Leakage current and breakdown electric-field studies on ultrathin atomic-layer-deposited Al2O3 on GaAs

Article abstract of DOI:10.1063/1.2120904

Atomic-layer deposition (ALD) provides a unique opportunity to integrate high-quality gate dielectrics on III-V compound semiconductors. We report detailed leakage current and breakdown electric-field characteristics of ultrathin Al2 O3 dielectrics on GaAs grown by ALD. The leakage current in ultrathin Al2 O3 on GaAs is comparable to or even lower than that of state-of-the-art Si O2 on Si, not counting the high- k dielectric properties for Al2 O3. A Fowler-Nordheim tunneling analysis on the GaAs Al2 O3 barrier height is also presented. The breakdown electric field of Al2 O3 is measured as high as 10 MVcm as a bulk property. A significant enhancement on breakdown electric field up to 30 MVcm is observed as the film thickness approaches to 1 nm.

Full text of DOI:10.1063/1.2120904

Leakage current and breakdown electric-field studies on ultrathin atomic-layer-
deposited Al 2 O 3 on GaAs
H. C. Lin, P. D. Ye, and G. D. Wilk
Citation: Applied Physics Letters 87, 182904 (2005); doi: 10.1063/1.2120904
View online: http://dx.doi.org/10.1063/1.2120904
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/87/18?ver=pdfcov
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APPLIED PHYSICS LETTERS 87, 182904 ͑2005͒
Leakage current and breakdown electric-field studies
on ultrathin atomic-layer-deposited Al O on GaAs
2
3
a͒
H. C. Lin and P. D. Ye
School of Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana 47907
G. D. Wilk
Advanced Semiconductor Materials (ASM) America, 3440 East University Drive, Phoenix, Arizona 85034
͑
Received 29 June 2005; accepted 1 September 2005; published online 25 October 2005͒
Atomic-layer deposition ͑ALD͒ provides a unique opportunity to integrate high-quality gate
dielectrics on III-V compound semiconductors. We report detailed leakage current and breakdown
electric-field characteristics of ultrathin Al O dielectrics on GaAs grown by ALD. The leakage
2
3
current in ultrathin Al O on GaAs is comparable to or even lower than that of state-of-the-art SiO
2
3
2
on Si, not counting the high-k dielectric properties for Al O . A Fowler-Nordheim tunneling
2
3
analysis on the GaAs/Al O barrier height is also presented. The breakdown electric field of Al O
2
3
2
3
is measured as high as 10 MV/cm as a bulk property. A significant enhancement on breakdown
electric field up to 30 MV/cm is observed as the film thickness approaches to 1 nm. © 2005
American Institute of Physics. ͓DOI: 10.1063/1.2120904͔
Al O is a widely used insulating material for gate di-
sured as high as 10 MV/cm for films thicker than 50 Å,
2
3
electric, tunneling barrier, and protection coating due to its
excellent dielectric properties, strong adhesion to dissimilar
materials, and its thermal and chemical stabilities. Al O has
which is near the bulk breakdown electric field for ALD
Al O . A significant enhancement on breakdown electric
2
3
field up to 30 MV/cm is observed as the film thickness ap-
proaches to 10 Å. The capability to deposit high-quality ul-
trathin insulating films on dissimilar materials by ALD opens
the way to explore novel device concepts away from the
traditional Si MOSFETs.
2
3
a high band gap ͑ϳ9 eV͒, a high breakdown electric field
5–10 MV/cm͒, a high permittivity ͑8.6–10͒, high thermal
͑
stability ͑up to at least 1000 °C͒, and remains amorphous
under typical processing conditions. Compared to the con-
ventional methods to form thin Al O films, i.e., by sputter-
The starting materials were 2 in. Si-doped GaAs wafers
2
3
1
7
3
ing, electron-beam evaporation, chemical vapor deposition,
or oxidation of pure Al films, the atomic-layer-deposited
with the doping concentration of ͑6–8͒ϫ10 /cm . Before
Al O deposition, substrates were treated with a diluted HF
2
3
͑
ALD͒ Al O is of much higher quality. ALD is an ultra-thin-
solution to remove the native oxide to eliminate the interfa-
cial layer sometimes existing at the Al O /GaAs interface.
2
3
film deposition technique based on sequences of self-limiting
surface reactions enabling thickness control on atomic scale.
The ALD high-k materials including Al O are the leading
2
3
The wafers were transferred immediately to an ASM
Pulsar2000™ ALD module to grow ALD films. An excess of
each precursor was supplied alternatively to saturate the sur-
face sites and ensure self-limiting film growth. Al O films
2
3
candidates to substitute SiO for sub-100 nm Si complimen-
tary metal-oxide-semiconductor field-effect transistor ͑MOS-
FET͒ applications. ALD also provides unique opportunity to
integrate high-quality gate dielectrics on non-Si semiconduc-
tor materials. We have applied the ALD grown high-k gate
dielectrics on high-mobility III-V compound semiconductors
and demonstrated GaAs and GaN MOSFETs with excellent
2
2
3
1
were grown using alternating pulses of Al͑CH ͒ ͑the Al
3 3
precursor͒ and H O ͑the oxygen precursor͒ in a carrier N
2 2
gas flow. Different Al O oxide layers of the thickness of 12,
2
3
15, 20, 25, 30, 40, 50, and 60 Å were deposited at a substrate
temperature of 300 °C. The number of ALD cycle was used
here to control the thickness of deposited films. All deposited
Al O films in this Letter are amorphous, which is favorable
2
–5
device performance.
The quality of ultrathin Al O on
2 3
GaAs is of great importance for exploring the ultimate ultra-
high-speed MOSFETs or terahertz MOSFETs which com-
bine the following three features: ͑1͒ high-k dielectrics ͑2͒
high-mobility carrier channels, and ͑3͒ ultrashort gate
lengths below 100 nm.
In this Letter, we report detailed leakage current and
breakdown electric-field characteristics of ultrathin Al O di-
electrics on GaAs grown by ALD. The leakage current in
ultrathin Al O on GaAs is comparable to or even lower than
2
3
for gate dielectrics. The 600 °C O anneals were performed
2
ex situ in a rapid thermal annealing chamber following film
deposition. A 1000-Å-thick Au film were deposited on the
back side of GaAs wafers to reduce the contact resistance
between GaAs wafers and the chuck of the measurement
setup. The oxide leakage currents were measured through
capacitors which were fabricated using 3000 Å Au top
electrodes.
2
3
2
3
that of the state-of-the-art SiO on Si, not counting the high-
The Al O dielectric films are highly electrically insulat-
2
2
3
k dielectric properties for Al O , which is more than a factor
ing, showing very low leakage current density of
2
3
ϳ10− –10 A/cm at zero bias, as shown in Fig. 1. This
could do with the fact that Al is electropositive +3 and has a
strong affinity to oxygen. We measured the dependence of
10
−9
2
of 2 higher compared to SiO . Through Fowler-Nordheim
2
͑
FN͒ tunneling analysis, the GaAs/Al O barrier height is
2 3
determined. The breakdown electric field of Al O is mea-
2
3
the leakage current density ͑J ͒ on the applied potential on
L
a͒Author to whom correspondence should be addressed; electronic mail:
yep@purdue.edu
the capacitor ͑V ͒ for a set of ALD Al O samples with the
g 2 3
oxide thickness systematically reduced from 50 to 12 Å. The
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003-6951/2005/87͑18͒/182904/3/$22.50 87, 182904-1 © 2005 American Institute of Physics
On: Tue, 02 Dec 2014 08:55:00
0

182904-2
Lin, Ye, and Wilk
Appl. Phys. Lett. 87, 182904 ͑2005͒
FIG. 1. Leakage current density J vs gate bias V for ALD Al O films on
GaAs with different film thickness from 12 to 50 Å.
L
g
2
3
2
FIG. 2. Fowler-Nordheim plot ͑J /E
vs 1/E ͒ of the I-V data for a
L
ox
ox
6
0-Å-thick Al O ALD film on GaAs. Inset: high-resolution TEM image of
2 3
this sample.
positive bias means that the metal electrode is positive with
respective to GaAs. The plot shows a decrease in current
density with increasing film thickness in general. Direct tun-
neling current is observed for film thickness ഛ30 Å, while
film with thickness ജ50 Å shows no significant direct tun-
neling. The 12 Å Al O with the equivalent oxide thickness
bution of the VBR is narrow but existing. By assuming that
the VBR follows the Gaussian distribution, we find its average
standard-deviation-to-mean ratio to be 7.6%. We choose the
fitted peak value as the final VBR after taking account the
2
3
of only 4.7 Å still shows well-behaved direct tunneling char-
acteristic and does not break down at ±3 V bias. Compared
0
.4 eV Schottky barrier height between Au and GaAs at the
backside, and 0.3 eV potential difference between Au work
function and GaAs affinity to vacuum. To measure the exact
physical thickness of an ultrathin insulating film is also chal-
lenging, in particular, as the film thickness approaches 10 Å.
We use ellipsometry, high-resolution TEM, and tunneling
currents to calibrate the physical thickness of these ALD
films previously determined by counting the number of ALD
reaction cycles, as shown in Fig. 3. A good 1:1 linear depen-
dence is obtained as the film thickness larger than 15 Å with
the error less than 5%. More deviation could occur as the
film thickness approaches 10 Å.
to the state-of-the-art SiO on Si, the leakage current density
2
of Al O on GaAs is one order of magnitude lower. The low
2
3
electrical leakage even for films as thin as 12 Å suggests that
ALD supplies a new technology to form unprecedented high-
quality insulating films on various substrates.
The rise in leakage current observed in the J -V , as
L
g
shown in Fig. 1, at higher applied bias is consistent with FN
tunneling. The electron tunneling through a triangular barrier
for this type of field-assisted tunneling is described as
J = ␣Eox2 exp͑− ␤/E ͒,
͑1͒
ox
where
␣
−
6
*
= 1.54 ϫ 10 /m ⌽
B
and
= 6.83 ϫ 10 ͑m ͒ ͑⌽B͒3/2.
7
* 1/2
͑2͒
␤
*
m is the effective electron mass in the insulator and ⌽ is
B
2
the barrier height. Figure 2 shows a plot of J/Eox vs 1/Eox
resulting from I-V measurement of a 60-Å-thick Al O film
2
3
as shown by a high-resolution transmission electron micros-
copy ͑TEM͒ image in the inset of Fig. 2. The negative slope
of −␤, before its breakdown at ϳ10 MV/cm, can be used to
calculate the Al O /GaAs barrier height, which is an impor-
2
3
*
tant parameter for device design. Using m =0.23 for Al O
2
3
and Fig. 2, the GaAs/Al O barrier height ⌽ is ϳ3.2 eV,
2
3
B
which is higher than the Si/Al O barrier height of 2.6 eV
2
3
6
determined by the similar method.
Breakdown electric field ͑EBR͒ is an important material
parameter to evaluate the strength of the insulating layer. EBR
is defined as VBR divided by T , where V is the break-
FIG. 3. The measured film thickness by ellipsometry, tunneling current, and
high-resolution TEM vs the film thickness determined by the ALD growth
or the number of reaction cycles. The measured thickness ͑upper triangles͒
of “tunneling current ͑A͒” is determined by the linear fitting of all data
points of ln͑J ͒ vs T , while the thickness ͑down triangles͒ of “tunneling
ox
BR
down electric voltage across the insulating layer and Tox is
the physical thickness of the insulating layer. Between 16–25
breakdown measurements were performed on different de-
L
ox
current ͑B͒” is determined by the linear line with two fixed points of 50 and
0 Å. Here, we assume that the thickness for 50 and 60 Å determined by
6
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vices on each thickness ranging from 12 to 60 Å. The distri-
ALD growth is accurate.
On: Tue, 02 Dec 2014 08:55:00

182904-3
Lin, Ye, and Wilk
Appl. Phys. Lett. 87, 182904 ͑2005͒
acteristics of ALD films on GaAs demonstrate the potential
to scale down the gate length of GaAs MOSFET below
1
00 nm with reliable ultrathin high-k oxide layers as
dielectrics.
In summary, we have systematically studied leakage cur-
rent and breakdown electric-field characteristics of ultrathin
Al O dielectrics on GaAs grown by ALD. The leakage cur-
2
3
rent in ultrathin Al O on GaAs, comparable to or even
2
3
lower than that of state-of-the-art SiO on Si, indicates the
2
extremely high quality of Al O on GaAs. The ALD Al O
2
3
2
3
film grows remarkably well and is an excellent choice for an
insulating or protective layer on a wide variety of device
applications. A significant enhancement on breakdown elec-
tric field up to 30 MV/cm opens the way to explore other
novel device concepts and exotic physics phenomena, which
requires extremely high electric field on insulating layers.
The authors would like to thank H. X. Wu for technical
assistance and B. Yang, K. K. Ng, J. D. Bude, and M. A.
Alam for valuable discussions.
FIG. 4. The breakdown electric fields vs different thicknesses of the ALD
films. The different signs represent the thickness determined by different
methods.
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mined by various methods. The electrical properties of ALD
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3
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BR
Al O films with the thickness of between 50–60 Å is
2
3
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higher breakdown electric field indicates the possibility of
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