Bismuth-metal nanocrystals self-embedded in high-k Bi-based pyrochlore dielectrics grown at room temperature

Article abstract of DOI:10.1002/pssa.201026226

Bismuth-metal nanocrystals (NCs) are self-embedded within high-k Bi ₂Mg_2/3Nb_4/3O₇ (BMNO) dielectrics grown at room temperature by rf magnetron sputtering were demonstrated for the low-voltage nonvolatile memory device. The BMNO pyrochlore films grown in Ar/O₂ ambient at room temperature showed the stable dielectric properties, together with high dielectric constant (～45), small dielectric loss (0.2%), and improved leakage current. Bismuth nanocrystals (a mean size of 2-3 nm) and a high-k BMNO films (～50 nm thickness) as a control oxide layer were continuously in situ formed at room temperature in Ar and Ar/O₂ ambient, respectively. A significant threshold voltage shift of 0.95 V is observed at a gate voltage of 5 V and the threshold voltage shift linearly increases with increasing gate voltage from 3 to 6 V. The self-embedded bismuth nanocrystals in high-k BMNO dielectrics at room temperature represent a viable candidate for low-voltage NFGM device applications.

Full text of DOI:10.1002/pssa.201026226

Phys. Status Solidi A 208, No. 4, 932–936 (2011) / DOI 10.1002/pssa.201026226
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applications and materials science
Bismuth-metal nanocrystals self-
embedded in high-k Bi-based pyrochlore dielectrics
grown at room temperature
,2,3
Nak-Jin Seong1, Hyun-June Jung2, and Soon-Gil Yoon*
1 Department of Nano and Bio Technology, Daegu-Gyeongbuk Institute of Science and Technology (DGIST), Daegu, Korea
2 School of Nano-Science and Technology, Chungnam National University, 220 Gung-Dong, Yuseong-gu, Daejeon 305-764, Korea
3 Graduate of Analytical Science and Technology, Chungnam National University, 220 Gung-Dong, Yuseong-gu, Daejeon 305-764,
Korea
Received 28 April 2010, revised 4 December 2010, accepted 7 December 2010
Published online 11 January 2011
Keywords bismuth, floating-gate memories, high-k dielectrics, nanocrystals, niobates, pyrochlore
* Corresponding author: e-mail sgyoon@cnu.ac.kr, Phone: þ82 42 821 6638, Fax: þ82 42 822 3206
Bismuth-metal nanocrystals (NCs) are self-embedded within control oxide layer were continuously in situ formed at room
high-k Bi2Mg2/3Nb4/3O7 (BMNO) dielectrics grown at room temperature in Ar and Ar/O2 ambient, respectively. A
temperature by rf magnetron sputtering were demonstrated for significant threshold voltage shift of 0.95 V is observed at a
the low-voltage nonvolatile memory device. The BMNO gate voltage of 5 V and the threshold voltage shift linearly
pyrochlore films grown in Ar/O2 ambient at room temperature increases with increasing gate voltage from 3 to 6 V. The self-
showed the stable dielectric properties, together with high embedded bismuth nanocrystals in high-k BMNO dielectrics at
dielectric constant (ꢀ45), small dielectric loss (0.2%), and room temperature represent a viable candidate for low-voltage
improved leakage current. Bismuthnanocrystals (amean size of NFGM device applications.
2–3 nm) and a high-k BMNO films (ꢀ50 nm thickness) as a
ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction The use of a floating gate composed of most vital issues in advanced complementary metal-oxide-
distributed nanocrystals reduces the problems of charge loss semiconductor (CMOS) technology. In some of our recent
encountered in conventional floating-gate electrically era- studies, Bi2Mg2/3Nb4/3O7 (BMNO) pyrochlore films have
sable programmable read-only memories, allowing for been successfully investigated for metal-insulator-metal
thinner tunnel oxide and, thereby, smaller operating capacitor applications [11–13]. The BMNO pyrochlore films
voltages, better endurance and retention, and faster pro- grown at room temperature showed stable dielectric proper-
gram/erase speed [1–4]. To improve the performance of the ties, together with high dielectric constant (ꢀ45), small
nanofloating-gate memory (NFGM), metal nanocrystals dielectric loss (ꢀ0.2%), and improved leakage current,
such as Au, Ag, Ni, Pt, and W were subsequently proposed which are the basic prerequisites for its extended application
due to their large density of states, strong coupling with the as possible gate dielectric in advanced CMOS technology
conduction channel, wide range of work function selectivity, [14]. BMNO films for high-k gate dielectrics show many
and small energy perturbation [5–9]. High-k dielectrics were advantageous characteristics, such as excellent surface
also introduced to facilitate the equivalent oxide thickness smoothness and morphology, high crystallization tempera-
(EOT) scaling, the leakage current reduction, and the field- ture, good interface properties with low oxide trap charge
sensitive tunneling [10]. and interface state densities, low leakage current, and high
The search for new high-k gate dielectric materials to breakdown strength.
replace SiO2 has drawn enormous attention because of their
For low-voltage memory applications of NFGM devices,
potential in reducing gate leakage current, thereby keeping the formation of a narrow band of nanocrystals within thin
the EOT thinner. Under these circumstances, the search for control oxide layers is needed and the controllability of the
suitable high-k gate dielectric materials is still one of the location of such nanocrystalline band from the tunnel oxide
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Phys. Status Solidi A 208, No. 4 (2011)
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layer/control oxide layer interface is necessary for the substrate by atomic-layer deposition (ALD), Fig. 1a and b.
optimization of device performance. Many promising The capacitance density of the Al2O3 layer as a function of
approaches for the achievement of the formation of metal applied electric field decreaseswith increasing filmthickness
nanocrystals have been reported. For silicon nanocrystals, from 4.5 to 12.6 nm and a minute hysteresis was observed in
the IBS technique for silicon implants into thin SiO2 was the films above thickness of 6.7 nm. Accordingly, the
performed [15] and for metal nanocrystals within the tunnel thickness of the tunneling oxide layer was maintained below
oxide, an ultrathin metal layer is deposited on the tunnel 6 nm without a hysteresis effect and an EOT of the films is
oxide and is then covered by a blocking layer, and about 2.06. The leakage current densities of the films are
subsequently annealed at high temperature for well-separ- plotted for various thicknesses in Fig. 1b. In Pt/Al2O3/Si
ated metal nanocrystals [16]. These methods require a capacitors, in order to investigate the interface effect
complicated process and are not suitable for preparation of between the Pt top electrode and the Al2O3 films, leakage
metal nanocrystals on flexible substrates for flexible characteristics for application of a negative electric field was
electronic device applications. In this study, the feasibility plotted. Leakage current densities of the films decrease with
for the embedment of metal nanocrystals in high-k dielectrics increasing film thickness and the films above a thickness of
is addressed. Bismuth-metal nanocrystals are self-embedded 6.7 nm exhibit a leakage current density of approximately
within the high-k BMNO films grown in an Ar ambient and 1 ꢂ 10ꢁ9 A/cm2 at ꢁ2 MV/cm. From the capacitance and
sequentially, the BMNO films as a control oxide layer were leakage current characteristics of the Al2O3 films with
formed in situ at room temperature in an Ar/O2 ambient thicknesses between 4.5 and 6.7 nm, Al2O3 films were
atmosphere. Because Bi metal nanocrystals have a work selected as the tunnel oxide layer of NFGM. The possibility
function of 4.34 eV, they have been proposed for nanocrys- of the existence of the bismuth nanocrystals in BMNO films
tal-based nonvolatile memory (NVM) applications.
grown in a pure argon atmosphere by radio-frequency
sputtering was presupposed by the thermodynamic equi-
2 Experimental For NFGM memory cells, the tunnel librium, as shown in Fig. 1c. The solid lines exhibit the DG–
oxide layer was formed by Al2O3 amorphous phases with a PO2 relationship for the formation of bismuth oxide or
thickness of 4–5 nm grown at 200 8C using Al(CH3)3 bismuth metal at an equilibrium state. From the figure, the
precursors and H2O oxidant by atomic layer deposition oxygen partial pressure for the formation of bismuth metal
(Lucida D100, NCD technology. Co., Ltd.). The bismuth by the reduction of bismuth oxide is below approximately
nanocrystals self-embedded within the BMNO films and the 10ꢁ58 atm. However, in practice, because a sputtering
high-k BMNO films as a control oxide layer were process is performed under an nonequilibrium state, the
continuously in situ formed at room temperature in Ar solid line exhibiting the stable state of bismuth metal in the
(20 sccm (standard cc minꢁ1)) and Ar/O2 (10:10 sccm) figure should be moved along the arrows, if thermodynamic
ambient atmosphere by rf magnetron sputtering, respect- expectation is considered together with experimental
ively. The rf power for both layers was maintained at 100 W. conditions such as deposition temperature and rf power.
A 100-nm thick Pt gate electrode was prepared at room Although the rf sputtering is performed without substrate
temperature by dc sputtering and then was patterned as heating, the substrate temperature lies in the range of 50–
100 ꢂ 100 mm2 area by a photolithography. The thickness of 80 8C by radio-frequency radiation. The addition of the
the Al2O3 tunnel oxide layer was indirectly identified from a energies such as temperature and radio-frequency power
deposition rate and was directly confirmed by a TEM cross- during the deposition in a pure argon atmosphere effectively
sectional image. Bismuth nanocrystals self-embedded in the induces the reduction of the bismuth oxide. As a result, the
high-k BMNO dielectric layer were identified using both a region where the bismuth metal is stable is enlarged toward
selective-area diffraction pattern (SADP) by TEM and the the higher oxygen partial pressure. Because an rf sputtering
chemical states by X-ray photoelectron spectroscopy (XPS). is performed in a pure argon ambient (purity of 99.9999%),
Bismuth nanocrystal embedded in Bi2O3 films by reduction the oxygen partial pressure in the case of BMNO deposition
of Bi2O3 in thermodynamic expectation was clearly using an argon ambient is approximately 10ꢁ12 atm, which is
identified by X-ray diffraction (XRD) using 300-nm thick indicated as a vertical dotted line in the figure. From the
Bi2O3 thin films grown in an argon ambient alone at room thermodynamic expectation including an experimental
temperature. Current–voltage (I–V) characteristics of the condition such as deposition temperature and rf power, the
Al2O3 tunnel oxide and BMNO control oxide layers were BMNO deposition in an argon ambient makes possible the
measured by HP4145B semiconductor parameter analysis. existence of the bismuth nanocrystal within the BMNO
The capacitance–voltage (C–V) measurements were per- phase. In order to support the thermodynamic result
formed by a HP 4194A impedance gain phase analyzer to practically, the XRD result of a 300-nm thick Bi2O3 thin
study the electron charging and discharging effects of the film grown by rf sputtering at room temperature using pure
bismuth nanocrystals.
argon ambient alone was shown in Fig. 1d. In the figure,
Bi(003), Bi(012), Bi(104), and Bi(110) peaks were clearly
3 Results and discussion For NFGM, electrical observed, while peaks exhibiting Bi2O3 were not observed,
properties of Al2O3 tunnel oxide layers were investigated indicating an amorphous phase of Bi2O3 films. This result
for various thicknesses in films grown at 200 8C on p-type Si suggests that the self-embedment of Bi metals in Bi2O3 films
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N.-J. Seong et al.: Bismuth-metal nanocrystals self-embedded in high-k Bi-based pyrochlore dielectrics
Figure 1 (onlinecolorat:www.pss-a.com)Variationsin(a)capacitancedensityand(b)leakagecurrentdensityversusappliedelectricfield
atdifferentAl2O3 thicknesses. (c)Thethermodynamicexpectationofmetallicbismuthandbismuthoxideformationasafunctionofoxygen
partialpressure. Theverticallineinthefigureindicatesanoxygenpartialpressurewhenanrfsputteringisperformedinapureargonambient
atmosphere. (d) XRD pattern of 300-nm thick Bi2O3 films grown by rf sputtering at room temperature in an argon ambient.
is possible by the deposition using rf sputtering in a pure metallic bismuth and bismuth oxide phases, indicating a
argon ambient and at room temperature.
metallic bismuth concentration higher than bismuth oxide.
TEM and XPS analysis were performed to investigate This result suggests that the bismuth nanocrystals were self-
the bismuth nanocrystals embedded in BMNO layers. embedded in the BMN layer. The Mg and Nb, but not Bi,
Figure 2 shows the cross-sectional TEM image of bismuth exist only as oxide phases in BMN (Ar) layer (not shown
nanocrystals self-embedded in a 3-nm thick BMNO gate here). In our previous studies, the BMNO films (ꢀ50 nm
dielectric grown in an argon atmosphere at room tempera- thickness) grown in an Ar/O2 atmosphere at room tempera-
ture. The 50-nm thick BMNO control oxide layers were ture were annealed at various temperatures and were
sequentially formed on 3-nm thick BMNO dielectrics at characterized for microstructure and thermal stability [14].
room temperature in an Ar/O2 atmosphere. The parts BMNO films are amorphous up to an annealing temperature
indicated by circles in 3-nm thick BMNO films look like of 800 8C in an O2 ambient and subsequently in N2 ambient,
crystalline phases. In order to ascertain the crystalline phases and the films are crystallized after annealing at 900 8C. The
clearly, high-resolution TEM analysis was performed for root mean square (rms) roughness of the films annealed at
100-nm thick BMNO films instead of 3-nm thick BMNO 900 8C was below 0.4 nm. For gate dielectric applications of
films grown in an argon ambient at room temperature. The 5-nm thick BMNO films, interface state density (Dit) and
high-resolution image of the parts crystallized in the 100-nm leakage current density were also investigated for the films
thick BMNO films was shown in Fig. 2b. Embedded phases annealed in an O2 ambient and subsequently in a N2 ambient
exhibit a crystalline structure with a mean size of 3 nm, at 700 8C for 1 min [17]. The annealed BMNO films exhibit
Fig. 2b. In order to clearly ascertain the bismuth nanocrystal, Dit of 4 ꢂ 1011 cmꢁ2 eVꢁ1, a dielectric constant of 38–42,
XPS Bi 4f doublets observed in a 100-nm thick BMN (Ar) and a leakage current density of about 3 ꢂ 10ꢁ6 A/cm2 at a
layer by XPS depth-profiles are shown in Fig. 2c. The BMN bias voltage of ꢁ1 V. As a result, BMNO films are enough to
layers including nanocrystal phases show a mixture of apply for the high-k gate dielectrics and for control oxide
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Phys. Status Solidi A 208, No. 4 (2011)
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Figure 2 (onlinecolorat:www.pss-a.com)(a)Cross-sectionalTEMimage
of the fabricated NFGM structure with bismuth nanocrystals embedded in
high-k BMNO dielectric layer and Al2O3 tunnel oxide layer. (b) HRTEM
image of bismuth nanocrystals, (c) XPS spectrum of bismuth nanocrystals
embedded in BMNO dielectric layer. Thicknesses of the BMNO films used
for (b) and (c) are about 100 nm.
layer in NFGM device applications. Before describing the observed. The relationship between the flat-band voltage
feasibility for NFGM, the C–V characteristics of the shift and the gate voltage is shown in Fig. 3c. As the gate
reference sample, which is a BMNO (50 nm)/Al2O3/Si voltage increases from 3 to 6 V, flat-band voltage shift
(MIS) structure without Bi NCs, were shown in Fig. 3a. The linearly increases and the shift was approximately 0.95 and
C–V curve of the 50-nm thick BMNO films shows a typical 1.27 V at a gate voltage of 5 and 7 V, respectively. As a result,
dielectric behavior observed in a MIS structure, which shows for NFGM devices using Bi NCs, the feasibility for the self-
a minute hysteresis. However, the hysteresis was clockwise embedded bismuth NCs in high-k BMNO dielectric prepared
when a voltage was applied from ꢁ5 V to þ5 V and vice in situ at room temperature was realized and furthermore, the
versa, which occurred via the interface charges rather than Bi NC embedded at almost room temperature is important
the memory effect. With respect to NFGM devices with the for the integration of metal NCs with high-k material that is
Bi NCs, the capacitance–voltage (C–V) hysteresis after generally known to crystallize at low temperatures forming
bidirectional sweeps, which implies the electron-charging leakage current paths and diffusion paths for metals and
and -discharging effects of bismuth nanocrystals self- other impurities [19].
embedded in BMNO layer is shown in Fig. 3b. The hysteresis
was counterclockwise which is due to substrate injection
4 Conclusions Bismuth nanocrystals self-embedded
from the electrons of the deep inversion layer and holes of the in high-k BMNO dielectrics grown at room temperature by rf
deep accumulation layer of Si substrate [18]. In Fig. 3b, with magnetron sputtering were demonstrated for the low-voltage
the voltage is swept from 2 to ꢁ2 V, the threshold voltage nonvolatile memory device. Bismuth nanocrystals (with a
shift was a minute value of about 19 mV. As the swept mean size of 2–3 nm) and the high-k BMNO films (ꢀ50 nm
voltage is increased to 5 V, a pronounced C–V shift is thickness) as a control oxide layer were continuously formed
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Acknowledgements This work was funded by the Korea
Science and Engineering Foundation (KOSEF) grant funded by
the Korea government (MOST) (R01-2007-000-21017-0), by the
KOSEF grant funded by the Korea Government (MEST) (No. 2009-
0079164). This work is also the outcome of a Manpower
Development Program for Energy & Resources supported by the
MinistryofKnowledge andEconomy(MKE), andwassupportedby
the Brain Korea 21 Project in 2006.
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in situ at room temperature in Ar and Ar/O2 ambient,
respectively. A significant threshold voltage shift of 0.95 V is
observed at a gate voltage of 5 V and the threshold-voltage
shift linearly increases with increasing gate voltage from 3
to 6 V.
ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.pss-a.com