Atomic-layer-deposited Al2O3 on Bi2Te 3 for topological insulator field-effect transistors

Article abstract of DOI:10.1063/1.3622306

We report dual-gate modulation of topological insulator field-effect transistors (TI FETs) made on Bi₂Te₃ thin flakes with integration of atomic-layer-deposited (ALD) Al₂O₃ high-k dielectric. Atomic force micros

Full text of DOI:10.1063/1.3622306

Atomic-layer-deposited Al2O3 on Bi2Te3 for topological insulator field-effect
transistors
Han Liu and Peide D. Ye
Citation: Applied Physics Letters 99, 052108 (2011); doi: 10.1063/1.3622306
View online: http://dx.doi.org/10.1063/1.3622306
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APPLIED PHYSICS LETTERS 99, 052108 (2011)
Atomic-layer-deposited Al O on Bi Te for topological insulator
2
3
2
3
field-effect transistors
a)
Han Liu and Peide D. Ye
School of Electrical and Computer Engineering and Birck Nanotechnology Center, Purdue University,
West Lafayette, Indiana 47907, USA
(Received 22 April 2011; accepted 5 July 2011; published online 4 August 2011)
We report dual-gate modulation of topological insulator field-effect transistors (TI FETs) made on
Bi Te thin flakes with integration of atomic-layer-deposited (ALD) Al O high-k dielectric.
Atomic force microscopy study shows that ALD Al O is uniformly grown on this layer-structured
2
3
2 3
2
3
channel material. Electrical characterization reveals that the right selection of ALD precursors and
the related surface chemistry play a critical role in device performance of Bi Te based TI FETs.
We realize both top-gate and bottom-gate control on these devices, and the highest modulation rate
2
3
of 76.1% is achieved by using simultaneous dual gate control. VC 2011 American Institute of
Physics. [doi:10.1063/1.3622306]
Three dimensional topological insulator (TI) materials,
such as Bi Te , Bi Se , and Sb Te , have recently attracted
based TI FETs with both top-gate and back-gate controls.
Bi Te thin flakes were peeled off from bulk ingots by stand-
2
3
2
3
2
3
2
3
1–5
12
ard 3M scotch tape techniques and were then transferred to
much attention due to their unique physical properties.
These structures are layered like graphene but appear to
behave like insulators having a band gap in the material
bulk. However, these materials show metallic behavior on
their surfaces. The surface states of TIs consist of an odd
number of massless Dirac cones, around which the linear dis-
persion of the electron spectrum is such that the carriers
reach extremely high surface carrier mobilities up to 9000-
a highly doped silicon wafer with 300 nm thick SiO . The
2
thickness of these ultrathin flakes is less than 50 nm. Ten
nanometer Al O high-k dielectric layers were deposited by
2
3
ꢀ
an ASM F-120 ALD system at 200 C by using tri-methyl-alu-
minum (TMA) and H O or TMA and O as precursors. The
pulse time is 0.8 s TMA and 1.2 s H O, or 1s TMA and 1 s
2
3
2
O , and the purge time is 6 s N for each precursor. Source/
3
2
2
6
1
0 000 cm /Vs. Moreover, these surfaces, protected by time
drain regions were defined by optical lithography. Al O was
2 3
7
reversal symmetry, result in a non-scattering carrier trans-
port regime, making TIs rather promising for future nanoe-
lectronics applications with ultralow power dissipation.
In order to realize practical TI field-effect transistors
etched away using buffered oxide etch (BOE) at these source/
drain regions, followed by e-beam evaporation of a 20 nm/40
nm Cr/Au metal and lift-off process for ohmic contacts. A
10 nm/50 nm Cr/Au layer was finally deposited as the top-
gate. Final device structure is shown in Figure 1(a), which is
similar to a traditional metal-oxide-semiconductor FET, but
with two conducting surfaces and one conducting bulk chan-
nel in the channel region.
(FETs), it is important to study the integration of gate dielec-
trics on these materials so that the channel current can be
controlled by the top-gate. In some early studies of Bi Te3
2
based TI devices, where a heavily doped silicon back gate
was used, no modulation was observed within a back-gate
The crystal structure of Bi Te has been shown to be
2
3
8
voltage sweep from À50 V to 50 V at room temperature.
similar to graphene, with stacking layers bonded by the Van
der Waals force. The lattice constant for hexagonal Bi Te is
13–16
0.4384 nm for the a-axis and 3.045 nm for the c-axis.
The highest modulation obtained is around 20% measured at
9
less than 10 K. For Bi Se , which has a larger bandgap of
2
3
2
3
0
.3 eV than 0.14 eV of Bi Te , minuscule gate modulation
3
Each layer of Bi Te , a quintuple layer with the thickness of
2 3
2
10
was reported using top-gate control at room temperature,
while much larger modulation was achieved by the back-
ꢁ1 nm, consists of the five layer sequence Te-Bi-Te-Bi-Te.
8
Three quintuple layers form one unit cell. On the top and
11
gate sweeps. Although these are inspiring results to
observe the field effect by using a global back-gate, this can-
not satisfy the requirement for real device applications,
which requires individual device top-gate control and room
temperature operation. Therefore, the realization of highly
efficient top-gate modulation for Bi Te and other TI materi-
bottom Te layers, there are no dangling bonds at the surface.
This may lead one to suspect that it is not possible to deposit
ALD Al O directly on these flakes because there would be
2
3
no chemical absorption due to the absence of dangling bonds
at surface, as generally seen in the case of graphene. Early
studies also revealed that ALD Al O fails to be deposited
2
3
2 3
als at room temperature is urgently needed.
In this letter, we demonstrate Al O growth by atomic-
on graphene surfaces but would only cluster and grow on
graphene edges and ultimately form nanoribbons.
1
7,18
How-
ever, our results show that this problem does not occur for
Bi Te . A Veeco Dimension 3100 AFM was used to study
2
3
layer deposition (ALD) with two different precursors on
Bi Te top surfaces. The uniformity of the surface morphol-
ogy after the ALD Al O deposition is also studied. Electrical
2
3
2
3
the flake surface and the layer edges after Al O deposition.
2 3
2
3
characterization has shown a strong modulation of Bi Te3
2
The pristine Bi Te surface was studied after peeling and
2 3
being transferred to SiO /Si substrates. We notice the surface
2
a)
is not as smooth as the graphene surface. This might be
attributed to the fact that Bi Te is not as highly air-stable as
Author to whom correspondence should be addressed. Electronic mail:
yep@purdue.edu.
2
3
0003-6951/2011/99(5)/052108/3/$30.00
99, 052108-1
VC 2011 American Institute of Physics
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1

052108-2
H. Liu and P. D. Ye
Appl. Phys. Lett. 99, 052108 (2011)
FIG. 1. (Color online) (a) Schematic device structure
of a Bi
surface with multiple quintuple layer terraces coated
with an ultrathin ALD Al layer. (c) Three-dimen-
2 3 2 3
Te based TI FET. (b) AFM image of Bi Te
2 3
O
sional profile of the same surface showing different
thicknesses of the broken layers with 1-3 unit cells. (d)
AFM height measurement of the smooth terraces along
the white line illustrated in Figure 1(b).
3
graphene; oxygen and water molecules in air can slightly
oxidize the Te-terminated surface. This oxidation facilitates
the following ALD process by creating nucleation spots for
precursor absorption. The smallest step observed is ꢁ1 nm
corresponding to the thickness of one quintuple layer. Most
of terraces have the heights of single or multiple unit cells.
Figure 1(b) shows the surface morphology of Bi Te surface
cm . For Device 1, a maximum of 45.6% modulation is
reached through back-gate control, where the back-gate volt-
age is swept from À50 V to 50 V. We calculate the transcon-
ductance (g ) to be 9.8 nS. Comparatively, the top gate g is
m
m
measured 60 nS at its maximum, six times larger than the
back-gate gm due to the thinner top dielectric thickness and
higher k value. Though we get an improved value for trans-
conductance by top-gate control, the improvement is not as
high as we expect considering the oxide thickness and dielec-
tric constant for both SiO as back-gate oxide and Al O for
2
3
after 20 cycles of ALD growth using TMA and H O, corre-
2
sponding to ꢁ1.8 nm Al O deposition. In the graphene
2
3
case, the surface of graphene remained intact while Al O
2
3
2
2 3
17
nanoribbons are clearly formed at the graphene edges. No
Al O nanoribbons or clusters can be observed at the Bi Te
layer edges. Figure 1(c) shows that ALD Al O is conformal
top-gate oxide. The top-gate gm should be around 60 times
larger than back-gate g , if we don’t consider top-gate partial
2
3
2
3
m
coverage of the channel and roughly estimate the practical
dielectric constant of Al O to be ꢁ7.8, twice as that of SiO .
2
3
and uniformly coated on a series of steps of the peeled
Bi Te surface with the relative step height remaining simi-
2
3
2
The stark contrast between predicted and measured values
indicates non-ideal Al O /Bi Te interfaces, which is consist-
2
3
lar before ALD. Figure 1(d) shows the measured heights of
3 nm, 9 nm, and 6 nm corresponding to 1–3 unit cells.
These observations show that the p-bond in pristine Bi Te
2
3
2
3
ꢁ
ent with the discussions above. Also, from the extrinsic trans-
conductances, we can estimate the low limit of the effective
2
3
2
is not chemically inert in atmosphere as graphene. This insta-
bility makes direct ALD growth of dielectrics on such lay-
ered structures possible; however, the surface reactions
definitely induce potential defects at the high-k/TI interface
and hence deteriorate device performance as well.
electron mobility with top-gate control to be ꢁ1.69 cm /Vs
2
and with back-gate control to be ꢁ17.0 cm /Vs. This effective
mobility contains two parts, the surface state mobility and the
bulk mobility. The results indicate that the low mobility bulk
channel is the dominant conducting channel and the high mo-
bility surface channels are degraded by the poor interfaces, in
Electrical characterization was performed after device
fabrication using a Keithley 4200. Two devices, one using
TMA/H O (Device 1) and another using TMA/O (Device 2)
2
3
as precursors for Al O are studied here. The typical geomet-
2
3
ric features are 2 lm in gate length and 1 lm in gate width.
The channel resistance, including the contact resistance, for
Devices 1 and 2 are 32 kX and 25 kX, respectively, indicating
that the flake thicknesses are similar. Linear I-V characteris-
tics indicate a good ohmic contact to the Bi Te flakes.
2
3
Figures 2 shows independent top-gate and back-gate modula-
tion for both devices with another gate floating. The top-gate
FIG. 2. (Color online) (a) Drain current vs top-gate bias of two Bi Te TI
2
3
À10
FETs with a 10 nm Al
gate bias of the same devices with a 300 nm SiO
2
O
3
as top-gate dielectric; (b) drain current vs back
as back-gate dielectric.
and back-gate leakage currents are less than 10
A. We do
2
not observe an electron-hole or ambipolar transition because
the strong stoichiometric doping of Bi Te makes it an n-type
Drain-source voltage is 0.1 V in both measurements. The observed minor
drain current change at the same bias condition is ascribed to the change of
contact resistance and interface traps after gate bias stress.
2
3
19
material with an estimated doping concentration of ꢁ10 /
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052108-3
H. Liu and P. D. Ye
Appl. Phys. Lett. 99, 052108 (2011)
a 20 V step. We achieve the highest modulation rate of 76.1%
for Device 1 and 61.8% for Device 2. All these indicate a sig-
nificant enhancement in modulation for Bi Te thin flakes at
2
3
room temperature, using Al O high-k as a top-gate dielectric
2
3
and dual gate control. Development of a perfect high-k/TI
interface is a must to realize real device applications based on
TI FETs. In particular, the truly attractive property of TI as a
channel material for device applications is its surface channel
with high carrier mobility and velocity. Any formation of top-
gate dielectric on semiconductors cannot be as important as on
TI since the conducting channel is on the surface.
FIG. 3. (Color online) Drain current vs dual gate modulation of two Bi
TI FETs. (a) Bi Te TI FET with a 10 nm Al deposited by TMA and
O and (b) Bi Te TI FET with 10 nm Al deposited by TMA and O
2 3
Te
2
3
2 3
O
H
2
2
3
2
O
3
3
.
particular, the top interface. Poor interface conditions easily
result in a strong degradation of field-effect modulation effi-
In conclusion, we have investigated ALD high-k oxide
formation on Bi Te as a top-gate dielectric. AFM studies
2
3
19
ciency as demonstrated in ALD high-k/III-V MOSFETs.
Compared to III-V MOSFETs, such interface degradation in
Bi Te could impose more serious problems in device per-
reveal the feasibility of direct ALD of high-k dielectrics on
this layered material. Electrical characterization shows a pro-
nounced modulation by both top-gate and back-gate with the
highest modulation of 76.1% achieved with simultaneous dual
gate control. However, at this point, the top-gate modulation
is not as effective as the back-gate modulation at the same
electrical field due to the degraded interface between the ALD
dielectric and the TI. Further studies on protecting the TI sur-
2
3
formance due to its unique carrier transport properties. It has
been stated in previous studies that the conductance of those
topological insulators is composed of two parts: the bulk con-
ductance and the surface conductance, including the top sur-
10
face conductance and bottom surface conductance. The two
surface conducting channels are of interests due to its high
mobility and non-scattering carrier transport. Considering the
large intrinsic carrier density in its bulk, the top surface is
more sensitive to the top gate control than the bottom surface,
so the two surfaces do not play symmetric roles under gate
bias. The bottom surface has a better interface condition with
21–23
face during ALD dielectric formation are on-going.
The authors would like to thank X. Xu, C. Liu, G. Q. Xu,
Y.P. Chen, A.T. Neal, and N. Conrad for valuable discussions
and E. Milligan, W.J. Qian, J.F. Tian, and M. Xu for technical
assistance. The work is supported by DARPAMESO program.
the back SiO dielectric as it has been left intact during the
2
1
fabrication process, resulting in better back-gate control at the
same electrical field. However, the top surface condition of
the flakes had been changed to some extent due to Al O dep-
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17
2
3
1
1
8
9
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20
2
2
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1