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RESEARCH ARTICLE
|
MAY 31 2023
Effect of praseodymium coating on electron emission from a
nanoscale gold field emitter array
L. B. De Rose
;
D. H. Catanzaro
;
C. Choi
;
A. Scherer
Journal of V
acuum Science & T
echnology B
41, 042801 (2023)
https://doi.org/10.1
116/6.0002700
05 July 2023 16:34:45
Effect of praseodymium coating on electron
emission from a nanoscale gold field emitter array
Cite as: J. Vac. Sci. Technol. B
41
, 042801 (2023);
doi: 10.1116/6.0002700
View Online
Export Citation
CrossMar
k
Submitted: 20 March 2023 · Accepted: 8 May 2023 ·
Published Online: 31 May 2023
L. B. De Rose,
1
,
a)
D. H. Catanzaro,
1
,
b)
C. Choi,
2
and A. Scherer
1
AFFILIATIONS
1
Department of Applied Physics, California Institute of Technology, Pasadena, California 91125
2
Department of Electrical Engineering, California Institute of Technology, Pasadena, California 91125
a)
Author to whom correspondence should be addressed:
lderose@caltech.edu
b)
Present address:
Department of Electrical Engineering, Stanford University, Stanford, California 94305.
ABSTRACT
Although field emission devices are inherently robust to high temperature and radiation environments as well as have high switching
speeds, their development has been hindered by high voltages that are typically required for their operation. In this work, we investigate the
effect of thin-film praseodymium (Pr) coating on the emission characteristics of a lateral gold (Au) field emitter array. Because Pr has a sig-
nificantly lower work function than Au, it is expected to increase the field emission measured current. Pr is deposited onto the device via
thermal angled evaporation in a custom-built vacuum chamber with
in situ
electrical characterization capability. Our experiments demon-
strate that a 10 nm-thick Pr layer reduces the turn-on voltage by almost half compared to the noncoated Au structure. These results are
promising for the development of power-efficient, low voltage field emission electronics.
© 2023 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license
(
http://creativecommons.org/licenses/by/4.0/
).
https://doi.org/10.1116/6.0002700
I. INTRODUCTION
Field emission is a type of electron emission mechanism by
which bound electrons are released from the surface of a solid
material into vacuum in the presence of a high external electrostatic
field. As a result of this applied field, the potential barrier becomes
sufficiently narrow, allowing the electrons to tunnel through it.
1
3
,
51
Devices based on field emission have been used in various applica-
tions, including x-ray sources,
4
flat panel displays,
5
,
6
electron beam
lithography systems,
7
and microwave amplifiers.
8
Compared to conventional semiconductor or thermionic
devices, field emission-based electronics have several advantages.
Vacuum technology is inherently immune to harsh operating
conditions such as radiation environments or high ambient tem-
peratures,
9
,
10
making it ideal for aerospace, military, or nuclear
applications. In addition, the use of vacuum as a carrier channel
allows for ballistic transport due to the lack of scattering
events.
11
,
12
Since device capacitance can be engineered by simply
changing the geometry, field emission devices are attractive for
high-frequency applications. Moreover, they do not require a sup-
plementary heating source or cooling systems and are easier and
cheaper to manufacture than thermionic devices.
13
,
14
One of the main drawbacks of field emission devices that has
limited their popularity is the relatively high threshold voltage
needed to generate a significant tunneling current compared to
solid-state devices.
11
,
15
Low voltage operation is desirable to
reduce power consumption, increase device lifetime by minimiz-
ing ion sputtering, and produce h
igh transconductance devices
that are essential for practical electronic devices.
16
,
17
Additionally,
low voltage operation can reduce leakage currents that arise at
high fields and compete with field emission, such as Frenkel
Poole emission or Ohmic conduction.
18
Despite these challenges,
there are several ways to improve the performance of field emis-
sion devices. One approach is to sharpen the cathode or employ
structures with high aspect ratios, which increases the local field
at emission sites compared to the applied macroscopic field.
19
22
This geometrically induced field enhancement causes the width of
the surface potential barrier to be further reduced, facilitating
electron emission. However, excessively high field enhancements
can lead to overheating and device failure.
23
Furthermore, the
spacing between the electrodes can be reduced to improve emis-
sion efficiency
24
although this method is ultimately limited by the
minimum feature sizes attainable with available fabrication techniques.
ARTICLE
avs.scitation.org/journal/jvb
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41
(4) Jul/Aug 2023; doi: 10.1116/6.0002700
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042801-1
©Author(s)2023
05 July 2023 16:34:45
Finally, emission can be optimized by applying a coating to the
cathode to lower the effective work function, which reduces the
height of the potential barrier.
25
Examples of thin-film coatings
include diamond,
26
28
diamondlike carbon,
29
31
nitrides,
32
,
33
car-
bides,
34
alkali metals,
35
37
silicides,
38
and some oxides.
39
43
In this study, we investigate the effect of a 10 nm-thick praseo-
dymium (Pr) coating on the emission characteristics of a lateral
gold (Au) field emitter array. Pr has a work function of 2.7 eV,
44
which is significantly lower than that of Au, which has a work
function of approximately 5.3 eV.
45
Moreover, Pr has a lower work
function than other commonly used field emitter coatings, such as
lanthanum (3.5 eV) or cerium (2.9 eV).
46
Although praseodymium
oxide has been shown to be an appealing choice for low work func-
tion coatings
47
as it may enable the usage of devices at atmospheric
pressure, it has a lower electrical conductivity than pure Pr. Thus,
the effective electron transport on terminals is not as efficient as in
Pr. Additionally, due to the high atomic number of Pr, more elec-
trons are available for emission, resulting in higher expected emis-
sion currents. The field emitter array is fabricated using Au because
of its chemical inertness, which provides emission stability. Yet, the
fabrication process can also be extended to other metals that can
be evaporated. Furthermore, an asymmetric design is employed, in
which the cathode consists of a 30-tip array while the anode is flat.
We opt for an in-plane geometry to leverage high-resolution lithog-
raphy, which simplifies the fabrication process and provides greater
control over main physical device dimensions, specifically spacing
between the electrodes.
II. EXPERIMENTAL PROCEDURE
A. Fabrication of a gold field emitter array
The devices were fabricated on a 500
μ
m-thick JGS2 fused
silica substrate, which was initially coated with a 25 nm layer of
chrome (Cr) via electron beam evaporation (CHA Industries
Mark 40). The purpose of this thin metal layer was to aid in
charge dissipation during subsequent electron beam lithography.
A 200 nm-thick layer of 950 poly(methyl methacrylate) (PMMA)
A4 was spin-coated onto the sample, which was then baked at
180

C for 4 min. Electron beam lithography at 100 keV was used
to pattern the devices, followed by development at room tempera-
ture for 60 s in a 1:3 solution of methyl isobutyl ketone (MIBK)
and isopropanol. Next, the Cr over the exposed pattern was
removed, and a 6 nm titanium adhesion layer, a 60 nm gold elec-
trode layer, and a 20 nm titanium etch mask layer were deposited
using electron beam evaporation at a pressure of

10

8
Torr
(Kurt J. Lesker Labline). The samples were left in acetone overnight
for the lift-off process, which was aided by sonication, resulting in
vacuum gaps of 40 nm on average between the electrodes. After
lift-off, any remaining Cr under the now-removed resist was etched
away. The next step was to remove any insulating material in the
area surrounding the emission sites to prevent charging effects that
cause hysteretic behavior as well as to avoid dielectric breakdown
and device failure. Undercutting the substrate near the emitter also
increases the length of possible leakage current pathways. A dry etch
using C
4
F
8
and O
2
(Oxford Instruments Plasmalab System 100
ICP-RIE 380) was first used to increase the exposed surface area, fol-
lowed by a brief wet etch in buffered hydrofluoric acid, resulting in

100 nm vertical undercut of the structures. Critical point drying
(Tousimis 915B) was employed to prevent any damaging effect as a
result of surface tension.
Figure 1
shows scanning electron micro-
graphs of a finished device.
FIG. 1.
Scanning electron micrographs of a lateral field emission multitip
device: (a) top view and (b) side view at 50

tilt. The cathode consists of a
30-tip field emitter array while the anode is flat. The separation between both
electrodes is about 40 nm.
ARTICLE
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41
(4) Jul/Aug 2023; doi: 10.1116/6.0002700
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042801-2
©Author(s)2023
05 July 2023 16:34:45
B. Pr evaporation and the measurement setup
The devices were ultrasonically wedge wire-bonded with alu-
minum wires to a ceramic pin grid array package (Spectrum
Semiconductor Materials CPG15504). A 10 nm-thick layer of Pr
was deposited via thermal evaporation in a custom deposition
chamber with a base pressure of 6

10

6
Torr. The angle of evap-
oration was set to 45

with respect to the sample plane to achieve a
conformal coating of emitter tips. To prevent oxidation or contami-
nation that could modify Pr work function due to air exposure, a
feedthrough with electrical connections for
in situ
measurements
was added to the chamber. The device current-voltage (I-V) char-
acteristics before and after Pr evaporation were measured using two
sourcemeters (Keithley 2450).
III. RESULTS AND DISCUSSION
To clean the devices from surface contaminants and obtain
stable emission, multiple I-V sweeps were performed until no sig-
nificant changes were observed between consecutive scans. The
cathode voltage was held at 0 V while the anode was positively
biased to promote electron emission from the cathode.
Figure 2
shows the I-V characteristics of the noncoated and Pr-coated 30-tip
field emitter array. To prevent excessive resistive heating that could
damage the sharp tips, a current limit of 100 nA was imposed. The
Pr coating reduced the turn-on voltage, defined as the voltage
required to measure a current of 10 nA, from 8.2 to 4.3 V. This
indicates that, for a given bias, the emission current is drastically
increased by the Pr layer compared to the bare device.
Field electron emission from metals is described using the
Fowler
Nordheim (FN) equation, which relates the measured
current
I
to the applied voltage
V
and the work function
f
as
follows:
I
¼
aS
(
β
V
)
2
f
exp

b
f
3
=
2
β
V
!
,
(1)
where
S
is the effective emission area,
β
is the field factor that
depends on the device geometry, and
a
and
b
are constants
given by
a
¼
e
3
m
16
π
2

h

1
:
54
μ
A

eV

V

2
,
b
¼
4
3
e
ffiffiffiffiffiffiffi
2
m

h
2
r

6
:
83 V

nm

1

eV

3
=
2
,
where
e
and
m
are the elementary charge and mass of the electron,
respectively. In practice, measured field emission data are often
analyzed through the so-called FN plot, which is obtained by line-
arizing
Eq. (1)
to
48
,
49
ln
I
V
2

¼
A
1
V

þ
B
,
(2)
where
A
and
B
are the slope and
y
-intercept, respectively. In terms
of physical parameters, these are given by
A
¼
b
f
3
=
2
β
,
(3a)
B
¼
ln
aS
β
2
f

:
(3b)
In the FN plot, the
y
-axis is given by ln
I
=
V
2
ðÞ
and the
x
-axis by
1
=
V
, so that a straight line corresponds to field emission. This tech-
nique allows us to distinguish our measured data from other emis-
sion mechanisms. Moreover, the FN plot can help us deduce
changes in device parameters, including the effective work function
and field factor.
32
,
50
Figure 3
shows the measured data using FN coordinates along
with fitted least squares regression lines. After the Pr coating, the
slope in the FN plot becomes significantly less steep. As the slope
is related to the work function and the field factor, a decrease in
the observed slope corresponds to either a reduction in the effective
work function, an increase in the field factor, or a combination of
both. The values for the measured slope,
y
-intercept, and R
2
value
obtained from regression lines fitted to the FN plot before and after
deposition of Pr on the sample are included in
Table I
.
From the FN emission equation given in
Eq. (1)
, if we reduce
the emitter work function
f
by a factor
c
(
c
.
1) while keeping the
field factor and emission area constant, the magnitude of the slope
decreases by a factor of
c
3
=
2
and the
y
-intercept increases by ln (
c
).
The values of the expected slope and
y
-intercept are also included
in
Table I
. The ratio of the slopes allows us to infer an
FIG. 2.
Effect of Pr coating in field emission I-V characteristics. A dashed line
at 10 nA is also included to determine the turn-on voltage.
ARTICLE
avs.scitation.org/journal/jvb
J. Vac. Sci. Technol. B
41
(4) Jul/Aug 2023; doi: 10.1116/6.0002700
41,
042801-3
©Author(s)2023
05 July 2023 16:34:45
experimentally measured
c
m
,
A
as follows:
c
m
,
A
¼
A
non
A
Pr

2
3
,
(4)
where
A
non
and
A
Pr
correspond to noncoated and Pr-coated FN
slopes, respectively.
Based on our experimental results, we obtain a value of
c
m
,
A
¼
1
:
65, which is slightly smaller than the expected
c
x
¼
1
:
96
that was calculated using the work functions of pristine Au and Pr.
However, if we consider the difference between the
y
-intercepts
before and after Pr coating, denoted as
B
non
and
B
Pr
, so that
c
m
,
B
¼
e
B
Pr

B
non
,
(5)
we obtain
c
m
,
B
¼
2
:
01. The difference between
c
m
,
A
and
c
m
,
B
likely
arises from the assumption of a constant field factor and emission
area, which may be oversimplification since the surface roughness
of the Pr film can easily modify them.
Let us assume that the field factor is modified after Pr evapo-
ration by a factor
f
so that
β
Pr
¼
f
β
non
:
(6)
Thus, we have the following relations between the noncoated and
Pr-coated regression coefficients:
A
Pr
A
non
¼
1
f
m
c
3
=
2
m
,
(7a)
B
Pr

B
non
¼
ln (
c
m
f
2
m
),
(7b)
where
f
m
corresponds to experimentally measured field factor mod-
ification. Solving the system of equations leads to values
c
m
¼
1
:
50
and
f
m
¼
1
:
16, indicating that the Pr coating not only reduced the
effective work function but also increased the field factor. Thus,
assuming a starting work function of 5.3 eV for Au, our field factor
increased from 2.12 to 2.45 nm

1
. Note that a 10 nm-thick layer of
material evaporated at 45

shrinks the distance between the termi-
nals to approximately 33 nm. This corresponds to an expected field
factor modification given by
f
x
¼
1
:
21, which agrees with the
experimentally measured field factor modification
f
m
.
IV. SUMMARY
A lateral 30-tip Au field emission array with a 40 nm vacuum
gap was fabricated via electron beam lithography, metal deposition,
and lift-off, and the effect of a 10 nm layer of thermally evaporated
Pr on the electron emission characteristics was investigated. The Pr
coating led to a significant enhancement in the measured emission
current, resulting in a reduction in almost a factor of two on the
turn-on voltage. Moreover, the FN plot analysis showed a decrease
in the magnitude of the slope and
y
-intercept, indicating that the
effective work function was reduced after Pr evaporation. These
results pave the way to building low voltage practical nanoscale
field emission electronics that could enable high temperature
robustness, radiation hardness, and high frequency operation.
ACKNOWLEDGMENTS
We wish to acknowledge several technical discussions from
Leora Peltz and Robert Frampton from The Boeing Company.
AUTHOR DECLARATIONS
Conflict of Interest
The authors have no conflicts to disclose.
Author Contributions
L. B. De Rose:
Data curation (lead); Formal analysis (equal);
Investigation (equal); Methodology (lead); Resources (equal);
Software (lead); Validation (equal); Visualization (lead); Writing
original draft (lead); Writing
review & editing (equal).
D. H.
Catanzaro:
Formal analysis (equal); Investigation (equal);
Validation (equal); Writing
review & editing (equal).
C. Choi:
Formal analysis (equal); Investigation (supporting); Validation
(equal); Writing
review & editing (equal).
A. Scherer:
Conceptualization (lead); Funding acquisition (lead); Project
administration (lead); Resources (equal); Supervision (lead);
Writing
review & editing (equal).
TABLE I.
Linear regression analysis data before and after Pr coating.
Parameter
Before Pr
Expected
After Pr
Slope (
A
)
39.26
15.11
18.45
y
-intercept (
B
)
17.79
17.15
17.09
R
2
value
0.966
NA
0.968
FIG. 3.
FN plots of Pr-coated and noncoated devices. The lines correspond to
least-squares regression.
ARTICLE
avs.scitation.org/journal/jvb
J. Vac. Sci. Technol. B
41
(4) Jul/Aug 2023; doi: 10.1116/6.0002700
41,
042801-4
©Author(s)2023
05 July 2023 16:34:45
DATA AVAILABILITY
The data that support the findings of this study are available
from the corresponding author upon reasonable request.
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05 July 2023 16:34:45