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Ferroelectric properties of hydrothermally prepared BaTiO3 thin films on Si(100) substrates by low-temperature processing
cm __
__ BB
May 1995
EIXMER
Materials Letters 23 ( 1995) 203-207
Ferroelectric properties of hydrothermally prepared BaTiO, thin films on Si( 100) substrates by low-temperature processing C.R. Cho a, M.S. Jang a, S.Y. Jeong a, S.J. Lee a, B.M. Limb aPhysics Department and Research CenterforDielectric and Advanced Matter Physics, Pusan National University, Pusan 609- 735. South Korea b Research Institute of Industrial Science and Technology, Pohang 709-600, South Korea
Received 5 December 1994; in final form 3 March
1995; accepted 7 March 1995
Abstract Polycrystalline BaTiO, thin films were prepared hydrothermally on Ti-deposited Si( 100) substrates at 220°C for 24 h using a 2.0 N Ba( OH) 2 solution. The surface morphology of the films observed by scanning electron microscopy indicated that the films were crack-free and uniform, and the thickness of the cross section was about 350 nm. From the AES analysis of the films, it was found that stoichiometrically uniform films exist and serious interdiffusion in the interface region did not appear. The dielectric constant and dielectric loss at 10 kHz at room temperature were about 150-200 and below 0.08, respectively. The capacitance dependence on applied dc voltage exhibited maximum value at -0.9 V and breakdown did not occur in the measurement range. The hydrothermally prepared films showed ferroelectric behavior and that was found by us for the first time. The measured value of remanent polarization (P,) and coercive field (E,) amounted to 21.6 pC/cm’ and 26 kV/cm, respectively.
In recent years, barium titanate thin films have been widely studied with respect to the application to dynamic random access memories [ 1,2]. New methods and processing techniques have been developed to produce good quality films for device applications. Generally, various methods like MOCVD [3,4], rf sputtering [ 51, sol-gel [ 61, and laser ablation [ 71 have been used for the fabrication of BaTiO, films. All of these techniques require temperatures higher than 500°C for crystallization during or after film formation, independent of the film fabrication method. The high temperature processing conditions often cause problems at the film-substrate interface [5], and hence, difficulties in integrating the films with silicon monolithic circuits.
0167-577x/95/$09.50
0 1995 Elsevier Science B.V. All rights reserved
SSDIO167-577x(95)00056-9
The hydrothermal method has been recently adopted for the preparation of BaTi03 thin films [ 8-101. The major advantage of this method is that it does not require a high-temperature treatment. However, because this synthesis involves the “dissolutionrecrystallization” process [ 1 I] of Ti metal in a Ba(OH), solution to deposit a crystalline layer of BaTi03 on the surface, the interface between the film and Ti layer was not clear, and the surface of the film was not uniform and the composition was also not homogeneous, so measurements of bias dependence were difficult. Bacsa et al. [ 121 reported that interdiffusion in the film interface region was reduced when hydrothermal synthesis was used with TiC as the barrier layer, but the C-V characteristics and ferroelectric properties of the film were not reported as to this date.
204
CR. Cho et al. /Moterids
---I
1 20
I
7
I 40
I
30 28
I 50
1
60
(dcprecs)
Fig. 1. XRD pattern of BaTiO, films processed N Ba(OH), solution for 24 h.
at 220°C in the 2.0
Fig. 2. SEM micrographs OFthe (a) surfaceand (b) inclined cross section of the BaTiO, film formed on SiO,/Si( 100) substrates at 220°C in the 2.0 N Ba(OH), for 24 h by the hydrothermal reaction.
In this Letter we present the results of a synthesis procedure, using SiOZ films as a diffusion barrier between
Letters 23 (1995) 203-207
the Si substrate and the Ti film, on which the reaction takes place. The crystallographic structure and composition of the polycrystalline BaTiO, thin films synthesized by hydrothermal reaction were investigated. The electrical properties, including the dependence of the capacitance on bias voltage and the hysteresis loop, were also measured for the thin film capacitor of MIM (metal/insulator/metal) structure. The silicon wafers were first thermally oxidized to form a SiO, layer, 70 nm thick, to prevent the formation of titanium silicides [ 131 at the interface during the reaction. Titanium layers 0.5 pm thick were deposited on both sides of boron-doped Si( 100) substrates at a deposition rate of 0.345 rim/s at 300°C by electron beam deposition. The wafers were then cut into 1 X 2 cm size pieces, and suspended onto a frame which was subsequently placed into a tube-type autoclave with a reaction chamber 30 cm in diameter and 55 cm in length. Three independent heaters surrounding the autoclave were used in order to establish an optimal temperature difference for inducing thermal convection of the reaction medium during the hydrothermal reaction. To confirm the nature of the phase of the hydrothermally synthesized film, an X-ray diffractometer with Cu KLYradiation was used with a Ni filter. Scanning electron microscopy (SEM) was used to examine the surface morphologies and cross section of the films. The composition and depth profile of BaTi03 were analyzed by Auger electron spectroscopy (AES, Perkin-Elmer PH610). Gold dots ( = 2.0~ lo-’ cm* area) were sputtered onto the BaTi03 films as a top electrode for the electrical measurements with the film temperature kept at 250°C in a vacuum of lo-” Torr. The unreacted Ti layer was used as the bottom electrode. C-Vmeasurements in the frequency range I kHz to 1 MHz were performed with an impedence analyzer (HP4194A). To examine the ferroelectric behavior of the film, polarization reversal was observed by using the modified Sawyer-Tower circuit at 60 Hz. The X-ray diffraction patterns of one of the thin films processed at 220°C in 2.0 N Ba( OH), for 24 h are given in Fig. 1. It showed that the formation ofBaTi0, was nearly complete with no reaction with SiOZ, the polycrystalline Ti films preferred to orient in the [ 0021 direction, and the high intensity of the Ti peak may be due to unreacted Ti. The polycrystalline BaTi03 thin film showed no preferred orientation. Peaks were sharp
CR. Cho et al. /Materials Letters 23 (1995) 203-207
90
1
Q
220
420
820
620
Kinetic
1020
energy
1220
1420
1620
18273 2020
(eV)
ioo
SO
g
60
ij
4 4o 20 0
0.0
7.0
14.0
21.0
Sputter
28.0
35.0
time
420
49.0
56.0
63.0
7‘0 0
(min.)
Fig. 3. (a) Auger spectrum of the surface and (b) depth profile of a BaTiO, film processed at 220°C in the 2.0 N Ba(OH), solution for 24 h.
and large enough to prove the crystallinity of barium titanate. Fig. 2 shows an SEM micrograph of the surface (Fig. 2a) and a inclined cross section (Fig. 2b) of the BaTiO, film formed on Ti/SiO,/Si substrates at 220°C in 2.0 N Ba( OH), for 24 h by hydrothermal reaction. The average grain size of the film was uniform and estimated to be around 0.2 Frn. The thickness of the BaTiO, thin film and the Ti layer was about 350 nm and 300 nm, respectively. We could obtain only an approximate value of the thickness, because the interface between the Ti substrate and BaTiO, film was not very well defined. Fig. 3 depicts the distribution of the elemental components and depth profiles obtained by AES measurements for the BaTiO, thin films. In the measurement the surface of the specimen, mounted on the holder, was bombarded by an Ar+ ion beam. The primary electron beam, 3.5 keV, 0.2 kA, with a scanning speed
205
of 20 eV/s was used for AES measurements. It is suggested that the carbon detected in the interface and the surface of the BaTiO, films is due to contamination caused by the Ba( OH) 2 solution, the chamber wall and sample handling. the AES signals for each element were located at 271 eV for C( KL,,,L,,,), 383 eV for Ti( L2M2,sM2.s), 417 eV for Ti (L,M,,M,,), 508 eV for 0( KL,,,L,,,), 582 eV for Ba( M,N*,N,,,), and 598 eV for Ba(M,N,,N,,,) as shown in Fig. 3a. The depth profile yields a uniform distribution of the components throughout the whole film as shown in Fig. 3b. In accordance with the results of XRD and SEM measurements the results of the depth profile showed the presence of the unreacted Ti layer. We also confirmed that an intermediate layer exists between the BaTiOs film and the Ti layer, even though it did not appear in the XRD results. The increase of the width of the interface layer will have a large effect on the electrical properties of the film because the intermediate layer has a low dielectric constant. In Fig. 3b, since the exact sputtering rate of BaTi03 and Ti by the 3.5 keV beam is not known, the sputtered depth was not obtained. But the atomic concentration of Ba, Ti and 0 in this film remained about 1: 1: 3 through the whole layer, which indicates the formation of a uniform layer of BaTiO,. Capacitance-frequency characteristics of the films with a thickness of 350 nm are shown in Fig. 4. The measured values of the capacitance and tan 6 at 10 kHz were 0.75 and 0.08 nF, respectively. With the decrease of the frequency to below 500 Hz, the capacitance and tan 6 increased little-by-little, which is due to a contact
1.2 c
220-C
24h
treatment
5 e:
0.9
s 5
0.6
z B
0.3
Fig. 4. Variation of the capacitance and the dielectric loss as a function of frequency at room temperature for BaTiO, films formed at 220°C in the 2.0 N Ba(OH), solution for 24 h.
206
C.R. Cho et al. /Materials
0.75, frequency
-6
-5
-4
IOOkHz
-3
-2
-1
Applied
0
1
2
3
4
5
6
Bias (V)
Fig. 5. Capacitance versus voltage curve of Au/BaTiO,/Ti/SiO,/ Si structure. The measured frequency and the film thickness are 100 kHz and 350 nm, respectively.
resistance between the probe and the electrode. No notable frequency dispersion of the capacitance and dielectric loss (tan 8) was observed in the measurement range. Bacsa [ 121 reported, for a hydrothermally synthesized film on a Si substrate with a barrier layer of Tic, that the dielectric constant was of the order of 450-500. But our sample showed that the dielectric constant was about 150 and is lower than that obtained by Bacsa. This result for our sample may be due to the formation of the intermediate layer between the BaTi03 film and the Ti layer. The bias voltage dependence of capacitance (C-V characteristics) at room temperature was measured for BaTiO, thin film capacitors with a MIM structure. Fig. 5 gives the results of C-V measurements according to the positive and the negative sweep direction, when the bias voltage was applied on the top electrode of an initially unpoled film. In addition, a ramp bias with ac signal of a low amplitude ( 100 mV) is applied to the BaTiO, film capacitors. The magnitude of the applied bias voltage was adjusted so that the same range of bias field form - 5 to 5V, was applied to the dielectric film. The capacitance variation with increasing of the applied bias indicates that the switching of the polarization in the ferroelectric film and the abrupt capacitance variation at the point of switching represents the points of the applied bias corresponding to the coercive electric field ( + E,) [ 14,151. In our case, the maximum value of the capacitance was obtained at a bias voltage of about -0.9 V. In the region 1 kHz-1 MHz, the maximum capacitance voltage as a function of bias field almost did not vary. These results may be due to the
Letters 23 (1995) 203-207
intermediate oxide layer between the film and Ti layer. A similar field dependence of the capacitance for SrTiO, thin films has been reported [ 16,171. The capacitance decrease and the increase in the dielectric loss with bias voltage are due to high leakage, especially at high voltage, as shown in Fig. 5. Fig. 6 shows a P-E hysteresis loop measured under an electric field of 43 kV/cm at 60 Hz for film with a thickness of 350 nm. The loop observed in the present investigations show incomplete saturation. A further increase of the electric field applied to the film caused electric breakdown. The saturation nature of the loop provides strong evidence for ferroelectric behavior of the film. The non-saturation loop [ 18,191 of the films may be attributed to a complex conduction process due to depolarizing fields and defects at the grain boundaries and at the interface. The measured values of remanent polarization (P,) and coercive field (E,) are 21.6 $/cm’ and 26 kV/cm respectively. The value of E, for the film prepared by hydrothermal reaction was about 4 times as low as that of the polycrystalline film grown by sol-gel processing [6], which may be due to the stress effect of the films during their growth. In summary, we synthesized for the first time polycrystalline ferroelectric barium titanate thin films on Ti-deposited Si substrates via hydrothermal reaction. From AES analysis we confirmed that an intermediate oxide layer between the film and the Ti layer was detected, and the oxide layer plays the role of a barrier so that it reduces diffusion at the interface. The thin film hydrothermally synthesized may have potential application for use in high-density memory devices
Fig. 6. Ferroelectric P-E hysteresis loop of a BaTiO, thin film, using a 60 Hz ac voltage. Scale: vertical=25 p,C/cm’/div.; horizontal = 57 kV/cm/div.
C.R. Cho et al. /Materials
because of the advantage of the low-temperature processing. The ferroelectric film synthesized by this method had a remanent polarization of 21.6 pC/cm’ and a coercive field of 26 kV/cm. The authors wish to thank Dr. E. Shi for useful discussion. This work was supported by the Korea Science and Engineering Foundation (KOSEF) through the Research Center for Dielectric and Advanced Matter Physics (RCDAMP) at Pusan National University.
References [ 11 M. Kitabatake and K. Wasa, Japan. J. Appl. (1985) 33. [2] L.H. Paker and Al F. Tasch, IEEE Circuits (1990) 17. [3] L.A. Willis, B.W. Wessels, D.S. Richelson Appl. Phys. Letters 60 (1992) 6. [4] P.C.V. Buskirk, R. Gokdiner, P.S. Kirlin and Res. 7 (1992) 542. [5] V.S. Dharmadhikari and W.W. Grannemann, (1982) 8988.
Pbys. Suppl. 22 Devices Mag. 6 and T.J. Marks, S. Nutt, J. Mater. J. Appl. Phys. 53
Letters 23 (1995) 203-207
201
[6] M.N. Kamala.sanan and S. Chandra, Appl. Phys. Letters 59 (1991) 3549. [ 71 K. Nashimoto, D.K. Fork and T.H. Geballe, Appl. Phys. Letters 60 (1992) 10. [8] K. Kajiyoshi, N. Ishizawa and M. Yoshimura, J. Am. Ceram. sot. 74 (1991) 349. [9] E. Shi, C.R. Cho, M.S. Jang and S.Y. Jeong, Integrated Ferroelectrics, to be published. [ 101 C.R. Cho, E. Shi, M.S. Jang, S.Y. Jeong and SC. Kim, Japan. J. Appl. Phys. 33 (1994) 4984. [ 111 E. Shi, C.R. Cho, M.S. Jang, S.Y. Yeong and H.J. Kim, J. Mater. Res. 9 (1994) 2914. [ 121 R. Bacsa, J.P. Dougherty and L.J. Pilione, Appl. Phys. Letters 63 (1993) 1653. [ 131 A. Zur,T.C. McGilland M.A. Nicolet, J. Appl. Phys. 57 ( 1985) 600. [ 141 G. Robrer, S. Narayan, L. McMillan and A. Kulkami, J. Vacuum Sci. Technol. A6 (1988) 1756. [ 151 T. Mihara, H. Watanabe, H. Yoshimori, C.A. Paz De Araujo, B. Melnick and L.D. Macmillan, Integrated Ferroelectrics 1 (1992) 269. [ 161 D. Roy, C.J. Peng and S.B. Kmpanidhi, Appl. Phys. Letters 60 (1992) 2478. [ 171 K. Abe and S. Komatsu, Japan. J. Appl. Phys. 31 ( 1992) 2985. [ 181 J.R. Slack and J.C. Burfoot, Thin Solid Films, 6 (1970) 233. [ 191 J.K.G. Panitz and C.C. Hu, J. Vacuum Sci. Tecbnol. 16 ( 1979) 315.
__ BB
May 1995
EIXMER
Materials Letters 23 ( 1995) 203-207
Ferroelectric properties of hydrothermally prepared BaTiO, thin films on Si( 100) substrates by low-temperature processing C.R. Cho a, M.S. Jang a, S.Y. Jeong a, S.J. Lee a, B.M. Limb aPhysics Department and Research CenterforDielectric and Advanced Matter Physics, Pusan National University, Pusan 609- 735. South Korea b Research Institute of Industrial Science and Technology, Pohang 709-600, South Korea
Received 5 December 1994; in final form 3 March
1995; accepted 7 March 1995
Abstract Polycrystalline BaTiO, thin films were prepared hydrothermally on Ti-deposited Si( 100) substrates at 220°C for 24 h using a 2.0 N Ba( OH) 2 solution. The surface morphology of the films observed by scanning electron microscopy indicated that the films were crack-free and uniform, and the thickness of the cross section was about 350 nm. From the AES analysis of the films, it was found that stoichiometrically uniform films exist and serious interdiffusion in the interface region did not appear. The dielectric constant and dielectric loss at 10 kHz at room temperature were about 150-200 and below 0.08, respectively. The capacitance dependence on applied dc voltage exhibited maximum value at -0.9 V and breakdown did not occur in the measurement range. The hydrothermally prepared films showed ferroelectric behavior and that was found by us for the first time. The measured value of remanent polarization (P,) and coercive field (E,) amounted to 21.6 pC/cm’ and 26 kV/cm, respectively.
In recent years, barium titanate thin films have been widely studied with respect to the application to dynamic random access memories [ 1,2]. New methods and processing techniques have been developed to produce good quality films for device applications. Generally, various methods like MOCVD [3,4], rf sputtering [ 51, sol-gel [ 61, and laser ablation [ 71 have been used for the fabrication of BaTiO, films. All of these techniques require temperatures higher than 500°C for crystallization during or after film formation, independent of the film fabrication method. The high temperature processing conditions often cause problems at the film-substrate interface [5], and hence, difficulties in integrating the films with silicon monolithic circuits.
0167-577x/95/$09.50
0 1995 Elsevier Science B.V. All rights reserved
SSDIO167-577x(95)00056-9
The hydrothermal method has been recently adopted for the preparation of BaTi03 thin films [ 8-101. The major advantage of this method is that it does not require a high-temperature treatment. However, because this synthesis involves the “dissolutionrecrystallization” process [ 1 I] of Ti metal in a Ba(OH), solution to deposit a crystalline layer of BaTi03 on the surface, the interface between the film and Ti layer was not clear, and the surface of the film was not uniform and the composition was also not homogeneous, so measurements of bias dependence were difficult. Bacsa et al. [ 121 reported that interdiffusion in the film interface region was reduced when hydrothermal synthesis was used with TiC as the barrier layer, but the C-V characteristics and ferroelectric properties of the film were not reported as to this date.
204
CR. Cho et al. /Moterids
---I
1 20
I
7
I 40
I
30 28
I 50
1
60
(dcprecs)
Fig. 1. XRD pattern of BaTiO, films processed N Ba(OH), solution for 24 h.
at 220°C in the 2.0
Fig. 2. SEM micrographs OFthe (a) surfaceand (b) inclined cross section of the BaTiO, film formed on SiO,/Si( 100) substrates at 220°C in the 2.0 N Ba(OH), for 24 h by the hydrothermal reaction.
In this Letter we present the results of a synthesis procedure, using SiOZ films as a diffusion barrier between
Letters 23 (1995) 203-207
the Si substrate and the Ti film, on which the reaction takes place. The crystallographic structure and composition of the polycrystalline BaTiO, thin films synthesized by hydrothermal reaction were investigated. The electrical properties, including the dependence of the capacitance on bias voltage and the hysteresis loop, were also measured for the thin film capacitor of MIM (metal/insulator/metal) structure. The silicon wafers were first thermally oxidized to form a SiO, layer, 70 nm thick, to prevent the formation of titanium silicides [ 131 at the interface during the reaction. Titanium layers 0.5 pm thick were deposited on both sides of boron-doped Si( 100) substrates at a deposition rate of 0.345 rim/s at 300°C by electron beam deposition. The wafers were then cut into 1 X 2 cm size pieces, and suspended onto a frame which was subsequently placed into a tube-type autoclave with a reaction chamber 30 cm in diameter and 55 cm in length. Three independent heaters surrounding the autoclave were used in order to establish an optimal temperature difference for inducing thermal convection of the reaction medium during the hydrothermal reaction. To confirm the nature of the phase of the hydrothermally synthesized film, an X-ray diffractometer with Cu KLYradiation was used with a Ni filter. Scanning electron microscopy (SEM) was used to examine the surface morphologies and cross section of the films. The composition and depth profile of BaTi03 were analyzed by Auger electron spectroscopy (AES, Perkin-Elmer PH610). Gold dots ( = 2.0~ lo-’ cm* area) were sputtered onto the BaTi03 films as a top electrode for the electrical measurements with the film temperature kept at 250°C in a vacuum of lo-” Torr. The unreacted Ti layer was used as the bottom electrode. C-Vmeasurements in the frequency range I kHz to 1 MHz were performed with an impedence analyzer (HP4194A). To examine the ferroelectric behavior of the film, polarization reversal was observed by using the modified Sawyer-Tower circuit at 60 Hz. The X-ray diffraction patterns of one of the thin films processed at 220°C in 2.0 N Ba( OH), for 24 h are given in Fig. 1. It showed that the formation ofBaTi0, was nearly complete with no reaction with SiOZ, the polycrystalline Ti films preferred to orient in the [ 0021 direction, and the high intensity of the Ti peak may be due to unreacted Ti. The polycrystalline BaTi03 thin film showed no preferred orientation. Peaks were sharp
CR. Cho et al. /Materials Letters 23 (1995) 203-207
90
1
Q
220
420
820
620
Kinetic
1020
energy
1220
1420
1620
18273 2020
(eV)
ioo
SO
g
60
ij
4 4o 20 0
0.0
7.0
14.0
21.0
Sputter
28.0
35.0
time
420
49.0
56.0
63.0
7‘0 0
(min.)
Fig. 3. (a) Auger spectrum of the surface and (b) depth profile of a BaTiO, film processed at 220°C in the 2.0 N Ba(OH), solution for 24 h.
and large enough to prove the crystallinity of barium titanate. Fig. 2 shows an SEM micrograph of the surface (Fig. 2a) and a inclined cross section (Fig. 2b) of the BaTiO, film formed on Ti/SiO,/Si substrates at 220°C in 2.0 N Ba( OH), for 24 h by hydrothermal reaction. The average grain size of the film was uniform and estimated to be around 0.2 Frn. The thickness of the BaTiO, thin film and the Ti layer was about 350 nm and 300 nm, respectively. We could obtain only an approximate value of the thickness, because the interface between the Ti substrate and BaTiO, film was not very well defined. Fig. 3 depicts the distribution of the elemental components and depth profiles obtained by AES measurements for the BaTiO, thin films. In the measurement the surface of the specimen, mounted on the holder, was bombarded by an Ar+ ion beam. The primary electron beam, 3.5 keV, 0.2 kA, with a scanning speed
205
of 20 eV/s was used for AES measurements. It is suggested that the carbon detected in the interface and the surface of the BaTiO, films is due to contamination caused by the Ba( OH) 2 solution, the chamber wall and sample handling. the AES signals for each element were located at 271 eV for C( KL,,,L,,,), 383 eV for Ti( L2M2,sM2.s), 417 eV for Ti (L,M,,M,,), 508 eV for 0( KL,,,L,,,), 582 eV for Ba( M,N*,N,,,), and 598 eV for Ba(M,N,,N,,,) as shown in Fig. 3a. The depth profile yields a uniform distribution of the components throughout the whole film as shown in Fig. 3b. In accordance with the results of XRD and SEM measurements the results of the depth profile showed the presence of the unreacted Ti layer. We also confirmed that an intermediate layer exists between the BaTiOs film and the Ti layer, even though it did not appear in the XRD results. The increase of the width of the interface layer will have a large effect on the electrical properties of the film because the intermediate layer has a low dielectric constant. In Fig. 3b, since the exact sputtering rate of BaTi03 and Ti by the 3.5 keV beam is not known, the sputtered depth was not obtained. But the atomic concentration of Ba, Ti and 0 in this film remained about 1: 1: 3 through the whole layer, which indicates the formation of a uniform layer of BaTiO,. Capacitance-frequency characteristics of the films with a thickness of 350 nm are shown in Fig. 4. The measured values of the capacitance and tan 6 at 10 kHz were 0.75 and 0.08 nF, respectively. With the decrease of the frequency to below 500 Hz, the capacitance and tan 6 increased little-by-little, which is due to a contact
1.2 c
220-C
24h
treatment
5 e:
0.9
s 5
0.6
z B
0.3
Fig. 4. Variation of the capacitance and the dielectric loss as a function of frequency at room temperature for BaTiO, films formed at 220°C in the 2.0 N Ba(OH), solution for 24 h.
206
C.R. Cho et al. /Materials
0.75, frequency
-6
-5
-4
IOOkHz
-3
-2
-1
Applied
0
1
2
3
4
5
6
Bias (V)
Fig. 5. Capacitance versus voltage curve of Au/BaTiO,/Ti/SiO,/ Si structure. The measured frequency and the film thickness are 100 kHz and 350 nm, respectively.
resistance between the probe and the electrode. No notable frequency dispersion of the capacitance and dielectric loss (tan 8) was observed in the measurement range. Bacsa [ 121 reported, for a hydrothermally synthesized film on a Si substrate with a barrier layer of Tic, that the dielectric constant was of the order of 450-500. But our sample showed that the dielectric constant was about 150 and is lower than that obtained by Bacsa. This result for our sample may be due to the formation of the intermediate layer between the BaTi03 film and the Ti layer. The bias voltage dependence of capacitance (C-V characteristics) at room temperature was measured for BaTiO, thin film capacitors with a MIM structure. Fig. 5 gives the results of C-V measurements according to the positive and the negative sweep direction, when the bias voltage was applied on the top electrode of an initially unpoled film. In addition, a ramp bias with ac signal of a low amplitude ( 100 mV) is applied to the BaTiO, film capacitors. The magnitude of the applied bias voltage was adjusted so that the same range of bias field form - 5 to 5V, was applied to the dielectric film. The capacitance variation with increasing of the applied bias indicates that the switching of the polarization in the ferroelectric film and the abrupt capacitance variation at the point of switching represents the points of the applied bias corresponding to the coercive electric field ( + E,) [ 14,151. In our case, the maximum value of the capacitance was obtained at a bias voltage of about -0.9 V. In the region 1 kHz-1 MHz, the maximum capacitance voltage as a function of bias field almost did not vary. These results may be due to the
Letters 23 (1995) 203-207
intermediate oxide layer between the film and Ti layer. A similar field dependence of the capacitance for SrTiO, thin films has been reported [ 16,171. The capacitance decrease and the increase in the dielectric loss with bias voltage are due to high leakage, especially at high voltage, as shown in Fig. 5. Fig. 6 shows a P-E hysteresis loop measured under an electric field of 43 kV/cm at 60 Hz for film with a thickness of 350 nm. The loop observed in the present investigations show incomplete saturation. A further increase of the electric field applied to the film caused electric breakdown. The saturation nature of the loop provides strong evidence for ferroelectric behavior of the film. The non-saturation loop [ 18,191 of the films may be attributed to a complex conduction process due to depolarizing fields and defects at the grain boundaries and at the interface. The measured values of remanent polarization (P,) and coercive field (E,) are 21.6 $/cm’ and 26 kV/cm respectively. The value of E, for the film prepared by hydrothermal reaction was about 4 times as low as that of the polycrystalline film grown by sol-gel processing [6], which may be due to the stress effect of the films during their growth. In summary, we synthesized for the first time polycrystalline ferroelectric barium titanate thin films on Ti-deposited Si substrates via hydrothermal reaction. From AES analysis we confirmed that an intermediate oxide layer between the film and the Ti layer was detected, and the oxide layer plays the role of a barrier so that it reduces diffusion at the interface. The thin film hydrothermally synthesized may have potential application for use in high-density memory devices
Fig. 6. Ferroelectric P-E hysteresis loop of a BaTiO, thin film, using a 60 Hz ac voltage. Scale: vertical=25 p,C/cm’/div.; horizontal = 57 kV/cm/div.
C.R. Cho et al. /Materials
because of the advantage of the low-temperature processing. The ferroelectric film synthesized by this method had a remanent polarization of 21.6 pC/cm’ and a coercive field of 26 kV/cm. The authors wish to thank Dr. E. Shi for useful discussion. This work was supported by the Korea Science and Engineering Foundation (KOSEF) through the Research Center for Dielectric and Advanced Matter Physics (RCDAMP) at Pusan National University.
References [ 11 M. Kitabatake and K. Wasa, Japan. J. Appl. (1985) 33. [2] L.H. Paker and Al F. Tasch, IEEE Circuits (1990) 17. [3] L.A. Willis, B.W. Wessels, D.S. Richelson Appl. Phys. Letters 60 (1992) 6. [4] P.C.V. Buskirk, R. Gokdiner, P.S. Kirlin and Res. 7 (1992) 542. [5] V.S. Dharmadhikari and W.W. Grannemann, (1982) 8988.
Pbys. Suppl. 22 Devices Mag. 6 and T.J. Marks, S. Nutt, J. Mater. J. Appl. Phys. 53
Letters 23 (1995) 203-207
201
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