Utilizing a high quality factor (Q~1.5×10^6) optical microresonator to provide sensitivity down to a fractional surface optical loss of alphas[prime]~10^–7, we show that the optical loss within Si microphotonic components can be dramatically altered by Si surface preparation, with alphas[prime]~1×10^–5 measured for chemical oxide surfaces as compared to alphas[prime]<=1×10^–6 for hydrogen-terminated Si surfaces. These results indicate that the optical properties of Si surfaces can be significantly and reversibly altered by standard microelectronic treatments, and that stable, high optical quality surface passivation layers will be critical in future Si micro- and nanophotonic systems

Borselli, Matthew

Johnson, Thomas J.

Painter, Oskar

English

arXiv

Caltech Authors - Main

APPLIED PHYSICS LETTERS 88, 131114 2006Measuring the role of surface chemistry in silicon microphotonics
Matthew Borselli,a Thomas J. Johnson, and Oskar Painter
Department of Applied Physics, California Institute of Technology,b Pasadena, California 91125
Received 13 December 2005; accepted 7 March 2006; published online 31 March 2006
Utilizing a high quality factor Q1.5106 optical microresonator to provide sensitivity down to
a fractional surface optical loss of s10
−7, we show that the optical loss within Si microphotonic
components can be dramatically altered by Si surface preparation, with s110
−5 measured for
chemical oxide surfaces as compared to s110
−6 for hydrogen-terminated Si surfaces. These
results indicate that the optical properties of Si surfaces can be significantly and reversibly altered
by standard microelectronic treatments, and that stable, high optical quality surface passivation
layers will be critical in future Si micro- and nanophotonic systems. © 2006 American Institute of
Physics. DOI: 10.1063/1.2191475Because of the predicted saturation of “Moore’s law”
scaling of transistor cost and density,1 silicon-based micro-
photonics is being explored for the routing and generation of
high-bandwidth signals.2–7 Historically, studies of Si surface
and interface states have primarily focused on their elec-
tronic properties.8,9 Three exceptions to this are deep-level
optical spectroscopy,10 cavity-ringdown spectroscopy,11 and
the ultrasensitive technique of photothermal deflection
spectroscopy12,13 PDS which can measure fractional optical
absorption down to l10−8. None of the aforementioned
techniques, however, is well suited for studying as-processed
microphotonic elements. In this work we utilize a specially
designed microdisk optical resonator to study the optical
properties of surfaces typical in silicon-on-insulator SOI
microphotonic elements in a noninvasive, rapid, and sensi-
tive manner. Shown in Fig. 1, the high quality factor Q Si
microdisk resonators used in this work provide surface-
specific optical sensitivity due to the strong overlap of the
top and bottom surfaces of the active Si layer with the elec-
tric field energy density of appropriately polarized bound
optical modes of the microdisk.
A normalized measure of surface sensitivity for a
guided-wave mode in a waveguide or resonator can be de-
fined as ss / ts, where s is the fractional electric field
energy overlap with a surface perturbation of physical depth
ts. If optical loss is dominated by interactions with the sur-
face, then the modal loss coefficient per unit length m
measured from experiment can be related to a fractional
loss per pass through the surface given by s=m /s
=2ng / 0Q, for a resonator with quality factor Q and
modal group index of refraction ng. As discussed in Ref. 14,
for a true two-dimensional surface in which the perturbation
depth is infinitesimal, s is the most relevant quantity de-
scribing the surface and is equivalent to the fraction of power
lost for a normal incident plane wave propagating across the
surface. From finite-element method FEM simulations,15
shown in Fig. 1, the transverse magnetic TM polarization
whispering gallery modes WGMs of the microdisk are
90 more sensitive to the top and bottom 100 Si surfaces
than the etched sidewall at the microdisk periphery; specifi-
cally, top =top =3.510
−3 nm−1 and side =8.110
−5 nm−1.
This implies that 0.2% of the optical mode exists in a
aElectronic mail: borselli@caltech.edu
bURL: http://copilot.caltech.edu
0003-6951/2006/8813/131114/3/$23.00 88, 13111
Downloaded 14 Apr 2006 to 131.215.225.174. Redistribution subject tosingle monolayer at the top bottom Si surface, while little
of the mode sees imperfections at the microdisk perimeter.
For the measured devices described below Q1.5106,
a surface absorption of one-tenth of the full linewidth
was measurable, corresponding to a sensitivity limit of
s10
−7.
The silicon microdisks in this work were fabricated from
a SOI wafer from SOITEC, consisting of a 217 nm thick
silicon device layer p-type, 14–22  cm resistivity, 100
orientation with a 2 m SiO2 buried oxide BOX layer.
Microdisks of 5 m radius were fabricated,16 finishing with
a 10 min acetone soak and piranha etch to remove organic
materials. A 1 h dilute hydrofluoric acid HF solution com-
prised of five parts 18.3 M de-ionized DI water to one
part concentrated aqueous HF 49% was used to remove a
protective SiNx cap and partially undercut the disk, as shown
in the scanning electron microscope SEM micrograph in
Fig. 1a. The wafer was then rinsed in de-ionized water,
dried with nitrogen N2, and immediately transferred into a
N2 purged testing enclosure.
The microdisk resonators were characterized using a
swept-wavelength external-cavity laser New Focus Velocity,
=1420–1498 nm, linewidth	300 kHz connected to a fi-
ber taper waveguide probe.17 A fiber-based Mach-Zehnder
interferometer was used to calibrate the high resolution, pi-
ezocontrolled wavelength scans to ±0.01 pm linewidth accu-
racy. The micron-scale fiber taper probe was formed from a
standard single-mode optical fiber and was used to evanes-
cently excite the WGMs of the microdisk with controllable
FIG. 1. Color online a SEM micrograph of a 5 m radius SOI micro-
disk. b Zoomed-in representation of disk edge white dashed box showing
a surface-sensitive TM polarized whispering gallery mode solved via FEM.
© 2006 American Institute of Physics4-1
 AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
131114-2 Borselli, Johnson, and Painter Appl. Phys. Lett. 88, 131114 2006loading. Figure 2a shows the normalized spectral transmis-
sion response of a 5 m radius microdisk resonator, illustrat-
ing clear families of modes having similar linewidth, 
, and
free spectral range FSR. By comparison to FEM simula-
tions of the Si microdisk, each mode in Fig. 2a was catego-
rized and labeled as TMp,m, where p and m are the radial and
azimuthal numbers, respectively.
Owing to their large surface sensitivity see Fig. 1, the
spectral signature of the TM1,m modes was used to determine
the quality of the optical surfaces. Figure 2b shows a high
resolution scan across the TM1,31 mode. The observed double
resonance dip, termed a doublet, is a result of surface rough-
ness coupling of the normally degenerate clockwise CW
and counter-clockwise CCW propagating WGMs.16,18,19
The rate at which photons are backscattered is quantified by
the doublet splitting, , while the rate at which photons are
lost from the resonator is quantified by the intrinsic line-
width, 
, of the individual doublet modes. From a fit to the
transmission spectrum of Fig. 2b, =11.9 pm and

=2.2 pm, corresponding to an intrinsic modal quality fac-
tor of Qi0 /
=6.8105 for this TM1,31 mode. This
should be contrasted with the electric field energy density
plot and transmission spectrum shown in Fig. 2c for a more
confined, and less surface sensitive, TE WGM of a much
larger 40 m radius microdisk top =bot =1.210
−3 nm−1
and side =2.310
−5 nm−1. From the fit parameters
=0.8 pm, 
=0.3 pm, the Q of the buried TE mode is
Qi=4.710
6, corresponding to a loss per unit length of
i=0.13 dB/cm. This is nearly an order of magnitude
smaller optical loss than that of the as-processed TM1,m
modes, and provides an upper bound on the bulk Si optical
loss of the SOI material.
The stark difference between the surface-sensitive TM
and bulk TE modes indicates that the as-processed Si sur-
faces are far from optimal. Etch-induced surface damage on
the microdisk sidewall can only account for a small fraction
of this difference due to the enhanced sensitivity of the
TM1,m to the top and bottom Si surfaces comparison of the
TM and TE modes in the same microdisk and with similar
modal overlap with the microdisk edge bear this out. Dam-
age to the top and bottom Si surfaces can stem from a variety
of possible sources including chemical mechanical polishing,
native oxide formation during storage, or adventitious or-
ganic matter.20 In an attempt to repair the Si surfaces a series
of chemical oxidation treatments were performed on the de-
vices. The well-known process9,21,22 of repeated chemical
FIG. 2. Color online Normalized spectral transmission response of Si m
microdisk with the fiber taper placed 0.6±0.1 m away from the disk edge
the fiber taper moved 3 m laterally away from the disk edge. b High re
CW/CCW mode splitting and individual mode linewidth, respectively. c El
showing the reduced loss of a bulk TE WGM.oxidation in piranha H2SO4/H2O2 and HF oxide stripping
Downloaded 14 Apr 2006 to 131.215.225.174. Redistribution subject towas employed to controllably prepare the Si surfaces. Three
cycles of the piranha/HF process, recipe shown in Table I,
were applied to the as-processed devices. From the blueshift
in the WGM resonances due to the three cycles of the
piranha/HF process, an estimated 1.9±0.1 nm of Si was re-
moved from the surface of the microdisk. The fit to the
TM1,31 transmission spectrum, shown in Fig. 3b, indicates
that a significant improvement to the surfaces has also taken
place, yielding a =7.2 pm and 
=1.1 pm.
To separate the effects of the piranha oxidation and the
HF etch, the sample was put through a piranha/HF/piranha
treatment. The first cycle of piranha/HF was used to “re-
fresh” the hydrogen passivation before reoxidizing the Si
surface with piranha. Figure 3c shows the fit to the now
barely resolvable TM1,31 doublet yielding =8.6 pm and

=5.6 pm. The fivefold increase in linewidth and a negli-
gible increase in doublet splitting is indicative of a signifi-
cant activation of absorbing surface states without an in-
crease in surface scattering. Removing the chemical oxide
with the HF dip listed in Table I and retesting indicated that
an oxide film equivalent to 2.8±0.1 nm of SiO2 had been
present. The fit to the transmission spectrum of the TM1,31
mode in Fig. 3d yielded fit parameters =9.7 pm and

=1.2 pm, showing that the optical damage to the Si sur-
faces caused by piranha oxidation was reversible.
As a final treatment to the 5 m radii microdisks, we
used the same 3 oxidation and stripping process as de-
scribed in Table I, but with a HCl based chemistry 8:1:2
H2O:HCl:H2O2, heated to 60 °C instead of the H2SO4
based chemistry. Figure 3e shows a graphical representa-
tion of the average behavior of all TM1,m modes in the
1420–1470 nm span after each chemical treatment. The re-
sults reveal that the HCl oxidation was slightly less effective
at passivating the silicon surface than the piranha oxidation;
however, it is expected that the optimum solution for chemi-
cal oxidation will depend upon the Si crystal orientation and
previous chemical treatments.23,24
TABLE I. Summary of piranha oxidation surface treatment.
Step Compositiona Temp. Time
Piranha 3:1 H2SO4/H2O2 100 °C 10 min
3 rinse DI H2O 23 °C 30 s
HF dip 10:1 H2O/HF 23 °C 1 min
2 rinse DI H2O 23 °C 15 s
a
sk resonators. a Broad scan across =1400 nm band for a 5 m radius
ptimized for TM coupling. The spectrum was normalized to the response of
on scan of the TM1,31 mode at =1459 nm in a.  and 
 indicate the
energy density plot and high resolution scan of a 40 m radius microdisk,icrodi
and o
soluti
ectricStandard concentration aqueous solutions.
 AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
131114-3 Borselli, Johnson, and Painter Appl. Phys. Lett. 88, 131114 2006Although it has recently been observed22 that repeated
chemical oxidation and removal of silicon can provide a
smoothing action on etched sidewalls, the large shifts in op-
tical loss with chemical treatment described above can be
linked to surface-state absorption as opposed to surface scat-
tering. Whereas the highly confined Si waveguide measure-
ments to date have been sensitive to changes in loss as low
as 1 dB/cm, the microdisks of this work are sensitive to
changes of loss more than an order of magnitude smaller
	0.03 dB/cm where surface chemistry is more likely to
play a role. Indeed, as mentioned above the TM-polarized
microdisk WGMs are selectively sensitive to the top and
bottom Si surfaces which are extremely smooth in compari-
son with etched surfaces. The negligible change in average
mode splitting, , with chemical treatment Fig. 3e is
also indicative of little change in surface roughness. A
complementary analysis25 of power dependent transmission
scans showed that 50% of residual optical loss, after
piranha/HF treatment and hydrogen surface passivation, is
still due to surface-state absorption bulk absorption is neg-
ligible at this level.
By comparing the cavity Q before and after the piranha
oxide removal, a fractional surface absorption loss per pass
of s,ox 110
−5 is estimated for the piranha oxide. This
large fractional absorption in the =1400 nm wavelength
band 0.85 eV is attributed to single-photon absorp-
tion by midgap interface states. Such electronic interface
states at the Si/ piranhaSiOx interface have been observed
in Ref. 26, with three sets of state-density maxima in the
band gap of silicon occurring at 0.3, 0.5, and 0.7 eV refer-
enced to the valence-band maximum, with a Fermi energy of
FIG. 3. Color online a–d Wavelength scan of the TM1,31 doublet mode
after each chemical treatment and accompanying schematic of chemical
treatment. a As-processed, b triple piranha/HF cycle Table I, c single
piranha/HF/piranha step allowing measurement of piranha oxide, and d HF
dip to remove chemical oxide from previous treatment and restore passiva-
tion. e Average intrinsic linewidth, 
, and average doublet splitting, ,
for all TM1,m modes within the 1420–1470 nm spectrum after each chemi-
cal treatment step.Downloaded 14 Apr 2006 to 131.215.225.174. Redistribution subject to0.4 eV. Thus, our observed surface absorption is most
likely dominated by the transition from the filled 0.3 eV
surface-state band to the conduction band at 1.1 eV. In com-
parison, the modal absorption loss of the hydrogen-
passivated Si surface was measured25 to be as small as m
H
0.08 cm−1, corresponding to a fractional surface absorp-
tion loss per pass of s,H 110
−6 for the top bottom Si
active layer surface.
All of the measurements described above were per-
formed in a N2 purged environment over several weeks.
Even in such an environment, however, changes in the hy-
drogen passivated surfaces were observed over times as short
as a few days. Left in an unprotected air environment, deg-
radation of the optical surface quality was evident in a matter
of hours. Research and development of stable surface passi-
vation techniques optimized for optical quality, akin to the
gate oxides of CMOS microelectronics, will be a key ingre-
dient in the future development of Si photonics. Our data
suggest that surface chemistry as much as surface roughness
will ultimately limit the performance of Si microphotonic
devices, and further development of Si passivation tech-
niques should be able to reduce optical losses by as much as
an order of magnitude towards the bulk c-Si limit while
improving the stability and manufacturability of future Si
photonic components.
This work was supported by DARPA through the EPIC
program. One of the authors M.B. thanks the Moore Foun-
dation, NPSC, and HRL Laboratories.
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Appl. Phys. 79, 7051 1996. AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp


Measuring the role of surface chemistry in silicon microphotonics

APPLIED PHYSICS LETTERS 88, 131114 2006Measuring the role of surface chemistry in silicon microphotonics
Matthew Borselli,a Thomas J. Johnson, and Oskar Painter
Department of Applied Physics, California Institute of Technology,b Pasadena, California 91125
Received 13 December 2005; accepted 7 March 2006; published online 31 March 2006
Utilizing a high quality factor Q1.5106 optical microresonator to provide sensitivity down to
a fractional surface optical loss of s10−7, we show that the optical loss within Si microphotonic
components can be dramatically altered by Si surface preparation, with s110−5 measured for
chemical oxide surfaces as compared to s110−6 for hydrogen-terminated Si surfaces. These
results indicate that the optical properties of Si surfaces can be significantly and reversibly altered
by standard microelectronic treatments, and that stable, high optical quality surface passivation
layers will be critical in future Si micro- and nanophotonic systems. © 2006 American Institute of
Physics. DOI: 10.1063/1.2191475Because of the predicted saturation of “Moore’s law”
scaling of transistor cost and density,1 silicon-based micro-
photonics is being explored for the routing and generation of
high-bandwidth signals.2–7 Historically, studies of Si surface
and interface states have primarily focused on their elec-
tronic properties.8,9 Three exceptions to this are deep-level
optical spectroscopy,10 cavity-ringdown spectroscopy,11 and
the ultrasensitive technique of photothermal deflection
spectroscopy12,13 PDS which can measure fractional optical
absorption down to l10−8. None of the aforementioned
techniques, however, is well suited for studying as-processed
microphotonic elements. In this work we utilize a specially
designed microdisk optical resonator to study the optical
properties of surfaces typical in silicon-on-insulator SOI
microphotonic elements in a noninvasive, rapid, and sensi-
tive manner. Shown in Fig. 1, the high quality factor Q Si
microdisk resonators used in this work provide surface-
specific optical sensitivity due to the strong overlap of the
top and bottom surfaces of the active Si layer with the elec-
tric field energy density of appropriately polarized bound
optical modes of the microdisk.
A normalized measure of surface sensitivity for a
guided-wave mode in a waveguide or resonator can be de-
fined as ss / ts, where s is the fractional electric field
energy overlap with a surface perturbation of physical depth
ts. If optical loss is dominated by interactions with the sur-
face, then the modal loss coefficient per unit length m
measured from experiment can be related to a fractional
loss per pass through the surface given by s=m /s
=2ng / 0Q, for a resonator with quality factor Q and
modal group index of refraction ng. As discussed in Ref. 14,
for a true two-dimensional surface in which the perturbation
depth is infinitesimal, s is the most relevant quantity de-
scribing the surface and is equivalent to the fraction of power
lost for a normal incident plane wave propagating across the
surface. From finite-element method FEM simulations,15
shown in Fig. 1, the transverse magnetic TM polarization
whispering gallery modes WGMs of the microdisk are
90 more sensitive to the top and bottom 100 Si surfaces
than the etched sidewall at the microdisk periphery; specifi-
cally, top =top =3.510−3 nm−1 and side =8.110−5 nm−1.
This implies that 0.2% of the optical mode exists in a
aElectronic mail: borselli@caltech.edu
bURL: http://copilot.caltech.edu
0003-6951/2006/8813/131114/3/$23.00 88, 13111
Downloaded 14 Apr 2006 to 131.215.225.174. Redistribution subject tosingle monolayer at the top bottom Si surface, while little
of the mode sees imperfections at the microdisk perimeter.
For the measured devices described below Q1.5106,
a surface absorption of one-tenth of the full linewidth
was measurable, corresponding to a sensitivity limit of
s10−7.
The silicon microdisks in this work were fabricated from
a SOI wafer from SOITEC, consisting of a 217 nm thick
silicon device layer p-type, 14–22  cm resistivity, 100
orientation with a 2 m SiO2 buried oxide BOX layer.
Microdisks of 5 m radius were fabricated,16 finishing with
a 10 min acetone soak and piranha etch to remove organic
materials. A 1 h dilute hydrofluoric acid HF solution com-
prised of five parts 18.3 M de-ionized DI water to one
part concentrated aqueous HF 49% was used to remove a
protective SiNx cap and partially undercut the disk, as shown
in the scanning electron microscope SEM micrograph in
Fig. 1a. The wafer was then rinsed in de-ionized water,
dried with nitrogen N2, and immediately transferred into a
N2 purged testing enclosure.
The microdisk resonators were characterized using a
swept-wavelength external-cavity laser New Focus Velocity,
=1420–1498 nm, linewidth	300 kHz connected to a fi-
ber taper waveguide probe.17 A fiber-based Mach-Zehnder
interferometer was used to calibrate the high resolution, pi-
ezocontrolled wavelength scans to ±0.01 pm linewidth accu-
racy. The micron-scale fiber taper probe was formed from a
standard single-mode optical fiber and was used to evanes-
cently excite the WGMs of the microdisk with controllable
FIG. 1. Color online a SEM micrograph of a 5 m radius SOI micro-
disk. b Zoomed-in representation of disk edge white dashed box showing
a surface-sensitive TM polarized whispering gallery mode solved via FEM.
© 2006 American Institute of Physics4-1
 AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
131114-2 Borselli, Johnson, and Painter Appl. Phys. Lett. 88, 131114 2006loading. Figure 2a shows the normalized spectral transmis-
sion response of a 5 m radius microdisk resonator, illustrat-
ing clear families of modes having similar linewidth, 
, and
free spectral range FSR. By comparison to FEM simula-
tions of the Si microdisk, each mode in Fig. 2a was catego-
rized and labeled as TMp,m, where p and m are the radial and
azimuthal numbers, respectively.
Owing to their large surface sensitivity see Fig. 1, the
spectral signature of the TM1,m modes was used to determine
the quality of the optical surfaces. Figure 2b shows a high
resolution scan across the TM1,31 mode. The observed double
resonance dip, termed a doublet, is a result of surface rough-
ness coupling of the normally degenerate clockwise CW
and counter-clockwise CCW propagating WGMs.16,18,19
The rate at which photons are backscattered is quantified by
the doublet splitting, , while the rate at which photons are
lost from the resonator is quantified by the intrinsic line-
width, 
, of the individual doublet modes. From a fit to the
transmission spectrum of Fig. 2b, =11.9 pm and

=2.2 pm, corresponding to an intrinsic modal quality fac-
tor of Qi0 /
=6.8105 for this TM1,31 mode. This
should be contrasted with the electric field energy density
plot and transmission spectrum shown in Fig. 2c for a more
confined, and less surface sensitive, TE WGM of a much
larger 40 m radius microdisk top =bot =1.210−3 nm−1
and side =2.310−5 nm−1. From the fit parameters
=0.8 pm, 
=0.3 pm, the Q of the buried TE mode is
Qi=4.7106, corresponding to a loss per unit length of
i=0.13 dB/cm. This is nearly an order of magnitude
smaller optical loss than that of the as-processed TM1,m
modes, and provides an upper bound on the bulk Si optical
loss of the SOI material.
The stark difference between the surface-sensitive TM
and bulk TE modes indicates that the as-processed Si sur-
faces are far from optimal. Etch-induced surface damage on
the microdisk sidewall can only account for a small fraction
of this difference due to the enhanced sensitivity of the
TM1,m to the top and bottom Si surfaces comparison of the
TM and TE modes in the same microdisk and with similar
modal overlap with the microdisk edge bear this out. Dam-
age to the top and bottom Si surfaces can stem from a variety
of possible sources including chemical mechanical polishing,
native oxide formation during storage, or adventitious or-
ganic matter.20 In an attempt to repair the Si surfaces a series
of chemical oxidation treatments were performed on the de-
vices. The well-known process9,21,22 of repeated chemical
FIG. 2. Color online Normalized spectral transmission response of Si m
microdisk with the fiber taper placed 0.6±0.1 m away from the disk edge
the fiber taper moved 3 m laterally away from the disk edge. b High re
CW/CCW mode splitting and individual mode linewidth, respectively. c El
showing the reduced loss of a bulk TE WGM.oxidation in piranha H2SO4/H2O2 and HF oxide stripping
Downloaded 14 Apr 2006 to 131.215.225.174. Redistribution subject towas employed to controllably prepare the Si surfaces. Three
cycles of the piranha/HF process, recipe shown in Table I,
were applied to the as-processed devices. From the blueshift
in the WGM resonances due to the three cycles of the
piranha/HF process, an estimated 1.9±0.1 nm of Si was re-
moved from the surface of the microdisk. The fit to the
TM1,31 transmission spectrum, shown in Fig. 3b, indicates
that a significant improvement to the surfaces has also taken
place, yielding a =7.2 pm and 
=1.1 pm.
To separate the effects of the piranha oxidation and the
HF etch, the sample was put through a piranha/HF/piranha
treatment. The first cycle of piranha/HF was used to “re-
fresh” the hydrogen passivation before reoxidizing the Si
surface with piranha. Figure 3c shows the fit to the now
barely resolvable TM1,31 doublet yielding =8.6 pm and

=5.6 pm. The fivefold increase in linewidth and a negli-
gible increase in doublet splitting is indicative of a signifi-
cant activation of absorbing surface states without an in-
crease in surface scattering. Removing the chemical oxide
with the HF dip listed in Table I and retesting indicated that
an oxide film equivalent to 2.8±0.1 nm of SiO2 had been
present. The fit to the transmission spectrum of the TM1,31
mode in Fig. 3d yielded fit parameters =9.7 pm and

=1.2 pm, showing that the optical damage to the Si sur-
faces caused by piranha oxidation was reversible.
As a final treatment to the 5 m radii microdisks, we
used the same 3 oxidation and stripping process as de-
scribed in Table I, but with a HCl based chemistry 8:1:2
H2O:HCl:H2O2, heated to 60 °C instead of the H2SO4
based chemistry. Figure 3e shows a graphical representa-
tion of the average behavior of all TM1,m modes in the
1420–1470 nm span after each chemical treatment. The re-
sults reveal that the HCl oxidation was slightly less effective
at passivating the silicon surface than the piranha oxidation;
however, it is expected that the optimum solution for chemi-
cal oxidation will depend upon the Si crystal orientation and
previous chemical treatments.23,24
TABLE I. Summary of piranha oxidation surface treatment.
Step Compositiona Temp. Time
Piranha 3:1 H2SO4/H2O2 100 °C 10 min
3 rinse DI H2O 23 °C 30 s
HF dip 10:1 H2O/HF 23 °C 1 min
2 rinse DI H2O 23 °C 15 s
a
sk resonators. a Broad scan across =1400 nm band for a 5 m radius
ptimized for TM coupling. The spectrum was normalized to the response of
on scan of the TM1,31 mode at =1459 nm in a.  and 
 indicate the
energy density plot and high resolution scan of a 40 m radius microdisk,icrodi
and o
soluti
ectricStandard concentration aqueous solutions.
 AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
131114-3 Borselli, Johnson, and Painter Appl. Phys. Lett. 88, 131114 2006Although it has recently been observed22 that repeated
chemical oxidation and removal of silicon can provide a
smoothing action on etched sidewalls, the large shifts in op-
tical loss with chemical treatment described above can be
linked to surface-state absorption as opposed to surface scat-
tering. Whereas the highly confined Si waveguide measure-
ments to date have been sensitive to changes in loss as low
as 1 dB/cm, the microdisks of this work are sensitive to
changes of loss more than an order of magnitude smaller
	0.03 dB/cm where surface chemistry is more likely to
play a role. Indeed, as mentioned above the TM-polarized
microdisk WGMs are selectively sensitive to the top and
bottom Si surfaces which are extremely smooth in compari-
son with etched surfaces. The negligible change in average
mode splitting, , with chemical treatment Fig. 3e is
also indicative of little change in surface roughness. A
complementary analysis25 of power dependent transmission
scans showed that 50% of residual optical loss, after
piranha/HF treatment and hydrogen surface passivation, is
still due to surface-state absorption bulk absorption is neg-
ligible at this level.
By comparing the cavity Q before and after the piranha
oxide removal, a fractional surface absorption loss per pass
of s,ox 110−5 is estimated for the piranha oxide. This
large fractional absorption in the =1400 nm wavelength
band 0.85 eV is attributed to single-photon absorp-
tion by midgap interface states. Such electronic interface
states at the Si/ piranhaSiOx interface have been observed
in Ref. 26, with three sets of state-density maxima in the
band gap of silicon occurring at 0.3, 0.5, and 0.7 eV refer-
enced to the valence-band maximum, with a Fermi energy of
FIG. 3. Color online a–d Wavelength scan of the TM1,31 doublet mode
after each chemical treatment and accompanying schematic of chemical
treatment. a As-processed, b triple piranha/HF cycle Table I, c single
piranha/HF/piranha step allowing measurement of piranha oxide, and d HF
dip to remove chemical oxide from previous treatment and restore passiva-
tion. e Average intrinsic linewidth, 
, and average doublet splitting, ,
for all TM1,m modes within the 1420–1470 nm spectrum after each chemi-
cal treatment step.Downloaded 14 Apr 2006 to 131.215.225.174. Redistribution subject to0.4 eV. Thus, our observed surface absorption is most
likely dominated by the transition from the filled 0.3 eV
surface-state band to the conduction band at 1.1 eV. In com-
parison, the modal absorption loss of the hydrogen-
passivated Si surface was measured25 to be as small as m
H
0.08 cm−1, corresponding to a fractional surface absorp-
tion loss per pass of s,H 110−6 for the top bottom Si
active layer surface.
All of the measurements described above were per-
formed in a N2 purged environment over several weeks.
Even in such an environment, however, changes in the hy-
drogen passivated surfaces were observed over times as short
as a few days. Left in an unprotected air environment, deg-
radation of the optical surface quality was evident in a matter
of hours. Research and development of stable surface passi-
vation techniques optimized for optical quality, akin to the
gate oxides of CMOS microelectronics, will be a key ingre-
dient in the future development of Si photonics. Our data
suggest that surface chemistry as much as surface roughness
will ultimately limit the performance of Si microphotonic
devices, and further development of Si passivation tech-
niques should be able to reduce optical losses by as much as
an order of magnitude towards the bulk c-Si limit while
improving the stability and manufacturability of future Si
photonic components.
This work was supported by DARPA through the EPIC
program. One of the authors M.B. thanks the Moore Foun-
dation, NPSC, and HRL Laboratories.
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Measuring the role of surface chemistry in silicon microphotonics

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