- Email: [email protected]
Si by ion beam bombardment
Available online at www.sciencedirect.com
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 73–78 www.elsevier.com/locate/nimb
Improvement on thermoelectric properties of multilayered Si1xGex/Si by ion beam bombardment B. Zheng *, Z. Xiao, B. Chhay, R. Zimmerman, D. Ila Center for Irradiation Materials (CIM), Department of Physics, Alabama A&M University, Normal, AL 35762, USA Received 3 April 2007; received in revised form 23 October 2007 Available online 23 November 2007
Abstract We made n-type nano-scale thin film thermoelectric (TE) devices that consist of multiple periodic layers of Si1xGex/Si. The period is about 10 nm. The structure was modified by 5 MeV Si ion bombardment that formed a nano-scale cluster structure. In addition to the effect of confinement of the phonon transmission, formation of nanoclusters by the ionization energy of incident MeV Si ions further increases the scattering of phonons, increasing the chance of inelastic interaction of phonons, resulting in more annihilation of phonons. This limits phonon mean free path. Phonons are absorbed and dissipated along the layers rather than in the direction perpendicular to the layer interfaces, therefore cross plane thermal conductivity is reduced. The increase of the density of electronic states due to the formation of nanocluster minibands increases the cross plane Seebeck coefficient and increases the cross plane electric conductivity of the film. Eventually, the thermoelectric figure of merit of the TE film increases. Ó 2007 Elsevier B.V. All rights reserved. PACS: 81.07.b Keywords: Thermoelectric multilayer superlattice; Ion bombardment; Rutherford backscattering spectroscopy; 3x measurement of thin film thermal conductivity
1. Introduction Semiconductor thermoelectric devices are widely used as the cooling device for integrated circuits and the electrical power generator device because of their higher efficiency, smaller volume and higher power density. While Si and Ge can be monolithically integrated with Si-based microelectronic device. And thin film multilayer superlattice thermoelectric device with quantum well structure has a lower thermal conductivity, In order to further decrease phonon transmission and meanwhile increase Seebeck coefficient and electric conductivity of thin film, in this research, MeV Si ion bombardment is used to modify the
*
Corresponding author. Tel.: +1 603 233 3250. E-mail addresses: [email protected], [email protected]. edu (B. Zheng). 0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.10.041
film to form nanoclusters among the superlattice. The phonon confinement effect is enhanced by ion bombardment that forms nanoclusters. The defects and disorder in the lattice caused by bombardment and the grain boundaries of the nano-scale clusters increase phonon scattering, increasing the probability of inelastic interactions of phonons and annihilation of phonons. This limits phonon mean free path. The existence of these nanoclusters enhances the horizontal dissipation and absorption of phonons along the superlattice interface rather than perpendicular to superlattice. The cross plane thermal conductivity of an ion bombardment thin film decreases. The cross plane electric conductivity and the cross plane Seebeck coefficient of the ion bombarded thin film also increases due to the increased density of electronic states. Hence, the thermoelectric figure of merit of the film increases. Si and Ge are very commonplace materials used in thermoelectric device, therefore study on ion beam modified
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B. Zheng et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 73–78
Si1xGex/Si superlattice is particularly significant for testing the effect of ion beam modification on thermoelectric device. The actual thermoelectric device consists of multiple n type and p type couples, while in this research, we just study one section (n type or p type) individually. As shown in Fig. 1, the Si/Si1xGex multilayer thin film thermoelectric (TE) devices prepared at CIM are periodic structures consisting of tens to hundreds of alternating layers, each with different band gap (Si: 1.12 eV, S1xGex: 0.8–0.9 eV). Each multilayer film is deposited between two metal layers. Due to the Peltier effect between the semiconductor film and the metal, an electric current causes heat to be absorbed at the negative terminal and released at the positive terminal. The performance of a thermoelectric device can be quantified by the dimensionless figure of merit ZT = S2rT/k [1]. Our aim is to obtain a high ZT value by increasing cross plane Seebeck coefficient S, cross plane electrical conductivity r and reducing cross plane thermal conductivity k by bombarding a multilayer structure with MeV Si ions. Ion bombardment induces the formation of nano-scale clusters. In addition to the Bragg reflection at the layer interfaces [1–3], the defects and disorder in the lattice caused by ion bombardment and the boundaries of these nano-scale clusters formed by bombardment increase phonon scattering, increasing the chance of an inelastic interaction and phonon annihilation. These effects inhibit heat transport perpendicular to the layer interfaces [4–7]. Phonons are absorbed and dissipated along the layer interfaces. Hence, the cross plane thermal conductivity decreases. The increase of the density of electronic states due to the periodic potential barriers of nanoclusters produced by ion bombardment also increases the cross plane Seebeck coefficient and the cross plane electric conductivity. The thermoelectric figure of merit of the multilayer structure increases.
Heats up
Metal Cu
2. Experiments 2.1. Fabrication of thin film thermoelectric sample At CIM, the electron beam evaporation deposition system with two guns was used to deposit Si1xGex/Si multilayer thin film. The vacuum chamber with a cryogenic pump was maintained at 2 106 Torr. The multilayer films were sequentially deposited on a Si substrate, which was coated with a SiO2 insulation layer and a metal (Cu) contact layer in advance. The thickness of the deposited layers was controlled by a crystal oscillator deposition monitor. Each Si layer was deposited at a constant rate by a single e-beam evaporator. The Si deposition rate was maintained constant while an additional evaporator periodically provided Ge atoms at a rate that determined x in alternate layers of Si1xGex. This technique produced a periodic structure consisting of 70 alternating layers of Si1xGex/Si. The period was about 10 nm. Five MeV Si ion bombardment was performed on this multilayered sample using the AAMU Pelletron ion beam accelerator. SRIM simulation software [8] shows that 5 MeV Si ions pass through Si1+xGex/Si multilayer film and terminate deep in the substrate. A second Cu contact layer was later deposited over the thin film after ion bombardment. The sample was cut by a diamond cutter to get a clean edge of each layer. In this way, a complete thermoelectric sample was made. In order to determine the stoichiometry of the Si1xGex layer grown in this condition, a single layer Si1xGex thin film was grown on glassy polymeric carbon (GPC) for the purpose of Rutherford backscattering spectroscopic (RBS) analysis. The RBS spectrum of this Si1xGex layer grown on GPC and the RUMP (RBS Analysis and Simulation Package) simulation results shown in Fig. 2 indicates that the Si1xGex may be characterized as Si0.84Ge0.16. The Raman scattering spectrum of the 70-layer Si/Si1xGex thin film (10 nm each layer) before and after Si ion bombardment shown in Fig. 3 indicates that ion bombardment breaks the Si1xGex bonds forming nano-scale Ge and Si polycrystals and a more disordered structure.
Q
2.2. Measurements of thermoelectric properties of thin films 10nm Si1-xGex/10nm Si 70 layers
Cool down
Metal Cu
Q
SiO2 Si substrate
Fig. 1. Schematic of thin film TE device.
2.2.1. Cross plane thermal conductivity We used the 3x technique to measure the cross plane thermal conductivity of the thin film as shown in Fig. 4. A narrow Pt strip, used both as heater and as a temperature sensor, is deposited on the thin film [9]. An AC current I sin xt heats the metal strip through two terminals by providing a known power I 2 R sin2 xt, where R is the resistance of the heater. A voltage signal IR is obtained through the other two terminals and compared in a Kelvin bridge " x # I 2 R ln x21 k¼a ð1Þ V 32 W=K m: 4pl V 31IR
B. Zheng et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 73–78
75
Energy (MeV) 0.5
6
1.0
2.0
2.1 MeV He+ 1700 backscattered angle
5
Normalized Yield
1.5
4
Experiment RUMP simulation line
3
2
Ge
Si 1
As Se 0 0
200
400
600
800
1000
Channel Fig. 2. Rutherford Backscattering Spectroscopy (RBS) spectrum of a single layer Si1xGex thin film on carbon and RUMP simulation results.
Ge 7000 6000
after 5 Mev 1×1014/cm2 Si ion bombardment
intensity
5000
before ion bombardment
4000 3000
Si-Ge amorphous Si
2000
polycrystalline Si 1000 0 188
288
388
488
588
wavenumber (cm-1) Fig. 3. Raman scattering spectrum of the 70 layer Si1xGex/Si thin film (10 nm each layer) before and after Si ion bombardment.
Circuit with the voltage on a standard resistance Rs that passes the same current. The large 1x voltage component and all other harmonic and anharmonic voltages generated by the current supply are cancelled using the bridge. A third harmonic voltage is generated across the Pt heater because the resistivity of Pt has a large temperature coefficient of resistance a K1 and the resistivity of Rs does not. R changes at a frequency 2x. With the known input power passing through the thin film deposited on a silica substrate, and temperature information from the 3x voltages for two AC current frequencies, the thermal conductivity k can be calculated [10–13]. The length of the Pt heater is l. The lock in amplifier in Fig. 4, operating with a variable frequency reference suppresses noise and allows the detection of the 3x signals V31 and V32 expressed in Eq. (1) relative to the 1x voltage
I 0 sin ω t
Pt strip Rs amplifier
thin film substrate
Lock-in amplifier
RS232
computer
Fig. 4. Schematic of the 3x measurement of thin film cross plane thermal conductivity.
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B. Zheng et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 73–78
V = IR of the current source. The fundamental frequencies were 15 and 30 Hz, well below the limit of operation of the Kelvin bridge [14]. The thermal conductivities of a 70-layer Si1xGex/Si structure before and after bombardment by 5 MeV Si ions with fluences of 0.5, 1, 5 and 10 1013/cm2 are shown in Fig. 5. The cross plane thermal conductivity decreases with increasing ion bombardment fluence.
30
thermal conductivity (mW/cmK)
25
20
15
10
5
0 0
2E+13
4E+13
6E+13
8E+13
1E+14
2.2.2. Cross plane Seebeck coefficient The experimental setup for measurement of the cross plane Seebeck coefficient at various temperatures is shown in Fig. 6. In order to measure the cross plane Seebeck coefficient, the film is deposited between two thermally and electrically conductive Cu layers as shown in Fig. 6. A reference wire (Constantan alloy) is used to eliminate the extraneous thermal offset voltage and to derive the Seebeck coefficient of the sample. Nitrogen gas cooling tubes are imbedded in the heat sink that is thermally connected to one side of the sample and to the Constantan reference wire. A heater on the other side of the sample and reference wire establishes a temperature. The nitrogen gas cooling and the heater resistor are controlled to adjust the temperature of the sample and reference wire when thermal equilibrium is reached. A silicon diode sensor monitors the temperature at the hot end. To measure the Seebeck coefficient of the sample versus temperature, two electric powers P1 and P2 are applied sequentially to produce two incremental temperatures DT(P1) and DT(P2). The Seebeck coefficient of the sample SS is calculated [14] according to SS ¼ SR
1.2E+14
5Mev Si ion bambardment fluence (#/cm2)
Fig. 5. Thermal conductivity of the 70 layer Si1xGex/Si thin film before and after 5 MeV Si ion bombardment with various fluences.
V S ðP 2 Þ V S ðP 1 Þ ; V R ðP 2 Þ V R ðP 1 Þ
where SR is the Seebeck coefficient of the Constatan reference wire. VS(P1), VS(P2), VR(P1) and VR(P2) are the
hot area heater reference material (constantan alloy wire)
sample
copper line
cold area VS
VR
SiO2 substrate Cu Sample Cu
ð2Þ
hot area cold area
Fig. 6. Experiment for the Seebeck coefficient measurement.
B. Zheng et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 73–78
2.2.3. Cross plane electric conductivity Cross plane electrical conductivity of a thin film can be measured as shown in Fig. 8. The power supply is adjusted to 1 mV using potential divider to prevent penetrating the thin film. Here we assume that the Schottky junction barrier between Cu and semiconductor and the resistance of the copper layer are negligible [14]. Temperature (K) 250
270
290
310
330
350
370
6
electric conductivity (/Ohm cm)
5 4 3 2 1 0 0
2E+13
4E+13
6E+13
8E+13
1E+14
1.2E+14
5 MeV Si ion bombardment fluence (#/cm2)
Fig. 9. Cross plane electric conductivity of 70 layer Si1xGex/Si thin film before and after 5 MeV Si ion bombardment with various fluences.
1
T=300K
figure of merit
Seebeck voltages of the sample and reference wire at the two different temperature gradients. The Seebeck coefficients of the 70-layer Si1xGex/Si sample before and after bombardment by 5 MeV Si ion with fluence of 0.5 1013/cm2, 1 1013/cm2, 5 1013/ cm2 and 1014/cm2 are shown in Fig. 7. Fig. 7 indicates that the amplitude of the Seebeck coefficient of the thin films increases with increasing ion bombardment fluence. For n-type material, the Seebeck 3 coefficient S ¼ DV ¼ keB ln NN DC , where N c ¼ 2ð2pmhc2KT Þ2 is DT the effective density of electron states in the conduction band, ND is the doping density of n type semiconductor. Here ND n, n is the density of the majority carrier in n type semiconductor. The negative value of the observed Seebeck coefficient indicates that our samples are n type.
77
0.1
0.01
390
0
Seebeck coefficient (uV/K)
-200 -400
0.001 0
-600
2E+13
4E+13
6E+13
8E+13
1E+14
1E+14
5 MeVsi ion bombardment fluence (#/cm2)
-800 no bombardment -1000
5.00E+12/cm2
-1200
1E+13/cm2
-1400
5E+13/cm2
-1600
Fig. 10. Figure of merit of 70 layer Si1xGex/Si thin film before and after 5 MeV Si ion bombardment with various fluences.
1E+14/cm2
Fig. 7. Seebeck coefficient of 70 layer Si1xGex/Si thin film at various temperatures before and after 5 MeV Si ion bombardment with various fluences.
2.2.4. Figure of merit The figure of merit (ZT = S2rT/k) of the 70-layer Si1xGex/Si sample at various bombardment fluences is shown in Fig. 10. Fig. 10 indicates that the figure of merit increases with increasing bombardment fluence.
A 70 layers 10 nm Si1-xGex 10 nm Si
The electric conductivities of the 70-layer Si1xGex/Si sample before and after bombardment by 5 MeV Si ion with fluences of 0.5 1013/cm2, 1 1013/cm2, 5 1013/ cm2 and 1014/cm2 are shown in Fig 9. The electric conductivity increases with increasing bombardment fluence.
Metal Cu
3. Discussion and conclusion V
Metal Cu SiO2
Si substrate Fig. 8. Measurement setup of thin film cross plane electric conductivity.
The TE thin film in our experiment was made with a 10 nm period. Only the phonons whose wave vector lies in the first Brillouin zone are significant for elastic waves (k 6 p/a or k P 2a). The phonon transmission can be minimized as long as the period a is more than or equal to half of the phonon wavelength 2a P k. If the period is too small 2a < k, the phonon transmission can not be minimized. If
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the multilayer period satisfies the Bragg condition 2a sin h = nk, here h = p/2, the acoustic impedance mismatch between the materials of each layer gives rise to selective Bragg scattering at the layer interfaces. The phonon will be confined within the structure of multilayer (so called phonon blocking). However, the layer period cannot be so large that electron tunneling phenomenon will disappear and decrease the electric conductivity. The acoustic wave will become the oscillation of atoms instead of layers. Because the distance between neighboring atoms a0 is small, meaning k P 2a0 , in this case, the phonon wave vector lies in the first Brillouin zone k 6 p/a0 , and phonon transmission can not be reduced. The phonon confinement effect is enhanced by ion bombardment that forms nanoclusters. The defects and disorder in the lattice caused by bombardment and the grain boundaries of the nano-scale clusters increase phonon scattering, increasing the probability of inelastic interactions of phonons and annihilation of phonons. This limits cross plane phonon mean free path. Because thermal conductivity is k = 1/3 (Cqvlmfp), where C is specific heat, q is density, v is velocity of phonon, lmfp is phonon mean free path, so cross plane thermal conductivity decreases. The existence of these nanoclusters enhances the horizontal dissipation and absorption of phonons along the superlattice interface rather than perpendicular to superlattice. The cross plane thermal conductivity of an ion bombardment thin film decreases. Although the disorder in the crystal produced by ion bombardment will decrease the electric conductivity, this tendency is overwhelmed by the increase of the density of electronic states in the structure formed by ion bombardment such that
cross plane electric conductivity of the thin film increases. The cross plane Seebeck coefficient of the ion bombarded thin film also increases due to the increased density of electronic states. Hence, the thermoelectric figure of merit of the film increases. The Si1xGex/Si semiconductor structure modified by Si ion bombardment shows promise as a high figure of merit thermoelectric material. 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] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
X. Fan, Electron. Lett. 37 (2001) 126. X. Fan, G. Zeng, Appl. Phys. Lett. 78 (2001) 1580. R. Singh, Mater. Res. Soc. Symp. Proc. 793 (2004) 1141. D.G. Cahill, M. Katiyar, Phys. Rev. B 50 (1994) 6007. R. Venkatasubramanian, Phys. Rev. B 61 (2000) 3091. J.L. Liu, Phys. Rev. B 67 (2003) 4781. P. Hyldgaard, Phys. Rev. 56B (1997) 118. J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985, 234. D.G. Cahill, Rev. Sci. Instrum. 61 (1990) 802. T. Borca-Tasciuc, Rev. Sci. Instrum. 72 (2001) 679. T. Scott, Appl. Phys. Lett. 80 (2002) 1737. S. M Lee, D.G. Cahill, Appl. Phys. Lett. 70 (1997) 1259. G. Zeng, Appl. Phys. Lett. 71 (2004) 1234. B. Zheng et al., MRS Symp. Proc. 929 (2006) 81.
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 73–78 www.elsevier.com/locate/nimb
Improvement on thermoelectric properties of multilayered Si1xGex/Si by ion beam bombardment B. Zheng *, Z. Xiao, B. Chhay, R. Zimmerman, D. Ila Center for Irradiation Materials (CIM), Department of Physics, Alabama A&M University, Normal, AL 35762, USA Received 3 April 2007; received in revised form 23 October 2007 Available online 23 November 2007
Abstract We made n-type nano-scale thin film thermoelectric (TE) devices that consist of multiple periodic layers of Si1xGex/Si. The period is about 10 nm. The structure was modified by 5 MeV Si ion bombardment that formed a nano-scale cluster structure. In addition to the effect of confinement of the phonon transmission, formation of nanoclusters by the ionization energy of incident MeV Si ions further increases the scattering of phonons, increasing the chance of inelastic interaction of phonons, resulting in more annihilation of phonons. This limits phonon mean free path. Phonons are absorbed and dissipated along the layers rather than in the direction perpendicular to the layer interfaces, therefore cross plane thermal conductivity is reduced. The increase of the density of electronic states due to the formation of nanocluster minibands increases the cross plane Seebeck coefficient and increases the cross plane electric conductivity of the film. Eventually, the thermoelectric figure of merit of the TE film increases. Ó 2007 Elsevier B.V. All rights reserved. PACS: 81.07.b Keywords: Thermoelectric multilayer superlattice; Ion bombardment; Rutherford backscattering spectroscopy; 3x measurement of thin film thermal conductivity
1. Introduction Semiconductor thermoelectric devices are widely used as the cooling device for integrated circuits and the electrical power generator device because of their higher efficiency, smaller volume and higher power density. While Si and Ge can be monolithically integrated with Si-based microelectronic device. And thin film multilayer superlattice thermoelectric device with quantum well structure has a lower thermal conductivity, In order to further decrease phonon transmission and meanwhile increase Seebeck coefficient and electric conductivity of thin film, in this research, MeV Si ion bombardment is used to modify the
*
Corresponding author. Tel.: +1 603 233 3250. E-mail addresses: [email protected], [email protected]. edu (B. Zheng). 0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.10.041
film to form nanoclusters among the superlattice. The phonon confinement effect is enhanced by ion bombardment that forms nanoclusters. The defects and disorder in the lattice caused by bombardment and the grain boundaries of the nano-scale clusters increase phonon scattering, increasing the probability of inelastic interactions of phonons and annihilation of phonons. This limits phonon mean free path. The existence of these nanoclusters enhances the horizontal dissipation and absorption of phonons along the superlattice interface rather than perpendicular to superlattice. The cross plane thermal conductivity of an ion bombardment thin film decreases. The cross plane electric conductivity and the cross plane Seebeck coefficient of the ion bombarded thin film also increases due to the increased density of electronic states. Hence, the thermoelectric figure of merit of the film increases. Si and Ge are very commonplace materials used in thermoelectric device, therefore study on ion beam modified
74
B. Zheng et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 73–78
Si1xGex/Si superlattice is particularly significant for testing the effect of ion beam modification on thermoelectric device. The actual thermoelectric device consists of multiple n type and p type couples, while in this research, we just study one section (n type or p type) individually. As shown in Fig. 1, the Si/Si1xGex multilayer thin film thermoelectric (TE) devices prepared at CIM are periodic structures consisting of tens to hundreds of alternating layers, each with different band gap (Si: 1.12 eV, S1xGex: 0.8–0.9 eV). Each multilayer film is deposited between two metal layers. Due to the Peltier effect between the semiconductor film and the metal, an electric current causes heat to be absorbed at the negative terminal and released at the positive terminal. The performance of a thermoelectric device can be quantified by the dimensionless figure of merit ZT = S2rT/k [1]. Our aim is to obtain a high ZT value by increasing cross plane Seebeck coefficient S, cross plane electrical conductivity r and reducing cross plane thermal conductivity k by bombarding a multilayer structure with MeV Si ions. Ion bombardment induces the formation of nano-scale clusters. In addition to the Bragg reflection at the layer interfaces [1–3], the defects and disorder in the lattice caused by ion bombardment and the boundaries of these nano-scale clusters formed by bombardment increase phonon scattering, increasing the chance of an inelastic interaction and phonon annihilation. These effects inhibit heat transport perpendicular to the layer interfaces [4–7]. Phonons are absorbed and dissipated along the layer interfaces. Hence, the cross plane thermal conductivity decreases. The increase of the density of electronic states due to the periodic potential barriers of nanoclusters produced by ion bombardment also increases the cross plane Seebeck coefficient and the cross plane electric conductivity. The thermoelectric figure of merit of the multilayer structure increases.
Heats up
Metal Cu
2. Experiments 2.1. Fabrication of thin film thermoelectric sample At CIM, the electron beam evaporation deposition system with two guns was used to deposit Si1xGex/Si multilayer thin film. The vacuum chamber with a cryogenic pump was maintained at 2 106 Torr. The multilayer films were sequentially deposited on a Si substrate, which was coated with a SiO2 insulation layer and a metal (Cu) contact layer in advance. The thickness of the deposited layers was controlled by a crystal oscillator deposition monitor. Each Si layer was deposited at a constant rate by a single e-beam evaporator. The Si deposition rate was maintained constant while an additional evaporator periodically provided Ge atoms at a rate that determined x in alternate layers of Si1xGex. This technique produced a periodic structure consisting of 70 alternating layers of Si1xGex/Si. The period was about 10 nm. Five MeV Si ion bombardment was performed on this multilayered sample using the AAMU Pelletron ion beam accelerator. SRIM simulation software [8] shows that 5 MeV Si ions pass through Si1+xGex/Si multilayer film and terminate deep in the substrate. A second Cu contact layer was later deposited over the thin film after ion bombardment. The sample was cut by a diamond cutter to get a clean edge of each layer. In this way, a complete thermoelectric sample was made. In order to determine the stoichiometry of the Si1xGex layer grown in this condition, a single layer Si1xGex thin film was grown on glassy polymeric carbon (GPC) for the purpose of Rutherford backscattering spectroscopic (RBS) analysis. The RBS spectrum of this Si1xGex layer grown on GPC and the RUMP (RBS Analysis and Simulation Package) simulation results shown in Fig. 2 indicates that the Si1xGex may be characterized as Si0.84Ge0.16. The Raman scattering spectrum of the 70-layer Si/Si1xGex thin film (10 nm each layer) before and after Si ion bombardment shown in Fig. 3 indicates that ion bombardment breaks the Si1xGex bonds forming nano-scale Ge and Si polycrystals and a more disordered structure.
Q
2.2. Measurements of thermoelectric properties of thin films 10nm Si1-xGex/10nm Si 70 layers
Cool down
Metal Cu
Q
SiO2 Si substrate
Fig. 1. Schematic of thin film TE device.
2.2.1. Cross plane thermal conductivity We used the 3x technique to measure the cross plane thermal conductivity of the thin film as shown in Fig. 4. A narrow Pt strip, used both as heater and as a temperature sensor, is deposited on the thin film [9]. An AC current I sin xt heats the metal strip through two terminals by providing a known power I 2 R sin2 xt, where R is the resistance of the heater. A voltage signal IR is obtained through the other two terminals and compared in a Kelvin bridge " x # I 2 R ln x21 k¼a ð1Þ V 32 W=K m: 4pl V 31IR
B. Zheng et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 73–78
75
Energy (MeV) 0.5
6
1.0
2.0
2.1 MeV He+ 1700 backscattered angle
5
Normalized Yield
1.5
4
Experiment RUMP simulation line
3
2
Ge
Si 1
As Se 0 0
200
400
600
800
1000
Channel Fig. 2. Rutherford Backscattering Spectroscopy (RBS) spectrum of a single layer Si1xGex thin film on carbon and RUMP simulation results.
Ge 7000 6000
after 5 Mev 1×1014/cm2 Si ion bombardment
intensity
5000
before ion bombardment
4000 3000
Si-Ge amorphous Si
2000
polycrystalline Si 1000 0 188
288
388
488
588
wavenumber (cm-1) Fig. 3. Raman scattering spectrum of the 70 layer Si1xGex/Si thin film (10 nm each layer) before and after Si ion bombardment.
Circuit with the voltage on a standard resistance Rs that passes the same current. The large 1x voltage component and all other harmonic and anharmonic voltages generated by the current supply are cancelled using the bridge. A third harmonic voltage is generated across the Pt heater because the resistivity of Pt has a large temperature coefficient of resistance a K1 and the resistivity of Rs does not. R changes at a frequency 2x. With the known input power passing through the thin film deposited on a silica substrate, and temperature information from the 3x voltages for two AC current frequencies, the thermal conductivity k can be calculated [10–13]. The length of the Pt heater is l. The lock in amplifier in Fig. 4, operating with a variable frequency reference suppresses noise and allows the detection of the 3x signals V31 and V32 expressed in Eq. (1) relative to the 1x voltage
I 0 sin ω t
Pt strip Rs amplifier
thin film substrate
Lock-in amplifier
RS232
computer
Fig. 4. Schematic of the 3x measurement of thin film cross plane thermal conductivity.
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B. Zheng et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 73–78
V = IR of the current source. The fundamental frequencies were 15 and 30 Hz, well below the limit of operation of the Kelvin bridge [14]. The thermal conductivities of a 70-layer Si1xGex/Si structure before and after bombardment by 5 MeV Si ions with fluences of 0.5, 1, 5 and 10 1013/cm2 are shown in Fig. 5. The cross plane thermal conductivity decreases with increasing ion bombardment fluence.
30
thermal conductivity (mW/cmK)
25
20
15
10
5
0 0
2E+13
4E+13
6E+13
8E+13
1E+14
2.2.2. Cross plane Seebeck coefficient The experimental setup for measurement of the cross plane Seebeck coefficient at various temperatures is shown in Fig. 6. In order to measure the cross plane Seebeck coefficient, the film is deposited between two thermally and electrically conductive Cu layers as shown in Fig. 6. A reference wire (Constantan alloy) is used to eliminate the extraneous thermal offset voltage and to derive the Seebeck coefficient of the sample. Nitrogen gas cooling tubes are imbedded in the heat sink that is thermally connected to one side of the sample and to the Constantan reference wire. A heater on the other side of the sample and reference wire establishes a temperature. The nitrogen gas cooling and the heater resistor are controlled to adjust the temperature of the sample and reference wire when thermal equilibrium is reached. A silicon diode sensor monitors the temperature at the hot end. To measure the Seebeck coefficient of the sample versus temperature, two electric powers P1 and P2 are applied sequentially to produce two incremental temperatures DT(P1) and DT(P2). The Seebeck coefficient of the sample SS is calculated [14] according to SS ¼ SR
1.2E+14
5Mev Si ion bambardment fluence (#/cm2)
Fig. 5. Thermal conductivity of the 70 layer Si1xGex/Si thin film before and after 5 MeV Si ion bombardment with various fluences.
V S ðP 2 Þ V S ðP 1 Þ ; V R ðP 2 Þ V R ðP 1 Þ
where SR is the Seebeck coefficient of the Constatan reference wire. VS(P1), VS(P2), VR(P1) and VR(P2) are the
hot area heater reference material (constantan alloy wire)
sample
copper line
cold area VS
VR
SiO2 substrate Cu Sample Cu
ð2Þ
hot area cold area
Fig. 6. Experiment for the Seebeck coefficient measurement.
B. Zheng et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 73–78
2.2.3. Cross plane electric conductivity Cross plane electrical conductivity of a thin film can be measured as shown in Fig. 8. The power supply is adjusted to 1 mV using potential divider to prevent penetrating the thin film. Here we assume that the Schottky junction barrier between Cu and semiconductor and the resistance of the copper layer are negligible [14]. Temperature (K) 250
270
290
310
330
350
370
6
electric conductivity (/Ohm cm)
5 4 3 2 1 0 0
2E+13
4E+13
6E+13
8E+13
1E+14
1.2E+14
5 MeV Si ion bombardment fluence (#/cm2)
Fig. 9. Cross plane electric conductivity of 70 layer Si1xGex/Si thin film before and after 5 MeV Si ion bombardment with various fluences.
1
T=300K
figure of merit
Seebeck voltages of the sample and reference wire at the two different temperature gradients. The Seebeck coefficients of the 70-layer Si1xGex/Si sample before and after bombardment by 5 MeV Si ion with fluence of 0.5 1013/cm2, 1 1013/cm2, 5 1013/ cm2 and 1014/cm2 are shown in Fig. 7. Fig. 7 indicates that the amplitude of the Seebeck coefficient of the thin films increases with increasing ion bombardment fluence. For n-type material, the Seebeck 3 coefficient S ¼ DV ¼ keB ln NN DC , where N c ¼ 2ð2pmhc2KT Þ2 is DT the effective density of electron states in the conduction band, ND is the doping density of n type semiconductor. Here ND n, n is the density of the majority carrier in n type semiconductor. The negative value of the observed Seebeck coefficient indicates that our samples are n type.
77
0.1
0.01
390
0
Seebeck coefficient (uV/K)
-200 -400
0.001 0
-600
2E+13
4E+13
6E+13
8E+13
1E+14
1E+14
5 MeVsi ion bombardment fluence (#/cm2)
-800 no bombardment -1000
5.00E+12/cm2
-1200
1E+13/cm2
-1400
5E+13/cm2
-1600
Fig. 10. Figure of merit of 70 layer Si1xGex/Si thin film before and after 5 MeV Si ion bombardment with various fluences.
1E+14/cm2
Fig. 7. Seebeck coefficient of 70 layer Si1xGex/Si thin film at various temperatures before and after 5 MeV Si ion bombardment with various fluences.
2.2.4. Figure of merit The figure of merit (ZT = S2rT/k) of the 70-layer Si1xGex/Si sample at various bombardment fluences is shown in Fig. 10. Fig. 10 indicates that the figure of merit increases with increasing bombardment fluence.
A 70 layers 10 nm Si1-xGex 10 nm Si
The electric conductivities of the 70-layer Si1xGex/Si sample before and after bombardment by 5 MeV Si ion with fluences of 0.5 1013/cm2, 1 1013/cm2, 5 1013/ cm2 and 1014/cm2 are shown in Fig 9. The electric conductivity increases with increasing bombardment fluence.
Metal Cu
3. Discussion and conclusion V
Metal Cu SiO2
Si substrate Fig. 8. Measurement setup of thin film cross plane electric conductivity.
The TE thin film in our experiment was made with a 10 nm period. Only the phonons whose wave vector lies in the first Brillouin zone are significant for elastic waves (k 6 p/a or k P 2a). The phonon transmission can be minimized as long as the period a is more than or equal to half of the phonon wavelength 2a P k. If the period is too small 2a < k, the phonon transmission can not be minimized. If
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the multilayer period satisfies the Bragg condition 2a sin h = nk, here h = p/2, the acoustic impedance mismatch between the materials of each layer gives rise to selective Bragg scattering at the layer interfaces. The phonon will be confined within the structure of multilayer (so called phonon blocking). However, the layer period cannot be so large that electron tunneling phenomenon will disappear and decrease the electric conductivity. The acoustic wave will become the oscillation of atoms instead of layers. Because the distance between neighboring atoms a0 is small, meaning k P 2a0 , in this case, the phonon wave vector lies in the first Brillouin zone k 6 p/a0 , and phonon transmission can not be reduced. The phonon confinement effect is enhanced by ion bombardment that forms nanoclusters. The defects and disorder in the lattice caused by bombardment and the grain boundaries of the nano-scale clusters increase phonon scattering, increasing the probability of inelastic interactions of phonons and annihilation of phonons. This limits cross plane phonon mean free path. Because thermal conductivity is k = 1/3 (Cqvlmfp), where C is specific heat, q is density, v is velocity of phonon, lmfp is phonon mean free path, so cross plane thermal conductivity decreases. The existence of these nanoclusters enhances the horizontal dissipation and absorption of phonons along the superlattice interface rather than perpendicular to superlattice. The cross plane thermal conductivity of an ion bombardment thin film decreases. Although the disorder in the crystal produced by ion bombardment will decrease the electric conductivity, this tendency is overwhelmed by the increase of the density of electronic states in the structure formed by ion bombardment such that
cross plane electric conductivity of the thin film increases. The cross plane Seebeck coefficient of the ion bombarded thin film also increases due to the increased density of electronic states. Hence, the thermoelectric figure of merit of the film increases. The Si1xGex/Si semiconductor structure modified by Si ion bombardment shows promise as a high figure of merit thermoelectric material. 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] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
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