JOURNAL OF APPLIED PHYSICSVOLUME 87, NUMBER 415 FEBRUARY 2000

Improvement in photoluminescence efficiency of SiO2 films containing Si
nanocrystals by P doping: An electron spin resonance study
Minoru Fujii,a) Atsushi Mimura, Shinji Hayashi, and Keiichi Yamamoto
Department of Electrical and Electronics Engineering, Faculty of Engineering, Kobe University, Rokkodai,Nada, Kobe 657-8501, Japan

Chika Urakawa and Hitoshi Ohta
Department of Physics, Faculty of Science, Venture Business Laboratory, Kobe University, Rokkodai, Nada,Kobe 657-8501, Japan

共Received 26 August 1999; accepted for publication 9 November 1999兲
SiO2 and phosphosilicate glass 共PSG兲 films containing Si nanocrystals 共nc-Si兲 as small as a few
nanometers were studied by electron spin resonance 共ESR兲 and photoluminescence 共PL兲, and thecorrelation between the two measurements was examined. It is shown that the incorporation of nc-Si
in SiO2 results in the drastic increase in the ESR signal; the signal is assigned to the Si dangling
bonds at the interfaces between nc-Si and matrices ( P b centers兲. The ESR signal becomes weakerby doping P into SiO2 matrices, i.e., by using PSG as matrices. By increasing the P concentration,
the ESR signal decreases further. By decreasing the ESR signal, the low-energy PL peak at 0.9 eVdecreases, while the band-edge PL at 1.4 eV increases. These results suggest that the 0.9 eV peakis related to P b centers, and that the decrease in the density of the P b centers by P doping brings
about an improvement in the band-edge PL efficiency of nc-Si. © 2000 American Institute ofPhysics. 关S0021-8979共00兲07104-8兴

I. INTRODUCTION

In our previous work, we have prepared nc-Si as small
as a few nanometers embedded in SiO2 matrices by a cosputtering method, and studied the photoluminescence 共PL兲properties.4,5 At room temperature, the nc-Si exhibited a PL
at around 1.4 eV. From the observed size dependence of thePL peak, we attributed the PL to the recombination of freeelectron–hole pairs in nc-Si 共band-edge PL兲.4,5 In addition tothe band-edge PL, an additional PL peak was observed at 0.9eV at low temperatures 共typically less than 100 K兲. At 5 K,the intensity of the low-energy peak was comparable to thatof the band-edge PL. The low energy peak is observed alsofor porous Si and is generally attributed to the recombinationof photoexcited carriers via the P b centers.5,7,8
In a previous paper,9 we reported a new method in improving the band-edge PL efficiency of oxidized nc-Si. We
employed phosphosilicate glass 共PSG兲, which is the mixtureof SiO2 and P2O5 , instead of pure SiO2 as surrounding matrices of nc-Si. It was shown that, as the P concentration
increases, the intensity of the band-edge PL increases drastically, while the P b center-related PL decreases. These results
suggest that the density of the P b centers decreases by P
doping and that this decrease results in improvement in theband-edge PL efficiency.
Although the optical data suggest a decrease in the P bcenters, direct evidence was not shown in the previous work.
Defects in Si nanostructures have been studied by electron
spin resonance 共ESR兲.6–8,10,11 The large surface area of Si
nanostructures allows the ESR study of surface and interface
defects under highly improved conditions compared withbulk samples. In this article, we will show the results of ESR
studies of the PSG films containing nc-Si. We will discuss
the effects of P doping on the density of paramagnetic de-Light emission in the near-infrared and visible regions
has been observed for a variety of Si nanostructures.1–5 Owing to the large surface area of Si nanostructures, proper
surface passivation is essential in obtaining efficient luminescence. Surface passivation controls the luminescence intensity through the elimination of competing nonradiative carrier relaxation pathways.6 Passivation by hydrogen andoxygen is reported to be effective in improving the luminescence efficiency.6 However, the hydrogen passivation is impractical for the device application, because hydrogen is easily dissociated from the surface by a low-temperature heattreatment.
Oxygen-passivated Si nanocrystals 共nc-Si兲, i.e., oxidizednc-Si, have been prepared by the oxidation of porous Si,6–8
and by plasma chemical deposition,1 cosputtering4,5,9 and ion
implantation10,11 methods. These oxidized nc-Si are reported
to exhibit strong luminescence, and the surface passivationby oxygen is considered to play an important role in thestrong luminescence. However, it is also reported that oxygen passivation induces some kinds of new defects.6–8 Oneof the well-known defects in oxidized nc-Si is the Si dangling bonds at the interface between nc-Si and SiO2 matrices( P b center兲.6–8 At room temperature, the P b center acts as anonradiative recombination center, thereby decreasing theband-edge luminescence efficiency. Therefore, by decreasing
the density of the P b center, further improvement in theband-edge luminescence efficiency of oxidized nc-Si is expected.
a兲Author to whom correspondence should be addressed; electronic mail:fujii@eedept.kobe-u.ac.jp

0021-8979/2000/87(4)/1855/3/$17.00

1855© 2000 American Institute of Physics

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J. Appl. Phys., Vol. 87, No. 4, 15 February 2000

Fujii et al.

FIG. 2. 共a兲 PL intensities of the 1.4 and 0.9 eV peaks at 5 K, and 共b兲
integrated ESR signal as a function of C P .
FIG. 1. Photoluminescence from nc-Si dispersed in PSG thin films 共a兲 atroom temperature and 共b兲 at 5 K. P concentration (C P) is changed from 0 to
1.7 mol %.

fects and reveal the mechanism of the improvement in theband-edge PL efficiency.
II. EXPERIMENTThe samples were prepared by the same method as usedin our previous work.9 Small pieces of Si chips were placedon a PSG sputtering target and they were cosputtered in Argas. The substrates were fused quartz. After the sputtering,
the films were annealed in an N2 gas atmosphere for 30 minat 1100 °C. P concentration in the films was controlled bychanging the P concentration in the PSG target. The average
P concentration in a whole film, i.e., the mol concentration ofP2O5 关C P 共mol%兲兴 was determined by an electron probe microanalysis 关JXA8900 共JEOL兲兴. The size of nc-Si was determined by cross-sectional high-resolution transmission electron microscopic 共HRTEM兲 observations 关JEM-2010
共JEOL兲兴. The HRTEM observations revealed that each nc-Si
was isolated from the others by PSG barriers several nanometers in thickness. The size of nc-Si was found to be notaffected by P doping. In this work, the diameter of nc-Si wasfixed to about 3.5 nm, and only C P was changed from 0 to
1.8 mol %.
PL spectra were measured using a Jobin Yvon HR-320
monochromator equipped with a Ge detector 关EO-817 共North
Coast兲兴. The excitation source was a 488.0 nm line of an Ar
ion laser. The spectra were measured in the temperature
range between 5 and 300 K in a continuous-flow He cryostat
关Optistat 共Oxford Instruments兲兴. X-band ESR was measured
by a conventional ESR spectrometer 关EMX 8/2.7 共Bruker兲兴at room temperature. A nuclear magnetic resonance Gauss
meter with 10⫺6 resolution was used for field monitoring.
III. RESULTS AND DISCUSSION

Figure 1共a兲 shows the room temperature PL spectra forthe samples with different C P . The spectra are the same as

those shown in Ref. 9. A peak can be observed at about 1.4eV. Although not shown here, the 1.4 eV peak shifted to ahigher energy and became intense with decreasing the size ofnc-Si. The peak energy versus size relation was in good
agreement with those reported for porous Si and nc-Si,1,2,4,5
suggesting that the PL arises from the recombination ofelectron–hole pairs confined in nc-Si 共band-edge PL兲. In Fig.
1, we can see that as C P increases, the PL intensity increases.
Figure 1共b兲 shows the PL spectra at 5 K. For the samples
with C P smaller than 1.3 mol %, a broad peak appears ataround 0.9 eV in addition to the 1.4 eV peak. The 0.9 eV
peak is generally assigned to the recombination of electron–hole pairs via P b centers. Detailed studies on the size dependence of the 0.9 eV peak were made in our previous work.5In Fig. 2, we can see that the 0.9 eV peak becomes weaker as
C P increases and almost disappears for C P⫽1.3 mol %. The
integrated intensities of the 0.9 and 1.4 eV peaks at 5 K areshown in Fig. 2共a兲 as a function of C P . The 1.4 eV peakincreases with C P , while the 0.9 eV peak decreases monotonously. The C P dependence of the 1.4 eV peak at room temperature was very similar to that at 5 K.
Figure 3 shows the ESR derivative spectra of a SiO2 film
共A兲, and SiO2 共B兲 and PSG 共C–F兲 films containing nc-Si; C P
is changed from 0 to 1.8 mol %. A factor of 10 is multiplied
to the spectra A, E and F. All the samples are annealed at1100 °C for 30 min. For the SiO2 film 共A兲, a weak ESR
signal is seen; the g factor and the linewidth are 2.0044 and7 G, respectively. The ESR signal becomes much stronger byincorporating nc-Si in the SiO2 film 共B兲, although the g factor 共2.0059兲 and the linewidth 共7.9 G兲 are not largely affected. The drastic increase in the ESR signal indicates thatthe signal arises from the defects induced by the incorporation of nc-Si in SiO2 . By doping P into samples, the ESR
signal changes gradually 共C–F兲. As C P increases, the signal
becomes weaker and for the sample with C P⫽1.8 mol % 共F兲,no ESR signals were detected. The integrated intensity of theESR signal is plotted in Fig. 2共b兲. The intensity is calibratedby simultaneously measuring the signal from a fixed amount

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Fujii et al.

J. Appl. Phys., Vol. 87, No. 4, 15 February 2000

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FIG. 4. g factor and linewidth of ESR signal as a function of C P .

FIG. 3. ESR derivative spectra of a SiO2 film 共A兲, and SiO2 共B兲 and PSG
共C兲–共F兲 films containing nc-Si. For the samples containing nc-Si, the sizeand concentration of nc-Si are fixed to almost the same and C P is changed
from 0 to 1.8 mol %.

of CuSO4 powder. We can see that the intensity decreases
drastically with C P . In Fig. 4, the g factor and the linewidth
are plotted as a function of C P . The g factor is almost independent of C P , while the linewidth increases with C P . The
reason for the increase in the linewidth is not known atpresent.
In the present samples, nc-Si are dispersed in SiO2 films.
Very similar films have been prepared by implanting Si in
SiO2 and postannealing.10,11 For the samples prepared by Si
implantation, the presence of three kinds of paramagnetic
defects is reported. These defects are Si E ⬘ centers, nonbridging oxygen hole centers 共NBOHCs兲 and P b centers.
Among three types of defects, the NBOHC- and E ⬘ -type
defects are annealed out above 500 °C, and only the P b centers remain in the samples annealed above 500 °C.10,11 Since
the present samples are annealed at 1100 °C, the NBOHCand E ⬘ -type defects are considered to be annealed out. We
can thus attribute the observed ESR signal to P b centers.
Therefore, Fig. 2 indicates that the number of P b centersdecreases by P doping.
In Fig. 2, we can see a clear correlation between theintensities of the ESR signal and those of the two PL peaks.
The 0.9 eV PL becomes weaker as the ESR signal becomes
weaker, i.e., as the number of P b centers decreases, while the1.4 eV PL becomes intense. These results clearly demonstrate that the 0.9 eV peak is related to P b centers as has
been suggested so far. Furthermore, the results suggest thatthe decrease in density of the P b centers by P doping brings
about an improvement in the band-edge PL efficiency ofnc-Si.
Although our results clearly demonstrate that the densityof P b centers decreases by P doping, its mechanism is notclear. One of the possible explanations is that the incorporation of P atoms in matrices reduces the stress at the interfaces

between nc-Si and the matrices, and that the reduction of thestress results in a decrease in the defect density.9
IV. CONCLUSION

SiO2 and PSG films containing nc-Si as small as a few
nanometers were studied by ESR and PL. It was found thatthe incorporation of nc-Si in SiO2 results in the drastic increase in the intensity of the ESR signal, which is assigned tothe P b centers. The ESR signal was found to be weak bydoping P and almost disappears for C P⫽1.8 mol %. As theESR signal became weak, the intensity of the low-energy PLat 0.9 eV decreased, while that of the band-edge PL at 1.4 eVincreased. These results suggest that the 0.9 eV peak is related to P b centers, and that the decrease in density of the P bcenters brings about a drastic improvement in the band-edge
PL efficiency of nc-Si.
ACKNOWLEDGMENTS

The authors are grateful to Kensaku Imamori for hisvaluable assistance in this work. This work was supported by
a Grant-in-Aid for Scientific Research from the Ministry ofEducation, Science, Sports and Culture, Japan, and a Grant
for Research for the Future Program from the Japan Society
for the Promotion of Science.
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