In this work, thermionic electron emission (TEE) from hot filament chemical vapour deposition polycrystalline diamond films deposited on p-type silicon substrates was recorded in the 25–650 8C temperature range. The studied surfaces were as deposited, as well as hydrogenated by atomic hydrogen under ultra-high vacuum conditions. The impact of substrate temperature during hydrogenation, TH, on TEE was studied. For TH ¼ 25 8C the TEE was found to display a broad maximum at substrate temperature around 300 8C followed by an exponential increase. Annealing at 700 8C results in irreversible changes in surface conditioning, and drastic reduction of TEE yield at low temperatures. For samples that underwent hydrogenation at TH ¼ 300 and 500 8C, the TEE yield is significant at higher temperatures only. The TEE from these samples is stable also after 700 8C annealing treatment. We associate these effects with irreversible thermal induced physicochemical changes of the hydrogen bonding configuration adsorbed on the polycrystalline diamond surface resulting in changes in its surface electronic structure which occur upon annealing to 300 8C.

1. Part of Topical Section
Recent Advances on Diamond Surfaces and Devices
The impact of surface hydrogenation
on the thermionic electron emission
from polycrystalline diamond films
S. Elfimchev*,1,2
, Sh. Michaelson2
, R. Akhvlediani2
, M. Chandran2
, H. Kaslasi2
, and A. Hoffman**,1,2
1
The Nancy and Stephen Grand Technion Energy Program (GTEP), Israel Institute of Technology, Haifa 32000, Israel
2
Schulich Faculty of Chemistry, Technion, Israel Institute of Technology, Haifa 32000, Israel
Received 13 March 2014, revised 1 July 2014, accepted 22 July 2014
Published online 22 August 2014
Keywords diamond, negative electron affinity, surface termination, thermionic emission, thin films
* Corresponding author: e-mail sergeyel@tx.technion.ac.il, Phone: þ9724 8293754, Fax: þ9724 8295703
** e-mail choffman@tx.technion.ac.il, Phone: þ9724 8293747/3727, Fax: þ9724 8295703
In this work, thermionic electron emission (TEE) from hot
filament chemical vapour deposition polycrystalline diamond
films deposited on p-type silicon substrates was recorded in
the 25–650 8C temperature range. The studied surfaces were
as deposited, as well as hydrogenated by atomic hydrogen
under ultra-high vacuum conditions. The impact of substrate
temperature during hydrogenation, TH, on TEE was studied.
For TH ¼ 25 8C the TEE was found to display a broad maximum
at substrate temperature around 300 8C followed by an
exponential increase. Annealing at 700 8C results in irreversible
changes in surface conditioning, and drastic reduction of TEE
yield at low temperatures. For samples that underwent
hydrogenation at TH ¼ 300 and 500 8C, the TEE yield is
significant at higher temperatures only. The TEE from these
samples is stable also after 700 8C annealing treatment. We
associate these effects with irreversible thermal induced
physicochemical changes of the hydrogen bonding configura-
tion adsorbed on the polycrystalline diamond surface resulting
in changes in its surface electronic structure which occur upon
annealing to $300 8C.
ß 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction Thermionic electron emission (TEE)
is the temperature-induced emission of electrons from a
surface. This occurs because the thermal energy given to the
electrons overcomes the binding potential, also known as
work function of the material. For semiconductors the work
function is a sum of electron affinity and band gap between
conduction band and Fermi level. The electron affinity is a
surface function and is affected by crystal orientation,
surface chemistry, doping, band bending etc. The first
demonstration of diamond negative electron affinity (NEA)
was performed by Himpsel in 1979 on (111) hydrogenated
diamond surface [1]. In the later studies it was found that
NEA may persist for diamond with other crystallographic
orientations [2–4].
The influence of different surface conditioning on NEA
and TEE from polycrystalline diamond was previously
studied [5].Many groups were focused on the influence of
doping of diamond with n-type atoms such as nitrogen and
phosphorous on its TEE properties. The most commonly
considered n-type dopant is nitrogen, which is a deep donor
with $1.7 eV below the conduction band minimum [6].
Koeck and Nemanich in 2009 reported on the low
temperature thermionic emitter from nitrogen incorporated
ultra-nanocrystalline diamond [7].They indicated very low
work function and observed electron emission at temper-
atures as low as 250 8C.More recent work reported on
photo-enhanced TEE from nitrogen doped diamond films
with an onset of about 220 8C [8]. These results indicate the
potential for a solar energy conversion device that takes
advantages of both photo and thermionic emission of
diamond films.
In this work, we have studied the influence of
polycrystalline diamond temperature during in situ hydro-
genation by a thermal cracker on TEE in the 30–650 8C
temperature range by recording the energy spectra of the
electrons emitted using a hemispherical energy analyser
under ultra-high vacuum (UHV) conditions. With this aim
in mind we examined the TEE from thin polycrystalline
Phys. Status Solidi A 211, No. 10, 2238–2243 (2014) / DOI 10.1002/pssa.201431168
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2. diamond ($200 nm) film bare surfaces that underwent
exposure to thermally activated hydrogen at film temper-
atures of 25 8C, 300 8C, and 500 8C.
2 Experimental Diamond films were grown by hot
filament CVD (HF CVD) method on a p-type silicon
substrate. The overall chamber pressure was 50 Torr held by
gaseous mixture of CH4/H2 1:99% flow of 100 sccm[9].
Silicon wafer was pretreated by diamond and alumina mixed
slurry, resulted in density >1010
cmÀ2
of diamond particles
on the surface [10]. The growth time of 20 min, at these
conditions, lead to $200 nm thick continuous polycrystal-
line diamond films with average grain size about 200 nm.
These films were deeply investigated by HRSEM, HR-
EELS, and Raman spectroscopy techniques in our previous
works [10, 11].The TEE measurements in this study were
carried out in the temperature range 25–6508 (TEE
temperature is named TTEE throughout this paper). The
surface hydrogenation was produces in situ at the prepara-
tion chamber, keeping 5 Â 10À7
Torr pressure during 30 min.
Hydrogen activation produced by tungsten filament heated
to 2000 8C. TEE measurements were recorded by hemi-
spherical energy analyzer (DATA-Phoibos 150 MCD) under
the base pressure of 10À9
Torr. Mg Ka radiation was used
for XPS experiments. The polycrystalline diamond film was
set on a ceramic heater disposed in the focal plane of the
analyzer (40 mm from the aperture). The temperature was
measured by infrared pyrometer MICRON M90-Q with
emissivity 0.65. The pyrometer was calibrated in our
laboratory by internal thermocouple in UHV conditions and
also in ambient conditions. Thermal electrons emitted from
diamond film were accelerated by 30 V negative bias.
HR-EELS experiments were performed in the UHV
system with a base pressure of about 8 Â 10À10
Torr
equipped with dual monochromator and one analyzer.
HR-EEL experiments were produced at room temperature in
0–600 meV loss energy range. The monochromatic electron
beam of 5 eV was recorded in 558 angle specular geometry.
The sample underwent in situ hydrogenation at film
temperatures (TH) of: (1) without intentional heating (25 8C),
(2) 300 8C, and (3) 500 8C. Before every hydrogenation the
sample was annealed to 700 8C. After each step the TEE was
recorded in the TTEE ¼ 25 À 650 8C range in up-ward and
down-ward temperature gradients (i.e., first during increas-
ing the sample temperature and then during decreasing it).
In Fig. 1 is shown the schematic cartoon representing
the sequence of surface treatments, TEE measurements and
in situ annealing applied to the polycrystalline diamond
sample.
3 Results and discussion In this section, we present
the TEE measurements and discussions from polycrystalline
diamond films in situ exposed to thermally activated
hydrogen at different substrate temperatures (TH). This is
followed by HR-EELS measurements of hydrogenated film
followed annealing at different temperatures. Finally, based
on these experimental results, a correlation between TEE and
surface adsorption/desorption phenomena is discussed.
3.1 TEE after hydrogenation at TH ¼ 25 8C As-
deposited diamond film without in situ hydrogenation
display insignificant TEE yields up to TTEE > 700 8C. In this
section are shown TEE spectra in the TTEE ¼ 25 À 650 8C
temperature range recorded on polycrystalline diamond
surface after annealing at 700 8C and in situ exposure to
activated hydrogen at TH ¼ 25 8C. The integral intensity of
the distribution curves after room temperature hydrogenation
is plotted as a function of temperature in Fig. 2.
From Fig. 2(a), the as-hydrogenated surfaces display a
measurable TEE for sample temperatures as low as $25 8C
and was found to exponentially increase with temperature
reaching a local maximum at about 300 8C followed by
decreasing by nearly two orders of magnitude towards
500 8C. Then TEE rises exponentially with temperature
towards 650 8C. After 650 8C the sample was annealed at
700 8C and the TEE measurement was performed in reverse
direction (Fig. 2(b)).
After annealing at 700 8C TEE yield was significant only
at temperatures above 500 8C and the phenomenon of low
temperature emission completely disappeared. As can be
learnt from Fig. 2(b), following annealing at 700 8C the TEE
yield does not return to its initial value. It can be suggested
Figure 1 In situ processes and temperature treatments of
polycrystalline thin diamond film.
100
10-1
100
101
102
103
104
105
(c)
(b)
Peakarea
(a)
200 300
Temperatu
400 500
ure(°C)
600
Figure 2 Peak area of TEE distribution vs. temperature after
hydrogenation (TH ¼ 25 8C) in forward and opposite directions ((a)
and (b), blue curves), in forward direction after 700 8C annealing
and cooling ((c), red curve).
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3. that the origin of low-temperature TEE is different and may
be associated with physicochemical changes that undergoes
the hydrogen terminated polycrystalline diamond surface
following the thermal process (discussed in Section 5). In
fact, following annealing at 700 8C, the TEE yield most
significantly decreases and completely disappears at low
temperatures. This dependency is indicative of irreversible
changes in the surface conditioning induced by the thermal
cycle. TEE measurements were repeated once more after
12 h (Fig. 2(c)) and very similar TEE values were measured
as those carried during cooling down (Fig. 2(b)).
A representative XP spectrum of the 200-nm-thick
diamond film carried out immediately after hydrogenation at
TH ¼ 25 8C is shown in Fig. 3(a). This spectrum reveals a
contamination-free diamond surface. Similar XP spectra
were obtained after each hydrogenation process (not shown).
The loss features to the C 1s photoelectron is shown in Fig. 3
(b). As can be seen from this figure characteristic surface and
bulk plasmons of diamond at 23 and 34 eV can be clearly
seen in the loss spectrum. These results show that the surface
of the film following the different surface conditioning
applied in this study is composed of high-quality crystalline
diamond phase.
For better understanding the phenomenon of local
minimum and maximum in TEE shown in Fig. 2,
hydrogenation was carried out at TH ¼ 300 8C and
TH ¼ 500 8C, described in the Sections 2 and 3, respectively.
3.2 TEE after hydrogenation at TH ¼ 300 8C The
second hydrogenation was conducted after annealing at
700 8C, while the polycrystalline diamond sample was
maintained at 300 8C during hydrogenation (TH ¼ 300 8C).
In Fig. 4 the peak area of TEE distribution is plotted as a
function of temperature. Comparing TH ¼ 300 8C shown in
Fig. 4(a) with room temperature hydrogenation (Fig. 2(a))
the TEE yield is strongly reduced at TTEE below 300 8C in
the case of TH ¼ 300 8C. At more elevated temperatures
(TTEE > 300 8C, Fig. 4(a)) the behavior of the curve is similar
to the room temperature hydrogenation (Fig. 2(a)). TEE
yield increases with temperature and reaches a local
maximum value at about 400 8C followed by a decrease
of nearly two orders of magnitude towards 500 8C. Then
TEE rises again with temperature. After 650 8C the sample
was annealed to 700 8C and then TEE measurement was
produced in reverse direction (Fig. 5).
Like in the case of room temperature hydrogenation
(shown in Section 1) the curves in reverse direction after
700 8C and after cooling (Figs. 4(b) and (c)) coincide with
each other.
3.3 TEE after hydrogenation at TH ¼ 500 8C The
third hydrogenation was conducted upon annealing to 7008,
while the polycrystalline diamond sample was maintained at
500 8C (TH ¼ 500 8C). TEE measurements were carried out
without any thermal pre-treatment. In Fig. 5 the peak area of
1000 800 600 400 200
CPS
C1S
(a)
Binding energy (eV)
45 40 35 30 25 20 15 10
CPS
Loss energy (eV)
(b)
Figure 3 (a) General XP spectrum of polycrystalline diamond
sample after hydrogenation. (b) Detailed view of diamond plasmon
loss induced by C (1s) photoelectron line.
100 200 300 400 500 600
100
101
102
103
104
105
(c)
(b)
Peakarea
Temperature(°C)
(a)
Figure 4 Peak area of TEE distribution vs. temperature after
hydrogenation (TH ¼ 300 8C) (a) in forward and (b) opposite
directions (blue curves); (c) in forward direction after 700 8C
annealing and cooling (red curve).
100 200 300 400 500 600
100
101
102
103
104
105
(c)
(b)
Peakarea
Temperature(°C)
(a)
Figure 5 Peak area of TEE distribution vs. temperature after
hydrogenation (TH ¼ 500 8C) (a) in forward and (b) opposite
directions (blue curves); (c) in forward direction after 700 8C
annealing and cooling (red curve).
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4. the TEE spectra is plotted as a function of temperature. In
this case, the TEE yield is strongly reduced at temperatures
below TTEE ¼ 500 8C and no local maximum was measured.
After TTEE ¼ 650 8C the sample was annealed to 700 8C and
then the TEE measurement was carried out in the reverse
direction (Fig. 5(c)). Similarly to the two previous cases
(discussed in Sections 1 and 2), the curves in reverse
direction after 700 8C and after 12 h cooling (Figs. 5(b) and
(c)) coincide with each other.
3.4 HR-EEL spectra In Fig. 6 are shown HR-EEL
spectra of HF CVD polycrystalline diamond sample
underwent different surface treatment. These series of
experiments were conducted on different samples than the
ones used for the TEE measurements. In Fig. 6(a) is shown a
HR-EEL spectrum corresponding to as-deposited sample
loaded to UHV chamber and annealed at 300 8C. HR-EEL
spectra of room temperature in situ hydrogenated film
immediately after hydrogenation procedure is shown in
Fig. 6(b), while spectra recorded following annealing to of
350 8C and 600 8C are shown in Fig. 6(c) and (d),
respectively. Detailed peak fitting associated to the C–C/
C–H band is shown in Fig. 6(e)–(h) and that associated to the
C–H stretching region in Fig. 6(i)–(l).
The shape of these spectra has been widely described in
our previous studies [12–15], while here we will describe
these features for convenience. Figure 6(i) receives detailed
peak fitting for C–H stretching mode. This peak consists of at
least three different components at $350, $360, and
$375 meV. The first and second peaks are associated with
(111) and (100) diamond crystallographic orientations [16].
The last one belongs to sp2
-hybridized carbon, disposed on
the film surface and also in a grain boundary region [17].
In situ hydrogenation by thermal cracker is accompanied
by 180 meV energy loss peak appearance (red arrow in
Fig. 6). This peak relates to CHx group vibrations [14, 18].
The modes of about 180 meV were widely investigated on
silicon surfaces by dissociative adsorption in the chem-
isorbed methyl group and also by shear vibrations of sp3
-
CH2 groups [19–21]. This peak has also been found on in
situ hydrogenated diamond surface [22–24]. The stretching
modes of these vibrations cannot be obtained because of
overlapping with C–H vibration modes at 360 meV [19–21].
No significant changes could be observed between the
HR-EELS measured after in situ hydrogenation (Fig. 6(b))
and HR-EELS followed annealing to 350 8C (Fig. 6(c)).
However, annealing to 600 8C (Fig. 6(d)) resulted in
remarkable changes in the spectrum of HR-EEL: 180 meV
modes disappears along with the reducing in relative
intensity of CHx stretching mode at $360 meV and the
increasing of 375 meV peak intensity associated with sp2
-
hybridized CHx modes. These groups are probably related to
the hydrogen bonding at surface defects presented on a
diamond [17, 25, 26].
The origin of changes in the HR-EEL spectra may be
associated with the thermal decomposition sp3
-CHx (x ¼ 2,
3) species, since these species are expected to desorb at
temperatures below 600 8C [27, 28]. Furthermore, the
intensity of the 300 meV and 450 meV peaks, which belong
to C–C diamond phonon, increases following annealing to
600 8C. That means that the diamond surface became
populated mostly by sp3
C–H species.
A summary of the TEE measurements presented in this
study is shown in Figs. 7(a) and (b). From these figures there
are orders of magnitude difference in TEE yield at
TTEE 300 8C for the surfaces that were hydrogenated at
different temperatures, while similar values were obtained at
TTEE ¼ 600 8C and 650 8C for all three treatments. It should
be noticed that at TTEE 300 8C the highest yield is obtained
for TH ¼ 25 8C hydrogenation and lowest for TH ¼ 500 8C.
However, very similar TEE values were obtained following
heating the surface to 700 8C.
Based on the HR-EELS and XPS results and analysis we
suggest that as-deposited film surface is characterized by
high crystalline structure, however, also by sp2
-hybridized
carbon, and positioned on the film surface (detected as a
contribution to C–H stretching mode at 375 meV, Fig. 6 (l)).
(b)
(a)
600 °C
350 °C
H-term.
(c)
0 100 200 300 400 500 600
(d)
as-dep.
300 °C
(e)
H-term.
(f)
350 °C
(g)
90 120 150 180 210
600 °C
(h)
as-dep.
300 °C
(i)
H-term.
(j)
Energy Loss [meV]
Energy Loss [meV]
as-dep.
300 °C
350 °C
(k)
HR-EELS of as-deposited and hydrogenated HF CVD film
330 345 360 375 390
600 °C
(l)
Energy Loss [meV]
Figure 6 HR-EEL spectra of HF CVD diamond sample (a)
as-deposited sample, loaded to UHV conditions and annealed at
300 8C; (b) in situ hydrogenated at room temperature, (c) annealed
at 350 8C and (d) annealed at 600 8C. (e)–(h) detailed view and
peak fitting of mixed C–C/C–H band (data originated from spectra
(a)–(d)); (i)–(l) detailed view and peak fitting of C–H stretching
peak (data originated from spectra (a)–(d)).
Phys. Status Solidi A 211, No. 10 (2014) 2241
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5. This sp2
carbon can increase electron affinity and therefore
the as-deposited film that was not in situ hydrogenated
displays insignificant TEE yield.
Following annealing and in situ hydrogenation, different
CHx (x ¼ 1,2,3) are adsorbed onto the diamond surface. This
is evidenced by the broad peak at 180 meV and also by the
reduced intensity of the first diamond overtone at 300 meV
(Fig. 6(b), (f), and (j)). This termination appears to result in
the enhanced TEE yield, shown in Fig. 2(a). Annealing to
elevated temperatures results in hydrogen/hydrocarbon
desorption detected by HR-EELS as appearance of sp2
features in the C–H peak (Fig. 6(l)) and disappearance of
CH3 related vibration at 180 meV (Fig. 6(h)). This
desorption could result in degradation in TEE value (local
minimum at TTEE ¼ 500 8C, Fig. 2(a)). Further increase in
TEE with temperature can be considered as an increasing
amount of thermally excited electrons.
4 Summary Polycrystalline diamond films underwent
in situ hydrogenation by thermal cracker at different sample
temperatures. TEE measurements were performed at 25 8C
TTEE 650 8C in forward and backward directions.
Significant TEE at low temperatures (25–500 8C) was
observed from hydrogenated polycrystalline diamond
surface for TH ¼ 25 8C, while TH ¼ 300 and 500 8C results
in order of magnitude lower TEE yield. For TH ¼ 25 8C, TEE
at TTEE ¼ 25–500 8C is irreversible process, which dis-
appeared after annealing to elevated temperatures. HR-
EELS analysis reveals that in situ hydrogenation at room
temperature causes the formation of sp3
-CHx adsorbed
groups located on the diamond surface. In situ annealing of
the hydrogenated surface to 600 8C results in desorption of
weakly bonded species, adsorbed during in situ hydrogena-
tion. The temperature dependence of TEE was discussed on
the light of hydrogen/hydrocarbon adsorption/desorption
phenomena.
Acknowledgements The authors acknowledge the support
from the Nancy and Stephen Grand Technion Energy Program
(GTEP) and European Community Seventh Framework Pro-
gramme ProMEThEUS (FP7, grant agreement No. 308975).
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