Metastable Amorphous SiO 2 created by Ion Bombardment
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infrared (IR) spectroscopy and electron spin resonance (ESR) using various kinds of MeV ion beams. The results are discussed in term of densification and oxygen vacancies. EXPERIMENT SiO 2 films were formed on (100) silicon wafers by thermal oxidation at 10000C for 3 hours. Commercial silicon wafers are usually doped with B or P and also contain oxygen impurities. To avoid oxygen impurity and dopants in silicon wafers, we used pure silicon wafers having high resistance and fabricated by the Floating Zone method and suitable for FlIR and ESR. This specially made silicon wafer was fabricated by Shin-Etsu Semiconductor Co. LTD.. Oxidation was performed to obtain -200nm thick oxide film. Ion bombardment was performed at room temperature and at a residual pressure below lx 10-4Pa. 10MeV H+, 15MeV He+, 2MeV Li+, 4MeV C+, 20MeV Si4+ , and 30MeV Si6+ beams were generated with the Universitd de Montral 6MV Tandem accelerator. Electron spin resonance (ESR) measurements were carded out with a standard X-band Brucker ESP-300E spectrometer at room temperature. Absolute spin concentrations were obtained by double numerical integration of the experimental first-derivative spectra. The optimum power needed to obtain the spectra was 20IsW. Relative 21 Mat. Res. Soc. Symp. Proc. Vol. 504 01998 Materials Research Society
error of the ESR signal intensity is within 30%. Infrared (IR) absorption was measured with a Fourier transform IR spectro-photometer using light at normal incidence. Isochronal annealing was performed at 350, 500, 600, and 800'C for 2 hours at a residual pressure below lx 10-4Pa to avoid the effect of oxidation by residual air. RESULTS Fig. I shows the IR absorption spectra of an a-Si02 film before and after irradiation with 2MeV Li+. There are two first order absorptions in the IR absorption spectra at 1078cm-1 and at 805cm- 1, which were labeled o4 and 0,3, respectively [1]. A decrease of frequency of a04 and an increase of frequency of "s were observed to occur simultaneously with increase of
dosage. Fig.2 shows the frequency shift of (o4 in the IR absorption spectra as a function of dosage. The closed circle stands for the frequency peak at 1078cm- 1 of a virgin sample as occurring at a dose of 1012 cm-2 . No change of the IR-spectrum contour was observed after 0.4 -
0 4
0.35 :
virgin ,3x1.0,,12
0.3
-
,.:o)4
-2 ------ M'..-"
2 3 x 1013cm-2"----•.,
,"
14
2
Fig.l IR spectra of a-SiO 2
3x 10 cm-
.
10 11511cm2C." :/""0.25"-2 is8ix
0 0.25
E o 0.2
I
films
2cm"2
S.
-Tew
before
and
after
ir-
of 2MeV Li+ ions. two aoi absorption pa peaks are
:."radiation
• ':-The
0.1 5
labeled 0)3 and 04.
0. 0.0 5
.•
1200
V1300
1100
1000
800
900
700
Wavenumber (cm-t)
1080
-.
----
1075
Virgin
1070
A,
1065
"t1
-- - --
\., 10MeV H"15MeV He+
-
"2MeV Li+ \
Fig.2 Frequency of
4MeVC
1060' 1055
30MeV Si64."
1050' 1045-104(OT . .... 1•0-•- . ....................... 3 tol14
W12
io1
0o4
as a
function of cumulative fluence of ion bombardment.
l i-... 5
o
e
2
Fluence (cm- )
22
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