Phonon-drag Contribution to Seebeck Coefficient of Ge-on

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Makara Journal of Technology Makara Journal of Technology

Volume 19 Number 1 Article 5

4-1-2015

Phonon-drag Contribution to Seebeck Coefficient of Ge-on- Phonon-drag Contribution to Seebeck Coefficient of Ge-on- insulator Substrate Fabricated by Wafer Bonding Process insulator Substrate Fabricated by Wafer Bonding Process

Veerappan Manimuthu

Department of Nanovision Technology, Graduate School of Science and Technology, Shizuoka University, 3-5-1 Johoku, Naka-Ku, Hamamatsu 432-8011, Japan, [email protected]

Shoma Yoshida

Department of Nanovision Technology, Graduate School of Science and Technology, Shizuoka University, 3-5-1 Johoku, Naka-Ku, Hamamatsu 432-8011, Japan

Yuhei Suzuki

Faiz Salleh

Mukannan Arivanandhan

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Recommended Citation Recommended Citation

Manimuthu, Veerappan; Yoshida, Shoma; Suzuki, Yuhei; Salleh, Faiz; Arivanandhan, Mukannan; Kamakura, Yoshinari; Hayakawa, Yasuhiro; and Ikeda, Hiroya (2015) "Phonon-drag Contribution to Seebeck

Coefficient of Ge-on-insulator Substrate Fabricated by Wafer Bonding Process," Makara Journal of Technology: Vol. 19 : No. 1 , Article 5.

DOI: 10.7454/mst.v19i1.3026

Available at: https://scholarhub.ui.ac.id/mjt/vol19/iss1/5

This Article is brought to you for free and open access by the Universitas Indonesia at UI Scholars Hub. It has been accepted for inclusion in Makara Journal of Technology by an authorized editor of UI Scholars Hub.

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Phonon-drag Contribution to Seebeck Coefficient of Ge-on-insulator Substrate Phonon-drag Contribution to Seebeck Coefficient of Ge-on-insulator Substrate Fabricated by Wafer Bonding Process

Fabricated by Wafer Bonding Process

Authors Authors

Veerappan Manimuthu, Shoma Yoshida, Yuhei Suzuki, Faiz Salleh, Mukannan Arivanandhan, Yoshinari Kamakura, Yasuhiro Hayakawa, and Hiroya Ikeda

This article is available in Makara Journal of Technology: https://scholarhub.ui.ac.id/mjt/vol19/iss1/5

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Makara J. Technol. 19/1 (2015), 21-24 doi: 10.7454/mst.v19i1.3026

April 2015 | Vol. 19 | No. 1 21

Phonon-drag Contribution to Seebeck Coefficient of Ge-on-insulator Substrate Fabricated by Wafer Bonding Process

Veerappan Manimuthu

^1,2*

, Shoma Yoshida

¹

, Yuhei Suzuki

¹

, Faiz Salleh

¹

, Mukannan Arivanandhan

¹

, Yoshinari Kamakura

³

, Yasuhiro Hayakawa

^1,2

, and Hiroya Ikeda

^1,2

1. Department of Nanovision Technology, Graduate School of Science and Technology, Shizuoka University, 3-5-1 Johoku, Naka-Ku, Hamamatsu 432-8011, Japan

2. Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Naka-Ku, Hamamatsu 432-8011, Japan 3. Graduate School of Engineering, Osaka University, 1-1 Yamadaoka, Suita, Osaka 565-0871, Japan

*e-mail: [email protected]

Abstract

In order to build high-sensitivity infrared photodetectors using SiGe nanowires, we investigate the thermoelectric characteristics of Ge-on-insulator (GOI) layers as a reference for SiGe. We fabricate p-type GOI substrates with an impurity concentration of 10¹⁶-10¹⁸cm^-3 by a wafer-bonding process using Ge and oxidized Si wafers. Annealing treatment is performed in order to further increase the bonding strength of Ge/SiO₂ interface. We measure the Seebeck coefficient in the temperature range of 290-350 K. The Seebeck coefficient of the GOI layers is very close to the theoretical value for Ge, calculated on the basis of carrier transport. Hence, there is a small phonon-drag effect in GOI.

On the other hand, the effect of phonon drag on the Seebeck coefficient of Si is usually significant. These results likely stem from the differences between phonon velocity, phonon mean-free-path, and hole mobility between Ge and Si.

Abstrak

Kontribusi Tarikan Fonon terhadap Koefisien Seebeck Substrat Ge-On-Insulator yang Difabrikasi dengan Proses Pengikatan Wafer. Untuk membuat sebuah fotodetektor infra merah bersensitivitas tinggi dengan kawat-kawat nano SiGe, kami menyelidiki ciri-ciri termoelektrik dari lapisan-lapisan Ge-on-insulator (GOI) sebagai rujukan untuk SiGe. Kami memfabrikasi substrat-substrat GOI tipe-p dengan konsentrasi impuritas sebesar 10¹⁶-10¹⁸cm^-3 melalui proses pengikatan wafer menggunakan wafer-wafer Ge dan Si yang teroksidasi. Perlakuan aniling diterapkan untuk semakin menambah kekuatan ikat antarmuka Ge/SiO₂. Kami mengukur koefisien Seeback pada rentangan suhu 290- 350 K. Koefisien Seebeck dari lapisan-lapisan GOI itu nyaris serupa dengan angka teoretis untuk Ge yang diperhitungkan berdasarkan transpor pembawa. Jadi, terdapat efek tarikan fonon kecil pada GOI. Di pihak lain, efek tarikan fonon pada koefisien Seebeck Si umumnya terjadi secara signifikan. Hasil yang demikian mungkin diakibatkan oleh perbedaan antara kecepatan fonon, jalur bebas purata fonon, dan mobilitas lubang antara Ge dan Si.

Keywords: Ge on insulator, infrared photodetector, phonon-drag effect, seebeck coefficient

1. Introduction

Like the power generator, the thermopile infrared (IR) photodetector is among the applications of thermoelectric materials. The thermopile is an array of n- and p-type semiconductor thermocouples that produces a thermoelectromotive force (TEMF) corresponding to the temperature difference arising from the absorption of IR radiation. The sensitivity of this IR photodetector R can be expressed as

G , R = nS η

(1)

where n is the number of thermocouples, S is the Seebeck coefficient, η is the IR absorbance

,

and G is the thermal conductance between the warm and cool regions [1]. Thus, a high-sensitivity thermopile IR photodetector requires a large number of thermocouples

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Manimuthu, et al.

Makara J. Technol. April 2015 | Vol. 19 | No. 1

22

composed of thermoelectric materials with high Seebeck coefficient and low thermal conductivity.

Nanostructured SiGe is expected to meet the thermoelectric demands posed by an increase in the number of thermocouples due to nanometer-scale miniaturization, an increase in Seebeck coefficient due to the carrier confinement effect [2-3] and a decrease in thermal conductivity caused by enhancing the boundary and defect scattering of phonons [4-5].

In order to investigate the thermoelectric properties of SiGe nanowires experimentally, a SiGe-on-insulator (SGOI) substrate is needed for nanowire fabrication.

Among the techniques that can be employed to fabricate such substrates, the wafer bonding process seems the most favourable one owing to its simplicity and the controllability of material parameters, such as thickness and impurity concentration. In this paper, therefore, we focus on the Seebeck coefficient of p-type Ge-on- insulator (GOI) layers fabricated by the wafer-bonding process and discuss its physical causes, such as carrier and phonon contribution, by comparing these GOI layers to p-type Si, with the aim of predicting the Seebeck coefficient of SGOI layers.

2. Methods

In this study, a p-type Ge (100) wafer and a thermally- oxidized p-type Si(100) wafer with an oxide thickness of 100 nm were used to fabricate a GOI substrate. After chemical cleaning, the Ge and Si wafers were treated with ammonia (28%) solution for 10 min and rinsed with deionized water. The surfaces of the Ge and oxidized Si wafers were brought into contact by the van der Waals attraction in a clean room at room temperature as shown in Fig. 1. Finally, the bonded wafers were annealed in N₂ atmosphere to further strengthen the bonding of the Ge/SiO₂ interface [6]. The resultant GOI substrate consisted of a sandwich structure with a top p-type Ge layer (GOI layer), buried oxide (BOX) layer, and p-type Si substrate. We cut the GOI substrate to a size of 1 × 1 cm² and made an ohmic contact with In to carry out Seebeck coefficient and Hall measurements.

Our experimental setup for Seebeck coefficient measurement is illustrated in Fig. 2. Two copper plates were placed side by side, with a 1-mm gap in between.

A resistive heater was placed below one of the copper plates. The sample was placed across the gap, in contact with both plates. Thus, by controlling the heater current, a temperature difference could be produced in a plane parallel to the sample surface. A pair of probes and two K-type thermocouples were directly attached to the sample surface. The time evolution of the TEMF was measured by a digital nanovoltmeter (Keithley 2182A) simultaneously with the temperature in the high- and

low-temperature regions by a digital multimeter (Keithley 2700) equipped with a switching module (Keithley 7700).

Seebeck coefficient was determined from the TEMF (∆V=V_H-V_L) and the temperature difference (∆T=T_H- T_L) via the equation S = -∆V/∆T over the range of temperature of 290-350 K [7]. The Hall measurements to determine the electrical resistivity, mobility, and carrier concentration were performed at a room temperature.

Figure 1. Wafer-Bonding Process for Fabrication of GOI Substrate

Figure 2. Seebeck Coefficient Measurement Setup

3. Results and Discussion

Figure 3 shows the measured electrical resistivity of the GOI layer as a function of measured carrier concentration. The solid line is an Irvin curve for p-type Ge reported in the literature [8]. As seen in Fig. 3, the measured GOI resistivity decreases along with increasing carrier concentration and the measured values are in nearly perfect agreement with the Irvin curve. Figure 4 plots the measured Hall mobility of the GOI layer as a function of carrier concentration. The solid line in this figure denotes the reported hole mobility for p-type Ge [8]. It is found that the GOI mobility is in a good agreement with the reported value.

On the basis of Figs. 3 and 4, it can be said that the measured electrical properties of the fabricated GOI layer are not influenced by the BOX layer and Si substrate.

Thermal oxidation of Si

wafer(100)

Wafer bonding of Ge(100) and

oxidized Si wafer(100)

Annealing the bonded GOI

substrate

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Phonon-drag Contribution to Seebeck Coefficient of

23

10¹⁵ 10¹⁶ 10¹⁷ 10¹⁸

10^-4 10^-3 10^-2 10^-1 10⁰ 10¹ 10²

GOI Ge (reported)

Resistivity[Ω-cm]

Carrier Concentration [cm-3]

Figure 3. Electrical Resistivity of GOI Layer as a Function of Carrier Concentration

10¹⁴ 10¹⁵ 10¹⁶ 10¹⁷ 10¹⁸ 10¹⁹ 10²

10³ 10⁴

GOI Ge (reported)

Mobility[cm2/V-s]

Carrier Concentration [cm-3]

Figure 4. Hall Mobility of GOI Layer as a Function of Carrier Concentration

Measured Seebeck coefficient values for the GOI layers are shown in Fig. 5 as a function of carrier concentration.

The reported values for p-type Ge wafers are also shown [9]. The solid line in this figure indicates the theoretical values calculated by using the equation [10],

( ) ( ) ( 1 ) ( ) ,

2

₁

 



 

 −

+

= +

⁺

η

η η

r r B

e

r F

F r e

S k

(2)

where r is the scattering parameter and r = 1 when both phonon scattering and impurity scattering are considered. e is the elementary charge and k_B is the Boltzmann’s constant. F_n(η) is the Fermi integral and η is the reduced Fermi energy given by

( )

^,

4 2 ₂ ₁_/₂

* η

π ^F

h T k n m ^B 









=  (3)

where m* is the effective mass, T is the absolute temperature, and h is Planck's constant. The measured Seebeck coefficient of GOI decreases along with

increasing carrier concentration and is in a good agreement with the reported values of Ge. These findings indicate that the measured Seebeck coefficient values are valid and that the BOX layer and the Si substrate have no effect on the Seebeck coefficient values of the GOI layer. In addition, the Seebeck coefficient values of GOI are very close to the theoretical values. This reveals that there is no phonon- drag effect in GOI, considering that Eq. (2) is based solely on the carrier (hole) transport, and not on the phonon transport. This result is consistent with a previous study [9,11], in which the contribution of phonons to the Seebeck coefficient of Ge in the carrier concentration range from 10¹⁴-10¹⁸cm^-3 above 250 K was found to be almost zero. In contrast, the phonon- drag effect is usually observed in Si, even at a room temperature [7,12-3].

This observed difference in phonon drag between Ge and Si can be explained by differences in their material properties. The Seebeck coefficient component arising from the phonon-drag contribution, S_ph, can be expressed as [14]

, 3

4

6 7

 





 



= 

K K T

l S u

l ph ph

ph

π µ

(4)

where u_ph is the sound velocity for longitudinal acoustic phonons, l_ph is the phonon mean free path including all scattering processes except for phonon-carrier (hole) scattering, and µ_l is the carrier mobility associated with phonon scattering. K_n is the parameter integral with an integer n, defined by

∫ ( )

∞ +

 



 

 +

= −

0 4

2 1

6 exp

i l n

n

K d

µ ξ µ

ξ ξ

ξ

(5)

10¹⁵ 10¹⁶ 10¹⁷ 10¹⁸ 10¹⁹ 10²⁰ 0.0

0.2 0.4 0.6 0.8 1.0 1.2

GOI Ge (reported) Calculation

Seebeck Coefficient [mV/K]

Carrier Concentration [cm^-3]

Figure 5. Seebeck Coefficient of p-type GOI Layer as a Function of Carrier Concentration. The Data Reported for p-type Ge is also Shown. The Solid Represents the Theoretical Values

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Manimuthu, et al.

24

and carrier mobility between Ge and Si. Consequently, by using these material parameters, it is possible to predict the Seebeck coefficient for SiGe.

Acknowledgement

This work was financially supported by a Grant-in- Aid for Challenging Exploratory Research (No.24651168) and a Grant-in-Aid for Scientific Research (No.25289087) from Japan Society for the Promotion of Science.

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