Carbon nanowalls in field emission cathodes
The carbon nanowall (CNW) layers were grown from a gas mixture of hydrogen and methane, activated by a DC glow discharge, on Si substrates (Si/CNW layered structure). The second layer of CNW was grown either on the first layer (Si/CNW/CNW structure) or on Ni or NiO films deposited on the first CNW l...
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nasplib_isofts_kiev_ua-123456789-1301122025-02-09T14:47:37Z Carbon nanowalls in field emission cathodes Углеродные наностенки в автоэмиссионных катодах Вуглецеві наностінки в автоемісійних катодах Belyanin, А.F. Borisov, V.V. Daghetsyan, S.A. Evlashin, S.A. Pilevsky, A.A. Samorodov, V.A. Материалы электроники The carbon nanowall (CNW) layers were grown from a gas mixture of hydrogen and methane, activated by a DC glow discharge, on Si substrates (Si/CNW layered structure). The second layer of CNW was grown either on the first layer (Si/CNW/CNW structure) or on Ni or NiO films deposited on the first CNW layer (Si/CNW/Ni/CNW and Si/CNW/NiO/CNW structures). The composition and structure of the resulting layered structures were studied using scanning electron microscopy, Raman spectroscopy, and X-ray diffractometry. It was found that annealing of Si/CNW structure in vacuum, growing of the second CNW layer on Si/CNW, as well as deposition of Ni or NiO films prior to the growing of the second CNW layer improve functional properties of field emission cathodes based on the electron-emitting CNW layers. Слои углеродных наностенок (СН) выращивали из газовой смеси водорода и метана, активированной тлеющим разрядом постоянного тока, на подложках из Si (слоистая структура Si/СН). Второй слой СН выращивали на первом слое (структура Si/СН/СН) или на пленках Ni или NiO, осажденных на первом слое СН (структуры Si/СН/Ni/СН и Si/СН/NiO/СН). Методами растровой электронной микроскопии, спектроскопии комбинационного рассеяния света и рентгеновской дифрактометрии исследованы состав и строение полученных слоистых структур. Установлено, что отжиг в вакууме структуры Si/СН, наращивание на Si/СН второго слоя СН, а также нанесение пленок Ni или NiO перед наращиванием второго слоя СН приводят к улучшению функциональных свойств автоэмиссионных катодов на основе слоев СН, эмитирующих электроны. Шари СН для досліджень вирощували з газової суміші Н₂ і СН₄, активованої тліючим розрядом постійного струму, на підкладках з Si. Перед нарощуванням СН на підкладках створювалися вуглецеві затравочні центри шляхом обробки поверхні іонами Н+ та СхНу+. Емісійні характеристики отриманих шаруватих структур Si/СН контролювали півгодинними випробуваннями. Піддані випробуванням та/або тривалому зберіганню на відкритому повітрі шаруваті структури Si/СН або відпалювали в вакуумі (1,5 години при 720 К), або на їх поверхні нарощували другий шар СН (Si/СН/СН) за тих же умов, що і перший. Другий шар СН нарощували також на поверхні першого шару СН, вкритого плівкою Ni або NiO (структури Si/СН/Ni/СН та Si/СН/NiO/СН). Плівки Ni отримували методом магнетронного розпилення, а плівки NiO — термічною обробкою в розчині Ni(NO₃)₂. Максимальна висота першого шару СН щодо підкладки становила 2—4 мкм, сумарна висота першого і другого шарів — 8,5 мкм. Склад і будову шаруватих структур досліджували з використанням растрової електронної мікроскопії, рентгенівської дифрактометрії і спектрометрії комбінаційного розсіювання світла. 2017 Article Carbon nanowalls in field emission cathodes / А.F. Belyanin, V.V. Borisov, S.A. Daghetsyan, S.A. Evlashin, A.A. Pilevsky, V.A. Samorodov // Технология и конструирование в электронной аппаратуре. — 2017. — № 6. — С. 34-43. — Бібліогр.: 17 назв. — англ. 2225-5818 DOI: 10.15222/TKEA2017.6.34 https://nasplib.isofts.kiev.ua/handle/123456789/130112 538.911: 538.975 en Технология и конструирование в электронной аппаратуре application/pdf Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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| topic |
Материалы электроники Материалы электроники |
| spellingShingle |
Материалы электроники Материалы электроники Belyanin, А.F. Borisov, V.V. Daghetsyan, S.A. Evlashin, S.A. Pilevsky, A.A. Samorodov, V.A. Carbon nanowalls in field emission cathodes Технология и конструирование в электронной аппаратуре |
| description |
The carbon nanowall (CNW) layers were grown from a gas mixture of hydrogen and methane, activated by a DC glow discharge, on Si substrates (Si/CNW layered structure). The second layer of CNW was grown either on the first layer (Si/CNW/CNW structure) or on Ni or NiO films deposited on the first CNW layer (Si/CNW/Ni/CNW and Si/CNW/NiO/CNW structures). The composition and structure of the resulting layered structures were studied using scanning electron microscopy, Raman spectroscopy, and X-ray diffractometry. It was found that annealing of Si/CNW structure in vacuum, growing of the second CNW layer on Si/CNW, as well as deposition of Ni or NiO films prior to the growing of the second CNW layer improve functional properties of field emission cathodes based on the electron-emitting CNW layers. |
| format |
Article |
| author |
Belyanin, А.F. Borisov, V.V. Daghetsyan, S.A. Evlashin, S.A. Pilevsky, A.A. Samorodov, V.A. |
| author_facet |
Belyanin, А.F. Borisov, V.V. Daghetsyan, S.A. Evlashin, S.A. Pilevsky, A.A. Samorodov, V.A. |
| author_sort |
Belyanin, А.F. |
| title |
Carbon nanowalls in field emission cathodes |
| title_short |
Carbon nanowalls in field emission cathodes |
| title_full |
Carbon nanowalls in field emission cathodes |
| title_fullStr |
Carbon nanowalls in field emission cathodes |
| title_full_unstemmed |
Carbon nanowalls in field emission cathodes |
| title_sort |
carbon nanowalls in field emission cathodes |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| publishDate |
2017 |
| topic_facet |
Материалы электроники |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/130112 |
| citation_txt |
Carbon nanowalls in field emission cathodes / А.F. Belyanin, V.V. Borisov, S.A. Daghetsyan, S.A. Evlashin, A.A. Pilevsky, V.A. Samorodov // Технология и конструирование в электронной аппаратуре. — 2017. — № 6. — С. 34-43. — Бібліогр.: 17 назв. — англ. |
| series |
Технология и конструирование в электронной аппаратуре |
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Tekhnologiya i Konstruirovanie v Elektronnoi Apparature, 2017, ¹ 6
34
MATERIALS OF ELECTRONICS
ISSN 2225-5818
UDC 538.911: 538.975
А. F. BELYANIN1, V. V. BORISOV2, S. A. DAGHETSYAN3, S. A. EVLASHIN2,
A. A. PILEVSKY2, V. A. SAMORODOV2
Russia, Moscow, 1Central Research Technological Institute “Technomash”,
2Skobeltsyn Institute of Nuclear Physics, 3Lomonosov Moscow State University
E-mail: belyanin@cnititm.ru
CARBON NANOWALLS IN FIELD EMISSION CATHODES
Carbon materials, including various crystalline
(diamond, graphite) and noncrystalline (fullerene,
nanotubes, graphene, etc.) ordered substances
with unique physicochemical properties are of
practical interest. Some carbon materials due to
the autoemission property are promising for use
as an emitting layer of field emission cathodes
(autocathodes). The presence of field emission
means a decrease in the electric field strength to
1—10 V/μm, which is required for the onset of
field emission of electrons. Autocathodes are used
in the development of X-ray tubes, microwave
devices, electron guns for exciting lasers,
cathodoluminescent lighting devices, flat displays
and other devices [1—5]. The most promising for
the creation of autocathodes with a low electron
emission barrier are the so-called carbon nanowalls
(CNW) — layers of a plate-like carbon material
with a predominant orientation of the plates per-
pendicular to the substrate [1—3].
The layers of carbon materials formed by plasma
methods, including CNW, are as a rule multiphase
layers [6—8]. The structure and concentration of
crystalline and X-ray amorphous phases depend on
the conditions for carbon materials formation and
affect their emission properties. The problems of
using CNW in autocathodes are associated with
the instability of emission parameters (magnitude
and density of the cathode current, as well as the
degree of electrical current uniformity over the
The carbon nanowall (CNW) layers were grown from a gas mixture of hydrogen and methane, activated
by a DC glow discharge, on Si substrates (Si/CNW layered structure). The second layer of CNW was
grown either on the first layer (Si/CNW/CNW structure) or on Ni or NiO films deposited on the
first CNW layer (Si/CNW/Ni/CNW and Si/CNW/NiO/CNW structures). The composition and
structure of the resulting layered structures were studied using scanning electron microscopy, Raman
spectroscopy, and X-ray diffractometry. It was found that annealing of Si/CNW structure in vacuum,
growing of the second CNW layer on Si/CNW, as well as deposition of Ni or NiO films prior to the
growing of the second CNW layer improve functional properties of field emission cathodes based on the
electron-emitting CNW layers.
Keywords: carbon nanowalls, layered structures, electron microscopy, Raman spectroscopy, field emission
cathodes.
cathode area) due to changes in composition and
structure during testing and operation [4, 6, 8].
Before being placed into electrovacuum
devices and soldered, autocathodes are always
preliminaryly tested in a vacuum chamber for
compatibility with the parametres of the device.
In some cases, preliminary tests are carried out to
achieve such required parameters as autoemission
current and its stability in time. Stability tests can
be performed both in the voltage stabilization mode
[4] and in the current stabilization mode. In the
first case we consider cathode current dependence
(decrease) on time at a fixed stabilized voltage, in
the second case — voltage dependence (growth)
on time at a fixed stabilized current. In both cases,
the graphs of the dependencies (hereinafter aging
curves) objectively characterize the degradation
(aging) of the autocathode, regardless of what
causes it.
Storing the tested autocathodes with CNW
layers on open air also leads to a deterioration of
their emission properties. This is caused by the
fact that during vacuum testing on the surface of
CNW plates, the layer of adsorbed hydrocarbons
is destroyed. This layer normally prevents
adsorption of the components of the air mixture
(water and nitrogen molecules, etc.) that impair
the emission characteristics of the autocathodes
[4, 15]. To recover the emission properties of
autocathodes that had passed preliminary tests,
they are annealing in vacuum or in an inert gas
DOI: 10.15222/TKEA2017.6.34
http://www.tkea.com.ua/tkea/2017/6_2017/pdf/06_rus.pdf
Tekhnologiya i Konstruirovanie v Elektronnoi Apparature, 2017, ¹ 6
35
MATERIALS OF ELECTRONICS
ISSN 2225-5818
atmosphere at a temperature of about 720 K [6].
Autocathodes that had not passed preliminary tests
(even without vacuum breakdown), as a rule are
not further used.
We can assume that autocathodes with a second
CNW layer should have better emission properties
than autocathodes with only the first CNW layer
annealed. Moreover, for autocathodes that had not
been preliminaryly tested, growing of the second
CNW layer on top of the first one should increase
the yield ratio for vacuum electronics.
This study researches how vacuum annealing
and growing of a second CNW layer affects the
emission properties of layered autocathodes based
on carbon nanostructures.
Samples used in the research
The CNW samples were grown on Si substrates
from a gas mixture of hydrogen (H2) and methane
(CH4) activated by a DC glow discharge [3, 6].
Before growing CNW, priming carbon centers were
created on the substrate. For this purpose, at a
temperature of 1020 K, the surface of the substrate
was bombarded with H+ and CxHy
+ ions (high
frequency discharge, 13.56 MHz, 40 W, 20 min),
formed in the microwave plasma mixture of hy-
drogen and methane (8—10 volume % CH4) at
pressure of 6,6∙103 Pa. Silicon substrates with seed
particles were treated in H2 plasma, after which a
CNW layer was grown at a substrate temperature
of 800—1300 K and a deposition rate of 6 μm/h.
Emission characteristics of the obtained Si/CNW
layered structures were tested for 0.5 hour.
Si/CNW structures that were tested and/or
stored in the open air for a long time (1 to
3 years) were either annealed in vacuum, or a sec-
ond CNW layer was grown on their surface under
the same conditions as the first layer. In a vacuum
(10–3—10–5 Pa), the samples were annealed for
1.5 hours at 720 K (Si/CNW(ann) structure).
When the second layer was grown, the crystal-
lites of the first layer acted as seed centers of the
second layer. The second layer of CNW was also
grown on the surface of the first layer of CNW
coated with Ni or NiO.
Ni films were obtained by magnetron sputter-
ing from a Ni target with a direct current in an
argon atmosphere (Si/CNW/Ni structure). The
conditions for obtaining Ni films are as follows:
Ar pressure 1.2—1.5 Pa; discharge power 900 W;
substrate temperature 420—570 K; deposition rate
1.5 μm/h. The thickness h of the resulting Ni
films was 10, 40, 80, and 160 nm (Si/CNW/Nih
structures).
NiO films were formed in two stages. At the
first stage, a 0.25% solution of Ni(NO3)2 was ap-
plied to the CNW layer in a 50% hydroalcoholic
(H2O + C2H5OH) mixture at room tempera-
ture, followed by heat treatment at 420 K. The
heat treatment caused the crystalline hydrate
Ni(NO3)2⋅6H2O formed during heat treatment
to decompose to NiO form at a temperature of
370—410 K.
Ni(NO3)2 was deposited at atmospheric pressure
either by immersing the substrate with a CNW
layer in a solution, followed by heat treatment
(the result was the Si/CNW/NiO structure) or
by aerosol precipitation (5—10 cycles of 1 minute
with heat treatment after each cycle, the result
was the Si/CNW/NiO* structure). To generate
the aerosol, the Albedo IN-8 (Альбедо ИН-8) ha-
logenator was used with an average mass median
aerodynamic diameter of the aerosol particles of
3.94 μm.
Research technique
The CNW composition and the layered
structures were studied using a Carl Zeiss Supra
40-30-87 scanning electron microscope (SEM), a
Rigaku D/MAX-2500/PC X-ray diffractometer
(Cukα radiation) and a LabRAM HR800 (HORIBA
Jobin-Yvon) laser Raman scattering spectrometer
(632.8 nm line of He-Ne laser, beam spot diameter
4 μm2, depth of the analyzed layer 3 μm).
Current-voltage (I-V) characteristics and
aging curves that determine the dependence of
voltage on time during long-term emission tests
at a given current in the regime of constant
current stabilization were obtained using a
Pw2500_v2_3kV_1a source of stabilized pulsed
current produced by SINP MSU and a Spellmann
Sl30 source of stabilized direct current. The
measurements were carried out in diode cells at a
pressure of 5⋅10–5 Pa.
To correctly compare the structures with the
second layer of CNW, each sample with the first
CNW layer was divided into two parts (A, B)
equal in area which were used to co-grow a second
layer of CNW in one charge. Part A was used as
control, and part B was covered with a Ni or NiO
layer, then the A:Si/CNW/CNW and B:Si/
CNW/Ni/CNW (or B:Si/CNW/NiO/CNW)
structures were compared. In the tests with
annealing, the second CNW layer was not
grown onto part B, while the first CNW layer
was annealed, then the A:Si/CNW/CNW and
B:Si/CNW(ann) structures were compared.
The I-V characteristic and the autoemission
parameters of the samples were recorded in the
pulsed mode of electric current measurement. The
aging curves and their parameters were measured
in the constant stabilized current mode.
Field emission tests were carried out on samples
with a surface having intrinsic conductivity.
Tekhnologiya i Konstruirovanie v Elektronnoi Apparature, 2017, ¹ 6
36
MATERIALS OF ELECTRONICS
ISSN 2225-5818
During measuring the I-V characteristic in the
pulsed mode, a glass plate with a conductive layer
of mixed indium-tin oxide (ITO, chemical formula:
(In2O3)0.9-(SnO2)0.1) was used as an anode. The
plate was covered with a luminophore layer. This
anode completely covered the emitting surface of
the sample. When measuring the aging curves in
the constant stabilized current mode (10 mA), a
water-cooled thick-walled (5 mm) copper anode
was used with a polished working surface in the
form of a 5×2×2 mm strip located above the 4×2 mm
rectangular area of the autocathode.
The gap (D) between the surface of the
autocathode and the anode was 250 μm when
measured in pulsed mode and 125 μm — in direct
current mode. The I-V characteristics were plotted
in the coordinates (E, J), where E = U/D is the
electric field strength in the gap between the
anode and the autocathode, J = I/S is the current
density, U is the potential difference between the
electrodes; I is the current of the autocathode, S is
the working area of the autocathode. According
to the I-V characteristics, Fowler—Nordheim
diagrams were plotted in (E–1, ln(J/E2))
coordinates.
Composition and structure of CNW
Carbon nanowalls are a porous material formed
by curved lamellar (scaly) clusters of X-ray
amorphous and crystalline phases of carbon (Fig. 1).
The CNW plates were 3—10 nm thick [3]. Apart
from bent carbon plates, the structure of CNW
samples also contain rods (plates folded into
tubes), nanotubes and equiaxed particles with an
average size of 40—50 nm (Fig. 1, b).
X-ray diffractometry shows that CNW contains
mainly graphite (P63/mmc spatial group) and
carbyne (hexagonal syngony), as well as phases
of diamond (Fd3-m), chaotite (P6/mmm spatial
group) and graphite modifications (R3 and P3
spatial groups) [6, 8]. The thickness of the CNW
plates corresponds to the size of the crystallites
(X-ray coherent scattering regions, LCSR) equal to
8.5—9.5 nm and calculated from the broadening
on X-rays of diffraction peaks of 0002 graphite.
In the Raman spectra of the Si/CNW layer struc-
ture, which explicitly reflect the composition and
structure of CNW [9, 10], we can observe intense D, G
and 2D bands, located at the Raman shift Dn, equal
to 1330—1343, 1577—1591 and 2660—2673 cm–1,
respectively. At the same time, weak bands are
fixed at Dn equal to 233—243, 863—879 and
1081—1167 cm–1 (x band); 1612—1627 (D' band);
2449—2482 (x+D band); 2909—2934 (D+G band)
and 3221—3248 cm–1 (2D’ band). (In this study
we denote CNW Raman spectral bands as D, G,
x, D', x+D, 2D, D+G и 2D' [11—13].)
Fig. 2 (curve 1) shows the Raman spectrum of
one of the Si/CNW samples. The values of the
intensity ratio of the main CNW Raman spectral
bands depending on their formation conditions
have a considerable spread: ID/IG = 0.32—2.03;
ID/I2D = 0.98—1.23; ID/ID+G = 14.1—17.6;
ID/I2D = 13.0—16.1 [6, 8].
The CNW Raman spectra were compared with
similar spectra of highly oriented pyrolytic UPV-1T
(УПВ-1Т) graphite (Fig. 2, curve 2). The 2D
graphite band consists of two components: 2D1
and 2D2 (Fig. 2, curves 3, 4) with an intensity
proportional to the one of the G band. In contrast
to the 2D graphite band, the 2D CNW band is
symmetrical, which is characteristic of graphene
[9, 13]. The differences in the 2D band in the
CNW and graphite Raman spectra are caused by
a significant curvature of individual regions of
the graphite atomic layers {0001}, which disrupts
atomic bonds inside and between the layers.
Depending on the degree of the CNW plates
curvature (curvature radius 590—770 nm), the
2D band changes shape, which reflects changes
in the electronic bands corresponding to the posi-
tions of atoms in the lattice. The layers in such a
crystallite (CNW plates) form a hexagonal lattice
(two-layer stacking of carbon atoms) [1, 6, 14].
If we consider the CNW crystallites as graphite
plates, then their size (plate thickness) calculated
Fig. 1. SEM image of the Si/CNW layer structure:
a — CNW growth surface; b — Si/CNW cleavage
a)
b) 200 nm
200 nm
Tekhnologiya i Konstruirovanie v Elektronnoi Apparature, 2017, ¹ 6
37
MATERIALS OF ELECTRONICS
ISSN 2225-5818
from the intensitiy ratio of the D and G (ID/IG)
Raman spectral bands would be L = 3.3—9.9 nm.
The obtained size is close to the values calculated
from the X-rays. Taking into account that the
interplanar distance of graphite (0001 plane)
is 0.335 nm, one can state that there are about
10—30 layers of graphene in a CNW plate.
The maximum height of the first CNW layer
starting from the substrate is 2—4 μm, the to-
tal height of the first and second layers is ap-
proximately 8.5 μm (Fig. 3, a, b). After the
heat treatment of the Si/CNW layered structure
covered with Ni(NO3)2·6H2O crystal hydrate, NiO
crystallites less than 2 μm in size were formed on
the surface of the CNW (Fig. 3, c). The deposi-
tion of a 10 nm thick Ni film on the CNW layer
(Si/CNW/Ni10 structure) by magnetron sputter-
ing resulted in the formation of an islet structure
with a cluster size less than 10 nm (Fig. 3, d).
A continuous Nih film (thickness h > 40 nm) was
formed mainly on the CNW edges located at a 90°
angle to the Ni particles flux during magnetron sput-
tering (Fig. 3, d). On all other CNW surfaces, the
Ni film thickness was by orders of magnitude smaller.
Fig. 4, a, b shows the globular structure of the
CNW layers with a globule diameter of 1.5—2 μm.
The second layer of CNW, deposited on a NiO
film, has a more dense packing of globules and a
larger thickness of the plates (Fig. 4, d). Samples
A:Si/CNW/CNW and B:Si/CNW/Ni10/CNW
(Fig. 4, a, b) contain a large number of nanotubes
with a diameter of 10—40 nm, while in samples
with Ni or NiO films their number does not exceed
1—2 per globule.
The structure of the second CNW layer is
characterized by the presence of carbon plates
on the crystallites (plates) of the first layer, in-
cluding multiwall nanotubes (Fig. 4, c). In the
Si/CNW/Ni160/CNW and Si/CNW/NiO*/CNW
samples, thickened crystallites of carbon plates
with rounded edges (not typical) were found
(Fig. 4, d, e). It was discovered that on average
Fig. 2. Raman spectra of Si/CNW (1) and graphite (2) layered structures.
Inset graph shows a fragment of the spectra in the Dn range of 2550—2800 cm–1
(3 and 4 are Lorentz distribution functions, which in sum approximate curve 2)
I,
r
el
.
un
.
500
300
100
1300
700
100
200 1000 1800 2600 3400 2500 2610 2670 2730 2790
Dn, cm–1 Dn, cm–1
a) b) c) d) e)
Fig. 3. SEM images of the samples:
a — Si/CNW (cleavage); b — Si/CNW/NiO*/CNW (cleavage); c — Si/CNW/NiO*; d — Si/CNW/Ni10;
e — Si/CNW/Ni80
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the maximum height of the second CNW layer was
2.4 times greater than that of the first; the globular
structure of the second CNW layer became more
dense and homogeneous; the number of multiwall
nanotubes decreased; on the Raman spectrum of
the second CNW layer with a globular structure, a
band appeared at Dn = 2285 cm–1 (Fig. 5, Table 1).
The Raman spectra shown in Fig. 5 are normal-
ized to the intensity of the 2D (I2D) band. On
the Raman spectra of the Si/СNW/NiO*/СNW
structure, the band intensity at Dn = 2285 cm–1
increased almost 4-fold. The wide band at
Dn = 2262—2286 cm–1 (broadening of the
Dn1/2 = 120—160 cm–1 band) was manifested in
the Raman spectra of СNW after annealing at
temperatures above 870 K [7]. A similar band
was also observed on the Raman spectra of poly-
cluster diamond films produced by the microwave
discharge method [3].
The crystallite size (LCSR), the number of
graphene layers (N), and the I2D/ID parameter
a) b)
c) d) e)
Fig. 4. SEM images of surfaces of layered structures:
a — А1:Si/СNW/СNW; b — В1:Si/СNW/Ni10/СNW;
c — В2:Si/СNW/Ni10/СNW; d — Si/CNW/Ni160/CNW; e — Si/CNW/NiO*/CNW
(right-hand images in a and b are scaled-up fragments of the surface of the second CNW layer)
Fig. 5. СNW Raman spectra before and after growing
the second СNW layer:
1 — А:Si/СNW; 2 — А:Si/СNW/СNW;
3 — В:Si/СNW/NiO*/СNW
Layered structure ∆ν1/2,
cm–1 ID IG ID′ I2D ID/IG LCSR N ID’/IG I2D/ID I2D/IG I2285
Si/СNW 19.4 228.3 388.8 113.4 202 0.6 7.5 22 0.3 0.9 0.5 0
Si/СNW/СNW 17.7 340.6 338.1 130.5 202 1.0 4.4 13 0.4 0.6 0.6 337
Si/СNW/NiO*/СNW 19.9 442.4 356.9 176.5 202 1.2 3.6 10 0.5 0.5 0.6 130.2
Table 1
Structural parameters of the upper layer of СNW layered structures
I,
r
el
.
un
.
1200 1600 2000 2400 2800 3200
Dn, cm–1
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of the samples А:Si/СNW; А:Si/СNW/СNW
and В:Si/СNW/NiO*/СNW (in this order)
decreased, while the ID’/IG, I2D/IG parameters
and I2285 band intensity increased (Table 1).
Emission properties of СNW-based
autocathodes
Simultaneously with the I-V measurement, a
sequence of images was registered on an lumino-
phore anode screen. Obviously, in this sequence,
you should select an image with the worst visual
uniformity, which is located in the middle part
of the sequence, while at its beginning and at its
end there are images that are visually evaluated
as homogeneous: almost black ones, obtained at
small electric fields, at the beginning, and lighted
ones, obtained at large electric fields, at the end
(Fig. 6).
To assess the uniformity of images, subjective
(visual) criteria are often used. Of the many objec-
tive homogeneity criteria based on digital image
processing, we chose the simplest criterion based
on variation coefficient. We used the intensity
(brightness) of a pixel in a gray raster image as
the random variable. Uniformity of the image (in
percent) was calculated from a sample consisting
of all pixels of the image, according to the formula
Н = 100V – 33,
where V is variation coefficient, V = s/ X ; s and
X are the mean square deviation and average
linear deviation in the sample, respectively.
In statistical data processing, the sample is
considered homogeneous if V ≤ 0.33 (H ≤ 0%).
For the sequence of images obtained from the
luminophore anode screen, maximum of H (the
worst homogeneity) was taken as homogeneity Н∆.
As can be seen from Fig. 6, the visual estimation
of homogeneity coincides with the maximum at
H250 = –16.8%. The Н∆ parameter can be regarded
as an estimate of the homogeneity of the cathode
current distribution in the working region of the
authocathode at the ∆ gap.
It is known [4, 15] that the electric field
around a pointed conductor is amplified and can
be represented as bE0, where b is the field gain
near a single emitter and approximately equals to
the aspect ratio (height/transverse dimension)
of the conductor; E0 is the ideal electric field
strength equal to U/D. Assuming that all emis-
sion centers have regular geometry (the same sizes
and relative position), the dependencies on the
Fowler—Nordheim (FN) diagrams are described
by the equation of the straight line y = Bx + C,
where x = 1/E, y = ln(J/E2). The slope ratio
of the straight line B is a value proportional to
b, while the density of the emissive centers DE is
proportional to the exp(C) value (C is the segment
cut off by the straight line on the ordinate axis).
For the given films, a linear region can be dis-
tinguished on the curves of the FN diagrams [17].
For a relative comparison of the values character-
izing emission properties of the autocathodes, it is
sufficient to assume that in this linear region only
emission centers with regular geometry generate
the electrical current, while the contribution of the
others to the resulting current is negligible [4, 15].
As emission characteristics of the autocathodes, we
considered the following: the autoemission thresh-
old, ET, is the minimum value of E at which the
emission current is registered; the estimation of the
aspect number of a single emitter in the regular
geometry b; the estimation of density of emission
centers in the regular geometry DE; homogeneity
Н250 in a sequence of emission images.
The aging curves (AC) for the structures with
the second CNW layer were obtained as the voltage
U on time T dependence at a constant current of
10 mA, measured in the current stabilization mode.
The 10 mA value of current was chosen due to the
capabilities of the equipment available, as well as
to current density limitations (J ≈ 0.12 mA/cm2),
at which undesirable vacuum breakdowns are un-
likely in the test cell. The aging curves allowed
determining the aging rate for 6 hours (V6h),
for 3 hours (V3h) and for the last hour (V1h) of
tests. A comparison of the aging curves of A and
B parts of samples with a second CNW layer was
carried out using the QAB parameter that takes
into account the relative position of these curves
–26.9 –23.1 –18.4 –16.8 –18.5 –18.8 –19.9
Fig. 6. Sequence of 2×2 mm emission images with different homogeneity H250
(the values in % are given in the images)
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along the ordinate (U) and the ratio of the areas
bounded by these curves and the abscissa axis:
QAB = (SА/(SA+SB)) – 0.5,
is the area under the ACB for В:Si/СNW/
Nih/СNW, В:Si/СNW/NiO/СNW,
В:Si/СNW(ann);
is the area under the ACA of the correspond-
ing B control structures А:Si/СNW/СNW
(or А:Si/СNW in the case of annealing).
where SB
SA
If the ACB is located above the ACA, then
–0.5 < QAB < 0 (part B has a higher voltage);
if ACB is located below ACA, then 0.5 > QAB > 0
(part B is less high-voltage). The smaller |QAB|,
the closer are the ACA and ACB areas. At the same
time, the |QAB| < 0.5 inequality is valid.
Fig. 7 presents the I-V characteristic curves
and the FN lines described above for the parts
of a single sample of layered structure, and their
parameters are given in Table 2. As can be seen
from the presented data, the parts without the
second CNW layer (А:Si/СNW и В:Si/СNW)
are characterized by a high autoemission thresh-
old (ET ≥ 5.6 V/μm), a large aspect ratio b
(which confirms the presence of a large number
of multiwall nanotubes on the first CNW layer),
a low density of emission centers DE, and a low
homogeneity of field emission images (H250 ≥ 0).
Parts with a second CNW layer (structures В:Si/
СNW/NiО*/СNW and А:Si/СNW/СNW)
are characterized by a lower autoemission threshold
(ET ≤ 3.6 V/μm), a smaller value of b, a higher
DE density, as well as a better images uniformity
(H250 ≤ 0). For a given sample, the part with the
А:Si/СNW/СNW structure is characterized by
better values of the parameters ET, b, H250, V3h,
V1h than the В:Si/СNW/NiО*/СNW structure,
and the values of the DE parameter for them are
virtually identical.
Examples of the arrangement of the I-V char-
acteristics and the aging curves of the samples
with the Si/СNW/NiO*/СNW (sample 1) and
Si/СNW(ann) (sample 2) structures are shown
in Fig. 8, 9.
As can be seen from Fig. 8, for sample 1, the I-V
characteristic of the В1:Si/СNW/NiO*/СNW
Structure
Measuring mode
Pulse Continuous
ET, V/μm b ln(DE) H250, % V6h, V/h V3h, V/h V1h, V/h QАВ
А:Si/СNW 6.1 42.7 1.9 39.07 — — — —
В:Si/СNW/NiО*/СNW 3.6 32.9 6.0 –10.52 50 33 10 0.18
В:Si/СNW 5.6 47.0 3.0 29.36 — — — —
А:Si/СNW/СNW 2.4 21.1 5.9 –7.18 10 0 0 —
Table 2
Example of emission characteristics of autocathode parts on layered structures
Fig. 7. Example of the I-V characteristics and their
linear representations in the FN coordinates (see inset)
obtained for the following layered structures:
1 — А:Si/СNW; 2 — В:Si/СNW/NiO*/СNW;
3 — В:Si/СNW; 4 — А:Si/СNW/СNW
60
40
20
0
J,
m
A
/
cm
2
2 4 6 8 E, V/mm
2
4
3
1
Fig. 8. I-V characteristics of layered structures of two
samples:
Sample 1 (solid lines): 2 — В1:Si/СNW/NiO*/СNW;
3 — А1:Si/СNW/СNW; 6 — А1:Si/СNW; 7 — В1:Si/СNW;
Sample 2 (dashed lines): 1 — А2:Si/СNW/СNW;
4 — В2:Si/СNW(ann); 5 — А2:Si/СNW
30
20
10
0
J,
m
A
/
cm
2
2 4 6 8 E, V/mm
6
7
5
4
3
21
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structure (curve 2) is shifted to the left by
4 V/μm relative to the I-V characteristics of the
А1:Si/СNW and В1:Si/СNW structures (curves
6, 7) and by 1.5 V/μm relative to the I-V char-
acteristic for the А1:Si/СNW/СNW (curve 3).
For sample 2, the I-V characteristic of the
В2:Si/СNW(ann) structure (curve 4) is shifted to
the left by 0.8 V/μm relative to the I-V character-
istic of the А2:Si/СNW (curve 5) and to the right
by 1.8 V/μm relative to the I-V characteristic of
the А2:Si/СNW/СNW (curve 1). This indicates
that the emission properties of the autocathode
with a second CNW layer deposited on the NiO*
oxide layer have improved in comparison with the
autocathode with the second CNW layer without
the oxide, as well as that the emission properties
of the autocathode with the second CNW layer
have improved in comparison with an autocathode
that had only undergone a restorative annealing
of the first CNW layer.
According to the data from Table 2 and Fig. 9, for
В1:Si/СNW/NiO*/СNW and А1:Si/СNW/СNW
structures, the QAB parameter is 0.1 for 6 hours of
testing, 0.18 for 3 hours and 0.08 for the last hour.
In this case, the ACB for В1:Si/СNW/NiO*/СNW
turns out to be less high-voltage than the ACA
for А1:Si/СNW/СNW, but it loses to the ACA
in the aging rate. The behavior of the ACA and
ACB during 6 hour tests (QAB = 0.1) shows their
asymptotic convergence at positive values of QAB,
which can be explained by the fact that the second
CNW layer on the A1 and B1 parts was grown
simultaneously during the same charge.
A summary data of the averaged characteristics
of the investigated autocathodes is presented in
Table 3. The averaging was carried out according
to the groups (samples) of the parts of the layered
structures indicated in the table. The sample sizes
corresponded to the number of parts in each of
the groups.
The analysis of the data from Table 3 shows
the following. Regardless of which film is used
Table 3
Mean values (M) and standard deviations (S) of the emission characteristics of parts of autocathodes on
layered structures
Fig. 9. Aging curves of layered structures of sample 1
for 6 hour tests:
1 — В1:Si/СNW/NiO*/СNW; 2 — А1:Si/СNW/СNW
U, V
850
750
0 120 240 T, min
1
2
Structure Sample
size
Measurement mode
Pulse Continuous
ET, V/μm b ln(DE) H250, % V3h, V/h V1h, V/h QАВ
А:Si/СNW; B:Si/СNW 26
М 4.5 60.6 7.2 11.8 — — —
S 1.3 24.7 2.5 19.9 — — —
А:Si/СNW/СNW 13
М 2.3 37.5 9.2 –2.5 20.0 10.8 —
S 1.1 20.5 3.0 17.7 21.3 13,2 —
B:Si/СNW (ann) 3
М 3.1 53.3 10.2 –14.2 11.1 13.3 –0.01
S 0.1 9.1 1.4 10.8 10.2 15.3 0.41
B:Si/СNW/NiO/СNW 2
М 3.5 58.2 13.0 –6.0 5.0 15.0 0.44
S 0.3 4.3 0.9 3.6 21.2 21.2 0.06
B:Si/СNW/NiO*/СNW 2
М 2.6 33.6 8.5 2.1 16.7 10.0 0.32
S 1.3 1.0 1.7 17.8 23.6 0 0.19
B:Si/СNW/Ni10/СNW 3
М 1.9 34.1 10.0 –16.4 14,4 3.3 0.14
S 0.3 11.4 1.7 2.8 5.1 5.8 0.32
B:Si/СNW/Ni40/СNW 1 M 2.2 36.5 6.8 –2.9 26.7 0 –0.48
B:Si/СNW/Ni80/СNW 1 M 2.9 35.4 7.4 –7.3 20.0 10.0 –0.50
B:Si/ СNW/Ni160/СNW 1 M 3.2 53.4 13.1 –3.8 13.3 0 –0.12
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(Ni or NiO), the presence of the second CNW
layer reduces the average value of the ET emission
threshold to about 2.3 V/μm as compared to the
average value of 4.5 V/μm for the first CNW layer.
The average ET values for the second CNW layer
structures with or without Ni, NiO film differ by
approximately 0.1%. On average, the best results
for the ET parameter (1.9 V/μm) were shown by
the Si/СNW/Ni10/СNW structures, for which
the aging rate is also minimal (V1h ≈ 3.3 V/h)
with a positive QAB value of 0.14. The best
V1h results had the Si/СNW/Ni40/СNW and
Si/СNW/Ni160/СNW structures (V1h = 0) at
low ET values, however for these structures the
QAB parameter turned out to be negative.
Because of the large number of multiwall
nanotubes [16], the autocathodes without both the
second CNW layer and restorative annealing of the
first CNW layer showed the best results for the
b parameter (60.5 on average) with a low density
of emission centers (DE ≈ 103, ln(DE) = 7.2).
Then the b parameter was decreasing in struc-
tures with a second layer in the following order:
Si/СNW/NiO/СNW and Si/СNW/Ni160/СNW.
These same structures with the minimum number
of multiwall nanotubes showed the best result for
the DE parameter (DE ≈ 105, ln(DE) = 13).
As to the homogeneity of the emission images,
the best results on the average were shown by the
Si/СNW/Ni10/СNW structures (Н250 = –16.4%).
Then the Н250 parameter was getting worse
(increasing) in structures in the following or-
der: Si/СNW(ann), Si/СNW/Ni80/СNW,
Si/СNWNiO/СNW, Si/СNW/Ni160/СNW,
Si/СNW/Ni40/СNW, Si/СNW/NiO*/СNW,
and finally the worst were Si/СNW structures.
Conclusion
Thus, the analysis of the I-V characteris-
tics and aging curves of autocathodes based on
Si/СNW, Si/СNW/СNW, Si/СNW/Ni/СNW
and Si/СNW/NiO/СNW layered structures,
as well as the assessment of homogeneity of the
images obtained on the luminiferous anode screen
allowed establishing the following. On average,
the emission properties of autocathodes with a
second СNW layer and the presence (optional)
of a Ni or NiO film between the СNW layers are
better than those of autocathodes that undergo
restorative vacuum annealing of the first layer
at 720 K. Of all the tested autocathodes with
a second layer, the best results on the emission
characteristics were shown by the autocathodes
with 10 nm thick islet Ni films, which indicates
the possibility to use similar cathodes in vacuum
electronics. However, for better statistical valid-
ity, it is necessary to collect a larger amount of
experimental data.
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Tekhnologiya i Konstruirovanie v Elektronnoi Apparature, 2017, ¹ 6
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MATERIALS OF ELECTRONICS
ISSN 2225-5818
Д. т. н. А. Ф. БЕЛЯНИН1, В. В. БОРИСОВ2, С. А. ДАГЕСЯН3,
к. ф-м. н. С. А. ЕВЛАШИН2, А. А. ПИЛЕВСКИЙ2, В. А. САМОРОДОВ2
Россия, г. Москва,1ЦНИТИ «Техномаш», 2НИИ ядерной физики им. Д. В. Скобельцына,
3Московский государственный университет им. М. В. Ломоносова
E-mail: belyanin@cnititm.ru
УГЛЕРОДНЫЕ НАНОСТЕНКИ В АВТОЭМИССИОННЫХ КАТОДАХ
Слои углеродных наностенок (СН) выращивали из газовой смеси водорода и метана, активированной
тлеющим разрядом постоянного тока, на подложках из Si (слоистая структура Si/СН). Второй слой
СН выращивали на первом слое (структура Si/СН/СН) или на пленках Ni или NiO, осажденных на пер-
вом слое СН (структуры Si/СН/Ni/СН и Si/СН/NiO/СН). Методами растровой электронной микро-
скопии, спектроскопии комбинационного рассеяния света и рентгеновской дифрактометрии исследова-
ны состав и строение полученных слоистых структур. Установлено, что отжиг в вакууме структуры
Si/СН, наращивание на Si/СН второго слоя СН, а также нанесение пленок Ni или NiO перед наращи-
ванием второго слоя СН приводят к улучшению функциональных свойств автоэмиссионных катодов на
основе слоев СН, эмитирующих электроны.
Ключевые слова: углеродные наностенки, слоистые структуры, электронная микроскопия, спектроско-
пия комбинационного рассеяния света, автоэмиссионные катоды.
А. Ф. БЄЛЯНІН1, В. В. БОРИСОВ2, С. А. ДАГЕСЯН3,
С. А. ЄВЛАШИН2, А. А. ПІЛЕВСЬКИЙ2, В. А. САМОРОДОВ2
Росія, м. Москва, 1ЦНДТІ «Техномаш», 2НДІ ядерної фізики ім. Д. В. Скобельцина МДУ;
3Московський державний університет ім. М. В. Ломоносова
E-mail: belyanin@cnititm.ru
ВУГЛЕЦЕВІ НАНОСТІНКИ В АВТОЕМІСІЙНИХ КАТОДАХ
Вуглецеві матеріали, що включають різні кристалічні (алмаз, графіт) і некристалічні (фуллерен, на-
нотрубки, графен та ін.) впорядковані речовини з унікальними фізико-хімічними властивостями, пред-
ставляють практичний інтерес. Деякі вуглецеві матеріали завдяки властивості автоеміссіі є перспек-
тивними для використання як емітуючого шару автоемісійних катодів (автокатодов). Найбільш пер-
спективними для створення автокатодів з низьким бар'єром емісії електронів вважаються так звані
вуглецеві наностінкі (СН) — шари пластинчастого вуглецевого матеріалу з переважним орієнтуванням
пластин перпендикулярно підкладці. Роботу присвячено дослідженню впливу відпалу в вакуумі і нароще-
ного другого шару СН на емісійні властивості шаруватих автокатодів на основі вуглецевих наностінок.
Шари СН для досліджень вирощували з газової суміші Н2 і СН4, активованої тліючим розрядом постійного
струму, на підкладках з Si. Перед нарощуванням СН на підкладках створювалися вуглецеві затравочні
центри шляхом обробки поверхні іонами Н+ та СхНу
+. Емісійні характеристики отриманих шаруватих
структур Si/СН контролювали півгодинними випробуваннями. Піддані випробуванням та/або тривало-
му зберіганню на відкритому повітрі шаруваті структури Si/СН або відпалювали в вакуумі (1,5 годи-
ни при 720 К), або на їх поверхні нарощували другий шар СН (Si/СН/СН) за тих же умов, що і перший.
Другий шар СН нарощували також на поверхні першого шару СН, вкритого плівкою Ni або NiO (струк-
тури Si/СН/Ni/СН та Si/СН/NiO/СН). Плівки Ni отримували методом магнетронного розпилення,
а плівки NiO — термічною обробкою в розчині Ni(NO3)2. Максимальна висота першого шару СН щодо
підкладки становила 2—4 мкм, сумарна висота першого і другого шарів — 8,5 мкм. Склад і будову ша-
руватих структур досліджували з використанням растрової електронної мікроскопії, рентгенівської
дифрактометрії і спектрометрії комбінаційного розсіювання світла.
Емісійні властивості представлено у вигляді статистичних оцінок порогу автоемісіі, коефіцієнту по-
силення електричного поля поблизу одиночного автоемітера і щільності еміcійних центрів в регулярній
геометрії, однорідності емісійних зображень, а також швидкості старіння автоемісійних катодів за
тривалих випробувань на постійному стабілізованому струмі. Розроблена методика випробувань до-
зволила проводити коректне порівняння емісійних характеристик автоемісійних катодів до і після ви-
рощування другого шару СН, а також після відпалу в вакуумі. Встановлено, що в середньому емісійні
властивості автокатодів з другим шаром СН і плівкою Ni або NiO між шарами СН є кращими, ніж
у автокатодів, які пройшли відновлювальний відпал у вакуумі першого шару за температури 720 К.
Відзначено, що з усіх досліджених автокатодів з другим шаром СН найкращі результати за емісійними
характеристиками в середньому показали автокатоди з острівковими плівками Ni товщиною 10 нм.
Проведені дослідження підтверджують можливість застосування шаруватих автокатодів на основі ву-
глецевих наностінок в пристроях вакуумної електроніки.
Ключові слова: вуглецеві наностінки, шаруваті структури, електронна мікроскопія, спектроскопія
комбінаційного розсіяння світла, автоемісійні катоди.
DOI: 10.15222/TKEA2017.6.34
УДК 538.911: 538.975
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