Improvement of Electrical Properties
of Ba
0.7
Sr
0.3
TiO
3
Capacitors With an Inserted
Nano-Cr Interlayer
Chia-Cheng Ho, Student Member, IEEE, Bi-Shiou Chiou, Senior Member, IEEE, and Li-Chun Chang
Abstract—The metal–insulator–metal (MIM) capacitors were prepared with Ba0 .7Sr0 .3TiO3/Cr/Ba0 .7Sr0 .3TiO3(BST/Cr/BST)
dielectric and Pt electrode. The multilayer BST/Cr/BST was sput-tered onto Pt/Ti/SiO2/Si substrate. The presence of nano-Cr
inter-layer affects the electrical properties of the capacitors. The temper-ature coefficient of capacitance (TCC) of capacitors with 2 nm Cr is about 69% of that of capacitors without Cr. In a previous work, the formation of the TiO2 secondary phase was found after the
BST/Cr/BST dielectrics were annealed at 1023 K in O2atmosphere
for 1 h. It is suggested that the nano-Cr interlayer as a catalyst leads to the TiO2formation during the annealing in O2atmosphere. The
negative value of TCC of BST can be compensated by the positive TCC of TiO2, and the temperature stability in the dielectric
con-stant can be realized for capacitors with nano-Cr interlayer. The voltage stability of BST is also improved with the insertion of nano-Cr interlayer, and the quadratic coefficient in voltage coefficient of capacitance (VCC) of Pt/BST/Cr(2 nm)/BST/Pt is about 30% of that of the BST capacitor without Cr. The effects of Cr thickness on TCC, VCC, dissipation factor, and leakage current density of Pt/BST/Cr/BST/Pt parallel plate capacitors are investigated.
Index Terms—Ba0 .7Sr0 .3TiO3/Cr/Ba0 .7Sr0 .3TiO3 (BST)
ca-pacitor, dissipation factor, nano-Cr interlayer, voltage coefficient of capacitance (VCC).
I. INTRODUCTION
I
N THE PAST decades, improvements in circuit perfor-mance and cost per function have been pursued by shrinking device geometries. In order to keep the storage capacitance, BaxSr1−xTiO3 dielectrics are extensively investigated forap-plications in the high-density dynamic random access memo-ries (DRAM) and the capacitors of the analog/mixed circuit. The high charge storage density, the low leakage current, high breakdown field, and high time-dependent dielectric breakdown (TDDB) [1]–[3] of BST are attractive for use in DRAM and the analog/mixed-signal circuit. However, the stability of the BST dielectric constant becomes a major concern while the applied voltage and/or operating temperature vary. The quadratic volt-age coefficient of capacitance (VCC) is a critical factor for the
Manuscript received June 16, 2007; revised January 6, 2008. This work was supported by the National Science Council, Taiwan, under Contract NSC 95-2221-E-009-085. The review of this paper was arranged by Associate Editor B. Yu.
C.-C. Ho and B.-S. Chiou are with the Department of Electronics Engineer-ing, National Chiao-Tung University, Hsinchu 300, Taiwan, R.O.C. (e-mail: doubles.ee89g@nctu.edu.tw; bschiou@mail.nctu.edu.tw).
L.-C. Chang is with the Department of Materials Engineering, Mingchi University of Technology, Taipei 243, Taiwan, R.O.C. (e-mail: lcchang@ edirect168.com).
Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TNANO.2008.920201
capacitor in DRAM and the analog/mixed-signal circuit [4], [5]. Besides, the dielectric constant of BST varies with the operating temperature [6]–[9], which affects the precise capacitance.
Previously, nanoferroelectric-based core-shell particles have been used for improving the characteristics of the ferroelectric dielectrics [10], and nanocomposite capacitor shows the promis-ing dielectric properties with the ceramic fillers [11]. Our pre-vious work [6]–[8] shows that parallel plate capacitors with an inserted nano-Cr interlayer exhibit lower leakage current and more temperature stable in the dielectric constant. However, there are few papers regarding the stabilities of the capacitors with the nano-Cr interlayer as the operation temperature and the applied voltage change. Besides, the formation of TiO2film was
found in the annealed specimens with the nano-Cr interlayer. In this study, the effects of the Cr thickness on the temperature and voltage stability of the dielectric behaviors of Pt/BST/Cr/BST/Pt capacitors are investigated.
II. EXPERIMENTALPROCEDURES
Specimens of Pt/Ba0.7Sr0.3TiO3/Cr/Ba0.7Sr0.3TiO3/Pt MIM
capacitors were employed. The starting p-type Si (1 0 0) wafers were cleaned by the standard Radio Corporation of America (RCA) cleaning process. After cleaning, a 100-nm-SiO2 films
were grown on the Si substrate by the dry-oxidation in 1273 K for 30 min. A 10-nm-Ti films were sputtered onto the SiO2layer.
The bottom electrodes, 100-nm-thick Pt films, were dc sputtered at room temperature. The Ba0.7Sr0.3TiO3(BST) films were then
deposited onto Pt electrodes using an RF magnetron sputtering at a substrate temperature of 625 K. The sputtering chamber was evacuated to a base pressure of 1× 10−5 torr. During the deposition of BST films, a constant pressure of 5× 10−3 torr was maintained by a mixture of argon and oxygen at 9 and 3 sccm, respectively. The RF power for the deposition of both the first and second BST layers was 120 W (power density was 2.7 W/cm2), and the total thickness of BST films was about
300 nm. Chromium films with various thicknesses (2, 5, 10, and 15 nm) were deposited at a dc power of 100 W after the deposition of the first BST films without the vacuum broken. The thickness of Cr films was monitored with a quartz crystal and a controller. The second BST layers were then deposited. The BST/Cr/BST/Pt specimens were annealed at 1023 K in O2
for 1 h, and then were bombarded by O2 plasma for 10 min
before the deposition of the top Pt electrode.
The high-resolution transmission microscopy (HRTEM) (JEM-3000 F, JEOL Ltd., Japan) was employed to observe
Fig. 1. Cross-sectional image on the Pt/BST/Cr/BST/Pt specimen with a Cr thickness of the following. (a) 0 nm. (b) 2 nm. (c) 5 nm. (d) 10 nm. (e) 15 nm.
the structure of specimens. The cross-sectional TEM specimen was prepared using the focused ion beam (FIB) system (Nova 200, FEI company, Japan). An LCR meter (HP-4285, Hewlett Packard Company, USA) was employed to measure the dielec-tric constant and dissipation factor of the samples in the tem-perature range from 298 K to 398 K. A semiconductor parame-ter analyzer (HP4155B, Hewlett Packard Company, USA) was employed to measure the leakage current, and the activation en-ergies were obtained in the temperature range from 298 K to 423 K.
III. RESULTS ANDDISCUSSION
Fig. 1(a)–(e) are the cross-sectional HRTEM images of BST dielectric structure. Specimens with mono BST layer annealed in O2 atmosphere for 1 h exhibit the uniform microstructure
and no thin layer formation, as shown in Fig. 1(a). Other BST/Cr/BST specimens indicate the formation of a thin layer on the top BST layer, as shown Fig. 1(b)–(e). Our previous work suggests that a β-TiO2 secondary phase is formed after
the BST/Cr/BST dielectrics are annealed in O2 for 1 h [7], [8].
Hence, the thin layer on top of the BST is TiO2. However, the
formation of TiO2 is not clear now. The possible mechanism is
that Cr layer as the catalyst leads to the TiO2formation during
the annealing in O2 atmosphere. In the case of mono BST
an-nealing in O2 atmosphere, there is no TiO2formation for mono
BST dielectric, as shown in Fig. 1(a). In our previous work [6], the second phase in BST/Cr(2 nm)/BST annealed in air was also found, but no second phase was appeared for BST/Cr(2 nm)/ BST annealed in N2 atmosphere. All agree with the formation
of TiO2 from the catalyst (Cr). Though the created new
com-pounds of BST films are possible, Cr+ 3 (0.130 nm) is difficult to replace Ti+ 4 (0.145 nm) in the pervoskite structure, and the shift of Ti binding energy is not obvious [8].
Fig. 2 shows the temperature dependence of the dielectric con-stant of specimens with various Cr thicknesses (tC r) as function
of the applied electric field at an oscillation voltage of 0.1 V and a measuring frequency of 100 kHz. The dielectric constant decreases as tC rincreases. Comparing with mono-BST
capac-itor (ε ∼ 456), the dielectric constant (ε) decreases to 371 for specimens with 2-nm-Cr layer, as exhibited in Fig. 2 and sum-marized in Table I. The temperature coefficient of capacitance (TCC) is defined as
TCC = CT − CTr
(T− Tr)× CTr
(1) where T is the temperature of interest (398 K in this study), Tr
is the temperature of reference (298 K in this study), and CT
and CT rare the capacitances at zero-bias measured at T and Tr,
respectively. The TCCs of the specimens are listed in Fig. 2. A TCC of−0.5 × 10−3/K is obtained for capacitors with 2 nm Cr as compared to that of−1.6 × 10−3/K for capacitors without nano-Cr interlayer, while positive TCCs are obtained for speci-mens with thicker tC rinterlayer of 10 nm. The BST employed in
this study has a negative TCC, while the TiO2exhibits a positive
TCC [12]. It is argued that the positive TCC of the TiO2
com-pensates the negative TCC of BST. The TiO2formation resulted
from the presence of Cr interlayer [8] improves the temperature stability of the dielectric constant of BST films. Fig. 3 gives the TiO2thickness (tT iO2) of specimens from Fig. 1(b)–(e). On
the basis of tT iO2, the TiO2 dielectric constants (εT iO2) based
on serial capacitor model were calculated with the following equation
εT iO2 =
tT iO2CεBST
aεBST− tBSTC
(2) where C is the measured capacitance, a is the electrode area, and εT iO2 (tT iO2) and εBST (tBST) are the dielectric constant
(thickness) of TiO2and BST, respectively. The estimated εT iO2,
as shown in Fig. 3, is almost keep constant, and the average value is 21.6, which is consistent with [13].
The nano-Cr interlayer also improves the voltage stability of the dielectric constant of BST capacitor. The voltage stability of the capacitors can be expressed in terms of the VCC obtained from a second-order polynomial equation
C(V ) = Co(AV2+ BV + 1) (3)
where Co is the capacitance at zero-bias, and A and B are the
quadratic and linear voltage coefficients of the capacitance, respectively. On the basis of Fig. 2, the VCCs of specimens are calculated and summarized in Table I. The BST/Cr/BST dielectric decreases the absolute value of the quadratic co-efficient (A) from 5.93× 10−3/V2 of the mono-BST
dielec-tric to 1.78× 10−3/V2 of BST/Cr(2 nm)/BST dielectric and 1.03× 10−3/V2 of BST/Cr(15 nm)/BST dielectric at 298 K. Among all measuring temperatures, BST/Cr/BST dielectric shows more voltage stable than the mono-BST dielectric, as indicated in Table I.
The dissipation factor, as shown in Fig. 4, decreases initially, and then, increases as tC r increases. The BST/Cr(2 nm)/BST
dielectric shows the smallest dissipation factor of 0.023 as com-pared to the 0.028 of mono-BST dielectric. Fig. 5(a) exhibits the leakage current density (J) versus electric field (E) of the
Fig. 2. Electric field dependence of the 100 kHz dielectric constant of the following as the function of the temperature. (a) Mono-BST. (b) BST/Cr(2mn)/BST. (c) BST/Cr(5 nm)/BST. (d) BST/Cr(10 nm)/BST. (e) BST/Cr(15 nm)/ BST. The TCC from 298 K to 398 K are indicated. The temperatures of measurement are 298 K (), 323 K (
◦
), 348 K (), 373 K (∇), and 398 K ().
Fig. 3. TiO2 thickness () from TEM image and the calculated dielectric
constant of TiO2() versus the Cr thickness.
capacitors at 298 K. Capacitors with tC r = 2 and 5 nm show
lower leakage current at electric fields below 50 kV/cm than other capacitors. A linear relationship is obtained on the Ln(J/E) and Ln(J/T2) versus E0.5, as shown in Fig. 5(b) and (c), respec-tively, which suggests Schottky emission (SE) or Poole–Frenkel (PF) emission mechanisms [14], [15]. The leakage current den-sities of the two conduction mechanisms are expressed as fol-lows:for SE: Ln JSE A∗T2 = βSEE 0.5− ϕ SE kBT (4) for PF: Ln JPF E = βPFE 0.5− ϕ PF kBT (5)
TABLE I
VOLTAGECOEFFICIENT OFCAPACITANCE(VCC)# ATVARIOUSTEMPERATURES OFPt/BST/Cr/BST/Pt CAPACITORWITHVARIOUSCr THICKNESSES(t C r)
Fig. 4. Dissipation factor as the function of the electric field of BST/Cr/BST with various Cr thicknesses (tC r). (), 0 nm; (
◦
), 2 nm; (), 5 nm;(∇), 10 nm; and (
), 15 nm.
where βSE= (e3/4πε0εd)0.5, A∗is the effective Richardson’s
constant (120 A/cm2/K2), φSE is the potential barrier height at
the surface, ε0 is the dynamic dielectric constant of free space,
εd is the dynamic relative dielectric constant in the infrared
region, e is the unit charge, kB is Boltzmann’s constant, T is
the absolute temperature, E is the external electric field, βPF =
(e3/πε0εd)0.5, and φPF is the potential barrier height of trap
potential well. The PF transport mechanism is a result of the lowering of the barrier height of traps in the dielectrics. The conduction mechanisms of metal/BST/metal capacitors are usu-ally interpreted as SE at lower electric fields and PF emission at higher fields [1]. The dynamic relative dielectric constant can be inferred from n2, where n is the refractive index. Therefore, the
dynamic relative dielectric constant of BST films is about 4 [16]. The dashed lines in Fig. 5(b) and (c) are the PF and SE fittings, respectively. The slope of the Ln(J/E) and Ln(J/T2) versus E0.5 curve, as summarized in Table II, is β/kBT . Both βSEand βPF
are calculated and listed in Table II for comparison. The β values obtained in this study are closer to βSEor βPF, and this suggests
that the dominant mechanism for charge carriers to transport is SE or PF emission in the different applied electric field. The PF fitting regions of the various Cr thicknesses (tC r) showing PF
behaviors are above 53 kV/cm (tC r= 0 nm), 37 kV/cm (tC r=
2 nm), 35 kV/cm (tC r = 5 nm), 120 kV/cm (tC r = 10 nm),
and 125 kV/cm (tC r = 15 nm). Lower values of PF
bound-ary are obtained for BST specimens containing Cr interlayer (tC r= 2 nm and tC r= 5 nm). Our previous work [1] suggests the
more oxygen vacancies and the higher interfacial space charge concentration, the smaller is the electric field boundary of SE/PF. It is suggested these defect appeared between BaxSr1−xTiO3
and TiO2. Especially for the thin TiO2 layer for the specimens
TABLE II
βS E, βP F, ACTIVATIONENERGY(Ea),ANDβ EXTRACTEDFROMCURVEFITTING(SLOPE: β/kBT )OF THESEORPF EMISSIONMECHANISM∗
OFPt/BST/Cr/BST/Pt CAPACITORWITHVARIOUSCr THICKNESSES(tC r)
Fig. 5. (a) Leakage current density (J) versus electric field (E); (b) PF emission plot of Ln(J/E ) versus E0 . 5; (c) SE plot of Ln(J/T2) versus E0 . 5of
spec-imens are of BST/Cr/BST with various Cr thicknesses (tC r). (), 0 nm; (
◦
), 2nm; (), 5 nm; (∇), 10 nm; and (
), 15 nm. The dashed lines are PF or SE fitting.
Fig. 6. Arrhenius plots of leakage current of BST/Cr/BST with various Cr thicknesses (tC r). (), 0 nm; (
◦
), 2 nm; (), 5 nm; (∇), 10 nm; and (),
15 nm.
of the oxygen vacancies and the interfacial space charges on the leakage current are comparatively obvious.
Fig. 6 shows the Arrhenius plot of leakage currents from 298 K to 423 K. The activation energies of hopping electrons, calculated on the basis of Fig. 6 and exhibited in Table II, are 0.039 eV (tC r = 0 nm), 0.064 eV (tC r = 2 nm), 0.049 eV
(tC r = 5 nm), 0.028 eV (tC r = 10 nm), and 0.016 eV
(tC r = 15 nm). The specimens with tC r = 2 nm and 5 nm
have higher activation energies. It could be because some de-fects could trap the charges to reduce the leakage current, but the thermal excitation of the trapped charge from one site to the other dominates the transport in the films [17], [18]. The reason may increase the activation energies of capacitors with tC r= 2 and
5 nm. Though the dielectric constant decreases with the increase of tC r, the BST specimen with 2-nm-Cr interlayer shows smaller
leakage current, lower dissipation factor, and better temperature and voltage stabilities.
IV. CONSLUSION
MIM capacitors of Pt/BST/Cr/BST/Pt are investigated in this study. A TiO2film was formed after the BST/Cr/BST dielectric
is annealed at 1023 K in O2 atmosphere. The implementation
temperature stability, and leakage current of the BST dielectrics. A dissipation factor of 0.023 at 100 kHz is obtained for BST/Cr (2 nm)/BST dielectric as compared to that of 0.028 for mono-BST dielectric. A TCC of−0.5 × 10−3/K from 298 K to 398 K is obtained for BST/Cr(2 nm)/BST dielectric as compared to that
of−1.6 × 10−3/K for mono-BST dielectric. The improvement
of the dielectric behaviors of the BST/Cr/BST is attributed to the formation of TiO2layer after the annealing. Besides, among all
measuring temperatures, the BST/Cr/BST dielectric also shows more voltage stable than the mono-BST dielectric. The leakage current fittings show BST specimens containing Cr interlayer (tC r = 2 and 5 nm) show lower values of PF boundary. The
more oxygen vacancies and the higher interfacial space charge concentration, the smaller is the electric field boundary of SE/PF.
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Chia-Cheng Ho (S’08) received the B.S. and M.S.
degrees in electronic engineering in 2000 and 2002, respectively, from National Chiao-Tung University, Hsinchu, Taiwan, R.O.C., where he is currently work-ing toward the Ph.D. degree in electrical institute.
From 2007 to 2008, he was a Visiting Assistant in Research (VAR) at Yale University, where he was engaged in the development and simulation of the in-elastic electron tunneling spectroscopy (IETS). His current research interests include high k material, nano device, and high frequency circuit.
Bi-Shiou Chiou (M’99–SM’03) received the B.S.
degree in nuclear engineering and the M.S. degree in health physics, both from the National Tsing Huang University, Hsinchu, Taiwan, R.O.C., in 1975 and 1977, respectively, and the Ph.D. degree in materi-als science and engineering from Purdue University, West Lafayette, IN, in 1981.
She is currently a Professor of electronics en-gineering at the National Chiao-Tung University, Hsinchu. Her current research interests include elec-tronic packaging, elecelec-tronic ceramics, thick and thin film technology, nano technology, and optoelectronic packaging.
Li-Chun Chang received the B.E. and M.E. degrees
in materials science and engineering from National Cheng Kung University, Tainan, Taiwan, R.O.C., in 1990 and 1992, and the Ph.D. degree in electronics engineering from National Chiao-Tung University, Hsinchu, Taiwan, R.O.C., in 2005.
During 2005 to 2007, she was an Assistant Profes-sor at Huafan University, Taipei, Taiwan. She is cur-rently an Assistant Professor at Mingchi University of Technology, Taipei. Her current research interests in-clude resistive random access memory (RRAM) and electronic ceramics.
