Nanoindentation pop-in effects of Bi2Te3 thermoelectric thin films

Journal of Alloys and Compounds 622 (2015) 601–605

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Nanoindentation pop-in effects of Bi2Te3 thermoelectric thin films Sheng-Rui Jian a,⇑, Cheng-Hsun Tasi a, Shiau-Yuan Huang a, Chih-Wei Luo b a b

Department of Materials Science and Engineering, I-Shou University, Kaohsiung 840, Taiwan Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan

a r t i c l e

i n f o

Article history: Received 4 September 2014 Received in revised form 16 October 2014 Accepted 25 October 2014 Available online 1 November 2014 Keywords: Bi2Te3 thin films PLD Pop-in Nanoindentation Hardness

a b s t r a c t The structural, surface morphological and nanomechanical characteristics of Bi2Te3 thin films are investigated by means of X-ray diffraction (XRD), atomic force microscopy (AFM) and nanoindentation techniques. The Bi2Te3 thin films are deposited on c-plane sapphire substrates using pulsed laser deposition (PLD). The XRD result showed that Bi2Te3 thin film had a c-axis preferred orientation and a smoother surface feature from AFM observation. Nanoindentation results exhibit the discontinuities (so-called multiple ‘‘pop-ins’’ event) in the loading segments of the load–displacement curves, indicative of the deformation behavior in the hexagonal-structured Bi2Te3 thin film is the nucleation and propagation of dislocations. Based on this scenario, an energetic estimation of nanoindentation-induced dislocation resulted from pop-in effects is made. Furthermore, the hardness and Young’s modulus of Bi2Te3 thin films were measured by a Berkovich nanoindenter operated with the continuous contact stiffness measurements (CSM) mode. The obtained values of the hardness and Young’s modulus are 5.7 ± 0.8 GPa and 158.6 ± 6.2 GPa, respectively. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Bismuth telluride, Bi2Te3, is a V–VI compound semiconductor (with an indirect band gap of 0.15 eV [1]) well-known for its pronounced hexagonal layered structure [2] and excellent thermoelectric properties [3,4]. Bi2Te3 is one of the most efficient thermoelectric materials in the applications of thermoelectric devices because it has a high figure-of-merit, ZT ¼ ðS2 rÞ=ðkTÞ, of about 1 at room temperature, where S is the Seebeck coefficient, r is the electrical conductivity, k is the thermal conductivity and T is the absolute temperature. Further, the fabrication of thermoelectric devices, such as cooling devices [5] and energy harvesting devices [6], by using the thin films techniques is very important for the practical applications. Various processing techniques, including sputtering [7,8], molecular beam epitaxy (MBE) [9,10], metal– organic chemical vapor deposition (MOCVD) [11] and pulsed laser deposition (PLD) [12,13], have been used to grow the Bi2Te3 thin films. Among the growth techniques, PLD is a technique widely used for the growth of multi-element materials due to its high-energy flux and capability of preserving the stoichiometries from the target materials. The growth rate achieved by PLD can be easily varied by adjusting the repetition rate and energy density of laser pulses, which is useful for both atomic level investigations ⇑ Corresponding author. Tel.: +886 7 6577711x3130; fax: +886 7 6578444. E-mail address: [email protected] (S.-R. Jian). http://dx.doi.org/10.1016/j.jallcom.2014.10.133 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.

and for growing thick layer of films within reasonable time durations. Furthermore, the PLD method can also offer the potential of growing high-quality thin films at relatively lower substrate temperatures as compared to other techniques. In addition to monitoring the thermoelectric [3,4] and optoelectronic properties [14] through careful control of the processing parameters, successful fabrication of devices based on Bi2Te3 thin films also requires better understanding of the mechanical properties since the contact loading during processing or/and packaging can significantly degrade the performance of these devices. Therefore, there is a growing demand of investigating the mechanical characteristics of materials, in particular in the nanoscale regime. To this respect, nanoindentation has been proven to serve as one of the powerful tools in revealing the important mechanical parameters, such as the hardness and elastic modulus [15–17], fracture behaviors [18–20] and creep resistance [21,22] of various nanostructured materials and thin films. Moreover, this method has other potential applications and studies of the indentationinduced phase transformation [23–25] and dislocation nucleation and propagation in materials [26–28]. During loading process, the initial segment of the load–displacement curve resulting from nanoindentation is usually the manifestation of the elastic behavior, whereas the onset of plastic deformation is generally associated with a significantly displacement discontinuity, i.e. ‘‘pop-in’’ event. The difference between the elastic and plastic deformation behaviors enables the analysis of the mechanical

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responses. The elastic behavior of materials can be analyzed by the elastic contact deformation relationship given by the Hertzian elastic contact model [29], as following:

P¼

4 pffiffiffiffiffiffi Er Rh 3

ð1Þ

Here P, R and h are denoted as the applied indentation load, the radius of indenter tip, and the corresponding indentation depth, respectively. The reduced elastic modulus, Er , is given by

Er ¼

1  v 2f Ef

1  v 2i þ Ei

!1 ð2Þ

where v is the Poisson’s ratio; E is the Young’s modulus and, the subscripts i and f are the indenter tip and indented films, respectively. The deviation of the experimental data from the Hertzian relationship is usually attributed to the onset of plastic deformation occurring when a load is applied to the material exceeds certain values [30]. No apparent features of pop-in event was observed and reported in the load–displacement curves of the Bi2Te3 materials and related thin films [31,32]. However, since the onset of nanoscale plasticity and mechanical properties of materials are strongly influenced by various factors, such as the temperature effects [33] and the crystal orientation [34], during nanoindentation testing, the interplays between the observed pop-in event and the onset of micro/nanoscale plasticity for Bi2Te3 thin films remain elusive. Therefore, it is not surprising to see that there have been some discrepancies in understanding the onset of plasticity on the nanometer-scale and, in some cases, the plastic deformation mechanisms of Bi2Te3 thin films were misinterpreted. Herein, the Berkovich nanoindentation-induced multiple pop-in events were observed and the mechanical properties of the hexagonal Bi2Te3 thin films were obtained by analyzing the nanoindentation load–displacement curves. Further, based on the classical dislocation theory [35], the number of Berkovich nanoindentation-induced dislocation loops formed in the pop-in event on Bi2Te3 thin films was also calculated. Consequently, the results shed some light on the understanding of the deformation behaviors of Bi2Te3 thermoelectric thin films on nanoscale. 2. Experimental details Experimentally, a pulsed KrF excimer laser (with the wavelength of 248 nm, the pulse duration of 20 ns, the pulse energy of 150 mJ, and the repetition rate of 2 Hz) was used to ablate the stoichiometric polycrystalline Bi2Te3 target. The laser power density on the target surface was about 5 J/cm2. The c-plane sapphire wafers were used as the substrates and fixed the substrate temperature at 250 °C. Before the deposition, the chamber was pumped down to less than 2  106 Torr by using a mechanical pump and a turbo molecular pump. The helium gas pressure was kept at 2  101 Torr. The thickness of Bi2Te3 thin films is about 300 nm. The structural properties of Bi2Te3 thin films are inspected by using X-ray diffraction (XRD; Bruker D8 Advance TXS with Cu Ka radiation, k = 1.5406 Å). The surface morphology is observed by atomic force microscopy (AFM; TopometrixAccures-II). Additionally, the mechanical properties (hardness and Young’s modulus) of Bi2Te3 thin films are investigated using an MTS Nano IndenterÒ XP instrument with a three-sided pyramidal Berkovich indenter tip. The indenter tip has a nominal radius of about 50 nm with the pyramidal faces forming an angle of 65.3° with the vertical axis. Prior to applying loads to the Bi2Te3 thin films indentations were conducted on the standard sample (fused silica with a Young’s modulus of 68–72 GPa) to obtain the reasonable loading range. The continuous stiffness measurements (CSM) [36] were carried out by superimposing a small-amplitude and 75-Hz oscillation on the force signal to record stiffness data along with load and displacement data dynamically. Firstly, the indenter was loaded and unloaded three times to ensure that the tip was properly in contact with the surface of the materials and that any parasitic phenomenon is released from the measurements. Then, the indenter was loaded for a fourth and final time at a strain rate of 0.05 s1, with a 60 s holding period inserted at peak load in order to avoid the influence of creep on unloading characteristics, which were used to compute mechanical properties of the specimen. Also, for the sake of obtaining steady mechanical properties and preventing interference from environmental fluctuation

factor, each test was performed when the thermal drift dropped down to 0.01 nm/s. The analytic method developed by Oliver and Pharr [37] was used to measure the hardness (H) and Young’s modulus (E) of Bi2Te3 thin films from the load–displacement curves. In order to reveal the Berkovich nanoindentation-induced features of Bi2Te3 thin films surface, cyclic nanoindentation tests were also performed in this study. These tests were carried out by the following sequences. First, the indenter was loaded to a chosen load and then unloaded by 90% of the previous load, which completed the first cycle. It then was reloaded to a larger chosen load and unloaded by 90% for the second cycle. Fig. 3(a) shows a typical cyclic indentation test repeated for 4 cycles, revealing features such as the multiple pop-ins phenomena in the loading segment of load–displacement curve. More detailed discussion on these features will be given later. It is noted that in each cycle, the indenter was hold for 10 s at 10% of its previous maximum load for thermal drift correction and for assuring that complete unloading was achieved. The thermal drift was kept below ±0.05 nm/s for all indentations considered in this study. The same loading/ unloading rate of 10 mN/s was used. After that, scanning electron microscopy (SEM) studies were performed with Hitachi S3400N, Japan, at 7 kV operating voltage in secondary electron mode.

3. Results and discussion The XRD result of Bi2Te3 thin films deposited on c-plane sapphire is shown in Fig. 1(a). The clear diffraction peaks of (0 0 3), (0 0 6), (0 0 1 5), (0 0 1 8) and (0 0 2 1) indicate that Bi2Te3 thin films are highly c-axis-oriented textured, as shown in Fig. 1(a). The obtained diffraction peaks are in good agreement with the previous study [38], confirming that the present films are indeed purely caxis-oriented hexagonal structured Bi2Te3. In addition, the average grain size is estimated to be 30 nm (calculated by using Scherrer’s formula [39] with the full-width at half maximum (FWHM) of the (0 0 1 5) peak). The AFM scan of a 1 lm  1 lm area displayed in Fig. 1(b) reveals a surface roughness [40] value of 0.75 nm, which is much smaller than the typical indentation depth thus should not have noticeable effects during nanoindentation. The typical nanoindentation load–displacement curve obtained for Bi2Te3 thin film is displayed in Fig. 2(a). The total penetration depth into the Bi2Te3 thin film was 50 nm with a peak load of 0.35 mN, which is well within the nanoindentation criterion suggested by Li et al. [41], which states that the nanoindentation depth should never exceed 30% of films thickness or the size of nanostructures under test. The results displayed in Fig. 2, thus, should reflect primarily the intrinsic properties of the present Bi2Te3 thin film. It is evident from Fig. 2(a) that there are several pop-ins events occurring along the loading segment of the load–displacement curve with the threshold loading of around 0.03 mN corresponding to the first pop-in point. On the other hand, as can be seen in both Figs. 2(a) and 3(a), the deformation between pop-ins is predominantly elastic even with load up to 10 mN, suggesting that slip process should play a prominent role in the deformation mechanisms of this layered material. Furthermore, since the multiple pop-ins are generally closely related to the sudden collective activities of dislocations (such as dislocation generation or movement bursts), it is suggestive that during the course of plastic flow preferential collective slips might be occurring by activating the pre-existing dislocations during thin film growth or following nucleation of dislocations when some critical strain has reached [42]. The penetration depth dependence of hardness and Young’s modulus in Bi2Te3 thin film are displayed in Fig. 3(b) and (c), respectively. In Fig. 3(b), it can be found that the penetration depth-dependent hardness of Bi2Te3 thin film can be roughly divided into two stages, namely the initial increase to a maximum value and an asymptotic decrease toward a nearly constant value after the penetration depth reaches 50 nm. The increase in hardness at small penetration depth is usually attributed to the transition between purely elastic to elastic/plastic contact whereby the hardness is really reflecting the mean contact pressure and is not representing the intrinsic hardness of Bi2Te3 thin film. Only

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Fig. 1. (a) XRD pattern obtained from 300-nm-thick Bi2Te3 thin film grown on c-plane sapphire by PLD. (b) AFM image of the surface of Bi2Te3 thin film. The surface roughness (Ra) is 0.75 nm.

Fig. 3. (a) The cyclic load–displacement curves obtained at an indentation load of 10 mN for Bi2Te3 thin film. Notice that the multiple ‘‘pop-ins’’ are observed (see the arrows), and (b) SEM micrograph of an indent at an applied load of 10 mN. The Berkovich indentation-induced pile-up event around the indented area is displayed.

Fig. 2. Nanoindentation results: (a) a typical load–displacement curve of Bi2Te3 thin film showing the multiple ‘‘pop-ins’’ during loading, (b) hardness-displacement curve and, and (c) Young’s modulus-displacement curve for Bi2Te3 thin film.

under a condition of a fully developed plastic zone does the mean contact pressure represent the hardness. When there is no plastic zone, or a partially formed plastic zone, the mean contact pressure (which is measured using the Oliver and Pharr method) is less than the nominal hardness. After the first stage, the hardness gradually decreases with increasing the penetration depth and eventually reaches a more or less constant value of 5.7 ± 0.8 GPa. It is also noted that within the penetration depth of 50 nm, the hardness appears to be quite ‘‘noisy’’ as a function of the penetration depth, presumably due to the extensive dislocation activities being activated in this stress range. Furthermore, in Fig. 3(c), Young’s

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modulus as a function of the penetration depth determined by using the method of Oliver and Pharr [37] also shows a similar tendency as that of films hardness. The obtained Young’s modulus for Bi2Te3 thin film is 158.6 ± 6.2 GPa. Both the hardness and Young’s modulus of PLD-derived Bi2Te3 thin films are much larger than the sputtering-derived Bi2Te3 thin films and bulk Bi2Te3 [31]. The reason for the apparent discrepancy is not clear at present. Now turning back to Fig. 2(a), the cyclic load–displacement curves obtained with the Berkovich indenter for a hexagonal Bi2Te3 thin film clearly displays multiple pop-ins in the indentation loading curves. In particular, due to the large indentation depth, the ‘‘pop-out’’ event observed in the last segment of unloading curve might be related to the indentation-induced phase transition occurring in single-crystal Si [25]. Moreover, as revealed by the SEM photograph shown in Fig. 3(b), the pile-up phenomena along the edges of the residual indented pyramid are observed. Therefore, the cause of the ‘‘later’’ multiple pop-ins may have been complicated by the involvement of the Berkovich indentation induced pile-up event on the surface of hexagonal Bi2Te3 thin film. Further, a closer look at the loading curves displayed in Figs. 2(a) and 3(a) reveals that the multiple pop-ins do not exactly coincide at the same indenter penetration depths. Since each curve is associated with a specific stress rate depending on the maximum indentation load, suggesting that the first pop-in event is not thermally activated. Instead, these phenomena have been ubiquitously observed in a wide variety of materials and are usually attributed to dislocation nucleation or/and propagation during loading [28,43], or micro-cracking [18–20] phenomenon. On the other hand, the pressure-induced structural phase transition in Bi2Te3 has been investigated by using diamond anvil cell (DAC) experiments [44] and first principle calculations based on the density function theory [45] previously. The magnitude of pressure required to induce phase transitions is significantly higher than the apparent room-temperature hardness of Bi2Te3 thin film measured in this study. Moreover, the multiple ‘‘pop-ins’’ have been reported previously in hexagonal structured sapphire [46] and GaN thin films [47,48], and evidences have indicated that the primary nanoindentation-induced deformation mechanism in these hexagonal-structured materials is the nucleation and the propagation of dislocations. It is thus quite plausible to state that similar mechanisms must have been prevailing in the present Bi2Te3 thin films. Within the scenario of the dislocation nucleation and propagation, the first pop-in event naturally reflects the transition from perfectly elastic to plastic deformation. That is, it is the onset of plasticity in Bi2Te3 thin film. Under this circumstance, the corresponding critical shear stress (smax ) under the Berkovich indenter at an indentation load, P c , where the load–displacement discontinuity occurs, can be determined by using the following relation [29]:

smax ¼

0:31

p

" 6P c

 2 #1=3 Er R

2

cdis ¼

Gb 8p

     2  vf 4r ln 2 rcore 1  vf

ð5Þ

where G is the shear modulus of Bi2Te3 thin film (=Ef/2(1 + vf)  64 GPa) and rcore is the radius of dislocation core. By using Eqs. (3) and (5), then Eq. (4) can be rewritten as:

F¼

  2  Gb r 2  v f 4r 2 ln  2  pbr sc 4 r core 1  vf

ð6Þ

Eq. (6) relates the material properties and experimentally observed pop-in load to the free energy responsible for dislocation nucleation; in addition, sc is the resolved shear stress of smax on the active slip system and is usually taken as half smax [51]. The nucleation of a dislocation loop is similar to the nucleation of a spherical new phase within a homogeneous matrix, where the corresponding function F contains terms with first and second power of r. F has a maximum at a critical radius, r c , above which the system gains energy by increasing r. According to Eq. (6), this maximum energy is decreased with increasing load and a pop-in, i.e., the homogeneous formation of circular dislocation loop, becomes possible without thermal energy at F ¼ 0 [52]. The F ¼ 0 condition allows sc to be found through Eqs. (4) and (5):

rc ¼

2cdis bsmax

Noting that and (7) yield:

rc ¼

ð7Þ

sc has a maximum value, when dsc =dr ¼ 0, Eqs. (5)

e3 r core  5r core 4

ð8Þ

Here, the value of rcore  0.9 nm is calculated; therefore, rc  4.5 nm can be calculated to obtain from Eq. (8). The number of dislocation loops formed during the first pop-in can, thus, be estimated from the work-done associated with the pop-in event. From the shaded area depicted in Fig. 4, this work

ð3Þ

where R is the radius of the indenter tip. The obtained maximum shear stress, smax , for Bi2Te3 thin film is about 2 GPa. This smax is responsible for the homogeneous dislocation nucleation within the deformation region underneath the indenter tip. It has been proposed that the first pop-in is associated with the homogeneous dislocation nucleation mechanisms during nanoindentation [30,49]. According to the classical dislocation theory [35], the shear stress required to initiate plastic deformation depends on the energy required to generate a dislocation loop. The free energy of a circular dislocation loop of radius, r, is given by:

F ¼ cdis 2pr  sbpr 2

where cdis , b and s are denoted as the line energy of the dislocation loop, the magnitude of Burgers vector (0.438 nm) [50] and the external shear stress acting on the dislocation loop, respectively. The first term on the right-hand side of Eq. (4) describes the energy required to create a dislocation loop in a defect-free lattice and is equal to the increase in lattice energy due to the formation of a dislocation loop. The second term gives the work done by the applied stress (s) as a result of the Burgers vector displacement and reflects the work done on the system to expand the dislocation. The line energy for a circular loop (cdis ), which results from the lattice strain in the vicinity of the dislocation for r > r core , is given by [35]:

ð4Þ

Fig. 4. The corresponding pop-in event (see the blue arrow) from Fig. 2(a) is zoomed in, where the plastic strain work is denoted as, W p (the critical loading times the sudden incremental displacement at constant load). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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is estimated to be 0.12  1012 Nm, implying that 3  103 dislocation loops with critical diameter might have been formed during the pop-in event. This number is low and is consistent with the scenario of homogeneous dislocation nucleation-induced pop-in, instead of activated collective motion of pre-existing dislocations [30]. When the total dissipation energy, namely the area between the loading and unloading curves shown in Fig. 3(a), is taken as the energy to generate dislocations with critical radius, as high as 8  107 dislocation loops may be formed within a loading– unloading curve. Although it is not realistic to assume that all the dissipated indentation energy was entirely transferred to generate dislocation loops, the estimation has, nevertheless, provided an upper limit for the number of dislocation loops with critical radius in the initial state. 4. Conclusion In summary, the XRD, SEM, AFM and nanoindentation techniques were used to investigate the structural and surface morphological features, and the nanomechanical properties of Bi2Te3 thin film deposited on c-plane sapphire by using PLD. The AFM observation and XRD results indicated that the Bi2Te3 thin films are highly uniform and the crystallinity of films grown on substrate with the hexagonal structure. In addition, the nanoindentation results indicate that the values of hardness and Young’s modulus of a hexagonal Bi2Te3 thin film are 5.7 ± 0.8 GPa and 158.6 ± 6.2 GPa, respectively. The number of the nanoindentation-induced dislocation loops estimated from the work-done within the course of the pop-in event suggested that the pop-in was mainly due to the homogeneous nucleation of dislocations. Similar to many hexagonal-structured materials, the primary deformation mechanism for Bi2Te3 thin films is the nucleation and the propagation of dislocations. Based on this scenario, the nanoindentation-induced generation of dislocation loops associated with the first pop-in event was estimated to be in the order of 103 with a critical radius of rc  4.5 nm. The obtained dislocation density is relatively low and is consistent with the scenario of homogeneous dislocation nucleation-induced in the first pop-in phenomenon. Acknowledgement Authors like to thank Dr. P.-F. Yang (Product Characterization, Advanced Semiconductor Engineering) for his technical supports. References [1] G.A. Thomas, D.H. Rapke, R.B. van Dover, L.F. Mattheis, W.A. Surden, L.F. Schneemaper, J.V. Waszczak, Phys. Rev. B 46 (1992) 1553. [2] O. Madelung, U. Rössler, M. Schulz, Group III Condensed Mater, Springer, Heidelberg, 1998. [3] D.M. Rowe, CRC Handbook of Thermoelectrics, CRC Press, Boca Raton, FL, 1995. [4] G.S. Nolas, J. Sharp, H.J. Goldsmid, Thermoelectrics Basic Principles and New Materials Development, Springer, Berlin, Germany, 2001. [5] S. Sharma, V.K. Dwivedi, S.N. Pandit, Int. J. Green Energy 11 (2014) 899.

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