Ferroelectric BaTiO3 films with a high-magnitude dielectric constant grown on p-Si by low-pressure metalorganic chemical vapor deposition

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Applied Surface Science 90 (1995) 75-80

Ferroelectric BaTiO 3 films with a high-magnitude dielectric constant grown on p-Si by low-pressure metalorganic chemical vapor deposition T.W. Kim

a, *,

Y.S. Yoon b, S.S. Yom b, C.O. Kim c

a Department of Physics, Kwangwoon University, 447-1 Wolgye-dong, Nowon-ku, Seou1139-701, South Korea b Applied Physics Laboratory, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul, South Korea c Department of Physics, Hanyang University, Seou1133-791, South Korea

Received 13 January 1995; accepted for publication 20 April 1995

Abstract

Metalorganic chemical vapor deposition of BaTiO3 using Ba(tmhd)2, Ti(OC3H7)4, and N20 via pyrolysis at relatively low temperature ( ~ 600°C) was performed in order to produce a BaTiO3 insulator gate with a high-quality BaTiO3/p-Si interface and with a dielectric constant of high magnitude. X-ray diffraction and transmission electron microscopy results showed that the as-grown BaTiO3 films on p-Si substrates were polycrystalline. The stoichiometry of the BaTiO3 films was observed by Auger electron spectroscopy. Room-temperature current-voltage and capacitance-voltage measurements clearly revealed a metal-insulator-semiconductor behavior for the samples with a BaTiO3 insulator gate, the interface state densities at the BaTiOa/p-Si were approximately in the low 1011 eV-1 cm-2 at the middle of the Si energy gap, and the dielectric constant determined from the 1 MHz C - V profile was as large as 200. These results indicate that the BaTiO3 layers grown by MOCVD can be used for high-density memories.

1. Introduction

Recently, the growth of insulator films on Si has been particularly attractive due to their many promising applications [1-5]. Many groups have investigated SiO 2 [6,7], Si3N 4 [8,9], AI20 3 [10-12], PbTiO 3 [13,14], and BaTiO 3 [15,16] as possible deposited insulator layers for Si metal-insulator-semiconductor (MIS) applications. Among the various insulator films, BaTiO 3 is attractive due to its pervskite-type ferroelectric characteristics such as its electro-optic properties, which have device applications [17,18]. * Corresponding author.

Some groups reported the epitaxial growth of BaTiO 3 thin fdms using the deposition techniques of molecular beam epitaxy [19], laser ablation [20], and metal organic chemical vapor deposition (MOCVD) [2124]. However, to the best of our knowledge, the possibility for applications of BaTiO 3 as an insulator gate with both a high-quality BaTiO3/p-Si interface and a high-magnitude dielectric constant has still not been investigated. This paper reports the growth of BaTiO 3 thin films on p-Si(100) substrates for temperature between 300 and 800°C using MOCVD. X-ray diffraction and transmission electron microscopy (TEM) was carried out in order to investigate the atomic

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76

T. W. Kim et aL /Applied Surface Science 90 (1995) 75-80

structure of the BaTiO3/P-Si, and Auger electron spectroscopy (AES) was performed to characterize the stoichiometry of the grown films. Room-temperature current-voltage (I-V) and capacitance-voltage (C-V) measurements were performed to investigate the possibility of MIS behavior for Ag/BaTiO3/p-Si diodes and to determine the interface state densities at the BaTiO3/p-Si as well as the dielectric constants of the BaTiO 3 thin films.

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(100)

3 10

210

310

410

i

50

70

80

Fig. 1. X-ray diffraction pattern of a BaTiO 3 film grown at 600°C on p-Si.

microscopy results appeared rounded in shape, indicating formation of granular polycrystalline BaTiO 3 films. Ellipsometric measurements showed that the thickness of the BaTiO 3 films was approximately 2000 ,~ and that the refractive index was 2.2 which was in good agreement with the value for BaTiO 3 films deposited by the sputtering methodo[25]. Thus, the typical growth rate was about a few A / s . The result of the XRD measurement for a BaTiO 3 film deposited at 600°C on Si(100) is shown in Fig. 1. The peaks at 31.25 ° and 66.1 ° correspond to the (110) and (220) BaTiO 3 planes, respectively, and by unfiltered CuK/3, the peak at 61.9 ° is attributed to (100) Si. The peaks corresponding to the BaTiO 3 planes show a highly oriented film growth with the [110] BaTiO 3 direction normal to the Si(100). The

3. Results and discussions

t BaTIOs / pSi

A

The BaTiO 3 films as-grown by MOCVD had mirror-like surfaces without any indication of pinholes, which was confirmed by Normarski optical microscopy. Although the growth of the BaTiO 3 films had been performed in the temperature range between 300 and 800°C, only the physical properties of the films grown at 600°C are repotted because they had the best surface morphology among the several samples grown at relatively low temperatures. The SEM results for the BaTiOa films indicated a very smooth and dense surface morphology. The morphology determined from the atomic force

60

20 (DEGREE)

2. Experimental details The fl-diketonate complex of Ba(tmhd) 2 (tmhd = 2,2,6,6-tetramethyl-3,5-heptanedionate), titanium iso-propoxide (TIP), and argon were used as organic sources and a carrier gas, respectively. The temperatures of the oil baths for the Ba(tmhd) 2 and TIP were maintained at 250 and 15°C, respectively. The flow rates of the argon carder gas for the Ba(tmhd) 2 and TIP were 30 and 5 sccm, respectively. Heating tape was wrapped around the organometallic-source vapor-transport lines at 250°C to prevent condensation from the source bath to the growth chamber. As soon as the chemical process was finished, the Si substrates were mounted onto a molybdenum susceptor. N20 gas was injected into the chamber at a system pressure of 1 Torr. The typical deposition was carded out for 60 min and was followed by a slow cooling to room temperature at a rate of 100°C/h in an oxygen atmosphere to prevent strain-induced microcracks [15].

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B I T I O s (110)

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i

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I

200

400

600

800

1000

KINETIC ENERGY { eV ) Fig. 2. Auger electron spectroscopy spectrum of a BaTiO a / p - S i structure grown at 600°C. The spectrum was obtained at a 0.02 /.~m depth.

T. W. Kim et al. /Applied Surface Science 90 (1995) 75-80

full-width at half-maximum (FWHM) of the BaTiO 3 (110) peak was approximately 0.23 °. This value is a little smaller than the 0.3 ° for BaTiO 3 epitaxially grown on SrTiO 3 [19]. However, it is impossible to explain unambiguously from the XRD measurements whether as-grown BaTiO 3 films form epitaxial films or textured grains. The composition of the BaTiO 3 thin layer Was investigated by AES measurements. The results of the AES showed that the film consisted of lead, titanium, oxygen, and carbon at the surface of BaTiO z as shown in Fig. 2. The existence of the carbon

77

impurities could be due to contamination from the source materials or from the growth chamber at the final stage of the film growth. The existence of the carbon impurity was also supported by X-ray photoelectron spectroscopy. There is no residual carbon above a 0.05 /.tm depth. The ratios of the peak-topeak intensities among the BaKLL, TiKLL, and O KLL peaks of the BaTiO 3 films were similar to those obtained by Wills et al. [21]. The detailed results of the AES measurements were reported previously [15]. TEM measurements were performed to investi-

Fig. 3. High-resolution transmission electron microscopy image of the BaTiO3/P-Si structure grown at 600°C.

78

T. W. Kim et al. /Applied Surface Science 90 (1995) 75-80

Fig. 4. Electron diffractionpatterns from transmissionelectron microscopyof the BaTiOa/p-Si structuregrown at 600°C.

gate the crystallinity of the BaTiO 3 grown on p-Si. The high-resolution TEM image for the BaTiOa/p-Si in Fig. 3 shows an interracial layer between the top BaTiO 3 layer and the bottom Si substrate. The electron diffraction patterns taken from a plan view of the sample are shown in Fig. 4. The strong and weak spots in the pattern indicate the Si substrate and the BaTiO 3 polycrystalline layer, respectively. The randomly distributed spots and the diffusion of the ring corresponding to the (110) peaks originate from the BaTiO 3 polycrystalline and the interfacial amorphous layers. In addition to the X-ray, AES, and TEM measurements, I - V and C - V measurements at room temperature were carried out to characterize the electrical properties of Au/BaTiO3/P-Si. Ohmic contacts were fabricated by gold evaporation on the front side and by indium soldering on the back side of the samples. T h e I - V characteristics for A u / B a T i O a / S i are shown in Fig. 5. The DC measurements were performed using an HP4141B picoammeter with a ramp rate of 0.05 V / s . At low voltages, an exponential increase of current with voltage was observed and was followed by a saturation region at higher voltages, The results of the I - V characteristics for both

forward and reverse biases showed similar behavior, except that the magnitude of the current under reverse bias was lower by one order of magnitude than that under forward bias, The I - V behavior is similar to those of a A u / S i 3 N 4 / S i diode [26] and an

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APPLIED VOLTAGE

( Volt )

Fig. 5. Gucrent-vo|tage characteristics for a Au/BaTiO 3/p-Si

diode.

T.W. Kim et al. /Applied Surface Science 90 (1995) 75-80

A1/Ta2Os/SiO2/Si capacitor [27]. The mechanism for the I - V behavior may be due to field-enhanced thermal excitation of trapped electrons into the conduction band. The expression for trap states with a Coulomb potential is virtually identical to that of Schottky emission [26]. The results from the C - V measurements for various frequencies are shown in Fig. 6, and this behavior is similar to those from the C - V measurements of an ordinarily prepared AI/SiO2/Si diode which have the states of the accumulation, depletion, and inversion with the applied bias [26]. The mechanism of the C - V measurements for various frequencies is described in detail in the literature [26]. The minimum values of the capacitances measured at low frequencies are smaller than that obtained at 1 MHz. Even though the capacitance goes through a minimum and then increases again as the inversion layer of electrons forms at the surface, the minimum capacitance behavior with different applied frequency might originate from the interface-trapped charge. The thickness of the BaTiO 3 gate insulator determined from the ellipsometic measurements was about 2000 A, and the diameter of the top electrode used for the C - V measurements was 0.5 mm. The results for the dielectric constant as a function of frequency for the Ag/BaTiO3/P-Si diode are shown in Fig. 7. Fig. 7 shows that the dielectric constant increases as

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the applied frequency decreases. The dielectric constant of ferroelectric BaTiO 3 oxide can be given by e = 8 ' - ie",

(1)

where e' is the real part and 6" is the imaginary part. The Debye equations give the following equations: e ' = 8= + (e0 - eo~)/(1 + a~2r2), e " = ( e 0 - E=) ~o~-/(1 + oJ2~'2),

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-5

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( Volt )

Fig. 6. Capacitance-voltage curves of a A u / B a T i O 3 / p - S i diode for various frequencies.

where e0 is the static dielectric constant, e= is the dielectric constant at high frequency, r is the relaxation time, and to is the applied frequency. Eqs. (1) and (2) show that the dielectric constant increases with the decrease of the applied frequency. Therefore, the frequency dependence of the dielectric constant for the BaTiO 3 thin film in Fig. 7 is in good agreement with the above equation. The dielectric constant determined from the 1 MHz C - V measurements was approximately 200. This value is almost the same as that of a polycrystalline BaTiO 3 film grown by the sputtering method [28]. The interface state density at the BaTiO3/Si interface determined by frequency-dependent C - V measurements and by the Terman method [29] was approximately in the high 1011 eV cm -2 at the middle of the Si energy gap.

80

T.W. Kim et al. /Applied Surface Science 90 (1995) 75-80

4. Summary and conclusions X-ray and TEM measurements showed that a BaTiO 3 polycrystalline film was grown by low-pressure MOCVD and that an interracial layer was formed between the BaTiO 3 and the Si. The result of the I - V and C - V measurements at room temperature clearly demonstrate MIS behavior for the A u / BaTiO3/p-Si substrates. The dielectric constant determined from the 1 MHz C - V measurements was about 200, and the interface state density determined by the C - V characteristic and by the Terman method was approximately in the high 1011 eV cm -2 at the middle of the Si energy gap. These results suggest that a BaTiO 3 polycrystalline layer grown by MOCVD on p-Si not only is good enough for highdensity dynamic-memory cells but also provides a good motivation for the fabrication of MIS field-effect transistors with BaTiO 3 insulator gates.

Acknowledgements This work was supported by the Basic Science Research Institute Program, the Korea Ministry of Education, in 1995.

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