IEEE ELECTRON DEVICE LETTERS, VOL. 39, NO. 6, JUNE 2018 847
Ultralow-Contact-Resistance Au-Free Ohmic
Contacts With Low Annealing Temperature
on AlGaN/GaN Heterostructures
Jinhan Zhang , Xuanwu Kang , Xinhua Wang, Sen Huang , Chen Chen, Ke Wei, Yingkui Zheng,
Qi Zhou , Wanjun Chen , Bo Zhang, and Xinyu Liu
Abstract
— We report on the electrical and microstruc-
tural characterization of Au-free Ti/Al/Ti/TiN contacts for
AlGaN/GaN heterostructures. Ultra-low Au-free ohmic con-
tact has been obtained with contact resistance and spe-
cific contact resistivity as low as 0.21 · mm and 1.16 ×
10
−6
· cm
2
, respectively. The ohmic alloy temperature is
reduced as low as 550 °C by pre-ohmic recess of the AlGaN
barrier and optimization of the thickness of bottom Ti layer.
We found that interfacial layer formation of AlN between the
ohmic metal and AlGaN surface is crucial to realize a low
contact resistance with reduced low annealing temperature
by a combination of electrical
I
–
V
characterization and
high-resolutiontransmissionelectron microscopy analysis.
Furthermore, we suggest a hypothesis that the bottom
Ti layer plays a catalytic role for the Al–N reaction with
optimized thickness.
Index Terms
— AlGaN/GaN, Au-free, contact resistance,
low temperature, uniformity.
I. INTRODUCTION
A
lGaN/GaN high-electron-mobility transistors (HEMTs),
by virtue of the polarization-induced high mobility and
density 2-D electron gas (2DEG) at the hetero-interface, are
highly attractive for radio frequency (RF)/microwave and
power switching applications [1], [2]. GaN grown on 6-in.-Si
Manuscript received January 30, 2018; revised March 27, 2018;
accepted March 30, 2018. Date of publication April 3, 2018; date of
current version May 22, 2018. This work was supported in part by the
National Key Research and Development Program of China under Grant
2016YFB0400100 and Grant 2017YFB0403000, in part by the Natural
Science Foundation of China under Grant 61474138, Grant 61534007,
Grant 11634002, Grant 61334002, Grant 61631021, Grant 61404163,
and Grant 61527816, in part by the Key Frontier Project of Chinese
Acadamy of Sciences under Grant QYZDB-SSW-JSC012, and in part by
the Opening Project of Key Laboratory of Microelectronic Devices and
Integrated Technology, Institute of Microelectronics of Chinese Academy
of Sciences. The review of this letter was arranged by Editor T. Palacios.
(Jinhan Zhang and Xuanwu Kang contributed equally to this work.)
(Corresponding author: Xuanwu Kang.)
J. Zhang, Q. Zhou, W. Chen, and B. Zhang are with the State Key
Laboratory of Electronic Thin Films and Integrated Devices, University
of Electronic Science and Technology of China, Chengdu 610054, China.
X. Kang, X. Wang, S. Huang, C. Chen, K. Wei, Y. Zheng, and X. Liu are
with the Key Laboratory of Microelectronic Devices and Integrated Tech-
nology, Institute of Microelectronics of Chinese Academy of Sciences,
Beijing 100029, China (e-mail: kangxuanwu@ime.ac.cn).
Color versions of one or more of the figures in this letter are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LED.2018.2822659
wafers is commercially available and enables the fabrica-
tion of AlGaN/GaN HEMTs in CMOS fabs used for the
processing of Si devices, offering great opportunities for
large scale production and on-wafer integration [3]. How-
ever, conventional AlGaN/GaN HEMTs are fabricated with
Au-based ohmic contacts (e.g., Ti/Al/Ni/Au) annealed at high
temperature (>800 °C) [4]–[6], which are not compatible with
CMOS fabs because Au is forbidden in a Si process platform.
Meanwhile, the conventional process of ohmic annealing at
above 800 °C has been revealed as the primary cause for
the formation of nitrogen deficiency and oxidation of bare
(Al)GaN surface, resulting in degradation of dynamic perfor-
mance of GaN-based HEMTs [7], [8]. In addition, the high
thermal budget limits the implementation of a self-aligned
gate-first CMOS process [9]. Therefore, it is highly desirable
to develop low-thermal-budget Au-free ohmic contacts for
AlGaN/GaN HEMTs.
Up to now, several Au-free metallization schemes have been
reported [10]–[14], [21]. However, the formation mechanism
of Au-free contacts with low annealing temperature is still
unclear. Moreover, large contact resistance (R
C
) limits the
practical application of Au-free ohmic contacts. To achieve
low R
C
and to reveal the formation mechanism of Au-free
ohmic contacts with low annealing temperature, we inves-
tigate Ti/Al/Ti/TiN metallization with varying thickness of
bottom layer Ti. Through a series of systematic experi-
ments, we demonstrate that results of excellent ohmic contacts
with ultralow R
C
(0.21 · mm), lower annealing tempera-
ture (550 °C) and improved process window and uniformity.
High-resolution transmission electron microscopy (HR-TEM)
is used to analyze the interfacial structure. At the interface of
alloy reaction, a continuous AlN layer is observed, which plays
a key role to achieve ultralow resistance of Au-free contacts
with low temperature annealing.
II. D
EVICE FABRICATION
The schematic cross section of the Ti/Al/Ti/TiN ohmic con-
tacts is depicted in Fig. 1. The wafers used in this work were
grown by MOCVD on a 2-inch Si substrate. The AlGaN/GaN
heterostructure consists of a 20-nm Al
0.25
Ga
0.75
N barrier layer
with a 2-nm GaN cap, a 1-nm AlN interlayer and a 4-μm
GaN buffer layer. 2DEG sheet resistance and Hall mobility
0741-3106 © 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
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