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Epitaxial optimization of atomically smooth Sr3Al2O6 for freestanding perovskite films by molecular beam epitaxy
Thin Solid Films 697 (2020) 137815
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Epitaxial optimization of atomically smooth Sr3Al2O6 for freestanding perovskite films by molecular beam epitaxy
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H.Y. Sun, C.C. Zhang, J.M. Song, J.H. Gu, T.W. Zhang, Y.P. Zang, Y.F. Li, Z.B. Gu, P. Wang, ⁎ Y.F. Nie National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210023, China
ABSTRACT
The epitaxial crystal quality of strontium aluminate (Sr3Al2O6) films under various Sr/Al ratios were systematically investigated by reactive molecular beam epitaxy. Efficient guiding rules for real-time optimization are summarized that a four-fold reconstructed reflection high-energy electron diffraction (RHEED) pattern and 4 periods of RHEED oscillations coinciding in the growth of one unit cell of Sr3Al2O6 is the key signature for the optimal growth condition. Following above rules, atomically smooth Sr3Al2O6 and freestanding SrTiO3 films with a full width at half maximum less than 0.03∘ (mainly limited by the substrates) were synthesized. The high-crystalline quality of freestanding SrTiO3 and atomically smooth interface between SrTiO3 film and Sr3Al2O6 were highlighted by the appearance of well-defined fringes from X-ray diffraction data and well-organized atomic distribution from electron microscopy. The epitaxial optimization of Sr3Al2O6 buffer layer with atomic flatness and high-crystalline quality will shed light on the synthesis of ultrathin freestanding oxide perovskite films, paving the way to the exploration of incorporating strongly correlated properties in conventional semiconductors for a generation of multifunctional electronic devices.
1. Introduction The synthesis of freestanding transition metal oxide films is of great interests in the exploration of intriguing low dimensional correlated phases. Compared with conventional low-dimensional materials [1–7], transition metal oxides exhibit more exotic phases derived from the strong correlation between d electrons [8–16]. Moreover, in the freestanding form, 1D and 2D strains can be exerted on oxide films to tune the lattice constants and symmetry continuously, which can not be achieved by the conventional epitaxial growth of films on substrates [17,18]. The large tunability of the lattice structure and symmetry will allow the exploration of intriguing quantum phases in strained freestanding films. In addition, high quality single-crystalline freestanding oxide films can be grown and transferred onto other substrates, including oxides and non-oxides, in order to form a wide variety of new interfaces that can not be realized by epitaxial growth. For example, the incorporation of strongly correlated properties in conventional semiconductors will avail to the appearance of a new generation of multifunctional electronic devices [19–25]. As such, a growth method for high quality freestanding transition metal oxide films is in high demand. Many techniques have been employed to synthesize freestanding crystalline films, including selective etching of the buffer layer using acids [26–29], dissolving the NaCl substrate by water [30], and thermal
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melting the film/substrate interface by laser heating [31] and ion implantations [32]. These techniques work well for certain materials but they are quite unique and difficult to be generalized to many other perovskite oxides. Recently, Di Lu et al. [33,34] reported the application of hygroscopic sacrificial strontium aluminate (Sr3Al2O6) as the buffer layer to grow and release freestanding perovskite films from the substrates. Since the Sr3Al2O6 buffer layer can be dissolved in water and there is no need to use acids, this method can be applied to synthesize freestanding films out of a wider range of perovskite oxides. So highcrystalline quality and atomically smooth Sr3Al2O6 layer is essential for the growth and transfer of low dimensional freestanding perovskite oxide films. Experimental results demonstrate that the quality of Sr3Al2O6 especially the surface smoothness, shows an extremely strong dependence on the Sr/Al ratio. Only by keeping the optimal Sr/Al ratio instead of the Sr-poor or Sr-rich cases during growth, the atomically smooth Sr3Al2O6 films can be synthesized efficiently. However, most of the previous studies were based on pulsed laser deposition growth, which did not allow a systematic adjustment on the Sr/Al ratios. Up to now, clear guidelines on how to grow atomically smooth Sr3Al2O6 are of great practical importance but still lacking, hindering the application of this synthesis technique in the exploration of low-dimensional correlated phenomena. From this perspective, epitaxial growth of Sr3Al2O6 with ideal crystalline quality is very challenging but highly desired.
Corresponding author. E-mail addresses: [email protected] (H.Y. Sun), [email protected] (Y.F. Nie).
https://doi.org/10.1016/j.tsf.2020.137815 Received 4 October 2019; Received in revised form 4 January 2020; Accepted 17 January 2020 Available online 18 January 2020 0040-6090/ © 2020 Elsevier B.V. All rights reserved.
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2. Experimental details In this paper, we report a systematical exploration of the optimized growth of Sr3Al2O6 by reactive molecular-beam epitaxy (MBE). Using separated effusion cells, MBE technique enables the flexible and realtime adjustment of the Sr/Al flux ratio to explore its impact on the crystalline quality and surface smoothness. The quality of Sr3Al2O6 film shows a strong dependence on the Sr/Al flux ratio and the optimal growth condition exhibits clear signatures in the reflection high-energy electron diffraction (RHEED) pattern and intensity oscillations. According to these summarized guiding rules, real-time optimization of growth condition for synthesizing atomically smooth Sr3Al2O6 film efficiently was demonstrated in this work. In addition, high quality SrTiO3 film as a prototype perovskite was grown on Sr3Al2O6 buffer layer and transferred with atomically flat terraces and well-defined Kessig fringes which haven’t reported in previous researches. Epitaxial Sr3Al2O6 films were grown on (001) SrTiO3 single-crystalline substrates using a DCA R450 MBE system at a growth temperature of 750 ∘C (measured by pyrometer) and 1.33 × 10 4 Pa of a mixed oxidant (10%O3+90%O2). SrTiO3 substrates were etched in buffered HF acid at room temperature and annealed in oxygen at 1000 ∘ C for 80 min to obtain TiO2-terminated step-and-terrace surfaces [35]. During film growth, an electron beam of 15 KeV kinetic energy was employed for RHEED measurements. The crystalline structures of Sr3Al2O6 films were examined by X-ray diffraction (XRD) using a Bruker D8 Discover diffractometer. The terraced micromorphology and atomic-scale microstructure of films were revealed by FM-Nanoview 1000 atomic force microscopy (AFM) and an FEI Titan Cubed G2 60300 scanning transmission electron microscope (STEM), respectively. The similar atomic arrangement as well as the close lattice match (4:1 unit cell size ratio) between Sr3Al2O6 (space group Pa3¯ , a = 15.844 Å) and SrTiO3 (space group Pm3¯m, a = 3.905 Å) allows for the epitaxial film growth of Sr3Al2O6 on SrTiO3 substrates (Fig. 1(a)). As stoichiometry is crucial for film lattice structure and crystallinity, the flux ratio of Sr and Al sources was calibrated roughly with by a quartz crystal microbalance (QCM) with an accuracy of about ± 5% before film growth. Then we fine calibrated the Sr (Al) fluxes by growing SrTiO3 (LaAlO3) films on SrTiO3 (LaAlO3) substrate, respectively. Once we obtain stable RHEED oscillations, the exact flux can be extracted from the period (deposition time per unit cell) [36]. For the optimal growth in Fig. 1, we adjusted the flux to be 3:2 with an accuracy <1%. For the off-stoichiometric growth, we adjusted the Al source temperature to increase or decrease its flux, and the amount of flux change were estimated by the thermodynamic relationship between vapor pressure and temperature [37], the relationship is shown as follows:
ln (P1/ P2) = ( Hvap/ R)((1/T2)
(1/ T1))
Fig. 1. (a) Schematic of the crystal structure of Sr3Al2O6 from the side view and top view. Oxygen atoms are omitted for simplicity. The ball-and-stick model of Sr3Al2O6 can be seen in Ref.[20]. In the top view, only 1/4 of the Sr3Al2O6 unit cell (a sublayer) is projected onto the (001) plane. (b) RHEED intensity oscillations and diffraction patterns taken in the growth of Sr3Al2O6 (8 u.c.) films at cases of Sr poor, optimal and Sr rich. Corresponding AFM images are also shown. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
even missing, resulting in a two-fold reconstructed pattern. One explanation is that the excess Sr atoms on the surface fill the A-site Sr vacancies as illustrated in the top view in Fig. 1(a). As all the Sr vacancy sites are filled, the in-plane unit cell size of Sr3Al2O6 is reduced by half. In the reciprocal space mapping, this results in the transition of RHEED patterns from four-folded to two-folded. In the Sr-poor case, RHEED diffraction patterns still show weak four-fold reconstructed stripes but become spotty, indicating the poor surface quality due to the formation of islands on the surface [39]. The surface morphologies of all the Sr3Al2O6 films were checked by AFM measurements after growth shortly. During AFM measurements, the air exposure was strictly controlled to minimize the film damage (formation of islands and particles) due to the adsorption of water vapor in air. Only the Sr3Al2O6 film grown with the optimal condition exhibits the minimum roughness and step-and-terrace surface. In order to show more clear hydrolysis process of the optimized Sr3Al2O6 film after exposure to air, AFM images in Fig. 2 were collected. The hydrolysis in air takes place very fast, resulting in islands and particles on the terraced surface. The bare Sr3Al2O6 film can be fully dissolved in deionized water within a few minutes while this process can take hours to
(1)
where P is the vapor pressure, ΔHvap is the heat of vaporization of the compound, R is the gas constant, and T is the equilibrium temperature (in Kelvin) corresponding to vapor pressure P. The flux is proportional to the vapor pressure. (ΔHvap/R) can be extracted from the data base [38]. For a typical growth, we can skip this time-consuming calibration process of the growth of SrTiO3 (LaAlO3). We mainly rely on the calibrated QCM values and fine adjust the Al source flux by varying its source temperature to optimize the RHEED intensity oscillations and diffraction patterns during the film growth. 3. Results and discussions RHEED diffraction patterns exhibit a strong dependence on the Sr/ Al flux ratio. As shown in the middle of Fig. 1(b), the Sr3Al2O6 film with optimal Sr/Al ratio exhibits four-fold reconstructed RHEED diffraction patterns taken along both [110] and [100] azimuths. These four-fold reconstructed patterns are consistent with the 1:4 ratio between the unit cell sizes of SrTiO3 and Sr3Al2O6. In comparison, when Sr is rich during growth, the four-fold reconstructed stripes become blurry or 2
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days if Sr3Al2O6 film is capped with other perovskite films. The recycled substrate and the released film are clean with atomically smooth surface after the Sr3Al2O6 layer is dissolved. Moreover, to reduce the effect of water vapor absorption in air, extra SrTiO3/Sr3Al2O6/ SrTiO3 hetrostructures were grown by only varying Sr/Al ratios during the growth of Sr3Al2O6 buffer layer while keeping other parameters constant. Results can be well imagined that only the heterostructure with the optimal Sr/Al ratio shows the atomically smooth terraces (Fig. 2(d)). Surface morphologies of other two heterostructures show the similar charecters with Sr3Al2O6 films in Fig. 1(b), so they are omitted here. In addition to RHEED diffraction patterns, RHEED intensity oscillations also exhibit a strong dependence on the Sr/Al flux ratio. Since RHEED intensity oscillations reflect the periodic variation of the surface layer, it has been routinely used in the layer-by-layer epitaxial thin film growth [36,39–41]. As shown in Fig. 1(b), only the deposition with the optimal condition shows 4 periods of intensity oscillations in the growth of one unit cell of Sr3Al2O6, in contrast to the 2 periods of RHEED intensity oscillations per unit cell in the Sr-poor and Sr-rich cases. The RHEED oscillations of Sr3Al2O6 are very sensitive to the Sr/ Al ratios and minor deviation of the ratio will result in a doubling of the oscillation period. When the Sr/Al ratio is severely deviated, no oscillations will be observed (not shown). From the side view in Fig. 1(a), each atomic sublayer repeats itself every 1/4 of the unit cell along the caxis with certain in-plane translation. As RHEED is not sensitive to the ordering along the out-of-plane direction [39], this repetition of equivalent atomic sublayers yields 4 periods of intensity oscillations in the growth of one Sr3Al2O6 unit cell. In the off-stoichiometric cases, the existence of excess atoms or vacancies will affect the ordering and consequently alter or even totally destroy the oscillation patterns. These observations indicate that the four-fold reconstructed RHEED diffraction patterns and 4-period RHEED oscillations are the important signatures for high quality layer-by-layer growth of Sr3Al2O6 films. Following these simple rules, the calibration of the growth condition becomes easy and reproducible. As shown in Fig. 3(a), we show how to efficiently calibrate the growth condition of Sr3Al2O6 film on SrTiO3 substrate by adjusting source flux rates in real time. In this particular case, the initial weak four-fold reconstructed and spotty RHEED diffraction patterns taken along [110] azimuth both indicate that Sr was slightly deficient at the beginning. By decreasing the Al source temperature to reduce Al flux slightly, a clear four-fold reconstructed RHEED diffraction pattern and 4-period intensity oscillation were obtained within the growth of a few unit cells. After the growth condition calibration, we grew a 16 u.c. Sr3Al2O6 film with the optimized parameters on a fresh SrTiO3 substrate for the crystallinity characterizations. High-resolution XRD 2 scan shows sharp (00L) diffraction peaks with clear Kiessig fringes, confirming the high crystallinity of the Sr3Al2O6 film (Fig. 3(b)). Reciprocal space mapping taken around the (103) substrate diffraction peak reveals that the Sr3Al2O6 film is coherently strained to the underlying SrTiO3 substrate in Fig. 3(c). Rocking curve measurements were performed on the (008) film and (002) substrate diffraction peaks showing the FWHM values of 0.015∘ and 0.013∘ for the film and substrate in Fig. 3(d), respectively. These similar FWHM values of film and substrate suggest that the film quality
Fig. 3. (a) Real-time optimization of the Sr/Al ratio during the growth of a 16 u.c. Sr3Al2O6 film. RHEED intensity oscillations and diffraction patterns were taken along the [110] azimuth direction. (b) High-resolution XRD 2 scan of the 16 u.c. Sr3Al2O6 film grown on SrTiO3 substrate with optimal growth condition. (c) Reciprocal space map around the (103) substrate peak. (d) Rocking curves of Sr3Al2O6 film and SrTiO3 substrate. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
is mainly limited by the substrate. As an example, we grew the prototype perovskite SrTiO3 film using optimized Sr3Al2O6 as the buffer layer. As high quality Sr3Al2O6 layer can dissolve in water even it is only a few unit cell thick, we intentionally grew thin Sr3Al2O6 buffer layer to avoid the formation of dislocations due to the structural relaxation of Sr3Al2O6 on SrTiO3. A 40 u.c. SrTiO3 film on 10 u.c. Sr3Al2O6 buffer layer was grown to form a coherent heterostructure. The SrTiO3 film was calibrated and grown with an efficient co-deposition technique by monitoring the RHEED oscillations [36]. After growth, the sample was glued onto Kapton tape using epoxy and the Sr3Al2O6 buffer layer was subsequently dissolved away by de-ionized water, leaving the SrTiO3 film on the Kapton tape. This sample transfer method can yield large and complete freestanding Fig. 2. (a),(b),(c) AFM image of the surface morphology of a 8 u.c. Sr3Al2O6/SrTiO3 (001) sample 8 min (a), 16 min (b) and 24 min (c) after exposure to air.(d) Surface morphology of the 15 u.c. SrTiO3/8 u.c.Sr3Al2O6/SrTiO3 heterostructure with optimal Sr/Al ratio after growth. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3
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Fig. 4. (a) High-resolution XRD 2 scan of a 40 u.c. SrTiO3/10 u.c. Sr3Al2O6/SrTiO3 heterostructure and the corresponding freestanding SrTiO3 film. The asterisk denotes the background diffraction from the kapton tape which is used for transferring SrTiO3 film. The inserted AFM image exhibits atomically smooth surface of the freestanding SrTiO3. (b) Corresponding zoom-in scans around SrTiO3(002) substrate peak show clear thickness fringes indicating the smooth surface of the films before and after sample release. (c) Rocking curves of (008) and (002) diffractions of Sr3Al2O6 film and SrTiO3 substrate, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
films for XRD measurements. The crystalline quality of the sample was checked by XRD before and after the film release. In Fig. 4, the sharp (00L) diffraction peaks with well-defined Kiessig fringes in the2 scans and narrow FWHM of the rocking curves indicate the high crystalline quality and smooth surfaces of the as-grown heterostructure and released film. Clear terraced surface of the released SrTiO3 film (the insert in Fig. 4(a)) indicates the atomically smooth interface between SrTiO3 and Sr3Al2O6. Besides, for characterizing the atomic-scale structural coherency of freestanding SrTiO3, we transferred the film onto silicon substrate and a holey carbon TEM grid for cross-sectional and plan-view high-angle annular dark-field (HAADF) STEM measurements, respectively. The corresponding sample preparation methods have been reported in our previous work [20]. Microstructure characterization including crosssectional HAADF image (Fig. 5(a)), plan-view HAADF image and corresponding selected area electron diffraction (Fig. 5(b)) indicate that the high crystalline quality of the SrTiO3 film in its freestanding form. Conclusions we report a systematic investigation of the growths of Sr3Al2O6 when varying Sr/Al ratios using reactive MBE. The crystal quality of Sr3Al2O6 film, especially the surface smoothness, exhibits an extremely strong dependence on the Sr/Al flux ratio. A four-fold reconstructed RHEED pattern and 4 periods of intensity oscillations coinciding in the growth of one unit cell of Sr3Al2O6 is the key signature for optimal growth of atomically smooth films. Practical guiding rules for synthesizing atomically smooth Sr3Al2O6 films were summarized and
Fig. 5. (a) Cross-sectional plan-view scanning transmission electron microscope (STEM)- high-angle annular dark-field (HAADF) image and (b) plan-view HAADF image of a released freestanding 5 u.c. SrTiO3 film. Corresponding selected area electron diffraction pattern and zoom-in STEM-HAADF image are also presented. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 4
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experimentally testified by reactive MBE. Our work solves the problem how to efficiently synthesize atomically smooth Sr3Al2O6 film, paving the way for subsequent fabrication of high quality freestanding perovskite films and the exploration of intriguing two-dimensional correlated phases in complex oxides [20].
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Epitaxial optimization of atomically smooth Sr3Al2O6 for freestanding perovskite films by molecular beam epitaxy
T
H.Y. Sun, C.C. Zhang, J.M. Song, J.H. Gu, T.W. Zhang, Y.P. Zang, Y.F. Li, Z.B. Gu, P. Wang, ⁎ Y.F. Nie National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210023, China
ABSTRACT
The epitaxial crystal quality of strontium aluminate (Sr3Al2O6) films under various Sr/Al ratios were systematically investigated by reactive molecular beam epitaxy. Efficient guiding rules for real-time optimization are summarized that a four-fold reconstructed reflection high-energy electron diffraction (RHEED) pattern and 4 periods of RHEED oscillations coinciding in the growth of one unit cell of Sr3Al2O6 is the key signature for the optimal growth condition. Following above rules, atomically smooth Sr3Al2O6 and freestanding SrTiO3 films with a full width at half maximum less than 0.03∘ (mainly limited by the substrates) were synthesized. The high-crystalline quality of freestanding SrTiO3 and atomically smooth interface between SrTiO3 film and Sr3Al2O6 were highlighted by the appearance of well-defined fringes from X-ray diffraction data and well-organized atomic distribution from electron microscopy. The epitaxial optimization of Sr3Al2O6 buffer layer with atomic flatness and high-crystalline quality will shed light on the synthesis of ultrathin freestanding oxide perovskite films, paving the way to the exploration of incorporating strongly correlated properties in conventional semiconductors for a generation of multifunctional electronic devices.
1. Introduction The synthesis of freestanding transition metal oxide films is of great interests in the exploration of intriguing low dimensional correlated phases. Compared with conventional low-dimensional materials [1–7], transition metal oxides exhibit more exotic phases derived from the strong correlation between d electrons [8–16]. Moreover, in the freestanding form, 1D and 2D strains can be exerted on oxide films to tune the lattice constants and symmetry continuously, which can not be achieved by the conventional epitaxial growth of films on substrates [17,18]. The large tunability of the lattice structure and symmetry will allow the exploration of intriguing quantum phases in strained freestanding films. In addition, high quality single-crystalline freestanding oxide films can be grown and transferred onto other substrates, including oxides and non-oxides, in order to form a wide variety of new interfaces that can not be realized by epitaxial growth. For example, the incorporation of strongly correlated properties in conventional semiconductors will avail to the appearance of a new generation of multifunctional electronic devices [19–25]. As such, a growth method for high quality freestanding transition metal oxide films is in high demand. Many techniques have been employed to synthesize freestanding crystalline films, including selective etching of the buffer layer using acids [26–29], dissolving the NaCl substrate by water [30], and thermal
⁎
melting the film/substrate interface by laser heating [31] and ion implantations [32]. These techniques work well for certain materials but they are quite unique and difficult to be generalized to many other perovskite oxides. Recently, Di Lu et al. [33,34] reported the application of hygroscopic sacrificial strontium aluminate (Sr3Al2O6) as the buffer layer to grow and release freestanding perovskite films from the substrates. Since the Sr3Al2O6 buffer layer can be dissolved in water and there is no need to use acids, this method can be applied to synthesize freestanding films out of a wider range of perovskite oxides. So highcrystalline quality and atomically smooth Sr3Al2O6 layer is essential for the growth and transfer of low dimensional freestanding perovskite oxide films. Experimental results demonstrate that the quality of Sr3Al2O6 especially the surface smoothness, shows an extremely strong dependence on the Sr/Al ratio. Only by keeping the optimal Sr/Al ratio instead of the Sr-poor or Sr-rich cases during growth, the atomically smooth Sr3Al2O6 films can be synthesized efficiently. However, most of the previous studies were based on pulsed laser deposition growth, which did not allow a systematic adjustment on the Sr/Al ratios. Up to now, clear guidelines on how to grow atomically smooth Sr3Al2O6 are of great practical importance but still lacking, hindering the application of this synthesis technique in the exploration of low-dimensional correlated phenomena. From this perspective, epitaxial growth of Sr3Al2O6 with ideal crystalline quality is very challenging but highly desired.
Corresponding author. E-mail addresses: [email protected] (H.Y. Sun), [email protected] (Y.F. Nie).
https://doi.org/10.1016/j.tsf.2020.137815 Received 4 October 2019; Received in revised form 4 January 2020; Accepted 17 January 2020 Available online 18 January 2020 0040-6090/ © 2020 Elsevier B.V. All rights reserved.
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2. Experimental details In this paper, we report a systematical exploration of the optimized growth of Sr3Al2O6 by reactive molecular-beam epitaxy (MBE). Using separated effusion cells, MBE technique enables the flexible and realtime adjustment of the Sr/Al flux ratio to explore its impact on the crystalline quality and surface smoothness. The quality of Sr3Al2O6 film shows a strong dependence on the Sr/Al flux ratio and the optimal growth condition exhibits clear signatures in the reflection high-energy electron diffraction (RHEED) pattern and intensity oscillations. According to these summarized guiding rules, real-time optimization of growth condition for synthesizing atomically smooth Sr3Al2O6 film efficiently was demonstrated in this work. In addition, high quality SrTiO3 film as a prototype perovskite was grown on Sr3Al2O6 buffer layer and transferred with atomically flat terraces and well-defined Kessig fringes which haven’t reported in previous researches. Epitaxial Sr3Al2O6 films were grown on (001) SrTiO3 single-crystalline substrates using a DCA R450 MBE system at a growth temperature of 750 ∘C (measured by pyrometer) and 1.33 × 10 4 Pa of a mixed oxidant (10%O3+90%O2). SrTiO3 substrates were etched in buffered HF acid at room temperature and annealed in oxygen at 1000 ∘ C for 80 min to obtain TiO2-terminated step-and-terrace surfaces [35]. During film growth, an electron beam of 15 KeV kinetic energy was employed for RHEED measurements. The crystalline structures of Sr3Al2O6 films were examined by X-ray diffraction (XRD) using a Bruker D8 Discover diffractometer. The terraced micromorphology and atomic-scale microstructure of films were revealed by FM-Nanoview 1000 atomic force microscopy (AFM) and an FEI Titan Cubed G2 60300 scanning transmission electron microscope (STEM), respectively. The similar atomic arrangement as well as the close lattice match (4:1 unit cell size ratio) between Sr3Al2O6 (space group Pa3¯ , a = 15.844 Å) and SrTiO3 (space group Pm3¯m, a = 3.905 Å) allows for the epitaxial film growth of Sr3Al2O6 on SrTiO3 substrates (Fig. 1(a)). As stoichiometry is crucial for film lattice structure and crystallinity, the flux ratio of Sr and Al sources was calibrated roughly with by a quartz crystal microbalance (QCM) with an accuracy of about ± 5% before film growth. Then we fine calibrated the Sr (Al) fluxes by growing SrTiO3 (LaAlO3) films on SrTiO3 (LaAlO3) substrate, respectively. Once we obtain stable RHEED oscillations, the exact flux can be extracted from the period (deposition time per unit cell) [36]. For the optimal growth in Fig. 1, we adjusted the flux to be 3:2 with an accuracy <1%. For the off-stoichiometric growth, we adjusted the Al source temperature to increase or decrease its flux, and the amount of flux change were estimated by the thermodynamic relationship between vapor pressure and temperature [37], the relationship is shown as follows:
ln (P1/ P2) = ( Hvap/ R)((1/T2)
(1/ T1))
Fig. 1. (a) Schematic of the crystal structure of Sr3Al2O6 from the side view and top view. Oxygen atoms are omitted for simplicity. The ball-and-stick model of Sr3Al2O6 can be seen in Ref.[20]. In the top view, only 1/4 of the Sr3Al2O6 unit cell (a sublayer) is projected onto the (001) plane. (b) RHEED intensity oscillations and diffraction patterns taken in the growth of Sr3Al2O6 (8 u.c.) films at cases of Sr poor, optimal and Sr rich. Corresponding AFM images are also shown. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
even missing, resulting in a two-fold reconstructed pattern. One explanation is that the excess Sr atoms on the surface fill the A-site Sr vacancies as illustrated in the top view in Fig. 1(a). As all the Sr vacancy sites are filled, the in-plane unit cell size of Sr3Al2O6 is reduced by half. In the reciprocal space mapping, this results in the transition of RHEED patterns from four-folded to two-folded. In the Sr-poor case, RHEED diffraction patterns still show weak four-fold reconstructed stripes but become spotty, indicating the poor surface quality due to the formation of islands on the surface [39]. The surface morphologies of all the Sr3Al2O6 films were checked by AFM measurements after growth shortly. During AFM measurements, the air exposure was strictly controlled to minimize the film damage (formation of islands and particles) due to the adsorption of water vapor in air. Only the Sr3Al2O6 film grown with the optimal condition exhibits the minimum roughness and step-and-terrace surface. In order to show more clear hydrolysis process of the optimized Sr3Al2O6 film after exposure to air, AFM images in Fig. 2 were collected. The hydrolysis in air takes place very fast, resulting in islands and particles on the terraced surface. The bare Sr3Al2O6 film can be fully dissolved in deionized water within a few minutes while this process can take hours to
(1)
where P is the vapor pressure, ΔHvap is the heat of vaporization of the compound, R is the gas constant, and T is the equilibrium temperature (in Kelvin) corresponding to vapor pressure P. The flux is proportional to the vapor pressure. (ΔHvap/R) can be extracted from the data base [38]. For a typical growth, we can skip this time-consuming calibration process of the growth of SrTiO3 (LaAlO3). We mainly rely on the calibrated QCM values and fine adjust the Al source flux by varying its source temperature to optimize the RHEED intensity oscillations and diffraction patterns during the film growth. 3. Results and discussions RHEED diffraction patterns exhibit a strong dependence on the Sr/ Al flux ratio. As shown in the middle of Fig. 1(b), the Sr3Al2O6 film with optimal Sr/Al ratio exhibits four-fold reconstructed RHEED diffraction patterns taken along both [110] and [100] azimuths. These four-fold reconstructed patterns are consistent with the 1:4 ratio between the unit cell sizes of SrTiO3 and Sr3Al2O6. In comparison, when Sr is rich during growth, the four-fold reconstructed stripes become blurry or 2
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days if Sr3Al2O6 film is capped with other perovskite films. The recycled substrate and the released film are clean with atomically smooth surface after the Sr3Al2O6 layer is dissolved. Moreover, to reduce the effect of water vapor absorption in air, extra SrTiO3/Sr3Al2O6/ SrTiO3 hetrostructures were grown by only varying Sr/Al ratios during the growth of Sr3Al2O6 buffer layer while keeping other parameters constant. Results can be well imagined that only the heterostructure with the optimal Sr/Al ratio shows the atomically smooth terraces (Fig. 2(d)). Surface morphologies of other two heterostructures show the similar charecters with Sr3Al2O6 films in Fig. 1(b), so they are omitted here. In addition to RHEED diffraction patterns, RHEED intensity oscillations also exhibit a strong dependence on the Sr/Al flux ratio. Since RHEED intensity oscillations reflect the periodic variation of the surface layer, it has been routinely used in the layer-by-layer epitaxial thin film growth [36,39–41]. As shown in Fig. 1(b), only the deposition with the optimal condition shows 4 periods of intensity oscillations in the growth of one unit cell of Sr3Al2O6, in contrast to the 2 periods of RHEED intensity oscillations per unit cell in the Sr-poor and Sr-rich cases. The RHEED oscillations of Sr3Al2O6 are very sensitive to the Sr/ Al ratios and minor deviation of the ratio will result in a doubling of the oscillation period. When the Sr/Al ratio is severely deviated, no oscillations will be observed (not shown). From the side view in Fig. 1(a), each atomic sublayer repeats itself every 1/4 of the unit cell along the caxis with certain in-plane translation. As RHEED is not sensitive to the ordering along the out-of-plane direction [39], this repetition of equivalent atomic sublayers yields 4 periods of intensity oscillations in the growth of one Sr3Al2O6 unit cell. In the off-stoichiometric cases, the existence of excess atoms or vacancies will affect the ordering and consequently alter or even totally destroy the oscillation patterns. These observations indicate that the four-fold reconstructed RHEED diffraction patterns and 4-period RHEED oscillations are the important signatures for high quality layer-by-layer growth of Sr3Al2O6 films. Following these simple rules, the calibration of the growth condition becomes easy and reproducible. As shown in Fig. 3(a), we show how to efficiently calibrate the growth condition of Sr3Al2O6 film on SrTiO3 substrate by adjusting source flux rates in real time. In this particular case, the initial weak four-fold reconstructed and spotty RHEED diffraction patterns taken along [110] azimuth both indicate that Sr was slightly deficient at the beginning. By decreasing the Al source temperature to reduce Al flux slightly, a clear four-fold reconstructed RHEED diffraction pattern and 4-period intensity oscillation were obtained within the growth of a few unit cells. After the growth condition calibration, we grew a 16 u.c. Sr3Al2O6 film with the optimized parameters on a fresh SrTiO3 substrate for the crystallinity characterizations. High-resolution XRD 2 scan shows sharp (00L) diffraction peaks with clear Kiessig fringes, confirming the high crystallinity of the Sr3Al2O6 film (Fig. 3(b)). Reciprocal space mapping taken around the (103) substrate diffraction peak reveals that the Sr3Al2O6 film is coherently strained to the underlying SrTiO3 substrate in Fig. 3(c). Rocking curve measurements were performed on the (008) film and (002) substrate diffraction peaks showing the FWHM values of 0.015∘ and 0.013∘ for the film and substrate in Fig. 3(d), respectively. These similar FWHM values of film and substrate suggest that the film quality
Fig. 3. (a) Real-time optimization of the Sr/Al ratio during the growth of a 16 u.c. Sr3Al2O6 film. RHEED intensity oscillations and diffraction patterns were taken along the [110] azimuth direction. (b) High-resolution XRD 2 scan of the 16 u.c. Sr3Al2O6 film grown on SrTiO3 substrate with optimal growth condition. (c) Reciprocal space map around the (103) substrate peak. (d) Rocking curves of Sr3Al2O6 film and SrTiO3 substrate. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
is mainly limited by the substrate. As an example, we grew the prototype perovskite SrTiO3 film using optimized Sr3Al2O6 as the buffer layer. As high quality Sr3Al2O6 layer can dissolve in water even it is only a few unit cell thick, we intentionally grew thin Sr3Al2O6 buffer layer to avoid the formation of dislocations due to the structural relaxation of Sr3Al2O6 on SrTiO3. A 40 u.c. SrTiO3 film on 10 u.c. Sr3Al2O6 buffer layer was grown to form a coherent heterostructure. The SrTiO3 film was calibrated and grown with an efficient co-deposition technique by monitoring the RHEED oscillations [36]. After growth, the sample was glued onto Kapton tape using epoxy and the Sr3Al2O6 buffer layer was subsequently dissolved away by de-ionized water, leaving the SrTiO3 film on the Kapton tape. This sample transfer method can yield large and complete freestanding Fig. 2. (a),(b),(c) AFM image of the surface morphology of a 8 u.c. Sr3Al2O6/SrTiO3 (001) sample 8 min (a), 16 min (b) and 24 min (c) after exposure to air.(d) Surface morphology of the 15 u.c. SrTiO3/8 u.c.Sr3Al2O6/SrTiO3 heterostructure with optimal Sr/Al ratio after growth. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. (a) High-resolution XRD 2 scan of a 40 u.c. SrTiO3/10 u.c. Sr3Al2O6/SrTiO3 heterostructure and the corresponding freestanding SrTiO3 film. The asterisk denotes the background diffraction from the kapton tape which is used for transferring SrTiO3 film. The inserted AFM image exhibits atomically smooth surface of the freestanding SrTiO3. (b) Corresponding zoom-in scans around SrTiO3(002) substrate peak show clear thickness fringes indicating the smooth surface of the films before and after sample release. (c) Rocking curves of (008) and (002) diffractions of Sr3Al2O6 film and SrTiO3 substrate, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
films for XRD measurements. The crystalline quality of the sample was checked by XRD before and after the film release. In Fig. 4, the sharp (00L) diffraction peaks with well-defined Kiessig fringes in the2 scans and narrow FWHM of the rocking curves indicate the high crystalline quality and smooth surfaces of the as-grown heterostructure and released film. Clear terraced surface of the released SrTiO3 film (the insert in Fig. 4(a)) indicates the atomically smooth interface between SrTiO3 and Sr3Al2O6. Besides, for characterizing the atomic-scale structural coherency of freestanding SrTiO3, we transferred the film onto silicon substrate and a holey carbon TEM grid for cross-sectional and plan-view high-angle annular dark-field (HAADF) STEM measurements, respectively. The corresponding sample preparation methods have been reported in our previous work [20]. Microstructure characterization including crosssectional HAADF image (Fig. 5(a)), plan-view HAADF image and corresponding selected area electron diffraction (Fig. 5(b)) indicate that the high crystalline quality of the SrTiO3 film in its freestanding form. Conclusions we report a systematic investigation of the growths of Sr3Al2O6 when varying Sr/Al ratios using reactive MBE. The crystal quality of Sr3Al2O6 film, especially the surface smoothness, exhibits an extremely strong dependence on the Sr/Al flux ratio. A four-fold reconstructed RHEED pattern and 4 periods of intensity oscillations coinciding in the growth of one unit cell of Sr3Al2O6 is the key signature for optimal growth of atomically smooth films. Practical guiding rules for synthesizing atomically smooth Sr3Al2O6 films were summarized and
Fig. 5. (a) Cross-sectional plan-view scanning transmission electron microscope (STEM)- high-angle annular dark-field (HAADF) image and (b) plan-view HAADF image of a released freestanding 5 u.c. SrTiO3 film. Corresponding selected area electron diffraction pattern and zoom-in STEM-HAADF image are also presented. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 4
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experimentally testified by reactive MBE. Our work solves the problem how to efficiently synthesize atomically smooth Sr3Al2O6 film, paving the way for subsequent fabrication of high quality freestanding perovskite films and the exploration of intriguing two-dimensional correlated phases in complex oxides [20].
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