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Properties of polycrystalline SrRuO3 thin films on Si substrates
h4aterids Research Buhtin, Vol. 32, No. 1. pp. 83-96.1997 Copyright 0 1996 Elswier Science Ltd Printedin tbe USA. All rights -cd 0025-5408/W $17.00 +.OO
Pergamon
PI1 SOO2~5408(96)00165-1
PR.OPERTIES OF POLYCRYSTALLINE SrRuOl THIN FILMS ON Si SUBSTRATES
Koji Watanabe*+, Miho Ami, and Masahiro Tanaka+ Sony Corporation Research Center, 174, Fujitsuka-cho, Hodogaya-ku, Yokohama-shi, Japan (Refereed) (Received July 15, 1996; Accepted July 16, 1996)
ABSTRACT The perovskite SrRu03 thin films were deposited on Si( 100) substrates using pulsed laser deposition. These thin films had columnar structures and exhibited good surface morphology and good barrier characteristics for Si. The films exhibited metallic conductivities, with a room temperature resistivity of 529 @cm. These thin films can be useful in many applications, such as electrodes for ferroelectric and electro-optic devices. KEYWORDS: A. oxides, A. thin films, B. laser deposition, electrical properties
D. diffusion,
D.
INTRODUCTION It is widely recognized that conductive metallic oxides are useful in many applications (l), such as ferroelectric devices and electro-optic devices. In the case of ferroelectric devices, F’t electrodes are usually used to maintain stability in an oxidized atmosphere at the high temperature ranges. But it is difficult to etch Pt using a dry process. This is a disadvantage in using Pt to achieve high density devices (2). Moreover, the problem of ferroelectric fatigue, which is loss of switchable polarization on repeated polarization reversal, is very serious for ferroelectric capacitors using metal electrodes.
*To whom correspondence should be addressed. +Present address: Yagi Microdevices Laboratory,
Research Center, Sony Corporation
Technology Center 4-14-1, Asahi-cho, Atugi-shi, Kanagawa-ken, Japan. 83
Atsugi
84
K. WATANABE
et al.
Vol. 32, No. 1
Several models for ferroelectric fatigue have been proposed. Scott et al. (3) reported that a region of low oxygen concentration, which exists at the ferroelectrics/metal interface, causes ferroelectric fatigue. Therefore, it is believed that conductive oxide electrodes make up for oxygen vacancies and improve ferroelectric fatigue. Several reports (45) indicate that use of conductive oxide electrodes improves ferroelectric fatigue. We believe that conductive oxide electrodes will be a key technology of memory devices. In previous papers (6,7), we have reported a study of RUOZ electrodes. SrRuO3 has the perovskite structure of which the lattice parameters are a = 0.553 nm, b = 0.557 nm, and c = 0.785 nm (8). Most ferroelectric materials have perovskite or perovskitelike structures. The similarity between the crystal structure of the ferroelectric materials and that of the conductive oxide electrodes will be an advantage in growing film. In fact, some reports (9,lO) show that SrRuOs electrodes are useful for preparing epitaxial (Ba, Sr)TiOa films. SrRuOx exhibits isotropic electric properties and the room temperature resistivity of single crystal is reported to be 280 p&&cm (8). The temperature dependencies of the resistivity show good metallic behavior, and a kink at about 160 K is observed in the resistivity curve, which corresponds to the Curie point. Pulsed laser deposition (PLD) (11) is a method used to fabricate thin films, in which atoms, molecules, and ions flown from the target by injection of pulsed laser are deposited on the substrate. Various thin films can be formed under many kinds of gases and a wide range of ambient pressure using simple vacuum hardware and laser devices. As laser pulse is injected on targets only from the outside of the vacuum chamber, no impurities exist in the films obtained. In this study, we used PLD to deposit SrRuOs thin films on Si(100). Although there have been reports about SrRu03 epitaxial films deposited on oxide single crystals (12,13), no report about SrRuOs thin films deposited on Si(100) substrates has previously been presented. The crystallinity, surface morphology, and conductivity of SrRuOs thin films, as well as the interdiffusion between a Si and a SrRuOx layer, were investigated in relation with the deposition conditions. And the film deposition conditions where SrRuOs thin films exhibited good metallic conductivities were examined. EXPERIMENTAL
PROCEDURE
SrRuO3 targets for thin film deposition were made from SrCO3 and RUOZ powders. These powders were mixed in the stoichiometric ratio and ground for 24 h. The mixture was then heated in air at 1200°C for 12 h. After a second grinding, the powder was pressed with a load of 1 ton/cm* to make disks 25 mm in diameter. The disks were heated in air at 1300°C for 24 h to obtain ceramic targets for PLD. SrRuO, thin films were deposited on Si(100) substrates in the form of 25 mm x 25 mm squares, using PLD. Laser pulse (248 nm) from a KrF excimer laser (LAMBDA PHYSIK LPXlOO) was focused on the target through the cylindrical lenses. The target was in a target holder which could be rotated, and the target was always rotated during an ablation. The average laser energy density was 2 J/cm* and repetition rates were 5-10 Hz. Gas pressures during deposition were controlled by Baratron sensors (MKS Co.). The oxygen pressure was changed in a range from 20 to 200 mTorr. The substrate temperature was measured from a thermocouple inserted into a hole of the heater block. The deposition temperatures were varied from 500 to 8OO’C. Structure analysis of the thin films was performed with X-ray diffraction measurements. Cu-Kol radiation using 28 scan mode with an RADIII-B system
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(Rigaku Co.) was used to obtain the diffraction data of thin films and 9-28 scan mode was used for ceramic targets. The surface morphology and grain sttucture of the thin films were observed with an atomic force microscope (AFM; Park Science Instruments, Autoprobe) and a scanning electron microscope (SEM; S-4000, Hitachi Co.). The root mean square surface roughness (RMS), maximum roughness (Rmax), and average grain size were estimated by AFM topo images over an area of 2 x 2 pm’. The film thickness was determined1 from the cross-sectional SEM images. Secondary ion mass spectroscopy (SIMS; CAMECA,, IMS-4F) was performed in order to examine the interdiffision of Si and SrRuO3 in the boundary region between the film and Si surface. The electrical resistance of the SrRuOs films and ceramic targets was measured using the conventional four-point probe method with a current source (Keithley 224 model), a digital multimeter (Keithley 196 model), and a cryo-system (CG308; Daikm Co.). Gold electrode stripes between 1 and 2 mm wide and Z!OOnm thick were evaporated onto 25 mm x 5 mm thin film samples and onto ceramic target samples cut from the disks in the form of 25 mm x 5 mm x 1 mm. RESULTS AND DISCUSSION Film Fabrication. Figure 1 shows the SEM photograph of the surface of the SrRuOa ceramic targets. From XRD data, impurity phases were not observed in SrRu03 ceramic targets. It is known that laser ablation of low-density pressed targets result in the ejection of large numbers of macroscopic particles from the surface of the target. The irradiation of the target surface by a pulsed laser results in rapid heating of the surface layer, in which the energy absorbed is not diffised from the surface region but is used mostly to excite the atoms (14). If there are many defects in the surface region, they interfere with the rapid and homogeneous heating of the surface layer. From these points of view, a higher surface density and defects smaller in size than a laser spot on the surface layer are thought to be desirable fix an ablation target. The averaged density of our target was 86.83% of theoretical density, but the surface density, the most important part as the ablation target, calculated from Figure 1 was 97.8%. The defect size (-5 pm’) estimated from Figure 1 was much smaller thtm the area of a laser spot (-10 mm2). Therefore, it may be concluded that the
FIG. 1 SEM photograph for the surface of the SrRuOp ceramic target.
K. WATANABE et al.
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surface layer of our SrRuO3 ceramic target can be heated rapidly and homogeneously by a pulsed laser. In addition to the points mentioned above, the sample showed good metallic conductivity, with a resistivity of 1.2 mQ*cm at 300 K. The observed resistivity is greater than that of a single crystal SrRu03 reported by Bouchard et al. (8), but lower than that of a sintered SrRu03 reported by Fukunaga and Tsuda (15). It may be concluded that the resistivity of a polycrystalline ceramic which includes grain boundaries is greater than that of a single crystal which does not include grain boundaries. This difference in the resistivities can be explained by the packaging density (15). The temperature dependences of the three samples are quite similar. A resistance ratio ~(300 K)/p(20 K) of our SrRuO3 ceramic targets is about 10 and that value is equal to that of single crystal. Electrical properties are thought to be more sensitive than XRD measurements to impurities. A resistance ratio becomes the measure of the amount of impurities when the contribution of grain boundary scattering effects is eliminated. So, our SrRuO, ceramic target contained as small an amount of scattering centers, impurities, and grain boundaries as the single crystal SrRuO3 reported by Bouchard et al. It therefore can be said that the SrRuO3 ceramic targets we obtained have good quality for PLD. Figure 2 shows the XRD patterns for the films, the deposition temperature of which was varied from 500 to 800°C. At the substrate temperature of 500°C, the crystal growth of SrRu03 had already started: The XRD pattern of a film deposited at 500°C was clearly fitted by using crystal parameters of SrRuO3 in ASTM cards (43-472). Miller indexes for SrRuO3 are also shown in Figure 2. Above 600°C, both the intensity and the sharpness of the XRD peaks corresponding to SrRuO, have increased, and the impurity peak is distinguishable. Figure 3 shows the AFM topo images of films deposited at 500, 600, 700, and 8OO’C. Root mean squares (RMS) roughness, maximum of roughness (Rmax), and average grain size are estimated using AFM data shown in Table 1. These data show that as the deposition temperature increases, the crystallinity and the average grain size of the films increase. This result coincides with the result from the XRD pattern measurements.
10
20
30
40 20
XRD patterns 800°C.
50
60
70
(‘1
FIG. 2 for the SrRuOs films whose deposition
temperature
was 500, 600, 700, and
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(b) FIG. 3 AFM topo images of the SrRuOs films deposited at (a) 500, (b) 600, (c) 700, and (d) 800°C.
K. WATANABE et al.
id) FIG. 3
(Continued).
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TABLE
1
Root Mean Squares (RMS) Roughness, Maximum of Roughness Grain Size Estimated Using AFM Data
-
89
(Rmax), and Average Average
Deposition temperature (“C)
RMS
(nm)
(a@
grain size (n@
800
15
111
200
700
6
42
90
600
4
30
70
500
4
37
40
Figure 4 shows the cross-sectional SEM photograph for the film, deposited at 800°C. It is seen that the film has the columnar structure. The average grain size of the film corresponds with the width of the columns. Column width estimated from Figure 4 was 170 nm and that value is not far from the value of the average grain size (200 nm) estimated using AFM data. Figure 5 shows the SIMS depth profiles for the SrRuOJSi films deposited at 600, 700, and 800°C. The interdiffision of elements at the SrRuOJSi interface was slightly larger in the films deposited at 800°C than in the other flhns. It is seen in Figure 5 that the slopes of the Sr and Ru profiles at the SrRuOJSi interface do not differ much at various deposition temperatures, but the Si on the SrRuOdSi interface at 800°C becomes more diffusive than those at lower temperatures. This means that Sr and Ru are difficult to diffise into Si, but Si can diffise into SrRuOs films. The mixing layer thickness at the SrRuOJSi interface at SOOT was estimated to be about 20 mu at most when the SrRuOJSi interface at 600°C was considered as a reference profile. From XRD data, it can be said that no silicides are formed at the SrRuWSi interface. It can be seen that most of the diffised Si atoms are segregated at
Cross-sectional
SEM photograph
FIG. 4 for the SrRu03 films deposited at SOOT.
K. WATANABE
90
et al.
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Si
‘“0”
L--_ ......___..r__r_ii_LO~~_.._..C~-.~--_(_-__.~.__-^_dlis
.
’
0.10
DEPTH
*
.
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0
20
(microns)
----
___I
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-.a...
. ..._L_
_A...._.
-,~_--r_-L___-1_-1_~_-._~-*---~0.05 0.10
0.1s
DEPTH hT&YOnS)
FIG. 5 SIMS depth profiles for the SrRuOJSi films deposited at (a) 600, (b) 700, and (c) 800°C.
Vol. 32, No. 1
STRON’I-IUh4 RUTIIRNATE FILMS
--
10~
91
-_-~-_-__ W
Si
‘S
01
*,;-..A.__.-
_..a.
.-L-L.
.-4-r._~_-._L._L-,-
0.05
.
0.10
DEPTH
0.1s
.
.
A-
C
20
(microns)
FIG. 5 Continued.
grain boundaries of SrRuOs columnar particles. The AFM data, which show the grain size and the maximum roughness of the films deposited at 800°C are larger than those at lower deposition temperature, support our speculation about Si diffusion: the films deposited at 800°C include the appropriate spacing among the grains where Si atoms can easily diffise. The depth profiles also show that there is a deficiency of oxygen at the surface of the film deposited at 800 ‘C. The diffused elements, mainly Si, and deficiency of oxygen at the surface can affect film resistivity (we discuss this point later). Given that the SrRuOp films on Si or SiOdSi wafers were grown at temperatures below 700°C, the interdiffusion of elements between the film and the wafer was not serious: the crystal growth of SrRuOj started at 500°C, and the crystallinity became better and the grain size greater as the deposition temperature increased. Electrical Properties. Figure 6 shows the temperature dependence of the electrical resistivities and of the electrical resistivities normalized at the values of 300 K for the SrRuO3 flhns deposited at 550,600,650,700, and 800°C. The film thickness of each sample was about 100-130 nm and the oxygen pressure during deposition was 100 mTorr. Above 6OO”C, the films show metallic conductivities, because the films become single phase of SrRuO,, as shown in Figure 2. The resistivity of the film decreases as the deposition temperature increases, and the resistivity of the film deposited at 800°C becomes 529 @*cm. Moreover, the slope of the resistivity-temperature plot becomes larger as the deposition temperature increases. Krusin-Elbaum (16) reported that the residual resistivity
92
Vol. 32, No. 1
K. WATANABE et al.
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100 150 200 250 Temperature(K)
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350
FIG. 6 Temperature dependence of the electrical resistivities (a) and of the electrical resistivities normalized at the values of300 K (b), for the SrRuOs films deposited at 550,600,650,700, and 800°C. ratio (RRR), defined as ~(300 K)/p(4.2 K), of the RuOz films increased with the average grain size, implying that the electron mean free path increased with fewer grain boundaries. The vertical axis of Figure 6b corresponds to the inverse of the RRR, indicating that the RRR of our SrRuO, films increases with deposition temperature. From the XRD patterns and the topo images from an AFM, it is known that grain size increased with increasing deposition temperature. This result agrees with the result for the RuO2 films reported by Krusin-Elbaum (16). Therefore, it can be seen that the effect of the grain boundary scattering decreases as the grain size increases and that the SrRu03 films show good metallic conductivities. Above 600°C, the films showed metallic conductivities and the resistivity of the film decreased as the grain size increased. The size effect and the thickness limit of films, at which the same properties are maintained as bulk materials, not only are interesting from the viewpoint of materials science, but also are important properties in electrical device applications. Figure 7a shows the room temperature electrical resistivities of films deposited at 700 and 800°C of substrate temperatures under 100 mTorr of oxygen, as the function of film thickness. Figure 7b shows the temperature dependence of the electrical resistivities of the same films deposited at 700°C, shown in Figure 7a, and Figure 7c shows the temperature dependence of the electrical resistivities of films deposited at SOO”C, also shown in Figure 7a. These figures
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STRONTIUM RUTHENATE FILMS
thickness(nm)
0
50
100
150
200
250
300
350
Temperature(K)
-0
50
100 150 200 250 Temperature(K)
300
350
FIG. 7
(a) Room. temperature electrical resistivities of the SrRuOp films deposited at 700°C (circles) and 8OO’C (squares) of substrate temperatures under 100 mTorr of oxygen, as the function of the film thickness. (b) Temperature dependence of the electrical resistivities of the same films deposited at 7OO”C, shown in (a), the film thickness of which was 40 nm (triangles), 60 nm (circles), 90 nm (rhombuses), and 120 nm (squares).‘(c) Temperature dependence of the electrical resistivities of the same films deposited at 8OO”C, shown in (a), the film thickness of which was 40 run (circles), 70 nm (squares), and 130 nm (triangles).
94
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Vol. 32, No. 1
Temperature(K)
FIG. 8 Temperature dependence of the electrical resistivities of the SrRuOx films whose deposition temperature was 65O’C and oxygen pressure was 20 mTorr (triangles), 100 mTorr (circles), and 200 mTorr (squares). clearly show that the resistivities markedly increase as film thickness decreases and that the 40 nm thick film shows no metallic conductivity. The high resistivity of the film can be attributed to the poor crystallinity of SrRuOx in the early stage of deposition (17). Furthermore, these figures show that each series of films deposited at 700°C and 800 “C has the characteristic p vs. T curves as the film thickness increases. When the film is thicker than a critical value, the temperature dependence of the film resistivity is dominantly determined by deposition temperature. The very thin (less than about 100 nm) films deposited at 8OO’C have a higher resistivity and smaller RRR value than those at 700°C. This phenomena can be thought to be caused by (1) Si atoms diffusing into the grain boundary of the films, which affects the electrical properties of the films and/or (2) a deficiency of oxygen at the surface, which affects the electrical properties of the films (18). The mixing layer thickness at the SrRuOJ/Si interface at SOO”C, which was estimated to be about 20 nm, seems reasonable, because the 70-nm-thick tihn deposited at 800°C has the pT characteristic which is somewhere between that of 40-nm-thick film and 60-nm-thick film deposited at 7OO’C. From our experimental point of view, it can be said that films thicker than 50 mn are needed to maintain metallic conductivities in the case of our SrRuOx films and that the diffused Si and the oxygen deficiency of films at the surface negatively affect the electrical properties, especially in the thinner region. Figure 8 shows the temperature dependence of the electrical resistivities of the SrRuOJ films for the deposition temperature of 650°C, with oxygen pressure varied from 20 to 200 mTorr. The film thickness of these samples was about 100 mn. To eliminate the influence of interdiffusion between Si and SrRuOa films completely, the deposition temperature of 650°C was chosen. Though the SrRuOp films deposited at 100 mTorr show good metallic conductivities, the SrRuOs films deposited at 20 and 200 mTorr do not show metallic conductivities. The XRD patterns show that the peak shifts of these films were larger than those of films deposited at 100 mTorr, where XRD patterns for the ceramic ablation target was used as a standard. Table 2 shows the diffraction angles of d(O04) peaks and the peak shifts. It is obvious that the peak shifts of the films deposited at 20 and 200 mTorr are much larger than those of the good conductive films deposited at 100 mTorr. Moreover, these peak shifts occur in the direction of lengthening c axes. The causes of lengthening c axes are
Vol. 32, No. 1
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Diffraction
95
TABLE 2 Angles of d(004) Peaks and the Peak Shifts d(film) d(target) run
d(004) am 0.19585
Conductivitv metallic
650 “C, 100 mTorr
0.19569
metallic
650 “C, 20 mTorr
0.19738
not metallic
0.00153
650 “C, 200 mTorr
0.19844
not metallic
0.00259
Samples Ceramic target
-0.00016
thought to be: (1) the lack of 02 ions between positive ions, which causes the Coulomb repulsion, and (2) the substitution of Sr ions in Ru-ion sites. The point here is that good metallic conductivities of the SrRuO3 films decline when the film composition shifts from the stoichiometric composition. To obtain good metallic conductive SrRuOJ films, we have to control the deposition temperature, the process oxygen pressure, and the resultant film thickness; the best properties are achieved by realizing the ideal stoichiometry of films. CONCLUSION Good crystalline and good conductive SrRuOs thin films have been successfully grown on silicon substrates using pulsed laser deposition. Metallic conductive films were obtained when the deposition temperature was greater than 6OO’C and the Iihn thickness was greater than 50 nm. It was found that the diffised Si and the deficiency of oxygen at the surface affected the electrical properties of the films, but these affects could be eliminated when the films were deposited below 700°C or/and were thicker than the critical values. The stoichiomletry of the film is essential to obtaining good metallic conductive SrRuOs films. The resistivity (300 K) of SrRuOs film deposited at 8OO’C was 529 #J-cm. ACKNOWLEDGMENTS We would like to thank Ms. Minatoya for her kind assistance in performing the AFM measurements, Mr. Nomachi for SIMS measurements, and Mr. Takasu and Mr. Yamauchi for SEM measurements. And we would like to thank Mr. Miyazawa for the design of our machine and Mr. Isobe for his kind assistance in starting up our machine. REFERENCES 1. L. Knrsin-Elbaum and M. Winmer, J. Electrochem. Sot. 135( lo), 2610 (1988). 2. K. Takemura, T. Sakuma and Y. Miyasakq Appl. P&s. Lett. 64(22), 135 (1992). 3. J. F. Scott, C.A. Araujo, B.M. Melnick, L.D. McMillaa aad R. Zuleeg, J. Appl. P&s. 70(l), 382 (1991). 4. D.P. Vijay and S.B. Desu, J. Electrochem. Sot. 140(9), 2640 (1993). 5. T. Nakamura, Y. Nakao, A. Kamisawa and H. Takasu, Jpn. J. Appl. P&s. 33(pt. 1, 9B), 5207 (1994). 6. J.F. Tressler, K. Watanabe aad M. Tanaka, J. Am. Cerum. Sot. 79(2), 525 (1996).
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et al.
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7. K. Watanabe, J.F. Tressler, M. Sadamoto, C. Isobe and M. Tanaka, < Electrochem. Sot. 143, 3008 (1996). 8. R.J. Bouchard and J.L. Gillson, Mater. Res. Bull. 7,873 (1972). 9. Q.X. Jia, X.D. Wu, S.R. Foltyn and P. Tiwari, Appf. Phys. Lett. 66 (17), 2197 (1995). 10. S.Y. Hou, J. Kwo, R.K. Watts, J.-Y. Cheng and D.K. Fork, Appl. Phys. Lett. 67(10), 1387 (1995). Il. D.B. Chrisey and G.K. Hubler, Pulsed Laser Deposition of Thin Films, Wiley t Sons, New York (1994). 12. C.B. Eom, R.J. Cava, R.M. Fleming, J.M. Phillips, R.B. van Dover, J.H. Marshall, J.W.P. Hsu, J.J. Krajewski and W.F. Peck, Jr., Science 258, 1766 (1992). 13. X.D. Wu, S.R. Foltyn, R.C. Dye, Y. Coulter and R.E. MuenchauseqAppZ. Phys. Lett. 62(19), 2434 (1993). 14. H.U. Habermeier, Appl. Surface Science 69,204 (1993). 15. F. Fukunaga and N. Tsuda, J. Phys. Sot. Jpn. 63( lo), 3798 (1994). 16. L. Krusin-Elbaum, Thin Solid Films 169, 17 (1989). 17. H. Maiwa, N. Ichinose and K. Okazaki, Jpn. J. Appl. Phys. 33(pt. 1,9B), 5223 (1994). 18. T.S. Kalkur and Y.C. Lu, Thin Solid Films 205,266 (1991).
Pergamon
PI1 SOO2~5408(96)00165-1
PR.OPERTIES OF POLYCRYSTALLINE SrRuOl THIN FILMS ON Si SUBSTRATES
Koji Watanabe*+, Miho Ami, and Masahiro Tanaka+ Sony Corporation Research Center, 174, Fujitsuka-cho, Hodogaya-ku, Yokohama-shi, Japan (Refereed) (Received July 15, 1996; Accepted July 16, 1996)
ABSTRACT The perovskite SrRu03 thin films were deposited on Si( 100) substrates using pulsed laser deposition. These thin films had columnar structures and exhibited good surface morphology and good barrier characteristics for Si. The films exhibited metallic conductivities, with a room temperature resistivity of 529 @cm. These thin films can be useful in many applications, such as electrodes for ferroelectric and electro-optic devices. KEYWORDS: A. oxides, A. thin films, B. laser deposition, electrical properties
D. diffusion,
D.
INTRODUCTION It is widely recognized that conductive metallic oxides are useful in many applications (l), such as ferroelectric devices and electro-optic devices. In the case of ferroelectric devices, F’t electrodes are usually used to maintain stability in an oxidized atmosphere at the high temperature ranges. But it is difficult to etch Pt using a dry process. This is a disadvantage in using Pt to achieve high density devices (2). Moreover, the problem of ferroelectric fatigue, which is loss of switchable polarization on repeated polarization reversal, is very serious for ferroelectric capacitors using metal electrodes.
*To whom correspondence should be addressed. +Present address: Yagi Microdevices Laboratory,
Research Center, Sony Corporation
Technology Center 4-14-1, Asahi-cho, Atugi-shi, Kanagawa-ken, Japan. 83
Atsugi
84
K. WATANABE
et al.
Vol. 32, No. 1
Several models for ferroelectric fatigue have been proposed. Scott et al. (3) reported that a region of low oxygen concentration, which exists at the ferroelectrics/metal interface, causes ferroelectric fatigue. Therefore, it is believed that conductive oxide electrodes make up for oxygen vacancies and improve ferroelectric fatigue. Several reports (45) indicate that use of conductive oxide electrodes improves ferroelectric fatigue. We believe that conductive oxide electrodes will be a key technology of memory devices. In previous papers (6,7), we have reported a study of RUOZ electrodes. SrRuO3 has the perovskite structure of which the lattice parameters are a = 0.553 nm, b = 0.557 nm, and c = 0.785 nm (8). Most ferroelectric materials have perovskite or perovskitelike structures. The similarity between the crystal structure of the ferroelectric materials and that of the conductive oxide electrodes will be an advantage in growing film. In fact, some reports (9,lO) show that SrRuOs electrodes are useful for preparing epitaxial (Ba, Sr)TiOa films. SrRuOx exhibits isotropic electric properties and the room temperature resistivity of single crystal is reported to be 280 p&&cm (8). The temperature dependencies of the resistivity show good metallic behavior, and a kink at about 160 K is observed in the resistivity curve, which corresponds to the Curie point. Pulsed laser deposition (PLD) (11) is a method used to fabricate thin films, in which atoms, molecules, and ions flown from the target by injection of pulsed laser are deposited on the substrate. Various thin films can be formed under many kinds of gases and a wide range of ambient pressure using simple vacuum hardware and laser devices. As laser pulse is injected on targets only from the outside of the vacuum chamber, no impurities exist in the films obtained. In this study, we used PLD to deposit SrRuOs thin films on Si(100). Although there have been reports about SrRu03 epitaxial films deposited on oxide single crystals (12,13), no report about SrRuOs thin films deposited on Si(100) substrates has previously been presented. The crystallinity, surface morphology, and conductivity of SrRuOs thin films, as well as the interdiffusion between a Si and a SrRuOx layer, were investigated in relation with the deposition conditions. And the film deposition conditions where SrRuOs thin films exhibited good metallic conductivities were examined. EXPERIMENTAL
PROCEDURE
SrRuO3 targets for thin film deposition were made from SrCO3 and RUOZ powders. These powders were mixed in the stoichiometric ratio and ground for 24 h. The mixture was then heated in air at 1200°C for 12 h. After a second grinding, the powder was pressed with a load of 1 ton/cm* to make disks 25 mm in diameter. The disks were heated in air at 1300°C for 24 h to obtain ceramic targets for PLD. SrRuO, thin films were deposited on Si(100) substrates in the form of 25 mm x 25 mm squares, using PLD. Laser pulse (248 nm) from a KrF excimer laser (LAMBDA PHYSIK LPXlOO) was focused on the target through the cylindrical lenses. The target was in a target holder which could be rotated, and the target was always rotated during an ablation. The average laser energy density was 2 J/cm* and repetition rates were 5-10 Hz. Gas pressures during deposition were controlled by Baratron sensors (MKS Co.). The oxygen pressure was changed in a range from 20 to 200 mTorr. The substrate temperature was measured from a thermocouple inserted into a hole of the heater block. The deposition temperatures were varied from 500 to 8OO’C. Structure analysis of the thin films was performed with X-ray diffraction measurements. Cu-Kol radiation using 28 scan mode with an RADIII-B system
Vol. 32, No. 1
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85
(Rigaku Co.) was used to obtain the diffraction data of thin films and 9-28 scan mode was used for ceramic targets. The surface morphology and grain sttucture of the thin films were observed with an atomic force microscope (AFM; Park Science Instruments, Autoprobe) and a scanning electron microscope (SEM; S-4000, Hitachi Co.). The root mean square surface roughness (RMS), maximum roughness (Rmax), and average grain size were estimated by AFM topo images over an area of 2 x 2 pm’. The film thickness was determined1 from the cross-sectional SEM images. Secondary ion mass spectroscopy (SIMS; CAMECA,, IMS-4F) was performed in order to examine the interdiffision of Si and SrRuO3 in the boundary region between the film and Si surface. The electrical resistance of the SrRuOs films and ceramic targets was measured using the conventional four-point probe method with a current source (Keithley 224 model), a digital multimeter (Keithley 196 model), and a cryo-system (CG308; Daikm Co.). Gold electrode stripes between 1 and 2 mm wide and Z!OOnm thick were evaporated onto 25 mm x 5 mm thin film samples and onto ceramic target samples cut from the disks in the form of 25 mm x 5 mm x 1 mm. RESULTS AND DISCUSSION Film Fabrication. Figure 1 shows the SEM photograph of the surface of the SrRuOa ceramic targets. From XRD data, impurity phases were not observed in SrRu03 ceramic targets. It is known that laser ablation of low-density pressed targets result in the ejection of large numbers of macroscopic particles from the surface of the target. The irradiation of the target surface by a pulsed laser results in rapid heating of the surface layer, in which the energy absorbed is not diffised from the surface region but is used mostly to excite the atoms (14). If there are many defects in the surface region, they interfere with the rapid and homogeneous heating of the surface layer. From these points of view, a higher surface density and defects smaller in size than a laser spot on the surface layer are thought to be desirable fix an ablation target. The averaged density of our target was 86.83% of theoretical density, but the surface density, the most important part as the ablation target, calculated from Figure 1 was 97.8%. The defect size (-5 pm’) estimated from Figure 1 was much smaller thtm the area of a laser spot (-10 mm2). Therefore, it may be concluded that the
FIG. 1 SEM photograph for the surface of the SrRuOp ceramic target.
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surface layer of our SrRuO3 ceramic target can be heated rapidly and homogeneously by a pulsed laser. In addition to the points mentioned above, the sample showed good metallic conductivity, with a resistivity of 1.2 mQ*cm at 300 K. The observed resistivity is greater than that of a single crystal SrRu03 reported by Bouchard et al. (8), but lower than that of a sintered SrRu03 reported by Fukunaga and Tsuda (15). It may be concluded that the resistivity of a polycrystalline ceramic which includes grain boundaries is greater than that of a single crystal which does not include grain boundaries. This difference in the resistivities can be explained by the packaging density (15). The temperature dependences of the three samples are quite similar. A resistance ratio ~(300 K)/p(20 K) of our SrRuO3 ceramic targets is about 10 and that value is equal to that of single crystal. Electrical properties are thought to be more sensitive than XRD measurements to impurities. A resistance ratio becomes the measure of the amount of impurities when the contribution of grain boundary scattering effects is eliminated. So, our SrRuO, ceramic target contained as small an amount of scattering centers, impurities, and grain boundaries as the single crystal SrRuO3 reported by Bouchard et al. It therefore can be said that the SrRuO3 ceramic targets we obtained have good quality for PLD. Figure 2 shows the XRD patterns for the films, the deposition temperature of which was varied from 500 to 800°C. At the substrate temperature of 500°C, the crystal growth of SrRu03 had already started: The XRD pattern of a film deposited at 500°C was clearly fitted by using crystal parameters of SrRuO3 in ASTM cards (43-472). Miller indexes for SrRuO3 are also shown in Figure 2. Above 600°C, both the intensity and the sharpness of the XRD peaks corresponding to SrRuO, have increased, and the impurity peak is distinguishable. Figure 3 shows the AFM topo images of films deposited at 500, 600, 700, and 8OO’C. Root mean squares (RMS) roughness, maximum of roughness (Rmax), and average grain size are estimated using AFM data shown in Table 1. These data show that as the deposition temperature increases, the crystallinity and the average grain size of the films increase. This result coincides with the result from the XRD pattern measurements.
10
20
30
40 20
XRD patterns 800°C.
50
60
70
(‘1
FIG. 2 for the SrRuOs films whose deposition
temperature
was 500, 600, 700, and
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(b) FIG. 3 AFM topo images of the SrRuOs films deposited at (a) 500, (b) 600, (c) 700, and (d) 800°C.
K. WATANABE et al.
id) FIG. 3
(Continued).
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TABLE
1
Root Mean Squares (RMS) Roughness, Maximum of Roughness Grain Size Estimated Using AFM Data
-
89
(Rmax), and Average Average
Deposition temperature (“C)
RMS
(nm)
(a@
grain size (n@
800
15
111
200
700
6
42
90
600
4
30
70
500
4
37
40
Figure 4 shows the cross-sectional SEM photograph for the film, deposited at 800°C. It is seen that the film has the columnar structure. The average grain size of the film corresponds with the width of the columns. Column width estimated from Figure 4 was 170 nm and that value is not far from the value of the average grain size (200 nm) estimated using AFM data. Figure 5 shows the SIMS depth profiles for the SrRuOJSi films deposited at 600, 700, and 800°C. The interdiffision of elements at the SrRuOJSi interface was slightly larger in the films deposited at 800°C than in the other flhns. It is seen in Figure 5 that the slopes of the Sr and Ru profiles at the SrRuOJSi interface do not differ much at various deposition temperatures, but the Si on the SrRuOdSi interface at 800°C becomes more diffusive than those at lower temperatures. This means that Sr and Ru are difficult to diffise into Si, but Si can diffise into SrRuOs films. The mixing layer thickness at the SrRuOJSi interface at SOOT was estimated to be about 20 mu at most when the SrRuOJSi interface at 600°C was considered as a reference profile. From XRD data, it can be said that no silicides are formed at the SrRuWSi interface. It can be seen that most of the diffised Si atoms are segregated at
Cross-sectional
SEM photograph
FIG. 4 for the SrRu03 films deposited at SOOT.
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Si
‘“0”
L--_ ......___..r__r_ii_LO~~_.._..C~-.~--_(_-__.~.__-^_dlis
.
’
0.10
DEPTH
*
.
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20
0
20
(microns)
----
___I
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10
-.a...
. ..._L_
_A...._.
-,~_--r_-L___-1_-1_~_-._~-*---~0.05 0.10
0.1s
DEPTH hT&YOnS)
FIG. 5 SIMS depth profiles for the SrRuOJSi films deposited at (a) 600, (b) 700, and (c) 800°C.
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STRON’I-IUh4 RUTIIRNATE FILMS
--
10~
91
-_-~-_-__ W
Si
‘S
01
*,;-..A.__.-
_..a.
.-L-L.
.-4-r._~_-._L._L-,-
0.05
.
0.10
DEPTH
0.1s
.
.
A-
C
20
(microns)
FIG. 5 Continued.
grain boundaries of SrRuOs columnar particles. The AFM data, which show the grain size and the maximum roughness of the films deposited at 800°C are larger than those at lower deposition temperature, support our speculation about Si diffusion: the films deposited at 800°C include the appropriate spacing among the grains where Si atoms can easily diffise. The depth profiles also show that there is a deficiency of oxygen at the surface of the film deposited at 800 ‘C. The diffused elements, mainly Si, and deficiency of oxygen at the surface can affect film resistivity (we discuss this point later). Given that the SrRuOp films on Si or SiOdSi wafers were grown at temperatures below 700°C, the interdiffusion of elements between the film and the wafer was not serious: the crystal growth of SrRuOj started at 500°C, and the crystallinity became better and the grain size greater as the deposition temperature increased. Electrical Properties. Figure 6 shows the temperature dependence of the electrical resistivities and of the electrical resistivities normalized at the values of 300 K for the SrRuO3 flhns deposited at 550,600,650,700, and 800°C. The film thickness of each sample was about 100-130 nm and the oxygen pressure during deposition was 100 mTorr. Above 6OO”C, the films show metallic conductivities, because the films become single phase of SrRuO,, as shown in Figure 2. The resistivity of the film decreases as the deposition temperature increases, and the resistivity of the film deposited at 800°C becomes 529 @*cm. Moreover, the slope of the resistivity-temperature plot becomes larger as the deposition temperature increases. Krusin-Elbaum (16) reported that the residual resistivity
92
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K. WATANABE et al.
(a)
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200
250
300
350
Temperature(K)
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1.5 Q 8 CT’
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?P 0.5
0 0
50
100 150 200 250 Temperature(K)
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350
FIG. 6 Temperature dependence of the electrical resistivities (a) and of the electrical resistivities normalized at the values of300 K (b), for the SrRuOs films deposited at 550,600,650,700, and 800°C. ratio (RRR), defined as ~(300 K)/p(4.2 K), of the RuOz films increased with the average grain size, implying that the electron mean free path increased with fewer grain boundaries. The vertical axis of Figure 6b corresponds to the inverse of the RRR, indicating that the RRR of our SrRuO, films increases with deposition temperature. From the XRD patterns and the topo images from an AFM, it is known that grain size increased with increasing deposition temperature. This result agrees with the result for the RuO2 films reported by Krusin-Elbaum (16). Therefore, it can be seen that the effect of the grain boundary scattering decreases as the grain size increases and that the SrRu03 films show good metallic conductivities. Above 600°C, the films showed metallic conductivities and the resistivity of the film decreased as the grain size increased. The size effect and the thickness limit of films, at which the same properties are maintained as bulk materials, not only are interesting from the viewpoint of materials science, but also are important properties in electrical device applications. Figure 7a shows the room temperature electrical resistivities of films deposited at 700 and 800°C of substrate temperatures under 100 mTorr of oxygen, as the function of film thickness. Figure 7b shows the temperature dependence of the electrical resistivities of the same films deposited at 700°C, shown in Figure 7a, and Figure 7c shows the temperature dependence of the electrical resistivities of films deposited at SOO”C, also shown in Figure 7a. These figures
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93
STRONTIUM RUTHENATE FILMS
thickness(nm)
0
50
100
150
200
250
300
350
Temperature(K)
-0
50
100 150 200 250 Temperature(K)
300
350
FIG. 7
(a) Room. temperature electrical resistivities of the SrRuOp films deposited at 700°C (circles) and 8OO’C (squares) of substrate temperatures under 100 mTorr of oxygen, as the function of the film thickness. (b) Temperature dependence of the electrical resistivities of the same films deposited at 7OO”C, shown in (a), the film thickness of which was 40 nm (triangles), 60 nm (circles), 90 nm (rhombuses), and 120 nm (squares).‘(c) Temperature dependence of the electrical resistivities of the same films deposited at 8OO”C, shown in (a), the film thickness of which was 40 run (circles), 70 nm (squares), and 130 nm (triangles).
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Temperature(K)
FIG. 8 Temperature dependence of the electrical resistivities of the SrRuOx films whose deposition temperature was 65O’C and oxygen pressure was 20 mTorr (triangles), 100 mTorr (circles), and 200 mTorr (squares). clearly show that the resistivities markedly increase as film thickness decreases and that the 40 nm thick film shows no metallic conductivity. The high resistivity of the film can be attributed to the poor crystallinity of SrRuOx in the early stage of deposition (17). Furthermore, these figures show that each series of films deposited at 700°C and 800 “C has the characteristic p vs. T curves as the film thickness increases. When the film is thicker than a critical value, the temperature dependence of the film resistivity is dominantly determined by deposition temperature. The very thin (less than about 100 nm) films deposited at 8OO’C have a higher resistivity and smaller RRR value than those at 700°C. This phenomena can be thought to be caused by (1) Si atoms diffusing into the grain boundary of the films, which affects the electrical properties of the films and/or (2) a deficiency of oxygen at the surface, which affects the electrical properties of the films (18). The mixing layer thickness at the SrRuOJ/Si interface at SOO”C, which was estimated to be about 20 nm, seems reasonable, because the 70-nm-thick tihn deposited at 800°C has the pT characteristic which is somewhere between that of 40-nm-thick film and 60-nm-thick film deposited at 7OO’C. From our experimental point of view, it can be said that films thicker than 50 mn are needed to maintain metallic conductivities in the case of our SrRuOx films and that the diffused Si and the oxygen deficiency of films at the surface negatively affect the electrical properties, especially in the thinner region. Figure 8 shows the temperature dependence of the electrical resistivities of the SrRuOJ films for the deposition temperature of 650°C, with oxygen pressure varied from 20 to 200 mTorr. The film thickness of these samples was about 100 mn. To eliminate the influence of interdiffusion between Si and SrRuOa films completely, the deposition temperature of 650°C was chosen. Though the SrRuOp films deposited at 100 mTorr show good metallic conductivities, the SrRuOs films deposited at 20 and 200 mTorr do not show metallic conductivities. The XRD patterns show that the peak shifts of these films were larger than those of films deposited at 100 mTorr, where XRD patterns for the ceramic ablation target was used as a standard. Table 2 shows the diffraction angles of d(O04) peaks and the peak shifts. It is obvious that the peak shifts of the films deposited at 20 and 200 mTorr are much larger than those of the good conductive films deposited at 100 mTorr. Moreover, these peak shifts occur in the direction of lengthening c axes. The causes of lengthening c axes are
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Diffraction
95
TABLE 2 Angles of d(004) Peaks and the Peak Shifts d(film) d(target) run
d(004) am 0.19585
Conductivitv metallic
650 “C, 100 mTorr
0.19569
metallic
650 “C, 20 mTorr
0.19738
not metallic
0.00153
650 “C, 200 mTorr
0.19844
not metallic
0.00259
Samples Ceramic target
-0.00016
thought to be: (1) the lack of 02 ions between positive ions, which causes the Coulomb repulsion, and (2) the substitution of Sr ions in Ru-ion sites. The point here is that good metallic conductivities of the SrRuO3 films decline when the film composition shifts from the stoichiometric composition. To obtain good metallic conductive SrRuOJ films, we have to control the deposition temperature, the process oxygen pressure, and the resultant film thickness; the best properties are achieved by realizing the ideal stoichiometry of films. CONCLUSION Good crystalline and good conductive SrRuOs thin films have been successfully grown on silicon substrates using pulsed laser deposition. Metallic conductive films were obtained when the deposition temperature was greater than 6OO’C and the Iihn thickness was greater than 50 nm. It was found that the diffised Si and the deficiency of oxygen at the surface affected the electrical properties of the films, but these affects could be eliminated when the films were deposited below 700°C or/and were thicker than the critical values. The stoichiomletry of the film is essential to obtaining good metallic conductive SrRuOs films. The resistivity (300 K) of SrRuOs film deposited at 8OO’C was 529 #J-cm. ACKNOWLEDGMENTS We would like to thank Ms. Minatoya for her kind assistance in performing the AFM measurements, Mr. Nomachi for SIMS measurements, and Mr. Takasu and Mr. Yamauchi for SEM measurements. And we would like to thank Mr. Miyazawa for the design of our machine and Mr. Isobe for his kind assistance in starting up our machine. REFERENCES 1. L. Knrsin-Elbaum and M. Winmer, J. Electrochem. Sot. 135( lo), 2610 (1988). 2. K. Takemura, T. Sakuma and Y. Miyasakq Appl. P&s. Lett. 64(22), 135 (1992). 3. J. F. Scott, C.A. Araujo, B.M. Melnick, L.D. McMillaa aad R. Zuleeg, J. Appl. P&s. 70(l), 382 (1991). 4. D.P. Vijay and S.B. Desu, J. Electrochem. Sot. 140(9), 2640 (1993). 5. T. Nakamura, Y. Nakao, A. Kamisawa and H. Takasu, Jpn. J. Appl. P&s. 33(pt. 1, 9B), 5207 (1994). 6. J.F. Tressler, K. Watanabe aad M. Tanaka, J. Am. Cerum. Sot. 79(2), 525 (1996).
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7. K. Watanabe, J.F. Tressler, M. Sadamoto, C. Isobe and M. Tanaka, < Electrochem. Sot. 143, 3008 (1996). 8. R.J. Bouchard and J.L. Gillson, Mater. Res. Bull. 7,873 (1972). 9. Q.X. Jia, X.D. Wu, S.R. Foltyn and P. Tiwari, Appf. Phys. Lett. 66 (17), 2197 (1995). 10. S.Y. Hou, J. Kwo, R.K. Watts, J.-Y. Cheng and D.K. Fork, Appl. Phys. Lett. 67(10), 1387 (1995). Il. D.B. Chrisey and G.K. Hubler, Pulsed Laser Deposition of Thin Films, Wiley t Sons, New York (1994). 12. C.B. Eom, R.J. Cava, R.M. Fleming, J.M. Phillips, R.B. van Dover, J.H. Marshall, J.W.P. Hsu, J.J. Krajewski and W.F. Peck, Jr., Science 258, 1766 (1992). 13. X.D. Wu, S.R. Foltyn, R.C. Dye, Y. Coulter and R.E. MuenchauseqAppZ. Phys. Lett. 62(19), 2434 (1993). 14. H.U. Habermeier, Appl. Surface Science 69,204 (1993). 15. F. Fukunaga and N. Tsuda, J. Phys. Sot. Jpn. 63( lo), 3798 (1994). 16. L. Krusin-Elbaum, Thin Solid Films 169, 17 (1989). 17. H. Maiwa, N. Ichinose and K. Okazaki, Jpn. J. Appl. Phys. 33(pt. 1,9B), 5223 (1994). 18. T.S. Kalkur and Y.C. Lu, Thin Solid Films 205,266 (1991).