Physicochemical processes involved in synthesis of thin films based on double oxides of the ZrO2-GeO2 system

Article abstract of DOI:10.1134/S0036023611060064

The major physicochemical processes underlying the preparation of thin films of the ZrO-GeO₂ system from film-forming solutions based on zirconium oxochloride (ZrOCl₂ · 8H₂O), germanium tetrachlo-ride (GeCl₄), and ethanol were studied. The phase composition, structure, and physicochemical properties of the films were determined. Pleiades Publishing, Ltd., 2011.

Full text of DOI:10.1134/S0036023611060064

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2011, Vol. 56, No. 6, pp. 835–840. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © L.P. Borilo, L.N. Borilo, 2011, published in Zhurnal Neorganicheskoi Khimii, 2011, Vol. 56, No. 6, pp. 888–893
.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Physicochemical Processes Involved in Synthesis of Thin Films Based
on Double Oxides of the ZrO2–GeO2 System
L. P. Borilo and L. N. Borilo
Tomsk State University, Tomsk, Russia
Received August 18, 2009
Abstract—The major physicochemical processes underlying the preparation of thin films of the ZrO–GeO2
system from filmꢀforming solutions based on zirconium oxochloride (ZrOCl2 · 8H2O), germanium tetrachloꢀ
ride (GeCl4), and ethanol were studied. The phase composition, structure, and physicochemical properties
of the films were determined.
DOI: 10.1134/S0036023611060064
In recent years there was a rise in interest to the in air in two steps: in a drier at 60–80°С for 30 min and
sol–gel manufacture of oxide materials. This was due in a muffle at 500–900°С for 1 h.
to the fact that sol–gel technology, which is a synthetic
The filmꢀforming capacities of solutions were evalꢀ
method involving chemical condensation in a liquid
uated by measuring their viscosities on a glass viscomꢀ
phase, is regarded as the most efficient and simplest
eter (VPZhꢀ2, capillary diameter of 0.99 mm, 25°С).
For studying the physicochemical processes involved
in formation of ZrO2, GeO2, and double oxides of the
ZrO2–GeO2 system in thinꢀfilm and disperse states
method for manufacturing nanoparticles [1, 2]. This
method can provide not only dispersed powders but
also thin films based on complex chemical systems
having layer thicknesses of 10 to 200 nm and multiꢀ
from filmꢀforming solutions, we used a set of the folꢀ
layer films having thicknesses up to 1 m [3]. The most
μ
lowing complementary methods: thermal analysis,
IR spectroscopy, and powder Xꢀray diffraction. Therꢀ
mal analysis for the precursors and powders of dried,
hydrolyzed filmꢀforming solutions was performed on a
widely used thinꢀfilm oxide systems are materials
based on Group IV elements, specifically, zirconium
oxides doped by oxides of other elements, allowing on
the one hand, wide and efficient control of the phase
composition and thickness of films and, on the other,
stabilization of the properties of the resulting materials
[4, 5]. In this context, topical is the manufacture of a
phase based on double oxides of the ZrO2–GeO2 sysꢀ
tem in a thinꢀfilm state.
Qꢀ1500 derivatograph (25 to 900°C, calcined
reference, air atmosphere).
α
ꢀAl2O3
IR spectra were studied for films on silicon subꢀ
strates annealed at various temperatures and recorded
in the range 400–4000 cm–1 on a PerkinꢀElmer Specꢀ
trum One spectrophotometer. The phase composiꢀ
tions of films were determined on a DRONꢀ3M difꢀ
fractometer using the characteristic copper anode
Here, we study the physicochemical processes
involved in manufacturing films of double oxides of
the ZrO2–GeO2 system by sol–gel technology from
filmꢀforming solutions and the phase composition and
optical properties of the resulting films.
radiation Cu
K
(
λ
= 1.5418 nm).
α
The thicknesses and refractive indices of films were
studied on an LEFꢀ3M laser ellipsometer ( = 632.8 nm)
λ
at five spots over the entire surface of a film for each
sample; optical parameters were calculated from the
model of a uniform nonꢀabsorbing layer on an isotroꢀ
pic substrate [6]. Film adhesion to substrates was meaꢀ
sured on a PMTꢀ3 microhardness tester. Reflection
and transmission spectra in the visible and UV were
recorded on an SFꢀ20 spectrophotometer. Dielectric
constants were calculated after Kramers–Kroning
from the reflection and transmission spectra; the
bandgap width was calculated from the absorption
edge position. Electrophysical properties of films were
EXPERIMENTAL
Thin films were manufactured by sol–gel technolꢀ
ogy from filmꢀforming solutions that were prepared
based on ethanol (96 wt %), germanium tetrachloride
GeCl4 (high purity grade), and zirconium oxochloride
ZrOCl2 · 8H2O (pure for analysis grade) with a fixed
overall concentration of 0.4 mol/L. The filmꢀforming
solutions were exposed at 25°С for a certain period of
time for the solution to acquire a certain viscosity.
Films were obtained on glass, singleꢀcrystal silicon, or studied on an E7ꢀ8 setup. Acid–base properties were
quartz substrates by centrifuging at 500–3000 rpm and determined using a 673M pH meter. Film surfaces
pulling at 1–5 mm/s. Film formation was carried out were examined and mechanical properties were studꢀ
835

836
L.P. BORILO, L.N. BORILO
, 103, Pa s
and solvent. This provides, on the one hand, rapid parꢀ
η
tial hydrolysis and polymerization in solution with the
retention of products in the sol form and, on the other,
final hydrolysis in a thin layer upon the application of
the filmꢀforming solution to the substrate. Viscosity
was taken to be the criterion of the filmꢀforming
capacity of solutions. For practice, it is important
whether filmꢀforming solutions are stable over time;
therefore, we experimentally determined a relationꢀ
ship between the solution viscosity, time, and feasibilꢀ
ity of obtaining films from them. These data are disꢀ
played in Fig. 1. After solutions are prepared, their visꢀ
cosities change strongly during the first 1 to 3 days.
Experiments showed that film formation occurred
1
4.5
4.0
5
4
3.5
3.0
2.5
2.0
1.5
1.0
0.5
3
2
5
10
15
20
25
30
Time, days
only after the solution aged to reach viscosity of 2.3
×
10–3 Pa s. Films obtained from such solutions have uniꢀ
form thicknesses and good adhesion to substrates.
When solution viscosity exceeds 5.5
are formed or clouding and precipitation occur; films
obtained from such solutions are thick and have nonꢀ
uniform thicknesses. In germanium tetrachloride
based solutions, hydrolytic polycondensation occurs
Fig. 1. Viscosity (
η
) of filmꢀforming solutions versus time
) GeO , ( ) ZrO ,
×
10–3 Pa s, gels
for preparing films of composition: (
1
2
2
2
(
3
) 1ZrO : 1GeO , (
4
) 2ZrO : 1GeO , and (
5
) 1ZrO :
2
2
2
2
2
2GeO .
2
to change the solution viscosity (Fig. 1, curve 1):
GeCl4 + 3C2H5OH + H2O
= Ge(C2H5O)3(ОН) + 4HCl,
1
2
480
720
(1)
310
590
2Ge(C2H5O)3OH = [Ge(C2H5O)3]2O + Н2О. (2)
120
330
Once a certain amount of coarse particles is piled
up in the solution, the solution becomes supersatuꢀ
rated with respect to the solid phase and the solution
abruptly changes from sol to gel, this gel being stable
for 30 days.
100
110
3
4
680
780
290
630
770
310
310
An ethanolic solution of ZrOCl2
viscosities than a germanium tetrachloride solution
(Fig. 1, curve ) and has a far longer period of time
⋅
8H2O has lower
5
110
110
580
760
2
wherein it retains filmꢀforming properties (longer than
20 months). This is explained by the liability of soluꢀ
tions containing zirconium compounds to heavy
hydrolysis, on account of the high charge and small
size (0.80 Å) of the Zr4+ ion, to form hydroxo comꢀ
plexes [Zr4(OH)8(H2O)16 – x(C2H5OH)x]8+ [1, 4, 5]
which are stable for long periods of time:
100 200 300 400 500 600 700
T, °C
Fig. 2. DTA curves for the decomposition of a filmꢀformꢀ
ing solution for preparing films of composition: ( ) ZrO ,
) GeO , and ( ) ZrO –GeO containing (
1
2
(2
3,
4,
5
3) 70, (4)
2
2
2
50, and (5) 30 mol % GeO .
2
4ZrOCl2 + xC2H5OH + (16 – x + 4)H2O
(3)
ied using an NtꢀMDT Ntegra Aura atomic force
microscope (with a silicon needle diameter of 2ꢀ5 mm).
[Zr4(OH)8(C2H5OH)x(H2O)16 – x]Cl8.
Solution viscosities for preparing films based on
zirconium and germanium double oxides (Fig. 1,
curves 3–5) have intermediate values (Fig. 1, curve 2).
RESULTS AND DISCUSSION
A filmꢀforming capacity is known to occur in
materials that are liable to form in solutions macroꢀ
molecules or associations, which adhere to the subꢀ
strate surface when applied to the substrate and
decompose to oxides when the solvent is removed at
higher temperatures [4, 5]. For films to be uniform in
composition and thickness and strongly adhere to the
substrate surface, the filmꢀforming solution should
contain an optimal ratio of the filmꢀforming precursor
In such solutions, condensation can occur to form –
Zr–O–Ge– bonds [7].
[Zr4(OH)12(C2H5OH)x(H2O)12 – x]Cl4
+ 4Ge(C2H5O)3OH
(4)
= [Zr4(OH)12(C2H5OH)x(H2O)8 – x]4O
× [Ge4(C2H5O)12] + 4H2O + 4HCl.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 6 2011

PHYSICOCHEMICAL PROCESSES INVOLVED
837
Table 1. Phase composition of films at various annealing temꢀ
peratures (as probed by powder Xꢀray diffraction)
Thermoanalytical curves for GeO2 formation (Fig. 2,
curve ) showed that the oxide formation process had
2
three steps. First (at 60–200°C), physically bound
water is removed; second (at 250–480°C), hydrolytic
polycondensation occurs to remove chemically bound
water and ethanol; these processes are accompanied
T
ann, °C
700
Composition, mol %
ZrO2
500
900
C + T
T + M
M
by changes in weight and endotherms. Above 500°С
,
an exotherm appears due to burning of organic prodꢀ 90ZrO2–10GeO2
C + T
C + T
C + T
Sch + T
Sch + T
Sch + T
A
ucts. IR spectra and powder Xꢀray diffraction data
(Table 1) showed that a GeO2 film was formed as an
amorphous phase, in accordance with [7].
70ZrO2–30GeO2
50ZrO2–50GeO2
30ZrO2–70GeO2
GeO2
A
A
A
A
T
T
T
A
Zirconium dioxide crystallizes, as shown by therꢀ
mal analysis (Fig. 2, curve 1), above 450°С with two
exotherms, at 480 and 720°С. The former is associated
with transition from an amorphous to cubic and tetꢀ
ragonal phases; the latter with transition to the monoꢀ
clinic phase. Powder Xꢀray diffraction data for ZrO2
films on silicon substrates show that at 500°С zircoꢀ
nium oxide was formed in cubic and tetragonal phases;
as temperature increases, transition from the tetragoꢀ
nal to monoclinic phase occurred. The transition to
the monoclinic phase ended at 900°С (Table 1). In IR
spectra, the absorption bands associated with the
vibrations of water and –OH groups disappeared as
temperature increases. The dehydration of filmꢀformꢀ
ing solutions on the substrate surface is accompanied
by polymerization and formation of infinite chains:
Phase notations: C = cubic, T = tetragonal, T = tetragonal, and
M = monoclinic ZrO phases; Sch = tetragonal structure of
2
scheelite GeZrO ; and Am = Xꢀray amorphous.
2
oxide formation in oxide systems (Table 2). The actiꢀ
vation energy for the first step is close to the heat of
water vaporization, which verifies removal of physꢀ
isorbed water at this step. The activation energies for
the subsequent steps have higher values, correspondꢀ
ing to a typical chemical reaction.
For films of the double oxides of the ZrO2–GeO2
system, exotherms are observed at higher temperaꢀ
tures (Fig. 2, curves 3–5), and the rateꢀcontrolling
step of oxide formation changes as evidenced by relaꢀ
tive process rates (Table 2), which is characteristic of
crystallization of chemical compounds in the systems
under consideration. Powder Xꢀray diffraction analyꢀ
sis (its results are compiled in Table 1) showed formaꢀ
tion of a chemical compound (tetragonal zirconium
⎯
Zr–O–H– having a vibration frequency of 1090 cm–1
and –Zr–O–Zr– having a frequency of ~620 cm–1. At
~700°С, the regular crystal lattice of ZrO2 is formed;
the vibrations of [ZrO4] tetrahedra appear in the IR
spectra in the regions of 871 and 584 cm–1.
Thermoanalytical data were used to calculate, by
the Horowitz–Metzger method [9], activation enerꢀ
gies for the main stages of the processes involved in germanate of the general formula ZrGeO4 having
Table 2. Kinetic parameters of oxide formation steps
Composition, mol %
Step
Calculated parameter
no.
GeO2
ZrO2
50ZrO2–50GeO2
I
Temperature range,
°
C
60–200
2.9
80–200
3.1
80–200
2.6
Relative process rate, g/min
Activation energy, kJ/mol
36
44
40
II
Temperature range,
°
C
250–480
2.8
250–350
2.9
250–450
2.9
Relative process rate, g/min
Activation energy, kJ/mol
72
89
92
III Temperature range,
°
C
500–700
3.2
400–520
2.6
500–720
3.8
Relative process rate, g/min
Activation energy, kJ/mol
132
127
164
IV Temperature range,
°
C
–
–
–
600–800
720–800
Relative process rate, g/min
Activation energy, kJ/mol
6.3
–
7.1
–
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 6 2011

838
L.P. BORILO, L.N. BORILO
(а)
(b)
μ
m
10
9
8
7
6
5
4
3
2
1
nm
30
nm
25
20
10
20
0
15
2
10
4
8
10
6
6
4
5
8
2
10
0
0
μ
m
0
1 2 3 4 5 6 7 8 9 10
20
18
16
14
12
10
8
6
4
2
0
1.4
1.2
1.0
0.8
0.6
0.4
0.2
nm
20
15
10
5
0
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0.2
0.4
0.6
0.8
1.0
1.2
1.4
μ
m
0
0.2
0.8
μ
0.4
1.2 1.4
0.6
1.0
m
nm
200
160
120
80
0
4.0
3.0
2.0
1.0
0
50
100
150
200
250
300
350
400
450
500
nm
4
40
2
0
0
100
200 300 400 500
nm
50
100
150
200
μ
m
Fig. 3. AFM images of films of the ZrO –GeO system obtained in (a) phase contrast and (b) surface topography modes.
2
2
scheelite (CaWO4) structure) and a second phase based
on tetragonal ZrO2
We have studied optical, acidity, and electrophysiꢀ
cal properties of the films, which is of applied imporꢀ
tance, as well as their adhesion to various substrates. In
constructing a composition–property diagram for the
ZrO2–GeO2 system, as the property we used the
refractive index (a structureꢀsensitive parameter) and
surface acidity. Selected properties of the films
obtained on silicon substrates are compiled in Table 3.
For a sample containing up to 10 mol % germanium,
an abrupt increase in refractive index is observed (up to
2.16); this is on account of formation of a solid soluꢀ
tion based on the cubic O2 phase which typically has
higher refractive index values [5]. In the range 30–
.
Thin films of the ZrO2–GeO2 system containing
50 mol % GeO2 were studied by atomic force microsꢀ
copy; the results are displayed in Fig. 3. Surface topogꢀ
raphy and phase contrast modes were used. The phase
contrast mode is employed for heterogeneous systems
and provides, apart from surface topography, measureꢀ
ments of mechanical properties and verifies the presꢀ
ence of phases [1]. The results of these studies showed
that the films were poreꢀfree, continuous, and uniform
and consisted of two phases in the form of annular and
rod species; the ring sizes were within 50 nm; the ledge
height was 2.6 nm.
70 mol % GeO2, ZrGeO4 is formed (as mentioned
above; see Table 1); this influences the properties of
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 6 2011

PHYSICOCHEMICAL PROCESSES INVOLVED
Table 3. Physicochemical properties of films of the ZrO2–GeO2 system
839
Film composition
mol % GeO2
Parameter
ZrO2
GeO2
5
10
30
50
70
90
88
Film thickness, nm
Refractive index
Adhesion F, MPa
60
94
65
68
77
78
82
,
n
2.03
1.93
8.16
5.1
1.69
0.86
5.52
4.5
2.11
1.87
10.12
2.16
1.85
11.35
1.97
1.65
7.22
1.96
1.63
6.71
1.95
1.60
6.20
1.72
1.08
5.85
Dielectric constant ε
Gap width E, eV
4.8–5.0
1010–12
Resistivity ρ, Ω cm
1010–12 108–10
the films. For ZrGeO4ꢀcontaining samples, refractive
indices are roughly equal (1.95–1.96). The acidity diaꢀ
gram shows that the acidity increases smoothly in
response to increasing germanium oxide content, and
its values are within the range 6.0–6.2 when concenꢀ
The resulting film thicknesses fall in the range 60–
94 nm. Thicker films have been obtained for germaꢀ
nium oxide on account of high viscosities in the filmꢀ
forming solutions. The least adhesion is intrinsic to
GeO2 films; the best adhesion is intrinsic to ZrO2 films,
which are characterized by better bonding to the surꢀ
face of silicon substrates on account of the greater
bond ionicity.
trations are within the range 30–70 mol % GeO2
.
The absence in films of a Zr3GeO8ꢀbased solid soluꢀ
tion, which is characteristic of systems of such the
composition [10], may be explained by the influence
of the substrate on the newly formed film structure
[111]. Germanate ZrGeO4 (which has scheelite strucꢀ
ture) crystallizing from solutions is characterized by
equal proportions of Zr and Ge atoms and a high tetꢀ
ragonal group symmetry and is formed via compleꢀ
mentary selfꢀassembly [12].
Depending on composition, the films have resistivꢀ
ities ranging between 108 and 1012
Ω
cm, bandgap valꢀ
ues of 4.5 to 5.1 eV, refractive indices of 1.69 to 2.16,
and radiofrequency dielectric constants of 5.52 to
11.35.
To summarize, we have showed that films having
reproducible properties based on double oxides of the
ZrO2–GeO2 system can be prepared from filmꢀformꢀ
ing solutions provided that solution viscosities are
n
pHsusp
2.2
2.1
between 2.3
×
10–3 and 5.5 10–3 Pa s. The formation of
×
simple and complex oxides has a number of consecuꢀ
tive steps and involve vaporization of physisorbed
water and ethanol, hydrolysis and condensation of
hydrolysis products, burning of organic remnants, and
crystallization. We have studied the phase composiꢀ
tions of the films and constructed composition–
refractive index and composition–acidity diagrams.
Films of the ZrO2–GeO2 system have high chemical
and thermal stability, have good adhesion to various
substrates, and are wideꢀgap semiconductors.
2.0
1.9
1.8
7
1.7
b
6
1.6
а
5
1.5
0 10 20 30 40 50 60 70 80 90 100
mol %
ZrO2
GeO2
ACKNOWLEDGMENTS
This study was supported by the Federal Target
Program “Research and Pedagogical Staffs of Innovaꢀ
tive Russia” for 2009⎯2012.
Fig. 4. Composition–property diagrams for films of the
ZrO –GeO system: ( ) acidity and ( ) refractive index.
a
b
2
2
T
= 900°C.
ann
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 6 2011

840
L.P. BORILO, L.N. BORILO
6. B. M. Komranov and B. A. Shapochnykh, Measureꢀ
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RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 6 2011