16.40 o5 p murmu

P.P. Murmu

NZIP 2011 Wellington, New Zealand

Investigation of structural, electronic and magnetic properties of Gd implanted ZnO single crystals

GNS ScienceMacDiarmid Institute VUW

ZnO applications


• II-VI compound semiconductor

• Direct band gap semiconductor, EB ~ 3.37eV

• Transparent conducting oxide

• Has n-type conductivity, due to zinc-interstitials and oxygen vacancies

• Availability of high quality bulk substrate (low defect densities) for homo-epitaxial film growth

• Suitable host material for spintronic applications ?

• ZnO based dilute magnetic semiconductors

c

a

Oxygen

ZincZnO properties

Spintronics cartoon courtesy to T. Jungwirth


Magnetic ions Cation Anion

H. Toyosaki et al. Nat. Mater. 3, 221 (2004)

Dilute Magnetic Semiconductors (DMSs)

• Semiconductor doped with magnetic elements (e.g. GaAs:Mn, GaN:Mn, GaN:Gd, TiO2:Co)

• Applications: Magnetic RAM, Spin FET, Spin LED etc.

• Room temperature ferromagnetism

• Origin: Metallic clusters / secondary phases ??

• Highly sensitive to defects

Datta and Das, APL 56, 665 (1990)


ZnO- Rare earth (RE) doping

• Large magnetic moments e.g. 7.94 μB for Gd

• Very high magnetic moment (4000 μB/Gd) in GaN:Gd*

• Aim: to investigate the electronic and magnetic properties of ZnO:Gd prepared by ion implantation

* Dhar et al. PRL 94, 037205 (2005)

Calculated and measured effective Bohr magneton for rare-earth ions


GNS ion implanter – New Zealand unique facility

Implantation parameters:

Energy: up to 100 keV

Ions: 12C+, 13C+, 14N+, 15N+,

&

Vacuum: < 2x10-7 mbar

Gd depth profiles calculated with DYNAMIC-TRIM for Gd implanted at 40 keV into ZnO in the fluence range from 6.7x1014 to 3.0x1016 ions.cm-2

0 5 10 15 20 25 30

0

2

4

6

8

10

12

14

F=6.7x1014 Gd.cm-2

F=3.9x1015 Gd.cm-2

F=6.4x1015 Gd.cm-2

F=1.1x1016 Gd.cm-2

F=3.0x1016 Gd.cm-2

Gd

conc

entr

atio

n (a

t.%

)Depth (nm)


Rutherford Backscattering spectrometry (RBS)

E. Rutherford, Philos. Mag. 6, vol. 21 (1911) p. 669-688

He++

He++

H. Geiger (left) and E. Rutherford (right)

Au-foil

"The making of the Rutherford documentary" by Dr John Campbell , 10.45 am Wed 19th Oct


Elastic interaction: RBS, channeling, recoiling ions

Inelastic interaction: X-rays, visible-UV photons, Nuclear reaction

RBS: kinematic factor, K α masscross section, σ α yieldenergy loss, ΔE α depth

RBS and channeling: drastic reduction in backscattered particles


Schematic illustration of RBS mechanism

High energy light ion interaction with target atoms


200 300 400 500 600 700 800 900

Channel

0

20

40

60

80

Nor

malized

Yield

1.0 1.5 2.0

Energy (MeV)

F=3e16 Gd/cm2 implanted ZnORUMP simulation

O Zn Gd

RBS performed with 2 MeV He+

Rutherford Universal Manipulation Program (RUMP) used to retrieve the composition, depth profile etc.

For the 1x1016 Gd cm-2 and higher fluence implantation, preferential sputtering occurs

Leads to a slightly lower retained dose for the implantation at higher fluences

0.0 5.0x1015 1.0x1016 1.5x1016 2.0x1016 2.5x1016 3.0x1016 3.5x1016

0.0

5.0x1015

1.0x1016

1.5x1016

2.0x1016

2.5x1016

3.0x1016

3.5x1016

fluence vs. reatained dose

Ret

ain

ed d

ose

(

ion

s.cm

-2 )

Fluence ( ions.cm-2 )

Zn

Gd

O

surface

3 MeV Particle Accelerator at GNS Science, Lower Hutt

RBS spectrum and RUMP fittingFluence vs. retained dose



RBS-Channeling

Random and <0001>-aligned RBS spectra of 3.9x1015 Gd cm-2 implanted and annealed ZnO (magnified Gd peaks in inset)

100 200 300 400 500 600 7000

100

200

300

400

500

600

560 570 580 590 600 610 620 630 6400

5

10

15

20

25

Gd peaks

Yie

ld (

arb

.u.)

Channel No.

Random Channeled

O

Gd

Zn

Yie

ld (

arb

.u.)

Channel No.

Random Channeled

Zn

RBS/C along <0001>

Zn minimum yield (ZnXmin) of ~7% indicates the high crystalline quality of un-doped ZnO

For 3.9x1015 Gd.cm-2, ZnXmin in as implanted ZnO is ~ 27%, which suggests the implantation causes only moderate structural damages

Vacuum annealing at 650 oC, the Xmin for Zn goes down to ~ 22%, which implies the annealing helps to regain the crystalline quality


RBS-Channeling

Angular scan around <0001> for 3.9x1015 Gd cm-2 (a) as-implanted and (b) annealed ZnO

Angular scans around <0001>

Zn and Gd scans followed same trend, suggests both subjected to similar disorders

Split between the scans implies some of Gd atoms at random sites

Using, GdZn= (1-GdXmin) / (1-ZnXmin), around 60% Gd atoms estimated to occupy the substitutional lattice sites

A small fraction of interstitial Gd atoms align with <0001> (a shadow-effect)

GdZn reduces to 47 % up on annealing, possibly due to the diffusion of Gd atoms

-3 -2 -1 0 1 2 30.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

No

rmal

ized

Yie

ld

Angle (deg.)

Zn Gd

(a)

-3 -2 -1 0 1 2 30.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1(b)

No

rmal

ized

Yie

ld

Angle (deg.)

Zn Gd


Raman Spectroscopy

• Raman effect: inelastic scattering of light (provides information about vibrational modes in a material)

• Discovered by C.V. Raman in 1928

• E2(high) and E2(low) modes from O and Zn vibrations

• 575 cm-1 attributed to the A1(LO) mode of ZnO

• A1(LO) mode primarily observed due to implantation induced disorder

• also correlated to the structural defects such as oxygen vacancies (Vo) and zinc interstitials (Zni) or their complexes

C.V. Raman demonstrating his Nobel Prize (1930) winning spectrometer

100 200 300 400 500 600 700

0.00

0.05

0.10

0.15

0.20

0.25

0.30

E2(low

)

2E2(M

)

A1(L

O)

E2(h

igh)

(a)

(b)

(a) as-implanted

(b) 650 oC (c) unimplanted ZnO

Norm

aliz

ed inte

nsi

ty

Raman shift (cm-1)

(c)

Raman spectra of unimplanted, 3.9x1015 Gd cm-2 implanted and annealed ZnO


SQUID results

• Diagmagnetic response observed in unimplanted and as-implanted ZnO

• Annealing enhanced the ferromagnetism in low fluence Gd implanted and annealed ZnO

• Ferromagnetism at room temperature !!

• FC curve shows combination of ferromagnetic and paramagnetic ordering

• Clustering or secondary phases ??

• Zero field-cooled (ZFC) magnetisation curve suggests contribution from super-paramagnetic particles

Hysteresis loop of 3.9x1015 Gd cm-2 implanted and annealed ZnO at 5 K & 300 K

-12 -9 -6 -3 0 3 6 9 12-4

-3

-2

-1

0

1

2

3

4

5 K 300 K

Mo

men

t (1

0-3 e

mu

/cm

3 )

Field (k Oe)

0 50 100 150 200 250 300 350 400

0.0

1.0x10-4

2.0x10-4

3.0x10-4

4.0x10-4

5.0x10-4

100 Oe

ZFC

Mo

men

t (e

mu

/cm

3 )

Temperature (K)

FC

FC and ZFC magnetisation curves of 3.9x1015 Gd cm-2

implanted and annealed ZnO at 100 Oe


SQUID results

• For 3.0x1016 Gd cm-2 implanted and annealed ZnO very small ferromagnetic ordering (even at 5 K)

• Ferromagnetism decreases at higher fluences

• Ferromagnetic interaction depends on the ions separation, and can have ferromagnetic or antiferromagnetic coupling

• Antiferromagnetic interaction among Gd atoms at high concentration may be responsible

(b) FC and ZFC magnetisation curves of 3.0x1016 Gd cm-2 implanted and annealed ZnO

0 50 100 150 200 250 300 350 400

-9.0x10-5

-6.0x10-5

-3.0x10-5

0.0

3.0x10-5

6.0x10-5

9.0x10-5

100 Oe

ZFC

Mom

ent (e

mu /cm

3)

Temperature (K)

FC

-12000 -9000 -6000 -3000 0 3000 6000 9000 12000

-4.0x10-3

-3.0x10-3

-2.0x10-3

-1.0x10-3

0.0

1.0x10-3

2.0x10-3

3.0x10-3

4.0x10-3

3.9x1015 Gd/cm2

3.0x1016 Gd/cm2

Mo

men

t (e

mu

/cm

3 )

Field (Oe)

(a) Hysteresis loops of 3.9x1015 & 3.0x1016 Gd cm-2 implanted and annealed ZnO at 5K


XANES results

• O K-edge (~ 538 eV) observed due to electronic transition from O 2p states to conduction band

• A pre-edge feature appears in ZnO thin film

• Usually assigned to intrinsic defects such as oxygen vacancies and zinc interstitials

• Gd atoms in 3+ state

1170 1180 1190 1200 1210 1220

0

2

4

6

8

Inte

nsi

ty (

arb

. u

.)

Energy (eV)

Gd-metal (ref.) pm-71: 0.007 Gd pm-84: 0.033 Gd

525 530 535 540 545 550 555 560 565 570

0

1

2

3

4

5 Si-607: as-dep ZnO film pm-103: as-rec. ZnO pm-71: 0.007 Gd pm-84: 0.033 Gd

Inte

nsi

ty (

arb

. u

.)

Energy (eV)

(a) O K-edge for as-deposited ZnO film, un-implanted and Gd-implanted ZnO single crystals

(b) Gd M-edge for Gd-metal and Gd-implanted ZnO single crystals


Conclusion

• RBS channeling along <0001> show ~60% Gd occupation in Zn sub-lattices

• A small fraction of the ions may be at interstitialy shadowed region aligned with <0001>

• Radiation damages related A1(LO) peak observed in Gd implanted ZnO

• Ferromagnetic ordering at room temperature

• Moment decreases at higher temperatures

• FC/ZFC curves suggest presence of small clusters

• O K-edge shows intrinsic defect related pre-edge feature

• Gd valency: 3+


• Ministry of Science and Innovation

• MacDiarmid scholarship

• GNS Science scholarship

• Australian synchrotron

Acknowledgement

Funding

Supervisors

• Dr A. Markwitz (GNS, New Zealand)

• Dr G.V.M. Williams (VUW, New Zealand)

• Dr S. Grenville (IRL, New Zealand)

• Dr S. Rubanov (University of Melbourne, Australia)

• Dr A. Suvorova (UWA, Australia)

Collaborators

• Dr B.J. Ruck (VUW, New Zealand)

• Dr J. Kennedy (GNS, New Zealand)


RBS and channeling

(a) Random (b) planar and (c) axial

16.40 o5 p murmu

Business

gd scans

bfor gd

low fluence gd

diffusion of gd atoms

implanted zno annealing

magnetic properties

shadoweffect gd zn reducesto

zno applications