Two-step ALD process for non-oxide ceramic deposition: the example of boron nitride

Atomic layer deposition (ALD) based on polymer-derived ceramics (PDCs) chemistry is used for the fabrication of boron nitride thin films from reaction between trichloroborazine and hexamethyldisilazane. The transposition of the PDCs route to ALD is highly appealing for depositing ceramics, especially non-oxide ones, as it offers various molecular precursors. From a two-step approach composed of an ALD process forming a so-called preceramic film and its subsequent ceramization, conformal and homogenous BN layers are successfully synthesized on various inorganic substrates. In the first stage, smooth polyborazine coatings are obtained at a temperature as low as 90 °C. The saturation and self-limitation of the ALD gas-surface reactions are verified. Intriguingly, three ALD windows seem to exist and are attributed to change in ligand exchange. After the ceramization stage using a heat treatment, conformal near-stoichiometric BN layers are obtained. Their structure in terms of crystallinity can be adjusted from amorphous to well-crystalline sp2 phase by controlling the treatment temperature. In particular, a crystallization onset occurs at 1000 °C and well defined sp2 crystalline planes oriented parallel to the surface are noted after ceramization at 1350 °C. Finally, side-modification of the substrate surface induced by the thermal treatment appears to impact on the final BN topography and defect generation.


Introduction
Atomic layer deposition (ALD) is a technique of choice for the fabrication of ultra-thin films with a thickness controlled at the nanometer scale.Based on sequential self-limited gas-surface reactions, it allows the uniform and conformal coating of different kinds of substrates ranging from planar to highly structured ones.Various materials, such as metal oxides, metal nitrides, metal sulfides, metals, organic-inorganic hybrids, can be fabricated by ALD [1][2][3][4][5][6][7], however some compounds remain difficult to achieve or require use of corrosive or pyrophoric precursors [8][9][10][11][12][13].The precursor chemistry is thus a key domain for extending the potential of ALD.In particular, safer and greener metallic precursors are currently investigated [14][15][16].Inspired from solution route, unusual chemical approaches can also be applied to ALD.For example, as an alternative to water-based processes, non-aqueous sol-gel chemistry has been implemented for oxide deposition, thus avoiding the use of an oxidizing agent as well as the formation of OH intermediates, enabling deposition on moisture-sensitive substrates [17][18][19].In the same way, polymer derived ceramics (PDCs) route [20][21][22][23][24] can offer new potentialities for ALD of ceramics [25,26].Indeed, deposition of non-oxide ceramics such as nitride requires generally relatively high deposition temperature Reaction between TCB and HMDS leading to polycondensation and formation of polyborazine, a BN preceramic polymer, which is then converted to hexagonal BN by heat treatment.Adapted from [23].
(>300 • C) as well as the use of ammonia and/or nitrogen-containing plasma as co-reactant [3,27,28], while PDCs route opens new possibilities in terms of precursors and deposition temperatures.
In PDCs pathway, tailored molecular precursors undergo polycondensation to form a so-called preceramic material which in turn is converted at high temperature into a dense or even crystalline final ceramic.A shaping can be done at the different stages like in the case of the sol-gel chemistry.The transposition of PDCs chemistry to ALD involves thus the following two-steps: (i) the deposition from molecular precursors of a preceramic film (ii) which is further ceramized into the desired material.Such two-step ALD processes have been successfully reported for synthesis of 2-dimensional (2D) materials as transition metal dichalcogenides (TMDs) [29,30] and boron nitride (BN) [26] as well as of silicon-based ceramics [25].In particular, from metal tetrakis(diethylamide) reacted with ethylenedithiol, MoS 2 [29] and TiS 2 [30] have been obtained after thermal treatment of the respective as-deposited metal thiolate thin films.ALD based on PDCs chemistry appears also suited for BN deposition.Especially, from trichloroborazine (TCB, Cl 3 B 3 N 3 H 3 ) and hexamethyldisilazane (HMDS, (CH 3 ) 3 SiNHSi(CH 3 ) 3 ), a film made of BN preceramic, so-called polyborazine, is deposited using ALD and then converted into dense or even crystalline BN layer [26,31], following the PDCs pathway schematized in figure 1. Due to the low-temperature ALD first step, polymer templates become accessible enabling easy fabrication of designed BN nanostructures [32].In the present paper, a systematic study of the ALD parameters of the two-step approach is reported.After demonstrating the saturated reaction conditions, the film characteristics on flat substrates are discussed.Particular attention is given to the impact of the thermal treatment on the structure of the final ceramic.

Two-step ALD of BN
ALD experiments are carried out using a home-made ALD reactor placed in a glove box and operated in exposure mode (figure 2).It consists on a hot-wall chamber with two separated precursor inlets at the opposite of the exhaust.A cold trap placed between the exhaust line and the pump allows condensation of the unreacted precursors and byproducts.
Trichloroborazine, which can be purified by sublimation either before use or directly during the process, and hexamethyldisilazane (98%, CarlRoth) are used as precursors for deposition of polyborazine preceramic film.TCB and HMDS are sequentially introduced by pneumatic ALD valves from their canisters, kept respectively at 70 • C-75 • C and 30 • C. Thanks to a pneumatic valve at the outlet of the reactor, each precursor can be held in the chamber with sufficient time to react with the surface.Pure nitrogen used as a carrier gas continuously flows at a rate of 5 sccm.
In order to optimize the ALD parameters, deposition is first performed on a small piece (typically 1 × 1 cm) of p-type silicon (100) substrates with native oxide layer.Based on previous results [26], the reactor temperature is initially set at 80 • C. The growth rate is investigated as a function of the opening (varying from 0.8 s to 1.8 s, 0.02 s to 0.05 s for TCB and HMDS, respectively), the residence (from 10 s to 120 s) and purge (range of 10 s to 30 s) times.
Once the above parameters optimized, the ALD window is determined.The depositions take place between 70 • C and 200 • C with 1.2 s (TCB) and 0.03 s (HMDS) pulse length, followed by 20 s of residence and purge.The number of cycles is varied from 100 to 500.
Then, 300 or 500 ALD cycles are carried out on various planar substrates: Si (100), Si (111), both with native oxide, 4 H-SiC on-axis substrates treated with HF solution to remove the native oxide, and sapphire cleaned with isopropanol.The conformality of the BN layer is verified using SiO 2 /Si patterned wafer as well as nanostructures such as 250 nm-silica nanoparticles synthesized by a modified Stöber method and Ni foam.In order to convert the deposited preceramic film into dense BN, annealing for 2 h under inert atmosphere is realized at a temperature ranging from 400 • C to 1400 • C depending on the support.

Characterization of the planar substrates coated with BN layer
The thickness of as-deposited and annealed films is determined by spectroscopic ellipsometry at an incident angle of 75 • using a Semilab (SE-2000) ellipsometer.Data fit is realized with the 'SAM suite' software using a Cauchy dispersion law.The growth per cycle (GPC) can then be calculated by dividing the extracted thickness by the number of ALD cycles performed.A minimum of five measures is realized per sample to get representative average thickness.
The surface topography is characterized by atomic force microscopy (AFM) using a commercial CSI-nano observer microscope operating in tapping mode at a resonant frequency of 60 kHz.As-deposited and annealed layers are further analyzed by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) using a Zeiss Merlin VP compact microscope operated at 2 and 5 kV and equipped with a SDD OXFORD X-Max EDXS detector.Samples are prepared without metallic coating.
The crystallinity of the annealed thin films is evaluated by x-ray diffraction (XRD).Measurements are recorded for 1 h at 2θ ranging from 20 • to 70 • with a benchtop 2D phaser bruker diffractometer equipped with a LYNXEYE XE-T detector (CuKα radiation; λ = 1.5406Å).Raman spectroscopy is also performed after annealing using a LabRAM HR Evolution (Horiba) spectrometer with a 532 nm excitation wavelength.
Fourier transform infra-red (FTIR) spectra are recorded on a Nicolet 380 FT-IR spectrometer (Thermo Electron Corporation) coupled with smart orbit attenuated total reflectance (ATR) with diamond crystal accessory.The spectra are recorded in ATR mode from 4000 cm −1 -400 cm −1 with a step of 4 cm −1 and using 150 scans to reduce the signal-to-noise ratio.The blank is performed on the bare substrate to enhance the signal arising from the thin film.X-ray photoelectron spectroscopy (XPS) is performed on BN-coated Si using a Phoibos (SPECS GmbH) and CLAM IV (Thermo VG Scientific) hemispherical energy analyzer with respectively Al k α and Mg k α radiation.The binding energy scale is calibrated by measuring C 1 s peak (BE = 284.8eV) from the surface contamination and the accuracy of the measure is ±0.1 eV.
Transmission electron microscopy (TEM) coupled with EDS is used to characterize the films on Si after annealing at 1000 • C and 1350 • C. Cross-section is performed by mechanical polishing.High-resolution TEM (HR-TEM) and scanning TEM (STEM) equipped with a high-angle annular dark-field (HAADF) imaging is recorded at 200 kV using a FEG-JEOL 2100 F equipped with a 80 mm 2 silicon drift detector EDS (Oxford Instrument) and a ThermoFisher Titan SPECTRA 200 transmission electron microscope operating at 200 kV equipped with a cold FEG (Field emission Gun) and a C s aberration probe corrector together with two large EDS detectors presenting a high sensibility corresponding to a solid angle of 1.8 sr.All the analyses are performed using a probe convergent semi-angle of 29.4 mrad and a collection angle between 109 and 200 mrad allowing STEM-HAADF (scanning TEM-high angle annular dark field imaging mode) Z-contrast imaging.During chemical analysis using EDS within the STEM-HAADF imaging mode, Si (K α = 1.74 keV), O (K α = 0.523 keV), B (K α = 0.183 keV), and N (K α = 0.392 keV) are identified as the key elements of interest.The total probe current for this analysis is set to about 110 pA.

ALD parameters
Before investigating the influence of the deposition temperature on the growth, the saturation conditions of the surface are verified at 80 • C, temperature at which controlled deposition has been established [26].The GPC is evaluated as a function of the precursor dosing with a fixed purge of 20 s and a residence time of 10 s for both precursors.Steady GPC is reached from HMDS and TCB pulses of respectively 0.03 s and 1.4 s (figures 3(a) and (b)), evidencing the effective surface saturation.Furthermore, purge times of 10 s and 12 s reveal sufficient (figures 3(c) and (d)) to ensure the total removal of unreacted HMDS and TCB, respectively.Regarding the residence time, when 20 s of residence time is used instead of 10 s (figure 3(a)), 1.2 s of TCB pulse is enough to complete the reaction, indicating a rather slow kinetic.As shown in figure 4(a) a minimum of 10 s is required to nearly reach a steady growth (GPC of 0.75-0.80Å.cycle −1 ), while for residence time longer than 20 s, linear dependence of the GPC to the residence time is observed that might be explained by physisorption of the TCB at the surface.In the following, to ensure the self-limiting regime, the pulse length of TCB and HMDS are set at 1.2 s and 0.03 s, respectively, each followed by 20 s of residence and 20 s of purge.
The polyborazine growth rate on Si (100) with native oxide is then investigated in the range of 70 • C-200 • C, where no decomposition is expected [33].Figure 4(b) displaying the obtained GPC as a function of the deposition temperature shows four distinct zones: (I) Below 80 • C, a high GPC value is noted and attributed to the physisorption of BN precursor on the surface.Indeed, the deposition temperature is close to the one of TCB sublimation [33].
(II) From 80 • C to 100 • C and (III) from 100 • C to 175 • C, two plateaus at, respectively, 0.79 Å.cycle −1 and 0.44 Å.cycle −1 are observed.Both could be considered as an ALD window.The difference in GPC values is assumed to be related to the anchoring geometry of TCB on the surface.
(IV) Above 170 • C, the GPC decreases and reaches a new plateau at 0.20-0.30Å.cycle −1 .It can be mentioned that activation of the support, by oxygen plasma or piranha treatment before the growth, does not influence the GPC.The diminution of the GPC from 170 • C can be explained either by a partial desorption of TCB and/or polyborazine, or by a change of TCB adsorption mode.A positioning of the BN precursor parallel to the surface will induce a reduced number of anchoring sites, which directly impacts the growth rate.
Nonetheless, these assumptions must be corroborated by investigating the involved reaction mechanism, which is out of the scope of this work.
The characteristic linear dependence of the film thickness on the number of ALD cycles is finally verified at 90 • C and 120 • C using the optimized parameters (figure 4(c)).The ellipsometry mapping recorded from a polyborazine-coated 4 ′′ Si wafer (figure 4(d)) reveals a non-uniformity of the ALD film of less than 8% after 300 cycles.It should be noted that it decreases on the 2 ′′ central part where less than 4% of thickness deviation is observed.The non-uniformity is attributed to a temperature gradient on the outer part of the chamber [34,35].To ensure the uniformity [35] of the coating, only the 2 ′′ central part of the reactor is used.

Polyborazine and BN thin films on inorganic substrates
As-deposited coating is uniform and smooth with a slight granular aspect, as revealed by AFM and SEM characterizations (figures 5(a) and (b)).The extracted root mean square roughness (RMS) of a 300 cycle film grown at 90 • C on Si(100) with native oxide (RMS = 0.7 nm) is close to that of the bare Si substrate (RMS = 0.3 nm), evidencing the smoothness of the layer.Similar film morphology is also observed on the various investigated substrates as Si (111), deoxidized SiC and sapphire as well as on SiO 2 /Si patterned wafer on which the as-deposited layer follows perfectly the step topography of the micro-patterned substrate (in supporting information, figure SI.1).The uniformity and conformality of the ALD process are also evidenced by the coating of nanostructures [31,32] such as silica nanoparticles and Ni foam (figures 5(c), (d) and SI.2).
The chemical bonding and composition of the preceramic polymer layer are then investigated by FTIR and XPS.No noticeable difference is noted in FTIR spectra as a function of the substrates (figures 6(a) and SI.3).As shown in figure 6(a), typical signature of the polyborazine is observed independently of the deposition temperature with a strong in-plane sp 2 B-N vibration (ν BN sp 2 ) band at 1400 cm −1 and characteristic bands of the polymer in the 1300-800 cm −1 region (grey area) [36].The bands at 3500-3200 cm −1 and 705 cm −1 are assigned, respectively, to N-H vibration (ν NH ) and bending (δ NH ).The width of the N-H vibrational band suggests different types of N-H environment, which is in agreement with the polyborazine structure (figure 1).No signature of B-H (∼2500 cm −1 ) [37-39] and B-Cl (∼765-740 cm −1 ) [40,41] bonds is identified.
XPS measurements confirm the BN bonding with peaks in the B 1 s and N 1 s spectra at, respectively, 191.0 eV and 398.9 eV [42].Less than 1% of residual chlorine is also noted due to unreacted Cl group of TCB in the polyborazine film.This contamination is below the detection limit of FTIR.It should be mentioned a partial oxidation of the film due to the air sensitivity of the polyborazine attributed to the presence of reactive N-H bonds.
The preceramic polyborazine ALD film on Si is then converted to dense BN in a subsequent thermal treatment under inert atmosphere.No evident change in thickness is observed as a function of the annealing temperature.XPS spectra recorded from films grown at 90 • C and 120 • C and then annealed at either 1000 • C or 1350 • C demonstrate the obtaining of near stoichiometric films (B/N ratio of 0.96-1.05)without noticeable chlorine contamination (figures 7, SI.4 and table SI.1).As shown by the B contribution at 192.1 eV assigned to B-O bonds [42,43], oxygen content measured between 5 and 10% could be attributed to an interface formation with the native oxide of the support, the substrate being observed (table SI.2).Furthermore, two binding energy loss features at 9 eV and 25 eV apart from the main B-N peak are observed in the B1s and N1s core level spectra.Respectively attributed to π plasmons (πP) and bulk plasmon (Bp), they are characteristics of sp 2 BN, i.e. rhombohedral (rBN) or hexagonal BN (hBN) [44][45][46][47].Similar results have been previously reported on BN nanostructures synthesized by this ALD process on oxygen-containing polymer substrate [26].Progressive removal of the N-H bonds occurs as a function of the annealing temperature, as demonstrated by FTIR (figure 6(b)).From 800 • C, conversion of polyborazine to BN is complete, which is evidenced by the total disappearance of ν NH and the evolution of the in-plane sp 2 B-N vibration band in terms of position and width.Above 1000 • C, sharpening of the ν BN sp 2 band indicates an improvement of the BN cycle ordering.Indeed, sharper is the band at 1376 cm −1 , better should be the BN crystallinity [48].After treatment at 1350 • C, a well-defined band is observed at 1376 cm −1 suggesting the formation of crystalline sp 2 BN; despite that the film thickness seems insufficient for full XRD confirmation (pattern in figure SI.5).The beginning of crystallization occurs around 1000 • C as few crystalline nano-domains, mainly parallel to the substrate and embedded in the amorphous BN layer, are revealed by HR-TEM performed in cross-section (figure 8(a)).After treatment at 1350 • C, crystalline BN layer with clear lattice fringes parallel to the surface is formed on top of the 1.2 nm thick native SiO x /Si substrate as visible in figure 8(b).Raman spectra recorded from as-deposited and thermally treated layers confirm this structural evolution (figure 7(d)).While polyborazine film and BN layers treated at 800 • C and 1000 • C do not show Raman vibration, ceramization at 1350 • C leads to a well-defined band at 1367.6 cm −1 , characteristic to the E 2g mode of sp 2 B-N [49,50].The narrow full width at half maximum (FWHM) of 14.6 cm −1 indicates the good crystalline quality of the layer [49][50][51].It should be mentioned that obtaining such FWHM value usually requires temperature higher than 1350 • C. In conventional PDCs route, a minimum of 1600 • C or additivation is required to reach the same crystalline quality [31,50,52].On the other hand, this value is in line with the lowest ones reported in the literature for thin films deposited by chemical [53][54][55] or physical vapor deposition [56][57][58][59].Regarding ALD films, FWHM around 40 cm −1 are reported for ALD performed at 750 • C [60] and 900 • C [51].Often post-annealing at high temperature and/or under high vacuum is performed.Nevertheless, the characteristic Raman band appears shifted compared to bulk hBN [61][62][63], which can be attributed to either lower crystallinity [49] or phase change.AFM imaging reveals that up to annealing at 1000 • C the BN film remains smooth (figure 8(c)); while at higher temperatures a strong increase in roughness is observed with a RMS value of 12.6 nm reached after treatment at 1200 • C (figure 8(d)).In particular, annealing conducts to the formation of ripples becoming hills and valleys with increasing temperature.This change of topography is confirmed by SEM imaging: after annealing at 1000 • C, only a few crumpling are visible (figures 8(e)), while at higher temperature pinholes and an irregular surface are observable (figures 8(f) and (g)).The roughness and topography evolution may result from a modification of the Si surface under heat treatment, rather than from crystallization of the deposited layer.Indeed, AFM imaging of bare substrate after annealing at 1000 • C and 1200 • C under inert atmosphere reveals the formation of wrinkles (figure SI.6) that arises from step bunching [64][65][66].While similar features are observed on the surface coated with BN, this phenomenon seems worsen by the presence of the coating, as noticed by formation of hills and valleys from 1200 • C.This aggravation can be attributed to the difference in local atmosphere: polyborazine ceramization releases ammonia and dihydrogen that may exacerbate this mechanism [65].Si step bunching is evidenced by the observation of atomic terraces in cross-section TEM images (figure SI.7(a)) driving, at low magnification, to apparent bending of the support (figure SI.7(b)), in agreement with the AFM topography.Furthermore, the presence of micrometer cavities (figure SI.7(c)) suggests an exo-diffusion of Si with uncompensated vacancies that may be attributed to the presence of the coating.
In figure 9(a), the EDS mapping nicely shows that B and N, respectively represented by cyan and purple color, are homogenously localized in the crystalline top layer.The thin silicon oxide SiO x layer is evidenced by the presence of oxygen, in green color at the interface between the BN film and the Si substrate.The deposited film follows the substrate surface (figures SI.7 and SI.9).Lattice spacing around 0.33 nm is measured in agreement with sp 2 BN crystalline phase (figures 9 and SI.8).HR-STEM-BF imaging demonstrates dots that are periodically spaced, corresponding to atomic columns.Looking at the perpendicular direction from the lattice fringes, the dots are perfectly aligned as highlighted in figure 9(b) and by the corresponding fast Fourier transform (FFT) (Inset).Such arrangement denotes AB stacking, characteristics of hBN [67].In Figures 9(b) and SI.8, the presence of crystalline defects as well as few irregularities in the planes number is also demonstrated.It should be mentioned that in some zones, a sharp interface between BN and Si is observed (figures SI.8 and SI.9) as the oxide layer vanishes (figure SI.9).It may originate from reaction of Si with SiO x at high temperature leading to the release of volatile SiO [68,69].This mechanism is exacerbated by the presence of the released H 2 and eventual trace of oxygen inside the furnace.A closer look reveals that the presence of step bunching causes defects in the organization of the BN planes.Especially in absence of the interlayer oxide, Si defects result to misalignment of BN planes that conducts to grain boundaries, which propagates along the thickness as pointed out in figure SI.8.No BN-Si or BN-SiO x mixed interface is evidenced suggesting the formation of a rather sharp interface between the ALD coating and the support, although atom diffusion and crystallization processes happened.
To study the impact of the substrate on the film crystallinity, 4H-SiC and sapphire single crystals are used as substrates.As discussed above, similar morphology of the polyborazine film is obtained independently of the substrate, whereas differences are noticed after the heat treatment, pointing to the role of the support on the conversion of the 300 ALD cycle preceramic layers into the final BN films.AFM and SEM images (figures 10(a)-(c)) recorded, after heat treatment up to 1000 • C, from BN layers on deoxidized SiC show uniform and smooth layers with a morphology similar than on Si with native oxide.After treatment at 1600 • C, cracks are visible on the surface (figure 10(d), inset) and a closer look reveals the formation of a rough and probably porous film evoking a surface reconstruction.Similarly to silicon, this phenomenon can be attributed to SiC step bunching induced by the thermal treatment [70,71].Interestingly, compared to deposition on Si, slight differences in chemical bonding are noticed as revealed by FTIR spectra (figures 6(c) and SI.3).On deoxidized SiC, figure 6(c) shows wide band in the 1400-1300 cm −1 region, assigned to the in-plane sp 2 B-N vibration, which narrows with the annealing temperature and centers at 1376 cm −1 for 3) that can be attributed to sp 3 BN mode [72,73], but are not yet fully understood.Indeed, presence of oxidation cannot be completely excluded as the bands associated to B-O appear in the 1000-1300 cm −1 range [74].
Similar topography evolution is observed for BN on sapphire.As demonstrated by the figures 10(e), (f) and SI.10, a progressive topography modification occurs from 1000 • C to 1600 • C evolving from a smooth surface to a reconstructed one, via a flower-like morphology at 1350 • C (figure SI.10).The 1350 • C treated layers on sapphire does not display characteristic ν BN sp 2 FTIR vibration band.Two bands at 1240 cm −1 and 1095 cm −1 (figures 6(c) and SI.3) are observed and can be assigned to sp 3 B-N bonding in the wurtzite structure (wBN) [75,76].These features tend to suggest an interaction with the substrate leading to a more complex structure of the final film with a transition from hBN to wBN, i.e. from sp 2 to sp 3 hybridized BN layer.In particular, in the literature, it has been demonstrated that sapphire stabilizes wBN rather than hBN even at moderate temperature (800 • C) below atmospheric pressure [77].Moreover, SEM imaging suggests a global surface reconstruction at high temperature (1200 • C-1600 • C) with BN layer, while without deposition, treatment under inert atmosphere of sapphire does not lead to topography modification (figure SI.11).

Conclusion
To sum up, during the ALD step, homogenous and conformal amorphous polyborazine films are deposited from TCB and HMDS in a controlled manner onto various substrates, even on structured one as nanoparticles.Interestingly, this process displays three distinct ALD windows that might be attributed to different numbers of B-Cl reacting with the surface group inducing thus a change in anchoring geometry of the aromatic BN precursor.Control of the thickness by the number of ALD cycles is evidenced for each plateau and saturation of surface reaction is verified with, respectively, TCB and HMDS pulse lengths of 1.2 and 0.03 s.No noticeable difference is noted in terms of preceramic morphology as a function of the support during this stage.During the 2nd step, thermal annealing under controlled atmosphere leads to the conversion of polyborazine films into smooth BN layers, maintaining their conformality.The chemical and structural analyses realized on the film at different stages converge to the following scenario: polymerization and condensation of the preceramic polymer take place between RT and 800 • C, yielding pure BN.This is supported by the progressive N-H bond disappearance.A further increase in annealing temperature results in crystallinity improvement.As supported by XPS, Raman spectroscopy and TEM, the annealed near stoichiometric films on Si substrate present at 1000 • C an amorphous BN phase with embedded crystallites while at 1350 • C hexagonal BN with basal planes parallel to the support are obtained.On the contrary to the ALD step, during the annealing process, the substrate may impact the final structure and topography due to its surface modification induced by the thermal annealing.Especially, step bunching may cause defects in the final layer.Moreover, the nature of the support, as sapphire, can favor an unexpected crystalline structure as wBN due to interactions with the formed layer.
In conclusion, based on PDCs chemistry, this two-step approach permits the fabrication of various controlled BN hetero-/nanostructures.In the first step, the low temperature of the ALD process enables using thermal sensitive supports as templates.For the second step, in the present study, we focused on conventional thermal annealing; but rapid thermal annealing or UV insolation could be envisaged to decrease the post-treatment temperature.The final structure of the BN layer can be adjusted by controlling the heat treatment and the nature of the substrate, ranging from amorphous to either pure sp 2 crystalline phases as hexagonal BN on silicon with native oxide, sp 3 -wurtzite on sapphire, or a mix of sp 2 -sp 3 phases.Taking advantage of this structure tuning, the proposed approach is highly attractive to master the properties of the final layers.Among others, it is interesting for the integration of BN in electronic devices.

Figure 1 .
Figure 1.Reaction between TCB and HMDS leading to polycondensation and formation of polyborazine, a BN preceramic polymer, which is then converted to hexagonal BN by heat treatment.Adapted from[23].

Figure 2 .
Figure 2. Scheme of the ALD reactor.

Figure 3 .
Figure 3. Growth per cycle of the polyborazine layer at 80 • C as a function of (a) TCB pulse length (HMDS pulse length set at 0.03 s and both purges at 20 s), (b) HMDS pulse length (with a TCB pulse of 1.2 s and purges and residences, respectively, of 20 s and 10 s), (c) TCB purge and (d) HMDS purge duration (HMDS and TCB pulse lengths, respectively, set at 0.03 s and 1.2 s, residence times at 10 s and the second purge fixed at 20 s).GPC values are the mean value of multiple trials under identical conditions.Error bars are determined by the standard deviation and correspond to statistical errors.

Figure 4 .
Figure 4. Growth per cycle of the polyborazine layer (a) depending on the residence time (deposition at 80 • C) and (b) as the function of the deposition temperature (residence times of 20 s).TCB and HMDS pulse lengths are set, respectively, at 1.2 s and 0.03 s, the purges are both fixed at 20 s.(c) Preceramic film thickness as a function of the number of ALD cycles performed at 90 • C (black squares) and 120 • C (dark cyan triangles).According to the linear regression, a respective GPC of 0.79 Å.cycle −1 and of 0.44 Å.cycle −1 is determined.In (b) and (c) the values are the mean value of multiple trials under identical conditions.The size of each set is not identical for each condition.Error bars correspond to the standard deviation.(d) Thickness map of a 300 cycle polyborazine film deposited at 90 • C on 4 ′′ Si wafer with native oxide recorded by ellipsometry.

Figure 5 .
Figure 5. (a) 10 × 10 µm AFM (measured RMS = 0.7 nm) and (b) SEM image of a polyborazine film grown at 90 • C on Si(100) after (a) 300 and (b) 250 cycles.(c) TEM image of BN coated SiO2 nanoparticles after ALD at 80 • C followed by an annealing at 1000 • C under inert atmosphere and (d) the corresponding dark field STEM image and EDS elemental mapping (color code: white: Si; violet: O; orange: B and green: N).

Figure 7 .
Figure 7. (a) XPS survey spectrum, recorded from a film obtained after 400 ALD cycles at 90 • C then annealed at 1350 • C, and the corresponding XPS high resolution scans with deconvolution across (b) B 1 s and (c) N 1 s edges.(d) Raman spectra of a polyborazine film grown on Si(100) at 90 • C (black) and annealed at 800 • C (orange), 1000 • C (light brown) and 1350 • C (red).

Figure 8 .
Figure 8. (a), (b) TEM images recorded on cross section prepared for (a) 40 nm-thick and (b) 17 nm thick polyborazine films deposited on Si followed by an annealing at (a) 1000 • C and (b) 1350 • C, respectively.In (a) the arrows point at the lattice fringes; the crystalline planes are embedded into the amorphous BN matrix and appear preferentially oriented parallel to the substrate.After treatment at 1350 • C, well defined lattice fringes parallel to the substrate are observed.(c), (d) 10 × 10 µm AFM images of BN layers on Si wafer after annealing at (c) 1000 • C and (d) 1200 • C reveal an increase of the roughness with the temperature.(e)-(g) SEM images of the top surface of the BN film on Si wafer annealed at (e) 1000 • C, (f) 1200 • C and (g) 1350 • C confirm a change in surface morphology.Insert in (g) presents the corresponding EDS spectrum.

Figure 9 .
Figure 9. (a) STEM-BF (top) and STEM-HAADF (bottom) images recorded simultaneously on an area on the TEM cross section prepared on the film deposited on Si followed by an annealing at 1350 • C and the corresponding EDS elemental maps of Si, B, O and N elements.The dash lines highlight the separation of the different components: the substrate Si, the native silicon oxide, and the deposited BN film, and the good agreement between the imaging and the maps.(b) HR-STEM-BF image recorded on an area of the BN film on Si with native oxide treated at 1350 • C. The arrow points at an incomplete BN sheet on the top.The yellow dotted lines highlight the position of atoms in planes and between adjacent planes.In inset, the Fourier Transform of the area delimited by the black dashed square.