Excitation effects and luminescence stability in porous SiO2:C layers

Article abstract of DOI:10.1002/pssa.201100815

White-light emitting porous SiO₂:C layers on silicon wafers have been fabricated by oxidation of carbonized porous silicon. The study was focused mainly on the identification of the mechanism of light emission and photo-induced degradation. The

Full text of DOI:10.1002/pssa.201100815

Phys. Status Solidi A, 1–7 (2012) / DOI 10.1002/pssa.201100815
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p s s
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applications and materials science
Excitation effects and luminescence
stability in porous SiO2:C layers
,
1
Andriy Vasin* , Andriy Rusavsky1, Alexei Nazarov1, Vladimir Lysenko1, Galyna Rudko1, Yurii Piryatinski2,
Ivan Blonsky2, Jarno Salonen3, Ermei Makila3, and Sergii Starik4
1 Lashkaryov Institute of Semiconductor Physics, pr. Nauki 41, 03028 Kiev, Ukraine
2 Institute of Physics, pr. Nauki 46, 03028 Kiev, Ukraine
3 Department of Physics, University of Turku, 20014 Turku, Finland
4 Bakul Institute of Superhard Materials, Avtzavodskaya 2, 04074 Kiev, Ukraine
Received 27 December 2011, revised 14 February 2012, accepted 15 February 2012
Published online 6 April 2012
Keywords carbon incorporated silicon oxide, white luminescence
* Corresponding author: e-mail andriy.vasin@gmail.com, Phone: þ38 044 5256395, Fax: þ38 044 5256177
White-light emitting porous SiO2:C layers on silicon wafers phenomena were observed: reversible and irreversible. The
have been fabricated by oxidation of carbonized porous silicon. irreversible degradation is suggested to be associated with
The study was focused mainly on the identification of the photo-induced chemical interaction of light emitting material
mechanism of light emission and photo-induced degradation. with atmospheric oxygen. It is demonstrated that irreversible
The effect of carbonization temperature and exposure to degradation can be reduced by encapsulation of the light-
intense ultraviolet irradiation on the photoluminescence (PL) emitting material. The structural configuration of light-emitting
properties was studied by steady state and time-resolved PL centers and mechanism of reversible degradation are discussed.
measurements. Two types of photo-induced degradation
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1 Introduction Light emitting materials are of great relatively narrow emission band, so the color-rendering
importance in modern optoelectronics, lighting, and light index (CRI) of these white light sources is low.
indication technologies. In the field of lighting applications,
The general approach to obtaining white color with high
the development of white-light emitting materials with high CRI is a combination of materials with complementary
luminous efficiency and good color properties attracts a great spectral characteristics. However, single material phosphor
interest. At present such materials (phosphors) are used in converting blue-and/or-UV into white light with high CRI is
luminescent lamps and light emitting diodes for converting more desirable because application of multicolor phosphors
of ultraviolet (UV) irradiation into visible white light. reduces the light emission efficiency. Moreover, the
Currently, all commercially available white light emitting emission spectrum of such multicomponent phosphors
phosphors contain heavy metals including transition and usually changes with operation time due to different
rare-earth metals. Hence, the development of new white light degradation properties of different phosphor components.
emitting materials free of expensive and toxic heavy metals
is an important task.
Recently, a new class of white-light emitting Si:O:C
composite materials free of heavy metals was found. The
During last decade, two alternatives to the phosphors material can be synthesized in form of sol–gel-derived SiOC
based on metal activators were suggested and studied: thin films [2], a-SiO(x)C(y):H thin films deposited by
(1) organic materials and (2) semiconductor nanoparticles. thermal chemical vapor deposition (CVD) [3], a-SiO(x)-
Despite the optimistic progress in the efficiency of organic C(y):H thin films synthesized by low temperature oxidation
white-light sources, the stability and long-term reliability of carbon-rich a-Si(x)C(y):H thin films [4], porous layers
issues of organic materials are still to be solved. On the other por-SiO2:C on silicon wafer [5, 6], or mesoporous nano-
hand, semiconductor quantum dots (QDs) have attracted powders [7]. Emission spectrum of thesematerials covers the
interest due to their high luminescence efficiency and size- whole visible range and is very similar to that of the scattered
tunable band-gap characteristics [1]. However, QDs exhibit sun radiation (natural day light).
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A. Vasin et al.: Excitation effects and luminescence stability in porous SiO2:C layers
In our previous work, it was demonstrated that photo-
luminescence (PL) band of por-SiO2:C layers is composed of
at least two bands, namely the blue band with maximum
intensity at about 400 nm and white-green band with
maximum at about 500–540 nm [6]. It was shown that the
white-green band is related to the presence of carbon
whereas the blue shoulder most likely originates from
defects in the surrounding SiO2 matrix [8]. Present report is a
logical continuation of the previous studies to understand
light emission mechanisms in carbon-incorporated silicon
oxide.
1050 0C
950 0C
850 0C
1000
1250
1500
1750
2 Experiment Porous silicon precursors (por-Si) were
fabricated by anodizing pþ-type Si(100) wafers (0.015–
0.025 V cm) in a hydrogen fluoride (40%):ethanol mixture
1:1. The por-Si samples were carbonized in N2 (1.5 l minꢀ1)/
C2H2 (1 l minꢀ1) flow at 850, 950, and 1050 8C resulting in
the formation of carbonized porous silicon (por-Si:C). The
-1
Raman shift, cm
Figure 1 Raman scattering spectra of por-Si:C layers: PS850;
PS950, and PS1050.
next sample preparation step was the thermal treatment at 850 8C resulted in the appearance of very weak carbon
atmospheric pressure in moisturized argon (Ar/H2O) flow at related RS band in the range of 1000–1800 cmꢀ1 (bottom
600 8C for 2 h resulting in the formation of light emitting spectrum in Fig. 1). This band originates from sp2-
carbon-incorporated porous silicon oxide (por-SiO2) layer. coordinated carbon clusters. Intensity of carbon-related RS
Hereafter, we have labeled the samples fabricated at band increased strongly with increasing of carbonization
different carbonization temperatures as PS850, PS950, and temperature up to 950 and 1050 8C (Fig. 1, upper spectra).
PS1050, respectively (see Table 1).
In these cases, RS band is composed by well-defined D-
PL measurements were carried out using nitrogen laser and G-bands inherent to graphite-like carbon structures.
(le ¼ 337.1 nm, pulse duration te ¼ 9 ns, pulse repetition Integrated intensities of RS bands in PS950 and PS1050 are
frequency f ¼ 80 Hz) for excitation. Time-resolved PL almost equal but the values of full width at half maximum of
measurements were performed using stroboscopic regis- D- and G-bands are smaller in PS1050 indicating more
tration system allowing PL registration with given delay ordered graphite-like structure in carbon clusters.
times (hereafter ‘‘pulse regime’’). We used the delay time
Samples of PS850 series were annealed in pure argon,
about 0.7 ns and strobe window 0.1 ns for measuring PL humid argon, or dry oxygen flow for 30 min after
spectra in the ‘‘pulse regime’’. The spectral slit width for carbonization. Integrated intensity of carbon RS band
measuring PL spectra was 0.2–0.4 nm. Obviously, contri- increased by several times after the annealing in pure
bution of light emission centers with longer radiative decay and humid argon while no carbon related RS band was
in PL spectrum is reduced when measuring in ‘‘pulse- detected after the annealing in oxygen (Fig. 2). The main
regime’’. It means that if the spectrum is composed of several conclusion derived from these observations is that large
components originating from recombination centers with fraction of carbon material in the porous layer after
different decay times it is possible to evaluate the relative carbonization procedure exists in Raman inactive form.
contribution of these centers by comparing spectra measured
in steady-state and ‘‘pulse regime’’.
Interatomic bonding structure of the layers was
700
examined by Raman scattering (RS, excitation by 514 nm
Ar laser radiation) and Fourier-transform infrared reflection
spectroscopy (FTIR).
600
500
400
300
200
100
0
3 Results and discussion
3.1 Raman scattering and FTIR RS spectra of as-
carbonized samples are presented in Fig. 1. Carbonization at
Table 1 Labeling of the samples.
carbonization temperature (8C)
por-Si:C
por-SiO2:C
850
950
1050
PS850
PS950
PS1050
PSO850
PSO950
PSO1050
Figure 2 Intensities of RS bands in the samples of PS850 series:
as-fabricated and thermally treated in dry argon, wet argon or dry
oxygen at 600 8C for 30 min.
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Phys. Status Solidi A (2012)
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60
40
20
0
Presumably, carbon is incorporated in the form of few atoms
clusters that cannot be ‘‘seen’’ by RS.
Si-O-Si
asym., TO
During the thermal treatment in dry or humid argon
migration and coalescence of carbon precipitates take place
resulting in formation of large sp2-coordinated clusters
detectable by RS spectroscopy. Annealing in oxygen results
in oxidation of carbon and its escape in the form of volatile
mono- and/or carbon oxide.
Unfortunately, identification of carbon clusters in light
emitting PSO series by RS spectra cannot be done because
Raman signal (at 514 nm excitation) is masked by strong
luminescence background in visible spectral range.
FTIR: due to the high conductivity of the Si substrate, the
transmission FTIR measurements were not possible. There-
fore, we used the reflection measurements. Despite strong
interference features modulating FTIR spectra some useful
information can be derived. First, no C–H bonds were
detected in as-carbonized series (Fig. 3, spectrum 1). It
means that during the carbonization procedure, carbon is
deposited inside porous layer in an unhydrogenated form.
Second, weak but well detectable Si–C related band is
Si-O-Si
asym., LO
C-Hn, str.
2700 2800 2900 3000
Si-O-Si
sym.
2
1
3
1
2
3
PSO850
PSO950
PSO1050
1000
1500
2000
2500
3000
3500
Wavenumber, cm-1
Figure 4 (onlinecolorat:www.pss-a.com)FTIRreflectionspectra
ofPSO850(1),PSO950(2),andPSO1050(3).Thespectraareshifted
in vertical direction. Inset illustrates C–Hn related bands.
observed at 800 cmꢀ1 accompanied by Si–O band at ing), 1110 cmꢀ1 (asymmetrical stretching TO mode) and
1060 cmꢀ1 (Fig. 3, spectra 2, 3, 4). The intensity of Si–C 1277 cmꢀ1 (asymmetrical stretching LO mode) [9].
related band increases with increasing of carbonization
temperature.
Formation of C–Hn bonds is identified by IR band at
2800–3000 cmꢀ1 (see Fig. 4).
The presence of thin SiC surface layer allows to explain
why carbonrelated RS band observed after short annealingin
3.2 Interference of PL inside por-SiO2:C
wet argon is significantly stronger than that measured after layer PL spectra of por-SiO2:C is very broad and covers
the annealing in dry argon (see middle bars in Fig. 2). Water the spectral range from near UV to near IR (350 to 800 nm).
vapor does not oxidize carbon at such temperature therefore The spectra of the layers with the thickness of about 1–4 mm
it is reasonable to suggest that carbon atoms that are released often exhibit pronounced regular features. The number and
after oxidation of SiC surface layer afterwards participate in width of these features vary from sample to sample-making
the formation of carbon clusters that are active in Raman analysis and interpretation of the spectra complicated.
scattering.
Distance between peaks is smaller in red spectral region in
Si–C band disappeared after oxidation and strong Si–O comparison with that in blue region. These observations
related bands appeared at 826 cmꢀ1 (symmetrical stretch- allowed us to suggest that these peaks originate from the
interference inside por-SiO2:C layer. To check this idea, the
sample with well-defined regular features in PL spectrum
was selected, and the spectra were measured at different
100
Si-C
4
3
2
1
PS1050
PS950
PS850
por-Si
angles between optical axis of the monochromator and
surface plane of the sample (Fig. 5). One can clearly see that
positions of the peaks are shifted with the change of the angle
and the peak shift is larger in the long wave spectral region.
This observation unambiguously identifies regular features
as the interference effects.
Similar interference effect was observed in PL spectra of
a-SiOC:H films [4] justifying the general character of such
artifact for measurements of broad visible PL band in thin
layers. The effect of interference on PL spectra is very
complicated and depends on the excitation wavelength,
spectral dependence of absorption and refraction coefficients
of the layer in UV–VIS range, geometry of optical system,
distribution of the density of light-emitting sites in depth
etc. We would like to emphasize that the analysis of the
spectral properties of broad PL generated in thin layers
should be performed very carefully to avoid possible
misinterpretations.
Si-O
80
60
40
20
0
700 800 900 1000 1100 1200
1000 1500 2000 2500 3000 3500 4000
Wavenumber, cm-1
Figure 3 (onlinecolorat:www.pss-a.com)FTIRreflectionspectra
of por-Si (1), PS850 (2), PS950 (3), and PS1050 (4). Spectra are
shifted in vertical direction for better visualization.
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A. Vasin et al.: Excitation effects and luminescence stability in porous SiO2:C layers
(a)
steady state
600
1
400
1050 0C
(3)
200
2
950 0C
(2)
0
(1)
850 C
0
600
400
200
0
(b)
"pulse"
3
400
500
600
700
Wavelength, nm
1050 0C (3)
950 0C (2)
850 0C (1)
Figure 5 TypicalPLspectrummeasuredatdifferentanglebetween
opticaxisofmonochromatorandsurfaceplaneofthesample:(1)208;
(2) 308; (3) 508.
400
500
600
700
800
3.3 Correlation of carbonization temperature
and spectral properties of PL As it was demonstrated in
Ref. [6] the broad PL spectrum of por-SiO2:C layers is
composed of two bands: (1) the broad band with maximum
PL intensity at 500–540 nm, and (2) relatively narrow blue
shoulder, with maximum intensity at about 400 nm. It was
suggested that the broad band is associated with carbonized
surface, while the blue shoulder is presumably originating
from point defects in porous silicon oxide matrix [8]. In
the present study, we carried out more detailed study of
correlations between carbonization temperature and relative
contribution of these two bands.
PL spectra of the samples PSO850, PSO950, and
PSO1050 measured in steady state and ‘‘pulse regimes’’
are presented in Fig. 6a and b, respectively. It is clearly seen
that relative contribution of the blue shoulder in steady state
spectra is increasing with decreasing of carbonization
temperature (Fig. 6a) so that color of the PL changes from
orange in PSO1050 to white in PSO850. In ‘‘pulse regime’’
measurements, the blue shoulder was strongly reduced
indicating the decay time of the blue band is significantly
longer than that of the carbon-related band. Elimination of
the blue shoulder by applying ‘‘pulse regime’’ demonstrates
that carbon-related band is shifted gradually to shorter
wavelength with the decrease of the carbonization tempera-
ture (Fig. 6b).
Wavelength, nm
Figure 6 PL spectraof thesamplescarbonized atdifferent temper-
atures and measured in steady state (a) and ‘‘pulse’’ (b) regimes:
(1) PSO850; (2) PSO950; (3) PSO1050.
Unprotected
(a)
800
600
400
200
0
1
2
(b)
Protected by SiC
800
600
400
200
0
1
2
3.4 Photo induced PL degradation Another aim
of the present research is examination of PL stability in por-
SiO2:C material under intense UV radiation. UV radiation of
Xe lamp was used as a UV source. We observed that the
exposure of the samples to UV radiation of Xe lamp for
20 min resulted in the decrease of PL intensity by about 50%
(Fig. 7a).
500
600
700
800
Wavelength, nm
Figure 7 PL spectra of uncoated (a) and a-SiC coated (b) por-
SiO2:C samples before (spectrum 1) and after (spectrum 2) 20 min
exposure to Xe-lamp.
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Phys. Status Solidi A (2012)
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As it was shown in Ref.[8], annealing in oxygen
atmosphere at the temperature as low as 600 8C for 30 min
leads to strong reduction of carbon related PL band. Such
quenching correlated with drastic decrease of carbon content
in por-SiO2:C layer. This observation gives a reasonable
suggestion that emitting sites are associated with carbon that
is removed from porous layer by atmospheric oxygen. It is
logical to suppose that rapid and strong photo-induced
degradation results from photo-induced oxidation of carbon
by atmospheric oxygen. To check this idea, some samples
have been coated with 100 nm thick amorphous SiC thin film
deposited by magnetron sputtering to protect light emitting
layer from direct contact with air. SiC film is thick enough to
reduce the effect of atmospheric oxygen and, on the other
hand, it is sufficiently thin to avoid strong absorption of UV
excitation and visible emission light. The substrate tempera-
ture during deposition process was as low as 200 8C to avoid
structural changes in por-SiO2:C.
800
600
400
1
2
unprotected
(a)
protected by SiC
Xe lamp "on"
Xe lamp "off"
Protected by SiC
(b)
N2 laser
N2 laser
N2 laser
+
600
Xe lamp
Such encapsulation by SiC reduced the degradation
effect significantly (Fig. 7b). Some residual degradation can
be attributed to incomplete protection due to the presence of
micro defects in a-SiC film as well as to the diffusion of
oxygen molecules through nanovoids in amorphous SiC
network. This observation is the additional evidence of
carbon-related origin of the broad PL band. However, the
structure of carbon-related light-emitting sites is still an open
question.
0
2
4
Time, minutes
Figure 8 (a) Evolution of PL amplitude at 500 nm under contin-
uous N2 laser and cyclic on/off Xe-lamp irradiation; (b) typical PL
evolution within single cycle of Xe-lamp irradiation.
3.5 Reversible degradation To analyze photo-
induced PL degradation under intense UV radiation in more
details we have performed the study of time dependent PL
evolution under cyclic Xe lamp radiation (on/off cycles). PL
amplitude at 500 nm emission wavelength was measured as a interaction of carbon with atmospheric oxygen. Height of
function of time (Fig. 8a). One can see that the degradation the step in ‘‘on’’ and ‘‘off’’ edge is approximately the same
rate in unprotected material is faster. In addition to gradual indicating full recovery of PL efficiency.
PL quenching step-like features can be seen at the moments
of Xe lamp switching on/off. After Xe lamp was switched on,
3.6 Discussion of local structure in por-SiO2:C
the intensity immediately dropped step-like for several Let us focus now on the discussion of possible origin of
percent followed by continuous monotonic reduction. After carbon-related emission band in por-SiO2:C layers. We will
the lamp was switched off, the PL amplitude raised step-like. analyze possible structural configuration that could be
At first sight the drop of the light emission intensity with responsible for the broadband light emission in carbon
increasing of the intensity of excitation radiation looks quite incorporated silicon oxide. Hereafter we will confine our
surprising. Additional UV radiation of Xe lamp obviously discussion to carbon related structural configurations.
results in the increase of total emission intensity (well seen
During the carbonization treatment of porous silicon
by eye) but in ‘‘pulse regime’’ the light detection system is precursor acetylene molecules interact with hydrogen-
synchronized with laser pulses so that only pulsed PL is terminated silicon surface resulting in the formation of the
detected. It means that intensity of pulsed component of PL SiCn surface layer:
decreased. It can be attributed to reduction of quantum
efficiency of the PL under intense UV radiation and this
reduction is reversible.
Si : Si ꢀ Hn þ C2H2 ! Si : Si ꢀ Cm þ H2" :
Typical evolution of PL amplitude within a single ‘‘on/
The presence of Si–C layer in por-Si:C is proved by the
off’’ cycle in SiC coated sample is illustrated in Fig. 8b. One presence of corresponding band at about 800 cmꢀ1 in FTIR
can see that under laser radiation PL intensity is almost spectra (see Fig. 3). Depending on carbonization time and
constant, but when Xe lamp was switched on the emission temperature the carbonized (or carbidized) surface of pore
intensity droped fast. Continuous exponential reduction of walls could consist of (i) monolayer of carbon covalently
PL intensity during the ‘‘on’’ cycle can now be attributed to bonded to silicon, (ii) carbon-rich SiCn surface layer
irreversible degradation associated with photo-stimulated containing few Si–C surface layers with attached multi-
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A. Vasin et al.: Excitation effects and luminescence stability in porous SiO2:C layers
atomic carbon clusters or (iii) SiCn layer covered by reports on optical, electrical, and chemical properties of this
continuous amorphous carbon layer a-C. material there are only few publications reporting strong
During the wet oxidation treatment, water molecules white PL. This may be an indication that effective
react with silicon but do not oxidize carbon material. As a luminescence is not inherent to hydrogenated silicon
result, porous silicon oxide layer is formed and surface- oxycarbide.
bonding reconstruction of carbonized surface occurs.
Structure of an ‘‘ideal’’ a-SiCO:H material is represented
Appearance of C–Hn stretching band at 2800–3000 cmꢀ1 by amorphous SiOx network terminated by –CH3 or –OH
in FTIR spectra indicates formation of C–Hn bonds after groups with possible substitution of Si–O–Si bridges by
oxidation step (Fig. 4). It means that some of hydrogen atoms –CH2– crosslinking [16]. Such ‘‘ideal’’ structure contains no
released after water molecule decomposition reacts with ‘‘defects’’ like homonuclear Si–Si and C–C bondsor Si–O–C
carbon dangling bonds.
bridges, but real materials contain variety of structural
There are two possible bonding ‘‘bridges’’ between ‘‘defects’’. It is logical to suggest that the reported light
silicon oxide matrix and carbonized surface:
emission of silicon oxycarbide films results from structural
fluctuations associated with the ‘‘defects’’.
The authors of Ref. [3] have found correlation of PL
intensity and position of the 437 cmꢀ1 IR absorption band in
a-SiO(x)C(y):H films deposited by CVD method. This band
is associated with bending (rocking) vibrations of Si–O–Si
Si : Si ꢀ Cn þ H2O ! SiOx: Si ꢀ O ꢀ Cn ꢀ Hm
or
Si : Si ꢀ Cn þ H2O ! SiOx: Si ꢀ Cn ꢀ Hm:
In Ref. [8], the traces of Si–C bonds in por-SiO2:C were bridges, and in thermally grown dense silicon oxide layers
observed by EELS spectroscopy. But in the present study this band is located at 457 cmꢀ1 [10]. Small shift of this band
FTIR spectra of por-SiO2:C do not contain any detectable in CVD a-SiO(x)C(y):H films was ascribed to the presence of
signs of Si–C.
Si–O–C bonds. Based on the correlation of the IR band
Oxidation of porous silicon leads to a strong increase of intensity and PL intensity, it was concluded that light
the volume of the material and to the decrease of the volume emission originates from localized electron states associated
of pores. Some very small pores can even totally collapse. with Si–O–C bonds. Unfortunately, no clear proof of this
Under these circumstances the interaction of carbon clusters interpretation has been presented. In fact, shift of the
with silicon oxide matrix increases. Some of the clusters can vibration frequency can be caused by several factors, such
be trapped and encapsulated in nanovoids.
as mechanical stresses, local Si–O–Si bonding distortions
Briefly summarizing, we suggest that three carbon- etc.
related structural configurations that possibly exist in porous
SiO2:C layer can be responsible for carbon-related light deposited by atmospheric pressure microplasma jet was
In Ref. [14], white luminescence of a-SiOC:H films
emission band: carbon clusters, Si–O–C and Si–C bonds.
ascribed to neutral oxygen vacancy (NOV) defects in SiOx
matrix. This interpretation was based on the PL intensity and
3.7 Overview SiO(x):C(y) compounds that exhibited hysteresis of C–V characteristics associated with trapped
broad visible PL can be divided into three general groups: (1) positive space charges. Authors assumed that all positive
carbon implanted silicon oxide layers; (2) silicon oxycarbide charges in a-SiOC:H/Si heterostructure are trapped by bulk
thin films and (3) nanostructured composites.
NOV defects. They excluded from consideration a large
number of other bulk and interface defects that are also able
(1) In Ref. [10, 11], it was suggested that PL of carbon to trap positive charge and, thus, to contribute to C–V
implanted silicon oxide in blue–yellow range can be hysteresis.
associated with the formation of carbon-related clusters in
Strong white light emission observed in sol–gel-derived
form of graphitic or SiC clusters. However in Refs. [12, 13], SiOC layers [2] was ascribed to the combination of emission
light emitting SiO2:C layers were fabricated by similar ion bands associated with Si, SiC, and C related precipitates. To
implantation method but no Si–C related bands in FTIR support their interpretation the authors referred to Ref. [11]
spectra were detected that makes SiC formation an unlikely but no detailed argumentation has been presented.
assumption. They also argued that Si–C formation is
energetically unfavorable.
Broadband light emission was observed in carbon
incorporated silicon oxide thin films deposited by RF-
magnetron sputtering [15]. Maximum intensity of PL band
(2) Broad visible light emission band was observed in was observed to be shifted from about 2.1 to 2.3 eV with
hydrogenated and non-hydrogenated SiO(x)C(y) layers: thin decreasing of carbon incorporation. It was suggested that PL
films deposited by thermal CVD technique [3], atmospheric is originating from carbon clusters.
pressure microplasma jet [14], sol–gel-derived layers [2] or
magnetron sputtering [15].
(3) Here, we will discuss another possible origin of light
Amorphous hydrogenated oxycarbide thin films a- emission in SiOC compound, i.e., radiative recombination in
SiCO:H are intensively investigated as a material for low-k carbon nanoparticles. In fact, the first report on luminescent
barrier dielectric for Cu-based multilevel interconnection in properties of individual carbon nanoparticles was published
ultralarge-scale integrated circuits. Despite huge number of in 2006 [17]. Thereafter, chemists have discovered and
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Phys. Status Solidi A (2012)
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developed several methods of fabrication of light emitting associated with photo-induced chemical reaction of light
nanoparticles, so called ‘‘carbon nanodots’’ (reviewed in emitting material (presumably carbon) with atmospheric
Ref. [18]). It was shown that carbon particles with a size of oxygen. It is demonstrated that irreversible degradation can
several nanometers after an appropriate surface passivation be reduced by encapsulation that prevents contact of the
become highly luminescent in broad visible spectral range. material with air. Reversible degradation is still under
Unfortunately physical mechanism of light emission has not discussion and need additional work will be required to
been studied properly. Nevertheless, the most important understand the phenomenon. But, on the basis of our results,
conclusion of the studies is that amorphous carbon we suggest that it is due to fluorescence intermittency effects.
nanoparticles can effectively emit visible light. It is also
important to note that light emitting carbon dots usually
contain very large amount of oxygen (tens of at. %)
and Technology Center of Ukraine, project No. 5513.
Acknowledgements This study was supported by Science
If the carbon-related PL band in por-SiO2:C is associated
with the presence of carbon clusters in porous layer, we
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First, in carbon-related molecular-like clusters the gap
between HOMO and LOMO of p-orbitals decreases with
increasing of the number of atoms. Obviously, with
increasing of carbonization temperature the average number
of atoms in carbon clusters inside porous layer increases;
leading to decrease in the energy of radiative transitions.
Secondly, on the basis of the carbon cluster hypothesis, it
is quite easy to explain irreversible photo-induced degra-
dation by photo-induced oxidation of carbon material in
porous layer.
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exposure is the mechanism of fluorescence intermittency (or
‘‘blinking’’), phenomenon, which is well known in light
emitting semiconductor nanoparticles [19]. Under irradia-
tion by energetic photons, some of nanoparticles can be
charged by electron jumping from core state to neighboring
energy state at the interface or in the surrounding wide band
gap matrix. In the charged state, the non-radiative Auger
recombination becomes the dominating recombination
path making the nanocrystal ‘‘dark’’ until neutrality will be
recovered. Obviously, the higher relative fraction of charged
particles the lower is the integral PL intensity. It has been
demonstrated that such charging of QDs by trapped electrons
is adominant process limiting the efficiency of light-emitting
devices based on organic materials and colloidal QDs [20].
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can be assigned to charging and discharging of fraction
of light emitting sites when photon flux is switching-on
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3.8 Summary Pulse laser excitation effects and photo-
induced degradation of PL of porous SiO2:C layers fabricated
by successive carbonization and oxidation of porous silicon
were studied. It was found that carbon-related emission band
was shifted toward longer wavelengths when the carboniz-
ation temperature was increase. Two types of photo-induced
degradation phenomena were observed: reversible and
irreversible. Irreversible degradation is suggested to be
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