A study on silicide semiconductors for high efficiency thin ...future low cost solar cells. But the problem for organic materials is the light degradation. Furthermore, conversion

2014 Master Thesis

A study on silicide semiconductors for

high efficiency thin film

photovoltaic devices

Taichi Inamura

12M36055

Department of Electrical and Electronic Engineering

Tokyo Institute of Technology

Supervisor

Professor: Hiroshi Iwai

Associate Professor: Kuniyuki Kakushima

Abstract

Semiconductor silicides have been attracted as thin film solar cell

material candidates for next generations, owing to its appropriate bandgaps

with large absorption coefficient and abundance of resources. Especially,

BaSi2/-FeSi2 tandem thin silicide solar cells are expected to achieve

efficiency 40 %.

One of the issues in semiconductor silicides is that the carrier density is

still in the order of 1018 cm-3, which is three orders of magnitude high

considering sufficient depletion width to absorb sun light with absorption

coefficient of 105 cm-1 at 1.0 eV.

Although, composition control is reported to be the key to achieve

-FeSi2 with low carrier density, the origin of carrier has not yet been

clarified.

The purpose of this thesis is to investigate the optimal Fe/Si composition

for carrier density reduction and to clarify the origin of carriers. Moreover,

light response of -FeSi2 films has been characterized by fabricating

Schottky-type -FeSi2 solar cell.

-FeSi2 films have been deposited by multi-sputtering process, which

consists of cyclic deposition of Fe and Si layers in a multi-target sputtering

system with subsequent crystallization annealing. As the thickness of each

film can be well controlled by sputtering time, the composition of -FeSi2

films can be easily controlled. The origin or carriers has been characterized

through temperature dependent resistivity measurement to extract the

activation energy.

By changing the composition of -FeSi2, it has been found that Si-rich

condition with Si /Fe ratio of 2.25 has shown the largest resistivity of ~0.6

cm. Low temperature measurements have revealed four kinds of defect

levels, which can be categorized into two types; one related to composition of

-FeSi2 and the other related to crystalline defects. The former type with

deep activation energy has been assigned though measurements of samples

with different compositions, and the latter one with shallow activation

energy by crystallization annealing temperature. Based on the above

measurements a carrier density of 1016 cm-3 can be achieved at 90 K.

-FeSi2/p+-Si Schottky solar cell measured at 90 K has shown a photovoltaic

response with open circuit voltage of 40 mV, which is low considering the

bandgap of ideal -FeSi2.

Equivalent circuit modeling has revealed the presence of shunt

resistance which suggests semiconductor nature within the film.

In the near future, more fine composition ratio control, improvement of

crystalline quality and increase of shunt resistance is required for -FeSi2

thin film photovoltaic device.

A study on semiconductor silicides for high efficiency thin film photovoltaic devices

Contents

Chapter1 Introduction

1.1 Thin film solar cell ...................................................................................................... 2

1.2 Introduction of semiconductor silicides .................................................................... 3

1.3 Introduction of BaSi2 / -FeSi2 tandem solar cell ..................................................... 5

1.4 Issues in semiconductor silicides ............................................................................. 10

1.5 Reports on semiconductor silicides .......................................................................... 11

1.6 Purpose of this study ................................................................................................ 14

1.7 Outline of this thesis ................................................................................................ 15

Reference ............................................................................................................................. 17

Chapter 2 Fabrication and characterization

2.1 Fabrication procedure .............................................................................................. 20

2.2 Experimental details ................................................................................................ 21

2.2.1 SPM cleaning and HF treatment ..................................................................... 21

2.2.2 RF magnetron sputtering .................................................................................. 21

2.2.3 Photolithography and Metal etching ................................................................ 22

2.2.4 Lift-off process .................................................................................................... 23

2.2.5 Rapid thermal annealing (RTA) ....................................................................... 23

2.2.6 Vacuum evaporation for Al deposition.............................................................. 23

2.3 Characterization Method ......................................................................................... 25

2.2.1 Fourier transform infrared spectroscopy (FT-IR) ................................................ 25

2.2.2 Four-point probe method ....................................................................................... 26

2.2.3 Transmission Line Model (TLM) ........................................................................... 27

2.2.4 Van der Pauw method ....................................................................................... 30

Reference ............................................................................................................................. 32

Chapter 3 Formation of -FeSi2 3.1 Introduction .............................................................................................................. 34

3.2 Infrared absorption characteristics of -FeSi2 ....................................................... 35

3.3 X-ray diffraction pattern of -FeSi2......................................................................... 38

A study on semiconductor silicides for high efficiency thin film photovoltaic devices

3.4 Conclusion ................................................................................................................. 38

References ............................................................................................................................ 39

Chapter 4 Electrical characteristics of -FeSi2

4.1 Introduction .............................................................................................................. 42

4.2 Resistivity measurement by four-point method ..................................................... 43

4.2.1 Resistivity control by Si/Fe composition ratio ...................................................... 43

4.2.2 Extraction of defect level position ......................................................................... 45

4.2.3 Extraction of defect concentration ........................................................................ 46

4.2.4 Effect of sputtering atmosphere ............................................................................ 48

4.3 Resistivity measurement by TLM ........................................................................... 49

4.4 Carrier density measurement by van der Pauw .................................................... 50

4.5 Conclusion ................................................................................................................. 54

Reference ............................................................................................................................. 55

Chapter 5 Demonstration of -FeSi2 Schottky solar cell

5.1 Introduction .............................................................................................................. 57

5.2 J-V characteristics of -FeSi2/Si Schottky solar cell .............................................. 58

5.3 Equivalent circuit of solar cell ................................................................................. 59

5.4 Conclusion ..................................................................................................................... 62

Reference ............................................................................................................................. 63

Chapter 6 Conclusion

Chapter 1 Introduction

1

Chapter 1

Introduction

1.1 Thin film solar cell

1.2 Introduction of semiconductor silicides

1.3 Issues in semiconductor silicides

1.4 Reports on semiconductor silicides

1.5 The purpose of this study

References


2

1.1 Thin film solar cell

Solar cells have been expected as clean energy replaced to thermal

power generation used coal, petroleum oil and liquefied natural gas or

nuclear power generation and so on. However, a solar cell amounts to only

~0.2 % of total power generation energy supply so far (Figure1.1). The cost

per Kilowatt-Hour of solar cell is much higher than that of thermal or

nuclear power generation. Therefore, to spread the usage of solar cells more

widely and rapidly, not only the improvement of conversion efficiency but

also the decrease of the production cost is indispensable. Figure 1.2 is

classification of solar cells according to the used materials [1.1]. The

crystalline silicon (c-Si) accounts for 90 % or more in a solar cell market [1.2].

However, there is a problem of being approaching limit of conversion

efficiency caused by the band gap. The materials with a variety of band gap

are needed for further conversion efficiency improvement.

Figure 1.1 Distribution of consumption of primary energy with respect to

coal, nuclear, hydro and renewable energy resources

Renewable

PV

Wind

Biomass

Geothermal65.4%

Coal

Nuclear24.8%

Hydro7.8%

0.4%

0.4%

0.3%

1.0%


3

Figure 1.2 Classification of solar cell according to materials

Thin film solar cells have been focused as important photovoltaic cells

for future generations, owing to several advantages. One is that we can limit

usage and save resources thanks to thin film structure. Therefore, the cost

for manufacturing can be kept low. The other is that these have various

bandgap. Hence, higher conversion efficiency than crystalline Si are

expected by tandem structure.

1.2 Introduction of semiconductor silicides

Various thin films have been reported so far. Table 1.1 shows some

examples of it.

Crystal line silicon

(c-Si)

monocrystalline / single crystalline

Polycrystalline / multi crystalline

amorphous silicon

(a-Si)Microcrystalline / m-Si

CIS (CIGS)

CdTe

GaAs

Dye sensitized

polymer

silicon base

compound base

organic based

solar cell


4

Table 1.1 Comparison of thin film solar cells

a-Si and CIGS are the leading thin film solar cells due to its high

absorption coefficient. Especially, a-Si solar cell could be applied to a liquid

crystal display production process. Therefore, a-Si solar cell was rapidly

developed and widely expanded in a world. Disadvantage of a- Si solar cell is

that theoretical conversion efficiency is near 20 % which is low for solar cells

for next generation solar cell. Furthermore, vacuum process such as plasma

CVD or sputtering is used in the fabrication process. Therefore, the process

cost is high. Similarly, CIGS also uses the vacuum process and the

production cost is high although conversion efficiency shows high value near

30 %. However, there is a problem of using Indium of rare metal. Also, a dye

sensitized solar cell or an organic thin film attracted recently attention as

future low cost solar cells. But the problem for organic materials is the light

degradation. Furthermore, conversion efficiency for this material is

insufficient to exceed 30 % or more. On the other hand, semiconductor

silicide materials are thought to be the powerful candidates to achieve low

cost and high conversion efficiency compared to above materials. Various

silicides have been reported to have bandgap. For example, the band gap of

BaSi2, -FeSi2 and Mg2Si are 1.4 eV, 0.8 eV and 0.75 eV respectively. In

addition, there are abundantly resources for these three silicides. Moreover,

the fact that BaSi2 and -FeSi2 are stable against light illumination is

reported. After all, in order to achieve above goal, semiconductor silicides

Bandgap

Eg (eV)

Transition

type

Absorption

coefficient

α (cm-1)

Resources DegradationEfficiency

(%)

a-Si 1.7 indirect 104 Excellent Good 20 [1.3]

CIGS 1.0～1.6 direct 105 Bad Excellent 29 [1.3]

Organic 1.0～ indirect 105 Good Bad 14 [1.3]

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

FeSi2 0.8 direct 105～ Excellent Excellent [1.4] 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.)


5

have been considered as suitable candidates.

1.3 Introduction of BaSi2 / -FeSi2 tandem solar cell

If BaSi2 and -FeSi2 are connected in series, tandem thin silicide film

solar cell can be realized (Figure 1.3).

Figure 1.3 Structure of BaSi2/-FeSi2 tandem solar cell

Indeed, we calculate the maximum efficiency of tandem solar cell. The

conversion efficiency, , of solar cell is calculated as the ratio between the

generated maximum power, Pm, generated by a solar cell and the incident

power, Pin. The incident power is equal to the irradiance of AM 1.5 spectrum,

normalized to 1000 W/m2. Therefore, the is given by

η =𝑃𝑖𝑛

𝑃𝑜𝑢𝑡=

𝐽𝑚𝑎𝑥𝑉𝑚𝑎𝑥

𝑃𝑖𝑛=

𝐽𝑠𝑐𝑉𝑜𝑐𝐹𝐹

𝑃𝑖𝑛 (1.1)

where Jmax is maximum current density, Vmax maximum voltage, Jsc short

circuit current density, Voc open circuit voltage, FF fill factor.

The irradiance of AM 1.5 spectrum can be calculated from the spectral

power density, P(), using the following equation:

𝑃𝑖𝑛 = ∫ 𝑃(𝜆)𝑑𝜆∞

0 (1.2)

BaSi2

-FeSi2

transmitted light

incident light


6

= ∫ 𝜙(𝜆)ℎ𝑐

𝜆𝑑𝜆

∞

0 (1.3)

where 𝜙(𝜆) is photon flux density, h Plank’s constant, c speed of light.

When we denote g as the wavelength of photons that corresponds to the

band gap energy of the absorber of the solar cell, only the photons with

energy higher than the bandgap are absorbed, it means photons with λ ≤ 𝜆𝑔.

The fraction of the incident power, pabs that is absorbed by a solar cell and

used for energy conversion is expressed as

𝑃𝑎𝑏𝑠 =∫ 𝜙(𝜆)

ℎ𝑐

𝜆

𝜆𝑔0

∫ 𝜙(𝜆)ℎ𝑐

𝜆

∞0

(1.4)

A part of the absorbed energy, the excess energy of photons, is lost due to

the thermalization of photo-generated electrons and holes in the absorber

material. The fraction of the absorbed energy that the solar cell can deliver

as useful energy, puse, is described by

𝑃𝑢𝑠𝑒 =𝐸𝑔 ∫ 𝜙(𝜆)𝑑𝜆

𝜆𝑔0


𝜆𝑑𝜆

𝜆𝑔0

(1.5)

We can determine the conversion efficiency limited by the spectral

mismatch

η = 𝑃𝑎𝑏𝑠𝑃𝑢𝑠𝑒 =∫ 𝜙(𝜆)

ℎ𝑐

𝜆𝑑𝜆𝐸𝑔 ∫ 𝜙(𝜆)𝑑𝜆

𝜆𝑔0

𝜆𝑔0


𝜆𝑑𝜆 ∫ 𝜙(𝜆)

ℎ𝑐

𝜆𝑑𝜆

𝜆𝑔0

∞0

(1.6)

Figure 1.4 illustrates the fraction of the AM 1.5 spectrum that can be

converted into a usable energy.


7

Figure 1.4 The fraction of AM 1.5 spectrum that can be converted into a

usable energy

In general, when light arrives on an interface between two media, a part

of the light is reflected from and the other part is transmitted through the

interface. This means that a part of the incident energy that can be

converted into a usable energy by the solar cell is lost by reflection. We shall

denote the total reflectance as R*, which can be considered as the effective

reflectance in the wavelength range of interest.

In most c-Si solar cells, one of the metal electrodes is placed on the front

side of the cell. The metal-covered area does not allow the light to enter the

solar cell because it totally reflects the light in wavelength range of interest.

When we denote the total area of the cell Atot and the cell area that is not

covered by the electrode Af, the active area of the cell is determined by the

ratio of Af / Atot. This ratio is called the coverage factor and determines the so

called shading losses. However, in the case of thin film solar cell, transparent

conductive film such as ITO is used for front side electrode. Therefore,

Af /A tot ~1.

When light penetrates into a material, it will be absorbed as it

propagates through the material. The absorption of light in the material

depends on its absorption coefficient. Incomplete absorption in the absorber

due to its limited thickness is an additional loss that lowers the efficiency of

Wavelength

Non absorption λg


8

the energy conversion. The incomplete absorption loss can be described by

the internal optical quantum efficiency, QEop.

Not all charge carriers that are generated in a solar cell are collected at

the electrodes. The photo-generated carriers are the excess carriers with

respect to the thermal equilibrium and are subjected to the recombination.

This can be expressed by the electrical quantum efficiency, QEel.

With these taken into consideration, the absolute external quantum

efficiency QE which is defined as the number of charge carriers collected per

incident photon at each wavelength can be approximated as

Q(E) = (1 − 𝑅∗)𝑄𝐸𝑜𝑝(𝜆)𝑄𝐸𝑒𝑙(𝜆) (1.7)

The maximum current density that the solar cell can deliver is

determined by the bandgap of the absorber layer that determines which

photons of the incident radiation can generate electron-hole pairs. The

maximum current density, Jmax is described as

𝐽𝑚𝑎𝑥 = 𝑞 ∫ 𝜙(𝜆)𝑑𝜆𝜆𝑔

0 (1.8)

The short-circuit current density is determined by the absolute external

quantum efficiency

𝐽𝑠𝑐 = 𝐽𝑚𝑎𝑥(1 − 𝑅∗)𝑄𝐸𝑜𝑝𝑄𝐸𝑒𝑙 (1.9)

= q(1 − 𝑅∗)𝑄𝐸𝑜𝑝𝑄𝐸𝑒𝑙 ∫ 𝜙(𝜆)𝑑𝜆𝜆𝑔

0 (1.10)

The open-circuit voltage depends on the saturation current and

short-circuit current density.

𝑉𝑜𝑐 =𝑘𝑇

𝑞𝑙𝑛 (

𝐽𝑠𝑐

𝐽0) (1.11)

where k is Boltzmann’s constant, Jo saturation current density. The

saturation current density depends on the recombination in the solar cell

that cannot be avoided and is referred to as the fundamental recombination

which is determined by the voltage factor, qVoc/Eg.


9

The maximum power generated by a solar cell is dependent on the fill

factor, FF. In case of a solar cell that behaves as an ideal diode only direct

recombination occurs and the maximum FF is a function of Voc.

By combining the previous equation, the conversion efficiency can be

written as

η =∫ 𝜙(𝜆)

ℎ𝑐

𝜆𝑑𝜆

𝜆𝑔0


𝜆𝑑𝜆

∞0

𝐸𝑔 ∫ 𝜙(𝜆)𝑑𝜆𝜆𝑔

0


𝜆𝑑𝜆

𝜆𝑔0

(1 − 𝑅∗)𝑄𝐸𝑜𝑝𝑄𝐸𝑒𝑙𝑞𝑉𝑜𝑐

𝐸𝑔𝐹𝐹 (1.12)

For simplicity, we assume that total reflectance is 0 %, quantum

efficiency is 100 %, and FF is 1. In the case of tandem solar cell, current is

limited by smaller value in each cell and voltage is sum of each cell because

they are connected in series.

Figure 1.5 shows the calculated efficiency of tandem solar cell. Assuming

1.4 eV of BaSi2 with 0.8 eV of -FeSi2, efficiency over 40 % could be expected.

Figure 1.6 shows I-V characteristics of BaSi2/-FeSi2 tandem solar cell. From

this, the open circuit voltage could exceed 1.5 V with short circuit current of

2.5 A/cm2. Therefore, tandem silicide solar cell could be expected as a new

thin film solar cell.

Figure 1.5 Conversion efficiency of tandem solar cell

45%40%

35%30%

Top : 1.4eV

Bottom : 0.8eV

top cell bandgap (eV)

bottom

ce

ll b

an

dg

ap

(eV

)

1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.00.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6


10

Figure 1.6 I-V Characterization of BaSi2/-FeSi2 tandem solar cell

1.4 Issues in semiconductor silicides

Thin film solar cells have been investigated for next generation energy

discussed in previous section. Especially, semiconductor silicides such as

BaSi2 and -FeSi2 are expected as material of tandem solar cell thanks to

some advantages. However, there are some challenges in semiconductor

silicides. One is high carrier density [1.5]. BaSi2 and -FeSi2 have large

absorption coefficient of over 105 cm-1. Therefore, the requirement for silicide

solar cell is that it should have depletion width of several hundred

nanometers to absorb most of the light from sun. Figure 1.7 shows depletion

layer width dependence of -FeSi2 on carrier density. This figure reveals that

carrier densities of 1015 ~1016 cm-3 are required. Therefore, low carrier

density of the film is required for thin film silicide solar cell.

0 0.5 1.0 1.5 2.0-0.5-3.0

-2.0

-1.0

0

1.0

2.0×102

cu

rre

nt d

en

sity（

A/m

2)

voltage (V)

Voc

Jsc


11

Figure 1.7 Target value of carrier density of -FeSi2 for sufficient absorption

of the light from sun

1.5 Reports on semiconductor silicides

Lots of works have reports on silicide formations. Such as ion

implantation of Fe into Si with annealing, reactive epitaxy method, MOCVD

have been used to form -FeSi2 [1.6, 1.7, 1.8]. As for film quality, there are a

lot of researches on carrier density of -FeSi2 formed by MBE. The carrier

density of the formed films is reported to depend on Si and Fe composition

ratio. In the case of Figure 1.8 (a), when the composition ratio is 1 to 2, the

type of semiconductor becomes n-type and below 1.8, -FeSi2 are p-type

semiconductor. On the other hand, in the case of Figure 1.8 (b), when the

composition ratio is 1 to 2, the type becomes p-type. Therefore, composition

control is the key to achieve -FeSi2 with low carrier density. However, the

origin of carrier has not yet been clarified.

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


12

Figure 1.8 Carrier density dependence of -FeSi2 on Si/Fe composition ratio

(a) Ref.[1.9], (b) Ref.[1.10]

K. Takakura et al.,

Jpn. J. Appl. Phys ,

Vol. 39, 790 (2000)

1.85

2.6

(a)

(b)


13

There have been limited reports on solar cells fabricated using -FeSi2.

The highest efficiency reported is 3.7 % obtained on a crystalline n-type

-FeSi2 film epitaxially grown on p-Si(111) (Figure 1.8) [1.11]. Minimizing

the ~40 % visible light reflectance from -FeSi2 with antireflection coating

may help to improve the efficiency up to ~6 % [1.12]. However, this value is

still much lower than the theoretical value. As it was previously mentioned,

it could be a possible cause that depletion layer isn’t sufficiently spread for

absorption from sun.

Figure 1.9 Schematic device structure of a thin-film n--FeSi2/p-Si

heterojunction solar cell (a) and a typical I-V characteristic curve under

sunlight of air mass 1.5, 100mW/cm2 illumination (b)


14

1.6 Purpose of this study

As mentioned in previous section, semiconductor silicides have been

attracted as new generation thin film photovoltaic materials. Based on the

above introduction, the purpose of this study is to develop a process to realize

fine control of Fe/Si composition ratio, to investigate the optimal Fe/Si

composition for carrier density reduction and to clarify the origin of carriers.

In this process, just Fe and Si layer are sputtered in situ, followed by proper

annealing as shown in Figure 1.10. Moreover, light response of -FeSi2 films

has been characterized by fabricating Schottky-type -FeSi2 soalr cell.

Figure 1.10 Schematic illustration of multi-sputtering process. A set of Fe/Si,

with arbitrary composition ratio, is cyclically stacked, followed by proper

annealing to form -FeSi2

FeSi

･･

･annealing FeSi2

Si substrate Si substrate

1 set


15

1.7 Outline of this thesis

Figure 1.11 shows the contents of this thesis. This thesis is consisted of 6

parts.

In chapter 1, the introduction of this thesis is stated.

In chapter 2, the fabrication process of devices and electrical

characterization are explained.

In chapter 3, -FeSi2 formed by our multi-stacked process is confirmed.

It becomes obvious that -FeSi2 with bandgap of 0.8 eV is formed.

In chapter 4, electrical characteristics of -FeSi2 are examined.

Resistivity of -FeSi2 is dependent on composition ratio. Moreover, four kinds

of activation energy for carriers can be extracted.

In chapter 5, -FeSi2 Schottky solar cell was demonstrated. It is testified

that -FeSi2 has ability of solar energy conversion.

Finally, chapter 6 summarizes this study.


16

Figure 1.11 Contents of this thesis

Chapter 1

Chapter 2

-FeSi2 Formation Electrical characteristics of -FeSi2

Demonstration of -FeSi2 Schottkysolar cell

Conclusion

Introduction

Fabrication and characterization

Chapter 3 Chapter 4

Chapter 5

Chapter 6


17

Reference

[1.1] AIST, Research Center for Photovoltaic Technologies “Classification of

solar cell”

[1.2] IEA, “Energy Technology Perspective, Pathway to a Clean Energy System” (2012)

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

[1.4] T. Suemasu, New Technology Presentation Meeting at University of

Tsukuba (2012)

[1.5] T. Suemasu, K. Takakura, C. Li, Y. Ozawa, Y. Kumagai, F. Hasegawa,

“Epitaxial growth of semiconducting -FeSi2 and its application to

ligh-emitting diodes”, Thin Solid Films, 461,209-218 (2004)

[1.6] M. Sugiyama, Y. Maeda, “Microstructure characterization of ion-beam

synthesized -FeSi2 phase by transmission electron microscopy”, Thin Solid

Films, 381, 256 (2001)

[1.7] T. Suemasu, T. Fujii, M. Tanaka, K. Takakura, Y. Iikura, F. Hasegawa,

Jpn. J. Appl. Phys, 36, 3620 (1997)

[1.8] M. Suzuno, K. Akutsu, H. Kawakami, K. Akiyama, T. Suemasu,

“Metalorganic chemical vapor deposition of -FeSi2 on -FeSi2 seed crystals

formed on Si substrates”, Thin Solid Fioms, 519, 24, 8473-8476 (2011)

[1.9] K.Takakura, T. Suemasu, Y. Ikura, F. Hasegawa, “ Control of the

Conduction Type of Nondoped High Mobility -FeSi2 Films Grown from Si/Fe

Multilayers by Change of Si/Fe Ratios”, Jpn. J. Appl. Phys, 39, 787-791

(2000)

[1.10] N. Seki, K. Takakura, T. Suemasu, F. Hasegawa, “Conduction type and

defect levels of -FeSi2 films grown by MBE with different Si/Fe ratios”,

Materials Science in Semiconductor Processing, 6, 5-6, 307-309 (2003)

[1.11] Z. Liu, S. Wang, N. Otogawa, Y. Suzuki, M. Osamura, Y. Fukuzawa, T.

Ootsuka, Y. Nakayama, H. Tonoue, Y. Makita, “ A thin-film solar cell of

high-quality -FeSi2/Si heterojunction prepared by sputtering”, Solar Energy


18

Materials&Solar Cells, 90, 276-282 (2006)

[1.12] Y. Makita, T. Ootsuka, Y. Fukuzawa, N. Otogwa, H. Abe, L. Zhengxin,

Y. Nakayama, “-FeSi2 as Kankyo (environmentally friendly) semiconductor

for solar cells in the space application”, Proc. SPIE, 6197, 61970O (2006)


19

Chapter 2

Fabrication and characterization

2.1 Fabrication procedure

2.2 Experimental details

2.3 Characterization Method

2.4

References


20

2.1 Fabrication procedure

Figure 2.1 shows fabrication procedure of the sample for -FeSi2

Schottky solar cell. The sample was fabricated on n-type (100)-oriented Si

substrate. The substrate impurity concentration is 3×1015 cm-3. To determine

the diode area, 400-nm-thick thermal SiO2 was formed. It was patterned by

photolithography and etched by buffered hydrofluoric acid (BHF). After SPM

cleaning and HF treatment, thin thermal SiO2 (2~3 nm) was formed because

of protection Si surface from resist and developers. Lift-off pattering and HF

treatment due to removing SiO2 for protection formed by thermal oxidation

were performed. FeSi2 was deposited by RF sputtering. After that, lift-off due

to removing FeSi2 which exists at excess area was performed. Rapid thermal

annealing (RTA) in F.G. (N2:97%, H2:3%) ambient was performed due to

silicidation. Finally, an Al film was formed as a backside electrode by

thermal evaporation. Finally,

Figure 2.1 Fabrication procedure of -FeSi2/Si heterojunction solar cell

p-Si(100) Sub (~1015 cm-3)

SPM and HF cleaning

Deposition by RF sputtering (silicide semiconductor)

Lift-off

Backside Al contact

Silicidation by RTA in F.G.

Diode patterning

BHF etching of SiO2

Thermal oxidation for isolation


21

2.2 Experimental details

2.2.1 SPM cleaning and HF treatment

Various contaminations such as particles and organic substances are

produced during semiconductor manufacturing process. They become a cause

of false operation. Therefore, surface treatment and cleaning are important

and unavoidable during device fabrication. SPM cleaning is one of the

effective cleaning methods. Hydrogen peroxide solution (H2O2) and sulfuric

acid (H2SO4) (H2O2 : H2SO4=1 : 4) are used as cleaning liquid. The substrates

were dipped in this liquid which is kept at 150 oC for 5 minutes. Because of

its oxidizability, particles and organic substance are oxidized and separated

from the surface of Si substrate. Then, the samples were rinsed in DI water.

After that, they were dipped in hydrofluoric acid (1% HF) for 1 min to remove

native and oxidized SiO2 during SPM.

2.2.2 RF magnetron sputtering

-FeSi2 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 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.


22

Figure 2.2 Schematic illustration of RF magnetron sputtering

2.2.3 Photolithography and Metal etching

The photolithography process during the device fabrication was utilized.

First of all, a thin uniform positive photoresist layer of S1805 was coated on

the samples by spin coating followed by baking at 115 oC for 5 minutes on a

hot plate. Next, the samples were aligned and exposed through e-beam

patterned hard-mask with high-intensity ultraviolet (UV) light at 405 nm

wavelength. For positive resists, exposure to the UV light changes the

chemical structure of the resist so that it becomes more soluble in the

developer. Exposed samples were developed by the specified developer

(NMD-3). The exposed resist is washed away by the developer solution,

leaving windows of the bare underlying material. Therefore, contains an

exact copy of the pattern which is to remain on the wafer. Post baking was

done at 130 oC for 10 minutes.

Ar+

Substrate

Plasma

Target

~

Permanent magnet

e-Ar+

e-


23

2.2.4 Lift-off process

Lift-off is the process which selectively removes deposited films.

Following photolithography and deposition, resists and deposited films

which exist on excess area are left by ultrasonic cleaning with acetone.

2.2.5 Rapid thermal annealing (RTA)

Rapid thermal annealing (RTA) is performed for silicidation. In this

study, QHC-P610CP (ULVAC RIKO Co. Ltd) is used as RTA equipment. The

annealing was performed by six infrared lamps surrounding the sample

stage made of carbon coated SiC. The heating temperature was controlled by

thermocouple feedback. Heating chamber is filled with F.G. to interfere with

oxidation. In this study, the time of elevated temperature is 30 seconds and

the time of annealing is 5 minutes.

2.2.6 Vacuum evaporation for Al deposition

Al for backside electrodes is deposited by vacuum evaporation in vacuum

chamber at a base pressure up to 1.0×10-3 Pa. Al source is set on tungsten

(W) boat and heated up to boiling point of Al by joule heating. However,

melting point of W is higher than boiling point of Al, W boat doesn’t melt.

Chamber pressure was kept under 4×10-3 Pa. Figure 2.3 shows illustration of

Al deposition.


24

Figure 2.3 Schematic illustration of vacuum evaporation

Figure 2.4 Schematic illustration of -FeSi2 Schottky solar cell fabrication

process

W boat

sample

Quartz thickness monitor

Al source

Large current

Al

Al

Al

Al

~ 10-3 Pa

p+-Si(100)

SiO2 SiO2 400 nm

p+-Si(100)

SiO2 SiO2SiO2

p+-Si(100)

SiO2 SiO2

p+-Si(100)

SiO2 SiO2

p+-Si(100)

SiO2 SiO2

semiconductor silicide

p+-Si(100)

SiO2 SiO2

p+-Si(100)

SiO2 SiO2

Al

Resist

SPM and HF cleaning

Diode patterning

BHF etching of SiO2

Thermal oxidation to protect Si surface

Lift-off patterning

HF treatment

Deposition by RF sputtering in Ar

Lift-off by ultrasonic cleaning with acetone

Backside Al contact by vacuum evaporation


25

2.3 Characterization Method

2.2.1 Fourier transform infrared spectroscopy (FT-IR)

In general, Fourier transform infrared spectroscopy (FT-IR) is used to

examine how molecules are formed by observing an infrared spectrum

originated from molecular vibration. However, we use this method to extract

the bandgap.

We measure absorption characteristics by transmission method. Figure

2.5 shows illustration of this method. Light is detected through air, sample

and stage. We measured the background absorption to remove the excess

absorption of air and stage before samples are measured.

Figure 2.5 FT-IR transmission method

The measured spectrum divided from background gives a transmittance.

Absorption coefficient is defined as

α = −1

𝑑𝑙𝑛 (

𝐼

𝐼0) =

1

𝑑𝑙𝑛 (

1

𝑇) (2.1)

where I0 and I are intensity of light before and after incidence on a sample, d

is thickness of a sample, T is transmittance.

Light source

Detector

stagesample


26

For direct transition semiconductor, absorption coefficient is given by

𝛼𝑑𝑖𝑟 =𝐴√ℏ𝜔−𝐸𝑔

ℏ𝜔 (2.2)

(𝛼𝑑𝑖𝑟ℏ𝜔)2 = 𝐴2(ℏ𝜔 − 𝐸𝑔) (2.3)

where ℏis photon energy, and Eg is bandgap.

Therefore, the bandgap is extracted by measuring absorption coefficient.

2.2.2 Four-point probe method

The most common method for measuring resistivity is the four-point

probe method [2.1, 2.2]. A small contact current is passed through the outer

two probes and the voltage is measured between the inner two probes. For a

thin wafer with thickness W which is much less than either the length a or

the width d (Figure 2.5), the sheet resistance Rsh is given by

𝑅𝑠ℎ =𝑉

𝐼𝐶𝐹 (2.4)

in units of W/sq., where CF is the correction factor.

The resistivity is then

ρ = 𝑅𝑠ℎ𝑊 (2.5)

In the limit of d ≫ S, where S is the probe spacing, the correction factor

CF becomes /ln2 (=4.54).


27

Figure 2.5 Current flow and voltage measurement of four-probe method

2.2.3 Transmission Line Model (TLM)

The transmission line model (TLM) is a well-known classical method for

measuring the sheet and contact resistance [2.3, 2.4]. This method

determines the specific contact resistivity which is not the resistance of the

metal-semiconductor interface alone, but it is practical quantity describing

the real contact.

When current flows from the semiconductor to metal, it encounters the

resistances c and Rsh in Figure 2.7, choosing the path of least resistance.

The potential distribution under the contact is determined by both c and Rsh

according to [2.5]

V(x) =𝐼√𝑅𝑠ℎ𝜌𝑐

𝑍

cosh[(𝐿−𝑥)/𝐿𝑇]

sinh(𝐿/𝐿𝑇) (2.6)

where L is the contact length, Z the contact width, and I the current flowing

into the contact.

The “1/e” distance of the voltage curve is defined as the transfer length

LT given by

𝐿𝑇 = √𝜌𝑐/𝑅𝑠ℎ (2.7)

a

d

W

V

S


28

This length can be thought of as that distance over which most of the

current transfers from semiconductor into the metal or from the metal into

the semiconductor.

Figure 2.7 Current transfer from semiconductor to metal represented by the

arrows. The semiconductor/metal contact is represented by the c-Rsh

equivalent circuit with the current choosing the path of least resistance.

Figure 2.8 shows TLM test structure. When the voltage is measured in

the ladder structure between contacts 1 and 4, for example, the current flow

may be perturbed by contacts 2 and 3. The effect of contacts 2 and 3 depends

on the transfer length LT and the contact length L. For L ≪ 𝐿𝑇, the current

does not penetrate appreciably into the contact metal and contacts 2 and 3

have no effect on the measurement. For L ≫ 𝐿𝑇, the current does flow into

the metal and the contact can be thought of as two contacts, each of length LT,

joined by a metallic conductor [2.6].


29

Figure 2.8 Basic structure of TLM

For contacts with L ≥ 1.5𝐿𝑇 and for a front contact resistance

measurement of the structure in Figure 2.8, the total resistance RT between

any two contacts is

𝑅𝑇 =𝑅𝑠ℎ𝑑

𝑍+ 2𝑅𝑐 (2.8)

The total resistance is measured for various contact spacing and plotted

versus d as illustrated in Figure2.9. Three parameters can be extracted from

such a plot. The slope Δ𝑅𝑇/Δd = 𝑅𝑠ℎ/𝑍 leads to the sheet resistance Rsh with

the contact width Z independently measured. The intercept at d=0 is RT=2Rc

giving the contact resistance Rc.

Figure 2.9 A plot of total resistance as a function of contact spacing

1 2 3 4 5

Z W

L d1 d2 d3 d4

0 d

RT

2Rc

Slope = Rsh/Z


30

2.2.4 Van der Pauw method

Van der Pauw method is a useful method to measure the resistivity,

carrier density, and mobility. Figure 2.10 shows general geometry for Van der

Pauw method. Required condition for this method is that the contacts are at

the circumference of the sample and are sufficiently small, the sample is

uniformly thick, and does not contain isolated holes.

Figure 2.10 Basic structure of Van der Pauw Hall sample

The sample of Figure 2.10, the resistivity is given by [2.7]

ρ =𝜋𝑡

𝑙𝑛2

𝑅12,34+𝑅23,41

2𝐹 (2.9)

where R12,34 = V34/I. The current I enters the sample through contact 1 and

leaves through contact 2 and V34 = V3-V4 is the voltage between contacts 3

and 4. R23,41 is similarly defined. Current enters the sample through two

adjacent terminals and the voltage is measured across the other two

adjacent terminals. F is a function of the ratio Rr = R12,34/R23,41 only,

satisfying the relation

𝑅𝑟−1

𝑅𝑟+1=

𝐹

𝑙𝑛2𝑎𝑟𝑐𝑜𝑠ℎ (

𝑒𝑥𝑝(ln 2/𝐹)

2) (2.10)

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

The hole and electron densities are given by

1 2

34


31

p =1

𝑞𝑅𝐻; 𝑛 = −

1

𝑞𝑅𝐻 (2.11)

where RH is Hall coefficient and is defined as

𝑅𝐻 =𝑑𝑉𝐻

𝐵𝐼=

𝑡Δ𝑅24,13

𝐵 (2.12)

Δ𝑅24,13 = |𝑉13−𝑉13

0

𝐼13| (2.13)

where VH is hall voltage, t is sample thickness, B is magnetic field applied in

a direction perpendicular to the sample. V13 and V013 are the voltage between

contacts 1 and 4 with and without a magnetic field.

The mobility m is defined as the product of the Hall coefficient and

conductivity

μ = |𝑅𝐻|𝜎 =|𝑅𝐻|

𝜌 (2.14)

where is conductivity, is resistivity.


32

Reference

[2.1] W.E. Beadle, J.C. C. Tsai, R.D. Plummer, Eds., “Quick Reference

Manual for Silicon Integrated Circuit Technology”, Wiley, New York (1985)

[2.2] F.M. Smits, “Measurement of Sheet Resistivities with the Four-Point

Probe”, Bell Syst. Tech. J., 37, 711 (1958)

[2.3] H. Murrman, D. Widmann, “Current crowding on metal contacs to

planar devices”, Electron Devices, IEEE Transactions on, 16, 12, 1022-1024

(1969)

[2.4] W. Shockley, “Research and investigation of inverse epitaxial uhf power

transistors”, Technical Documentary Report AT TDR, 64-207, AF Avionics

Laboratory, Research and Technology Division, Air Force Systems Command,

Wright-Patterson AFB (1969)

[2.5] H.H. Berger, “Models for contacts to planar devices”, Solid State

Electronics, 15, 145-158 (1972)

[2.6] L.K. Mak, C.M. Rogers, D.C. Northrop, “Specific Contact Resistance

Measurement on Semiconductor”, J. Phys. E: Sci. Instr. 22, 317-321 (1989)

[2.7] L.J. van der Pauw, “A Method of Measuring Specific Resistivity and

Hall Effect of Discs of Arbitrary Shape”, Phill. Tech. Rep., 13, 1-9 (1958)

Chapter 3 Formation of -FeSi2

33

Chapter 3

Formation of -FeSi2

3.1 Introduction

3.2 Infrared absorption characteristics of -FeSi2

3.3 X-ray diffraction pattern of -FeSi2

3.4 Conclusion

References


34

3.1 Introduction

As described in the chapter 1, semiconductor -FeSi2 has attracted much

attention in recent years because of its prominent photovoltaic properties

[3.1, 3.2, 3.3]. For -FeSi2 formation process, lots of works have reports. For

example, MOCVD [3.4], EB (Electron Beam) vapor [3.5], ion implantation of

Fe into Si with annealing [3.6], RDE (Reactive Deposition Epitaxy) [3.7], and

sputtering by FeSi2 target [3.8] have been used to form -FeSi2. For film

quality, there are a lot of reports about the samples formed by MBE

(Molecular Beam Epitaxy) [3.9, 3.10]. They report that the carrier density of

the formed films depends on Si and Fe composition ratio. Therefore, to

realize fine control of Fe/Si composition ratio is the key for reducing carrier

density. However, it is difficult to control Fe/Si composition ratio. Based on

the above introduction, we propose a simple multi sputtering process to form

-FeSi2 to realize fine control of Fe/Si composition ratio. The process is

simple, just multi-stacking Fe and Si layer in situ sputtering with proper

annealing. As the thickness of each film can be well controlled by sputtering

time, the composition of -FeSi2 films can be easily controlled. The purpose of

this chapter is to confirm formation of -FeSi2 by multi-stacking process.

Figure 3.1 Schematic illustration of multi-stacking process

FeSi

･･

･

annealing FeSi2

Si substrate Si substrate

1 set


35

3.2 Infrared absorption characteristics of -FeSi2

Figure 3.2 shows infrared absorption characteristics of -FeSi2. For the

sample (a) which is reactively formed by 20 nm-thick Fe, conspicuous

absorption cannot be observed. On the other hand, for the sample (b) with

Fe/Si stacked sputtered process and sample (c) with FeSi2 target, increase in

absorption edge around 6400 cm-1 in the spectrum can be observed while

increasing the annealing temperature from 600 to 900 oC. For the samples

(b) and (c) annealed at 1000 oC, absorption could not be confirmed. -FeSi2 is

reported to transform phase (-FeSi2) which has metal nature at high

annealing temperature [3.11]. Therefore, these samples annealed at 1000 oC

could not be -FeSi2 but -FeSi2.

We calculate difference absorption spectrum between the sample with 20

nm- thick and with 10 nm-thick.

𝐼20(𝑘) − 𝐼10(𝑘) = 𝐼𝐹𝑒𝑆𝑖2(k) (3.1)

where I20(k) and I10(k) are the spectrum intensity of the sample with

20-nm-thick and 10-nm-thick respectively.

Figure 3.3 shows the subtracted absorption spectrum ((a) is Fe/Si

stacked sample, (b) is FeSi2 target sample). Increase in the absorption edge

around 6400 cm-1 in the spectrum can be more clearly observed.


36

Figure 3.2 Infrared absorption characteristics of -FeSi2

(a) Fe mono layer (b) Fe/Si stacked (c) FeSi2 target

Figure 3.3 Difference absorption spectrum as a difference between the

sample with 20 nm- thick and with 10 nm-thick

(a) Fe/Si stacked (b) FeSi2 target

Wavenumber (cm-1)

4000500060007000

as depo

400℃

500℃

600℃

700℃

800℃

900℃

1000℃

Ab

so

rba

nce

(a

.u)

No

absorption edge

Wavenumber (cm-1)

α-FeSi2

4000500060007000

as depo

400℃

500℃

600℃

700℃

800℃

900℃

1000℃

absorption edge

as depo

400℃

500℃

600℃

700℃

800℃

900℃

1000℃

Wavenumber (cm-1)

4000500060007000

α-FeSi2

absorption edge

(a) (b) (c)

500250045006500

Wavenumber (cm-1)

Ab

so

rba

nce

(a

.u)

Wavenumber (cm-1)

as depo

400℃

500℃

600℃

700℃

800℃

900℃

as depo

400℃

500℃

600℃

700℃

800℃

900℃

500250045006500

6400 cm-1

6400 cm-1

(a) (b)

FeSi2 – 10 nm

Si substrate

FeSi2 – 20 nm

Si substrate

FeSi2 – 10 nm


37

-FeSi2 is reported to be direct transition semiconductor [3.12] and its

absorption coefficient is given by

𝛼𝑑𝑖𝑟 =𝐴√ℏ𝜔−𝐸𝑔

ℏ𝜔 (3.2)

(𝛼𝑑𝑖𝑟ℏ𝜔)2 = 𝐴2(ℏ𝜔 − 𝐸𝑔) (3.3)

where ℏis photon energy, Eg is bandgap. Therefore, the bandgap is

extracted by measuring absorption coefficient. Figure 3.4 shows magnified

absorption edge of samples annealed at 600 oC, where band gap of 0.8 eV can

be extracted.

Figure 3.4 Bandgap calculation of -FeSi2 (a) Fe/Si stacked (b) FeSi2 target

(600 oC, 5min

annealing in N2)

0

1

2

3

4

5

6

7

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Photon energy (eV)

Eg=0.8eV

×

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Photon energy (eV)

0

1

2

3

4

5

6

7

(600 oC, 5min

annealing in N2)

Eg=0.8eV

×

Fe/Si stacked FeSi2 target

(a) (b)


38

3.3 X-ray diffraction pattern of -FeSi2

Figure 3.5 shows θ-2θ x-ray diffraction (XRD) pattern of the sample

annealed at 600 oC. Strong peaks related to -FeSi2 can be observed.

Figure 3.5 X-ray diffraction pattern of -FeSi2

3.4 Conclusion

In order to achieve the fine control of Fe and Si composition ratio, we

propose multi-sputtering process. The formation of -FeSi2 formed by our

process was confirmed by FT-IR and XRD measurement. From FT-IR result

of the films formed by Fe/Si stacking and FeSi2 target, absorption edge of 0.8

eV was obtained. And also, from XRD, strong peaks related to -FeSi2 were

observed. Therefore, we can conclude that -FeSi2 can be formed with our

multi-sputtering process.

2θ (deg)

10 20 30 40 50 60 70 80 90 100

(600℃ 5min

annealing in N2)

Inte

nsity (

a.u

) FeS

i 2(2

02)/

(220)

FeS

i 2 (4

22)

FeS

i 2 (004)/

(040)

FeS

i 2 (133)

orthorhombus


39

References

[3.1] Z. X. Liu, M. Watanabe, M. Hanabusa, “Electrical and photovoltaic

properties of iron-silicide/silicon heterostructure formed by pulsed laser

deposition”, Thin Solid Films, 381, 262, (2001)

[3.2] Y. Fukuzawa, T. Ootsuka, N. Otogawa, H, Abe, Y. Nakayama, Y. Makita,

Proc. SPIE, 6197, 61970N (2006)

[3.3] M. Shaban, K. Nomoto, S. Izumi, T. Yoshitake, IEEE Electron Device

Lett, 31, 1428 (2010)

[3.4] M. Suzuno, K. Akutsu, H. Kawakami, K. Akiyama, T. Suemasu,

“Metalorganic chemical vapor deposition of -FeSi2 on -FeSi2 seed crystals

formed on Si substrates”, Thin Solid Fioms, 519, 24, 8473-8476 (2011)

[3.5] D.H. Tassis, C.L. Mitsas, T.T. Zorba, M. Angelekeris, C.A. Dimitriadis,

O. Valassiades, Di.I. Siapkas, G. Kiriakidis, Semicond. “Optical and

electrical characterization of high quality -FeSi2 thin films grown by solid

phase epitaxy”, Applied Surface Science, 102, 178-183 (1996)

[3.6] M. Sugiyama, Y. Maeda, “Microstructure characterization of ion-beam

synthesized -FeSi2 phase by transmission electron microscopy”, Thin Solid

Films, 381, 256 (2001)

[3.7] M. Tanaka, Y. Kumagaya, T. Suemasu, F. Hasegawa, “Formation of

-FeSi2 Layers on Si(001) Substrates”, Jpn. J. Appl. Phys, 36, 3620-3624

(1997)

[3.8] D. Tan, C.T. Chua, G.K. Dalapati, D.Chi, “Effect of Al incorporation on

the crystallization kinetics of amorphous FeSi2 into poly -FeSi film on


40

SiO2/Si(100) substrate”, Thin Solid Films, 98, 013507 (2011)

[3.9] T. Suemasu, K. Takakura, C. Li, Y. Ozawa, Y. Kumagai, F. Hasegawa,

“Epitaxial growth of semiconducting -FeSi2 and its application to

light-emitting diodes”, Thin Solid Films, 461, 209-218 (2004)

[3.10] N. Seki, K. Takakura, T. Suemasu, F. Hasegawa, “Conduction type and

defect levels of -FeSi2 films grown by MBE with different Si/Fe ratios”,

Materials Science in Semiconductor Processing, 6, 5-6, 307-309 (2003)

[3.11] F.X. Zhang, S. Saxena, “Phase stability and thermal expansion

property of FeSi2”, Scripta Materialia, 54, 1375-1377 (2006)

[3.12] L. Wang, C. Lin, X. Chen, S. Zou, L. Qin, H. Shi, W.Z, Shen, M. Osling,

“A clarification of optical transition of -FeSi2 film”, Solid State

Communications, 97, 5, 385-388 (1996)


41

Chapter 4

Electrical characteristics of -FeSi2

4.1 Introduction

4.2 Resistivity measurement by four-point method

4.3 Resistivity measurement by TLM

4.4 Carrier density measurement by van der Pauw

4.5 Conclusion

References


42

4.1 Introduction

One of the issues of -FeSi2 is that sufficient depletion width cannot be

formed because carrier density of -FeSi2 is too high [4.1]. Moreover, the

origin of carriers has not yet been clarified. In this chapter, we discuss the

origin of carriers.

The energy level of the carriers and its concentration can be extracted by

measuring the temperature dependent resistivity. The resistivity and

carrier density n are given by

ρ =1

𝑞𝑛𝜇 (4.1)

n = √𝑁𝑐𝑁𝑑𝑒−

𝐸𝑐−𝐸𝑑2𝑘𝑇 = √𝑁𝑐𝑁𝑑𝑒

−𝐸𝑎

2𝑘𝑇 (4.2)

where q is electronic charge, m mobility, Nc effective density of state of

conduction band, Nd donor impurity density, k Boltzmann’s constant, T

absolute temperature, and Ea activation energy.

Therefore, ln is expressed by

𝑙𝑛𝜌 =𝑞𝐸𝑎

2𝑘

1

𝑇− (𝑙𝑛𝑞 +

1

2𝑙𝑛𝑁𝑐𝑁𝑑 + 𝑙𝑛𝜇) (4.3)

Figure 4.1 shows an example of resistivity on inverse absolute

temperature (Arrhenius’ plot). The slope of the resistivity corresponds to the

activation energy and the intercept to y-axis is the relative concentration of

the level. Therefore, we can speculate the origin of defect and its

concentration by changing the composition or annealing condition.


43

Figure 4.1 Extraction of defect level position and concentration

4.2 Resistivity measurement by four-point method

4.2.1 Resistivity control by Si/Fe composition ratio

Table 4.1 shows prepared samples whose thickness of Si layer was

changed from Fe-rich condition to Si-rich to confirm compositional control of

-FeSi2. The film resistivity was measured by four point probe method.

10-1

1

10R

esis

tivity (

･cm

)

1000/T (K-1)102 4 6 8 12 14 16 18 20 22

1

2

Arrhenius’ plot

slope : activation energy

intercept : relative defect concentration (under constant m)


44

Table 4.1 Compositional control of -FeSi2

Figure 4.2 shows resistivity control of the films after annealing at 800 oC.

By changing the composition ratio, it was confirmed that the resistivity is

dependent on composition ratio. When Fe and Si ratio is 1 to 2.25, a largest

resistivity of -FeSi2 has been obtained.

Figure 4.2 Resistivity dependence of -FeSi2 on Si/Fe composition ratio

sampleFe thickness

(nm)

Si thickness

(nm)

sets

(number of)

Si/Fe

composition

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

Assumed Si/Fe atomic ratio1.0 1.5 2.0 2.5 3.0

Re

sis

tivity(･cm

)

10-1

1

1：1.50

1：2.00

1：2.25

1：2.50

A

B

C

D

(800℃ 5min

annealing in F.G.)


45

4.2.2 Extraction of defect level position

The energy level of the carriers and its concentration can be extracted by

measuring the temperature dependent resistivity. Figure 4.3 shows

resistivity change of -FeSi2 film with composition ratio of 2.25, annealed at

800 oC. By fitting resistivity change on temperature, we can extract 4 levels;

shallow level of 13 and 51 meV and 0.11 and 0.2 eV.

Figure 4.3 Resistivity dependence of -FeSi2 on temperature

Theoretical calculation has revealed that the lowest position of the

defective levels of Si vacancy exist ~ 0.2 eV higher from the top of the valence

band, while the highest position of the defective levels of Fe vacancy exist ~

0.3 eV lower from the bottom of the conduction band [4.2]. On the other hand,

the formation energy is given by [4.3, 4.4]

Δ𝐸𝑓𝑜𝑟𝑚𝑎𝑡𝑖𝑜𝑛 = [𝐸𝑇(𝑑𝑒𝑓𝑒𝑐𝑡𝑖𝑣𝑒) + 𝜇𝑥] − 𝐸𝑇(𝑝𝑒𝑟𝑓𝑒𝑐𝑡) (4.4)

where ET (defective) is total energy of the defective unit cell with a vacancy of

Si or Fe, mx (x=Si, Fe) is the chemical potential of Si or Fe, ET (perfect) is the

total energy of the perfect crystal. The chemical potentials can be varied

within a range limited by the three constraints:

1000/T (K-1)102 4 6 8 12 14 16 18 20 22

10-1

1

10

Resis

tivity (

･cm

)

Ea2 = 110 meV

Ea3 = 51 meV

Ea4 = 13 meV

Ea1 = 200 meV

Fe : Si = 1 : 2.25

800 oC, 5min annealing in F.G.

Ea4

Ec

EvEa1 Ea2 Ea3

defect activation energy

Ea1 200 meV

Ea2 110 meV

Ea3 51 meV

Ea4 13 meV


46

𝜇𝑆𝑖 ≤ 𝜇𝑆𝑖(𝑏𝑢𝑙𝑘) (4.5)

𝜇𝐹𝑒 ≤ 𝜇𝐹𝑒(𝑏𝑢𝑙𝑘) (4.6)

2𝜇𝑆𝑖 + 𝜇𝐹𝑒 = 𝜇𝐹𝑒𝑆𝑖2(𝑏𝑢𝑙𝑘) (4.7)

where 𝜇𝐹𝑒𝑆𝑖2(bulk), the chemical potential of the bulk FeSi2, is a constant

value calculated as the total energy per FeSi2 unit formula.

The formation energies were calculated under the two extreme

conditions, the Si-rich limit [𝜇𝐹𝑒 = 𝜇𝐹𝑒𝑆𝑖2(𝑏𝑢𝑙𝑘) − 2𝜇𝑆𝑖(𝑏𝑢𝑙𝑘) 𝑎𝑛𝑑 𝜇𝑆𝑖 = 𝜇𝑆𝑖(𝑏𝑢𝑙𝑘)]

and Fe-rich limit [𝜇𝑆𝑖 = 1/2(𝜇𝐹𝑒𝑆𝑖2(𝑏𝑢𝑙𝑘) − 𝜇𝐹𝑒(𝑏𝑢𝑙𝑘) 𝑎𝑛𝑑 𝜇𝐹𝑒 = 𝜇𝐹𝑒(𝑏𝑢𝑙𝑘))] . The

formation energies of the native point defects depend largely on the atomic

chemical potentials of Fe and Si. The silicon vacancy exhibits the lowest

formation energy at the Fe-rich limit and the iron vacancy exhibits the

lowest formation energy at the Si-rich limit. At the Si-rich limit, the

calculated Δ𝐸𝑓𝑜𝑟𝑚𝑎𝑡𝑖𝑜𝑛 values are 2.258, 2.249, 3.133 and 2.180 eV for

removing SiⅠ, SiⅡ, FeⅠ, and FeⅡ from the perfect crystal, respectively. At the

Fe-rich limit, the calculated Δ𝐸𝑓𝑜𝑟𝑚𝑎𝑡𝑖𝑜𝑛 values are 1.172, 1.163, 5.306 and

4.353 eV for removing SiⅠ, SiⅡ, FeⅠand FeⅡ from the perfect crystal,

respectively. The formation energies of Si vacancy in -FeSi2 are smaller

than those of Fe vacancy except near the region of the Si-rich limit, which

implies the formation of Si vacancy in -FeSi2 can be more favorable than the

formation of Fe vacancy. Therefore, the observed defect level of 0.2 eV can be

thought to be originated from Si vacancy.

4.2.3 Extraction of defect concentration

Figure 4.4 shows Arrhenius’s plot dependency of -FeSi2 on annealing

temperature. As the annealing temperature increases, the intercept of

Arrhenius’s plot increases keeping its slope constant. This indicates that the

concentration of each defect reduces by annealing process. However, the

increased amounts of intercept vary by each level.


47

Figure 4.4 Change of Arrhenius’s plot by annealing

Indeed, Figure 4.5 shows changes of each defect concentration. The

defect level of 0.2 eV and 0.11 eV showed slight reduction by 23 % with

higher annealing temperature, while shallow level showed large reduction

by 63 and 96 % for 51 and 13 meV, respectively. As all the films have the

same composition ratio, the first two levels are estimated to be related to

composition of -FeSi2. On the other hand, the shallow levels can be thought

to be related to crystalline defects, which can be recovered by higher

temperature annealing.

2 4 6 8 10 12 14 16 18 20 22

1000/T (K-1)

10-1

1

10

102

103

Re

sis

tivity (

･cm

)

Ea2 = 110 meV

Ea3 = 51 meV

Ea4 = 13 meV

Ea1 = 200 meV

High

Low

annealing

temp

Fe : Si = 1 : 2.25


48

Figure 4.5 Reduction of defect concentration by annealing

4.2.4 Effects of sputtering atmosphere

To extract the effect of sputtering atmosphere, we deposit -FeSi2 films

in various atmosphere; (i) Ar (40 sccm), (ii) Ar (40sccm) + O2 (0.17 sccm), (iii)

Ar + N2 (0.15 sccm), (iv) Kr (7 sccm). Figure 4.6 shows oxygen, nitrogen and

krypton effects on resistivity of -FeSi2. To introduce either oxygen or

nitrogen causes the resistivity of -FeSi2 to be decreased by two orders of

magnitude. These results suggest that residual oxygen or nitrogen species

are one possible cause of high carrier density of -FeSi2. On the other hand,

the resistivity of -FeSi2 showed two orders of magnitude higher values by

changing sputtering gas from Ar to Kr. As for Si, it is reported that

sputtering rate of Kr is lower than Ar [4.5]. Moreover, recoil particle is

provided and dense film is formed by Kr sputtering [4.6]. It could be one of

the key to reduce residual gas in the film for low carrier density of -FeSi2.

750 800 825 8500

0.2

0.4

0.6

0.8

1

Re

lative

de

fect co

nce

ntr

atio

n

Annealing temperature (oC)

a1a2a3a4

-23%

-63%

-96%


49

Figure 4.6 Sputtering gas effects on resistivity of -FeSi2

4.3 Resistivity measurement by TLM

In this section, the resistivity of -FeSi2 films was measured by TLM

method. Figure 4.7 shows the total resistance RT at 2 V as a function of

contact spacing d. A linear approximation of the measured resistance (black

line) is then extrapolated. From the slope and intercepts to y-axis of this

line, sheet resistance Rsh (≈ 3.7×105 /sq.) and contact resistivity c (≈ 63.5

･cm) were extracted. This result of sheet resistance matches that measured

by previous four-point method. Therefore, contact resistivity can be ignored

in the result measured by four-point method.

Ar only Ar + O2 Ar + N2 Kr

1

10-1

10-2

10-3

10-4

10

102

Re

sis

tivity (

･cm

)

Sputtering atmosphere

800 oC, 5min

annealing in F.G.


50

Figure 4.7 Total resistance as function of contact spacing

4.4 Carrier density measurement by van der Pauw

In this section, the carrier density and resistivity of the -FeSi2 films were

evaluated by Hall measurement using the van der Paw method. The applied

magnetic field was 0.3 T. Figure 4.8 shows typical hall voltage. The hall

coefficient RH is defined as [4.7]

𝑅𝐻 =𝑑𝑉𝐻

𝐵𝐼 (4.8)

We measured hall voltage by changing the direction of the magnetic field

and calculated hall coefficient. From hall coefficient, it is confirmed that the

formed -FeSi2 is n-type and the carrier density of -FeSi2 is 7.66×1018 cm-2.

-FeSi2

d

20 40 60 80 10000

2

4

6

10

12

14

16To

tal r

esis

tan

ce @

2V

(

)

Contact spacing (mm)

800 oC, 5min annealing in F.G.

Thickness : 80 nm


51

Figure 4.8 Typical I-V characteristics for carrier density measurement using

van der Pauw

By using same patterned sample, we can measure resistivity too. Figure

4.9 shows typical I-V characteristics for resistivity measurement. Current

enters the sample through two adjacent terminals and the voltage is

measured across the other two adjacent terminals. Resistivity of 3.45 ･cm

was extracted by measuring I-V characteristics for four times as it was

mentioned in chapter 2.

0 1 2 3-1-2-3Current (mA)

0

2

4

6

8

-2

-4

-6

-8

Hal

l vo

ltag

e (m

V)

800 oC, 5minannealing in F.G.

Magnetic field : 0.3T


52

Figure 4.9 Typical I-V characteristics for resistivity measurement using van

der paw

The mobility m is defined by

μ =|𝑅𝐻|

𝜌 (4.9)

From above results, electron mobility of -FeSi2 is 23.6 cm2/Vs.

Figure 4.10 shows resistivity of -FeSi2 dependency on film thickness.

Resistivity ranges from 3.45 to 16.8 ･cm in all samples. Figure 4.11 shows

carrier density and mobility of -FeSi2 dependency on film thickness. Carrier

density is reduced with thicker film thickness, while mobility is increased.

0 1 2 3 4 5-1-2-3-4-5-6

-4

-2

0

2

4

6

Current (mA)

Volta

ge (

V)

800 oC, 5minannealing in F.G.

×10-1


53

Figure 4.10 Resistivity of -FeSi2 dependence on film thickness

Figure 4.11 Carrier density and mobility of -FeSi2 dependence on film

thickness

0 100 200 300 400 500 600

Film thickness (nm)

0

4

8

12

16

Res

isti

vity

(

･cm

)

80 nm

300 nm

500 nm

0 100 200 300 400 500 600

Film thickness (nm)

1017

1018

1019

Car

rier

den

sity

(cm

-2)

20

25

30

35

40

Mo

bili

ty (

cm2/V

s)

80 nm

300 nm

500 nm

mobility

carrier density


54

4.5 Conclusion

In order to confirm compositional control of -FeSi2, we changed the

thickness of Si and Fe layer and measured the film resistivity by four point

probe method. It is revealed that the resistivity is dependent on composition

ratio. With Si and Fe ratio of 2.25, a largest resistivity of -FeSi2 has been

obtained. From temperature dependency, four kinds of defect levels have

been extracted. Moreover, it is detected that they can be categorized into two

types; one related to composition of -FeSi2 and the other related to

crystalline defects. For sputtering atmosphere, oxygen and nitrogen caused

decrease of resistivity of -FeSi2. Therefore, to control the composition ratio

and reduce residual gases in the film are the key to achieve low carrier

density of -FeSi2.

The resistivity of -FeSi2 was measured by not only four-point method

but also TLM method. The results measured by four-point method and TLM

match. Therefore, it is no problem to discuss resistivity of films measured by

four-point method.

Carrier density and mobility were evaluated by Hall measurement using

the van der Paw method. Carrier density and mobility of -FeSi2 with 80 nm

thickness were 7.66× 1018 cm-2 and 23.6 cm2/Vs respectively. For film

thickness dependency, carrier density is decreased and mobility is increased

with thicker thickness.


55

Reference

[4.1] K. Takakura, T. Suemasu, Y. Ikura, F. Hasegawa, “Control of the

control type of nondoped high mobility b-FeSi2 films grown from Si/Fe

multilayers by change of Si/Fe ratios, Jpn J. Appl. Phys, 309, 233-236 (2000)

[4.2] J. Tani, H. Kido, “First-principle study of native point defects in

-FeSi2”, Journal of Alloys and Compounds, 352 153-157 (2003)

[4.3] S.B. Zhang, J.E. Northrup, “Chemical potential dependence of defect

formation energies in GaAs: Application to Ga self-diffusion, Phys. Rev. Lett,

67, 2339 (1991)

[4.4] D.B. Laks, V. Meregalli, “Theory of FeSi2 direct gap semiconductor on

Si(100)”, Phys. Rev. B, 45,10965 (1992)

[4.5] R. Ohki, Y. Hoshi, “Properties of ITO thin films sputter-deposited by

using various kinds of rare gases”, Technical report of IEICE, CPM 99-91

(1999)

[4.6] J.A. Thornton, D. W. Hoffman, “ Internal stresses in amorophous silicon

films deposited by cylindrical magnetron sputtering using Ne, Ar, Kr, Xe, and

Ar+H2”, J. Vac. Sci. Technol., 18, 203 (1981)

[4.7] D.K. Schroder, “SEMICONDUCTOR MATERIAL AND DEVICE

CHARACTERIZATION Third Edition”, 466-471 (2006)


56

Chapter 5

Demonstration of -FeSi2

Schottky solar cell

5.1 Introduction

5.2 I-V characteristics of -FeSi2/p+-Si Schottky solar cell

5.3 Equivalent circuit

5.4 Conclusion

Reference


57

5.1 Introduction

As described in chapter 1, -FeSi2 has been attracted as solar cell

material. The highest efficiency of -FeSi2 is 3.7 % under AM 1.5, 100

mW/cm2 illumination so far [5.1]. The open circuit voltage was 0.45 V, short

circuit current density is 14.8 mA/cm2, and fill factor is 0.55. However, it is

-FeSi2/Si heterojunction solar cell; fabricated on p-type silicon substrate. An

expanse of depletion layer of -FeSi2 is much smaller than that of Si because

the carrier density of Si and -FeSi2 are ~1015 cm-3 and ~1018 cm-3

respectively. In other words, light response of this sample could be caused by

Si rather than -FeSi2.

We fabricated Schottky solar cell to confirm light response of -FeSi2

itself. In this study, we used p+-Si whose impurity concentration is ~1018 cm-3

instead of metal. An activation energy of Si is ~20 meV. On the other, an

activation energy of -FeSi2 is ~ 200 meV. Therefore, if -FeSi2/p+-Si Schottky

solar cell sample is cooled, the carrier density of -FeSi2 decreases and that

of Si is not changed.

Figure 5.1 Schematic device structure of -FeSi2/p+-Si schottky solar cell

p+-Si

SiO2 SiO2

Al

-FeSi2

Schottky


58

5.2 J-V characteristics of -FeSi2/Si Schottky solar cell

For 200×200 mm2 without any surface protection or anti-reflection

coating, Figure 5.2 compares the J-V characteristics of -FeSi2/p+-Si Schottky

solar cell under dark and light conditions at 90 K. Not only the forward

current but also reverse current were increased by light illumination. This

could be because band-to-band tunneling probability of carriers was

increased by light excitation. The open-circuit voltage Voc and short-circuit

current density Jsc were not verified. Therefore, we cannot confirm power

generation of -FeSi2/p+-Si solar cell.

Figure 5.2 J-V characteristics of -FeSi2/p+-Si Schottky solar cell

0 0.2-0.2-0.4-0.6-0.8-1.0

Voltage (V)

0

-40

-30

-20

-10

Cu

rrent density (

mA

/cm

2)

light

dark

800 oC, 5min

annealing in F.G.

90 K


59

5.3 Equivalent circuit of solar cell

The J-V characteristic of an illustrated solar cell that behaves as the

ideal diode is described by [5.2]

J = 𝐽0 [𝑒𝑥𝑝 (𝑞𝑉

𝑘𝑇) − 1] − 𝐽𝑝ℎ (5.1)

This behavior can be described by a simple equivalent circuit, in which a

diode and a current source are connected in parallel. The firs term in Eq.

(5.1) describes the dark diode current density and the second term describes

the photo-generated current density. In practice, the FF is influenced by the

shunt resistance, Rp, of a solar cell. The leakage current which is

characterized by the shunt resistance causes the voltage drop. The influence

of this parameter on the J-V characteristic of the solar cell can be studied

using the equivalent circuit presented in Figure 5.3. The J-V characteristic of

the one-diode equivalent circuit with the shunt resistance is described by

J = 𝐽0 [𝑒𝑥𝑝 (𝑞𝑉

𝑘𝑇− 1)] +

𝑉

𝑅𝑝− 𝐽𝑝ℎ (5.2)

Figure 5.3 The equivalent circuit of a solar cell with shunt resistance Rp

V

+

-A

Iph Rp


60

In order to testify the light response of -FeSi2 thin film, we subtracted

the value of shunt resistance Rp from the measured current. That is

𝐽𝑖𝑑𝑒𝑎𝑙 = 𝐽𝑡𝑜𝑎𝑙 −𝑉

𝑅𝑝 (5.3)

where Jtotal is measured current.

Figure 5.4 shows subtracted current density characteristic. This result

revealed that -FeSi2 film has the ability of solar energy conversion. The

leakage current which passes the shunt resistance is caused by the current

through local defects in junction. Figure 5.5 shows shunt resistance

dependency on measurement temperature. The shunt resistance showed

reduction with lower measurement temperature. This indicates that it has

semiconductor nature. However, conversion efficiency is much smaller than

theoretical efficiency. In a practical solar cell, the series resistance Rs is

included in addition to shunt resistance. Figure 5.6 shows the equivalent

circuit including the series resistance. The series resistance is introduced by

the resistance of the main current path through which the photo-generated

carriers arrive to the external circuit. The contribution to the series

resistance comes from the bulk resistance of the junction, the contact

resistance between the junction and electrodes, the resistance of the

electrodes. The four defect levels extracted in chapter 4 could work as

efficient trap centers for photo-generation carriers [5.3, 5.4]. This results in

high series resistance and large leakage current. Therefore, it could be the

key to reduce the defect concentration for high conversion efficiency.


61

Figure 5.4 J-V characteristic of -FeSi2/p+-Si Schottky solar cell. In this

result, we subtracted the current which passes the shunt resistance.

Figure 5.5 Shunt resistance dependence on temperature

0-10-20-30-40-50

Voltage (mV)-60 10

0

2

4

6

8

-2

-4

-6

-8

-10

Cu

rre

nt d

en

sity (m

A/c

m2)

light

Voc Jsc

V

+

-A

IphRp

ideal solar cell

0 100 200 300

40K

90K

300K

104

105

106

107

Temperature (K)

Shu

nt

resi

stan

ce (

)


62

Figure 5.6 The equivalent circuit of a solar cell with shunt resistance Rp and

series resistance Rs

5.4 Conclusion

We measured J-V characteristic of -FeSi2/p+-Si Schottky solar cell at 90

K to confirm light response of -FeSi2 film. However, the open-circuit voltage

and the short-circuit current can be rarely obtained. In order to testify the

solar energy conversion ability of -FeSi2 thin film, we consider the J-V

characteristic of ideal solar cell. The open-circuit voltage and the

short-circuit current is slightly obtained. Therefore, it is revealed that

-FeSi2 has solar energy conversion ability. However, the value is much

smaller than theoretical value. It is essential to increase the shunt and to

reduce the series resistance for higher performance of -FeSi2.

V

+

-A

Iph Rp

Rs


63

Reference

[5.1] Z. Liu, S. Wang, N. Otogawa, Y. Suzuki, M. Osamura, Y. Fukuzawa, T.

Ootsuka, Y. Nakayama, H. Tanoue, Y. Makita, “A thin-film solar cell of

high-quality -FeSi2/Si heterojunction prepared by sputtering”, Solar Energy

Materials & Solar Cells, 90, 276-282 (2006)

[5.2] S.M. Sze, Kwok K. Ng, “Physics of Semiconductor Devices Third

Edition”, 719-736, (2007)

[5.3] K. Wunstel, P. Wagner, “IRON-RELATED DEEP LEVELS IN

SILICON”, Solid State Communications, 40, 797-799 (1981)

[5.4] T. Suemasu, T. Fujii, K. Takakura, F. Hasegawa, “Dependence of

photoluminescence from -FeSi2 and induced deep levels in Si on the size of

-FeSi2 balls embedded in Si crystals”, Thin Solid Films, 381, 209-213 (2001)


64

Chapter 6

Conclusion


65

In this thesis, we studied semiconductor silicides for high efficiency thin

film photovoltaic devices. In this chapter, the studies are summarized below.

In chapter 1, maximum conversion efficiency of BaSi2/-FeSi2 tandem

silicide solar cell is calculated. It ideally achieves 40 %. However, carrier

density of semiconductor silicides is too high to be used for photovoltaic

devices.

In chapter 3, multi-sputtering process to form -FeSi2 is investigated in

order to control composition ratio. Absorption edge of 0.8 is obtained in the

film formed by our Fe/Si stacked process same as one formed by FeSi2 target

sputtering. And -FeSi2 crystalline phase can be detected. Therefore, -FeSi2

with bandgap of 0.8 eV can be formed by our multi-sputtering process.

In chapter 4, electrical characteristics of -FeSi2 are investigated. It is

confirmed that resistivity of -FeSi2 depends on Fe/Si composition ratio. In

our case, Si-rich condition with Si /Fe ratio of 2.25 has shown the largest

resistivity of ~0.6 cm. From temperature dependency, two types of defect

levels, one related to composition of -FeSi2 and the other related to

crystalline defects have been detected. Carrier density and mobility of

-FeSi2 are 7.66×1018 cm-2 and 23.6 cm2/Vs respectively.

In chapter 5, light response of -FeSi2 has been characterized by

fabricating Schottky-type -FeSi2 solar cell. However, the open-circuit

voltage and short-circuit current cannot be obtained. And so, we subtract the

value of shunt resistance from the measured current. In the result, the

open-circuit voltage and the short-circuit current is slightly obtained.

Equivalent circuit modeling has revealed the presence of shunt resistance

which suggests semiconductor nature within the film. In the future, more

fine composition ratio control, improvement of crystalline quality and

increase of shunt resistance is required for -FeSi2 thin film photovoltaic

device.

Acknowledgements

66

Acknowledgements

First of all, I would like to express my gratitude to my supervisor Prof.

Hiroshi Iwai for his continuous encouragement and advices for my study. He

also gave me many chances to attend conferences. The experiences are

precious for my present and future life.

I also thank Prof. Yoshinori Kataoka and Associate Prof. Kuniyuki

Kakushima for many kindness supports.

I deeply thank Prof. Takeo Hattori, Prof. Kenji Natori, Prof. Kazuo

Tsusui, Prof. Prof. Hitoshi Wakabayashi, Prof. Nobuyuki Sugii, Prof. Akira

Nishiyama, and Associate Prof. Parhat Ahmet for useful advice and great

help whenever I met difficult problem.

I would like to thank Mr. Katsuaki Aoki and Mr. Akito Sasaki at Toshiba

Materials Co., LTD. for giving me chance of cooperative research.

Also, I thank research colleagues of Iwai Lab for their friendship, active

discussions and many of encouraging words. Especially, I have good

friendships with Hiroshi Oomine, Takuya Seki, Atsushi Takemasa, Shuhei

Hosoda, Ryo Yoshihara and Song Jinhan.

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

Ms. Matsumoto.

Finally, I appreciate to my parents Tetsuji and Yoshimi, my brother Yuji

and my sister Shiori. I can keep researching thanks to my family

cooperation.

Taichi Inamura

February, 2014

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A study on silicide semiconductors for high efficiency thin ...future low cost solar cells. But the problem for organic materials is the light degradation. Furthermore, conversion