Structural, magnetic and transport study of DBPLD fabricated magnetic semiconductors 2

With increasing Co concentration, part of the host lattice changes from wurtzite structure to rock-salt structure.. Besides the peaks of substrate, diffraction peaks corresponding to the

In this chapter, we will study the crystal structures, chemical state and electronic

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

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

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)

5.3 Crystal Structures of Zn1-xCoxO Thin Films

5.3.1 Crystal structures of Zn1-xCox O thin film with x = 0.015

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

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

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

14.2 14.4 14.6 14.8 15.0 15.2 15.4 15.6 15.8 16.0

-1000

0 1000

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

TEM diffractions and imaging studies, the epitaxial relationship between the film (f) and substrate (s), was established as follows:

s

)0001

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

)0006(

(b)

)0211(

)0002(

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

In summary, the Zn1-xCox O (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 (1010)f //(1120)s

5.3.2 Dependence of crystal structures of Zn1-xCoxO thin film on Co concentration

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.5198

0.5200 0.5202 0.5204 0.5206 0.5208 0.5210 0.5212

Co concentration x

(b)

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

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

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

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

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,

were given in Fig 5-6

Figures 5-6(a) and 5-6(b) show HRTEM cross section images and diffraction

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

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

imagies show high-quality lattice structure The specimen was observed with the

imaging studies, the epitaxial relationship between the film (f) and substrate (s), was

found to be

(0001)f //(0001)s and (1010)f //(1120)s (5-2)

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

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

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

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

Sapphire Film

Fig 5-6 HRTEM images of the Zn1-xCoxO films, the substrates and the interfaces (a, c,

e) and selected area electron diffraction pattern of the film (b, d, f) and substrates

(insets) with different Co concentrations: (a, b) for x = 0.015, (c, d) for x = 0.16 and

(e, f) for x = 0.46 The circle in Fig 6(e) shows a defect

With increasing Co concentration, the host lattice tends to change For a higher Co concentration, the diffraction pattern revealed that the regular diffraction pattern is changed to a less regular pattern, that is, most defects are polycrystal structure instead

of single crystal These results could be explained by the deposition theory [4] It is known that ZnO has a higher surface energy of {0001} facet on c-plane sapphire And CoO has the rock salt structure, the crystal structure and anisotropy of Co – O are different with that of Zn – O The change of deposition anisotropy makes the

“developing ZnO facets” more difficult In addition, the energy released due to the different radius of Co ions and Zn ions will lead to the distortion of the lattice [5] The change of surface energy leads to the change of deposition anisotropy Therefore, the preferred growth orientation is dispersed and wurtzite crystal structures are destroyed

Another reason is that at relatively low Co concentration x, Co ions substitute the

Zn-site in ZnO and locate at the tetrahedron centers, the structures are single crystals

With increasing x, the strain in the lattice increases and more O - Co - O bonds are

established in the films which are different with that of O – Zn – O, schematically as shown in Fig 5-8 The bond angle tend to change and the host lattice is distorted As a result, some Co ions are located at the octahedral centers As more Co ions are located

at the octahedron centers, the host lattice is distorted further and the preferred orientation of (0002) of the film is changed Consequently, the lattice constant did not

follow a linear dependence on x which is usually a optical probe whether the dopants

change the host lattice In our studies, no perceptible precipitates were observed at low

Co concentration (x < 0.1) At higher Co concentration, precipitates may be observed

These results of structure studies coincide with those of property studies which will be discussed in next chapter

(a)

(b)

Fig 5-7(a) HRTEM image and (b) corresponding selected area electron diffraction of

the film with x = 0.16 showing dispersed Co clusters with the diameter about 10 nm

The patterns were designated to be Co clusters

Fig 5-8 Schematic graphs of Zn-O bond in ZnO and Co-O bond in CoO (not to scale)

composition of the film can be obtained by 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 1020.8 eV, which coincides with Zn in ZnO [6] There is no

peak at 1021.9 eV, indicating that no trace of pure Zn was detected in the film [6] It is

well known that the Co – O bonding and Co – Co bonding peaks occur at 780 eV and

778 eV, respectively From Fig 6-9(b), the Co 2p3/2 peak appears at 780 eV,

indicating that only Co – O bonding exists in the film and no Co – Co bonding exists

in the film The O (1s) peak centered at 529.4 eV is in agreement with the binding energy for O 1s of 528.1–531.05 eV for a metal oxide [6], which may be attributed to

oxide ions in ZnO or CoO These results show that Co has replaced Zn, resulting in Co – O bonding in the host lattice Moreover, the film is a single crystal metal oxide compound with more oxygen vacancies but less Zn interstitials Under our experimental conditions, the high vacuum pressure leads to more oxygen vacancies in the film The oxygen vacancies act as donors, and contribute to the semiconductor properties of the film [7]

1015 0 1020 1025 1030 1035 1040 20000

40000 60000 80000 100000 120000

The XPS spectra of the films with different Co concentrations is shown in Fig 5-10

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

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

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

Fig 5-10 XPS spectra for Zn1-xCo x O films: (a) O 1s XPS spectrum of Zn1-xCo x O (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

of the host semiconductor is well preserved The optical band gap energy of the films

= 0 gives Eg = 3.31 eV, which represents the optical bandgap of the film

250 300 350 400 450 500 550 600 650 700 750 800 850

0.0 0.5 1.0 1.5 2.0 2.5

Wavelength (nm)

Fig 5-11 Optical absorption spectra of Zn0.985Co0.015O film

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 3 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 4 in Fig 5-12(a)], band tail is shifted to a larger wavelength (red shift) The