Physical Review Accelerators And Beams 22, 093402 (2019)

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Physical Review Accelerators And Beams 22, 093402 (2019)

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PHYSICAL REVIEW ACCELERATORS AND BEAMS 22, 093402 (2019)

Electron beams produced by innovative photocathodes based on nanodiamond layers
L. Velardi ,1,2,,* V. Turco,1 L. Monteduro,1 G. Cicala,2,3 A. Valentini,3 and V. Nassisi1
1Laboratorio di Elettronica Applicata e Strumentazione LEAS, INFN, Department of Physics, University of Salento–Lecce, Via Provinciale Lecce-Monteroni, 73100 Lecce, Italy 2CNR-ISTP, Via Amendola 122/D, 70126 Bari, Italy
3INFN-Sezione di Bari, Department of Physics, University of Bari, Via Orabona 4, 70126 Bari, Italy
(Received 21 December 2018; published 27 September 2019)
The investigation of two different photocathodes (PCs) based on nanodiamond (ND) layers, irradiated by a KrF nanosecond excimer laser (wavelength, λ ¼ 248 nm; photon energy, EPh ¼ 5 eV) is reported. The ND layers were deposited by means of a pulsed spray technique. Specifically, the active layer of each PC consisted of untreated (as-received) and hydrogenated ND particles, 250 nm in size, sprayed on a pdoped silicon substrate. The ND-based photocathodes were tested in a vacuum chamber at 10−6 mbar and compared to a Cu-based one, used as reference. All the photocathodes were irradiated at normal incidence. The quantum efficiency (QE) of the photocathodes was assessed. QE values of the ND-based photocathodes were higher than that of the reference one. In particular, the hydrogenated ND-based PC exhibited the highest QE due to the negative electron affinity that results from the surface terminated by hydrogen. Additionally, the photocathode surface/local temperature and the multiphoton process contribution to the electron emission were studied.
DOI: 10.1103/PhysRevAccelBeams.22.093402

I. INTRODUCTION
The production of electron beams by photo and thermionic emission is becoming increasingly important and of growing interest in the scientific community. To date, the cathode represents a key component of electron injectors in synchrotrons, free electron lasers [1–3], linear accelerators (for example energy recovery linacs, ERLs) [4] and systems for imaging and diffraction. Many large-scale particle-accelerator facilities have been and are today employed in the development and characterization of cathodes, such as BNL [5–7], CERN [8], DESY [8] and SLAC [7,9]. In Italy, the National Institute of Nuclear Physics (INFN) is the biggest research organization involved in the design, production and characterization of cathodes for FLASH [3] and PITZ [9] facilities at DESY and for the SPARK_LAB [1,10–12] laboratory.
For applications, a cathode is required to be not only a good electron emitter (high quantum efficiency, QE), but also it must have a good lifetime/robustness, fast response time and low emittance. In general, photocathode (PC) technology is based on metals (Cu, Mg) [2,8,10,12–14] and
*[email protected]
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

semiconductors (CsI, GaAs, Cs3Sb, Cs2Te, K2CsSb, GaN, diamond, etc.) [2,8,13,15–19]. Metal photocathodes are characterized by low efficiency, good response time and are resistant to contamination. On the contrary, those based on semiconductors show high efficiency and slower response time but are sensitive to contamination.
A method used to reduce the PC work function is surface cesiation (i.e., covering the surface with cesium, Cs), which provides a negative electron affinity condition (NEA, i.e., the vacuum level shifted beneath of conduction band minimum) and therefore a photoemission enhancement. NEA photocathodes are bright electron sources and have high quantum efficiency because the electrons injected in the conduction band are emitted directly into the vacuum, without any energy barrier at the surface.
Wide band gap (≥5 eV) semiconductors (e.g., AlN, BN, CsI, diamond, etc.) are an important class of materials because they match the energy of ultraviolet (UV) photons, making them very attractive solar blind UV detectors. In particular, diamond is a very interesting wide gap material of 5.47 eV with a low electron affinity of 0.35–0.5 eV [20] that becomes negative (−1.27 eV) when its surface is hydrogenated [21,22]. Moreover, it has superior chemical-physical properties such as high chemical inertness, high mechanical hardness, high radiation hardness, good thermal conductivity and high secondary electron emission coefficient that allows it to work under harsh external conditions.
The production of diamond films is based on chemical vapor deposition (CVD) techniques, such as hot filament

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L. VELARDI et al.
CVD (HFCVD) [23,24] and microwave plasma enhanced CVD (MWPECVD) [25,26], working at high substrate temperatures (≥500 °C). These high temperatures are not compatible with many devices, for example microchannel plates where the photocathode is implemented directly on top of them to increase the QE. Usually, microchannel plates are made of glass and cannot be heated to temperatures higher than 350 °C.
In recent years, new techniques have been developed in diamond technology, allowing different possibilities in terms of deposition conditions and materials utilized as carbon sources. Besides CVD deposition techniques, which generally produce diamond films with excellent physical and chemical properties, starting from CH4 highly diluted in H2, another cheap and straightforward method is the pulsed spray technique [27,28] which allows the deposition of layers from a dispersion of nanodiamond (ND) particles in water.
This last technique was used to produce high QE UV photocathodes based on nanodiamond layers [18,19,29] in the vacuum ultraviolet range (140–210 nm). The QE was evaluated using a deuterium lamp coupled to a monochromator for the wavelength selection and a calibrated National Institute of Standards and Technology (NIST) Si photodiode.
In this paper, the photoemission characteristics of the ND-based photocathodes produced by pulsed spray technique were measured using a KrF excimer laser (monocromatic wavelength λ ¼ 248 nm). The QE was compared with that of a reference Cu photocathode. The local PC temperature (due to the laser-surface interaction) and the multiphoton absorption process were also investigated.
II. MATERIALS AND METHODS
A graphite rich nanodiamond powder [30] with an average grain size of 250 nm was utilized to fabricate the active layer of the photocathodes. The powder was dispersed in deionized water and sonicated for 30 min by a Bandelin Sonoplus HD2070 system and then sprayed on pdoped silicon substrates, 12 × 12 mm2. The deposition temperature was set at 150 °C, which is a temperature much lower than those used in CVD techniques (500–900 °C). For additional details about the setup and the parameters used for the pulsed spray technique, see [18,19].
Two types of devices were manufactured using untreated (as received, NDar) and hydrogenated (treated, H-ND) nanodiamond particles. The H-ND particles were obtained by treating the NDar powder in H2 microwave plasma. The plasma was generated in a homemade cylindrical stainless steel ASTeX-type reactor. During the hydrogenation process, the working pressure, the microwave power, the H2 flow rate and the process time were held constant at 50 mbar, 1250 W, 200 sccm and 1 h, respectively. Details about the hydrogenation process can be found in [19].

PHYS. REV. ACCEL. BEAMS 22, 093402 (2019)
III. ELECTRON EMISSION FROM PHOTOCATHODES
A. Experimental apparatus for emission measurements Figure 1 shows a sketch of the experimental apparatus in
which the laser-induced photoemitted charge measurements were performed. The apparatus includes a KrF excimer laser (photon energy of 5 eV) with pulse time duration τ of about 20 ns measured as full width at half maximum (FWHM) and a vacuum stainless-steel chamber equipped with a quartz window, a turbomolecular pump, capacitors, anode and a holder for photocathodes. The measurements were carried out setting the anode-cathode distance at 3 mm and applying an accelerating voltage up to 6 kV which corresponds to an electric field of 2 MV=m. The maximum value of 6 kV was due to the occurrence of electrical breakdown in the accelerating gap responsible for plasma discharge. The KrF laser energy was varied in the range from 10−3 to 10−2 J=cm2, higher values were not achieved because of the plasma formation.
The laser beam was divided by a beam splitter. One laser beam was then focused by a thin lens (100 cm focal length) producing a spot dimension of 40 mm2 on the photocathode. The other laser beam was sent to a fast Hamamatsu R1328U-02 photodiode in order to control the evolution time of the laser pulse, and to use it as trigger. The photocathodes were fixed on a holder connected to the chamber by an insulating flange (Fig. 1).
Four capacitors (350 pF=40 kV) were connected between the anode and ground (chamber walls) in order to stabilize the applied voltage (which accelerates the electrons) and allow measurement of long pulses.
To analyze the beam current, it is necessary to consider the cathode holder together with the chamber walls as a transmission line. The value of characteristic impedance was calculated to be approximately 50 Ω. Therefore, to avoid signal reflections and to record the real current pulses, the cathode was connected to ground by 12 parallel
FIG. 1. Experimental apparatus: KrF, excimer laser; Ph, photodiode; B, beam splitter; M, mirror; L, lens; W, quartz window; TMP, turbomolecular pump; C, capacitors; Va, anode voltage connector; A, anode; PC, photocathode.

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The anode was a stainless steel mesh with four grids per mm2. The anode grid passed approximately 60% of the
incident laser light.

FIG. 2. Shunt calibration.

resistors of approximately 600 Ω each, used as shunt with a total load resistance (Rload) of 50 Ω. One of these, a 550 Ω resistor R2, was connected in series to a 50 Ω coaxial cable (Rcable), as shown in Fig. 2. The Rcable was used to transmit the current signal to the oscilloscope.
The shunt calibration was performed by simulating an
electron beam and, for this purpose, a rectangular voltage
pulse V0 was applied to the cathode holder through a 46x attenuator to avoid the oscilloscope damage. It was
generated by a fast switch with rise time of a few ns.
The resulting output voltage Vout was

50

Vout ¼ 550 þ 50 Vo:

ð1Þ

The value of the current i0 was

i ¼ Vo ¼ 550 þ 50 Vout :

ð2Þ

o Rload

50 Rload

Figure 3 shows the calibration results. The temporal
evolution of the input pulse V0 is represented by the upper trace (yellow), whereas the Vout signal is shown by the bottom trace (pink) with a rise time of 2.5 and 5 ns,
respectively. According to Eq. (1), Vout is less than V0 and the attenuation factor obtained by Eq. (2) was 0.247 A=V.
This calibration allows us to measure the photoelectric
charge as a function of the voltage, for the QE calculation.

FIG. 3. Experimental results of the temporal response time
measurement of apparatus. Yellow trace: input pulse V0; pink trace: output signal Vout.

B. Theory of photoemission
The electron emission induced by laser-matter interaction is governed by the generalized Richardson equation [31]:

J ¼ J0 þ JnÀphoton;

ð3Þ

where the term Jo represents the contribution due to the thermionic mechanism. The current density JnÀphoton is related to the n-photon absorption processes, given by

X Nþ1

nhν − ϕ

JnÀphoton ¼ anInðtÞAð1 − RÞnT2ðtÞf

; ð4Þ

n¼1

kT

where an is the nth quantum coefficient related to the matrix element of the quantum n-photon process, I is the incident
laser irradiance, A is the Richardson constant, R is the PC optical reflection, T is the temperature, ϕ the work function
of the emitter, k is the Boltzmann constant,N is the largest integer less than φ=hυ, fðnhkυT−φÞ is the Fowler function with n number of photons and hν the photon energy.
Note that the n-photon absorption process for n > 1 is
relevant when the incident photon energy is less than the
band gap.

C. Temperature temporal profile
In this section, it is shown that the thermionic component J0 is negligible because the maximum increase in temperature of the cathode, due to the laser irradiation, is of the order of few Kelvin.
From Eq. (4), one can observe that the temporal behavior of J is governed by the variables irradiance IðtÞ and temperature TðtÞ that depend on time. The temporal profile of the irradiance I is easily deduced by the laser source, while the temporal shape T on the photocathode surface is more complicated. Indeed, the temporal behavior of the temperature is described by the heat equation:

∂Tðt; x⃗ Þ ¼ ∇2Tðt; x⃗ Þ;

ð5Þ

∂t

where x is the depth from the surface. The TðtÞ solution is expressed in terms of the following
integral [32–34],

Zt

TðtÞ ¼ T0 þ CI0 gðt − t0Þt0−1=2dt0;

ð6Þ

0

where To is the initial temperature, C is a constant dependent on the cathode material and Io is the laser irradiance.

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FIG. 4. Temporal evolution of the cathode surface temperature (blue trace) and the laser irradiance (dashed burgundy trace), approximated by a triangular piecewise function.

The triangular piecewise function is employed to define

gðtÞ:

8

>><

t a

;

0≤t≤a

gðtÞ

¼

>>:

ðaþb−tÞ b

;

a≤t≤aþb

ð7Þ

0;

otherwise;

although the real temporal profile of the laser pulse has a more complicated shape.
Figure 4 shows the ΔT variation of the Cu and diamond cathode surface (blue trace) and the laser pulse temporal profile (dashed burgundy trace) measured and calculated, respectively, at the maximum laser energy (2.5 mJ) employed in this work. The a and b parameters, shown in Fig. 4, govern the shape of the triangle. This approximation, however, apart from being good enough to this paper’s aim, allows to obtain analytical well-known solutions of the heat equation (5) [34]. It is worth noting that the temperature value does not change significantly over the entire duration of the pulse. These results are in agreement with the assumption of constant temperature in Eq. (4). The higher ΔT variation found for the Cu surface is due to its specific heat capacity [385 J=ðkg · KÞ] lower than that of the diamond [509 J=ðKg · KÞ] [35].

IV. RESULTS AND DISCUSSION
Figure 5 shows the extracted photocharge, measured in coulomb, of Cu, NDar and H-ND photocathodes as a

FIG. 5. Photoemitted extracted charge of the Cu, NDar and H-ND-based cathodes and the Child-Langmuir (C-L) red curve as
a function of the accelerating voltage.

function of the accelerating voltage, at three different laser energy values. The red curve (circles) in each graphic represents the Child Langmuir (C-L) curve, which is a theoretical upper bound of current density J allowed in the space-charge regime for a plane diode. The generic form of the C-L equation is

J ¼ 2.43 × 10−6 V3=2 ;

ð8Þ

d2

where V is the accelerating voltage and d is the anodecathode distance.
From the results shown in Fig. 5, it is seen that at low accelerating voltage, the charge extracted from the NDbased photocathodes exceeds the predictions of the ChildLangmuir law. This peculiar result is due to the plasma formation near the cathode surface [33,36]. Although the utilized laser energies are lower than the ablation threshold,

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TABLE I. Quantum efficiency (QE) values of the Cu, NDar and H-ND-based photocathodes at three different values of laser
energy (Elaser).

Elaser (mJ)
0.5 1.0 2.4

Laser Intensity (J=cm2)
1.25 × 10−3 2.50 × 10−3 6.00 × 10−3

QECu (electrons/ photons)
4.20 × 10−6 4.10 × 10−6 4.30 × 10−6

QENDar (electrons/ photons)
1.65 × 10−5 1.70 × 10−5 1.80 × 10−5

QEHÀND (electrons/ photons)
5.00 × 10−5 4.00 × 10−5 3.70 × 10−5

the emitted electrons can ionize the atoms of contaminants and adsorbates, forming a plasma layer on the diamond surface. This plasma layer thickens with increasing laser energy, decreasing the anode-cathode distance d and thus modifying the current density values from the C-L equation (8).
At higher bias voltages, none of the photocathodes provide the level of charge predicted by C-L law. The charge from the Cu and NDar cathodes saturates for each of the three laser energies at voltages higher than 2000 V, however charge saturation of the H-ND cathode is achieved only at low laser energy of 0.5 mJ and not at 1 and 2.5 mJ (this statement is more evident when results are plotted on a linear scale).
The QE is calculated as the ratio of emitted electrons and incident photons. The QE values are reported in Table I.
The QE values of Table I are plotted in Fig. 6, to better represent the trend of the quantum efficiency of the three cathodes as a function of the laser energy.
As expected, the photocathodes based on NDar and H-ND are more efficient than that of the Cu one, used as reference. This high performance in the UV range is due to the good optical and electrical properties of the diamond [37–39]. Moreover, the graphite component of the ND

powder plays a fundamental role in the enhancement of the photoemission, as well shown in several previous works [18,19,40,41]. The highest QE found for the H-ND-based photocathode is due to the NEA character of its hydrogenated surface.
For the case of Cu and NDar cathodes, the QE is constant as a function of increasing laser energy, consistent with the expected one-photon absorption process. But for the H-ND cathode, the QE decreases with increasing laser energy. As already noted, the extracted charge from the H-ND surface does not reach complete saturation. Thus, it would be necessary to increase the accelerating voltage to collect the maximum number of electrons.
Figure 7 shows the temporal response of the three cathodes. The laser signals (dashed yellow) show the typical bell-shaped curve with a FWHM about 20 ns that corresponds to the KrF pulse duration; the electron signals (dotted brown) show curves with different width for the three cathodes. The shape of these curves generally depends on the presence of defects and/or impurity states in the crystalline lattice, on the absorption/reflection mechanisms and on the multiphoton processes that occur

FIG. 6. Quantum efficiency (QE) of the Cu, NDar and H-NDbased photocathodes vs laser energy.

FIG. 7. Temporal response for the three photocathodes.

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during the laser-cathode interaction. A narrower curve means the contribution of the two-photon process (J2) is added to the one-photon process (J1), increasing the output and reducing the width of the total current signal (J ¼ J1 þ J2). This is observed for the Cu and NDar photocathodes (the width at FWHM of electron signals is less than that of the corresponding laser pulse), whereas the H-ND cathode shows a wider electron signal compared with the laser pulse. From these data, we postulate that the NEA character of its surface diminishes the gap that the electrons have to overcome for their emission to vacuum, so the J1 contribution is the highest for this PC.
Despite these considerations, the one-photon process remains the dominant mechanism for all the investigated PCs, under the KrF irradiation at the wavelength of 248 nm.
V. CONCLUSIONS
A comparative study of three photocathodes, two based on ND layers (NDar and H-ND) and Cu as reference, was carried out. The photoemission measurements showed that the ND-based photocathodes exhibit quantum efficiencies higher than that of Cu. In particular, the highest photoemission was found in H-ND-based photocathode thanks to the NEA character of its hydrogenated surface. By the analysis of photocurrent signals, we postulate that the contribution of the one-photon process (under KrF irradiation) is higher for the H-ND because of the NEA character of its surface, which reduces the energy gap between the valence band maximum and the vacuum level.
The paper also shows how it is possible to measure directly the quantum efficiency of a photocathode, without the use of a calibrated NIST photodiode. In the experimental section, we showed how the shunt calibration assures the recording of signals with rise time of a few ns.
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