Ohmic contacts to InN-based materials
The key aspects of ohmic contact formation to InN-based materials were investigated. Detailed analysis of studies conducted over the past three decades, allows determining the basic principles of such contacts. The contact structure properties and optimal conditions for them are presented. Different...
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Sai, P.O. 2017-04-09T17:30:39Z 2017-04-09T17:30:39Z 2016 Ohmic contacts to InN-based materials / P.O. Sai // Технология и конструирование в электронной аппаратуре. — 2016. — № 4-5. — С. 3-14. — Бібліогр.: 52 назв. — англ. 2225-5818 DOI: 10.15222/TKEA2016.4-5.03 https://nasplib.isofts.kiev.ua/handle/123456789/115688 538.91 The key aspects of ohmic contact formation to InN-based materials were investigated. Detailed analysis of studies conducted over the past three decades, allows determining the basic principles of such contacts. The contact structure properties and optimal conditions for them are presented. Different types of metallization are considered, the advantages and disadvantages of each are determined, including the basic requirements that such contact must meet. There is emphasis on the using multilayer metallization with the barrier layers. In the case of the InAlN/GaN systems, the general approaches of forming ohmic contacts were considered. Рассмотрены ключевые моменты в формировании омических контактов к нитрид индиевых пленок, фокусируясь на n-InN и InAlN/GaN гетероструктурах. Детальный анализ исследований, проведенных за последние три десятилетия, позволяет определить основные принципы формирования подобных контактов. Приведены параметры контактов и оптимальные условия их достижения, рассмотрены различные типы металлизации и определены преимущества и недостатки каждого из них, учитывая основные требования, которым подобные контакты должны отвечать. Сделан акцент на перспективах использования многослойной металлизации с диффузионными барьерами. Рассмотрены общие подходы к формированию омических контактов к InAlN/GaN-гетероструктур. В даній роботі розглянуто ключові моменти в формуванні омічних контактів до нітрид-індієвих плівок, фокусуючись на гетероструктурах n-InN і InAlN/GaN. Детальний аналіз досліджень, проведених за останні три десятиліття, дозволив визначити основні принципи формування подібних контактів. Наведено параметри контактів та оптимальні умови їх досягнення, розглянуто різні типи металізації і визначено переваги та недоліки кожного з них, враховуючи основні вимоги, яким подібні контакти мають відповідати. Наголошено на перспективах використання багатошарової металізації з дифузійними бар’єрами. Розглянуто загальні підходи формування омічних контактів до InAlN/GaN-гетероструктур. en Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України Технология и конструирование в электронной аппаратуре Новые компоненты для электронной аппаратуры Ohmic contacts to InN-based materials Омические контакты к материалам на основе нитрида индия Омічні контакти до матеріалів на основі нітриду індію Article published earlier |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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| title |
Ohmic contacts to InN-based materials |
| spellingShingle |
Ohmic contacts to InN-based materials Sai, P.O. Новые компоненты для электронной аппаратуры |
| title_short |
Ohmic contacts to InN-based materials |
| title_full |
Ohmic contacts to InN-based materials |
| title_fullStr |
Ohmic contacts to InN-based materials |
| title_full_unstemmed |
Ohmic contacts to InN-based materials |
| title_sort |
ohmic contacts to inn-based materials |
| author |
Sai, P.O. |
| author_facet |
Sai, P.O. |
| topic |
Новые компоненты для электронной аппаратуры |
| topic_facet |
Новые компоненты для электронной аппаратуры |
| publishDate |
2016 |
| language |
English |
| container_title |
Технология и конструирование в электронной аппаратуре |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| format |
Article |
| title_alt |
Омические контакты к материалам на основе нитрида индия Омічні контакти до матеріалів на основі нітриду індію |
| description |
The key aspects of ohmic contact formation to InN-based materials were investigated. Detailed analysis of studies conducted over the past three decades, allows determining the basic principles of such contacts. The contact structure properties and optimal conditions for them are presented. Different types of metallization are considered, the advantages and disadvantages of each are determined, including the basic requirements that such contact must meet. There is emphasis on the using multilayer metallization with the barrier layers. In the case of the InAlN/GaN systems, the general approaches of forming ohmic contacts were considered.
Рассмотрены ключевые моменты в формировании омических контактов к нитрид индиевых пленок, фокусируясь на n-InN и InAlN/GaN гетероструктурах. Детальный анализ исследований, проведенных за последние три десятилетия, позволяет определить основные принципы формирования подобных контактов. Приведены параметры контактов и оптимальные условия их достижения, рассмотрены различные типы металлизации и определены преимущества и недостатки каждого из них, учитывая основные требования, которым подобные контакты должны отвечать. Сделан акцент на перспективах использования многослойной металлизации с диффузионными барьерами. Рассмотрены общие подходы к формированию омических контактов к InAlN/GaN-гетероструктур.
В даній роботі розглянуто ключові моменти в формуванні омічних контактів до нітрид-індієвих плівок, фокусуючись на гетероструктурах n-InN і InAlN/GaN. Детальний аналіз досліджень, проведених за останні три десятиліття, дозволив визначити основні принципи формування подібних контактів. Наведено параметри контактів та оптимальні умови їх досягнення, розглянуто різні типи металізації і визначено переваги та недоліки кожного з них, враховуючи основні вимоги, яким подібні контакти мають відповідати. Наголошено на перспективах використання багатошарової металізації з дифузійними бар’єрами. Розглянуто загальні підходи формування омічних контактів до InAlN/GaN-гетероструктур.
|
| issn |
2225-5818 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/115688 |
| citation_txt |
Ohmic contacts to InN-based materials / P.O. Sai // Технология и конструирование в электронной аппаратуре. — 2016. — № 4-5. — С. 3-14. — Бібліогр.: 52 назв. — англ. |
| work_keys_str_mv |
AT saipo ohmiccontactstoinnbasedmaterials AT saipo omičeskiekontaktykmaterialamnaosnovenitridaindiâ AT saipo omíčníkontaktidomateríalívnaosnovínítriduíndíû |
| first_indexed |
2025-11-25T01:18:06Z |
| last_indexed |
2025-11-25T01:18:06Z |
| _version_ |
1850503593266774016 |
| fulltext |
Tekhnologiya i Konstruirovanie v Elektronnoi Apparature, 2016, ¹ 4–5 3
NEW COMPONENTS FOR ELECTRONIC EQUIPMENT
ISSN 2225-5818
UDC 538.91
P. O. SAI
Ukraine, Kyiv, V. E. Lashkaryov Institute of Semiconductor Physics of NAS of Ukraine
E-mail: pavel_sai@mail.ua
OHMIC CONTACTS TO InN-BASED MATERIALS
Nowadays, indium nitride (InN) as III-nitride
compound (А3N) attracts rapid interest among
researchers from around the world. Mostly this is
due to the breakthrough in InN growth. The most
quality material is grown by metalorganic vapour
phase epitaxy (MOVPE) and plasma-activated
molecular beam epitaxy (PAMBE). Considering
the fact that the synthesis of epitaxial InN films
originated in the second half of the 70s, however
only in 2002 the group of researchers headed by
Davydov V. Yu et al. [1] found that this is a
semiconductor with a narrow band gap of 0.7 eV
in contrast to 1.9 eV, as previously thought. It
follows that only InxGa1-xN can span the entire
visible wavelength range, and InxAl1–xN overlap
the wavelength range from infrared to ultraviolet
(Fig. 1).
The attention focused on InN has increased
significantly at the beginning of the XXI century.
The key aspects of ohmic contact formation to InN-based materials were investigated. Detailed analysis
of studies conducted over the past three decades, allows determining the basic principles of such contacts.
The contact structure properties and optimal conditions for them are presented. Different types of
metallization are considered, the advantages and disadvantages of each are determined, including the
basic requirements that such contact must meet. There is emphasis on the using multilayer metallization
with the barrier layers. In the case of the InAlN/GaN systems, the general approaches of forming ohmic
contacts were considered.
Keywords: ohmic contact, Indium Nitride, contact resistivity, rapid thermal annealing.
The various studies conducted in several papers
[1—8] prevented the re-evaluation of the most
important electrical InN parameters (Table 1).
These data indicate the lowest effective mass for
electrons among A3N semiconductors [2], high
saturation velocity [6] and high mobility [7]. All
the superior electric properties of this material
make InN a highly potential semiconductor for the
fabrication of high-speed electronic devices. The
terahertz (THz) emission with the maximum
at the 3—5 THz is observed under electrical
pumping from InN epilayers [9, 10], it makes
this material promising for portable THz emitters.
Most A3N films are grown on substrates such
as sapphire (Al2O3), silicon (Si) or silicon carbide
(SiC), because single crystals of III-nitride cannot
be grown easily. The absence of homogeneous
(crystallographically coordinated) substrates
for (In,Ga,Al)N growth is one of the distinctive
feature of A3N growth. The epitaxial InN growth
on GaN buffer layer is the best case, however
significant mismatch of the lattice parameters a and
thermal expansion coefficients a of the InN/GaN
heterostructure must be considered. There are
Dа/а = +11% and Daа/aа = –38%, respectively
(Fig. 1). Such heteroepitaxial growth usually
results in high level of structural defect density,
due to the relaxation of local mechanical stresses
on the InN/substrate interface.
X-ray diffraction (XRD), transmission electron
microscopy (TEM), atomic force microscopy
(AFM) are the most commonly encountered
methods of investigating structural defects density
in А3N films. According to the recent structural
studies [11—14] of epitaxial indium nitride films
grown by MOVPE or PAMBE onto different
DOI: 10.15222/TKEA2016.4-5.03
Fig. 1. A3N band gap and a-parameter of crystal lattice
Eg, eV
6
4
2
0
3,1 3,2 3,3 3,4 3,5 3,6
a-lattice constant, nm
l, nm
200
300
400
500
600
700
1000
2000Dа/а = 2,4% Dа/а = 11% InN
GaN
AlN
Visible
Range
Tekhnologiya i Konstruirovanie v Elektronnoi Apparature, 2016, ¹ 4–54
NEW COMPONENTS FOR ELECTRONIC EQUIPMENT
ISSN 2225-5818
substrates such as Si, SiC and Al2O3, provide
that threading dislocations are prevailing type
among all known defects in InN layers. This
type of defects can grow deeper and extend the
semiconductor device active areas. Meanwhile,
threading dislocations significantly affect both
the characteristics of InN-based devices and the
parameters of contacts to them. The density of
dislocations usually is characterized by the wide
range of high value from 109 cm–2 to 1011 cm–2,
depending on the growth parameters and film
thickness.
The next important problem is creating ohmic
contacts to the indium nitride films with high
density of structural defects. Such contacts must
satisfy a large set of requirements. There are
technological requirements for their production
process, requirements for reliability of such
contacts and requirements to achieve an excellent
electrical parameters of future contacts.
It should be pointed out that during the
past years, a set of previous researches [17—52]
uncovered important useful data about the ohmic
contacts to In-based materials. Unfortunately,
none of these researches could offer the complete
picture. Therefore, in this study, our objective is
to investigate them in more detail, and to explore,
in the light of all these, principles of metal/InN
ohmic contact formation. We plan to consider
different types of metallization and to determine
the advantages and disadvantages of each. At the
end of present paper, the common approaches for
ohmic contacts to InAlN/GaN heterostructures
will be considered.
Conditions of ohmic contact formation
An ohmic contact is a metal/semiconductor
contact exhibit linear current-voltage (I-V)
characteristics in a range of operating currents. The
contact resistivity (ρc), temperature dependence
of contact resistivity (ρc(T)), maximum working
temperature are the main characteristics of ohmic
contact.
Depending on the purpose of ohmic contacts,
in other words on the complexity and type of
load during operation, these contacts consist of
a single layer (single-layer) or several layers
(multilayer) of metallization, each of them has
its own functionality:
1. Contact layer — metallization layer that
is directly responsible for the formation of a
potential metal-semiconductor barrier, because it
formed in the immediate vicinity of semiconductor.
Moreover, it should limit the diffusion of the upper
metals onto the semiconductor surface;
2. Doping layer — thin layer between a
semiconductor and a contact layer used for
additional doping of the semiconductor near-surface
layer, which is commonly used to implement the
tunneling mechanism of current flow in ohmic
contacts and reduce the contact resistivity;
3. Over layer — layer that is used for
compensating of mechanical stresses caused by the
significant mismatch of the lattice parameters and
thermal expansion coefficients;
4. Barrier layer — refractory metallic layer,
limiting inter-diffusion between contact and outer
metallization layers;
5. Adhesion layer — metallic layer formed
between the outer layers and contact layer, it is
used to improve the wetting of the lower layer of
material that is applied after the adhesion layer.
As a result increases mechanical strength of general
ohmic contact metallization;
6. Cap layer — metallic layer is designed to
connect contact with external terminals to switch
on the device in the electrical circuit. On the other
hand, it acts as protective layer to minimize or
prevent the oxidation of the underlying metals.
Table 1
Comparison of common semiconductor parameters [1—8, 15, 16]
Parameters at 300 К InN
(wurtzite)
AlN
(wurtzite)
GaN 4H-SiC
(wurtzite)
Si
(diamond)(wurtzite) (zinc bland)
Band gap Eg, eV 0.70 6.20 3.51 3.30 3.25 1.12
Effective electron mass, mе/m0 0.04 0.40 0.20 0.13 0.42 0.19
Mobility of electrons μ, см2/(V∙s) 2500 1100 1300 1000 800 1350
Saturation velocity vs, ⋅107 cm/s 5.6 1.9 3.0 3.0 2.0 1.0
Crystal lattice parameters a,c, nm a = 0.355
c = 0.570
a = 0.275
c = 0.498
a = 0.319
c = 0.519 a = 0.452 a = 0.308
c = 1.510 a = 0.543
Linear thermal expansion
coefficient a, ⋅10–6 K–1
aа= 3.80
aс= 2.90
aа= 4.15
aс= 5.27
aа= 5.59
aс= 3,17 — aа= 4.30
aс= 4.70 aа= 2.60
Melting point, Т, °С 1100 3000 2500 2500 2830 1420
Dielectric constant, e 15.3 9.1 8.9—9.5 9.7 9.7 11.8
Tekhnologiya i Konstruirovanie v Elektronnoi Apparature, 2016, ¹ 4–5 5
NEW COMPONENTS FOR ELECTRONIC EQUIPMENT
ISSN 2225-5818
Ohmic contact can be formed in the following
cases:
1. Absence of the metal—semiconductor
potential barrier. In this case an electron work
function of a metal (jm) must be less than an
electron affinity of a n-type semiconductor (c).
In case of p-type semiconductor reversed condition
(jm > c) must be satisfied.
Implementation of the first case by selecting
contact metal with a required work function for
indium nitride is almost impossible due to the
existence of high-density surface charges [17].
It can significantly exceed the concentration of
majority carriers. As a result, the surface charges
significantly affect the energy diagram of the
metal—semiconductor heterojunction that is
virtually independent of the work function of the
contact metal [18]. Managing the concentration
of surface states usually carried out using
various technological treatments semiconductor
films. There are the processes of preparing the
semiconductor surface before the metal deposition,
process of metal deposition, annealing process of
formed contacts. Therefore given processes will
significantly affect the ohmic contact resistivity —
one of the most important parameters.
2. The presence of the narrow metal—
semiconductor potential barrier that allows
electrons to tunnel through the barrier. To form
ohmic contact in this case the additional doping of
contact layer is often used. As a result the space
charge region is so thin that quantum-mechanical
tunneling of charge carriers is possible. However,
InN films are usually characterized by the high
electron concentration in the range of 1017—1020 сm–3
due to the growth specifics [5]. This fact
contributes to the formation of low-resistance
ohmic contact to the semiconductor without
additional doping of near-surface semiconductor
layers. Moreover, the significant concentration of
surface states facilitates a solution to the problem
of formation of a narrow potential barrier.
3. The case of the sufficiently low metal—
semiconductor potential barrier. It is necessary
for possibility of carriers to flow over the barrier.
The low-barrier contacts usually are formed due
to the realization of surface pretreatment and
subsequent correct selection of metallization layers
for deposition onto the semiconductor surface.
4. The presence of semiconductor layer
shorted by metal shunts that can be caused by
the deposition of metal atoms on dislocations or
other structural defects [19]. This case is high
probably for indium nitride films with high density
of structural defects. It was confirmed that the
increasing temperature dependences of the contact
resistivity ρc(T) obtained for ohmic contacts to
InN can be explained by current flow through
dislocations associated with metal shunts [20].
To date, the temperature dependence of ohmic
metal/InN contacts resistivity (ρc) is not fully
investigated. For fixed values of the barrier
height and carrier concentration, the temperature
dependence of contacts resistivity determines the
carrier transport mechanism through the metal/
semiconductor interface. The field emission is one
of the frequently occurred transport mechanisms
due to heavily doped semiconductor films [21—24].
The anomalous increase in the temperature
dependence of contact resistivity, ρc(T), was
obtained by the authors of [25, 26], who attempted
to explain the increase by the metallic conductivity
in degenerate InN [26]. However, no direct
measurements of ρc(T) were performed in this
work. In addition, the affect of a high density
of structural defects was not taken into account.
The necessity of structural studies for fully
understanding the carrier transport mechanisms
of ohmic contacts to InN-based films was
demonstrated in [27]. Investigation of Ti/Al/Au
ohmic contacts to InAlN/AlN/GaN found a
significant influence the TiN contact inclusions
(spike) in GaN layers on a current flow.
Prior to metal deposition the surface treatment
of InN films usually can be carried out in several
steps each of which performs a separate problem
[22, 28—30]. In many cases the first step is the
removing native oxides by dipping the samples
in H2O:NH4OH(20:1) for 1 min. Subsequently,
the second step is the etching in HCl:H2O (1:3)
solution to remove the possible In on the top
surface. For the third step HF:H20 (1:50) are used.
Finally, InN films are rinsed with deionizer water.
For the correct selection of contact metal a
host of factors must be considered. There are the
distinctive features of semiconductor that define
the bending of energy bands in the surface region
of the semiconductor; the adhesion of metal to
a semiconductor; the lattice mismatch effect in
metal/semiconductor interface (the parameters of
commonly used metals for creating ohmic contacts
to InN, are shown in the Table 2), etc.
However, all of the above requirements are
insufficient for creating the low resistance ohmic
contact. One possibility is that the rectifying
contact can be formed after metal deposition onto
unwarmed substrate. In this case rapid thermal
annealing (RTA) is used for manipulating of the
height and width of potential barrier due to the
forming n+ and p+ layers. However, the relatively
low temperature of InN dissociation (630°С in
vacuum and 500°С in an atmosphere of N2 [3])
must be consider during the thermal treatments.
Investigation of surface morphology and elemental
composition of single crystal indium nitride surface
[31, 32] confirm low thermal stability compared
to the other compounds of III-nitride group. The
degradation of structure was observed during
thermal treatments in a nitrogen atmosphere at
temperatures above 550°C due to InN dissociation
and subsequent N loss from the nitride surface are
found to occur.
Thus, the formation of ohmic contacts to
InN is a sequence of complex processes that is
Tekhnologiya i Konstruirovanie v Elektronnoi Apparature, 2016, ¹ 4–56
NEW COMPONENTS FOR ELECTRONIC EQUIPMENT
ISSN 2225-5818
caused by the necessity to take into account the
distinctive features of a semiconductor and wide
set of requirements that the contact must meet.
The main requirements can be classified as follows:
1. Technological requirements:
— avoidance of device characteristics changing
during the formation of contacts;
— possibility of selective etching of metals in
the process of photolithography;
— ability to use of technological impacts
for controlled change of electrophysical contact
characteristics;
— manufacturing process of that contacts must
be as simple as possible and consistent with the
production of semiconductor.
2. Electric requirements:
— sufficiently low contact resistivity for
avoidance of a significant voltage drop at the
contact and its additional heating;
— symmetric and linear I-V characteristics in
a range of operating currents;
— avoidance of minority-carrier injection.
3. Requirements for reliability:
— stability of contact properties to prolonged
electrical and thermal loads during operation;
— reasonable adhesion of contact metal to InN;
— use of materials with similar thermal
expansion coefficient and crystal lattice parameters;
— preservation of the contact structural
homogeneity during long-term operation of the
device for avoiding substantial change in contact
parameters.
Minimal ohmic contact resistivity limits to n-InN
The contact resistivity is an important parameter
characterizing the metal/semiconductor interfaces.
It consists of near-contact area resistivity of
semiconductor and series connecting resistivity
caused by current flow over the potential
barrier. Minimal ohmic contact resistivity limits
(ρc min) to widely used n-type semiconductors
was derived in [33], giving the lowest possible
ohmic contact resistivity for nondegenerate and
degenerate metal—semiconductor. It was assumed
that the potential barrier is absent during the
semiconductor—metal current flow and probability
of tunneling in the same direction tends to 1.
In accordance with this assumption the final
minimal contact resistivity ρc min is:
min * *exp c
c
d
k qV k N
qAm T kT qAm T N
,
(1)
Table 2
Parameters* of metals and alloys used for forming ohmic contacts to InN
* W — work function of the metal, ρ — resistivity.
Metal W, еV Melting
point,°С a0, nm a,
⋅10–6/К–1 ρ, W⋅μm
Ag 4.30 960 0.409 18.90 0.015
Al 4.25 660 0.405 22.30 0.026
Au 4.25 1063 0.408 14.00 0.023
Hf 3.80 2230 0.319 6.00
Mo 4.30 2620 0.315 5.27 0.050
Nb 3.95—4.87 2468 0.330 0.152
Ni 4.50 1453 0.352 13.20 0.068
Pb 4.20 327 0.495 28.30 0.190
Pd 4.80 1550 0.389 11.75 0.108
Pt 5.32 1770 0.392 9.50 0.098
Ta 4.12 2996 0.331 6.60 0.124
Ti 3.95 1608 0.295 8.10 0.470
Zr 3.90 1855 0.323 7.36 — 4.99 0.410
V 4.30 1887 0.302 10.60 0.248
W 4.54 3400 0.316 4.40 0.055
WSi2 4.05 2160
TiB2 3.80 2790 0.323 5.50
TiN 2.92—4.09 2950 4.70
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Boltzmann's constant;
elementary electric charge;
Richardson constant (A = 120 А•сm–2•К–2);
me/m0 , me and m0 — effective mass and
free electron mass respectively;
temperature;
2(2pkTm*/h2)3/2 — effective density of
states in the conduction band;
electrically active bulk doping concentration.
where k —
q —
A —
m*=
T –
Nc=
Nd —
In compliance with [33], we estimated the
minimal ohmic contact resistivity limits in case
of n-InN:
1/2
9 1 1/2
min 3
[K ]
1.15 10 [Ohm cm ·K ] .
[cm ]c
d
T
N
(2)
For instance, the obtained minimal contact
resistivity is 3.91∙10–8 Ohm∙сm2 for Nd = 5∙1017 cm–3
at T = 300 K.
Contacts based on a single-layer metallization
The most simple in terms of technology and
low-cost variant is the formation of ohmic contacts
based on single-layer metal structures. For selection
of such metal structures one gives the most
important attention to the next parameters. There
are conductivity, adhesion to a semiconductor,
electron work function and metal melting point.
The wolframium (W) and wolframium silicide
(WSix) layers are the most common single-layer
ohmic contacts to the n-InN. Previously these
metal layers take priority during choosing the
material for forming single-layer ohmic contacts
to the n-InN due to the low resistivity and high
melting point (Table 2).
The conducted in [21, 22, 28, 29, 34, 35]
studies of W/n-InN and WSix/n-InN indicate
the high thermal stability of these structures.
The physical properties of the obtained contacts
to InN, InxAl1–xN and InxGa1–xN films degraded
after RTA at temperature higher than 400°С, 500°С
and 600°С, respectively [28, 29]. It was confirmed
by the sharp contact resistivity increasing after
appropriate thermal treatments (Fig. 2).
Fig. 2. Contact resistivity for W, WSix contacts as a
function of annealing temperature [28, 29]
100
10–1
10–2
10–3
10–4
10–7
0 200 400 600 800 1000
RTA temperature, °C
ρ c
,
O
hm
⋅c
m
2
W/InN
WSi0,44/In0,75Al0,25N
W/In0,65Ga0,35N
Рис. 3. XRD data for the W/InGaN samples as-deposited (a) and annealed at 500°C (b), 700°C (c), and
950°C (d) [34]
107
106
105
104
103
102
101
30 40 50 60 70 80 2q, °
In
te
ns
it
y,
c
ou
nt
s/
s
Al2O3
InGaN
GaN(0004)
GaN(0002)
InGaN
W(110)
108
107
106
105
104
103
102
101
100
30 40 50 60 70 80 2q, °
In
te
ns
it
y,
c
ou
nt
s/
s
Al2O3
InGaN
GaN(0004)
GaN(0002)
InGaN
W(110)
W(220)
W2N(222)W2N(311)
W2N(200)
W2N(111)
30 40 50 60 70 80 2q, °
In
te
ns
it
y,
c
ou
nt
s/
s
108
107
106
105
104
103
102
101
100
Al2O3
InGaN
GaN(0004)
GaN(0002)
InGaN
W(110)
W(220)
W2N(222)W2N(311)
W2N(200)
W2N(111)
108
107
106
105
104
103
102
101
30 40 50 60 70 80 2q, °
In
te
ns
it
y,
c
ou
nt
s/
s
Al2O3
InGaN
GaN(0004)
GaN(0002)
InGaN
W(110)
W(220)
W2N(222)W2N(311)
W2N(200)
W2N(111)
a) b)
c) d)
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Metal
layers
Semiconductor hetero-
structure
Layer thickness,
nm
Donor
concentration,
сm–3
Minimal
resistivity,
Ohm⋅cm2
Treatment Reference
Ti/Al n+InN/GaAs 20/100/200 1020 1.2⋅10–7 RTA 500°С,
15 s [20]
Ti/Al n+In0,65Al0,35N/GaAs 20/100/200 8⋅1018 1.0⋅10–4 RTA 450°С,
15 s [20]
Ti/Al n+In0.75Ga0,25N/GaAs 20/100/200 1019 2.0⋅10–7 RTA 600°С,
15 s [21]
Ti/Al InN/GaN 20/100/100 1019 6.0⋅10–5 RTA 500°С [35]
Ti/Au InN/AlN 100/200/
1000/250 2⋅1018 1.4⋅10–7 non-annealed [17]
Al/Au InN/GaN/Al2O3
100/200/
1000/220
1.49⋅1018 1.9⋅10–6 non-annealed [17]
Ni/Au InN/AlN/Al2O3
100/200/
1000/200
2.28⋅1018 1.0⋅10–6 non-annealed [17]
Ni/Ag InN/GaN/Al2O3 – — 3.5⋅10–2 TA 400°С,
30 min [29]
Fig. 4. AES depth profiles of W/InN after RTA at
500°C [21]
0 2 4 6 8 10 12 14
Sputter time, min
In
te
ns
it
y,
%
100
80
60
40
20
0
W
One of the main causes of contact resistance
decreasing during RTA for contacts based on
single-layer metallization (W, WSix) could be
forming of the interfacial phase of wolframium
nitride (W2N), its appearance was confirmed by
X-ray diffraction studies (Fig. 3). W2N was formed
after RTA at 500°С. In addition, the interfacial
W2N phase became better defined with increasing
temperature, indicating the occurrence of the more
extensive interfacial reactions. This indicates that
the nitrogen is outdiffused from a semiconductor,
resulting in the accumulation of nitrogen vacancies
near the semiconductor surface. These nitrogen
vacancies are likely to act as donors. Thus, the
increase in the carrier concentrations near the
surface layer is responsible for the improved I-V
characteristics.
Table 3
Survey of literature data on bilayer ohmic contacts to n-type InN
A major disadvantage of such contact-layer
formation is the considerable thickness of
heterogeneous interface formed after RTA, it is
confirmed by AES depth profiles (Fig. 4) [21].
Therefore, these contacts are not suitable for all
modern semiconductor devices that often require
the unfloatable ohmic contacts with thin boundary
dividing metal and semiconductor. The main work
directions on these problems are introduction
additional layers of metallization and reduction the
temperature of annealing treatment or complete
exclusion of RTA.
Bilayer ohmic contacts based on Al, Ni, Ti
Ti-based ohmic contacts to n-InN are widely
distributed due to high melting point of this
metal (1608°С), low resistance and crystal growth
parameters related with InN. However, Ti layer
has high propensity to oxidation (the formation
of high-resistance titanium oxide compounds).
Thus, it is used with Al or Au top layers to
prevent diffusion of titanium on the surface
and its oxidation. Minimal contact resistivity
1.2⋅10–7 Ohm⋅сm2 of such structures was obtained
after 500°C RTA for 15 s (Table 3) [21].
A further increase of the annealing temperatures
(600°C) leads to abrupt increase of the values of
ρc. A more detailed analysis of Ti/InN interface
found the diffusion of Ti into semiconductor and
significant diffusion of In via metallization after
thermal treatments at 300°C for 60 s [25]. The
limited temperature stability could be caused by
a high tendency of these metals to oxidation or
a relatively low melting point Al (660°C) and
high probability formations of Al droplets on the
surface.
In
N
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NEW COMPONENTS FOR ELECTRONIC EQUIPMENT
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Low values of the contact resistivity were
achieved in [17] for non-annealed contacts. The
authors associated this data to the existence of
high-density surface charge in InN films, it is
about 4.3•1013 сm–2. Probably, it is a crucial
point in the formation of the ohmic contacts
with Al, Ni, Ti contact layers immediately after
the deposition of metallization on InN without
subsequent annealing. However, in that work
any structural research and temperature studies
of electrical properties were not carried out thus
the reliability and stability of such contacts is not
completely investigated.
Barrier layer as part of multilayer contact
metallization to n-InN
Modern researches of ohmic contacts argue
about the feasibility and prospects of using
multilayer contact structure [37]. As has been
noted above, similar structure may consist of
several metallic layers for various purposes.
The barrier layer is one of the most important
part of multilayer contact metallization. It is
generally a polycrystalline layer of refractory
metal and alloy (Ni, Ti, Pt) [25, 26, 38, 39] or
boride nanocrystalline layer of refractory metal
(for example, TiB2) [23].
Ti/Al/Ni/Au and Ti/Al/TiB2/Ti/Au
ohmic contacts on n-type InN were compered
[23]. These structures differ from each other
only by the barrier layer. The minimum
values of contact resistivity of 1.6⋅10–6 and
6.0⋅10–6 Ohm⋅cm2 were obtained for the TiB2-
based and Ni-based ohmic contacts, respectively.
However, significant differences could be observed
examining AES depth profiles after RTA at 400°C
(Fig. 5). According to Fig. 5, Ni layer is an unable
to cope with its main purpose, which leads to a
significant mass transfer between the metallization
layers and the semiconductor. In contrast, the
TiB2-based ohmic contacts displayed thermal
stability, suggesting that it is a better diffusion
barrier than Ni. After RTA the TiB2-based
contact structure much better maintains layered
homogeneity. Thus thermal stability and reliability
of the investigated contacts were generally defined
by the barrier layer properties.
Comparing the optimal values of the contact
resistivity for structures using barrier layers and
other contact metallization (Fig. 6), we notice low
values of the contact resistivity in a wide range of
doping concentration obtained for ohmic contacts
with barrier layers.
Fig. 5. AES depth profiles of multilayer metallization with barrier layers Ti/Al/Ni/Au (a, b)
and Ti/Al/TiB2/Au (c, d) as-deposited (а, c) and after RTA at 400°С (b, d) [23]
100
80
60
40
20
0
0 100 200 300 400 500 600
Sputter depth, nm
A
to
m
ic
c
on
se
nt
ra
ti
on
,
%
100
80
60
40
20
0
0 50 100 150 200
Sputter depth, nm
A
to
m
ic
c
on
se
nt
ra
ti
on
,
%
100
80
60
40
20
0
0 100 200 300 400
Sputter depth, nm
A
to
m
ic
c
on
se
nt
ra
ti
on
,
%
100
80
60
40
20
0
0 100 200 300 400 500
Sputter depth, nm
A
to
m
ic
c
on
se
nt
ra
ti
on
,
%
Ti
Au
In
N
Ni
Al
Ti
Au
In
N
Ni
Al
Ti
Au
In
N
B
Al
Ti
Au
In
N
B
Al
a) b)
c) d)
Tekhnologiya i Konstruirovanie v Elektronnoi Apparature, 2016, ¹ 4–510
NEW COMPONENTS FOR ELECTRONIC EQUIPMENT
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—W [21, 22, 28, 29, 34, 35];
— WSi [21, 22, 34, 35];
— Ti/Al [36];
— Ti/Au [17];
— Ni/Au [17];
— Al/Au [17];
— Ti/Pt/Au [25, 38, 39];
— Pd/Ti/Au [20];
— Pd/Ti/Pt/Au [26];
— Ti/Al/Ni/Au [23];
— Ti/Al/TiB2/Au [23]
10–1
10–3
10–5
10–7
10–9
10–11
1016 1017 1018 1019 1020 1021
Nd, cm–3
ρ c
,
O
hm
⋅c
m
2
1016 1017 1018 1019 1020 1021
10-11
10-9
10-7
10-5
10-3
10-1
1016 1017 1018 1019 1020 1021
10-11
10-9
10-7
10-5
10-3
10-1
1016 1017 1018 1019 1020 1021
10-11
10-9
10-7
10-5
10-3
10-1
1016 1017 1018 1019 1020 1021
10-11
10-9
10-7
10-5
10-3
10-1
1016 1017 1018 1019 1020 1021
10-11
10-9
10-7
10-5
10-3
10-1
1016 1017 1018 1019 1020 1021
10-11
10-9
10-7
10-5
10-3
10-1
1016 1017 1018 1019 1020 1021
10-11
10-9
10-7
10-5
10-3
10-1
1016 1017 1018 1019 1020 1021
10-11
10-9
10-7
10-5
10-3
10-1
1016 1017 1018 1019 1020 1021
10-11
10-9
10-7
10-5
10-3
10-1
1016 1017 1018 1019 1020 1021
10-11
10-9
10-7
10-5
10-3
10-1
1016 1017 1018 1019 1020 1021
10-11
10-9
10-7
10-5
10-3
10-1
1016 1017 1018 1019 1020 1021
10-11
10-9
10-7
10-5
10-3
10-1
ρ c,
Oh
m
·c
m
2
Nd, cm-3
Pd/Ti/Au [20]
ρ c,
Oh
m
·c
m
2
Nd, cm-3
W [21, 22, 28, 29, 34, 35]
ρ c,
Oh
m
·c
m
2
Nd, cm-3
WSi [21, 22, 34, 35]
ρ c,
Oh
m
·c
m
2
Nd, cm-3
Ti/Al [36]
ρ c,
Oh
m
·c
m
2
Nd, cm-3
Ti/Au [17]
ρ c,
Oh
m
·c
m
2
Nd, cm-3
Ni/Au [17]
ρ c,
Oh
m
·c
m
2
Nd, cm-3
Al/Au [17]
ρ c,
Oh
m
·c
m
2
Nd, cm-3
Ti/Pt/Au [25, 38, 39]
ρ c,
Oh
m
·c
m
2
Nd, cm-3
Pd/Ti/Pt/Au [26]
ρ c,
Oh
m
·c
m
2
Nd, cm-3
Ti/Al/Ni/Au [23]
ρ c,
Oh
m
·c
m
2
Nd, cm-3
Ti/Al/TiB2/Au [23]
ρ c,
Oh
m
·c
m
2
Nd, cm-3
Table 4
Survey of literature data on ohmic contacts to InAlN/GaN HEMTs
* Layer thickness are presented in nanometers;
** ICP-RIE – Inductively Coupled Plasma Reactive Ion Etching.
Metal
layers
Semiconductor
heterostructures*
Layer thick-
ness, nm
Minimal
resistivity Treatment Reference
Ti/Al/
Ti/Au
In0.15Al0.85N(22.7)/
GaN(2000)/SiC
20/100/
45/55 430 Ohm⋅μ RTA 850°С, 30 s [41]
Ti/Au
InN(10)/InGaN(40)/
GaN(20)/AlGaN/
GaN(n-face)
27 Ohm⋅μ [42]
Ti/Au InAlN(2.5)/AlN(1.5)/
GaN(200)/GaN(1600)/SiC 160 Ohm⋅μ [43]
Ti/Al/
Mo/Au
GaN(2)/
In0.134Al0.866N(8)/AlN(1)/
GaN(2000)/AlN(70)/SiC
16/64/
30/50 300 Ohm⋅μ RTA 860°С [44]
Ti/Al/
Ni/Au
In0.17Al0.83N(5)/
AlN(1)/GaN(21)/
AGalN(800)/SiC
410 Ohm⋅μ RTA 830°С, 30 s [46]
Ti/Al/
Ni/Au
In0.18Al0.82N(9)/
AlN(1)/GaN(2500μm)/Al2O3
30/160/
40/50
1.1⋅10-7
Ohm⋅cm2
RTA 600°C, SiCl4
RIE [46]
Mo/Al/
Mo/Au
In0.17Al0.83N(5.6)/
AlN(1)/GaN/6H-SiC
15/60/
35/50
7.8⋅10-7
Ohm⋅cm2
RTA 650°C 30 s,
SiCl4 RIE [48]
Ti/Al/
Ni/Au
In0.17Al0.83N(10.2)/
GaN(50)/GaN(1900)/Al2O3
2⋅10-5
Ohm⋅cm2
RTA 800°C 30 s,
ICP-RIE [49]
Ti/Al/
Ni/Au
GaN(2.5)/
In0.17Al0.83N(10.2)/
GaN(50)/GaN(1900)/Al2O3
3⋅10-5
Ohm⋅cm2
RTA 800°C 30 s,
ICP-RIE** [50]
Ti/Al/
Ni/Au
GaN(2)/In0.16Al0.84N()/
AlN(1)/GaN(3000)/
AlN(300)/Al2O3
30/100/
40/50 0.45 Ohm RTA 800°C [45]
Ti/Al/Au
In0.18Al0.82N(20)/
AlN(1)/GaN(3600)/
GaN(1000)/Al2O3
30/70/70 5.28⋅10-4
Ohm⋅cm2 RTA 850°C [26]
Ti/Al/
Ni/Au
In0.18Al0.82N (9)/
AlN(1)/GaN(1300)/Si
25/200/
40/100
4.75⋅10-6
Ohm⋅cm2 RTA 800°C 60 s [47]
Hf/Al/Ta In0.18Al0.82N (9)/
AlN(1)/GaN(1300)/Si 15/200/20 6.75⋅10-6
Ohm⋅cm2 RTA 600°C 60 s [50]
Fig. 6. Contact resistivity as a function of doping concentration: experimental data (dots) and theoretically
calculated values of minimal contact resistivity (line)
Tekhnologiya i Konstruirovanie v Elektronnoi Apparature, 2016, ¹ 4–5 11
NEW COMPONENTS FOR ELECTRONIC EQUIPMENT
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Ohmic contacts to InAlN/GaN HEMTs
During the last decade the intensive research of
InAlN/GaN heterostructure electrical parameters
was conducted [27, 41—52], which is promising to
create high electron mobility transistor (HEMT).
Due to the better values of electron mobility and
the possibility of high-density two-dimensional
electron gas formation (2DEG), InAlN/GaN
heterostructure is considered as an alternative
to AlGaN/GaN using for creating the basis for
stronger HEMTs. Since these transistors work with
high current density, the ohmic contacts to them
firs-of-all must withstand respective load. HEMT
performance substantially depends on the parasitic
resistance presence. Hence, as one of the priorities
is reduction of the ohmic contact resistance.
One option for reducing the contact resistivity
was the deposition of ohmic contacts onto thin
10 nm InN layer, which was epitaxial grown on
GaN/InAlN heterostructure using graded InхGa1–хN
(40 nm, х = 0.01—0.26) [43]. In this way, due
to the existence of high-density surface charges in
InN films, the reduction of Ti/Au ohmic contact
resistivity was achieved to 27 Ohm⋅m compared
to 160 Ohm⋅m [44] for similar contacts without
the InN top layer.
Another approach to reduction of contact
resistance was using SiCl4 reactive ion etching
(SiCl4-RIE) InAlN surface before metal deposition
[45, 46]. In case of Ti/Al/Ni /Au, it is possible
to significantly reduce the ohmic contact resistivity
(1.1⋅10–7 Ohm⋅cm2 after RTA at 600°С [47])
compering to similar contacts without using
SiCl4-RIE (2⋅10–5 Ohm⋅cm2 after RTA at 800°С
[47]). That was related to following SiCl4-RIE
advantages: the removing of the natural oxide from
semiconductor surface; the removing of carbon
impurities that accumulate on the surface during
the epitaxial growth of InAlN by MOCVD; the
thickness reduction of the potential barrier that
allows electron tunneling through it.
Analyzing known contact metallization schemes
to InAlN/GaN (Table 4), it is worth noting
that the Ti/Al-based contact structures are
widely spread. However, one of their distinctive
feature is the possibility of contact inclusions
or spikes formation after the high-temperature
annealing which is required for the low-resistance
ohmic contacts. The presence of such spikes was
confirmed by transmission electron microscopy
studies [27, 50]. According to the authors, Ti
diffuse through InAlN layer, consequently TiN
local inclusions were formed in GaN layer. The
concentration of such inclusions exceed 108 cm–2
[27]. Whereas, that greatly affects the carrier
transport mechanisms, since TiN acts as a carrier
path leading between the metallization and the
area of two-dimensional electron gas.
A sharp metal-semiconductor interface and
avoidance significant interdiffusion between
contacting layers were achieved by reduction
RTA temperature to 600°C using metallization of
refractory metals Hf/Al/Ta [51, 52].
Conclusion
In this study, the key issues for ohmic contact
formation to InN-based materials have been
reviewed, focusing on the cases of n-type InN and
InAlN/GaN heterostructures. A critical analysis
of the main literature results reported in the last
three decades allowed identifying the most suitable
metallization structures and optimal annealing
conditions for ohmic contact formation.
Two main stages can be associated with the
developing of the contact metallization to n-InN
that might be separated by the period of the
revaluation of semiconductor band gap and other
essential parameters (effective electron mass,
saturation velocity, mobility of electrons, etc.). In
the first phase (90 years of XX century) interest in
indium nitride was due to promising applications
in the developing of some active elements for
optoelectronics. Single-layer ohmic contacts based
on W and WSi became the largest distribution
during the first stage. Such type of metallization
characterized by lowest optimal contact resistivity,
however it requires high temperature annealing.
Since the beginning of the XXI century,
indium nitride has been regarding as promising
for high-speed semiconductor devices. Multi-layer
metallization type with the barrier layers was used
in case of InN. Thermal stability and reliability
of whole metallization structure were generally
defined by the barrier layer properties. The low
contact resistance and superior thermal stability
were achieved by using TiB2 layers.
The mechanisms of current transport in
the metal—InN structure aren’t completely
investigated due to the lack of information
connected with temperature dependencies of
contact resistivity and extended structural studies
of semiconductor films.
The formation principles of ohmic contacts to
InAlN/GaN heterostructures have the distinctive
features. Using typical metallization based on
Ti/Al mostly leads to the formation of contact
inclusions or spikes that penetrate the InAlN layer.
To avoid this in the current studies the following
approaches were considered: using reactive
ion etching InAlN surface before deposition
ohmic contacts; search for alternative materials
for contact metallization; using thin layers of
refractory metals (Hf, Mo, Pd) to form a sharp
boundary dividing «metal—semiconductor».
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Received 12.07 2016.
Tekhnologiya i Konstruirovanie v Elektronnoi Apparature, 2016, ¹ 4–514
NEW COMPONENTS FOR ELECTRONIC EQUIPMENT
ISSN 2225-5818
П. А. САЙ
Украина, г. Киев, Институт физики полупроводников им. В. Е. Лашкарёва НАН Украины
E-mail: pavel_sai@mail.ua
ОМИЧЕСКИЕ КОНТАКТЫ К МАТЕРИАЛАМ
НА ОСНОВЕ НИТРИДА ИНДИЯ
Рассмотрены ключевые моменты в формировании омических контактов к нитрид индиевых пленок, фо-
кусируясь на n-InN и InAlN/GaN гетероструктурах. Детальный анализ исследований, проведенных за
последние три десятилетия, позволяет определить основные принципы формирования подобных контак-
тов. Приведены параметры контактов и оптимальные условия их достижения, рассмотрены различные
типы металлизации и определены преимущества и недостатки каждого из них, учитывая основные тре-
бования, которым подобные контакты должны отвечать. Сделан акцент на перспективах использова-
ния многослойной металлизации с диффузионными барьерами. Рассмотрены общие подходы к формиро-
ванию омических контактов к InAlN/GaN-гетероструктур.
Ключевые слова: омический контакт, нитрид индия, удельное контактное сопротивление, быстрая тер-
мическая обработка.
П. О. САЙ
Україна, м. Київ, Інститут фізики напівпровідників
ім. В. Є. Лашкарüова НАН України
E-mail: pavel_sai@mail.ua
ОМІЧНІ КОНТАКТИ ДО МАТЕРІАЛІВ НА ОСНОВІ НІТРИДУ ІНДІЮ
Нітрид індію, що є представником групи тринітридів, останнім часом викликає бурхливий інтерес серед
дослідників з усього світу. Здебільшого це спричинено зростанням якості InN завдяки таким технологіям
його вирощування, як металоорганічна газофазна епітаксія та молекулярно-пучкова епітаксія з плазмо-
вою активацією азоту. Найнижче серед тринітридів значення ефективної маси електронів в поєднанні
з високими значеннями швидкості насичення та рухливості електронів робить нітрид індію перспек-
тивним для розвитку високошвидкісної напівпровідникової електроніки. Використання InxGa1–xN
дозволяє перекрити весь видимий діапазон довжини хвиль, а InxAl1–xN — діапазон довжини хвиль від
інфрачервоного випромінювання до ультрафіолетового.
Критичною проблемою, що гальмує розвиток мікроелектроніки на основі InN, залишається складність
створення омічного контакту до напівпровідникових плівок нітриду індію з високою густиною струк-
турних дефектів. Подібні контакти мають задовольняти широкому спектру вимог. Насамперед, це
технологічні вимоги до процесу їх виготовлення, до їх надійності та електрофізичних параметрів. Однак
до тепер не було сформовано узагальненої картини фізичних процесів, що відбуваються в структурі
«метал — InN» під час струмоперенесення, та не проведено огляд впливу технологічних умов на якість
омічного контакту.
В даній роботі розглянуто ключові моменти в формуванні омічних контактів до нітрид-індієвих плівок,
фокусуючись на гетероструктурах n-InN і InAlN/GaN. Детальний аналіз досліджень, проведених за
останні три десятиліття, дозволив визначити основні принципи формування подібних контактів.
Наведено параметри контактів та оптимальні умови їх досягнення, розглянуто різні типи металізації
і визначено переваги та недоліки кожного з них, враховуючи основні вимоги, яким подібні контакти ма-
ють відповідати. Наголошено на перспективах використання багатошарової металізації з дифузійними
бар’єрами. Розглянуто загальні підходи формування омічних контактів до InAlN/GaN-гетероструктур.
Ключові слова: омічний контакт, нітрид індію, питомий контактний опір, швидка термічна обробка.
DOI: 10.15222/TKEA2016.4-5.03
УДК 538.91
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