Infrared Gratings Based on SiC/Si-heterostructures
C. Rockstuhl
1,a
, H.P. Herzig
1,b
, Ch. Förster
2,c
, A. Leycuras
3,d
, O. Ambacher
2,e
,
and J. Pezoldt
2,f
1
Institute of Microtechnology, University of Neuchâtel, Rue A.-L. Breguet 2, CH-2000 Neuchâtel,
Switzerland
2
FG Nanotechnologie, Zentrum für Mikro- und Nanotechnologien, TU Ilmenau, Postfach 100565,
98684 Ilmenau, Germany
3
CRHEA, CNRS, Parc de Sophia Antipolis, Rue Bernard Gregory, 06560 Valbonne, France
a
carsten.rockstuhl@unine.ch,
b
HansPeter.Herzig@unine.ch,
c
christian.forester@tu-ilmenau.de,
d
Andre.Leycuras@crhea.cnrs.fr,
e
oliver.ambacher@tu-ilmenau.de,
f
joerg.pezoldt@tu-ilmenau.de
Keywords: infrared, optical properties, optical device, 3C- SiC, Si.
Abstract. The fabrication process and the spectral properties of gratings for the infrared wavelength
region on the basis of 3C-SiC layers grown by CVD on (100) oriented Si substrates are demon-
strated. The formed 3C-SiC gratings on Si support two phonon polaritons as a function of the geo-
metrical properties excited between 10.3 and 11.4 µm. They appear as a dip in the transmission
spectrum. A third minimum in the transmission spectrum is caused by the substrate – grating inter-
action. The obtained resonances were polarization sensitive, i.e. they appeared only under TM-
polarized illumination.
Introduction
The classical application for a material such as SiC is in the field of devices for high temperatures,
sensors and high frequencies as well as high power electronics. Beside those primary applications,
which exploit the electronic qualities of the material, other properties of SiC attracted the interest of
researchers. One of them is its dielectric constant in the infrared region of the spectrum between 10
µm and 12 µm (1000 cm
-1
and 833 cm
-1
). In this spectral region independent of the polytype the
real part of the dielectric constant takes a negative value between –1 and –10 and the imaginary part
is small [1, 2]. This behaviour is caused by the phonon frequencies of SiC. If the frequency of the
illuminating wave field matches the eigen frequencies of the SiC lattice, a phonon polariton is ex-
cited in resonance. Due to the small remaining imaginary part of the dielectric constant of SiC, the
phonon experiences only slight damping. This leads to a large near-field amplitude in the vicinity of
the particle and a strong scattering cross section at the resonance frequency. Due to this singular
property, SiC can be used as a coherent, directed emitter in the corresponding spectral region. Such
a device can be realised by heating a well designed grating made of SiC, which will excite a vibra-
tional state in the SiC lattice. By choosing appropriate geometrical parameters for the morphology
of the grating, the phonon mode can be coupled into a radiative mode that propagates into the far-
field. Furthermore, it was shown, that the design of the surface morphology allows the modification
of the radiation direction and the value of the emission of the structure for a given wavelength [3].
The aim of the presentation is to report on the fabrication and spectral characterisation of SiC
based gratings in the infrared wavelength region.
Experimental
The gratings were fabricated by using (100) oriented ß-SiC grown on (100)Si substrates. The
thicknesses of the 3C-SiC layers were 1.9, 2.4 and 2.7 µm. On the basis of the grown heterostruc-
tures, different gratings were fabricated. The binary gratings consist of SiC bars located on the Si
Published in Materials Science Forum (MSF) 483-485, 433-436, 2005
which should be used for any reference to this work
1
substrate having different periods. The height of the SiC bars equals the SiC layer thickness. The
SiC material between the neighboring SiC bars was removed completely in the fabrication process.
The periods of the gratings were chosen to be 6, 8, 10 and 14 µm. For each period eight different
widths of the SiC bars were designed leading to different grating fill-factors. The fill-factor is de-
fined as the ratio between the width of the SiC bar and the period of the grating. The process se-
quence of the grating fabrication was as follows: (a) 3C-SiC epitaxial growth, (b) formation of the
metall mask by lift-off processing with i-line photolithography, (c) anisotropic dry etching of SiC
by an ECR-plasma etch process, (d) removal of the metal mask by selective wet etching (Fig.1).
The metal mask consists of a 100 nm thick Al layer deposited by magnetron sputtering. Subse-
quently to the Al lift-off process, the SiC in the unmasked region was completely removed by using
an ECR-plasma etch system described in [4]. A 8:4:1 mixture of Ar, SF
6
and O
2
at a process
pressure of 1.8x10
-3
mbar was used to etch the silicon carbide layer, with a pronounced anisotropic
etch behaviour. The etch process was carried out at 720 W. A low DC bias voltage of 70 V on the
substrate was applied to obtain a smooth and straight step profile as well as low etching damage on
the bottom trenches and the side walls. To remove the etch mask a high selective mixture of H
3
PO
4
,
HNO
3
and H
2
O was used.
The geometry of the fabricated gratings was evaluated by scanning electron microscopy (SEM)
and profilometry. The smallest achieved SiC bar width was 0.8 µm. The largest bar width was 1 µm
below the grating periodicity. SEM pictures of fabricated SiC gratings on Si (100) are shown in Fig.
2a and 2b.
Fig. 2: ß-SiC gratings fabricated on (100)Si: (a) 2.7 µm (height) x 3.0 µm (width) x 2 m
m
(length) and 14 µm periodicity, (b) 2.7 µm (height) x 0.8 µm (width) x 2 mm (length) and 14
µ
m
p
eriodicit
y
(a) (b)
Si
SiC
(a)
Si
SiC
Al
(b)
Si
Al
(c)
Si
(d)
SiC
Fig.1: Process Steps of the gratings: (a) heteroepitaxy, (b) metal mask formation, (c) anisotropic
dry etching, (d) selective wet metal etching.
2
The measurements of the transmission properties of the gratings in the infrared were carried out
using an FTIR spectrometer Magna IR 860 from Nicolet. The structures were illuminated with a
slightly focused beam and the wavelength dependent transmission in the 0th order was measured.
As the reference spectrum, used for the normalization of the results, the transmission by the bare
silicon substrate was applied. The spectral resolution was chosen such that Fabry-Perot oscillations
caused by multiple reflections of the light within the substrate were avoided.
Results and discussion
A typical result of a measured transmission spectrum (design values: 2.7 µm height and 4.0 µm
width) in comparison with a simulation is shown in Fig. 3. The simulation was carried out by using
the Fourier Modal Method [5] and we assumed a slightly different value for the width (3 µm) for a
better accordance. The measured width of the bars was 3.3 µm. The scattering signature for single
objects without a periodic boundary have been calculated for comparison with the Boundary Ele-
ment Method[6]. The illuminating wave field was TM-polarized, meaning that the magnetic field
vector oscillates parallel to the grating structure. For comparison, the calculated spectral transmis-
sions of nonstructured SiC/Si heterostructures with different SiC thicknesses are shown in Fig. 4.
The measured transmission spectrum in Fig. 3 displays three pronounced minima, which do not
appear in the transmission spectrum for the case of the plane heterostructures. These dips in the
transmission are due to excited phonon resonances. Two well defined minima at lower wavelengths
appear at 10.4 µm and 10.9 µm. They are associated with the rectangular geometrical cross-section
of each bar of the SiC grating. The minimum in the transmission at 11.85 µm is attributed to the
substrate – object interaction and has no counterpart in the spectrum of the freestanding object (ß-
SiC). The difference in the resonance wavelength of the measured and the simulated phonon polari-
tons is probably due to an incertitude in the assumed dielectric constant for the calculation. The
reason for the deviation in the dielectric constants can be mainly caused by the residual stress in the
grown SiC layer. This residual tensile stress shifts the TO phonon position to lower wave numbers
and therefor lead to changes in the dielectric function in the infrared region of the spectra. Due to
the fact that each of the resonances is associated with a specified value of the real part of the dielec-
tric function, a change in this parameter will shift the wavelength for which the phonon is excited.
In the case of TE-polarized light no resonances could be measured, in accordance with the theoreti-
cal prediction. The wavelengths for which the phonon polaritons are excited depend primarily on
the axis ratio of the SiC bar, defined as film thickness divided by the width of the bar [7].
Fig. 4: Spectral transmission of a plane
SiC/Si heterostructure at normal incidence
calculated for different thicknesses
Fig. 3: Comparison between a measured and
simulated transmission signal for a ß-SiC
grating (2.7 µm height and 4.0 µm width)
3
In Fig. 5 the wavelength of the second surface
phonon polariton resonance is shown as a func-
tion of the axis ratio and compared with theoreti-
cally predicted values. Fig. 5 summarizes the re-
sults obtained for gratings with a period of 10 µm
prepared on heterostructures based on different
SiC layer thicknesses grown by CVD on Si sub-
strates. As can be seen, changing the geometrical
properties of the grating allows the possibility to
tune the resonance frequency of the grating. If the
axis ratio decreases, i.e. if the width of the SiC
bars increases for a given layer thickness and a
constant period of the grating, a red shift of the
resonance wavelength can be obtained. The dis-
continuity at axis ratios around 0.63/0.67 in the
function of the resonance wavelength versus the
axis ratio is caused by the fact that different film
thicknesses (2.7, 2.4 and 1.9 µm) have been used
for the measurements and the absolute size of the
structure has an influence on the phonon wavelength. The slight systematic deviation between the
simulated and measured phonon wavelengths is attributed to the incertitude in the dielectric con-
stant for the SiC structures in the calculation, as already outlined. Another ambiguity is the precise
width of each fabricated grating structure.
Summary
SiC gratings were fabricated by processing SiC/Si heterostructures. These gratings show theoreti-
cally predicted surface phonon polariton resonances in the infrared spectra. The resonance frequen-
cies can be tuned by changing the geometrical properties of the SiC grating. The formed gratings
are suitable for wavelength and polarization filters in the infrared spectra.
Acknowledgement
This work was supported by the European Union within the framework of the Future and Emerging
Technologies-SLAM program under grant No. IST-2000-26479. The authors would like to thank
the group of J. Faist at the Institute of Physics from the University of Neuchâtel for providing the
equipment for the infrared measurements. C. Rockstuhl thanks the Canon-Foundation Europe for
supporting his research.
References
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[4] Ch. Förster, V. Cimalla, R. Kosiba, G. Ecke, P. Weih, O. Ambacher and J. Pezoldt: Mater. Sci.
Forum Vol. 457-460 (2004), p. 821.
[5] J. Turunen in H. P. Herzig: Micro-Optics (Taylor & Francis, 1997)
[6] C. Rockstuhl, M. Salt and H.P. Herzig: J. Opt. Soc. Am. Vol. A 20 (2003), p. 1969.
Fig. 5: Measured and simulated wave-
length of the second surface phonon po-
lariton resonance versus the axis ratio of
the SiC bars.
4
