0
are clearly resolved. The presence of Ge peaks implies that the
main body of the Ge NW is composed of pure Ge. Note that
the higher population of oxidized Ge species observed at low
photon energy irradiation implies that rapid oxidation of Ge
NWs occurs predominantly on the surface in ambient conditions.
0
On the contrary, no Si 2p peak was detected from the Ge
NWs (Fig. 4c and d). Instead, oxide Si 2p peaks from SiO
4
2
+
bonds (Si 2p, B103 eV) were defined. The peaks at the high
binding energy regions (105–106 eV) in Fig. 4c seem to
Fig. 5 (a) IDS–VGS and (b) IDS–VDS characteristic curves of a Ge NW
field effect transistor (FET) device having 1.7 mm of channel length.
The bias voltage applied in (a) is 0.1 V, and the gate voltages in (b) are
1
originate from the charging effect of non-conducting oxides
6
1
7
or the existence of the excess oxygens at the oxide layers.
Note that for the inspection of Si 2p peaks, Ge NWs grown on
a Ge(100) substrate instead of Si(111) were examined to avoid
possible confusion. These results indicate that the silicon
content exists only as an oxide form in Ge NWs. We further
confirmed that Si was indeed involved as an impurity rather
than as a crystalline skeletal component from micro-Raman
V
GS = À30 to 30 V (from top to bottom) in 5 V steps. The inset in (a)
is an optical image of the Ge NW FET device (scale bar: 10 mm).
grown Ge crystals were obtained. The key conclusion that
SiCl must be continuously supplied together with GeCl for
4
4
the synthesis of long Ge NWs was also confirmed by observing
the formation of Ge tadpoles at the end of Ge NWs when the
À1
spectroscopy, by which neither a Si–Ge band (400 cm ) nor a
À1
4 4
SiCl supply was terminated while GeCl was further supplied.
Si–Si band (510 cm ) was observed, while only a Ge–Ge band
À1
EDX, XPS and Raman spectroscopy studies revealed that Si
existed on the surface of Ge NWs at impurity levels, and the
FET devices fabricated with as-synthesized Ge NWs showed
p-type semiconductor transport characteristics at room
temperature.
(
300 cm ) was detected in Ge NWs. This result implies again
that no direct chemical bonding of Si to Ge or Si to Si exists in
1
8
Ge NWs (Fig. S2 in ESIw).
To evaluate the quality of Ge NWs synthesized using liquid
GeCl as a precursor, individual Ge NWs were fabricated into
4
This work was supported by the Nano/Bio Science &
Technology Program of MEST (2005-01325), KOSEF
through EPB center (R11-2008-052-02000). KOSEF
the form of a field effect transistor (FET) device, and their
electrical transport properties were studied at room temperature.
As shown in Fig. 5, the Ge NW FET device shows a p-type
semiconductor characteristic with low degree hysteresis upon
(
(
2008-04306, 2007-8-1158), Korean Research Foundation
MOEHRD, KRF-2005-005-J13103) H.C.C thanks the World
IDS–VGS scanning, even without extra doping. Such an intrinsic
Class University (WCU) program through the KSEF
R31-2008-000-10059-0). The authors thank Sung Woo Heo
hole doping is attributed to the accumulated holes trapping
9, 20
(
1
negative charges at the surface of the Ge NWs.
2
The on/off
for taking Raman spectra.
ratio is high (B10 ) with a high on-state current level. Because
the electrode metal (Cr/Au)–Ge NW contact is not ohmic, the
Notes and references
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DS–VDS curves show asymmetric contact behaviors.
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Fig. 4 XPS spectra of Ge NWs grown on (a, b) Si(111) and (c, d)
Ge(100) substrates. For the inspection of Si 2p peaks, Ge NWs were
grown on a Ge(100) substrate instead of Si(111) to avoid any possible
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126 | Chem. Commun., 2009, 5124–5126
This journal is ꢀc The Royal Society of Chemistry 2009