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Laser ablation of manganite thin films monitored by in situ RHEED
Journal of Magnetism and Magnetic Materials 211 (2000) 9}15
Laser ablation of manganite thin "lms monitored by in situ RHEED J. Klein, C. HoK fener, L. Al!*, R. Gross II. Physikalisches Institut, Universita( t zu Ko( ln, Zu( lpicher Str. 77 50937 Ko( ln, Germany
Abstract Re#ection high-energy electron di!raction (RHEED) during crystal growth has become a standard analytical tool in the fabrication of semiconducting materials by molecular-beam epitaxy in an ultrahigh vacuum environment. For the fabrication of high-quality epitaxial and heteroepitaxial thin "lm structures of oxide materials such as perovskite manganites or high-temperature superconductors, it is also highly desirable to use RHEED technique during epitaxial growth. However, the complex oxide materials are commonly fabricated by pulsed laser deposition (PLD) at high oxygen pressure above 10 Pa preventing the use of standard RHEED systems. A two-stage di!erential pumping system can serve to circumvent this problem [1]. In this way, the electron path in the high-pressure oxygen atmosphere within the deposition chamber can be reduced considerably. Then the specular beam intensity of the RHEED system is su$ciently high to allow the observation of intensity oscillations during the growth of oxide materials. We have used high-pressure RHEED and in situ atomic force microscopy (AFM) for the investigation of the surface morphology of oxide thin "lms prepared by PLD. ( 2000 Elsevier Science B.V. All rights reserved. Keywords: Manganite thin "lms; RHEED; Pulsed laser deposition
1. Introduction Epitaxial thin "lms and heterostructures of complex oxide materials are required for a large number of devices showing interesting properties both with respect to applications and basic physics. Typical examples are Josephson junctions made from the cuprate high-temperature superconductors [2] or magnetic tunnel junctions based on doped manganites [3,4]. In order to control the epitaxial growth, the interface properties, and the
* Corresponding author. Tel.: #49-221-470-3700; fax: #49221-470-5178. E-mail address: [email protected] (L. Al!)
layer thickness in heterostructures, RHEED represents a powerful tool. In the thin "lm growth of metallic or semiconducting materials under ultrahigh vacuum (UHV) conditions RHEED is a wellestablished method (for overviews see Refs. [5}7]). However, the high oxygen pressure (a few 10 Pa) used during the growth of oxide materials prevents the use of standard RHEED systems due to the small mean free path of the beam electrons under these conditions. One way to overcome this problem is the reduction of oxygen pressure during "lm deposition. In order to achieve an oxidation potential corresponding to 30 Pa molecular oxygen, atomic oxygen or ozone instead of molecular oxygen has to be used [8]. The molecular oxygen pressure then can be reduced to below 1 Pa in
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J. Klein et al. / Journal of Magnetism and Magnetic Materials 211 (2000) 9}15
a standard PLD process [9]. However, these modi"ed fabrication processes have in common the problem that the oxygen partial pressure is far from the usual values. Therefore, in our setup we use a two-stage di!erential pumping system similar to that proposed by Rijnders et al. [1]. In this article we show how the modi"ed RHEED system can be used for the in situ monitoring of the growth process of the colossal magnetoresistance (CMR) materials La D MnO with D"Ca and Ba. 1@3 2@3 3 2. RHEED system A schematic view of our high-pressure RHEED system is shown in Fig. 1. In the deposition chamber the oxygen pressure can be as large as 30 Pa. A double di!erential pumping system decreases the
Fig. 1. Schematic view of the high-pressure RHEED system.
pressure between the two coaxial tubes to below 0.1 Pa and at the "lament of the RHEED gun and inside the inner tube to below 10~3 Pa. Along their path from the "lament to the substrate the beam electrons have to pass two apertures at the front end of the tubes with a diameter of 500 lm. In the high vacuum regime (p(10~3 Pa) between the RHEED "lament and the "rst aperture the scattering probability for the beam electrons is vanishingly small. However, in the high-pressure regime inside the deposition chamber, the small mean free path of the beam electrons results in a decrease of the specular beam intensity. The mean free path l of the electrons is given by 1 k ¹ l" " B . p 2n p 2p O O Here, n is the oxygen molecular number density and l depends on p and the energy-dependent scattering cross-section p 2 of oxygen. O With p"10 Pa and p +10~20 m2 the O mean free path is estimated to be2 5 cm. In our setup the actual path length d of the beam electrons in )1 the high-pressure regime is about 20 cm. A further reduction of the path length was not required in order to have enough re#ected beam intensity for the monitoring of RHEED oscillations. This is shown in Fig. 2, where image (a) shows a RHEED pattern of a 3.2 nm thick La Ba MnO "lm 1@3 2@3 3 under UHV conditions. For comparison, image (b) shows the RHEED pattern from the same surface but at an oxygen pressure of 6.67 Pa
Fig. 2. RHEED patterns recorded in (a) UHV environment and (b) at 6.67 Pa (50 m Torr) O . 2
J. Klein et al. / Journal of Magnetism and Magnetic Materials 211 (2000) 9}15
(50 m Torr) and a 15 times longer exposure time. This is in agreement with the expected exponential decay of the electron beam intensity, I, during its path through the high-pressure deposition chamber following IJe~d)1 @l. Instead of increasing the exposure time, in practice, an increase in electron beam intensity is used to obtain a su$cient specular beam intensity on the RHEED screen.
3. RHEED oscillations during the growth of oxide 5lms The oxide thin "lms (La Ba MnO :LBMO, 2@3 1@3 3 La Ca MnO :LCMO) were fabricated on 2@3 1@3 3 SrTiO substrates by PLD. The substrate temper3 ature was 7603C and the "lms were deposited at an oxygen pressure of 26.66 Pa (200 m Torr) using a 248 nm KrF-eximer laser with a pulse energy of 450 mJ resulting in an energy density of 2 J/cm2. During the growth of the "lms the intensity of the specular beam was monitored. Oscillations in the intensity have been clearly observed. Each intensity oscillation corresponds to the growth of a single monolayer [10}12]. Fig. 3 shows the RHEED intensity oscillations during the growth of a La Ba MnO "lm on SrTiO . The angle of 2@3 1@3 3 3 incidence and the energy of the electron beam was 2.23 and 15 keV, respectively. Seven maxima with decreasing intensity as indicated by the arrows can be seen corresponding to the deposition of seven
Fig. 3. RHEED oscillations observed during the growth of La Ba MnO on SrTiO . 2@3 1@3 3 3
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monolayers of La Ba MnO . Note that the 2@3 1@3 3 spikes superimposed on the RHEED oscillations are caused by the laser pulses. About 10 pulses are required for the completion of a single monolayer. The occurrence of RHEED oscillations during the growth of a thin "lm is generally attributed to layer-by-layer growth. In this growth mode, also called Frank}van der Merwe growth, a single layer is completed before the following layer starts to grow. The surface morphology of the growing crystal changes from smooth over rough to smooth because of nucleation and coalescence of predominantly two-dimensional growth islands. For a completed layer the surface is smooth again because of the absence of steps. Thus, the specular beam intensity will be maximum. After the deposition of half a monolayer the areal density of surface steps or growth islands will be maximum thereby enhancing di!usive scattering at the step edges. As a result, the detected intensity becomes minimum. We note, however, that in spite of the comprehensibility of this step density model a more detailed description of RHEED oscillations and even the applicability of the step density model itself is still under discussion [13,14]. 3.1. Complementary xlm and surface characterization by X-ray and AFM Counting the number of RHEED oscillations during growth immediately allows the precise determination of the "lm thickness d. For example, for a La Ca MnO "lm grown on SrTiO 2@3 1@3 3 3 using 1600 laser pulses according to the number of RHEED oscillations we expect d"66.3 nm. Another method for the determination of the "lm thickness is X-ray di!raction at grazing angles of incidence. In Fig. 4 the di!raction pattern of the La Ca MnO "lm is shown. Here, by using the 2@3 1@3 3 Bragg condition with d given by the "lm thickness, we determine d"66.3$0.4 nm. The exact agreement of both methods in this case is of course coincidence. Usually, the disagreement between the two methods is within the scattering of the data. We note that the observation of a large number of re#ected X-ray intensity maxima and the weak decay with increasing angle of incidence give clear evidence for a well-de"ned interface between
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J. Klein et al. / Journal of Magnetism and Magnetic Materials 211 (2000) 9}15
layers is shown. The root mean square roughness of the surface is about 0.4 nm con"rming the results of the RHEED and X-ray analysis. Note that the grazing incidence X-ray analysis and the AFM study give complementary information. Whereas AFM gives spatially resolved information on a small, freely selectable part of the surface (typically 1 lm2), the X-ray analysis provides a spatially averaged value of a several 100 lm2 large area. 3.2. Correlation between RHEED pattern and interface properties Fig. 4. X-ray di!raction pattern of a 66 nm thick LBMO "lm obtained at grazing incidence.
Fig. 5. AFM image of the surface of a 20 nm thick LCMO/LBMO/LCMO/LBMO heterostructure.
substrate and "lm as well as a very low roughness of the "lm [15]. A well-established technique for measuring the surface roughness is atomic force microscopy (AFM). In our deposition system, the sample can be transferred from the deposition chamber to an UHV-AFM system without breaking the vacuum. This allows the investigation of interface morphology in heterostructures by interrupting the "lm growth between two subsequent layers for AFM analysis. In Fig. 5 the surface topology of a 20 nm thick heterostructure consisting of 4 manganite
In this subsection, we want to demonstrate the power of RHEED for the analysis of the interface properties in heteroepitaxial thin "lms. In Fig. 6 a comparison of the RHEED patterns and AFM images obtained for a La Ba MnO "lms with 2@3 1@3 3 di!erent surface/interface properties is shown. In the upper row the RHEED pattern of a smooth, 8 ML thick "lm is shown. The surface roughness is well below 1 nm as demonstrated by the AFM image (upper right). A single scan along the marked line in the AFM surface topology image is shown below the AFM image. The corresponding RHEED pattern shows sharp spots on the zeroth Laue circle and also Kikuchi lines are visible. In the middle row, a 54 ML thick "lm with a surface roughness above 1 nm as determined from the AFM image is shown. The corresponding RHEED pattern looks markedly di!erent from the pattern of the smooth surface. Here, the spots are still present but are surrounded by a streaky di!use part. It has been shown that such a pattern is indicative for island growth where the FWHM of the di!use intensity roughly is proportional to 1/¸ with L being the average size of the growth islands [12,16]. A third kind of RHEED pattern is shown in the bottom row of Fig. 6. Here, a regular point lattice is observed which results from transmitted electron interference and is called transmission pattern. This, of course, indicates the presence of large three-dimensional growth islands. From the corresponding AFM images (only the lateral force image is shown in Fig. 6) an average island diameter of about 50}100 nm and an average island height of about 10 nm is obtained. We note that the RHEED patterns shown in Fig. 6 are taken under UHV
J. Klein et al. / Journal of Magnetism and Magnetic Materials 211 (2000) 9}15
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Fig. 6. Comparison of RHEED patterns (left column) and AFM images (right column) of smooth (top row) and rough (middle and bottom row) LBMO surfaces. Further explanation in the text.
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J. Klein et al. / Journal of Magnetism and Magnetic Materials 211 (2000) 9}15
Fig. 7. RHEED oscillations observed during the growth of La Ca MnO on a 60 nm thick epitaxial La Ba MnO 2@3 1@3 3 2@3 1@3 3 "lm.
Fig. 8. Detailed view of a specular beam oscillation recorded during the growth of LBMO on STO. The triangles mark the instant of the laser pulse.
conditions for better image quality. Of course, these images can also be obtained in a high-pressure oxygen environment. Thus, RHEED provides useful information about the interfaces properties during epitaxial growth.
La Ca MnO (0 0 2) peak was 0.0183. Note 2@3 1@3 3 that the mosaic spread of the "lm in this case already is limited by the quality of the SrTiO 3 substrate [17,18]. Due to the almost perfect lattice match between La Ba MnO and SrTiO , the 2@3 1@3 3 3 La Ba MnO peaks are completely super2@3 1@3 3 imposed by the substrate peaks. However, it is evident that the La Ba MnO layer serving as 2@3 1@3 3 the substrate for the subsequent La Ca MnO 2@3 1@3 3 layer is expected to have a similarly small mosaic spread as the La Ca MnO layer. 2@3 1@3 3
3.3. RHEED study of manganite heterostructures For the fabrication of multilayer devices the control of interfaces between subsequently grown layers of di!erent materials is crucial. For example, in tunnel junctions using the CMR materials (e.g. L a C a M n O -S r T i O - L A C a M n O 2@3 1@3 3 3 2@3 1@3 3 junctions) the growth properties and precise thickness of the insulator, as well as the interfaces between the subsequent layers have to be known and optimized. In Fig. 7 the RHEED intensity oscillations observed for the growth of La Ca MnO 2@3 1@3 3 on a 60 nm thick La Ba MnO "lm are shown. 2@3 1@3 3 In order to be able to measure a clear RHEED intensity modulation, the surface of the completed La Ba MnO layer was annealed for about 2@3 1@3 3 20 min in the deposition system allowing the material to form a smooth surface layer. Also, the beam intensity was increased by a factor of three compared to the oscillation shown in Fig. 3. Sixteen oscillations corresponding to the deposition of 16 monolayers can be clearly distinguished in Fig. 7. The completed "lm structure also was studied by X-ray di!raction. The h}2h-scan showed only (0 0 l ) peaks and the FWHM of the rocking curve of the
3.4. Detailed study of the RHEED signal In this subsection we show how the RHEED analysis can be used to reveal more detailed information on the growth process. As an example we choose the growth of the "rst monolayer of a La Ba MnO "lm on SrTiO as shown in 2@3 1@3 3 3 Fig. 8. We "rst consider the behavior of the intensity measured between the start of the deposition process and the "rst minimum. Evidently, after each laser pulse the intensity sharply drops due to the amount of material deposited on the surface leading to di!use scattering. Then, the deposited atoms start to rearrange on the surface forming mostly two-dimensional islands. This causes a reduction of the number of steps, and, thus, an increase of RHEED intensity until the next laser pulse. An exponential approximation to an equilibrium value of the beam intensity is observed, when
J. Klein et al. / Journal of Magnetism and Magnetic Materials 211 (2000) 9}15
the time between two laser pulses is su$ciently long. This indicates a thermodynamic surface relaxation process from which the surface relaxation time constants can be obtained. This issue is not further discussed here. Now, we consider the intensity measured between the "rst minimum and the following maximum. Here, immediately after the laser pulse the intensity increases without any initial drop. Within the step edge model, this indicates that the deposited material moves right into the holes between growth islands in the half completed monolayer thereby enhancing the surface smoothness and the specular beam intensity. The described behavior is periodically repeated. In order to further prove the validity of our interpretation within the step density model, a clear correlation to AFM surface morphology images has still to be provided.
4. Conclusions We have developed a high pressure RHEED system suitable for the typical deposition conditions used for the fabrication of oxide materials. Intensity oscillations due to the layer-by-layer growth of epitaxial thin "lms and heterostructures have been observed for the manganites. By independent complementary measurement methods such as X-ray di!raction at grazing incidence and in situ AFM surface morphology studies consistent information of the high quality of the epitaxy was obtained. In particular, the surface roughness of "lms with a typical "lm thickness of up to 100 nm was low enough to allow the observation of growth oscillations in subsequently deposited layers. The mosaic spread of the manganite "lms was found to be limited by the substrate quality. In summary, we have shown that high-pressure RHEED supplemented by other techniques such as X-ray di!raction and in situ AFM analysis represents a power-
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ful tool in the advanced processing of complex oxide materials similar to the wide spread use of UHV-RHEED in semiconducting device fabrication. References [1] G.J.H.M. Rijnders, G. Koster, D.H.A. Blank, H. Rogalla, Appl. Phys. Lett. 70 (1997) 1888. [2] R. Gross, L. Al!, A. Beck, O.M. FroK hlich, D. KoK lle, A. Marx, IEEE Trans. Appl. Supercond. 7 (1997) 2929. [3] J.Z. Sun, W.J. Gallagher, P.R. Duncombe, L. KrusinElbaum, R.A. Altman, A. Gupta, Yu Lu, G.Q. Gong, Gang Xiao, Appl. Phys. Lett. 69 (1996) 3266. [4] T. Obata, T. Manako, Y. Shimakawa, Y. Kubo, Appl. Phys. Lett. 74 (1999) 290. [5] M.G. Lagally, D.E. Savage, MRS Bull. XVIII (1) (1993) 24. [6] I. Bozovic, J.N. Eckstein, MRS Bull. XX (1995) 32. [7] W. Braun, Applied RHEED, in: Springer Tracts in Modern Physics, Vol. 154, Springer, Berlin, 1999. [8] T. Terashima, Y. Bando, K. Iijima, K. Yamamoto, K. Hirata, K. Hayashi, K. Kamigaki, H. Terauchi, Phys. Rev. Lett. 65 (1990) 2684. [9] H. Karl, B. Stritzker, Phys. Rev. Lett. 69 (1992) 2939. [10] J.J. Harris, B.A. Joyce, P.J. Dobson, Surf. Sci. 103 (1981) L90. [11] J.H. Neave, B.A. Joyce, P.J. Dobson, N. Norton, Appl. Phys. A 31 (1983) 1. [12] M.G. Lagally, D.E. Savage, M.C. Tringides, in: P.K. Larsen, P.J. Dobson (Eds.), Re#ection High Energy Electron Di!raction and Re#ection Electron Imaging of Surfaces, Plenum, London, 1989, pp. 139}174. [13] U. Korte, P.A. Maksym, Phys. Rev. Lett. 78 (1997) 2381. [14] Z. Mitura, S.L. Dudarev, M.J. Whelan, Surf. Rev. Lett. 5 (1998) 701. [15] S.K. Sinha, Jour. de Physique. III 4 (1994) 1543. [16] M.G. Lagally, W. Moritz, in: P.K. Larsen, P.J. Dobson (Eds.), Re#ection High Energy Electron Di!raction and Re#ection Electron Imaging of Surfaces, Plenum, London, 1989, pp. 175}191. [17] B. Wiedenhorst, C. HoK fener, Yafeng Lu, J. Klein, L. Al!, R. Gross, B.H. Freitag, W. Mader, Appl. Phys. Lett. 74 (24) (1999) 3636. [18] B. Wiedenhorst, C. HoK fener, Yafeng Lu, J. Klein, B.H. Freitag, W. Mader, L. Al!, R. Gross, J. Magn. Magn. Mater. this volume.
Laser ablation of manganite thin "lms monitored by in situ RHEED J. Klein, C. HoK fener, L. Al!*, R. Gross II. Physikalisches Institut, Universita( t zu Ko( ln, Zu( lpicher Str. 77 50937 Ko( ln, Germany
Abstract Re#ection high-energy electron di!raction (RHEED) during crystal growth has become a standard analytical tool in the fabrication of semiconducting materials by molecular-beam epitaxy in an ultrahigh vacuum environment. For the fabrication of high-quality epitaxial and heteroepitaxial thin "lm structures of oxide materials such as perovskite manganites or high-temperature superconductors, it is also highly desirable to use RHEED technique during epitaxial growth. However, the complex oxide materials are commonly fabricated by pulsed laser deposition (PLD) at high oxygen pressure above 10 Pa preventing the use of standard RHEED systems. A two-stage di!erential pumping system can serve to circumvent this problem [1]. In this way, the electron path in the high-pressure oxygen atmosphere within the deposition chamber can be reduced considerably. Then the specular beam intensity of the RHEED system is su$ciently high to allow the observation of intensity oscillations during the growth of oxide materials. We have used high-pressure RHEED and in situ atomic force microscopy (AFM) for the investigation of the surface morphology of oxide thin "lms prepared by PLD. ( 2000 Elsevier Science B.V. All rights reserved. Keywords: Manganite thin "lms; RHEED; Pulsed laser deposition
1. Introduction Epitaxial thin "lms and heterostructures of complex oxide materials are required for a large number of devices showing interesting properties both with respect to applications and basic physics. Typical examples are Josephson junctions made from the cuprate high-temperature superconductors [2] or magnetic tunnel junctions based on doped manganites [3,4]. In order to control the epitaxial growth, the interface properties, and the
* Corresponding author. Tel.: #49-221-470-3700; fax: #49221-470-5178. E-mail address: [email protected] (L. Al!)
layer thickness in heterostructures, RHEED represents a powerful tool. In the thin "lm growth of metallic or semiconducting materials under ultrahigh vacuum (UHV) conditions RHEED is a wellestablished method (for overviews see Refs. [5}7]). However, the high oxygen pressure (a few 10 Pa) used during the growth of oxide materials prevents the use of standard RHEED systems due to the small mean free path of the beam electrons under these conditions. One way to overcome this problem is the reduction of oxygen pressure during "lm deposition. In order to achieve an oxidation potential corresponding to 30 Pa molecular oxygen, atomic oxygen or ozone instead of molecular oxygen has to be used [8]. The molecular oxygen pressure then can be reduced to below 1 Pa in
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a standard PLD process [9]. However, these modi"ed fabrication processes have in common the problem that the oxygen partial pressure is far from the usual values. Therefore, in our setup we use a two-stage di!erential pumping system similar to that proposed by Rijnders et al. [1]. In this article we show how the modi"ed RHEED system can be used for the in situ monitoring of the growth process of the colossal magnetoresistance (CMR) materials La D MnO with D"Ca and Ba. 1@3 2@3 3 2. RHEED system A schematic view of our high-pressure RHEED system is shown in Fig. 1. In the deposition chamber the oxygen pressure can be as large as 30 Pa. A double di!erential pumping system decreases the
Fig. 1. Schematic view of the high-pressure RHEED system.
pressure between the two coaxial tubes to below 0.1 Pa and at the "lament of the RHEED gun and inside the inner tube to below 10~3 Pa. Along their path from the "lament to the substrate the beam electrons have to pass two apertures at the front end of the tubes with a diameter of 500 lm. In the high vacuum regime (p(10~3 Pa) between the RHEED "lament and the "rst aperture the scattering probability for the beam electrons is vanishingly small. However, in the high-pressure regime inside the deposition chamber, the small mean free path of the beam electrons results in a decrease of the specular beam intensity. The mean free path l of the electrons is given by 1 k ¹ l" " B . p 2n p 2p O O Here, n is the oxygen molecular number density and l depends on p and the energy-dependent scattering cross-section p 2 of oxygen. O With p"10 Pa and p +10~20 m2 the O mean free path is estimated to be2 5 cm. In our setup the actual path length d of the beam electrons in )1 the high-pressure regime is about 20 cm. A further reduction of the path length was not required in order to have enough re#ected beam intensity for the monitoring of RHEED oscillations. This is shown in Fig. 2, where image (a) shows a RHEED pattern of a 3.2 nm thick La Ba MnO "lm 1@3 2@3 3 under UHV conditions. For comparison, image (b) shows the RHEED pattern from the same surface but at an oxygen pressure of 6.67 Pa
Fig. 2. RHEED patterns recorded in (a) UHV environment and (b) at 6.67 Pa (50 m Torr) O . 2
J. Klein et al. / Journal of Magnetism and Magnetic Materials 211 (2000) 9}15
(50 m Torr) and a 15 times longer exposure time. This is in agreement with the expected exponential decay of the electron beam intensity, I, during its path through the high-pressure deposition chamber following IJe~d)1 @l. Instead of increasing the exposure time, in practice, an increase in electron beam intensity is used to obtain a su$cient specular beam intensity on the RHEED screen.
3. RHEED oscillations during the growth of oxide 5lms The oxide thin "lms (La Ba MnO :LBMO, 2@3 1@3 3 La Ca MnO :LCMO) were fabricated on 2@3 1@3 3 SrTiO substrates by PLD. The substrate temper3 ature was 7603C and the "lms were deposited at an oxygen pressure of 26.66 Pa (200 m Torr) using a 248 nm KrF-eximer laser with a pulse energy of 450 mJ resulting in an energy density of 2 J/cm2. During the growth of the "lms the intensity of the specular beam was monitored. Oscillations in the intensity have been clearly observed. Each intensity oscillation corresponds to the growth of a single monolayer [10}12]. Fig. 3 shows the RHEED intensity oscillations during the growth of a La Ba MnO "lm on SrTiO . The angle of 2@3 1@3 3 3 incidence and the energy of the electron beam was 2.23 and 15 keV, respectively. Seven maxima with decreasing intensity as indicated by the arrows can be seen corresponding to the deposition of seven
Fig. 3. RHEED oscillations observed during the growth of La Ba MnO on SrTiO . 2@3 1@3 3 3
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monolayers of La Ba MnO . Note that the 2@3 1@3 3 spikes superimposed on the RHEED oscillations are caused by the laser pulses. About 10 pulses are required for the completion of a single monolayer. The occurrence of RHEED oscillations during the growth of a thin "lm is generally attributed to layer-by-layer growth. In this growth mode, also called Frank}van der Merwe growth, a single layer is completed before the following layer starts to grow. The surface morphology of the growing crystal changes from smooth over rough to smooth because of nucleation and coalescence of predominantly two-dimensional growth islands. For a completed layer the surface is smooth again because of the absence of steps. Thus, the specular beam intensity will be maximum. After the deposition of half a monolayer the areal density of surface steps or growth islands will be maximum thereby enhancing di!usive scattering at the step edges. As a result, the detected intensity becomes minimum. We note, however, that in spite of the comprehensibility of this step density model a more detailed description of RHEED oscillations and even the applicability of the step density model itself is still under discussion [13,14]. 3.1. Complementary xlm and surface characterization by X-ray and AFM Counting the number of RHEED oscillations during growth immediately allows the precise determination of the "lm thickness d. For example, for a La Ca MnO "lm grown on SrTiO 2@3 1@3 3 3 using 1600 laser pulses according to the number of RHEED oscillations we expect d"66.3 nm. Another method for the determination of the "lm thickness is X-ray di!raction at grazing angles of incidence. In Fig. 4 the di!raction pattern of the La Ca MnO "lm is shown. Here, by using the 2@3 1@3 3 Bragg condition with d given by the "lm thickness, we determine d"66.3$0.4 nm. The exact agreement of both methods in this case is of course coincidence. Usually, the disagreement between the two methods is within the scattering of the data. We note that the observation of a large number of re#ected X-ray intensity maxima and the weak decay with increasing angle of incidence give clear evidence for a well-de"ned interface between
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layers is shown. The root mean square roughness of the surface is about 0.4 nm con"rming the results of the RHEED and X-ray analysis. Note that the grazing incidence X-ray analysis and the AFM study give complementary information. Whereas AFM gives spatially resolved information on a small, freely selectable part of the surface (typically 1 lm2), the X-ray analysis provides a spatially averaged value of a several 100 lm2 large area. 3.2. Correlation between RHEED pattern and interface properties Fig. 4. X-ray di!raction pattern of a 66 nm thick LBMO "lm obtained at grazing incidence.
Fig. 5. AFM image of the surface of a 20 nm thick LCMO/LBMO/LCMO/LBMO heterostructure.
substrate and "lm as well as a very low roughness of the "lm [15]. A well-established technique for measuring the surface roughness is atomic force microscopy (AFM). In our deposition system, the sample can be transferred from the deposition chamber to an UHV-AFM system without breaking the vacuum. This allows the investigation of interface morphology in heterostructures by interrupting the "lm growth between two subsequent layers for AFM analysis. In Fig. 5 the surface topology of a 20 nm thick heterostructure consisting of 4 manganite
In this subsection, we want to demonstrate the power of RHEED for the analysis of the interface properties in heteroepitaxial thin "lms. In Fig. 6 a comparison of the RHEED patterns and AFM images obtained for a La Ba MnO "lms with 2@3 1@3 3 di!erent surface/interface properties is shown. In the upper row the RHEED pattern of a smooth, 8 ML thick "lm is shown. The surface roughness is well below 1 nm as demonstrated by the AFM image (upper right). A single scan along the marked line in the AFM surface topology image is shown below the AFM image. The corresponding RHEED pattern shows sharp spots on the zeroth Laue circle and also Kikuchi lines are visible. In the middle row, a 54 ML thick "lm with a surface roughness above 1 nm as determined from the AFM image is shown. The corresponding RHEED pattern looks markedly di!erent from the pattern of the smooth surface. Here, the spots are still present but are surrounded by a streaky di!use part. It has been shown that such a pattern is indicative for island growth where the FWHM of the di!use intensity roughly is proportional to 1/¸ with L being the average size of the growth islands [12,16]. A third kind of RHEED pattern is shown in the bottom row of Fig. 6. Here, a regular point lattice is observed which results from transmitted electron interference and is called transmission pattern. This, of course, indicates the presence of large three-dimensional growth islands. From the corresponding AFM images (only the lateral force image is shown in Fig. 6) an average island diameter of about 50}100 nm and an average island height of about 10 nm is obtained. We note that the RHEED patterns shown in Fig. 6 are taken under UHV
J. Klein et al. / Journal of Magnetism and Magnetic Materials 211 (2000) 9}15
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Fig. 6. Comparison of RHEED patterns (left column) and AFM images (right column) of smooth (top row) and rough (middle and bottom row) LBMO surfaces. Further explanation in the text.
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Fig. 7. RHEED oscillations observed during the growth of La Ca MnO on a 60 nm thick epitaxial La Ba MnO 2@3 1@3 3 2@3 1@3 3 "lm.
Fig. 8. Detailed view of a specular beam oscillation recorded during the growth of LBMO on STO. The triangles mark the instant of the laser pulse.
conditions for better image quality. Of course, these images can also be obtained in a high-pressure oxygen environment. Thus, RHEED provides useful information about the interfaces properties during epitaxial growth.
La Ca MnO (0 0 2) peak was 0.0183. Note 2@3 1@3 3 that the mosaic spread of the "lm in this case already is limited by the quality of the SrTiO 3 substrate [17,18]. Due to the almost perfect lattice match between La Ba MnO and SrTiO , the 2@3 1@3 3 3 La Ba MnO peaks are completely super2@3 1@3 3 imposed by the substrate peaks. However, it is evident that the La Ba MnO layer serving as 2@3 1@3 3 the substrate for the subsequent La Ca MnO 2@3 1@3 3 layer is expected to have a similarly small mosaic spread as the La Ca MnO layer. 2@3 1@3 3
3.3. RHEED study of manganite heterostructures For the fabrication of multilayer devices the control of interfaces between subsequently grown layers of di!erent materials is crucial. For example, in tunnel junctions using the CMR materials (e.g. L a C a M n O -S r T i O - L A C a M n O 2@3 1@3 3 3 2@3 1@3 3 junctions) the growth properties and precise thickness of the insulator, as well as the interfaces between the subsequent layers have to be known and optimized. In Fig. 7 the RHEED intensity oscillations observed for the growth of La Ca MnO 2@3 1@3 3 on a 60 nm thick La Ba MnO "lm are shown. 2@3 1@3 3 In order to be able to measure a clear RHEED intensity modulation, the surface of the completed La Ba MnO layer was annealed for about 2@3 1@3 3 20 min in the deposition system allowing the material to form a smooth surface layer. Also, the beam intensity was increased by a factor of three compared to the oscillation shown in Fig. 3. Sixteen oscillations corresponding to the deposition of 16 monolayers can be clearly distinguished in Fig. 7. The completed "lm structure also was studied by X-ray di!raction. The h}2h-scan showed only (0 0 l ) peaks and the FWHM of the rocking curve of the
3.4. Detailed study of the RHEED signal In this subsection we show how the RHEED analysis can be used to reveal more detailed information on the growth process. As an example we choose the growth of the "rst monolayer of a La Ba MnO "lm on SrTiO as shown in 2@3 1@3 3 3 Fig. 8. We "rst consider the behavior of the intensity measured between the start of the deposition process and the "rst minimum. Evidently, after each laser pulse the intensity sharply drops due to the amount of material deposited on the surface leading to di!use scattering. Then, the deposited atoms start to rearrange on the surface forming mostly two-dimensional islands. This causes a reduction of the number of steps, and, thus, an increase of RHEED intensity until the next laser pulse. An exponential approximation to an equilibrium value of the beam intensity is observed, when
J. Klein et al. / Journal of Magnetism and Magnetic Materials 211 (2000) 9}15
the time between two laser pulses is su$ciently long. This indicates a thermodynamic surface relaxation process from which the surface relaxation time constants can be obtained. This issue is not further discussed here. Now, we consider the intensity measured between the "rst minimum and the following maximum. Here, immediately after the laser pulse the intensity increases without any initial drop. Within the step edge model, this indicates that the deposited material moves right into the holes between growth islands in the half completed monolayer thereby enhancing the surface smoothness and the specular beam intensity. The described behavior is periodically repeated. In order to further prove the validity of our interpretation within the step density model, a clear correlation to AFM surface morphology images has still to be provided.
4. Conclusions We have developed a high pressure RHEED system suitable for the typical deposition conditions used for the fabrication of oxide materials. Intensity oscillations due to the layer-by-layer growth of epitaxial thin "lms and heterostructures have been observed for the manganites. By independent complementary measurement methods such as X-ray di!raction at grazing incidence and in situ AFM surface morphology studies consistent information of the high quality of the epitaxy was obtained. In particular, the surface roughness of "lms with a typical "lm thickness of up to 100 nm was low enough to allow the observation of growth oscillations in subsequently deposited layers. The mosaic spread of the manganite "lms was found to be limited by the substrate quality. In summary, we have shown that high-pressure RHEED supplemented by other techniques such as X-ray di!raction and in situ AFM analysis represents a power-
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