Spatially selective formation of microcrystalline germanium by laser-induced pyrolysis of organogermanium nanocluster film

Article abstract of DOI:10.1246/cl.2002.662

A new method to prepare microcrystalline Ge film by laser-induced pyrolysis of spin-coated organogermanium nanocluster film was developed, which is applicable to the spatially selective formation of microcrystalline Ge micropatterns.

Full text of DOI:10.1246/cl.2002.662

662
Chemistry Letters 2002
Spatially Selective Formation of Microcrystalline Germanium by Laser-Induced Pyrolysis of
Organogermanium Nanocluster Film
Akira Watanabe,ꢀ Masashi Unno, Fusao Hojo,y and Takao Miway
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Aoba-ku, Sendai 980-8577
yHitachi Research Laboratory, Hitachi Ltd., Hitachi, Ibaraki 319-1292
(Received February 18, 2002; CL-020153)
A new method to prepare microcrystalline Ge film by laser-
induced pyrolysis of spin-coated organogermanium nanocluster
film was developed, which is applicable to the spatially selective
formation of microcrystalline Ge micropatterns.
300 cmꢂ1, respectivelly.5{8 In Figure 1(a), a shoulder around
280cm ꢂ1 is attributed to the a-Ge lattice of the OGE, where other
bands are assigned to Ge–C (517 cmꢂ1) and C–C (789, 1170, and
1416 cmꢂ1) bonds. By preheating at 200 ꢁC, the Raman band of a-
Ge at 280cm ꢂ1 becomes significant accompanying the disap-
pearance of the Raman band above 500 cmꢂ1 as shown in Figure
1(b). The unheated and preheated OGE films were irradiated with
a 355 nm laser pulse (Nd:YAG laser, FWHM 6 ns). The laser
energy density is ca. 60mJ/cm 2 and the number of the laser pulse
irradiation is one. For both films, a sharp Raman band is observed
at 300 cmꢂ1 as shown in Figure 1(c) and (d). Judging from the
Raman band tailing to lower wavenumbers than 300 cmꢂ1, the
Raman band should be assigned to that of ꢀc-Ge.
In the case of the unheated precursor film, Raman bands of
carbon in the region of 1200 to 1600 cmꢂ1 are scarcely observed
as shown in Figure 1(c). On the other hand, preheated precursor
film shows Raman bands of carbon at 1329 and 1553 cmꢂ1 as
shown in Figure 1(d). Judging from the broadness of the Raman
bands, it is assigned to an amorphous carbon.7 Such a remarkable
difference can be explained by considering the precursor
In recent years, the Si–Ge alloy system has received
increased attention for possible applications in the semiconductor
devices due to its interesting properties. The structural and
chemical properties of Si–Ge are similar to Si, which makes this
material system compatible with the Si technology.1 The interest
in Ge is stimulated by the application to Si–Ge alloys. In previous
papers, we have reported the formation of Si, Ge, and Si–Ge
alloys using organo-silicon and germanium nanoclusters as
precursors.2{4 These hyperbranched organometallic polymers
have a Si (or Ge) core surrounded by organic side chains, which
give the solubility in common organic solvents.2{7 The inorganic
films were prepared by heat treatment of spin-coated films of
organo-silicon (or germanium) nanocluster to eliminate the
organic side chains. In this paper, we report a new method to form
Ge film by combination of organogermanium nanocluster (OGE)
and laser-induced pyrolysis technique, where laser-irradiated
area of OGE film is converted to microcrystalline Ge (ꢀc-Ge)
film with spatial selectivity.
The Ge cluster was prepared as dispersion in THF solution by
the reaction of GeCl4 with Mg metal under ultrasonic field. By
addition of tert-butyl bromide to the solution, Cl groups around
the Ge cluster were replaced by tert-butyl groups and the tert-
butyl-substituted germanium cluster was dissolved into the
solution. The detailed procedures are described elsewhere.4 The
molecular weight and the distribution determined by gel
permeation chromatography (GPC) analysis are 1130and 1.14,
respectively. By elemental analysis, the composition of the OGE
is determined to be Ge1(C4H9)0:78. Thin films of OGE (0.27 ꢀm
in thickness) were prepared by spin coating on a quartz substrate
from a 10wt% toluene solution. The preheating of the film was
carried out at 200 ꢁC for 30min in vacuo ( 10ꢂ6 torr). The color of
the thin film changed from pale yellow to brownish yellow after
the preheating. Raman spectra were measured by a micro-Raman
apparatus with a spectrometer and a liquid nitrogen-cooled
charge-coupled device detector. All samples were excited with
the 514.5 nm line available from an argon ion laser. A
metallograph with an objective (ꢃ100) was used for focusing
the excitation laser beam on a sample and for collecting the
Raman scattering.
The structural changes of the OGE by pyrolysis were studied
by Raman spectroscopy. It is known that the transverse optical
(TO)-phonon bands of Ge-Ge lattice of amorphous germanium
(a-Ge) and crystalline germanium (c-Ge) are observed at 280and
Figure 1. Raman spectra of OGE films unheated (a),
preheated at 200 ꢁC (b), laser-induced pyrolyzed
without preheating (c), and laser-induced pyrolyzed
after preheating at 200 ꢁC (d).
Copyright Ó 2002 The Chemical Society of Japan

Chemistry Letters 2002
663
structure. In Figure 1(b), the preheated film shows only a weak
Raman band of Ge–C bond at 537 cmꢂ1 accompanying the
disappearance of the bands attributed to C–C bonds. This must be
due to the conversion of tert-butyl group to Ge–C network
structure because the cross section of the vibration of Ge–C
network is very small.9 The Ge–C structure is expected to be a
cross-link structure where carbon enters the germanium network.
The laser-induced pyrolysis converted the Ge–C structure to
amorphous carbon. Contrary to this, the organic substituent is
effectively eliminated and the crystallization of Ge is observed in
the case of the laser-induced pyrolysis of unheated OGE film. The
Ge–C network structure is the reason why the elimination of the
carbon from the preheated precursor film is more difficult than
that from the unheated precursor film.
Figure 2(a) and (b) show the optical micrographs of films
obtained by laser-induced pyrolysis using unheated and preheated
precursor films, respectively. Although the formation of the large
grains which correspond to ꢀc-Ge is observed in both cases, the
size of the grain is different. The unheated OGE gives lager grain
than that of the preheated OGE. This difference is due to the cross-
linkage of preheated OGE. The cross-linkage of Ge–C network
structure reduces the melting of the precursor film during laser
irradiation and then decreases the grain size.
Figure 3. Raman spectra of the film obtained by Ar
ion laser-induced pyrolysis of OGE after preheating at
200 ꢁC. (a) unirradiated region, (b) irradiatted region.
film. The width of the line corresponds to the diameter of the
focused laser beam. Grains are formed symmetrically around the
centerline where the energy density is maximum. The Raman
spectra of irradiated and unirradiated regions are shown in Figure
3. In the measurements, the excitation energy density of laser
beam was lowered enough to inhibit the laser-induced structural
change. The sharp band at 300 cmꢂ1 of irradiated region [Figure
3(b)] is assigned to ꢀc-Ge, while broad band at 280cm ꢂ1 of
unirradiated region [Figure 3(a)] is assigned to a-Ge. This result
suggests that spatially selective formation of ꢀc-Ge can be
achieved by laser-induced pyrolysis of OGE film. In this
preliminarily experiment, a preheated OGE film was used as a
precursor because the unheated OGE film (0.27 ꢀm in thickness)
has low absorbance at 514.5 nm. The optical densities of the
unheated and the preheated OGE films in this study are 0.045 and
0.079 at 514.5 nm, respectively. The optimization of the
wavelength of the CW laser beam and the optical instruments
enables to depict the micropattern of ꢀc-Ge by scanning the laser
beam on the OGE precursor film.
The laser-induced pyrolysis of OGE film is also caused by
focusing the CW (continuous wave) laser beam. Figure 2(c)
shows the optical micrograph of preheated OGE film irradiated at
514.5 nm line of an Ar ion laser, where the laser beam was focused
on the film using an objective lens (ꢃ100) with the energy density
of ca. 1 MW/cm2. The horizontal line in the micrograph was
drawn by scanning the focused laser beam on the flat precursor
In conclusion, the spatially selective formation of ꢀc-Ge by
laser-induced pyrolysis of OGE film is demonstrated. This is a
new method to form a micropattern of ꢀc-Ge, and the formation
of a micropattern of Si–Ge alloy is expected by laser-induced
pyrolysis of spin-coated OGE on Si substrate.
This work was supported by a Grant-in-Aid for Scientific
Research (Nos. 1155900, 13650942) from the Ministry of
Education, Science and Culture of Japan.
References and Notes
1
2
F. Schaffler, Semicond. Sci. Technol., 12, 1515 (1997).
A. Watanabe, M. Unno, F. Hojo, and T. Miwa, J. Mater. Sci. Lett., 20,
491 (2001).
3
A. Watanabe, M. Unno, F. Hojo, and T. Miwa, Mater. Lett., 47, 89
(2001).
4
5
A. Watanabe, M. Unno, F. Hojo, and T. Miwa, Chem. Lett., 2001, 1092.
A. Watanabe, M. Fujitsuka, O. Ito, and T. Miwa, Jpn. J. Appl. Phys., 36,
L1265 (1997).
6
A. Watanabe, M. Fujitsuka, O. Ito, and T. Miwa, Mol. Cryst. Liq. Cryst.,
316, 363 (1998).
Figure 2. Optical micrographs of films obtained by
YAG laser-induced pyrolysis of OGE without preheat-
ing (a), by YAG laser-induced pyrolysis of OGE after
preheating at 200 ꢁC (b), and by Ar ion laser-induced
pyrolysis of OGE after preheating at 200 ꢁC (c).
7
8
A. Watanabe, M. Fujitsuka, and O. Ito, Thin Solid Films, 354, 13 (1999).
Y.-G. Baek, T. Ikuno, J.-T. Ryu, S. Honda, M. Katayama, K. Oura, and
T. Hirao, Appl. Surf. Sci., 1–5, 7546 (2001).
J. Vilcarromero and F. C. Marques, Thin Solid Films, 343/344, 440
(1999).
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