Low-field electron emission and cathode luminescence of piezoelectric films of oxides and chalcogenides
Basic parameters of effective low-field emitters of electrons of piezoelectric films for bright flat cathode luminescent display and other activity are optimised. Specimens of electron emitters of cathode luminescent screens are based on compounds of SiO₂, ZnO, ZnS, solid solution of Zn1-xCdxS. High...
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Інститут хімії поверхні ім. О.О. Чуйка НАН України
2002
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| Назва журналу: | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| Цитувати: | Low-field electron emission and cathode luminescence of piezoelectric films of oxides and chalcogenides / A.A. Dadykin, A.G. Naumovets, P.P. Gorbik, I.V. Dubrovin, V.M. Ogenko, M.N. Filonenko // Поверхность. — 2002. — Вип. 7-8. — С. 163-176. — Бібліогр.: 14 назв. — англ. |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine| _version_ | 1859983503931736064 |
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| author | Dadykin, A.A. Naumovets, A.G. Gorbik, P.P. Dubrovin, I.V. Ogenko, V.M. Filonenko, M.N. |
| author_facet | Dadykin, A.A. Naumovets, A.G. Gorbik, P.P. Dubrovin, I.V. Ogenko, V.M. Filonenko, M.N. |
| citation_txt | Low-field electron emission and cathode luminescence of piezoelectric films of oxides and chalcogenides / A.A. Dadykin, A.G. Naumovets, P.P. Gorbik, I.V. Dubrovin, V.M. Ogenko, M.N. Filonenko // Поверхность. — 2002. — Вип. 7-8. — С. 163-176. — Бібліогр.: 14 назв. — англ. |
| collection | DSpace DC |
| container_title | Поверхность |
| description | Basic parameters of effective low-field emitters of electrons of piezoelectric films for bright flat cathode luminescent display and other activity are optimised. Specimens of electron emitters of cathode luminescent screens are based on compounds of SiO₂, ZnO, ZnS, solid solution of Zn1-xCdxS. High efficient "cold" emitters of electrons were prepared on the basis of mono- and polycrystalline films. The emission and cathode-luminescent properties of the films were investigated. It is shown that developed films are perspective like the flat displays with the brightness of 300 Cd/m².
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| first_indexed | 2025-12-07T16:27:20Z |
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163
Surface properties of inorganic materials
LOW-FIELD ELECTRON EMISSION AND CATHODE
LUMINESCENCE OF PIEZOELECTRIC FILMS OF OXIDES
AND CHALCOGENIDES
A.A. Dadykin1, A.G. Naumovets1, P.P. Gorbik2, I.V. Dubrovin2,
V.M. Ogenko2, and M.N. Filonenko2
1Physics Institute, National Academy of Sciences
Prospekt Nauki 46, 03028 Kyiv-28, UKRAINE
2Institute of Surface Chemistry, National Academy of Sciences
Gen. Naumov Str. 17, 03680 Kyiv-164, UKRAINE
Abstract
Basic parameters of effective low-field emitters of electrons of piezoelectric films for
bright flat cathode luminescent display and other activity are optimised. Specimens of
electron emitters of cathode luminescent screens are based on compounds of SiO2, ZnO, ZnS,
solid solution of Zn1-xCdxS. High efficient "cold" emitters of electrons were prepared on the
basis of mono- and polycrystalline films. The emission and cathode-luminescent properties of
the films were investigated. It is shown that developed films are perspective like the flat
displays with the brightness of 300 Cd/m2.
Introduction
According to the classical theory of autoemission of metals and semiconductors,
electric fields near a surface of emitters to produce measurable currents are needed with
E>107 V/cm. However, in a series of runs it was registered appearance of the electron
emission for E>105 V/cm estimated from macrogeometry of electrodes (so-called LFEE -
low-field electron emission). It was established experimentally that the appearance of LFEE
can be explained neither a simple geometric strengthening of an electric field over
microprojections of surface nor a local diminution of work function, nor a suggested, in the
case of diamond, presence of areas on the emitting surface, having the natural negative
electron affinity (NEA).
The direct experiments have established [1] that a main, insertion less in fact,
component of LFEE is due to the presence on the emitting surface of filmy dielectric
formations with a great ratio of longitudinal sizes to thickness. Visual observations and a
secondary ion mass-spectrometry have shown that the formations consisted of commination
products of cathode luminophores fabricated from the compounds of AIIBVI type, from anode
screen of the experimental diode cell, or from oxides of SiO2 type remaining on the chemical
etching out of cathodes. Both silicon dioxide and cathode luminophores, as it is known, are
good piezoelectrics. In this connection it was proposed, that had found an experimental
support, that as a result of deformations under an electric field there appear contact electric
fields on faces of piezoelements exceeding 107 V/cm. For such external fields in the
near-surface region of emitters, in case of a low dielectric constant (in piezoelectrics ε ~ 5),
internal fields reach values of 106 - 107 V/cm too, for which the Zener effect is possible, and
164
electrons can exit to vacuum immediately out of the valence zone of a piezoelectric.
Evidently, in order to realise this situation it is necessary an enough perfection of specimens -
a general requirement for technology of piezoelectric.
It is interesting to trace a relationship of LFEE with piezoeffect on the example of
specimens of the AIIBVI-type compounds and silicon dioxide which are relatively readily can
be prepared in a form of glasses, ceramics, textures and monocrystals with a sharply differing
piezoelectric activity. Along with it, owing to the temperature and temporal stability of
physical parameters, a development of technologies of field emitters on their basis presents a
practical interest. Preliminary runs have shown that current density at the quartz cathode
exceeds 10 A/cm2 (the best barium oxide cathodes give ~1 A/cm2), and such cathodes can be
used in fabrication of bright flat cathode-luminescent panels-displays.
Procedure of measurements and experimental specimens
Experimental devices are made as autoelectronic Muller projectors to study emission
from separate areas of points and diode cells with flat cathodes and anode-screens coated with
cathode luminophores of the AIIBVI-type for visualisation of emission. Cathode-anode
distances were ordinarily of ~100 microns, and operating voltages - of several kV, so that
average electric fields in a cell reached 106 V/cm. Upon such field mechanical stresses appear
in piezoactive films that exceed considerably the electrostriation ones resulting in degradation
of the cathode luminophore and pollution of the cathode. In this connection, there were
designed and prepared, by the method of evaporation in a quasi-closed volume, filmy cathode
luminophores of ZnO and films of solid solution of Zn1-xCdxS with a good adhesion enduring
without a marked degradation electron flows over 1 W/cm2. Efficiency of cathode
luminescence at a voltage of 400 V was about 3 Lu/W that is comparable to the efficiency of
low-voltage powdered luminophores of the ZnO:Zn type. The objects of studies were:
1. Oxide films on the surface of silicon points that were prepared by treating blanks in nitric
acid.
2. Submicronic layers of SiO2 formed in electroadsorption of oxygen on silicon at
~106 V/cm.
3. "Mushroom-shaped" SiO2/SiO structures that were formed while fabricating multipoint
silicon cathodes.
4. Multielement matrix structures of a textured SiO2 that were produced by a final oxidation
applied through masks onto metalled glass plates.
5. Monocrystalline quartz plates (piezoresonance quartz pick - up of the industry production).
6. Quartz and sapphire glasses.
7. Polycrystalline filmy textured structures of zinc oxide produced by the method of
oxidation of zinc selenide and sulfide.
8. Textured films of zinc sulfide.
9. Textured films of solid solution of Zn1-xCdxS.
Field electron emission from the piezoactive SiO2
Fig. 1 gives an autoemission image of the silicon point with an atomically clean apex
and hardly stable to vaporisation oxide film at its foot. Autoemission characteristics in
coordinates lgi=f(1/V) (FN-characteristics) of different areas of the point differ sharply both
qualitatively and quantity. The current of films at the points foot (E~105 V/cm) is many orders
higher than that of the face (111) on the apex (E>107 V/cm), and a FN-characteristic of
emission has a well-defined fracture reflecting differences in mechanisms of emission from
several of the emitter.
165
Transformation of FN characteristics as the silicon point with the atomically clean
surface oxidises by the way of electroadsorption of oxygen at E~106 V/cm are presented in
Fig. 2. The fracture appears when the continuous oxide film of ~ 500Å thickness is formed.
Fig. 1. Autoemission characteristics of the field emission from the atomically clean face (111)
of silicon (a) and from the oxide formation at the points foot (b).
Fig. 2. Transformation of characteristics of field emission of the Si-cathode as oxygen is
electroadsorbed: 1- the atomically clean surface, 2-a monolayer of oxygen, 3 - for the
thickness of a SiO2 film ~500Å.
Of special interest are emission properties of "mushroom-shaped" system of SiO2
given in Fig. 3. Emission is observed from edges of oxide "hats", but completely corroded
points with the hats removed do not emit.
Geometry of emitting elements is distinguished by a great proportion of longitudinal
sizes to thickness and can be easily reproduced in a filmy variant. Film elements in a form of
166
matrices of the textured SiO2 having a varied shape and dimension are prepared by the final
oxidation of SiO in air at T=700K. Emission was observed from edges of elements with the
greatest sizes in the matrix with elements of different sizes.
.
Fig. 3. Characteristics of field emission from the end of the oxide film
on a silicon leg of the "mushroom-shaped" emitter.
Because it has been established that for LFEE were essential not absolute sizes of
elements, but their ratios, so elements of centimetre sizes were tested. For this purpose,
monocrystalline quartz piezoresonance pickups were used (Fig. 4) with metallic electrodes for
supply of a polarising voltage. Emission was not detected up to values of electric field near
the surface of piezoelement exceeding 106 V/cm in the absence of the polarising voltage. With
the supply of a polarising voltage as early as for the polarisation field of <105 V/cm there was
recorded a stationary electron emission from the end of piezoelement in direction of one of
electric axes of quartz. The emission image of the piezoelement in the anode - screen coated
with ZnS cathode luminophore is shown in Fig. 4.
Fig. 4. A characteristic of field emission from the end of quartz piezoresonator at voltage in
the anode - screen 2kV. Vp - a polarising voltage in plates of resonator.
167
With a change in polarity of the voltage in electrodes, emission is observed in the
direction of another electric axis turned about 1200. When the value of a polarising voltage is
changed, the image turns smoothly in the plane of piezoelement that indicates to the existence
of shear piezopolarisation. FN-characteristic as in all above-mentioned cases has a fracture,
and a current density estimated from the geometry of specimen, exceeds 10 A/cm.
In the case of specimens fabricated from evidently non-piezoactive fused silica and
sapphire, emission is not detected even for polarising field of >106 V/cm. Attention is
attracted to a high stability of LFEE in the field of great emission currents even for
p~10-5 Torr, that is important for practical applications. Fig. 5, a gives a simple explanation of
the results obtained.
Fig. 5. (a) - On a mechanism of piezogeometrical amplification of the electric field near the
end of piezoelement, П - a factor of piezogeometrical amplification; (b) - An
energetic scheme of the piezofield emission in the field of large emission
currents.
The piezoelement in a form of plate with sizes l and thickness d in a polarising field Ep
deforms so that end faces and side surfaces find themselves under different potentials,
proportional to absolute deformations. Here
V/Vp = Δl / Δd = σ l / d (1)
where σ is the Poisson coefficient.
The contact field arising at the end may be estimated as
Е ~ (V-Vp)/d=4πk2 σ l/d Еp=ПЕp (2)
where k - a coefficient of the electromechanical bond. Under experimental conditions σ~0.5;
k~0.5; l/d~103, so that П>100. It means that the E field can exceed 107 V/cm for the polarising
field Ep~105 V/cm. In this case an internal field E/ε for low ε can attain values of 107 V/cm
sufficient to develop the internal electric breakdown (Zener effect), and the condition of the
effect NEA is realised on the emitter surface with the field-penetrating inside. The
energetically scheme of the field emitter in such a situation is shown in Fig. 5, b. This model
of emitter explains both a course of FN characteristics and a high stability of emission for
high currents: for low voltages the emission is limited by a potential barrier on the boundary
to vacuum and considerable fluctuations of current appear due to adsorption of residual gases,
but for high ones - the source of electrons (a valence band) becomes hidden of the external
medium by the protective film of piezoelectric.
168
Emission and cathodoluminescent properties of zinc oxide films prepared
by the method of oxidation of zinc selenide and sulfide
Zinc oxide possesses a number of physical and chemical properties and finds a wide
application in the world-wide production of adhesives, paints, glasses, piezoelements etc [2].
Of the particular interest is its use in the electrotechnical and electron industry as piezoelectric
in the form of ceramics, films and crystals [3-5]. Manufacture of the materials indicated is
connected with certain difficulties, since zinc oxide possesses a high melting temperature and
a low pressure of saturating vapours. Of considerable interest is the use of polycrystalline
films of zinc oxide as components of photodetectors of UV radiation [6] and luminescent
materials [7,8]. To produce films, high-temperature technologies are used such as evaporation
and condensation of the starting material [9] by an electron beam [10], by the method of
magnetron sputtering [11]. The use of these methods complicates a control of physical
properties, disturbs stoichiometry of the material. Particular difficulties appear in the synthesis
of films and coats in the case of using profiled supports as well as supports with a low
temperature of melting. We studied physicochemical conditions of formation of
polycrystalline layers of zinc oxide from zinc-containing precursor - zinc sulfide and selenide
by means of oxidation. Also were studied the possibilities of using the prepared films as
emitters and anode - screens of cathodoluminescent displays. The essence of the chosen
technological method consists in the use of light-volatile zinc compounds as the precursor.
These properties belong to its chalcogenides, which, along with a well-developed technology
of their production (in particular, sulfide, selenide and solid solution on their basis), possess
an insignificant (<1 mol %) mutual solubility with zinc oxide at temperatures of over
1000oC [12].
The starting films of zinc sulfide and selenide were prepared by evaporation and
condensation under vacuum of 10-5 mm Hg. Temperatures of stable to vaporisation and
support varied in the ranges of 800-850oC and 100-150oC, respectively.
Oxidation of films of zinc chalcogenides was performed in a flow system with oxygen
at temperature of 300-450oC. The oxygen underwent the preliminary drying and purification.
Temperatures of processes of synthesis and oxidation were controlled with accuracy to ±0.1K
by means of a precision programmable regulator of RIF-101 type.
The chemical composition and morphology of films was studied by a scanning
electron microscope with a X-ray spectral microanalyzer of ICXA-733 type and
Auger-spectrometer IAMP-IOS (JEOL firm).
Study of the structure and the phase composition of films were carried out by means of
diffractometer DRON-3M with CuKα-radiation.
Flat and profiled plates of glass were used as supports, including the glass coated with
a transparent electroconducting layer of tin oxide.
Optimum regimes were selected in order to sputter films of zinc chalcogenides of
0.3-10 μ thick using the above-mentioned method, which possessed a minimum number of
microdefects. It was established that the composition of films (in limits of the measuring
error) was close to the composition of starting compounds.
During the oxidation reaction of chalcogenide films, the transformation of a sphalerite
lattice of chalcogenides into a wurtzite one - a typical of zinc oxide and having changes in
parameters of a crystal unit cell. This, as well, possibly, the lack of agreement between
coefficients of thermal expansion of a support and film resulted in the formation of a
polycrystalline coating of zinc oxide. At this point a quality of films, i.e., the presence of
macrodefects and transverse pores increases. In this connection, oxidative process of
chalcogenide films for different temperatures was studied in detail. Optimum results have
been obtained for the isothermal oxidation of zinc selenide at temperature of 400oC.
169
Fig. 6 gives dynamics of the oxidative process of the zinc selenide film. The absence
of coinciding, the most intense, reflexes of zinc selenide and oxide made it possible to observe
well the kinetics of a topochemical oxidation reaction:
2ZnSe+3О2 = 2ZnO+2SeO2 (3)
Fig. 6. Dynamics of oxidation of zinc selenide by data of X-ray phase analysis.
During the course of experiment it was established that a dependence of a
transformation degree in the oxidation reaction of ZnSe on time at t=4000C has typical
sigmoid form.
The transformation degree was determined by a calibration curve obtained for
processing the data on measurement of intensity of reflexes for a mechanical mixture of the
ZnO-ZnSe system. Kinetic regularities were analysed using equations by B.V. Erofeev [13]
for the growth of spherical nuclei which number increases at a constant rate in a form
a = 1 - exp (-Ktn) (4)
where α - is a degree of transformation, K - is a constant, t - time, n = σ +a, σ - has a meaning
of the number of elementary stages when a nucleus transforms in the actively growing
nucleus, and depends on the number of directions in which nuclei grow. An n - constant
entering the equation was found by treatment of experimental data using the relationship
ln [- ln (1 - a)] = lnK + n lnt (5)
Since ln[-ln(1-a)] depends linearly on lnt, then by a tangent of angle of slope of this
straight line it can be determined the parameter n, which value allows to judge about
mechanism of oxidation of zinc chalcogenides.
The value of n=4.2 close to 4 is the evidence that the growth of nuclei develops in
three directions for the one stage change of a nucleating centre into the actively growing
nucleus. The deviation of experimental dependence from linear in the initial stage of the
transformation process points to a complex nature of oxidative mechanism for this stage. In
order to improve adhesion of ZnO films etchers for supports have been selected. The best
results for the thermoadding have been obtained in case of films of zinc oxide synthesised on
a glass support pretreated by boiling in bichromate, and then by treatment for 5 min in 5%
hydrofluoric acid at the standard temperature as well as glass specimens coated with tin oxide
by boiling for 5 min in 10% alkaline solution followed by careful washing in the distilled
water and drying.
The use of films of zinc sulfide and selenide as precursors makes possible to produce
while oxidation them with oxygen in a flow system, quality textured films of zinc oxide,
devoid of macrodefects and with a controlled stoichiometry.
As is known, textured ZnO films under certain technological conditions of their
production have high values of piezomodules and coefficients of the electromechanical
170
coupling. According to [1] highly efficient piezofield emitters operating steadily even for
p@10-5 Torr can be produced on the basis of these films. On the other hand, zinc oxide is an
efficient cathode luminophor. So, studies of emission and luminescent properties of ZnO have
been performed in devices, in which a cathode and anode are made of the same material, with
regard to requirements to its stoichiometry. Such a performance of the checking diode allows
avoiding a vacuum possible contaminations during measurements. The measurements have
been carried out in a flat diode cell of 1´1 cm size and with gap of @ 100 μ to ensure average
field strengths of > 105 V/cm. The results of measurements are presented in Fig. 7 with the
Fauler-Nordheim dependence (FN characteristic).
It attracts attention a high stability of emission in the range of large emission currents
even for p=10-5 Torr. It is due to the fact that large (@107 V/cm) electric fields are created in
complete films of piezoelectric's in a near-surface field of @100 Å, great enough to develop
the Zener effect, for which a source of electrons becomes a valence band of piezoelectric (the
electron source is hidden from an outer medium with a protective film of the piezoelectric).
A potential barrier on a boundary limits the emission with vacuum in the range of low
currents. Here, the emission is not stable because of great fluctuations of work function
caused by adsorption of residual gases. A current density from separate elements of the matrix
exceeded 10 A/cm2; a full current to anode - screen reached 1 mA for Ecp@105 V/cm. The
emission differed also by a high spatial uniformity for a high stability in the region of high
currents (>90% elements emitted simultaneously).
The ZnO-Zn film of cathode luminophor for a high adhesion endured without visible
breakdown electron flows more that 1 W/cm2. The efficiency of film cathode luminescence is
comparable with efficiency of a powdery cathode luminophor (≈ 3 Lu/W). For such
parameters of a powdery cathode and anode it is readily attained a brightness of the
cathodoluminescent screen, ≈ 300 Cd/m2 that is two orders more than the brightness of the
electroluminescent screen.
Fig. 7. A FN-characteristic of 100-element film ZnO emitter prepared by oxidation of zinc
selenide.
Changes in emission and luminescent properties of ZnO films were not found during
pretreatment of operating mock-ups of displays for hundreds of hours.
It should be noted that in the case of ZnO films as well AlN films, which were
amorphous by data of X-ray phase analysis, no electron emission was found even for middle
electric fields exceeding 106 V/cm.
The similar situation appears as well in the case of films of cubic modification for
normal to a support orientation of axis of the fourth-order symmetry which is nonpiezoactive
171
that confirms too the fact of emergence of the low-field electron emission as a result of the
piezoelectric amplification of electric field.
Thus, the described technology can be used in development of highly efficient
piezofield cathodes for devices of vacuum micro- and nanoelectronics of different purposes.
Flat cathodoluminescent displays based on ZnS:CdS thin films
Films of a solid solution Zn1-xCdxS of a different composition (0<x<1) were prepared
on dielectric supports (glass, glass with a thin layer of SnO2 or a metal) using a thermal
evaporation in a quasi-closed volume from autonomous sources: polycrystalline tablets of
zinc and cadmium sulfide. As starting components, powders of ZnS and CdS were used that
the chemical industry releases. Chambers of a vacuum apparatus maintained a pressure of
1.33·10-2–6.65·10-3 Pa in the working volume. A choice of geometric sizes of the reactor was
determined by sizes of the working chamber of the vacuum aggregate and a temperature field
of oven. The chemical composition of the resulting films was determined by results of the
X-ray phase analysis, by a spectral distribution of photoconduction as well as by a spectral
distribution of photosensitivity of specially fabricated heterostructures. Temperatures of
evaporators and supports during evaporation were controlled by means of
platinum-platinum/rhodium and chromel-alumel thermocouples. The temperature of supports
was changed within 150-300oC. The temperature of sources was changed within 700-950oC
for cadmium sulfide and within 800-1080oC for zinc sulfide. Horizontal screens with
incompatible openings were situated in the reactor between the support and evaporator.
The crystalline structure of the resulting films was found to depend on their
composition. The films of zinc sulfide had a cubic structure despite the fact that in the initial
state the wurtzite phase was predominant in the ZnS structure. The value of parameter for the
cubic unit cell a ZnS films that is equal to 5.412 Å was consistent well to the cited references
(a=5.406 Å). The films of solid solutions of Zn1-xCdxS on the part of ZnS had a sphalerite
structure and, if make a note of intensity of reflexes, 1-2% wurtzite phase. For higher
concentrations of cadmium sulfide a structure of Zn1-xCdxS films were a wurtzite one. All the
films examined were oriented partially what can be judged by the ratio of reflexes and their
quantity. It should be noted that diffractograms of films obtained under the same
technological regimes are identical that is the evidence of a sufficient reproduction of the
elaborated procedure of Zn1-xCdxS film production.
The composition of the films studied was determined by values of lattice parameters of
solid solutions using the graphical dependence for polycrystalline samples as well as by the
spectral dependence of photosensitivity of heterojunction of pCu2 - nZn1-xCdxS (with this in
view there have been made heterojunctions which photosensitivity was due to the intrinsic
absorption of emission in Zn1-xCdxS). It is known that the width of a forbidden gap of solid
solutions of Zn1-xCdxS varies depending on the composition by the linear law. The
composition of films was determined, basing on this the position of a "red" boundary of
photoeffect in heterojunction. The results obtained are fairly consistent with the data of X-ray
phase analysis on the investigation of photoconductivity of Zn1-xCdxS films. Lattice
parameters of solid solution of Zn1-xCdxS films being studied and their chemical composition
are given in Table 1.
The study of microstructure of produced films by means of the metallography
microscope revealed the existence of the explicitly expressed polycrystalline structure, the
character of which depended on preparative conditions and composition of solid solution.
Thus, Zn1-xCdxS films of a differing composition but approximately of equal thickness (~10μ)
deposited on the same supports at the same temperatures, had a differing microstructure. The
structure of films was granular one, the grain size was decreased as a concentration of zinc
sulfide in a solid solution was increased. The grains had a crystalline facet. One has found that
172
the grain size depended apparently on the support temperature and increased with the
temperature rise. The study in the polarised light hag showed that these films were textured
partly. Texturization of the films prepared might be due to the predominant growth of
favourably oriented nuclei.
Table 1. The structure of Zn1-xCdxS as function of composition.
Films
number
Composition
(in mol %)
Type of crystal
structure
Lattice
parameters,Å,
а+0.006 ZnS CdS
1 — 100 wurtzite 4.142
2 14 86 wurtzite 4.092
3 24 76 wurtzite 4.066
4 27 73 wurtzite 4.060
5 47 53 wurtzite 3.990
6 57 43 wurtzite 3.962
7 78 22 sphalerite 5.508
8 100 — sphalerite 5.412
It was established for a transverse chip that crystallites had in height a columnar form,
their thickness increased with the growth of a film thickness. Surface of the transverse
microsection of films do not have microcracks, void inclusions or micropores. Grain
boundaries have a dense cohesion and a sufficiently high mechanic strength that allows the
film together with a conducing sublayer to be deformed without any failure. Optimal for
making matrix luminescent displays turned out to be the films of Zn1-xCdxS, which have a
polycrystalline structure with a middle size of crystallites, 2-6 μ. At this point, the height of
columnar crystallites was commensurate with the film thickness.
The Zn1-xCdxS films have electron conductance (that was checked by the method of
thermoprobe and by the Hall effect) because, perhaps, that in the process of growth there
occurs self-activation of the solid solution by excessive cadmium. The specific resistance of
films is ~104 to 1010 Ohm·cm, increasing for an increased concentration of ZnS in the solid
solution.
Measurements of the Hall effect were carried out for Zn1-xCdxS films that contain in
the solid solution up to 20 mol % zinc sulfide (when concentrations of zinc sulfide in the solid
solution are large, the resistively of the films was increased abruptly that prevented from
making measurements).
The experimental results demonstrate that the n concentration of free carriers (of
electrons) in films depends substantially on the temperature of warming - up a support. On
rising the support temperature, n diminishes. The highest-resistant films of Zn1-xCdxS are
produced at temperatures of the support of ~ 180-220oC. The optimum temperatures are about
200 - 220oC for which are observed a favourable combination of polycrystalline structure and
electrophysical parameters of films Zn1-xCdxS solid solution. In the investigated area of
composition of base films, the concentration of free electrons depends weakly on the
concentration of zinc sulfide in a solid solution and amounts for different samples
1011-5×1016 cm-3. The value of mobility ranges from 10 to 15 cm/Vs. Low values of carriers
mobility can be explained by the high density of grain boundaries and other defects of the
crystal structure of the Zn1-xCdxS films (dislocation, intrinsic and exemplary defects).
The experimental dependence of the width of forbidden gap of Zn1-xCdxS on the
composition is given in Table 2. The synthesised solid solutions have a great practical interest
for needs of vacuum micro- and nanoelectronics: on the one hand, these, as
173
piezoelelectronics, can be used for fabricating highly efficient cold sources of electrons for
different purposes, and on the other hand, as efficient cathode luminophors for full-colour
screens of television sets of a high sharpness.
Table 2. The dependence of the width of forbidden gap of Zn1-xCdxS on the composition.
Composition
(in mole %)
Eg, eV
ZnS CdS
0 100 2.25
20 80 2.50
40 60 2.75
60 40 3.00
80 20 3.25
100 0 3.50
The visualization and measurements of the electron emission have been carried out in a
flat diode cell having a size 1x1 cm2 and an inter electrode gap of ~100 mm. The average
electric fields in the gap could be brought up to ~106 V/cm. The thin film cathode and anode
(screen) were prepared from solid solutions Zn1-x CdxS obtained by thermal evaporation from
autonomous ZnS and CdS sources heated in a quasi-closed chamber. The thickness of the
films was 0.3 to 3 mm. In another version of the technique, the films were deposited by
evaporation of a mixture of ZnS and CdS powders filling a platinum crucible
Choosing the content of the components as well as the temperatures of the evaporators
and substrates could control the ratio between the wurtzite (piezoactive) and sphalerite phase
in the films. The phase composition was determined by X-ray phase analysis and by spectral
measurements of the photoconductivity. The microstructure of the films was investigated by
optical and scanning electron microscopy.
Depending on the deposition conditions, the resistively of the films could be varied
within the range 0.01 to 1010 Ohm cm. The resistively as well as the adhesion of the films
depended also on the properties of the conducting metal sublayer. We tested Ta, Cr, Mo, and
Ni. Suitable properties were found for the sublayers prepared by evaporation of a Cu-Ag
alloy.
To fabricate the matrices of “single-unit” emitting elements (0.3x0.3 mn2), the glass
plates previously coated with a metal sublayer and Zn1-x Cdx S thin film, were locally abraded
in such a way as to create the corresponding “pattern”.
Fig. 8. Field emission pattern of a 100 element matrix cathode (1´1 cm2).
The matrices with the elements of the “pixel” type (100 emitters per pixel) were
prepared by evaporation through metal masks. The noise and response characteristics of the
electron emission were measured using a high-sensitive fluctuation technique developed in
our laboratory.
Fig. 8 shows a typical emission image of a 100-element matrix cathode visualised on a
Zn1-xCdxS thin film cathodoluminescent screen. Varying the film composition could vary the
174
colour of the image from blue to red. The film sustained, without any appreciable destruction,
the electron fluxes up to 1 W/cm2, which allowed us to obtain the brightness of the
luminescence above 300 Cd/m2 comparable to the brightness of powder phosphors. Such a
high brightness may be caused by the mosaic structure of the film.
The cathode film whose emission is depicted in Fig. 11 contained about 90% of the
wurtzite modification and its resistively amounted to 109 Ohm cm. The electrically active axis
had a predominant orientation normal to the surface. The separate elements of the matrix
shaped as squares 0.3´0.3 mm2 in size were deposited onto conducting Cu stripes, which gave
the possibility to scan the image line by line. Fig. 8 shows the emission distribution when all
the elements are switched on simultaneously. It is estimated that above 90% of the elementary
emitters give comparable currents. To attain such a high degree of the spatial uniformity of
emission in the case of metal tip arrays, it is necessary to provide each tip with an individual
current-limiting resistor. This requirement results in additional complication of technology.
Fig. 9 depicts a typical current-voltage dependence of the total emission current from a
cathode matrix plotted in the Fauler-Nordheim (FN) coordinates. The dependence was
recorded under the pressure of residual gases of ~10-5 Torr, and exhibited a remarkable
emission stability in the region of high emission currents. To attain such a high stability
within the whole range of the currents, a vacuum of~10-10 Torr is necessary. The total current
from the matrix amounted to 1 mA and was limited by cracking of the glass substrate of the
screen. It is estimated that the current density from an individual emitting element of the
matrix is ³10 A/cm2 at the field (measured macroscopically) E£105 V/cm. The operating
parameters of the model display described above demonstrate the possibility of fabrication of
both single-element and many-element (pixel) displays of this type. The specific choice
depends on the requirements for image brightness, dimensions and information capacity of the
display. To fabricate a matrix of single-element emitters, a technology should be used which
ensures a narrower spread in the size of the elements (e.g. photolithography).
Fig. 9. F-N plot of a matrix field emitter at P=10-5 Torr. Integrated current from matrix is
1 mA at Eav=3 105 V/cm. Current density from one emitting spot is j>10 A/cm2.
The emission characteristics of the piezoactive thin film cathodes can be interpreted in
terms of a potential diagram shown in Fig. 10. At low voltages, the emission is limited by the
transparency of the potential barrier at the boundary thin of film-vacuum. The strong current
fluctuations observed in this case are caused by adsorption of residual gases. At high voltages,
when the electric field penetrates the film, the condition of the effective negative electron
affinity is realized at the surface.
At high fields, due to penetration of the field into emitter and band bending, the EA
becomes negative (cef >0, the diagram on the left) and the current becomes stable (see the
upper segment of the FN plot). The electric field within the film due to its piezogeometrical
175
enhancement can attain a value of ~107 V/cm, which is sufficient for the Zener effect in the
dielectric. Such a situation can arise near the edges of a thin film with a piezoelectric activity
where a considerable amplification of the electric field should come into effect. This
possibility was exemplified in [1] for SiO2, ZnS, ZnO and CdS piezoelectric films. Due to
field-induced inclination of the energy bands the operating source of electrons is protected by
the film and the surrounding gas atmosphere does not practically affect the emitter operation.
Fig. 10. At low fields, the electron affinity (EA) of the emitter is positive (cef >0, see the
diagram on the right), and the emission current shows powerful fluctuations (see the
strong scatter of the points in the FN plot at low currents).
The spatial uniformity of the emission is probably provided by the circumstance that
the emitting films itself, which can be readily prepared, rather uniform over the entire emitter
surface, serves a current limiting resistor.
Conclusions
This study, after carrying out many investigations, basing on examples of specimens of
a different configuration and chemical composition, has established that a stationary low
inertial (ε<10-7 s) macroscopically low-field (Ecp~105 V/cm) electron emission appeared only
in the case of samples being made of piezoelectric materials of a high degree of stoichiometry
and purity. In the case of non-piezoactive materials (glass, fused silica, sapphire, amorphous
films of AIIBVI), or the films of AIIBVI produced, from data of X ray diffraction analysis, in a
sphalerite modification at a normal to support orientation a non-piezoactive axis of the fourth
order, no emission was registered for even middle fields exceeding 106 V/cm. In the case of
non-piezoactive diamond - like films with a high content of donor defects, although emission
is registering for Emiddle~105 V/cm, however it differs by a big (~10-3 s) time lag and a high
level of flicker – noise [14]. From the results above-mentioned it is seen that for emergence of
the insertion less emission not absolute sizes of samples are essential, rather a ratio of their
longitudinal sizes to a thickness. Strong electric fields (>107 V/cm) appear for a high (~100)
ratio of these sizes near edges of the samples fabricated from piezoelectric materials, and in
the near surface region for a low (ε~10) dielectric constant, electron fields are enough to
develop Zener effect. The energetic scheme of emission corresponding to this case explains
insensitivity of emission to vacuum condition: a source of electrons - a valence band - is
hidden from an external medium.
In addition, methods for preparation of films used in this study - after oxidation of
sulfides as well as the method of evaporation in a quasi-closed volume - give the possibility to
produce highly effective film cathode luminophors with a controllable chromacity.
The 100-element matrix display that has been made through these methods, for a high
homogeneity of emission had a brightness of cathode luminescence exceeding 300 Cd/m2.
High (~10 A/cm2) local densities of currents from cathodes of piezoelectric materials give the
176
possibility for making on their basis also other articles of vacuum microelectronics such as
powerful miniature reception - amplifying and generator valves, cathodes of electron
microscopes, etc. Thus, the results of this study make it possible to deepen understanding of
the mechanism of a mysterious phenomenon of quick response low field emission and can be
used for development of various articles of electrovacuum technique based on prospective
piezoelectric.
Acknowledgement
This work has been supported by the Ministry of Ukraine for Education and Science
(Project N5.1.04453).
References
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Introduction
Introduction
ZnS
CdS
CdS
Acknowledgement
References
|
| id | nasplib_isofts_kiev_ua-123456789-126364 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | XXXX-0106 |
| language | English |
| last_indexed | 2025-12-07T16:27:20Z |
| publishDate | 2002 |
| publisher | Інститут хімії поверхні ім. О.О. Чуйка НАН України |
| record_format | dspace |
| spelling | Dadykin, A.A. Naumovets, A.G. Gorbik, P.P. Dubrovin, I.V. Ogenko, V.M. Filonenko, M.N. 2017-11-20T18:51:26Z 2017-11-20T18:51:26Z 2002 Low-field electron emission and cathode luminescence of piezoelectric films of oxides and chalcogenides / A.A. Dadykin, A.G. Naumovets, P.P. Gorbik, I.V. Dubrovin, V.M. Ogenko, M.N. Filonenko // Поверхность. — 2002. — Вип. 7-8. — С. 163-176. — Бібліогр.: 14 назв. — англ. XXXX-0106 https://nasplib.isofts.kiev.ua/handle/123456789/126364 Basic parameters of effective low-field emitters of electrons of piezoelectric films for bright flat cathode luminescent display and other activity are optimised. Specimens of electron emitters of cathode luminescent screens are based on compounds of SiO₂, ZnO, ZnS, solid solution of Zn1-xCdxS. High efficient "cold" emitters of electrons were prepared on the basis of mono- and polycrystalline films. The emission and cathode-luminescent properties of the films were investigated. It is shown that developed films are perspective like the flat displays with the brightness of 300 Cd/m². This work has been supported by the Ministry of Ukraine for Education and Science (Project N5.1.04453). en Інститут хімії поверхні ім. О.О. Чуйка НАН України Поверхность Surface properties of inorganic materials Low-field electron emission and cathode luminescence of piezoelectric films of oxides and chalcogenides Article published earlier |
| spellingShingle | Low-field electron emission and cathode luminescence of piezoelectric films of oxides and chalcogenides Dadykin, A.A. Naumovets, A.G. Gorbik, P.P. Dubrovin, I.V. Ogenko, V.M. Filonenko, M.N. Surface properties of inorganic materials |
| title | Low-field electron emission and cathode luminescence of piezoelectric films of oxides and chalcogenides |
| title_full | Low-field electron emission and cathode luminescence of piezoelectric films of oxides and chalcogenides |
| title_fullStr | Low-field electron emission and cathode luminescence of piezoelectric films of oxides and chalcogenides |
| title_full_unstemmed | Low-field electron emission and cathode luminescence of piezoelectric films of oxides and chalcogenides |
| title_short | Low-field electron emission and cathode luminescence of piezoelectric films of oxides and chalcogenides |
| title_sort | low-field electron emission and cathode luminescence of piezoelectric films of oxides and chalcogenides |
| topic | Surface properties of inorganic materials |
| topic_facet | Surface properties of inorganic materials |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/126364 |
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