enhanced methods for growing boron nitride
films from diborane and ammonia
Cite as: J. Vac. Sci. Technol. A 38, 033411 (2020); https://doi.org/10.1116/6.0000132
Submitted: 19 February 2020 . Accepted: 01 April 2020 . Published Online: 23 April 2020
Ramazan O. Apaydin, Arnoud J. Onnink, Xingyu Liu, Antonius A. I. Aarnink, Michel P. de Jong, Dirk J. Gravesteijn, and Alexey Y. Kovalgin
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Comparative study of thermal and
radical-enhanced methods for growing boron nitride films
from diborane and ammonia
Cite as: J. Vac. Sci. Technol. A38, 033411 (2020);doi: 10.1116/6.0000132
View Online Export Citation CrossMark
Submitted: 19 February 2020 · Accepted: 1 April 2020 · Published Online: 23 April 2020
Ramazan O. Apaydin, Arnoud J. Onnink, Xingyu Liu, Antonius A. I. Aarnink, Michel P. de Jong, Dirk J. Gravesteijn, and Alexey Y. Kovalgina)
AFFILIATIONS
MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands a)Electronic mail:a.y.kovalgin@utwente.nl
ABSTRACT
This work studies the deposition of boron/boron nitride (B/BN) composite films at low substrate temperature (275–375 °C) by alternating pulses of diborane (B2H6) and ammonia (NH3) with argon purging in between to avoid gas-phase reactions of the precursors. This
process is similar to atomic layer deposition in which the dominance of surface reactions simplifies the growth mechanism. However, non-self-limiting decomposition of B2H6and incomplete nitridation lead to the incorporation of pure boron (pure-B), causing deviation
from the desired 1:1 B:N stoichiometry. Using the pure-B fraction as a measure of incomplete nitridation, this article describes consecu-tive experiments to control this effect and ultimately understand it in the context of a surface reaction model. First, it is demonstrated that, in a purely thermal mode, the growth of the layers and their composition strongly depend on the total gas pressure. The pure-B content (not to be confused with the total boron content) could thus be varied in the range of∼6–70 vol. %. Next, enhancement of nitri-dation by the dissociation of NH3 into reactive radicals using a hot-wire was found to be insufficient to produce stoichiometric BN.
Finally, plasma-assisted deposition at 310 °C resulted in nearly stoichiometric polycrystalline BN with an interplane distance matching that of hexagonal BN; the material was stable in air for at least six months. The pressure dependence in the purely thermal mode is con-sistent with a growth model of BN from B2H6and NH3via the so-called surface-adduct mechanism. The effects of the radical-enhanced
methods on nitridation are explained using this model.
Published under license by AVS.https://doi.org/10.1116/6.0000132
I. INTRODUCTION
Boron nitride (BN), a wide-bandgap group III–V compound, has attracted attention because of its remarkable combination of properties.1–3 Among them, high thermal conductivity, wide-bandgap, high temperature stability and mechanical strength, low dielectric constant, and high chemical stability in air make BN an attractive material for scientific research and potential applications.1–10 BN can be synthesized in amorphous, rhombohedral, wurtzitic, cubic, and hexagonal phases, of which the cubic and hexagonal phases are the most studied.
Several methods have been used to deposit BN, including molec-ular beam epitaxy,7,11ion beam assisted deposition,12,13 magnetron sputtering,14RF sputtering,15pulsed laser deposition,16and chemical vapor deposition (CVD).6,10,17–19 Among these, CVD is the most
intensively studied, commonly using diborane (B2H6) and
ammonia (NH3) gas mixtures as precursors.20–23 However, the
high growth temperatures (close to 1000 °C) required to form crystalline films limit many (potential) applications.20,24In addi-tion, different thermal expansion coefficients of the substrate and of the deposited film negatively affect processing at high temperatures. BN films deposited at temperatures below 800 °C are usually amorphous.20,25
This work studies low-temperature (275–375 °C) CVD of BN films by alternating pulses of B2H6 and NH3 interspersed with
argon (Ar) purging. Similar to atomic layer deposition (ALD) and in contrast to continuous CVD, the pulsed approach suppresses gas-phase reactions to yield a simplified and more easily modeled mechanism of layer growth, governed by only surface reactions. However, the low substrate temperature and non-self-limiting
decomposition of B2H6 present a challenge to the production of
stoichiometric BN by this approach, as the NH3 step may fail to
nitridize all of the growth from the preceding B2H6step. Such an
incomplete nitridation results in the incorporation of pure boron (pure-B) in the layers, as shown inSec. III C. The present work addresses this issue by experimentally studying the degree of nitri-dation in pulsed low-temperature CVD of BN, both with and without methods to enhance nitridation (see below), and incorpo-rating the results in a model of layer growth by surface reactions. The fraction of pure-B in the layers is used as the measure of incomplete nitridation to be minimized.
Several radical-enhanced deposition methods have been employed to increase the degree of nitridation. In plasma-enhanced CVD or PECVD, NH3 is dissociated in a plasma discharge to
produce a variety of nitrogen-containing (NHx, where x = 0–2) radicals that react more readily with the low-temperature layer surface than NH3 itself. Surface bombardment with energetic
plasma species can additionally enhance the crystallization of the growing film. For instance, several groups have observed an improvement in the crystallinity by PECVD of aluminum nitride (AlN), zinc oxide (ZnO), hafnium oxide (HfO2), and vanadium
oxide (V2O5) films.26–30
Another approach to generate radicals is by utilizing a hot-wire (HW). In this method, a hot tungsten (W) filament heated up to ≈2700 K can be used to dissociate gas molecules such as molecular hydrogen (H2) and ammonia (NH3) into atomic hydrogen (at-H)
and NHx radicals.31–37 Compared to plasmas, this method is purely chemical as it excludes the presence of energetic charged species and (UV) photons, reducing poorly controlled chemical reactions, surface bombardment effects, and photoactivation.36–44 Differentiating between the contributions of the chemical (both heterogeneous and homogeneous) reactions by radicals (chemical process) and of the bombardment effects (physical process) can clarify the film growth mechanism. In our previous work,45 we had already started exploring the effects of using HW for the dep-osition of BN films, thereby enabling a pure-chemical but still radical-enhanced process.
The novelty of this work is in making consecutive efforts to optimize and model the film growth mechanism, with specific focus on the nitridation of the growing surface by NH3 at low
temperatures. The study was carried out in steps, with every next step increasing the degree of nitridation by various means. First, the deposition was performed in a purely thermal mode, meaning no other (except by temperature) enhancement of nitridation. This expectedly gave the lowest share of nitrogen in the films. It, however, appeared that the gas pressure remarkably played a crucial role in the nitridation process. For a particular precursor system, this finding was considered to be practically important and scientifically interesting, since, combined with the previous literature findings, it allowed to hypothesize the film deposition mechanism via the surface-adduct formation. Furthermore, we gradually increased the efficiency of the nitridation reaction by dissociating NH3into reactive species, from soft by HW to hard
by plasma. The corresponding observations were in line with the proposed mechanism. Finally, the crystalline and stable-in-air BN films were obtained with plasma at a temperature of just 310 °C. The latter was an important practical outcome.
II. EXPERIMENT A. Reactor design
Thin film growth experiments were carried out using two dif-ferent reactor systems. The purely thermal and HW-assisted deposi-tion experiments were performed using a home-built ALD/CVD cluster system equipped with three single wafer reactors connected via a loadlock, which allowed to keep the reactors under high vacuum continuously. This enabled wafer transfer without vacuum break, thus preventing surface/interface deterioration. The reactor for BN consisted of a 32 cm3 inner chamber and a path of 25– 30 cm between the radical generation region (i.e., the HW position) and the wafer. The HW was installed in a small tool above the chamber, allowing to place the HW both in and out of the line-of-sight with the substrate. More detailed information on the reactor design can be found in our previous publication.45In this study, HW-assisted experiments were performed with the HW posi-tion in the line-of-sight with the substrate, in order to minimize the recombination of generated radicals upon their delivery to the sub-strate. A resistively heated W filament was used as the hot-wire.
To perform plasma-assisted film deposition experiments, a Picosun Plasma ALD/CVD R-200 reactor was used. The precursors (B2H6and NH3) were carried by Ar gas and introduced through the
gas inlet on the top of the reactor. During the deposition process, the plasma was switched on only during the ammonia pulses. B. Substrate preparation
Prior to deposition, silicon (Si) (100) wafers were cleaned in an ultraclean environment using a standard procedure: (i) a 1-min dip into 1% HF to strip the native oxide, (ii) rinsing in de-ionized water, and (iii) ozone-steam cleaning, followed by another HF dip and rinsing.
After the cleaning process, wafers were placed in the home-built ALD/CVD reactor for thermally and HW-assisted experi-ments. For the experiments performed in the Picosun reactor, the as-cleaned wafers were first oxidized with oxygen gas at 1050 °C to grow 100 nm silicon dioxide (SiO2) layers.
C. Compositional and crystallinity analysis
The deposited films were characterized by x-ray photoelectron spectroscopy (XPS) to obtain the elemental composition and type of chemical bonding. In situ spectroscopic ellipsometry (SE) was used to monitor the actual surface temperature,46 film thickness, and optical functions in real time. Additionally, ex situ SE was employed to determine the uniformity of the thickness and optical functions across the wafer. Films deposited with plasma were further characterized by high resolution transmission electron microscopy (HRTEM), energy filtered TEM (EFTEM), grazing inci-dence angle x-ray diffraction (GIXRD), and x-ray reflectivity (XRR) techniques. XPS spectra were acquired with a Quantera SXM (scan-ning XPS microprobe, from Physical Electronics) machine using monochromatic Al Kα radiation at 1486.6 eV. The binding energies of the photoelectron lines of the samples, as received, are referenced to the C1s line of adventitious hydrocarbons at 284.8 eV. For com-positional analysis, XPS sputter depth profiling was performed using Ar ions (1–3 keV). SE measurements were carried out with a
Woollam M2000UI spectroscopic ellipsometer, operating in the wavelength (λ) range between 0.72 and 5.05 eV, in combination
withCOMPLETEEASE 5.19 modeling software. A brief explanation of
the procedure used to interpret the SE data is given inSec. II D. TEM measurements were performed with a CM300ST-FEG TEM from Philips, while GIXRD and XRR analyses were carried out with an X’pert Powder XRD system from Malvern Panalytical. D. Spectroscopic ellipsometry analysis
SE measures the change in the polarization of a beam of light reflected from the substrate, as a function of the wavelength.47–49 The change in the polarization can be related to the parameters of interest (i.e., refractive index n and extinction coefficient k) by a least-square fitting to an optical model.46,50Thus, to interpret the measured data, a reliable optical model is required. For our studies, the optical model comprised of an Si or SiO2 substrate with a
deposited layer on top.
The initial fits of the SE data were performed via Kramers– Kronig consistent B-Spline parameterization with a node spacing (control points) of 0.3 eV to determine the optical functions of any layer without assuming a certain spectral shape.51 The Sellmeier model was used to describe n(λ)-dependencies of stoichiometric BN films,46,52 while the Tauc–Lorentz46,53 oscillator model was used to describe the optical functions of amorphous boron (a-B). For layers thinner than 5 nm, an additional feature was observed and described by the Lorentz oscillator model.46This feature does not match any electronic transitions of BN in the theoretical and experimental works that have been surveyed,54–58 suggesting the presence of a second compound. Based on SE, this compound was attributed to the pure boron (B) phase (later on also mentioned as excess-B), which was then confirmed by XPS; the latter indicated boron rich B/BN composites. The parameter correlation in the SE fits was minimized by following the approach explained else-where.59,60 Sensitivity tests following Ref. 61 were performed to confirm that the SE fits represented the unique global solutions.
In our study, the Bruggeman effective medium approximation (EMA)46,62,63model was employed to describe the optical functions of a composite layer (i.e., B and BN), by using the optical functions of the two components. For this, we used the dielectric functions of a pure boron layer deposited by CVD and a stoichiometric BN layer deposited by pulsed PECVD. The validity of the composite assumption is the subject of another publication64and is not dis-cussed in this paper.
III. RESULTS AND DISCUSSION A. Purely thermal deposition
Thermal decomposition of B2H6 is not expected to be
self-limiting in the studied temperature (T) range. This was confirmed experimentally and shown in the supplementary material.130The non-self-limiting nature of B2H6chemisorption implies two
coex-isting processes in the BN formation mechanism. First, (dissocia-tive) chemisorption of B2H6 should occur during a B2H6 pulse,
presumably leading to the formation of the pure boron (pure-B) phase. The growth surface can be BHx(x = 0–3) terminated after this step. This step is not self-limiting since the pure-B layer will
keep growing with time (Fig. S1).130Second, during the subsequent exposure to NH3, the as-formed surface is expected to react with
NH3, thereby incorporating nitrogen into the layers. This step is
self-limiting in the sense that the nitrogen content in BN can hardly exceed a certain share.
In the subsequent experiments, the effects of substrate tem-perature, diborane dose, and total pressure (ranging from 5 × 10−3 to 12 mbar) on the growth rate (growth per cycle, GPC) of the films and on the amount of excess-B were investigated. The exper-imental conditions can be found in Table S1 in the supplementary material.130It was observed that the nitrogen content was hardly affected by lowering the substrate temperature from 375 to 290 °C, whereas the GPC decreased from 0.045 to 0.021 nm/cycle. These results suggest that the temperature rather affects the B2H6
(disso-ciative) chemisorption than the pure-B nitridation. Strong enhancement of the GPC with increasing B2H6 pulse time, as
shown in Fig. 1(a), implies that the GPC is limited by the B2H6
dose. The B2H6pulse time also crucially determines the excess-B
in the layers [Fig. 1(b)].
Importantly, increasing the total pressure (Ptot) leads to higher
nitrogen content and a correspondingly lower amount of excess-B [Fig. 2(a)]. However, a gradual decrease in the GPC (Ptot< 1 mbar)
was observed with increasing the pressure [Fig. 2(b)]. This rather unusual result can be explained by a decreased delivery of B2H6to
the wafer at a higher pressure (i.e., lower diffusion rate of B2H6) or
a suppressed (dissociative) chemisorption of B2H6(the rate limiting
step) on the as-nitridized NHx-terminated (x = 1, 2) surface com-pared to the BHx-terminated surface. Since increasing Ptot has a
remarkably large impact on both GPC and excess-B, we have expanded the pressure range up to 12 mbar (seeFig. 2).
Although all data were obtained at somewhat different B2H6-pulse, Ar-purge, and NH3-pulse durations [see Table SI for
details (Ref. 130)], the data given inFig. 2(a)exhibit a clear trend of GPC versus Ptot. Particularly, the GPC slightly decreases with
increasing the pressure for Ptot < 1 mbar and starts to increase
rapidly for Ptot exceeding 1 mbar. The excess-boron share (as
obtained by SE) gradually decreases with increasing the Ptot,
being as high as∼70 vol. % in the low-pressure range and as low as ∼6 vol. % in the high-pressure range [Fig. 2(b)]. In spite of showing the cumulative data of different reactors (experiments at Ptot≥ 1 mbar were carried out in the Picosun reactor, in the
purely thermal mode, whereas the low-pressure data were obtained using the home-built cluster system) and substrates (Si and SiO2),Fig. 2 indicates a consistent and clear effect of Ptot.
We consider these findings to be a strong point, indicating a general trend, weakly dependent on the actual reactor geometry and the type of substrate used.
Obviously, changing Ptot affects the partial pressures of both
diborane (PB2H6) and ammonia (PNH3). However, it should be
noted that PNH3 was one order of magnitude higher than PB2H6.
Particularly, for Ptot≥ 1 mbar, PB2H6 was in the range of 0.01–
0.1 mbar, whereas PNH3 ranged from 0.1–2 mbar. Likewise, for
Ptot< 1 mbar, the partial pressures were of the orders 10−4–
10−3mbar (B2H6) and 10−3–10−2mbar (NH3). Considering the
large difference in the partial pressures, in combination with the significant decay of excess-B with increasing Ptot, a dominant role
provides a quantitative description of the relation between pressure and excess-B.
As stated above, the film growth occurs via the sequential steps of (i) growing pure-B during each B2H6pulse and (ii) nitridation of
the as-formed B during the subsequent NH3pulse. The rather strong
increase of the GPC at Ptot≥ 1 mbar, which coexists with the
gradu-ally decreasing amount of excess-B in the films, suggests nitridation to control the growth at Ptot≥ 1 mbar. The nitridation may in turn
enhance the B formation by changing the termination toward getting more NHx(x = 1, 2) surface groups.
Summarizing, thermal depositions performed at Ptot< 0.1 mbar
indicate a crucial role of PB2H6 in the growth rate and film
compo-sition, whereas the impact of PNH3 dominates at Ptot≥ 1 mbar (see
Fig. 2), increasing the GPC and lowering the excess-B. Decreasing the excess-B below 6 vol. % is, however, challenging in purely thermal mode and at low temperatures. We, therefore, continued the study to explore two radical-enhanced approaches. To explore the effects of radicals on both the film growth rate and composition, and thereby to indirectly confirm the proposed deposition mecha-nism, we gradually changed the degree of dissociation of NH3from
soft by HW to hard by plasma. FIG. 1. Dependence of GPC (a) and excess-B share (b) on B2H6pulse
dura-tion, as measured byin situ SE. Refer to Table SI in the supplementary material (Ref. 130) for deposition conditions. The error bars for (a) are within the symbol size, not exceeding 0.001.
FIG. 2. SE measurements of GPC (a) and excess-B (b) plotted vs total gas pressure. Symbols: squares for 2 s B2H6-pulse duration, diamonds for 0.2 s,
and triangles for 0.1 s B2H6-pulse duration. The experiments atPtot≥ 1 mbar
were carried out in the Picosun reactor, whereas forPtot< 1 mbar, the cluster
system was used. Refer to Table SI in the supplementary material (Ref. 130) for other deposition conditions. The error bars for (a) are within the symbol size, not exceeding 0.001. The dashed line in (b) represents the model further detailed inSec. IV B.
B. Radical-enhanced deposition 1. Hot-wire assisted deposition
The impact of an HW on the dissociation of NH3 has been
investigated by several groups, confirming the formation of mainly NH2radicals.36,44This also means providing a corresponding flux
of atomic hydrogen (at-H) to the substrate. For NH3 dissociation,
the at-H flux is expected to be larger than that of NH2, due to a
higher recombination probability of the latter. The generation, recombination, and delivery of radicals to the growth surface were previously studied in our experiments on tellurium etching (for at-H) and silicon nitridation (for NHx).45
The HW-assisted deposition led to a dramatic decrease in the GPC with increasing HW temperature (THW). In particular, being
∼0.01 nm/cycle in the HW-off mode (i.e., HW at room tempera-ture), the GPC decreased to∼0.001 nm/cycle at THW= 2100 K. [an
efficient dissociation of NH3was already observed at THW> 1600 K
(Ref. 45)]. In spite of the lower GPC, more nitrogen was incorpo-rated into the layers with increasing THW. Under specific
condi-tions [see Table SI (Ref. 130)], the excess-B decreased from ∼50 vol. % at THW= 300 K to∼37 vol. % at THW= 1800 K, as
deter-mined by SE and later confirmed by XPS. Furthermore, a clear shift in the XPS B1s peak position toward that of BN was observed with increasing THW(seeSec. III C).
Although utilizing an HW has a clear effect on both GPC and stoichiometry, the concentration of N-containing radicals seems to be insufficient to nitridize the growing film to the extent required for the production of stoichiometric BN. Additionally, the competing interaction of at-H with the surface was hypothesized to decrease the GPC, as schematically illustrated inFig. 5(b).
We must bear in mind that the gas pressure crucially affects both the film growth rate and the film composition in a purely thermal mode at low temperatures (Sec. III A). Further decreasing the excess-B to below 6 vol. %, as shown inFig. 2(b), to approach stoichiometric BN, is however problematic. The soft dissociation of NH3by HW does not sufficiently enhance the film nitridation. To
obtain stoichiometric and crystalline BN at only 310 °C, we there-fore proceed with a plasma radical source to ensure a higher degree of NH3dissociation.
2. Plasma-assisted deposition
The depositions were performed on thermal SiO2 (100 nm),
with the Picosun reactor at a substrate temperature of 310 °C. We must bear in mind that the previously described experiments were performed at comparable substrate temperatures of 290 ± 20 °C. The total gas pressure was kept at 1 mbar for all the plasma experi-ments, corresponding to the excess-boron share of ∼6 vol. % obtained in a purely thermal mode [Fig. 2(b)]. The other process conditions were kept identical to the thermal counterpart. Remarkably, the GPC in plasma (0.017 nm/cycle) was hardly changed compared to that in the thermal mode (0.02 nm/cycle), meaning that the plasma merely enhanced the nitridation process. The latter was confirmed by both SE and XPS. The XPS revealed nearly stoichiometric BN films on the surface (see Fig. S3).130The in-depth increasing difference between the B and N shares is possi-bly due to preferential sputtering.65
C. Comparative analysis of the film properties 1. XPS analysis
A comparison of B1s and N1s spectra showed a significant influence of the growth methods. As mentioned, the amount of excess-B, as estimated by SE and confirmed by XPS, decreased from∼50 to ∼37 vol. % for the thermally and HW-deposited films. The plasma enhancement resulted in 46 at. % of N, 50 at. % of B, and 4 at. % of oxygen.
The B1s spectra were obtained after sputtering the initial 0.5– 1 nm of the film surface to prevent the contribution of various surface contaminants. A comparison of the B1s spectra of pure-B as well as thermally, HW-, and plasma-deposited B/BN films is shown inFig. 3.
Going from thermal through HW- to plasma-assisted depo-sition, one can clearly see that the B1s peak broadens toward higher binding energies and that the binding energy of the main peak gradually shifts from 188.4 eV (i.e., BZB bonding66) to 190.6 eV (i.e., BZN bonding17,18,67). This confirms that effective nitridation is enabled by external energy sources. In addition to the B1s spectra, we analyzed the N1s binding energies [Fig. 3(b)]. For all the samples, the N1s peak positions corresponded to
FIG. 3. (a) XPS B1s spectra of pure-B (i), thermally, and HW- deposited B/BN [(ii) and (iii)], and plasma-deposited BN films (iv). (b) XPS N1s spectra of films (ii)–(iv) from (a). For deposition conditions, please refer to Table SI (Ref. 130).
stoichiometric BN; no shift toward higher or lower binding ener-gies was observed.
To support the increase in the BN share in the B1s peak with an increasing degree of nitridation, B1s peak fitting was performed (Fig. S4).130It was concluded that the broad features seen in the thermally and HW-deposited films could adequately be described by two Gaussian peaks fixed at the binding energies of pure-B and stoichiometric BN. On the other hand, the B1s peak of the plasma-deposited film was solely fitted by a single Gaussian peak at the BZN binding energy. These results are consistent with the forma-tion of mixed pure-B and stoichiometric BN phases in the ther-mally and HW-deposited films, whereas plasma assistance leads to growing nearly stoichiometric BN.
2. SE analysis
Applying the EMA model (seeSec. II Dfor details) confirmed the presence of excess-B in the samples shown in Fig. 2. The plasma samples showed no detectable amount of excess-B, which was further confirmed by EFTEM elemental mapping, indicating the presence of only nitrogen and boron with some trace amounts of oxygen (see Fig. S5).130
3. Stability in air
The environmental stability of BN is of prime importance for practical applications and occurs for films with a sufficient degree of crystallinity. It is known that (partially) amorphous and low-crystal-order BN films can quickly degrade in air due to their interaction with oxidizing species,20whereas h-BN layers are chem-ically inert in many environments.68Degradation in air is, there-fore, a measure of the film crystallinity and nonstoichiometry.
The degradation can be investigated by ex situ SE. The mea-surements were performed for the as-deposited (i.e., measured within 20 min) samples and after their exposure to air for more than 6 months. To draw conclusions regarding the degradation of films, a comparison of the delta values was carried out, since delta is rather sensitive to changes in their thickness and optical functions.46It can be concluded from Fig. S6130that the thermally and HW-deposited samples significantly changed their optical responses after the 6-month exposure, presumably due to their oxidation, whereas the plasma-deposited samples showed no changes. The latter might be the first indication of having a crystalline structure.20,24
4. HRTEM, GIXRD, and XRR analyses
The crystalline structure of the plasma-deposited films is con-firmed by HRTEM.Figure 4(a)shows a lamellar-type structure with the crystal planes perpendicular to the substrate surface. A line profile analysis (the inset) reveals that the interplane distance (d-spacing) ranges between 3.4 and 3.8 Å. These values fall in the range previ-ously reported for hexagonal BN (3.3–3.7 Å).6,10,19,25,69–71
Further confirmation of the interplane distance was carried out by fast Fourier transform (FFT) analysis of the three selected sections (FFT1, FFT2, and FFT3), using radial profile analysis with
IMAGEJ1.52i (Ref. 72) and RADIAL PROFILE EXTENDED (Ref. 73)
soft-ware. The three boxed areas shown inFig. 4(a)revealed an average interplane distance of 3.57 Å, with a second periodicity at 2.16 Å,
as shown in Fig. 4(b). The former matches with the (002) d-spacing of h-BN (3.3–3.7 Å), while the latter corresponds to the (100) d-spacing of h-BN (2.17 Å). This is close to the values of sphalerite β-BN (2.09 Å)74and wurtziteγ-BN (2.20 and 2.10 Å).75 These two structures, however, do not have any d-spacing above FIG. 4. (a) HRTEM image of a plasma-deposited BN layer (after being stored in air for 4 months) on thermally grown SiO2. The three boxes indicate the
areas for FFT analysis. Inset: line profile inside the area FFT1, showing that the interlayer distance ranges between 3.4 and 3.8 Å (with a pixel size of 0.2 Å). (b) Radial profiles of the FFT power spectra of the three boxed areas in (a).
2.2 Å, and so cannot cause the broad peak centered at 3.57 Å. In the 3.3–3.7 Å range reported for BN, larger d-spacings have been attributed to curved76 and turbostratic77 BN. The d-spacing may increase due to the presence of ZNHx (x = 1, 2) groups in BN, similar to the effect of the hydroxyl or carboxyl groups reported for graphite.78,79
The GIXRD analysis of the plasma-deposited film shown in
Fig. 4confirmed the occurrence of the h-BN phase. The diffraction peak positions coincide well with the values reported for h-BN (Ref. 80) (Fig. S7).130XRR revealed a thickness of 14.3 nm, which is in accordance with the TEM and SE results. Fitting the mass density revealed 1.82 g/cm3, which is close to 1.89 g/cm3reported for h-BN films.81
IV. THE MECHANISM OF GROWTH AND NITRIDATION A model of the layer growth and nitridation by surface reac-tions is required to understand the demonstrated dependence of both GPC and excess-B share on Ptot (recallFig. 2).Section IV A
hypothesizes such a model based on the literature. Section IV B
applies the model specifically to incomplete nitridation described inSec. III A.Section IV Cdetails how the model can incorporate the observed effects of hot-wire and plasma enhancement on nitri-dation. The agreement with these observations identifies the pro-posed model as a candidate to be confirmed by more direct methods, such as in situ vibrational spectroscopy of surface groups. A. Surface-adduct pathway for purely thermal
deposition
It is well known that the so-called Lewis-acid and Lewis-base compounds can react with each other, resulting in the formation of a gas-phase adduct.82The formation of adducts on the surface was confirmed for ALD of AlN83–85and suggested to play a role for ALD of gallium nitride (GaN).86–88 Details can be found in the supplementary material.130
Since B2H6is also a known Lewis acid that forms adducts,89–96
in this study, we attempt to adopt the adduct-assisted pathway for growing BN from B2H6and NH3. The dependence of both the GPC
and the composition (degree of nitridation) on the total pressure, as demonstrated inFig. 2, may also be interpreted as the dependence on PNH3, indeed suggesting the adduct-assisted mechanism. To the
best of our knowledge, hypothesizing the role of the surface-adduct for growing BN from B2H6 and NH3at low temperatures has not
been done so far.
The formation of the gas-phase H6B2:NH3compound from
B2H6 and NH3has been suggested to be the most energetically
favorable reaction, based on the calculations performed by Nguyen et al.93Furthermore, through a subsequent release of H2,
first H2BvNH2and then HBvNH species can be produced. As
the reaction temperature increases, the gas-adduct complex pro-gressively loses hydrogen, and at temperatures above 500 °C, BN can finally be obtained.21,22,92,97–100 Gómez-Aleixandre et al.21 and Rodriguez et al.101 showed that, as a result of the limited (partial) dissociation of ammonia at temperatures below 850 °C, the interaction between diborane and ammonia is strongly tem-perature dependent.
The gas-phase decomposition of B2H6was studied by many
researchers.102–109 On the surface, B
2H6 is known to cause a
non-self-limiting formation of solid boron.109–112 The studies of Fehlner,113–116 Baylis et al.,117 and Söderlund et al.118 suggested that BH3species dominate in the gas phase. (Since BH3is also a
known Lewis acid,119 it readily forms the gas-phase H 3B:NH3
adduct91by reacting with NH3.) Mohammadi et al.111proposed a
growth model based on the chemisorption of BH3and the
subse-quent elimination of H2. A similar mechanism for the interaction
of B2H6 with silicon oxide surfaces has been proposed by
Gillis-D’Hamers et al.120,121The authors showed that, upon chem-isorption, diborane forms BH- or BH2-terminated surface sites. To
simplify the reaction schematic in our case, we have only drawn BH-terminations, as shown inFig. 5.
B. Excess B fraction described by the surface-adduct model
Applying the model to our work, the initial step is likely the decomposition of B2H6 (in the gas phase or on the surface),
forming BHx-terminations (x may be 1 and/or 2) accompanied by
the release of H2. Because this decomposition is not self-limited,
additional species of B2H6 and/or BH3 may already react with
BHx-terminated sites, encapsulating the underlying B and releasing its hydrogen termination as H2. These B atoms can then no longer
be nitridized in the subsequent NH3step, since the latter requires
an accessible BHxgroup at the surface (see below). Let F denote the fraction of B atoms bonded to hydrogen at the surface of the growing layer when the NH3pulse begins.
FIG. 5. Proposed schematic of the nitridation step in the formation of BN from pulses of B2H6 and NH3. (a) In thermal mode, via the formation of a
surface-adduct and then its conversion into BZNH2ZB linkages. (b) In
radical-enhanced mode (HW or plasma), via additional removal of surface species by atomic hydrogen (at-H) and involvement of NHx(x = 1, 2) radicals, and via direct
nitridation by atomic nitrogen (for plasma only). According to Gillis-D’Hamers (Refs. 120 and 121), upon chemisorption, diborane forms BH- or BH2-terminated surface sites. To simplify the schematic reaction diagram, we
In the next step, we hypothesize the chemisorption of NH3to
BHx-terminated surface sites yielding a surface-adduct, analogous
to the known91gas-phase H
3B:NH3adduct. Modeling the
chemi-sorption with a Langmuir isotherm,122Eq. (1)gives the fractionθ of the available BHxsites that are covered with NH3in equilibrium,
θ ¼ KeqPNH3
1þ KeqPNH3, (1)
where Keqis an equilibrium constant with dimensions of inverse
pres-sure, related but not equal123to the dimensionless equilibrium cons-tant. As also in the thermal ALD of GaN,124the adsorption of NH
3is
expected to reach equilibrium much faster than the duration of the pulse. Taking into consideration that the chemisorption of NH3leads
to nitridation (see below) and the partial pressure PNH3 scales up with
the throttle-regulated total pressure Ptot,Eq. (2)provides an expression
for the atomic fraction of excess B, fB,
fB¼ 1 Fθ ¼ 1 F KPtot
1þ KPtot
, (2)
where K is the product of Keqand the pressure scaling factor (0.182 in
this work). We show elsewhere64that f
Bmay be approximated by the
volume fraction of pure-B determined by SE. The model ofEq. (2)
describes the trend of Fig. 2(b), using K = 200 mbar-1 and F = 0.92. The Langmuir isotherm (and thus K) dominates the trend at low pres-sures, whereas F determines the lowest pure-B fraction possible without the radical enhancement of nitridation (seeSec. IV C). The model may be improved by including the dependency of F on the duration of the B2H6pulse.
After the adsorption of NH3, the reaction of the adduct-NH3
with hydrogen of a neighboringZBHx-termination occurs, releas-ing H2[Fig. 5(a)]. This forms—NH2- bridges between the
neigh-boring B atoms (i.e., BZNH2ZB linkages), analogous to
AlZNH2ZAl linkages in AlN ALD.84 Such BZNH2ZB linkages
laterally expand over the entire growth surface in a self-limiting fashion. Exposure to B2H6during the subsequent precursor pulse
restores the required BH-terminations (not a self-limiting process), once again releasing H2. It should be noted that direct thermal
nitridation of pure-B is thermodynamically inhibited at tempera-tures below 900 °C.125However, via the proposed adduct-assisted pathway, nitrogen can still be effectively incorporated at much lower temperatures.
C. A comparison of purely thermal and radical-enhanced modes
Applying HW or plasma is expected to alter the growth mech-anism and, therefore, can be considered to be an indirect confirma-tion of the proposed model. In particular, the dissociaconfirma-tion of NH3
into NHx(x = 0–2) radicals36,37,126,127and at-H may facilitate alter-native chemical routes and thus suppress the adduct-assisted pathway. Furthermore, plasma enhancement of nitridation includes surface bombardment by energetic particles that may enable the nitridation of subsurface boron [effectively increasing F inEq. (2)], whereas hot-wire enhancement lacks this effect.
The effect of radicals from a hot-wire or plasma on nitridation greatly depends on the efficiency with which various radical species are generated and transported to the growing layer. First, the gener-ated at-H can remove hydrogen of the BH-terminations (the so-called hydrogen abstraction128,129), thereby eliminating the B-H surface sites which are essential to nitridation by the adduct-assisted mechanism [Fig. 5(b)]. If reactive NHxradicals are present in suffi-cient quantities, they can still support the formation of BZNH2ZB
and/or BZNHZB linkages. Atomic nitrogen can further directly nitridize elemental boron. Therefore, for optimized process condi-tions, radical-enhanced methods increase the nitrogen share, still having diborane chemisorption as the rate limiting step.
On the other hand, nonoptimized conditions (i.e., not enough NHxradicals but too much at-H) may lead to lower GPC
(seeSec. III B) and nonstoichiometric B share. The latter is due to a decreased efficiency of nitrogen incorporation via the thermally assisted surface-adduct mechanism, as schematically shown in
Fig. 5(b). A deficiency of NHxradicals at the growth surface and
an excess of at-H, for example, due to a low-power radical source or because of recombination on the way to the substrate,36may result in B-rich samples.
Our experiments show that the highest degree of nitrogen incorporation is provided by the plasma-assisted growth; this gives nearly stoichiometric, crystalline, and air-stable BN films at a sub-strate temperature of 310 °C. Enhancement of nitridation by using a hot-wire was much less effective, presumably due to a suboptimal flux of radicals (e.g., too much at-H) and/or a lack of surface bom-bardment to nitridize subsurface B.
V. CONCLUSIONS
This work studied the formation of B/BN composites using purely thermal, hot-wire, and plasma-assisted deposition methods by sequentially pulsing B2H6and NH3precursors. It has been
dem-onstrated that, in the purely thermal mode, increasing the total gas pressure considerably enhances the GPC, incorporating a bigger share of nitrogen in the layers and thereby reducing the pure-B content from∼70 to 6–7 vol. %. However, a further decrease in the excess-B share, to obtain a stoichiometric BN in thermal mode, is challenging. To improve both stoichiometry and crystallinity, HW-and plasma-assisted methods have been explored. Utilizing the HW had a clear effect on both the GPC and the film composition, although the concentration of NH2 radicals was not sufficiently
high enough to effectively decrease the excess-B share. Therefore, switching to plasma provided nearly stoichiometric, crystalline, and air-stable BN films at just 310 °C.
Concerning the growth mechanism in the purely thermal mode, we hypothesized a surface-adduct-assisted reaction pathway, analogous to the earlier suggested surface-adduct mechanisms for growing AlN and GaN by thermal ALD at low temperatures. In par-ticular, BH-terminated surface sites react with NH3, forming surface
adducts. The adduct-NH3 consequently reacts with hydrogen of a
neighboring BHx-termination (x = 1, 2), forming BZNH2ZB
link-ages and releasing H2. A subsequent B2H6pulse restores the original
BHx-terminations, once again eliminating H2.
The experimentally measured excess-B share was modeled based on the surface-adduct mechanism, describing the adsorption
of NH3that leads to nitridation with a Langmuir isotherm. Both
the model and the data show that the adduct-assisted pathway enables effective incorporation of nitrogen into BN films at low temperatures and sufficiently high partial pressures of NH3.
However, subsurface B is not nitridized as it lacks the BHxgroup
required for the formation of the adduct. Applying HW or plasma provides additional reaction pathways and alters the growth mecha-nism, further enhancing nitridation under optimal conditions. ACKNOWLEDGMENTS
This work was financially supported by The Netherlands Organization for Scientific Research (NWO), Domain Applied and Engineering Sciences (TTW), Project No. 13929. The authors thank G. A. M. Kip and E. G. Keim of the MESA+ Institute for performing XPS and (EF)TEM analysis, respectively. We further thank Seda Kizir (XUV group, University of Twente) and Sourish Banerjee (IDS group) for their help with XRR and GIXRD mea-surements, respectively. Jurriaan Schmitz (IDS) is acknowledged for the fruitful discussions. Toyota Europe and ASM International are acknowledged for their partial financial support of this project. REFERENCES
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addi-tional information on pure boron CVD, list of deposition conditions, XPS sputter depth profiles and peak deconvolution, EFTEM elemental mapping, air-stability tests, GIXRD data, and the surface-adduct mechanism.
