Current transport and maximum dielectric strength of atomic-layer-deposited ultrathin Al2 O3 on GaAs

Article abstract of DOI:10.1063/1.2535528

Current-voltage characteristics have been measured for an ultrathin atomic-layer-deposited Al2 O3 on GaAs as a function of film thickness, ambient temperature, and electric field. Current transport measurements indicate direct tunneling for 12-25 A ultrathin films and typical Frenkel-Poole emission for 50-60 A thick films. The maximum dielectric strength, which is of considerable importance for metal-oxide-semiconductor device applications, is studied thoroughly in this letter. The square root of the maximum dielectric strength is found to be roughly reversely proportional to the temperature from 500 to 300 K, and becomes constant below 200 K. The enhancement in the dielectric field strength observed from 300 to 200 K for the ultrathin insulating films can be adequately described using classical models.

Full text of DOI:10.1063/1.2535528

Current transport and maximum dielectric strength of atomic-layer-deposited ultrathin
Al 2 O 3 on GaAs
Y. Q. Wu, H. C. Lin, P. D. Ye, and G. D. Wilk
Citation: Applied Physics Letters 90, 072105 (2007); doi: 10.1063/1.2535528
View online: http://dx.doi.org/10.1063/1.2535528
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APPLIED PHYSICS LETTERS 90, 072105 ͑2007͒
Current transport and maximum dielectric strength of atomic-layer-
deposited ultrathin Al2O3 on GaAs
Y. Q. Wu, H. C. Lin, and P. D. Yea͒
School of Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana 47907
and Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907
G. D. Wilk
ASM America, 3440 East University Drive, Phoenix, Arizona 85034
͑Received 16 October 2006; accepted 13 January 2007; published online 14 February 2007͒
Current-voltage characteristics have been measured for an ultrathin atomic-layer-deposited Al2O3
on GaAs as a function of film thickness, ambient temperature, and electric field. Current transport
measurements indicate direct tunneling for 12–25 Å ultrathin films and typical Frenkel-Poole
emission for 50–60 Å thick films. The maximum dielectric strength, which is of considerable
importance for metal-oxide-semiconductor device applications, is studied thoroughly in this letter.
The square root of the maximum dielectric strength is found to be roughly reversely proportional to
the temperature from 500 to 300 K, and becomes constant below 200 K. The enhancement in the
dielectric field strength observed from 300 to 200 K for the ultrathin insulating films can be
adequately described using classical models. © 2007 American Institute of Physics.
͓DOI: 10.1063/1.2535528͔
In order to further drive complementary metal-oxide-
semiconductor ͑CMOS͒ integration beyond the 22 nm node,
novel channel materials and device structures must be devel-
oped to improve its functional density, speed, and power dis-
sipation. One emerging research field involves the use of
III-V compound semiconductor materials as conduction
channels to replace traditional Si or strained Si.1 These high
mobility materials could be integrated with novel high-k di-
electrics and heterogeneously built on silicon or silicon on
insulator. The research in III-V materials and devices for
logic applications represents a great opportunity but also a
significant challenge, since the heterogeneous integration of
III-V compound semiconductors on Si and the growth of
high-quality gate dielectrics on III-V are both “historically”
difficult problems.2,3
Atomic layer deposition ͑ALD͒ is an ultrathin-film depo-
sition technique based on sequences of self-limiting surface
reactions, which enables thickness control on the atomic
scale. The recent development of high-quality ALD gate di-
electrics on Si or Ge justifies some optimism regarding the
integration of deposited oxides on III-V compound semicon-
ductors. We have applied ALD to grow Al2O3 gate dielec-
trics on III-V compound semiconductors and demonstrated
and its exceptional thermal and chemical stabilities. Al2O3
has a high band gap ͑ϳ9 eV͒, a high breakdown electric
field ͑5–30 MV/cm͒,7 a high permittivity ͑8.6–10͒, high
thermal stability ͑up to at least 1000 °C͒, and remains amor-
phous under typical processing conditions. The leakage cur-
rent observed in ultrathin Al2O3 on GaAs is equivalent to or
lower than that of the state-of-the-art SiO2 on Si. The break-
down electric field of Al2O3 films thicker than 50 Å can be
up to ϳ10 MV/cm; this value is near the bulk breakdown
electric field for SiO2. A significant enhancement in the
breakdown electric field ͑up to ϳ30 MV/cm͒ has been ob-
served for smaller film thicknesses ͑12 Å͒.7 In this letter, we
report detailed maximum dielectric strength studies of ultra-
thin ALD Al2O3 dielectrics on GaAs. Based on our findings,
the low-temperature breakdown electric field in 12 Å thick
Al2O3 can be as high as 60 MV/cm.
2 in. Si-doped GaAs wafers with a doping concentration
of ͑6–8͒ϫ1017/cm3 were initially pretreated with a dilute
HF solution to remove the native oxide present on its sur-
face. The wafers were then transferred immediately to an
ASM Pulsar2000™ ALD module to grow the Al2O3 films.
The films were grown using alternating pulses of Al͑CH3͒3
͑the aluminum precursor͒ and H2O ͑the oxygen precursor͒ in
the presence of N2 carrier gas flow. An excess of each pre-
cursor was supplied alternatively to saturate the surface sites
and to ensure self-limiting film growth. The number of ALD
cycles was varied to modulate the thickness of Al2O3 films.
Various Al2O3 oxide layers of thicknesses of 12, 15, 20, 25,
30, 40, 50, and 60 Å were deposited at a substrate tempera-
ture of 300 °C. All of the deposited films were amorphous,
making them favorable to be used as a gate dielectric. Fol-
lowing the deposition, the films were annealed at 600 °C
with oxygen gas in a rapid thermal annealing chamber. A
1000 Å thick Au film was then deposited on the backside of
the GaAs wafer to reduce the contact resistance between the
wafer and electrical probes. The oxide leakage current was
measured by 3000 Å of Au, which served as the top elec-
trode of the capacitor.
high-performance
depletion-mode
MOS
field-effect
transistors.4–6 Current research thrusts are focused on III-V
CMOS technology for large-scale integration or digital ap-
plications. This requires not only improved oxide/
semiconductor interface properties but also high-k gate di-
electrics with further scalability to sub-1.0 nm effective
inversion gate-oxide thicknesses. Nevertheless, the current
transport and maximum dielectric strength studies of ultra-
thin high-k oxides on GaAs are of utmost importance. Al2O3
is a widely used insulating material for gate dielectrics, tun-
neling barriers and protection coatings due to its excellent
dielectric properties, strong adhesion to dissimilar materials,
a͒
Author to whom correspondence should be addressed; electronic mail:
yep@purdue.edu
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0003-6951/2007/90͑7͒/072105/3/$23.00 90, 072105-1 © 2007 American Institute of Physics
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072105-2
Wu et al.
Appl. Phys. Lett. 90, 072105 ͑2007͒
FIG. 2. Square root of the maximum dielectric strength Em vs ambient
temperature T for six different thickness ALD Al2O3 films on GaAs. The
dashed lines are calculated via Eqs. ͑1͒–͑3͒ on film thicknesses of 12, 40,
and 60 Å using the experimental data except for C1 /G as a fitting parameter.
tunneling or Fowler-Nordheim cold emission is also found to
be a straight line over many decades of current density. The
dynamic dielectric constant ͑␧ ͒, obtained from the slopes of
d
curves at high fields and high temperatures in Fig. 1͑b͒, is
found to be on the same order of magnitude as the static
dielectric constant for Al2O3. The value obtained from the
slopes of curves in Fig. 1͑a͒ is one order of magnitude higher
than the static value. This result is of paramount importance
to confirm that the dynamic dielectric constant is self-
consistent and the dominant transport mechanism is indeed
Frenkel-Poole emission. As the ramp voltage or electric field
increases, the conduction current or current density increases
accordingly. Eventually, the electric field reaches a maxi-
mum value, beyond which destructive breakdown occurs.
The above maximum voltage or breakdown voltage normal-
ized to the film thickness is defined as the maximum dielec-
tric strength ͑Em͒. The difference between the metal work
function and GaAs affinity is considered to determine the
breakdown voltage across the insulating film. We use ellip-
sometry, high-resolution transmission electron microscopy,
and tunneling currents to calibrate the physical thickness of
these ALD films. The results were compared to the predeter-
mined target thicknesses based on our selected number of
ALD reaction cycles ͑see Fig. 3 in Ref. 7͒. A clear 1:1 linear
dependency is obtained for film thicknesses larger than 15 Å
with an error of less than 5%. Some evidence for accelerated
initial Al2O3 growth on HF-pretreated GaAs surface, re-
ported in Ref. 9, is also observed in our experiments. The
error bars are given on Em in Fig. 2, considering a 5%–20%
uncertainty on film thicknesses and the near Gaussian distri-
bution of breakdown voltages measured on assembly of
10–20 devices at the same film thickness.
FIG. 1. ͑a͒ Measured leakage current density J vs square root of electric
field E for 12 Å thick ALD Al2O3 films on GaAs at five different tempera-
tures ͑300 KഛTഛ500 K͒. ͑b͒ The similar measurement performed at 60 Å
thick ALD Al2O3 films on GaAs.
Figure 1 shows the leakage current density ͑J͒ versus the
square root of the electric field ͑E͒ on 12 and 60 Å thick
Al2O3 films at five different ambient temperatures. A voltage
bias was applied on the metal electrodes with respect to the
grounded GaAs substrate. The Al2O3 dielectric films are
highly electrically insulating. A very low leakage current
density ͑ϳ10−9–10−8 A/cm2͒ is exhibited near zero bias at
the limit of instrumentation. Figs. 1͑a͒ and 1͑b͒ show signifi-
cantly different features. Assuming that the dielectric con-
stant of Al2O3 is 8.6 without any interfacial layer, the 12 Å
Al2O3 film with an equivalent oxide thickness of only 5.4 Å
shows a well-behaved direct tunneling characteristic. Dielec-
tric breakdown transpires at an electric field near 30 MV/cm
at 300 K. Compared to the state-of-the-art SiO2 on Si using a
polysilicon gate, the leakage current density of Al2O3 on
GaAs is equivalent to or even one order of magnitude lower
at a similar bias range.7 The plot also shows a slight increase
in current density with increasing temperature; this is un-
equivocal to a pure tunneling mechanism, which lacks tem-
perature dependence. Figure 1͑b͒ indicates typical Frenkel-
Poole emission features. It has been pointed out by
O’Dwyer8 that a linear dependence of log͑J͒ vs E1/2 quite
accurately over several decades does not necessarily imply a
Following Klein and Gafni’s work, assuming that Em is
limited by the thermal instability of the structure10 and Sze’s
classical model on Si3N4,11 an approximate expression for
the maximum dielectric strength as a function of the ambient
temperature is
Em Ϸ ͑␲␧ ␧d/q͓͒⌽1 − CT͔2,
͑1͒
0
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Frenkel-Poole emission since the plot of log͑J͒ vs E1/2 for
where
128.59.171.71 On: Thu, 11 Dec 2014 08:59:34

072105-3
Wu et al.
Appl. Phys. Lett. 90, 072105 ͑2007͒
C ϵ ͑k/q͒ln͓q͑⌽1 − aE1m/2͒C1Em2 Ad/⌫kT2͔,
͑2͒
In summary, we have systematically studied current-
transport characteristics in conjunction with the maximum
dielectric strength of ultrathin atomic-layer-deposited Al2O3
dielectrics grown on GaAs as a function of oxide thickness,
ambient temperature, and electric field strength. The Al2O3
films were grown remarkably well on GaAs using ALD. This
makes them an excellent choice for its use in passivation
layers or dielectrics for III-V MOS devices. Current transport
measurements indicate direct tunneling for 12–25 Å ultra-
thin films and typical Frenkel-Poole emission for 50–60 Å
thick films. The maximum dielectric strength of 12 Å ALD
Al2O3 on GaAs is ϳ30 MV/cm at room temperature and
reaches ϳ60 MV/cm at low temperatures.
which is a slowly varying function of T and Em. ⌽1 is the
barrier height for Frenkel-Poole emission, ␧ is the permit-
0
tivity of free space, ␧d is the dynamic dielectric constant, q is
the charge of electrons, aϵ͑q/␲␧ ␧ ͒1/2, C1 is the propor-
0
d
tionality constant for Frenkel-Poole emission dependant on
density of the trapping centers, A is the area of the structure,
d is the thickness of the film, and ⌫ is the thermal conduc-
tance. At low temperatures, the longitudinal-phonon energy
h␯»kT, Em becomes the intrinsic dielectric strength indepen-
dent of T as
Em͑T͒ Ϸ E͑0͒.
͑3͒
The authors would like to thank M. A. Alam and Y. Xuan
for their valuable discussions, and M. DaSilva for critical
reading of this letter. This work is supported in part by NSF
͑Grant No. ECS-0621949͒ and the MARCO MSD Focus
Center.
The plot of E1m/2 vs T, as shown in Fig. 2, verifies that the
maximum dielectric strength of ultrathin ALD films on GaAs
could be surprisingly depicted by the classical model, Eqs.
͑1͒–͑3͒, developed for thick insulating films.10,11 The value
of Em at a given temperature is virtually independent of film
thickness for thick films. The bulk value for ALD Al2O3 on
GaAs at room temperature is around 10 MV/cm as shown in
Fig. 2 for 60 Å film. The bulk value of Em at low tempera-
tures becomes independent of the temperature and ap-
proaches a value of ϳ15 MV/cm; this is predicted by Eq.
͑3͒. The value of Em is strongly dependent on the film thick-
ness for values of 40 Å or less due to the well-known Em
enhancement for the ultrathin insulating films.11 Based on
our findings, this enhancement becomes significant once the
film thickness approaches 25 Å. The value of Em increases to
30 MV/cm at room temperature and 60 MV/cm at low tem-
peratures for 12 Å thick Al2O3 on GaAs. The “anomalous”
or rapid increase of Em with the decrease of temperature
observed only for ultrathin films can be expected since Em is
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largest value for the thinnest film, as described in Eq. ͑2͒.
The dashed lines are calculated curves on three representa-
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fitting parameter for the appropriated regions.
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