Master thesis

Electrical contact characteristics of

n-type diamond with Ti, Ni, NiSi2, and Ni3P

electrodes

Department of Electronics and Applied Physics Interdisciplinary Graduate School of Science and Engineering

Tokyo Institute of Technology

January, 2014

Student ID : 12M36240

Atsushi Takemasa

Supervisor:

Professor Nobuyuki Sugii and Hiroshi Iwai

Abstract of the thesis

Semiconductor diamonds have possibilities in future semiconductor materials for

power devices. The main issue of the diamonds for the power devices is high resistance

of the metal/n-diamond contact. It was reported that this is caused by the Fermi level

pinning at the contacts. In this thesis, to solve this problem, interfacial reactions at

metal/n-diamond interfaces were investigated. Four metal electrodes, Ni, Ti, and

TiN/NiSi2, and Ni3P were investigated in this study. A diamond substrate used in this

study contains phosphorous impurities of 5 x 1019 cm-3. The electrodes have a circular

transmission line model (CTLM) pattern. A highlight of these four electrodes is Ni3P. It

was reported that the P impurities at the metal/n-Si contacts can tune Schottky barrier

height (SBH). In this prior research, it was described that P impurities form dipoles at

the interface and modulate the SBH. In this study, ohmic contacts for n-diamond were

not achieved: measured contact resistances under high bias around 9-10 V of these four

electrodes were 10-1~100 cm2 although the contact resistances of metal/n-diamond are

required to be 10-5 cm2 or less. However, a slight improvement was observed. The

Ni3P/n-diamond contact annealed at 800oC flowed larger current than others under the

low bias (<2 V) condition. TEM observation revealed that a thin graphite layer formed

at the Ni3P/n-diamond interface during annealing at 800oC. This interfacial reaction

was not observed at Ni/n-diamond interface annealed at 800oC, so it is conceivable that

P caused graphite formation. There are three possible causes increasing current under

low bias voltage at Ni3P/n-diamond annealed at 800oC, one is defects in the diamond

substrate associated with phosphorus diffusion into the diamond, second is P doping

into the diamond, and third is SBH modulation due to graphite insertion. In summary,

- 2 -

low-resistance contact formation of metal/n-diamond was investigated. The best value

in this study was still high and about four orders of magnitude higher than the required

value of 10-5 cm2. Slight improvement of current increase at Ni3P/n-diamond contact

under low bias voltage and graphite formation at the interface were newly observed. To

find out the improvement clearly, further researches are required, though in this study,

Ni3P showed the new interfacial reaction forming graphite at metal/ n-diamond

interface during annealing at 800oC and possibilities to decrease contact resistances for

n-diamond.

- 3 -

CONTENTS

Abstract of the thesis ...........................................................................................................- 2 -

Chapter 1 Introduction .........................................................................................................- 5 -

1.1 Backgrounds of the thesis .......................................................................................- 6 -

1.1.1 Characteristics of semiconductor diamond..........................................................- 6 -

1.1.2 Issue of semiconductor diamond .......................................................................- 9 -

1.1.3 Contact resistance ..........................................................................................- 11 -

1.1 Purpose of the study ............................................................................................- 14 -

References........................................................................................................................- 15 -

Chapter 2 Experiment ........................................................................................................- 16 -

2.1 Experimental process ...........................................................................................- 17 -

2.1.1 Diamond substrate used in this thesis ...............................................................- 19 -

2.1.2 Treatments for the diamond substrate...............................................................- 20 -

2.1.3 Photolithography ...........................................................................................- 21 -

2.1.4 RF magnetron-sputtering ................................................................................- 24 -

2.1.5 Lift off..........................................................................................................- 26 -

2.1.6 Thermal annealing process .............................................................................- 27 -

2.2 Measurement method ...........................................................................................- 28 -

2.2.1 I-V (Current - Voltage) measurement ...............................................................- 28 -

2.2.2 Circular Transmission Line Model (CTLM) .....................................................- 29 -

2.2.3 Multi-stacking Process for NiSi2 electrode .......................................................- 33 -

2.2.4 Ni3P electrode and the effect of dipoles ............................................................- 34 -

2.2.5 X-ray photoelectron spectroscopy (XPS) measurement......................................- 35 -

2.2.6 Surface coverage ...........................................................................................- 37 -

References........................................................................................................................- 38 -

Chapter 3 Result and disscution ..........................................................................................- 40 -

3.1 Contact resistance ................................................................................................- 41 -

3.2 I-V characteristics ................................................................................................- 42 -

3.3 Physical analysis of Ni3P/n-diamond contact...........................................................- 46 -

3.4 Discussion about low bias current at the Ni3P/n-diamond contact..............................- 50 -

3.5 Research about treatments for a diamond substrate ..................................................- 52 -

References........................................................................................................................- 54 -

Conclusion .......................................................................................................................- 54 -

Acknowledgment ..............................................................................................................- 55 -

- 4 -

Chapter 1 Introduction

- 5 -

1.1 Backgrounds of the thesis

In this section 1.1, backgrounds of this thesis are explained, what

diamond is and what problems of semiconductor diamond are.

1.1.1 Characteristics of semiconductor diamond

In general, diamonds are known as jewels and the hardest material in the

world. Although in the electrical engineers, the diamonds is known as future

materials for semiconductors. Semiconductor diamonds draw an attention as

a future semiconductor material for power devices. There are some

candidates for power devices, Si, SiC, GaN, and the diamond, whose physical

properties are shown in Table 1 [1.1].

Table 1 Properties of semiconductor materials

5.59.09.711.8Dielectric constant

202.04.91.5Thermal conductivity (W/cm-K)

1 x 1073.3 x 1062.5 x 1063.0 x 105Breakdown field (V/cm)

1,600150115600Hole mobility (cm2/V-s)

2,2009001,0001,400Electron mobility (cm2/V-s)

2.7 x 1072.7 x 1072.2 x 1071.0 x 107Saturation velocity (cm/s)

5.473.393.261.12Bandgap (eV)

DiamondGaNSiC (4H)Si

5.59.09.711.8Dielectric constant

202.04.91.5Thermal conductivity (W/cm-K)

1 x 1073.3 x 1062.5 x 1063.0 x 105Breakdown field (V/cm)

1,600150115600Hole mobility (cm2/V-s)

2,2009001,0001,400Electron mobility (cm2/V-s)

2.7 x 1072.7 x 1072.2 x 1071.0 x 107Saturation velocity (cm/s)

5.473.393.261.12Bandgap (eV)

DiamondGaNSiC (4H)Si

- 6 -

It is shown that the diamond has the wide bandgap, the high breakdown

field and the high thermal conductivity as compared with other materials.

There are figures which show how diamond is better than other materials in

power devices. First one is named Baliga figure of merit (BFOM) derived by

Baliga in 1983 [1.2], which defines material parameters to minimize the

conduction losses in power FETs. Second one is named Baliga high-frequency

figure of merit (BHFFOM) derived by Baliga in 1989 [1.2], which defines material

parameters to minimize the conduction losses in high frequency.

These FOMs are listed below.

(1)

Where, e is the electron mobility and EB is the breakdown field of a

semiconductor.

(2)

There is tab. 2 which shows these two figures of merit for each material in

Fig. 1. In Table 2, for simplicity, these two FOMs are normalized to the Si

FOMs.

3BeEBFOM

2Be EBHFFOM

- 7 -

Table 2 Figures of merit of materials in tab. 1

Si SiC GaN Diamond

BFOM 1 340 653 27,128

BHFFOM 1 50 78 1,746

According to brilliant physical properties of the diamond in table 1, two

FOMs show diamond’s superiority in power devices. Although, the diamond

has high potential electrical properties for high-voltage or high frequency

applications, as shown above, there is very difficult problem in application

for diamond devices. This is shown in the next section.

- 8 -

1.1.2 Issue of semiconductor diamond

In the section 1.1.1, it was shown that the semiconductor diamond has

possibilities in power devices, but the diamond has an issue in contact

resistance. To fabricate electrical devices, low ohmic contact is necessary. It

was achieved low ohmic contact of 10-5 cm2 for boron-doped p-type diamond,

but ohmic contact for phosphorus-doped n-type diamond with impurity

concentration of ~1020 cm-3 has not achieved yet, which is Shottky contact of

10-3 cm2 [1.3]. It is reported that this high resistance is caused by Fermi

level pinning at metal/n-diamond contact, which formed a Schottky barrier

height of ~4.3eV [1.4]. The Fermi level pinning causes the situation that

Schottky barrier height cannot be controlled by the metal work function, as

shown in Fig 1.1. The band diagram of metal/n-type diamond contact is

shown in Fig 1.2.

- 9 -

Figure 1.1 Schottky barrier height as a function of metal work fu

P-doped diamond

nction for

Figure 1.2 Band diagram of metal/n-type diamond contact

Schottky barrier height~4.3eV

- 10 -

Band gap 5.5eV

n-type diamondMetal

EC

EV

EF

~0.6 eV

1.1.3 Contact resistance

In this section, it is described about contact resistance. The band diagram

of metal/n-type semiconductor at forward bias is shown in Fig 1.3. All the

abbreviations were referred by [1.5].

Figure 1.3 Schematic energy-band diagrams under forward bias

Where qBn is Schottky barrier height, qbi is Built-in potential, Vapp is

the applied voltage.

Schottky barrier height is defined as

n-type semiconductor

Metal

mBn qq . (3)

EC

EF

EF

qBn

qbi-qV

qVapp

app

- 11 -

Where qm is the metal work function and q is the electron affinity of

semiconductors. If you would like to think about the case at reverse bias, you

should put minus values into Vapp.

There are two types of contact, ohmic and Schottky contacts, when metal

and semiconductors are contacted. For ohmic contact, the current density

varies as

00

expE

VqJ appBn

ohmic

, (4)

where

s

d

m

NqhE

*400 . (5)

Where h is Planck’s constant, Nd is the donor impurity density, m* is the

electron effective mass, and s is the semiconductor permittivity. At ohmic

contact, contact resistance is defined as

1

0

appVapp

c V

J . (6)

- 12 -

The contact resistance is defined only in ohmic contacts, but in order to

discuss how high the experimental results are, and how far the one is from

prior research and required values, the contact resistance value under a

specific bias condition is used for the Schottky contact in this study by TLM

method which is described later.

- 13 -

1.1 Purpose of the study

As described in the section 1.1, the diamond draws an attention as a

future semiconductor material for power devices, but high contact resistance

of the metal/n-diamond contact makes it difficult to be used in the practical

application. Therefore, the purpose of this study is to search for ways which

solve this issue, discussing the metal/n-diamond interfaces, especially

Ni3P/n-diamond. Besides, the diamond has been researched for long time,

but researches as experimental process for diamond aren’t enough. To widely

discuss experimental process of diamond is needed, so we discussed

treatment process for the diamond in this thesis.

- 14 -

References

[1.1] S. Takashi, “SiC Power Devices,” pp. 49–53

[1.2] B. J. Baliga, “Power semiconductor device figure of merit for

high-frequency applications”, IEEE Electron Device Lett., vol. 10, no.

10, pp. 455_457, 1989

[1.3] H. Kato, et al., Appl. Phys. Lett. 93, 202103 (2008)

[1.4] M. Suzuki, Phys. Stat. sol. (a) 203, No. 12 (2006)

[1.5] Yuan Taur and Tak H. Ning (2009) Modern VLSI Devices,

The U.S.A: Cambridge University Press

- 15 -

Chapter 2 Experiment

- 16 -

2.1 Experimental process

In this section, an experimental process of this thesis is described. The

main stream of this process is in Fig 2.1. We used four types of metal

electrodes: Ti, Ni, TiN/NiSi2, and Ni3P with thickness of 50, 50, 50/38, and

50nm, respectively. These electrodes were deposited with an rf sputter on a

diamond substrate treated with a hot mixture of H2SO4 and HNO3, the ratio

of H2SO4 and HNO3 is three to one. NiSi2 was formed by amorphous Si /Ni

layers of 1.9/0.5nm thick which turned into NiSi2 during annealing at 500oC

[2.1]. The substrate contains phosphorous impurities of 5 x 1019 cm-3. The

electrodes have circular-transmission-line model (CTLM) patterns [2.2]

formed through the lift off process. After that, these samples were annealed

at a variety of temperatures in N2 atmosphere for 1 min. And then, current

voltage characteristics were measured. In order to reveal whether the Felmi

level pinning at the interface is relaxed or not, contact resistance under high

voltage around 9-10V was evaluated with the CTLM method [2.2].

- 17 -

A diamond substrate with phosphorus concentration of 5 x 1019 cm-3

Hot H2SO4 and HNO3 (3:1) treatment

Photoresist coating and photolithography

Metal deposition with RF sputtering

(Ti, Ni, NiSi2, Ni3P)

Forming electrodes on a pattern of Circular Transmission Line Model (CTLM) by lift off process

Annealing in N2 atmosphere at a variety of temperature

Measuring current-voltage characteristics and calculating contact resistance

Circular TLM pattern

metal

n-diamond

- 18 -

Figure 2.1 the experimental process of this study

19 -

2.1.1 Diamond substrate used in this thesis

The diamond substrate used in this study is n-type diamond with

phosphorus doping density of 5 x 1019 cm-3, which was grown on the synthetic

Ib diamond substrate by High Pressure High Temperature (HPHT) method

at National Institute of Advanced Industrial Science and Technology (AIST).

Carbon source of CVD was CH4 and Phosphorus source was PH3 [2.3]-[2.5].

This substrate has a doped layer with thickness of 1.1 m on top its crystal

orientation is (111) and the area of 2 x 2 mm. This substrate is depicted in

Fig 2.2.

Figure 2.2 the diamond substrate used in this study

Ib (111) Diamond

Phosphorus doping density of 5 x 1019 cm-3

2.0 mm1.1 m

-

2.1.2 Treatments for the diamond substrate

The semiconductor diamond has a unique treatment process different

from this conventional semiconductor like Si. In general, for semiconductors

such as Si, H2O2 and HSO4 (SPM) treatment is applied, but, for diamond, hot

H2SO4 and HNO3 (3:1) (mixed acid) treatment is used. This is because

oxidation on diamond substrate is necessary to fabricate electrodes with high

adhesion strength correctly and forming patterns [2.6]. In section 3.3, results

about treatments for diamond substrate is shown it is found out difference

about oxygen on the diamond substrate between SPM and hot mixed acid

treatments.

- 20 -

2.1.3 Photolithography

Photolithography is a process used in microfabrication to selectively

remove parts of a thin film or bulk of a substrate. This uses light to transfer

a geometric pattern from a photomask to a light-sensitive chemical

“photoresist”, or simply “resist”, on the substrate. A series of chemical

treatment then either engraves the exposure pattern on, create extremely

small patterns (down to a few tens of nanometers in size), it affords exact

control over the shape and size of the objects it creates.

In this thesis, the photolithography was used as a method to make

pattern of electrodes. The apparatus is MJB4 of Karl Süss contact-type mask

aligner. The substrates were coated with thicker or thinner positive type

photoresists were baked at 115oC for 5 minutes on electrical hotplate. After

that, spin-coated photoresist layers were exposed through e-beam patterned

hard-mask with high-intensity ultraviolet (UV) light with the wavelength of

405nm. The exposure duration was set respectively to 4 sec and 10 sec for

thinner photoresist and thicker one. And then, exposed warfers were

developed by the specified tetra-methyl-ammonium-hydroxide (TMAH)

developer named NMD-3 (Tokyo Ohka Co. Ltd). The wafers were dipped into

the solvent for 1 to 2 minute.

- 21 -

- 22 -

Figure 2.3 Process flow of photolithografhy

Photoresisit (S1805) spin-coating by 4000 rpm

Baking at 115oC for 5 min

Exposure 4 sec

Photoresisit (S1818) spin-coating by 4000 rpm

Exposure 10 sec under the mask

Development (NMD3)

- 23 -

Figure 2.4 Photo of photolithography apparatus

2.1.4 RF magnetron-sputtering

After the process of photolithogarafy, metal were deposited by radio

frequency (RF) magnetron sputtering.

Sputtering is one of the vacuum processes to deposit ultra thin film on a

substrate. A high voltage across a low-pressure gas (usually argon at about

10mTorr) is applied to create “plasma”, which consists of electrons and gas

ions in a high-energy state. Then the energized plasma ions strike “target”,

composed of the desired coating material, and cause atoms of the target to be

ejected with enough energy to reach the substrate surface.

Figure 2.5 Photo of UHV Multi Target Sputtering System ES-350SU

- 24 -

Figure 2.6 Schematic internal structure of RF sputtering system

- 25 -

26 -

2.1.5 Lift off

In lift off process as fig 2.7, photoresist pattern is formed on the substrate.

After that, metal is deposited by RF sputtering. And then, photoresist

pattern is cleaned up by acetone and creation of metal electrode is complete.

In this process, metal deposition has to be done at room temperature,

because if substrate is too warmed resist pattern can be deformed.

Figure 2.7 the diagram of lift off process

Photoresist pattern Metal deposition Removing resist

-

2.1.6 Thermal annealing process

Thermal annealing is the process to densify the electrode, increase

adhesion strength and make a chemical reaction at metal/semiconductor

interfaces. In this thesis, rapid thermal annealing (RTA) were used. The

ambient gas in furnace was evacuated adequately before annealing and N2

gas was introduced with flow rate of 1.5/min during annealing preserving the

furnace pressure at atmospheric pressure. All annealed samples were taken

out from the chamber under 100oC.

- 27 -

28 -

2.2 Measurement method

2.2.1 I-V (Current - Voltage) measurement

In order to get I-V characteristics of metal/n-type diamond,

semiconductor-parameter analyzer (HP415A, Hewlett-Packard) were used.

Figure 2.8 Schematic drawing of I-V measurement

-

2.2.2 Circular Transmission Line Model (CTLM)

Performance index of the ohmic contact is expressed in contact resistance

(C). The contact resistance can be written as

0

V

C I

V ( cm2). (6)

TLM method is often used to measure the contact resistance [2.7-2.9].

This method is considered as equivalent to the transmission line circuit

electrodes with the semiconductor layer below. The forms of the electrodes

are circular or rectangular generally used. In the method for measuring the

resistance of the rectangle electrode, current can affect the results of the

resistance measurement at electrode edge. It is necessary mesa structure to

remove the edge current. Process is complicated for that. However, the edge

does not affect if circular pattern is used, it enables the analysis more

accurate.

In this study, the circular pattern was used shown in figure 2.9, where a2 -

a1 is equal to spacing d.

- 29 -

Figure 2.9 CTLM pattern used in this thesis

The first step is measuring the characteristics between the outer and

inner electrodes. The second step is calculating the resistance using Ohm’s

law from I-V characteristics. The gap area which is between electrodes can

be written as

daaaaaaaaS 12121221

22 . (7)

Thus the area is proportional to d. Propagation length can be determined by

linear approximation of the characteristic R-d as shown in figure 2.10. There

are so -2Lt in the d-axis intercept where the line extrapolated to the zero

resistance.

- 30 -

Figure 2.10 an image of R-d characteristics

Where it is a2, a1>>Lt. Resistance R which is measured is written as

2

ln

221

2

12

a

aR

a

LR

a

LRR

SH

tSKtSK

, (8)

where RSK is a sheet resistance of semiconductors right under electrodes,

RSH is a sheet resi

presents the

stance in semiconductors except area of RSK. First and

second terms of the right side of this equation represent the resistance of the

semiconductor layer under the electrode. This indicates that resistance is

inversely proportional to the circumference, which is proportional to the

propagation length in the case of CTLM. The third term re

resistance of the semiconductor layer other than the electrode immediately

- 31 -

below. It is expressed as th

measuring the sum of the resistance which was under electrode and the

order. Assuming RSH=RSK, a R can be expressed as (9),

e resistance of expansion (8) is determined by

1

2

12

ln11

2 a

a

aaL

RR t

SH

(9)

V the voltage drop between the electrodes, and the current value I. Using

Ohm’s law, V is expressed as (10),

1

2

12

ln11

2 a

a

aaL

RIV t

SH

. (10)

Sheet resistance is obtained from this equation.

Since the sheet resistance is obtained and the propagation length can be

obtained from, the contact resistance c is expressed as

. (11)

2tSHC LR

- 32 -

2.2.3 Multi-stacking Process for NiSi2 electrode

NiSi2 electrode used in this research was formed by amorphous Si/Ni

layers of 1.9/0.5nm thick which turned into NiSi2 during annealing at 500oC,

which is called as the multi-stacking process [2.10].

NiSi

・・・

substrate substrate

NiSi2

Annealedat 500oC

The atomic ratio of Ni and Si is 1 to1

Alternately deposited Ni and Si

Formed NiSi2

Figure 2.11 Fabrication of NiSi2 with multi-stacking process

- 33 -

2.2.4 Ni3P electrode and the effect of dipoles

In a prior research, it was reported that the P impurities at the

metal/n-Si contact can tune Schottky barrier height and this was described

that P impurities form dipoles at the interface and the Schottky barrier

height is modulated [2.10].

Figure 2.12 J-V characteristics of Schottky diode

- 34 -

2.2.5 X-ray photoelectron spectroscopy (XPS)

measurement

In order to investigate widely about the contact formation process of

semiconductor diamond, it was found out what difference between SPM

treatment and hot mixed acid (H2SO4 and HNO3) are. For diamond, in order

to form patterns of electrodes, it is important to apply oxidation process to

diamond. Some oxidation processes were investigated but this mixed acid

process is generally used. However, SPM has oxidizability and is widely used

as a treatment process in semiconductors such as silicon. There is the fact

that only SPM process can’t form patterns of electrodes, but there aren’t

enough researches which show how much oxygen actually are at diamond

surface after SPM treatment, and how much difference of oxygen between

these two treatment processes. So, for a reference, the XPS investigation was

done with supports by Nohira laboratory in Tokyo City University. XPS is

one of the most effective surface analysis method of determining the

elements. XPS spectra are obtained by irradiating a material with a beam of

X-ray while simultaneously measuring the kinetic energy (Ek) and number of

electrons, which escape from the material being analyzed. The relation of

energies as follow:

bk EEh (12)

- 35 -

Where h is energy of the X-ray, Ek is the kinetic energy of the emitted

electron and Eb is binding energy of emitted electron. This determined by Ek

and h is incident X-ray energy, which is constant. Eb is observed to energy

peak, which is determined by composition of sample.

Figure 2.13 Illustration of the XPS system

- 36 -

2.2.6 Surface coverage

A surface coverage is a physical quantity which shows how many atoms

are at adsorption sites on surface of materials, and has a unit named

monolayer (ML) which finds out the ratio of ideal numbers of atoms forming

a surface to numbers of absorbed atoms on the surface, assuming the ratio of

numbers of atoms forming the surface is 1.

- 37 -

References

[2.1] Y. Tamura, et al, Abstract #2663, Honolulu PRiME (2012)

[2.2] D. K. Schroder, Semicinductor Material and Device Characterization,

3rd edition, Wiley-Interscience

[2.3] S. Koizumi, M. Kamo, Y. Sato, H. Ozaki, and T. Inuzuka, “Growth

and characterization of phosphorous doped {111} homoepitaxial

diamond thin films,” Applied Physics Letters, vol. 71, no. 8, p. 1065,

1997

[2.4] S. Koizumi, T. Teraji, and H. Kanda, “Phosphorus-doped chemical

vapor deposition of diamond,” Diamond and Related Materials, vol.

9, no. 3–6, pp. 935–940, Apr. 2000

[2.5] S.-G. Ri, H. Kato, M. Ogura, H. Watanabe, T. Makino, S. Yamasaki,

and H. Okushi, ”Electrical and optical characterization of

boron-doped (111) homoepitaxial diamond films”, Diamond and

Related Materials, vol. 14, no 11-12, pp. 1964-1968, Nov. 2005

[2.6] H. Kato, et al., Phys. stat. sol. (a) 205, No. 9, 2195-2199 (2008)

[2.7] O. F. Ring, S. For, and C. Resistance, “3OL,, at all points on the ring,

and where,” vol. 146, pp. 15–20, 1987

- 38 -

[2.8] H. B. Harrison, “Transmission Line,” no. May, pp. 111–113, 1982

[2.9] P. W. Ulrich Goesele, Pierre Laveant, Rene Scholz, Norbert Engler,

“Diffusion Engineering by Carbon in Silicon,” Materials Research

Society, vol. 35, no. 2, pp. 2–6, 1992

[2.10] Y. Tamura, et al, Abstract #2663, Honolulu PRiME (2012)

- 39 -

Chapter 3 Result and disscution

- 40 -

41 -

3.1 Contact resistance

There is a relationship between contact resistance of four electrodes and annealing

temperature as shown in Fig. 3.1. These contact resistance were evaluated under higher

voltage range around 9-10V, because no ohmic contacts formed in this investigation. In

the prior research (sec. 2.3.3 Ni3P electrode and the effect of dipoles), it was reported

that P impurities at metal/n-Si contacts can tune Schottky barrier height and this was

described that P impurities form dipoles at the interface and the Schottky barrier height

is modulated, but this behavior wasn’t confirmed in the current experiment with the

Ni3P/n-diamond contact. Furthermore, Fig. 3.1 shows contact resistance is still high

(10-1-100 cm2).

Figure 3.1 Relationship between annealing temperature and

contact resistances under high voltage (9-10 V)

10

Con

tact

res

ista

nce c

[cm

2 ]

10-1

2000

Annealing temperature T [Co]

600400 800

A diamond has P density of 5×1019cm-3

100

10-2

Ti/n-diamond

Ni/n-diamond

Ni3P/n-diamondTiN/NiSi2/n-diamond

-

42 -

3.2 I-V characteristics

In this section, I-V characteristics of Ti, Ni, NiSi2 and Ni3P/n-diamond are shown.

There are I-V characteristics of n-type diamond contact with Ti, Ni, NiSi2 and Ni3P at

various annealing temperatures in Fig. 3.2-3.6. According to these investigations, a

slight improvement of metal/n-diamond contact property was confirmed.

Figure 3.2 I-V characteristics of the Ti/n-diamond contact

-2 0

Voltage (V)

-6-10 -4-8

Cu

rren

t (

A)

12

0

8

4

as deposited

200oC

400oC600oC

P density of 5 x 1019cm-3

Ti

n-diamond

Ti

n-diamond

m

180m

-

- 43 -

Figure 3.3 I-V characteristics of the Ni/n-diamond contact

Figure 3.4 I-V characteristics of the NiSi2/n-diamond contact

-2 0

Voltage (V)

-6 -4-10 -8

Cu

rren

t |I

|(A

)12

0

8

4

as deposited

200oC

400oC600oC800oC

Ni

n-diamond

Ni

n-diamond

P density of 5 x 1019cm-3

180m

m

-2 0

Voltage (V)

-6 -4-10 -8

Cu

rren

t |I

| (A

)

12

0

8

4

as deposited

200oC

400oC600oC800oC

P density of 5 x 1019cm-3

・・n-diamond

Ti

×

N(50nm)Si(1.9nm)/Ni(0.50nm)16 layers

180m

m

- 44 -

Figure 3.5 I-V characteristics of the Ni3P/n-diamond contact

Figure 3.6 I-V characteristics of n-type diamond contacts with three electrodes annealed

at 800oC and as deposited Ni

-2 0

Voltage V (V)

-6 -4-10 -8

Cur

rent

|I| (A

)

12

0

8

4

as deposited

200oC

400oC600oC800oC

P density of 5 x 1019cm-3

Ni3P

n-diamond

-1 0

Voltage (V)

-2

180m

m

Cur

rent

|I| (A

)

0.3

0

0.2

0.1

P density of 5 x 1019cm-3

Ni/n-diamond

Ni3P/n-diamond

TiN/NiSi2/n-diamond

Ni/n-diamond as deposited

According to Figs 3.2-3.6, it was shown little improvement of electrodes having no

impurity and n-diamond contacts with thermal annealing. In fig 3.6, it was shown that

the Ni3P electrode annealed at 800oC flowed lager current than others under low bias

(<approximately 2 V) condition. Since the Ni/n-diamond contact didn’t show this

current increase, P might modify the Ni3P/n-diamond interface during annealing at

800oC and then increased current only under low bias voltage. According to these

results and discussions, modification at the interface with P could be a possible cause to

occur current increase under low bias voltage (<2 V). To investigate what happened at

the Ni3P/n-diamond interface, the transmission electron microscopy (TEM).

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3.3 Physical analysis of Ni3P/n-diamond contact

The sample of the Ni3P/n-diamond contact has been treated with hot H2SO4 and

HNO3, but pattern remained as can be seen in fig 3.7, while other sample’s patterns

were vanished with this treatment. In order to investigate what changes are at the

Ni3P/n-diamond interface, TEM images of this interface in the area where the pattern of

the electrode remained was taken.

Figure 3.7 the diamond surface of the Ni3P/n-diamond after mixed acid treatment

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- 47 -

Figure 3.8 TEM image of the diamond surface of J-V measured the Ni3P/n-diamond

after mixed acid treatment

.

a = 3.35 x 10-10 m

b = 1.42 x 10-10 m

Figure 3.9 Crystal structure of graphite

10nm

(111) diamond

10nm

RTA 800oC 1min

10nm

(111) diamond

Strain of diamond surface

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Figure 3.10 Strain of diamond surface in the TEM image

In Fig 3.8, a layered structure on top the diamond substrate surface was confirmed. The

layer spacing ( ~0.34 nm ) in Fig. 3.8 corresponded to that of graphite. This observation

indicates that Ni3P/n-diamond interface formed graphite during annealing at 800oC and

then the Ni3P/n-diamond contact turned to be the graphite/n-diamond contact as

schematically shown in Fig 3.11. It is reported that the graphite formation temperature is

1300oC or higher [3.1]. With only thermal annealing process, the formation of 800oC in

this study might be the lowest one. Further study, including the graphite formation and

possibility of low resistance, is strongly needed.

n-diamond

Ni3P

n-diamond

Ni3P

graphite

Annealedat 800oC

Ni3P/n-diamond graphite/n-diamond

Figure 3.11 Formation of graphite on top of diamond during annealing 800oC

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3.4 Discussion about low bias current at the Ni3P/n-diamond

contact

It was shown that the Ni3P/n-daimond contact during annealing at 800oC flows

larger current under low bias voltage (<approximately 2 V) than other contacts, and in

Fig 3.5, electrode after annealed at 800oC flowed larger current than at other annealing

temperatures. There are three possible causes to increase current under low bias voltage

at the Ni3P/n-diamond annealed at 800oC, one is defects in the diamond substrate

associated with phosphorus diffusion into the diamond, second is P diffusion into the

diamond, and third is Schottky barrier height (SBH) modulation due to graphite

insertion. The Ni/n-diamond contact annealed at 800oC did not show such graphite

formation which the Ni3P/n-diamond contact showed, but it should need to reveal

whether P actually diffuse into diamond or not at the Ni3P/n-diamond interface with a

physical analysis. It is plausible that P diffusion into the diamond is such a trigger of the

reaction to term graphite at surface of diamond, and made a model which indicates what

happened at the Ni3P/n-diamond interface during annealing at 800oC in Fig 3.12.

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- 51 -

Figure 3.12 A possible model for reaction to form graphite at diamond surface during

annealing at 800oC for Ni3P/diamond.

Ni3P

diamond

PPP P

Ni3P

diamondP

PP

P

Annealing

graphite

The phosphorus diffusion into diamond gives damages to the diamond surface transforming this surface into graphite.

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17B3.5 Research about treatments for a diamond substrate

What discussed in this section is difference between SPM and hot H2SO4 and HNO3

treatment on diamond substrate forcussing on oxdation. The result of XPS measurement

is shown in Fig 3.13.

SPM

Hot H2SO4 & HNO3

Figure 3.13 O (1s) spectra of diamond substrate with SPM or hot H2SO4 & HNO3

treatment. Intensities are normalized by C (1s).

- 53 -

Fig 3.13 indicates that the SPM sample get about a half or less amount of oxygen on

substrate than the hot H2SO4 & HNO3 (hot mixed acid sample). To discuss quantitatively,

surface coverages of oxygens on the diamond with these two treatments were

calicurated.

Table 3.1 Surface coverage of oxygens on the diamond substrate

Treatment methods Surface coverages [ML]

SPM 0.97

Hot

H2SO4 & HNO3

1.75

Table 3.1 shows SPM put almost 1 ML oxygen on diamond substrate, and hot mixed

acid put ~2 ML oxygen on diamond substrate. Considering the result which has only

SPM treatmet can’t form electrodes as correct patterns, the significant amount of

oxygen probably leaves from the diamond surface for the SPM sample. This implies

that surface coverage with hot mixed acid of 1.75 ML shouldn’t enough to form good

electrodes. There could be a room to improve fablication process by further discussing

the oxidation process of the diamond surface.

- 54 -

6BReferences

[3.1] T. Matsumoto, et al., Reduction of n-type diamond contact resistance by

graphite electrode, 2013.

7BConclusion

In this study, low-resistance contact formation of metal/n-diamond was investigated.

The best value in this study was still high and about four orders of magnitude higher

than the required value of 10-5 cm2, which calculated under high bias voltage around

9-10V. Slight improvement of current increase at the Ni3P/n-diamond contact under low

bias voltage and graphite formation at the interface were newly observed. In order to

find out the improvement clearly, further researches are required. Nonetheless, it was

shown that a metal electrode containig impurity has a possibility to improve the contact

resistance with n-diamond. Besides, although, hot H2SO4 and HNO3 treatment is used as

treatment and oxidation process for the diamond substrates, there could be room to

improve fablication process of electrodes by disscussing oxidation process.

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8BAcknowledgment

A part of this research was supported by Nohira laboratory in Tokyo City University

and Advanced Industrial Science and Technology (AIST), I would like to express my

gratitude for members of the Nohira lab and AIST.

I really appreciate for my professors. Professor Hiroshi Iwai, Associate Professor

Kuniyuki Kakushima, and my Supervising Professor Nobuyuki Sugii, they not only let

me get knowledge about semiconductor and how to approach difficulties as an

engineering scientist, but also give me opportunities to attend domestic and

international conferences. Moreover, Professor Takeo Hattori who retired last year,

Professor Kenji Natori, Professor Kazuo Tsutsui, Professor Akira Nishiyama, Professor

Yoshinori Kataoka and Professor Hitoshi Wakabayashi gave me a lot of technical

advices.

I would like to appreciate for Ms. Nishizawa and Ms. Matsumoto, who are

secretaries in Iwai laboratory and helped us with supports of office jobs.

Mates in Iwai lab encouraged and enlightened me many times. Thank you.

Finally, I want to thank my parents Nobuo and Yukari, and my sisters Mizuho and

Midori for their endless support and encouragement.

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