- Email: [email protected]
SiO2+Ge Multi-nanolayer thin Films
Available online at www.sciencedirect.com
ScienceDirect Physics Procedia 66 (2015) 321 – 328
C 23rd Conference on Application of Accelerators in Research and Industry, CAARI 2014
Effects of Mev Si Ions and Thermal Annealing on Thermoelectric and Optical Properties of SiO2/SiO2+Ge Multi-Nanolayer Thin Films S. Budak1*, M. A. Alim1, S. Bhattacharjee2, C. Muntele3 1
2
Department of Electrical Engineering & Computer Science, Alabama A&M University, Normal, AL USA Department of Mechanical and Civil Engineering, Alabama A&M University, Normal, AL USA 3 Cygnus Scientific Services, Huntsville, AL 35815, USA
Abstract Thermoelectric generator devices have been prepared from 200 alternating layers of SiO 2/SiO2+Ge superlattice films using DC/RF magnetron sputtering. The 5 MeV Si ions bombardment has been performed using the AAMU Pelletron ion beam accelerator to form quantum dots and / or quantum clusters in the multi-layer superlattice thin films to decrease the cross-plane thermal conductivity, increase the cross-plane Seebeck coefficient and increase the cross-plane electrical conductivity to increase the figure of merit, ZT. The fabricated devices have been annealed at the different temperatures to tailor the thermoelectric and optical properties of the superlattice thin film systems. While the temperature increased, the Seebeck coefficient continued to increase and reached the maximum value of -25 μV/K at the fluence of 5x1013 ions/cm2. The decrease in resistivity has been seen between the fluence of 1x1013 ions/cm2 and 5x1013 ions/cm2. Transport properties like Hall coefficient, density and mobility did not change at all fluences. Impedance spectroscopy has been used to characterize the multi-junction thermoelectric devices. The loci obtained in the C*-plane for these data indicate non-Debye type relaxation displaying the presence of the depression parameter. © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2014 The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and peer-review under responsibility of the Organizing Committee of CAARI 2014. Selection and peer-review under responsibility of the Organizing Committee of CAARI 2014 Keywords: Ion bombardment; thermal annealing; thermoelectric and optical properties; multi-nanolayers; figure of merit.
*Corresponding author: Tel.: 256-372-5894; Fax: 256-372-5855 Email: [email protected]
1875-3892 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and peer-review under responsibility of the Organizing Committee of CAARI 2014 doi:10.1016/j.phpro.2015.05.040
322
S. Budak et al. / Physics Procedia 66 (2015) 321 – 328
1. Introduction The theory of thermoelectric power generation and thermoelectric refrigeration was first presented by Altenkirch in 1990 [1]. The demand for electrical energy is growing rapidly with increasing world population and diminution of conventional energy sources [2]. Thermoelectric devices are solid state devices and they are reliable energy converters. Since they do not have mechanical moving parts, they do not have noise or vibration [3]. Thermoelectric power generation could convert heat to electricity directly [4]. The efficiency of the thermoelectric generator highly depends on the operating temperatures, figure of merit, and the design configuration including the external load parameter of the device [5]. The efficiency of the thermoelectric devices is limited by the material properties of ntype and p-type semiconductors [6]. Thermoelectric energy conversion is a field that can greatly benefit from the nanoscale heat transport phenomenon [7]. The best thermoelectric materials were succinctly summarized as “phonon-glass electron-crystal”, which means that the materials should have a low lattice thermal conductivity as in glass, and high electrical conductivity as in crystals [8]. The efficiency of the thermoelectric devices is determined by the figure of merit ZT [9]. The figure of merit is defined by ZT=S2σT/K, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and K is the thermal conductivity. ZT could be increased by increasing S and σ, or by decreasing K. In order to compete with conventional refrigerators, a ZT of 3 is required. Due to their limited energy conversion efficiencies (i.e. ZT is ~1), thermoelectric devices currently have a rather narrow set of applications. However, there is a reinvigorated interest in the field of thermoelectrics due to classical and quantum mechanical size effects providing additional ways to enhance energy conversion efficiencies in nano-structured materials [10]. Metallic nanoclusters formed within silica (SiO 2) have several interesting optical and electrical properties with applications in optoelectronics and as thermoelectric materials [11]. Similar studies have been performed on the similar SiO2/SiO2+Ge thin film systems at 50 and 100 multilayers at the different thickness of 117 nm and 190 nm, respectively in the past [12]. While the 50 multilayers of SiO2/SiO2+Ge at the thickness of 117 nm were showing similar Seebeck coefficients at the similar ion fluences, the 100 multilayers of SiO2/SiO2+Ge at the thickness of 190 nm were showing about eight times higher Seebeck coefficients at the similar fluences. In this study, we reported the fabrication and some thermoelectrical and optical characterization for the 200 multilayers of SiO2/SiO2+Ge at the thickness of 1076 nm. 2. Experimental We have deposited the multilayer thin films with 200 alternating layers of SiO2 and SiO2+Ge nano-layers on silicon dioxide (SiO2) and fused silica (suprasil) substrates by DC/RF magnetron sputtering. These thin films form a periodic quantum-well structure consisting of 200 alternating layers of total thickness of 1076 nm. The DC gun was used to sputter Ge material while RF gun was used to sputter SiO 2. Ar gas was used to form plasma in the DC/RF magnetron sputtering chamber. The chamber was pumped down to about 5x10-5 Torr. The deposition was performed when the pressure was at about 3x10-3 Torr. The substrates were mounted on the substrate holder and rotated during the whole deposition process. The growth rate was monitored by an INFICON Quartz Crystal Microbalance (QCM). In order to form nano-structures (nano dots and / or nano clusters) in the multilayers, 5 MeV Si ions bombardment has been performed with the Pelletron ion beam accelerator at the Alabama A&M University’s Materials Research Laboratory (AAMU-MRL). The energy of the bombarding Si ions was chosen using the Stopping Range Ions in Matter (SRIM) simulation software. SRIM shows that 5 MeV Si ions pass through the multilayer thin films and terminate deep in the substrate. The fluences used for the bombardment were selected 12 2 14 2 between 1x10 ions / cm and 1x10 ions / cm . Cross-plane Seebeck coefficients, in-plane electrical conductivity, carrier density, mobility and Hall Effect coefficients as a function of temperature under the different applied fluences have been measured by MMR Seebeck and van der Pauw - Hall Effect measurement system. In addition to the thermoelectrical and transport properties, Atomic force microscopy (AFM), impedance measurements, and optical absorption measurements have also been performed AFM was used to see the effects of the thermal annealing on the surface of multilayer thin films. The LF4192A (Hewlett-Packard) Impedance Analyzer was used in understanding the ac “small-signal” electrical behavior of the entire thermoelectric device. This is a two-terminal measurement yielding underlying operative mechanism via an equivalent circuit model.
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3. Results and Discussion Figure 1 shows the temperature dependence of Seebeck coefficients of 200 multilayers of SiO 2/SiO2+Ge thin films at the different fluences. As seen from figure 1, the ion beam bombardment at the fluence of 5x10 13 ions/cm2 affected the Seebeck coefficients and increased the Seebeck coefficients in the negative direction. While the temperature increased, the Seebeck coefficient continued to increase and reached the maximum value of -25 μV/K at the fluence of 5x1013 ions/cm2. Higher Seebeck coefficient is one of the expected quantities for the high efficiency thermoelectric materials and devices. The reason for high Seebeck coefficient might arise from the higher charge carrier concentration from the 5 MeV Si ions bombardment at the fluence of 5x1013 ions/cm2. While the temperature increased for the thin film bombarded at the fluence of 5x1013 ions/cm2, the Seebeck coefficient increased in negative direction due to the higher charge carrier mobility at the higher temperature. The fluence of 5x1013 ions/cm2 could bring some useful output for the future optimization studies for the future thermoelectric devices. The future fluence studies should go beyond or close the fluence of 5x1013 ions/cm2 for getting higher Seebeck coefficients. Similar studies have been performed on the similar SiO2/SiO2+Ge thin film systems at 50 and 100 multilayers at the different thickness of 117 nm and 190 nm, respectively in the past [12, 10]. While the 50 multilayers of SiO2/SiO2+Ge at the thickness of 117 nm were showing similar Seebeck coefficients at the similar ion fluences, the 100 multilayers of SiO2/SiO2+Ge at the thickness of 190 nm were showing about eight times higher Seebeck coefficients at the similar fluences. The higher Seebeck coefficients might arise from the quantum dots or clusters formation in the thinner layers of the thermoelectric material systems.
Figure 1. Temperature dependence of Seebeck coefficients of 200 multilayers of SiO2/SiO2+Ge thin films at the different fluences.
Figure 2 shows the fluence dependence of van der Pauw resistivity values of 200 multilayers of SiO 2/SiO2+Ge thin films at room temperature. As seen from figure 2, the resistivity increased until the fluence of 1x10 13 ions/cm2, then decreased up to the fluence of 5x1013 ions/cm2. After the fluence of 5x1013 ions/cm2, the resistivity increased again. The expected effects of the high energy ion bombardment are to cause a decrease in the resistivity since the ion bombardment could form nanodots and / or nanoclusters in the SiO2/SiO2+Ge thin films. The decrease in resistivity has been seen between the fluence of 1x1013 ions/cm2 and 5x1013 ions/cm2. This might tell us that the interval between 1x1013 ions/cm2 and 5x1013 ions/cm2 could be suitable for reaching low resistivity values. Low resistivity or high electrical conductivity is one of the expected parameters for the high efficiency thermoelectric materials and devices. Since the electrical resistivity remains low only at certain interval in the figure 2, the future thin film systems could be bombarded with the values along with this interval for the similar thin film systems. Low electrical resistivity (high electrical conductivity) and high Seebeck coefficient have been reached at the same fluence of 5x1013 ions/cm2. The increase in charge carrier concentration might increase both the Seebeck coefficient and the electrical conductivity at the suitable energy and fluences.
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Figure 3 shows the temperature dependence of van der Pauw resistivity values of 200 multilayers of SiO2/SiO2+Ge thin films bombarded at the fluence of 1x1012 ions/cm2. As seen from figure 3, the electrical resistivity increased while the temperature increased at the fluence of 1x10 12 ions/cm2. The increase in the electrical resistivity is not expected for the highly efficient thermoelectric materials and devices. We expect to see increase in the electrical conductivity while the samples are being bombardment since the high energy ion bombardment increases the charge carrier concentration the multilayer thin films. The increase in the charge carrier concentration could increase Seebeck coefficient and the electrical conductivity. 800
V an der P auw R es is tivity @ 300K 200 M ultilayers S iO 2/S iO 2+G e T hin F ilms T hic knes s =1067 nm
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Figure 2. Fluence dependence of van der Pauw Resistivity values of 200 multilayers of SiO2/SiO2+Ge thin films at room temperature.
4x10
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0 300
350
400
450
500
T (K )
Figure 3. Temperature dependence of van der Pauw resistivity values of 200 multilayers of SiO2/SiO2+Ge thin film bombarded at the fluence of 1x1012 ions/cm2.
Figure 4 shows the fluence dependence of Hall coefficients of 200 multilayers of SiO2/SiO2+Ge thin films at room temperature. As seen from figure, the Hall coefficient stayed at the same amount except of the value where the Hall coefficient increased at the fluence of 1x1013 ions/cm2. Figure 5 shows the fluence dependence of Mobility coefficients of 200 multilayers of SiO2/SiO2+Ge thin films at room temperature. As seen from figure 5, mobility increased at the fluence of 5x10 12 ions/cm2. The mobility values stay very close to the each other at other fluences except for the fluence of 5x10 12 ions/cm2. The increase in mobility might come from the increase of the charge carrier concentration due to the high energy ion bombardment. Figure 6 shows the fluence dependence of Carrier Density coefficients of 200 multilayers of SiO2/SiO2+Ge thin
S. Budak et al. / Physics Procedia 66 (2015) 321 – 328
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-3
H all coefficient (cm /C )
films at room temperature. As seen from the figure 6, when the thin film was bombarded, the density value decreased first and then increased while the fluence was increased. When the fluence continued to increase, the density showed slight increments and decrements without so much deviation. The first decrement might be due to the thermal stabilization effect due to high energy ions bombardment. Then, the increment looks acceptable at the suitable fluences since the 5 MeV ions bombardment could increase the density in the multilayer thin film systems. 7x10
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7
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3x10
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H all E ffec t C oeffic ient M eas urements @ 300K 200 M ultilayers S iO 2/S iO 2+G e T hin F ilms T hic knes s =1067 nm
0 -1x10
7
1
0
50
10
5 2
F luenc e (ions /c m )x10
12
2
M obility (cm /V s)
Figure 4. Fluence dependence of Hall coefficients of 200 multilayers of SiO2/SiO2+Ge thin films at room remperature. 2.5x10
6
2.0x10
6
1.5x10
6
1.0x10
6
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M obility M easurements@ 300K 200 M ultilayers S iO 2/S iO 2+G e T hin F ilms T hickness =1067 nm
0.0 -5.0x10
5
0
1
5
10 2
F luence (ions /cm )x10
50
100
12
Figure 5. Fluence dependence of Mobility coefficients of 200 multilayers of SiO2/SiO2+Ge thin films at room temperature. 2x10
13
1x10
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-1x10
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-3x10
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D ensity (cm )
0 D ens ity M eas urements @ 300K 200 M ultilayers S iO 2/S iO 2+G e T hin F ilms T hic knes s =1067 nm
0
1
5
50
10 2
F luenc e (ions /c m )x10
100
12
Figure 6. Fluence dependence of Carrier Density coefficients of 200 multilayers of SiO2/SiO2+Ge thin films at room temperature.
Figure 7 shows the impedance plot and figure 8 shows the admittance plot of unbombarded 200 multilayers of SiO2/SiO2+Ge thin film for the frequency range 20 Hz – 13 MHz. The ac small-signal electrical data were acquired using HP4192A Impedance Analyser for the unbombarded 200-layered stack comprising repeated Si+Ge monolayer
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layer on to SiO2 layer for the frequency range 20 Hz – 13 MHz. These data are analysed via complex plane formalisms. Skewed semi-circular response (via equal grid scaling) is observed in the complex capacitance (C* = Cʹ - j C" = Cp – j Gp/ω) plane. The loci obtained in the C*-plane for these data indicate non-Debye type relaxation displaying the presence of the depression parameter. The skewed semi-circular behavior also shows the contribution of the shunt dc resistance (conductance) at the low frequency end [13-15].
200
150
100
Zcc (: )
50
0
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-100
-150
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250 Zc (: )
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Figure 7. Impedance plot of unbombarded 200 multilayers of SiO2/SiO2+Ge thin film.
8 6 4
Ycc (u 10-3 : -1)
2 0 -2 -4 -6 -8 -10
0
5
10
15
20
Y c (u 10-3 : -1)
Figure 8. Admittance plot of unbombarded 200 multilayers of SiO2/SiO2+Ge thin film.
Figure 9 shows the Optical absorption spectra of 200 multilayers of SiO2/SiO2+Ge thin films for unannealed and annealed samples at 100 oO C. As seen from figure 9, the absorption shows increment between 350-800 nm. This might be due to the diffusing of Ge ions into the multilayers and more absorption due to Ge ions. Figure 10 shows the AFM images of 200 multilayers of SiO2/SiO2+Ge thin films for unannealed and annealed samples at different temperatures. The thin films were annealed at different temperatures and AFM images were recorded to see the annealing effect on the surface. The purpose of the annealing on the thin films is to reach more smooth surfaces. Figure 10 shows this effect.
S. Budak et al. / Physics Procedia 66 (2015) 321 – 328
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0.14
Absorption (a.u.)
0.12
O ptical A bs orption S pectra 200 M ultilayers S iO 2/S iO 2+G e T hin F ilm
0.10
U nannealed o A nnealed @ 100 C
0.08 0.06 0.04 300
400
500
600
700
800
Wavelength (nm)
Figure 9. Optical Absorption spectra of 200 multilayers of SiO2/SiO2+Ge thin films for unannealed and annealed at 100 O C.
A F M of U nannealed 200 M L T hin films
o
A nnealed @ 400 C
o
A nnealed @ 700 C
o
A nnealed @ 1000 C
Figure 10. AFM images of 200 multilayers of SiO2/SiO2+Ge thin films for unannealed and annealed at different temperatures.
4. Conclusion Thermoelectric multilayer thin films for thermoelectric generator devices have been prepared from 200 alternating layers of SiO2/SiO2+Ge superlattice films using DC/RF magnetron sputtering. The 5 MeV Si ions bombardment has been performed using the AAMU Pelletron ion beam accelerator to tailor the thermoelectric properties. The ion beam bombardment at the fluence of 5x1013 ions/cm2 affected the Seebeck coefficients,increasing them in the negative direction. While the temperature increased, the Seebeck coefficient continued to increase and reached the maximum value of -25 μV/K at the fluence of 5x1013 ions/cm2. A higher Seebeck coefficient is one of the expected
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quantities for the high efficient thermoelectric materials and devices. The decrease in resistivity has been seen between the fluence of 1x1013 ions/cm2 and 5x1013 ions/cm2. This might tell us that the interval between 1x1013 ions/cm2 and 5x1013 ions/cm2 could be suitable for the low resistivity values. Low resistivity or high electrical conductivity is one of the expected parameters for the high efficiency thermoelectric materials and devices. Transport properties like Hall coefficient, carrier density and mobility did not change at all fluences. For the future studies, more optimization studies are needed to be performed to see significant changes in the transport properties. Acknowledgements Research sponsored by Materials Research Laboratory (MRL), National Science Foundation under NSF-EPSCOR R-II-3 Grant No. EPS-1158862, DOD under Nanotechnology Infrastructure Development for Education and Research through the Army Research Office # W911 NF-08-1-0425, and DOD Army Research Office # W911 NF12-1-0063, U.S. Department of Energy National Nuclear Security Admin with grant# DE-NA0001896 and grant# DE-NA0002687, , NSF-REU with Award#1156137. References [1] Xi, H, Luo, L and Fraisse, G, 2007, Development and application of solar-based thermoelectric technologies, Renewable & Sustainable Energy Reviews 11, 8103-8117. [2] Budak, S., Smith, C., Muntele, C., Chhay, B., Heidary, K., Johnson, R. B., ILA, D., 2013, Thermoelectric Properties of SiO 2/SiO2+CoSb Multi-nanolayered Thin Films Modified by MeV Si Ions, Journal of Intelligent Material Systems and Structures, 24(11), 1350-1356. [3] Riffat, S.B., Xiaoli, 2003. Applied Thermal Engineering 23, 913-935. [4] Budak, S., Guner, S., Minamisawa, R.A., Muntele, C. I., Ila, D., Thermoelectric Properties of Zn4Sb3/CeFe(4-x)CoxSb12 Nano-layered Superlattices Modified by MeV Si ions Beam, Applied Surface Science, 310, 226-229 (2014). [5] Sahin, A.Z. and Yilbas, B.S., 2013, The thermoelement as thermoelectric power generator: effect of leg geometry on the efficiency and power generation, Energy Conversion and Management 65, 26-32. [6] Scales, B.C., 2002, Smaller is cooler, Science 295, 1248. [7] Goldsmid, H., 1964, Thermoelectric Refrigeration, Plenum Press, New York. [8] Slack, G., 1995, In: Rowe, D. M. (Ed.), CRC Handbook of Thermoelectrics, CRC Press, pp.407. [9] Guner, S., Budak, S., Minamisawa, R.A., et al., 2008, Thickness and MeV Si ions bombardment effects on the thermoelectric properties of Ce3Sb10 thin films, Nucl. Instr. and Meth. in Phys. Res. B 266, 1261-1264. [10] Budak, S., Parker, R., Smith, C., Muntele, C., Heidary, K., Johnson, R. B., ILA, D., 2013, Superlattice Multi-nanolayered Thin Films of SiO2/SiO2+Ge for Thermoelectric Device Applications, Journal of Intelligent Material Systems and Structures, 24(11), 1357-1364. [11] Guner, S., Budak, S., Gibson, B., ila, D., Optical properties of Ag nanoclusters formed by irradiation and annealing of SiO2/SiO2:Ag thin films, Applied Surface Science 310 ( 2014) 180-183. [12] Budak, S., Smith, C., Pugh, M., Heidary, K., Colon, T., Johnson, R.B., ila, D., MeV Si Ions Bombardment Effects on Thermoelectric Properties of Thermoelectric of SiO2/SiO2+Ge Nanolayers, Radiation Physics and Chemistry, 81 (2012) 410-413. [13] Alim, M. A, 1996, Electrical Characterization of Engineering Materials, Active and Passive Electronic Components 19, 139-169. [14] Wang, C. C., Chen, W. H., Akbar, S. A., and Alim, M. A., 1997, High-Temperature a.c. Electrical Behaviour of Polycrystalline Calcium Zirconate, Journal of Materials Science 32, 2305-2312. [15] Azad, A.M., Shyan, L. L. W., and Alim, M. A., 1999, Electrical Characterization of the Solid-State Reaction Derived CaSnO3”, Journal of Materials Science 34, 3375-3396.
ScienceDirect Physics Procedia 66 (2015) 321 – 328
C 23rd Conference on Application of Accelerators in Research and Industry, CAARI 2014
Effects of Mev Si Ions and Thermal Annealing on Thermoelectric and Optical Properties of SiO2/SiO2+Ge Multi-Nanolayer Thin Films S. Budak1*, M. A. Alim1, S. Bhattacharjee2, C. Muntele3 1
2
Department of Electrical Engineering & Computer Science, Alabama A&M University, Normal, AL USA Department of Mechanical and Civil Engineering, Alabama A&M University, Normal, AL USA 3 Cygnus Scientific Services, Huntsville, AL 35815, USA
Abstract Thermoelectric generator devices have been prepared from 200 alternating layers of SiO 2/SiO2+Ge superlattice films using DC/RF magnetron sputtering. The 5 MeV Si ions bombardment has been performed using the AAMU Pelletron ion beam accelerator to form quantum dots and / or quantum clusters in the multi-layer superlattice thin films to decrease the cross-plane thermal conductivity, increase the cross-plane Seebeck coefficient and increase the cross-plane electrical conductivity to increase the figure of merit, ZT. The fabricated devices have been annealed at the different temperatures to tailor the thermoelectric and optical properties of the superlattice thin film systems. While the temperature increased, the Seebeck coefficient continued to increase and reached the maximum value of -25 μV/K at the fluence of 5x1013 ions/cm2. The decrease in resistivity has been seen between the fluence of 1x1013 ions/cm2 and 5x1013 ions/cm2. Transport properties like Hall coefficient, density and mobility did not change at all fluences. Impedance spectroscopy has been used to characterize the multi-junction thermoelectric devices. The loci obtained in the C*-plane for these data indicate non-Debye type relaxation displaying the presence of the depression parameter. © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2014 The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and peer-review under responsibility of the Organizing Committee of CAARI 2014. Selection and peer-review under responsibility of the Organizing Committee of CAARI 2014 Keywords: Ion bombardment; thermal annealing; thermoelectric and optical properties; multi-nanolayers; figure of merit.
*Corresponding author: Tel.: 256-372-5894; Fax: 256-372-5855 Email: [email protected]
1875-3892 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and peer-review under responsibility of the Organizing Committee of CAARI 2014 doi:10.1016/j.phpro.2015.05.040
322
S. Budak et al. / Physics Procedia 66 (2015) 321 – 328
1. Introduction The theory of thermoelectric power generation and thermoelectric refrigeration was first presented by Altenkirch in 1990 [1]. The demand for electrical energy is growing rapidly with increasing world population and diminution of conventional energy sources [2]. Thermoelectric devices are solid state devices and they are reliable energy converters. Since they do not have mechanical moving parts, they do not have noise or vibration [3]. Thermoelectric power generation could convert heat to electricity directly [4]. The efficiency of the thermoelectric generator highly depends on the operating temperatures, figure of merit, and the design configuration including the external load parameter of the device [5]. The efficiency of the thermoelectric devices is limited by the material properties of ntype and p-type semiconductors [6]. Thermoelectric energy conversion is a field that can greatly benefit from the nanoscale heat transport phenomenon [7]. The best thermoelectric materials were succinctly summarized as “phonon-glass electron-crystal”, which means that the materials should have a low lattice thermal conductivity as in glass, and high electrical conductivity as in crystals [8]. The efficiency of the thermoelectric devices is determined by the figure of merit ZT [9]. The figure of merit is defined by ZT=S2σT/K, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and K is the thermal conductivity. ZT could be increased by increasing S and σ, or by decreasing K. In order to compete with conventional refrigerators, a ZT of 3 is required. Due to their limited energy conversion efficiencies (i.e. ZT is ~1), thermoelectric devices currently have a rather narrow set of applications. However, there is a reinvigorated interest in the field of thermoelectrics due to classical and quantum mechanical size effects providing additional ways to enhance energy conversion efficiencies in nano-structured materials [10]. Metallic nanoclusters formed within silica (SiO 2) have several interesting optical and electrical properties with applications in optoelectronics and as thermoelectric materials [11]. Similar studies have been performed on the similar SiO2/SiO2+Ge thin film systems at 50 and 100 multilayers at the different thickness of 117 nm and 190 nm, respectively in the past [12]. While the 50 multilayers of SiO2/SiO2+Ge at the thickness of 117 nm were showing similar Seebeck coefficients at the similar ion fluences, the 100 multilayers of SiO2/SiO2+Ge at the thickness of 190 nm were showing about eight times higher Seebeck coefficients at the similar fluences. In this study, we reported the fabrication and some thermoelectrical and optical characterization for the 200 multilayers of SiO2/SiO2+Ge at the thickness of 1076 nm. 2. Experimental We have deposited the multilayer thin films with 200 alternating layers of SiO2 and SiO2+Ge nano-layers on silicon dioxide (SiO2) and fused silica (suprasil) substrates by DC/RF magnetron sputtering. These thin films form a periodic quantum-well structure consisting of 200 alternating layers of total thickness of 1076 nm. The DC gun was used to sputter Ge material while RF gun was used to sputter SiO 2. Ar gas was used to form plasma in the DC/RF magnetron sputtering chamber. The chamber was pumped down to about 5x10-5 Torr. The deposition was performed when the pressure was at about 3x10-3 Torr. The substrates were mounted on the substrate holder and rotated during the whole deposition process. The growth rate was monitored by an INFICON Quartz Crystal Microbalance (QCM). In order to form nano-structures (nano dots and / or nano clusters) in the multilayers, 5 MeV Si ions bombardment has been performed with the Pelletron ion beam accelerator at the Alabama A&M University’s Materials Research Laboratory (AAMU-MRL). The energy of the bombarding Si ions was chosen using the Stopping Range Ions in Matter (SRIM) simulation software. SRIM shows that 5 MeV Si ions pass through the multilayer thin films and terminate deep in the substrate. The fluences used for the bombardment were selected 12 2 14 2 between 1x10 ions / cm and 1x10 ions / cm . Cross-plane Seebeck coefficients, in-plane electrical conductivity, carrier density, mobility and Hall Effect coefficients as a function of temperature under the different applied fluences have been measured by MMR Seebeck and van der Pauw - Hall Effect measurement system. In addition to the thermoelectrical and transport properties, Atomic force microscopy (AFM), impedance measurements, and optical absorption measurements have also been performed AFM was used to see the effects of the thermal annealing on the surface of multilayer thin films. The LF4192A (Hewlett-Packard) Impedance Analyzer was used in understanding the ac “small-signal” electrical behavior of the entire thermoelectric device. This is a two-terminal measurement yielding underlying operative mechanism via an equivalent circuit model.
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3. Results and Discussion Figure 1 shows the temperature dependence of Seebeck coefficients of 200 multilayers of SiO 2/SiO2+Ge thin films at the different fluences. As seen from figure 1, the ion beam bombardment at the fluence of 5x10 13 ions/cm2 affected the Seebeck coefficients and increased the Seebeck coefficients in the negative direction. While the temperature increased, the Seebeck coefficient continued to increase and reached the maximum value of -25 μV/K at the fluence of 5x1013 ions/cm2. Higher Seebeck coefficient is one of the expected quantities for the high efficiency thermoelectric materials and devices. The reason for high Seebeck coefficient might arise from the higher charge carrier concentration from the 5 MeV Si ions bombardment at the fluence of 5x1013 ions/cm2. While the temperature increased for the thin film bombarded at the fluence of 5x1013 ions/cm2, the Seebeck coefficient increased in negative direction due to the higher charge carrier mobility at the higher temperature. The fluence of 5x1013 ions/cm2 could bring some useful output for the future optimization studies for the future thermoelectric devices. The future fluence studies should go beyond or close the fluence of 5x1013 ions/cm2 for getting higher Seebeck coefficients. Similar studies have been performed on the similar SiO2/SiO2+Ge thin film systems at 50 and 100 multilayers at the different thickness of 117 nm and 190 nm, respectively in the past [12, 10]. While the 50 multilayers of SiO2/SiO2+Ge at the thickness of 117 nm were showing similar Seebeck coefficients at the similar ion fluences, the 100 multilayers of SiO2/SiO2+Ge at the thickness of 190 nm were showing about eight times higher Seebeck coefficients at the similar fluences. The higher Seebeck coefficients might arise from the quantum dots or clusters formation in the thinner layers of the thermoelectric material systems.
Figure 1. Temperature dependence of Seebeck coefficients of 200 multilayers of SiO2/SiO2+Ge thin films at the different fluences.
Figure 2 shows the fluence dependence of van der Pauw resistivity values of 200 multilayers of SiO 2/SiO2+Ge thin films at room temperature. As seen from figure 2, the resistivity increased until the fluence of 1x10 13 ions/cm2, then decreased up to the fluence of 5x1013 ions/cm2. After the fluence of 5x1013 ions/cm2, the resistivity increased again. The expected effects of the high energy ion bombardment are to cause a decrease in the resistivity since the ion bombardment could form nanodots and / or nanoclusters in the SiO2/SiO2+Ge thin films. The decrease in resistivity has been seen between the fluence of 1x1013 ions/cm2 and 5x1013 ions/cm2. This might tell us that the interval between 1x1013 ions/cm2 and 5x1013 ions/cm2 could be suitable for reaching low resistivity values. Low resistivity or high electrical conductivity is one of the expected parameters for the high efficiency thermoelectric materials and devices. Since the electrical resistivity remains low only at certain interval in the figure 2, the future thin film systems could be bombarded with the values along with this interval for the similar thin film systems. Low electrical resistivity (high electrical conductivity) and high Seebeck coefficient have been reached at the same fluence of 5x1013 ions/cm2. The increase in charge carrier concentration might increase both the Seebeck coefficient and the electrical conductivity at the suitable energy and fluences.
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Figure 3 shows the temperature dependence of van der Pauw resistivity values of 200 multilayers of SiO2/SiO2+Ge thin films bombarded at the fluence of 1x1012 ions/cm2. As seen from figure 3, the electrical resistivity increased while the temperature increased at the fluence of 1x10 12 ions/cm2. The increase in the electrical resistivity is not expected for the highly efficient thermoelectric materials and devices. We expect to see increase in the electrical conductivity while the samples are being bombardment since the high energy ion bombardment increases the charge carrier concentration the multilayer thin films. The increase in the charge carrier concentration could increase Seebeck coefficient and the electrical conductivity. 800
V an der P auw R es is tivity @ 300K 200 M ultilayers S iO 2/S iO 2+G e T hin F ilms T hic knes s =1067 nm
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Figure 2. Fluence dependence of van der Pauw Resistivity values of 200 multilayers of SiO2/SiO2+Ge thin films at room temperature.
4x10
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T (K )
Figure 3. Temperature dependence of van der Pauw resistivity values of 200 multilayers of SiO2/SiO2+Ge thin film bombarded at the fluence of 1x1012 ions/cm2.
Figure 4 shows the fluence dependence of Hall coefficients of 200 multilayers of SiO2/SiO2+Ge thin films at room temperature. As seen from figure, the Hall coefficient stayed at the same amount except of the value where the Hall coefficient increased at the fluence of 1x1013 ions/cm2. Figure 5 shows the fluence dependence of Mobility coefficients of 200 multilayers of SiO2/SiO2+Ge thin films at room temperature. As seen from figure 5, mobility increased at the fluence of 5x10 12 ions/cm2. The mobility values stay very close to the each other at other fluences except for the fluence of 5x10 12 ions/cm2. The increase in mobility might come from the increase of the charge carrier concentration due to the high energy ion bombardment. Figure 6 shows the fluence dependence of Carrier Density coefficients of 200 multilayers of SiO2/SiO2+Ge thin
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-3
H all coefficient (cm /C )
films at room temperature. As seen from the figure 6, when the thin film was bombarded, the density value decreased first and then increased while the fluence was increased. When the fluence continued to increase, the density showed slight increments and decrements without so much deviation. The first decrement might be due to the thermal stabilization effect due to high energy ions bombardment. Then, the increment looks acceptable at the suitable fluences since the 5 MeV ions bombardment could increase the density in the multilayer thin film systems. 7x10
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H all E ffec t C oeffic ient M eas urements @ 300K 200 M ultilayers S iO 2/S iO 2+G e T hin F ilms T hic knes s =1067 nm
0 -1x10
7
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10
5 2
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M obility (cm /V s)
Figure 4. Fluence dependence of Hall coefficients of 200 multilayers of SiO2/SiO2+Ge thin films at room remperature. 2.5x10
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M obility M easurements@ 300K 200 M ultilayers S iO 2/S iO 2+G e T hin F ilms T hickness =1067 nm
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10 2
F luence (ions /cm )x10
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100
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Figure 5. Fluence dependence of Mobility coefficients of 200 multilayers of SiO2/SiO2+Ge thin films at room temperature. 2x10
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0 D ens ity M eas urements @ 300K 200 M ultilayers S iO 2/S iO 2+G e T hin F ilms T hic knes s =1067 nm
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1
5
50
10 2
F luenc e (ions /c m )x10
100
12
Figure 6. Fluence dependence of Carrier Density coefficients of 200 multilayers of SiO2/SiO2+Ge thin films at room temperature.
Figure 7 shows the impedance plot and figure 8 shows the admittance plot of unbombarded 200 multilayers of SiO2/SiO2+Ge thin film for the frequency range 20 Hz – 13 MHz. The ac small-signal electrical data were acquired using HP4192A Impedance Analyser for the unbombarded 200-layered stack comprising repeated Si+Ge monolayer
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layer on to SiO2 layer for the frequency range 20 Hz – 13 MHz. These data are analysed via complex plane formalisms. Skewed semi-circular response (via equal grid scaling) is observed in the complex capacitance (C* = Cʹ - j C" = Cp – j Gp/ω) plane. The loci obtained in the C*-plane for these data indicate non-Debye type relaxation displaying the presence of the depression parameter. The skewed semi-circular behavior also shows the contribution of the shunt dc resistance (conductance) at the low frequency end [13-15].
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Figure 7. Impedance plot of unbombarded 200 multilayers of SiO2/SiO2+Ge thin film.
8 6 4
Ycc (u 10-3 : -1)
2 0 -2 -4 -6 -8 -10
0
5
10
15
20
Y c (u 10-3 : -1)
Figure 8. Admittance plot of unbombarded 200 multilayers of SiO2/SiO2+Ge thin film.
Figure 9 shows the Optical absorption spectra of 200 multilayers of SiO2/SiO2+Ge thin films for unannealed and annealed samples at 100 oO C. As seen from figure 9, the absorption shows increment between 350-800 nm. This might be due to the diffusing of Ge ions into the multilayers and more absorption due to Ge ions. Figure 10 shows the AFM images of 200 multilayers of SiO2/SiO2+Ge thin films for unannealed and annealed samples at different temperatures. The thin films were annealed at different temperatures and AFM images were recorded to see the annealing effect on the surface. The purpose of the annealing on the thin films is to reach more smooth surfaces. Figure 10 shows this effect.
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0.14
Absorption (a.u.)
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O ptical A bs orption S pectra 200 M ultilayers S iO 2/S iO 2+G e T hin F ilm
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U nannealed o A nnealed @ 100 C
0.08 0.06 0.04 300
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500
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Figure 9. Optical Absorption spectra of 200 multilayers of SiO2/SiO2+Ge thin films for unannealed and annealed at 100 O C.
A F M of U nannealed 200 M L T hin films
o
A nnealed @ 400 C
o
A nnealed @ 700 C
o
A nnealed @ 1000 C
Figure 10. AFM images of 200 multilayers of SiO2/SiO2+Ge thin films for unannealed and annealed at different temperatures.
4. Conclusion Thermoelectric multilayer thin films for thermoelectric generator devices have been prepared from 200 alternating layers of SiO2/SiO2+Ge superlattice films using DC/RF magnetron sputtering. The 5 MeV Si ions bombardment has been performed using the AAMU Pelletron ion beam accelerator to tailor the thermoelectric properties. The ion beam bombardment at the fluence of 5x1013 ions/cm2 affected the Seebeck coefficients,increasing them in the negative direction. While the temperature increased, the Seebeck coefficient continued to increase and reached the maximum value of -25 μV/K at the fluence of 5x1013 ions/cm2. A higher Seebeck coefficient is one of the expected
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quantities for the high efficient thermoelectric materials and devices. The decrease in resistivity has been seen between the fluence of 1x1013 ions/cm2 and 5x1013 ions/cm2. This might tell us that the interval between 1x1013 ions/cm2 and 5x1013 ions/cm2 could be suitable for the low resistivity values. Low resistivity or high electrical conductivity is one of the expected parameters for the high efficiency thermoelectric materials and devices. Transport properties like Hall coefficient, carrier density and mobility did not change at all fluences. For the future studies, more optimization studies are needed to be performed to see significant changes in the transport properties. Acknowledgements Research sponsored by Materials Research Laboratory (MRL), National Science Foundation under NSF-EPSCOR R-II-3 Grant No. EPS-1158862, DOD under Nanotechnology Infrastructure Development for Education and Research through the Army Research Office # W911 NF-08-1-0425, and DOD Army Research Office # W911 NF12-1-0063, U.S. Department of Energy National Nuclear Security Admin with grant# DE-NA0001896 and grant# DE-NA0002687, , NSF-REU with Award#1156137. References [1] Xi, H, Luo, L and Fraisse, G, 2007, Development and application of solar-based thermoelectric technologies, Renewable & Sustainable Energy Reviews 11, 8103-8117. [2] Budak, S., Smith, C., Muntele, C., Chhay, B., Heidary, K., Johnson, R. B., ILA, D., 2013, Thermoelectric Properties of SiO 2/SiO2+CoSb Multi-nanolayered Thin Films Modified by MeV Si Ions, Journal of Intelligent Material Systems and Structures, 24(11), 1350-1356. [3] Riffat, S.B., Xiaoli, 2003. Applied Thermal Engineering 23, 913-935. [4] Budak, S., Guner, S., Minamisawa, R.A., Muntele, C. I., Ila, D., Thermoelectric Properties of Zn4Sb3/CeFe(4-x)CoxSb12 Nano-layered Superlattices Modified by MeV Si ions Beam, Applied Surface Science, 310, 226-229 (2014). [5] Sahin, A.Z. and Yilbas, B.S., 2013, The thermoelement as thermoelectric power generator: effect of leg geometry on the efficiency and power generation, Energy Conversion and Management 65, 26-32. [6] Scales, B.C., 2002, Smaller is cooler, Science 295, 1248. [7] Goldsmid, H., 1964, Thermoelectric Refrigeration, Plenum Press, New York. [8] Slack, G., 1995, In: Rowe, D. M. (Ed.), CRC Handbook of Thermoelectrics, CRC Press, pp.407. [9] Guner, S., Budak, S., Minamisawa, R.A., et al., 2008, Thickness and MeV Si ions bombardment effects on the thermoelectric properties of Ce3Sb10 thin films, Nucl. Instr. and Meth. in Phys. Res. B 266, 1261-1264. [10] Budak, S., Parker, R., Smith, C., Muntele, C., Heidary, K., Johnson, R. B., ILA, D., 2013, Superlattice Multi-nanolayered Thin Films of SiO2/SiO2+Ge for Thermoelectric Device Applications, Journal of Intelligent Material Systems and Structures, 24(11), 1357-1364. [11] Guner, S., Budak, S., Gibson, B., ila, D., Optical properties of Ag nanoclusters formed by irradiation and annealing of SiO2/SiO2:Ag thin films, Applied Surface Science 310 ( 2014) 180-183. [12] Budak, S., Smith, C., Pugh, M., Heidary, K., Colon, T., Johnson, R.B., ila, D., MeV Si Ions Bombardment Effects on Thermoelectric Properties of Thermoelectric of SiO2/SiO2+Ge Nanolayers, Radiation Physics and Chemistry, 81 (2012) 410-413. [13] Alim, M. A, 1996, Electrical Characterization of Engineering Materials, Active and Passive Electronic Components 19, 139-169. [14] Wang, C. C., Chen, W. H., Akbar, S. A., and Alim, M. A., 1997, High-Temperature a.c. Electrical Behaviour of Polycrystalline Calcium Zirconate, Journal of Materials Science 32, 2305-2312. [15] Azad, A.M., Shyan, L. L. W., and Alim, M. A., 1999, Electrical Characterization of the Solid-State Reaction Derived CaSnO3”, Journal of Materials Science 34, 3375-3396.