# Enhanced quality of single crystal CrBx/TiBy diboride superlattices by controlling boron stoichiometry during sputter deposition

Samira Dorri, Naureen Ghafoor, Justinas Palisaitis, Sjoerd Stendahl, Anton Devishvili, Alexei Vorobiev, Fredrik Eriksson, Per O.Å. Persson, Jens Birch

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## ► To cite this version:

Samira Dorri, Naureen Ghafoor, Justinas Palisaitis, Sjoerd Stendahl, Anton Devishvili, et al. Enhanced quality of single crystal CrBx/TiBy diboride superlattices by controlling boron stoichiometry during sputter deposition. Applied Surface Science, 2024, 655, pp.159606. 10.1016/j.apsusc.2024.159606. hal-05441986

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https://hal.science/hal-05441986v1

Submitted on 5 Jan 2026

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Full Length Article

# Enhanced quality of single crystal CrBₓ/TiBᵧ diboride superlattices by controlling boron stoichiometry during sputter deposition

Samira Dorri<sup>a, *</sup>, Naureen Ghafoor<sup>a</sup>, Justinas Palisaitis<sup>a</sup>, Sjoerd Stendahl<sup>b</sup>, Anton Devishvili<sup>c</sup>,
Alexei Vorobiev<sup>b, c</sup>, Fredrik Eriksson<sup>a</sup>, Per O.Å. Persson<sup>a</sup>, Jens Birch<sup>a</sup>

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ARTICLE INFO

Keywords:
Superlattice
Diboride
Thin film
Co-sputtering
Nanostructure

A B S T R A C T

Single-crystal CrB<sub>2</sub>/TiB<sub>2</sub> diboride superlattices with well-defined layers are promising candidates for neutron optics. However, excess B in sputter-deposited TiB<sub>y</sub> using a single TiB<sub>2</sub> target deteriorates the structural quality of CrB<sub>x</sub>/TiB<sub>y</sub> (0001) superlattices. We study the influence of co-sputtering of TiB<sub>2</sub> + Ti on the stoichiometry and crystalline quality of 300-nm-thick TiB<sub>y</sub> single layers and CrB<sub>x</sub>/TiB<sub>y</sub> (0001) superlattices on Al<sub>2</sub>O<sub>3</sub>(0001) substrates grown by DC magnetron sputter epitaxy at growth-temperatures T<sub>s</sub> ranging from 600 to 900 °C. By controlling the relative applied powers to the TiB<sub>2</sub> and Ti magnetrons, y could be reduced from 3.3 to 0.9. TiB<sub>2.3</sub> grown at 750 °C exhibited epitaxial domains about 10x larger than non-co-sputtered films. Close-to-stoichiometry CrB<sub>1.7</sub>/TiB<sub>2.3</sub> superlattices with modulation periods Λ = 6 nm grown at 750 °C showed the highest single crystal quality and best layer definition. TiB<sub>2.3</sub> layers display rough top interfaces indicating kinetically limited growth while CrB<sub>1.7</sub> forms flat and abrupt top interfaces indicating epitaxial growth with high adatom mobility.

**1. Introduction**

Single crystal, hetero-epitaxially grown periodic multilayers, also known as superlattices (SLs), have great potential for highly efficient interference neutron mirrors at large-scale neutron facilities owing to the possibility of achieving extremely abrupt interfaces (±½ atomic layer). CrB<sub>2</sub> and TiB<sub>2</sub> exhibit a small lattice mismatch (1.92 % along the a-axis) and a large difference in neutron scattering cross section which make CrB<sub>2</sub>/TiB<sub>2</sub> SLs a suitable candidate for neutron optics. Hence, for this application, it is crucial to obtain high-quality single crystal SLs with periodicities (Λ = D<sub>CrB2</sub> + D<sub>TiB2</sub>) in the range of 1–100 nm with as abrupt interfaces as possible.

We previously demonstrated the possibility of growing relatively high-crystalline-quality CrBₓ/TiBᵧ (0001) diboride SLs [x,y ∈ 1.7–3.3] on Al₂O₃(0001) substrates by magnetron sputter epitaxy (MSE) [1]. However, over-stoichiometric TiB₃, and the formation of B-rich tissue phases, particularly for SLs with layer-thickness ratios Γ = $\frac{D_{TiB_y}}{D_{CrB_x} + D_{TiB_y}} \ge 0.5$, was found to cause deterioration of the epitaxy and interface quality [1]. Hence, controlling the B stoichiometry is key to obtain high-quality

epitaxial growth of CrBₓ/TiBᵧ SLs with flat and abrupt interfaces. Due to the differences between sputter-ejected target constituents, where sputtered B atoms are preferentially ejected along the target surface normal, while sputtered Ti atoms have a wider angular ejection distribution with a relatively higher off-normal flux [2], sputter-deposited TiBᵧ are typically over-stoichiometric (B/Ti > 2) [3–6]. Some methods were proposed to control the compositions of over-stoichiometric monolithic TiB₂ thin films [2,7–9]. However, they merely focus on tuning the composition of polycrystalline and nanostructured single-phase thin films rather than the crystalline quality and are not suited for growing CrBₓ/TiBᵧ SL neutron mirrors engineered with sub-nm layer thickness precision. A method to control the TiBᵧ stoichiometry should not negatively affect the epitaxial growth and composition of the CrBₓ layers, for which good growth conditions for the composition (x = B/Cr ratio = 1.7 for CrB₁.₇ single layers) and crystalline quality have already been established [1]. Recently, the interest in transition metal diboride SLs for hard-coating applications has also increased owing to a possible improvement in mechanical properties through the hindering of dislocation mobility at the interfaces [10]. Mechanical properties of TiB₂/

<footnote><sup>a</sup> Thin Film Physics Division, Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-581 83 Linköping, Sweden</footnote>

<footnote><sup>b</sup> Department of Physics and Astronomy, Material Physics, Uppsala University, SE-751 20 Uppsala, Sweden</footnote>

<footnote><sup>c</sup> Institut Laue-Langevin, 71 avenue des Martyrs, CS 20156, 38042 Grenoble cedex 9, France</footnote>

<footnote>* Corresponding author.
E-mail address: samira.dorri@liu.se (S. Dorri).</footnote>
<figure><img src="image_5.png" /><figcaption>Fig. 1. (a, b, c) show HAADF-STEM micrographs of non-co-sputtered TiB$_{3.3}$ at T$_{s}$ = 600 °C and (d, e, f) co-sputtered TiB$_{2.3}$ at T$_{s}$ = 750 °C: (a and d) low-magnification, (b and e) lattice-resolved plan-view micrographs, and (c and f) EELS map of Ti-L$_{23}$ (red), B-K (green), and O-K (blue) in TiB$_{3.3}$ and TiB$_{2.3}$. (g) corresponding XRD θ-2θ scans for 300-nm-thick TiB$_{3.3}$ and TiB$_{2.3}$ single layers grown at T$_{s}$ = 600 and 750 °C, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

WB$_{2}$ and TiB$_{2}$/ZrB$_{2}$ polycrystalline SLs have been studied which showed over-stoichiometric TiB$_{2.66}$ [11], further underlining the importance of controlling the B stoichiometry for better crystalline quality metal diboride SLs. Hellgren et al. [12] showed that the B/Ti ratio of TiB$_{y}$ thin films co-sputtered from Ti and TiB$_{2}$ targets can be tuned from y = 2.8 to 1.2 by increasing the Ti-target power while maintaining the TiB$_{2}$-target power constant. However, the as-deposited layers were amorphous, and a high-temperature (1100 °C) vacuum-annealing process was essential to achieve polycrystalline thin films. In this work, we explore a strategy for achieving high crystal quality CrB$_{2}$/TiB$_{2}$ SLs, with emphasis on obtaining the highest possible layer definition, by employing co-sputtering from TiB$_{2}$ and Ti targets to control the B stoichiometry, y, in TiB$_{y}$, at substrate temperatures between 600 and 900 °C.

## 2. Material and methods

0001-oriented TiB$_{y}$ single layers and CrB$_{x}$/TiB$_{y}$ superlattices (SLs) were grown in a direct-current MSE system on Al$_{2}$O$_{3}$(0001) substrates. The deposition system is equipped with two type II 7.5-cm-diameter unbalanced magnetrons with a tilt angle of 25° and one 5-cm-diameter balanced magnetron with a tilt angle of 50° to the substrate normal, which are mounted in a confocal geometry at the top of the chamber. Stoichiometric 7.5-cm CrB$_{2}$ and TiB$_{2}$ compound targets, and a 5-cm Ti metal target (99.9 % purity) were used. The substrates were mounted on a substrate table below the magnetrons at a target-to-substrate distance of about 12 cm, rotating at a constant rate of 17 rpm in order to ensure good lateral thickness uniformity of the deposited layers. All three magnetrons were continuously running during the deposition, and fast computer-controlled shutters were used in front of each target to obtain the desired layer thicknesses. The base pressure for the deposition system was $3 \times 10^{-7}$ Torr. Polished Al$_{2}$O$_{3}$(0001), $1.0 \times 1.0$ cm$^2$, substrates were chemically cleaned sequentially in trichloroethylene (3 min), acetone (5 min), and isopropanol (5 min) in an ultrasonic bath, and then blown dry with pure nitrogen before inserting the substrates through a load-lock into the deposition chamber.

All substrates were pre-heated in vacuum at 900 °C for about one hour before deposition to produce a clean well-ordered surface. A negative bias voltage of -30 V was applied to the substrate to attract

ions from the plasma. To further enhance the ion flux a solenoid surrounding the substrate which dynamically coupled to the magnetic field of each of the type II magnetrons was utilized [13]. A series of 300-nm-thick single-layer thin films were synthesized by co-sputtering of Ti and TiB$_{2}$ at T$_{s}$ = 600 °C and p$_{Ar}$ = 4 mTorr which are the optimum growth conditions for depositing CrB$_{x}$/TiB$_{y}$ SLs [1]. The relative powers to the TiB$_{2}$ and Ti magnetrons were varied from 120 to 70 W and 0 to 50 W, respectively, at a constant total power of P$_{TiB2}$ + P$_{Ti}$ = 120 W. Selected TiB$_{y}$ single layers were grown at T$_{s}$ = 750 °C and 900 °C. A first series of CrB$_{x}$/TiB$_{y}$ SLs with a modulation period of $\Lambda = 6$ nm, thickness ratio $\Gamma = \frac{D_{TiB_y}}{\Lambda} \approx 0.7$, and N = 50 bilayers was studied using co-sputtering for TiB$_{y}$ layers with P$_{Ti}$ = 20 and 25 W, at two different growth temperatures T$_{s}$ = 600 °C and 750 °C. A second series of SLs with different thickness ratios $\Gamma \approx 0.3, 0.5$, and 0.7 was deposited at a substrate temperature of T$_{s}$ = 750 °C with $\Lambda = 6$ nm, N = 50, at the optimum applied power of P$_{TiB2}$/P$_{Ti}$ = 100 W/20 W. For growing SLs, the applied power to the CrB$_{2}$ target was 80 W, yielding a CrB$_{x}$ deposition rate of 0.066 nm/s. For TiB$_{y}$ with P$_{Ti}$ = 20–25 W and a constant total power of P$_{TiB2}$ + P$_{Ti}$ = 120 W, the deposition rate was 0.055 ± 0.005 nm/s. Information about how to calculate the deposition rate in SL structures is described in [14].

X-ray diffraction (XRD) θ-2θ was performed using a Panalytical X'Pert Bragg-Brentano diffractometer with a Cu X-ray source, operating at 45 kV and 40 mA, generating Cu- Kα X-rays. On the primary side a Bragg-Brentano HD mirror was used with a ½° divergence slit and a ½° antiscatter slit, and on the secondary side a 5-mm antiscatter slit was used together with an X'celerator detector operating in scanning line mode. The elemental compositions of the films were determined using time-of-flight elastic recoil detection analysis (ToF-ERDA) and was carried out with a 36 MeV $^{127}$I$^{8+}$ probe beam incident at 67.5° with respect to the sample surface normal and the recoils were detected at an angle of 45° [15]. The Linköping double-corrected FEI Titan³ 60–300 microscope, operated at 300 kV was used for (scanning) transmission electron microscopy ((S)TEM) studies. The microstructure and local composition were investigated by STEM high angle annular dark field (HAADF-STEM) imaging and electron energy loss spectroscopy (EELS). Cross-sectional TEM sample preparation was performed using mechanical polishing followed by Ar ion milling. Plan-view TEM samples were prepared by conventional cutting and cleaving followed by a gentle
focused ion beam (FIB) milling [16]. X-ray reflectivity (XRR) was performed using a Panalytical Empyrean diffractometer with a Cu X-ray tube, operating at 45 kV and 40 mA. On the primary side, a parallel beam X-ray mirror was used in combination with a 1/32° divergence slit and a parallel plate collimator was used in combination with a collimator slit at the reflected beam using a PiXcel-3D area detector in OD mode. Neutron reflectivity (NR) was conducted at the Institut Laue-Langevin in Grenoble with the Swedish neutron reflectometer Super-ADAM [17]. The measurement was performed using a monochromatic wavelength of 5.183 Å and a sample-to-detector distance of 250 cm. Footprint correction was applied using dedicated data reduction software at the beamline.

### 3. Results and discussion

To explore if co-sputtering of TiB₂ and Ti can dilute the amount of B in the over-stoichiometric TiB₃ layers without causing any amorphization, a series of TiB₃ single layers were grown with different relative applied power to the Ti and TiB₂ targets at Tₛ = 600 °C, and evaluated with respect to composition and quality as presented in Supplementary Materials (SM). The y value in TiB₃ was gradually decreased from 3.3 to 0.9 by increasing the relative applied power to the Ti vs. TiB₂ target. TiB₂.₃ gave optimal crystalline quality and stoichiometry (Fig. S1 in SM). The crystalline quality of TiB₂.₃ further improved by increasing Tₛ up to 750 °C, as discussed and shown in Fig. S2 in SM.

Fig. 1(a-f) compare plan-view HAADF-STEM micrographs and color-coded Ti-L₂₃ (red), B-K (green), and O-K (blue) EELS elemental maps from the non-co-sputtered TiB₃.₃ grown at Tₛ = 600 °C and co-sputtered TiB₂.₃ grown at Tₛ = 750 °C. Low-magnification and lattice-resolved plan-view micrographs of TiB₃.₃ in Fig. 1(a) and 1(b) show bright contrast crystalline domains embedded in a matrix of low-density. The corresponding EELS map in Fig. 1(c) indicate that the excess B in this single layer segregates to the column boundaries, forming an amorphous B-rich tissue phase. This is a typical nanostructure of TiB₂ thin films grown by magnetron sputtering [18,19].

Fig. 1(a) and 1(d) show that crystalline domains in TiB₂.₃ are approximately ten times larger across, than those in the highly over-stoichiometric TiB₃.₃. The lattice-resolved images in Fig. 1(b) and 1(e) reveal that the larger domains in TiB₂.₃ are single crystalline in nature, exhibiting clear lattice fringes in all domains, showing a higher degree of crystalline order than in TiB₃.₃. Fig. 1(f) shows EELS elemental map from a junction between multiple domain boundaries in TiB₂.₃. The map indicates that the boundaries are B rich, while the junctions are O rich, proving the formation of large porosities along boundary junctions where atmospheric oxygen can enter the porous TiB₂.₃, possibly deteriorating the film properties [20]. This is supported by ToF-ERDA that shows 9 at. % O in the TiB₂.₃ single layer grown at Tₛ = 750 °C, as compared to 1.6 at. % O in the TiB₃.₃ layer.

Fig. 1(g) shows the corresponding XRD 0-2θ scans of TiB₃.₃ and TiB₂.₃ single layers. Comparatively higher intensity of the 000l peaks in TiB₂.₃ and absence of 1011-oriented crystallites indicate higher crystalline quality than for the TiB₃.₃ owing to the reduced amount of B in the film.

After achieving high crystalline quality TiB₂.₃ by co-sputtering at Tₛ = 750 °C, we investigated its impact on improving epitaxial growth of 000l-oriented CrBₓ/TiBᵧ SLs on c-plane sapphire. Without employing Ti co-deposition, CrBₓ/TiBᵧ SLs with highly non-stoichiometric individual layers were obtained at an optimum temperature of Tₛ = 600 °C [1]. The established, considerably higher, optimum growth temperature for TiB₂.₃ by Ti co-sputtering, implies that a higher Tₛ may be expected also for SLs. In addition to y = 2.3, the lower B/Ti ratio y = 2.1, established for TiB₃ single layers, would be of interest for SLs for achieving a closer-to-stoichiometry composition. Thus, a series of CrBₓ/TiBᵧ SLs were grown using TiB₂ + Ti co-sputtering with the optimal deposition parameters for TiB₂.₃ and TiB₂.₁, at two different growth temperatures Tₛ = 600 and 750 °C with the modulation period Λ = 6 nm and thickness

<figure><img src="image_6.png" /><figcaption>Fig. 2. XRD 0-2θ scans for superlattices with different y values corresponding to TiB₃ single layers, grown at Tₛ = 600 and 750 °C. Modulation period Λ = 6 nm, thickness ratio Γ ≈ 0.7, and number of bilayers N = 50 were kept constant. The average 000l superlattice peaks are marked by (*).</figcaption></figure>

ratio of Γ = D<sub>TiB<sub>3</sub></sub>/Λ ≈ 0.7. For comparison, a SL with the same Λ and Γ was grown without Ti co-sputtering at Tₛ = 600 °C (the same growth conditions as in [1]). The large relative thickness of TiB₃ Γ ≈ 0.7 was chosen to give a large impact on reducing excess B leading to an improved nanostructure by co-sputtering. Fig. 2 shows the XRD 0-2θ data of CrBₓ/TiBᵧ SLs for y = 3.3, 2.3, and 2.1, corresponding to TiB₃ single layers. The intensities of the 0001 and 0002 SLs average peaks (marked by * in the figure) and the intensities and number of visible satellite peaks, indicate that the SL grown at Tₛ = 750 °C with TiB₂.₃ has the highest crystalline quality.

Compositional analyses show that the average B/transition metal (TM) ratio for the co-sputtered SLs is about 2.1, and for the non-co-sputtered SL is 2.7. In addition, there is a considerable decrease in the oxygen concentration (see in Fig. 2) from 4.6 at. % to 0.3 at. % when increasing Tₛ from 600 °C to 750 °C in SLs with TiB₂.₃. A slightly higher oxygen concentration with a similar decreasing effect (from 6.1 at. % to 1.3 at. %) upon increasing Tₛ is observed for SLs grown with optimum deposition parameters for TiB₂.₁ layers. The reduced oxygen content is attributed to a higher crystalline quality of SLs grown at 750 °C (for both y = 2.3 and 2.1) compared to those grown at 600 °C. This is in qualitative agreement with the observations of reduced oxygen content with improved crystal quality upon increased growth temperature as observed in ZrB₂ epilayers [21].

The SL grown from a single TiB₂ target exhibits an oxygen concentration of 0.9 at. % which is much lower than SLs synthesized at the same temperature Tₛ = 600 °C using co-sputtering. This may be explained by the elimination of O-rich free volume by segregation of excess B from the TiB₂.₃ layers to triple points and domain boundaries which is known to occur in CrBₓ/TiBᵧ SLs grown under these conditions [1]. A higher layer definition in SLs synthesized using single TiB₂ target compared to the SLs co-sputtered at Tₛ = 600 °C is supported by the observation of more intense and higher order satellite peaks around the 000l average crystal peaks. However, this SL exhibits a poor crystalline quality as evidenced by low intensities of the average SL crystal peaks, the emergence of an additional peak indicating partial 1011 orientation, and with a B/TM ratio of 2.7. Hence, it is unfavourable compared to the close-to-stoichiometric SL grown at Tₛ = 750 °C which, based on the XRD, exhibits superior crystal quality and SL layer definition. Therefore, SLs
<figure><img src="image_7.png" /><figcaption>Fig. 3. (a) XRD θ-2θ scans for CrB<sub>1.7</sub>/TiB<sub>2.3</sub> superlattices with different thickness ratios Γ ≈ 0.7, 0.5, and 0.3. Period Λ = 6 nm and number of layers N = 50 were kept constant. The average 0002 superlattice peaks are marked by (*) and the order of satellites are shown by ± integers. (b, c, d) cross-sectional HAADF-STEM micrographs of CrB<sub>1.7</sub>/TiB<sub>2.3</sub> superlattices (corresponding to (a)) with Γ ≈ 0.3, 0.5, and 0.7.</figcaption></figure>

with high structural quality and close-to-stoichiometry composition can be achieved by applying the optimized Ti and TiB₂ growth conditions for synthesizing TiB_y single layers with y = 2.3 at T_s = 750 °C. The oxygen concentration in this SL (0.3 at. %) is significantly less than that in single layer TiB₂.₃ (9.0 at. %), which is due to a change from porous columnar epitaxial growth to a dense 2D epilayer growth, by the periodic introduction of CrBₓ layers which are grown with a relatively higher adatom mobility, as discussed below.

Fig. 3(a) shows XRD θ-2θ patterns of CrB<sub>1.7</sub>/TiB<sub>2.3</sub> SLs with different thickness ratios Γ ≈ 0.7, 0.5, and 0.3, and a nominal periodicity of Λ = 6 nm for 50 bilayers N = 50, grown at the established optimal T<sub>S</sub> = 750 °C. The high intensity of the average 0002 Bragg-peaks reveals high crystalline quality for all three thickness ratios. The Γ ≈ 0.3 SL exhibits a significantly higher 0002 Bragg-peak, surrounded by a higher number and more intense satellites, indicative of higher structural quality with more well-defined layers than the SLs with relatively thicker TiB<sub>2.3</sub> layers (Γ ≥ 0.5). In Fig. 3(b-d), cross-sectional HAADF-STEM micrographs of SLs with Γ ≈ 0.3, 0.5, and 0.7 are shown. The micrographs show the presence of flat layers with relatively abrupt interfaces between the individual layers in the SLs with different thickness ratios where bright and dark layers correspond to CrB<sub>1.7</sub> and TiB<sub>2.3</sub>, respectively. The darker regions (one of them marked with an arrow in Fig. 3 (d)) are B-rich tissue phases which are attributed to B segregation in the over-stoichiometric TiB<sub>2.3</sub> layers [1]. In Fig. 3(b) the SL with Γ ≈ 0.3 exhibits only faint indication of B segregation and overall very flat layers which, together with the strong XRD SL peaks (Fig. 3(a)), is evidence of significant improvements in both crystal quality and layer definition of the SL by reducing the B stoichiometry in the TiB_y layers to y = 2.3, as compared to y = 3.3 in previous work. By the same comparison, B segregation and crystal quality are significantly improved also for Γ ≈ 0.5 and 0.7, although it is seen in Fig. 3(c and d) that the excess B in the TiB<sub>2.3</sub> layers causes more severe B segregation and crystal quality

deterioration as the overall B content in the SLs increases with higher TiB<sub>2.3</sub> relative thickness. No roughness accumulation is observed in the low-magnification STEM micrographs (not shown) of these SLs. For all three Γs, it is clearly seen in Fig. 3(b-d) that interfaces where CrB<sub>1.7</sub> is over-grown by TiB<sub>2.3</sub> are more abrupt than the TiB<sub>2.3</sub>-to-CrB<sub>1.7</sub> interfaces.

In contrast to HAADF-STEM, XRR and NR measurements include a large sample volume and the interface quality can be quantified by corresponding simulations. In Fig. 4, the XRR and NR data of the SL with Λ = 6 nm and Γ ≈ 0.3 (same as the HAADF-STEM shown in Fig. 3(b)) are shown along with the best fit simulations using the GenX code [22]. High intensity Bragg-peaks in both XRR (Fig. 4(a)) and NR (Fig. 4(b)) indicate well defined SL layers with abrupt interfaces which is in qualitative agreement with the observed XRD pattern in Fig. 3(a). The simulations were performed using the same model for both XRR and NR with coupled structural parameters for simultaneous fitting, and they show an excellent agreement with the experimental data regarding the period Λ and thickness ratio Γ. To obtain a good fit, an increasing interface width σ for each subsequent period throughout the SL stack was included in the model in order to take into account accumulated roughness resulting in reduced reflected intensity. The initial interface widths σ near the substrate, averaged over the 1.0 x 1.0 cm<sup>2</sup>, of the CrB<sub>1.7</sub>-to-TiB<sub>2.3</sub> and TiB<sub>2.3</sub>-to-CrB<sub>1.7</sub> interfaces are found to be 0.4 Å and 1.0 Å, respectively, with interface width evolutions of 0.27 Å/period for CrB<sub>1.7</sub>-to-TiB<sub>2.3</sub> and 0.66 Å/period for TiB<sub>2.3</sub>-to-CrB<sub>1.7</sub>.

The asymmetric interface widths and their evolution during SL growth observed by TEM and revealed by XRR and NR, indicate significantly different conditions for nucleation and growth of TiB<sub>2.3</sub> and CrB<sub>1.7</sub>. The CrB<sub>1.7</sub> exhibits 2D growth which provides flat surfaces for TiB<sub>2.3</sub> nucleation forming abrupt interfaces. Each TiB<sub>2.3</sub> layer is observed to develop a rough surface, which is most pronounced for thicker TiB<sub>2.3</sub> layers, which then are smoothened by subsequent 2D
<figure><img src="image_8.png" /><figcaption>**Fig. 4.** XRR (a) and NR (b) data for a CrB<sub>1.7</sub>/TiB<sub>2.3</sub> superlattice with thickness ratios Γ ≈ 0.3, modulation period Λ = 6 nm and number of layers N = 50. The obtained best fits for both XRR and NR simulations are shown in red. The simulations were performed simultaneously for both X-ray and neutron data with coupled structural parameters (see main text for details). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

growth of CrB<sub>1.7</sub>. We submit that such a growth scenario is possible by noting a large difference in homologous temperature (T<sub>H</sub>) between CrB<sub>2</sub> and TiB<sub>2</sub> at T<sub>S</sub> = 750 °C (T<sub>H</sub>CrB<sub>2</sub> = 0.4 and T<sub>H</sub>TiB<sub>2</sub> = 0.3), where T<sub>H</sub> = (T<sub>S</sub>/T<sub>M</sub>) and T<sub>M</sub> = absolute melting temperature, that would lead to a kinetically limited growth of TiB<sub>2.3</sub>, while the CrB<sub>1.7</sub> growth is characterized by a high adatom mobility. That would place the CrB<sub>1.7</sub> close to the sweet spot for ion-assisted epitaxial growth in the Thornton structure zone diagram [23] modified by Anders [24], while the high T<sub>M</sub> for TiB<sub>2.3</sub> places it in the region characterized by columnar grain growth. This model may also explain how high-quality epitaxial growth of CrB<sub>1.7</sub>/TiB<sub>2.3</sub> SLs are obtained while growth of TiB<sub>2.3</sub> single layer occurs in a porous columnar epitaxial fashion under the same conditions.

## 4. Conclusions

Co-sputtering from Ti and TiB<sub>2</sub> targets provides significant control over the B content in TiB<sub>y</sub> (0.9 ≤ y ≤ 3.3), enabling the growth of high crystal quality CrB<sub>x</sub>/TiB<sub>y</sub> (0001) diboride superlattices (SLs). Close-to-stoichiometry single layer TiB<sub>2.3</sub> grown at 750 °C represents the optimum growth conditions producing a columnar epitaxial film with a high (9 at. %) oxygen content due to porous column boundaries. The same growth conditions yield single crystal growth of CrB<sub>1.7</sub>/TiB<sub>2.3</sub> (0001)/Al<sub>2</sub>O<sub>3</sub>(0001) SLs with an oxygen content of only 0.4 at. % which is attributed to a transition from a 3D columnar growth mode for single layer TiB<sub>2.3</sub>, to a dense epitaxial 2D growth when introducing nm-thin CrB<sub>1.7</sub> layers, featuring a relatively high adatom mobility, to form SLs. Within the SLs, TiB<sub>2.3</sub> layers display rough top interfaces indicating a kinetically limited growth, while CrB<sub>1.7</sub> forms layers with flat and abrupt top interfaces with a smoothening effect, indicating epitaxial

growth with high adatom mobility. The rough top surfaces of the TiB<sub>2.3</sub> layers results in locally abrupt interfaces with an accumulating roughness evolution. The structural analyses show that SLs grown at 750 °C exhibit the highest crystalline quality and the highest layer definition.

### CRediT authorship contribution statement

**Samira Dorri:** Writing – review & editing, Writing – original draft, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Naureen Ghafoor:** Writing – review & editing, Data curation. **Justinas Palisaitis:** Writing – review & editing, Investigation, Data curation. **Sjoerd Stendahl:** Data curation, Investigation, Writing – review & editing. **Anton Devishvili:** Formal analysis, Investigation, Writing – review & editing. **Alexei Vorobiev:** Data curation, Formal analysis, Writing – review & editing. **Fredrik Eriksson:** Writing – review & editing, Data curation. **Per O. Å. Persson:** Writing – review & editing, Funding acquisition, Data curation. **Jens Birch:** Writing – review & editing, Supervision, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Data availability

Data will be made available on request.

### Acknowledgments

The authors acknowledge financial support from Swedish National Graduate School in Neutron Scattering (SwedNess), Swedish Foundation for Strategic Research (SSF) GSn15 - 0008, Swedish Government Strategic Research Area in Materials Science on Advanced Functional Materials (AFM) at Linköping University (Faculty Grant SFO Mat LiU No. 2009 00971), Center in Nanoscience and technology at LiTH CeNano, Hans Werthén Foundation (IVA), Society of Vacuum Coaters Foundation (SVCF), Swedish Research Council (VR) Grant numbers 2019-00191 (for accelerator-based ion-technological center in tandem accelerator laboratory in Uppsala University), VR and SSF for access to ARTEMI, the Swedish National Infrastructure in Advanced Electron Microscopy (2021-00171 and RIF21-0026), Knut and Alice Wallenberg Foundation for the support of the electron microscopy laboratory at Linköping University (KAW 2015.0043), VR 2021-00159 for the Swedish neutron reflectometer SuperADAM at ILL 10.5291/ILL-DATA.CRG-2964, and Plansee GmbH for providing diboride targets.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.apsusc.2024.159606.

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