Transcript document

Germanium Nanowire Growth and Device Applications
Lauren Klein, Tong Wang, Eric Garfunkel
Department of Chemistry, Rutgers University, Piscataway, NJ
Introduction: Germanium nanowires were grown via the vapor-liquid-solid (VLS) method in a hot-wall chemical vapor
deposition reactor. Directionality was obtained by epitaxial growth on single-crystal Si and Ge substrates, aided by
appropriate etch chemistries and growth conditions. The structure, chemistry and properties of nanowires have also
been investigated. The unstable native oxide on Ge nanowires results in poor electrical and optical properties. We are
investigating various chemical passivation routes, including chlorination, alkylation, H-termination, and thiol methods,
as well as more robust high-k dielectrics, such as HfO2, by atomic layer deposition. The nanowires and their coatings
have been characterized by scanning and transmission electron microscopies, x-ray photoelectron spectroscopy, x-ray
diffraction, and Rutherford backscattering spectrometry. Ge nanowires were also grown on a variety of other substrates
for potential applications as transistors, solar cell and sensors.
Nanowires are grown via a vapor-liquid-solid (VLS) mechanism in our
chemical vapor deposition reactor. Germane gas is used as a precursor,
and substrates are functionalized with 20nm gold nanoparticle catalyst
seeds. The gold nanoparticles catalyze the decomposition of the GeH4 and
forming a liquid eutectic with the germanium as it dissolves in the metal.
As the influx of Ge to the particles continues, the gold-germanium solution
becomes supersaturated leading to the eventual precipitation of the
germanium nanowire from the particle. Most wires grow from the substrate
with the catalyst particle remaining on top. We tune growth times, partial
pressure, flow rate, and temperature to achieve optimal growth on various
substrates.
Epitaxial Growth on Si and Ge
Substrate Preparation
To functionalize single crystal silicon and
germanium substrates with Au, wafers were
first etched in a 2% HF solution to remove
any native oxide, then in a 0.1 M solution of 3aminopropyltriethoxysilane (APTES) in
methanol for 10 minutes. When
subsequently submerged in a colloidal
solution of Au, the wafers become uniformly
coated with Au nanoparticles.
a)
10 cycles of HfO2 ALD
measured equivalent HfO2 thickness: ~ 12.6 Å
Ge areal density: 21.6E15 atoms/cm2 (equivalent
thickness: 49 Å)
Ge coverage: 3.6%
Si
a): before and b): after 10 cycles of atomic layer
deposition of HfO2 on Ge nanowires grown on Si (100)
substrate.
The Au-functionalized wafers are again
submerged in the HF solution for an
additional 2 minutes before immediately
being loaded into the reactor chamber. SiO2
and ITO substrates are functionalized under
similar conditions.
c): after 10 cycles of HfO2 ALD on Ge nanowires
grown on Si (111) substrate.
Ge
Hf
c)
b)
ZnO substrates were prepared with gold
nanoparticles through a slightly different
chemistry. The wafers are soaked in a 0.1 M
solution of 1,4-diaminobutane for 2 hours and
treated with gold colloids for 30 min.
d) Ge nanowires grown on SiO2 at 360°C and 3.2
torr germane partial pressure for 5 min.
Growth on Oxides
Dense, randomly-oriented nanowire growth
is achieved on oxide substrates. We
continue to explore substrate materials
such as silicon dioxide, indium tin oxide,
and zinc oxide for further technological
applications, such as the solar cell shown
to the right.
Energy [keV]
200
400
600
800
1000
1200
1400
1600
1800
7,000
6,500
d)
Ge areal density: 434E15 atoms/cm2 (equivalent
thickness: 98.4 nm)
Ge coverage: 33.5%
6,000
5,500
5,000
70116_Tong_GeNW_25D_BA_20kr2.AS1
Simulated
H
O
Si
Ge
Au
4,500
Ge
4,000
Counts
b)
a)
VLS growth in LPCVD reactor
Nanowire Growth and High-k Dielectric Layer Characterization by
Rutherford Backscattering Spectrometry (RBS, 2MeV He)
3,500
O
3,000
2,500
a)
Si
2,000
1,500
a) A polymer/nanowire hybrid solar cell using n-type
polypyrrole derivatives and p-type germanium nanowires.
Wires grown on ITO and coated in a polymer matrix.
1,000
Au
500
0
0
20
40
60
80
100
120
140
160
180
200
220
240
260
280
300
320
Channel
Passivation of GeNW
a) Top and b) side view of epitaxially grown wires on germanium <111> substrate with
schematic illustrating growth directions
c) Top and d) side view of epitaxially grown wires on silicon <100> substrate with schematic
Illustrating growth directions
After 400C anneal, without gate bias
The unstable native oxide of germanium results in an
electrically poor interface, making germanium unsuitable
for many device applications. In order to use germanium
nanowires for future technological applications, the
passivation problem must be resolved.
a)
c)
4
3
2
Ids (X 10 nA) _
a) Ge Nanowires grown on SiO2. Tapering of wires is
evident due to uncatalyzed radial CVD growth at high
temperatures. Wires are randomly oriented on oxide
substrates.
d)
c)
Single-Nanowire FETs: Two-Probe and Four-Probe Transport Measurement
1
0
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
-1
-2
b)
-3
In addition to the high-k deposition noted here, we have
begun to explore wet chemical passivation of our
nanowires.
-4
Vds (V)
After 400C anneal, Vds=0.1 V
0.7
0.6
0.5
b)
Ids (X 10 nA) _
Germanium surfaces can be passivated by hydrogen
termination after exposure to dilute hydrofluoric acid.
Similarly, surfaces can be chlorine terminated through
exposure to hydrochloric acid.
0.8
0.4
d)
0.3
0.2
Ge nanowires on Si100, near Si111 Bragg peak
Ge nanowires on Si100, near Si220 Bragg peak
c)
Si 220
Ge111
Si111
Ge 220
In order to achieve a more robust wet-chemical
passivation, we have functionalized our wires using 1,8octanedithiol. We intend to further explore
hydrogermylation and ammonia sulfide passivation
schemes.
a) As-fabricated unpassivated Ge nanowire FET
exhibiting large hysteresis by sweeping gate
voltage measured in air immediately after a
rapid thermal anneal in N2 at 400°C for 5 min
to obtain ohmic contact. Gate oxide is 400 nm
thick and electrodes are 40 nm thick Ti.
0
-10
-5
0
5
Current work involves FTIR and XPS studies of these
and other passivation chemistries.
Si 311
Ge220
Ge311
Vg (V)
Crystallinity and epitaxy evident from XRD spectra
Ge lattice constant = 5.661 +/- 0.01Å
Bulk value = 5.657Å
Acknowledgements: D. Olaya, M. Gershenson, N. Zhitenev,
L. Wielunski, T. Emge, A. Ermakov, C.L. Hsueh, J. Zhu, Y. Lu,
G. Fanchini, M. Chhowalla, R. Lorber, H.W. Zimmass, S.
Guha
References:
c) 4-terminal GeNW device
d) 4-terminal GeNW schematic
Ge311
b) Top view and c) side view of Ge nanowires grown on
ITO
10
-0.1
b) Schematic of GeNW transistor
Ge nanowires on Si100, near Si311 Bragg peak
Au111
0.1
Applications: We have begun to explore four type of Ge
nanowire (NW) devices: single NW transistors, multi-NW
thin film transistors, multi-NWs as components of
photovoltaic cells, and NW sensors.
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