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Imposed quasi-layer-by-layer homoepitaxial growth of SrTiO3 films by large area pulsed laser deposition
Thin Solid Films 519 (2011) 6330–6333
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Imposed quasi-layer-by-layer homoepitaxial growth of SrTiO3 films by large area pulsed laser deposition Y.Z. Chen ⁎, N. Pryds Fuel Cells and Solid State Chemistry Division, Risø National Laboratory for Sustainable Energy, Technical University of Denmark, DK-4000 Roskilde, Denmark
a r t i c l e
i n f o
Article history: Received 24 June 2010 Received in revised form 25 January 2011 Accepted 6 April 2011 Available online 13 April 2011 Keywords: Pulsed laser deposition Layer-by-layer Strontium titanate Homoepitaxy Reflective high energy electron diffraction (RHEED)
a b s t r a c t The homoepitaxial growth of SrTiO3 (STO) films was investigated by a large-area pulsed laser deposition (PLD), which was in-situ monitored by a high pressure reflective high energy electron diffraction. By combining a conventionally continuous film deposition with a followed interval relaxation, a persistent layerby-layer (LBL) film growth of more than 100 unit cells STO films was achieved. This interrupted PLD technique could realize persistent LBL film growth at any laser frequency between 1 and 10 Hz and provides an effective way to fabricate high quality complex oxide films on unit cell scale. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The broad spectrum of electronic and ionic properties of complex oxide materials offers tremendous opportunities for application in oxide electronic devices and solid oxide fuel cells, especially when a combination of various properties in a single device is achieved [1,2]. Recent developments in thin-film techniques, such as the pulsed laser deposition (PLD) equipped with reflective high energy electron diffraction (RHEED), have made it possible to grow complex oxide heterostructures and superlattices/multilayers with designed properties [3–5]. To obtain high-quality artificial multilayers, controlled interfaces at unit-cell-scale in addition with atomically flat film surfaces are preferred. In these cases, film growth in a layer-by-layer (LBL) mode or a step flow mode is a prerequisite. As for the step flow film growth, though it can result in smooth and uniform grown layers, the absence of periodic changes in surface characteristics makes it difficult to monitor the film growth in situ and determine the precise thickness of grown layers. In contrast, periodic changes in surface roughness can occur during the LBL film growth. This can result in, for example, periodic oscillations in RHEED intensities. By monitoring the time dependent RHEED intensities, the unit cells of grown films can be accurately determined from the number of oscillations. At the meantime, a wealth of information about growth parameters can also be obtained [6]. Thus, a LBL film growth is highly desirable for
⁎ Corresponding author. Tel.: + 45 4677 5614; fax: + 45 4677 5858. E-mail address: [email protected] (Y.Z. Chen). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.04.023
precisely fabrication of superlattice/multilayer structures at nanoscale. To achieve the goal of growing complex oxide films in LBL mode, the PLD process, one of the most promising technique to grow oxide films [3–11], has been investigated extensively and various approaches have been explored [7–9]. Due to the fact that the nucleation of new layers always starts before a monolayer is completely finished in conventionally continuous deposition processes, a technique known as pulsed laser interval deposition (PLID) has been developed to enable the persistence of the LBL film growth to larger film thickness [7]. In PLID, the exact amount of material for completing one unit-cell layer is firstly deposited by a high laser pulse frequency (f), which is then followed by an interval of no deposition to allow enough time for the relaxation of the deposited material. The value of f is suggested to be as high as possible with the aim to form small islands by completing the unit-cell film growth within the characteristic relaxation time (typically, 0.2–0.5 s) [6]. However, the f (1–10 Hz) often used in conventional PLD process can hardly achieve this requirement. Note that the energetic impinging ablation species during PLD process could break up surface islands by inserting themselves into the islands and promote a two-dimensional film growth [12], the film deposition of more than one unit cell during a PLID process may provide additional force for interlayer diffusion and thus optimized LBL film growth. Here, a modification to the PLID technique is investigated by depositing more than one-unit cell films during the deposition stage. It is found that a persistent LBL growth of homoepitaxial SrTiO3 (STO) film can be realized by combining conventionally continuous film deposition with a followed interval relaxation, which is feasible at all the investigated f between 1 and 10 Hz.
Y.Z. Chen, N. Pryds / Thin Solid Films 519 (2011) 6330–6333
2. Experimental details
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The homoepitaxial STO films were deposited on (001)-oriented STO substrates in a large-area PLD system [13,14]. A KrF (λ = 248 nm) laser with repetition rate f of 1–9 Hz and laser fluence of 1.5 J/cm2 was used. A commercial STO single crystal was used as a target. The distance between the substrate and the target was kept constant at about 6.0 cm. During film deposition, the substrate temperature was kept at 750 °C and various oxygen pressures PO2 between 1.0 × 10− 6 mbar and 1.0 × 10− 2 mbar were used. The film growth process was in-situ monitored by a high pressure RHEED with settled electron beam energy of 30 keV and electron incident direction along [100] of the STO substrate. All STO substrates were specially prepared to obtain a single TiO2-terminated surface with regular unit-cell steps [15,16], which were achieved by chemical etching and subsequent annealing in air at 1000 °C for 1 h. During the chemical etching process, the STO substrates were firstly ultrasonically cleaned in 80 °C deionized water for 30 min to create soluble Sr-hydroxide complex at STO surface [16], then the SrOcontaining layer was further removed by ultrasonically cleaning in acidic solution of HCl–HNO3 (3:1) instead of the conventional buffered HF acid [15,16] for 20 min at 70 °C. The substrates and films were also investigated by atomic force microscopy (AFM) and X-ray diffraction (XRD). 3. Results and discussion Fig. 1(a) shows a 2 μm × 2 μm AFM morphology of the obtained TiO2-terminated STO substrates. Similar to the results with buffered HF acid [15,16], the etching acidic solution of HCl–HNO3 can also result in a regular atomically flat terrace surface as shown in the figure, which has a root mean roughness (RMS) of about 0.14 nm. The terraces are rather uniform with a height of about one unit-cell STO (≈0.4 nm) as shown in Fig. 1(b). The terrace width is about 220 nm, which implies a miscut angle of 0.1° for the substrate. The high quality of the treated STO is also confirmed by the RHEED pattern. As shown in the inset of Fig. 2, the diffraction pattern exhibits bright 1 × 1 streaks and spots lying on the 0th Laue circle, which indicates an atomically smooth surface. Fig. 2 shows the timedependent RHEED intensity during a typical continuous deposition of homoepitaxial STO films at f = 1 Hz and PO2 = 2.0 × 10− 6 mbar. Clear oscillations in the RHEED intensity are observed at the beginning of deposition, which indicates a LBL film growth mode. Generally, every intensity oscillation corresponds to a completed layer growth of the film. The intensity recovery indicates that a layer is fully grown before the next layer is nucleated. Thus, the grown film thickness is about 24 unit cells STO in this process, with a growth rate of about ~0.1 Å/s. However, as the subsequent film deposition proceeds, a gradual decrease in the RHEED peak intensity presents. This indicates a tendency of increased surface roughness in addition with a deviation of film growth mode from LBL one, which is probably due to the accumulation
Intensity (arb. units)
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0
400
1200
Fig. 2. A typical time-dependent RHEED intensity during a continuous film deposition at f = 1 Hz and PO2 = 2.0 × 10− 6 mbar. The inset shows a RHEED pattern of the treated STO substrate surface.
of large-diameter islands during deposition. When the film deposition is stopped, the surface becomes so rough that the RHEED intensity can hardly recover and even goes further down, which most likely results from the nucleation of three-dimensional islands. However, it is found that the RHEED intensity can recover to its initial value if the continuous film deposition is interrupted before such a serious surface deterioration is reached. For our experiments, this means that the film deposition should be interrupted before the film is thicker than 10 unit cells. Fig. 3(a) shows a time-dependent RHEED intensity during deposition of a 9 unit cells STO film at f = 1 Hz and PO2 = 2.0 × 10− 6 mbar. When the film deposition is interrupted, the RHEED intensity exhibits an exponential increase toward the initial value, which can be well fitted by two or multiple relaxation processes with the relaxation time, τ, between 0.2 and 30 s. Thus, a reconstruction of an atomically smooth surface could be achieved by a waiting interval of more than 30 s for non deposition. Fig. 3(b) shows the time dependent RHEED intensity during the followed film deposition by combining a period of continuous film deposition with a subsequent interrupted non deposition. It is remarkable that clear oscillations in the RHEED intensity are observed even after deposition of 100 unit cells STO film. The bright streaky RHEED patterns after film deposition [Fig. 3(d)] resemble greatly the case of the bare STO [Fig. 3(c)]. These results clearly demonstrate that a persistent LBL film growth could be achieved by using a modified PLID method with conventional deposition conditions. The effect of f on the LBL film growth mode by this interrupted deposition was further investigated at PO2 = 2.0 × 10− 6 mbar. Fig. 4(a) shows the film deposition of 5 unit cells STO at different f of 3, 5, 7, 9 Hz. It is interesting to note that persistent LBL film growth could be realized by reproducing such continuous deposition at any f with a followed interval of non deposition. Fig. 4(b) shows the frequency dependence of
10 nm 3.0 (nm)
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1.5 0
800
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2 (μm)
0 0
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Fig. 1. (a) A 2 μm × 2 μm AFM morphology of an atomically flat TiO2-terminated SrTiO3 substrate with regular terrace steps. (b) A line profile in (a) showing the step height of one unit cell of STO (0.39 nm) and the step width of about 220 nm.
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b
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d
Continuous deposition plus waiting interval 0
500
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Time (s) Fig. 3. (a) A layer-by-layer homoepitaxial growth of 9 unit cell STO layers with recovered RHEED intensity at f = 1 Hz and PO2 = 2.0 × 10− 6 mbar. (b) The followed persistent layerby-layer film growth by interrupted deposition. (c) and (d) show the RHEED patterns before and after film deposition, respectively.
σ, which is defined by the averaged relative changes between the minimum and maximum amplitudes of RHEED oscillations. The σ, thus the oscillation damping, is nearly constant at f b 5 Hz, while a clear increase in σ thus a suppression of oscillation damping is observed at higher frequencies. This is different from the case in the conventionally continuous film deposition, where a higher f generally results in a faster damping of RHEED intensity, as illustrated in Fig. 4(c) for a continuous deposition at f = 5 Hz in comparison with that in Fig. 2 for f = 1 Hz. However, this suppression of the oscillation damping by increasing f is quite similar to the situation in the conventional PLID technique, which suggests a significant suppression of forming multilevel films [7]. Fig. 5(a) and (b) shows the AFM morphology of two STO films grown under continuous and interrupted deposition, respectively, at f = 1 Hz and PO2 = 2.0 × 10− 6 mbar. The film thickness is about 100 unit cells. As shown in Fig. 5(a), the film surface obtained by continuous deposition exhibits a smooth surface with a RSM of 0.24 nm. However, the surface
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Fig. 4. (a) The film deposition of 5 unit cells STO at different f of 3, 5, 7, 9 Hz with PO2 = 2.0 × 10− 6 mbar. (b) The corresponding frequency dependence of oscillation damping, σ. (c) The time dependent RHEED intensity during a continuous film deposition at f = 5 Hz and PO2 = 2.0 × 10− 6 mbar.
consists of mainly small islands with diameters of 30–50 nm, instead of terrace steps. In contrast, the film surface deposited by the interval deposition, as shown in Fig. 5(b), exhibits atomically flat terraces with step height of 0.4 nm, though the terrace step is not as sharp as that of the bare substrates. It should be noted that similar surface characteristics are also observed for films deposited at f = 5 Hz. These results further confirm the fact that the suppression of island accumulation, which is accompanied by a persistence of LBL film growth to thicker films, could be realized by interval deposition. Additionally, the obtained films, which were in situ heated at 600 °C under 0.8 mbar for 30 min after deposition, were also checked by X-ray diffraction (XRD). No obvious difference in the lattice parameters was observed between the films and substrates. Since the out-of-plane lattice parameter of STO films will be enhanced in non-stoichiometric films, independent of whether they were Ti or Sr rich [8,17], the obtained STO films in our case therefore may be a stoichiometric one. The effect of PO2, which is an important factor in determining the properties of complex oxide film, on the LBL film growth was also investigated. It should be mentioned that the oxygen pressure dominates the plasma expansion dynamics during PLD process [18,19]. Due to the collisions between the expanding plasma plume and the surrounding oxygen molecules, the plume front is generally confined to a distance smaller than 4.0 cm when the background PO2 is higher than 5.0 × 10− 2 mbar [18]. As a consequence, for realizing LBL film growth in large area PLD, where a rather large distance between targets and substrates (larger than 6.0 cm) exists, the film growth is better to be performed in a PO2 lower than 1.0× 10− 2 mbar to maintain a high cluster mobility for the interlayer transfer during the relaxation intervals. Fig. 6(a) shows the RHEED oscillations obtained under various PO2 between 1.0 × 10− 6 mbar and 1.0× 10− 2 mbar during the STO homoepitaxial growth. As shown in the figure, LBL film growth mode could be realized in all the investigated oxygen pressure range. Fig. 6(b) and (c) shows the dependence of the film growth rate and the σ, respectively, on the oxygen pressure. The film growth rate is about 38 laser pulses for growing one unit cell STO, ~0.1 Å/s, in the PO2 range of 1.0 ×10− 6 − 1.0 ×10− 3 mbar. When PO2 is increased to 1.0× 10− 2 mbar, a decrease of about 10% in growth rate is observed. The constant film growth rate between 1.0× 10− 6 and 1.0 ×10− 3 mbar seems to be consistent with the free expansion of the plasma at these oxygen pressures [18,19]. The reduction of the film growth rate at PO2 = 1.0 ×10− 2 mbar may indicate the presence of the collisions between the plasma and the oxygen molecules thus the scattering and attenuation of the fast plume. The plasma attenuation at PO2 = 1.0 ×10− 2 mbar can also account for the
Y.Z. Chen, N. Pryds / Thin Solid Films 519 (2011) 6330–6333
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5 μm
10 nm
a
b 0
0 5 (μm)
0
Fig. 5. The 5 μm × 5 μm AFM morphology of STO films grown under continuous (a) and interrupted (b) deposition with a thickness of about 100 unit cells at f = 1 Hz and PO2 = 2.0 × 10− 6 mbar.
Time (s)
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even in a large area PLD. This modified PLID technique could realize persistent LBL film growth at any laser frequency between 1 and 10 Hz and provides an effective way to fabricate high quality complex oxide films on unit cell scale.
750 -2
1×10 mbar -3 1×10 mbar 1×10 mbar -5 1×10 mbar -4
Acknowledgments The authors thank F. B. Saxild, J. Geyti, K. V. Hansen, S. Linderoth, J. Schou, K. Rodrigo and K. M. Kant for their valuable help. -6
1×10 mbar
References
0.105 0.15
σ
Growth rate( /s)
0.20
0.100
b -6
10
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10
-4
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-3
10
-2
10
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-6
10
-5
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-4
10
-3
10
-2
0.10
10
PO2 (mbar)
Fig. 6. (a) The RHEED oscillations obtained under various oxygen pressures between 1.0 × 10− 6 and 1.0 × 10− 2 mbar for STO homoepitaxial growth at f = 1 Hz. (b) and (c) The film growth rate and the oscillation damping σ, respectively, as a function of oxygen pressure.
increased σ as shown in Fig. 6(c), since a decrease in the plasma species energy could lead to a decrease in the surface diffusion mobility of the adatoms and the formation of large-diameter islands. However, the obvious decrease in oscillation damping at PO2 =1.0 ×10− 3 mbar is unexpected, which needs to be further investigated. In summary, by a modified interval deposition, which relies on a combination of the normal continuous film deposition of more than one unit cell with a followed interval relaxation, a persistent layer-bylayer film growth is achieved during SrTiO3 homoepitaxial growth
[1] X.X. Guo, J. Maier, Adv. Mater. 21 (2009) 2619. [2] Y. Tokura, H.Y. Hwang, Nature Mater. 7 (2008) 694. [3] A.J.H.M. Rijnders, G. Koster, D.H.A. Blank, H. Rogalla, Appl. Phys. Lett. 70 (1997) 1888. [4] H.M. Christen, G. Eres, J. Phys. Condens. Matter 20 (2008) 264005. [5] Y.Z. Chen, J.R. Sun, X.Y. Xie, D.J. Wang, W.M. Lu, S. Liang, B.G. Shen, Appl. Phys. Lett. 90 (2007) 143508. [6] G. Koster, A.J.H.M. Rijnders, D.H.A. Blank, H. Rogalla, Mater. Res. Soc. Symp. Proc. 526 (1998) 33. [7] G. Koster, G.J.H.M. Rijnders, D.H.A. Blank, H. Rogalla, Appl. Phys. Lett. 74 (1999) 3729. [8] T. Ohnishi, T. Yamamoto, S. Meguro, H. Koinuma, M. Lippmaa, J. Phys. Conf. Ser. 59 (2007) 514. [9] Y.Y. Tse, S.R.C. Mcmitchell, T.J. Jackson, Y.H. Liu, I.P. Jones, Ferroelectrics 368 (2008) 49. [10] M. Hiratani, Y. Tarutani, T. Fukazawa, M. Okamoto, K. Takagi, Thin Solid Films 227 (1993) 100. [11] J.H. Gao, J. Gao, H.K. Wong, Thin Solid Films 515 (2006) 559. [12] P.R. Willmott, R. Herger, C.M. Schleputz, M. Martoccia, B.D. Patterson, Phys. Rev. Lett. 96 (2006) 176102. [13] N. Pryds, J. Schou, S. Linderoth, Appl. Surf. Sci. 253 (2007) 8231. [14] N. Pryds, D. Cockburn, K. Rodrigo, I.L. Rasmussen, J. Knudsen, J. Schou, Appl. Phys. A 93 (2008) 705. [15] M. Kawasaki, K. Takahashi, T. Maeda, R. Tsuchiya, M. Shinohara, O. Ishiyama, T. Yonezawa, M. Yoshimoto, H. Koinuma, Science 266 (1994) 1540. [16] G. Koster, B.L. Kropman, G.J.H.M. Rijnders, D.H.A. Blank, H. Rogalla, Appl. Phys. Lett. 73 (1998) 2920. [17] B. Jalan, R. Engel-Herbert, N.J. Wright, S. Stemmer, J. Vac. Sci. Technol. A 27 (2009) 461. [18] S. Amoruso, A. Sambri, X. Wang, J. Appl. Phys. 100 (2006) 013302. [19] A.N. Khodan, S. Guyard, J.-P. Contour, D.-G. Crété, E. Jacquet, K. Bouzehouane, Thin Solid Films 515 (2007) 6422.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Imposed quasi-layer-by-layer homoepitaxial growth of SrTiO3 films by large area pulsed laser deposition Y.Z. Chen ⁎, N. Pryds Fuel Cells and Solid State Chemistry Division, Risø National Laboratory for Sustainable Energy, Technical University of Denmark, DK-4000 Roskilde, Denmark
a r t i c l e
i n f o
Article history: Received 24 June 2010 Received in revised form 25 January 2011 Accepted 6 April 2011 Available online 13 April 2011 Keywords: Pulsed laser deposition Layer-by-layer Strontium titanate Homoepitaxy Reflective high energy electron diffraction (RHEED)
a b s t r a c t The homoepitaxial growth of SrTiO3 (STO) films was investigated by a large-area pulsed laser deposition (PLD), which was in-situ monitored by a high pressure reflective high energy electron diffraction. By combining a conventionally continuous film deposition with a followed interval relaxation, a persistent layerby-layer (LBL) film growth of more than 100 unit cells STO films was achieved. This interrupted PLD technique could realize persistent LBL film growth at any laser frequency between 1 and 10 Hz and provides an effective way to fabricate high quality complex oxide films on unit cell scale. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The broad spectrum of electronic and ionic properties of complex oxide materials offers tremendous opportunities for application in oxide electronic devices and solid oxide fuel cells, especially when a combination of various properties in a single device is achieved [1,2]. Recent developments in thin-film techniques, such as the pulsed laser deposition (PLD) equipped with reflective high energy electron diffraction (RHEED), have made it possible to grow complex oxide heterostructures and superlattices/multilayers with designed properties [3–5]. To obtain high-quality artificial multilayers, controlled interfaces at unit-cell-scale in addition with atomically flat film surfaces are preferred. In these cases, film growth in a layer-by-layer (LBL) mode or a step flow mode is a prerequisite. As for the step flow film growth, though it can result in smooth and uniform grown layers, the absence of periodic changes in surface characteristics makes it difficult to monitor the film growth in situ and determine the precise thickness of grown layers. In contrast, periodic changes in surface roughness can occur during the LBL film growth. This can result in, for example, periodic oscillations in RHEED intensities. By monitoring the time dependent RHEED intensities, the unit cells of grown films can be accurately determined from the number of oscillations. At the meantime, a wealth of information about growth parameters can also be obtained [6]. Thus, a LBL film growth is highly desirable for
⁎ Corresponding author. Tel.: + 45 4677 5614; fax: + 45 4677 5858. E-mail address: [email protected] (Y.Z. Chen). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.04.023
precisely fabrication of superlattice/multilayer structures at nanoscale. To achieve the goal of growing complex oxide films in LBL mode, the PLD process, one of the most promising technique to grow oxide films [3–11], has been investigated extensively and various approaches have been explored [7–9]. Due to the fact that the nucleation of new layers always starts before a monolayer is completely finished in conventionally continuous deposition processes, a technique known as pulsed laser interval deposition (PLID) has been developed to enable the persistence of the LBL film growth to larger film thickness [7]. In PLID, the exact amount of material for completing one unit-cell layer is firstly deposited by a high laser pulse frequency (f), which is then followed by an interval of no deposition to allow enough time for the relaxation of the deposited material. The value of f is suggested to be as high as possible with the aim to form small islands by completing the unit-cell film growth within the characteristic relaxation time (typically, 0.2–0.5 s) [6]. However, the f (1–10 Hz) often used in conventional PLD process can hardly achieve this requirement. Note that the energetic impinging ablation species during PLD process could break up surface islands by inserting themselves into the islands and promote a two-dimensional film growth [12], the film deposition of more than one unit cell during a PLID process may provide additional force for interlayer diffusion and thus optimized LBL film growth. Here, a modification to the PLID technique is investigated by depositing more than one-unit cell films during the deposition stage. It is found that a persistent LBL growth of homoepitaxial SrTiO3 (STO) film can be realized by combining conventionally continuous film deposition with a followed interval relaxation, which is feasible at all the investigated f between 1 and 10 Hz.
Y.Z. Chen, N. Pryds / Thin Solid Films 519 (2011) 6330–6333
2. Experimental details
6331
The homoepitaxial STO films were deposited on (001)-oriented STO substrates in a large-area PLD system [13,14]. A KrF (λ = 248 nm) laser with repetition rate f of 1–9 Hz and laser fluence of 1.5 J/cm2 was used. A commercial STO single crystal was used as a target. The distance between the substrate and the target was kept constant at about 6.0 cm. During film deposition, the substrate temperature was kept at 750 °C and various oxygen pressures PO2 between 1.0 × 10− 6 mbar and 1.0 × 10− 2 mbar were used. The film growth process was in-situ monitored by a high pressure RHEED with settled electron beam energy of 30 keV and electron incident direction along [100] of the STO substrate. All STO substrates were specially prepared to obtain a single TiO2-terminated surface with regular unit-cell steps [15,16], which were achieved by chemical etching and subsequent annealing in air at 1000 °C for 1 h. During the chemical etching process, the STO substrates were firstly ultrasonically cleaned in 80 °C deionized water for 30 min to create soluble Sr-hydroxide complex at STO surface [16], then the SrOcontaining layer was further removed by ultrasonically cleaning in acidic solution of HCl–HNO3 (3:1) instead of the conventional buffered HF acid [15,16] for 20 min at 70 °C. The substrates and films were also investigated by atomic force microscopy (AFM) and X-ray diffraction (XRD). 3. Results and discussion Fig. 1(a) shows a 2 μm × 2 μm AFM morphology of the obtained TiO2-terminated STO substrates. Similar to the results with buffered HF acid [15,16], the etching acidic solution of HCl–HNO3 can also result in a regular atomically flat terrace surface as shown in the figure, which has a root mean roughness (RMS) of about 0.14 nm. The terraces are rather uniform with a height of about one unit-cell STO (≈0.4 nm) as shown in Fig. 1(b). The terrace width is about 220 nm, which implies a miscut angle of 0.1° for the substrate. The high quality of the treated STO is also confirmed by the RHEED pattern. As shown in the inset of Fig. 2, the diffraction pattern exhibits bright 1 × 1 streaks and spots lying on the 0th Laue circle, which indicates an atomically smooth surface. Fig. 2 shows the timedependent RHEED intensity during a typical continuous deposition of homoepitaxial STO films at f = 1 Hz and PO2 = 2.0 × 10− 6 mbar. Clear oscillations in the RHEED intensity are observed at the beginning of deposition, which indicates a LBL film growth mode. Generally, every intensity oscillation corresponds to a completed layer growth of the film. The intensity recovery indicates that a layer is fully grown before the next layer is nucleated. Thus, the grown film thickness is about 24 unit cells STO in this process, with a growth rate of about ~0.1 Å/s. However, as the subsequent film deposition proceeds, a gradual decrease in the RHEED peak intensity presents. This indicates a tendency of increased surface roughness in addition with a deviation of film growth mode from LBL one, which is probably due to the accumulation
Intensity (arb. units)
Laser on
Laser off
0
400
1200
Fig. 2. A typical time-dependent RHEED intensity during a continuous film deposition at f = 1 Hz and PO2 = 2.0 × 10− 6 mbar. The inset shows a RHEED pattern of the treated STO substrate surface.
of large-diameter islands during deposition. When the film deposition is stopped, the surface becomes so rough that the RHEED intensity can hardly recover and even goes further down, which most likely results from the nucleation of three-dimensional islands. However, it is found that the RHEED intensity can recover to its initial value if the continuous film deposition is interrupted before such a serious surface deterioration is reached. For our experiments, this means that the film deposition should be interrupted before the film is thicker than 10 unit cells. Fig. 3(a) shows a time-dependent RHEED intensity during deposition of a 9 unit cells STO film at f = 1 Hz and PO2 = 2.0 × 10− 6 mbar. When the film deposition is interrupted, the RHEED intensity exhibits an exponential increase toward the initial value, which can be well fitted by two or multiple relaxation processes with the relaxation time, τ, between 0.2 and 30 s. Thus, a reconstruction of an atomically smooth surface could be achieved by a waiting interval of more than 30 s for non deposition. Fig. 3(b) shows the time dependent RHEED intensity during the followed film deposition by combining a period of continuous film deposition with a subsequent interrupted non deposition. It is remarkable that clear oscillations in the RHEED intensity are observed even after deposition of 100 unit cells STO film. The bright streaky RHEED patterns after film deposition [Fig. 3(d)] resemble greatly the case of the bare STO [Fig. 3(c)]. These results clearly demonstrate that a persistent LBL film growth could be achieved by using a modified PLID method with conventional deposition conditions. The effect of f on the LBL film growth mode by this interrupted deposition was further investigated at PO2 = 2.0 × 10− 6 mbar. Fig. 4(a) shows the film deposition of 5 unit cells STO at different f of 3, 5, 7, 9 Hz. It is interesting to note that persistent LBL film growth could be realized by reproducing such continuous deposition at any f with a followed interval of non deposition. Fig. 4(b) shows the frequency dependence of
10 nm 3.0 (nm)
a
b
0.4 nm
1.5 0
800
Time (s)
2 (μm)
0 0
1.0
2.0 (μm)
Fig. 1. (a) A 2 μm × 2 μm AFM morphology of an atomically flat TiO2-terminated SrTiO3 substrate with regular terrace steps. (b) A line profile in (a) showing the step height of one unit cell of STO (0.39 nm) and the step width of about 220 nm.
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b
0
250
Intensity (arb. units)
Intensity (arb. units)
a
500
Time (s)
c
d
Continuous deposition plus waiting interval 0
500
1000
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Time (s) Fig. 3. (a) A layer-by-layer homoepitaxial growth of 9 unit cell STO layers with recovered RHEED intensity at f = 1 Hz and PO2 = 2.0 × 10− 6 mbar. (b) The followed persistent layerby-layer film growth by interrupted deposition. (c) and (d) show the RHEED patterns before and after film deposition, respectively.
σ, which is defined by the averaged relative changes between the minimum and maximum amplitudes of RHEED oscillations. The σ, thus the oscillation damping, is nearly constant at f b 5 Hz, while a clear increase in σ thus a suppression of oscillation damping is observed at higher frequencies. This is different from the case in the conventionally continuous film deposition, where a higher f generally results in a faster damping of RHEED intensity, as illustrated in Fig. 4(c) for a continuous deposition at f = 5 Hz in comparison with that in Fig. 2 for f = 1 Hz. However, this suppression of the oscillation damping by increasing f is quite similar to the situation in the conventional PLID technique, which suggests a significant suppression of forming multilevel films [7]. Fig. 5(a) and (b) shows the AFM morphology of two STO films grown under continuous and interrupted deposition, respectively, at f = 1 Hz and PO2 = 2.0 × 10− 6 mbar. The film thickness is about 100 unit cells. As shown in Fig. 5(a), the film surface obtained by continuous deposition exhibits a smooth surface with a RSM of 0.24 nm. However, the surface
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Fig. 4. (a) The film deposition of 5 unit cells STO at different f of 3, 5, 7, 9 Hz with PO2 = 2.0 × 10− 6 mbar. (b) The corresponding frequency dependence of oscillation damping, σ. (c) The time dependent RHEED intensity during a continuous film deposition at f = 5 Hz and PO2 = 2.0 × 10− 6 mbar.
consists of mainly small islands with diameters of 30–50 nm, instead of terrace steps. In contrast, the film surface deposited by the interval deposition, as shown in Fig. 5(b), exhibits atomically flat terraces with step height of 0.4 nm, though the terrace step is not as sharp as that of the bare substrates. It should be noted that similar surface characteristics are also observed for films deposited at f = 5 Hz. These results further confirm the fact that the suppression of island accumulation, which is accompanied by a persistence of LBL film growth to thicker films, could be realized by interval deposition. Additionally, the obtained films, which were in situ heated at 600 °C under 0.8 mbar for 30 min after deposition, were also checked by X-ray diffraction (XRD). No obvious difference in the lattice parameters was observed between the films and substrates. Since the out-of-plane lattice parameter of STO films will be enhanced in non-stoichiometric films, independent of whether they were Ti or Sr rich [8,17], the obtained STO films in our case therefore may be a stoichiometric one. The effect of PO2, which is an important factor in determining the properties of complex oxide film, on the LBL film growth was also investigated. It should be mentioned that the oxygen pressure dominates the plasma expansion dynamics during PLD process [18,19]. Due to the collisions between the expanding plasma plume and the surrounding oxygen molecules, the plume front is generally confined to a distance smaller than 4.0 cm when the background PO2 is higher than 5.0 × 10− 2 mbar [18]. As a consequence, for realizing LBL film growth in large area PLD, where a rather large distance between targets and substrates (larger than 6.0 cm) exists, the film growth is better to be performed in a PO2 lower than 1.0× 10− 2 mbar to maintain a high cluster mobility for the interlayer transfer during the relaxation intervals. Fig. 6(a) shows the RHEED oscillations obtained under various PO2 between 1.0 × 10− 6 mbar and 1.0× 10− 2 mbar during the STO homoepitaxial growth. As shown in the figure, LBL film growth mode could be realized in all the investigated oxygen pressure range. Fig. 6(b) and (c) shows the dependence of the film growth rate and the σ, respectively, on the oxygen pressure. The film growth rate is about 38 laser pulses for growing one unit cell STO, ~0.1 Å/s, in the PO2 range of 1.0 ×10− 6 − 1.0 ×10− 3 mbar. When PO2 is increased to 1.0× 10− 2 mbar, a decrease of about 10% in growth rate is observed. The constant film growth rate between 1.0× 10− 6 and 1.0 ×10− 3 mbar seems to be consistent with the free expansion of the plasma at these oxygen pressures [18,19]. The reduction of the film growth rate at PO2 = 1.0 ×10− 2 mbar may indicate the presence of the collisions between the plasma and the oxygen molecules thus the scattering and attenuation of the fast plume. The plasma attenuation at PO2 = 1.0 ×10− 2 mbar can also account for the
Y.Z. Chen, N. Pryds / Thin Solid Films 519 (2011) 6330–6333
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Fig. 5. The 5 μm × 5 μm AFM morphology of STO films grown under continuous (a) and interrupted (b) deposition with a thickness of about 100 unit cells at f = 1 Hz and PO2 = 2.0 × 10− 6 mbar.
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even in a large area PLD. This modified PLID technique could realize persistent LBL film growth at any laser frequency between 1 and 10 Hz and provides an effective way to fabricate high quality complex oxide films on unit cell scale.
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Acknowledgments The authors thank F. B. Saxild, J. Geyti, K. V. Hansen, S. Linderoth, J. Schou, K. Rodrigo and K. M. Kant for their valuable help. -6
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References
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Fig. 6. (a) The RHEED oscillations obtained under various oxygen pressures between 1.0 × 10− 6 and 1.0 × 10− 2 mbar for STO homoepitaxial growth at f = 1 Hz. (b) and (c) The film growth rate and the oscillation damping σ, respectively, as a function of oxygen pressure.
increased σ as shown in Fig. 6(c), since a decrease in the plasma species energy could lead to a decrease in the surface diffusion mobility of the adatoms and the formation of large-diameter islands. However, the obvious decrease in oscillation damping at PO2 =1.0 ×10− 3 mbar is unexpected, which needs to be further investigated. In summary, by a modified interval deposition, which relies on a combination of the normal continuous film deposition of more than one unit cell with a followed interval relaxation, a persistent layer-bylayer film growth is achieved during SrTiO3 homoepitaxial growth
[1] X.X. Guo, J. Maier, Adv. Mater. 21 (2009) 2619. [2] Y. Tokura, H.Y. Hwang, Nature Mater. 7 (2008) 694. [3] A.J.H.M. Rijnders, G. Koster, D.H.A. Blank, H. Rogalla, Appl. Phys. Lett. 70 (1997) 1888. [4] H.M. Christen, G. Eres, J. Phys. Condens. Matter 20 (2008) 264005. [5] Y.Z. Chen, J.R. Sun, X.Y. Xie, D.J. Wang, W.M. Lu, S. Liang, B.G. Shen, Appl. Phys. Lett. 90 (2007) 143508. [6] G. Koster, A.J.H.M. Rijnders, D.H.A. Blank, H. Rogalla, Mater. Res. Soc. Symp. Proc. 526 (1998) 33. [7] G. Koster, G.J.H.M. Rijnders, D.H.A. Blank, H. Rogalla, Appl. Phys. Lett. 74 (1999) 3729. [8] T. Ohnishi, T. Yamamoto, S. Meguro, H. Koinuma, M. Lippmaa, J. Phys. Conf. Ser. 59 (2007) 514. [9] Y.Y. Tse, S.R.C. Mcmitchell, T.J. Jackson, Y.H. Liu, I.P. Jones, Ferroelectrics 368 (2008) 49. [10] M. Hiratani, Y. Tarutani, T. Fukazawa, M. Okamoto, K. Takagi, Thin Solid Films 227 (1993) 100. [11] J.H. Gao, J. Gao, H.K. Wong, Thin Solid Films 515 (2006) 559. [12] P.R. Willmott, R. Herger, C.M. Schleputz, M. Martoccia, B.D. Patterson, Phys. Rev. Lett. 96 (2006) 176102. [13] N. Pryds, J. Schou, S. Linderoth, Appl. Surf. Sci. 253 (2007) 8231. [14] N. Pryds, D. Cockburn, K. Rodrigo, I.L. Rasmussen, J. Knudsen, J. Schou, Appl. Phys. A 93 (2008) 705. [15] M. Kawasaki, K. Takahashi, T. Maeda, R. Tsuchiya, M. Shinohara, O. Ishiyama, T. Yonezawa, M. Yoshimoto, H. Koinuma, Science 266 (1994) 1540. [16] G. Koster, B.L. Kropman, G.J.H.M. Rijnders, D.H.A. Blank, H. Rogalla, Appl. Phys. Lett. 73 (1998) 2920. [17] B. Jalan, R. Engel-Herbert, N.J. Wright, S. Stemmer, J. Vac. Sci. Technol. A 27 (2009) 461. [18] S. Amoruso, A. Sambri, X. Wang, J. Appl. Phys. 100 (2006) 013302. [19] A.N. Khodan, S. Guyard, J.-P. Contour, D.-G. Crété, E. Jacquet, K. Bouzehouane, Thin Solid Films 515 (2007) 6422.