APPLIED PHYSICS LETTERSVOLUME 85, NUMBER 16

18 OCTOBER 2004

Photodegradation of porous silicon induced by photogenerated
singlet oxygen molecules
D. Kovalev,a) E. Gross, and J. Diener
Technische Universität München, Physik-Department E16, D-85747 Garching, Germany

V. Yu. Timoshenko
Faculty of Physics, Moscow State M. V. Lomonosov University, 119992 Moscow, Russia

M. Fujii
Department of Electrical and Electronics Engineering, Faculty of Engineering, Kobe University,Rokkodai, Nada, Kobe 657-8501, Japan(Received 16 February 2004; accepted 5 August 2004)
We report on the mechanism of photodegradation of porous silicon luminescence in ambient
containing molecular oxygen. Energy transfer from excitons confined in silicon nanocrystallites tomolecular oxygen results in the generation of highly chemically reactive singlet oxygen molecules.
The subsequent interaction of singlet oxygen molecules with the hydrogenated surface of silicon
nanocrystals results in its photooxidation and the creation of additional nonradiative defects, i.e., theluminescence fatigue effect. © 2004 American Institute of Physics. [DOI: 10.1063/1.1804241]
Excitons confined in silicon nanocrystals have a very
long radiative lifetime due to their indirect nature and competition between radiative and various nonradiative processes results in a relatively low quantum yield of silicon
nanocrystal assemblies. After the discovery of the photoluminescence (PL) from porous silicon1 (Si) it has been recognized that the ultimate limitation is an issue of stability. Almost all groups involved in the study of as-preparedhydrogen-passivated porous Si layers at normal ambient conditions have encountered a significant reduction in the quantum yield: depending on the optical excitation level and exposure time, the initial quantum yield can be reduced byorders of magnitude.2 Tischler et al.3 found that the photodegradation is present only in oxygen ambient and is a resultof photooxidation of the porous Si surface, which introduces
an additional nonradiative recombination channel: Si dangling bonds. In contrast, light emission from naturally oxidized porous Si layers or Si nanocrystals embedded in a wideband-gap matrix4,5 is more efficient and was found to bequite stable.2 Recently, several alternative approaches for thephotostabilization of porous Si via its surface modification
have been suggested.6,7 However, no distinct microscopic
mechanism of the porous Si photodegradation and photooxidation has yet been proposed.
The electronic energy transfer from excitons confined in
Si nanocrystals to adsorbed oxygen molecules was found tobe very efficient at cryogenic temperatures.8 Spectroscopic
experiments clarified the details of the energy transfer
mechanism. The energy transfer is mediated by a resonant
direct electron exchange mechanism and results in the efficient generation of a chemically reactive species: singlet
oxygen molecules.9 It this Letter we demonstrate that PL
photodegradation of H-terminated porous Si layers in oxygenambients at room temperature is dependent on their PL quantum yield and is directly governed by the interaction of photosensitized singlet oxygen molecules with the surface of Si
nanocrystals. For low quantum yield H-terminated and higha)

Electronic mail: dkovalev@ph.tum.dequantum yield O-terminated surfaces, this effect is absent
due to a strongly reduced single oxygen generation rate.
Thus, the photodegradation mechanism is very similar to thatof light-emitting polymers10 or dye molecules.
Experiments were carried out using various porous Si
layers having a different PL quantum yield. The porous Si
layers were prepared by standard electrochemical etching1 of(100)-oriented, boron-doped bulk Si substrates with a typicalresistivity of 5 ⍀ cm. The etching current density was
30 mA/ cm2. To obtain samples with a high quantum yield,
porous Si layers remained in the etching solution for one
additional hour.1 Thermal oxidation of Si nanocrystal surfaces was performed at 200 °C in oxygen ambient for 10 h.
PL was excited by the 488-nm line of an Ar+ laser and detected by a single monochromator equipped with a Si chargecoupled device. The samples were mounted in a He flow
cryostat with temperatures between 5 and 300 K. The chemical composition of Si nanocrystal surfaces was analyzed by aBohmem DA-3 FTIR spectrometer. All optical measurements were performed at room temperature.
Figure 1 shows the PL spectra of H-terminated samples
having a high (sample A) and a low (sample B) quantum
yield measured in vacuum (open circles). The featurelessemission bands located in the near-infrared and visible region reflect the wide band-gap distribution of the Si nanocrystal assembly. The injection of molecular oxygen results
in a partial PL quenching which depends on the oxygen ambient pressure and is stronger for samples having higher
quantum yield. The prompt removal of oxygen molecules
leads to a complete recovering of the initial emission properties of porous Si. The strength of PL quenching (defined asthe PL intensity measured in vacuum divided by the PL intensity measured in oxygen ambient) at 1 bar for sample A isshown in the inset of Fig. 1 (open circles). The spectral dependence of the PL quenching strength does not depend on
the PL band position and is most efficient at ⬃1.63 eV,
which coincides with the excitation energy of the 3⌺ → 1⌺
transition in oxygen molecules.11 Thus, excitons having energies in the vicinity of the 3⌺ → 1⌺ transition contributemost efficiently to the singlet oxygen generation. The inte-0003-6951/2004/85(16)/3590/3/$22.00
3590© 2004 American Institute of Physics
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Appl. Phys. Lett., Vol. 85, No. 16, 18 October 2004

Kovalev et al.

3591

FIG. 1. Open circles: PL spectra of porous Si samples having a high (A) anda low (B) quantum yield in vacuum 共10−5mbar兲. Solid circles: PL spectra ofporous Si samples having high (A) and low (B) quantum yield in vacuum
after photodegradation in oxygen ambient at 1 bar. T = 300 K. Eex.
= 2.54 eV, Iex. = 100 mW/ cm2, irradiation time is 1 h. Note the logarithmic
scale of the plot. Inset: Spectral dependence of the strength of PL quenchingfor sample A induced by oxygen ambient (open circles) and that due tophotodegradation (divided by factor of 10, solid circles).

FIG. 2. IR absorbance of porous Si. Solid curve: H-terminated layer. Dottedcurve: Sample B after illumination in oxygen ambient at 1 bar. T = 300 K.
Iex. = 100 mW/ cm2. Eex. = 2.54 eV; irradiation time is 1 h. Dashed curve:Sample A after irradiation at the same conditions. Arrows label surface
vibration bonds. Inset: Integral absorbance due to O–Si–O bonds (solidcircles) and due to 共Si兲Si-Hx and 共O-兲Si-Hx stretching bond (between 2050
and 2300 cm−1, open circles) vibrations.

gral level of PL quenching allows us to estimate the generation rate of singlet oxygen in porous Si layers. For sample A
for 1 bar oxygen pressure, room temperature, and 1 W / cm2
PL excitation intensity it is ⬃1019 1O2 / cm3 sec. Because thesinglet oxygen generation rate scales with the PL quantum
yield, it is 3 orders of magnitude smaller for sample B. Efficiently emitting oxidized PSi layers are not able to generate
singlet oxygen8 and their PL quantum yield is not affected by
the oxygen ambient.
A large difference in the singlet oxygen generation rate
results in a distinct photodegradation behavior, depending on
the PL quantum yield of the porous Si layers and their surface termination. While all H-terminated samples illuminatedin vacuum possess completely stable PL, in oxygen ambient
the PL intensity of efficiently luminescing samples exhibits
fast photodegradation for large Si nanocrystals emitting at
the low-energy part of the PL spectrum (Fig. 1, sample A,solid circles). Again, we define the strength of the persistent
PL quenching as the ratio between initial PL intensity andthat after photodegradation, both being measured in vacuum(inset of Fig. 1, solid circles). The behavior of porous Si
layers having low quantum yield is completely different.
Their PL intensity above 1.5 eV slightly rises with illumination time and only at low emission energies a small PL photodegradation is detected (Fig. 1, sample B, solid circles). We
note that the quantum yield of oxidized samples is constantover time.
These observations indicate that the PL degradation efficiency directly correlates with the singlet oxygen generation rate and scales with the surface area of nanocrystals.
Singlet oxygen molecules are known to have high chemical
reactivity and can oxidize the surface of silicon nanocrystals.
To confirm this conjecture we performed photooxidation experiments with samples having a different quantum yieldand, therefore, a different singlet oxygen generation rate.
Figure 2 shows the IR absorption spectra of porous Si layers
in the spectral range of the Si-Hx共x = 1 , 2 , 3兲 and the O-Si-O
vibration bands.12 While as-prepared layers exhibit only
Si-Hx surface bonds, the illumination in the oxygen ambient
results in the oxidation of the nanocrystal surfaces. The oxidation is always more efficient for samples with high quantum yield. A complete rearrangement of the Si-Hx stretching

bond configuration shows that oxygen atoms backbonded tosurface silicon atoms are introduced during illumination(Fig. 2, dashed line). The additional manifestation of oxidation can be seen as an efficient suppression of the S-Hx wagging mode at ⬃650 cm−1 as well as the Si-H2 scissor modeat ⬃912 cm−1.12 This effect is almost absent for the samples
with a small quantum yield and the only noticable signatureof oxidation appears due to strongly absorbing O-Si-O
stretching bonds (Fig. 2, dotted line).
The time evolution of the Si-O bond concentration during illumination in oxygen ambient at 1 bar is shown in theinset of Fig. 2 (closed circles). On the initial stage of oxidation the decrease in the number of sites available for oxidation can be neglected. Instead, the sublinear rate of oxidationis governed by a reduced singlet oxygen generation rate dueto photodegradation: again there is a clear anticorrelation between the oxidation rate and the PL quantum yield. The integral absorbance governed by the surface 共O-兲Si-Hx and共Si-兲Si-Hx bond vibrations12 does not change during photooxidation and evidences a constant number of surface H atoms (inset of Fig. 2, open circles).
In order to get oxygen backbonded a Si–Si bond should
first be broken. The excitation energies of the 1⌺ and 1⌬
singlet oxygen states being 1.63 and 0.98 eV, respectively,11
are smaller than the Si-Si bond enthalpy (1.8 eV).13 Therefore, the energy stored in a singlet oxygen molecule is notenough to break Si–Si bonds and to incorporate backbonded
oxygen. Thus, the process of photooxidation similar to thermal oxidation is expected to be strongly temperature dependent. Figure 3 shows the IR absorption spectra of porous Silayers photooxidized at the same experimental conditions but
at different temperatures. The rate of photooxidation varies
significantly with temperature, indicating the presence of anadditional energy barrier for the photooxidation reaction. Unfortunately, temperature-dependent measurements do not allow us to determine this value because the quantum yield of
H-terminated samples and its temporal evolution, as well as
the number of adsorbed oxygen molecules, are temperaturedependent. We note that at 5 K, despite the highest singlet
oxygen generation rate,8 oxidation is completely absent atDownloaded 18 Nov 2004 to 133.30.106.15. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

3592

Kovalev et al.

Appl. Phys. Lett., Vol. 85, No. 16, 18 October 2004

To conclude, we demonstrate that the efficient photodegradation of H-terminated porous Si layers is mediated byphotosensitized singlet oxygen molecules. This effect is very
similar to that known for light-emitting polymers or dye molecules. Therefore, H-terminated porous Si light-emitting devices will self-destruct during operation if fabricated without
special attention to the elimination of molecular oxygen contamination. An alternative way of emission stabilization isthe intentional oxidation of the surface of silicon nanocrystals which prevents the singlet oxygen formation.

FIG. 3. IR absorbance of porous Si layers. Sample A after irradiation atdifferent temperatures in oxygen ambient at 1 bar. Iex. = 100 mW/ cm2. Eex.
= 2.54 eV; irradiation time is 1 h.

This work is partly supported by an Industrial Technology Research Grant Program in 2003 from New Energy andIndustrial Technology Development Organization (NEDO),
Japan. V. Yu. T. is grateful to the Alexander von Humboldt
Foundation.
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2any photoexcitation levels (up to 10 W / cm2) and any irradiation time (up to 10 h).
Oxidation mediated by singlet oxygen drastically modifies the emission properties of porous Si. Contrary to naturaloxidation, which usually improves the quantum yield of porous Si layers due to passivation of dangling bonds,14 theinteraction of singlet oxygen with Si nanocrystals creates
new dangling bonds.3 Since singlet oxygen is generated in
the gas phase it can interact with any nanocrystal assembling
porous Si layers. The probability of interaction scales withthe surface area of particular nanocrystals and the strongest
degradation level is found as expected for the largest nanocrystals (see Fig. 1). A small increase of the PL intensity athigher energies is a direct result of photooxidation: the band
gap of the oxidized nanocrystals shifts towards higher values
due to quantum confinement.14

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