Laser CVD of nanodisperse Ge-Sn alloys obtained by dielectric breakdown of SnH4/GeH4 mixtures

Article abstract of DOI:10.1002/ejic.200801172

TEA CO₂ laser-induced dielectric breakdown of an equi-molar mixture of gaseous stannane and germane in Ar allows decomposition of both metal hydrides and chemical vapour deposition of the nanostructured Ge/Sn films. Analysis of the films by FTI

Full text of DOI:10.1002/ejic.200801172

SHORT COMMUNICATION
DOI: 10.1002/ejic.200801172
Laser CVD of Nanodisperse Ge–Sn Alloys Obtained by Dielectric Breakdown
of SnH /GeH Mixtures
4
4
Tomá sˇ K rˇ enek,[a,b] Petr Bezdi cˇ ka, Nataliya Murafa, Jan Subrt, and Josef Pola*
[c]
[c]
ˇ
[c]
[a]
Keywords: Germanium / Tin / Stannanes / Nanostructures / Alloys / Metastable compounds
TEA CO
2
laser-induced dielectric breakdown of an equi-
amorphous Ge/Sn phase. The crystalline Ge/Sn nanoalloys
molar mixture of gaseous stannane and germane in Ar allows
decomposition of both metal hydrides and chemical vapour
deposition of the nanostructured Ge/Sn films. Analysis of the
films by FTIR and Raman spectroscopy, X-ray diffraction
analysis and electron microscopy revealed nanoregions of
crystalline Sn and Sn-rich Ge/Sn alloys surrounded by an
contain different amounts of Sn incorporated into the Ge lat-
tice and undergo changes in composition at temperatures as
low as 200 °C.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
MBE yielded[15] crystalline d-c films (x = 0.3–0.34) on Ge,
Introduction
ultrahigh-vacuum chemical vapour deposition from Ge H6
2
Search for novel alloys, especially their metastable forms
and multiphase micro- and nanostructures, their formation,
growth and morphological changes represents an active sci-
entific area of advanced materials[1] and is expected to lead
to new discoveries in fundamental and highly applied
topics.
and SnD precursors allowed synthesis of crystalline strain-
4
[
16–18]
free films (x Յ 0.20) on silicon substrates
and, finally,
pulsed laser annealing of amorphous Ge/Sn films resulted
in the formation of metastable crystalline features with x =
[
5]
[19]
0
.22 on glass and even x = 0.55 on Ge substrates.
Another laser technique enabling kinetic rather than
The formation of bulk Sn Ge
alloys[2] with the solid
x
1–x
thermodynamic control of the process is IR laser-induced
gas-phase codecomposition of two different molecules,[
which is capable of simultaneously generating clusters of
several (two or more different) metals that will interact in
the gas phase to yield “mixed nanosized systems” and de-
posit as such from the gas phase. This process experiences
solubility limit of Ge in Sn diamond-cubic structure Ͼ1%
is difficult due to (i) large lattice mismatch (15%) between
Ge and α-Sn and (ii) instability of d-c α-Sn phase at ambi-
ent temperature. There is, however, great interest in the
20,21]
Sn Ge
alloys with nonequilibrium composition of both
x
1–x
elements (higher Ge content in the solid phase), because
these alloys have a great potential (e.g., see ref.[3–5]) of a
tunable direct energy gap. Such alloys can be fabricated
only as metastable nanosized films by using special tech-
niques allowing nonequilibrium growth.
[
22,23]
very high heating and cooling rates,
occurs within the
order of microseconds and is feasible for the formation of
metastable structures.
Here we report on a novel approach to chemical vapour
deposition of Sn/Ge metastable alloys through gas-phase
Up to now, coevaporation of Sn and Ge and deposition
codecomposition of SnH and GeH by IR laser-induced
to substrates kept at 100 K allowed[6] formation of amorph-
4
4
dielectric breakdown and show that this technique is a suit-
able method to obtain nanostructured films composed of
crystalline Sn and Ge/Sn alloy nanoobjects in the amorph-
ous Ge/Sn phase.
ous single phase (x Յ 0.15) films, sputtering resulted in de-
position of amorphous[ and single-crystal or poly-
crystalline films (x Յ 0.15),[8] molecular beam epitaxy
7]
(
MBE) enabled the growth of homogeneous crystalline
[
9–11]
[12]
(x = 0.2–0.5)
and fully strained (x Յ 0.26) films on
[
13]
[14]
Ge, InSb
(x Յ 0.1) and CdTe
substrates, ion-assisted
Results and Discussion
[
a] Laboratory of Laser Chemistry, Institute of Chemical Process
Fundamentals, Academy of Sciences of the Czech Republic,
The unfocused transversely excited atmospheric (TEA)
CO laser irradiation of SnH and GeH (both 5 Torr) in
1
6502 Prague, Czech Republic
2
4
4
E-mail:pola@icpf.cas.cz
[
[
b] University of West Bohemia,
Ar (total pressure 110 Torr) does not lead to direct
multiphoton excitation and decomposition of these com-
pounds, as both molecules do not possess absorption bands
in the laser emission region (Figure 1).
30614 Plzen, Czech Republic
c] Institute of Inorganic Chemistry, Academy of Sciences of the
Czech Republic,
ˇ
2
5068 Re zˇ , Czech Republic
1464
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 1464–1467

Laser CVD of Nanodisperse Ge–Sn Alloys
the (Si–Si)* and (Ge–Ge)* bands (Figure 3) thus indirectly
reveal the presence of Si/M (M = Sn, Ge) and Ge/Sn alloys,
although Si–Ge, Si–Sn and Ge–Sn Raman modes are not
observed.
Figure 1. FTIR spectrum of SnH
total pressure 110 Torr).
4 4
and GeH (both 5 Torr) in Ar
(
However, the highly focused radiation induces nonreso-
nant interaction leading to dielectric breakdown[
24,25]
(a vis-
ible spark) and results in decomposition of both SnH and
4
GeH , yielding elemental metals in the gas phase and al-
4
lowing deposition of ultrafine solid particles on the Si sub-
strate. The absorption bands of the Ge and Sn precursors
diminish at a similar rate and, respectively, deplete by 35
Figure 3. Typical Raman spectrum of the deposit. [The bands des-
ignated as (Si–Si)* and (Ge–Ge)*, respectively, occur in Si/M (M
and 45% with 50 pulses, which indicates similar consump- = Sn, Ge) and Ge/Sn alloys.].
tion of GeH and SnH for the formation of the solid de-
posit.
SEM images of the deposit (Figure 2) reveal fluffy mor-
phology with submicrometre-sized bodies merging into sev-
eral micrometre-large agglomerates. EDX-SEM analyses
indicate small contamination by O and C elements and
4
4
Both vibrational spectra do not indicate the presence of
interstitial hydrogen. [Although this species has not yet
been detected for Sn and Ge/Sn, it is infrared active in Ge
–1
[29]
at 700 and 1880 cm (ref. ) and Raman active at 745,
1
794 and 3630 cm–1 (refs.[29,30])].
µ-XRD analysis (Figure 4a) reveals the presence of two
reveal stoichiometries (Sn1.00Ge0.95Si10.60C4,50O1,94 for
2
crystalline phases, one with the unit cell parameters a =
.5831 nm and c = 0.3182 nm, which corresponds to tetrag-
3
00 µm area and Sn1.00Ge0.88Si0.15 C0,51O0,89 for several
0
micrometre-sized particles) that agree with the relative de-
composition of both metal hydrides as determined by FTIR
spectra. The analyses also show that Sn/Ge particles do not
cover the Si substrate completely.
onal β-Sn (designated as T and existing at temperatures
Ͼ13.25 °C) and another corresponding to the Ge/Sn alloy
(designated as G and GЈ). The bifurcated signal of the alloy
can be deconvoluted to two peaks corresponding to the unit
cell parameter a = 0.57552 and 0.59181 nm. These values
are significantly higher than the unit cell parameter of pure
diamond-type cubic Ge (a = 0.5657 nm) and they reflect,
[
27]
in agreement with the literature, approximately 10% and
far more (above 25%) incorporation of Sn atoms into the
Figure 2. Typical SEM images of the deposit.
FTIR spectroscopy of the deposited film does not give
[
26]
any evidence on Sn–H and Ge–H bonds typical
for
Sn Ge1–x:H films and confirms complete elimination of H
x
from both precursors.
The Raman spectrum of the deposit (Figure 3) shows a
–
1
broad band at 282–288 cm and a sharp band at 518–
–
1
–1
5
20 cm with a shoulder at 490 cm , which are, respec-
[
11]
tively, assigned to the Ge–Ge bond in a Sn/Ge alloy, the
Si–Si bond in the silicon substrate and the Si–Si bond in Si/
M (M = Sn,
[
27]
[28]
Ge ) alloys. The Ge–Ge band being
–
1
[11]
shifted from that of pure Ge (ca. 300 cm ) corresponds
Figure 4. µ-XRD analysis of the deposit before (a) and after (b)
to the Sn Ge alloy with x = 0.17–0.25. The positions of heating to 200 °C.
x
1–x
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ˇ
T. K rˇ enek, P. Bezdi cˇ ka, N. Murafa, J. Subrt, J. Pola
SHORT COMMUNICATION
Ge lattice. The bifurcated signal becomes single after heat- fore and after heating to 200 °C is consistent with moderate
ing to 200 °C (Figure 4b), in which case the peak corre- temperature relaxation of nanoalloy structures and compo-
sponds to the unit cell parameter a = 0.57201 nm and ap- sition changes leading to separation of both alloy constitu-
proximately 7% incorporation of Sn atoms into the Ge lat- ents, which occur rather as surface than bulk process.[
tice. We note that weight fractions of the crystalline Sn (ca.
31,32]
30%) and Ge/Sn alloy (ca. 50%), as well as amorphous con-
tent (ca. 20%), estimated by Rietveld refinement procedure Conclusions
before and after heating are virtually the same. These results
are supported by analysis by transmission electron micro-
scopy.
In conclusion, we have demonstrated that IR laser dielec-
tric breakdown in an equimolar gaseous mixture of SnH4/
GeH in Ar allows gas-phase deposition of nanoparticles
4
The TEM image (Figure 5a) reveals irregular agglomer-
ates consisting of uniform plate-like crystalline nanopar-
ticles (10–50 nm). The marked places are point-out areas
from which the EDS analyses, indicating great amounts of
Ge and Sn elements and small contaminations of O and C,
were carried out. The HRTEM images (Figure 5b–d) are
consistent with a blend of crystalline and amorphous phase.
Figure 5b shows the HRTEM micrograph of a crystalline
nanoparticle with the interlayer spacing of d = 0.279 nm,
the value corresponding to tetragonal Sn (PDF 04-0673).
Figure 5c shows the HRTEM micrograph of a crystalline
nanoparticle with the interlayer spacing of d = 0.339 nm,
containing crystalline Sn and metastable Sn-rich Ge/Sn al-
loys along with a Sn/Ge amorphous phase. These nanopar-
ticles have to be formed upon decomposition of both metal
hydrides and intermixing/clustering of extruded metal
atoms in the gas phase. The Ge/Sn nanoalloys possess dif-
ferent contents of Sn incorporated in the Ge lattice and are
modified in composition at temperatures as low as 200 °C.
Experimental Section
4 4
Laser irradiation of an equimolar mixture of SnH /GeH (both
which corresponds to cubic Ge with d = 0.326 nm (PDF 04- 5 Torr) in Ar (total pressure 110 Torr) leading to dielectric break-
545). As agreed with µ-XRD analysis, the slight increase in down in this mixture was accomplished with a TEA CO2 laser
the interlayer spacing can be explained by the Sn atoms (Plovdiv University, model 1300 M) operating with a repetition fre-
0
0
0
quency of 1 Hz at the P(20) line of the 00 1Ǟ10 0 transition
(
incorporated into the Ge lattice. A very interesting
HRTEM micrograph (Figure 5d) reveals the occurrence of
a crystalline nanoparticle with two different values of inter-
layer spacing (d = 0.330 nm and d = 0.349 nm) and it is
in line with different extents of Sn incorporation in the Ge
lattice.
–
1
944.19 cm ) and pulse energy 1.8 J. The gaseous mixture was irra-
diated in a Pyrex reactor (520 mL in volume, Figure 6) that con-
sisted of two orthogonally positioned Pyrex tubes (both 3 cm in
diameter, one 9 cm and the other 13 cm long) fitted with KBr win-
dows and furnished with a PTFE valve connected to a vacuum
manifold and pressure transducer. The laser pulse was focused with
a NaCl lens (f.l. 15 cm) to generate a spark in the centre of the
tube, above which was accommodated a silicon substrate. The mix-
ture was irradiated with 45 pulses, and the thin film on the Si sub-
strate was deposited through 3 cycles.
1
2
Figure 6. Top view of the irradiated Pyrex reactor: (1) KBr window;
(2) PTFE valve; (3) substrate; (4) spark; (5) laser pulse; (6) lens.
Figure 5. TEM (a) and HRTEM (b–d) micrographs of the deposit.
We assume that the presence of different Ge/Sn alloys
is in keeping with a nonuniform temperature and possible
transient (both neutral and charged) particles profiles in di- using absorption bands at 677 cm and 1902 cm (SnH ) and
The progress of codecomposition of SnH
4 4
and GeH was moni-
tored by FTIR spectroscopy (Nicolet Impact spectrometer) by
–
1
–1
4
–
1
–1
electric breakdown. The µ-XRD analysis of the deposit be- 816 cm and 2108 cm (GeH
4
). Thereafter, the reactor was evacu-
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Eur. J. Inorg. Chem. 2009, 1464–1467

Laser CVD of Nanodisperse Ge–Sn Alloys
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Received: December 2, 2008
Published Online: March 12, 2009
This work was supported by Ministry of Education, Youth and
Sports of the Czech Republic (Grant no. LC523).
3
[
[
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