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Kota Hasegawa, [Takao Shimizu](https://orcid.org/0000-0001-9508-7601), [Naoki Ohashi](https://orcid.org/0000-0002-4011-0031)

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[<i>In-plane</i> lattice orientation in aluminum scandium nitride epitaxial films deposited on Nb-doped SrTiO<sub>3</sub>(111) substrates via reactive magnetron sputtering](https://mdr.nims.go.jp/datasets/4ca9ea0b-6420-46cc-84d5-413fe71ee925)

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In-plane lattice orientation in aluminum scandium nitride epitaxial films deposited on Nb-doped SrTiO3(111) substrates via reactive magnetron sputteringFULL PAPERIn-plane lattice orientation in aluminum scandium nitrideepitaxial films deposited on Nb-doped SrTiO3(111) substratesvia reactive magnetron sputteringKota Hasegawa1,2, Takao Shimizu2,3,³ and Naoki Ohashi1,2,41Interdisciplinary Graduate School of Engineering Sciences, Kyushu University,6–1 Kasuga-koen, Kasuga, Fukuoka 816–8580, Japan2National Institute for Materials Science, 1–1 Namiki, Tsukuba, Ibaraki 305–0044, Japan3JST-PRESTO, Japan Science and Technology Agency, 4–1–8 Honcho, Kawaguchi, Saitama 332–0012, Japan4MDX Research Center for Element Strategy, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226–8503, JapanWurtzite-type aluminum scandium nitride [WZ-(Al1¹xScx)N] thin films were grown on 0.5wt% Nb-dopedSrTiO3(111) (Nb:STO) single crystal substrates using the radio frequency reactive magnetron sputtering methodwith Al and Sc targets. WZ-(Al1¹xScx)N thin films with 0 ¯ x ¯ 0.49 were epitaxially grown on Nb:STO sub-strates. Films with x ¯ 0.3 exhibited multi-domain in-plane orientation. The coexistence of two rotation domains,(Al,Sc)N[100]//Nb:STO½1�10� and (Al,Sc)N[100]//Nb:STO½11�2�, was observed. The abundance of these twodomains varied with x, and films with x > 0.3 were single-crystal-like single-domain films. Although the latticeparameters and domain structure intricately changed with x, the calculated unit cell volume was in accordancewith the Vegard’s law. These results indicate that the unit cell volume is determined by the chemical composition.©2023 The Ceramic Society of Japan. All rights reserved.Key-words : (Al,Sc)N, SrTiO3(111), Reactive magnetron sputtering, Epitaxial films[Received January 7, 2023; Accepted April 19, 2023]1. IntroductionWurtzite-type aluminum nitride, which possesses a widebandgap, high dielectric strength, high thermal conduc-tivity, and low acoustic and dielectric losses, has beenextensively developed for applications in optoelectronics,1)electric insulation substrates,2) and microelectromechani-cal system (MEMS) filters.3) Solid-solution compounds,such as (Al,Ga)N, have also attracted significant attentionin power electronics and ultraviolet lighting devicesbecause of their potential for bandgap engineering.4) Thisstudy focuses on wurtzite-type solid-solution systems ofaluminum scandium nitride [WZ-(Al1¹xScx)N]. This solid-solution system has significant potential for MEMS appli-cations because its electromechanical performance can beenhanced by substituting Al with Sc.5),6) WZ-(Al1¹xScx)Nis also known as the first ferroelectric wurtzite-type crys-tal; Fichtner et al. observed polarity switching in WZ-(Al1¹xScx)N in 2019.7) Before their discovery, WZ-typecompounds were regarded as pyroelectric materials but notferroelectric materials. The ferroelectric behavior of WZ-(Al1¹xScx)N is different from that of other conventionalferroelectric compounds, e.g., significantly high sponta-neous polarization (80­110¯C/cm2) and a high coercivefield (1.8­5MV/cm).The metal/dielectric/metal (MIM) structure is the mostfundamental structure used in dielectric and piezoelectricdevices. For instance, a Pt-based MIM, Pt/oxide/Pt, is awidely employed conventional structure for characterizingdielectric and piezoelectric materials. Note that the bottomelectrodes act as an electric contact to introduce the electricfield and electric current and as a structural template todeposit the insulator layer. As the crystallinity of the insu-lator layer depends on the interfacial structure between thebottom electrode and the dielectric layer, the structure andchemical composition of the bottom electrode layer arekey factors in the synthesis of high-quality insulator layersin MIM structures. Significant research has been conduct-ed on the effects of the bottom electrodes on the propertiesand structures of ferroelectrics, aiming to improve theferroelectricity of titanates for the development of MIMstructures.8)­10) The selection of the bottom electrode layerused in MIM structures involving WZ-(Al1¹xScx)N thin-film insulator layers is important. For instance, Nie et al.reported that MIM structures with highly c-axis-alignedWZ-(Al1¹xScx)N films, obtained using a platinum bottomelectrode, possessed a high breakdown field strength and³ Corresponding author: T. Shimizu; E-mail: SHIMIZU.Takao@nims.go.jp‡ Preface for this article: DOI https://doi.org/10.2109/jcersj2.131.P7-1Journal of the Ceramic Society of Japan 131 [7] 242-247 2023DOI https://doi.org/10.2109/jcersj2.23002 JCS-Japan©2023 The Ceramic Society of Japan242This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/),which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.https://doi.org/10.2109/jcersj2.131.P7-1https://doi.org/10.2109/jcersj2.131.P7-1https://doi.org/10.2109/jcersj2.23002https://creativecommons.org/licenses/by/4.0/low leakage current compared to those obtained using amolybdenum electrode.11)The selection of the bottom electrode is essential for thehigh performance of MIM structure devices. Here, the im-portance of the in-plane lattice parameters must be empha-sized. In our previous study, we investigated the effect ofthe lattice parameters of stressed AlN on its ferroelectricproperties.12) The ratio of c- and a-axis lengths (c/a) is anessential parameter for determining ferroelectric switch-ing behavior in WZ-(Al1¹xScx)N, based on the ferroelec-tric switching mechanism proposed by Moriwake et al.13)We hypothesized that both the out-of-plane (c-axis) andin-plane (a-axis) lattice parameters should be consideredwhen investigating ferroelectricity in WZ-(Al1¹xScx)N.Yazawa et al. reported that the coercive field of WZ-(Al1¹xScx)N varies with its c-axis length,14) attributing thevariation in the c-axis length to structural constraints atthe film/electrode interfaces. In order to study in-plane lat-tice parameters as well as electric properties of dielectriclayers, usage of highly conductive single crystal substrateappropriate to obtain epitaxial layer is the most conven-tional way. Hence, in this study, a heavily Nb-dopedSrTiO3 single crystal with a very low electrical resistivityof 0.001 (³·cm) was used to study the crystal structure ofWZ-(Al1¹xScx)N films.Several reports have been published on the epitaxialgrowth of WZ-AlN on a SrTiO3(111) single-crystal sub-strates using molecular beam epitaxy,15) pulsed laser depo-sition,16) and magnetron sputtering.17),18) Although SrTiO3has a cubic rather than a hexagonal lattice, three-foldsymmetry exists along the ©111ª axis, and the oxygensublattice is considered suitable for the epitaxial growth offilms with hexagonal symmetry. The epitaxial growth ofZnO, which has a WZ-type structure, was achieved usingthe (111) face of a cubic lattice.19) This finding motivatedus to study the epitaxial growth of WZ-(Al1¹xScx)N onNb:STO(111) single-crystal substrates. Previous studies onWZ-AlN on SrTiO3(111) revealed changes in the domainstructure with increasing deposition temperature. Ferro-electric properties, particularly switching dynamics, arestrongly affected by the domain structure. Therefore,investigating the in-plane epitaxial relationship and do-main structure of WZ-(Al1¹xScx)N on Nb:STO(111) single-crystal substrates is of considerable significance.20),21)In this study, we successfully deposited epitaxial WZ-(Al1¹xScx)N thin films with 0 ¯ x < 0.5, which are appro-priate for studying the relationships between the crystalstructure and chemical composition (x), on Nb:STO(111)single-crystal substrates using radio frequency (RF) reac-tive magnetron sputtering. We also discuss the in-planerotation domains of the WZ-(Al1¹xScx)N films and thein-plane lattice parameters.2. Experimental methods(Al1¹xScx)N thin films were deposited on Nb:STO(111)single-crystal substrates via RF reactive magnetron sput-tering. Commercially available single crystals with 0.5wt% Nb and an electron concentration above 2.0 © 1020cm¹3 at room temperature were used.22) The error of orien-tation was estimated as «0.1°. The substrate was heated to450 °C and rotated during the deposition. The chamberpressure was maintained at 0.7 Pa by introducing high-purity argon (99.999%) and nitrogen (99.999%) gases ata ratio of 1:1. The background pressure in the depositionchamber was maintained below 2.0 © 10¹6 Pa to avoid theunintentional oxidation of the films and targets. Solid-solution films were deposited via co-sputtering using anindependently controlled dual-cathode source of pure alu-minum (99.999%) and scandium (99.99%). The compo-sition x in (Al1¹xScx)N was controlled by changing the RFpower from 45 to 100W for the aluminum source andfrom 0 to 55W for the scandium source. Before deposi-tion, pre-sputtering was performed under deposition condi-tions for over 30min to stabilize the discharge and removesurface contamination on each target.The film thickness and composition were determinedvia X-ray fluorescence spectroscopy using a ZSX Primusspectrometer (Rigaku Co., Ltd., Tokyo, Japan). The crystalstructures of the (Al1¹xScx)N thin films were investigatedvia X-ray diffraction (XRD) using a SmartLab diffrac-tometer (Rigaku Co., Ltd., Tokyo, Japan) with Cu-K¡radiation. Phase identification and the determination of theout-of-plane lattice parameters were performed using con-ventional ª­2ª mode scans. Additionally, º-scan measure-ments using the (101) diffraction of WZ-(Al1¹xScx)N wereperformed to determine the epitaxial relationship betweenthe crystalline lattices of the films and substrates. Further-more, ½­2ª mode scans were performed to evaluate thed-spacing corresponding to the (101) diffraction of WZ-(Al1¹xScx)N and to deduce the in-plane lattice parameters.3. Results and discussionThe deposition conditions and the characteristics of theresulting films are summarized in Table 1. The film thick-ness and composition are shown as functions of the RFinput power during the deposition. The cation compositionx in the deposited films was highly dependent on the inputpower and was successfully controlled in the range of0 ¯ x ¯ 0.49. Reproducibility of the film composition wasconfirmed by repeating the experiment for several timesunder the same deposition conditions.Figure 1 shows the XRD ª­2ª patterns of the(Al1¹xScx)N samples. Only the diffraction peak assignableto the 00l diffraction of WZ-(Al1¹xScx)N was observedin the diffraction pattern with 111 diffraction peak fromTable 1. Sample specifications, RF power conditions, and thick-ness and composition of (Al1¹xScx)N thin filmsSampleRF powerfor Al (W)RF powerfor Sc (W)Thickness(nm)x in(Al1¹xScx)N(a) 100 0 150 0(b) 74 26 128 0.09(c) 70 30 128 0.18(d) 60 40 124 0.30(e) 53 47 111 0.37(f ) 45 55 109 0.49Journal of the Ceramic Society of Japan 131 [7] 242-247 2023 JCS-Japan243Nb:STO. The 002 peak of WZ-(Al1¹xScx)N is observed atapproximately 36.01° for x ¯ 0.37. For x = 0.49, the peakattributed to this diffraction was observed at a higher dif-fraction angle, indicating a decrease in the lattice parame-ter. The peak at 36.01° for the film with x = 0 correspondsto the 002 diffraction peak of bulk AlN (c = 4.980¡)generally detected at 36.04°.23) Our results also indicatethat the c-axis lattice parameter of WZ-(Al1¹xScx)N is notsensitive to the chemical composition x for x ¯ 0.37. Basedon the table of ionic radii,24) Sc3+ ions are significantlylarger than Al3+ ions. Although the ionic radius of Sc3+ atfour-fold coordination sites is not listed in the table, Sc3+is substantially larger than Al3+ when comparing the ionicradii at six-fold coordination sites. Therefore, the latticeshould expand with an increase in x. However, the c-latticeparameter did not change significantly with x for x ¯ 0.37.Considering the ionic radii of Sc3+ and Al3+, the reductionin the lattice parameter for x = 0.49 is notable. The abruptchange in the peak position give rise to suspicion that theWZ structure could not be maintained at this compositionor that Sc is not sufficiently solved. The evaluation of thein-plane (a-axis) lattice parameter is important for con-firming the formation of solid-solution WZ-(Al1¹xScx)N forthe full range of x and determining its crystal structure.Figure 2 shows the XRD º scans of the 101 diffractionof WZ-(Al1¹xScx)N. The dashed red lines in the figure cor-respond to the projection of the ©001ª axis of the Nb:STOsubstrate. The (Al1¹xScx)N samples with 0 ¯ x ¯ 0.3showed 12 peaks separated by 30°. However, only sixpeaks should have been detected in the º-scan because ofthe six-fold symmetry of the WZ structure. These resultsindicate the presence of in-plane rotation domains. TheWZ-(Al1¹xScx)N samples with 0.3 < x < 0.5 showed sixpeaks separated by 60°, suggesting that WZ-(Al1¹xScx)Nis well aligned to form a single-crystal-like structure. Forsamples with x ¯ 0.3, the epitaxial relationships are ex-pressed as WZ-(Al,Sc)N[100]//Nb:STO½11�2� for one do-main and WZ-(Al,Sc)N[100]//Nb:STO½1�10� for the otherdomain, as illustrated in Fig. 3. The triangles represent thesize of the oxygen sublattice at the (111) face of Nb:STO,and the hexagons represent the nitrogen sublattice in WZ-type nitrides. We expect that the interfacial structures ofAlN and Nb:STO will match as shown in Fig. 3(B), con-sidering that the size of the oxygen sublattice on theNb:STO(111) face and the nitrogen sublattice at the (001)face of AlN should lead to good lattice matching. However,our results suggest that the structure of the AlN/Nb:STOinterface is as illustrated in Fig. 3(A), which is consis-tent the previous reports on AlN growth on SrTiO3(111)single-crystal substrates, although Nb:STO is used in thisstudy.16)­18) Therefore, Nb doping into STO negligiblyaffected the in-plane orientation relationship.As shown in Fig. 3, the abundance of the two domainsvaries with the cation composition x. The fraction of thedomain with the WZ-(Al,Sc)N[100]//Nb:STO½1�10� rela-tionship gradually increased with the concentration ofSc. A single-domain film with the WZ-(Al,Sc)N[100]//Nb:STO½1�10� relationship was observed for x > 0.3.These characteristics are summarized in Fig. 4. I(A) andI(B) represent the integrated intensities of the domainswith (Al,Sc)N[100]//Nb:STO½11�2� and (Al,Sc)N[100]//Nb:STO½1�10�, respectively. The fraction I(B)/(I(A) + I(B))increased from 20 to 100% as the Sc content increasedfrom 0 to 0.37, revealing change in a domain preferencefor Sc in WZ-(Al1¹xScx)N. Considering that the fraction ofthe domain changed gradually with the chemical compo-x=0.49x=0.37x=0.30x=0.18x=0.09x=0.0WZ (002)NSTO(111)454035CuKα2θ (deg.)log(intensity) (arb. unit)Fig. 1. XRD ª­2ª patterns for (Al1¹xScx)N thin films with0 ¯ x < 0.5, measured at room temperature. ‘WZ(002)’ and‘NSTO(111)’ denote the 002 diffraction peak of WZ-(Al1¹xScx)Nand 111 diffraction peak of Nb-doped SrTiO3, respectively.0.490.370.300.180.09x=0.0Intensity (arb. unit)φ (deg.)036030024018012060Fig. 2. XRD ¤ scans of the 101 diffraction of WZ-(Al1¹xScx)N(0 ¯ x < 0.5). Dashed lines indicate the projection of the ©100ªdirection of the Nb-doped SrTiO3 substrate.Hasegawa et al.: In-plane lattice orientation in aluminum scandium nitride epitaxial films deposited on Nb-doped SrTiO3(111)substrates via reactive magnetron sputteringJCS-Japan244sition, the change in the interfacial structure was negli-gible. We assume that the presence of two domains resultsfrom a combination of several parameters. The change inthe volume fraction of the domain by deposition tem-perature was reported.17),18) That study has shown thatsingle domain films with (Al,Sc)N[100]//Nb:STO½11�2�were obtained at 270 and 370 °C and films with mixeddomains were grown above 470 °C; namely, domain with(Al,Sc)N[100]//Nb:STO½1�10� increases with the increasein deposition temperature.17),18) However, the domain with(Al,Sc)N[100]//Nb:STO½1�10� was still minor in the filmsdeposited at the highest temperature of 570 °C.17),18) Bycontrast, single domain with (Al,Sc)N[100]//Nb:STO½1�10�was achieved by Sc addition with x ² 0.37 in this study.Figure 5 shows the composition dependence of the c-axis lattice constant (c) and a-axis lattice constant (a). Fora hexagonal structure, a and c can be calculated from thed-spacing of the plane indexed by h, k, and l as follows:1d2ðhklÞ¼ 43h2 þ k2 þ hka2þ l2c2ð1ÞIn this study, we determined c from out-of-plane measure-ments of the 002 diffraction. We then determined a fromthe calculated c and the results of 101 diffraction measure-ments. As mentioned previously, the parameter c remainedapproximately unchanged for 0 ¯ x ¯ 0.3 and decreased[100](Al,Sc)N║[112]STO(A)3√ 6 aSTO[100](Al,Sc)N║[110]STOa(Al,Sc)N= 3.188Å(B)<111>STO[110]STO[112]STO√ 2aSTO(C)Fig. 3. Schematic of the epitaxial relationship between the (001) face of the (Al1¹xScx)N film and the (111) faceof the Nb-doped SrTiO3 substrate. Both (A) WZ-(Al,Sc)N[100]//Nb:STO½1�10� and (B) WZ-(Al,Sc)N[100]//Nb:STO½11�2� relationships are illustrated with (C) the (111) face of the Nb-doped SrTiO3 substrate. Here, thein-plane (a-axis) lattice parameter of the (Al1¹xScx)N film was assumed to be 3.188¡, referring to the size of theoxygen sublattice on the (111) face of the Nb-doped SrTiO3 substrate.0.01.00.80.60.40.25.00.0 0.40.30.20.1x in (Al1-xScx)NI (B)/[I (A)+I (B)]Fig. 4. Fraction of the rotation domains, WZ-(Al,Sc)N[100]//Nb:STO½11�2� (I(A)) and WZ-(Al,Sc)N[100]//Nb:STO½1�10� (I(B)),deduced from the results of ¤ scan measurements for(Al1¹xScx)N.Fig. 5. Structural parameter of WZ-(Al1¹xScx) thin films with0 ¯ x < 0.5. Blue closed circles indicate the experimental resultsof this study, and the open green circles indicate reported theo-retical values.9) Dashed red line assumes the complete restrictionof the a-axis parameter by the Nb-doped SrTiO3 substrate withthe (Al,Sc)N[100]//Nb:STO½1�10� configuration. Dashed blueline is a visual guide.Journal of the Ceramic Society of Japan 131 [7] 242-247 2023 JCS-Japan245with increasing Sc content for 0.3 < x < 0.5. This behav-ior is unconventional considering the ionic radii. The aparameter increased almost linearly with the Sc content, asexpected from ionic radii of Al and Sc. The theoretical aand c values calculated by Furuta et al.25) are also shownin Fig. 5. Notably, the change in the calculated latticeparameters with x also showed the two features observedin this study. A non-monotonic change in c-axis length,namely, a slight increase in value for x ¯ 0.3 and a rapiddecrease for 0.3 < x < 0.5, was deduced from the calculat-ed lattice parameters. Furthermore, a monotonic increasein a-axis length with increasing x was noted in both theirsimulations and our experimental results. Figure 5 plotsthe cubic root of the lattice volume, in order to validatesolubility of scandium. The cubic root of the lattice vol-ume exhibited an almost linear change with x, followingthe Vegard’s law. This suggests that a WZ-(Al,Sc)N solidsolution is obtained for all values of x in this study despitethe abrupt change in the position of the XRD peaks shownin Fig. 2. Notably, Akiyama et al.5) reported a steep de-crease in the c-axis length with increasing Sc concentra-tion for high x values in WZ-(Al1¹xScx)N deposited on aplatinum electrode. Hence, the x-dependence of the latticeparameters shown in Fig. 5 probably represents a generaltrend for any bottom electrode material.Compared with the lattice parameter of Nb:STO, latticemismatch does not strongly affect the epitaxial relationship.The lattice mismatch between WZ-(Al1¹xScx)N (x μ 0.3)and Nb:STO for the (Al,Sc)N[100]//Nb:STO½1�10� rela-tionship was small, as illustrated in Figs. 3 and 5. Thenitrogen sublattice at WZ-(Al1¹xScx)N(001) face shouldmatch the oxygen sublattice at the Nb:STO(111) face fora WZ-(Al1¹xScx)N a-axis length of 3.188¡ under the(Al,Sc)N[100]//Nb:STO½1�10� epitaxial relationship. How-ever, the domain with the (Al,Sc)N[100]//Nb:STO½11�2�configuration is retained despite a large lattice mismatchof approximately 15%. Therefore, other factors, includingthe adsorption energy26) at the interface between WZ-(Al1¹xScx)N(001) and Nb:STO(111), determine the epitax-ial relationship. As previously mentioned, the in-planelattice parameter changed monotonically with x, indicatingthat the WZ-(Al1¹xScx)N lattice was well relaxed in the filmprepared in this study.4. ConclusionsWZ-(Al1¹xScx)N thin films were epitaxially grown onNb:STO(111) single-crystal substrates via RF reactivemagnetron sputtering. c-axis-oriented WZ-(Al1¹xScx)Nthin films were obtained for 0 ¯ x ¯ 0.49. The investi-gation of the epitaxial relationship revealed two coexist-ing domains, expressed as (Al,Sc)N[100]//Nb:STO½11�2�and (Al,Sc)N[100]//Nb:STO½1�10�. Their relative abun-dance changed systematically with the cation composi-tion. The domain with the (Al,Sc)N[100]//Nb:STO½11�2�configuration was dominant in the AlN film, and filmswith x > 0.3 had a single domain with the (Al,Sc)N[100]//Nb:STO½1�10� configuration. These results imply that thelattice alignment of the WZ-(Al1¹xScx)N thin films is notstrongly restricted by the crystal structure of the Nb:STOsubstrate. The c-axis length was maximum at x μ 0.3,whereas the a-axis length monotonically increased withan increase in x. Despite the non-monotonic change in thec-axis length with the chemical composition, the x-dependency of the unit cell volume showed that thissolid-solution system follows the Vegard’s law.The crystal structure of the WZ-(Al1¹xScx)N solid solu-tion on the Nb:STO substrate is well relaxed and notstrongly restricted by the substrate lattice. Considering thatthe c/a ratio of WZ-(Al1¹xScx)N is an essential parameterfor determining the ferroelectric behavior of this solid-solution system and that the restriction of c/a with the sub-strate lattice allows the control of their ferroelectric prop-erties, further investigations of bottom electrode materialsare required to achieve strain engineering and control thec/a ratio of this solid-solution system.Acknowledgments Part of this study was performed atthe Tokodai Institute for Elemental Strategy (TIES) and sup-ported by the grant number JPMXP0112101001. This workwas also supported by MEXT Program: Data Creation andUtilization Type Material Research and Development ProjectGrant Number JPMXP1122683430. This work was partlysupported by JST and PRESTO under the grant numberJPMJPR20B3.References1) R. Yu, G. Liu, G. Wang, C. Chen, M. Xu, H. Zhou, T.Wang, J. Yu, G. Zhao and L. Zhang, J. Mater. Chem. C,9, 1852­1873 (2021).2) Z. Valdez-Nava, D. Kenfaui, M. L. Locatelli, L.Laudebat and S. 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