Optical and structural characteristics of yttrium doped ... - Springer Link

J Sol-Gel Sci Technol (2011) 58:42–47 DOI 10.1007/s10971-010-2352-0

ORIGINAL PAPER

Optical and structural characteristics of yttrium doped ZnO films using sol–gel technology Po-Tsung Hsieh • Ricky Wen-Kuei Chuang Chao-Qun Chang • Chih-Ming Wang • Shoou-Jinn Chang

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Received: 1 April 2010 / Accepted: 19 October 2010 / Published online: 30 October 2010 Ó Springer Science+Business Media, LLC 2010

Abstract Yttrium-doped ZnO gel was spin-coated on the SiO2/Si substrate. The as-prepared ZnO:Y (YZO) thin films then underwent a rapid thermal annealing (RTA) process conducted at various temperatures. The structural and photoluminescence characteristics of the YZO films were discussed thereafter. Our results indicated that the grain size of YZO thin films being treated with various annealing temperatures became smaller as compared to the ones without being doped with yttrium. Furthermore, unlike other ZnO films, the grains of YZO thin films appeared to separate from one another rather than aggregating together as both types of the films were annealed under the same environment. The photoluminescence characteristic measured showed that the UV emission was the only radiation obtained. However, the UV emission intensity of YZO thin film was much stronger than that of the ZnO thin film after annealing them with the same condition. It was also found that the intensity increased with an increase in the annealing temperature, which was caused by the exciton generated and the texture surface of the YZO thin film. Keywords ZnO Yttrium Sol–gel processes UV emission Exciton

P.-T. Hsieh (&) R. W.-K. Chuang C.-Q. Chang S.-J. Chang Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan, Taiwan e-mail: [email protected] R. W.-K. Chuang S.-J. Chang Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan C.-M. Wang Department of Electrical Engineering, Cheng Shiu University, Kaohsiung County, Taiwan

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1 Introduction White light is usually generated by combining the three emission bands of blue, green and red [1]. However, three chips with different colors are needed which inevitably brings up the overall fabrication cost of the light source. Other alternatives have also been proposed including the one of using the ultraviolet (UV) emission as an excitation source to excite the phosphors for white light generation. These UV sources are predominantly made with III–V materials such as GaN [2, 3]. It was not until recently other potential materials such as II–VI semiconductors have started to attract much attention from the research community for a possible replacement of GaN. In particular, ZnO has already been made famous for its promising applicability in the fields of microelectronics [4] and surface acoustic wave (SAW) devices [5]. Due to its large exciton binding energy of 60 meV and the wide band-gap of 3.3 eV, ZnO thin films have been applied to many optoelectronic devices including the short wavelength light emitting diodes (LEDs) [6] and also as the transparent electrodes of these LEDs for possible light emission enhancement [7]. There are many preparation methods which could be applied to produce the ZnO thin films, such as spray pyrolysis, sol gel technique, metal organic chemical vapor deposition (MOCVD), sputtering, electron beam evaporation (EBE), pulsed laser deposition (PLD), and molecular beam epitaxy (MBE) [8–13]. Among them, sol–gel technique is by far the least inexpensive and can be doped rather easily. A thorough investigation of the photoluminescence property is considered vitally important for supplying the intrinsic emission characteristics of the ZnO thin film. Generally, ZnO typically exhibits luminescence bands in the UV and visible regions [14, 15]. It was reported that the

J Sol-Gel Sci Technol (2011) 58:42–47

UV emission originated from the band-to-band transition and exciton transition [16–21]. However, it has also been learnt that the doping with selective elements could be used to adjust the physical, chemical, and optical properties of semiconductors [22–24]. Minami et al. [24] have investigated the electrical and optical properties of Sc-doped ZnO thin films prepared by DC magnetron sputtering technique. Kaur and Mehra et al. [25, 26] have reported the relevant results of the structural, electrical and optical characteristics of Y-doped ZnO films, however, the optical property of the Y-doped ZnO films has only focused on the transmittance alone. Yu et al. [27] proposed the structural, electrical, and photoluminescence (PL) properties of ZnO films doped with various yttrium concentration. The article mentioned that Y-doped ZnO films annealed in air for 550°C for 2 h obtained a blueshift of the NBE emission peak with the increasing Y concentration. Moreover, the NBE emission peak intensity became weaker as Y concentration was raised from 0 to 5%. According to our preliminary research [28], the crystalline status and the related characteristics were different as the annealing temperature of ZnO thin films is close to the melting point (419.5°C) and the gasification point (907°C) of zinc. Obvious variations in the structural and optical properties were observed as ZnO thin film were annealed at higher temperature of 900°C. The grain size and the UV emission intensity of ZnO thin film became larger and stronger respectively. In this study, yttrium-doped ZnO thin films were prepared by sol–gel technique and then annealed with various annealing conditions. The crystalline and structural characteristics of the as-grown ZnO thin film and yttriumdoped ZnO thin film with various preparation parameters were investigated. The optical properties of the Y-doped ZnO films were also analyzed, including a discussion on the observation of the enhanced UV emission of YZO thin films.

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Fig. 1 Flow chart of sol–gel technique for YZO thin films

yielded. The precursor solution was then dropped onto SiO2/Si substrates and then spun at 1,000 rpm for the first 10 s before spinning it at 2,000 rpm for an additional 20 s. After the deposition by spin coating, the films were dried at 270°C for 3 min to evaporate the solvent and remove the organic residuals. The spin-coating and preheating procedures were repeated four times in order to obtain the desired thickness of YZO films. The films were then postheated for 2 min with temperature ranging from 600 to 900°C under an ambient atmosphere using the rapid thermal annealing (RTA) process. Crystalline characteristics of thin films were studied by X-ray diffraction (XRD) using Siemens D-5000 diffractometer with the X-ray source of Cu-Ka. A field emission scanning electron microscope (FESEM F7001) was used to analyze the surface morphologies and grain structures. The luminescent characteristics of ZnO:Y films annealed at various temperatures were probed through the photoluminescence (PL) measurement with a He-Cd laser of 325 nm. All measurements were performed under room temperature.

2 Experimental 3 Results and discussion ZnO and YZO thin films were deposited on the SiO2/Si substrates by using the sol–gel technique. Figure 1 shows the flow chart detailing the preparation steps of ZnO and YZO films. Solutions of 2-methoxyethanol (C3H8O2) and monoethanolamine (C2H7NO, MEA) were used as the solvent in this study. Zinc acetate dihydrate and yttrium acetate were first dissolved in a mixture of 2-methoxyethanol and MEA solution at room temperature. The molar ratio for MEA to zinc acetate was fixed at 1.0 and the doping ratio for metallic yttrium to metallic zinc was maintained at 1 wt%. The resultant solution was stirred at 120°C for 1 h and 80°C for another 1 h, and then a clear and homogeneous liquid as the coating solution can be

Figure 2 show the X-ray diffraction patterns of sol– gel-synthesized YZO thin film after the post-annealing conducted at 900°C. Various peaks were identified corresponding to (100), (002), (101), (102) and (110) plane of ZnO. No significant peaks ascribed to yttrium or yttria were discovered, which could be well explained by the small doping ratio of yttrium to zinc implemented earlier during the experiment. Furthermore, the (002) diffraction located at 34.5° is by far the most apparent and intensive among all the diffraction peaks observed. The spectrum reveals a preferential ZnO (002) orientation of YZO thin film existed after annealing it at high temperature. The inset of Fig. 2

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Fig. 2 X-ray diffraction pattern of YZO thin film after annealing at 900°C

illustrates the (002) diffraction peak position of ZnO and YZO samples. According to the figure, the (002) peak for ZnO sample locates at 34.55° and shifted to 34.4° for YZO thin film annealed at 600°C. Yu et al. [27] reported that the increasing yttrium concentrations resulted in the position shift of the (002) peak of the doped ZnO thin film to lower diffraction angles. The phenomenon may be owing to the ionic radius of Y3? is larger than that of Zn2?. Furthermore, it can be noticed that the (002) diffraction peak shifted to 34.5° when the annealing temperature increased from 600 to 900°C. The variation of ZnO structure with yttrium doping can be easily observed. Figures 3(a–d) show the surface morphologies of YZO films annealed at 600, 700, 800 and 900°C, respectively. The particle size of YZO film annealed at 600°C is nearly 50 nm and its grain boundary is not clearly identified. Moreover, most grains appear to be discrete rather than sticking together. As the annealing temperature was increased to 700°C, the corresponding grain size also increased up to nearly 100 nm while the grain boundary appeared to be more sharply featured when compared to the one obtained at 600°C. The grain size became even larger and the grain boundary became more pronounced as the annealing temperature was increased to 800 and 900°C. Note that each grain of YZO thin film after annealing at high temperatures is limited in size. Many voids are discovered on the surface of YZO films being treated with various annealing temperatures. Figure 3(e) presents the morphology of ZnO thin film annealed at 900°C. The grain shape is clearly defined and, moreover, each grain is closely connected with the other neighboring grains. As compared with Fig. 3(d), the difference in grain shape for ZnO thin films prepared with or without the doping of

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yttrium is very apparent. Based on the EDS analysis for YZO thin films annealed at various annealing temperatures, as shown in Fig. 4, it is apparent that yttrium existed in ZnO thin film and the Y doping concentration decreased with the increasing annealing temperature. The Y doping concentration of YZO thin film annealed at 900°C is approximate to 1%, which is the same with YZO sol. As described in the introduction, the PL properties of ZnO films doped with various yttrium concentrations and annealed at 550°C were discussed [27]. In this study, PL properties of YZO films (with a fixed yttrium concentration) annealed at various high annealing temperatures were investigated. The PL spectra for YZO thin films measured are shown in Fig. 5. Two ultraviolet (UV) emission peaks located at 375 and 380 nm are identified. The intensities of both UV emission peaks increase gradually as the annealing temperature is increased. Based on the earlier study of Chatterjee et al. [18], the grain size and crystal orientation were in fact closely related to an increase in the UV emission intensity. In this study, the grain size of YZO thin film, the ZnO (002) diffraction intensity and the UV intensity all increase in response to an increase in the annealing temperature from 600 to 900°C. Our results are in fact consistent with Chatterjee’s earlier report. However, by comparing the two emission peaks, the intensity of 380 nm appears to be much sharper and stronger compared with that of 375 nm and it reaches the maximum for film annealed at 900°C. Moreover, the increase in intensity of the 375 nm signal is not as strong as the 380 nm signal. Many studies have been carried out to investigate the UV emission mechanism of ZnO thin films and some reports indicated that the UV emission might be due to the bandto-band transition and the free exciton recombination [17–20]. The near band edge emission of 375 nm was believably due to the band-to-band transition since the corresponding energy is about 3.3 eV, which was close to the band gap of ZnO thin films. Another UV emission at 380 nm was considered to be due to the free exciton recombination in ZnO as discussed [28]. We believe that the probability of the bonding between zinc and oxygen atoms and/or ions was increased and an improvement in the crystallinity of ZnO thin film could be achieved by annealing the films at higher temperature, all of which can be confirmed by the XRD spectrum shown in Fig. 2. Nevertheless, some zinc or oxygen ions are still existed as interstitials in ZnO film, potentially acting as non-radiative recombination centers which could reduce the emission intensity. Therefore, as the excited electrons return from the conduction to valence band, they could temporarily land on the exciton state with an energy level little bit lower than the bottom of conduction band before making another transition down to the valence band. As a result, the emission of exciton recombination is also UV-related.


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Fig. 3 Surface morphologies of YZO films annealed at a 600°C, b 700°C, c 800°C, d 900°C, and e ZnO film annealed at 900°C. (The scale bar is 500 nm)

In fact, other report also documented about the visible light emission with a similar process [29], from which the authors indicated that the O/Zn ratio of as-grown sample preheated at 275°C was 1.044 (oxygen-rich), indicating the post-annealing might generate more defects which could lead to the visible light emission. In this study, the O/Zn ratio of ZnO thin film annealed at 900°C is approximately equalled to 1, indicating a stoichiometric thin film is obtained and capable of emitting radiation in pure UV regime. Based on the analysis above, the strong UV emission at 380 nm most likely is the direct consequence of the free exciton recombination and the diminution of non-radiative centers. Furthermore, the intensity of UV emission at the same wavelength of 380 nm of YZO thin film is nearly 10 times stronger compared with that of ZnO

thin film, as shown in Fig. 6. Therefore, the bandgap of YZO film is 3.26 eV, as well as the ZnO thin film. It is clearly explained that the doping concentration of yttrium do not influenced by the variation of annealing temperatures. As the doping of yttrium is implemented, more electrons could be released and this in turn could also reduce the generation of zinc interstitial. Furthermore, the dissolved yttrium ions may diminish diffusion coefficients of Zn and/or O in the ZnO lattice and the grain size of YZO becomes limited, as discussed earlier. The reduced grain size somehow formed a texture surface morphology of YZO film, which can enhance the UV emission. However, the crystallinity of YZO thin film appears to be better compared with the others. As reported earlier [30, 31], the improvement in crystallinity potentially could

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Fig. 4 EDS analysis of YZO thin films annealed at various temperatures

Fig. 6 The comparison of UV emission intensity of ZnO and YZO thin films annealed at 900°C

4 Conclusions In summary, ZnO:Y thin films were prepared by sol–gel technique at room temperature and annealed at various temperatures from 600 to 900°C. The intensity of UV emission centered at 375 and 380 nm is enhanced by increasing the annealing temperature, which is consistent with a trend of increase in the grain size of ZnO:Y films. The emission peak at 375 nm was due to the band-to-band transition. The excitons were considered playing a dominant role for UV peak emission. A rather strong UV emission of YZO thin film may be jointly caused by several important factors including the release of more electrons and the generation of more excitons, and also the unique texture structure formed by the separated grains of YZO film. Fig. 5 PL spectra of YZO films annealed at various temperatures

reduce the likelihood of the non-radiation recombination, thereby increasing the probability of the radiation recombination of the carriers and facilitating the existence of the free exciton. According to the discussion above, the strong UV emission of YZO film obtained after annealing at high temperature may be caused by the improvement of film crystallinity, the increase in the number of free excitons and the better texture structure, which could all be obtained by reducing grain size shown in the surface.

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