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Nanoscale electrical and crystallographic properties of ultra-thin dielectric films
Thin Solid Films 518 (2010) S17–S21
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Nanoscale electrical and crystallographic properties of ultra-thin dielectric films Yuan-Chang Liang a,⁎, Yung-Ching Liang b a b
Institute of Materials Engineering, National Taiwan Ocean University, Keelung 20224, Taiwan Department of Physics and Astronomy, University of Southern California, Los Angeles, California 90089, USA
a r t i c l e
i n f o
Available online 19 March 2010 Keywords: Characterization X-ray scattering Surface morphology Nanoscale electrical properties
a b s t r a c t Thin SrTiO3 (STO) films with a thickness of 14 nm were grown on the Pt/Ti/SiO2/Si (STO/Pt) and Ru/SiO2/Si (STO/Ru) substrates at room temperature by radio-frequency magnetron sputtering. The as-deposited STO films were then post-annealed at 450–650 °C in an oxygen atmosphere in order to obtain various crystallographic features. The crystalline STO/Pt and STO/Ru films were obtained at an annealing temperature above 550 °C. The results of X-ray photoelectron spectroscope reveal that the crystalline STO films have a microscopically homogeneous film structure. Besides, secondary ion mass spectrometry depth profiles show an increase of interface roughness with an increasing annealing temperature of the STO films. Atomic force microscope images also show that the surface of the crystalline STO/Ru film is smoother than that of the STO/Pt film at a given annealing temperature. The effective improvement in the chemical homogeneity and surface roughness of the STO films results in a considerable decrease in the nanostructural leakage current of the STO films upon the use of a Ru electrode. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The study of thin-film structures of the SrTiO3 (STO) is motivated by the integration of the STO with semiconductors like silicon, and the raised interest for unusual effects of the STO thin films differing substantially from those in a corresponding bulk material [1]. High dielectric constant of the STO provides a capacitance density of an order of magnitude higher than the conventional gate dielectrics, such as SiO2 and Ta2O5. However, crystallographic characteristics of the STO thin film make a profound effect on the electrical properties of the film. Grain boundaries of the polycrystalline STO film tend to produce high leakage currents and the amorphous film has a low dielectric constant further limiting its application in memory devices. Up to now, the characterization of current transportation across the STO thin films on nanoscale has not been investigated, which is an important issue for the application of the ultra-thin STO film in nanodevices. Among all the metals, Pt, Ru, and Ir are mostly used as an electrode material due to their low resistivity and high chemical durability [2,3]. The trend towards miniaturization of devices depends on the thinning of films. Increasing the electrical resistivity of the crystalline thin STO films is very important for realizing its applications in devices. Furthermore, the understanding of leakage phenomena in the STO thin films grown on the metal electrodes would benefit greatly from
⁎ Corresponding author. E-mail address: [email protected] (Y.-C. Liang). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.03.021
the detection of leakage current at a spatial resolution that is similar to the length scale of the structural non-uniformities. From the conventional current–voltage characterization it is not possible to obtain any information on the nanoscale leakage current properties [4–6]. With this respect, an already well-established technique to study electrical properties on a lateral scale of a few nanometers is the conductive atomic force microscope (CAFM) [7]. However, the application of CAFM to investigate the nanoscale leakage current properties of high dielectric perovskite oxides is still lacking. In this work, the correlation between the crystallographic characteristics and the nanostructural leakage current properties of the ultra-thin STO films was studied. 2. Experimental The 14 nm-thick STO films were grown on the Pt(150 nm)/Ti/SiO2/ Si(100) and Ru(150 nm)/SiO2/Si(100) substrates. The Ti layer was used to increase the adhesion between the Pt and the silicon substrate. Deposition was performed using a radio-frequency magnetron sputtering system. During deposition of the STO thin films, the substrate temperature was kept at room temperature and the gas pressure of deposition was fixed at 15 mTorr with an Ar/O2 ratio of 4:1. The STO thin films were post-deposition annealed at 450, 550, and 650 °C for 10 min in an oxygen ambient pressure of 200 Torr to obtain various degrees of crystallographic features. The crystallographic structures of the STO thin films were analyzed by X-ray diffraction (XRD). The symmetrical theta-2 theta setting was used for the XRD measurement. The thickness of the STO films was examined by X-ray reflectivity. The composition depth profile was
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examined by secondary ion mass spectrometry (SIMS) with an oxygen ion source. The chemical bonding state of oxygen in the STO films was determined using an X-ray photoelectron spectroscope (XPS). The surface morphology of the STO films was investigated with an atomic force microscope (AFM). The surface current image of the STO films was observed by conductive atomic force microscope (CAFM) with PtIr tips. These observations were conducted on an area of 0.75 μm × 0.75 μm. 3. Results and discussion Fig. 1 shows the XRD patterns of the STO thin films on the Pt/Ti/ SiO2/Si (STO/Pt) and Ru/SiO2/Si (STO/Ru) substrates. No Bragg reflection that originated from the STO film is detected in Fig. 1(a), revealing that the STO/Pt thin film annealed at 450 °C is characterized with an amorphous feature. The Bragg reflections of the STO thin films appear with the increase of the annealing temperature above 550 °C (Fig. 1(b)). However, the crystalline quality of the STO/Pt film declined at the annealing temperature of 650 °C (Fig. 1(c)). The (110) Bragg reflection dominates the crystallographic features in the crystalline STO/Pt thin films and this is in agreement with the literature [8]. On the other hand, Fig. 1(d) shows that the STO/Ru thin film annealed at 450 °C exhibits an amorphous feature. The crystalline STO/Ru thin film was formed at the annealing temperature above 550 °C. Of particular interest, the crystalline STO films self-organized to produce an almost (111)-oriented feature on the Ru electrodes herein. The complete (111)-oriented perovskite Pb(Zr,Ti)O3 film could be actually obtained under an appropriate control of film thickness [9]. Moreover, other perovskite oxide films with selforganized growth of (111)-orientation have been attributed to the difference of nucleation and growth rates in various orientations [10]. The growth mechanism of the thin (111)-oriented STO film on the Ru electrode remains unclear herein. Further investigation using in-situ X-ray scattering with synchrotron radiation will help to elucidate the
growth mechanism of the ultra-thin STO/Ru film [11]. The intensity of Bragg reflections of the STO/Ru film increases with the annealing temperature. The dependence of peak intensity on annealing temperature can be understood by the increase of grain size and crystallinity of the STO/Ru film annealed at a higher temperature. The XRD results demonstrate that a high annealing temperature of 550 °C is necessary for the crystallization of the sputtered amorphous STO films on the lattice mismatched metal substrates herein. Fig. 2 shows the XPS core-level spectra of the O1s of the STO/Ru films. A symmetric main peak at ∼ 530.4 eV was observed for the films that were annealed at 550–650 °C. The main O1s peak displayed in Fig. 2(a) and (b) is ascribed to the lattice oxygen. Sayers and Armstrong [12] reported a main oxygen peak at 530.1 eV for the STO single crystal and this is in good agreement with the data shown in the figures. Furthermore, the STO film annealed at 450 °C shows an asymmetrical spectrum as displayed in Fig. 2(c), revealing that the multiple bonding states of oxygen exist in the STO film [13,14]. The XPS result reveals an inhomogeneous bonding state of the amorphous STO/Ru film. The composition modulations of the STO/Pt and STO/Ru films annealed at 550 and 650 °C were shown in Fig. 3. The compositional profiles near the STO/metal electrode interface become sharper at a relatively lower annealing temperature. From the SIMS analysis, the ratio of the area of the interface to the area of the STO film is large when the STO film was annealed at a relatively higher temperature. Notably, it is apparent that oxygen atoms had diffused into Ru at high annealing temperatures (Fig. 3(c) and (d)). The Ru layer was not fully oxidized because the oxygen concentration gradually drops down at the end of the Ru layer. However, the thickness that is associated with the partial oxidation of Ru electrode is not thick enough to contribute to the appearance of diffraction peaks for RuOx phases in the XRD spectra. The formation of RuOx that might roughen the STO/Ru interface further affects the electrical properties of the STO thin films [15].
Fig. 1. XRD patterns of the STO/Pt and STO/Ru films annealed at various temperatures.
Y.-C. Liang, Y.-C. Liang / Thin Solid Films 518 (2010) S17–S21
Fig. 2. XPS spectra of the O1s core levels of the STO/Ru films annealed at various temperatures.
Fig. 4 shows the AFM surface morphology of the STO/Pt and STO/ Ru films. The surface of the STO/Pt films annealed at 550–650 °C contains large three-dimensional grains. Their surfaces appear rather rough. The surface of the STO/Pt film annealed at 450 °C shows a smooth surface. The root-mean-square (rms) surface roughness of the STO/Pt films is 1.1, 3.4, and 3.8 nm annealed at 450, 550, and 650 °C, respectively. The annealing temperature dependence of coarsening in surface features of the STO/Ru films was also observed in Fig. 4. The rms surface roughness of the STO/Ru films is 1.1, 2.1, and 2.7 nm annealed at 450, 550, and 650 °C, respectively. Moreover, the crystalline STO/Ru films that were annealed at 550–650 °C exhibit a denser and smoother film surface than that of the STO/Pt films annealed at the same temperature range. Notably, some holes with depth ∼15 nm were found on the surface of the STO/Pt films annealed above 550 °C. These holes account for the rough surface of the annealed STO/Pt films. Moreover, from the SIMS analysis, interdiffusion of Ti occurred between the STO and the Pt electrode. This may also account for an increase of surface roughness in the annealed STO/ Pt films. Comparatively, the AFM images show that the use of Ru electrode markedly reduces the surface roughness of the crystalline STO films. The crystalline STO film has a higher dielectric constant than that of the amorphous one and is considered for practical applications of devices. The current maps of the crystalline STO films annealed at 550–650 °C, presented on the micro- and/or nanoscale under the various biases using CAFM were shown in Figs. 5 and 6. Fig. 5 plots the spatial distribution of current spots of the STO/Pt films measured at various applied biases. The STO/Pt films annealed at the high temperature yield a film thickness inhomogenity. As the bias is increased to 1.0 V, some white spots (with current larger than 2pA) were detected in the measured area. Since the depth of some holes in the annealed STO/Pt film corresponds with the STO film thickness from the AFM analysis, these holes might contribute to the short circuit during the CAFM measurement. Moreover, the number of the spots with minor currents (current lower than 2pA) in the STO/Pt films increases with the annealing temperature. It is reported that a considerable out-diffusion of Ti ions in the dielectric film will occur
Fig. 3. SIMS depth profiles of the STO films grown on various substrates and annealed at various temperatures. (a) STO/Pt film at 550 °C, (b) STO/Pt film at 650 °C, (c) STO/Ru film at 550 °C, and (d) STO/Ru film at 650 °C.
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Fig. 4. AFM surface topography of the STO/Pt and STO/Ru films annealed at various temperatures.
when the film was grown on Pt at a high temperature [7]. An excess of Ti ions in the dielectric film worsens the leakage current performance of the film. The spatial distribution of leakage spots of the STO/Ru films annealed at 550–650 °C was shown in Fig. 6. The crystalline STO/ Ru films show high resistance under an applied field of 0.5 V. A trace amount of leakage spots with minor leakage currents was detected in the measured area at 1.0 V. Comparatively, the crystalline STO/Ru films are more resistant than the crystalline STO/Pt films at 1.0 V. This might be associated with the fact that the surface of the crystalline STO/Ru film is dense; therefore, prohibits current tunneling from the surface defects at the small applied bias. Further increasing bias to breakdown voltage (2.0 V) the STO/Ru films have a homogeneously spatial distribution of leakage spots over the measured area. From the analysis of current statistics, the leakage current performance of the STO/Ru film that was annealed at 650 °C is poorer than that of the film annealed at 550 °C and this is associated with the rough surface and interface structures of the film annealed at a relatively high temperature. The experimental results show that the surface of the STO/Ru film is smoother than that of the STO/Pt film at a given annealing temperature. The sputtering deposited Ru has a rougher surface than that of chemical vapor deposited (CVD) Ru. However, the dielectric film grown on the sputtering deposited Ru does not show the drawbacks of the dielectric film on the CVD-Ru electrode [16,17]. The thickness of Ru is 150 nm in this work. For most Ru metal gates, the thickness may be reduced to 10–50 nm. The decrease of Ru thickness
for electrode application might further decrease the roughness of the thin STO film with an annealing procedure. More studies will be conducted to evaluate the effect of Ru thickness on the surface topography of thin dielectric films in the near future. 4. Conclusions A high annealing temperature of 550 °C is necessary to obtain the crystalline STO/Pt and STO/Ru films. The thin crystalline STO/Pt films exhibit mixed crystallographic orientations; moreover, the thin crystalline STO/Ru films show a nearly (111)-oriented feature. In addition, SIMS depth profiles show that the higher annealing temperature results in a rougher interface between the STO and the metal electrode. The crystalline STO/Ru films have a relatively smooth film surface with respect to the crystalline STO/Pt films at a given annealing temperature. Both the decrease of surface roughness and the lack of Ti out-diffusion make the crystalline STO/Ru film exhibit better nanostructural leakage current performance than that of the STO/Pt film. References [1] [2] [3] [4]
P.C. Joshi, S.B. Krupanidhi, Appl. Phys. Lett. 61 (1992) 1525. M.W. Cole, W.D. Nothwang, C. Hubbard, E. Ngo, M. Ervin, J. Appl. Phys. 93 (2003) 9218. M.S. Tsai, T.Y. Tseng, J. Am. Ceram. Soc. 83 (1999) 351. Y.C. Liang, Y.C. Liang, J. Electrochem. Soc. 154 (2007) G193.
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Fig. 5. The current images of the STO/Pt films annealed at various temperatures.
Fig. 6. The current images of the STO/Ru films annealed at various temperatures. [5] [6] [7] [8] [9] [10] [11]
Y.C. Liang, Y.C. Liang, J.P. Chu, Electrochem. Solid State Lett. 11 (2008) G41. Y.C. Liang, J.P. Chu, Jpn. J. Appl. Phys. 47 (2008) 257. Y.C. Liang, Y.C. Liang, J. Electrochem. Soc. 156 (2009) G84. N. Sugii, K. Takagi, Thin Solid Films 323 (1998) 63. C.S. Liang, L.J. Lin, C.M. Wu, Electrochem. Solid State Lett. 8 (2005) F29. S. Yokoyama, T. Ozeki, T. Oikawa, H. Funakubo, Integr. Ferroelectr. 59 (2003) 1429. Y.C. Liang, H.Y. Lee, H.J. Liu, Y.W. Hsieh, Y.C. Liang, J. Synchrotron Radiat. 14 (2007) 163.
[12] [13] [14] [15] [16]
C.N. Sayers, N.R. Armstrong, Surf. Sci. 77 (1978) 301. Y.C. Park, Y.S. Kim, H.K. Seo, S.G. Ansari, H.S. Shin, Surf. Coat. Technol. 161 (2002) 62. P.V. Nagarkar, P.C. Searson, F.D. Gealy, J. Appl. Phys. 69 (1991) 459. J.H. Huang, Y.S. Lai, J.S. Chen, J. Electrochem. Soc. 148 (2001) F133. S.K. Dey, J. Goswami, D. Gu, H. Waard, S. Marcus, Chris Werkhoven, Appl. Phys. Lett. 84 (2004) 1606. [17] J. Lin, T. Suzuki, D. Matsunaga, K. Eguchi, Appl. Phys. Lett. 88 (2006) 062902.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Nanoscale electrical and crystallographic properties of ultra-thin dielectric films Yuan-Chang Liang a,⁎, Yung-Ching Liang b a b
Institute of Materials Engineering, National Taiwan Ocean University, Keelung 20224, Taiwan Department of Physics and Astronomy, University of Southern California, Los Angeles, California 90089, USA
a r t i c l e
i n f o
Available online 19 March 2010 Keywords: Characterization X-ray scattering Surface morphology Nanoscale electrical properties
a b s t r a c t Thin SrTiO3 (STO) films with a thickness of 14 nm were grown on the Pt/Ti/SiO2/Si (STO/Pt) and Ru/SiO2/Si (STO/Ru) substrates at room temperature by radio-frequency magnetron sputtering. The as-deposited STO films were then post-annealed at 450–650 °C in an oxygen atmosphere in order to obtain various crystallographic features. The crystalline STO/Pt and STO/Ru films were obtained at an annealing temperature above 550 °C. The results of X-ray photoelectron spectroscope reveal that the crystalline STO films have a microscopically homogeneous film structure. Besides, secondary ion mass spectrometry depth profiles show an increase of interface roughness with an increasing annealing temperature of the STO films. Atomic force microscope images also show that the surface of the crystalline STO/Ru film is smoother than that of the STO/Pt film at a given annealing temperature. The effective improvement in the chemical homogeneity and surface roughness of the STO films results in a considerable decrease in the nanostructural leakage current of the STO films upon the use of a Ru electrode. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The study of thin-film structures of the SrTiO3 (STO) is motivated by the integration of the STO with semiconductors like silicon, and the raised interest for unusual effects of the STO thin films differing substantially from those in a corresponding bulk material [1]. High dielectric constant of the STO provides a capacitance density of an order of magnitude higher than the conventional gate dielectrics, such as SiO2 and Ta2O5. However, crystallographic characteristics of the STO thin film make a profound effect on the electrical properties of the film. Grain boundaries of the polycrystalline STO film tend to produce high leakage currents and the amorphous film has a low dielectric constant further limiting its application in memory devices. Up to now, the characterization of current transportation across the STO thin films on nanoscale has not been investigated, which is an important issue for the application of the ultra-thin STO film in nanodevices. Among all the metals, Pt, Ru, and Ir are mostly used as an electrode material due to their low resistivity and high chemical durability [2,3]. The trend towards miniaturization of devices depends on the thinning of films. Increasing the electrical resistivity of the crystalline thin STO films is very important for realizing its applications in devices. Furthermore, the understanding of leakage phenomena in the STO thin films grown on the metal electrodes would benefit greatly from
⁎ Corresponding author. E-mail address: [email protected] (Y.-C. Liang). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.03.021
the detection of leakage current at a spatial resolution that is similar to the length scale of the structural non-uniformities. From the conventional current–voltage characterization it is not possible to obtain any information on the nanoscale leakage current properties [4–6]. With this respect, an already well-established technique to study electrical properties on a lateral scale of a few nanometers is the conductive atomic force microscope (CAFM) [7]. However, the application of CAFM to investigate the nanoscale leakage current properties of high dielectric perovskite oxides is still lacking. In this work, the correlation between the crystallographic characteristics and the nanostructural leakage current properties of the ultra-thin STO films was studied. 2. Experimental The 14 nm-thick STO films were grown on the Pt(150 nm)/Ti/SiO2/ Si(100) and Ru(150 nm)/SiO2/Si(100) substrates. The Ti layer was used to increase the adhesion between the Pt and the silicon substrate. Deposition was performed using a radio-frequency magnetron sputtering system. During deposition of the STO thin films, the substrate temperature was kept at room temperature and the gas pressure of deposition was fixed at 15 mTorr with an Ar/O2 ratio of 4:1. The STO thin films were post-deposition annealed at 450, 550, and 650 °C for 10 min in an oxygen ambient pressure of 200 Torr to obtain various degrees of crystallographic features. The crystallographic structures of the STO thin films were analyzed by X-ray diffraction (XRD). The symmetrical theta-2 theta setting was used for the XRD measurement. The thickness of the STO films was examined by X-ray reflectivity. The composition depth profile was
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examined by secondary ion mass spectrometry (SIMS) with an oxygen ion source. The chemical bonding state of oxygen in the STO films was determined using an X-ray photoelectron spectroscope (XPS). The surface morphology of the STO films was investigated with an atomic force microscope (AFM). The surface current image of the STO films was observed by conductive atomic force microscope (CAFM) with PtIr tips. These observations were conducted on an area of 0.75 μm × 0.75 μm. 3. Results and discussion Fig. 1 shows the XRD patterns of the STO thin films on the Pt/Ti/ SiO2/Si (STO/Pt) and Ru/SiO2/Si (STO/Ru) substrates. No Bragg reflection that originated from the STO film is detected in Fig. 1(a), revealing that the STO/Pt thin film annealed at 450 °C is characterized with an amorphous feature. The Bragg reflections of the STO thin films appear with the increase of the annealing temperature above 550 °C (Fig. 1(b)). However, the crystalline quality of the STO/Pt film declined at the annealing temperature of 650 °C (Fig. 1(c)). The (110) Bragg reflection dominates the crystallographic features in the crystalline STO/Pt thin films and this is in agreement with the literature [8]. On the other hand, Fig. 1(d) shows that the STO/Ru thin film annealed at 450 °C exhibits an amorphous feature. The crystalline STO/Ru thin film was formed at the annealing temperature above 550 °C. Of particular interest, the crystalline STO films self-organized to produce an almost (111)-oriented feature on the Ru electrodes herein. The complete (111)-oriented perovskite Pb(Zr,Ti)O3 film could be actually obtained under an appropriate control of film thickness [9]. Moreover, other perovskite oxide films with selforganized growth of (111)-orientation have been attributed to the difference of nucleation and growth rates in various orientations [10]. The growth mechanism of the thin (111)-oriented STO film on the Ru electrode remains unclear herein. Further investigation using in-situ X-ray scattering with synchrotron radiation will help to elucidate the
growth mechanism of the ultra-thin STO/Ru film [11]. The intensity of Bragg reflections of the STO/Ru film increases with the annealing temperature. The dependence of peak intensity on annealing temperature can be understood by the increase of grain size and crystallinity of the STO/Ru film annealed at a higher temperature. The XRD results demonstrate that a high annealing temperature of 550 °C is necessary for the crystallization of the sputtered amorphous STO films on the lattice mismatched metal substrates herein. Fig. 2 shows the XPS core-level spectra of the O1s of the STO/Ru films. A symmetric main peak at ∼ 530.4 eV was observed for the films that were annealed at 550–650 °C. The main O1s peak displayed in Fig. 2(a) and (b) is ascribed to the lattice oxygen. Sayers and Armstrong [12] reported a main oxygen peak at 530.1 eV for the STO single crystal and this is in good agreement with the data shown in the figures. Furthermore, the STO film annealed at 450 °C shows an asymmetrical spectrum as displayed in Fig. 2(c), revealing that the multiple bonding states of oxygen exist in the STO film [13,14]. The XPS result reveals an inhomogeneous bonding state of the amorphous STO/Ru film. The composition modulations of the STO/Pt and STO/Ru films annealed at 550 and 650 °C were shown in Fig. 3. The compositional profiles near the STO/metal electrode interface become sharper at a relatively lower annealing temperature. From the SIMS analysis, the ratio of the area of the interface to the area of the STO film is large when the STO film was annealed at a relatively higher temperature. Notably, it is apparent that oxygen atoms had diffused into Ru at high annealing temperatures (Fig. 3(c) and (d)). The Ru layer was not fully oxidized because the oxygen concentration gradually drops down at the end of the Ru layer. However, the thickness that is associated with the partial oxidation of Ru electrode is not thick enough to contribute to the appearance of diffraction peaks for RuOx phases in the XRD spectra. The formation of RuOx that might roughen the STO/Ru interface further affects the electrical properties of the STO thin films [15].
Fig. 1. XRD patterns of the STO/Pt and STO/Ru films annealed at various temperatures.
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Fig. 2. XPS spectra of the O1s core levels of the STO/Ru films annealed at various temperatures.
Fig. 4 shows the AFM surface morphology of the STO/Pt and STO/ Ru films. The surface of the STO/Pt films annealed at 550–650 °C contains large three-dimensional grains. Their surfaces appear rather rough. The surface of the STO/Pt film annealed at 450 °C shows a smooth surface. The root-mean-square (rms) surface roughness of the STO/Pt films is 1.1, 3.4, and 3.8 nm annealed at 450, 550, and 650 °C, respectively. The annealing temperature dependence of coarsening in surface features of the STO/Ru films was also observed in Fig. 4. The rms surface roughness of the STO/Ru films is 1.1, 2.1, and 2.7 nm annealed at 450, 550, and 650 °C, respectively. Moreover, the crystalline STO/Ru films that were annealed at 550–650 °C exhibit a denser and smoother film surface than that of the STO/Pt films annealed at the same temperature range. Notably, some holes with depth ∼15 nm were found on the surface of the STO/Pt films annealed above 550 °C. These holes account for the rough surface of the annealed STO/Pt films. Moreover, from the SIMS analysis, interdiffusion of Ti occurred between the STO and the Pt electrode. This may also account for an increase of surface roughness in the annealed STO/ Pt films. Comparatively, the AFM images show that the use of Ru electrode markedly reduces the surface roughness of the crystalline STO films. The crystalline STO film has a higher dielectric constant than that of the amorphous one and is considered for practical applications of devices. The current maps of the crystalline STO films annealed at 550–650 °C, presented on the micro- and/or nanoscale under the various biases using CAFM were shown in Figs. 5 and 6. Fig. 5 plots the spatial distribution of current spots of the STO/Pt films measured at various applied biases. The STO/Pt films annealed at the high temperature yield a film thickness inhomogenity. As the bias is increased to 1.0 V, some white spots (with current larger than 2pA) were detected in the measured area. Since the depth of some holes in the annealed STO/Pt film corresponds with the STO film thickness from the AFM analysis, these holes might contribute to the short circuit during the CAFM measurement. Moreover, the number of the spots with minor currents (current lower than 2pA) in the STO/Pt films increases with the annealing temperature. It is reported that a considerable out-diffusion of Ti ions in the dielectric film will occur
Fig. 3. SIMS depth profiles of the STO films grown on various substrates and annealed at various temperatures. (a) STO/Pt film at 550 °C, (b) STO/Pt film at 650 °C, (c) STO/Ru film at 550 °C, and (d) STO/Ru film at 650 °C.
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Fig. 4. AFM surface topography of the STO/Pt and STO/Ru films annealed at various temperatures.
when the film was grown on Pt at a high temperature [7]. An excess of Ti ions in the dielectric film worsens the leakage current performance of the film. The spatial distribution of leakage spots of the STO/Ru films annealed at 550–650 °C was shown in Fig. 6. The crystalline STO/ Ru films show high resistance under an applied field of 0.5 V. A trace amount of leakage spots with minor leakage currents was detected in the measured area at 1.0 V. Comparatively, the crystalline STO/Ru films are more resistant than the crystalline STO/Pt films at 1.0 V. This might be associated with the fact that the surface of the crystalline STO/Ru film is dense; therefore, prohibits current tunneling from the surface defects at the small applied bias. Further increasing bias to breakdown voltage (2.0 V) the STO/Ru films have a homogeneously spatial distribution of leakage spots over the measured area. From the analysis of current statistics, the leakage current performance of the STO/Ru film that was annealed at 650 °C is poorer than that of the film annealed at 550 °C and this is associated with the rough surface and interface structures of the film annealed at a relatively high temperature. The experimental results show that the surface of the STO/Ru film is smoother than that of the STO/Pt film at a given annealing temperature. The sputtering deposited Ru has a rougher surface than that of chemical vapor deposited (CVD) Ru. However, the dielectric film grown on the sputtering deposited Ru does not show the drawbacks of the dielectric film on the CVD-Ru electrode [16,17]. The thickness of Ru is 150 nm in this work. For most Ru metal gates, the thickness may be reduced to 10–50 nm. The decrease of Ru thickness
for electrode application might further decrease the roughness of the thin STO film with an annealing procedure. More studies will be conducted to evaluate the effect of Ru thickness on the surface topography of thin dielectric films in the near future. 4. Conclusions A high annealing temperature of 550 °C is necessary to obtain the crystalline STO/Pt and STO/Ru films. The thin crystalline STO/Pt films exhibit mixed crystallographic orientations; moreover, the thin crystalline STO/Ru films show a nearly (111)-oriented feature. In addition, SIMS depth profiles show that the higher annealing temperature results in a rougher interface between the STO and the metal electrode. The crystalline STO/Ru films have a relatively smooth film surface with respect to the crystalline STO/Pt films at a given annealing temperature. Both the decrease of surface roughness and the lack of Ti out-diffusion make the crystalline STO/Ru film exhibit better nanostructural leakage current performance than that of the STO/Pt film. References [1] [2] [3] [4]
P.C. Joshi, S.B. Krupanidhi, Appl. Phys. Lett. 61 (1992) 1525. M.W. Cole, W.D. Nothwang, C. Hubbard, E. Ngo, M. Ervin, J. Appl. Phys. 93 (2003) 9218. M.S. Tsai, T.Y. Tseng, J. Am. Ceram. Soc. 83 (1999) 351. Y.C. Liang, Y.C. Liang, J. Electrochem. Soc. 154 (2007) G193.
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Fig. 5. The current images of the STO/Pt films annealed at various temperatures.
Fig. 6. The current images of the STO/Ru films annealed at various temperatures. [5] [6] [7] [8] [9] [10] [11]
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