Influence of annealing temperature on the structural ...

Influence of annealing temperature on the structural and optical properties of highlyoriented Al and Er co-doped ZnO films Ji-Zhou Kong, Zheng Wang, Chun-Yan Luan, Mei-Ling Wang, Fei Zhou, XueMei Wu, Wen-Jun Zhang, Kong-Jun Zhu, Jin-Hao Qiu, Juan-Antonio Zapien, et al. Journal of Materials Science: Materials in Electronics ISSN 0957-4522 Volume 24 Number 10 J Mater Sci: Mater Electron (2013) 24:3868-3874 DOI 10.1007/s10854-013-1331-y

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Author's personal copy J Mater Sci: Mater Electron (2013) 24:3868–3874 DOI 10.1007/s10854-013-1331-y

Influence of annealing temperature on the structural and optical properties of highly-oriented Al and Er co-doped ZnO films Ji-Zhou Kong • Zheng Wang • Chun-Yan Luan • Mei-Ling Wang • Fei Zhou • Xue-Mei Wu • Wen-Jun Zhang • Kong-Jun Zhu • Jin-Hao Qiu Juan-Antonio Zapien • Shuit-Tong Lee

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Received: 12 March 2013 / Accepted: 1 June 2013 / Published online: 11 June 2013 Ó Springer Science+Business Media New York 2013

Abstract High-quality c-axis oriented 7 mol% Al and 1.5 mol% Er co-doped ZnO films (ZEAO) were prepared on the quartz glass substrates by using sol–gel method. The influence of the annealing temperature on the crystal orientation, microstructure and optical properties of the ZEAO thin films were studied by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and UV–vis transmittance spectrophotometer, respectively. XRD results revealed that all the samples were polycrystalline with hexagonal structure and exhibited (002) preferential orientation. With increasing the annealing temperature, the grain size and orientation extent increased. The optical studies showed each ZEAO film had a relatively high transmittance above 85 %. The transmittance as high as 95 % was obtained at the annealing temperature of 800 °C, and the corresponding average grain size was about 50 nm. The cathodoluminescence (CL) spectra of these films were also used to characterize the luminescence properties. Strong UV emission centered at 380 nm was observed in the CL spectra taken for the pure ZnO. For the

J.-Z. Kong Z. Wang M.-L. Wang F. Zhou (&) K.-J. Zhu J.-H. Qiu State Key Laboratory of Mechanics and Control of Mechanical Structures, Nanjing University of Aeronautics and Astronautics, No. 29 Yudao Street, Nanjing 210016, People’s Republic of China e-mail: [email protected] C.-Y. Luan F. Zhou W.-J. Zhang J.-A. Zapien S.-T. Lee Department of Physics and Materials Science, Center of SuperDiamond and Advanced Films (COSDAF), City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, China X.-M. Wu School of Physical Science and Technology, Suzhou University, Suzhou 215006, People’s Republic of China

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ZEAO sample, the blue-green emission is related to the 4f shell transition in the Er3? ions of ZnO matrices, corresponding to a transition from the excited states (4F5/2).

1 Introduction Zinc oxide (ZnO) is considered as a promising technological material, due to its excellent optical and electrical properties in the wide field, such as the flat-panel displays, photovoltaic cells, and pyroelectric and piezoelectric material [1–3], especially, its potential application in the solar cell [4, 5]. In order to optimize the optical properties of ZnO for specific applications, a variety of different dopants including Al [6, 7], Ga [8], and Er [9] have been determined to be the most suitable materials. Al-doped ZnO (ZAO) films are well known to have low resistivity and high transmittance in the visible region to enable ZAO thin films to be used in a wide range of technological applications, such as transparent conducting electrodes in solar cells, window materials in display, and various optoelectronic devices [10, 11]. Dghoughi et al. [12] reported that 5 mol% Al doped ZnO films prepared by spray pyrolysis exhibited (002) preferential orientation, had a high transmittance, and minimum resistivity. However, the growth of high-quality ZnO films on the Si and/or amorphous quartz glass substrate is still a challenge due to the lattice and thermal mismatch between the film and the substrate [13, 14]. In addition, researchers have modified ZnO by doping with Er to tune the optical bandgap modulation and enhance photoluminescence properties. Lamrani et al. [15] found the addition of erbium can effectively control the film surface morphology and its cathodoluminescent properties, and the best crystalline and morphological quality were obtained at lower erbium concentration.

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In our previous work, high-quality oriented Al and Er co-doped ZnO films were prepared on the quartz glass by the sol–gel spin-coating method. And an L16 (45) orthogonal experimental design was chosen to obtain the optimal processing conditions. It is found that 1.5 % Er and 7 % Al co-doped ZnO (ZEAO) films with six layers showed the largest texture coefficient [16]. In the present work, the effect of annealing temperature on the structural, and optical properties of these ZEAO films is investigated. The surface element’s chemical valence and cathodoluminescence property are also studied.

field-emission scanning electron microscope (SEM, Philips FEG XL30) at 20 kV operating voltage. The UV–vis transmittance spectra of the samples were recorded by a spectrophotometer (HP8453, HP). The bonding structure of the annealed film was validated by X-ray photoelectron spectroscopy (XPS), and all of the spectra were calibrated by assigning the adventitious carbon peak to 284.8 eV. The cathodoluminescence (CL) spectra were measured by a scanning electron microscope (SEM, Philips FEG XL-30) with an electron beam energy of 10 keV at room temperature in the UV–visible spectral range.

2 Experimental details

3 Results and discussion

High-quality c-axis oriented Al and Er co-doped ZnO thin films were prepared on the quartz glass substrates by the sol–gel dip-coating method. Zinc acetate [Zn(CH3COO)2 2H2O], 2-methoxy ethanol and monoethanolamine (MEA) were chosen as the zinc precursor, solvent and sol stabilizer, respectively. The dopant sources of Al and Er were aluminum nitrate [Al(NO3)39H2O] and erbium nitrate [Er(NO3)36H2O], respectively. In a typical procedure, 3 mmol zinc acetate was dissolved in 10 mL 2-methoxyethanol solution containing a certain amount of MEA, and then the certain dopant sources were also added. The molar ratios of MEA/Zn2?, and MEA/Al3?, MEA/Er3? were maintained at 1:1. The homogeneous solution was stirred at 80 °C for 2 h to yield a clear, stable and homogeneous sol, and then aged for 24 h at the room temperature. The quartz glass (size: 2.5 9 2.5 cm2) used as a substrate was ultrasonically cleaned by detergent, methanol and acetone for 10 min, respectively. And then, the substrates were rinsed with deionized water and dried in nitrogen gas flow. Due to the centrifugal force applied to the specimen, a uniform film can be obtained by controlling the rotation speed. The transparent sol was spin-coated at a rotation speed of 3000 rpm for 20 s onto the quartz glass substrates. After coating each layer, the wet films were dried at 100 °C to evaporate the solvent, and then pre-annealed at 300 °C for 10 min to remove the organic residuals with the low boiling point. The procedures were repeated for six times to achieve the desired thickness. Finally, the films were annealed at various temperature ranged from 600 to 900 °C (heating rate of 10 °C/min) for 30 min in air in order to eliminate any undesired species and to achieve the undoped and doped ZnO thin films. Finally, they were cooled to room temperature still in the furnace before investigating their properties. This process was performed for all the films. The structural identification of the thin films was carried out using X-ray diffractometer (XRD, Bruker D8 Advance) using the Cu Ka radiation (k = 0.15406 nm) in the h/2h geometry. The surface morphology was characterized by a

The crystal structure and crystallinity of these thin films were identified by X-ray diffraction. These XRD patterns correspond to the diffraction peaks of crystalline ZnO (JCPDS card No. 75-0576). Therefore, it can be deduced that all as-prepared films had a hexagonal wurtzite structure. Typical XRD patterns of the pure ZnO thin films annealed at various temperatures are shown in Fig. 1a, and no secondary phases such as Al2O3/Er2O3 and amorphous ZnO exist. When the film was annealed at 600 °C, (002)

Fig. 1 XRD patterns of the annealed samples: a pure ZnO and b ZEAO

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and (101) peaks were observed, indicating that the deposited ZnO films are crystallized in the hexagonal structure with a preferred orientation along the (002) direction perpendicular to the substrate. In addition, a broadened peak located at *22° can be also observed, which is attributed to amorphous carbon [17, 18]. As increasing the annealing temperature, only the (002) peak was observed in the XRD patterns, showing that higher annealing temperature could enhance the preferred orientation extents [16]. From Fig. 1b, it is readily observed that all samples exhibit only the (002) peak, indicating that they have c-axis preferred orientation, that is, vertical growth with respect to the substrate, due to the self-texturing phenomenon [19]. Not only did the (002) peak intensity increase sharply with increasing annealing temperature, but the full-width at half-maximum (FWHM) for the (002) peak decreased (Table 1). These tendencies above indicates that the quality of the ZEAO film improved when annealed at a higher temperature. In addition, the intensity of the peak (002) about the ZEAO films is lower than that of the pure ZnO sample. This can be explained that the excess dopant could deteriorate the crystallinity of films, due to the formation of stresses by the difference in ion size between zinc and the dopants and the segregation of dopants in grain boundaries for high doping concentrations [19]. It is noted that the diffraction peaks of (002) are located at 2h = 34.49° and 34.64° for the undoped and doped ZnO thin films, respectively. Above values are very close to that of the standard ZnO crystal (34.45°). And the small shift of the diffraction peak (002), suggesting that aluminium and erbium replace zinc substitutionally in the hexagonal lat˚ ) comtice, due to the small ionic radius of Al3? (0.54 A 2? ˚ pared with that of Zn (0.74 A). The grain sizes of the ZEAO films are deduced from the (002) peak width in the XRD patterns by using the Scherrer’s formula. And the calculated grain sizes are increasing with improving the annealing temperature, as shown in Table 1. Figure 2 illustrates SEM images for surface morphology of the undoped and doped films annealed at different temperatures. It can be clearly observed that the crystallite size of ZnO and ZEAO films increases with increasing annealing temperature, supporting the results of the XRD

Table 1 c-axis length, FWHM, and grain size along the c-axis of ZEAO films Annealing temperature (°C)

2h (°)

c-axis length (nm)

FWHM

Grain size (nm)

600

34.45

0.5206

0.518

16.4

700

34.40

0.5210

0.435

19.6

800 900

34.36 34.34

0.5214 0.5215

0.374 0.305

23.1 28.9

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analysis above. As shown in Fig. 2a, c, e, and g, all of the ZnO films show the relative rough surfaces, and the surface texture become bigger when the annealing temperature increases, meaning that the films annealed at a high temperature could dominate more textural surface [20]. In contrast with the pure ZnO sample, the particle sizes of the ZEAO films annealed at the same temperature are smaller (see Table 2), indicating that the low-solubility dopant could restrain the growth of crystal and minish the particle size [21, 22]. In addition, the doped films exhibit relatively a porous structure, compared with the pure ZnO samples. The particle sizes measured by SEM (in Table 2) donot contradict with the values estimated from XRD patterns using the Scherrer equation (in Table 1), because the particle sizes obtained from SEM analysis corresponds to a dimension which parallels to the substrate, while the values calculated by XRD analysis corresponds to a dimension perpendicular to the substrate [12]. Above difference between SEM and XRD analysis was also observed in CeO2 films [23]. In addition, the corresponding RMS roughness values are listed in Table 2. RMS roughness of the films increases with increasing the annealing temperature. It is interesting that the RMS values of all the ZEAO samples are relatively lower than the pure ZnO that is relatively rough. The optical properties of the ZnO and ZEAO thin films were determined from the transmission measurements in the wavelength range of 350–700 nm. The examination was done for the film–substrate combination, the transmission about the substrate was also measured and subtracted. Figure 3a, b show the optical transmittances for the ZnO and ZEAO films prepared at different annealing temperature. For the ZnO samples, the transmittance increases with increasing the annealing temperature. In contrast, for the ZEAO films, it is clear that all the samples show higher transmittance than the corresponding ZnO films in the visible range. Table 3 also lists the average transmittance values of ZnO and ZEAO films for wavelengths from 400 to 750 nm. And the average value of all ZEAO samples is above 85 %, indicating a good optical quality of the deposited films. High transparency for Aldoped ZnO films was also reported by Chang et al. [24], which were deposited by the radio frequency reactive magnetron sputtering method. In addition, the absorption edge shifts to the longer wavelength region when the temperature increases. The red shift of the absorption edge is caused by the decreased Burstein-Moss shift [25], which could be attributed to the decrease of carrier concentration (ne) [26]. At the same time, it could be found that the maximum of transmittance value for the ZEAO films is up to 95 % with the annealing temperature of 800 °C. This may be mainly due to the surface morphology [27]. According to above discussion, 800 °C is considered as the


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Fig. 2 SEM images of ZnO and ZEAO thin films annealed at various temperatures

Table 2 Average crystallite size determined from SEM images and RMS roughness of ZnO and ZEAO films Annealing temperature (°C)

Crystalline size (nm)

RMS roughness (nm)

ZnO

ZnO

ZEAO

ZEAO

600

36.8

20.6

7.86

6.09

700

58.5

38.5

8.94

8.59

800 900

84.1 100.3

49.7 64.3

9.47 11.5

8.86 9.22

optimal annealing temperature to obtain the ZEAO film with high transmittance. As a direct band gap semiconductor, the optical bandgap (Eg) of the ZnO and ZEAO films can be determined by the extrapolation methods from

(ahm)2 = A(hm - Eg), where a is the absorption coefficient determined from the scattering and reflectance spectra according to Kubelka–Munk theory, hm is the photon energy, and A is a constant for direct transition, respectively [28]. Thus, from the transmittance results in Fig. 3, we can calculate the bandgaps about 3.23 and 3.29 eV for ZnO and ZEAO, respectively. Compared with pure ZnO films, Al, Er co-doping will result in a widening of the bandgap. This demonstrates that Al3? and Er3? ions have entered into the lattice of ZnO, in consistent with the XRD results. The surface element’s chemical valence was analyzed by using X-ray photoelectron spectroscopy on 800 °Cannealed ZEAO film detailed in Fig. 4. The binding energies in the XPS spectra presented in Fig. 4 are calibrated

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Fig. 3 Optical properties of the thin films annealed at various temperatures: a pure ZnO and b ZEAO

Table 3 Average optical transmittances of ZnO and ZEAO films annealed at various temperatures Annealing temperature (°C)

ZnO

ZEAO

600

81 %

89 %

700

83 %

92 %

800

84 %

95 %

900

85 %

87 %

by using that of C 1 s (284.8 eV). In Fig. 4a, the O 1 s profile is asymmetric and fitted with the non-linear least square fit program using Gauss–Lorentzian peak shapes. After deconvolution, two O 1 s peaks located at 530.5 and 531.9 eV appear, indicating two different kinds of O species. The main peak centered at 530.5 eV can be ascribed to the O2- ions in ZnO lattice (the lattice oxygen, OL). M. Wei et al. [29] reported that the binding energy component of O2- in pure ZnO crystal lattice is located at 530.27 eV. For the ZEAO sample, it is obvious that O1 s spectra’s main peak shifts to high energy side due to the dopants into ZnO lattice. As reported [29], the binding energy of O2- in Al2O3 is about 531 eV, higher that in ZnO lattice. The high binding energy component centered at 531.9 eV is related to the oxygen deficiencies and/or specific chemisorbed adsorbed oxygen (Os, 531.9 eV),

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Fig. 4 X-ray photoelectron spectroscopy of the ZEAO film annealed at 800 °C: a O 1 s, b Zn 2p3/2 and c Al 2p

caused by the surface adsorbed hydroxyl, H2O, –CO3 [28, 30]. The symmetric peak appeared at 1022.7 eV in Fig. 4b is attributed to Zn 2p3/2 [28]. The peak of Al 2p located at 74.5 eV corresponds to the Al–O bonding, as shown in Fig. 4c, confirming the doping of Al enters into the ZnO crystal lattice [31]. Due to the limitation of resolution and signal sensitive factor, the signal of Er element cannot be observed. Figure 5 shows the room temperature CL spectra of the pure ZnO and ZEAO samples, at the electron beam energy of 10 keV. As shown in Fig. 5a, for the pure ZnO film, the narrow UV emission peak centered at 380 nm (Ek = 3.27 eV), close to that of the ZnO band gap transition, is attributed to the near band-edge recombination of free


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4 Conclusions In summary, highly c-axis oriented ZEAO thin films have been prepared by sol–gel technique. The influence of annealing temperature on the crystal orientation, microstructure and optical properties of the ZEAO thin films was investigated. XRD results revealed that all the samples were polycrystalline with hexagonal structure ZnO with (002) preferential orientation. With increasing annealing temperature, the grain size and orientation extent increased. The optical studies showed each ZEAO films had a relatively high transmittance above 85 %. The transmittance as high as 95 % was obtained at the annealing temperature of 800 °C, and the corresponding average grain size was about 50 nm. CL spectra of these films were also used to characterize the luminescence properties. Strong UV emission centered at 380 nm was observed in the CL spectra taken for the pure ZnO. For the ZEAO sample, the blue-green emission is related to the 4f shell transition in the Er3? ions of ZnO matrices, corresponding to a transition from the excited states (4F5/2). Therefore, Al and Er co-doped ZnO films might be a promising candidate for further photonic applications. Fig. 5 Cathodoluminescent spectra of a undoped ZnO film, and b ZEAO film annealed at 800 °C

excitons through exciton–exciton collision [32]. The pure ZnO sample exhibits a very high ratio of intensity of UV emission, revealing that the sample is highly crystalline with few oxygen deficiencies [33], which is consistent with the report about the optical properties of Al:ZnO nanowires [34]. For the ZEAO sample, the UV emission peak is also present. The incorporation of erbium and aluminum in ZnO apparently generates another peak at approximately 457 nm. The new luminescent band originate from the 4f shell transition in the Er3? ions of ZnO matrices, and correspond to a transition from the excited states (4F5/2) [35]. The bivalent Zn2? ions are substituted by the trivalent Er3? and Al3? ions, the defects or vacancies will be increased. These defects or vacancies would greatly transfer the enhanced energy from the band edge to the defect states responsible for the visible emission. Another green emission band centered at 581 nm is attributed to the mixed transitions 2H11/2, 4S3/2 ? 4I15/2 of Er3? ions. In addition, the emission with low intensity centred at around 645 nm is probably due to the crystalline defect, such as the lattice distortion, electron–phonon coupling, etc. Furthermore, it can be seen that the peak localized at 380 nm and the appearance of the peaks at 457 and 645 nm in Fig. 5b suggest that the incorporation of small amount of Al3? and Er 3? ions in the ZnO lattice, which is in good agreement with the results obtained by XRD and SEM analysis.

Acknowledgments This work is financially supported by ‘‘Weaponry Equipment Pre-research Foundation of China’’ (08HK0206), ‘‘Opening Funds of Key Laboratory of Thin Films of Jiangsu’’ (KJS0833), ‘‘the Fundamental Research Funds for the Central Universities’’, No. NS2012068; ‘‘China Postdoctoral Science Foundation’’, No. 2012M511749; ‘‘Jiangsu Postdoctoral Science Research Foundation’’, No. 1102056C and the Talent Program of Nanjing University of Aeronautics and Astronautics (NUAA) (2012). We would like to acknowledge them for the financial support.

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