GYU-CHUL YI [email protected] POSTECH Dept. of Materials Science and Engineering 1 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]
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GYU-CHUL [email protected]
POSTECHDept. of Materials Science and Engineering 1
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
GYU-CHUL [email protected]
POSTECHDept. of Materials Science and Engineering 2
Catalyst-free MOCVD of ZnO nanorods
- Well-developed semiconductor technology: MOCVD- High purity- Nanodevices
Catalyst-free MOCVDAppl. Phys. Lett. 80, 4232 (’02)
DEZn O2
500 oC
- Many different substrates: Sapphire, Si, quartz and glass
- MOCVD: easy scale-up
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 c-axis direction, forming vertically aligned ZnO nanorods.
GYU-CHUL [email protected]
POSTECHDept. of Materials Science and Engineering 3
High purity ZnO nanorods
- Well-developed semiconductor technology - High purity- Nanodevices
TEM images
Temperature-dependent PL spectra
3.30 3.32 3.34 3.36 3.38
XB
XA
I2-3I2-2I2-1
200 K180 K160 K140 K120 K100 K80 K60 K40 K30 K20 K10 K
Photon energy (eV)
PL
inte
nsity
(arb
. uni
ts)
Almost defect-free single crystallinity in TEM images
Low temperature PL spectroscopy: sensitive to small amount of impuritiesPeak separation with small FWHM in the range of 1-3 meV: high optical qualityFree exciton peak at 3.376 eVFree exciton observation at 10 K: high purity
Appl. Phys. Lett. 82, 964 (’03).
GYU-CHUL [email protected]
POSTECHDept. of Materials Science and Engineering 4
Fabrication of ZnO nanorod MOSFETs
5 μm
Gate: heavily doped n-Si
-A 250-nm-thick silicon oxide layer as an insulating gate oxide layer on a heavily doped n-type silicon substrate.
Source Drain
-Metal contact pattern: e-beam lithography and metal evaporation & lift-off-Au/Ti electrodes (good ohmic)
-Single crystal ZnO nanorod
Polymer coating: surface passivationfor reducing surface effect and enhancinggate effect
- Well-developed semiconductor technology - Nanodevices
GYU-CHUL [email protected]
POSTECHDept. of Materials Science and Engineering 5
Electrical characteristics of surface-passivated FETs
Surface passivation
-5 0 5 100.0
2.0x10-6
4.0x10-6
6.0x10-6
8.0x10-6
1.0x10-5
Vsd: 1.0 V
0.2 V
0.6 V
0.4 V
0.8 V
Gate voltage (V)
Dra
in c
urre
nt (A
)
10-9
10-8
10-7
10-6
1x10-5
Vsd: 1.0 V
- The electrical characteristics of ZnO nanorod FETs were significantly improved by coating polyimide on ZnO nanorod surfaces.
1.4 V/decade –Subthreshold swing (S)104-105~7Current ON/OFF ratio
20 μS/μm1-2 μS/μmTransconductance (gm/W)
~ –5 V> –20 VTurn on voltageAfterBefore
-20 -10 0 10 200.0
2.0x10-6
4.0x10-6
6.0x10-6
Vsd:
0.2 V
0.6 V
0.4 V
0.8 V
1.0 V
Gate voltage (V)
Dra
in c
urre
nt (A
)
Appl. Phys. Lett., 85, 5052 (’04).
GYU-CHUL [email protected]
POSTECHDept. of Materials Science and Engineering 6
Schottky contacts for Schottky diodes and MESFETs
Schottky contact: gate electrode- 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
- local gate: individual control of transistor for integration
Schottky diodes, MESFETs
Schottky gate
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
Au:Schottky contact
Ti/Au:ohmic contact
ZnO nanorod Schottky diode2 µm
GYU-CHUL [email protected]
POSTECHDept. of Materials Science and Engineering 7
Current-voltage characteristics of ZnO nanorod Schottky diodes
-3 -2 -1 0 1 2 30.0
1.0x10-6
2.0x10-6
3.0x10-6
Drain voltage (V)
Dra
in c
urre
nt (A
)
-3 -2 -1 0 1 2 3
10-11
1x10-9
1x10-7
1x10-5
Drain voltage (V)
Dra
in c
urre
nt (A
)
Excellent rectifying current-voltage characteristicForward-to-reverse bias current ratio (If/Ir): ~105–106
Ideality factor (η): ~1.2: well-defined interface between a ZnO nanorod and a Au layer
−= 1exp
TKqVIIb
s η
GYU-CHUL [email protected]
POSTECHDept. of Materials Science and Engineering 8
Electrical characteristics of ZnO nanorod MESFETs
-3 -2 -1 00.0
5.0x10-7
1.0x10-6
1.5x10-6
2.0x10-6
Vd = 1 V
Vd = 2 V
Vd = 3 V
Vg (V)
I sd (A
)
0 1 2 30.0
5.0x10-7
1.0x10-6
1.5x10-6
Vg = -2.0 V
Vg = -1.5 V
Vg = -1.0 V
Vg = 0.0 V
Vg = -0.5 V
Vsd (V)
I sd (A
)
ZnO nanorod MESFETs-3 -2 -1 0 1 2 3
-2.0x10-6
0.0
2.0x10-6
4.0x10-6
S-G
S-D
D-G
V (V)
I (A
)
D
S
GG: Au Schottky contact
S, D: Au/Ti ohmic contact
S D
G
-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
GYU-CHUL [email protected]
POSTECHDept. of Materials Science and Engineering 9
ZnO nanorod logic devices
V2
V1
Vo V2V1
Vo
VC
Vi
Vo
VC
V1
Vo
V2
0
1
2
31,1
0,11,00,0
Time (arb. units)
V 0 (V
) AND
NOT NOR
V2
V1
VoV2
V1
Vo
Vc
Vi Vo
Vc
V1 Vo
Vc
V2
0
-1
-2
-3
1
0
Time (arb. units)
V 0 (V
)
0
-1
-2
-3
1,10,11,0
0,0
Time (arb. units)
V 0 (V
)
OR
0
1
2
31,10,11,0
0,0
Time (arb. units)
V 0(V
)Advanced Materials, 17, 1393 (’05).
GYU-CHUL [email protected]
POSTECHDept. of Materials Science and Engineering 10
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.
Catalyst-free MOVPE0 1 2 3
0.0
5.0x10-7
1.0x10-6
1.5x10-6
Vg = -2.0 V
Vg = -1.5 V
Vg = -1.0 V
Vg = 0.0 V
Vg = -0.5 V
Vsd (V)
I sd (A
)
-3 -2 -1 0 1 2 30.0
1.0x10-6
2.0x10-6
3.0x10-6
Drain voltage (V)
Dra
in c
urre
nt (A
)
5 μm-5 0 5 10
0.0
2.0x10-6
4.0x10-6
6.0x10-6
8.0x10-6
1.0x10-5
Vsd: 1.0 V
0.2 V
0.6 V
0.4 V
0.8 V
Gate voltage (V)
Dra
in c
urre
nt (A
)
10-9
10-8
10-7
10-6
1x10-5
Vsd: 1.0 V
Appl. Phys. Lett., 85, 5052 (’04).
VC V1
VoV2
NOR
V1 Vo
Vc
V2
0
-1
-2
-3
1,10,11,0
0,0
Time (arb. units)
V 0 (V
)
Advanced Materials, 17, 1393 (’05).
GYU-CHUL [email protected]
POSTECHDept. of Materials Science and Engineering 11
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)
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