JOURNAL OF APPLIED PHYSICSVOLUME 90, NUMBER 10

15 NOVEMBER 2001

Effects of P doping on photoluminescence of Si1À x Gex alloy nanocrystals
embedded in SiO2 matrices: Improvement and degradationof luminescence efficiency
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

Kazuyuki Moriwaki
Graduate School of Science and Technology, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan

共Received 14 May 2001; accepted for publication 28 August 2001兲
The effects of P doping on photoluminescence 共PL兲 properties of Si1⫺x Gex alloy nanocrystals (ncSi1⫺x Gex ) in SiO2 thin films were studied. P doping drastically decreases the electron spinresonance 共ESR兲 signals that are assigned to the Si and Ge dangling bonds at the interfaces between
nc-Si1⫺x Gex and SiO2 matrices 共Si and Ge P b centers兲. With increasing P concentration, the signal
from the Ge P b centers are first quenched, and then the signal from the Si P b centers start to bequenched. The quenching of the ESR signals is accompanied by a drastic enhancement of the PL
intensity. The PL intensity has a maximum at a certain P concentration, which depends on the Si:Ge
ratio. By further increasing the P concentration, the PL intensity becomes weaker. In this Pconcentration range, optical absorption emerges due to the intravalley transition of free electrons
generated by the P doping. The observation of the free-electron absorption provides direct evidence
that carriers in nanometer-sized Si1⫺x Gex alloy crystals can be controlled by impurity doping.
© 2001 American Institute of Physics. 关DOI: 10.1063/1.1413486兴

I. INTRODUCTION

During the past decade, nanometer-sized Si crystals 共ncSi兲 have been intensively investigated because they are believed to be promising materials for novel optical or electrooptical devices and offer a good model system to study zerodimensional quantum size effects in indirect-gap
semiconductors. A continuous shift of the photoluminescence
共PL兲 energy from the bulk band gap to the visible region has
been observed with a decrease of the nanocrystal size.1– 6 The
observed strong size dependence indicates that the PL is dueto the recombination of excitons confined in zerodimensional Si quantum dots.
The quantum confinement effect enhances the possibilityof no-phonon 共NP兲 quasi-direct optical transitions. However,
the indirect band-gap nature of bulk Si crystal is still preserved even for nc-Si several nanometers in diameter.7 This
results in a relatively long PL lifetime, which is one of the
obstacles in achieving Si-based light-emitting devices.
Therefore a new approach to shorten the radiative exciton
lifetime further is highly desired.
NP transitions can be enhanced by Si1⫺x Gex alloy formation. In a bulk Si1⫺x Gex alloy, the enhancement of the NP
transition oscillator strength can be observed due to thebreakdown of the k-conservation rule arising from the violation of the translational symmetry of the crystalline lattice.8a兲

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

0021-8979/2001/90(10)/5147/5/$18.00

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Similar effects have been successfully observed in
nanometer-sized Si1⫺x Gex alloy crystals 共nc-Si1⫺x Gex ).9–11
In resonant PL spectra of nc-Si1⫺x Gex , structures corresponding to momentum conserving phonons are smeared out
by increasing the Ge concentration.9 Furthermore, the radiative lifetime is shortened with this increasing Ge
concentration.9–12 These results provide strong evidence thatthe oscillator strength of nc-Si is enhanced by Si1⫺x Gex alloy
formation.
However, in actual nc-Si1⫺x Gex , the luminescence efficiency is smaller than that of pure nc-Si.12 In a previous
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.13 The ESR data indicated that Ge dangling bonds
are generated at the interfaces between nc-Si1⫺x Gex and
SiO2 matrices 共Ge P b centers兲, and that the number of Ge P bcenters increases with the Ge concentration. From a comparison between these two measurements, we could conclude
that Ge P b centers act as efficient nonradiative recombination centers for photogenerated carriers, resulting in thequenching of the PL. In order to make good use of the advantages of nc-Si1⫺x Gex mentioned above, the eliminationof surface defects is indispensable.
For pure nc-Si in SiO2 matrices, the passivation of Si
dangling-bond defects 共Si P b centers兲 by hydrogen and oxygen has been reported to be effective in improving the luminescence efficiency.14,15 Recently, we demonstrated that Si
P b centers can be effectively passivated by P doping, leading© 2001 American Institute of Physics

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J. Appl. Phys., Vol. 90, No. 10, 15 November 2001

Toshikiyo et al.

to the significant enhancement of the PL efficiency.16 –18 For
nc-Si1⫺x Gex , the passivation of Ge P b centers in a similar
way might be possible.
In this work we prepared P-doped nc-Si1⫺x Gex as small
as 4 nm in diameter with different P concentrations embedded in phosphosilicate glass 共PSG兲 thin films and studied the
ESR and PL properties. It will be shown that the density of
not only Si but also Ge P b centers decreases with increasing
P concentration. The decrease in the defect density results ina dramatic improvement of the PL efficiency. However, byfurther increasing the P concentration, the PL intensity decreases. The quenching of the PL is accompanied by theappearance of optical absorption in the infrared range. The
mechanism causing this improvement and degradation of thePL efficiency by P doping will be discussed.
II. EXPERIMENT

P doped Si1⫺x Gex alloy nanocrystals were prepared by
the same method as used in our previous work.9,12 The Si,
Ge, SiO2 , and PSG 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 measurements was about 700
nm, and that for ESR and optical absorption measurements
about 8.4 ␮ m. The Ge and P concentrations 共mol %兲 in thefilms were controlled by changing the sputtering power andthe distance between the substrate and the target for the four
sputtering guns independently. After the deposition, the films
were annealed in an N2 gas atmosphere for 30 min at
1100 °C. During the annealing nc-Si1⫺x Gex were grown in
PSG matrices. The P and Ge concentrations were determinedbyelectronprobe
microanalysis
and
Raman
spectroscopy,12,19 respectively. The P concentration varied
from 0 共without P doping兲 to about 1.0 mol %. The Ge concentration (x) varied from 0.1 to 0.3. The size of ncSi1⫺x Gex was estimated from cross-sectional high-resolution
transmission electron microscopic 共HRTEM兲 images. These
HRTEM observations revealed that spherical nanocrystals as
small as 4 to 5 nm in diameter grow in amorphous PSG
matrices. Each nanocrystal was found to be isolated from theothers by PSG barriers several nanometers in thickness. The
PL spectra were measured using a single monochromatorequipped with a liquid N2 cooled Ge detector. The excitation
source was the 488.0 nm line of an Ar ion laser. The spectra
were measured in the temperature range between 10 and 300
K in a continuous-flow He cryostat. The X-band ESR was
measured by using a conventional ESR spectrometer at room
temperature. The intensity of the ESR signals was calibrated
by simultaneously measuring the signals from a fixedamount of Mn2⫹/MgO powder. The optical absorption spectra were measured in the range between 0.19 and 3.2 ␮ m. To
obtain the absorption coefficients of nc-Si1⫺x Gex the measured absorption spectra 共absorbance兲 were divided by thefilm thickness and the volume fraction of nc-Si1⫺x Gex .
III. RESULTS

Figure 1共a兲 shows PL spectra of nc-Si0.9Ge0.1 for various
P concentrations at 10 K. PL peaks can be observed at

FIG. 1. Photoluminescence from nc-Si1⫺x Gex with 共a兲 x⫽0.1 and 共b兲 x⫽0.3
dispersed in PSG thin films at 10 K.

around 1.44 and 0.9 eV. The 1.44 eV peak corresponds to theband-edge PL. The 0.9 eV peak can only be observed at low
temperatures and is generally assigned to the recombinationof photoexcited carriers via Si P b centers.16,17,20,21 Figure
1共b兲 shows PL spectra of nc-Si0.7Ge0.3 at 10 K. A band-edge
PL peak can be observed at around 1.40 eV. The integrated
intensities of the band-edge PL and the 0.9 eV PL are shownin Fig. 2 as a function of the P concentration. The verticalaxis represents the intensity with respect to the band-edge PL
intensity of nc-Si0.9Ge0.1 in SiO2 共without P doping兲. First ofall, it should be noted that the intensity of the band-edge PL

FIG. 2. Intensities of the band-edge PL (䉭 and 䉱) and Si-P b -center-related
PL (䊐 and 䊏) at 10 K as a function of P concentration. Open symbols arefor nc-Si0.9Ge0.1 and closed symbols are for nc-Si0.7Ge0.3 . The vertical axis
represents the PL intensity with respect to the band-edge PL intensity of ncSi0.9Ge0.1 in SiO2 .

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J. Appl. Phys., Vol. 90, No. 10, 15 November 2001

Toshikiyo et al.

5149

FIG. 4. Integrated intensities of ESR signals from Si (䉭 and 䉱) and Ge (䊐and 䊏) P b centers as a function of P concentration. Open symbols are for
nc-Si0.9Ge0.1 and closed symbols are for nc-Si0.7Ge0.3 . The vertical axis
represents the ESR intensity with respect to the intensity of Ge P b centers in
nc-Si0.9Ge0.1 in SiO2 .
FIG. 3. ESR derivative spectra of nc-Si1⫺x Gex dispersed in PSG thin films
at room temperature.

for nc-Si0.7Ge0.3 in SiO2 is much smaller than that for ncSi0.9Ge0.1 . This Ge concentration dependence of the PL intensity is generally observed for nc-Si1⫺x Gex in pure SiO2 .13
With increasing P concentration, the intensities of the bandedge PL for both samples first increase and then decrease.
This behavior can be observed over the entire temperature
range 共10–300 K兲. The maximum PL intensities are almost
the same for both series of samples and are also almost thesame as the maximum PL intensity measured on P doped
pure nc-Si.16,17 Since the PL intensity of nc-Si0.7Ge0.3 in pure
SiO2 is smaller than that of nc-Si0.9Ge0.1 , the degree of improvement in the band-edge PL efficiency is higher for ncSi0.7Ge0.3 ; the PL intensity of nc-Si0.9Ge0.1 is improved by afactor of 3 while that of nc-Si0.7Ge0.3 by a factor of 14. In
Fig. 2, in contrast to the band-edge PL, the P concentration
dependence of the 0.9 eV PL intensity is qualitatively different between the two samples. For nc-Si0.9Ge0.1 , the intensityof the 0.9 eV PL decreases monotonously, while for ncSi0.7Ge0.3 it has a maximum at 0.6 mol %.
Figures 3共a兲 and 3共b兲 show ESR derivative spectra of ncSi0.9Ge0.1 and nc-Si0.7Ge0.3 doped for various P concentrations, respectively. For the samples without P doping, asymmetric ESR signals can be observed; the g value and thepeak-to-peak linewidth are 2.0058 and 10.7 G for ncSi0.9Ge0.1 , and 2.0103 and 36.6 G for nc-Si0.7Ge0.3 , respectively. These signals can be assigned to a superposition of
signals from Si and Ge P b centers.13 As the P concentration
increases, the intensity of the ESR signals decreases, while
the g values and the peak-to-peak linewidths are almost constant over the entire P concentration range. Figure 4 shows
integrated intensities of ESR signals for Si and Ge P b centers
as a function of the P concentration. The intensities are obtained by deconvoluting an ESR spectrum into two Lorentzian functions. The vertical axis represents the ESR intensitywith respect to the intensity of the Ge P b centers in ncSi0.9Ge0.1 in pure SiO2 . It is worth noting that the ESR in-

tensities of nc-Si0.7Ge0.3 in pure SiO2 共in particular the intensity of the Ge P b centers兲 are much larger than those of ncSi0.9Ge0.1 in pure SiO2 . In nc-Si0.9Ge0.1 , the intensities of the
Si and Ge P b centers decrease at almost the same rate withincreasing P concentration, indicating that the number of Si
and Ge P b centers decreases monotonously with the P doping level. On the other hand, in nc-Si0.7Ge0.3 , the Si and Ge
P b centers exhibit a different P concentration dependence;
the intensity of the signal from the Ge P b centers decreases
rapidly with increasing P concentration, while that from the
Si P b centers remains almost constant within a low P concentration range and then decreases gradually with further
increases in the P concentration.
Figure 5 shows optical absorption spectra for samples
with different P concentrations. The periodic structures are

FIG. 5. Optical absorption spectra for nc-Si1⫺x Gex dispersed in PSG thinfilms at room temperature.

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5150

J. Appl. Phys., Vol. 90, No. 10, 15 November 2001

FIG. 6. Schematic band diagrams of nanocrystals 共a兲 without defects, 共b兲
with a Si P b center, 共c兲 with a Ge P b center, and 共d兲 with Si and Ge P bcenters.

due to the interference of the incidence light inside the films.
For those samples with P concentrations smaller than 0.6 mol%, the absorption decreases monotonically towards the infrared region. This absorption is due to the valence-toconduction-band transitions in nc-Si1⫺x Gex and/or transitions related to band tails or defects levels. On the other
hand, for those samples with P concentrations larger than 0.6
mol %, a smoothly rising absorption continuum appears in
the infrared region. A very similar absorption in the infrared
range has been observed for P-doped nc-Si.18 The infraredabsorption is assigned to the intravalley transition of electrons generated by P doping in the conduction band, i.e.,
free-electron absorption in nc-Si. The free-electron absorption is characterized by a monotonic, structureless spectrum
that grows as ␭ p , where ␭ is the photon wavelength and thepower p depends on the mode of scattering. The present
infrared absorption can be well fitted with ␭ 1.5, suggestingthat this absorption is accompanied by scattering by acoustic
phonons.22
IV. DISCUSSION

The observed P concentration dependences of the PL and
ESR properties as well as optical absorption spectra can consistently be explained by the following model. From thepoint of view of PL properties, nc-Si embedded in SiO2 matrices 共without Ge doping兲 can be classified into two categories. One is nc-Si without nonradiative recombination centers关Fig. 6共a兲兴, which shows a slow band-edge PL at around 1.4
eV.5 The other is nc-Si having at least one Si P b center 关Fig.
6共b兲兴. In these nanocrystals, photoexcited carriers are always
trapped at the Si P b centers and recombine via the centers.
These nanocrystals do not show the band-edge PL but show
only the 0.9 eV PL at low temperatures.

Toshikiyo et al.

Through Si1⫺x Gex alloy formation, Ge P b centers are
introduced into both kinds of nanocrystals. As a result,nanocrystals with Ge P b centers 关Fig. 6共c兲兴 and those with Si
and Ge P b centers 关Fig. 6共d兲兴 are formed. In these nanocrystals, photoexcited carriers are preferentially trapped at the Ge
P b centers and recombine nonradiatively even at low
temperatures.13 The present samples are ensembles of thesefour kinds of nanocrystals. With increasing Ge concentration,
the number of nanocrystals with Ge P b centers 关Figs. 6共c兲and 6共d兲兴 increases, resulting in the quenching of both the
band-edge PL and Si-P b -center-related PL. In the present
work we studied two series of samples with different Ge
concentrations 共Si0.9Ge0.1 and Si0.7Ge0.3). In the sample withthe low Ge concentration, nanocrystals without defects 关Fig.
6共a兲兴 and with only Si P b centers 关Fig. 6共b兲兴 were considered
to be present to some extent. On the other hand, in thesample with the high Ge concentration, almost all of thenanocrystals were considered to have Ge P b centers, resulting in an extremely weak band-edge PL and SiP b -center-related PL 共see Fig. 2兲.
As can be clearly seen in Fig. 4, not only Si P b centers
but also Ge P b centers are passivated by P doping. The passivation is considered to be made electrically, i.e., electrons
supplied by the P doping become trapped at the P b centersand disactivate them. In nc-Si0.7Ge0.3 , the quenching of the
ESR signal from a Ge P b center is much faster than thatfrom a Si P b center. This result indicates that, if both Si and
Ge P b centers exist in one nc-Si1⫺x Gex , electrons suppliedby P doping are preferentially captured by the Ge P b centers.
After the passivation of the Ge P b centers is completed, thepassivation of the Si P b centers starts.
This model can well explain the P concentration dependence of PL properties. Preferential passivation of Ge P bcenters increases the number of nanocrystals that do not haveelectrically active Ge P b centers. As a result, in ncSi0.7Ge0.3 , the intensities of both the band-edge PL and SiP b -center-related PL increase by P doping, i.e., the Si P bcenters remain active even if the P concentration is relatively
high. On the other hand, in nc-Si0.9Ge0.1 , the number ofnanocrystals having Ge P b centers is small. Therefore thepassivation of the Si P b centers starts at a low P concentration, resulting in the quenching of the Si-P b -center-related
PL within the low P concentration region.
At a high P concentration, after the passivation of boththe Si and Ge P b centers is completed, electrically active P
atoms supply free electrons in nc-Si1⫺x Gex . The free-carrier
absorption in Fig. 5 at a high P concentration gives direct
evidence that free electrons are generated in nc-Si1⫺x Gex . Ifsuch free electrons are supplied, three-body Auger recombination becomes possible among the electrons and photoexcited electron–hole pairs 共eeh Auger process兲, i.e., the recombination energy of an electron–hole pair can betransferred to an electron with its following excitation to ahigher energy in the conduction band. The lifetime of the eeh
Auger recombination process in nc-Si is in the range of 0.1
and 1 ns.23 This lifetime is several orders of magnitude
shorter than the radiative lifetime of excitons in ncSi1⫺x Gex 9,12 and is also shorter than the Si-P b -center-related
PL.24 Therefore the generation of free electrons results in a

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Toshikiyo et al.

J. Appl. Phys., Vol. 90, No. 10, 15 November 2001

significant decrease in the PL efficiency. The observed
quenching of the PL intensity at a high P concentration canbe explained by this model.
In Fig. 2 the value of the P concentration having the
maximum PL intensity depends on the Ge concentration. The
PL intensity of nc-Si0.9Ge0.1 has a maximum at 0.3 mol %,while that of nc-Si0.7Ge0.3 has one at 0.6 mol %. This indicates that a larger number of P atoms are required for ncSi0.7Ge0.3 than for nc-Si0.9Ge0.1 to passivate Si and Ge P bcenters. This result is consistent with ESR data. As can beseen in Fig. 4, the total number of Si and Ge P b centers for
nc-Si0.7Ge0.3 in pure SiO2 is much larger than that for ncSi0.9Ge0.1 in pure SiO2 . Therefore to complete the passivation of all of the centers, a larger number of P atoms arenecessary, resulting in a shift of the PL maximum to a high P
concentration region.
V. CONCLUSION

The effects of P doping on the PL properties of ncSi1⫺x Gex in SiO2 thin films were studied. It was found that
doped P atoms can play several different roles depending on
the surface termination conditions. If P atoms are doped in
nc-Si1⫺x Gex with Si and Ge P b centers, electrons suppliedby the P doping are preferentially captured by the Ge P bcenters and disactivate the centers. After the passivation of
the Ge P b centers is completed, the passivation of the Si P bcenters starts. This results in an improvement in the PL efficiency. Since the number of Ge P b centers increases withincreasing Ge concentration, a larger number of P atoms arenecessary to completely passivate all of the P b centers, i.e.,
to obtain the maximum PL efficiency. The present results
demonstrate that P doping is an effective method for passivating the P b centers and improving the PL efficiency of
Si1⫺x Gex alloy nanocrystals. After the passivation of all P bcenters, further doped P atoms supply free electrons in ncSi1⫺x Gex . The free-electron absorption observation provides
direct evidence that carriers in nanometer-sized Si1⫺x Gex alloy crystals can be supplied by impurity doping.
VI. ACKNOWLEDGMENTS

The authors are grateful to Kazutomi Shigeeda for his
valuable assistance in this work. This work was supported by
a Grant-in-Aid for Science Research from the Ministry of
Education, Culture, Sports, Science and Technology, Japan,

5151

and a Grant for Research for the Future Program from theJapan Society for the Promotion of Science 共JSPS-RFTF98P-01203兲. K. T. would like to thank the Japan Society for
the Promotion of Science for their financial support.
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