The epitaxial relationships of lattices and crystalline qualities of LSMO/ZnO/sapphire double-hetero systems were thoroughly analyzed using X-ray diffraction techniques with a modern high resolution XRD system. It was revealed that the epitaxial growth of the LSMO (110) phase was promoted under higher temperature accompanying the suppression of other competitive growth of the LSMO (001) and (111) phases. Supplying the plasma oxygen accelerated the suppression of the LSMO (111) phase. The complex epitaxial orientational relationships in these three growth modes were revealed from the precise analyses with the high resolution out-of-plane XRD and the in-plane XRD measurements, pole figure measurements, and reciprocal space mappings measurements, as [112ˉ](111)LSMO//[11ˉ00](0001)ZnO//[112ˉ0](0001)Sap, [110ˉ](111LSMO)//[11ˉ00](0001)Zno//[112ˉ0](0001)Sap, and [110](001)LSMO//[11ˉ00](0001)Zno//[112ˉ0](0001)Sap. The validity of this model for the epitaxial orientational relationships in LSMO/ZnO/sapphire double heteroepitaxial layers was confirmed identically with the data ana- lyses of the out-of-plane wide-range Reciprocal Space Mapping using the 2-dimensional X-ray detector.
Various combinations of oxide thin films have been studied for fabrications of new functional devices. Perovskite (La,Sr)MnO3 (LSMO) manganate shows versatile electrical and magnetic properties such as colossal magnetoresistance [
ZnO system whose characteristics can be modified by magnetic field, temperature and UV light [
Bhosle and Narayan [
. In this article, we define (110) and (0001) as the surface planes which show the out-of-plane crystalline orientations of the grown thin film, and and as the crystalline axes parallel to the surface planes which show the inplane crystalline orientations of the films, respectively, hereafter. The lattice matching between the LSMO film and the sapphire substrate for the case of Bhosle and Narayan is expected to be good since = 0.13753 nm (= 0.3890 nm) is very close to = 0.13746 nm (= 0.4762 nm). However, the lattice mismatch in the perpendicular direction within the surface plane is as high as 19%. They reported that this relatively large mismatch was accommodated by matching of four (200) planes of the LSMO films with five planes of sapphire.
In our case, the lattice mismatch should be considered between LSMO films and ZnO underlayers. The interplanar distances of major lattice planes perpendicular to the surface plane are = 0.27506 nm and = 0.19450 nm for the LSMO films, and = 0.16250 nm and = 0.28146 nm (= 0.3250 nm) for the ZnO underlayer. A small lattice mismatch of −2.2% is expected by the coincidence of and, and also a lattice mismatch of 20% is expected by the coincidence of and. Thus, the epitaxial growth of the LSMO film on the ZnO underlayer with the epitaxial orientation of is expected. A relatively large mismatch along LSMO [
In this study, we try to extensively analyze lattice relationships of epitaxial growth and crystalline qualities in the LSMO/ZnO/sapphire double-hetero system. We use here X-ray Diffraction (XRD) technique with the modern high resolution XRD systems, such as out-of-plane 2θ-θ XRD scans, ω rocking curves, in-plane XRD scans, f rocking curves, pole figure measurements, and reciprocal space mappings.
The thin films of (La,Sr)MnO3 (LSMO) overlayer were epitaxially grown on the ZnO underlayer on the (0001) sapphire (Sap) substrate by ion beam sputtering technique. Molecular oxygen (ML) or plasma oxygen (PL) was supplied during the growth. Deposition conditions for the LSMO overlayer were varied as a function of the substrate temperature (Ts: 650˚C - 750˚C) and the oxygen partial pressure (Po). Details of the film fabrication can be referred in the previous papers [1,4].
Four LSMO/ZnO/Sap double-hetero epitaxial films were mainly studied in this article. Two of them are termed as PL-LT and PL-HT in which the LSMO films were grown at the low Ts (650˚C) and high Ts (750˚C) with the supply of plasma oxygen, respectively. The other two are termed as ML-LT and ML-HT in which the LSMO films were grown at the low Ts (650˚C) and high Ts (700˚C) with the supply of molecular oxygen, respectively. Many other double-layer films grown at different deposition conditions were also characterized to give deliberate and correct discussions, while the experimental results are not shown for these samples in this paper.
All the XRD measurements were performed with the “SmartLab” diffractometer system (Rigaku Corp.) [
For the high resolution XRD analysis with double axis geometry, a Ge220-2bounce channelcut monochromating collimator was employed in the incident optics to use the CuKα1 radiation (λ = 0.154059 nm) [6,7]. A resolution of optics for XRC was confirmed to be better than 0.004 deg by the rocking curve measurement for the sapphire 0006 reflection. The 2θ-θ scans were performed to detect diffraction signals arising from the lattice planes parallel to the sample surface plane as shown in
The in-plane XRD is a powerful technique for thin film characterizations, since this technique is geometrically sensitive to film surface layers. Further we can directly access to the lattice planes perpendicular to the film surface which is illustrated in
Regarding the surface sensitivity, the divergence of the incident X-ray beam in the normal direction to the sample surface greatly affects the sensitivity, which is indicated by vertical arrows in
A resolution of the in-plane XRD profiles should depend on the optical elements employed, such as the Parallel Slit Collimator (PSC) in the incident side, and the Parallel Slit Analyzer (PSA) in the receiving side (
The pole figure (PF) measurement is an indispensable technique for the characterization of orientational relationships in the complex heteroepitaxial thin film systems. A sample is generally manipulated with tilting and rotated with f-axis while keeping the 2θ-θ configuration. This geometry is called as a skew geometry, where the angles of the incident X-rays and exit X-rays to the sample surfaces should be kept identical. The χ-axis is commonly used for tilting a sample in usual PF measurements in a case of conventional XRD systems. However, in the present PF measurement using the in-plane axis, the lattice planes inclined to the surface plane can be accessed by the tilting motion through the combining motions of 2θ and 2θχ, without tilting the sample. As can be easily seen from the schematic illustration of the goniometer
motion in
The RSM measurements were complementarily done to cover the PF measurements for the characterization of orientational relationships in the complex heteroepitaxial thin film systems. The RSMs of wide angular ranges can be measured using the 2D detector and point-shaped incident X-ray beam as shown in
Both of the RSM and the PF measurements should access to the lattice plane inclined with ψ with respect to the sample surface as illustrated in
ometry (
The wide range RSM data can be obtained by iterative motions of the goniometer for small χ steps and 2θ/ω scans. The 2D data should be shown either in the goniometer coordinates (the two coordinate axes are tilting angles (χ) and 2θ angles) or in the reciprocal space coordinates. The two orthogonal axes are employed in a scale of the reciprocal unit (1/Å is used in this study), those are the direction along the surface plane and the direction normal to the surface plane.
It is quite clear that the (110) phase growth is much promoted at the higher temperature than at the lower temperature, contrary the (001) phase growth is promoted at the lower temperatures than at the higher temperatures. This must be resulted from a different lattice matching between the LSMO overlayer and ZnO underlayer. Probably the LSMO (110) phase has the better lattice matching at the higher temperatures, while the LSMO (001) phase has the better lattice matching at the lower temperature due to different thermal expansion rates of these lattices. It was partially reported by our previous paper [
LSMO (111) phase can be grown faintly under the molecular supply, while it cannot be grown at all under the plasma supply. We can recognize an outstanding rule of the plasma effect for the three phases. The growth is always enhanced by the plasma when the temperature is low, on the contrary, the growth is always suppressed by the plasma when the temperature is high. This clearly indicates that the crystalline formation energy is supplemented by the plasma energy when the thermal energy is insufficient. The plasma energy, however, gives excess energy when the thermal energy is sufficient, leading to decomposition of LSMO crystal structure. This must be the reason why the LSMO (111) and (001) phases cannot be grown at the higher temperature of 750˚C under the plasma supply.
It should be noted that as shown in
The orientational distribution of the crystallite axis of the (110) phase can be analyzed from the tilt spreading in the ω rocking curve profiles for the LSMO 110 reflections shown in
(a) LSMO 110 reflections (b) LSMO 111 reflections (c) LSMO 002 reflections
(d) LSMO 220 reflections (e) LSMO 222 reflections
shortage of the thermal energy when the thermal energy is insufficient, leading to the better mosaic crystallinity.
An inset in
In-plane Reciprocal Space Mapping (RSM) data [11,16, 17] for the four samples are shown in Figures 9(a)-(d) with the reciprocal space coordinates in the unit of 1/Å. In these figures, the horizontal axis is aligned to axis of the sapphire substrate and also to axis of the ZnO underlayer, and the vertical axis is aligned to axis of the sapphire substrate and also to axis of the ZnO underlayer.
The 2θχ-f coupled scans, which should correspond to the 2θ-θ scans for the out-of-plane XRD, sweep the directions in the reciprocal space radially from the origin of reciprocal space. That is, we sweep by the iterative motions of incremental step of Δf and 2θχ-f coupled scans. (the 2θχ scan under various fixed f with infinitesimally small interval). The 2θχ corresponds to the distance between the vertical crystalline plane, and the f corresponds to the in-plane direction of this crystalline plane. An example of the traces of 2θχ-f coupled scans is demonstrated by a red broken arrow in
Here we mention first on the mosaic spread of twisting, that can be evaluated by the in-plane rocking curve (f-scan) profiles in this measurement system. The in-plane rocking curve profiles traced from these 2D figures in Figures 9(a)-(d) for the 110 reflections of LSMO are shown in
In contrast to the wide spread of FWHM observed in the samples of PL-LT and ML-LT, the FWHM values in the samples of PL-HT and ML-HT grown at the higher temperatures are rather small as around 2.0 deg, and it is close to the value for reflections of the ZnO underlayer (not shown here). It is supposed from these facts that the twisting of LSMO film follows that of ZnO underlayer during the film growth of LSMO at the higher temperatures. However, such constraint of the LSMO lattice from the ZnO lattice is not so strong at the lower temperature. Due to this relaxation, the LSMO films grown at the lower temperature have the larger spread of twisting. This gives a very important knowledge concerning the in-plane orientations in the epitaxial growth.
The lattice constant of LSMO along the in-plane direction can be determined as a = 0.389 - 0.390 nm from the 110 reflections. These values are almost the same with
(a) Sample PL-LT (b) Sample PL-HT
(c) Sample ML-LT (d) Sample ML-HT those estimated by the out-of-plane 2θ-θ measurement mentioned above. We would like to mention here, the crystallinity (i.e., twist) of the LSMO overlayer can be improved by inserting the ZnO underlayer with the smaller lattice mismatch between the LSMO overlayer and the sapphire substrate with the larger lattice mismatch, compared with the case of direct growth of LSMO on the sapphire substrate [
indicated in Figures 9(a)-(d) appearing on each equidistant circumference. However, it should be noted that each grain does not always have all these three vertical planes, for example, the (001) grain does not have the vertical (111) plane. The LSMO has the cubic symmetry and the (001) phase should show four-fold symmetry within the surface plane, basically. However, as mentioned later, the LSMO has three equivalent alignments on the (0001) ZnO with the hexagonal symmetry showing six-fold symmetry. Then the LSMO does not always show the exact four-fold symmetry, but shows apparent six-fold like symmetry.
The 111 reflection is only related to the (110) grain, then it is efficient to analyze the in-plane orientational relationships using the 111 reflection first. The azimuthal angles β corresponding to the diffraction spots from the cubic LSMO, should appear at Bhosle and Narayan [
. The in-plane relation of the lattice orientation of our
is the same with that of their [1-10](110)LSMO//[1-100](0001)Sap in the system of rectangular on hexagonal lattices. However, the relation between LSMO and sapphire substrate is different. Then it is important that we can control the orientation of LSMO against the sapphire substrate by
inserting the intermediate buffer layer.
For our visual comprehension, we show in
Faint spots marked by white broken circles in Figures 9(b) and (d) can be identified as the diffraction spots from ZnAl2O4 spinel phase. Dovienko et al., [
In order to further clarify the epitaxial orientations of the (111) phase of LSMO overlayer, we performed the pole figure (PF) measurements for the LSMO 111 reflections. We should remind here that the (111) phase does not have the vertical (111) plane in its grain, then we cannot adopt the in-plane measurement. That is a reason why we adopt the pole figure measurement to detect the 111 reflection from the (111) phase. The choice of 111 reflections is based on two conditions. One reason is that the orientational relationships can be easily deduced from their diffraction spot positions in the PF data with respect to the lattices of ZnO underlayer and sapphire lattices. The other reason is that there are no reflections in the vicinity of 2θ value for this reflection (around 40 deg). The only exception is the sapphire 0006 reflection whose 2θ value is 41.68 deg. But its position to be appeared in
the PF data is already known well (at the center position of the PF data), and this might not be an obstacle for the orientational analysis of the LSMO overlayer.
We focused on comparison of the PF measurements for the samples of ML-LT and ML-HT to analyze this problem. The experimental results of diffraction spots are shown in Figures 13(a) and (b) for the 111 reflections from the (111) phase of ML-LT and ML-HT, respectively. These 2D data are shown in polar coordinates [
The symbols used in these figures are the same as shown, for example, in
Lastly we tried to confirm the validity of the epitaxial orientational relationships mentioned above by an alternative measurement. That is, out-of-plane wide-range RSM measurements were performed using the 2D detector for the samples of ML-LT and ML-HT. The results are shown in Figures 14(a) and (b) for the ML-LT and
ML-HT, respectively. The horizontal axisqxcorresponds to the directions of axis of sapphire substrate and axis of the ZnO underlayer, and the vertical axis qzcorresponds to the directions of axis of sapphire and ZnO. The detected intensity is scaled in log scale.All the diffraction peaks in Figures 14(a) and (b) can be indexed by the above mentioned epitaxial orientational relationships as shown in the figures.The positions of all the peaks are in good coincidence with the peak positions determined above.Consequently this measurement provides the validity of the above measurements and analyses..
All the peaks in Figures 14(a) and (b) can be indexed with a present model of epitaxial orientational relationships, and the positions of all these peaks are in good coincidence with peak positions estimated from the model, proving the validity of this model.
The LSMO overlayer was grown on the (0001) ZnO underlayer on the (0001) sapphire substrate by the ion beam sputtering at the growth temperatures from 650˚C to 750˚C with the supply of molecular oxygen or plasma oxygen. The out-of-plane and in-plane epitaxial orientational relationships of the lattices and crystalline qualities were analyzed using various X-ray diffraction techniques for the LSMO/ZnO/Sap double-hetero epitaxial systems. The growth of out-of-plane (110) oriented phase of LSMO is promoted at the higher temperatures, accompanying the suppression of other competitive growth of the LSMO (001) and (111) phases. The growth of LSMO (111) phase is remarkably suppressed by supplying the plasma oxygen at the higher temperature due to the excess energy. The slight contraction of lattice constants along the growth direction was found in the samples grown at the higher temperature. The tilting of lattices of the LSMO (110) phase was evaluated by the peak breadth of ω rocking curves as around 2.0 deg. This value is almost the same for the all samples investigated. The twisting of LSMO lattices was evaluated by the in-plane f rocking curves, indicating the improvement of crystalline quality for the samples grown at the higher temperatures.
The complex epitaxial orientaional relationships of the three LSMO phases were clarified by the precise analyses using the high resolution measurements of the out-of-plane XRD, the in-plane XRD, the in-plane RSM, and PF. They are summarized as
(μ-mode).
Fiure 14. The Out-of-plane wide-range reciprocal space mappings obtained by using the 2D X-ray detector for the samples (a) ML-LT and (b) ML-HT. The horizontal axis corresponds to the directions of axis of the ZnO underlayer and axis of the sapphire substrate. The vertical axis corresponds to the directions of [
The validity of these analyses for the epitaxial orientations of LSMO/ZnO/Sap double heteroepitaxial layers were confirmed by the alternative analysis of the outof-plane wide-range reciprocal space mappings using the 2D X-ray detector.