Growth and characterisation of cobalt doped zinc oxide 2

The origin of stabilization of ferromagnetic state by electron doping comes from electrons participating in a double exchange mechanism, lowering the energy of the ferromagnetic state..

it a potential candidate for room temperature DMS Theoretical models1,2 predicted a T c

above room temperature for TM-doped ZnO films, which has stimulated numerous experimental works in obtaining ZnO-based DMSs using various approaches Although most of the experimental work has reported some sort of magnetic properties in TM-doped ZnO (mostly Co and Mn), its origin still remains debatable In fact, the magnetic properties reported so far for TM-doped ZnO range from intrinsic ferromagnetism with

various T c,3-10 extrinsic ferromagnetism,11-13 paramagnetism or superparamagnetism14-16

to anti-ferromagnetism.17-19 In the rest of this chapter, I will give an overview of both the theoretical and experimental work reported so far on TM-doped ZnO A very brief introduction to Mn-doped ZnO and Co-doped TiO2 will also be presented

2.2 Theoretical predictions on TM-doped ZnO

The research on ZnO-based DMS was mainly stimulated by the theoretical predictions of above room temperature ferromagnetism in 5% Mn-doped p-ZnO with an acceptor concentration of 3.5 × 1020 cm−3 by Dietl et al.1 These theoretical predictions were based on the Zener mean-field model, which assumes hole-mediated exchange interactions between the Mn local moments in which the Mn dopants provide both the

local moments and holes T c is obtained through minimizing the Ginzburg-Landau energy functional, F, with respect to magnetization, M, at given temperature, T, and hole concentration, p The free-energy functional, F, consists of two parts: Fc[M] and

free-FS[M] The former is due to carrier contribution which is computed based on a six-band Luttinger–Kohn Hamiltonian together with the p-d exchange contribution The latter is parameterized by an exchange energy, N0β, the so-called p–d exchange term The free

energy functional due to the magnetic spins, FS[M], is given by 0 0

0M dM H M( )

H(Mo) is the inverse function of the experimental dependence of the magnetization on

the magnetic field, H, in the absence of the carriers, which is parameterized by the

Brillouin function T c determined is thus given by,

[ ]

N

GaAs N

eV N

x A T T x

F o

o o

eff F AF F

05.0)

A more general model to describe the carrier-spin interactions is the RKKY model which takes into account both the itinerant nature of carriers and Friedel oscillations of the electron spin polarization around the localized spins Although the oscillation usually averages out, due to small Fermi wavevectors in DMSs, the RKKY is useful tool to model the random distributions of magnetic spins Jalbout et al have

carried out Monte Carlo simulations on ZnO:Co based on a three-dimensional RKKY model.20 The dependence of RKKY exchange coupling on the density of both localized spins and itinerant electrons is accounted for by

where C is a positive constant independent of R, F(2kFR) is the oscillating function, kF

is the wave vector at the Fermi surface, and G(R) is a distribution function of the average number of Co2+ ions that can be found in a three-dimensional material with the relative distance R from the central Co2+ ion Ferromagnetism is favoured when the above function is negative; otherwise the system will be non-ferromagnetic This requires that 2kFRnearest < 4.5, where Rnearest is the average distance of nearest-neighbouring magnetic atoms The simulation results showed that ferromagnetism is favoured in Zn0.85Co0.15O and Zn0.75Co0.25O, with carrier concentration of 2.9×1020cm-3and 1.2×1018 cm-3, respectively

In addition to the Zener and RKKY models, there were also significant amount

of work devoted to first principles calculations of the T c of TM-doped ZnO A recently proposed density-functional theory (DFT) calculation for (Co, Al)-co-doped ZnO suggests that the RKKY interaction is dominant when the distance between Co and Al

is large and double exchange interaction is dominant when this distance decreases.21This calculation was performed for a supercell consisting of 32 atoms, with two Co atoms and one Al atom The results showed that Co-doped ZnO favours a spin-glass

state with anti-ferromagnetic ordering, which is 70 meV per Co ion more stable than the ferromagnetic state However, with the addition of Al, which acts as an electron dopant, the stability of ferromagnetic states is enhanced It is argued that when the Al-3p orbital

is hybridized with Co-3d orbitals, the doped electrons can go into the Co 3d state and stabilize the ferromagnetic state, through double-exchange interactions In this state, the doped electrons become localized in the Co 3d orbital and cannot be described by the RKKY model However, when the Al dopant ion is far from Co, no hybridization can occur and the dopant electron goes to the host conduction band instead of the Co 3d orbital This doped electron thus stabilizes the ferromagnetic state through the RKKY interactions Therefore, there is a critical distance between Co and Al atoms which acts

as a boundary between RKKY and double exchange induced ferromagnetism

Using Korringa-Kohn-Rostoker (KKR) Green function calculations based on local density approximation, Sato and Katayama-Yoshida predicted that V, Cr, Fe, Co and Ni doped ZnO are ferromagnetic, Mn doped ZnO is antiferromagnetic, whereas Ti and Cu doped ZnO remains paramagnetic.2 The total energy per unit supercell is calculated by considering a cell with four primitive unit wurzite structure cells, with two

of eight Zn atoms being substituted by two transition metal atoms The holes are from N dopants substituting O atoms, whereas the electrons are from Ga dopants replacing Zn atoms Fig 2-1 shows the energy difference between the anti-ferromagnetic state and ferromagnetic state with no additional carrier dopant Considering Mn-doped ZnO with

d5 configuration, there is no additional carrier, and thus resulting in the ferromagnetic configuration being more stable than the ferromagnetic state As the electronic configuration differs from Mn, ferromagnetic becomes favourable again From these observations, it is predicted that the resulting mobile carriers stabilizes the ferromagnetic state through the double exchange mechanism, which prompts the study

anti-of dependence anti-of carrier concentration on the stability anti-of ferromagnetic states

Figure 2-1 Chemical trend of the magnetic states for 3d transition metal atom doped ZnO Total magnetic moments per one transition metal atom are also shown [After K Sato, 2001, Ref 2]

Further studies showed that hole-doping stabilizes the ferromagnetic ordering of

Mn, whereas electron-doping stabilizes the ferromagnetic ordering of Fe, Co, or Ni doped ZnO (see Fig 2-2).22 This is of great importance because for practical applications, n-type ZnO is easier to be produced compared to p-type ZnO The stability between spin glass state and ferromagnetic state is compared in ZnO-based DMS as a function of magnetic ions and carrier dopants The origin of stabilization of ferromagnetic state by electron doping comes from electrons participating in a double exchange mechanism, lowering the energy of the ferromagnetic state In these doped materials, the 3d up or down spin states are not fully occupied Thus, a 3d transition metal electron in a partially occupied 3d orbital can hop onto the 3d orbital of a

neighbouring transition metal, lowering the energy of the system, favouring the ferromagnetic state

Figure 2-2 Stability of ferromagnetic state in (a) Mn-, (b) Fe-, (c) Co- and (d) Ni-doped ZnO based DMS

as a function of carrier concentration A positive energy indicates ferromagnetic state is more stable than the spin glass state [After K Sato, 2001, Ref 22]

In another study of the effect of electron-doping, the first-principles spin-density functional calculations by Lee and Chang predicted that heavy electron doping and high

Co concentration are required for obtaining ferromagnetism in ZnO:Co.23 Without electron doping, anti-ferromagnetic coupling is favoured over ferromagnetic coupling, resulting in a spin-glass state The double exchange interaction, similar to that suggested above by Sato, is said to stabilize the ferromagnetic state Ferromagnetic ordering in the system has a short range of about 3 Å and ferromagnetic interaction does not depend on the direction of Co ion alignment and Co-Co distances Calculations suggest that samples should be prepared with very high electron carrier density of about 1020 cm-3

This is because the ferromagnetic state is favoured when Co-doped ZnO is doped with above 0.5 electron per Co atom A large amount of Co doping should also

electron-be used in order to reduce the Co-Co distance and to promote short-range ferromagnetic ordering When the Co-Co distance is too large, electron-doping might not be able to affect the magnetic coupling, thus leading to a less favourable ferromagnetic state

On the other hand, Spaldin argues theoretically that only hole-doping promotes ferromagnetism in both ZnO:Co and ZnO:Mn.24 The calculations were carried out on a

32 atom wurtzite supercell with a single Co atom and a single Zn vacancy, using DFT

In an undoped ZnO:Co system, the energies of ferromagnetic and anti-ferromagnetic configurations are similar However with p-type doping, the ferromagnetic state is strongly stabilized and is 60 meV lower in energy than the anti-ferromagnetic state Interaction with holes causes strong ligand field effects that overcome crystal field splitting It is also observed that when two Co ions are in a unit cell, there is an increase

in hybridization of majority electrons with the oxygen p states, compared to an undoped system

In contrast to the above findings, Sluiter et al predicted that both the hole- and

electron-doping are required to promote ferromagnetic ordering in ZnO:Co and ZnO:Mn.25 The DFT calculation was carried out on a supercell with 40 formula units of ZnO, with two Zn atoms being replaced with transition metals, as shown in Fig 2-3 From the figure, Ni-, Cr- and Ti- doped ZnO looks like suitable candidates for ferromagnetism at low concentrations This is not true in reality, suggesting the need to modify Mn-, Fe- or Co-doped systems Li is introduced as a co-dopant in ZnO:Co system, as it does not contain d electrons to interfere with magnetic calculations and also has a suitable ionic radius for the system Li co-doping is found to enhance the ferromagnetic state as it amplifies the Co pair coupling Through the use of electron

doping with Zn interstitials and hole doping with Zn vacancies or Li, strong ferromagnetic properties has been predicted and confirmed through experimental studies, up to room temperature

Figure 2-3 Magnetic coupling J between substitutional TM atom pairs formed by site 0 and site N, labelled on the x axis and in the wurzite ZnO supercell (right) where large blue (small red) spheres are designated Zn (O) atoms In the supercell, the directions through atoms 1 and 3, 1 and 5, and 0 and 4 correspond to <100>, <010>, and <001>, respectively Positive J favors FM alignment [After M H F Sluiter, 2005, Ref 25]

Hydrogen-mediated spin-spin interaction was also predicted to be able to induce high temperature ferromagnetism in ZnO:Co.26 Through first-principle pseudopotential total-energy calculations within the local spin density appproximation, interstitial H was found to help mediate a short-range ferromagnetic spin-spin interaction which affects the magnetic properties of ZnO:Co in two ways: structurally by forming highly stable

Co dimmers on nearest neighbouring Zn sites and electronically by opening a channel for strong ferromagnetic spin-spin interaction between the Co dimmers, as shown in Fig 2-4 below The highly stable Co dimmer forms a Co-H-Co complex when it reacts with

an ionized H atom This induces strong ferromagnetic spin-spin interaction between Co atoms, resulting in room temperature ferromagnetism The parallel spin pairing state of (TM-H-TM) is 0.21 eV and 0.26 eV more stable than the anti-parallel state for Co and

Mn respectively Also, the low carrier concentration of a ZnO:Co system is due to a formation of a deep level by the H in a Co-H-Co complex, resulting in no contribution

Coey et al., however, argues that conventional superexchange or

double-exchange interactions cannot produce long-range magnetic order at low concentrations

of magnetic doping It is proposed that ferromagnetism in DMS is mediated by a donor impurity band.27 This spin-split impurity band is formed by oxygen vacancies that form

bound magnetic polarons (BMPs), as shown in Fig 2-5(a) The T c, determined by field approximation, is dependent on concentration of magnetic cations and donors The

mean-expression for T c is given as

B

o

eff c o sd

r

r f J n

x s S T

3 2

1 2

d exchange parameter, fo is the oxygen packing fraction, rceff is the effective cation radius, ro is the oxygen radius and kB is the Boltzmann constant

Figure 2-5 (a) Representation of magnetic polarons A donor electron in its hydrogenic orbit couples with its spin antiparallel to impurities with a 3d shell that is half-full or more than half-full Cation sites are represented by small circles Oxygen is not shown; the unnoccupied oxygen sites are represented by squares; (b) The magnetic phase diagram for dilute ferromagnetic semiconductors The electrons are localized in the shaded area x p and p are the cation and donor polaron percolation thresholds, respectively is the ratio of the radius of the hydrogenic donor orbital to the Bohr radius [After J M D Coey, 2005, Ref 27]

(a)

(b)

A high T c is only possible when hybridization and charge transfer occurs between donor-derived impurity band to unoccupied minority-spin or majority 3d states

at Fermi level Fig 2.5(b) shows a phase diagram for a DMS system from which various magnetic phases has been observed, depending on the dopant concentration and radius of donor orbitals Polaron percolation threshold, δp, and the cation percolation threshold, χp, are the critical parameters of the phase diagram When the concentration

of magnetic cation is sufficiently high, ferromagnetism can occur when δ>δp and χ<χp

Very recently, it is shown that that spinodal decomposition may occur when the system is quenched.28 Spinodal decomposition leads to a complicated random pattern of high concentration regions connecting to each other Compared to DMS with homogeneous impurity distribution, this network could lead to magnetic percolations,

resulting in a high T c Spinodal decomposition is simulated by the Monte Carlo method,

dictating the distance between impurity sites The T c was then calculated using the random phase approximation for disordered systems.29 In a uniform DMS system,

magnetic impurities are distributed randomly and the system T c is low, due to the lack

of magnetic percolation paths In a quasi-one-dimensional structure, large clusters with high anisotropic shape can form, even with low dopant concentrations Spinodal decomposition, in a layer by layer growth condition, lead to formation of quasi-one-dimensional structures in DMS, making delta-doping a favourable technique to obtain room temperature ferromagnetism One delta-doped layer, with impurity concentration

of 5 %, can lead to T c of 346 K

2.3 Experimental Work on Co doped ZnO

2.3.1 Early observations

Ueda reported transition-metal Co-, Mn-, Cr-, Ni-doped ZnO on sapphire substrates deposited by the pulsed laser deposition (PLD) technique.3 This growth technique was used because it was believed to be able to obtain a higher solubility of magnetic ions in the ZnO host matrix The TM-doped ZnO films were deposited at 350 – 600 oC in 2 – 4 ×10 -5 Torr oxygen environment, on Al2O3 (1120) substrates Ceramic targets with 1 wt% of Al were used to produce n-type ZnO From XRD studies, as shown in Fig 2-6 (a), the Co-doped ZnO films prepared were single phase, with a c-axis preferred orientation The Co atoms systematically substituted the Zn atoms, without changing the ZnO wurtzite structure, as seen from the linear increase of d(002) values with Co concentration illustrated in Fig 2-6(b) Their results also showed that the Co-doped films has the highest solubility limit, of <50 % The substitution of Zn was confirmed by the d-d transitions of Co ions, from absorptions peaks occurring at

570, 620 and 660 nm, in the optical transmission spectra Moving on to magnetic

studies, the temperature dependence of magnetism showed that the films have a T c

around room temperature and a Zn0.95Co0.05O film has a coercivity of 50 Oe at 6K, as show in Fig 2-6(c) and (d) respectively The presence of weak ferromagnetism from CoO had been ruled out as an origin of the observed ferromagnetism They believe that the ferromagnetic properties of the higher carrier concentration samples are carrier-mediated, through RKKY or double exchange mechanism These samples exhibited

ferromagnetic properties with higher T c and saturation magnetization, whilst other samples exhibiting spin glass behaviour The Cr, Ni and Mn doped films prepared did not exhibit any ferromagnetic behaviour

(a) (b)

Figure 2-6 (a) 2θ-θ XRD scan of Zn 0.95 Co 0.05 O film; (b) c-axis lattice constant d(002) dependance on Co concentration; (c) Magnetization vs temperature curves measured in field of 0.1 T for various samples; (d) hysteresis curves measured at 6 K of Zn1–xCoxO (x = 0.05 and 0.15) films [After K Ueda, 2001, Ref 3]

After this pioneer work, a lot of work has been done on TM-doped ZnO In what follows, an overview of the growth techniques and conditions used in depositing the films is given Some typical or interesting results and problems encountered during characterizations are also discussed

2.3.2 Growth of Co-doped ZnO

The various common growth techniques that have been used for fabricating doped ZnO films include PLD, metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), sputtering and sol-gel methods (see Table 2-1) The PLD, MOCVD and MBE methods were known to produce undoped ZnO with a better crystalline quality as compared to the other two techniques; however their advantages are not reflected in the highly doped DMS samples, specifically in the case of Co-doped

Co-ZnO The ultimate aim of these different growth techniques is to obtain Co-doped ZnO with a high concentration and uniformly distributed Co, without the formation of any secondary phases With the different deposition methods, it is found that the structural and physical properties of Co-doped ZnO oxides were strongly dependent on the growth conditions, dopant types, and post-deposition annealing, which will be discussed further

in the next sections It is important to take note that the thermal solubility limit of Co in ZnO ranges from 10% to 25%, depending on the growth techniques and conditions used

Table 2-1 Co doped ZnO systems

Films Fabrication Technique Magnetic Properties Reference

Zn0.964Co0.036O Spin coating Intrinsic ferromagnetism 10

2.3.2.1 Growth conditions

For every growth method, fine-tuning of the growth conditions is crucial to obtain films with good homogeneity and reproducible properties Many groups studied the influence of substrate temperature and oxygen pressure on the magnetic properties

of obtained films Generally, a lower substrate temperature (600 oC) and higher oxygen pressure (> ×10 -5 Torr) are able to produce films with homogeneous Co distribution Fig 2-7 shows how the substrate temperature and atmospheric environment affect the structural properties of the films prepared by PLD.12 The homogeneity of the film is generally improved by lowering the substrate temperature The oxygen partial pressure during the growth affects the formation of oxygen vacancies, which in turn may affect the magnetic properties of the film In Co-doped ZnO, the experimental results have shown that the oxygen vacancies can enhance ferromagnetism through the formation of BMPs.30 Although a low oxygen pressure improves the ferromagnetic exchange interactions,31 it has also been reported that a high oxygen pressure can suppress the formation of Co clusters.32

Figure 2-7 Growth phase diagram of Zn 0.75 Co 0.25 O films in relation to substrate temperature and O 2

pressure Open circles and crosses denote homogeneous (Zn0.75Co0.25O) and inhomogeneous (Zn 0.75 Co 0.25 O + CoO + Co) structures, respectively [After J H Kim, 2002, Ref 12]

P-Figure 2-8 300 K magnetization data for 0.2% Mn 2+ :ZnO and 3.5% Co 2+ :ZnO films prepared by direct chemical synthesis with or without addition of nitrogen (a) 0.2% Mn 2+ : ZnO with added nitrogen, (b) 0.2% Mn 2+ :ZnO without added nitrogen, (c) 3.5% Co 2+ :ZnO with added nitrogen, and (d) 3.5% Co 2+ :ZnO without added nitrogen [After K R Kittilstved, 2006, Ref 33]

Other than the dopants used above, Cu doping was found to improve ferromagnetism in Co-doped ZnO system.34 , 35 The results showed that Cu doping drastically changes the magnetic properties of the films, as seen in Fig 2-9 The amount

of Cu doping is about 1-4 at% and in both works, the films are believed to be homogeneous and free of secondary phases and nanoclusters The improvement of ferromagnetic properties from additional Cu-doping is attributed to the enhancement of carrier concentrations

Figure 2-9 M-H curves of (a) Zn 0.95 Co 0.05 O and (b)Zn 0.94 Co 0.05 Cu 0.01 O at room temperature (RT) and 78 K Inset in (b) shows the M-H curve at RT with its y-axis enlarged [After O D Jayakumar, 2005, Ref 35]

2.3.2.3 Post-growth annealing

Different post-growth annealing in different atmosphere has also been carried out by various groups to try to reveal the origin of ferromagnetism in Co-doped ZnO In

the study by Khare et al., the presence of Zn interstitial or oxygen vacancies is believed

to introduce free carriers, which promote ferromagnetism. 36 By annealing their sample

in Zn vapour atmosphere, they observed ferromagnetism in their films, although there is

no noticeable change in resistivity (see Fig 2-10) On the other hand, annealing the samples in oxygen atmosphere leads to a reduction in Ms and similar to Zn annealing, the resistivity of the film remains constant Their results show that ferromagnetism is

not linked to conductivity although the annealing atmosphere does show an effect on ferromagnetism

Figure 2-10 Room temperature M-H curves for Zn 0.98 Co 0.02 film before and after annealing in Zn vapour

at 410 o C [After N Khare, 2006, Ref 36]

In another post-growth annealing study, it was found that the magnetisation varies with the amount of oxygen vacancies in ZnO, which in turn affects the formation

of BMPs.30 In this study, the samples were annealed in air, Ar and Ar/H2 atmospheres, producing films which have Ms values of 0.2, 0.9 and 1.5 µB/Co respectively In comparison, the as-deposited films have an Ms of 0.5 µB/Co at room temperature (see Fig 2-11) In Ar/H2 and Ar annealing atmospheres, resistivity decreases due to the presence of more oxygen vacancies, as determined from x-ray near edge spectroscopy studies On other hand, films annealed in air have less oxygen vacancies, making them highly insulating With the ferromagnetism varying with oxygen vacancies, the model

of BMPs can be used to describe the origin of ferromagnetism in these Co-doped ZnO samples

Figure 2-11 M-H curves for as-deposited and annealed Co:ZnO films at room temperature [After H S Hsu, 2006, Ref 30]

2.3.3 Characterization of Co-doped ZnO

The common methods used in the characterization of Co-doped ZnO are listed

in Table 2-2 and those used in this study will be described further in Sections 3.2 Although these methods have been used in one way or another to characterize Co-doped ZnO, a systematic method to conclude intrinsic ferromagnetism has yet to be established A general approach adopted is to “prove” the absence of clusters/secondary phases through the use of XRD and TEM, coupled with observation of d-d transitions of

Co2+ in optical measurements, and subsequently any ferromagnetism observed is deemed intrinsic The study of transport properties has often been left out in many papers, which is important because the correlation between magnetic and transport properties is critical in determining if they exhibit carrier-mediated ferromagnetism In addition, one should also carry out experiments on one set of samples, instead of samples prepared in different periods with similar chemical compositions, as reproducibility of samples is very much dependent on the condition of the growth

system In the following sections, the commonly observed characterization results and also various interesting findings will be explored

Table 2-2 Properties and their corresponding characterization methods

Properties Characterization method

Structural • XRD

• TEM

• X-ray photoelectron spectroscopy (XPS)

• Atomic force microscopy (AFM)

• Auger electron spectroscopy (AES)

• Energy dispersive x-ray analysis (EDX, EDS)

• Rutherford backscattering spectrometry (RBS)

• Secondary ion mass spectrometry (SIMS)

• Electron energy loss spectroscopy (EELS) Optical • Optical transmission

• Photoluminescence (PL) Magnetic • Superconducting quantum interference device (SQUID)

• Vibrating sample mangetometer (VSM)

• MCD Transport • Magnetoresistance (MR)

• Resistivity measurements

• Conductance measurements

• Hall effect

2.3.3.1 Structural and optical studies

As mentioned above from Ueda’s work, XRD is commonly used to show that

Co is able to substitute Zn without changing the ZnO wurtzite structure Table 2-3 summarized the peak position of ZnO (002) for various Co-doped ZnO films on Al2O3

(0001) substrates The peak position of pure ZnO (002) is 34.422o, however, with the

substitution of Zn with Co, strain is introduced into the lattice, leading to a shift of the peak position From the plot of d(002) versus Co concentration, one can roughly estimate the solubility limit of Co in ZnO, i.e., the composition at which the breakdown

of Vegard’s law occurs The absence of any additional peaks in a XRD scan is also often used to eliminate the presence of Co cluster and secondary phases, which are further confirmed through high-resolution TEM observations Although XRD and TEM are able to probe the crystalline structure of the films grown, it is sceptical that they can

“confirm” the absence of clusters of very small size, which has often been used as a support of intrinsic ferromagnetism in Co-doped ZnO films It should thus be noted that not all particles that contribute to ferromagnetism are detectable by XRD or TEM techniques, especially when the particles are very small or have a lattice structure which

is almost identical to that of the host material

Table 2-3 XRD results from various groups

Sample Growth Method 2θ peak position (o) Reference

global characterization techniques are used A film characterized via TEM might be homogeneous due to the small area studied, however for XRD characterization, it is likely to be considered inhomogeneous as the area studied is large Also, given the amount of Co doped into ZnO is of small quantity, compositions of films determined must have significant amount of accuracy

In determining the composition of the film, XPS, RBS and EDX are commonly used, although these techniques have limited accuracy After obtaining films with good structural quality and homogeneity, it is important to confirm Co substitution into the ZnO host matrix

One method to “confirm” if Co has replaced Zn is to check the valence state of

Co which is normally determined by XPS, AES and EELS Although the valence state

of Co in the films can be determined from XPS, AES and EELS, it should be noted that secondary phases might be responsible for the valence state

Optical transmission can also be used to show Co substitution of Zn in the host lattice, through the observation of characteristic peaks (see Fig 2-12) Absorption bands observed at 571, 618 and 665 nm are characteristic of d-d transition of tetrahedrally coordinated Co2+ of 4A2(F) → 2A1(G), 4A2(F) → 4T1(P), and 4A2(F) → 2E(G) transitions respectively.40 These peaks observed from optical transmission spectrum can be used together with results from EELS and XPS to confirm Cosubstitution of Zn, but they should not be used to confirm Co2+ is the origin of ferromagnetism The observation of these peaks can only confirm the presence of tetrahedrally coordinated Co2+ ions in the ZnO host matrix The correlation of the above structural results with those obtained from magnetic and transport techniques are necessary before confirming the origin of ferromagnetism