ZnCo2O4: A transparent, p-type, ferromagnetic semiconductor

relevant to spintronics and wide bandgap electronics

Norton Group Meeting

4/1/08

Joe Cianfrone

Proposal Review

• ZnCo2O4 is ferromagnetic semiconductor

• Can be p- or n-type

• Applications to spintronics and wide bandgap electronics

• Investigation of cation and anion substitution to better understand the optical, electronic and magnetic properties

ZnCo2O4: Structure and composition

• Spinel structure, A+2(B+3)2O4

• Space group Fd3m

• Tetrahedral A sites

• Octahedral B sites

(From O’Handley, from Woodward)

ZnCo2O4: Electronic and optical properties

• Carrier type depends on oxygen pressure during growth

• Indirect Bandgap of 2.63 eV (472 nm)

(From Kim et al.)

Background: Origins of ferromagnetism

• ZnCo2O4 can be both ferromagnetic and antiferromagnetic!

• Antiferromagnetic Co-O-Co superexchange

• Ferromagnetic Co-Co hole mediated exchange

• For a large enough number of holes, films are ferromagnetic

Experiment Methodology: Characterization Scheme

XRD θ-2θscans

Structural characterization

Vary T, P to optimizeGrowth region for spinel phase epitaxy

Tools usedExperiment Goal

Hall effect, SQUID,optical absorption, SE, AFM

XRD Ω-RCs, φ-scans, XPS

Quality of epitaxy

Vary Laser pulse E,f to optimize crystallinity

Hall effect, PPMS, SQUIDoptical absorption, SE, AFM

Magnetic, electronic, optical, surface properties

Film growth at optimum temperature, vary growth pressure

Film growth at optimum temperature, vary growth pressure

Magnetic, electronic, optical, surface properties

Film growth at optimum temperature, vary growth pressure

Hall effect, PPMS, SQUIDoptical absorption, SE, AFM

Magnetic, electronic, optical, surface properties

Film growth at optimum temperature, vary growth pressure

θ-2θ scan results of ZnCo2O4 grown on Sapphire

15 20 25 30 35 40 45 50 55 60

2θ

400 C

500 C

600 C

750 C

Sapphire (0001)ZnCo2O4 (222)

ZnO (200)

ZnCo2O4 (111) ZnCo2O4 (333)

CoO (200)

ZnCo2O4 (220)

Structural Characterization: X-Ray Diffraction

Structural Characterization: X-Ray Photoelectron Spectroscopy

XPS spectrum for a ZnCo2O4 film grown at 400 C, 150 mTorr

1.00E+05

1.50E+05

2.00E+05

2.50E+05

3.00E+05

3.50E+05

4.00E+05

4.50E+05

5.00E+05

5.50E+05

02004006008001000

Measured Energy (eV)

Inte

ns

ity

Zn peaks

Co peaks

O peak

O peak

C peak

Co peak symmetry resolves into two peaks indicating Co3O4 present in addition to ZnCo2O4

2.00E+05

2.05E+05

2.10E+05

2.15E+05

2.20E+05

2.25E+05

465470475480485490495500

Energy (eV)

Inte

ns

ity

Electronic Characterization: Hall effect

Resistivity vs. Oxygen Growth Pressure for ZnCo2O4

films grown at 400 C

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

0 50 100 150 200Oxygen Growth Pressure (mTorr)

Re

sis

tiv

ity

(O

hm

-cm

)

Carrier Density vs. Oxygen Growth Pressure for ZnCo2O4

films grown at 400 C

1.E+17

1.E+18

1.E+19

1.E+20

0 20 40 60 80 100 120 140 160 180 200

Oxygen Growth Pressure (mTorr)

Ca

rrie

r D

en

sit

y (

cm

-3)

n

p

Mobility vs. Oxygen Growth Pressure for ZnCo2O4 films grown at 400 C

0.001

0.01

0.1

1

0 20 40 60 80 100 120 140 160 180 200

Oxygen Growth Pressure (mTorr)

Mo

bili

ty (

V/s

/cm

2 )

Optical Characterization: UV-Vis spectra

Optical Absorption data for a ZnCo2O4 film grown at 400ºC, 150 mTorr

0

10

20

30

40

50

60

70

80

90

100

1 2 3 4 5 6

Energy (eV)

Tra

ns

mit

tan

ce

(%

)

Tauc plot for bandgap estimation indicates Eg=2.48 eV

0

50

100

150

200

250

300

350

400

450

500

1 1.5 2 2.5 3 3.5 4 4.5

Energy (eV)

(ah

ν)2

Magnetic Properties:SQUID

Magnetization vs. Field showing hystersis curve

-5.E-04

-4.E-04

-3.E-04

-2.E-04

-1.E-04

0.E+00

1.E-04

2.E-04

3.E-04

4.E-04

5.E-04

-1.E+04 -8.E+03 -6.E+03 -4.E+03 -2.E+03 0.E+00 2.E+03 4.E+03 6.E+03 8.E+03 1.E+04

Applied Field (G)

Mag

net

izat

ion

(O

e)

10 K

100 K

Magnetization vs. Temperature indicating TC>300 K

2.3E-05

2.4E-05

2.5E-05

2.6E-05

2.7E-05

2.8E-05

2.9E-05

3.0E-05

3.1E-05

3.2E-05

0 50 100 150 200 250 300

Temperature (K)

Mag

net

izat

ion

(O

e)

Zero Field Cooled

Field Cooled

• Ferromagnetism exhibited inZnCo2O4 sample grown at 400 C, 150 mTorr O2

• TC>300 K

Experiment status

• Known:– Optimum temperature and substrate isolated (400 C,

sapphire)– Film grown at 400 C, 150 mTorr was ferromagnetic

• Unknown:– Effect of pressure on electronic, magnetic, optical

properties– Origin of ferromagnetism– Effect of laser pulse frequency on crystal quality and

how this affects electronic, magnetic, optical properties

Planned Experiments

• Films to be grown at 400 C and pressures:– 10, 50, 100, 150, 200, 300 mTorr O2

• Characterization:– Electronic: Hall, PPMS (AHE, MR)– Magnetic: SQUID, PPMS (χ vs. H, χ vs. T)– Optical: Spectroscopic Ellipsometry, Optical

Absorption– Surface: AFM

Goals• Electronic properties as a function of Oxygen growth pressure:

– Carrier type

– Resistivity

– Carrier density

• Optical properties dependence on growth pressure:

– (α)1/2 and (αhν)2 vs. E

– Size and type of bandgap

– Refractive index, extinction coefficient vs. wavelength

Goals

• Magnetic properties dependence on growth pressure:– M (per Co atom) vs. pO2

– TC vs pO2

• Origin of magnetic nature– Carrier dependent Co-Co exchange?

• AHE?

– Co clusters?• TEM to look for Co precipitates• XPS to look for Co-Co atoms

CTT

C

H

M