The document discusses the preparation and characterization of zinc oxide (ZnO) thin films using two-step chemical methods, focusing on their applications in gas sensors and electronic displays. The films were synthesized using Chemical Bath Deposition (CBD) on seeded substrates with structural, optical, and electrical properties analyzed through various techniques. Results indicate that the films exhibit n-type conductivity, high crystallinity, and suitable optical characteristics, including a reflectance of 18% and transmittance of 58% for as-prepared films.

1. STUDIES ON ZINC OXIDE THIN
FILMS PREPARED BY TWO -
STEP CHEMICAL METHODS
By
NAVYASREE .M

2. ZINC OXIDE( ZnO) THIN FILMS
A thin film is a layer of material ranging from
fractions of a nanometer to several micrometers in
thickness on a clean substrate.
ZnO is a transparent conducting oxide (TCO) which
has recently been studied. It is an n-type
semiconductor with a direct energy wide gap of about
3.37 eV at room temperature, that could be obtained
with very low resistivities.
Zno based elements have attracted much attention
as gas sensors because of their chemical sensitivity
to volatile and other radical gases, their high chemical
stability, suitably to doping, non-toxicity, and low cost .
It is used in electronic displays , fabrication of blue
light emitting diode, dye sensitized solar cells and
window plates in solar cells.

3. GROWTH TECHNIQUES OF
NANOMATERIALS AND THIN FILMS
In general these techniques can be broadly classified
into two groups: vacuum-based deposition
process and solution-based deposition process. In
vacuum –based synthesis, deposition takes place
under high vacuum (~10-8 to 10-11 mbar). Therefore
the process is very clean; probability of deposition of
foreign and undesired materials is very low. But the
vacuum based systems require costly and
complicated equipment. Solution based synthesis
process, on the other hand are based on chemical
reactions in liquid phase. Therefore, these technique
are simple and cost-effective and most importantly,
large range of materials can be synthesized by these

4. ZINC OXIDE THIN FILMS BY TWO-STEP
CHEMICAL PROCESSES
In seeding process, a thin layer of ZnO is
deposited as seed on the substrate prior to the
second stage, usually the solution growth technique.
Usually in all the two-step chemical methods that
have been employed, wet solution method like CBD
was used for secondary nucleation.
Seeding by SILAR
A complete deposition cycle of SILAR comprises of
four processes involving subsequent immersion of
substrates in cationic and anionic precursor solutions
for adsorption and reaction along with rinsing the
substrate in between to remove the loosely held
species in distilled water kept at room temperature.

6. The aqueous zinc-ions solution complexed with
ammonia (25%) and diethanolamine (DMA) C2H7N
was used as the cationic precursor, in which 100 ml of
0.35 M of zinc nitrate was (Zn(NO3)2.6H2O) used. .
100 ml of the complex solution of cations and distilled
water for intermediate rinsing was kept at room
temperature while the beaker with distilled water as
anionic precursor was kept at 368K during deposition.
Immersion time in cationic and anionic solution was
equal, which is optimized to 2 s and the immersion
time for rinsing the substrates was 5 s.
5 deposition cycles were repeated to form a
very thin seed layer of ZnO on the substrate. The
seeded substrates were washed in distilled water and
dried in hot air.

7. Film synthesis by CBD
Chemical bath deposition (CBD) include the
controlled precipitation from solution of a compound
on a suitable substrate
ZnO films were prepared on seeded glass
substrates by CBD.

8. In the CBD process, the same cationic precursor
prepared for synthesizing the seeded substrate was
used as the reaction solution. The ZnO seeded
substrates were immersed into the aqueous solution
and tilted against the wall of beaker. The beaker is
kept on a hot plate and temperature is maintained at
950C. The substrate is kept in the solution for 2 hours.
After the film growth, the substrate was removed from
the solution, rinsed with distilled water and then dried
blowing hot air. Film was named as ZnO. And some
films are annealed at 573K, and film was named as
ZnO A.

9. CHARACTERIZATION TECHNIQUES
Film thickness measurement
General (gravimetric method)
These are methods which are based on the
determination of a mass. The film thickness d can be
calculated from the mass of the coating m if the density
ρ and the area A on which the material is deposited are
known:
D = m/ (A ρ) (1)

10. Structural characterization
X-ray diffraction (XRD) studies were carried out to
study the crystallographic properties of the thin films.
Crystallite size
The mean crystallite sizes of the films can be
determined using the FWHM of the diffraction peaks
by the Scherrer’s equation given by,
0.94 λ/βcosӨ (2)
where 0.94 is the value of the shape factor, ‘ ’ is
the wavelength of X-rays which is 1.5406 nm for CuK
, ‘ ’ is the FWHM of diffraction peak measured in
radians and ‘ ’ is the Bragg’s angle.

11. • The internal relaxation parameter, bond length, the
bond angles of the films, and volume of the unit cell
The internal relaxation parameter (u), bond length (b) and
the bond angles ( and ’) of the ZnO films were calculated
using the Eqs given below.
2
2
1 1
3 4
a
u
c
  
   
  
b cu
1
2 2
1
arccos 1 3
2 2
c
u
a



  
                 
   
(3)
(4)
(5)

12. Microstrain
The microstrain (ε) can be determined
using the tangent formula
4 tan




2
cV = a c
The photon energy dependence of the absorption
coefficient for direct allowed transition is given by,
α = (A/h)(h −Eg)1/2
The defects in the film is quantified by computing
the dislocation density using Williamson Smallman
relation,
δ =1/D2 (9)
(7)
(6)
(8)

13. Electrical resistivity
According to ohm’s law, the electrical resistance is
proportional to the sample’s length ‘L’ and the
resistivity ‘’ and inversely proportional to the
sample’s cross sectional area ‘S’ given by the product
of the film thickness and the width of the film.
Therefore, the electrical resistivity (ρ) of the films can
be determined using Eq.given below.
ρ= R S/L. (10)
it is inversely proportional to the carrier mobility (m),
carrier concentration (N) and electronic charge (e)
given by the relation,
ρ=1/Nem (11)
In the present work, electrical studies were carried out
by the two-probe method.

14. Results and discussion
1 Thickness of the film
The film thickness was computed by gravimetric
method using Eq. (1). The estimated average value
of thickness is 1850 nm.
2 Structural analysis
. XRD pattern of ZnO film shows peaks corresponds
to reflections from (100), (002), (101), (102), (110),
(103), (112), (210) crystal planes. Maximum intensity
is observed for reflections from (101) crystal plane.
Reflections from these planes have intensity counts
(arbitrary unit) 13403 at the angle (Ө) 18.16 degree.

15. XRD pattern of as prepared film
20 30 40 50 60 70 80
-2000
0
2000
4000
6000
8000
10000
12000
14000
(210)
(112)
(103)
(110)
(102)
(101)
(002)
(100)
ZnO
intensity(arb.unit)
2 (degree)

16. XRD pattern of annealed film
XRD patterns of ZnO A film also containing peaks corresponds to
same crystal planes as in ZnO. But maximum intensity is
observed for reflections from (101) crystal plane have intensity
counts (arbitrary unit) 13812 at the angle (Ө) 18.39345 degree.
FWHM of the highest peak of ZnO film is 0.7217 degree and
that of Zn0 A film is 0.96734 degree .
20 30 40 50 60 70 80
-2000
0
2000
4000
6000
8000
10000
12000
14000
16000
(210)
(112)
(103)
(110)
(102)
(101)
(002)
(100)
Intensity(arb.unit)
2 (degree)
ZnO A

17. The very high intensity and sharpness of the diffraction peaks
indicates the high crystallinity of the as-grown samples. No
characteristic peak of other phases or impurities was observed in
the XRD patterns.
The grains, particles, or nanorods of the films are comprised of
small crystallites. The mean crystallite sizes ZnO films are
calculated using by the Debye-Scherrer’s equation given by Eq.
(2).
For ZnO film crystalline size D is found to be 26 nm for the
as-prepared film and is 29 nm for the annealed film.
Dislocation is a type of defect in crystals. Dislocations are areas
were the atoms are out of position in the crystal structure. The
defects in the ZnO films was evaluated by computing the
dislocation density using Williamson Smallman relation given by
Eq. (9)
As- prepared ZnO film is obtained as 1.4792x 10^15 lines/m.
Annealed ZnO A film is obtained as 1.189x10^15 lines/m.

18. Microstrain in the nanocrystals of ZnO films was computed by
using the tangent formula given by Eq. (7).
As-prepared film: 0.000457
Annealed film: 0.000420
Lattice constants, c/a ratio, internal relaxation parameter and
bond angles of ZnO films are given in the table 4.1.
a c c/a u b=cu
ALPH
A
BEETA
3.251 5.211 1.602 0.3797 1.978 108.47 110.51
3.248 5.203 1.601 0.3798 1.976 108.43 110.54

19. Morphological analysis
The SEM micrographs of the ZnO films are depicted in
Figs. 4.2. Morphology is obtained as nanoflakes. The substrate
coverage is excellent for the films.

20. 4 Optical analysis
Fig. 1. shows the optical transmittance and reflectance spectra of
as-grown and annealed films. As –prepared film exhibit 18 %
reflectance and 58% transmittance. Annealed film exhibits 17.5%
reflectance and 60% transmittance in the entire visible region of
the spectrum. Both transmittance and reflectance spectra exhibit
a wavy nature.
600 800 1000 1200 1400 1600 1800
15
16
17
18
19
20
b
a
Reflectance(%)
Wavelenght(nm)
a:ZnO
b:ZnO A

21. .
400 600 800 1000 1200 1400 1600 1800
25
30
35
40
45
50
55
60
65
b
a
Transmittance(%)
Wavelenght (nm)
a: ZnO
b: ZnO A
Fig.1 Transmitted and reflectance spectra of ZnO films

22. Fig. 2 depicts the variation of refractive index and extinction
coefficient with wavelength of the as-synthesized and annealed
ZnO films. Approximate values of refractive index for as prepared
film is 2.23 and that of annealed film is 2.2 in the entire VIS-NIR
spectrum.
600 800 1000 1200 1400 1600 1800
1.9
2.0
2.1
2.2
2.3
2.4
b
a
Refractiveindex
Wavelenght(nm)
a: ZnO
b: ZnO A

23. .
400 600 800 1000 1200 1400 1600 1800
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.040
b
a
Extinctioncoefficent
Wavelenght(nm)
a: ZnO
b: ZnO A
Fig.2 Variation of refractive index and extinction
coefficient

24. In Fig. 3, the relationship between (αhν)2 versus hν is
plotted. The optical band gap value can be obtained
by extrapolating the linear portion to the photon
energy axis.
0 1 2 3 4 5 6
0.0
2.0x10
8
4.0x10
8
6.0x10
8
8.0x10
8
1.0x10
9
1.2x10
9
1.4x10
9
b a
(h)
2
(cm
-1
eV)
2
h (eV)
a: ZnO
b:ZnO A
Fig.3 . The (h)2 vs h plot of as-synthesized
and annealed ZnO films

25. From the plot it is clear that optical band gap value for
as-prepared ZnO film is 3.0 eV and for annealed
ZnO A it is 2.5 eV. .
5 Electrical studies
All the films exhibited n-type conductivity, which
was determined by hot-probe method. The electrical
resistivity of the ZnO films was examined and the value
of resistivity of the films determined is
As-prepared film: 10-1Ωm
Annealed film: 10-2Ωm

26. CONCLUTION
Adherent, transparent and semiconducting ZnO thin films were
synthesized on pre-coated glass substrates with ZnO seed layer.
Seeding of substrate is done by SILAR (successive ionic layer
adsorption reaction). Film preparation is done by CBD (chemical
bath deposition). The high intensity and sharp diffraction peaks
indicates the poly crystallinity of the ZnO films. Films have
predominantly (101) preferred orientation. Strain developed in
the ZnO films is minimum and negligible.
As –prepared film exhibit 18 % reflectance and 58%
transmittance. Annealed film exhibits a maximum of 17.5%
reflectance and 60% transmittance in the entire visible region of
the spectrum. Film possesses dense packed nanoflake
morphology with high internal surface area. Approximate values
of refractive index for as prepared film is 2.23 and that of
annealed film is 2.2 in the entire ViS-NIR spectrum. Films exhibit
low electrical resistivity of the order of 10-1 for as- prepared film
and 10-2 for annealed film.