Thermoelectric property of Al-doped ZnO thin films based on solution synthesized by wet chemical reaction

TẠP CHÍ KHOA HỌC SỐ 2/2016 137 
THERMOELECTRIC PROPERTY OF AL-DOPED ZNO THIN 
FILMS BASED ON SOLUTION SYNTHESIZED BY WET 
CHEMICAL REACTION 
Trinh Quang Thong1
(1)
, Dinh Van Dzung
2
, Vu Van Doanh
3
, Le Hai Dang
3 
1
 Hanoi University of Science and Technology 
2
 University of Education – Vietnam National University Hanoi 
3
 Hanoi National University of Education 
Abstract: The multilayer Al-doped ZnO thin films are fabricated using the solution 
prepared by sol-gel technique at.1% and 2% Al concentrations on Silic and glass 
substrates. The crystal structure information structure and average grain size of the 
samples are characterized by X-ray diffraction (XRD). The rough surface with crystal 
grains for the heat treated films also is observed by the FESEM image. The grain sizes 
were determined in the range of 30 and 50 nm. The electrical conductivity was measured 
by means of four-point probe showing small electrical resistivity of Al doping. The 
thermoelectric property is studied between room temperature and around 673 K to 
evaluate the Seebeck coefficients and compared with each other showing the advantages 
and disadvantages of the samples depending on Al concentration, number of deposition 
layers and substrate materials. 
Keywords: Thin films, Al doped ZnO, sol-gel, XRD, SEM, Seebeck coefficient. 
1. INTRODUCTION 
Recently, much attention has been focused on the thermoelectric (TE) effects and 
materials for their ability to convert heat directly into electricity with a number of 
advantages over traditional electric generators, including design simplicity, the absence of 
moving parts, low-noise performance, high reliability, and miniaturization with no loss in 
efficiency. These devices have the potential to increase the utilization of industrial or home 
waste heat by recapturing the energy from these sources to generating electricity applied 
for power sources for spacecraft, wrist watches, portable household refrigerators, 
electronic, medical, and research equipment or vice versa like seat conditioning devices in 
(1)
 Nhận bài ngày 17.01.2016 gửi phản biện và duyệt đăng ngày 25.01.2016 
 Liên hệ tác giả: Trịnh Quang Thông; Email: thong.trinhquang@hust.edu.vn 
138 TRƯỜNG ĐẠI HỌC THỦ ĐÔ HÀ NỘI 
high-end cars. The efficiency of a TE converter is determined by the quantity Z or so-
called the figure of merit ZT which is proportional to the material electrical conductivity 
(), Seebeck coefficient (S), and inversely thermal conductivity () [1 -2]. 
In general, all materials exhibit TE effects. In this context, the semiconductors were 
found with adequate S, acceptable resistance that can be tuned by doping and low thermal 
conductivity and consequently possible large Z value. Among them the transparent ZnO 
was considered a popular n-type semiconductor having several favorable properties for TE 
application including high electron mobility and wide band-gap. In particular, recent 
studies of Al-doped ZnO (denoted AZO) showed the advanced potentials to be the novel 
TE materials with relatively good Seebeck coefficient at room temperature and belonging 
to a new research trend of oxide family which is the thermally and electrically stable in air 
at high temperature [3-10]. 
In fact, AZO thin films can be fabricated by physical vapor deposition, chemical vapor 
deposition [2-10]. In this paper, the multilayered and hetero-layered films were synthesized 
by spin and also dip coating methods using the solution prepared by low cost sol-gel 
technique at1% and 2% Al-doped concentrations on glass and SiO2/Si substrates. The 
characterization of crystal structure, surface morphology, electric conductance, and 
Seebeck coefficient of AZO thin films were measured and investigated. 
2. MATERIALS AND EXPERIMENTAL METHOD 
The precursor solution was synthesized using sol-gel process by mixing, in appropriate 
proportions, 0.5M of zinc acetate dihydrate (CH3COO)2 Zn-2H2O and aluminum nitrates 
9-hydrate Al(NO3)3-9H2O as dopant source. The Zn/Al ratios in the solution were chosen 
99/1 and 98/2 at % relying on the reported works [3-11] that showed the best TE property 
corresponding to those ratios. In our work, ethylene glycol C2H6O2, isopropanol 
(CH3)2CHOH, diethanolamine glyerin OHCH2CHOHCH2OH, and triethylenamin C6H15N 
were used as solvent and catalyst. The Al-doped ZnO solutions were kept under continuous 
magnetic stirring for 10 hour at room temperature. The detail process for synthesis of 
solution is shown in Fig.1. 
TẠP CHÍ KHOA HỌC SỐ 2/2016 139 
Fig.1: Flow chart showing sol-gel synthesis of AZO solution 
SiO2/Si and Corning 1737F glasses are used as substrates for studies. Spin and dip 
coating method were used for making multilayered or heterolayered films by repeated 
depositions on SiO2/Si substrate at room temperature at a ramp-up rate of 1500 rpm for 
30s. The multilayered films were fabricated by using only solution of 2% Al doping. 
Whereas, the heterolayered films were made by deposition of alternatively one or double 
layer of solution with doped Al concentration at 1% and the other one of solution with 
doped Al concentration at 2%. The dip coating was performed with the withdrawing speed 
of the glass substrate was set at 20 mm/min. The glass laminas with dimension of 50mm x 
10mm x 1mm were strongly washed and dried at 250
0
C for several hours at ambient air 
before utilization. For both methods, after each deposition, the AZO films were preheated 
in air at 250 C for 20 min. The processing step was repeated in those ways between 2 to 6 
times for a desired thickness. After the multiple coatings, the AZO coated samples were 
finally post-heated at 550
0
C in air using a furnace for 4 hrs to set the crystal structures. 
X-ray diffraction (Cu–Kα, Siemen D5005 Brucker,  = 1.54056 A) was employed to 
identify the crystal structure of the samples at room temperature. SEM Hitachi S-4800 
system was used to examine the grain size and morphology. The sheet resistivity of the 
AZO thin films was analyzed by DC four-terminal method, and its TE characterization was 
investigated by using a home-made Seebeck measurement setup (Fig.2). 
140 TRƯỜNG ĐẠI HỌC THỦ ĐÔ HÀ NỘI 
Fig.2: (a) Schematic representation of the whole instrumental for arrangement 
Seebeck coefficient, and (b) holder with installed sample (small image at top left corner) 
3. RESULTS AND DISCUSSION 
The X-ray diffraction (XRD) patterns of the AZO films are displayed in Fig. 3. All 
samples had the same crystal phase and hexagonal wurtzite structure, peak intensity 
increased for thicker films and Al concentration at 2% compared to thinner ones and at 1%. 
Three XRD diffraction peaks can be found at 31.8
0
, 34.4
0
 and 36.2
0
, corresponding to 
(100), (002) and (101) planes in the AZO films, respectively. The presence of these 
principal peaks reveals a random orientation of the nanocrystals. For the films deposited on 
glass substrate, the structure was preserved with (002) preferred crystal orientation. It is 
because of (002) plane surface low free energy which makes the growth along the c axis 
faster than those of the others crystallographic directions and confirms that the films have 
the existence of both columnar and polygon surface structure. In case of the films 
deposited on SiO2/Si there was a different preferential in (101) orientation meaning that the 
film has a textured pyramid-like structures for both multilayered and heterolayered films. It 
also can be seen the presence of the amorphous phase on these XRD patterns that may be 
due to the influence of substrate structures. 
TẠP CHÍ KHOA HỌC SỐ 2/2016 141 
(a) (b) 
Fig.3: XRD patterns of AZO thin films on glass (a) and SiO2/Si substrate (b) 
Generally, it can be said that the alternative structure of heterolayered films did not 
change their crystal structure. Based on the measured XRD patterns, the average grain size 
can be calculated using famous Scherrer's formula. The grain size of the AZO crystallite 
calculated using that equation is in range of 12 to 14 nm. Fig. 4 shows the film morphology 
obtained by the scanning electron microscopy (SEM) images. The typical clusters of the 
coherent grains can be easily observed in these pictures. Visually, the AZO thin films 
fabricated in this study do not have the round-shaped grains but the elongated ones with 
length of about 30 to 50 nm and size of about 15 to 20 nm. It can be seen that the two-layer 
films include many pores whereas the four-layer films are more connected and dense. 
Fig.4: SEM images of AZO films consisting of two (a) and four (b) layers 
Both the electrical and TE property of AZO thin films were investigated at the 
temperature range between 300 K and 673 K. the measurement data of sheet resistivity is 
presented in Fig.5. 
142 TRƯỜNG ĐẠI HỌC THỦ ĐÔ HÀ NỘI 
Fig.5: Sheet resistivity of AZO thin films 
Commonly, the samples exhibit the higher sheet resistivity for films with 2 at % Al 
doping compared to that with 1 at % Al doping. Its value is of the order 10
6
 /sq at room 
temperature but possibly down to the order of 10
5
 /sq at 673K, especially for films with 2 
at % Al doping. In general, these values are still large may be due to the small grain size 
and existence of pores in films. It is because the electron mobility in thin films is limited 
by grain boundaries. Accordingly, in order to decrease the resistance for free electrons the 
crystalline size should be increased because of the lower grain-boundary scattering. The 
Seebeck coefficient is determined by measuring V(T) at two temperatures which are hot 
temperature (TH) and cold one (TC). The plots of Seebeck coefficient (S) versus 
temperature (T) are shown in Fig.6. 
Fig.6: Seebeck coefficient vs temperature characterization of the AZO thin film at 
different operating temperatures 
TẠP CHÍ KHOA HỌC SỐ 2/2016 143 
Here, the values of S are in the negative range showing that AZO is n-type. Its 
absolute value at every measurement point increases as temperature increases. Namely, the 
lowest and highest value is about 125 V/K and 220 V/K for the films with 1 at % Al 
doping and 100 V/K and 165 V/K for the films with 2 at % Al doping, respectively. 
Clearly, it is a little bit decreased with more doped Al concentration. Although the value of 
S of fabricated AZO thin films obtained in this study is relatively high however the 
resistance is still large and accordingly the efficiency may not demonstrate the best 
performance. 
4. CONCLUSION 
In this work, the ZnO solution with 2 at % and 1% Al doping were synthesized by sol-
gel method. The AZO thin films were formed on SiO2/Si and glass substrates. The material 
characterizations of crystal structure, morphology and electrical and TE properties were 
investigated. The phase formation of ZnO with hexagonal wurtzite structure and the grain 
growth of AZO thin films were observed. The obtained films provided pretty good values 
of Seebeck coefficient which is a typically expected characterization for TE applications. 
However, the film resistance is too high then the electrical conductance would be small 
and consequently the small figure of merit. Therefore, the TE characterization of AZO 
films in the future study should be improved may be by using other catalyst or doping 
elements. 
REFERENCES 
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films prepared by sol-gel dip-coating, Journal of the European Ceramic Society 24, pp.1009-1013. 
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Magnetron Sputtering and Their Physical Properties, Journal of the Korean Physical Society, Vol. 49, 
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TÍNH CHẤT NHIỆT ĐIỆN CỦA MÀNG ZNO PHA TẠP AL SỬ DỤNG 
DUNG DỊCH TỔNG HỢP BẰNG PHẢN ỨNG HÓA Ở PHA ƯỚT 
Tóm tắt: Màng ZnO pha tạp Al đa lớp đã được chế tạo sử dụng dung dịch tổng hợp bằng 
phương pháp sol-gel với nồng độ Al là 1% và 2% phủ trên đế silic và thủy tinh Pyrex. 
Cấu trúc tinh thể của màng tạo thành đã được kiểm tra bằng phép đo nhiễu xạ tia X 
(XRD). Hình thái học bề mặt của màng đã được khảo sát bằng ảnh hiển vi điện tử quét 
FESEM. Kích thước trung bình hạt tinh thể đã được xác định trong khoảng 30 đến 50 nm. 
Độ dẫn điện của màng đã được xác định thông qua phép đo điện trở theo phương pháp 
bốn mũi dò cho thấy điện trở thấp của màng pha tạp. Tính chất nhiệt điện đã được nghiên 
cứu dựa trên phép đo xác định hệ số Seebeck. Các phép đo điện này đã được nghiên cứu 
trong dải nhiệt độ từ nhiệt độ phòng đến 400°C (673 K) để đánh giá khả năng chịu nhiệt 
của vật liệu để xem xét ưu và nhược điểm của mẫu tùy thuộc nòng độ Al được pha tạp và 
số lần phủ màng trên đế. 
Từ khóa: Màng mỏng, ZnO pha tạp Al, sol-gel, XRD, SEM, hệ số Seebeck.