Texture evolution in rhombohedral boron carbide ﬁ lms grown on 4H-SiC(0001¯) and 4H-SiC(0001) substrates by chemical vapor deposition †

Boron carbide in its rhombohedral form (r-B x C), commonly denoted B 4 C or B 13 C 2 , is a well-known hard material, but it is also a potential semiconductor material. We deposited r-B x C by chemical vapor deposition between 1100 °C and 1500 °C from triethylboron in H 2 on 4H-SiC(0001) and 4H-SiC(0001¯). We show, using ToF-ERDA, that pure B 4 C was grown at 1300 °C, furthermore, using XRD that graphite forms above 1400 °C. The ﬁ lms deposited above 1300 °C on 4H-SiC(0001¯) were found to be epitaxial, with the epitaxial relationships B 4 C(0001)[101¯0] k 4H-SiC(0001¯)[101¯0] obtained from pole ﬁ gure measurements. In contrast, the ﬁ lms deposited on 4H-SiC(0001) were polycrystalline. We suggest that the di ﬀ erence in growth mode is explained by the di ﬀ erence in the ability of the di ﬀ erent surfaces of 4H-SiC to act as carbon sources in the initial stages of the ﬁ lm growth.


Introduction
Due to its high hardness of about 35 GPa, 1 rhombohedral boron carbide, commonly denoted B 4 C or B 13 C 2 , here referred to as r-B x C (4 ≤ x ≤ 10.5), is mainly used as a super hard material in applications such as abrasive or plate armour. 2 Additionally, owing to the self-healing capabilities of the boron icosahedra under energetic bombardment 3 and due to the high neutron cross-section of the isotope 10 B, it is a promising material for boron-based neutron detectors. 4][7] Semiconducting r-B x C could be used in combination with the wide bandgap materials boron subphosphide (B 12 P 2 ) 8 and boron subarsenide (B 12 As 2 ), 9 with which it shares structural similarities, and would enable the fabrication of novel radiation resistant heterostructures.In addition, r-B x C is interesting for thermoelectric devices due to its high temperature thermoelectric properties, with a high thermal conductivity (4-10 W m −1 K −1 ) and high Seebeck coefficient (150 to 300 μV K −1 between 500-1000 °C). 4,10Furthermore, another peculiarity of boron carbide is its ability to form crystals with five-fold symmetry due to crystal twinning. 11,12Controlled formation of these twinned crystals could be used to approximate and study boron-carbide-based quasicrystals.However, to fully explore r-B x C as a semiconductor, thin films with high crystalline quality, preferably epitaxially grown are needed, but the growth of epitaxial r-B x C crystals has proved to be challenging 13 using vapor phase deposition techniques.
Using chemical vapor deposition (CVD) with the singlesource precursor triethylboron (TEB) in an H 2 ambient in the temperature range of 1100 to 1500 °C, we investigate the growth conditions for r-B x C films on the Si-terminated (4H-SiC (0001)) and C-terminated (4H-SiC(0001 ˉ)) faces.We report epitaxial growth on the 4H-SiC(0001 ˉ) at 1300 °C, while deposition on 4H-SiC(0001) yields polycrystalline films at the same temperature.

Film deposition
Thin films of r-B x C were deposited on 4H-SiC substrates in a hot-wall horizontal CVD reactor from pyrolysis of 0.7 SCCM triethylboron (TEB, B(CH 2 CH 3 ) 3 , 99.99%, SAFC HiTech) in 5700 SCCM hydrogen gas (H 2 , Palladium membrane purified) for 180 min.The process pressure was regulated to 7000 Pa by a throttle valve and the deposition temperature of 1100-1500 °C, monitored by a pyrometer (Heitronics KT81R, calibrated by silicon melting).The influence of the residence time was studied by either changing the total flow or by utilizing the position of the substrates in the SiC-coated elliptical graphite susceptor.
The temperature distribution and the residence times in the susceptor were evaluated using COMSOL Multiphysics (version 5.2).The susceptor was modelled using a 2D axisymmetric approximation.Since TEB is highly diluted in H 2 , a flow of pure H 2 was considered in these calculations.The temperature profile inside the susceptor (without flow) was described by a parabolic function, as previously calculated for this reactor by Danielsson. 14The residence time t res at a given point x i was defined using the formula: where x is a variable coordinate along the length of the susceptor, x inlet is the coordinate at the inlet of the susceptor and v(x) is the calculated gas velocity at the distance.
Prior to deposition, the 4H-SiC wafers were sawn into 10 × 10 mm 2 pieces.The orientations of the 4H-SiC substrates used were 4H-SiC(0001) and 4H-SiC(0001 ˉ) which conventionally refers to the Si-terminated and C-terminated sides, respectively.The offcut relative to the c-axis was ±0.5°(commonly referred as on-axis).Prior to deposition, the substrates were degreased in acetone for five minutes followed by ethanol for five minutes, both using an ultrasonic bath.This cleaning step was followed by a standard cleaning procedure for semiconductors with etching in oxidating alkaline solution (ammonia solution (25%), hydrogen peroxide solution (48%) and water in respective proportions 1 : 1 : 5) at 80 °C for five minutes followed by a rinse in deionized water and further treatment in hot oxidizing acidic solution (hydrochloric acid solution (37%), hydrogen peroxide (48%) and water in respective proportions 1 : 1 : 6) at 80 °C for five minutes and finally a rinse in deionized water.Additionally, the substrates used for the lowest deposition (1100 °C) temperature were dipped in 2% HF solution to ensure the removal of the native silicon dioxide.

Film characterization
X-ray diffraction (XRD) was performed using Cu Kα radiation.The 2θ/ω diffractograms were recorded using an X'Pert Pro diffractometer equipped with a Bragg-Brentano HD optics with 1/2°divergence slit as primary optics and a X'celerator detector with 5 mm anti-scatter slit, 0.4°Soller slits and Kβ Ni filter as secondary optics.XRD ω-scan (rocking curve measurements) are obtained using the PANAlytical Empyrean MRD and the measurements were performed by using capillary optics (X-ray lens) with a 2 × 2 mm mask on primary optics and a parallel plate collimator with 0.27°slit and a Ni filter as secondary optics.Since the X-Ray lens is not a monochromatic optic module, the use of a Ni filter was necessary to suppress the Cu Kβ line.The integral breadth was obtained by fitting the rocking curves with Pearson VII functions using the fityk software (version 1.3.1).XRD φ-scans and pole figures were recorded using an X'Pert MPD diffractometer equipped with 2 × 2 mm 2 cross-slits and a Kβ Ni filter as primary optics and a proportional detector (PW1711/96) with a parallel plate collimator and a 0.27°slit as secondary optics.
Fourier transform infrared spectroscopy (FTIR) reflectance spectra were measured in a Bruker VERTEX 70 equipment with incident s-polarized light at an angle of 60°with respect to the sample surface normal.The spectra were acquired in reflectance configuration at incidence angle of 30°with respect to the surface normal, a resolution of 2 cm −1 and s-polarization.The measurements were conducted at room temperature, after a 30 min N 2 purge, with 2 cm −1 resolution and averaged over 50 scans.A thin film of gold was used as reference.
Scanning electron microscopy (SEM) was used to study the surface morphology of the films.The microscope used was a Zeiss Gemini.All the micrographs were acquired using an accelerating voltage of 5 kV and an in-lens detector.SEM was also used for thickness measurements of the B x C films deposited on the 4H-SiC(0001); these measurements were performed on the aligned cross sections of the samples.The samples were scratched using a diamond tipped pen and then cracked.The thickness of B x C film grown at 1100 °C was determined to be around 900 nm, those grown at 1200 °C, 1300 °C was around 1.5 µm, the film grown at 1400 °C was about 1.6 µm and the film grown at 1500 °C was determined around 3 µm.
The compositional analysis of selected films was performed using time-of-flight energy elastic recoil detection analysis (ToF-ERDA).The measurements were done using a 36 MeV 127 I +8 beam.The incidence angle of primary ions and exit angle of recoils were both 67.5°to the sample surface normal constituting a recoil angle of 45°.The time-of-flight vs energy loss map is also utilized, this has better separation for lighter elements (B and C).The measured ToF-ERDA spectra data was then converted into relative atomic concentration profiles using the Potku code. 15

Results
Boron carbide deposition on C-face 4H-SiC(0001 ˉ) From the XRD diffractograms in Fig. 1, it is evident that films deposited at 1100 and 1200 °C are polycrystalline, as seen from the additional reflections from crystal planes other than (0003) and higher order diffractions.For higher temperatures, the films are oriented along [0003] since no reflections from crystal planes other than (0003) and higher order diffractions are visible in the diffractograms.metry of the hexagonal 4H-SiC substrate compared to the r-B x C crystal of rhombohedral symmetry.FTIR further supports the XRD data; as observed, the chain stretching mode disappears for r-B x C grown on the C-face compared to r-B x C grown on the Si-face in Fig. S1, ‡ which is consistent with the triatomic chains being perpendicular to the electric field (s-polarized light).The onset of graphite formation is observed from the appearance of sp 2 -Cð000'Þ peaks at 1400 °C in Fig. 1. r-B x C and pyrolytic graphite where found to grow to a similar extent and in a similar fashion, as seen from a comparison of the intensity of r-B x C(0003) and sp 2 -Cð000'Þ) in Fig. 1, and from the top-view micrograph in Fig. 4. Rocking curve measurements of the (0003) planes for samples deposited above 1300 °C (Fig. 3) show a sharp component whose integral breadth is comparable to the substrate as well as a much broader component which decreases as the deposition temperature increases.Rocking curve measurements allows evaluation of the overall degree of alignments of the crystallites along the [001] and [101] directions.The dispersion [001] direction is associated to the tilting of the grains with respect to the vicinal surface normal while [101] comprises an additional rotational component referred as twisting.Further information about the differences in crystal quality between the films grown at the examined temperatures, as determined using rocking curves, is expressed in Fig. 3d.
Plan view SEM (Fig. 4) show that the films deposited on C-face 4H-SiC at 1100 °C (Fig. 4a) and 1200 °C (Fig. 4b) were highly facetted and polycrystalline while the films deposited at higher temperatures were smoother.The films deposited at 1300 °C (Fig. 4c) and 1400 °C (Fig. 4) constituted of percolated islands and continuous films were obtained at 1500 °C (Fig. 4e).The continuous films comprised distinct areas of boron carbide and graphite and hexagonal defects were observed for both materials (Fig. 5).
The sample deposited at 1300 °C was selected for compositional analysis as it was not contaminated with graphite (as seen from Fig. 1).ToF-ERDA yielded B to C ratio of 4 : 1 (B -79.2 at% ± 0.8 at%, C -20.5 at% ± 0.4 at%) and with around 0.3 at% of O and H below the detection limit of the technique.

Boron carbide deposition on Si-face 4H-SiC(0001)
Deposition on Si-face 4H-SiC(0001) substrates resulted in polycrystalline rhombohedral boron carbide as seen from the 2θ/ω diffractogram (Fig. 6) and FT-IR reflectance spectra (Fig. S2 ‡) of a film deposited at 1400 °C.Pyrolytic graphite could also be detected under some conditions (Fig. 7).The orientation of the r-B x C crystals and formation of graphite were found to be strongly dependent on the growth temperature and on the residence time of the gas in the susceptor, as presented in Fig. 7.
The residence time at a given temperature was varied by placing the sample at different positions in the susceptor.
Regardless the deposition temperature, the B x C films deposited at low residence times (below 0.10 s) were polycrystalline with no preferred orientation in contrast with the films deposited at longer residence times which showed preferred orientation.For residence times between 0.10 and 0.12 s this orientation evolved from [022 ˉ1] at 1200 °C, to [0003] at 1300 °C to [101 ˉ4] at 1400 °C, with a minor contribution of the (202 ˉ5) planes in all cases, as shown in Fig. 7.For longer residence times (0.14 s), only the [0003] direction is favored at all temperatures.Finally, we found that the onset of pyrolytic graphite formation occurs at around 1400 °C and for residence times around 0.09 s and becomes the dominant phase at higher temperatures and residence times.the crystal structure of r-B x C is rhombohedral and (2) the polycrystalline nature of the textured films, which would usually exhibit a fiber texture without symmetry.The five-fold symmetry originates from the tendency of r-B x C to form fivefold twinning (Fig. S3 ‡). 11,12,16However, upon closer inspection of the five broad poles at ψ ≈ 60°, reveals the further division into three distinct peaks within each of these five broad poles.This is consistent with the fact that five and six are nondivisible: for a given family of the directions of a five-fold crystal, can only be aligned with one of the 〈112 ˉ0〉 directions of 4H-SiC.As such, an alignment along [112 ˉ0], [12 ˉ10] and [2 ˉ110], will result in three adjacent 5-fold patterns as can be seen in Fig. 8. Furthermore, due to the lack of axial symmetry of five-fold objects (e.g., a pentagon) there are two ways two align a five-fold crystal along a given direction (e.g., the apex of a pentagon pointing towards either 4H-SiC[112 ˉ0] or 4H-SiC½1120), so that one pole pattern is centrosymmetric with respect to the other.In our case, this is observed in Fig. 8 where poles of much fainter intensity can be found between the more intense poles of the five-fold patterns.As a result, the ring pattern comprises 5 (r-B x C twinning) × 3 (4H-SiC symmetry axes) × 2 (lack of axial symmetry of 5-fold objects) = 30 poles (Fig. S4 ‡).The fortuitous varying intensity of the pattern brings to light that these five-fold twinned crystals or domains grow along each 112 ˉ0 axes of the 4H-SiC surface in a heteroepitaxial fashion.The third pattern at the center of the pole figure is the superpositions of the last of the three B x C{101 ˉ4} poles (ψ ≈ 7°) but are shadowed by the strong 4H-SiC(0001) pole (ψ ≈ 0°).
From top-view scanning electron microscopy (Fig. 9), the films deposited at 1100 °C appear to comprise nodular grains (Fig. 9a).The films deposited between 1200 and 1400 °C (Fig. 9b-d) show highly twinned grains with a more pronounced faceting with increasing temperature.The grain size is around 1 µm.The surface morphology of these films is consistent with the texture evolution of the film.The crystallites of the films deposited at 1200 °C (Fig. 9b) and 1300 °C (Fig. 9c) appear as square-based pyramids.This is associated with films with a strong [022 ˉ1] texture.At 1400 °C (Fig. 9d), the films appear porous and some of the grains with 5-fold twinning are visible, as expected from the pole figure of the [101 ˉ4] orientated films.At 1500 °C (Fig. 9e), the boron carbide grains do not show preferred orientation.
Finally for comparison, the elemental composition of the film on the Si-face at 1300 °C determined using ToF-ERDA also yielded a ratio of 4 : 1 (B -79.4% ± 0.7%, C -20.5% ± 0.3%) with around 0.1% of O incorporation and the absence of H as it is below the detection limit of the instrument.

Discussion
As observed, the orientation of the boron carbide films on silicon carbide depends on the surface termination, the temperature, and the residence time.The demonstration of epitaxial growth of r-B 4 C on 4H-SiC(0001 ˉ) from 1300 °C supports this, while all films deposited on 4H-SiC(0001) were found to be polycrystalline at the studied temperatures.In comparison to structurally similar materials such as rhombohedral subphosphide (B 12 2 ) and subarsenide (B 12 As 2 ), the threshold temperature for epitaxial growth for r-B x C is comparable to the one reported for the CVD of B 12 P 2 17 and similar structural variants such as twinning was observed in B 12 As 2 [18][19][20] and B 12 P 2 17 epitaxy, albeit on on-axis Si-face 4H-SiC(0001).A notable difference is observed in the evolution of crystalline quality with temperature as seen from rocking curves measurements: the crystalline quality of B 12 P 2 and B 12 As 2 was reported to degrade from 1350 °C (ref.21) and 1450 °C, 19 respectively, while the crystalline quality of r-B x C continues improving even up to 1500 °C.This could be attributed to the formation of volatile P or PH x (respectively As or AsH x ) species from the deposited film, which is less prone to occur in the case of carbon, albeit at the cost of forming graphite.Finally, the presence of two components in our rocking curves measurements further suggests that the crystallites of the epitaxial r-B 4 C films initially grow well aligned with the substrate at the early stages of the growth and deviate slightly later on, as in ref. 22.However, in spite of the similarities with B 12 P 2 and B 12 As 2 , which are typically grown epitaxially on the Si-terminated face, epitaxial growth of r-B 4 C was not achieved on 4H-SiC (0001). 17,19,23,24With the exception of their orientation on either SiC termination, the r-B 4 C films show identical crystal structures, composition at the same growth temperature (1300 °C) and temperature thresholds for graphite formation (1400 °C), which indicates that the observed differences can only be attributed to nucleation and not to the steady-state growth.To understand these observations, we consider the differences that are already known between the two terminations of silicon carbide and are relevant to the early stage of the deposition process, such as surface energy, and graphene formation or surface stability in H 2 .
From a thermodynamics perspective, the surface energy of the C-face of silicon carbides polytype is lower (7.5 J m −2 ) than that of the Si-face (18 J m −2 ). 25 To our knowledge, there are no computed data of the surface energy for all r-B 4 C facets.Nevertheless, considering the surface energy for boron carbide for the (101 ˉ1) planes (3.21 J m −2 , (ref.26) 3.27 J m −2 (ref.27)) and the surface energy for α-boron facets (1.9 to 3.6 J m −2 ), 28 having a similar structure as r-B 4 C, differing only in the absence the triatomic chains, one can conclude that they are lower than the surface energy of the C-face of 4H-SiC.Assuming the interfacial energy to be small for (0001)-oriented B 4 C crystals on SiC(0001) and SiC(0001 ˉ) due to the small lattice mismatch, one would expect that both surfaces would allow epitaxial growth; although it can be argued that the surface energy of SiC(0001) is so high that multiple orientations of B 4 C could grow in-spite of a higher interfacial energy, hence leading to polycrystalline growth.The issue with these energetic considerations is that they might not be valid at our growth conditions.
A consequence of the high energy of the Si-face is that it is highly unstable in pure and carbon-enriched hydrogen ambient between 1400-1580 °C, [29][30][31] which is similar to our experimental conditions.0][31] In contrast, the C-face is reported to be etched smoothly at these conditions forming only micro steps with an average height of 2.5 nm, 30 equivalent to 2.5 times the c-axis of a 4H-SiC unit cell and about twice the c-axis of r-B 4 unit cell (12.065-12.175Å). 32 These aspects could the different results obtained between the two SiC faces, the etching of the Si-face results in a complex topography, facilitating polycrystalline growth, while the steps after etching the C-face are close to lattice-matched to both the c-and a-axes of the r-B 4 C unit cell.
Besides thermodynamic and morphological aspects, the chemistry at the SiC surfaces is also relevant, especially the role of carbon.Our growth conditions (1100-1500 °C, 7000 Pa and hydrocarbon by-product to hydrogen ratio in the order of 135 to 406 ppm, assuming 1 to 3 mole of ethylene (C 2 H 4 ) per mole of TEB 33 ) seem to be adequate for the growth of graphene at the 4H-SiC(0001) surface. 34Formation of graphene is known to be even easier on 4H-SiC(0001 ˉ) surface due to the absence of the partially sp 3 -bonded interfacial layer (so-called buffer layer) between graphene and SiC. 35However, only the Si-face affords oriented graphene layers (from 1350 °C at our CVD conditions), 34 while the graphene layers grown on the C-face are usually randomly rotated with respect to the substrate. 35,36If a few layers of graphene would be the enabler for epitaxial growth of r-B x C, one would expect epitaxial growth on the Si-face and not on the C-face, which is contrary to our observations, suggesting that graphene does not play a role in the process.
We instead consider the possibility that our deposition process operates in a window where the substrate, especially the C-face, may act as a carbon source but at a rate that prevents the formation of graphene.][39] From our results, we note that when the Si-terminated surface is used with a relatively short residence time, the texture of the films varied strongly with temperature, while for longer residence time the [0003] orientation prevails, until graphite forms.We suggest that this can be explained by the fact that a delicate balance of carbon concentration at the 4H-SiC(0001) surface may be needed to promote the growth of boron carbide along the c-axis.At lower temperatures (1200 °C and below), the Si-face is not an efficient carbon source on its own and the decomposition rate of the C 2 H 4 byproducts is slow, leading to growth along the [0003] orientation only for the longest residence times.Around 1300 °C the carbon concentration is adequate to promote nucleation along the c-axis, although the films remain polycrystalline.Above 1300 °C, graphite forms and consumes the carbon provided by the C 2 H 4 byproducts and again prevents the films from growing along the c-axis.In contrast, the C-face would be a more efficient C-source, making the growth less depending on the carbon coming from the gas phase and less sensitive to the formation of graphite.While the details of how carbon is involved in the nucleation mechanisms of boron carbide are yet to be understood, the carbon at the SiC surfaces might play a more significant role in r-B x C epitaxy than the surface energy considerations and step formation mentioned above, as epitaxial B 12 As 2 19,23,24 and B 12 P 2 17 can be prepared on 4H-SiC (0001).

Conclusion
Our study shows that rhombohedral boron carbide with a composition with preferred orientation and amount of co-deposited graphite varying with the deposition temperature and the gas residence time in the CVD reactor.We suggest that these results are explained by the ability of the SiC surface to supply carbon for the initial growth of r-B x C.These results give further insight on the influence of growth conditions for this material system and enable the use of epitaxial boron carbide thin films for their intended applications in electronic and thermoelectric devices.
As the information provided by the 2θ/ω scans is confined to planes that are parallel to the sample surface, XRD pole figure measurement was performed to assess the degree of in-plane orientation with respect to the substrate.The distinct poles in the pole figure measurement in Fig. 2, shows that the r-B x C films are not only c-axis oriented, but grown epitaxially on the 4H-SiC(0001 ˉ).The epitaxial relationships were determined as r-B x C(0001)[101 ˉ0]k 4H-SiC(0001 ˉ)[101 ˉ0].The six poles, separated by 60°, show that the films are twinned.This is expected due to the higher sym-Dalton Transactions Paper This journal is © The Royal Society of Chemistry 2022 Dalton Trans., 2022, 51, 15974-15982 | 15975 Open Access Article.Published on 30 September 2022.Downloaded on 11/9/2022 7:56:05 AM.This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.

Fig. 3 X
Fig. 3 X-ray rocking curve (in black) of the r-B x C(0003) planes r-B x C film deposited at (a) 1300 °C, (b) 1400 °C and (c) 1500 °C on 4H-SiC (0001 ¯).The sharp and broad components are shown in red and orange, respectively.(d) Integral breadth of the broad (orange squares) and sharp (red circles) components; the integral breadth of the asymmetric broad component at 1300 °C was calculated from the convolution of the two components shown in (a).The dotted grey line represents the averaged integral breadth for the rocking curves of 4H-SiC(0004) planes measured on each sample.

Fig. 6
Fig. 6 2θ/ω diffractogram of a polycrystalline B x C deposited at 1400 °C close to the inlet of the susceptor, corresponding to a residence time of 0.06 s.The sharp low-intensity non-indexed peaks come from the substrate.

Fig. 7
Fig. 7 Evolution of the orientation, from 2θ/ω XRD, of the r-B x C films deposited on 4H-SiC(0001) substrates placed at the center and the backside of the susceptor.Bold face marks the dominating orientations.The corresponding residence times and temperatures were estimated by modeling.
B 4 C (ToF-ERDA) can grow epitaxially on 4H-SiC(0001 ˉ) at 1300 °C by CVD using triethylboron as single source precursor in H 2 .The epitaxial relationships were r-B 4 C(0001)[101 ˉ0]k 4H-SiC(0001)[101 ˉ0] as seen from pole figure measurements.XRD and SEM revealed that higher deposition temperature results in co-deposition of graphite, and lower deposition temperature results in polycrystalline boron carbide.Deposition on 4H-SiC(0001) resulted in polycrystalline r-B x C