- Email: [email protected]
SrTiO3 interfaces via rare earth doping
Solid State Communications 156 (2013) 35–37
Contents lists available at SciVerse ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Tuning the two-dimensional carrier density at LaAlO3/SrTiO3 interfaces via rare earth doping Frank Schoofs, Mehmet Egilmez, Thomas Fix, Judith L. MacManus-Driscoll, Mark G. Blamire n Department of Materials Science & Metallurgy, University of Cambridge, Pembroke Street, CB2 3QZ Cambridge, United Kingdom
a r t i c l e i n f o
abstract
Article history: Received 16 November 2012 Accepted 18 November 2012 by F. Peeters Available online 27 November 2012
The transport properties of LaAlO3/SrTiO3 (LAO/STO) heterostructures with rare earth modified RE0.5Sr0.5TiO3 (RE¼ La, Nd, Sm, Dy) interlayers are studied. Although it was found that the sheet carrier density of the heterostructure follows the trend of the bulk films, purely extrinsic doping effects are excluded based on observed structural distortions as well as detailed resistance versus temperature analysis. Detailed resistance versus temperature analysis revealed a strong interface confinement of the conduction for a single LSTO unit cell between LAO and STO, which tends toward bulk Fermi-liquid behavior as the LSTO thickness is increased. & 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Perovskite heterostructures C. Transport properties D. Two-dimensional transport
1. Introduction Carrier density modulation of the much studied conductive heterointerface between the insulating oxides LaAlO3 and SrTiO3 (LAO//STO) has been achieved through different methods. Electrostatic modulation was observed in field effect devices with bottom [1,2] and top [3] gating. Chemically induced modulation by transition metal doping of the STO B-site [4–6] resulted in a reduced carrier density compared to undoped interfaces, especially for Sc and Fe substitution. On the other hand, A-site doping, for example with trivalent rare earth ions, such as La3 þ , has not been investigated. The bulk properties of LaxSr1 xTiO3 (LSTO) have however been studied extensively: the compound is metallic with electron-like carriers for a wide range of x [7–12]. Here we report on how the transport properties of the 2DEG can be manipulated by introducing one or more unit cells (uc) of RE0.5Sr0.5TiO3 (with RE¼La, Nd, Sm, Dy) at the LAO/STO interface.
2. Materials and methods Two types of samples were prepared: quasi-bulk 120 uc RE0.5Sr0.5TiO3//STO plain films and 15 uc LAO/n uc RE0.5Sr0.5TiO3// STO heterostructures where nZ1. Multiple heterostructures of the same composition confirmed reproducibility across samples. All films were grown on TiO2-terminated STO (001) substrates (Crystal GmbH) by PLD using a KrF laser (l ¼248 nm). The repetition rate was 10 Hz at a target to substrate distance of 80 mm. The laser n
Corresponding author. E-mail address: [email protected] (M.G. Blamire).
0038-1098/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ssc.2012.11.016
fluence was 1 J cm 2—the optimum value for maximizing carrier density in standard LAO//STO interfaces in our deposition system [13]. The substrate heater temperature was 850 1C and the oxygen pressure (pO2) during depositing and cooling down was around 10 2 mbar. This relatively high pO2 ensures that the interface conduction is not dominated by the presence of oxygen vacancies in the STO [14–16] and reduces the extent of intermixing [17]. The growth was monitored in situ by reflection high-energy electron diffraction (RHEED). Commercial STO and LAO targets were used. The RE0.5Sr0.5TiO3 (RE:STO) targets were prepared in-house by milling and pre-sintering powders of high purity (99.99%) SrCO3, TiO2, La2O3, Nd2O3, Sm2O3 and Dy2O3 powders (Alfa Aesar) in the appropriate ratios at 900 1C for 6 h, followed by pressing and sintering at 1100 1C for 6 h. Lattice parameters were obtained by x-ray diffraction with a PANalytical high-resolution X’Pert PRO diffractometer with a 4-bounce monochromator and a PANalytical Empyrean with a 2-bounce monochromator. Electrical contacts were made with Al wires using a bonder. The Hall effect was measured in van der Pauw configuration. No non-linearities arising from dual band conduction could be observed up to 9 T at 10 K in any sample. Resistance versus temperature (R–T) curves were obtained during warming up in a 4-point contact measurement with a spacing of approximately 5 mm between the positive and negative terminals. All R–T curves are fitted with a power law of the type: R ¼ R0 þAUT a
ð1Þ
where R0 is the residual resistivity due to elastic scattering, A is a temperature-independent coefficient related to a quasi-particle effective mass and the exponent a provides more information on the nature of the metallic state [18].
36
F. Schoofs et al. / Solid State Communications 156 (2013) 35–37
3. Results & discussion The carrier density of our plain RE:STO films was measured: as expected, the films were conductive at room temperature, but the carrier density at 200 K is observed to significantly decrease as the ion size decreases from La3 þ to Dy3 þ (Fig. 1a), which might be related to the position of the dopant level within the bandgap. The interface-doped LAO/STO heterostructures are all conducting, with a similar trend in carrier density versus rare earth ion as for the bulk (Fig. 1b). The electron mobility at this temperature is approximately the same for all samples, i.e. 10 cm2 V 1 s 1. The equivalent 1 uc carrier density observed in the LAO/STO heterostructures exceeds the value derived from the bulk RE:STO measurement by two orders of magnitude, as can be appreciated by comparing Fig. 1a and b. This implies that the presence of the LAO capping layer strongly modifies the conductive properties of these layers. A more direct demonstration of this effect is that a single LSTO unit cell capped with 15 uc STO (15 uc STO/1 uc LSTO// STO) or 2 uc LAO (2 uc LAO/1 uc LSTO//STO) was insulating at all temperatures. The absence of conduction in these samples indicates that a single LSTO unit cell cannot support or explain conduction at the LAO/STO interface or an LSTO/STO interface. The sensitivity of the conduction to the interface properties is proven by offset-doping: inserting an STO unit cell between LAO
Fig. 1. (a) Carrier density at 200 K for RE0.5Sr0.5TiO3//SrTiO3 thin films. (b) Sheet carrier density at 200 K of undoped and 1 uc RE0.5Sr0.5TiO3-interfaced LaAlO3/SrTiO3 heterostructures.
Fig. 2. Evolution of unit cell volume versus rare earth element of (a) 120 uc RE0.5Sr0.5TiO3films (on SrTiO3) and (b) 15 uc LaAlO3 films (on 1 uc RE0.5Sr0.5TiO3// SrTiO3).
and a single LSTO unit cell (15 uc LAO/1 uc STO/1 uc LSTO//STO), results in a reduced carrier density similar to those for undoped LAO/STO heterointerfaces. Similar results were reported with offset transition metal doping [5]. All these observations stress the importance of the interface of the STO-based perovskite as well as the requirement of sufficient LAO capping in order to support conduction—purely extrinsic intermixing effects cannot explain these results. In order to understand the nature of the LAO/STO interface, doped by the addition of the RE:STO single uc interlayers, further transport measurements and structural characterizations were carried out. The evolution of the unit cell volume versus RE-dopant is shown in Fig. 2a. All RE0.5Sr0.5TiO3films are fully strained to the STO substrate (as evidenced from reciprocal space maps) and the volume changes are entirely due to an expansion of the out-of-plane lattice parameter. The unit cell volume shows a gradual increase (Fig. 2a), but it is clear that the Nd0.5Sr0.5TiO3 film deviates from this trend. However, the unit cell volume of the LAO on the single uc of RE0.5Sr0.5TiO3 follows a trend more or less inverse of increasing average A-site ion size, which increases from Sr2 þ to La3 þ but then decreases toward Dy3 þ . The results clearly indicate that the presence of a single uc at the interface has a significant impact on the structural distortion of the LAO, in agreement with other reports of RE:STO layers sandwiched between STO [19]. As LSTO is a well-studied metallic oxide, the more in-depth transport measurements focused on the La-doped samples. In agreement with the results on bulk LSTO samples [8], doped STO [20] and oxygen deficient LAO//STO heterostructures [16], the resistance versus temperature (R–T) behavior of a 120 uc thin film of LSTO (Fig. 3b) shows mostly a T2 dependence, indicative of electron–electron (Fermi-liquid) scattering. The R–T behavior of LAO/n uc LSTO//STO (nZ1) heterostructures is shown in Fig. 3a. We have reported before on structural phase transitions of LAO// STO heterostructures manifesting themselves in the R–T measurements [16], in agreement with Siemons et al. [21]. The same transition temperatures are clearly present also in these LSTOinterfaced samples and are therefore not associated with the incorporation of the LSTO but are intrinsic to the LAO/STO heterointerface layer. One transition (at T0 E85 K) also appears in the LSTO//STO sample (Fig. 3b). A STO-capped 15 uc STO/3 uc LSTO//STO sample then was studied in order to understand the nature of the conduction mechanism in the absence of LAO (Fig. 3b). Similar to the case of
Fig. 3. (Color online) Resistance versus temperature with power law fits for (a) STO/ 3 uc LSTO//STO, LAO/3 uc LSTO//STO and LAO/1 uc LSTO//STO heterostructures and (b) 120 uc LSTO//STO. Transition temperatures are marked by a dashed line.
F. Schoofs et al. / Solid State Communications 156 (2013) 35–37
37
as the ion size of the RE is decreased from Nd to Sm and to Dy, following the trend seen in the bulk RE-doped films. Structural distortions occur in the LAO, in line with the ion size of the interface unit cell. Detailed resistance versus temperature analysis revealed a strong interface confinement for a single LSTO unit cell between LAO and STO, which tends toward bulk Fermi-liquid behavior as the LSTO thickness is increased. This implies that purely extrinsic intermixing effects can be excluded as the source of conduction at LAO/STO interfaces.
Acknowledgments This research was partially funded by the ERC Advanced Investigator Grant (247276, NOVOX) and EPSRC (Grant no. EP/E026206/1). References Fig. 4. (Color online) Overview of power law exponents from the resistance versus temperature fits for various heterostructures containing LSTO as well as an undoped 15 uc LAO//STO heterostructure.
4 uc of Nb:STO capped by STO [22], this sample shows a conductive behavior, but only above 180 K. The power law dependence in this region is approximately T2, with a comparable exponent appearing for the 15 uc LAO/3 uc LSTO//STO sample in the same temperature region, again pointing toward bulk-like Fermi-liquid behavior. With LAO as the capping layer, the resistance is significantly lowered, as mentioned before. Upon reducing the thickness of the LSTO from 3 to 1 uc, the resistance is lowered further by an order of magnitude, confirming a different mechanism than purely extrinsic doping to generate the mobile carriers. In order to observe general trends in conduction mechanism with varying LSTO thickness, the power law exponent (ai) values, were plotted against the number of LSTO unit cells for each temperature region (Fig. 4). As the LSTO thickness increases, both a1 and a3 tend toward bulk doped-like behavior, i.e. a E2. Put differently, as the LSTO thickness decreases, these values tend toward a E1, which implies a more two-dimensional confinement of the carriers (non-Fermi-liquid behavior, i.e. a ¼ 1) [23]. This detailed R–T analysis confirms that the two-dimensional confinement of electron transport is preserved for 1 uc doped LAO/ STO interfaces, but that extrinsic doping effects start to dominate when the thickness of the RE0.5Sr0.5TiO3layer is increased. 4. Conclusion We have shown that the sheet carrier density of LAO/STO interfaces can be effectively modified by rare earth substitution on the A-site of the interface-STO. The transport properties are enhanced for a single unit cell of LSTO at the interface, but decline
[1] S. Thiel, G. Hammerl, A. Schmehl, C.W. Schneider, J. Mannhart, Science 313 (2006) 1942. [2] A.D. Caviglia, S. Gariglio, N. Reyren, D. Jaccard, T. Schneider, M. Gabay, S. Thiel, G. Hammerl, J. Mannhart, J.-M. Triscone, Nature 456 (2008) 624. ¨ [3] B. Forg, C. Richter, J. Mannhart, Appl. Phys. Lett. 100 (2012) 053506. [4] T. Fix, J.L. MacManus-Driscoll, M.G. Blamire, Appl. Phys. Lett. 94 (2009) 172101. [5] T. Fix, F. Schoofs, J.L. MacManus-Driscoll, M.G. Blamire, Phys. Rev. Lett. 103 (2009) 166802. [6] T. Fix, F. Schoofs, J.L. MacManus-Driscoll, M.G. Blamire, Appl. Phys. Lett. 97 (2010) 072110. [7] J.E. Sunstrom, S.M. Kauzlarich, P. Klavins, Chem. Mater. 4 (1992) 346. [8] Y. Tokura, Y. Taguchi, Y. Okada, Y. Fujishima, T. Arima, K. Kumagai, Y. Iye, Phys. Rev. Lett. 70 (1993) 2126. [9] A. Ohtomo, J. Nishimura, Y. Murakami, M. Kawasaki, Appl. Phys. Lett. 88 (2006) 232107. [10] J. Son, P. Moetakef, B. Jalan, O. Bierwagen, N.J. Wright, R. Engel-Herbert, S. Stemmer, Nat. Mater. 9 (2010) 482. [11] D.J. Keeble, B. Jalan, L. Ravelli, W. Egger, G. Kanda, S. Stemmer, Appl. Phys. Lett. 99 (2011) 232905. [12] M. Gu, C.R. Dearden, C. Song, N.D. Browning, Y. Takamura, Appl. Phys. Lett. 99 (2011) 261907. [13] F. Schoofs, T. Fix, A.S. Kalabukhov, D. Winkler, Y. Boikov, I. Serenkov, V. Sakharov, T. Claeson, J.L. MacManus-Driscoll, M.G. Blamire, J. Phys.: Condens. Matter 23 (2011) 305002. ¨ [14] A.S. Kalabukhov, R. Gunnarsson, J. Borjesson, E. Olsson, T. Claeson, D. Winkler, Phys. Rev. B 75 (2007) 121404. [15] G. Herranz, et al., Appl. Phys. Lett. 94 (2009) 012113. [16] F. Schoofs, M. Egilmez, T. Fix, J.L. MacManus-Driscoll, M.G. Blamire, Appl. Phys. Lett. 100 (2012) 081601. ¨ [17] A.S. Kalabukhov, A. Boikov, I.T. Serenkov, V.I. Sakharov, J. Borjesson, N. Ljustina, E. Olsson, D. Winkler, T. Claeson, Europhys. Lett. 93 (2011) 37001. [18] S.S. Grigera, R.S. Perry, A.J. Schofield, M. Chiao, R.S. Julian, G.G. Lonzarich, S.I. Ikeda, Y. Maeno, A.J. Millis, A.P. Mackenzie, Science 294 (2001) 329. [19] H.W. Jang, et al., Science 331 (2011) 886. [20] D. van der Marel, J.L.M. van Mechelen, I.I. Mazin, Phys. Rev. B 84 (2011) 205111. [21] W. Siemons, G. Koster, H. Yamamoto, T.H. Geballe, D.H.A. Blank, M.R. Beasley, Phys. Rev. B 76 (2007) 155111. [22] Y. Kozuka, M. Kim, H. Ohta, Y. Hikita, C. Bell, H.Y. Hwang, Appl. Phys. Lett. 97 (2010) 222115. [23] S.W. Tozer, A.W. Kleinsasser, T. Penney, D. Kaiser, F. Holtzberg, Phys. Rev. Lett. 59 (1987) 1768.
Contents lists available at SciVerse ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Tuning the two-dimensional carrier density at LaAlO3/SrTiO3 interfaces via rare earth doping Frank Schoofs, Mehmet Egilmez, Thomas Fix, Judith L. MacManus-Driscoll, Mark G. Blamire n Department of Materials Science & Metallurgy, University of Cambridge, Pembroke Street, CB2 3QZ Cambridge, United Kingdom
a r t i c l e i n f o
abstract
Article history: Received 16 November 2012 Accepted 18 November 2012 by F. Peeters Available online 27 November 2012
The transport properties of LaAlO3/SrTiO3 (LAO/STO) heterostructures with rare earth modified RE0.5Sr0.5TiO3 (RE¼ La, Nd, Sm, Dy) interlayers are studied. Although it was found that the sheet carrier density of the heterostructure follows the trend of the bulk films, purely extrinsic doping effects are excluded based on observed structural distortions as well as detailed resistance versus temperature analysis. Detailed resistance versus temperature analysis revealed a strong interface confinement of the conduction for a single LSTO unit cell between LAO and STO, which tends toward bulk Fermi-liquid behavior as the LSTO thickness is increased. & 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Perovskite heterostructures C. Transport properties D. Two-dimensional transport
1. Introduction Carrier density modulation of the much studied conductive heterointerface between the insulating oxides LaAlO3 and SrTiO3 (LAO//STO) has been achieved through different methods. Electrostatic modulation was observed in field effect devices with bottom [1,2] and top [3] gating. Chemically induced modulation by transition metal doping of the STO B-site [4–6] resulted in a reduced carrier density compared to undoped interfaces, especially for Sc and Fe substitution. On the other hand, A-site doping, for example with trivalent rare earth ions, such as La3 þ , has not been investigated. The bulk properties of LaxSr1 xTiO3 (LSTO) have however been studied extensively: the compound is metallic with electron-like carriers for a wide range of x [7–12]. Here we report on how the transport properties of the 2DEG can be manipulated by introducing one or more unit cells (uc) of RE0.5Sr0.5TiO3 (with RE¼La, Nd, Sm, Dy) at the LAO/STO interface.
2. Materials and methods Two types of samples were prepared: quasi-bulk 120 uc RE0.5Sr0.5TiO3//STO plain films and 15 uc LAO/n uc RE0.5Sr0.5TiO3// STO heterostructures where nZ1. Multiple heterostructures of the same composition confirmed reproducibility across samples. All films were grown on TiO2-terminated STO (001) substrates (Crystal GmbH) by PLD using a KrF laser (l ¼248 nm). The repetition rate was 10 Hz at a target to substrate distance of 80 mm. The laser n
Corresponding author. E-mail address: [email protected] (M.G. Blamire).
0038-1098/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ssc.2012.11.016
fluence was 1 J cm 2—the optimum value for maximizing carrier density in standard LAO//STO interfaces in our deposition system [13]. The substrate heater temperature was 850 1C and the oxygen pressure (pO2) during depositing and cooling down was around 10 2 mbar. This relatively high pO2 ensures that the interface conduction is not dominated by the presence of oxygen vacancies in the STO [14–16] and reduces the extent of intermixing [17]. The growth was monitored in situ by reflection high-energy electron diffraction (RHEED). Commercial STO and LAO targets were used. The RE0.5Sr0.5TiO3 (RE:STO) targets were prepared in-house by milling and pre-sintering powders of high purity (99.99%) SrCO3, TiO2, La2O3, Nd2O3, Sm2O3 and Dy2O3 powders (Alfa Aesar) in the appropriate ratios at 900 1C for 6 h, followed by pressing and sintering at 1100 1C for 6 h. Lattice parameters were obtained by x-ray diffraction with a PANalytical high-resolution X’Pert PRO diffractometer with a 4-bounce monochromator and a PANalytical Empyrean with a 2-bounce monochromator. Electrical contacts were made with Al wires using a bonder. The Hall effect was measured in van der Pauw configuration. No non-linearities arising from dual band conduction could be observed up to 9 T at 10 K in any sample. Resistance versus temperature (R–T) curves were obtained during warming up in a 4-point contact measurement with a spacing of approximately 5 mm between the positive and negative terminals. All R–T curves are fitted with a power law of the type: R ¼ R0 þAUT a
ð1Þ
where R0 is the residual resistivity due to elastic scattering, A is a temperature-independent coefficient related to a quasi-particle effective mass and the exponent a provides more information on the nature of the metallic state [18].
36
F. Schoofs et al. / Solid State Communications 156 (2013) 35–37
3. Results & discussion The carrier density of our plain RE:STO films was measured: as expected, the films were conductive at room temperature, but the carrier density at 200 K is observed to significantly decrease as the ion size decreases from La3 þ to Dy3 þ (Fig. 1a), which might be related to the position of the dopant level within the bandgap. The interface-doped LAO/STO heterostructures are all conducting, with a similar trend in carrier density versus rare earth ion as for the bulk (Fig. 1b). The electron mobility at this temperature is approximately the same for all samples, i.e. 10 cm2 V 1 s 1. The equivalent 1 uc carrier density observed in the LAO/STO heterostructures exceeds the value derived from the bulk RE:STO measurement by two orders of magnitude, as can be appreciated by comparing Fig. 1a and b. This implies that the presence of the LAO capping layer strongly modifies the conductive properties of these layers. A more direct demonstration of this effect is that a single LSTO unit cell capped with 15 uc STO (15 uc STO/1 uc LSTO// STO) or 2 uc LAO (2 uc LAO/1 uc LSTO//STO) was insulating at all temperatures. The absence of conduction in these samples indicates that a single LSTO unit cell cannot support or explain conduction at the LAO/STO interface or an LSTO/STO interface. The sensitivity of the conduction to the interface properties is proven by offset-doping: inserting an STO unit cell between LAO
Fig. 1. (a) Carrier density at 200 K for RE0.5Sr0.5TiO3//SrTiO3 thin films. (b) Sheet carrier density at 200 K of undoped and 1 uc RE0.5Sr0.5TiO3-interfaced LaAlO3/SrTiO3 heterostructures.
Fig. 2. Evolution of unit cell volume versus rare earth element of (a) 120 uc RE0.5Sr0.5TiO3films (on SrTiO3) and (b) 15 uc LaAlO3 films (on 1 uc RE0.5Sr0.5TiO3// SrTiO3).
and a single LSTO unit cell (15 uc LAO/1 uc STO/1 uc LSTO//STO), results in a reduced carrier density similar to those for undoped LAO/STO heterointerfaces. Similar results were reported with offset transition metal doping [5]. All these observations stress the importance of the interface of the STO-based perovskite as well as the requirement of sufficient LAO capping in order to support conduction—purely extrinsic intermixing effects cannot explain these results. In order to understand the nature of the LAO/STO interface, doped by the addition of the RE:STO single uc interlayers, further transport measurements and structural characterizations were carried out. The evolution of the unit cell volume versus RE-dopant is shown in Fig. 2a. All RE0.5Sr0.5TiO3films are fully strained to the STO substrate (as evidenced from reciprocal space maps) and the volume changes are entirely due to an expansion of the out-of-plane lattice parameter. The unit cell volume shows a gradual increase (Fig. 2a), but it is clear that the Nd0.5Sr0.5TiO3 film deviates from this trend. However, the unit cell volume of the LAO on the single uc of RE0.5Sr0.5TiO3 follows a trend more or less inverse of increasing average A-site ion size, which increases from Sr2 þ to La3 þ but then decreases toward Dy3 þ . The results clearly indicate that the presence of a single uc at the interface has a significant impact on the structural distortion of the LAO, in agreement with other reports of RE:STO layers sandwiched between STO [19]. As LSTO is a well-studied metallic oxide, the more in-depth transport measurements focused on the La-doped samples. In agreement with the results on bulk LSTO samples [8], doped STO [20] and oxygen deficient LAO//STO heterostructures [16], the resistance versus temperature (R–T) behavior of a 120 uc thin film of LSTO (Fig. 3b) shows mostly a T2 dependence, indicative of electron–electron (Fermi-liquid) scattering. The R–T behavior of LAO/n uc LSTO//STO (nZ1) heterostructures is shown in Fig. 3a. We have reported before on structural phase transitions of LAO// STO heterostructures manifesting themselves in the R–T measurements [16], in agreement with Siemons et al. [21]. The same transition temperatures are clearly present also in these LSTOinterfaced samples and are therefore not associated with the incorporation of the LSTO but are intrinsic to the LAO/STO heterointerface layer. One transition (at T0 E85 K) also appears in the LSTO//STO sample (Fig. 3b). A STO-capped 15 uc STO/3 uc LSTO//STO sample then was studied in order to understand the nature of the conduction mechanism in the absence of LAO (Fig. 3b). Similar to the case of
Fig. 3. (Color online) Resistance versus temperature with power law fits for (a) STO/ 3 uc LSTO//STO, LAO/3 uc LSTO//STO and LAO/1 uc LSTO//STO heterostructures and (b) 120 uc LSTO//STO. Transition temperatures are marked by a dashed line.
F. Schoofs et al. / Solid State Communications 156 (2013) 35–37
37
as the ion size of the RE is decreased from Nd to Sm and to Dy, following the trend seen in the bulk RE-doped films. Structural distortions occur in the LAO, in line with the ion size of the interface unit cell. Detailed resistance versus temperature analysis revealed a strong interface confinement for a single LSTO unit cell between LAO and STO, which tends toward bulk Fermi-liquid behavior as the LSTO thickness is increased. This implies that purely extrinsic intermixing effects can be excluded as the source of conduction at LAO/STO interfaces.
Acknowledgments This research was partially funded by the ERC Advanced Investigator Grant (247276, NOVOX) and EPSRC (Grant no. EP/E026206/1). References Fig. 4. (Color online) Overview of power law exponents from the resistance versus temperature fits for various heterostructures containing LSTO as well as an undoped 15 uc LAO//STO heterostructure.
4 uc of Nb:STO capped by STO [22], this sample shows a conductive behavior, but only above 180 K. The power law dependence in this region is approximately T2, with a comparable exponent appearing for the 15 uc LAO/3 uc LSTO//STO sample in the same temperature region, again pointing toward bulk-like Fermi-liquid behavior. With LAO as the capping layer, the resistance is significantly lowered, as mentioned before. Upon reducing the thickness of the LSTO from 3 to 1 uc, the resistance is lowered further by an order of magnitude, confirming a different mechanism than purely extrinsic doping to generate the mobile carriers. In order to observe general trends in conduction mechanism with varying LSTO thickness, the power law exponent (ai) values, were plotted against the number of LSTO unit cells for each temperature region (Fig. 4). As the LSTO thickness increases, both a1 and a3 tend toward bulk doped-like behavior, i.e. a E2. Put differently, as the LSTO thickness decreases, these values tend toward a E1, which implies a more two-dimensional confinement of the carriers (non-Fermi-liquid behavior, i.e. a ¼ 1) [23]. This detailed R–T analysis confirms that the two-dimensional confinement of electron transport is preserved for 1 uc doped LAO/ STO interfaces, but that extrinsic doping effects start to dominate when the thickness of the RE0.5Sr0.5TiO3layer is increased. 4. Conclusion We have shown that the sheet carrier density of LAO/STO interfaces can be effectively modified by rare earth substitution on the A-site of the interface-STO. The transport properties are enhanced for a single unit cell of LSTO at the interface, but decline
[1] S. Thiel, G. Hammerl, A. Schmehl, C.W. Schneider, J. Mannhart, Science 313 (2006) 1942. [2] A.D. Caviglia, S. Gariglio, N. Reyren, D. Jaccard, T. Schneider, M. Gabay, S. Thiel, G. Hammerl, J. Mannhart, J.-M. Triscone, Nature 456 (2008) 624. ¨ [3] B. Forg, C. Richter, J. Mannhart, Appl. Phys. Lett. 100 (2012) 053506. [4] T. Fix, J.L. MacManus-Driscoll, M.G. Blamire, Appl. Phys. Lett. 94 (2009) 172101. [5] T. Fix, F. Schoofs, J.L. MacManus-Driscoll, M.G. Blamire, Phys. Rev. Lett. 103 (2009) 166802. [6] T. Fix, F. Schoofs, J.L. MacManus-Driscoll, M.G. Blamire, Appl. Phys. Lett. 97 (2010) 072110. [7] J.E. Sunstrom, S.M. Kauzlarich, P. Klavins, Chem. Mater. 4 (1992) 346. [8] Y. Tokura, Y. Taguchi, Y. Okada, Y. Fujishima, T. Arima, K. Kumagai, Y. Iye, Phys. Rev. Lett. 70 (1993) 2126. [9] A. Ohtomo, J. Nishimura, Y. Murakami, M. Kawasaki, Appl. Phys. Lett. 88 (2006) 232107. [10] J. Son, P. Moetakef, B. Jalan, O. Bierwagen, N.J. Wright, R. Engel-Herbert, S. Stemmer, Nat. Mater. 9 (2010) 482. [11] D.J. Keeble, B. Jalan, L. Ravelli, W. Egger, G. Kanda, S. Stemmer, Appl. Phys. Lett. 99 (2011) 232905. [12] M. Gu, C.R. Dearden, C. Song, N.D. Browning, Y. Takamura, Appl. Phys. Lett. 99 (2011) 261907. [13] F. Schoofs, T. Fix, A.S. Kalabukhov, D. Winkler, Y. Boikov, I. Serenkov, V. Sakharov, T. Claeson, J.L. MacManus-Driscoll, M.G. Blamire, J. Phys.: Condens. Matter 23 (2011) 305002. ¨ [14] A.S. Kalabukhov, R. Gunnarsson, J. Borjesson, E. Olsson, T. Claeson, D. Winkler, Phys. Rev. B 75 (2007) 121404. [15] G. Herranz, et al., Appl. Phys. Lett. 94 (2009) 012113. [16] F. Schoofs, M. Egilmez, T. Fix, J.L. MacManus-Driscoll, M.G. Blamire, Appl. Phys. Lett. 100 (2012) 081601. ¨ [17] A.S. Kalabukhov, A. Boikov, I.T. Serenkov, V.I. Sakharov, J. Borjesson, N. Ljustina, E. Olsson, D. Winkler, T. Claeson, Europhys. Lett. 93 (2011) 37001. [18] S.S. Grigera, R.S. Perry, A.J. Schofield, M. Chiao, R.S. Julian, G.G. Lonzarich, S.I. Ikeda, Y. Maeno, A.J. Millis, A.P. Mackenzie, Science 294 (2001) 329. [19] H.W. Jang, et al., Science 331 (2011) 886. [20] D. van der Marel, J.L.M. van Mechelen, I.I. Mazin, Phys. Rev. B 84 (2011) 205111. [21] W. Siemons, G. Koster, H. Yamamoto, T.H. Geballe, D.H.A. Blank, M.R. Beasley, Phys. Rev. B 76 (2007) 155111. [22] Y. Kozuka, M. Kim, H. Ohta, Y. Hikita, C. Bell, H.Y. Hwang, Appl. Phys. Lett. 97 (2010) 222115. [23] S.W. Tozer, A.W. Kleinsasser, T. Penney, D. Kaiser, F. Holtzberg, Phys. Rev. Lett. 59 (1987) 1768.