Charge storage characteristics of atomic layer deposited RuOx nanocrystals

Abstract

The charge storage characteristics of atomic layer deposited Ru O _x nanocrystals embedded in high- k Hf O ₂/ Al ₂ O ₃ films in a metal/ Al ₂ O ₃/ Ru O _x/ Hf O ₂/ Si O ₂/n- Si structure have been investigated. The size and density of Ru O _x nanocrystals have been measured using transmission electron microscopy. The Ru O _x nanocrystals show a density of ∼1×10¹²/ cm ² and a diameter of 5–8 nm . A large hysteresis memory window of ∼13.3 V at a gate voltage of 9 V has been observed for Ru O _x nanocrystal memory capacitors. A hysteresis memory window of 0.7 V has also been observed under a small sweeping gate voltage of 1 V . A promising memory window of Ru O _x nanocrystals has been observed as compared with those of pure Hf O ₂ and Al ₂ O ₃ charge trapping layers, due to charge storage in the Ru O _x metal nanocrystals. The Ru O _x nanocrystal memory c-
-
apacitor has similar leakage current with the pure Hf O ₂ and Al ₂ O ₃ charge trapping layers. The Ru O _x memory capacitor has a large breakdown voltage of ∼13.8 V .

Full text

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Charge storage characteristics of atomic layer deposited RuOxnanocrystals
S. Maikapa?
Department of Electronic Engineering, Chang Gung University, Tao-Yuan, Taiwan 333, Republic of China
T. Y. Wang
Department of Material Science Engineering, National Taiwan University, Taipei, Taiwan 106,
Republic of China
P. J. Tzeng, C. H. Lin, and L. S. Lee
Electronic and Opto-electronic Research Laboratories, Industrial Technology Research Institute, Hsinchu,
Taiwan 310, Republic of China
J. R. Yang
Department of Material Science Engineering, National Taiwan University, Taipei, Taiwan 106,
Republic of China
M. J. Tsai
Electronic and Optoelectronic Research Laboratories, Industrial Technology Research Institute, Hsinchu,
Taiwan 310, Republic of China
?Received 2 April 2007; accepted 24 May 2007; published online 19 June 2007?
The charge storage characteristics of atomic layer deposited RuOxnanocrystals embedded in high-
k HfO2/Al2O3films in a metal/Al2O3/RuOx/HfO2/SiO2/n-Si structure have been investigated. The
size and density of RuOxnanocrystals have been measured using transmission electron microscopy.
The RuOxnanocrystals show a density of ?1?1012/cm2and a diameter of 5–8 nm. A large
hysteresis memory window of ?13.3 V at a gate voltage of 9 V has been observed for RuOx
nanocrystal memory capacitors. A hysteresis memory window of 0.7 V has also been observed
under a small sweeping gate voltage of 1 V.Apromising memory window of RuOxnanocrystals has
been observed as compared with those of pure HfO2and Al2O3charge trapping layers, due to charge
storage in the RuOxmetal nanocrystals. The RuOxnanocrystal memory capacitor has similar
leakage current with the pure HfO2and Al2O3charge trapping layers. The RuOxmemory capacitor
has a large breakdown voltage of ?13.8 V. © 2007 American Institute of Physics.
?DOI: 10.1063/1.2749857?
Nonvolatile memory devices with a low gate voltage op-
eration, consuming less power and allowing higher integra-
tion with high-speed writing and erasing of data have an
important role in semiconductor industry for future nanos-
cale flash memory device applications. Silicon nitride
?Si3N4? charge trapping layers in a polycrystalline-silicon–
oxide–silicon-nitride–oxide–silicon ?SONOS? structure with
poor retention and scaling problem have been reported.1The
nonvolatile memory devices with high-k charge trapping lay-
ers in SONOS structure have been reported by several
researchers.2–5To improve the device performance, memory
device structures with nanocrystals ?or quantum dots? have
been reported for the possible solution of next generation of
nonvolatile memory device applications.6–21However, for
the integration of nanocrystals into the memory device struc-
ture, it is a challenging task to control the highly reproduc-
ible memory device with a high spatial density, small size,
and narrow size distribution of the nanocrystals. Recently,
the memory structure with ruthenium ?Ru? nanocrystals has
also been reported.17To get high density, small size, and
narrow size distribution of nanocrystals, the memory devices
with metal nanocrystals formed by atomic layer deposition
?ALD? have not yet been reported. In this letter, the memory
device structure with ruthenium oxide ?RuOx? nanocrystals
formed by atomic layer deposition has been investigated.
The RuOxis an attractive candidate for metal nanocrystal
memories because it has a large work function of ?4.8 eV to
bring about deep quantum well. Furthermore, high-k materi-
als with a large barrier height, such as Al2O3film, are inter-
esting alternatives as a blocking oxide to improve the device
performance and scaling. A large memory window with a
low gate voltage ?Vg?5 V?, small size ?5–8 nm?, high den-
sity ??1?1012/cm2?, and good uniformity have been ob-
served for atomic layer deposited RuOxnanocrystals in a
platinum/Al2O3/RuOx/HfO2/SiO2/n-Si structure for nanos-
cale high-performance flash memory device applications.
The pure HfO2and Al2O3charge trapping memory devices
have also been fabricated for comparison.
N-type Si
?100?
substrate
?1 Ohm-cm was cleaned by the RCA process to remove
native oxide from the surface. After cleaning the n-type Si
substrate, a tunneling oxide ?SiO2? with a thickness of 3 nm
was grown by rapid thermal oxidation system at 1000 °C for
15 s. The high-k HfO2film as a wetting layer was grown by
ALD using hafnium tetrachloride ?HfCl4? precursor at a sub-
strate temperature of 300 °C. The thickness of HfO2film
was ?2 nm. Then, the ruthenium oxide ?RuOx? layer with a
thickness of ?2 nm was grown by ALD using di-
ethylcyclopentadienyl ruthenium ?Ru?EtCp?2? precursor at a
substrate temperature of 350 °C. Then, the high-k Al2O3film
as a blocking oxide was grown by ALD using trimethylalu-
witha resistivityof
a?Author to whom correspondence should be addressed; electronic mail:
sidhu@mail.cgu.edu.tw
APPLIED PHYSICS LETTERS 90, 253108 ?2007?
0003-6951/2007/90?25?/253108/3/$23.00 © 2007 American Institute of Physics
90, 253108-1

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minum ?Al?CH3?3? precursor at a substrate temperature of
300 °C. The H2O precursor was used for oxygen. The thick-
ness of Al2O3film was ?20 nm. The precursor temperatures
were 185 °C for HfCl4, 100 °C for Ru?EtCp?2and 23 °C for
Al?CH3?3. Due to an unoptimized process, the oxygen can be
included into the Ru film, resulting in a RuOxlayer, by ALD.
To form the RuOxnanocrystals, a postdeposition annealing
?PDA? treatment at a temperature of 900 °C for 1 min was
performed in N2?90%? and O2?10%? gases. The platinum
?Pt? metal gate electrode ?gate area: 1.12?10−4cm2? was
used for all memory capacitors. The postmetal annealing was
performed with a temperature of 400 °C for 5 min in N2
?90%? and H2?10%? gases. To investigate the charge storage
characteristics, the memory capacitor structures were de-
signed such as S1: n-Si/SiO2?3 nm?/Al2O3?20 nm?/Pt, S2:
n-Si/SiO2?3 nm?/HfO2?2 nm?/Al2O3?20 nm?/Pt,
S3:
n-Si/SiO2?3 nm?/HfO2?2 nm?/RuOx?2 nm?/Al2O3?20
nm?/Pt. The memory capacitors ?S1 and S2? were fabricated
for comparison. To probe the size and microstructure of
RuOxnanocrystals, high-resolution transmission electron mi-
croscopy was carried out using a FEI Tecnai F30 field emis-
sion system with an operating voltage of 300 kV and a reso-
lution of 0.17 nm. Electrical characteristics of all memory
capacitors were performed using a HP 4284A LCR meter and
HP4156B semiconductor analyzer systems.
Figures 1?a? and 1?c? show the cross-sectional TEM im-
ages of the n-Si/SiO2/HfO2/RuOx/Al2O3 ?sample: S3?
structure for as deposited and after PDA treatment, respec-
tively. The thicknesses of SiO2, HfO2, RuOx, and Al2O3
films are found to be 3.0, 2.0, 2.0, and 20 nm, respectively,
for the as-deposited sample. The HfO2and RuOxfilms show
partial crystallinity while the Al2O3film shows amorphous
nature. After the annealing treatment, clear RuOxnanocrys-
tals embedded in HfO2and Al2O3films have been observed.
and
The average diameter of RuOxnanocrystals is 5–8 nm and
the thickness is about 3 nm. The thicknesses of SiO2, HfO2,
and Al2O3films are found to be 3.0, 1.0, and 17 nm, respec-
tively. The lattice constants of Ru film ?hexagonal? are cal-
culated: a=0.275 nm, b=0.275 nm, c=0.443 nm. The lattice
constants of monoclinic HfO2 films are found to be
a=0.511 nm, b=0.517 nm, and c=0.529 nm, while those
values are found to be a=0.448 nm, b=0.443 nm, and
c=0.309 nm for the orthorhombic RuO2films. Note that the
HfO2film as a wetting layer has been used because the RuOx
film cannot be directly deposited by ALD on SiO2without
HfO2film. The reason for RuOxfilm deposition on HfO2
layers is unclear. It is also beneficial that the high-HfO2layer
can be used as a part of tunneling oxide.
Theelemental
Si/SiO2/HfO2/RuOx/Al2O3?sample: S3? structure were in-
vestigated by energy dispersive x-ray spectroscopy ?EDS?
analysis with a spot size of ?0.5 nm and a spatial resolution
of ?1 nm. Figures 1?b? and 1?d? show the elemental concen-
trations of O, Hf, Si, Ru, and Al measured by EDS for the as
deposited and after annealing treatment. The numbers indi-
cated on the curve in Figs. 1?b? and 1?d? correspond to the
numbers shown in the TEM image. It is estimated that the
SiO2, HfO2, and Al2O3films show close stoichiometric for
the as-deposited and annealing treated samples. Average con-
centrations of Hf, Ru, and O atoms are found to be 20, 14,
and 54 at. %, respectively, for the as-deposited sample, while
those values are found to be 20, 18, and 47 at. %, respec-
tively, for the annealed sample. After annealing treatment, it
is shown that the Ru-rich RuOxnanocrystal is formed in our
memory structure. The high density of ?1?1012/cm2for
RuOxnanocrystals measured by plan-view TEM is observed
?Fig. 2?. The diameter of nanocrystals is 5–8 nm. The thick-
ness of nanocrystal is about 3 nm. It indicates that the shape
of nanocrystal is likely a thick disk. The density and size of
RuOxnanocrystals can be controlled by changing the thick-
ness of the RuOxlayer.
Figure 3?a? shows a good clockwise hysteresis of RuOx
nanocrystal memory capacitors with different sweeping gate
voltages ?Vg?. A small capacitance equivalent thickness is
found to be ?9.3 nm. The high-frequency ?1 MHz?
capacitance-voltage ?C-V? has been measured with a hold
time of 100 ms. A large hysteresis memory window of
13.3 V at sweeping gate voltage of Vg=9 V is observed, due
to the high density of RuOxmetal nanocrystals. Yim et al.17
reported the hysteresis memory window of ?6.7 V at a large
sweeping gate voltage of 10 V for Ru nanocrystal memory
device. A hysteresis memory window of ?0.7 V is also ob-
served under an extremely low gate voltage of ±1 V, due to
deep quantum well ?high work function of ?4.8 eV? of
RuOx nanocrystals and small conduction band offset
compositionsof
n-
FIG. 1. ?Color online? High-resolution transmission electron microscopy
?TEM? images of Al2O3/RuOx/HfO2/SiO2/n-Si structure for ?a? as-
deposited and ?c? 900 °C, 1 min samples. Average elemental concentrations
of oxygen ?O?, silicon ?Si?, hafnium ?Hf?, ruthenium ?Ru?, and aluminum
?Al? for the ?b? as-deposited and ?d? annealed samples have been shown.
Clear RuOxmetal nanocrystals have been observed for annealed sample.
FIG. 2. ?a? Plan-view transmission electron microscopy image of RuOx
nanocrystals in Al2O3/RuOx/HfO2/SiO2/n-Si structure and ?b? high-
resolution TEM image of a single RuO2nanocrystal.
253108-2 Maikap et al.Appl. Phys. Lett. 90, 253108 ?2007?

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??Ec?1.7 eV? of HfO2films. It indicates that the charge can
be stored in the RuOxnanocrystals under small positive gate
voltage and the stored charges can be erased easily under
small negative gate voltage applications. The RuOxmetal
nanocrystal memory devices formed by ALD show the best
hysteresis memory characteristics as compared with reported
nanocrystal memory devices in the literatures.17,18The
amount of stored charges in RuOxnanocrystals can be esti-
matedusingthe relation
??3.7?10−7F/cm2? is the capacitance density at accumula-
tion region and +VFB??4.4 V? is the flatband voltage shift
under a positive gate voltage of Vg?6 V. Thus, the electron
density stored in RuOx nanocrystals is estimated to be
?1?1013/cm2. It indicates that one RuOxnanocrystal can
store about ten electrons, which is similar to the reported
results on HfO2nanocrystals.11The hysteresis memory win-
dow of RuOxnanocrystal capacitor increases with an in-
crease of the sweeping gate voltage up to 9 V ?Fig. 3?b??.
The nanocrystal capacitor has a large hysteresis memory
window as compared with those of the pure HfO2and Al2O3
charge trapping layers. Large memory windows with a low
gate voltage operation of RuOxnanocrystal memory capaci-
tor can be used in future nanoscale flash memory device
applications.
The leakage current density of RuOxnanocrystal capaci-
tor is similar to those of the pure HfO2and Al2O3charge
trapping layers up to a gate voltage of 9 V ?Fig. 4?. The
RuOxnanocrystal capacitor shows the breakdown voltage of
−13.8 V and leakage current density of 5?10−10A/cm2at a
Q=Coxx?+VFB?, where
Cox
gate voltage of −5 V. A breakdown voltage ?−13.8 V? of
RuOxnanocrystal is lower ?slightly? as compared with that of
the breakdown voltage of 17 V for pure HfO2charge trap-
ping layer, and it may be due to the contamination ?slight? of
RuOxmetals in the Al2O3blocking oxide after annealing
treatment.
In conclusion, the excellent charge storage characteris-
tics of atomic layer deposited RuOxnanocrystal capacitors
have been observed. A large hysteresis memory window of
13.3 V at a sweeping gate voltage of 9 V, low leakage cur-
rent density of 5?10−10/cm2at a gate voltage of −5 V, and
high breakdown voltage of −13.8 V have been investigated.
The atomic layer deposited RuOxnanocrystal memory ca-
pacitor can be used in future nanoscale high-speed flash
memory device applications.
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FIG. 3. ?Color online? ?a? Capacitance vs sweeping gate voltage character-
istics of RuOxnanocrystal memory capacitors. ?b? The hysteresis memory
window increases with increasing the sweeping gate voltage.
FIG. 4. ?Color online? Leakage current densities of RuOxnanocrystal, pure
HfO2, and Al2O3charge trapping layers.
253108-3 Maikap et al.Appl. Phys. Lett. 90, 253108 ?2007?

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