Phase identification of boron nitride thin films by polarized infrared
reflection spectroscopy
a)
M. F. Plass, W. Fukarek, S. M a¨ ndl, and W. M o¨ ller
Forschungszentrum Rossendorf e.V., Institut f u¨ r Ionenstrahlphysik und Materialforschung,
Postfach 51 01 19, D-01314 Dresden, Germany
͑
Received 26 February 1996; accepted for publication 30 April 1996͒
Six different types of boron nitride films were investigated by polarized infrared reflection
spectroscopy. Films with a highly cubic, mixed cubic and noncubic, and exclusively noncubic phase
composition were synthesized using ion beam assisted deposition. Additionally, postdeposition
argon ion irradiated cubic and noncubic boron nitride films as well as a nitrogen implanted boron
sample were analyzed. Using this technique, besides the cubic phase, two different noncubic
modifications, layered anisotropic and amorphous, could be distinguished. A preferential orientation
2
of the normal axis of the sp -bonded basal planes parallel to the substrate surface was
observed. © 1996 American Institute of Physics. ͓S0003-6951͑96͒03627-3͔
Infrared spectroscopy is the main characterization tool
for boron nitride thin films as the peak positions in the IR
spectra of cubic and hexagonal BN are well separated.
Mostly absorption measurements in transmission mode1
have been reported, and only a few were performed in re-
lower the amorphization threshold as reported by Kalish and
7
2
Prawer. As the average power density is 120 mW/cm or
less, the temperature during the implantations remained be-
low 100 °C. For this energy, the mean projected range, as
,2
8
calculated with TRIM, is 26 and 41 nm in cubic ͑density
3,4
3
3
flection geometry on metals using unpolarized light. With
ϭ3.5 g/cm ͒ and noncubic ͑ϭ2.2 g/cm ) BN, respec-
2
13
Ϫ2
15
these techniques, different sp -bonded phases have not been
tively. The dose was varied between 10 cm and 10
Ϫ2
identified. According to cross-sectional transmission electron
cm . It can be assumed that at least the highest dose results
in an amorphization of the surface region. A 200 nm thick
boron film was implanted using PIII with a nitrogen plasma
and 20 kV pulses at a dose of 10 cm to form an amor-
phous boron nitride surface layer by direct ion implantation.
The projected range corresponds to a quarter of the film
thickness. All doses were calibrated by RBS measurements
on silicon reference samples.
3
microscopy ͑XTEM͒ viewgraphs, c-BN containing films ex-
hibit a layered structure. A few nanometers of randomly ori-
2
17
Ϫ2
ented sp -bonded BN are formed at the substrate interface,
2
followed by a vertically oriented sp -bonded structure and
the cubic phase on top. Albeit, the surface is terminated by
2
an sp -bonded layer with a thickness of a few nanometers,
1
as seen in in-situ electron loss spectra. The aim of the
present communication is twofold. First, we will show that
polarized infrared reflection ͑PIRR͒ spectroscopy is capable
of identifying different, i.e., layered or amorphous,
PIRR spectra were recorded at angles of incidence ⌽ of
5° or 60° using a wire grid polarizer on a BaF or CaF2
2
4
substrate. The angle ␣ between the plane of incidence of the
IR beam and the plane formed by ion source, substrate, and
evaporator during deposition ͑ISE plane͒ was rotated in steps
of 45°. The PIRR setup and the data acquisition and reduc-
tion have been described in Ref. 9.
2
sp -bonded phases in BN films. Second, spectroscopic evi-
dence will be presented for the vertical basal planes found by
XTEM. However, it is neither possible to resolve the sub-
strate interface nor a superficial layer with a thickness of
several monolayers using this technique.
PIRR spectra at different angles ␣ are shown of a non-
cubic BN film in Fig. 1͑a͒, a film containing a cubic fraction
in Fig. 1͑b͒, and a highly cubic film in Fig. 1͑c͒. The spectra
recorded with s-polarized light show only the response of the
transversal optical ͑TO͒ modes and are similar to published
transmission spectra,1 whereas the p-polarized spectra ex-
hibit a larger dynamic range and additional features due to
longitudinal optical ͑LO͒ modes. The BN bending mode
The BN samples were synthesized by ion beam assisted
deposition ͑IBAD͒,1 using an ion energy of 500 eV and
normal angle of incidence, on ͗100͘ silicon wafers heated to
,3
400 °C. The ion ͑nitrogen and argon͒ to boron atom (I/A)
,2
arrival ratio was varied between 1.6 and 2.0, resulting in an
increasing cubic fraction in the films with increasing I/A-
ratio. Additionally, one sample was prepared by evaporating
boron without ion assistance.
Ϫ1
shows up at 760 cm and the TO and LO in-plane stretch-
Using plasma immersion ion implantation ͑PIII͒,5 some
BN samples were post-deposition implanted with 40 keV
argon ions for modifying the near surface region of the films.
In this PIII experiment, the sample is immersed in a plasma
and pulsed with a high voltage at repetition rate of 100 Hz
or less. This results in a high dose rate of 1.67
,6
Ϫ1
ing mode around 1400 and 1600 cm , respectively. The
shoulder in the spectra measured with s-polarized light at
Ϫ1
about 1500 cm is due to the anisotropy of the h-BN
10
crystal. In the following, the changes in the reflection spec-
tra related to the bending mode are not discussed as it is hard
to draw definite conclusions from the small changes. Beyond
that, in Fig. 1͑b͒ the structure between 1000 and 1200
1
6
Ϫ2 Ϫ1
ϫ10 cm
s
during the 30 s pulses, far beyond the
range of commercial ion implanters. A high dose rate may
Ϫ1
cm arises from the c-BN TO mode. Structures due to the
c-BN LO mode cannot be seen clearly in this figure. Com-
pared to the noncubic film, the low energy side of the sharp
a͒Electronic mail: plass@hera.fz-rossendorf.de
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Appl. Phys. Lett. 69 (1), 1 July 1996 0003-6951/96/69(1)/46/3/$10.00 © 1996 American Institute of Physics
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Plass
