Temperature-Induced Four-Fold-on-Six-Fold Symmetric Heteroepitaxy

Article pubs.acs.org/crystal

Temperature-Induced Four-Fold-on-Six-Fold Symmetric Heteroepitaxy, Rocksalt SmN on Hexagonal AlN Jay R. Chan,*,† Stéphane Vézian,‡ Joe Trodahl,† Mohamed Al Khalfioui,‡,§ Benjamin Damilano,‡ and Franck Natali*,† †

The MacDiarmid Institute for Advanced Materials and Nanotechnology, School of Chemical and Physical Sciences, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand ‡ Centre de Recherche sur l’Hétéro-Épitaxie et ses Applications (CRHEA), Centre National de la Recherche Scientifique, Rue Bernard Gregory, F-06560 Valbonne, France § Université de Nice Sophia Antipolis, Parc Valrose, 06102 Nice cedex 2, France ABSTRACT: We report on the occurrence of rotational domains involving 4-fold symmetric epitaxy on a hexagonal net, a rare case in the literature. A temperature-driven crossover from fully (111)- to (001)-oriented samarium nitride grown by molecular beam epitaxy on hexagonal (0001) aluminum nitride is observed by means of in situ reflection high energy electron diffraction, scanning tunnelling microscopy studies, and ex situ X-ray diffraction. Using an especially rich set of growth conditions and nitrogen precursors, we observe the key role played by the growth kinetics, with a strong thermal cracking of the ammonia source and a re-evaporation of Sm adatoms occurring in the same temperature range of the orientation crossover.

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INTRODUCTION Combining different materials through the heteroepitaxial growth process involves many materials science challenges, notably the adaptability of interfacial and surface structures, and the chemical stability between the epilayer and the substrate. The generally accepted picture of the heteroepitaxial process follows the idea that the dissimilarities between the epilayer and the substrate will be accommodated by misfit dislocations,1−3 interfacial atomic arrangement,4−6 and rotational domains.7−11 The latter has been investigated extensively, with recently a seminal contribution in which a general expression for the minimum number of rotational domains NRD of the epilayer on the substrate has been derived. The rotational symmetry at the interface can be defined using the notation of Grundmann7,8 NRD = lcm(n , m)/m

face on a hexagonal net, leading to NRD = 3; in this case the cubic epilayer will have three domains rotated by 120° with respect to each other. Such systems are however rare in practice as the symmetry matching of close-packed planes usually results in a preferential (111)-oriented growth resulting in twinned domains (see below). Nevertheless, (001)-oriented epitaxial growth on C6 or C3 nets has been observed for a few cases reported in the literature, MgO(001)/ZnO(0001),15 and more exotic systems such as SrB6(001) on ultrasmooth α-Al2O3(0001) substrates16 and tetragonal BaTiO3(001) on a ZnO(0001) surface.7 However, the fundamental mechanisms driving this orientation crossover have not yet received attention and still remain elusive, which motivates our present study. Our work reveals a new system exhibiting a relatively sharp temperature-driven crossover from a fully (111)- to fully (001)-oriented rocksalt SmN epilayer grown on a hexagonal AlN(0001) surface. The structure of the films was determined by in situ reflection high energy electron diffraction (RHEED) and scanning tunnelling microscopy (STM) studies, supported by RHEED simulations and ex situ X-ray diffraction (XRD). The SmN epitaxial thin films grown by molecular beam epitaxy (MBE) under controlled growth temperature and a choice of two nitrogen precursors demonstrate the key role played by the growth kinetics on the orientation crossover. More than just an intriguing phenomenon, the heteroepitaxy of SmN on AlN comes into broad investigations on combining

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where n and m denote the Cn and Cm rotational symmetries of the substrate and the epilayer nets, respectively, and lcm(n, m) is the least common multiple of n and m. Here we report the growth of an FCC (rocksalt) structure onto a hexagonal net (n = 6), in which the growth is remarkably switched from (111) (m = 3) to (001) (m = 4), controlled by the deposition parameters. Although there are a total of 25 possible symmetry combinations, by far the most common are those for which the nets have the same symmetry (m = n). However, it is not uncommon to exploit the (111) face of a cubic material (n = 3) as a substrate for a hexagonal films (m = 6), in which case there is only one rotational variant.12−14 The reverse, a hexagonal net but a (111) cubic film, has two variants rotated by 180°. A particularly interesting case is n = 6 and m = 4, a square cubic © 2016 American Chemical Society

Received: July 31, 2016 Revised: September 9, 2016 Published: September 13, 2016 6454

DOI: 10.1021/acs.cgd.6b01133 Cryst. Growth Des. 2016, 16, 6454−6460

Crystal Growth & Design

Article

Figure 1. RHEED images taken for 100 nm thick samples grown at (a) 700 °C and (b) 800 °C using NH3 as the nitrogen precursor along the ⟨11̅00⟩ azimuth of AlN (0001). Simulations of the respective RHEED patterns from (c) (111) oriented SmN film with 180° rotated twin domains along the ⟨112̅⟩ SmN azimuth and (d) (001) oriented SmN film with 120° rotated triplet domains along the ⟨110⟩ SmN azimuth. Contributions from each domain are indicated by red, green, and blue components. equivalent pressure (BEP) of 2.7 × 10−5 Torr, 1.9 × 10−5 Torr, and 1 × 10−7 Torr for N2, NH3, and Sm, respectively. Note that the growth of REN is generally performed under nitrogen-rich conditions in order to limit the formation of nitrogen vacancies.17 In situ RHEED was used to monitor the epitaxial nature of the growth and the RHEED observation of the well-known (7 × 7) ↔ (1 × 1) Si(111) surface phase transition was used to calibrate the temperature measurement by adjusting the emissivity of the optical infrared pyrometer to get a transition at 830 °C. In situ STM was performed to analyze the SmN surface morphology. Ex situ XRD scans in both symmetric and asymmetric geometries were used to characterize the crystallinity of the SmN layers as well as the heteroepitaxial relationship between the hexagonal AlN layer and SmN epilayer. For XRD measurements, the SmN layers were capped with a 100−150 nm thick GaN layer in order to prevent decomposition in air. Film growth rates were inferred from ex situ thickness measurements via cross-section scanning electron microscopy on a freshly cleaved edge of the samples.

rare-earth nitride (REN) and group-III nitride compounds to unite the best characteristic of both materials and to demonstrate novel semiconductor-based spintronics applications.17−20 Even in the company of strongly contrasting ferromagnetism among the RENs, SmN stands out for its unusual physical properties.21 It is the only known ferromagnetic semiconductor with a nearly vanishing net magnetic moment directed opposite to the fully aligned spin moment.22 Even more peculiar is the recent discovery that heavily donor-doped SmN by nitrogen vacancies becomes superconducting with ferromagnetism remaining intact at low temperature.23 In this framework the importance of the heteroepitaxy of SmN on AlN does not only arise from addressing the control of cubic-on-hexagonal growth, but also stems from the potential it offers for the development of innovative applications for spintronics, and even further, superconducting spintronics.24

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RESULTS AND DISCUSSION RHEED patterns of 100 nm thick SmN layers grown with NH3 are shown in Figure 1. Double spots characteristic of rotational twin domains are seen on the SmN layers grown at 700 °C and lower temperatures (Figure 1a) when measured along the [11̅00] azimuth of AlN (0001).17,18 In contrast, at growth temperatures higher than 750 °C a completely new RHEED pattern (Figure 1b), 15° off the [11̅00] azimuth emerged, though the films are still epitaxial in nature. The streakier nature of the RHEED

EXPERIMENTAL SECTION

The growth of SmN was performed by MBE on 100 nm thick hexagonal (0001)-oriented AlN which were themselves grown on (111) silicon substrates, as described more fully elsewhere.25 Prior to the growth of SmN, the AlN surface was cleaned using a thermal annealing at 880 °C under NH3 for 15 min. Either NH3 or molecular N2 were used as the nitrogen precursors, while a conventional effusion cell was used for Sm solid source. The SmN layers were grown at a substrate temperature ranging from 300 to 800 °C under N-rich conditions, with a beam 6455



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disappears for a growth temperature of 800 °C. A similar temperature-driven crossover seems also to take place when the SmN films are grown with N2, though a fully (001)-oriented films cannot be achieved (Figure 2b) as the growth rate fell to zero for temperatures higher than 600 °C. The SmN lattice constant measured at room temperature for all samples average to aSmN = 5.035 ± 0.008 Å using both 111 and 002 reflections, suggesting that all samples are fully relaxed,21 and thus does not allow any conclusions about the effect of the strain on the orientation crossover. In order to further investigate the SmN crystal orientation, asymmetric ϕ scans were performed on several samples to identify the epitaxial relationship between the different orientations of SmN grown with NH3 on AlN (0001) (Figure 3a,b). (111)-oriented growth, performed at 700 °C, shows the same epitaxial relationship as was first identified for GdN on GaN and AlN heteroepitaxial systems17,18 with a ϕ scan of the 002 reflection showing six peaks spaced 60° equally apart characteristic of the twin rotational domains rotated 180° relative to one another (Figure 3a). A schematic of the inferred epitaxial relationship between (fully relaxed) SmN and AlN is shown in Figure 3c. Specifically we observe twin rotational domains (labeled a and b in Figure 3c) with SmN(111) || AlN(0001) and SmN(111)⟨11̅0⟩ || AlN(0001)⟨112̅0⟩. A 7/8 coincidence site lattice (CSL) matching between AlN (0001) and SmN (111) planes is observed along the three equivalent ⟨112̅0⟩ AlN directions (Figure 4a), i.e., 7 × aSmN(111) = 8 × aAlN where aSmN(111) = aSmN/√2 and aAlN are the in-plane lattice parameters of SmN(111) and AlN(0001), respectively (aSmN = 5.035 Å and aAlN = 3.111 Å at 300 K). In contrast, for a fully (001)-oriented film, a ϕ scan of the 022 reflection would produce a square symmetry with four peaks equally spaced 90° apart. The ϕ scans on (001)-oriented SmN samples reveal 12 peaks spaced 30° apart on top of the six peaks from the AlN 101̅2 ϕ scan (Figure 3b). The inferred epitaxial relationship for this growth orientation is shown in Figure 3d; three rotational domains (labeled a, b, and c) rotated by 120° relative to each other with the relationships SmN(001) ||

pattern at elevated growth temperatures in Figure 1b compared to Figure 1a suggests also a smoother surface morphology. This dramatic change in the RHEED pattern is only observed for growth under NH3 as the growth rate of SmN under N2 drops to zero at high substrate temperatures as discussed below. An important question is whether or not the change of the surface morphology coincides with a change in the growth orientation and crystallographic structure. We have thus precisely performed XRD measurements in both symmetric and asymmetric geometries (Figures 2 and 3a,b). For samples

Figure 2. XRD 2θ−ω scans for varying growth temperatures of SmN using (a) NH3 and (b) N2 as a nitrogen precursor. The SmN layer thickness is typically about 100 nm. At 600 °C using N2 and 750 °C using NH3, mixed (111)- and (001)-oriented growth of SmN occurs. At the highest growth temperature of 800 °C with NH3, only the SmN 002 reflection is seen indicating a transition to fully (001)-oriented growth.

grown at moderate temperatures (up to 700 °C) with NH3, only (111)-oriented SmN growth is obtained on top of the AlN (0001) surface in the symmetric 2θ−ω scans (Figure 2a). At higher growth temperatures, a 002 SmN reflection appears, and interestingly the 111 SmN peak intensity weakens and totally

Figure 3. XRD ϕ scans from SmN samples grown using (a) NH3 at 700 °C and (b) at 800 °C. The 6-fold symmetry of the AlN 101̅2 reflection is compared to (a) the 6-fold symmetry from two rotational domains (labeled a and b) of (111)-oriented SmN layer and (b) the 12-fold symmetry from three rotational domains (labeled a, b, and c) of (001)-oriented SmN layer Schematic atomic models of the respective in-plane epitaxial relationships between (c) (111)-oriented and (d) (001)-oriented SmN epilayers on (0001) AlN as inferred from the XRD ϕ scans. 6456



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Figure 4. 2D schematic illustration highlighting the coincidence networks site for the (a) (111) and (b) (001) SmN planes (blue dots) on AlN (0001) (red dots). Only one rotational domain is shown for each orientation for clarity’s sake. The corresponding surface unit cells are indicated and coincidence sites are highlighted in green. (a) A domain matching of 7/8 exists along AlN⟨112̅0⟩, 7 × aSmN(111) = 8 × aAlN. (b) Two distinct domain matchings exist along AlN [1120̅ ] and AlN[11̅ 00]; 8 × aSmN(001) = 13 × aAlN and AlN[11̅ 00], 16 × aSmN(001) = 15 √3 × aAlN respectively.

AlN(0001) and SmN(001)⟨100⟩ || AlN(0001)⟨11̅00⟩. Triple rotational domain epitaxial systems like this are rare with only a few specific cases of cubic materials reported in the literature.7,15,16 For this growth orientation, we have identified a separate CSL matching for a single domain between the square SmN (001) and hexagonal AlN (0001) lattices (Figure 4b). Along AlN ⟨112̅0⟩, 8 × aSmN = 13 × aAlN while along AlN ⟨1̅100⟩, 16 × aSmN = 15 √3 × aAlN. Note that, contrary to (111)-oriented SmN films which result in a hexagonal CSL, the 6-fold symmetry is broken, and the ⟨112̅0⟩ family of directions is no longer equivalent with respect to SmN directions on the (111) face, resulting instead in a centered rectangular CSL. It is important to note that the epitaxial growth of SmN using NH3 as nitrogen precursor occurs both below and above the NH3 thermal cracking threshold (∼450 °C).26 Clearly there is a strong catalytic breakdown of ammonia molecules by RE atoms on the growing surface. While a strong reactivity of RE ions and pure molecular N2 has been reported leading to the growth of SmN and some of the other REN under pure N2,17 this was not previously known for NH3. With the aim of qualitatively confronting the epitaxial relationships inferred from XRD results and the experimental RHEED patterns, we have performed kinematic simulations of the RHEED patterns. Following a kinematical scattering approach derived from the one proposed by Braun,27 the diffracted intensity of an incident electron beam from an infinite SmN lattice is calculated from the reciprocal lattice for different given crystal orientations determined from the XRD measurements. Here we assume (i) single scattering events and (ii) diffraction arising from an atomically rough 3D surface to compute the intensity spot maxima which are most relevant to the experimentally observed diffraction patterns. The intensity of a diffraction spot is calculated as I = F2 where F = ||F⃗|| is the structure factor given by

j=n

F(G⃗) =

j=n

∑ f j exp(iG⃗ · rj⃗) = ∑ f j exp(i(kd⃗ − ki⃗ )· rj⃗) j=1

j=1

(2)

where the sum is over all atoms j located at rj⃗ within the unit cell and f j is the atomic form factor of the jth atom. The Laue condition for the 3D diffraction is incorporated in eq 2 and given by kd⃗ − ki⃗ = G⃗ where kd⃗ , ki⃗ , and G⃗ are the diffracted beam wavevector, the incident beam wavevector and the 3D reciprocal lattice vector, respectively. For this simulation, a SmN lattice constant of 0.503 nm was used for the three space directions, with an electron beam energy of 20 keV. The computational results are displayed in Figure 1c,d and are in excellent agreement with the recorded images. The contributions from each rotational domain are indicated by intensity in components of red, green, and blue (RGB color model), showing the overlap among the patterns. It is significant that the RHEED patterns for a given growth temperature and nitrogen precursor do not change during the deposition suggesting that the nucleation in the first few monolayers of deposition determines the overall growth orientation of the rest of the film. We turn now to an interpretation of crystalline orientation, especially regarding how the growth dynamics might lead to such a sharply controlled symmetry crossover. The fundamental issue that determines the epitaxial growth relationship relates to the thermally driven diffusion of ions on the substrate in comparison to the deposition rate; it is the competition between these two phenomena, in the presence of primarily interface energetics, that is expected to ultimately determine the crystalline quality and orientation. Indeed that is a controlling factor in at least some of orientation transitions.15,28−31 The present system involves a richer set of controlling influences, for which variation with substrate temperature of SmN growth rate, shown in Figure 5, offers substantial insight. There is a larger background chamber pressure when growing with N2 compared to NH3 which decreases the effusion rate from the Sm effusion cell, causing a difference of 20% at low growth temperatures between NH3 and 6457



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difference, two main features can be seen on the images. First, there is a clear decrease in surface roughness when increasing the growth temperature from 700 to 800 °C; the fully (001)-oriented SmN with a root-mean-square (RMS) roughness of 1.46 nm contrasts with the 3.20 nm for the (111)-oriented sample, confirming the previous RHEED observations. Then there is also a clear increase in grain size when increasing the growth temperature from 700 to 800 °C; the (001)-oriented SmN, with an average grain size of (33 ± 2) nm compared to (19 ± 2) nm for the (111)-oriented 700 °C sample. It is remarkable that even in the presence of the 3-fold rotational variants implied by the square-on-hexagon growth, STM studies of the films show a factor of nearly 2 larger crystallite diameters in the (001) versus the (111) growth conditions, a clear advantage in most device applications. This is not entirely unexpected, as it is well-known that the (111) surface of ionic rocksalt materials consisting entirely of either cations or anions is unstable due to a diverging surface energy.33 The surface must be stabilized in some way; hydroxyl termination is often observed in rocksalt oxides, as well as surface reconstructions or faceting of the surface. These processes inevitably lead to a roughening of the surface. In addition, and even more importantly, one-lattice-parameter steps corresponding to the half of the lattice constant (2.52 Å), i.e., one molecular monolayers, with smooth terraces edges are present for the fully (001)-oriented SmN layer (Figure 6 (inset)). It is important to note that our observations are similar to those reported on SrB6 showing that fully (001)-oriented films have to be grown on a vicinal-like ultrasmooth sapphire substrate in order to replicate the straight atomic steps from the substrate.16 These experimental results are in agreement with a recent theoretical model on the preferred orientation formation in rocksalt materials.29 Their model suggests that a decrease of the deposition rate and a higher surface diffusion of adatoms would result in a (001) preferential growth orientation. There are key differences that must be noted with a heteroepitaxial system compared to the development of a preferential orientation on an amorphous substrate. As mentioned previously, RHEED during the early stages of deposition suggests that the nucleation determining the epitaxial growth orientation occurs within the first few monolayers, while the kinetic model assumes an initial equated existence of (111) and (001) planes (guaranteed by using an amorphous substrate) and considers the interdiffusion of adatoms between (111) and (001) planes. The growth of SmN also requires the reaction of metallic Sm and the nitrogen precursor, while the MgO study has controlled adatom interactions by evaporating a MgO pellet source. These

Figure 5. SmN growth rate as a function of the growth temperature for films grown using NH3 (blue squares) and N2 (red dots) as nitrogen precursors (left axis) compared to the cracking efficiency of NH3 (green triangles) from ref 26 (right axis). The SmN growth rate was determined from cross-section scanning electron microscopy.

N2. Such screening of metal flux has already been reported for NH3-rich growth of InGaN.32 As the substrate temperature increases the vapor pressure of Sm rises, driving re-evaporation of adsorbed Sm before it is incorporated into the SmN film and causing the growth rate to drop sharply from 500 °C. The coincidence of this falling growth rate with the onset of the (111)-to-(001) transition immediately suggests that (001) orientation represents the lower energy configuration in fully stoichiometric films. The growth rate falls to zero under N2, signaling that essentially all of the Sm re-evaporates before reacting with N2, but with NH3 it drops by only 30% of the maximum growth rate. This is driven by a second coincidence; the thermal cracking efficiency of NH3 increases sharply above 500 °C, thus providing enhanced access to atomic nitrogen that is available from only catalytic cracking at lower temperatures. Note that thermal cracking is near 0% below 500 °C but increases to a maximum of 4% at 600 °C for the reported case of GaN-NH3 MBE.26 To get more insight into the growth kinetics, one should consider the surface morphology of the different samples. As the substrate temperature increases, the two main effects are the increased diffusion length as well as the increased desorption rates of Sm adatoms. A comparison of in situ STM images of SmN surfaces for samples grown at 700 °C (Figure 6a) and 800 °C (Figure 6b) using NH3 as nitrogen precursor underline two different types of morphologies. Among the overall morphology

Figure 6. 200 nm × 200 nm STM images for samples grown at 700 °C (a) and 800 °C (b) with NH3 after deposition of 100 nm of SmN. Acquisition parameters were 1 nA, and 1.5 and 2 V (sample biases) respectively. (Inset) Height profile indicated by the line in (b). 6458



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Rotation Mismatch Control of the Symmetry and Properties of SrRuO3. ACS Appl. Mater. Interfaces 2016, 8, 14871−14878. (11) Takahashi, R.; Misumi, H.; Lippmaa, M. Self-Template Growth of Orientation-Controlled Fe3O4 Thin Films. Cryst. Growth Des. 2012, 12, 2679−2683. (12) Lazarov, V.; Zimmerman, J.; Cheung, S.; Li, L.; Weinert, M.; Gajdardziska-Josifovska, M. Selected Growth of Cubic and Hexagonal GaN Epitaxial Films on Polar MgO (111). Phys. Rev. Lett. 2005, 94, 216101. (13) Dadgar, A.; Schulze, F.; Wienecke, M.; Gadanecz, A.; Bläsing, J.; Veit, P.; Hempel, T.; Diez, A.; Christen, J.; Krost, A. Epitaxy of GaN on silicon−impact of symmetry and surface reconstruction. New J. Phys. 2007, 9, 389. (14) Wei, X.; Li, Y.; Zhu, J.; Huang, W.; Zhang, Y.; Luo, W.; Ji, H. Epitaxial properties of ZnO thin films on SrTiO3 substrates grown by laser molecular beam epitaxy. Appl. Phys. Lett. 2007, 90, 151918. (15) Xiao, B.; Yang, Q.; Walker, B.; Gonder, C. A.; Romain, G. C.; Mundle, R.; Bahoura, M.; Pradhan, A. Competition between (001) and (111) MgO thin film growth on Al-doped ZnO by oxygen plasma assisted pulsed laser deposition. J. Appl. Phys. 2013, 113, 214102. (16) Kato, Y.; Shiraishi, N.; Tsuchimine, N.; Kobayashi, S.; Yoshimoto, M. Fabrication of semiconducting SrB6−δ thin films on ultrasmooth sapphire substrates by laser molecular beam epitaxy. J. Cryst. Growth 2010, 312, 378−381. (17) Natali, F.; Plank, N.; Galipaud, J.; Ruck, B.; Trodahl, H.; Semond, F.; Sorieul, S.; Hirsch, L. Epitaxial growth of GdN on silicon substrate using an AlN buffer layer. J. Cryst. Growth 2010, 312, 3583−3587. (18) Scarpulla, M. A.; Gallinat, C.; Mack, S.; Speck, J.; Gossard, A. GdN (111) heteroepitaxy on GaN (0001) by N2 plasma and NH3 molecular beam epitaxy. J. Cryst. Growth 2009, 311, 1239−1244. (19) Kent, T.; Yang, J.; Yang, L.; Mills, M.; Myers, R. Epitaxial ferromagnetic nanoislands of cubic GdN in hexagonal GaN. Appl. Phys. Lett. 2012, 100, 152111. (20) Krishnamoorthy, S.; Kent, T. F.; Yang, J.; Park, P. S.; Myers, R. C.; Rajan, S. GdN Nanoisland-Based GaN Tunnel Junctions. Nano Lett. 2013, 13, 2570−2575. (21) Natali, F.; Ruck, B. J.; Plank, N. O.; Trodahl, H. J.; Granville, S.; Meyer, C.; Lambrecht, W. R. Rare-earth mononitrides. Prog. Mater. Sci. 2013, 58, 1316−1360. (22) Anton, E.-M.; Ruck, B.; Meyer, C.; Natali, F.; Warring, H.; Wilhelm, F.; Rogalev, A.; Antonov, V.; Trodahl, H. Spin/orbit moment imbalance in the near-zero moment ferromagnetic semiconductor SmN. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 134414. (23) Anton, E.-M.; Granville, S.; Engel, A.; Chong, S.; Governale, M.; Zülicke, U.; Moghaddam, A.; Trodahl, H.; Natali, F.; Vézian, S.; Ruck, B. Superconductivity in the ferromagnetic semiconductor samarium nitride. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 94, 024106. (24) Linder, J.; Robinson, J. W. Superconducting spintronics. Nat. Phys. 2015, 11, 307−315. (25) Le Louarn, A.; Vézian, S.; Semond, F.; Massies, J. AlN buffer layer growth for GaN epitaxy on (111) Si: Al or N first? J. Cryst. Growth 2009, 311, 3278−3284. (26) Mesrine, M.; Grandjean, N.; Massies, J. Efficiency of NH3 as nitrogen source for GaN molecular beam epitaxy. Appl. Phys. Lett. 1998, 72, 350. (27) Braun, W. Applied RHEED: Reflection High-Energy Electron Diffraction During Crystal Growth; Springer Science & Business Media: Berlin, 1999; Vol. 154. (28) Ahn, K. H.; Baik, S.; Kim, S. S. Change of growth orientation in Pt films epitaxially grown on MgO (001) substrates by sputtering. J. Mater. Res. 2002, 17, 2334−2338. (29) Yu, H. K.; Lee, J.-L. Understanding of Preferred Orientation Formation in Rock-Salt Materials: The Case of MgO. Cryst. Growth Des. 2016, 16, 1978−1983. (30) Tang, C. Y.; Chen, Y. Y.; Li, W.; Sun, L. J.; Yao, X.; Jirsa, M. Supersaturation-Controlled Growth Orientation and Grain Boundary Transition in REBa2Cu3O7−δ (RE = Sm, Sm1−x Yx) Liquid-Phase Epitaxial Films. Cryst. Growth Des. 2010, 10, 575−579.

differences highlight the complexity of factors affecting the possible epitaxial growth orientations of dissimilar crystal structures.

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CONCLUSION We have studied the orientation relationship between MBE grown rocksalt SmN epilayers on a hexagonal AlN surface. Structural and surface investigations show a temperature-driven crossover from fully (111)- to (001) oriented SmN films, i.e., from a 3- to 4-fold symmetric rocksalt SmN. This case, rare by essence as rocksalt materials have the tendency of a (111) preferred orientation on a hexagonal net, is understood on the basis of a change of the growth kinetics. Importantly we observe a concomitance between the crossover and a growth regime where a re-evaporation of the adsorbed Sm occurs. In situ STM images suggest also a step flow growth mode for fully (001) oriented SmN whose origin can be attributed to a change in the diffusion length of adatoms with the increase of the growth temperature.

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AUTHOR INFORMATION

Corresponding Authors

*(J.C.) Email: [email protected]. *(F.N.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge funding from the Marsden Fund (Grant No. 08-VUW-1309) and the MacDiarmid Institute for Advanced Materials and Nanotechnology, funded by the New Zealand Centres of Research Excellence Fund. We acknowledge support from GANEX (ANR-11-LABX-0014). GANEX belongs to the public funded “Investissements d’Avenir” program managed by the French ANR agency.

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REFERENCES

(1) Van der Merwe, J. H. Misfit dislocation generation in epitaxial layers. Crit. Rev. Solid State Mater. Sci. 1991, 17, 187−209. (2) Chen, C.; Wang, Z.; Kato, T.; Shibata, N.; Taniguchi, T.; Ikuhara, Y. Misfit accommodation mechanism at the heterointerface between diamond and cubic boron nitride. Nat. Commun. 2015, 6, 6327. (3) Nabarro, F. R. Theory of Crystal Dislocations; Clarendon Press: Gloucestershire, U.K., 1967. (4) Klenov, D. O.; Zide, J. M.; Zimmerman, J. D.; Gossard, A. C.; Stemmer, S. Interface atomic structure of epitaxial ErAs layers on (001) In0.53Ga0.47As and GaAs. Appl. Phys. Lett. 2005, 86, 241901. (5) Sivaramakrishnan, S.; Wen, J.; Scarpelli, M. E.; Pierce, B. J.; Zuo, J.M. Equilibrium shapes and triple line energy of epitaxial gold nanocrystals supported on TiO2 (110). Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82, 195421. (6) Gubo, M.; Ebensperger, C.; Meyer, W.; Hammer, L.; Heinz, K.; Mittendorfer, F.; Redinger, J. Tuning the Growth Orientation of Epitaxial Films by Interface Chemistry. Phys. Rev. Lett. 2012, 108, 066101. (7) Grundmann, M.; Böntgen, T.; Lorenz, M. Occurrence of Rotation Domains in Heteroepitaxy. Phys. Rev. Lett. 2010, 105, 146102. (8) Grundmann, M. Formation of epitaxial domains: Unified theory and survey of experimental results. Phys. Status Solidi B 2011, 248, 805− 824. (9) Stemmer, S.; Pirouz, P.; Ikuhara, Y.; Davis, R. Film/Substrate Orientation Relationship in the AIN/6H-SiC Epitaxial System. Phys. Rev. Lett. 1996, 77, 1797. (10) Gao, R.; Dong, Y.; Xu, H.; Zhou, H.; Yuan, Y.; Gopalan, V.; Gao, C.; Fong, D. D.; Chen, Z.; Luo, Z.; Martin, L. Interfacial Octahedral 6459



Article

(31) Paisley, E. A.; Losego, M. D.; Gaddy, B. E.; Tweedie, J. S.; Collazo, R.; Sitar, Z.; Irving, D. L.; Maria, J.-P. Surfactant-enabled epitaxy through control of growth mode with chemical boundary conditions. Nat. Commun. 2011, 2, 461. (32) Lang, J.; Speck, J. NH3-rich growth of InGaN and InGaN/GaN superlattices by NH3-based molecular beam epitaxy. J. Cryst. Growth 2012, 346, 50−55. (33) Tasker, P. The stability of ionic crystal surfaces. J. Phys. C: Solid State Phys. 1979, 12, 4977.

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Temperature-Induced Four-Fold-on-Six-Fold Symmetric Heteroepitaxy

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