3d5/2 /Mo 3d3/2 ratios show three dis-
tinct temperature regimes. The transition temperatures be-
tween the first and second ͑ϳ525 °C͒ and between the sec-
ond and third ͑ϳ800 °C͒ regimes agree with the reported
desorption temperatures for MoO2 ͑Ref. 13͒ and MoO3.14
Examination of the C1s structure below 775 °C shows sev-
eral peaks attributed to various states of adsorbed CO on
Figure 2 shows a Fowler–Nordheim plot and Fig. 3
shows FEED spectra for the same nonannealed tip. During
the measurements, the tip bias was Ϫ89.99 V with an anode
voltage that ranged from 1000 to 1200 V in 50 mV steps and
TABLE I. Peak ratios with annealing temperature.
T ͑°C͒
C/Mo5/2
O/Mo5/2
O/C
Mo5/2 /Mo3/2
26
500
650
725
775
825
875
900
0.5
0.4
0.7
0.6
0.7
0.2
0.2
0.2
1.5
1.5
1.9
1.5
1.1
0.3
0.1
0.1
3.1
3.7
2.7
2.6
1.7
1.1
0.6
0.3
1.2
1.1
0.6
0.8
0.7
1.4
1.5
1.5
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FIG. 2. Fowler–Nordheim plot for a nonannealed Mo2C tip.
129.12.234.99 On: Mon, 15 Dec 2014 01:15:23

Appl. Phys. Lett., Vol. 76, No. 18, 1 May 2000
Rouse et al.
2585
FIG. 3. Feed for a nonannealed compared to an annealed tip.
an analyzer resolution of 0.1 eV. The FEED in Fig. 3 is the
average of normalized energy distributions obtained at each
voltage on the I–V plot. Three well-defined peaks are seen
that have been fitted with exponentially modified Gaussian
functions using the PEAKFIT program.10 The peaks have in-
flection points at 3.7, 4.7, and 5.7 eV, respectively. FE
thresholds are given by the energies at inflection points of
the FEED contributions.7,9 The first two energies are in good
agreement with the accepted values for the work functions of
Mo2C ͑Ref. 5͒ and Mo,6,7 respectively. The 5.7 eV threshold
is attributed to a deeper state of Mo. Figure 3 also shows a
FEED for the same Mo2C tip heated to 800 °C and exposed
to 360 L of air at 10Ϫ7 Torr for 1 h in the processing cham-
ber attached to the VG-ESCALAB system. These data are
plotted on the same scale as used for the nonannealed data
and have been fitted with three peaks that have points of
inflection at 3.7, 4.7, and 5.6 eV, respectively. Thus, the
work function of Mo2C does not vary with an exposure of
360 L of air. This is much different than the 6.4% and 12.8%
increases in work function found for Mo with 10 and 100 L
exposures to O2.3
Low-energy UPS spectra were obtained using 5.64, 5.06,
and 4.49 eV incident photons. The data and best fits to the
data using the PEAKFIT program10 with Gaussian functions
are shown plotted in Fig. 4 as a function of binding energy
͑energy difference between photon energy and the ejected
electron kinetic energy͒ at each photon energy used. The
energy scale measures the energy difference between the
vacuum level and populated levels below the vacuum level.
Peaks at averaged energies of 2.9, 3.8, and 4.3 eV are ob-
tained at all three photon energies. A peak at 1.7 eV is ob-
tained with 4.49 eV photons, a peak at 4.7 eV with 5.06 and
5.64 eV photons, and a peak at 5.2 eV with 5.64 eV photons.
The 3.8 eV result can be attributed to Mo2C ͑Ref. 5͒ and the
4.7 eV result can be attributed to Mo,6,7 while the peaks at
1.7, 2.9, and 4.3 eV are attributed to Mo oxides. The 2.9 eV
peak could be due to MoO that has a reported transition at
2.6 eV in its photoelectron spectra.17 The peak at 5.2 eV is
likely due to the deeper state of Mo seen at 5.6 eV in the
FEED discussed above.
FIG. 4. UPS for 4.49, 5.06, and 5.64 eV photons.
have a 17% lower work function than Mo and make ex-
tremely good field emitter surfaces, even with significant
oxygen content.
The authors would like to thank Professor Teresa Golden
of the Chemistry Department at the University of North
Texas for allowing them to do the XRD studies in her labo-
ratory. This work is supported in part by the NSF through
Grant No. DMR-9705187.
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