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ФИЗИЧЕСКАЯ ЭЛЕКТРОНИКА
DOI 10.5862/JPM.225.5 UDC 537.533.2
G.G. Sominski', V.E. Sezonov ', Yu.M. Zadiranov2
1 Peter the Great St. Petersburg Polytechnic University 2 Ioffe Physical Technical Institute
FIELD EMITTERS MADE OF THE CONTACTED YTTERBIUM AND CARBON NANOLAYERS
The operation of field emitters of a new type prepared from contacted nanolayers of ytterbium and carbon has been investigated. The performed calculations and experiments allowed to optimize the emission characteristics of the emitters. The calculations took into account the existence of a transition zone between the layers of Yb and C. Emission characteristics of the cathodes including up to 40 pairs of layers of carbon and ytterbium with optimum thicknesses of 5 and 2 nm respectively were measured. The created multilayered emitters provide the average emission current density over the surface of the emitter up to 10 - 20 A/cm2 and show promise for use in miniature electronic devices.
FIELD EMITTER, CONTACT POTENTIAL DIFFERENCE, YTTERBIUM, CARBON, CALCULATION, EXPERIMENT.
1. Introduction
One of the intractable problems of any field emitter is the need to obtain fields of the
order or even more than 2* 107 V/cm near their surface at moderate voltages. Earlier we have
demonstrated the possibility of using the fields near the nanocontacts of materials with different work functions eq [1, 2] for this purpose. Field
emitters prepared from contacted layers of materials with greatly different work functions were developed. When first such cathodes were fabricated, thermal evaporation was used to
create thin layers of contacting materials. This technology allows to make the emitters consisting
of no more than three or four pairs of layers with different work functions. It is obvious that to obtain intensive field emission we are bound to have a system with a large number of pairs of layers and to collect current from such a layered cathode. In this paper, we report the results on the development and investigation of multilayer cathodes including up to 40 pairs of layers with varying work functions.
2. Numerical computations
We present here the calculation data obtained for the structure prepared from ytterbium (Yb, eq = 3.1 eV) and carbon (C, eq = 4.7 eV) layers.
Calculations necessary to optimize the layered field emitters and determine their emission characteristics have been carried out using the Comsol software. Electric fields, electron trajectories, current density distributions over the surface of the field emitter and emitter currents were found. The calculations took into account the existence of a transition zone between the layers of Yb and C, where the mixture of these materials exists [3]. The current to the anode was determined by the Fowler-Nordheim equation (see for example Ref. [4]).
Emission of the layered cathode is conditioned by the fields that exist due to the difference in work functions of contacted materials, as well as by the 'external' field associated with the supply of voltage U between
а)
Anode
b)
■ 1 k>' /
с —:- Yb
В 4 jj > 3 's 2 til ^ 5 > 4 tS 3
Xç 1
2
jr. nm
Fig. 1. Schematic representation of the problem
statement: a region of the diode system (a) and the distributions of potential U and electric
field E near the contact between the Yb and C layers (b). Trajectories of the electrons e are shown; d is a distance, x is a position of the critical point
the cathode and the anode. The distribution of the potential U(x) in the transition zone due to the difference in the work functions of contacted materials was described by the function:
U(x) = 3.1 + 1.6 • cos
( / yll AA2 n I x A
2 Ч 71
where l is a width of the transition zone.
The shape of this distribution can be varied by changing the coefficients A1 and A2. The coefficients were chosen so as to ensure the best possible agreement between the calculation and the experimental results on the emission characteristics of the layered cathode.
Fig. 1, a schematically shows the contact region of adjacent Yb and C layers. The vertical dashed lines indicate the boundaries of the transition zone between the layers. Fig. 1, b demonstrates typical distributions (used in the calculations) of the potential U and the total electric field E in the contact area defined at Ua = 6 kV and the given value Ae^ = 1.6 eV of the work function difference for these layers in the diode with a gap of 1 mm between the cathode and the anode. Typical electron trajectories (e) are shown in Fig. 1, a as well.
As follows from the calculation results of electron trajectories, electrons emitted by the cathode region x < xc reach the anode at a fixed voltage Ua, whereas those from the region x > x return to the cathode at the same
c
voltage.
The performed calculations revealed that the anode current of the layered cathodes depends on the thickness of the contacted layers. Typical calculated dependencies of the anode current / upon the thickness values of the ytterbium (dYb) and the carbon (dC) layers are shown in Fig. 2.
According to the information in literature (see for example Ref. [3, 5]), the width of the transition zone is typically about 0.6 — 0.8 nm. Therefore, we can probably take dC = 1 — 2 nm as the optimal thickness of a carbon layer, as it is only a little more than the transition zone dimension. Thickness dYb of the ytterbium layers should be much more, and may be taken, for example, as about 5 nm.
Two current-voltage characteristics of the cathodes including 20 pairs of Yb-C layers are shown in Fig. 3. The former was obtained for
Fig. 2. Plots of the anode currents versus thicknesses of Yb (a) and C (b) layers at two values of the C (a) and Yb (b) layer thickness dCYb, nm: 2 (1), 5 (2), 3 (3) and 5 (4). In the calculations, we put Ua = 6 kV
^ Научно-технические ведомости СПбГПУ. Физико-математические науки № 3(22 5) 201 5
Fig. 3. Calculated current-voltage characteristics for the emitter with 20 pairs of layers structure with d„ = 5 nm, d' = 2nm (1) and d„ = 2nm,
dC = 2nm (1) and dC = 5nm (2)
optimal thickness values dYb = 5 nm and dC = 2 nm, the latter was calculated for the cathode with different thickness values of the Yb and C layers: dYb = 2 nm and dC = 5 nm. It can be seen that the deviation from the optimal dimensions leads to a significant drop of current.
3. Experimental investigation
Layered cathodes were made using magnetron sputtering. The carbon and ytterbium layers were sequentially deposited on a single crystal substrate of gallium arsenide. Two— three cathode systems, differing in the number N of pairs of layers (ytterbium-carbon) and/or in layer thicknesses, were located on a surface of a single substrate. After the procedure of the layers application was completed, cleavage of the single crystal and of the bottom part of the layered cathodes was accomplished. Thus, an atomically smooth emitting surface of the cathode was formed. The cathode surface morphology was examined with a scanning electron microscope Supra 45 WDXC.
The triode system for the investigation of the layered cathodes emission characteristics is shown schematically in Fig. 4. Measurements of the field emission currents were carried out in pulsed mode (2 ^s, 200 Hz).
Fig. 4 shows the cathode system with three layered structures as an object. A pulse of negative voltage was supplied to the cathode system (C) through a metallization layer (M) (200 nm of titanium). Flows of electrons from all three layered structures penetrating the grid ( G) to the corresponding collector 1, 2 or 3 were
Fig. 4. The triode system scheme to measure the emission characteristics of a sandwich-layered cathode: G is a grid; C is a cathode system: 1—3 are collectors; N is the quantity of pairs of layers in the layered structures; M is a metallization layer to apply a voltage to the cathode system; power supply units (on the right) and measuring equipment (at the top) are shown as well
detected simultaneously. A negative voltage of 100 — 200 V to earth was applied to the grid in order to reduce the flow of secondary electrons from the collectors.
Typical current-voltage characteristics of the cathodes with different quantities (N) of pairs of layers are shown in Fig. 5. As expected, the field emission current was built up with the N value. The dashed line in Fig. 5 demonstrates the calculated current-voltage characteristic of the cathode including 40 pairs of layers. Emission currents from the cathode with 40 pairs of layers reached 50 — 100 ^A. At higher
4 60
ЦА
40 -
20
A3
Л
s2
4 6 8
Ua, kV
Fig. 5. The experimental (1—3) and the calculated
(4) current-voltage characteristics of layered systems with the different quantities of layers N: 10 (1), 20 (2), 40 (3, 4)
Fig. 6. The experimental current-voltage characteristics of emitter with 20pairs of layers structure with d„ = 5 nm
Yb
d„ = 2nm,
Yb
dC = 2nm (1) and dC = 5nm (2)
currents, destruction of the cathodes occurred. The emission current densities averaged over the surface of the layered cathode reached values of ~(1 - 2) • 10 A/cm2.
To test the validity of the calculation results on the optimal layer thicknesses in the layered structure, we measured emission characteristics of two cathodes with the same quantity (N = 20) of pairs of layers but with significantly different layer thicknesses (see Fig. 6).
The curve 1 was obtained for the cathode with optimal (according to our calculations) thicknesses of the ytterbium layers dYb = 5 nm and the carbon ones dC = 2 nm. The curve 2 was derived for the cathode with nonoptimal layer thicknesses of ytterbium and carbon, respectively equal to 2 and 5 nm. The
obtained data revealed that the currents from optimized layered structure were substantially higher than those from the structure with non-optimal layer thicknesses. Thus, these measurements prove the correctness of the conducted optimization.
4. Summary
We have investigated the operation of field emitters of a new type made of contacted nanolayers of ytterbium and carbon. The most important results of this study are the following:
The technology of multilayer cathodes production from the contacted layers of materials with different work functions was developed;
The calculation technique for multilayer structures and their optimization was worked out;
An experimental method to investigate multilayer structures was developed and achievable emission characteristics of multilayer cathodes were obtained;
The emission characteristics of the cathodes including 10, 20 and 40 pairs of layers of carbon and ytterbium were determined. The possibility of obtaining emission current densities (averaged over the cathode surface) up to 10 — 20 A/cm2 was demonstrated.
The investigated cold cathodes showed considerable promise as a means of producing miniature vacuum devices with high emission current densities.
REFERENCES
[1] G.G. Sominski, V.E. Sezonov, Yu.M. Za-diranov, Multilayer Field Emitters of New Type, Proc. of 10th Int. Vacuum Electron Sources Conf. and 2nd Int. Conf. on Emission Electronics, St. Petersburg, Russia, June 30 - July 4, 2014. (2014) 228.
[2] G.G. Sominski, V.E. Sezonov, T.A. Tumare-va, E.P. Taradaev, «Field emitter» Patent of the Russian Federation № 118119. 17.02.2012.
[3] I.A. Zhuravel, E.A. Bugaev, L.E. Konotopskii, et al., Structural and Phase Transformations in C/Si Multilayers during Annealing, Technical Phys-
ics, Physical Science of Materials. 59 (2014) 701707.
[4] N.V. Egorov, E.P. Sheshin, Field emission. The principles and instruments. Dolgoprudny: Publishing house "Intelligence", 2011. P.704.
[5] I.A. Zhuravel, E.A. Bugaev, A.Y. Devizenko, Y.P. Pershin, L.E. Konotopskii, Investigation of the structure of the interlayer boundaries in multilayer periodic compositions Cr / Sc and Co / C by the method of X-ray diffuse scattering, Physical surface engineering. 2 (2011) 134-141.
THE AUTHORS
SOMINSKI Gennadi G.
Peter the Great St. Petersburg Polytechnic University
29 Politekhnicheskaya St., St. Petersburg, 195251, Russian Federation
sominski@rphf.spbstu.ru
^ Научно-технические ведомости СПбГПУ. Физико-математические науки № 3(225) 201 5
SEZONOV Vyacheslav E.
Peter the Great St. Petersburg Polytechnic University
29 Politekhnicheskaya St., St. Petersburg, 195251, Russian Federation
sezonovve@mail.ru
ZADIRANOV Yuri M.
Ioffe Physical-Technical Institute
26 Politekhnicheskaya St., St. Petersburg, 194021, Russian Federation
Соминский Г.Г., Сезонов В.Е., Задиранов Ю.М. ПОЛЕВЫЕ ЭМИТТЕРЫ, ИЗГОТОВЛЕННЫЕ ИЗ ПРИВЕДЕННЫХ В КОНТАКТ НАНОСЛОЕВ ИТТЕРБИЯ И УГЛЕРОДА.
Изучена работа полевых эмиттеров нового типа, изготовленных из нанослоев иттербия и углерода, которые были приведены в контакт. Проведены расчеты, позволяющие оптимизировать характеристики такого слоистого катода. В расчетах было учтено наличие переходной области между слоями УЬ и С. Определены эмиссионные характеристики катодов, включающих до 40 пар слоев с оптимальной толщиной слоев УЬ и С 5 и 2 нм соответственно. Показана возможность получения плотностей тока полевой эмиссии до 10 — 20 А/см2.
ПОЛЕВОЙ ЭМИТТЕР, ПОЛЕ КОНТАКТНОЙ РАЗНОСТИ ПОТЕНЦИАЛОВ, ИТТЕРБИЙ, УГЛЕРОД, РАСЧЕТ, ЭКСПЕРИМЕНТ.
СПИСОК ЛИТЕРАТУРЫ
[1] Sominski G.G., Sezonov V.E., Zadiranov
Yu.M. Multilayer field emitters of new type // Proc. of 10th Int. Vacuum Electron Sources Conf. and 2nd Int. Conf. on Emission Electronics. St. Petersburg, Russia, June 30 - July 4, 2014. P. 228.
[2] Соминский Г.Г., Сезонов В.Е., Тумарева Т.А., Тарадаев Е.П. «Полевой эмиттер». Патент Российской Федерации № 118119. 17.02.2012.
[3] Zhuravel I.A., Bugaev E.A., Konotopskii L.E., et al., Structural and Phase Transformations in C/Si Multilayers during Annealing // Technical Physics. Physical Science of Materials. 2014. Vol.
59. Pp.701-707.
[4] Егоров Н.В., Шешин Е.П. Автоэлектронная эмиссия. Принципы и приборы. Долгопрудный: Издательский дом «Интеллект», 2011. 704 с.
[5] Журавель И.А., Бугаев Е.А., Девизенко А.Ю., Першин Ю.П., Кондратенко В.В. Исследование структуры межслоевых границ раздела в многослойных периодических композициях Cr/Sc И Co/C методом рентгеновского диффузного рассеяния // Физическая инженерия поверхности. 2011. Т. 2. С. 134-141.
СВЕДЕНИЯ ОБ АВТОРАХ
СОМИНСКИЙ Геннадий Гиршевич — доктор физико-математических наук, профессор кафедры физической электроники Санкт-Петербургского политехнического университета Петра Великого. 195251, Российская Федерация, г. Санкт-Петербург, Политехническая ул., 29. sominski@rphf.spbstu.ru
СЕЗОНОВ Вячеслав Евгеньевич — аспирант кафедры физической электроники Санкт-Петербургского политехнического университета Петра Великого.
195251, Российская Федерация, г. Санкт-Петербург, Политехническая ул., 29. sezonovve@mail.ru
ЗАДИРАНОВ Юрий Михайлович — кандидат физико-математических наук, старший научный сотрудник Физико-технического института им. А.Ф.Иоффе РАН.
194021, Российская Федерация, г. Санкт-Петербург, Политехническая ул., 26.
© Санкт-Петербургский политехнический университет Петра Великого, 2015
