Investigation of resistance formation mechanisms for contacts to n-AlN and n-GaN with a high dislocation density
We studied temperature dependences of the resistivity, Pc(T) , of Pd-Ti-Pd-Au ohmic contacts to wide-gap semiconductors n - GaN and n - AlN with a high dislocation density. BothPc(T) curves have portions of exponential decrease, as well as those with very slight Pc(T) dependence at higher temp...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2012
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| Цитувати: | Investigation of resistance formation mechanisms for contacts to n-AlN and n-GaN with a high dislocation density / A.V. Sachenko, A.E. Belyaev, N.S. Boltovets, Yu.V. Zhilyaev, L.M. Kapitanchuk, V.P. Klad’ko, R.V. Konakova, Ya.Ya. Kudryk, A.V. Kuchuk, A.V. Naumov, V.V. Panteleev, V.N. Sheremet // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2012. — Т. 15, № 4. — С. 351-357. — Бібліогр.: 19 назв. — англ. |
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Sachenko, A.V. Belyaev, A.E. Boltovets, N.S. Zhilyaev, Yu.V. Kapitanchuk, L.M. Klad’ko, V.P. Konakova, R.V. Kudryk, Ya.Ya. Kuchuk, A.V. Naumov, A.V. Panteleev, V.V. Sheremet, V.N. 2017-05-31T05:22:26Z 2017-05-31T05:22:26Z 2012 Investigation of resistance formation mechanisms for contacts to n-AlN and n-GaN with a high dislocation density / A.V. Sachenko, A.E. Belyaev, N.S. Boltovets, Yu.V. Zhilyaev, L.M. Kapitanchuk, V.P. Klad’ko, R.V. Konakova, Ya.Ya. Kudryk, A.V. Kuchuk, A.V. Naumov, V.V. Panteleev, V.N. Sheremet // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2012. — Т. 15, № 4. — С. 351-357. — Бібліогр.: 19 назв. — англ. 1560-8034 PACS 73.40.Cg, 73.40.Ns, 85.40.-e https://nasplib.isofts.kiev.ua/handle/123456789/118725 We studied temperature dependences of the resistivity, Pc(T) , of Pd-Ti-Pd-Au ohmic contacts to wide-gap semiconductors n - GaN and n - AlN with a high dislocation density. BothPc(T) curves have portions of exponential decrease, as well as those with very slight Pc(T) dependence at higher temperatures. Besides, the Au-Pd-TiPd-n-GaN contacts have a portion of Pc(T) flattening out in the low-temperature region. This portion appears only after rapid thermal annealing (RTA). In principle, its appearance may be caused by preliminary heavy doping of the near-contact region with a shallow donor impurity as well as doping in the course of contact formation owing to RTA, if the contact-forming layer involves a material atoms of which serve as shallow donors in III N compounds. The obtained Pc(T) dependences cannot be explained by the existing mechanisms of current transfer. We propose other mechanisms explaining the experimental Pc(T) curves for ohmic contacts to n - GaN and n -AlN . This work was supported by the State Target Scientific and Technical Program of Ukraine “Nanotechnologies and nanomaterials” for 2010-2014. en Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України Semiconductor Physics Quantum Electronics & Optoelectronics Investigation of resistance formation mechanisms for contacts to n-AlN and n-GaN with a high dislocation density Article published earlier |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| title |
Investigation of resistance formation mechanisms for contacts to n-AlN and n-GaN with a high dislocation density |
| spellingShingle |
Investigation of resistance formation mechanisms for contacts to n-AlN and n-GaN with a high dislocation density Sachenko, A.V. Belyaev, A.E. Boltovets, N.S. Zhilyaev, Yu.V. Kapitanchuk, L.M. Klad’ko, V.P. Konakova, R.V. Kudryk, Ya.Ya. Kuchuk, A.V. Naumov, A.V. Panteleev, V.V. Sheremet, V.N. |
| title_short |
Investigation of resistance formation mechanisms for contacts to n-AlN and n-GaN with a high dislocation density |
| title_full |
Investigation of resistance formation mechanisms for contacts to n-AlN and n-GaN with a high dislocation density |
| title_fullStr |
Investigation of resistance formation mechanisms for contacts to n-AlN and n-GaN with a high dislocation density |
| title_full_unstemmed |
Investigation of resistance formation mechanisms for contacts to n-AlN and n-GaN with a high dislocation density |
| title_sort |
investigation of resistance formation mechanisms for contacts to n-aln and n-gan with a high dislocation density |
| author |
Sachenko, A.V. Belyaev, A.E. Boltovets, N.S. Zhilyaev, Yu.V. Kapitanchuk, L.M. Klad’ko, V.P. Konakova, R.V. Kudryk, Ya.Ya. Kuchuk, A.V. Naumov, A.V. Panteleev, V.V. Sheremet, V.N. |
| author_facet |
Sachenko, A.V. Belyaev, A.E. Boltovets, N.S. Zhilyaev, Yu.V. Kapitanchuk, L.M. Klad’ko, V.P. Konakova, R.V. Kudryk, Ya.Ya. Kuchuk, A.V. Naumov, A.V. Panteleev, V.V. Sheremet, V.N. |
| publishDate |
2012 |
| language |
English |
| container_title |
Semiconductor Physics Quantum Electronics & Optoelectronics |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| format |
Article |
| description |
We studied temperature dependences of the resistivity, Pc(T) , of Pd-Ti-Pd-Au
ohmic contacts to wide-gap semiconductors n - GaN and n - AlN with a high
dislocation density. BothPc(T) curves have portions of exponential decrease, as well as
those with very slight Pc(T) dependence at higher temperatures. Besides, the Au-Pd-TiPd-n-GaN
contacts have a portion of Pc(T) flattening out in the low-temperature region.
This portion appears only after rapid thermal annealing (RTA). In principle, its
appearance may be caused by preliminary heavy doping of the near-contact region with a
shallow donor impurity as well as doping in the course of contact formation owing to
RTA, if the contact-forming layer involves a material atoms of which serve as shallow
donors in III N compounds. The obtained Pc(T) dependences cannot be explained by
the existing mechanisms of current transfer. We propose other mechanisms explaining
the experimental Pc(T) curves for ohmic contacts to n - GaN and n -AlN .
|
| issn |
1560-8034 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/118725 |
| citation_txt |
Investigation of resistance formation mechanisms for contacts to n-AlN and n-GaN with a high dislocation density / A.V. Sachenko, A.E. Belyaev, N.S. Boltovets, Yu.V. Zhilyaev, L.M. Kapitanchuk, V.P. Klad’ko, R.V. Konakova, Ya.Ya. Kudryk, A.V. Kuchuk, A.V. Naumov, V.V. Panteleev, V.N. Sheremet // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2012. — Т. 15, № 4. — С. 351-357. — Бібліогр.: 19 назв. — англ. |
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2025-11-24T02:23:33Z |
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2025-11-24T02:23:33Z |
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| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2012. V. 15, N 4. P. 351-357.
© 2012, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
351
PACS 73.40.Cg, 73.40.Ns, 85.40.-e
Investigation of resistance formation mechanisms for contacts
to n-AlN and n-GaN with a high dislocation density
A.V. Sachenko1, A.E. Belyaev1, N.S. Boltovets2, Yu.V. Zhilyaev3, L.M. Kapitanchuk4, V.P. Klad’ko1,
R.V. Konakova1, Ya.Ya. Kudryk1, A.V. Kuchuk1, A.V. Naumov1, V.V. Panteleev3, V.N. Sheremet1
1V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine, 03028 Kyiv, Ukraine
Phone: 38(044) 525-61-82; fax: 38(044) 525-83-42; e-mail: konakova@isp.kiev.ua
2State Enterprise Research Institute “Orion”, 03057 Kyiv, Ukraine
3Ioffe Physical-Technical Institute, Russian Academy of Sciences, 194021 Sankt-Peterburg, Russia
4Paton Institute of Electric Welding, NAS of Ukraine, 03068 Kyiv, Ukraine
Abstract. We studied temperature dependences of the resistivity, Tc , of Pd-Ti-Pd-Au
ohmic contacts to wide-gap semiconductors GaNn and AlNn with a high
dislocation density. Both Tc curves have portions of exponential decrease, as well as
those with very slight Tc dependence at higher temperatures. Besides, the Au-Pd-Ti-
Pd-n-GaN contacts have a portion of Tc flattening out in the low-temperature region.
This portion appears only after rapid thermal annealing (RTA). In principle, its
appearance may be caused by preliminary heavy doping of the near-contact region with a
shallow donor impurity as well as doping in the course of contact formation owing to
RTA, if the contact-forming layer involves a material atoms of which serve as shallow
donors in NIII compounds. The obtained Tc dependences cannot be explained by
the existing mechanisms of current transfer. We propose other mechanisms explaining
the experimental Tc curves for ohmic contacts to GaNn and AlNn .
Keywords: contact resistivity, III-N compounds, dislocation density.
Manuscript received 22.08.12; revised version received 25.09.12; accepted for
publication 17.10.12; published online 12.12.12.
1. Introduction
In recent years, both the materials science of III-N
compounds and manufacturing technology of various
devices based on these compounds (light-emitting and
Schottky barrier diodes, field-effect transistors, etc.) are
actively elaborated. The corresponding elemental base is
predominantly developed with GaN and its solid
solutions [1-6]. InN and solid solutions in the InN-GaN-
AlN system are also promising materials for
optoelectronic devices. And AlN serves as material for
substrates in a number of device structures.
Formation of ohmic contacts to GaN and InN (as
well as to AlN with its higher resistivity) still makes a
complicated problem. According to classic models,
making the ohmic contact requires that the electron work
function of metal, m , be less than the electron affinity
of AlNn [7]. Since AlN is a wide-gap material
with low electron affinity ( AlN = 0.6 eV) [8], its
electron work function m exceeds AlN . Therefore,
formation of ohmic contacts to AlN within the
framework of classic models presents some difficulties.
Most of III-N compounds are wide-gap. That is why
the present-day devices made on their basis demonstrate
operation temperatures 1.5-2 times higher than those of
silicon and gallium arsenide devices [4, 5]. Therefore,
ohmic contacts are obtained at rather high temperatures of
contact-forming layers firing. This favors appearance of
structurally-imperfect metal-semiconductor interface, in
particular, with a high dislocation density in the near-
contact region of the NIII heterostructures grown
predominantly on foreign substrates. The authors of [9-
11] noted that metal shunts associated with dislocations
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2012. V. 15, N 4. P. 351-357.
© 2012, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
352
can form in the near-contact regions of such structures. As
a result, the contact resistivity c increases with
temperature. This is related to temperature dependence of
metal resistivity and is very undesirable for devices
operating at elevated temperatures.
In [12] it was reported on a mechanism of shunt
formation owing to indium diffusion via dislocations
from the contact formed with indium-tin alloy in gallium
nitride light-emitting diodes. In our papers [13, 14], it
was shown that temperature dependence of contact
resistivity Tc in such ohmic contacts is more
complicated than linear. In this case, growing as well as
decreasing with temperature Tc curves are possible.
As to the ohmic contacts to high-resistance AlN,
there is no detailed information on them in literature,
except for the paper [15] where AlNIn alloyed
contact was considered. It was noted that the above
contact was high-resistance, with linear current-voltage
characteristic, and its resistance was close to the bulk
one. We did not manage to find literature data on Tc
dependence of ohmic contacts to AlNn .
The objective of this work is investigation of the
features of Tc curves of ohmic contacts to GaNn
and AlNn grown on foreign substrates.
2. Experimental procedure
The single-crystalline GaN and AlN films were prepared
at the Ioffe Physical-Technical Institute, using chloride
vapor-phase epitaxy, at a standard setup with a
horizontal reactor [16]. The GaN layers (thickness of
~30 m, donor concentration 318cm10 ) were grown
on an AlN “template” on sapphire. The high-resistant
AlN layers (thickness of ~3.5 m) were grown on a
heavily doped SiCn substrate.
X-ray diffraction measurements on the GaN (AlN)
layers showed that half-width of X-ray diffraction peak
was 0~5 arcmin (30 arcmin). The mean linear dislocation
density was 15cm10 . Using consecutive vacuum
deposition onto substrates heated to 350 С, the Au-Pd-Ti-
Pd-n-GaN (AlN) contact metallization was prepared on
both samples. Then test structures with linear and radial
geometry of templates were formed to measure contact
resistivity with the transmission line method (TLM) [17]
in the 100 to 380 K temperature range.
Ohmic contacts on the Au-Pd-Ti-Pd-n-GaN
structures were made on the substrate heated to 350 С
as well as with further rapid thermal annealing (RTA) at
Т = 900 С for 30 s. Ohmic contact to the Au-Pd-Ti-Pd-
n-AlN structure was formed after such RTA only.
We used Auger electron spectroscopy (LAS 2000
spectrometer) in combination with ion etching (1 keV
Ar+ ions) to study component concentration depth
profiles in the metallization layers. The cleavages of
ohmic contacts and morphological features of GaN and
AlN film surfaces were studied using a Field Emission
Auger Microprobe JAMP 9500F in the scanning electron
microscopy mode.
Shown in Fig. 1а are component concentration
depth profiles in the Au-Pd-Ti-Pd-n-GaN contact
metallization. One can see that this contact system has a
layered structure of metallization with a rather abrupt
interface between contact-forming metal (palladium) and
gallium nitride. This interface was formed in the course
of Pd deposition onto the substrate (GaN surface) heated
to 350 С. Fig. 2а presenting a fragment of Au-Pd-Ti-
Pd-n-GaN contact cleavage confirms presence of a
rather abrupt metal-GaN interface. One can also see
columnar structure of the GaNn film at the cleavage.
However, the GaN film has a rather high density of
hexagonal defects (see Figs 2b to 2f), in particular, with
the lateral dimension close to 10 m (Figs 2с and 2d)
and funnel-shaped deepening with metallized surface
after contact deposition (Fig. 2е), at the depth of 4 to
6 m from the surface at a fragment of TLM structure
(i.e., a test structure intended for measurement of c
using TLM). A fragment of GaN surface with such
defects is presented in Fig. 2f. Similar hexagonal defects
were observed in [6] on thick GaN layers grown on
GaAs substrates. The resistivity of Au-Pd-Ti-Pd-n-GaN
ohmic contacts measured at room temperature was
25 cmOhm107.6 ( 25 cmOhm108.7 ) for linear
(radial) TLM topology.
0 5 10 15 20 25
0
20
40
60
80
100
N
Ga
Ti
Pd
Au
O
A
t.
%
Time of etching, min
C
a)
0 5 10 15 20 25
0
20
40
60
80
100
Ga
N
Ti
Au
O
A
t.
%
Time of etching, min
C
b)
Fig. 1. Component concentration profiles in the Au-Pd-Ti-Pd-
n-GaN contact metallization: а – initial, b – after RTA at
900 С for 30 s.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2012. V. 15, N 4. P. 351-357.
© 2012, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
353
a) b)
c) d)
e) f)
Fig. 2. a – pattern of an Au-Pd-Ti-Pd-n-GaN contact cleavage fragment; b – morphology of GaN film surface with hexagonal
defects; c – fragment of a TLM structure with hexagonal defects; d – morphology of a hexagonal defect; e – a funnel-shaped
metallization defect; f – 3D fragment of a sample with hexagonal defects.
Fig. 3а presents component concentration depth
profiles in the Au-Pd-Ti-Pd-n-AlN contact metallization
formed on a substrate heated to 350 С. One can see that
the metal-AlN interface is considerably smeared. This is
a region of mixing the AlN components, Pd and Ti with
a portion (up to 10 atomic percent) of oxygen in the Ti
and Pd layers. A cleavage of the Au-Pd-Ti-Pd-n-AlN
contact structure (Fig. 4) confirms structural
nonuniformity of the metal-AlN interface. In this case,
one can see from the morphology of the initial AlNn
film (Fig. 5) that this film has pores ~40 nm in diameter.
X-ray diffraction data (XRD) for contact structures
Au-Pd-Ti-Pd-n-GaN(AlN) are shown in Fig. 6. In the
XRD spectra from the initial samples, the reflections of
Au (111, 200) and Pd (111) from polycrystalline metal
layers are observed. The absence of reflections from the
Ti film is apparently related to the so-called X-ray
amorphous state of titanium, which has a metallic
conductivity. (Structural quality (half-width and intensity
of the peaks) of Au and Pd films is much better on GaN
substrate than on the AlN substrate). After annealing, the
following phase transitions are observed. For AlN
substrate, annealing leads to the interdiffusion of metal
atoms plating and the formation of Aux(Pd, Ti)1–x solid
solution. The observed peak at ~38.2° indicates that some
fraction of pure Au is remained.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2012. V. 15, N 4. P. 351-357.
© 2012, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
354
0 5 10 15 20
0
20
40
60
80
100
Al
N
Ti
Pd
C
A
t.
%
Time of etching, min
O
Au a)
0 5 10 15 20 25 30
0
20
40
60
80
100
Al
N
Ti
Pd
Au
O
C
A
t.
%
Time of etching, min
b)
Fig. 3. Component concentration profiles in the Au-Pd-Ti-Pd-
n-AlN contact metallization: а – initial; b – after RTA at
900 С for 30 s.
For GaN substrate, annealing also leads to
interdiffusion of metal atoms. However, in this case, the
formation of Pd3Ti phases, as well as two
nonstoichiometric phases (solid solutions) based on Au
with lattice constant smaller and larger than in pure gold.
We assume that closer to the interface the Ga-rich solid
solution of x1x TiPd,GaAu is formed. And closer to
the surface, the Pd-rich solid solution of
x1x Ti,GaPdAu is formed. So, in the case of GaN
substrate, interaction between metallization and substrate
is observed.
Fig. 4. Pattern of an Au-Pd-Ti-Pd-n-AlN contact cleavage
fragment.
Fig. 5. Morphology of n-AlN film surface.
Fig. 6. Fragments of X-ray diffraction patterns for Au-Pd-Ti-
Pd/AlN/SiC and Au-Pd-Ti-Pd/GaN/Al2O3 structures.
The Au-Pd-Ti-Pd-n-AlN contact formed after metal
deposition onto a substrate heated up to 350 С is a high-
resistance non-ohmic contact. It becomes ohmic after
RTA at Т = 900 С for 30 s only. Resistivity of such a
contact measured at room temperature on a radial test
TLM structure was 0.05 Ohmcm2. It should be noted
that an alloyed GaN-In n ohmic contact with n ≈ 1018
and 318cm108 demonstrated c of the same order at
Т = 300 K [15]. It was also found that, after RTA at
900 С for 30 s, metallization components of contact
structures on AlN and GaN were intermixed (see Figs 1c
and 3c, respectively).
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2012. V. 15, N 4. P. 351-357.
© 2012, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
355
a) b)
c) d)
Fig. 7. Morphology of Au film surface in Au-Pd-Ti-Pd-n-GaN contact metallization (а – initial; b – after RTA at
900 С for 30 s) and in Au-Pd-Ti-Pd-n-AlN contact metallization (с – initial; d – after RTA at 900 С for 30 s).
Intense mass transport in contact metallization
manifested itself also at the surface of the upper metal
layer (Au films) – see Figs 7a to 7d. One can see from
comparison of initial samples with those subjected to
RTA that after RTA gold coating on the contact
metallization to GaN is more non-uniform than that on
AlN. The morphology data on the Au films correlate
with the contact concentration depth profiles in those
contacts. However, the cleavage structure in the Au-Pd-
Ti-Pd-n-GaN ( AlNn ) contacts did not change
qualitatively after RTA.
2. Experimental results and discussion
Shown in Fig. 8 are the temperature dependences of the
contact resistivity, Tc , for annealed ohmic contacts
Au-Pd-Ti-Pd-n-AlN (curve I) and Au-Pd-Ti-Pd-n-GaN
(curve II). A comparison of Tc curves for ohmic
contacts to AlNn and GaNn showed that in both
cases, at low temperatures of measurement, c
decreases as temperature grows from 160 up to 200 K
(250 K) for contacts to GaNn ( AlNn ). At further
growth of temperature up to 375 K, c varies but
slightly. The considerable distinction between the above
dependences is that at room temperature the c value
for ohmic contact to AlNn is three orders higher than
that to GaNn . The reason is related with the lower
doping level of AlNn . Besides, as one can see from
Fig. 8 (curve II), at low temperatures (100 to 150 K) the
c value for ohmic contact to GaNn practically does
not depend on temperature. The current-voltage
characteristics of all the contacts under investigation are
linear in the 100 to 375 K temperature range.
To explain these effects, a novel concept of current
transfer (adequate in the case of high dislocation density
in semiconductor) was proposed in [13, 14]. This
concept takes into account current flow through metal
shunts (associated with dislocations) and current
limitation by diffusion supply of electrons. The currents
flowing between dislocations were neglected. The
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2012. V. 15, N 4. P. 351-357.
© 2012, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
356
100 150 200 250 300 350 400
10-6
10-5
0,1
1
3
2
1
T, K
II
1
2
I
c,
•c
m
2
Fig. 8. Temperature dependence c(T) after RTA at 900 С
for 30 s in ohmic contacts Au-Pd-Ti-Pd-n-AlN (curve I) and
Au-Pd-Ti-Pd-n-GaN (curve II).
contact resistivity c may increase or decrease with
temperature, depending on the predominant mechanism
of electron scattering. However, Tc presented in
Fig. 8 cannot be explained by either the existing models
for current transport in ohmic contacts [7] or the concept
proposed in [13, 14].
Let us start discussion of the results obtained from
the curve II (Fig. 8). It presents temperature dependence
of the contact resistivity for ohmic contacts to GaN on
the Al2O3 substrate. One can see that curve II has three
portions: 1) very weak dependence c(T) in the 200-
380 K temperature range, 2) exponential dependence,
with activation energy about 0.1 eV, in the 160-200 K
temperature range, and 3) c does not depend on
temperature at T < 160 K.
In principle, the portion 1 may correspond to the
mechanism of electron current flow through the metal
shunts associated with dislocations, with current
limitation by diffusion supply of electrons to the contact
[13, 14]. Growth of c(T) in the portion 2 as temperature
decreases is purely exponential. This cannot be
explained by power dependence for electron mobility
decrease due to scattering on dislocations or low-
temperature “freezing-out” of electrons. In our case,
silicon is a shallow donor with ionization energy below
20 meV, and this has to result in increasing c(T) curve
at temperatures below 50 K only. Therefore, to explain
the portion 2 one should assume that this is related to the
structural disorder effect on metal shunt conductivity at
low temperatures, which changes the conduction type
from metallic to semiconductor one with a
corresponding activation energy. This seems plausible if
one takes into account that metal shunt diameter is close
to atomic sizes and, in principle, a single defect could
lead to activation-type conduction at low
temperatures [18].
The currents flowing through dislocations
associated with metal shunts as well as currents flowing
between dislocations correspond to parallel connection
of resistances. Therefore, the plateau of the Tc curve
in the portion 3 at T < 160 K is to be related to the
current flowing between dislocations in the case of
heavily doped near-contact region and, correspondingly,
strong degeneracy. Then the contact resistance is
determined by the thermionic emission mechanism and
practically does not depend on temperature (see
[13, 14]). The portion 3 appears for the Au-Pd-Ti-Pd-n-
GaN structures under investigation after RTA only.
In principle, appearance of portion 3 may be related
to previous heavy doping of the near-contact region with
a shallow donor impurity as well as doping in the course
of contact formation due to RTA, if the contact-forming
layer involves material serving as a shallow donor in the
N-III compound. The relatively high c value in the
portion 3 (of the order of 25 cmOhm10 ) may be
explained by potential nonuniformity at the interface
plane within the ShklovskiiEfros theory [19].
In the case of contact to AlN (curve I in Fig. 8), the
Tc curve has only two portions, 1 and 2 (without
portion 3), whose presence is explained just as in the
case of contact to GaN. In principle, portion 3 (plateau)
may appear in the case of contact to AlN at lower
temperatures than in the case of contact to GaN.
The obtained results make it possible to draw some
conclusions concerning specific character of the physical
mechanisms responsible for current flow in ohmic
contacts to wide-gap semiconductors with a high density
of structural defects.
4. Conclusion
Investigation of the temperature dependence for the
contact resistivity, Tc , was performed for Au-Pd-Ti-
Pd-n-GaN ( AlNn ) ohmic contacts. It was found that,
in the 380-200 K range, Tc is determined by current
flow through metal shunts associated with dislocations
and is limited by diffusion supply of electrons. In the
250-100 K (200-160 K) temperature range for ohmic
contacts to AlNn ( GaNn ), contact resistivity c
grows exponentially as temperature decreases. This
behavior cannot be described by either thermionic
emission or field emission. For the ohmic contact to
GaNn , in the 160-100 K temperature range, c does
not depend on temperature. This is due to thermionic
mechanism of current transport (at strong degeneracy)
owing to current transfer between dislocations.
It is supposed that the exponential dependence
Tc at low temperatures can be explained by taking
into account the effects of structural disorder on
conductivity of metal shunts having atomic sizes.
According to [19], these effects are responsible for
changing the conduction type from metallic to
semiconductor one.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2012. V. 15, N 4. P. 351-357.
© 2012, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
357
Acknowledgements
This work was supported by the State Target Scientific
and Technical Program of Ukraine “Nanotechnologies
and nanomaterials” for 2010-2014.
References
1. H. Morkoç, Handbook of Nitride Semiconductors
and Devices, Vol. 2. WILEY-VCH Verlag GmbH
&Co, KGaA, Weinheim, 2008.
2. F.E. Shubert, Light-Emitting Diodes, 2nd Ed.
Cambridge University Press, 2006.
3. Yu.G. Shreter, Yu.T. Rebane, V.A. Zykov,
V.G. Sidorov, Wide-Gap Semiconductors. Nauka,
Sankt-Peterburg, 2001 (in Russian).
4. R. Quay, Gallium Nitride Electronics. Springer-
Verlag, Berlin-Heidelberg, 2008.
5. A.G. Vasil’ev, Yu.V. Kolkovskii, Yu.A. Kontsevoi,
Microwave Wide-Gap Semiconductor Transistors.
Tekhnosfera, Moscow, 2011 (in Russian).
6. Technology of Gallium Nitride Crystal Growth,
Eds. D. Ehrentraut, E. Meissner, M. Bockowski.
Springer-Verlag, Berlin, 2010.
7. S.M. Sze, K.K. Ng, Physics of Semiconductor
Devices, 3rd Ed. John Wiley and Sons, 2007.
8. Properties of Advanced Semiconductor Materials,
Eds. M. Levinshtein, S. Rumyantsev, M. Shur.
John Wiley and Sons, 2001.
9. T.V. Blank, Yu.A. Gol’dberg // Semiconductors,
41(11), p. 1263 (2007).
10. T.V. Blank, Yu.A. Gol’dberg, E.A. Posse //
Semiconductors, 40(10), p. 1173 (2006).
11. T.V. Blank, Yu.A. Gol’dberg, E.A. Posse //
Semiconductors, 43(9), p. 1164 (2009).
12. Сhin-Yuan Hsu, Wen-How Lan, Yew Chung
Sermon Wu // Jap. J. Appl. Phys. 44(10), p. 7424
(2005).
13. A.V. Sachenko, A.E. Belyaev, A.V. Bobyl,
N.S. Boltovets, V.N. Ivanov, L.M. Kapitanchuk,
R.V. Konakova, Ya.Ya. Kudryk, V.V. Milenin,
S.V. Novitskii, I.S. Tarasov, V.N. Sheremet,
M.Ya. Yagovkina // Semiconductors, 46(3), p. 334
(2012).
14. A.V. Sachenko, A.E. Belyaev, N.S. Boltovets,
R.V. Konakova, Ya.Ya. Kudryk, S.V. Novitskii,
V.N. Sheremet, J. Li, S.A. Vitusevich // J. Appl.
Phys. 111(8), 083701 (2012).
15. V.N. Bessolov, T.V. Blank, Yu.A. Gold’berg,
O.V. Konstantinov, E.A. Posse // Semiconductors,
42(11), p. 1315 (2008).
16. Yu.V. Zhilyaev, S.N. Rodin // Techn. Phys. Lett.
36(5), p. 397 (2010).
17. D.K. Schroder, Semiconductor Material and
Device Characterization. Wiley, New Jersey, 2006.
18. Y. Imry, Introduction to Mesoscopic Physics.
Oxford University Press, 2002.
19. B.I. Shklovskii, A.L. Efros, Electronic Properties
of Doped Semiconductors. Springer, Berlin, 1984.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2012. V. 15, N 4. P. 351-357.
PACS 73.40.Cg, 73.40.Ns, 85.40.-e
Investigation of resistance formation mechanisms for contacts
to n-AlN and n-GaN with a high dislocation density
A.V. Sachenko1, A.E. Belyaev1, N.S. Boltovets2, Yu.V. Zhilyaev3, L.M. Kapitanchuk4, V.P. Klad’ko1,
R.V. Konakova1, Ya.Ya. Kudryk1, A.V. Kuchuk1, A.V. Naumov1, V.V. Panteleev3, V.N. Sheremet1
1V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine, 03028 Kyiv, Ukraine
Phone: 38(044) 525-61-82; fax: 38(044) 525-83-42; e-mail: konakova@isp.kiev.ua
2State Enterprise Research Institute “Orion”, 03057 Kyiv, Ukraine
3Ioffe Physical-Technical Institute, Russian Academy of Sciences, 194021 Sankt-Peterburg, Russia
4Paton Institute of Electric Welding, NAS of Ukraine, 03068 Kyiv, Ukraine
Abstract. We studied temperature dependences of the resistivity,
(
)
T
c
r
, of Pd-Ti-Pd-Au ohmic contacts to wide-gap semiconductors GaN
-
n
and AlN
-
n
with a high dislocation density. Both
(
)
T
c
r
curves have portions of exponential decrease, as well as those with very slight
(
)
T
c
r
dependence at higher temperatures. Besides, the Au-Pd-Ti-Pd-n-GaN contacts have a portion of
(
)
T
c
r
flattening out in the low-temperature region. This portion appears only after rapid thermal annealing (RTA). In principle, its appearance may be caused by preliminary heavy doping of the near-contact region with a shallow donor impurity as well as doping in the course of contact formation owing to RTA, if the contact-forming layer involves a material atoms of which serve as shallow donors in
N
III
-
compounds. The obtained
(
)
T
c
r
dependences cannot be explained by the existing mechanisms of current transfer. We propose other mechanisms explaining the experimental
(
)
T
c
r
curves for ohmic contacts to GaN
-
n
and AlN
-
n
.
Keywords: contact resistivity, III-N compounds, dislocation density.
Manuscript received 22.08.12; revised version received 25.09.12; accepted for publication 17.10.12; published online 12.12.12.
1. Introduction
In recent years, both the materials science of III-N compounds and manufacturing technology of various devices based on these compounds (light-emitting and Schottky barrier diodes, field-effect transistors, etc.) are actively elaborated. The corresponding elemental base is predominantly developed with GaN and its solid solutions [1-6]. InN and solid solutions in the InN-GaN-AlN system are also promising materials for optoelectronic devices. And AlN serves as material for substrates in a number of device structures.
Formation of ohmic contacts to GaN and InN (as well as to AlN with its higher resistivity) still makes a complicated problem. According to classic models, making the ohmic contact requires that the electron work function of metal,
m
j
, be less than the electron affinity
c
of AlN
-
n
[7]. Since AlN is a wide-gap material with low electron affinity (
AlN
c
= 0.6 eV) [8], its electron work function
m
j
exceeds
AlN
c
. Therefore, formation of ohmic contacts to AlN within the framework of classic models presents some difficulties.
Most of III-N compounds are wide-gap. That is why the present-day devices made on their basis demonstrate operation temperatures 1.5-2 times higher than those of silicon and gallium arsenide devices [4, 5]. Therefore, ohmic contacts are obtained at rather high temperatures of contact-forming layers firing. This favors appearance of structurally-imperfect metal-semiconductor interface, in particular, with a high dislocation density in the near-contact region of the
N
III
-
heterostructures grown predominantly on foreign substrates. The authors of [9-11] noted that metal shunts associated with dislocations can form in the near-contact regions of such structures. As a result, the contact resistivity
c
r
increases with temperature. This is related to temperature dependence of metal resistivity and is very undesirable for devices operating at elevated temperatures.
In [12] it was reported on a mechanism of shunt formation owing to indium diffusion via dislocations from the contact formed with indium-tin alloy in gallium nitride light-emitting diodes. In our papers [13, 14], it was shown that temperature dependence of contact resistivity
(
)
T
c
r
in such ohmic contacts is more complicated than linear. In this case, growing as well as decreasing with temperature
(
)
T
c
r
curves are possible.
As to the ohmic contacts to high-resistance AlN, there is no detailed information on them in literature, except for the paper [15] where AlN
In
-
alloyed contact was considered. It was noted that the above contact was high-resistance, with linear current-voltage characteristic, and its resistance was close to the bulk one. We did not manage to find literature data on
(
)
T
c
r
dependence of ohmic contacts to AlN
-
n
.
The objective of this work is investigation of the features of
(
)
T
c
r
curves of ohmic contacts to GaN
-
n
and AlN
-
n
grown on foreign substrates.
2. Experimental procedure
The single-crystalline GaN and AlN films were prepared at the Ioffe Physical-Technical Institute, using chloride vapor-phase epitaxy, at a standard setup with a horizontal reactor [16]. The GaN layers (thickness of ~30 (m, donor concentration 3
18
cm
10
-
>
) were grown on an AlN “template” on sapphire. The high-resistant AlN layers (thickness of ~3.5 (m) were grown on a heavily doped
SiC
-
+
n
substrate.
X-ray diffraction measurements on the GaN (AlN) layers showed that half-width of X-ray diffraction peak was (0~5 arcmin (30 arcmin). The mean linear dislocation density was 1
5
cm
10
-
£
. Using consecutive vacuum deposition onto substrates heated to 350 (С, the Au-Pd-Ti-Pd-n-GaN (AlN) contact metallization was prepared on both samples. Then test structures with linear and radial geometry of templates were formed to measure contact resistivity with the transmission line method (TLM) [17] in the 100 to 380 K temperature range.
Ohmic contacts on the Au-Pd-Ti-Pd-n-GaN structures were made on the substrate heated to 350 (С as well as with further rapid thermal annealing (RTA) at Т = 900 (С for 30 s. Ohmic contact to the Au-Pd-Ti-Pd-n-AlN structure was formed after such RTA only.
We used Auger electron spectroscopy (LAS 2000 spectrometer) in combination with ion etching (1 keV Ar+ ions) to study component concentration depth profiles in the metallization layers. The cleavages of ohmic contacts and morphological features of GaN and AlN film surfaces were studied using a Field Emission Auger Microprobe JAMP 9500F in the scanning electron microscopy mode.
Shown in Fig. 1а are component concentration depth profiles in the Au-Pd-Ti-Pd-n-GaN contact metallization. One can see that this contact system has a layered structure of metallization with a rather abrupt interface between contact-forming metal (palladium) and gallium nitride. This interface was formed in the course of Pd deposition onto the substrate (GaN surface) heated to 350 (С. Fig. 2а presenting a fragment of Au-Pd-Ti-Pd-n-GaN contact cleavage confirms presence of a rather abrupt metal-GaN interface. One can also see columnar structure of the GaN
-
n
film at the cleavage.
However, the GaN film has a rather high density of hexagonal defects (see Figs 2b to 2f), in particular, with the lateral dimension close to 10 (m (Figs 2с and 2d) and funnel-shaped deepening with metallized surface after contact deposition (Fig. 2е), at the depth of 4 to 6 (m from the surface at a fragment of TLM structure (i.e., a test structure intended for measurement of
c
r
using TLM). A fragment of GaN surface with such defects is presented in Fig. 2f. Similar hexagonal defects were observed in [6] on thick GaN layers grown on GaAs substrates. The resistivity of Au-Pd-Ti-Pd-n-GaN ohmic contacts measured at room temperature was 2
5
cm
Ohm
10
7
.
6
×
×
-
(
2
5
cm
Ohm
10
8
.
7
×
×
-
) for linear (radial) TLM topology.
0
5
10
15
20
25
0
20
40
60
80
100
N
Ga
Ti
Pd
Au
O
At. %
Time of etching, min
C
a)
0
5
10
15
20
25
0
20
40
60
80
100
Ga
N
Ti
Au
O
At. %
Time of etching, min
C
b)
Fig. 1. Component concentration profiles in the Au-Pd-Ti-Pd-n-GaN contact metallization: а – initial, b – after RTA at 900 (С for 30 s.
100
150
200
250
300
350
400
10
-6
10
-5
0,1
1
3
2
1
T, K
II
1
2
I
r
c
,
W
•cm
2
Fig. 3а presents component concentration depth profiles in the Au-Pd-Ti-Pd-n-AlN contact metallization formed on a substrate heated to 350 (С. One can see that the metal-AlN interface is considerably smeared. This is a region of mixing the AlN components, Pd and Ti with a portion (up to 10 atomic percent) of oxygen in the Ti and Pd layers. A cleavage of the Au-Pd-Ti-Pd-n-AlN contact structure (Fig. 4) confirms structural nonuniformity of the metal-AlN interface. In this case, one can see from the morphology of the initial AlN
-
n
film (Fig. 5) that this film has pores ~40 nm in diameter.
X-ray diffraction data (XRD) for contact structures Au-Pd-Ti-Pd-n-GaN(AlN) are shown in Fig. 6. In the XRD spectra from the initial samples, the reflections of Au (111, 200) and Pd (111) from polycrystalline metal layers are observed. The absence of reflections from the Ti film is apparently related to the so-called X-ray amorphous state of titanium, which has a metallic conductivity. (Structural quality (half-width and intensity of the peaks) of Au and Pd films is much better on GaN substrate than on the AlN substrate). After annealing, the following phase transitions are observed. For AlN substrate, annealing leads to the interdiffusion of metal atoms plating and the formation of Aux(Pd, Ti)1–x solid solution. The observed peak at ~38.2° indicates that some fraction of pure Au is remained.
0
5
10
15
20
0
20
40
60
80
100
Al
N
Ti
Pd
C
At. %
Time of etching, min
O
Au
a)
0
5
10
15
20
25
30
0
20
40
60
80
100
Al
N
Ti
Pd
Au
O
C
At. %
Time of etching, min
b)
Fig. 3. Component concentration profiles in the Au-Pd-Ti-Pd-n-AlN contact metallization: а – initial; b – after RTA at 900 (С for 30 s.
For GaN substrate, annealing also leads to interdiffusion of metal atoms. However, in this case, the formation of Pd3Ti phases, as well as two nonstoichiometric phases (solid solutions) based on Au with lattice constant smaller and larger than in pure gold. We assume that closer to the interface the Ga-rich solid solution of
(
)
x
1
x
Ti
Pd,
Ga
Au
-
is formed. And closer to the surface, the Pd-rich solid solution of
(
)
x
1
x
Ti
,
Ga
Pd
Au
-
is formed. So, in the case of GaN substrate, interaction between metallization and substrate is observed.
Fig. 4. Pattern of an Au-Pd-Ti-Pd-n-AlN contact cleavage fragment.
Fig. 5. Morphology of n-AlN film surface.
Fig. 6. Fragments of X-ray diffraction patterns for Au-Pd-Ti-Pd/AlN/SiC and Au-Pd-Ti-Pd/GaN/Al2O3 structures.
The Au-Pd-Ti-Pd-n-AlN contact formed after metal deposition onto a substrate heated up to 350 (С is a high-resistance non-ohmic contact. It becomes ohmic after RTA at Т = 900 (С for 30 s only. Resistivity of such a contact measured at room temperature on a radial test TLM structure was 0.05 Ohm(cm2. It should be noted that an alloyed
GaN
-
In
n
-
ohmic contact with n ≈ 1018 and
3
18
cm
10
8
-
×
demonstrated
c
r
of the same order at Т = 300 K [15]. It was also found that, after RTA at 900 (С for 30 s, metallization components of contact structures on AlN and GaN were intermixed (see Figs 1c and 3c, respectively).
Intense mass transport in contact metallization manifested itself also at the surface of the upper metal layer (Au films) – see Figs 7a to 7d. One can see from comparison of initial samples with those subjected to RTA that after RTA gold coating on the contact metallization to GaN is more non-uniform than that on AlN. The morphology data on the Au films correlate with the contact concentration depth profiles in those contacts. However, the cleavage structure in the Au-Pd-Ti-Pd-n-GaN (AlN
-
n
) contacts did not change qualitatively after RTA.
2. Experimental results and discussion
Shown in Fig. 8 are the temperature dependences of the contact resistivity,
(
)
T
c
r
, for annealed ohmic contacts Au-Pd-Ti-Pd-n-AlN (curve I) and Au-Pd-Ti-Pd-n-GaN (curve II). A comparison of
(
)
T
c
r
curves for ohmic contacts to AlN
-
n
and GaN
-
n
showed that in both cases, at low temperatures of measurement,
c
r
decreases as temperature grows from 160 up to 200 K (250 K) for contacts to GaN
-
n
(AlN
-
n
). At further growth of temperature up to 375 K,
c
r
varies but slightly. The considerable distinction between the above dependences is that at room temperature the
c
r
value for ohmic contact to AlN
-
n
is three orders higher than that to GaN
-
n
. The reason is related with the lower doping level of AlN
-
n
. Besides, as one can see from Fig. 8 (curve II), at low temperatures (100 to 150 K) the
c
r
value for ohmic contact to GaN
-
n
practically does not depend on temperature. The current-voltage characteristics of all the contacts under investigation are linear in the 100 to 375 K temperature range.
To explain these effects, a novel concept of current transfer (adequate in the case of high dislocation density in semiconductor) was proposed in [13, 14]. This concept takes into account current flow through metal shunts (associated with dislocations) and current limitation by diffusion supply of electrons. The currents flowing between dislocations were neglected. The contact resistivity (c may increase or decrease with temperature, depending on the predominant mechanism of electron scattering. However,
(
)
T
c
r
presented in Fig. 8 cannot be explained by either the existing models for current transport in ohmic contacts [7] or the concept proposed in [13, 14].
Let us start discussion of the results obtained from the curve II (Fig. 8). It presents temperature dependence of the contact resistivity for ohmic contacts to GaN on the Al2O3 substrate. One can see that curve II has three portions: 1) very weak dependence (c(T) in the 200-380 K temperature range, 2) exponential dependence, with activation energy about 0.1 eV, in the 160-200 K temperature range, and 3) (c does not depend on temperature at T < 160 K.
In principle, the portion 1 may correspond to the mechanism of electron current flow through the metal shunts associated with dislocations, with current limitation by diffusion supply of electrons to the contact [13, 14]. Growth of (c(T) in the portion 2 as temperature decreases is purely exponential. This cannot be explained by power dependence for electron mobility decrease due to scattering on dislocations or low-temperature “freezing-out” of electrons. In our case, silicon is a shallow donor with ionization energy below 20 meV, and this has to result in increasing (c(T) curve at temperatures below 50 K only. Therefore, to explain the portion 2 one should assume that this is related to the structural disorder effect on metal shunt conductivity at low temperatures, which changes the conduction type from metallic to semiconductor one with a corresponding activation energy. This seems plausible if one takes into account that metal shunt diameter is close to atomic sizes and, in principle, a single defect could lead to activation-type conduction at low temperatures [18].
The currents flowing through dislocations associated with metal shunts as well as currents flowing between dislocations correspond to parallel connection of resistances. Therefore, the plateau of the
(
)
T
c
r
curve in the portion 3 at T < 160 K is to be related to the current flowing between dislocations in the case of heavily doped near-contact region and, correspondingly, strong degeneracy. Then the contact resistance is determined by the thermionic emission mechanism and practically does not depend on temperature (see [13, 14]). The portion 3 appears for the Au-Pd-Ti-Pd-n-GaN structures under investigation after RTA only.
In principle, appearance of portion 3 may be related to previous heavy doping of the near-contact region with a shallow donor impurity as well as doping in the course of contact formation due to RTA, if the contact-forming layer involves material serving as a shallow donor in the
N
-
III
compound. The relatively high
c
r
value in the portion 3 (of the order of
2
5
cm
Ohm
10
×
-
) may be explained by potential nonuniformity at the interface plane within the Shklovskii(Efros theory [19].
In the case of contact to AlN (curve I in Fig. 8), the
(
)
T
c
r
curve has only two portions, 1 and 2 (without portion 3), whose presence is explained just as in the case of contact to GaN. In principle, portion 3 (plateau) may appear in the case of contact to AlN at lower temperatures than in the case of contact to GaN.
The obtained results make it possible to draw some conclusions concerning specific character of the physical mechanisms responsible for current flow in ohmic contacts to wide-gap semiconductors with a high density of structural defects.
4. Conclusion
Investigation of the temperature dependence for the contact resistivity,
(
)
T
c
r
, was performed for Au-Pd-Ti-Pd-n-GaN (AlN
-
n
) ohmic contacts. It was found that, in the 380-200 K range,
(
)
T
c
r
is determined by current flow through metal shunts associated with dislocations and is limited by diffusion supply of electrons. In the 250-100 K (200-160 K) temperature range for ohmic contacts to AlN
-
n
(GaN
-
n
), contact resistivity
c
r
grows exponentially as temperature decreases. This behavior cannot be described by either thermionic emission or field emission. For the ohmic contact to GaN
-
n
, in the 160-100 K temperature range,
c
r
does not depend on temperature. This is due to thermionic mechanism of current transport (at strong degeneracy) owing to current transfer between dislocations.
It is supposed that the exponential dependence
(
)
T
c
r
at low temperatures can be explained by taking into account the effects of structural disorder on conductivity of metal shunts having atomic sizes. According to [19], these effects are responsible for changing the conduction type from metallic to semiconductor one.
Acknowledgements
This work was supported by the State Target Scientific and Technical Program of Ukraine “Nanotechnologies and nanomaterials” for 2010-2014.
References
1. H. Morkoç, Handbook of Nitride Semiconductors and Devices, Vol. 2. WILEY-VCH Verlag GmbH &Co, KGaA, Weinheim, 2008.
2. F.E. Shubert, Light-Emitting Diodes, 2nd Ed. Cambridge University Press, 2006.
3. Yu.G. Shreter, Yu.T. Rebane, V.A. Zykov, V.G. Sidorov, Wide-Gap Semiconductors. Nauka, Sankt-Peterburg, 2001 (in Russian).
4. R. Quay, Gallium Nitride Electronics. Springer-Verlag, Berlin-Heidelberg, 2008.
5. A.G. Vasil’ev, Yu.V. Kolkovskii, Yu.A. Kontsevoi, Microwave Wide-Gap Semiconductor Transistors. Tekhnosfera, Moscow, 2011 (in Russian).
6. Technology of Gallium Nitride Crystal Growth, Eds. D. Ehrentraut, E. Meissner, M. Bockowski. Springer-Verlag, Berlin, 2010.
7. S.M. Sze, K.K. Ng, Physics of Semiconductor Devices, 3rd Ed. John Wiley and Sons, 2007.
8. Properties of Advanced Semiconductor Materials, Eds. M. Levinshtein, S. Rumyantsev, M. Shur. John Wiley and Sons, 2001.
9.
T.V. Blank, Yu.A. Gol’dberg // Semiconductors, 41(11), p. 1263 (2007).
10. T.V. Blank, Yu.A. Gol’dberg, E.A. Posse // Semiconductors, 40(10), p. 1173 (2006).
11. T.V. Blank, Yu.A. Gol’dberg, E.A. Posse // Semiconductors, 43(9), p. 1164 (2009).
12. Сhin-Yuan Hsu, Wen-How Lan, Yew Chung Sermon Wu // Jap. J. Appl. Phys. 44(10), p. 7424 (2005).
13. A.V. Sachenko, A.E. Belyaev, A.V. Bobyl, N.S. Boltovets, V.N. Ivanov, L.M. Kapitanchuk, R.V. Konakova, Ya.Ya. Kudryk, V.V. Milenin, S.V. Novitskii, I.S. Tarasov, V.N. Sheremet, M.Ya. Yagovkina // Semiconductors, 46(3), p. 334 (2012).
14. A.V. Sachenko, A.E. Belyaev, N.S. Boltovets, R.V. Konakova, Ya.Ya. Kudryk, S.V. Novitskii, V.N. Sheremet, J. Li, S.A. Vitusevich // J. Appl. Phys. 111(8), 083701 (2012).
15. V.N. Bessolov, T.V. Blank, Yu.A. Gold’berg, O.V. Konstantinov, E.A. Posse // Semiconductors, 42(11), p. 1315 (2008).
16. Yu.V. Zhilyaev, S.N. Rodin // Techn. Phys. Lett. 36(5), p. 397 (2010).
17. D.K. Schroder, Semiconductor Material and Device Characterization. Wiley, New Jersey, 2006.
18. Y. Imry, Introduction to Mesoscopic Physics. Oxford University Press, 2002.
19. B.I. Shklovskii, A.L. Efros, Electronic Properties of Doped Semiconductors. Springer, Berlin, 1984.
� EMBED Origin50.Graph ���
Fig. 8. Temperature dependence (c(T) after RTA at 900 (С for 30 s in ohmic contacts Au-Pd-Ti-Pd-n-AlN (curve I) and Au-Pd-Ti-Pd-n-GaN (curve II).
� �
a) b)
� �
c) d)
Fig. 7. Morphology of Au film surface in Au-Pd-Ti-Pd-n-GaN contact metallization (а – initial; b – after RTA at 900 (С for 30 s) and in Au-Pd-Ti-Pd-n-AlN contact metallization (с – initial; d – after RTA at 900 (С for 30 s).
� �
a) b)
� �
c) d)
� �
e) f)
Fig. 2. a – pattern of an Au-Pd-Ti-Pd-n-GaN contact cleavage fragment; b – morphology of GaN film surface with hexagonal defects; c – fragment of a TLM structure with hexagonal defects; d – morphology of a hexagonal defect; e – a funnel-shaped metallization defect; f – 3D fragment of a sample with hexagonal defects.
© 2012, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
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