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Study on fatigue and breakdown properties of Pt/(Pb,Sr)TiO3/Pt capacitors
View the table of contents for this issue, or go to the journal homepage for more 2006 J. Phys.: Condens. Matter 18 10457
Chun-Chien Tsai1, Chuan-Ping Juan1, Chueh-Kuei Jan1and
Huang-Chung Cheng1
1Department of Electronics Engineering and Institute of Electronics, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu 30010, Taiwan
2Department of Materials Science and Engineering, National United University, Miaoli 36003, Taiwan
E-mail:[email protected]
Received 5 September 2006, in final form 11 October 2006 Published 3 November 2006
Online atstacks.iop.org/JPhysCM/18/10457
Abstract
Pulsed-laser deposited (Pb, Sr)TiO3 (PSrT) films on Pt/SiO2/Si substrate at
various ambient oxygen pressures (PO2) are investigated in this work. Films
deposited at PO2 below 100 mTorr exhibit the (100) preferred orientation and
a tetragonal structure with larger tetragonality. In addition, films deposited at 80 mTorr exhibit the most apparent ferroelectric properties in contrast to those deposited at 200 mTorr. Moreover, films deposited at higher PO2 also exhibit
longer lifetimes and higher breakdown fields due to their smaller leakage current density, in terms of the reduction of defects, compensation of oxygen vacancies (OVs), an improved interface and small cluster sizes. An energy band model reveals that fatigue properties of PSrT films are dominated by interfacial states at low PO2and by deep trapping states at high PO2, which could be ascribed to
OVs located at the interfaces and inside films, respectively.
1. Introduction
(Pb, Sr)TiO3 (PSrT) is feasible for memory, sensor, frequency tuning devices and microwave
applications due to its large electric-field-dependent dielectric constant and composition-dependent Curie temperature [1–6]. The PSrT film is constituted by a solid solution of PbTiO3
(PTO) and SrTiO3 (STO). PTO and STO films, at room temperature, behave as a tetragonal
structure (ferroelectric phase) and a cubic structure (paraelectric phase), respectively, because PTO has the Curie temperature (Tc) at 490◦C and STO has the Tcat−220◦C. The effects of
lead (Pb) substituted by strontium (Sr) in the PTO film decrease crystallization temperature and offer a good control of the dielectric properties at room temperature [1,2].
In this work, PSrT films were prepared using the pulsed-laser deposition (PLD) because of its simplicity, versatility, and capability of growing a wide variety of stoichiometric oxide films without subsequent high-temperature annealing. Thus, PLD is a potential technique which could be excellent for fabricating ceramic films with complex compounds and integrated into low-temperature semiconductor processing to protect the formerly fabricated structure from damage and eliminate the volatilization of PbO in lead titanate-based thin films, which always degrades the crystallinity of perovskite phases and the electric properties of ferroelectric devices [7]. The pulsed-laser deposition process consists of three steps [8]: (1) vaporization of a target material by laser beam, (2) transport and interaction of a vapour plume with a background ambient, and (3) condensation of the ablated material onto a substrate where a thin film nucleates and grows. Hence, the structural and the electrical characteristics of PLD ferroelectric films are strongly affected by the processing parameters, such as substrate temperature, laser energy fluence (laser power density) and oxygen ambience.
Lots of studies on ferroelectric films have focused on the platinum electrode system which gives good stability to lead-based perovskite materials [9]. The noble metal platinum (Pt) is considered as the electrode of the Pt/PSrT/Pt capacitor also because of its low power consumption and low RC delay [6], compared to the conductive oxides [9], such as (La, Sr)CoO3, (La, Sr)MnO3, or RuO2. Nevertheless, the polarization degradation of
lead-based ferroelectric films has been constantly observed for a metal electrode when Pt is employed [7,9,10]. Some studies about the effect of ambient oxygen pressure ( PO2) on PLD
ferroelectric films have been reported [11–14]. However, the effect of PO2on PLD PSrT films
prepared on Pt/SiO2substrate and their according reliability properties have not been addressed
much. Therefore, the purpose of this study is focused on the properties of polarization switching degradation (fatigue) and breakdown of PSrT films deposited with various ambient oxygen pressures by the low-temperature PLD technique.
2. Experimental procedures
The 100 nm thick platinum (Pt) film was sputtered onto SiO2/p-type Si as the bottom electrode
followed by annealing at 450◦C for 30 min in N2 ambient. Thin PSrT films with 200 nm in
thickness were deposited on Pt/SiO2/Si substrate electrodes with a KrF pulsed-laser deposition
system (Lambda Physik Excimer Laser LPX 200i,λ = 248 nm). A set of optical lenses was used to focus the excimer laser beam onto the(Pb0.6, Sr0.4)TiO3 target in vacuum. The PSrT
target was prepared with a conventional ceramic fabrication process [8]. The vacuum chamber was pumped down to a base pressure of 0.1 mTorr and then refilled with O2as reactive gas. The
vaporized species of the target transferred and deposited on the substrate heated by a thermal heater. The target to substrate distance was 4 cm. The deposition temperature was fixed at a low substrate temperature of 400◦C, calibrated at the wafer upper surface. The ambient oxygen pressure(PO2) was varied from 50 to 200 mTorr. During the PLD process, the laser pulse rate
and the average energy fluence were 5 Hz and 1.55 J cm−2per pulse, respectively.
The film thickness and the surface morphology of PSrT films were examined by field emission scanning electron microscopy (FESEM) (S-4000, Hitachi). An Auger electron spectroscope (AES) (Auger 670 PHI Xi, Physical Electronics) was used to analyse the element ratios on the surface of PSrT films. The crystallinity of the film was analysed by x-ray diffractometer (D5000, Siemens, using Cu Kα, λ ∼ 0.154 nm).
After the physical examinations, the Pt top electrodes, with a thickness of 100 nm and a diameter of 75μm, were deposited by sputtering and patterned by a shadow mask process to form a Pt/PSrT/Pt capacitor structure. An automatic measurement system that combines an IBM PC/AT, a semiconductor parameter analyser (4156C, Agilent Technologies) and a
Figure 1. Deposition rate of PLD PSrT films prepared at various ambient oxygen pressures ( PO2) on Pt/SiO2/Si. 80 mTorr 50 mTorr 100 mTorr 300 nm 200 mTorr 300 nm 300 nm 300 nm
Figure 2. SEM surface morphologies of PLD PSrT films deposited at various ambient oxygen pressures on Pt/SiO2/Si.
probe station was used to measure the leakage current (I –V characteristics) and breakdown properties. The ferroelectric polarization versus electric field (P–E) characteristics of the PSrT film were determined directly by virtual ground circuits (RT-66A standardized ferroelectric testing system, Radiant Technologies). A pulse generator (8110A, Hewlett Packard) and a pulse/function generator (8116A, Hewlett Packard) were connected together with low noise BNC cables to generate a +3 V/−3 V bipolar wave pulsed at 1 MHz, confirmed by an oscilloscope (54645A, Hewlett Packard), as an input signal for the measurement of polarization switching degradation (fatigue).
3. Results and discussion
PLD PSrT films were prepared at various ambient oxygen pressures(PO2). Figure1shows that
the deposition rate of PSrT films is maximum at PO2 = 50 mTorr and then decreases as PO2
increases due to the increasing collisions between ejected species and the ambient oxygen gas. Moreover, film thickness could be controlled and roughly considered as a time-based function for different PO2. In general, the situation of collisions affects the stacking of clusters during
PLD, which directly associates with morphologies and structural characteristics of films [8]. Figure 2shows SEM surface morphologies of PLD PSrT films deposited at various PO2 on
Figure 3. Relative element ratios of PSrT films deposited at various ambient oxygen pressures on Pt/SiO2/Si.
Figure 4. X-ray diffraction spectra of PSrT films deposited at various ambient oxygen pressures on Pt/SiO2/Si.
Pt/SiO2/Si (100) wafers. As the PO2increases, the morphology becomes denser and less porous,
and the clusters gradually become more uniform, rounded and small, which reflects the better stacking of clusters and fewer defects. Usually, the grain size may be confined by the size of a cluster. Thus, a larger cluster may connect to larger grain size and fewer grain boundaries. We will address later, as a result of the electrical characteristics, that the dense morphology of PSrT films deposited at higher PO2exhibits fewer interfacial states and fewer deep trapping
states inside films. Figure 3 presents the element ratios, calibrated with the stoichiometric composition of the(Pb0.6, Sr0.4)TiO3target, on the surface of PSrT films analysed by the AES
technique, and reveals that the film composition is a function of PO2. It can be seen that the
element ratio of Ti/(Pb + Sr) increases markedly with the increase of PO2 as opposed to the
element ratio of Pb/(Pb + Sr). Besides, the element ratio of Sr/(Pb + Sr) slightly varies with
PO2. It also indicates that the element ratio of O/(Pb + Sr) increases from 2.41 to 3.69 as PO2
increases. This suggests that the oxygen content of films can be strongly compensated at higher
PO2.
Figure4displays the x-ray diffraction (XRD) spectra of PSrT films deposited at various
Figure 5. Current density versus electric field ( J –E) characteristics of Pt/PSrT/Pt capacitors applied at positive bias.
(100), (110), (210) and (211) planes of(Pb1−xSrx)TiO3 perovskite phases [1,2,6,15–18].
The crystalline PSrT films are observed at such low temperature (400◦C) because (1) the addition of strontium induces a lower crystallization temperature of PSrT than that of PZT [1] and (2) the PLD technique can preserve the crystalline phase and stoichiometric ratio of the target material at low substrate temperature [8]. Moreover, the intensity of the (100) and (110) diffraction peaks varies significantly as PO2increases. In figure4, the relative intensity of the
(110) orientation increases with increasing PO2 and shows a maximum at PO2 = 200 mTorr.
In contrast, the relative intensity of the (100) orientation exhibits a little variation when
PO2 100 mTorr and then decreases noticeably as PO2 increases from 100 to 200 mTorr.
A transition from (100) preferred orientation to (110) preferred orientation of PSrT films is observed as PO2 increases above 100 mTorr. Furthermore, the PSrT films have a tetragonal
structure with a clear (100) diffraction peak for x < 0.5 and a cubic structure with a strong (110) diffraction peak for x 0.5, as reported in previous works [1,2]. On the other hand, the tetragonalities (c/a), i.e. the ratios of c-axis/a-axis lattice constant obtained by electron diffraction patterns (not shown in this work), of the PSrT films deposited at 50 and 200 mTorr are∼1.033 and ∼1, respectively. The larger tetragonality for films deposited at lower PO2
may be attributed to the oxygen deficiency [19–21]. This suggests that the structure of PSrT films deposited with PO2below 100 mTorr is tetragonal, consistent with the low element ratio
of O/(Pb + Sr) (figure 3). In short, it is clear that the surface morphology, stoichiometric composition, preferred orientation, and tetragonality of PSrT films could apparently be affected by ambient oxygen pressures during PLD.
Figure 5 displays the current density versus electric field ( J –E) characteristics of Pt/PSrT/Pt capacitors under positive bias. It is found that the leakage current is reduced when
PO2 increases. The inhibited leakage current observed in PSrT films deposited at higher PO2
will be correlated with the more compensated oxygen vacancies (OVs) due to the larger oxygen element ratio as shown in figure3. The OVs will act as charged defects to degrade leakage currents as expressed by the following reaction [22–24]:
Oo↔ V••o + 2e+
1
2O2, (1)
where Oo, V••o , and e represent the oxygen ion on its normal site, the oxygen vacancy, and
Figure 6. TDDB characteristics (time to breakdown as a function of electric field) for PSrT films deposited at various PO2.
breakdown (TDDB) to predict the 10 year lifetime. TDDB is regarded as the resistance degradation of dielectric films, which slowly increases the leakage current under constant temperature and dc field stress. The mechanism of resistance degradations in perovskite films could be categorized into the grain boundary model and the reduction model [22–24]. The grain boundary model indicates a large potential drop across the high-resistivity grain boundary. Films with smaller grain sizes exhibit more grain boundaries, resulting in the shared drop in voltage and the suppressed resistance degradation. On the other hand, the reduction model suggests that OVs and injection electrons cause resistance degradation. Consequently, PSrT films deposited at higher PO2 exhibit longer lifetime and higher breakdown field due to their
smaller leakage current density (figure5), in terms of the reduction of defects, compensation of OVs (i.e. larger oxygen concentration shown in figure3), an improved interface (i.e. the denser, less porous and more uniform morphology shown in figure2), and small cluster sizes.
Figure 7 presents the saturation polarization (2Ps), remnant polarization (2Pr), and
coercive field (2Ec) values of PSrT films vary with PO2. The maximum values of films
deposited at 400◦C and 80 mTorr are 29.08 μC cm−2, 6.65 μC cm−2, and 90.0 kV cm−1, respectively, which are comparable to those of PSrT films prepared by the sol-gel process at temperatures higher than 700◦C [1, 6]. In contrast, films deposited at 200 mTorr reveal extremely small and indistinct ferroelectricities. The polarization of the lead titanate-based crystal is maximum in the [100] direction, so the polarizations of the films enhanced with the preferred (110) orientation are weaker than those of the films with the preferred (100) orientation [9,25]. As mentioned above, PSrT films deposited with PO2 below 100 mTorr
exhibit the (100) preferred orientation and a tetragonal structure with larger tetragonality. Because the ferroelectric dipole originates from ionic displacement in the c-axis direction, large spontaneous polarization is obtained with the elongated c-axis, i.e. the larger tetragonality. In other words, the ferroelectricity of PSrT films is the combined effect of the preferred orientation and oxygen content. Thus, the PSrT film deposited at 80 mTorr exhibits the most apparent ferroelectric properties, which are connected with the enhanced (100) preferred orientation (figure 4) and the larger oxygen concentration (figure 3). Table 1 summarizes the physical properties and electrical characteristics of PLD PSrT films deposited at various ambient oxygen pressures on Pt/SiO2/Si (100) wafers.
Figure 7. Saturation polarization (2 Ps), remnant polarization (2 Pr) and coercive field (2Ec) of Pt/PSrT/Pt capacitors prepared at various PO2.
Table 1. Summarized characteristics of PLD PSrT films deposited at various ambient oxygen pressures on Pt/SiO2/Si (100) wafers.
Ambient oxygen pressure ( PO2, mTorr) 50 80 100 200
Oxygen element ratioa 2.41 2.52 3.00 3.69 Leakage current density (μA cm−2)b 0.812 0.416 0.187 0.119 Time-zero breakdown field (kV cm−1)c 721 976 1042 1117 Saturation polariztion (2 Ps,μC cm−2)d 16.67 29.08 27.13 2.87 Remanent polarization (2 Pr,μC cm−2)d 4.36 6.65 4.82 0.09 Coercive field (2Ec, kV cm−1)d 87.5 90.0 82.0 9.6 aEvaluated from AES analysis.
bEvaluated from J –E curves biased at+100 kV cm−1. cEvaluated from TDDB curves.
dEvaluated from P–E hysteresis loops.
Figure8 demonstrates the properties of remnant polarization(Pr) fatigue of Pt/PSrT/Pt
capacitors after the switching operation. This fatigue examination does not include the film deposited at 200 mTorr because of its indistinct ferroelectricities. Films deposited at 80 mTorr start to fatigue from 108 switching cycles and reveal a slight degradation in P
r (<17%) after
1010 switching cycles. From previous works [26–29], it has been pointed out that the space
charge of OVs can be the major fatigue contributor. The fatigue process will alter the space charge distribution and reduce the Schottky barrier height at the interface of electrodes, leading to the change of leakage currents. Later in the text, the leakage current data before/after fatigued switching operation are interpreted as Schottky emission (SE) at lower electric fields and Poole–Frenkel emission (PF) at higher electric fields to analyse the space charge correlated to interface-limited and bulk-limited characteristics, accordingly [24,30,31]. The SE and PF behaviour are expressed as
SE: log(JSE/T2) = −q[ϕB− (q E/4πεdε0)1/2]/(kT ln 10) + log(A∗), (2)
PF: log(JPF/E) = −q[ϕt− (q E/πεdε0)1/2]/(kT ln 10) + log(B), (3)
where A∗ is the effective Richardson’s constant, B is a constant,ϕBis the potential barrier
Figure 8. The fatigue behaviours of remnant polarization ( Pr) versus accumulative switching cycles of Pt/PSrT/Pt capacitors prepared at various PO2.
ferroelectric material in the infrared region, q is the unit charge, k is Boltzmann’s constant, T is absolute temperature, and E is the external electric field. If the conduction current follows SE behaviour, then a log(J/T2) against E1/2plot should be linear. Similarly, a log(J/E) against
E1/2plot can be made for PF. Figure9(a) presents the SE plot of fresh and fatigued Pt/PSrT/Pt capacitors deposited at various PO2 and the dashed lines are the fitted results. The inhibited
leakage current biased at lower electric fields of PSrT films deposited at higher PO2is correlated
with the lower Schottky barrier, fewer interfacial states, and fewer space charges (charged OVs) due to the improved interface (figure2) and compensated OVs. This suggests that the films deposited at higher PO2 have fewer interface states before being fatigued and exhibit nearly
fatigue-free J –E characteristics at lower electric fields. Figure9(b) shows the PF plot using the same experimental current–voltage (I –V ) data. Likewise, the inhibited leakage current of films deposited at higher PO2and biased at higher electric fields is associated with less deep trapping
states and fewer space charges (charged OVs) inside films due to compensated OVs. It indicates that the onset of PF behaviours of fresh Pt/PSrT/Pt capacitors appears when the bias exceeds
+130 kV cm−1. In contrast, the onset of PF behaviours of fatigued Pt/PSrT/Pt capacitors
changes from above+130 to +90 kV cm−1when PO2 increases from 50 to 100 mTorr. This
suggests that films deposited at higher PO2 exhibit fewer interfacial states with an improved
interface, which inhibit OVs generating and accumulating at the interface of electrodes, but prefer to create OVs inside films after fatigue.
Figure10illustrates the electron energy band of PSrT films and shows that the interfaces of substrate electrodes act as n-type semiconductors due to the generation of oxygen vacancies in ABO3 perovskites [24,26]. It reveals that PSrT films deposited at lower PO2 have porous
surface and more OVs than those deposited at higher PO2, yielding more interfacial states
and more trapping states inside films. As a result, there are more OVs accumulated at the interface and the fatigue properties are dominated by the interfacial states. Conversely, PSrT films deposited at higher PO2exhibit improved interfaces and compensated OVs, yielding fewer
interfacial states and fewer deep trapping states inside films. It suggests that OVs are generated inside films and the fatigue properties are dominated by deep trapping states. Hence, films deposited at 80 mTorr exhibit nearly fatigue-free J –E characteristics at lower electric fields and a higher onset field of PF behaviours of fatigued Pt/PSrT/Pt capacitors. The results could be connected with the compensated OVs and a balance distribution of OVs located at the interfaces
Figure 9. (a) Schottky emission plot fitting of log(J/T2) versus E1/2and (b) Poole–Frenkel
emission plot fitting of log(J/E) versus E1/2for the fresh and fatigued Pt/PSrT/Pt capacitors
prepared at various PO2.
and inside films, corresponding to the optimum fatigue resistance with fatigue starting from 108
Figure 10. Schematic drawings of the electron energy band for the fresh and fatigued Pt/PSrT/Pt capacitors deposited at low and high PO2.
4. Conclusions
The surface morphology, film composition, crystallographic orientation and electrical characteristics of PSrT films are found to be influenced by ambient oxygen pressures during low-temperature PLD. PSrT films deposited with PO2 below 100 mTorr exhibit the (100)
preferred orientation and a tetragonal structure with larger tetragonality. Thus, films deposited at 80 mTorr have the most apparent ferroelectric properties. In contrast, films deposited at 200 mTorr reveal extremely small and indistinct ferroelectricities. Films deposited at higher
PO2also exhibit longer lifetime and higher breakdown field due to their smaller leakage current
density, in terms of the reduction of defects, compensation of OVs, an improved interface and small cluster sizes. It also suggests that PSrT films deposited at lower PO2have porous surface
and more OVs than those deposited at higher PO2, yielding more interfacial states and more
trapping states inside films, and the fatigue properties are dominated by the interfacial states. Conversely, PSrT films deposited at higher PO2 show improved interfaces and compensated
OVs, yielding fewer interfacial states and fewer deep trapping states inside films, and the fatigue properties are dominated by deep trapping states.
Acknowledgments
This work was supported in part by the National Science Council of ROC under the contract NSC94-2218-E-009-028. Thanks are also due to the Nano Facility Center (NFC) in National
[6] Hou C S, Pan H C, Chou C C and Cheng H F 1999 Ferroelectrics 232 129
[7] Lim D G, Park Y, Moon S I and Yi J 2000 Applications of ferroelectrics 2000 ISAF 2000: Proc. 2000 12th IEEE
Int. Symp. (Honolulu) (Piscataway, NJ: IEEE) p 599
[8] Chrisey D B and Huber G K 1992 Pulsed Laser Deposition of Thin Films (New York: Wiley–Interscience) pp 55–87, 167–98
[9] De Araujo C P, Scott J F and Taylor G W 1996 Ferroelectric Thin Films: Synthesis and Basic Properties,
Ferroelectric and Related Phenomena vol 10 (Netherlands: Gordon and Breach) pp 193–226, 447–78
[10] Wang Y, Wang K F, Zhu C and Liu J M 2006 J. Appl. Phys.99 044109
[11] James A R and Prakash C 2004 Appl. Phys. Lett.84 1165
[12] Scarisoreanu N, Craciun F, Dinescu G, Verardi P and Dinescu M 2004 Thin Solid Films453/454 399
[13] Rodrigueza R C, Sanchezb S P, Wattsb B E and Leccabue F 2003 Mater. Lett.57 3958
[14] Gao X S, Xue J M, Li J, Ong C K and Wang J 2003 Microelectron. Eng.66 926
[15] Chou C C, Chang H Y, Lin I N, Shaw B J and Tan J T 1998 Japan. J. Appl. Phys.37 5269
[16] Pan H C, Chang G C, Chou C C and Cheng H F 1999 Integr. Ferroelectr. 25 179 [17] Chung H J, Kim J H and Woo S I 2001 Chem. Mater.13 1441
[18] Chung H J and Woo S I 2001 J. Vac. Sci. Technol. B19 275
[19] Li C L, Chen Z H, Zhou Y L and Cui1 D F 2001 J. Phys.: Condens. Matter13 5261
[20] Fuchs D, Adam M, Schweiss P, Gerhold S, Schuppler S, Schneider R and Obst B 2000 J. Appl. Phys.88 1844
[21] Cui D F, Wang H S, Chen Z H, Zhou Y L, Lu H B and Yang G Z 1997 J. Vac. Sci. Technol. A15 275
[22] Waster R, Baiatu R T and Haratl K H 1990 J. Am. Ceram. Soc.73 1645
[23] Tsai M S and Tseng T Y 2000 IEEE Trans. Compon. Packag. Technol.23 128
[24] Shye D C, Chiou B S, Lai M J, Hwang C C, Jiang C C, Chen J S, Cheng M H and Cheng H C 2003 J. Electrochem.
Soc.150 F20
[25] Xu Y 1991 Ferroelectric Materials and Their Applications (Amsterdam: Elsevier Science Publishers B. V.) pp 101–62
[26] Scott J F 2000 Ferroelectric Memories (Berlin: Springer) pp 79–85, 134–43 [27] Jiang A Q, Scott J F, Dawber M and Wang C 2002 J. Appl. Phys.92 6756
[28] Dawber M and Scott J F 2000 Appl. Phys. Lett.76 1060
[29] Bratkovsky A M and Levanyuk A P 2001 Phys. Rev. B63 132103
[30] Vorobiev A, Rundqvist P, Khamchahe K and Gevorgian S 2004 J. Appl. Phys.96 4642
