Sb2Te3 nano-layers

NIM B Beam Interactions with Materials & Atoms

Nuclear Instruments and Methods in Physics Research B 261 (2007) 608–611 www.elsevier.com/locate/nimb

Effects of MeV Si ions bombardments on thermoelectric properties of sequentially deposited BixTe3/Sb2Te3 nano-layers q S. Budak, C.I. Muntele, R.A. Minamisawa, B. Chhay, D. Ila

*

Center for Irradiation of Materials, Department of Physics, Alabama A&M University, Normal, AL 35762, USA Available online 28 March 2007

Abstract We made 50 periodic nano-layers of BixTe3/Sb2Te3 superlattice films with Au layers deposited on both sides as metal contacts. Each alternating layer is 5 nm thick. The performance of the thermoelectric materials and devices is shown by a dimensionless figure of merit, ZT. The purpose of this study is to tailor the figure of merit of layered structures used as thermoelectric generators. The superlattices were bombarded by 5 MeV Si ions at seven different fluences to form nano-cluster structures. Rutherford backscattering spectrometry (RBS) was used to determine the film thickness and stoichiometry. To get the figure of merit before and after MeV bombardments, we measured the cross plane thermal conductivity by 3rd harmonic method, cross plane Seebeck coefficient and electrical conductivity using Van der Pauw method. The electronic energy deposited due to ionization by the MeV Si beam in its track produces nano-scale structures. This disrupts and confines phonon transmission thereby reducing thermal conductivity and increasing electron density, which in turn increases the Seebeck coefficient and the electrical conductivity. These combine to then increase the figure of merit for these superlattice films.  2007 Elsevier B.V. All rights reserved. PACS: 81.07. b Keywords: Ion bombardment; Multi-nano-layers; Figure of merit (ZT)

1. Introduction Thermoelectric materials are being important due to their application in both thermoelectric power generation and microelectronic cooling [1]. The major problem with the thermoelectric devices is poor efficiency. This efficiency of the thermoelectric devices is limited by the properties of n- and p-type semiconductors. Effective thermoelectric materials have a low thermal conductivity and a high electrical conductivity [2]. A high thermal conductivity causes too much heat leakage through heat conduction [3]. The performance of the thermoelectric materials and devices is shown by a dimensionless figure of merit, ZT = S2rT/j, where S is the Seebeck coefficient, r is the electrical conducq *

Patent is pending. Corresponding author. Tel.: +1 256 372 5866; fax: +1 256 372 5868. E-mail address: [email protected] (D. Ila).

0168-583X/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.03.051

tivity, T is the absolute temperature and j is the thermal conductivity [4]. ZT can be increased by increasing S, increasing r, or decreasing j. The large bulk value of the ZT made systems containing Bi2Te3 is a good choice to demonstrate the benefits of using low-dimensional materials in thermoelectric applications. Early studies predicting the ZT of superlattices containing Bi2Te3 showed that effects of carrier confinement cause an enhancement in ZT of more than an order of magnitude over that of the bulk samples. These studies also showed that superlattices with smaller periods have a higher figure of merit [5]. In this study, we report on the growth of 50 periodic nano-layers of BixTe3/Sb2Te3 superlattice films on the fused silica (quartz) and silicon substrates using an ionbeam assisted deposition (IBAD) system and high energy Si ions bombardments of the films for reducing thermal conductivity and increasing electrical conductivity.

S. Budak et al. / Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 608–611

609

2. Experimental

3. Results and discussion

We have grown 50 periodic nano-layers of BixTe3/ Sb2Te3 superlattice films on the fused silica (suprasil) and silicon substrates with the IBAD system. The multilayer nano-films were sequentially deposited to have a periodic structure consisting of alternating BixTe3 and Sb2Te3 layers. The two electron-guns were used for evaporating the two solids of BixTe3 and Sb2Te3. These two e-guns were turned on and off alternately to make nano-multilayers. The thickness of the layers was controlled by an INFICON deposition monitor. The film geometry used for the deposition of BixTe3/Sb2Te3 nano-layers films is shown in Fig. 1. The geometry in Fig. 1(a) shows two Au contacts on the top and bottom of the multilayers. These two Au contacts were used in the Seebeck coefficient measurements. As seen from Fig. 1(b), the edge of the film was etched using the Arion etching in IBAD system to prevent short-circuit connections among the multilayers. The electrical conductivity was measured by the Van der Pauw system and the thermal conductivity was measured by the 3x technique. The thermal conductivity measurement was performed at a room temperature of 22 C. One could find detailed information about 3x technique in [6–9]. The 5 MeV Si ions bombardments were used by the Pelletron ion beam accelerator at the Alabama A&M University’s Center for Irradiation of Materials (AAMU-CIM). The fluences used for the bombardments were 1 · 1014 ions/cm2, 3 · 1014 ions/cm2, 5 · 1014 ions/cm2, 7 · 1014 ions/cm2, 10 · 1014 ions/cm2, 13 · 1014 ions/cm2 and 15 · 1014 ions/cm2. The lowest thermal conductivity was found at the fluence of 7 · 1014 ions/cm2 and the highest electrical conductivity was found at the same fluence. RBS measurement was performed using 2.1 MeV He+ ions in an IBM scattering geometry with the particle detector placed at 170 from the incident beam to monitor the film thickness and stoichiometry before and after 5 MeV Si ions bombardments [1,10,11].

Fig. 2(a) shows RBS spectra of 50 periodic nano-layers of BixTe3/Sb2Te3 superlattice films on glassy polymeric carbon (GPC) substrate when the sample is at the normal angle and tilted by 45. Fig. 2(b) shows RBS spectra and RUMP simulation [12] of 50 periodic nano-layers of BixTe3/Sb2Te3 superlattice films on GPC substrate when RBS measurement was performed using 2.1 MeV He+ ions. As seen from Fig. 2(b), RUMP simulation matches RBS spectra very well. This RUMP simulation gives information about the amount of the elements in the deposited samples. The RUMP simulation parameters for Bi and Te gave us no Bi2Te3 deposition but there are Bi and Te depositions. This is due to the excess amount of current applied to Bi2Te3 crucible. The melting points of Bi2Te3 and Sb2Te3 are 573 C and 629 C, respectively [13]. We used e-beam currents of 16 mA for Bi2Te3 and 8 mA for Sb2Te3. As seen from the melting points, the melting point of Bi2Te3 is less than that of Sb2Te3. During the growth process the bond between Bi and Te was broken due to more amount of the applied e-beam current. Thus, more amount of Bi elements sublimates on the film while the stoichiometry was kept at 2:3 ratio between Sb and Te. While the applied e-beam current for Sb2Te3 was sufficient to keep 2:3 ratio between Sb and Te, the applied e-beam current for Bi2Te3 was more than sufficient. Excess amount of the current for Bi2Te3 caused more deposition of Bi instead of BixTe3. By using these films, we reached a quite higher figure of merit like 1.38. Fig. 3 shows thermoelectric properties of 50 periodic nano-layers of BixTe3/Sb2Te3 superlattice films. Fig. 3(a) shows the Seebeck coefficient dependence on the applied fluences. As seen from Fig. 3(a), the Seebeck coefficient started to increase at the initial fluence and continued while the bombardment was being continued. The increase in the Seebeck coefficient is one of the expected properties of

Fig. 1. The film geometry used for the deposition of BixTe3/Sb2Te3 nano-layers films on the fused silica substrate (a) from the cross-section and (b) from the top.

610

S. Budak et al. / Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 608–611

S(microV/K)

+

RBS Spectra using 2.1 MeV He 50 period of BixTe3/Sb2Te3 multilayer film Before bombardment

2000 1500 1000

Bi

48 44

Theta=45

500

36 28 3000 0

0

0 150

12

14

225

300

375

450

525

600

675

Channels Fig. 2. (a) RBS spectra of 50 periodic nano-layers of BixTe3/Sb2Te3 superlattice films on glassy polymeric carbon (GPC) substrate when the sample is at the normal angle and tilted by 45. (b) RBS spectra and RUMP simulation of 50 periodic nano-layers of BixTe3/Sb2Te3 superlattice films on GPC substrate when RBS measurement was performed using 2.1 MeV He+ ions.

-1

2

4

6

8

10

12

14

0.0090 0.0075

Electrical Conductivity

c

0.0060 0.0045

0

2

4

0.00018

6

8

10

12

14

12

14

Thermal Conductivity

0.00012 0.00006 0.00000 1.6 1.40 1.2 1.0 0.8 0.6 0.4 0.2 0.0

ZT (Figure of Merit)

Thermal Conductivity (mW/cmK)

C

75

10

Square of Seebeck Coefficient

1200

0.00024

RBS Spectra RUMP simulation

0

8

b

1800

0

1000 500

6

2400

2

Bi

Electrical Conductivty

o

+

Random 2.1 MeV He 170 Backscattered angle of 50 period of BixTe3/Sb2Te3multilayer film Before bombardment

1500

4

0.0105

Sb Te

b

2000

2

2

S (microV/K)

600 667 67 133 200 267 333 400 467 533 60

Channels 2500

Seebeck Coefficient

32

C

0

a

40

0

0

Normalized Yield(a.u)

52

Theta=0

(ohm*cm)

Normalized Yield(a.u)

56

Sb Te

a

2500

0

d 2

4

6

8

10

Figure of Merit

e 2

4

6

8 14

10

12

14

2

Fluence(x10 ions/cm )

thermoelectric materials. The requirement of a high Seebeck coefficient is natural since S is a measure of the average thermal energy which is carried per charge (electron or hole) [14]. Fig. 3(b) shows the square of the Seebeck coefficient change depending on the fluences of the bombardments. As seen from Fig. 3(b), the graph of the square of the Seebeck coefficient tends to increase all the time. Fig. 3(c) shows the electrical conductivity change depending on the fluences. As seen from Fig. 3(c), the electrical conductivity started to increase when the first bombardment of 1 · 1014 ions/cm2 was applied. This increase in the electrical conductivity continued until the fluence of 7 · 1014 ions/cm2. After the fluence of 7 · 1014 ions/cm2, the electrical conductivity started to decrease. This shows that ion bombardment caused an increase in the electrical conductivity until one certain fluence was reached (in our case, this fluence was found as 7 · 1014 ions/cm2). While the virgin sample is being bombarded with the 5 MeV Si ions, the numbers of the charge carriers in both the conduction and valence bands increase. This causes shorter energy gap between the conduction and valence bands. The shorter energy gap causes increase in the electrical conductivity. As seen from Fig. 3(c), when we continued to bombard our samples after the fluence of 7 · 1014 ions/cm2, we

Fig. 3. Thermoelectric properties of 50 periodic nano-layers of BixTe3/ Sb2Te3 superlattice films.

saw that the electrical conductivity started to decrease. The decrease in the electrical conductivity might be due to the degenarations in both the conduction and the valence bands as seen in Fig. 3(c) after the fluence of 7 · 1014 ions/cm2. The increase in the electrical conductivity is one of the desired conditions for both the thermoelectric materials and the devices. Fig. 3(d) shows the thermal conductivity change depending on the fluences. As seen from Fig. 3(d), the thermal conductivity started to decrease when the 5 MeV Si ions bombardments were introduced until the fluence of 7 · 1014 ions/cm2. After the fluence of 7 · 1014 ions/cm2, the thermal conductivity started to increase. While the samples are being bombarded with 5 MeV Si ions, the heat energy of 50 periodic nano-layers of BixTe3/Sb2Te3 superlattice films increases. This causes some interface scattering and absorption of phonons. Scattering and absorption of phonons cause decrease in thermal conductivity. If you continue to bombard the samples, the amount of the thermal conductivity may increase due to the increase of the discrete energy levels in between

S. Budak et al. / Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 608–611

the conduction and valence bands. As seen from the figures of thermal conductivity and electrical conductivity, the high energy ion bombardments can produce nanostructures and modify the property of thin films [15], resulting in lower thermal conductivity and higher electrical conductivity. Fig. 3(e) shows the fluence dependence of figure of merit of the 50 periodic nano-layers of BixTe3/Sb2Te3 superlattice films. As seen from Fig. 3(e), the figure of merit value started to increase when the 5 MeV Si ions bombardments were introduced until the fluence of 7 · 1014 ions/cm2 like the electrical conductivity. After the fluence of 7 · 1014 ions/cm2, the figure of merit started to decrease. This effect is coming from both the electrical conductivity and the thermal conductivity of the 50 periodic nano-layers of BixTe3/Sb2Te3 superlattice films. Both the good thermoelectric materials and devices should have higher figure of merit. The figure of merit for 50 periodic nano-layers of BixTe3/Sb2Te3 superlattice films increases from 0.004938 at zero fluence to 1.38 at 7 · 1014 ions/cm2 fluence. After 1.38 at 7 · 1014 ions/cm2, the figure of merit starts to decrease until 0.042. 4. Conclusion The RUMP simulation parameters for Bi and Te gave us no Bi2Te3 deposition but there are Bi and Te depositions. The melting points of Bi2Te3 and Sb2Te3 are 573 C and 629 C, respectively [13]. We used e-beam currents of 16 mA for Bi2Te3 and 8 mA for Sb2Te3. During the growth process the bond between Bi and Te was broken due to the excess amount of the applied e-beam current. Thus, more amount of Bi elements sublimates on the film while the stoichiometry was kept at 2:3 ratio between Sb and Te. The applied e-beam current for Sb2Te3was sufficient enough to keep 2:3 ratio between Sb and Te. The applied e-beam current for Bi2Te3 was more than sufficient.

611

By using these films, we reached a quite higher figure of merit like 1.38. We will try to deposit the suitable stoichiometry (x = 0.1–2.0) for BixTe3 for the following study in near future. Thus, we could reach a higher figure of merit for the nano-layers of BixTe3/Sb2Te3 superlattice system. Acknowledgements Research sponsored by the Center for Irradiation of Materials, Alabama A&M University and by the AAMURI Center for Advanced Propulsion Materials under the contract number NAG8-1933 from NASA and by National Science Foundation under Grant No. EPS-0447675. References [1] Z. Xiao, R.L. Zimmerman, L.R. Holland, B. Zheng, C.I. Muntele, D. Ila, Nucl. Instr. and Meth. B 242 (2006) 201. [2] B.C. Scales, Science 295 (2002) 1248. [3] G. Chen, A. Shakouri, Trans. ASME 124 (2002) 242. [4] Z. Xiao, R.L. Zimmerman, L.R. Holland, B. Zheng, C.I. Muntele, D. Ila, Nucl. Instr. and Meth. B 241 (2005) 568. [5] M.N. Touzelbaev, P. Zhou, R. Venkatasubramanian, K.E. Goodson, J. Appl. Phys. 90 (2001) 763. [6] L.R. Holland, R.C. Simith, J. Appl. Phys. 37 (1966) 4528. [7] D.G. Cahill, M. Katiyar, J.R. Abelson, Phys. Rev. B 50 (1994) 6077. [8] T.B. Tasciuc, A.R. Kumar, G. Chen, Rev. Sci. Instr. 72 (2001) 2139. [9] L. Lu, W. Yi, D.L. Zhang, Rev. Sci. Instr. 72 (2001) 2996. [10] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping Range of Ions in Solids, Pergamon Press, New York, 1985. [11] W.K. Chu, J.W. Mayer, M.-A. Nicolet, Backscattering Spectrometry, Academic Press, New York, 1978. [12] L.R. Doolittle, M.O. Thompson, RUMP, Computer Graphics Service, 2002. [13] MSDS Search, Alfa Aesar Company, . [14] G. Chen, A. Narayanaswamy, C. Dames, Superlattice. Microstr. 35 (2004) 161. [15] D. Ila, R.L. Zimmerman, C.I. Muntele, P. Thevenard, F. Orucevic, C.L. Santamaria, P.S. Guichard, S. Schiestel, C.A. Carosella, G.K. Hubler, D.B. Poker, D.K. Hensley, Nucl. Instr. and Meth. B 191 (2002) 416.