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J. Phys. Chem. B 1999, 103, 4264-4268
Structural Order of Nanocrystalline ZnO Films Nguyen H. Tran, Andreas J. Hartmann, and Robert N. Lamb* Surface Science and Technology, School of Chemistry, UniVersity of New South Wales, Sydney, NSW 2052, Australia ReceiVed: September 8, 1998; In Final Form: March 3, 1999
Long- and short-range structural order of nanocrystalline ZnO films grown using single source chemical vapor deposition (SS-CVD) of basic zinc acetate Zn4O(CH3COO)6 have been investigated. The addition of a low background pressure of H2O during SS-CVD provides a mechanism for varying the structural properties through control of impurities occluded within the film. The structure of the resultant films can be varied from amorphous (10 at. % impurities) to highly crystalline with preferred c-axis orientation (less than 1 at. % impurities). Transmission electron microscopy, X-ray diffraction, and electron spectroscopy measurements have been used to investigate the structure of the film grains and to explain the observed correlation between impurity concentration and size of the film crystallites (3-40 nm). The results of extended X-ray absorption fine structure studies indicate that the orientation of the crystallites (random or c-axis) is dependent on the atomic short-range structural properties. Self-texturing, an intrinsic property of ZnO, is inhibited for high defect densities.
I. Introduction Ordered nanocrystalline ZnO films exhibit important piezoand optoelectric properties.1,2 The evolution of structure in such films is critical. We have previously discussed the growth of ZnO films using single source chemical vapor deposition (SSCVD) of basic zinc acetate (BZA).3-5 In this high vacuum and relatively low-energy deposition process, the BZA decomposes to form ZnO. An important feature of this work was the addition of a low concentration of H2O in the background during decomposition. This results in enhanced crystallinity and orientation of the resultant films.3 It is the emphasis of the present work to elucidate the correlation between film topography and structure beyond the short-range order limitations of X-ray diffraction (XRD). In particular, we are concerned with the structure of the film grains which are not necessarily associated with single crystallites. To this end, we used extended X-ray absorption fine structure (EXAFS) for short-range structural studies and transmission electron microscopy (TEM) for film topography and long-range order grain structure. Together, these should also allow rapid optimization of film growth parameters for structural device quality films by providing an indication of “evolution” of film structure prior to XRD measurement. II. Experimental Section The single source CVD precursor (basic zinc acetate) was sublimed in a high vacuum deposition chamber (base pressure ∼10-8 mbar) using a modified Knudsen cell.3 The cell temperature was adjusted so that the partial pressure arising from the sublimation of the precursor was approximately 1 × 10-5 mbar. The precursor was deposited onto heated (450 °C) Si(100) substrates. Film growth was performed using different water ambient pressure conditions (1 × 10-3, 10-4, 10-5 mbar). To provide optimal growth conditions,3 the Knudsen cell and * Corresponding author. Fax: 61 2 9662 1697. E-mail: r.lamb@unsw. edu.au.
substrate temperature were kept constant at 200 and 450 °C, respectively. Details of the chemistry involved in the decomposition of basic zinc acetate were discussed previously.3,4 XRD scans in the θ-2θ mode were performed using a Siemens D-500 Kristalloflex diffractometer with unmonochromated Cu KR radiation. TEM micrographs on cross-sectioned samples were performed using a Philips EM430 electron microscope operated at 300 kV. The EXAFS data were collected at the Australian National Beamline Facility (ANBF, BL-20B at the Photon Factory, Tsukuba, Japan).6 The Zn 1s EXAFS spectra were obtained by measuring the fluorescence yield in air. III. Results and Discussion Figure 1 shows XRD patterns for ZnO films which were grown using different H2O partial pressure conditions, while all other film growth parameters (e.g., substrate and source temperatures) remained unchanged. For comparison, the XRD scan for ZnO powder is also shown. For a water ambient pressure of 10-3 mbar only the (002) reflection is detectable, which indicates that the hexagonal film is c-axis oriented. The average crystallite size was calculated from the full width at half-maximum of this (002) reflection using the Debye-Scherrer method.7 This averaged 40 nm (c-dimension) for 10-3 mbar H2O and was reduced to 15 nm for 10-4 mbar. Below this, no XRD peaks were visible. Figure 2 shows TEM cross-sectioned micrographs of these ZnO films. The bright-field and the corresponding dark-field micrographs show the film topographies and grain structures. Due to the diffraction contrast, bright areas in the dark-field micrographs correspond to single crystalline areas8 which have diffraction reflections within the aperture of the microscope (if the aperture of the microscope is moved slightly, other areas appear bright). The micrograph taken from the c-axis oriented film (Figure 2a, 10-3 mbar H2O) shows that the film consists of columnar grains with a length of approximately 300 nm. In particular, the dark-field micrographs show that one column
10.1021/jp9836426 CCC: $18.00 © 1999 American Chemical Society Published on Web 05/04/1999
Structural Order of Nanocrystalline ZnO Films
Figure 1. XRD patterns for ZnO powder and ZnO films grown at different H2O ambient pressures.
cannot be associated with only one single crystal but is composed by a number of them. As indicated by XRD, all crystallites within a grain have c-axis orientation, but the TEM results show that they may have different a,b orientations (otherwise the entire grain would have the same brightness in the dark-field micrograph). The crystallites within the columnar grains have a size up to 150 nm with the average size being ∼40 nm which is in agreement with the XRD results discussed above. The randomly oriented polycrystalline film (Figure 2b, 10-4 mbar H2O) has similar topography. There is some indication for the formation of columns, but as indicated by XRD, the crystallites have no preferred orientation. The average size of the crystallites is roughly in agreement with the value derived from XRD (15 nm). The film shown in Figure 2c is amorphous in XRD terms (see Figure 1), but the micrograph shows randomly oriented crystallites not in columnar arrangement with an average size of approximately 3 nm. As discussed previously, the role of H2O ambient pressure during film growth and BZA decomposition is two-fold: it favors the formation of volatile reaction byproducts,3 and it is also required as an additional oxygen source for the minimization of oxygen vacancies in the resulting ZnO film.4 X-ray photoemission spectroscopy (XPS) studies have confirmed that the carbon concentration in the films and therefore the amount of nonvolatile reaction byproducts is systematically dependent on the water ambient pressure.3 For the films studied here, quantitative XPS analysis indicated that the carbon content increased steadily from approximately 1 to 10 at. % as the water pressure was minimized (starting with 10-3 mbar). TEM micrographs together with XPS quantification results indicate that the size of the crystallites is correlated with the amount of reaction byproducts occluded within the films. The nonvolatile hydrocarbon molecules, which form during the precursor decomposition, are larger than the zinc or oxygen ions, and therefore, they are probably at interstitial lattice positions. During the growth of the crystallites, they diffuse possibly toward the surface (similar observations have been made previously with large impurity molecules in ZnO).9 As these reaction byproducts are nonvolatile, they are accumulated at
J. Phys. Chem. B, Vol. 103, No. 21, 1999 4265 the surface and form a layer which (i) acts as a sandwich-layer between the underlying crystallite and subsequently adsorbed ZnO molecules and (ii) reduces the mobility of the ZnO molecules on the surface of the crystallite (hydrocarbon molecules themselves might also act as nucleation centers). This would suggest that above a certain surface coverage with reaction byproducts the growth of the underlying crystallite is inhibited and new ZnO crystallites would form. For large crystals, the surface-to-volume ratio is smaller than for small crystals. Thus, in the case of high water ambient pressure, where the concentration of reaction byproducts is lower, the resulting crystallites grow larger as then the surface coverage with reaction byproducts is associated with more volume of a ZnO crystallite. Films grown at 10-3 mbar water ambient pressure are preferred c-axis oriented. This can be explained in terms of the low surface free energy of the (001) plane which leads to selftexturing.10 As discussed above, the columns of c-axis oriented films are composed by a number of small crystallites, all of which have preferred orientation even though they are separated by grain-boundary layers. In contrast, for lower water ambient pressure, no preferred orientation of the crystallites was observed. It is possible that the random orientation of these smaller crystallites is related to crystallographic deviations which could inhibit self-texturing. We employed EXAFS to investigate the influence of the H2O partial pressure on the short-range order of the crystallites. Figure 3 shows the Zn 1s EXAFS functions χ(k) for ZnO films, which were grown using different water ambient pressure conditions, and for ZnO powder. The EXAFS function was extracted from the raw data using standard procedures.11,12 The corresponding radial distribution functions (RDF), shown in Figure 4 (solid lines), were obtained by Fourier transforming the χ(k) function for the k range 2-10 Å-1. Phase corrections were not taken into account at this stage. Figure 5 shows a schematic of the ZnO unit cell. The main peaks in the curves shown in Figure 4 are centered at approximately 1.5 and 2.4 Å and are related to the nearest oxygen and zinc neighbors (first and second shell), respectively, while the following smaller peaks are related to further oxygen and zinc neighbors. The FEFF5 code13,14 was used to calculate the EXAFS model structure for intrinsic ZnO (including phase shifts), and local structural parameters were obtained using established fitting parameters.15,16 The fitted curves are shown in Figure 4 (doted lines). The data were fitted for the range 0-3.5 Å which corresponds to the first and second shells. For ZnO powder, the fitting result for the Debye -Waller factor (σ2), a measure for static and dynamic disorder, was 0.001 Å2 for the first shell and 0.007 Å2 for the second shell, respectively, which is in agreement with references.17,18 As the temperature remained unchanged for all measurements, the dynamic disorder component of the Debye-Waller factor is also expected to remain unchanged for all film measurements. To minimize the fitting parameters, the static component of the Debye-Waller factor was also kept constant. It was assumed that the probable increase of static disorder of the ZnO thin films (compared with ZnO powder) could be expressed in terms of a reduced number of nearest neighbors at intrinsic positions. Therefore, however, the fitted number of nearest neighbors cannot be associated with the true number of nearest neighbors but is a measure for both the density of vacancies and differences of static disorder. As a second and alternative approach, we fitted the Debye-Waller factor and kept the number of nearest neighbors constant.
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Figure 2. Bright-field and dark-field TEM micrographs of ZnO films grown at different H2O ambient pressures.
The fitting results indicated that for all thin film samples the average nearest Zn- O and Zn-Zn atomic distances are 1.97
( 0.05 Å and 3.25 ( 0.07 Å, respectively, which is, within the error of the experiment, the value expected for bulk ZnO.15 For
Structural Order of Nanocrystalline ZnO Films
Figure 3. Zn 1s EXAFS functions χ(k) for ZnO powder and ZnO films grown at different H2O ambient pressures.
Figure 4. Zn 1s EXAFS radial distribution function (solid lines) and fitted spectra (dotted lines) for ZnO powder and ZnO films grown at different H2O ambient pressures. Phase shifts Φ(k) and backscattering amplitudes F(k) were calculated using the FEFF5 code,13,14 and the best-fit value for the amplitude reduction factor S02(k) was 0.98. The edge energy shift parameter ∆E0 was between -1 and -9 eV for all films. The Debye-Waller factor σ2 was 0.001 Å2 and 0.007 Å2 for the first and second shells, respectively (see text for details).
intrinsic ZnO of infinite extension, the number of nearest oxygen neighbors (N1) of every zinc ion is 4 and the number of nearest zinc neighbors (N2) is 12 (see Figure 5). In the case of the crystallites being free from any defects but of finite size, the average number of nearest neighbors is directly related to the size of the crystallite as atoms at the crystal boundaries have only a fraction of the nearest neighbor number compared with atoms in the bulk of the material. The ratio of the number of atoms at the boundaries to atoms in the bulk is decreasing for increasing crystal size. This size dependency of the average
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Figure 5. Wurtzite (hexagonal) structure of the ZnO unit cell. Every Zn atom is surrounded by 4 oxygen atoms (1st shell) and 12 zinc atoms (2nd shell).
Figure 6. Calculated curves showing the dependency of the average number of nearest neighbors on the size of perfect ZnO single crystallites.
number of nearest zinc neighbors has been used to calculate crystallite size as function of N1 or N2 (for simplicity, the shapes of the film crystallites were assumed to be spherical), and the results are shown in Figure 6. These data were then used to estimate the average number of nearest neighbors associated with the different sizes of the crystallites in the thin film samples as derived from TEM and XRD (see above). The results are shown in Figure 7 (top) compared with the number of nearest neighbors as estimated from the EXAFS fitting procedure (Figure 7, bottom). The differences between the top and the bottom curves are a measure for extrinsic crystallographic deviations. For the film which was grown using the highest water ambient pressure, N1 and N2 are approximately 50% smaller than expected for intrinsic ZnO. (In our alternative approach which involved the fitting of σ2 using N1) 4 and N2
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Tran et al. associated increase of disorder, expected for the incorporation of the impurity molecules into the lattice, was not observed. This local structural observation coincides with the change of the crystallographic long-range order; for 10-3 mbar H2O pressure the films had preferred c-axis orientation, whereas for 10-4 mbar and below the grain orientation was random. The growth of c-axis oriented ZnO films was explained in terms of the low surface free energy for the (001) plane 10 which is determined by the atomic arrangement within this plane. The density of the surface free energies of the low index planes is 0.099, 0.123, and 0.209 eV/Å2 for the (001), (110), and (101) planes, respectively.10 For crystallites with a large density of vacancies and defects the charge distribution within the crystallographic planes at the surfaces of the crystallites would be quite different from that of intrinsic ZnO. In particular, this would suggest that the surface free energy of the (001) surface planes of the crystallites is not necessarily always smaller than the surface free energy associated with other planes. Consequently, self-texturing would be inhibited and the crystallites would have random orientation.
Figure 7. Variation of the number of nearest neighbors for ZnO films grown at different H2O ambient pressures. The top curves show the theoretical results which were calculated using the data shown in Figure 6 and crystallite sizes estimated from XRD and TEM. The bottom curves represent the EXAFS fitting results.
) 12, the fitted values for σ2 increased to 0.005 and 0.01 Å2 for the first and second shell, respectively.) This can be explained in terms of a large number of oxygen vacancies which ZnO tends to grow with and the associated static disorder which may also be a result of strain or stress between the large grains of the film. For the remaining films, the values of N1 and N2 are significantly smaller again; N1 is reduced to approximately 35%, and N2 is reduced to approximately 20% (or alternatively, for N1 ) 4 and N2 ) 12, σ2 increased further to 0.01 and to 0.02 Å2, respectively). It is interesting to note that once the water pressure is below 10-3 mbar the average number of nearest neighbors and disorder remain virtually unchanged. Oxygen originating from H2O is expected to fill the vacancies.4 Figure 7 indicates that the density of vacancies and disorder increases abruptly when the H2O partial pressure was decreased from 10-3 to 10-4 mbar but then remained steadily for lower H2O partial pressures. This would indicate that oxygen vacancies by far outnumber the amount of oxygen provided by the decomposition. The size of the crystalline areas as derived from TEM and XRD (40, 15, and 3 nm) and the impurity concentration (see above), however, are still highly dependent on the H2O partial pressure (10-3 mbar, 10-4 mbar, and none) which would suggest that a larger proportion of H2O is required to fill the oxygen vacancies than for the decomposition of the precursor. Zinc desorption from the surface of the crystallites is also an expected source of defects.9 The latter would cause more defects for smaller crystals as the relative number of surface atoms is larger for smaller crystallites. However, as below 10-4 mbar H2O partial pressure the static disorder and the relative number of vacancies appear to be independent of the size of the crystallites, this can only be a minor effect. This observation also confirms that the nonvolatile reaction byproducts are diffusing to the surface of the crystallites (as assumed in the above discussion). As indicated by XPS,3 the carbon concentration of the films increased significantly if the H2O ambient pressure was reduced below 10-4 mbar, but an
IV. Conclusion Structural variations of nanocrystalline ZnO films have been investigated using the low-energy SS-CVD approach. Through the variation of a chemical film growth parameter, the H2O partial pressure, both grain size and grain orientation can be altered. Self-texturing is an intrinsic property of ZnO, and it results in the formation of c-axis oriented film grains which are composed of a number of single crystallites. However, local extrinsic structural properties, such as atomic vacancies, can be controlled by the film growth chemistry and a large number of atomic defects inhibits self-texturing. Local structural properties therefore have significant influence on the long-range order of the ZnO films. Acknowledgment. The authors thank D. McCulloch for the TEM analysis, G.J. Foran for supporting the EXAFS experiments, and G. L. Mar for stimulating discussions. References and Notes (1) Van De Pol, F. C. M. Ceram. Bull. 1990, 69 (12), 1959. (2) Koch, M.; Timbrell, P. Y.; Lamb, R. N. Semiconductor Sci. Technol. 1995, 10, 1523. (3) Mar, G. L.; Timbrell, P. Y.; Lamb, R. N. Chem. Mater. 1995, 7 (10), 1890. (4) Koch, M. H.; Hartmann, A. J.; Lamb, R. N.; Neuber, M.; Grunze M. J. Phys. Chem. B 1997, 101 (41), 8231. (5) Mar, L. G.; Timbrell, P. Y.; Lamb, R. N. Thin Solid Films 1993, 223, 341. (6) Foran, G. J.; Cookson, D. J.; Garrett, R. F. In Synchrotron Radiadion Facilities in Asia; Ohta, T., Kikuta, S., Eds.; Ionics Publishing: Tokyo, 1994; p 119. (7) Culity, B. D. Elements of X-ray Diffraction, 2nd ed.; AddisonWesley: Reading, MA, 1978; p 102. (8) Fryer, J. R. The Chemical Applications of Transmission Electron Spectroscopy; Academic Press: London, New York, San Francisco, 1979. (9) Watari, T.; Bradt, R. C. J. Ceram. Soc. Jpn. 1993, 101, 1056. (10) Fujimura, N.; Nishihara, T.; Goto, S.; Xu, J.; Ito, T. J. Cryst. Growth 1993, 130, 269. (11) Stern, E. A.; Sayers, D. E.; Lytle, F. W. Phys. ReV. B 1975, 11, 4836. (12) Teo, B. K. In EXAFS Spectroscopy; Teo, B. K., Joy, D. C., Eds.; Plenum Press: New York, London, 1981. (13) Rehr, J. J.; Mustre de Leon, J.; Zabinsky, S. I.; Albers, R. C. J. Am. Chem. Soc. 1991, 113, 5135. (14) Mustre de Leon, J.; Rehr, J. J.; Zabinsky, S. I.; Albers, R. C. Phys. ReV. B 1991, 44, 4146. (15) Sabine, T. M.; Hogg, S. Acta Crystallogr. 1969, B52, 2254. (16) Abrahams, S. C.; Bernstein, J. L. Acta Crystallogr. 1969, B25, 1233. (17) Yoshiasa, A.; Koto, K.; Maeda, H.; Ishii, T. Jpn. J. Appl. Phys. 1997, 36, 781. (18) Yoshiasa, A.; Maeda, H.; Ishii, T.; Emura, S.; Moriga, T.; Koto, K. Solid State Ionics 1995, 78, 31.
