# Presentation: 394451

## Figure of Merit for Thermoelectric Power Generation Estimated from FeSi2 Thin Film

- ○Hirofumi Kakemoto, Hajime Shibata*and Takaaki Tsurumi

- Tokyo Inst. Tech. (Tokyo, JAPAN)
- *National Institute for Advanced Science and Technology (Tsukuba, JAPAN)

- Abstract: **FeSi2 has been attracted to be applied to the thermoelectric device, for instance, the Seebeck coefficient shows the maximum value about 500oC that it is good for thermoelectric power generation. However low figure of merit (Z) has been reported about 5x10-4 K-1. The Z is represented as *m**2/3**/**), where *m**, ** and ** are effective mass, mobility and thermal conductivity, respectively. Although ** is good for thermoelectric power, low ** has been reported as polaronic-conduction in **FeSi2 crystal. In 3D electron density distribution of **FeSi2 crystal, Si layer in the crystal shows covalent bonding network with Si atoms, and it suggests the new possibility for enhancement of **. In this report, the objective is to exhibit the possibility for enhancement of Z in order to control the crystallographic orientation of **FeSi2 crystal by means of film formation. **FeSi2 thin film was prepared on Si(100) substrate using molecular beam epitaxy method. The crystallographic orientation of sample showed about **70**% of [100] direction from x-ray diffraction pattern. The transport properties were investigated using Hall measurement with van der Pauw electrode configuration. The resistivity and ** were also measured, and they were compared with **FeSi2 polycrystal. In addition, enhancement of Z was estimated using above formula.

- Figure of Merit for Thermoelectric Power Generation Estimated from **FeSi2 Thin Film

- American Physics Society

- Friday, Oct. 26, 2007

- Barkeley, California

- APS 2007 Fall meeting

**Notes:**
Thank you chair man, Good morning!

My name is Hirofumi Kakemoto come from Tokyo institute of Technology,Tokyo,  Japan

The presentation is entitled “Figure of merit for thermoelectric power generation estimated from FeSi2 thin film”

## 1.Introduction

- Fe

- Si

**FeSi2 crystal structure

- Modified CaF2 structure
*a*=98.63, *b*=77.91, *c*=78.33 nm

- Epitaxial growth on Si substrate.

**FeSi2

- A thermoelectric material
- reported as **0.85eV** energy band-gap.

- Highly optical absorption coefficient ***(h****)* larger than 1.0x105 cm-1 at 1.0eV.

- Problem: low Z

**Notes:**
At first, Introduction

Beta-FeSi2 is a modified CaF2 structure, and has three times bigger primitive cell as shown lattice constants

Beta-FeSi2 is a thermoelectric material showing of 0.85eV energy band-gap. 

Recently, high optical absorption coefficient that is larger than 1.0x10^5 cm^-1 at 1.0eV has been reported.
Epitaxial growth on Si substrate.

Thermoelectric property is well known from polycrystal.

Seeback coefficient shows maximum value at 500oC, it is good temperature dependence for thermoelectronics.
However, efficiency is not good.

So, strong-Urbach-tail is difficult to discuss the nature of band-gap.

********************************************
イントロダクションです。

Beta-FeSi2はxです。
Seeback係数は、xです。
Figure of meritは、xです。
予稿原稿参考

## Objective in this study

- 3D-electron density distribution of **FeSi2 crystal.

- (band-calc. APW method)

**Problem: Small polaron **

- Correlation between electric structure and transport properties

- Fe
- Si
- Fe
- Si
- Fe
- Si
- Fe
- Si

**Notes:**
So, figure of merit, thermoelectric generation, can be represented as Z=xxx or xxx.

This is other representation as B factor.

We can say, that 

In this presentation, we focus increasing mobility in this crystal.

So, Objective is ..

Preparation a-axis oriented beta-FeSi2 thin film on Si(100) substrate, and fin the methods for obtaining the optical constants of beta-FeSi2 thin film on Si(100) substrate, and find the methods for obtaining the optical constants of beta-FeSi2 thin film on Si(100) substrate removing the Urbach-tail. In addition, investigate the transport properties of beta-FeSi2 thin film.

*********************************************************************************
性能指数:Figure of meritはxと示されます。

従って、移動度で表せます。

しかし、移動度はあまり高くないことが分かっています。
それは、

研究目的です。

「」です。

## 2.Experimental

**2.1 Sample preparation**

**Notes:**
Next is the Experimental

The samples were prepared by molecular beam epitaxy method.
Actually, the chamber was initially a pulsed laser deposition,
In this study modified as molecular beam epitaxy using an Fe source.

This is Preparation Condition
Substrate was used un-doped Si(100) substrate.

Pressure was : 10^-9 Torr.
Substrate temperature was kept about 500oC
And, then, annealed same temperature for 2 hours

Characterization(s) are
Reflection high energy  electron beam diffraction
X-ray diffraction
FTIR and Hall measurement

## 3.Results & Discussion

- Crystallographic orientation

- Epitaxial relation

**FeSi2

- Si substrate

- abeta

- Si(100)

- aSi=54.31nm

- bbeta

- cbeta

- Reflection high energy electron beam diffraction (RHEED) pattern.

- Si(100) substrate

- Deposition condition & film orientation

- Annealing time

- Fe deposition time

- 2h

- 4h

- 30min

- 60min

- Fe depo. rate 2 (>10mA of E.C.)

- Annealing time

- Fe deposition time

- 2h

- 4h

- 30min

- 60min

- Fe depo. rate 1 (<10mA of E.C.)

- 2/x

- 10/6

- 10/3

- 9/22

- 

- X

- 

- X

- As-depo. X

- 1/31

- 1/8

- orientation

- Random

- Orientation ratio:

- 2x1

**Notes:**
Results and Discussion

First, .. the surface of Si substrate was monitored RHEED (like this)
The surface of Si substrate shows 2x1 pattern.

Epitaxial relation between Si and beta-FeSi2 film is like this.

Firstly, Fe rate, namely, emission current of Fe source was used less than 10mA.
But, crystallinity of sample is not good.

Hence, second, emission current was used larger than 10mA.

Crystallographic orientation is, in this case, defined as..

*******************************************************************

実験結果です。

まず、Siの表面を加熱して活性化したのち、
Feを蒸着しました。

## Semiconductor or metal?

- 202/220

- (422)

- Si

- Si

- Si

- Random

- Fe

- Fe

- 600

- 800

- 400

**Semiconductor or metal?**

- Urbach-tail

- Si-substrate

**FeSi2 film

- Photon Energy (eV)

- 1.1

- 0.85

- Reflectance spectrum: must be performed **Kramers-Kronig** (KK) Transformation

- KK transformation also needs wide frequency data (** →0, ∞)

- Urbach-tail of Si substrate can not be separated from **FeSi2 film by only transmittance spectrum.

- without annealing

- with annealing **2h**

***(h****)  *(cm-1)

**Notes:**
This is the XRD patterns of samples

Before annealing
Fe peaks are apparently observed.

However, adjusting preparation conditions,

Although, 202/220 diffraction peak is strongly ..
[100] oriented beta-FeSi2 peaks 
So, 400, 600, 800 peaks are clearly observed.

A partly, we success that orientation of this crystal is controlled by MBE method.

Next we are decided band gap of sample.

One of the sample was provided to FT-IR measurements, and analyzed 
, the answer is the Simultaneous Equation (SE) analysis

As mentioned in introduction

In this material, strong Urbach-tail is observed around 0.85eV

Kramer-Kronig transformation

## Simultaneous Equation analysis

**Simultaneous Equation analysis**

- Schematic illustration of sample.

- Si substrate

**FeSi2

- Analysis software

*z*=*d*0

*z*=0

*z*=*d*0+*d*1

- Mathematica (Wolfram Research ): Solving SEs

- Igor Pro (WaveMetrics):Basic calculation

*r*S=(*R*)0.5, *t*S=(*T*)0.5

- Measurement data

**Model**

**Notes:**
This is the schematic illustration of sample.

The boundary conditions are defined like this

Anaysis software is

Igor pro
Mathematica

*******************************************************

試料の評価です。
を行いました。

また、FTIRの解析には、
連立解法を行いました。

## Reflectance and transmittance spectra of sample (oriented film)

*R*

*T*

*R*

*T*

- Si

**FeSi2

- Si sub.

**FeSi2/Si sub.

**Reflectance and transmittance spectra of sample (oriented film)**

- Reflectance

- Reflectance

- Very good analysis method!

- (nm)

**Notes:**
Reflectance and transmittance spectra of sample

These are reflectance and transmittance data of Si substrate and sample.

The onsets are clearly observed in transmittance spectra. (like this)

At first, extinction coefficient of transmittance data only is calculated as shown in Exp..
Strong Urbach-tail is shown around band-edge..

On the other hand, simultaneous equation analysis was carried out for sample.

The Urbach tail is removed, and abrupt extinction coefficient is calculated using SE analysis.

**************************************************************

今回、演題とは、関係ないが、
前回の講演で、しっかり発表できなかったので、
報告します。

## Predicted properties

- Fe

- Si

**FeSi2 crystal structure

- Modified CaF2 structure
*a*=98.63, *b*=77.91, *c*=78.33 nm

- Highly optical absorption coefficient ***(h****)* larger than 1.0x105 cm-1 at 1.0eV

- Epitaxial growth on Si substrate

**FeSi2

- Semiconductor showing of **0.85eV** energy band-gap

- Thermoelectric property

- a-axis

- Fe

- Si

- Si

**Fe**

- Si

**Fe**

**Fe**

**Fe**

**Fe**

**Fe**

- Si

**Si**

**Si**

- Fe
- Si
- Fe
- Si
- Fe
- Si
- Fe
- Si

- 2D & 3D electron density distributions of **FeSi2 crystal.

- Si-network

- Maximum entropy refinement

- Si-layer

**Along a-axis,** Fe and Si layers exist alternately.

- Improve mobility?

*Focus*

- Conduction-pass is clearly observed with only Si atoms.

- Conduction pass

## Hall measurement

- Van der Pauw config.

- Table 1 The electrical measurement data of samples.

- Polycrystal         15.6             1.36             5.68x1017;      reported by H.Kakemoto et al., JJAP.
- Random                                  2.6
- Oriented                                  9.6

**・cm	    **(cm2/Vs)**cm-3;Remarks

**Notes:**
The Hall measurement was carried out using Van der Pauw method.

Hole mobility of samples is increased with increasing orientation of sample.

Figure of merit of sample is represented as Z=xx

So this is B-factor representation, these parameters are independent parameters.

## 4.Summary

**Acknowledgment**
-   Author would like to thank A.Obara for technical supports. The study was supported by Sumitomo foundation for basic science, Venture Business Laboratory foundation (Tokyo Inst. of Tech.) and Nippon Sheet Glass foundation for materials science and engineering.

- Future study

- 1.**FeSi2 film on Si(100) substrate was prepared by molecular beam epitaxy method using a Fe source. Using x-ray diffraction and scanning electron microscopy, the structural properties, namely, crystallographic orientation, surface morphology and film thickness were estimated and observed. For the sake of further improvement, the preparation condition for thinner with highly crystallinity and [100] orientation of **FeSi2 film should be found.
- 2.Optical properties were measured by a Fourier transform infrared spectroscopy, and measured reflectance and transmittance data were transformed to optical constants of **FeSi2 film using simultaneous equations analysis. From obtained extinction coefficient, the abrupt onset of band-edge of **FeSi2 film was evaluated with energy band-gap of 0.85eV.
- 3.The transport property was characterized by Hall measurement. The enhancements of hole mobility and figure of merit for thermoelectric power were observed as a function of crystallographic orientation of **FeSi2 film.

- 1. Preparation of perfect [100] oriented film
- 2. Its Hall measurement
- 3. Seebeck coefficient measurement (in future)
- N. Oriented ceramics (in future)

- [100]

- [100] with [101]/[110]

**Notes:**
Summary of this presentation

Is like this

And

Future study is

1
2
3

N

## Future Study

- Very high mobility of **FeSi2 thin film on Si substrate has been reported by Takakura et al. (up to 104 cm2/Vs), anomalies or not?

- Takakura et al., J.Appl.Phys.**97** (2005) 093716.

- We have a possibility to find some quantum effects in transport properties (in interface) and Seebeck effect around 50K.