1

2014 Bachelor Thesis

Electrical characteristic of -FeSi2

Department of Electrical and Electronic Engineering

Tokyo Institute of Technology

10_06200

Takafumi Kato

Supervisor: Prof. Hiroshi Iwai

Associate Prof. Kuniyuki Kakushima

February,2014

2

3

Abstract of Bachelor Thesis

Electrical characteristic of FeSi2

Supervisor: Prof. Hiroshi Iwai

Associate Prof. Kuniyuki Kakushima

Tokyo Institute of Technology

Department of Electrical and Electronic Engineering

10_06200 Takafumi Kato

February, 2014

-FeSi2 has been drawn attention because of its direct band gap (~0.85eV), high optical

absorption coefficient (>105 cm-1 at 1.0eV), non-toxicity and abundance of Fe and Si on

earth. In this study, I search for electrical characteristic of -FeSi2 such as carrier density

and resistivity dependent thickness, sheet resistance dependent Fe/Si atomic ratio etc.

4

Content Chapter 1 Introduction ................................................................................................... 6

1.1 Present situation of energy in Japan .................................................................................... 7

1.2 Solar photovoltaics .............................................................................................................. 8

1.3 Introduction of -FeSi2 ...................................................................................................... 10

1.4 Issue of -FeSi2 ................................................................................................................ 11

1.5 Purpose of this study ......................................................................................................... 12

1.6 Reference ........................................................................................................................... 13

Chapter 2 Experiment .................................................................................................. 15

2.1 Fabrication procedure ........................................................................................................ 16

2.2 Experimental details .......................................................................................................... 18

2.2.1 SPM cleaning and HF treatment ................................................................................ 18

2.2.2 RF magnetron sputtering ............................................................................................ 19

2.2.3 Rapid temperature annealing (RTA) .......................................................................... 20

2.2.4 Photolithography ........................................................................................................ 20

2.2.5 Wet etching by H2O2 .................................................................................................. 21

2.2.6 Liftoff process ............................................................................................................ 21

2.2.7 Ion implantation ......................................................................................................... 21

2.2.8 Scanning Electron Microscope (SEM) ....................................................................... 23

2.2.9 4-point probe method ................................................................................................. 23

2.2.10 Van der pauw method .............................................................................................. 25

2.2.11 Transmission Line Method (TLM)........................................................................... 27

2.3 Reference ............................................ エラー! ブックマークが定義されていません。

Chapter 3 Iron disilicide using stacked sputtered process ........................................... 30

3.1 Introduction ....................................................................................................................... 31

5

3.2 Sheet resistance dependent Fe/Si atomic ratio .................................................................. 31

3.3 Activation energy of -FeSi2 dependent temperature ....................................................... 33

3.4 Reference ........................................................................................................................... 39

Chapter4 FeSi2 target deposition ................................................................................. 40

4.1 Introduction ....................................................................................................................... 41

4.2 Sheet resistance dependent anneal temperature and anneal time ...................................... 41

4.3 P doping for -FeSi2 .......................................................................................................... 43

4.4 -FeSi2 on high resistance n-Si .......................................................................................... 44

4.5 Thickness dependence of carrier density and resistivity ................................................... 45

4.6 Reference........................................................................................................................... 46

Chapter 5 Conclusion .................................................................................................. 47

Chapter 6 Acknowledgements ..................................................................................... 49

6

Chapter 1

Introduction

1.1 Present situation of energy in Japan

1.2 Solar photovoltaics

1.3 Introduction of -FeSi2

1.4 Issue of -FeSi2

1.5 Purpose of this study

1.6 Reference

7

1.1 Present situation of energy in Japan

Figure 1.1 shows a change of generated energy’s supply in Japan (1973~2011).[1.7]

According to Figure 1.1, Power generation amount has been increasing year by year.

Thus our country has consumed enormous energies. But self-sufficiency ratio in the

primary energy supply is 4.8% in 2010 (without nuclear energies). This value is

remarkably lower than major developed countries (America, China, United Kingdom,

German and so on). Therefore this fact is serious problem for Japan because if resource

import was stopped, energy supply could not be done stably. Figure1.2 shows details of

self-sufficiency energy in Japan. Figure1.2 shows greenhouse gas emissions in Japan

from 1990 to 2011. According to Figure2.1, greenhouse gas emissions increased from

1990 to 2007. Decreasing in 2008 and 2009, greenhouse emissions also raised in 2010

and 2011. In Kyoto protocol to the United Nations framework convention on climate

change, we should meet standard value of greenhouse gas emissions in 1990.[1.9]. From

environment point view, we need to select Non-emitted CO2 energy.

Figure 1.1 Change of generated energy’s supply in Japan [1.7]

8

Figure1.2 Trends of greenhouse gas emissions and removals in Japan [1.8]

1.2 Solar photovoltaics

In my study, I focus on photovoltaics in recyclable energy. Kind of photovoltaics are

shown in Figure1.3. Photovoltaics is roughly divided into silicon, compound, and organic.

Silicon has two type organizations which are crystalline Si and Amorphous Si. Crystalline

Si has regularly arranged Si construction, while amorphous Si has irregularly arranged Si

by fabricating in low temperature. In compound, there are three type organizations which

areⅢ-Ⅴ family multi-junctions, CIGS, and CdTe. Ⅲ-Ⅴ family multi-junctions is

composed by Ⅲ family (Gallium, Indium) and Ⅴ family (Phosphorus, Arsenic).

Current high efficiency is performed by Ⅲ-Ⅴ family multi-junctions photovoltaics

(made by Sharp:37.9% [1.10]). CIGS is made from Copper, Indium, Gallium, and Selenium.

Organic photovoltaics is paid attention to in flexibility, colorability, lightweight, and low-

9

cost.

Now, I pay attention to thin-film solar cell. Table 1.4 shows comparison of thin film

solar cells such as a-Si, CIGS, organic, and silicide. Because of abundant silicon, a-Si

resources are abundant, and it have less-degradation and 20% of efficiency. While CIGS

has high efficiency (29%) although it has poor abundance. Organic solar cell is flexible,

light, and colorable. On the other hand, Organic deteriorates more than other solar cells.

In regard to silicide solar cells, they are abundant than CIGS and organic photovoltaics

(without ReSi2). Among their silicides, FeSi2 and BaSi2 have less degradation and large

absorption (=105cm-1 at 1.5eV .[1.1]-[1.4]). In this study, I focus on iron disilicides (FeSi2)

in silicide photovoltaics.

Figure 1.3 Kind of photovoltaics

Photovoltaics

Silicon

Compound

Organic

Crystalline Si

Amorphous Si

Single crystal Si

Polycrystalline Si

Microcrystalline Si

Ⅲ-Ⅴfamily multi-junction

CIGS

(Thin-film Si)

CdTe

10

Figure 1.4 Comparison of thin-film solar cells[1.6]

1.3 Introduction of -FeSi2

Due to its remarkable optical and electrical properties, the semiconducting iron disilicide

-FeSi2 has recently attracted considerable attention from both scientists and engineers.

-FeSi2 has a large absorption coefficient (=105cm-1 at 1.5eV), which is 200 times larger

than that of crystalline silicon (Figure 1.5 Optical absorption coefficients of various

single-crystal semiconductors), and a direct optical band gap of about 0.8eV (Figure 1.6

Variations of the absorption coefficient a vs the photon energy). It is also compatible with

silicon technology. From the ecological point of view, -FeSi2 is a nontoxic material, and

its elements (Fe and Si) are abundant in nature. Therefore, -FeSi2 is one of the most

promising materials for various applications such as light-emitting diodes, infrared

sensors, and solar cells. In view of photovoltaic cell, -FeSi2 theoretical energy

convention efficiency is about 16-23%.[1.1]-[1.4]

Bandgap

Eg (eV)

Transition

type

Absorption

coefficient

α (cm-1)

Resources DegradationEfficiency

(%)

a-Si 1.7 indirect 104 Excellent Good 20

CIGS 1.0～1.6 direct 105 Bad Excellent 29

Organic 1.0～ indirect 105 Good Bad 14

BaSi2 1.4 indirect 105～ Excellent Excellent 32 (cal.)

FeSi2 0.8 direct 105～ Excellent Excellent 24 (cal.)

Silicide Mg2Si 0.75 indirect 105~ Excellent No data 22 (cal.)

CrSi2 0.3 indirect 105 ~ Good No data 8 (cal.)

ReSi2 0.1 direct 104~ Bad No data 1 (cal.)

[1.11]

[1.11]

[1.11]

[1.12]

[1.12]

11

Figure 1.5 Optical absorption coefficients of various single-crystal semiconductors[1.5]

Figure 1.6 Variations of the absorption coefficient a vs the photon energy[1.6]

1.4 Issue of -FeSi2

My study is to fabricate -FeSi2 solar cells. But -FeSi2 has high defect amounts[1.13]

Photon Energy (eV)0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

0

2

4

6

8

10

12

Eg = 0.79eV

14600℃,5min annealing

(αћω

)2 (

10

cm-2

eV2)

12

and -FeSi2 high carrier densities (about 1018cm-3)[1.14] so that we cannot make -FeSi2

solar cells. High defect amounts causes interruption of generated carrier. In other words,

carrier generated by light cannot extract as light-current. Also high carrier densities leads

narrow depletion layer, so -FeSi2 hole and electron can’t be generated enough in -FeSi2

depletion layer, and -FeSi2 can’t generated electricity sufficiently. Figure1.7 shows

depletion of -FeSi2 dependent its carrier density estimated by calculation.[1.6] Current

status of -FeSi2 carrier density is 1018 cm-3 and its depletion is 20nm. And

Figure1.7 Depletion of -FeSi2 dependent its carrier density[1.6]

1.5 Purpose of this study

This study of purpose is decreasing -FeSi2 carrier densities and investigating the origin

of defects. Figure1.8 shows flow of this bachelor thesis’ chapter.

1014 1015 1016 1017 1018 1019 10201

10

102

103

Carrier density (cm-3)

De

ple

tio

n -

laye

r w

idth

(n

m)

the area around hereCurrent statusTarget

13

In Chapter3, -FeSi2 was fabricated by using Fe and Si stacked deposition to measure

sheet resistance and calculate carrier density under constant mobility. Then, -FeSi2

activation energy was extracted from temperature dependence, and investigate the origin

of its defects.

In Chapter4, -FeSi2 was fabricated by using FeSi2 target deposition to decrease its

carrier density. In addition to 4-point probe method, the sample was measured by van der

pauw and TLM. To lower carrier density, experiment that is changing annealing

temperature and time, substrate. And also Ion implantation of phosphorus was performed.

Figure1.8 Flow of Chapter in this bachelor thesis

1.6 Reference

[1.1] R.H. Bube, Photovoltaic Materials, Imperial College Press, Amsterdam, 1998,

p.3

[1.2] M. Powalla, K. Herz, Appl. Surf. Sci. 65/66 (1993) 482.

[1.3] Z. Yang, K.P. Homewood, M.S. Finney, M.A. Harry, K.J. Reeson, J. Appl.

Phys. 78 (1995) 1958.

[1.4] Y. Makita, Proceedings of the First NREL Conference, 1997, p. 3.

Chapter1: Introduction

Chapter2: Experiment

Chaper3: Iron disiliside using

stacked sputtered processChapter4: FeSi2 target deposition

Chapter5: Conclusion

14

[1.5] K. Yamaguchi, et al., Int. J. Hydrogen Energy,Vol. 32, pp. 2723 (2007).

[1.6] T.Inamura, et al., The Electrochemical Society Proceedings Series,1851 (2013)

[1.7] IEA, Electricity information 2013 (2012)

[1.8] GIO, National Greenhouse Gas Inventory Report of JAPAN

[1.9] UNITED RATIONS, Kyoto Protocol to the united nations framework convention

on climate change (1998)

[1.10] NEDO, Recyclable energy technology report, Vol.2, pp.9 (2013)

[1.11] E.Arvizu, World Future Energy Summit, NREL (2013)

[1.12] T.Suemasu, New thechnology presentation meetings at University of Tsukuba

(2012)

[1.13] K.Okajima et.al., Thin Solid Films Vol.381 267 (2001)

[1.14] K. Takakura et.al., Jpn.J Appl.Phys, vol.39, 790 (2000)

15

Chapter 2

Experiment

2.1 Fabrication procedure

2.2 Experimental details

2.2.1 SPM cleaning and HF treatment

2.2.2 RF magnetron sputtering

2.2.3 Rapid temperature annealing (RTA)

2.2.4 Photolithography

2.2.5 Wet etching by H2O2

2.2.6 Liftoff process

2.2.7 Ion implantation

2.2.8 Scanning Electron Microscope (SEM)

2.2.9 Current-Voltage characteristics

2.2.10 4-point probe method

2.2.11 Van der pauw method

2.2.12 Transmission Line Method (TLM)

16

2.1 Fabrication procedure

Two fabrication procedure of -FeSi2 was performed in this study. Figure2.1 shows

fabrication process of 4-point probe method. The sample of 4-point probe method for -

FeSi2 was fabricated on SiO2 (400nm) with n-Si (100) substrate. First, the substrate was

cleaned by SPM and HF treatment. After -FeSi2 was deposited by RF sputtering in Ar,

rapid temperature annealing (RTA) was performed in F.G. atmosphere because of

activation. Then 4 electrodes of W (tungsten) was fabricated by deposition of RF

sputtering in Ar, photolithography, and H2O2 etching. These electrodes were shapes of

circle. After all fabrication was finished, 4-point probe method was performed for

measuring sheet resistance.

Figure2.1 Fabrication process of -FeSi2 4-point probe method

n-Si Sub with SiO2 (400nm)

SPM and HF treatment

-FeSi2 Deposition by RF sputtering in Ar

RTA 5min in F.G (H2:3% + N2:97%)

Photolithography

H2O2etching

Measurement

W(tungsten) Deposition by RF spattering in Ar

Fabrication

4-point probe method

SiO2

-FeSi2

(-FeSi2 surface)

f=200mm

17

Figure2.2 shows fabrication process of TLM and van der pauw method. And Figure 2.3

shows a mask of TLM and van der pauw method. In Figure2.3, Red areas are electrodes

(tungsten), yellow areas are -FeSi2, and black areas are SiO2 surface. The sample of TLM

and van der pauw method was also fabricated on SiO2 (400nm) with n-Si (100) substrate.

The substrate was cleaned by SPM and HF treatment and W (tungsten) was deposited by

RF sputtering in Ar. And Photolithography and H2O2 etching was performed due to

patterning electrodes. After -FeSi2 was deposited by RF sputtering in Ar, excess areas of

-FeSi2 was removed by liftoff process. Finally, annealing process was performed in F.G.

atmosphere. Then TLM and van der pauw method were carried out for measuring

resistivity, carrier density and mobility of -FeSi2.

Figure2.2 Fabrication process of TLM and van der pauw method

n-Si Sub with SiO2 (400nm)

SPM and HF treatment

W(tungsten) Deposition by RF sputtering in Ar

Photolithography and H2O2etching

Liftoff

TLM method, van der pauw method

-FeSi2 Deposition by RF spattering in Ar

Annealing in F.G

Measurement

Fabrication

18

Figure2.3 A mask of TLM and van der pauw method

2.2 Experimental details

2.2.1 SPM cleaning and HF treatment

Particles and organic substance at the surface of SiO2 substrate become a cause of false

operation. So, it is important to clean the surface of Si substrate. SPM cleaning is one of

the effective cleaning methods. SPM is made from H2O2 and H2SO4 (H2O2:H2SO4 = 1:3).

SPM cleaning is performed by the oxidation effect (H2SO4 + H2O2 → H2SO5 + H2O)

Because of its oxidizability, particles and organic substance are oxidized and separated

from the surface of Si substrate. HF treatment is made by use of below 1% HF for

19

eliminating oxide films. In this study, SPM cleaning was done at 180 degrees for 5

minutes. HF treatment was done at room temperature for 1 minute.

2.2.2 RF magnetron sputtering

Metal is deposited by radio frequency (RF) magnetron sputtering with Ar gas. An RF

with 13.56 MHz is applied between substrate side and target side. Because of the

difference of mass, Ar ions and electrons are separated. A magnet is set underneath the

target, so that the plasma damage is minimized. Electrons run through the circuit from

substrate side to target side, because substrate side is subjected to be conductive and target

side is subjected to be insulated. Then, target side is negatively biased and Ar ions hit the

target.

Figure 2.2 Schematic illustration of RF magnetron sputtering

20

2.2.3 Rapid temperature annealing (RTA)

Rapid thermal annealing (RTA) is performed for activation. Heating chamber is filled

with F.G to be terminated by hydrogen. In this study, the time of elevated temperature is

30 seconds, and anneal time is 5, 10, 15, 20, 30 min respectively.

2.2.4 Photolithography

Fig 2.3 shows photolithography equipment (MJB4 of Karl Süss contact-type mask

aligner) and its process flow. At first, samples were coated with positive type photoresists

by spin-coating method. The thicker photoresist called S1818 and thinner one called

S1805 were used to selectively pattern metal electrodes and -FeSi2. Secondly, the coated

photoresists were baked at 115 oC for over 5 min by using electrical hotplate. Then, spin-

coated photoresist layers were exposed through e-beam patterned hard-mask with high-

intensity ultraviolet (UV) light. The exposure duration was set to 3 sec and 5 sec for

thinner photoresist and thicker one, respectively. Thirdly, exposed wafers were developed

using the specified tetra-methyl-ammonium-hydroxide (TMAH) developer called NMD-

3. The wafers were dipped into the solvent for 1~2 minutes. And finally, the samples are

heated to fixate the resist at 130oC at 10min. This is called post-bake.

21

Figure2.3 Photolithography equipment and process flow

2.2.5 Wet etching by H2O2

The samples were putted into hydrogen peroxide (H2O2) in order to eliminate parts

with the exception of pattern. The samples were dipped into H2O2 for 5 minutes.

2.2.6 Liftoff process

Liftoff is the process which selectively removes deposited films. Following

photolithography and deposition, resists and deposited films which exist on excess area

are left by using acetone.

2.2.7 Ion implantation

Figure2.4 shows schematic illustration of ion implantation. Ion implantation consists

of an ion source which has a filament of generating thermoelectron, mass spectrometry

1

2

3

4

5

Photoresist spin-coating

Pre-baking at 115oC

for over 5 min.

Post-baking at 125oC

for 5 min.

Exposure

Development

130o

for 10min

22

by which a great variety of ions are separated, analysis slit which selects ions of necessary

element and leads to an accelerating pipe, faraday cup which measures current value of

ions to confirm the number of ion. First a thermoelectrons are generated in ion source by

applying current. The source of molecules and atoms collides with these thermo electrons

and ionize. Ionized elements is extracted by extracting voltage, and led to mass

spectrometry. Ionized elements are curved in mass spectrometry and selected in analysis

slit. Selected ions are lead to an accelerating pipe and they are accelerated by accelerating

voltage. After all of the above process is finished, these ions are driven into the sample in

farady cup.

Figure2.4 Schematic illustration of ion implantation

ion source

extracting voltage

accelerating voltage

sample

ion beam

analysis slit

narrowing-down of beam

mass spectrometry

faraday cup

23

2.2.8 Scanning Electron Microscope (SEM)

The -FeSi2 samples were microscopically observed in order to measure the size of

FeSi2 film thickness by scanning electron microscope (SEM). Figure.2.4 shows SEM

system S-4800 (HITACHI High- Technologies Corporation) and Figure2.5 shows its

schematic internal configuration. The image of SEM is produced by scanning the samples

with a focused beam of electrons and detecting the secondary electrons, back-scattered

electrons, characteristic X-rays, light, and transmitted electrons.

Figure2.5 Schematic view of internal configuration of SEM equipment

2.2.9 4-point probe method

In the resistance measurement, four-point method is one of the most basic methods.

The resistance including a contact resistance between the probe and the sample would be

Virtual Source

First Condenser Lens

Condenser Aperture

Objective Aperture

Second Condenser Lens

Scan Coils

Objective Lens

Sample

24

obtained in the two probe resistance measurement. In order to measure resistance of the

material with low resistance such as metal, the measurement of resistance that doesn’t

include the contact resistance is required. The measurement of resistance without

including the contact resistance becomes possible by using four-point probe method. In

this study, four-point probe method is used to measure the resistance of -FeSi2. Figure

2.6 shows the schematic illustrations of the electrode structure to use this method.

Figure 2.6 Schematic illustrations of the electrode structure

(-FeSi2 surface)

I

V

electrode

25

2.2.10 Van der pauw method

Van der pauw method is one of the Hall effect, which is used to measure the resistivity

and the hall coefficient of a sample. This method is proposed by L.J. van der Pauw. He

showed how the resistivity, carrier density, and mobility of a flat sample of arbitrary shape

can be determined without knowing the current pattern if the following conditions are

met: the contacts are at the circumference of the sample and are sufficiently small, the

sample is uniformly thick, and does not contain isolated holes.

For the sample of Figure2.7, the resistivity is given by

ρ =𝜋𝑡

ln(2)

𝑅12,34 + 𝑅23,412

F

Figure2.7 Thin film-type van der Pauw Hall sample

Where R12,34 = V3,4 / I. The current I enters the sample through contact 1 and leaves

through contact 2 and V3,4 = V3 – V4 , is the voltage between contacts 3 and 4. R23,41 is similarly

1

2

3

4

(2.1)

26

defined. F is a function of the ratio Rr = R12,34 / R23,41 only, satisfying the relation

𝑅𝑟 − 1

𝑅𝑟 + 1=

𝐹

ln(2)𝑎𝑠𝑐𝑜𝑠ℎ(

exp(ln(2) 𝐹⁄ )

2)

For symmetric samples (circles or squares) F=1.

The van der Pauw Hall mobility is determined by measuring the resistance R24,13 with

and without a magnetic field. R24,13 is measured by forcing the current intone and out of

the opposite terminal, for example, terminals 2 and 4 in Figure2.7, with the voltage

measured across terminals 1 and 3. The Hall mobility is then given by

𝜇𝐻 =𝑑𝛿𝑅24,13

𝐵𝜌

Where R24,13 is the resistance change of R24,13 due to the magnetic field.

Carrier densities of the sample is given by

𝑛 =𝑟

𝑞|𝑅𝐻|

RH is hall coefficient which id given experimentally by

𝑅𝐻 =𝑑𝑉𝐻𝐵𝐼

VH is the Hall voltage, B is the magnetic field, d is the sample thickness, and I is the

current. Equation (2.4) and (2.1) are for carrier densities per unit volume and for

resistivity (･cm) For uniformly doped samples of thickness d, the sheet Hall

coefficient resistance RHsh is defined as

𝑅𝐻𝑠ℎ =𝑅𝐻𝑑

(2.2)

(2.3)

(2.5)

(2.4)

(2.6)

27

And

𝜇𝐻 =|𝑅𝐻𝑠ℎ|

𝑅𝑠ℎ

where Rsh =d .

The thickness is well defined for bulk samples. For semiconducting films on

insulating substrates, the mobility is frequently observed to decrease toward the

substrate. Surface depletion forces the current to flow in the low- mobility portion of the

film, giving apparent mobility lower than true mobility.

2.2.11 Transmission Line Method (TLM)

Transmission line Method (TLM) is often used to specify contact resistance and sheet

resistance of materials. In This study, TLM was measured for sheet resistance and

resistivity of -FeSi2. Figure2.7 shows schematic of transmission line method pattern.

As shown in Figure2.7 wide length and contact gap are W, d respectively. The contact

gap is different distance, for example, d = 5, 15, 25, 35, 45, 55, 65, 75, 85, 95 m. And in

this study, wide length is 300m.Figure2.8 shows a cross section of transmission line

method pattern. In Figure2.8, contact resistance and sheet resistance are Rc and Rsh. The

relational expression of total resistance (RT), contact resistance, and sheet resistance is as

follows;

𝑅𝑇 = 2𝑅𝑐 +𝑑

𝑊𝑅𝑠ℎ

The slope of (2.1) equals a value including sheet resistance. Figure2.9 shows total

resistance-contact gap relation. When total resistance is zero (RT =0), transfer length (LT)

(2.2)

(2.1)

28

is obtained as follow;

𝐿𝑇 = −𝑑

2

Figure2.7 Schematic of transmission line method pattern

Fig2.8 A cross section of transmission line method pattern

electrode (tungsten) -FeSi2

W

d

SiO2

Rsh

Rc

d

29

Figure2.9 Total resistance-contact gap relations

0

2Rc

2LT d (m)

RT

(

)

slope:

30

Chapter 3

Iron disilicide using stacked sputtered process

3.1 Introduction

3.2 Sheet resistance dependent Fe/Si atomic ratio

3.3 Activation energy of -FeSi2 dependent temperature

3.4 Reference

31

3.1 Introduction

Now, stacked sputtered process is explained in Figure 3.1. The stacked silicidation is to

deposit Fe target and Si target alternately on SiO2 with n-Si (100) substrate. Then

annealing stacked sample, -FeSi2 is formed. The merit of this experimental method is to

control -FeSi2 ratio easily by changing a ratio of Fe and Si. In this chapter, characteristics

of changing Fe and Si ratio of -FeSi2 using stacked sputtered process are investigated.

Furthermore, one of the -FeSi2 fabricating by stacked sputtered process are measured.

Figure3.1 Schematic illustration of stacked silicidation process. A set of Si/Fe is cyclically stacked

on SiO2 with n-Si (100) substrates, followed by annealing in F.G atmosphere to form -FeSi2 film.

3.2 Sheet resistance dependent Fe/Si atomic ratio

Figure3.2 shows sheet resistance (sh) on Fe/Si atomic ratio. Fe thickness and Si

thickness are following table 3.3 respectively. Sample A has Fe/Si atomic ratio of 1.50

and Sample B has 2.00, Sample C has 2.25, SampleD has 2.50 respectively. All of the

Samples performed 800oC annealing in F.G for 5minites. According to Figure3.2, sheet

Si

Substrate

Fe

・・・

Si/Fe

Annealing

Substrate

-FeSi2

SiFe

32

resistances of sample A, B, C, D are changed. Therefore atomic ratio can be controlled

by stacked sputtered process. The highest sheet resistance in Sample A-D is Sample C

which has Fe/Si atomic ratio of 2.25.

Figure3.2 sheet resistance of -FeSi2 dependent Fe/Si atomic ratio

Table3.3 Atomic ratio Si/Fe for Fe and Si thickness

1.00 1.50 2.00 2.50 3.00

105

104

Sh

eet

resi

stan

ce [Ω/sq.]

Si/Fe atomic ratio

1.00:1.50

1.00:2.00

1.00:2.25

1.00:2.50

800 oC 5min annealing in F.G.

sampleFe thickness

(nm)

Si thickness

(nm)Set

Atomic ratio

Si/Fe

A 2.0 5.1 10 1.50

B 2.0 6.8 10 2.00

C 2.0 7.6 10 2.25

D 2.0 8.5 10 2.50

33

3.3 Activation energy of -FeSi2 dependent temperature

Figure3.4 shows temperature dependence of sheet resistance about sample C (Fe/Si

atomic ratio equals to 2.25). The horizontal line of Figure 3.4 represents 1000 divided by

temperature T, and the vertical represents a logarithm of sheet resistance. According to

Figure3.4, sheet resistance increases when temperature T decreases. This Figure3.4

include inclination equal to activation energy and intercept equal to defect amount in

Figure3.5. This activation energy and defect amount were extracted by Arrhenius’ plot,

which was proposed by Svante Arrhenius in 1889[3.1]. Concretely, a function of Arrhenius’

plot is given by

lnρ =𝐸𝑎

2𝑘

1

𝑇− (𝑙𝑛𝑞 +

1

2𝑙𝑛𝑁𝑐𝑁𝑑 + 𝑙𝑛𝜇) (3.1)

34

Figure3.4 Sheet resistance of -FeSi2 (Fe:Si = 1:2.25) dependent tempuraure

As the inclination and intercept were extracted in Figure3.5, there were 4 activation

energies and intercepts respectively in Figure3.4 (as Figure 3.6). Therefore it is found that

the sample C (Fe:Si = 1:2.25) had 4 detect level in its band gap. Each 4 activation energies

is regarded as Ea1, Ea2, Ea3, and Ea4 and Figure3.7 shows an enlarged view of Ea3 and Ea4.

It was derived by calculation that Ea1 = 14 meV, Ea2 = 51meV, Ea3 = 1.1×102 meV, and Ea4

= 1.4×102 meV respectively.

12

13

14

15ln

(sh

eet

resi

stan

ce)

(/s

q.)

11

10

9

8

7

6

102 4 6 8 10 12 14 16 18 20 22

1000/T (K-1)

SiO2(400nm)

FeSiFe

Si

FeSi

Si 7.6nm/Fe 2.0nm

x 10set

･･･

800oC 5min annealing in F.G

35

Figure3.5 Extraction of defect level position and amounts by Arrhenius’ plot

2 4 6 8 10 12 14 16 18 20 2210

11

12

13

14

15

1000/T (K-1)

ln(s

hee

t re

sist

ance

) (

/sq

.)

800 oC

② intercept : defect amount

① inclination : activation energy

(defect level position)

800oC 5min annealing in F.G

36

Figure3.6 Sheet resistance of-FeSi2 (Fe:Si = 1:2.25) dependent temperature (defect

level position and amounts extraction version)

12

13

14

15

ln(s

hee

t re

sist

ance

) (

/sq.)

11

10

9

8

7

6

52 4 6 8 10 12 14 16 18 20 22

1000/T (K-1)

Ea4=

1.4×102 meV

Ea3=

1.1×102 meV

Ea1=14

meV

Ea2=51 meV

Ea4Ea3 Ea2 Ea1

Ec

Ev

Fe:Si=1:2.25

800oC 5min annealing in F.G.

37

Figure3.7 Enlarged view of Ea3 and Ea4

When anneal temperature was raised from 800oC to 850oC, defect amount of inclination

increased as shown in Figure3.8 which included Ea1 and Ea2. Also defect amount increased

as shown as Figure3.9 which included Ea3 and Ea4. As compared to Figure3.8, defect

amount more increased in Figure3.8. In other words, there are two defect levels which

are affected greatly by annealing and affected rarely by annealing. In addition, it is found

that the defect levels which are affected greatly by annealing is related to composition in

-FeSi2 film while the defect levels which are affected rarely by annealing is related to

crystalline defects which can be recovered by annealing[3.2].

3.0 3.2 3.4 3.6 3.8 4.0

1000/T (K-1)

11.0

11.2

11.4

12.0

11.8

11.6

shee

t re

sist

ance

(

/sq

.)

Ea4=1.4×102 meV

Ea3=1.1×102 meV

Fe:Si=1:2.25

800oC 5min annealing in F.G.

38

Figure3.8 Increase of defect amount by raising anneal temperature (Ea1 and Ea2)

2 4 6 8 10 12 14 16 18 20 22

1000/T (K-1)

ln(s

hee

t re

sist

ance

) (

/sq.)

Ea1=13 meV

Ea2=51 meV

12

14

15

16

17

13

850℃

800℃Fe:Si=1:2.25

5min annealing in F.G.

11.2

11.4

11.6

11.8

12

12.2

12.4

12.6

12.8

2.8 3 3.2 3.4 3.6 3.8 4 4.2

11.2

11.4

11.6

11.8

12

12.2

12.4

12.6

12.8

2.8 3 3.2 3.4 3.6 3.8 4 4.2

850℃

800℃

Ea4=1.4×102 meV

Ea3=1.1×102 meV

Fe:Si=1:2.25

5min annealing in F.G.

12.8

12.6

12.4

12.2

12.0

11.8

11.6

11.4

11.22.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2

1000/T (K-1)

ln(s

hee

t re

sist

ance

) (

/sq

.)

39

Figure3.9 Increase of defect amount by raising anneal temperature (Ea3 and Ea4)

3.4 Reference

[3.1] Laidler, K.J. (1987) Chemical Kinetics,Third Edition, Harper & Row, p.42

[3.2] J. Tani, et al, Journal of Alloys and Compounds, 352 153-157 (2003)

40

Chapter4

FeSi2 target deposition

4.1 Introduction

4.2 Sheet resistance dependent annealing temperature and time

4.3 P doping for -FeSi2

4.4 -FeSi2 on high resistance n-Si

4.5 Thickness dependence of carrier density and resistivity

4.6 Reference

41

4.1 Introduction

In this chapter, FeSi2 target was used in RF sputtering. Without high resistance n-type

Si substrate, SiO2 with n-Si was used as substrate (SiO2:400nm). 4-point probe method

procedure conforms to Figure2.2. On the other hand, TLM and van der pauw method

conform to Figure2.3.

4.2 Sheet resistance dependent annealing temperature and time

Figure4.1 shows sheet resistance of -FeSi2 dependent anneal temperature. As shown

in Figure4.1, sheet resistance increased by arising anneal temperature from 500oC to

900oC. This reason was that the -FeSi2 and -FeSi2 mixed region in the upper side of the

film became thinner with increasing the annealing temperatures[4.1]. Since the resistivity

of -FeSi2 is lower than that of -FeSi2[4.3]

.

And it decreased in 950oC anneal temperature. This is because metal of -FeSi2 was

formed by changing phase in 937oC and over[4.3].

Figure4.3 shows sheet resistance of -FeSi2 dependent anneal time. As shown in

Figure4.3, sheet resistance did not changed by altering annealing time (5min ~ 30min).

42

Figure4.1 Sheet resistance of -FeSi2 dependent anneal temperature

Figure4.2 Fe/Si atomic ratio dependence on temperature[4.3]

1.00E+04

1.00E+05

1.00E+06

400 500 600 700 800 900 1000

104

105

106S

hee

t re

sist

ance

[Ω

/sq

.]

Anneal temperature [℃]

43

Figure4.3 Sheet resistance of -FeSi2 dependent anneal time

4.3 P doping for -FeSi2

Now in this part, -FeSi2 doped P (phosphorus) by ion implantation is explained.

Figure4.4 shows dose amounts dependence of sheet resistance. The dose amounts are

1×1013, 1×1014, 1×1015cm-1 respectively. As shown in Figure4.4, sheet resistance of P

doped -FeSi2 increases slightly. But carrier density of -FeSi2 has been high yet under

constant mobility. Sheet resistance of -FeSi2 dependent anneal time is shown in

Figure4.5. Anneal time is 5, 10, 15, 20, 30 for each dose amounts. As anneal time

increased,

1.00E+05

1.00E+06

0 10 20 30 40

105

106S

hee

t re

sist

ance

[Ω/s

q.]

Aneeal time[min]

44

Figure 4.4 Dose amounts dependence of sheet resistance

4.4 -FeSi2 on high resistance n-Si

Figure 4.6 shows comparison between sheet resistance of -FeSi2 on high resistance n-

Si and on SiO2. As shown in Figure 4.6, sheet resistance on high resistance n-Si is smaller

than it on SiO2. Although this aimed diffusion of Si from n-Si to -FeSi2, carrier density

of -FeSi2 decreased under constant mobility.

1.E+05

2.E+05

3.E+05

1.00E+12 1.00E+13 1.00E+14 1.00E+15 1.00E+161

2

3(×105)

Sh

eet

resi

stan

ce [

Ω/s

q.]

1013 10151014

Dose amounts[cm-2]

45

Figure 4.6 Sheet resistance of high resistance n-Si/-FeSi2

4.5 Thickness dependence of carrier density and resistivity

Figure 4.7 shows -FeSi2 film thickness dependence of carrier density and resistivity.

The horizontal line of Figure 4.7 represents -FeSi2 film thickness, and the vertical

represents carrier densities and resistivity. As -FeSi2 film thickness increased, carrier

density of -FeSi2 decreased. The carrier density of -FeSi2 were 1.62× 1020cm-2 at 5nm,

6.47× 1018cm-2 at 80nm, and1.68×1018 cm-2 at 300nm respectively. On the other hand,

resistivity of -FeSi2 raised as film thickness increased.

0.00E+00

5.00E+04

1.00E+05

1.50E+05

2.00E+05

2.50E+05

3.00E+05

SiO2/FeSi2 800oC 高抵抗n-Si /FeSi2 800oC

×105

0

0.5

1.0

1.5

2.0

2.5

3.0

SiO2/-FeSi2 High resistance n-Si/-FeSi2

Shee

t re

sist

ance

[Ω

/sq

.]

46

Figure 4.7 Thickness dependence of carrier density and resistivity of-FeSi2

4.6 Reference

[4.1] K.Hiehata, et.al, e-J.Surf.Sci.Nanotech.Vol.10 190 (2012)

[4.2] Ch. Kloc, E. Arushanov, M. Wendl, H. Hohl, U. Malang, and E. Bucher, J. Alloys

Compd. 219, 93 (1995).

[4.3] K.nogi, et.al, Journal of Material Science, 35, 5845 (2000)

0

1

2

3

4

5

6

1.00E+18

1.00E+19

1.00E+20

1.00E+21

1 10 100 1000

carrier density[cm^-3]

resistivity[Ω・cm]

80nm

300nm

5nm

1019

1018

1020

Carr

ier

den

sity［

cm-3］

-FeSi2 film thickness [nm]

Res

isti

vit

y［・

cm］

1021

47

Chapter 5

Conclusion

5.1 Conclusion

48

5.1 Conclusion

In this study, Fe/Si ratio of -FeSi2 could be controlled by Fe and Si stacked sputtering.

And 4 activation energies could be extracted in graph of sheet resistance dependent

temperature. These activation energies were divided into two type energies, affected by

annealing and by not annealing. Also it is conformed that carrier density of -FeSi2

changed by altering annealing temperature and its film thickness. Concretely, its carrier

density decreases in 900oC annealing temperature. In regard to film thickness, I need to

perform more experiment and discussion because I could not specify where an electrical

current applied in -FeSi2 flowed. Change of -FeSi2 carrier density by altering anneaing

time and substrate less changed than something changing anneling temperature and film

thickness. Implantation of phosphorus also less changed.

49

Chapter 6

Acknowledgements First of all, I would like to express the deepest appreciation to my supervisor

Prof.Hiroshi Iwai for his generous supports and advices for my study. He also gave me

chances to attend conferences. I had a precious experience for my present and future life.

I am also grateful to Associate Prof. Kuniyuki Kakushima for many kindness, supports,

and encouragements. He gave me many useful and important advice for my study.

I deeply thank to Prof. Yoshinori Kataoka, Prof. Kenji Natori, Prof. Kazuo Tsutsui, Prof.

Nobuyuki Sugii, Prof. Akira Nishiyama, and Prof. Hitoshi Wakabayahi for useful advice

and great help whenever I met difficult problem.

I would like to thank Prof. Hiroshi Kastumata of Meiji University for advice of -FeSi2

and his kindness.

Also I thank to colleagues of Iwai Lab for their friendship, active many discussions and

many of encouragement. Especially, my senior, Mr. Taichi Inamura gave me much of help.

I can`t thank you enough. Mr. Takamasa Kawanago, Mr. Wu Yan, Mr. Tomoya Shouji,

Mr. Masaaki Motoki, Mr. Akinori Hasegawa Mr. Syu Munekiyo advised me various

approach. This advice was very useful and important for my study.

I am thankful to Yoshihiro Mastukawa, Takumi Ohashi and Kou Ishikawa for many

active discussion, their friendship and cooperation. It is thanks to them that I have come

this far.

I would like to appreciate the support of secretaries, Ms.Nishizawa and Ms.Matsumoto.

50

Finally, I would like to thank my parents Hiromitsu and Kumiko and my sister Nanami

for their endless support and encouragement.

Takafumi Kato

February, 2014