High quality Ge epitaxial layers in narrow channels on Si „001… substrates
G. Wang,
1,2,a
E. Rosseel,
1
R. Loo,
1
P. Favia,
1
H. Bender,
1
M. Caymax,
1
M. M. Heyns,
1,2
and W. Vandervorst
1,3
1
IMEC, Kapeldreef 75, B-3001 Leuven, Belgium
2
Department of MTM, KULeuven,Kasteelpark Arenberg 44-bus 2450, B-3001 Leuven, Belgium
3
Instituut voor Kern-en Stralingsfysica, KULeuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium
Received 18 January 2010; accepted 22 February 2010; published online 16 March 2010
We demonstrate the selective growth of high quality Ge epitaxial layers in channels as narrow as
10 nm on patterned Si 001 substrates by a combination of low temperature growth and selective
recrystallization using Ge melt and regrowth during a millisecond laser anneal. Filling narrow
trenches at high growth temperature as required for obtaining high quality layers was shown to be
prohibited by Ge outdiffusion due to the high Ge chemical potential in such narrow channels. At low
temperature, a hydride-terminated surface is maintained which counteracts the outdiffusion of the
Ge adatoms and provides excellent trench filling. The resulting low crystalline quality can be
restored by a selective Ge melt and epitaxial regrowth using a millisecond laser anneal.
© 2010 American Institute of Physics. doi:10.1063/1.3360231
As the downscaling of Si-based complementary metal
oxide semiconductor devices is approaching the physical
limit, an alternative to further boost the device performance
is to introduce high carrier mobility materials as well as
three-dimensional device structures, like Fin field effect tran-
sistors FinFETs.
1,2
Most of the high carrier mobility mate-
rials, such as Ge and III-V compound semiconductors, have
different thermal-mechanical properties and lead to higher
costs compared with Si. To overcome this bottleneck, a
widely accepted route is to integrate Ge and III-V materials
on Si substrates using epitaxial growth.
3–8
Whereas this has
been demonstrated for blanket layers and large area devices,
epitaxial growth in very narrow trenches as required for Ge
FinFETs formation becomes a problem due to the absence
of trench filling at the required high temperatures. Obviously
when the device dimensions reach tens of nanometers, the
material properties such as thermal stability in relation to
device size become a point of concern.
The thermal stability of low dimensional crystals has
been well studied in terms of their melting points and a melt-
ing point decrease has been found for crystal dimension ap-
proaching 10–20 nm.
9,10
The thermal stability of semicon-
ductor materials in extremely narrow channels against
surface diffusion during growth is not yet well established.
And fast surface diffusion driven by the large chemical po-
tential as a result of the reduced device dimension may be
invoked as a mechanism prohibiting the epitaxial growth of
Ge in narrow channels with shallow trench isolation STI.
In this paper, we report successful Ge selective epitaxial
growth in 10–20 nm wide channels and explain our results
based on the large chemical potential of Ge in narrow chan-
nels which prevents the direct growth of high quality Ge at a
temperature higher than 450 °C. Clearly, a low temperature
is required for successful Ge growth in channels as narrow as
10 nm. However, these layers are defective and a selective
melt and regrowth step is added to obtain high quality Ge
materials. The process window time-temperature is set by
the interplay between Ge-melting, potential Ge/Si interdiffu-
sion and Ge surface migration, and epitaxial regrowth quality
can be reached using millisecond ms laser anneal.
To make narrow channels on Si 001 substrates, SiO
2
STI patterning was used. The patterned Si wafers with STI
side walls in 110 directions were etched with hydrogen
chloride HCl vapor to a 50–120 nm depth from the STI
surface. After the Si recess, Ge was deposited in the chan-
nels. The detailed growth conditions can be found in Ref. 8
but the main focus in this work is the 10–20 nm wide chan-
nels. The Ge morphology and crystalline quality were char-
acterized with a scanning electron microscope SEM and
transmission electron microscopy TEM, respectively.
In order to understand the limitations in filling narrow
channels one needs to calculate the variation of Ge chemical
potentials during the epitaxial growth as a function of chan-
nel width. In the following calculation, a source/drain S/D
dimension of 100 100 nm
2
was considered and the chan-
nel in-between has a length of 100 nm with different channel
widths, as shown in Fig. 1a. If a Ge bulk substrate is taken
as the reference, the total Gibbs free energy in the Ge grown
in the S/D is,
G
S/D
= C
11
+ C
12
S/D
2
wlh +
110_SiO
2
2l +2w - w
0
h
+
001
wl , 1
where C
11
and C
12
are the Ge bulk elastic constants;
S/D
is
the remaining strain in the S/D; w, l, and h are the S/D width,
length, and Ge thickness, respectively; w
0
is the channel
width;
110_SiO
2
is the interface energy between the Ge and
the STI side wall;
001
is the Ge 001 surface energy. The
first term on the right accounts for the elastic energy while
the second and third terms represent the surface energy. For a
given device structure, the lateral dimensions are fixed and
the variable is the thickness, h, upon Ge growth. The elastic
strain relaxation of Ge was not considered and only plastic
strain relaxation was taken into account. This assumption is
justified in our experiments since the Ge growth started at a
low temperature 450 °C, 750 Torr. At this condition, a
smooth layer was obtained due to the limited surface diffu-
sion. The remaining elastic strain decreases strongly with Ge
thickness during growth. Therefore, the remaining strain was
a
Electronic mail: wangga@imec.be.
APPLIED PHYSICS LETTERS 96, 111903 2010
0003-6951/2010/9611/111903/3/$30.00 © 2010 American Institute of Physics 96, 111903-1