Bi3.25La0.75Ti3O12 thin films on ultrathin Al2O3 buffered Si for ferroelectric memory application

(1)

Bi 3.25 La 0.75 Ti 3 O 12 thin films on ultrathin Al 2 O 3 buffered Si for ferroelectric

memory application

San-Yuan Chen, Chia-Liang Sun, Shi-Bai Chen, and Albert Chin

Citation: Applied Physics Letters 80, 3168 (2002); doi: 10.1063/1.1471937 View online: http://dx.doi.org/10.1063/1.1471937

View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/80/17?ver=pdfcov

Published by the AIP Publishing

Articles you may be interested in

Characteristics of Pt/Bi 3.25 La 0.75 Ti 3 O 12 / ZrO 2 / Si structures using ZrO 2 as buffer layers for ferroelectric-gate field-effect transistors

J. Vac. Sci. Technol. A 22, 1739 (2004); 10.1116/1.1759352

Physical characteristics and electrical properties of Sr 0.8 Bi 2+x Ta 2 O 9 films on Al 2 O 3 / Si annealed at high temperature

J. Appl. Phys. 94, 6735 (2003); 10.1063/1.1621716

Effect of annealing temperature on physical and electrical properties of Bi 3.25 La 0.75 Ti 3 O 12 thin films on Al 2 O 3 -buffered Si

Appl. Phys. Lett. 80, 1984 (2002); 10.1063/1.1459115

Fatigue-free samarium-modified bismuth titanate ( Bi 4x Sm x Ti 3 O 12 ) film capacitors having large spontaneous polarizations

Appl. Phys. Lett. 79, 3137 (2001); 10.1063/1.1415353

Bi 3.25 La 0.75 Ti 3 O 12 thin films prepared on Si (100) by metalorganic decomposition method

Appl. Phys. Lett. 78, 1733 (2001); 10.1063/1.1355012

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 140.113.38.11 On: Thu, 01 May 2014 06:22:28

(2)

Bi

_3.25

La

_0.75

Ti

₃

O

₁₂

thin films on ultrathin Al

₂

O

₃

buffered Si for ferroelectric

memory application

San-Yuan Chena)and Chia-Liang Sun

Department of Materials Science and Engineering, National Chiao Tung University, 300 Hsinchu, Taiwan, Republic of China

Shi-Bai Chen and Albert Chin

Department of Electronics Engineering, National Chiao Tung University, 300 Hsinchu, Taiwan, Republic of China

共Received 4 June 2001; accepted for publication 20 February 2002兲

We have investigated the physical and electrical properties of Bi3.25La0.75Ti3O12共BLT兲 thin films on Pt/Ti/SiO2/Si and on Al2O3(6 nm)/Si, which are used for one-transistor-one-capacitor and one-transistor ferroelectric memory, respectively. The BLT thin films on both substrates show good capacitance–voltage characteristics and the same threshold voltage shift of 1.6 V at applied⫾10 V bias. However, the leakage current of BLT on Al2O3/Si at⫺100 kV/cm is two orders of magnitude lower than that on Pt. The comparable memory characteristics and much reduced leakage current of BLT on Al2O3/Si are the strong advantages as compared with BLT on Pt because it is directly related to switching energy and device scaling down. © 2002 American Institute of Physics.

关DOI: 10.1063/1.1471937兴

Ferroelectric random access memory 共FRAM兲 has at-tracted much attention recently because of better speed per-formance than Flash memory and smaller size than static RAM.1The current cell structure of FRAM is one transistor and one capacitor 共1T-1C兲 type where the data are memo-rized in the metal/ferroelectric/metal共MFM兲 capacitor.2– 6To utilize the full advantage of FRAM, one transistor共1T兲 ferro-electric metal-oxide-semiconductor field effect transistor

共FeMOSFET兲-type memory is desirable because of the same

small 1T cell structure as Flash memory. Further, the ferro-electric effect induced voltage can be amplified by the tran-sistor in 1T MOSFET cell that gives higher sensitivity than 1T-1C cell. Unfortunately, the progress of 1T FeMOSFET memory is obstructed by the strong interface reaction be-tween most ferroelectric materials and Si that greatly de-grades the device characteristics.7,8Recently, we have devel-oped 1T FeMOSFET memory using Al2O3

9,10

as both gate dielectric for MOSFET and interface reaction barrier be-tween ferroelectric Pb(Zr,Ti)O3共PZT兲 and Si. Good transis-tor and memory characteristics are obtained simultaneously because of good gate dielectric integrity and interface prop-erty of Al2O3.9,10 In this letter, we have investigated the stacked gate dielectric of Bi3.25La0.75Ti3O12(BLT)/Al2O3 on Si because BLT is another promising ferroelectric material with excellent nonfatigue behavior on Pt electrode.11

Four-inch, p-type共100兲 Si wafers with ⬃10 ⍀ cm resis-tivity were used in this study. For the MFM structure, a 150-nm-thick Pt was first deposited on Ti(20 nm)/ SiO2(200 nm)/Si as the bottom electrode. For the FeMOS structure, a 6-nm-thick Al2O3gate dielectric was first formed on Si and the detailed formation procedure was reported previously.9,10 Then BLT was deposited on both

Pt/Ti/SiO2/Si and Al2O3/Si by chemical solution deposition using spin coating at 4000 rpm for 30 s.5,6For the chemical solution synthesis, bismuth acetate, lanthanum acetate hy-drate, and titanium n-butoxide were used as precursors and dissolved in the solvents composed of acetic acid, 2-methoxythanol, and glycerol in sequence. Excess 10% Bi precursor was added to compensate for the Bi loss during annealing. After each coating, the wet films were pyrolyzed for several minutes and the formed multilayer films were annealed at 650 °C for 1 h. Au was used as upper electrode for both capacitor structures and Al bottom electrode was used for FeMOS capacitor at the bake side of Si substrates. The total thickness of BLT is about 420 nm as measured by cross-sectional scanning electron microscopy 共SEM兲 images and the capacitor area is 9⫻10⫺4 cm2. The phase and sur-face microstructure of BLT films were observed by x-ray diffraction 共XRD兲 and SEM. Transmission electron micros-copy 共TEM兲 was used to investigate the interface character-istics of BLT film grown on Al2O3/Si. The electrical prop-erties were characterized by capacitance–voltage (C – V) and current density–voltage (J – V) by using Hewlett Packard 4284 and 4155B, respectively.

Figures 1共a兲 and 1共b兲 show the XRD patterns of BLT thin films deposited on Pt/Ti/SiO2/Si and Al2O3/Si, respec-tively. As shown in Fig. 1共a兲, BLT on Pt is polycrystalline without preferred orientation,12 even though the bottom Pt has preferred共111兲 orientation. In the case of BLT deposited on amorphous Al2O3shown in Fig. 1共b兲, the structure is also polycrystalline and the XRD pattern is almost the same as that on Pt. Thus, high quality ferroelectric BLT can be formed on both polycrystalline Pt and amorphous Al2O3.

a兲_{Electronic mail: [email protected]}

APPLIED PHYSICS LETTERS VOLUME 80, NUMBER 17 29 APRIL 2002

3168

0003-6951/2002/80(17)/3168/3/$19.00 © 2002 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

(3)

We have also examined the microstructures of BLT thin films deposited on Pt/Ti/SiO2/Si and Al2O3/Si by SEM, and the images are shown in Figs. 2共a兲 and 2共b兲, respectively. The grain sizes of BLT on Pt/Ti/SiO2/Si and Al2O3/Si are almost the same, about 50 nm, even though the structure of Pt substrate is polycrystalline that is quite different from amorphous Al2O3. The near same grain size of BLT on Pt/Ti/SiO2/Si or Al2O3/Si observed by SEM is consistent with the XRD analysis shown in Fig. 1, which suggests that BLT on both substrates may have close material quality.

Figure 3 shows the cross-sectional TEM picture of the BLT/Al2O3/Si structure and reveals only a slight interface layer on the top of Al2O3. One possible reason may be due to the fact that the atoms in BLT diffuse to Al2O3 gate di-electric. This result is in sharp contrast to the obvious thick intermediate amorphous layer reported by Choi et al.,13 which may be due to the difference in substrate temperature between pulsed ablation deposition and chemical solution deposition. Without the appearance of thick amorphous layer, the ferroelectric BLT film could directly come in contact with the buffer layer this indicates the better interface be-tween them since the memory performance of capacitors may be improved.

We have further studied the memory properties using high frequency共1 MHz兲 C – V measurement, and Figs. 4共a兲 and 4共b兲 show the C – V characteristics of BLT on Pt/Ti/SiO2/Si and Al2O3/Si, respectively. As shown in Fig. 4共a兲 the C – V characteristics of BLT on Pt have the typical butterfly pattern, which is similar to that of other ferroelec-tric materials.14After⫾10 V applied voltages, a peak voltage difference of 1.6 V is obtained that is due to the polarization effect of BLT. For BLT on Al2O3/Si, the hysteresis C – V

curves shown in Fig. 4共b兲 are related to the ferroelectric properties of BLT because Al₂O₃ gate dielectric has negli-gible hysteresis.9,10 The decrease of capacitance value at positive gate bias in the FeMOS structure is due to the small series capacitance in Si depletion region. The phenomenon is different from that in the MFM structure because of the neg-ligible depletion width in metal. A threshold voltage shift of 1.6 V is measured for BLT on Al2O3/Si that is the same as that in MFM capacitor. The same voltage shift suggests that the electric memory property of BLT on Al2O3/Si is at least the same or even better than that on Pt because of the addi-tional voltage drop in Al2O3gate dielectric and reduced elec-tric field in BLT.

FIG. 1. XRD patterns of BLT on共a兲 Pt/Ti/SiO2/Si and共b兲 Al2O3(6 nm)/Si.

FIG. 2. SEM images of BLT on共a兲 Pt/Ti/SiO2/Si and共b兲 Al2O3(6 nm)/Si.

FIG. 3. Cross-sectional TEM image of BLT on Al2O3(6 nm)/Si.

3169

Appl. Phys. Lett., Vol. 80, No. 17, 29 April 2002 Chenet al.

(4)

We have also investigated the capacitor leakage current because it is one of the most important properties for a memory capacitor. Figures 5共a兲 and 5共b兲 show J – V charac-teristics of BLT on Pt/Ti/SiO2/Si and on Al2O3/Si, respec-tively. The leakage current density of BLT on Pt is 1.8

⫻10⫺6 _A/cm2 _at_{⫺100 kV/cm that is comparable to that of}

共118兲-oriented Bi4Ti3O12,15 and the high dielectric break-down field of ⫺450 kV/cm is also comparable to that of

共001兲-oriented Bi4Ti3O12. These results indicate the good quality of BLT on Pt.15 On the other hand, the leakage cur-rent density of BLT on Al2O3/Si at ⫺100 kV/cm is 1.8

⫻10⫺8 _A/cm2 _{that is almost two orders of magnitude lower} than BLT on Pt. This low leakage current is comparable with that of vanadium-doped Bi4Ti3O12

16

and smaller than our previous data of PZT/Al2O3/Si FeMOS.

16 –17

In addition, no dielectric breakdown up to ⫺500 kV/cm also implies that BLT on Al2O3/Si has better dielectric integrity than that on Pt/Ti/SiO₂/Si. This smaller leakage current and the larger breakdown field may be due to the additional large energy barrier of Al₂O₃ dielectric9,10 and no intermediate phase in the interface. The much reduced leakage current of BLT on Al2O3/Si is the strong advantage as compared with BLT on Pt because it is directly related to the applied switching en-ergy and technology scaling down.

In conclusion, BLT films on Pt/Ti/SiO2/Si and Al2O3/Si substrates were investigated in this work. Both MFM and FeMOS capacitors show good C – V characteristics and have the same threshold voltage shift of 1.6 V at ⫾10 V applied voltages. The leakage current density of BLT FeMOS capaci-tors is about 1.8⫻10⫺8 A/cm2 at ⫺100 kV/cm that is two

orders of magnitude lower than that of MFM capacitors. The excellent memory characteristics and very low leakage cur-rent of BLT on Al2O3/Si are the merits for 1T ferroelectric memory as compared with 1T-1C cell.

This work has been supported by the National Science Council Contract Nos. NSC90-2215-E-009-061 and NSC89-2215-E-009-100 of Taiwan.

1_{J. F. Scott, Phys. World 8, 46}_共1995兲. 2

W. Wu, K. H. Wong, C. L. Mak, C. L. Choy, and Y. Z. Zhang, J. Appl. Phys. 88, 2068共2000兲.

3_{D. Bao, N. Mizutani, X. Yi, and L. Zhang, Appl. Phys. Lett. 77, 1041} 共2000兲.

4

S. H. Kim, D. J. Kim, J. P. Maria, A. I. Kingon, S. K. Streiffer, J. Kim, O. Auciello, and A. R. Krauss, Appl. Phys. Lett. 76, 496共2000兲.

5_{S. Y. Chen and V. C. Lee, J. Appl. Phys. 87, 8024}_共2000兲. 6_{S. Y. Chen and V. C. Lee, J. Appl. Phys. 87, 3050}_共2000兲.

7_{K. Sugibuchi, Y. Kurogi, and N. Endo, J. Appl. Phys. 46, 2877}_共1975兲. 8

Y. Shichi, S. Tanimoto, T. Goto, K. Kuroiwa, and Y. Tarui, Jpn. J. Appl. Phys., Part 1 33, 5172共1994兲.

9_{A. Chin, Y. H. Wu, S. B. Chen, C. C. Liao, and W. J. Chen, Symposium on} Very Large Scale Integrated Technology, Honolulu, HI, 2000, p. 16. 10

A. Chin, C. C. Liao, C. H. Lu, W. J. Chen, and C. Tsai, Symposium on Very Large Scale Integrated Technology, Kyoto, Japan, 1999, p. 135. 11_{B. H. Park, B. S. Kang, S. D. Du, T. W. Noh, J. Lee, and W. Jo, Nature}

共London兲 401, 682 共1999兲.

12_{Y. Hou, X. H. Wu, H. Wang, M. Wang, and S. X. Shang, Appl. Phys. Lett.} 78, 1733共2001兲.

13_{T. Choi, Y. S. Kim, C. W. Yang, and J. Lee, Appl. Phys. Lett. 79, 1516} 共2001兲.

14_{M. Sedlar and M. Sayer, J. Appl. Phys. 80, 372}_共1996兲.

15_{T. Watanabe, H. Funakubo, and K. Saito, J. Mater. Res. 16, 303}_共2001兲. 16

Y. Noguchi and M. Miyayama, Appl. Phys. Lett. 78, 1903共2001兲.

17_{A. Chin, M. Y. Yang, C. L. Sun, and S. Y. Chen, IEEE Electron Device}

Lett. 22, 336共2001兲.

FIG. 5. J – V characteristics of BLT on 共a兲 Pt/Ti/SiO2/Si and 共b兲

Al2O3(6 nm)/Si.

FIG. 4. C – V characteristics of BLT on 共a兲 Pt/Ti/SiO2/Si and 共b兲

Al2O3(6 nm)/Si.

3170 Appl. Phys. Lett., Vol. 80, No. 17, 29 April 2002 Chenet al.