IEEE ELECTRON DEVICE LETTERS, VOL. 32, NO. 5, MAY 2011
623
AlGaN/GaN High-Electron-Mobility Transistors
Fabricated Through a Au-Free Technology
Hyung-Seok Lee, Member, IEEE, Dong Seup Lee, and Tomas Palacios, Member, IEEE
Abstract—This letter reports undoped AlGaN/GaN highelectron-mobility transistors (HEMTs) fabricated with a
Si-CMOS-compatible technology based on Ti/Al/W ohmic and
Schottky contacts. The use of ohmic recess is key to reduce the
contact resistance of this Au-free metallization below 0.5 Ω · mm.
Comparison of HEMTs fabricated on the same wafer with and
without ohmic recess shows that the recess provides a tenfold
reduction in contact resistance, resulting in a fivefold lower forward voltage drop at IDS = 100 mA/mm. The reported Au-free
AlGaN/GaN HEMT fabrication technology provides similar performance (i.e., contact resistance, leakage current, and breakdown
voltage) than state-of-the-art Au-based AlGaN/GaN HEMTs and
can be used in standard Si fabs without the risk of contamination.
Index Terms—GaN, high-electron-mobility transistor (HEMT),
ohmic contact, recess etch, 2-D electron gas (2DEG).
I. I NTRODUCTION
D
UE to their combination of low conduction losses and
fast switching speed, AlGaN/GaN high-electron-mobility
transistors (HEMTs) are attractive for applications in power
electronics [1]. In addition to the excellent material properties,
GaN-on-Si wafers with a diameter of 6 inches are commercially
available, which enables the fabrication of GaN high-voltage
transistors in some of the many 6-inch fabs traditionally used
for the processing of Si devices [2]. This flexibility is an important consideration for these devices to be able to compete with
Si power devices in terms of cost. Conventional metallization
in III–V semiconductors is, however, based on Au contacts,
which are not allowed in Si fabs due to contamination issues.
Therefore, the development of CMOS-compatible metals with a
sufficiently low contact resistance (< 0.5 Ω · mm) [3] in GaNbased HEMTs is a key step to enable the processing of these
devices in Si fabs.
A Ti/Al/XY /Au stack, where XY is a diffusion layer for
Au (e.g., Ni, Pd, or Mo), is the most widely used ohmic metal
scheme for the fabrication of GaN devices [4]–[6]. A contact
resistance (Rc ) below 0.6 Ω · mm is commonly obtained in
this metallization [6]. The Au layer is believed to improve
the contact resistance by forming Ga vacancies in the semiconductor [5] and by preventing the oxidation of the metal
surface. However, long-term Au diffusion has been proposed
Manuscript received January 13, 2011; revised February 2, 2011; accepted
February 3, 2011. Date of publication March 21, 2011; date of current version
April 27, 2011. This work was supported in part by the ARPA-E ADEPT
program. The review of this letter was arranged by Editor G. Meneghesso.
The authors are with the Department of Electrical Engineering and Computer
Science, Massachusetts Institute of Technology, Cambridge, MA 02139-4307
USA (e-mail: hslee75@mit.edu; dongseup@mit.edu; tpalacios@mit.edu).
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.2011.2114322
Fig. 1. (a) Schematic cross section of the fabricated undoped AlGaN/GaN
HEMTs with S/D ohmic recess. (b) SEM image of the cross section of a
Ti/Al/W contact after annealing. Before metallization, the ohmic contact region
was recessed ∼30 nm.
as an important degradation mechanism for ohmic contacts [5].
Therefore, in addition to enabling device fabrication in Si fabs,
a Au-free metallization scheme would also help to avoid longterm contact degradation.
Au-free ohmic contacts can be obtained by intentionally
doping the AlGaN/GaN structure [7]. However, a highly doped
AlGaN/GaN layer increases the electric field under the gate,
degrading both the breakdown voltage and the leakage current [8]. Ion implantation is another approach to get Au-free
contacts, although the required high-dose implants and high
dopant activation temperatures create defects that can influence
the device characteristics [9]. Selective regrowth of the ohmic
contacts has also been used to form Au-free contacts by enhancing tunneling of carrier through the highly doped regrown
layers [10]. However, the regrowth step significantly increases
the fabrication complexity and cost.
In this letter, we use recessed ohmic contacts to demonstrate the fabrication of AlGaN/GaN HEMTs with a Au-free
Si-compatible technology. A low Rc of 0.49 Ω · mm is obtained, and its influence on the device characteristics is also
described.
II. E XPERIMENTAL P ROCEDURE
Fig. 1(a) shows a simplified cross-sectional view of the fabricated undoped AlGaN/GaN HEMTs. The AlGaN/GaN epilayer
was grown by metal-organic chemical vapor deposition on a
4-inch Si (111) wafer and consists of an ∼1.8-μm undoped GaN
buffer layer and an ∼17.5-nm Al0.26 Ga0.74 N barrier with an
∼2-nm unintentionally doped GaN cap layer. Mesa isolation
was formed by electron cyclotron resonance (ECR) reactive ion
etching with a 100-W BCl3 /Cl2 plasma. The region underneath
the ohmic contacts was recessed 15 and 30 nm by using ECR
etching with an etch rate of ∼1 nm/min and an RF bias of
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IEEE ELECTRON DEVICE LETTERS, VOL. 32, NO. 5, MAY 2011
Fig. 2. Contact resistance as a function of annealing temperature for Ti/Al/W
with different recess depths. The inset shows the I–V characteristics across a
10-μm gap in as-deposited and annealed Ti/Al/W contacts with ∼30-nm recess.
∼75 V [11]. A Veeco Dimension 3100 atomic force microscope
was used to measure the etch depth. A Ti/Al/W (600/1000/
300 Å) metal stack was then deposited by electron beam evaporation. The thin W layer prevents the oxidation of the Al layer.
Linear transmission line method (LTLM) structures were fabricated at the same time with the transistor devices. The ohmic
metal stack was annealed in a rapid thermal processing chamber
at 600 ◦ C–1000 ◦ C in nitrogen ambient. Fig. 1(b) shows a scanning electron microscope (SEM) image of the recessed contact
after annealing at 870 ◦ C. To study the effect of the ohmic
recess on the source/drain (S/D) contact resistance, a sample
was processed with the same metal stack (Ti/Al/W) but without
recessed contacts. As a control experiment, an additional sample with conventional ohmic metallization (Ti/Al/Ni/Au metal
stack annealed at 870 ◦ C) [11] was fabricated. All samples have
nominally the same epitaxial structure and fabrication process,
except for the S/D ohmic contact. After the formation of the
ohmic contacts, a 3-nm-thick Ga2 O3 gate dielectric was grown
by exposing the gate region to an O2 plasma treatment, and then
a gate electrode with a Ti/Al/W metal stack was deposited by
electron beam evaporation. Finally, the devices were passivated
by a 20-nm-thick Al2 O3 dielectric formed by atomic layer
deposition [12].
III. M EASUREMENT R ESULTS
Fig. 2 shows the Rc of Ti/Al/W metallization as a function
of annealing temperature for samples with different ohmic
recesses. While the contacts annealed below 700 ◦ C exhibit
nonlinear characteristics (see the inset in Fig. 2), a clear ohmic
behavior has been obtained for contacts annealed at 800 ◦ C
or above. The samples without ohmic recess showed ohmic
behavior after a 900 ◦ C anneal, and the lowest Rc in these
samples was 2.02 Ω · mm in the contacts annealed at 950 ◦ C
for 30 s. The Rc was decreased to 1.71 Ω · mm in the samples
with ∼15-nm ohmic recess etch and a 30-s anneal at 870 ◦ C.
The lowest Rc (0.49 Ω · mm) was obtained in a fully recessed
sample (∼30-nm recess) annealed at 870 ◦ C for 30 s. It should
Fig. 3. (a) Total resistance for different ohmic metallizations as a function of
contact distance. (b) Optical micrograph images of the surface in (left) Ti/Al/W
recessed contacts and (right) standard Ti/Al/Ni/Au contacts.
Fig. 4. DC I–V output characteristics of undoped AlGaN/GaN HEMTs with
Ti/Al/W contacts ( ) with and ( ) without a 30-nm ohmic recess. As a
reference, the I–V characteristics of a standard transistor ( ) with nonrecessed
Ti/Al/Ni/Au ohmic metallization are also shown.
be noted that, in the ∼30-nm recessed samples, the entire
AlGaN barrier, including the 2-D electron gas (2DEG), has
been removed underneath the contact region. This indicates
that the Ti/Al/W metallization needs to be in direct contact
with the 2DEG at the sidewall of the recess region in order
to minimize Rc . Also, the formation of a (Ti, Al)N [5] layer
after the anneal helps to reduce the energy barrier between the
ohmic metal and the 2DEG channel. Fig. 3(a) shows the total
resistance versus TLM contact spacing as measured in a sample
annealed under optimized conditions. As a reference, a control
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LEE et al.: AlGaN/GaN HEMTs FABRICATED THROUGH A Au-FREE TECHNOLOGY
625
IV. S UMMARY
A Si-compatible fabrication technology for undoped
AlGaN/GaN HEMT has been successfully demonstrated using
recessed ohmic contacts and gate dielectrics. The optimum annealing temperature for recessed Ti/Al/W ohmic metallization
was 870 ◦ C. The recessed contacts had over one order of magnitude lower contact resistivity (∼6.5 × 10−6 Ω · cm2 ) than the
nonrecessed contacts (∼8.7 × 10−5 Ω · cm2 ). By comparison
with a HEMT with conventional Ti/Al/Ni/Au S/D contacts
(∼3.1 × 10−6 Ω · cm2 ), a HEMT with recessed Ti/Al/W contacts showed less than 15% difference in the forward voltage
drop at IDS = 100 mA/mm. In addition, the surface morphology of the Ti/Al/W contacts was much smoother. These results
enable the fabrication of high-performance GaN power devices
in Si fabs without the risk of contaminating the Si wafers, which
enables the large-scale production of GaN power electronics.
Fig. 5. O FF-state IDS −VDS characteristics of undoped AlGaN/GaN HEMTs
with and without ohmic recess.
sample with Ti/Al/Ni/Au metallization annealed at 870 ◦ C is
also shown.
The specific contact resistivities (ρc ) on the recessed and
then nonrecessed Ti/Al/W contacts were ∼6.5 × 10−6 Ω · cm2
(Rc ≈ 0.49 Ω · mm) and ∼8.7 × 10−5 Ω · cm2 (Rc ≈ 2.1 Ω ·
mm), respectively, as measured on LTLM structures. It should
be noted, however, that the definition of specific contact resistivity is questionable in the case of sidewall contacts where the
metal is in direct contact with the 2DEG channel. The ρc of the
control sample (Ti/Al/Ni/Au) was ∼ 3.1 × 10−6 Ω · cm2 (Rc ≈
0.38 Ω · mm).
Fig. 3(b) shows optical micrographs of the surface morphology of Ti/Al/W recessed contacts (left) and conventional
Ti/Al/Ni/Au contacts (right). The Ti/Al/W metallization shows
a much smoother morphology, which has been attributed to
the lack of AlAu4 alloys [4]. The smooth surface of Ti/Al/W
contacts is an additional advantage when scaling down device
dimensions [14].
The current–voltage characteristics of transistors fabricated
with the optimized recessed contacts, nonrecessed contacts,
and the control sample are shown in Fig. 4. The measured
device had a gate length Lg = 3 μm, gate–source distance
Lgs = 1.5 μm, and gate–drain distance Lgd = 1.5 μm. All
the devices have a similar threshold voltage of ∼ −2.2 V,
gate leakage current of < 10−1 mA/mm, and three-terminal
breakdown voltage of ∼87 V (see Fig. 5). The lower contact
resistance of the optimized Au-free structure allows much
lower voltage drop for the same output current level, a very
important characteristic for high-voltage power electronics. For
example, the forward voltage drops at IDS = 100 mA/mm are
VDS = 0.78 V and VDS = 1.31 V for the recessed and nonrecessed samples, respectively. As a comparison, the forward
voltage drop of the Ti/Al/Ni/Au control transistor is VDS =
0.68 V at IDS = 100 mA/mm. These results show that undoped
AlGaN/GaN HEMTs can be fabricated with a Si-compatible
contact technology without degradation in device performance.
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