Effect of Terminating Species on the Initial Growth of BN on Diamond

Article pubs.acs.org/JPCC

Effect of Terminating Species on the Initial Growth of BN on Diamond Substrates Anna Pallas* and Karin Larsson Department of Chemistry − Ångström Laboratory, Uppsala University, Box 523, Uppsala, 751 20 Sweden ABSTRACT: The details in the layer-by-layer formation of H (or F)-terminated boron nitride onto diamond (100) have been theoretically studied using ab initio density functional theory under periodic conditions. Cubic boron nitride, c-BN, is a very interesting and promising material due to its extreme properties. However, there are severe problems during the vapor phase synthesis of c-BN because of the formation of noncubic phases in the initial grow steps, why a gentle large area chemical vapor deposition (CVD) deposition is needed. The substrate material has been experimentally shown to be very important for an ideal growth of c-BN in the initial grow process. Diamond is a material that has been found to be a good substrate material for this purpose. By alternating H (or F)-terminated B and N layers, and calculating the resulting interfacial binding strengths and geometrical structures, the initial growth has been studied and compared to earlier theoretical results that have been made without terminating species. Two different structural alignments, with respect to the underlying diamond substrate, were initially constructed. One model was heterostructurally positioned on top of the diamond (100) substrate, while the other model had the x-axis of the c-BN lattice aligned with the y-axis of the diamond lattice. For the situation with a terminated monatomic B layer on top of diamond, the heteroepitaxially built structure was the most energetically favored. This result is just the opposite from the nonterminated situation, where the nonheteroepitaxial structures were favored. The binding energy for the heteroepitaxial terminated monolayer of N was calculated even stronger than the terminated B monolayer. When applying a second atomic layer of c-BN on top of the monatomic B layer, the heteroepitaxially build structure is energetically preferred. For the nonheteroepitaxial growth the adlayer actually bonds to the diamond substrate when Hterminated. Without terminating species the nonheteroepitaxial adlayer did not bind at all. Also for the situation with two atomic adlayer with N closest to the diamond substrate, the heteroepitaxial structure is energetically preferred over the nonheteroepitaxial structure. The opposite is true without any terminating species. When four, six, and eight atom layers are applied, with N atoms closest to the diamond substrate, the heteroepitaxially built structures are favored. The results showed that the terminating species helped to uphold the cubic sp3-formation. When using terminating species, the heteroepitaxially built interfaces had generally higher binding strengths than the nonheteroepitaxially built interfaces. While, when not using terminating species, the nonheteroepitaxially built interfaces were preferred instead. The c-BN structure seems to be more easily formed when several BN layers already are formed, and the interfacial binding strength is stabilized.

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INTRODUCTION Cubic boron nitride, c-BN, exhibits several excellent physical and chemical properties, of which some are comparable (and even superior) to diamond. Examples of properties are extreme hardness, chemical stability, large band gap, wear resistance, and chemical inertness. 1−3 Furthermore, the oxidation and graphitization temperatures for c-BN are much higher compared to diamond (1200 and 1500 °C vs 600 and 1400 °C).4 Hence, c-BN is a promising material as a tool for coating machining of steel, cast iron, and ferrous alloys. It is also a promising material for cutting tools, thermal, optical, hightemperature, and high-frequency electronic devices.3−5 Being associated with the deposition techniques, severe problems within the vapor phase synthesis of c-BN have led to high compressive stress and film delimitation. When using techniques based on surface bombardment with highly energetic ions, a mixture of BN phases (amorphous, turbostratic, and cubic) is often the result for the first atomic layers of c-BN (in the interface between the substrate material and BN).6 Also, high compressive stresses (up to 20 GPa), poor crystalline quality, small grain sizes, and high defect © 2014 American Chemical Society

density are usually the result when using these types of methods.4,7−9 As a result, the film will easily peel off from the substrate at thicknesses over 200 nm.4,6−8 Cubic BN films have, however, recently been deposited without the presence of any distinctive interlayer of turbostratic or amorphous phases between the diamond substrate and cBN. This was accomplished by using fluorine-assisted plasmaenhanced chemical vapor deposition (PECVD) on singlecrystalline diamond substrates.6 The bias voltage applied on the substrate during the deposition was only about −20 V, which led to a low film stress of only 2 GPa and, hence, also to good film/substrate adhesion and energetic stability.6 There is at present an urgent need to develop more gentle vapor phase methods whereby it is possible to grow defect-free c-BN films without noncubic interlayers. The role of the substrate material has earlier been found to be very crucial. It must not only help improving the binding between the Received: August 20, 2013 Revised: January 14, 2014 Published: January 28, 2014 3490

dx.doi.org/10.1021/jp4083213 | J. Phys. Chem. C 2014, 118, 3490−3503

The Journal of Physical Chemistry C

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substrate material and the film. It should also be able to control the phase ratio (within the BN film) in favor of the cubic phase.10 The lattice mismatch is a third factor of ultimate importance for heteroepitaxial c-BN growth. The most common substrate materials of today falls within the following categories: hard (diamond, β-SiC, Si),11−15 soft (metals), and ionic (KBr, KI).16,17 The initial nucleation stage during vapor phase deposition of thin films is expected to have a major effect on the quality of the film. This is especially the situation for c-BN growth, for which the initial nucleation stage of the cubic phase is much more difficult to achieve than the continuous film growth (i.e., the parameter window for the initial nucleation stage is much more narrow than for the later stage of growth).6,16,18 In this respect, the combined effect of substrate and surface termination, on the c-BN nucleation, is of a large importance to study more thoroughly. The purpose with the present study was therefore to theoretically investigate this initial growth process of c-BN (100) on a diamond (100) substrate by using density functional theory (DFT) under periodic boundary conditions. The effect of a diamond substrate, in combination with variously terminated surfaces, was accomplished by using either H- or F-terminated BN adlayers. Further, also the adsorption and abstracting processes of the terminating species are very important steps for the c-BN film growth. These studies have been performed earlier by Karlsson et al.19 The reason to use diamond (100) as a substrate was twofold: (i) there is a very small lattice mismatch between diamond (100) and c-BN (100), and (ii) recent successful growth results have shown it possible to grow the cubic phase of BN directly onto the diamond surface (without any intermediate noncubic phases of BN).6−8 For this combinatorial choice of substrate material/terminating species, the effect on adlayer structural phase and stability has been carefully investigated by calculating the substrate/BN adlayer binding energies and geometrical structures. The main goal with the present investigation has thereby been twofold: (i) to get a deeper knowledge about the effect of H (or F)-termination on the initial layer-by-layer growth of c-BN and (ii) to understand the underlying structural and chemical factors that makes diamond an efficient substrate for phase-pure growth of c-BN.

populations within interfacial C−B (or C−N) bonds were calculated. Also, the degree of electron transfer within these bonds was estimated by calculating individual atomic charges. The DFT method used in the present investigation is based on an ultrasoft pseudopotential plane-wave approach using the Cambridge Sequential Total Energy Package (CASTEP) program package from Accelrys, Inc.20−22 The electronic exchange and correlation corrections were approximated using the Generalized Gradient Approximation (GGA-PW91) developed by Perdew and Wang.23 The plane wave kinetic energy cutoff was set to 240 eV, and the k-point mesh generated from Monkhorst Pack was set to 2 × 3 × 1 (yielding 3 k-points).24 The Mulliken analysis method was used to partitioning the electron density to individual atoms in calculating the atomic charges and electron bond populations in the region between two atoms. The plane wave states were thereby projected onto the local atomic orbital basis sets.25,26 The degree of electron transfer between the diamond substrate and the BN structure was estimated from the atomic charges, from which ion binding information is received. The electron bond population is generally regarded to be a measure of the covalent bond strength.27 The interfacial models were constructed as repeated supercells. The diamond (100) substrate was a (2 × 1) reconstructed surface comprised of a 5-layer slab of 12 atoms per layer. The BN (100) adlayer consisted of alternate layers of N and B atoms, also with 12 atoms per layer. The upper BN surface atoms were terminated by either F or H atoms. The bottom C layer of the diamond substrate was H-terminated and kept fixed during the geometry optimization, in order to simulate a continuous bulk structure during the calculations. All other layers, including the BN surface terminating species, were allowed to fully relax by using the BFGS algorithm (Broyden− Fletcher−Goldfarb−Sharmo).28 Careful test calculations showed that the total energy difference for the situations with one or two fixed bottom C layers is less than 0.002%. In addition, earlier test calculations have shown that a vacuum slab thickness of 7.5 Å is optimal to use for the present type of investigation.29 Hence, it was found adequate to use one fixed bottom C layer, and a vacuum level of 10 Å, in the present study. Four different types of interfaces have been included in the present investigation: with either N or B attached to the diamond (100) substrate and with the BN adlayer being either F (or H)-terminated. The here obtained theoretical results were all compared with earlier results where no terminating species were used.30 The initial layer-by-layer process could sequentially be studied by modeling, and optimizing, 1, 2, 4, 6, and 8 atomic layers of BN, where all were either ideally heteroepitaxially, or nonheteroepitaxially, attached to a diamond (100) substrate. With the exception for a single BN layer, all thicker BN adlayers were built by adding new layers ontop of a non-geometry-optimized interface. Different BN//diamond substrate interfacial matching were also used as initial models. The heteroepitaxial matching was done by aligning the x-axis of c-BN with the x-axis of the diamond surface. For the nonheteroepitaxial situation, the cubic BN structure was rotated with respect to diamond so that the overlayer x-axis was aligned with the y-axis of the substrate (i.e., a 90° rotation).

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METHODS The main focus in the present investigation was to study the initial layer-by-layer growth of c-BN. Hence, the development of interfacial binding energies during this sequence of growth was of greatest importance to study. The interfacial binding energy between the diamond (100) surface and the BN adlayer has been calculated using eq 1 ΔE binding = Etot − Ediamond − E BN

(1)

where Etot is the calculated total energy, obtained from the geometry optimization calculation. Ediamond and EBN are the calculated total energies for the diamond substrate and the adlayer (also including terminating species), respectively. Both values were obtained by performing single-point calculations for the corresponding optimized geometries as obtained from the Etot calculations. The interfacial bonding energies, as calculated by eq 1, are generally regarded to be a measure of adhesion strength between the diamond substrate and the growing BN film. For further validation and analyses of the structural evolution in the interfacial region, the electron bond 3491



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RESULTS AND DISCUSSION A. General. The possibility for an initial cubic BN growth on a diamond (100) substrate has earlier been investigated by the present authors, but without the assumption of a terminated growing BN film. The purpose was to simulate the deposition of BN using high-energy ion bombarding methods like ion-beam assisted evaporation PVD, where the total gas phase pressure is so low that termination of the BN surface with species like H, or F, is not expected to take place to any larger extent (at least much less than 100%).30 The obtained results in this work were found to strongly support the experimental observations made in refs 31 and 32 where the nucleation of c-BN is proceeded by a soft interlayer of amorphous and turbostratic BN. It was clear that it is most favorable to start the deposition with N directly onto the diamond substrate. In fact, the energetically most preferred interfacial structure was a heteroepitaxial one. However, for the following B layer, a more amorphous-like structure became energetically favored. When sequentially increasing the adlayer thickness to a four atomic layer thick, a heteroepitaxial cubic phase of BN was again found to be the most energetically favorable one. In contrast to the situation with N, when starting the initial growth with B closest to the diamond substrate, the nonheteroepitaxial structure becomes energetically the most preferred one. In addition, the next-coming N layer (applied onto the first B layer) will result in a complete structural “liftoff” of the whole BN adlayer from the surface. In summary, these earlier studies showed that, for a situation with bare (i.e., nonterminated) surface areas, an initial growth of BN (100) onto diamond (100) will most probably result in an amorphous-like structure closest to the diamond substrate. As presented above, this is a result that strongly supports the experimental findings using highly energetically PVD methods, where not completely terminated surface are expected due to low gas phase pressures in the reactor. In addition to “brute force” PVD methods, a rather successful c-BN growth has been obtained using rough and faceted diamond (100) substrates at low substrate bias (−20 V) and by using an electron-cyclotron-resonance microwave plasma CVD setup.6 The gas mixture contained N2, BF3, and H2 as reactant gases, with a pressure kept at about 2 × 10−3 Torr. This resulted in 2 μm thick c-BN films with a low film stress of about 2 GPa.6 Even though rather good results have been obtained with CVD methods based on low-energy ion bombardment, it has never been shown possible to use more gentle CVD (or ALD) methods in the growth of phase pure c-BN thin films. Especially the ALD method should be highly versatile for the development of a precursor design for c-BN growth. The atomic layer deposition (ALD) technique is a thin film deposition method that sequentially introduces different gaseous precursors into the reaction chamber, thereby depositing a monatomic layerby-layer thin film of desired form and with a high and even thickness control. It would be highly feasible to tailor-make the necessary surface termination design by using this specific deposition technique in the growth of c-BN. The purpose with the present theoretical investigation was to outline the combinatorial effect of substrate and surface termination type on the structural evolution of c-BN during a layer-by-layer ALD growth of BN onto diamond (100). By starting the deposition with either a monolayer of N (or B)

closest to the diamond substrate, and thereafter increase the layers up to a maximum of eight atom layers the, the layer-bylayer growth process has been carefully studied. Either H (or F) has here been used to terminate the BN surface. Because of previous experimental results, only H was used as terminating specie on the N atoms, while both H and F species were used on the B atoms.31 For all adlayer scenarios, both interfacial binding energies and the geometrical structures of the interfaces were carefully calculated. For all of these models, either an N- or a B-layer was attached directly to the diamond surface. In addition, two different structural orientations of the BN adlayer (versus the diamond substrate) were used: heteroepitaxial and nonheteroepitaxial. The results obtained in this article have been compared to the recent study by the present authors where the initial layerby-layer growth of BN onto diamond (100) was studied, but without the presence of surface terminating species. As discussed above, we are here mainly concerned about the possibility to steer the growth of BN to the cubic phase, merely by using H and/or F as surface terminating species in combination with diamond (100) as the substrate surface. The focus of the present study is to achieve an atomic-level understanding about how the combination of (i) substrate// BN bond formations and (ii) surface−adsorbate interactions may chemically and structurally induce the formation of cubic BN. B. Structural Geometries and Energetic Surface Stabilization. B.1. A Monolayer N (or B) on Diamond (100). For one monolayer of N, perfectly heteroepitaxially positioned on top of a diamond (100) surface, the termination by H did not render any major changes when compared with the nontermination scenario. As can be seen in Table 1, the Table 1. Interfacial Binding Energies ΔE (eV per Binding Adlayer Atom) for Geometry-Optimized Structuresa adlayer

no termination

H-termination

Ne Be B NBe NB BNe BN NBNB NBNBe BNBNe BNBN NBNBNBe NBNBNB NBNBNBNBe NBNBNBNB

7.8 4.6 5.4 3.9 5.8 5.2

7.4 5.8 3.8 6.1 4.9 5.3 5.0 5.8 6.4 5.9 4.5 9.4 4.6 5.2 4.6

5.8 6.5 6.8 4.1 3.3 4.4 4.7 4.5

F-termination 6.1 4.3 6.0 4.2

5.0 6.3

a The suffix e (in e.g. Ne) means that the BN adlayer is heteroepitaxial with respect to the diamond (100) substrate.

interfacial binding energy for a nonterminated N surface is 7.8 eV per binding N atom, which is to be compared with a value of 7.4 eV per binding N atom for the situation with Htermination. The corresponding results for the heterostructural B monolayer are, however, quite different. Stronger interfacial binding energies were obtained when terminating by H (or F) 3492



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became slightly surface reconstructed but still crystalline. For every other row along the X-axes, the F atoms are moved slightly in the negative Y-direction, resulting in a (2 × 1)reconstruction (see Figure 1e,f). The H-terminated N-rich surface has also undergone this type of (2 × 1)-reconstruction (see Figure 1a,b). However, for the nonheteroepitaxial interface, the F-terminated B-rich surface became much more ordered than the H-terminated one. The F-terminated B-rich surface has undergone a (4 × 1) reconstruction (see Figure 2a− d).

species (Table 1). For the situation where no terminating species were used, the nonheteroepitaxial interfacial structure was calculated to be energetically more favored than the heterostructural counterpart (Table 1). Hence, surface termination will induce stabilization of the interfacial adhesion strength for the heteroepitaxial structure. However, the various types of terminating species will stabilize differently: 6.1 eV (Htermination) vs 5.8 eV (F-termination) per B atom. It is also noteworthy that, except for the role by the terminating species to induce a strong heteroepitaxial adhesion to the surface, it will also secure sp3 hybridization (i.e., cubic structure) of the upper surface atoms (see Figure 1a−d).

Figure 2. Periodic slabs containing five atomic layers of C in a diamond (100)-2 × 1 surface. The bottom C layer is H-terminated. The figure demonstrates a nonheteroepitaxial B monolayer which is either terminated with H (a, b) or with F (c, d). All structures are geometry optimized. Two different side views are presented: either along the y-axis (a, c) or from above (b, d).

B.2. A Two-Atomic BN Adlayer on Diamond (100). An additional monatomic layer of N (or B) was added to the initial surface configuration of one monolayer of N (or B) on the diamond (100) substrate. The results obtained from the geometrical optimizations, where a B layer was added heteroepitaxially onto the first optimized N layer, showed a clear difference for the nonterminated and terminated adlayer scenario. As can be seen in Table 1, the terminated heteroepitaxial BN adlayers were found to attach much more strongly to the diamond substrate (than the nonterminated ones). As a result of the geometry optimization, both the H- and F-terminated B surfaces became (2 × 1)-reconstructed (see Figure 3). The Hterminated surface showed B−B dimer rows, while the Fterminated surface showed B−B dimer formations in zigzag patterns. The most probable explanation to this difference in dimer formation is the more pronounced electron repulsion between the chemisorbed F species (as compared with H adsorbates). For the initially nonheteroepitaxial interfacial structure, totally different results were obtained when comparing H (or

Figure 1. Periodic slabs containing five atomic layers of C in a diamond (100)-2 × 1 surface. The bottom C layer is H-terminated, and all structures are geometry optimized. The figure demonstrates (a, b) an H-terminated monolayer of heteroepitaxial N, (c, d) an Hterminated monolayer of heteroepitaxial B, and (e, f)an F-terminated monolayer of heteroepitaxial B. Two different side views are presented: either along the y-axis (a, c, e) or from above (b, d, f).

It is also interesting to note that F-termination (compared to H-termination) will for both structures of the B monolayer formation result in slightly improved interfacial bond strengths (see Table 1). Various changes in surface geometrical structure were, however, obtained, depending on the matching of the BN adlayer to the underlying diamond substrate. For the heteroepitaxial interface, the F-terminated B-rich surface 3493



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Figure 4. Geometry-optimized BN/diamond interfaces, including a nonheteroepitaxial BN adlayer with N closest to the diamond substrate. Parts a and b show the H-terminated interface, using two different side-views (either along the y-axis (a) or along the x-axis (b)). The corresponding geometrical structure for the F-terminated BN adlayer is shown in parts c and d.

Figure 3. A geometry-optimized two-atomic layer of initially heteroepitaxial BN attached to diamond (with N closest to the substrate). Parts a, b, and c show three different side-views of the Hterminated interface: either along the y-axis (a), along the x-axis (b), or from the top (c). The corresponding geometrical structures for Fterminated structures are shown in parts d, e, and f.

F)-termination with bare surfaces. The nonterminated BN adlayer was here found to adhere stronger (Table 1). As a conclusion, the use of surface-termination may steer the c-BN growth toward an ideal heterostructural adlayer onto diamond (100). As a result of the geometry optimizations, both the Fterminated BN surfaces became amorphous-like, while the Hterminated one still showed a crystalline structure (although somewhat disordered; see Figure 4). Since this conclusion is solely based on thermodynamic considerations, it is highly likely that the preference for a cubic, but nonheteroepitaxial, growth of a H-terminated c-BN thin film will be enhanced at higher temperatures (i.e., since activation barriers have to be overcome). When adding a second layer of N onto the first attached nonheterostructural B monolayer (being the energetically most stable form), the calculated interfacial binding energy becomes much higher for H-termination, compared to the nontermination scenario (see Figure 5c,d). In fact, a nonterminated adlayer was found not to bind to the diamond substrate at all.30 As can be seen in Table 1, H-termination will result in a two-

Figure 5. Geometry-optimized BN/diamond interfaces, including heteroepitaxial or nonheteroepitaxial BN adlayers with B closest to the diamond substrate. Parts a and b show the heteroepitaxial structure, using two different side views (either along the y-axis (a) or along the x-axis (b)). The corresponding geometrical structures for the nonheteroepitaxial BN adlayer are shown in parts c and d.

atomic BN adlayer that is even more strongly attached to the diamond substrate than the first attached B layer. 3494



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When adding a second monolayer of N onto the first attached heterostructural B monolayer (see Figure 5a,b), there were only a minor difference obtained in interfacial binding energy when using H-termination versus nontermination (see Table 1). Hence, it is possible to draw the conclusion that surface termination with H will, not for this very early stage of growth, control the growth toward the preferred cubic phase. Instead, a mixture of phases is expected to be formed. B.3. A Four-Atomic BN Adlayer on Diamond (100). A heteroepitaxial and nonheteroepitaxial BN adlayer, respectively, was thereafter built on top of the two-atomic thick hetero- vs nonheteroepitaxial BN adlayer (forming four-atomic thick BN adlayers). H- or F-terminated BN/diamond interfaces were also here compared with the corresponding nonterminated ones.30 For the situation with the initial heteroepitaxial four-atomic BN adlayer, with N closest to the diamond substrate, there were no significant difference in interfacial binding energy when comparing a nonterminated surface with an H-terminated surface or F-terminated one (Table 1). The evolution of interfacial binding energy, when going from a two-atomic to a four-atomic BN layer, is almost nonexisting for the terminated adlayers: H-termination (6.1−6.4 eV) and F-termination (6.0− 6.3 eV) per binding N. An increased adlayer thickness from a two-atomic to a four-atomic BN adlayer will not induce any change in interfacial binding energy when using surface termination. However, a different situation occurs when using nontermination. As can be seen in Table 1, a four-atomic thick BN adlayer will then bind with approximately a factor twice as strong as compared with a two-atomic-thick BN adlayer. As already mentioned, there was no significant difference in interfacial binding energy when comparing the nonterminated surface with the H-terminated surface or F-terminated one. The structural difference after the geometry optimization, however, was much more significant. The terminated BN adlayer becomes much more ordered, though reconstructed, in the form of a cubic BN structure for the heterostructural interface (see Figure 6). The terminating H (or F) atoms do apparently succeed to uphold the cubic structure of the surface atoms. On the other hand, the corresponding nonterminated BN adlayer show a less ordered structure. The B atoms on the bare surface reconstruct since they strive toward a three-valence environment and hence a more planar structure. The strive by the B atoms to flatten out and bind to only three N neighbors is even more visible for the second B layer. In addition, the N atoms prefer a bonding situation with five valence electrons. Both of these tendencies for B and N will jointly cause the disruptor of some B−N bonds in the BN structure. The nonheteroepitaxial structures show slightly lower interfacial binding energies compared to the corresponding heteroepitaxial ones. In addition, the interfacial binding energies for the nonterminated and terminated nonheteroepitaxial structures are very similar. The resulting geometrical structures of the H- and F-terminated surfaces where both amorphous like and very similar (see Figure 7). Hence, the formation of a heteroepitaxial BN adlayer with N closest to the diamond substrate seems to be the energetically most preferred result when using nonterminated BN adlayers under the assumption of a layer-by-layer growth process, Moreover, the formation of heteroepitaxial cubic BN adlayers also seems to be energetically favored when terminating with either H or F. Also, a four-atomic c-BN adlayer, with B closest to the diamond substrate, was initially built as either a heteroepitaxial

Figure 6. Geometry-optimized heteroepitaxial interfaces of a fouratomic thick adlayer of BN with N attached to the diamond. Parts a and b show the nonterminated interfaces, using two different side views (either along the y-axis (a) or along the x-axis (b)). Parts c and d show the H-terminated interfaces, using two different side views (either along the y-axis (c) or along the x-axis (d)). The corresponding geometrical structures for F-terminated structures are shown in parts e and f.

or nonheteroepitaxial (but cubic) continuation of the diamond lattice. The H-termination did not induce any difference in interfacial binding energy when going from a two-atomic to a four-atomic BN adlayer (Table 1). These results are to be compared with a corresponding nonterminated heteroepitaxial structure, for which a slightly stronger interfacial binding energy was observed. This was also the situation for the nonheteroepitaxial structures: 5.0 eV per binding B (two-atomic) vs 4.5 eV per binding B (four-atomic) (see Table 1). However, the nonheteroepitaxial cubic adlayer structure showed a completely different result for a nonterminated surface, in which case the two-atomic thick BN adlayer did not bind at all to the diamond (100) surface. The four-atomic thick BN adlayer was observed to bind with an interfacial binding energy of 4.1 eV per binding B. As can be seen in Figure 8, both the heterostructural and nonheterostructural structures became crystalline as a result of the geometry optimization. From all of the energy values presented in Table 1, one can also draw the conclusion that only small differences in adlayer binding energies are observed for the H-terminated and nonterminated nonheteroepitaxial surfaces. For the hetero3495



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epitaxial c-BN structure, the nonterminated surface showed a slightly stronger adlayer binding energy. B.4. Six- and Eight-Atomic BN Adlayers on Diamond (100). When continuing up to six and eight atomic BN layers attached onto the diamond substrate, the nonterminated scenario was also here of interest for a comparison with the terminated ones. Only H-terminated surfaces was studied since this termination type was found to give very similar BN adlayer binding energies, as compared to F-termination, for the situation with one, two, and four atomic layers, respectively. Only interfaces with N closest to the diamond substrate were calculated for adlayers built from three BN layers (i.e., six atomic layers). The initially heteroepitaxial H-terminated interface resulted in a cubic, heteroepitaxial BN adlayer as a result of the geometry optimization (see Figure 9). The

Figure 7. Geometry-optimized nonheteroepitaxial interfaces of a fouratomic adlayer layer of BN, with N attached to the diamond substrate. Parts a and b show the H-terminated interfaces, using two different side views (either along the y-axis (a) or along the x-axis (b)). The corresponding geometrical structures for F-terminated structures are shown in parts c and d.

Figure 9. Geometry optimized interfaces of BN/diamond obtained from heteroepitaxially constructed six-atomic layers (with N attached to the diamond substrate). Parts a and b show the H-terminated interfaces, using two different side views (either along the y-axis (a) or along the x-axis (b)).

interfacial binding energy was strong, even stronger than for corresponding situations with one, two, and four layers with N closest to the substrate (see Table 1). All of these interfacial binding energies are much stronger than for the nonterminating interface built from three BN layers. For the nonheteroepitaxial six-atomic thick H-terminated BN adlayer, an appreciably weaker interfacial binding energy was obtained as compared with the heteroepitaxial counterpart (see Figure 10). However, the binding energy was almost identical to the corresponding nonterminated BN adlayer. Both the initially nonheteroepitaxial nonterminated and H-terminated surfaces, resulted in crystalline cubic BN adlayers with a minor tendency for B to bind to only three N (due to the preferred number of valence electronsthree) and N to also bind to three B (and thereby fulfill a stable five-valence electron shell) (see Figure 10). It is from these calculations possible to draw the conclusion that the most energetically stable BN//diamond interfacial conf iguration is for the H-terminated BN adlayer, being heteroepitaxial attached to the diamond substrate. In addition, the H-termination will induce an even stronger interfacial binding energy, as compared to the nonterminated scenario. When approaching a BN film thickness of four BN layers (resulting in an eight-atomic-thick adlayer) the interfacial binding energy obtained for the heteroepitaxial, H-terminated interface, is 5.2 eV per binding N atom (see Figure 11). As can be seen in Table 1, this bond strength is much weaker compared to the corresponding six-atomic-thick adlayer. However, it is at the same time somewhat stronger than the

Figure 8. Geometry-optimized interfaces of BN/diamond with B attached to diamond. All interfaces are H-terminated. Parts a and b show structures obtained from initially heteroepitaxially constructed four-atomic BN adlayer, using two different side views (either along the y-axis (a) or along the x-axis (b)). Parts c and d show structures obtained from initially nonheteroepitaxially constructed four-atomic adlayer, using two different side views (either along the y-axis (c) or along the x-axis (d).

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situation with a corresponding nonterminated BN adlayer. The initially heteroepitaxial nonterminated H-terminated surfaces showed here an even larger tendency for creating a crystalline cubic BN structure, although somewhat reconstructed, with more or less perfect sp3-hybridization for both B and N (see Figure 11). For the nonheteroepitaxial BN//diamond interface situation, the H-terminated adlayer resulted in an interfacial binding energy of 4.6 eV per binding N atom (see Figure 12). As can be

Figure 10. Geometry-optimized interfaces of BN/diamond obtained from nonheteroepitaxially constructed six-atomic adlayers (with N attached to the diamond substrate). Parts a and b show the nonterminated interfaces, using two different side views (either along the y-axis (a) or along the x-axis (b)). The corresponding geometrical structures for H-terminated structures are shown in parts c and d.

Figure 12. Geometry-optimized interfaces of BN/diamond obtained from optimization of nonheteroepitaxially constructed eight-atomic adlayers (with N attached to the diamond substrate). Parts a and b show the nonterminated interfaces, using two different side views (either along the y-axis (a) or along the x-axis (b)). The corresponding geometrical structures for H-terminated structures are shown in parts c and d.

seen in Table 1, this result is identical to the result obtained for the six-atomic adlayer interface. It is also almost identical to the situation with no terminating species. Both the nonterminated and H-terminated surfaces also here resulted in crystalline cubic BN adlayers with minor surface reconstructions and otherwise more or less perfect sp3-hybridization of B and N. Even though the dif ferences in binding energy is somewhat small, it can also here be concluded that the most energetically stable BN//diamond interfacial conf iguration is for the H-terminated BN adlayer, being heteroepitaxial attached to the diamond substrate. In addition, the H-termination will induce an even stronger interfacial binding energy, as compared to the nonterminated scenario. C. Electron Bond Populations. Calculation of electron bond populations is generally expected to give an indication of covalent bond strengths. The electron bond population describes the electron density between two atoms and are here presented as the calculated values for the bonds between the diamond surface C atoms and the first atomic layer of either N (or B) attached to the diamond substrate (see Tables 2 and 3). The values of the electron bond population in the present

Figure 11. Geometry-optimized interfaces of BN/diamond obtained from heteroepitaxially constructed eight-atomic adlayers (with N attached to the diamond substrate). Parts a and b show the nonterminated interfaces, using two different side views (either along the y-axis (a) or along the x-axis (b)). The corresponding geometrical structures for H-terminated structures are shown in parts c and d.

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study are all numerically large (see Table 2 and 3 and Figures 13 and 14).25,26 Table 2. Electron Bond Population Values for GeometryOptimized Structures between the C Atoms in the Interface and the Contiguous N and C Atoms N on top heteroepitaxial

nonheteroepitaxial

H-terminated

F-terminated

H-terminated

F-terminated

atomic layers

C−N

C−C

C−N

C−C

C−N

C−C

C−N

C−C

1 2 4 6 8

0.67 0.67 0.67 0.62 0.58

0.80 0.82 0.81 0.80 0.80

0.67 0.66

0.80 0.80

0.77 0.79 0.80 0.81

0.77 0.92 0.92

0.67 0.66

0.80 0.81

Table 3. Electron Bond Population Values for GeometryOptimized Structures between the C atoms in the Interface and the Contiguous B and C Atoms B on top heteroepitaxial

Figure 14. Electron bond population values vs bond length between the diamond surface C atoms and the directly attached B atoms in the BN adlayer. The suffix e (in e.g. Be) means that the adlayer is heteroepitaxial with respect to the diamond (100) substrate.

nonheteroepitaxial

H-terminated

F-terminated

H-terminated

F-terminated

atomic layers

C−B

C−C

C−B

C−C

C−B

C−C

C−B

C−C

1 2 4

0.89 0.69 0.80

0.73 0.84 0.84

0.92

0.77

0.89 0.92 0.93

0.79

0.90

0.77

lengths is generally expected. For H-terminated BN adlayers, with N being directly attached to the diamond substrate, the calculated results for the heteroepitaxially and nonheteroepitaxially built interlayers were observed to be completely different (Figure 13). As can be seen in Figure 13 and Table 2, the nonheteroepitaxial interfaces showed large C−N bond populations (around 0.8), with corresponding bond lengths of 1.4−1.5 Å. The bond populations do apparently increase with adlayer thickness, with an overall quite acceptable correlation with the bond lengths. The exception to the otherwise very good correlation between the bond population and bond length was observed for the four-atomic BN adlayer. There are different plausible explanations to this exception. One is the fact that the calculated bond populations will only give information about the covalent nature of the bond, and other contributions are hence not included (e.g., ionic bond strengths). In addition, the geometrical structures of these interfaces are very complex in nature, showing very irregular patterns when increasing the BN adlayer thickness. The Fterminated nonheteroepitaxial interfaces do not, however, show the same type of trend as the H-terminated ones, with increasing electron bond population with increasing BN adlayer thickness. In fact, the bond populations were in principle identical (0.67−0.66), while the bond lengths varied (from 1.49 to 1.54 Å) (see Figure 13). The most plausible explanations are also here a combination of nonperfect covalent bonds and a too irregular bond formation. When comparing the results for F-termination with Htermination, it is though clear that the F-terminated interfaces show smaller electron bond population values and longer bond lengths (see Figure 13 and Table 2). This difference in results may be explained by the larger electronegativity of F compared to H (3.98 vs 2.20). This conclusion is further strengthened by the observation that the interfacial bond populations will increase with an increase in BN adlayer thickness. The explanation to this difference in bond population (for F and

Figure 13. Electron bond population values vs bond length between the diamond surface C atoms and the directly attached N atoms in the BN adlayer. The suffix e (in e.g. Ne) means that the adlayer is heteroepitaxial with respect to the diamond (100) substrate.

C.1. BN Adlayer with N Attached to the Diamond Substrate. C.1.1. H- or F-Terminated Nonheteroepitaxial Adlayer. Since the bond populations are a clear indication of covalent bond strengths, a strong correlation to covalent bond 3498



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values and bond lengths. Instead, there is a constant C−B bond length of 1.59 Å (Figure 14). It must again be stressed that the electron bond population results only give information on the covalent contribution of the bond and other additions, such as ionic, are not included. In addition, the geometries for these nonheteroepitaxial structures show very irregular patterns when the adlayer thickness increases, and hence, the bond length distributions are likely to follow these irregular patterns. The result for the F-terminated nonheteroepitaxial BN// diamond interfaces is indeed very similar to the H-terminated one (see Figure 14). The corresponding C−B bond lengths are 1.59 Å for both types of termination, which is identical to all the other nonheteroepitaxially built interfaces with B directly attached to the diamond surface (i.e., H- and F-terminated two- and four atomic BN adlayers). C.2.2. H- or F-Terminated Heteroepitaxial Adlayer. As was the situation for nonheteroepitaxial H-terminated BN// diamond interfaces (with B atoms directly attached to the diamond surface), corresponding heteroepitaxial interfaces also gave high electron bond populations (Table 3). As can be seen in Figure 14, a good correlation between electron bond population and bond length was observed, with shorter bond lengths for a larger bond population. However, there is no correlation found for the electron bond population when increasing the adlayer thickness with increasing electron bond population. There is also a more or less perfect correlation between the interfacial binding energy and bond populations vs bond lengths. When comparing results for different choices of atomic types closest to the diamond substrate, the numerical values of the electron interfacial bond populations were found to be larger for the structures with B directly bonded to the diamond substrate (compared to the corresponding structures with N binding to diamond). In addition, both interface types showed a clear correlation between electron bond populations and bond lengths. Calculations for adlayers starting with B closest to the diamond substrate were only performed up to four atomic layers. Since termination with F atoms where only made for the B surface sites, there is only one result for F-terminated heteroepitaxial interfaces starting with B closest to the diamond substratethe monatomic B adlayer. It shows an electron bond population of 0.92, which is close to the situation with the corresponding H-terminated structure (0.89) (see Figure 14). The corresponding C−B bond length for the F-terminated adlayer is also very similar to the corresponding H-terminated adlayer. The difference in bond length is though expected since the bond population for the F terminated B-layer is somewhat larger than for the H-terminated one. It could be concluded from the comparison between the electron bond populations and the interfacial binding energies that also here a clear trend can be observed. This is also the situation for the bond lengths versus interfacial binding energies. When compared to the situation with N closest to the diamond substrate, the calculated C−B bond populations for an F-terminated interface was found to be larger for the situation with B closest to the diamond substrate. The bond lengths were, on the other hand, very similar (see Figures 13 and 14). In summary, all of the heteroepitaxially built diamond//BN interfaces showed good correlation between the electron bond populations and the interfacial binding energies. However, this is not the situation for the nonheteroepitaxially built structures. As discussed above, the most plausible explanation to this

H) is that the more electronegative F species will have the capacity to withdraw more electrons from the C−N bond, thereby weakening it. This is an effect that will be further diminished for an increased BN film thickness, where the terminating species will get a smaller influence on the interfacial bond region. When trying to correlate interfacial binding adhesion energies, various factors come into play. For instance, the adhesion energies are calculated for the whole supercell and contain contributions from various bonding types. Hence, it is not straightforward to project it to specific bonds. We have hence in the present chapter chosen to correlate electron bond population with bond length, since both can be strictly located to a specific bond and since both are strongly coupled to bond strengths. C.1.2. H- and F-Terminated Heteroepitaxial Adlayer. The heteroepitaxially built H-terminated interfaces all showed much smaller electron bond populations than the nonheteroepitaxial counterparts (in the region of 0.58−0.67 versus approximately 0.8). As expected, the corresponding bond lengths were also longer (1.52−1.55 Å versus 1.4−1.5 Å) (Figure 13). In addition, the trends for these heteroepitaxially built interfaces were opposite from the nonheteroepitaxially built ones; the electron bond population was observed to decrease with an increasing adlayer thickness (Table 2). Even though we have above explained why it is not easy to project total adhesion energy to individual bonds, we can here just inform that, with one exception, these results correlates quite well with the interfacial binding energies; a decrease in interfacial binding energy is accompanied by a decrease in electron bond population. The F-terminated heteroepitaxial interfaces were found to follow the same trend (though rather weak) as the Hterminated ones, with decreasing bond population with increasing thickness of the BN adlayer (see Figure 13). The heteroepitaxial two-atomic BN adlayer showed an identical electronic bond population as the H-terminated counterpart, but with a slightly shorter C−N bond length. The heteroepitaxial four-atomic BN-layer showed a bond population of 0.66 (corresponding to a bond length of 1.54 Å), which is almost identical to the results obtained for the corresponding H-terminated four-atomic BN adlayer (Table 2 and Figure 13). As a conclusion, the nonheteroepitaxial adlayer structures showed slightly larger electron bond populations within the interfacial C−N bonds (as compared to the corresponding heteroepitaxially built structures) that also increased with adlayer thickness. The C−N electron bond populations for the heteroepitaxially built structures decreased with increasing adlayer thickness. The bond lengths correlated perfectly with the bond populations, with longer bond lengths for smaller values of electron bond populations. C.2. BN Adlayer with B Attached to the Diamond Substrate. C.2.1. H- or F-Terminated Nonheteroepitaxial Adlayer. For the situation with H-terminated nonheteroepitaxial BN//diamond interfaces, where B atoms are directly attached to the diamond substrate, high bond populations were observed between the C and B atoms across the interface. The electron bond population increased with BN adlayer thickness, which is identical to the corresponding result with N directly attached to the diamond substrate. The electron bond populations can be seen in Table 3. As was the situation with an F-terminated nonheteroepitaxial BN layer (see section C.1.1), there is no correlation observed between these bond population 3499



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discrepancy is that also other bond types (than covalency) are present in these bonds. Or alternatively, there are more general electrostatic interactions over the diamond//BN interface. It must here be stressed that the nonheteroepitaxial structures show very irregular pattern and complex structures, which is not at all the situation for the heteroepitaxial structures. D. Degree of Electron Transfer. The degree of electron transfer, between the diamond substrate and attached BN adlayer in the interface, was estimated from the calculated atomic charges. This electron transfer is a strong indication for a chemical interaction over the interface and will for instance give an indication of degree of ionic bonding. It is thereby useful as a complement to the electron bonding population calculations, from which it is possible to get information about covalent bond strengths. Both of these tools will, hence, be invaluable in achieving a deeper understanding about the underlying causes to the differences in interfacial binding energies as well as for the resulting geometrical structures. The interaction between diamond surface C atoms and attached N (or B) atoms is dependent on the degree of spherical orbital overlaps and orbital energy difference. Perfect covalent bonds do not show any electron transfer at all, while the more polar bonds show a much higher degree of electron transfer. The interfacial binding interactions of interest in the present study must have a certain degree of polarized contribution, to the otherwise covalent bonds, since the electronegativity values for C, N, and B are 2.55, 3.04, and 2.04, respectively.33 D.1. BN Adlayer with N Directly Attached to Diamond (100). D.1.1. H- or F-Terminated Heteroepitaxial Adlayer. For the situation with an H-terminated monatomic N adlayer, heteroepitaxially attached onto a diamond (100) surface, a partial electron transfer from the diamond surface to the N adlayer (of 2.61 e per super cell) was found. This is the expected direction of electron transfer since N has a higher electronegativity compared to C (3.04 vs 2.55). When increasing the number of BN adlayers, the electron transfer was, with some exceptions, found to decrease (see Figure 15). The more general picture is, hence, that the binding energy will decrease with adlayer thickness, which correlates quite well with the values for partial electron transfer. This is an expected result since a larger interaction between two materials in an interface usually implies larger adhesion energy and a larger degree of electron transfer (due to overlapping surface orbitals from the two materials constituting the interface). For the situation with the F-terminated and heteroepitaxially built adlayers, with N directly attached to the diamond (100) surfaces, similar values for partial electron transfer, as for the Hterminated interfaces, were observed (see Figure 15). As can be seen from this figure, the F-termination electron transfer values are only slightly larger than the corresponding H-termination ones. The corresponding adlayer adhesion energies are 6.0 vs 6.1 eV/binding N atom for a two-atomic adlayer and 6.3 vs 6.4 eV/binding N atom for a four-atomic adlayer. The values of the electron transfer clearly show that the effect by the terminating species is much more pronounced for a thinner BN adlayer. The larger effect by the F-terminated interface is though expected since the F atoms are much more electronegative than the H atoms (3.98 vs 2.20). However, this influence by the terminating species becomes rapidly smaller when increasing the adlayer thickness to a four-atomic one. The difference in electron transfer for F versus H is 0.09 for the two-atomic adlayer and 0.03 for the four-atomic one.

Figure 15. Electron transfer values from the whole adlayer to the diamond substrate vs number of atomic layers. Nitrogen atoms are directly attached to the diamond substrate. The notation “he” means that the adlayer is heteroepitaxial with respective to the diamond (100) substrate.

D.1.2. H- or F-Terminated Nonheteroepitaxial Adlayer. For the nonheteroepitaxially built H-terminated adlayer, an almost constant degree of electron transfer was observed for somewhat thicker BN adlayers (see Figure 15). On the contrary, the very thin two-atomic BN adlayer shows an electron transfer of only 1.05 e. Hence, also for these structures, the types of terminating species (H or F) were observed to have a more pronounced effect for thinner BN adlayers. These results thus indicate that the degree of electron transfer over the interface will cease to a constant value for a BN thickness of at least four atomic layers. The F-terminated interfaces follow the same trend, although the electron transfer for the two atomic adlayer is higher than for the corresponding H-terminated one. The four-atomic adlayer shows an electron transfer of 2.11 e (as compared with 2.03 e for the H-terminated interface) (see Figure 15). Because of the electronegativity difference between H and F, the Fterminated adlayers will also here show a larger degree of electron transfer, which becomes more dominant for the thinner adlayers. The difference in electron transfer for F versus H is 0.70 for the two-atomic adlayer and 0.08 for the fouratomic one. It is hence obvious that the terminating species has a much higher influence on the degree of electron transfer for nonheteroepitaxial adlayers compared to heteroepitaxial ones (see section D.1.1). D.2. BN Adlayer with B Directly Attached to Diamond (100). D.2.1. H- or F-Terminated Heteroepitaxial Adlayer. For the situation with B closest to the C atoms in the interface, the electron transfer direction is the opposite. The B species will partially donate electrons to the C atoms in the diamond substrate because of the difference in the electronegativity values between C (2.55) and B (2.04). For the H-terminated heteroepitaxially built adlayer, the partial electron transfer (within each supercell) can be seen in Figure 15. It is hard to notice any clear trend with such few values, but there is a tendency for the electron transfer values to decrease when the 3500



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electron transfer and binding situations when the adlayer thickness is as small as one atomic thick only. As was the situation with a monatomic thick and heteroepitaxial B adlayer, the electron transfer from the Fterminated adlayer to the diamond substrate was observed to be larger than for the corresponding H-terminated interface. There are two plausible explanations to this result. One possible explanation is that the geometry optimized H- and Fterminated structures are very different (see Figures 2a−d), resulting also in different adhesion energies. And the second plausible explanation is, as was also discussed for heteroepitaxial interfaces above (in section D.2.1), the repulsion between the highly electron density rich F adsorbates. The degree of electron transfer for the H- and F-terminated interface is 1.70 vs 2.10 e. The corresponding interfacial binding energies are 4.3 vs 3.8 eV/binding B atom. This result is expected since a larger degree of electron transfer should result in more pronounced adhesion energy.

adlayer thickness increases from one atomic layer to four atomic layers. For only one atomic layer thick adlayer, the influence from the terminating species is most probably the dominating factor for the degree of electron transfer, which can be explained by the differences in electronegativity between H (2.20) and N (3.04). Hence, the smaller electronegativity value for the H atom will allow more electrons to be donated from the B atoms to the substrate C atoms than the N atoms would do. As can be seen in Figure 16, the one-atomic-thick, Fterminated adlayer, the electron transfer result is very similar to

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SUMMARY AND CONCLUSIONS It is a well-known fact that the initial growth of phase-pure cubic BN films is very difficult to experimentally accomplish. The initial BN layers often results in a mixture of phases, such as hexagonal, amorphous, turbostratic, and cubic. These BN phases lead to films with higher compressive stress, poor crystalline quality, small grain size, and a high defect density. The film will most often peel off from the substrate material, even at lower thicknesses. Moreover, the initial growth process has been observed to be very important for the film adhesion to the substrate material. The main purpose with the present study has been to outline the possibility for capturing the initial cubic BN phase directly onto the substrate, by combining the choice of substrate material with terminating species. The first choice of substrate materials, in a forthcoming series of investigations, has naturally been diamond since it has been shown to be the only substrate up to date onto where it is possible to grow c-BN directly without any amorphous or hexagonal phases in between. The initial growth of c-BN (100) onto a (2 × 1)reconstructed diamond (100) surface has therefore in the present work been theoretically investigated using density functional theory (DFT) under periodic boundary conditions. Four initial structures were used: with either B or N closest to the diamond substrate and where the adlayer was either H- or F-terminated. Five different BN adlayer thicknesses were considered: one, two, four, six, and eight atomic layers. The initially built cubic form of BN adlayers were either constructed as a perfect heterostructural continuation of the diamond structure or rotated 90° with respect to the diamond substrate (called nonheteroepitaxial adlayer). The results showed that nitrogen is energetically the most favorable precursor to start the growth processes with, as a first atomic layer directly binding to the diamond substrate in heteroepitaxial positionsboth with and without the terminating species H and F. A single atomic layer of B, directly attached to the diamond substrate, was also found to energetically prefer a heteroepitaxial continuation of the diamond substrate. But this was only the situation for an Hor F-terminated B adlayer. In the absence of terminating species, the energetically most preferred structure will be more amorphous-like. For a two-atomic-thick BN adlayer, with N closest to the diamond substrate, it was found that surface termination with either H or F will steer toward a heterostructural cubic phase of

Figure 16. Electron transfer values from the whole adlayer to the diamond substrate vs number of atomic layers. Boron atoms are directly attached to the carbon atoms in the diamond substrate. he means that the adlayer is heteroepitaxial with respective to the diamond (100) substrate.

the H-termination scenario. According to the differences in electronegativity values between H (2.20) and F (3.98), the degree of electron transfer from the adlayer (with B closest to the diamond surface) to the diamond substrate is expected to be lower for the F-terminated adlayer than for the Hterminated one. One plausible explanation to the observation that also F-termination renders an electron transfer from B to C (over the interface) is the indication of sterical repulsions between the terminating F species. As can be seen in Figure 1f, an on-top position of F is expected, but repulsions among the highly electron density rich adsorbates causes them to undergo a surface reconstruction. For this very thin adlayer, this repulsion may also result in an induced electron transfer from the adlayer to the diamond C atoms. D.2.2. H- or F-Terminated Nonheteroepitaxial Adlayer. The H-terminated nonheteroepitaxial BN//diamond interfaces show partial electron transfers that correlate well with the interfacial binding energies (see Table 1 and Figure 16). Also here, the electron transfer results will converge toward a fairly constant value for thicker adlayers. This convergence occurs very fast, already for a two-atomic-thick adlayer. This is also the situation for the interfacial binding energies. These results indicate that the terminating species will be able to inf luence the 3501



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(6) Zhang, W.; Bello, I.; Lifshitz, Y.; Chan, K.; Wu, Y.; Chan, C.; Meng, X.; Lee, S. Thick and Adherent Cubic Boron Nitride Films Grown on Diamond Interlayers by Fluorine-Assisted Chemical Vapor Deposition. Appl. Phys. Lett. 2004, 85, 1344−1346. (7) Zhang, W.; Bello, I.; Lifshitz, Y.; Chan, K.; Meng, X.; Wu, Y.; Chan, C.; Lee, S. Epitaxy on Diamond by Chemical Vapor Deposition: A Route to High-Quality Cubic Boron Nitride for Electronic Applications. Adv. Mater. 2004, 16 (16), 1405−1408. (8) Zhang, W.; Meng, X.; Chan, C.; Chan, K.; Wu, Y.; Bello, I.; Lee, S. Interfacial Study of Cubic Boron Nitride Films Deposited on Diamond. J. Phys. Chem. B 2005, 109 (33), 16005−16010. (9) Chong, Y.; Ma, K.; Leung, K.; Chan, C.; Ye, Q.; Bello, I.; Zhang, W.; Lee, S. Synthesis and Mechanical Properties of Cubic Boron Nitride/Nanodiamond Composite Films. Chem. Vap. Deposition 2006, 12 (1), 33−38. (10) Zhang, F.; Guo, Y.; Song, Z.; Chen, G. Deposition of High Quality Cubic Boron Nitride Films on Nickel Substrates. Appl. Phys. Lett. 1994, 65, 971−973. (11) Friedmann, T.; Mirkarimi, P.; Medlin, D.; McCarty, K.; Klaus, E.; Boehme, D.; Johnsen, H.; Mills, M.; Ottesen, D.; Barbour, J. IonAssisted Pulsed Laser Deposition of Cubic Boron Nitride Films. J. Appl. Phys. 1994, 76, 3088−3101. (12) Yamamoto, K.; Keunecke, M.; Bewilogua, K.; Czigany, Z.; Hultman, L. Structural Features of Thick c-Boron Nitride Coatings Deposited Via a Graded B−C−N Interlayer. Surf. Coat. Technol. 2001, 142, 881−888. (13) Kester, D.; Ailey, K.; Lichtenwalner, D.; Davis, R. Growth and Characterization of Cubic Boron Nitride Thin Films. J. Vac. Sci. Technol. 1994, 12, 3074−3081. (14) Mirkarimi, P.; Medlin, D.; McCarty, K.; Barbour, J. Growth of Cubic BN Films on ß-SiC by Ion-Assisted Pulsed Laser Deposition. Appl. Phys. Lett. 1995, 66, 2813−2815. (15) Hofsass, H.; Ronning, C.; Griesmeier, U.; Gross, M.; Reinke, S.; Kuhr, M.; Fischer, R.; Zweck, J. Characterization of Cubic Boron Nitride Films Grown by Mass Separated Ion Beam Deposition. Nucl. Instrum. Methods, B 1995, 106 (1), 153−158. (16) Mirkarimi, P.; McCarty, K.; Medlin, D. Review of Advances in Cubic Boron Nitride Film Synthesis. Mater. Sci. Eng., R 1997, 21 (2), 47−100. (17) Cameron, D. C.; Karim, M. Z. Properties of Mixed-Phase BN Films Deposited by r.f. PACVD. Thin Solid Films 1993, 236 (1−2), 96−102. (18) Zhang, W. J.; Bello, Y.; Lifshitz, Y.; Lee, S. T. Recent Advances in Cubic Boron Nitride Deposition. MRS Bull. 2003, 28 (3), 184−188. (19) Karlsson, J.; Larsson, K. Hydrogen-Induced De/Reconstruction of the c-BN(100) Surface. J. Phys. Chem. C 2010, 114 (8), 3516−3521. (20) Hohenberg, P.; Kohn, W. Inhomogeneous Electron Gas. Phys. Rev. 1964, 136, B864−871. (21) Kohn, W.; Sham, L. Self-Consistent Equations Including Exchange and Correlation Effects. Phys. Rev. 1965, 140 (4A), A1133−A1138. (22) Clark, S. J.; Segall, M. D.; Pickard, C. J.; Hasnip, P. J.; Probert, M. I. J.; Refson, K.; Payne, M. C. First Principles Methods Using CASTEP. Z. Kristallogr., B 2005, 220 (5−6-2005), 567−570. (23) Perdew, J.; Wang, Y. Accurate and Simple Analytic Representation of the Electron-Gas Correlation Energy. Phys. Rev. B 1992, 45 (23), 13244−13249. (24) Monkhorst, H.; Pack, J. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13 (12), 5188−5192. (25) Segall, M. D. Population Analysis in Plane Wave Electronic Structure Calculations. Mol. Phys. 1996, 89 (2), 571−577. (26) Segall, M. D.; Shah, R.; Pickard, C. J.; Payne, M. C. Population Analysis of Plane-Wave Electronic Structure Calculations of Bulk Materials. Phys. Rev. B 1996, 54 (23), 16317−16320. (27) Sanchez-Portal, D. Projection of Plane-Wave Calculations Into Atomic Orbitals. Solid State Commun. 1995, 95 (10), 685−690. (28) Fischer, T. H.; Almlof, J. General Methods for Geometry and Wave Function Optimization. J. Phys. Chem. 1992, 96 (24), 9768− 9774.

BN. There was a very similar result observed for the two-atomic thick BN adlayer, with B closest to the diamond substrate. A terminated heteroepitaxial c-BN//diamond interface was found to be the energetically most preferred one (in this case with H). For the four-atomic-thick BN adlayer, with N closest to the diamond substrate, the crystalline heteroepitaxial structure is the most energetically preferred one. Also here it is possible to draw the conclusion that the terminating species may uphold the cubic structure and steer the c-BN growth toward a crystalline, heterostructural adlayer. For the four-atomic-thick BN adlayer, with B closest to the diamond substrate, the crystalline heteroepitaxial structure is the most energetically preferred, with and without terminating species. When continuing to six and eight atomic thick BN adlayers, with N closest to the diamond substrate, all structures became crystalline after the geometry optimization. However, there is an energetic preference for the H-terminated heterostructural cBN//diamond interface. All results have indicated that it is most important to use surface termination for the initial growth of the first BN atomic layers in order to avoid more amorphous and noncubic material. After four atomic layers, also nonperfect surface termination will render a phase-pure growth of c-BN directly onto the diamond substrate. In addition, at these thicker layers, the tendency for three-valence for B and five-valence for N was no longer that apparent. This means that the strive by B and N to bind to three atoms each is not so dominating at these thicknesses. The effect by the terminating species H (or F) was also clearly present for the thinnest adlayers, showing a chemical influence on both the electron transfer over the interface and on the adhesion energy. This was an influence that was diminished for thicker atomic layers than two to four.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (A.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the, Göran Gustafsson Foundation and the Swedish Foundation for Strategic Research (SSF); “Materials Science for Nanoscale Engineering”. The computational results were obtained using the software programs from Accelrys, Inc. (first principles calculations were done with the CASTEP program within the Materials Studio program package).

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REFERENCES

(1) Olander, J.; Larsson, K. Cubic Boron Nitride Growth from NH3 and BBr3 Precursors: a Theoretical Study. Diamond Relat. Mater. 2002, 11 (3−6), 1286−1289. (2) Sachdev, H. Comparative Aspects of the Homogeneous Degradation of c-BN and Diamond. Diamond Relat. Mater. 2001, 10 (3−7), 1390−1397. (3) Larsson, K. Precursor Design in c-BN Growth. Diamond Relat. Mater. 2002, 11 (3−6), 1300−1304. (4) Zhang, W.; Chong, Y.; Bello, I.; Lee, S. Nucleation, Growth and Characterization of Cubic Boron Nitride (cBN) Films. J. Phys. D: Appl. Phys. 2007, 40 (20), 6159−6174. (5) Olander, J.; Larsson, K. Initial Growth of Hexagonal and Cubic Boron Nitride: A Theoretical Study. Phys. Rev. B 1995, 68 (75), 075411−075418. 3502



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(29) Carbone, M.; Larsson, K.; Carlsson, J. Diamond Nucleation on Hexagonal Boron Nitride: A Theoretical Investigation of the Influence of Nearest Substituents. J. Phys. Chem. A 1997, 101 (49), 9445−9448. (30) Pallas, A.; Larsson, K. Initial Growth of BN on Diamond Substrates: A Theoretical Approach. J. Phys. Chem. C 2010, 114 (26), 11448−11455. (31) Zhang, W. J.; Chan, C. Y.; Meng, X. M.; Fung, M. K.; Bello, I.; Lifshitz, Y.; Lee, S. T.; Jiang, X. The Mechanism of Chemical Vapor Deposition of Cubic Boron Nitride Films from Fluorine-Containing Species. Angew. Chem., Int. Ed. 2005, 30, 4749−4753. (32) Shtansky, D. V.; Yamada-Takamura, Y.; Yoshida, T.; Ikuhara, Y. Mechanism of Nucleation and Growth of Cubic Boron Nitride Thin Films. Sci. Technol. Adv. Mater. 2000, 1 (4), 219−225. (33) Aylward, G.; Findlay, T. SI Chemical Data, 3rd ed.; Jacaranda Wiley LTD: Milton, 1994.

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Effect of Terminating Species on the Initial Growth of BN on Diamond

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