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BaTiO3 superlattices on Si substrates using a TiN buffer layer by pulsed laser deposition method
ARTICLE IN PRESS
Journal of Crystal Growth 289 (2006) 540–546 www.elsevier.com/locate/jcrysgro
Integration of artificial SrTiO3/BaTiO3 superlattices on Si substrates using a TiN buffer layer by pulsed laser deposition method Tae-Un Kima, Bo Ram Kima, Won-Jae Leeb, Jong Ha Moona, Byung-Teak Leea, Jin Hyeok Kima, a
Photonic and Electronic Thin Films Laboratory, Department of Materials Science & Engineering, Chonnam National University, 300 Yongbong-Dong, Puk-Gu, Kwangju, 500-757, Republic of Korea b Department of Information Material Engineering, Dongeui University, 995 Eomgwangno, Busanjin-Gu, Busan, 614-714, Republic of Korea Received 15 August 2005; received in revised form 4 November 2005; accepted 24 November 2005 Available online 7 February 2006 Communicated by M. Kawasaki
Abstract Epitaxial SrTiO3(STO)/BaTiO3(BTO) artificial superlattices, STO, BTO, and (Ba0.5,Sr0.5)TiO3 (BSTO) thin films have been grown on TiN-buffered Si (0 0 1) substrates by pulsed laser deposition method and their dielectric properties were studied. The crystal orientation, epitaxy nature, and microstructure of epitaxial oxide thin films were investigated using X-ray diffraction and transmission electron microscopy. Thin films were prepared with laser fluence of 3 and 2 J/cm2, repetition rate of 8 and 10 Hz, substrate temperature of 700 and 650 1C for TiN and oxide, respectively. The TiN buffer layer and oxide thin films were grown with cube-on-cube epitaxial orientation relationship of [1 1 0](0 0 1)filmsJ[1 1 0](0 0 1)TiNJ[1 1 0](0 0 1)Si. The dielectric constants of BTO, STO, BSTO, and STO/BTO superlattice epitaxial thin films with 1 nm/1 nm periodicity were shown to be as high as 300, 410, 520, and 680 at the frequency of 100 kHz, respectively. r 2006 Elsevier B.V. All rights reserved. PACS: 68.65.Cd; 77.55.+f; 81.15.Fg; 68.55.Jk; 68.37.Lp Keywords: A3. Laser epitaxy; B1. Oxides; B2. Dielectric materials
1. Introduction Oxide artificial superlattices have attracted much attention because the superlattices have the potential to provide a new function or enhanced performance to existing devices by controlling lattice strain, dimensionality, and stacking periodicity. A number of papers concerning the growth of artificial superlattices of BaTiO3(BTO)/ SrTiO3(STO) and related materials [1–3], PbZrO3/PbTiO3 [4], SrZrO3/SrTiO3 [5], Bi-based layered oxides [6], and LaFeO3/LaCrO3 [7] have been reported to improve their dielectric, ferroelectric, electro-optic, and magnetic properties. Especially, (0 0 1) oriented STO/BTO superlattices are of great interest because it is relatively well known material Corresponding author. Tel.: 82 62 530 1709; fax: 82 62 530 1699.
E-mail address: [email protected] (J.H. Kim). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.11.119
system and their dielectric and ferroelectric properties can be enhanced by controlling the lattice strain along the polarized direction. For the growth of STO/BTO epitaxial superlattices, oxide single-crystal substrates, such as, STO [2,8], and MgO [3], have been used because they have low lattice mismatch and similar crystal structure with film materials, which gives easy of fabrication. However, oxide substrates are expensive and incompatible with existing mature Si technology. Therefore, in practical viewpoint, it is more desirable to fabricate highly oriented or epitaxial oxide thin films on Si substrates to open the door to fabricate Si-based novel devices with enhanced functionality and flexibility [9,10]. There have been many previous studies on oxide epitaxy on Si substrates. Various kinds of material systems, such as BTO [11], STO [12,13], CeO2 [14], NdNiO3 [15], and ZnO [16,17] thin films were prepared on Si substrates by various
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processing methods of MOCVD, PLD, and MBE technique. In preparing epitaxial oxide thin films on Si substrates, there are several major difficulties of possible oxidation of Si surface, interdiffusion at the interface, and large lattice mismatches between oxide films and Si substrates, which result in the formation of amorphous or poor crystalline oxide films on Si substrates. These problems can be avoided if a proper buffer layer is introduced between the film and substrate interface. Various kinds of buffer layers, such as, TiN [12], SiC [16], AlN [17], and CaF2 [18] have been used to prepare high-quality epitaxial oxide thin films on Si substrates. Among them, TiN is the most promising candidate because it has very strong corrosion and erosion resistance, good diffusion resistance, low resistivity, and good adhesion property to most oxide materials [19–22]. In addition, the epitaxy nature of the TiN thin film on a Si substrate has been well known by domain matching epitaxy theorem even though they have very large lattice mismatch of 25% [19]. The purpose of this study is to integrate artificial STO/ BTO superlattices on Si substrates for the possible application of functional superlattices in the novel devices. We also investigated the dielectric property of the superlattice and compared it with those of epitaxial STO, BTO, (Ba0.5Sr0.5)TiO3 (BSTO) oxide thin films on Si substrates. 2. Experimental procedure Epitaxial TiN and oxide thin films were fabricated on Si (0 0 1) substrates using a multitarget PLD system equipped with a high-vacuum chamber up to 5 106 Torr and a KrF excimer laser (l ¼ 248 nm). Si (0 0 1) substrates were ultrasonically cleaned in acetone, methanol, isopropyl alcohol, and deionized water sequentially followed by dipping into 10 wt% HF solution for 1 min to remove the surface oxide layer. The cleaned substrate was loaded into the processing chamber immediately to minimize formation of native oxide. After loading the substrate into the chamber, the substrate was heated at a desired temperature at a base vacuum pressure 5 106 Torr. The stoichiometric polycrystalline TiN target (99.5%, CERAC, USA) and oxide targets (BTO, STO, BSTO), prepared by solidstate reaction of high-purity SrCO3, BaCO3, and TiO2 powders (Sigma-Aldrich, USA), were ablated by focusing the KrF excimer laser beam into the targets. The target–substrate distance was kept at 50 mm. The TiN buffer layer was prepared in vacuum condition. First few atomic layers of the artificial superlattices and oxide thin films were prepared in vacuum condition to prevent possible oxidation of the TiN buffer layer and then they were fabricated in oxygen partial pressure of 1 mTorr. The laser repetition rate was 8 Hz for the TiN film and 10 Hz for oxide films. Laser energy densities irradiated on the TiN and oxide targets were 3 and 2 J/cm2, respectively. The deposition temperature for the TiN buffer layer was 700 1C and for the oxide thin films was 650 1C. Oxide thin films, of which thickness was controlled by changing the deposition
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time, were successively prepared without breaking vacuum using the multitarget holders. The phase of the film was characterized using X-ray diffraction (XRD) (X’pert PRO, Philips, Eindhoven, The Netherlands) operated at 40 kV and 40 mA. Microstructure, crystallographic orientation, and epitaxy of the films were analyzed using conventional TEM (JEM 2000EX, JEOL, Tokyo, Japan) and high-resolution TEM (HRTEM) (Tecnai, FEI, Eindhoven, The Netherlands) at 200 kV. Samples for TEM were prepared by a conventional wedge technique for cross-sectional view observation. The relative dielectric constants of the superlattice and epitaxial oxide thin films were measured using an impedance analyzer (HP4192A, HP, USA) at 100 kHz and at room temperature. The thickness of the oxide thin films for the electrical property measurement was fixed to 100 nm. Pt top electrodes with 100 mm 100 mm in size were deposited using sputtering method and the TiN buffer layer, 100 nm in thickness, was used as a bottom electrode. 3. Results and discussion Heteroepitaxial TiN buffer layers were deposited at optimized processing conditions such as substrate temperature of 700 1C, base pressure of 5 106 Torr, and laser energy density of 3 J/cm2. The XRD and TEM results in our previous study showed that the TiN film has cubeon-cube epitaxial relationship with a Si substrate [23]. Figs. 1(a–c) show XRD y22y scans of STO, BSTO, and BTO thin films grown on Si substrates with TiN buffer layer, 50 nm in thickness, using the multitarget PLD system. Diffraction peaks marked with ‘‘*’’ are from the TiN (0 0 2) plane and indicated by ‘‘arrows’’ are from oxide (0 0 2) planes. The diffraction pattern shows very strong {0 0 1} family of planes of oxides and TiN indicating that the films are highly textured along the film normal even though very weak peaks from BSTO (1 1 1) in Fig. 1(b) and BTO (1 1 0) in Fig. 1(c) are observed. It is also clearly observed that the position of STO, BSTO, and BTO (0 0 2) peaks shifts from high angle (2y ¼ 45:811 for STO) to low angle (2y ¼ 45:011 for BSTO and 44.021 for BTO) reflecting the increasing the lattice planar distances of oxide (0 0 2) planes. In order to characterize in-plane epitaxy nature of oxide films, off-axis XRD f-scans were performed on oxide/TiN/Si specimens. Figs. 2(a–c) show off-axis XRD f-scans of the BSTO {1 0 1} planes, TiN {1 0 1} planes, and Si {2 0 2} planes, respectively, where the BSTO thin film was grown on the TiN-buffered Si(0 0 1) substrate. Peaks are only observed at every 901 at the same f-angle position indicating a good alignment of the a and b axes of the BSTO and TiN films with Si (0 0 1) substrate for cube-on-cube orientation, i.e., the crystallography of the heterostructure is revealed to be [1 0 0]BSTOJ[1 0 0]TiNJ[1 0 0]Si and [0 1 0]BSTOJ[0 1 0]TiNJ0 1 0]Si. No other peak from misoriented crystals was observed in the f-scans. XRD f-scans from the STO/TiN/Si and the BTO/TiN/Si specimens also showed same in-plane orientation relation-
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Fig. 1. X-ray diffraction (XRD) y22y scans of SrTiO3 (a), (Ba0.5,Sr0.5)TiO3 (b), and BaTiO3 (c) thin films grown on TiN-buffered Si substrates using the multitarget PLD system at 650 1C and oxygen partial pressure of 1 mTorr.
Fig. 2. XRD off-axis f-scans of the (Ba0.5,Sr0.5)TiO3 {1 0 1} planes (a), TiN {1 0 1} planes (b), and Si {2 0 2} planes (c), where the (Ba0.5,Sr0.5)TiO3 thin film was grown on the TiN-buffered Si(0 0 1) substrate at 650 1C and oxygen partial pressure of 1 mTorr.
ship as shown in Fig. 2. These XRD results of y22y scans and off-axis f-scans show the epitaxial orientation relationship of [1 1 0](0 0 1)filmsJ[1 1 0](0 0 1)TiNJ[1 1 0](0 0 1)Si. Figs. 3(a–c) are bright-field TEM micrographs of STO, BTO, and BSTO thin films grown on TiN/Si substrates, respectively. Insets are corresponding selected area diffraction (SAD) patterns obtained at the film/substrate interface. All the images show that interfaces of oxides/TiN and TiN/Si are sharp without any indication of interfacial reaction and any formation of interfacial compounds. SAD patterns obtained along the Si [1 1 0] zone axis show a typical diffraction pattern from an interface between a heteroepitaxial thin film and a substrate even though they have slight mosaic nature shown by the formation of arc around diffraction spots. Indexing of these patterns shows an epitaxial orientation relationship of [1 1 0](0 0 1)films J[1 1 0](0 0 1)TiNJ[1 1 0](0 0 1)Si, which is well consistent with the XRD results in Figs. 1 and 2. Our experimental results shown above describe that oxide thin films of STO, BTO, and BSTO were grown successfully on TiN-buffered Si substrates with cube-oncube epitaxy nature in this study. It is generally observed in perovskite oxide films on Si substrates that epitaxial perovskite oxide unit cell rotates 451 on Si [1 0 0] with
small pffiffiffi lattice mismatch because the diagonal length ( 2a; a ¼ 0:3820:40 nm) of lattice of perovskite oxides agrees well with the lattice constant of Si (0.543 nm) and then the [0 1 0] axis of perovskite oxide films tends to have 451 rotation with respect to that of Si substrate [24,25]. However, the cube-on-cube in-plane epitaxy nature, observed in this study, has been previously reported by others where a non-oxide TiN buffer layer was introduced between oxide films and Si substrates as explained in the BTO/TiN/Si [11] and the STO/TiN/Si system [20,26]. Our experimental results of the epitaxy nature of oxide films on Si substrates are well consistent with these previous reports, where the TiN buffer layer grown with cube-oncube epitaxy nature plays an important role in the formation of cube-on-cube epitaxy of oxide thin films on Si substrates. After success in the growth of heteroepitaxial oxide thin films of BTO, STO, and BSTO on TiN buffered Si substrates as described above, it was tried to grow alternating STO and BTO thin layers to form an artificial STO/BTO superlattice with 2 nm/2 nm periodicity on a Si substrate. The thickness of each layer was controlled by changing the number of pulses irradiated on each target during the deposition process. For the growth of 1 nm thick
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Fig. 3. Bright-field TEM micrographs of SrTiO3 (a), BaTiO3 (b), and (Ba0.5,Sr0.5)TiO3 (c) thin films grown on TiN/Si substrates at 650 1C and oxygen partial pressure of 1 mTorr. Insets are corresponding SAD patterns obtained at the film/substrate interface.
STO and BTO layers, STO and BTO targets were irradiated for 170 and 120 pulses, respectively, in this study. Figs. 4(a) and (b) show XRD y22y scan and f-scan of the multilayer containing the STO/BTO superlattice grown on a TiN-buffered Si substrate. XRD y22y scan result shows only {0 0 1} family of planes of the superlattice
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and TiN indicating that the films are highly textured along the film normal. Peaks marked by ‘‘*’’ in the range between 401 and 501 shows the characteristic of superlattice structure, which is very well consistent with the peaks in typical XRD pattern of superlattice structures reported by others. [3,27] XRD peaks, in the f-scan result of Fig. 4(b), are only observed at every 901 at the same f-angle position indicating the crystallography of the heterostructure is revealed to be [1 0 0]superlatticeJ[1 0 0]TiNJ[1 0 0]Si and [0 1 0]superlatticeJ[0 1 0]TiNJ[0 1 0]Si. These XRD results of y22y scans and off-axis f-scans show the epitaxial orientation relationship of [1 1 0](0 0 1)superlattice J[1 1 0](0 0 1)TiNJ[1 1 0](0 0 1)Si, which is well consistent with the results of the single oxide layer thin films of BTO, STO, and BSTO in Figs. 1 and 2. The microstructure of the STO/BTO superlattice was also investigated using TEM as shown in Fig. 5. Figs. 5(a)–(d) are a bright-field TEM micrograph of the STO/BTO superlattice specimen, a high-resolution image showing the STO/BTO layers on an atomic level, the SAD pattern obtained at the superlattice/TiN/Si interface along the Si [1 1 0] zone axis, and a schematic drawing of the SAD pattern indexed, respectively. The insets in Figs. 5(a) and (c) are the magnified view of the high-resolution image and the magnified diffraction pattern of the boxed part in Fig. 5(c), respectively. The bright-field image shows periodic alternating layers with bright and dark contrast in the oxide layer. These bright and dark contrast results from the difference in the electron scattering factor between barium (Ba) and strontium (Sr). Because Ba in the BTO layer is a heavier element compare with Sr in the STO layer, the BTO layer scatters more electrons than the STO layer, which results in dark contrast in the bright-field image (Fig. 5(a)). Therefore, bright-field TEM result of periodic layers of bright and dark contrast in Fig. 5(a) indicates that STO and BTO alternating layers are successfully deposited. Alternating layers of bright and dark contrast are also observed in the high-resolution image (Fig. 5(b)), which indicates that the STO and BTO alternating layers are formed on an atomic level even though the interface between the STO and BTO is not atomically flat and sharp. The average thickness of each layer is about 2.4 nm, which is thicker than the initially intended oxide thickness of 2 nm. These bright-field and high-resolution TEM results show that alternating heteroepitaxial STO and BTO layers were formed successfully on the TiN-buffered Si substrate, even though controlling the thickness of depositing species and formation of very abrupt interfaces between each layers has not been successfully achieved yet. The SAD pattern in Fig. 5(c) shows a typical diffraction pattern observed at the interface between a heteroepitaxial thin film and a substrate. It is clearly observed that there are three sets of diffraction patterns as indicated by rectangles and circles in the indexed schematic drawing of the diffraction pattern (Fig. 5(d)). The rectangles with a dash-line and gray circles, a continuous-line and closed circles, and a dot-line and
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Fig. 4. XRD y22y scan (a) and off-axis f-scans (b) of the SrTiO3/BaTiO3 superlattice grown on a TiN-buffered Si substrate using the multitarget PLD system at 650 1C and oxygen partial pressure of 1 mTorr.
open circles represent diffraction patterns of superlattice, Si, and TiN, respectively. It is also clearly observed in the magnified diffraction pattern where ‘‘T’’ indicates transmitted beam (the inset in Fig. 5(c)) that there are distinct satellite spots, marked by a white arrow, indicating the formation of superlattice structure. This SAD pattern result of satellite spots is very well consistent with the XRD extra peaks in Fig. 4. Indexing of the SAD pattern shows that the superlattice and TiN-buffered Si has the cube-on-cube epitaxial orientation relationship of [1 1 0](0 0 1)superlatticeJ[1 1 0](0 0 1)TiNJ[1 1 0](0 0 1)Si, which is well consistent with XRD results of y22y scan and off-axis f-scans in Fig. 4. The lattice parameters of STO and BTO layers in the superlattice were studied to investigate the strain in the superlattice film. The in-plane lattice parameter (a) of the superlattice was calculated from the SAD pattern in Fig. 5(c) by using the Si diffraction pattern as a reference. Because the split of spots from between BTO and STO was not observed in the SAD pattern, only one in-plane lattice parameter (a) was obtained in this study. The calculated inplane lattice parameter (a) is 0.395 nm, which is different from 0.391 nm of bulk STO and 0.399 nm of bulk BTO, and it is similar to the reported values of 0.393 nm of BTO and 0.392 nm of STO in the STO/BTO superlattice with
periodicity of 2 nm [27]. The out-of-plane lattice parameters (c), calculated from the XRD result in Fig. 4(a), of BTO and STO in the superlattice structure are 0.407 and 0.388 nm, respectively. These values are different from those of 0.403 nm of bulk BTO and 0.391 nm of bulk STO and show similar tendency to the reported values of 0.416 and 0.386 nm in STO/BTO superlattice with periodicity of 2 nm [27]. These results indicate that the superlattice fabricated in this study is in in-plane strained state compare with the bulk BTO and STO crystals, which results in that the BTO layer expands and the STO layer contracts along the c-axis. Fig. 6 shows the relative dielectric constants of various heteroepitaxial oxide thin films fabricated on TiN-buffered Si substrates. The relative dielectric constants were evaluated from the capacitance value of zero bias voltage at 100 kHz and room temperature. The relative dielectric constants of BTO, STO, BSTO, and superlattice with the intended stacking periodicity of 1 nm/1 nm thin films are about 300, 410, 520, and 680, respectively. These values are not the best results compared with the reported high dielectric constants of epitaxial BTO (er ¼ 803 at 1 MHz) [11], STO (er ¼ 312 at 1 MHz) [20], and STO/BTO superlattice (er ¼ 1600 at 1 MHz) [3] thin films, however, our experimental values fall within standard values for BTO,
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Fig. 5. Bright-field TEM micrograph of the STO/BTO superlattice specimen (a), a high-resolution image showing the STO/BTO layers on an atomic level (b), the SAD pattern obtained at the superlattice/TiN/Si interface along the Si [1 1 0] zone axis (c), and a schematic diagram of the SAD pattern indexed (d). The insets in (b) and (c) are the magnified view of the high-resolution image and the magnified diffraction pattern of the boxed part in (c), respectively.
STO, BSTO, and STO/BTO superlattice thin films. It has been well known that the dielectric property of a thin film very depends on the microstructures of the films [28–30], interfacial quality [31], and film texture [32]. Therefore, the dielectric constant values of our oxide thin films could be increased by optimizing processing conditions to improve the crystal quality and interfacial property. Generally, the dielectric constant of the BTO is higher than that of the STO. However, the dielectric constant of the BTO thin film is lower compare with that of the STO thin film in this study. This may be explained by the poor crystal quality of the BTO thin film (Fig. 3(a)) compare with that of the STO thin film (Fig. 3(b)). The higher dielectric constant of the STO/BTO superlattice specimen compared with other
BTO, STO, and BSTO thin films in this study could be explained by the strain induced by the lattice mismatch between the STO and BTO layers as explained above and observed by many other previous studies on the STO/BTO superlattices [2,3,27,33]. Although the STO/BTO interface is neither atomically abrupt nor perfectly coherent, the lattice planes between STO and BTO crystals are mostly connected across the interface as shown in Fig. 5(b), which seems to give the change in the a- and c- lattice parameters of BTO and STO and results in the increase of the dielectric constant. Further study on the effect of the strain change on the dielectric constant of the superlattice integrated on a Si substrate by varying the STO/BTO periodicity is being investigated.
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References
1000 Film thickness: 100 nm Frequency: 100 kHz Room Temp.
Dielectric constant (εr)
800
STO/BTO Superlattice
600
(Ba0.5Sr0.5)TiO3
400 SrTiO3
BaTiO3 200
0 Thin Film Materials Fig. 6. Relative dielectric constants of various heteroepitaxial BaTiO3, SrTiO3, (Ba0.5,Sr0.5)TiO3, and SrTiO3/BaTiO3 superlattice thin films fabricated on TiN-buffered Si substrates at 650 1C and oxygen partial pressure of 1 mTorr.
4. Conclusion Artificial STO/BTO superlattices, STO, BTO, and BSTO thin films were epitaxially grown on TiN-buffered Si substrates using the multitarget PLD system. Oxide thin films were grown with a cube-on-cube epitaxy nature which has an epitaxial orientation relationship of [1 1 0](0 0 1)filmsJ[1 1 0](0 0 1)TiNJ[1 1 0](0 0 1)Si. The dielectric constants of BTO, STO, BSTO, and the STO/BTO superlattice with 1 nm/1 nm periodicity were shown to be as high as 300, 410, 520, and 680 at the frequency of 100 kHz, respectively.
Acknowledgements Authors acknowledge the support from Korea Ministry of Science and Technology through the National Research Laboratory program.
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Journal of Crystal Growth 289 (2006) 540–546 www.elsevier.com/locate/jcrysgro
Integration of artificial SrTiO3/BaTiO3 superlattices on Si substrates using a TiN buffer layer by pulsed laser deposition method Tae-Un Kima, Bo Ram Kima, Won-Jae Leeb, Jong Ha Moona, Byung-Teak Leea, Jin Hyeok Kima, a
Photonic and Electronic Thin Films Laboratory, Department of Materials Science & Engineering, Chonnam National University, 300 Yongbong-Dong, Puk-Gu, Kwangju, 500-757, Republic of Korea b Department of Information Material Engineering, Dongeui University, 995 Eomgwangno, Busanjin-Gu, Busan, 614-714, Republic of Korea Received 15 August 2005; received in revised form 4 November 2005; accepted 24 November 2005 Available online 7 February 2006 Communicated by M. Kawasaki
Abstract Epitaxial SrTiO3(STO)/BaTiO3(BTO) artificial superlattices, STO, BTO, and (Ba0.5,Sr0.5)TiO3 (BSTO) thin films have been grown on TiN-buffered Si (0 0 1) substrates by pulsed laser deposition method and their dielectric properties were studied. The crystal orientation, epitaxy nature, and microstructure of epitaxial oxide thin films were investigated using X-ray diffraction and transmission electron microscopy. Thin films were prepared with laser fluence of 3 and 2 J/cm2, repetition rate of 8 and 10 Hz, substrate temperature of 700 and 650 1C for TiN and oxide, respectively. The TiN buffer layer and oxide thin films were grown with cube-on-cube epitaxial orientation relationship of [1 1 0](0 0 1)filmsJ[1 1 0](0 0 1)TiNJ[1 1 0](0 0 1)Si. The dielectric constants of BTO, STO, BSTO, and STO/BTO superlattice epitaxial thin films with 1 nm/1 nm periodicity were shown to be as high as 300, 410, 520, and 680 at the frequency of 100 kHz, respectively. r 2006 Elsevier B.V. All rights reserved. PACS: 68.65.Cd; 77.55.+f; 81.15.Fg; 68.55.Jk; 68.37.Lp Keywords: A3. Laser epitaxy; B1. Oxides; B2. Dielectric materials
1. Introduction Oxide artificial superlattices have attracted much attention because the superlattices have the potential to provide a new function or enhanced performance to existing devices by controlling lattice strain, dimensionality, and stacking periodicity. A number of papers concerning the growth of artificial superlattices of BaTiO3(BTO)/ SrTiO3(STO) and related materials [1–3], PbZrO3/PbTiO3 [4], SrZrO3/SrTiO3 [5], Bi-based layered oxides [6], and LaFeO3/LaCrO3 [7] have been reported to improve their dielectric, ferroelectric, electro-optic, and magnetic properties. Especially, (0 0 1) oriented STO/BTO superlattices are of great interest because it is relatively well known material Corresponding author. Tel.: 82 62 530 1709; fax: 82 62 530 1699.
E-mail address: [email protected] (J.H. Kim). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.11.119
system and their dielectric and ferroelectric properties can be enhanced by controlling the lattice strain along the polarized direction. For the growth of STO/BTO epitaxial superlattices, oxide single-crystal substrates, such as, STO [2,8], and MgO [3], have been used because they have low lattice mismatch and similar crystal structure with film materials, which gives easy of fabrication. However, oxide substrates are expensive and incompatible with existing mature Si technology. Therefore, in practical viewpoint, it is more desirable to fabricate highly oriented or epitaxial oxide thin films on Si substrates to open the door to fabricate Si-based novel devices with enhanced functionality and flexibility [9,10]. There have been many previous studies on oxide epitaxy on Si substrates. Various kinds of material systems, such as BTO [11], STO [12,13], CeO2 [14], NdNiO3 [15], and ZnO [16,17] thin films were prepared on Si substrates by various
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processing methods of MOCVD, PLD, and MBE technique. In preparing epitaxial oxide thin films on Si substrates, there are several major difficulties of possible oxidation of Si surface, interdiffusion at the interface, and large lattice mismatches between oxide films and Si substrates, which result in the formation of amorphous or poor crystalline oxide films on Si substrates. These problems can be avoided if a proper buffer layer is introduced between the film and substrate interface. Various kinds of buffer layers, such as, TiN [12], SiC [16], AlN [17], and CaF2 [18] have been used to prepare high-quality epitaxial oxide thin films on Si substrates. Among them, TiN is the most promising candidate because it has very strong corrosion and erosion resistance, good diffusion resistance, low resistivity, and good adhesion property to most oxide materials [19–22]. In addition, the epitaxy nature of the TiN thin film on a Si substrate has been well known by domain matching epitaxy theorem even though they have very large lattice mismatch of 25% [19]. The purpose of this study is to integrate artificial STO/ BTO superlattices on Si substrates for the possible application of functional superlattices in the novel devices. We also investigated the dielectric property of the superlattice and compared it with those of epitaxial STO, BTO, (Ba0.5Sr0.5)TiO3 (BSTO) oxide thin films on Si substrates. 2. Experimental procedure Epitaxial TiN and oxide thin films were fabricated on Si (0 0 1) substrates using a multitarget PLD system equipped with a high-vacuum chamber up to 5 106 Torr and a KrF excimer laser (l ¼ 248 nm). Si (0 0 1) substrates were ultrasonically cleaned in acetone, methanol, isopropyl alcohol, and deionized water sequentially followed by dipping into 10 wt% HF solution for 1 min to remove the surface oxide layer. The cleaned substrate was loaded into the processing chamber immediately to minimize formation of native oxide. After loading the substrate into the chamber, the substrate was heated at a desired temperature at a base vacuum pressure 5 106 Torr. The stoichiometric polycrystalline TiN target (99.5%, CERAC, USA) and oxide targets (BTO, STO, BSTO), prepared by solidstate reaction of high-purity SrCO3, BaCO3, and TiO2 powders (Sigma-Aldrich, USA), were ablated by focusing the KrF excimer laser beam into the targets. The target–substrate distance was kept at 50 mm. The TiN buffer layer was prepared in vacuum condition. First few atomic layers of the artificial superlattices and oxide thin films were prepared in vacuum condition to prevent possible oxidation of the TiN buffer layer and then they were fabricated in oxygen partial pressure of 1 mTorr. The laser repetition rate was 8 Hz for the TiN film and 10 Hz for oxide films. Laser energy densities irradiated on the TiN and oxide targets were 3 and 2 J/cm2, respectively. The deposition temperature for the TiN buffer layer was 700 1C and for the oxide thin films was 650 1C. Oxide thin films, of which thickness was controlled by changing the deposition
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time, were successively prepared without breaking vacuum using the multitarget holders. The phase of the film was characterized using X-ray diffraction (XRD) (X’pert PRO, Philips, Eindhoven, The Netherlands) operated at 40 kV and 40 mA. Microstructure, crystallographic orientation, and epitaxy of the films were analyzed using conventional TEM (JEM 2000EX, JEOL, Tokyo, Japan) and high-resolution TEM (HRTEM) (Tecnai, FEI, Eindhoven, The Netherlands) at 200 kV. Samples for TEM were prepared by a conventional wedge technique for cross-sectional view observation. The relative dielectric constants of the superlattice and epitaxial oxide thin films were measured using an impedance analyzer (HP4192A, HP, USA) at 100 kHz and at room temperature. The thickness of the oxide thin films for the electrical property measurement was fixed to 100 nm. Pt top electrodes with 100 mm 100 mm in size were deposited using sputtering method and the TiN buffer layer, 100 nm in thickness, was used as a bottom electrode. 3. Results and discussion Heteroepitaxial TiN buffer layers were deposited at optimized processing conditions such as substrate temperature of 700 1C, base pressure of 5 106 Torr, and laser energy density of 3 J/cm2. The XRD and TEM results in our previous study showed that the TiN film has cubeon-cube epitaxial relationship with a Si substrate [23]. Figs. 1(a–c) show XRD y22y scans of STO, BSTO, and BTO thin films grown on Si substrates with TiN buffer layer, 50 nm in thickness, using the multitarget PLD system. Diffraction peaks marked with ‘‘*’’ are from the TiN (0 0 2) plane and indicated by ‘‘arrows’’ are from oxide (0 0 2) planes. The diffraction pattern shows very strong {0 0 1} family of planes of oxides and TiN indicating that the films are highly textured along the film normal even though very weak peaks from BSTO (1 1 1) in Fig. 1(b) and BTO (1 1 0) in Fig. 1(c) are observed. It is also clearly observed that the position of STO, BSTO, and BTO (0 0 2) peaks shifts from high angle (2y ¼ 45:811 for STO) to low angle (2y ¼ 45:011 for BSTO and 44.021 for BTO) reflecting the increasing the lattice planar distances of oxide (0 0 2) planes. In order to characterize in-plane epitaxy nature of oxide films, off-axis XRD f-scans were performed on oxide/TiN/Si specimens. Figs. 2(a–c) show off-axis XRD f-scans of the BSTO {1 0 1} planes, TiN {1 0 1} planes, and Si {2 0 2} planes, respectively, where the BSTO thin film was grown on the TiN-buffered Si(0 0 1) substrate. Peaks are only observed at every 901 at the same f-angle position indicating a good alignment of the a and b axes of the BSTO and TiN films with Si (0 0 1) substrate for cube-on-cube orientation, i.e., the crystallography of the heterostructure is revealed to be [1 0 0]BSTOJ[1 0 0]TiNJ[1 0 0]Si and [0 1 0]BSTOJ[0 1 0]TiNJ0 1 0]Si. No other peak from misoriented crystals was observed in the f-scans. XRD f-scans from the STO/TiN/Si and the BTO/TiN/Si specimens also showed same in-plane orientation relation-
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Fig. 1. X-ray diffraction (XRD) y22y scans of SrTiO3 (a), (Ba0.5,Sr0.5)TiO3 (b), and BaTiO3 (c) thin films grown on TiN-buffered Si substrates using the multitarget PLD system at 650 1C and oxygen partial pressure of 1 mTorr.
Fig. 2. XRD off-axis f-scans of the (Ba0.5,Sr0.5)TiO3 {1 0 1} planes (a), TiN {1 0 1} planes (b), and Si {2 0 2} planes (c), where the (Ba0.5,Sr0.5)TiO3 thin film was grown on the TiN-buffered Si(0 0 1) substrate at 650 1C and oxygen partial pressure of 1 mTorr.
ship as shown in Fig. 2. These XRD results of y22y scans and off-axis f-scans show the epitaxial orientation relationship of [1 1 0](0 0 1)filmsJ[1 1 0](0 0 1)TiNJ[1 1 0](0 0 1)Si. Figs. 3(a–c) are bright-field TEM micrographs of STO, BTO, and BSTO thin films grown on TiN/Si substrates, respectively. Insets are corresponding selected area diffraction (SAD) patterns obtained at the film/substrate interface. All the images show that interfaces of oxides/TiN and TiN/Si are sharp without any indication of interfacial reaction and any formation of interfacial compounds. SAD patterns obtained along the Si [1 1 0] zone axis show a typical diffraction pattern from an interface between a heteroepitaxial thin film and a substrate even though they have slight mosaic nature shown by the formation of arc around diffraction spots. Indexing of these patterns shows an epitaxial orientation relationship of [1 1 0](0 0 1)films J[1 1 0](0 0 1)TiNJ[1 1 0](0 0 1)Si, which is well consistent with the XRD results in Figs. 1 and 2. Our experimental results shown above describe that oxide thin films of STO, BTO, and BSTO were grown successfully on TiN-buffered Si substrates with cube-oncube epitaxy nature in this study. It is generally observed in perovskite oxide films on Si substrates that epitaxial perovskite oxide unit cell rotates 451 on Si [1 0 0] with
small pffiffiffi lattice mismatch because the diagonal length ( 2a; a ¼ 0:3820:40 nm) of lattice of perovskite oxides agrees well with the lattice constant of Si (0.543 nm) and then the [0 1 0] axis of perovskite oxide films tends to have 451 rotation with respect to that of Si substrate [24,25]. However, the cube-on-cube in-plane epitaxy nature, observed in this study, has been previously reported by others where a non-oxide TiN buffer layer was introduced between oxide films and Si substrates as explained in the BTO/TiN/Si [11] and the STO/TiN/Si system [20,26]. Our experimental results of the epitaxy nature of oxide films on Si substrates are well consistent with these previous reports, where the TiN buffer layer grown with cube-oncube epitaxy nature plays an important role in the formation of cube-on-cube epitaxy of oxide thin films on Si substrates. After success in the growth of heteroepitaxial oxide thin films of BTO, STO, and BSTO on TiN buffered Si substrates as described above, it was tried to grow alternating STO and BTO thin layers to form an artificial STO/BTO superlattice with 2 nm/2 nm periodicity on a Si substrate. The thickness of each layer was controlled by changing the number of pulses irradiated on each target during the deposition process. For the growth of 1 nm thick
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Fig. 3. Bright-field TEM micrographs of SrTiO3 (a), BaTiO3 (b), and (Ba0.5,Sr0.5)TiO3 (c) thin films grown on TiN/Si substrates at 650 1C and oxygen partial pressure of 1 mTorr. Insets are corresponding SAD patterns obtained at the film/substrate interface.
STO and BTO layers, STO and BTO targets were irradiated for 170 and 120 pulses, respectively, in this study. Figs. 4(a) and (b) show XRD y22y scan and f-scan of the multilayer containing the STO/BTO superlattice grown on a TiN-buffered Si substrate. XRD y22y scan result shows only {0 0 1} family of planes of the superlattice
543
and TiN indicating that the films are highly textured along the film normal. Peaks marked by ‘‘*’’ in the range between 401 and 501 shows the characteristic of superlattice structure, which is very well consistent with the peaks in typical XRD pattern of superlattice structures reported by others. [3,27] XRD peaks, in the f-scan result of Fig. 4(b), are only observed at every 901 at the same f-angle position indicating the crystallography of the heterostructure is revealed to be [1 0 0]superlatticeJ[1 0 0]TiNJ[1 0 0]Si and [0 1 0]superlatticeJ[0 1 0]TiNJ[0 1 0]Si. These XRD results of y22y scans and off-axis f-scans show the epitaxial orientation relationship of [1 1 0](0 0 1)superlattice J[1 1 0](0 0 1)TiNJ[1 1 0](0 0 1)Si, which is well consistent with the results of the single oxide layer thin films of BTO, STO, and BSTO in Figs. 1 and 2. The microstructure of the STO/BTO superlattice was also investigated using TEM as shown in Fig. 5. Figs. 5(a)–(d) are a bright-field TEM micrograph of the STO/BTO superlattice specimen, a high-resolution image showing the STO/BTO layers on an atomic level, the SAD pattern obtained at the superlattice/TiN/Si interface along the Si [1 1 0] zone axis, and a schematic drawing of the SAD pattern indexed, respectively. The insets in Figs. 5(a) and (c) are the magnified view of the high-resolution image and the magnified diffraction pattern of the boxed part in Fig. 5(c), respectively. The bright-field image shows periodic alternating layers with bright and dark contrast in the oxide layer. These bright and dark contrast results from the difference in the electron scattering factor between barium (Ba) and strontium (Sr). Because Ba in the BTO layer is a heavier element compare with Sr in the STO layer, the BTO layer scatters more electrons than the STO layer, which results in dark contrast in the bright-field image (Fig. 5(a)). Therefore, bright-field TEM result of periodic layers of bright and dark contrast in Fig. 5(a) indicates that STO and BTO alternating layers are successfully deposited. Alternating layers of bright and dark contrast are also observed in the high-resolution image (Fig. 5(b)), which indicates that the STO and BTO alternating layers are formed on an atomic level even though the interface between the STO and BTO is not atomically flat and sharp. The average thickness of each layer is about 2.4 nm, which is thicker than the initially intended oxide thickness of 2 nm. These bright-field and high-resolution TEM results show that alternating heteroepitaxial STO and BTO layers were formed successfully on the TiN-buffered Si substrate, even though controlling the thickness of depositing species and formation of very abrupt interfaces between each layers has not been successfully achieved yet. The SAD pattern in Fig. 5(c) shows a typical diffraction pattern observed at the interface between a heteroepitaxial thin film and a substrate. It is clearly observed that there are three sets of diffraction patterns as indicated by rectangles and circles in the indexed schematic drawing of the diffraction pattern (Fig. 5(d)). The rectangles with a dash-line and gray circles, a continuous-line and closed circles, and a dot-line and
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Fig. 4. XRD y22y scan (a) and off-axis f-scans (b) of the SrTiO3/BaTiO3 superlattice grown on a TiN-buffered Si substrate using the multitarget PLD system at 650 1C and oxygen partial pressure of 1 mTorr.
open circles represent diffraction patterns of superlattice, Si, and TiN, respectively. It is also clearly observed in the magnified diffraction pattern where ‘‘T’’ indicates transmitted beam (the inset in Fig. 5(c)) that there are distinct satellite spots, marked by a white arrow, indicating the formation of superlattice structure. This SAD pattern result of satellite spots is very well consistent with the XRD extra peaks in Fig. 4. Indexing of the SAD pattern shows that the superlattice and TiN-buffered Si has the cube-on-cube epitaxial orientation relationship of [1 1 0](0 0 1)superlatticeJ[1 1 0](0 0 1)TiNJ[1 1 0](0 0 1)Si, which is well consistent with XRD results of y22y scan and off-axis f-scans in Fig. 4. The lattice parameters of STO and BTO layers in the superlattice were studied to investigate the strain in the superlattice film. The in-plane lattice parameter (a) of the superlattice was calculated from the SAD pattern in Fig. 5(c) by using the Si diffraction pattern as a reference. Because the split of spots from between BTO and STO was not observed in the SAD pattern, only one in-plane lattice parameter (a) was obtained in this study. The calculated inplane lattice parameter (a) is 0.395 nm, which is different from 0.391 nm of bulk STO and 0.399 nm of bulk BTO, and it is similar to the reported values of 0.393 nm of BTO and 0.392 nm of STO in the STO/BTO superlattice with
periodicity of 2 nm [27]. The out-of-plane lattice parameters (c), calculated from the XRD result in Fig. 4(a), of BTO and STO in the superlattice structure are 0.407 and 0.388 nm, respectively. These values are different from those of 0.403 nm of bulk BTO and 0.391 nm of bulk STO and show similar tendency to the reported values of 0.416 and 0.386 nm in STO/BTO superlattice with periodicity of 2 nm [27]. These results indicate that the superlattice fabricated in this study is in in-plane strained state compare with the bulk BTO and STO crystals, which results in that the BTO layer expands and the STO layer contracts along the c-axis. Fig. 6 shows the relative dielectric constants of various heteroepitaxial oxide thin films fabricated on TiN-buffered Si substrates. The relative dielectric constants were evaluated from the capacitance value of zero bias voltage at 100 kHz and room temperature. The relative dielectric constants of BTO, STO, BSTO, and superlattice with the intended stacking periodicity of 1 nm/1 nm thin films are about 300, 410, 520, and 680, respectively. These values are not the best results compared with the reported high dielectric constants of epitaxial BTO (er ¼ 803 at 1 MHz) [11], STO (er ¼ 312 at 1 MHz) [20], and STO/BTO superlattice (er ¼ 1600 at 1 MHz) [3] thin films, however, our experimental values fall within standard values for BTO,
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Fig. 5. Bright-field TEM micrograph of the STO/BTO superlattice specimen (a), a high-resolution image showing the STO/BTO layers on an atomic level (b), the SAD pattern obtained at the superlattice/TiN/Si interface along the Si [1 1 0] zone axis (c), and a schematic diagram of the SAD pattern indexed (d). The insets in (b) and (c) are the magnified view of the high-resolution image and the magnified diffraction pattern of the boxed part in (c), respectively.
STO, BSTO, and STO/BTO superlattice thin films. It has been well known that the dielectric property of a thin film very depends on the microstructures of the films [28–30], interfacial quality [31], and film texture [32]. Therefore, the dielectric constant values of our oxide thin films could be increased by optimizing processing conditions to improve the crystal quality and interfacial property. Generally, the dielectric constant of the BTO is higher than that of the STO. However, the dielectric constant of the BTO thin film is lower compare with that of the STO thin film in this study. This may be explained by the poor crystal quality of the BTO thin film (Fig. 3(a)) compare with that of the STO thin film (Fig. 3(b)). The higher dielectric constant of the STO/BTO superlattice specimen compared with other
BTO, STO, and BSTO thin films in this study could be explained by the strain induced by the lattice mismatch between the STO and BTO layers as explained above and observed by many other previous studies on the STO/BTO superlattices [2,3,27,33]. Although the STO/BTO interface is neither atomically abrupt nor perfectly coherent, the lattice planes between STO and BTO crystals are mostly connected across the interface as shown in Fig. 5(b), which seems to give the change in the a- and c- lattice parameters of BTO and STO and results in the increase of the dielectric constant. Further study on the effect of the strain change on the dielectric constant of the superlattice integrated on a Si substrate by varying the STO/BTO periodicity is being investigated.
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References
1000 Film thickness: 100 nm Frequency: 100 kHz Room Temp.
Dielectric constant (εr)
800
STO/BTO Superlattice
600
(Ba0.5Sr0.5)TiO3
400 SrTiO3
BaTiO3 200
0 Thin Film Materials Fig. 6. Relative dielectric constants of various heteroepitaxial BaTiO3, SrTiO3, (Ba0.5,Sr0.5)TiO3, and SrTiO3/BaTiO3 superlattice thin films fabricated on TiN-buffered Si substrates at 650 1C and oxygen partial pressure of 1 mTorr.
4. Conclusion Artificial STO/BTO superlattices, STO, BTO, and BSTO thin films were epitaxially grown on TiN-buffered Si substrates using the multitarget PLD system. Oxide thin films were grown with a cube-on-cube epitaxy nature which has an epitaxial orientation relationship of [1 1 0](0 0 1)filmsJ[1 1 0](0 0 1)TiNJ[1 1 0](0 0 1)Si. The dielectric constants of BTO, STO, BSTO, and the STO/BTO superlattice with 1 nm/1 nm periodicity were shown to be as high as 300, 410, 520, and 680 at the frequency of 100 kHz, respectively.
Acknowledgements Authors acknowledge the support from Korea Ministry of Science and Technology through the National Research Laboratory program.
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