Bulk And Thin Films Of Zncoo By Pld

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 ZnO and GaN would exhibit ferromagnetism above room temperature on doping with Mn. According to the theory, ferromagnetism between magnetic dopant ions is mediated by holes in the valence band through indirect exchange. Dietl's theory has proven useful in understanding the experimental results for GaMnAs but it appears to be inconsistent for the experimental results of transition metal doped wide bandgap semiconductors, such as ZnO and GaN. It is based on many reasons, including the difficulty in experimentally preparing p-type ZnO material and the observations of ferromagnetism in n-type ZnO DMS.

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

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Coey’s Model Acoording to coey, donor defects which could arise from oxygen vacancies or zinc interstitials in the case of ZnO, overlapped and form an impurity band. This impurity band can interact with 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 and can align the spins of the magnetic dopants parallel to one another. Ferromagnetism is achieved when the BMPs start to overlap to form a continuous chain throughout the material, thus percolating ferromagnetism in the DMS.

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

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

Experimental Setup

Pulsed Laser Deposition System

Nd:YAG Laser

Characterization Techniques

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SIEMENS D5000 X-ray Diffractometer

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

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Cary 50 UV-VIS Spectrophotometer

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SHIMADZU UV-VIS 2501 Spectrophotometer

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Lakeshore 7400 Vibrating Sample Magnetometer

Results for Bulk Samples

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XRD Results of bulk samples

Quantitative analysis 

Rietveld Method

Material

a

c

Volume

Å

Å

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 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.

1/ n =

( F ( R))

h

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.

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A weak ferromagnetic behaviour indicates that the presence of cobalt ions in the ZnO lattice is not enough to overcome paramagnetic signal of ZnO which is essentially due to low doping concentration of Co used in the present experiment. However, if we increase the Co doping concentration to increase the Co+2 ion substitution in ZnO lattice to increase the ferromagnetic component we might end up having Co clusters and spinel phases which will also contribute to ferromagnetic component and then it that case the origin of ferromagnetism in samples cannot be singly attributed to ZnCoO phase. In the present study less doping % of Co was preferred to avoid Co clusters and spinel phase formation and their contribution to ferromagnetic component. Our XRD and XPS results also confirm the formation of homogeneous ZnCoO phase and hence the observation of ferromagnetism, though albeit on the weaker side, is only due to Co+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 fullwidth 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 all samples. The Co 2p3/2 peak occurs at 780.6, 780.4, 780.1 and 780.1 eV, while the Co 2p1/2 peak is located at 796.2 , 796.1, 796.0 and 796.0eV for 1,2,3 and 5% Co doped ZnO thin film samples respectively, showing chemical shifts compared to that of pure Co metal. The difference between Co 2p1/2 and Co 2p3/2 is 15.5 eV which indicate that Co ions have a valance of 2+ in a rather high probability. Satellite peaks appeared at about 786.0 and 802.4 eV for Co 2p3/2 and Co 2p1/2, respectively, for almost all samples. The very intense satellite structure results from the chargetransfer band structure characteristic of the late 3d transition metal monoxides. The differences between the main peaks and the corresponding satellites further prove that Co ions are surrounded by oxygen atoms and have 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 energetic ions of up to several 100 eV energy and lower energetic atoms and ions (10–50 eV).

penetration in the (growing film) surface

activation of surface reactions with e.g. physiosorbed (reactive gas) atoms or molecules

High Energetic Plasma

activation of surface mobility (diffusion)

re-sputtering of both impinging and loosely bonded species from the surface.

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

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However, surface, interface, and strain energy minimization do not necessarily favour the same orientation. Therefore, one could expect different textures depending on whether surface, interface, or strain energy minimization is the dominant factor.

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

Deposition of thin films at different temperatures

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

Conclusions

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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.

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