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 significantly 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 demonstrate 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.

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 applications, 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 temperatures,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 supplementary 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 minimizing ion sputtering, and produce high 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 emission 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 emission efficiency24 although this method is ultimately limited by the minimum feature sizes attainable with available fabrication techniques. 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 carbides,34 alkali metals,35–37 silicides,38 and some oxides.39–43 

In this study, we investigate the effect of a 10 nm-thick praseodymium (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 function coatings47 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 electrons are available for emission, resulting in higher expected emission 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 lithography, which simplifies the fabrication process and provides greater control over main physical device dimensions, specifically spacing between the electrodes.

The devices were fabricated on a 500  μm-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 temperature 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 electrode 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 C4F8 and O2 (Oxford Instruments Plasmalab System 100 ICP-RIE 380) was first used to increase the exposed surface area, followed 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 micrographs 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.

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.

Close modal

The devices were ultrasonically wedge wire-bonded with aluminum 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 evaporation was set to 45 ° with respect to the sample plane to achieve a conformal coating of emitter tips. To prevent oxidation or contamination 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) characteristics before and after Pr evaporation were measured using two sourcemeters (Keithley 2450).

To clean the devices from surface contaminants and obtain stable emission, multiple I-V sweeps were performed until no significant 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.

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.

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.

Close modal
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 ϕ as follows:
I = a S ( β V ) 2 ϕ exp ( b ϕ 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 = e 3 m 16 π 2 1.54 μ A eV V 2 , b = 4 3 e 2 m 2 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 linearizing Eq. (1) to48,49
ln ( I V 2 ) = A ( 1 V ) + B ,
(2)
where A and B are the slope and y-intercept, respectively. In terms of physical parameters, these are given by
A = b ϕ 3 / 2 β ,
(3a)
B = ln ( a S β 2 ϕ ) .
(3b)
In the FN plot, the y-axis is given by ln ( I / V 2 ) and the x-axis by 1 / V, so that a straight line corresponds to field emission. This technique allows us to distinguish our measured data from other emission 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.

FIG. 3.

FN plots of Pr-coated and noncoated devices. The lines correspond to least-squares regression.

FIG. 3.

FN plots of Pr-coated and noncoated devices. The lines correspond to least-squares regression.

Close modal
TABLE I.

Linear regression analysis data before and after Pr coating.

ParameterBefore PrExpectedAfter Pr
Slope (A−39.26 −15.11 −18.45 
y-intercept (B−17.79 −17.15 −17.09 
R2 value 0.966 NA 0.968 
ParameterBefore PrExpectedAfter Pr
Slope (A−39.26 −15.11 −18.45 
y-intercept (B−17.79 −17.15 −17.09 
R2 value 0.966 NA 0.968 
From the FN emission equation given in Eq. (1), if we reduce the emitter work function ϕ 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 experimentally measured c m , A as follows:
c m , A = ( A n o n A P r ) 2 3 ,
(4)
where A n o n and A P r correspond to noncoated and Pr-coated FN slopes, respectively.
Based on our experimental results, we obtain a value of c m , A = 1.65, which is slightly smaller than the expected c x = 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 n o n and B P r, so that
c m , B = e B P r B n o n ,
(5)
we obtain c m , B = 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 evaporation by a factor f so that
β P r = f β n o n .
(6)
Thus, we have the following relations between the noncoated and Pr-coated regression coefficients:
A P r A n o n = 1 f m c m 3 / 2 ,
(7a)
B P r B n o n = ln ( c m f m 2 ) ,
(7b)
where f m corresponds to experimentally measured field factor modification. Solving the system of equations leads to values c m = 1.50 and f m = 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 terminals to approximately 33 nm. This corresponds to an expected field factor modification given by f x = 1.21, which agrees with the experimentally measured field factor modification f m.

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.

We wish to acknowledge several technical discussions from Leora Peltz and Robert Frampton from The Boeing Company.

The authors have no conflicts to disclose.

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).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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