Attempts of boron nitride deposition on different substrates under the influence of the electric field present in the reaction zone

Article abstract of DOI:10.1016/S0025-5408(01)00575-X

Attempts were undertaken to obtain deposits of hexagonal boron nitride under the influence of either the electric field or current of various frequency flowing through the substrate. It was demonstrated that magnetic field as well as electrical potential difference created in the reaction zone both influence the way BN layers are formed. The layers grow with the formation of fine crystallites in the center of the substrate with their size increasing while moving from the center of the substrate to its edges. Distinct correlation between the electric fields line density and the sizes of the crystallites was observed. The distribution of the crystallites depends on current frequency used. It was found that there is an optimal current frequency for which the substrate is covered with the homogeneous layer of BN. The adhesion of this deposit depends strongly on the kind of the substrate used.

Full text of DOI:10.1016/S0025-5408(01)00575-X

Materials Research Bulletin 36 (2001) 915–923
Pergamon
Attempts of boron nitride deposition on different
substrates under the influence of the electric field present
in the reaction zone
Ewa Pawlas-Forysta, Witold Przybyłoa,
Marek Kopytoa, Krzysztof Fitznerb,*
aInstitute of Metallurgy and Materials Science Polish Academy of Sciences 25 Reymonta St.,
30-059 Krako´w, Poland
bFaculty of Non-Ferrous Metals, Dept. of Physical Chemistry and Electrochemistry Academy of Mining and
Metallurgy, 30 Mickiewicza Ave., 30-057 Krako´w, Poland
(Refereed)
Received 13 June 2000; accepted 30 August 2000
Abstract
Attempts were undertaken to obtain deposits of hexagonal boron nitride under the influence of either the
electric field or current of various frequency flowing through the substrate. It was demonstrated that
magnetic field as well as electrical potential difference created in the reaction zone both influence the way
BN layers are formed. The layers grow with the formation of fine crystallites in the center of the substrate
with their size increasing while moving from the center of the substrate to its edges. Distinct correlation
between the electric fields line density and the sizes of the crystallites was observed. The distribution of the
crystallites depends on current frequency used. It was found that there is an optimal current frequency for
which the substrate is covered with the homogeneous layer of BN. The adhesion of this deposit depends
strongly on the kind of the substrate used. © 2001 Elsevier Science Ltd. All rights reserved.
Keywords: A. Thin film; B. Vapor deposition
1. Introduction
Boron nitride is an inorganic compound possessing a number of interesting physicochem-
ical properties [1,2], which in turn result from its several crystallographic modifications. Two
* Corresponding author.
E-mail address: mmfitzne@imim-pan.krakow.pl (K. Fitzner).
0025-5408/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0025-5408(01)00575-X

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of them seem to be potentially most useful: hexagonal and cubic BN structure. Boron nitride
is chemically a very stable compound (even at a high temperature), it exhibits high and
strongly anisotropic thermal conductivity (similarly to graphite), and being an excellent
dielectric is highly insulating. That’s why hexagonal BN was proposed as a good dielectric
for the use in the III-V semiconductors. Attempts have been made to deposit BN on InP in
order to develop new technology for insulator formation during the fabrication of semicon-
ductor devices at a low temperature [3–5]. Being chemically inert it can be used as a
protective layer resistant against high temperature corrosion. For example, the incorporation
of silicon during the GaAs crystals growth from crucibles and ampoules made of silica can
be avoided if this process is conducted in BN crucibles or if BN is deposited on SiO2 as a
film to prevent direct contact with the silica wall [6]. Hexagonal BN may also be used in
ceramic protective coatings while mixed with B4C. Since B4C is an exceptionally hard
material, the introduction of softer hexagonal BN phase should increase the fracture tough-
ness and may act as a solid lubricant [7,8]. It can be also introduced into ceramic-matrix
composites replacing carbon matrix. Studies of infiltration of porous substrates were under-
taken to produce BN coatings on fibers [9,10].
In turn cubic boron nitride is the second hardest material known after diamond. Apart from
the possibility of using it as cutting tool material it can be deposited as an insulator on Si.
Having high thermal conductivity it may be superior to the other known epitaxial insulators
on silicone [11].
Our investigations which we carried out over the last several years using different
substrates and NH3 ϩ BCl3 gas mixtures let us determine conditions necessary to deposit
hexagonal boron nitride from the gas phase. These investigations were carried out in an open
system. It was found [12–13] that temperature, pressure ratio of the reagents, amount of
argon and hydrogen added, as well as the geometry of the reaction zone, may influence the
process of the deposition. Thermodynamic analysis of this process was conducted in our
previous paper [12]. Theoretical efficiency of the process was calculated as well as the rate
of the deposition. Both of these parameters depend strongly on the temperature and partial
pressure of reagents in the gas phase. Results showed that the amorphous BN phase formed
during deposition process at a low temperature disappears above 1400 K. This result is in
agreement with the conclusions of Takahashi’s et al. [14] work.
Deposition of the homogeneous layer of BN requires lower flow rates, mainly BCl3, while
keeping the ratio of BCl3 / NH3 in the range 0.2–0.5. Under higher flow rates the layer
becomes nonhomogeneous. In general, the adhesion of the deposit depends on the kind of the
substrate used.
In the present paper we report the attempts of hexagonal BN deposition under the influence of
either electric or magnetic fields present in the reaction zone. It was decided to check if such
external force can influence the structure of the deposit formed on the surface of the substrate.
2. Experimental
The scheme of the apparatus and the experimental procedure were described in the previous
paper [13]. The main part consists of the reaction tube in which the reaction zone is placed. From

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Fig. 1. The schematic diagram of the experimental arrangement (a). (b) gases introduced separately from below.
(c) gases introduced separately from above. (d) hydrogen addition into the gas mixtures.
the previous study it was clear that mutual position of radially placed nozzles which introduce
gases into the zone, may influence the structure of the deposit. That’s why the scheme of the
reaction zone shown in Fig. 1 displays several modifications of the gas-inlet system.
Experiments were run under identical conditions of temperature and pressure. The gas
mixture with the ratio PBCl3/PNH3 ϭ 0.5 flowed through the system with the rate 140
cm3/min and under total pressure of 1 atm. The temperature was kept constant at 1323K. The
substrates made of pure nickel and 18/8 stainless steel were cleaned and polished in order to
obtain clean and flat surface. Then the sample was placed inside the tube at constant distance
from the nozzle which introduced the mixture of gases into the reaction zone. For all the
experiments the time of deposition varied from 3,5 to 4.5 min. After this time the flow of
BCl3 was stopped while the obtained deposit was further kept at constant temperature under
the stream of Ar ϩ 7% H2 gas mixture (with NH3 flow reduced by half) for another 30 min.
Then, the total flow rate was reduced and the furnace with the sample was cooled to the room
temperature. X-ray diffraction of the deposit obtained in this way is shown in Fig. 2. It
demonstrates that hexagonal boron nitride was formed under given experimental conditions.
Motojima et al [15] suggested that the heat treatment performed at 1273K for 5h in argon
stabilized the hexagonal structure of CVD deposits obtained in the temperature range
873–973K. It seems however, that under these conditions only impurities are entirely
removed and the transition into hexagonal structure cannot be completed.
Having experimental conditions of hexagonal BN deposition determined, the influence of
an external field on CVD process may be investigated. At first, AC current source (I ϭ 0.2A)

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Fig. 2. X-ray diffraction pattern of the vapor deposition BN on Ni substrate at 1323K.
was connected with the nickel substrate. The expected effect was not visible except in the
area of the connections of the electric leads to the substrate. The obtained deposit showed
weak stripes in this region of the substrate. This effect suggested that the structure can be
modified by external factor not directly connected with the chemistry of CVD process itself.
Next, the electric potential difference was imposed upon the substrate. Schematic arrangement
of the reaction zone is shown in Fig. 3. Negative potential was connected to the substrate against
the positive potential which was connected to the metallic ring introduced into the reaction zone
in such a way that the created electric field was perpendicular to the surface of the substrate. Since
the current may bring about glowing, and moreover the current flow through imperfect silica
insulation is not known, one cannot suggest the direction of the magnetic field in this case. The
estimated field density under potential difference of 500V and given electrodes distance is about
60000V/m. In this case the magnetic field effect can be neglected.
Finally, alternating current was used, and the arrangement of the experimental setup is
shown in Fig. 4. Maximum current was 0.5A, while substrate resistance is negligibly small.
The resulting potential drop across the substrate is of the order of tens of millivolts. This field
is not strong since the current is small and there is only one conductor. The current used had
the frequency of 50Hz as well as the acoustic frequency. The direction of the field was
changing according to sin - function. The deposition process was carried out with the current
frequency of: 50, 530, 1060 and 1670Hz. An electronic device assembled and connected to

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Fig. 3. Distribution of the electric field lines perpendicular to the surface of the substrate.
the circuit let the frequency be changed from 30Hz to 3000Hz with the amplitude changing
from sin to step-function. One can assume that electric field has no influence on the process,
while created magnetic field acts parallel to the surface of the substrate.
3. Results
All experiments were run at constant temperature of 1323K. Also, the flow of the gas
mixture was kept the same for all experiments. Attempts have been made to keep the distance
between electrodes identical for all experiments.
Fig. 4. Distribution of the magnetic field lines perpendicular to the axis of the substrate placed in the reaction
zone.

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Fig. 5. Scanning electron micrograph of vapor-deposited BN on Ni surface obtained in the presence of the electric
current.
Fig. 6. Surface of the BN deposit obtained at AC current of the frequency 530Hz.

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Fig. 7. Surface of the BN deposit obtained at AC current of frequency 1070Hz.
Deposit shown in Fig. 5 was obtained under the electric field perpendicular to the surface
of nickel substrate. Minus sign potential was connected to the substrate. It is seen that this
deposit is distributed in the shape of coaxial circles with greater amount of deposit concen-
trated in the region with higher density of field lines. Change in the potential sign on the
substrate (i.e. plus connected to nickel) gave no visible effect on the deposit.
Deposits obtained under AC current flowing through nickel substrate are shown in Figs.
6 and 7. The frequency of the current was 530 and 1070 Hz, respectively. Deposit obtained
on stainless steel with the current of 50 Hz is shown in Fig. 8. The surface of all samples was
prepared in the same manner prior to experiments. In all cases the influence of the field is
visible. Deposits grow in the similar centrifugal manner and the current with higher fre-
quency tends to introduce a less uniform distribution of deposit’s grains. It was also observed
that deposit adheres better to steel than to nickel and its grains are smaller. It seems that there
exists optimal field’s frequency which yields uniform distribution of the deposit over the
surface of the substrate.
The obtained results indicate that there is a possibility to influence the structure of the
deposit using external field. It is also clear that the field exerts influence on the mechanism
of the reaction of BN formation. Since the effects of the field are seen with minus potential
connected to the substrate one may speculate that at least one of elementary steps causing BN
formation must involve positively charged particles. Perhaps, it is a radical formation with
positive charge.

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Fig. 8. Scanning electron micrograph of vapor-deposited BN surface obtained on the stainless steel substrate in
the presence of the current (50Hz).
The application of the field with variable frequency may cause mechanical vibrations of
the substrate and resulting acoustic vibrations in the reaction zone. This may also be the
reason of nonuniform deposit distribution. However, since the acoustic wave length is at least
several times longer than the distance between grains, it does not seem likely that this wave
is responsible for the created structures. It seems that the electric as well as magnetic field
are indeed responsible for their formation.
4. Conclusions
a) Attempts to deposit boron nitride in an open system from the gas phase containing the
mixture BCl3 ϩ NH3 showed that the structure of the deposit may be influenced by an
external field.
b) The shape and distribution of this deposit depends on the frequency of the current. It
seems that there exists an optimal frequency which yields uniform distribution of the
deposit over the substrate.
c) Results obtained under electric field may suggest the mechanism of the reaction
leading to the formation of the deposit.
d) It was observed that stainless steel substrate is characterized with better adhesion than
nickel.

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