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«MODERN SCIENCE АND RESEARCH»
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996
EFFECTS OF THERMAL ANNEALING AND PLASMA TREATMENT ON THE
PROPERTIES OF ZNO
M. Rasulova
S. Zaynobidinov
Sh. Ermatov
L. Abdulfayizov
Andijan State University.
https://doi.org/10.5281/zenodo.12589131
Abstract.
This work presents a comparative study of the influence of thermal annealing
and plasma treatment in hydrogen plasma on the properties of polycrystalline thin films of ZnO
obtained by metalorganic chemical vapor deposition and obtained by the hydrothermal method
Key words:
Sol-gel method, hydrodynamic conditions, surfactant, homogeneous, thermal
annealing, plasma treatment.
ВЛИЯНИЕ ТЕРМИЧЕСКОГО ОТЖИГА И ПЛАЗМЕННОЙ ОБРАБОТКИ НА
СВОЙСТВА ZNO
Аннотация.
В работе представлено сравнительное исследование влияния
термического отжига и плазменной обработки в водородной плазме на свойства тонких
поликристаллических пленок ZnO, полученных методом металлоорганического
химического осаждения из газовой фазы и полученных гидротермальным методом.
Ключевые слова:
золь-гель метод, гидродинамические условия, ПАВ, гомогенный,
термический отжиг, плазменная обработка.
Sol-gel technology is one of the common methods for producing ultrafine particles. Using
this method, it is possible to obtain nanoparticles, porous structures with ordered and disordered
pores, nanocoatings, fiber and monolithic structures, etc. The sol-gel method consists of two
stages. At the first stage, a sol is synthesized, and then the resulting sol is converted into a gel [1].
Changing the pH of the reaction medium, varying the hydrodynamic conditions in which
the sol is formed, controlling the viscosity of the reaction medium by selecting a solvent, choosing
a surfactant, the composition of the reaction medium are the parameters on which the morphology
of the resulting ultrafine zinc oxide particles depends. Works [2-3] show the influence of the pH
of the reaction medium on the morphology of ultrafine zinc oxide particles. It was found that at a
pH of 9, particles with sizes of 14 and 25 nm were obtained, and a further increase in pH leads to
a decrease in the size of crystallites. In order to control the process of sol formation during
synthesis, various structure-forming agents are used in addition to the sol former. In work [4],
citric acid is used as a sol former, and ethylene glycol, polyethylene glycol and polyvinyl alcohol
are used as structure-forming agents. It has been shown [5] that the morphology of zinc oxide
nanoparticles is influenced not only by the nature of the structure-forming agent used, but also by
its molar ratio with the sol former [5]. The advantages of the sol-gel method are: control of the
structure of the final product at the stage of gel formation; elimination of numerous stages of
washing sediments from interfering ions; obtaining a homogeneous product at the molecular level;
high purity of the finished product. The disadvantages of the method are the production of
agglomerated particles of zinc oxides and the need to use reagents based on salts of organic acids,
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2024
International scientific journal
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VOLUME 3 / ISSUE 6 / UIF:8.2 / MODERNSCIENCE.UZ
997
which are produced in limited quantities on the world market, which is unprofitable from an
economic point of view. Recently, new promising directions for the synthesis of zinc oxide have
been developed, based on the thermal decomposition of organic and inorganic salts. The main
advantage of this production method is the possibility of synthesizing predominantly pure zinc
oxide, and adjusting the parameters of the thermolysis process allows the formation of powders
with a given dispersion and morphology. Salts are used as starting reagents in thermal
decomposition processes, the anions of which can be easily removed during heat treatment:
sulfates, carbonates, acetates, oxalates, nitrates, etc. However, the method of producing zinc oxide
by thermolysis of inorganic compounds also has significant disadvantages, the main of which is
the need recycling of gaseous products and the use of technological equipment made of corrosion-
resistant materials. Therefore, it is advisable to use zinc carbonates, hydroxides or basic carbonates
as starting compounds, which can be synthesized by the hydrothermal method.
Next, we will conduct comparative studies of the influence of thermal annealing and
plasma treatment in hydrogen plasma on the properties of polycrystalline thin films of ZnO
obtained by metal-organic chemical vapor deposition (MOCVD - ZnO) and obtained by the
hydrothermal method (HT - ZnO). Two series of boron-doped ZnO films, n-type, were studied: I)
films on glass substrates, synthesized by the MOCVD method at a temperature of ~ 200
o
C, II)
films on glass and crystalline silicon substrates, which were synthesized by the hydrothermal
method (HT - ZnO) in real work. To obtain NT–ZnO samples, a thin ZnO seed layer was initially
grown on a substrate by the sol–gel method. To prepare the seed layer, 0.4 g of zinc acetate
Zn(CH
3
COO)
2
·2H
2
O was dissolved in 10 ml of ethanol for 30 min at room temperature using a
magnetic stirrer to obtain a homogeneous and transparent sol. Uniform distribution of the sol on
the surface of the substrate was achieved by applying several drops to the substrate mounted on a
horizontal table, followed by rotating the table at a speed of about 2000 rpm for 5 minutes. Final
annealing at 450°C for 60 minutes in a muffle furnace resulted in the formation of a uniform seed
layer. With continuous stirring, prepare an equimolar (0.3M) aqueous solution of zinc nitrate
hexahydrate Zn(NO
3
)
2
6H
2
O and hexamethylenetetramine (CH
2
)
6
N
4
. Hydrothermal synthesis was
carried out in a fluoroplastic beaker for 2 hours with stirring at 90°C. The samples were placed
vertically, with the seed layer towards the wall of the glass. To obtain n-type ZnO films doped
with boron, boric acid H
3
BO
3
was added to the solution at a concentration of 0.6 at.% to 3 at.%.
Aluminum nitrate Al(NO
3
)
3
9H
2
O or aluminum chloride AlCl
3
6H
2
O from 0.6 at. % up to
3 atm. % was added to the solution to obtain n-type ZnO films doped with aluminum. After growth,
the resulting samples were removed from the vessel, washed with distilled water in an ultrasonic
bath for 10 minutes to remove residual particles adhering to the surface, and dried in an oven at
100 - 110
o
C for 5 minutes. The typical thickness of HT – Zn films doped with boron or aluminum
after synthesis for 2 hours was 0.8 – 1 µm, provided that the concentration of impurities in the
solution was less than 1 at. %. Increasing the concentration of impurities in the solution led to
slower growth. The samples were annealed in a quartz tube furnace. Annealing at atmospheric
pressure was carried out in air or in a nitrogen flow with a purity of 99.999%. Annealing in a
vacuum of ~ 2·10 – 2 mbar was carried out during pumping with an oil rotary pump. Annealing in
an oil-free vacuum of ~ 1·10 – 5 mbar was carried out in an alundum furnace using a
turbomolecular pump. Plasma treatment was carried out in a quartz reactor in a hydrogen
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2024
International scientific journal
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VOLUME 3 / ISSUE 6 / UIF:8.2 / MODERNSCIENCE.UZ
998
atmosphere at a pressure of 0.5 mbar with an RF power of 30 Wt (27.12 MHz). Measurements
showed that during H – treatment for 5 minutes, the temperature of the samples increased to 50 –
70°C due to microwave absorption. The morphology of the samples was examined by scanning
electron microscopy using a Quanta 200I 3D microscope (FEI). The crystal structure was studied
using an MPD X-ray diffractometer. Optical transmission spectra in the range 190 – 1100 nm were
measured with a Lambda spectrophotometer. Photoluminescence (PL) spectra were studied with
a spectrophotometer in the wavelength range 300 – 850 nm when excited with a xenon flash lamp
through a bandpass filter FF01 – 292/27 – 25 Semrock). An additional edge filter BLP01 – 325R
– 25 (Semrock) was used to cut off scattered radiation from the xenon lamp. The carrier
concentration, mobility, and resistivity of the films were measured at room temperature using the
van der Pauw four-probe method using an HMS-3000 setup (Ecopia) with a 0.56 T magnet. Figure
1 shows the XRD patterns of HT–ZnO and MOCVD–ZnO thin films, indicating that all ZnO
samples exhibit a hexagonal wurtzite structure. The HT–ZnO samples are polycrystalline with a
preferred (002) orientation, and the MOCVD–ZnO sample is also polycrystalline with a preferred
(110) orientation. Figure 2 shows carrier concentration and mobility in MOCVD–ZnO films
annealed for 40 minutes in air, nitrogen, and oil vacuum. Typical room temperature parameters of
MOCVD – ZnO films with a thickness of 1.7 μm are the following: free electron concentration ~
5·10
19
cm
–3
, carrier mobility 22 – 24 cm
2
/(V s), resistivity ~ 0.005 Om cm. The deviation of these
parameters in batches of samples cut from a 100 mm·100 mm glass substrate did not exceed 10%.
The electrical parameters of the film did not change after annealing in air at temperatures below
200ºC. At higher annealing temperatures, the carrier concentration gradually decreases, reaching
~ 7 × 10
17
cm–3 after annealing at 550°C. Likewise, carrier mobility decreases to 0.2 – 0.4 cm
2
/(V
s). It can be concluded that strong scattering occurs at grain boundaries. In addition, carriers are
captured by oxygen at grain boundaries, and the concentration of free electrons in the bulk of the
ZnO film decreases. This explains the decrease in carrier mobility and concentration.
Figure 1 – XRD – samples of HT – ZnO samples doped with a sample of boron or
aluminum and MOCVD – ZnO (B). The line diagram at the bottom of the figure shows the
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JCPDS #01 map for ZnO with a hexagonal wurtzite-type structure
Figure 2 – carrier concentration and mobility in MOCVD – ZnO films depending on
the annealing temperature in air (1), in a nitrogen atmosphere (2) and in an oil vacuum (3)
On the contrary, the temperature dependences of carrier concentration and mobility during
annealing of MOCVD – ZnO films in vacuum and in nitrogen are significantly different (Figure
48). The mobility increases from 22 – 24 cm
2
/(V s) to 32 – 36 cm
2
/(V s) at annealing temperatures
of 150 – 160°C, the carrier concentration also increases slightly. At higher annealing temperatures,
the carrier concentration decreases and reaches 4.8 × 10
18
cm
–3
upon annealing at 350–550°C,
while the carrier mobility is restored to the initial value. This behavior may be due to the fact that
annealing in a vacuum and in an inert atmosphere leads to an oxygen deficiency in the surface
regions. This promotes the formation of oxygen vacancies and interstitial zinc atoms in ZnO.
Interstitial Zn atoms, which have high mobility and low transport barrier energy, can
diffuse into the sample, so they may be responsible for the observed change in electrical properties.
The decrease in electron concentration during annealing may be due to the compensation
of boron donors by interstitial zinc atoms. Figure 49a shows the absorption spectra of ZnO samples
annealed in oil vacuum. The corresponding energy positions of the Fermi level, calculated from
the carrier concentration in these samples, are shown in Figure 3 b as a function of the annealing
temperature. It can be seen that at low annealing temperatures (100 and 150
o
C) absorption
increases with increasing photon energies, and at higher annealing temperatures the absorption
edge shifts towards low photon energies. These shifts are accompanied by a shift in the position
of the Fermi level towards the conduction band, starting with annealing above 150°C, and the
energy Eg, corresponding to optical absorption 5·10
5
cm–1, decreases to 3.31 eV. This can be
explained by the formation of oxygen vacancies with states near the bottom of the band gap, and
the main contribution to the shift of the optical absorption edge during annealing is associated with
the movement of the Fermi level.
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Figure 3 - (a) Absorption spectra of MOCVD samples - ZnO: initial sample (1) and
after annealing in vacuum for 40 minutes at a temperature of 100 (curve 2), 150 (3), 200 (4),
250 (5), 370 (6), 350 (7) and 410oC (8); (b) Dependence of the Fermi level (EF – Ec) on the
annealing temperature in ZnO samples during annealing in vacuum (1) and nitrogen (3),
calculated from the measured Hall and energy Eg, which corresponds to an optical
absorption of 5·105 cm–1, at annealing in oil vacuum (2) and nitrogen (4)
Figure 4 shows the effect of annealing on the electrical parameters of two sets of ZnO
samples. The first 12 samples were subjected to preliminary annealing in air (curve 1) at 200 –
550
o
C. Then they were annealed in an oil vacuum for 1 hour at 500ºC (curve 2). It can be seen that
after annealing, all samples, including the control sample not annealed in air, have approximately
the same parameters: mobility ~ 23 cm
2
/(V s), resistivity 0.044 Ohm cm, carrier density about 6
10
18
cm
–3
. Thus, annealing in an oil vacuum eliminates the effect of pre-annealing in air. The
second group of 5 samples was annealed in air at 200 – 550
o
C (the electrical parameters of the
samples were the same as for the first group), and then annealed in a high vacuum without oil for
1 hour at 500
o
C (Figure 4, curve 3). It can be seen that the effect of annealing in an oil-free vacuum
in terms of the electrical parameters of the samples differs significantly from annealing in vacuum
oil. Then the samples were treated in hydrogen plasma (Figure 4, curve 4), and their electrical
parameters returned to the same values as after annealing in an oil vacuum.
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Figure 4 – Carrier concentration (a), mobility (b) and resistivity (c) in MOCVD – ZnO
films, annealed for 40 minutes in air (1) and in vacuum at 2·10
– 2
mbar, 1 hour at 500
o
C (2),
ZnO films annealed in air for 30 minutes and in an oil-free vacuum of 1·10
–5
mbar for 1 hour
at 500
o
C (3), followed by treatment in hydrogen plasma for 4 minutes at room temperature
(4)
In both groups, there is an oxygen deficiency on the ZnO surface. However, during oil-free
vacuum annealing, excess zinc atoms may evaporate from the surface, and the formation of
nonequilibrium interstitial zinc atoms in the surface region becomes difficult. During annealing in
an oil vacuum, oil molecules can be absorbed by the catalytically active zinc oxide surface and
initiate the carbothermic reduction of zinc oxide. It was previously shown that the exchange of
oxygen with the gas phase affects the surface states of ZnO and plays an important role in its
electrical conductivity. Thus, in the case of annealing in an oil vacuum, the reduction of ZnO on
the surface by organic molecules creates an excess of zinc atoms, which can diffuse into the bulk
of the film and recombine with zinc vacancies created under oxygen-enriched conditions.
Probably, the carrier concentration (4–8)·10
18
cm
–3
, achieved by annealing, corresponds to
the equilibrium established by the diffusion of zinc from the surface into the bulk. The typical
resistivity of the synthesized hydrothermal samples was about 50 Om cm, so the behavior of the
electrical parameters during annealing in air was not studied. Figure 5 shows the change in
electrical parameters of HT – ZnO samples doped with boron and aluminum during annealing in
vacuum. Films typically show a carrier concentration of about 2 × 10
18
cm
–3
, a mobility of 25 – 27
cm
2
/V s, and a resistivity of 0.1 Ω cm after annealing at 450°C for 1 hour. It should be noted that
undoped HT–ZnO samples also showed similar mobility and resistivity behavior during vacuum
annealing, but the low resistivity value after annealing under oxygen deficiency conditions was
unstable. The resistance of samples that were stored at room temperature in air for 1 - 2 weeks
increased significantly due to exposure to oxygen. In contrast, the electrical parameters of HT –
ZnO samples doped with B or Al were stable after annealing (Figure 5). It can be concluded that
annealing of HT – ZnO leads to activation of the electrical activity of impurity atoms of boron and
aluminum impurities.
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Figure 5 – Carrier mobility and resistivity in HT – ZnO films doped with B (1) and
Al (2) as a function of annealing temperature in an oil vacuum
Figure 6 shows the PL spectra of HT – ZnO samples annealed in vacuum. The intensity of
the band near the absorption edge (NBE) increases after annealing at temperatures of 250 – 400°C
and decreases after annealing above 400°C, as in MOCVD – ZnO samples. The dependence of the
intensity of the emission band through deep levels (DLE) is non-monotonic upon annealing at 250
– 400°C and decreases upon annealing above 400°C. The intensity first decreases and then
increases slightly, and the peak of the band shifts to longer wavelengths. Note that an increase in
DLE intensity upon annealing is typical for ZnO samples grown at low temperatures (<200°C).
Hydrogen is known to readily diffuse into ZnO during low temperature plasma treatment
(eg, 30 minutes hydrogen plasma treatment at 100oC gives a penetration depth of about 10
microns) and effectively passivates electrically active defects. Hydrogen plasma treatment has a
strong effect on the electrical properties of ZnO samples grown by MOCVD and annealed in air.
As can be seen from Figure 5, hydrogen treatment completely eliminates scattering at grain
boundaries, that is, H – treatment eliminates the effect of oxygen
adsorption
.
Figure 6 – PL spectra of initial samples of NT – ZnO films doped with boron (a) and
aluminum (b), and samples of NT – ZnO annealed in air. Also shown are the spectra of
samples after annealing at 450°C followed by plasma treatment in hydrogen; the intensity of
these spectra must be multiplied by 50 for comparison
Hydrogen treatment was also carried out for ZnO films with a thickness of ~2 microns
grown by the hydrothermal method. After a short (4 – 5 minutes) hydrogen plasma treatment at
room temperature, the carrier concentration reached 5.5 10
18
cm
–3
, the mobility increased to ~ 30
cm2/V s, and the resistivity was in the range of 0.035 – 0.040 Om cm. We investigated the effect
of hydrogen plasma treatment on the photoluminescence of samples grown by MOCVD and
hydrothermal methods. The PL spectra of ZnO samples at room temperature before treatment
consisted of a narrow (~0.25 eV) NBE peak at ~380 nm and a broad deep level emission (DLE)
band ranging from 450 to 800 nm with a maximum around 500 nm. Typical PL spectra of
synthesized ZnO samples, as well as samples annealed at 400°C and treated in hydrogen plasma,
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are shown in Figure 7, along with the dependence of the NBE band intensity on the annealing
temperature shown in the inset in Figure 7.
Figure 7 – PL spectra of samples HT – ZnO (a) and MOCVD – ZnO (b): 1 – initial
spectra, 2 – after annealing in air at 400
o
C for 30 min and 3 – after treatment in hydrogen
plasma at room temperature for 4 min. The inset shows the dependence of the intensity of
the NBE band on T
revers
in samples annealed in air for 30 minutes at the appropriate
annealing temperature and after treating these samples for 4 minutes in hydrogen plasma
It can be seen that at higher annealing temperatures the NBE intensity increases, passes
through a maximum at an annealing temperature of about 300oC, and begins to decrease. At the
same time, the DLE intensity begins to increase above 200 – 300
o
C. This is consistent with the
results of the work. Treatment with hydrogen plasma increases the intensity of NBE and leads to
the disappearance of the DLE band. Passivation of the DLE strip occurs at room temperature and
in a short time. This effect has been noted in a number of studies. The average increase in NBE
band intensity after hydrogen plasma treatment was 25 – 35 times in hydrothermal and MOCVD
grown samples. The effect of hydrogen plasma treatment was found to depend on the pre-annealing
temperature in air, and the intensity of the NBE band increased in pre-annealed samples. The inset
in Figure 7 shows that although the intensity of the NBE band changes slightly with increasing
annealing temperature, it increases very strongly after hydrogen treatment in both HT–ZnO and
MOCVD–ZnO samples. If the PL intensity increases after treatment with hydrogen 50–60 times
in the initial samples, then for samples after preliminary annealing the PL intensity increases after
hydrogen treatment by three orders of magnitude. Pre-annealing in air has little effect on the
intensity of the NBE band at annealing temperatures below 200
o
C. At higher annealing
temperatures, the PL intensity increases, reaching a maximum at 400 – 550
o
C. In the case of
annealing in vacuum and nitrogen, the dependences of the PL intensity are the same and differ
from annealing in air. Preliminary annealing already at 100 – 160
o
C has a significant effect on the
dependences. The maximum effect of preliminary annealing in vacuum and nitrogen on the
intensity of the NBE band is observed at ~ 250
o
C, while above 350
o
C the PL intensity changes
slightly.
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Conclusion
Thus, in this work, the electrical properties (concentration and mobility of carriers,
resistivity), optical absorption spectra and photoluminescence of polycrystalline ZnO films were
studied depending on thermal and plasma treatment. It has been shown that annealing of MOCVD
– ZnO films in an oxygen atmosphere leads to a decrease in the concentration of free carriers due
to oxygen chemisorption and the formation of depletion layers at grain boundaries. Carrier
mobility decreases due to scattering at charged grain boundaries.
The electrical properties of ZnO samples can be significantly restored by subsequent
annealing, and the result of recovery annealing depends on the gas atmosphere during annealing.
Annealing in a nitrogen atmosphere, as well as in an oil vacuum of 2·10
–2
mbar, makes it
possible to completely restore carrier mobility, and the carrier concentration stabilizes at the level
of (6 – 8)·10
18
cm
–3
. On the contrary, annealing in an oil-free vacuum of 1·10
–5
mbar improves the
electrical properties only slightly. Short-term treatment in hydrogen plasma dramatically improves
the electrical and photoluminescent properties, and the effect of plasma treatment on the PL
intensity depends significantly on the pretreatment of MOCVD–ZnO and HT–ZnO films.
Preliminary annealing in vacuum, as well as in an inert atmosphere, leads to a slight
increase in the efficiency of hydrogen treatment, however, annealing in an oxidizing atmosphere
at 400 - 500
o
C followed by treatment in hydrogen plasma leads to the highest NBE intensity. A
correlation was discovered between the Fermi level and the shift of the optical absorption edge
during thermal annealing [6].
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