Ultrabroadband transmission measurements on waveguides of silicon-rich silicon dioxide

Article abstract of DOI:10.1063/1.1631065

Waveguides of luminescent (annealed) and nonluminescent (unannealed) SRSO were fabricated using PECVD. Unannealed waveguides exhibited good transmission qualities throughout the visible spectrum, while annealed waveguides were absorbing below 700 nm. The

Full text of DOI:10.1063/1.1631065

Ultrabroadband transmission measurements on waveguides of silicon-rich silicon
dioxide
R. T. Neal, M. D. C. Charlton, G. J. Parker, C. E. Finlayson, M. C. Netti, and J. J. Baumberg
Citation: Applied Physics Letters 83, 4598 (2003); doi: 10.1063/1.1631065
View online: http://dx.doi.org/10.1063/1.1631065
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APPLIED PHYSICS LETTERS
VOLUME 83, NUMBER 22
1 DECEMBER 2003
Ultrabroadband transmission measurements on waveguides of silicon-rich
silicon dioxide
R. T. Neal, M. D. C. Charlton,a) and G. J. Parkera)
Department of Electronics and Computer Science, University of Southampton, Southampton
SO17 1BJ, United Kingdom
C. E. Finlayson,b) M. C. Netti,a) and J. J. Baumberga)
Department of Physics and Astronomy, University of Southampton, Southampton
SO17 1BJ, United Kingdom
͑
Received 17 July 2003; accepted 8 October 2003͒
We report ultrabroadband measurements on waveguides of photoluminescent silicon-rich silicon
dioxide produced by plasma enhanced chemical vapor deposition. Material absorption below 700
nm and waveguide loss above 1300 nm leave a broad spectral region of good transmission
properties, which overlaps with the photoluminescence spectrum of the core material. Proposed
mechanisms for the material absorption and photoluminescence are discussed based on our findings.
©
2003 American Institute of Physics. ͓DOI: 10.1063/1.1631065͔
The use of silicon as the basis of the microelectronics
industry has led to it being the best characterized and under-
stood material in the world. Silicon is so widely used prima-
rily because of silicon dioxide’s excellent properties as an
electrical insulator, although the relatively inexpensive na-
ture of the material must also be considered an incentive.1
While bulk silicon possesses good electrical properties, it is
an indirect band gap material, resulting in very low band
edge luminescence efficiency.2
waveguides of PECVD SRSO for wavelengths from approxi-
mately 600 to 1600 nm.
To form a waveguide, silicon wafers were thermally oxi-
dized to a thickness of over 2 ␮m. A layer of silicon-rich
silicon dioxide 450-nm-thick was deposited upon the ther-
mally grown silicon dioxide by PECVD. The level of silicon
incorporation in the layer was controlled by varying the ratio
of the composite gases, SiH and N O. To complete the
4
2
structure, a cladding layer of 200 nm standard PECVD sili-
con dioxide was deposited on top ͑shown in the inset of Fig.
2͒. To activate the photoluminescent properties of the SRSO,
some of the waveguides were annealed at 1150 °C for dura-
tions ranging from 30 min to 6 h. Table I shows how the
refractive index and layer thickness ͑measured by ellipsom-
etry at 633 nm for separate thin film calibration samples͒ of
the core layer of each waveguide changed with the activation
anneal. The postanneal refractive index was constant for all
durations of anneal.
The motivation for silicon light emitting devices is so
great that many methods have been developed to attempt to
3
overcome this problem. These include doping with carbon,
4
implantation with rare earth elements, dislocation
engineering, and recently, reverse solar cells. Since the dis-
covery by Canham in 1990 of strong photoluminescence
5
6
7
from porus silicon, there has been strong interest in light
emitting nanoscale silicon structures. This was further fu-
8
elled by an article by Pavesi in which optical gain was re-
ported in waveguides of silicon nanocrystals implanted into
silicon dioxide. Much work has continued on silicon nano-
In order to determine the photoluminescent properties of
the samples, a 30 mW beam from an argon ion laser ͑␭ϭ514
nm͒ was focused into a spot of approximately 20 ␮m diam-
eter at the edge of the waveguide and used to photoexcite the
SRSO. This power density was used because the active layer
is so thin at normal incidence, and the absorbing Si nanopar-
ticles fill Ͻ1% of the volume ͓see Fig. 4͑a͔͒, leading to in-
efficient optical pumping in this geometry. The resulting
photoluminescence was collected at the edge of the wave-
guide and collimated using a 90ϫ microscope objective.
This light was then coupled into a spectrometer. Although
there was little difference between the TE and TM polarized
luminescence spectra, all reported measurements were car-
ried out in the TE polarization.
Results for the photoluminescence measurements carried
out are shown in Fig. 1. The spectrometer used was a
nitrogen-cooled silicon charge coupled device ͑CCD͒ detec-
tor, giving a spectral resolution of a few nanometers with an
effective wavelength range of 450 nm–1.1 ␮m. Figure 1͑a͒
demonstrates how the peak intensity of the luminescence
shifts to the longer wavelengths with increasing the silicon
9
10
crystals, with ion implantation, sputtering, and plasma en-
hanced chemical vapor deposition ͑PECVD͒ being the fa-
vored fabrication techniques. Following initial work by Ia-
11
cona et al. on the growth of silicon nanocrystals, we have
now succeeded in fabricating strongly luminescent planar
optical waveguides based on nanocrystaline silicon material.
Any optical circuit to be made from luminescent silicon
nanocrystals will have the waveguide as its fundamental el-
ement. However, there are no broadband measurements
available in the current research literature setting out the
critical transmission properties of waveguides of silicon-rich
silicon dioxide ͑SRSO͒. These properties, essential to the
fabrication of integrated circuits in SRSO, are reported in
this letter. Ultrabroadband transmission measurements have
been carried out which present the transmission properties of
a͒Also at: Mesophotonics Ltd., 2 Venture Rd., Chilworth Science Park,
Southampton SO16 7NP, United Kingdom.
b͒
Electronic mail: cef@phys.soton.ac.uk
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0

Appl. Phys. Lett., Vol. 83, No. 22, 1 December 2003
Neal et al.
4599
TABLE I. Key parameters for the SRSO waveguides used in these experiments, both before and after the
annealing stage. The refractive index and film thickness values were measured by ellipsometry.
Ratio
n
Thickness
Annealing time
n
Thickness
Wafer type
(SiH N O)
͑before͒
͑before, Å͒
͑h͒
͑after͒
͑after, Å͒
4
2
A
B
C
12
9
6
1.605
1.575
1.547
4345
4524
4621
0.5,1,3,6
0.5,1,3,6
0.5,1,3,6
1.685
1.576
1.528
5720
3850
4047
incorporation in the SRSO. We also found clear evidence
that as the anneal time increases, so does the intensity of the
luminescence from the SRSO. This is shown in Fig. 1͑b͒ for
wafer type ‘‘A.’’ These results would appear to be in contrast
with those reported by Brongersma et al.12
Waveguide transmission measurements were carried out
using an ultrabroadband continuum generated from a nonlin-
ear photonic crystal ‘‘holey’’ fiber. The fiber was pumped
using the 150 fs, ␭ϭ1100 nm idler output from a pulsed
optical parametric amplifier derived from a regeneratively
amplified Ti:sapphire femtosecond-pulsed laser. The result-
ing supercontinuum was observed to range from around 600
to 1600 nm in wavelength and was tightly focused into the
core layers of the SRSO waveguides. The transmitted light
was measured using both a peltier-cooled Si CCD ͑500–
nealed waveguides begin to guide is not dependent upon
either the duration of the anneal ͓Fig. 3͑a͔͒ or upon the sili-
con incorporation of the sample ͓Fig. 3͑b͔͒. We attribute the
waveguide losses below 700 nm to optical absorption by
silicon nanocrystals, which have formed during the thermal
anneal. Figure 4͑a͒ shows an absorption model for a silicon
dioxide waveguide13 with a silicon incorporation of 0.04%
plotted with our experimental results. As no silicon could
possibly have been added postdeposition, it is interesting to
note that the silicon only becomes actively absorbing after
the anneal stage. This is consistent with the occurrence of
characteristic silicon nanocrystal photoluminescence only in
those samples which have been annealed. We believe that the
relatively broad material absorption band below 700 nm il-
lustrates the cluster size range, and the invariant edge at 700
nm suggests a band gap energy of 1.77 eV for the largest
optically active silicon nanocrystals. This is in agreement
with work done by Ma et al. in 1999,14 who reported a simi-
lar consistent absorption edge between 1.65 and 1.8 eV, cor-
responding to 690–776 nm.
1100 nm͒ and an InGaAs infrared detector ͑900–1800 nm͒,
in order to cover the full spectral range. A reference beam
was routed around the waveguide to allow the relative trans-
mittance spectra to be accurately derived.
Figure 2 shows the spectra for transmission through a
waveguide of wafer type ‘‘B’’ ͑core thickness 385 nm͒ which
has been annealed for 1 h ͑solid line͒, transmission through
an unannealed waveguide ͑short dashed line͒ and the photo-
luminescence, all for the same waveguide sample. It may be
seen that the unannealed waveguide has much better trans-
mission properties in the visible portion of the spectrum,
although it exhibits no photoluminescence. It is also clear
that the annealed waveguides have improved waveguide
transmission in the IR portion of the spectrum. The photolu-
minescence of the sample has also been appended to Fig. 2
to illustrate how closely the absorption losses at shorter
wavelengths matches the onset of the photoluminescence.
Focusing upon the visible and near IR portion of the
spectrum, we observe that the wavelength at which the an-
We also believe that the improved waveguide transmis-
sion in the IR after annealing is as a result of the raised
refractive index of the core layer. Figure 4͑b͒ shows calcu-
lations of how the effective refractive index of the wave-
guide core layer varies as a function of wavelength.15 As all
samples measured exhibited similar IR losses ͑shown for wa-
fer type B in Fig. 2͒, it is not probable that waveguide cutoff
is the sole loss mechanism. We would expect that absorption
losses in the SiO cladding layers will cause attenuation of
2
the waveguide modes in the core layer. It is noted that the
losses above 1300 nm are very similar to the characteristic
IR absorption losses, which are among the major loss factors
1
6
in SiO optical fibers.
2
FIG. 1. Photoluminescence spectra for waveguides of SRSO showing ͑a͒
the redshift in peak intensity wavelength with increasing silicon incorpora-
tion into the SRSO core layer, and ͑b͒ increasing photoluminescence inten-
FIG. 2. Full ultrabroadband transmission spectra for both annealed and
unannealed SRSO waveguide samples together with the photoluminescence
spectrum of the core material. The inset shows a schematic diagram of the
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sity as a function of anneal duration.
waveguide structure.
On: Mon, 01 Dec 2014 00:26:05

4600
Appl. Phys. Lett., Vol. 83, No. 22, 1 December 2003
Neal et al.
wavelength of peak intensity may be considered a result of
different cluster size distributions as the silicon incorporation
of the SRSO changes, we believe the increase in photolumi-
nescence intensity as a function of anneal time is either due
to the continued formation of radiative interface states or to
the annealing out of nonradiative recombination pathways
associated with defects formed during the amorphous
PECVD process. Detailed luminescence lifetime measure-
ments may in the future allow this issue to be clarified.
In conclusion, waveguides of luminescent ͑annealed͒
and nonluminescent ͑unannealed͒ SRSO were fabricated us-
ing PECVD. Unannealed waveguides exhibited good trans-
mission qualities throughout the visible spectrum, while an-
nealed waveguides were absorbing below 700 nm. The onset
wavelength for this absorption was invariant with changes to
the anneal time and silicon incorporation in the waveguide
core layer. This demonstrates that waveguides of SRSO ex-
hibit favorable optical properties for the portion of the elec-
tromagnetic spectrum in which the core material is photolu-
minescent. This work prepares the way for integrated devices
incorporating luminescent SRSO waveguides as a fundamen-
tal component.
FIG. 3. Visible transmission spectra for waveguides of SRSO around the
onset wavelength of nanocrystal absorption which ͑a͒ is invariant with re-
spect to anneal duration, and ͑b͒ is invariant with respect to silicon incor-
poration in the core layer.
Despite strong absorption below 700 nm and above 1300
nm, waveguides of luminescent SRSO are efficient
waveguides for the near IR portion of the electromagnetic
spectrum. This is especially pertinent for the 700–1000 nm
portion of the spectrum, in which SRSO generates photolu-
minescence. The waveguide will, however, reabsorb the
higher energy portion of the photoluminescence spectrum as
it passes through the material.
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a different mechanism. This is attributed to radiative recom-
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FIG. 4. Models for ͑a͒ transmission of a bulk silicon dioxide waveguide
with a 0.04% incorporation of silicon, plotted with experimental results for
a waveguide of wafer type A and ͑b͒ the refractive index of each waveguide
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