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SrTiO3 interfaces studied with scanning tunneling microscopy
Current Applied Physics 14 (2014) 1692e1695
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Modified gap states in Fe/MgO/SrTiO3 interfaces studied with scanning tunneling microscopy Hyung-Joon Shin a, *, Seong Heon Kim b, Heejun Yang c, Young Kuk d, * a School of Materials Science and Engineering, KIST-UNIST Ulsan Center for Convergent Materials, Center for Multidimensional Carbon Materials and Low Dimensional Carbon Materials Center, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Republic of Korea b Samsung Advanced Institute of Technology, Gyeonggi-do 443-803, Republic of Korea c Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea d Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 August 2014 Received in revised form 17 September 2014 Accepted 22 September 2014 Available online 7 October 2014
The geometric and electronic structures of Fe islands on MgO film layers were studied with scanning tunneling microscopy and spectroscopy. The MgO layers were grown on a Nb-doped single crystal SrTiO3 (100) surface. Deposited Fe atoms aggregate into islands, the height and diameter of which are about 2.5 and 9.4 nm respectively. Fe islands modify the electronic structure of MgO surface; a ring type depression in the scanning tunneling microscope topography appears by lowered local electron density of states around Fe islands. We find that adsorbed Fe atoms reduce the gap states of MgO layers around Fe islands, which is attributed to the reason for the depletion of the electronic density of states. © 2014 Elsevier B.V. All rights reserved.
Keywords: MgO SrTiO3 Interface state STM STS
1. Introduction MgO insulating layer has been widely used for various purpose, because it has unique electrical, magnetic and chemical characteristics [1e5]. An anomalously large oneoff ratio of the tunneling magnetoresistance (TMR) was reported when an ultrathin epitaxial MgO film used was as an insulating layer between two ferromagnetic layers [1,2]. It also can be used as a substrate for enhanced catalytic seeds of metal nanoparticles [3,4]. It has been widely believed that the electronic structures of metal/MgO interface may be the reason why these unique characteristics appear in an MgO film. There have been a lot of studies to clarify the detailed mechanism, both experimentally and theoretically [5e10]. In most of these experiments, including scanning tunneling microscopy (STM) studies, geometric and electronic structures of MgO films grown on metal surfaces, such as Fe (100), Ag (100), or Mo (100), have been studied since it is
rather straightforward to grow epitaxial MgO films on those substrates. In order to understand the local modification of the geometric and electronic structures of MgO layer by small amount of metal atoms, an STM study on the reverse geometry, metal adsorbed MgO substrate, can be beneficial. In this study, we grew a thin MgO film on a Nb-doped SrTiO3 (STO) (100) single crystal, and studied the geometric and electronic structures with or without Fe nanoclusters using STM and scanning tunneling spectroscopy (STS). The STO (100) is an unique semiconducting substrate on which the epitaxial MgO films can be grown [11]. Furthermore, metal nanodots on MgO/STO can be a model system of metaleinsulatoresemiconductor (MIS) structure in nanoscale, which is one of the most important junctions in electronic devices. We find that adsorbed Fe atoms reduce the gap states of MgO layers around Fe islands, by which the bias dependent ring type depressions can be observed around the islands. 2. Materials and methods
* Corresponding authors. E-mail addresses: [email protected] (H.-J. Shin), [email protected] (Y. Kuk). http://dx.doi.org/10.1016/j.cap.2014.09.012 1567-1739/© 2014 Elsevier B.V. All rights reserved.
To prepare MgO thin films, we used a 0.5% Nb-doped STO (100) single crystal for the substrate. An STO single crystal
H.-J. Shin et al. / Current Applied Physics 14 (2014) 1692e1695
(15 3 0.5 mm3) was dipped into deionized water for 15 min, and etched in a buffered NH3 F-HF (BHF) solution for 5 min to obtain TiO2 terminated surface [12,13]. After chemical cleaning, the sample was introduced into a preparation chamber which is attached to a home-made STM chamber [14]. The base pressure of this system was less than 1.0 1010 torr. The sample was Ar ion sputtered (15 min, 1.0 keV) and annealed at 1223 K several times. MgO films were deposited by evaporating Mg under an oxygen pressure of 2.0 107 torr. The temperature of the substrate was kept at 1023 K during deposition. Post-annealing was followed for 15 min after deposition. After the growth of the MgO layer, we deposited Fe nanodots using an e-beam evaporator while keeping the sample at 923 K. All STM measurements were performed at 77 K with an electrochemically-etched tungsten tip. Because of low tunneling conductance at negative sample bias voltage, we performed the most STM imaging at positive sample biases. 3. Results and discussion Fig. 1(a) is an STM image of an STO (100) surface. The measured step height is about 4 Å, which is almost the same as the lattice constant of STO (3.98 Å). Atomically-resolved reconstruction structures are visible with corrugated atomic rows and atomic defects. There are several reconstruction phases on the STO (100) surface depending on the sample preparation method, as reported by other groups [15,16]. Fig. 1(b) shows 2-ML-thick MgO films grown on an STO (100) surface. The details of the growth mode of the first monolayer of MgO films on STO (100) have been discussed in the previous paper [12]. For the case of 2-ML-thick MgO films, we cannot find any 1D feature of MgO in the STM topography. Since the monolayer of MgO is lack of its own crystallinity along the vertical direction, the 1D structure is much more pronounced. As thickness of the MgO films increases, however, it seems that the increased
Fig. 1. STM images of (a) STO (100) clean surface (Vbias ¼ 2.0 V, It ¼ 0.22 nA), (b) MgO film grown on STO (100) surface (Vbias ¼ 3.4 V, It ¼ 0.10 nA), and (c) Fe nanodots deposited on MgO/STO (Vbias ¼ 3.0 V, It ¼ 0.10 nA), (d) Height profile along the dashed line marked in (c).
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number of ionic bonds per Mg2þ or O2 ions does not permit the relaxation of the surface layer any more. Consequently, the layerby-layer growth mode prevails from the second layer instead of the 1D growth mode like as other MgO films grown on metal substrates. The topographic image of Fe nanodots grown on an MgO film is shown in Fig. 1(c). The clusters were distributed evenly on the surface, with no obvious preferential nucleation site along a step edge or on a flat terrace. The height and the diameter of the dots vary and they are 2.5 ± 0.4 nm and 9.4 ± 2.3 nm in Fig. 1(d), respectively. It was well known that Fe grows epitaxially on MgO (100) with the 45 in a plain rotational relation, since the lattice constants of Fe and MgO are 2.866 and 4.216 Å. Fig. 1(c) shows that Fe has self-assembled into nanodots. The surface energy of Fe (100) (2.9 J/m2) [17] is larger than that of MgO (100) (1.1 J/m2) [18], thus Fe deposited at elevated temperature exhibits 3D island growth to minimize the surface energy of the whole system thermodynamically. Each dot has a similar dome shape, because the surface energy anisotropy becomes smaller at high temperatures. As previously observed in other nanotdots on the MgO films [19], we can find the Fe nanodots in registry with square oxide lattice when the substrate temperature is lowered below 773 K during Fe deposition. Interestingly, one can observe a ring-type depression of
Fig. 2. STM images of Fe dots measured with various biases. (a) Vbias ¼ 0.6 V, It ¼ 0.10 nA, (b) Vbias ¼ 1.0 V, It ¼ 0.10 nA, (c) Vbias ¼ 1.5 V, It ¼ 0.10 nA, (d)Vbias ¼ 2.0 V, It ¼ 0.10 nA, (e) Vbias ¼ 2.5 V, It ¼ 0.10 nA. (f) Bias dependence of depression around Fe dots.
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the substrate around the Fe nanodots in topography [white arrows in Fig. 1(c)]. The depth of the depression was about 0.27 nm and the width was about 10 nm. These depressions were found around any dot. Fig. 2(a)e(e) shows STM images measured at several biases. As the applied bias increased from 0.6 V, the depth decreased gradually. The depth showed a minimum of 1.7 Å at the sample bias of 2.0 V and increased again slightly above this bias [Fig. 2(f)]. Such a bias dependent topographic change implies that these are due to the change of the electronic structure. Similar depressions were reported on surfaces with metal nanoclusters using STM measurement by other groups previously [20e22]. Carroll et al. deposited Cu clusters on a TiO2 (110) surface, and observed the depressions around the Cu clusters by STM [20]. They attributed this behavior to the electronic effect arising from Schottky barrier formation. However it seems that ring-type depressions around metal dots are not the general case on TiO2 surfaces. Lai et al. reported the STM results of various metal nanoclusters (Au, Pd, Ag, and Al) on a TiO2 (110) surface [21,22]. Among those metals, only aluminum showed depressions around nanodots. They concluded that these depressions arose from the strong chemical interaction between the clusters and the TiO2 substrate. We could not observe any depression around Fe nanodots, when we deposited them on the STO (100) surface directly without MgO films. In addition, various metal nanodots grown on STO (100) do not result in any depression around clusters [23e26]. Therefore, it seems that the role of the MgO film is important in this behavior. In the case of single metal atoms on ultrathin insulating films over metal substrates, it has been reported that the charged metal atoms are imaged as protrusions surrounded by a depression in STM topography [27,28]. However, the whole area of depressed region is much larger in our results than the above results. In order to understand the electronic structure of this system, we measured the local density of states (LDOS) by measuring spatially dI/dV signals. Since the tunneling current (I) of an STM reflects the LDOS of the sample, from the Fermi level to the sample bias voltage, the value of the dI/dV is approximately proportional to the LDOS at this energy level [29]. In this experiment, dI/dV was obtained with phase sensitive detection using a lock-in amplifier. The fixed tunneling gap for dI/dV was set by the tunneling current during the sampling time period, in the sample-and-hold mode. Fig. 3(b) shows the LDOS spectra measured at region A and B marked in Fig. 3(a). The spectrum in Fig. 3(c) represents the difference of LDOS, that is to say the LDOS of B subtracted from that of A. The dI/dV measured at A represents the typical STS of the normal MgO/STO. The position of the conduction band edge is estimated to be about 4.0 V. At the depressed region, the differential conductance curve B shows that the density of states (DOS)
at 3.6 V is reduced in comparison with that of the MgO surface (curve A). However, there are additional reductions of the DOS at sample biases of 0.4 and 1.7 including 3.6 V. The peak intensities at these biases at B are reduced to almost half the value of the undepressed MgO film. The peak intensities of these states in differential conductance are relatively weak, because they exist inside the bandgap region of MgO film. Despite the weak intensity, we can observe the difference in electronic structures between the depressed and undepressed regions for other ten different Fe islands. In Fig. 3(c), we were able to observe decreases of the DOS of the depressed region more clearly. It is thought that these states are originated from the interfacial states of MgO/STO. The depression in STM topography means that the unoccupied DOS is reduced, because we performed STM measurement with a positive sample bias. In Fig. 2(f), the depth of depression was the largest at a bias of 0.6 V. The DOS at 0.4 V might contribute to such a deep depression. Since, there are only a few states in the energy level between 0.0 V and 0.6 V, the contribution of DOS at 0.4 V in the total tunneling current is relatively very important. As the sample bias increases, the depth of the depression decreases gradually, because the weight of the DOS at 0.4 V in the total tunneling current also decreases. The slight increase of depth at 1.7 V might be due to the DOS at 1.7 V [Fig. 3(b) and (c)]. Finally, the total differences in DOS between MgO and the depressed region at biases up to 3.0 V result in a slow increase in the depth of depressions. There are some possible reasons for explaining the change of electronic structures of MgO films around nanodots. First, tipinduced band bending around the dot can result in the depression of STM topography [20]. If this is the main reason for the depression, we should observe depressions for the case of nanodots on the bare STO surface. However, it should be noted that the depression has not been observed for metal/STO structures so far [23e26]. We can also estimate the depth of the depression from a simple electrostatic model calculation, and the observe results do not support this scenario. Second, the compositional change around the dot can be also one of the reasons. If Fe atoms can diffuse into the MgO film from nanodot, it can modify the electronic structure of the MgO films. This can be confirmed by annealing temperature and duration dependence of the observed depression, whereas the present observation does not conclusively support this scenario. In addition, we can consider dielectric screening charge with charge transfer. The Fe/MgO/STO is a small capacitor in a sense. As the tip bias is negative, electrons in the STO substrate just beneath the dot can be depleted, whereby the screening charge would fill the interfacial or trapped states of MgO/STO around the dot. Consequently, interfacial state cannot work as a conduction channel any more, because it is already occupied by screening
Fig. 3. (a) An STM image of Fe dot (Vbias ¼ 0.6 V, It ¼ 0.10 nA). (b) Differential conductance spectra measured at A and B marked in (a). (c) The difference of LDOS at B subtracted by that at region A. The arrows in (b) and (c) indicate the positions of 0.4, 1.7, and 3.6 V. The dashed line in (c) indicates zero respectively.
H.-J. Shin et al. / Current Applied Physics 14 (2014) 1692e1695
charge. It should be noted that there is a shift of the band onset about 60 meV toward the negative energy region at depressed region (Fig. 3), indicating the difference of the band bending between depressed and undepressed regions due to the screening charge. This can be proved by performing the CeV dependence of the metal-MgO-STO capacitor; this experiment has not been performed yet. Among these factors, we cannot conclude which is the main reason for the modification of the gap states around the dots. At present, STM data alone is not sufficient to identify these states precisely without the help of theoretical considerations. However, it is certain that the bias dependent depressions around nanodots originate in the change of electronic structures. We hope that further theoretical considerations on the interfacial states STO/MgO be performed in future for better understanding of this system. 4. Conclusions To summarize, we studied the electronic structure of Fe/MgO/ STO with STM and STS. The Fe nanodot exhibited a 3D island growth mode on MgO film, and we were able to observe the depression of the MgO film around Fe dots. The depth of the depression varied with the bias. The STS results showed that the unoccupied DOS around the dot is reduced, which results in a depression in topography. It is thought that the dielectric screening around the dot and the chemical compositional change are attributed to depletion of the DOS around the dots. We believe that our results show one of the possible phenomena we could face, especially for MIS structures at the nanometer scale. In designing or developing electronic devices of small dimension, it should be noted that the scale of problems discussed here is not negligible in dimension for nano-electronics. Acknowledgment This work was supported in part by the Korean Science and Engineering Foundation and Korea Research Foundation Grant funded by the Korean Government (KRF-2006-C00013), in part by the KIST-UNIST Ulsan Center (KUUC) for Convergent Materials, in
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part by the year of 2011 Research Fund of UNIST (1.110009.01) and in part by the National Research Foundation of Korea Grant (NRF2006-0093847, NRF-2010-00349).
References [1] S.S.P. Parkin, C. Kaiser, A. Panchula, P.M. Rice, B. Hughes, M. Samant, S.H. Yang, Nat. Mater. 3 (2004) 862. [2] S. Yuasa, T. Nagahama, A. Fukushima, Y. Suzuki, K. Ando, Nat. Mater. 3 (2004) 868. €kkinen, R.N. Barnett, [3] A. Sanchez, S. Abbet, U. Heiz, W.-D. Schneider, H. Ha U. Landman, J. Phys. Chem. A 103 (1999) 9573. [4] U. Heiz, W.-D. Schneider, J. Phys. D Appl. Phys. 33 (2000) R85. [5] H.-J. Shin, J. Jung, K. Motobayashi, S. Yanagisawa, Y. Morikawa, Y. Kim, M. Kawai, Nat. Mater. 9 (2010) 442. [6] M.C. Gallagher, M.S. Fyfield, J.P. Cowin, S.A. Joyce, Surf. Sci. 339 (1995) L909. [7] S. Altieri, L.H. Tjeng, G.A. Sawatzky, Phys. Rev. B 61 (2000) 16948. [8] M. Klaua, D. Ullmann, J. Barthel, W. Wulfhekel, J. Kirschner, R. Urban, T.L. Monchesky, A. Enders, J.F. Cochran, B. Heinrich, Phys. Rev. B 64 (2001) 134411. [9] S. Schintke, S. Messerli, M. Pivetta, F. Patthey, L. Libioulle, M. Stengel, A. De Vita, W.-D. Schneider, Phys. Rev. Lett. 87 (2001) 276801. [10] J. Goniakowski, C. Noguera, Interface Sci. 12 (2004) 93. [11] M. Sterrer, M. Heyde, M. Novicki, N. Nilius, T. Risse, H.-P. Rust, G. Pacchioni, H.J. Freund, J. Phys. Chem. B 110 (2006) 46. [12] H.-J. Shin, S.H. Kim, H.J. Yang, Y. Kuk, Nanotechnology 18 (2007) 175304. [13] M. Kawasaki, K. Takahashi, T. Maeda, R. Tsuchiya, M. Shinohara, O. Ishiyama, T. Yonezawa, M. Yoshimoto, H. Koiuma, Science 266 (1994) 1540. [14] J. Lee, H. Kim, S.-J. Kahng, G. Kim, Y.-W. Son, J. Ihm, H. Kato, Z.W. Wang, T. Okazaki, H. Shinohara, Y. Kuk, Nature 415 (2002) 1005. [15] M.R. Castell, Surf. Sci. 505 (2002) 1. [16] M.R. Castell, Surf. Sci. 516 (2002) 33. [17] M.J.S. Spencer, A. Hung, I.K. Snook, I. Yarovsky, Surf. Sci. 513 (2002) 389. [18] M.B. Taylor, C.E. Sims, G.D. Barrera, N.L. Allan, W.C. Mackrodt, Phys. Rev. B 59 (1999) 6742. [19] P. Myrach, N. Nilius, H.-J. Freund, J. Phys. Chem. C 113 (2009) 18740. [20] D.L. Carroll, M. Wagner, M. Ruhle, D.A. Bonnell, Phys. Rev. B 55 (1997) 9792. [21] X. Lai, T.P. St. Clair, M. Valden, D.W. Goodman, Prog. Surf. Sci. 59 (1998) 25. [22] X. Lai, C. Xu, D.W. Goodman, J. Vac. Sci. Technol. A 16 (1998) 2562. [23] F. Silly, M.R. Castell, Phys. Rev. Lett. 94 (2005) 046103. [24] F. Silly, M.R. Castell, Appl. Phys. Lett. 87 (2005) 53106. [25] F. Silly, M.R. Castell, Appl. Phys. Lett. 87 (2005) 63106. [26] F. Silly, M.R. Castell, Phys. Rev. Lett. 96 (2006) 086104. [27] J. Repp, G. Meyer, F.E. Olsson, M. Persson, Science 305 (2004) 493. [28] M. Sterrer, T. Risse, U.M. Pozzoni, L. Giordano, M. Heyde, H.-P. Rust, G. Pacchioni, H.-J. Freund, Phys. Rev. Lett. 98 (2007) 096107. [29] J. Tersoff, D.R. Hamann, Phys. Rev. B 31 (1985) 805.
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Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Modified gap states in Fe/MgO/SrTiO3 interfaces studied with scanning tunneling microscopy Hyung-Joon Shin a, *, Seong Heon Kim b, Heejun Yang c, Young Kuk d, * a School of Materials Science and Engineering, KIST-UNIST Ulsan Center for Convergent Materials, Center for Multidimensional Carbon Materials and Low Dimensional Carbon Materials Center, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Republic of Korea b Samsung Advanced Institute of Technology, Gyeonggi-do 443-803, Republic of Korea c Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea d Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 August 2014 Received in revised form 17 September 2014 Accepted 22 September 2014 Available online 7 October 2014
The geometric and electronic structures of Fe islands on MgO film layers were studied with scanning tunneling microscopy and spectroscopy. The MgO layers were grown on a Nb-doped single crystal SrTiO3 (100) surface. Deposited Fe atoms aggregate into islands, the height and diameter of which are about 2.5 and 9.4 nm respectively. Fe islands modify the electronic structure of MgO surface; a ring type depression in the scanning tunneling microscope topography appears by lowered local electron density of states around Fe islands. We find that adsorbed Fe atoms reduce the gap states of MgO layers around Fe islands, which is attributed to the reason for the depletion of the electronic density of states. © 2014 Elsevier B.V. All rights reserved.
Keywords: MgO SrTiO3 Interface state STM STS
1. Introduction MgO insulating layer has been widely used for various purpose, because it has unique electrical, magnetic and chemical characteristics [1e5]. An anomalously large oneoff ratio of the tunneling magnetoresistance (TMR) was reported when an ultrathin epitaxial MgO film used was as an insulating layer between two ferromagnetic layers [1,2]. It also can be used as a substrate for enhanced catalytic seeds of metal nanoparticles [3,4]. It has been widely believed that the electronic structures of metal/MgO interface may be the reason why these unique characteristics appear in an MgO film. There have been a lot of studies to clarify the detailed mechanism, both experimentally and theoretically [5e10]. In most of these experiments, including scanning tunneling microscopy (STM) studies, geometric and electronic structures of MgO films grown on metal surfaces, such as Fe (100), Ag (100), or Mo (100), have been studied since it is
rather straightforward to grow epitaxial MgO films on those substrates. In order to understand the local modification of the geometric and electronic structures of MgO layer by small amount of metal atoms, an STM study on the reverse geometry, metal adsorbed MgO substrate, can be beneficial. In this study, we grew a thin MgO film on a Nb-doped SrTiO3 (STO) (100) single crystal, and studied the geometric and electronic structures with or without Fe nanoclusters using STM and scanning tunneling spectroscopy (STS). The STO (100) is an unique semiconducting substrate on which the epitaxial MgO films can be grown [11]. Furthermore, metal nanodots on MgO/STO can be a model system of metaleinsulatoresemiconductor (MIS) structure in nanoscale, which is one of the most important junctions in electronic devices. We find that adsorbed Fe atoms reduce the gap states of MgO layers around Fe islands, by which the bias dependent ring type depressions can be observed around the islands. 2. Materials and methods
* Corresponding authors. E-mail addresses: [email protected] (H.-J. Shin), [email protected] (Y. Kuk). http://dx.doi.org/10.1016/j.cap.2014.09.012 1567-1739/© 2014 Elsevier B.V. All rights reserved.
To prepare MgO thin films, we used a 0.5% Nb-doped STO (100) single crystal for the substrate. An STO single crystal
H.-J. Shin et al. / Current Applied Physics 14 (2014) 1692e1695
(15 3 0.5 mm3) was dipped into deionized water for 15 min, and etched in a buffered NH3 F-HF (BHF) solution for 5 min to obtain TiO2 terminated surface [12,13]. After chemical cleaning, the sample was introduced into a preparation chamber which is attached to a home-made STM chamber [14]. The base pressure of this system was less than 1.0 1010 torr. The sample was Ar ion sputtered (15 min, 1.0 keV) and annealed at 1223 K several times. MgO films were deposited by evaporating Mg under an oxygen pressure of 2.0 107 torr. The temperature of the substrate was kept at 1023 K during deposition. Post-annealing was followed for 15 min after deposition. After the growth of the MgO layer, we deposited Fe nanodots using an e-beam evaporator while keeping the sample at 923 K. All STM measurements were performed at 77 K with an electrochemically-etched tungsten tip. Because of low tunneling conductance at negative sample bias voltage, we performed the most STM imaging at positive sample biases. 3. Results and discussion Fig. 1(a) is an STM image of an STO (100) surface. The measured step height is about 4 Å, which is almost the same as the lattice constant of STO (3.98 Å). Atomically-resolved reconstruction structures are visible with corrugated atomic rows and atomic defects. There are several reconstruction phases on the STO (100) surface depending on the sample preparation method, as reported by other groups [15,16]. Fig. 1(b) shows 2-ML-thick MgO films grown on an STO (100) surface. The details of the growth mode of the first monolayer of MgO films on STO (100) have been discussed in the previous paper [12]. For the case of 2-ML-thick MgO films, we cannot find any 1D feature of MgO in the STM topography. Since the monolayer of MgO is lack of its own crystallinity along the vertical direction, the 1D structure is much more pronounced. As thickness of the MgO films increases, however, it seems that the increased
Fig. 1. STM images of (a) STO (100) clean surface (Vbias ¼ 2.0 V, It ¼ 0.22 nA), (b) MgO film grown on STO (100) surface (Vbias ¼ 3.4 V, It ¼ 0.10 nA), and (c) Fe nanodots deposited on MgO/STO (Vbias ¼ 3.0 V, It ¼ 0.10 nA), (d) Height profile along the dashed line marked in (c).
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number of ionic bonds per Mg2þ or O2 ions does not permit the relaxation of the surface layer any more. Consequently, the layerby-layer growth mode prevails from the second layer instead of the 1D growth mode like as other MgO films grown on metal substrates. The topographic image of Fe nanodots grown on an MgO film is shown in Fig. 1(c). The clusters were distributed evenly on the surface, with no obvious preferential nucleation site along a step edge or on a flat terrace. The height and the diameter of the dots vary and they are 2.5 ± 0.4 nm and 9.4 ± 2.3 nm in Fig. 1(d), respectively. It was well known that Fe grows epitaxially on MgO (100) with the 45 in a plain rotational relation, since the lattice constants of Fe and MgO are 2.866 and 4.216 Å. Fig. 1(c) shows that Fe has self-assembled into nanodots. The surface energy of Fe (100) (2.9 J/m2) [17] is larger than that of MgO (100) (1.1 J/m2) [18], thus Fe deposited at elevated temperature exhibits 3D island growth to minimize the surface energy of the whole system thermodynamically. Each dot has a similar dome shape, because the surface energy anisotropy becomes smaller at high temperatures. As previously observed in other nanotdots on the MgO films [19], we can find the Fe nanodots in registry with square oxide lattice when the substrate temperature is lowered below 773 K during Fe deposition. Interestingly, one can observe a ring-type depression of
Fig. 2. STM images of Fe dots measured with various biases. (a) Vbias ¼ 0.6 V, It ¼ 0.10 nA, (b) Vbias ¼ 1.0 V, It ¼ 0.10 nA, (c) Vbias ¼ 1.5 V, It ¼ 0.10 nA, (d)Vbias ¼ 2.0 V, It ¼ 0.10 nA, (e) Vbias ¼ 2.5 V, It ¼ 0.10 nA. (f) Bias dependence of depression around Fe dots.
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the substrate around the Fe nanodots in topography [white arrows in Fig. 1(c)]. The depth of the depression was about 0.27 nm and the width was about 10 nm. These depressions were found around any dot. Fig. 2(a)e(e) shows STM images measured at several biases. As the applied bias increased from 0.6 V, the depth decreased gradually. The depth showed a minimum of 1.7 Å at the sample bias of 2.0 V and increased again slightly above this bias [Fig. 2(f)]. Such a bias dependent topographic change implies that these are due to the change of the electronic structure. Similar depressions were reported on surfaces with metal nanoclusters using STM measurement by other groups previously [20e22]. Carroll et al. deposited Cu clusters on a TiO2 (110) surface, and observed the depressions around the Cu clusters by STM [20]. They attributed this behavior to the electronic effect arising from Schottky barrier formation. However it seems that ring-type depressions around metal dots are not the general case on TiO2 surfaces. Lai et al. reported the STM results of various metal nanoclusters (Au, Pd, Ag, and Al) on a TiO2 (110) surface [21,22]. Among those metals, only aluminum showed depressions around nanodots. They concluded that these depressions arose from the strong chemical interaction between the clusters and the TiO2 substrate. We could not observe any depression around Fe nanodots, when we deposited them on the STO (100) surface directly without MgO films. In addition, various metal nanodots grown on STO (100) do not result in any depression around clusters [23e26]. Therefore, it seems that the role of the MgO film is important in this behavior. In the case of single metal atoms on ultrathin insulating films over metal substrates, it has been reported that the charged metal atoms are imaged as protrusions surrounded by a depression in STM topography [27,28]. However, the whole area of depressed region is much larger in our results than the above results. In order to understand the electronic structure of this system, we measured the local density of states (LDOS) by measuring spatially dI/dV signals. Since the tunneling current (I) of an STM reflects the LDOS of the sample, from the Fermi level to the sample bias voltage, the value of the dI/dV is approximately proportional to the LDOS at this energy level [29]. In this experiment, dI/dV was obtained with phase sensitive detection using a lock-in amplifier. The fixed tunneling gap for dI/dV was set by the tunneling current during the sampling time period, in the sample-and-hold mode. Fig. 3(b) shows the LDOS spectra measured at region A and B marked in Fig. 3(a). The spectrum in Fig. 3(c) represents the difference of LDOS, that is to say the LDOS of B subtracted from that of A. The dI/dV measured at A represents the typical STS of the normal MgO/STO. The position of the conduction band edge is estimated to be about 4.0 V. At the depressed region, the differential conductance curve B shows that the density of states (DOS)
at 3.6 V is reduced in comparison with that of the MgO surface (curve A). However, there are additional reductions of the DOS at sample biases of 0.4 and 1.7 including 3.6 V. The peak intensities at these biases at B are reduced to almost half the value of the undepressed MgO film. The peak intensities of these states in differential conductance are relatively weak, because they exist inside the bandgap region of MgO film. Despite the weak intensity, we can observe the difference in electronic structures between the depressed and undepressed regions for other ten different Fe islands. In Fig. 3(c), we were able to observe decreases of the DOS of the depressed region more clearly. It is thought that these states are originated from the interfacial states of MgO/STO. The depression in STM topography means that the unoccupied DOS is reduced, because we performed STM measurement with a positive sample bias. In Fig. 2(f), the depth of depression was the largest at a bias of 0.6 V. The DOS at 0.4 V might contribute to such a deep depression. Since, there are only a few states in the energy level between 0.0 V and 0.6 V, the contribution of DOS at 0.4 V in the total tunneling current is relatively very important. As the sample bias increases, the depth of the depression decreases gradually, because the weight of the DOS at 0.4 V in the total tunneling current also decreases. The slight increase of depth at 1.7 V might be due to the DOS at 1.7 V [Fig. 3(b) and (c)]. Finally, the total differences in DOS between MgO and the depressed region at biases up to 3.0 V result in a slow increase in the depth of depressions. There are some possible reasons for explaining the change of electronic structures of MgO films around nanodots. First, tipinduced band bending around the dot can result in the depression of STM topography [20]. If this is the main reason for the depression, we should observe depressions for the case of nanodots on the bare STO surface. However, it should be noted that the depression has not been observed for metal/STO structures so far [23e26]. We can also estimate the depth of the depression from a simple electrostatic model calculation, and the observe results do not support this scenario. Second, the compositional change around the dot can be also one of the reasons. If Fe atoms can diffuse into the MgO film from nanodot, it can modify the electronic structure of the MgO films. This can be confirmed by annealing temperature and duration dependence of the observed depression, whereas the present observation does not conclusively support this scenario. In addition, we can consider dielectric screening charge with charge transfer. The Fe/MgO/STO is a small capacitor in a sense. As the tip bias is negative, electrons in the STO substrate just beneath the dot can be depleted, whereby the screening charge would fill the interfacial or trapped states of MgO/STO around the dot. Consequently, interfacial state cannot work as a conduction channel any more, because it is already occupied by screening
Fig. 3. (a) An STM image of Fe dot (Vbias ¼ 0.6 V, It ¼ 0.10 nA). (b) Differential conductance spectra measured at A and B marked in (a). (c) The difference of LDOS at B subtracted by that at region A. The arrows in (b) and (c) indicate the positions of 0.4, 1.7, and 3.6 V. The dashed line in (c) indicates zero respectively.
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charge. It should be noted that there is a shift of the band onset about 60 meV toward the negative energy region at depressed region (Fig. 3), indicating the difference of the band bending between depressed and undepressed regions due to the screening charge. This can be proved by performing the CeV dependence of the metal-MgO-STO capacitor; this experiment has not been performed yet. Among these factors, we cannot conclude which is the main reason for the modification of the gap states around the dots. At present, STM data alone is not sufficient to identify these states precisely without the help of theoretical considerations. However, it is certain that the bias dependent depressions around nanodots originate in the change of electronic structures. We hope that further theoretical considerations on the interfacial states STO/MgO be performed in future for better understanding of this system. 4. Conclusions To summarize, we studied the electronic structure of Fe/MgO/ STO with STM and STS. The Fe nanodot exhibited a 3D island growth mode on MgO film, and we were able to observe the depression of the MgO film around Fe dots. The depth of the depression varied with the bias. The STS results showed that the unoccupied DOS around the dot is reduced, which results in a depression in topography. It is thought that the dielectric screening around the dot and the chemical compositional change are attributed to depletion of the DOS around the dots. We believe that our results show one of the possible phenomena we could face, especially for MIS structures at the nanometer scale. In designing or developing electronic devices of small dimension, it should be noted that the scale of problems discussed here is not negligible in dimension for nano-electronics. Acknowledgment This work was supported in part by the Korean Science and Engineering Foundation and Korea Research Foundation Grant funded by the Korean Government (KRF-2006-C00013), in part by the KIST-UNIST Ulsan Center (KUUC) for Convergent Materials, in
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part by the year of 2011 Research Fund of UNIST (1.110009.01) and in part by the National Research Foundation of Korea Grant (NRF2006-0093847, NRF-2010-00349).
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