CVD Growth of cubic Boron Nitride; A Theoretical/Experimental Approach
Karin Larsson
Department of Materials Chemistry, Uppsala University
Box 538, SE 75121 Uppsala, Sweden
Cubic boron nitride (c-BN) exhibits an extraordinary combination of physical and chemical
properties (hardness, high thermal conductivity, transparency) which is comparable or even
superior to diamond in what concerns its ability to be doped as both n- and p-type semiconductor,
and its higher chemical stability and lower reactivity at high temperatures. All these properties
strongly suggest that c-BN is an extremely promising multifunctional material, which could be
tailored for a very large range of advanced applications, such as micro- and opto-electronics,
electron emission devices, radiation detection, biosensing, high temperature and/or radiation
resistant devices.
However, the synthesis of c-BN is still very little understood. Energetic vapour phase deposition
techniques have been found to give c-BN, but then in a mixture with amorphous and hexagonal
BN. More gentle methods, were no bombardment of ions can detrimentally affect crystallinity
and/or film stress content, are at present being looked for. To be successful in the low-energetic
synthesis of c-BN it is very important to obtain a deeper knowledge at the molecular level of the
surface growth processes. This can be obtained by using a blend of experiments and quantum
mechanical calculations. An overview is made here over the experimental and theoretical
approaches within this field during the last decade.
2
Introduction
Boron nitride (BN) is a compound that occurs in several structures. In a similar way as carbon,
BN can occur in hexagonal sp2-bonded (h-BN), cubic sp3-bonded (c-BN), and in nanotube1 and
fullerene2,3 structures. Cubic boron nitride is a synthetic wide band gap crystalline compound
with many excellent properties similar to diamond (being isoelectronic to c-BN). It has a low
density (3.48 g cm-3), extreme hardness second only to diamond, high thermal conductivity (13
Wcm-1K-1 for T=300K), wide band gap (6-6.4 eV), large resistivity (1016 cm), high resistance to
oxidation (up to 1300 K) and chemical stability, transparency from near ultraviolet to infrared
(refractive index n=2.1 for =600 nm), and it can be easily n- and p-type doped. However, there
are also large dissimilarities between c-BN and diamond. Cubic boron nitride possesses a higher
thermal stability, both in oxidising environments and in contact with Fe, Co, and Ni.4 It is, hence,
promising as a tool coating for machining of steel, cast iron and ferrous alloys. Furthermore, cBN is even superior to diamond in what concerns its ability to be doped as both n- and p-type
semiconductor, suitable for p-n junction diodes. All these properties strongly suggest that c-BN is
an extremely promising multifunctional material, which could be tailored for a very large range
of advanced applications (e.g., micro- and opto-electronics, electron emission devices, radiation
detection, biosensing, high temperature and/or radiation resistant devices). However, most of
these applications cannot be exploited with c-BN powder synthesized by the traditional high
pressure and high temperature (HPHT) method. Instead, the deposition of c-BN films in a large
area is required.
There are at present severe problems in the synthesis of c-BN. Compared to diamond, which can
be easily grown by a HPHT catalytic conversion of graphite to homogeneous single diamond
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crystals of up to cm3 in size, the largest c-BN single crystals reported so far are of a few mm3 in
size.5 The difference is even more striking when considering the vapour growth at low pressure.
Compared to diamond (which can be relatively easily grown as thick polycrystalline films
containing mm sized crystallites), the c-BN obtained at low pressure has always become a
mixture with other polymorphic forms of BN. A maximum size of only 20 nm for the c-BN
crystallites was recently obtained using ion assisted deposition.6 The dissimilarity in vapour
phase growth of c-BN vs. diamond is at first surprising since the cubic phase of c-BN is the
thermodynamical most stable one, whilst the hexagonal phase of carbon (graphite) is the
thermodynamical most stable phase at low pressure and low temperature.7-9
Energetical Vapour Phase Deposition
General
It is well known that c-BN does not exist in nature. Like man-made diamond, it was first
synthezised at high pressure and high temperature.10 However, c-BN crystals prepared by the
HPHT method have very limited applications due to costly facilities, small particle formation and
the difficulty in forming thin films. Low-pressure gas phase synthesis of a c-BN film was later on
presented.11,12 From then on, an impressive progress within c-BN thin film technology has been
made, including mainly all sorts of chemical vapour deposition (CVD) and physical vapour
deposition (PVD) technologies.13-16 This is true not only for the development of deposition
techniques, but also for the theoretical understanding of the underlying mechanisms and the
characterization of film properties.17-21
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4
Surface bombardment with energetic ions
A common feature is noticed in reviewing the literature on c-BN deposited using PVD. That is,
the c-BN phase is being created by “brute force” under the influence of surface bombardment
with highly energetic ions of nitrogen and argon. “Brute force” has also been found necessary for
successful CVD routes. For this process, a bias voltage is usually applied to the substrate to
extract ions from plasma. Examples of energetic vapour phase deposition processes are: plasmaassisted chemical vapour deposition (PACVD)22-23, ion beam assisted deposition (IBAD)24-27, ion
assisted pulsed laser deposition28, bias sputtering29,30, and mass separated ion beam deposition26.
By using these methods, c-BN growth occurs by a mechanism in which the structural changes are
accomplished by several features such as high compressive stress of several GPa, and growth of a
layered structure consisting of an amorphous (a-BN) interface layer, followed by textured sp2bonded BN and a final layer of c-BN.
It is generally believed that ion bombardment favours the formation of a dense phase. On the
other hand, excessive ion bombardment and excessively high bombardment energy have both
been found to be detrimental.19,24,31,32 Well-known stress problem, restricting maximum film
thickness to a few 100 nm18,19,21,33, has prevented any industrial application of c-BN films.
However, important progress has recently been achieved in overcoming this limitation, allowing
the deposition of films with thicknesses of up to 2 m.34-39 More generally, the break-through in
reducing the compressive stress of c-BN films during growth has been achieved using both biasenhanced CVD34,40 and PVD37,39,41 techniques. A second break-through towards application of cBN has also recently been discovered for a large area heteroepitaxial growth of c-BN on top of
diamond by IBAD, resulting in c-BN layers suitable for electronic and optoelectronic devices.42
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Parameter windows
The existence of a parameter window for c-BN growth has earlier been defined. It became
evident that c-BN deposition is governed by at least five primary parameters which are highly
interdependent.43 Ion energy, ions/neutral flux ratio, ion mass, angle of ion incidence, as well as
substrate temperature are identified to be decisive for the nature of BN films deposited by ion
assisted techniques. Growth of c-BN takes place for ion energies from as low as 50 eV37,39, up to
at least 20 keV44. Several different growth and nucleation mechanisms are covered by these very
broad parameter windows.
As presented in Section Surface bombardment with energetic ions, the deposition of c-BN films
is distinguished by a rather unique nucleation sequence45, which is observed on almost all
substrate types27. The cubic form of BN does not grow directly on the substrates. An amorphous
mixed layer (with a thickness in the order of the ion range) is firstly formed, followed by a
textured h-BN layer with the c-axis parallel to the substrate. Cubic BN starts to nucleate on top of
this nucleation layer (h-BN). The thickness of the amorphous BN and t-BN interface was in the
order of 3 nm and 20 nm, respectively.46 It has turned out that the parameter windows for c-BN
nucleation and growth (continuation of growth on a c-BN nucleus) are totally different47,48, with a
decisive parameter threshold for c-BN growth being lower than for nucleation. This means that
once the nucleation has taken place, each of the five highly different parameters can be
(interdependently) reduced. Hence, a change in any of the parameters can be compensated by the
change of one or several of the other parameters (within certain limits).43 The exception is
substrate temperature, which can be lowered at least to room temperature independent of the
other parameters.
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Induced film stresses
It has experimentally49,50 been shown that the impact of energetic ions during the growth of BN
films has mainly two effects. It generates a large amount of defects and it favours the growth of the
cubic phase relative to the hexagonal. The induced high film stresses will limit the thicknesses of
the films (in addition to poor crystalline quality, small grain size, and high density of defects and
grain boundaries). Accordingly, a delamination from the substrates will be attributed to these high
compressive stresses. The c-BN film will peel off from the substrate even for rather low thicknesses
(0.1-0.2 m), which will obstruct its applications for commercial purposes. As a consequence, the
intrinsic stress of BN thin films51,52, the evolution of stress51,53,54, and the post-deposition of the film
stress55 have carefully been examined. Recent studies39 have shown that post-deposition ion
bombardment over high quality c-BN thin films will lead to a strong relaxation of the compressive
stresses without destroying the cubic phase.
Substrate effects
It has recently been reported that the substrate material can control the phase ratio within the BN
films.56 A study of the nucleation behaviour of BN on various substrates is thus of major
importance in improving the adhesion of the films. Quite a number of studies57,58 have therefore
been performed aimed at this nucleation behaviour. A second factor that will affect the growth of
high-quality c-BN is the interfacial mismatch (or strain) at the film/substrate interface. Hence, a
knowledge and control over the interfacial chemical/physical reactions is extremely important in
order to enhance the nucleation and growth process of c-BN. Although many of the c-BN films
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have been deposited on silicon substrates, other substrates have also been used. High c-BN
fraction (<85%) has been obtained on hard substrates like diamond27, SiC28 and Si25, whereas
generally medium and low c-BN fraction (20-70%) have been obtained on soft substrates like
metallic substrates (Ni, Cu, Al)27,59 and ionic bond (10%) substrates (KBr, KI)60. Thick and
stable c-BN films up to 2.7 m were earlier deposited on Si substrates.61 The c-BN films were
then deposited on a compositionally graded interlayer, which consisted of B, C and N. The Ni
substrate has been found to have a catalytic effect on the nucleation and growth of c-BN films,
which thereby will prevent the formation of c-BN in the low pressure CVD process.57,62
However, it is generally difficult to deposit c-BN films with higher phase purity on metal
substrates (e.g., Al, Ag, Cu, Mo, Ni, steel) by using PVD methods.12,27 A smaller content of the
c-BN phase are formed on metal substrates than on Si, and the c-BN content decreases with
decreasing substrate micro-hardness.60
Many reports also suggest that Ni can play an important role in the nucleation and growth of cBN films under vapour phase growth conditions.56 The beneficial effect of the Ni substrate on
phase stability is expected of two reasons. One is the strong chemical affinity between B and
Ni.22 The other is the low lattice mismatch between Ni (a=3.524 Å) and c-BN (a=3.616 Å).63 The
effect of the lattice parameter and crystal structure of the substrate, in BN phase selection, has
recently been experimentally and theoretically
investigated for the Cu and Ni0.8Cu0.2
substrates.64 From ab initio calculations, the estimated elastic energy (from the lattice mismatch)
was found to be consistent with the energy difference between c-BN and h-BN. These
experimental and calculated results imply that the smaller the lattice-mismatch, the more stable
the cubic phase of BN. Hence, the strain energy at the interface was assumed to be responsible
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for the BN phase selection. In contrast to this conclusion, it has recently been shown65 that the
decisive parameters for c-BN growth are the physical properties of the substrate and its surface
(like hardness and roughness) rather than lattice parameter matching between c-BN and the
substrate.
Chemical Vapour Phase Deposition
Low energetic CVD techniques
The CVD techniques usually operate at the low energy regime under the influence of several gas
phase species. Besides boron- and nitrogen-containing species, hydrogen28,44 (or fluorine40,66)
contribute to the growth process, resulting in a selective etching of h-BN in favour of c-BN67.
Examples of typical CVD deposition techniques base on plasma activation of the gas phase
mixture are: electron-cyclotron-resonance-enhanced plasma CVD68,69, ICP CVD43, bias enhanced
Plasma CVD70, and DC Plasma Jet71.
Thick c-BN films (>20 m) have recently been deposited by plasma jet chemical vapour
deposition in an Ar-N2-BF3-H2 gas system.34 The films were found to have a cubic content
higher than 90% and low residual stresses of 1-2 GPa. However, the drawback of plasma jet
CVD in the controllability of the deposition conditions, and high cost of the operating gas and
electrical power, will limit the use of such a process for industrial applications. Microwave
plasma CVD is one of the most popular methods that are used for the synthesis of diamond films,
and it may therefore be a good candidate in the deposition of c-BN films. There are some reports
on the deposition of c-BN films by microwave plasma CVD where different reactant sources
including solid NaBH4, borane-dimethylammine, BH3NH3, BF3, NH3, N2 and H2 have been
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used.72-76 In addition, c-BN have recently been deposited on (001) Si substrates by rf- or dc-bias
assisted microwave plasma CVD in a He-N2-BF3-H2 system, with a phase purity of about 50%
for the rf-biased sample, and 70% for the dc-biased sample.66 Fluorine and the negative substrate
bias were assumed to play important roles in the deposition of c-BN.
The introduction of fluorine chemistry into the plasma enhanced chemical vapor deposition
(PECVD) process has been found to reduce the film stress.34,77 Moreover, a direct growth of
thick, adherent c-BN films with excellent mechanical properties on diamond substrates have very
recently been presented. The c-BN deposition was performed in an electron-cyclotron-resonance
microwave plasma CVD system using a fluorine based gas mixture (He–Ar–N2–BF3–H2).77,78
The c-BN film was then grown at a low substrate bias of -20 V (compared to 50–100 V typically
used in PVD). The CVD method presented in Ref. 78 was shown to give a) a direct c-BN growth
on diamond without non-cubic BN interface layers, and (b) a synthesis of c-BN films with
extraordinary adhesion to the substrates and good mechanical properties.
Gentle Chemical Vapour Deposition
As presented above, the CVD techniques usually operate at the low energy regime. The ions will
normally not penetrate the surface upon impact when the ion energies are well below 50 eV.
This raises the question why the ion energy cannot be set to zero. It is at present an urgent need
for a process development of more gentle CVD methods, were no bombardment of ions can
detrimentally affect crystallinity and/or film stress content. In addition, great efforts to increase
crystallite perfection, to control stoichiometry, to increase adherence and to eliminate undesirable
h-BN still remain. BN deposition with a thermally activated CVD reactor (without plasma
activation) have recently been carried out.79 Tris(dimetylamino)borane (((CH3)2N)3B) was used
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as single source precursor which was decomposed in a cold wall reactor. An IR-spectrum clearly
showed the peak for c-BN at 1088.3 cm-1, and an additional peak at 1551.5 cm-1 which
corresponds to sp2 carbon. Moreover, x-ray diffraction showed a clear peak at 2=43.2O
corresponding to c-BN (1 1 1).
Within a traditional CVD technique, chemical reactions are assumed to be the controlling
mechanism on the growing c-BN surface. In this case, B- and N-containing precursors are being
used in the gaseous mixture, which diffuse to and self-assemble on the growing c-BN surface
without the impact of high-energy heavy ions. Surface dominated growth mechanisms are also
assumed for those CVD techniques where the ion energies are well below 50 eV. However, a
synthesis process based on these more gentle CVD methods, resulting in the production of pure
and well-crystallized c-BN films, have major difficulties since well-controlled chemical routes
are not yet available. When comparing to diamond growth, the severe c-BN growth problems
using these types of methods are not easily understood. Although c-BN and diamond are very
similar regarding both structure and properties, there are very large dissimilarities in what
concerns their gas phase synthesis at low pressure. The metastable phase of carbon (diamond) is
easily and reproducibly grown by ordinary CVD, whilst it has shown to be extremely difficult to
grow the more stable form of BN (c-BN) with the same type of method.
Chemical processes during thin film growth
Vapour growth of thin films is a dynamic process with a large degree of complexity and
dynamics. The growth of c-BN films from vapour phase is a very complicated physical and
chemical process, and the detailed growth mechanism is still not clear. A deeper understanding
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of these surface reactions is of greatest importance in order to develop well-controlled chemical
routes for the CVD growth process of c-BN. The CVD techniques involve chemical reactions in
both the gaseous phase and on the growing surface. Generation of surface vacancies by gaseous
species and adsorption of surface terminating species, as well as of growth species, are especially
involved. In addition, this complex dynamic process is assumed to include surface processes like
migration. For the purpose to optimize the deposition processes of c-BN, it is very important to
obtain a deeper knowledge at the molecular level of all surface processes. Theoretical modelling
and simulations are useful tools in gaining this type of atomic level knowledge, and thereby
provide useful information to the experimentalists. A. Bartl et al.80 have presented some
conditions that have to be fulfilled in order to achieve a successful c-BN formation using CVD.
They include (i) stabilization of the c-BN surface during growth, (ii) sufficient high mobility of
the precursor at temperatures in the stability range of c-BN, (iii) preferential etching of h-BN and
other ”non-c-BN phases”, and (iv) preventing secondary nucleation of h-BN during growth of cBN.
Towards a Chemical Route for Synthesis of c-BN
Surface energies
It has experimentally been observed that (111) or (100) low index planes are formed during the
synthesis of c-BN.80,81 As demonstrated in Fig. 1, the (111) and (100) surfaces consist of alternate
B- and N- layers, whilst the (110) surface contains both types of atoms in each layer. An
investigation of the various properties of these different surface planes can give important
information that is useful for the development of novel c-BN vapour deposition processes. One
important property of a surface is its reactivity, i.e. the tendency to form bonds with gas phase
adsorbates. One parameter that is generally regarded to be a measure of the surface reactivity is
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the surface energy (i.e. the energy of formation of a surface from the bulk). The surface
reactivities of the three low-index planes [(111), (100), and (110)] of c-BN have very recently
been studied theoretically, using density functional theory (DFT) under periodic boundary
conditions.82 Surface energies have in that study been calculated for the three different c-BN
surfaces, and their corresponding reactivities were accordingly compared and discussed.
The surface energies (J/m2) for non-terminated relaxed and unrelaxed (i.e., ideal bulk structure)
surfaces were in Ref. 82 calculated by using the following equation:
 = (Esurf – Ebulk )/2A
(1)
where Ebulk is the the energy of a bulk unit cell, Esurf is the energy of a surface slab (scaled to
the unit cell size), and A is equal to the surface area (a2) (a = side of a unit cell).
The calculated surface energies for the three low-index c-BN planes are presented in Table I. The
values for ideal (bulk) structures and relaxed structures of the non-terminated surfaces are
presented. The order of surface energies for the three non-terminated, ideal c-BN surfaces was
found to be (110) > (100) > (111). However, the energy difference between the (110) and (100)
surfaces is very small. This order of surface energies became somewhat different after geometry
optimization; (100) > (110) > (111). Hence, the relaxation that takes place in order to lower the
surface energy of the system, will result in a real energy loss which was found to be most
pronounced for the (110) surface (1.4, 1.7 vs. 0.5 J/m2). As a consequence, when the sp3
hybridization is no longer retained by surface terminating species, the surface atoms will more or
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less collapse to surface atoms with sp2 hybridization. This is especial true for the B-rich (111)
and (100) planes, whilst the B sites on the mixed (110) plane show similar degree of
hybridization for both B and N surface atoms. Hence, the radical (non-terminated) N surface
atom was found to maintain the sp3 structure of the surface better than a corresponding B surface
atom does. However, this difference between the two adsorption sites was small at the “mixed“
surface (110).82 The reason to this behaviour may be that the electron configuration (and
chemical reactivity) of the B atoms is very special since B has only 3 electrons, and hence a small
tendency to bind tetrahedrally to other elements. Grossner, Furthmüller, and Bechhstedt83 have
also calculated the surface energy of the (110) plane of group-III nitrides, using a DFT planewave pseudopotential method. A surface energy of 3.4 J/m2 was then obtained for a relaxed and
non-terminated c-BN(110) surface, which is very close to the corresponding one obtained in Ref.
84 (3.0 J/m2).
Surface stabilization
Adsorption with surface terminating species
Common to all of the ordinary CVD methods are dilute mixtures of simple growth species in an
excess of a reactive surface terminating species. H and F have generally been considered to be
very important reactants for diamond growth since one of their dominant roles during the growth
process is to maintain the sp3 configuration of the surface carbon atoms.84-87 The stability of
various halogenated diamond (111) surfaces has been investigated theoretically in Ref. 84. These
calculations showed that hydrogen, H, and the halogen F and Cl species, will sustain the bulk sp
3
structural configuration of the surface carbon atoms. The adsorption energy of F to a Fterminated diamond (111) surface was found to be very similar to the corresponding adsorption
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energy of H to an H-terminated surface. Furthermore, it was expected that F- and H-terminated
diamond (111) surfaces should passivate surfaces from being chemically attacked better than, for
instance, Cl- or Br-terminated surfaces do.
Cubic BN growth is more complicated due to its binary nature. As can be seen in Fig. 1, the
planes consist of either B- or N-atoms [(111) and (100)], or there is a mixture of both B and N in
each plane (110). This will most probably lead to restrictions in the thin film growth. Widany et
al.88 have performed MD simulations of clean (non-terminated) c-BN (111) surfaces at room
temperature. Similar to clean diamond (111), transformation to sp2 hybridization was then
observed. The influence of the addition of hydrogen to the gas phase using an inductively
coupled plasma CVD process to deposit c-BN, was investigated by Kuhr et al.89 It was found that
H2 in addition to the hydrogen, introduced with the source gas trimethyl borazine, prevents the
formation of c-BN but not that of the sp2 nucleation layer. The surface structures of single-crystal
c-BN (100) and (111) have been exposed to argon ion sputtering and then to a prolonged
exposure of hydrogen-plasma in a work by Loh et al.90,91 The low-energy argon ion sputtering of
c-BN (100) resulted in local disrupture on the surface (from sp3 to sp2 type bonding). An
additional H etching resulted in an H-terminated and restructured c-BN (100)-(2x1) surface with
sp3 type bonding. Prolonged exposure in H plasma lead to a H-terminated (111)-(1x1) surface.
Furthermore, the (100) surface was, as a result of a prolonged exposure with H, etched to become
an unreconstructed (111) surface. In the work by Bohr et al.92, the influence of F2 on c-BN in
thermodynamic equilibrium was found to be similar to that of atomic hydrogen. Hence, the
reaction of F2 with the solid phase is thermodynamically possible. The positive effects of
hydrogen have also clearly been observed during vapour phase deposition of BN. It has been
found that CVD growth of c-BN with the presence of H2 will show a better stability than without
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H2.93 Even in PVD techniques, the addition of small percentage of hydrogen to the gas phase
during the formation of the h-BN buffer layer has been found to improve the adhesion of c-BN
films even under humid conditions.94
The challenge in the CVD process is now to find a way to stabilize the c-BN surface that is
equivalent to the use of atomic hydrogen (or fluorine) in diamond CVD. In a previous theoretical
work95,96, the calculated adsorption energy for F shows that this species will (compared to H) be
much stronger bonded to a B-rich c-BN(111) surface (653 vs. 442 kJ/mol) (see Table II). The
situation is opposite for adsorption to an N-terminated surface (251 vs. 465 kJ/mol). The angles
formed by the surface B atom and two of its binding neighbours (N-B-N) are also shown in Table
II for the different adsorption processes. This angle is expected to be numerically close to the
tetrahedral angle (109.4) for an sp3-hybridized surface atom. The angles obtained for the H- and
F-terminating surfaces in the present investigation are 110.0 vs. 111.7. Especially the former one
is rather close to the tetrahedral angle. For the latter one, there is a minor influence of sp 2
hybridization. Both the large adsorption energies for H and F, and the optimized structural
geometries for the closest environments to the adsorbates, will hence support the conclusion that
F, as well as H, will be effective in stabilizing the c-BN B(111) and N(111) surfaces, i.e., protect
the sp3-hybridized surface atoms from being sp2-hybridized to any larger extent. However, the
B(111) surface has a more pronounced tendency to become terminated by F species due to the
larger adsorption energy of F compared to H.
The efficiency of surface termination of the species H to the (110) surface has been calculated to
be very similar to the (111) situation (Table II). Hydrogen H has been found to be strongly
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bonded to the c-BN (110) surface, with almost identical strengths to both B and N surface sites
(about 420 kJ/mole). Hence, the H species will become efficient as a surface terminating species
for both the (111) and (110) c-BN surfaces. Stabilization of the c-BN surface during growth is
one of the conditions that according to Bartl et al.80 has to be fulfilled in order to achieve a
successful c-BN formation.
Abstraction of surface terminating species
Different abstraction reactions including the surface terminating species have earlier been
investigated theoretically for the B-terminated c-BN(111) surface.95 Both the H- and Fterminated B(111) surface were then assumed to undergo abstraction reactions with gaseous H or
F species. As can be seen in Table II, the energetically most favourable abstraction reactions are
obtained for the surface-terminating H species. An abstraction of terminating H species by
gaseous F was found to be the most exothermic process (-104 kJ/mol). This is to be compared
with the endothermic abstraction of surface terminating F species with gaseous H (+108 kJ/mol).
The energetically least favourable abstraction reaction included no hydrogen species. The
abstraction of surface-terminating F species by gaseous F was in fact an extreme endothermic
process (+512 kJ/mol). The numerical value of the abstraction energy for the corresponding
process including only H species was situated somewhere in between these above presented
extremes (+18 kJ/mol). This is a weakly endothermic process at 0 K, which however will be
possible at higher temperatures and at the supersaturation condition of hydrogen in the chemical
vapour deposition of c-BN thin films.
Based on this energetic investigation, it is possible to draw the conclusion that an H-terminated cBN B(111) surface will be energetically favourable in the most gentle CVD synthesis of c-BN,
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where chemical reactions will be responsible for abstraction reactions including gaseous H and F
species, respectively. Ion bombardment has recently been shown needed to activate such a c-BN
surface bonded to fluorine (see Chemical Vapour Phase Deposition).
Continued growth of c-BN on c-BN
Chemisorption of growth species on (111) surfaces. The adsorption of H- or F-terminated
growth species on H- or F-terminated (111) surfaces of diamond and c-BN, respectively, have
earlier been theoretically studied.95,97-100 It was then assumed that CH3 and the C2H fragment of
acetylene are the major contributing precursors in the growth of diamond.101 Hence, it is of a
large interest to also study the effect of the corresponding B- and N-containing species on the
growth of c-BN (111). All of these species are monoradicals and expected to be found during
CVD of these materials. Mixtures of boron-containing precursor and nitrogen (or ammonia) have
generally been applied for c-BN deposition.102 Diborane B2H6 is widely used as a boroncontaining precursor. Other examples include boron chloride80 and boron fluoride.103 The
BF3/N2/H2 gas mixture was successfully used to deposit BxNx films under CVD conditions in a
work by Hentschel et al.104 Single-source precursors have also been used.102 One major difference
between a single-source precursor and, e.g., NX2 (X=H,F) is that the former species contains both
B and N, being a growth species for growth of two layers simultaneously. When using NX2 as a
growth species, only a N(111) surface is possible to grow. A boron-containing species also has to
be introduced into the CVD reactor in order to be able to grow a second layer of c-BN B(111). As
was discussed in Section Abstraction of Surface Terminating Species, a mixture of F and H
species in the gas phase will result in energetically favourable abstraction reactions. It is hence of
a large interest to also study the effect of F (versus H) on the adsorption reactions during thin film
growth of c-BN.
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It has in an earlier theoretical study been shown that the most strongly adsorption occurs on the
H-terminated surfaces (Fig. 2).105 This figure includes the adsorption of species to the B-rich
surface of c-BN(111). It is then understood that one has to look for a preferential adsorption of Ncontaining adsorbates to this specific surface. The NH2 species adsorbed on a H-terminated
surface is the most strongly bonded one (459 kJ/mol). It is even more strongly bonded to the
surface compared to the H species (442 kJ/mol). The NF2 species adsorbed on a F-terminated
surface was found to be the weakest bonded (17 kJ/mol). One of the reasons for the smaller
adsorption energy obtained for the F-terminated surface is most probably the sterical hindrances
induced by the larger F atoms (both in NF2 and on the surface) (Fig. 3). A bonding situation in
which the gaseous species BX2 (X=H or F) will be bonded to the B(111) surface of c-BN will,
hence, result in a much smaller range of adsorption energies compared to the corresponding
adsorption of NX2 (range of 85 vs. 442 kJ/mol). As can further be seen in Fig. 2, the adsorption
of BX2 to a F-terminated surface will result in much larger bond strengths compared to the
adsorption of NX2 to a F-terminated surface. On the contrary, the adsorption of BX2 to a Hterminated surface will result in somewhat smaller adsorption energies compared to the
adsorption of NX2 to a H-terminated surface. As a conclusion drawn form these energetical
calculations, a mixture of the boron- (BX2) and nitrogen-containing (NX2) species in the gas
phase during chemical vapour deposition of c-BN thin films, will most probably result in a
mixture of these species as well on the B(111) surfaces. This is an unfavorable result since only
nitrogen-containing species are the expected growth precursors on this type of surface. The
situation is even worse for a F-terminated surface, for which the boron-containing species (BX2)
was found to energetically be much more strongly bonded to the surface compared to the
nitrogen-containing species (NX2).
18
19
In Ref. 99, adsorption of the NH2 and BH2 species on the H-terminated c-BN B(111) surface did
result in adsorption energies (equivalent to binding energies) numerically very similar to the
corresponding adsorption of CH2 (459 and 394 vs. 416 kJ/mol) on an H-terminated diamond
(111) surface.4 This was also the situation for C2H (diamond) and NBX (c-BN)) (X=H or F).105
What is most interesting is the observation (Fig. 2) that there is a large energetical preference for
adsorption of NBX to the c-BN surface (of about 220 kJ/mol compared to BNX). The
possibilities for co-adsorption (of N- and B-containing species) to the growing c-BN surface can
be diminished by using an Atomic Layer Deposition (ALD) technique instead of ordinary CVD,
where gases are mixed in the reaction chambers (see Section Experimental growth of BN using
atomic layer deposition). Another alternative is to look for growth precursors like XNBX
(including both B and N). BNX and NBX are assumed to be formed by introducing XBNX into
the reactor. Hence, it is only this precursor (of those tested in Ref. 95) that will give any
possibility for non-mixing of B- and N-containing species on the growing B(111) surface of cBN.
Surface migration of growth species during c-BN growth. The complex dynamic processes
occurring during chemical vapour deposition of c-BN is assumed to include surface processes
like migration of surface terminating species (e.g., H, F) and different types of growth species. As
was discussed by Bartl et al.80, the challenge in the CVD of c-BN is to grow the stable phase by
either suppressing the nucleation of h-BN or to favour the nucleation of c-BN. A high mobility of
the growth precursors (e.g., NH2) is then needed for the continuous growth of the stable c-BN
phase.
19
20
Surface migration is a very important elementary surface reaction step for an ideal growth of a
Frank-Van Der Merwe type, and has earlier been studied theoretically in comparising the c-BN
growth with diamond growth using CVD techniques.95 Single jumps between two neighbouring
surface atoms were then assumed in the calculations of energy barriers of migration. The
migration was of an anisotropic character since every surface boron has six symmetrically
equivalent boron neighbours on the surface. As was discussed above in Section Chemisorption of
growth species on (111) surfaces, the NH2 species has a somewhat larger tendency (compared to
BH2) for adsorption to a monoradical H-terminated c-BN B(111) surface (adsorption energy of
459 vs. 394 kJ/mol). On the other hand, the smallest barrier of energy has theoretically been
obtained for the BH2 species (120 (BH2) vs. 276 (NH2) kJ/mol). These barriers of energies are
relatively large, and will thus prevent an adsorbed NH2 (or BH2) species to move more or less
freely over the surface. The barriers of migration are, however, considerable more energetic
favorable (by about 183(NH2) and 274(BH2) kJ/mol) compared to any desorption process.
Chemisorption of growth species on (110) surfaces. Adsorption energies for H and NHx (x=1-3)
at B and N surface sites on c-BN (110) have also earlier been theoretically investigated (see Fig.
4).106 It was obvious that both NH and NH2 will strongly favour a boron surface site. The
difference in adsorption energy for NH is 258 kJ/mole (449 (B) and 191 (N) kJ/mole). The
corresponding values for NH2 are 303 kJ/mole (463 (B) and 160 (N) kJ/mole). Another
observation in Figure 4 is that both NH and NH2 will bind somewhat stronger than H to a B
surface site. The relative adsorption energy is 31 and 45 kJ/mole for NH and NH2, respectively.
This feature is very important in thin film growth since the incoming growth species must be able
to replace a terminating H atom for growth to occur. As can further be seen in Figure 4, the
gaseous NH3 species does not have any tendency to bind to either of the surface sites on c-BN
20
21
(110) (B or N). Hence, a decomposition of the into the CVD reactor incoming NH3 gaseous
precursor is very crucial for adsorption and, thereby, growth to occur. This can experimentally be
achieved by using various means of gas phase activation (e.g. thermally, laser or plasma
activation).
When comparing the adsorption energies for NH3 to a (110) and (111)-surface, respectively, it is
clear that the non-decomposed NH3 adsorbs easier to a B site on the (111) surface (236 kJ/mole
compared to 12 kJ/mole for a corresponding site on the (110) surface). This circumstance implies
that it is more important with decomposition for growth of (110) compared to (111) surfaces of cBN. If, however, the precursors (NH3 and BBr3) are decomposed to NH, NH2 and BBr, these
fragments will experience larger binding energies to the (110) surface (compared to the (111)
surface).96
The adsorption energies for H vs. BHx (x=1-3) to a B and N surface sites on c-BN (110) have
also been calculated in Ref. 106. Some obvious trends can be drawn when considering relative
adsorption energies (see Fig. 5). The B-containing adsorbates with the largest numbers of H
ligands (BH3) were found to not have any tendency to bind to either of the surface B- or N-sites
on c-BN (110). As shown in Fig. 4, this was also the situation for NH3. Another observation is
that there is only one adsorbate BH2 that energetically favour the binding to a surface N site, and
hence creates a B-N bond. The difference in adsorption energy is 110 kJ/mole (310 (B) vs. 420
(N) kJ/mole). A conclusion that can be drawn from Fig. 5 is that a gas mixture including BH will
most probably result in BH species adsorbed to N surface sites, and with a preference for BH on
the corresponding B sites. The only B-containing adsorbates that will have the possibility to
compete with H (when considering adsorption reactions on surface N sites) are BH and BH2. The
21
22
corresponding adsorption energies are 416 (H), 390 (BH) and 420 (BH2). As presented above,
gaseous BH2 does experience a rather strong discrimination between the various surface sites in
favour of the N site. In addition, since it has an adsorption energy of a similar strength compared
to an H atom, it cannot be ruled out as a plausible contributor for c-BN growth on the (110)
plane.
Several striking observations are made when studying Figures 4 and 5. First of all, nondecomposed BH3 and NH3 species do not have any tendency to become chemically bonded to the
c-BN (110) surface (which strongly supports the need for a gas phase activation and/or a use of
controlled chemical gas phase reactions). The here proposed plausible H-containing growth
species (BH2, NH, NH2) are within a specific range of adsorption energies (420-460 kJ/mole),
being somewhat larger than for the corresponding H species (416 kJ/mole). However, other
species than the plausible growth species are calculated to also bind very strongly to the surface
B site. This is especially the situation with the adsorption of the B-containing adsorbates BH and
BH2, for which the adsorption energies are 364 and 306 kJ/mole. These species will most
probably have the possibility to compete with our preferred N-containing growth precursor if
they are allowed to be present in the gas mixture during the film deposition of c-BN. This
problem can be circumvented by using an ALD method instead of an ordinary CVD method (as
will be described below).
Experimental growth of BN using atomic layer deposition (ALD)
As presented in Section Continued growth of c-BN on h-BN, ammonia is widely used as the Nsupplying growth precursor for CVD of BN.102 It can also supply H atoms to the process. B2H6 is
analogously a good B supplier, but it is difficult to handle experimentally. BF3 , BCl3, and BBr3
22
23
are other boron-source precursors used for CVD of BN.109,110 Of these three molecules, BBr3 is
the thermodynamically least stable according to the standard enthalpy of formation which is
2136, 2404, and 2240 kJ/mol for BF3, BCl3, and BBr3, respectively.109 BBr3 is, hence, expected
to be the most reactive of the three molecules at low temperatures and it was in Ref. 110 chosen
as the B-supplying precursor.
The precursors are usually thermally activated in traditional CVD, and the temperature is hence
an important process parameter. The precursor molecules can also be activated by e.g. a hotfilament, UV light photolysis, and the formation of a plasma. Different types of activation can
then be used to increase (or decrease) the gaseous content of specific precursor species. This can
be a useful tool in designing experiments where certain growth processes are enhanced and others
discriminated. Various chemical reactions occur in a CVD reactor, depending on the precursor
species present in the gas mixture. Consequently, the adsorption of many different species will
take place during CVD. One way to diminish the possibility for unwanted adsorption
competitions is to use ALD. This method is a sophisticated type of CVD. A major difference is
that the precursor gases within ALD are separately introduced into the reaction chamber.111 Gas
phase reactions between different precursors are then avoided. Moreover, the gaseous precursors
can be differently activated in order to increase the concentration of a specific species in the
reaction chamber. The great advantage with ALD is that c-BN growth can be controlled on an
atomic level by using this type of gas phase precursor design. The main difference between the
CVD and ALD techniques is that different growth precursors, and hence different growth
mechanisms, can be used in the vapour growth of materials like BN. By laser-activation of the
precursors in the CVD processes, the deposition temperature can generally be decreased and/or
23
24
the deposition rate increased.108 However, a very limited number of laser-assisted ALD (LALD)
studies have until now been reported in the literature.107
Atomic layer deposition of BN has earlier been reported twice, and then without any laseractivation of the precursor gas.112,
113
The boron nitride films were in Ref. 112 found to be
turbostratic. Hexagonal BN has a c-axis lattice parameter of 0.67 nm, which is twice the interlayer spacing between two successive sheets. In turbostratic BN, the hexagonal sheets are less
ordered, which induces longer inter-layer distances.114 NH3 and BBr3 are two precursors that
earlier have been used for both ALD and CVD of BN.105,112 BN thin films have in a very recent
work been grown by ALD, with or without laser-activation of the NH3 and BBr3 precursors.110
The experimental setup is shown in Fig. 6. It is well-known that photo-dissociation of NH3 by
using an ArF excimer laser (which operates at 193 nm) produces NH and NH2.109 Moreover,
BBr3 was in Ref. 110 observed to dissociate when irradiated by this wavelength (see Fig. 7).
Radical species of dissociated molecules are generally more reactive than the parent molecules.
As can be seen in Fig. 8, the dissociated fragments of NH3 (i.e., NH2, NH, and N) and BBr3 (i.e.,
BBr2, BBr, and B) have theoretically been found to adsorb much stronger to BN surfaces than the
non-fragmented molecules themselves.115 Moreover, H species will also be formed during the
dissociation of the NH3 precursors, and has theoretically been predicted to stabilize the (111)
surface of c-BN.97,116 The gas precursorsNH3 and BBr3 were hence in Ref. 110, hence, considered
to be a promising combination of precursors for LALD of BN thin films.
As can be seen in Ref. 110, the BN growth rate increased by more than 100 % (from 0.045 – 0.12
nm/cycle) by using photochemical activation of the BBr3 gaseous precursor (in the temperature
range 250-600 °C). However, no change in deposition rate was correspondingly observed when
24
25
activating the NH3 pulse (Fig. 6). Light emission from NH was detected using optical emission
spectroscopy (OES) on the gas phase during laser irradiation of NH3, while emission from B and
BBr was observed during the BBr3 gas sequence. All films deposited contained predominantly
turbostratic BN, and the c-axis lattice parameters were measured to be 0.71 - 0.79 nm (compared
to 0.67 nm for h-BN). The lower c-axis parameters within the here presented range were obtained
for films deposited by LALD, or by ALD at the higher temperature range (400-750 °C). This
result will also be attributed to the results of the photolytic activation using LALD (see above),
which gives highly reactive growth precursors that favour the growth path towards h-BN.
Nanocrystalline BN grains where formed in the films, which were more extended in the intraplane (a-/b-) direction compared to in the interplanar (c-) direction. The nanocrystalline grains
were larger in the films obtained by LALD, compared to the films obtained by ALD, for
temperature up to about 600 °C.
Growth of c-BN vs. h-BN: a theoretical approach
One way of searching for optimal precursors during ALD growth of BN (with a special focus on
the cubic phase) is by performing theoretical modelling of surface processes that take part in the
layer-by-layer growth. It is then of an outermost importance to identify those precursors that
favours the cubic phase over the hexagonal phase of BN. The initial steps of c-BN and h-BN
vapour growth from NH3 and BBr3 precursors, have earlier been theoretically investigated using
DFT calculations under periodic boundary conditions.115 This has been done by studying the
adsorption of NH3 and BBr3, and their decomposed fragments, onto c-BN(111) surfaces and hBN(10-10) edges, respectively (Figs. 1 and 9). An understanding of the growth processes would
make it possible to favour (or discriminate) the growth of either c-BN (or h-BN) during BN
vapour deposition. As a result, almost all of the adsorption reactions were found to be exothermic
25
26
(see Fig. 8). The endothermic reactions include adsorption of NH3 to the N sites of either c-BN or
h-BN, as well as BBr3 adsorption to the B-rich (111) surface of c-BN. The most weakly bonded
species to the cubic phase of BN correspond to the non-fragmented NH3 and BBr3. Adsorption of
gaseous N, NH, and BBr species were found to be unfavourable, as is clearly seen in Fig. 8. N
prefers energetically an N surface site on c-BN, whilst NH prefers either a cubic N site or a
hexagonal B site. BBr2 favours all surface site types, except the cubic B site. Hence, there is an
apparent risk for N-N and B-B bond formation with these species. Furthermore, adsorption of
NH2, B, or BBr were predicted to preferably form bonds to the hexagonal N sites, with a large
probability for NH2 to adsorb to a cubic N site.
The present results suggest that there is a large probability to obtain a mixture of B-N, N-N, and
B-B bonds when using activation of the NH3/BBr3 gas mixture within a CVD reactor. Separate
introductions of precursors into the reactor (e.g. by using ALD) can prevent such a situation. The
most optimal precursor fragments to use for c-BN growth is N in addition to the parent molecules
NH3 and BBr3 themselves (Fig. 8).
It is very interesting to note that the result reported in Ref. 115 will strongly support the
experimental results in Ref. 110, where turbostratic BN/h-BN was obtained when using laseractivated ALD on BBr3 and NH3 gas pulses. As can be seen in Fig. 8, the calculated adsorption
energies for the N fragment (in addition to BBr3 and NH3) that were present in the gas phase
above the growing BN film are especially marked. Based on the numerical values of the
adsorption energies, it is then obvious that these species will favour a hexagonal growth instead
of a cubic. It can also be seen in Fig. 8 that the species that will favour a c-BN growth (over the
hexagonal counterpart) were not detected in the ALD chamber in Ref. 110. These species (the N
26
27
fragment and the parent molecules BBr3 and NH3) would then be ideal in discrimination between
c-BN and h-BN. Unfortunately, a high degree of radical surface terminating species have earlier
been found needed in stabilising the growing c-BN surface, and hence some type of
fragmentation is non-avoidable.95,96
Initial nucleation of c-BN on h-BN
As was presented in Section Parameter windows, the cubic form of BN does generally grow on
the edge atoms of the hexagonal BN. The initial nucleation of c-BN on the zigzag edges (100)
and (-100), as well as on the armchair edge (110), of the basal plane of h-BN has earlier been
theoretically investigated (based on DFT methodology) under the influence of either H or F
termination of the edge atoms.116 It was then observed that a nucleation of c-BN on the zigzag
edge is energetically the most favourable one for both H- and F-termination (compared to a
continued growth of h-BN). The wurtzitic structure of BN was most favourable on the armchair
edge. It was for all cases shown that the F atoms possess a significantly larger ability to stabilize
the c-BN (compared to the H atoms). This result support the experimental findings in Ref. 91 and
94, that states that H will even improve the adhesion of c-BN on h-BN since the basal planes of
h-BN are orthogonal to the growth planes of c-BN.
The possibility to also nuclei by c-BN on the basal plane of h-BN, has earlier been theoretically
investigated using the DFT method and a cluster approach.117 It was then found that the h-BN
basal plane B and N atoms will not undergo any chemical reaction with H (i.e., no possibility for
adsorption of H). This is to be compared with the strong chemisorption of H to the B and N sites
on the zigzag edges of h-BN (432 vs. 490 kJ/mol). On the contrary, the F species was found to
chemisorb rather strongly to these basal plane atoms (B; -313 and N; -100 kJ/mol).
27
28
Geometrically, the adsorption of F lead to a local sp3 hybridization, and the F species are thereby
able to provoke the formation of embryonic c-BN nucleus on h-BN (001). Conclusively, gaseous
F radicals are expected to be more favourable (compared to H) in the etching process of h-BN, as
well as in the nucleation of c-BN on the basal planes of h-BN.
Acknowledgment. The work was supported by the Swedish Research Council (VR) and Göran
Gustafsson Foundation. The computational results were obtained using the software programs
from Accelrys, Inc. (first principle calculations were done with the CASTEP program within the
Cerius2™ program package).
28
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Figure captions
1. Low-index surface planes of c-BN [(111), (110) and (100)].
2. Calculated adsorption energies of plausible H- or F-terminated growth species to an
H- or F-terminated B-rich surface of c-BN (111).
3. Models showing the adsorbate geometry for NX2, BX2 and BNX (or NBX).
4. Calculated adsorption energies (in kJ/mol)of plausible N-containing growth species to an
H-terminated B-rich surface of c-BN (110).
5. Calculated adsorption energies (in kJ/mol) of plausible B-containing growth species to an
H-terminated N-rich surface of c-BN (110).
6. Experimental setup for ALD growth of BN.
7. Results from optical emission spectroscopy of the gas phase directly over the growing
BN film using ALD.
8. Calculated adsorption energies (in kJ/mol) for adsorption of various B- and Ncontaining species to the B and N edge atoms of h-BN vs. the B and N atoms on the
(111) surfaces of c-BN.
9. Model of the adsorption of growth species to the B- and N-edge atoms of h-BN.
33
34
Table I. Surface energies for the various low index planes of c-BN.
(J/m2)
Bulk surface
Relaxed surface
(111)
3.1
2.6
(110)
4.7
3.0
(100)
4.6
3.2
34
35
Table II. Adsorption energies (in kJ/mol) for the adsorption of H (or F) to a B- or Nterminated (111) and (110) c-BN surface. The energy for gas phase abstraction of surface
terminating H (or F) by gaseous H (or F) is also demonstrated.
H-terminated (111)
F-terminated (111)
H-terminated (110)
Adsorption energy
Abstraction energy
(kJ/mol)
(kJ/mol)
-442 (B), -465 (N)
N-B-N = 110.00 (B)
-653 (B), -251 (N)
N-B-N = 111.70 (B)
-418 (B), -416 (N)
+18 (H:B), +45 (H:N)
-104 (F:B)
+108 (H:B)
+512 (F:B), +28 (H:N)
----
35
36
Fig. 1
(111)
(110)
(100)
36
37
Fig. 2
Eads (kJ/mol)
800
700
600
500
H
F
400
300
200
6
C2H CH3
5
BH2
4
NH2
3
2
NBH BNH
1
0
0
7
100
37
38
Fig. 3
NBH
NH2
BH2
38
39
Fig. 4
39
40
Fig. 5
40
41
Fig. 6
Lens
LASER
BBr3
Pump
NH3
Growth rate (Å/cycle)
T [C]
P [torr]
Substrate
Carrier gas
Linear gas flow velocity [m/s]
Typical pulse sequence:
NH3 : N2 : BBr3 : N2
Typical nr of cycles
250-750
10
SiO2
N2
~1
1,5
Laser
BBr3
Laser
NH3
0
0
2:3:5:3
5
10
Pulse length at 400 °C
1000
41
42
Fig. 7
NH
[nm]
0
310
330
350
B
BBr
[nm]
0
170
220
270
320
42
43
Fig. 8.
ads (kJ/mol)
cBN(111), B-plane
hBN(100), B-site
N-plane
N-site
750
0
Experimental species
Optimal species to use
43
44
Fig. 9.
44