Microstructure evolution in amorphous Hf-B-Si-C-N high temperature resistant coatings after annealing to 1500 °C in air

Recently, amorphous Hf-B-Si-C-N coatings found to demonstrate superior high-temperature oxidation resistance. The microstructure evolution of two coatings, Hf7B23Si22C6N40 and Hf6B21Si19C4N47, annealed to 1500 °C in air is investigated to understand their high oxidation resistance. The annealed coatings develop a two-layered structure comprising of the original as-deposited film followed by an oxidized layer. In both films, the oxidized layer possesses the same microstructure with HfO2 nanoparticles dispersed in an amorphous SiOx-based matrix. The bottom layer in the Hf6B21Si19C4N47 coating remains amorphous after annealing while Hf7B23Si22C6N40 recrystallized partially showing a nanocrystalline structure of HfB2 and HfN nanoparticles separated by h-Si3N4 and h-BN boundaries. The HfB2 and HfN nanostructures form a sandwich structure with a HfB2 strip being atomically coherent to HfN skins via (111)-Hf monolayers. In spite of the different bottom layer structure, the oxidized/bottom layer interface of both films was found to exhibit a similar microstructure with a fine distribution of HfO2 nanoparticles surrounded by SiO2 quartz boundaries. The high-temperature oxidation resistance of both films is attributed to the particular evolving microstructure consisting of HfO2 nanoparticles within a dense SiOx-based matrix and quartz SiO2 in front of the oxidized/bottom layer interface acting as a barrier for oxygen and thermal diffusion.

elemental composition, structure, mechanical, electrical and optical properties, and oxidation resistance of the films 32 . Moreover, the effects of shortened (50 µs compared to 85 µs) voltage pulses on the film properties were investigated for selected N 2 fractions in the gas mixture. Here, it should be mentioned that the negative voltage pulses on the sputtered target need to be sufficiently short to avoid micro-arcing at the target and thus, to produce high-quality defect-free films with low surface roughness. In paper 32 , we demonstrated very high oxidation resistance in air up to 1500 °C for two sufficiently hard (20)(21)(22) Hf-B-Si-C-N films deposited at the voltage pulse length of 50 µs: the electrically conductive Hf 7 B 23 Si 22 C 6 N 40 film prepared at the 15% N 2 fraction in the gas mixture, and the optically transparent Hf 6 B 21 Si 19 C 4 N 47 film prepared at the 25% N 2 fraction in the gas mixture, both with a contamination level <3 at.%.
In this work, the Hf 7 B 23 Si 22 C 6 N 40 and Hf 6 B 21 Si 19 C 4 N 47 films were prepared on SiC substrates and annealed in air up to 1500 °C. Comprehensive high-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED) studies were conducted to determine the microstructures that develop during high temperature exposure in these films and to understand their superior high-temperature oxidation resistance and the important effect of nitrogen incorporation (not investigated in 13 ).
This study is a part of an overall program of our laboratories at UTA and UWB conducted to develop new, sufficiently hard, thin-film materials in the Hf-B-Si-C-N system with a controlled electrical conductivity and optical transparency, and with ultrahigh thermal stability in air. When the excellent oxidation resistance at elevated temperatures of thin-film materials is combined with a high optical transparency (e.g., as in the case of Si-B-C-N films 30,31 ), they can be used for high-temperature protective coatings of electronic and optical elements. Alternatively, a combination of the high oxidation resistance and electrical conductivity is desirable for harsh-environment sensors, such as capacitive pressure, vibration and tip clearance sensors (e.g., for advanced gas turbine engines [33][34][35][36]. The pulse magnetron sputter technique used makes it possible to deposit densified films onto substrates at a floating potential (i.e., without any substrate bias) due to increased kinetic energies of sputtered target material and process gas ions at the substrates, caused by the high positive voltage overshoots after the negative voltage pulses 32 . This is of key importance for industrial applications, particularly for production of thin-film materials on large area non-conductive substrates (no rf-induced bias needed). experimental Material preparation. The Hf 7 B 23 Si 22 C 6 N 40 and Hf 6 B 21 Si 19 C 4 N 47 films were synthesized in a Balzers BAS 450PM sputtering system. The films were deposited on polished and ultrasonically cleaned SiC single crystal substrates using reactive magnetron sputtering from a single B 4 C-Hf-Si target in an argon-nitrogen gas mixture. The target (127 mm × 254 mm) was prepared by positioning p-type Si and Hf stripes on a B 4 C plate (6 mm thick) with fixed 65% B 4 C + 15% Hf + 20% Si fractions in the target erosion area. A pulsed dc power supply (Rübig MP120) was used to drive the magnetron operating at a 10 kHz repetition frequency with a 500 W average target power and a fixed 50 µs negative-voltage pulse length with a short-lived high positive voltage overshoot (higher than 200 V, as shown in 32 ) after the negative voltage pulses. During sputtering, the voltage pulse duration of 50 µs is sufficiently short to avoid micro-arcing at the non-conductive layer formed on the B 4 C-Hf-Si target (for details see 32 ). The base pressure prior to deposition was 1 × 10 −3 Pa. The total pressure of the gas mixture during the deposition was 0.5 Pa with a N 2 fraction being 15% and 25% for Hf 7 B 23 Si 22 C 6 N 40 and Hf 6 B 21 Si 19 C 4 N 47 film, respectively. The distance from target to substrate was 100 mm and the substrate temperature was maintained at 450 °C. The substrates were held at a floating potential. These conditions produced a film thickness of 1450 nm and 1240 nm for the Hf 7 B 23 Si 22 C 6 N 40 and Hf 6 B 21 Si 19 C 4 N 47 , respectively. Rutherford backscattering spectrometry (RBS) and elastic recoil detection (ERD) with a Van de Graaf generator with a linear electrostatic accelerator were used to determine the film composition.
The annealing (annealing denotes a high temperature exposure and oxidation in air) experiments were conducted from room temperature up to 1500 °C using a symmetrical high resolution Setaram TAG 2400 system in synthetic atmospheric air (flow rate of 1 l/h). The heating and cooling rate was 10 °C/min and 30 °C/min, respectively.
Microstructure characterization. The crystallographic structure of the annealed films was first studied by X-ray diffraction (XRD) θ-2θ measurements utilizing a Bruker D8 Advance Diffractometer with Cu Kα radiation at 40 kV voltage and 40 mA current. The film microstructures were then studied by cross-section and plan-view TEM. Thin foil specimens were prepared by the procedure of mechanical grinding, polishing, and dimpling, followed by Ar-ion milling. A Hitachi H-9500 electron microscope was utilized operated at 300 keV with a point resolution of 0.18 nm.

Results and Discussion
XRD analysis. The as-deposited and annealed films were first examined by θ-2θ XRD analysis. Both as-deposited Hf-B-Si-C-N films have a pure amorphous structure as identified by XRD and further confirmed by TEM studies (see later section). Figure 1  www.nature.com/scientificreports www.nature.com/scientificreports/ The XRD spectrum of the annealed Hf 7 B 23 Si 22 C 6 N 40 film shows additional peaks besides those present in the spectrum of the annealed Hf 6 B 21 Si 19 C 4 N 47 film. The peak at an angle of 39.48°, and broad peaks at angles of 56.70° and 68.00° match very well with the (200), (220) and (311) of face centered cubic (fcc) HfC 0.5 N 0.5 (PDF#04-002-2469, a = 4.586 Å, Fm-3m), respectively. Note that the (111) HfC 0.5 N 0.5 peak at 33.8° is on the left shoulder of the mHfO 2 (020) peak. Also, the peaks at 25.80°, 42.03° and 58.79° match well with the (001), (101) and (110) of HfB 2 (PDF#38-1398, hexagonal, a = 3.14 Å, c = 3.47 Å, P6/mmm) 37 . It is noted that the peak at 25.80° can also match a turbostratic structure of BN or B(C)N 38 as reported previously 27 . Thus, in addition to HfO 2 , the annealed Hf 7 B 23 Si 22 C 6 N 40 film showed formation of Hf(C)N solid solution, HfB 2 and possibly BN. The XRD data and corresponding phases are summarized in Table 1. teM studies. Overall film structure. We have employed cross-section TEM for a detailed microstructure study of the annealed films. Figure 2(a,b) presents cross-section TEM images of the annealed Hf 6 B 21 Si 19 C 4 N 47 and Hf 7 B 23 Si 22 C 6 N 40 films, respectively. Both annealed films have a two-layered structure: a nanocomposite layer with discrete HfO 2 nanoparticles embedded in an amorphous SiO x -based matrix on the top surface followed by the remaining of the original as-deposited film at the bottom. A similar film morphology with some distinct differences however (see later section), has also been observed in the annealed Hf 7 B 23 Si 17 C 4 N 45 film studied previously 12,13 . The top composite layer in the present annealed films has nearly the same thickness of ~360 nm and exhibits nearly the same microstructure. The entire film thickness (the total of both layers) is ~1300 nm and ~1600 nm for the Hf 6 B 21 Si 19 C 4 N 47 and Hf 7 B 23 Si 22 C 6 N 40 film, respectively. Figure 2(c) is a SAED pattern of the top  www.nature.com/scientificreports www.nature.com/scientificreports/  www.nature.com/scientificreports www.nature.com/scientificreports/ As noted earlier, the top oxidized layer in both annealed films exhibited a similar microstructure. Figure 3(a) is a plan-view TEM image of the top nanocomposite oxidized layer of the annealed Hf 7 B 23 Si 22 C 6 N 40 film showing a large number of HfO 2 nanoparticles dispersed in a dense SiO x -based matrix. The HfO 2 nanoparticles are mostly spherical with their size varying from ~15 nm to ~80 nm. It can also be seen that in some cases, nanoparticles merge together producing an ellipsoidal shape. HRTEM studies demonstrated the clear matrix has an amorphous structure. The presence of the m-HfO 2 and o-HfO 2 nanoparticles in this layer was further verified by HRTEM imaging. For example, the nanoparticle P 1 in Fig. 3(b) presents two sets of lattice fringes with a spacing of 2.82 Å and 2.63 Å corresponding to (111) and (002) of m-HfO 2 , respectively. Similarly, the nanoparticle P 2 , Fig. 3(b), presents a set of lattice fringes with a spacing of 2.96 Å corresponding to (111) of o-or t-HfO 2 .
Upon annealing of these films, HfO 2 nanoparticles are first nucleated at the oxidized/bottom layer interface followed by particle coarsening as the top surface is approached, Fig. 2. Subsequently, Si and more than likely B are oxidized forming a protective borosilicate matrix 13 . It has been shown that above 1100 °C, B 2 O 3 reacts with SiO 2 forming protective borosilicate glass that fills the pores in the amorphous SiO 2 structure 39 resulting in a dense amorphous SiO x -based matrix as shown in Fig. 3(a). Figure 4(a) is a typical HRTEM image of the bottom layer of the annealed Hf 6 B 21 Si 19 C 4 N 47 film showing a pure amorphous structure. The regions with darker contrast more than likely correspond to Hf-rich areas and the brighter regions to light element (B, Si, C, N) rich regions. Figure 4(b) is a bright-field TEM image of the bottom layer of the annealed Hf 7 B 23 Si 22 C 6 N 40 film presenting a quite different microstructure to that shown in Fig. 4(a). Faceted nanoparticles with clear and sharp edges were formed in this bottom film layer. The size of these faceted nanoparticles varies from a few nm up to 20 nm. The nanoparticles are expected to be Hf-rich structures (i.e., HfN, HfB 2 ) whereas the boundary regions are rich in light elements. The  www.nature.com/scientificreports www.nature.com/scientificreports/ boundary regions have a very complex structure and HRTEM was intensively used for a comprehensive study of its microstructure as described in the following section.  Fig. 4(b).

Microstructure of the bottom layer.
Besides the presence of h-Si 3 N 4 , curved lattice fringes with similar characteristics to an onion structure were frequently observed in the boundary areas, as shown in the S 2 region in Fig. 5(a). The spacing between the curved lattice fringes is about 3.4 to 3.5 Å, which is very close to the lattice spacing of the (002) of hexagonal BN, which is about 3.33 Å. The nucleation and formation of h-BN has been found in our previous studies of Si-B-C-N film annealed up to 1700 °C in air 28 . The spacing of the curved BN lattice fringes in the latter film was about 3.42 Å, 3% larger than the (002) lattice fringes in the bulk BN, which matches well with the result in the current study. In fact, the somewhat larger lattice spacing can be due to incorporation of C into BN forming a turbostratic B(C)N structure as reported earlier 27,38 . Figure 5(c) presents a HRTEM image and the corresponding FFT from another boundary region showing the curved lattice fringes of BN, in which in addition to the primary lattice fringes of (002)-BN, another set of fringes with a spacing of 2.19 Å corresponding to (100) of BN were observed. This provides solid evidence for the formation of BN in the boundary region in the bottom layer of the annealed Hf 7 B 23 Si 22 C 6 N 40 film.
The thin, long strip designated by S 3 in Fig. 5(a), corresponding to a dark area in Fig. 4(b), was identified to be HfB 2 viewed along its [010] direction. The horizontal lattice fringes have a spacing of 3.45 Å corresponding to (001) HfB 2 while the vertical fringes have a spacing of 2.71 Å corresponding to (100) HfB 2 . The crystal S 4 underneath the S 3 HfB 2 strip was identified to be fcc HfN viewed along its [110] direction. One set of the {111} planes are parallel to the (001) of HfB 2 . The two regions on the top of the HfB 2 strip designated as S 5 and S 6 are also HfN domains. The lattice fringes in the two regions have a spacing of 2.67 Å, corresponding to (111) of HfN. Figure 5(a) shows that the HfN nanoparticles S 4 , S 5 , and S 6 are atomically coherent to the HfB 2 strip S 3 . After intensive examinations of several other areas, we found that the HfB 2 crystals were formed in a strip shape and always co-existed with HfN nanoparticles via atomically coherent interfaces. It was interesting to note that even though the HfN nanoparticles can nucleate independently, the HfB 2 nanostructures were always formed coherently on interfaces with HfN. This tends to indicate that HfB 2 forms first out of the amorphous structure and its (001) acts as a low energy nucleation site for the HfN phase. Such a sequence of formation is consistent with the lower enthalpy of formation of HfB 2 compared to HfN 40 . www.nature.com/scientificreports www.nature.com/scientificreports/ This coherent joining of the two different structures is not quite surprising and can be understood. HfB 2 is a close packed hexagonal structure and can be obtained by sequentially packing the close-packed (001) monolayers of Hf (label as "A Hf ") and B (label as "B B ") along the [001] direction, A Hf B B A Hf B B A Hf B B …, Fig. 6(b). Due to its small size, B atoms can fill the two types of interstices on the top of the Hf monolayer, resulting in the atomic B:Hf ratio of 2:1. HfN is a fcc structure and can be obtained by sequentially packing the closed-packed (111) monolayer of Hf (labelled as A Hf , B Hf , C Hf ) and N (labelled as A N , B N and C N ) in the way of A Hf B N C Hf A N B Hf C N A Hf B N C Hf A N B Hf C N …, Fig. 6(c). The Hf monolayers in both HfB 2 and HfN have the same arrangement. The shortest Hf-Hf bonds (also the major bonds) are 3.21 Å in HfN and 3.14 Å in HfB 2 (with a small mismatch of ~2%), indicating that the Hf monolayers in HfN and HfB 2 can be interchangeable. Therefore, it is quite conceivable that HfB 2 and HfN can coherently join to each other by sharing the Hf monolayer, for example,---A Hf B B A Hf B B A Hf B N C Hf A N B Hf C N ---.  Fig. 6(f). Actually, the small size of the HfB 2 strip (~2 nm) and the misfit coherency strain at the HfN/HfB 2 interface can provide an explanation for the somewhat larger lattice spacing of the (101) HfB 2 (2.2 Å vs 2.14 Å) observed in the SAED pattern for this phase, Fig. 2(e). www.nature.com/scientificreports www.nature.com/scientificreports/ Thus, the present results reveal an interesting crystallization in the bottom layer of the annealed Hf 7 B 23 Si 22 C 6 N 40 film with formation of HfB 2 and HfN nanoparticles surrounded by Si 3 N 4 and BN boundaries. It is more interesting that this crystallization was not observed in the Hf 6 B 21 Si 19 C 4 N 47 film with higher N content. The results clearly show that even a small difference in the N content can stabilize the amorphous structure at high temperatures.

Oxidized/bottom layer interface. The interfaces between the bottom and oxidized layers in both annealed
Hf-B-Si-C-N films were further studied using cross-section HRTEM. In the annealed Hf 6 B 21 Si 19 C 4 N 47 film, densely packed ultra-fine size nanoparticles were nucleated at the interface between the bottom and oxidized layer, Fig. 7(a). These small nanoparticles gradually coarsened into spherical particles (~20 to 30 nm) towards the top surface. Figure 7(c) is a HRTEM image of the interface showing the presence of both m-HfO 2 (particles T 1 and T 3 with (111) lattice fringes and spacing of 2.84 Å) and o-HfO 2 (particles T 2 and T 4 with (101) lattice fringes and spacing of 2.96 Å). The small particles have a size of ~6 nm with an irregular shape (T 1 , T 2 and T 3 ) indicating an early nucleation stage. The larger particle T 4 that has a size of ~14 nm possesses a spherical shape to minimize surface energy. It was interesting to note that the boundaries between HfO 2 nanoparticles and matrix were no longer purely amorphous. High temperature, β-quartz SiO 2 (PDF #11-0252, a = 5.002 Å, c = 5.454 Å, P3221) formation was observed at the boundary area of the HfO 2 particles in the interface region, as evidenced by the presence of (101) lattice fringes with a spacing of ~3.4 Å. Thus, the present evidence suggests that as oxygen diffuses and reaches the interface, it reacts with Hf atoms in the amorphous structure and nucleates HfO 2 nanoparticles. As the amorphous structure is depleted from Hf, Si atoms react with oxygen at the depleted HfO 2 / matrix boundaries forming quartz SiO 2 . In fact, this formation process at the boundaries of the spherical HfO 2 particles seems consistent with the exhibited (101) SiO 2 curved lattice fringes instead of straight lines observed in all HRTEM images. It should also be noted that due to their ultrafine scale and curved lattice planes, detection of such a structure by XRD or even SAED is very difficult. It is also important to note that formation of quartz was observed only in the oxidized/bottom layer interface region and not in the upper oxidized layer where more intense oxidation conditions prevail. Figure 7(b) is a cross-section TEM image of the oxidized/bottom layer interface in the annealed Hf 7 B 23 Si 22 C 6 N 40 film. As shown earlier, the bottom layer in this film has an entirely different microstructure. www.nature.com/scientificreports www.nature.com/scientificreports/ During annealing, the amorphous structure was partially crystalized and resulted in the formation of HfB 2 , HfN, Si 3 N 4 , and BN. Thus, as oxygen diffuses in this film and reaches the interface, oxidation of the individual crystalline phases takes place. Due to differences in the oxidation kinetics of the various crystalline phases in the bottom layer of this film, the oxidized/bottom layer interface is not sharp as the one in the Hf 6 B 21 Si 19 C 4 N 47 film, Fig. 7(a). As shown in Fig. 7(b), a partially oxidized zone develops in the Hf 7 B 23 Si 22 C 6 N 40 film. Just above the interface, spherical HfO 2 particles form along with much enriched SiO 2 crystalline structures, Fig. 7(d), similar to those observed in the Hf 6 B 21 Si 19 C 4 N 47 film, Fig. 7(a). A closer observation of the image in Fig. 7(d) shows extensive formation of curved quartz lattice fringes that could not have been produced with the same process as that in the Hf 6 B 21 Si 19 C 4 N 47 film. Quartz formation in this film is attributed to oxidation of pre-existing hexagonal Si 3 N 4 boundaries around HfN and HfB 2 nanodomains. Thus, a similar microstructure involving a fine distribution of HfO 2 nanoparticles surrounded by quartz boundaries developed in the oxidized/bottom layer interface region of both films.
It is important to note that X-ray energy-dispersive spectroscopy analysis showed that both bottom layers in the annealed films are oxygen free. Thus, two entirely different oxidation processes occur in the two films; the first involves oxidation of individual elements out of the amorphous structure and the second, oxidation of nanocrystalline phases (HfB 2 , HfN, Si 3 N 4 , and BN) formed during annealing. In spite of the differences in the aforementioned oxidation process, it should be emphasized that both films exhibit similar microstructures at the oxidized/bottom layer interface and in the top oxidized layer. Furthermore, both films have been found to possess high oxidation resistance with no mass change after annealing up to 1300 °C 32 . It is interesting to note that even though annealing up to 1500 °C produces an oxidized layer of the same thickness (360 nm) in both films, it also results in some small differences in the oxidation behavior that can be attributed to their microstructures. Film Hf 7 B 23 Si 22 C 6 N 40 exhibits a slow mass gain (less than 10 μg/cm 2 ) while film Hf 6 B 21 Si 19 C 4 N 47 shows negligible mass change 32 . This behavior confirms that both oxidation and volatilization occur simultaneously during annealing. The latter as-deposited film contains more nitrogen and as such, more nitrogen will be released during oxidation resulting in a lower or no mass gain. On the other hand, the former film crystallizes during annealing and therefore, oxygen can penetrate more easily into the film. In this case, oxygen can be present not only in the top 360 nm oxidized layer but also in the partially oxidized interface zone resulting in a mass gain.
Based on the present results, the high temperature oxidation resistance of both films can be attributed to the similar microstructures that develop in the oxidized layer and its interface with the bottom layer in spite of the differences in the oxidation process. In both films, oxidation results in a high density of HfO 2 nanoparticles within a dense amorphous SiO x -based matrix and quartz SiO 2 formation in front of the base layer. In view of the presence of low thermal conductivity HfO 2 (0.49-0.95 Wm −1 K −1 ) nanoparticles, dense SiO x -based matrix and high density SiO 2 nanoquartz (2.5 g/cm 3 ), an effective oxygen and thermal diffusion barrier is produced at the interface that can result in high temperature oxidation resistance. It is important to note that the oxidation behavior of the present films produced with the present new deposition process shows significant improvements from previously studied Hf 7 B 23 Si 17 C 4 N 45 film 13 . Note that the latter film annealed up to 1500 °C exhibited a significantly thicker (544 nm) oxide layer composed of two sublayers. One next to the as-deposited film, composed of a dense population of HfO 2 nanoparticles (produced by nucleation and growth) and a top, surface sublayer with coarsened and dispersed HfO 2 nanoparticles (produced by Ostwald ripening). It is interesting that the top sublayer was absent in the oxide layer of the present films that exhibited a much smaller thickness (360-370 nm, more than 30% reduction). Another distinct difference is that a continuous array of HfO 2 nanoparticles is present at the top surface of the present oxide layers, Fig. 2(a,b). This clearly indicates the high stability of the HfO 2 nanoparticles in the present microstructures more than likely due to high quality, defect-free films produced by the present deposition process.
Finally, it should be noted that the crystallization observed in the Hf 7 B 23 Si 22 C 6 N 40 film can actually be utilized as a route to enhance the hardness of the as-deposited film. Hf 7 B 23 Si 22 C 6 N 40 films annealed in He up to 1300 °C (no oxidation) were found to exhibit a hardness of about 26 GPa due to the evolved nanocrystalline microstructure. Thus, this annealing procedure in an inert environment can be utilized to produce films with high hardness along with excellent high temperature oxidation resistance.

summary
Both annealed Hf-B-Si-C-N films exhibit a two-layered structure comprising the original as-deposited film at the bottom and an oxidized layer at the top. In both films, the oxidized layer is composed of m-HfO 2 and/or o-HfO 2 embedded in an amorphous SiO x -based matrix. The bottom layer in the Hf 6 B 21 Si 19 C 4 N 47 film remains amorphous while that in the Hf 7 B 23 Si 22 C 6 N 40 film is partially crystallized exhibiting a nanocomposite structure of HfB 2 and HfN nanoparticles surrounded by h-Si 3 N 4 and h-BN boundary phases. The HfB 2 and HfN nanoparticles found to form a sandwich structure with a HfB 2 strip core being atomically coherent to HfN skins via a (111) Hf monolayer. The oxidized/bottom layer interface of the Hf 6 B 21 Si 19 C 4 N 47 film is characterized by a high density of HfO 2 nuclei surrounded by SiO 2 quartz boundaries resulting from the oxidation of the amorphous structure. The interface of the Hf 7 B 23 Si 22 C 6 N 40 film shows a similar HfO 2 /SiO 2 quartz boundary microstructure that is a result of oxidation of the crystalline phases (HfN, HfB 2 and h-Si 3 N 4 ) produced during annealing. The high temperature oxidation resistance of these films is attributed to the particular microstructure involving formation of HfO 2 nanoparticles in a SiO x -based matrix along with quartz formation at the oxidized/bottom layer interface that acts as a barrier for oxygen and thermal diffusion.

Data Availability
All experimental deposition conditions and characterization procedures, methods and data are provided in the text. Any clarifications will be available by contacting the corresponding author.