AuxSiO2(1−x) nano-layers

NIM B Beam Interactions with Materials & Atoms

Nuclear Instruments and Methods in Physics Research B 261 (2007) 1167–1170 www.elsevier.com/locate/nimb

MeV Si ions bombardment effects on thermoelectric properties of sequentially deposited SiO2/AuxSiO2(1 x) nano-layers q S. Budak, C. Muntele, B. Zheng, 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 prepared 50 periodic nano-layers of electro-cooling system consisting of SiO2/AuxSiO2(1 x) super lattice with Au layer deposited on both sides as metal contacts. The deposited multilayer films have a periodic structure consisting of alternating layers where each layer is 10 nm thick. The purpose of this research is to tailor the figure of merit of layered structures used as thermoelectric generators. The super lattices were then bombarded by 5 MeV Si ions at three different fluences to form nano-cluster structure. Rutherford backscattering spectrometry (RBS) was used to monitor the film thickness and stoichiometry before and after MeV bombardment. We measured the thermoelectric efficiency of the fabricated device before and after 5 MeV bombardment measuring the cross plane thermal conductivity by third harmonic method, measuring cross plane Seebeck coefficient, and measuring electric conductivity using Van der Pauw method. As predicted the electronic energy deposited due to ionization by MeV Si beam in its track produces nano-scale structures which disrupt and confine phonon transmission therefore reducing thermal conductivity, increasing electron density of state so as to increase Seebeck coefficient, and electric conductivity, thus increasing figure of merit.  2007 Elsevier B.V. All rights reserved. PACS: 81.07. b Keywords: Ion bombardment; Multi-nano-layers; Figure of merit

1. Introduction Thermoelectric materials are increasingly important due to their applications in thermoelectric power generation and microelectronic cooling [1]. Thermoelectric devices are quite simple. They do not have moving parts and do not generate greenhouse gases. The main problem with the thermoelectric devices is poor efficiency. This efficiency is limited by the material properties of n- and p-type semiconductors. Effective thermoelectric materials have a low thermal conductivity and a high electrical conductivity [2]. 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 q *

Patent is pending for this study. 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.2007.03.049

electric conductivity, T is the absolute temperature, and j is the thermal conductivity [3]. ZT can be increased by increasing S, increasing r, or decreasing j. In this study we report on the growth of a SiO2/AuxSiO2(1 x) super lattice on fused silica and silicon substrates using an ion-beam assisted deposition (IBAD) system, and high energy Si ion bombardments of the films for reducing thermal conductivity and increasing electrical conductivity. 2. Experimental We have grown SiO2/AuxSiO2(1 x) super lattice nanolayers films on the silica and silicon substrates with the IBAD system. The multilayer films were sequentially deposited to have a periodic structure consisting of alternating SiO2 and AuxSiO2(1 x) layers. The two electrongun evaporators for evaporating the two solids were turned on and off alternately to make multilayers. The thickness of

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S. Budak et al. / Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 1167–1170

Fig. 1. Film geometry of the sample (a) from the cross-section and (b) from the top.

3. Results and discussion Fig. 2 shows RBS spectra and RUMP simulation of 50 periodic nano-layers of SiO2/AuxSiO2(1 x) film on Si substrate when the source was helium. Before we started to grow multilayer films, we did calibration of IBAD for SiO2 + Au growth. The amount of the gold in the formula of SiO2/Aux: (SiO2)1 was found depending on the e-beam currents. According to the calibration we found as x = 0.04 at a certain current. The same parameters were

400 +

Normalized Yield(a.u)

350

Random 2.1 MeV He

Experimental line

o

170 Backscattered angle of 50 periodic SiO2/SiO2+Au multilayer film

300 250 200 150 O

100

Si

Au

Rump simulation

50 0 0

67

133 200 267 333 400 467 533 600 667

Channel Fig. 2. RBS spectra and RUMP simulation of 50 periodic nano-layers of SiO2/AuxSiO2(1 x) film on Si substrate when the source was helium.

kept the same for the growth of the multilayer films. Using the calibration we prepared 50 periodic nano-layers of SiO2/AuxSiO2(1 x) super lattice films. Fig. 3 shows the temperature dependence of the Seebeck Coefficient of the 50

10

Seebeck Coefficient (micro V/K)

the layers was controlled by an INFICON deposition monitor. The film geometry used in this study is shown in Fig. 1. The multilayer films on the silica substrates were used for Seebeck coefficient measurements and the films on the silicon substrates were used for electrical conductivity and thermal conductivity measurements. The geometry in Fig. 1 shows two Au contacts on the top and bottom of the multilayers. These contacts were used in the Seebeck coefficient measurement system and were also used when electric contact was needed. As seen from Fig. 1(b) the edge of the film was etched using the Ar-ion 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 22 C. One can find detailed information about this technique elsewhere [4–7]. The 5 MeV Si ion bombardments were performed by the Pelletron ion-beam accelerator at the Alabama A&M University’s Center for Irradiation of Materials (AAMUCIM). The amount of the energy of the bombarding Si ions was chosen by the SRIM simulation software (SRIM). The fluences used for the bombardment were 1 · 1013 ions/cm2, 5 · 1013 ions/cm2, and 1 · 1014 ions/cm2. The lowest thermal conductivity was found at the fluence of 1 · 1014 ions/cm2, and the highest electrical conductivity was found at the same fluence. Rutherford backscattering spectrometry (RBS) was used to monitor the film thickness and stoichiometry before and after MeV bombardments [1,8,9].

0 -10 before bombardment

-20

13

2

13

2

14

2

after 1x10 ions/cm

-30

after 5x10 ions/cm after 1x10 ions/cm

-40 -50 -60 300

320

340

360

380

T(K) Fig. 3. The temperature dependence of the Seebeck coefficients of the 50 periodic nano-layers of SiO2/AuxSiO2(1 x) super lattice.

S. Budak et al. / Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 1167–1170

S (microV/K)

2

S(microV/K)

0

a

-20 -40

Seebeck Coefficient

-60 4000 0 3000

b

2

2000

4

6

8

10

6

8

10

6

8

10

6

8

10

6

8

10

Square of Seebeck Coefficient

-1

2

1000

(ohm*cm)

Electric Conductivity

1169

0 1.6 0 1.2 0.8 0.4 0.0

Thermal Conducrivity (mW/cmK)

0 0.02

c

2

4 Electrical Conductivity

d

2

4

0.01 Thermal Conductivity 0.00

ZT Figure of Merit

0 2

e

2

4

Figure of Merit 0 0

2

4

Fluence (x1013ions/cm2) Fig. 4. Thermoelectric properties of 50 periodic nano-layers of SiO2/AuxSiO2(1

periodic nano-layers of SiO2/AuxSiO2(1 x) super lattice films. As seen from Fig. 3, the Seebeck coefficients showed some dependence on the fluences while the temperature is increasing. When the fluence was at a maximum, we achieved the highest Seebeck coefficient at the room temperature. As the temperature increased, the Seebeck coefficient increased in the positive direction and after 320 K the Seebeck coefficients for each fluence reached almost the same value. Fig. 4 shows thermoelectric properties of the 50 periodic nano-layers of SiO2/AuxSiO2(1 x) super lattice films. Fig. 4(a) shows the Seebeck coefficient dependence on the fluence. As seen from Fig. 4(a), the Seebeck coefficient started to increase at the initial fluence and decreased after the fluence of 5 · 1013 ions/cm2. The figure of merit contains the square of the Seebeck coefficient and the Fig. 4(b) shows the square of the Seebeck coefficient dependence on the fluences of the bombardment. As seen clearly, the graph tends to increase. Even some numbers are negative, the square of the seebeck coefficient makes them positive. Fig. 4(c) shows the electrical conductivity change depending on the fluences of the bombardment. The behavior of the electrical conductivity is the same as the behavior of the square of the Seebeck coefficient. This shows that ion bombardment caused an increase in the electrical conductivity. While the virgin sample is being bombarded with 5 MeV Si ions, the number of the charge carriers in the conduction and valence bands increases. This causes

x)

super lattice.

shorter energy gap in between the conduction and valence bands. This results in the increase in electrical conductivity. The increase in the electrical conductivity is one of the desired conditions for both the thermoelectric materials and the devices. Fig. 4(d) shows the thermal conductivity change depending on the fluences. Fig. 4(d) shows that the thermal conductivity decreases as the bombardment fluence increases. While the samples are being bombarded with 5 MeV Si ions, the heat of the samples increases. This causes some interface scattering and absorption of phonons. This effect results in decrease in thermal conductivity. This is also another desired situation for both the thermoelectric materials and the devices. Similar effects were seen in [10–12]. The nano-clusters in the layers caused the decrease in the thermal conductivity and the phonon scattering caused the increase in the electrical conductivity. We see the opposite competing in between the thermal and electrical conductivities. Fig. 4(e) shows the change of figure of merit depending on the ion bombardment. As seen from Fig. 4(e) the figure of merit increases as the fluence increases. This is the solution to the major problem of thermoelectric materials. The ion bombardment showed that the Figure of Merit increases as the fluence of the bombardment increases when suitable fluences are chosen. The figure of merit for this sample increased from 0.66 · 10 4 to 2.52 at fluences of 0 and 1 · 1014 ions/cm2, respectively.

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4. Conclusion We have fabricated an electro-cooling super lattice system consisting of 50 periodic nano-layers of SiO2/ AuxSiO2(1 x), super lattice films with Au layers deposited on both sides as metal contacts, by ion-beam assisted deposition (IBAD). The multilayer films were sequentially deposited to have a periodic structure consisting of alternating SiO2 and AuxSiO2(1 x) layers. The thickness of the layers was controlled by an INFICON deposition monitor. 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. The 5 MeV Si ion bombardments were performed by the Pelletron ion-beam accelerator at AAMU-CIM. The energy of the bombarding Si ions was chosen by SRIM simulation. The fluences used for the bombardment were 1 · 1013 ions/cm2, 5 · 1013 ions/cm2, and 1 · 1014 ions/cm2. The lowest thermal conductivity was found at the fluence of 1 · 1014 ions/cm2, and the highest electrical conductivity was found at the same fluence. Rutherford backscattering spectrometry (RBS) was used to monitor the film thickness and stoichiometry before and after MeV bombardments. The high energy ion bombardment can produce nanostructures and modify the property of thin films [10–12], therefore resulting in lower thermal conductivity and higher electrical conductivity. The nano-clusters in the layers caused the decrease in the thermal conductivity and the phonon scattering caused the increase in the electrical conductivity. The figure of merit increased as the fluence increased. This is the solution to the major problem of thermoelectric materials. The ion bombardment showed that the figure of merit increased when the bombardment fluence increased when suitable fluences are chosen. Preliminary studies on the figure of merit were shown in [13]. The figure of merit for this sample increased from 0.66 · 10 4 to 2.52 at fluences of 0 and 1 · 1014 ions/cm2, respectively. The major problem in the thermoelectric materials was to increase the electrical conductivity, decrease the thermal

conductivity and increase the Seebeck coefficient. As a result of these effects, one could get higher figure of merit. In this study we reached a quite higher figure of merit of 2.52. Although SiO2 is an insulator, the 50 periodic nanolayers of SiO2/AuxSiO2(1 x) super lattice films behave like thermoelectric materials due the high energy 5 MeV Si ion bombardment. Acknowledgments 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. EPS0447675. 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] Brian C. Scales, Science 295 (2002) 1248. [3] Z. Xiao, R.L. Zimmerman, L.R. Holland, B. Zheng, C.I. Muntele, D. Ila, Nucl. Instr. and Meth. B 241 (2005) 568. [4] L.R. Holland, J. Appl. Phys. 34 (1963) 2350. [5] L.R. Holland, R.C. Simith, J. Appl. Phys. 37 (1966) 4528. [6] D.G. Cahill, M. Katiyar, J.R. Abelson, Phys. Rev. B 50 (1994) 6077. [7] L. Lu, W. Yi, D.L. Zhang, Rev. Sci. Instr. 72 (2001) 2996. [8] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping Range of Ions in solids, Pergamon Press, New York, 1985. [9] W.K. Chu, J.W. Mayer, M.-A. Nicolet, Backscattering Spectrometry, Academic Press, New York, 1978. [10] 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. [11] R.L. Zimmerman, D. Ila, E.K. Williams, B. Gasic, A. Elsamedicy, A.L. Evelyn, D.B. Poker, D.K. Hensley, D.J. Larkin, Nucl. Instr. and Meth. B 166 (2000) 892. [12] D. Ila, E.K. Williams, S. Sarkisov, C.C. Smith, D.B. Poker, D.K. Hensley, Nucl. Instr. and Meth. B 141 (1998) 289. [13] S. Budak, B. Zheng, C. Muntele, Z. Xiao, I. Muntele, B. Chhay, R.L. Zimmerman, R.L. Holland, D. Ila, Mater. Res. Soc. Symp. Proc. 929 (2006).