Electrical Transport in Individual Metal and Semiconductor Nanowires

林志忠台灣交通大學物理所及電子物理系

Nano and Quantum Nano and Quantum Phenomena LaboratoryPhenomena Laboratory

NTHU 量子輸運實驗講習班 19-22 December 2006

林志忠教授 (交通大學) 簡紋濱教授 (交通大學)陳福榮、開執中教授(清華大學)

NTHU

HRTEM & SEM

Low TemperaturePhysics

Quantum Electron Transport

Omicron - LTSPM

Quantum Design- SQUID

Electron-Beam Lithography

Topics of Importance and Interest

small samples + low temperatures + quantum properties— small samples made by bottom-up & top-down methods— challenging experiments on low-temperature physics— novel quantum phenomena and new states of matter

H Kamerlingh Onnes:

知識來自測量Through Measurement to Knowledge

知識來自精密的測量精雕細琢、臻於極限

Four-probe measurements on individual semiconductor nanowires: ZnO

Four-probe measurements on individual metal nanowires: ITO (Sn-doped In2O3) and RuO2

Two-probe measurements: quantitative characterizations of electronic contact resistances

NTHU

Fabrication of Nanowires Characterization of Structures

]0011[]0002[

]0211[

Growth of small diameter metal and semiconductor nanowiresFabrication of DMS nanowires, e.g., Co-ZnO and Mn-ZnOHRTEM and EELS studies of atomic and electronic structures

Electrical Transport Measurements on Single Nanowires

• Fabrication of submicron electrodes

• Detection of small signals

• Low Temperatures and Magnetic fields

Semiconductor nanowires: ZnO

Metal nanowires: RuO2 and ITO (In2-xSnxO3)

Fabrication of Submicron Metal Electrodes

T/Au or cr/Au electrodes

120 µm

90 µm

3 µm

Standard photolithographye-beam lithography

李滄州

Fabrication of Submicron Metal ElectrodesTi/Au or Cr/Au electrodes

Substrate G, 054, Ru-t-1

abc d

17 µm

13 µm

Substrate H, 057

3 µm

2 µm

oxidized Si substrate

I or V

I or V

李滄州

Contact resistances may dominate the measured resistance

0 50 100 150 200 250 300 3500.2

0.3

0.4

0.5

500

1000

5000

10000

R (k

Ω)

T (K)

R2p= Rnanowire + 2Rcontacts + 2Relectrodes≈ 2Rcontacts

R4p = Rnanowire

70-nm diameter RuO2 nanowire

2-probe measurementEverything in the measurement loop contributes to the measured resistance

4-probe measurementOnly the nanowire section between the voltage probes contributes to the measured resistance

Appl. Phys. Lett. 90, 013105 (2007)

Semiconducting ZnO Nanowires

ZnO is a prototypical nanowire material system with a direct band gap of 3.35 eV

ZnO is widely used as a host material for the fabrication of diluted magnetic semiconductor devices ⇒ Spintronics

Electrical properties of ZnO nanowires are barely known

SEM image of a 210-nm diameter ZnO nanowire with six Ti/Au electrodes

Experimental method and parameters:

Electrical contacts are made of Ti(20 nm)/Au(130 nm)

As-deposited electrical contacts at 300 K are typically ~ tens kΩ to ~ hundreds kΩ

Annealing in vacuum at 300°C for a few tens min can reduce the contact resistance by ~ an order of magnitude (e.g., ~ a few kΩ)

-0.4 -0.2 0.0 0.2 0.4-12

-8

-4

0

4

8

12

I (nA

mp)

V (Volt)

I-V , ZnO_f-R_4P 300 K 70 K 2 K

Typical I-V curves measured with 4-probe configuration

Intrinsic Electrical Transport in ZnO Nanowires

As-grown ZnO NWs are n-type. The electron doping is attributed to Zn interstitials, oxygen vacancies, or hydrogen (Oxygen vacancies facilitate the occurrence of ferromagnetism in Co-ZnO ⇒ SQUID and magneto-transport studies)

The resistance at each temperature was determined from the “ohmic” dI/dV curve near zero bias

0.1 1 10 100

0.1

1

10

100

ρ (Ω

cm

)

T (K)

ZnO_k-1_A20, 4p ρ -T

B = 0

B = 1.5 T

Semiconductor ZnO Nanowires

0exp( / )AE Tρ ρ=

Thermally activated conduction near room temperature (e.g., 200-300 K)

250 nm

127 nm

87 nm

The electrical-transport around room temperature reveals thermal activation behavior. The activation energy EA may be determined

Magneto-transport in Single ZnO Nanowires

0 4 8 12 16 2010.0

10.1

10.2

10.3

10.4

10.5

T = 4.2 K

2005/6/11Zn-L0114-probefields up & down

R (M

Ω)

B2 (T2)

2( ) 2 2 *(0)

eR B B BmRτ µ∆ ⎛ ⎞= =⎜ ⎟

⎝ ⎠

1n eρ µ=*emτµ =

• The positive magnetoresistance of single-crystalline ZnO NWsreveals parabolic field dependence

⇒ The electron mean free time, mobility and carrier concentrationcan be extracted. The values are sensitive to individual nanowires

ZnO Nanowires: Comparison with other works

Our dataThin films

APL 83, 1128 (2003)

Single nanorodsAPL 85, 2002

(2004)

Single nanowiresNonotech. 16,

746 (2005)

Resistivityρ (300 K) (Ω cm)

~ 0.02~ 0.16

~ 0.1 ~ 4(2-probe)

~ 7(2-probe)

Mean free time τ (4 K) (s)

~ 8 × 10-14~ 1 × 10-13

--- --- ---

Electron mobilityµ (4 K) (cm2/V s)

~ 500~ 700

--- --- ---

carrier concentration n (cm-3)

~ 6 × 1017~ 1 × 1014

--- --- ---

Semiconductor ZnO Nanowires

1 10 10010-3

10-1

101

103

105

107

ρ (Ω

cm

)

T (K)

ZnO bulk_4p, single crystalZnO L011_4p, circular cross sectionZnO_d-L_4p, circular cross sectionZnO_f-R_4p, circular cross sectionZnO_g-L_4p, hexagonal cross sectionZnO_h-R_4p, hexagonal cross section

single crystalline

hexagonal

circular

heavily doped

As-grown ZnO NWs often contain high levels of defects, leading to significant n-type doping. ZnO is very sensitive to various gases

邱劭斌、李滄州、林永翰

Summary: ZnO Nanowires

Four-probe measurements have been performed on individual ZnO nanowires over a wide range of temperature and in magnetic fields

Microscopic carrier parameters (mean free time, mobility, carrier concentration) at low temperatures have been determined

Magneto-transport measurements may now be performed on diluted magnetic semiconductor NWs(e.g., Co-ZnO)

Magnetic Properties of Co-doped ZnO Nanowires

Magnetic Field (Oe)

Mag

netiz

atio

n (e

mu)

-1500 -1000 -500 0 500 1000 1500-8.0x10-5

-4.0x10-5

0.0

4.0x10-5

8.0x10-5

annealed in high vacuum second annealing in oxygen

Direct observation of structure and oxygen vacancy effects on ferromagnetism: carrier-mediated mechanism

Growth of nanowiresCo, Mn ion implantationThermal annealingHRTEMSQUID magnetometry

Magneto-transport measurements on individual nanowires would be a powerful probe for DMS NWs’properties

Phys Rev B 73, 233308 (2006) Nanotechnology 74, 172201 (2006)

Metallic RuO2 Nanowires

The electrical resistances of RuO2 single crystals decrease with decreasing temperature. The resistance ratio R(300 K)/R(4 K) is ~ 100–1000, indicating the high quality of the crystals

RuO2 single crystals

IrO2 single crystals

θD ≈ 400 K

θD ≈ 290 K

λBG = 0.13

λE = 0.26

θE = 810 K

Bulk RuO2 is a metal with ρ(300 K) ≈ 50–100 µΩ cm JPCM 16, 8035 (2004)

Metallic RuO2 Nanowires

Four-probe measurements on individual RuO2metal NWs

The electrical properties can be learned though quantitative measurements over a wide range of temperature and in magnetic fields

Quantum-interference transport may be expected at low temperatures

a b

c

de

≈ 0.5 μm

≈ 1 μm

≈ 40 nm

a b

c

de

≈ 0.5 μm

≈ 1 μm

≈ 40 nm

Resistance of a 40-nm diameter and 0.5-µm long RuO2 nanowire

The resistance decreases with decreasing temperature

R(300K)/R(5K) ≈ 1.5–1.6, indicating strong defect scattering

ρ (300K) ≈ 160 µΩ cm ⇒ le ≈ 7.5 Ǻ

林永翰

The electrical resistivity reveals typical metallic behavior

The thermoelectric power (Seebeck coefficient) indicates free-electron-like characteristic: a linear dependence on temperature

ITO is a metal with ρ(300 K) ~ 200 µΩ cm

0 100 200 300

-8

-6

-4

-2

0

S (µ

V/K)

T (K)

slope ~ EF-1

RF-sputtered polycrystalline ITO Films

Sn-doped In2O3-x Nanowires

JAP 96, 5918 (2004)

ITO (indium tin oxide) — the atomic ratio of Sn/In ~ 3–4 %

ITO nanowires reveal typical metallic behavior

At low temperatures, 3D electron-electron interaction effect causes a resistance rise with reducing temperature

19 µm

14 µm

0 40 80 120 160 200 240 280 320120

140

160

180

200

220

170 nm

240 nm

110 nm

ρ (µ

Ω c

m)

T (K)

metallic behavior

0 1 2 3 4 5 6 7 8 9 10 11159.5

160.0

160.5

161.0

161.5

ρ (uΩ

cm

)

T 1/2 (K1/2)

ρ-sqrT ; ITO_c-R_ab,cd ; LTC11_LR700 ; 941209

T 1/2

ρ0 ≈ 160 µΩ cm

邱劭斌

In 3D weakly disordered systems, electron-electron interaction effect causes a T1/2 resistance rise with decreasing temperature. This correction to the Drudeconductivity is due to the suppression of electronic density of states at the Fermi level

T 1/2

3000 Ǻ

sputtered thick RuO2 films

-10 -5 0 5 10

104

105

106

107

108

109

110

2005/06/30

AC measurement050630_Al/AlOx/AlRj=7 kΩAl film, 150A ρ(300K)=66 µΩ cmRsq(300K)=44 Ω/sq

0.36K, H=1.5T 1.8K, H=1.5T 5.3K, H=1.5T 10K, H=1.5T 20K, H=1.5T 5.3K, H=0T 10K, H=0T 20K, H=0T

G(V

)(µS)

V(mV)

dI/dV measurements at low temperatures

The suppression in DOS can be directly measured in metal-insulator-metal tunnel

junctions (葉勝玄、林志忠: unpublished)

Quantitative Characterizations of Electronic Contacts

• Miniature building blocks (nanowires) must be electrically connected to make into functional nanodevices

• The properties of electronic contacts need be understood and, hopefully, controlled

• Electron transmission through narrow constrictions are of fundamental interest

Two-Probe Experiment: RuO2 Metal Nanowires

• Very often, the measured resistance of a (presumably metal) nanowire increases with decreasing temperature, due to contact resistance being dominant in two-probe configuration

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0-16

-12

-8

-4

0

4

8

12

16

2004/10/15~16

Ru-w-1

Dia. ~ 300 nmLength ~ 300 nmρ(300K) ~ 3.4*104 µΩcm

I (n

A)

V (mV)

300 K 207 K 110 K 50 K 12 K

RuO2 nanowire

Cr/Au (10/90 nm) electrodes

黃鶯聲教授

Two-Probe Measurement on RuO2 Nanowires

• The properties of electronic contacts can be quantitativelycharacterized through two-probe measurements on an individual metal nanowire

• In the case of single metal nanowires:

• As the temperature reduces, the resistance does not increase as fast as expected for the thermally activated conduction

( ) 2 2 2 ( )mea NW contact leads contactR T R R R R T= + + ≈

0 0( ) exp( / )R T R T T=

thermal activation

elastic tunneling

two-probe method

three-probe method

Rc thermally fluctuation-induced tunneling conduction

The temperature behavior can be well ascribed to the “thermally fluctuation-induced tunneling conduction” previously proposed by Ping Sheng (1978, 1980)

• In the presence of thermal noise voltages, the total electric field across the junction is: E = EA ± ET

• The net tunneling current is given:

net ( ) ( )T TA Aj j E E j E E= + + −

rmstotal applied TV V V= ±〈 〉

Fluctuation-induced tunneling (FIT)According to the thermally fluctuation-induced tunneling model, the temperature dependence of the resistance is given by

10

0 exp TR R T T

⎛ ⎞⎜ ⎟⎜ ⎟⎜ ⎟⎝ ⎠

= +

mass effective electron : heightbarrier the: area junction the: widthjunction the:

0

mVAw

32

0 00 2 2

16 ,2B

AVTe k w mε

π= , 8 2

200

1 wkeAVT

B

ε=

• Typical values are: w ~ a few nm, V0 ~ a few meV

Electrical transport throughIrO2 nanowire contacts

In this case, the contact resistance is described by the “charging-effect hopping model”

Ref. Ping Sheng (1973)

Temperature dependence of contact resistances

-50 0 50 100 150 200 250 300 350-20

0

20

40

60

80

100

120

140

R (k

Ω)

T (K)

2p

3p

• Diameter ≈ 110 nm

• Length ≈ 1.4 µm

• R2p(300 K) ≈ 5.6 kΩ

• Rnanowire ≈ 0.4 kΩfor the whole measured temperature range

-1.0 -0.5 0.0 0.5 1.0-10

-5

0

5

10

-60 -30 0 30 60-10

-5

0

5

10

2p T = 12 K

Cur

rent

(nA)

Voltage (mV)

2p T = 300 K T = 200 K T = 88 K

Cur

rent

(nA)

Voltage (µV)

0.00 0.02 0.04 0.06 0.08 0.10

10

100

R (k

Ω)

T -1 (K -1)

2p

3p

T = 20 K

Simple thermal activation:

10

log

)/exp()(−∝⇒

∆=

TR

TkERTR B

0.2 0.3 0.4 0.5 0.6

10

100

R (k

Ω)

T -1/4 (K -1/4)

2p

3p

T ≈ 40 K

3-D Mott variable range hopping:

4/1

4/10

log

])/exp[()(−∝⇒

=

TR

TTRTR M

T -1/2 dependence of logR is observed over a very wide temperature range

2/12/100 log ])/exp[()(

−∝⇒= TRTTRTR

0.05 0.10 0.15 0.20 0.25 0.30

10

100

3pR0 = 1.9 kΩT0 = 113 K

2pR0 = 4.3 kΩT0 = 109 K

R (k

Ω)

T -1/2 (K -1/2)

100 K

In this case, the electronic conduction through the contacts can be understood in terms of electrons hopping through fine metal grains formed at the interface between the NW and the electrode

The maximum conductivity occurs at a dominant separation of neighboring metal grains of ∼5 (∼ 2) Å at 10 (100) K

Corresponding charging energy ∼ 3 (∼ 8) meV

CONCLUSION

• Controllable growth of semiconductor and metal nanowires

• Successful electron-beam lithography technique generating electrical-transport measurements on individual nanowires

• Successful four-probe measurements down to liquid-helium temperatures and in magnetic fields

• Electronic contact resistances, under certain configurations, can be quantitatively characterized

• As-grown nanowires often contain high levels of defects. The carrier mean free path is very short

• Further directionsMagneto-transport in DMS nanowiresElectron-interference transport in nanostructuresNew phases/states of matter in nanostructures

台灣交通大學

低溫及介觀物理實驗室林志忠教授Email: jjlin@mail.nctu.edu.tw

Magnetic Properties of Co-doped ZnO NanowiresTemperature dependence of contact resistancesT -1/2 dependence of logR is observed over a very wide temperature range