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SURFACE AND INTERFACE ANALYSIS
Surf. Interface Anal. 28, 204–207 (1999)
Raman Scattering and Room-temperature
Visible Photoluminescence from Si Nanocrystals
Embedded in SiO2 Thin Films
Yinyue Wang,* Hengxiang Gong, Yinhu Yang, Yongping Guo and Runjin Gan
Department of Physics, Lanzhou University, Lanzhou 730000, People’s Republic of China
Silicon nanocrystals (nc-Si) embedded in SiO2 glassy mixture have been prepared on glass substrates and
silicon and germanium wafers by r.f. co-sputtering and post-annealing in a vacuum. Using Raman spectrometry, photoluminescence and electrical conductivity measurements, we have investigated the structures
and the optical properties of the Si/SiO2 composite films. The results show that Si nanocrystals could be
formed in Si/SiO2 composite films by thermal annealing. The growth of nc-Si is associated with annealing
temperature and silicon content. As the size of nc-Si reaches 5 nm, the intensity of photoluminescence at
room temperature is at a maximum and the emitting photon energy is 2.15 eV. Copyright  1999 John
Wiley & Sons, Ltd.
KEYWORDS: nanosilicon (nc-Si); r.f. co-sputtering technique; Raman scattering; visible room-temperature photoluminescence
INTRODUCTION
Visible room-temperature photoluminescence from porous
silicon was discovered by Canham1 in 1990. The discovery not only shows the potency of silicon in photoelectron technology, but also leads to the possibility of
a full silicon photoelectron circuit. Recently, porous silicon has been studied in increasingly greater depth, and
although no single model exists to account for the results,
the low-dimensional nano-grain is observed by scanning
electron microscopy (SEM) to be the main mechanism of
photoluminescence. Porous silicon fabricated by the wet
method is unstable and ideal electrical contact is hard to
obtain. Therefore, it is of great importance to look for a
new approach for fabricating nanosilicon and nanosiliconbased alloys to obtain strong visible photoluminescence at
room temperature.
Nanocrystal semiconductor/isolator composite films
fabricated as embedded nanocrystal (nc) semiconductor in
isolator substrate and having characteristics of nano-grain
and thin films show some distinctive optical properties
and have the potential for a wide range of applications.
Some experimental and theoretical results on the optical
properties of nano-Si2,3 and nano-Ge4 embedded in SiO2
thin films have been reported.
In the present study, we have prepared nc-Si embedded
in SiO2 thin films on glass substrates and Si wafer,
respectively, by r.f. co-sputtering and a post-annealing
treatment that was introduced to reduce silicon oxides
and result in nc-Si of different sizes in SiO2 matrices.
Raman spectrometry, photoluminescence and electrical
conductivity measurements were used to measure the
structures and the optical properties of the composite
* Correspondence to: Y. Wang, Department of Physics, Lanzhou
University, Lanzhou 730000, P.R. China.
E-mail: wangyy@lzu.edu.cn
Contract/grant sponsor: National Nature Science Foundation, China.
CCC 0142–2421/99/130204–04 $17.50
Copyright  1999 John Wiley & Sons, Ltd.
films and to determine the nc-Si size dependence of the
photoluminescence spectra.
EXPERIMENTAL
The Si/SiO2 composite films were deposited on quartz
substrates, high-resistance Si wafer and Ge wafer by an r.f.
cosputtering technique. Some SiO2 chips of 99.99% purity
placed on 99.999% pure polycrystal Si plates (100 mm
in diameter, 4 mm in thickness) were employed as the
sputtering target. To obtain a series of similar films of different Si content in the polycrystalline SiO2 , we adjusted
the distance between the target and the substrates and the
area ratio of the composite target; the substrates were also
rotated. In the deposition process, ultrahigh-purity argon
(Ar) was introduced to the reactor as the sputtering gas
and the partial Ar pressure was maintained at 30 Pa. The
power density of the r.f. plasma was maintained at 3.82 W
cm 2 . The substrate temperature was kept at ¾30 ° C by
circulating water and the thickness of the deposited films
was ¾500 nm.
The films have been grown uniformly over large samples and the isothermal annealing process of the asdeposited samples was performed by: dividing each large
uniform sample into many pieces; annealing these small
samples in a vacuum for 30 min at different temperatures; and slow cooling to room temperature in order to
reduce the ‘frozen-in’ effect caused by fast cooling. The
annealing temperature was 300–800 ° C and the temperature fluctuation was controlled within š3 ° C.
RESULTS AND DISCUSSION
Raman spectra
Figures 1 and 2 show the Raman spectra of the Si/SiO2
composite films at different annealing temperatures. From
Received 30 November 1998
Accepted 8 March 1999
Si NANOCRYSTALS IN SiO2 THIN FILMS
205
employed as a substrate. As shown in Fig. 2, the Raman
peak of Ge is at ¾300 cm 1 . The TO mode of nc-Si, which
is at 814 cm 1 for Si substrate, ranges from 802 cm 1 to
814 cm 1 with increasing Si content in the films (there
were six Si/SiO2 mixtures in our study, of which the sixth
was 100% silicon; see Fig. 4).
Room-temperature photoluminescence spectra
Figure 1. Raman spectra of a sample deposited on Si wafer at
different annealing temperatures.
Room-temperature photoluminescence was obtained using
a SPEX-140 laser spectrophotometer with an argon ion
laser ( D 514.5 nm) as the excitation source. It can be
seen in Fig. 3 that for annealing temperatures Ta > 700 ° C
and Ta < 600 ° C no useful signal exists. However, a good
signal was obtained for Ta D 650 ° C. The photoluminescence intensity dependence on the Si content is shown in
Fig. 4.
The thermal annealing process is necessary because Si
exists in the amorphous state (a-Si) in the as-deposited
Si/SiO2 composite films, with no photoluminescence signals. After thermal annealing, the metastable a-Si and SiOx
.x < 2/ in the films form an Si cluster that can transform
to small-size nc-Si through thermodynamic decomposition, diffusion and aggregation of the silicon atoms. The
amount and size of the nc-Si are related to Ta and the
silicon content. Different Si/SiO2 area ratios of the composite sputtering targets lead to different Si content in
the films. In our study, the fractions of Si in the six target compositions are: 40% (sample 1), 50% (sample 2),
60% (sample 3), 70% (sample 4), 80% (sample 5) and
100% (sample 6). From Fig. 5, it can be seen that the
Si content does not lead to any change in the position of
the photoluminescence peak, but strongly influences the
PL integral intensity. Samples 2, 3 and 4 show greater
photoluminescence integral intensity, with sample 2 displaying the highest peak intensity at 2.15 eV. The 100%
Figure 2. Raman spectra of samples deposited on Ge wafers
with different silicon content after annealing at 650 ° C.
Fig. 1 it can be seen clearly that the nc-Si (annealed at
800 ° C) longitudinal-optical (LO) and transverse-optical
(TO) modes are at ¾520 cm 1 and 814.5 cm 1 , respectively. For the LO mode, higher annealing temperature
increases the integral intensity and shifts the position to
higher values but causes a decrease in the full width at
half-maximum (FWHM). This behaviour accords with the
relation to the phonon limitation function and the intensity
of the Raman peaks.5 Upon annealing at 650 and 800 ° C,
the average sizes of the nc-Si are calculated5 to be ¾49 nm
and 10 nm, with peaks are located at 516 cm 1 and
519.5 cm 1 . To study the Raman spectra of the Si/SiO2
composite films in more detail, high-resistance Ge was
Copyright  1999 John Wiley & Sons, Ltd.
Figure 3. Photoluminescence spectra of Si/SiO2 composite films
deposited on Si wafer after annealing at different temperatures.
Surf. Interface Anal. 28, 204–207 (1999)
206
Y. WANG ET AL.
Figure 4. Photoluminescence spectra of Si/SiO2 composite films
deposited on Si wafer with different silicon content after
annealing at 650 ° C.
silicon sample (sample 6) only shows a weak photoluminescence peak.
The amount and size of the nc-Si formed after annealing
at 650 ° C may be related to the Si content. It is possible
that the average size of nc-Si in the full silicon sample is large, which accounts for the lack of an obvious
photoluminescence peak. On the other hand, the average
size in sample 1 (with the least Si content) was small,
leading to relatively weak photoluminescence intensity.
Evident peaks (in the vicinity of 17140 cm 1 with energy
of 2.12 eV) were only observed in samples 2–4, especially in sample 2. We speculate that this sample has the
right Si content to produce an average size of nc-Si in
large quantities for a strong photoluminescence.
Conductance–temperature relationship
Figure 6 shows the conductance–temperature relationship
for sample 2. The conductance of the as-deposited sample
is the greatest in value. After long annealing at 300 ° C, the
conductance decreases to a minimum. The conductance
increases after subsequent annealing at 500 ° C for 30 min,
and then decreases slightly after annealing at 700 ° C. All
the conductance–temperature relations follow the rule for
normal semiconductors
ln I / 1/T C constant
Figure 6 shows also the relation between activation energy
.Ea / and annealing temperature .Ta /. The results obtained
for samples 3 and 4 are similar to those of sample 2.
In the as-deposited sample the silicon exists as a-Si and
SiOx oxide .x < 2/, so the films are amorphous, resulting
in greater conductance. After the long anneal at 300 ° C,
Surf. Interface Anal. 28, 204–207 (1999)
Figure 5. Photoluminescence spectra of Si/SiO2 composite films
deposited on Ge wafer with different silicon content after
annealing at 650 ° C.
the Si clusters form nc-Si. The average sizes and amounts
of nc-Si increase with Ta , whereas the conductance of
the films decreases sharply, which is expected from the
conduction mechanism of the discontinuous films. When
Ta D 650 ° C the quantity of nc-Si is largest, with an
average size of ¾5 nm, leading to higher conductance
and also greater photoluminescence intensity. By contrast,
when Ta D 700 ° C, the average size increases further
but the amount decreases, causing the distance among
the Si grains to increase and hence the conductance to
decrease (the photoluminescence intensity is weak). By
contrasting Fig. 6 with Figs 3 and 5, this inherent relation
would be observed between the conductance and the
photoluminescence.
CONCLUSION
The Si nanocrystals embedded in SiO2 thin films have
been obtained by r.f. co-sputtering and post-annealing
Copyright  1999 John Wiley & Sons, Ltd.
Si NANOCRYSTALS IN SiO2 THIN FILMS
207
Figure 6. The I T relation of sample 2: silicon content was 50% after thermal annealing at different temperatures. The insert in Fig. 6
shows the activation energy .Ea / Ta relation.
treatment in a vacuum. After thermal annealing, nc-Si
forms as a result of silicon atoms diffusing and aggregating in SiO2 . The size and amount of nc-Si increase with
suitable annealing conditions .Ta D 650 ° C/. At 650 ° C
the average size is ¾5 nm and the amount is the greatest,
so that stronger visible emission (2.15 eV) is observed at
room temperature. The emission mechanism is in accordance with the quantum limitation effect.
Acknowledgements
The authors would like to thank the National Nature Science Foundation,
People’s Republic of China for financial support.
REFERENCES
1. L. T. Canham, Appl. Phys. Lett. 57, 1046 (1990).
2. Y. Maeda, Phys. Rev. B51, 1658 (1995).
3. Y. Y. Wang, Y. H. Yang and Y. P. Guo, Acta Phys. Sinica 46,
203 (1997).
Copyright  1999 John Wiley & Sons, Ltd.
4. O. Zhang, S. C. Bayliss and D. A. Hutt, Appl. Phys. Lett. 66,
1997 (1995).
5. I. H. Campbell and P. M. Fauchet, Solid State Commun. 58,
793 (1986).
Surf. Interface Anal. 28, 204–207 (1999)
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