Materials Transactions, Vol. 54, No. 6 (2013) pp. 890 to 894
Special Issue on Nanojoining and Microjoining
© 2013 The Japan Institute of Metals and Materials
Interfacial Nanostructure and Electrical Properties
of Ti3SiC2 Contact on p-Type Gallium Nitride
Aiman bin Mohd Halil1, Masakatsu Maeda2 and Yasuo Takahashi2
1
Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, Suita 565-0871, Japan
Joining and Welding Research Institute, Osaka University, Suita 565-0871, Japan
2
In the present study, the interfacial nanostructure and electrical properties of Ti3SiC2 formed by depositing a Ti­Si­C ternary film with a
composition stoichiometrically equivalent to Ti3SiC2 on p-type GaN and subsequent annealing at 1073 K were analyzed by X-ray diffraction,
transmission electron microscopy and direct current conduction test. The results reveal that structural changes occur by the annealing.
Polycrystalline Ti3SiC2 is formed at most of the contact interface area and single crystal Ti3SiC2 at small area of the contact interface.
Furthermore, other than Ti3SiC2 phase, polycrystalline Ti5Si3 and TiSi2 with different grain sizes are also formed, resulting in a formation of
three-layered film after the annealing. By all these structural change, the electric conduction profiles show that the Schottky barrier height (SBH)
is reduced. The estimated SBH of the Ti3SiC2 contact on p-type GaN is 0.70 eV, which is 1.73 eV lower than the theoretically predicted value.
[doi:10.2320/matertrans.MD201206]
(Received December 17, 2012; Accepted February 5, 2013; Published March 23, 2013)
Keywords: p-type gallium nitride, Ti3SiC2, stoichiometrically equivalent amorphous, crystallization, Schottky barrier height, ohmic contact
1.
Introduction
Due to the physical limitation of silicon, which is still used
in most of today’s power electronic devices, it is important to
seek for better alternative materials for use in the nextgeneration power electronic devices. Gallium nitride (GaN)
is one of the most promising candidates. Compared with
silicon, this wide band-gap semiconductor can be operated at
higher temperatures, frequencies, voltages and power densities with lower leak current.1) With all these advantages,
GaN-based power electronic devices are promising higher
energy efficiency with capability of handling higher power
and longer service life compared to silicon-based power
electronic devices. However, in spite of having a significant
development in the growth and processing technology of
light emitting diodes (LED) and laser diodes (LDs), the
application of GaN semiconductor as power electronic
devices is still far from real.
One of the main problems in realizing GaN-based power
electronic devices is related to the fact that the only effective
p-type dopant known for GaN is magnesium. Although
some decent annealing methods were developed2,3) after the
breakthrough discovery of Mg-activation by electron-beam
irradiation,4) not much recent development of p-type GaN
conductivity has been accomplished. It is known that only
approximately one percent of doped magnesium atoms are
activated during Mg-activation.5,6) To achieve reasonable
p-type conductivity, a relatively high Mg-doping level is
needed. However, such high level of impurities in GaN
leads to growth disturbances such as stacking faults, which
consequently degrade the quality of crystalline.
The other important problem is the difficulties in formation
of low resistance ohmic contact to p-type GaN. GaN have to
be connected with metallic materials to form an electric
circuit. However, due to the very deep valance band edge for
p-type GaN (7.50 eV under the vacuum level), there is no
pure metallic element which can directly form ohmic contact
with p-type GaN. Without the formation of ohmic contact,
the interference of carrier will occur at the contact interface,
which generates Joule heat and deteriorates the energy
efficiency and the reliability of the devices. Hence, to form
an efficient and reliable power electronic device, a clear
understanding on formation of low-resistance ohmic contact
between semiconductors and metallic materials by lowering
the Schottky barrier height (SBH) and/or by thinning the
carrier depleted zone at the contact interface is needed.
Unfortunately, there is no appropriate contact material for
p-type GaN to form low-resistance ohmic contact has been
recognized so far. Jang et al. suggested that with a Au/
Ni/Ru/Ag/Ni multilayered contact on p-type GaN, lowresistance and thermally stable ohmic characteristic is
achieved.7) In another paper, Chen et al. suggested that a
low-resistance ohmic contact is achieved with the formation
of NiO and a specific microstructure by annealing Au/Ni/
p-type-GaN in an oxidative ambient.8) Despite a lot of
p-type GaN ohmic contact solutions have been reported,
most of the solutions are difficult to be reproduced, even if
the procedures are exactly followed. This suggests that the
reported procedures do not correspond solely to the formation
of the favored contact interface structure. Thus, it is important
to establish a technology to control the interfacial structure
based on knowledge of the mechanism of low-resistance
ohmic contacts formation between p-type GaN and contact
materials. However this is a difficult subject: the suggested
contacts are formed by complex interfacial reaction between
the p-type GaN and multilayer contact materials.7)
In researches related to p-type GaN contacts, to seek for
appropriate contact materials, a lot of considerations are
made based on p-type GaAs contact materials such as Ni, Au
and Pt. These contact materials have been empirically proven
to form good electrical properties with p-type GaAs and have
been used for a wide variety of devices such as optoelectronic
devices, metal semiconductor field-effect transistors, heterojunction FET and heterojunction bipolar transistors.9) However, in this study, a different approach to seek an appropriate
contact material for p-type GaN has been used. Compared
with GaAs, GaN shows more similarity with SiC, as shown
in Table 1.10­13) GaN shows close similarity with SiC in
Interfacial Nanostructure and Electrical Properties of Ti3SiC2 Contact on p-Type Gallium Nitride
GaN
Crystal system
Lattice parameter, a/nm
GaAs
4H-SiC
Hexagonal
0.3189
Cubic
0.5653
Hexagonal
0.3073
0.5186
®
1.0053
Coefficient of thermal
expansion, ¡/K¹1
5.59 © 10¹6
6.00 © 10¹6
4.47 © 10¹6
Band gap, Eg/eV
3.39
1.42
3.26
Electron affinity, »/eV
4.11
4.07
3.20
c/nm
lattice parameters, coefficient of thermal expansion and band
gap, which are the three most important factors which
directly affect the contact structure and electrical properties
of the formed contact. Hence, there is a high possibility that,
SiC contact materials are more suitable than GaAs contact
materials for formation of low resistance ohmic contact to
GaN.
The present study demonstrates this idea by forming
Ti3SiC2 on p-type GaN and examining the properties of the
contact. It have been suggested by many researchers that
Ti3SiC2 achieve good ohmic contact on p-type SiC.14­19)
In the present study, Ti3SiC2 is formed on p-type GaN by
depositing a Ti­Si­C ternary film with a composition
stoichiometrically equivalent to Ti3SiC2 on p-type GaN and
subsequent annealing at 1073 K.
2.
Experimental Procedure
Substrates used in the present study were 2.0-µm-thick,
4.0 mm-square p-type GaN epitaxially grown on a 330-µmthick sapphire (0001) wafer with a 2.3-µm-thick GaN buffer
layer. The surface orientation and carrier density of the
substrate were (0001) Ga-face and 3 © 1017 cm¹3, respectively. Before the deposition process, the substrates were
cleaned with acetone applying ultrasonic vibration. Then, the
substrates were fixed in a radio-frequency (RF) magnetron
sputter deposition apparatus using 1.0-mm-wide Al masking
ribbons. Before the sputter deposition, the surfaces of Ti­Si­
C alloy target and the substrates were sputter-cleaned. The
sputter deposition of Ti­Si­C ternary films on the substrates
was performed immediately after the sputter-cleaning. Both
of sputter-cleaning and sputter deposition were performed in
0.8 Pa of 99.9999% Ar under the RF power of 200 W. The
target used in the sputter deposition process was a Ti­Si­C
ternary alloy sintered disk with the composition of Ti65Si22C13. Sputter deposition for 600 s with this target forms
successfully a 500-nm-thick film with the composition of
Ti49Si18C33 on all p-type GaN substrates. The composition of
the ternary films was close to the stoichiometric composition
of Ti3SiC2. Some of the deposited samples were subjected
to annealing at 1073 K in vacuum of 1.3 © 10¹3 Pa. The
specimens were cooled down immediately after reaching the
temperature of 1073 K.
The contact structure and electrical properties of the asdeposited and annealed specimens were then analyzed by
X-ray diffraction (XRD), transmission electron microscopy
(TEM) and direct current (DC) conduction test at room
temperature.
GaN
Sapphire
Ti3SiC2
0.5
Properties
Square root of intensity, (C/C0) /a.u.
Table 1 Properties of GaN, GaAs and 4H-SiC.
30°
891
(b)
(a)
40°
50°
60°
70°
80°
Diffraction angle, 2θ (CuKα)
90°
Fig. 1 XRD patterns of specimens. (a) as-deposited, (b) after annealing at
1073 K.
3.
Result and Discussion
Figure 1 shows the structural change of the specimens by
annealing at 1073 K. In the XRD pattern of the as-deposited
specimens, all the identified peaks correspond to GaN and
sapphire. No other peak related to the Ti­Si­C phase in the
as-deposited film is identified. This analysis indicates that the
deposited film consist only of an amorphous phase. The XRD
pattern of the specimens after the annealing appears different
from that of the as-deposited specimen. Peaks corresponding
to various crystallographic planes of Ti3SiC2 are identified,
indicating that Ti3SiC2 has a randomly oriented polycrystalline structure. Hence, it is proven that the Ti3SiC2 can be
formed on p-type GaN substrates by deposition of the Ti­
Si­C ternary film stoichiometrically close to Ti3SiC2 and
subsequent annealing at 1073 K. On the other hand, it is
suggested that p-type SiC/Ti/Al need to be annealed up to
1273 K for 120 s or longer to form Ti3SiC2 phase on p-type
SiC.14­19) Therefore, the Ti3SiC2 formation method employed
in the present study has advantages in lowering the annealing
temperature and shortening the holding time. The lower
annealing temperature was achieved due to the fact that
the deposited film is in thermodynamically unstable state,
triggering the crystallization process to form Ti3SiC2 phase
at lower temperature. Lowering of the annealing temperature
is strongly demanded, in order to prevent surface roughening
of the contact and deterioration of capacitances of gate
insulation layers. Furthermore, processes with lower annealing temperature and shorter hold time are preferred for
production cost suppression.
The structure change during annealing has been analyzed
further by TEM. Figure 2 shows the structure of the asdeposited specimen. Figure 2(a) shows a bright field image
(BFI) of the specimen. In the image, the deposited film
of which thickness is approximately 500 nm appears in a
monotonous contrast showing no grain boundaries within the
film. Figure 2(b) shows the selected area electron diffraction
(SAED) pattern of the specimen. The SAED pattern consists
of ½1100
zone axis net pattern corresponding to GaN single
crystal and a halo ring corresponding to amorphous phase.
The results indicate that the deposited Ti­Si­C ternary film
is in amorphous state, which agrees with the XRD pattern
892
A. M. Halil, M. Maeda and Y. Takahashi
(a)
deposited
film
GaN
500 nm
(b)
Fig. 2 Structure of an as-deposited specimen. (a) BFI, (b) SAED pattern.
shown in Fig. 1. Figure 3 shows the structure of the
specimen after annealing at 1073 K. In the BFI shown in
Fig. 3(a), the dislocation line of p-type GaN stops near the
contact interface between the p-type GaN and deposited film,
indicating that the p-type GaN substrate near the contact
interface have structurally transformed by annealing at
1073 K, forming a layer of intermediate-phase. The highmagnification BFI shown in Fig. 3(b) clearly shows that
the single-layer Ti­Si­C ternary film of the as-deposited
specimen has changed into three-layered film, distincted by
the grain sizes. The grain sizes increase from coarse-grained
near the contact interface to fine-grained near the film
surface, indicating that the structural change of the film such
as crystallization and nucleation start form near the contact
interface. Figure 3(c) shows the SAED pattern taken from
the area shown in Fig. 3(a). This pattern consists of net
patterns of GaN and Ti3SiC2 single crystals and Debye­
Scherrer rings of Ti5Si3 and TiSi2 polycrystals. The net
pattern of GaN and Ti3SiC2 show a close correlation with
each other as
ð0002ÞGaN == ð1102ÞTi
3 SiC2
ð1Þ
ð1120ÞGaN
== ð1120ÞTi
3 SiC2
ð2Þ
and
Therefore, it is considered that the Ti3SiC2 is formed adjacent
to GaN epitaxially. On the other hand, the Debye­Scherrer
rings of Ti5Si3 and TiSi2 appear differently. The rings
corresponding to Ti5Si3 are continuous, indicating that the
grains of Ti5Si3 are very fine. The rings corresponding to
TiSi2 appear as discontinuous scattered spots, indicating that
only a few grains of TiSi2 are involved in the diffraction.
Contrary with the analysis from XRD pattern, which
shows that the film is consisting of polycrystals Ti3SiC2, the
TEM observation shows that the selected area of the
specimen shown in Fig. 3(a) is consisting of single crystal
Ti3SiC2. These observations reveal that most of the formed
Ti3SiC2 in the film is polycrystalline Ti3SiC2. Single crystal
Ti3SiC2 failed to be formed in most of the contact interface
area likely due to the mismatch in lattice parameter between
p-type GaN and Ti3SiC2.
Figure 4 shows the profile of electrical conduction and
its change by annealing at 1073 K. The relations between
voltage and current for both as-deposited and annealed
specimens are indicated by non-linear curves. This result
shows that both specimens fail to achieve ohmic contact.
However, the voltage to raise the current is lowered by the
annealing. The change indicates that the SBH is reduced
by some amount after the annealing, i.e., after the formation
of Ti3SiC2 on the p-type GaN. To evaluate the reduction
of SBH after the formation of Ti3SiC2, the conduction
profiles are analyzed by the thermionic emission model20)
given by
eº
I0 ¼ AA T 2 exp SB ;
ð3Þ
kT
where I0, A, A*, T and k are the saturated current, the
contact area, the effective Richardson contact of 1.038 ©
106 Am¹2 K¹2 for p-type GaN,20) the absolute temperature of
the specimen and the Boltzmann constant, respectively. The
estimated SBH of the as-deposited and annealed specimens
are 0.89 and 0.70 eV, respectively. Therefore, a reduction of
0.19 eV of SBH is attributed to the structural change in the
contact film from the amorphous to polycrystalline Ti3SiC2.
In addition to that, compared to the value predicted by
Schottky­Mott model21) given by
eº SB ¼ » þ Eg eº Ti3 SiC2 ;
ð4Þ
where the work function of Ti3SiC2 ðº Ti3 SiC2 Þ is taken as
5.07 eV,18) the SBH of the p-type GaN and Ti3SiC2 interface
(the annealed specimen) is 1.73 eV lower. Mohammad have
discussed such lowering of the SBH in detail, and suggested
that such lowering of the SBH is caused by band gap
narrowing and image force lowering effects.21)
4.
Conclusion
The interfacial nanostructure and electrical properties of
Ti3SiC2 formed by depositing a Ti­Si­C ternary film with a
composition stoichiometrically equivalent to Ti3SiC2 on ptype GaN and subsequent annealing at 1073 K were analyzed
by XRD, TEM and DC conduction test. The following points
were clarified:
(1) Ti3SiC2 phase can be formed adjacent to p-type GaN
by deposition of an amorphous Ti­Si­C ternary film
stoichiometrically close to Ti3SiC2 and subsequent
annealing at 1073 K for a short time.
Interfacial Nanostructure and Electrical Properties of Ti3SiC2 Contact on p-Type Gallium Nitride
(a)
893
(b)
Fine-grained (5.0 nm)
(b)
deposited
film
Medium-grained (6.8 nm)
Coarse-grained (12.8 nm)
GaN
Dislocation
Intermediate-phase
500 nm
200 nm
(c)
1122GaN
1102 Ti3SiC2
0002GaN
2024 Ti3SiC2
1120 Ti3SiC2
1120GaN
Ti 5Si3
200
TiSi 2
130
060
210
300
Fig. 3 Structure of the specimen after annealing at 1073 K. (a) BFI, (b) high magnification BFI if the area marked on Fig. 3(a) with a
dashed rectangle, (c) SAED pattern of the whole area shown in Fig. 3(a).
3
2
Current, I/mA
(4) The electric conduction profiles show that the SBH is
reduced by the formation of Ti3SiC2 phase at the contact
interface. The SBH of the Ti3SiC2 contact on p-type
GaN is 0.70 eV, which is 1.73 eV lower than the
theoretically predicted value.
as-deposited
annealed
1
0
Acknowledgement
-1
The authors express their gratitude to Prof. Em. H. Mori
and Mr. E. Taguchi for their kind permission and assistance
to use facilities in the Research Center for Ultra-High Voltage
Electron Microscopy, Osaka University, Japan.
-2
-3
-10
-8
-6
-4
-2 0
2
Voltage, V/V
4
6
8
10
Fig. 4 Electric conduction profile of specimens before and after annealing
at 1073 K.
(2) The XRD and TEM results reveal that the structural
changes during the annealing occur. Polycrystalline
Ti3SiC2 is formed at most of the contact interface area
and single crystal Ti3SiC2 at small area of the contact
interface.
(3) The TEM results reveal that other than Ti3SiC2
phase near the interface, polycrystalline Ti5Si3 and
TiSi2 with different grain sizes are also formed,
resulting in a formation of three-layered film after the
annealing.
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