Visible photoluminescence in carbon-implanted thermal SiO2 films

Article abstract of DOI:10.1002/(SICI)1521-396X(199808)168:2<531::AID-PSSA531>3.0.CO;2-V

The structures formed after the implantation of carbon in thermal SiO₂ and annealing presented visible photoluminescence (PL) bands at room temperature. The peak and intensity of PL bands depended strongly on the temperature of annealing. In ad

Full text of DOI:10.1002/(SICI)1521-396X(199808)168:2<531::AID-PSSA531>3.0.CO;2-V

Y.H. Yu et al.: Visible Photoluminescence in Carbon-Implanted Thermal SiO2 Films
531
phys. stat. sol. (a) 168, 531 (1998)
Subject classification: 61.72.Ww; 78.55.Hx; S10.1
Visible Photoluminescence
in Carbon-Implanted Thermal SiO2 Films
Y.H. Yu (a, b), S.P. Wong (b), and I.H. Wilson (b)
(a) Ion Beam Laboratory, Shanghai Institute of Metallurgy, Chinese Academy of Sciences,
Shanghai 200050, People's Republic of China
(b) Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin,
N.T., Hong Kong, People's Republic of China
(Received July 29, 1997; in revised form March 10, 1998)
The structures formed after the implantation of carbon in thermal SiO2 and annealing presented
visible photoluminescence (PL) bands at room temperature. The peak and intensity of PL bands
depended strongly on the temperature of annealing. In addition, very weak PL bands were ob-
served after the implantation of argon in thermal SiO2 and similar annealing. HRTEM was also
used to characterize the microstructure of the carbon or argon implanted thermal SiO2 films. These
observations showed that carbon aggregates were probably the origin of visible PL bands.
1. Introduction
In recent years, there has been considerable interest in semiconductor nanostructures
such as Ge [1], C [2] or Si ultrafine clusters [3 to 6], which can be synthesized by
several methods and present strong visible photoluminescence. Nanometer sized clus-
ters show unique electrical and optical properties which are not observed in bulk mate-
rials. Although these nanoclusters have been studied extensively, the origin and mech-
anism of the photoluminescence is still controversial.
Another novel approach to produce nanoclusters is ion implantation. Important ad-
vantages of this method are exact control over the quantities and depth distributions of
the implanted ions and its full compatibility with Si technology. Ion beam synthesis of
nanoclusters is a potential candidate for the manufacture of nanosystems not only for
basic research but also for the application for optoelectronic devices.
In this paper, we introduce, for the first time, the observation of visible PL bands in
carbon implanted thermal SiO2 films. The results are compared to those obtained for
argon implanted SiO2 films, where only defects are formed. Furthermore, experiments
have been performed in detail to find out whether the visible PL comes from carbon
nanoclusters, or defects, or thermal SiO2 films after high temperature annealing.
2. Experimental
SiO2 films of 400 nm thickness were grown on silicon h100i wafers by wet oxidation at
ꢀ
950 C. One set of samples was implanted with carbon at an energy of 50 keV and a
dose of 6 Â 1016cm±±2. The choice of energy ensured that the carbon ions come to rest
within the SiO2 films as the mean projected range of 50 keV carbon ions in SiO2 films

532
Y.H. Yu, S.P. Wong, and I.H. Wilson
is about 170 nm, as determined by TRIM [7]. The other set was implanted with
1.4 Â 1016cm±±2 of 150 keV Ar ions. The energy and dose was chosen using TRIM so
that the projected range, the straggle and the peak concentration of atomic displace-
ments would closely resemble the atomic displacement profiles created by the carbon
ion implantation. Pieces of the samples were annealed at different temperatures in a
conventional furnace under nitrogen atmosphere. The temperature was between 800
ꢀ
and 1000 C and annealing time was 20 min. The PL experiments were performed at
room temperature exciting the sample with a He±Cd laser working at a wavelength of
325 nm. For structural characterization, HRTEM (High Resolution Transmission Elec-
tron Microscopy) was performed.
3. Results and Discussion
Fig. 1 shows the room-temperature PL spectra of thermal SiO2 films implanted with
carbon. The spectra are plotted with the annealing temperature as parameter. The an-
nealing time was of 20 min for all samples. As shown in Fig. 1, the as-implanted sample
showed a broad band centered at 490 nm, and the intensity of this band increased
ꢀ
rapidly with the increase of annealing temperature. After annealing at over 800 C, a
lower-intensity peak below 400 nm and another broad peak of much reduced intensity
at 720 nm appeared. The cut-off on the short-wavelength side below 400 nm was
caused by using an edge filter during the measurements. For comparison, the PL spec-
tra of the SiO2 films annealed at the corresponding temperature for 20 min was also
measured. As shown in Fig. 1, the as-grown one showed a weak band centered at
460 nm, and its intensity decreased rapidly with increasing annealing temperature.
ꢀ
After annealing at 1000 C, no PL band was measured. It was found that prior to the
ion implantation, the thermal SiO2 films exhibited a weak PL band at 460 nm due to
pre-existing defects and the intensity of this band decreased after annealing as repair-
ing of the pre-existing defects
was expected.
It is well known that ion im-
plantation creates a defect den-
sity in the SiO2 network that far
exceeds the equilibrium value. As
is already known that some de-
fects in SiO2 exhibit visible lumi-
nescence [8], it is very important
to characterize whether the
photoluminescence observed in
Fig. 1. PL spectra of carbon-im-
planted SiO2 films at room tempera-
ture

Visible Photoluminescence in Carbon-Implanted Thermal SiO2 Films
533
Fig. 2. PL spectra of argon-implanted
SiO2 films at room temperature
carbon implanted thermal SiO2
films is due to these defects or
not. To address this case, 150 keV
argon ion implantation into 400 nm
thermal SiO2 films on Si at a dose
of 1.4 Â 1016 cm±±2 was performed.
The energy and dose were care-
fully chosen using the TRIM, such
that the projected range, the strag-
gle, and the peak concentration of
atomic displacement profile is cre-
ated by the carbon ion implanta-
tion. Fig. 2 shows the room-tem-
perature PL spectra of thermal
SiO2 films implanted with argon
ions and annealed at the same conditions as shown in Fig. 1. As shown in Fig. 2, the as-
implanted one showed a broad band with a peak at 650 nm. The origin of this band
was related to the damage and defects produced by ion implantation (Non-Bridging
ꢀ
Oxygen Hole center, NBOH) [9]. This band disappeared after annealing over 800 C as
a result of repairing the irradiation-induced damage and defects.
ꢀ
Meanwhile, after annealing over 800 C, a weak and clearly defined band occurred at
the same wavelength as in thermal SiO2 films. The intensity of the band at 460 nm kept
unchanged when the annealing temperature varies from 800 to 1000ꢀC. It seems as if
the irradiation-induced damage passive the pre-exꢀisting defects in the thermal SiO2
more pre-existing defects exist even after the 1000 C treatment. Based upon the data
shown in Figs. 1 and 2, it was found that the structures formed after carbon implanta-
tion in thermal SiO2 and after annealing, which present visible photoluminescence, are
not due to the microstructure change of thermal SiO2 caused by high temperature an-
nealing and defects produced by ion implantation. As the blue PL emission from the
SiC nanocrystals of the porous materials was reported [10, 11], it is also important for
us to discuss whether SiC clusters exist in carbon implanted SiO2 or not. No discernible
traces of Si±C bonds could be detected by infrared (FTIR) measurements. This result
suggests that under these experimental conditions, SiC clusters are not formed or at
least, the amount of the clusters is so small to be undetectable. Furthermore, if we
consider the energy of Si±Si, Si±C and C±C bonds (2.36, 3.21 and 3.70 eV, respec-
tively) and maximum atomic concentrations of the implanted ions in the projected
range, phase separation in SiC and carbon aggregates is thermodynamically favorable
(i.e. Si±C bonds are stronger than Si±Si ones). However, we have to consider the slow
kinetics for SiC formation at 1000 ꢀC from the possible reactions between implanted
ions and the matrix [12]. Then, it was found that the annealing temperature should be
as high as 1300 ꢀC so that the reaction could be kinetically possible. HRTEM was
carried out on the samples similar to those in Fig. 1 in order to find out microstructures

534
Y.H. Yu et al.: Visible Photoluminescence in Carbon-Implanted Thermal SiO2 Films
of the carbon implanted thermal SiO2. No precipitates could be observed in both the
as-implanted and the annealed samples. It may be concluded that there are amorphous
carbon clusters existing in thermal SiO2 after carbon implantation. Our analysis gives
evidence that visible photoluminescence may be related to the amorphous carbon clus-
ters.
4. Conclusion
Visible photoluminescence at room temperature in carbon-implanted thermal SiO2 was
observed. The peak and intensity of PL bands depended strongly on the temperature of
annealing. There are only the very weak PL bands observed after the implantation of
argon in thermal SiO2 and similar annealing. HRTEM measurements revealed that car-
bon ion implantation into thermal SiO2 films gave rise to the formation of amorphous
carbon clusters. These observations showed that carbon aggregates were probably the
origin of visible PL bands in carbon-implanted thermal SiO2 films.
References
[1] Y. Maeda, N. Tsukamoto, Y. Yazawa, Y. Kanemitsu, and Y. Masumoto, Appl. Phys. Lett. 59,
3168 (1991).
[2] S. Hayashi, M. Kataoka, and K. Yamamoto, Jpn. J. Appl. Phys. 32, L274 (1993).
[3] L.T. Canham, Appl. Phys. Lett. 57, 1046 (1990).
[4] W. Skorupa, R.A. Yankov, I.E. Tyschenko, H. Frob, T. Bohme, and K. Leo, Appl. Phys. Lett.
68, 2410 (1996).
[5] T. Komoda, J. Weber, K.P. Homewood, P.L.F. Hemment, and B.J. Sealy, Nucl. Instrum. and
Methods B120, 93 (1996).
[6] T. Fischer, V. Petrova-Koch, K. Shcheglov, M.S. Brandt, and F. Koch, Thin Solid Films 276,
100 (1996).
[7] J.F. Ziegler, J.P. Biersack, and U. Littmark, The Stopping and Range of Ions in Solids, Perga-
mon Press, New York 1985.
[8] K. Awazu, K. Muta, and H. Kawazoe, J. Appl. Phys. 74, 2237 (1993).
[9] S. Bota, B. Garrido, A. Baravan, P.P. Koronov, and J.R. Morante, Solid State Electronics 39,
355 (1996).
[10] L.S. Liao, X.M. Bao, Z.F. Yang, and N.B. Min, Appl. Phys. Lett. 64, 226 (1994).
[11] V. Petrova-Koch, O. Sreseli, G. Polisski, D. Kovalev, T. Muschik, and F. Koch, Thin Solid
Films 255, 107 (1995).
[12] B. Garrido, M. Lopez, S. Ferre, A. Romano-Rodriguez, A. Perez-Rodriguez, P. Ruterana, and
J. R. Morante, Nucl. Instrum. and Methods B120, 101 (1996).