Recently, there is a growing interest in applying ZnO thin films on silicon buffer substrates for p-n junction devices, optical wave guide, etc.
A sol gel process is very attractive technique for obtaining oxide thin films, due to easy control of film composition, easy fabrication of large area thin films with low cost and the ability to coat-specific shapes substrates.
This paper presents a kinetic investigation of the crystallization (550-650 oC) of high preferential c-axis oriented ZnO thin films on p-type (100) silicon wafer substrate, from XRD data

Catalina ITICESCU

Joao CANEJO

Viorica MUŞAT

The Annals of “Dunarea de Jos” University of Galati Fascicle IX Metallurgy and Materials Science

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THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI 
FASCICLE IX METALLURGY AND MATERIALS SCIENCE, ISSN 1453 – 083X. NR 2 – 2005 
 
 
 
 
ON THE KINETICS OF SOL GEL Al:ZnO THIN FILMS 
CRYSTALLIZATION  ON SILICON SUBSTRATE 
 
Viorica MUŞAT1, Joao CANEJO2
Catalina ITICESCU1
1“Dunarea de Jos “University of Galati 
2 Faculty of Science and Technology, CENIMAT, New University of Lisbon 
email: Viorica.Musat@ugal.ro
 
ABSTRACT 
 
Recently, there is a growing interest in applying ZnO thin films on silicon buffer 
substrates for p-n junction devices, optical wave guide, etc. 
A sol gel process is very attractive technique for obtaining oxide thin films, due to 
easy control of  film composition,  easy fabrication of large area thin films with low 
cost and the ability to coat-specific shapes substrates.  
This paper presents a kinetic investigation of the crystallization (550-650oC) of 
high preferential c-axis oriented ZnO thin films on p-type (100) silicon wafer 
substrate, from XRD data. 
 
KEYWORDS: Al-doped ZnO, thin films, sol-gel, X-ray diffraction, atomic force 
microscopy, kinetic curves 
 
 
1. Introduction 
 
Zinc oxide (ZnO) based thin films are 
inexpensive n-type, wide band gap (3,2eV) 
semiconductor materials with high transmittance in 
VIS-NIR region and electrical (resistive or 
conductive) properties depending on the doping 
element(s) and microstructure. Transparent and 
conductive Al:ZnO thin films on glass substrate with 
high c-axis orientated crystalline structure along (002) 
plane are extensively studied for various applications 
including transparent conducting electrode for 
different electronic devices such as solar cells and  
electroluminescence displays [1-5]. Recently, there is 
a growing interest in applying n-type conductive 
doped-ZnO thin films on p-type silicon buffer 
substrate for p-n junction devices [6], optical wave 
guide [7] and ultraviolet (UV) photoconductive 
detector [8]. Between the most important applications 
of UV detection are missile warning system, air 
quality monitoring, accurate measurement of 
radiation for the treatment of UV irradiated skin, etc 
[9-10] With the use of wide band-gap semiconductors 
such as ZnO (UV) photoconductive detector, the need 
for costly filters to attenuate unwanted and IR 
radiation would be eliminated [11]. As transparent 
conductive oxide (TCO), doped ZnO films are very 
promising alternative materials to tin oxide and 
indium tin oxide because of their superior abundance 
in nature, nontoxicity and the excellent stability in 
hydrogen plasma which is an unavoidable processing 
ambient in silicon-related fields [12]. 
A sol gel process is very attractive technique for 
obtaining oxide thin films, because  the advantages of 
easy control of  film composition,  easy fabrication of 
large area thin films with low cost and the ability to 
coat-specific shapes substrates.  
Only few quantitative kinetic studies concerning 
the crystallization of oxide thin films are presented in 
the literature. The kinetic parameters of the 
crystallization of Sr0.7Bi2.3Ta2O9 (SBT) [13-14], 
Sr0.7Bi2.3Ta2O9-BiTiTaO9 (SBT-BTT) [13], 
Pb0.53Zr0.47Ti (PZT) [15-16] and TiO2  [17] thin films 
were evaluated. To date, no papers about the kinetic 
parameters of sol-gel Al:ZnO thin films 
crystallization have been found.  
This paper presents, based on XRD data, 
qualitative kinetic study (variation of XRD curves 
parameters of annealed films as a function of the 
annealing time) of the crystallization (550-650oC) of 
Al(2wt%):ZnO thin films deposed on silicon  
substrate by sol-gel method.  
 
2. Experimental 
 
The thin films used as samples for the kinetic 
study were prepared via a non-alkoxide route and a 
spin-coating technique at 1500 rpm (rotation per 
minute) on p-type (100) silicon wafer substrate.  
 
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THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI 
FASCICLE IX METALLURGY AND MATERIALS SCIENCE, ISSN 1453 – 083X. NR 2 – 2005 
 
 
 
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The thin films deposition was performed using a 
sol prepared with Zn(CH3COO)2·2H2O 99.5%, 
AlCl3·6H2O 98% as cation sources and  ethanol as 
solvent. The concentration of metal ions in the 
solution was 0.50 mol l-1. In order to study the 
kinetics of crystallization, the as-deposed thin films 
samples were stabilized by preheated in air for 5 min 
at 400ºC. In order to study the kinetics of 
crystallization, different pieces of stabilized thin films 
were annealed for different times (10, 20, 30, 40, 50, 
60, 90 and 120 minutes) at 550, 600 or 650oC;  three 
series of eight samples corresponding to the three 
values of the crystallization temperature have been 
obtained .  
After the annealing (crystallization) treatment, 
the X-ray patterns of the samples were recorded in 2θ 
= 30-37 degree range, at room temperature using a 
Rigaku diffractometer (model RAD IIA) with CuKα 
radiation.  
The kinetic plots present the values of (002) 
peak intensity,  net area and FWHM of the annealed 
samples that were normalized taking into 
consideration the ratio between its surface area and 
the surface area of the  biggest sample. 
The thickness of the crystallized thin films was 
measured using a Sloan Dektak 3D surface 
profilmeter. The final postheated films have a 
thickness of about 180 and 200 nm for samples 
annealed at 650 and 550 oC, respectively.  
The morphology on the surface of the films was 
analyzed using an AFM   microscope. Tapping mode 
AFM experiments were performed in a Nanoscope 
IIIa Multimode AFM microscope (Digital 
Instruments, Veeco). Commercial etched silicon tips 
with typical resonance frequency of ca. 300 Hz 
(RTESP,  Veeco) have been used as AFM probes.  
The electrical resistivity of the films was 
measured in dark, using a KEITHLEY 617 Model 
Programmable Electrometer.  
 
3. Results and discussions 
  
Figure 1 shows the XRD patterns of the films 
crystallized at 550, 600 and 650oC in air for different 
periods between 10 min and 120 min, and the normal 
random orientated  ZnO pattern.  The XRD patters of 
the crystallized films (Fig. 1) show a most important 
(002) peak, indicating a preferential c-axis orientated 
würtzite type, comparative to the normal random 
orientated wurtzite structure characterized by (101) 
most intense peak.  
The film crystallized at 600oC show the highest  
preferential c-axis orientation. Characteristic to c-axis 
orientated crystalline structure are the grains 
uniformly perpendicular to the substrate surface.  
The AFM micrographs (Fig. 2) confirm this 
orientation and show non-porous and cracks free 
films morphology with average grain size depending 
on the speed of film deposition. Higher speed 
deposition, higher grain size.  
The surface roughness mean square (rms) of the 
films, estimated from AFM measurements, rises from 
~12 nm to ~26 nm when the deposition speed 
increases from 1500 rpm to 3000 rpm, respectively. 
 
 
    10 min (1)
 
 
 
 
 
 
 
 
 
 
0
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 1. XRD patterns of the thin films 
annealed in air at different temperature values. 
 
Figure 3 shows the variations, during annealing, 
of the net area, FWHM and inter-planes distance (d) 
related to the most intense (002) peak for samples 
annealed (crystallized) at 550, 600 and 650oC. 
   20 min(2)
   30 min (3)
   40 min (4)
   50 min (5)
   60 min (6)
   90 min (7)
 120 min (8)
In
te
ns
ity
 (a
.u
.)
X2 Theta (dergrees)
(100) (002) (101)
 
30 32 34 36
0
200
400
600
800
1000
In
te
ns
ity
 (a
.u
.)
2 Theta (dergrees)
   10 min (1)
   20 min(2)
   30 min (3)
   40 min (4)
   50 min (5)
   60 min (6)
   90 min (7)
 120 min (8)
(100)
(002)
(101)
30 32 34 36
0
200
400
600
800
1000
In
te
ns
ity
 (a
.u
.)
2 Theta (degrees) 
 B
 C
 D
 E
 F
 G
 H
 I
(100)
(002)
(101)
 
55
o
60
o
65
o
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THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI 
FASCICLE IX METALLURGY AND MATERIALS SCIENCE, ISSN 1453 – 083X. NR 2 – 2005 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 2. AFM images, 3D surface topology, of 
thin films deposed at 1500rpm (a) and  3000rpm 
(b) after crystallization in air  at 650C. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 3. The variation of the net area related to 
the most intense (002) peak, during the 
annealing at 550, 600 and 650oC. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 4. The variation of the half width   
FWHM of the most intense (002) peak, 
 during the annealing at 550, 600 and 650oC. 
 
 
 
 
 
 
 
 
 
 
 
 
  
 
 
Fig. 5. The variation of the interplanar distance, 
d, related to the most intense (002) peak, during 
the annealing at 550, 600 and 650oC. 
 
From figure 3 one can see that during the first 
period of annealing (0-40 min), the net area of (002) 
peak continuously increase. The increase is the most 
important for the sample annealed at 600oC, smallest 
for the sample annealed at 550oC. The increase of 
(002) net area during annealing is an indication that 
the films crystallized at 600oC consist of a 
significantly larger amount of volume of crystalline 
phase, crystalline phase that is oriented perpendicular 
to the surface. Between 50-60 min annealing, the net 
area of the peaks decreases and after that, no 
significant variation occurs. 
Generally, the decrease in FWHM of (002) peak 
confirms the improvement in the texture and quality 
(grain size) of c-axis orientated crystalline structure 
of the film. Figure 4 shows FWHM variation curves 
with minimum values and a shift of these minimum 
values depending on the crystallization/annealing 
temperature. Higher the annealing temperature, lower 
the minimum FWHM values (higher crystallite size) 
reached at shorter annealing time.   
(
(
0 20 40 60 80 100 120
0 
100 
200 
300 
400 
500 
600 
700 
  550 o C (1)
 600 o C  (2)
  650 o C (3)
1 
2 
3 
(0
02
)  
P
ea
k 
N
et
 A
re
a 
 (a
.u
.) 
 Time (min) 
550o  (1)
oC ( )
650o  (3)
0 20 40 60 80 100 120
0,40
45
50
55
60
65
70
75
80
85
90
95
0,
0,
0,
0,
0,
0,
0,
0,
0,
0,
0,
Time (min)
FW
H
M
 (2
 th
et
ao
 )
1
2
3
( b)550
oC (1) 
600oC (2) 
650oC (3) 
0 20 40 60 80 100 120
2,580
2,585
2,590
2,595
2,600
2,605
2,610
2,615
d 
(A
)
Time (min)
2
3
1
( c  )
550oC (1)
600oC (2)
650oC (3)
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THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI 
FASCICLE IX METALLURGY AND MATERIALS SCIENCE, ISSN 1453 – 083X. NR 2 – 2005 
 
 
The variation of the interplanar distance 
according to (002) peak as a function of annealing 
time is presented in figure 5. This variation differs 
from the variation of the others parameters, each 
curve is characterized by two minimum and two 
maximum values situated at the same time values, for 
all the three series of samples annealed at 550-600 
and 650oC. This variation shows that the mechanism 
of crystallization is complex and the rate determining 
step changes several times during annealing. The 
decrease of d during the first 20 minutes can be 
attributed to the total release of the organics from the 
stabilized films. 
The film annealed in air at 600oC show the 
highest degree of crystallization and the highest 
preferential c-axis orientated crystalline structure. 
These results are in agreement with the literature data 
that recommend  one hour annealing at 600oC in air 
for the crystallization of ZnO thin films deposed on 
silicon substrate [7-12]. 
The electrical measurements on the crystallized 
films showed very resistive films. A resistivity of  
3.9×106, 8,7×106 and 3.9×107 Ωcm  were obtained for 
the films crystallized at 550, 600 and 650oC, 
respectively.  
The thickness of the thin films ranges between 
250-300 nm and 200-250 nm for samples postheated 
at 450 and 500ºC respectively. 
 
4. Conclusions 
 
A qualitative kinetic study of isothermal 
crystallization, in the temperature range 550-650oC, 
of ZnO:Al (2wt%) thin films deposed on p-type (100) 
silicon wafer substrate was performed, based on X-
ray diffraction patterns recorded at room temperature. 
The crystallized films show a preferential c-axis 
orientated würtzite type structure with dominating 
(002) peak. 
The variation of the net area, FWHM and inter-
planes distance (d) related to the most intense (002) 
peak imply that the mechanism of films 
crystallization is complex and the rate determining 
step changes during annealing. 
The film annealed in air at 600oC shows the 
highest degree of crystallization and the highest 
preferential c-axis orientated crystalline structure, in 
good agreement with the literature data that 
recommend one hour at 600oC annealing for the 
crystallization of ZnO thin films deposed on silicon 
substrate. 
The crystallized films in air are resistive. 
Resistivity values of 3.9×106, 8,7×106 and 3.9×107 
Ωcm were obtained for the films crystallized at  550, 
600 and 650oC, respectively. After annealing in 
reducing atmosphere, the films turn in conductive. 
 
References 
 
[1]. S. Fujihara, C. Sasaki, T. Kimura, Applied Surface Sci. 180 
(2001) 341. 
[2]. M. Ohyama, J. Am. Ceram. 81 (1998) 1622. 
[3]. D. Bao, H. Gu, A. Kuang, Thin Solid Films 312 (1998) 37.  
[4]. J.F. Chang, W.C. Lin, M.H. Hon, Applied Surface Sci. 183 
(2001) 18. 
[5]. E. Fortunato, P. Nunes, A. Marques, D. Costa, H. Aguas, I. 
Ferreira, M.E.V. da Costa, M.H. Godinho, P.L. Almeida, R. 
Martins, Adv. Eng. Mater. 4 ( 2002) 610. 
[6]. M. Purica, E. Budianu, E. Rusu, , Thin Solid Films, vol. 383, 
2001, pg. 283. 
[7].  F.C.M. Van de Pol, Ceram Bull., 69 (1990) 1959. 
[8]. Z. Q. Xu, H. Deng, J. Xie, Y. Li and X.T. Zu, Appl. Surf. 
Sci. (2006, in press, www.sciencedirect.com). 
[9].  H. Otha and H. Hosono, Mater. Today 7(2004) 42. 
[10]. T.H. Moon, M.C. Jeong, W. Lee and J.M. Myoung, Appl. 
Surf. Sci. 240 (2005) 280. 
[11]. Y.C. Lee et al, Appl. Surf. Sci. 249 (2005) 91. 
[12]. S.Y. Kuo et al, Journal of  Crystal  Growth, 285 (2006) 78-84 
[13]. W.C. Kwak and Y.M. Sung., J. Mater. Res., 17(2002),1463. 
[14]. I. M. Sung, J. Mater. Res., 7(2001), 2039. 
[15]. Z. Huang, Q. Zhuang and E.W. Whatmore, J. Appl.  Phys., 
86 (1999), 1662. 
[16] Z. Huang , Q. Zhuang and E.W. Whatmore, J. Appl. Phys., 
85 (1999), 7355. 
[17]. G.J. Exarhos and.M. Aloi, Thin Solid Films, 193 (1990), 42. 
 
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On the Kinetics of Sol Gel Al:ZnO Thin Films Crystallization on Silicon Substrate

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On the Kinetics of Sol Gel Al:ZnO Thin Films Crystallization on Silicon Substrate

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