Growth and characterisation of cobalt doped zinc oxide 4

Chapter Structural Characterization CHAPTER STRUCTURAL CHARACTERIZATION AND CHEMICAL VALENCY ANALYSIS 4.1 Introduction In this chapter, the structural properties of Co-doped ZnO:Al characterized by XRD and TEM are discussed. The results of chemical valence analysis using XPS and optical transmission spectroscopy are also presented. Throughout this thesis, the Co composition is given as a nominal value which is determined by XPS. As this technique only has an accuracy of about 5%, caution need to be taken when comparing the properties of the films obtained in this work with those reported in literature. It is noted that the Co composition determined in this work tend to be higher than those reported in literature. This is particularly true when the MR data of samples with similar nominal Co compositions are being compared. 4.2 XRD patterns The growth of ZnO films was first optimized without any doping. The optimum growth conditions for obtaining ZnO films with a good structural quality were found to be as following: Ar pressure, mTorr, substrate temperature, 500oC, and ZnO sputtering power, 150W. As the non-doped ZnO films were highly resistive, the study of their transport properties impossible. Thus, all the films prepared were co-doped with Al through the use of an Al2O3 target. While maintaining a reasonably good structural quality, the ZnO films were doped using an Al2O3 sputtering power of 30W and the film has a low resistivity of 1.3 mΩ⋅cm. The Al composition as determined by XPS was less than 0.1%. For the Al-doped ZnO samples, only ZnO (002) and (004) 102 Chapter Structural Characterization peaks were detected in the θ−2θ scan of XRD patterns in the range of 30-90o. This indicates that the films prepared were well textured, with the c-axis pointing in the normal direction of the substrate. The lattice parameter determined for the Al-doped ZnO film is 3.361Å (a) and 5.488Å (c). This is comparable to pure ZnO films with values of 3.250Å (a) and 5.207Å (c). As shown in Fig. 4-1 is a typical XRD pattern of Al-doped ZnO. The FWHM for Al-doped ZnO is about 0.77o which is comparable to the results reported in literature.1,2 5000 ZnO (002) 4000 3000 Al2O3 (006) Intensity (counts/s) 6000 2000 1000 30 35 40 45 50 o 2θ θ( ) Figure 4-1 XRD pattern of an Al-doped ZnO film. In all samples prepared, ZnO was doped with Al. For the co-doped samples, the Co composition, x, was controlled by varying the sputtering power of the Co target. In the specific setup, the minimum controllable power was about W, which gives a Co composition of about at.% as determined by XPS. As for δ-doped samples, Co was doped into the Al-doped ZnO films digitally, at a Co sputtering power of 10 W for a specific duration of time. In general, the co-doped samples have a better structural 103 Chapter Structural Characterization quality than their δ -doped counterparts, as revealed from the XRD results (not shown here). Generally, the crystalline quality of the films degraded as the Co doping amount was increased. As observed from Fig.4-2, the FWHM of ZnO (002) peak decreased with an increase in Co concentration initially. It began to increase as the Co composition exceeded 0.2 and decreased again after reaching a maximum at about 0.3. The initial decrease of FWHM was somewhat unexpected. It indicated that Co was soluble in the ZnO host matrix with Co composition of less than 0.2. The decrease of FWHM indicated that the ZnO:Co film had a better quality than ZnO when both were grown on the sapphire substrates under the specific conditions used in this study. This could be attributed to either the difference in thermodynamic properties of ZnO and ZnO:Co or the slightly smaller diameter of the Co2+ ions. The upturn at Co composition of 0.2 was due to the formation of secondary phases, as would be discussed later and also in following chapters. As the Co sputtering power was further increased, the film becomes increasingly less textured which results in a peak of FWHM when the Co content was around 0.3. A further increase of Co composition beyond this value lead to precipitation of Co nanoparticles, which results in an improvement of the crystallinity of the ZnO and ZnO:Co phases. 104 Chapter Structural Characterization 2.0 o FWHM ( ) 1.5 1.0 0.5 0.0 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 Co composition Figure 4-2 ZnO (002) XRD FWHM versus cobalt composition for co-sputter films. Figs. 4-3 ((a) –(c)) show the XRD patterns of ZnO:Co with different Co compositions in the scan range of = 30 - 50o. The data displayed was divided into three different ranges of x values, i.e., (a) x < 0.2, (b) 0.2 < x < 0.3 and (c) x 0. 3. For lightly doped samples, x < 0.2, the XRD spectra showed peaks of ZnO (002) and those associated with the substrate; no other peaks due to secondary phases were observed. These results, in combination with the TEM results that would be discussed later, suggest that the Co was soluble in the ZnO host matrix and no impurity phases were present at x < 0.2. 105 Chapter Structural Characterization Figure 4-3 XRD scan of co-doped ZnO films. As the ionic radius of Co2+ is about 96% of that of Zn2+, the in-plane lattice constant of relaxed ZnO:Co film was expected to decrease when Zn atoms are replaced by Co atoms, leading to an increase of out-of-plane lattice constant due its large Poisson’s ratio. This explained why the (002) peak of ZnO:Co shifts to the lower angle side of the original (002) peak of ZnO, as shown in Fig. 4-3 (a). This was also an indication that within this composition range, the Co atoms were soluble in the ZnO host matrix. The solubility limit of Co in ZnO was found to be about 0.25 in literature.3,4 Shown in Fig. 4-4 is the peak position at ~ 34o plotted as a function of the Co composition. It could be seen that up to 0.2 of Co content, there was not much changes in the peak position, which was expected as the films are composed of single 106 Chapter Structural Characterization phase co-doped ZnO. As the Co content was further increased towards 0.25, a left shift in peak position was observed, possibly due to the formation of Co-rich ZnO:Co. A further increase of Co beyond 0.3 leads to the recovery of the peak to positions near to those for films with x < 0.2, indicating the onset of phase segregation. o Peak position ( ) 34.6 34.4 34.2 34.0 33.8 33.6 33.4 0.05 0.1 0.15 0.2 0.25 0.3 0.35 Co composition Figure 4-4 XRD peak position (around 34o) versus Co composition. For samples with x > 0.2, as shown in Fig. 4-3 (b), in addition to the (002) ZnO peak, new peaks appear at 31.8o, 35.8-36o, 40.6, 42.4 and 44.5o, respectively. The assignment of these peaks was nontrivial because Co might exist in the material in question in at least five different forms: ZnO:Co, CoO, Co3O4, ZnCo2O4 and Co. The peak at around 31.8o and 36.253o could be assigned to ZnO (100) and (101), respectively. The CoO nanoparticles might exist in both cubic and hexagonal structures in the ZnO:Co host matrix. Therefore, the peak around 36o may be assigned to either one of the following peaks due to secondary phases: CoO (111) at 36.493o for cubic CoO, CoO (101) at 36.3o for hexagonal CoO, ZnCo2O4 (311) at 36.803o and Co3O4 (311) at 36.853o. Also, the peak at 44.5o was near peak positions of CoO (200) 107 Chapter Structural Characterization FCC at 40.6o and 42.4o, Co (111) at 44.217o, ZnCo2O4 (400) at 44.74o and Co3O4 (400) at 44.81o. For 0.2 < x < 0.3, the onset of secondary phase formation occured, with the secondary phase being presumably hexagonal CoO:Zn or CoO; Other phases like Co, Co3O4 and ZnCo2O4 may also exist, though they were not dominating based on the EELS results to be discussed shortly. At this high Co composition, the texture of ZnO:Co could change, as reflected in the appearance of a broad peak around 31.8o. For sample E (x = 0.24), the peak around 36o and 38o could be due to CoO (111). The shift of peak position to lower angles could be a result of Zn incorporation into the CoO matrix. As the sputtering power was further increased, in sample F (x = 0.25), the CoO (111) peak disappeared and Co (111) peak appeared. It should be noted that the lattice constants of hexagonal CoO are very similar to those of ZnO; thus it was difficult to differentiate between the two using XRD, especially if CoO grew pseudmorphically inside ZnO. Pole figure measurements (Fig. 4-5) had also been carried out for this sample and it could be seen from the results that the film was epitaxially grown and (002) textured. The pole diagram showed that this film was still dominantly single crystalline, suggesting that the films with Co content less than 0.25 were indeed single crystalline films with a good texture. 108 Chapter Structural Characterization Figure 4-5 Pole figure diagram for sample F (Zn0.75Co0.25O). As shown in Fig. 4-3 (c), as the Co concentration increased further, peaks at 31.8o and 36.1o start to disappear, with the appearance of peaks around 44.5o. The peak at 44.5o was due to Co clusters, though again the existence of other secondary phases such as ZnCo2O4 and Co3O4 could not be excluded. The formation of Co clusters was more probable because the formation of ZnCo2O4 and Co3O4 needed an oxygen-rich environment instead of more Co atoms. Before ending this section, there is a need to make the remark that any attempt to find peaks exactly at the same positions of pure ZnO or CoO is meaningless because ZnO would incorporate Co and, vice versa, CoO would contain Zn. 4.3 TEM observations The samples with different Co compositions had been examined by HRTEM. The samples with Co compositions lower than the solution limit were found to be homogeneously grown on the substrate. As an example, Fig. 4-6 shows the crosssectional TEM image, EDS mapping and diffraction images of the Co15W sample. As 109 Chapter Structural Characterization can be seen from the TEM image and diffraction pattern, the film was homogeneous and there were no detectable precipitates of Co. (b) (a) Al2O3 (006) ZnO :Co 100 nm Al2O3 (c) Al Growth direction 50 nm Co Zn O Figure 4-6 TEM results of co-doped Co15W (Zn0.80Co0.20O) sample; (a) Cross-sectional TEM image; (b) electron diffraction pattern of the same region; (c)EDS mapping of Al, Co, O and Zn of films, with direction of film growth as indicated. In Fig. 4.6 (c), the film’s diffraction pattern showed a single crystalline phase with no impurity spots. From XRD studies above, peaks had been observed in the vicinity where CoO (200) is expected. This could be due to the fact that the XRD pattern was from a large area of sample, whereas the TEM results were from a very small spot. 110 Chapter Structural Characterization When increasing the Co content to 0.24, it was observed that, similar to the Co15W sample, the Co20W sample also showed the absence of Co-agglomeration or precipitation (Fig.4-7(a)). The film, in general, was still homogeneous, though some external spots were detected in the diffraction pattern, as shown in Fig. 4-7 (b). These spots could be due to CoO (200), FCC Co (111), HCP Co (002) or even ZnO (101), as observed in the XRD patterns. The homogeneity of the film was found to degrade significantly after the Co content exceeds 0.25, the onset composition of secondary phase formation. As a result, the film quality of Co25W was much poorer as compared to the above-mentioned two samples. Also, the TEM cross-sectional image (Fig. 4-8 (a)) showed an inhomogeneous film. In Fig. 4-8 (b), the diffraction spots of secondary phases were seen to increase in quantity. Fig. 4-9 (a) shows the TEM images of the Co 32W sample, which illustrated clearly the inhomogeneous nature of the sample. Columnar growth, similar to that reported by Schaedler et. al., was observed, together with nanosized secondary phases, as shown by a dark particle in Fig. 4-9(b). For the corresponding δ-doped sample, the film structure was found to be very irregular, as shown in Fig. 4-10 (a), and it could be observed that Co clusters started to form throughout the film. Through analysis of fast-Fourier transform pattern and also EELS measurements on that particular particle shows that it exists as FCC Co. (b) (a) Extra spots ZnO : Co 100 nm Al2O3 111 ZnO (002) Chapter Structural Characterization (c) Al Growth direction 25 nm O Co Zn Figure 4-7 TEM results of co-doped Co20W (Zn0.76Co0.24O) sample; (a) Cross-sectional TEM image; (b) electron diffraction pattern of the same region; (c)EDS mapping of Al, Co, O and Zn of films, with direction of film growth as indicated. (a) ZnO :Co Al2O3 100 nm Growth direction (b) CoO (220) CoO (111) CoO (200) Figure 4-8 TEM results of co-doped Co25W (Zn0.75Co0.25O) sample; (a) Cross-sectional TEM image; (b) electron diffraction pattern of the same region. 112 Chapter Structural Characterization (a) ZnO:Co 100nm Al2O3 (b) (c) ZnO:Co 4nm Al2O3 Figure 4-9 TEM results of co-doped Co32W (Zn0.71Co0.29O) sample; (a) Cross-sectional TEM image; (b) HRTEM image of a selected region; (c) electron diffraction pattern of the same region. (a) ZnO:Co 100nm Al2O3 (b) ZnO:Co (c) Co 4nm Al2O3 Figure 4-10 TEM results of δ-doped Co98s ([(ZnO:Al (2.38 nm)/Co (1.0 nm)]× 60) sample; (a) Crosssectional TEM image; (b) HRTEM image of a selected region; (c) electron diffraction pattern of the same region. 113 Chapter Structural Characterization The EELS analysis was carried out on co-doped Co32W and δ-doped Co98s samples. The valence of Co detected from different locations in Fig. 4-9 (a) turned out all to be 2+ (Fig. 4-11(a)). The results showed that the film consists of mainly Coincorporated ZnO and/or Zn-incorporated CoO because the valence state of Co in ZnCo2O4 and Co3O4 was 3+ and 4+, and that in Co clusters was 0, respectively. In comparison, the corresponding δ-doped Co98s sample, showed a variation of and 2+ valence state (Fig. 4-11 (b)), at different positions of the sample. These results along with the electron and x-ray diffraction data confirmed that the δ-doped samples contain both substitutional Co and Co clusters, whereas the co-doped samples had Co in valence +2 state, in the form of either Co2+ ions doped in ZnO or in the form of Co2+ in CoO. Figure 4-11 EELS spectra of L3/L2 of Co for (a) co-doped Co 32W (Zn0.71Co0.29O) at four different positions and (b) δ-doped Co 98s samples. 114 Chapter Structural Characterization Selected area electron diffraction of a particle in the above two heavily Codoped samples, as shown in Fig. 4-9 (c) and 4-10 (c), respectively, confirmed the presence of secondary phases. Detailed study of the diffraction pattern and HRTEM images showed the presence of hexagonal closed packed (HCP) Co, face center cubic (FCC) Co, hexagonal CoO and also ZnCo2O4 phase, distributed in the ZnO matrix of the co-doped sample. This did not contradict the EELS result which suggested that the sample was dominantly composed of phases containing Co2+ ions, which could either be ZnO:Co or CoO:Zn. Before ending this section, it should be stressed again that the TEM observation could only provide information about the material in a very localized region; the results don’t necessarily reflect the macroscopic properties of the materials detected by other techniques. 4.3 XPS, AES and UPS studies The XPS was used to analyze the chemical environment experienced by Co in lightly doped samples. Fig.4-12 shows the XPS spectra of samples with different Co compositions (note that the spectra have not corrected for charge-shift; therefore, the O 1s peak was found at 531 eV). As could be seen from the figure, the Co 2p3/2 peak position was at 781.8 eV, while the splits of Co 2p3/2 and 2p1/2 were about 15.4 eV for the samples understudy. As the split for Co metal was about 15.05 eV, one can rule out the existence of metallic Co clusters in the samples measured. If Co was surrounded by oxygen, the split would be about 15.5 eV.3,6 The above results suggested again that samples with a Co composition less than 0.25 were dominantly ZnO:Co or CoO:Zn, while those at very high doping levels can possibly contain other secondary phases such as Co. 115 Chapter Structural Characterization 2P1/2 sat. sat. 2P3/2 (a) Intensity(arb.unit) (b) (c) (d) 810 805 800 795 790 785 780 775 Binding Energy(eV) Figure 4-12 XPS spectra measured with photon energy h =900 eV for co-doped samples (a)Co 10W(Zn0.84Co0.16O), (b) Co 20W (Zn0.76Co0.24O), (c) Co 25W(Zn0.75Co0.25O) and (d) Co 30W(Zn0.73Co0.27O). From the AES spectra shown in Fig. 4-13, a gradual appearance of a satellite feature at the high-energy shoulder at about 781 eV was observed as the Co content increased. This could be due to electron-correlations of some Co ions, possibly attributed to some trivalent Co ion formation or lattice distortion in the outer surface. 116 Chapter Structural Characterization a b c d Intensity(arb.unit) 2.0 1.5 1.0 0.5 770 775 780 785 790 795 800 805 Photon Energy(eV) Figure 4- 13 XAS spectra recorded by LMM AES signal from photoelectrons for samples (a) Co10W and (d) Co30W (Zn0.84Co0.16O), (b) Co 20W (Zn0.76Co0.24O) , (c) Co 25W(Zn0.75Co0.25O) (Zn0.73Co0.27O). Satellite features at the shoulder are marked by a line. hν=60eV O 2p Intensity(arb.unit) a b c d Zn 3d -30 -25 -20 -15 -10 -5 Binding energy(eV) Binding energy(eV) Figure 4-14 Valence-band UPS spectra, photon energy h =60 eV for samples (a) Co 10W (Zn0.84Co0.16O), (b) Co 20W (Zn0.76Co0.24O), (c) Co 25W (Zn0.75Co0.25O) and (d) Co30W (Zn0.73Co0.27O). The component indicated by an arrow below Ef about 0.92 eV is developing as Co content increases. 117 Chapter Structural Characterization In Fig.4-14, another feature that becomes distinctive as the Co concentration increased was found at about 0.92 eV below Ef. It might be attributed to either O 2p or a minor amount of Co 3d component where a trivalent Co oxide compound was found to have a feature below Ef at about eV.7 The trivalent Co oxide was an indication of the presence of ZnCo2O4, as it was also observed from the XRD. Thus, it could be concluded that the films under this study consist of mainly Co2+ - containing phases at x < 0.25 and a mixture of Co2+, Co3+ ions and Co atoms as the Co composition exceeded 0.25. 4.5 Optical transmission studies The observation of d-d transitions in the optical transmission spectra is one of the common “criteria” often used to “prove” that Co atoms have replaced Zn to form substitutional dopants. The d-d transitions, assigned as 4A2(F) → 2A1(G), 4A2(F) → T1(P), and 4A2(F) → 2E(G) transitions in high spin state Co2+(d7), were known to have a wavelength (photon energy) of 571, 618, and 665 nm (2.17, 2.00 and 1.86 eV), respectively. Fig.4-15 shows the transmittance of samples with different Co compositions, normalized to their respective value at 800 nm. With the increase of Co content, the absolute strength of the absorption bands increased almost linearly, indicating Co atoms substituting Zn to form Co2+(d7) ions. The amplitude of the absorption fringes initially increased and then decreased after x reaches 0.25, possibly due to the fluctuations in local crystal field surrounding different Co ions, in particular due to the formation of Co-Co bonds and /or Co clusters at very high Co compositions. 118 Chapter Structural Characterization Al-doped ZnO L J C D F E H Figure 4-15 Optical transmission spectra of various samples. ‘*’ marks energy levels used to determine hysteresis MCD curves in Figure 5-9 for co-doped Co32W, (Zn0.71Co0.29O). To show the different absorption bands more clearly, the transmittance (T) with respect to wavelength ( ) are differentiated, as shown in Fig. 4-16 (dT/d – ). Peaks observed in this plot corresponded to the maximum rate of change of transmittance with respect to wavelength. For Al-doped ZnO, without Co doping, a strong peak due to band edge absorption appears around 345.5 nm. Note that this value is not directly corresponding to the bandgap; therefore, it should not be compared with the bandgap of ZnO which is 370 nm. Similar to the analysis of XRD patterns, here again the samples were divided into groups to facilitate the discussion. Both a blue-shift (A) and a red-shift (B) of the band edge absorption for the Co-doped samples when compared to the Al-doped ZnO were observed. The blue and red shifts varied with the Co composition. Discussion would begin at the range of x < 0.2, where Co was shown to be soluble in the ZnO host matrix from the XRD data; the wavelength of peak A decreased, while that of peak B 119 Chapter Structural Characterization increased. The energy differences between peak A and peak B are 0.41, 0.76, 0.77 and 0.75 eV for samples A, B, C and E, respectively. Figure 4-16 Differiential transmittance dependence on wavelength (dT/d – ) of various samples, showing blue-shift peak wavelength (A) and red-shift peak wavelength (B). Kittilstved et al. observed an absorption band in ZnO:Co (x = 0.035) at an energy of 0.32 eV below the excitonic transition line of ZnO and assigned it to ligand valence band to metal charge transfer transitions. Qiu et. al. had observed an abnormal bandgap narrowing in ZnO:Co nanorods with x = ~ 0.1 and attributed it to lattice volume expension of ZnO induced by Co-doping. The redshift was found to follow the relationship ∆Eg = 0.54(e − x / 0.03 − 1) eV.10 As shown by the solid-line in Fig. 4-17, the wavelength of peak B followed this relationship below x = 0.23, i.e., λB = 1239 / 3.589 + 0.54(e − x / 0.03 − 1) . On the other hand, the wavelength of peak A could be fitted well by the equation λA = 1239 /(3.589 + 1.69 x) and the result was shown by the lower solid curve in the Fig. 4-12. Here, the coefficient of x (1.69 eV) was taken from the fitting result of excitonic transitions in lightly doped samples discussed by Pacuski et. al 11 120 Wavelength (nm) Chapter Structural Characterization 600 Peak A Peak B 500 Qiu et al Qiu et al 400 300 20 40 Co composition (at. %) Figure 4-17 Fitting of blue and red shift wavelength to Qiu et. al. prediction versus Co composition. [After X. Qiu, 2006, Ref. 10] By combining the XRD, EELS and optical transmission data, it is argued that peak A, as shown in Fig. 4-17, was due to Co-rich ZnO:Co phase, while peak B was due to Zn-rich ZnO:Co phase. Considering the facts that the XRD diffraction peaks are close to those of ZnO or hexagonal CoO, the wavelength of peak A was close to the bandgap of CoO12 (~ 2.9 eV), and the valence of Co was dominantly +2 when x was below 0.29, the Co-rich ZnO:Co phase was most likely Zn-incorporated CoO. With a further increase of Co, the Zn-rich phase gradually disappeared; instead the Co-rich phase and secondary phase clusters become dominant, as shown by the XRD patterns discussed above. 121 Chapter Structural Characterization 4.6 Summary As have discussed in this chapter, ZnO:Co is a very complicated material system; it is definitely not an easy task to identify all the phases and reveal their detailed distributions inside the samples with different Co compositions. Nevertheless, based on the systematic studies carried out using XRD, TEM, XPS, AES, UPS and optical transmission spectroscopy, the findings can be summarized as following: (1) Samples with x < 0.2 In this composition range, the Co was soluble in ZnO. At very low doping levels, the films were dominantly single phase of ZnO doped with Co. However, when x increased to near 0.2, Co-rich ZnO:Co may form which eventually evolved into other secondary phases when x increased further. (2) Samples with 0.2 < x < 0.3 This is the region where onset of secondary phase formation occured. The Co-rich region of ZnO:Co might start to evolve into Zn-incorporated CoO. Polycrystalline films with columnar structures started to form, though there was still very low level of secondary phase precipitation. (3) Samples with x > 0.3 In this region, the Co began precipitating to form secondary phase nanoclusters. Secondary phases could be in the form of Co, CoO or ZnCo2O4. These results would be further supported by magnetic and electrical properties to be discussed in the next two chapters. 122 Chapter Structural Characterization Co2+ Co2+ Co2+ Co Co2+ Single phased ZnO doped with Co Co2+ Co2+ Co2+ Co2+ Co2+ Al-doped ZnO Single phase, (002) 2+ Co2+ Co2+ Co-rich ZnCoO Formation ZnO:Co 15 Co2+ of Co-rich 20 Co composition (%) Co2+ Co CoO 2+ Co Co2+ Co Co 2+ 2+ CoO Co-rich ZnCoO evolves to form CoO CoO CoO 2+ Co2+ CoO CoO CoO Co2+ Columnar structure starts to form 25 30 Co composition (%) Figure 4-18 Schematic summarizing characteristic of samples prepared. 123 Co 2+ Co ZnCo2O4 CoO Co ZnCo2O4 Co2+ Secondary phase (Co, CoO or ZnCo2O4) formation 33 Chapter Structural Characterization References: N. Khare, M. J. Kappers, M. Wei, M. G. Blamire, and J. L. MacManus-Driscoll, “Defect induced ferromagnetism in Co-doped ZnO”, Adv. Mater. 18, 1449 (2006). A. Dinia, G. Schmerber, V. Pierron-Bohnes, C. Meny, P. Panissod, and E. Beaurepaire, “Magnetic perpendicular anisotropy in sputtered (Zn0.75Co0.25)O dilute magnetic semiconductor”, J. Mag. Mag. Mater. 286, 37 (2005). H. J. Lee, S. Y. Jeong, C. R. Cho, and C. H. Park, “Study of diluted magnetic semiconductor: Co-doped ZnO”, Appl. Phys. Lett. 81, 4020 (2002). Z. Jin, M. Murakami, T. Fukumura, Y. Matsumoto, A. Ohtomo, M. Kawasaki, and H. Koinuma, “Combinatorial laser MBE synthesis of 3d ion doped epitaxial ZnO thin films”, J. Cryst. Growth 214/215, 55 (2000). T. A. Schaedler, A. S. Gandhi, M. Saito, M. Ruhle, R. Gambino, and C. G. Levi, “Extended solubility of CoO in ZnO and effects on magnetic properties”, J. Mat. Res. 21, 791 (2006). C. D. Wagner, W. M. Riggs, L. E. Davis, and J. F. Moulder, Handbook of X-ray Photoelectron Spectroscopy, (Perkin-Elmer Co., 1979), pp. 78. M. Kobayashi, Y. Ishida, J. I. Hwang, T. Mizokawa, A. Fujimori, K. Mamiya, J. Okamoto, Y. Takeda, T. Okane, Y. Saitoh, Y. Muramatsu, A. Tanaka, H. Saeki, H. Tabata, and T. Kawai, “Characterization of magnetic components in the diluted magnetic semiconductor Zn1–xCoxO by x-ray magnetic circular dichroism”, Phys. Rev. B 72, 201201(R) (2005). P. Koidl, “Optical absorption of Co2+ in ZnO”, Phys. Rev. B 15, 2493 (1977). K. R. Kittilstved, W. K. Liu, and D. R. Gamelin, “Electronic structure origins of polarity-dependent high-TC ferromagnetism semiconductors”, Nat. Mater. 5, 291 (2006). 124 in oxide-diluted magnetic Chapter Structural Characterization 10 X. Qiu, L. Li, and G. Li, “Nature of the abnormal band gap narrowing in highly crystalline Zn1–xCoxO nanorods”, Appl. Phys. Lett. 88, 114103 (2006). 11 W. Pacuski, D. Ferrand, J. Cibert, C. Deparis, J. A. Gaj, P. Kossacki, and C. Morhain, “Effect of the s,p-d exchange interaction on the excitons in Zn1–xCoxO epilayers”, Phys. Rev. B. 73, 035214 (2006). 12 G. W. Pratt and R. Coelho, “Optical absorption of CoO and MnO above and below the Néel temperature”. Phys. Rev. 116, 281 (1959). 125 [...]... out all to be 2+ (Fig 4- 11(a)) The results showed that the film consists of mainly Coincorporated ZnO and/ or Zn-incorporated CoO because the valence state of Co in ZnCo2O4 and Co3O4 was 3+ and 4+ , and that in Co clusters was 0, respectively In comparison, the corresponding δ -doped Co98s sample, showed a variation of 0 and 2+ valence state (Fig 4- 11 (b)), at different positions of the sample These results... electron and x-ray diffraction data confirmed that the δ -doped samples contain both substitutional Co and Co clusters, whereas the co -doped samples had Co in valence +2 state, in the form of either Co2+ ions doped in ZnO or in the form of Co2+ in CoO Figure 4- 11 EELS spectra of L3/L2 of Co for (a) co -doped Co 32W (Zn0.71Co0.29O) at four different positions and (b) δ -doped Co 98s samples 1 14 Chapter 4 Structural...Chapter 4 Structural Characterization (c) Al Growth direction 25 nm O Co Zn Figure 4- 7 TEM results of co -doped Co20W (Zn0.76Co0.24O) sample; (a) Cross-sectional TEM image; (b) electron diffraction pattern of the same region; (c)EDS mapping of Al, Co, O and Zn of films, with direction of film growth as indicated (a) ZnO :Co Al2O3 100 nm Growth direction (b) CoO (220) CoO (111) CoO (200) Figure 4- 8 TEM... Co 4nm Al2O3 Figure 4- 10 TEM results of δ -doped Co98s ([(ZnO:Al (2.38 nm)/Co (1.0 nm)]× 60) sample; (a) Crosssectional TEM image; (b) HRTEM image of a selected region; (c) electron diffraction pattern of the same region 113 Chapter 4 Structural Characterization The EELS analysis was carried out on co -doped Co32W and δ -doped Co98s samples The valence of Co detected from 4 different locations in Fig 4- 9... electron diffraction of a particle in the above two heavily Codoped samples, as shown in Fig 4- 9 (c) and 4- 10 (c), respectively, confirmed the presence of secondary phases Detailed study of the diffraction pattern and HRTEM images showed the presence of hexagonal closed packed (HCP) Co, face center cubic (FCC) Co, hexagonal CoO and also ZnCo2O4 phase, distributed in the ZnO matrix of the co -doped sample This... 1 24 in oxide- diluted magnetic Chapter 4 Structural Characterization 10 X Qiu, L Li, and G Li, “Nature of the abnormal band gap narrowing in highly crystalline Zn1–xCoxO nanorods”, Appl Phys Lett 88, 1 141 03 (2006) 11 W Pacuski, D Ferrand, J Cibert, C Deparis, J A Gaj, P Kossacki, and C Morhain, “Effect of the s,p-d exchange interaction on the excitons in Zn1–xCoxO epilayers”, Phys Rev B 73, 0352 14 (2006)... 2p or a minor amount of Co 3d component where a trivalent Co oxide compound was found to have a feature below Ef at about 1 eV.7 The trivalent Co oxide was an indication of the presence of ZnCo2O4, as it was also observed from the XRD Thus, it could be concluded that the films under this study consist of mainly Co2+ - containing phases at x < 0.25 and a mixture of Co2+, Co3+ ions and Co atoms as the... 0.25 4. 5 Optical transmission studies The observation of d-d transitions in the optical transmission spectra is one of the common “criteria” often used to “prove” that Co atoms have replaced Zn to form substitutional dopants The d-d transitions, assigned as 4A2(F) → 2A1(G), 4A2(F) → 4 T1(P), and 4A2(F) → 2E(G) transitions in high spin state Co2+(d7), were known to have a wavelength (photon energy) of. .. XRD data; the wavelength of peak A decreased, while that of peak B 119 Chapter 4 Structural Characterization increased The energy differences between peak A and peak B are 0 .41 , 0.76, 0.77 and 0.75 eV for samples A, B, C and E, respectively Figure 4- 16 Differiential transmittance dependence on wavelength (dT/d – ) of various samples, showing blue-shift peak wavelength (A) and red-shift peak wavelength... absorption band in ZnO:Co (x = 0.035) at an energy of 0.32 eV below the excitonic transition line of ZnO and assigned it to ligand valence band to metal charge transfer transitions 9 Qiu et al had observed an abnormal bandgap narrowing in ZnO:Co nanorods with x = 0 ~ 0.1 and attributed it to lattice volume expension of ZnO induced by Co-doping The redshift was found to follow the relationship ∆Eg = 0. 54( e . at 44 .5 o was near peak positions of CoO (200) Chapter 4 Structural Characterization 108 FCC at 40 .6 o and 42 .4 o , Co (111) at 44 .217 o , ZnCo 2 O 4 (40 0) at 44 . 74 o and Co 3 O 4 (40 0). phase segregation. 33 .4 33.6 33.8 34. 0 34. 2 34. 4 34. 6 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 Co composition Peak position ( o ) Figure 4- 4 XRD peak position (around 34 o ) versus Co composition ZnCo 2 O 4 and Co 3 O 4 could not be excluded. The formation of Co clusters was more probable because the formation of ZnCo 2 O 4 and Co 3 O 4 needed an oxygen-rich environment instead of more