CHAPTER 5: STUDIES ON STRUCTURES IN Zn1-xCoxO THIN FILMS CHAPTER STUDIES ON STRUCTURES IN Zn1-xCoxO THIN FILMS 5.1 Introduction For materials research, structure study is also the basis. Motivated by the desire for illuminating the physical mechanisms underlying DMS ferromagnetism [1], we studied the structures of the materials, as it has a very high potential for elucidating the physics behind the magnetic behaviors in Zn1-xCoxO thin films. In this chapter, we will study the crystal structures, chemical state and electronic structures of the Zn1-xCoxO thin films. To study the crystal structures, we will employ HRTEM to determine whether Co clusters exist in the lattice on the nanometer scale. Investigations of chemical states and electronic structures including valence band PES were used to provide indirect and direct information on the electronic structures. According to our experimental results, the Zn1-xCoxO thin films are single crystals without apparent precipitates at relative low Co concentrations. With increasing Co concentration, part of the host lattice changes from wurtzite structure to rock-salt structure. Co 3d high spin states were observed. National University of Singapore 66 CHAPTER 5: STUDIES ON STRUCTURES IN Zn1-xCoxO THIN FILMS 5.2 Surface Morphology (a) 0.015 (b) 0.02 (c) 0.16 (d) 0.27 Fig. 5-1 AFM micrographs of Zn1-xCoxO with different Co concentrations [(a) x = 0.015, (b) x = 0.02, (c) x = 0.16 and (d) x = 0.27] obtained at the growth temperature of 650 ℃ for the films. Figures 5-1(a-d) exhibit the AFM images of Zn1-xCoxO thin films of different Co concentrations obtained at the optimum growth condition mentioned in Chapter 4. The thin films were granular at low Co concentration. For example, for Co concentration of x = 0.015 in Fig.5-1(a), the grain size seemed to be uniform. The surface roughness was about 0.8 nm, and the grain size was about 150 nm. This might be due to the formation of a solid solution with a crystal structure. In contrast, irregular features may be observed at a higher Co concentration (x = 0.27), as shown in Fig. 5-1(d). National University of Singapore 67 CHAPTER 5: STUDIES ON STRUCTURES IN Zn1-xCoxO THIN FILMS 5.3 Crystal Structures of Zn1-xCoxO Thin Films 5.3.1 Crystal structures of Zn1-xCoxO thin film with x = 0.015 We have given the typical XRD pattern of Zn1-xCoxO film with x = 0.015, also as shown in Fig. 5-2. Besides the peaks of substrate, diffraction peaks corresponding to the (0002) and (0004) planes of the film with a hexagonal structure can be clearly observed. No peak of other phases was detected. Consequently, it is a wurtzite structure with the c-axis of the film aligned with that of the substrate. The (0002) and (0004) peaks of the film are located at 34.46◦ and 72.64◦, respectively, very close to those of ZnO. From the position of the reflection peaks, the lattice parameter of the film was determined to be c = 0.520 nm, which is similar to the reported values for ZnO. The inset of Fig. 5-2 shows an enlarged plot of the (0002) peak of the film. It can be seen that two peaks due to Cu Kα1 and Cu Kα2 radiation with wavelength 1.5406 and 1.5444 Å, respectively, were revealed clearly. This shows that the film is of good crystallinity. As the peak is sharp, it is reasonable to consider that the dispersion of the lattice parameters of the film is small. National University of Singapore 68 (0002) Kβ Co-doped ZnO (0002) Kα Zn O (0 00 4) 34.0 34.2 34.4 34.6 34.8 35.0 2θ (Degree) C odo pe d c-Al2O3 (0006) Intensity (a.u.) Co-doped ZnO (0002) Co 0.015 Intensity (a.u.) CHAPTER 5: STUDIES ON STRUCTURES IN Zn1-xCoxO THIN FILMS 10 20 30 40 50 60 70 80 90 2θ (Degree) Fig. 5-2 XRD pattern of the Zn0.985Co0.015O film grown on c-plane sapphire substrate. A typical high-resolution XRD ω-rocking curve around the (0002) peak of the Zn1-xCoxO films obtained using the XDD beam line at the SSLS is shown in Fig. 5-3. A sharp peak with a FWHM of only 0.03° (central upper part) is observed, indicating that the film orientation is very close to the direction perpendicular to the plane of substrate, and the dispersion of the orientation is small. However, we can see a tail at the base of the sharp peak. Its FWHM is about 0.2°. This tail is probably due to the small population of texture distribution or lattice distortion. In general, FWHM of the (0002) ω-rocking curve of the sample is small, showing a relatively good crystallinity of the film. National University of Singapore 69 CHAPTER 5: STUDIES ON STRUCTURES IN Zn1-xCoxO THIN FILMS 10000 9000 Co 0.015 8000 Intensity (a.u.) 7000 6000 5000 4000 3000 2000 1000 -1000 14.2 14.4 14.6 14.8 15.0 15.2 15.4 15.6 15.8 16.0 θ (Degree) Fig. 5-3 A typical X-ray rocking curve of the Zn0.985Co0.015O film grown on c-plane sapphire substrate at the growth temperature of 650 °C. HRTEM cross-section images of Zn1-xCoxO (x = 0.015) near the surface and at the interface is shown in Figures 5-4(a) and 5-4(b). The interfaces between Zn1-xCoxO films and substrates were found to be smooth and clear. The images reveal high quality lattice structures with few defects. No precipitates were observed. The sample was observed with the < 10 > axis of the film normal to the specimen surface. From the TEM diffractions and imaging studies, the epitaxial relationship between the film (f) and substrate (s), was established as follows: (0001) f //(0001) s and (1120) f //(10 0) s . National University of Singapore (5-1) 70 CHAPTER 5: STUDIES ON STRUCTURES IN Zn1-xCoxO THIN FILMS In addition, both the films at the interface and near the surface reveal clear lattice with the same orientation, suggesting that the film was formed to be a single crystal. Film Surface (a) (0006) Sapphire Film (20 0) (0002) (11 20) (b) Fig. 5-4 HRTEM images of the Zn0.985Co0.015O films, the substrates and the interfaces (a) near surface and (b) at interface, showing the epitaxial relationship of (0001) f //(0001) s and (1120) f //(10 0) s . National University of Singapore 71 CHAPTER 5: STUDIES ON STRUCTURES IN Zn1-xCoxO THIN FILMS In summary, the Zn1-xCoxO (x = 0.015) film has the wurtzite structure, showing good crystallinity without obvious clusters. The film grows along a preferred direction, following the epitaxial relationship of (0001) f //(0001) s and (10 0) f //(1120) s . 5.3.2 Dependence of crystal structures of Zn1-xCoxO thin film on Co concentration 0.5212 Lattice constant (nm) (a) 0.5210 0.5208 0.5206 0.5204 0.5202 0.5200 0.5198 0.00 0.05 0.10 0.15 0.20 0.25 Co concentration x 0.30 0.30 FWHM (Degree) 0.25 (b) 0.20 0.15 0.10 0.05 0.00 0.05 0.10 0.15 0.20 0.25 Co concentration x Fig. 5-5(a) Variation of c-axis lattice constant c(0002) with Co concentration x; (b) FWHM of the (0002)f rocking curves dependence on Co concentration x. National University of Singapore 72 CHAPTER 5: STUDIES ON STRUCTURES IN Zn1-xCoxO THIN FILMS The Co concentration x dependence of the c-axis lattice constant is given in Fig. 5-5(a). The lattice constant c value does not increase linearly with Co concentration x. This behavior is due to the different ionic radii (r) of the tetrahedral coordinated Zn2+ r = 0.60 Å and r = 0.58 Å for Co2+ [2]. In addition, with increasing x, the Co ions are not always homogeneously distributed in ZnO. With increasing Co concentration x, apart from Co atoms substituting the Zn-site in ZnO, some Co atoms locate at the center of the octahedral site rather than at tetrahedral sites. Namely, part of the host lattice gradually varies from wurtzite structure to rock-salt structure. As we know CoO has a rock-salt type structure with lattice parameter of 0.426 nm [3] which is smaller than that of wurtzite structure of ZnO. The dependence of the FWHM of the (0002)f rocking curves on Co concentration is shown in Fig. 5-5(b), where (0002)f is the (0002) plane of Zn1-xCoxO thin film, f assigns the film. With increasing Co concentration, the top sharp peak mentioned above reduces gradually, and the FWHM tends to increase. The rocking curves tend to broaden, indicating dispersion of the orientation. The broadening in the rocking curves of the films may be due to the distortion of the host lattice, which could be due to strain induced by the occupation of Zn ion sites by Co ions, or the presence of Co precipitates or clusters. In the case of the occupation of Co ions at Zn ion sites, because there exists some differences between Co ions and Zn ions radii [2], strains will be induced in the lattice. This probably results in the distortion of the host lattice. Some Co ions locating at the center of octahedron also causes the distortion of the host lattice. However, generally speaking, our experimental results of XRD and the values of the National University of Singapore 73 CHAPTER 5: STUDIES ON STRUCTURES IN Zn1-xCoxO THIN FILMS FWHM of the rocking curves indicate that the film obtained by this DBPLD method grow along a preferred direction well. To present a whole picture of the trend of crystal structures on Co concentration, the HRTEM results of the Zn1-xCoxO thin films with x ranging from 0.015 to 0.046 were given in Fig. 5-6. Figures 5-6(a) and 5-6(b) show HRTEM cross section images and diffraction pattern of Zn1-xCoxO (x = 0.015) film. Figure 5-6(a) show the image of the film with the lattice clear and straight. The corresponding selected area electron diffraction shows a very clear dot pattern, as shown in Fig. 5-6(b). No smeared or circle patterns were observed. Hence, based on our experimental results, the Zn1-xCoxO (x = 0.015) film reveal a high quality lattice structure with few defects. Figures 5-6(c) and 5-6(d) show HRTEM cross section imagies and diffraction pattern of Zn1-xCoxO (x = 0.16) film, the substrates and interface. The interfaces between Zn1-xCoxO films and substrates were found to be smooth and clear. The imagies show high-quality lattice structure. The specimen was observed with the < 11 > axis of the film normal to the sample surface. From the TEM diffractions and imaging studies, the epitaxial relationship between the film (f) and substrate (s), was found to be (0001) f //(0001) s and (10 0) f //(1120) s (5-2) HRTEM cross section images of Zn1-xCoxO (x = 0.015) near the interface and its diffraction pattern are also shown in Fig. 5-6(a) and 5-6(b). From it, the epitaxial relationship between the film (f) and substrate (s) is the same as Exp. (5-2) above. National University of Singapore 74 CHAPTER 5: STUDIES ON STRUCTURES IN Zn1-xCoxO THIN FILMS Hence, based on our experimental results, the epitaxial relationship between the film and the substrate is concluded that the lattice of the film rotates 30° to that of the substrate. Figures 5-6(e) and 5-6(f) show high-resolution cross-section images and diffraction pattern of Zn1-xCoxO thin film with x = 0.46. A lattice could still be observed at this high Co concentration, however, defects were also observed in this film [the circle area in Fig. 5-6(e)]. The diffraction pattern became less regular. The diffraction pattern revealed well that most defects are polycrystal structure instead of single crystal, as shown in Fig. 5-6(f). The light circles on the diffraction pattern indicate that the film contains precipitates. Based on our experimental results, we conclude that high quality wurtzite crystal structure films have been fabricated, particularly for the films with relative low Co concentrations. In all of the specimens with Co concentration x < 0.1, TEM images display good lattice without obvious clusters. But for some samples with relatively high Co concentrations, sometimes, we observe secondary phase precipitates. Figure 5-7 shows the Co-metal clusters (about 10nm) in the Zn1-xCoxO thin films with Co concentration about 0.16. We need to note that clusters may not be observed for some samples with the Co concentration x > 0.1. This is due to the non-uniformity of the DBPLD method. National University of Singapore 75 CHAPTER 5: STUDIES ON STRUCTURES IN Zn1-xCoxO THIN FILMS 120000 (a) Zn2p3 100000 c/s 80000 60000 40000 20000 1015 1020 1025 1030 1035 1040 Binding energy (eV) 13800 (b) Co2p3 13600 13400 13200 c/s 13000 12800 12600 12400 12200 12000 11800 760 770 780 790 800 810 820 Binding energy (eV) 30000 (c) O1s 25000 c/s 20000 15000 10000 5000 520 525 530 535 540 545 Binding energy (eV) Fig. 5-9 XPS spectra for Zn0.985Co0.015O0.67 film: (a) Zn 2p3/2 XPS spectrum, (b) Co 2p3/2 XPS spectrum, (c) O 1s XPS spectrum. National University of Singapore 81 CHAPTER 5: STUDIES ON STRUCTURES IN Zn1-xCoxO THIN FILMS The XPS spectra of the films with different Co concentrations is shown in Fig. 5-10. A typical O1s peak of Zn1-xCoxO thin film is given in Fig. 5-10(a). The O1s peak is observed to reveal a shoulder at higher binding energy. By fitting the peak with gaussians, the O1s can be separated into two peaks. The lower energy peak (labeled O1) located at 529.3 eV, corresponds to O - Zn bonds, while the higher energy peak, located at 531.6 eV (O2), can be attributed to O - H bonds resulting from exposure to air. In our study, we focus on the lower energy peak O1 rather than the higher energy peak O2. For our synthesized materials, the shift of the peak O1 toward higher binding energy is observed with increasing Co concentration, especially when Co concentration x exceeds 0.03 [see Fig. 5-10(b)]. In the case of the occupation of Co ions at Zn sites, it is reasonable that the substitution atoms affect the surrounding O Zn binding energies through the effect of electron charges. The position of peak O1 increases from 529.3 to 529.7 eV as the Co concentration x of Zn1−xCoxO increases from 0.03 to 0.25. With higher Co concentration, the O1s peak attributed to O - Zn bonds (peak O1) shifts toward higher binding energy. However, there is no obvious change in the position of Zn 2p3/2 peaks, as shown in Fig. 5-10(c). In Fig. 5-10(d), the Co 2p3/2 peaks correspond to the Co - O bonding [6]. Under our experimental conditions, the intensity of Co 2p3/2 peaks increases with increasing Co concentration. In order to show the effect of Co precipitates, a Co 2p3/2 spectrum of the Zn1−xCoxO film with x = 0.41 is also plotted. The excessive Co content results in the occurrence of a peak at 778 eV, indicating the presence of Co precipitates. National University of Singapore 82 CHAPTER 5: STUDIES ON STRUCTURES IN Zn1-xCoxO THIN FILMS 28000 28000 Intensity (c/s) 24000 22000 26000 Experimental data Fitting data Backing ground O1 O2 (a) Co 0.03 O1s 20000 16000 O1 O2 12000 O1 22000 18000 14000 Co 0.015 Co 0.03 Co 0.05 Co 0.09 Co 0.25 (b) 24000 Intensity(a.u.) 26000 20000 18000 16000 14000 12000 10000 10000 8000 8000 524 526 528 530 532 534 536 538 540 6000 526 542 527 Binding energy (eV) 528 529 530 531 532 533 534 Binding Energy (eV) 24000 Co 0.015 Co 0.03 Co 0.05 Co 0.09 Co 0.25 (c) Intensity (c/s) 100000 Zn 2p3/2 80000 60000 40000 22000 Intensity (c/s) 120000 (d) Co 0.015 Co 0.03 Co 0.05 Co 0.09 Co 0.25 Co 0.41 CoO Co 2p3/2 20000 18000 16000 Co 14000 20000 12000 1017 1018 1019 1020 1021 1022 1023 1024 1025 776 Binding Energy (eV) 778 780 782 784 786 788 790 Binding Energy (eV) Fig. 5-10 XPS spectra for Zn1-xCoxO films: (a) O 1s XPS spectrum of Zn1-xCoxO (x = 0.03) film with Gauss fitting results (thinner lines), (b) Step scanned data of the deconvoluted O1 peaks, (c) Zn 2p3/2 XPS spectra, (d) Co 2p3/2 XPS spectra. Note that in (b) the peaks have been shifted by constant offset for clarity. 5.5 Electronic Structure Study 5.5.1 Absorption Spectra of Zn1−xCoxO Thin Films The optical absorption spectrum of the film for x = 0.015 is shown in Fig. 5-11. The film is observed to be transparent in the visible region from 400 nm and it has a narrow absorption band around 380 nm, from which the bandgap energy of 3.31 eV can be obtained. It is very close to that of pure ZnO. When electrons transit from initial state to final state, photons of a certain frequency will be absorbed. The absorption coefficient is proportional to the density of the electrons in the initial and final states [8]. Therefore, the clear absorption edge also demonstrates that the electrical structure National University of Singapore 83 CHAPTER 5: STUDIES ON STRUCTURES IN Zn1-xCoxO THIN FILMS of the host semiconductor is well preserved. The optical band gap energy of the films was obtained by plotting α2 versus wavelength λ (hν), where α and λ ( hν) are absorption coefficient and wavelength, respectively. The intercept on the hν-axis at α2 = gives Eg = 3.31 eV, which represents the optical bandgap of the film. 2.5 Abs. (a.u.) 2.0 1.5 1.0 0.5 0.0 250 300 350 400 450 500 550 600 650 700 750 800 850 Wavelength (nm) Fig. 5-11 Optical absorption spectra of Zn0.985Co0.015O film. The optical absorption spectra of Zn1−xCoxO thin films with different Co concentrations are shown in Fig. 5-12(a). The values of optical bandgap can be obtained from these spectra. The absorption spectrum of the film with x = 0.05 is given by the curve in Fig. 5-12(a). It is transparent to light when the wavelength is larger than 413 nm (band tail), and there is a very rapidly rising absorption edge (absorption onset) at round 373 nm. The sharp absorption edge of the sample indicates high crystal quality. As for the sample with higher Co concentration [for example, 0.09, as shown as curve in Fig. 5-12(a)], band tail is shifted to a larger wavelength (red shift). The National University of Singapore 84 CHAPTER 5: STUDIES ON STRUCTURES IN Zn1-xCoxO THIN FILMS relatively larger band tail may be attributed to the energy levels near the band edge. From it, the absorption edge is not well defined, which is expected for some component of indirect-bandgap semiconductor transmission. This is agreement with the previous result that the host lattice gradually changes from wurtzite structure of ZnO to rock-salt type structure of CoO [9]. On the other hand, there is a blue shift for the absorption onset. In short, broadening behavior near the absorption onset can be observed with higher Co concentration. A blue shift of the absorption edge with increasing dopant concentration indicates an increase in the optical bandgap of the system. This is due to the addition of Co ions into the Zn sites in ZnO which affects the electronic band structure of the material: more Co-O bondings and indirect transition. The optical bandgap energy dependence on Co concentration is shown in Figure 5-12(b). National University of Singapore 85 CHAPTER 5: STUDIES ON STRUCTURES IN Zn1-xCoxO THIN FILMS 550 A2(F)--- E1(G) Black Co 0.015 Green Co 0.02 Red Co 0.05 Blue Co 0.09 Navy Co 0.25 Intensity (a.u.) Absorption (a.u.) A2(F)--- A1(G) (a) A2(F)--- T1(P) 600 650 700 Wavelength (nm) 750 300 400 500 600 700 Wavelength (nm) 3.70 3.65 (b) Bandgap (eV) 3.60 3.55 3.50 3.45 3.40 3.35 3.30 0.00 0.05 0.10 0.15 0.20 0.25 Co concentration x Fig. 5-12(a) Optical absorption spectra of Zn1-xCoxO films grown on c-plane sapphire substrates at different Co concentrations. Inset shows an enlarged plot of the absorption spectra around 650 nm. A, E and T are designations of the intermediate energy bands. (b) Variation in the optical band-gap of Zn1-xCoxO films with Co concentration. National University of Singapore 86 CHAPTER 5: STUDIES ON STRUCTURES IN Zn1-xCoxO THIN FILMS The absorption coefficient α is closely related to the density of electrons in the initial and final states [10]. In our samples, the absorption edge becomes less defined with increasing Co concentration. The broadening of the absorption spectra near the absorption onset suggested that probably there are some components of indirect transmission. Some peaks corresponding to midgap-like absorption were also observed. This phenomenon, together with the results of XRD and XPS, is consistent with Zn ions being replaced by Co ions. Inset of Fig. 5-12(a) is an enlarged plot of the absorption spectra near 650 nm. We observed that for higher Co concentration, the dominant midgap-like absorption peaks were around 569 nm (2.18 eV), 616 (2.01 eV) and 662 nm (1.87 eV). Using the high spin d7 electron configuration of the tetrahedrally coordinated Co2+ ions, these absorption are expressed by A2 ( F )→ 2A1 (G ) , A2 ( F )→ T1 ( P ) and A2 ( F )→2E1 (G ) . Here, A, E and T are generally designations of intermediate energy bands [11,12]. The midgap absorption around these energy develops as Co concentration increases. The absorption spectra of Co2+ in ZnO can be explained by static crystal-field theory [12] as follows. Clearly, the energy level of Co2+ (3d7) in ZnO are determined by Coulomb, crystal field, and spin-orbit interactions. In this inset, the observation of the ∧ spin-allowed transition to 4T1 ( P) indicates the trigonal splitting of 4T1 ( P) with E ∧ ∧ ∧ below A ( E and A are defined as energy bands in Fig. of Ref. [12] shown in Appendix 1) caused by the dependence of the trigonal field splitting of the 4T1 states ∧ ∧ (energy E - energy A ) on υ and υ ′ , where υ and υ ′ are trigonal crystal-field parameters. In our case, υ ′ > National University of Singapore υ is expected. From our absorption results, the 87 CHAPTER 5: STUDIES ON STRUCTURES IN Zn1-xCoxO THIN FILMS many electron ground states are singlet A2. We observed the absorption band to the d-d transition of the Co2+ impurity in Zn1−xCoxO, indicating the band splitting, i.e., the trigonal splitting of 4T1 ( P) . Moreover, the absorption takes place over a relatively wide energy range rather than at a sharp peak. Therefore, it is reasonable to assign this absorption to the charge-transfer transition between donor and/or acceptor ionization levels of Co ions in Zn1−xCoxO crystal field with trigonal and tetrahedral symmetry [13]. We know the host lattice changes from wurtzite structure to rock-salt structure with increasing Co concentration. 5.5.2 Photoluminescence of Zn1-xCoxO Thin Films Under our experimental conditions, near-band emission can be found at low Co concentration. Figure 5-13(a) shows a typical near band PL emission spectrum. In this figure, the emission band centered around 379 nm (near bandgap) with a FWHM of about 30 eV. It is in agreement with the results of absorption spectra. The green band near 520 nm, reported to be associated with the point defects and impurities in the films [14], are generally quenched in our studies. Here, we concentrate on the free exciton peaks around 380 nm. The near band PL emission spectra at different doping concentration are also shown in Fig. 5-13 (b) and 5-13(c). With increasing Co concentration [Fig. 5-13(b)], the intensity of PL decreases. Further increase in Co concentration may even quench the near-band emission at room temperature, as shown in Fig. 5-13(c). The decrease in the intensity of PL spectra indicates that defects may probably increase with increasing Co National University of Singapore 88 CHAPTER 5: STUDIES ON STRUCTURES IN Zn1-xCoxO THIN FILMS concentration. This is consistent with the XRD data mentioned earlier. We claim that defects may suppress the excitonic emission of the films. The blue shift of the PL peak position with increasing Co concentration coincides with the absorption spectra results. 150 (a) 100 Intensity (a.u.) 50 40 (b) 20 30 (c) 20 10 360 380 400 420 440 460 480 Wavelength (nm) Fig. 5-13 Emission spectra for Zn1-xCoxO films with different Co concentration x. (a) x = 0.015, (b) x = 0.02, (c) x = 0.12. 5.5.3 Valence Band PES of Zn1-xCoxO Thin Films The valence band PES spectrum of Zn1-xCoxO (x = 0.02) is shown in Fig. 5-14(b). From the figure, the valence band PES spectra of Zn1-xCoxO (x = 0.02) can be fitted with four Gauss lines with the peak position of binding energy around 11 eV (P1), eV (P2), 7.5 eV (P3) and eV (P4), respectively. The peaks are labeled as P1, P2, P3 and P4, represent the four fitting peaks in Fig. 5-14(b) in turn. National University of Singapore 89 CHAPTER 5: STUDIES ON STRUCTURES IN Zn1-xCoxO THIN FILMS The theoretical calculated density of states of ZnO near EF by FEFF801 is shown in Fig. 5-14(a). The dotted curves present the contribution of s band, p band and d band to the DOS. They have been calculated on the basis of the results from one-panel calculations with s, p and d muffin-tin orbitals placed on TM and oxygen sites. The inset shows an enlarged part of 3d, p and s partial density of states. From this, we can observe the components of calculated Zn 3d, mixed s and p (O 2p) density of states. The features of Fig. 5-14(a) are basically similar to the experimental result shown in Fig. 5-14(b) except that there is some shift of the peaks. Though the agreement between theory and experiment is imperfect, in the present analyses, comparing the spectra in Fig. 5-14(a) and 5-14(b), P1 corresponds to Zn 3d, P2 represents O 2p, P3 represents the mixture of O 2p and Zn 4s. P4 is on the top of O 2p, a feature of the spectra of the Zn1-xCoxO thin films that is different from that of ZnO, reflecting the effect of the Co. Ref [15] pointed out that in CoO, the center of 3d band is on the top of the O 2p band. Hence, it is reasonable to consider that this peak is related to Co 3d band with the feature of non-bonding O 2p in Zn1-xCoxO. From our experimental results, Zn1-xCoxO thin films have similar band structures to that of ZnO, in particular when x is small (such as x = 0.02 in Fig. 5-14). It is also known Zn1-xCoxO has a wurtzite-type lattice, for which both the lowest conduction band and uppermost valence bands have extremal points at the canter of the Brillouin zone. The Γ8 valence states involve mostly anion (O2-) p orbitals. In the Zn1-xCoxO thin films, Co ions occupy Zn-sites with the tetrahedral symmetry. For this symmetry, the hybridization related to Γ8 is important. Later we will discuss that, in a tetrahedral National University of Singapore 90 CHAPTER 5: STUDIES ON STRUCTURES IN Zn1-xCoxO THIN FILMS field, the fivefold degenerate d orbitals of Co2+ are split into triply degenerate levels (t2g) and doubly degenerate levels (eg) [16]. The t2g orbitals are hybridized with Γ8 -band state [17]. This is the basis of our discussion about the origin of magnetism for the Zn1-xCoxO films in next Chapter. Ref [18] shows a sharp peak near EF using local spin-density approximation, and the result of Co 3d state and O 2p interaction, namely, spin-orbit interaction. DOS (electrons/eV) DOS (e/eV) 1.0 (a) 15 Theoretical ZnO sum d p s 10 d 0.8 0.6 p s 0.4 0.2 0.0 -18 -16 -14 -12 -10 -8 -6 -4 -2 Energy (eV) Intensity (a.u.) (b) Experimental Zn0.98Co0.02O P2 P3 -18 -16 -14 Exp Fit P1 P2 P3 P4 P1 -12 -10 -8 P4 -6 -4 -2 Binding energy (eV) Fig. 5-14(a) Theoretical calculated valence band spectrum of ZnO with d, p and s partial density of states. Light curves describe the contribution of s band, p band and d band to the density of states, as shown in its inset. The inset gives an enlarged part of 3d, p and s partial density of states; (b) Valence band photoemission spectra of Zn1-xCoxO (x = 0.02) thin film with Gauss fitting results P1, P2, P3 and P4. National University of Singapore 91 CHAPTER 5: STUDIES ON STRUCTURES IN Zn1-xCoxO THIN FILMS We now discuss the valence band PES of Zn1-xCoxO thin films with different Co concentrations, as shown in Fig. 5-15(a). All the spectra have been normalized by the incident intensity after subtraction of background. In order to show variance in Peak P2, we set P _ ratio = P _ area . It is known that the exchange interaction between P1 _ area p band in the valence band and d electrons is mainly derived from the p-d hybridization. Especially at the Γ point, the top of the valence band is constructed purely from the anion p orbitals that can only hybridize with the d orbitals of t2 symmetry [19]. The P _ ratio reflects the variance of the band related to the p-d hybridization. The inset of Fig. 5-15(a) shows the variation in P _ ratio as a function of Co concentration. It can be seen that P _ ratio (O 2p) increases even after a small Co concentration in Zn1-xCoxO film. Further increasing Co concentration dose not result in a drastic further increase of the P _ ratio , even when Co concentration reaches 0.27. Our observations indicate that the effect of Co 3d state, explained by the spin–orbit interaction through Co 3d state and O 2p interaction in Zn1-xCoxO thin film, is limited. Namely, although the mixing of the p-band and d-electrons can be realized in Zn1-xCoxO system, the enhancement of the p-d hybridization is not observed. Consequently, a long range ferromagnetic order is not produced. From Fig. 5-15(b), one can see that, with increasing Co concentration, compared with P2, P3 and P4, the variance in the binding energy of Zn 3d (P1) is smaller. With increasing Co concentration, this result of less variance in the binding energy of Zn 3d National University of Singapore 92 CHAPTER 5: STUDIES ON STRUCTURES IN Zn1-xCoxO THIN FILMS coincides with the XPS result that there is no obvious change in the position of Zn 2p3/2 peaks [Fig. 5-10(c)]. In Fig. 5-15(b), it can also been seen that with increasing Co concentration, the binding energy of O 2p and the mixed band increases. The energy for P4 increases too. The variance in the energies with increasing Co concentration is due to more Co ions substituting the Zn-site in ZnO and more Co-O bond substituting Zn-O bond. Similar to ZnO, the valence band of the host Zn1-xCoxO is formed by occupied O 2p levels [7]. Hence, the increase in the binding energy of O 2p could be an evidence for more mixing of cation (Co2+) and anion (O2-) p orbitals when more Co ions are doped. With increasing Co concentration, the increase in the binding energy of O 2p contributes to the increase in the optical band gap. For the Co concentration less than 0.1, a small peak (P4) located around 1-3 eV on the top of the O 2p valence band. It is probably related to the Co 3d band with the feature of non-bonding O 2p in Zn1-xCoxO. National University of Singapore 93 CHAPTER 5: STUDIES ON STRUCTURES IN Zn1-xCoxO THIN FILMS 2000 0.6 P2_ratio (a) Intensity (a.u.) 1500 1000 0.4 0.2 0.0 0.1 0.2 0.3 Co concentration x Co 0.015 Co 0.02 Co 0.09 Co 0.27 500 -14 Binding energy (eV) -12 -10 -8 -6 -4 Binding energy (eV) (b) -2 P1 P2 P3 P4 -2 -4 -6 -8 -10 -12 10 15 20 25 Co concentration (%) Fig. 5-15(a) Valence band photoemission spectra of Zn1-xCoxO with different Co concentrations. Inset shows the variation in P _ ratio as a function of Co concentration; (b) Zn 3d (P1), O 2p (P2), mixture of Zn 4s and O 2s (P3) and P4 energy levels dependence on Co concentrations. National University of Singapore 94 CHAPTER 5: STUDIES ON STRUCTURES IN Zn1-xCoxO THIN FILMS 5.6 Summary z DBPLD grown Zn1-xCoxO films showed good crystallinity, suggesting substitution of Zn-sites in ZnO by Co ions. z The films have the wurtzite structure. The lattice parameters are closely distributed. The films grow along a preferred direction, following the epitaxial relationship of (0001) f //(0001) s and (10 0) f //(1120) s , which is particularly visible at low Co concentration (x < 0.1). HRTEM images show good lattice without obvious clusters for the films less Co concentrations (x < 0.1). However for some samples with relatively high Co concentrations, we sometimes observe clusters on the scale of nanometer. z With increasing Co concentration x, apart from Co atoms substituting the Zn-site in ZnO, some Co atoms locate at the center of the octahedral site rather than at tetrahedral sites. That is, the host lattice tends to change. Part of the host lattice gradually varies from wurtzite structure to rock-salt structure. For a higher Co concentration, the diffraction pattern revealed that the regular diffraction pattern is changed to a less regular pattern, namely, most defects are polycrystal structure in stead of single crystal. z The shift of absorption edge and broadening of the absorption onset increase as Co concentration increases, indicating that more impurity levels have developed within the bandgap, and reflecting some components of indirect transitions. z The midgap absorption develops as Co concentration increases. The observation of the spin-allowed transition indicates the trigonal splitting in the films. National University of Singapore 95 CHAPTER 5: STUDIES ON STRUCTURES IN Zn1-xCoxO THIN FILMS z Near-band emission can also be observed in the films with relative low Co concentrations. z Substitution atoms affect the surrounding O - Zn binding energies. With increasing Co concentration, the O1s peak attributed to O - Zn bonds (peak O1) shifts toward higher binding energy. The excessive Co content results in the appearance of a peak at 778 eV, indicating the presence of Co precipitates. z The peak P2 represents O 2p. Our observations indicate that the effect of Co 3d state, explained by the spin–orbit interaction through Co 3d state and O 2p interaction in Zn1-xCoxO thin film, is limited. The peak P4 locates near the top of the O 2p valence band. It is probably related to the Co 3d band with the feature of non-bonding O 2p in Zn1-xCoxO. National University of Singapore 96 [...]... 524 526 528 530 5 32 534 536 538 540 6000 526 5 42 527 Binding energy (eV) 528 529 530 531 5 32 533 534 Binding Energy (eV) 24 000 Co 0.015 Co 0.03 Co 0.05 Co 0.09 Co 0 .25 (c) Intensity (c/s) 100000 Zn 2p3 /2 80000 60000 40000 22 000 Intensity (c/s) 120 000 (d) Co 0.015 Co 0.03 Co 0.05 Co 0.09 Co 0 .25 Co 0.41 CoO Co 2p3 /2 20000 18000 16000 Co 14000 20 000 120 00 0 1017 1018 1019 1 020 1 021 1 022 1 023 1 024 1 025 ... presence of Co precipitates National University of Singapore 82 CHAPTER 5: STUDIES ON STRUCTURES IN Zn1-xCoxO THIN FILMS 28 000 28 000 Intensity (c/s) 24 000 22 000 26 000 Experimental data Fitting data Backing ground O1 O2 (a) Co 0.03 O1s 20 000 16000 O1 O2 120 00 O1 22 000 18000 14000 Co 0.015 Co 0.03 Co 0.05 Co 0.09 Co 0 .25 (b) 24 000 Intensity(a.u.) 26 000 20 000 18000 16000 14000 120 00 10000 10000 8000 8000 524 ... p s 0.4 0 .2 0.0 -18 -16 -14 - 12 -10 -8 -6 -4 -2 0 2 Energy (eV) Intensity (a.u.) 0 (b) Experimental Zn0.98Co0.02O P2 P3 -18 -16 -14 Exp Fit P1 P2 P3 P4 P1 - 12 -10 -8 P4 -6 -4 -2 0 2 Binding energy (eV) Fig 5-14(a) Theoretical calculated valence band spectrum of ZnO with d, p and s partial density of states Light curves describe the contribution of s band, p band and d band to the density of states,... vacancies act as donors, and contribute to the semiconductor properties of the film [7] National University of Singapore 80 CHAPTER 5: STUDIES ON STRUCTURES IN Zn1-xCoxO THIN FILMS 120 000 (a) Zn2p3 100000 c/s 80000 60000 40000 20 000 0 1015 1 020 1 025 1030 1035 1040 Binding energy (eV) 13800 (b) Co2p3 13600 13400 1 320 0 c/s 13000 128 00 126 00 124 00 122 00 120 00 11800 760 770 780 790 800 810 820 Binding energy... University of Singapore 93 CHAPTER 5: STUDIES ON STRUCTURES IN Zn1-xCoxO THIN FILMS 20 00 0.6 P2_ratio (a) Intensity (a.u.) 1500 1000 0.4 0 .2 0.0 0.1 0 .2 0.3 Co concentration x Co 0.015 Co 0. 02 Co 0.09 Co 0 .27 500 0 -14 Binding energy (eV) 2 - 12 -10 -8 -6 -4 Binding energy (eV) (b) -2 0 P1 P2 P3 P4 0 -2 -4 -6 -8 -10 - 12 5 10 15 20 25 Co concentration (%) Fig 5-15(a) Valence band photoemission spectra of Zn1-xCoxO... midgap-like absorption peaks were around 569 nm (2. 18 eV), 616 (2. 01 eV) and 6 62 nm (1.87 eV) Using the high spin d7 electron configuration of the tetrahedrally coordinated Co2+ ions, these absorption are expressed by 4 A2 ( F )→ 2A1 (G ) , 4 A2 ( F )→ 4 T1 ( P ) and 4 A2 ( F )→2E1 (G ) Here, A, E and T are generally designations of intermediate energy bands [11, 12] The midgap absorption around these energy... contribution of s band, p band and d band to the DOS They have been calculated on the basis of the results from one-panel calculations with s, p and d muffin-tin orbitals placed on TM and oxygen sites The inset shows an enlarged part of 3d, p and s partial density of states From this, we can observe the components of calculated Zn 3d, mixed s and p (O 2p) density of states The features of Fig 5-14(a)... affects the electronic band structure of the material: more Co-O bondings and indirect transition The optical bandgap energy dependence on Co concentration is shown in Figure 5- 12( b) National University of Singapore 85 CHAPTER 5: STUDIES ON STRUCTURES IN Zn1-xCoxO THIN FILMS 1 550 1 4 2 A2(F) - E1(G) 2 2 1 Black Co 0.015 2 Green Co 0. 02 3 Red Co 0.05 4 Blue Co 0.09 5 Navy Co 0 .25 4 2 4 5 4 4 3 Intensity... some shift of the peaks Though the agreement between theory and experiment is imperfect, in the present analyses, comparing the spectra in Fig 5-14(a) and 5-14(b), P1 corresponds to Zn 3d, P2 represents O 2p, P3 represents the mixture of O 2p and Zn 4s P4 is on the top of O 2p, a feature of the spectra of the Zn1-xCoxO thin films that is different from that of ZnO, reflecting the effect of the Co Ref... calculating of the relative areas for specific binding energies The composition of the film is Zn0.985Co0.015O0.67 The binding energies of Zn 2p3 /2 as shown in Fig 5-9(a), and Co 2p3 /2 in Fig 5-9(b), and O 1s in Fig 5-9(c), provide a complete picture of the elements’ chemical states The Zn 2p3 /2 XPS peak appeared at 1 020 .8 eV, which coincides with Zn in ZnO [6] There is no National University of Singapore . 534 6000 8000 10000 120 00 14000 16000 18000 20 000 22 000 24 000 26 000 28 000 Co 0.015 Co 0.03 Co 0.05 Co 0.09 Co 0 .25 Intensity(a.u.) Binding Energy (eV) (b) O1 1017 1018 1019 1 020 1 021 1 022 1 023 1 024 1 025 0 20 000 40000 60000 80000 100000 120 000 . 5 42 8000 10000 120 00 14000 16000 18000 20 000 22 000 24 000 26 000 28 000 O2 Intensity (c/s) Binding energy (eV) Experimental data Fitting data Backing ground O1 O2 O1 (a) Co 0.03 O1s 526 527 528 529 530 531 5 32 533 534 6000 8000 10000 120 00 14000 16000 18000 20 000 22 000 24 000 26 000 28 000 . 1015 1 020 1 025 1030 1035 1040 0 20 000 40000 60000 80000 100000 120 000 c/s Binding energy (eV) Zn2p3 (a) 760 770 780 790 800 810 820 11800 120 00 122 00 124 00 126 00 128 00 13000 1 320 0 13400 13600 13800