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(Sr1−x,Lax)TiO3 superlattices grown by laser molecular beam epitaxy
Surface & Coatings Technology 201 (2007) 5374 – 5377 www.elsevier.com/locate/surfcoat
Electrical properties of SrTiO3/(Sr1−x,Lax)TiO3 superlattices grown by laser molecular beam epitaxy Juho Kim a , Young Sung Kim b , Jaichan Lee a,⁎ a
b
Department of Materials Science and Engineering, Center for Advanced Plasma Surface Technology, Sung Kyun Kwan University, Suwon, 440-746, Republic of Korea Advanced Material Process of Information Technology, Sung Kyun Kwan University, Suwon, 440-746, Republic of Korea Available online 14 August 2006
Abstract We have successfully fabricated SrTiO3/(Sr1−x,Lax)TiO3 (STO/SLTO) superlattices on step-terrace structured SrTiO3, in which La defect is distributed in a regular manner of one dimension. The superlattices contain one unit cell thick SLTO layer with various stacking sequences. The charge carrier density of the superlattices was in the range of 1.7 × 1019–2 × 1016 cm− 3. In addition, the mobility and resistivity of STO/SLTO superlattice were 11 cm2/Vs and 0.18 Ω cm at the STO1/SLTO1 stacking sequence, respectively. The charge carrier density of STO/SLTO superlattices increased significantly with the change in a stacking sequence, while the mobility is relatively insensitive to the change in the stacking sequence. The resulting resistivity of the superlattics changed significantly with the change in the stacking sequence due to mainly the change of the charge carrier density. The oxide superlattice approach provides an useful way of nonlinear control of charge carrier density and electrical resistivity. © 2006 Elsevier B.V. All rights reserved. PACS: 73.21.Cd; 73.50.-h Keywords: STO/SLTO superlattices; RHEED; Electrical properties; Carrier density; Mobility; Resistivity
1. Introduction Perovskite-type ABO3 oxides have long been studied due to their wide physical properties, such as dielectric, ferroelectric, superconductivity, ferromagnetics, antiferromagnetics, colossal magnetoresistance, metal-insulator transition behavior, etc [1–3]. The electrical properties of the perovskite are closely related to its crystal structure and defects including oxygen vacancies, which is modified by doping or annealing in different oxygen atmosphere due to the fact that point defects are electrically charged. Specifically, strontium titanate (SrTiO3) is a prototype of the ABO3 perovskite oxides and exhibits various interesting physical properties. For examples, SrTiO3 exhibits n-type superconductivity at low temperatures when it is doped with Nb or oxygen vacancy [4,5]. Impurity doping of La on Sr site or Nb on Ti site forms donor states which generate extra electrons for maintaining
⁎ Corresponding author. Tel./fax: +82 31 290 7397/7410. E-mail address: [email protected] (J. Lee). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.07.042
of electric neutrality, leading to the change from insulator to an ntype semiconductor and finally to a metallic behavior [6]. Hwang et al. reported the electrical properties of SrTiO3 (STO) thin films through various experimental approaches such as, delta doping, oxygen stoichiometry, and polar/non-polar heterointerface, in which high mobility as high as 104 cm2/Vs at low temperatures was obtained [7]. In this study, we have considered SrTiO3 doped with La (i.e., Sr1−xLaxTiO3). SrTiO3 is a band insulator with d0 configuration, whereas LaTiO3 is a Mott insulator with d1 configuration [8]. Sr1−xLaxTiO3 between d1 and d0 configurations has a transition from band to Mott insulator via metallic state, in which a number of d electrons is varied from 0 to 1 with a change of x. In addition, Vilquin et al. reported that the carrier type was first found to be hole below 10% Sr doping level, while electron over 15% Sr doping level [9]. Meanwhile, oxide superlattice has attracted much attention since the oxide superlattice has been reported to provide new functionality or enhanced physical properties over single layer oxide thin films [10–12]. The oxide superlattice is, in general, meant by a multilayered structure consisting of two or more
J. Kim et al. / Surface & Coatings Technology 201 (2007) 5374–5377
alternating oxide layer. Layer-by-layer growth technique of oxide thin film allows successful fabrication of the oxide superlattice, which is composed of alternating two or more epitaxial thin layers. Fabrication of the oxide superlattice also requires understanding the epitaxial growth mode as well as substrate with stepterrace surface structure for successful atomic scale control on growing surface. To realize physical property of oxide superlattice, it is critically important to grow the oxide superlattice via two dimensional (2D) growth. Two dimensional growth modes, such as layer-by-layer growth or step-flow growth, has been incorporated in thin film growth, specifically the growth of oxide superlattice, by the molecular beam epitaxy (MBE) or pulsed laser deposition (PLD) [13,14]. There has been no prior report of the electrical conductivity of doped STO thin film via the superlattice approach. In this study, we have incorporated point defect such as La into SrTiO3, and fabricated (Sr1−x,Lax)TiO3/SrTiO3 (STO/SLTO) superlattice by laser molecular beam epitaxy (laser-MBE), in which the La point defect is incorporated in regular manner. We report an investigation on the growth and electrical properties in STO/ SLTO superlattices with various stacking sequences.
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3. Results and discussions Fig. 1 shows the RHEED intensity oscillation and diffraction patterns of STO3/SLTO1 and STO6/SLTO1 superlattices, and AFM topography image of STO6/SLTO1 superlattice (100 nm thick). The in-situ RHEED intensity oscillation and diffraction pattern provide useful information of the surface structure, morphology and growth mode. In the superlattice, the thickness of STO and SLTO layers were in-situ controlled for various unit cell thicknesses by monitoring the RHEED intensity oscillation,
2. Experimental The SrTiO3/(Sr1−x,Lax)TiO3 (STO/SLTO) superlattices have been deposited onto stepped-SrTiO3 substrate using laser-molecular beam epitaxy (laser-MBE). Ceramic SrTiO3 and (Sr1−x, Lax)TiO3 with doped La (x = 12 at.%) in A-site were used as a target. The STO substrate was wet-etched by NH4F-buffered HF (BHF) solution, followed by annealing to obtain a step-andterrace structure. This well-defined STO substrate is important to obtain two-dimensional (2D) growth (layer-by-layer or step-flow mode) with atomic-level control. Prior to the superlattice deposition, homoepixial SrTiO3 thin film with 10 monolayers was deposited for high quality thin films. The STO/SLTO superlattices were deposited at 650 °C and 10− 5 Torr of oxygen pressure (containing 8 wt.% O3). The energy density and the pulse rate of KrF laser were 1.2 J/cm2 and 2 Hz, respectively. For the STO/SLTO superlattices with various stacking sequences, the thickness of the STO layer was varied from one unit cell to 6 unit cells while the SLTO layer was kept at one unit cell thickness, i.e., STO1 unit cell/SLTO1 unit cell–STO6 unit cell/SLTO1 unit cell. Then the periodic structure was repeated for total thickness of 100 nm. Hereafter, the superlattices with the stacking sequences of STO1 unit cell/SLTO1 unit cell–STO6 unit cell/SLTO1 unit cell are represented by STO1/SLTO1–STO6/SLTO1. The stacking period of the superlattice was controlled by in-situ monitoring the intensity oscillation of specular spot of reflection high energy electron diffraction (RHEED). The structural analysis of STO/ SLTO superlattice was carried out by high resolution X-ray diffraction (HRXRD) and synchrotron X-ray scattering. The synchrotron scattering experiments were performed at beamline 3C2 at the Pohang Light Source (PLS) in Korea. The electrical properties, such as the resistivity, carrier density and hall mobility were measured at room temperature by the van der Pauw method through Hall measurement (Hall effect measurement system, ECOPIA, HMS-3000).
Fig. 1. RHEED intensity oscillation and diffraction patterns of (a) STO3/SLTO1, (b) STO6/SLTO1 superlattice, and (c) AFM topography image (image size: 2 × 2 μm2) of STO6/SLTO1 superlattice (100 nm thick).
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J. Kim et al. / Surface & Coatings Technology 201 (2007) 5374–5377
as shown in Fig. 1 (a) and (b). It is well known that the RHEED intensity oscillation is observed in layer-by-layer two dimensional (2D) growth. The intensity oscillation observed for both SLTO and STO layers indicates that 2D growth was obtained for both SLTO and STO layers. The RHEED intensity oscillation of STO/SLTO superlattice and streak diffraction pattern was preserved to the final state of the growth, indicating that two dimensional (2D) growth is obtained throughout the growth of whole structure. The RHEED intensity oscillation was obtained in the growth of all STO/SLTO superlattices studied in this work (STO1 unit cell/SLTO1 unit cell–STO6 unit cell/SLTO1 unit cell). Fig. 1 (c) shows AFM image of the STO6/SLTO1 superlattice. The corresponding step height and terrace width of the superlattice were one unit cell (4 Å) and 350 nm, respectively, which were the same as those of step-terrace structured STO substrate. Therefore, the layer-by-layer growth of superlattices with various stacking sequences was confirmed by AFM mage as well as RHEED intensity oscillation. Fig. 2 shows X-ray diffraction patterns of STO/SLTO superlattices with various stacking sequences. As the stacking sequence of the superlattice was changed from STO1/SLTO1 to STO6/SLTO1, the main peaks of the superlattice shifted toward low angles. All the STO/SLTO superlattices have a good epitaxial nature with (001) growth orientation along the surface normal direction and complete lattice coherence at the interface of STO and SLTO layers. Those results indicate that the degree of the lattice strain developed in the superlattice changes with the stacking sequence. Detailed analysis of the lattice strain of STO/ SLTO superlattices is reported in an earlier publication [15]. Fig. 3 shows the carrier density and mobility of STO/SLTO superlattices with various stacking sequences obtained from Hall effect measurement at room temperature. As the stacking sequence of STO/SLTO superlattice was changed from STO1/ SLTO6 to STO1/SLTO1, the carrier density of STO/SLTO superlattices increased nonlinearly, as shown in Fig. 3. This behavior is different from bulk Sr1−xLaxTiO3, in which the carrier density increased linearly with increasing La content [16]. Consequently, the superlattice showed metallic behavior upon the doping and exhibited the maximum carrier density of
1.7 × 1019 cm− 3 at a stacking sequence of STO1/SLTO1. In the perovskite structure, La ion substitutes for Sr site in a solid solution since La ion and Sr ion are similar each other in ionic radius. When La3+ ion is located at the Sr2+ site, Ti4+ ions are partially reduced to Ti3+ for the maintenance of electric neutrality, in which an excess electron in SLTO layer resides on Ti 3d conduction band and influences electrical conductivity. This was proved by Ohtomo et al., by electron energy loss spectra (EELS), in which the extra electrons are spread up to 2 nm in the fractional Ti3+ state [17]. The mobility of the charge carrier in the superlattice is also given in Fig. 3. The mobility of STO/ SLTO superlattices increased slightly as the stacking sequence was changed from STO1/SLTO6 to STO1/SLTO1. It is noted that the mobility is relatively insensitive to the stacking sequence while the carrier density of superlattices changes significantly with the change in the stacking sequence. As a result, the mobility of STO/SLTO superlattices was obtained in the range of 3–11 cm2/Vs, which is a typical range of doped STO thin films at the room temperature. The corresponding electrical resistivity is shown in Fig. 4. The resistivity of STO/SLTO superlattices decreased significantly with the change in the stacking sequence from STO1/SLTO6 to STO1/SLTO1, which is
Fig. 2. X-ray diffraction patterns of STO/SLTO superlattices with various stacking sequences.
Fig. 4. Electrical resistivity of STO/SLTO superlattices with a stacking sequence.
Fig. 3. Charge carrier density and mobility of STO/SLTO superlattices with a stacking sequence.
J. Kim et al. / Surface & Coatings Technology 201 (2007) 5374–5377
mainly attributed to the significant change of the carrier density. The resistivity as low as 0.18 Ω cm was obtained at the STO1/ SLTO1 stacking sequence. Therefore, we have successfully fabricated STO/SLTO superlattices, in which the La defects are regularly arranged in one dimension. Further, we have obtained a wide range of electrical properties by varying the stacking sequence of STO/SLTO superlattices. 4. Conclusions We have fabricated the STO/SLTO superlattices with various stacking sequences by laser-MBE. The STO/SLTO superlattices were epitaxially grown on step-and-terrace structured STO substrate with an aid of in-situ monitoring by RHEED intensity oscillation. The superlattices have well defined distribution of La defect, i.e., one unit cell thick SLTO layer in one dimension. A wide range of the carrier density was obtained in specifically La modified SrTiO3 (SLTO) layer by varying the stacking sequence of the STO/SLTO superlattices. The carrier density of STO/SLTO superlattices was in the range of 1.7 × 1019–2 × 1016 cm− 3. We were able to manipulate the electrical properties in STO/SLTO superlattices by changing a stacking sequence. Acknowledgements This works is supported by the Korea Ministry of Commerce, Industry and Energy through the National Research Program for 0.1 Terabit Non-volatile Memory Development and National Research Laboratory (NRL) Program.
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References [1] D. Fuchs, C.W. Schneider, R. Schneider, H. Rietschel, J. Appl. Phys. 85 (1999) 7362. [2] D. Olaya, F. Pan, C.T. Rogers, J.C. Price, Appl. Phys. Lett. 84 (2004) 4020. [3] K. Ueda, H. Tabata, T. Kawai, Science 280 (1998) 1064. [4] T. Tomio, H. Miki, H. Tabata, T. Kawai, S. Kawai, Appl. Phys. Lett. 76 (1994) 5886. [5] D. Olaya, F. Pan, C.T. Rogers, J.C. Price, Appl. Phys. Lett. 80 (2002) 2928. [6] A. Leitner, C.T. Rogers, J.C. Price, Appl. Phys. Lett. 72 (1998) 3065. [7] H.Y. Hwang, A. Ohtomo, N. Nakagawa, D.A. Muller, J.L. Grazul, Physica E 22 (2004) 712. [8] Y. Tokura, Y. Taguchi, Y. Okada, Y. Fujishima, T. Arima, Phys. Rev. Lett. 70 (1993) 2126. [9] B. Vilquin, T. Kanki, T. Yanagida, H. Tanaka, T. Kawai, Appl. Surf. Sci. 244 (2005) 494. [10] J. Kim, L. Kim, Y. Kim, Y.S. Kim, D. Jung, J. Lee, Appl. Phys. Lett. 80 (2002) 3581. [11] H.N. Lee, H.M. Christen, M.F. Chisholm, C.M. Rouleau, D.H. Lowndes, Nature 433 (2005) 395. [12] K. Lee, J. Lee, J.W. Kim, J. Korean Phys. Soc. 46 (2005) 112. [13] H.N. Lee, H.M. Christen, M.F. Chisholm, C.M. Rouleau, D.H. Lowndes, Appl. Phys. Lett. 84 (2004) 4107. [14] T. Choi, J. Lee, J. Korean Phys. Soc. 46 (2005) 116. [15] J. Kim, L. Kim, D. Jung, J. Lee, Ferroelectrics 336 (2006) 255. [16] T. Okuda, K. Nakanishi, S. Miyasaka, Y. Tokura, Phys. Rev. B 63 (2001) 113104. [17] A. Ohomo, D.A. Muller, J.L. Grazul, H.Y. Hwang, Nature 419 (2002) 378.
Electrical properties of SrTiO3/(Sr1−x,Lax)TiO3 superlattices grown by laser molecular beam epitaxy Juho Kim a , Young Sung Kim b , Jaichan Lee a,⁎ a
b
Department of Materials Science and Engineering, Center for Advanced Plasma Surface Technology, Sung Kyun Kwan University, Suwon, 440-746, Republic of Korea Advanced Material Process of Information Technology, Sung Kyun Kwan University, Suwon, 440-746, Republic of Korea Available online 14 August 2006
Abstract We have successfully fabricated SrTiO3/(Sr1−x,Lax)TiO3 (STO/SLTO) superlattices on step-terrace structured SrTiO3, in which La defect is distributed in a regular manner of one dimension. The superlattices contain one unit cell thick SLTO layer with various stacking sequences. The charge carrier density of the superlattices was in the range of 1.7 × 1019–2 × 1016 cm− 3. In addition, the mobility and resistivity of STO/SLTO superlattice were 11 cm2/Vs and 0.18 Ω cm at the STO1/SLTO1 stacking sequence, respectively. The charge carrier density of STO/SLTO superlattices increased significantly with the change in a stacking sequence, while the mobility is relatively insensitive to the change in the stacking sequence. The resulting resistivity of the superlattics changed significantly with the change in the stacking sequence due to mainly the change of the charge carrier density. The oxide superlattice approach provides an useful way of nonlinear control of charge carrier density and electrical resistivity. © 2006 Elsevier B.V. All rights reserved. PACS: 73.21.Cd; 73.50.-h Keywords: STO/SLTO superlattices; RHEED; Electrical properties; Carrier density; Mobility; Resistivity
1. Introduction Perovskite-type ABO3 oxides have long been studied due to their wide physical properties, such as dielectric, ferroelectric, superconductivity, ferromagnetics, antiferromagnetics, colossal magnetoresistance, metal-insulator transition behavior, etc [1–3]. The electrical properties of the perovskite are closely related to its crystal structure and defects including oxygen vacancies, which is modified by doping or annealing in different oxygen atmosphere due to the fact that point defects are electrically charged. Specifically, strontium titanate (SrTiO3) is a prototype of the ABO3 perovskite oxides and exhibits various interesting physical properties. For examples, SrTiO3 exhibits n-type superconductivity at low temperatures when it is doped with Nb or oxygen vacancy [4,5]. Impurity doping of La on Sr site or Nb on Ti site forms donor states which generate extra electrons for maintaining
⁎ Corresponding author. Tel./fax: +82 31 290 7397/7410. E-mail address: [email protected] (J. Lee). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.07.042
of electric neutrality, leading to the change from insulator to an ntype semiconductor and finally to a metallic behavior [6]. Hwang et al. reported the electrical properties of SrTiO3 (STO) thin films through various experimental approaches such as, delta doping, oxygen stoichiometry, and polar/non-polar heterointerface, in which high mobility as high as 104 cm2/Vs at low temperatures was obtained [7]. In this study, we have considered SrTiO3 doped with La (i.e., Sr1−xLaxTiO3). SrTiO3 is a band insulator with d0 configuration, whereas LaTiO3 is a Mott insulator with d1 configuration [8]. Sr1−xLaxTiO3 between d1 and d0 configurations has a transition from band to Mott insulator via metallic state, in which a number of d electrons is varied from 0 to 1 with a change of x. In addition, Vilquin et al. reported that the carrier type was first found to be hole below 10% Sr doping level, while electron over 15% Sr doping level [9]. Meanwhile, oxide superlattice has attracted much attention since the oxide superlattice has been reported to provide new functionality or enhanced physical properties over single layer oxide thin films [10–12]. The oxide superlattice is, in general, meant by a multilayered structure consisting of two or more
J. Kim et al. / Surface & Coatings Technology 201 (2007) 5374–5377
alternating oxide layer. Layer-by-layer growth technique of oxide thin film allows successful fabrication of the oxide superlattice, which is composed of alternating two or more epitaxial thin layers. Fabrication of the oxide superlattice also requires understanding the epitaxial growth mode as well as substrate with stepterrace surface structure for successful atomic scale control on growing surface. To realize physical property of oxide superlattice, it is critically important to grow the oxide superlattice via two dimensional (2D) growth. Two dimensional growth modes, such as layer-by-layer growth or step-flow growth, has been incorporated in thin film growth, specifically the growth of oxide superlattice, by the molecular beam epitaxy (MBE) or pulsed laser deposition (PLD) [13,14]. There has been no prior report of the electrical conductivity of doped STO thin film via the superlattice approach. In this study, we have incorporated point defect such as La into SrTiO3, and fabricated (Sr1−x,Lax)TiO3/SrTiO3 (STO/SLTO) superlattice by laser molecular beam epitaxy (laser-MBE), in which the La point defect is incorporated in regular manner. We report an investigation on the growth and electrical properties in STO/ SLTO superlattices with various stacking sequences.
5375
3. Results and discussions Fig. 1 shows the RHEED intensity oscillation and diffraction patterns of STO3/SLTO1 and STO6/SLTO1 superlattices, and AFM topography image of STO6/SLTO1 superlattice (100 nm thick). The in-situ RHEED intensity oscillation and diffraction pattern provide useful information of the surface structure, morphology and growth mode. In the superlattice, the thickness of STO and SLTO layers were in-situ controlled for various unit cell thicknesses by monitoring the RHEED intensity oscillation,
2. Experimental The SrTiO3/(Sr1−x,Lax)TiO3 (STO/SLTO) superlattices have been deposited onto stepped-SrTiO3 substrate using laser-molecular beam epitaxy (laser-MBE). Ceramic SrTiO3 and (Sr1−x, Lax)TiO3 with doped La (x = 12 at.%) in A-site were used as a target. The STO substrate was wet-etched by NH4F-buffered HF (BHF) solution, followed by annealing to obtain a step-andterrace structure. This well-defined STO substrate is important to obtain two-dimensional (2D) growth (layer-by-layer or step-flow mode) with atomic-level control. Prior to the superlattice deposition, homoepixial SrTiO3 thin film with 10 monolayers was deposited for high quality thin films. The STO/SLTO superlattices were deposited at 650 °C and 10− 5 Torr of oxygen pressure (containing 8 wt.% O3). The energy density and the pulse rate of KrF laser were 1.2 J/cm2 and 2 Hz, respectively. For the STO/SLTO superlattices with various stacking sequences, the thickness of the STO layer was varied from one unit cell to 6 unit cells while the SLTO layer was kept at one unit cell thickness, i.e., STO1 unit cell/SLTO1 unit cell–STO6 unit cell/SLTO1 unit cell. Then the periodic structure was repeated for total thickness of 100 nm. Hereafter, the superlattices with the stacking sequences of STO1 unit cell/SLTO1 unit cell–STO6 unit cell/SLTO1 unit cell are represented by STO1/SLTO1–STO6/SLTO1. The stacking period of the superlattice was controlled by in-situ monitoring the intensity oscillation of specular spot of reflection high energy electron diffraction (RHEED). The structural analysis of STO/ SLTO superlattice was carried out by high resolution X-ray diffraction (HRXRD) and synchrotron X-ray scattering. The synchrotron scattering experiments were performed at beamline 3C2 at the Pohang Light Source (PLS) in Korea. The electrical properties, such as the resistivity, carrier density and hall mobility were measured at room temperature by the van der Pauw method through Hall measurement (Hall effect measurement system, ECOPIA, HMS-3000).
Fig. 1. RHEED intensity oscillation and diffraction patterns of (a) STO3/SLTO1, (b) STO6/SLTO1 superlattice, and (c) AFM topography image (image size: 2 × 2 μm2) of STO6/SLTO1 superlattice (100 nm thick).
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J. Kim et al. / Surface & Coatings Technology 201 (2007) 5374–5377
as shown in Fig. 1 (a) and (b). It is well known that the RHEED intensity oscillation is observed in layer-by-layer two dimensional (2D) growth. The intensity oscillation observed for both SLTO and STO layers indicates that 2D growth was obtained for both SLTO and STO layers. The RHEED intensity oscillation of STO/SLTO superlattice and streak diffraction pattern was preserved to the final state of the growth, indicating that two dimensional (2D) growth is obtained throughout the growth of whole structure. The RHEED intensity oscillation was obtained in the growth of all STO/SLTO superlattices studied in this work (STO1 unit cell/SLTO1 unit cell–STO6 unit cell/SLTO1 unit cell). Fig. 1 (c) shows AFM image of the STO6/SLTO1 superlattice. The corresponding step height and terrace width of the superlattice were one unit cell (4 Å) and 350 nm, respectively, which were the same as those of step-terrace structured STO substrate. Therefore, the layer-by-layer growth of superlattices with various stacking sequences was confirmed by AFM mage as well as RHEED intensity oscillation. Fig. 2 shows X-ray diffraction patterns of STO/SLTO superlattices with various stacking sequences. As the stacking sequence of the superlattice was changed from STO1/SLTO1 to STO6/SLTO1, the main peaks of the superlattice shifted toward low angles. All the STO/SLTO superlattices have a good epitaxial nature with (001) growth orientation along the surface normal direction and complete lattice coherence at the interface of STO and SLTO layers. Those results indicate that the degree of the lattice strain developed in the superlattice changes with the stacking sequence. Detailed analysis of the lattice strain of STO/ SLTO superlattices is reported in an earlier publication [15]. Fig. 3 shows the carrier density and mobility of STO/SLTO superlattices with various stacking sequences obtained from Hall effect measurement at room temperature. As the stacking sequence of STO/SLTO superlattice was changed from STO1/ SLTO6 to STO1/SLTO1, the carrier density of STO/SLTO superlattices increased nonlinearly, as shown in Fig. 3. This behavior is different from bulk Sr1−xLaxTiO3, in which the carrier density increased linearly with increasing La content [16]. Consequently, the superlattice showed metallic behavior upon the doping and exhibited the maximum carrier density of
1.7 × 1019 cm− 3 at a stacking sequence of STO1/SLTO1. In the perovskite structure, La ion substitutes for Sr site in a solid solution since La ion and Sr ion are similar each other in ionic radius. When La3+ ion is located at the Sr2+ site, Ti4+ ions are partially reduced to Ti3+ for the maintenance of electric neutrality, in which an excess electron in SLTO layer resides on Ti 3d conduction band and influences electrical conductivity. This was proved by Ohtomo et al., by electron energy loss spectra (EELS), in which the extra electrons are spread up to 2 nm in the fractional Ti3+ state [17]. The mobility of the charge carrier in the superlattice is also given in Fig. 3. The mobility of STO/ SLTO superlattices increased slightly as the stacking sequence was changed from STO1/SLTO6 to STO1/SLTO1. It is noted that the mobility is relatively insensitive to the stacking sequence while the carrier density of superlattices changes significantly with the change in the stacking sequence. As a result, the mobility of STO/SLTO superlattices was obtained in the range of 3–11 cm2/Vs, which is a typical range of doped STO thin films at the room temperature. The corresponding electrical resistivity is shown in Fig. 4. The resistivity of STO/SLTO superlattices decreased significantly with the change in the stacking sequence from STO1/SLTO6 to STO1/SLTO1, which is
Fig. 2. X-ray diffraction patterns of STO/SLTO superlattices with various stacking sequences.
Fig. 4. Electrical resistivity of STO/SLTO superlattices with a stacking sequence.
Fig. 3. Charge carrier density and mobility of STO/SLTO superlattices with a stacking sequence.
J. Kim et al. / Surface & Coatings Technology 201 (2007) 5374–5377
mainly attributed to the significant change of the carrier density. The resistivity as low as 0.18 Ω cm was obtained at the STO1/ SLTO1 stacking sequence. Therefore, we have successfully fabricated STO/SLTO superlattices, in which the La defects are regularly arranged in one dimension. Further, we have obtained a wide range of electrical properties by varying the stacking sequence of STO/SLTO superlattices. 4. Conclusions We have fabricated the STO/SLTO superlattices with various stacking sequences by laser-MBE. The STO/SLTO superlattices were epitaxially grown on step-and-terrace structured STO substrate with an aid of in-situ monitoring by RHEED intensity oscillation. The superlattices have well defined distribution of La defect, i.e., one unit cell thick SLTO layer in one dimension. A wide range of the carrier density was obtained in specifically La modified SrTiO3 (SLTO) layer by varying the stacking sequence of the STO/SLTO superlattices. The carrier density of STO/SLTO superlattices was in the range of 1.7 × 1019–2 × 1016 cm− 3. We were able to manipulate the electrical properties in STO/SLTO superlattices by changing a stacking sequence. Acknowledgements This works is supported by the Korea Ministry of Commerce, Industry and Energy through the National Research Program for 0.1 Terabit Non-volatile Memory Development and National Research Laboratory (NRL) Program.
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References [1] D. Fuchs, C.W. Schneider, R. Schneider, H. Rietschel, J. Appl. Phys. 85 (1999) 7362. [2] D. Olaya, F. Pan, C.T. Rogers, J.C. Price, Appl. Phys. Lett. 84 (2004) 4020. [3] K. Ueda, H. Tabata, T. Kawai, Science 280 (1998) 1064. [4] T. Tomio, H. Miki, H. Tabata, T. Kawai, S. Kawai, Appl. Phys. Lett. 76 (1994) 5886. [5] D. Olaya, F. Pan, C.T. Rogers, J.C. Price, Appl. Phys. Lett. 80 (2002) 2928. [6] A. Leitner, C.T. Rogers, J.C. Price, Appl. Phys. Lett. 72 (1998) 3065. [7] H.Y. Hwang, A. Ohtomo, N. Nakagawa, D.A. Muller, J.L. Grazul, Physica E 22 (2004) 712. [8] Y. Tokura, Y. Taguchi, Y. Okada, Y. Fujishima, T. Arima, Phys. Rev. Lett. 70 (1993) 2126. [9] B. Vilquin, T. Kanki, T. Yanagida, H. Tanaka, T. Kawai, Appl. Surf. Sci. 244 (2005) 494. [10] J. Kim, L. Kim, Y. Kim, Y.S. Kim, D. Jung, J. Lee, Appl. Phys. Lett. 80 (2002) 3581. [11] H.N. Lee, H.M. Christen, M.F. Chisholm, C.M. Rouleau, D.H. Lowndes, Nature 433 (2005) 395. [12] K. Lee, J. Lee, J.W. Kim, J. Korean Phys. Soc. 46 (2005) 112. [13] H.N. Lee, H.M. Christen, M.F. Chisholm, C.M. Rouleau, D.H. Lowndes, Appl. Phys. Lett. 84 (2004) 4107. [14] T. Choi, J. Lee, J. Korean Phys. Soc. 46 (2005) 116. [15] J. Kim, L. Kim, D. Jung, J. Lee, Ferroelectrics 336 (2006) 255. [16] T. Okuda, K. Nakanishi, S. Miyasaka, Y. Tokura, Phys. Rev. B 63 (2001) 113104. [17] A. Ohomo, D.A. Muller, J.L. Grazul, H.Y. Hwang, Nature 419 (2002) 378.