Effects of Ti-doped concentration on the microstructures and optical properties of ZnO thin films

Superlattices and Microstructures 52 (2012) 765–773

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Effects of Ti-doped concentration on the microstructures and optical properties of ZnO thin films J. Liu, S.Y. Ma ⇑, X.L. Huang, L.G. Ma, F.M. Li, F.C. Yang, Q. Zhao, X.L. Zhang College of Physics and Electronics Engineering, Key Laboratory of Atomic and Molecular Physics & Functional Materials of Gansu Province, Northwest Normal University, Lanzhou, Gansu 730070, China

a r t i c l e

i n f o

Article history: Received 9 April 2012 Received in revised form 19 June 2012 Accepted 20 June 2012 Available online 28 June 2012 Keywords: ZnO:Ti thin films RF magnetron sputtering X-ray diffraction Optical properties

a b s t r a c t The Ti-doped ZnO (ZnO:Ti) thin films have been deposited on glass substrates by radio frequency (RF) reactive magnetron sputtering technique with different Ti doping concentrations. The effect of Ti contents on the crystalline structure and optical properties of the as-deposited ZnO:Ti films was systematically investigated by X-ray diffraction (XRD), scanning electronic microscopy (SEM) and fluorescence spectrophotometer. The XRD measurements revealed that all the films had hexagonal wurtzite type structure with a strong (100) preferential orientation and relatively weak (002), (101), and (110) peaks. It was found that the intensity of the (100) diffraction peaks was strongly dependent on the Ti doping concentration. And the full width at half-maximum (FWHM) of (002) diffraction peaks constantly changed at various Ti contents, which decreased first and then increased, reaching a minimum of about 0.378° at 1.43 at.% Ti. The morphologies of ZnO:Ti films with 1.43 at.% Ti showed a denser texture and better smooth surface. All the films were found to be highly transparent in the visible wavelength region with an average transmittance over 90%. Compared with Eg = 3.219 eV for pure ZnO film, all the doping samples exhibited a blue-shift of Eg. It can be attributed to the incorporation of Ti atoms and raising the concentration of carriers. Five emission peaks located at 412, 448, 486, 520, and 550 nm were observed from the photoluminescence spectra measured at room temperature and the origin of these emissions was discussed. Ó 2012 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +86 15117268830; fax: +86 9317971503. E-mail address: [email protected] (S.Y. Ma). 0749-6036/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.spmi.2012.06.021

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1. Introduction As is well known, ZnO is a II–VI semiconductor material with wide and direct band gap (about 3.37 eV), excellent chemical, thermal stability, specific electrical, and a large exciton binding energy of 60 meV [1]. Due to its unique properties, it has become a very popular material. So ZnO have been extensively researched in recent years for the breadth of their technological applications, such as UV absorption [2], optoelectronic devices [3], and acoustic-wave devices [4] and so on. Furthermore, ZnO is the most favorable material because of its abundance in nature, relatively low cost, good stability in hydrogen plasma process, and non-toxicity [5,6]. Therefore, there is a considerable interest in understanding the microstructure and optical properties of doped ZnO films, which will give more chance for potential applications. At present, a number of ZnO films doped with various metallic ions have been widely studied for the manipulation of their optical and electrical properties, Such as Al, Mg, Co, Ga, Sn, S, and Cu. But few paper discussed the structural, optical properties of the Ti-doping ZnO(ZnO:Ti), especially prepared by using radio frequency (RF) reactive magnetron sputtering technique. Among the various types of doped ZnO thin films, Ti doped ZnO films have been investigated recently for their unique electrical, magnetic, and sensing properties [7–9]. It has been demonstrated that larger preferential c-axis orientation of ZnO films doped with Ti have a better optical properties than pure ZnO films [10,11]. However, there have been only a few studies on their microstructure, optical, and luminescence properties of Ti-doped ZnO films with a strong (100) preferential orientation in detail. In this paper, Ti-doped ZnO (ZnO:Ti) thin films with a strong (100) preferential orientation were prepared on glass substrates by RF reactive magnetron sputtering technique with different Ti doping concentrations. The effect of Ti contents on the structural and optical properties of ZnO thin films was systematically investigated by X-ray diffraction (XRD), scanning electronic microscopy (SEM), UV/VIS and a fluorescence spectrophotometer. 2. Experiment ZnO:Ti thin films with different Ti doping concentration were deposited on glass (Corning 7105) substrates using radio frequency reactive magnetron sputtering technique. High-purity Zn target (99.9999%, 60 mm in diameter) and Ti foils (purity 99.9%) were used in the experiments. High purity Ar and O2 were employed as the sputtering and reactive gas, respectively. To conduct Ti doping, numbers of Ti foils were pasted to Zn target, which with the coverage area of Ti foil about 1%, 2%, and 3% of the effective sputtering area of Zn target. The corresponding Ti concentration of ZnO:Ti films were 1.43, 2.03, and 2.77 at.%, determined by energy dispersive spectrometer (EDS, JSM-6701F). The distance between target and substrate was 50 mm. The deposition chamber was initially evacuated down to a residual pressure of 2  104 Pa, and then the O2 and Ar gas mixture were introduced into the chamber to maintain reactant pressure for 2 Pa. The Zn target was pre-sputtered for 10 min to remove surface contamination and maintain system stability. The substrate temperature was maintained at 200 °C with RF power of 150 W. During sputtering for 1 h, the O2:Ar ratio was 10:10 sccm. Finally, the samples were annealed under vacuum at 500 °C for 1 h. The crystallinity and orientation of ZnO:Ti films were studied by X-ray diffraction (XRD, D/Max-2400) using the Cu Ka radiation with k = 0.15406 nm. The surface morphology was characterized by scanning electronic microscopy (SEM, JSM-6701F). The photoluminescence (PL) were measured by using the excitation of Xe Lamp (RF-5301, wavelength 325 nm). The transmission and absorption spectra were measured by using Lambda35UV/VIS, and all spectra were measured at room temperature in air. 3. Results and discussion 3.1. Microstructure properties of ZnO and ZnO:Ti films Fig. 1 shows XRD patterns of ZnO:Ti thin films grown at different Ti contents: 0%, 1%, 2%, and 3%. All films exhibit four main diffraction peaks of crystallized ZnO: (100), (002), (101), and (110). This result

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Fig. 1. XRD patterns of ZnO:Ti/glass thin films with different Ti contents: (a) 0%, (b) 1%, (c) 2%, and (d) 3%.

indicates that all of the ZnO:Ti thin films are polycrystalline with a hexagonal wurtzite structure, no phases corresponding to other oxides are detected. It implies that the Ti ions may substitute the Zn sites or incorporate interstitially in the lattice. It can be found in the XRD pattern that the intensity of the (100) diffraction peaks far exceed others, such as (002), (101), and (110). As the Ti content increases, the intensity of (100) peaks decreases, and the intensity of (002) peaks increases first and then decreases, reaching a maximum at the Ti doping concentration about 1.43 at.%. This experimental result is different from the report of Lu et al. [7]. Although a (002) diffraction peak is also seen in XRD patterns of our sample, the integrated intensity of the (002) to (100) peak is weaker. It is known that conditions (e. g. composition of the atmosphere Ar–O2, substrate temperature, ion bombardment of the surface and annealing processes) have great influence on the microstructure of ZnO films. Moreover, the strong (100) preferential orientation of ZnO films appeared in our XRD measurements. Thus, we suggest that growth conditions of these films may lead to the evolution of crystalline face energy due to imbalances of the components (Zn and O) in the area of nucleation of the crystallites. The crystallite size of ZnO films with different Ti contents were calculated using the Scherrer equation [12]:

D¼

0:9k b cos h

ð1Þ

where k, h, and b are the X-ray wavelength (0.15406 nm), diffraction angle and the full width at halfmaximum (FWHM) of the ZnO:Ti (002) peak, respectively. Fig. 2 shows the FWHM of (002) diffraction peaks and crystallite size of ZnO films as a function of different Ti doping concentration. For ZnO films, there is an important factor judging the crystal quality that is FWHM of (002) diffraction peak. From our results, we find that FWHM of (002) diffraction peaks change constantly when the films are prepared at various Ti contents. As shown in Fig. 2, FWHM of (002) diffraction peaks decreases first and then increases, reaching a minimum of about 0.378° at 1%, which means that ZnO:Ti film with a doping concentration of 1.43 at.% has the maximum crystallite size. According to the Bragg formula: k = 2dsinh, where k is the X-ray wavelength (0.15406 nm), h is the diffraction angle of the peak and d denotes the crystalline plane distance of indices (h,k,l), which are the Miller indices. d002 and d100 were calculated. Considering the interplanar spacing for the hexagonal system [13]: 2

dh;k;l ¼

2

2

4 h þ hk þ k l þ 2 3 a2 c

!12 ð2Þ

J. Liu et al. / Superlattices and Microstructures 52 (2012) 765–773

25.0

0.55

FWHM / deg.

0.50

22.5

0.45 20.0 0.40

Grain size/nm

768

17.5 0.35

0.30

15.0 0

1

2

3

Ti /% Fig. 2. The FWHM of (002) diffraction peak and grain size of ZnO films as a function of different Ti doping concentration.

We also calculate the unit cell parameters (a and c) of the ZnO:Ti thin films and the values for these parameters are shown in Table 1. It is found that the c value of ZnO:Ti at 2% is smaller than that of standard ZnO powder c0 (0.5206 nm), while others are all larger than c0. The lattice mismatch between ZnO film and substrate can result in varying degrees of stress during the deposition process of ZnO thin films [14]. The strain also affects significantly the structures and properties of the film to some extent. Thus, the analysis of stress of the ZnO:Ti films is very important. For ZnO films with wurtzite structure, the average uniform strain (e002, e100) in the lattice along the c-axis and the a-axis in the randomly oriented ZnO films deposited with different Ti contents have been estimated from the lattice parameters using the following expression [15],

e002 ¼

c  c0 ; c0

e100 ¼

a  a0 a0

ð3Þ

where ‘‘c’’, ‘‘a’’ is the lattice parameter of the ZnO film calculated from (002), (100) peak of XRD pattern and the c0, a0 is the lattice parameter (c0 = 0.5206 nm, a0 = 0.3249 nm) for the ZnO bulk. As shown in Fig. 3, an obvious transition from compressive to tensile stress was observed when the Ti content was varied from 0% to 3%. Moreover, Table.1 shows other parameters of the ZnO:Ti films at various Ti doping concentration. In order to further characterize the effect of doping concentration on the microstructural properties of films, we measured the SEM images of ZnO:Ti thin films prepared at the different concentration: (a) 0%,(b) 1%, (c) 2%,(d) 3%, as shown in Fig. 4. It is found that the Ti concentration has a great influence on the surface structure of ZnO film. The morphologies of ZnO:Ti thin films are found to be continuous and dense, and the grain size have been changed by Ti doping. This is consistent with the grain size calculated in Fig. 2. ZnO:Ti thin film with 1.43 at.% Ti shows a denser texture and better smooth

Table 1 The parameters of ZnO/glass thin films at various Ti doping concentration. ZnO:Ti

(h,k,l)

2h (°)

dh,k,l (nm)

a (nm)

c (nm)

I002/I100

Pure

(002) (100) (002) (100) (002) (100) (002) (100)

34.23 31.73 34.25 32.67 34.46 31.94 34.24 31.70

0.2618 0.2818 0.2616 0.2823 0.2601 0.2799 0.2617 0.2820

0.3254

0.5236

136/394

0.3260

0.5232

202/358

0.3232

0.5202

143/257

0.3256

0.5234

141/251

1% 2% 3%

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a c

0.006

Strain (arb. units)

0.004 0.002 0.000 -0.002 -0.004 -0.006 0

1

2

3

Ti/% Fig. 3. The average strain (e002, e100) in the lattice along the c-axis and the a-axis in ZnO films deposited with different Ti contents: (a) 0%, (b) 1%, (c) 2%, and (d) 3%.

Fig. 4. SEM images of ZnO:Ti films with different concentration: (a) 0%, (b) 1%, (c) 2%, (d) 3%.

surface. As the Ti doping concentration is further increased, the surface of ZnO:Ti thin film becomes more denser and cluster together, which was in good agreement with the grain strain. We concluded that the distortion of the lattice might stem from the increase of residual stress due to more Ti ions incorporated into the Zn sites.

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3.2. Optical properties Fig. 5 shows the transmission spectra of the ZnO:Ti thin films with different Ti doping concentration. All the films are found to be highly transparent in the visible wavelength region with an average transmittance over 90%. The observed high transparency in the visible spectral region makes these films suitable for transparent conducting electrode applications. From the inset of Fig. 5, we can also see that the UV absorption edge shifts to a shorter wavelength when Ti doped into ZnO films. Both ZnO and ZnO:Ti film have a direct band gap. So the relative absorption edge can be given by the Tauc relationship [16] as follows:

ahv ¼ Aðhv  Eg Þn

ð4Þ

where a is the absorption coefficient, A is a constant, h is Planck’s constant, m is the photon frequency, Eg is the optical band gap, and n is 1/2 for direct band gap semiconductors. Fig. 6 shows the plot of (ahm)2 vs. hm of ZnO:Ti films prepared at different doping contents (a) 0%, (b) 1%, (c) 2%, and (d) 3%, respectively. The straight-line portion of the curve, when extrapolated to zero, gives the optical band gap Eg. From the results of Fig. 6, Eg for the ZnO:Ti thin films with various contents of Ti are 3.219, 3.238, 3.234, and 3.227 eV. Compared to Eg = 3.219 eV for pure ZnO film, all the other samples exhibit a blue-shift of Eg. The results reveal that the change of the Eg for the ZnO:Ti thin films is likely due to Ti atoms induced into and raising the concentration of carriers. But the values of Eg is found to be lower than the report of Lu et al. [7] who observed for Ti-doped ZnO films. The result may due to the post-annealing temperature that reduces the defects (defect concentration) in all films and decreases the optical energy gap [17,18]. In semiconductors with broad band gap, the optical gap can be increased or decreased by varying the carrier concentration. In our experiments, the optical band gap shifts to higher energy, which is related to an increase in the carrier concentration according to the Burstein–Moss effect [19]. Actually, our results indicate that only proper content of Ti could result in a bigger blue-shift of the band-gap energy. We think that the band gap shift is naturally related to the increase in carrier concentration as the result of introduction of Ti. Fig. 7 shows PL spectra of ZnO:Ti thin films at various Ti doping concentration: (a) pure ZnO, (b) 1%, (c) 2%, and (d) 3%, respectively. When pure ZnO film was prepared on the glass substrate, five weak emission peaks located at 412, 448, 486, 520, and 550 nm are observed from PL spectra. When Ti elements were brought into, no other new emission peak occurred in ZnO:Ti films. It is also five main emission peaks observed from PL spectra, but the intensity of the emission peaks with the change of Ti doping concentration constantly undergoes corresponding change. The emission peaks centered

a

100

c

b

d

60

Transmittance(%)

Transmittance(%)

80

40

38

d cb

a

36

20 387

390

393

Wavelength(nm) 0 350

400

450

500

550

600

650

700

Wavelength(nm) Fig. 5. Transmission spectrum of different Ti doping concentration of ZnO films: (a) pure ZnO, (b) 1%, (c) 2%, (d) 3%.

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J. Liu et al. / Superlattices and Microstructures 52 (2012) 765–773

30 90

(α hν) 2 (μ m-1 eV)

25

75

a

20

b

c

60

d 15

45

10

30

5

15

0 3.10

3.15

3.20

3.25

0 3.35

3.30

hν (ev) Fig. 6. Plot of (ahm)2 vs. hm of the ZnO:Ti films with different Ti contents prepared at (a) 0%, (b) 1%, (c) 2%, and (d) 3%.

Intensity(counts)

500 400 300 200 412

100

486

0 350

400

450

600

(b)

500 400 300

412 455

200

520 570

550

550

600

0 350

650

400

Wavelength (nm) 700 600

700 600

Intensity(counts)

Intensity(counts)

500 400 486 451

200

520

412

100 0 350

450

500

Wavelength (nm)

500

550

600

650

550

600

650

(d)

500 400 448

300 200 520

100

563

400

450

Wavelength (nm)

(c)

300

486

100

520

500

700

Intensity(counts)

600

700 b 600 d 500 400 c 300 200 a 100 0 350 400 450 500 550 600 650 448 Wavelength (nm)

(a)

intensity(counts)

700

0 350

405

400

565

486

450

500

550

600

650

Wavelength (nm)

Fig. 7. PL spectra of ZnO:Ti/glass thin films with different Ti doping concentration: (a) pure ZnO, (b) 1%, (c) 2%, and (d) 3%.

at 486 and 520 nm not shift, but peaks located at 412 nm blue shift to 405 nm at 3% Ti and the 448 nm peak red shift to 455 nm first then shift to 451, 448 nm with Ti doping concentration increased. Meanwhile, the emission peak centered at 550 nm shift to 570, 563 nm and then shift to 565 nm as the concentration increased. In addition, when Ti content at 1%, all the emission peaks are at the strongest. It is found that some of emission peaks are shifted depending on the Ti concentration. We concluded

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that the energy interval of defect states might be changed by the different of doping Ti concentration. Though some of PL emission peaks are affected by doping Ti concentration, the shift principle of emission peaks at different deposited conditions is still a matter to study. It is well known that there are various defects in ZnO films, such as oxygen vacancies (Vo), zinc vacancies (VZn), interstitial oxygen (Oi), interstitial zinc (Zni), antisite oxygen (OZn) and so on [20]. The origins of the defect-related deep-level PL band have been investigated for a long time. The blue emission might come from the intrinsic defects. Zni and Vo are main donor defects, while VZn and Oi are main acceptor defects in intrinsic ZnO [21]. Zhang et al. [22] found a 446 nm (2.78 eV) blue emission peak in their ZnO films fabricated on corning 7059 glass substrate using radio frequency reactive magnetron sputtering. In our samples, blue peaks occurred at about 448 nm that corresponding phonon energy was about 2.77 eV, which was attributed to the electron transition from the shallow donor level of Vo to the top of the valence band [23]. Kohan et al. [24] have calculated the energy levels of defects in ZnO films by the full-potential linear muffin-tin orbit method, and they have shown that the energy interval from the Zni to VZn was about 2.54 eV, which were well consistent with the energy of the blue peaks at 486 nm (2.55 eV) appeared in our experiment. The green peaks occurred at about 520 nm that corresponding phonon energy was about 2.38 eV. According to the theoretical analysis, we concluded that the green emission located at 520 nm might due to the electron transition from deep oxygen vacancy level to the top of valance band [25]. A violet emission band (centered at 412 nm, 3.01 eV) can also be observed in our experiment. We think that this emission band may be caused by the transition from shallow donor level (Vo) to valence band and radiation transition related interface traps existing at the grains boundaries [26,27]. The origins of the weaker green bands (about 550–570 nm) are attributed to oxygen interstitials and oxygen vacancies [28]. The valence of Ti could be +2 and +4 in ZnO:Ti films[29,30]. The radius of Ti2+, Ti4+, and Zn2+ ions is 0.094, 0.068, and 0.074 nm, respectively. At the doping concentration of 1%, Ti2+ substitute will lead to increase of interstitial Zn and O vacancies defects, so strong emission peaks are expected to be observed. However, excess Ti incorporation into ZnO shows obviously degraded the emission peaks intensity due to the increase in the probability of Ti2+ substitute and Ti4+ interstitial coexist in ZnO films, which will affect the concentration of the interstitial Zn and O vacancies. So the Ti doping concentration apparently affects the emission peaks intensity of ZnO films. 4. Conclusions Pure and ZnO:Ti films were deposited on glass substrates using RF reactive magnetron sputtering technique with different Ti doping concentrations. All ZnO films show a strong (100) peak of preferred orientation and relatively weak (002), (101) and (110) peaks. And the intensity of the (100) diffraction peaks is strongly dependent on the Ti doping concentration. The strongest ZnO:Ti (002) peak, the minimum FWHM and the least residual stress can be obtained at the doping concentration of 1.43 at.%. SEM images show that the grain size of the sample doping with Ti is continuous, and denser than that of pure ZnO. This is consistent with the grain size calculated by FWHM of XRD. Our results indicate that the band gap shift is naturally related to the increase in carrier concentration as the result of introduction of Ti, and only proper content of Ti could provide a blue-shift of the band-gap energy. Five main emission peaks located at 412, 448, 486, 520, and 550 nm are observed from PL spectra. The origin of these emissions is discussed and concluded. It can be believed that the defect centers are mainly ascribed to oxygen vacancies (Vo) and interstitial Zn (Zni) in ZnO:Ti films. The Ti doping concentration had a great influence on the PL spectra of the film. Acknowledgments This work was supported by the National Natural Science Foundations of China (Grant No. 10874140) and Natural Science Foundational of Gansu Province (Grant no. 0710RJZA105). References [1] R.E. Marotti, P. Giorgi, G. Machado, E.A. Dalchiele, Sol. Energy Mater. Sol. Cells 90 (2006) 2356.

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