Growth and characterisation of cobalt doped zinc oxide 2

Chapter Cobalt doped zinc oxide CHAPTER COBALT DOPED ZINC OXIDE 2.1 Introduction Zinc oxide (ZnO) is a wide direct bandgap (3.2eV) semiconductor with a large exciton binding energy (60meV), which has potential applications in UV light emitters and detectors. In recent years, there is also an increasing interest in making ZnO a dilute magnetic semiconductor. As the valence of Zn in ZnO is +2, it can be readily replaced by transition metal (TM) ions, enabling interesting magnetic, optical and electrical properties. In contrast to the low solubility of TM in III-V semiconductors, ZnO acting as a host material can accommodate more transition metal dopants, making it a potential candidate for room temperature DMS. Theoretical models1,2 predicted a Tc 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 TMdoped 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 Tc,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. 36 Chapter Cobalt doped zinc oxide 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. Tc is obtained through minimizing the Ginzburg-Landau freeenergy 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 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 − M dM H ( M ) , where 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. Tc determined is thus given by, Tc ( x) = TF − T AF = AF xeff 0.05 (βN o [eV ])2 N o (GaAs ) nor TF − T AF ( x ) (2.1) No where AF is the Fermi-liquid parameter, xeff is the effective spin concentration, β is the p-d exchange integral, No is the concentration of cation sites, TFnor is the normalized hole-induced ferromagnetic temperature and TAF is the anti-ferromagnetic temperature. The above results suggest that, in general, higher concentrations of holes and magnetic ions would lead to DMSs with higher Tc. 37 Chapter Cobalt doped zinc oxide 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 ex H eff =C F (2k F R)G ( R) , (2.2) R 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 nearestneighbouring 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-3 and 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 Tc 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.21 This 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 38 Chapter Cobalt doped zinc oxide 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 antiferromagnetic 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 39 Chapter Cobalt doped zinc oxide of dependence of carrier concentration on the stability 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 40 Chapter Cobalt doped zinc oxide 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 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. 41 Chapter Cobalt doped zinc oxide This is because the ferromagnetic state is favoured when Co-doped ZnO is electrondoped with above 0.5 electron per Co atom. A large amount of Co doping should also 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 42 Chapter Cobalt doped zinc oxide 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 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 and 3, and 5, and and correspond to , , and , 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 43 Chapter Cobalt doped zinc oxide 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 to carrier transport. Figure 2-4(a) Atomic geometry of a Co-O bond-centered H [HBond-Center-Co]; (b) the most stable geometry of H bound to a Co dimer (Co-HAnti-bonding-Co) complex and (c) The energy diagram for a (Co-H-Co) complex in the spin-parallel state shows that the hydrogenic level is located deep below the Co-eg level. [After C. H. Park, 2005, Ref. 26] Coey et al., however, argues that conventional superexchange or doubleexchange 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 Tc, determined by meanfield approximation, is dependent on concentration of magnetic cations and donors. The expression for Tc is given as (S + 1)s xδ Tc 3n J sd f o kB rceff ro (2.3) where S is the spin of 3d cation, s is the donor electron spin, x is the dopant (cation) concentration, δ is the donor concentration, n is 1(monoxide) or (dioxide), Jsd is the s- 44 Chapter Cobalt doped zinc oxide 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. (a) (b) 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. xp 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] 45 Chapter Cobalt doped zinc oxide (d) (e) (f) Figure 2-17 Normalized magnetoresistance of (a) Cr, (b) Mn, (c) Fe ,(d) Co, (e) Ni and (f) Cu-doped ZnO thin films, magnetic field is parallel to c-axis. Indicated is carrier concentration at 300 K. [After Z. W. Jin, 2001, Ref. 55] Figure 2-18 (a) Butterfly MR of Zn0.75Co0.25O film at 4.2 K, (b) the insert shows the temperature dependence of resistivity of various films studied. [After P. Stamenov, 2006, Ref. 37] 64 Chapter Cobalt doped zinc oxide Transport measurements can also determine the type of semiconductor of the films, carrier concentration of films and whether they are insulating in nature. Obtaining the carrier concentration is very important as it further determines if the material exhibits carrier-mediated ferromagnetism. This information can be obtained via Hall measurements, which are similar to setups in characterizing semiconductors. Hall effect measurement results give information of the type of semiconductor and the presence or absence of AHE. AHE is used as a strong evidence for intrinsic ferromagnetism in DMSs systems.57,58 When carrying out Hall measurements, it is also important to take note of the presence of Co clusters. Small single-domain Co particles can behave like a single localized spin below their blocking temperature and contribute to the Hall measurement results.59 Also, considering the fact that AHE has also been reported in ferromagnetic clusters,59 granular materials 60 - 62 and inhomogeneous DMS in the hopping transport regime,63 observation of AHE alone cannot support the claim that the DMS under study is indeed a ferromagnet of intrinsic origin, unless it is correlated with ferromagnetism observed by other means and furthermore, secondary phases and precipitates must be absent in the sample. Transport studies between DMS/superconductor (SC) are a relatively new topic in the field of DMS materials. Work has been carried out for GaAs:Mn and also InSb:Mn through the use of point contact Andreev reflection (PCAR) spin spectroscopy measurements.64,65 Measurements were able to determine the spin polarization of these materials to be about 83% and 52% respectively. Attempts to study polarization using this method are hindered by the extreme sensitivity of results to the nature of the DMS/SC interface. 65 Chapter Cobalt doped zinc oxide Figure 2-19 Typical normalized conductance curves for two different Sn superconductor contacts with In1-xMnxSb epitaxial films: (a) contact resistance Rc ~57 Ω, T=1.2 K, ∆=0.52 mV; fitting parameters: Z=0.19 and P=54%; (b) contact resistance Rc~36 Ω, T=1.6 K, ∆ (1.6 K)=0.5 mV; fitting parameters: Z=0.20 and P=52%. [After R. P. Panguluri, 2004, Ref. 65] 2.4 Other related DMSs 2.4.1 ZnO: Mn Theoretical work shows that for ferromagnetic Mn doped ZnO, p-type doping is necessary, otherwise anti-ferromagnetic properties will be prominent1,2. Ionic radius of Mn2+ (0.66 Å), is similar to Zn2+ (0.60 Å), making Mn easily soluble in ZnO matrix. Room temperature ferromagnetic Mn doped ZnO has been achieved via pulsed laser deposition, as shown in Fig. 2-20, with Tc estimated to be above 400 K.66 Table 2-4, below, summarises common growth methods, with corresponding magnetic properties of Mn-doped ZnO films. Similar to its Co-doped counterpart, growth techniques used are numerous and magnetic properties of films produced varied. The equilibrium solubility limit of Mn in ZnO is approximately 13 a.t.%, 67 which can be overcome by non-equilibrium nature of certain growth techniques. Possible impurity phases that can be present in this system includes, Mn3O4 and (Zn, Mn) Mn2O4, which are ferromagnetic, and MnO and MnO2, which are antiferromagnetic. Mn doped ZnO tetrapod nanostructures, prepared by evaporating Zn 66 Chapter Cobalt doped zinc oxide metal and Mn doping by diffusion, has shown presence of a ferromagnetic (Zn, Mn)Mn2O4 surface layer.68 Figure 2-20 M-H curve of Zn0.978Mn0.022O pulsed laser deposited thin film on fused quartz at 300K obtained via SQUID measurements after subtracting diamagnetic contribution arising from the fused quartz (Insert shows as-obtained results) . [After P. Sharma, 2004, Ref. 66] Table 2-4 Mn doped ZnO systems. Films Fabrication Technique Magnetic Properties Reference Zn0.93Mn0.07O Reactive RF Paramagnetic 69 Magnetron sputtering Zn0.64Mn0.36O Pulsed laser deposition Anti-ferromagnetic 70 Zn1-xMnxO Pulsed laser deposition Paramagnetic 71 Laser molecular beam Ferromagnetic at Tc = 30K 72 epitaxy and 45K for x = 0.1 and 0.3 (0 ≤ x ≤ 0.35) Zn1-xMnxO respectively Zn1-xMnxO Vapor Phase growth Ferromagnetic with Tc 73 vapour Ferromagnetic with Tc ~ 74 =37K, x = 0.13 Mn implanted ZnO:Sn ZnO:Sn transport, via Mn ions 250 K, 3at.%Mn implanted 67 Chapter Cobalt doped zinc oxide 2.4.2 TiO2 : Co Co-doped TiO2 system has drawn much attention as it has been reported to be ferromagnetic and has Tc above room temperature (see Fig. 2-21 for M-H curves for films prepared via MBE). LaAlO3 (001) and SrTiO3 (001) are common substrates used for this system. The former has a smaller lattice mismatch (-0.2%) with the anatase phase, allowing growth of films with better structure. Above 575 oC, anatase would undergo an irreversible phase transition to rutile phase. In Co-doped anatase TiO2, Co would substitute the Ti site, and the Co ions would carry a +2 oxidation state. Figure 2-21 M-H curve of Co0.07Ti0.93O2 on SrTiO3 at 300K, magnetic field applied parallel to film surface. [After Y. Matsumoto, 2001, Ref. 75] The growth methods which are commonly used and the magnetic properties of Co-doped TiO2 are summarised in Table 2-5. As observed, the magnetic properties reported vary with different growth method used. Growth method, microstructure and Co distribution in films is said to be the cause of this wide range of ferromagnetic properties. Choice of substrate, growth conditions, annealing temperature, oxygen vacancies has been investigated thoroughly. To date, source of magnetism still remains debatable, but can be divided into carrier-mediated and nanocluster ferromagnetism. 68 Chapter Cobalt doped zinc oxide Table 2-5 Co doped TiO2 systems. Films Fabrication Technique Magnetic Properties Reference µB/Co 75 Anatase Ti1-xCoxO2 Combinatorial laser MBE 0.32 (x ≤ 0.08) room temperature Anatase Ti1-xCoxO2 Oxygen-plasma-assisted 1.26 µB/Co at room (x ≤ 0.1) temperature MBE above Anatase Ti1-xCoxO2 Soft Chemical Processing Ferromagnetic at room (0.0 ≤ x ≤ 1.0) temperature Anatase Ti1-xCoxO2 Pulsed laser deposition 0.23 (0.07 ≤ x ≤ 0.12) room temperature Anatase Ti1-xCoxO2 Laser MBE 0.16 (x = 0.05) room temperature µB/Co µB/Co above above 76 77 78 79 Chambers76 observed carrier-mediated ferromagnetism in anatase films prepared and only semiconducting films exhibit ferromagnetic properties. Anomalous Hall effect has also been used to implicate carrier-mediated ferromagnetism.58,80 However, carriermediated ferromagnetism is overshadowed with reports of Co nanocluster formation in Co-doped TiO2.32,81 Impurity phase for Co-doped TiO2 systems could be in the form of CoOx and CoTiOx. Oxygen vacancies in anatase TiO2 films fabricated at low O2 partial pressure would help diffusion of Co ions, encouraging formation of Co nanoclusters. 82 Considering the Co metal, the saturation magnetic moment is known to be about 1.7 µ B/Co. Thus, when ferromagnetism of Co-doped TiO2 systems is about this value, the presence of Co nanoclusters should not be excluded. As can be seen from Mn-doped ZnO and Co-doped TiO2 system, numerous methods of growth methods and results has been reported, similar to that in Co-doped ZnO system. But the aim of studying all these system is still similar and that is to determine accurately the origin of ferromagnetism for these materials, in hope of realising practical room temperature spintronic devices. 69 Chapter Cobalt doped zinc oxide 2.5 Summary In this chapter, I have given an overview to Co-doped ZnO oxide systems through covering both the theoretical and experimental works. As it was previously mentioned in Chapter 1, although many theoretical and experimental studies have been carried out, the origin of ferromagnetism is still under debate. Initial studies tended to support the intrinsic origin of ferromagnetism, but further studies showed that extrinsic origin is more likely to be responsible for the magnetic properties observed in this system. Although many factors may result in the scattered results, one of the factors is the lack of systematic study on a series of samples grown under same conditions. More attention should also be paid to the correlation of results obtained by different characterization techniques rather than focusing on one or two results. In the next chapter, the experimental details of this work including both the sample preparation and characterization will be covered. References: T. Dietl, H. Ohno, F. Matsukura, J. Cibert, and D. 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Kim, 20 02, Ref 12] 50 Chapter 2 Cobalt doped zinc oxide 2. 3 .2. 2 Choice of dopants As ZnO can be both p-type and n-type doped, the choice of electrical dopant would be important especially if ferromagnetism is dependent on donor energy levels Kittilstved et al demonstrated that the control of electronic structure and polarity is critical to achieving high Tc ferromagnetism of transition metal doped. .. K magnetization data for 0 .2% Mn2+:ZnO and 3.5% Co2+:ZnO films prepared by direct chemical synthesis with or without addition of nitrogen (a) 0 .2% Mn2+: ZnO with added nitrogen, (b) 0 .2% Mn2+:ZnO without added nitrogen, (c) 3.5% Co2+:ZnO with added nitrogen, and (d) 3.5% Co2+:ZnO without added nitrogen [After K R Kittilstved, 20 06, Ref 33] 51 Chapter 2 Cobalt doped zinc oxide Other than the dopants... 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 57 Chapter 2 Cobalt doped zinc oxide Figure 2- 12 Optical transmission of d-d transitions in Ni, Fe, Mn and Co -doped ZnO films [After Z W Jin, 20 00, Ref 41] 2. 3.3 .2 Magnetic... and other mechanisms. 52, 53,54 62 Chapter 2 Cobalt doped zinc oxide Figure 2- 16 Conductance curve of a 20 0 nm La0.7Ca0.3MnO3 device at zero magnetic field [After M Paranjape, 20 03, Ref 53] MR measurement can also give insights of sp-d exchange interactions, spinsplitting, formation of BMPs, localization etc Fig 2- 17 illustrates the assortment of MR behaviour of transition metal (Cr, Mn, Fe, Co, Ni and. .. largest effect for Codoped ZnO Co -doped ZnO also show MCD structures near 2 eV, due to d-d transitions of Co2+ ions The MCD spectrum shape can also change with temperature, providing more information of the magnetic properties of the samples 60 Chapter 2 Cobalt doped zinc oxide Figure 2- 14 MCD spectra of Zn1-xTMxO (TM Sc, Ti, V, Cr, Mn, Co, Ni and Cu) films at 5 K [After K Ando, 20 01, Ref 50] By interpreting... concentration at 300 K [After Z W Jin, 20 01, Ref 55] Figure 2- 18 (a) Butterfly MR of Zn0.75Co0 .25 O film at 4 .2 K, (b) the insert shows the temperature dependence of resistivity of various films studied [After P Stamenov, 20 06, Ref 37] 64 Chapter 2 Cobalt doped zinc oxide Transport measurements can also determine the type of semiconductor of the films, carrier concentration of films and whether they are insulating... includes, Mn3O4 and (Zn, Mn) Mn2O4, which are ferromagnetic, and MnO and MnO2, which are antiferromagnetic Mn doped ZnO tetrapod nanostructures, prepared by evaporating Zn 66 Chapter 2 Cobalt doped zinc oxide metal and Mn doping by diffusion, has shown presence of a ferromagnetic (Zn, Mn)Mn2O4 surface layer.68 Figure 2- 20 M-H curve of Zn0.978Mn0. 022 O pulsed laser deposited thin film on fused quartz at 300K... ≤ 0.10) 49 Chapter 2 Cobalt doped zinc oxide 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... 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 (0 02) for various Co -doped ZnO films on Al2O3 (0001) substrates The peak position of pure ZnO (0 02) is 34. 422 o, however, with the 55 Chapter 2 Cobalt doped zinc oxide. .. 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 Co substitution of Zn, but they should not be used to confirm Co2+ is the origin of ferromagnetism The observation of . Chapter 2 Cobalt doped zinc oxide 36 CHAPTER 2 COBALT DOPED ZINC OXIDE 2. 1 Introduction Zinc oxide (ZnO) is a wide direct bandgap (3.2eV) semiconductor with a large. (Zn 0.75 Co 0 .25 O + CoO + Co) structures, respectively. [After J. H. Kim, 20 02, Ref. 12] Chapter 2 Cobalt doped zinc oxide 51 2. 3 .2. 2 Choice of dopants As ZnO can be both p-type and n-type doped, . Chapter 2 Cobalt doped zinc oxide 50 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