Electron microscopy study of thermoelectric n-type Bi2(Te0.9Se0.1)3 film deposited by dc sputtering
Introduction
Transformation of electrical and thermal energy in cooling systems is very important. The material, which converts the thermal energy to electrical energy or vice versa, is called as thermoelectric material [1]. Thermoelectric materials are of interest for applications in solid-state cooling and electrical power generation devices based on the Seebeck and Peltier effects due to their many attractive features, such as long life, no moving parts, no emission of toxic gases, low maintenance and high reliability [2]. Binary and ternary semiconductor V–VI compounds, such as Bi2Te3 and its alloys with Sb2Te3 and Bi2Se3, are the best materials for thermoelectric applications at room temperature so far reported [3], [4]. Bi2Te3 and Sb2Te3 are narrow-band-gap semiconductors and possess promising potential applications for thermoelectric micro-devices [5]. Thermoelectric devices can directly convert waste heat to electricity, or electricity into thermal energy [6].
However, small size and thickness and integration with other new devices cannot be solved with existing bulk materials. The primary approach to miniaturize thermoelectric devices is to employ thin film technology. Traditional thermoelectric devices have been fabricated from sintered blocks of the materials. But, there are certain difficulties and limitations in making highly miniaturized devices due to the cutting and assembling processes. For low dimensional applications, development and further investigations of thermoelectric thin films are necessary [7], [8], [9]. The performance of thermoelectric devices depends on the figure of merit (ZT) of the material. In comparison with bulk thermoelectric materials, thin film thermoelectric materials offer remarkable scope for ZT enhancement [10].
Because the physical properties of materials depend on their microstructures, microstructural characterizations are of great importance. Especially, detailed microstructural analysis performed by TEM offers great advantages in understanding the characteristic behaviour of materials. In recent years, thermoelectric film materials have been grown and studied intensively by many researchers [11], [12], [13], [14], [15]. The values of three parameters are very important for a thermoelectric material and affect device performance. These parameters are the Seebeck coefficient (S; V/K), the electrical conductivity (σ; Ω−1 m−1) and the thermal conductivity (κ; W m−1 K−1). Therefore, the dependence of these parameters on the type of substrate material, temperature, chemical composition, film thickness, heat treatment temperature and, most importantly, microstructural properties of the film are investigated. The microstructure of thermoelectric material is the most distinctive feature that determines the performance and use for electronic or optical devices.
Thermoelectric ternary n-type Bi2(Te0.9Se0.1)3 film was used in the present work to investigate the microstructure in plan-view and cross-section samples. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques were used.
Section snippets
Experimental procedures
The n-type Bi2(Te0.9Se0.1)3 film was deposited by direct current (dc) sputtering technique on polyimide foil at substrate temperature of 200 °C. Before deposition, the base pressure in the vacuum chamber was lower than 1 × 10−4 Pa. The film was deposited at a sputtering rate of 6.051 nm/s from Bi2Te2.7Se0.3 target in Ar gas atmosphere. Surface morphology of the film was observed with a field emission scanning electron microscope (FESEM, JEOL JSM7001F). The film thickness was determined to be ∼14.2 μm
Results
Fig. 1a displays a low-magnification SEM image of plan-view Bi2(Te0.9Se0.1)3 sample. The film surface contains mound-like structures ranging from ∼0.6 μm to ∼2.8 μm in size. These mound-like structures are crystal agglomerates. The film surface is rough. EDX spectrum was taken from the whole surface of the sample in order to perform chemical element analysis of the film surface. The surface structure of the film is composed of 56.35 at.% Te, 37.33 at.% Bi and 6.32 at.% Se. Fig. 1b shows more
Discussions
In order to determine the film stoichiometry, EDX spectra were taken from different points and regions of the film surface. The EDX spectra results indicate that the average atom proportions of Bi:Te:Se in Bi2Te2.7Se0.3 is 37.03:56.49:6.48, which are in a good agreement with nominal composition for the film. The stoichiometry of the film is close to the initial composition (atom proportions Bi:Te:Se in sputtering target material is 40:54:6). The average percentage of deviation from
Conclusions
Thermoelectric film of n-type Bi2(Te0.9Se0.1)3 was deposited on polyimide foil substrate by dc sputtering. The surface morphology, elemental composition and microstructure of the film were investigated by SEM, EDX and TEM. SEM images showed that the film is polycrystalline in nature with rough surface causing crystal agglomerates similar to each other. EDS studies indicated that the average atom proportions of Bi:Te:Se in Bi2Te2.7Se0.3 is 37.03:56.49:6.48 and the film stoichiometry was
Acknowledgements
This experimental work was partially carried out through a project (No.: 1386) at Firat University Electron Microscopy (FUEM) Laboratory, TR. Therefore, Firat University and its former rector, Professor M. Hamdi Muz, are acknowledged for the financial supports.
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