Morphology of germanium nanowires grown in presence of B2H 6

Article abstract of DOI:10.1063/1.2165089

We study the Au-catalyzed chemical vapor growth of germanium (Ge) nanowires in the presence of di-borane (B2 H6), serving as doping precursor. Our experiments reveal that, while undoped Ge nanowires can be grown epitaxially on Si(111) substrates with very long wire lengths, the B2 H6 exposure renders the Ge nanowires significantly tapered. As we describe here, this peculiar morphology stems from the combination of the acicular, one-dimensional nanowire growth and a dramatically enhanced, B-induced conformal Ge deposition. The combination of acicular and conformal Ge growth mechanisms results in cone-shaped Ge nanostructures.

Full text of DOI:10.1063/1.2165089

Morphology of germanium nanowires grown in presence of B 2 H 6
E. Tutuc, S. Guha, and J. O. Chu
Citation: Applied Physics Letters 88, 043113 (2006); doi: 10.1063/1.2165089
View online: http://dx.doi.org/10.1063/1.2165089
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APPLIED PHYSICS LETTERS 88, 043113 ͑2006͒
Morphology of germanium nanowires grown in presence of B2H6
E. Tutuc,a͒ S. Guha, and J. O. Chu
I.B.M. T. J. Watson Research Center, Yorktown Heights, New York 10598
͑Received 2 October 2005; accepted 8 December 2005; published online 26 January 2006͒
We study the Au-catalyzed chemical vapor growth of germanium ͑Ge͒ nanowires in the presence of
di-borane ͑B2H6͒, serving as doping precursor. Our experiments reveal that, while undoped Ge
nanowires can be grown epitaxially on Si͑111͒ substrates with very long wire lengths, the B2H6
exposure renders the Ge nanowires significantly tapered. As we describe here, this peculiar
morphology stems from the combination of the acicular, one-dimensional nanowire growth and a
dramatically enhanced, B-induced conformal Ge deposition. The combination of acicular and
conformal Ge growth mechanisms results in cone-shaped Ge nanostructures. © 2006 American
Institute of Physics. ͓DOI: 10.1063/1.2165089͔
There has been significant interest lately in the metal-
catalyzed growth of semiconductor nanowires. Experimen-
tally, group IV semiconductors Si1 and Ge,2 as well as III–V
compounds, e.g., InAs, InP,3 GaAs,4 have been shown to
exhibit metal catalyzed, one-dimensional crystal growth. The
interest in these structures is fuelled in part by the promise of
novel devices, relevant to complementary metal-oxide-
semiconductor ͑CMOS͒ technology scaling.5 Among semi-
conductor nanowire systems expected to be compatible with
silicon technology, germanium is of particular interest,
stimulated by the prospect of replacing Si as a semiconduct-
ing channel in MOS field effect transistors, due to its higher
carrier mobilities. Several groups have recently reported Au-
catalyzed, chemical vapor growth of Ge nanowires.2,6 A po-
tential advantage of semiconductor nanowires is that elec-
tronic doping in these devices should be a straightforward
extension of semiconductor bulk doping. Therefore, the
study of nanowire growth in presence of dopants, as well as
dopant incorporation during growth, is of importance to
nanowire device design.
the wires grow at an average rate of 4 ␮m/h ͑Ref. 7͒ and
with diameters ranging from 20 to 50 nm. The one-
dimensional, nanowire crystal growth is generally attributed
to the vapor-liquid-solid phase mechanism.8 In this model
the metal catalyst and the semiconductor form an eutectic
alloy. At temperatures higher than the eutectic temperature,
the catalyst becomes liquid and facilitates the transport of
semiconductor atoms from the vapor ambient to the crystal.
In our experiments, the Ge nanowires distinguish themselves
because they grow at temperatures significantly lower the
Au–Ge eutectic temperature ͑360 °C͒, therefore the Au cata-
lyst remains in the solid phase.
The noteworthy feature of Fig. 1͑a͒ is that the Ge nano-
wires align with the ͓111͔ directions of the Si substrate: they
Here we examine the low-temperature, chemical vapor
growth of Au-catalyzed Ge nanowires in the presence of di-
borane B2H6, serving as our doping precursor. Our experi-
ments reveal that B2H6 dramatically enhances the conformal
deposition of Ge at low temperatures, enhancing lateral
growth rates and thereby resulting in cone-like and not rod-
like nanostructures. These findings impose constraints on
modulating the Ge nanowire doping along the wire length
during the growth, but open the tantalizing possibility of us-
ing the low-temperature, highly B-doped Ge layers in con-
junction with a dielectric as gate/dielectric stack for Ge
nanowire transistors.
In Fig. 1͑a͒ we show one example of Ge nanowires epi-
taxially grown on a Si substrate. Prior to loading into the
chemical vapor deposition ͑CVD͒ reactor, a 10Å-thick Au
film was deposited on the H-terminated Si͑111͒ substrate.
The substrate was annealed in situ at a temperature T
=500 °C in an atmosphere of H2 in order for the Au film to
form droplets, followed by a cool-down to the optimal, T
=285 °C growth temperature. The 2 h long nanowire growth
was done at a total pressure of 5 Torr, using a 60 sccm flow
of GeH4 ͑10% in a helium mixture͒. Under these conditions
FIG. 1. ͑Color online͒ ͑a͒ Scanning electron micrograph ͑SEM͒ showing Ge
nanowires epitaxially grown on a Si͑111͒ substrate. The wires grow mainly
along the different ͓111͔ crystal directions, indicated by arrows in the figure.
͑b͒ X-ray diffraction k-space mapping of the Ge nanowires grown on Si.
a͒
Electronic mail: etutuc@us.ibm.com
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0003-6951/2006/88͑4͒/043113/3/$23.00 88, 043113-1 © 2006 American Institute of Physics
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043113-2
Tutuc, Guha, and Chu
Appl. Phys. Lett. 88, 043113 ͑2006͒
FIG. 2. Top-down ͑a͒ and cross-section ͑b͒ SEM view of Ge nanowires
grown in presence of B2H6. The Ge nanowires are markedly different than
those grown without B2H6. The cone dimensions agree well with estimates
based on the B-enhanced Ge planar deposition rate.
FIG. 3. ͑a͒ SEM view of regular, undoped Ge nanowires. ͑b͒ Ge nanowires
grown initially without B2H6 followed by a second growth stage during
which both GeH4 and B2H6 are present in the CVD chamber. Both growths
were performed at 285 °C. Note that the second growth of ͑b͒ yields a
conformal 160–180nm-thick shell which wraps around the undoped Ge
core, resulting in 400-nm-thick rods.
can grow either normal to the substrate, or along the other
three crystallographic ͓111͔ directions which form 70.5°
angles with the normal.
ence of an electronic dopant such as diborane clearly cata-
lyzes and enhances the Ge CVD growth process, a phenom-
enon long known for the case of Si CVD.10
The B-enhanced Ge two-dimensional deposition trans-
lates into a nanowire radius increase during growth. If Rv and
Rl are the vertical and lateral growth rates, respectively, then
the diameter ͓d͑y͔͒ of a wire as a function of distance from
the base at time t is given by
To further substantiate the findings of Fig. 1͑a͒ we have
performed an x-ray diffraction k-space mapping of the Ge
nanowires on Si͑111͒ samples. The experiments were done
by measuring a series of x-ray rocking curves around the
Si͑111͒ diffraction peak, at various incidence angles between
the x-ray beam and the substrate. The data is presented in Fig
1͑b͒. Here, ␻ denotes the angle between the incident x-ray
beam and the substrate and 2␪ is the angle formed by the
directions of the incident and reflected x-ray beams. These
data show clearly the Si͑111͒ and Ge͑111͒ diffraction peaks,
corresponding to the fully relaxed Si and Ge lattice con-
stants. The data bear similarities with x-ray rocking curves
from planar germanium on insulator substrates.9 The vertical,
2° extension of the Ge͑111͒ peak along the ␻ axis indicates
that Ge nanowires axis’ can slightly deviate from the ͓111͔
directions of the substrate. The Ge͑111͒ diffraction peak full
width at half maximum along the ␻/2␪ direction is 0.12°.
In Fig. 2 we show an example of Ge nanowires on Si
substrates grown in the presence of B2H6. The substrate
preparation and growth conditions are identical with those
used for the sample of Fig. 1, with the only difference that
during the growth we introduced a small, 2 sccm, flow of
1%-diluted B2H6. As it can be readily seen from the images
of Fig. 2, the Ge nanowires are rendered conical with a typi-
cal cone base diameter of 400 nm. Further experiments show
that the Ge cone growth and dimensions depends little on
temperature in the 255–320 °C range, except for the “cone”
density which increases at higher temperatures.
d͑y͒ = 2͑t − y/Rv͒Rl + d0,
͑1͒
where d0 is the initial diameter of the wire determined by the
seed. The radius change is proportional to the exposure time,
the net result being a cone rather than a constant radius nano-
wire. For a 2.7 nm/min Ge planar growth rate, a nanowire
which starts out at 20 nm, will see an increase of 160 nm of
its radius during a 1 h growth. The cone geometry of Fig. 2,
which is representative of our results for Ge wire grown in
the presence of B2H6, corresponds to a ratio Rv /RlХ13. Cor-
roborated with our lateral growth rate Rl=160 nm/h, we ob-
tain a vertical growth rate of Rv =2 ␮m/h for Ge nanowires
grown in the presence of B2H6. This rate is in good agree-
ment, albeit slightly lower than the typical 2–4 ␮m/h growth
rate of undoped Ge nanowires of Fig. 1. As a comparison,
Rv /Rl for undoped wires is տ500.
To further demonstrate the effect of B-enhanced Ge
deposition we have carried out a 1 h, undoped Ge nanowire
growth, followed by a 1 h growth during which both GeH4
and B2H6 were introduced in the CVD chamber. The results
are shown in Fig. 3. Panel ͑a͒ of Fig. 3 shows the result of a
regular, 1 h undoped Ge nanowire growth from Au film cata-
lyst, which exhibits typical Ge nanowires of 20 nm diameter.
In panel ͑b͒ we show the result of a regular nanowire growth
followed by a 1 h growth during which both GeH4 and B2H6
were used. Growth conditions for the latter were the same as
except for the presence of boron: 60 sccm GeH4 ͑10%͒, 2
sccm B2H6 ͑1%͒, T=285 °C, 5 Torr. The data of Fig. 3͑b͒
clearly show that the second stage growth simply results in
coating the undoped Ge nanowires with a shell of B-doped
germanium. The 400 nm diameter of this core/shell structure
is consistent with the 2.7 nm/min planar Ge deposition rate
in the presence of boron.
As we show here, this unusual growth morphology re-
sults from the combination of the acicular, one-dimensional
nanowire growth and the conformal, B-enhanced Ge deposi-
tion, which results in two-dimensional lateral growth.
B-dependence Ge CVD growth rates data at low tempera-
tures are scarce in the literature. In order to test the confor-
mal Ge deposition enhancement in presence of boron, we
have carried out a growth at normal nanowire growth condi-
tions, but using a bare Si substrate, without the Au catalysts.
Our experiments show that at T=285 °C, 60 sccm GeH4
͑10%͒ flow, 2 sccm B2H6 ͑1%͒ flow, and total pressure 5
Torr, the B-doped germanium deposits at a rate of
2.7 nm/min. Note that the normal CVD growth of Ge at
Next we address the question of boron incorporation in
the Ge shells. To quantify this, we have grown a core/shell
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such low temperatures is otherwise not possible. The pres-
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043113-3
Tutuc, Guha, and Chu
Appl. Phys. Lett. 88, 043113 ͑2006͒
structure similar to that of Fig. 3͑b͒, except that the shell
thickness was 12 nm thick. The nanowires were dispersed on
a highly conductive Si substrate with a 30nm-thick thermally
grown oxide. Using standard electron-beam lithography fol-
lowed by lift-off we patterned Ni leads in order to contact the
nanowires. Our measurements show that B-doped Ge nano-
wires exhibit resistor-like I−V characteristics and show very
little change to a voltage applied to Si substrate, which acts
as a backgate here. Since undoped Ge nanowires are insulat-
ing in the same transport configuration, we conclude that the
transport in the B-doped Ge nanowire occurs in the B-doped
shell. In order to quantify the dopant concentration in the
B-doped Ge shells, we have scaled the measured resistance
of the B-doped Ge nanowires by the wire length as well as
by the shell cross-section area, in order to measure the shell
resistivity. The result is ␳=0.001–0.004 ⍀ cm. These num-
bers correspond to a carrier concentration of 0.7–4
ϫ1019 cm−3 for the B-doped Ge shell.11
responding to a ratio Rv /RlӍ180. While this ratio still indi-
cates a significant wire tapering compared to the undoped
case, the lower B2H6 dilution clearly results in less of a
nanowire tapering.
In summary we have presented an unusual growth
mechanism of Ge nanowires exposed to B2H6, which results
in a highly doped shell growth, and also quantified the B
incorporation in the Ge shell. Our study point out that the
doping of Ge nanowires may not be straightforward, and
substantially complicate attempts to modulate the doping
profile along the wire. On the other hand, these findings
open-up possibilities of a low temperature core/dielectric/
gate in situ field effect transistor growth.
The authors thank J. Hannon, F. M. Ross, Guy Cohen,
M. C. Reuter, and J. Ott for illuminating discussions and
technical assistance. This work was supported in part by
DARPA/SPAWAR contract No. N66001-05-C-8043.
The B-enhanced, highly doped Ge conformal growth
shown in Fig. 3 opens up the tantalizing possibility of a low
temperature, Ge nanowire based, cylindrical metal-oxide-
semiconductor field effect transistor growth. For example, in
the first stage an undoped Ge nanowire growth is used to
create the Ge core which serves as the transistor’s channel.
Next, a low temperature conformal dielectric growth, e.g.,
atomic layer depositions of Al2O3, followed by a highly
B-doped Ge shell, will create the dielectric and the gate,
respectively. The undoped wire acts a the semiconducting
channel, while the highly doped Ge shell acts as gate, much
like the highly doped, polycrystalline Si gates used in current
CMOS technology.
Finally, we comment on the role of B2H6 concentration
in enhancing the conformal Ge deposition at low temperature
and the ensuing nanowire tapering. Our experiments were
focused on a relatively high, 1% concentration of the B2H6
precursor. To explore the effect of B2H6 dilution, we have
performed a similar growth to that of Fig. 2 except for a
lower B2H6 concentration: namely T=285 °C, 60 sccm
GeH4 ͑10%͒, 2 sccm B2H6 ͑20 ppm͒, and total pressure 5
Torr. In these conditions, our results show conformal Ge
deposition of about 20 nm/h, and Ge nanowire tapering cor-
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