Field effect experiments on Field effect experiments on transition metal oxidestransition metal oxides

E. E. BellingeriBellingeri

CNRCNR--INFM INFM -- LAMIA LAMIA CorsoCorso PerronePerrone 24 16152 24 16152 GenovaGenova ItalyItaly

Oxide Electronics Group Oxide Electronics Group

Director Director A.S. A.S. SiriSiri**

D. MarrD. Marréé**, , I. Pallecchi, L. PellegrinoI. Pallecchi, L. Pellegrino

G. Canu, A. G. Canu, A. CavigliaCaviglia

**UniversitUniversitàà didi GenovaGenova DipartimentoDipartimento didi FisicaFisica

Via Via DodecanesoDodecaneso 33 16146 33 16146 GenovaGenova

ItalyItaly

0

5

1015

20

2530

3540

45

50

Film 1 Film 2 Film 3 Film 4 Film 5 Film 6 Film 7 Film 8

μ 0Hc2

// ( T

esla

)

0 5 10 15 20 25 30 35 400

5

10

15

20

25

μ 0Hc2⊥ (

Tes

la )

T ( K )

0.0 0.2 0.4 0.6 0.8

1.5

2.0

2.5

3.0

Ani

sotro

py

T / TC

0

5

1015

20

2530

3540

45

50

Film 1 Film 2 Film 3 Film 4 Film 5 Film 6 Film 7 Film 8

μ 0Hc2

// ( T

esla

)

0 5 10 15 20 25 30 35 400

5

10

15

20

25

μ 0Hc2⊥ (

Tes

la )

T ( K )

0.0 0.2 0.4 0.6 0.8

1.5

2.0

2.5

3.0

Ani

sotro

py

T / TC

Superconductivity & technological transferThin films of superconductingmaterials – MgB2 with extremely high upper critical fields

Superconducting fault current limiterfabricated at LAMIA for CESI SpA

Superconducting resonator fabricatedat LAMIA for ESAOTE SpA

Spin-off company for MgB2 :Columbus Superconductors srl:

Km length MgB2 superconducting tapes and wires

Synthesis of superconducting oxidesStructural refinement and HRTEM analysis

Single crystal growth of superconducting

and magnetic oxides

HPHT Combustionsynthesisof nitrides Mechanical alloying

of metallic and ceramicnanopowders

Synthesis of intermetallic alloysfor hydrogen storage

and ceramics for fuel cells

Oxide Electronics: new materials and nanodevices

Channel dim: 0.7 μm x 8 μm

120 130 140 150

5.4x107

5.6x107

5.8x107

TMI=134.33KVgate = +/- 40 Volts => ΔTMI~ 1K

R (o

hm)

T (K)

Field Effect Nanotransistorson Functional Oxides

0 50 150 2001.0x104

1.5x104

2.0x104

2.5x104

3.0x104

3.5x104

Cha

nnel

Res

ista

nce

(Ω)

Time (min.)

30

-15

15

-30

0 Gat

e Vo

ltage

FerroelectricMemories Colossal

Magnetoresistance based devices

Outline

• Transition metal oxides

• Field effect geometries

• Experiments

Traditional

Ferroelectric

StackedBack GateSide Gate

Stacked

Local (AFM)Oxide Semiconductor

Manganites

Superconductor

Perovskite

Wurzite

Transition Metal oxides

• Transparent semiconductor • High mobility• Wide-band gap• Doped with magnetic ions (Co, Mn…): dilute magnetic semiconductors

ZnO HexagonalLattice parametersa=3.24 Å, c=5.19 Å

A: Alkaline Earth B: Transition Metal

Lattice Parameter (3.9 ± 0.1)Å

(Pseudo)cubicMany physical properties depending on cations• High Tc superconductivity• Ferroelectricity• Ferromagnetism, Colossal Magnetoresistance• Semiconductors, Insulators

ABO3

Wurzite

Perovskite

Dielectrics and semiconductors:

SrTiO3, LaVO3 , ZnO

0 50 100 150 200 250 30010-8

10-7

10-6

10-5

10-4

10-3

10-2

ρ (O

hm c

m)

Temperature (K)

M-I transition in titanates

Colossal magnetoresistance:

La1-xSrxMnO3, La1-xBaxMnO3

120 180 240 3000

10

20

30

40

50

60

70

80

90

R (K

Ω)

T (K)

B=0 B=0.1 T B=0.2 T B=0.5 T B=1 T B=2 T B=3 T B=5 T B=7 T B=9 T

0 50 100 150 200 250 3000.0

5.0x105

1.0x106

1.5x106

2.0x106

2.5x106

3.0x106

3.5x106

4.0x106

4.5x106

R (Ω

)

T (K)

Continuos flow P(O2) 10-2 mbar P(O2) 10-3 mbar P(O2) 10-4 mbar

Superconductivity:

YBCO, LaSCO, infinite layers…0 20 40 60 80 100

0

5

10

15

20

(La1.85Sr0.15)CuO4 thin film

Res

ista

nce

[Ω]

Temperature [K]

n Band filling factor proportional to the chargeW Band width proportional to d and p orbital overlapping

BANDWIDTH CONTROL

FILLING CONTROL

Isovalent atomic substitution(different radius)

Pressure

Orbital overlappingEterovalent atomic substitution

Field effect

Fermi level variationCell deformation

How to change n and W?

Properties are controlled by:

Problem: the phase diagram depends not only on the carrier concentration but also on the lattice structure; chemical doping affects both carrier concentration and lattice structure and it is very difficult to discriminate between these two contributions.Field effect tunes the carrier concentration only, thereby it is a powerful tool to study this correlated system

Illustration of the zero-temperature behaviour of various correlatedmaterials as a function of charge density. Silicon is shown as a reference. The examples for high-Tc superconductors and for colossal magnetoresistive (CMR) manganitesreflect YBa2Cu3O7- and (La,Sr)MnO3, respectively. AF, antiferromagnetic; FM, ferromagnetic; I, insulator; M, metal; SC, superconductor; FQHE, fractional quantum Hall effect; Wigner, Wigner crystal.

Electric field effect in correlated oxide systemsC. H. Ahn, J.-M. Triscone and J. Mannhart

Nature 424, 1015-1018 (28 August 2003) doi: 10.1038/nature01878

102010181014 101610121010 1022 (e cm-3)

Many of interesting physical properties in these material occur at 1019-1021 e-/cm3

(for 100 nm film => 1014 – 1016 e/cm2 => 10 – 1000 μC/cm2)Very high q value!

High polarizationUltrathin films

λε ε

DS

ext

kTe n

= 02

20

2

0

2

342

3

neE

k

Enek

Fr

TFTF

FrTF

εεππλ

εε

==

=

The characteristic width of accumulation or depletion layer is given by the electrostatic screening length

Thomas-Fermi for metal

Debye for semiconductors

≈1 A

≈10 nm

Lower carrier density larger λ

3.8 nm12 nm20 nm1018 (e/cm3)

1.2 nm3.8 nm6.5 nm1019 (e/cm3)

0.38 nm1.2 nm2.0 nm1020 (e/cm3)

εr =10 εr =100 εr =300 N(e/cm3)

3.8 nm12 nm20 nm1018 (e/cm3)

1.2 nm3.8 nm6.5 nm1019 (e/cm3)

0.38 nm1.2 nm2.0 nm1020 (e/cm3)

εr =10 εr =100 εr =300 N(e/cm3)

Debye length at room temperature

Field effect devices: Stacked Geometry

MIS heterostructureMetal-Insulator-Semiconductor

• Growth of the dielectric layer on the channel• Compatibility problems• Leakage

ChannelSTO single crystalGate

Source Drain

Gate

Back gate geometry

• Very good dielectric properties of the oxide (single x-tal)

• Very thick High voltage

Side gate devices

E

I

Source Drain

gatesgates

I

E

gategate

Active Active channelchannel

Source

Drain

Gate

Gate

Source Drain

Advantages of the side gate geometry

• Easy: 2-layers structure (film/substrate)

• High quality substrates Best Dielectric Properties

• Low voltage required

•Possibility to study the surface by Scanning Probe Microscopy

Planar Geometry Side Gate Geometry

Side gate field effect devices fabrication by AFM

First step: patterning by optical lithography and wet etching in 10-20 μm wide crossing channels

with bond pads

Conducting silicon W2C coated tip

0.12N/m

Second step: sub-micron patterning by Atomic Force Microscope anodization: the AFM biased tip triggers a local chemical and morphological transformation. Nanoxidation of siliconJ.A.Dagata,et al., Appl. Phys. Lett. 56, 2001 (1990)

Fabrication of constrictions as narrow as 40 nm.

The modified regions are swollen, porous and electrically insulating

•Nanoscale GaAs/AlGaAsheterostructures

Local “anodic oxidation” of oxides:

11 μm x 11 μm

11 μm x 11 μm

11 μm x 11 μm

Voltage controlled etching depthVoltage controlled etching depth !!!!!!

Modified regions are selectively etched in HCl solutionModified regions are selectively etched in HCl solution

HCl HCl solutionsolution

11 μm x 11 μm11 μm x 11 μm

Etched sub-micron wide insulating barriers can sustain applied voltages up to ±80 Volts without appreciable leakage (<100pA).

-80 -40 0 40 80

-0.4

-0.2

0.0

0.2I g (

nA)

Vg (Volts)

T=20K T=100K T=200K

Gate Gate

Source

Drain

Side gate field effect devices by AFM

-400 -300 -200 -100 0 100 200 300 400

107

108

X [nm]

Elec

tric

Fiel

d (V

/m)

-1 nm -20 nm -50 nm

AIR εr =1

Substrate εr

Gate Gate

Channel

Channel width 600 nm

Gap 1200 nm, Thickness 10 nm

Applied Voltage V = 10V

EstimatedEstimated capacitancecapacitance:C/m = 8.8 * εr pF/m

)/(10 210 VcmedV

dnr

g

sheet ×= ε

εr = 25 (LAO)

dnsheet= 2.5 1012 (e/cm2)

εr = 1000 (STO)

dnsheet= 1014 (e/cm2)

Vg=10Volts

Electric field profile and capacitance calculation

Ferroelectric Field effect Induced charge proportional to the ferroelectric remnant polarization

Ferroelectric

Channel

GatePulse

Ferroelectric properties:

-10 -5 0 5 10-40

-20

0

20

40

P (μ

C/c

m2 )

V (V)

Pb(Zr0.20,Ti0.80)O3

Remnant polarizationPr ~ 10÷60 μC/cm2

Coercive fieldEc~ 100 kV/cm

Pr~ 20 μC/cm2

σ~1014charges/cm2

c-axis= 4.13-4.16 Å

Pb

Pb

O

Pb

O

Pb

O

Ti

O

Pb

O

Pb

O

Pb

Pb

• No V applied during measurements• No leackage

• Only 2 states available

Perovskite films growth and structural properties

0 20 40 60 80 100 120 140180

190

200

210

220

230

240

Inte

nsity

(a.u

.)

Time (sec.)

Layer by layer growth

0 2 4 μmAtomically

smooth surfaces“Sharp” interface reflectivity

0 45 90 135 180 225 270 315

LAO 110

Cou

nts

[a.u

.]

φ [Deg]

STO 110

“Cube on cube”

epitaxial growth

20 25 30 35 40 45 500

1x104

2x104

3x104

4x104

5x104

6x104

7x104

8x104

44 45 4 6

10 3

10 4

10 5

2θ [D e g ]

Cou

nts

2Θ [deg]

Oxides Semiconductors

ZnOSTO single crystalAg (ZnO)

Source Drain

Gate

Easy case :ZnOBack-gate devices

Double side polished SrTiO3 110 substrate

10 20 30 40 50 60 70 80 90 100105

106

107

108

RS

D [Ω

]

T[K]

VG [V] +100 +50 0 -50

-15 -10 -5 0 5 10 150

100

200

300

400

T=300 K

RS

D (k

Ω)

E G (kV /cm )-5 -4 -3 -2 -1 0 1 2 3 4 5

1 0 5

1 0 6

1 0 7

1 0 8

1 0 9

1 0 1 0

RS

D (Ω

)

E G (K V /c m )

IS D 1 n A 1 0 n A 1 0 0 n A

Channel thickness 70 nm

T=50 K

Channel thickness 270 nm

• More then 5 order of magnitude RSD modulation at 50K

n = 5 1016 e/cm3

0 2 4 60

2

4

6

8

10

-3 -2 -1 0 1 2 3

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

1 V

2 V

3 V

4 V

VG=5 V

I SD [μ

A]

VSD [V]

0.25 0.5 1 2 3

Vg [V] -1 -0.5 -0.25 0

VSD [V]

I SD [μ

A]

0 5 10 15 20 2505101520253035

10-11

10-10

10-9

10-8

10-7

10-6

0

2

4

6

8

10

12

-20 -10 0 1010-10

10-9

10-8

10-7

10-6

μFE

[cm

2 /Vs]

Vg [V]

77K

I SD [A

]

μ EF [c

m2 /V

s]

VG [V]

300K

I SD [A

]Gate

Gate

Source Drain

5 μm

Vds= 0.3 V

n = 6 1018 e/cm3

77 K

Transfer characteristics & field effect mobility

Characteristic curves

ZnOSide gate devices

20 nm thick ZnOfilm by two-step

method

0 10 20 30 40 50 60 70 80 90 100106

107

108

109

R

[Ω]

T [K]

-4 -2 0 2 4 6 8 10 12

0.00

0.05

0.10

0.15

0.20

ΔE [e

V]

Vg [V]

C ExpDec1 fit of Data1_C

Thermal activated behaviour

MIT driven εr of the STO substrateCompetition between T and V dependences of εr Estimation of the density of state

(impurity band)

D(E)∝1/(E-Ec)

Time stable 20 % change in channel resistance

modulated by polarization pulses

Ferroelectric Field Effect0 50 150 2001.0x104

1.5x104

2.0x104

2.5x104

3.0x104

3.5x104

Ferroelectric field effect on SrTiO3 channel

Cha

nnel

Res

ista

nce

(Ω)

Time (min.)

30

-15

15

-30

0 Gat

e Vo

ltage

Consistent withchannel thickness

and channel carrierdensity

Strontium titanate resistance modulation by ferroelectric field effectD. Marré, A. Tumino, E. Bellingeri, I. Pallecchi, L. Pellegrino, A.S. Siri,

J. Phys. D: Appl. Phys. 36 No 7 896-900 (2003)

Ferroelectric

SrTiO3:La Channel

GatePulse

Substrate

S D

First example of side gate devices (SrTiOFirst example of side gate devices (SrTiO33 on LaAlOon LaAlO33))

Fabrication of submicron-scale SrTiO3 devices by an atomic force microscope L. Pellegrino, at al Appl. Phys. Lett. 81, 3849 (2002)

0 50 100

15.6

15.7

15.8 d)

10 nA

100 nA1 μA

Res

ista

nce

(MO

hm)

Time (s)

1 μm x 1 μm

200 nm

Drain

Source Gate

Ground

3 μm x 3 μm

Successive Modification of the channel by the AFM tip

950 1000 1050 1100 1150 1200

-10-505

10

23.0

23.2

23.4

Gat

e V

olta

ge (V

)

Time (Sec)

Res

ista

nce

(MO

hm)

Temperature Temperature characterizationcharacterization of the side FET of the side FET

0 30 60 90 120 150 180 210 240 270 3000.92

0.94

0.96

0.98

1.00

1.02

1.04

1.06

1.08

1

10

100

Res

ista

nce

(MO

hm)

R(V

) / R

(0)

Temperature (K)

+44V/0V -44V/0V

0 V +44V - 44V

-60 -40 -20 0 20 40 600.80

0.85

0.90

0.95

1.00

1.05

1.10

1.15

1.20

R(V

)/R(0

)

Applied Gate Voltage (V)

300K (1micAmp) 200K (100nA) 100K (100nA) 80K (20nA) 60K (20nA)

-40 -20 0 20 400.92

0.94

0.96

0.98

1.00

1.02

1.04

1.06

1.08

1.10

R(V

)/R(0

)

Applied Voltage (V)

300 K 200 K 100 K 50 K 30 K 20 K 10 K 11 μm x 11 μm11 μm x 11 μm

HCl HCl solutionsolution

Nonlinearities in the I vs V channel behavior

VVgategate=0=0

-100V

+100V

Linear to Nonlinear

characteristics at low Ttransition of the SD

Gate modulation of Gate modulation of the SD linearitythe SD linearity

-20 -15 -10 -5 0 5 10 15 20

-0.6

-0.4

-0.2

0.0

0.4

0.6

0.2

Sour

ce D

rain

Vol

e (V

)

Source Drain Current (nA)

tag

200K 100K 50K 20K

-50 -40 -30 -20 -10 0 10 20 30 40 50-0.4-0.3-0.2-0.10.00.10.20.30.4 50 K

Sour

ce D

rain

Vol

tage

(V)

Source Drain Current (nA)

0V +22V +44V -22V -44V

-20 -10 0 10 20-0.8-0.6-0.4-0.20.00.20.40.60.8

20 KS

ourc

e- D

rain

Vol

tage

(V

)

Source Drain Current (nA)

-100V -44V 0 V +44V +100V

0 100 200 300 400 500 60020.0

22.5

25.0

27.5

30.0

32.5

35.0

37.5

- 100 V

0 V

- 100 V

- 88 V- 100 V

- 88 V

- 66V

- 44 V

-22 V0 V

Res

ista

nce

(MO

hm)

Time (s)

Observation of EObservation of E--Field Induced DriftField Induced Drift

Low Gate fieldLow Gate field High Gate field High Gate field

No No hysteresishysteresis Memory effectsMemory effects

20 Kelvin

0 200 400 600 800 1000 1200 1400 160018

20

22

24

26

28

3020 Kelvin

0 V

- 44 V - 44 V

0 V

+ 44 V

0 V

-44 V

0 V

+ 44 V

0 V

Res

ista

nce

(MO

hm)

Time (s)

……ExploitingExploiting the the DielectricDielectric ConstantConstant of the of the SubstrateSubstrateMaximumMaximum resistanceresistance modulationmodulation observedobserved on on

homoepitaxialhomoepitaxial STO STO devicesdevicesSTO:LaSTO:La on STOSTO

STO substrate

-10 -8 -6 -4 -2 0 2 4 6 8 10

-50

0

50

100

150

200

ΔR/R

(%)

VGate (V)

2K 5K 10K 20K 50K

0 10 20 30 40 500

5

10

15

20

25

30

-10 -5 0 5 100.1

1

10

R (M

Ω)

Temperature (K)

VGate (V) +9 0 -9

50K

20K

10K

5K

2K

Res

ista

nce

(MΩ

)

VGate (V)

• Homoepitaxial thin films have better conductivity

• Higher effect at lower gate voltages

Drain-source current IDS plotted against the drain-source bias VDS of the Al2O3/SrTiO3 FET at 300 K. A channel length and a width of the FET device were 25 and 300 µm, respectively. The inset shows the blow-up of the IDS–VDS curve for VGS = 0 V

. (a) The gate-source bias VGS dependence of the drain-source current IDS for a fixed drain-source bias VDS = +1 V of the same device used for Fig. 2. The on–off ratiobetween VGS of 0 and 4 V for VDS of 1 V exceeds 100. (b) VGS dependence of the field effect mobility µFE. µ and µ were deduced from Fig. 3(a) by using Eqs. (1) and (2), respectively. Both increase monotonically with VGSand no saturation was observed even for large gate bias

Fig. 1. (a) Drain-source currentIDS plotted against the drain-source bias VDS of the Al2O3/KTaO3 FET for various gate voltages VGS at 300 K. TheKTaO3 single crystal was annealed at 700 °C prior to thedevice fabrication.

Manganites

SideSide--gate devices in a Lagate devices in a La0.670.67BaBa0.330.33MnOMnO33 exhibiting metallic behaviorexhibiting metallic behavior

Reversible shift of the metal-semiconductor transition temperature by 3.2K

120 128 136 144 152 160 168 1762021222324252627282930

Vgate = -66 V

0 VVgate = +66 V

TMI=147.84 K at Vgate =0ΔTMI=3.2 K at Vgate =+/- 66 V

R (M

Ω)

T (K)Comparison of the effects of electric and magnetic fields

Channel width 2.3 mmGate barrier width 1.4 mm

100 110 120 130 140

30

40

50

30 60 90 120 150

Vg = 0 V

T (K)

Vg = 0 VVg = -40 V

Vg = +40 V

R (M

Ω)

T (K)

B=0ΔTMI≈1.3K

190 200 210 220

3.9

4.0

4.1

4.2

90 120 150 180 210

Vg = 0 V

T (K)

Vg = -40 VVg = 0 V

Vg = +40 V

R (M

Ω)

T (K)

B=9TΔTMI≈1K

For εr≈1000, the equivalent charge per unit volume accumulated/depleted by a gate voltage

of ±66V in the channel is 6.2•1019 cm-3

Reversible shift of the transition temperature of manganites in planar field-effect devices

patterned by atomic force microscope I. Pallecchi et al., Appl. Phys. Lett. 83, 4435 (2003)

Film thickness 22 nm ⇒C≈7.6•εr pF/m

Field effect on planar devices made of epitaxial manganite perovskites I. Pallecchi et al., J. Appl. Phys. 95, 8079 (2004)

-80 -60 -40 -20 0 20 40 60 80

2832364044

30

33

3636

37

3817.0

17.2

17.4

17.6

Rds

(MΩ

)

Vg (Volts)

T=50K

Rds

(MΩ

)

T=100K

Rds

(MΩ

)

T=150K

Rds

(MΩ

)

T=190KThe relative change in channel resistance behavior as a function of the applied electric field is odd and linear at T>100K; at lower temperature the observed non-linearitiesmay be due to non-linear dielectric permittivity of the SrTiO3 substrate.

Sheet charge or volume charge?How much in depth does the electric field penetrate in a film with more than 1020 carriers/cm3? Shall we invoke a phase separationscenario?

Is this true field effect?

Side-gate devices in a La0.7Sr0.3MnO3 below the percolation threshold

0 50 100 150 200 2502

4

6

8

10

B=9T

B=6T

B=3T

B=1T B=0

Vg=0

R (M

Ω)

T (K)

50 100 150 2000.0

0.1

0.2

0.3

0.4

0 3 6 90.0

0.1

0.2

0.3

0.4

ΔR/R

(T=1

00K)

B (T)

B=9T

B=6TB=3T

B=1TB=0

ΔR/R

T (K)

The electric field enlarges or shrinks the metallic ferromagnetic domains, while the magnetic field

enlarges and also polarizes them.

50 100 150 2002

4

6

4

6

8

4

6

8

4

6

8

104

6

8

10

B = 9 T

Vg= - 70 V

Vg= + 70 V

Vg= 0

R (M

Ω)

T (K)

B = 6 T

Vg= - 70 V

Vg= + 70 VVg= 0

R (M

Ω)

B = 3 T

Vg= - 70 V

Vg= + 70 V

Vg= 0

R (M

Ω)

B = 1 T

Vg= - 70 V

Vg= + 70 V

Vg= 0

R (M

Ω)

B = 0

Vg= - 70 V

Vg= + 70 V

Vg= 0

R (M

Ω)

Semiconducting behavior with incipient metallic

transition

In a phase-separation scenario, metallic ferromagnetic regions are embedded in a semiconducting

paramagnetic matrix and their volume fraction is below the percolation threshold

I.Pallecchi et al.

High Temperature Superconductors

Field effects in superconducting films. Changeof the DS resistance of an ,8-nm-thick YBa2Cu3O7-d channel with a ,300-nm-thick Ba0.15Sr0.85TiO3 gate insulator. The blue curve corresponds to depletion of the carrier density, and the red curve correspondstoenhancement of the carrier density in the DS (drain–source) channel.

Temperature and electric field dependence of the capacitance and dielectric constant of the STO single-crystal gate insulator. Inset: schematic of the device

Resistivity as a function of temperature of the PBCO/NBCO heterostructure close to the foot of the transition and over the wholetemperature range (inset) for different applied fields across the gate dielectric. The resistivity was calculated using the thickness of the NBCO layer. The arrow indicates the value of TKT for zero appliedfield.

Critical temperature TKT (the Kosterlitz–Thoulesstemperature), and Tc0, the temperature at whichR=0.1 of the NBCO layer as a function of the measured polarization at TKT or Tc0

http://www.sciencemag.org/content/vol284/issue5417/images/large/se1897491001.jpeg

Temperature dependence of the resistivity of Nd1.2Ba1.8Cu3Oy films having different thicknesses: 4 u.c. (closed squares), 8 u.c. (open squares), 10 u.c. (closed circles), and 110 cells (open triangles). In the inset a sketch of the field effect device is shown.

Sheet resistance measured as a function of temperature on an 8 u.c. FET for Vg = 0 (closed circles), Vg = –30 V (open diamonds), and Vg

= –34 V (open circles). The dashed line indicates the value of the quantum resistance RQ = 6.45 k . In the inset the insulating–

superconducting transition is shown

Conclusions• Field effect in transition metal oxides is possible

adopting new geometries and high-κ materials.

• Back and side gate geometry allow direct access to the channel under FE

• In ZnO the application is close

• FE proved to be a powerful tools for the study of strongly correlated electron system:– Magnetic transition and phase separation in manganites– Superconducting properties in HTCS