Effect of annealing temperature on the structure of ZnO films

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TRƯỜNG ĐẠI HỌC THỦ ĐÔ HÀ NỘI

EFFECT OF ANNEALING TEMPERATURE ON THE

STRUCTURE OF ZnO FILMS

Dang Tran Chien¹⁽¹⁾, Ta Anh Tan²

¹Hanoi University of Natural Resources and Environment

²Hanoi 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: [email protected].

TẠP CHÍ KHOA HỌC  SỐ 2/2016

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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^-2torr. 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.

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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

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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

Sample Z4 (450

Sample Z5 (500

°C)

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TRƯỜNG ĐẠI HỌC THỦ ĐÔ HÀ NỘI

Zn

O bulk

JCPDS

card 36-

1451

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] :

1

d²_hkl

4 h²+hk+k²l²

+

=

3

a²

c²

(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]:

c_film- c

=

^bulk×100%

c_bulk

(2)

Where c_filmis the lattice parameter of c axis for the ZnO films, and c_bulkis 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]:

2C₁²₃-C₃₃(C₁₁+C₁₂)

σ_film

=

×ε

2C₁₃

(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.

Fig.3. Variation of the stress induced in ZnO

Fig.4. XRD pattern of ZnO at different

films as a funtion of the annealing temperature.

thickness.

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).

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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)

0.32

600

800

5.1992

5.1996

5.2020

0.30

1200

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.

REFERENCES

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aluminum and indium incorporation on the structural and optical properties of ZnO thin films

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2. Chung J.-L., Chen J.-C., and Tseng C.-J. (2008), Electrical and optical properties of TiO₂-

doped ZnO films prepared by radio-frequency magnetron sputtering. Journal of Physics and

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optical properties of sputtered zinc oxide film. Journal of Applied Physics 80, pp.1063-1073.

4. Hwang K.S., Jeon Y. S, and Kang B.A. (2005), Crystallinity, Surface Morphology and Optical

Properties of Nanocrystalline ZnO Films Prepared with a Metal Naphthenate. Journal of the

Korean Physical Society 46, pp.521-526.

5. Jiang Y., Wu M., Wu X., Sun Y., and Yin H.: Low-temperature hydrothermal synthesis of

flower-like ZnO microstructure and nanorod array on nanoporous TiO2 film. Materials Letters

63, pp.275-278.

6. Lin J.-P. and Wu J.-M. (2009), The functional sequence of hydrogen in electronic conduction

of sol–gel derived Al-doped ZnO films as determined via impedance spectroscopy. Scripta

Materialia 60, pp.313-316.

7. Lupan O., Pauporté T., Chow L., Viana B., Pellé F., Ono L. K., Roldan Cuenya B., and

Heinrich H. (2010), Effects of annealing on properties of ZnO thin films prepared by

electrochemical deposition in chloride medium. Applied Surface Science 256, pp.1895-1907.

8

Rao T. P., Kumar M. C. S., Angayarkanni S. A., and Ashok M. (2009), Effect of stress on

optical band gap of ZnO thin films with substrate temperature by spray pyrolysis. Journal of

Alloys and Compounds 485. pp.413-417.

9. Tran Chien Dang, Duy Long Pham, Huu Lam Nguyen, and Van Hoi Pham (2010), CdS

sensitized ZnO electrodes in photoelectrochemical cells. Advances in Natural Sciences:

Nanoscience and Nanotechnology 1, pp.035010-6.

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11. Wang M. C., Lin H. J., and Yang T. S. (2009), Characteristics and optical properties of iron ion

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and Compounds 473, pp.394-400.

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

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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.