Thin films of ZnO:M synthesized by ultrasonic spray pyrolysis

Article abstract of DOI:10.1134/S0036023611100056

High optical quality ZnO:M@Si nanocomposites (where M is the doping element) were obtained by ultrasonic spray pyrolysis. The variation of experimental conditions, the use of various precursors and dopants demonstrated that the morphology of zinc oxide nanoparticles is mainly determined by the sort of the doping element. The luminescence spectra confirm indirectly the isomorphous incorporation of the dopant ions into the zinc oxide lattice.

Full text of DOI:10.1134/S0036023611100056

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2011, Vol. 56, No. 10, pp. 1509–1516. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © L.N. Demyanets, V.V. Kireev, L.E. Li, V.V. Artemov, 2011, published in Zhurnal Neorganicheskoi Khimii, 2011, Vol. 56, No. 10, pp. 1587–1594.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Thin Films of ZnO:M Synthesized by Ultrasonic Spray Pyrolysis
†
L. N. Demyanets , V. V. Kireev, L. E. Li, and V. V. Artemov
Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskii pr. 59, Moscow, 117333 Russia
Received May 26, 2010
Abstract—High optical quality ZnO:M@Si nanocomposites (where M is the doping element) were obtained
by ultrasonic spray pyrolysis. The variation of experimental conditions, the use of various precursors and
dopants demonstrated that the morphology of zinc oxide nanoparticles is mainly determined by the sort of
the doping element. The luminescence spectra confirm indirectly the isomorphous incorporation of the
dopant ions into the zinc oxide lattice.
DOI: 10.1134/S0036023611100056
†
Zinc oxide is a wellꢀknown wide bandgap semiconꢀ
ductor having a unique set of physical properties,
which determine its practical value for many fields of
technology. Particular attention is attracted by nanocꢀ
rystalline zinc oxide as a material of modern nanoꢀ
technology in the fields of photonics, spintronics, laser
physics, and others [1–4]. The scope of applications of
smallꢀsized ZnO is markedly extended by using isoꢀ or
heteroisomorphous substitution in the oxide structure.
EXPERIMENTAL
Synthesis. ZnO films were prepared by crystallizaꢀ
tion of ZnO upon thermal decomposition of the aeroꢀ
sol obtained by ultrasonic dispersion of a precursor
solution (ultrasonic spray pyrolysis, USP).
The ultrasonic aerosol generator produces an aeroꢀ
sol consisting of ultradisperse drops of several
micrometer size above the solution surface. Fast heatꢀ
ing of the drops induces evaporation of the solvent,
heating and decomposition of the precursor, and
chemical reaction between the starting compounds
+
2+
The
М
(Li, H), M (Be, Mg, Fe, Co, Ni, Mn, etc.),
3
+
5+
M (In, Ga,Cr, Ln, Al, etc.), M (Sb, P, N, V), and
6+
M ions (S) ions have been admixed to zinc oxide [5–
8]. The incorporation of dopant ions into the lattice
(
Fig. 1). It is very important that in this case, homogꢀ
1
enization occurs at the molecular level, because a true
solution is atomized. Thus, using precursors of a comꢀ
plex composition, it is possible to obtain doped partiꢀ
cles.
changes the band gap width, gives rise to new physical
characteristics (magnetic, piezoelectric, optical, specꢀ
troscopic, and chemical characteristics). The dopants
function as either donors or acceptors. The key substiꢀ
tution dopants that function as donors include the
An experimental setup was designed for sputtering
of zinc oxide crystallites. The schematic view of the
setup is presented in Fig. 2. A Liiot humidifier (Korea)
was used as the ultrasonic aerosol generator. The sputꢀ
tering chamber was a 1.5 mꢀlong quartz tube heated
with a tube furnace at the bottom. Owing to the suffiꢀ
cient height of the tube and high temperature in the
sputtering chamber, a convective flow is formed,
which ensures the vertical migration of the aerosol
mixture. The substrates are placed in the tube on a
heatꢀresistant alloy wire. This setup allows one to perꢀ
form deposition on numerous substrates in one experꢀ
iment; for each substrate, the path lengths of the aeroꢀ
sol drops and nascent particles (the distance from the
3+
Group III elements M , which occupy cationic sites,
Group VII elements (F, C1), which occupy anionic
sites of the crystal lattice, and
duction of these ions into the zinc oxide lattice
increases the electrical conductivity. For the formation
of acceptors in the compounds A B , it is necessary to
replace a Group II element by a Group I cation and to
replace a Group VI element by a Group V anion. Such
a doping suppresses the
actually observed upon doping of zinc oxide with Sb,
P, N giving rise to ꢀtype conductivity.
+
М
(Li, H). The introꢀ
II VI
nꢀtype conductivity; this is
р
Nanocrystalline ZnO doped with various elements
is prepared using the same set of methods as are used
to obtain a nominally pure material, in particular,
crystallization methods from the liquid, gas, or solid
phase [3, 19–25].
generator nozzle to the substrate, L) and temperature
are known. Since the temperature is changed along the
height of the sputtering chamber, the dependence of
the substrate temperature on the substrate position in
The purpose of this work was to obtain Inꢀ, Gaꢀ, the chamber (and, hence, on
and Pꢀdoped ZnO nanocrystals and films in order to ent rated temperatures measured by the control therꢀ
elucidate the effect of dopants on the morphology and mocouple. The results of this study for some values
are presented in Fig. 3. Using this plot, it is possible to
L) was studied at differꢀ
T
c
physical properties of ZnO:M.
estimate the real temperature of each substrate in the
experiment.
†
Deceased.
1509

1510
DEMYANETS et al.
Crystalline
material
Crystallization
and grain growth
1
Amorphous precursor
Decomposition
and chemical
reaction
2
3
Reactant and
product mixture
Heating
Reactant
mixture
Solvent
evaporation
4
5
Aerosol
drop
Fig. 1. Scheme of crystallite formation by USP.
Experimental conditions. The experiments were
6
7
carried out with the following variable parameters:
zinc acetate, formate, or nitrate as the precursor; preꢀ
cursor concentration, 0.02–0.05 mol/L; Si(111),
Si(100) as the substrate; Tc (measured by the control
thermocouple), 550–770
determined from the plot in Fig. 3), 200–665
tering time, 8–28 h; nozzle–substrate distance
5 cm; doping additives, Ga, In, P, Li. The atomic
ratio of zinc to the doping element Zn : M was 25 : 1 to
0 : 1.
A solution of zinc acetate was prepared by dissolvꢀ
ing Zn(CH COO) 2H O in distilled water and addiꢀ
°С
; substrate temperature
; sputꢀ
= 7–
T
(
°
С
L
3
9
5
8
·
3
2
2
tion of acetic acid (5 to 10 drops per liter) to avoid
hydrolysis. A solution of zinc nitrate was prepared by
dissolving Zn(NO3)2 · 6H O in distilled water. A soluꢀ
2
tion of zinc formate was prepared by dissolving zinc
oxide in formic acid.
Zinc oxide was doped by indium or gallium by disꢀ
solving Ga(CH COO) OH or presynthesized In(OH)3
3
2
in a small amount of acetic or formic acid. For phosꢀ
phorus or phosphorus–lithium doping, solutions of
Fig. 2. Schematic diagram of the USP sputtering setup
with convective transfer of aerosol. ( ) carrier arm for the
wire, ( ) refractory wire, ( ) quartz tube, ( ) direction of
the aerosol migration, ( ) substrates, ( ) control thermoꢀ
) ultrasonic aerosol generator, (
1
ammonium dihydrogen phosphate (NH H PO4) or
4
2
2
3
4
lithium dihydrogen phosphate (LiH PO4) were preꢀ
2
5
6
pared. An appropriate amount of the solution containꢀ
ing a doping element was added to the zincꢀcontaining
solution.
couple, (
load.
7) furnace, (
8
9)
Characterization of products. The obtained prodꢀ
—
401F);
scanning electron microscopy (JEOL JSMꢀ
ucts were characterized by the following procedures:
7
—powder Xꢀray diffraction (XRDꢀ6000 Shimadzu
diffractometer, Cu radiation);
K
—optical spectroscopy.
α
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 10 2011

THIN FILMS OF ZnO:M SYNTHESIZED
1511
RESULTS AND DISCUSSION
800
A series of ZnO:M@Si(111) nanocomposites was
prepared as crystallites of zinc oxide activated by
dopant ions and supported on the substrate. The charꢀ
acter of the coating formed on the substrates by sputꢀ
tering is substantially affected by the sample position
in the furnace (sputtering chamber). This is related to
specific features of particle formation by the USP
700
1
600
500
400
2
3
4
5
(
Fig. 1). In the lower part of the sputtering chamber,
the solvent (water) has not been entirely removed and
the particles are deposited on the substrate as a soluꢀ
tion, and crystallization takes place on the surface and
does not differ from usual crystallization taking place
on evaporation. No nanoparticle film is formed in this
way. In the middle part of the sputtering chamber, both
the temperature and the residence time of a particle in
the hot area are greater, the solvent has been removed,
and the particles are at the stage of formation and have
not yet completely crystallized. The particles have an
active surface and interact with one another to give a
dense film, which usually looks like a tarnish. The
upper part of the sputtering chamber contains many
particles whose residence time in the furnace is suffiꢀ
6
300
00
2
1
0
15
20
25
, cm
30
35
L
Fig. 3. Substrate temperature
vs. distance from the substrate to the ultrasonic generator
nozzle ( ) at the nominal temperature of the experiment
Tc: ( ) 800, ( ) 750, ( ) 700, ( ) 650, ( ) 600, ( ) 550°C.
T in the sputtering chamber
L
1
2
3
4
5
6
cient for the formation of wellꢀcrystallized particles the substrate temperature was raised to 450
С, the parꢀ
°
with a passive surface. On deposition on the substrate, ticle morphology was retained; however, pyramid
they do not form a thin film but form a white bloom (or
{
1
000 } became the most developed simple form along
yellowish one in the case of indium doping). This
bloom has low adhesion to the substrate and can be
readily manually removed, whereas the film formed in
the middle area is rather dense and cannot be manuꢀ
ally removed.
The composition of the obtained films was deterꢀ
mined directly by energyꢀdispersive analysis (EDA)
and indirectly based on variation of the emission specꢀ
tra as compared with the spectra of nominally pure
zinc oxide, indicating incorporation of the dopant.
The fact that the obtained films correspond to the hexaꢀ
gonal ZnO modification, wurtzite structural type, was
confirmed by powder Xꢀray diffraction and EDA (Fig. 4).
The morphology of the crystallites constituting the
film (pyramidal, bipyramidal, prismatic, elongated,
and platy ones) is governed by the experimental
parameters. The typical morphology of the ZnO:M
crystallites is shown in Figs. 5–7. The main crystalloꢀ
graphic simple forms forming the faceting are monoꢀ
with monohedron (1011); the c axis was perpendicular
or inclined to the substrate surface (Fig. 5b), no partiꢀ
cle aggregation took place. On further increase in the
substrate temperature to 580 С, the pyramid faces disꢀ
°
appeared, 250–500 nm regular (0001) aggregates of
plate crystals are formed (Fig. 5c). Crystallization of zinc
formate solutions gives rise to a film of 200–300 nm
closeꢀpacked plate crystals and several
gates of plate crystals (Fig. 5d).
The introduction of gallium compounds to the
growth system results in the formation of crystallites
mainly shaped as truncated pyramids (Fig. 6). The
most clearꢀcut pyramidal faceting is observed when
nitrate precursors are used (Figs. 6a, 6b). The particle
size is 100–300 nm, and the (0001) surface is composed
of numerous layers with thickness of up to 10 nm, which
is indicative of twoꢀdimensional nucleation on these
surfaces. As the temperature is raised from 440 to
µm large aggreꢀ
5
10°С, the monohedron (0001) faces almost disappear
hedron {0001}, pyramid {1011}, and prism {1010}.
(the growth rate along the [0001] direction increases),
Figure 5 shows the typical morphology of indiumꢀ and simple pyramid shape {1011} develops. At 510
activated ZnO crystallites obtained using aqueous a film composed of closeꢀpacked zinc oxide particles
solutions of zinc acetate and formate as the precursors. shaped like hexagonal pyramids is obtained, the axis
°С,
c
On attempted synthesis of similar crystallites from being perpendicular to the substrate surface (Fig. 6b).
nitrate precursors, the solution grew turbid during the When galliumꢀdoped zinc oxide is synthesized from
experiments and, hence, the solution composition formate and acetate solutions, the characteristic partiꢀ
changed and the crystal formation pattern could not cle size is 1
be properly interpreted. At temperatures of ~200 formed with nitrate precursors (Figs. 6c–6f). The
dispersion of acetate solutions afforded substrateꢀ pyramidal faceting of the particles obtained from forꢀ
deposited aggregates composed of ZnO particles with mate solutions at 560 was not very clearly defined
an average size of ~200 nm. The particles had a tabular and, hence, the particles looked rounded (Fig. 6c). As
habit with feebly developed pyramid faces (Fig. 5a). As the temperature was raised to 655 , the pyramidal
µ
m, which is several times larger than that
°С,
°С
°С
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 10 2011

1512
DEMYANETS et al.
I
1
/
00
I0, %
(а)
(b)
I
1
, pulse
000
900
800
700
600
500
400
300
200
100
80
60
40
20
0
0
1
2
3
4
Е
5
, keV
6
7
8
9
10
10
20
30
40
2
50
, deg
60
70
80
θ
Fig. 4. Data of (a) energy dispersive analysis and (b) powder Xꢀray diffraction analysis.
1
2
μ
m
m
1
00 nm
μ
300 nm
(а)
(b)
5 μm
250 nm
500 nm
10 μm
(c)
(d)
Fig. 5. Photomicrographs of ZnO:In samples synthesized under different conditions. (a) Zn(CH COO) as the precursor; conꢀ
3
2
centration
Rat = 50 : 1,
precursor, = 0.05 mol/L, Ratt = 25 : 1,
c
= 0.02 mol/L; atomic ratio Rat = Zn : In = 50 : 1, = 0.02 mol/L,
T
= 200°C; (b) Zn(CH COO) as the precursor,
c
3
2
T
= 450°C; (c) Zn(CH COO) as the precursor, c = 0.02 mol/L, Rat = 25 : 1, T = 580°C; (d) Zn(HCOO) as the
3
2
2
c
T
= 565°C.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 10 2011

THIN FILMS OF ZnO:M SYNTHESIZED
1513
100 nm
100 nm
(а)
(b)
1
μ
m
1 μm
(c)
(d)
1
μ
m
1 μm
(e)
(f)
Fig. 6. Photomicrographs of ZnO:Ga samples synthesized under different conditions. (a) Zn(NO ) as the precursor,
с
T
= 0.02 mol/L,
= 510°C; (c)
3
2
atomic ratio Rat = Zn : Ga = 50 : 1,
Zn(HCOO) as the precursor, = 0.02 mol/L, Rat = 50 : 1,
= 655°C; (e) Zn(CH COO) as the precursor, = 0.02 mol/L, Rat = 25 : 1,
T
= 440°C; (b) Zn(NO ) as the precursor,
c
= 0.02 mol/L, Rat = 50 : 1,
3 2
c
T
= 560°C; (d) Zn(HCOO) as the precursor,
c
= 0.02 mol/L, Rat
3
=
2
2
2
5 : 1,
T
c
c
T
= 515°C; (f) Zn(CH COO) as the preꢀ
3
2
2
cursor,
= 0.02 mol/L, Rat = 25 : 1, T = 640°C.
faceting was clearly defined (Fig. 6d); however, in this observed in other cases, in particular, the pyramid
case, apart from large (>1
m) particles, numerous faces are complicated by additional faceting to form a
particles with sizes of hundreds nanometers were cone with a corrugated surface (Figs. 6e, 6f).
present on the substrate. The particles synthesized
Using phosphorus (and phosphorus together with
µ
from acetate solutions have a specific feature not lithium) as the doping element, samples obtained
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 10 2011

1514
DEMYANETS et al.
50 nm
200 nm
300 nm
(а)
(b)
1 μm
5
μ
m
1 μm
(c)
(d)
1
μ
m
10 μm
(e)
(f)
Fig. 7. Photomicrographs of ZnO:P samples synthesized under different conditions; atomic ratio Rat = Zn : P = 50 : 1. (a)
Zn(CH COO) as the precursor, concentration = 0.02 mol/L, = 520°C; (b) Zn(CH COO) as the precursor, = 0.02 mol/L,
= 520°C, sapphire as the substrate; (c) Zn(HCOO) as the precursor, = 575°C; (d) Zn(HCOO) as the preꢀ
c
T
c
3
2
3
2
T
c
c
= 0.05 mol/L,
= 0.02 mol/L,
T
T
2
2
2
2
cursor,
cursor,
c
c
= 0.02 mol/L,
= 0.02 mol/L,
T
T
= 515°C; (e) Zn(HCOO) as the precursor,
= 600°C; (f) Zn(HCOO) as the preꢀ
= 640°C.
from acetate and formate solutions were synthesized similar morphology were formed. The aggregate diamꢀ
and studied. On attempted preparation of nitrate preꢀ eter was ~100 nm on sapphire substrates and several
cursors, a precipitate separated from the solution; hundreds nm on silicon substrates. Particles of the
therefore, sputtering was not performed in this case. same morphology are formed in the synthesis from the
Phosphorus doping was found to give rise to polysynꢀ formate precursor and combined lithium and phosꢀ
thetic twins with (0001) intergrowth plane; this gives phorus doping (Fig. 7c). In this case, the elongated
rise to longꢀprismatic aggregates ending with a pyraꢀ aggregates often grow from the same site, while in the
mid or a positive monohedron (Fig. 7). For the synꢀ case of phosphorus doping and use of acetate solutions
thesis from acetate solutions, sputtering was carried considered previously, the aggregates are arranged
out not only on silicon (Fig. 7a) but also on sapphire randomly on the substrate. Doping with phosphorus
substrates (Fig. 7b). In both cases, crystallites with and the use of formate solutions at 515 С affords
°
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 10 2011

THIN FILMS OF ZnO:M SYNTHESIZED
1515
Figure 8 shows typical luminescence spectra of the
doped nanocomposites ZnO:M. The presence of
intense exciton luminescence attests to the high optiꢀ
cal quality of the obtained material and to the absence
of quenching centers at the dopant concentrations
used. The luminescence spectra of activated zinc
oxide show a new longꢀwavelength peak (Fig. 8). This
peak is related to recombination of excitons localized
on the dopant centers, as noted in a previous publicaꢀ
tion [26]. The appearance of this band is indirect eviꢀ
dence for the isomorphous incorporation of dopant
ions into the zinc oxide lattice.
This is of the greatest interest for the case of zinc
oxide activation by phosphorus ions, as the activation
with phosphorus forms ZnO material with pꢀtype conꢀ
ductivity, which opens up broad prospects for the
design of light emitting semiconductor devices based
on this composite.
Thus, ZnO:M@Si nanocomposites (M is a doping
element, In, Ga, or P) of high optical quality were
produced by ultrasonic spray pyrolysis of aqueous
solutions of the precursors. The morphology of ZnO
crystallites formed on Si(111) and the appearance of
the film are determined by the type of the precursor,
the presence of doping additives, the substrate temperꢀ
ature, and the distance from the substrate to the ultraꢀ
sonic generator nozzle. The EDA and luminescence
spectral data confirm the incorporation of the dopant
ions into the zinc oxide lattice.
(
а)
5
4
3
2
1
00
00
00
00
00
0
380
400
(b)
420
440
3000
2000
1000
0
380
400
c)
420
(
400
300
200
100
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