The stability of an amorphous GexSi1−xO2 in contact with an epitaxial (100)GexSi1−x layer obtained by partially oxidizing an epitaxial GexSi1−x layer on a (100)Si substrate in a wet ambient at 700 °C is investigated for x=0.28 and 0.36 upon annealing in vacuum at 900 °C for 3 h, aging in air at room temperature for 5 months, and immersion in water. After annealing at 900 °C, the oxide remains amorphous and the amount of GeO2 in the oxide stays constant, but some small crystalline precipitates with a lattice constant similar to that of the underlying GeSi layer emerge in the oxide very near the interface for both x. Similar precipitates are also observed after aging for both x. The appearance of these precipitates can be explained by the thermodynamic instability of GexSi1−xO2 in contact with GexSi1−x. In water at RT, 90% of GeO2 in the oxide is dissolved for x=0.36, while the oxide remains conserved for x=0.28

Arbet-Engels, V.

Chen, J. S.

Liu, W. S.

Nicolet, M-A.

Wang, K. L.

Caltech Authors - Main

COMMUNICATIONS 
Instability of a Gesi 1-x02 film on a Ge&-, layer 
W. S. Liu, J. S. Chen, and M.-A. Nicolet 
California Institute of Technology, Pasadena, California 91IZ.5 
V. Arbet-Engels and K. L. Wang 
University of California, Los Angeles, Califortnia 90024 
(Received 13 April 1992, accepted for publication 22 July 1992) 
The stability of an amorphous Ge.$i t-,0, in contact with an epitaxial (lOO)Ge,Si,-, layer 
obtained by partially oxidizing an epit.axial Ge,Si,-, layer on a (1OO)Si substrate in a wet 
ambient at 700 “C is investigated for x=0.28 and 0.36 upon annealing in vacuum at 900 “C for 
3 h, aging in air at room temperature for 5 months, and immersion in water. After annealing at 
900 “C!, the oxide remains amorphous and the amount of GeO, in the oxide stays constant, but 
some small c.rystalline precipitates with a lattice constant similar to that of the underlying GeSi 
layer emerge in the oxide very near the interface for both x. Similar precipitates are also 
observed after aging for both x. The appearance of these precipitates can be explained by the 
thermodynamic instability of Ge,Si,-,O, in contact with Ge,Si,-,. In water at RT, 90% of 
GeO, in the oxide is dissolved for x=0.36, while the oxide remains conserved for x=0.28. 
The oxidation of Ge,Si,-, has become of interest re- 
cently,‘-‘2 due to the potential of GeSi devices and the 
importance of oxidation in device technology. It was ini- 
tially reported that for GeSi samples of low Ge content 
(x < 0.2) at high temperature (T> 800 "C), a pure SiOZ 
layer grows with Ge piling up behind it.lb6 This resulting 
pileup of Ge greatly increases the interface trap density of 
the oxide3 and thus should be avoided if the oxide is to be 
used in a device environment. At low temperatures (T 
< 700 C)‘-i* or for high Ge content,7V8 the Si supply at the 
reaction interface is restricted, and Ge can also be oxi- 
dized, resulting in the formation of a GeSi oxide. The oxide 
that formed at low temperature in wet ambient shows a 
uniform composition with the same Ge/Si ratio as that of 
the virgin GeSi epilayer. No pileup of Ge is observed in 
that case and the oxidation rate is very large.‘* The conse- 
quent low thermal budget of this wet oxidation process of 
GeSi at low temperature is of practical interest. 
It has been reported that GeO,-SiO, glasses can, in the 
proper composition range, exhibit thermal coefficients of 
expansion that approximate that of the silicon (approxi- 
mately 40X lO-7/Y between 25 and 700 ‘C).13 The inter- 
facial stress between these two materials, indeed is less 
than 10’ dyn/cm2 (almost stress free) over a wide compo- 
sition range of Ge0,.14 The thermal coefficient of expan- 
sion of Ge is about 70X lo-‘/“C in the same temperature 
range. The Ge,Si, -x02/Ge$i1 --1 system therefore exhibits 
much closer thermal coefficients of expansion between the 
protective insulating layer and the underlying GeSi film 
than the SiOz/Ge,Si, w-X system does. This fact is of interest 
especially for epitaxial GeSi films, whose metastability 
might be affected by this interfacial stress. However, the 
properties of the oxide, and especially its stability, need to 
be examined since the resulting interface between GeSiOz 
and GeSi is thermodynamically unstable and germanium 
oxide is also known to be unstable at high temperature or 
in moisture. We present here the results of experiments 
that investigate the stability of Ge,Si,-,O, on GeSi upon 
annealing, aging, and exposure to moisture. 
The GeSi samples for this study were grown epitaxially 
o&o a ( 100)Si substrate by molecular beam epitaxy with- 
out intentional doping. One sample is a layer of Ges.3,Sia.M 
about 470 nm thick, the other a layer of Ge0.28Si0.72 about 
550 nm thick, Both films are elastically relaxed as deter- 
mined from x-ray double-crystal diffractometry. Samples 
from the wafers were oxidized in a tube furnace at 700 “C 
for 1 h in wet ambient obtained by bubbling nitrogen gas 
through 95 “C! de-ionized water. For the annealing test, the 
oxidized samples were vacuum annealed at 900 “C! for 3 h. 
Other oxidized samples were stored in air at room temper- 
ature for about 5 months. Additional oxidized samples 
were placed in de-ionized water at room temperature from 
several hours up to several days. The water sometimes was 
heated up to the boiling point to accelerate the test. The 
oxide film of al1 these samples was investigated by trans- 
mission electron microscopy and backscattering spectrom- 
etry before and after the tests. 
The backscattering spectra for the oxide of Geu36Si0.64 
before and after the vacuum annealing test are indistin- 
guishable. The oxide thus remains uniform. No Ge is lost 
during annealing. Similar results are also found in the case 
of the oxide of Gqo.2sSi,,,. This is noteworthy because ger- 
manium dioxide tends to volatize at elevated temperatures 
in the presence of pure germanium due to the following 
reaction: GeOZ + Ge = 2 GeO(,] . *s The conservation of Ge 
in the present case could be due to the absence of pure Ge 
or the action of the oxide as a protective cap. 
Figure 1 shows cross-sec.tional transmission electron 
micrographs of the oxide of Ge0,36Si0.64 after 3 weeks and 5 
months aging in air at room temperature and after the 
4444 J. Appl. Phys. 72 (9), 1 November 1992 0021-0979/92/214444-03.$04.00 @ 1992 American institute of Physics 4444 
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3 weeks 5 months 3 h @ 900°C in vacuum 
FIG. 1. Cross-sectional transmission electron microgranhs of the GeSi oxide/GeSi layer interface for (a) 3 weeks aging in the air, (b) 5 months aging 
in the air, and (c) 300 ‘C annealing for 3 h after o&&ion. 
vacuum annealing test. Small precipitates emerge in the 
oxide near the interface and alter the initially sharp inter- 
face after 5 months aging and 900 “C annealing. In both 
casa, the precipitat.es are mostly located at about 10 nm 
from the interface. The size of the precipitate after 900 “C 
annealing is generally larger than that after 5 months aging 
at room temperature. The precipitates are crystalline with 
a lattice constant similar to that of the underlying G&i 
layer, as shown in Fig. 2 which is a high-resolution micro- 
graph of the oxide/GeSi region after 5 months aging. Sim- 
ilar features are also observed in the oxide of Geo.zsSiO,iz 
after the vacuum annealing and aging tests. By the lattice 
FIG. 2 High-resolution cross-sectional transmission electron micrograph 
of a GeSiOa/GcSi in&facial region atIer aging for 5 months in air at 
room temperature. Crystalline precipitates with a lattice constant similar 
to that of the underlying G&i layer arc ohsrrved near the interface as the 
arrows indicate. 
4445 J. Appl. Whys., Vol. 72, No. 9, 1 November 1992 
constants, these precipitates could be elemental Ge, ele- 
mental Si, G&i solution, or even tetragonal Ge&, which 
can grow epitaxially on the ( f 10) plane of germanium.16 
Thermodynamic arguments given as follows predict that 
these precipitates should be Ge$i,-, solid solution or in 
the limit, elemental Ge. 
The Ge-Si-0 ternary phase diagram calculated by Pain 
et a1.l’ has tielines connecting Si02 with GeO, and 
Ge$i,-, at 1000 K. The phase diagram is similar from 
room temperature to 900 “C according to our calculation 
similar to Pain et al. Any compound below the Ge02-Si02 
tieline, i.e., of composition Ge$il .O,..., (06~~1, O<S 
< l), will decompose into SiOz plus GeOz and Ge, or into 
SiOz and a Ge2il --y solution, depending on what the ratio 
of 01 ( Ge + Si ) is. Cross-sectional transmission electron 
micrographs show that the interface between the GeSi 
layer and the Si substrate is still sharp after 900 “C snneal- 
ing, indicating only insignificant interdiffusion between 
them. We can thus exclude the Si substrate from the sys- 
tem and only consider the equilibrium between the oxide 
and the remaining GeSi layer. Thermodynamically, the ox- 
ide then becomes unstable when it contacts GeSi, since the 
ratio of Oi( Ge -+ Si) for this system is less than 2 and will 
locate thii system below the GeO.+iO, tieline in the Ge- 
Si-0 phase diagram. To explain the particular location of 
the precipitates seen in Fig. 1 requires a model of the ki- 
netics involved with nucleation and growth. This instabil- 
ity also exists with pure Si and is unacceptable from a 
device point of view. 
The backscattering spectra of the oxide before and af- 
ter immersion in water at room temperature for one day 
are shown in Fig. 3. After this test, the oxide remains 
uniform but with a Ge content that decreases from 
Ge0,36Si0,6402 to about Geo,05Sio~9~02, or equivalently, a re- 
duction from 2000 to 1400 f.u./cm’. This result means that 
about 90% of the GeO> is lost in the water. To see if GeO, 
could be totally dissolved in t.he water, t.he sample of Fig. 3 
Liu” ef a/. 4445 
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3.01 I , I , 1 , 1 , I , I , -y 
i; k  (Si)/Geo.36Sio.s4(470 nm), 2 MeV 4He+ 4  
ii 2.5 - 
2 
F 
as deposited 
aal3 700 ‘C/wet oxidation 1  h  
“0 2.0 
xxx RT water immersion for 1  day afterwards 
5  
9 
y 1.5 
P 
B 1.0 
E 
3 
g 0.5 
3 
2 00 600 800 1000 1200 1400 1600 
ENERGY (keV) 
FIG. 3. 2 MeV 4He+ backscattering spectra of a Gec,,&,,,O, layer 
before and after immersion in water at room temperature for 1 day (Near- 
normal radiation beam incidence; the scattering angle of detected parti- 
cles: 170’). 
was placed back in fresh de-ionized water for a week with 
water being replaced every day. No significant change was 
observed by backscattering spectrometry. The whole oxide 
including SiO, is totally lost after 16 h immersion in boil- 
i11g water. For Ge0&5i0.7202, the oxide is conserved after 
immersion in the water at room temperature for one day 
according to the backscattering spectrometry measure- 
ments, indicating the solubility of this oxide in the room 
temperature water is negligible. However, some amount of 
GeOz is lost after 16 h immersion in boiling water. 
Hexagonal GeOz is known to be rapidly soluble in 
water; tetragonal GeOa, on the other hand, is only very 
slightly soluble in the room-temperature water ( <0.005 
g/R. ” According to IR absorption spectrometry, the state 
of GeOa in the present Geo,,,Sio,,O, and Ge0,28Si0.7202 
films is mostly in the from of tetrahedrally coordinated 
[GeO,l germanium (hexagonal form of GeOz) with a little 
octahedrally coordinated [Ge06] germanium (tetragonal 
form of Ge02).‘* By that observation, most of the Ge02 
should be water soluble in both cases. Most of the GeO, in 
G~.&3&.,0, is in fact lost upon immersion in water at 
* room temperature. G~,~sS1e,,~ 2 0 , however, is completely 
water insoluble. Similar water tests have been performed 
on the GeO?-Si02 glasses deposited on Si substrates by low 
pressure chemical vapor deposition.‘4 They also found a 
limit (about 20% GeO,) below which the glasses is water 
i11soluble. To what extent the unequal solubility of 
Ge$i1-,O, films can simply be explained by a tetragonal 
versus hexagonal coordination of Ge thus remains an open 
question. 
Compositional and structural inhomogeneities can also 
affect the solubility. If enough SiO, surrounds GeOz, it can 
be shielded from attack by water even though it may be 
tetrahedrally coordinated. The ratio of Ge02/SiOZ below 
which the oxide is completely room-temperature water in- 
soluble is then decided by the detailed structure of the 
oxide, which could be affected by the ways the oxide is 
formed. The effect of hot water could be to accelerate the 
breakup of this shielding until the remaining oxide is so 
porous that even the SiOZ is eroded away. 
The interface between Ge,Si,-,02 films and Ge,Si1-, 
or Si is thermodynamically unstable. This instability leads 
to the formation of GeSi precipitates by a reduction of 
GeOl near the interface. The phenomenon is detectable a 
few months after the formation of the interface and storage 
at room temperature meaning that it is a major consider- 
ation in any proposed application of Ge,Si, -,02 in 
Ge,Si,-, devices. The mixed oxides are also sensitive to 
moisture, in ways not explained yet. 
This work was supported by the Semiconductor Re- 
search Corporation under a coordinated research program 
between Caltech (92-SJ-100) and UCLA (92-SJ-088). We  
thank N. M. Abuhadba and Dr. C. Aita at the University 
of W isconsin-Milwaukee for IR analyses of some samples. 
The technical assistance of R. Gorris is also thankfully 
acknowledged. 
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4446 J. Appl. Phys., Vol. 72, No. 9, 1 November 1992 Liu et a/. 4446 
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Instability of a GexSi1−xO2 film on a GexSi1−x layer

https://authors.library.caltech.edu/11547/1/LIUjap92c.pdf

Instability of a GexSi1−xO2 film on a GexSi1−x layer

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