Effect of annealing temperature on the structure of ZnO films

16 TRƯỜNG ĐẠI HỌC THỦ ĐÔ HÀ NỘI 
EFFECT OF ANNEALING TEMPERATURE ON THE 
STRUCTURE OF ZnO FILMS 
Dang Tran Chien
1(1)
, Ta Anh Tan
2 
1
 Hanoi University of Natural Resources and Environment 
2 
Hanoi Metropolitan University 
Abstract: In this work, ZnO nanocrystalline thin films were obtained by evaporating Zn 
using the thermal evaporation technique and then thermal treatment. The results show that 
after annealed at temperature of 400°C, the porous structure becomes visible and the ZnO 
film has a particle-like structure. At an annealing temperature of 450°C, the film has a 
flower-like structure. The smoother and approximately uniform surface can be observed at 
an annealing temperature of 500°C. The film goes into state of compression with decrease 
in annealing temperature from 500ºC to 350ºC and becomes completely stress free at about 
485ºC. The lattice constant c increases from 5.1992 Å to 5.2020 Å on increasing the 
thickness from 600 nm to 1200 nm and the stress gradually decreases. This implies that the 
stress film can be relaxed by increasing the film thickness. The obtained ZnO films were 
characterized by X-ray diffraction (XRD) and a field emission scanning electron 
microscope (FE-SEM). 
Keywords: ZnO films, flower-like structure, porous structure, film thickness. 
1. INSTRUCTIONS 
Zinc oxide has been widely investigated due to its properties piezoelectricity, 
conductivity, optical absorption and emission. ZnO thin films have many applications in gas 
sensors, photocatalysis, dye-sensitized solar cells [5]. There are many different techniques 
used for thin films growth: radio frequency (RF) magnetron sputtering [2], sol-gel [6], pulsed 
laser deposition [10], spay pyrolysis technique [1], etc. In this work we used the vacuum 
evaporation technique combined with thermal process for the fabrication of ZnO films. In 
comparision with other techniques, thermal evaporation is a ralatiely simple one for large-
sacle uniform coating to produce clean, dense and strong adhesion to substrate thin films [9]. 
Studying of strain/stress in the film can provide useful information regarding deffect 
(1)
 Nhận bài ngày 11.01.2016 gửi phản biện và duyệt đăng ngày 20.01.2016. 
 Liên hệ tác giả: Đặng Trần Chiến; Email: dtchien@hunre.edu.vn. 
TẠP CHÍ KHOA HỌC SỐ 2/2016 17 
evolution, which is very important for a better understanding to improve the quality of the 
film and for fabrication of ZnO based devices [8]. In this paper, we study the effect of 
annealing temperature on the ZnO crystalline structure, strain and stress of the thin films. 
2. EXPERIMENTAL SETUP 
Silicon substrates were ultrasonically cleaned in a series of organic solvents (ethanol, 
methanol and acetone) and deionized water. All the substrates were subsequently treated 
by a glow discharge technique. A layer of Zn was vacuum-deposited by a thermal 
evaporation technique at a pressure of 10
-2
 torr. The deposition rate was controlled at 1.5 
nms
-1
. The samples named Z1, Z2, Z3, Z4, Z5 were annealed at 300°C, 
350°C, 400°C, 450°C, and 500°C in air for 6 hours, respectively. The thickness of all 
samples is about 600 nm. The surface morphology of the samples was investigated using a 
Hitachi Field Emission Scanning Electron Microscopy (FE-SEM). X-ray diffractograms 
were recorded on a XRD-5000 diffractometer using CuKα radiation with the wavelength 
of 1.5406 Å. 
3. RESULTS AND DISCUSSION 
3.1. Effect of annealing temperature on morphology of the ZnO thin films 
Figure 1 shows FE-SEM images of the annealed ZnO thin films at temperatures of 
400°C (sample Z3), 450°C (sample Z4), and 500°C (sample Z5). From the figure it is 
apparent that at annealing temperature of 400°C, the ZnO film has a particle-like structure. 
The porous structure becomes visible. The cross-section of Z3 shows a strong adhesion of 
the film to the substrate. At an annealing temperature of 450°C, the film has a flower-like 
structure. The smoother and approximately uniform surface can be observed at an annealing 
temperature of 500°C (sample Z5). The increase in smoothness of ZnO thin film with 
increased annealing temperature might originate from the increase in the grain size in the 
film due to the thermal treatment. Therefore, the surface roughness becomes smoother 
when the annealing temperature increases. 
18 TRƯỜNG ĐẠI HỌC THỦ ĐÔ HÀ NỘI 
Fig.1. FE-SEM images of surface and cross-section of samples named Z3, Z4, Z5 at 
annealed temperature of 400°C, 450°C, 500°C, respectively. 
3.2. Effect of annealing temperature on structural characterization of the ZnO 
thin films 
Figure 2 shows the XRD patterns of annealed ZnO thin films at different temperatures. 
It can be seen from figure 2(a) that the high intensity of diffraction peaks at the 2θ position 
of about 36.3º, 39.0º, and 43.2º from zinc are found in the sample annealed at 300°C, 
wheareas for the film annealed at 350°C, diffraction peaks at the 2θ position of 39.0º and 
43.2º are shown with very low intensity. Especially, the peaks at the 2θ position of 36.3º 
TẠP CHÍ KHOA HỌC SỐ 2/2016 19 
and 54.3º disappeared (figure 2(b)). As can be seen from figures 2(c) and (d), no 
diffraction peaks from zinc or other impurities are detected. The diffraction peaks 
at the 2θ values of approximately 31.7º, 34.3º, 36.3º, 47.6º, 56.6º and 62.8º 
correspond to the (100), (002), (101), (102), (110) and (103) planes, respectively. 
Fig.2. XRD pattern of samples named Z1, Z2, Z3, Z4, Z5 at annealed temperature of 
350°C, 400°C, 450°C and 500°C, respectively. 
Table 1: Lattice parameters at different temperature. 
Sample Z2 (350 °C) Sample Z3 (400 
°C) 
 Sample Z4 (450 
°C) 
 Sample Z5 (500 
°C) 
(h
k
l) 
d
h
k
l (Å
) 
a (Å
) 
c (Å
) 
c/a 
d
h
k
l (Å
) 
a (Å
) 
c (Å
) 
c/a 
d
h
k
l (Å
) 
a (Å
) 
c (Å
) 
c/a 
d
h
k
l 
(Å
) 
a (Å
) 
c (Å
) 
c/a 
(1
0
0
) 
2
.8
1
2
1
3
.2
4
7
1
5
.1
8
0
4
1
.5
9
5
4
2
.8
0
7
2
3
.2
4
1
5
5
.1
9
5
4
1
.6
0
2
8
2
.8
1
4
9
3
.2
5
0
4
5
.1
9
9
2
1
.5
9
9
6
2
.8
1
6
9
3
.2
5
2
7
5
.2
0
9
4
1
.6
0
1
6
(0
0
2
) 
2
.5
9
0
2
2
.5
9
7
7
 2
.5
9
9
6
 2
.6
0
4
7
20 TRƯỜNG ĐẠI HỌC THỦ ĐÔ HÀ NỘI 
Zn
O bulk 
JCPDS 
card 36-
1451 
3
.2
4
9
8
5
.2
0
6
6
1
.6
0
2
1
 3.2
4
9
8
5
.2
0
6
6
1
.6
0
2
1
3
.2
4
9
8
5
.2
0
6
6
1
.6
0
2
1
3
.2
4
9
8
5
.2
0
6
6
1
.6
0
2
1
 They are indexed as a wurtzite structure of ZnO (JCPDS card No.36-1451). These results 
indicate that the conversion to ZnO increases with the increase in annealing temperature. It is 
clear that the zinc films were completely transformed to ZnO. Furthermore, increasing the 
annealing temperature leads to enhancement of the intensity of XRD peaks and the peaks 
become shaper (figure 2(d)). Hence, their crystallinity is improved.The lattice parameters 
were evaluated using the equation [7] : 
2 2 2
2 2 2
hkl
1 4 h +hk+k l
= +
d 3 a c
 (1) 
Where d is the measured space of (hkl) planes, which is determined by Bragg’s Law. 
h,k,l are the Miller’s indexes of crystalline plane relevant to the measured d-spacing. a and c 
are the lattice parameters of a wurtzite structure. The results are given in table 1. The strain 
in the direction of c axis can be calculated from equation [4]: 
film bulk
bulk
c - c
 = ×100%
c

 (2)
Where cfilm is the lattice parameter of c axis for the ZnO films, and cbulk is the lattice 
parameter of ZnO in bulk. The strain in the direction of c axis of the samples Z2, Z3, Z4, Z5 is 
-0.50 %; -0.22 %; - 0.14 %; 0.05 %, respectively. The negative sign indicates that the films 
are in a state of compression along c axis. For hexagonal crystals, the stress (σ) in the plane 
of the film can be calculated using the biaxial strain model[8]: 
2
13 33 11 12
film 
13
2C -C (C +C )
σ = ×ε
2C
 (3)
Where C11=209,7 GPa; C12=121.1 GPa; C13=105.1 GPa; C33=210.9 Gpa are the elastic 
stiffness constant of bulk ZnO. The results are Z2= 1.13 GPa; Z3= 0.49 GPa; Z4= 0.31 
Gpa; Z5= -0.11 Gpa. The stress in the film is plotted as a funtion of the annealing 
temperature in figure 3. The negative sign for the film annealed at 500°C indicates that the 
lattice constant is elongated in comparision with bulk ZnO. This result shows that at 
annealed temperature of 500°C, the film is in a state of elongation. The film goes into state 
TẠP CHÍ KHOA HỌC SỐ 2/2016 21 
of compression with decrease in annealing temperature from 500°C to 350°C and becomes 
completely stress free at about 485°C. These results can be explained as follows: It is well 
known that the defects cause the lattice disorder, which develops the stress in the films. 
After annealed, the interstitial oxygen is probably removed and the defects in the lattice are 
eliminated [3]. Furthermore, the increasing annealing temperature increases the atomic 
mobility and reduces the structural defects. The average crystallite size of the ZnO thin 
films is estimated by the Scherrer’s equation [11] as follows: D = 0.89λ/βcosθ, where D 
denotes the everage crystallite size of the ZnO thin films, λ= 0.15405 nm is the X-ray 
wavelength of CuKα, β is the width of the peak measured at half maximum intensity 
(FWHM) and θ is the Bragg’s angle of the peak. The crystallite sizes of the samples Z2, Z3, 
Z4, Z5 are 19 nm, 22 nm, 25 nm, 38 nm, respectively. The crystallite size of the films 
increases with the annealing temperature. 
3.3. Effect of thickness on the structure of ZnO thin films 
 Figure 4 shows the XRD pattern of ZnO thin films annealed at 450°C with thickness 
of 600 nm, 800 nm, and 1200 nm. As shown in figure 4.a, the full width at half maximum 
(FWHM) of (002) diffraction peak decrease with an increase in thickness, correspondingly the 
grain size of films increases with an increase in thickness. Lattice c can be calculated from 
the peak position (002) using the relation inferred from equation (1) c = 2d002. The strain in 
the direction of c axis of different thickness can be calculated from equation (2) and (3). 
Fig.3. Variation of the stress induced in ZnO 
films as a funtion of the annealing temperature. 
Fig.4. XRD pattern of ZnO at different 
thickness. 
temperature. 
22 TRƯỜNG ĐẠI HỌC THỦ ĐÔ HÀ NỘI 
The results are given in table 2. As can be seen from table 2, the lattice constant c increases 
from 5.1992 Å to 5.2020 Å on increasing the thickness from 600 nm to 1200 nm and the 
stress gradually decreases. This implies that the stress in the films can be relaxed by 
Table 2: Dependence of lattice constant on ZnO film thickness. 
Film thickness (nm) c (Å) Stress (Gpa) 
600 5.1992 0.32 
800 5.1996 0.30 
1200 5.2020 0.20 
increasing the film thickness. It is known that stress has a great influence on the crystal 
lattice development of materials. When the film has large stress, it tends to create particles 
of small size and vice versa. As mentioned above, increase annealing temperature leads to 
increase in particle size and the stress of the film relaxes. Similarly, the grain size of films 
increases with an increase in thickness leading the stress to be relaxed in the thicker films. 
4. CONCLUSIONS 
ZnO thin films have been successfully prepared by the thermal evaporation 
technique combined with thermal process. At an annealing temperature of 400°C, the 
ZnO films have a particle-like structure. At an annealing temperature of 450°C, the ZnO 
films have a flower-like structure. The smoother and approximately uniform surface can be 
observed at an annealing temperature of 500°C. The film goes into state of compression 
with decrease in annealing temperature from 500°C to 350°C and becomes completely 
stress free at about 485°C. At annealed temperature of 500°C, the film is in a state of 
elongation. The stress in the films can be relaxed by increasing the films thickness. In the 
next work, we will study the effect of annealing temperature on optical and photoelectronic 
properties of ZnO thin films fabricated by the thermal technique combined with thermal 
treatment. 
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Heinrich H. (2010), Effects of annealing on properties of ZnO thin films prepared by 
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ẢNH HƯỞNG CỦA NHIỆT ĐỘ Ủ LÊN CẤU TRÚC 
CỦA MÀNG MỎNG ZnO 
Tóm tắt: Trong bài báo, màng mỏng ZnO được chế tạo bằng cách làm bay hơi Zn sử 
dụng kỹ thuật bốc hơi chân không kết hợp ủ nhiệt. Các kết quả cho thấy sau khi ủ ở nhiệt 
độ 400°C, xuất hiện cấu trúc nano xốp và màng ZnO có cấu trúc hạt nano. Tại nhiệt độ ủ 
450°C, màng ZnO có cấu trúc hoa nano. Bề mặt màng phẳng hơn khi ủ tại nhiệt độ 
500°C. Màng ở trạng thái nén khi giảm nhiệt độ ủ từ 500ºC đến 350ºC và hoàn toàn tự 
do ở nhiệt độ ủ 485ºC. Các hằng số mạng c tăng từ 5.1992 Å đến 5.2020 Å khi tăng độ 
dày từ 600 nm đến 1200 nm và ứng suất giảm dần. Điều đó có nghĩa là ứng suất của 
24 TRƯỜNG ĐẠI HỌC THỦ ĐÔ HÀ NỘI 
màng giảm khi tăng độ dày màng. Cấu trúc của màng mỏng ZnO được nghiên cứu bằng 
phương pháp nhiễu xạ tia X (XRD). Hình thái màng được nghiên cứu bằng kính hiển vi 
điện tử quét phát xạ trường (FE-SEM). 
Từ khóa: màng ZnO, cấu trúc hoa nano, cấu trúc xốp, màng dày.