Heat-resistant barrier and ohmic contacts based on TiBx and ZrBx interstitial phases to microwave diode structures
We investigated thermal stability of Au–TiBx (ZrBx) barrier contacts, as well as ohmic contacts with a TiBx diffusion barrier to n-Si (GaAs, InP, GaP, GaN, SiC). The electrophysical measurements of Schottky barrier diodes and ohmic contacts were performed both before and after rapid thermal annea...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
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| Zitieren: | Heat-resistant barrier and ohmic contacts based on TiBx and ZrBx interstitial phases to microwave diode structures / A.E. Belyaev, N.S. Boltovets, V.N. Ivanov, V.P. Kladko, R.V. Konakova, Ya.Ya. Kudryk, V.V. Milenin, V.N. Sheremet // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2008. — Т. 11, № 3. — С. 209-216. — Бібліогр.: 39 назв. — англ. |
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Belyaev, A.E. Boltovets, N.S. Ivanov, V.N. Kladko, V.P. Konakova, R.V. Kudryk, Ya.Ya. Milenin, V.V. Sheremet, V.N. 2017-06-01T04:37:21Z 2017-06-01T04:37:21Z 2008 Heat-resistant barrier and ohmic contacts based on TiBx and ZrBx interstitial phases to microwave diode structures / A.E. Belyaev, N.S. Boltovets, V.N. Ivanov, V.P. Kladko, R.V. Konakova, Ya.Ya. Kudryk, V.V. Milenin, V.N. Sheremet // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2008. — Т. 11, № 3. — С. 209-216. — Бібліогр.: 39 назв. — англ. 1560-8034 PACS 73.23.+y, 73.40.Sx, 73.40.Gk https://nasplib.isofts.kiev.ua/handle/123456789/118902 We investigated thermal stability of Au–TiBx (ZrBx) barrier contacts, as well as ohmic contacts with a TiBx diffusion barrier to n-Si (GaAs, InP, GaP, GaN, SiC). The electrophysical measurements of Schottky barrier diodes and ohmic contacts were performed both before and after rapid thermal annealing (RTA) up to 600 °С for the structures on Si, GaAs, InP and GaP, as well as up to higher temperatures for GaN (~900 °C) and SiC (~1000 °C). The concentration depth profiles of contact components were taken using Auger electron spectrometry, while phase composition and surface morphology of the metallization layers on test structures were determined using x-ray diffraction and atomic force microscopy. It was shown that the silicon, indium phosphide, gallium phosphide and gallium arsenide contact structures retained their properties and layer structure after RTA up to 600 °С. Contact degradation occurred at a temperature of 800 °С. The structures based on SiC (GaN) remained stable at temperatures up to 1000 °С (900 °С). en Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України Semiconductor Physics Quantum Electronics & Optoelectronics Heat-resistant barrier and ohmic contacts based on TiBx and ZrBx interstitial phases to microwave diode structures Article published earlier |
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Heat-resistant barrier and ohmic contacts based on TiBx and ZrBx interstitial phases to microwave diode structures |
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Heat-resistant barrier and ohmic contacts based on TiBx and ZrBx interstitial phases to microwave diode structures Belyaev, A.E. Boltovets, N.S. Ivanov, V.N. Kladko, V.P. Konakova, R.V. Kudryk, Ya.Ya. Milenin, V.V. Sheremet, V.N. |
| title_short |
Heat-resistant barrier and ohmic contacts based on TiBx and ZrBx interstitial phases to microwave diode structures |
| title_full |
Heat-resistant barrier and ohmic contacts based on TiBx and ZrBx interstitial phases to microwave diode structures |
| title_fullStr |
Heat-resistant barrier and ohmic contacts based on TiBx and ZrBx interstitial phases to microwave diode structures |
| title_full_unstemmed |
Heat-resistant barrier and ohmic contacts based on TiBx and ZrBx interstitial phases to microwave diode structures |
| title_sort |
heat-resistant barrier and ohmic contacts based on tibx and zrbx interstitial phases to microwave diode structures |
| author |
Belyaev, A.E. Boltovets, N.S. Ivanov, V.N. Kladko, V.P. Konakova, R.V. Kudryk, Ya.Ya. Milenin, V.V. Sheremet, V.N. |
| author_facet |
Belyaev, A.E. Boltovets, N.S. Ivanov, V.N. Kladko, V.P. Konakova, R.V. Kudryk, Ya.Ya. Milenin, V.V. Sheremet, V.N. |
| publishDate |
2008 |
| language |
English |
| container_title |
Semiconductor Physics Quantum Electronics & Optoelectronics |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| format |
Article |
| description |
We investigated thermal stability of Au–TiBx (ZrBx) barrier contacts, as well
as ohmic contacts with a TiBx diffusion barrier to n-Si (GaAs, InP, GaP, GaN, SiC). The
electrophysical measurements of Schottky barrier diodes and ohmic contacts were
performed both before and after rapid thermal annealing (RTA) up to 600 °С for the
structures on Si, GaAs, InP and GaP, as well as up to higher temperatures for GaN
(~900 °C) and SiC (~1000 °C). The concentration depth profiles of contact components
were taken using Auger electron spectrometry, while phase composition and surface
morphology of the metallization layers on test structures were determined using x-ray
diffraction and atomic force microscopy. It was shown that the silicon, indium
phosphide, gallium phosphide and gallium arsenide contact structures retained their
properties and layer structure after RTA up to 600 °С. Contact degradation occurred at a
temperature of 800 °С. The structures based on SiC (GaN) remained stable at temperatures
up to 1000 °С (900 °С).
|
| issn |
1560-8034 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/118902 |
| citation_txt |
Heat-resistant barrier and ohmic contacts based on TiBx and ZrBx interstitial phases to microwave diode structures / A.E. Belyaev, N.S. Boltovets, V.N. Ivanov, V.P. Kladko, R.V. Konakova, Ya.Ya. Kudryk, V.V. Milenin, V.N. Sheremet // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2008. — Т. 11, № 3. — С. 209-216. — Бібліогр.: 39 назв. — англ. |
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2025-11-25T20:29:45Z |
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| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2008. V. 11, N 3. P. 209-216.
© 2008, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
209
PACS 73.23.+y, 73.40.Sx, 73.40.Gk
Heat-resistant barrier and ohmic contacts based
on TiBx and ZrBx interstitial phases to microwave diode structures
A.E. Belyaev1*, N.S. Boltovets2**, V.N. Ivanov2, V.P. Kladko1,
R.V. Konakova1, Ya.Ya. Kudryk1, V.V. Milenin1, V.N. Sheremet1
1V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine
41, prospect Nauky, 03028 Kyiv, Ukraine
*Phone: (380-44) 525-24-47; e-mail: belyaev@isp.kiev.ua
2State Scientific & Research Institute “Orion”, 8a Eugene Pottier str., 03057 Kyiv, Ukraine
**Phone: (380-44) 456-05-48; e-mail: bms@i.kiev.ua
Abstract. We investigated thermal stability of Au–TiBx (ZrBx) barrier contacts, as well
as ohmic contacts with a TiBx diffusion barrier to n-Si (GaAs, InP, GaP, GaN, SiC). The
electrophysical measurements of Schottky barrier diodes and ohmic contacts were
performed both before and after rapid thermal annealing (RTA) up to 600 °С for the
structures on Si, GaAs, InP and GaP, as well as up to higher temperatures for GaN
(~900 °C) and SiC (~1000 °C). The concentration depth profiles of contact components
were taken using Auger electron spectrometry, while phase composition and surface
morphology of the metallization layers on test structures were determined using x-ray
diffraction and atomic force microscopy. It was shown that the silicon, indium
phosphide, gallium phosphide and gallium arsenide contact structures retained their
properties and layer structure after RTA up to 600 °С. Contact degradation occurred at a
temperature of 800 °С. The structures based on SiC (GaN) remained stable at tempera-
tures up to 1000 °С (900 °С).
Keywords: Schottky barrier, ohmic contact, diffusion barrier, wide-gap semiconductors.
Manuscript received 09.06.08; accepted for publication 20.06.08; published online 15.09.08.
1. Introduction
To make highly reliable solid-state microelectronic
devices that would be tolerant to active actions, such
contact metallization systems have to be used that could
restrict the effect of factors stemming from migration
and corrosion processes. Based on both the results of our
previous complex investigations of physico-chemical
mechanisms of formation and degradation of contact
structures characteristics [1-4] and literature data [5-10],
the following factors are to be mentioned:
• appearance at boundaries between phases (in the
course of metal−semiconductor contact formation) of a
thermodynamically non-equilibrium interlayer of complex
composition whose characteristics vary under operation of
contact structures and active actions on them;
• generation of structural defects near semi-
conductor surface due to reactions between contacting
layers of different physical nature;
• formation of local nonuniformities in the
junction region of metal−semiconductor contact due to
distinctions of solid-phase interactions between metal
and semiconductor in different interfacial areas;
• presence of oxide layers at the boundary
between phases of metal−semiconductor contact;
• appearance of intrinsic stresses due to distinc-
tions in crystal structures and thermodynamic parameters
of contact-forming materials.
Thus, the problem of developing reliable contacts is
determination of interrelation between the above factors,
design-technological approaches used when making metal−
semiconductor contacts, and active actions on the contacts
that make it possible to minimize their negative effect.
The attempts to stabilize the contact electronic
characteristics (with concurrent reduction of contact area
and depth) by applying the traditional thermodynamic
and kinetic approaches failed. The way based on
introduction of stabilizing layers – diffusion barriers
(DBs) (that are not products of interactions between
phases in contacting layers) – in the contact structure
[11] turned out to be more promising. Such an approach
enables one to make essentially novel versions of
smaller contact structures for solid-state semiconductor
devices that are tolerant to various extreme actions.
The requirements imposed on contact metallization
are as follows. The materials for DBs must have high
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2008. V. 11, N 3. P. 209-216.
© 2008, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
210
Table 1. Effect of RTA on SB height φB and ideality factor n of Au−TiBx (ZrBx)−n-Si (GaAs, InP, GaP, GaN, SiC) SBDs.
ϕB, V n
SBD structures initial 400 оС 600 оС 800 оС 1000 оС initial 400 оС 600 оС 800 оС 1000 оС
Au−TiBx−n-n+-Si 0.55 0.6 0.56 0.58 1.2 1.2 1.28 1.77
Au−ZrBx−n-n+-Si 0.54 0.55 0.55 0.55 1.08 1.08 1.1 1.2
Au−TiBx−n-n+-GaAs 0.8 0.79 0.78 1.18 1.2 1.23
Au−TiBx−n-n+-InP 0.5 0.5 0.53 1.1 1.2 1.5
Au−TiBx−n-n+-GaP 0.89 0.9 0.9 0.9 1.16 1.18 1.18 1.2
Au−TiBx−n-GaN 0.9 0.9 1.3 1.3
Au−ZrBx−n-GaN 0.89 0.89 1.28 1.28
Au−TiBx−n-SiC 6H 0.82 0.82 0.82 1.2 1.2 1.2
Au−ZrBx−n-SiC 6H 0.79 0.8 0.8 1.2 1.2 1.2
Au−ZrBx−n-n+-SiC 4H 0.83 0.83 0.83 1.2 1.2 1.2
Au−ZrBx−n-SiC 15R 0.78 0.78 0.78 1.5 1.5 1.5
conductivity (as well as thermal and chemical stability)
and small coefficient of thermal expansion. Besides,
formation of layers of these materials having various
structures and morphology should not lead to consi-
derable technological problems. It was found [12-15]
that nitrides and borides of IV, V and VI group metals
meet the above requirements. They demonstrate such
properties of materials with predominant covalent
bonding as high thermal stability, rigidity and melting
temperature and are chemically inactive. At the same
time, their electrical, thermal and optical properties are
close to those of materials with metallic bonding. They
belong to the close-packed structures whose composition
and crystalline state can be varied over wide limits.
Following are the results of our investigations of
various contact systems (involving layers of titanium
and zirconium diborides) for microwave diode structures
based on Si, GaAs, InP, GaP and SiC 4H epitaxial n-n+
structures, as well as GaN−i-Al2O3 heterostructures and
SiC 6H and SiC 15R bulk wafers. Most attention is paid
to the features of the physico-chemical processes
occurring at contact formation, as well as the factors that
restrict thermal stability of the contacts.
2. Formation of contact systems
The TiBx, ZrBx and Au films (each ~0.1 µm thick) for
barrier contacts were obtained using magnetron
sputtering in an argon atmosphere (~0.7 Pa pressure in
the chamber). TiBx and ZrBx were sputtered from
powder targets of stoichiometric composition [16-19].
The ohmic contacts for n+-Si were prepared using
magnetron sputtering of Ti followed by its burning-in at
a temperature Т = 450 °С and deposition of TiBx and Au
buffer layer. The ohmic contacts to GaAs, GaP and InP
were made with gold−germanium eutectic Au:Ge
(88:12) or Au:Ge (97:3) alloy; those to n-SiC were
formed with silicide phase of nickel Ni2Si, while the
ohmic contacts to n-GaN were formed with titanium
metallization. The DBs were made using sputtering of
TiBx (in the case of ohmic contacts to Si, GaP, InP and
SiC) or TiBx and Mo (TiBx and Al) for GaAs (GaN).
The substrates were as follows: the Si, GaAs, InP
and GaP epitaxial n-n+ structures (~3-5 µm layers, donor
concentration in the n-layers of ~1016 cm-3), ~1 µm n-
GaN layer (donor concentration of ~1017 cm-3) grown on
Al2O3, wafers of bulk n-SiC 6H and n-SiC 15R (dopant
concentration of ~1018 cm-3) and n-n+-SiC 4H epitaxial
structures (thickness of ~1.5 µm, impurity concentration
in the n-layer of ~1017 cm-3).
We studied, both before and after rapid thermal
annealing (RTA) at Т = 400, 600, 800 and 1000 °С for
60 s, the samples of two types: test structures and
Schottky barrier diode (SBD) structures (diameter of
~100 µm). For the test structures with continuous
metallization, we investigated concentration depth profiles
in the contact structures (with Auger electron spectro-
metry), as well as phase composition and surface
morphology of metallization layers using x-ray diffraction
(XRD) technique and atomic force microscopy. For the
test TLM (transmission line method) structures with
different metallization layers, we studied contact
resistivity ρc (in the 77−400 К temperature range), both
before and after RTA at Т = 400, 500, 600, 800, 900 and
1000 °С. For the SBD structures, we took I−V curves
from which the Schottky barrier (SB) height ϕВ and
ideality factor n were determined (see Table 1).
The electronographic studies showed that the
sputtered TiBx and ZrBx layers were nanostructured; the
size of ordered areas was below 3 nm [20]. RTA did not
lead to changes in TiBx film structure. This result was
typical of all the structures studied. The nanostructured
state of the TiBx and ZrBx films was retained after RTA
at Т = 900 °С (for gallium nitride samples) and 1000 °С
(for silicon carbide samples) [21, 22]. The feature of
such nanostructured TiBx and ZrBx layers was
considerable slowing-down of grain boundary diffusion
in the metal−semiconductor contact region. This factor
leads to essential distinction of the properties of barrier
contacts based on amorphous TiBx and ZrBx phases from
those of contacts formed by polycrystalline metal or
alloy films using the traditional technique.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2008. V. 11, N 3. P. 209-216.
© 2008, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
211
Table 2. Strains ε in TiBx-n-GaAs (InP, GaP) contacts.
RTA modes ε
(TiBx−GaAs)
ε
(TiBx−InP)
ε
(TiBx−GaP)
initial
400 °С, 60 s
600 °С, 60 s
800 °С, 60 s
4×10-5
3.8×10-5
0.6×10-5
0.8×10-5
2×10-5
1.5×10-5
0.7×10-5
0.2×10-5
8×10-4
7×10-4
5×10-4
3. Structure and physico-chemical properties
of TiBx and ZrBx films on semiconductor substrates
before and after RTA
Au–TiBx (ZrBx)–n-n+-Si. The concentration depth
profiles for components of the Au–TiBx–n-n+-Si and
Au–ZrBx–n-n+-Si contact metallization were obtained
with Auger electron spectroscopy before and after RTA
at Т = 400, 600 and 800 °С. An analysis of these profiles
and metallization surface morphology showed that, at
annealing temperatures ≤600 °С, the layer structure of
metallization is retained for both types of contact
metallization, and no considerable redistribution of
structure components occurs [23]. Increase of RTA
temperature to 800 °С leads to damage of contact layer
structure, and topographic nonuniformity of the contact
surface does not obey the Gaussian distribution. The
above facts indicate an essential role of activation
processes at the interfaces between phases. The
interfacial microrelief formed under such conditions is
determined by chemical reactions between the compo-
nents of metal and semiconductor.
Presence of different phases and interfacial
roughness related to it favor some impairment of electro-
physical characteristics, in particular, considerable
increase of the ideality factor n (see Table 1).
Thus, the thermal degradation threshold of the Au–
TiBx–n-n+-Si and Au–ZrBx–n-n+-Si contact structures is
determined by tolerance of the TiBx and ZrBx layers for
heat actions.
Au–TiBx–n-n+-InP. The results of layer-by-layer
Auger analysis of the Au–TiBx–n-n+-InP contacts
before and after RTA at Т = 400 and 600 °С also
indicate absence of considerable intermixing at the
interfaces between phases, both in the initial sample
and those after RTA at temperatures up to 600 °С.
They are also confirmed by the XRD patterns for the
same samples [24]. An analysis of those patterns taken
before and after RTA up to 600 °С showed presence of
a quasi-amorphous TiBx film. The processes occurring
in the contact structure exposed to RTA at T >600 °С
indicate intensification of chemical reactions between
the components of the contact-forming pair. In this
case, there exists no mechanism of restriction of
interactions between phases related to ingress of atoms
to the reaction area and alloy formation. Mass transfer
intensification after RTA at Т = 800 °С is due to rela-
xation of intrinsic stresses in the contacts accompanied
with cracking of the contact system. The measurements
of SB parameters support the above conclusion (see
Table 1).
The Au–TiBx–n-n+-GaAs (GaP) contact systems
demonstrated similar structural, physico-chemical and
electrophysical properties both before and after RTA. In
this case, the common physical process leading to
variation of electrophysical parameters of the TiBx–
InP (GaAs, GaP) barrier contacts was RTA-induced
relaxation of intrinsic stresses in the contact systems
[19]. Given in Table 2 are our experimental data on the
effect of RTA on strain ε in the TiBx–InP (GaAs, GaP)
contact systems. One can see that relaxation processes
are most pronounced after RTA at Т = 600 °С; whatever
the distinction in thermal expansion coefficients of TiBx
and III−V semiconductor compounds, the ε values
decrease as RTA temperature is increased. Relaxation of
intrinsic stresses induces atomic interdiffusion from TiBx
film to the substrate and from semiconductor to the film
that increases after cracking of the contact system. A
typical morphology of the TiBx–GaAs contact after RTA
at Т = 800 °С is shown in Fig. 1. Such mechanism of
intrinsic stress relaxation is supported by the data on
layer-by-layer Auger analysis performed for the TiBx–
InP (GaAs, GaP) contact systems [19].
The electrophysical parameters of the diode
structures under investigation demonstrated high resistan-
ce to RTA up to temperatures of ~600 °С (see Table 1).
The Au–TiBx (ZrBx)–n-SiC 6H contact systems
(contrary to the TiBx–InP (GaAs, GaP) contacts) did not
demonstrate changes in component distributions at the
interfaces between phases after RTA up to Т = 1000 °С
(Figs. 2 and 3). The thickness (~20 nm) of junction layer
at the Au−TiBx (ZrBx)–n-SiC 6H interface, as well as its
composition, remained practically the same after RTA.
This supported the conclusion about thermal stability of
contact barrier properties (see Table 1) and indicated
absence of intense interaction between the contact
components and SiC. The latter statement is supported
also by the results of XRD analysis showing retention of
quasi-amorphous state of the TiBx and ZrBx films after
RTA at 1000 °С (otherwise intense grain boundary
diffusion would be observed). In this case, the electro-
physical parameters of SBs practically did not change
after RTA at 1000 °С (see Table 1).
Fig. 1. Typical morphology of Au−TiBx–GaAs contact after
RTA at Т = 800 °С.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2008. V. 11, N 3. P. 209-216.
© 2008, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
212
0 2 4 6 8 10 12
0
20
40
60
80
100
à
C
on
ce
nt
ra
tio
n,
a
t.
%
Sputtering time, min
Au
B
Ti
Si
C
O
0 2 4 6 8 10 12
0
20
40
60
80
100
b
C
on
ce
nt
ra
tio
n,
a
t.
%
Sputtering time, min
Au
B
Ti
C
Si
O
Fig. 2. Concentration depth profiles for Au−TiBx−n SiC 6H
contacts before (a) and after (b) RTA at Т = 1000 оС for 90 s.
0 2 4 6 8 10
0
20
40
60
80
100
a
C
on
ce
nt
ra
tio
n,
a
t.
%
Sputtering time, min
Au
B
Zr
Si
C
O
0 2 4 6 8 10
0
20
40
60
80
100
b
C
on
ce
nt
ra
tio
n,
a
t.
%
Sputtering time, min
Au
B
Zr
Si
C
O
Fig. 3. As in Fig. 2 but for Au−ZrBx−n-SiC 6H contacts.
20 30 40 50 60 70
100
102
104
106
A
u
(2
00
)
In
te
ns
ity
, a
rb
. u
.
2θ, deg
G
aN
(0
00
2)
A
u
(1
11
)
α
-A
l 2O
3(0
00
6)
2
1
Fig. 4. .XRD patterns for Au−TiBx–n-GaN samples: 1 – initial,
2 – after RTA at Т = 900 °С for 30 s.
One can see from the above results that the
threshold of thermal degradation of contact systems
based on TiBx and ZrBx on SiC 6H may be related to
tolerance of TiBx and ZrBx films for heat actions, as well
as in the case of similar systems on Si.
The Au–TiBx (ZrBx)–n-GaN contact systems were
studied with XRD technique, both before and after RTA
at Т = 900 °С. The XRD patterns for the initial Au–
TiBx–n-GaN sample, as well as that after RTA at Т =
900 °С for 30 s, were obtained by us earlier [25]; they
are presented in Fig. 4. In both cases, the TiBx film was
x-ray-amorphous. No formation of other phases due to
metallurgical processes in the metallization layers was
detected. This indicated their thermal stability. An
increase of the Au (111) peak intensity after RTA
indicates presence of texture [111] related to thermally
activated growth of Au grains. The essential distinction
of those contact systems from the above-mentioned is
structural nonuniformity of the GaN film grown on
sapphire. This resulted from considerable lattice mis-
match between GaN and Al2O3. Therefore, determi-
nation of the thermal threshold of degradation of such
contact structure needs further structural and physico-
chemical investigations of those objects. The results on
parameters of the Au–TiBx–n-GaN SBs, both initial and
after RTA at temperatures up to 900 °C, show their
thermal stability, thus indicating thermal stability of the
TiBx–n-GaN interface (see Table 1).
Our Auger analysis of the Au–TiBx (ZrBx)–n-GaN
contact systems confirmed presence of layered structure
in both initial and exposed to RTA up to 900 °С
samples.
4. Electrophysical properties of contacts before and
after RTA
Barrier contacts. The electrophysical parameters of SBs
of all barrier contacts under investigation were
determined from the measurements of forward branches
of I−V curves; they are presented in Table 1. The feature
of these I−V curves was that they could be described in
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2008. V. 11, N 3. P. 209-216.
© 2008, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
213
Table 3. Parameters of ohmic contacts with DBs before and after RTA.
ρc, Ohm⋅cm2
Contacts NВ, cm-3
initial 400 °С 500 °С 600 °С 800 °С 900 °С 1000 °С
ϕB, V Ref.
Si−TiSi2−TiBx−Au 2×1019 2.1×10-6 2.1×10-6 1.3×10-6 8×10-6 0.04−0.05 [21]
GaAs−AuGe−TiBx−Au (5-9)×1015 5.2×10-5 1.9×10-5 2.5×10-5 3.6×10-4 0.1−0.14 [32]
InP−AuGe−TiBx−Mo−Au 1018 1.03×10-4 2.2×10-5 2.59×10-5 2.25×10-4 0.1−0.15 [34]
SiC−Ni2Si−TiBx−Au 1.2×1018 rectifying 10-4 0.3 [35]
GaN−Ti−Al−TiBx−Au 1017 rectifying 10-4 0.33* [36]
GaP−AuGe−TiBx−Au 1017 1.4×10-3 1.4×10-3 1.2×10-4 1.1×10-4 ϕВ1 = 0.038
ϕВ2 = 0.078 [37]
the terms of the theory of thermionic emission because
the ideality factors of all diode structures (the initial ones
and those after RTA) lied within 1.08-1.2.
As to the mechanism of current flow in the
forward-biased SBDs based on n-SiC 6H and n-GaN,
their over-barrier current dominates at high temperatures
only. In the 77−400 К temperature range, tunneling
according to the dislocation mechanism prevails [38].
The SB height dependence on the semiconductor
ionicity also agreed with that predicted by the theory of
thermionic emission. One can conclude from the results
given in Table 1 that, whatever the results of structural
and physico-chemical analysis, the interfaces of the
contact systems under investigation remained thermally
stable after RTA at temperatures up to 600 °С (for Si,
GaAs, InP and GaP SBs) and even higher temperatures,
~900 °C (for GaN SBs) and ~1000 °C (for SiC SBs).
This result also indicates high heat-resistance of the TiBx
and ZrBx barrier-forming films. Some authors proposed
to apply those films when making ohmic contacts with
DBs (e.g., from ZrB2 and W2B layers to GaAs) [26], for
improvement of thermal stability of light-emitting diodes
based on the InGaN/GaN heterojunctions with TiB2 DBs
[27], ohmic contacts to p-GaN with W2B2 [28], ohmic
and barrier contacts to n-GaN with ZrB2 and CrB2 [29-
32].
Ohmic contacts. The contact resistivity ρс was
determined for all contact systems studied with the TLM
technique, while the barrier height ϕВ for ohmic contacts
was determined from the results of measurements of
temperature dependence of contact resistance. The ρс
and ϕВ values measured for both the initial samples and
those after RTA are given in Table 3. One can see that
the parameters of ohmic contacts with TiBx DBs on Si,
InP, GaP and GaAs do not change considerably after
RTA at 400, 500 and 600 °С. This is due to absence of
interaction with neighboring metallization layers. This
effect is retained also after RTA at Т = 800 °С; however,
relaxation of intrinsic stresses at cracking intensifies
mass transfer between the metallization and metal layers
and semiconductor and thus results in increase of ρс,
while φВ does not change considerably.
As to the parameters of Au–TiBx–AuGe–n-GaP
ohmic contacts, an analysis of the contact resistivity ρс
vs temperature curve shows that it has two sections with
different slopes (φВ1 ≈ 0.038 V and φВ2 ≈ 0.078 V). This
indicates contact nonuniformity. RTA up to Т = 600 °С
does not change substantially the contact parameters.
This shows that ohmic contact formation is proceeding
in the course of component sputtering at Т = 450 °С.
Some incompleteness of reactions between phases at the
contact-forming film−GaP interface is removed during
RTA at Т = 500 (600) °С for 1 min. This was supported
by the results of XRD analysis.
The XRD patterns for both initial Au–TiBx–AuGe–
n-GaP sample and those after RTA at Т = 500−600 °С
for 60 s are shown in Fig. 5. For all samples studied, the
TiBx film was x-ray-amorphous. Along with the gold
peaks, those of germanium, GeP3 and (weak) AuGaO2
are observed in the XRD for the initial sample. No other
phases resulting from metallurgical processes in the
metallization layers were found. The portions of
AuGaO2 and GeP3 phases increased after RTA at Т =
500 оС. A new phase (Au0.72Ge0.28) appeared after RTA
at Т = 600 °С. Its appearance is accompanied with a
drastic drop of both the GeP3 phase portion and pure
gold peaks. A decrease of the intensity of Au(111) and
Au(200) peaks after RTA at 600 °С indicates redis-
tribution of gold in the layers due to its diffusion and
formation of new phases. These results correlate well
with those obtained from the Auger spectra.
It turned out also that the mechanism of current
flow in ohmic contacts to heavily doped semiconductors
(NB ≥ 1018 cm-3 for Si and InP) is determined, over a
wide temperature range, by tunneling of charge carriers.
The thermal threshold in the ohmic metallization with
TiBx DBs to those materials is determined by TiBx
tolerance for heat actions, as in the case of barrier
contacts to Si, GaAs, GaP and InP formed with
interstitial phases.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2008. V. 11, N 3. P. 209-216.
© 2008, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
214
10 20 30 40 50 60 70
102
103
104
105
2
3
1
G
e(
01
2)
In
te
ns
ity
, a
rb
. u
.
A
uG
aO
2(0
04
)
G
eP
3(2
02
)
A
u 0.
72
G
e 0.
28
(0
11
)
G
e(
20
0)
te
tra
g.
A
u(
11
1)
2θ, deg
A
u(
20
0)
Fig. 5. XRD patterns for Au–TiBx–AuGe–n-GaP samples:
1 – initial, 2 (3) – after RTA at Т = 500 °C (600 °С) for 60 s.
20 40 60 80 100 120 140 160
6
7
8
9
10
11
12
ϕB1=0,33 эВ
ln
(∆
R
T)
q/kT, V-1
Fig. 6. Contact resistance as function of inverse temperature
for Au–TiBx–Al–Ti–n-GaN contacts.
The ohmic contacts to GaN and n-SiC were
exposed to RTA at 900 and 1000 °С. In this case, ρс
value for SiC did not exceed 10-4 Ohm⋅cm2, while for
GaN it was 10-6 Ohm⋅cm2 (the best samples). The
mechanism of current flow in the ohmic contact to GaN
was determined by tunneling in the wide temperature
range 77−200 К and by thermionic emission in the
220−280 К temperature range (Fig. 6). Thermal stability
of ohmic contacts with DBs to wide-gap semiconductors
SiC and GaN, as well as that of the TiBx (ZrBx)–n-SiC
and TiBx (ZrBx)–n-GaN barrier contacts, is determined
by tolerance of TiBx films for high-temperature actions.
8. Conclusion
An analysis of the Refs. [26-32], as well as our above
results, shows that an interest in borides of refractory
metals (in particular concerning their application in
contact systems to wide-gap semiconductors) stems, first
of all, from thermal and chemical inactivity of those
contact components. These features, as well as high
conductivity, are retained after high-temperature
treatments that simulate both short-term overloads and
long-term extreme operation modes [39].
One should note, however, that practical applica-
tion of these contact materials with unique properties in
the manufacturing technology for semiconductor devices
is still at its start, and there are many questions to be
answered. Among such items are (i) radiation resistance
of contact systems involving borides of refractory metals
serving as DBs or barrier-forming layers, (ii) the role of
presence of many phases in such systems, and (iii) the
effect of the above factor on structural and electro-
physical properties of the contacts (or, ultima analysi, on
the heat- and radiation-resistance and reliability of the
corresponding devices and systems).
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