Electrical contact characteristics of n-type diamond with Ti, Ni, NiSi 2 ...
Transcript of Electrical contact characteristics of n-type diamond with Ti, Ni, NiSi 2 ...
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,
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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.
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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 -
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Chapter 1 Introduction
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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
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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
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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.
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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.
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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
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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
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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)
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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.
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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.
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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
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Chapter 2 Experiment
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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].
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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.
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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.
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- 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)
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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
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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.
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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
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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.
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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
- 48 -
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.
- 50 -
- 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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