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Laser molecular beam epitaxy growth and properties of SrTiO3 thin films for microelectronic applications
Thin Solid Films 515 (2006) 559 – 562 www.elsevier.com/locate/tsf
Laser molecular beam epitaxy growth and properties of SrTiO3 thin films for microelectronic applications J.H. Hao a,⁎, J. Gao a , H.K. Wong b a b
Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong Department of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong Available online 24 January 2006
Abstract Dielectric SrTiO3 thin films were deposited on LaAlO3 and Si substrates using laser molecular beam epitaxy. The correlations between the deposition parameters of SrTiO3 thin films, their structural characteristics, and dielectric properties were studied. The conditions for achieving epitaxial SrTiO3 thin films were found to be limited to deposition conditions such as deposition temperature. We show that the SrTiO3 films with single (110) orientation can be grown directly on Si substrates. The nature of epitaxial growth and interfacial structures of the grown films were examined by various techniques, such as Laue diffraction and X-ray photoelectron spectroscopy. The SrTiO3/Si interface was found to be epitaxially crystallized without any SiO2 layer. Furthermore, we have measured dielectric properties of the grown SrTiO3 multilayer suitable for tunable microwave device. A large tunability of 74.7%, comparable to that of SrTiO3 single-crystal, was observed at cryogenic temperatures. Such STO thin films will be very promising for the development of microelectronic device applications. © 2005 Elsevier B.V. All rights reserved. PACS: 77.55.+f; 81.15Fg Keywords: Strontium titanate; Heteroepitaxy; Tunability; Silicon; Microelectronics
1. Introduction The ABO3 perovskite titanates like SrTiO3 (STO) possess a wide range of physical properties. The study of heteroepitaxial structures of STO is motivated by the integration of STO with semiconductors like silicon (Si), and the boosted interest for unusual effects of STO thin films differing substantially from those in corresponding bulk materials. The formation of epitaxial oxide structures on semiconductors represents a promising means to integrate their unique properties on a semiconductor platform for both existing and future microelectronic device concepts [1,2]. The resulting system like STO/Si can find many potential applications such as alternative gate oxides to SiO2, ferroelectric random access memories, pseudo substrates for the subsequent heteroepitaxy of other various functional perovskite oxides. On the other hand, STO bulk is an incipient ferroelectric material which has large dielectric nonlinearity at cryogenic temperature [3,4]. Those properties make it an ideal material for ⁎ Corresponding author. Tel.: +852 2859 2361; fax: +852 2559 9152. E-mail address: [email protected] (J.H. Hao). 0040-6090/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.12.297
frequency and phase agile microwave devices. Such devices are of interest for many applications, especially in the wireless communication. The tunable microwave devices based on thin films have advantages of small size and weight, low RF loss and fast switching time. However, the poorer properties of STO thin films reported so far are the main technical bottleneck facing this technology. The study of structural atomic-scale imperfections, oxygen vacancy and interface effects on the physical properties of the films is much needed to meet the requirement of microelectronic applications. It is typically necessary to select suitable deposition temperature, deposition rate and specific growth sequences for STO material. Laser molecular beam epitaxy (L-MBE) combines the advantages of pulsed laser deposition for oxide film growth and MBE for film growth [5]. Consequently, L-MBE provides a range of solutions to optimize growth conditions, and tailor the STO film/substrate interface. In this work, dielectric STO thin films were deposited on various substrates with LMBE technique. The correlations between the deposition parameters of STO thin films and their physical properties were investigated.
J.H. Hao et al. / Thin Solid Films 515 (2006) 559–562
3. Results and discussion Fig. 2 shows a typical XRD θ–2θ scan curve of STO thin films directly grown on various substrates, including LAO and (100)-oriented Si substrates. In the θ–2θ scan curve of STO thin films grown on LAO in Fig. 2 (a), only the (k00) peaks of the STO are present along with those peaks of LAO substrate. It
Fig. 1. Experimental setup for Laser-MBE technique.
STO(200)
LAO(100)
STO/LAO
STO(100)
STO/Si
STO(110)
STO(200)
(b) XRD Intensity (a.u.)
Ts=650 °C
STO/Si Ts=760 °C
STO(220) Si(400)
(c)
STO(110)
A single-crystal STO target was used in our experiment. The STO films were deposited by the Laser-MBE system as shown in Fig. 1. The system consists mainly of an ultra-high vacuum (UHV) chamber containing a multi-target holder and a substrate holder, a pulsed KrF excimer laser (λ = 248 nm) with optics, and in situ monitoring reflection high energy electron diffraction (RHEED) with high-sensitivity CCD camera-based video recording system and an image processor connected to a computer. An infrared pyrometer located outside the chamber, will be used for measuring the substrate temperature. The LaAlO3 (LAO) and Si substrates were used. In particular, the Si wafers were etched in a dilute HF solution to remove the native oxide from the Si surface. A variety of substrate temperature and oxygen partial pressure was employed during the growth process. After deposition, the deposited films were cooled to room temperature in an ambient of high oxygen pressure. The film thickness was 50–1200 nm from a measurement of a Dektax3ST surface profile. The X-ray diffraction (XRD) analysis was made using θ–2θ scan on the Siemens D5000 X-ray diffractometer. The in-plane alignments of the films were studied by low-angle Laue diffraction [6]. The interfacial phase was studied using X-ray photoelectron spectroscopy (XPS). A top Ag electrode and bottom SrRuO3 (SRO) electrode were formed a parallel-plate capacitor structure for dielectric measurement. Dielectric properties were measured at 1 KHz using an Agilent 4284A Precision LCR Meter with option adding ± 40 V internal dc bias voltages. The voltage dependence of the capacitance and dielectric loss (tanδ) was measured in a closed-cycle cryogenic system, allowing for a continuous temperature sweep within the temperature range from 10 to 300 K.
LAO(200)
(a)
2. Experimental
STO(100)
560
20
30
40
50
60
70
2θ (deg.) Fig. 2. XRD patterns of STO thin films grown on (a) LAO substrate. (b) Si substrate at 650 °C. (c) Si substrate at 760 °C.
shows the STO films grow with a axis normal to the substrate. Further studies for Φ scans indicated that only four peaks, 90° apart, were observed for the STO films. This indicates that the STO film is in-plane aligned with the substrate. Fig. 2 (b) and (c) suggested that the out-of-plane orientation for the STO films grown on Si was related to the deposition temperature. Deposition temperature at 650 °C resulted in the formation of polycrystalline film, in which two peaks of (110) and (200) of STO were observed as shown in Fig. 2 (b). Fig. 2 (c) demonstrated that the STO films deposited at 760 °C exhibit strong (110) diffraction peaks in addition to the silicon substrate diffraction, with no extra XRD peaks from other crystalline orientations of the STO films. Low-angle Laue diffraction method was used to examine the in-plane epitaxial relationship and determine precisely the orientation in STO/Si. Fig. 3 shows the resulting Laue diffraction patterns of the STO thin films on Si. The spots for STO/Si(100) were labeled. Analysis of the Laue diffraction pattern for STO/ Si(100) reveals that the in-plane alignments are STO [001]// ¯ Si [001] and STO [110]//Si [010]. It suggested that the epitaxial
J.H. Hao et al. / Thin Solid Films 515 (2006) 559–562
growth of STO thin films on Si without any buffer layer has been achieved through rotating the STO lattice by 45° with respect to the Si lattice. It is known that not only the quite large lattice mismatch between STO and Si but also the formation of an amorphous oxide over Si surface presents a challenge for epitaxial growth. Those STO films grown directly on Si using various deposition techniques were usually polycrystalline with randomly oriented grains. The sputtering process was used in a depth profile study of XPS spectra. The confirmation of the absence of amorphous SiO2 layer in our sample was established in XPS core levels of Si 2p taken at the STO/Si interface as shown in Fig. 4. Apart from the Si 2p peak at 99.6 eV for pure Si from Si substrate, a broad peak centered about 102.6 eV formed. It is known that the binding energy of Si 2p for SiO2 is at 103.6 eV. Consequently, no peak corresponding to the binding energy of amorphous SiO2 was found at the interface.
It is well accepted that the use of appropriate templates or buffer layers between STO and Si substrate is required for the epitaxial growth of STO films [7–9]. Thus, it is unusual that the epitaxy of STO (110) films grown directly on Si was achieved. Growth mechanism may be related to the structure and bonding at the interface between the grown STO thin films and Si. Furthermore, the (110)-oriented STO structure in the work is expected to be useful for practical applications [10,11], such as the preparation of ferroelectric-insulator–semiconductor devices. In Fig. 5, low-frequency dielectric constant (ε) and loss tangent are plotted as a function of applied voltage for STO films deposited on SRO electrode layer on LAO substrate at T = 10 K. The hystereses exist in both the ε and tanδ measurements, which may be associated to the ferroelectric ordering and/or the interfacial space charge. As can be seen, both ε and tanδ changed with increasing applied voltage. It demonstrated that the dielectric constants of the STO thin films are tunable by applied dc voltage. For the STO single crystal, the dielectric constant can be suppressed by more than 80% by applying an electric voltage [12], and this is often characterized by the tuning as defined by (ε(0) − ε(V)) / ε(0), where ε(V) and ε(0) are the dielectric constants with and without the applied voltage. However, the tuning properties of STO thin films, which are necessary for device applications, are not as good as those of single crystal. Earlier studies demonstrated that the Lyddane–Sachs–Teller relation between the optical-phonon eigenfrequencies and the dielectric constant is fully maintained, as is the case in the bulk material [13,14]. In the STO films, the soft mode revealed hardening
1.0
(a) T=10 K Tuning=74.7%
0.8 ε/εmax
Fig. 3. Laue diffraction patterns of the STO thin films on Si. X-ray is incident at the film surface at ∼8°. Si peaks are indexed as normal numbers while STO peaks are broader and indexed as italic numbers.
561
0.6 0.4 0.2
Si substrate
1.2
STO/Si interface
(b) T=10 K
Intensity (a.u.)
tan δ/(tan δ)max
1.0 0.8 0.6 0.4 0.2 0.0 -20 98
100
102
-10
0 V (V)
10
20
104
Binding energy (eV) Fig. 4. XPS core levels of Si 2p taken at the STO/Si interface.
Fig. 5. The dependence of (a) dielectric constant and (b) loss tangent as a function of applied voltage for STO films deposited on SRO electrode layer on LAO substrate at T = 10 K. The measurement was carried out at 1 kHz.
562
J.H. Hao et al. / Thin Solid Films 515 (2006) 559–562
compared to that in single crystal. On the other hand, many factors, such as strain, oxygen vacancies and interfacial effects have very significant impact on the dielectric nonlinearity and loss of titanate films [15,16]. At any rate, a tuning of 74.7% of STO films is obtained in Fig. 5(a). It should be kept in mind that the bias electric field for comparable tuning is much larger than that in single crystal as thin film is generally much thinner than single crystal. Nonetheless, it is evident that similar tunability compared to that of STO single crystal can be achieved in thin films. Also, the tunability was found to decrease considerably upon increasing the temperature. Further studies are planned to investigate the effects of growth mode, crystallinity, and interface layer on tunability and losses in atomically controlled epitaxy of STO thin films. 4. Conclusions In summary, we have investigated the epitaxial growth and properties of STO thin films on LAO and Si substrates. The STO films deposited on Si at 760 °C exhibit single (110) diffraction peaks. The epitaxial growth of STO thin films on Si without any buffer layer has been achieved through rotating the STO lattice by 45° with respect to the Si lattice. The (110)-oriented STO structure in the work should be useful for microelectronic applications. Also, the dc electric field dependence of dielectric properties has been studied at cryogenic temperatures. The high tunability (74.7%) compared to that of STO single-crystal was observed in STO thin films. Such STO films are much needed for the development of tunable microwave device applications. We expect that the Laser-MBE technique will enable not only to form STO thin films but also to construct artificially designed new lattices suitable for various microelectronic devices in our planned work.
Acknowledgments This work has been supported by the Research Grant Council of Hong Kong (Grant No. HKU 7025/04P). References [1] Z. Yu, Y. Liang, C. Overgaard, X. Hu, J. Curless, H. Li, Y. Wei, B. Craigo, D. Jordan, R. Droopad, J. Finder, K. Eisenbeiser, D. Marshall, K. Moore, J. Kulik, P. Fejes, Thin Solid Films 462–463 (2004) 51. [2] C.J. Först, C.R. Ashman, K. Schwarz, P.E. Blöchi, Nature 427 (2004) 53. [3] M.A. Saifi, L.E. Cross, Phys. Rev. B 2 (1970) 677. [4] H.-M. Christen, J. Mannhart, E.J. Williams, C. Gerber, Phys. Rev. B 49 (1994) 12095. [5] H. Koinuma, N. Kanda, J. Nishino, A. Ohtomo, H. Kubota, M. Kawasaki, M. Yoshimoto, Appl. Surf. Sci. 109/110 (1997) 514. [6] G.A. Walker, J. Vac. Sci. Technol. 7 (1970) 465. [7] D.P. Norton, C. Park, Y.E. Lee, J.D. Budai, J. Vac. Sci. Technol. B 20 (2002) 257. [8] T. Yamada, N. Wakiya, K. Shinozaki, N. Mizutani, Appl. Phys. Lett. 83 (2003) 4815. [9] Y.Z. Yoo, P. Ahmet, Z.W. Jin, K. Nakajima, T. Chikyow, M. Kawasaki, Y. Konishi, Y. Yonezawa, J.H. Song, H. Koinuma, Appl. Phys. Lett. 82 (2003) 4125. [10] S. Migita, S. Sakai, J. Appl. Phys. 89 (2001) 5421. [11] Y. Mukunoki, N. Nakagawa, T. Susaki, H.Y. Huang, Appl. Phys. Lett. 86 (2005) 171908. [12] R.C. Neville, B. Hoeneisen, C.A. Mead, J. Appl. Phys. 43 (1972) 2124. [13] A.A. Sirenko, C. Bernhard, A. Golnik, A.M. Clark, J.H. Hao, W. Si, X.X. Xi, Nature 404 (2000) 373. [14] I.A. Akimov, A.A. Sirenko, A.M. Clark, J.H. Hao, X.X. Xi, Phys. Rev. Lett. 84 (2000) 4625. [15] J.H. Hao, W.D. Si, X.X. Xi, R. Guo, A.S. Bhalla, L.E. Cross, Appl. Phys. Lett. 76 (2000) 3100. [16] A.M. Clark, J.H. Hao, W.D. Si, X.X. Xi, Integr. Ferroelectr. 29 (2000) A53.
Laser molecular beam epitaxy growth and properties of SrTiO3 thin films for microelectronic applications J.H. Hao a,⁎, J. Gao a , H.K. Wong b a b
Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong Department of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong Available online 24 January 2006
Abstract Dielectric SrTiO3 thin films were deposited on LaAlO3 and Si substrates using laser molecular beam epitaxy. The correlations between the deposition parameters of SrTiO3 thin films, their structural characteristics, and dielectric properties were studied. The conditions for achieving epitaxial SrTiO3 thin films were found to be limited to deposition conditions such as deposition temperature. We show that the SrTiO3 films with single (110) orientation can be grown directly on Si substrates. The nature of epitaxial growth and interfacial structures of the grown films were examined by various techniques, such as Laue diffraction and X-ray photoelectron spectroscopy. The SrTiO3/Si interface was found to be epitaxially crystallized without any SiO2 layer. Furthermore, we have measured dielectric properties of the grown SrTiO3 multilayer suitable for tunable microwave device. A large tunability of 74.7%, comparable to that of SrTiO3 single-crystal, was observed at cryogenic temperatures. Such STO thin films will be very promising for the development of microelectronic device applications. © 2005 Elsevier B.V. All rights reserved. PACS: 77.55.+f; 81.15Fg Keywords: Strontium titanate; Heteroepitaxy; Tunability; Silicon; Microelectronics
1. Introduction The ABO3 perovskite titanates like SrTiO3 (STO) possess a wide range of physical properties. The study of heteroepitaxial structures of STO is motivated by the integration of STO with semiconductors like silicon (Si), and the boosted interest for unusual effects of STO thin films differing substantially from those in corresponding bulk materials. The formation of epitaxial oxide structures on semiconductors represents a promising means to integrate their unique properties on a semiconductor platform for both existing and future microelectronic device concepts [1,2]. The resulting system like STO/Si can find many potential applications such as alternative gate oxides to SiO2, ferroelectric random access memories, pseudo substrates for the subsequent heteroepitaxy of other various functional perovskite oxides. On the other hand, STO bulk is an incipient ferroelectric material which has large dielectric nonlinearity at cryogenic temperature [3,4]. Those properties make it an ideal material for ⁎ Corresponding author. Tel.: +852 2859 2361; fax: +852 2559 9152. E-mail address: [email protected] (J.H. Hao). 0040-6090/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.12.297
frequency and phase agile microwave devices. Such devices are of interest for many applications, especially in the wireless communication. The tunable microwave devices based on thin films have advantages of small size and weight, low RF loss and fast switching time. However, the poorer properties of STO thin films reported so far are the main technical bottleneck facing this technology. The study of structural atomic-scale imperfections, oxygen vacancy and interface effects on the physical properties of the films is much needed to meet the requirement of microelectronic applications. It is typically necessary to select suitable deposition temperature, deposition rate and specific growth sequences for STO material. Laser molecular beam epitaxy (L-MBE) combines the advantages of pulsed laser deposition for oxide film growth and MBE for film growth [5]. Consequently, L-MBE provides a range of solutions to optimize growth conditions, and tailor the STO film/substrate interface. In this work, dielectric STO thin films were deposited on various substrates with LMBE technique. The correlations between the deposition parameters of STO thin films and their physical properties were investigated.
J.H. Hao et al. / Thin Solid Films 515 (2006) 559–562
3. Results and discussion Fig. 2 shows a typical XRD θ–2θ scan curve of STO thin films directly grown on various substrates, including LAO and (100)-oriented Si substrates. In the θ–2θ scan curve of STO thin films grown on LAO in Fig. 2 (a), only the (k00) peaks of the STO are present along with those peaks of LAO substrate. It
Fig. 1. Experimental setup for Laser-MBE technique.
STO(200)
LAO(100)
STO/LAO
STO(100)
STO/Si
STO(110)
STO(200)
(b) XRD Intensity (a.u.)
Ts=650 °C
STO/Si Ts=760 °C
STO(220) Si(400)
(c)
STO(110)
A single-crystal STO target was used in our experiment. The STO films were deposited by the Laser-MBE system as shown in Fig. 1. The system consists mainly of an ultra-high vacuum (UHV) chamber containing a multi-target holder and a substrate holder, a pulsed KrF excimer laser (λ = 248 nm) with optics, and in situ monitoring reflection high energy electron diffraction (RHEED) with high-sensitivity CCD camera-based video recording system and an image processor connected to a computer. An infrared pyrometer located outside the chamber, will be used for measuring the substrate temperature. The LaAlO3 (LAO) and Si substrates were used. In particular, the Si wafers were etched in a dilute HF solution to remove the native oxide from the Si surface. A variety of substrate temperature and oxygen partial pressure was employed during the growth process. After deposition, the deposited films were cooled to room temperature in an ambient of high oxygen pressure. The film thickness was 50–1200 nm from a measurement of a Dektax3ST surface profile. The X-ray diffraction (XRD) analysis was made using θ–2θ scan on the Siemens D5000 X-ray diffractometer. The in-plane alignments of the films were studied by low-angle Laue diffraction [6]. The interfacial phase was studied using X-ray photoelectron spectroscopy (XPS). A top Ag electrode and bottom SrRuO3 (SRO) electrode were formed a parallel-plate capacitor structure for dielectric measurement. Dielectric properties were measured at 1 KHz using an Agilent 4284A Precision LCR Meter with option adding ± 40 V internal dc bias voltages. The voltage dependence of the capacitance and dielectric loss (tanδ) was measured in a closed-cycle cryogenic system, allowing for a continuous temperature sweep within the temperature range from 10 to 300 K.
LAO(200)
(a)
2. Experimental
STO(100)
560
20
30
40
50
60
70
2θ (deg.) Fig. 2. XRD patterns of STO thin films grown on (a) LAO substrate. (b) Si substrate at 650 °C. (c) Si substrate at 760 °C.
shows the STO films grow with a axis normal to the substrate. Further studies for Φ scans indicated that only four peaks, 90° apart, were observed for the STO films. This indicates that the STO film is in-plane aligned with the substrate. Fig. 2 (b) and (c) suggested that the out-of-plane orientation for the STO films grown on Si was related to the deposition temperature. Deposition temperature at 650 °C resulted in the formation of polycrystalline film, in which two peaks of (110) and (200) of STO were observed as shown in Fig. 2 (b). Fig. 2 (c) demonstrated that the STO films deposited at 760 °C exhibit strong (110) diffraction peaks in addition to the silicon substrate diffraction, with no extra XRD peaks from other crystalline orientations of the STO films. Low-angle Laue diffraction method was used to examine the in-plane epitaxial relationship and determine precisely the orientation in STO/Si. Fig. 3 shows the resulting Laue diffraction patterns of the STO thin films on Si. The spots for STO/Si(100) were labeled. Analysis of the Laue diffraction pattern for STO/ Si(100) reveals that the in-plane alignments are STO [001]// ¯ Si [001] and STO [110]//Si [010]. It suggested that the epitaxial
J.H. Hao et al. / Thin Solid Films 515 (2006) 559–562
growth of STO thin films on Si without any buffer layer has been achieved through rotating the STO lattice by 45° with respect to the Si lattice. It is known that not only the quite large lattice mismatch between STO and Si but also the formation of an amorphous oxide over Si surface presents a challenge for epitaxial growth. Those STO films grown directly on Si using various deposition techniques were usually polycrystalline with randomly oriented grains. The sputtering process was used in a depth profile study of XPS spectra. The confirmation of the absence of amorphous SiO2 layer in our sample was established in XPS core levels of Si 2p taken at the STO/Si interface as shown in Fig. 4. Apart from the Si 2p peak at 99.6 eV for pure Si from Si substrate, a broad peak centered about 102.6 eV formed. It is known that the binding energy of Si 2p for SiO2 is at 103.6 eV. Consequently, no peak corresponding to the binding energy of amorphous SiO2 was found at the interface.
It is well accepted that the use of appropriate templates or buffer layers between STO and Si substrate is required for the epitaxial growth of STO films [7–9]. Thus, it is unusual that the epitaxy of STO (110) films grown directly on Si was achieved. Growth mechanism may be related to the structure and bonding at the interface between the grown STO thin films and Si. Furthermore, the (110)-oriented STO structure in the work is expected to be useful for practical applications [10,11], such as the preparation of ferroelectric-insulator–semiconductor devices. In Fig. 5, low-frequency dielectric constant (ε) and loss tangent are plotted as a function of applied voltage for STO films deposited on SRO electrode layer on LAO substrate at T = 10 K. The hystereses exist in both the ε and tanδ measurements, which may be associated to the ferroelectric ordering and/or the interfacial space charge. As can be seen, both ε and tanδ changed with increasing applied voltage. It demonstrated that the dielectric constants of the STO thin films are tunable by applied dc voltage. For the STO single crystal, the dielectric constant can be suppressed by more than 80% by applying an electric voltage [12], and this is often characterized by the tuning as defined by (ε(0) − ε(V)) / ε(0), where ε(V) and ε(0) are the dielectric constants with and without the applied voltage. However, the tuning properties of STO thin films, which are necessary for device applications, are not as good as those of single crystal. Earlier studies demonstrated that the Lyddane–Sachs–Teller relation between the optical-phonon eigenfrequencies and the dielectric constant is fully maintained, as is the case in the bulk material [13,14]. In the STO films, the soft mode revealed hardening
1.0
(a) T=10 K Tuning=74.7%
0.8 ε/εmax
Fig. 3. Laue diffraction patterns of the STO thin films on Si. X-ray is incident at the film surface at ∼8°. Si peaks are indexed as normal numbers while STO peaks are broader and indexed as italic numbers.
561
0.6 0.4 0.2
Si substrate
1.2
STO/Si interface
(b) T=10 K
Intensity (a.u.)
tan δ/(tan δ)max
1.0 0.8 0.6 0.4 0.2 0.0 -20 98
100
102
-10
0 V (V)
10
20
104
Binding energy (eV) Fig. 4. XPS core levels of Si 2p taken at the STO/Si interface.
Fig. 5. The dependence of (a) dielectric constant and (b) loss tangent as a function of applied voltage for STO films deposited on SRO electrode layer on LAO substrate at T = 10 K. The measurement was carried out at 1 kHz.
562
J.H. Hao et al. / Thin Solid Films 515 (2006) 559–562
compared to that in single crystal. On the other hand, many factors, such as strain, oxygen vacancies and interfacial effects have very significant impact on the dielectric nonlinearity and loss of titanate films [15,16]. At any rate, a tuning of 74.7% of STO films is obtained in Fig. 5(a). It should be kept in mind that the bias electric field for comparable tuning is much larger than that in single crystal as thin film is generally much thinner than single crystal. Nonetheless, it is evident that similar tunability compared to that of STO single crystal can be achieved in thin films. Also, the tunability was found to decrease considerably upon increasing the temperature. Further studies are planned to investigate the effects of growth mode, crystallinity, and interface layer on tunability and losses in atomically controlled epitaxy of STO thin films. 4. Conclusions In summary, we have investigated the epitaxial growth and properties of STO thin films on LAO and Si substrates. The STO films deposited on Si at 760 °C exhibit single (110) diffraction peaks. The epitaxial growth of STO thin films on Si without any buffer layer has been achieved through rotating the STO lattice by 45° with respect to the Si lattice. The (110)-oriented STO structure in the work should be useful for microelectronic applications. Also, the dc electric field dependence of dielectric properties has been studied at cryogenic temperatures. The high tunability (74.7%) compared to that of STO single-crystal was observed in STO thin films. Such STO films are much needed for the development of tunable microwave device applications. We expect that the Laser-MBE technique will enable not only to form STO thin films but also to construct artificially designed new lattices suitable for various microelectronic devices in our planned work.
Acknowledgments This work has been supported by the Research Grant Council of Hong Kong (Grant No. HKU 7025/04P). References [1] Z. Yu, Y. Liang, C. Overgaard, X. Hu, J. Curless, H. Li, Y. Wei, B. Craigo, D. Jordan, R. Droopad, J. Finder, K. Eisenbeiser, D. Marshall, K. Moore, J. Kulik, P. Fejes, Thin Solid Films 462–463 (2004) 51. [2] C.J. Först, C.R. Ashman, K. Schwarz, P.E. Blöchi, Nature 427 (2004) 53. [3] M.A. Saifi, L.E. Cross, Phys. Rev. B 2 (1970) 677. [4] H.-M. Christen, J. Mannhart, E.J. Williams, C. Gerber, Phys. Rev. B 49 (1994) 12095. [5] H. Koinuma, N. Kanda, J. Nishino, A. Ohtomo, H. Kubota, M. Kawasaki, M. Yoshimoto, Appl. Surf. Sci. 109/110 (1997) 514. [6] G.A. Walker, J. Vac. Sci. Technol. 7 (1970) 465. [7] D.P. Norton, C. Park, Y.E. Lee, J.D. Budai, J. Vac. Sci. Technol. B 20 (2002) 257. [8] T. Yamada, N. Wakiya, K. Shinozaki, N. Mizutani, Appl. Phys. Lett. 83 (2003) 4815. [9] Y.Z. Yoo, P. Ahmet, Z.W. Jin, K. Nakajima, T. Chikyow, M. Kawasaki, Y. Konishi, Y. Yonezawa, J.H. Song, H. Koinuma, Appl. Phys. Lett. 82 (2003) 4125. [10] S. Migita, S. Sakai, J. Appl. Phys. 89 (2001) 5421. [11] Y. Mukunoki, N. Nakagawa, T. Susaki, H.Y. Huang, Appl. Phys. Lett. 86 (2005) 171908. [12] R.C. Neville, B. Hoeneisen, C.A. Mead, J. Appl. Phys. 43 (1972) 2124. [13] A.A. Sirenko, C. Bernhard, A. Golnik, A.M. Clark, J.H. Hao, W. Si, X.X. Xi, Nature 404 (2000) 373. [14] I.A. Akimov, A.A. Sirenko, A.M. Clark, J.H. Hao, X.X. Xi, Phys. Rev. Lett. 84 (2000) 4625. [15] J.H. Hao, W.D. Si, X.X. Xi, R. Guo, A.S. Bhalla, L.E. Cross, Appl. Phys. Lett. 76 (2000) 3100. [16] A.M. Clark, J.H. Hao, W.D. Si, X.X. Xi, Integr. Ferroelectr. 29 (2000) A53.