ZnO nanorod electronic devices
April 3, 2006
Gyu-Chul Yi
National Creative Research Initiative (CRI) Center for Semiconductor Nanorods &
Department of Materials Science and Engineering,
Pohang University of Science and Technology (POSTECH)
Pohang 790-784, Korea
*[email protected]
POSTECH
Dept. of Materials Science and Engineering
1
GYU-CHUL YI
[email protected]
Catalyst-free MOCVD of ZnO nanorods
- Well-developed semiconductor technology: MOCVD
- High purity
Catalyst-free MOCVD
- Nanodevices
Appl. Phys. Lett. 80, 4232 (’02)
DEZn
O2
500 oC
„
„
- Many different substrates:
Sapphire, Si, quartz and glass
- MOCVD: easy scale-up
POSTECH
Dept. of Materials Science and Engineering
2
No use of metal catalysts even
without any special substrate
treatment
Extreme case of island growth: at the
initial stage of growth, c-axis oriented
ZnO nano-crystallites are formed by
random nucleation process, then the
nano-crystals are elongated due to a
higher growth rate along the ZnO caxis direction, forming vertically
aligned ZnO nanorods.
GYU-CHUL YI
[email protected]
High purity ZnO nanorods
- Well-developed semiconductor technology
- High purity
- Nanodevices
TEM images
Temperaturedependent
PL spectra
PL intensity (arb. units)
Almost defect-free single
crystallinity in TEM images
I2-1
I2-2
I2-3
Low temperature PL spectroscopy:
sensitive to small amount of impurities
„ Peak separation with small FWHM in the
range of 1-3 meV: high optical quality
„ Free exciton peak at 3.376 eV
„ Free exciton observation at 10 K: high
purity
Appl. Phys. Lett. 82, 964 (’03).
XB
10 K
20 K
30 K
40 K
60 K
80 K
100 K
120 K
140 K
160 K
180 K
200 K
3.30
„
XA
3.32
3.34
3.36
3.38
Photon energy (eV)
POSTECH
Dept. of Materials Science and Engineering
3
GYU-CHUL YI
[email protected]
Fabrication of ZnO nanorod MOSFETs
- Well-developed semiconductor technology
- Nanodevices
Source
-A 250-nm-thick silicon oxide layer as an
insulating gate oxide layer on a heavily
doped n-type silicon substrate.
Drain
Gate: heavily doped n-Si
-Single crystal ZnO nanorod
-Metal contact pattern: e-beam
lithography and metal evaporation
& lift-off
-Au/Ti electrodes (good ohmic)
5 μm
POSTECH
Dept. of Materials Science and Engineering
Polymer coating: surface passivation
for reducing surface effect and enhancing
gate effect
4
GYU-CHUL YI
[email protected]
Electrical characteristics of surface-passivated FETs
4.0x10
Appl. Phys. Lett., 85, 5052 (’04).
-5
1x10
Vsd: 1.0 V
Drain current (A)
Drain current (A)
6.0x10
-6
0.8 V
-6
0.6 V
2.0x10
0.4 V
-6
Surface passivation
-5
8.0x10
-6
6.0x10
-6
4.0x10
-6
2.0x10
-6
Vsd: 1.0 V
-6
10
Vsd: 1.0 V
0.8 V
-7
10
0.6 V
0.4 V
-8
10
0.2 V
0.2 V
-9
0.0
-20
1.0x10
10
-10
0
10
20
-5
Gate voltage (V)
0
5
0.0
10
Gate voltage (V)
- The electrical characteristics of ZnO nanorod FETs were significantly improved by coating
polyimide on ZnO nanorod surfaces.
Before
After
Turn on voltage
> –20 V
~ –5 V
Transconductance
(gm/W)
1-2 μS/μm
20 μS/μm
Current ON/OFF ratio
~7
10 -10
Subthreshold swing (S)
–
1.4 V/decade
POSTECH
Dept. of Materials Science and Engineering
4
5
5
GYU-CHUL YI
[email protected]
Schottky contacts for Schottky diodes and MESFETs
ƒ Schottky contact: gate electrode
Schottky diodes, MESFETs
- no insulating layer between a Schottky gate and a
channel
- large capacitive coupling
- high signal power gain
- fast switching useful for high frequency device and
circuit applications
Schottky gate
- local gate: individual control of transistor for integration
Au:Schottky contact
Ti/Au:ohmic contact
ZnO nanorod Schottky diode
2 µm
ƒ
Metal oxide semiconductors:
- air stable surface without formation of an insulating native oxide layers
- clean and abrupt M/SC interface without any specific etching process
POSTECH
Dept. of Materials Science and Engineering
6
GYU-CHUL YI
[email protected]
3.0x10
-6
2.0x10
-6
1.0x10
Drain current (A)
Drain current (A)
Current-voltage characteristics of ZnO nanorod Schottky diodes
-6
1x10
-5
1x10
-7
1x10
-9
10
-11
0.0
-3
-2
-1
0
1
2
3
-3
-2
Drain voltage (V)
-1
0
1
2
3
Drain voltage (V)
ƒ Excellent rectifying current-voltage characteristic
ƒ Forward-to-reverse bias current ratio (If/Ir): ~105–106
ƒ Ideality factor (η): ~1.2: well-defined interface between
a ZnO nanorod and a Au layer
POSTECH
Dept. of Materials Science and Engineering
7
 qV

I = I s exp
− 1
 ηK b T 
GYU-CHUL YI
[email protected]
Electrical characteristics of ZnO nanorod MESFETs
-6
4.0x10
D-G
D
S
D
-6
2.0x10
S-G
I (A)
G
S
S-D
0.0
S, D: Au/Ti ohmic contact
G
G: Au Schottky contact
-6
-2.0x10
ZnO nanorod MESFETs
1.5x10
2.0x10
Vg = 0.0 V
-6
0
Vg = -2.0 V
1
2
1.0x10
2
3
V (V)
-6
-6
-6
Vd = 1 V
5.0x10
Vg = -1.5 V
0
1
Vd = 2 V
Vg = -1.0 V
-7
0.0
-1
Vg = -0.5 V
-6
Isd (A)
Isd (A)
5.0x10
-2
Vd = 3 V
1.5x10
1.0x10
-3
-7
0.0
-3
3
Vsd (V)
-2
-1
0
Vg (V)
-Conductance response to the gate bias voltage has been significantly enhanced:
low turn-on voltage of – 1.5 V.
- Individual MESFET operation for logic gate operation
POSTECH
Dept. of Materials Science and Engineering
8
GYU-CHUL YI
[email protected]
ZnO nanorod logic devices
Advanced Materials, 17, 1393 (’05).
1,0
0,1
1,1
V1
V0 (V)
V1
1
0
V0 (V)
-3
-2
Vo
0,0
-3
Vi Vo
1,0
Vi
-1
V2
Time (arb. units)
0,0
V1 V2 Vo
Vc
VC
0,1
V1
0
Time (arb. units)
NOT
V2
1
Vo
0
V1
AND
2
V2
0,0
Vo
Vc
Vo
V2
OR
2
1,1
3
V0 (V)
V0 (V)
3
Vc
NOR
-2
-1
Vo
VC
1
0
1,0
Vo
0
Time (arb. units)
POSTECH
Dept. of Materials Science and Engineering
9
0,1
1,1
V1 V2
Time (arb. units)
GYU-CHUL YI
[email protected]
Summary
High quality ZnO nanorods grown by catalyst-free MOCVD can be applied for
electronic nanodevices including field effect transistors, Schottky diodes, and
logic gates.
-5
Drain current (A)
Vsd: 1.0 V
10
-6
10
-7
0.8 V
-5
8.0x10
-6
6.0x10
-6
10
-8
0.4 V
4.0x10
-6
2.0x10
-6
-6
Vg = 0.0 V
-6
Vg = -0.5 V
1.5x10
0.6 V
1.0x10
Vg = -1.0 V
-7
10
5 μm
Vsd: 1.0 V
1.0x10
Isd (A)
1x10
0.2 V
-9
-5
0
5
Gate voltage (V)
Appl. Phys. Lett., 85, 5052 (’04).
5.0x10
Vg = -1.5 V
0.0
10
0.0
Vg = -2.0 V
0
1
2
3
Vsd (V)
Catalyst-free MOVPE
-6
-3
-6
0,0
V1 V2 Vo
2.0x10
V0 (V)
Drain current (A)
3.0x10
-6
1.0x10
Vc
NOR
-2
-1
0.0
-3
-2
-1
0
1
Drain voltage (V)
2
V
3
Advanced Materials, 17, 1393 (’05).
POSTECH
Dept. of Materials Science and Engineering
10
1,0
0
0,1
1,1
C
V
V1 V2
o
Time (arb. units)
GYU-CHUL YI
[email protected]
Research activities
Nanomaterials
„
Catalyst-free MOCVD of ZnO nanorods
„
ZnO/ZnMgO nanorod quantum structures with composition modulation along either
radial or axial direction.
„
Catalyst-free and catalyst-assisted growth of GaN nanorods
„
Chemical solution method to grow nanoparticles, nanorods, and nanodisks
„
Structural, optical, and electrical characterizations of nanomaterials
Nanorod device applications
„
Metallization on semiconductor nanorods (either ohmic or Schottky contact)
„
ZnO nanorod photonic devices (light emitting devices)
„
ZnO nanorod electronic devices (FETs, Schottky diodes, and logic gates)
POSTECH
Dept. of Materials Science and Engineering
11
GYU-CHUL YI
[email protected]