DOI: 10.1021/cg1010693
Control of the Microstructure and Crystalline Orientation of ZnO Films on a Seed-free Glass Substrate by Using a Spin-Spray Method
2010, Vol. 10 4968–4975
Hajime Wagata,*,† Naoki Ohashi,‡ Takaaki Taniguchi,† Ken-ichi Katsumata,† Kiyoshi Okada,† and Nobuhiro Matsushita*,† †
Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Midori, Nagatsuta, Yokohama, Japan, and ‡National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Received August 16, 2010; Revised Manuscript Received September 24, 2010
ABSTRACT: ZnO films on nonseeded substrates with controllable microstructures and crystal orientations were fabricated with a spin-spray method employing trisodium citrate as an additive. The microstructure of the films was made to change from a rod array to a dense film by increasing the trisodium citrate concentration in the solution. This change in structure was accompanied by a weakening of the 002 peak in the XRD pattern. After postdeposition heating, the ZnO rod arrays showed a decrease in visible luminescence, whereas dense ZnO films showed sharp UV luminescence and a weak broad visible luminescence, thus demonstrating a difference in the defect structure of the samples. We tuned the microstructures of ZnO films on nonseeded substrates by using the spin-spray method and analyzed the formation mechanism.
Introduction Zinc oxide (ZnO), a wide band gap (3.37 eV) semiconductor, has potential applications in many fields because of its outstanding optical and electronic properties.1 These properties strongly depend on the nano-/microstructure of ZnO, and hence, precise structural control is necessary for practical applications. ZnO films with porous structures such as rods, wires, and plates are useful for dye-sensitized solar cells,2-4 hydrophilic/hydrophobic surface coatings,5,6 ultraviolet light emitting devices,7,8 and piezoelectric nanogenerators,9,10 while those with dense structures are applicable to transparent electrodes,11,12 thin film transistors,13,14 and piezoelectric devices.15,16 Epitaxial techniques17-19 are considered to be promising ways to make ZnO films for optoelectronic applications. Moreover, solution-deposition techniques utilizing aqueous chemistry, such as hydrothermal,20,21 electrochemical,22,23 and chemical bath deposition (CBD),24,25 have recently attracted attention because of their low process temperature, lowenvironmental load, and cost-effectiveness. ZnO films prepared in solution processes often have porous structures. Since ZnO has a hexagonal lattice with polarity, it tends to crystallize in hexagonal plates or needle-like shapes, and this habit causes the films to have low density. Although a porous structure is useful for applications requiring large surface area, it is less useful for other applications. Moreover, most of the solution-deposition techniques for making ZnO films require seed layers such as metals,26,27 metal oxides (generally ZnO itself),5,28,29 metal hydroxides,30 or surface functional organic groups.21,31 Consequently, the synthesis process is expensive and time-consuming. What is needed is a simple seed-free synthetic route that makes it easy to control the nano-/microstructure. In this study, we examined seed-free deposition of ZnO films by using a spin-spray technique to control the microstructure *To whom correspondence should be addressed. Telephone: þ81-045924-5369. Fax: þ81-045-924-5358 E-mail addresses:
[email protected]. ac.jp (H.W.);
[email protected] (N.M.). pubs.acs.org/crystal
Published on Web 10/07/2010
and crystalline orientation. The spin-spray process itself was described in our previous report.32 This process has been used for syntheses of ferrite films at low temperature ( 200 cm-1, a bidentate chelate has Δ < 110 cm-1, and bridging ligands have intermediate values of Δ ranging between 140 and 200 cm-1.47,48 Since the observed Δ for the ZnO cit 10 mM sample was 190 cm-1, the ligand configuration in that sample is assignable to the bridging configuration. These results are consistent with our speculation that citrate acts as a surfactant inhibiting growth of ZnO on the (0001) facet and is responsible for the polycrystalline nature of the resultant films. The peaks for v(CO), vs(COO-), and vas(COO-) were diminished after a heat treatment at 400 °C, indicating that the organics remaining in ZnO cit 10 mM were completely decomposed in the heat treatment. Optical Transmittance of As-Prepared Films. Figure 6 shows the optical transmittance spectra of the films prepared from different citrate concentrations: ZnO cit 0, 0.50, 2.0, and 10 mM. All spectra showed adsorption edges around 380 nm, which corresponds to the optical band gap of ZnO. The film with a rod array structure (ZnO cit 0 mM) had very low transmittance due to scattering. In contrast, the films grown from solutions with relatively high citrate concentrations (ZnO cit 0.50, 2.0, and 10 mM) were transparent in the visible region. Higher citrate concentrations resulted in higher film transmittances. In particular, the transmittance of ZnO cit 10 mM was 68-85% in the visible range. These phenomena are attributed to the change of the microstructure. The low transmittance of ZnO cit 0 mM was ascribed to light scattering of sparse needle arrays with gaps; the dense and continuous structures with smooth surfaces had less scattering. Note that the appearance of an interference
Figure 7. Photoluminescence spectra of ZnO films: (a) as-prepared and annealed ZnO cit 0 mM, (b) as-prepared and annealed ZnO cit 0 mM, and (c) as-prepared ZnO cit 0 mM and ZnO cit 10 mM before and after UV-irradiation for 5 h.
fringe pattern for ZnO cit 10 mM is evidence that this sample had a relatively smooth surface and a uniform film thickness. Film Quality Evaluation Using Photoluminescence. Figure 7 shows typical PL spectra of the as-deposited ZnO cit 0 mM and 10 mM. Also shown in the PL spectra is the effect of the postdeposition treatments. Note that the discontinuity at 600 nm was due to the spectrometer without a quantum efficiency correction. The as-deposited and annealed ZnO cit 0 mM had relatively intense visible luminescence, whereas their band-edge luminescence in the near UV region was considerably weaker than the visible luminescence. The thermal treatment caused a slight increase in the band-edge luminescence intensity, and this was accompanied by a red-shift and a suppression of the visible luminescence.
Article
ZnO cit 10 mM showed a totally different behavior from that of ZnO cit 0 mM. The as-prepared ZnO cit 10 mM had neither band-edge nor visible luminescence, but it had bandedge luminescence after the thermal treatment (Figure 7b). Note that UV irradiation also enhanced the band-edge luminescence, similar to the effect of thermal treatment. On the other hand, the PL spectra of ZnO cit 0 mM showed no obvious change before and after UV irradiation. These results indicated that the citrates added to the reaction solution caused a change in the recombination process of the excited carriers. Our tentative explanation for the effect of the UV irradiation treatment is that ZnO worked as a photocatalyst to remove residual organic compounds in the as-deposited film and this catalytic activity acted like a thermal treatment. The UV irradiation did not affect the luminescence properties of ZnO cit 0 mM because organic compounds were not present in ZnO cit 0 mM. In principle, the band-edge emission intensity is a measure of film quality because the band-edge luminescence originates from recombination of free or bound excitons.1,49,50 Visible emissions are also a measure to determine whether deep centers (causing visible emission) are present or not. Visible emissions have been associated with oxygen vacancies,51 interstitial Zn ions,52 cationic impurities,53,54 and damaged crystals.55,56 Therefore, increased band-edge luminescence associated with suppression of visible luminescence is evidence for the removal of recombination centers by the thermal treatment. Here, we should not conclude that ZnO cit 10 mM after the postdeposition treatment had a lower total defect concentration compared to ZnO cit 0 mM, although its visible luminescence was much less intense than that from ZnO cit 0 mM. ZnO cit 0 mM had a higher integrated luminescence intensity, and the band-edge luminescence from ZnO cit 10 mM was very weak in comparison with the luminescence from bulk ZnO or epitaxial ZnO film grown by physical deposition.38,57 These results suggest that the concentration of nonradiative recombination centers in ZnO cit 0 mM was lower than that in ZnO cit 10 mM. We will need to conduct further investigations before we can make a final conclusion about the total defect concentration in the films. At present, we do not have sufficient results to discuss the effect of the microstructure on the luminescence properties. A high-density film with a relatively smooth surface was achieved by adding trisodium citrate to the reaction solution, but it caused the film to have a polycrystalline structure. If the grain boundaries, i.e., the incoherency of the crystalline lattice, lower the luminescence efficiency, ZnO cit 10 mM with a microstructure composed of many small grains may not be suitable for applications requiring bright luminescence. Film Quality Evaluation Using Thermal Desorption Spectroscopy (TDS). Figure 8 shows the TDS spectra of ZnO cit 0 mM and 10 mM. The plots are for mass fragments (m/z) of 2, 18, and 44 corresponding to H2, H2O, and CO2 molecules, respectively. The amount of desorption from ZnO cit 10 mM was much larger than that from ZnO cit 0 mM. H2 started to desorb around 100 °C, and the desorption reached a peak at 275 °C. Another huge desorption began at 325 °C (Figure 8a). H2O also started to desorb at about 100 °C. There were several peaks around 175, 250, and 325 °C, but the desorption finished around 350 °C (Figure 8b). These peaks may correspond to water and/or hydroxyls removed from the film or decomposition of zinc hydroxide. Since no zinc hydroxide reflections were apparent in the XRD patterns, the H2O desorption seems to be due to water or hydroxyl groups from the films. Since the desorption peaks
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Figure 8. TDS spectra of ZnO cit 0 mM and 10 mM: (a) H2, (b) H2O, and (c) CO2.
for H2 at 275 °C and above 350 °C do not correspond to the desorption peak of H2O, it is presumable that the hydrogen desorption was not due to zinc hydroxide decomposition (eq 1) but due to removal of protons from the ZnO lattice (eq 2). 1 ð1Þ ZnðOHÞ2 f ZnO þ H2 v þ O2 v 2 1 ZnO : H f ZnO þ H2 v 2
ð2Þ
The CO2 desorption, starting at 225 °C, only occurred in the films prepared with trisodium citrate in the reaction solution (Figure 8c). The corresponding CO2 desorption peak was in accordance with the FT-IR results showing incorporation of citric groups in the film. The FT-IR spectrum of ZnO cit 0 mM included the vibration modes of the
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CO2 molecule, but it contained no evidence of CO2 desorption. This means that the CO2 desorption observed in the TDS measurements is not due to desorption of CO2 molecules from the film’s surface but due to decomposition of citric groups. Since the water desorption at 300-400 °C seemed to correspond to the CO2 desorption, it is presumable that the water desorption in this range is related to thermal decomposition of citric groups. Previous studies31,58,59 indicated that hydrogen, hydroxyl groups, or water have important effects on ZnO’s luminescence properties. For example, Saito et al.31 reported the effect of hydrogen or water desorption on the luminescence properties of ZnO films prepared by electroless deposition: they also found that TDS peaks for m/z = 18 corresponding to desorption of H2O indicated an enhancement of bandedge luminescence and that TDS peaks for m/z = 2 corresponding to the desorption of H2 indicated suppression of UV luminescence. Since Ohashi et al.60,61 has seen evidence that active recombination centers are passivated by inserting hydrogen into the ZnO lattice, it is presumable that hydrogen impurities are a cause of the enhanced luminescence of ZnO. In the present study, desorption of H2 and H2O was observed in both samples, although the amount of desorption from the sample prepared with trisodium citrate was much more than that in the one prepared without citrate. The behavior of ZnO cit 0 mM was rather similar to the previously reported behavior, if we assume that H2 desorption during the thermal treatment removed the active recombination centers. Since hydrogen can passivate acceptors causing yellow luminescence,57 the red-shift of the visible luminescence of ZnO cit 0 mM and the slight lowering of its intensity during the thermal treatment is likely due to the removal of hydrogen, as reported in the literature.31 In contrast, the thermal treatment obviously removed hydrogen from ZnO cit 10 mM, and it enhanced rather than suppressed the bandedge luminescence. This effect is reasonable because the H2 and H2O desorption from ZnO cit 10 mM was mostly due to citric groups. Summary ZnO films were prepared by using a spin-spray method adding trisodium citrate to the starting solution. The microstructure of the films depended on the citrate concentration. Increasing the citrate concentration resulted in a dense polycrystalline ZnO film with a smooth surface. The XRD results showed that crystal growth along the c-axis was inhibited by a high citrate concentration. The TEM observation revealed that one-column particles consisted of nanocrystals (∼10 nm) in the film/substrate interface region and that the crystalline size increased to about 50 nm as the film thickness increased. The FT-IR spectra showed that citric groups remained in the films but they could be removed by thermal treatment at 400 °C. These results imply that adsorption of citric ions to the growing grains modified the growth mechanism of ZnO. Postdeposition treatment with heat or UV light decomposed the remaining organics in the film, and as a result, the ZnO films prepared with citrate additives showed relatively strong UV emission and weak visible emission. This implies that the addition of citrate to the reaction solution changes not only the morphology but also the defect structure of the resultant films. Acknowledgment. We gratefully acknowledge the help of Prof. Yoshitake Kitamoto for his training on the FIB technique
Wagata et al.
of preparing the samples for the TEM measurements, Mr. Akira Genseki and Mr. Katsuaki Hori (Center for Advanced Materials Analysis, Tokyo Institute of Technology) for making the TEM observations, and Mr. Yuichi Higashikawa and Mr. Kunimitsu Maejima (ESCO, Ltd., Japan) for conducting the TDS analysis. This work was supported by the Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists (JSPS Research Fellow 22-7316). Supporting Information Available: FFT images, SEM images, XRD patterns, and photographs of films. This material is available free of charge via the Internet at http://pubs.acs.org.
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