Phase Transformation of Germanium Ultrafine Particles at High Temperature
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223 Mat. Res. Soc. Symp. Proc. Vol. 405 0 1996 Materials Research Society
EXPERIMENTAL The cluster-beam evaporation technique to deposit Ge nanostructures was described in detail elsewhere [1]. During the deposition, the substrate temperature was kept either at room or liquid nitrogen temperature. Hereafter, we refer to the Ge films deposited at room and liquid nitrogen temperature as Ge-RT and Ge-LNT, respectively Some of Ge-RT and Ge-LNT samples were annealed in argon (Ar) atmosphere at various temperatures for 60 min. P-type (100) Si substrates were used for Raman spectroscopy, electron spectroscopy for chemical analysis (ESCA) and photoluminescence (PL) measurements, while quartz substrates were used for optical absorption measurements. The Raman spectroscopy was carried out by using the 514.5 nm line of Ar ion laser with an intensity of 400 mW. The 313 nm line of a mercury lamp selected with an optical bandpass filter, whose full width at half maximum (FWHM) was 10 nm, was used as irradiation light for photo-oxidation [4]. and an excitation source for PL measurements. The rise in the sample temperature during the Raman and PL measurements was negligible. Both measurements were carried out in the air at room temperature. RESULTS AND DISCUSSION The Raman spectra of the as-deposited Ge-RT and Ge-LNT are shown in Fig. 1. The double peak at 267 and 258 cm- 1 for Ge-RT and Ge-LNT suggests that the structure is not the ordinary diamond but the tetragonal structure, which is present in a high-pressure form of Ge (known as ST-12). The results of Raman spectroscopy are consistent with our earlier report [1]. In Ge, the tetragonal structure can accommodate Ge atoms by 20 % more than the diamond structure [6] and is known to be metastable.
(a) "C:
_(b) CZ,
(C C:
200
300
Raman Shift (cm- 1) Fig. 1. Raman spectra of (b) Ge-RT and (c) Ge-LNT. The Raman spectra of (a) bulk Ge and (d) a high-pressure form of Ge [5] are also shown for comparison.
224
Figure 2 shows the Raman spectra of Ge-RT and Ge-LNT annealed in Ar atmosphere for 60 min. No Raman peaks of Ge-LNT are observed for an annealing temperature of 700 'C or higher. A decrease in the film thickness of Ge-LNT was found due to evaporation of GeOx during
deposited -Ia
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as-deposited 'i)
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CO-
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'
•,• 0 ., 700 C
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0
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C
700 °C
200
400
360
200
360 .0..
Raman Shift (cm-')
Raman Shift (cm-1)
(a)
(b)
400
Fig. 2. Raman spectra of (a) Ge-RT and (b) Ge-LNT annealed in Ar atmosphere for 60 min. The annealing temperatures are shown in the figures. the anneal. It is speculated from volatility of GeO that GeOx is volatile particularly at temperatures higher than 400 'C As seen in the ESCA spectra of the surfaces of Ge-RT and Ge-LNT in Fig. 3,
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Ge
2
.-.
GeO2
Ge
CC
C__
as-deposited
.CO
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-
a
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CnC CC
34
30 32 Binding Energy (eV)
34
28
30 32 Binding Energy (eV)
28
(b)
(a)
Fig. 3. Ge3d ESCA spectra of the surfaces of (a) Ge-RT and (b) Ge-LNT. The ESCA spectra are shown for both as-deposited and annea
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