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

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THERMOELECTRIC PROPERTY OF AL-DOPED ZNO THIN

FILMS BASED ON SOLUTION SYNTHESIZED BY WET

CHEMICAL REACTION

(1)

Trinh Quang Thong¹, Dinh Van Dzung², Vu Van Doanh³, Le Hai Dang³

¹Hanoi University of Science and Technology

²University of Education – Vietnam National University Hanoi

³Hanoi 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

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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)₂Zn-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.

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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 250⁰C 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 550⁰C 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).

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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 31.8⁰, 34.4⁰and 36.2⁰, 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.

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(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.

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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 10⁶/sq at room

temperature but possibly down to the order of 10⁵/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 (T_H) and cold one (T_C). 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

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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

1. D.M. Rowe (1995), CRC Handbook of Thermoelectrics, CRC Press, New York, NY, USA.

2. H. Julian Goldsmid (2010), Introduction to Thermoelectricity, e-ISBN 978-3-642-00716-3, Springer

Heidelberg Dordrecht London New York.

3. G.G. Valle, P. Hammer, S.H. Pulcinelli, C.V. Santilli (2004), Transparent and conductive ZnO:Al thin

films prepared by sol-gel dip-coating, Journal of the European Ceramic Society 24, pp.1009-1013.

4. Z.-Q. Xu, H. Deng, Y. Li, and H. Cheng (2006), “Al-doping effects on structure, electrical and optical

properties of c-axis-orientated ZnO:Al thin films, Materials Science in Semiconductor Processing, vol.

9, No.1–3, pp.132-135.

5. Jeung Hun Park (2006), Deposition-Temperature Effects on AZO Thin Films Prepared by RF

Magnetron Sputtering and Their Physical Properties, Journal of the Korean Physical Society, Vol. 49,

pp.584-588.

6. Seoung-Soo Lee et al. (2008), Thermal Degradation Behavior of Aluminum-Doped Zinc-Oxide Thin

Films Prepared by Using a Sol-Gel Process, Journal of the Korean Physical Society, Vol. 53, No. 1,

pp.181-191.

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7. L.Li, et al. (2008), Influence of oxygen argon ratio on the structural, electrical, optical and

thermoelectrical properties of Al-doped ZnO thin films, Physica E, 41, pp.169-174.

8. P. Mele et al. (2013), Effect of substrate on thermoelectric properties of Al-doped ZnO thin films, Appl.

Phys. Lett. 102, 253903; doi: 10.1063/1.4812401-1-4.

9. S. Saini, et al. (2014), Thermoelectric Properties of Al-Doped ZnO Thin Films, Journal of Electronic

Materials, DOI: 10.1007/s11664-014-2992-1-6.

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properties, Journal of Materials Chemistry A, The Royal Society of Chemistry.

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 mꢀi 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.