JOURNAL OF APPLIED PHYSICSVOLUME 89, NUMBER 9

1 MAY 2001

Electron spin resonance study of defects in Si1À x Gex alloy nanocrystals
embedded in SiO2 matrices: Mechanism of luminescence quenching
Kimiaki Toshikiyo, Masakazu Tokunaga, and Shinji Takeoka
Graduate School of Science and Technology, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan

Minoru Fujiia) and Shinji Hayashi
Department of Electrical and Electronics Engineering, Faculty of Engineering, Kobe University, Rokkodai,
Nada, Kobe 657-8501, Japan

共Received 14 November 2000; accepted for publication 14 February 2001兲
Dangling bond defects in Si1⫺x Gex alloy nanocrystals (nc-Si1⫺x Gex ) as small as 4 nm in diameter
embedded in SiO2 thin films were studied by electron spin resonance 共ESR兲, and the effects of thedefects on photoluminescence 共PL兲 properties were discussed. It was found that the ESR spectrum
is a superposition of signals from Si and Ge dangling bonds at the interfaces between nc-Si1⫺x Gex
and SiO2 matrices 共Si and Ge P b centers兲. As the Ge concentration increased, the signal from the
Ge P b centers increased, while that from the Si P b centers was nearly independent of Ge
concentration. The increase in the number of Ge P b centers was accompanied by strong quenchingof the PL. The observed correlation between the two measurements suggests that the Ge P b centers
act as efficient nonradiative recombination centers for photogenerated carriers, resulting in thequenching of the main PL. © 2001 American Institute of Physics. 关DOI: 10.1063/1.1362410兴
I. INTRODUCTION

responding to the momentum conserving phonons are
smeared out by increasing the Ge concentration.8 Furthermore, shortening of the radiative lifetime with increasing Ge
concentration has been commonly observed.8,10,11 These results provide strong evidence that the oscillator strength ofnc-Si is greatly improved by Si1⫺x Gex alloy formation.
However, in actual nc-Si1⫺x Gex , the luminescence efficiency is smaller than that of pure nc-Si.7 One of the possible explanations for the low PL efficiency is the generationof defects caused by alloying at the interfaces between
nc-Si1⫺x Gex and SiO2 matrices, and the defects act as nonradiative recombination centers. Although some kinds of defects have been reported to exist at the interface between the
Si1⫺x Gex alloy and their oxide,12,13 it has not yet been clarified which defects act mainly as nonradiative recombination
centers. In order to make good use of the advantages of thenc-Si1⫺x Gex mentioned, detailed studies on the mechanismof PL quenching are indispensable.
In this work, we prepared nc-Si1⫺x Gex as small as 4 nm
in diameter embedded in SiO2 thin films with different Ge
concentrations and studied the electron spin resonance 共ESR兲
and PL properties. It will be shown that both properties depend on the Ge concentration. From a comparison between
these two measurements, we will discuss the mechanism of
PL quenching.

In recent years, nanometer-sized Si and Ge crystals(nc-Si and nc-Ge兲 have been extensively studied because
they offer new possibilities for indirect band gap semiconductors as new materials in photoelectric applications. It has
been demonstrated that the photoluminescence 共PL兲 energyof nc-Si is tunable from the bulk band gap to the visible
region by simply controlling the size.1–6 This wide tunabilityof PL energy indicates that the PL is due to the recombination of excitons confined in zero-dimensional Si quantum
dots. The tuning range can be expanded by Si1⫺x Gex alloy
formation. In a bulk Si1⫺x Gex alloy, it is well known that Si
and Ge resolve each other over the entire concentration
range, and the band gap energy changes from a Si one to a
Ge one when the concentration is changed. Similar effects
have been successfully observed in nanometer-sized
Si1⫺x Gex alloy crystals (nc-Si1⫺x Gex ); 7,8 the band gap energy 共luminescence energy兲 of nc-Si1⫺x Gex changes fromthe widened band gap of nc-Si to that of nc-Ge as the Geconcentration increases.7
The other advantage of Si1⫺x Gex alloy formation is thebreakdown of the k-conservation rule and the enhancementof the oscillator strength due to the violation of the translational symmetry of the crystalline lattice. In pure nc-Si, it
has been demonstrated that the indirect band gap nature ofbulk Si crystals is preserved even for nc-Si several nanometers in diameter, although the probability of the no-phonon
optical transition is much larger than that of bulk crystals.9
The indirect band gap nature results in a relatively long PL
lifetime, which is one of the obstacles to realizing Si-based
light-emitting devices. In our previous work on resonant PLfrom nc-Si1⫺x Gex , we demonstrated that the structures cor-

II. EXPERIMENT

Si1⫺x Gex alloy nanocrystals were prepared by the same
method used in our previous work.7,8 Si, Ge, and SiO2 sputtering targets were simultaneously sputtered in Ar gas of 0.3
Pa using a multitarget sputtering apparatus. The substrates
were fused quartz plates. The thickness of the films for PL
and ESR measurements were about 700 nm and 8.4 ␮ m,respectively. Ge concentration in the films was controlled bychanging the rf power and the distance between the substratea兲

Author to whom correspondence should be addressed; electronic mail:fujii@eedept.kobe-u.ac.jp

0021-8979/2001/89(9)/4917/4/$18.00

4917© 2001 American Institute of Physics

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4918

J. Appl. Phys., Vol. 89, No. 9, 1 May 2001

FIG. 1. PL from nc-Si1⫺x Gex dispersed in SiO2 thin films 共a兲 at room
temperature and 共b兲 at 10 K. Ge concentration is changed from 0 to 0.30.

and the target during cosputtering. After the deposition, films
were annealed in a N2 gas atmosphere for 30 min at 1100 °C.
During the annealing, nc-Si1⫺x Gex were grown in SiO2 matrices. Ge concentration (x) was changed from 0 to 0.41. The
values of x were determined from Raman spectra.7,14 The
size of nc-Si1⫺x Gex was estimated by cross sectional highresolution transmission electron microscopic 共HRTEM兲 observations. HRTEM observations revealed that sphericalnanocrystals as small as 4–5 nm in diameter were grown inamorphous SiO2 matrices. Each nanocrystal was isolated
from the others by SiO2 barriers several nanometers in thickness. The PL spectra were measured with a single monochromator equipped with a liquid N2 cooled Ge detector 共0.8–1.9eV兲 and a liquid N2 cooled InSb detector 共0.4–0.8 eV兲. The
excitation source was the 488.0 nm line of an Ar ion laser.
The PL spectra were measured in the temperature range between 10 and 300 K in a continuous-flow He cryostat.
X-band ESR was measured with a conventional ESR spectrometer at room temperature. The intensity of ESR signals
was calibrated by simultaneously measuring the signal froma fixed amount of Mn2⫹/MgO powder.

Toshikiyo et al.

FIG. 2. 共a兲 PL intensities of the 1.4 and 0.9 eV peaks at 10 K, and 共b兲 theintegrated intensities of the ESR signals from Si and Ge P b centers as afunction of Ge concentration.

ally assigned to the recombination of photoexcited carriers
via Si dangling bonds at the interfaces between nc-Si and
SiO2 matrices 共Si P b centers兲.15–18 The intensity of this peakshows the same Ge concentration dependence as that of themain band. The integrated PL intensities of the 0.9 and 1.4
eV peaks at 10 K are shown in Fig. 2共a兲 as a function of Ge
concentration. Both the peaks become weaker at almost thesame rate by increasing Ge concentration.
Figure 3 shows the ESR derivative spectra ofnc-Si1⫺x Gex with various Ge concentrations. For purenc-Si, a sharp ESR signal is observed; the g value and thepeak-to-peak linewidth are 2.0055 and 10 G, respectively.

III. RESULTS

Figure 1共a兲 shows the room temperature PL spectra ofnc-Si1⫺x Gex with various Ge concentrations. For the sample
with x⫽0 共pure nc-Si兲, a PL peak is observed around 1.43
eV. This PL arises from the recombination of electrons andholes confined in nc-Si.3,4 We can see that, as the Ge concentration increases, the PL peak shifts to lower energies andreaches 1.29 eV for the sample with x⫽0.30. This shift isaccompanied by the quenching of the PL by about one orderof magnitude. By further increasing the Ge concentration, PL
was completely quenched. Figure 1共b兲 shows the PL spectra
at 10 K. In addition to the main PL band, an additional PL
peak is observed around 0.9 eV. The 0.9 eV peak is gener-

FIG. 3. ESR derivative spectra of nc-Si1⫺x Gex dispersed in SiO2 thin films
at room temperature. Ge concentration is changed from 0 to 0.41.

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J. Appl. Phys., Vol. 89, No. 9, 1 May 2001

Toshikiyo et al.

4919

back-bonding sites increases, the g value of a dangling bond
becomes larger.13,20 Therefore, the observed increase in gvalues with Ge concentration indicates that the number of P bcenters with a larger number of Ge back bonds increases
with Ge concentration.
Another factor that might be taken into account is thenarrowing of the band gap with increasing Ge concentration.
It is expected that the g values of the Si and Ge P b centerswould increase by narrowing the band gap because the deviation of the g value from that of a free electron is inversely
proportional to the band gap of a Si1⫺x Gex alloy.20,21 However, this effect is generally much smaller than that discussed
above, so it is considered negligible.

B. Correlation between ESR and PL
FIG. 4. g values of the HMF (䉱) and LMF (䊏) signals as a function of Ge
concentration. The data for Si P b centers 关 䉭 共Ref. 12兲, 䉮 共Ref. 16兲, and 䉲
共Ref. 19兲兴 and Ge P b centers 关 䊐 共Ref. 12兲, and 䊊 共Ref. 13兲兴 are alsoplotted. The broken curves are drawn to guide the eyes.

This signal can be assigned to Si P b centers.16,19 For purenc-Si, a clear correlation has been observed between theintensity of this signal and that of the 0.9 eV PL.16 For the
sample containing Ge atoms, in addition to the signal from Si
P b centers, a broad signal appears at the low-magnetic-fieldside 共LMF兲 共e.g., x⫽0.24). The intensity of the broad signal
increases as the Ge concentration increases. For the sample
with x⫽0.41, the g value and the peak-to-peak linewidth of
the broad signal are 2.0199 and 47 G, respectively.
IV. DISCUSSION
A. Origin of the ESR signal

Although the observed ESR spectrum apparently consists of more than two peaks, we tentatively deconvoluted
the ESR spectrum into two components. All the spectra
could be well deconvoluted by two Lorentzian functions.
Figure 4 shows the g values of the high magnetic field side
共HMF兲 signal and the LMF signal as a function of Ge concentration. The broken curves are drawn to guide the eyes.
By increasing the Ge concentration, the g values of bothsignals increase. For comparison, we plotted the data for Si
P b centers and Ge P b centers from the literature on surface
oxidized porous Si1⫺x Gex , 12 an oxygen implanted Si1⫺x Gex
layer,13 surface oxidized porous Si,19 and nc-Si in SiO2 prepared by a cosputtering method.16 We can see that thepresent results agree well with the previous results, suggesting that the HMF and LMF signals can be assigned to Si and
Ge P b centers, respectively.
Since Si and Ge are mixed with each other randomly,
each signal is assumed to consist of four components due tothe difference in the arrangement of back-bond atoms. The
signals from Si P b centers consist of – Si⬅Si3 , – Si
⬅Si2 Ge, – Si⬅SiGe2 , and – Si⬅Ge3 , while those from Ge
P b centers consist of – Ge⬅Si3 , – Ge⬅Si2 Ge, – Ge
⬅SiGe2 , and – Ge⬅Ge3 . Since the spin–orbit coupling
constant of Ge 共940 cm⫺1 ) is larger than that of Si 共142cm⫺1 ) by a factor of 7, as the number of Ge atoms at the

The integrated intensities of the ESR signals from Si and
Ge P b centers are plotted in Fig. 2共b兲. The ESR intensity of
the Ge P b centers increases with Ge concentration, whilethat of the Si P b centers is nearly independent of Ge concentration, indicating that the number of Ge P b centers increases, while that of Si P b centers is constant. We can see in
Fig. 2 that the increase in the number of Ge P b centers is
associated with the quenching of the PL, suggesting that theincrease in the number of Ge P b centers is responsible for
the PL quenching.
In Fig. 2共a兲, not only the main PL band but also the 0.9
eV PL becomes weaker. As mentioned above, the 0.9 eV PL
is assigned to the recombination of photoexcited carriers via
Si P b centers. For pure nc-Si, a clear correlation has been
observed between the PL intensity and the intensity of the
ESR signal from Si P b centers; the intensity of the 0.9 eV PL
is nearly proportional to that of the ESR signal of Si P bcenters.16 Similarly, for nc-Si1⫺x Gex , it is expected that theintensity of the 0.9 eV PL shows the same Ge concentration
dependence as that of the ESR signal from Si P b centers.
However, in Fig. 2共b兲, in spite of the constant intensity of thesignal from the Si P b centers, the 0.9 eV PL becomes weakerwith Ge concentration. This result suggests the following
model for PL quenching by Si1⫺x Gex alloy formation. Ge P bcenters form defect levels in the band gap of nc-Si1⫺x Gex .
The levels are deeper in energy than those of Si P b centers.
Photoexcited electron–hole pairs in nc-Si1⫺x Gex are trappedat the Ge P b centers and recombine via the centers, if there is
at least one Ge P b center in a single nanocrystal. The recombination is made nonradiatively at temperatures higher than
10 K, because no PL signal was detected in the range between 0.4 and 0.8 eV. Under this assumption, nc-Si1⫺x Gex
in a sample can be classified into three categories. One isnc-Si1⫺x Gex which do not have any P b centers. These
nanocrystals show a band edge PL around 1.4 eV. The second kind of nanocrystals are those having at least one Si P bcenter but no Ge P b center. Since the lifetime of the bandedge PL is very long 共several tenth of ␮ s to ms兲,5,8,22 photoexcited carriers in these nanocrystals are always trapped at
the Si P b centers. Therefore, these nanocrystals do not show
a band edge PL but only the 0.9 eV PL. The last kind ofnanocrystal are those containing at least one Ge P b center. In
these nanocrystals, despite whether there exist Si P b centers,

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Toshikiyo et al.

J. Appl. Phys., Vol. 89, No. 9, 1 May 2001

photoexcited carriers always recombine via Ge P b centers.
The observed PL spectra are an ensemble of those from thesethree kinds of nanocrystals. As the Ge concentration increases, the ratio of nanocrystals having Ge P b centers increases, resulting in the quenching of both the main PL and
the PL from Si P b centers.
For pure nc-Si, passivation of dangling bond defects byhydrogen and oxygen has been reported to be effective in
improving luminescence efficiency.23,24 Recently, we demonstrated that doping of P atoms into SiO2 films containing
nc-Si brings about drastic improvement in the PL ofnc-Si.15,16 Similarly, for nc-Si1⫺x Gex , passivation of Ge P bcenters by similar approaches might be possible. These studies are now underway in our laboratory.
V. CONCLUSION

The dangling bond defects in nc-Si1⫺x Gex embedded in
SiO2 matrices were studied by ESR, and the correlation of
the defects with the PL properties was revealed. It was foundthat the ESR spectrum is a superposition of signals from Si
and Ge P b centers. The signal from the Ge P b centers increased with the Ge concentration, while that of the Si P bcenters was nearly independent of Ge concentration. By increasing the number of Ge P b centers, the PL intensity wasstrongly quenched. From the observed clear correlation between the two data, it can be concluded that Ge P b centers
act as efficient nonradiative recombination centers for photogenerated carriers, resulting in the quenching of the mainPL.
ACKNOWLEDGMENTS

This work was supported by a Grant-in-Aid for Science
Research from the Ministry of Education, Science and Culture, Japan, and a Grant for Research for the Future Programfrom Japan Society for the Promotion of Science 共JSPSRFTF 96P-00305 and JSPS-RFTF-98P-01203兲. Two of theauthors 共K.T. and S.T.兲 would like to thank the Japan Society
for the Promotion of Science for financial support.

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