Bulk and Thin Films of ZnCoO by PLD

S. Karamat, R.S. Rawat and Paul Lee

NIE/NTU

Introduction

• Dilute Magnetic Semiconductors (DMSs)

The semiconductors in which the lattice is made up in part of substitution magnetic

ions.

Applications

Spintronics: Spin LEDs, Spin Transistors, Spin Valves, Magnetic recorders ----------

Challenge

Synthesis of a material exhibiting both semiconducting as well as magnetic

properties, a prerequiste for spin- electronic devices. The incompatibility between

non-magnetic semiconductors and magnetic materials is a big hinderance to combine

them in the form of one material having both properties.

One of the approaches to combine the spin and charge of the carriers in a material

having both semiconducting as well as magnetic properties is to introduce magnetic

ions like Mn, Cr, Co and Fe into non-magnetic semiconductors.

Synthesis Methods

Solid state reaction, Ball-milling, Sol-gel, RF magnetron sputtering, Chemical vapor

deposition (CVD), Metalorganic chemical vapor deposition (MOCVD), Molecular beam

epitaxy (MBE), Pulsed laser deposition (PLD), etc.

Motivation

Dietl. predicted on a theoretical basis that ZnOand GaN would exhibit ferromagnetism above

room temperature on doping with Mn.According to the theory, ferromagnetismbetween magnetic dopant ions is mediated byholes in the valence band through indirectexchange. Dietl's theory has proven useful inunderstanding the experimental results forGaMnAs but it appears to be inconsistent forthe experimental results of transition metaldoped wide bandgap semiconductors, such asZnO and GaN. It is based on many reasons,including the difficulty in experimentallypreparing p-type ZnO material and theobservations of ferromagnetism in n-type ZnODMS.

T. Dietl, H. Ohno, F. Matsukura, J. Cibert and D. Ferrand, Science, 287 1019 (2000)

Coey’s ModelAcoording to coey, donor defects which could arise from oxygen vacancies or zinc interstitialsin the case of ZnO, overlapped and form an impurity band. This impurity band can interactwith local magnetic moments through the formation of bound magnetic polarons (BMP).Within each BMP, the bound carrier interacts with the magnetic dopants inside its radius andcan align the spins of the magnetic dopants parallel to one another. Ferromagnetism isachieved when the BMPs start to overlap to form a continuous chain throughout the material,thus percolating ferromagnetism in the DMS.

Kittilstved confirmationKittilstved spectroscopic experiment of cobalt-doped ZnO showed that the singly ionizedCo + state lies close to the conduction band having almost the same energy as in a shallowdonor state. It showed if the energies are similar, charge transfer can take place between thecobalt atoms and the donor impurities which lead towards the hybridization necessary forferromagnetism. It showed an inherent polarity difference for ferromagnetism in cobalt dopedZnO.

Ueda ExperimentsUeda showed promising results and it was found that the ZnCoO become FM above 280 K with 5–25% Co doping.

Experimental Setup

Nd:YAG

Laser

Pulsed Laser

Deposition

System

Characterization Techniques

SIEMENS D5000 X-ray Diffractometer

Kratos Axis Ultra X-ray Photoelectron Spectroscopy (XPS) system

Cary 50 UV-VIS Spectrophotometer

SHIMADZU UV-VIS 2501 Spectrophotometer

Lakeshore 7400 Vibrating Sample Magnetometer

Results for Bulk Samples

XRD Results of bulk samples

Quantitative analysis

Rietveld Method

Material a

Å

c

Å

Volume

Å3

(ZnO)0.99(Co3O4)0.01 3.2510 5.2019 47.61526

(ZnO)0.98(Co3O4)0.02 3.2513 5.2015 47.62114

(ZnO)0.97(Co3O4)0.03 3.2517 5.2014 47.63133

(ZnO)0.95(Co3O4)0.05 3.2517 5.2014 47.63133

XPS Survey Scans for Bulk Samples

Zn 2p and Co 2p core peaks

The Co 2p doublet, Co 2p3/2 and Co 2p1/2 is observed at 779.9 and 795.8 eV, respectively. The Co

2p core peaks showed only the presence of Co+2 valance ions with their shaking satellites. The

energy splitting between the doublets is almost around 15.9 eV which indicates that the Co2+ ion is

in high spin state. The information about spin state helps us to know the coordination of Co+2 ions

with other ions. High spin state of Co+2 ions has the probability to acquire tetrahedral coordination

as well as octahedral coordination.

Band gap measurements in bulk sample

nRF /1))(( h

The optical band gap

measurements has been done

using the Kubelka–Munk

function F(R) = (1 − R)2/2R

where R is the diffuse

reflectance of the pellets. To

measure the band gap we

plotted data.

=

M-H curves for (ZnO)1-x(Co3O4)x≤0.05 bulk samples

Hysteretic behavior observed in VSM signals of samples showed mixing of

paramagnetic (indicated by non-saturation of magnetization) and ferromagnetic

(indicated by finited coercivity) behavior. The magnified spectra reveal the weak

ferromagnetic nature (finite coercivity) of samples.

A weak ferromagnetic behaviour indicates that the presence of cobaltions in the ZnO lattice is not enough to overcome paramagnetic signalof ZnO which is essentially due to low doping concentration of Coused in the present experiment. However, if we increase the Codoping concentration to increase the Co+2 ion substitution in ZnOlattice to increase the ferromagnetic component we might end uphaving Co clusters and spinel phases which will also contribute toferromagnetic component and then it that case the origin offerromagnetism in samples cannot be singly attributed to ZnCoOphase.

In the present study less doping % of Co was preferred to avoid Coclusters and spinel phase formation and their contribution toferromagnetic component. Our XRD and XPS results also confirm theformation of homogeneous ZnCoO phase and hence the observationof ferromagnetism, though albeit on the weaker side, is only due toCo+2 ion substitution in ZnO lattice in ZnCoO phase.

Results for Thin films

XRD for films grown in Vacuum

XRD for films grown in Ar-O2

The θ–2θ XRD patterns for a series of films grown at 350 °C in vacuum (base pressure

~ 7×10–5 mbar) using pellets with varying cobalt doping concentration. The peaks

correspond to the wurtzite ZnO (002) indicating good texture with the c-plane of the

sapphire substrate.

Particle Size in thin films

Scherer’s equation

t=0.9λ/β cos θ

where λ is the X-ray

wavelength, β is the full-

width at half-maximum of

the (002) diffraction line,

and θ is the diffraction angle

of the XRD spectra.

XPS core peak spectrum for Zn, Co and O elements

XPS survey scans were done for all Co doped

ZnO thin film samples which show the presence

of zinc, cobalt and oxygen clearly.

Zn 2p core peaks showed quiet symmetrical

behaviour in BE for different samples. Here

shows for Zn 2p spectrum only for

(ZnO)0.98(Co3O4)0.02 thin film sample. Zn 2p3/2

and Zn 2p1/2 core peaks have BE peaks at

1021.08 and 1044.1 eV, respectively which is in

agreement with the previous reports.

The O1s core peak exhibits a slight

asymmetrical behaviour. This profile can be fit

by two symmetrical peaks, having binding

energy at 530.0 and 531.2 eV.

Co 2p doublet was observed for allsamples. The Co 2p3/2 peak occurs at780.6, 780.4, 780.1 and 780.1 eV,while the Co 2p1/2 peak is located at796.2 , 796.1, 796.0 and 796.0eV for1,2,3 and 5% Co doped ZnO thin filmsamples respectively, showingchemical shifts compared to that ofpure Co metal.The difference between Co 2p1/2 andCo 2p3/2 is 15.5 eV which indicate thatCo ions have a valance of 2+ in arather high probability.Satellite peaks appeared at about786.0 and 802.4 eV for Co 2p3/2 andCo 2p1/2, respectively, for almost allsamples. The very intense satellitestructure results from the charge-transfer band structure characteristicof the late 3d transition metalmonoxides.The differences between the main

peaks and the correspondingsatellites further prove that Co ionsare surrounded by oxygen atoms andhave a chemical valence of 2+.

M-H curves for (ZnO)1-x(Co3O4)x≤0.05 thin films

deposited in vacuum

Magnetization measurements display a distinct ferromagnetic behaviour. It is useful to mention here that the

magnetic background of the substrate has been subtracted from all of the magnetization data. All the loops show

the features of ferromagnetism at room temperature (∼300 K). The ferromagnetic ordering is indeed intrinsic to

Co:ZnO films based on the substitutional behaviour of Co in the wurtzite lattice of ZnO, and the donor defects as

well as the electrons are important to the enhancement of FM. Magnetization data taken at 300 K for a series of

films grown under vacuum with different amounts of cobalt show smaller magnetization due to very small doping

percentage.

Deposition of thin film on Si substrate

Reasons

PLD plasma consists of two fraction of species: high energeticions of up to several 100 eV energy and lower energetic atomsand ions (10–50 eV).

High Energetic Plasma

penetration in the (growing film)

surface

activation of surface mobility

(diffusion)

re-sputtering ofboth impinging andloosely bondedspecies from thesurface.

activation ofsurface reactionswith e.g.physiosorbed(reactive gas)atoms or molecules

The total energy in any film deposited on a substrate is the sum ofthree components: surface energy of the film, the film–substrateinterface energy, and the strain energy in the film. Films grow insuch a way that the total energy is minimized.

However, surface, interface, and strain energy minimization do notnecessarily favour the same orientation. Therefore, one couldexpect different textures depending on whether surface, interface,or strain energy minimization is the dominant factor.

ZnO has tetragonal coordination formed by the sp3 hybridized orbit.As it has a wurtzite hexagonal structure, the direction of each apexis parallel to the c-axis. According to the calculation of Fujimara etal, the (002) lattice plane has the lowest density of surface energy.Therefore, surface energy minimization favours (002) texture in ZnOfilms. Thus, in most cases, films grow with the (002) plane parallelto the surface of the substrate, thus, minimizing the surface freeenergy of the film. In the case of polycrystalline films grown onsingle crystalline substrates, interface energy minimization can leadto the dominance of epitaxial orientations.

Deposition of thin films at different temperatures

M-H curves for Zn0.97Co0.03O thin films

Conclusions

Co doped ZnO films showed only the (002) peak in the XRD patterns,

indicating that all of them had preferential orientation along the (002)

reflection plane of ZnO.

The films grown in vacuum are highly crystalline as compare to films grown in

Ar-O2 environment.

The crystallite size of the films grown in vacuum showed a consistent

increase with the increase in doping % of cobalt while the particle size of films

grown in Ar-O2 showed inconsistency.

M-H curves showed the ferromagnetic behaviour for the films grown in

vacuum and for the films grown in Ar-O2, M-H curves are not very developed.

Compositional analysis of thin films done by XPS showed the presence of

Co+2 ions which is the source of ferromagnetism in our thin film samples.

Thank You