Nanomechanical and wettability properties of Bi2Te3 thin films: Effects of postannealing Sheng-Rui Jian, Phuoc Huu Le, Chih-Wei Luo, and Jenh-Yih Juang
Citation: Journal of Applied Physics 121, 175302 (2017); doi: 10.1063/1.4982911 View online: http://dx.doi.org/10.1063/1.4982911 View Table of Contents: http://aip.scitation.org/toc/jap/121/17 Published by the American Institute of Physics
JOURNAL OF APPLIED PHYSICS 121, 175302 (2017)
Nanomechanical and wettability properties of Bi2Te3 thin films: Effects of post-annealing Sheng-Rui Jian,1,a) Phuoc Huu Le,2 Chih-Wei Luo,3 and Jenh-Yih Juang3 1
Department of Materials Science and Engineering, I-Shou University, Kaohsiung 840, Taiwan Faculty of Basic Sciences, Can Tho University of Medicine and Pharmacy, 179 Nguyen Van Cu Street, Can Tho, Vietnam 3 Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan 2
(Received 8 March 2017; accepted 20 April 2017; published online 3 May 2017) In this study, Bi2Te3 thin films were deposited on SiO2/Si(100) substrates by pulsed laser deposition (PLD) at 250 C. The films were then annealed in-situ in the deposition chamber at various annealing temperatures (Ta) ranging from 200 to 300 C. The microstructural, morphological, and nanomechanical properties of the Bi2Te3 thin films were investigated by X-ray diffraction (XRD), scanning electron microscopy, and nanoindentation techniques, respectively. The XRD results indicated that all the Bi2Te3 thin films have high crystalline quality with predominant (0015) texture. Nano-indentation measurements performed with a Berkovich nanoindenter operating under the continuous contact stiffness measurement mode revealed that both the hardness and Young’s modulus of the Bi2Te3 films decreased with increasing Ta. In addition, the water contact angle measurements were carried out to delineate the effects of annealing on the changes in the surface energy and wettability of the films. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4982911]
I. INTRODUCTION
The V-VI compound semiconductor Bi2Te3 and its alloys have long been recognized as the most promising thermoelectric (TE) materials for near room temperature TE device applications.1–4 The performance of TE materials is determined by a figure of merit, ZT, given by ZT ¼ (S2r/j)T, where S, r, j, and T are the Seebeck coefficient, electrical conductivity, thermal conductivity, and temperature, respectively. Since materials with large r usually have large j at the same time, one of the major tasks to raise ZT is to engineer the microstructure such that j can be significantly reduced while keeping r relatively intact. For instance, Bhama et al.5 demonstrated that the spark plasma textured bulk n-type Bi2Te2.7Se3 and p-type Bi0.5Sb1.5Te3 could have 42% and 33% ZT enhancements over conventional plasma sintering samples, respectively. On the other hand, thermal annealing process has been demonstrated to be a viable way in enhancing the TE properties of Bi2Te3 thin films and bulks.6–8 In addition to improving the TE properties of materials, mechanical properties are also of critical importance when designing and fabricating the practical TE devices are concerned.9 Thus, understanding the mechanical properties of TE materials has been of great interest for fabricating the efficient and endurable devices. In this respect, nanoindentation has been used ubiquitously for characterizing the nanomechanical properties of a wide variety of film/substrate systems10–15 due to its high efficiency and convenience. In a typical load-displacement curve obtained from indentation measurement, one can easily extract the nanomechanical properties (such as hardness, elastic modulus, and fracture toughness)10–15 and also the tribological characteristics14,15 a)
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and elastic/plastic deformation behaviors16,17 of the indented materials. The other aspect that has been of increasing interest is the water wettability of the surface of the material, which is mainly governed by the chemical composition and microstructures.18 In particular, the hydrophobic surface has been one of the critical factors in many optoelectronic devices19 and solar cell20 applications. Therefore, how the annealing processes affect the behavior of hydrophobicity or hydrophilicity of the resultant surface is also of great importance in realizing the designed functionality for device applications. In this work, the nanomechanical properties of a series of Bi2Te3 thin films were investigated by nanoindentation using the continuous stiffness mode (CSM). The films were prepared by pulsed laser deposition (PLD) and then treated at various annealing temperatures (Ta ¼ 200, 250, and 300 C) for 1 h. The microstructural properties and surface morphology of the as-grown and annealed Bi2Te3 thin films were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The changes in nanomechanical properties of Bi2Te3 thin films are attributed to the effects of post-annealing on film crystallinity and grain size. It is also interesting to note that annealing seemed to have driven the film surface from being hydrophobic to hydrophilic. II. EXPERIMENTS
Bi2Te3 thin films were deposited on SiO2/Si (100) substrates at 250 C under an argon ambient pressure of 220 mTorr using PLD. UV pulses (with a pulse duration of 20 ns and a pulse energy of 280 mJ) from a KrF excimer laser (k ¼ 248 nm; repetition rate: 10 Hz) were focused on a stoichiometric polycrystalline Bi2Te3 target. The corresponding pulse fluence was approximately 6.5 J/cm2, and the target-to-substrate distance was set at 40 mm. The number of laser pulses delivered was 15 000 (deposition time, 25 min), yielding a film thickness of
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approximately 1.0–1.2 lm. The average growth rate was esti˚ /pulse. Annealing was carried mated to be approximately 0.77 A out immediately after the deposition by keeping the as-grown films in the PLD chamber at 200, 250, or 300 C for 1 h under an argon atmosphere of 220 mTorr. The microstructure and surface morphology of the obtained Bi2Te3 thin films were ana˚ ) and field lyzed by XRD (Panalytical X’Pert, CuKa, k ¼ 1.54 A emission SEM (JEOL JSM-6500), respectively. All the nanoindentation measurements were conducted at room temperature using the MTS NanoXPV system (MTS Corporation, Nano Instruments Innovation Center, Oak Ridge, TN, USA). The resolutions of the loading force and displacement are 50 nN and 0.1 nm, respectively. A Berkovich diamond indenter was pressed into the films up to a depth of 100 nm. The strain rate was varied from 0.01 to 1 s1. An additional harmonic movement, with the amplitude and frequency being set at 2 nm and 45 Hz, respectively, was simultaneously applied on the indenter to perform the continuous stiffness measurements (CSM) technique.21 During the nanoindentation process, the indenter was held at the peak load for 10 s before it was completely withdrawn from the specimen to prevent the influence of creep from interpreting unloading characteristics, which were of essential importance in computing the mechanical properties of the specimen. Before each test, it is important to wait until the thermal drift had reduced to below 0.01 nm/s. At least 20 indents were conducted on each sample in order to achieve statistical significance. The hardness (H) and Young’s modulus (E) of Bi2Te3 thin films were obtained from the load-displacement curves by using the analytic method developed by Oliver and Pharr,22 with the following relations: R
hc ¼ ht e
Pm ; S
(1)
A ¼ Aðhc Þ ¼ 24:56h2c ;
(2)
H ¼ Pm =A; rffiffiffi 1 p Eef f ¼ S ; 2 A
(3)
1 1 v2i 1 v2 ; ¼ þ Ei E Eef f
(4) (5)
where e, Pm, A, Eeff, S, and v are denoted as a constant depending upon the geometry of the indenter (¼ 0.75 for a Berkovich indenter), the peak indentation load, the projected area, the effective elastic modulus, the unloading stiffness, and Poisson’s ratio of the indented material, respectively. Ei (¼1141 GPa) and vi (¼0.07) are the elastic modulus and Poisson’s ratio for the diamond indenter.22 In addition, the surface wettability of all Bi2Te3 thin films under ambient conditions was monitored by using a Ramehart Model 200 contact angle (CA) goniometer with deionized water as the liquid at room temperature. III. RESULTS AND DISCUSSION
Figure 1 shows the XRD patterns of the as-deposited and annealed Bi2Te3 thin films. Only the Si substrate and
FIG. 1. XRD patterns of the as-deposited and annealed Bi2Te3 thin films at annealing temperatures (Ta) of 200 C, 250 C, and 300 C.
(00 l)-family peaks of Bi2Te3 thin films are observed, indicating that all the films are single phased without any discernible impurity and/or second phases. Moreover, it is clearly evident that the intensity of the (00 15) diffraction peak increases substantially and the full width at half maximum (FWHM) becomes narrower with increasing Ta, a clear signature of improving crystalline quality. We used Scherrer’s formula23 (Eq. (6) shown below) to extract the crystalline sizes of Bi2Te3 thin films from the XRD data DXRD ¼
0:9k ; b cos h
(6)
where b is the FWHM of the (00 15) peak in radian, k is the ˚ for the Cu Ka line source), wavelength of the X-ray (1.54 A and h is Bragg’s angle. The crystallite sizes are 36 nm, 37 nm, 38 nm, and 45 nm for as-deposited, 200 C-, 250 C-, and 300 C-annealed Bi2Te3 thin films, respectively. Although the results showed that the crystallite size increased only slightly, the overall crystallinity of the films was substantially improved with increased annealing temperature, presumably due to higher thermal energy provided. The enhanced thermally driven atomic diffusion is expected to facilitate the repairing of the dislocated atomic occupancies and even promote the coalescence of adjacent grains, leading to the improved crystallinity of the films.24 Fig. 2 shows the surface morphology of the as-deposited and annealed Bi2Te3 films. The SEM images revealed that, except for the as-deposited film (Fig. 2(a)), all the films are polycrystalline featuring a uniform fine-grained morphology with size in the submicrometer range. It is noted that the apparent grain size seen from SEM images is evidently significantly larger than the crystallite size estimated from the XRD results shown in Fig. 1. We believe that the apparent grain seen from the surface morphology might have consisted of many crystallites and even include some amorphous regions. Consequently, the sizes of particles observed in SEM images
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FIG. 2. SEM images of (a) as-deposited, (b) 200 C–annealed, (c) 250 C– annealed, and (d) 300 C– annealed Bi2Te3 thin films.
can be larger than the crystallite sizes estimated from the XRD results. The typical CSM load-displacement curves of the asdeposited and annealed Bi2Te3 thin films are shown in Fig. 3. As being indicated in Eqs. (1)–(5), the nanoindentation curves provide rich information about the elastic and plastic deformation behaviors, and the material and prominent parameters such as hardness and the Young’s modulus can be readily obtained. The values of hardness and Young’s modulus of the as-deposited and annealed Bi2Te3 thin films obtained from the load-displacement curves are listed in Table I. It is evident that both the hardness and Young’s modulus decrease significantly with increasing Ta. The suppression of the film hardness coincides with the increased crystallite size when Ta is increased from 200 to 300 C (see Fig. 4 and Table I). This result consistently follows the well-known Hall-Petch
FIG. 3. Nanoindentation CSM load-displacement curves of the as-deposited Bi2Te3 thin films and 200 C-annealed, 250 C-annealed, and 300 C-annealed Bi2Te3 thin films.
relationship,13 indicating that dislocation activities play the primary role in determining the mechanical properties and deformation behaviors of these films. This can also explain the apparent “pop-in” events in load-displacement curves for all films investigated in the present study (Fig. 3), wherein the occurrence of pop-in has been linked to abrupt plastic flow triggered by massive dislocation nucleation.16,17,27 It could also be observed from the curves that the apparent load for the initiation of pop-in events during nanoindentation is decreased for films annealed at higher Ta. Alternatively, recent studies on nanowires of various materials have evidently indicated that the surface stress state could significantly influence the Young’s modulus of the materials.28,29 Namely, in nanowires with diameters below around 30–50 nm, when the surface is under the compressive stress state, the Young’s modulus tends to decrease and vice versa. In the present case, since the crystallite sizes estimated by Scherrer’s formula are within the range of 35–45 nm, similar surface stress state relaxation effects might have also played a role in the reduction of Young’s modulus and hardness for films annealed at 300 C. As will be seen below, the surface energy of the present films indeed exhibits substantial changes after annealing. As being mentioned above that the surface energy can also be significantly influenced by the chemical composition and microstructure of the films, it is, thus, interesting and heuristic to investigate how annealing affects the surface properties of the Bi2Te3 films, which has been largely overlooked in this field. The results of wettability tests are shown in Fig. 5. It is evident from Fig. 5 that, while the as-deposited Bi2Te3 thin film exhibits the strongest hydrophobic behavior with a largest contact angle of 75.2 , the annealed Bi2Te3 thin films become more hydrophilic with the decreased contact angles (for example, 51.7 for Ta ¼ 300 C). In general, surface wettability is a measure of surface energy and is most commonly quantified
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TABLE I. The estimated crystallite size obtained from XRD, nanoindentation results, and contact angle and surface energy of the as-deposited and annealed Bi2Te3 thin films. PLD-derived Bi2Te3 thin films as-depositeda @ 220 mTorr annealeda @ 200 C @ 250 C @ 300 C PLD25 @ 2 103–2 105 Torr Bulk26
D (nm) (00 15)-peak
H (GPa)
Ef (GPa)
hCA
cds (mJ/m2)
36
3.4 6 0.3
82.4 6 9.7
75.2
22.8
37 38 45
3.3 6 0.7 2.7 6 0.6 2.2 6 0.4
61.3 6 4.1 55.8 6 4.8 43.1 6 3.7
73.1 72.5 51.7
23.4 23.6 29.5
11–20 —
2.9–4.1 1.26
106–127 41.80
— —
— —
a
This work.
cls ¼ cs þ cl 2
qffiffiffiffiffiffiffiffiffiffi cds cdl ;
(7)
where cdl and cds are denoted as the dispersive portions of the surface tension for the liquid and solid surfaces, respectively. Using the Young’s equation20 combined with Eq. (7) and employing nonpolar liquid deionized water (72.8 mJ/m2) as a testing liquid, cdl is equal to cl , and the Girifalco-GoodFowkes-Young equation becomes cds ¼
FIG. 4. Hardness and Young’s modulus of the as-deposited and annealed Bi2Te3 thin films.
by the contact angle, hCA.20 The surface energy for all Bi2Te3 thin films was calculated using the Fowkes-Girifalco-Good (FGG) theory.30 According to the analysis of the FGG method, the considered critical interaction is the dispersive force or the van der Waals force across the interface existing between the water droplet and the solid surface. The FGG equation is given as
1 c ðcos hCA þ 1Þ; 4 l
(8)
where cdS is the surface energy of the calculated materials. Hence, by straightforward analyses, the surface energy obtained for the as-deposited Bi2Te3 thin film is 22.8 mJ/m2 and that for films annealed at Ta of 200 C, 250 C, and 300 C is 23.4, 23.6, and 29.5 mJ/m2, respectively, consistent with the monotonically decreasing contact angle (or increasing hydrophilicity) with increasing Ta seen in the measurements. Several explanations for the thermal-induced hydrophilicity have been proposed, including the oxygen vacancy sites,31 surface structures,32 and changes in surface roughness.33 In our case here, since the surface structure and roughness (Fig. 2) seemed to remain rather similar between the annealed films, the healing of atomic impairment seems to be more relevant. Nevertheless, more systematic investigations are certainly needed to delineate what actually happens.
IV. CONCLUSIONS
FIG. 5. Contact angle and surface energy of the as-deposited and annealed Bi2Te3 thin films. The inset shows image of a water droplet on the surface of the Bi2Te3 thin films.
We report the structural, morphological, nanomechanical, and wetting properties of the as-deposited and in-situ annealed Bi2Te3 thin films grown by PLD. All Bi2Te3 thin films exhibited a highly (00 l)-preferred growth orientation. The films are polycrystalline, and their grain sizes increase considerably with increasing Ta. The hardness and Young’s modulus of the Bi2Te3 thin films decrease significantly from 3.3 6 0.7 GPa to 2.2 6 0.4 GPa and from 61.3 6 4.1 GPa to 43.1 6 3.7 GPa when Ta increased from 200 to 300 C, respectively. In addition, the contact angles of Bi2Te3 thin films decrease remarkably from 73.1 for Ta ¼ 200 C to 51.7 for Ta ¼ 300 C. This indicates that the annealed
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Bi2Te3 films become more highly hydrophilic and thus have the higher surface energies with increasing Ta. ACKNOWLEDGMENTS
Financial support from the Ministry of Science and Technology, Taiwan, under Contract Nos. MOST105-2112 M-214-001 and MOST 103-2112 -M-009-015-MY3 and Vietnam National Foundation for Science and Technology Development (NAFOSTED) under Grant No. 103.99–2015.17 is gratefully acknowledged. 1
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